6B-4 RF Filter using Boundary Acoustic Wave

Authors report a new structure of a shear horizontal (SH) type boundary acoustic wave for cellular phone applications. Such a structure composed of electrodes with a low shear wave velocity between two materials, namely, the SiO₂ film/high density electrode/LiNbO ₃ substrate, is proposed. The unique feature of this structure is that it does not require a cavity on the substrate surface. The ladder filter and the longitudinal coupled multiple mode filter composed of this structure is reported. By changing the propagation angle of the acoustic wave, the electromechanical coupling factor k² ranged from 0 to 16% as well as a normalized bandwidth of 0.67 to 1.85 fold as large as that of a 36~46deg Y-X LiTaO₃ leaky surface acoustic wave (LT-LSAW) filter. In addition, an excellent temperature coefficient of delay time (TCD = 25 ppm/degC) and a large mutual coupling coefficient kappa₁₂ (=0.15) were also obtained. The newly developed RF filter using this structure was very small and thin