AN ULTRASENSITIVE SILICON PRESSURE-BASED MICROFLOW SENSOR

An ultrasensitive silicon pressure-based flowmeter has been developed for use in measuring sub-SCCM gas flows in semiconductor process equipment. The device utilizes a capacitive pressure sensor to measure the pressure drop induced by flow across a micromachined silicon flow channel. The flow-meter is fabricated using a single-sided dissolved-wafer process and requires only six masks. The capacitive pressure sensor uses a thin (2.9-mu-m) stress-compensated membrane, which enables the sensor to monitor differential pressures as low as 1 mtorr while withstanding overpressures greater than 700 torr. Creep and fatigue change the offset by < 0.2% full scale and alter the pressure sensitivity by < 0.03 fF/mtorr; the hysteresis observed on all devices has also been < 0.2% full scale, where "full-scale" is defined to be the pressure required to deflect the membrane half the gap distance. The results reported here indicate that it may be possible to extend the pressure range of these devices by an order of magnitude beyond full scale, which would increase the flow resolution from 12 to 16 b and cover a flow range of 10(-8) to 10(-3) SCCM.