AVS 47th International Symposium
    MEMS Wednesday Sessions
       Session MM-WeM

Paper MM-WeM2
The Use of Micromachined Arrays to Develop Processing/Performance Databases for Metal/Oxide Sensing Materials

Wednesday, October 4, 2000, 8:40 am, Room 309

Session: Microfabricated Sensors
Presenter: J.E. Tiffany, National Institute of Standards and Technology
Authors: J.E. Tiffany, National Institute of Standards and Technology
R.E. Cavicchi, National Institute of Standards and Technology
S. Semancik, National Institute of Standards and Technology
Correspondent: Click to Email
We describe the efficient study of multiple metal/oxide microsamples on micromachined platforms called microhotplate arrays. These platforms include addressable temperature control of 36 individual elements which is employed in fabricating and evaluating varied sensing films being examined for solid state conductometric gas microsensors. Each 100 micron array element consists of a suspended structure with a buried heater and surface electrodes. We present results of screening experiments (metal coverage and type, annealing and sensing temperature, gas type) designed to generate a response database of sensitivity and selectivity. Tin oxide was deposited on all array elements via a Ni seeded, self-lithographic MOCVD process. Low coverage (25-100 Å) catalytic metal s (Ni, Pd, Pt, Cu, Ag, Co, Rh, Ir, Ru) were then deposited on select elements using masked evaporation or sputtering. We describe the response of these array elements to a wide variety of gases (2-butanone, acetone, toluene, benzene, methanol, ethanol, hydrogen, and carbon monoxide). Response data was collected for bare and catalyst modified sensors. The bare tin oxide films showed a normally distributed (10%) conductance response when exposed to the test gases at fixed temperature, demonstrating the statistical stability of the screening approach. Relative sensitivities for the different metal catalysts are reported as a function of sensing temperature for each test gas. We observed, for example, that the addition of Ni catalyst decreased hydrogen response, whereas 2-butanone response was increased. Benzene response was also observed to cross over from negative to positive at a specific temperature due to competing surface reactions both with and without Ni catalyst. Such cases of increased sensitivity, selectivity and crossover response can be extracted from the materials screening response data and be used in customizing microsensors for specific tasks.