Composite substrate for acoustic wave device

A composite substrate for an acoustic wave device, said composite substrate comprising:

• a piezoelectric material layer;
• a supporting substrate;
• and
• x layers (x represents an integer of 3 or larger) of intermediate layers between said piezoelectric material layer and said supporting substrate,
• wherein said piezoelectric material layer, said supporting substrate and said intermediate layers satisfy the following formula (1),
• wherein the following formula (2) is satisfied when x is an even number, and
• wherein the following formula (3) is satisfied when x is an odd number.
• Rn<Rn+1 (1)
• (In the formula (1),
• n represents all of integers of 1 to x,
• Rn represents an arithmetic average roughness of a surface, on a side of said piezoelectric material layer, of an n′th-order intermediate layer from said piezoelectric material layer, and
• Rx+1 represents an arithmetic average roughness of a surface, on a side of said piezoelectric material layer, of said supporting substrate.)
• Vn−1<Vn (2)
• (In the formula (2),
• n represents all of even numbers of 2 or larger and x or smaller, and
• Vn represents an acoustic velocity of said n′th-order intermediate layer from said piezoelectric material layer.)
• Vn−1>Vn (3)
• (In the formula (3),
• n represents all of odd numbers of 1 or larger and x or smaller,
• Vn represents said acoustic velocity of said n′th-order intermediate layer from said piezoelectric material layer, and
• V0 represents an acoustic velocity of said piezoelectric material layer.)