An integrated amperometric microsensor

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Sensors and Actuators, BI (1990) 303-307

An Integrated Amperometric Microsensor WILLIAM J. BUTTNER Center for Environmental Research, Biological, Enviromental and Medical Research Division, Argonne National Laboratory, Argonne, IL 60439 (U.S.A.) G. JORDAN MACLAY Microelectronics Laboratory, Department of Electrical Engineering and Computer Science, University of Illinois, Chicago, IL 6M80 (U.S.A.) J. R. STETTER Transducer Research, Inc., 1288 Olympus Drive, Naperville, IL 60540 (U.S.A.)

Abstract An amperometric microsensor has been fabricated using techniques adapted from conventional microelectronic technology. The device was tested with 100 ppm of NO in humidified air, and gave a response an order of magnitude greater than a commercially available sensor and two orders of magnitude greater than a previously fabricated microsensor with special electrodes which were defined ‘with metal masks’ (G. J. Maclay, W. J. Buttner and J. R. Stetter, IEEE Trans., 35 (1988) 793). This previous work indicated that improved signal/noise would be expected for an electrode with a larger ratio of the square of the perimeter to the area (P2/A). The sensor was fabricated on a thermally oxidized layer of silicon dioxide on a silicon wafer. A gold layer of 3000 8, thick was deposited by thermal evaporation and then patterned by photolithography and etching. In order to obtain good adhesion of the gold to the silicon dioxide it was necessary to first deposit a thin layer of chromium. The working electrode was etched to contain many windows about 50 micrometers square in order to obtain an improved sensor geometry with large P2/A. A thin lilm of Nafion (a Du Pont trade mark) was spin-coated over the electrodes and served as the electrolyte (G. J. Maclay and J. R. Stetter, U.S. Patent applied for (1988)). A fixture was built to expose the Nafion surface of the sensor to gases. The response of the integrated microsensor to NO per gram of gold catalyst was about 5000 times greater than in the commercial sensor. Unfortunately the presence of the chromium adhesion layer led to corrosion in the electrodes which eventually destroyed the sensor operation. Introduction Amperometry is a method of electrochemical analysis in which the signal of interest is a current 09254005/90/$3.50

that is linearly dependent upon the concentration of the analyte. As the chemical species approaches the working electrode (W.E.), electrons are catalytically transferred from the analyte to the W.E. (oxidation) or to the analyte from the W.E. (reduction). The direction of flow of electrons is dependent upon the properties of the analyte and can be controlled by the electric potential applied to the W.E. The transfer of charge is measured as a current. For sensor application, the W.E. is often referred to as the sensing electrode. To maintain charge neutrality within the sample, a counter reaction occurs at a second electrode, the counter electrode (C.E.). The current flows between the W.E. and C.E. in the external circuit in the measured sensor output. The C.E. and W.E. are in mutual contact with an ion-conducting electrolyte which is required to complete the electrical circuit internally. Amperometry has numerous industrial, medical, and environmental monitoring applications. There are numerous advantages associated with the implementation of microfabrication technology for the construction of amperometric sensors including reduced cost, reduced sensor size, smaller sample size, faster response time, and well-defined geometric features. While microfabricated potentiometric sensors have been built [ 1,2], the development of microamperometric sensors has been slow. These microdevices consist of microfabricated electrodes which are dipped into a liquid electrolyte. In this report, we describe the development of a sensor with an integrated design in which the electrodes were photolithographically etched on a silicon dioxide surface of a 2” silicon wafer and a thin film of Nafion (Du Pont trade mark), a solid organic polymer which conducts protons when wetted with water, served as the electrolyte [3]. Since in our previous work, we demonstrated that the single/noise (S/N) was dependent upon the ratio of the square of the 0 Elsevier Sequoia/Printed

in The Netherlands

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perimeter to A (P’/A) [3], the W.E. of the integrated sensor was constructed in the shape of an ultrafine grid consisting of uniformly spaced 50 micron size holes. We know of only two previous reports of microfabricated integrated amperometric structures [ 5, 61. Morita et al. [6] applied a drop of the electrolyte over the electrode array and immersed the structure into an acetonitrile solution.

Materials and Methods Sensor Design and Fabrication During the development of the integrated sensor, three different sensor designs were fabricated which were evaluated in two series of experiments. The various sensor designs were: (1) A ‘standard sensor’ was constructed in which an existing amperometric gas sensor design was used. These sensors used a gold mesh/gold powder W.E. in conjunction with a platinized platinum C.E. and a platinum-air reference electrode (R.E.). The three electrodes were pressed into l/16” thick Nafion membrane as illustrated in Fig. l(a). The performance of these ‘macro’ sensors is known and can be used for comparison to the microelectrode structure studied herein [7l. (2) Several different sensors were built having microfabricated W.E. but with the C.E. and R.E. identical to the standard sensor described above. The W.E. were vacuum evaporated on to sheets of l/16” thick Nafion in a square grid (Fig. l(b)) using photoetched shadow masks. Three grid densities were used. Since the C.E. and R.E. were identical to the standard sensor, performance difference can be attributed to the W.E. fabrication process and geometric configuration [3]. (3) Sensors were built with microfabricated electrodes etched from a thin film of gold which had been deposited onto a SiO, surface of a silicon wafer. A 0.3 to 0.5 micron thick layer of Nafion solution was spin-coated over the electrodes. An ultrafine grid geometry was used for the W.E. surface. A Au-air R.E. was used in this sensor design [8]. Sensor Evaluation Sensors were mounted in a housing that provided a gas exposure chamber over the sensing electrodes and contacts for the electronic signal. For the sensors fabricated using Nafion sheets (types 1 and 2, shown in Fig. l(a) and (b)), the housing allowed for the wetting of the Nafion sheets through contact with a reservoir of water. For the integrated sensor (type 3, Fig. l(c)), the Nafion film was wetted by flowing humidified air over the W.E. prior to and during electrochemical measurements. Humidification of the air was

W.E

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Fig. 1. Schematic of the three sensors used in this study. (a) The standard sensor with platinized platinum mesh counter electrodes (C.E.) and reference electrodes (R.E.). The working electrode (W.E.) was gold mesh (8.33 mm diameter). The three electrodes were pressed as shown into a 23.8mm diameter circular sheet of Nafion. (b) Sensor with microfabricated gold working electrode (6mm on the edge) but with standard C.E. and R.E. positioned as shown on a 23.8 mm diameter circular sheet of Nafion. Three different grid densities were utilized in this study: fine (17 holes along an edge), medium (5 holes along an edge), and coarse (a single hole). (c) Integrated sensor with microfabricated W.E., C.E., R.E. and spin-coated Nafion electrolyte. The W.E. consisted of an ultrafine grid and 50 micron size holes uniformly spaced throughout the 5 x 5 mm square W.E. surface.

achieved by flowing dry cylinder air through a 14 cm length of Gortex tubing immersed in distilled water. At a flow rate of 100 cm/min this procedure results in nearly 100% relative humidity in the gas sample stream [9]. The sensors were maintained under potentiostat control using a PAR 273 potentiostat, and were biased to 1.3 V versus the standard hydrogen electrode in acid electrolyte (0.3 V versus a Pt-air R.E. or 0.5 V versus a Au-air R.E.).

305 TABLE 1. Physical parameters and response of sensors to 100 ppm of NO

sensors

Area (mm2)

Grid spacing (mm)

Number of holes

P2/A

Signal 01A)

Coarsea Medium’ Fine” Ultr&neb Standard’

32 26 26 18.75 100

2 0.55 0.20 0.05

I 25 289 2500

0.15 1.0 11.6 74.6


%ensor with W.E. microfabricated using metal masks. bIntegrated sensor with microfabricated W.E., C.E., R.E., and spin-coated electrolyte. cConventional sensor with noble metal mesh/powder W.E., C.E. and R.E.

Results and Discussion Determination of the Design Parameters for the W.E. The development of our integrated microamperometric gas sensor proceeded through two steps. Initially our emphasis was on determining the critical geometric design parameters for the W.E. The standard gas sensor design depicted in Fig. l(a) was modified in that instead of a noble metal mesh/powder W.E., a grid structure W.E. was vapor deposited on to the Nafion sheets as illustrated in Fig. l(b). Three different grid densities were utilized (see Table 1). The behavior of the sensors with the microfabricated W.E. was compared to the standard sensor. The qualitative response was identical for all sensors tested as illustrated in Fig. 2. The similarity of the results indicate that the electrochemistry is essentially the same for all of the gold electrodes. Thus the

structural differences are primarily altering the kinetic process on the electrode surface. There were however significant variances in the signal intensity for the various sensors. Experimental curves of the response of the sensor when exposed to H2S are depicted in Fig. 3. Comparable behavior was observed for other vapors [3]. By varying the size of the mesh of the W.E. structure, it was observed that as the grid density increased, so did the signal to noise ratio. This observation suggests that the faradaic or analytical current is preferentially generated along the perimeter of the electrode where the gas, electrolyte and electrocatalyst (the electrode) meet. This triple-phase boundary at the electrode edge should be the most catalytically active area because (i) edges, steps and other defects are more catalytically active

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Fig. 2. Normalized response (to 83 ppm of H,S) of the standard sensor (Fig. l(a)) and the medium and fine grid Sensors (Fig. l(b)) to the indicated vapors.

Fig. 3. Comparison of the response induced by 83 ppm of H,S on the fine grid sensor and the medium grid sensor (see Fig. l(b) and Table I) as measured three days after fabrication. The bias potential was + 300mV (vs. a Pt-air reference electrode) and the gas was flowed at 100 c&in.

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than flat surfaces; (ii) the gas must diffuse through the Nafion over a longer path in order to react at the center of the electrode than to react at the edge; and (iii) the electric field density is inversely proportional to the radius of curvature of the electrode and, therefore, is much larger at the edge than along the flat surface. To extend this discussion in a more analytical manner, let us assume that the geometrical dependence of the signal to background ratio (S/B) for the microsensor is a function of only the total perimeter P and the area A of the electrode (i.e. the differences observed are assumed to be structure dependent since the chemistry of all the systems is constant). The ratio of the signal to background S/B is a dimensionless quantity and, therefore, S/B may be expressed as a function of P*/A, which is the only independent dimensionless quantity that can be formed from P and A. In the simplest case, S/B would be directly proportional to P2/A S/B = K(P2/A) where K depends on the gas detected. The Integrated Microamperometric

Sensor

On the basis of these initial experiments, a second-generation Nafion-based microfabricated amperometric sensor was designed. Because enhanced sensitivity was predicted by increased P2/A, an ultrafine grid geometry was used for the W.E. surface. Photolithographic technology was required to fabricate the precise grid structure used in this study. The extension of this geometric model dependence to this integrated sensor with spin-coated Nafion over the thin-film electrode is not straightforward, because, unlike the sensors depicted in Fig. l(b), the perimeter is not easily defined. In the earlier microsensor design, the electrodes were vapor deposited directly onto sheets of Nafion. Thus a well-defined gas/electrode/electrolyte triple-phase boundary existed. For the present sensor, thin films of gold were deposited onto a Si02 substrate, and a dilute solution of Nafion was spin-coated to cover the total surface of the electrode. Since a linear triplephase boundary does not exist, the perimeter does not have the same meaning as with the sensors fabricated on Nafion sheet, and the previously developed model may not be directly applicable. It is not possible to formally calculate P2/A for the electrode. The reaction kinetics would be expected to differ from those of the previous sensor designs because diffusion differs. In the integrated microsensor design, all the reacting gas must diffuse through a uniform, thin film of Nafion less than 1 pm thick.

Nevertheless, edge effects clearly still appear to play a significant role in controlling sensor sensitivity. Upon exposure to 100 ppm of nitric oxide, the sensor exhibited a rapid, large, and reversible response. As the humidification period increased, the sensitivity increased even more. Presumably, this was due to the improved conductivity of the Nafion. The maximum response of this sensor occurred after approximately 45 min of humidification. The maximum response to 100 ppm of NO at 100 cc/min was 490 PA. Table 1 compares these responses to the net signal obtained from the previously fabricated microsensors and to a highoutput ‘conventional’ or commercial sensor. The integrated microsensor output dwarfs all others. The response is even more impressive when one considers that the microsensor has a sensing electrode consisting of less than 0.2 mg of gold, while conventional designs for commercial sensors typically use between 10 to 75 mg of electrocatalyst consisting of gold powder and mesh [7J The response per unit weight of electrode material is about 5000 times greater for the microsensor than the standard sensor (i.e. the catalyst is about 5000 times more efficiently used). This microsensor exhibited, unfortunately, a short lifetime. A highly unstable response began to occur within 2 h of the humidification and test procedure. Micrographic analysis revealed the probable cause of sensor failure [S]. This corrosion can be attributed to the use of chromium in the structure. While chromium improves adhesion of the gold thin film to the silicon substrate, etching of the gold and chromium films exposes the chromium to the Nafion. Under the applied potential in the aqueous environment, the chromium corrodes. This degrades the gold/silicon interface and causes the ultimate failure of the sensor. Morita et al. [6], who also used chromium as an adhesion layer, did not observe corrosion with a non-aqueous electrolyte system consisting of poly(ethylene oxide) with LiCF,S03 and acetonitrile. Similarly, when we used a water free electrolyte (LiC104 in propylene carbonate) instead of humidified Nafion, the chromium corrosion did not occur and the sensor exhibited a much longer lifetime. References 1 J. N. Zemel, Ion-sensitive field effect transistors and related devices, AMY. Gem., 56 (1975) 255A. 2 J. Van der Spiegel, I. Lauks, P. Chan and D. Babic, The extended gate chemically sensitive field effect transistor as multi-species microprobe, Sensors and Actuators, 4 (1983) 291. 3 G. J. Maclay, W. J. Buttner and J. R. Stetter, Microfabricated amperometric gas Sensors, IEEE Trans., 35 (1988) 193.

307 4 G. J. Maclay, and J. R. Stetter, U.S. Patent Appfic, (1988). 5 W. J. Buttner, G. M. Flanagan, G. J. Maclay, V. Stamoudis, S. Zaromb and J. R. Stetter, Proc. 1987 Army Chemical Research, Development, and Engineering Center Conf. Chemical Defence Research, Aberdeen, U.K., Nov., 1987, p. 1011. 6 M. Morita, M. L. Longmire and R. L. Murray, Solid-state voltammetry in a three electrode electrochemical cell-on-achip with a microlithographically defined microelectrode, Anal. Chem., 60 (1988) 2770.

7 J. M. Sedlak and K. F. Blurton, The electrochemical reactions of carbon monoxide, nitric oxide and nitrogen dioxide at gale electrodes, .I. Electrochem. Sot. 123 (1976) 1476. 8 W. J. Buttner, J. G. Maclay and J. R. Stetter, Microfabricated amperometric gas sensors with an integrated design, Sensors and Actuators, to be published. 9 S. Zaromb, C. S. Woo, K. Quandt, L. M. Rice, A. Fermaint and L. Mitnaul, Simple permeation absorber for sampling and preconcentrating hazardous air contaminants, J. Chromatogr., 439 (1988) 283.