Microarray study of temperature dependent sensitivity and selectivity of metal/oxide sensing interfaces

J. Tiffany, R. E. Cavicchi, S. Semancik

National Institute of Standards and Technology Gaithersburg, MD 20899

ABSTRACT

Conductometric gas microsensors offer the benefits of ppm-level sensitivity, real-time data, simple interfacing to electronics hardware, and low power consumption. The type of device we have been exploring consists of a sensor film deposited on a "microhotplate"- a 100 micron platform with built-in heating (to activate reactions on the sensing surface) and thermometry. We have been using combinatorial studies of 36-element arrays to characterize the relationship between sensor film composition, operating temperature, and response, as measured by the device's sensitivity and selectivity. Gases that have been tested on these arrays include methanol, ethanol, dichloromethane, propane, methane, acetone, benzene, hydrogen, and carbon monoxide, and are of interest in the management of environmental waste sites. These experiments compare tin oxide films modified by catalyst overlayers, and ultrathin metal seed layers. The seed layers are used as part of a chemical vapor deposition process that uses each array element's microheater to activate the deposition of SnO2, and control its microstructure. Low coverage (2 nm) catalytic metals (Pd, Cu, Cr, In, Au) are deposited on the oxides by masked evaporation or sputtering. This presentation demonstrates the value of an array-based approach for developing film processing methods, measuring performance characteristics, and establishing reproducibility. It also illustrates how temperature-dependent response data for varied metal/oxide compositions can be used to tailor a microsensor array for a given application.

1. INTRODUCTION

As the power of computing microchips continues to improve, increasingly sophisticated, inexpensive data analysis tools are becoming available for chemical sensing technologies that gather complex data from the environment. Examples include chemometric and neural-network based pattern recognition algorithms, which enable the response from an array of non- chemically-specific sensors to be converted to useful information such as the concentration and composition of the components of a gas mixture1-5, or the match to an identified odor in an "electronic nose" application6-8. The demand for more detailed data in fields as diverse as process sensing, sensing for transportation and building air quality, etc., coupled with this computing capability, now places even more pressure on sensor developers to produce a greater variety of sensor types and materials to produce the required data. Similar to the challenges of the development of new catalysts and new pharmaceuticals, this situation is amenable to the use of combinatorial synthesis methods to efficiently search a multidimensional phase space for useful sensing materials.

Of the many approaches available to develop a gas sensing technology, conductometric sensors offer the advantages of being a simple measurement, using robust materials, and providing good sensitivity, and fast response times. For example, a nanoparticle tin oxide film was shown to have sub nanogram/gram (ppB) sensitivity to methanol in air and produced a stable response to over 100 hours of gas-on / gas-off cycling9. Conductometric sensors may be fabricated from a variety of materials combinations, including conducting polymers such as polypyrole10 or polythiophene11 or carbon-loaded polymers12, and metal oxides with or without catalysts13-18. The sensing materials often exhibit strong temperature-dependent properties, which can enhance the sensitivity to a particular gas, and change the relative sensitivities to a set of different gases. Metal oxide sensing materials have been fabricated for decades as sintered pastes of ceramic material surrounding a platinum heater operated at temperatures in the range of 200 oC to 400 oC. New materials are developed by fabricating discrete devices, one by one, and evaluating their performance in a gas testing system.

Miniaturized versions of the discrete conductometric sensors were developed by combining microelectromechanical systems (MEMS) and complementary metal oxide semiconductor (CMOS) technologies to produce a testing platform termed a "microhotplate19." These devices are fabricated in silicon, using the layer sequences available to CMOS technology, typically consisting of silicon oxide, polycrystalline silicon (polysilicon), and a metal layer (aluminum, tungsten) and then subjected to a surface-micromachining etch step, which produces a pit underneath suspended layers of these materials. A typical device size is 100 µm. The pit provides a high degree of thermal isolation, allowing the suspended portion of the devices to achieve the high temperatures (>500 oC) needed for controlling the sensor material microstructure as well as activating certain sensing reactions. In addition, the thermal isolation and miniaturization allow the devices to operate at these temperatures with relatively low power consumption (tens of milliwatts), making battery-operated devices possible. The small size also results in a rapid thermal time constant, so the heaters can be switched to different temperatures in a few milliseconds, which opens up new sensing modes based on dynamic temperature programming20-21.

Most importantly, the batch fabrication used to produce the CMOS layers can be readily employed to fabricate arrays with hundreds of microhotplates. For sensors, the limit on array size is set by practical lead counts to get signals in and out. For example, a 48-element array can be produced with 98 leads: 48 heater drivers, 48 sensing highs and two commons. In the future, on-chip multiplexing will eliminate this practical restriction. Previously we have described the use of a 36-element array to investigate the effect of different seed layers on the microstructure of chemical vapor deposited tin oxide22. In the work described here, we use 4 and 36 element arrays to survey the effects of some different catalyst materials added to the top surface of tin oxide for a set of nine test gases. Although the larger arrays require more process steps to produce a variety of materials over the array, once fabricated, they allow simultaneous testing to be easily performed. When considering new test materials, we have found the four-element arrays are useful because of the reduced fabrication time, and reduced risk of a single process step spoiling the entire array. The trade-off here is the increase in effort and time required to test only four elements at a time, and the larger number of factors from fabrication and test conditions that can affect reproducibility.

The goal of these experiments is to demonstrate a combinatorial approach for developing a database that will relate the effect of the catalyst additives to the gas sensitivity for different gases. Such a database might eventually be used as a lookup table in which a sensing need is identified, a promising materials combination is selected from the database, and additional experiments are performed to further customize the material, and its use (operating temperature or temperature program) for the application.

2. EXPERIMENT

The basic microhotplate device configuration is illustrated schematically in Figure 1. The device consists of a polysilicon heater, a metal heat distribution plate, and metal contacts separated by layers of insulating SiO2. The devices were fabricated at Lincoln Laboratories at the Massachusetts Institute of Technology23 using mask designs configured at NIST. The metal layers for this process consisted of a sandwich of titanium/ aluminum/ titanium-tungsten. The fabrication process used chemical-mechanical polishing between layers, to produce a smooth deposition surface with a root mean square roughness of 1.1 nm as determined by atomic force microscopy. Figure 1. Schematic of layers within a microhotplate. Wafers received from the fab were diced into chips. Chips were subjected to an anisotropic silicon etch consisting of either ethylenediamene-pyrochatechol water or tetramethylammonium

100 µm 340 µm

Figure 2. 4-element array of microhotplates. Figure 3. 48-element array of microhotplates. hydroxide. If formulated correctly, the latter etch is less aggressive on exposed metal layers. Examples of a four-element and a 48-element array are shown in Figures 2 and 3. The etched chips were packaged in 40-pin dual-in line packages (4-element array) and 84-pin packages (48-element array.) Only 36 of the 48 elements were used.

Tin oxide was grown by chemical vapor deposition (CVD) onto a nickel-seeded surface. The nickel seed layer, 1.6 nm thick, speeds the nucleation process and produces a finer grain tin oxide film as compared to films grown on an unseeded hotplate. The seed layer was prepared by the vacuum evaporation of nickel onto the entire array. The CVD process was performed in a cold-wall system operating at about 500 Pa. The sources were produced by flowing argon through a tetramethyltin bubbler operating at temperature of -40 oC at a flow rate of 20 standard cubic centimeters per second (sccm), flowing oxygen at 300 sccm, and using a balance of argon flowing at 300 sccm. The CVD process uses the microheater of an array element to provide the thermal energy for decomposition of the precursor, and reaction with oxygen to grow tin oxide on that element24- 26. Each array element was monitored to record the electrical resistance of the growing film, using the electrical contacts on the top of the microhotplate. Growth proceeded under computer-control and was stopped, by turning off the microheater, when that element reached a desired resistance value. In these experiments the resistance value for terminating growth was set at 5 kΩ. The apparatus is capable of separately controlling 16 simultaneous growths.

After growth of the tin oxide, the devices were annealed in air at 450 oC for 1500 s. Catalyst layers were deposited on selected elements using vacuum evaporation. The catalysts used were Pd, Cu, Au, Cr, and In, all grown to a thickness of 2 nm as measured by a quartz crystal monitor in the vacuum system. Four element arrays were masked so that 2 out of the four elements were exposed to the deposition. Nine such devices were prepared. The 48-element array was masked to expose three elements to a selected metal catalyst deposition.

Prior to sensor testing, array elements were annealed at 450 oC in dry air for 1500 s. The sensor test system was configured to switch one of nine possible test gases into a stream of zero-grade dry air that was flowing over the sample for a cycle consisting of 150 s gas-on and 150 s gas-off. The gases tested were methanol, ethanol, dichloromethane, propane, methane, acetone, benzene, hydrogen, and carbon monoxide. These test gases were obtained from commercial sources as calibrated gas mixtures with dry air. The sensors were operated at constant temperature during the gas flow cycles. Sensors were tested at operating temperatures from 150 oC to 400 oC. The apparatus for testing is able to monitor the conductance of all 36 elements simultaneously with a measuring time of a few ms for each set of 36 conductance values.

3. CVD GROWTH

Figure 4 shows the resulting tin oxide film on one of the elements of the 48-element array. The presence of the film is indicated by the appearance of interference fringes on the microhotplate. The fringe lines are effectively contours of constant film thickness and therefore constant temperature during the growth. The growth of tin oxide appears to terminate along the legs of the structure, at points where the temperature was insufficient to decompose tetramethyltin that was flowing over the entire array. This method of growth is well-suited for combinatorial studies in that it does not require masking or other lithographic steps to localize the material. There is also a growing number of CVD precursors for both oxides and catalytic metals that can be used for such studies. Growth Film Growth

Nucleation/InductionGrain Continuous 0.20

0.15 ) !

0.10

0.05 Conductance (1/k

0.00 60 µm 0 100 200 300 400 500 Time (s)

Figure 4. CVD film of tin oxide on one of the array Figure 5. Conductance measured during the CVD elements of the 48-element array. growth of tin oxide.

Figure 5 shows the change in electrical conductance during 0.20 the growth of the film. Three phases of growth are (a) identified: nucleation, where the film resistance exceeds the measurement limit; percolation-grain growth, where the conductance increases more rapidly than linearly; and 0.15 continuous growth, where the conductance increases

] linearly with time, as would be expected for a film with a Ω linear increase in film thickness. A fourth region is sometimes encountered (although not shown in this example), where the conductance increase becomes slower 0.10 than linear. In this fourth range, the contact resistance becomes comparable to or exceeds the film resistance. The conductance curve exhibits a step-drop at the preset value

Conductance [1/k Conductance corresponding to a resistance of 5kΩ. Here, the microheater 0.05 has turned off, stopping the growth of tin oxide, and causing the thermally-activated conductance to drop to its room- temperature value.

0.00 While it is efficient to simultaneously grow the films of an 0 100 200 300 400 500 array in this way, an interesting cross-talk effect was noted Time (s) that caused a widened distribution in the growth times and final room temperature resistances of the films. In Figure 6 0.20 (b) (a), the conductance of 16 simultaneously grown films is shown. Inspection of Figure 6 (b), a detail, shows that when individual heaters are shut-off to terminate the growth on 0.18 that element, there is a step-like change on the conductance ]

Ω of elements that are still growing. This is a proximity effect, in that the growing film is acting as a gas sensor. Sudden 0.16 changes in a nearby heater cause a change in the gas environment of the growing film, leading to a shift in the 0.14 conductance. These shifts will therefore influence the timing of the shutoff of that element. To circumvent this effect, yet maintain the efficiency of simultaneous growth, the films

Conductance [1/k Conductance 0.12 were grown on all 16 elements simultaneously for a prescribed time that was smaller than the time to reach the desired conductance value. Then films were grown 0.10 sequentially, with only one heater switched on at any time. This resulted in a much narrower distribution of growth 380 400 420 440 460 480 500 times and room temperature film resistances. For example, Time (s) the mean growth time and standard deviation of growth time Figure 6. (a) Conductance of 16 elements measured during were 459 s and 72 s, respectively for simultaneously grown simultaneous growth. (b) Detail of (a) highlighting the films, while for the films on a second large array grown in effect of other heaters shutting off on the bold conductance the simultaneous-then-sequential manner just described, curve. these respective times were 483 s. and 19 s. The catalyst experiments using a 48-element array were performed on a chip processed with the latter method.

4. SENSOR TESTING

An example of the sensor testing results is shown in Figure 7 for the 36 elements on the large array. Shown here are the conductance responses to ethanol, methanol, dichloromethane, propane, and methane at 100 µg/g (100 ppm) in air with all sensors run at 400 oC. Data was collected in this manner for the remaining four gases listed in the Experiment section for both the large array with catalysts, and the 9 4-element arrays at operating temperatures from 150 oC to 400 oC. Figure 8 shows the response of one of the gold-catalyzed sensors at different temperatures.

Ethanol Methanol Dichloromethane Propane Methane

Air Gas Air Air Gas Air Air Gas Air Air Gas Air Air Gas Air Air -6 160x10

140

120

100 )

] Ω ! 80 1/ ( 60

40 Conductance [1/

Conductance 20

0 0 150 300 450 600 750 900 1500 1800 1950 2100 1050 1650 1200 1350 2250 2400 TimeTime [s](s)

Figure 7. Sensor measurements on the large array for 5 of the test gases.

Figure 8. Sensor measurements on a gold-catalyzed sensor element for 5 test gases at 6 different operating temperatures indicated in οC.

Figure 9 shows the response to the three gold-catalyzed -6 sensors on the large array. To 90x10 -6 C 60x10 Cmaxmax facilitate the analysis, a

] sensitivity parameter S is ! 55 Cmax-Cmin S defined as the ratio (Cmax- 80 = 50 Cmin Cmin)/Cmin, where Cmax is the conductance with the test gas 45 C min

] on, and Cmin the conductance ! 70 Conductance[1/ just prior to turning the test 1350 Time [s] 1650 gas on as defined in the inset to Figure 9. 60

Figure 10 shows the sensitivity Conductance[1/ of the different catalyzed surfaces to the nine test gases, 50 with Fig. 10 (a) displaying the results from the 4-element arrays, and Figure 10 (b) 40 displaying the result from the single large array. The trends in the relationship between the 0 300 600 900 1200 1500 1800 2100 2400 sensitivity to each gas for the Time [s] various catalysts are similar for the two sets of data. Most Figure 9. Sensor measurements on the 3 gold-catalyzed elements of the large array at 400 apparent is the enhanced οC, (taken from Figure 7). Inset: definition of sensitivity S. sensitivity to hydrogen from

50 2.5 Acetone Benzene

40 Hydrogen 2 CO Ethanol 30 Methanol 1.5 Dichloromethane Sensitivity Propane Sensitivity 20 1 Methane

10 0.5

0 0 Pd Cu Cr In Au none (b) Pd Cu Cr In Au none (a) Catalyst Metal Type Catalyst Metal Type

Figure 10. Sensitivity as a function of catalyst metal type for the 9 test gases. (a) results from 9 4-element array chips. (b) results from the 36-elements of the large array. 120 the gold-catalyzed % error (4-elements) surface. One significant % error (36-element) difference between the 100 two sets of data is the markedly lower overall sensitivity of the data 80 from the large array. We determined that this was due to the 60 difference in the contact arrangement

40 between the array elements on the 4- and 48- element arrays and 20

(Standard Deviation/Mean (Standard )% the procedure used to terminate the CVD growth. Specifically, 0 the contacts are

CO actually much closer to each other on the 4- Ethanol Benzene Acetone Propane

Methane element array, so that Methanol Hydrogen when growth is terminated at a set resistance value, the 4- Dichloromethane element array will have Figure 11. Spread in results for the 4-element and 36-element arrays for each of the 9 test gases. a thinner film on each element than is on each element of the 48- element array. The thinner film has a higher proportion of its current flow near the surface region, thereby producing the enhanced sensitivity over the results from the large array where the sensing films are thicker.

The large array has the benefit of reducing the number of variables which can affect results in a combinatorial survey. As can be seen in Figure 11, results from the large array show considerably less scatter, defined here as the standard deviation of the group of samples. Factors which might reduce the scatter include the following: 1) the large array elements come from the same portion of the silicon wafer, and are etched at the same time resulting in more identical thermal characteristics; 2) the CVD process conditions were identical for all array elements, compared to small differences in pressure and flow rates for the sequentially processed 4-element arrays; and 3) gas test conditions were also identical for the large array, compared to variations that might be present from the sequential tests on 4-element arrays.

5. CONCLUSIONS

The results of this type of study are the beginnings of the creation of a four-dimensional database of sensitivity, material, test gas, and operating temperature for conductometric oxide gas sensors. Large arrays offer important advantages connected to increased efficiency in the generation of such data by reduction of the number of factors that can cause sample-to-sample variation, as shown here by the reduced dispersion of results from the large array compared to the 4-element array data. This lowered variation will allow more accurate comparisons to be made using the response database.

ACKNOWLEDGEMENTS

The authors would like to acknowledge technical support from Jim Allen on chip fabrication, and Balaji Panchapakesan for discussions on CVD growth of seeded tin oxide. The authors also acknowledge partial support from DOE Grant #07- 98ER62709.

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