Patent ID: 8049395

Claim:
A boundary acoustic wave device comprising: a piezoelectric layer; a dielectric layer laminated on the piezoelectric layer; an IDT electrode arranged at an interface between the piezoelectric layer and the dielectric layer; and a low thermal expansion medium layer laminated on a surface of the piezoelectric layer opposite to the interface and made of a material having a linear thermal expansion coefficient less than that of the piezoelectric layer; wherein the boundary acoustic wave device has a structure the propagates a SH boundary acoustic wave at the boundary between the piezoelectric layer and the dielectric layer; and an acoustic velocity of a SH wave at the piezoelectric layer satisfies Expression (1), and when λ represents a wavelength of the IDT electrode, (an acoustic velocity of a transverse wave at the dielectric layer)/λ with respect to a response frequency of the SH boundary acoustic wave and to a response frequency in a high order mode satisfies Expression (2) as follows: (acoustic velocity of transverse wave at dielectric layer)<(acoustic velocity of SH wave at piezoelectric layer)<(acoustic velocity of transverse wave at low thermal expansion medium layer) Expression (1); and (response frequency of boundary acoustic wave)<(acoustic velocity of transverse wave at dielectric layer)/λ<(response frequency of high order mode) Expression (2).