Experimental and Theoretical Investigations of Some Useful Langasite Cuts for High-Temperature SAW Applications

Passive high-temperature sensors are a most promising area of use for SAW devices. Langasite (La3Ga5SiO14; LGS) has been identified as promising piezoelectric material to meet high-temperature SAW challenges. Because it is necessary to know the material behavior for an accurate device design, the frequency-temperature behavior of Rayleigh SAW (R-SAW) and shear-horizontal SAW (SH-SAW) LGS cuts is investigated on delay line and resonator test structures up to 700 degrees C by RF characterization. In the range of the 434-MHz ISM band, the (0 degrees, 22 degrees, 90 degrees) SH-SAW cut shows thermal behavior similar to the (0 degrees, 138.5 degrees, 26.7 degrees) R-SAW cut. Associated with the (0 degrees, 22 degrees, 31 degrees) cut, in which SAWs present mixed types of polarization, the (0 degrees, 22 degrees, 90 degrees) SH-SAW orientation might allow differential measurements on a single substrate. In the temperature range of 400 to 500 degrees C, delay line test devices using the SH-SAW cut show a considerable drop of signal. Theoretical analysis indicates that this newly described behavior might be a result of anisotropy effects in this cut, occurring in case of any slight misorientation of electrode alignment.