Enabling very high temperature acoustic wave devices for sensor & frequency control applications

The introduction of piezoelectric crystals capable of acoustic wave (AW) excitation at high temperatures (> 600 degrees C) has opened new possibilities for harsh environment applications, such as combustion engines, industrial processes, and gas/oil extraction. Significant remaining challenges are the fabrication of electrode thin films as well as appropriate packaging capable of withstanding such harsh environments. Thin film electrodes utilizing platinum over zirconium (Pt/Zr) developed by the University of Maine research team for surface acoustic wave gas sensors proved to be inappropriate for long term operation above 700 degrees C, due to the de-wetting phenomenon of thin film Pt. In this paper the fabrication and testing of thin film electrodes and AW devices for longer term operation (from a few hours to months) in high temperature environments (up to 1000 degrees C) have been investigated. The techniques used to overcome the problem of AW device electrode failure at temperatures above 600 degrees C include: multilayered film architectures, alloy compositions, high temperature processing, and protective ceramic overlay films. In particular Pt, zirconium (Zr), ZrO2, Pt/Rhodium, and Pt/Au films have been examined alone or in combinations as the electrode materials, and ultra-thin SiAION coatings have been used to extend electrode lifetime and to provide device protection in harsh environments. It has been found that the combination of layered and alloy electrodes retarded or prevented de-wetting of the Pt film, and extended the long-term AW device operation from 600 degrees C to at least 950 degrees C. These results indicate the feasibility of very high temperature AW device operation, and open up new opportunities for AW device applications in harsh environments.