A Pirani vacuum sensor has been fabricated by the silicon micromachining technique. A square glass membrane was formed on (100) silicon substrate with a platinum-film resistor coated. The membrane is suspended by its four leads extended to the corners of an etched cavity. This structure can provide both low thermal loss through leads to the substrate and large active area for gaseous heat conduction. It thus can be used as a highly sensitive vacuum sensor. The fabricated sensor has shown a linear response of pressure from 8×10−5 to 6 Torr with constant-temperature operation. It is found that the low-pressure limit of the vacuum sensor was caused by the noise of the instrument used in the experiment. A new terminology called ‘‘noise equivalent pressure’’ thus is definable due to this finding. The physical limit of the noise equivalent pressure is analyzed. Optimization of the device structure and the noise equivalent pressure are also discussed in detail. A new method of ambient-temperature compensation is also proposed and analyzed here.

As an extension of previous work in our laboratory, a wide-range Pirani gauge that is capable of measuring vacuum pressure down to 10 Ϫ7 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 V/°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 Ϫ5 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 Ϫ7 Torr, which is only limited by the signal readout resolution ͑ϳ1 V͒. This is three orders of magnitude more sensitive than the conventional vacuum gauges of the thermal conductivity type.

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