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Diss. ETH No. 14992

Shaping Ceramics in Small Scale – from Microcomponents to Gas Sensors A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH for the degree of DOCTOR OF NATURAL SCIENCES

presented by MARTIN HEULE Dipl. Chem. ETH born November 2, 1973 citizen of Zurich and Widnau (SG), Switzerland

accepted on the recommendation of Prof. Dr. Ludwig J. Gauckler, examiner Dr. Nicolae Barsan, co-examiner Zurich, 2003

In loving memory of my mother, Margrit Heule-Fuchs 1947-2001

5

Acknowledgements

I am very grateful to my advisor, Prof. Dr. Ludwig J. Gauckler, for the confidence he placed in me and his valuable guidance throughout the duration of the thesis. By sending us PhD students around the world early on in our projects, we have numerous opportunities to meet the important researchers in the field. In addition, I would like to thank Dr. Nicolae Barsan from the University of Tübingen (Germany) for all his advice on gas sensing, his spontaneous offer to perform joint experiments and for accepting to be co-examiner. Without external technical support, this project would not have been possible. I am very much indebted to Dr. Stefan Blunier and Prof. Dr. Jürg Dual from the Institute of Mechanical Systems IMES not only for allowing me to use their clean room equipment continuously, but also for giving me true microfabrication support. Dr. Daniel Bächi and Dr. Nicolas Szita taught me how to operate in a clean room in the best possible manner. Membrane-based microdevices would not work without silicon nitride layers of outstanding quality. These were coated for me by Dr. B. Ketterer and F. Glaus from PSI Villigen. A very cordial ‘thank you’ goes to all my collegues, especially Sibylle Vuillemin and Urs Schönholzer, who together with me constituted the powerful micropatterning group at Nonmetallic Materials. I thank the people who shared their office with me, Nicholas Grundy, Bengt Hallstedt and Beate Balzer, Lorenz Meier for extensive and refreshing discussions among chemists, the people working on SOFC for sharing electrochemical knowhow and equipment, Michael Jörger, Christoph Kleinlogel, Anja Bieberle, Eva Jud, Dainius Perednis, Michel Prestat, Brandon Bürgler, Jennifer Rupp for her assistance concerning the Tübingen-measurements, Kurosch Rezwan for helping in the enzyme-microreactor project, Hans Wyss for Hg-porosimetry measurements and finally Peter Kocher for his high-precision workshop pieces.

6 Luana Cavalli was courageous enough to perform her diploma thesis exploring one of my fancy ideas. Thanks for pushing it through! I was also very lucky to have numerous undergraduate students choosing my topic for their semester projects: Michael Werner, Lukas Pfister, Pascal Jud, Roger Bachmann, Julia Schell, Beatrice Sutter (all ETHZ), Lauren Ellery (Corpus Christi College, Cambridge, UK) and Nathan Yoder (Purdue University, West Lafayette, IN, USA). In addition, there were many people supporting my work, I would like to recognise in particular: • Roger Michel for continuous discussion and scientific exchange about soft lithography and help with ToF-SIMS measurements. • Didier Falconnet, Prof. Dr. Marcus Textor, Laboratory of Surface Science and Technology, ETHZ Schlieren also for ToF-SIMS measurements. • Prof. Dr. Nicholas Spencer, LSST, for allowing us to sneak constantly inside his laboratories in order to use the plasma cleaner and the contact angle measurement setup. • Dr. Alexander Gurlo for spending almost one week of his precious time for joint measurements in Tübingen. • Dr. Thomas Bürgi (Technical Chemistry) for access to their e-beam evaporation machine with which early versions of the gas sensors were coated with Pt and SiO2. • Hans-Ruedi Scherrer and his apprentices (Physics Dept Workshop) for coating some microhotplate wafers with excellent Pt-layers. • Anna Mezzacasa and Prof. Dr. Ari Helenius (Institute of Biochemistry, ETHZ), for letting me alienate their DNA-injecting equipment for the selective doping of microceramics. • Yves Kaufmann for helping me to correct this thesis. My family have supported me in all my decisions and never failed to support me in need. This is an excellent opportunity to say ‘thank you for all’! Finally, I would like to thank Mirjam Holderegger for all her constant support and patience during the sometimes awkward writing process.

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Contents Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.

General Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.1. Miniaturisation Science and Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.2. Photolithography and IC industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.3. Soft Lithography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 1.4. Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 1.5. Metal Oxide Gas Sensors on Micromachined Substrates . . . . . . . . . . . . 33 1.6. Aim of the Study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 1.7. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.

Overview on Powder-based Ceramic Meso- and Microscale Fabrication Processes . . . . . . . . . . . . . . . . . . . . . . . 47 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 2.2. Direct writing methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 2.3. Downsizing Mechanical Processing Methods . . . . . . . . . . . . . . . . . . . . . . 54 2.4. Lithography-based Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 2.5. Self-Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 2.6. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 2.7. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.

Powder-Based Microcomponents on Silicon Substrates fabricated by Micromolding in Capillaries . . . . . . . . . . . . . 69

4.

Patterning Colloidal Suspensions by Selective Wetting of Microcontact-Printed Surfaces . . . . . . . . . . . . . . . 85

5.

Casting of Suspensions into Photoresist Structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

10

CONTENTS

6.

Miniaturised Enzyme Reactor Based on Hierarchically Shaped Porous Ceramic Microstruts . . . . . . . . . . . . . . 109

7.

Gas Sensors Fabricated from Ceramic Suspensions by Micromolding in Capillaries . . . . . . . . . . . . . . . . . . . . . 121

8.

Miniaturised Tin Oxide Gas Sensors on Microhotplates by Micromolding in Capillaries. . . . . . . . . . . . . . . 133

9.

Increasing the Integration Density by Vertically Separating the Heater of the Microhotplate Design . . . . . . . . . . . . . 149

10.

Validating the Concept of Miniaturising Resistive-type Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . 157

11.

Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

A.

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

A.1. Microsystem Design Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.2. Improved Clean Room Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.3. Thermal Characterisation of Microhotplates . . . . . . . . . . . . . . . . . . . . . A.4. Powder Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.5. ToF-SIMS Spectra of Microcontact-Printed Surfaces . . . . . . . . . . . . . . A.6. List of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.7. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

171 181 189 199 204 211 213

Curriculum Vitae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

11

S

ummary

Thin film ceramic layers are an integral part of microsystems technology. However, colloidal dispersions as used in classical ceramic powder processing have not been considered as building blocks for microdevices until recently. Ceramics have several advantages over other materials also in microsystems, e.g. heat resistance, hardness, corrosion resistivity or functional properties. The purpose of this interdisciplinary thesis is to integrate colloidal systems into microfabrication technology. As a main result, the prototype fabrication of miniaturized semiconducting gas sensor arrays based on tin oxide powders is demonstrated. A single gas sensor is barely visible by the human eye and covers only 10 by 30 µm2 which is two orders of magnitude smaller than most of today’s microsensor designs. The miniaturized sensors could be integrated on state-of-the-art microhotplate substrates based on silicon micromachining. The techniques which make possible the direct use of a colloidal dispersion for generating functional ceramic microstructures are derived from soft lithography and photolithography. The backbone of the thesis is a series of articles. It is organised as follows: Chapter 1 contains general introductions to microfabrication technologies as they are used in this work, soft lithography and the current state in the field of solid state gas sensing devices. In Chapter 2, a review article deals with processing techniques that possibly allow for the introduction of powder-based ceramic components into microelectromechanical systems. In the first experimental part in Chapter 3, techniques from soft lithography are

12

SUMMARY

adapted to colloidal systems. The technique which is mainly used is micromolding in capillaries (MIMIC). Formed in polydimethylsiloxane elastomers, microchannels served as molds which then were spontaneously filled with suspensions of 0.1 - 40% solids loading owing to capillary forces. The resulting micro-thickfilm structures have a height of several micrometers and therefore differ from usual ceramic thin film coatings. By adjusting only the solid content of the suspensions, even smaller lines of 1 - 2 µm width could easily be prepared. The other important soft lithography scheme, microcontact printing (µCP), is discussed in Chapter 4. Micropatterns of ceramic powders can be obtained by selective wetting of microcontact-printed surfaces. Aqueous colloidal dispersions adhered only to the hydrophilic micropatterns whereas they were repelled by the hydrophobic surroundings in a simple dip coating process. Printing and selective wetting were carried out successfully on two different ink/substrate combinations with a resolution of 5 µm. The third microfabrication approach is photoresist casting (PRC, Chapter 5). Ceramic microstructures with cross sectional areas of 5 by 10 µm2 were obtained. By using low-cost mask lithography, the PRC procedure could be tested in a microfluidic application, as presented in Chapter 6. A microfluidic continuous flow enzyme reactor consisting of aluminum oxide microstruts mounted inside PDMS channels was built. Owing to the microdesign of the struts, a two-fold hierarchical structure was obtained. First, the placement of the struts enforces mixing due to the multiple splitting of the laminar flow and subdivides the main channel into many 15 µm wide channels. Second, the struts consist of loosely sintered particle networks exhibiting a well defined porosity of 60 nm. Compared to a microchannel system without these struts, a five-fold increase in enzymatic product formation was obtained. The second part of the thesis beginning with Chapter 7 deals exclusively with gas sensing applications. Miniaturized tin oxide gas sensors on bulk substrates were fabricated and characterised first. The MIMIC procedure was integrated in a sequence of conventional photolithography steps. Miniaturized gas sensors on sapphire single crystal substrates showed responses for 100 ppm of hydrogen and 600 ppm for carbon monoxide obtained from an active sensing area of only 10 by 40 µm2. In Chapter 8, miniaturized gas sen-

SUMMARY

sors are fabricated on microhotplates which consume only mW heating power. Additionally, the concept of placing one gas sensor on one microhotplate is extended towards a gas sensor array consisting of several sensors on one hotplate. Twelve miniaturised gas sensors of nanoparticulate tin oxide were integrated as an array on a single microhotplate by using micromolding in capillaries. The integration density of gas sensors on microhotplates was even more increased, as discussed in Chapter 9. The heater structures were buried in a more complex microfabrication scheme using 5 masks. Processing steps to prepare a 20 sensor array on microhotplates are presented and discussed with regard to processing sequence, sensitivity to 1000 to 1500 ppm hydrogen and power consumption. Finally, the concept of miniaturising semiconducting gas sensors could be tested and validated in collaboration with the Institute of Physical and Theoretical Chemistry of the University of Tübingen (IPTC). Gas sensors were prepared with MIMIC-microlines of In2O3 powder on substrates from IPTC. It is possible to miniaturize semiconducting gas sensors without affecting the basic sensitivity of the sensors.

Fig. 1. Ceramic microline structures on a microhotplate compared with a

human hair lying across.

13

14

SUMMARY

Zusammenfassung

Dünne keramische Filme sind integrale Bestandteile in den Mikrofabrikationstechnologien. Hingegen wurden kolloidale Dispersionen, wie sie in der klassischen keramischen Pulververarbeitung verwendet werden, bisher nicht als mögliche Ausgangsmaterialien für Mikrosystemkomponenten erachtet. Keramik bietet auch in Mikrosystemen diverse Vorteile gegenüber anderen Materialien, z.B. Hochtemperaturbeständigkeit, Härte, Korrosionsbeständigkeit oder diverse funktionelle Eigenschaften. Diese interdisziplinäre Arbeit befasst sich mit der Integration von Kolloidalen Suspensionen in die Mikrofabrikationstechnologie. Das wichtigste Resultat dieser Arbeit besteht in der Entwicklung von miniaturisierten Gassensor-Arrays, welche aus halbleitendem Zinnoxidpulver hergestellt wurden. Ein einzelner Gassensor ist kaum noch sichtbar für das menschliche Auge und benötigt nur eine Fläche von 10 mal 30 µm2, was zwei Grössenordnungen kleiner ist als die der meisten modernen Mikrosensoren. Die miniaturisierten Sensoren wurden erfolgreich auf modernen, mikrosystemtechnisch hergestellten Microhotplates integriert. Soft lithography und Photolithographie sind Techniken, die die direkte Erzeugung von Mikrostrukturen aus kolloidalen Systemen erlauben. Das Rückgrat dieser Arbeit bildet eine Reihe von Veröffentlichungen. Sie sind wie folgt organisiert: Kapitel 1 enthält generelle Einführungen zu Mikrofabrikationstechniken, wie sie verwendet wurden, zu Soft Lithography und zu halbleitenden Gassensoren. Im nächsten Kapitel 2 werden in einem Übersichtsartikel alternative

SUMMARY

Prozesse besprochen, die pulver-basierte Komponenten in der Mikrosystemtechnologie ermöglichen. Im ersten experimentellen Kapitel 3 wird Micromolding in Capillaries (MIMIC) behandelt. In poly-dimethylsiloxan geformte Mikrokanäle können spontan mit Hilfe der Kapillarkraft mit Suspensionen gefüllt werden, welche Volumenanteile von 0.1 - 40% aufwiesen. Die resultierenden Mikrostrukturen haben eine Höhe von einigen µm und unterscheiden sich daher wesentlich von herkömmlichen keramischen Dünnfilmbeschichtungen. Noch dünnere Linien von 1 - 2 µm Breite können mit denselben Mikrokanälen durch das Anpassen des Volumengehalts der Suspension erzeugt werden. Das andere wichtige Verfahren aus der Soft Lithography ist eine einfache Stempeltechnik, das Microcontact Printing (Kapitel 4). Mikrostrukturen aus keramischem Pulver wurden durch selektives Benetzen von gestempelten Oberflächen erzeugt. Eine wasserbasierte kolloidale Dispersion kann nur hydrophile Mikrostrukturen benetzen, während sie von hydrophoben Flächen abperlt. Der Stempelprozess mit anschliessendem Beschichten mit Suspension konnte mit zwei Tinte-Substrat Kombinationen erfolgreich mit einer Auflösung von 5 µm durchgeführt werden. Der dritte Ansatz zur Mikrostrukturierung ist das Füllen von Photolackstrukturen, Photoresist Casting genannt (Kapitel 5). Mikrolinien mit Querschnitten von 5 mal 10 µm2 wurden erzeugt. Mit dieser Technik wird in Kapitel 6 eine erste Anwendung demonstriert. Ein Mikro-Enzymreaktor mit kleinsten Strukturen aus Aluminiumoxid in einem PDMS-Kanal wurde gebaut. Dieser Aufbau hat eine zweifache Hierarchische Struktur: Erstens sorgt die versetzte Platzierung der keramischen Strukturen für eine erzwungene Durchmischung der laminar fliessenden Substratlösung und unterteilte den Hauptkanal in viele kleine, 5-15 µm breite Subkanäle. Zweitens bestehen die Mikroelemente aus locker gesinterten Partikeln, welche eine gut definierte Porengrösse von 60 nm aufweisen. Verglichen mit einem Kanalsystem ohne diese Mikrokeramiken, hat dieser Enzymreaktor eine fünffach höhere Umsatzrate. Der zweite Teil der Arbeit befasst sich ausschliesslich mit der Anwendung dieser Mikrokomponenten als Gassensoren. In Kapitel 7 wird die Herstellung von miniaturisierten Zinnoxidsensoren auf Saphirsubstraten beschrieben. Mittels MIMIC wurde ein Zinnoxid Sensorarray hergestellt. MIMIC

15

16

SUMMARY

wurde in eine Sequenz von konventionellen Photolithographieschritten integriert. Die miniaturisierten Gassensoren mit einer aktiven Fläche von nur 10 mal 40 µm2 zeigten deutliche Signale für 100 ppm Wasserstoff und für 600 ppm Kohlenmonoxid. Im folgenden Kapitel 8 wurden die miniaturisierten Gassensoren auf mikrofabrizierten, heizbaren Substraten, sogenannten Microhotplates platziert. Zwölf miniaturisierte Gassensoren aus SnO2 Nanopulver wurden mit MIMIC auf einem Microhotplate als Array integriert. Kapitel 9 zeigt, wie die Integrationsdichte von Gassensoren durch die vertikale Trennung von Heizstrukturen und Messelektroden weiter erhöht werden kann. In Zusammenarbeit mit dem Institut für Physikalische und Theoretische Chemie der Universität Tübingen (IPTC) bot sich die Gelegenheit, die vorgestellten Miniaturisierungskonzepte zu validieren (Kapitel 10). Gassensoren mit MIMIC-Linien aus In2O3 wurden auf IPTC-eigenen Substraten hergestellt und ausgemessen. Damit konnte gezeigt werden, dass die gassensitiven Eigenschaften trotz Miniaturisierung erhalten bleiben.

Abb. 1. Keramische Mikrolinien auf einem Microhotplate im Vergleich zu

einem schräg darüberliegenden menschlichen Haar.

17

1.

General Introduction

1.1. Miniaturisation Science and Technology As miniaturisation of electronic circuits progresses, other disciplines follow and new areas of application arise. Based on established microfabrication processes from the IC industry, systems generally called Microelectromechanical Systems (MEMS) have been developed.[1,2] MEMS are tiny sytems that are not ‘only’ able to calculate or to store data but to move, which constitutes the classical MEMS area, to sense and to manipulate light,[3,4] to send and to receive,[5,6] to detect and to dispense chemicals in liquid[7] and gaseous form. As this list of abilities indicates, the course towards small and intelligent autonomous systems for a variety of purposes is set. Examples of more complex systems are a micromachined scanning confocal optical microscope[8] or the Digital Micromirror Device (DMD) projection chip by Texas Instruments.[9,10] Up to 1.3 million movable micromirrors (1280 x 1024 pixel, SXGA format), each 16 µm square, are integrated on one CMOS chip. By tilting each mirror individually, a light beam can be switched on and off. These DMD-chips are successfully used for projection display applications. The sheer reduction of size is not the only rationale of device miniaturisation. Other properties like volume (~ l3), surface (~ l2), diffusion (~ l0.5) or time (~ l0) scale differently with respect to length scale l. Thus, one can make

18

CHAPTER 1

not only smaller but more powerful devices. To illustrate this, the micromotors fabricated by Sandia National Laboratories rotate at 500’000 rpm - a car engine rotates at a few thousand rpm.[11] The fabrication methods of MEMS allow a massive parallel production of multiple devices in one processing step. Owing to energy-intensive cleaning steps and expensive clean room facilities, MEMS processes in general are much more expensive than conventional manufacturing. However, the mass production of MEMS devices can bring down the cost of a single microdevice considerably, in certain cases down to only a few cents per device. Since the number of available microsystems is not a limiting factor, the distribution of MEMS functionality to the single user becomes possible. Medical analysis procedures can be made considerably faster by using microfluidic chips, and physicians are sometimes enabled to carry out an analysis themselves rather than having to send blood or tissue to medical laboratories. This development would not be possible without the use of MEMS technology. 1.2. Photolithography and IC industry The basic process in microfabrication is photolithography. Its origins date back to 1822 when Nicéphore Niépce transferred an image to a glass plate covered with a mixture of lavender oil and bitumen (asphalt).[12] He placed an oil-painted paper on top and exposed the plate to the sun. After a few hours, the illuminated areas were hardened and the unexposed areas could be dissolved in terpentine/lavender oil solution. This was the first application of a process later to be called ‘negative’ photolithography. Today, photolithography is the basis of all electronic processes of the IC industry. With some delay, photolithography has also been used for the fabrication of MEMS. For further reference, the book Fundamentals of Microfabrication - the Science of Miniaturisation by Marc Madou gives an excellent overview on all MEMS-relevant processes.[13] It has been used for reference throughout this work. MEMS and electronics, although based on almost identical processes, are virtually separated. The CMOS[14] (Complementary Metal Oxide Semiconductor) and VLSI (Very Large Scale Integrated Circuit) technologies producing the current generation of 90 nm chip layouts[15] are sophisticated to such an

INTRODUCTION

extent that no deviations from the predefined standard processing routes are possible. Additionally, there is Moore’s law first stated in 1965 predicting the progress of miniaturisation in electronics.[16] Moore claimed that it would be possible to increase the integration density in ICs by a factor of two every year. So, the industry is struggling to keep up with that pace. Intel Corp. expects to produce 45 nm circuits in 2007.[17] The website of Intel provides a very good source for current information on the state-of-the-art in the IC industry. Another series of review articles concerning the future of electronics have been published in Nature.[18-21] Whereas the IC industry has long switched to exposing photoresists with contactless direct projection optics,[22] the MEMS community predominantly uses photolithography with hard masks for exposure. Fig. 1–1 displays the basic photolithography processing as it was used also in this work. Positive resists are normally based on novolak resin mixed with a diazoquinone photosensitizer,[23-25] negative resists on crosslinking epoxide resins. It is commercially available as SU-8.[26]

19

20

CHAPTER 1

expose through patterned Cr/glass mask by UV illumination spin coat photoresist

Cr

silicon SiO2/Si3N4 or similar layer

a) additive

Lift-off

b) subtractive

Etching

Surface/Bulk Micromachining

Fig. 1–1. Basic photolithography. After exposing a photoresist with UV-

light through a patterned mask (Cr metal patterned on a glass plate), the image is developed by dissolving the unexposed resist areas (negative resist, not shown), respectively the exposed areas (positive resist). For further processing, there are two approaches. a) lift-off process: Additive process by depositing another layer of material on top, subsequent removal of the resist and thus releasing the patterned material. b) surface micromachining: subtractive approach by dry or wet chemical etching in which the photoresist protects areas from the etchant. Furthermore, the underlying layer, e.g. SiO2 may serve as additional mask for deep etching into silicon, called bulk micromachining.

INTRODUCTION

1.3. Soft Lithography Soft lithography is the enabling micropatterning technology for the direct shaping of colloidal dispersions in this work. G. M. Whitesides proposed soft lithography as low cost alternative to standard photolithography as early as 1993.[27-30] The principal element is an elastomeric, transparent silicone polymer for microstructure transfer. A commercially available elastomer is polydimethylsiloxane (PDMS) which was initially proposed by the inventors. It was also used throughout this study. Patterned PDMS molds are prepared by casting a prepolymer onto a master structure and then crosslinking it. Finally, it can be peeled off the master and becomes a negative replica of the master topography (Fig. 1–2a). Masters can be obtained from various sources such as conventional photoresist structures and they can be reused several times without the necessity for clean room equipment. This is a very important issue, since it opens the door for microfabrication also to researchers without clean room access. There were several variations of microstructure transfer presented (Fig. 1– 2b) in a ground-breaking review in Angew. Chem.[31] The focus is on a stamping method called microcontact printing (µCP). A PDMS mold is simply used as a stamp to transfer ink molecules onto a surface. An enormous range of ink molecules are applicable, which makes it a true cross-disciplinary micropatterning technique. Applications range from transferring simple SAM-forming molecules to catalysts[32] and proteins or living cells.[33,34] In Micromolding in Capillaries (MIMIC), a PDMS mold is placed upside down on a substrate. The microstructures are cut open at the side, where a droplet of liquid can be applied for spontaneous filling of capillaries.[35] This method was primarily aimed at patterning organic polymers, but can be extended to other liquids, provided that they wet the PDMS capillaries and have a sufficiently low viscosity. The other two approaches are less known. Microtransfer molding (µTM) is the analogue to gravure printing, where the recessed structures are first filled with an ink material which is then transferred to the substrate. A solvent-soaked PDMS mold is placed on a solid layer of soluble polymer in Sol-

21

22

CHAPTER 1

vent Assisted Micromolding (SAMIM). The mold sinks down into the liquefied layer, thereby shaping the surface into a replica of the master by filling the recessed microcavities.

a) PDMS mold preparation CH3 O

Si

CH3

Master structure

n

Cure PDMS prepolymer

Demold PDMS

b) pattern Transfer

Micromolding in Capillaries (MIMIC)

Microcontact Printing (µCP)

Micro-transfer molding (µTM)

Solvent-assisted Micromolding (SAMIM)

Fig. 1–2. Soft lithography with its variations. a) preparation of PDMS molds

by cross-linking PDMS prepolymer over a master structure. b) main procedures for microstructuring liquid materials.

INTRODUCTION

At this point, the properties of PDMS are introduced in order to demonstrate its versatility in soft lithography. Apart from the commercial PDMS formulation that has become a de facto standard in soft lithography (Sylgard 184, Dow Corning), Schmid et al. prepared other blends using different molecular weights and different cross-linking molecules. They were thus able to optimize the surface hardness and Young’s modulus for nanometer resolution µCP.[36] Sylgard 184 was used throughout this work. The curing chemistry and components of Sylgard 184 are shown in Fig. 1–3. The main component is a prepolymer of 18.500 D with vinyl terminal groups. A hydrosilane/dimethylsiloxane (684 D) copolymer is the cross-linking agent. The resulting elastomer has interesting properties. The most important is the spontaneous wetting of surfaces, i.e. the PDMS stamp spontaneously establishes a conformal contact to the surface. The work of adhesion to glass and gold was determined to be 0.1 J/m2 and 0.5 J/m2 respectively.[37] This property allows to seal the microcapillaries in MIMIC effectively even on substrates exhibiting a certain roughness, which is key to many applications. After structure transfer, the molds can be easily removed in a similar manner like the well known Post-It notes. In addition, PDMS is biocompatible and permeable for gases which allows for patterning of living cells. By oxygen plasma treatment, the surface can be functionalized with hydroxyl functions, rendering it hydrophilic or enabling the coupling of other molecules. Two plasma treated molds can be joined irreversibly by just pressing them together. All these properties make it extremely versatile not only in soft lithography but also as rapid prototyping material in microfluidic applications: Ismagilov et al. for example prepared complex-shaped microfluidic valves of PDMS.[38]

23

24

CHAPTER 1

O Si

O

cat.

n = 250

PDMS prepolymer

Si

O

O O

Si

n

+ Pt

Si

Si

H O Si

O Si 5

Si O Si

Si O

Si

5

Hydrosilane cross-linker

O Si

O Si

O Si 5

5

Cross-link formed by hydrosilylation reaction

Fig. 1–3. Components of Sylgard 184 PDMS. Cross-linking is achieved by a

Pt-catalyzed hydrosylilation reaction.

1.4. Gas Sensors The term “sensor” is used very often, in everyday life as well as in science and technology. If a definition of “sensor” is sought, one can choose from a variety of differing suggestions. One concise suggestion for a definition states: “The sensor is the primary part of a measuring chain which converts the input variable into a signal suitable for measurement.”[39] Without including or ruling out too many technicalities, this definition reflects the principal function of a sensor, namely to convert an input variable such as temperature, pressure or in the case of chemical sensors, the presence of a substance into a measurable signal. Typically, this is an electrical signal which can be processed electronically. There is much discussion whether other parts in that measurement chain like signal amplification or conditioning etc. should also be considered as a part of the sensor or not. For referring to the central part physically responsible for signal conversion solely, the term “transducer” is also used frequently.[40] There are a lot of complementary and competing gas sensor technologies relying on different physical/chemical principles. See the book of Madou and Morrison[41] or the Sensors series from VCH[42] for further reference. A brief summary is given in Table 1–1.

INTRODUCTION

Table 1–1. Various devices suitable for gas sensing. Adapted from [42] and

from other sources where indicated. Class

Gas Sensor

Detection Principle

Solid State Sensors[41]

Semiconducting

Surface-catalytic combustion/interaction changes semiconductor electronic states, resp. conductivity

Potentiometric

Nernst-potential on an electrode system, one electrode interacts with analyte

ChemFET[43]

Field effect transistors with a gate material interacting with the analyte. I,V-curves become chemically sensitive

Amperometric

Diffusion limited current of an ionic conductor

Calorimetric

Heat of analyte reaction, e.g. catalytic combustion

Optodes

IR, UV-VIS absorption spectroscopy

Fluorescence

Fluorescence excitation or quenching

SPR, Surface Plasmon Resonance

Interaction of an analyte layer with an evanescent field

Optical Sensors[42]

... Mass Sensitive Devices[42]

Acoustic[44] Quartz Microbalance QMB, etc.

SAW, Surface-acoustic waves or BAW (bulk acoustic waves) excited on a quartz or piezoelectric substrate change in phase/frequency upon ad- or absorption of analyte in a suitable sorption layer (e.g. metals, polymers)

MEMS sensors

Micro- or nanocantilevers bend mechanically upon adsorption of analyte[45-48]

25

26

CHAPTER 1

Table 1–1. Various devices suitable for gas sensing. Adapted from [42] and

from other sources where indicated. Class

Gas Sensor

Detection Principle

Polymerbased[49,50]

QMB, mass sensors

A polymeric sorption layer increases in mass when analytes absorb

Calorimetric

Heat of solution is measured when analytes absorb into polymer layer

For this work, semiconducting metal oxide sensors for measuring reducible gases were miniaturised using a novel combination of standard and innovative microfabrication processes. In 1962, Seiyama et al. first presented the discovery that semiconducting thin films of ZnO changed their electrical resistance (signal) as a function of varying gas concentrations (input variable). This simple device was employed as a novel detector for gas chromatography.[51] The layer had to be heated to 200°C and higher for maximum sensitivity. In the subsequent research, the focus has been drawn on SnO2, which proved to be one of the most sensitive, in the case of oxidizing gases down to the sub-ppm range[52] and versatile semiconductors for gas sensing.[53] Other semiconducting metal oxides have also been used as summarized in the examples of Table 1–2. The list is not complete as there is an ongoing search for novel gas sensitive materials.[54] The fields of possible practical applications becomes apparent by looking at the analyte gases in Table 1–2. Gas leakage detection in homes and in industrial facilities (H2, CH4, etc.), fire warning systems (CO), environmental monitoring (NOx), in automotive applications: cabin air quality monitoring (VOC, hydrocarbons), just to mention the most important ones. More demanding applications are still a domain of standard methods in analytical chemistry, e.g. gas chromatography and mass spectrometry.

INTRODUCTION

Table 1–2. Examples of Metal oxides used in conductivity type gas

sensors for different analyte gases. Metal Oxide

Analyte Gas(es)

Reference

SnO2

CO, H2, CnH2n+2

[53]

WO3

NOx, O3

[55]

In2O3

CO (“selective”)

[56]

CdO-Fe2O3

C2H5OH

[57]

TiO2

CO and NO2

[58]

TiO2-WO3

CH3OH

[59]

WO3-Bi2O3

NOx

[60]

ZnGaO4

Hydrocarbon mixtures (liquid petroleum gas)

[61]

Al-Fe2O3

CO, CH4

[62]

1.4.1. Tin Oxide as Gas Sensing Material The electrical resistance of SnO2 reacts to reducing species, e.g. CO, H2, Hydrocarbons, Ethanol and some oxidising gases like NOx and O3. At first glance, tin oxide seems to be extremely versatile. However, the main problem is selectivity. The reducing gases mentioned cause a decrease in electrical resistance, the oxidating ones an increasing resistance which makes it difficult to discern between different molecules with the same sign of electrochemical potential.[63] There are two main approaches for selectivity improvement: to exploit the temperature-dependence[64-67] of the sensitivity of different molecules and to dope the tin oxide with different catalytically active metals, mostly Pd,[66] Pt, In, Cu,[68,69] Nb, Mn[70] or Si.[71] Again, these are only a few successful examples as there are many researchers more or less aimlessly trying to find new additives for improving metal oxide gas sensing. There have also been examples of mixing different metal oxides, like mixing TiO2

27

28

CHAPTER 1

and SnO2 for butane sensing.[72] The selectivity for small molecules can be enhanced by depositing a diffusion filter layer such as SiO2 on top of the semiconductor.[73] Another more recent strategy was to come to terms with the selectivity problem of semiconducting gas sensors.[74-81] Biological olfactory systems inspired the idea of combining the signals of several imperfect sensors. In human and mammalian noses, there are thousands of taste buds with bad selectivity to different odors, but the brain is capable to derive a specific odor identification by “processing” the signals from all taste buds. The technological analogue is hence often called “electronic nose” and it is tried to mimick the natural olfactory process, see Fig. 1–4. An array of sensors with different functionality is employed and their data are processed by statistical mathematics[82] or neural networks that are used to predict partial pressures of gas mixtures.[83] Massart and Vanderginste wrote a mile-stone textbook about these multivariate mathematical methods, also called chemometrics.[84] It is important to note that only those species can be classified by electronic noses that produce a useful signal in at least one sensor of the array. Therefore, the sensitivity range of all the sensors in the array defines the application range. Classification of odors and taste becomes possible which is important for food industries. Additionally, electronic nose systems have to be trained and calibrated, e.g. the array needs to have measured the head space of “fresh” odor from fruit before being able to classify fruit with unknown freshness. Currently, the systems are far behind the natural capabilities of e.g. a dog’s nose. One of the pioneers in the field is Lundström who used arrays of MOSFET (Metal Oxide Field Effect Transistors).[43,85] Hong et al. used a micromachined 4-sensor array of metal oxide sensing layers for detecting CO and EtOH mixtures.[86] Complex resistance data were evaluated as well.[87,88] There has also been a potentiometric approach presented by Reinhard, a multielectrode setup on a ZrO2 ionic conductor.[89]

INTRODUCTION

Highlighted cut through a tongue's taste bud. ("array" of several receptor cells)

Neural tissue

Multiple olfactory cells each with limited capability

Neural preprocessing

Brain: derives overall impression of smell / taste.

Array of sensor elements

Data storage, matrix representation, statistical representation

Pattern recognition PCA Neuronal Network

Fig. 1–4. Electronic noses as a biomimetic approach.

1.4.2. Mechanisms of Gas Sensitivity An understanding of the gas sensing mechanism on the molecular level is extremely difficult to obtain. There are few analytical methods that offer sufficient sensitivity for identifying molecules on hot surfaces or methods that offer the time resolution to capture reaction intermediates and kinetic data.[90] Under real sensing conditions there is already a complex gas mixture, air, humidity and the analyte gas. Therefore, most measurements were done under better defined laboratory conditions, but lack to include the various additional effects from water. Therefore, the current mechanistic picture still is very sketchy and lacks portability to real world gas sensors.

29

30

CHAPTER 1

H2O oxygen depletion layer

H

water activation

+

OH

carbon monoxide sensing

CO2

CO

O2-

2-

O

OSn4+

bulk oxygen

Fig. 1–5. Schematic of molecular species on a tin oxide surface.

Tin oxide is a semiconductor with a band-gap of 3.6 eV which makes it almost insulating at room temperature. Its semiconducting properties are brought about by oxygen vacancies. As schematically shown in Fig. 1–5, tin oxide crystallizes in the rutile structure. The species noted in print are the principal ones that have been characterised spectroscopically among other, more complicated molecular assemblies by IR and EPR (electron paramagnetic resonance) and by TPD (temperature programmed desorption).[91] A recent feature article by Barsan and Weimar summarizes the current state of knowledge.[92] Tin oxide activates atmospheric oxygen, incorporating it into its surface by reduction and building up a negatively charged layer. This equilibrium begins to establish above 100°C.

β/2 O2,g + α e- + S

O-αβS

(1)

31

INTRODUCTION

where α can take values of 1 or 2 depending on the oxygen reduction state, one or two-fold. β also may have values of 1 for single atom/ionic form and 2 for molecular forms of oxygen. S denotes a free surface site bridging two tin ions and g stands for gaseous. The electron(s) have to cross the negatively charged barrier at the surface layer to reduce the oxygen molecule. In the energy band structure diagram, this depletion layer leads to a bending of states towards the tin oxide surface (see Fig. 1–6). This is also a trapping of electrons and hence the electrical resistance of the topmost layer is increased. The situation is further complicated by the fact that water is similarly activated (the resistance of tin oxide in dry atmosphere is lower than in wet conditions). By measuring the work function of the tin oxide, the effects of CO and water could be characterised independently[93]

H2Og + SnSn + OO

(Sn+Sn - OH-) + (OH)+O + e-

(2)

Indices Sn and O indicate ionic sites of the elements in the rutile lattice. There are at least two different species formed, a “hydroxylated” Sn-ion and a “protonated” oxygen. There may be an additional mechanism consuming a lattice oxygen and creating an oxygen vacancy, two hydroxylated tin species and two electrons instead.

H2Og + 2 SnSn + OO

2 (Sn+Sn - OH-) + V++O + 2 e-

(3)

Processes like this presumably are also involved in ageing and unwanted signal drift effects over longer times.[94,95] An analyte molecule like CO reacts with the chemisorbed oxygen or hydroxyl groups and forms CO2 and electrons that are injected into the conduction band. Macroscopically, a decrease in electrical resistance is measured.

β COg + O-aβS

β CO2,g + α e- + S

(4)

32

CHAPTER 1

IR absorption experiments reveiled the existence of intermediate carbonate species, coordinated in both unidentate or bidentate form. Subsequently, one has to take into account the effects on grain size and features like porosity. In powder-based sensors, it is estimated that the most gas sensitive resistance lies in the necks, where particles are connected. Necks often are very small, in the range of a few atomic layers. By model calculations, different power laws for the electrical conductivity for different configurations were obtained. However, the results are difficult to compare with real gas sensing, since most sensors are doped with transition metals. Pd or Pt induce a high density of new surface states which interact with the atmosphere similar to the properties discussed above. It was shown that Pd and Pt both are oxidized at the tin oxide surface.[96] Among the analytical techniques used were XPS (x-ray photoelectron spectroscopy), Raman spectroscopy and TEM (transmission electron microscopy).[97] Before these results, most people argued that the catalytic enhancing of sensitivity or selectivity relied on the catalytic activation of the analyte molecule by a metallic cluster which then diffuses towards the semiconductor for reaction with the oxygen layer. This diffusion effect is called spill-over effect. It has been directly observed by Bennet et al. using STM microscopy.[98] Additional effects may be induced by the electrode setup[99] for measuring the resistance which are difficult to characterise,[100] the contact resistance or other factors. In conclusion, one can not accurately predict neither which sensitive material nor which sensor setup would be the best for a given application. 1.4.3. Nanoscaled Powders Improve Sensitivity A significant transition is encountered when using tin oxide nanopowders. As the radius r of nanoparticles approaches the depletion layer depth z0, which might be a few atomic layers (see Fig. 1–6), the higher band energy of the surface charge layer becomes dominant throughout the material. The observed resistance is much more dependent on the state of the surface layer, i.e. dependent on the atmosphere. Additionally, the higher specific surface area should increase the sensitivity as well.

INTRODUCTION

E Econduction band 3.6 eV

Ef Evalence band

0

z0 depth from surface z

Fig. 1–6. Energy band diagram for tin oxide surfaces.

As a matter of fact, nanoscaled powders were indeed found to have a large beneficial impact on gas sensing.[101-104] Whether this model explanation is sufficient for explaining all effects from the use of nanoscaled powders and thin films, is largely unknown. 1.4.4. Classic Sensor Designs For standard resistive-type gas sensors, metal oxides are prepared as sintered pellets that have been contacted with Pt wire. Resistance measurement and heating are done using the same electrodes. The predominant design consists of a thin ceramic substrate onto which heating wires and sensing wires of noble metals have been screen-printed. This is the case in so-called Figaro type sensors (Figaro Engineering, Inc., Tokyo, Japan). Whereas they are cheap to produce, they have a significant disadvantage. The power consumption to maintain the substrate and sensing layer at temperatures of 200800°C is rather high, typically a few watts. If the power should be cut to save energy, the times to heat up and stabilize the signal are prohibitingly low. The key for overcoming these problems is miniaturisation. 1.5. Metal Oxide Gas Sensors on Micromachined Substrates The main problem to integrate metal oxide based gas sensors with micromachined substrates are the high operating temperatures. Typical temperatures in the sensing layers are up to 500°C which have to be maintained. Semancik et al. solved this problem in 1993 by embedding thin film heating filaments

33

34

CHAPTER 1

on a freestanding dielectric membrane and introduced the term microhotplates.[105] Their design based on a standard CMOS process which could be important for integration with electronics and for a cost-effective mass production owing to the wide availability of CMOS processes. How far the integration of sensor and electronics has progressed, is impressively demonstrated by Hagleitner et al.[106] Their CMOS-based chip features an array of three different sensors, a capacitive, a calorimetric and a cantilever gas sensor, temperature controller, preamplification, digitalisation of the signals and communication via a standard protocol. The concept for lowering the heating power by orders of magnitude was to minimize the mass to be heated and to thermally insulate it as effectively as possible from the surrounding frame that is to remain at ambient temperature. As membrane materials, thin film Si3N4, SiO2,[107] thin Si by CMOSetch stop processes, even amorphous SiC films[108] or a combination of these are used. Even 250 nm thin Si3N4 membranes exhibit astonishing mechanical strength and a low mass that gives the devices robustness even when exposed to hard mechanical shocks. For the embedded heating filament structures and measuring electrodes, Pt or polysilicon are popular. In more advanced designs, there are also interlayers with good thermal conductivity, e.g. Al or a Si plug on the backside of the membrane to obtain a uniform temperature distribution over the whole microhotplate area.[109] Integrated temperature measurement was also used to determine the changes in temperature of -0.1°C to -4°C of the oxide layer upon gas exposure.[110] A comparison of a standard classic design with the microhotplate developed within this work is shown in Table 1–3, and measured power consumption data are given in Fig. 1–7.

INTRODUCTION

Table 1–3. Comparison of a classic sensor design versus microhotplate

sensors. Classic

Microhotplate

Steinel Hydrogen Sensor

12 Sensor Array M. Heule

Sensing Material

Ga2O3

SnO2

Size of heated element

2.1 · 1.3 · 0.7 mm3

0.9 · 0.9 mm2 , 250 nm

Mass /mg

12.3

0.016

Thermal Mass /JK-1 5.3 · 10-3

3.6 · 10-6

Heating Time constant /s

5-10

0.005

Power consumption /W

Min. 0.6 Max. 1

Min. 0.01 Max. 0.10

The metal oxide layer is typically sputtered or evaporated since these are processes compatible to standard MEMS and CMOS processes.[111-113] But also powders have been deposited, either by droplet coating[107] or by screen printing.[114] The layers are annealed on the chip by the integrated heater in all the referenced examples. Alternatively, the use of laser annealing was suggested by Steiner et al.[115]

35

CHAPTER 1

Temperature /˚C

800 700

Microhotplate no. 1 Microhotplate no. 2 Sapphire Microheater

600

fitted curves, T = a p

b

500 400 300 200 100 0 0.0

0.5

1.0 1.5 Power /W

2.0

2.5

800 700 Temperature /˚C

36

600 500 400 300 200 100 0 0.001

0.01

0.1 power /W

1

Fig. 1–7. Temperatures as a function of power consumption from bulk-sub-

strate sensors and microhotplates.

In terms of micromachining, there are two approaches to generate the freestanding membranes (see Fig. 1–8). The first approach requires a photolithography step on the backside of the wafer to define the membrane openings (Fig. 1–8a). Subsequently, the silicon wafer is etched until the membrane layer on the frontside is released. This etching process through the

INTRODUCTION

whole wafer thickness is usually done using KOH or using deep reactive ion etching (DRIE). In the second approach, the membrane layers themselves are patterned (Fig. 1–8b). Silicon is underetched through the resulting openings until the membrane is released. In that design, the membrane is suspended only by four thin beams. Extensive thermal characterisation data of the microhotplates fabricated within this project are discussed in Appendix A3.

a

b

Side Top

Fig. 1–8. Two microfabrication schemes to generate freestanding mem-

branes for microhotplates. a) backside etching through the whole wafer thickness. b) frontside underetching through openings in the membrane layer. c) microhotplate chips. d) microhotplate glowing red hot at approx. 600 mW.

37

38

CHAPTER 1

1.6. Aim of the Study The goal of this thesis is to explore processing techniques for the fabrication of miniaturized devices featuring tiny ceramic structures with dimensions in the µm range. Materials and methods from microfabrication disciplines should be combined with classical colloidal powder processing of ceramics. As example for possible microdevice applications, the fabrication of a miniaturized array of semiconducting gas sensors was chosen.

INTRODUCTION

1.7. References [1] S. D. Senturia, "Microsystem design", Kluwer Academic, Boston, 2001. [2] A. E. Franke, T.-J. King, R. T. Howe, Integrated MEMS Technologies, MRS Bulletin, 2001, 26, 4, 291-295. [3] C. R. Giles, D. Bishop, V. Aksyuk, MEMS for Light-Wave Networks, MRS Bulletin, 2001, 26, 4, 328-332. [4] N. Clark, A "Silicon Eye" Using MEMS Micromirrors, MRS Bulletin, 2001, 26, 4, 320-324. [5] C. T.-C. Nguyen, L. P. B. Katehi, G. M. Rebeiz, Micromachined Devices for Wireless Communications, Proc. IEEE, 1998, 86, 8, 1756-1768. [6] D. J. Young, Micromachining for rf Communications, MRS Bulletin, 2001, 26, 4, 331-332. [7] D. S. Peterson, T. Rohr, F. Svec, J. M. Frechet, Enzymatic Microreactor-on-a-Chip: Protein Mapping Using Trypsin Immobilized on Porous Polymer Monoliths Molded in Channels of Microfluidic Devices, Anal. Chem., 2002, 74, 4081-4088. [8] D. L. Dickensheets, G. S. Kino, Silicon Micromachined Scanning Confocal Optical Microscope, IEEE J. MEMS, 1998, 7, 1, 38-47. [9] L. J. Hornbeck, The DMD Projection Display Chip: A MEMS Based Technology, MRS Bulletin, 2001, 26, 4, 325-327. [10] P. F. Van Kessel, L. J. Hornbeck, R. E. Meier, M. R. Douglass, A MEMS based Projection Display, Proc. IEEE, 1998, 86, 8, 1687-1704. [11] Sandia National Laboratories, Silicon Micromachines and other MEMS devices, http://mems.sandia.gov, 2002. [12] T. W. Harris, "Chemical Milling", Clarendon Press, Oxford, 1976. [13] M. J. Madou, "Fundamentals of Microfabrication", 2nd ed., CRC Press, Boca Raton, 2002. [14] U. Hilleringmann, "Mikrosystemtechnik auf Silizium", B. G. Teubner, Stuttgart, 1995, 10. [15] Intel Corp., M. Bohr, Presentation at the Intel Developer Forum, Sept 12, http://www.intel.com/research/silicon/Bohr_IDF_0902.pdf, 2002. [16] G. E. Moore, Cramming more components onto integrated circuits, Electronics, 1965, 38, 8. [17] Intel Corp., Microarchitecture and Circuits Website, http://www.intel.com/labs/ microarch, 2002. [18] T. Ito, S. Okazaki, Pushing the Limits of lithography, Nature, 2000, 406, 10271031. [19] A. I. Kingon, J. P. Maria, S. K. Streiffer, Alternative dielectrics to silicon dioxide for memory and logic devices, Nature, 2000, 406, 1032-1038. [20] S. Loyd, Ultimate physical limits to computation, Nature, 2000, 406, 1047-1054. [21] M. H. Devoret, R. J. Schoelkopf, Amplifying quantum signals with the single-electron transistor, Nature, 2000, 406, 1039-1046. [22] R. F. Pease, Imprints offer Moore, Nature, 2002, 417, 6891, 802-803.

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[23] A. Reiser, H. Y. Shih, T. F. Yeh, J. P. Huang, Novolak-Diazoquinone Resists: The Imaging Systems of the Computer Chip, Angew. Chem. Int. Ed., 1996, 35, 24282440. [24] A. Reiser, Z. L. Yan, Y. K. Han, M. S. Kim, Navolak-diazonaphthoquinone resists: The central role of phenolic strings, J. Vac. Sci. Technol. B, 2000, 18, 3, 12881293. [25] A. Reiser, J. P. Huang, X. He, T. F. Yeh, S. Jha, H. Y. Shih, M. S. Kim, Y. K. Han, K. Yan, The molecular mechanism of novolak-diazonaphthoquinone resists, European Polymer Journal, 2002, 38, 619-629. [26] H. Lorenz, M. Despont, N. Fahri, J. Brugger, P. Vettiger, P. Renaud, High aspectratio, ultrathick, negative-tone-near-UV-photoresist and its applications for MEMS, Sens. Actuators A, 1998, 64, 33-39. [27] A. Kumar, G. M. Whitesides, Features of Gold Having Micrometer to Centimeter Dimensions Can Be Formed through a Combination of Stamping with an Elastomeric Stamp and an Alkanethiol Ink Followed by Chemical Etching, Appl. Phys. Lett., 1993, 63, 14, 2002-2004. [28] J. L. Wilbur, A. Kumar, E. Kim, G. M. Whitesides, Microfabrication by Microcontact Printing of Self-Assembled Monolayers, Adv. Mater., 1994, 6, 7-8, 600-604. [29] R. J. Jackman, J. L. Wilbur, G. M. Whitesides, Fabrication of Submicron Features on Curved Substrates by Microcontact Printing, Abstr. Pap. Am. Chem. Soc., 1995, 210, 212. [30] R. J. Jackman, J. L. Wilbur, G. M. Whitesides, Fabrication of Submicrometer Features on Curved Substrates by Microcontact Printing, Science, 1995, 269, 5224, 664-666. [31] Y. Xia, G. M. Whitesides, Soft Lithography, Angew. Chem. Int. Ed., 1998, 37, 550575. [32] P. C. Hidber, W. Helbig, E. Kim, G. M. Whitesides, Microcontact printing of palladium colloids: Micron-scale patterning by electroless deposition of copper, Langmuir, 1996, 12, 5, 1375-1380. [33] R. S. Kane, S. Takayama, E. Ostuni, D. E. Ingber, G. M. Whitesides, Patterning proteins and cells using soft lithography, Biomaterials, 1999, 20, 23-24, 23632376. [34] G. M. Whitesides, E. Ostuni, S. Takayama, X. Y. Jiang, D. E. Ingber, Soft lithography in biology and biochemistry, Annu. Rev. Biomed. Eng., 2001, 3, 335-373. [35] E. Kim, Y. N. Xia, G. M. Whitesides, Polymer Microstructures Formed by Molding in Capillaries, Nature, 1995, 376, 581-584. [36] H. Schmid, B. Michel, Siloxane Polymers for High-Resolution, High-Accuracy Soft Lithography, Macromolecules, 2000, 33, 8, 3042-3049. [37] A. Bietsch, B. Michel, Conformal contact and pattern stability of stamps used for soft lithography, J. Appl. Phys., 2000, 88, 4310-4318. [38] R. F. Ismagilov, D. Rosmarin, P. J. A. Kenis, D. T. Chiu, W. Zhang, H. A. Stone, G. M. Whitesides, Pressure-Driven Laminar Flow in Tangential Microchannels: an Elastomeric Microfluidic Switch, Anal. Chem., 2001, 73, 4682-4687. [39] International Electrotechnical Committee, Terms and Definitions in Industrial Process Measurement and Control, IEC draft 65/84, 1982.

INTRODUCTION

[40] W. Göpel, K. D. Schierbaum, "Chemical and Biochemical Sensors", edited by W. Göpel, J. Hesse, J. N. Zemel, Sensors - A Comprehensive Survey, VCH, Weinheim, 1991, 2. [41] M. Madou, S. R. Morrison, "Chemical Sensing with Solid State Devices", Academic Press, Boston, 1989. [42] "Chemical and Biochemical Sensors", edited by W. Göpel, J. Hesse, J. N. Zemel, Sensors - A Comprehensive Survey, VCH, Weinheim, 1991. [43] T. Eklov, I. Lundstrom, Distributed Sensor System for Quantification of Individual Components in a Multiple Gas Mixture, Anal. Chem., 1999, 71, 3544-3550. [44] A. Hierlemann, A. J. Ricco, K. Bodenhöfer, W. Göpel, Effective Use of Molecular Recognition in Gas Sensing: Results from Acoustic Wave and in Situ FT-IR Measurements, Anal. Chem, 1999, 71, 3022-3035. [45] C. B. Freidhoff, R. M. Young, S. Sriram, T. T. Braggins, T. W. O'Keefe, J. D. Adam, H. C. Nathanson, R. R. A. Syms, T. J. Tate, M. M. Ahmad, S. Taylor, J. Tunstall, Chemical sensing using nonoptical microelectromechanical systems, J. Vac. Sci. Technol. A, 1999, 17, 4, 2300-2307. [46] M. K. Baller, H. P. Lang, J. Fritz, C. Gerber, J. K. Gimzewski, U. Drechsler, H. Rothuizen, M. Despont, P. Vettiger, F. M. Battiston, J. P. Ramseyer, P. Fornaro, E. Meyer, H. J. Guntherodt, A cantilever array-based artificial nose, Ultramicroscopy, 2000, 82, 1-4, 1-9. [47] F. M. Battiston, J. P. Ramseyer, H. P. Lang, M. K. Baller, C. Gerber, J. K. Gimzewski, E. Meyer, H. J. Guntherodt, A chemical sensor based on a microfabricated cantilever array with simultaneous resonance-frequency and bending readout, Sens. Actuator B, 2001, 77, 1-2, 122-131. [48] J. Fritz, M. K. Baller, H. P. Lang, H. Rothuizen, P. Vettiger, E. Meyer, H. J. Guntherodt, C. Gerber, J. K. Gimzewski, Translating biomolecular recognition into nanomechanics, Science, 2000, 288, 5464, 316-318. [49] H. N. McMurray, J. Albadran, Colorimetric and Fluorimetric Polymer Membrane Gas-Sensing Materials, MRS Bulletin, 1999, 24, 6, 55-59. [50] M. Cole, J. W. Gardner, A. W. Y. Lim, P. K. Scivier, J. E. Brignell, Polymeric resistive bridge gas sensor array driven by a standard cell CMOS current drive chip, Sens. Actuators B, 1999, 58, 518-525. [51] T. Seiyama, A. Kato, K. Fujiishi, M. Nagatani, A New Detector for Gaseous Components Using Semiconductive Thin Films, Anal. Chem., 1962, 34, 1502-1503. [52] C. Cobianu, C. Savaniu, A. Arnautu, R. Iorgulescu, D. Dascalu, G. Leo, M. Mazzer, R. Rella, P. Siciliano, S. Capone, L. Vasanelli, A SnO2 microsensor device for sub-ppm NO2 detection, Sens. Actuators B, 1999, 58, 552-555. [53] K. Ihokura, J. Watson, "The Stannic Oxide Gas Sensor", CRC Press, New York, 1994. [54] J. Watson, K. Ihokura, Gas-Sensing Materials, MRS Bulletin, 1999, 24, 6, 14-17. [55] W. Zhang, H. Uchida, T. Katsube, T. Nakatsubo, Y. Nishioka, A novel semiconductor NO gas sensor operating at room temperature, Sens. Actuators B, 1998, 49, 58-62. [56] H. Yamaura, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Selective CO Detection by Using Indium Oxide-based semiconductor gas sensor, J. Electrochem. Soc., 1996, 143, 2, L36.

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[57] X. Liu, Z. Xu, Y. Liu, Y. Shen, A novel high performance ethanol gas sensor based on CdO-Fe2O3 semiconducting materials, Sens. Actuators B, 1998, 52, 270-273. [58] V. Guidi, M. C. Carotta, M. Ferroni, G. Martinelli, L. Paglialonga, E. Comini, G. Sberveglieri, Preparation of nanosized titania thick and thin films as gas sensors, Sens. Actuators B, 1999, 57, 197-200. [59] M. Ferroni, D. Boscarino, E. Comini, D. Gnani, V. Guidi, G. Martinelli, P. Nelli, V. Rigato, G. Sberverglieri, Nanosized thin films of tungsten-titanium mixed oxides as gas sensors, Sens. Actuators B, 1999, 58, 01. Mrz, 289-294. [60] A. A. Tomchenko, Structure and Gas Sensing Properties of WO3-Bi2O3 Mixed Thick-Films, Eurosensors XIII Conference Proceedings, 1999, The Hague, Netherlands, 69. [61] L. Satyanarayana, C. V. G. Reddy, S. V. Manorama, V. J. Rao, Liquid-petroleumgas sensor based on a spinel semiconductor, ZnGaO4, Sens. Actuators B, 1998, 46, 01. Jul. [62] J. S. Han, T. Bredow, D. E. Davey, A. B. Yu, D. E. Mulcahy, The effect of Al addition on the gas sensing properties of Fe2O3 based sensors, Sens. Actuators B, 2001, 75, 18-23. [63] M. Fleischer, H. Meixner, Selectivity in high-temperature operated semiconductor gas-sensors, Sens. Actuators B, 1998, 52, 179-187. [64] N. Barsan, A. Tomescu, The temperature dependence of the response of SnO2based gas sensing layers to O2, CH and CO, Sens. Actuators B, 1995, 26, 3, 45-48. [65] T. Takada, A new method for gas identification using a single semiconductor sensor, Sens. Actuators B, 1998, 52, 45-52. [66] G. Tournier, C. Pijolat, R. Lalauze, B. Patissier, Selective detection of CO and CH4 with gas sensors using SnO2 doped with palladium Pd, Sens. Actuators B, 1995, 26, 1-3, 24-28. [67] M. Jaegle, J. Wöllenstein, T. Meisinger, H. Böttner, G. Müller, T. Becker, C. Bosch-v.Braunmühl, Micromachined thin film SnO2 gas sensors in temperaturepulsed operation mode, Sens. Actuators B, 1999, 57, 130-134. [68] A. Galdikas, A. Mironas, A. Setkus, Copper-doping level effect on sensitivity and selectivity of tin oxide thin-film gas sensor, Sens. Actuators B, 1995, 26, 1-3, 29-32. [69] A. Galdikas, A. Mironas, D. Senuliene, A. Setkus, W. Göpel, K. D. Schierbaum, Copper on-top sputtering induced modification of tin dioxide thin film gas sensors, Sens. Actuators B, 1999, 58, 1-3, 330-337. [70] G. Behr, W. Fliegel, Electrical properties and improvement of the gas sensitivity in multiple-doped SnO2, Sens. Actuators B, 1995, 26, 3, 33-37. [71] E. Comini, G. Faglia, G. Sberveglieri, CO and NO2 response of tin oxide silicon doped thin films, Sens. Actuators B, 2001, 76, 1-3, 270-274. [72] W.-Y. Chung, D.-D. Lee, B.-K. Sohn, Effects of added TiO2 on the characteristics of SnO2 based thick-film gas sensors, Thin Solid Films, 1992, 221, 304-310. [73] C. D. Feng, Y. Shimizu, M. Egashira, Effect of Gas Diffusion Process on Sensing Properties of SnO2 Thin Film Sensors in a SiO2/SnO2 Layer-Built Structure Fabricated by Sol-Gel Process, J. Electrochem. Soc., 1994, 141, 221. [74] A. D'Amico, C. D. Natale, Chemical Portraits of Gasses and Liquids: Trends and Perspectives, Eurosensors XIII Conference Proceedings, 1999, The Hague, Netherlands, 999.

INTRODUCTION

[75] H. Baltes, D. Lange, A. Knoll, The electronic nose in Liliput, IEEE Journal, 1998, 9, 35-38. [76] H. T. Nagle, R. Gutierrez, S. S. Schiffman, The HOW and WHY of electronic noses, IEEE Spectrum, 1998, 9, 22-34. [77] M. Faia, M. A. Pereira, A. M. Nunes, C. S. Furtado, Electronic Noses, a different Approach to the Sensitivity and Selectivity Issues, J. Europ. Ceram. Soc., 1999, 19, 883-886. [78] J. V. Hatfield, A. R. Daniels, D. Snowden, K. C. Persaud, P. A. Payne, Development of a Hand Held Electronic Nose (H2EN), Eurosensors XIII Conference Proceedings, 1999, The Hague, Netherlands, 215. [79] S. M. Reddy, P. A. Payne, Effect of unmodified and derivatised poly(vinyl chloride) overlayers on the response of an electronic nose based on conducting polymers, Sens. Actuators B, 1999, 58, 536-543. [80] F. Winquist, I. Lundström, P. Wide, The combination of an electronic tongue and an electronic nose, Sens. Actuators B, 1999, 58, 512-517. [81] C. Delpha, M. Siadat, M. Lumbreras, An electronic nose using time reduced modelling parameters for a reliable discrimination of Forane 134a, Sens. Actuators B, 2001, 77, 1-2, 517-524. [82] E. Llobet, J. Rubio, X. Vilanova, J. Brezmes, X. Correig, J. W. Gardner, E. L. Hines, Electronic nose simulation tool centred on PSpice, Sens. Actuators B, 2001, 76, 1-3, 419-429. [83] M. A. Martin, J. P. Santos, J. A. Agapito, Application of artificial neural networks to calculate the partial gas concentrations in a mixture, Sens. Actuators B, 2001, 77, 1-2, 468-471. [84] D. L. Massart, B. G. M. Vandeginste, "Chemometrics: a textbook", Elsevier, Amsterdam, 1988. [85] T. Eklov, I. Lundstrom, Gas mixture analysis using a distributed chemical sensor system, Sens. Actuators B, 1999, 57, 274-282. [86] H. K. Hong, H. W. Shin, H. S. Park, D. H. Yun, C. H. Kwon, K. Lee, S. T. Kim, T. Moriizumi, Gas identification using oxide semiconductor gas sensor array and neural-network pattern recognition, Transducers 95, Eurosensors IX, 1995, 687. [87] B. S. Joo, N. J. Choi, Y. S. Lee, J. W. Lim, B. H. Kang, D. D. Lee, Pattern recognition of gas sensor array using characteristics of impedance, Sens. Actuators B, 2001, 77, 1-2, 209-214. [88] D. S. Lee, J. K. Jung, J. W. Lim, J. S. Huh, D. D. Lee, Recognition of volatile organic compounds using SnO2 sensor array and pattern recognition analysis, Sens. Actuators B, 2001, 77, 1-2, 228-236. [89] G. Reinhardt, S. I. Somov, U. Schönauer, U. Guth, W. Göpel, Solid electrolytes for gas sensing at high temperatures. Multielectrode setup to analyze gas mixtures, Transducers, Eurosensors IX, 1995, 799. [90] P. G. Harrison, N. C. Lloyd, W. Daniell, C. Bailey, W. Azalee, Evolution of Microstructure during the Thermal Activation of Chromium-Promoted Tin(IV) Oxide Catalysts: An FT-IR, FT-Raman, XRD, TEM and XANES/EXAFS Study, Chem. Mater., 1999, 11, 896-909.

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[91] N. Barsan, M. Schweizer-Berberich, W. Göpel, Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report, Fresenius J. Anal. Chem., 1999, 365, 287-304. [92] N. Barsan, U. Weimar, Conduction Model of Metal Oxide Gas Sensors, J. Electroceram., 2001, 7, 143-167. [93] N. Barsan, A. Heilig, J. Kappler, U. Weimar, W. Göpel, CO-Water Interaction with Pd-doped SnO2 Gas Sensors: Simultaneous Monitoring of Resistances and Work Functions, Eurosensors XIII Conference Proceedings, 1999, The Hague, Netherlands, 367. [94] R. Ionescu, Ageing and p-type conduction in SnO 2 gas sensors, Sens. Actuators B, 1999, 58, 1-3, 375-379. [95] S. Nicoletti, L. Dori, F. Corticelli, M. Leoni, P. Scardi, Tin Oxide Thin-Film Sensors for Aromatic Hydrocarbons Detection: Effect of Aging Time on Film Microstructure, J. Am. Ceram. Soc., 1999, 82, 1201. [96] A. Dieguez, J. L. Alay, A. Cabot, A. Romano-Rodriguez, J. R. Morante, J. Kappler, N. Barsan, U. Weimar, W. Göpel, Influence on the gas sensor performance of the metal chemical states introduced by impregnation of SnO2 sol-gel nanocrystals, Eurosensors XIII Conference Proceedings, 1999, The Hague, Netherlands, 109. [97] J. Kappler, N. Barsan, U. Weimar, A. Dieguez, J. L. Alay, A. Romano-Rodriguez, J. R. Morante, W. Göpel, Correlation between XPS, Raman and TEM measurements and the gas sensitivity of Pt and Pd doped SnO2 based gas sensors, Fresenius J Anal Chem, 1998, 361, 110-114. [98] R. A. Bennett, P. Stone, M. Bowker, Pd nanoparticle enthanced re-oxidation of non-stoichiometric TiO2 STM imaging of spillover and a new form of SMSI, Catalysis Letters, 1999, 59, 99-105. [99] P. Ingleby, J. W. Gardner, P. N. Bartlett, Effect of micro-electrode geometry on response of thin-film poly(pyrrole) and poly(aniline) chemoresistive sensors, Sens. Actuators B, 1999, 57, 17-27. [100] S. B. Basame, H. S. White, Scanning Electrochemical Microscopy of Metal/Metal Oxide Electrodes. Analysis of Spatially Localized Electron-Transfer Reactions during Oxide Growth, Anal. Chem., 1999, 71, 3166-3170. [101] A. Chiorino, G. Ghiotti, F. Prinetto, M. C. Carotta, D. Gnani, G. Martinelli, Preparation and characterization of SnO2 and MoOx-SnO2 nanosized powders for thick-film gas sensors, Sens. Actuators B, 1999, 58, 3, 338-349. [102] M. C. Carotta, M. Ferroni, D. Gnani, V. Guidi, M. Merli, G. Martinelli, M. C. Casale, M. Notaro, Nanostructured pure and Nb-doped TiO2 as thick film gas sensors for environment gas monitoring, Sens. Actuators B, 1999, 58, 3, 310-317. [103] G. Martinelli, M. C. Carotta, E. Traversa, G. Ghiotti, Thick-Film Gas Sensors Based on Nano-Sized Semiconducting Oxide Powders, MRS Bulletin, 1999, 24, 6, 30-36. [104] G.-J. Li, S. Kawi, High Surface SnO2: a novel semiconductor-oxide gas sensor, Mat. Lett., 1998, 34, 99-102. [105] J. S. Suehle, R. E. Cavicchi, M. Gaitan, S. Semancik, Tin Oxide Gas Sensor Fabricated Using CMOS Micro-Hotplates and In-Situ Processing, IEEE Electron Device Letters, 1993, 14, 3, 118-120. [106] C. Hagleitner, A. Hierlemann, D. Lange, A. Kummer, N. Kerness, O. Brand, H. Baltes, Smart single-chip gas sensor system, Nature, 2001, 414, 293-296.

INTRODUCTION

[107] D. Briand, A. Krauss, B. van der Schoot, U. Weimar, N. Barsan, W. Gopel, N. F. de Rooij, Design and fabrication of high-temperature micro-hotplates for dropcoated gas sensors, Sens. Actuators B, 2000, 68, 1-3, 223-233. [108] F. Solzbacher, C. Imawan, H. Steffes, E. Obermeier, H. Möller, A modular system of SiC-based microhotplates for the application in metal oxide gas sensors., Sens. Actuators B, 2000, 64, 95-101. [109] D. Briand, B. v. d. Schoot, N. F. d. Rooij, H. Sundgren, I. Lundström, A LowPower Micromachined MOSFET Gas Sensor, IEEE J. MEMS, 2000, 9, 3, 303308. [110] A. Heilig, N. Barsan, U. Weimar, W. Göpel, Selectivity enhancement of SnO2 gas sensors: simultaneous monitoring of resistance and temperatures, Sens. Actuators B, 1999, 58, 1-3, 302-309. [111] V. Guidi, G. C. Cardinali, L. Dori, G. Faglia, M. Ferroni, G. Martinelli, P. Nelli, G. Sberveglieri, Thin-film gas sensor implemented on a low-power-consumption micromachined silicon structure, Sens. Actuators B, 1998, 49, 88-92. [112] M. C. Horrillo, I. Sayago, L. Ares, J. Rodrigo, J. Gutierrez, A. Götz, I. Garcia, L. Fonseca, C. Cane, E. Lora-Tamayo, Detection of low NO2 concentrations with low power micromachined tin oxide gas sensors, Sens. Actuators B, 1999, 58, 1-3, 325-329. [113] Z. Tang, S. K. H. Fung, D. T. W. Wong, P. C. H. Chan, J. K. O. Sin, P. W. Cheung, An integrated gas sensor based on tin oxide thin-film and improved microhotplate, Sens. Actuators B, 1998, 46, 174-179. [114] D. Vincenzi, M. A. Butturi, M. Stefancich, C. Malagu, V. Guidi, M. C. Carotta, G. Martinelli, V. Guarnieri, S. Brida, B. Margesin, F. Giacomozzi, M. Zen, A. Vasiliev, A. V. Pisliakov, Low-power thick-film gas sensor obtained by a combination of screen printing and micromachining techniques, Thin Solid Films, 2001, 391, 288-292. [115] K. Steiner, G. Sulz, E. Neske, E. Wagner, Laser annealing of SnO2 thin-film gas sensors in single chip packages, Sens. Actuators B, 1995, 26 - 27, 64-67.

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47 M. Heule, S. Vuillemin, L. J. Gauckler, submitted as review article.

2.

Overview on Powder-based Ceramic Meso- and Microscale Fabrication Processes

Processing techniques are reviewed that allow the introduction of ceramic components made from powders into microelectromechanical systems (MEMS). Ceramics have several advantages over other materials also in microsystems, e.g. heat resistance, hardness, corrosion resistivity or functional properties. The range of available materials in microfabrication technology is being increased beyond those deposited by thin-film technology. Top-down approaches like mechanical and laser-based direct writing processes, ink-jet printing, microextrusion and lithography-based methods are presented. They are complemented by some more fundamental work in the field of bottom-up synthesis of micro- and nanoscaled ceramic materials. 2.1. Introduction Ceramic materials deposited as thin films have been used extensively in electronics and MEMS (micro-electromechanical systems). However, ceramic powders have not been considered for microfabrication processes until recently. Ceramics have several advantages over other materials in MEMS:[1] heat resistance, hardness, corrosion resistivity even in harsh environments, chemical inertness for biological applications and abundant functional properties like piezoelectricity,[2,3] pyroelectricity or catalytic activity of surfaces. Many MEMS devices could be improved if ceramic thick-film coatings as microstructures were readily available, e.g. Harris et al. modeled and fabri-

48

CHAPTER 2

cated cross flow micro heat exchangers. The need for an analogous LIGA structure made of ceramics to the present PMMA polymer structure was clearly stated. Better heat exchange properties and durability of ceramics could be expected.[4] In another project concerning the fabrication of MEMS-based microthrusters for application in small space crafts, ceramics would be much better thermal insulators and enhance lifetime.[5,6] Currently, the microthrusters are fabricated entirely in silicon. The aim of this work is to review recently introduced microfabrication processes for small ceramic objects in the range from 1 mm down to the µm size range. Most methods presented rely on a top-down approach, i.e. they are based on scaling down an existing larger-scale fabrication method. A summary of the presented processes is given in Table 2–1.

Table 2–1. Overview on microfabrication processes with ceramic powders.

The list is ordered by the lowest achievable resolution that was reported or demonstrated in the referenced articles. Smallest Feature Resolution / µm

Aspect ratio achieved

2D-layers or 3Dbodiesa

References

STM tip electrochemical etching

0.01

-

2D

[9]

Casting suspensions into standard photolithography masks

1-5

1-2

2D

[72], [73]

Soft Lithography

1-5

1-3

2D/3D

[74], [75], [76], [77], [80], [81]

Microstereolithography (UV-curable polymer solution filled with alumina)

2

high

3D

[42]

Co-extrusion (µm resolution in two dimensions, rods mm long)

5-16

high

3D

[49], [50]

MAPLE direct write

10

low

2D

[36], [37], [38]

LIGA

10-20

10

2D

[65]

LTTC-ML (low-temperature co-fired ceramic multilayer) technology

25-100

variable (stacking)

3D

[61], [62], [64]

Method

MICROSCALE CERAMIC PROCESSES

Table 2–1. Overview on microfabrication processes with ceramic powders.

The list is ordered by the lowest achievable resolution that was reported or demonstrated in the referenced articles. Smallest Feature Resolution / µm

Aspect ratio achieved

2D-layers or 3Dbodiesa

References

Direct Ceramic Machining DCM of presintered bodies

50

variable

3D

[47]

Screen Printing

100

low

2D

[44]

Pulsed Laser Ablation (depending on the optics used)

30-200

high

2D

[29], [30]

Precision grinding (microcylindrical shapes)

50

high

3D

[57]

Metal embossing

50

high

2D

[52]

Ink-jet printing of suspensions

70

low

2D

[14]

Freeform Ink-jet printing of suspensions (smallest wall thickness)

170

high

3D

[13]

3DPTM process (ink-jet printing of binder solution into dried powder)

200

variable

3D

[15], [16]

Micropen Writing (freestanding 3D Periodic Structures)

250

1

3D

[21], [25], [26], [27]

Method

a. 2D-layers: substrate-supported structures, 3D-bodies: possibility to create freestanding microparts.

49

50

CHAPTER 2

2.2. Direct writing methods Direct writing systems consist of an automated translation stage which moves a pattern generating device like an ink-jet head or laser writing optics. A general review on the state of the art in ink-jet printing of different materials can be found elsewhere.[7] Structures are drawn in sequence and formed continuously. In contrast to parallel processes like photolithography, each sample has to be fabricated individually which puts certain limits on the fabrication speed and efficiency. On the other hand, each device can be built with an individual shape with computer-controlled movement. Certain approaches allow the fabrication of complex shapes in all three dimensions which is often termed freeform fabrication.[8] As an example of ultimately small direct writing, Hung et al. used an STM tip to electrochemically etch a square-shaped hole of exactly 1 µm side length into Tl2O3 thin films.[9] When a bias voltage of -2.5 V is applied between tip and surface, etching of Tl2O3 is performed. Within one minute, a 200 nm grain under the tip could be dissolved. 2.2.1. Direct writing of powders using ink jet systems Ainsley et al. reported a method for solid freeform fabrication (the manufacturing of a small part in 3D) by controlled droplet deposition of powder filled melts. Al2O3 powders were suspended in n-alkane mixtures of melting temperatures from 50-60°C. Deposited by an ink-jet system, free-standing bodies like a rotation wheel of 1-2 cm,[10,11] or test structures with a minimal wall thickness of 100 µm in the green state were demonstrated.[12] The solids loading could be increased up to 40 vol%. Zhao et al. similarly used ZrO2 powders suspended in organic matrices for the generation of maze structures with vertical side walls within 170 ± 10 µm in the sintered state (Fig. 2–1a).[13] The same group was also able to ink-jet print aqeous lead zircon titanate (PZT) suspensions with a particle size of 37.5 µm in two dimensions onto filter paper. There, the resolution of 360 dpi was given by the use of a commercial ink-jet printer. Other limitations were found in the layer thickness, samples thicker than 20 layers could not be dried in good quality.[14]

MICROSCALE CERAMIC PROCESSES

Alternatively, only binder solution without particles can be ink-jet printed to consolidate a well defined region in a dried layer of powder.[15-17] The next layer is then sprayed as a suspension and left to dry before binder solution is printed again for the definition of the part shape. The process is called 3DPTM (Three Dimensional Printing) and has been developed at MIT.[18] Complex-shaped 3D parts similar to those from stereolithography can be manufactured using 3DP. The exact conditions of binder solution soaking a porous powder bed had to be characterised thoroughly.[19] Typical dimensions of such a disk-shaped volume element formed by one droplet of binder solution lie in the range of 50 µm for the thickness and 200 µm for the disk diameter. This volume element is also the smallest feature size that can be reproduced by the process. For removal of the powder regions not consolidated by binder solution, the part is immersed in a water bath. [20]

a) ink-jet printing of colloids

b) MAPLE direct writing

170 µm hν ink-jet head

coating material

objective

ribbon ZrO2 substrate

substrate

c) dip-pen writing 250 µm SiO2 oil immersion

substrate

Fig. 2–1. a) ink jet printing of ZrO2 structures with a 170 µm resolution. The

inks have a solid loading of 14 vol%.[13] b) MAPLE-direct-write process (matrix-assisted pulsed-laser evaporation). A variety of oxides, metals for electrical contacts and polymers can be transferred.[36] c) threedimensionally layered mesostructures of silica colloids fabricated by dip-pen writing.[21]

51

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CHAPTER 2

2.2.2. Micropen writing Another direct writing technique is called micropen fabrication in which a powder-filled paste is extruded through a small orifice. Smay et al. demonstrated the power of the approach by writing 3D-periodic structures which consist of layers of gratings stacked perpendicularly on each other (Fig. 2– 1c).[21] This requires that the deposited structures of 250 µm width have to support their own weight as deposited for bridging the gaps in the underlying layer(s). Therefore, specifically tailored colloidal systems[22-24] to satisfy that need were developed by using e.g. poly-ethylenimine coated silica microspheres as a viscoelastic gel obtained by setting the pH to the isoelectric point at pH 10. In order to slow the drying, the deposition was carried out immersed in a non-wetting oil. In a similar fashion, these results were transferred to aqeous PZT suspensions under different pH conditions.[25] For assessment of the maximum spanning distance, angled test structures with increasing width were deposited. The strongest colloidal gel at pH 6.15 was able to span up to 2 mm distance with a buckling of less than 500 µm. By using gel-casting (stabilisation is reached by in-situ polymerisation of the paste after deposition), freestanding structures of 250 µm thickness spanning several mm have been micropen-deposited.[26] As first applications, Pb(Nb,Zr,Ti)O3 pastes were deposited as thick-film capacitors,[27] and ZnO varistors written by this method were characterised compared to standard ZnO varistors.[28] 2.2.3. Laser-based direct writing Using pulsed laser systems, enough energy can be focused on a spot to ablate even the hardest materials. Kruusing et al. used Nd:YAG lasers to cut out magnetic ceramics NdFeB and MnZn ferrites.[29] Cut sizes ranged from 3050 µm. Oliveira et al. observed the formation of very small columns while ablating Al2O3 ceramics with a KrF excimer laser.[30] The typical crater formed by one laser pulse is of cylindric shape with a diameter of approx. 200 µm and a depth of 30 µm. They also characterised the plasma plumes formed of the ablated ceramic material using optical emission spectroscopy.[31] Laser ablation may leave a different chemical composition of the surface behind as Laude et al. have shown upon ablating Y-Si-Al-O-N ceram-

MICROSCALE CERAMIC PROCESSES

ics.[32] They observed melting, Si3N4 decomposition and enrichment of SiO2 on the surface. On the application level, Hanreich et al. fabricated ultra-thin pick-up coils for magnetic flux detection by filling laser-drilled holes in an Al2O3 substrate with electrode material.[33] This drilling made it possible to create flat alumina-integrated electrodes which can be put into close contact with a magnetic surface to measure its flux. The holes are funnel-shaped with an outer diameter of 700 µm and an inner diameter of 210 µm. Furthermore, this drilling method was used for creating human skin humidity sensors requiring a flat, integrated electrode design to contact skin.[34] Since many ceramic materials are chemically inert, etching is a difficult approach. However, Horisawa et al. enhanced the etching of Al2O3 ceramics immersed in H3PO4 locally by a pulsed laser and were able to create small holes of 0.5 mm diameter.[35] A very elegant method to transfer ceramic materials from one substrate to another has been developed by the Naval Research Laboratories in Washington DC. It was termed MAPLE direct write technique (matrix assisted pulsed laser evaporation).[36-38] The material to be deposited is coated on a UVtransparent sheet of quartz serving as a ribbon. The ribbon is then brought into close contact with an arbitrary, flat substrate of metal, plastics or ceramics (Fig. 2–1b). With a pulsed UV-laser, the interface between ribbon and coating is quickly heated, causing a rapid ablation of matrix material which in turn propels the coating material towards the substrate. If ceramic powders are to be deposited, the powder is embedded in an organic binder matrix. This has been demonstrated using BaTiO3 and Au particles on two separate ribbons. The gold was used to write electrodes, subsequently, a small capacitor structure of 15 µm thickness was deposited with the barium titanate. The range of materials available is impressive,[39] also polymers and even biomolecules and eukaryotic cells were transferred.[40] These complex molecules and cells are not harmed since they are embedded in a polymer matrix which absorbs almost all laser power and is pyrolysed. The expanding plume of gases carries the complex molecules into the gas phase. The same mechanism is used for soft ionisation in MALDI mass spectrometry (Matrix Assisted Laser Desorption and Ionisation). Materials limitations of the method arise if

53

54

CHAPTER 2

complex oxides are to be transferred. They may partially be decomposed upon ablation. Additionally, the same laser was used to perform laser-induced annealing or trimming of the deposited layers. Fitz-Gerald et al. demonstrated the possibility to deposit phosphor powders consisting of Y2O3:Eu or Zn2SiO4:Mn for high-definition display applications.[41] In stereolithography, a laser is scanned across a UV-curable polymer solution. Complex-shaped 3D parts can be manufactured if the hardened part is lowered, exposing fresh solution into which the next layer is written. Such polymer solutions were filled by Zhang et al. with ceramic powders, e.g. 33 vol% Al2O3 with a diameter of 200 nm.[42] The curing reaction then forms a green body. The lateral resolution was improved down to 2 µm using appropriate focussing optics. 15 µm thick single layers on a substrate could be obtained. Full ceramic microgears with diameters of 400 to 1000 µm (20 µm thick) were generated and could be sintered at 1400°C, exhibiting a low shrinkage. Provin et al. used a similar scheme to fabricate very smooth 3D-shaped parts in the mm range.[43] In contrast to the previously cited work, UV-light from a Hg lamp is projected via a LCD screen with VGA resolution (640x480 pixels) which is used to define the pattern of a single layer in one exposure step. Solids loading of alumina was 24 vol%. It was observed that the UV-absorption increases due to the alumina filling which limits the curing depth. Consequently, a depth resolution of < 10 µm was reached. 2.3. Downsizing Mechanical Processing Methods Silk screen printing is a process of choice for many thick-film coating applications since it is available up to industrial scale. Typical film thicknesses range from 5 to 50 µm. The main drawback is the limited lateral resolution around 100 µm which is a function of mesh size and ceramic paste properties.[44] Thiele et al. examined process compatibility issues between screenprinted PZT layers and silicon substrates.[45] Traditional machining of sintered ceramics is done with diamond-cutting tools. Conventional tools can be used when using a soft presintered ceramic body. If the presintered blank is homogeneous, it shrinks linearly in all dimensions and makes it possible to precompensate the sintering shrinkage

MICROSCALE CERAMIC PROCESSES

by machining enlarged features. A final pattern reproduction quality of 0.1% length deviation in 50 µm structures can be obtained by this Direct Ceramic Machining (DCM) process.[46,47] The application of piezoelectric ceramics as ultrasonic transducers has been the driving force for the development of various methods to create polymer-filled arrays of small ceramic rods with a high aspect ratio.[48] It was shown that the resolution in medical ultrasonic imaging and related technologies can be improved by miniaturizing the rod size. Among the techniques are rod placing, dice and fill, lost mold processes (mechanically engineered molds), injection molding, laser cutting, solvent-air jet machining of green tapes and ceramic fiber processing. The smallest rods were 20 - 50 µm. The group of Halloran has developed a multistage co-extrusion process which was used to extrude alumina, PZT and Pb-Mg-Nb-TiO3 pastes (Fig. 2–2a).[49] Embedded in a carbon-black matrix, M-shaped ceramic extrudates with a cross section of 2 mm were stacked 5 by 5 to be extruded again. Together, these 25 M-shapes were thereby reduced in size 5:1. If this process of reassembly and extrusion is repeated, the size of the M-shape is reduced by a factor of five each time n, whereas the number of M-shapes increases by 25n. After four reduction steps, an extrudate containing 390,625 M-shapes of 16 µm height were obtained. Using the same size reduction process, other ceramics-metal composites with a specific square micro-configuration in the order of 15 µm for the ceramic phase and 5 µm for the AgPd metallic interlayers were produced.[50] On a larger scale, Knitter et al. presented a rapid prototyping for ceramic microreactors by low-pressure injection molding.[51] The molds with smallest features of 500 µm were fabricated using stereolithography and molding them in silicone. In another work, they present PZT microrod arrays (50 µm wide, 300 µm high) for ultrasonic transducers using a combination of tape casting and metal mold embossing.[52] The group of Lange used a process of colloidal isopressing against the 130 µm features of a coin which were reproduced as an accurate surface replica in Al2O3 ceramics.[53,54] Similar surface reproduction processes include the fabrication of titania scaffolds for hepatocyte cell cultures[55] and the micro-embossing with a polymeric stamp into shrinkage-free ZrSiO4 ceramics.[56] Alternatively, Yeo et al. produced micro-

55

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cylindrical parts of 50 µm diameter with a precision grinding process.[57] A reactive hot isostatic pressing route using powders was adapted to manufacture SiC microrotors[58] and again PZT microstructures for ultrasonic transducers.[59] Molds were fabricated by standard micromachining of a silicon wafer. 2.3.1. Meso-scaled devices based on co-fired ceramic tape technology An important example of advanced ceramic technology suited for manufacturing miniature devices is the LTTC process (Low Temperature Co-fired Ceramic).[60] It is based on ceramic green tapes (“GT”) of 100-400 µm thickness which are soft and can be readily cut, ground or dissolved. They are commercially available by Dupont Electronic Materials. Holes and grooves for a microsystem are cut into such tape layers and by stacking them, complex structures can be built. Alternatively, smaller holes can be etched by dissolving a photolithographically defined pattern. Electrical connections are integrated by screen-printing on tape surfaces and by filling holes with metal paste in the vertical direction. For final hardening, the green tape layers of the stack are first laminated by applying pressures up to 3600 psi at 60-80°C, then sintered at 875°C. In a recent overview by Gongora-Rubio, several aspects of versatility and device applications are presented.[61] Small holes of 25 µm can be “etched” into GT tapes by using an acetone containing jet of nitrogen out of a small nozzle. Whereas binder organics are dissolved by the acetone, the force of the jet drives the filling powders away. One problem that has to be addressed is the sagging of tape in the sealing process of empty cavities during the lamination process. Among the devices built were proximity sensors, gas flow sensors, electrochemical sensors and pressure sensors. Vojak et al. created a ceramic microdischarge device with LTTC technology, housing a stable Ne-discharge in a cylindrical cavity of 140 µm diameter and 230 µm length at a voltage of 137 V.[62] For organic synthesis, a ceramic microreactor for the methoxylation of methyl-2-furoate has been demonstrated.[63] Finally, Wilcox et al. demonstrated that the technology is mature by integrating a PEM (polymer electrolyte membrane) fuel cell design including methanol gas reforming.[64] They also entered the field of microfluidics,

MICROSCALE CERAMIC PROCESSES

a) extruded M-Shaped rods of Al2O3 paste in carbon black matrix.

MM MM Stack and repeat extrusion process.

M M M M

M M M M

M M M M

MM MM

1:5 Extrusion

M M M M 1:5 Extrusion

MM MM MM MM MM MM MM MM

b) slip pressing with high aspect ratio LIGA mold. 20 µm

Al2O3 suspension

Al2O3 ceramics demold, fire

Fig. 2–2. a) Alumina M-shape fabricated by co-extrusion in a carbon-black

matrix in the final sintered state.[49] b) Alumina micro-rods prepared by slip pressing of a LIGA metal mold into powder.[67]

producing channel, valve and pumping systems, that either worked magnetohydrodynamically or by piezo actuation. A continuous flow PCR (polymerase chain reaction) device was also presented. 2.4. Lithography-based Processes The LIGA technology (German acronym for Lithography, Galvanoformung and Abformtechnik) was the first to produce high-aspect ratio structures in microfabrication using x-rays for pattern exposure. There are instances, where LIGA-produced molds were filled with ceramic materials.[65] To reproduce the shapes in a casting process accurately, shrinkage and cracking

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problems have to be solved. Chan et al. used a TEOS (tetraethoxysilane) sol filled with 50 nm titania particles for casting into PMMA-molds. The microgears obtained had single teeth of 50 µm.[66] Using the high-aspect ratio features of LIGA structures in a slip pressing scheme, Bauer et al. made Al2O3 structures such as profile nozzles and posts in the order of 100 µm with absolutely vertical side walls and sharp edges (~ 1 µm, Fig. 2–2b).[67,68] Ruzzu et al. combined ceramic and metallic LIGA-structures for advanced electronic packaging applications.[69] Feiertag et al. made use of an unique feature of xray lithography, the possibility to expose from different angles.[70] By this scheme, angled and 3-dimensionally crossed structures for creating photonic crystals were obtained. More and more, high-aspect ratio lithography based on UV-curable resists like the epoxy-based SU-8 negative resists is beginning to replace LIGA.[71] Standard UV-photoresists may also be used to cast ceramic structures directly into photoresist masks.[72,73] Small thick-film structures of Al2O3 or SnO2 with an aspect ratio of 1 could readily be fabricated using a resist with higher viscosity that forms layers of 5-15 µm in one spin-coating step. A few years ago, Whitesides suggested the use of elastomeric silicones (mostly polydimethylsiloxane PDMS) for micropattern transfer, called soft lithography.[74-77] There are several variations available and suited for direct patterning of liquids. Microcontact printing is used for stamping self-assembled monolayers serving as resist or as functional layers. In micromolding in capillaries (MIMIC), a fluid is patterned by spontaneous filling of PDMS microchannels. MIMIC was used to pattern polymeric precursor solutions for SiBNC ceramics[78] or Sr2Nb2O7 ceramics by a sol-gel route,[79] as well as to structure suspensions of tin oxide,[80,81] using channel sizes down to 10 µm. Such microlines of tin oxide were then integrated to form tiny semiconducting gas sensors.[82,83] Yang et al. made pattern of porous oxides using MIMIC.[84] By exploiting the flow characteristics in small channels, such systems were used to guide the growth of tubular silica structures.[85] PDMS molds were also used for filling with ceramic suspensions, either generating accurately textured surfaces of bulk ceramics[86] or freestanding parts.[87]

MICROSCALE CERAMIC PROCESSES

Using pre-ceramic polymers, a commercially available mixture (Ceraset) that forms SiCN ceramics upon pyrolysis, the group of Raj exploited several microfabrication schemes. Microgears were obtained by filling SU-8 pattern.[88] By adding pressure in a similar process called microforging, 20 µm wide lines could be shaped.[89] Moreover, Ceraset could be exposed directly through a photomask for photopolymerisation, if mixed with an appropriate photoinitiator.[90] Feature sizes obtained are in the 100 µm range. There are doped Li2O-Al2O3-SiO2 glasses that are photo-curable. Salim et al. successfully exposed and etched microgrippers a few mm long with features of approx. 300 µm.[91] Due to their chemical inertness, ceramics are not an ideal material for photolithography/etching processes. Makino et al. etched alumina, silicon nitride, SiC and others in phosphoric acid. For masking, non-standard UVresists like polybutadiene rubbers were investigated. As a result, smooth trenches of 100 µm to 600 µm and a depth of 30-40 µm could be etched into alumina in 30 min.[92] Wang et al. used a modern gas phase etching process for machining PZT with a SF6-based deep reactive ion etching process.[93] 2.5. Self-Assembly So far, all approaches presented rely on top-down strategies, i.e. they are based on miniaturising existing techniques of larger scales. Alternatively, bottom-up approaches have also been successfully applied, in which the ability to form the desired microstructure is inherent to the smaller building blocks, e.g. colloidal particles. The particles “find” their place in the structure by selfassembly without the need for intervention from the outside when mixed appropriately. Although that self-assembly processes are especially fascinating, they have not been used much for MEMS fabrication. Self-assembly still is a field of science rather than engineering. There might also be a lack of crossdisciplinary communication between MEMS development and colloidal chemistry research. Photonic band gap materials are often prepared by self-assembly processes of colloidal crystals which are subsequently backfilled with a high refractive index material, e.g. sol-gel ceramics.[94] It is also important to note that for

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application as useful photonic bandgap materials, a sufficiently large difference in refractive index between particles and the surrounding phase in the crystal is necessary. A property which again calls for the use of ceramic materials.[95] Colloidal crystals consist of ordered colloidal particles in a cubic closest package with a periodicity in the wavelength range of visible light (Fig. 2–3). Hayward et al. have further directed the assembly of colloidal crystals by electrophoretic deposition into microstructures defined by the underlying structured electrodes.[96,97] They also patterned BaTiO3 layers using self-assembling block-copolymers that form nanoscale modulations on a surface.[98] In a recent review, Dabbs and Aksay show the abundant possibilities of shaping ceramic materials on a true nanometer scale. Parallels to bio-mineralisation and the hierarchical construction of biominerals are drawn.[99] Vlasov et al. observed that a spherical shape of particles and a narrow distribution of sphere size seems to be sufficient for spontaneously forming colloidal crystals.[100] In this study, silica spheres were used. In Xia’s group, a flow directed assembly of particles into colloidal crystals is presented.[101-103]

Fig. 2–3. Silica particles in hexagonal close packing as templates for photo-

nic band gap materials prepared by micromolding in capillaries.

MICROSCALE CERAMIC PROCESSES

Out of all various approaches for the fabrication of photonic crystals, the strikingly easy to reproduce convection self-assembly method suggested by Colvin’s group seems to have become a standard method. The surface to be coated is placed into a dilute suspension of microspherical particles in ethanol. As the solvent dries away under defined conditions, the particles assemble at the receding ethanol meniscus and form closest packed structures.[104109]

2.6. Summary There are abundant approaches for generating small ceramic elements using powders as building blocks. Due to the scientific and technological experience in synthesis and control of many functional properties, powders may offer enhanced functionalities also for MEMS devices compared to thin films. However, MEMS devices with successful integration of ceramic powders in any form remain scarce. The LTTC green tape process is sophisticated enough for making useful meso-scaled ceramic devices. For objects in the range of mm to 0.1 mm there are many processes available. Components with high aspect ratios of 100 or more are best molded using LIGA or related processes. Precision mechanical machining enables the fabrication of components with features of 50 to 100 µm with high aspect ratios at low cost. Considerably smaller structures in ceramics than 50 µm can be generated by MAPLE direct writing, soft lithography, standard photolithography and STM tip etching. Photonic band gap materials with periodicities in the range of 100 to 1000 nm can be generated by a number of schemes. In most other self-assembly approaches at the nanometer level, more development work is needed for generating useful coatings and structures.

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2.7. References [1] L. A. Liew, W. G. Zhang, L. N. An, S. Shah, R. L. Luo, Y. P. Liu, T. Cross, M. L. Dunn, V. Bright, J. W. Daily, R. Raj, K. Anseth, Ceramic MEMS - New materials, innovative processing and future applications, Am. Ceram. Soc. Bull., 2001, 80, 5, 25-30. [2] X. H. Zhu, J. M. Zhu, S. H. Zhou, Q. Li, Z. G. Liu, Microstructures of the monomorph piezoelectric ceramic actuators with functional gradients, Sens. Actuator APhys., 1999, 74, 1-3, 198-202. [3] Y. H. Lim, V. V. Varadan, V. K. Varadan, Finite-element modeling of the transient response of MEMS sensors, Smart Mater. Struct., 1997, 6, 1, 53-61. [4] C. Harris, M. Despa, K. Kelly, Design and fabrication of a cross flow micro heat exchanger, J. Microelectromech. Syst., 2000, 9, 4, 502-508. [5] C. Rossi, T. Do Conto, D. Esteve, B. Larangot, Design, fabrication and modelling of MEMS-based microthrusters for space application, Smart Mater. Struct., 2001, 10, 6, 1156-1162. [6] C. Rossi, S. Orieux, B. Larangot, T. Do Conto, D. Esteve, Design, fabrication and modeling of solid propellant microrocket-application to micropropulsion, Sens. Actuator A-Phys., 2002, 99, 1-2, 125-133. [7] P. Calvert, Inkjet printing for materials and devices, Chem. Mater., 2001, 13, 10, 3299-3305. [8] J. W. Halloran, Freeform fabrication of ceramics, British Ceramic Transactions, 1999, 98, 6, 299-303. [9] C. J. Hung, J. N. Gui, J. A. Switzer, Scanning probe nanolithography of conducting metal oxides, Appl. Phys. Lett., 1997, 71, 12, 1637-1639. [10] C. Ainsley, N. Reis, B. Derby, Freeform fabrication by controlled droplet deposition of powder filled melts, J. Mater. Sci., 2002, 37, 15, 3155-3161. [11] B. Derby, Materials opportunities in layered manufacturing technology, J. Mater. Sci., 2002, 37, 15, 3091-3092. [12] K. A. M. Seerden, N. Reis, J. R. G. Evans, P. S. Grant, J. W. Halloran, B. Derby, Ink-jet printing of wax-based alumina suspensions, J. Am. Ceram. Soc., 2001, 84, 11, 2514-2520. [13] X. Zhao, J. R. G. Evans, M. J. Edirisinghe, Direct Ink-Jet Printing of Vertical Walls, J. Am. Ceram. Soc., 2002, 85, 8, 2113-2115. [14] J. Windle, B. Derby, Ink jet printing of PZT aqueous ceramic suspensions, J. Mater. Sci. Lett., 1999, 18, 2, 87-90. [15] R. K. Holman, M. J. Cima, S. A. Uhland, E. Sachs, Spreading and infiltration of inkjet-printed polymer solution droplets on a porous substrate, J. Colloid Interface Sci., 2002, 249, 2, 432-440. [16] S. A. Uhland, R. K. Holman, S. Morissette, M. J. Cima, E. M. Sachs, Strength of green ceramics with low binder content, J. Am. Ceram. Soc., 2001, 84, 12, 28092818. [17] J. Moon, J. E. Grau, V. Knezevic, M. J. Cima, E. Sachs, Ink-Jet Printing of Binders for Ceramic Components, J. Am. Ceram. Soc., 2002, 85, 4, 755-62. [18] J. Moon, J. E. Grau, V. Knezevic, M. J. Cima, E. M. Sachs, Ink-jet printing of binders for ceramic components, J. Am. Ceram. Soc., 2002, 85, 4, 755-762.

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[87] B. Su, D. Zhang, T. W. Button, Micropatterning of fine scale ceramic structures, J. Mater. Sci., 2002, 37, 15, 3123-3126. [88] L. A. Liew, W. G. Zhang, V. M. Bright, L. N. An, M. L. Dunn, R. Raj, Fabrication of SiCN ceramic MEMS using injectable polymer- precursor technique, Sens. Actuator A-Phys., 2001, 89, 1-2, 64-70. [89] Y. P. Liu, L. A. Liew, R. L. Luo, L. N. An, M. L. Dunn, V. M. Bright, J. W. Daily, R. Raj, Application of microforging to SiCN MEMS fabrication, Sens. Actuator APhys., 2002, 95, 2-3, 143-151. [90] L. A. Liew, Y. P. Liu, R. L. Luo, T. Cross, L. N. An, V. M. Bright, M. L. Dunn, J. W. Daily, R. Raj, Fabrication of SiCN MEMS by photopolymerization of preceramic polymer, Sens. Actuator A-Phys., 2002, 95, 2-3, 120-134. [91] R. Salim, H. Wurmus, A. Harnisch, D. Hulsenberg, Microgrippers created in microstructurable glass, Microsyst. Technol., 1997, 4, 1, 32-34. [92] E. Makino, T. Shibata, Y. Yamada, Micromachining of fine ceramics by photolithography, Sens. Actuator A-Phys., 1999, 75, 3, 278-288. [93] S. A. Wang, X. H. Li, K. Wakabayashi, M. Esashi, Deep reactive ion etching of lead zirconate titanate using sulfur hexafluoride gas, J. Am. Ceram. Soc., 1999, 82, 5, 1339-1341. [94] J. D. Joannopoulos, R. D. Meade, J. N. Winn, "Photonic Crystals - Molding the Flow of Light", Princeton University Press, Princeton, NJ, 1995. [95] O. D. Velev, E. W. Kaler, Structured Porous Materials via Colloidal Crystal Templating: From Inorganic Oxides to Metals, Adv. Mater., 2000, 12, 7, 531. [96] R. C. Hayward, D. A. Saville, I. A. Aksay, Electrophoretic assembly of colloidal crystals with optically tunable micropatterns, Nature, 2000, 404, 56-58. [97] M. Trau, D. A. Saville, I. A. Aksay, Assembly of Colloidal Crystals at Electrode Interfaces, Langmuir, 1997, 13, 6375-6381. [98] T. Lee, N. Yao, I. A. Aksay, Nanoscale Patterning of Barium Titanate on Block Copolymers, Langmuir, 1997, 13, 3866-3870. [99] D. M. Dabbs, I. A. Aksay, Self-Assembled Ceramics Produced by Complex Fluid Templation, Annu. Rev. Phys. Chem., 2000, 51, 601-622. [100] Y. A. Vlasov, X.-Z. Bo, J. C. Sturm, D. J. Norris, On-chip natural assembly of silicon photonic bandgap crystals, Nature, 2001, 414, 289. [101] S. H. Park, Y. Xia, Assembly of Mesoscale Particles over Large Areas and Its Application in Fabricating Tunable Optical Filters, Langmuir, 1999, 15, 266-273. [102] S. H. Park, D. Qin, Y. Xia, Crystallization of Mesoscale Particles over Large Areas, Adv. Mater., 1998, 10, No. 13, 1028-1032. [103] S. H. Park, Y. Xia, Macroporous Membranes with Highly Ordered and ThreeDimensionally Interconnected Spherical Pores, Adv. Mater., 1998, 10, No. 13, 1045-1048. [104] J. F. Bertone, P. Jiang, K. S. Hwang, D. M. Mittleman, V. L. Colvin, Thickness dependence of the optical properties of ordered silica-air and air-polymer photonic crystals, Phys. Rev. Lett., 1999, 83, 2, 300-303. [105] V. L. Colvin, From opals to optics: Colloidal photonic crystals, MRS Bull., 2001, 26, 8, 637-641.

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[106] P. Jiang, G. N. Ostojic, R. Narat, D. M. Mittleman, V. L. Colvin, The fabrication and bandgap engineering of photonic multilayers, Adv. Mater., 2001, 13, 6, 389393. [107] P. Jiang, J. F. Bertone, V. L. Colvin, A lost-wax approach to monodisperse colloids and their crystals, Science, 2001, 291, 5503, 453-457. [108] R. Rengarajan, P. Jiang, D. C. Larrabee, V. L. Colvin, D. M. Mittleman, Colloidal photonic superlattices, Phys. Rev. B, 2001, 6420, 20, art. no.-205103. [109] M. E. Turner, T. J. Trentler, V. L. Colvin, Thin films of macroporous metal oxides, Adv. Mater., 2001, 13, 3, 180-183.

69 M. Heule, J. Schell, L. J. Gauckler, J. Amer. Ceram. Soc., 2003, in press.

3.

Powder-Based Microcomponents on Silicon Substrates fabricated by Micromolding in Capillaries

With the introduction of soft lithography and micromolding in capillaries, low cost microfabrication with liquid materials has become possible. In this article, we demonstrate how to fabricate porous ceramic lines of 10 µm width and several mm in length on silicon wafer substrates by using colloidal suspensions of tin oxide. Microchannels of polydimethylsiloxane (PDMS) served as molds that were spontaneously filled owing to capillary forces with suspensions of 0.1 - 40 vol% solid loading. The resulting ceramic lines have a height of about 7 µm and therefore differ from usual ceramic thin film coatings. The capillary filling characteristics were observed under the microscope, the implications of rheology and suspension chemistry are discussed and evaluated. Using the same capillaries, even smaller lines (2 - 3 µm width) of powder particles could easily be prepared by adjusting only the solid content of the suspensions. 3.1. Introduction In addition to traditional semiconducting materials, the integration of ceramic and organic materials into microsystems often termed microelectromechanical Systems (MEMS) has advanced enormously in recent years. The functionality of such novel microsystems has increased beyond the capabilities of electronic circuits and even beyond electromechanical funtionality, including chemical, thermal and optical sensing or actuating. The most important advantages of ceramics in microfabrication are their heat resistance

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allowing for high-temperature microdevices and their functional properties like piezo- and pyroelectricity and catalytic capabilities. Ceramics like SiC can play an important role in novel MEMS applications, either as microdevice substrate for harsh environments[1] or as microrotors.[2] Microdischarge reactions were performed inside new monolithic ceramic microdevices.[3] Functional coatings for instance are applied in micromachined gas sensors with tin oxide coatings or other catalytically active oxidic layers.[4,5] Microstructured coatings of functional ceramics may be realised as thin films using high-vacuum or sol-gel deposition processes in conjunction with photolithography. To our knowledge, however, there have been no comparable processes with true micrometer resolution in ceramic thick-film processing based on colloidal dispersions until recently. The lower resolution limit of screen printing typically is about 50 µm. The introduction of ceramic powders in microsystem technology may have significant advantages over thin film processes. First, there is much experience in powder synthesis. Exact control of powder grain size, size distribution and morphology is often possible. Second, in cases where dopants are to be controlled, dopant stoichiometry and its distribution on, respectively inside the grain may be used to finetune functional properties. Finally, powder networks with well defined porosity can be integrated to exhibit a larger specific surface area.

MICROMOLDING IN CAPILLARIES a

d

b

e

c

Fig. 3–1. Schematic of the micromolding in capillaries process with

ceramic suspensions. a) place PDMS mold on substrate. b) apply suspension droplet at end of channels. c) capillary forces fill microchannels within 10-15s. d) wash off excess suspension droplet, leave to dry. e) remove PDMS mold: microceramic green body structures for sintering.

One method to fabricate powder-based ceramic microstructures is to cast a suspension into appropriate photoresist structures.[6] Alternatively and more cost efficient is the use of soft lithography which was developed as a low cost route to use liquid materials in microfabrication.[7] The use of liquid inorganic precursor polymers in soft lithography has already been demonstrated to fabricate microstructured ceramics[8] as well as photonic band gap materials and other hierarchically ordered oxides.[9] In soft lithography, micropatterns are transferred by casting a silicone rubber, poly-dimethylsiloxane (PDMS), against a master structure (see Fig. 3–1). The PDMS is then peeled off, cut and used as a mold that forms microcapillaries on a substrate which can be filled with a liquid. This technique is referred to as micromolding in capillaries (MIMIC). The most striking feature of using PDMS as mold material is, that this elastomeric material readily establishes a reversible conformal contact on the molecular level on a variety of substrates, thus sealing

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the capillaries optimally. Additionally, the master structures may be reused many times to cast PDMS molds and performing MIMIC itself does not require clean room conditions. We have extended the technique to use colloidal dispersion of ceramic powders with solid contents of 0.1 to 40 vol%. These methods enabled the fabrication of microstructured ceramic lines with a spatial resolution of 10 µm.[10] In another publication we demonstrated how such microceramic lines of tin oxide can be integrated into a functional microdevice and applied them as miniaturized gas sensors.[11] A commercially available tin oxide powder was used for the present article to demonstrate the capabilities and limitations of MIMIC. By varying the solids loading and exploiting the drying characteristics inside the microchannels, continuous lines of only 1-3 µm width were obtained. The filling and drying of the dispersion inside the capillaries was further characterised by using video microscopy for direct observation of the processes. 3.2. Experimental Master Structures: The master containing a positive relief of the mold structures were prepared by standard photolithography., AZ 4562 photoresist (Clariant Inc., Wiesbaden, Germany) was spun onto a silicon test wafer at 3500 min-1, resulting in a resist thickness of approx. 7 µm. The structures were transferred by UV illumination through a chromium mask using a Suss MA-6 mask aligner. The exposed sample was developed in a Microposit 351 (Shipley Inc., Marlborough, MA, USA) - water mixture (ratio 1:3) and could be used as master structure several times without further treatment. PDMS molds: 9 g of Sylgard 184 PDMS prepolymer/catalyst mixture (Dow Corning Inc., Midland, MI, USA) were poured over a master structure and cured for 26 hrs. After peeling off and cutting to pieces containing the pattern accessible from two opposite edges, the PDMS molds were oxygen plasma-treated for 2 min using a 100 W Harrich PDC-32G sterilizer (Harrich Scientific, Ossinning, NY, USA). Tin oxide suspension: The tin oxide powder (Cerac Inc., Milwaukee, WI, USA, grain size of d50 = 280 nm as determined by a Microtrac UPA 150 particle sizer) was used as received. In 6.65 g of distilled water (18MOhmcm,

MICROMOLDING IN CAPILLARIES

Millipore), 1.84 g polyacrylic acid sodium salt 2100 (Fluka AG, Buchs, Switzerland) was dissolved and 0.06 g ammonia (25% in H2O) added, adjusting the pH to 9.9. The solution was filtered through a 200 nm pore size teflon filter and subsequently used as dispersing agent. 33 vol% suspension: to 6.01 g of water and 150 µl of dispersing agent, 21.1 g of tin oxide powder were subsequently added, frequently interrupted by ball milling sequences. For other solid loadings, the amounts of powder and dispersing agent were varied accordingly. The suspensions were ballmilled for 20 hrs. The viscosity curves were recorded using a Bohlin CS 50 rheometer (Bohlin Inc., Lund, Sweden). Micromolding in Capillaries: A plasma-treated mold was placed on a silicon test wafer. 10 µl of tin oxide suspension were dispensed at the entrance of the structures. After the capillary filling, the sample was left to dry at room temperature. The dried suspension droplet at the entrance was then removed by pressing the filled PDMS mold tightly to the wafer and rinsing the outside region with plenty of water, immediately followed by acetone. After 30 min, the PDMS mold was lifted and the sample was fired at 800°C for 5 hrs. Video Microscopy: A very thin PDMS stamp was cast by sandwiching the master structure, PDMS prepolymer mixture and a plastic transparency foil under a weight of ca. 50 g for curing. Its thickness was defined by using pieces of silicon wafer (0.38 mm) as spacers. The stamp was demolded carefully and placed on a silicon wafer substrate with the channel entrances overlapping the substrate edge by approx. 0.5 mm. The setup was mounted on a microscope equipped with a video camera using the 50x magnification objective. The tin oxide suspensions were injected through a PE tube from below and dispensed as close to the capillary entrances as possible. 3.3. Results and Discussion Microlines of tin oxide ceramics, only 10 µm wide were obtained by MIMIC. A schematic of the process is given in Fig. 3–1. The length of each line is determined by the characteristics of the capillary filling process and varied between 0.5 mm (40 vol% suspension) and 5 mm (15 vol%). Spontaneous filling is thermodynamically driven and occurs when the interfacial free energies can be minimized by wetting the capillary surface. Kim et al.

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have presented a thermodynamic model that describes the filling of a capillary with square cross-section.[12, 13] Driving force is the wetting of the channel walls. In order to maximize the driving force of capillary filling, low contact angles to PDMS are required. The wetting force is then sufficient to overcome the larger contact angles even of non-wetting substrates. Experimentally, we treated the PDMS surface in an RF oxygen plasma to replace surface methyl groups by hydroxyl groups.[14] The water contact angle of 120° is reduced to < 3° after the treatment. The silicon wafer substrate was not treated for better wetting with water as its contribution to the free energy is less significant. Although filling of channels is possible in spite the use of a hydrophobic substrate, the possibility of tearing non-adhesive material off the substrate later in the process upon removing the PDMS mold has to be taken into account. In the real filling process, the capillary forces have to overcome the viscous drag of the suspension. For simple cylindrically shaped capillaries, the filling velocity as function of filling length is dependent on the volume to surface ratio of the channel, the contact angles and the viscosity.[15] Solving for filling length z results in a root law z =

r ( γSV – γ SL ) ----------------------------- ⋅ t 2η

(1)

where r is the hydraulic radius r = V/A (ratio of volume to surface area), γ the surface tensions for solid-vapour (SV) and solid-liquid (SL), η the viscosity of the liquid, and t the time. However, multiple factors were not considered in this simple model. First, the viscosity of the suspension itself as a non-Newtonian fluid is a function of filling velocity or of the shear rate, respectively. Second, because of continuous drying of the suspension during MIMIC, the time before the suspension coagulates is limited. This time frame was estimated to be about 30-60 s for a 33 vol% suspension. Third, the channel geometry is not cylindrical. In the present case, the cross-section of the PDMS capillaries were semi-circular - a shape that resulted from the photoresist profile of the master structures. Fig. 3–2 shows micrographs at different scales of annealed ceramic microstructures obtained from 33 vol% tin oxide suspensions. Obviously, there is a

MICROMOLDING IN CAPILLARIES

statistical distribution of the final microline length due to the various influences just mentioned. Suspensions of higher solids content exhibit much larger viscosities than polymer solutions or prepolymer mixtures which were diluted in many cases. Therefore, microchannels of a total length of 5 mm often may not fill up to their full length. The microchannel geometry with its sharp corners, where PDMS and substrate are joined, also distorts the shape of the suspension surface during filling. Around those corners, the suspension seems to travel faster. Therefore, two fork-shaped spikes of 1-2 µm width emerging from the solid ceramic microstructures are usually observed (Fig. 3–2c). The same behavior also was reported for similar microstructures fabricated using polymer materials.[12] If the samples are left to dry for a sufficiently long time (minimal 30 min), the removal of the PDMS mold by gently tilting it over one edge using tweezers is usually successful, i.e. the structures are not damaged by the PDMS removal. The structures were then annealed at 800°C for 5 hrs.

Fig. 3–2. a) top binocular view on annealed microstructured lines of tin oxide on a

silicon wafer substrate as obtained by spontaneous capillary filling from a 33% suspension. b) SEM close-up of a line showing its porous microstructure. c) structures at the end of the microlines. The fork-like spikes originate from the wetting characteristics of suspensions inside non-circular microchannels.

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The lines consist of an annealed powder network exhibiting an estimated porosity of 40-50%[10] with high specific surface which was intended for the application of such microstructures as miniaturized semiconducting gas sensors. The effect of solid loading was evaluated by preparing suspensions of solid contents ranging from 40% to 0.1%. We distinguished between high solid loadings > 15 vol% and low solid loadings < 1 vol%. For high solid loadings, viscosity curves, the average filling length and an evaluation of filling length versus viscosity are shown in Fig. 3–3. When preparing suspensions for MIMIC, the viscosity needs to be minimized. Tin oxide powder was in the present case sterically stabilized using a ammonium polyacrylate liquidifying agent, while the pH of the suspension was set to 8. Using this compositions, it was possible to obtain viscosities lower than 100 mPas (100s-1) up to 33 vol%, which is reasonably close to water (1 mPas, see Fig. 3–3a). Even these comparably low values for the viscosity lead to a viscous drag in the range of the capillary forces as stated in equation (1). Fig. 3–3b shows the mean final lengths of 10 µm wide lines versus the solid loading of the suspension. The error bars mark the standard deviation of 20 lines evaluated for each suspension. As expected, the filling length decreases with higher solid loadings. Only the 15 vol% suspension reached the other end after approx. 5 mm and filled the available channel length of the mold fully. On the other hand, the scattering of the lengths at high solid loadings becomes smaller, the MIMIC process more reproducible. It is important to note that these results apply only to the spontaneous filling process and to this powder. Other powders will result in different rheology behavior and hence, in different filling characteristics. As other experiments showed, MIMIC can be performed using a variety of different powders (Al2O3, ZrO2) provided that the grain size is sufficiently small (roughly 1/10 of the capillary diameter) and that lowviscosity suspensions can be prepared. No external aids to further extend the filling length were applied. Possible methods could consist of slowing down the drying of the suspension by cooling the suspension to lower temperatures or of performing MIMIC under ultrasonic agitation. Alternatively, the elastomeric properties of the PDMS polymer can be of use to keep the filling process running by gently compressing and releasing the PDMS mold. Occasionally, it was possible to fill a capillary of 5 mm length with 33% sus-

MICROMOLDING IN CAPILLARIES

pension using this PDMS compressing method. However, it proved difficult to quantify the impact of such length-enhancing methods. A maximum viscosity of 0.1 - 0.2 Pa s was estimated from the results shown in Fig. 3–3c to obtain filling lenghts of at least 1 mm. We consider a filling length in the range of 1 mm well suited for most prototype MEMS applications. Qualitative agreement with the model from Eq. (1) was observed. The real process seems to be even more sensitive to viscosity as the filling length decreases more rapidly as the fitted η-1/2 curve. Based on observed initial filing velocities of 60-75 µm s-1 and the dimensions of the capillary, a shear rate of about 15 s-1 was calculated for this MIMIC process. Therefore, the viscosity values evaluated in Fig. 3–3c were taken at 15 s-1. Video microscopy on capillary filling of 33% suspensions also revealed that the filling is a discontinuous process. The running front of the suspension stops for a few ms after every entering 40 - 100 µm before continuing to advance inside the capillary. With time, the number of phases of movement becomes more and more scarce until the suspension begins to coagulate due to drying.

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a

b

c

Fig. 3–3. a) viscosity curves measured on the tin oxide suspensions steri-

cally stabilized with a poly-acrylate additive. b) final filling length of 10 mm wide PDMS capillaries as function of the solids loading of the tin oxide suspensions used. The value of 15 vol% suspensions is bracketed because the approx. 5 mm capillary length was filled up to its end, whereas all other suspensions did not fill the capillaries completely. c) plot of filling length versus viscosities taken at shear rates of 15 and 100 s-1.

MICROMOLDING IN CAPILLARIES

Fig. 3–4. Scanning electron microscope cross-sections of microlines prepared by

snapping the silicon wafer substrates. The samples were imaged without conductive caotings.

A second, equally important issue is the reproduction quality of the ceramic microlines. Although the 15 vol% suspension filled the channels completely, the distribution of powder after drying was very uneven. Many lines even exhibited gaps, where not enough powder was available to fully coat the substrate surface. Again, video microscopy served as an important tool to gain a better understanding of the process. As the solid loading is reduced to 15 vol% and lower, the suspension inside the capillary takes much longer to dry. Eventually, the suspension remainder droplet outside the PDMS mold dries before the suspension inside the capillaries. In those cases, the liquid phase inside the capillaries was observed to be drawn out of the capillary at very high speed due to capillary forces in the porous network of the dried droplet outside the PDMS. The stabilized fraction of particles is drawn along until it hits areas of precoagulated particles where they block the channel quickly, leading to an uneven distribution of powder inside the capillary. The

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receding suspension front again becomes fork-like shaped. Therefore, the corners of the capillaries dry last and thereby the remaining particles are aligned along the edges of the capillary. Such powder movements during drying were observed up to 25 vol% suspensions, although the impact on the final microstructure with 25% suspensions was not as pronounced. In that case, it only textured the surface of the line. Having discussed width and filling length in suspension-based MIMIC, the resulting structure height and cross-sections of the microlines are assessed in Fig. 3–4. The cross-sections were prepared by scoring and snapping the silicon wafer substrates perpendicular to the microlines. Only 40% and 33% suspensions were able to fill the full height of the capillaries of 6.5 -7 µm, while the 15% microline has a typical thickness of only 2 µm. The bell-shaped outline is attributed to drying shrinkage. This may also be of advantage for the removal of the PDMS mold, since the green body is assumed to break contact with the PDMS surface. For the application as miniaturized gas sensors, the inclined side walls also facilitated the electrical contacting by sputtering Pt metal contacts directly onto the microlines. Should a steeper side wall profile be required, an alternative microfabrication approach of casting ceramic suspensions directly into photo resist structures can be taken.[6,16] MIMIC with suspensions of very low solid loading can produce even smaller structures than the capillary. Lines of only 2 - 3 µm width were produced by exploiting the spikes of the drying suspension front along the PDMS-substrate corners (see Fig. 3–5). An appropriate solid content that leads to continuous lines was found at 1%. Lower solid loadings (e.g., 0.1%) resulted in unconnected dots of powder, higher solid loadings to unspecifically coated areas within the capillaries. Without changing the capillary size, line structures a factor of 5 smaller were obtained. A 1% suspension was observed to remain liquid inside the channels up to 30 min before drying off fast. In previous publications we have exploited a similar process for the alignment of V2O5 nanotubes by MIMIC.[17]

MICROMOLDING IN CAPILLARIES

Fig. 3–5. Very small double microlines of 1-2 µm width produced using a

1% tin oxide suspension. Higher solid contents lead to powder depositions on the whole substrate surface inside the microchannels, lower solid loadings to disconnected lines. The inside edge of a double line is clearly resolved, whereas the suspension penetrated the interface between PDMS mold and substrate somewhat, leading to rough outside edges.

3.4. Summary A low cost method to introduce new line-shaped ceramic coatings made from tin oxide powders was presented. Unlike traditional thin films, these structures feature an aspect ratio about 1 and, depending on the powder and annealing conditions used, may exhibit a controlled porosity. The full range of available ceramic powders is applicable, provided that the powder can be stabilized in a colloidal suspension to yield sufficiently low viscosities. Using micromolding in capillaries, ceramic components with typical cross-sections of 3 - 10 x 10 µm2 and lengths up to 5 mm are easily reproduced. However, the method lacks exact control on the filling length. MIMIC with ceramic suspensions is a rapid prototyping method before resorting to more sophisti-

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cated microfabrication technology. Possible fields of application include functional ceramic coatings in sensors, or as structural elements, possibly as heat resistant spacers. Such microstructures may also be of service in ceramic materials science, creating addressable microstructures, e.g. where single grains transforming during a sintering process may be found and reanalyzed afterward. Different ceramic powders deposited close to each other may be subjected to identical treatment conditions (thermal history, etc.) due to their proximity.

MICROMOLDING IN CAPILLARIES

3.5. References [1] M. Mehregany, C. A. Zorman, N. Rajan, C. Hung Wu, Silicon Carbide MEMS for Harsh Environments, Proc. IEEE, 1998, 86, 8, 1594-1610. [2] J. F. Li, S. Sugimoto, S. Tanaka, M. Esashi, R. Watanabe, Manufacturing silicon carbide microrotors by reactive hot isostatic pressing within micromachined silicon molds, J. Am. Ceram. Soc., 2002, 85, 1, 261-263. [3] B. A. Vojak, S. J. Park, C. J. Wagner, J. G. Eden, R. Koripella, J. Burdon, F. Zenhausern, D. L. Wilcox, Multistage, monolithic ceramic microdischarge device having an active length of similar to 0.27 mm, Appl. Phys. Lett., 2001, 78, 10, 13401342. [4] H. L. Tuller, Advanced Sensor Technology Based on Tin Oxide Thin Film MEMS Integration, J. Electroceram., 2000, 4, 2, 415-425. [5] N. Barsan, M. Schweizer-Berberich, W. Göpel, Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report, Fresenius J. Anal. Chem., 1999, 365, 287-304. [6] U. Schönholzer, R. Hummel, L. J. Gauckler, Microfabrication of Ceramics by Filling of Photoresist Molds, Adv. Mater., 2000, 12, 17, 1261-1263. [7] Y. Xia, G. M. Whitesides, Soft Lithography, Angew. Chem. Int. Ed., 1998, 37, 550575. [8] H. Yang, P. Deschatelets, S. T. Brittain, G. M. Whitesides, Fabrication of high performance ceramic microstructures from a polymeric precursor using soft lithography, Adv. Mater., 2001, 13, 1, 54. [9] P. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B. F. Chmelka, G. M. Whitesides, G. D. Stucky, Hierarchically Ordered Oxides, Science, 1998, 282, 2244. [10] M. Heule, L. Meier, L. J. Gauckler, Micropatterning of Ceramics on Substrates towards Gas Sensing Applications, Mat. Res. Soc. Symp. Proc., 2001, Vol. 657, EE9.4. [11] M. Heule, L. J. Gauckler, Gas Sensors Fabricated from Ceramic Suspensions by Micromolding in Capillaries, Adv. Mater., 2001, 13, 23, 1790. [12] E. Kim, Imbibition and Flow of Wetting Liquids in Noncircular Capillaries, J. Phys. Chem. B, 1997, 101, 855-863. [13] E. Kim., Y. Xia, G. M. Whitesides, Polymer microstructures formed by moulding in capillaries, Nature, 1995, 376, 581-584. [14] M. J. Owen, "Surface chemistry and applications" in “Siloxane Polymers”; edited by S. J. Clarson, J. A. Semlyen, PTR Prentice Hall, Englewood Cliffs, NJ, 1993, 309-372. [15] E. Kim, Y. Xia, G. M. Whitesides, Micromolding in Capillaries: Applications in Materials Science, J. Am. Chem. Soc., 1996, 118, 5722-5731. [16] M. Heule, L. J. Gauckler, Microfabrication of Ceramics based on Colloidal Suspensions and Photoresist Masks, J. Photopolym. Sci. Technol., 2001, 14, 3, 449. [17] H. J. Muhr, F. Krumeich, U. P. Schonholzer, F. Bieri, M. Niederberger, L. J. Gauckler, R. Nesper, Vanadium oxide nanotubes - A new flexible vanadate nanophase, Adv. Mater., 2000, 12, 3, 231.

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85 M. Heule, U. P. Schönholzer, L. J. Gauckler, submitted to Adv. Mater, 2003.

4.

Patterning Colloidal Suspensions by Selective Wetting of Microcontact-Printed Surfaces

Micropatterns of ceramic powders can be obtained by selective wetting of microcontact-printed surfaces. A large wetting contrast between hydrophilic micropatterns and hydrophobic areas was created. Aqeous colloidal dispersions of aluminum oxide and tin oxide adhered only to the hydrophilic micropatterns whereas they were repelled from the hydrophobic areas in a simple dip coating process. We examined two molecular ink/substrate systems: thiol self-assembled monolayers (SAM) on gold and octadecyltrichlorosilane (OTS) SAM on silicon wafer substrates. Corresponding contact angles obtained under varying printing conditions are presented. The chemical compositions of the printed layers were characterized by ToF-SIMS mass spectrometry. The thiol-gold SAM readily forms in microcontact printing whereas the OTS layer also contains a significant amount of PDMS residues. However, printing and selective wetting could be carried out successfully on both ink/substrate systems. Ceramic micropatterns with a resolution of 5 µm are shown. 4.1. Introduction Microcontact Printing (µCP) is an extremely versatile alternative to standard photolithography for direct micropatterning of a variety of substrates. It was introduced as one of a whole set of techniques for alternate microfabrication (Soft Lithography) using elastomeric molds for pattern transfer.[1] Elastomeric stamps are easily prepared by casting a prepolymer of polydimethylsiloxane (PDMS) against the topography of a master structure which

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themselves may be prepared by photolithography or other microfabrication methods. The stamps containing recessed microstructures are inked and subsequently brought into contact with substrates to transfer molecules in micropatterns by a simple printing process (Fig. 4–1a). The best characterized ink system is the printing of alkylthioles that form self-assembled monolayers (SAM) on gold surfaces.[2,3] The SAM may be used as an ultimatively thin mask for etching the gold layer selectively.[4,5] This has been demonstrated by using µCP for creating microelectrodes for cyclic voltammetry.[6] The achievable resolution may be in the submicron range, even on large areas.[7] Substrates do not necessarily need to be flat, as the work of Jackman et al. demonstrates.[8] Other printable substances include an enormous range of both ink and different substrates, for instance the printing of Pd catalysts for electroless deposition of patterned metal films,[7] zeolite layers[9] or biofunctional layers like patterned protein repellent polylysine-polyethyleneglycol co-polymers for cell adhesion studies.[10,11] Protein microarrays have also been presented.[12] Since µCP can be carried out without clean room equipment, the method is spreading quickly among various disciplines. One of the most striking features of µCP is the possibility to scale up the process for continuous mass fabrication since the process is the microfabrication equivalent of printing on paper. The group of Michel at IBM Corporate Research has undertaken first steps in that direction,[13,14] thereby synthesizing more stable PDMS-analog elastomers than the commercially available standard blend.[15]

MICROCONTACT PRINTING 2.

a

5.

1.

4.

PDMS stamp 3.

Substrate b

Alkylthiol-Au SAM formation C16H33

C16H33

S

+

S

Au

H

HDT

Cl

+

3 HCl

H2

Si-Wafer

C 18 H37 Cl

1/ 2

Au

Alkylsiloxane-SiO2 SAM formation Si

+

Cl

+

OH Si

C18 H37

(O H2O

Si O

)n

Si

OTS Fig. 4–1. a) principle of microcontact printing. (1) PDMS stamp is inked

with a SAM-forming molecule in solution and dried (2). The stamp is brought into contact with a substrate (3), transferring its pattern by forming the SAM (4). Substrates are then coated with colloidal suspensions by dip coating, yielding a surface with micropatterned thick films (5). b) reactions of SAM formation with hexadecanethiol (HDT) on gold and octadecyltrichlorosilane (OTS) on SiO2 surfaces.

In this article, we propose to supplement µCP with a scheme for the microfabrication of ceramic thick films. We use µCP for generating contrasting hydrophilic and hydrophobic areas which are then selectively wetted by a colloidal suspension to form micropatterns of ceramics. The article is part of a series in which other soft lithographic techniques have been used to fabricate small elements of ceramic powders.[16-20] Powders may offer several advan-

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tages over standard thin film ceramics, e.g. higher sensitivity and controlled porosity for gas sensing applications.[21,22] We examine two ink/substrate systems, the thiol-gold and the metal-less alkanetrichlorosilane chemistry which allows to print directly onto bare silicon wafer substrates, see Fig. 4– 1b. The layers obtained by µCP have been characterized by water contact angle measurements and more sensitively by ToF-SIMS (time-of-flight secondary ion mass spectrometry). 4.2. Experimental The masters containing a positive relief of the mold structures were prepared by standard photolithography using AZ4562 positive resist (Clariant GmbH, Wiesbaden, Germany). 9 g of Sylgard 184 PDMS prepolymer/catalyst mixture (Dow Corning Inc., Midland, MI, USA) were poured over a master structure and cured for 26 hrs. The substrates were silicon wafer pieces with a freshly evaporated layer of gold prepared by thermal evaporation in a Balzers MED 020 system at a pressure of 1.7 ·10-5 mbar, thickness ranging from 10.3 to 33.7 nm, including a layer of Cr (thickness 6.7 nm) in between to promote adhesion. For direct printing of octadecyltrichlorosilanes, bare Si wafer pieces were cut, rinsed with water (18 MΩ cm, Millipore), cleaned in an ultrasonic bath and subsequently treated in an oxygen plasma sterilizer for 2 min (Harrick PDC32G). µCP on Au layers. The PDMS stamp was inked for 30 min in 25 mL of 1-hexadecanethiol solution (Fluka AG, Buchs, Switzerland, used as received) in ethanol (38.2 µL, 1.25 x 10-4 mol, 5 mM). After this time the stamp was taken out of the solution and dried with a stream of nitrogen. Then it was placed on the substrate for 20 s before being removed. The printed substrate was immersed in a 25 mL solution of cysteamine hydrochloride HS(CH2)2NH2HCl (Fluka) in ethanol (0.0142 g, 1.25 x 10-4 mol, 5 mM) for 30 min, after which it was removed, washed in ethanol and dried with a stream of nitrogen.

MICROCONTACT PRINTING

µCP on Si. A 5 mM solution of Octadecyltrichlorosilane OTS (Fluka) in either hexane or ethanol was prepared just minutes before printing. Microcontact printing was carried out the same way as detailed above, with a contact time of 60 s. The substrates were heated to 60°C for 15 min and then rinsed with the solvent in use. Al2O3 suspensions (190 nm median particle size, 45 vol. %; Taimei Industries, Tokyo, Japan, TM-DAR 2831). The Al2O3 suspension (27.87 mL) was prepared in pure water, NH4Cl (0.0745g, 1.39 mmol, 0.05 M) and HCl (2 M, Titrisol, Fluka). An arbitrary amount of HCl was used here to reduce the pH of the suspension into the region 4-5. This suspension was ball milled overnight before use. SnO2 suspensions (220 nm median particle size, 33 vol%; Cerac Inc., Milwaukee, WI,USA). 21.1 g of SnO2 were added to 6.01 g of water containing 150 µL of dispersing agent which was prepared by mixing 6.65g water, 1.84 g polyacrylic acid sodium salt 2100 (Fluka) and 0.06g NH3, 25% in H2O). Again, the suspensions were homogenized by ball milling sequences throughout the powder adding procedure. Coating the printed substrates. The substrates were coated with suspension either by dip-coating or by dropping the suspension onto a tilted substrate (tilt angle approx. 70°). Sintering was done by heating the samples to 800°C for 5 hrs for SnO2, resp. 1100°C for 2 hrs for Al2O3 coatings. Surface characterisation. The static contact angle of water was determined on all samples. ToF-SIMS mass spectrometry was carried out in a PHI 7200 analyzer using a Cs+ primary ion beam with an incident energy of 8 keV. Samples were microcontact-printed not more than 3 hrs before measurement using blank PDMS stamps without microstructures. Reference samples were prepared by immersion in the ink solutions over night for the full SAM coverage references, and treating bare Si, resp. Au-coated pieces only with oxygen plasma for the blank reference samples. The ion dose was below the static limit (