Fully CMOS-compatible, surface-micromachined polysilicon microbridges have ben designed, fabricated, and tested for use in catalytic, calorimetric gas sensing. To improve sensor behavior, extensive electro-thermal modeling efforts were undertaken using SPICE. The validity of the SPICE model was verified by comparing its simulated behavior with experimental results. The temperature distribution of an electrically-heated microbridge was measured using an IR microscope. Comparisons among the measured distribution, the SPICE simulation, and distributions obtained by analytical methods show that heating at the ends of a microbridge has important implications for device response. Additional comparisons between measured and simulated current-voltage characteristics, as well as transient response characteristics, further support the accuracy of the model. A major benefit of electro-thermal modeling with SPICE is the ability to simultaneously simulate the behavior of a device and its control/sensing electronics. Results for the combination of a unique constant-resistance control circuit and microbridge gas sensor ar given. Models of in situ techniques for monitoring catalyst deposition are shown to be in agreement with experiment. Finally, simulated chemical response of the detector is compared with the data, and methods of improving response through modifications in bridge geometry are predicted.

This paper reports on the processing steps in a silicon foundry leading to improved performance of the Radiation Sensing Field Effect Transistor (RadFET) and the use of multiple RadFETs in a handheld, battery operated, combination spectrometer/dosimeter.

A post-processing, selective micro-chemical vapor deposition ("micro-CVD") technology for the deposition of catalytic films on surface-micromachined, nitride-passivated polysilicon filaments has been investigated.Atmospheric pressure deposition of platinum on microfilaments was accomplished by thermal decomposition of platinum acetylacetonate, an organometallic Pt precursor; deposition occurs selectively only on those filaments which are electrically heated.Catalyst morphology, characterized by scanning electron microscopy, can be controlled by altering deposition time, filament temperature, and through the use of pulsed heating of the filament during deposition.Morphology plays an important role in determining the sensitivity of these devices when used as combustible gas sensors.

A dual-dielectric pMOS dosimeter (RADFET) has been recently designed at Sandia. The RADFET consists of a thermally grown oxide and a CVD deposited nitride. With a negatively applied bias, holes are generated in the SiO/sub 2/ transport and are trapped at the SiO/sub 2//Si/sub 3/N/sub 4/ interface producing a measurable threshold-voltage shift. Because holes are trapped away from the Si/SiO/sub 2/ interface, hole neutralization by tunneling and interface-trap buildup are minimized resulting in little fade or annealing of the RADFET output response. RADFETs were irradiated at dose rates from 0.002 to 50 rad(Si)/s with biases from -5 to -20 V. RADFETs were also annealed for times up to 10/sup 7/ s at temperatures up to 100/spl deg/C. Within experimental uncertainty, no difference in RADFET output response at a given bias was observed over the dose rate range examined and for 25/spl deg/C anneals. At an anneal temperature of 100/spl deg/C only a 20% decrease in RADFET output response was observed. These results show that Sandia's RADFETs exhibit little or no fade of their output characteristics and are ideal for low dose rate space applications.

The Sandia Robust Hydrogen (SRH) sensor can detect partial pressures of H{sub 2} over a wide range ({approximately}1 ppm to pure H{sub 2}) because it employs two different kinds of catalytic metal sensors. A Field Effect Transistor (FET) with a catalytic gate detects low H{sub 2} concentrations and a resistor of the same metal is accurate for the higher concentrations. The use of Pd alloys with Ni preserves sensor reversibility by preventing the hydride phase transition under normal operating conditions. Mixing O{sub 2} with H{sub 2} does change the sensor signal under some conditions and comparisons of the behavior of a pure Pd device with a Pd/Ni device is given. The H{sub 2} concentration in flammable mixtures can be obtained, but an additional sensor is needed to measure the concentration of oxidant.