An experimental study on high-temperature metallization for micro-hotplate-based integrated gas sensors

Metallization for MEMS devices for high-temperature applications is more complex than metallization for integrated circuits. The additional challenges for MEMS metallization come from the fact it can be attacked by the silicon etchant during micro-machining process and from thermal damage due to high-temperature post-metallization steps and device operation at high temperatures. This paper examines the metallization schemes for fabricating an integrated gas sensor with micro-hotplate operating at temperatures above 300 °C. Several types of composite multi-layer metal electrodes were investigated. The surface morphology of the metal electrodes after high-temperature processing were analyzed with optical and scanning electron microscopy (SEM) and X-ray photo-electronic spectroscopy (XPS). The multi-layer metallization systems for micro-hotplate gas sensor applications were experimentally studied for suitability for high-temperature operation. Considering CMOS technology compatibility and long-term reliability at high temperature, the multi-layer metallizations: Al/Ti-W/Pt/SnO2 for the sensor and Al/Ti-W/polysilicon for the heater and temperature sensor are the best choices among the systems we have experimented.