Composite substrate for surface acoustic wave device and manufacturing method thereof

A composite substrate for a surface acoustic wave device, comprising a piezoelectric single crystal thin film, a support substrate, and a first intervening layer between the piezoelectric single crystal thin film and the support substrate, wherein:

• the first intervening layer is in contact with the piezoelectric single crystal thin film;
• and
• an acoustic velocity of a transverse wave of the first intervening layer is faster than an acoustic velocity of a fast transverse wave of the piezoelectric single crystal thin film,
• wherein the bonding interface between the first intervening layer and at least one layer adjacent to the first intervening layer has an uneven structure, and
• the ratio of the average length RSm of the element in the sectional curve of the uneven structure and the wavelength λ of the surface acoustic wave when used as the surface acoustic wave device is 0.2 or more and 5.0 or less.