Acoustic wave device, filter, multiplexer, and wafer

An acoustic wave device comprising:

• a substrate;
• a piezoelectric layer provided on the substrate;
• at least one pair of comb-shaped electrodes provided on the piezoelectric layer, the at least one pair of comb-shaped electrodes including a plurality of electrode fingers having an average pitch equal to or greater than 0.5 times a thickness of the piezoelectric layer;
• a first insulating layer provided between the substrate and the piezoelectric layer, the first insulating layer having an acoustic velocity of a bulk wave higher than an acoustic velocity of a bulk wave in the piezoelectric layer;
• and
• a second insulating layer provided between the first insulating layer and the piezoelectric layer, the second insulating layer having an acoustic velocity of a bulk wave higher than the acoustic velocity of the bulk wave in the first insulating layer,
• wherein an acoustic velocity of a bulk wave in the substrate is higher than the acoustic velocity of the bulk wave in the second insulating layer,
• wherein no other layer having a thickness equal to or greater than 1/16 times the average pitch is provided between the piezoelectric layer and the second insulating layer,
• wherein the piezoelectric layer is a rotated Y-cut X-propagation lithium tantalate substrate, and
• wherein the acoustic velocity of the bulk wave in the second insulating layer is equal to or less than 1.1 times the acoustic velocity of the bulk wave in the first insulating layer.

An acoustic wave device comprising:

• a substrate that is a monocrystalline sapphire substrate;
• a piezoelectric layer provided on the substrate, the piezoelectric layer being a rotated Y-cut X-propagation lithium tantalate substrate;
• at least one pair of comb-shaped electrodes provided on the piezoelectric layer, the at least one pair of comb-shaped electrodes including a plurality of electrode fingers having an average pitch equal to or greater than 0.5 times a thickness of the piezoelectric layer;
• a first insulating layer provided between the substrate and the piezoelectric layer, the first insulating layer being a polycrystalline or amorphous aluminum oxide layer;
• a second insulating layer provided between the first insulating layer and the piezoelectric layer, the second insulating layer having a thickness equal to or less than three times the average pitch, the second insulating layer being a polycrystalline or amorphous aluminum oxynitride layer, a polycrystalline or amorphous aluminum nitride layer, a polycrystalline or amorphous silicon layer, a polycrystalline or amorphous silicon nitride layer, or a polycrystalline or amorphous titanium nitride layer;
• and
• a third insulating layer provided between the piezoelectric layer and the second insulating layer, a distance between an interface between the third insulating layer and the second insulating layer and an interface between the piezoelectric layer and the at least one pair of comb-shaped electrodes being equal to or less than two times the average pitch, the third insulating layer being a silicon oxide layer or a silicon oxide layer to which fluorine is added.