Patent Publication Number: US-2023155563-A1

Title: Atomic layer deposition in acoustic wave resonators

Description:
BACKGROUND 
     The piezoelectric effect is exhibited by certain materials, and it is related to the electromechanical interaction between the mechanical and electrical states in the materials. Materials that exhibit the piezoelectric effect also exhibit the reverse piezoelectric effect. For example, lead zirconate titanate crystals generate piezoelectricity (i.e., generate an electric field potential) when mechanical forces are applied to deform the shape of the crystals. Lead zirconate titanate crystals will also deform or change in shape when an external electric field is applied to the crystals. 
     Piezoelectric materials can be categorized as either crystalline, ceramic, or polymeric materials. Lead zirconate titanate, barium titanate, and lead titanate are examples of piezoelectric ceramics materials. Certain semiconducting piezoelectric materials are compatible with semiconductor devices and integrated circuits. Gallium nitride and zinc oxide, among others, are examples of piezoelectric materials that are compatible with semiconductor devices and integrated circuits. 
     The electromechanical coupling coefficient of a piezoelectric material is a metric of the conversion efficiency between the electric and mechanical energy in the piezoelectric material. The electromechanical coupling coefficient can include parameters, such as the surfaces upon which electric potential is applied or formed and the direction along which mechanical energy is applied or developed in the material. 
     SUMMARY 
     Various examples of the use of atomic layer deposition in the manufacture of semiconductor devices, and particularly acoustic wave resonators, are described, along with a number of new acoustic wave resonator devices incorporating one or more layers of material deposited using atomic layer deposition. In one example, a method of manufacturing an acoustic resonator includes providing a substrate, depositing a layer of piezoelectric material over the substrate by atomic layer deposition, and forming an electrode in contact with the layer of piezoelectric material. 
     In certain aspects of the embodiments, the electrode is a first electrode, and the method also includes forming a second electrode in contact with the piezoelectric material. The first electrode and the second electrode are formed by sputtering metal in one example. In another example, the first electrode is formed by atomic layer deposition of metal, and the second electrode is formed by sputtering metal. In still another example, the first electrode and the second electrode are both formed by atomic layer deposition of metal. 
     In other aspects, the first electrode and the second electrode are both formed at over the layer of piezoelectric material in a stack of material layers of the acoustic resonator. In another case, the first electrode is formed under the layer of piezoelectric material and the second electrode is formed over the layer of piezoelectric material in the stack of material layers of the acoustic resonator. 
     In other aspects, the method also includes forming an acoustic reflector over the substrate, between the substrate and the layer of piezoelectric material. The reflector includes a plurality of layers of material. The layers include alternating layers of material having varying refractive indexes. In one case, the method also includes forming a supporting layer over the substrate, between the substrate and the layer of piezoelectric material. The method can also include forming an air cavity in the substrate in a region below the piezoelectric material. The cavity includes a plurality of supporting pillars in one example. 
     In still other aspects, the method also includes, after depositing the layer of piezoelectric material by atomic layer deposition, trimming the layer of piezoelectric material. The method can also include forming an encapsulation layer over the electrode by atomic layer deposition. The piezoelectric material comprises aluminum nitride in one case, although other types of piezoelectric material can be relied upon. 
     An acoustic resonator is described in another example. The acoustic resonator includes a substrate, a layer of piezoelectric material deposited over the substrate by atomic layer deposition, and an electrode in contact with the layer of piezoelectric material. The layer of piezoelectric material includes a layer of aluminum nitride that is equal to or less than 100 nm in thickness in one example. The acoustic resonator also includes a second electrode in contact with the piezoelectric material in some cases. At least one of the electrode, the second electrode, or both electrodes are formed by atomic layer deposition of metal in one example. The acoustic resonator can include an acoustic reflector over the substrate, between the substrate and the layer of piezoelectric material. The acoustic reflector can also include a supporting layer over the substrate, between the substrate and the layer of piezoelectric material, among other layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure can be better understood with reference to the following drawings. It is noted that the elements in the drawings are not necessarily to scale, with emphasis instead being placed upon clearly illustrating the principles of the embodiments. 
         FIG.  1    is a perspective view of an example bulk acoustic wave (BAW) resonator structure according to various embodiments described herein. 
         FIG.  2    is a perspective view of an example surface acoustic wave (SAW) resonator structure according to various embodiments described herein. 
         FIG.  3    is a cross-sectional view of an example solidly mounted bulk acoustic wave resonator structure according to various embodiments described herein. 
         FIG.  4    illustrates an example method of manufacture of the resonator structure shown in  FIG.  3    according to various embodiments described herein. 
         FIG.  5    is a cross-sectional view of an example thin-film bulk acoustic resonator according to various embodiments described herein. 
         FIG.  6    illustrates an example method of manufacture of the resonator structure shown in  FIG.  5    according to various embodiments described herein. 
         FIG.  7    illustrates an example method of manufacture of the SAW resonator shown in  FIG.  2    according to various embodiments described herein. 
         FIG.  8    is a cross-sectional view of an example laterally-excited bulk acoustic wave resonator according to various embodiments described herein. 
         FIG.  9    is a cross-sectional view of another example laterally-excited bulk acoustic wave resonator according to various embodiments described herein. 
         FIG.  10    illustrates an example method of manufacture of the resonator structures shown in  FIGS.  8  and  9    according to various embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     An acoustic resonator can be formed as a structure including a layer of piezoelectric material with electrodes in contact with one or more surfaces of the piezoelectric material. Characteristics for high performance acoustic resonators include accurate frequency response, high quality factor, high piezoelectric coupling or bandwidth, and small temperature coefficient of frequency, among others. 
     Different types and structures of acoustic resonators have been relied upon as oscillators, radio frequency (RF) filters, duplexers, and transformers in electric circuits, as components in micro-electromechanical systems (MEMS), and for other purposes. Examples of acoustic resonators include bulk acoustic wave (BAW) resonators and surface acoustic wave (SAW) resonators. Examples of BAW resonators include solidly mounted resonators (SMR) and thin-film bulk acoustic resonators (FBAR) as described in further detail below. Like SAW resonators, the operation of BAW resonators is based on the piezoelectric effect exhibited by the layer of piezoelectric material. 
     A number of different materials can be relied upon as the piezoelectric material in a BAW or SAW. As one example, zinc oxide (ZnO) is a relatively common piezoelectric material for high-frequency FBAR structures. For some material processing techniques, the stoichiometry of two-compound materials, such as ZnO, can be easier to control as compared to three-compound materials, when manufactured by thin film methods. Relatively thin layers of piezoelectric materials have been formed by sol-gel wet-chemical techniques, sputtering, pulsed laser deposition, and other techniques. 
     Today, many cellular communications devices include duplexers, filters, and other RF circuits including one or more acoustic resonators. Cellular communications devices can include several such RF circuits, and acoustic resonators are being adopted and relied upon at a larger scale due to the increased complexity of radio frequency front end electronics. A common application of acoustic wave structures, for example, is in RF filters for cellular phones, global positioning systems, Wi-Fi® systems, and other systems that rely upon RF signals for data communications. Such RF filters are often formed using a network of acoustic resonators, in a ladder, lattice, or stacked topology, and are designed to prevent the transmission of certain frequencies or frequency bands and to permit the reception of certain frequencies or frequency bands. BAW filter technology is complementing SAW filter technology in areas where increased power handling capability is needed. Further, BAW structures can be manufactured on silicon substrates in high volumes and are widely supported by current semiconductor device fabrication methods. 
     Advancements are needed in acoustic resonator technology, however, as new applications will rely upon even higher frequencies in the RF spectrum for communications. Newer communications devices and standards demand components capable of suitable operation at higher frequencies, with less variation in characteristic response over wide ranges of temperature and power levels. 
     In the context outlined above, aspects of acoustic resonators and methods of manufacture of acoustic resonators are described, including acoustic resonators with thinner layers of piezoelectric material, thinner electrode layers, and other features that facilitate higher performance. In one example, a method of manufacturing an acoustic resonator includes providing a substrate, depositing a layer of piezoelectric material over the substrate by atomic layer deposition (ALD), and forming an electrode in contact with the layer of piezoelectric material. ALD is used to deposit highly uniform and conformal thin films of piezoelectric material and, in some cases, electrodes and encapsulation layers. The acoustic resonators described herein are better suited for the demands of new RF filters, duplexers, transformers, and other components in front-end radio electronics and other applications. 
     Turning to the drawings,  FIG.  1    illustrates an example BAW resonator  10  according to various embodiments described herein. The illustration of the BAW resonator  10  is representative in  FIG.  1   . The positions, shapes, dimensions, and relative sizes of the layers and features of the BAW resonator  10  are not necessarily drawn to scale in  FIG.  1   . Example dimensions of the BAW resonator  10  are provided below, but the dimensions of the BAW resonator  10  are not specifically limited. The layers and other features shown in  FIG.  1    are also not exhaustive, and the BAW resonator  10  can include other layers, features, and elements that are not separately illustrated. Additionally, the BAW resonator  10  can be formed as part of a larger integrated structure or circuit in combination with other devices and circuit elements. Such a larger integrated structure may include several resonators similar to the BAW resonator  10 , among other integrated components. 
     The BAW resonator  10  includes a substrate  11 , an intermediate region  12  over the substrate  11 , a layer of piezoelectric material  13  over the substrate  11 , a first electrode  14 , and a second electrode  15 . The first electrode  14  is in contact with a first surface (i.e., bottom surface) of the piezoelectric material  13  and positioned at least in part under the piezoelectric material  13 , between the piezoelectric material  13  and the substrate  11 . The second electrode  15  is in contact with a second surface (i.e., top surface) of the piezoelectric material  13  and positioned at least in part over the piezoelectric material  13 . The BAW resonator  10  can also include additional layers described below but not illustrated in  FIG.  1   , such as a temperature compensation layer, an encapsulation layer, and other others. 
     As depicted in  FIG.  1   , the BAW resonator  10  can be embodied as a solidly mounted resonator, a thin-film bulk acoustic resonator, or a related type of BAW resonator. An example solidly mounted resonator (SMR) is described in greater detail below with reference to  FIG.  3   , and an example thin-film bulk acoustic resonator (FBAR) is described in greater detail below with reference to  FIG.  5   . For an SMR, the intermediate region  12  can be embodied as an acoustic mirror or reflector, such as a Bragg reflector, as further described below. For an FBAR, the intermediate region  12  can be embodied as a supporting layer of silicon or other material, and the FBAR can also include a cavity or opening under the piezoelectric material  13  for acoustic wave isolation. In either case, due to the piezoelectric properties of the layer of piezoelectric material  13  and the structural arrangement of the BAW resonator  10 , the BAW resonator  10  can generate a bulk acoustic or mechanical wave when an alternating electric potential input signal, such as an RF input, is applied across the electrodes  14  and  15 . The bulk acoustic or mechanical wave can travel or translate in the “Z” direction down into the BAW resonator  10 , as shown in  FIG.  1   , in the direction the thickness of the piezoelectric material  13  is measured. 
     The substrate  11  can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. A silicon substrate may be preferred as being relatively low-cost, scalable for manufacturing, and compatible with manufacturing and processing steps, but other substrates can be relied upon. As noted above, the intermediate region  12  over the substrate  11  can be embodied as an acoustic mirror or a supporting layer, depending upon the type of resonator formed. Examples of the intermediate region  12  are described below with reference to  FIGS.  3  and  5   . 
     The electrodes  14  and  15  can be embodied as layers of highly conductive material, such as a metal or metal alloy, including copper, silver, gold, molybdenum, tungsten, titanium, platinum, or aluminum, alloys thereof, and other conductive layers. One factor in the material selection for the electrodes  14  and  15  is the desired thickness of the electrodes  14  and  15 , which is also a factor in the response characteristics of the BAW resonator  10 . Considerations in the selection of the conductive material for the electrodes  14  and  15  and the manner of forming the electrodes  14  and  15  are described below. 
     The layer of piezoelectric material  13  can be embodied as a layer of lead zirconate titanate (PZT), barium strontium titanate (BST), barium titanate (BaTiO3), aluminum scandium nitride (AlScN), aluminum nitride (AlN), ZnO, or another piezoelectric material. Despite the lower electromechanical coupling coefficient compared to ZnO, AlN has a wider band gap and is compatible with the silicon integrated circuit technology used in FBAR and other structures. AlN is also compatible with the ALD processing techniques as described herein. Thus, in one preferred embodiment, the layer of piezoelectric material  13  is a layer of AlN, although other piezoelectric materials can be relied upon. In some cases, the layer of piezoelectric material  13  can include one or more layers of piezoelectric material, including two or more different types of piezoelectric material (e.g., a stack of two or more of layers of AlN, PZT, BST, BaTiO3, and AlScN), each of which can be formed using ALD processing techniques according to the embodiments. The piezoelectric material  13  can also include a layer of AlN having a certain crystal orientation. In various embodiments, the layer of AlN can be formed to have a crystal structure c-axis orientation in the “X,” “Y,” or “Z” directions shown in  FIG.  1   . In one example, the layer of AlN can have a crystal structure c-axis orientation to excite a bulk acoustic or mechanical wave in the “Z” direction down into the BAW resonator  10 . 
     The operating characteristics of the BAW resonator  10 , including the frequency response, the accuracy of the frequency response, and the quality factor are determined in part by the thickness, conformity, and uniformity of the layer of piezoelectric material  13 . Various sputtering techniques have been relied upon to form layers of piezoelectric material having a thickness of greater than about 300 nm or more. Current sputtering techniques cannot be reliably used to form layers of piezoelectric material that are thinner (e.g., such as 200 nm, 150 nm, or 100 nm in thickness or thinner) and also uniform and conformal. Thus, according to one aspect of the embodiments, the layer of piezoelectric material  13  can be embodied as a layer of AlN deposited by ALD. Forming the layer of AlN using ALD rather than sol-gel wet-chemical techniques, sputtering, pulsed laser deposition, or other techniques can result in thinner, more uniform, and more conformal layers of piezoelectric material. 
     As noted above, ALD is a process for depositing highly uniform and conformal thin films of material. The ALD process deposits thin films of material on a surface by the exposure of the surface to two chemical reactants. In one example, the process can be started with an initiation of the surface for the deposit of materials. The initiation can include annealing the surface, etching the surface, exposing the surface to one or more gases, or other steps to remove contaminants from the surface or otherwise prepare the surface for the deposit of materials. 
     After initiation, ALD processes typically proceed with the exposure of a surface to two or more precursor chemicals or reactants, in a repeating sequence. The precursors can be gaseous species introduced to an ALD chamber separately over time. The precursors react with the surface, respectively in time, in a self-limiting way (i.e., until the finite number of sites for the reaction are exhausted). Excess or remaining reactant of a precursor is removed before the next precursor or ALD cycle is applied or repeated. ALD processes are characterized by both dose times, which are the times that the surface is exposed to a respective precursor, and purge times, which are the times between doses during which the ALD chamber is evacuated for the next step. For a two reactant ALD process, a first reactant dose, first purge, second reactant dose, and second purge are steps in one ALD cycle. 
     A thin film is slowly deposited on the surface by the repeated exposure of the surface to the precursors, separately over time, with intermediate flushing steps in ALD processes. There is a maximum amount of material that can be deposited on the surface in a single ALD cycle, and it is determined by the precursor-surface interaction. The overall thickness of the thin film can be determined by the number of ALD cycles used, and the number of cycles can be tailored to grow uniform and conformal layers of material at a certain thickness with very high precision, even on complex surfaces. ALD processes are often characterized by the growth of material per ALD cycle, in nanometers or another suitable metric. 
     According to aspects of the embodiments, ALD can be used to form the layer of piezoelectric material  13  in the BAW resonator  10 . The layer of piezoelectric material  13  can be formed as a thinner layer of piezoelectric material than in conventional BAW resonators, such as when sputtering is used. In one example, the layer of piezoelectric material  13  can be formed as an AlN layer of 100 nm in thickness or less by ALD. The layer of piezoelectric material  13  can be formed to be a thinner layer of AlN, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. The layer of piezoelectric material  13  is not limited to a layer of AlN, as other thin films of piezoelectric materials can be formed using ALD. 
     The specific thickness of the layer of piezoelectric material  13  can be highly controlled or tailored in the BAW resonator  10 , sometimes to within less than 10 nm of a target thickness, to within less than 5 nm of a target thickness, or even closer to a target thickness by using ALD. The layer of piezoelectric material  13  can be formed as thin as possible using a minimal number of ALD cycles in some embodiments. The use of ALD rather than sputtering to deposit the layer of piezoelectric material  13  offers a number of advantages. For example, the use of ALD reduces crystal damage in the layer of piezoelectric material  13 , reduces dangling bonds, and reduces surface traps commonly attributed to sputter or plasma damage, resulting in better resonator performance. 
     The crystal orientation of the layer of piezoelectric material  13  can also be selected, in some cases, to tailor the operating characteristics of the BAW resonator  10 . As one example, a thin film of ZnO having a crystal structure c-axis that is normal to (i.e., perpendicular to) a top surface of a supporting substrate excites longitudinal waves in the thin film. A thin film of ZnO having a crystal structure c-axis that is tilted to (i.e., not perpendicular to) a top surface of a supporting substrate can excite a shear or transversal wave in the thin film. It is also possible, depending on the crystal structure orientation, to excite a combination of both longitudinal and shear waves. Similar to a thin film of ZnO, the crystal orientation of a thin film of AlN also impacts the excitation of waves in the thin film and surrounding layers. 
     The crystal orientation of a thin film of piezoelectric material depends on various factors, including the materials processing techniques used to form the film or layer of material, the materials selected, the surface on which the film is grown or deposited, and the conditions in which the film is grown or deposited, such as the temperatures, pressures, gases, vacuum conditions, and other factors. The crystal orientation of the layer of piezoelectric material  13  in the BAW resonator  10  can be directed using one or more ALD processing steps according to aspects of the embodiments. For example, one or more initiation steps of the ALD process, and the ALD growth process itself, can be relied upon to direct the crystal structure c-axis orientation of the layer of piezoelectric material  13 . 
     In other aspects of the embodiments, the electrode  14  can be formed using ALD, the electrode  15  can be formed using ALD, or both the electrodes  14  and  15  can be formed using ALD. As one example, the electrodes  14  and  15  can be formed as thin copper layers using ALD, but other highly conductive layers can be formed using ALD. If ALD is used to form the electrodes  14  and  15 , the type of conductive material used to form the electrodes  14  and  15  can be determined in part based on the ALD process available for the material. ALD processes for depositing thin films of pure metal substances, such as pure copper, are known, although additional processes are known for depositing metal alloys. 
     As noted above, one factor in the material selection for the electrodes  14  and  15  is the desired thickness of the electrodes  14  and  15 , which is also a factor in the response characteristics of the BAW resonator  10 . For example, if highly conductive and thin electrodes  14  and  15  are desired, then ALD processing can be selected to deposit copper for the electrodes  14  and  15 . The electrodes  14  and  15  can also be formed from other metals and metal alloys using ALD processing steps. In other cases, the electrodes  14  and  15  can be formed by sputtering, physical vapor deposition (PVD), chemical vapor deposition (CVD), or related techniques to deposit the conductive material layers for the electrodes  14  and  15 . When using a sputtering or other process technique besides ALD, the electrodes  14  and  15  may be relatively thicker. The shapes, sizes, and positions of the electrodes  14  and  15  are representative in  FIG.  1   . The electrodes  14  and  15  can be formed to have any suitable shape and size. 
     The thickness of the BAW resonator  10  can be significantly reduced by using ALD to form the layer of piezoelectric material  13  as compared to sputtering. The overall thickness can be further reduced by using ALD to form one or both of the electrodes  14  and  15 . The uniformity and conformity of the layer of piezoelectric material  13  and the electrodes  14  and  15  can also be improved by using ALD. As compared to other BAW resonators, the BAW resonator  10  can be tailored by ALD for use in an RF passband filter capable of operation in the 5-20 Ghz range or higher, for steeper stopband attenuation, lower insertion loss, and other improved characteristics. The BAW resonator  10  can also be tailored by ALD for more accurate frequency response, higher quality factor, higher piezoelectric coupling or bandwidth, and smaller temperature coefficient of frequency, among other improved characteristics. 
     A number of trimming steps may be required after sputtering, but trimming can be reduced or even avoided in many cases when ALD is used. Particularly, a layer of material deposited by sputtering may lack uniformity in thickness to some extent, and trimming is often used to achieve better uniformity in thickness across the surface of a material layer. The lack of uniformity in thickness can be especially pronounced for sputtering when considered across an entire wafer. Trimming can be a relatively costly and time-consuming processing, including the measurement of layer thickness over a wafer, forming a thickness map, and multiple trimming steps using an ion bean or other technique, to smooth the profile of the layer. This trimming process may be needed for both piezoelectric, electrode, and other layers (e.g., temperature compensation layers and/or encapsulation layers) when sputtering or other deposition techniques are relied upon. These trimming steps can be reduced or eliminated in some cases when ALD is used to form the layer of piezoelectric material  13  and the electrodes  14  and  15 , saving time and costs. These benefits are even greater when considered over an entire wafer of integrated devices. Other benefits to using ALD processing steps are described below. 
     Turning to other embodiments,  FIG.  2    illustrates an example SAW resonator  20  according to various embodiments described herein. The illustration of the SAW resonator  20  is representative in  FIG.  2   . The positions, shapes, dimensions, and relative sizes of the layers and features of the SAW resonator  20  are not necessarily drawn to scale in  FIG.  2   . Example dimensions of the SAW resonator  20  are provided below, but the dimensions of the SAW resonator  20  are not specifically limited. The layers and other features shown in  FIG.  2    are also not exhaustive, and the SAW resonator  20  can include other layers, features, and elements that are not separately illustrated. Additionally, the SAW resonator  20  can be formed as part of a larger integrated structure or circuit in combination with other devices and circuit elements. A larger integrated structure of this type may include several resonators similar to the SAW resonator  20 , among other integrated components. 
     The SAW resonator  20  includes a substrate  21 , a layer of piezoelectric material  23  over the substrate  21 , a first electrode  24 , a second electrode  25 , a first reflection grating  26 , and a second reflection grating  27 . The first electrode  24  is positioned over the piezoelectric material  23  and in contact with a first surface (i.e., top surface) of the piezoelectric material  23 . The second electrode  25  is also positioned over the piezoelectric material  23  and in contact with a first surface of the piezoelectric material  23 . The first electrode  24  and the second electrode  25  include a number of interdigitated fingers, extending laterally next to each other. The first reflection grating  26  and the second reflection grating  27  are positioned at opposite sides of the first electrode  24  and the second electrode  25 , as shown. 
     The SAW resonator  20  can also include additional layers described below but not illustrated in  FIG.  2   , such as a temperature compensation layer, an encapsulation layer, and other others. The SAW resonator  20  can also include an additional structure between the substrate and the layer of piezoelectric material  23  in some cases, such as an acoustic mirror or reflector (e.g., a Bragg reflector). The acoustic mirror or reflector can be relied upon to tailor the operating characteristics of the SAW resonator  20  to account for any unwanted or designed—for BAW-type resonance in the SAW resonator  20 . In some cases, the piezoelectric material  23  can be formed to have a crystal structure c-axis orientation in the “X,” “Y,” or “Z” directions, and some BAW-type resonances can be generated using a structure similar to the SAW resonator in some of those cases. Additional examples of such structures are described below with reference to  FIGS.  8 A and  8 B . 
     Due to the piezoelectric properties of the layer of piezoelectric material  23  and the structural arrangement of the SAW resonator  20 , the SAW resonator  20  can generate an acoustic or mechanical wave when an alternating electric potential input signal, such as an RF input, is applied across the electrodes  24  and  25 . The acoustic or mechanical wave can travel or translate in the “Y” direction across or along the top surface of the SAW resonator  20 , as shown in  FIG.  1   . The acoustic or mechanical wave can be reflected by the first reflection grating  26  and the second reflection grating  27 , according to the operation of the SAW resonator  20 . 
     The substrate  21  can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. A silicon substrate may be preferred as being relatively low-cost, scalable for manufacturing, and compatible with manufacturing and processing steps, but other substrates can be relied upon. 
     The electrodes  24  and  25  can be embodied as layers of highly conductive material, such as a metal or metal alloy, including copper, silver, gold, molybdenum, tungsten, titanium, platinum, or aluminum, alloys thereof, and other conductive layers. One factor in the material selection for the electrodes  24  and  25  is the desired thickness of the electrodes  24  and  25 , which is also a factor in the response characteristics of the SAW resonator  20 . The first reflection grating  26  and the second reflection grating  27  can also be embodied as layers of highly conductive material, such as metals or metal alloys. 
     The layer of piezoelectric material  23  can be embodied as a layer of PZT, BST, BaTiO3, AlScN, AlN, ZnO, or another piezoelectric material. In one preferred embodiment, the layer of piezoelectric material  23  is a layer of AlN, although other piezoelectric materials can be relied upon. In some cases, the layer of piezoelectric material  23  can include one or more layers of piezoelectric material, including two or more different types of piezoelectric material, each of which is formed using ALD processing techniques according to the embodiments. The piezoelectric material  23  can also include a layer of AlN having a certain crystal orientation. In various embodiments, the layer of AlN can be formed to have a crystal structure c-axis orientation in the “X,” “Y,” or “Z” directions. In one example, the layer of AlN can have a crystal structure c-axis orientation to excite the acoustic or mechanical wave in the “Y” direction across or along the top surface of the SAW resonator  20 . 
     The operating characteristics of the SAW resonator  20 , including the frequency response, the accuracy of the frequency response, and the quality factor are determined in part by the conformity and uniformity of the layer of piezoelectric material  23 . Although to a lesser extent than for the BAW resonator  10 , the operating characteristics of the SAW resonator  20  are also determined in part by the thickness of the layer of piezoelectric material  23 . According to one aspect of the embodiments, the layer of piezoelectric material  23  can be embodied as a layer of AlN deposited by ALD. Forming the layer of AlN using ALD rather than sol-gel wet-chemical techniques, sputtering, pulsed laser deposition, or other techniques can result in a layer of piezoelectric material that is more uniform and conformal. Forming the layer of AlN using ALD can also result in a thinner layer of piezoelectric material. 
     The overall thickness of the layer of piezoelectric material  23  can be determined by the number of ALD cycles used. In some cases, the layer of piezoelectric material  23  can be formed as a thinner layer of piezoelectric material than in conventional SAW resonators, such as when sputtering is used. In one example, the layer of piezoelectric material  23  can be formed as an AlN layer of 100 nm in thickness or less by ALD. The layer of piezoelectric material  23  can be formed to be a thinner layer of AN, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. The layer of piezoelectric material  23  is not limited to a layer of AlN, as other thin films of piezoelectric materials can be formed using ALD. 
     The specific thickness of the layer of piezoelectric material  23  can be tailored in the SAW resonator  20 , sometimes to within less than 10 nm of a target thickness, to within less than 5 nm of a target thickness, or even closer to a target thickness by using ALD. The layer of piezoelectric material  23  can be formed as thin as possible using a minimal number of ALD cycles in some embodiments. The use of ALD rather than sputtering to deposit the layer of piezoelectric material  23  offers a number of advantages. For example, the use of ALD reduces crystal damage in the layer of piezoelectric material  23 , reduces dangling bonds, and reduces surface traps commonly attributed to sputter or plasma damage, resulting in better resonator performance. 
     The thickness, uniformity, and conformality of the other layers in the SAW resonator  20 , including the electrodes  24  and  25  and any temperature compensation and encapsulation layers, can be particularly important in the SAW resonator  20 . Thus, in other aspects of the embodiments, the electrode  24  can be formed using ALD, the electrode  25  can be formed using ALD, or both the electrodes  24  and  25  can be formed using ALD. As one example, the electrodes  24  and  25  can be formed as thin copper layers using ALD, but other highly conductive layers can be formed using ALD. If ALD is used to form the electrodes  24  and  25 , the type of conductive material used to form the electrodes  24  and  25  can be determined in part based on the ALD process available for the material. ALD processes for depositing thin films of pure metal substances, such as pure copper, are known, although additional processes are known for depositing metal alloys. The first reflection grating  26  and the second reflection grating  27  can also be formed using ALD. 
     The use of ALD to form the electrodes  24  and  25  can be particularly beneficial in the SAW resonator  20 , as the acoustic or mechanical wave generated by the SAW resonator  20  travels in the “Y” direction across or along the top surface of the SAW resonator  20  at an interface between the layer of piezoelectric material  23  and the electrodes  24  and  25 , as shown in  FIG.  2   . Thus, the thickness, conformity, and uniformity of the electrodes  24  and  25  can influence or impact the operating characteristics of the SAW resonator  20  more than the thickness, conformity, and uniformity of the electrodes  14  and  15  in the BAW resonator  10 . As one example, forming thinner and more uniform electrodes  24  and  25  can help to reduce the effects of unwanted BAW mode resonance in the SAW resonator  20 . Additionally, the thickness of any temperature compensation and encapsulation layers formed over the layer of piezoelectric material  23  and the electrodes  24  and  25  can also have a larger impact on the operating characteristics of the SAW resonator  20  than in the BAW resonator  10 . Forming thinner and more uniform temperature compensation and encapsulation layers can also help to reduce the effects of unwanted BAW mode resonance in the SAW resonator  20 . 
     One factor in the material selection for the electrodes  24  and  25  is the desired thickness of the electrodes  24  and  25 , which is also a factor in the response characteristics of the SAW resonator  20 . For example, if highly conductive and thin electrodes  24  and  25  are desired, then ALD processing can be selected to deposit copper for the electrodes  24  and  25 . The electrodes  24  and  25  can also be formed from other metals and metal alloys using ALD processing steps. In other cases, the electrodes  24  and  25  can be formed by a sputtering process, PVD, or related techniques to deposit the conductive material layers for the electrodes  24  and  25 . When using a sputtering or other process technique besides ALD, the electrodes  24  and  25  may be relatively thicker. The shapes, sizes, and positions of the electrodes  24  and  25  are representative in  FIG.  2   . The electrodes  24  and  25  can be formed to have any suitable shape and size. In other cases, the first reflection grating  26  and the second reflection grating  27  can be formed by a sputtering process, PVD, or related techniques to deposit the conductive material layers for the reflection gratings  26  and  27 . The shapes, sizes, and positions of the reflection gratings  26  and  27  are representative in  FIG.  2   . The reflection gratings  26  and  27  can be formed to have any suitable shape and size. 
     The overall thickness of the SAW resonator  20  can be significantly reduced by using ALD to form the layer of piezoelectric material  23  as compared to sputtering. The overall thickness can be further reduced by using ALD to form one or both of the electrodes  24  and  25 . The uniformity and conformity of the layer of piezoelectric material  23  and the electrodes  24  and  25  can also be improved by using ALD. As compared to other SAW resonators, the SAW resonator  20  can be tailored by ALD for use in an RF passband filter capable of operation in the 10-20 Ghz range or higher, for steeper stopband attenuation, lower insertion loss, and other improved characteristics. The SAW resonator  20  can also be tailored by ALD for more accurate frequency response, higher quality factor, higher piezoelectric coupling or bandwidth, and smaller temperature coefficient of frequency, among other improved characteristics. 
       FIG.  3    illustrates an example SMR  30  according to various embodiments described herein, in a cross-sectional view. The illustration of the SMR  30  is representative in  FIG.  3   . The positions, shapes, dimensions, and relative sizes of the layers and features of the SMR  30  are not necessarily drawn to scale in  FIG.  3   . Example dimensions of the SMR  30  are provided below, but the dimensions of the SMR  30  are not specifically limited. The layers and other features shown in  FIG.  3    are also not exhaustive, and the SMR  30  can include other layers, features, and elements that are not separately illustrated. Additionally, the SMR  30  can be formed as part of a larger integrated structure or circuit in combination with other devices and circuit elements. Such a larger integrated structure may include several resonators similar to the SMR  30 , among other integrated components. 
     The SMR  30  includes a substrate  31 , an acoustic mirror  32  over the substrate  31 , a layer of piezoelectric material  33  over the substrate  31 , a first electrode  34 , a second electrode  35 , and an encapsulation layer  36 . The first electrode  34  is in contact with a first surface (i.e., bottom surface) of the piezoelectric material  33  and positioned at least in part under the piezoelectric material  33 , between the piezoelectric material  33  and the substrate  31 . The second electrode  35  is in contact with a second surface (i.e., top surface) of the piezoelectric material  33  and positioned at least in part over the piezoelectric material  33 . The substrate  31  can be similar to the substrate  11  shown in  FIG.  1    and can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. 
     The acoustic mirror  32  is one example of the intermediate region  12  in the embodiment shown in  FIG.  1   . The acoustic mirror  32  can be embodied as a reflector of acoustic waves, such as a Bragg reflector. As one example, the acoustic mirror  32  can be embodied as an odd number of layers of material having high and low acoustic impedance, with the high and low acoustic impedance layers being alternated in the layer stack. In some cases, the acoustic mirror  32  can be formed with ALD, with one or more layers of material in the acoustic mirror  32  being formed with ALD. The thickness of the layers in the stack can be optimized to the quarter wavelength, for example, of the acoustic waves being generated by the SMR  30 , to increase acoustic reflectivity. The acoustic mirror  32  provides acoustic isolation between the substrate  31  and the resonator formed by the piezoelectric material  33  and the electrodes  34  and  35 . 
     Due to the piezoelectric properties of the layer of piezoelectric material  33  and the structural arrangement of the SMR  30 , the SMR  30  can generate an acoustic or mechanical wave when an alternating electric potential input signal, such as an RF input, is applied across the electrodes  34  and  35 . The acoustic or mechanical wave can travel or translate in the “Z” direction, and it can be substantially reflected by the acoustic mirror  32 . 
     The electrodes  34  and  35  can be embodied as layers of highly conductive material, such as a metal or metal alloy, including copper, silver, gold, molybdenum, tungsten, titanium, platinum, or aluminum, alloys thereof, and other conductive layers. The layer of piezoelectric material  33  can be embodied as a layer of PZT, BST, BaTiO3, AlScN, AlN, ZnO, or another piezoelectric material. In one preferred embodiment, the layer of piezoelectric material  33  is a layer of AlN, although other piezoelectric materials can be relied upon. The piezoelectric material  33  can also include a layer of AlN having a certain crystal orientation. In one example, the layer of AlN can have a crystal structure c-axis orientation to excite the acoustic or mechanical wave in the “Z” direction. The encapsulation layer  36  can be embodied as a thin film of material to protect the SMR  30 . The encapsulation layer  36  can be a layer of aluminum oxide (Al 2 O 3 ) for example, another oxide material, or another suitable material to protect the SMR  30 . 
     The operating characteristics of the SMR  30 , including the frequency response, the accuracy of the frequency response, and the quality factor are determined in part by the thickness, conformity, and uniformity of the layer of piezoelectric material  33 . According to one aspect of the embodiments, the layer of piezoelectric material  33  can be embodied as a layer of AlN deposited by ALD. Forming the layer of AlN using ALD rather than sol-gel wet-chemical techniques, sputtering, pulsed laser deposition, or other techniques can result in thinner, more uniform, and more conformal layers of piezoelectric material. 
     The layer of piezoelectric material  33  can be formed as a thinner layer of piezoelectric material than in conventional SMR resonators, such as when sputtering is used. In one example, the layer of piezoelectric material  33  can be formed as an AlN layer of 100 nm in thickness or less by ALD. The layer of piezoelectric material  33  can be formed to be a thinner layer of AlN, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. The layer of piezoelectric material  33  is not limited to a layer of AlN, as other thin films of piezoelectric materials can be formed using ALD. In some cases, the layer of piezoelectric material  33  can include one or more layers of piezoelectric material, including two or more different types of piezoelectric material (e.g., a stack of two or more of layers of AlN, PZT, BST, BaTiO3, and AlScN), each of which can be formed using ALD processing techniques according to the embodiments. 
     The specific thickness of the layer of piezoelectric material  33  can be highly controlled or tailored in the SMR  30 , sometimes to within less than 10 nm of a target thickness, to within less than 5 nm of a target thickness, or even closer to a target thickness by using ALD. The layer of piezoelectric material  33  can be formed as thin as possible using a minimal number of ALD cycles in some embodiments. The use of ALD rather than sputtering to deposit the layer of piezoelectric material  33  offers a number of advantages. For example, the use of ALD reduces crystal damage in the layer of piezoelectric material  33 , reduces dangling bonds, and reduces surface traps commonly attributed to sputter or plasma damage, resulting in better resonator performance. 
     In other aspects of the embodiments, the electrode  34  can be formed using ALD, the electrode  35  can be formed using ALD, or both the electrodes  34  and  35  can be formed using ALD. As one example, the electrodes  34  and  35  can be formed as thin copper layers using ALD, but other highly conductive layers can be formed using ALD. If ALD is used to form the electrodes  34  and  35 , the type of conductive material used to form the electrodes  34  and  35  can be determined in part based on the ALD process available for the material. ALD processes for depositing thin films of pure metal substances, such as pure copper, are known, although additional processes are known for depositing metal alloys. 
     As noted above, one factor in the material selection for the electrodes  34  and  35  is the desired thickness of the electrodes  34  and  35 , which is also a factor in the response characteristics of the SMR  30 . For example, if highly conductive and thin electrodes  34  and  35  are desired, then ALD processing can be selected to deposit copper for the electrodes  34  and  35 . The electrodes  34  and  35  can also be formed from other metals and metal alloys using ALD processing steps. In other cases, the electrodes  34  and  35  can be formed by a sputtering process, PVD, or related techniques to deposit the conductive material layers for the electrodes  34  and  35 . When using a sputtering or other process technique besides ALD, the electrodes  34  and  35  may be relatively thicker. The shapes, sizes, and positions of the electrodes  34  and  35  are representative in  FIG.  3   . The electrodes  34  and  35  can be formed to have any suitable shape and size. 
     The encapsulation layer  36  can also be formed using ALD according to one aspect of the embodiments. The encapsulation layer  36  can be formed as a uniform and conformal thin film. In one example, the encapsulation layer  36  can be formed as a layer of Al 2 O 3  that is 100 nm or thinner, using ALD. The encapsulation layer  36  can also be formed to be a thinner layer of Al 2 O 3 , such as a layer of less than 10 nm, 5 nm, or thinner in some cases using ALD. The encapsulation layer  36  is not limited to a layer of Al 2 O 3 , as other thin films of protective materials can be formed using ALD. 
     The overall thickness of the SMR  30  can be significantly reduced by using ALD to form the layer of piezoelectric material  33  as compared to sputtering. The overall thickness can be further reduced by using ALD to form one or both of the electrodes  34  and  35  and the encapsulation layer  36 . The uniformity and conformity of the layer of piezoelectric material  33 , the electrodes  34  and  35 , and the encapsulation layer  36  can also be improved by using ALD. As compared to other SMR structures, the SMR  30  can be tailored by ALD for use in an RF passband filter capable of operation in the 5-20 Ghz range or higher, for steeper stopband attenuation, lower insertion loss, and other improved characteristics. The SMR  30  can also be tailored by ALD for more accurate frequency response, higher quality factor, higher piezoelectric coupling or bandwidth, and smaller temperature coefficient of frequency, among other improved characteristics. 
       FIG.  4    illustrates an example method of manufacture of the SMR  30  shown in  FIG.  3    according to various embodiments described herein. Although the method is described in connection with the SMR  30  shown in  FIG.  3   , the method can also be relied upon to manufacture solidly mounted resonators similar to that shown in  FIG.  3   . Additionally, although the method illustrates a specific order of steps in  FIG.  4   , the order of the steps can differ from that which is depicted. For example, two or more steps shown in succession can be performed, at least in part, at the same time. In some cases, one or more of the steps can be skipped or omitted. In other cases, additional steps not shown in  FIG.  4    can be relied upon, such as steps among or after the steps shown in  FIG.  4   . 
     At step  100 , the process includes providing a substrate for the SMR  30 . Referring to the example shown in  FIG.  3   , the substrate  31  is illustrated as one example of a substrate that can be provided at step  100 . The substrate  31  can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. The substrate  31  can be manufactured, sourced from a vendor, or formed or provided in any other suitable way at step  100 . 
     At step  102 , the process includes forming the acoustic mirror  32  over the substrate  31 . As one example, the acoustic mirror  32  can be formed as alternating layers of material having high and low acoustic impedance or refractive indexes, such as a Bragg reflector. The acoustic mirror  32  provides acoustic isolation between the substrate  31  and the resonator formed by the piezoelectric material  33  and the electrodes  34  and  35 , which are formed in later process steps. 
     At step  104 , the process includes forming the first electrode  34  over the acoustic mirror  32 . The first electrode  34  can be embodied as a layer of highly conductive material as described herein. In one example, the electrode  34  can be formed as a thin layer or film of copper using ALD. The electrode  34  can be formed at a thickness of 300 nm, 200 nm, or 100 nm or less using ALD. The electrode  34  can be formed to be a thinner layer of copper, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. 
     One factor in the selection of the material for the electrode  34  is the desired thickness of the electrode  34 , which is also a factor in the response characteristics of the SMR  30 . For example, if a highly conductive and thin electrode  34  is needed, then ALD processing can be selected to deposit copper for the electrode  34 . The electrode  34  can also be formed from other metals and metal alloys using ALD processing at step  104 . In other cases, the electrode  34  can be formed as molybdenum or another metal or metal alloy by sputtering, PVD, CVD, or related techniques at step  104 . When using a process technique besides ALD, the electrode  34  may be formed relatively thicker than when using ALD. 
     At step  106 , the process includes depositing the layer of piezoelectric material  33  over the substrate  31  by ALD processing steps. The layer of piezoelectric material  33  can be deposited on the electrode  34  as shown in  FIG.  3   . The ALD process can be started with an initiation of the surface of the electrode  34  for the deposit of piezoelectric material, such as AlN, on the electrode  34 . In a reaction chamber for ALD processing steps, the initiation can include annealing the electrode  34  or the top surface of the electrode  34 , etching the electrode  34 , exposing the electrode  34  to one or more gases, or other steps to remove contaminants from the top surface of the electrode  34  or otherwise prepare the surface of the electrode  34  for the deposit of materials. The crystal orientation of the layer of piezoelectric material  33  can also be directed at step  106 . For example, the initiation can be tailored to form the layer of piezoelectric material  33  having a crystal structure c-axis orientation for acoustic or mechanical waves in the “Z” direction shown in  FIG.  3   . 
     After initiation, the ALD process at step  106  can proceed with the exposure of the electrode  34  to two or more precursor chemicals or reactants, in a repeating sequence. The precursors can be gaseous species introduced to an ALD chamber separately over time. The precursors react, respectively in time, in a self-limiting way. Any excess or remaining reactant of a precursor is flushed away before the next precursor or ALD cycle is applied or repeated. ALD processes are characterized by both dose times, which are the times that the surface is exposed to a respective precursor, and purge times, which are the times between doses during which the ALD chamber is evacuated for the next step. For a two reactant ALD process, a first reactant dose, first purge, second reactant dose, second purge sequence are steps in one ALD cycle. 
     For the first reactant dose cycle, a first precursor can be introduced into the ALD reaction chamber, which exposes the top surface of the electrode  34  to the first precursor, with the application of heat. The time for the first reactant dose cycle can be selected to saturate the top surface of the electrode  34  with the first precursor. The ALD reaction chamber can then be purged in a first purge cycle to remove any byproducts of the first reactant dose cycle. The ALD reaction chamber can be purged by introducing an inert gas, for example, evacuated using a vacuum, or by other steps. 
     For the second reactant dose cycle, a second precursor can be introduced into the ALD reaction chamber. The time for the second reactant dose cycle can be sufficient for the second precursor to fully or substantially react with the first precursor, until sites for the reaction between the precursors are exhausted. The ALD reaction chamber can then be purged in a second purge cycle to remove any byproducts of the second reactant dose cycle. The ALD reaction chamber can be purged by introducing an inert gas, for example, evacuated using a vacuum, or by other steps. Then, another ALD cycle can begin. 
     The thickness of the layer of piezoelectric material  33  can be determined by the number of ALD cycles used at step  106 , and the number of cycles can be tailored to grow the layer of piezoelectric material  33  in a uniform and conformal way, with very high precision. In one example, the layer of piezoelectric material  33  can be formed as an AlN layer of 100 nm in thickness or less by ALD. The layer of piezoelectric material  33  can be formed to be a thinner layer of AlN, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD at step  106 . The layer of piezoelectric material  33  is also not limited to a layer of AlN, as other thin films of piezoelectric materials can be formed using ALD at step  106 . 
     In some cases, step  106  can also include trimming the layer of piezoelectric material  33 , to further improve the uniformity of the layer, tailor the thickness of the layer, or otherwise modify the layer of piezoelectric material  33 . Although trimming can be a relatively costly and time-consuming processing, any trimming performed at step  106  can be relatively minor as compared to if sputtering were used to form the layer of piezoelectric material  33 . 
     At step  108 , the process includes forming the second electrode  35  on the layer of piezoelectric material  33 . The second electrode  35  can be embodied as a layer of highly conductive material as described herein. In one example, the electrode  35  can be formed as a thin layer or film of copper using ALD. The electrode  35  can be formed at a thickness of 300 nm, 200 nm, or 100 nm or less using ALD. The electrode  35  can be formed to be a thinner layer of copper, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. 
     One factor in the selection of the material for the electrode  35  is the desired thickness of the electrode  35 , which is also a factor in the response characteristics of the SMR  30 . For example, if a highly conductive and thin electrode  35  is needed, then ALD processing can be selected to deposit copper for the electrode  35 . The electrode  35  can also be formed from other metals and metal alloys using ALD processing at step  108 . In other cases, the electrode  35  can be formed as molybdenum or another metal or metal alloy by sputtering, PVD, CVD, or related techniques at step  108 . When using a process technique besides ALD, the electrode  35  may be formed relatively thicker than when using ALD. 
     At step  110 , the process includes forming the encapsulation layer  36 . The encapsulation layer  36  can be formed as a uniform and conformal thin film. In one example, the encapsulation layer  36  can be formed as a layer of Al 2 O 3  that is 100 nm or thinner, by ALD processing steps. The encapsulation layer  36  can also be formed to be a thinner layer of Al 2 O 3 , such as a layer of less than 10 nm, 5 nm, or thinner in some cases using ALD. The encapsulation layer  36  is not limited to a layer of Al 2 O 3 , as other thin films of protective materials can be formed using ALD. 
     As compared to other deposition processes, the encapsulation layer  36  can be formed very thin using ALD, to tailor the frequency response, the accuracy of the frequency response, the quality factor, and the insertion losses of the SMR  30 . The encapsulation layer  36  can cover and encapsulate the layer of piezoelectric material  33 , the first electrode  34 , and the second electrode  35 , as shown in  FIG.  3   . In some cases, the encapsulation layer  36  can cover more or less of an area as compared to that shown in  FIG.  3   . 
     In some cases, the process shown in  FIG.  4    can include additional steps, such as forming one or more temperature compensation layers in the SMR  30 . For example, the process can include forming a temperature compensation layer, such as a layer of silicon dioxide (SiO2) or other material for temperature compensation, between steps  104  and  106 . In that case, a layer of SiO2 can be formed between the first electrode  34  and the piezoelectric material  33  using ALD, and the first electrode can be formed on the temperature compensation rather than on the piezoelectric material  33 . A layer of SiO2 can also be formed between steps  106  and  108 , between the piezoelectric material  33  and the second electrode  35  using ALD. Due to the use of ALD processing steps, the temperature compensation layer or layers can be more uniform, more conformal, and thinner than they would be if formed using other deposition techniques. As some examples, the layer of SiO2 can be formed to be a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness. 
       FIG.  5    illustrates an example FBAR  40  according to various embodiments described herein. The illustration of the FBAR  40  is representative in  FIG.  5   . The positions, shapes, dimensions, and relative sizes of the layers and features of the FBAR  40  are not necessarily drawn to scale in  FIG.  5   . Example dimensions of the FBAR  40  are provided below, but the dimensions of the FBAR  40  are not specifically limited. The layers and other features shown in  FIG.  5    are also not exhaustive, and the FBAR  40  can include other layers, features, and elements that are not separately illustrated. Additionally, the FBAR  40  can be formed as part of a larger integrated structure or circuit in combination with other devices and circuit elements. Such a larger integrated structure may include several resonators similar to the FBAR  40 , among other integrated components. 
     The FBAR  40  includes a substrate  41 , a supporting layer  42  over the substrate  41 , a layer of piezoelectric material  43  over the substrate  41 , a first electrode  44 , a second electrode  45 , an encapsulation layer  46 , and an isolation cavity  47 . The first electrode  44  is in contact with a first surface (i.e., bottom surface) of the piezoelectric material  43  and positioned at least in part under the piezoelectric material  43 , between the piezoelectric material  43  and the substrate  41 . The second electrode  45  is in contact with a second surface (i.e., top surface) of the piezoelectric material  43  and positioned at least in part over the piezoelectric material  43 . The substrate  41  can be similar to the substrate  11  shown in  FIG.  1    and can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. 
     The supporting layer  42  is one example of the intermediate region  12  in the embodiment shown in  FIG.  1   . The supporting layer  42  can be embodied as a layer of supporting material, such as silicon, formed over the substrate  41 . The supporting layer  42  supports the resonator formed by the piezoelectric material  43  and the electrodes  44  and  45 , as further described below. 
     Due to the piezoelectric properties of the layer of piezoelectric material  43  and the structural arrangement of the FBAR  40 , FBAR  40  can generate an acoustic or mechanical wave when an alternating electric potential input signal, such as an RF input, is applied across the electrodes  44  and  45 . The acoustic or mechanical wave can travel or translate in the “Z” direction, and it can be isolated by the isolation cavity  47 , as also described below. 
     The electrodes  44  and  45  can be embodied as layers of highly conductive material, such as a metal or metal alloy, including copper, silver, gold, molybdenum, tungsten, titanium, platinum, or aluminum, alloys thereof, and other conductive layers. The layer of piezoelectric material  43  can be embodied as a layer of PZT, BST, BaTiO3, AlScN, AlN, ZnO, or another piezoelectric material. In one preferred embodiment, the layer of piezoelectric material  43  is a layer of AlN, although other piezoelectric materials can be relied upon. The piezoelectric material  43  can also include a layer of AlN having a certain crystal orientation. In one example, the layer of AlN can have a crystal structure c-axis orientation to excite the acoustic or mechanical wave in the “Z” direction. The encapsulation layer  46  can be embodied as a thin film of material to protect the FBAR  40 . The encapsulation layer  46  can be a layer of Al 2 O 3  for example, another oxide material, or another suitable material to protect the FBAR  40 . 
     The operating characteristics of the FBAR  40 , including the frequency response, the accuracy of the frequency response, and the quality factor are determined in part by the thickness, conformity, and uniformity of the layer of piezoelectric material  43 . According to one aspect of the embodiments, the layer of piezoelectric material  43  can be embodied as a layer of AlN deposited by ALD. Forming the layer of AlN using ALD rather than sol-gel wet-chemical techniques, sputtering, pulsed laser deposition, or other techniques can result in thinner, more uniform, and more conformal layers of piezoelectric material. 
     The layer of piezoelectric material  43  can be formed as a thinner layer of piezoelectric material than in conventional FBAR resonators, such as when sputtering is used. In one example, the layer of piezoelectric material  43  can be formed as an AlN layer of 100 nm in thickness or less by ALD. The layer of piezoelectric material  43  can be formed to be a thinner layer of AlN, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. The layer of piezoelectric material  43  is not limited to a layer of AlN, as other thin films of piezoelectric materials can be formed using ALD. In some cases, the layer of piezoelectric material  43  can include one or more layers of piezoelectric material, including two or more different types of piezoelectric material (e.g., a stack of two or more of layers of AlN, PZT, BST, BaTiO3, and AlScN), each of which can be formed using ALD processing techniques according to the embodiments. 
     The specific thickness of the layer of piezoelectric material  43  can be highly controlled or tailored in the FBAR  40 , sometimes to within less than 10 nm of a target thickness, to within less than 5 nm of a target thickness, or even closer to a target thickness by using ALD. The layer of piezoelectric material  43  can be formed as thin as possible using a minimal number of ALD cycles in some embodiments. The use of ALD rather than sputtering to deposit the layer of piezoelectric material  43  offers a number of advantages. For example, the use of ALD reduces crystal damage in the layer of piezoelectric material  43 , reduces dangling bonds, and reduces surface traps commonly attributed to sputter or plasma damage, resulting in better resonator performance. 
     In other aspects of the embodiments, the electrode  44  can be formed using ALD, the electrode  45  can be formed using ALD, or both the electrodes  44  and  45  can be formed using ALD. As one example, the electrodes  44  and  45  can be formed as thin copper layers using ALD, but other highly conductive layers can be formed using ALD. If ALD is used to form the electrodes  44  and  45 , the type of conductive material used to form the electrodes  44  and  45  can be determined in part based on the ALD process available for the material. One factor in the material selection for the electrodes  44  and  45  is the desired thickness of the electrodes  44  and  45 , which is also a factor in the response characteristics of the FBAR  40 . For example, if very thin and conductive electrodes  44  and  45  are desired, then ALD processing can be selected to deposit copper for the electrodes  44  and  45 . In other cases, the electrodes  44  and  45  can be formed by a sputtering process, PVD, or related techniques. When using a sputtering or other process technique besides ALD, the electrodes  44  and  45  may be relatively thicker. 
     The encapsulation layer  46  can also be formed using ALD according to one aspect of the embodiments. The encapsulation layer  46  can be formed as a uniform and conformal thin film. In one example, the encapsulation layer  46  can be formed as a layer of Al 2 O 3  that is 100 nm or thinner, using ALD. The encapsulation layer  46  can also be formed to be a thinner layer of Al 2 O 3 , such as a layer of less than 10 nm, 5 nm, or thinner in some cases using ALD. The encapsulation layer  46  is not limited to a layer of Al 2 O 3 , as other thin films of protective materials can be formed using ALD. 
     The isolation cavity  47  is a space or cavity formed under the supporting layer  42 , the layer of piezoelectric material  43 , and the electrodes  44  and  45 . The isolation cavity  47  can vary in size and proportions as compared to that shown in  FIG.  5   . In some cases, the isolation cavity  47  can be wider than that shown in  FIG.  5   , and the isolation cavity  47  can be larger (i.e., in a top-down view of length and width dimensions) than the layer of piezoelectric material  43 . In other cases, at least a portion of the layer of piezoelectric material  43  can extend over the substrate  41 , beyond the isolation cavity  47 , as shown in  FIG.  5   . 
     The isolation cavity  47  can be formed by etching or other material-removal process steps, typically after the supporting layer  42  and other layers of the FBAR  40  are formed. In some cases, the isolation cavity  47  can be formed with one or more supporting pillars  48  remaining in the isolation cavity  47 . The substrate  41  can be selectively etched to form the isolation cavity, such that the supporting pillars  48  remain in the isolation cavity  47 . The supporting pillars  48  can provide additional support to the supporting layer  42 . The number and positions of the supporting pillars  48  can vary as compared to that shown in  FIG.  5   , and the supporting pillars  48  can be omitted in some cases. 
     The overall thickness of the FBAR  40  can be significantly reduced by using ALD to form the layer of piezoelectric material  43  as compared to sputtering. The overall thickness can be further reduced by using ALD to form one or both of the electrodes  44  and  45  and the encapsulation layer  46 . The uniformity and conformity of the layer of piezoelectric material  43 , the electrodes  44  and  45 , and the encapsulation layer  46  can also be improved by using ALD. As compared to other FBAR structures, the FBAR  40  can be tailored by ALD for use in an RF passband filter capable of operation in the 10-20 Ghz range or higher, for steeper stopband attenuation, lower insertion loss, and other improved characteristics. The FBAR  40  can also be tailored by ALD for more accurate frequency response, higher quality factor, higher piezoelectric coupling or bandwidth, and smaller temperature coefficient of frequency, among other improved characteristics. 
       FIG.  6    illustrates an example method of manufacture of the example FBAR  40  shown in  FIG.  5    according to various embodiments described herein. Although the method is described in connection with the FBAR  40  shown in  FIG.  5   , the method can also be relied upon to manufacture thin-film bulk acoustic resonators similar to that shown in  FIG.  5   . Additionally, although the method illustrates a specific order of steps in  FIG.  6   , the order of the steps can differ from that which is depicted. For example, two or more steps shown in succession can be performed, at least in part, at the same time. In some cases, one or more of the steps can be skipped or omitted. In other cases, additional steps not shown in  FIG.  6    can be relied upon, such as steps among or after the steps shown in  FIG.  6   . 
     At step  200 , the process includes providing a substrate for the FBAR  40 . Referring to the example shown in  FIG.  5   , the substrate  41  is illustrated as one example of a substrate that can be provided at step  200 . The substrate  41  can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. The substrate  41  can be manufactured, sourced from a vendor, or formed or provided in any other suitable way at step  200 . 
     At step  202 , the process includes forming the supporting layer  42  over the substrate  41 . As one example, the supporting layer  42  can be embodied as a layer of supporting material, such as silicon, formed over the substrate  41 . The supporting layer  42  can be deposited using a suitable deposition technique, such as PVD, CVD, or a related technique. The supporting layer  42  can be formed to any suitable thickness, and it is not necessary that the supporting layer  42  be formed as a thin film. The supporting layer  42  supports the resonator formed by the piezoelectric material  43  and the electrodes  44  and  45 , as further described below. 
     At step  204 , the process includes forming the first electrode  44  over the supporting layer  42 . The first electrode  44  can be embodied as a layer of highly conductive material as described herein. In one example, the electrode  44  can be formed as a thin layer or film of copper using ALD. The electrode  44  can be formed at a thickness of 300 nm, 200 nm, or 100 nm or less using ALD. The electrode  44  can be formed to be a thinner layer of copper, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. 
     One factor in the selection of the material for the electrode  44  is the desired thickness of the electrode  44 , which is also a factor in the response characteristics of the FBAR  40 . For example, if a highly conductive and thin electrode  44  is needed, then ALD processing can be selected to deposit copper for the electrode  44 . The electrode  44  can also be formed from other metals and metal alloys using ALD processing at step  204 . In other cases, the electrode  44  can be formed as molybdenum or another metal or metal alloy by sputtering, PVD, CVD, or related techniques at step  204 . When using a process technique besides ALD, the electrode  34  may be formed relatively thicker than when using ALD. 
     At step  206 , the process includes depositing the layer of piezoelectric material  43  over the substrate  41  by ALD processing steps. The layer of piezoelectric material  43  can be deposited on the electrode  44  as shown in  FIG.  5   . The ALD process can be started with an initiation of the surface of the electrode  44  for the deposit of piezoelectric material, such as AlN, on the electrode  44 . In a reaction chamber for ALD processing steps, the initiation can include annealing the electrode  44  or the top surface of the electrode  44 , etching the electrode  44 , exposing the electrode  44  to one or more gases, or other steps to remove contaminants from the top surface of the electrode  44  or otherwise prepare the surface of the electrode  44  for the deposit of materials. The crystal orientation of the layer of piezoelectric material  43  can also be directed at step  206 . For example, the initiation can be tailored to form the layer of piezoelectric material  43  having a crystal structure c-axis orientation for acoustic or mechanical waves in the “Z” direction shown in  FIG.  5   . 
     After initiation, the ALD process at step  206  can proceed with the exposure of the electrode  44  to two or more precursor chemicals or reactants, in a repeating sequence. The precursors can be gaseous species introduced to an ALD chamber separately over time. The precursors react, respectively in time, in a self-limiting way. Any excess or remaining reactant of a precursor is flushed away before the next precursor or ALD cycle is applied or repeated. ALD processes are characterized by both dose times, which are the times that the surface is exposed to a respective precursor, and purge times, which are the times between doses during which the ALD chamber is evacuated for the next step. For a two reactant ALD process, a first reactant dose, first purge, second reactant dose, second purge sequence are steps in one ALD cycle. 
     For the first reactant dose cycle, a first precursor can be introduced into the ALD reaction chamber, which exposes the top surface of the electrode  44  to the first precursor, with the application of heat. The time for the first reactant dose cycle can be selected to saturate the top surface of the electrode  44  with the first precursor. The ALD reaction chamber can then be purged in a first purge cycle to remove any byproducts of the first reactant dose cycle. The ALD reaction chamber can be purged by introducing an inert gas, for example, evacuated using a vacuum, or by other steps. 
     For the second reactant dose cycle, a second precursor can be introduced into the ALD reaction chamber. The time for the second reactant dose cycle can be sufficient for the second precursor to fully or substantially react with the first precursor, until sites for the reaction between the precursors are exhausted. The ALD reaction chamber can then be purged in a second purge cycle to remove any byproducts of the second reactant dose cycle. The ALD reaction chamber can be purged by introducing an inert gas, for example, evacuated using a vacuum, or by other steps. Then, another ALD cycle can begin. 
     The thickness of the layer of piezoelectric material  43  can be determined by the number of ALD cycles used at step  206 , and the number of cycles can be tailored to grow the layer of piezoelectric material  43  in a uniform and conformal way, with very high precision. In one example, the layer of piezoelectric material  43  can be formed as an AlN layer of 100 nm in thickness or less by ALD. The layer of piezoelectric material  43  can be formed to be a thinner layer of AlN, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD at step  206 . The layer of piezoelectric material  43  is also not limited to a layer of AlN, as other thin films of piezoelectric materials can be formed using ALD at step  206 . 
     In some cases, step  206  can also include trimming the layer of piezoelectric material  43 , to further improve the uniformity of the layer, tailor the thickness of the layer, or otherwise modify the layer of piezoelectric material  43 . Although trimming can be a relatively costly and time-consuming processing, any trimming performed at step  206  can be relatively minor as compared to if sputtering were used to form the layer of piezoelectric material  43 . 
     At step  208 , the process includes forming the second electrode  45  on the layer of piezoelectric material  43 . The second electrode  45  can be embodied as a layer of highly conductive material as described herein. In one example, the electrode  45  can be formed as a thin layer or film of copper using ALD. The electrode  45  can be formed at a thickness of 300 nm, 200 nm, or 100 nm or less using ALD. The electrode  45  can be formed to be a thinner layer of copper, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. 
     One factor in the selection of the material for the electrode  45  is the desired thickness of the electrode  45 , which is also a factor in the response characteristics of the FBAR  40 . For example, if a highly conductive and thin electrode  45  is needed, then ALD processing can be selected to deposit copper for the electrode  45 . The electrode  45  can also be formed from other metals and metal alloys using ALD processing at step  208 . In other cases, the electrode  45  can be formed as molybdenum or another metal or metal alloy by sputtering, PVD, CVD, or related techniques at step  208 . When using a process technique besides ALD, the electrode  45  may be formed relatively thicker than when using ALD. 
     At step  210 , the process includes forming the encapsulation layer  46 . The encapsulation layer  46  can be formed as a uniform and conformal thin film. In one example, the encapsulation layer  46  can be formed as a layer of Al 2 O 3  that is 100 nm or thinner, by ALD processing steps. The encapsulation layer  46  can also be formed to be a thinner layer of Al 2 O 3 , such as a layer of less than 10 nm, 5 nm, or thinner in some cases using ALD. The encapsulation layer  46  is not limited to a layer of Al 2 O 3 , as other thin films of protective materials can be formed using ALD. 
     As compared to other deposition processes, the encapsulation layer  46  can be formed very thin using ALD, to tailor the frequency response, the accuracy of the frequency response, the quality factor, and the insertion losses of the FBAR  40 . The encapsulation layer  46  can cover and encapsulate the layer of piezoelectric material  43 , the first electrode  44 , and the second electrode  45 , as shown in  FIG.  5   . In some cases, the encapsulation layer  46  can cover more or less of an area as compared to that shown in  FIG.  4   . 
     At step  212 , the process includes forming the isolation cavity  47  under the supporting layer  42 , the layer of piezoelectric material  43 , and the electrodes  44  and  45 . The isolation cavity  47  can be formed to any suitable size for the purpose of isolating the acoustic waves generated by the layer of piezoelectric material  43 . In some cases, the isolation cavity  47  can be wider than that shown in  FIG.  5   , and the isolation cavity  47  can be larger (i.e., in a top-down view of length and width dimensions) than the layer of piezoelectric material  43 . In other cases, at least a portion of the layer of piezoelectric material  43  can extend over the substrate  41 , beyond the isolation cavity  47 , as shown in  FIG.  5   . The isolation cavity  47  can be formed by etching or other material-removal process steps. In some cases, the isolation cavity  47  can be formed with one or more supporting pillars  48  ( FIG.  5   ) remaining in the isolation cavity  47  to provide additional support to the supporting layer  42 . The number and positions of the supporting pillars  48  can vary as compared to that shown in  FIG.  5   , and the supporting pillars  48  can be omitted in some cases. 
     In some cases, the process shown in  FIG.  6    can include additional steps, such as forming one or more temperature compensation layers in the FBAR  40 . For example, the process can include forming a temperature compensation layer, such as a layer of SiO2 or other material for temperature compensation, between steps  204  and  206 . In that case, a layer of SiO2 can be formed between the first electrode  44  and the piezoelectric material  43  using ALD. A layer of SiO2 can also be formed between steps  206  and  208 , between the piezoelectric material  43  and the second electrode  45  using ALD. Due to the use of ALD processing steps, the temperature compensation layer or layers can be more uniform, more conformal, and thinner than they would be if formed using other deposition techniques. As some examples, the layer of SiO2 can be formed to be a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness. 
       FIG.  7    illustrates an example method of manufacture of the SAW resonator  20  shown in  FIG.  2    according to various embodiments described herein. Although the method is described in connection with the SAW resonator  20  shown in  FIG.  2   , the method can also be relied upon to manufacture SAW resonators similar to that shown in  FIG.  2   . Additionally, although the method illustrates a specific order of steps in  FIG.  7   , the order of the steps can differ from that which is depicted. For example, two or more steps shown in succession can be performed, at least in part, at the same time. In some cases, one or more of the steps can be skipped or omitted. In other cases, additional steps not shown in  FIG.  7    can be relied upon, such as steps among or after the steps shown in  FIG.  7   . 
     At step  300 , the process includes providing a substrate for the SAW resonator  20 . Referring to the example shown in  FIG.  2   , the substrate  21  is illustrated as one example of a substrate that can be provided at step  300 . The substrate  21  can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. The substrate  21  can be manufactured, sourced from a vendor, or formed or provided in any other suitable way at step  300 . 
     At step  302 , the process includes depositing the layer of piezoelectric material  23  over the substrate  21  by ALD processing steps. The ALD process can be started with an initiation of the surface of substrate  21  for the deposit of piezoelectric material, such as AlN, on the substrate  21 . Ina reaction chamber for ALD processing steps, the initiation can include etching the substrate  21 , exposing the substrate  21  to one or more gases, or other steps to remove contaminants from the top surface of the substrate  21  or otherwise prepare the surface of the substrate  21  for the deposit of materials. The crystal orientation of the layer of piezoelectric material  23  can also be directed at step  302 . For example, the initiation can be tailored to form the layer of piezoelectric material  23  having a crystal structure c-axis orientation for acoustic or mechanical waves in the “Y” direction shown in  FIG.  2   . 
     After initiation, the ALD process at step  302  can proceed with the exposure of the substrate  21  to two or more precursor chemicals or reactants, in a repeating sequence. The precursors can be gaseous species introduced to an ALD chamber separately over time. The precursors react, respectively in time, in a self-limiting way. Any excess or remaining reactant of a precursor is flushed away before the next precursor or ALD cycle is applied or repeated. ALD processes are characterized by both dose times, which are the times that the surface is exposed to a respective precursor, and purge times, which are the times between doses during which the ALD chamber is evacuated for the next step. For a two reactant ALD process, a first reactant dose, first purge, second reactant dose, second purge sequence are steps in one ALD cycle. 
     For the first reactant dose cycle, a first precursor can be introduced into the ALD reaction chamber, which exposes the top surface of the substrate  21  to the first precursor, with the application of heat. The time for the first reactant dose cycle can be selected to saturate the top surface of the substrate  21  with the first precursor. The ALD reaction chamber can then be purged in a first purge cycle to remove any byproducts of the first reactant dose cycle. The ALD reaction chamber can be purged by introducing an inert gas, for example, evacuated using a vacuum, or by other steps. 
     For the second reactant dose cycle, a second precursor can be introduced into the ALD reaction chamber. The time for the second reactant dose cycle can be sufficient for the second precursor to fully or substantially react with the first precursor, until sites for the reaction between the precursors are exhausted. The ALD reaction chamber can then be purged in a second purge cycle to remove any byproducts of the second reactant dose cycle. The ALD reaction chamber can be purged by introducing an inert gas, for example, evacuated using a vacuum, or by other steps. Then, another ALD cycle can begin. 
     The thickness of the layer of piezoelectric material  23  can be determined by the number of ALD cycles used at step  302 , and the number of cycles can be tailored to grow the layer of piezoelectric material  23  in a uniform and conformal way, with very high precision. In one example, the layer of piezoelectric material  23  can be formed as an AlN layer of 100 nm in thickness or less by ALD. The layer of piezoelectric material  23  can be formed to be a thinner layer of AlN, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD at step  206 . The layer of piezoelectric material  23  is also not limited to a layer of AlN, as other thin films of piezoelectric materials can be formed using ALD at step  302 . 
     In some cases, step  302  can also include trimming the layer of piezoelectric material  23 , to further improve the uniformity of the layer, tailor the thickness of the layer, or otherwise modify the layer of piezoelectric material  23 . Although trimming can be a relatively costly and time-consuming processing, any trimming performed at step  306  can be relatively minor as compared to if sputtering were used to form the layer of piezoelectric material  23 . 
     At step  304 , the process includes forming the first electrode  24 , the second electrode  25 , and the reflection gratings  26  and  27  on the layer of piezoelectric material  23 . The electrodes  24  and  25  can be embodied as layers of highly conductive material. In one example, the electrodes  24  and  25  can be formed as thin layers or films of copper using ALD. The electrodes  24  and  25  can be formed at a thickness of 300 nm, 200 nm, or 100 nm or less using ALD. The electrodes  24  and  25  can be formed to be a thinner layer of copper, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. The reflection gratings  26  and  27  can be omitted in some cases. When included, the reflection gratings  26  and  27  can also be formed using ALD, at the same or similar thickness as the electrodes  24  and  25 . 
     One factor in the selection of the material for the electrodes  24  and  25  is the desired thickness of the electrodes  24  and  25 , which is also a factor in the response characteristics of the SAW resonator  20 . For example, if highly conductive and thin electrodes  24  and  25  are needed, then ALD processing can be selected to deposit copper for the electrodes  24  and  25 . The electrodes  24  and  25  can also be formed from other metals and metal alloys using ALD processing at step  304 . In other cases, the electrodes  24  and  25  can be formed as molybdenum or another metal or metal alloy by sputtering, PVD, CVD, or related techniques at step  304 . When using a process technique besides ALD, the electrodes  24  and  25  may be formed relatively thicker than when using ALD. 
     At step  306 , the process includes forming an encapsulation layer over the electrodes  24  and  25 , the reflection gratings  26  and  27 , and the layer of piezoelectric material  23 . Although an encapsulation layer is not shown in  FIG.  2   , an encapsulation layer similar to the encapsulation layers  36  and  46  shown in  FIGS.  3  and  5    can be formed as a uniform and conformal thin film over the electrodes  24  and  25 , the reflection gratings  26  and  27 , and the layer of piezoelectric material  23 . The encapsulation layer  46  can be formed as a layer of Al 2 O 3  that is 100 nm or thinner, by ALD processing steps. The encapsulation layer  46  can also be formed to be a thinner layer of Al 2 O 3 , such as a layer of less than 10 nm, 5 nm, or thinner in some cases using ALD. The encapsulation layer  46  is not limited to a layer of Al 2 O 3 , as other thin films of protective materials can be formed using ALD. 
     The structures and methods described herein can be used to fabricate a wide variety of useful integrated circuits. For example, the acoustic resonators described herein can be integrated with various components in a monolithic circuit format suitable for microwave circuit applications. Although embodiments have been described herein in detail, the descriptions, including the dimensions states, are by way of example. 
     In some cases, the process shown in  FIG.  7    can include additional steps, such as forming one or more temperature compensation layers in the SAW resonator  20 . For example, the process can include forming a temperature compensation layer, such as a layer of SiO2 or other material for temperature compensation, between steps  302  and  304 . In that case, a layer of SiO2 can be formed between the layer of piezoelectric material  23  and the electrodes  24  and  25  using ALD. Due to the use of ALD processing steps, the temperature compensation layer or layers can be more uniform, more conformal, and thinner than they would be if formed using other deposition techniques. As some examples, the layer of SiO2 can be formed to be a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness. 
       FIG.  8    is a cross-sectional view of an example laterally-excited bulk acoustic wave resonator  50  (“resonator  50 ”) according to various embodiments described herein. The illustration of the resonator  50  is representative in  FIG.  8   . The positions, shapes, dimensions, and relative sizes of the layers and features of the resonator  50  are not necessarily drawn to scale in  FIG.  8   . Example dimensions of the resonator  50  are provided below, but the dimensions are not specifically limited. The layers and other features shown in  FIG.  8    are also not exhaustive, and the resonator  50  can include other layers, features, and elements that are not separately illustrated. Additionally, the resonator  50  can be formed as part of a larger integrated structure or circuit in combination with other devices and circuit elements. Such a larger integrated structure may include several resonators similar to the resonator  50 , among other integrated components. 
     The resonator  50  includes a substrate  51 , a layer of piezoelectric material  53  over the substrate  51 , first electrodes  54 A- 54 C interdigitated with second electrodes  55 A- 55 C, an encapsulation layer  56 , and an isolation cavity  57 . The interdigitated electrodes  54 A- 54 C and  55 A- 55 C are similar to the interdigitated electrodes  24  and  25  shown in  FIG.  2   , although a cross section of the electrodes  54 A- 54 C and  55 A- 55 C is shown in  FIG.  8   . A limited number of the electrodes  54 A- 54 C and  55 A- 55 C are shown in  FIG.  8   , but it should be appreciated that a larger number of electrodes can be relied upon in practice. The electrodes  54 A- 54 C and  55 A- 55 C are in contact with a top surface of the piezoelectric material  53 . The substrate  51  can be similar to the substrate  11  shown in  FIG.  1    and can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. Although not shown in  FIG.  8   , the resonator  50  can also include a supporting layer similar to the supporting layer  42  in  FIG.  5    in some cases. Such a layer of supporting material can be formed from silicon, for example, and be positioned over the substrate  51  and under the layer of piezoelectric material  53 . 
     The resonator  50  can generate an acoustic or mechanical wave when an alternating electric potential input signal is applied across the first electrodes  54 A- 54 C and the second electrodes  55 A- 55 C. The electrodes  54 A- 54 C and  55 A- 55 C of the resonator  50  are not positioned on two different, opposing surfaces of the piezoelectric material  53 , as in the SMR  30  shown in  FIG.  3    and the FBAR  40  shown in  FIG.  5   . Instead, both the first electrodes  54 A- 54 C and the second electrodes  55 A- 55 C are formed on the top surface of the piezoelectric material  53 , which is similar to the SAW resonator  20  shown in  FIG.  2   . However, the resonator  50  is not designed to excite an acoustic or mechanical wave along the top surface of the resonator  50  like the SAW resonator  20 . Instead, the crystal structure c-axis orientation of the piezoelectric material  53  is oriented to excite a bulk acoustic or mechanical wave in the “Z” direction. That is, the c-axis orientation of the piezoelectric material  53  is oriented in the “Z” direction, perpendicular to the top surface of the piezoelectric material  53 . Thus, the resonator  50  is referenced as a laterally-excited bulk acoustic wave resonator. 
     The electrodes  54 A- 54 C and  55 A- 55 C can be embodied as layers of highly conductive material, such as a metal or metal alloy, including copper, silver, gold, molybdenum, tungsten, titanium, platinum, or aluminum, alloys thereof, and other conductive layers. The electrodes  54 A- 54 C and  55 A- 55 C are formed to have a width “W,” with a pitch “P” separating them. The width “W” is smaller than the pitch “P” in the resonator  50 . For example, the width “W” can be about 100 nm and the pitch “P” can be between 1-5 μm, although other dimensions can be relied upon. 
     The layer of piezoelectric material  53  can be embodied as a layer of PZT, BST, BaTiO3, AlScN, AlN, ZnO, or another piezoelectric material. In one preferred embodiment, the layer of piezoelectric material  53  is a layer of AlN, although other piezoelectric materials can be relied upon. In some cases, the layer of piezoelectric material  13  can include one or more layers of piezoelectric material, including two or more different types of piezoelectric material (e.g., a stack of two or more of layers of AlN, PZT, BST, BaTiO3, and AlScN), each of which can be formed using ALD processing techniques according to the embodiments. The encapsulation layer  56  can be embodied as a thin film of material to protect the resonator  50 . The encapsulation layer  56  can be a layer of Al 2 O 3  for example, another oxide material, or another suitable material to protect the resonator  50 . 
     The operating characteristics of the resonator  50 , including the frequency response, the accuracy of the frequency response, and the quality factor are determined in part by the thickness, conformity, and uniformity of the layer of piezoelectric material  53 . According to one aspect of the embodiments, the layer of piezoelectric material  53  can be embodied as a layer of AlN deposited by ALD. Forming the layer of AlN using ALD rather than sol-gel wet-chemical techniques, sputtering, pulsed laser deposition, or other techniques can result in thinner, more uniform, and more conformal layers of piezoelectric material. 
     The layer of piezoelectric material  53  can be formed as a thinner layer of piezoelectric material than in conventional resonators, such as when sputtering is used. In one example, the layer of piezoelectric material  53  can be formed as an AlN layer of 100 nm in thickness or less by ALD. The layer of piezoelectric material  53  can be formed to be a thinner layer of AlN, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. The layer of piezoelectric material  53  is not limited to a layer of AlN, as other thin films of piezoelectric materials can be formed using ALD. 
     The specific thickness of the layer of piezoelectric material  53  can be highly controlled or tailored in the resonator  50 , sometimes to within less than 10 nm of a target thickness, to within less than 5 nm of a target thickness, or even closer to a target thickness by using ALD. The layer of piezoelectric material  53  can be formed as thin as possible using a minimal number of ALD cycles in some embodiments. 
     The electrodes  54 A- 54 C and  55 A- 55 C can also be formed using ALD. As one example, the electrodes  54 A- 54 C and  55 A- 55 C can be formed as thin copper layers using ALD, but other highly conductive layers can be formed using ALD. If ALD is used to form the electrodes  54 A- 54 C and  55 A- 55 C, the type of conductive material used can be determined in part based on the ALD process available for the material. One factor in the material selection for the electrodes  54 A- 54 C and  55 A- 55 C is the desired thickness of the electrodes  54 A- 54 C and  55 A- 55 C, which is also a factor in the response characteristics of the resonator  50 . For example, if very thin and conductive electrodes  54 A- 54 C and  55 A- 55 C are desired, then ALD processing can be selected to deposit copper. In other cases, the electrodes  54 A- 54 C and  55 A- 55 C can be formed by a sputtering process, PVD, or related techniques. 
     The encapsulation layer  56  can also be formed using ALD according to one aspect of the embodiments. The encapsulation layer  56  can be formed as a uniform and conformal thin film. In one example, the encapsulation layer  56  can be formed as a layer of Al 2 O 3  that is 100 nm or thinner, using ALD. The encapsulation layer  56  can also be formed to be a thinner layer of Al 2 O 3 , such as a layer of less than 10 nm, 5 nm, or thinner in some cases using ALD. The encapsulation layer  56  is not limited to a layer of Al 2 O 3 , as other thin films of protective materials can be formed using ALD. 
     The isolation cavity  57  is a space or cavity formed under the layer of piezoelectric material  53 . The isolation cavity  57  can vary in size and proportions as compared to that shown in  FIG.  8   . In some cases, the isolation cavity  57  can be wider than that shown in  FIG.  8   , and the isolation cavity  57  can be larger (i.e., in a top-down view of length and width dimensions) than the layer of piezoelectric material  53 . The isolation cavity  57  can be formed by etching or other material-removal process steps. In some cases, the isolation cavity  57  can be formed with one or more supporting pillars, similar to the supporting pillars  48  shown in  FIG.  5   . 
     The overall thickness of the resonator  50  can be significantly reduced by using ALD to form the layer of piezoelectric material  53  as compared to sputtering. The overall thickness can be further reduced by using ALD to form electrodes  54 A- 54 C and  55 A- 55 C and the encapsulation layer  56 . The uniformity and conformity of the layer of piezoelectric material  53 , the electrodes  54 A- 54 C and  55 A- 55 C, and the encapsulation layer  56  can also be improved by using ALD. 
       FIG.  9    is a cross-sectional view of another example laterally-excited bulk acoustic wave resonator  60  (“resonator  60 ”) according to various embodiments described herein. The illustration of the resonator  60  is representative in  FIG.  9   . The positions, shapes, dimensions, and relative sizes of the layers and features of the resonator  60  are not necessarily drawn to scale in  FIG.  9   . Example dimensions of the resonator  60  are provided below, but the dimensions are not specifically limited. The layers and other features shown in  FIG.  9    are also not exhaustive, and the resonator  60  can include other layers, features, and elements that are not separately illustrated. Additionally, the resonator  60  can be formed as part of a larger integrated structure or circuit in combination with other devices and circuit elements. Such a larger integrated structure may include several resonators similar to the resonator  60 , among other integrated components. 
     The resonator  60  includes a substrate  61 , a layer of piezoelectric material  63  over the substrate  61 , first electrodes  64 A- 64 C interdigitated with second electrodes  65 A- 65 C, an encapsulation layer  66 , an isolation cavity  67 , and a floating metal plate  68 . The interdigitated electrodes  64 A- 64 C and  65 A- 65 C are similar to the interdigitated electrodes  24  and  25  shown in  FIG.  2   , although a cross section of the electrodes  64 A- 64 C and  65 A- 65 C is shown in  FIG.  9   . A limited number of the electrodes  64 A- 64 C and  65 A- 65 C are shown in  FIG.  9   , but it should be appreciated that a larger number of electrodes can be relied upon in practice. The electrodes  64 A- 64 C and  65 A- 65 C are in contact with a top surface of the piezoelectric material  63 . The floating metal plate  68  is in contact with the bottom surface of the piezoelectric material  63 . 
     The substrate  61  can be similar to the substrate  11  shown in  FIG.  1    and can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. Although not shown in  FIG.  9   , the resonator  60  can also include a supporting layer similar to the supporting layer  42  in  FIG.  5    in some cases. Such a layer of supporting material can be formed from silicon, for example, and be positioned over the substrate  61  and under the floating metal plate  68 . 
     The resonator  60  can generate an acoustic or mechanical wave when an alternating electric potential input signal is applied across the first electrodes  64 A- 64 C and the second electrodes  65 A- 65 C. The electric potential is not applied to the floating metal plate  68 . The electrodes  64 A- 64 C and  65 A- 65 C of the resonator  50  are not positioned on two different, opposing surfaces of the piezoelectric material  63 , as in the SMR  30  shown in  FIG.  3    and the FBAR  40  shown in  FIG.  5   . Instead, both the first electrodes  64 A- 64 C and the second electrodes  65 A- 65 C are formed on the top surface of the piezoelectric material  63 , which is similar to the SAW resonator  20  shown in  FIG.  2   , and the floating metal plate  68  is in contact with the bottom surface of the piezoelectric material  63 . The crystal structure c-axis orientation of the piezoelectric material  53  is oriented to excite a bulk acoustic or mechanical wave in the “X” direction, which is into the page in  FIG.  9   . That is, the c-axis orientation of the piezoelectric material  53  is oriented in the “X” direction, parallel to the top surface of the piezoelectric material  63  and into the page in  FIG.  9   . 
     The electrodes  64 A- 64 C and  65 A- 65 C and the floating metal plate  68  can be embodied as layers of highly conductive material, such as a metal or metal alloy, including copper, silver, gold, molybdenum, tungsten, titanium, platinum, or aluminum, alloys thereof, and other conductive layers. The electrodes  64 A- 64 C and  65 A- 65 C are formed to have a width “W,” with a pitch “P” separating them. The width “W” is larger than the pitch “P” in the resonator  60 . For example, the width “W” can be about 50-300 μm nm and the pitch “P” can be between 1-20 μm, although other dimensions can be relied upon. 
     The layer of piezoelectric material  63  can be embodied as a layer of PZT, BST, BaTiO3, AlScN, AlN, ZnO, or another piezoelectric material. In one preferred embodiment, the layer of piezoelectric material  63  is a layer of AlN, although other piezoelectric materials can be relied upon. In some cases, the layer of piezoelectric material  63  can include one or more layers of piezoelectric material, including two or more different types of piezoelectric material (e.g., a stack of two or more of layers of AlN, PZT, BST, BaTiO3, and AlScN), each of which can be formed using ALD processing techniques according to the embodiments. The encapsulation layer  66  can be embodied as a thin film of material to protect the resonator  60 . The encapsulation layer  66  can be a layer of Al 2 O 3  for example, another oxide material, or another suitable material to protect the resonator  60 . 
     The operating characteristics of the resonator  60 , including the frequency response, the accuracy of the frequency response, and the quality factor are determined in part by the thickness, conformity, and uniformity of the layer of piezoelectric material  63 . According to one aspect of the embodiments, the layer of piezoelectric material  63  can be embodied as a layer of AlN deposited by ALD. Forming the layer of AlN using ALD rather than sol-gel wet-chemical techniques, sputtering, pulsed laser deposition, or other techniques can result in thinner, more uniform, and more conformal layers of piezoelectric material. 
     The layer of piezoelectric material  63  can be formed as a thinner layer of piezoelectric material than in conventional resonators, such as when sputtering is used. In one example, the layer of piezoelectric material  63  can be formed as an AlN layer of 100 nm in thickness or less by ALD. The layer of piezoelectric material  53  can be formed to be a thinner layer of AlN, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. The layer of piezoelectric material  63  is not limited to a layer of AlN, as other thin films of piezoelectric materials can be formed using ALD. 
     The specific thickness of the layer of piezoelectric material  63  can be highly controlled or tailored in the resonator  60 , sometimes to within less than 10 nm of a target thickness, to within less than 5 nm of a target thickness, or even closer to a target thickness by using ALD. The layer of piezoelectric material  63  can be formed as thin as possible using a minimal number of ALD cycles in some embodiments. 
     The electrodes  64 A- 64 C and  65 A- 65 C and the floating metal plate  68  can also be formed using ALD. As one example, the electrodes  64 A- 64 C and  65 A- 65 C and the floating metal plate  68  can be formed as thin copper layers using ALD, but other highly conductive layers can be formed using ALD. If ALD is used to form the electrodes  64 A- 64 C and  65 A- 65 C and the floating metal plate  68 , the type of conductive material used can be determined in part based on the ALD process available for the material. One factor in the material selection for the electrodes  64 A- 64 C and  65 A- 65 C and the floating metal plate  68  is the desired thickness of the electrodes  64 A- 64 C and  65 A- 65 C and the floating metal plate  68 , which is also a factor in the response characteristics of the resonator  60 . In other cases, the electrodes  64 A- 64 C and  65 A- 65 C and the floating metal plate  68  can be formed by a sputtering process, PVD, or related techniques. 
     The encapsulation layer  66  can also be formed using ALD according to one aspect of the embodiments. The encapsulation layer  66  can be formed as a uniform and conformal thin film. In one example, the encapsulation layer  66  can be formed as a layer of Al 2 O 3  that is 100 nm or thinner, using ALD. The encapsulation layer  66  can also be formed to be a thinner layer of Al 2 O 3 , such as a layer of less than 10 nm, 5 nm, or thinner in some cases using ALD. The encapsulation layer  66  is not limited to a layer of Al 2 O 3 , as other thin films of protective materials can be formed using ALD. 
     The isolation cavity  67  is a space or cavity formed under the layer of piezoelectric material  63 . The isolation cavity  67  can vary in size and proportions as compared to that shown in  FIG.  9   . In some cases, the isolation cavity  67  can be wider than that shown in  FIG.  9   , and the isolation cavity  67  can be larger (i.e., in a top-down view of length and width dimensions) than the layer of piezoelectric material  63 . The isolation cavity  67  can be formed by etching or other material-removal process steps. In some cases, the isolation cavity  67  can be formed with one or more supporting pillars, similar to the supporting pillars  48  shown in  FIG.  5   . 
     The overall thickness of the resonator  60  can be significantly reduced by using ALD to form the layer of piezoelectric material  63  as compared to sputtering. The overall thickness can be further reduced by using ALD to form electrodes  64 A- 64 C and  65 A- 65 C and the encapsulation layer  66 . The uniformity and conformity of the layer of piezoelectric material  63 , the electrodes  64 A- 64 C and  65 A- 65 C, and the encapsulation layer  66  can also be improved by using ALD. 
       FIG.  10    illustrates an example method of manufacture of the resonator structures  50  and  60  shown in  FIGS.  8  and  9    according to various embodiments described herein. Although the method is described in connection with the resonator structures  50  and  60  shown in  FIGS.  8  and  9   , the method can also be relied upon to manufacture resonator structures similar to those shown. Additionally, although the method illustrates a specific order of steps in  FIG.  10   , the order of the steps can differ from that which is depicted. For example, two or more steps shown in succession can be performed, at least in part, at the same time. In some cases, one or more of the steps can be skipped or omitted. In other cases, additional steps not shown in  FIG.  10    can be relied upon, such as steps among or after the steps shown in  FIG.  10   . 
     At step  400 , the process includes providing a substrate. Referring to the examples shown in  FIGS.  8  and  9   , the substrates  51  and  61  are illustrated as example substrates that can be provided at step  400 . The substrate can be embodied as a silicon, silicon carbide, lithium niobate, sapphire, glass, ceramic, or another suitable type of substrate for the application. The substrate can be manufactured, sourced from a vendor, or formed or provided in any other suitable way at step  400 . 
     At step  402 , the process includes forming the floating metal plate  68  over the substrate. As one example, the floating metal plate  68  can be embodied as a layer of highly conductive material, such as a metal or metal alloy, including copper, silver, gold, molybdenum, tungsten, titanium, platinum, or aluminum, alloys thereof, and other conductive layers. The floating metal plate  68  can be formed using ALD. As one example, the floating metal plate  68  can be formed as a thin copper layer using ALD, but other highly conductive layers can be formed using ALD. In other cases, the floating metal plate  68  can be formed by a sputtering process, PVD, or related techniques. It is also noted that step  402  can be omitted in some cases, such as when forming the resonator  50  in  FIG.  8   . 
     At step  404 , the process includes depositing a layer of piezoelectric material by ALD processing steps. For example, the layer of piezoelectric material  53  can be deposited on or over the substrate  51  as shown in  FIG.  8   . In another example, the layer of piezoelectric material  63  can be deposited on or over the floating metal plate  68  as shown in  FIG.  9   . The ALD process can be started with an initiation for the deposit of piezoelectric material, such as AlN. In a reaction chamber for ALD processing steps, the initiation can include annealing, etching, or exposing surfaces to one or more gases, or other steps to remove contaminants or otherwise prepare surfaces for the deposit of materials using ALD. The crystal orientation of the layer of piezoelectric material can also be directed at step  404 . For example, the initiation can be tailored to form the layer of piezoelectric material having a crystal structure c-axis orientation for acoustic or mechanical waves in the “Z” direction shown in  FIG.  8    or in the “Y” direction shown in  FIG.  9   . The layer of piezoelectric material can also be formed to have another crystal structure c-axis orientation in some cases. After initiation, the ALD process at step  404  can proceed with the exposure using precursor chemicals or reactants, in a repeating sequence, as described herein. 
     The thickness of the layer of piezoelectric material formed at step  404  can be determined by the number of ALD cycles used, and the number of cycles can be tailored to grow the layer of piezoelectric material in a uniform and conformal way, with very high precision. In some cases, step  404  can also include trimming the layer of piezoelectric material formed, to further improve the uniformity of the layer, tailor the thickness of the layer, or otherwise modify the layer of piezoelectric material. Although trimming can be a relatively costly and time-consuming processing, any trimming performed at step  404  can be relatively minor as compared to if sputtering were used. 
     At step  406 , the process includes forming first and second electrodes on the layer of piezoelectric material. For example, the first electrodes  54 A- 54 C and second electrodes  55 A- 55 C can be formed on the layer of piezoelectric material  53 , as shown in  FIG.  8   . As another example, the first electrodes  64 A- 64 C and second electrodes  65 A- 65 C can be formed on the layer of piezoelectric material  63 , as shown in  FIG.  9   . The first and second electrodes can be formed as a thin layer or film of highly conducting material using ALD. The first and second electrodes can be formed at a thickness of 300 nm, 200 nm, or 100 nm or less using ALD. The first and second electrodes can be formed to be a thinner, however, such as a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness in some cases using ALD. 
     At step  408 , the process includes forming an encapsulation layer over the first and second electrodes. As examples, the encapsulation layer  56  shown in  FIG.  8    or the encapsulation layer  66  shown in  FIG.  9    can be formed. The encapsulation layer can be formed as a uniform and conformal thin film of Al 2 O 3  that is 100 nm or thinner, by ALD processing steps. The encapsulation layer  46  can also be formed to be a thinner layer of Al 2 O 3 , such as a layer of less than 10 nm, 5 nm, or thinner in some cases using ALD. The encapsulation layer  46  is not limited to a layer of Al 2 O 3 , as other thin films of protective materials can be formed using ALD. 
     At step  410 , the process includes forming an isolation cavity, such as one of the isolation cavities  57  or  67  shown in  FIG.  8  or  9   . The isolation cavity can be formed to any suitable size for the purpose of isolating the acoustic waves. In some cases, the isolation cavity can be wider than those shown in  FIGS.  8  and  9   , and the isolation cavity can be larger (i.e., in a top-down view of length and width dimensions) than the layer of piezoelectric material above it. The isolation cavity can be formed by etching or other material-removal process steps. In some cases, the isolation cavity can be formed with one or more supporting pillars to provide additional support, as described herein. 
     In some cases, the process shown in  FIG.  10    can include additional steps, such as forming one or more temperature compensation layers. For example, the process can include forming a temperature compensation layer, such as a layer of SiO2 or other material for temperature compensation, between steps  404  and  406 . In that case, a layer of SiO2 can be formed between the first and second electrodes and the piezoelectric material using ALD. Due to the use of ALD processing steps, the temperature compensation layer or layers can be more uniform, more conformal, and thinner than they would be if formed using other deposition techniques. As some examples, the layer of SiO2 can be formed to be a layer of less than 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm in thickness. 
     In other cases, the process can include forming a supporting layer over the substrate, after step  400  and before step  402 . The supporting layer can be embodied as a layer of silicon, although other supporting layers can be used. The supporting layer can be deposited using a suitable deposition technique, such as PVD, CVD, or a related technique. The supporting layer can be formed to any suitable thickness, and it is not necessary that the supporting layer be formed as a thin film. 
     Additionally, the process can include forming an acoustic mirror over the substrate, after step  400  and before step  402 . The acoustic mirror can be formed as alternating layers of material having high and low acoustic impedance or refractive indexes, such as a Bragg reflector. The acoustic mirror can provide acoustic isolation between the substrate and the resonator formed by the piezoelectric material that is later formed in step  404 . 
     The features, structures, or characteristics described above may be combined in one or more embodiments in any suitable manner, and the features discussed in the various embodiments can be interchangeable among the embodiments. In the foregoing description, certain details are provided to fully present the embodiments of the present disclosure. However, a person skilled in the art will appreciate that the technical solution of the present disclosure may be practiced without one or more of the specific details, or other methods, components, materials, and the like may be employed. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the present disclosure. 
     Although relative terms such as “on,” “below,” “upper,” “lower,” “top,” “bottom,” “right,” and “left” may be used to describe the relative spatial relationships of certain structural features, these terms are used for convenience only, as a direction in the examples. It should be understood that if the device is turned upside down, the “upper” component will become a “lower” component. When a structure or feature is described as being “on” (or formed on) another structure or feature, the structure can be positioned directly on (i.e., contacting) the other structure, without any other structures or features intervening between the structure and the other structure. When a structure or feature is described as being “over” (or formed over) another structure or feature, the structure can be positioned over the other structure, with or without other structures or features intervening between them. When two components are described as being “coupled to” each other, the components can be electrically coupled to each other, with or without other components being electrically coupled and intervening between them. When two components are described as being “directly coupled to” each other, the components can be electrically coupled to each other, without other components being electrically coupled between them. The “thickness” of the layers described herein can be measured from the top to the bottom of the page (i.e., in the “Z” direction) in the cross-sectional views. 
     Terms such as “a,” “an,” “the,” and “said” are used to indicate the presence of one or more elements and components. The terms “comprise,” “include,” “have,” “contain,” and their variants are used to be open ended and may include or encompass additional elements, components, etc., in addition to the listed elements, components, etc., unless otherwise specified. The terms “first,” “second,” etc., are used only as labels, rather than a limitation for a number of the objects. 
     Although embodiments have been described herein in detail, the descriptions are by way of example. The features of the embodiments described herein are representative and, in alternative embodiments, certain features and elements can be added or omitted. Additionally, modifications to aspects of the embodiments described herein can be made by those skilled in the art without departing from the spirit and scope of the present invention defined in the following claims, the scope of which are to be accorded the broadest interpretation so as to encompass modifications and equivalent structures.