Acoustic wave device and wafer with support substrate having an uneven surface, and manufacturing method of wafer

1. An acoustic wave device comprising:

a support substrate having an uneven surface;

a piezoelectric layer provided on the uneven surface of the support substrate;

an electrode that excites an acoustic wave in the piezoelectric layer; and

an insulating layer that is provided between the uneven surface of the support substrate and the piezoelectric layer, and has an air gap located in a recess part of the uneven surface, an upper portion of the air gap being covered with the insulating layer and a side portion of the air gap being surrounded by the insulating layer.

2. The acoustic wave device according to claim 1 , wherein a height of the air gap is equal to or greater than 0.2 times an average height of unevenness of the uneven surface.

3. The acoustic wave device according to claim 1 , wherein a width of the air gap is equal to or greater than 0.01 times an average period of unevenness of the uneven surface and equal to or less than 0.2 times the average period of the unevenness of the uneven surface, the width being a largest length in a short direction when the air gap is viewed in a plan view.

4. The acoustic wave device according to claim 1 ,

wherein the uneven surface has protruding portions regularly arranged, and

wherein the air gap is provided in plural, and a regular interval between each air gap is substantially equal to a regular interval between the protruding portions.

5. The acoustic wave device according to claim 1 ,

wherein the uneven surface has recessed portions regularly arranged, and

wherein the air gap is provided in plural, and a regular interval between each air gap is substantially equal to a regular interval between the recessed portions.

6. The acoustic wave device according to claim 1 , wherein the insulating layer includes a first insulating film mainly composed of silicon oxide, and a second insulating film provided between the first insulating film and the uneven surface of the support substrate, the second insulating film being composed of a material that causes an acoustic velocity of a bulk wave propagating through the second insulating film to be different from an acoustic velocity of a bulk wave propagating through the first insulating film.

7. The acoustic wave device according to claim 6 , wherein the air gap is provided so as to extend across the second insulating film, and extend to only a part of the first insulating film.

8. The acoustic wave device according to claim 6 , wherein the air gap is provided only in the second insulating film of the first and second insulating films.

9. The acoustic wave device according to claim 8 , wherein the air gap is provided in the second insulating film while having a length equal to or greater than 0.5 times a thickness of the second insulating film in a thickness direction of the second insulating film.

10. The acoustic wave device according to claim 1 , wherein the electrode is a pair of comb-shaped electrodes that is disposed on a surface of the piezoelectric layer opposite to the support substrate and includes a plurality of electrode fingers.

11. A wafer comprising:

a support substrate having an uneven surface;

a piezoelectric layer provided on the uneven surface of the support substrate; and

an insulating layer that is provided between the uneven surface of the support substrate and the piezoelectric layer, and has an air gap located in a recess part of the uneven surface, an upper portion of the air gap being covered with the insulating layer and a side portion of the air gap being surrounded by the insulating layer.

12. The acoustic wave device according to claim 1 , wherein a height of the air gap is greater than a width of the air gap, the width being a largest length in a short direction when the air gap is viewed in a pian view.

13. The acoustic wave device according to claim 1 , wherein the upper portion and the side portion of the air gap are in contact with the insulating layer.