WIDE-BAND ACOUSTICALLY COUPLED THIN-FILM BAW FILTER

• US 20130057360A1
• Filed: 04/21/2011
• Published: 03/07/2013
• Est. Priority Date: 04/23/2010
• Status: Active Grant

• Current Assignee: Teknologian tutkimuskeskus VTT Oy
• Original Assignee: Teknologian tutkimuskeskus VTT Oy
• Inventors: Meltaus, Johanna, Pensala, Tuomas
• Granted Patent: US 9,294,069 B2
• Time in Patent Office: Days
• US Class Current: 333/187
• CPC Class Codes: H03H 2003/0414 Resonance frequency
• CPC Class Codes: H03H 9/0095 using bulk acoustic wave devices
• CPC Class Codes: H03H 9/02007 of bulk acoustic wave devices
• CPC Class Codes: H03H 9/02125 of parasitic elements
• CPC Class Codes: 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
• CPC Class Codes: H03H 9/175 Acoustic mirrors
• CPC Class Codes: H03H 9/177 of the energy-trap type
• CPC Class Codes: H03H 9/205 having multiple resonators crystal tuning forks H03H9/21
• CPC Class Codes: H03H 9/564 implemented with thin-film techniques
• CPC Class Codes: H03H 9/584 Coupled Resonator Filters [CFR]
• CPC Class Codes: Y10T 29/42 Piezoelectric device making

Abstract

In a bulk acoustic wave (BAW) filter based on laterally acoustically coupled resonators on piezoelectric thin films, one can utilize two distinct acoustic plate wave modes of different nature, for example the thickness extensional (longitudinal) TE1 and the second harmonic thickness shear (TS2) mode to form a bandpass response. The invention is based on the excitation of at least two lateral standing wave resonances belonging to different plate wave modes that facilitate the transmission of signal. The passband is designed by tailoring the wave propagation characteristics in the device such that the resonances are excited at suitable frequencies to form a passband of a desired shape. The bandwidth of the filter described herein may therefore be more than twice that of the existing state-of-the-art microacoustic filters. Consequently, it has significant commercial and technological value.

Claims

1. An acoustic filter comprising;

a stack of materials suitable for an acoustic filter, the stack comprising at least one piezoelectric layer,

at least two resonators having a width and a gap between them and comprising electrodes on different sides of the piezoelectric layer, where the electrodes on at least one side of the piezoelectric layer are separated to form at least two resonator regions,

wherein the stack of materials, width and gap between the separated electrodes being adapted to produce at least two distinct acoustic plate wave modes each contributing at least one lateral standing wave resonance in the acoustic filter.

2. The acoustic filter according to claim 1, wherein the electrodes on only one side of the piezoelectric layer are separated to form at least two resonator regions and there is provided a continuous electrode on the other side of the piezoelectric layer.

3. The acoustic filter according to claim 1, wherein the electrodes on both sides of the piezoelectric layer are separated to form at least two resonator regions.

4. The acoustic filter according to claim 3, wherein at least two of the electrodes are electrically connected to provide a balanced electrical port.

5. The acoustic filter according to claim 1, wherein the structure is adapted to trap at least two distinct plate wave modes having a total of at least three lateral standing wave resonances in the acoustic filter.

6. The acoustic filter according to claim 5, wherein at least two of said at least three lateral standing wave resonances arise for one plate wave mode and at least one of said at least three lateral standing wave resonances arise for another plate wave mode.

7. The acoustic filter according to claim 1, wherein the structure is adapted to trap at least two distinct plate wave modes having a total of at least four lateral standing wave resonances in the acoustic filter.

8. The acoustic filter according to claim 1, wherein the at least two distinct acoustic plate wave modes comprising at least one longitudinal mode and at least one shear mode.

9. The acoustic filter according to claim 1, comprising at least two piezoelectric layers in stacked configuration.

10. The acoustic filter according to claim 1, wherein the piezoelectric layer(s) is (are) selected from symmetry group 6 mm, with their c-axis in vertical direction.

11. The acoustic filter according to claim 1, wherein the piezoelectric layer(s) is (are) selected from symmetry group 6 mm, with their c-axis in tilted orientation.

12. The acoustic filter according to claim 11, wherein a electric vertical field is coupled to both longitudinal acoustic wave and shear wave.

13. The acoustic filter according to claim 1, wherein symmetry group and crystal orientation of the piezoelectric layer is selected so as to form an intrinsic coupling with vertical electric field in both longitudinal and shear acoustic thickness modes.

14. The acoustic filter according to claim 1, wherein the piezoelectric layer is formed from AlN or ZnO.

15. A method of manufacturing an acoustic wide-band filter comprising the steps of;

selecting a structure design and materials for the filter,

designating a desired center frequency and bandwidth,

whereby designing a stack using the selected structure design and materials along with a set of resonators having a width and gap between them so that coupling to at least two distinct laterally propagating thickness vibration plate wave modes is achieved having a total of at least two resonances, and

fabricating the designed filter stack so that it couples the at least two distinct laterally propagating thickness vibration plate wave modes having a total of at least two resonances.

16. A method of manufacturing in accordance with claim 15,

wherein designing the stack so that the designated center frequency is between the onset frequencies of the at least two distinct plate wave modes, and the frequency difference between the onset frequencies two distinct plate modes is approximately one third the desired bandwidth.

17. (canceled)