# US 10,998,877 B2 · Film bulk acoustic resonator fabrication method with frequency trimming based on electric measurements prior to cavity etch

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- Number: US 10,998,877 B2
- Document ID: US10998877B2
- Status: Granted
- Kind: B2
- Application number: 16/988,202
- Filed: 2020-08-07
- Published: 2020-12-24
- Granted: 2021-05-04
- Earliest claimed: 2018-06-15
- Assignees: Resonant Inc.
- Applicants: Resonant Inc.
- Inventors: Patrick Turner; Ventsislav Yantchev; Robert B. Hammond
- Web page: https://patentgrep.com/patent/US10998877B2

## CPC

- H03H 9/02015: {Characteristics of piezoelectric layers, e.g. cutting angles}
- H03H 9/174: {Membranes}
- H03H 3/02: for the manufacture of piezoelectric or electrostrictive resonators or networks (H03H3/08 takes precedence)
- H03H 3/04: for obtaining desired frequency or temperature coefficient
- H03H 9/02228: {Guided bulk acoustic wave devices or Lamb wave devices having interdigital transducers situated in parallel planes on either side of a piezoelectric layer}
- H03H 9/205: having multiple resonators (crystal tuning forks H03H9/21)
- H03H 2003/023: {the resonators or networks being of the membrane type}
- H03H 2003/0442: {of a non-piezoelectric layer}
- H03H: IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS (waveguides, resonators, lines or other devices of the waveguide type H01P)

## Abstract

Methods of fabricating acoustic resonators are disclosed. A back surface of a single-crystal piezoelectric plate is bonded to a surface of a substrate. A conductor pattern is formed on the front surface of the piezoelectric plate, the conductor pattern including a plurality of interdigital transducers (IDTs) of a plurality of resonators. A dielectric passivation/tuning layer is formed over the conductor pattern and the front surface of the piezoelectric plate. Electrical measurements are made on at least some of the plurality of resonators. Material is selectively removed from the dielectric passivation/tuning layer in accordance with the electrical measurements. After removing material from the dielectric passivation/tuning layer, cavities are formed in the substrate such that interleaved fingers of each IDT are disposed on a respective diaphragm spanning a respective cavity

## Claims

### Claim 1

A method of fabricating acoustic resonators on a single crystal piezoelectric plate having opposing front and back surfaces, comprising:
bonding the back surface of the piezoelectric plate to a surface of a substrate;
forming a conductor pattern on the front surface of the piezoelectric plate, the conductor pattern including a plurality of interdigital transducers (IDTs) of a plurality of resonators;
forming a dielectric passivation/tuning layer over the conductor pattern and the front surface of the piezoelectric plate;
performing electrical measurements on some or all of the plurality of resonators;
selectively removing material from the dielectric passivation/tuning layer in accordance with the electrical measurements; and
after removing material from the dielectric passivation/tuning layer, forming cavities in the substrate such that interleaved fingers of each IDT are disposed on a respective diaphragm spanning a respective cavity.

### Claim 2 (depends on 1)

The method of claim 1, wherein the front surface of the piezoelectric plate is attached to a sacrificial substrate during the bonding, the method further comprising:
removing the sacrificial substrate to expose the front surface of the piezoelectric plate after the bonding.

### Claim 3 (depends on 2)

The method of claim 2, further comprising:
polishing the front surface of the piezoelectric plate after removing the sacrificial substrate and before performing thickness measurements.

### Claim 4 (depends on 1)

The method of claim 1, wherein performing electrical measurements comprises:
performing one or more electrical measurements on resonators distributed over the area of the piezoelectric plate.

### Claim 5 (depends on 4)

The method of claim 4, where selectively removing material from the dielectric passivation/tuning layer further comprises:
converting the measured electrical parameters of the plurality of resonators into a contour map indicating an amount of material to be removed from the dielectric passivation/tuning layer as a function of position on the piezoelectric plate; and
removing material in accordance with the contour map.

### Claim 6 (depends on 5)

The method of claim 5, wherein converting the measured electrical parameters of the plurality of resonators into a contour map further comprises:
using a trained neutral network to convert the measured electrical parameters of each of the plurality of resonators into a thickness of material to be removed from the dielectric passivation/tuning layer at the corresponding position on the piezoelectric plate.

### Claim 7 (depends on 6)

The method of claim 6, wherein the measured electrical parameters comprise an admittance of each resonator as a function of frequency over a frequency range from 0 to 1 GHz.

### Claim 8 (depends on 6)

The method of claim 6, wherein the measured electrical parameters comprise an admittance of each measured resonator as a function of frequency over a frequency range from 0 to 1 GHz.

### Claim 9 (depends on 1)

The method of claim 1, wherein removing excess material is performed with a scanning ion mill.

### Claim 10 (depends on 1)

The method of claim 1, wherein the piezoelectric plate and all of the plurality of IDTs are configured such that a respective radio frequency signal applied to each IDT excites a respective shear primary acoustic mode within the respective diaphragm.

### Claim 11 (depends on 10)

The method of claim 10, wherein the single-crystal piezoelectric plate is one of lithium niobate and lithium tantalate.

### Claim 12 (depends on 1)

The method of claim 1, wherein selectively removing material is performed with a scanning ion mill.

### Claim 13 (depends on 12)

The method of claim 12, wherein selectively removing material further comprises:
removing material from the dielectric passivation/tuning layer over shunt resonators through a first mask; and
removing material from the dielectric passivation/tuning layer over series resonators through a second mask.

### Claim 14

A method of fabricating filters on a single-crystal piezoelectric plate having opposing front and back surfaces, comprising:
bonding the back surface of the piezoelectric plate to a surface of a substrate;
forming a conductor pattern on the front surface, the conductor pattern including a plurality of ladder filter circuits, each ladder filter circuit including at least one shunt resonator and at least one series resonator, each of the shunt and series resonators including an interdigital transducer (IDT);
forming a dielectric passivation/tuning layer over the conductor pattern and the front surface of the piezoelectric plate;
performing electrical measurements of at least some of the resonators;
selectively removing material from the dielectric passivation/tuning layer in accordance with the electrical measurements; and
after removing material from the dielectric passivation/tuning layer, forming cavities in the substrate such that interleaved fingers of each IDT are disposed on a respective diaphragm spanning a respective cavity.

### Claim 15 (depends on 14)

The method of claim 14, wherein the front surface of the piezoelectric plate is attached to a sacrificial substrate during the bonding, the method further comprising:
removing the sacrificial substrate to expose the front surface of the piezoelectric plate after the bonding.

### Claim 16 (depends on 15)

The method of claim 15, further comprising:
polishing the front surface of the piezoelectric plate after removing the sacrificial substrate and before performing thickness measurements.

### Claim 17 (depends on 14)

The method of claim 14, wherein performing electrical measurements further comprises:
measuring one or more electrical parameters of resonators distributed over the area of the piezoelectric plate.

### Claim 18 (depends on 17)

The method of claim 17, where selectively removing material from the dielectric passivation/tuning layer further comprises:
converting the measured electrical parameters into a contour map indicating an amount of material to be removed from the dielectric passivation/tuning layer as a function of position on the piezoelectric plate; and
removing material in accordance with the contour map.

### Claim 19 (depends on 18)

The method of claim 18, wherein converting the measured electrical parameters of the into a contour map further comprises:
using a trained neutral network to convert the measured electrical parameters of each measured resonator into a thickness of material to be removed from the dielectric passivation/tuning layer at the corresponding position on the piezoelectric plate.

### Claim 20 (depends on 14)

The method of claim 14, wherein the piezoelectric plate and all of the IDTs are configured such that a respective radio frequency signal applied to each IDT excites a respective shear primary acoustic mode within the respective diaphragm.

### Claim 21 (depends on 14)

The method of claim 14, wherein the single-crystal piezoelectric plate is one of lithium niobate and lithium tantalate.