Abstract
Different cuts of langasite were investigated theoretically and experimentally as candidates for high temperature sensors utilizing SAW resonators and reflecting SAW tags. Single port SAW resonators with central frequencies close to 170, 200 and 433 MHz with W, Ir and Pt electrodes of different thickness were fabricated and their characteristics were measured as functions of temperature. An approach to SAW device encapsulation, which helps to avoid stress development during heating to high temperatures, was suggested and tested. The damage of the langasite surface during heating to high temperature is strong in the cut family with Euler angles (0o, 138.5o, ψ), while orientations with Euler angles (0o, 90o, ψ) and (0o, 22o, ψ) were found to be more tolerable to heating. In the latter cut family, two cuts, (0o, 22o, 31.5o) and (0o, 22o, 90o) can provide zero power flow angle and sufficient piezoelectric coupling. COM parameters of SAW resonators with W, Ir and Pt electrodes, including complex reflection coefficient, were calculated as functions of electrode thickness and measured in the wide temperature range. The frequency responses were described by COM equations and the fitted COM-parameters were compared to their simulated values. Resonators show high Q and their performance is perfectly described by COM equations
Conclusion
Langasite has an essentially non-linear temperature behavior, close to parabolic with the turnover point depending on the substrate orientation. Some orientations, such as (0o, 22o, 31.5o) have a vertex at about 400oC – in the middle of available operation temperature range. They may serve as a reference for measurement in wide temperature range but at the same time they present relatively high TCF at low and room temperature as well as at high temperature thus enabling development of temperature sensors with high sensitivity in these ranges. The (0o, 22o, 90o) cut of langasite parabolic dependence has a vertex at very low temperatures and shows a close to linear branch from room to high temperatures serving universally as a temperature sensor in all frequency ranges. These two orientations differ by the propagation direction on the same cut and may be used together to create a pair of sensors with different temperature behavior serving as a reference to each other. The frequency difference is then the parameter determining the measured temperature.
In systems restricted to work in ISM bands, the application of resonator based systems determine the need in relatively low TCF in order to keep the frequency inside a narrow band and the solutions with resonators on langasite require the division of measured range in several sub ranges.
Application of orientations (0o, 22o, 31.5o) and (0o, 22o, 90o) is preferable compared to commonly used in filters (0o, 138.5o, 26.6o) cut, due to higher tolerance to heating.
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