Patent Publication Number: US-2021184642-A1

Title: Piezoelectric acoustic resonator manufactured with piezoelectric thin film transfer process

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 16/433,849, titled “PIEZOELECTRIC ACOUSTIC RESONATOR MANUFACTURED WITH PIEZOELECTRIC THIN FILM TRANSFER PROCESS,” filed Jun. 6, 2019, which claims priority to and is a continuation of U.S. patent application Ser. No. 15/784,919, filed Nov. 16, 2017, now U.S. Pat. No. 10,355,659, issued July 16, 2019, which claims priority to and is a continuation in part of U.S. patent application Ser. No. 15/068,510, filed Mar. 11, 2016, now U.S. Pat. No. 10,217,930, issued Feb. 26, 2019. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to electronic devices. More particularly, the present invention provides techniques related to a method of manufacture and a structure for bulk acoustic wave resonator devices, single crystal bulk acoustic wave resonator devices, single crystal filter and resonator devices, and the like. Merely by way of example, the invention has been applied to a single crystal resonator device for a communication device, mobile device, computing device, among others. 
     Mobile telecommunication devices have been successfully deployed world-wide. Over a billion mobile devices, including cell phones and smartphones, were manufactured in a single year and unit volume continues to increase year-over-year. With ramp of 4G/LTE in about 2012, and explosion of mobile data traffic, data rich content is driving the growth of the smartphone segment—which is expected to reach 2 B per annum within the next few years. Coexistence of new and legacy standards and thirst for higher data rate requirements is driving RF complexity in smartphones. Unfortunately, limitations exist with conventional RF technology that is problematic, and may lead to drawbacks in the future. 
     With 4G LTE and 5G growing more popular by the day, wireless data communication demands high performance RF filters with frequencies around 5 GHz and higher. Bulk acoustic wave resonators (BAWR) using crystalline piezoelectric thin films are leading candidates for meeting such demands. Current BAWRs using polycrystalline piezoelectric thin films are adequate for bulk acoustic wave (BAW) filters operating at frequencies ranging from 1 to 3 GHz; however, the quality of the polycrystalline piezoelectric films degrades quickly as the thicknesses decrease below around 0.5 um, which is required for resonators and filters operating at frequencies around 5 GHz and above. Single crystalline or epitaxial piezoelectric thin films grown on compatible crystalline substrates exhibit good crystalline quality and high piezoelectric performance even down to very thin thicknesses, e.g., 0.4 um. Even so, there are challenges to using and transferring single crystal piezoelectric thin films in the manufacture of BAWR and BAW filters. 
     From the above, it is seen that techniques for improving methods of manufacture and structures for acoustic resonator devices are highly desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention, techniques generally related to electronic devices are provided. More particularly, the present invention provides techniques related to a method of manufacture and structure for bulk acoustic wave resonator devices, single crystal resonator devices, single crystal filter and resonator devices, and the like. Merely by way of example, the invention has been applied to a single crystal resonator device for a communication device, mobile device, computing device, among others. 
     In an example, the present invention provides a method and structure for a transfer process using a sacrificial layer for single crystal acoustic resonator devices. In this example, a BAWR with an air reflection cavity is formed. A single crystalline or epitaxial piezoelectric thin film is grown on a crystalline substrate. A first electrode is deposited on the surface of the piezoelectric film and patterned. A first passivation layer or etch-protection layer is deposited over the patterned first electrode layer. A sacrificial layer is deposited over the passivation layer and is then etched. A support layer is deposited over the sacrificial layer. The support layer is planarized or polished and then bonded to a substrate wafer. The crystalline substrate is removed via grinding and/or etching to expose the second surface of the single crystalline piezoelectric film. The piezoelectric film is etched to form VIA&#39;s and etch access holes for the sacrificial layer. A second electrode is deposited over the second surface of the piezoelectric film. A second passivation layer is deposited over the second electrode layer and patterned. A contact layer for proving and electrical connection to other circuits is deposited and patterned. The sacrificial layer can then be etched to make the air reflection cavity at one side of the BAW resonator. 
     In an example, the present invention provides a method and structure for a cavity bond transfer process for single crystal acoustic resonator devices. In this example, a BAW resonator with an air reflection cavity is formed. The process is similar to that previously described, except that the air cavity is etched inside the support layer after deposition of the support layer rather than using a sacrificial layer. 
     In an example, the present invention provides a method and structure for a solidly mounted transfer process for single crystal acoustic resonator devices. In this example, a BAW resonator with a reflector structure (e.g., Bragg-type reflector) is formed with single crystalline or epitaxial piezoelectric film. Compared to the previous examples, the reflector structure is deposited and patterned after the patterning of the first electrode and the first passivation layer. The support layer is deposited over the reflector structure and is planarized for bonding. The reflector structure can be multilayer with alternating low and high impedance layers. 
     In each of the preceding examples, energy confinement structures can be formed on the first electrode, second electrode, or both. In an example, these energy confinement structures are mass loaded areas surrounding the resonator area. The resonator area is the area where the first electrode, the piezoelectric layer, and the second electrode overlap. The larger mass load in the energy confinement structures lowers a cut-off frequency of the resonator. The cut-off frequency is the lower or upper limit of the frequency at which the acoustic wave can propagate in a direction parallel to the surface of the piezoelectric film. Therefore, the cut-off frequency is the resonance frequency in which the wave is travelling along the thickness direction and thus is determined by the total stack structure of the resonator along the vertical direction. In piezoelectric films (e.g., AlN), acoustic waves with lower frequency than the cut-off frequency can propagate in a parallel direction along the surface of the film, i.e., the acoustic wave exhibits a high-band-cut-off type dispersion characteristic. In this case, the mass loaded area surrounding the resonator provides a barrier preventing the acoustic wave from propagating outside the resonator. By doing so, this feature increases the quality factor (Q) of the resonator and improves the performance of the resonator and, consequently, the filter. 
     In addition, the top single crystalline piezoelectric layer can be replaced by a polycrystalline piezoelectric film. In such films, the lower part that is close to the interface with the substrate has poor crystalline quality with smaller grain sizes and a wider distribution of the piezoelectric polarization orientation than the upper part of the film close to the surface. This is due to the polycrystalline growth of the piezoelectric film, i.e., the nucleation and initial film have random crystalline orientations. Considering AlN as a piezoelectric material, the growth rate along the c-axis or the polarization orientation is higher than other crystalline orientations that increase the proportion of the grains with the c-axis perpendicular to the growth surface as the film grows thicker. In a typical polycrystalline AlN film with about a 1 um thickness, the upper part of the film close to the surface has better crystalline quality and better alignment in terms of piezoelectric polarization. By using the thin film transfer process contemplated in the present invention, it is possible to use the upper portion of the polycrystalline film in high frequency BAW resonators with very thin piezoelectric films. This can be done by removing a portion of the piezoelectric layer during or after the growth substrate removal process. 
     In an example, the present invention provides a method for fabricating a bulk acoustic wave resonator device. This method can include providing a piezoelectric substrate having a substrate surface region. This piezo electric substrate can have a piezoelectric layer formed overlying a seed substrate. A topside metal electrode can be formed overlying a portion of the substrate surface region. The method can include forming a topside micro-trench within a portion of the piezoelectric layer and forming one or more bond pads overlying one or more portions of the piezoelectric layer. A topside metal can be formed overlying a portion of the piezoelectric layer. This topside metal can include a topside metal plug, or a bottom side metal plug, formed within the topside micro-trench and electrically coupled to at least one of the bond pads. 
     In an example, the method can include thinning the seed substrate to form a thinned seed substrate. A first backside trench can be formed within the thinned seed substrate and underlying the topside metal electrode. A second backside trench can be formed within the thinned seed substrate and underlying the topside micro-trench. Also, the method includes forming a backside metal electrode underlying one or more portions of the thinned seed substrate, within the first backside trench, and underlying the topside metal electrode; and forming a backside metal plug underlying one or more portions of the thinned substrate, within the second backside trench, and underlying the topside micro-trench. The backside metal plug can be electrically coupled to the topside metal plug and the backside metal electrode. The topside micro-trench, the topside metal plug, the second backside trench, and the backside metal plug form a micro-via. In a specific example, both backside trenches can be combined in one trench, where the shared backside trench can include the backside metal electrode underlying the topside metal electrode and the backside metal plug underlying the topside micro-trench. 
     In an example, the method can include providing a top cap structure, wherein the top cap structure including an interposer substrate with one or more through-via structures electrically coupled to one or more top bond pads and one or more bottom bond pads. The top cap structure can be bonded to the piezoelectric substrate, while the one or more bottom bond pads can be electrically coupled to the one or more bond pads and the topside metal. A backside cap structure can be bonded to the thinned seed substrate such that the backside cap structure is configured underlying the first and second backside trenches, and one or more solder balls formed overlying the one or more top bond pads. 
     In an alternative example, the method can include providing a top cap structure, wherein the top cap structure including an interposer substrate with one or more blind via structures electrically coupled to one or more bottom bond pads. The top cap structure can be bonded to the piezoelectric substrate, while the one or more bottom bond pads are electrically coupled to the one or more bond pads and the topside metal. The method can include thinning the top cap structure to expose the one or more blind vias. One or more top bond pads can be formed overlying and electrically coupled to the one or more blind vias, and one or more solder balls can be formed overlying the one or more top bond pads. 
     In an alternative example, the method can include providing a top cap structure, wherein the top cap structure including a substrate with one or more bottom bond pads. The top cap structure can be bonded to the piezoelectric substrate, while the one or more bottom bond pads are electrically coupled to the one or more bond pads and the topside metal. A backside cap structure can be bonded to the thinned seed substrate such that the backside cap structure is configured underlying the first and second backside trenches. The method can include forming one or more backside bond pads within one or more portions of the backside cap structure. These one or more of the backside bond pads can be electrically coupled to the backside metal plug. One or more solder balls can be formed underlying the one or more backside bond pads. 
     In an alternative example, the present invention can provide a method for fabricating a top cap or bottom cap free structure, wherein the thinned device is assembled into the final package on the die level. Compared to the examples previously described, the top cap or bottom cap free structure may omit the steps of bonding a top cap structure to the piezoelectric substrate or the steps of bonding a backside cap structure to the thinned substrate. 
     One or more benefits are achieved over pre-existing techniques using the invention. In particular, the present device can be manufactured in a relatively simple and cost effective manner while using conventional materials and/or methods according to one of ordinary skill in the art. Using the present method, one can create a reliable single crystal based acoustic filter or resonator using multiple ways of three-dimensional stacking through a wafer level process. Such filters or resonators can be implemented in an RF filter device, an RF filter system, or the like. Depending upon the embodiment, one or more of these benefits may be achieved. 
     A further understanding of the nature and advantages of the invention may be realized by reference to the latter portions of the specification and attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to more fully understand the present invention, reference is made to the accompanying drawings. Understanding that these drawings are not to be considered limitations in the scope of the invention, the presently described embodiments and the presently understood best mode of the invention are described with additional detail through use of the accompanying drawings in which: 
         FIG. 1A  is a simplified diagram illustrating an acoustic resonator device having topside interconnections according to an example of the present invention. 
         FIG. 1B  is a simplified diagram illustrating an acoustic resonator device having bottom-side interconnections according to an example of the present invention. 
         FIG. 1C  is a simplified diagram illustrating an acoustic resonator device having interposer/cap-free structure interconnections according to an example of the present invention. 
         FIG. 1D  is a simplified diagram illustrating an acoustic resonator device having interposer/cap-free structure interconnections with a shared backside trench according to an example of the present invention. 
         FIGS. 2 and 3  are simplified diagrams illustrating steps for a method of manufacture for an acoustic resonator device according to an example of the present invention. 
         FIG. 4A  is a simplified diagram illustrating a step for a method creating a topside micro-trench according to an example of the present invention. 
         FIGS. 4B and 4C  are simplified diagrams illustrating alternative methods for conducting the method step of forming a topside micro-trench as described in  FIG. 4A . 
         FIGS. 4D and 4E  are simplified diagrams illustrating an alternative method for conducting the method step of forming a topside micro-trench as described in  FIG. 4A . 
         FIGS. 5 to 8  are simplified diagrams illustrating steps for a method of manufacture for an acoustic resonator device according to an example of the present invention. 
         FIG. 9A  is a simplified diagram illustrating a method step for forming backside trenches according to an example of the present invention. 
         FIGS. 9B and 9C  are simplified diagrams illustrating an alternative method for conducting the method step of forming backside trenches, as described in  FIG. 9A , and simultaneously singulating a seed substrate according to an embodiment of the present invention. 
         FIG. 10  is a simplified diagram illustrating a method step forming backside metallization and electrical interconnections between top and bottom sides of a resonator according to an example of the present invention. 
         FIGS. 11A and 11B  are simplified diagrams illustrating alternative steps for a method of manufacture for an acoustic resonator device according to an example of the present invention. 
         FIGS. 12A to 12E  are simplified diagrams illustrating steps for a method of manufacture for an acoustic resonator device using a blind via interposer according to an example of the present invention. 
         FIG. 13  is a simplified diagram illustrating a step for a method of manufacture for an acoustic resonator device according to an example of the present invention. 
         FIGS. 14A to 14G  are simplified diagrams illustrating method steps for a cap wafer process for an acoustic resonator device according to an example of the present invention. 
         FIGS. 15A-15E  are simplified diagrams illustrating method steps for making an acoustic resonator device with shared backside trench, which can be implemented in both interposer/cap and interposer free versions, according to examples of the present invention. 
         FIGS. 16A-16C  through  FIGS. 31A-31C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. 
         FIGS. 32A-32C  through  FIGS. 46A-46C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a cavity bond transfer process for single crystal acoustic resonator devices according to an example of the present invention. 
         FIGS. 47A-47C  though  FIGS. 59A-59C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a solidly mounted transfer process for single crystal acoustic resonator devices according to an example of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, techniques generally related to electronic devices are provided. More particularly, the present invention provides techniques related to a method of manufacture and structure for bulk acoustic wave resonator devices, single crystal resonator devices, single crystal filter and resonator devices, and the like. Merely by way of example, the invention has been applied to a single crystal resonator device for a communication device, mobile device, computing device, among others. 
       FIG. 1A  is a simplified diagram illustrating an acoustic resonator device  101  having topside interconnections according to an example of the present invention. As shown, device  101  includes a thinned seed substrate  112  with an overlying single crystal piezoelectric layer  120 , which has a micro-via  129 . The micro-via  129  can include a topside micro-trench  121 , a topside metal plug  146 , a backside trench  114 , and a backside metal plug  147 . Although device  101  is depicted with a single micro-via  129 , device  101  may have multiple micro-vias. A topside metal electrode  130  is formed overlying the piezoelectric layer  120 . A top cap structure is bonded to the piezoelectric layer  120 . This top cap structure includes an interposer substrate  119  with one or more through-vias  151  that are connected to one or more top bond pads  143 , one or more bond pads  144 , and topside metal  145  with topside metal plug  146 . Solder balls  170  are electrically coupled to the one or more top bond pads  143 . 
     The thinned substrate  112  has the first and second backside trenches  113 ,  114 . A backside metal electrode  131  is formed underlying a portion of the thinned seed substrate  112 , the first backside trench  113 , and the topside metal electrode  130 . The backside metal plug  147  is formed underlying a portion of the thinned seed substrate  112 , the second backside trench  114 , and the topside metal  145 . This backside metal plug  147  is electrically coupled to the topside metal plug  146  and the backside metal electrode  131 . A backside cap structure  161  is bonded to the thinned seed substrate  112 , underlying the first and second backside trenches  113 ,  114 . Further details relating to the method of manufacture of this device will be discussed starting from  FIG. 2 . 
       FIG. 1B  is a simplified diagram illustrating an acoustic resonator device  102  having backside interconnections according to an example of the present invention. As shown, device  101  includes a thinned seed substrate  112  with an overlying piezoelectric layer  120 , which has a micro-via  129 . The micro-via  129  can include a topside micro-trench  121 , a topside metal plug  146 , a backside trench  114 , and a backside metal plug  147 . Although device  102  is depicted with a single micro-via  129 , device  102  may have multiple micro-vias. A topside metal electrode  130  is formed overlying the piezoelectric layer  120 . A top cap structure is bonded to the piezoelectric layer  120 . This top cap structure  119  includes bond pads which are connected to one or more bond pads  144  and topside metal  145  on piezoelectric layer  120 . The topside metal  145  includes a topside metal plug  146 . 
     The thinned substrate  112  has the first and second backside trenches  113 ,  114 . A backside metal electrode  131  is formed underlying a portion of the thinned seed substrate  112 , the first backside trench  113 , and the topside metal electrode  130 . A backside metal plug  147  is formed underlying a portion of the thinned seed substrate  112 , the second backside trench  114 , and the topside metal plug  146 . This backside metal plug  147  is electrically coupled to the topside metal plug  146 . A backside cap structure  162  is bonded to the thinned seed substrate  112 , underlying the first and second backside trenches. One or more backside bond pads ( 171 ,  172 ,  173 ) are formed within one or more portions of the backside cap structure  162 . Solder balls  170  are electrically coupled to the one or more backside bond pads  171 - 173 . Further details relating to the method of manufacture of this device will be discussed starting from  FIG. 14A . 
       FIG. 1C  is a simplified diagram illustrating an acoustic resonator device having interposer/cap-free structure interconnections according to an example of the present invention. As shown, device  103  includes a thinned seed substrate  112  with an overlying single crystal piezoelectric layer  120 , which has a micro-via  129 . The micro-via  129  can include a topside micro-trench  121 , a topside metal plug  146 , a backside trench  114 , and a backside metal plug  147 . Although device  103  is depicted with a single micro-via  129 , device  103  may have multiple micro-vias. A topside metal electrode  130  is formed overlying the piezoelectric layer  120 . The thinned substrate  112  has the first and second backside trenches  113 ,  114 . A backside metal electrode  131  is formed underlying a portion of the thinned seed substrate  112 , the first backside trench  113 , and the topside metal electrode  130 . A backside metal plug  147  is formed underlying a portion of the thinned seed substrate  112 , the second backside trench  114 , and the topside metal  145 . This backside metal plug  147  is electrically coupled to the topside metal plug  146  and the backside metal electrode  131 . Further details relating to the method of manufacture of this device will be discussed starting from  FIG. 2 . 
       FIG. 1D  is a simplified diagram illustrating an acoustic resonator device having interposer/cap-free structure interconnections with a shared backside trench according to an example of the present invention. As shown, device  104  includes a thinned seed substrate  112  with an overlying single crystal piezoelectric layer  120 , which has a micro-via  129 . The micro-via  129  can include a topside micro-trench  121 , a topside metal plug  146 , and a backside metal  147 . Although device  104  is depicted with a single micro-via  129 , device  104  may have multiple micro-vias. A topside metal electrode  130  is formed overlying the piezoelectric layer  120 . The thinned substrate  112  has a first backside trench  113 . A backside metal electrode  131  is formed underlying a portion of the thinned seed substrate  112 , the first backside trench  113 , and the topside metal electrode  130 . A backside metal  147  is formed underlying a portion of the thinned seed substrate  112 , the second backside trench  114 , and the topside metal  145 . This backside metal  147  is electrically coupled to the topside metal plug  146  and the backside metal electrode  131 . Further details relating to the method of manufacture of this device will be discussed starting from  FIG. 2 . 
       FIGS. 2 and 3  are simplified diagrams illustrating steps for a method of manufacture for an acoustic resonator device according to an example of the present invention. This method illustrates the process for fabricating an acoustic resonator device similar to that shown in  FIG. 1A .  FIG. 2  can represent a method step of providing a partially processed piezoelectric substrate. As shown, device  102  includes a seed substrate  110  with a piezoelectric layer  120  formed overlying. In a specific example, the seed substrate can include silicon, silicon carbide, aluminum oxide, or single crystal aluminum gallium nitride materials, or the like. The piezoelectric layer  120  can include a piezoelectric single crystal layer or a thin film piezoelectric single crystal layer. 
       FIG. 3  can represent a method step of forming a top side metallization or top resonator metal electrode  130 . In a specific example, the topside metal electrode  130  can include a molybdenum, aluminum, ruthenium, or titanium material, or the like and combinations thereof. This layer can be deposited and patterned on top of the piezoelectric layer by a lift-off process, a wet etching process, a dry etching process, a metal printing process, a metal laminating process, or the like. The lift-off process can include a sequential process of lithographic patterning, metal deposition, and lift-off steps to produce the topside metal layer. The wet/dry etching processes can includes sequential processes of metal deposition, lithographic patterning, metal deposition, and metal etching steps to produce the topside metal layer. Those of ordinary skill in the art will recognize other variations, modifications, and alternatives. 
       FIG. 4A  is a simplified diagram illustrating a step for a method of manufacture for an acoustic resonator device  401  according to an example of the present invention. This figure can represent a method step of forming one or more topside micro-trenches  121  within a portion of the piezoelectric layer  120 . This topside micro-trench  121  can serve as the main interconnect junction between the top and bottom sides of the acoustic membrane, which will be developed in later method steps. In an example, the topside micro-trench  121  is extends all the way through the piezoelectric layer  120  and stops in the seed substrate  110 . This topside micro-trench  121  can be formed through a dry etching process, a laser drilling process, or the like.  FIGS. 4B and 4C  describe these options in more detail. 
       FIGS. 4B and 4C  are simplified diagrams illustrating alternative methods for conducting the method step as described in  FIG. 4A . As shown,  FIG. 4B  represents a method step of using a laser drill, which can quickly and accurately form the topside micro-trench  121  in the piezoelectric layer  120 . In an example, the laser drill can be used to form nominal 50 um holes, or holes between 10 um and 500 um in diameter, through the piezoelectric layer  120  and stop in the seed substrate  110  below the interface between layers  120  and  110 . A protective layer  122  can be formed overlying the piezoelectric layer  120  and the topside metal electrode  130 . This protective layer  122  can serve to protect the device from laser debris and to provide a mask for the etching of the topside micro-via  121 . In a specific example, the laser drill can be an 11 W high power diode-pumped UV laser, or the like. This mask  122  can be subsequently removed before proceeding to other steps. The mask may also be omitted from the laser drilling process, and air flow can be used to remove laser debris. 
       FIG. 4C  can represent a method step of using a dry etching process to form the topside micro-trench  121  in the piezoelectric layer  120 . As shown, a lithographic masking layer  123  can be forming overlying the piezoelectric layer  120  and the topside metal electrode  130 . The topside micro-trench  121  can be formed by exposure to plasma, or the like. 
       FIGS. 4D and 4E  are simplified diagrams illustrating an alternative method for conducting the method step as described in  FIG. 4A . These figures can represent the method step of manufacturing multiple acoustic resonator devices simultaneously. In  FIG. 4D , two devices are shown on Die # 1  and Die # 2 , respectively.  FIG. 4E  shows the process of forming a micro-via  121  on each of these dies while also etching a scribe line  124  or dicing line. In an example, the etching of the scribe line  124  singulates and relieves stress in the piezoelectric single crystal layer  120 . 
       FIGS. 5 to 8  are simplified diagrams illustrating steps for a method of manufacture for an acoustic resonator device according to an example of the present invention.  FIG. 5  can represent the method step of forming one or more bond pads  140  and forming a topside metal  141  electrically coupled to at least one of the bond pads  140 . The topside metal  141  can include a topside metal plug  146  formed within the topside micro-trench  121 . In a specific example, the topside metal plug  146  fills the topside micro-trench  121  to form a topside portion of a micro-via. 
     In an example, the bond pads  140  and the topside metal  141  can include a gold material or other interconnect metal material depending upon the application of the device. These metal materials can be formed by a lift-off process, a wet etching process, a dry etching process, a screen-printing process, an electroplating process, a metal printing process, or the like. In a specific example, the deposited metal materials can also serve as bond pads for a cap structure, which will be described below. 
       FIG. 6  can represent a method step for preparing the acoustic resonator device for bonding, which can be a hermetic bonding. As shown, a top cap structure is positioned above the partially processed acoustic resonator device as described in the previous figures. The top cap structure can be formed using an interposer substrate  119  in two configurations: fully processed interposer version  601  (through glass via) and partially processed interposer version  602  (blind via version). In the  601  version, the interposer substrate  119  includes through-via structures  151  that extend through the interposer substrate  119  and are electrically coupled to bottom bond pads  142  and top bond pads  143 . In the  602  version, the interposer substrate  119  includes blind via structures  152  that only extend through a portion of the interposer substrate  119  from the bottom side. These blind via structures  152  are also electrically coupled to bottom bond pads  142 . In a specific example, the interposer substrate can include a silicon, glass, smart-glass, or other like material. 
       FIG. 7  can represent a method step of bonding the top cap structure to the partially processed acoustic resonator device. As shown, the interposer substrate  119  is bonded to the piezoelectric layer by the bond pads ( 140 ,  142 ) and the topside metal  141 , which are now denoted as bond pad  144  and topside metal  145 . This bonding process can be done using a compression bond method or the like.  FIG. 8  can represent a method step of thinning the seed substrate  110 , which is now denoted as thinned seed substrate  111 . This substrate thinning process can include grinding and etching processes or the like. In a specific example, this process can include a wafer backgrinding process followed by stress removal, which can involve dry etching, CMP polishing, or annealing processes. 
       FIG. 9A  is a simplified diagram illustrating a step for a method of manufacture for an acoustic resonator device  901  according to an example of the present invention.  FIG. 9A  can represent a method step for forming backside trenches  113  and  114  to allow access to the piezoelectric layer from the backside of the thinned seed substrate  111 . In an example, the first backside trench  113  can be formed within the thinned seed substrate  111  and underlying the topside metal electrode  130 . The second backside trench  114  can be formed within the thinned seed substrate  111  and underlying the topside micro-trench  121  and topside metal plug  146 . This substrate is now denoted thinned substrate  112 . In a specific example, these trenches  113  and  114  can be formed using deep reactive ion etching (DRIE) processes, Bosch processes, or the like. The size, shape, and number of the trenches may vary with the design of the acoustic resonator device. In various examples, the first backside trench may be formed with a trench shape similar to a shape of the topside metal electrode or a shape of the backside metal electrode. The first backside trench may also be formed with a trench shape that is different from both a shape of the topside metal electrode and the backside metal electrode. 
       FIGS. 9B and 9C  are simplified diagrams illustrating an alternative method for conducting the method step as described in  FIG. 9A . Like  FIGS. 4D and 4E , these figures can represent the method step of manufacturing multiple acoustic resonator devices simultaneously. In  FIG. 9B , two devices with cap structures are shown on Die # 1  and Die # 2 , respectively.  FIG. 9C  shows the process of forming backside trenches ( 113 ,  114 ) on each of these dies while also etching a scribe line  115  or dicing line. In an example, the etching of the scribe line  115  provides an optional way to singulate the backside wafer  112 . 
       FIG. 10  is a simplified diagram illustrating a step for a method of manufacture for an acoustic resonator device  1000  according to an example of the present invention. This figure can represent a method step of forming a backside metal electrode  131  and a backside metal plug  147  within the backside trenches of the thinned seed substrate  112 . In an example, the backside metal electrode  131  can be formed underlying one or more portions of the thinned substrate  112 , within the first backside trench  113 , and underlying the topside metal electrode  130 . This process completes the resonator structure within the acoustic resonator device. The backside metal plug  147  can be formed underlying one or more portions of the thinned substrate  112 , within the second backside trench  114 , and underlying the topside micro-trench  121 . The backside metal plug  147  can be electrically coupled to the topside metal plug  146  and the backside metal electrode  131 . In a specific example, the backside metal electrode  130  can include a molybdenum, aluminum, ruthenium, or titanium material, or the like and combinations thereof. The backside metal plug can include a gold material, low resistivity interconnect metals, electrode metals, or the like. These layers can be deposited using the deposition methods described previously. 
       FIGS. 11A and 11B  are simplified diagrams illustrating alternative steps for a method of manufacture for an acoustic resonator device according to an example of the present invention. These figures show methods of bonding a backside cap structure underlying the thinned seed substrate  112 . In  FIG. 11A , the backside cap structure is a dry film cap  161 , which can include a permanent photo-imageable dry film such as a solder mask, polyimide, or the like. Bonding this cap structure can be cost-effective and reliable, but may not produce a hermetic seal. In  FIG. 11B , the backside cap structure is a substrate  162 , which can include a silicon, glass, or other like material. Bonding this substrate can provide a hermetic seal, but may cost more and require additional processes. Depending upon application, either of these backside cap structures can be bonded underlying the first and second backside vias. 
       FIGS. 12A to 12E  are simplified diagrams illustrating steps for a method of manufacture for an acoustic resonator device according to an example of the present invention. More specifically, these figures describe additional steps for processing the blind via interposer “ 602 ” version of the top cap structure.  FIG. 12A  shows an acoustic resonator device  1201  with blind vias  152  in the top cap structure. In  FIG. 12B , the interposer substrate  119  is thinned, which forms a thinned interposer substrate  118 , to expose the blind vias  152 . This thinning process can be a combination of a grinding process and etching process as described for the thinning of the seed substrate. In  FIG. 12C , a redistribution layer (RDL) process and metallization process can be applied to create top cap bond pads  160  that are formed overlying the blind vias  152  and are electrically coupled to the blind vias  152 . As shown in  FIG. 12D , a ball grid array (BGA) process can be applied to form solder balls  170  overlying and electrically coupled to the top cap bond pads  160 . This process leaves the acoustic resonator device ready for wire bonding  171 , as shown in  FIG. 12E . 
       FIG. 13  is a simplified diagram illustrating a step for a method of manufacture for an acoustic resonator device according to an example of the present invention. As shown, device  1300  includes two fully processed acoustic resonator devices that are ready to singulation to create separate devices. In an example, the die singulation process can be done using a wafer dicing saw process, a laser cut singulation process, or other processes and combinations thereof. 
       FIGS. 14A to 14G  are simplified diagrams illustrating steps for a method of manufacture for an acoustic resonator device according to an example of the present invention. This method illustrates the process for fabricating an acoustic resonator device similar to that shown in  FIG. 1B . The method for this example of an acoustic resonator can go through similar steps as described in  FIGS. 1-5 .  FIG. 14A  shows where this method differs from that described previously. Here, the top cap structure substrate  119  and only includes one layer of metallization with one or more bottom bond pads  142 . Compared to  FIG. 6 , there are no via structures in the top cap structure because the interconnections will be formed on the bottom side of the acoustic resonator device. 
       FIGS. 14B to 14F  depict method steps similar to those described in the first process flow.  FIG. 14B  can represent a method step of bonding the top cap structure to the piezoelectric layer  120  through the bond pads ( 140 ,  142 ) and the topside metal  141 , now denoted as bond pads  144  and topside metal  145  with topside metal plug  146 .  FIG. 14C  can represent a method step of thinning the seed substrate  110 , which forms a thinned seed substrate  111 , similar to that described in  FIG. 8 .  FIG. 14D  can represent a method step of forming first and second backside trenches, similar to that described in  FIG. 9A .  FIG. 14E  can represent a method step of forming a backside metal electrode  131  and a backside metal plug  147 , similar to that described in  FIG. 10 .  FIG. 14F  can represent a method step of bonding a backside cap structure  162 , similar to that described in  FIGS. 11A and 11B . 
       FIG. 14G  shows another step that differs from the previously described process flow. Here, the backside bond pads  171 ,  172 , and  173  are formed within the backside cap structure  162 . In an example, these backside bond pads  171 - 173  can be formed through a masking, etching, and metal deposition processes similar to those used to form the other metal materials. A BGA process can be applied to form solder balls  170  in contact with these backside bond pads  171 - 173 , which prepares the acoustic resonator device  1407  for wire bonding. 
       FIGS. 15A to 15E  are simplified diagrams illustrating steps for a method of manufacture for an acoustic resonator device according to an example of the present invention. This method illustrates the process for fabricating an acoustic resonator device similar to that shown in  FIG. 1B . The method for this example can go through similar steps as described in  FIG. 1-5 .  FIG. 15A  shows where this method differs from that described previously. A temporary carrier  218  with a layer of temporary adhesive  217  is attached to the substrate. In a specific example, the temporary carrier  218  can include a glass wafer, a silicon wafer, or other wafer and the like. 
       FIGS. 15B to 15F  depict method steps similar to those described in the first process flow.  FIG. 15B  can represent a method step of thinning the seed substrate  110 , which forms a thinned substrate  111 , similar to that described in  FIG. 8 . In a specific example, the thinning of the seed substrate  110  can include a back side grinding process followed by a stress removal process. The stress removal process can include a dry etch, a Chemical Mechanical Planarization (CMP), and annealing processes. 
       FIG. 15C  can represent a method step of forming a shared backside trench  113 , similar to the techniques described in  FIG. 9A . The main difference is that the shared backside trench is configured underlying both topside metal electrode  130 , topside micro-trench  121 , and topside metal plug  146 . In an example, the shared backside trench  113  is a backside resonator cavity that can vary in size, shape (all possible geometric shapes), and side wall profile (tapered convex, tapered concave, or right angle). In a specific example, the forming of the shared backside trench  113  can include a litho-etch process, which can include a back-to-front alignment and dry etch of the backside substrate  111 . The piezoelectric layer  120  can serve as an etch stop layer for the forming of the shared backside trench  113 . 
       FIG. 15D  can represent a method step of forming a backside metal electrode  131  and a backside metal  147 , similar to that described in  FIG. 10 . In an example, the forming of the backside metal electrode  131  can include a deposition and patterning of metal materials within the shared backside trench  113 . Here, the backside metal  131  serves as an electrode and the backside plug/connect metal  147  within the micro-via  121 . The thickness, shape, and type of metal can vary as a function of the resonator/filter design. As an example, the backside electrode  131  and via plug metal  147  can be different metals. In a specific example, these backside metals  131 ,  147  can either be deposited and patterned on the surface of the piezoelectric layer  120  or rerouted to the backside of the substrate  112 . In an example, the backside metal electrode may be patterned such that it is configured within the boundaries of the shared backside trench such that the backside metal electrode does not come in contact with one or more side-walls of the seed substrate created during the forming of the shared backside trench. 
       FIG. 15E  can represent a method step of bonding a backside cap structure  162 , similar to that described in  FIGS. 11A and 11B , following a de-bonding of the temporary carrier  218  and cleaning of the topside of the device to remove the temporary adhesive  217 . Those of ordinary skill in the art will recognize other variations, modifications, and alternatives of the methods steps described previously. 
     As used herein, the term “substrate” can mean the bulk substrate or can include overlying growth structures such as an aluminum, gallium, or ternary compound of aluminum and gallium and nitrogen containing epitaxial region, or functional regions, combinations, and the like. 
     One or more benefits are achieved over pre-existing techniques using the invention. In particular, the present device can be manufactured in a relatively simple and cost effective manner while using conventional materials and/or methods according to one of ordinary skill in the art. Using the present method, one can create a reliable single crystal based acoustic resonator using multiple ways of three-dimensional stacking through a wafer level process. Such filters or resonators can be implemented in an RF filter device, an RF filter system, or the like. Depending upon the embodiment, one or more of these benefits may be achieved. Of course, there can be other variations, modifications, and alternatives. 
     With 4G LTE and 5G growing more popular by the day, wireless data communication demands high performance RF filters with frequencies around 5 GHz and higher. Bulk acoustic wave resonators (BAWR), widely used in such filters operating at frequencies around 3 GHz and lower, are leading candidates for meeting such demands. Current bulk acoustic wave resonators use polycrystalline piezoelectric AlN thin films where each grain&#39;s c-axis is aligned perpendicular to the film&#39;s surface to allow high piezoelectric performance whereas the grains&#39; a- or b-axis are randomly distributed. This peculiar grain distribution works well when the piezoelectric film&#39;s thickness is around 1 um and above, which is the perfect thickness for bulk acoustic wave (BAW) filters operating at frequencies ranging from 1 to 3 GHz. However, the quality of the polycrystalline piezoelectric films degrades quickly as the thicknesses decrease below around 0.5 um, which is required for resonators and filters operating at frequencies around 5 GHz and above. 
     Single crystalline or epitaxial piezoelectric thin films grown on compatible crystalline substrates exhibit good crystalline quality and high piezoelectric performance even down to very thin thicknesses, e.g., 0.4 um. The present invention provides manufacturing processes and structures for high quality bulk acoustic wave resonators with single crystalline or epitaxial piezoelectric thin films for high frequency BAW filter applications. 
     BAWRs require a piezoelectric material, e.g., AlN, in crystalline form, i.e., polycrystalline or single crystalline. The quality of the film heavy depends on the chemical, crystalline, or topographical quality of the layer on which the film is grown. In conventional BAWR processes (including film bulk acoustic resonator (FBAR) or solidly mounted resonator (SMR) geometry), the piezoelectric film is grown on a patterned bottom electrode, which is usually made of molybdenum (Mo), tungsten (W), or ruthenium (Ru). The surface geometry of the patterned bottom electrode significantly influences the crystalline orientation and crystalline quality of the piezoelectric film, requiring complicated modification of the structure. 
     Thus, the present invention uses single crystalline piezoelectric films and thin film transfer processes to produce a BAWR with enhanced ultimate quality factor and electro-mechanical coupling for RF filters. Such methods and structures facilitate methods of manufacturing and structures for RF filters using single crystalline or epitaxial piezoelectric films to meet the growing demands of contemporary data communication. 
     In an example, the present invention provides transfer structures and processes for acoustic resonator devices, which provides a flat, high-quality, single-crystal piezoelectric film for superior acoustic wave control and high Q in high frequency. As described above, polycrystalline piezoelectric layers limit Q in high frequency. Also, growing epitaxial piezoelectric layers on patterned electrodes affects the crystalline orientation of the piezoelectric layer, which limits the ability to have tight boundary control of the resulting resonators. Embodiments of the present invention, as further described below, can overcome these limitations and exhibit improved performance and cost-efficiency. 
       FIGS. 16A-16C  through  FIGS. 31A-31C  illustrate a method of fabrication for an acoustic resonator device using a transfer structure with a sacrificial layer. In these figure series described below, the “A” figures show simplified diagrams illustrating top cross-sectional views of single crystal resonator devices according to various embodiments of the present invention. The “B” figures show simplified diagrams illustrating lengthwise cross-sectional views of the same devices in the “A” figures. Similarly, the “C” figures show simplified diagrams illustrating widthwise cross-sectional views of the same devices in the “A” figures. In some cases, certain features are omitted to highlight other features and the relationships between such features. Those of ordinary skill in the art will recognize variations, modifications, and alternatives to the examples shown in these figure series. 
       FIGS. 16A-16C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a piezoelectric film  1620  overlying a growth substrate  1610 . In an example, the growth substrate  1610  can include silicon (S), silicon carbide (SiC), or other like materials. The piezoelectric film  1620  can be an epitaxial film including aluminum nitride (AlN), gallium nitride (GaN), or other like materials. Additionally, this piezoelectric substrate can be subjected to a thickness trim. 
       FIGS. 17A-17C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a first electrode  1710  overlying the surface region of the piezoelectric film  1620 . In an example, the first electrode  1710  can include molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials. In a specific example, the first electrode  1710  can be subjected to a dry etch with a slope. As an example, the slope can be about 60 degrees. 
       FIGS. 18A-18C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a first passivation layer  1810  overlying the first electrode  1710  and the piezoelectric film  1620 . In an example, the first passivation layer  1810  can include silicon nitride (SiN), silicon oxide (SiOx), or other like materials. In a specific example, the first passivation layer  1810  can have a thickness ranging from about 50 nm to about 100 nm. 
       FIGS. 19A-19C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a sacrificial layer  1910  overlying a portion of the first electrode  1810  and a portion of the piezoelectric film  1620 . In an example, the sacrificial layer  1910  can include polycrystalline silicon (poly-Si), amorphous silicon (a-Si), or other like materials. In a specific example, this sacrificial layer  1910  can be subjected to a dry etch with a slope and be deposited with a thickness of about 1 um. Further, phosphorous doped SiO 2  (PSG) can be used as the sacrificial layer with different combinations of support layer (e.g., SiNx). 
       FIGS. 20A-20C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a support layer  2010  overlying the sacrificial layer  1910 , the first electrode  1710 , and the piezoelectric film  1620 . In an example, the support layer  2010  can include silicon dioxide (SiO 2 ), silicon nitride (SiN), or other like materials. In a specific example, this support layer  2010  can be deposited with a thickness of about 2-3 um. As described above, other support layers (e.g., SiNx) can be used in the case of a PSG sacrificial layer. 
       FIGS. 21A-21C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of polishing the support layer  2010  to form a polished support layer  2011 . In an example, the polishing process can include a chemical-mechanical planarization process or the like. 
       FIGS. 22A-22C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate flipping the device and physically coupling overlying the support layer  2011  overlying a bond substrate  2210 . In an example, the bond substrate  2210  can include a bonding support layer  2220  (SiO 2  or like material) overlying a substrate having silicon (Si), sapphire (Al 2 O 3 ), silicon dioxide (SiO 2 ), silicon carbide (SiC), or other like materials. In a specific embodiment, the bonding support layer  2220  of the bond substrate  2210  is physically coupled to the polished support layer  2011 . Further, the physical coupling process can include a room temperature bonding process followed by a 300 degree Celsius annealing process. 
       FIGS. 23A-23C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of removing the growth substrate  1610  or otherwise the transfer of the piezoelectric film  1620 . In an example, the removal process can include a grinding process, a blanket etching process, a film transfer process, an ion implantation transfer process, a laser crack transfer process, or the like and combinations thereof. 
       FIGS. 24A-24C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming an electrode contact via  2410  within the piezoelectric film  1620  (becoming piezoelectric film  1621 ) overlying the first electrode  1710  and forming one or more release holes  2420  within the piezoelectric film  1620  and the first passivation layer  1810  overlying the sacrificial layer  1910 . The via forming processes can include various types of etching processes. 
       FIGS. 25A-25C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a second electrode  2510  overlying the piezoelectric film  1621 . In an example, the formation of the second electrode  2510  includes depositing molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials; and then etching the second electrode  2510  to form an electrode cavity  2511  and to remove portion  2511  from the second electrode to form a top metal  2520 . Further, the top metal  2520  is physically coupled to the first electrode  1720  through electrode contact via  2410 . 
       FIGS. 26A-26C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a first contact metal  2610  overlying a portion of the second electrode  2510  and a portion of the piezoelectric film  1621 , and forming a second contact metal  2611  overlying a portion of the top metal  2520  and a portion of the piezoelectric film  1621 . In an example, the first and second contact metals can include gold (Au), aluminum (Al), copper (Cu), nickel (Ni), aluminum bronze (AlCu), or related alloys of these materials or other like materials. 
       FIGS. 27A-27C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a second passivation layer  2710  overlying the second electrode  2510 , the top metal  2520 , and the piezoelectric film  1621 . In an example, the second passivation layer  2710  can include silicon nitride (SiN), silicon oxide (SiOx), or other like materials. In a specific example, the second passivation layer  2710  can have a thickness ranging from about 50 nm to about 100 nm. 
       FIGS. 28A-28C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of removing the sacrificial layer  1910  to form an air cavity  2810 . In an example, the removal process can include a poly-Si etch or an a-Si etch, or the like. 
       FIGS. 29A-29C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to another example of the present invention. As shown, these figures illustrate the method step of processing the second electrode  2510  and the top metal  2520  to form a processed second electrode  2910  and a processed top metal  2920 . This step can follow the formation of second electrode  2510  and top metal  2520 . In an example, the processing of these two components includes depositing molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials; and then etching (e.g., dry etch or the like) this material to form the processed second electrode  2910  with an electrode cavity  2912  and the processed top metal  2920 . The processed top metal  2920  remains separated from the processed second electrode  2910  by the removal of portion  2911 . In a specific example, the processed second electrode  2910  is characterized by the addition of an energy confinement structure configured on the processed second electrode  2910  to increase Q. 
       FIGS. 30A-30C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to another example of the present invention. As shown, these figures illustrate the method step of processing the first electrode  1710  to form a processed first electrode  2310 . This step can follow the formation of first electrode  1710 . In an example, the processing of these two components includes depositing molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials; and then etching (e.g., dry etch or the like) this material to form the processed first electrode  3010  with an electrode cavity, similar to the processed second electrode  2910 . Air cavity  2811  shows the change in cavity shape due to the processed first electrode  3010 . In a specific example, the processed first electrode  3010  is characterized by the addition of an energy confinement structure configured on the processed second electrode  3010  to increase Q. 
       FIGS. 31A-31C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to another example of the present invention. As shown, these figures illustrate the method step of processing the first electrode  1710 , to form a processed first electrode  2310 , and the second electrode  2510 /top metal  2520  to form a processed second electrode  2910 /processed top metal  2920 . These steps can follow the formation of each respective electrode, as described for  FIGS. 29A-29C and 30A-30C . Those of ordinary skill in the art will recognize other variations, modifications, and alternatives. 
       FIGS. 32A-32C  through  FIGS. 46A-46C  illustrate a method of fabrication for an acoustic resonator device using a transfer structure without sacrificial layer. In these figure series described below, the “A” figures show simplified diagrams illustrating top cross-sectional views of single crystal resonator devices according to various embodiments of the present invention. The “B” figures show simplified diagrams illustrating lengthwise cross-sectional views of the same devices in the “A” figures. Similarly, the “C” figures show simplified diagrams illustrating widthwise cross-sectional views of the same devices in the “A” figures. In some cases, certain features are omitted to highlight other features and the relationships between such features. Those of ordinary skill in the art will recognize variations, modifications, and alternatives to the examples shown in these figure series. 
       FIGS. 32A-32C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a piezoelectric film  3220  overlying a growth substrate  3210 . In an example, the growth substrate  3210  can include silicon (S), silicon carbide (SiC), or other like materials. The piezoelectric film  3220  can be an epitaxial film including aluminum nitride (AlN), gallium nitride (GaN), or other like materials. Additionally, this piezoelectric substrate can be subjected to a thickness trim. 
       FIGS. 33A-33C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a first electrode  3310  overlying the surface region of the piezoelectric film  3220 . In an example, the first electrode  3310  can include molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials. In a specific example, the first electrode  3310  can be subjected to a dry etch with a slope. As an example, the slope can be about 60 degrees. 
       FIGS. 34A-34C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a first passivation layer  3410  overlying the first electrode  3310  and the piezoelectric film  3220 . In an example, the first passivation layer  3410  can include silicon nitride (SiN), silicon oxide (SiOx), or other like materials. In a specific example, the first passivation layer  3410  can have a thickness ranging from about 50 nm to about 100 nm. 
       FIGS. 35A-35C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a support layer  3510  overlying the first electrode  3310 , and the piezoelectric film  3220 . In an example, the support layer  3510  can include silicon dioxide (SiO 2 ), silicon nitride (SiN), or other like materials. In a specific example, this support layer  3510  can be deposited with a thickness of about 2-3 um. As described above, other support layers (e.g., SiNx) can be used in the case of a PSG sacrificial layer. 
       FIGS. 36A-36C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the optional method step of processing the support layer  3510  (to form support layer  3511 ) in region  3610 . In an example, the processing can include a partial etch of the support layer  3510  to create a flat bond surface. In a specific example, the processing can include a cavity region. In other examples, this step can be replaced with a polishing process such as a chemical-mechanical planarization process or the like. 
       FIGS. 37A-37C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming an air cavity  3710  within a portion of the support layer  3511  (to form support layer  3512 ). In an example, the cavity formation can include an etching process that stops at the first passivation layer  3410 . 
       FIGS. 38A-38C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming one or more cavity vent holes  3810  within a portion of the piezoelectric film  3220  through the first passivation layer  3410 . In an example, the cavity vent holes  3810  connect to the air cavity  3710 . 
       FIGS. 39A-39C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate flipping the device and physically coupling overlying the support layer  3512  overlying a bond substrate  3910 . In an example, the bond substrate  3910  can include a bonding support layer  3920  (SiO 2  or like material) overlying a substrate having silicon (Si), sapphire (Al 2 O 3 ), silicon dioxide (SiO 2 ), silicon carbide (SiC), or other like materials. In a specific embodiment, the bonding support layer  3920  of the bond substrate  3910  is physically coupled to the polished support layer  3512 . Further, the physical coupling process can include a room temperature bonding process followed by a 300 degree Celsius annealing process. 
       FIGS. 40A-40C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of removing the growth substrate  3210  or otherwise the transfer of the piezoelectric film  3220 . In an example, the removal process can include a grinding process, a blanket etching process, a film transfer process, an ion implantation transfer process, a laser crack transfer process, or the like and combinations thereof. 
       FIGS. 41A-41C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming an electrode contact via  4110  within the piezoelectric film  3220  overlying the first electrode  3310 . The via forming processes can include various types of etching processes. 
       FIGS. 42A-42C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a second electrode  4210  overlying the piezoelectric film  3220 . In an example, the formation of the second electrode  4210  includes depositing molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials; and then etching the second electrode  4210  to form an electrode cavity  4211  and to remove portion  4211  from the second electrode to form a top metal  4220 . Further, the top metal  4220  is physically coupled to the first electrode  3310  through electrode contact via  4110 . 
       FIGS. 43A-43C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a first contact metal  4310  overlying a portion of the second electrode  4210  and a portion of the piezoelectric film  3220 , and forming a second contact metal  4311  overlying a portion of the top metal  4220  and a portion of the piezoelectric film  3220 . In an example, the first and second contact metals can include gold (Au), aluminum (Al), copper (Cu), nickel (Ni), aluminum bronze (AlCu), or other like materials. This figure also shows the method step of forming a second passivation layer  4320  overlying the second electrode  4210 , the top metal  4220 , and the piezoelectric film  3220 . In an example, the second passivation layer  4320  can include silicon nitride (SiN), silicon oxide (SiOx), or other like materials. In a specific example, the second passivation layer  4320  can have a thickness ranging from about 50 nm to about 100 nm. 
       FIGS. 44A-44C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process for single crystal acoustic resonator devices according to another example of the present invention. As shown, these figures illustrate the method step of processing the second electrode  4210  and the top metal  4220  to form a processed second electrode  4410  and a processed top metal  4420 . This step can follow the formation of second electrode  4210  and top metal  4220 . In an example, the processing of these two components includes depositing molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials; and then etching (e.g., dry etch or the like) this material to form the processed second electrode  4410  with an electrode cavity  4412  and the processed top metal  4420 . The processed top metal  4420  remains separated from the processed second electrode  4410  by the removal of portion  4411 . In a specific example, the processed second electrode  4410  is characterized by the addition of an energy confinement structure configured on the processed second electrode  4410  to increase Q. 
       FIGS. 45A-45C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to another example of the present invention. As shown, these figures illustrate the method step of processing the first electrode  3310  to form a processed first electrode  4510 . This step can follow the formation of first electrode  3310 . In an example, the processing of these two components includes depositing molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials; and then etching (e.g., dry etch or the like) this material to form the processed first electrode  4510  with an electrode cavity, similar to the processed second electrode  4410 . Air cavity  3711  shows the change in cavity shape due to the processed first electrode  4510 . In a specific example, the processed first electrode  4510  is characterized by the addition of an energy confinement structure configured on the processed second electrode  4510  to increase Q. 
       FIGS. 46A-46C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process using a sacrificial layer for single crystal acoustic resonator devices according to another example of the present invention. As shown, these figures illustrate the method step of processing the first electrode  3310 , to form a processed first electrode  4510 , and the second electrode  4210 /top metal  4220  to form a processed second electrode  4410 /processed top metal  4420 . These steps can follow the formation of each respective electrode, as described for  FIGS. 44A-44C  and  45 A- 45 C. Those of ordinary skill in the art will recognize other variations, modifications, and alternatives. 
       FIGS. 47A-47C  through  FIGS. 59A-59C  illustrate a method of fabrication for an acoustic resonator device using a transfer structure with a multilayer mirror structure. In these figure series described below, the “A” figures show simplified diagrams illustrating top cross-sectional views of single crystal resonator devices according to various embodiments of the present invention. The “B” figures show simplified diagrams illustrating lengthwise cross-sectional views of the same devices in the “A” figures. Similarly, the “C” figures show simplified diagrams illustrating widthwise cross-sectional views of the same devices in the “A” figures. In some cases, certain features are omitted to highlight other features and the relationships between such features. Those of ordinary skill in the art will recognize variations, modifications, and alternatives to the examples shown in these figure series. 
       FIGS. 47A-47C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a piezoelectric film  4720  overlying a growth substrate  4710 . In an example, the growth substrate  4710  can include silicon (S), silicon carbide (SiC), or other like materials. The piezoelectric film  4720  can be an epitaxial film including aluminum nitride (AlN), gallium nitride (GaN), or other like materials. Additionally, this piezoelectric substrate can be subjected to a thickness trim. 
       FIGS. 48A-48C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a first electrode  4810  overlying the surface region of the piezoelectric film  4720 . In an example, the first electrode  4810  can include molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials. In a specific example, the first electrode  4810  can be subjected to a dry etch with a slope. As an example, the slope can be about 60 degrees. 
       FIGS. 49A-49C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a multilayer mirror or reflector structure. In an example, the multilayer mirror includes at least one pair of layers with a low impedance layer  4910  and a high impedance layer  4920 . In  FIGS. 49A-49C , two pairs of low/high impedance layers are shown (low:  4910  and  4911 ; high:  4920  and  4921 ). In an example, the mirror/reflector area can be larger than the resonator area and can encompass the resonator area. In a specific embodiment, each layer thickness is about ¼ of the wavelength of an acoustic wave at a targeting frequency. The layers can be deposited in sequence and be etched afterwards, or each layer can be deposited and etched individually. In another example, the first electrode  4810  can be patterned after the mirror structure is patterned. 
       FIGS. 50A-50C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a support layer  5010  overlying the mirror structure (layers  4910 ,  4911 ,  4920 , and  4921 ), the first electrode  4810 , and the piezoelectric film  4720 . In an example, the support layer  5010  can include silicon dioxide (SiO 2 ), silicon nitride (SiN), or other like materials. In a specific example, this support layer  5010  can be deposited with a thickness of about 2-3 um. As described above, other support layers (e.g., SiNx) can be used. 
       FIGS. 51A-51C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of polishing the support layer  5010  to form a polished support layer  5011 . In an example, the polishing process can include a chemical-mechanical planarization process or the like. 
       FIGS. 52A-52C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate flipping the device and physically coupling overlying the support layer  5011  overlying a bond substrate  5210 . In an example, the bond substrate  5210  can include a bonding support layer  5220  (SiO 2  or like material) overlying a substrate having silicon (Si), sapphire (Al 2 O 3 ), silicon dioxide (SiO 2 ), silicon carbide (SiC), or other like materials. In a specific embodiment, the bonding support layer  5220  of the bond substrate  5210  is physically coupled to the polished support layer  5011 . Further, the physical coupling process can include a room temperature bonding process followed by a 300 degree Celsius annealing process. 
       FIGS. 53A-53C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of removing the growth substrate  4710  or otherwise the transfer of the piezoelectric film  4720 . In an example, the removal process can include a grinding process, a blanket etching process, a film transfer process, an ion implantation transfer process, a laser crack transfer process, or the like and combinations thereof. 
       FIGS. 54A-54C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming an electrode contact via  5410  within the piezoelectric film  4720  overlying the first electrode  4810 . The via forming processes can include various types of etching processes. 
       FIGS. 55A-55C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a second electrode  5510  overlying the piezoelectric film  4720 . In an example, the formation of the second electrode  5510  includes depositing molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials; and then etching the second electrode  5510  to form an electrode cavity  5511  and to remove portion  5511  from the second electrode to form a top metal  5520 . Further, the top metal  5520  is physically coupled to the first electrode  5520  through electrode contact via  5410 . 
       FIGS. 56A-56C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to an example of the present invention. As shown, these figures illustrate the method step of forming a first contact metal  5610  overlying a portion of the second electrode  5510  and a portion of the piezoelectric film  4720 , and forming a second contact metal  5611  overlying a portion of the top metal  5520  and a portion of the piezoelectric film  4720 . In an example, the first and second contact metals can include gold (Au), aluminum (Al), copper (Cu), nickel (Ni), aluminum bronze (AlCu), or other like materials. This figure also shows the method step of forming a second passivation layer  5620  overlying the second electrode  5510 , the top metal  5520 , and the piezoelectric film  4720 . In an example, the second passivation layer  5620  can include silicon nitride (SiN), silicon oxide (SiOx), or other like materials. In a specific example, the second passivation layer  5620  can have a thickness ranging from about 50 nm to about 100 nm. 
       FIGS. 57A-57C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to another example of the present invention. As shown, these figures illustrate the method step of processing the second electrode  5510  and the top metal  5520  to form a processed second electrode  5710  and a processed top metal  5720 . This step can follow the formation of second electrode  5710  and top metal  5720 . In an example, the processing of these two components includes depositing molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials; and then etching (e.g., dry etch or the like) this material to form the processed second electrode  5410  with an electrode cavity  5712  and the processed top metal  5720 . The processed top metal  5720  remains separated from the processed second electrode  5710  by the removal of portion  5711 . In a specific example, this processing gives the second electrode and the top metal greater thickness while creating the electrode cavity  5712 . In a specific example, the processed second electrode  5710  is characterized by the addition of an energy confinement structure configured on the processed second electrode  5710  to increase Q. 
       FIGS. 58A-58C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to another example of the present invention. As shown, these figures illustrate the method step of processing the first electrode  4810  to form a processed first electrode  5810 . This step can follow the formation of first electrode  4810 . In an example, the processing of these two components includes depositing molybdenum (Mo), ruthenium (Ru), tungsten (W), or other like materials; and then etching (e.g., dry etch or the like) this material to form the processed first electrode  5810  with an electrode cavity, similar to the processed second electrode  5710 . Compared to the two previous examples, there is no air cavity. In a specific example, the processed first electrode  5810  is characterized by the addition of an energy confinement structure configured on the processed second electrode  5810  to increase Q. 
       FIGS. 59A-59C  are simplified diagrams illustrating various cross-sectional views of a single crystal acoustic resonator device and of method steps for a transfer process with a multilayer mirror for single crystal acoustic resonator devices according to another example of the present invention. As shown, these figures illustrate the method step of processing the first electrode  4810 , to form a processed first electrode  5810 , and the second electrode  5510 /top metal  5520  to form a processed second electrode  5710 /processed top metal  5720 . These steps can follow the formation of each respective electrode, as described for  FIGS. 57A-57C and 58A-58C . Those of ordinary skill in the art will recognize other variations, modifications, and alternatives. 
     In each of the preceding examples relating to transfer processes, energy confinement structures can be formed on the first electrode, second electrode, or both. In an example, these energy confinement structures are mass loaded areas surrounding the resonator area. The resonator area is the area where the first electrode, the piezoelectric layer, and the second electrode overlap. The larger mass load in the energy confinement structures lowers a cut-off frequency of the resonator. The cut-off frequency is the lower or upper limit of the frequency at which the acoustic wave can propagate in a direction parallel to the surface of the piezoelectric film. Therefore, the cut-off frequency is the resonance frequency in which the wave is travelling along the thickness direction and thus is determined by the total stack structure of the resonator along the vertical direction. In piezoelectric films (e.g., AlN), acoustic waves with lower frequency than the cut-off frequency can propagate in a parallel direction along the surface of the film, i.e., the acoustic wave exhibits a high-band-cut-off type dispersion characteristic. In this case, the mass loaded area surrounding the resonator provides a barrier preventing the acoustic wave from propagating outside the resonator. By doing so, this feature increases the quality factor of the resonator and improves the performance of the resonator and, consequently, the filter. 
     In addition, the top single crystalline piezoelectric layer can be replaced by a polycrystalline piezoelectric film. In such films, the lower part that is close to the interface with the substrate has poor crystalline quality with smaller grain sizes and a wider distribution of the piezoelectric polarization orientation than the upper part of the film close to the surface. This is due to the polycrystalline growth of the piezoelectric film, i.e., the nucleation and initial film have random crystalline orientations. Considering AlN as a piezoelectric material, the growth rate along the c-axis or the polarization orientation is higher than other crystalline orientations that increase the proportion of the grains with the c-axis perpendicular to the growth surface as the film grows thicker. In a typical polycrystalline AlN film with about a  1  um thickness, the upper part of the film close to the surface has better crystalline quality and better alignment in terms of piezoelectric polarization. By using the thin film transfer process contemplated in the present invention, it is possible to use the upper portion of the polycrystalline film in high frequency BAW resonators with very thin piezoelectric films. This can be done by removing a portion of the piezoelectric layer during the growth substrate removal process. Of course, there can be other variations, modifications, and alternatives. 
     While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. As an example, the packaged device can include any combination of elements described above, as well as outside of the present specification. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.