Analysis and Modeling of Surface Acoustic Wave Chemical Vapor Sensors

Abstract

Surface Acoustic Wave (SAW) sensors demonstrate superior selectivity for the detection of chemical agents. Due to their solid state design and fabrication, compatible with other modern technologies such as MIC (microwave integrated circuits), MEMS (micro-electromechanical-systems), CMOS, CCD (charge coupled devices) and integrated optic devices, SAW chemical sensors are extremely reliable. They have compact structure, high sensitivity, small size, outstanding stability, low cost, fast real-time response, passivity, and above all the ability to be incorporated in complex data processing systems. They can be used for in situ monitoring and sensing systems [Ho et al., 2003; Wohltjen & Dessy, 1979; Wohltjen, 1984; Comini, 2009] and for wireless sensing and monitoring in harsh environment [Pohl, 2000] including the detection of chemical warfare agents [Data Sheet, 2005] and land mine detection [Kannan et al., 2004]. It is interesting that a SAW-based sensor system is used as a volatile organic contamination monitoring system for the satellite and space vehicle assembly rooms in NASA. SAW sensors can distinguish organophosphates, chlorinated hydrocarbons, ketones, alcohol, aromatic hydrocarbons, saturated hydrocarbons, and water [Ho et al., 2003]. Surface acoustic waves were discovered in 1885 by Lord Rayleigh and are often named after him as Rayleigh waves [Rayleigh, 1885]. A surface acoustic wave is a type of mechanical wave motion which travels along the surface of a solid material, referred to as substrate. The amplitude of the wave decays exponentially with distance from the surface into the substrate, so that the most of the wave energy is confined to within one wavelength from the surface [Farnell, 1977; Martin et al., 1994]. A basic SAW device was originally developed in 1965 [White & Voltmer, 1965] by White and Voltmer when they found out how to launch a SAW in a piezoelectric substrate by an electrical signal. The basic SAW device consists of two interdigital transducers (IDTs) on a piezoelectric substrate such as quartz, Fig. 1. Each IDT is a reversible transducer made of interleaved metal electrodes, which are used to convert an electrical signal to an acoustic wave and vice versa. An IDT is a bidirectional transducer: it radiates energy equally on both sides of the electrodes. Consequently, theoretical insertion loss introduced by an IDT is at least 6 dB. SAW devices work in the frequency range of 10 MHz to several GHz.