# Patent

## Title
Impedance transforming bulk acoustic wave baluns

## Abstract

A bulk acoustic wave device includes an acoustic decoupler between first and second film bulk acoustic resonators (FBARs). The first FBAR is resonant at a resonant frequency of the device and includes first and second planar electrodes abutting opposite sides of a first resonator volume free of any intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to a propagation axis normal to the first and second electrodes. The first FBAR has a first electrical impedance parallel to the propagation axis. The second FBAR is resonant at the resonant frequency and includes third and fourth planar electrodes abutting opposite sides of a second resonator volume free of any intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis. The second FBAR has a second electrical impedance parallel to the propagation axis and different from the first electrical impedance.

## Background
BACKGROUND
FIG. 1 shows a block diagram of a balun 10, which is a circuit that performs a signal conversion between a single-ended (unbalanced) port 12 and two differential (balanced) ports 14, 16. In some implementations, the single-ended port 12 is a signal input and the two differential ports 14 16 are signal outputs. In other implementations, the single-ended port 12 is a signal output and the two differential ports 14, 16 are signal inputs. The single-ended or unbalanced port of a balun is a signal port that receives a signal over a cable (e.g., a coaxial cable) that uses a single conductor to transmit the signal, which typically is referenced to the device ground potential. The differential or balanced ports of a balun are signal ports that receive a signal over a cable (e.g., a twisted-pair cable) that has two identical conductors for carrying voltages that are opposite in polarity but equal in magnitude with respect to ground. The balun 10 typically is used to connect components with unbalanced inputs or outputs (e.g., an antenna) to signal lines and components with balanced inputs or outputs (e.g., a differential amplifier), which typically are characterized by superior immunity to electromagnetic interference, power supply noise and ground noise relative to unbalanced components. The balun 10 also typically passively transforms the impedance between the single-ended port 12 and the two differential ports 14, 16.
Baluns commonly are implemented by transformers that have a first winding connected to the single-ended port and a second winding connected to the two differential ports. Recently, baluns have been implemented by bulk acoustic wave (BAW) resonators.
Known BAW resonators include one or more piezoelectric layers disposed between two electrodes. For example, thin film bulk acoustic wave resonators (FBARs) typically include a single piezoelectric layer between two electrodes. Stacked thin film bulk acoustic wave resonators (SBARs), on the other hand, typically include two or more piezoelectric layers disposed between top and bottom electrode layers and separated from each other by one or more intervening electrodes. Some acoustic resonator devices include multiple acoustic resonators that are isolated from one another by respective decouplers, which are formed by one or more dielectric layers that provide only weak acoustic coupling between the acoustic resonators. A BAW resonator has a resonant frequency that is determined by the thickness of the piezoelectric layer and by the thicknesses and the materials used for the other layers. A BAW resonator typically is acoustically isolated from the supporting substrate by an acoustic isolator, which may include a cavity formed under a membrane supporting a BAW resonator or an acoustic mirror that includes of a stack of layers alternately formed of high and low acoustic impedance materials and having respective thicknesses of approximately one-quarter of the wavelength corresponding to the target resonant frequency of the device. A BAW resonator that is disposed on an acoustic mirror often is referred to as a solidly mounted resonator (SMR).
FIG. 2 shows an example of a thin film bulk acoustic wave resonator (FBAR) 18 that includes a single piezoelectric layer 20 between two electrodes 22, 24. The FBAR 18 is acoustically isolated from the underlying substrate by an acoustic isolator 26. In operation, a time-varying electrical signal at the resonant frequency of the FBAR 18 is applied across the electrodes 22, 24. The applied electrical signal induces acoustic vibrations parallel to a propagation axis 28, which is normal to the electrodes 22, 24.
FIG. 3 shows an example of a prior art balun 30 that is implemented using FBARs. The balun 30 includes a dielectric layer 32 separating a first FBAR 34 and a second FBAR 36. The first FBAR 34 consists of the overlapping regions of a piezoelectric layer 38, a top electrode 40, and a bottom electrode 42. The second FBAR 36 consists of the overlapping regions of a piezoelectric layer 44, a top electrode 46, and a bottom electrode 48. The active area of the balun 30 corresponds to the overlapping regions of the first and second FBARs 34, 36 (shown by the dashed box). In this example, the bottom electrode 42 of the first FBAR 34 is connected to the single-ended port 12, the top electrode 40 of the first FBAR 34 is connected to the device ground, the bottom electrode 48 of the second FBAR 36 is connected to the first differential port 14, and the top electrode 46 of the second FBAR 36 is connected to the second differential port 16. In this design, the material compositions and thicknesses of the constituent layers of the first and second FBARs 34, 36 are the same. Consequently, the balun 30 produces a 1:1 impedance transformation between the single-ended port 12 and the differential ports 14, 16.
FIG. 4 shows another example of a prior art balun 50 that includes a first SBAR 52 and a second SBAR 54. The first SBAR 52 is formed from a dielectric layer 55 between a first FBAR 56 and a second FBAR 58. The second SBAR 54 is formed from a dielectric layer 59 between a third FBAR 60 and a fourth FBAR 62. The bottom electrode of the first FBAR 56 is connected to the single-ended port 12 and to the top electrode of the third FBAR 60. The top electrode of the first FBAR 56, the bottom electrode of the second FBAR 58, the bottom electrode of the third FBAR 60, and the bottom electrode of the fourth FBAR 62 all are connected to the device ground. The top electrode of the second FBAR 58 is connected to the first differential port 14 and the top electrode of the fourth FBAR 62 is connected to the second differential port 16. In this design, the first, second, third, and fourth FBARs 56, 58, 60, 62 are the same. Consequently, the balun 50 produces a 1:4 impedance transformation from the single-ended port 12 to the differential ports 14, 16.
Although baluns that provide 1:1 and 1:4 impedance transformations have a wide variety of useful applications, some applications would benefit from baluns that may be tailored to provide customized impedance transformations that are different from 1:1 and 1:4. What are needed are impedance transforming BAW baluns that are capable of flexibly providing a wide variety of different impedance transformations between the balanced ports and the unbalanced ports without increasing losses within the devices.
SUMMARY
In one aspect, the invention features a bulk acoustic wave (BAW) device that includes a stacked bulk acoustic wave resonator (SBAR) that includes an acoustic decoupler between first and second film bulk acoustic resonators (FBARs). The first FBAR is resonant at the resonant frequency and includes first and second planar electrodes abutting opposite sides of a first resonator volume free of any intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to a propagation axis normal to the first and second electrodes. The first FBAR has a first electrical impedance parallel to the propagation axis. The second FBAR is resonant at the resonant frequency and includes third and fourth planar electrodes abutting opposite sides of a second resonator volume free of any intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis. The second FBAR has a second electrical impedance parallel to the propagation axis and different from the first electrical impedance.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.



DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram of a prior art balun.
FIG. 2 is a diagrammatic sectional view of a prior art FBAR.
FIG. 3 is a diagrammatic sectional view of a prior art 1:1 impedance transforming balun that is formed from a pair of FBARs.
FIG. 4 is a diagrammatic sectional view of a prior art 1:4 impedance transforming balun that is formed from a pair of electrically interconnected SBARs.
FIG. 5 is a diagrammatic sectional view of a balun that is formed from a pair of FBARs in accordance with an embodiment of the invention.
FIG. 6 is a diagrammatic sectional view of a balun that is formed from a pair of electrically interconnected SBARs in accordance with an embodiment of the invention.
FIG. 7 is a diagrammatic sectional view of a balun that is formed from a pair of electrically interconnected SBARs in accordance with an embodiment of the invention.
FIG. 8 is a diagrammatic sectional view of a balun that is formed from a stack of three FBARs in accordance with an embodiment of the invention.



DETAILED DESCRIPTION
In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.
The impedance transforming BAW baluns that are described in detail below are capable of flexibly providing a wide variety of different impedance transformations between the balanced ports and the unbalanced ports without increasing losses within the devices.
FIG. 5 shows a first impedance transforming balun 70 in accordance with an embodiment of the invention. The balun 70 includes and SBAR formed from an acoustic decoupler 72 that is disposed between a first FBAR 74 and a second FBAR 76 on a substrate 78. The balun 70 is acoustically isolated from the substrate 78 by an acoustic isolator 80, which may include a cavity formed under a membrane supporting the balun 70 or an acoustic mirror that includes of a stack of layers alternately formed of high and low acoustic impedance materials and having respective thicknesses of approximately one-quarter of the wavelength corresponding to the target resonant frequency of the balun 70. The balun 70 may be used in a wide variety of application areas, including communication lines, devices, and circuits (e.g., frequency conversion mixers, radars, transmitters, satellites, telephony network devices and circuits, and wireless network modems and routers).
The acoustic decoupler 72 electrically isolates the first FBAR 74 from the second FBAR 76 and provides only weak acoustic coupling between the first FBAR 74 and the second FBAR 76. In some implementations, the acoustic decoupler 72 is formed of a single layer of dielectric material (e.g., a SiLK dielectric polymer, available from The Dow Chemical Company of Midland, Mich., U.S.A.). In other implementations, the acoustic decoupler 72 is formed of a stack of layers alternately formed of high and low acoustic impedance materials and having respective thicknesses of approximately one-quarter of the wavelength corresponding to the target resonant frequency of the balun 70.
The first FBAR 74 is defined by the overlapping regions of a first planar electrode 82 and a second planar electrode 84 that abut opposite sides of a first resonator volume 86. The second FBAR 76 is defined by the overlapping regions of a third planar electrode 88 and a fourth planar electrode 90 that abut opposite sides of a second resonator volume 92.
The first, second, third, and fourth electrodes 82, 84, 88, 90 typically are formed of one or more layers of Mo, Al, Au, Pt, Ti, and alloys thereof. Each of the first and second resonator volumes 86, 92 is free of any intervening electrodes and contains piezoelectric material disposed for acoustic vibrations parallel to a propagation axis 94 that is normal to the planar electrodes 82, 84, 88, 90. The piezoelectric material in each of the first and second resonator volumes 86, 92 typically is in the form of one or more layers of piezoelectric material, such as a wurtzite-type hexagonal crystal (e.g., cadmium sulfide, cadmium selenide, zinc oxide, beryllium oxide, aluminum nitride, and wurtzite zinc sulfide, and solid solutions thereof) and lead-zirconate-titanate (PZT).
The active area of the balun 70 corresponds to the overlapping regions of the first and second FBARs 74, 76 (shown by the dashed box). The first electrode 82 of the first FBAR 74 is connected to the single-ended port 12, the second electrode 84 of the first FBAR 74 is connected to the device ground, the third electrode 88 of the second FBAR 76 is connected to the first differential port 14, and the fourth electrode 90 of the second FBAR 76 is connected to the second differential port 16. As used herein, the term “device ground” refers to a common return path in a device that is the zero voltage reference level for the device.
In this embodiment, the first FBAR 74 has a first electrical impedance parallel to the propagation axis 94 and the second FBAR 76 has a second electrical impedance parallel to the propagation axis 94 that is different from the first electrical impedance. As used herein, the term “electric impedance” refers to a measure of the impediment to the flow of alternating current caused by a combination of resistance and reactance and typically is measured in ohms at a given frequency. The different electrical impedances of the first and second FBARs 74, 76 are set to achieve a wide variety of different impedance transformations between the unbalanced port 12 and the unbalanced ports 14, 16. In general, the impedance transformation is given by Z1:Z2, where Z1 is the electrical impedance of the first FBAR 74 and Z2 is the electrical impedance of the second FBAR 76.
In some embodiments, the different electrical impedances of the first and second FBARs are set by forming one or more of the corresponding constituent layers of the first and second FBARs 74, 76 with different material compositions.
In the illustrated embodiment, the material compositions of the electrodes 82, 84, 88, 90 are substantially the same and the compositions of the piezoelectric materials in the first and second resonator volumes 86, 92 are substantially the same. The different electrical impedances of the first and second FBARs 74, 76 are set by forming the first and second resonator volumes 86, 92 with different thicknesses t1,P, t2,P. For example, in the exemplary embodiment shown in FIG. 5, the thickness t1,P of the first resonator volume 86 is smaller than the thickness t2, P of the second resonator volume 92. In this way, the electrical impedance of the second FBAR 76, which is proportional to the thickness of the piezoelectric material in the second resonator volume 92, is larger than the electrical impedance of the first FBAR 74, which is proportional to the thickness of the piezoelectric material in the first resonator volume 86. In order to maintain the same resonant frequencies in the first and second FBARs 74, 76, the overall acoustic thicknesses t1,R, t2,R of the first and second FBARs 74, 76 are made substantially equal to one-half of the wavelength corresponding to the resonant frequency of the balun 70. In the illustrated embodiment, this is achieved by making the acoustic thicknesses of the first and second electrodes 82, 84 larger than the acoustic thickness of the third and fourth electrodes 88, 90 by an amount that compensates for the smaller acoustic thickness of the first resonator volume 86 relative to the acoustic thickness of the second resonator volume 92.
In the illustrated embodiment, the overlapping areas of the first and second FBARs 74, 76 normal to the propagation axis 94 are substantially equal. In this way, acoustic signals may be transmitted efficiently between the first and second FBARs 74, 76 without increased loss of acoustic energy within the balun 70.
FIG. 6 shows a second impedance transforming balun 100 in accordance with an embodiment of the invention. The balun 100 includes first and second SBARs 102, 104 formed over respective acoustic isolators 106, 108 in a substrate 110. The acoustic isolators 106, 108 may include a cavity formed under a membrane supporting the balun 100 or an acoustic mirror that includes of a stack of layers alternately formed of high and low acoustic impedance materials and having respective thicknesses of approximately one-quarter of the wavelength corresponding to the target resonant frequency of the balun 100. The balun 100 may be used in a wide variety of application areas, including communication lines, devices, and circuits (e.g., frequency conversion mixers, radars, transmitters, satellites, telephony network devices and circuits, and wireless network modems and routers).
The first SBAR 102 is formed from an acoustic decoupler 112 that is disposed between a first FBAR 114 and a second FBAR 116. The acoustic decoupler 112 is formed the same way as the decoupler 72, which is described above and shown in FIG. 5. The first FBAR 114 is defined by the overlapping regions of a first planar electrode 118 and a second planar electrode 120 that abut opposite sides of a first resonator volume 124. The second FBAR 116 is defined by the overlapping regions of a third planar electrode 126 and a fourth planar electrode 128 that abut opposite sides of a second resonator volume 130.
The second SBAR 104 is formed from an acoustic decoupler 132 that is disposed between a third FBAR 134 and a fourth FBAR 136. The acoustic decoupler 132 is formed the same way as the decoupler 72, which is described above and shown in FIG. 5. The third FBAR 134 is defined by the overlapping regions of a fifth planar electrode 138 and a sixth planar electrode 142 that abut opposite sides of a third resonator volume 144. The fourth FBAR 136 is defined by the overlapping regions of a seventh planar electrode 146 and an eighth planar electrode 148 that abut opposite sides of a fourth resonator volume 150.
The electrodes of the first, second, third, and fourth FBARs 114, 116, 134, 136 typically are formed of one or more layers of Mo, Al, Au, Pt, Ti, and alloys thereof. Each of the first, second, third, and fourth resonator volumes 124, 130, 144, 150 is free of any intervening electrodes and contains piezoelectric material disposed for acoustic vibrations parallel to a propagation axis 152 that is normal to the planar electrodes 118, 120, 126, 128, 138, 142, 146, 148. The piezoelectric material in each of the first, second, third, and fourth resonator volumes 124, 130, 144, 150 typically is in the form of one or more layers of piezoelectric material, such as a wurtzite-type hexagonal crystal (e.g., cadmium sulfide, cadmium selenide, zinc oxide, beryllium oxide, aluminum nitride, and wurtzite zinc sulfide, and solid solutions thereof) and lead-zirconate-titanate (PZT).
The active area of the first SBAR 102 corresponds to the overlapping regions of the first and second FBARs 114, 116 (shown by the left dashed box) and the active area of the second SBAR 104 corresponds to the overlapping regions of the third and fourth FBARs 134, 136 (shown by the right dashed box). The second electrode 120 of the first FBAR 114 is connected to the single-ended port 12. The first electrode 118 of the first FBAR 114 is connected to the fifth electrode 138 of the third FBAR 134. The third electrode 126 of the second FBAR 116, the seventh electrode 146 of the fourth FBAR 136, and the sixth electrode 142 of the third FBAR 134 are connected to the device ground. The fourth electrode 128 of the second FBAR 116 is connected to the first differential port 14, and the eighth electrode 148 of the fourth FBAR 136 is connected to the second differential port 16.
In this embodiment, the first, second, third, and fourth FBARs 114, 116, 134, 136 respectively have first, second, third, and fourth electrical impedances Z1, Z2, Z3, Z4 parallel to the propagation axis 152. In general, at least one of the first, second, third, and fourth electrical impedances is different from the others. The differences in the electrical impedances of the first, second, third and fourth FBARs 114, 116, 134, 136 are set to achieve a wide variety of different impedance transformations between the unbalanced port 12 and the unbalanced ports 14, 16.
In the illustrated embodiment, the first and third electrical impedances (i.e., the electrical impedances of the first and third FBARs 114, 134) are substantially equal (Z1=Z3=Z13), the second and fourth electrical impedances (i.e., the electrical impedances of the second and fourth FBARs 116, 136) are substantially equal (Z2=Z4=Z24), and the first and third electrical impedances are different from the second and fourth electrical impedances (i.e., Z13≠Z24). In general, the impedance transformation is given by Z13:Z24, where Z13 is the electrical impedance of the first and third FBARs 114, 134 and Z24 is the electrical impedance of the second and fourth FBARs 116, 136.
In some embodiments, the different electrical impedances of the FBARs 114, 116, 134, 136 are set by forming one or more of the corresponding constituent layers of the FBARs 114, 116, 134, 136 with different material compositions.
In the illustrated embodiment, the material compositions of the electrodes of the FBARs 114, 116, 134, 136 are substantially the same and the compositions of the piezoelectric material in the resonator volumes of the FBARs 114, 116, 134, 136 are substantially the same. The different electrical impedances of the FBARs 114, 116, 134, 136 are set by forming the first and third resonator volumes 124, 144 with thicknesses t3,P, t5,P that are different than the thicknesses t4,P, t6,P of the second and fourth resonator volumes 130, 150. For example, in the exemplary embodiment shown in FIG. 6, the thickness t3,P=t5,P of the first and third resonator volumes 124, 144 is smaller than the thickness t4,P=t6,P of the second and fourth resonator volumes 130, 150. In this way, the electrical impedance of the second and fourth FBARs 116, 136, which is proportional to the thickness of the piezoelectric material in the second and fourth resonator volumes 130, 150, is larger than the electrical impedance of the first and third FBARs 114, 134, which is proportional to the thickness of the piezoelectric material in the first and third resonator volumes 116, 136. In order to maintain the same resonant frequencies in the first, second, third and fourth FBARs 114, 116, 134, 136, the overall acoustic thicknesses t3,R, t4,R, t5,R, t6,R of the first, second, third and fourth FBARs 114, 116, 134, 136 are made substantially equal to one-half of the wavelength corresponding to the resonant frequency of the balun 100. In the illustrated embodiment, this is achieved by making the acoustic thicknesses of the first, second, fifth, and sixth electrodes 118, 120, 138, 142 larger than the acoustic thicknesses of the third, fourth, seventh, and eighth electrodes 126, 128, 146, 148 by amount that compensates for the smaller acoustic thicknesses of the first and third resonator volumes 124, 144 relative to the acoustic thicknesses of the second and fourth resonator volumes 130, 150.
In the illustrated embodiment, the overlapping areas of the first and second FBARs 114, 116 normal to the propagation axis 152 are substantially equal and the overlapping areas of the third and fourth FBARs 134, 136 normal to the propagation axis 152 are substantially equal. In this way, acoustic signals may be transmitted efficiently between the first and second FBARs 114, 116 and between the third and fourth FBARs 134, 136 without increased loss of acoustic energy within the balun 100. In the illustrated embodiment, the overlapping areas of the first and second FBARs 114, 116 also are substantially equal to the overlapping areas of the third and fourth FBARs 134, 136 in order to maintain the symmetry between the first and second differential ports 14, 16.
FIG. 7 shows a second impedance transforming balun 200 in accordance with an embodiment of the invention. The balun 200 includes first and second SBARs 202, 204 formed over respective acoustic isolators 206, 208 in a substrate 210. The acoustic isolators 206, 208 may include a cavity formed under a membrane supporting the balun 200 or an acoustic mirror that includes of a stack of layers alternately formed of high and low acoustic impedance materials and having respective thicknesses of approximately one-quarter of the wavelength corresponding to the target resonant frequency of the balun 200. The balun 200 may be used in a wide variety of application areas, including communication lines, devices, and circuits (e.g., frequency conversion mixers, radars, transmitters, satellites, telephony network devices and circuits, and wireless network modems and routers).
The first SBAR 202 is formed from an acoustic decoupler 212 that is disposed between a first FBAR 214 and a second FBAR 216. The acoustic decoupler 212 is formed the same way as the decoupler 72, which is described above and shown in FIG. 5. The first FBAR 214 is defined by the overlapping regions of a first planar electrode 218 and a second planar electrode 220 that abut opposite sides of a first resonator volume 224. The second FBAR 216 is defined by the overlapping regions of a third planar electrode 226 and a fourth planar electrode 228 that abut opposite sides of a second resonator volume 230.
The second SBAR 204 is formed from an acoustic decoupler 232 that is disposed between a third FBAR 234 and a fourth FBAR 236. The acoustic decoupler 232 is formed the same way as the decoupler 72, which is described above and shown in FIG. 5. The third FBAR 234 is defined by the overlapping regions of a fifth planar electrode 238 and a sixth planar electrode 242 that abut opposite sides of a third resonator volume 244. The fourth FBAR 236 is defined by the overlapping regions of a seventh planar electrode 246 and an eighth planar electrode 248 that abut opposite sides of a fourth resonator volume 250.
The electrodes of the first, second, third, and fourth FBARs 214, 216, 234, 236 typically are formed of one or more layers of Mo, Al, Au, Pt, Ti, and alloys thereof. Each of the first, second, third, and fourth resonator volumes 224, 230, 244, 250 is free of any intervening electrodes and contains piezoelectric material disposed for acoustic vibrations parallel to a propagation axis 252 that is normal to the planar electrodes 218, 220, 226, 228, 238, 242, 246, 248. The piezoelectric material in each of the first, second, third, and fourth resonator volumes 224, 230, 244, 250 typically is in the form of one or more layers of piezoelectric material, such as a wurtzite-type hexagonal crystal (e.g., cadmium sulfide, cadmium selenide, zinc oxide, beryllium oxide, aluminum nitride, and wurtzite zinc sulfide, and solid solutions thereof) and lead-zirconate-titanate (PZT).
The active area of the first SBAR 202 corresponds to the overlapping regions of the first and second FBARs 214, 216 (shown by the left dashed box) and the active area of the second SBAR 204 corresponds to the overlapping regions of the third and fourth FBARs 234, 236 (shown by the right dashed box). The second electrode 220 of the first FBAR 214 and the fifth electrode 238 of the third FBAR 234 are connected to the single-ended port 12. The first electrode 218 of the first FBAR 24, the third electrode 226 of the second FBAR 216, the seventh electrode 246 of the fourth FBAR 236, and the sixth electrode 242 of the third FBAR 234 are connected to the device ground. The fourth electrode 228 of the second FBAR 216 is connected to the first differential port 14, and the eighth electrode 248 of the fourth FBAR 236 is connected to the second differential port 16.
In this embodiment, the first, second, third, and fourth FBARs 214, 216, 234, 236 respectively have first, second, third, and fourth electrical impedances Z′1, Z′2, Z′3, Z′4 parallel to the propagation axis 252. In general, at least one of the first, second, third, and fourth electrical impedances is different from the others. The differences in the electrical impedances of the first, second, third and fourth FBARs 214, 216, 234, 236 are set to achieve a wide variety of different impedance transformations between the unbalanced port 12 and the unbalanced ports 14, 16.
In the illustrated embodiment, the first and third electrical impedances (i.e., the electrical impedances of the first and third FBARs 214, 234) are substantially equal (Z′1=Z′3=Z′13), the second and fourth electrical impedances (i.e., the electrical impedances of the second and fourth FBARs 216, 236) are substantially equal (Z′2=Z′4=Z′24), and the first and third electrical impedances are different from the second and fourth electrical impedances (i.e., Z′13≠Z′24). In general, the impedance transformation is given by




  
    
      
        
          4
          ·
          
            
              Z
              24
              ′
            
            
              Z
              13
              ′
            
          
        
        ⁢
        
          :
        
        ⁢
        1
      
    
    
      
        (
        1
        )
      
    
  




where Z′13 is the electrical impedance of the first and third FBARs 214, 234 and Z′24 is the electrical impedance of the second and fourth FBARs 216, 236.

In some embodiments, the different electrical impedances of the FBARs 214, 216, 234, 236 are set by forming one or more of the corresponding constituent layers of the FBARs 214, 216, 234, 236 with different material compositions.
In the illustrated embodiment, the material compositions of the electrodes of the FBARs 214, 216, 234, 236 are substantially the same and the compositions of the piezoelectric material in the resonator volumes of the FBARs 214, 216, 234, 236 are substantially the same. The different electrical impedances of the FBARs 214, 216, 234, 236 are set by forming the first and third resonator volumes 224, 244 with thicknesses t7,P, t9,P that are different than the thicknesses t8,P, t10,P of the second and fourth resonator volumes 230, 250. For example, in the exemplary embodiment shown in FIG. 7 the thickness t7,P=t9,P of the first and third resonator volumes 224, 244 is smaller than the thickness t8,P=t10,P of the second and fourth resonator volumes 230, 250. In this way, the electrical impedance of the second and fourth FBARs 216, 236, which is proportional to the thickness of the piezoelectric material in the second and fourth resonator volumes 230, 250, is larger than the electrical impedance of the first and third FBARs 214, 234, which is proportional to the thickness of the piezoelectric material in the first and third resonator volumes 216, 236. In order to maintain the same resonant frequencies in the first, second, third and fourth FBARs 214, 216, 234, 236, the overall acoustic thicknesses t7,R, t8,R, t9,R, t10,R of the first, second, third and fourth FBARs 214, 216, 234, 236 are made substantially equal to one-half of the wavelength corresponding to the resonant frequency of the balun 200. In the illustrated embodiment, this is achieved by making the acoustic thicknesses of the first, second, fifth, and sixth electrodes 218, 220, 238, 242 larger than the acoustic thicknesses of the third, fourth, seventh, and eighth electrodes 226, 228, 246, 248 by amount that compensates for the smaller acoustic thickness of the first and third resonator volumes 224, 244 relative to the acoustic thicknesses of the third and fourth resonator volumes 230, 250.
In the illustrated embodiment, the overlapping areas of the first and second FBARs 214, 216 normal to the propagation axis 252 are substantially equal and the overlapping areas of the third and fourth FBARs 234, 236 normal to the propagation axis 252 are substantially equal. In this way, acoustic signals may be transmitted efficiently between the first and second FBARs 214, 216 and between the third and fourth FBARs 234, 236 without increased loss of acoustic energy within the balun 200. In the illustrated embodiment, the overlapping areas of the first and second FBARs 214, 216 also are substantially equal to the overlapping areas of the third and fourth FBARs 234, 236 in order to maintain the symmetry between the first and second differential ports 14, 16.
FIG. 8 shows a third impedance transforming balun 300 in accordance with an embodiment of the invention. The balun 300 includes an SBAR 302 formed over respective acoustic isolator 304 in a substrate 306. The acoustic isolator 304 may include a cavity formed under a membrane supporting the balun 300 or an acoustic mirror that includes of a stack of layers alternately formed of high and low acoustic impedance materials and having respective thicknesses of approximately one-quarter of the wavelength corresponding to the target resonant frequency of the balun 300. The balun 300 may be used in a wide variety of application areas, including communication lines, devices, and circuits (e.g., frequency conversion mixers, radars, transmitters, satellites, telephony network devices and circuits, and wireless network modems and routers).
The SBAR 302 is formed from a first acoustic decoupler 308 that is disposed between a first FBAR 310 and a second FBAR 312 and a second acoustic decoupler 314 that is disposed between the second FBAR 312 and a third FBAR 316. The acoustic decouplers 308, 314 are formed the same way as the decoupler 72, which is described above and shown in FIG. 5. The first FBAR 310 is defined by the overlapping regions of a first planar electrode 318 and a second planar electrode 320 that abut opposite sides of a first resonator volume 322. The second FBAR 312 is defined by the overlapping regions of a third planar electrode 324 and a fourth planar electrode 326 that abut opposite sides of a second resonator volume 328. The third FBAR 316 is defined by the overlapping regions of a fifth planar electrode 330 and a sixth planar electrode 332 that abut opposite sides of a second resonator volume 334.
The electrodes of the first, second, and third FBARs 310, 312, 316 typically are formed of one or more layers of Mo, Al, Au, Pt, Ti, and alloys thereof. Each of the first, second, and third resonator volumes 322, 328, 334 is free of any intervening electrodes and contains piezoelectric material disposed for acoustic vibrations parallel to a propagation axis 336 that is normal to the planar electrodes 318, 320, 324, 326, 330, 332. The piezoelectric material in each of the first, second, and third resonator volumes 322, 328, 334 typically is in the form of one or more layers of piezoelectric material, such as a wurtzite-type hexagonal crystal (e.g., cadmium sulfide, cadmium selenide, zinc oxide, beryllium oxide, aluminum nitride, and wurtzite zinc sulfide, and solid solutions thereof) and lead-zirconate-titanate (PZT).
The active area of the SBAR 302 corresponds to the overlapping regions of the first, second, and third FBARs 310, 312, 316 (shown by the dashed box). The fourth electrode 326 of the second FBAR 312 is connected to the single-ended port 12. The second electrode 320 of the first FBAR 310, the third electrode 324 of the second FBAR 312, and the fifth electrode 330 of the third FBAR 316 are connected to the device ground. The first electrode 318 of the first FBAR 310 is connected to the second differential port 16, and the sixth electrode 332 of the third FBAR 316 is connected to the first differential port 14.
In this embodiment, the first, second, and third FBARs 310, 312, 316 respectively have first, second, and third electrical impedances Z′′1, Z′′2, Z′′3 parallel to the propagation axis 336. In general, at least one of the first, second, and third electrical impedances is different from the others. The differences in the electrical impedances of the first, second, and third FBARs 310, 312, 316 are set to achieve a wide variety of different impedance transformations between the unbalanced port 12 and the unbalanced ports 14, 16.
In the illustrated embodiment, the first and third electrical impedances (i.e., the electrical impedances of the first and third FBARs 310, 316) are substantially equal (Z′′1=Z′′3=Z′′13) and different from the second electrical impedance (i.e., the electrical impedance of the second FBAR 312, referred to by Z′′2). In general, the impedance transformation is given by 2×Z′′13:Z′′2, where Z′′13 is the electrical impedance of the first and third FBARs 310, 316 and Z′′2 is the electrical impedance of the second FBAR 312.
In some embodiments, the different electrical impedances of the FBARs 310, 312, 316 are set by forming one or more of the corresponding constituent layers of the FBARs 310, 312, 316 with different material compositions.
In the illustrated embodiment, the material compositions of the electrodes of the FBARs 310, 312, 316 are substantially the same and the compositions of the piezoelectric material in the resonator volumes of the FBARs 310, 312, 316 are substantially the same. The different electrical impedances of the FBARs 310, 312, 316 are set by forming the first and third resonator volumes 322, 334 with thicknesses t11,P, t13,P that are different than the thickness t12,P of the second resonator volume 328. For example, in the exemplary embodiment shown in FIG. 8 the thickness t11,P=t13,P of the first and third resonator volumes 322, 334 is smaller than the thickness t12,P of the second resonator volume 312. In this way, the electrical impedance of the second FBAR 312, which is proportional to the thickness of the piezoelectric material in the second resonator volume 328, is larger than the electrical impedance of the first and third FBARs 310, 316, which is proportional to the thickness of the piezoelectric material in the first and third resonator volumes 322, 334. In order to maintain the same resonant frequencies in the first, second, and third FBARs 310, 312, 316, the overall acoustic thicknesses t11,R, t12,R, t13,R of the first, second, and third FBARs 310, 312, 316 are made substantially equal to one-half of the wavelength corresponding to the resonant frequency of the balun 300. In the illustrated embodiment, this is achieved by making the acoustic thicknesses of the first, second, fifth, and sixth electrodes 318, 320, 330, 332 larger than the acoustic thicknesses of the third and fourth electrodes 324, 326 by an amount that compensates for the smaller acoustic thicknesses of the first and third resonator volumes 322, 334 relative to the acoustic thickness of the second resonator volume 328.
In the illustrated embodiment, the overlapping areas of the first, second, and third FBARs 310, 312, 316 normal to the propagation axis 336 are substantially equal. In this way, acoustic signals may be transmitted efficiently between the second FBAR 312 and the first and third FBARs 310, 316 without increased loss of acoustic energy within the balun 300. This feature also maintains the symmetry between the first and second differential ports 14, 16.
Any of the above-described impedance transforming balun embodiments in accordance with the invention may by supplemented by one or more reactive elements (i.e., inductances and capacitances) that are electrically coupled to one or more of the single-ended port 12 and the differential ports 14, 16 in order to improved the performance of these devices.
Other embodiments are within the scope of the claims.
For example, the embodiments in accordance with the invention described above and shown in the drawings provide an impedance transformation between a single-ended port and two differential ports. Other embodiments in accordance with the invention provide an impedance transformation between two single-ended ports.

## Claims

1. A bulk acoustic wave (BAW) device comprising:
a stacked bulk acoustic wave resonator (SBAR) comprising an acoustic decoupler between first and second film bulk acoustic resonators (FBARs),
the first FBAR being resonant at a resonant frequency and comprising first and second planar electrodes abutting opposite sides of a first resonator volume free of intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to a propagation axis normal to the first and second electrodes, the first FBAR having a first electrical impedance parallel to the propagation axis,
the second FBAR being resonant at the resonant frequency and comprising third and fourth planar electrodes abutting opposite sides of a second resonator volume free of intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis, the second FBAR having a second electrical impedance parallel to the propagation axis and different from the first electrical impedance,
wherein the first resonator volume has a first thickness dimension between the first and second planar electrodes parallel to the propagation axis and the second resonator volume has a second thickness dimension between the third and fourth planar electrodes parallel to the propagation axis, the second thickness dimension being different from the first thickness dimension, and
wherein each of the first and second planar electrodes has a thickness parallel to the propagation axis that is different from a thickness of each of the third and fourth planar electrodes parallel to the propagation axis, compensating for the difference between the first and second thickness dimensions, such that the first and second FBARs have substantially the same overall acoustic thickness.

2. The BAW device of claim 1, wherein the overall acoustic thickness of the first FBAR and the second FBAR is substantially equal to one-half of a wavelength corresponding to the resonant frequency.

3. The BAW device of claim 1, wherein the first resonator volume is characterized by a first normal dimension that is normal to the propagation axis and the second resonator volume is characterized by a second normal dimension that is normal to the propagation axis and is substantially equal to the first normal dimension.

4. The BAW device of claim 1, further comprising a single-ended signal port, a first differential signal port, a second differential signal port, and a device ground, wherein the first electrode is electrically coupled to the device ground, the second electrode is electrically coupled to the single-ended signal port, the third electrode is electrically coupled to the first differential signal port, and the fourth electrode is electrically coupled to the second differential signal port.

5. The BAW device of claim 1, further comprising a second SBAR electrically coupled to the first SBAR and comprising a second acoustic decoupler between third and fourth FBARs, wherein
the third FBAR is resonant at the resonant frequency and comprises fifth and sixth planar electrodes abutting opposite sides of a third resonator volume free of intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis, the third FBAR having a third electrical impedance parallel to the propagation axis, and
the fourth FBAR is resonant at the resonant frequency and comprises seventh and eighth planar electrodes abutting opposite sides of a fourth resonator volume free of intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis, the fourth FBAR having a fourth electrical impedance parallel to the propagation axis and different from the third electrical impedance.

6. The BAW device of claim 5, wherein the first electrical impedance and the third electrical impedance are substantially equal and the second electrical impedance and the fourth electrical impedance are substantially equal.

7. The BAW device of claim 5, wherein the third resonator volume has a third thickness dimension parallel to the propagation axis and the fourth resonator volume has a fourth thickness dimension parallel to the propagation axis and different from the third thickness dimension.

8. The BAW device of claim 7, wherein the first and third thickness dimensions are substantially equal and the second and fourth thickness dimensions are substantially equal.

9. The BAW device of claim 8, wherein the third resonator volume is characterized by a third normal dimension that is normal to the propagation axis, the fourth resonator volume is characterized by a fourth normal dimension that is normal to the propagation axis, and the third and fourth normal dimension are substantially equal to the first normal dimension.

10. The BAW device of claim 5, further comprising a single-ended signal port, a first differential signal port, a second differential signal port, and a device ground, wherein the first electrode is electrically coupled to the device ground, the second electrode and the fifth electrode are electrically coupled to the single-ended signal port, the fourth electrode is electrically coupled to the first differential signal port, and the eighth electrode is electrically coupled to the second differential signal port.

11. The BAW device of claim 5, wherein the first SBAR and the second SBAR correspond to different respective material stacks on a common substrate.

12. The BAW device of claim 1, wherein the SBAR additionally comprises a second decoupler between the second FBAR and a third FBAR, wherein the third FBAR is resonant at the resonant frequency and comprises fifth and sixth planar electrodes abutting opposite sides of a third resonator volume free of intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis.

13. The BAW device of claim 12, wherein the third FBAR has a third electrical impedance parallel to the propagation axis that is substantially equal to the first electrical impedance and different from the second electrical impedance.

14. The BAW device of claim 12, wherein the first resonator volume has a first thickness dimension parallel to the propagation axis, the second resonator volume has a second thickness dimension parallel to the propagation axis and different from the first thickness dimension, the third resonator volume has a third thickness dimension parallel to the propagation axis that is substantially equal to the first thickness dimension.

15. The BAW device of claim 14, wherein the first, second, and third resonator volumes are respectively characterized by first, second, and third normal dimensions that are normal to the propagation axis and are substantially equal to each other.

16. The BAW device of claim 12, further comprising:
a single-ended signal port, a first differential signal port, a second differential signal port, and a device ground, wherein the first electrode is electrically coupled to the first differential signal port, the third electrode is electrically coupled to the device ground, the fourth electrode is electrically coupled to the single-ended signal port, and the sixth electrode is electrically coupled to the second differential signal port.

17. The BAW device of claim 16, wherein the second electrode and the fifth electrode are electrically coupled to the device ground.

18. A bulk acoustic wave (BAW) device, comprising:
a first stacked bulk acoustic wave resonator (SBAR) comprising a first acoustic decoupler between first and second film bulk acoustic resonators (FBARs),
the first FBAR being resonant at a resonant frequency and comprising first and second planar electrodes abutting opposite sides of a first resonator volume free of intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to a propagation axis normal to the first and second electrodes, the first FBAR having a first electrical impedance parallel to the propagation axis, and
the second FBAR being resonant at the resonant frequency and comprising third and fourth planar electrodes abutting opposite sides of a second resonator volume free of intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis, the second FBAR having a second electrical impedance parallel to the propagation axis and different from the first electrical impedance; and

a second SBAR electrically coupled to the first SBAR and comprising a second acoustic decoupler between third and fourth FBARs,
the third FBAR being resonant at the resonant frequency and comprising fifth and sixth planar electrodes abutting opposite sides of a third resonator volume free of intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis, the third FBAR having a third electrical impedance parallel to the propagation axis, and
the fourth FBAR being resonant at the resonant frequency and comprising seventh and eighth planar electrodes abutting opposite sides of a fourth resonator volume free of intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis, the fourth FBAR having a fourth electrical impedance parallel to the propagation axis and different from the third electrical impedance,

wherein the first resonator volume has a first thickness dimension between the first and second planar electrodes parallel to the propagation axis and the second resonator volume has a second thickness dimension between the third and fourth planar electrodes parallel to the propagation axis, the second thickness dimension being different from the first thickness dimension, and
wherein the first electrode is electrically coupled to the fifth electrode, the second electrode is electrically coupled to a single-ended signal port, the fourth electrode is electrically coupled to a first differential signal port, the eighth electrode is electrically coupled to a second differential signal port, and the third, sixth and seventh electrodes are electrically coupled to a device ground.

19. An impedance transforming balun, comprising:
a first film bulk acoustic resonator (FBAR) resonant at a resonant frequency and having a first electrical impedance, the first FBAR comprising a first planar electrode, a second planar electrode, and a first resonator volume between the first and second planar electrodes, the first resonator volume having a first thickness dimension between the first and second planar electrodes and comprising piezoelectric material arranged for acoustic vibrations parallel to a propagation axis normal to the first and second planar electrodes;
a second FBAR resonant at the resonant frequency and having a second electrical impedance different from the first electrical impedance, the second FBAR comprising a third planar electrode, a fourth planar electrode, and a second resonator volume between the third and fourth planar electrodes, the second resonator volume having a second thickness dimension between the third and fourth planar electrodes and comprising piezoelectric material arranged for acoustic vibrations parallel to the propagation axis, the second thickness dimension being different from the first thickness dimension; and
an acoustic decoupler between the first FBAR and the second FBAR,
wherein the first and second planar electrodes have a thickness parallel to the propagation axis that is different from a thickness of the third and fourth planar electrodes parallel to the propagation axis, compensating for the difference between the first and second thickness dimensions, so that the second FBAR has substantially the same overall acoustic thickness as the first FBAR parallel to the propagation axis.