HIGH-PERFORMANCE PIRANI VACUUM GAUGE

As an extension of previous work in our laboratory, a wide-range Pirani gauge that is capable of measuring vacuum pressure down to 10(-7) Torr reproducibly has been built, The micromachined Pirani sensor used in the experiments has a suspended membrane that is supported by the nearly radiation-limited, thermally insulating beam leads crossing over a V-groove cavity. A method of partial dummy compensation, as proposed previously by Weng and Shie for eliminating the ambient drift, is proved here to be very effective with a thermal drift as small as only 5.7 mu V/degrees C. It has also been found that a thermal-stress-induced piezoresistive effect, which has a profound influence on the limitation of measurement, appears in the constant-bias operation wherein the sensor temperature rises with the reduction of gas pressure and therefore thermal conduction. This effect causes the irreproducibility of pressure measurements by the device below 10(-5) Torr. In addition to its inherently higher sensitivity, a constant-temperature circuit together with a thermoelectric stabilization of the sensor substrate temperature can eliminate the induced piezoresistive error. The constant-temperature circuit operating on the micro-Pirani sensor together with the above-mentioned temperature compensation and the stabilization methods have extended gauge capability down to 10(-7) Torr, which is only limited by the signal readout resolution (similar to 1 mu V). This is three orders of magnitude more sensitive than the conventional vacuum gauges of the thermal conductivity type. (C) 1995 American Vacuum Society