Design optimization of surface acoustic wave devices and investigation of fabricated devices under temperature, humidity and steam environments

Abstract

Surface Acoustic Wave (SAW) technique is one of the most promising Micro Electro Mechanical Systems (MEMS) based detection methods for a variety of applications. SAW sensor is based on the modulation of Rayleigh waves that travel near the surface of a piezoelectric substrate. Advent of MEMS technology has led to the development of SAW sensors which are widely used in chemical sensing, medical, industrial, telecommunication and biological applications. The main focus of this paper is to devise and characterize a generic SAW structure and study the acoustic properties at various environmental conditions before depositing the sensing layer/analyte for any particular application. In this work, a SAW device was designed using COMSOL Multiphysics for a resonant frequency of 100 MHz with 40 µm wavelength on Lithium Niobate (LiNbO3) piezoelectric substrate. The optimized device structure was fabricated using thin film deposition and UV photolithography. The resonant frequency of the fabricated device (97.63 MHz) matched closely with the simulation results (100 MHz) and theoretical results (100 MHz). The device was subjected to various environmental conditions to study the influence on the resonant frequency. The SAW structure showed a resonant frequency shift of 90 KHz in steam environment, ~ 1MHz when heated to 150°C and ~120 KHz when exposed to 80% RH. The device exhibited a linear shift in resonant frequency under all tested environmental conditions with a consistent baseline and good repeatability.