Extension of operating range towards lower pressures of MEMS-based thermal vacuum gauges by laser-induced heating

摘要

Bulk micromachined Pirani pressure gauges have been processed on 1 um thick silicon nitride membranes and on 1 um thick nitride bridges resembling an H-shaped structure with the meander-shaped heating element positioned in the crossbar region of the H. For the membrane device, the resistor element is fabricated from 20 nm Cr/200 run Au and for the H-shaped device from 30 nm Ni. The choice of using an H-shaped device and thinner, different metal was governed by the desire to increase the thermal resistances of the silicon nitride support structure and of the metal lines connecting the resistor element to the silicon cold junction, thus obtaining larger sensitivity and increased pressure range. It will be demonstrated that by heating the resistor elements via photon absorption using a green solid state laser as compared to Joules I~2 R heating, the relative resistance change versus pressure dependency shifts by about one order of magnitude towards lower pressures for both device structures. This fact is desirable for measuring pressures in the 1 x 10~(-3) to 1 x 10~(-6) Torr range using these miniaturized gauges.