LGX pure shear horizontal SAW for liquid sensor applications

This paper reports predicted and measured properties of the pure shear horizontal (SH) mode for the LGX family of crystals, which includes langasite (LGS), langanite (LGN), and langatate (LGT). These crystals are of the trigonal class 32 group, as quartz, and they exhibit the SH symmetry type uncoupling for the Euler angles (0°, θ, 90°). This surface acoustic mode, also known as surface transverse wave (STW), is especially attractive for liquid sensing due to the moderate damping observed in liquid or viscous environments. Numerical and experimental propagation data presented for the SH mode on LGX (0°, θ, 90°) includes phase velocity (v_p), electromechanical coupling coefficient (K²), temperature coefficient of delay (TCD), fractional change in frequency with respect to temperature (Δf/fo), penetration depth, metal strip reflectivity, and excitation of spurious plate modes as a function of θ. High electromechanical coupling and zero temperature coefficient of delay (TCD) along LGX Euler angles (0°, θ, 90°), θ between 10° and 25°, with penetration depths comparable to surface acoustic wave (SAW) devices are disclosed. In particular, along LGT (0°, 13.5°, 90°), the experimental results reported with resonators and delay line structures verify the high electromechanical coupling (0.8%) for a SH SAW mode, about 10 times stronger than the 36° Y rotated quartz SH orientation, and the existence of zero TCD around 140°C. The phase velocity of 2660 m/s is within 0.2% of the calculated value, which is about 55% below the phase velocity of 36° Y quartz, thus leading to smaller STW devices. The penetration depth of 6.5 wavelengths is eight times more shallow than 36° Y quartz, thus providing significant SH mode energy trapping close to the surface. With such positive predicted and measured coupling and propagation characteristics, these orientations are appropriate for the fabrication of high coupling, zero TCD, smaller, and highly sensitive STW devices for filtering, frequency control, and liquid sensor applications.