Film acoustically-coupled transformer with reverse C-axis piezoelectric material

• US 20050093657A1
• 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 6,987,433 B2
• Time in Patent Office: Days
• US Class Current: 333/191
• 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

An embodiment of the acoustically-coupled transformer has first and second stacked bulk acoustic resonators (SBARs) each having a stacked pair of film bulk acoustic resonators (FBARs) with an acoustic decoupler between the FBARs. Each FBAR has opposed planar electrodes with piezoelectric material between the electrodes. A first electrical circuit connects one FBARs of the first SBAR to one FBAR of the second SBAR, and a second electrical circuit connects the other FBAR of the first SBAR to the other FBAR of the second SBAR. The c-axis of the piezoelectric material of one of the FBARs is opposite in direction to the c-axes of the piezoelectric materials of the other three FBARs. This arrangement substantially reduces the amplitude of signal-frequency voltages across the acoustic decouplers and significantly improves the common mode rejection of the transformer. This arrangement also allows conductive acoustic decouplers to be used, increasing the available choice of acoustic decoupler materials.

Claims

1. An acoustically-coupled transformer, comprising:

a first stacked bulk acoustic resonator and a second stacked bulk acoustic resonator (SBAR), each SBAR 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 a c-axis, and

an acoustic decoupler between the FBARs;

a first electrical circuit connecting one of the FBARs of the first SBAR to one of the FBARs of the second SBAR; and

a second electrical circuit connecting the other of the FBARs of the first SBAR to the other of the FBARs of the second SBAR,

in which the c-axis of the piezoelectric material of one of the FBARs is opposite in direction to the c-axes of the piezoelectric material of the other three FBARs.

2. The transformer of claim 1, in which:

the first electrical circuit connects the one of the FBARs of the first SBAR in parallel with the one of the FBARs of the second SBAR; and

the second electrical circuit connects the other of the FBARs of the first SBAR in series with the other of the FBARs of the second SBAR.

3. The transformer of claim 2, in which:

the transformer additionally comprises first terminals and second terminals;

the first electrical circuit additionally connects the ones of the FBARs to the first terminals; and

the second electrical circuit additionally connects the others of the FBARs to the second terminals.

4. The transformer of claim 3, in which the first terminals constitute primary terminals and the second terminals constitute secondary terminals.

5. The transformer of claim 2, in which:

the transformer additionally comprises a substrate arranged support the SBARs;

in the one of the FBARs of each the SBARs, one of the electrodes thereof is a first electrode, the other of the electrodes thereof is a second electrode, and the first electrode is closer to the substrate than the second electrode; and

the first electrical circuit comprises;

an electrical connection between the first electrode of the first SBAR and the first electrode of the second SBAR, and

an electrical connection between the second electrode of the first SBAR and the second electrode of the second SBAR.

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

7. The transformer of claim 6, 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.

8. The transformer of claim 7, 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.

9. The transformer of claim 6, in which the acoustic decoupling material has an acoustic impedance in the range from about 2 Mrayl to about 16 Mrayl.

10. The transformer of claim 6, in which the acoustic decoupling material comprises plastic.

11. The transformer of claim 6, in which the acoustic decoupling material comprises polyimide.

12. The transformer of claim 6, in which the acoustic decoupling material comprises poly(para-xylylene).

13. The transformer of claim 6, in which the acoustic decoupling material is electrically conducting.

14. The transformer of claim 6, 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.

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

16. The transformer of claim 1, additionally comprising an electrical connection between ones of the electrodes located on opposite sides of the acoustic decouplers.

17. The transformer of claim 16, in which the acoustic decouplers provide the electrical connection.

18. 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), upper FBARs and acoustic decouplers between the lower FBARs and the upper FBARs, each of the FBARs comprising opposed planar electrodes and a layer of piezoelectric material therebetween, the piezoelectric material having a c-axis, the forming comprising setting the c-axis of the piezoelectric material of one of the FBARs opposite in direction to the c-axes of the piezoelectric material of the other three FBARs,

electrically connecting one of the FBARs of the first SBAR to one of the FBARs of the second SBAR; and

electrically connecting the other of the FBARs of the first SBAR to the other of the FBARs of the second SBAR.

19. The method of claim 18, in which:

the forming 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.

20. The method of claim 18, in which depositing the seed layer comprises:

depositing the seed layer on both electrodes; and

removing the seed layer from the other of the electrodes.

21. The method of claim 20, 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.

22. The method of claim 21, in which the first deposition conditions comprise an oxygen-rich environment and the second deposition conditions comprise a nitrogen-rich environment.

23. The method of claim 18, in which:

the forming 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.

24. The method of claim 23, 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.

25. The method of claim 24, in which one of the deposition conditions comprises an oxygen-rich environment and the other of the deposition conditions comprises a nitrogen-rich environment.

26. The method of claim 24, in which the removing is performed by etching.

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

28. The method of claim 18, in which:

the forming 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 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 a poling voltage of an opposite polarity between the other of the first electrodes and the other of the second electrodes;

29. The method of claim 28, 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.

30. The method of claim 29, additionally comprising singulating the wafer to separate the transformers from one another, the singulating comprising severing the interconnections between the electrodes.

31. The method of claim 18, additionally comprising electrically connecting ones of the electrodes located on opposite sides of the acoustic decouplers.

32. The method of claim 31, additionally comprising electrically connecting the ones of the electrodes using the acoustic decouplers.