Mass sensitivity of dual mode SAW delay lines on AlN=sapphire structure

Introduction: There have been growing demands in recent years for highly sensitive surface acoustic wave (SAW) sensors that are able to detect minute quantities, especially in the chemical and biological fields. AlN is a promising material to fulfil such a task because of its high SAW velocity and good piezoelectricity [1]. Compared to conventional bulk piezoelectric materials such as quartz and LiTaO3, its thin film structure provides additional flexibility to integrate with substrate materials to modulate the phase velocity v, electromechanical coupling coefficient K, etc., and excite higher velocity SAW modes [2]. High velocity leaky SAWs have been reported on anisotropic thin film structures [3] and some may possess special features such as SH polarisation [4, 5], which makes them suitable for aqueous biosensing due to much less attenuation in liquid than SAWs. In this Letter we quantitively study the mass sensitivities of the coexisting SAW and SH-SAW on the AlN(0 0 0 1)Al2O3(1 1 2̄ 0) structure by loading the device with an Al thin film. The results would serve as good references for the development of highly sensitive dual mode sensors which can work in either gas or liquid environments.