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Abstract-We report the first monolithic tin oxide (SnO,) gas sensor realized by commercial CMOS foundry fabrication (MO-SIS) and post-fabrication processing techniques. The device is composed of a sensing film that is sputter-deposited on a silicon micromachined hotplate. The fabrication technique requires no masking and utilizes in-situ process control and monitoring of film resistivity during film growth. Micro-hotplate temperature is controlled from ambient to 500°C with a thermal efficiency of 8"C/mW and thermal response time of 0.6 ms. Gas sensor responses of pure SnO, films to H, and 0, with an operating temperature of 350°C are reported. The fabrication methodology allows integration of an array of gas sensors of various films with separate temperature control for each element in the array, and circuits for a low-cost CMOS-based gas sensor system.

We have sought to enhance the sensitivity of conductometric gas microsensors through the design and fabrication of porous, three-dimensional tin oxide nanoparticle structures. Electrostatically controlled layer-by-layer processing in aqueous solutions was used to decorate sacrificial latex microspheres with Sb:SnO2 nanoparticles. To evaluate their sensing performance, these structures were then deposited as films, via micropipetting, on MEMS micro-hot-plate platforms with interdigitated electrodes. Prior to gas testing, rapid heating of the micro-hot-plates was used to remove the sacrificial latex templates, thereby revealing a 3-D structure composed of interconnected spherical tin oxide nanoparticle shells with porous ultrathin walls. Changes in film conductance, caused by exposure to test gases (methanol, carbon monoxide, benzene, water) in a dry air background, were measured at different temperatures. Hollow nanoparticle microsphere films exhibited partial selectivity for these different gases, good dynamic range at different temperatures and gas concentrations, and good repeatability and stability over long runs. These films also yielded approximately 3-fold and 5-fold increases in sensitivity to methanol when compared to SnO2 polycrystalline chemical vapor deposition films and Sb:SnO2 microporous nanoparticle films, respectively. Gains in sensitivity are attributed to the multiscale porous architecture of the hollow microsphere films. This architecture promotes gas diffusion and increases the active surface area.

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