PLATE MODE MICROACOUSTIC FILTERS INCLUDING BILATERAL TEMPERATURE COEFFICIENT OF FREQUENCY (TCF) LAYERS
A temperature coefficient of frequency (TCF) compensation layer may be disposed on a second surface of a piezoelectric layer opposite to a first surface to reduce the TCF of a microacoustic filter. The frequency response of a microacoustic filter with the TCF compensation layer on only the second surface of the piezoelectric layer includes spurious modes. In an exemplary microacoustic filter, another TCF compensation layer is disposed on the first surface of the piezoelectric layer between the electrode structure and the piezoelectric layer. Providing TCF compensation layers bilaterally on the first and second surfaces of the piezoelectric layer reduces spurious modes in the frequency response of the microacoustic filter. In some examples, the electrode structure is disposed on a substrate and may be isolated from the TCF compensation layer on the first surface of the piezoelectric layer by an air gap to reduce acoustic losses.
1 . A microacoustic filter comprising:
a piezoelectric layer having a crystalline structure operative to laterally excite a plate mode;
an electrode structure disposed adjacent a first surface of the piezoelectric layer;
a first temperature coefficient of frequency (TCF) compensation layer disposed on the first surface of the piezoelectric layer between the electrode structure and the piezoelectric layer; and
a second TCF compensation layer disposed on a second surface of the piezoelectric layer opposite to the first surface.
2 . The microacoustic filter of claim 1 , further comprising a substrate, wherein the electrode structure is between the substrate and the first TCF compensation layer.
3 . The microacoustic filter of claim 2 , further comprising a dielectric under-layer disposed between the electrode structure and the substrate.
4 . The microacoustic filter of claim 3 , further comprising an interstitial dielectric material, wherein:
the electrode structure comprises electrode fingers disposed on the dielectric under-layer and extending in a first direction; and
the interstitial dielectric material is disposed on the dielectric under-layer between the electrode fingers in a second direction orthogonal to the first direction.
5 . The microacoustic filter of claim 4 , wherein:
the first direction is parallel to the first surface of the piezoelectric layer; and
each of the interstitial dielectric material and the electrode fingers have a first thickness orthogonal to the first surface of the piezoelectric layer.
6 . The microacoustic filter of claim 1 , further comprising a first passivation layer disposed on the second TCF compensation layer.
7 . The microacoustic filter of claim 6 , further comprising a second passivation layer disposed on the first TCF compensation layer.
8 . The microacoustic filter of claim 7 , wherein:
the first passivation layer and the second passivation layer each comprise a layer of silicon nitride (Si 3 N 4 ).
9 . The microacoustic filter of claim 1 , wherein:
the first TCF compensation layer and the second TCF compensation layer each comprise a layer of carbon-doped silicon dioxide (SiO 2 C).
10 . The microacoustic filter of claim 1 , wherein:
the first TCF compensation layer has a thickness orthogonal to the first surface of the piezoelectric layer in a range of five (5) to forty (40) nanometers (nm).
11 . The microacoustic filter of claim 1 , further comprising a cavity formed between the first TCF compensation layer and the electrode structure.
12 . The microacoustic filter of claim 11 , wherein:
the cavity is at least partially filled with a gas; or
the gas comprises air.
13 . The microacoustic filter of claim 11 , wherein:
the electrode structure comprises a second surface that faces the first surface of the piezoelectric layer; and
the cavity is between the first surface of the piezoelectric layer and the second surface of the electrode structure.
14 . The microacoustic filter of claim 4 , wherein:
the first direction of the electrode fingers of the electrode structure is parallel to a second axis that is perpendicular to a first axis;
a third axis is perpendicular to the first axis and the second axis;
an orientation of the first axis, the second axis, and the third axis is relative to the crystalline structure of the piezoelectric layer as defined by Euler angles lambda, mu, and theta; and
the piezoelectric layer comprises lithium niobate with the Euler angle lambda being approximately 0°, the Euler angle mu being approximately 32.5°, and the Euler angle theta being approximately 0°, or at least one symmetrical equivalent thereof.
15 . The microacoustic 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; an avionics system; a drone; and a multicopter.
16 . A method of manufacturing a microacoustic filter, the method comprising
forming a piezoelectric layer having a crystalline structure operative to laterally excite a plate mode;
forming a first temperature coefficient of frequency (TCF) compensation layer on a first surface of the piezoelectric layer;
forming a second TCF compensation layer on a second surface of the piezoelectric layer opposite to the first surface; and
forming an electrode structure adjacent to the first surface of the piezoelectric layer.
17 . The method of claim 16 , wherein:
forming the electrode structure further comprises forming the electrode structure on a substrate and between the substrate and the first TCF compensation layer.
18 . The method of claim 17 , further comprising:
forming a dielectric under-layer between the electrode structure and the substrate;
forming the electrode structure comprising electrode fingers disposed on the dielectric under-layer and extending in a first direction; and
forming an interstitial dielectric material on the dielectric under-layer between the electrode fingers in a second direction orthogonal to the first direction.
19 . The method of claim 16 , further comprising forming a cavity between the first TCF compensation layer and the electrode structure.
20 . A microacoustic filter comprising:
a piezoelectric layer having a crystalline structure operative to laterally excite a plate mode;
a first dielectric layer disposed on a first surface of the piezoelectric layer;
a second dielectric layer disposed on a second surface of the piezoelectric layer opposite to the first surface; and
an electrode structure disposed on a carrier stack and spaced from the piezoelectric layer by a gap.