Comparison of Packaging Concepts for High-Temperature Pressure Sensors at 500 °C

Abstract

In industry there is an increasing demand for reliable pressure sensors for applications in harsh environment. Pressure sensors working at high temperatures of 500 °C impose new challenges in packaging due to thermal cross sensitivity and temperature induced stresses on the package. Other major issues are to identify stable materials at high temperature and stress-tolerant sensor mounting techniques. In this research work, a comparison of two different pressure sensing approaches is established. The first one involves a resistive pressure sensor: A thin-film platinum metallization was sputtered on a SiO2- membrane and structured as a Wheatstone bridge. The membrane is fabricated by bulk micromachining (RIE) of the Si/SiO2 substrate. The sensor is mounted onto the ceramic substrates (LTCC, AIN, Si3N4) using flip-chip contacts and glass solder die attachment techniques. The second sensing approach is using a Surface Acoustic Wave (SAW) device. Lithium Niobate (LiNbO3) has been proved as an excellent material for SAW sensors. It can operate at temperatures up to 500°C. A micro strain gauge is patterned onto a lithium niobate (LiNbO3) crystal and attached to a ceramic substrate (Al2O3/AIN) like a cantilever by flip-chip interconnection and glass solder underfill. The ceramic substrate has a membrane structure fabricated by Electric Discharge Machining (EDM). The deforming membrane will strain the crystal inside the package on the free end of the cantilever. The strain produced on the cantilever is measured by the change of resistance of the micro strain gauge. Since lithium niobate is a well-known material for SAW applications, later this micro strain gauge will be replaced by a SAW element. A special design concept aims at the elimination of thermal stresses between membrane and sensing device. A comparative study is done on both the approaches with influences like material, assembly and die attachment technique.