Bulk acoustic wave devices with sandwich electrodes for higher resonant frequencies, and related fabrication methods
A bulk acoustic wave (BAW) device comprises a piezoelectric layer disposed between a first electrode layer and a sandwich electrode. The sandwich electrode includes a first layer of a first material having a first acoustic impedance and a second layer of a second material having a second acoustic impedance that is less than the first acoustic impedance of the first layer. The second layer of the sandwich electrode having the lower acoustic impedance is disposed between the first layer and the piezoelectric layer. The sandwich electrode combined with the piezoelectric layer and first electrode can cause the BAW device to resonate at a frequency whose wavelength corresponds to an acoustic cavity length of the BAW device, depending on an acoustic mirror included on one side of the BAW device. In one example, the acoustic cavity length is about 1.5 times of the resonant frequency wavelength.
1. An acoustic filter comprising:
a first bulk acoustic wave (BAW) device; and
a second BAW device;
wherein each of the first BAW device and the second BAW device comprises:
a piezoelectric layer;
a first electrode on a first side of the piezoelectric layer; and
a sandwich electrode on a second side of the piezoelectric layer, the sandwich electrode comprising:
a first layer of a first material having a first acoustic impedance; and
a second layer of a second material having a second acoustic impedance less than the first acoustic impedance, the second layer disposed between the first layer and the piezoelectric layer, wherein a first ratio of a thickness of the first layer to a thickness of the second layer of the sandwich electrode in the first BAW device is different than a second ratio of a thickness of the first layer to a thickness of the second layer of the sandwich electrode in the second BAW device.
2. The acoustic filter of claim 1 , wherein in one of the first BAW device and the second BAW device, a thickness of the second layer of the sandwich electrode is greater than a thickness of the first layer of the sandwich electrode.
3. The acoustic filter of claim 1 , wherein in each of the first BAW device and the second BAW device, the first electrode comprises:
a third layer of the first material; and
a fourth layer of the second material disposed between the third layer and the piezoelectric layer.
4. The acoustic filter of claim 1 , further comprising a ladder filter, wherein:
the first BAW device is a series-coupled device; and
the second BAW device is a shunt-coupled device.
5. The acoustic filter of claim 1 , wherein an acoustic thickness of the first sandwich electrode in at least one of the first BAW device and the second BAW device substantially corresponds to half of a wavelength of a resonant frequency of the at least one of the first BAW device and the second BAW device.
6. The acoustic filter of claim 1 , further comprising, in at least one of the first BAW device and the second BAW device, an acoustic mirror disposed between a substrate and one of the first electrode or the first sandwich electrode, wherein at least one layer of the acoustic mirror has an acoustic thickness substantially corresponding to three-quarters of the wavelength of the resonant frequency of the at least one of the first BAW device and the second BAW device.
7. The acoustic filter of claim 6 , wherein the acoustic mirror comprises:
a first mirror layer comprising one of a first dielectric layer and a first metal layer; and
a second mirror layer comprising one of a second dielectric layer and a second metal layer.
8. The acoustic filter of claim 7 , wherein the first dielectric layer comprises silicon dioxide (SiO 2 ), and the first metal layer comprises tungsten (W).
9. The acoustic filter of claim 1 , wherein, in at least one of the first BAW device and the second BAW device:
the first material and the first electrode comprise molybdenum (Mo); and
the second material comprises aluminum and copper (AlCu).
10. The acoustic filter of claim 1 , wherein, in at least one of the first BAW device and the second BAW device, the first electrode comprises a second sandwich electrode, further comprising:
a third layer of the first material; and
a fourth layer of the second material disposed between the third layer and the piezoelectric layer.
11. The acoustic filter of claim 1 integrated into a semiconductor die.
12. The acoustic filter 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.