Method for manufacturing composite substrate

A method for manufacturing composite substrate comprising:

• preparing a piezoelectric material substrate having a first rough surface and a damaged layer in a vicinity of the first rough surface, wherein a surface roughness of the first rough surface is greater than about 0.03 μm in arithmetic mean roughness Ra, and wherein cracks exist within the damaged layer;
• removing the damaged layer by etching the first rough surface of the piezoelectric material substrate using a chemical process;
• depositing an intervening layer on a second rough surface of the piezoelectric material substrate from which the damaged layer has been removed, wherein a surface roughness of the second rough surface is greater than about 0.03 μm in arithmetic mean roughness Ra;
• flattening a surface of the deposited intervening layer;
• bonding the piezoelectric material substrate to a support substrate having a lower thermal expansion coefficient than the piezoelectric material substrate, wherein the deposited intervening layer is in between the piezoelectric material substrate and the support substrate;
• and
• thinning the piezoelectric material substrate after bonding the piezoelectric material substrate to the support substrate,
• wherein the piezoelectric material substrate is Lithium tantalate (LT) or lithium niobate (LN).