Robust thin films for high temperature surface acoustic wave devices

Abstract

Surface Acoustic Wave (SAW) devices are nowadays most commonly used in the telecommunication sector as frequency filters or as delay lines. Besides these standard applications, such devices can also be used as key component in battery-less and wireless sensor applications. This may include temperature measurements, both in high temperature as well as aggressive gaseous environments. Current solutions for high temperature stable SAW devices are based on substrates like langasite, langatate oder langanite. The main disadvantage of all these crystals is the low phase velocity (~2700 m/s) and the high transition losses of the signal at elevated temperatures. To overcome these drawbacks, aluminum nitride (AlN) sputterdeposited as piezoelectric thin film in combination with sapphire as substrate show a promising alternative. Phase velocities of around 5700 m/s can be reached, which is substantially higher compared to other systems, named above. Therefore, with the same device geometry even higher resonance frequencies can be achieved. In this thesis the modifications of the electro-mechanical properties of aluminum nitride thin films due to the influence of temperature as well as inert and aggressive gases are investigated. The results showed that AlN layers are stable without changing their device relevant properties such as crystallographic structure, surface roughness, intrinsic stress, piezoelectricity and leakage current up to 700 °C independent of the surrounding gas atmosphere. At higher temperatures, the AlN layer gets oxidized in an oxygen-containing atmosphere, resulting in the loss of piezoelectric properties. Therefore, the impact of different passivation coatings on the oxidation resistance of the active AlN thin film was investigated. A promising solution is reactively sputtered-deposited silicon nitride. The oxidation resistance of this layer was evaluated by continuously monitoring with a high temperature X-ray diffraction equipment any changes and hence, any oxidation of the underlying AlN for 24 hours at 1000 °C in pure oxygen atmosphere. Furthermore, possible electrode materials are examined, such as platinum and iridium. To measure the impact of temperature load the modification in film resistance of the metallization lines having a width between 1 μm and 50 μm was investigated. Finally, SAW devices based on aluminum nitride on sapphire substrates were fabricated. For a better device understanding the thickness of the piezoelectric layer and the wave propagation direction with respect to the crystallographic orientation of the sapphire substrate were varied systematically. In addition, the influence of the electrode material on the phase velocity was examined. As a result, not the thickness, but predominantly the density of the electrode material has an impact and reduces the phase velocity. Finally, SAW devices were measured during high temperature load up to 800 °C in an inert gas atmosphere. The measurements showed that the temperature sensitivity is almost linear in the investigated temperature range. An additional passivation layer has only a minor influence on the latter device parameter.