Abstract:
An electronic package includes a die mounted on a first substrate; a second substrate disposed over the first substrate; a pillar wall extending between a surface of the die and an opposing surface of the second substrate to provide separation between the die and the second substrate, the pillar wall extending about a perimeter bounding the die and enclosing a cavity between the first and second substrates; and an encapsulating layer disposed over the first and second substrates and around the pillar wall. Substantially none of the encapsulating layer ingresses into the cavity.

Description:
BACKGROUND 
       [0001]    In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (rf) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators. 
         [0002]    As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications. 
         [0003]    One type of piezoelectric resonator is a bulk acoustic wave (BAW) resonator. Typically, there are two types of BAW resonators: a Film Bulk Acoustic Resonator (FBAR) and a solidly mounted bulk acoustic resonator (SMR). Both the FBAR and the SMR comprise acoustic stacks that are disposed over a reflective element. The reflective element of an FBAR is a cavity, normally in a substrate over which the acoustic stack is mounted. The reflective element of an SMR is a Bragg reflector comprising alternating layers of high acoustic impedance and low acoustic impedance layers. 
         [0004]    The BAW resonator has the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The FBAR includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed 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. 
         [0005]    Generally, a bulk acoustic wave (BAW) resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. The piezoelectric material may be a thin film of various materials, such as aluminum nitride (AlN), zinc oxide (ZnO), or lead zirconate titanate (PZT), for example. Thin films made of AlN are advantageous since they generally maintain piezoelectric properties at high temperature (e.g., above 400° C.). However, AlN has a lower piezoelectric coefficient d 33  than both ZnO and PZT, for example. 
         [0006]    In FBAR (film bulk acoustic resonators) devices, strain sensors, mechanical oscillators, and other electronic and microelectromechanical systems (MEMS) devices, it can be necessary to keep the device isolated mechanically and chemically from its surrounding environment. For example, the performance of an FBAR device is severely degraded if the motional device is in contact with the overmold compound of a typical microelectronics package. To this end, many devices have complicated and expensive encapsulation processes and methodologies. 
         [0007]    What is needed, therefore, is a structure that overcomes at least the shortcomings of known structures described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The illustrative 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. 
           [0009]      FIG. 1  is a cross-sectional view of an encapsulated electronic structure  100  in accordance with a representative embodiment. 
           [0010]      FIG. 2A  is a top view of a device substrate in accordance with a representative embodiment. 
           [0011]      FIG. 2B  is a cross-sectional view of a device substrate taken along the line A-A′ in  FIG. 2A . 
           [0012]      FIG. 3  is a cross-sectional view of an electronic structure in accordance with a representative embodiment. 
           [0013]      FIG. 4A  is a cross-sectional view of an encapsulated electronic structure in accordance with a representative embodiment. 
           [0014]      FIG. 4B  is a cross-sectional view of device substrate in accordance with a representative embodiment. 
           [0015]      FIG. 5  is a cross-sectional view of an encapsulated electronic structure in accordance with a representative embodiment. 
           [0016]      FIG. 6A  is a top view of a device substrate in accordance with a representative embodiment. 
           [0017]      FIG. 6B  is a cross-sectional view of a device substrate taken along the line A-A′ in  FIG. 6A . 
           [0018]      FIG. 7A  is a cross-sectional view of an electronic structure in accordance with a representative embodiment, and prior to sealing and encapsulating. 
           [0019]      FIG. 7B  is a cross-sectional view of an electronic structure in accordance with a representative embodiment. 
           [0020]      FIG. 7C  is a cross-sectional view of an electronic structure in accordance with a representative embodiment, and after deposition of an encapsulating layer. 
       
    
    
     DEFINED TERMINOLOGY 
       [0021]    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. The 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. 
         [0022]    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. 
         [0023]    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. 
         [0024]    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. 
       DETAILED DESCRIPTION 
       [0025]    In the following detailed description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of illustrative embodiments according to 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 illustrative embodiments. Such methods and apparati are clearly within the scope of the present teachings. 
         [0026]    Generally, it is understood that the drawings and the various elements depicted therein are not drawn to scale. Further, relative terms, such as “above,” “below,” “top,” “bottom,” “upper” and “lower” are used to describe the various elements&#39; relationships to one another, as illustrated in the accompanying drawings. It is understood that 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. 
         [0027]    The present teachings relate generally to packaged electronic devices and circuits. More specifically, the electronic devices and circuits comprise bulk acoustic wave (BAW) resonators including film bulk acoustic wave resonators (FBARs) and surface mount resonators (SMRs) in various applications. Moreover, BAW resonators of the present teachings may also comprise stacked bulk acoustic resonator (SBAR) device, a double bulk acoustic resonator (DBAR) device, or a coupled resonator filter (CRF) device. 
         [0028]    Contemplated applications of the BAW resonators of the present teachings include, but are not limited to communication filter applications and MEMs applications. For example, the bulk acoustic wave (BAW) resonators of the present teachings may be arranged in a ladder-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. 
         [0029]    Certain details of BAW resonators, including materials and methods of fabrication, may be found in one or more of the following commonly owned U.S. patents and patent applications: U.S. Pat. No. 6,828,713 to Bradley, et al.; U.S. Pat. No. 6,107,721, to Lakin; U.S. Pat. Nos. 5,587,620, 5,873,153, 6,384,697, 6,507,983, 7,275, 292, 7,388,454 and 7,629,865 to Ruby, et al.; U.S. Pat. No. 7,280,007 to Feng, et al.; U.S. Patent Application Publication No. 2007/0205850 to Jamneala, et al.; U.S. Pat. No. 8,248,185 to Choy, et al.; U.S. Patent Application Publication No. 2010/0327994 to Choy, et al.; U.S. patent application Ser. No. 13/662,460 entitled BULK ACOUSTIC WAVE RESONATOR HAVING PIEZOELECTRIC LAYER WITH MULTIPLE DOPANTS to John Choy, et al. and filed on Oct. 27, 2012; U.S. Patent Application Publications 20110180391 and 20120177816 to Larson, et al. The respective disclosures of the above patents, patent application publications and patent applications are specifically incorporated herein by reference. It is emphasized that the components, materials and method 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 contemplated. 
         [0030]    Examples of stacked bulk acoustic resonators, as well as their materials and methods of fabrication, may be found in U.S. Pat. No. 7,889,024 to Paul Bradley et al., U.S. Patent Application Publication No. 2012/0248941 to Shirakawa et al., and U.S. Patent Application Publications Nos. 2012/0218056, 2012/0280767, and 2012/0293278 to Burak et al. U.S. patent application Ser. No. 13/658,024 to Nikkel et al.; U.S. patent application Ser. No. 13/663,449 to Burak et al.; U.S. patent application Ser. No. 13/660,941 to Burak et al.; U.S. patent application Ser. No. 13/654,718 to Burak et al.; U.S. Patent App. Pub. No. 2008/0258842 to Ruby et al.; and U.S. Pat. No. 6,548,943 to Kaitila et al. Certain details of temperature compensation in the context of acoustic resonators are described in U.S. Pat. No. 7,345,410 to Grannen et al. and U.S. Pat. No. 7,408,428 filed Oct. 29, 2004 to Larson et al. The respective disclosures of the above patents and patent applications are specifically incorporated herein by reference. It is emphasized that the components, materials and method 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 contemplated. 
         [0031]      FIG. 1  is a cross-sectional view of an encapsulated electronic structure  100  in accordance with a representative embodiment. The encapsulated electronic structure  100  comprises a device substrate  101 . The device substrate  101  comprises a first BAW resonator  102  disposed over a first cavity  103  formed in the device substrate  101 , a second BAW resonator  104  disposed over a second cavity  105  formed in the device substrate  101 , and a third BAW resonator  106  disposed over a third cavity  107  formed in the device substrate  101 . As appreciated by one of ordinary skill in the art, the first, second and third BAW resonators  102 ,  104 ,  106  are FBARs. It is emphasized that first, second and third BAW resonators  102 ,  104 ,  106  may be SMRs with the first, second and third cavities  103 ,  105 ,  107  being replaced by an acoustic reflector (not shown) such as a Bragg reflector comprising alternating layers (not shown) of high and low acoustic impedance materials. 
         [0032]    A printed circuit board (PCB)  108  is disposed opposing the device substrate  101 . The PCB  108  may be, for example, a product module substrate, that is packaged in accordance with representative embodiments described herein. In a representative embodiment, the PCB  108  comprises a plurality of layers  109 ,  110 ,  111  and  112 . The PCB  108  and constituent layers  109 ,  110 ,  111  and  112  may be of a known material selected for the application of the circuits supporting the application of the first, second and third BAW resonators  102 ,  104 ,  106 . Illustratively, the constituent layers  109 ,  110 ,  111  and  112  may be FR4, epoxy glass or Teflon® PCB. 
         [0033]    Between the device substrate  101  and the PCB  108 , a first region  113 , a second region  114 , a third region  115  and a fourth region  116  are formed by the separation provided by a first pillar  117 , a second pillar  118 , a third pillar  119 , a fourth pillar  120  and a fifth pillar  121  as depicted in  FIG. 1 . Notably, upon encapsulation by a molding compound, the first through fourth regions  113 ˜ 116  form a cavity between opposing inner surfaces of the device substrate  101  and the PCB  108 . The first and second pillars  117 ,  118  provide structural support in the creation of space between the device substrate  101  and the PCB  108 . As appreciated by one of ordinary skill in the art, the first and second regions  113 ,  114  provide the “topside” cavities to the first, second and third BAW resonators  102 , 104 ,  106  required for their proper mechanical oscillation. Generally, as described below, a pillar wall is disposed circumferentially around the first, second and third BAW resonators  102 ,  104 ,  106  and their attendant circuitry. As can be appreciated, first and second pillars  117 ,  118  are two sides or walls of the circumferentially disposed pillar wall. The third, fourth and fifth pillars  119 ˜ 121  provide electrical connections between the first, second and third BAW resonators  102 ,  104 ,  106  and first, second and third electrical connections  122 ˜ 124  provided in the PCB  108 . As depicted for purposes of illustration, the first, second and third electrical connections  122 ˜ 124  are in electrical contact with first and second bond pads  125 ,  126  which are used for electrically connecting the first, second and third BAW resonators  102 ,  104 ,  106  to circuitry (not shown) upon mounting of the encapsulated electronic structure  100  to another substrate (not shown). 
         [0034]    An encapsulating layer  127  is provided over a side  128  of the device substrate  101  opposing the side of the device substrate  101  over which the first, second and third BAW resonators  102 , 104 , 106  are disposed. The encapsulating layer also extends along first and second sides  129 , 130  of the device substrate  101 , along side  132  of first pillar  117 , and along side  133  of second pillar  118 . Finally, the encapsulating layer  127  is provided over a top surface  134  of layer  112  of PCB  108 . 
         [0035]    The encapsulating layer  127  may be one of a number of known electronic molding compounds, and, as its name suggests, encapsulates the device substrate  101  with the PCB  108 . Illustratively, the encapsulating layer comprises a commercially available glass-filled epoxy, and has a thickness in the range of approximately 0.5 mm to approximately 4.0 mm. Notably, in the depicted embodiment the first and second pillars  117 ,  118  extend between the device substrate  101  and the PCB  108 , and prevent the ingress of the encapsulating layer  127  into any of the first through fourth regions  113 ˜ 116 , and thus the cavity formed therefrom. As such, the molding compound from which it is formed is prevented from contacting the first, second and third BAW resonators  102 , 104 , 106  or any of their attendant circuitry. As appreciated by one of ordinary skill in the art, the contacting of the molding compound can be detrimental to the performance of the BAW resonator circuit of which first, second and third BAW resonators  102 , 104 , 106  are a part. 
         [0036]    In certain embodiments, the encapsulating layer  127  may provide a hermetic seal of the first, second and third BAW resonators  102 , 104 , 106  or any of their attendant circuitry. However, this is not essential to ensure hermeticity. For example, the first and second pillars  117 ,  118  may form a hermetic seal through bonding to first solder pads  135  disposed over top surface  134  of the PCB  108 . Similarly, third and fourth pillars  119 ,  120  are bonded over top surface  134  of layer  112  of PCB  108  through second solder pads  136 , and are configured to make electrical contact to electrical connections (e.g., electrical connection  122 ) of the PCB  108 . Illustratively, the first and second solder pads  135 , 136  comprise Sn, SnAg, or a SnAgCu alloy deposited using a known method. In certain representative embodiments, a sealing layer  137  is provided over the first, second and third BAW resonators  102 , 104 , 106  and attendant circuitry, and provides a hermetic seal thereof. Illustratively, the sealing layer may be silicon nitride (Si 3 N 4 ), silicon carbide (SiC), silicon oxynitride (SiO x N y ), diamond-like carbon (DLC), or other suitable material within the purview of one of ordinary skill in the art. The sealing layer has a thickness between approximately 500 Å to approximately 4000 Å. 
         [0037]    The first through fifth pillars  117 ˜ 121  may be as described in U.S. Pat. Nos. 6,681,982, 6,592,019, 6,578,754 and 6,550,666; and commonly owned U.S. Pat. Nos. 8,314,472 and 8,344,504. The disclosures of these patents are specifically incorporated herein by reference. Generally, the first through fifth pillars  117 ˜ 121  are patterned using a thick spin-on resist or a thick dry film resist. The first through fifth pillars  117 ˜ 121  are then constructed by plating an appropriately conductive material. Illustratively, the first through fifth pillars  117 ˜ 121  are copper, but other materials are contemplated. Generally, the first through fifth pillars  117 ˜ 121  are metal or a metal alloy. The first through fifth pillars  117 ˜ 121  have a height “h” of approximately 10.0 μm to approximately 70.0 μm and a width “w” of approximately 10.0 μmm to 200.0 μm. 
         [0038]      FIG. 2A  is a top view of a device substrate  200  in accordance with a representative embodiment. Many aspects of the device substrate  200  are common to those of the device substrate  101  and are not necessarily repeated in order to avoid obscuring the description of the currently described representative embodiment. 
         [0039]    First through ninth BAW resonators  201 ˜ 209  are provided over the device substrate  200 . First pillar  210  is provided circumferentially around the first through ninth BAW resonators  201 ˜ 209  and their attendant circuitry (depicted by the dotted line in  FIGS. 2A ˜ 2 B and often referred to as the “BAW resonator active area”) forming a pillar wall to provide a barrier to the encapsulating layer (not shown in  FIG. 2A ) disposed over the device substrate  200 . Second through ninth pillars  211 ˜ 218  are provided over the device substrate  200  and provide electrical connections from the first through ninth BAW resonators  201 ˜ 209  to circuitry external to the device substrate (e.g., to circuitry on an opposing PCB such as PCB  108 ). Additionally, electrical traces can be provided over the device substrate  200  to enable selective electrical connections between components of the device substrate  200  to provide a desired electrical circuit (e.g., a ladder filter). For example, a first circuit trace  219  provides an electrical connection between the third BAW resonator  203  and the second pillar  211 . Similarly, second circuit trace  220  provides an electrical connection between the second BAW resonator  202 , the third BAW resonator  203 , the fourth BAW resonator  204  and the fourth pillar  213 . Finally, in the depicted embodiment, third circuit trace  221  is provided between seventh pillar  217  and ninth BAW resonator  209 . 
         [0040]    In accordance with representative embodiments, the first pillar  210  is formed over the device substrate  200  during standard processing using a method known to those practicing the art. Generally, the first pillar  210  comprises the same material (e.g., copper) and is fabricated at the same time as the second through ninth pillars  211 ˜ 218 , which generally provide electrical interconnectivity. Moreover, the second through ninth pillars  211 ˜ 218  are fabricated at the same time as the first pillar  210 . Notably, however, the second through ninth pillars  211 ˜ 218  are not necessarily formed of the same material as the first pillar  210  or as each other, and do not need to be formed at the same time. Furthermore, in a representative embodiment, the first through ninth pillars  206 ˜ 2   14  are formed near the end of the processing steps for creating the first through ninth BAW resonators  201 ˜ 209  over the device substrate  200 . 
         [0041]      FIG. 2B  is a cross-sectional view of device substrate  200  taken along the line A-A′ in  FIG. 2A . Notably, after formation of the first through ninth pillars  210 ˜ 218 , a solder  222  is provided (e.g., plated) over each of the first through ninth pillars  210 ˜ 218 . This solder can be Sn, SnAg, a SnAgCu alloy, or other suitable eutectic material, and illustratively has a thickness of approximately 10 μm to approximately 40 μm. After completion of BAW resonator die processing, the substrate (wafer) is singulated into individual die (i.e., device substrate  200 ) using a method known to one of ordinary skill in the art. The device substrate  200  can then be attached to the PCB (not shown in  FIG. 2B ). 
         [0042]      FIG. 3  is a cross-sectional view of an electronic structure  300  in accordance with a representative embodiment. Many aspects of the electronic structure  300  are common to those of the encapsulated electronic structure  100 , and the device substrate  200  described above, and are not necessarily repeated in order to avoid obscuring the description of the currently described representative embodiment. In essence, the embodiments of  FIG. 3  depict the electronic structure after bonding of a device substrate  301  to a PCB  308 , but prior to application of a suitable molding compound to encapsulate the electronic structure. 
         [0043]    The device substrate  301  comprises a first BAW resonator  302  disposed over a first cavity  303  formed in the device substrate  301 , a second BAW resonator  304  disposed over a second cavity  305  formed in the device substrate  301 , and a third BAW resonator  306  disposed over a third cavity  307  formed in the device substrate  301 . As appreciated by one of ordinary skill in the art, the first, second and third BAW resonators  302 ,  304 ,  306  are FBARs. It is emphasized that first, second and third BAW resonators  302 ,  304 ,  306  may be SMRs with the first, second and third cavities  303 ,  305 ,  307  being replaced by an acoustic reflector (not shown) such as a Bragg reflector comprising alternating layers (not shown) of high and low acoustic impedance materials. 
         [0044]    The printed circuit board (PCB)  308  is disposed opposing the device substrate  301 . The PCB  308  may be, for example, a product module substrate, that is packaged in accordance with representative embodiments described herein. In a representative embodiment, the PCB  308  comprises a plurality of layers  309 ,  310 ,  311  and  312 . The PCB  308  and constituent layers  309 ,  310 ,  311  and  312  may be of a known material such as described above, and selected for the application of the circuits supporting the application of the first, second and third BAW resonators  302 ,  304 ,  306 . 
         [0045]    Between the device substrate  301  and the PCB  308 , a first region  313 , a second region  314 , a third region  315  and a fourth region  316  are formed by the separation provided by a first pillar  317 , a second pillar  318 , a third pillar  319 , a fourth pillar  320  and a fifth pillar  321  as depicted in  FIG. 1 . Notably, upon encapsulation by a molding compound, the first through fourth regions  313 ˜ 316  form a cavity between opposing inner surfaces of the device substrate  301  and the PCB  308 . Moreover, the first and second pillars  317 ,  318  provide structural support in the creation of space between the device substrate  101  and the PCB  108 . As appreciated by one of ordinary skill in the art, the first and second regions  313 ,  314  provide the “topside” cavities to the first, second and third BAW resonators  102 , 104 ,  106  required for their proper mechanical oscillation. Generally, as noted above, a pillar wall is disposed circumferentially around the first, second and third BAW resonators  102 ,  104 ,  106  and their attendant circuitry. As can be appreciated, first and second pillars  317 ,  318  are two sides or walls of the circumferentially disposed pillar wall (e.g., two walls of first pillar  210  of the embodiment of  FIG. 2A ). The third, fourth and fifth pillars  319 ˜ 321  provide electrical connections between the first, second and third BAW resonators  102 , 104 , 106  and first, second and third electrical connections  322 ˜ 324  provided in the PCB  108 . As depicted for purposed of illustration, the first, second and third electrical connections  322 ˜ 324  are in electrical contact with first, and second bond pads  325 ,  326  which are used for electrically connecting the first, second and third BAW resonators  102 ,  104 ,  106  to circuitry (not shown) upon mounting of the encapsulated electronic structure  100  to another substrate (not shown). 
         [0046]    During fabrication, the device substrate  301  is attached to the PCB  308 . Notably, the device substrate  301  is flipped upside-down. First through fifth pillars  317 ˜ 321  are placed in contact with the PCB  308  and selectively in electrical contact with first, second and third electrical connections  322 ˜ 324  on the PCB  308  and subjected to high temperature (e.g., approximately 240° C.) to reflow the solder and make electrical and mechanical connections between the PCB  308  and the first through fifth pillars  317 ˜ 321 , resulting in the electronic structure  300 . As noted above, in a representative embodiment, the first and second pillars illustratively comprise copper with a layer of solder (e.g., first and second solder pads  335 ,  336 ) disposed thereover thereby creating a hermetic eutectic bond with the PCB  308 . 
         [0047]    After bonding of the device substrate  301  to the PCB  308  is completed, a molding compound (not shown in  FIG. 3 ) is provided over top surface  328  of the device substrate  301 , along side  332  of first pillar  317 , and along side  333  of second pillar  318  and over a top surface  334  of layer  312  of PCB  308 . In another representative embodiment, the molding compound (not shown in  FIG. 3 ) is disposed along side  332  of first pillar  317 , and along side  333  of second pillar  318  and over a top surface  334  of layer  312  of PCB  308 , but not over top surface  328 , which remains uncovered. 
         [0048]      FIG. 4A  is a cross-sectional view of an encapsulated electronic structure  400  in accordance with a representative embodiment. Many aspects of the encapsulated electronic structure  400  are common to those of the encapsulated electronic structure  100 , the device substrate  200  and the electronic structure  300  described above, and are not necessarily repeated in order to avoid obscuring the description of the currently described representative embodiment. 
         [0049]    As noted above, the pillars of representative embodiments illustratively comprise copper with a layer of solder disposed thereover thereby creating a hermetic eutectic bond with the PCB. Alternatively, the pillar wall that is disposed circumferentially around the BAW resonators and attendant circuitry may not contact the PCB, but rather is disposed close enough to avoid mold compound to ingress close to the BAW resonators and attendant circuitry. The encapsulated electronic structure  400  is fabricated in this manner. 
         [0050]    The encapsulated electronic structure  400  comprises a device substrate  401 . The device substrate  401  comprises a first BAW resonator  402  disposed over a first cavity  403  formed in the device substrate  401 , a second BAW resonator  404  disposed over a second cavity  405  formed in the device substrate  401 , and a third BAW resonator  406  disposed over a third cavity  407  formed in the device substrate  401 . As appreciated by one of ordinary skill in the art, the first, second and third BAW resonators  402 ,  404 ,  406  are FBARs. It is emphasized that first, second and third BAW resonators  402 ,  404 ,  406  may be SMRs with the first, second and third cavities  403 ,  405 ,  407  being replaced by an acoustic reflector (not shown) such as a Bragg reflector comprising alternating layers (not shown) of high and low acoustic impedance materials. 
         [0051]    A printed circuit board (PCB)  408  is disposed opposing the device substrate  401 . The PCB  408  may be, for example, a product module substrate, that is packaged in accordance with representative embodiments described herein. In a representative embodiment, the PCB  408  comprises a plurality of layers  409 ,  410 ,  411  and  412 . The PCB  408  and constituent layers  409 ,  410 ,  411  and  412  may be of a known material (e.g., FR4) selected for the application of the circuits supporting the application of the first, second and third BAW resonators  402 ,  404 ,  406 . 
         [0052]    Between the device substrate  401  and the PCB  408 , a first region  413 , a second region  414 , a third region  415  and a fourth region  416  are formed by the separation provided by a first pillar  417 , a second pillar  418 , a third pillar  419 , a fourth pillar  420  and a fifth pillar  421  as depicted in  FIG. 4A . Notably, upon encapsulation by a molding compound, the first through fourth regions  413 ˜ 416  form a cavity between opposing inner surfaces of the device substrate  401  and the PCB  408 . Moreover, the first and second pillars  417 ,  418  provide structural support in the creation of space between the device substrate  401  and the PCB  408 . As appreciated by one of ordinary skill in the art, the first and second regions  413 ,  414  provide the “topside” cavities to the first, second and third BAW resonators  402 ,  404 ,  406  required for their proper mechanical oscillation. Generally, as described above, a pillar wall is disposed circumferentially around the first, second and third BAW resonators  402 ,  404 ,  406  and their attendant circuitry (depicted by the dotted line in  FIGS. 4A ˜ 4 B and, as noted above, often referred to as the “BAW resonator active area”). As can be appreciated, first and second pillars  417 ,  418  are two sides or walls of the circumferentially disposed pillar wall. The third, fourth and fifth pillars  419 ˜ 421  provide electrical connections between the first, second and third BAW resonators  402 ,  404 ,  406  and first, second and third electrical connections  422 ˜ 424  provided in the PCB  408 . As depicted for purposes of illustration, the first, second and third electrical connections  422 ˜ 424  are in electrical contact with first and second bond pads  425 ,  426  which are used for electrically connecting the first, second and third BAW resonators  402 ,  404 ,  406  to circuitry (not shown) upon mounting of the encapsulated electronic structure  400  to another substrate (not shown). 
         [0053]    An encapsulating layer  427  is provided over a side  428  of the device substrate  401  opposing the side of the device substrate  401  over which the first, second and third BAW resonators  402 ,  404 ,  406  are disposed. The encapsulating layer also extends along first and second sides  430 ,  431  of the device substrate  401 , along side  432  of first pillar  417 , and along side  133  of second pillar  418 . Finally, the encapsulating layer  427  is provided over a top surface  134  of layer  414  of PCB  408 . 
         [0054]    The encapsulating layer  427  may be one of a number of known electronic molding compounds, and, as its name suggests, encapsulates the device substrate  401  with the PCB  408 . Notably, in the depicted embodiment the first and second pillars  417 ,  418  do not contact the PCB  408 , but rather a first gap  438  and a second gap  439  exist between the first and second pillars  417 ,  418 , respectively, and the PCB  408 . The first and second gaps  438 ,  439  are comparatively small having a height of approximately 250 nm to approximately 2000 nm so that encapsulating layer  427  cannot ingress into any of the first through fourth regions  413 ˜ 416 . As depicted in  FIG. 4A , the encapsulating layer does not extend across the width of the first gap  438  or the second gap  439 . As such, the molding compound is prevented from contacting the first, second and third BAW resonators  402 ,  404 ,  406  or any of their attendant circuitry. As appreciated by one of ordinary skill in the art, the contacting of the molding compound can be detrimental to the performance of the BAW resonator circuit of which first, second and third BAW resonators  402 ,  404 ,  406  are a part. 
         [0055]      FIG. 4B  is a cross-sectional view of device substrate  401  comprising first through fifth pillars  417 ˜ 421  prior to bonding to the PCB  408 . In the representative embodiment, first and second pillars  418 ,  419 , which form two sides of a pillar wall extending circumferentially around the first, second and third BAW resonators  402 ,  404 ,  406  and their attendant circuitry. In the representative embodiment, the first and second pillars  418 ,  419  (and all other pillars that form the pillar wall) are made of a different material than the third through fifth pillars  419 ˜ 421 . Alternatively, the first and second pillars  418 ,  419  (and all other pillars that form the pillar wall) are made of the same material as the third through fifth pillars  419 ˜ 421 . Notably, whereas the third through fifth pillars  419 ˜ 421  form electrical interconnections between the first, second and third BAW resonators  402 ,  404 ,  406  and their attendant circuitry, and first, second and third electrical connections  422 ˜ 424  of the PCB  408 , the first and second pillars  417 ,  418  may be formed from a photo-definable polymer that has been permanently cured to the device substrate  401 . Moreover, the first and second pillars  417 ,  418  have a height h 1  that is smaller than a height h 2  of the third through fifth pillars  419 ˜ 421 . As the height h 2  is less than the height h 1 , first and second gaps  438 ,  439  depicted in  FIG. 4A  exist between the respective ends of the first and second pillars  418 ,  419  and the upper surface  434  of the PCB  108 . The first and second pillars  417 ,  418  may form a hermetic seal through bonding to first solder pads  535  disposed over surface  534  of the PCB  408 , and electrical contact (e.g., to second bond pad  425 ) can be made via second solder pads  436 . As noted above after attachment of the device substrate  401  to the PCB  408 , a typical overmold process using a microelectronic molding compound known to one of ordinary skill in the art is carried out and results in the encapsulated electronic structure  400  depicted in  FIG. 4A . 
         [0056]      FIG. 5  is a cross-sectional view of an encapsulated electronic structure  500  in accordance with a representative embodiment. Many aspects of the encapsulated electronic structure  500  are common to those of the encapsulated electronic structure  100 , the device substrate  200 , the electronic structure  300 , and the encapsulated electronic structure  400  described above, and are not necessarily repeated in order to avoid obscuring the description of the currently described representative embodiment. 
         [0057]    The encapsulated electronic structure  500  comprises a device substrate  501 . The device substrate  501  comprises a first BAW resonator  502  disposed over a first region  503  formed in the device substrate  501 , a second BAW resonator  504  disposed over a second region  505  formed in the device substrate  501 , and a third BAW resonator  506  disposed over a third region  507  formed in the device substrate  501 . As appreciated by one of ordinary skill in the art, the first, second and third BAW resonators  502 ,  504 ,  506  are FBARs. It is emphasized that first, second and third BAW resonators  502 ,  504 ,  506  may be SMRs with the first, second and third regions  503 ,  505 ,  507  being replaced by an acoustic reflector (not shown) such as a Bragg reflector comprising alternating layers (not shown) of high and low acoustic impedance materials. 
         [0058]    A printed circuit board (PCB)  508  is disposed opposing the device substrate  501 . The PCB  508  may be, for example, a product module substrate, that is packaged in accordance with representative embodiments described herein. In a representative embodiment, the PCB  508  comprises a plurality of layers  509 ,  510 ,  511  and  512 . The PCB  508  and constituent layers  509 ,  510 ,  511  and  512  may be of a known material (e.g., FR4) selected for the application of the circuits supporting the application of the first, second and third BAW resonators  502 ,  504 ,  506 . 
         [0059]    Between the device substrate  501  and the PCB  508 , a first region  513 , a second region  514 , a third region  515  and a fourth region  516  are formed by the separation provided by a first pillar  517 , a second pillar  518 , a third pillar  519 , a fourth pillar  520  and a fifth pillar  521  as depicted in  FIG. 1 . Notably, upon encapsulation by a molding compound, the first through fourth regions  513 ˜ 516  form a cavity between opposing inner surfaces of the device substrate  501  and the PCB  508 . Moreover, the first and second pillars  517 ,  518  provide structural support in the creation of space between the device substrate  501  and the PCB  508 . As appreciated by one of ordinary skill in the art, the first and second regions  513 ,  514  provide the “topside” cavities to the first, second and third BAW resonators  502 ,  504 ,  506  required for their proper mechanical oscillation. Generally, as described below, a pillar wall is disposed circumferentially around the first, second and third BAW resonators  502 ,  504 ,  506  and their attendant circuitry. As can be appreciated, first and second pillars  517 ,  518  are two sides or walls of the circumferentially disposed pillar wall. The third, fourth and fifth pillars  519 ˜ 521  provide electrical connections between the first, second and third BAW resonators  502 ,  504 ,  506  and first, second and third electrical connections  522 ˜ 524  provided in the PCB  508 . As depicted for purposes of illustration, the first, second and third electrical connections  522 ˜ 524  are in electrical contact with first, and second bond pads  525 ,  526  which are used for electrically connecting the first, second and third BAW resonators  502 ,  504 ,  506  to circuitry (not shown) upon mounting of the encapsulated electronic structure  500  to another substrate (not shown). 
         [0060]    An encapsulating layer  527  is provided over a side  528  of the device substrate  501  opposing the side of the device substrate  501  over which the first, second and third BAW resonators  502 ,  504 ,  506  are disposed. The encapsulating layer also extends along first and second sides  530 ,  531  of the device substrate  501 , along side  532  of first pillar  517 , and along side  533  of second pillar  518 . Finally, the encapsulating layer  527  is provided over a top surface  534  of layer  512  of PCB  508 . 
         [0061]    The encapsulating layer  527  may be one of a number of known electronic molding compounds, and, as its name suggests, encapsulates the device substrate  501  with the PCB  508 . Notably, in the depicted embodiment the first and second pillars  517 ,  518  extend between the device substrate  501  and the PCB  508 , and prevent the ingress of the encapsulating layer  527  into any of the first through fourth regions  513 ˜ 516  and thus prevent the molding compound from which it is formed from contacting the first, second and third BAW resonators  502 ,  504 ,  506  or any of their attendant circuitry. As appreciated by one of ordinary skill in the art, the contacting of the molding compound can be detrimental to the performance of the BAW resonator circuit of which first, second and third BAW resonators  502 ,  504 ,  506  are a part. 
         [0062]    In certain embodiments, the encapsulating layer  527  may provide a hermetic seal of the first, second and third BAW resonators  502 ,  504 ,  506  or any of their attendant circuitry. However, this is not essential to ensure hermeticity. For example, the first and second pillars  517 ,  518  may form a hermetic seal through bonding to first and second solder pads  535 ,  536  disposed over surface  534  of the PCB  508 . In other representative embodiments, a sealing layer  537  is provided over the first, second and third BAW resonators  502 ,  504 ,  506  and attendant circuitry. 
         [0063]    The encapsulated electronic structure  500  is configured to have a first recess  538  and a second recess  539  formed in the PCB  508  through the removal of certain layers thereof. For example, in the representative embodiment depicted in  FIG. 5 , the first and second recesses  538 ,  539  are formed by the selective removal of portions of layers  511 ,  512  of the PCB  508 . Illustratively, the first and second recesses  538 ,  539  are formed using a known drilling or milling technique, but other ways of forming the recesses within the purview of one of ordinary skill in the art are contemplated. Moreover, the first and second recesses  538 ,  539  are located to be aligned with the BAW active area. Beneficially, and among other attributes, the first and second recesses  538 ,  539  accommodate devices, or components, or both (not shown) disposed on either the device substrate  501  or the PCB  508 , or both, while maintaining a substantially “flush” package structure. 
         [0064]      FIG. 6A  is a top view of a device substrate  600  in accordance with a representative embodiment. Many aspects of the device substrate  600  are common to those of the device substrate  200  and are not necessarily repeated in order to avoid obscuring the description of the currently described representative embodiment. 
         [0065]    Notably, in the representative embodiments described in connection with  FIGS. 6A ˜ 7 C, a pillar wall is not provided around the BAW resonators of the device substrate  600  and their attendant circuitry (depicted by the dotted line in  FIGS. 6A ˜ 6 B and often referred to as the “BAW resonator active area”). Rather, and as described in connection with embodiments of  FIGS. 7A and 7B , a polymer is dispensed and cured to create a topside cavity for the device substrate  600 . Similar to representative embodiments described above, the electrical interconnect pillars, and any mechanical pillars are provided on the device substrate  600 . After standard singulation, the device substrate  600  is placed face down on a PCB and attached using standard die attach techniques (e.g., a sequence of flux coating, die placement, and reflow). However, in the embodiments described presently, a viscous polymer is dispensed around the perimeter of the device substrate  600  and cured. 
         [0066]    First through ninth BAW resonators  601 ˜ 609  are provided over the device substrate  600 . First through ninth BAW resonators  601 ˜ 609  and their attendant circuitry are depicted by the dotted line in  FIGS. 6A ˜ 6 B and are often referred to as the “BAW resonator active area.” First through eighth pillars  610 ˜ 617  are provided over the device substrate  600  and provide electrical connections from the first through ninth BAW resonators  601 ˜ 609  to circuitry external to the device substrate (e.g., to circuitry on an opposing PCB such as PCB  108 ). Additionally, electrical traces can be provided over the device substrate  600  to enable selective electrical connections between components of the device substrate  600  to provide a desired electrical circuit (e.g., a ladder filter). For example, a first circuit trace  618  provides an electrical connection between the third BAW resonator  603  and the first pillar  610 . Similarly, second circuit trace  619  provides an electrical connection between the second BAW resonator  602 , the third BAW resonator  603 , the fourth BAW resonator  604  and the fourth pillar  613 . Finally, in the depicted embodiment, third circuit trace  620  is provided between sixth pillar  615  and ninth BAW resonator  609 . 
         [0067]    In accordance with representative embodiments, the first pillar  210  is formed over the device substrate  600  during standard processing using a method known to those practicing the art. Generally, the first through eighth pillars  610 ˜ 617  are formed near the end of the processing steps for creating the first through ninth BAW resonators  601 ˜ 609  over the device substrate  600 . 
         [0068]      FIG. 6B  is a cross-sectional view of device substrate  600  taken along the line A-A′ in  FIG. 6A . Notably, after formation of the first through eighth pillars  610 ˜ 617 , a solder  621  is provided (e.g., plated) over each of the first through eighth pillars  610 ˜ 217 . This solder can be Sn, SnAg, a SnAgCu alloy, or other suitable eutectic material, and illustratively has a thickness of approximately 10 μm to approximately 40 μm. After completion of BAW resonator die processing, the substrate (wafer) is singulated into individual die (i.e., device substrate  600 ) using a method known to one of ordinary skill in the art. The device substrate  600  can then be attached to the PCB (not shown in  FIG. 6B ). 
         [0069]      FIG. 7A  is a cross-sectional view of an electronic structure  700  in accordance with a representative embodiment, and prior to sealing and encapsulating. Many aspects of the electronic structure  700  are common to those of the encapsulated electronic structure  100 , the device substrate  200 , the electronic structure  300 , the encapsulated electronic structure  400 , the encapsulated electronic structure  500 , and the device substrate  600  described above, and are not necessarily repeated in order to avoid obscuring the description of the currently described representative embodiment. 
         [0070]    The electronic structure  700  comprises a first device substrate  701  and a second device substrate  702 . The first device substrate  701  comprises a first BAW resonator  703  disposed over a first cavity  704  formed in the first device substrate  701 . The second device substrate  702  comprises a second BAW resonator  705  disposed over a second cavity  706  formed in the second device substrate  702 . As appreciated by one of ordinary skill in the art, the first and second BAW resonators  703 ,  705  are FBARs. It is emphasized that first and second BAW resonators  703 ,  705  may be SMRs with the first and second cavities  704 , 706  being replaced by an acoustic reflector (not shown) such as a Bragg reflector comprising alternating layers (not shown) of high and low acoustic impedance materials. 
         [0071]    A printed circuit board (PCB)  707  is disposed opposing the first device substrate  701 . The PCB  707  may be, for example, a product module substrate, that is packaged in accordance with representative embodiments described herein. In a representative embodiment, the PCB  707  comprises a plurality of layers  708 ,  709 ,  710  and  711 . The PCB  707  and constituent layers  708 ,  709 ,  710  and  711  may be of a known material selected for the application of the circuits supporting the application of the first and second BAW resonators  703 ,  705 . 
         [0072]    Between the first device substrate  701  and the PCB  708 , a first region  712  and a second region  713  are formed by the separation provided by a first pillar  714 , a second pillar  715 , a third pillar  716 , a fourth pillar  717  and a polymer (not shown in  FIG. 7A ) described below. As appreciated by one of ordinary skill in the art, the first and second regions  712 ,  713  provide the “topside” cavities to the first and second BAW resonators  703 ,  705  required for their proper mechanical oscillation. Generally, as described below, a sealing wall is formed by first pillar  714 , a second pillar  715 , a third pillar  716 , a fourth pillar  717  and the polymer circumferentially around the first and second BAW resonators  703 ,  706  and their attendant circuitry (‘the active region’ such as outlined by the dotted line in  FIGS. 2A ˜ 2 B,  6 A˜ 6 B). As can be appreciated, first through fourth pillars  714 ˜ 717  provide electrical connections between the first and second BAW resonators  703 ˜ 705  and first, second and third electrical connections  719 ˜ 721  provided in the PCB  707 . First through fourth pillars  714 ˜ 717  also provide mechanical support in maintaining the separation between the first and second device substrates  701 ,  702  and the PCB  707 . As depicted for purposes of illustration, the first, second and third electrical connections  719 ˜ 721  are in electrical contact with first through second bond pads  721 ˜ 723  which are used for electrically connecting the first and second BAW resonators  703 ,  705  and circuitry on the device substrate  200  to circuitry (not shown) upon mounting of the electronic structure  700  to another substrate (not shown). 
         [0073]      FIG. 7B  is a cross-sectional view of an electronic structure  700  after deposition of a polymer  727  in accordance with a representative embodiment. Notably, the polymer  727  is dispensed over the PCB  707  in a pattern around the respective perimeters of each of the first and second device substrates  701 ,  702 , and then cured. In a representative embodiment, the polymer  727  comprises a suitable epoxy material, silicone or polyimide provided a at a height and width sufficient to create a seal around the first and second regions  712 ,  713  as depicted. As will be appreciated by one of ordinary skill in the art, the width of the polymer  727  measured from first through fourth pillars  714 ˜ 717  depends on the viscosity of the material selected for the polymer  727 . Illustratively, the polymer has a width of approximately 1 mm, and a height that extends above each of first through fourth pillars  714 ˜ 717  as depicted in  FIG. 7B . After deposition of the polymer  727 , it is cured by a known technique for the particular material selected for the polymer  727 . Beneficially, after curing, the polymer  727  serves as a seal ring to substantially prevent overmold compound from the subsequent high pressure overmolding step to ingress into the BAW resonator active area. 
         [0074]    The encapsulating layer  727  may be one of a number of known electronic molding compounds, and, as its name suggests, encapsulates the first and second device substrates  701 ,  702  with the PCB  707 . Notably, after deposition of the polymer  727  and encapsulation by the encapsulation layer  727 , the first and second regions  712 ,  713  form respective cavities between opposing inner surfaces of first and second device substrates  701 ,  702  and the PCB  107 . In the depicted embodiment the first through fourth pillars  714 ˜ 717  extend between the first and second device substrates  701 ,  702  and the PCB  707 , and prevent the ingress of the molding compound (not shown in  FIG. 7B ) into any of the first and second regions  712 ,  713  and thus prevent the molding compound from which they are formed from contacting the first and second BAW resonators  704 ,  706  or any of their attendant circuitry. As appreciated by one of ordinary skill in the art, the contacting of the molding compound can be detrimental to the performance of the BAW resonator circuit of which first and second BAW resonators  704 ,  706  are a part. 
         [0075]    In certain embodiments, the encapsulating layer  727  may provide a hermetic seal of the first and second BAW resonators  704 ,  706  or any of their attendant circuitry. However, this is not essential to ensure hermeticity. For example, the first through fourth pillars  714 ˜ 717  may form a hermetic seal through bonding to first and second solder pads  725 ,  726  disposed over surface  734  of the PCB  707 . In other representative embodiments, a sealing layer  737  is provided over the first and second BAW resonators  704 ,  706  and attendant circuitry. 
         [0076]      FIG. 7C  is a cross-sectional view of an electronic structure  700  after deposition of an encapsulating layer  729 . The encapsulating layer  729  is provided over first and second sides  730 ,  731  and over a top surface  732  of the first device substrate  701 ; over first and second sides  733 ,  734  and over a top surface  735  of the second device substrate  702 ; and over exposed portions of the polymer  727  to substantially completely encapsulate the first and second device substrates  701 ,  702 . 
         [0077]    Notably, piezoelectric layers consisting of both undoped and doped portions of piezoelectric material have been discussed herein with reference to BAW resonator devices, including FBARs and SMRs, as examples. However, it is understood that such piezoelectric layers may be formed in resonator stacks of various other types of resonator devices, without departing from the scope of the present teachings. For example, piezoelectric layers consisting of undoped and doped portions of piezoelectric material may be formed in resonator stacks of a stacked bulk acoustic resonator (SBAR) device, a double bulk acoustic resonator (DBAR) device, or a coupled resonator filter (CRF) device. 
         [0078]    In accordance with illustrative embodiments, bulk acoustic wave (BAW) resonators for various applications such as in electrical filters are described having an electrode comprising a cantilevered portion. Additionally, bulk acoustic wave (BAW) resonators for various applications such as in electrical filters are described having an electrode comprising a cantilevered portion and a bridge. One of ordinary skill in the art appreciates that many variations that are in accordance with the present teachings are possible and remain within the scope of the appended claims. These and other variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.