The Nanomechanical NOSE H.P. Langa,b, M.K. Ballera,b, F.M. Battistonb, J. Fritz a,b, R. Bergerc, J.-P. Ramseyerb P. Fornarob, E. Meyerb, H.-J. Güntherodtb, J. Bruggera, U. Drechslera, H. Rothuizena, M. Desponta, P. Vettigera, Ch. Gerbera, J.K. Gimzewskia a
IBM Research Division, Zurich Research Laboratory Säumerstrasse 4, CH-8803 Rüschlikon (Switzerland) b
Institute of Physics, University of Basel
Klingelbergstrasse 82, CH-4056 Basel (Switzerland) c
IBM SSD GmbH,
Dept. 4119, Hechtsheimer Strasse 2, D-55131 Mainz (Germany) e-mail
[email protected]; Tel: ++41 1 724 8407
Abstract We present a novel chemical sensor based on a microfabricated array of silicon cantilevers. Individual cantilevers are sensitized for the detection of analytes using metal coatings. Analyte molecules chemisorbing or physisorbing on the cantilever coating and chemical reactions produce a change in interfacial stress between analyte molecules and cantilever. This leads to a nanomechanical response of the cantilever, i.e. bending. The bending is read out using a time-multiplexed optical beamdeflection technique. From magnitude and temporal evolution of the bending, quantitative information on analyte species and concentration is derived. Here, we demonstrate the detection of ethene and water vapor with such a nanomechanical nose.
Introduction Present and future applications in quality and process control, biomedical analysis, medical diagnostics, environmental control, continuous and long-term monitoring of pollutants, forensics, fragrance design, and oenology require small, fast-responding, versatile, polyfunctional, multi purpose sensors that can be easily tailored to customer’s demands. Sensors produced cheaply may be disposable, especially for diagnostic applications. The approach presented here is based on microelectro-mechanical systems (MEMS) and nanotechnology: a microfabricated array of silicon cantilevers that can be easily mass-produced. Each cantilever is sensitized to a specific chemical transduction mechanism by coating it with an individual sensor layer. Exposing the sensor array to an analyte, the cantilevers bend in the nanometer range because changes in surface stress occur due to adsorption of analyte molecules on the sensor surface. Both physisorption or chemisorption processes are transduced into a
nanomechanical response, with the latter having a much greater effect on bending. Based on pioneering experiments on catalysis using micromechanical cantilevers for detection [1] it has been demonstrated that tiny quantities can be measured with cantilever-type sensors. Cantilever bending has been observed due to surface stress change upon adsorption of attomolar quantities of alkane thiols onto gold-coated cantilevers. This involves a surface stress change for formation of a complete self-assembled monolayer of alkane thiols scaling with alkane chain length [2]. A phase transition of alkanes has been found to produce heat changes in the nanojoule range, which can be monitored by the bending of a bimaterial cantilever [3]. Driving the cantilever at its resonance by a piezoelectric actuator has allowed mass changes in the picogram range to be tracked during water adsorption in nanoporous zeolite single crystals [4]. Devices based on single cantilevers have been used for photothermal spectroscopy [5], surface stress [6], and infrared detection [7-9]. Integration of such cantilevers in arrays allows the combination of various sensing capabilities within a compact analysis chamber with a small volume. Some of the cantilevers may serve as reference sensors, i.e. sensors that do not react with the analyte. Such a setup allows the operation of the sensor array even under conditions that impose disturbances on the whole array, e.g. turbulent gas flow, abrupt pressure changes, or mechanical vibrations. By evaluating the difference in responses from a sensor cantilever and a reference cantilever, a small sensor response can be extracted in a noisy environment [10,11]. Furthermore, by averaging signal responses, thermal noise or electronic noise originating from an amplifier can be reduced. By operating the sensor array in several operation modes, such as cantilever bending (static mode) or cantilever resonance frequency shift tracking (dynamic mode), we can
obtain ‘orthogonal’ responses to distinguish between similar compounds.
Experiment We have developed a modular and expandable system for the optical readout of cantilever sensor deflection, data processing, and acquisition. The optical sensor deflection unit (NOSE, Nanotechnology Olfactory Sensors) consists of a sensor array with individually coated cantilevers, which uses the sensor layers to transform physical and chemical reactions of the analyte into a nanomechanical response. The deflection of the cantilevers is optically read out by recording the position of an incident light spot reflected off the cantilever's apex by using a position-sensitive detector (PSD) as shown in Fig. 1. The photocurrents flowing from opposite electrodes of the PSD are individually converted into electrical voltages and amplified in a first preamplifier stage mounted directly on the PSD. The output signals are fed into a differential amplifier for further amplification, followed by analog-todigital conversion (ADC). A regulating circuit ensures constant light intensity at the surface of the PSD by adjusting the operating voltage of the light sources after reflection off the apex of the cantilever. An additional offset voltage is added to the signal to enable optimal signal amplification and full use of the dynamic range of the ADC. To facilitate read out of a large number of cantilevers in a sequential manner, a time multiplexing scheme [10] is applied. Each of the eight light sources can be switched on and off individually.
An automated analyte dosing system is used for programmed exposure of the sensor array to various gaseous analytes. Commercial digital mass flow controllers (Bronkhorst, The Netherlands) controlled by the data acquisition software are able to adjust gas flows in the range of 2 to 100 ml/min. Using a set of two mass flow controllers it is possible to mix the carrier gas (dry nitrogen, purity: 99.999990%, dried using a “Drierite” water-free CaSO4 column) with well-defined amounts of analyte. In the experiments presented in the Results section, the analytes investigated are high-purity (99.9995%) ethene and nitrogen saturated with water. The corresponding experimental setups are shown in Figs. 2 and 3.
Figure 2: Setup of the gas handling system for detection of ethene. F1 and F2 denote mass flow controllers.
Figure 1: Schematic setup of a NOSE module.
Figure 3: Setup of the gas handling system for detection of humidity.
NOSE modules and ADCs are controlled by a microcontroller board (µCB) via a bus. The µCB adjusts the offset of the amplifiers and transfers data periodically to a personal computer (PC).
The sensor array consists of eight beam-type silicon cantilevers with a length of 500 µm, a width of 100 µm, and a thickness of ~ 1 µm. The cantilever pitch is 250 µm (Fig. 4). The surface of the cantilevers is naturally coated with the native oxide of silicon. No further surface preparation has
been performed before coating the cantilevers with a sensor layer. Figure 5 shows a schematic cross section through the cantilever sensors employed in this work. A platinum layer (thickness: 30 nm) was deposited on the Si cantilever by rf sputtering in a commercially available sputtering apparatus (Balzers Med 010, Switzerland).
film growth rates were 0.02 nm/s for Ti, and 0.14 nm/s for Au. No further treatment such as annealing was applied to the layer. The layer sequence shown in Fig. 5c was achieved in a two-step gold deposition process. After deposition of the first gold layer, the cantilever was turned over for evaporation of the second gold layer. The adsorption of analyte on the cantilever imposes the formation of surface stress on the cantilever surface, leading to a bending of the cantilever (Fig. 6). Hence, a chemical process is transduced into a nanomechanical response.
Figure 4: Optical micrograph of a cantilever sensor array. The pressure during deposition was 0.05 mbar, the sputtering current was 10 mA. The distance between sputtering target (Pt, 99.99%) and the cantilever was 5 cm. These conditions were found to yield uniform platinum cantilever coatings appropriate for our experiments (Fig. 5a).
Figure 6: Schematic representation of the transduction of a chemical process into a nanomechanical response. The front side of the cantilever is coated by a sensor layer, whereas the backside is uncoated. (a) Cantilever before exposure to the analyte. (b) Bending of the cantilever during adsorption of an analyte layer due to formation of surface stress.
Results and Discussion
Figure 5: Schematic cross section of the cantilevers used in the experiments presented in this work. (a) Pt on one side, (b) Au on one side, (c) Au on both sides. Figure 5b schematically depicts a cantilever coated on one side by gold. The gold coating (thickness: 30 nm) was prepared in an electron beam evaporator at a pressure of 2.10-6 to 8.10-6 mbar. To improve adherence of gold, a titanum layer (thickness: 1.5 nm) was employed before growing the gold layer (without breaking the vacuum). The
To test the performance of the sensor array, we used an array of Pt-coated cantilevers, which had been exposed to well-defined mixtures of dry nitrogen and ethene using the setup displayed in Fig. 2. The total mass flow was 100 ml/min. As ethene is known to chemisorb on platinum surfaces, one expects a surface stress change and a bending of the cantilevers associated with the adsorption process [12]. Ethene absorption on platinum is reversible, i.e. stress is relieved after purging the analysis chamber with dry nitrogen. An ethene concentration profile of 0% to 100% in steps of 20% with intermittent concentrations of 0% of ethene was cycled. For clarity only the trace of nanomechanical deflection of one of the platinum-coated cantilevers is shown in Figure 7 (static mode deflection). The
sensor response is approximately linear with ethene concentration. The time scale of the response is of the order of a few minutes, e.g. one minute for the 0% to 20% step, or five minutes for the 0% to 60% ethene concentration step. Note that the deflection response of the sensor is reversible when switching back to dry nitrogen purge gas.
cantilever response. Using the setup proposed above, reference cantilevers can be employed on the same chip together with the sensor cantilevers. This ensures that the reference and the sensor cantilevers are imposed to identical experimental procedures within a very small analysis chamber volume.
Figure 7: Reaction of platinum with ethene (after baseline correction). The percentage values indicate the amount of ethene gas flow in dry nitrogen carrier gas. A signal of 1 V corresponds to a deflection of ~1 µm. The setup described in Fig. 3 was used to demonstrate the detection of water vapor using four cantilevers coated with a gold layer on one side. The remaining four cantilevers of the array were coated on both sides with a 30-nm-thick layer of gold to serve as reference cantilevers. Figure 8 shows traces of the eight cantilevers of the array on exposure to a mixture of dry nitrogen with nitrogen saturated with water to yield nitrogen at a relative humidity of 80%. Because the gold coating is inert to water vapor no deflection response occurs for the cantilevers coated on both sides with gold (lower four traces in Figs. 8a and 8b). However, the traces of the four cantilevers that were only coated on one side with gold do show a deflection response (upper traces in Figs. 8a and 8b). Here, the uncoated back surface of the cantilevers, which consists of native silicon oxide, acts as a sensor. The response is reproducible over time (Fig. 8a) while cycling relative humidity values of 0% and 80%. The response is even proportional to the amount of humidity present in nitrogen. Figure 8b shows a sequence of relative humidity between 20% and 100% with intermittent intervals of 0% relative humidity (upper four traces). The reference cantilevers (coated on both sides with gold) show no bending at all (lower four traces in Fig. 8b). These results indicate that a chemical sensor can be built using a microfabricated array of cantilever sensors. Both quantitative (signal magnitude) and qualitative (signal shape) imformation can be obtained. The use of reference cantilevers is essentially important for extracting true signals from a nanomechanical
Figure 8: Static cantilever bending response of an array of eight cantilevers (1 V corresponds to ~0.2 µm). Four of them are coated on one side with a gold layer (four upper traces) and four are coated with gold on both sides for reference (four lower traces). (a) Cyclic exposure to nitrogen at 80% relative humidity. (b) Cyclic exposure to various mixtures of dry nitrogen and nitrogen saturated with water (total flow rate: 100 ml/min).
Conclusion In conclusion, we have demonstrated that a chemically functionalized micromechanical cantilever array can be employed as a versatile, ultrasensitive detector for various gaseous analytes. Its major advantage is its capability to use reference sensors for differential measurements (background subtraction). The device is designed to operate in various exchangeable media such as ambient air, vacuum, gases, and liquids. The micromechanical design of the sensors implies short response times and high sensitivity over a wide range of operating temperatures in addition to the capability to eventually be integrated seamlessly into microelectronic devices.
Acknowledgments We thank P. Guéret and P. Seidler for their support. We are grateful to E. Delamarche for helpful discussions. This work is partially funded by the Swiss National Science Foundation program MINAST (Micro- and Nanostructure System Technology) Project 7.04 NOSE and the program NFP 36 (Nanoscience).
References [1]
[2]
[3]
[4]
[5]
J.K. Gimzewski, Ch. Gerber, E. Meyer and R.R. Schlittler, ’Observation of a chemical reaction using a micromechanical sensor‘, Chem. Phys. Lett. 217, 1994, pp. 589-594. R. Berger, E. Delamarche, H.P. Lang, Ch. Gerber, J.K. Gimzewski, E. Meyer and H.-J. Güntherodt, ’Surface stress in the self-assembly of alkanethiols on gold‘, Science 276, 1997, pp. 2021-2024. R. Berger, Ch. Gerber, J.K. Gimzewski, E. Meyer and H.-J. Güntherodt, ’Thermal analysis using a micromechanical cantilever‘, Appl. Phys. Lett. 69, 1996, pp. 40-42. R. Berger, Ch. Gerber and J.K. Gimzewski, ’Nanometers, picowatts, femtojoules: thermal analysis and optical spectroscopy using micromechanics‘, Analytical Methods & Instrum., Special Issue µTAS'96, 1996, pp. 74-77. J.R. Barnes, R.J. Stephenson, M.E. Welland, Ch. Gerber and J.K. Gimzewski, ’Photothermal spectroscopy with femtojoule sensitivity using a micromechanical device‘, Nature 372, 1994, pp.79-81.
[6]
G.Y. Chen, T. Thundat, E.A. Wachter and R.J. Warmack, ’Adsorption induced surface stress and its effects on resonance frequency of microcantilevers‘, J. Appl. Phys. 77, 1995, pp. 3618-3622. [7] T. Thundat, S.L. Sharp, W.G. Fischer, R.J. Warmack and E.A. Wachter, ’Micromechanical radiation dosimeter‘, J. Appl. Phys. 66, 1995, pp. 1563-1565. [8] J. Varesi, J. Lai, Z. Shi, T. Perazzo and A. Majumdar, ’Photothermal measurements at picowatt resolution using uncooled micro optomechanical sensors‘, Appl. Phys. Lett. 71, 1997, pp. 306-308. [9] S.R. Manalis, S.C. Minne, C.F. Quate, G.G. Yaralioglu and A. Atalar, ’Two dimensional micromechanical bimorph arrays for detection of thermal radiation‘, Appl. Phys. Lett. 70, 1997, pp. 3311-3313. [10] H.P. Lang, R. Berger, C. Andreoli, J. Brugger, M. Despont, P. Vettiger, Ch. Gerber, J.K. Gimzewski, J.-P Ramseyer, E. Meyer and H.-J. Güntherodt, ‘Sequential position readout from arrays of micromechanical cantilever sensors’, Appl. Phys. Lett. 72, 1998, pp. 383-385. [11] H.P. Lang, R. Berger, F. Battiston, J.-P. Ramseyer, E. Meyer, C. Andreoli, J. Brugger, P. Vettiger, M. Despont, T. Mezzacasa, L. Scandella, H.-J. Güntherodt, Ch. Gerber and J.K. Gimzewski, ’A chemical sensor based on a micromechanical cantilever array for the identification of gases and vapors‘, Appl. Phys. A 66, 1998, pp. S61-S64. [12] F.C. Tompkins, Chemisorption of Gases on Metals (Academic, London, New York, San Francisco, 1978).
