Patent Publication Number: US-2022231660-A1

Title: Surface acoustic wave (saw) devices with a diamond bridge enclosed wave propagation cavity

Description:
BACKGROUND 
     I. Field of the Disclosure 
     The field of the disclosure relates to devices for filtering one or more ranges of frequencies in analog signals in radio-frequency (RF) electronic devices. 
     IL Background 
     Mobile wireless device manufacturers pack ever-increasing capabilities into hand-held sized packages. Increasing capability means that more electronic components must fit into the package. This trend drives a size reduction of electronic components used for radio-frequency (RF) signal processing. A challenge to miniaturizing electronic components is finding a way to provide the same function in a physically smaller electronic device. Another challenge to miniaturizing electronic components is created by a physically smaller device dissipating the same or similar amount of power, leading to the same or similar heat generation. Heat generated within a physically smaller device leads to higher operating temperatures in a smaller package, which increases the potential to affect device performance and its life span. Thus, there is a desire to find ways for more effectively dissipating heat when reducing the device size. 
     One device that has been employed in RF signal processing circuits provided in smaller electronic devices for signal filtering is a surface acoustic wave (SAW) filter. The SAW filter removes or reduces the energy in one or more bands of frequencies from an input analog signal. A SAW filter filters frequencies by transforming electromagnetic wave propagation into mechanical wave propagation on the surface of a substrate material. SAW filters can be implemented in die sized SAW packages (DSSPs) for use in a mobile device, as an example. SAW DSSP technology has been significant in the reduction of mobile device sizes. However, there is continued demand for further size reduction of RF electronic devices. 
     SUMMARY OF THE DISCLOSURE 
     Aspects disclosed herein include surface acoustic wave (SAW) devices with a diamond bridge enclosed wave propagation cavity. Related fabrication methods are also disclosed. The SAW devices include a first interdigital transducer (IDT) and a second IDT each including interdigital electrodes disposed on a first surface of a substrate of piezoelectric material. The first IDT converts analog electrical radio-frequency (RF) signals into mechanical waves that propagate in a wave propagation region of the first surface of the substrate from the electrodes of the first IDT to the electrodes of the second IDT. The second IDT converts the mechanical waves back into analog electrical signals. The SAW device includes an enclosure that forms an air cavity above the first surface of the wave propagation region. The air cavity is provided to avoid interference with propagation of the mechanical waves in the substrate. The enclosure affects the overall height of the SAW device and also dissipates heat generated within the SAW device. 
     In exemplary aspects disclosed herein, the SAW device includes a diamond bridge enclosing an air cavity over the wave propagation region on the first surface of the substrate. The diamond bridge has a reduced height as compared to an enclosure formed by a cap substrate, for example, which enables miniaturization of RF circuits employing the SAW device as a filter for use in mobile devices. The thermal conductivity of the diamond bridge provides improved heat dissipation to avoid a reduction in performance and/or life span caused by heat generated in the SAW device. 
     In another exemplary aspect, processes of fabricating a SAW device including a diamond bridge are also disclosed. Disclosed processes include growing a diamond. layer over a buffer layer that is patterned to create a void to allow formation of a perimeter base of the diamond layer on the first surface of the substrate and around the wave propagation region. In a first process, the buffer layer is removed by deploying a buffer etch through the diamond material to create the air cavity. In a second process, a hole is formed in the diamond bridge to allow deployment of an etchant and removal of the etched buffer material through the hole. 
     In another exemplary aspect, a SAW device is disclosed. The SAW device includes a substrate comprising a piezoelectric material and a first surface. The SAW device includes a first IDT on the first surface of the substrate and a second IDT on the first surface of the substrate. The SAW device also includes a diamond bridge disposed over a wave propagation region between the first IDT and the second IDT in the first surface of the substrate and enclosing an air cavity above the wave propagation region. 
     In another exemplary aspect, a method of fabricating a SAW device is disclosed. The method includes forming a first IDT and a second IDT in a metal layer on a first surface of a substrate comprising a piezoelectric material, the first IDT and the second IDT disposed in a wave propagation region of the first surface of the substrate. The method also includes forming a diamond bridge disposed over the wave propagation region. 
     In another exemplary aspect, a circuit package including a package substrate and a SAW device coupled to the package substrate is disclosed. The SAW device in the circuit package includes a substrate comprising a piezoelectric material and a first surface. The SAW device includes a first IDT on the first surface of the substrate and a second IDT on the first surface of the substrate. The SAW device also includes a diamond bridge disposed over a wave propagation region between the first IDT and the second IDT in the first surface of the substrate and enclosing an air cavity above the wave propagation region. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGS. 
         FIG. 1  is a perspective view of interdigital transducers (IDTs) in a wave propagation region on a surface of a substrate in a surface acoustic wave (SAW) device without an enclosure forming an air cavity; 
         FIG. 2  is a cross-sectional side view of an exemplary SAW device including a diamond bridge enclosing a wave propagation region of a first surface of a substrate and. an air cavity above the wave propagation region for a reduced total device height, improved heat dissipation capability, and reduced mechanical deformation due to heating; 
         FIG. 3  is a cross-sectional side view of a conventional SAW device in which an air cavity is formed over a wave propagation region of a first surface of a functional substrate by a cap layer of a cap substrate bonded to a polymer frame; 
         FIG. 4A  is a diagram illustrating a process for fabricating the conventional SAW device in  FIG. 3 ; 
         FIG. 4B  is a cross-sectional side view of the conventional SAW device in a stage of fabrication according to the process in  FIG. 4A ; 
         FIGS. 5A-5H  illustrate exemplary fabrication stages in an exemplary process for fabricating the SAW device in  FIG. 2 ; 
         FIG. 6  is a flowchart illustrating an exemplary process for fabricating the SAW device in  FIG. 2  corresponding to the exemplary fabrication stages illustrated in  FIGS. 5A-5E  and continuing in either  FIGS. 5F-5H  or  FIGS. 7A-7E ; 
         FIGS. 7A-7E  illustrate a set of exemplary fabrication stages, in a second process option proceeding from the stages in  FIG. 5A-5E , for fabricating of a second example of the SAW device in  FIG. 2 ; 
         FIG. 8  is a top plan view of the SAW device shown in  FIG. 7A-7B  illustrating an exemplary location of a release hole in the diamond bridge; 
         FIG. 9  is a cross-sectional side view of an exemplary circuit package in which a plurality of the SAW devices in  FIG. 2  are mounted on a substrate; 
         FIG. 10  is a block diagram of an exemplary wireless communications device that includes a radio-frequency (RF) module including the SAW device in  FIG. 2 ; and 
         FIG. 11  is a block diagram of an exemplary processor-based system that includes a SAW device including a diamond bridge enclosing an air cavity over a wave propagation region of a substrate for a reduced total device height and an improved heat dissipation capability, as illustrated in  FIG. 2 , and according to any of the aspects disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to the drawing figures, several exemplary aspects of the present disclosure are described. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects. 
     Aspects disclosed herein include surface acoustic wave (SAW) devices with a diamond bridge enclosed wave propagation cavity. Related fabrication methods are also disclosed. The SAW devices include a first interdigital transducer (IDT) and a second IDT each including interdigital electrodes disposed on a first surface of a substrate of piezoelectric material. The first IDT converts analog electrical radio-frequency (RF) signals into mechanical waves that propagate in a wave propagation region of the first surface of the substrate from the electrodes of the first IDT to the electrodes of the second IDT. The second IDT converts the mechanical waves back into analog electrical signals. The SAW device includes an enclosure that forms an air cavity above the first surface of the wave propagation region. The air cavity is provided to avoid interference with propagation of the mechanical waves in the substrate. The enclosure affects the overall height of the SAW device and also dissipates heat generated within the SAW device. 
     In exemplary aspects disclosed herein, the SAW device includes a diamond bridge enclosing an air cavity over the wave propagation region on the first surface of the substrate. The diamond bridge has a reduced height as compared to an enclosure formed by a cap substrate, for example, which enables miniaturization of RF circuits employing the SAW device as a filter for use in mobile devices. The thermal conductivity of the diamond bridge provides improved heat dissipation to avoid a reduction in performance and/or life span caused by heat generated in the SAW device. 
     In another exemplary aspect, processes of fabricating a SAW device including a diamond bridge are also disclosed. The processes include growing a diamond layer over a buffer layer that is patterned to create a void to allow formation of a perimeter base of the diamond layer on the first surface of the substrate and around the wave propagation region. In a first process, the buffer layer is removed by deploying a buffer etch through the diamond material to create the air cavity. In a second process, a hole is formed in the diamond bridge to allow deployment of an etchant and removal of the etched buffer material through the hole. 
       FIG. 1  is a perspective view of a conventional SAW device  100  provided for comparison and to explain the exemplary aspects discussed below. The SAW device  100  includes first and second IDTs  102  and  104  in a wave propagation region  106  on a first surface  108  of a substrate  110 . The first IDT  102  includes contacts  112 A and  112 B for receiving a signal  114  provided on wires  116 A and  116 B. The contact  112 A is coupled to electrodes  118 A and the contact  112 B is coupled to electrodes  118 B. The electrodes  118 A are interdigitated or interleaved with the electrodes  118 B. The signal  114  creates a voltage Vi between the electrodes  118 A and  118 B in the first surface  108 . The substrate  110  is formed of a piezoelectric material  120 , which expands and contracts in the presence of the voltage V 1  based on the signal  114 . The expansion and contraction of the piezoelectric material  120  generates mechanical waves (not shown). Mechanical waves that propagate in a direction through the wave propagation region  106  to the second IDT  104  create a voltage V 2  between electrodes  122 A and  122 B, which are coupled, respectively, to contacts  124 A and  124 B. An output signal  126  is supplied on wires  128 A and  128 B based on the signal  114 . Propagation of the mechanical waves in the first surface  108  of the substrate  110  would be impeded by a protective layer disposed on the first surface  108 , but is not impeded by air immediately above the first surface  108 , as shown in  FIG. 1 . When the SAW device  100  is employed in a package, such as a die sized SAW package (DSSP) (not shown), an enclosure is provided on the first surface  108  to maintain an air cavity above the wave propagation region  106 . 
       FIG. 2  is a cross-sectional side view of an exemplary SAW device  200  including a first IDT  202 , a second IDT  204 , and a wave propagation region  206  between the first IDT  202  and the second IDT  204  with a diamond bridge  208  disposed over the wave propagation region  206 . The SAW device  200  has similar electrical function to the SAW device  100  in  FIG. 1 . The illustration in  FIG. 2  is provided for reference in the discussion of exemplary aspects presented below. The SAW device  200  includes a substrate  210  including a piezoelectric material  212 . The piezoelectric material  212  is a material with a high electromechanical coupling coefficient, such as lithium tantalate (LiTaO 3 ) or lithium niobate (LiNbO 3 ). Other options for the piezoelectric material  212  include aluminum nitride and scandium carbide. The substrate  210  includes a first surface  214  that extends in an X-axis direction and a Y-axis direction that is orthogonal to the X-axis direction. The SAW device  200  includes the first IDT  202  on the first surface  214  of the substrate  210 . The first IDT  202  includes a first plurality of electrodes  216 A interleaved with a second plurality of electrodes  216 B. The SAW device  200  includes the second IDT  204  on the first surface  214 , and the second IDT  204  includes a third plurality of electrodes  218 A interleaved with a fourth plurality of electrodes  218 B. The first IDT  202  and the second IDT  204  are formed in a patterned metal layer  215  disposed. on the first surface  214  of the substrate  210 . 
     The diamond bridge  208  is disposed over the wave propagation region  206  in the first surface  214  of the substrate  210 . The wave propagation region  206  is between the first IDT  202  and the second IDT  204 . The diamond bridge  208  also encloses an air cavity  220  above the wave propagation region  206 , A height H CAV  of the air cavity  220  extends in a Z-axis direction orthogonal to the X-axis and Y-axis directions. The diamond bridge  208  provides a reduced total device height of the SAW device  200 , improved heat dissipation capability, and reduced mechanical deformation compared to a conventional SAW device, as explained below. 
     In one example, the SAW device  200  may be a SAW filter that receives an input signal  222 , which is an RF signal. The SAW device  200  may be integrated into an RF front end module and configured to block frequencies of the input signal  222 . The input signal  222  applies a time-varying voltage V IN  between a solder bump  224  and another solder bump (not shown). The solder bump  224  is coupled to the first plurality of electrodes  216 A by a contact  225  and a conductive element  226 , and the other (not shown) solder bump is similarly coupled to the second plurality of electrodes  216 B. The first plurality of electrodes  216 A and the second plurality of electrodes  216 B transmit the input signal  222  to the piezoelectric material  212  of the substrate  210 . The piezoelectric material  212  expands or contracts in the presence of the voltage V IN . When the voltage V IN  changes periodically, the voltage V IN  causes time-varying expansion and contraction of the piezoelectric material  212 , which generates mechanical waves (not shown). Mechanical waves that propagate through the wave propagation region  206  to the second IDT  204  create a voltage V OUT  between the third plurality of electrodes  218 A and the fourth plurality of electrodes  218 B. In this example, the SAW device  200  generates an output signal  228  based on the input signal  222 . 
     Transmission of the input signal  222  through the first plurality of electrodes  216 A and the second plurality of electrodes  216 B and transmission of the output signal  228  through the third plurality of electrodes  218 A and the fourth plurality of electrodes  218 B causes thermal heating of the substrate  210 , especially near the first and second IDTs  202  and  204 . Heating of the substrate  210  increases electrical resistance, which wastes power, Heating of the substrate  210  also causes the piezoelectric material  212  to expand in the wave propagation region  206 . Expansion of the substrate  210  due to heating can change a distance between the first plurality of electrodes  216 A and the second. plurality of electrodes  216 B and between the third plurality of electrodes  218 A and the fourth plurality of electrodes  218 B. Such change in distance affects the dimensions of the waves and causes transmission phase loss, altering performance of the SAW device  200 . Excessive heating can also cause premature device failure. 
     To manage the generated heat, an exemplary aspect of the SAW device  200  is the diamond bridge  208  that encloses the wave propagation region  206  and the air cavity  220  above the first surface  214  of the substrate  210 . The diamond bridge  208  is formed of a diamond material  230 . Allotropes of carbon, such as graphite and diamond, are usually credited with having the highest thermal conductivities of any materials at room temperature. Thus, the diamond bridge  208  is an excellent thermal conductor for moving heat out of the substrate  210 . The diamond bridge  208  may be thermally coupled to a thermal interface material (TIM), a heat sink, or an air interface, for example, to effectively move excess heat away from the SAW device  200 . 
     The diamond bridge  208  includes a perimeter base  232  extending around the wave propagation region  206  of the first surface  214 . The diamond bridge  208  also includes a span portion  234  extending in the X-axis and Y-axis directions above the wave propagation region  206  of the first surface  214  from a first side of the perimeter base  232  to a second side of the perimeter base  232 . The perimeter base  232  is disposed on the patterned metal layer  215  and on the first surface  214  of the substrate  210 . The perimeter base  232  is between 45 and 55 micrometers (μm) in width. 
     The diamond bridge  208  has a total height H DB  in the range of 25-35 μm from the first surface  214  of the substrate  210  to a surface  236  of the diamond bridge  208 . A thickness of the substrate  210  is in the range of 50-70 μm. Thus, the height H DB  of the diamond bridge  208  is between 35% and 65% of a thickness of the substrate  210  in the Z-axis direction. The height H CAV  of the air cavity  220  is between 4 and 6 μm, to allow the mechanical waves to propagate in the first surface  214  unimpeded. Thus, the height H CAV  of the air cavity  220  is between 12% and 25% of the height H DB  of the diamond bridge  208  from the first surface  214  of the substrate  210  to the surface  236  of the diamond bridge  208  (i.e., of the span portion  234 ). Outside dimensions of the perimeter base  232  of the diamond bridge  208  extend about 1 millimeter (mm) along a first side (e.g., in the Y-axis direction) and about  1  mm along a second side orthogonal to the first side (e.g., in the X-axis direction). 
     The diamond material  230  provides the additional benefits of high rigidity and a low co-efficient of thermal expansion (CTE). Thus, in response to heating of the substrate  210 , as the heat from within the substrate  210  is conducted through the diamond bridge  208 , the diamond bridge  208  expands at a much lower rate than the substrate  210 . The rigidity of the diamond bridge  208 , which is affixed to the substrate  210 , inhibits mechanical deformation (i.e., due to heating) of the substrate  210 , thereby reducing the negative performance effects caused by heating in the SAW device  200 . 
       FIG. 3  is a cross-sectional side view of a conventional SAW device  300  provided for purposes of comparison to the SAW device  200  employing the diamond bridge  208  enclosing the wave propagation region  206  in  FIG. 2 . The SAW device  300  includes a wave propagation region  302  in a first surface  304  of a substrate  306 . An air cavity  308  over the wave propagation region  302  is enclosed by a cap substrate  310  bonded to a polymer frame  312 , which is fabricated in a process summarized in  FIGS. 4A and 4B  below. The polymer frame  312  is disposed on first and second IDTs  314  and  316  and the substrate  306  of the SAW device  300 . In contrast to the perimeter base  232  of the diamond bridge  208  in the SAW device  200 , the polymer frame  312  is a poor thermal conductor that does not effectively transfer heat away from the substrate  306 . In addition, as described below, the cap substrate  310  is formed of a piezoelectric material similar to the substrate  306  or may be a silicon (Si) wafer. Thus, the cap substrate  310  does not effectively move heat from the polymer frame  312  and out of the SAW device  300  like the diamond bridge  208  in  FIG. 2 . Furthermore, a CTE of the cap substrate  310  is not significantly lower than that of the substrate  306  and may be the same, so the cap substrate  310  does not inhibit mechanical deformation of the substrate  310  in the presence of internal heating. 
       FIG. 4A  is a diagram illustrating a process  400  for fabricating the conventional SAW device  300  in  FIG. 3  for comparison to distinguish the exemplary processes disclosed herein. An acoustic substrate  402  of the piezoelectric material for the substrate  306  is subjected to processes  404  to partially form a plurality of SAW devices  300 , the processes  404  producing a processed wafer  406 . The processes  404  include forming the IDTs  314  and  316  including electrodes  318  (shown in  FIG. 4B ). The processes  404  also include forming the polymer frames  312  around the wave propagation regions  302  of each of the SAW devices  300  as shown in  FIG. 3 . Next, a cap wafer  408  is disposed on top of the processed wafer  406 . The cap wafer  408  is bonded to the polymer frames  312  in each of the SAW devices  300  of the processed wafer  406  in a bonding step  410  to create a wafer assembly  412 . The wafer assembly  412  is diced to reduce portions of the cap wafer  408  in each of the SAW devices  300  to an area of the cap substrate  310  shown in  FIG. 4B . The wafer assembly  412  is also diced or cut through the processed wafer  406  to cingulate the SAW devices  300 . 
       FIG. 4B  is a cross-sectional side view of the conventional SAW device  300  in a stage of fabrication according to the process in  FIG. 4A . The SAW device  300  shown in  FIG. 4B  shows the electrodes  318  of the IDTs  314  and  316  on the substrate  306 , the polymer frame  312  formed on the IDTs  314  and  316 , and the cap substrate  310  bonded onto the polymer frame  312  to create the air cavity  308 . As noted above, the polymer frame  312  and the cap substrate  310  are formed of materials with much lower thermal conductivity than the diamond bridge  208  in  FIG. 2 . A height of the polymer frame  312  is about 50 μm, and the height of the cap substrate  310  above the first surface  304  of the substrate  306  is in the range of 50-70 μm. Thus, a total height H CAP  of the enclosure in the conventional SAW device  300  is in the range of 100-120 μm. 
     In contrast to the process shown in  FIG. 4A ,  FIGS. 5A-5H  are diagrams illustrating cross-sectional side views of a SAW device  500  in fabrication stages during an exemplary process  600  illustrated in a flowchart in  FIG. 6 . The process  600  is employed for fabricating a SAW device  500  that includes the diamond bridge  208  enclosing the wave propagation region  206  as shown in  FIG. 2 . The SAW device  500  may be the SAW device  200  including the diamond bridge  208  in  FIG. 2 , providing increased thermal conductivity, reduced mechanical deformation in response to heating, and reduced height for a smaller package size. In a first exemplary stage  500 A in  FIG. 5A , a substrate  502  is formed of a piezoelectric material with a high electromechanical coupling coefficient, such as LiTaO 3  or LiNbO 3 , for example. 
       FIG. 5B  illustrates an exemplary fabrication stage  500 B of step  602  of the process  600  in  FIG. 6  including forming a first IDT  504  and a second IDT  506  in a metal layer  508  on a first surface  510  of the substrate  502  comprising the piezoelectric material, the first IDT  504  and the second IDT  506  disposed in a wave propagation region  512  of the first surface  510  of the substrate  502 . 
     Forming the first MT  504  and the second IDT  506 , in one example, includes forming the metal layer  508  on the first surface  510  of the substrate  502 . The metal layer  508  may be formed of aluminum (Al) or copper (Cu). The metal layer  508  may also be implemented by a layer of non-metal conductive material such as doped polysilicon or silicide. Forming the first  504  includes patterning the metal layer  508  using photolithography and etching processes, for example, to remove portions of the metal layer  508 . The metal layer  508  is patterned to form first electrodes  514 A interleaved with second electrodes  514 B to form the first IDT  504 . The metal layer  508  is also patterned to form third electrodes  516 A interleaved with fourth electrodes  516 B of the second IDT  506 . The first and second IDTs  504  and  506  are formed in a wave propagation region  512  of the first surface  510  of the substrate  502 . Depending on the type of SAW device  500  (e.g., filter, oscillator, transformer, etc.) the metal layer  508  may include other structures in addition to the first IDT  504  and the second IDT  506  in the wave propagation region  512 . An insulation material  518  is disposed between the first and second electrodes  514 A,  514 B and the third and fourth electrodes  516 A,  516 B. 
       FIG. 5C  illustrates an exemplary fabrication stage  500 C of step  604  of the process  600  in  FIG. 6  including forming a diamond bridge  520  (shown in stage  500 E) over the wave propagation region  512 . In this regard, the illustration of fabrication stage  500 C of  FIG. 5C  also show the step  606  in process  600  in  FIG. 6  of forming the diamond bridge  520  including forming a buffer layer  522  on the metal layer  508  and on the first surface  510  of the substrate  502 . In the example in  FIG. 5C , the buffer layer  522  is formed by first depositing an oxide layer  524 , such as a layer of silicon dioxide (SiO 2 ). Thus, the terms buffer layer  522  and oxide layer  524  may be used interchangeably regarding this example. In one example, forming the buffer layer  522  includes treating the buffer layer  522  to damage a surface  526  of the buffer layer  522 . For example, treating the buffer layer  522  may include inducing ultrasonic damage to the surface  526  of the oxide layer  524  by methanol agitation. Other known methods for inducing damage to a layer of SiO 2  are also within the scope of the present disclosure. The damage to the surface  526  of the oxide layer  524  (buffer layer  522 ) reduces a rate of growth of a diamond. material  528  (see stage  500 E) on the buffer layer  522  compared to a rate of growth of the diamond material  528  on an undamaged surface. 
     As shown in the illustration of fabrication stage  500 D of  FIG. 5D  of the step  608  of the process  600  in  FIG. 6 , forming the diamond bridge  520  further includes patterning the buffer layer  522  to create a void  530  corresponding to a perimeter base  532  of the diamond bridge  520  disposed around the wave propagation region  512 . The illustration of stage  500 E shows that the buffer layer  522  has a thickness H CAV  which is the height of the air cavity  308  in  FIG. 3 . The void  530  in the buffer layer  522  exposes the metal layer  508  and the first surface  510  of the substrate  502 . The void  530  extends around the perimeter of the wave propagation region  512  and is created where the perimeter base  532  of the diamond bridge  520  will be formed, as shown in  FIG. 5E . 
       FIG. 5E  illustrates fabrication stage  500 E in the step  610  of process  600  in  FIG. 6  including forming the diamond material  528  of the diamond bridge  520 . Forming the diamond material  528  includes forming the perimeter base  532  of the diamond material  528  in the voids  530  of the butler layer  522  (step  612  of process  600  in  FIG. 6 ) and forming a span portion  534  of the diamond bridge  520  on the buffer layer  522  over the wave propagation region  512  (step  614  of process  600  in  FIG. 6 ). Forming the perimeter base  532  and the span portion  534  of the diamond bridge  520  includes growing the diamond material  528 , which may be achieved by chemical vapor deposition (CVD). In particular, the diamond material  528  may be formed in a plasma-enhanced CVD process using a direct-current (DC) discharge to generate the plasma. Alternatively, a hot-filament CVD (HFCVD) process may be used to form the diamond material  528  in the void  530  and on the buffer layer  522 . Due to the damage induced on the surface  526  of the oxide layer  524 , a rate of formation of the diamond material  528  is slower than a rate of formation of the diamond material  528  in the void  530  (i.e., on the first surface  510  of the substrate  502  and on the metal layer  508 ). Due to this difference in growth rate, the diamond bridge  520  may be grown to a desired height H DB  on the buffer layer  522  in approximately the same time that the perimeter base  532  is grown to the height H DB  in the void  530 , The diamond material  528  is thinned and/or planarized in a chemical mechanical polishing (CMP) process, using an ion beam or a laser. 
     The process  600  in  FIG. 6  further includes, as illustrated in fabrication stage  500 F shown in  FIG. 5F , the step  616  of removing the buffer layer  522  from under the span portion  534  to leave an air cavity  536  separating the span portion  534  from the wave propagation region  512 . In this regard, a first option for removing the buffer layer  522  is illustrated among further fabrication stages in  FIG. 5F .  FIGS. 7A and 7B  below illustrate further fabrication stages including an alternative option for removing the buffer layer  522 . 
     The fabrication stage  500 F illustrated in  FIG. 5F  shows the step of removing the buffer layer  522  under the span portion  534  of the diamond bridge  520  further comprises etching out the buffer layer  522  under the diamond bridge  520  by employing a buffer oxide etch process. According to such process, an etchant penetrates the diamond material  528 , chemically decomposes the buffer layer  522  and removes the buffer layer  522  residue through the diamond material  528 . As a result, the buffer layer  522  is removed from under the span portion  534  of the diamond bridge  520  to leave the air cavity  536  separating the span portion  534  from the wave propagation region  512  of the first surface  510  of the substrate  502 . An enclosed air cavity  536  protects the wave propagation region  512  from any materials that would interfere with propagation of mechanical waves in or on the first surface  510 . 
     In the fabrication stage  500 G in  FIG. 5G , the diamond material  528  outside the perimeter base  532  is removed to singulate the diamond bridge  520  enclosing the air cavity  536 . Fabrication stage  500 H in  FIG. 5H  illustrates solder bumps  538 A and  538 B on respective contacts  540 A and  540 B. The solder bumps  538 A and  538 B are coupled to the metal layer  508  in the first and second IDTs  504  and  506  by conductive elements  542 A and  542 B. The solder bumps  538 A and  538 B are employed to mount the SAW device  500  to a package (not shown), for example. The conductive elements  542 A and  542 B are formed in a process including depositing a titanium (Ti) adhesion layer and Cu seed layer, followed by patterned copper nickel (CuNi) traces. The contacts  540 A and.  54013  are formed as a patterned gold (Au) under bump metal (UBM). The solder bumps  538 A,  538 B are tin-silver-copper (Sn—Ag—Cu) solder balls. Other connective materials and structures could also be employed for coupling the SAW device  500  to a package. 
     Alternative fabrication stages  700 A- 700 E shown in  FIGS. 7A-7E  illustrate alternative steps for removing the buffer layer according to step  616  in the process  600  in  FIG. 6 . The fabrication stage  700 A in  FIG. 7A  is an alternative next fabrication stage after the fabrication stage  500 E in  FIG. 5E .  FIG. 7A  shows that a release hole  702  is formed in the span portion  534  of the diamond bridge  520 . In one example, the release hole  702  may be formed by a masked etch of the diamond material  528 , where the etch is an inductively coupled plasma (ICP) reactive ion etch (RIE) using a ratio of argon (Ar) and oxygen (O 2 ) in the plasma. The process for forming the release hole  702  is selective, stopping at the buffer layer  522 . 
     As shown in the fabrication stage  700 B in  FIG. 7B , removing the buffer layer  522  under the span portion  534  of the diamond bridge  520  in the alternative process further comprises deploying a buffer hydrofluoric acid (HF) etch through the release hole  702  and removing the buffer layer  522 . The buffer etch process described with reference to  FIG. 5A  is able to penetrate the diamond material  528 , but employing the release hole  702  enables more complete removal of the decomposed buffer layer  522 . (oxide layer  524 ), reducing an amount of residual SiO 2  or etch byproducts in the air cavity  536  that could interfere with wave propagation in the first surface  510  of the substrate  502 . 
     In fabrication stage  700 C in  FIG. 7C , the illustration shows that the release hole  702  is filled using a physical vapor deposition (PVD) fill of tungsten (W), Cu, or SiO 2  followed by planarization using CMP. Fabrication stage  700 D in  FIG. 7D  corresponds to fabrication stage  500 G, in which the diamond material  528  outside the perimeter base  532  is removed to singulate the diamond bridge  520 . Fabrication stage  700 E in  FIG. 7E  corresponds to fabrication stage  500 H, in which diamond solder bumps  704 A,  704 B, contacts  706 A,  706 B, and conductive elements  708 A,  708 B are disposed on the SAW device  500 . 
       FIG. 8  is a top plan view of the SAW device  500  fabricated according to the fabrication stages  500 A- 500 E in  FIGS. 5A-5E  and in the fabrication stage  700 A in  FIG. 7A , illustrating a location of the release hole  702  for deploying an etchant under the diamond bridge  520  and for removal of the buffer layer  522  (see  FIG. 5C ). As shown in this view, the release hole  702  may be formed outside the air cavity  536  to avoid interference with the air cavity  536  and the wave propagation region  512 . 
       FIG. 9  is an illustration of a circuit package  900  in which SAW devices  902  and  904 , corresponding to the SAW device  200  in  FIG. 2 , are coupled to a package substrate  906 . In one example, the circuit package  900  may further comprise an RF signal processing circuit (not shown). In such example, the SAW devices  902  and  904  may be SAW filters configured to block frequencies of an RF signal. Diamond bridges  908  of the SAW devices  902  and  904  reduce a height H DEV  of the SAW devices  902  and  904  extending above a package substrate  906  and also provide improved thermal conduction of heat generated in piezoelectric substrates  910 . 
       FIG. 10  illustrates an exemplary wireless communications device  1000  that includes RF components formed from one or more integrated circuits (ICs)  1002 , wherein any of the ICs  1002  can include a SAW device including a diamond bridge enclosing a wave propagation region of a first surface of a substrate and an air cavity above the wave propagation region for a reduced total device height, improved heat dissipation capability, and reduced mechanical deformation due to heating, as illustrated in any of  FIGS. 2, 5H, and 7E , and according to any of the aspects disclosed herein. The wireless communications device  1000  may include or be provided in any of the above-referenced devices, as examples. As shown in  FIG. 10 , the wireless communications device  1000  includes a transceiver  1004  and a data processor  1006 . The data processor  1006  may include a memory to store data and program codes. The transceiver  1004  includes a transmitter  1008  and a receiver  1010  that support bi-directional communications. In general, the wireless communications device  1000  may include any number of transmitters  1008  and/or receivers  1010  for any number of communication systems and. frequency bands. All or a portion of the transceiver  1004  may be implemented on one or more analog ICs, radio-frequency ICs (RFICs), mixed-signal ICs, etc. 
     The transmitter  1008  or the receiver  1010  may be implemented with a super-heterodyne architecture or a direct-conversion architecture. In the super-heterodyne architecture, a signal is frequency-converted between RF and baseband in multiple stages, e.g., from RF to an intermediate frequency (IF) in one stage, and then from IF to baseband in another stage. In the direct-conversion architecture, a signal is frequency-converted between RF and baseband in one stage. The super-heterodyne and direct-conversion architectures may use different circuit blocks and/or have different requirements. In the wireless communications device  1000  in  FIG. 10 , the transmitter  1008  and the receiver  1010  are implemented with the direct-conversion architecture. 
     In the transmit path, the data processor  1006  processes data to be transmitted and provides I and Q analog output signals to the transmitter  1008 . In the exemplary wireless communications device  1000 , the data processor  1006  includes digital-to-analog converters (DACs)  1012 ( 1 ),  1012 ( 2 ) for converting digital signals generated by the data processor  1006  into the I and Q analog output signals, e.g., I and Q output currents, for further processing. 
     Within the transmitter  1008 , lowpass filters  1014 ( 1 ),  1014 ( 2 ) filter the I and Q analog output signals, respectively, to remove undesired signals caused by the prior digital-to-analog conversion. Amplifiers (AMPs)  1016 ( 1 ),  1016 ( 2 ) amplify the signals from the lowpass filters  1014 ( 1 ),  1014 ( 2 ), respectively, and provide I and Q baseband signals. An upconverter  1018  upconverts the I and Q baseband signals with I and Q transmit (TX) local oscillator (LO) signals from a TX LO signal generator  1022  through mixers  1020 ( 1 ),  1020 ( 2 ) to provide an upconverted signal  1024 . A filter  1026  filters the upconverted signal  1024  to remove undesired signals caused by the frequency upconversion as well as noise in a receive frequency band. A power amplifier (PA)  1028  amplifies the upconverted signal  1024  from the filter  1026  to obtain the desired output power level and provides a transmit RF signal. The transmit RF signal is routed through a duplexer or switch  1030  and transmitted via an antenna  1032 . 
     In the receive path, the antenna  1032  receives signals transmitted by base stations and provides a received RF signal, which is routed through the duplexer or switch  1030  and provided to a low noise amplifier (LNA)  1034 . The duplexer or switch  1030  is designed to operate with a specific receive (RX)-to-TX duplexer frequency separation, such that RX signals are isolated from TX signals. The received RF signal is amplified by the LNA  1034  and filtered by a filter  1036  to obtain a desired RF input signal. Downconversion mixers  1038 ( 1 ),  1038 ( 2 ) mix the output of the filter  1036  with I and Q RX LO signals (i.e., LU_I and LO_Q) from an RX LO signal generator  1040  to generate I and Q baseband signals. The I and Q baseband signals are amplified by AMPs  1042 ( 1 ),  1042 ( 2 ) and further filtered by lowpass filters  1044 ( 1 ),  1044 ( 2 ) to obtain I and Q analog input signals, which are provided to the data processor  1006 . In this example, the data processor  1006  includes analog-to-digital converters (ADCs)  1046 ( 1 ),  1046 ( 2 ) for converting the analog input signals into digital signals to be further processed by the data processor  1006 . 
     In the wireless communications device  1000  of  FIG. 10 , the TX LO signal generator  1022  generates the I and Q TX LO signals used for frequency upconversion, while the RX LO signal generator  1040  generates the I and Q RX LO signals used for frequency downconversion. Each LO signal is a periodic signal with a particular fundamental frequency. A TX phase-locked loop (PLL) circuit  1048  receives timing information from the data processor  1006  and generates a control signal used to adjust the frequency and/or phase of the TX LO signals from the TX LO signal generator  1022 . Similarly, an RX PLL circuit  1050  receives timing information from the data processor  1006  and generates a control signal used to adjust the frequency and/or phase of the RX LO signals from the RX LO signal generator  1040 . 
     Wireless communications devices  1000  that each include a SAW device including a diamond bridge enclosing a wave propagation region of a first surface of a substrate and an air cavity above the wave propagation region for a reduced total device height, improved heat dissipation capability, and reduced mechanical deformation due to heating, as illustrated in any of  FIGS. 2, 5H, and 7E , and according to any of the aspects disclosed herein, may be provided in or integrated into any processor-based device. Examples, without limitation, include a set top box, an entertainment unit, a navigation device, a communications device, a fixed location data unit, a mobile location data unit, a global positioning system (GPS) device, a mobile phone, a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a tablet, a phablet, a server, a computer, a portable computer, a mobile computing device, a wearable computing device (e.g., a smart watch, a health or fitness tracker, eyewear, etc.), a desktop computer, a personal digital assistant (PDA), a monitor, a computer monitor, a television, a tuner, a radio, a satellite radio, a music player, a digital music player, a portable music player, a digital video player, a video player, a digital video disc (DVD) player, a portable digital video player, an automobile, a vehicle component, avionics systems, a drone, and a multicopter. 
     In this regard,  FIG. 11  illustrates an example of a processor-based system  1100  including a SAW device including a diamond bridge enclosing a wave propagation region of a first surface of a substrate and an air cavity above the wave propagation region for a reduced total device height, improved heat dissipation capability, and reduced mechanical deformation due to heating, as illustrated in any of  FIGS. 2, 5H, and 7E , and according to any aspects disclosed herein. In this example, the processor-based system  1100  includes one or more central processor units (CPUs)  1102 , which may also be referred to as CPU or processor cores, each including one or more processors  1104 . The CPU(s)  1102  may have cache memory  1106  coupled to the processor(s)  1104  for rapid access to temporarily stored data. As an example, the processor(s)  1104  could include a SAW device including a diamond bridge enclosing a wave propagation region of a first surface of a substrate and an air cavity above the wave propagation region for a reduced total device height, improved heat dissipation capability, and reduced mechanical deformation due to heating, as illustrated in any of  FIGS. 2, 5H, and 7E , and according to any aspects disclosed herein. The CPU(s)  1102  is coupled to a system bus  1108  and can intercouple master and slave devices included in the processor-based system  1100 . As is well known, the CPU(s)  1102  communicates with these other devices by exchanging address, control, and data information over the system bus  1108 . For example, the CPU(s)  1102  can communicate bus transaction requests to a memory controller  1110  as an example of a slave device. Although not illustrated in  FIG. 11 , multiple system buses  1108  could be provided, wherein each system bus  1108  constitutes a different fabric. 
     Other master and slave devices can be connected to the system bus  1108 . As illustrated in  FIG. 11 , these devices can include a memory system  1112  that includes the memory controller  1110  and one or more memory arrays  1114 , one or more input devices  1116 , one or more output devices  1118 , one or more network interface devices  1120  and one or more display controllers  1122 , as examples. Each of the memory system  1112 , the one or more input devices  1116 , the one or more output devices  1118 , the one or more network interface devices  1120 , and the one or more display controllers  1122  can include a SAW device including a diamond bridge enclosing a wave propagation region of a first surface of a substrate and an air cavity above the wave propagation region for a reduced total device height, improved heat dissipation capability, and reduced mechanical deformation due to heating, as illustrated in any of  FIGS. 2, 5H, and 7E , and according to any of the aspects disclosed herein. The input device(s)  1116  can include any type of input device, including, but not limited to, input keys, switches, voice processors, etc. The output device(s)  1118  can include any type of output device, including, but not limited to, audio, video, other visual indicators, etc. The network interface device(s)  1120  can be any device configured to allow exchange of data to and from a network  1124 . The network  1124  can be any type of network, including, but not limited to, a wired or wireless network, a private or public network, a local area network (LAN), a wireless local area network (WLAN), a wide area network (WAN), a BLUETOOTH™ network, and the Internet. The network interface device(s)  1120  can be configured to support any type of communications protocol desired. 
     The CPU(s)  1102  may also be configured to access the display controller(s)  1122  over the system bus  1108  to control information sent to one or more displays  1126 . The display controller(s)  1122  sends information to the display(s)  1126  to be displayed via one or more video processors  1128 , which process the information to be displayed into a format suitable for the display(s)  1126 . The display(s)  1126  can include any type of display, including, but not limited to, a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display, a light emitting diode (LED) display, etc. The display controller(s)  1122 , displays)  1126 , and/or the video processor(s)  1128  can include a SAW device including a diamond bridge enclosing a wave propagation region of a first surface of a substrate and an air cavity above the wave propagation region for a reduced total device height, improved heat dissipation capability, and reduced mechanical deformation due to heating, as illustrated in any of  FIGS. 2, 5H, and 7E , and according to any of the aspects disclosed herein. 
     Those of skill in the art will further appreciate that the various illustrative logical blocks, modules, circuits, and algorithms described in connection with the aspects disclosed herein may be implemented as electronic hardware, instructions stored in memory or in another computer readable medium and executed by a processor or other processing device, or combinations of both. The master and slave devices described herein may be employed in any circuit, hardware component, IC, or IC chip, as examples. Memory disclosed herein may be any type and size of memory and may be configured to store any type of information desired. To clearly illustrate this interchangeability, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. How such functionality is implemented depends upon the particular application, design choices, and/or design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. 
     The various illustrative logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices (e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration). 
     The aspects disclosed herein may be embodied in hardware and in instructions that are stored in hardware, and may reside, for example, in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, a hard disk, a removable disk, a CD-ROM, or any other form of computer readable medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a remote station. In the alternative, the processor and the storage medium may reside as discrete components in a remote station, base station, or server. 
     It is also noted that the operational steps described in any of the exemplary aspects herein are described to provide examples and discussion. The operations described may be performed in numerous different sequences other than the illustrated sequences. Furthermore, operations described in a single operational step may actually be performed in a number of different steps. Additionally, one or more operational steps discussed in the exemplary aspects may be combined. It is to be understood that the operational steps illustrated in the flowchart diagrams may be subject to numerous different modifications as will be readily apparent to one of skill in the art. Those of skill in the art will also understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations. Thus, the disclosure is not intended to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 
     Implementation examples are described in the following numbered clauses:
     1. A surface acoustic wave (SAW) device, comprising:
       a substrate comprising a piezoelectric material and a first surface;   a first interdigital transducer (IDT) on the first surface of the substrate;   a second IDT on the first surface of the substrate; and   a diamond bridge disposed over a wave propagation region between the first IDT and the second IDT of the first surface of the substrate and enclosing an air cavity above the wave propagation region.   
       2. The SAW device of clause 1, wherein:
       the first surface of the substrate extends in a first direction and a second direction orthogonal to the first direction; and   the diamond bridge comprises:
           a perimeter base extending around e wave propagation region of the first surface; and   a span portion extending in the first and second directions above the wave propagation region of the first surface from a first side of the perimeter base to a second side of the perimeter base,   
           
       3. The SAW device of clause 2, wherein:
       the first IDT and the second IDT are formed in a patterned metal layer disposed on the first surface of the substrate;   the first IDT comprises a first plurality of electrodes interleaved with a second plurality of electrodes;   the second IDT comprises a third plurality of electrodes interleaved with a fourth plurality of electrodes; and   the perimeter base of the diamond bridge is disposed on the patterned metal layer and on the first surface of the substrate.   
       4. The SAW device of any one of clauses 2 to 3, wherein the perimeter base has a width of 45-55 micrometers (μm).   5. The SAW device of any one of clauses 2 to 4, wherein:
       a height of the air cavity extends in a third direction orthogonal to the first surface between the first surface of the substrate and the span portion of the diamond bridge; and   the height of the air cavity is between 12% and 25% of the height of the diamond bridge from the first surface of the substrate to a surface of the span portion.   
       6. The SAW device of any one of clauses 1 to 5, wherein a height of the diamond bridge is between 35% and 65% of a thickness of the substrate.   7. The SAW device of any one of clauses 2 to 5, wherein the perimeter base extends 1 millimeter (mm) in the first direction and 1 mm in the second direction.   8. The SAW device of any one of clauses 1 to 7, integrated into a radio-frequency (RF) front end module.   9. The SAW device of any one of clauses 1 to 8 integrated into a device selected from the group consisting of: a set top box; an entertainment unit; a navigation device; a communications device; a fixed location data unit; a mobile location data unit; a global positioning system (GPS) device; a mobile phone; a cellular phone; a smart phone; a session initiation protocol (SIP) phone; a tablet; a phablet; a server; a computer; a portable computer; a mobile computing device; a wearable computing device; a desktop computer; a personal digital assistant (PDA); a monitor; a computer monitor; a television; a tuner; a radio; a satellite radio; a music player; a digital music player; a portable music player; a digital video player; a video player; a digital video disc (DVD) player; a portable digital video player; an automobile; a vehicle component; avionics systems; a drone; and a multicopter.   10. A method of fabricating a surface acoustic wave (SAW) device, the method comprising:
       forming a first interdigital transducer (IDT) and a second IDT in a metal layer on a first surface of a substrate comprising a piezoelectric material, the first IDT and the second IDT disposed in a wave propagation region of the first surface of the substrate; and   forming a diamond bridge disposed over the wave propagation region.   
       11. The method of clause 10, wherein forming the diamond bridge disposed over the wave propagation region comprises:
       forming a buffer layer on the metal layer and on the first surface of the substrate;   patterning the buffer layer to create voids corresponding to a perimeter base of the diamond bridge disposed around the wave propagation region;   forming a diamond material of the diamond bridge comprising:
           forming the perimeter base comprising the diamond material in the voids of the buffer layer; and   forming a span portion of the diamond bridge on the buffer layer over the wave propagation region; and   
           removing the buffer layer from under the span portion to leave an air cavity separating the span portion from the wave propagation region.   
       12. The method of clause 11, wherein forming the buffer layer further comprises treating the buffer layer to reduce a rate of formation of the diamond material.   13. The method of any one of clauses 10 to 12, wherein forming the first IDT and the second IDT comprises:
       forming the metal layer on the first surface of the substrate; and   patterning the metal layer to form:
           the first IDT comprising a first plurality of electrodes interleaved with a second plurality of electrodes; and   the second IDT comprising a third plurality of electrodes interleaved with a fourth plurality of electrodes.   
           
       14. The method of clause 12, wherein:
       forming the buffer layer comprises depositing an oxide layer; and   treating the buffer layer further comprises damaging a surface of the oxide layer.   
       15. The method of clause 14, wherein:
       depositing the oxide layer comprises forming a silicon dioxide (SiO 2 ) layer; and   damaging the surface of the oxide layer comprises inducing ultrasonic damage to the oxide layer by methanol agitation.   
       16. The method of any one of clauses 10 to 15, wherein forming the diamond bridge further comprises thinning and/or planarizing a surface of the diamond bridge.   17. The method of any one of clauses 11, 12, 14, and 15, wherein removing the buffer layer under the span portion of the diamond bridge further comprises etching out the buffer layer under the diamond bridge by a buffer oxide etch process.   18. The method of any one of clauses 11, 12, 14, 15, and 17, wherein:
       removing the buffer layer under the span portion of the diamond bridge further comprises:   forming a release hole in the span portion of the diamond bridge;   etching out the buffer layer through the release hole to form the air cavity; and   plugging the release hole to seal the air cavity.   
       19. The method of clause 18, wherein:
       forming the release hole in the span portion of the diamond bridge comprises etching the diamond bridge by inductively coupled plasma reactive ion etching with an argon (Ar) and oxygen (O 2 ) plasma.   
       20. A circuit package, comprising:
       a package substrate; and   a surface acoustic wave (SAW) device coupled to the package substrate, the SAW device comprising:
           a substrate comprising a piezoelectric material and a first surface;   a first interdigital transducer (IDT) on the first surface of the substrate;   a second IDT on the first surface of the substrate; and   a diamond bridge disposed over a wave propagation region between the first IDT and the second IDT of the first surface of the substrate and enclosing an air cavity above the wave propagation region.