# US 2013/0278356 A1 · Wide-band acoustically coupled thin-film baw filter

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- Number: US 2013/0278356 A1
- Document ID: US20130278356A1
- Status: Published application
- Kind: A1
- Application number: 13/879,028
- Filed: 2011-10-14
- Published: 2013-10-24
- Earliest claimed: 2010-10-14
- Assignees: Teknologian tutkimuskeskus VTT Oy
- Inventors: Johanna Meltaus; Tuomas Pensala
- Web page: https://patentgrep.com/patent/US20130278356A1

## CPC

- H03H 9/547: {Notch filters, e.g. notch BAW or thin film resonator filters}
- H03H 9/564: {implemented with thin-film techniques}
- H03H 9/568: {consisting of a ladder configuration}
- H03H: IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS (waveguides, resonators, lines or other devices of the waveguide type H01P)

## Abstract

The invention relates to an acoustically coupled thin-film BAW filter, comprising a piezoelectric layer, an input-port on the piezoelectric layer changing electrical signal into an acoustic wave (SAW, BAW), and an output-port on the piezoelectric layer changing acoustic signal into electrical signal. In accordance with the invention the ports include electrodes positioned close to each other, and the filter is designed to operate in first order thickness-extensional TE1 mode.

## Claims

### Claim 1

An acoustically coupled thin-film Bulk Acoustic Wave (BAW) filter, comprising;
a piezoelectric layer,
an input-port on the piezoelectric layer changing electrical signal into an acoustic wave, and
an output-port on the piezoelectric layer changing acoustic signal into electrical signal,
wherein
the ports include electrodes positioned such that acoustic coupling is achieved,
the filter is capable of operating in the first order thickness-extensional TE1 mode, and
wherein the mass loading by the top electrode (plateback) is such that the frequency difference between the k=0 frequency of the electrode region's TE1 mode and the outside region's TS2 mode is relatively small.

### Claim 2 (depends on 1)

A filter in accordance with claim 1, wherein the k=0 frequency of the outside region's TS2 mode is between 93% and 99.9 of the electrode region's TE1 cutoff frequency.

### Claim 3 (depends on 2)

A filter in accordance with claim 2, wherein the k=0 frequency of the outside region's TS2 mode is between 98% and 99.9% of the electrode region's TE1 cutoff frequency.

### Claim 4 (depends on 1)

A filter in accordance with claim 1, wherein the electrodes are positioned such that acoustic vibration in the lateral direction from one electrode to the other acoustically couples the electrodes.

### Claim 5 (depends on 1)

A filter in accordance with claim 1, wherein the filter structure that has interdigital electrode structure with two ports, such that the electrodes are connected alternatingly to the input port and the output port.

### Claim 6 (depends on 1)

A filter in accordance with claim 1, wherein the electrode topology is such that gap width G ensures good coupling at the even mode.

### Claim 7 (depends on 6)

A filter in accordance with claim 6, wherein the gap width is between 20% and 120% of the evanescent acoustic wave's decay length in the gap at the desired even resonance mode, where the wave's decay length is expressed as the length at which amplitude A=A0*1/e of the original amplitude A0.

### Claim 8 (depends on 1)

A filter in accordance with claim 1, wherein the electrode width W is such that more than one half-wavelength of the lateral acoustic wave's wavelength cannot fit within the electrode width.

### Claim 9 (depends on 1)

A filter in accordance with claim 1, wherein the electrode width W is smaller than the lateral acoustic wave's wavelength λodd at the desired odd resonance mode.

### Claim 10 (depends on 1)

A filter in accordance with claim 1, wherein the number of electrodes N, electrode width W and gap width G are designed such that the desired wavelength of the lateral acoustic wave at the even mode resonance frequency is achieved.

### Claim 11 (depends on 10)

A filter in accordance with claim 10, wherein N*W+N*G=λeven/2, where λeven is the wavelength of the lateral acoustic wave at the even mode resonance frequency, and that the highest-order mode trapped in the structure is the desired odd mode resonance.

### Claim 12 (depends on 1)

A filter in accordance with claim 1, wherein the electrode width W is such that the wavelength of the lateral acoustic wave at the desired odd mode resonance frequency, λodd, is obtained.

### Claim 13 (depends on 12)

A filter in accordance with claim 12, wherein W is between 25% and 50% of λodd.

### Claim 14 (depends on 1)

A filter in accordance with claim 1, wherein matching to the system impedance level is achieved, while retaining a desired loss level within the passband by the combination of N, W, and electrode length L.

### Claim 15 (depends on 1)

A filter in accordance with claim 1, wherein the piezoelectric layer is formed on an acoustic Bragg structure formed as a thin film stack or on an air-gap structure.

### Claim 16 (depends on 1)

A filter in accordance with claim 1, wherein by there being one or more resonators added to the filter in parallel and/or series.

### Claim 17 (depends on 13)

A filter in accordance with claim 13, wherein one or more of the resonators are coupled before and/or after the filter.

### Claim 18

(canceled)