There is known a surface acoustic wave device as one of acoustic wave devices using an acoustic wave. The surface acoustic wave device has a comb-like electrode formed on a surface of a piezoelectric substrate. The comb-like electrode has a metal strip. The metal strip receives and excites a surface acoustic wave, resonates a surface acoustic wave, or reflects a surface acoustic wave. The surface acoustic wave device is used as a filter of a wireless device such as a mobile phone unit, because the surface acoustic wave device is small and light, and greatly attenuates a signal out of a predetermined frequency band.
As the performance of the mobile phone unit gets higher, improvement of temperature characteristics of the surface acoustic wave device is needed, and reduction of temperature dependence of frequency such as a pass band of a filer and/or a resonance frequency of a resonator is needed. However, a piezoelectric single-crystal material such as lithium tantalate (LiTaO3) of which electromechanical coefficient is large lacks temperature stability.
Japanese Patent Application Publication No. 2004-343359 (hereinafter referred to as Document 1) discloses that a lithium tantalate substrate is jointed on a support substrate made of a sapphire substrate as an element substrate on which a surface acoustic wave element is provided. With the structure, temperature stability of the surface acoustic wave device is improved. On the other hand, Japanese Patent Application Publication No. 2002-9584 (hereinafter referred to as Document 2) discloses that a support substrate and an element substrate are made of a lithium tantalate substrate, a propagation direction of an acoustic wave of the surface acoustic wave element is an X-axis of the element substrate, and an axis direction of the support substrate in parallel with the propagation direction of the acoustic wave is a Z-axis.