D1.4 - High-Temperature Stable Electrodes for Langasite Based Surface Acoustic Wave Devices

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
D1 - High Temperature Sensors I
D. Richter, H. Fritze - Technische Universität Clausthal, Goslar (Germany)
532 - 537


Piezoelectric single crystals like langasite, langatate or gallium orthophosphate enable the development of high-temperature stable surface acoustic wave devices for sensor applications in harsh environments. Although e.g. langasite shows piezoelectric behavior up to its melting point at 1470 °C, the operation of piezoelectric devices is limited by the hightemperature stability of electrode materials and packaging.
The electrodes should remain stable under harsh conditions at high temperatures while exhibiting a good adhesion on langasite as well as a high conductivity. In particular, the films should be stable in a temperature range from 650 °C to 900 °C. Further, the electrode thickness should not exceed 100 nm due to the influence of the electrode mass on the acoustic waves.
In this work, potentially suited electrode materials and materials systems are investigated with respect to their applicability for langasite based SAW devices at high temperatures.
Among the metal based films, Pt/Rh (10 %) exhibits the best high temperature stability for a given film thickness. Minor degradation of the films is observed up to temperatures of 900 °C. Common Ti/Pt films are stable up to 800 °C. Stability for few hours is given up to 900 °C. Therefore those electrodes are suitable for short-term operations.
Palladium films show only a poor stability and should therefore not be considered as electrode material. Measurements with iridium thin films showed a degradation at temperatures of 700 °C, which is dominated by the oxidation of the material. The degradation process of the metal-based films is predominantly caused by agglomeration. This process can be significantly shifted to higher temperatures by deposition of an additional alumina protection layer.
Promising results are obtained for thin LSM (La0.65Sr0.35MnO3) films. These films are stable up to 900 °C and show a good conductivity in the temperature range from 600 °C to 900 °C. The comparable low conductivity can be improved by performing a codeposition of LSM with platinum.