Analysis of the thermal and temporal stability of Ta and Ti thin films onto SAW–substrate materials (LiNbO₃ and LiTaO₃) using AR‐XPS

Abstract

Modern surface‐acoustic‐wave devices are characterised by their trend to higher frequencies, power densities, and new applications. For this, a shrinking of the dimensions is necessary but hardly possible to achieve with standard Al‐based metallisation because of increased stress‐induced damaging (acoustomigration). In this context additional barrier interlayers between substrates and electrodes are necessary, especially for high‐temperature applications. For this, we present results of a detailed chemical interface analysis of deposited Ta and Ti thin‐film growth onto surface acoustic wave–substrate materials (LiNbO3 and LiTaO3) with respect to their interface layer structure and its temporal and thermal stability in vacuum. The analysis was mainly performed using non‐destructive and surface sensitive angle‐resolved X‐ray photoelectron spectroscopy. Besides a slight oxidation (oxygen adsorption) at the surface of these highly reactive metal thin films all interface sequences are stable up to 24 h. We also report higher thermal stability of Ta thin films up to 600 °C in contrast to Ti thin films (300 °C) because of presumed diffusion of Ti along grain boundaries. Copyright © 2016 John Wiley & Sons, Ltd.