Langasite 2.45 GHz ISM band SAW resonator for harsh environment wireless applications

The operation of a wireless sensor based solely on the radio frequency (RF) signal received, i.e., under no constraint of battery or alternate power source is very attractive for multiple industrial, aerospace and military applications. Above a few hundred °C, battery and energy scavenging schemes pose serious challenges regarding sensor system implementation and performance, thus increasing the interest in entirely passive RF sensors. In addition, for several applications, it is desirable to use the Industrial, Scientific and Medical (ISM) radio bands, since they are open to commercial applications. High-temperature harsh-environment surface acoustic wave (SAW) devices employing langasite crystals have been reported to frequencies up to several hundred MHz. In this paper, measured results from SAW resonators (SAWRs) operating in the ISM band of 2.45 GHz are presented and discussed. The SAWR devices were fabricated using both electron beam (e-beam) and ion milling patterning techniques on 4 inch diameter langasite (LGS) wafers. The interdigital transducers (IDTs) and reflectors have a minimum feature size of 0.185 µm, and employ a nanocomposite Pt-Al
2
O
3
/ Zr film architecture. The tested SAWR devices have been heated up to 800°C, cycled for several weeks in the 300°C to 800°C range to verify stable performance, and also wireless interrogated in a furnace environment. The results obtained and presented in this paper indicate the feasibility of using LGS SAWRs for wireless operation in the 2.45 GHz ISM band.