Film acoustically-coupled transformers with two reverse c-axis piezoelectric elements

• US 20050093396A1
• Filed: 04/29/2004
• 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, Oshmyansky, Yury
• Granted Patent: US 7,091,649 B2
• Time in Patent Office: Days
• US Class Current: 310/320
• CPC Class Codes: H01L 2224/48091 Arched
• CPC Class Codes: H01L 2224/49175 Parallel arrangements
• CPC Class Codes: H01L 2924/00014 the subject-matter covered by the group, the symbol of which is combined with the symbol of this group, being disclosed without further technical details
• 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

Abstract

Embodiments of an acoustically-coupled transformer have a first stacked bulk acoustic resonator (SBAR) and a second SBAR. Each of the SBARs has a lower film bulk acoustic resonator (FBAR) and an upper FBAR, and an acoustic decoupler between the FBARs. The upper FBAR is stacked atop the lower FBAR. Each FBAR has opposed planar electrodes and a piezoelectric element between the electrodes. The piezoelectric element is characterized by a c-axis. The c-axes of the piezoelectric elements of the lower FBARs are opposite in direction, and the c-axes of the piezoelectric elements of the upper FBARs are opposite in direction. The transformer additionally has a first electrical circuit connecting the lower FBAR of the first SBAR to the lower FBAR of the second SBAR, and a second electrical circuit connecting the upper FBAR of the first SBAR to the upper FBARs of the second SBAR.

Claims

1. An acoustically-coupled transformer, comprising:

a first stacked bulk acoustic resonator (SBAR) and a second SBAR, each SBAR comprising;

a lower film bulk acoustic resonator (FBAR) and an upper FBAR, the upper FBAR stacked atop the lower FBAR, each FBAR comprising opposed planar electrodes and a piezoelectric element between the electrodes, the piezoelectric element characterized by a c-axis,

an acoustic decoupler between the FBARs, in which the c-axes of the piezoelectric elements of the lower FBARs are opposite in direction and the c-axes of the piezoelectric elements of the upper FBARs are opposite in direction;

a first electrical circuit connecting the lower FBAR of the first SBAR to the lower FBAR of the second SBAR; and

a second electrical circuit connecting the upper FBAR of the first SBAR to the upper FBARs of the second SBAR.

2. The transformer of claim 1, in which:

the transformer additionally comprises first terminals and second terminals;

the first electrical circuit additionally connects the lower FBARs to the first terminals; and

the second electrical circuit additionally connects the upper FBARs to the second terminals.

3. The transformer of claim 1, in which:

the first electrical circuit connects the lower FBARs in series; and

the second electrical circuit connects the upper FBARs in series.

4. The transformer of claim 1, in which

the first electrical circuit connects the lower FBARs in one of anti-series, parallel and anti-parallel;

and

the second electrical circuit connects the upper FBARs in one of anti-series, parallel and anti-parallel.

5. The transformer of claim 1, in which the acoustic decouplers each comprise a layer of acoustic decoupling material.

6. The transformer of claim 5, 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.

7. The transformer of claim 6, in which the acoustic decoupling material has an acoustic impedance intermediate between the acoustic impedance of the piezoelectric material and the acoustic impedance of air.

8. The transformer of claim 5, in which the acoustic decoupling material has an acoustic impedance in the range from about 1 Mrayl to about 16 Mrayl.

9. The transformer of claim 5, in which the acoustic decoupling material comprises plastic.

10. The transformer of claim 5, in which the acoustic decoupling material comprises polyimide.

11. The transformer of claim 5, in which the acoustic decoupling material comprises poly(para-xylylene).

12. The transformer of claim 5, in which:

the transformer has a pass band 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.

13. The transformer of claim 1, in which the acoustic decouplers each comprise a Bragg structure.

14. The transformer of claim 1, in which one of the electrical circuits comprises a center tap.

15. A method of making an acoustically-coupled transformer, the method comprising:

fabricating a first stacked bulk acoustic resonator (SBAR) and a second SBAR, the fabricating comprising;

forming lower film bulk acoustic resonators (FBARs) and upper FBARs, each of the FBARs comprising opposed planar electrodes and a piezoelectric element therebetween, the piezoelectric element characterized by a c-axis, the forming comprising setting the c-axes of the piezoelectric elements of the lower FBARs opposite in direction and setting the c-axes of the upper FBARs opposite in direction, and

locating an acoustic decoupler between the lower FBAR and the upper FBAR of each SBAR;

electrically connecting the lower FBAR of the first SBAR to the lower FBAR of the second SBAR; and

electrically connecting the upper FBAR of the first SBAR to the upper FBAR of the second SBAR.

16. The method of claim 15, in which:

the forming additionally comprises;

depositing and patterning a metal layer to define a pair of the electrodes, and

depositing a layer of piezoelectric material over the electrodes; and

the setting comprises depositing a seed layer of reverse c-axis piezoelectric material on one of the electrodes prior to depositing the layer of piezoelectric material.

17. The method of claim 16, in which depositing the seed layer comprises:

depositing the seed layer on the pair of electrodes; and

removing the seed layer from the other of the electrodes.

18. The method of claim 16, in which:

depositing the seed layer is performed under first deposition conditions that promote the formation of reverse c-axis material; and

depositing the layer of piezoelectric material is performed under second deposition conditions that promote the formation of normal c-axis material, the portion of the layer deposited on the seed layer being reverse c-axis material notwithstanding its deposition under the second deposition conditions.

19. The method of claim 18, in which the first deposition conditions comprise an oxygen-rich environment and the second deposition conditions comprise a nitrogen-rich environment.

20. The method of claim 15, in which:

the forming additionally comprises;

depositing and patterning a metal layer to define a pair of the electrodes; and

depositing a layer of piezoelectric material on the electrodes, the layer comprising a region of reverse c-axis material on one of the electrodes and a region of normal c-axis material on the other of the electrodes; and

the setting comprises depositing the regions using different deposition conditions.

21. The method of claim 20, in which depositing the layer of piezoelectric material comprises:

depositing a first layer under first deposition conditions to form one of (a) reverse c-axis piezoelectric material and (b) normal c-axis piezoelectric material;

removing a portion of the first layer to expose one of the electrodes; and

depositing a second layer under second deposition conditions to form the other of (a) reverse c-axis piezoelectric material and (b) normal c-axis piezoelectric material; and

removing a portion of the second layer to expose the first layer.

22. The method of claim 21, in which one of the deposition conditions comprises an oxygen-rich environment and the other of the deposition conditions comprises a nitrogen-rich environment.

23. The method of claim 21, in which the removing is performed by etching.

24. The method of claim 21, in which the removing is performed by a lift-off process.

25. The method of claim 15, in which:

the forming additionally comprises;

depositing a metal layer and patterning same to define a pair of first electrodes,

depositing a layer of ferroelectric material over the first electrodes, and

depositing an additional metal layer and patterning same to define a pair of second electrodes opposite the first electrodes; and

the setting comprises applying a poling voltage of a nominal polarity between one of the first electrodes and an opposed one of the second electrodes and applying a poling voltage of an opposite polarity between the other of the first electrodes and the other of the second electrodes;

26. The method of claim 25, in which:

the method additionally comprises;

providing a wafer, and

fabricating an array of the transformers on the wafer; and

the applying comprises electrically interconnecting corresponding ones of the electrodes of the transformers.

27. The method of claim 26, additionally comprising singulating the wafer to separate the transformers from one another, the singulating comprising severing the interconnections between the electrodes.