Effects of layer thickness for GSAW, PSAW and HVPSAW devices

Considerable efforts in recent SAW device design and development have aimed at low loss, high performance and high frequency operation. A large number of applications relate to cellular and mobile telephony, pagers, local area networks, cordless phones, global positioning systems, and security systems. Pseudo-SAW (PSAW) and more recently the High Velocity Pseudo-SAW (HVPSAW) have received a great deal of attention due to their high phase velocities, and therefore the high frequencies of operation these modes may provide. In addition to high phase velocities, the pseudo-modes must also present low propagation losses and considerably high electromechanical coupling coefficients to be considered for practical devices. This paper verifies that the metallic layer thickness is a relevant SAW device parameter in order to achieve lower losses, for high frequency, high electromechanical coupling, low loss SAW devices. Popular PSAW and HVPSAW material orientations such as 64/spl deg/ YX LiNbO/sub 3/(0/spl deg/ 26/spl deg/ 0/spl deg/), 36/spl deg/ YX LiTaO/sub 3/ (0/spl deg/-54/spl deg/ 0/spl deg/), LiNbO/sub 3/ (90/spl deg/ 90/spl deg/ 36/spl deg/), LiTaO/sub 3/ (90/spl deg/ 90/spl deg/ 31/spl deg/), quartz ST-X (0/spl deg/ 132.75/spl deg/ 0/spl deg/), Li/sub 2/B/sub 4/O/sub 7/ (0/spl deg/ 45/spl deg/ 90/spl deg/), among others, are considered. In addition to the lower losses and the dispersion analysis for the pseudo modes, the present work discusses the transitions with layer thickness from HVPSAWs to higher order PSAWs modes, and the transitions from PSAWs to the generalized SAWs (GSAWs) modes. Besides contributing to the knowledge of the pseudo-modes behavior with layer thickness, this mode transition analysis enlightens the situation where the losses in the pseudo modes go to zero, due to the merging of the pseudo modes into the GSAWs. The fact that as a function of thickness the GSAWs are a natural continuation for the pseudo modes may be conveniently used in the fabrication of low loss devices. For completeness the effects of heavy layer metals, such as gold, in reducing the layer thickness where the pseudo modes merge to the GSAWs are discussed. Numerical results are compared to some experimental data available in the literature, and the present analysis elucidates experimentally observed higher order pseudo modes, and values of layer thickness where lower losses are achieved.