**6. Conclusion**

438 Acoustic Waves – From Microdevices to Helioseismology

Cu thicknesses between 0.12*λ* and 0.2*λ*, the modes SAW1 and SAW2, which have been determined as the lower-velocity and higher velocity modes, exchange their polarizations. After the second interaction, which occurs at *h*SiO2=*h*Cu=0.2*λ*, SAW1 and SAW2 turn back into Rayleigh-type and SH-type waves, respectively. However, at *h*Cu=0.2*λ* (Fig.11,c,d) both waves still have mixed polarizations. With increasing SiO2 film thickness, SAW1 and SAW2 transform into the boundary waves BW1 and BW2, respectively (Fig. 11, g, h), with acoustic waves localized in Cu grating and around it. The boundary waves have mixed polarizations, which would be impossible in isotropic substrate with isotropic thin film, but due to specific features of the analyzed LT orientation, BW1 is nearly sagittally polarized wave and BW2 is nearly pure SH wave. BW2 penetrates deeper into SiO2 film than into LT substrate. A numerical analysis reveals that SiO2 thickness about 1.5*λ* is sufficient for

*h*SiO2=3*λ*

 **Cu**  LT

transformation of SAW into the boundary wave.

*p*=*λ*/2

(a) (b)

Fig. 12. Acoustic fields associated with two higher-order modes propagating in 42ºYX LT

The acoustic fields associated with propagation of the two higher-order modes (Fig.10,b) have been also investigated. These modes have leaky wave nature. Fig.12 illustrates the structure of these modes at *h*SiO2=3*λ*. The first mode, which exists when *h*SiO2>0.7*λ*, has SH polarization deeply penetrating into SiO2 (Fig.12,a). With increasing SiO2 thickness, this mode degenerates into the SH BAW propagating in SiO2. The second mode, which exists at *h*SiO2>1.6*λ*, looks as a combination of SH-type SAW in LT substrate with Cu grating and sagittally polarized quasi-bulk wave propagating in SiO2 (Fig.12,b), with the amplitude of SH polarization component much higher than the amplitudes of two other components. This example demonstrates the effect of anisotropy on the propagation of acoustic waves in

with Cu grating and SiO2 film when *h*SiO2=3*λ*. (a) 1st mode; (b) 2nd mode

  In this chapter, some layered and multilayered structures, which look promising as substrates for modern SAW devices developed for applications in cellular phones, communication and navigation systems have been overviewed. A universal numerical technique, which enables fast and accurate analysis of these and other structures have been presented and, by way of example, applied to some multilayered structures of practical interest. The physical insight into the mechanisms of SAW transformation with increasing film thickness in a multilayered structure was provided via simulation of acoustic fields in one of the structures.
