Pure SH SAW on single crystal KNbO3 for liquid sensor applications

This paper reports on the design, fabrication, and testing of a surface acoustic wave (SAW) delay line on single crystal KNbO₃ Euler angles (0°,90°, 0°). This particular orientation of KNbO₃ is of symmetry Type 4, which is known to support an electromechanically active shear horizontal (SH) SAW mode, and has a very large electromechanical coupling coefficient of 53%. The SH-SAW mode is especially attractive for liquid sensing applications because the SAW is not significantly attenuated by the adjacent aqueous media, unlike that of a generalized SAW (GSAW), which contains particle displacement normal to the surface. 100 MHz SH-SAW delay line devices with 20% bandwidths were fabricated. Experimental results are reported relevant to liquid sensing applications, such as the electromechanical coupling coefficient, K², Δf/f₀ versus temperature, and the attenuation effect on the magnitude of the transmission coefficient, |S₂₁|, with and without de-ionized (DI) water loaded surface. Results show a difference of only about 2 dB in |S₂₁| when the surface is loaded with DI water, thus verifying the suitability of the pure SH-SHAW mode on KNbO₃ for liquid sensor applications.