A high thermal resistance MEMS-based Pirani vacuum sensor chip

The performance of thermal conductivity vacuum gauges can be improved by a well-designed geometry. The lower measurement range limit is determined by the size of the active sensing area and the thermal conduction heat losses through the supporting structures. The upper measurement range is limited by the distance between the heated element and the cold reference plane. Silicon based MEMS-technology gives the possibility to fabricate both sensing structures with suitable areas out of low thermal conductive materials and narrow gaps in order to extend the measurement range in both directions. In this work we present a MEMS-process to fabricate high thermal resistance sensor structures. The rectangular sensitive areas are anchored by four beams and are structured out of low thermal conductive PECVD-siliconnitride films with 1 mu m in thickness. The metallic heating structure is completely embedded in the SiN-layer. Both sensitive area and its support beams were released from the silicon bulk material by anisotropic underetching. In this way a free-supporting structure with a gap of 150 mu m to the silicon substrate was formed. The influence of the filament geometry and temperature was systematically investigated to determine the properties of the chips as thermal conductivity vacuum gauges. The temperature of the sensitive area was held constant by a self-balancing bridge circuit and the heating power was measured by Delta-Sigma-ADC. The average solid state thermal conductivity is in the order of 10(-6) W K-1. The measuring range of the most sensitive structures covers 8 orders of magnitude from 10(-5) mbar to 1000 mbar.