MICRO-PIRANI VACUUM GAUGE

A Pirani vacuum sensor has been fabricated by the silicon micromachini ng technique. A square glass membrane was formed on (100) silicon subs trate with a platinum-film resistor coated. The membrane is suspended by its four leads extended to the corners of an etched cavity. This st ructure can provide both low thermal loss through leads to the substra te and large active area for gaseous heat conduction. It thus can be u sed as a highly sensitive vacuum sensor. The fabricated sensor has sho wn a linear response of pressure from 8X10(-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 e xperiment. A new terminology called ''noise equivalent pressure'' thus is definable due to this finding. The physical limit of the noise equ ivalent pressure is analyzed. Optimization of the device structure and the noise equivalent pressure are also discussed in detail. A new met hod of ambient-temperature compensation is also proposed and analyzed here.