Optimal orientation function for SAW devices

The traditional electromechanical coupling coefficient (K²) and temperature coefficient of delay (TCD); more recently velocity (v_p), for compact or high frequency devices; and now power flow angle (PFA), diffraction (γ), amplitude (|Γ|) and phase (∠Γ) of a metal strip reflection coefficient are equally important in surface acoustic wave (SAW) device design and operation. In this paper a novel function which simultaneously analyzes seven SAW propagation properties (v_p, K², TCD, PFA, γ, |Γ|, and ∠Γ) is discussed and implemented. This flexible function allows the search and selection of optimal propagation directions along arbitrary piezoelectric substrates by employing user defined criteria for each property analyzed which: (i) specify target values; (ii) set acceptable variations from the target values; (iii) assign desired weights; and (iv) normalize to the maximum property value variations for the orientations under study. Given a substrate and a set of basic propagation properties covering a region in the space or in a plane, the function outputs a scalar number for each orientation. The minima indicate optimal orientations with respect to the user defined search criteria and weights. Contour plots are given showing the function results with respect to orientations in space for different substrates, the historic quartz, and those of more recent interest, the LGX family of crystals. The results validate the usefulness of the optimal orientation function in locating optimal SAW propagation directions and regions in space. Using quartz, for example, attractive propagation properties are confirmed along Euler angles (0°, 43.8°, 23.2°): K² is 19% above ST-X, diffraction is minimal, γ and PFA are zero, and the reflection coefficient is almost four times higher than ST-X. Comparisons with previously reported experimental results on quartz and the LGX family of crystals are given.