Bulk acoustic wave (BAW) device with oppositely polarized piezoelectric layers for higher order resonance and method of manufacture
A bulk acoustic wave (BAW) device comprises a layer stack including first and second electrodes, a first piezoelectric layer between the electrodes, and a second piezoelectric layer between the first piezoelectric layer and the second electrode. A polarization of a crystal structure of the second piezoelectric layer is opposite to a polarization of a crystal structure of the first piezoelectric layer to achieve higher order resonant frequencies in the BAW device by means other than merely thinning layers in the layer stack. In some examples, the BAW device is a two-terminal device and does not include a metal layer configured to be a third electrode. In some examples, the BAW device includes at least one intermediate layer between the first and second piezoelectric layers, and a total combined thickness of the at least one intermediate layer is less than 4% of a total thickness of the layer stack.
1 . A two-terminal bulk acoustic wave (BAW) device, comprising:
a layer stack comprising:
a first electrode configured to couple to a first terminal;
a second electrode configured to couple to a second terminal;
a first piezoelectric layer between the first electrode and the second electrode, the first piezoelectric layer comprising a first crystalline structure having a first polarization;
a second piezoelectric layer between the first piezoelectric layer and the second electrode, the second piezoelectric layer comprising a second crystalline structure having a second polarization opposite to the first polarization; and
at least one intermediate layer between the first piezoelectric layer and the second piezoelectric layer,
wherein the at least one intermediate layer comprises:
a first amorphous layer of a first amorphous material; and
a seed layer configured to determine the second polarization of the second piezoelectric layer.
2 . The two-terminal BAW device of claim 1 , wherein the at least one intermediate layer further comprises a second amorphous layer of a second amorphous material different than the first amorphous material and disposed between the first amorphous layer and the seed layer.
3 . The two-terminal BAW device of claim 2 , wherein the first amorphous layer is in direct contact with the first piezoelectric layer and the seed layer is in direct contact with the second piezoelectric layer.
4 . The two-terminal BAW device of claim 3 , wherein the second amorphous layer is in direct contact with the first amorphous layer and the seed layer.
5 . The two-terminal BAW device of claim 1 , wherein:
between the first piezoelectric layer and the second piezoelectric layer, the layer stack does not comprise a metal layer configured to be an electrode.
6 . The two-terminal BAW device of claim 1 , wherein:
the first piezoelectric layer and the second piezoelectric layer comprise a same piezoelectric material.
7 . The two-terminal BAW device of claim 1 , wherein the first amorphous material comprises titanium nitride (TiN).
8 . The two-terminal BAW device of claim 1 , wherein the first amorphous material comprises silicon dioxide (SiO 2 ).
9 . The two-terminal BAW device of claim 1 , wherein:
the second piezoelectric layer comprises aluminum scandium nitride (AlScN); and
the seed layer comprises aluminum nitride (AlN).
10 . The two-terminal BAW device of claim 1 , wherein:
the first electrode is in direct contact with the first piezoelectric layer; and
the second electrode is in direct contact with the second piezoelectric layer.
11 . The two-terminal BAW device of claim 1 , wherein:
the first piezoelectric layer comprises a first thickness; and
the second piezoelectric layer comprises a second thickness in a range from 90% to 110% of the first thickness.
12 . The two-terminal BAW device of claim 1 , further comprising an acoustic mirror coupled to the first electrode.
13 . The two-terminal BAW device of claim 1 , further comprising:
a substrate;
an air cavity between the first electrode and the substrate; and
a membrane between the air cavity and the first electrode.
14 . The two-terminal BAW device of claim 1 integrated into a device selected from the group consisting of: a set-top box; an entertainment unit; a navigation device; a communications device; a fixed location data unit; a mobile location data unit; a global positioning system (GPS) device; a mobile phone; a cellular phone; a smartphone; a session initiation protocol (SIP) phone; a tablet; a phablet; a server; a computer; a portable computer; a mobile computing device; a wearable computing device; a desktop computer; a personal digital assistant (PDA); a monitor; a computer monitor; a television; a tuner; a radio; a satellite radio; a music player; a digital music player; a portable music player; a digital video player; a video player; a digital video disc (DVD) player; a portable digital video player; an automobile; a vehicle component; avionics systems; a drone; and a multicopter.
15 . A bulk acoustic wave (BAW) device, comprising:
a layer stack, comprising:
a first electrode;
a second electrode;
a first piezoelectric layer between the first electrode and the second electrode, the first piezoelectric layer comprising a first crystalline structure having a first polarization;
a second piezoelectric layer between the first piezoelectric layer and the second electrode, the second piezoelectric layer comprising a second crystalline structure having a second polarization opposite to the first polarization; and
at least one intermediate layer between the first piezoelectric layer and the second piezoelectric layer,
wherein:
a distance between the first piezoelectric layer and the second piezoelectric layer is less than four percent (4%) of a total thickness of the layer stack; and
the at least one intermediate layer comprises:
a first amorphous layer of a first amorphous material; and
a seed layer configured to determine the second polarization of the second piezoelectric layer.
16 . The BAW device of claim 15 , wherein:
the first piezoelectric layer and the second piezoelectric layer comprise a same piezoelectric material.
17 . The BAW device of claim 15 , wherein the distance between the first piezoelectric layer and the second piezoelectric layer is equal to a thickness of the at least one intermediate layer.
18 . The BAW device of claim 15 , wherein the first amorphous layer comprises a layer of titanium nitride (TiN).
19 . The BAW device of claim 15 , wherein the first amorphous layer comprises a layer of silicon dioxide (SiO 2 ).
20 . The BAW device of claim 15 , wherein:
the second piezoelectric layer comprises aluminum scandium nitride (AlScN); and
the seed layer comprises aluminum nitride (AlN).
21 . The BAW device of claim 15 , further comprising an acoustic mirror coupled to the first electrode.
22 . The BAW device of claim 15 , further comprising:
a substrate;
an air cavity between the first electrode and the substrate; and
a membrane between the air cavity and the first electrode.
23 . A method of fabricating a two-terminal bulk acoustic wave (BAW) device, the method comprising:
forming a layer stack comprising:
forming a first electrode configured to couple to a first terminal;
forming a second electrode configured to couple to a second terminal;
forming a first piezoelectric layer between the first electrode and the second electrode, the first piezoelectric layer comprising a first crystalline structure having a first polarization;
forming a second piezoelectric layer between the first piezoelectric layer and the second electrode, the second piezoelectric layer comprising a second crystalline structure having a second polarization opposite to the first polarization; and
forming at least one intermediate layer between the first piezoelectric layer and the second piezoelectric layer, wherein the at least one intermediate layer comprises:
a first amorphous layer of a first amorphous material; and
a seed layer configured to determine the second polarization of the second piezoelectric layer.
24 . The method of claim 23 , wherein:
forming the first piezoelectric layer comprises forming a piezoelectric material on a sacrificial substrate;
forming the first electrode comprises forming a first metal layer configured to be an electrode on an outer surface of the first piezoelectric layer;
forming the second piezoelectric layer comprises removing the sacrificial substrate from the first piezoelectric layer and forming the piezoelectric material on an inner surface of the first piezoelectric layer; and
forming the second electrode comprises forming a second metal layer configured to be an electrode on the second piezoelectric layer.
25 . The method of claim 24 , wherein forming the second piezoelectric layer further comprises:
forming a second seed layer on the inner surface of the first piezoelectric layer; and
forming the piezoelectric material on the second seed layer.
26 . The method of claim 25 , further comprising:
forming a first amorphous layer in direct contact with the inner surface of the first piezoelectric layer; and
forming the second seed layer on the first amorphous layer.
27 . The method of claim 26 , further comprising:
forming a second amorphous layer in direct contact with the first amorphous layer; and
forming the second seed layer in direct contact with the second amorphous layer.
28 . The method of claim 27 , wherein:
forming the first amorphous layer, the second amorphous layer, and the second seed layer further comprises forming the first amorphous layer, the second amorphous layer, and the second seed layer to a total combined thickness of less than four percent (4%) of a total thickness of the layer stack.
29 . The method of claim 23 , wherein:
forming the layer stack does not comprise forming, between the first piezoelectric layer and the second piezoelectric layer, a metal layer configured to be an electrode.
30 . The method of claim 23 , further comprising:
forming an acoustic mirror on the first electrode; and
forming a target substrate on the acoustic mirror.
31 . The method of claim 23 , further comprising:
forming a membrane on the first electrode;
forming a substrate on the first electrode; and
forming an air cavity between the first electrode and the substrate.
32 . The method of claim 24 , wherein forming the first piezoelectric layer further comprises:
forming a first seed layer on the sacrificial substrate; and
forming the piezoelectric material on the first seed layer.