Stacked bulk acoustic resonator band-pass filter with controllable pass bandwidth

• US 20050093653A1
• Filed: 10/30/2003
• Published: 05/05/2005
• Est. Priority Date: 10/30/2003
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Avago Technologies Wireless IP Singapore Pte Limited (Broadcom, Inc.)
• Inventors: Larson, John D. III
• Granted Patent: US 7,019,605 B2
• Time in Patent Office: Days
• US Class Current: 333/187
• CPC Class Codes: H03H 9/132 characterized by a particular shape
• CPC Class Codes: H03H 9/584 Coupled Resonator Filters [CFR]
• CPC Class Codes: H03H 9/587 Air-gaps
• CPC Class Codes: H03H 9/605 consisting of a ladder configuration
• CPC Class Codes: Y10T 29/42 Piezoelectric device making
• CPC Class Codes: Y10T 29/49005 Acoustic transducer
• CPC Class Codes: Y10T 29/49117 Conductor or circuit manufacturing

Abstract

The band-pass filter has a stacked pair of film bulk acoustic resonators (FBARs) and an acoustic decoupler between the FBARs. Each of the FBARs has opposed planar electrodes and a layer of piezoelectric material between the electrodes. The acoustic decoupler controls the coupling of acoustic energy between the FBARs. Specifically, the acoustic decoupler couples less acoustic energy between the FBARs than would be coupled by direct contact between the FBARs. The reduced acoustic coupling gives the band-pass filter desirable in-band and out-of-band properties.

Claims

1. A band-pass filter, comprising:

a stacked pair of film bulk acoustic resonators (FBARs), each of the FBARs comprising opposed planar electrodes and a layer of piezoelectric material between the electrodes; and

an acoustic decoupler between the FBARs.

2. The band-pass filter of claim 1, in which the acoustic decoupler is structured to provide substantially critical coupling of acoustic energy between the FBARs.

3. The band-pass filter of claim 1, in which the acoustic decoupler comprises a layer of acoustic decoupling material.

4. The band-pass filter of claim 3, in which:

the piezoelectric material has an acoustic impedance; and

the acoustic decoupling material has an acoustic impedance less than the acoustic impedance of the piezoelectric material.

5. The band-pass filter of claim 3, in which:

the piezoelectric material has an acoustic impedance; and

the acoustic decoupling material has an acoustic impedance intermediate between the acoustic impedance of the piezoelectric material and the acoustic impedance of air.

6. The band-pass filter of claim 3, in which the acoustic decoupling material has an acoustic impedance in the range from about 2 Mrayl to about 16 Mrayl.

7. The band-pass filter of claim 3, in which the acoustic decoupling material comprises plastic.

8. The band-pass filter of claim 3, in which the acoustic decoupling material comprises polyimide.

9. The band-pass filter of claim 3, in which the acoustic decoupling material comprises poly(para-xylylene).

10. The band-pass filter of claim 3, in which:

the band-pass filter is characterized by a center frequency; and

the layer of acoustic decoupling material has a nominal thickness equal to an odd integral multiple of one quarter of the wavelength in the acoustic decoupling material of an acoustic wave having a frequency equal to the center frequency.

11. The band-pass filter of claim 10, in which the acoustic decoupling material comprises plastic.

12. The band-pass filter of claim 10, in which the acoustic decoupling material comprises polyimide.

13. The band-pass filter of claim 10, in which the acoustic decoupling material comprises poly(para-xylylene).

14. The band-pass filter of claim 3, in which:

the band-pass filter is characterized by a center frequency; and

the layer of acoustic decoupling material has a nominal thickness equal to one quarter of the wavelength in the acoustic decoupling material of an acoustic wave having a frequency equal to the center frequency.

15. The band-pass filter of claim 1, in which the acoustic decoupler comprises a Bragg structure.

16. The band-pass filter of claim 15, in which the Bragg structure comprises one or more low acoustic impedance Bragg elements interleaved with high acoustic impedance Bragg elements.

17. The band-pass filter of claim 16, in which two of the high acoustic impedance Bragg elements additionally serve as one of the electrodes of each of the FBARs.

18. The band-pass filter of claim 16, in which:

the band-pass filter is characterized by a center frequency; and

each of the Bragg elements comprises a layer having a nominal thickness equal to an odd integral multiple of one quarter of the wavelength in the respective material of an acoustic wave having a frequency equal to the center frequency.

19. The band-pass filter of claim 1, additionally comprising an electrical connection between adjacent ones of the electrodes of the FBARs.

20. The band-pass filter of claim 19, in which the acoustic decoupler is located between the adjacent ones of the electrodes.

21. The band-pass filter of claim 1, additionally comprising a ladder filter electrically connected in series with the stacked pair of FBARs.

22. The band-pass filter of claim 21, in which the ladder filter comprises additional FBARs.

23. The band-pass filter of claim 21, in which:

the band-pass filter additionally comprises an electrical connection between adjacent ones of the electrodes of the stacked pair of FBARs and the ladder filter; and

the remaining ones of the electrodes of the stacked pair of FBARs provide the output terminals of the band-pass filter.

24. A band-pass filter characterized by a center frequency, the band-pass filter comprising:

a stacked pair of film bulk acoustic resonators (FBARs), each of the FBARs comprising opposed planar electrodes and a layer of piezoelectric material between the electrodes, the piezoelectric material having an acoustic impedance; and

between the FBARs, a layer of acoustic decoupling material having a nominal thickness equal to an odd integral multiple of one quarter of the wavelength in the acoustic decoupling material of an acoustic wave having a frequency equal to the center frequency, the acoustic decoupling material having an acoustic impedance less than the acoustic impedance of the piezoelectric material.

25. The band-pass filter of claim 24, in which the acoustic decoupling material comprises one of polyimide and poly(para-xylylene).

26. An electrical filtering method, comprising:

providing a pair of film bulk acoustic resonators (FBARs);

applying an input electrical signal to one of the FBARs;

coupling less acoustic energy between the FBARs than would be coupled by direct contact between the FBARs; and

outputting a filtered output electrical signal from the other of the FBARs.

27. The filtering method of claim 26, in which:

the coupling establishes a first pass bandwidth; and

the method additionally comprises, prior to the applying, filtering the input electrical signal with a second pass bandwidth narrower than the first pass bandwidth.

28. A method of fabricating an acoustically-coupled device, the method comprising:

fabricating a first film bulk acoustic resonator (FBAR);

fabricating an acoustic decoupler on the first FBAR;

fabricating a second FBAR on the acoustic decoupler, including subjecting the acoustic decoupler to a maximum temperature; and

prior to fabricating the second FBAR, baking the first FBAR and the acoustic decoupler at a temperature not lower than the maximum temperature.

29. The method of claim 28, in which the acoustic decoupler comprises a layer of polyimide.

30. The method of claim 29, in which fabricating the acoustic decoupler comprises depositing the layer of polyimide by spin coating.