Patent Publication Number: US-2019190483-A1

Title: Bulk acoustic wave resonator

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation under 37 C.F.R. § 1.53(b) of commonly owned U.S. patent application Ser. No. 15/336,277, entitled “Bulk Acoustic Wave Resonator” filed on Oct. 27, 2016. Priority is claimed under 35 U.S.C. § 130 to U.S. patent application Ser. No. 15/336,277. The entire disclosure of U.S. patent application Ser. No. 15/336,277 is specifically incorporated by reference. 
    
    
     BACKGROUND 
     Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used in filters, such as filters having electrically connected series and shunt resonators forming ladder and lattice structures. The filters may be included in a duplexer (diplexer, triplexer, quadplexer, quintplexer, notch filters, etc.) for example, connected between an antenna and a transceiver for filtering received and transmitted signals. 
     Various types of filters use mechanical resonators, such as bulk acoustic wave (BAW) resonators, including film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs), or surface acoustic wave (SAW) resonators. The resonators convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. A BAW resonator, for example, is an acoustic device comprising a stack that generally includes a layer of piezoelectric material between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack and the thickness of each layer (e.g., piezoelectric layer and electrode layers). One type of BAW resonator includes a piezoelectric film as the piezoelectric material, which may be referred to as an FBAR as noted above. FBARs resonate at GHz frequencies, and are thus relatively compact, having thicknesses on the order of microns and length and width dimensions of hundreds of microns. 
     Among other uses, acoustic resonators may be used as notch filters or band-pass filters with associated passbands providing ranges of frequencies permitted to pass through the filters. With increasing power requirements placed on devices (e.g., mobile phones), ever increasing power demands are placed on filters, and particularly the resonators of the filters. These increasing power demands can have adverse impacts on the performance and reliability of the resonators. For example, as radio frequency (RF) signals with greater electrical power are applied to known RF resonators, excessive self-heating can occur near the geometric center of the active acoustic stack, which is the farthest from the points where the active acoustic stack contacts the substrate (so-called anchor points where power is dissipated). As can be appreciated, the size of the hot spot depends on the frequency and power applied and absorbed. 
     The temperature gradient in the hot spot creates an active area divided into multiple resonators resonating at different frequencies, and with different acoustic properties. This temperature gradient also impacts the physical properties of the material (e.g., material stiffness), and creates acoustic discontinuities in the active acoustic stack. These acoustic discontinuities in the region of the hot spot results in further energy confinement, which is manifest in further heating at the hot spot. Ultimately, the confinement of acoustic waves and attendant concentration of thermal energy at the hot spot can cause at least bowing of the active acoustic stack in FBARs, adversely impacting the acoustic response of the resonator; and at most rupturing of the active acoustic stack and catastrophic loss of the acoustic resonator. 
     What is needed, therefore, is a BAW resonator that overcomes at least the shortcomings of known BAW resonators described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements. 
         FIG. 1A  is a top view of a bulk acoustic wave (BAW) resonator in accordance with a representative embodiment. 
         FIG. 1B  is a cross-sectional view of a BAW resonator in accordance with a representative embodiment. 
         FIG. 1C  is a cross-sectional view of a BAW resonator in accordance with a representative embodiment. 
         FIG. 1D  is a simplified schematic block diagram of an electrical filter  120  in accordance with a representative embodiment. 
         FIG. 2  is a top view of a BAW resonator in accordance with a representative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the representative embodiments. Such methods and apparatuses are clearly within the scope of the present teachings. 
     It is to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. Any defined terms are in addition to the technical and scientific meanings of the defined terms as commonly understood and accepted in the technical field of the present teachings. 
     As used in the specification and appended claims, the terms ‘a’, ‘an’ and ‘the’ include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, ‘a device’ includes one device and plural devices. 
     As used in the specification and appended claims, and in addition to their ordinary meanings, the terms ‘substantial’ or ‘substantially’ mean to with acceptable limits or degree. For example, ‘substantially cancelled’ means that one skilled in the art would consider the cancellation to be acceptable. 
     As used in the specification and the appended claims and in addition to its ordinary meaning, the term ‘approximately’ means to within an acceptable limit or amount to one having ordinary skill in the art. For example, ‘approximately the same’ means that one of ordinary skill in the art would consider the items being compared to be the same. 
     Relative terms, such as “above,” “below,” “top,” “bottom,” “upper” and “lower” may be used to describe the various elements&#39; relationships to one another, as illustrated in the accompanying drawings. These relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings. For example, if the device were inverted with respect to the view in the drawings, an element described as “above” another element, for example, would now be “below” that element. Similarly, if the device were rotated by 90° with respect to the view in the drawings, an element described “above” or “below” another element would now be “adjacent” to the other element; where “adjacent” means either abutting the other element, or having one or more layers, materials, structures, etc., between the elements. 
     According to representative embodiments described below, a bulk acoustic wave (BAW) resonator comprises: an acoustic reflector disposed in a substrate; a lower electrode disposed over the acoustic reflector; a piezoelectric layer disposed over the lower electrode; and an upper electrode disposed over the piezoelectric layer. A contacting overlap of the lower electrode, the piezoelectric layer and the upper electrode over the acoustic reflector comprising an active area of the BAW resonator. An opening exists in the upper electrode in a region of the BAW resonator susceptible to unacceptable overheating. 
     When connected in a selected topology, a plurality of the resonators can act as an electrical filter. For example, the acoustic resonators may be arranged in a ladder-filter or lattice-filter arrangement, such as described in U.S. Pat. No. 5,910,756 to Ella, and U.S. Pat. No. 6,262,637 to Bradley, et al., the disclosures of which are specifically incorporated herein by reference. The electrical filters may be used in a number of applications, such as in duplexers (diplexers, triplexers, quadplexers, quintplexers, etc.). 
     A variety of devices, structures thereof, materials and methods of fabrication are contemplated for the BAW resonators of the apparatuses of the present teachings. Various details of such devices and corresponding methods of fabrication may be found, for example, in one or more of the following U.S. patent publications: U.S. Pat. No. 6,107,721, to Lakin; U.S. Pat. Nos. 5,587,620, 5,873,153, 6,507,983, 7,388,454, 7,714,684, and 8,436,516 to Ruby et al.; U.S. Pat. Nos. 7,369,013, 7,791,434, and 8,230,562 to Fazzio, et al.; U.S. Pat. Nos. 8,188,810, and 7,280,007 to Feng et al.; U.S. Pat. Nos. 8,248,185, and 8,902,023 to Choy, et al.; U.S. Pat. No. 7,345,410 to Grannen, et al.; U.S. Pat. No. 6,828,713 to Bradley, et al.; U.S. Pat. Nos. 7,561,009, 7,358,831, 9,243,316 to Larson, III et al.; U.S. Pat. No. 9,197,185 to Zou, et al., U.S. Patent Application Publication No. 20120326807 to Choy, et al.; U.S. Pat. No. 7,629,865 to Ruby; U.S. Patent Application Publications Nos. 20110180391, and 20120177816 to Larson III, et al.; U.S. Patent Application No. 20140132117 to Larson III; U.S. Patent Application Publication No. 20070205850 to Jamneala et al.; U.S. Patent Application Publication No. 20110266925 to Ruby, et al.: U.S. Patent Application Publication No. 20130015747 to Ruby, et al.; U.S. Patent Application Publication No. 20130049545 to Zou, et al.; U.S. Patent Application Publication No. 20140225682 to Burak, et al.; U.S. Patent Publication Nos.: 20140118090 and 20140354109 to Grannen, et al.; U.S. Patent Application Publication Nos. 20140292150, and 20140175950 to Zou, et al.; U.S. Patent Application Publication No. 20150244347 to Feng, et al.; U.S. Patent Application Publication 20150311046 to Yeh, et al.; and U.S. Patent Application Publication 20150207489 to Bi, et al. The entire disclosure of each of the patents, and patent application publications listed above are hereby specifically incorporated by reference herein. It is emphasized that the components, materials and methods of fabrication described in these patents and patent applications are representative, and other methods of fabrication and materials within the purview of one of ordinary skill in the art are also contemplated. 
       FIG. 1A  is a top view of a bulk acoustic wave (BAW) resonator  100  in accordance with a representative embodiment. 
     The BAW resonator  100  comprises a lower electrode (not shown in  FIG. 1A ) disposed over a substrate  101 , and an upper electrode  104  disposed over a piezoelectric layer  103 . A passivation layer (not shown in  FIG. 1A ) may be provided over the upper electrode, as may other components (not shown) such as mass loading layers that are useful in improving the performance of the BAW resonator  100 . 
     The BAW resonator  100  comprises an interconnect side  112  for providing a signal input or signal output to the upper electrode  104 . A bridge (not shown in  FIG. 1A ) may be provided between the upper electrode  104  and the interconnect  112 ; and cantilevered portions (not shown in  FIG. 1A ) may be provided over one or more of the sides of the upper electrode  104 , excepting the side of the interconnect  112 . 
     As depicted more clearly in  FIGS. 1B and 1C , the region of contacting overlap of the lower electrode (not shown in  FIG. 1A ), upper electrode  104 , the piezoelectric layer  103  and the cavity (not shown in  FIG. 1A ), or other acoustic reflector (e.g., Bragg reflector (see  FIG. 1C )) is referred to as an active area of the BAW resonator  100 . The acoustic motion of particles is launched and propagated in this area. This acoustic motion contributes to the self-heating of the BAW resonator  100  described below. By contrast, an inactive area of the BAW resonator  100  comprises a region of overlap between lower electrode (not shown in  FIG. 1A ), or upper electrode  104 , or both, and the piezoelectric layer  103  is not disposed over the cavity, or other acoustic reflector—(e.g., Bragg reflector). 
     An opening  117  is provided in the upper electrode  104 . As described more fully below, the opening  117  is not made in layers beneath the upper electrode (e.g., the piezoelectric layer  103 , or the lower electrode (not shown in  FIG. 1A ). As will become clearer as the present description continues, the opening  117  is generally located substantially at the geometric center or central portion of the active area of the BAW resonator  100 , and has an areal dimension that approximates the area of the region of a BAW resonator, which does not include the opening, where self-heating is the greatest. More generally, and as will become clearer as the present description continues, a center of the opening  117  is located at substantially the greatest distance from an anchor point of BAW resonator  100 . 
     As described below in connection with various representative embodiments, the opening  117  is located in a region of the BAW resonator  100  that is susceptible to unacceptable levels of overheating caused by self-heating of the BAW resonator  100 . Generally, this region is comparatively far from a thermal ground, or anchor point, which is a portion of the inactive area that contacts the substrate  101 . Notably, the shape of the opening  117  is chosen to somewhat match the shape of the region where self-heating can be unacceptably high in known BAW resonators (i.e., BAW resonators without the opening). Illustratively, the opening  117  has a substantially circular shape, as shown. However, this is not essential, and other shapes are contemplated. 
     Because the thermal resistance is greater from the geometric center of a BAW resonator to the edge (anchor point) of the BAW resonator than it is from points on the BAW resonator closer to the edge (anchor point) of the BAW resonator, the propensity for unacceptable levels of self-heating is comparatively great in known BAW resonators. By thermal conduction (interaction between phonons-electrons) the heat wave is partially evacuated from the active area of the known BAW resonator farther away into the substrate, which helps to cool down the active area. As air is a comparatively poor thermal conductor, there is no significant heat conduction through the air, and, as such, no heat flow out of the top of or beneath the membrane of the known BAW resonator. However, the heat can be evacuated from the active area only by flowing through the anchor points. Thus, a thermal gradient is generated in the x-y plane. As noted, in the known BAW resonator, the center or central portion of the active area (membrane when over a cavity), which is located closer to the anchor point with substrate, is hotter than the perimeter of the active area. As such, the distance the heat has to travel from the center of the BAW resonator to the edge is comparatively large, and then thermal resistance degrades. In addition, there is potentially more non-uniform stress/strain in the membrane as it gets larger. Ultimately, the BAW resonator can operate at unacceptably high temperatures, which can reduce its electrical performance (mainly manifest in a reduced quality factor (Q) and electromechanical coupling (kt 2 )); reduce its power handling; degrade its insertion loss; and shift the passband of a filter comprising known BAW resonators. 
     By contrast, by removing a portion of the upper electrode  104  of the BAW resonator  100  to provide the opening  117 , the BAW resonator  100  does not have an active area at the opening  117 . Stated somewhat differently, the region of the active area in known BAW resonators, which is susceptible to higher levels of heating, is removed in the BAW resonators of the present teachings. Accordingly, substantially no electric field is supported in the region of the BAW resonator  100  at the opening, and energy cannot be absorbed in the region of the opening  117 . Beneficially, therefore, eliminating the ability for the piezoelectric effect to be supported in the opening  117  eliminates the propagation of acoustic waves in the region of the opening  117 , and thus eliminates the incidence of self-heating in this region of the BAW resonator  100 . As noted above, according to a representative embodiment, the center of the opening  117  is located at substantially the greatest distance from an anchor point of BAW resonator  100  to provide beneficial elimination of the region of the active area of the BAW resonator that is susceptible, if not most susceptible, to unacceptable overheating. 
     In order to maintain the impedance of the BAW resonator  100  with the region of the opening  117  not contributing to the active area of the device, the dimensions of the BAW resonator  100  are increased outside of the region of the opening  117  by an amount substantially equal to the area of the opening  117 . Because the distance from any point on the active area of the BAW resonator  100  to the anchor point is necessarily less by the elimination of the portion of the upper electrode  104  to form the opening  117 , the overall thermal resistance of the BAW resonator  100  is lower, and energy is dissipated from points on the active area closer to the anchor point. While points in the active area of the BAW resonator  100  that are farther from the edges (anchor points) experience greater self heating than those points closer to the edges, because the portion of the BAW resonator  100  that is most susceptible to extreme self-heating has been removed, the magnitude of the thermal resistance and thereby thermal heating of the BAW resonator  100  is reduced. Accordingly, the overall thermal profile of the BAW resonator  100  is beneficially reduced. This reduction in self-heating enables application of RF signals to the BAW resonator  100  having greater electrical power, with a substantially reduced incidence of unacceptable levels of self-heating. As can be appreciated, this improves the overall performance and reliability of the BAW resonator  100  compared to known BAW resonators. Specifically, when compared to known BAW resonators, BAW resonator  100  has an improved Q, acoustic coupling coefficient (kt 2 ); improved power handling; less degradation of insertion loss; and less, if any, shift in the passband of a filter comprising BAW resonators of the present teachings. 
       FIG. 1B  depicts a cross-sectional view of BAW resonator  100  contemplated for use in the various apparatuses of the present teachings. As can be appreciated, the BAW resonator  100  comprises an FBAR. It is emphasized that the BAW resonator  100  is merely illustrative, and that other known BAW resonators are contemplated for use in the various apparatuses of the present teachings. 
     The BAW resonator  100  comprises a substrate  101 , a lower electrode  102  disposed beneath a piezoelectric layer  103 , which comprises a first surface in contact with a lower electrode  102  and a second surface in contact with the upper electrode  104 . An optional passivation layer  105  is provided over the upper electrode  104 . As will become clearer as the present description continues, the substrate  101  comprises a material that is not only amendable to known microfabrication and semiconductor processing methods, but also has a comparatively good thermal conductivity. Generally, the substrate  101  comprises silicon (i.e., polycrystalline or monocrystalline), but other materials, such as gallium arsenide (GaAs) and indium phosphide (InP), are contemplated. 
     A cantilevered portion  106  of the upper electrode  104  is provided on at least one side of the upper electrode  104 . The cantilevered portion  106  may also be referred to as a ‘wing.’ It is emphasized that the use of the cantilevered portion  106  is merely illustrative, and other structures useful in improving the performance of the BAW resonator  100 ′ (e.g., a frame element comprising a metal or a dielectric material, and disposed adjacent to the perimeter of the active area  110 ) are contemplated for use in addition to, or instead of the cantilevered portion  106 . 
     The lower and upper electrodes  102 ,  104  each comprise one or two (bi-electrode) electrically conductive materials (e.g., molybdenum (Mo), W, Pt, Ru, Al, Ta, Cu, or Ru) and provide an oscillating electric field in the z-direction of the coordinate system shown (i.e., the direction of the thickness of the substrate  101 ). In the illustrative embodiment described presently, the z-axis is the axis for the TE (thickness-extensional or “longitudinal”) mode(s) for the resonator. In a representative embodiment, the piezoelectric layer  103  and lower and upper electrodes  102 ,  104  are suspended over a cavity  107  that substantially provides acoustic isolation with the substrate  101 . Accordingly, the BAW resonator  100  is a mechanical resonator, which can be electrically coupled via the piezoelectric layer  103 . Other configurations that foster mechanical resonance by FBARs are contemplated. For example, as described in connection with  FIG. 1C , rather than cavity  107 , the BAW resonator  100  can be located over an acoustic Bragg reflector, such as a mismatched acoustic Bragg reflector (not shown in  FIG. 1B ) formed in or on the substrate  101  to provide acoustic isolation. 
     The cantilevered portion  106  of the upper electrode  104  extends over a gap  108 , which illustratively comprises air. In a representative embodiment, a sacrificial layer (not shown) is deposited by a known technique over the lower electrode  102  and a portion of the piezoelectric layer  103 . 
     The BAW resonator  100  comprises a bridge  114  along the interconnection side  112 . The bridge  114  provides a gap  115 , which may be a void (e.g., air) or may be filled with a low acoustic impedance material (e.g., non-etchable borosilicate glass (NEBSG), carbon doped silicon dioxide (CDO), or silicon carbide (SiC)). The bridge  114  is described in above-referenced U.S. Pat. No. 8,248,185, and as such many of the details of the bridge  114  are not repeated in the present application to avoid obscuring the description of the representative embodiments of the BAW resonator  100 . As depicted in  FIG. 1B , the cavity  107  has an edge  116 , and the bridge  114  extends past the edge  116  of the cavity  107  (or similar reflective element, such as a mismatched Bragg reflector) and over the substrate  101 . As such, in a representative embodiment, the bridge  114  is disposed partially over the cavity  107 , extends over the edge  116  of the cavity  107 , and is disposed partially over the substrate  101 . 
     As noted above, the cantilevered portion  106  provides an improvement in the Q-factor. Similarly, the bridge  114  also provides an improvement in the Q-factor. Beneficially, the combination of the cantilevered portion  106  and the bridge  114  provides a further improvement in the Q-factor of the BAW resonator  100 . To this end, inclusion of the bridge  114  with the cantilevered portion  106  in the BAW resonator  100  results in an improvement in the Q-factor at parallel resonance (Qp) and some impact on the Q-factor at series resonance (Qs). This is somewhat expected since the bridge  114  predominantly impacts Qp, as described in above-referenced U.S. Pat. No. 8,248,185 to Choy, et al. 
     As described above, the region of contacting overlap of the lower and upper electrodes  102 ,  104 , the piezoelectric layer  103  and the cavity  107 , or other acoustic reflector (e.g., Bragg reflector (see  FIG. 1C )) is referred to as the active area of the BAW resonator  100 . The acoustic motion of particles is launched and propagated in this area. This acoustic motion contributes to the self-heating of the BAW resonator  100  described above. By contrast, an inactive area of the BAW resonator  100  comprises a region of overlap between lower electrode  102 , or upper electrode  104 , or both, and the piezoelectric layer  103  is not disposed over the cavity  107 , or other acoustic reflector (e.g., Bragg reflector). 
     The portion of the inactive area that contacts the substrate  101  may be referred to collectively as an anchor point of the BAW resonator  100  (in this case FBAR). The anchor point on the substrate  101  first ensures the mechanical robustness and support of the entire membrane formed by the acoustic stack over the cavity  107 . Notably, when the acoustic reflector is a cavity (e.g., cavity  107 ), the active area is often referred to as a membrane. 
     Opening  117  is provided in the upper electrode  104  and the passivation layer  105 . The opening  117  is not made in layers beneath the upper electrode (e.g., the piezoelectric layer  103 , or the lower electrode  102 . The opening  117  provided during fabrication of the upper electrode  104  and passivation layer  105  using known masking methods. As noted above, the opening  117  generally located substantially at the geometric center or central portion of the active area of the BAW resonator  100 , and has an areal dimension that approximates the area of the region of a BAW resonator, which does not include the opening, where self-heating is the greatest. 
     The cantilevered portion  106  extends beyond an edge of the active area  110  by a width  109  as shown. The electrical contact  111  is connected to a signal line (not shown) and electronic components (not shown) selected for the particular application of the BAW resonator  100 . This portion of the BAW resonator  100  comprises an interconnection side  112  of the BAW resonator  100 . The interconnection side  112  of the upper electrode  104  to which the electrical contact  111  is made does not comprise a cantilevered portion. By contrast, one or more non-connecting sides of the BAW resonator  100  may comprise cantilevered portions  106  that extend beyond the edge of the active area  110 . 
     The piezoelectric layer  103  comprises a highly textured piezoelectric layer (e.g., AlN), and thus has a well-defined C-axis. As described more fully below, in an apparatus comprising a plurality of BAW resonators  100 , the polarization of each BAW resonator impacts the type of the connection (e.g., series connection, anti-series connection) that is made between the BAW resonators  100 . As will be appreciated by one of ordinary skill in the art, the growth of piezoelectric material along a C-axis of the material dictates the polarization of the BAW resonator, and thus the type of connection to be implemented. As such, providing a highly-textured piezoelectric layer  103 , such as by methods described in the above-referenced U.S. Pat. No. 9,243,316 and U.S. Patent Application Publication No. 20120177816 to Larson III, et al., is useful in apparatuses comprising BAW resonator  100 . 
     In addition to being highly-textured, the piezoelectric layer  103  of representative embodiments may also comprise one or more rare-earth (e.g., scandium (Sc)) doped layers of piezoelectric material (e.g., aluminum nitride (AlN)) as described in certain patent applications incorporated by reference above (e.g., U.S. Patent Application Publication 20140132117 to John L. Larson III; and U.S. Patent Application Publication No. 20150244347 to Feng, et al.). 
       FIG. 1C  shows a cross-sectional view of a BAW resonator  100 ′ in accordance with a representative embodiment. Many of the features of the BAW resonator  100 ′ are common to those of BAW resonator  100  described in connection with representative embodiments in  FIGS. 1A and 1B . The details of common features, characteristics and benefits thereof are not repeated in order to avoid obscuring the presently described embodiments. 
     The BAW resonator  100 ′ comprises bridge  114  along the interconnection side  112 . The bridge  114  provides a gap  115 , which may be a void (e.g., air) or may be filled with a low acoustic impedance material. The bridge  114  is described in above-referenced U.S. Pat. No. 8,248,185, and as such many of the details of the bridge  114  are not repeated in the present application to avoid obscuring the description of the representative embodiments of the BAW resonator  100 . As depicted in  FIG. 1C , an acoustic Bragg reflector  107 ′ comprises alternating high acoustic impedance layers and low acoustic impedance layers. The acoustic Bragg reflector  107 ′ has an edge  116 ′, and the bridge  114  extends past the edge  116 ′ of the acoustic Bragg reflector  107 ′ and over the substrate  101 . As such, in a representative embodiment, the bridge  114  is disposed partially over the acoustic Bragg reflector  107 ′, extends over the edge  116 ′ of the acoustic Bragg reflector  107 ′, and is disposed partially over the substrate  101 . 
     As described above, the cantilevered portion  106  provides an improvement in the Q-factor. Similarly, the bridge  114  also provides an improvement in the Q-factor. Beneficially, the combination of the cantilevered portion  106  and the bridge  114  provides a further improvement in the Q-factor of the BAW resonator  100 ′. To this end, inclusion of the bridge  114  with the cantilevered portion  106  in the BAW resonator  100 ′ results in an improvement in the Q-factor at parallel resonance (Qp) and some impact on the Q-factor at series resonance (Qs). This is somewhat expected since the bridge  114  predominantly impacts Qp, as described in above-referenced U.S. Pat. No. 8,248,185 to Choy, et al. As noted above, the cantilevered portion  106  may also be referred to as a ‘wing.’ It is emphasized that the use of the cantilevered portion  106  is merely illustrative, and other structures useful in improving the performance of the BAW resonator  100 ′ (e.g., a frame element disposed adjacent to the perimeter of the active area are contemplated for use in addition to, or instead of the cantilevered portion  106 . 
     In a representative embodiment, the piezoelectric layer  103  and lower and upper electrodes  102 ,  104  are disposed over an acoustic Bragg reflector  107 ′, such as a mismatched acoustic Bragg reflector formed in or on the substrate  101 . FBARs provided over an acoustic Bragg reflector are sometimes referred to as solid mount resonators (SMRs) and, for example, may be as described in above-referenced U.S. Pat. No. 6,107,721 to Lakin. Accordingly, the BAW resonator  100 ′ is a mechanical resonator, which can be electrically coupled via the piezoelectric layer  103 . 
     The region of contacting overlap of the lower and upper electrodes  102 ,  104 , the piezoelectric layer  103  and the acoustic Bragg reflector  107 ′ is referred to as the active area  110  of the BAW resonator  100 ′. By contrast, the inactive area of the BAW resonator  100 ′ comprises a region of overlap between lower electrode  102  or upper electrode  104 , and the piezoelectric layer  103  is not disposed over the acoustic Bragg reflector  107 ′. 
     Opening  117  is provided in the upper electrode  104  and the passivation layer  105 . The opening  117  is not made in layers beneath the upper electrode (e.g., the piezoelectric layer  103 , or the lower electrode  102 . As noted above, the opening  117  providing during fabrication of the upper electrode  104  and passivation layer  105  using known masking methods. As noted above, the opening  117  generally located substantially at the geometric center of the active area of the BAW resonator  100 , and has an areal dimension that approximates the area of the region of a BAW resonator, which does not include the opening, where self-heating is the greatest. 
     As alluded to above, and as noted below, the BAW resonators and apparatuses including the BAW resonators of the present teachings are contemplated for use in electrical filter applications, for example. A basic filter design of either a ladder or a lattice topology is constituted of several sections. The number of sections is not limited but selected to trade off performances in terms of insertion loss, roll-off and rejection of the filter.  FIG. 1D  is a simplified schematic block diagram of an electrical filter  120  in accordance with a representative embodiment. The electrical filter  120  comprises series BAW resonators  121  and shunt BAW resonators  122 . By way of illustration, the series BAW resonators  121  and shunt BAW resonators  122  may comprise the acoustic resonators described in connection with the representative embodiments of  FIGS. 1A-1C . Notably, however, and as can be appreciated by one of ordinary skill in the art, self-heating is more problematic in known series resonators than in known shunt resonators. As such, in certain embodiments, only series BAW resonators  121  incorporate the teachings of the representative embodiments of  FIGS. 1A-1C . 
     The electrical filter  120  is commonly referred to as a ladder filter, and may be used for example in duplexer applications. Further details of a ladder-filter arrangement may be as described for example in U.S. Pat. No. 5,910,756 to Ella, and U.S. Pat. No. 6,262,637 to Bradley, et al. The disclosures of these patents are specifically incorporated by reference. It is emphasized that the topology of the electrical filter  120  is merely illustrative and other topologies are contemplated. Moreover, the acoustic resonators of the representative embodiments are contemplated in a variety of applications besides duplexers. 
     Referring to  FIG. 2 , a top view of a BAW resonator  200  is depicted. Many aspects of the BAW resonators  100 ,  100 ′ and terminology used in their descriptions are common to the BAW resonator  200 , and are often not repeated to avoid obscuring the presently described representative embodiments. 
     The BAW resonator  200  comprises an interconnect side  212  for providing a signal input or signal output to the upper electrode  204 . A bridge (not shown in  FIG. 1A ) may be provided between the upper electrode  204  and the interconnect  212 ; and cantilevered portions (not shown in  FIG. 1A ) may be provided over one or more of the sides of the upper electrode  204 , excepting the side of the interconnect  212 . 
     As depicted more clearly in  FIGS. 1B and 1C , the region of contacting overlap of the lower electrode (not shown in  FIG. 1A ), upper electrode  204 , the piezoelectric layer  203  and the cavity (not shown in  FIG. 1A ), or other acoustic reflector (e.g., Bragg reflector (see  FIG. 1C )) is referred to as an active area of the BAW resonator  200 . The acoustic motion of particles is launched and propagated in this area. This acoustic motion contributes to the self-heating of the BAW resonator  200  described below. By contrast, an inactive area of the BAW resonator  200  comprises a region of overlap between lower electrode (not shown in  FIG. 1A ), or upper electrode  204 , or both, and the piezoelectric layer  203  is not disposed over the cavity, or other acoustic reflector (e.g., Bragg reflector). 
     An opening  217  is provided in the upper electrode  204 . As described more fully below, the opening  217  is not made in layers beneath the upper electrode (e.g., the piezoelectric layer  203 , or the lower electrode (not shown in  FIG. 1A )). As will become clearer as the present description continues, the opening  217  is generally located substantially at the geometric center of the active area of the BAW resonator  200 , and has an areal dimension that approximates the area of the region of a BAW resonator, which does not include the opening, where self-heating is the greatest. 
     As described above, the opening  217  is located in a region of the BAW resonator  200  that is susceptible to unacceptable levels of overheating caused by self-heating of the BAW resonator  200 . Generally, this region is comparatively far from a thermal ground, or anchor point, which is a portion of the inactive area that contacts the substrate  201 . Notably, the shape of the opening  217  is chosen to somewhat match the shape of the region where self-heating can be unacceptably high in known BAW resonators (i.e., BAW resonators without the opening). Illustratively, the opening  217  has a substantially hexagonal shape, as shown. However, this is not essential, and other shapes are contemplated as noted above. 
     The various components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.