CMOS MEMS oscillator for gas chemical detection

The paper presents the fabrication and demonstration of a CMOS/MEMS electrostatically self-excited resonator gas detector under various gas exposures. The cantilever resonator includes a 100 /spl mu/m square platform for the chemically sensitive layer, polystyrene. Polymer is deposited using a drop-on-demand inkjet. The electrostatically actuated resonator has approximately 967 pico-grams of polystyrene which was deposited in a 2 mg/ml 1 toluene:1 xylene solution. An on-chip preamplifier converts the high impedance sensor output to a voltage usable off-chip. External feedback electronics complete a free-running oscillator at the mechanical resonant frequency of 5475 Hz with 40 dB SNR, a calculated 23.6 pg/Hz sensitivity, and a 1 Hz 3 dB width in a 1 Hz resolution measurement bandwidth. Initial tests demonstrate sensitivity to using an external bubbler with 1 l/min N/sub 2/ flow. Frequency shift when the polystyrene is saturated with methanol, ethanol, 2-propanol, and acetone was 8.2, 8.0, 5.2 and 25.7 Hz, respectively.