synthesis of nanoscale semiconducting titanium

21. – 23. 9. 2011, Brno, Czech Republic, EU

SYNTHESIS OF NANOSCALE SEMICONDUCTING TITANIUM OXIDE PILLARS ARRAY AND INVESTIGATION OF ITS STRUCTURAL AND HUMIDITY PROPERTIES Dmitry SOLOVEI, Jaromir HUBALEK Department of Microelectronics, Brno University of Technology/Faculty of Electrical Engineering and Communication, a. s., Technická 3058/10, CZ-616 00, Brno, Czech Republic, EU, [email protected] Abstract This work presents methods for the formation of semiconducting metaloxide titanium nanopillars arrays and investigation results of its morphological, structural and resistive humidity properties for biological applications. The arrays of titanium oxide nanopillars were formed by using the electrochemical anodization of titanium through a porous alumina mask. The crystal modification of rutile titanium oxide was obtained by thermal annealing in the air. The resistive properties of semiconducting nanostructural titanium oxide layer were observed under the influence of different relative humidity level. Keywords: titanium oxide nanopillar arrays, rutile, porous anodic alumina, resistive humidity sensor INTRODUCTION Recently, with the development of nanotechnology, arrays of nanostructured oxide (nanodots, nanopillars, nanowires, nanopores) system of valve metals take on increasing popularity as a basis for sensitive layers of different sensors [1]. The main valve metals to produce nanostructured oxides for sensor applications are titanium, tungsten, zirconium, niobium, tin and etc. [2 – 6]. Nanostructures prepared on the surface with very simple technique have promising properties as for sensing ions in vapors and gases, and also for electronic components. The gradual development of science in the area of nanoelectronics can significantly reduce the size of such sensors and their power consumption, which open the possibility of using them as in-situ or wearable sensors e.g. for human biological processes monitoring [7 – 8] close to the skin or vapours, humidity and gases measuring in mobile applications. We have developed a sensing device based on arrays of metaloxide titanium pillars for rapid relative humidity measurement. FORMATION METHODS AND RESULTS The initial structure was a vacuum magnetron sputtered two-layer Ti/Al metal system on oxidized silicon substrate. Titanium layer has a thickness of 200 nm and it is placed under a layer of aluminum with thickness of 2 µm. An array of nanopillars of titanium oxide was formed by electrochemical anodization through the porous anodic alumina mask. Anodization of aluminum and titanium layers was performed in 0.9 M solution of oxalic acid at the potential of 32 V for aluminum layer. A formed porous matrix of aluminum oxide has pore diameter of about 15 nm, arranged in steps of 75 – 80 nm and a height of 2.8 microns. When anodization front reached titanium underlayer, the anodization potential began to increase and its growth continued up to a certain limited value at 37 volts (see Fig. 1, a). Then the anodizing mode was switched to the constant potential at which lasted electrochemical anodization of titanium underlayer. In this case, the current in the electrochemical system began to decline exponentially, driven by the rising pillars of titanium oxide at the bottom of each pore of the matrix of anodic aluminum oxide (see Fig. 1, b). The process of titanium anodization lasted up to 30 minutes and after reaching constant minimum value of anode current (of about 2 50 µA/cm ) it switched off and the samples were washed and dried. The whole electrochemical process was operated by computer and carried out by GPIB interfaces power supply and multimeters. −1

−1

Afterwards, the porous alumina matrix was selectively removed in 20 g l CrO3, 35 ml l H3PO4 solution kept at 65 °C for 15 min opening the array of titan ium oxide nanopillars (Fig. 1). Obtained pillars had the following dimensions: the average diameter 40 ± 5 nm, height 70 – 75 nm and spacing step 80 nm. Titanium oxide pillars were separated by intervals of metallic titanium width of about 30 – 40 nm (Fig. 2).


35

0.0007

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Potential, V

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20 15 10 5

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Fig. 1 Anodizing kinetics of Ti/Al metallic system: a – current constant mode, b – potential constant mode The resulting nanostructured titanium oxide layer was used as a basis for subsequent process steps such as photolithography, electrochemical deposition of gold and thermal annealing. Using photolithographic techniques on the surface of Ti/TiO2 layer, a photoresist mask was obtained for the formation of gold electrodes that provide electrical contact to the sensor structure. The mask was made in the form of an interdigital structure with electrode width of 100 microns separated by a layer of Ti/TiO2 in the width of 50 microns. The samples with the mask of photoresist were used for electrochemical deposition of gold, which was conducted in an aqueous solution of K[Au(CN2)] with the addition of H3BO3. Electrochemical 2 deposition was performed at 50 °C in a constant cur rent mode with the density of about 4 mA/cm for 5 min and the potential in an electrochemical system at 1.3 – 1.1 volts. After performing the electrochemical deposition of gold, plasma etching of photoresist was carried out in oxygen containing plasma for 60 min. Then the samples were annealed in air at 600 °C for 2 hours. This operation is necessary to improve the adhesion of electrochemically deposited gold layer to titanium, for the formation of crystalline phases of titanium oxide – rutile and for the complete oxidation of the remaining metallic titanium between titanium oxide nanopillars.

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b

Fig. 2 SEM images of anodized titanium oxide pillars sensing layer: a – lower magnification, b – higher magnification


The crystalline structure of titanium oxide nanopillars film was determined by X-ray diffractions studies (XRD). The XRD measurements indicated that the technology was employed for the growth of the TiO2 nanopillars film led to the formation of rutile nanocrystalline structures with one strong peak at 2θ = 69° (see Fig 3). 1800000 1600000 1400000

TiO2 - rutile

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2-Theta (degree) Fig. 3 X-ray diffractogram of the TiO2 nanopillars array film

After thermal annealing, the samples were fixed in the case and held by wiring conclusions. Fig. 4 shows images of a sensor prototype obtained by the above described manufacturing process. Then measurements of the sensor resistance change influenced by various air humidity levels were provided.

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Fig. 4 Sensor prototype images: a – sensor structures in the case, b – SEM images of inter-digital gold electrodes formed onto sensing layer, c – geometrical parameters of inter-digital gold electrodes

Resistance measurement of a sensor at various values of humidity control To measure the resistance of a sensor at various levels of air humidity we used the saturated salt solutions providing stable air humidity near a surface. We used the following saturated salt solutions: Lithium Chloride (relatively humidity 11 %), Magnesium Chloride (relatively humidity 33 %), Potassium Carbonate (relatively humidity 43 %), Magnesium Nitrate (relatively humidity 57 %), Sodium Chloride (relatively humidity 75 %), Potassium Chloride (relatively humidity 85 %), and Potassium Sulphate (relatively humidity 97 %). During the


measurements sensor was placed at the distance of 3 – 4 mm from the surface of the salt solution, the resistance was measured by a multimeter Agilent 34410A and recorded for 3 min. The measurement results are presented in Table 1 and graph (Fig. 5).

Table 1 Relatively humidity measurements Number of measurement

Relative humidity, %

Resistance, MOhms

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11

> 1500

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33

1200

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43

420

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57

200

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75

40

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85

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97

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RH, % Fig. 5 Sensor resistance response to relative humidity As can be seen from the graph, with increasing relative humidity sensor resistance is greatly reduced from the values of several GOhms to a value of 0.5 MOhms. The total change in resistance sensor was about 3 orders, so for the convenience of view a resistance plot on the relative humidity is constructed in logarithmic coordinates, in which the plot dependence is of an almost linear form. Most relevant sensor resistance is within a relative humidity of 30 to 97 %. Hypothetical mechanism of interaction of nanostructured titanium oxide with water molecules is as follows. Water is expected to absorb at exposed fivefold coordinated Ti sites in each case with the hydrogen atoms pointing away from the surface. Since hydrogen bonding interactions between the absorbed species and the bridging oxygen atoms of the substrate are expected to facilitate proton transfer. An H2O molecule adsorbed in a vacancy would provide a geometrically particularly well-suited adsorption site for O–H interaction and dissociation [9].


The data obtained by measuring the resistance of the sensor depending on the humidity must be subsequently refined during the measurements directly close to the measured object e.g. human skin, but the order of the resistance must be located in the same range if possible with only ± 10 %. CONCLUSION Based on electrochemical technology, as well as on the use of microelectronical methods, a prototype of a planar relative humidity sensor has been designed and manufactured. The sensor constructed to be sensitive to water molecules is based on nanostructured titanium oxide layer, located in the crystalline phases – rutile. The basis of the sensor is a sensitive layer consisting of nanosized pillars of titanium oxide on the surface whereas electrodes are deposited in the form of an inter-digital transducer. When measuring resistivity of the sensor using different values of relative humidity to obtain adequate resistance, data vary 9 6 from 10 to 10 Ohms when the ambient humidity is of 11 to 97 %. Adsorption of water molecules on the surface of nanostructured crystalline titanium oxide was presumably carried out by well-known mechanism of oxygen vacancies. Thus, studies of the dependencies concerning the resistance of humidity sensor showed that the fabricated sensor prototype can be used to control the humidity level in ambient or several specific applications. ACKNOWLEDGEMENTS The work has been supported by the grants GACR GA102/09/1601 and in the frame or Research Plan MSM0021630503. Many thanks also belong to SCHOTT Electronic Packaging Lanskroun s.r.o. for their packages used in the experimental work. LITERATURE [1.]

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