Abstract:
A device includes: a lead frame having an aperture in a central portion thereof; at least one acoustic transducer mounted on the lead frame above the aperture and configured to convert between acoustic energy and an electrical signal with low signal losses; a housing connected to the lead frame and including a base portion on a same side of the lead frame as the acoustic transducer; an amplifier is provided on a base portion of the housing in close proximity to the acoustic transducer; and a lid configured together with the base portion of the housing to define a cavity, wherein the acoustic transducer and the amplifier are closely positioned within the MEMS device cavity.

Description:
BACKGROUND 
       [0001]    Small acoustic devices, including acoustic transducers, are being employed in a number of applications, including gas flow detectors, and structural flaw detectors for buildings, bridges, pressure piping. In some applications, an acoustic transducer only transmits acoustic signals. In other applications, an acoustic transducer only receives acoustic signals. In still other applications, an acoustic transducer transmits acoustic signals and receives acoustic signals. Generally, acoustic transducers convert received electrical signals to acoustic signals when operating in a transmit mode, and/or convert received acoustic signals to electrical signals when operating in a receive mode. In particular, in many devices and applications, the acoustic signal that is transmitted and/or received is an ultrasonic signal. 
         [0002]    Acoustic transducers are manufactured using a variety of different technologies, including piezoelectric ultrasonic transducers and microelectromechanical system (MEMS) transducers. In the past, acoustic transducers have been manufactured with processes where the acoustic transducer element is placed in a metal, ceramic, or plastic package and a lid is bonded to the package. In a typical configuration, an electrical signal produced by the acoustic transducer is provided through a lead or wire from the package to an external amplifier provided on an external circuit board to which the packaged acoustic transducer is attached or connected. 
         [0003]    However, the electrical signal transmitted via the lead or wire from the packaged acoustic transducer device to the external amplifier is subject to loss, noise and/or interference due to the length of the interconnect lead length, all of which can reduce the receiving sensitivity of the acoustic device. 
         [0004]    What is needed, therefore, is an arrangement which can more efficiently couple electrical signals between an acoustic transducer and an amplifier. 
       SUMMARY 
       [0005]    In an example embodiment a device comprises: a lead frame having an aperture in a central portion thereof; a semiconductor die mounted on the lead frame, and including at least one acoustic transducer disposed above the aperture and configured to convert between acoustic energy and an electrical signal; an acoustic horn integrally connected to the lead frame, the horn extending from the lead frame and comprising a throat positioned adjacent to the acoustic transducer and a mouth opening at an opposite end of the acoustic horn from the throat; a substrate mounted on a base portion of the acoustic horn; an amplifier mounted on the substrate and being electrically connected to the acoustic transducer; and a lid configured together with the base portion of the housing to define a cavity, wherein the acoustic transducer and the amplifier are positioned within the cavity. 
         [0006]    In another example embodiment a device includes: a lead frame having an aperture in a central portion thereof; at least one acoustic transducer mounted on the lead frame above the aperture and configured to convert between acoustic energy and an electrical signal; a housing connected to the lead frame and including a base portion on a same side of the lead frame as the acoustic transducer; an amplifier provided on a base portion of the housing; and a lid configured together with the base portion of the housing to define a cavity, wherein the acoustic transducer and the amplifier are positioned within the cavity. 
         [0007]    In yet another example embodiment, a device comprises: a lead frame; an housing having a base portion integrated with the lead frame, and a protruding portion extending from the lead frame; a lid configured together with the base portion of the housing to define a cavity; and an acoustic transducer and an amplifier electrically connected together and both disposed within the cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    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 shown in the drawings may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements. 
           [0009]      FIG. 1  shows one embodiment of a semiconductor die including an acoustic device. 
           [0010]      FIG. 2  shows one embodiment of a semiconductor die including a plurality of acoustic devices. 
           [0011]      FIG. 3  shows another embodiment of a semiconductor die including a plurality of acoustic devices. 
           [0012]      FIG. 4  shows a top cutaway view of one embodiment of a packaged acoustic device. 
           [0013]      FIG. 5  shows a side view of a portion of one embodiment of a packaged acoustic device. 
           [0014]      FIG. 6  shows a side cutaway view of one embodiment of a packaged acoustic device. 
           [0015]      FIGS. 7A-F  illustrate various stages in on embodiment of a process of manufacturing a packaged acoustic device. 
           [0016]      FIG. 8  shows a side view of one embodiment of a packaged acoustic device. 
           [0017]      FIG. 9  shows a bottom view of one embodiment of a packaged acoustic device. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment 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 apparati and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparati are clearly within the scope of the present teachings. 
         [0019]    Unless otherwise noted, when a first device is said to be connected to a second device, this encompasses cases where one or more intermediate devices may be employed to connect the two devices to each other. However, when a first device is said to be directly connected to a second device, this encompasses only cases where the two devices are connected to each other without any intermediate or intervening devices. Similarly, when a signal is said to be coupled to a device, this encompasses cases where one or more intermediate devices may be employed to couple the signal to the device. However, when a signal is said to be directly coupled to a device, this encompasses only cases where the signal is directly coupled to the device without any intermediate or intervening devices. As used herein, “approximately” means within 10%, and “substantially” means at least 75%. As used herein, when a first structure, material, or layer is said to cover a second structure, material, or layer, this includes cases where the first structure, material, or layer substantially or completely encases or surrounds the second structure, material or layer. 
         [0020]      FIG. 1  shows one embodiment of a semiconductor die  100  including an acoustic device  110 . Semiconductor die also includes first electrode pads  130  connected to a first electrode of acoustic device  110 , and second electrode pads  135  connected to a second electrode of acoustic device  110 . In a beneficial embodiment, acoustic device  110  is a microelectromechanical system (MEMS) acoustic transducer having a diaphragm or membrane structure. A through-hole  112  is provided beneath the diaphragm of acoustic device  110 . 
         [0021]    For illustration purposes only, in one embodiment semiconductor die  100  has dimensions of approximately 2 mm on each side, the diaphragm of acoustic device  110  has a diameter of 540-743 μm, and through hole  112  has a diameter of 410-613 μm. 
         [0022]    Operationally, in some embodiments, acoustic device  110  may operate as a transmitting acoustic transducer to receive an electrical signal and to produce therefrom a corresponding acoustic signal or wave which is transmitted. In other embodiments, acoustic device  110  may operate as a receiving acoustic transducer to receive an acoustic signal or wave and to produce therefrom a corresponding electrical signal which is received. In still other embodiments, acoustic device may operate as both a transmitting acoustic transducer and a receiving acoustic transducer. 
         [0023]      FIG. 2  shows one embodiment of a semiconductor die  200  including a plurality of acoustic devices  210 . Semiconductor die also includes first electrode pads  230  connected to first electrodes of acoustic devices  210 , and second electrode pads  235  connected to second electrodes of acoustic devices  210 . In a beneficial embodiment, acoustic devices  210  are MEMS acoustic transducers each having a diaphragm or membrane structure. Through-holes  212  are provided beneath the diaphragms of acoustic devices  210 . 
         [0024]    For illustration purposes only, in one embodiment semiconductor die  200  has dimensions of approximately 2 mm on each side, the diaphragms of acoustic devices  210  each have a diameter of 525-779 μm, and through hole  112  has a diameter of 395-649 μm. 
         [0025]      FIG. 3  shows another embodiment of a semiconductor die  300  including a plurality of acoustic devices  310 . Semiconductor die also includes first electrode pads  330  connected to first electrodes of acoustic devices  310 , and second electrode pads  335  connected to second electrodes of acoustic devices  310 . In a beneficial embodiment, acoustic devices  310  are MEMS acoustic transducers each having a diaphragm or membrane structure. Through-holes  312  are provided beneath the diaphragms of acoustic devices  310 . 
         [0026]    For illustration purposes only, in one embodiment semiconductor die  300  has dimensions of approximately 2 mm on each side, the diaphragms of acoustic devices  310  each have a diameter of 525-779 μm, and through hole  112  has a diameter of 395-649 μm. 
         [0027]      FIG. 4  shows a top cutaway view, and  FIG. 5  shows a side cutaway view, of a portion of one embodiment of a packaged acoustic device  400 . Packaged acoustic device  400  includes a housing  410 , on which is mounted a substrate  420 , and a plurality of terminal leads  430 . 
         [0028]    As shown in  FIG. 5 , terminal leads  430  are integrally connected to a lead frame  510  on which is mounted semiconductor die  200  having one or more (e.g., three) acoustic transducers. Of course in other embodiments, other semiconductor dies, for example semiconductor dies  100  or  300 , having different numbers and/or configurations of acoustic transducers could be employed instead of semiconductor die  200 . As shown in  FIG. 5 , lead frame  510  includes an aperture  520  passing therethrough located in a central region of lead frame  510 . Semiconductor die  200  is disposed above aperture  520  so as to facilitate communication of acoustic waves or signals between the acoustic transducer(s) of semiconductor die  200  and an exterior of packaged acoustic device  400 . In one embodiment, semiconductor die  200  is attached to lead frame  510  by an adhesive  530  such as an epoxy. 
         [0029]    Lead frame  510  is formed from an electrically conductive material, such as various metals and metal alloys, including copper, nickel, aluminum, brass, copper/zinc alloys, and the like, or a combination thereof, for example. The material may be etched to form separate conductors and terminal leads  430 , as well as other features, such as aperture  520  and pads  435 . Lead frame  510  may also be plated for wirebonding, for example, using an optimized plating material, such as nickel and/or gold, to permit gold or aluminum wirebond attachment. 
         [0030]    Housing  410  further includes a base portion  415  on which substrate  420  is mounted. The base portion  415  of housing  410  has an aperture  417  therethrough in the area where semiconductor die  200  is mounted on lead frame  510 . 
         [0031]    Housing  410  is formed from a non-conductive material, such as various plastics or polymers, including liquid crystal polymer (LCP), polybutylene terephthalate (PBT), polypropylene (PP), polyphthalamide (PPA), and the like, for example. 
         [0032]    In a beneficial embodiment, housing  410  includes an acoustic horn (not shown in  FIGS. 4 and 5 ) on an opposite side of lead frame  510  from semiconductor die  200 , for coupling acoustic waves between the ambient air atmosphere and the acoustic transducer(s) of semiconductor die  200 . Further description of such an acoustic horn will be provided below with respect to  FIG. 6 . 
         [0033]    Substrate  420  is mounted on base portion  415  of housing  410 , for example by means of an adhesive  540  such as an epoxy. In the illustrated embodiment, substrate  420  is a printed circuit board. Beneficially, substrate  420  may be a ceramic or alumina ceramic substrate with electrically conductive pads and traces formed thereon, for example by a thick film printing metallization process. 
         [0034]    Substrate  420  has mounted thereon an amplifier, which may be an operational amplifier. In the illustrated embodiment, the amplifier includes an integrated circuit device  422  with active elements, and one or more external components  424  (e.g., resistor(s), capacitor(s), etc.) for setting at least one operating parameters (e.g., gain, bandwidth, etc.) of the amplifier, and/or for filtering one or more supply voltages provided to the amplifier. In the illustrated example, integrated circuit device  424  is a packaged semiconductor die with leads connected to metal traces on substrate  420 . However in other embodiments, integrated circuit device  424  may comprise an unpackaged semiconductor die. In some embodiments, the parameter-setting resistor(s)/capacitor(s) may be incorporated within the semiconductor die. 
         [0035]    Bond wires  440  electrically and operationally connect the amplifier to the acoustic transducer(s) of semiconductor die  200 , directly and/or via intermediate connections to pads  435  of lead frame  510 . Also, bond wires  440  connect the amplifier to one or more supply voltages, including ground, supplied via terminal leads  440 . Such connections may be made via one or more pads  435 . 
         [0036]    As shown in  FIG. 5 , packaged acoustic device  400  further includes a lid or cap  550 . Lid  550  is attached to the combined lead frame  510  and housing  410 . Lid  550  may be previously formed, for example, using a molding process. As shown in  FIG. 5 , lid  550  fits over lead frame  510  and housing  410 , and together with base portion  415  of housing  410  defines a cavity  560 . Semiconductor die  200  including its acoustic transducer(s), and the amplifier including integrated circuit device  422 , are both disposed within cavity  560 . Terminal leads  430  extend from the encasement formed by lead frame  510 , lid  550 , and base portion  415  of housing  410 , to enable electrical contact between external circuits and the amplifier and/or acoustic transducer(s) of packaged acoustic device  400 . In one embodiment, lid  550  is mechanically attached to base portion  415  of housing  410  by press fitting, for example. Alternatively or in addition, lid  550  may be attached to base portion  415  of housing  410  using an epoxy adhesive, for example, creating a hermetically sealed environment. Of course, other means of attachment, such as soldering, clamping, and the like, may be incorporated without departing from the scope of the present teachings. 
         [0037]      FIG. 6  shows a side cutaway view of packaged acoustic device  400 . As can be seen in  FIG. 6 , housing  410  includes an acoustic horn  610  disposed on an opposite side of lead frame  510  from semiconductor device  220  and cavity  560 . 
         [0038]    Generally, horns may be used to amplify acoustic signals, making them louder, as indicated by the incorporation of horns in various musical instruments and early hearing aids, for example. Horns may also be used to manipulate radiation patterns of acoustic emitters, generally referred to as beam forming or beam shaping, thus affecting dispersion of the acoustic signals. In addition, horns may provide impedance matching, rendering an acoustic transducer more compatible with the medium through which the acoustic signals travel. 
         [0039]    In the depicted embodiment, acoustic horn  610  is integral with housing  410  including base portion  415  that abuts a first side (e.g., bottom side) of lead frame  510  and a protruding portion that extends from base portion  415  along a center axis in a direction substantially perpendicular to lead frame  510 . In a representative embodiment, housing  410  including acoustic horn  610  is formed from plastic transfer molded to lead frame  510 , as discussed below. 
         [0040]    In the depicted embodiment, acoustic horn  610  has a generally hyperbolic or exponential cross-sectional shape, such that an inner dimension of acoustic horn  610  extends outwardly from an inner aperture or throat  612  to a flared outer aperture or mouth  614 . For example, the throat  612  may be circular with a diameter of about 2 mm and the mouth  614  may likewise be circular with a diameter of about 8 mm. However, the sizes and shapes of acoustic horn  610  and corresponding throat  612  and mouth  614 , as well as the respective configurations of the base portion  415  and the protruding portion, may vary to provide unique benefits for any particular situation or to meet application specific design requirements of various implementations, as would be apparent to one skilled in the art. For example, the cross-sectional shape of the protruding portion may be substantially conical, tubular, rectangular or trapezoidal, without departing from the scope of the present teachings. 
         [0041]    Acoustic horn  610  may be molded in the shape depicted, for example, in  FIG. 6 , using transfer molding or other molding techniques, to support different environmental and operating conditions. 
         [0042]    Beneficially, semiconductor die  200  includes an acoustic transducer having a suspended portion or membrane. In a beneficial embodiment, the membrane is exposed to the exterior of packaged acoustic device  400  through a back-etched portion of semiconductor chip  200 , and aperture  520  in lead frame  510 , which are substantially aligned with throat  612  of acoustic horn  610 . The back-etched portion may be formed in a substrate of semiconductor die  200 , which may include various types of materials, such as glass, sapphire, alumina, or the like, or any semiconductor material, such as silicon, gallium arsenide (GaAs), indium phosphide (InP), or the like, by machining or by chemically etching the substrate using photolithography, although various alternative techniques may be incorporated. In an embodiment, by being formed on the bottom of the lead frame  510 , acoustic horn  610  provides optimized low acoustic loss based on the mounting of semiconductor die  200  through the back-etched portion and aperture  520 . 
         [0043]    As stated above, the acoustic transducer may be a MEMS transducer, for example, for converting electronic signals to acoustic signals (e.g., ultrasonic signals) and/or for converting acoustic signals to electronic signals. In an embodiment, the acoustic transducer may be a thin film piezoelectric device and may include a stacked structure of a bottom electrode, a piezoelectric film, and a top electrode. The piezoelectric film can be formed from a material, such as aluminum nitride, lead zirconate titanate (PZT), or other film compatible with semiconductor processes. In another embodiment, acoustic transducer may include a piezoelectric crystal. The bottom and top electrodes may be formed from a metal compatible with semiconductor processes, such as molybdenum, tungsten, aluminum or a combination thereof. 
         [0044]    In an embodiment, a protective mesh or barrier screen  616  covers mouth  614  of acoustic horn  610 . Beneficially, screen  616  may include a pattern of apertures (not shown) for communicating acoustic signals between the acoustic transducer(s) of semiconductor die  200  and the exterior of packaged acoustic device  400 . For example, each of the apertures of screen  616  may be substantially smaller than the size of aperture  520  in lead frame  510 . Screen  616  may include acoustically transparent solid material to allow acoustic signals to exit and/or enter aperture  520 , but limiting debris, contaminants and/or moisture that can enter aperture  520 . In an embodiment, screen  616  is positioned directly in mouth  612  of the protruding portion of acoustic horn  610 . Screen  616  may be applied after assembling the packaged acoustic device  400 , including attachment of lid  550 . 
         [0045]      FIGS. 7A-F  illustrate various stages in one embodiment of a process of manufacturing packaged acoustic device  400 . 
         [0046]      FIG. 7A  shows lead frame  510  including electrical leads  430  and aperture  520  provided in a central region thereof. As discussed above, lead frame  510  may be plated for wirebonding, for example, using an optimized plating material, such as nickel and/or gold, to permit gold or aluminum wirebond attachment. 
         [0047]      FIG. 7B  shows a next intermediate product where housing  410  has been attached to lead frame  510 . 
         [0048]    In an example embodiment, a molding operation is performed on lead frame  510 . The molding operation includes placing lead frame  510  in a transfer mold previously formed to define the shape of housing  410 , including for example base portion  415  and acoustic horn  610 . A polymer, e.g., LCP, PBT, PP, or PPA, is then transfer molded, for example, to encapsulate lead frame  510  and to simultaneously form housing  410 , for example including acoustic horn  610 . The polymer is typically a solid at room temperature, and melted prior to transfer to the mold. The shape of acoustic horn  610  is defined by the shape of the machined transfer mold. The cooled (after melting) mold plastic will assume the horn shape within the transfer mold. Accordingly, housing  410 , including for example plastic acoustic horn  610  as shown in  FIG. 6 , is integrally formed to surround lead frame  510  during the molding operation. 
         [0049]      FIG. 7C  shows a next intermediate product where semiconductor die  200  including acoustic transducer(s) are mounted on lead frame  510  above aperture  520 , for example by an adhesive bond. 
         [0050]      FIG. 7D  shows a next intermediate product where substrate  420  including the amplifier (e.g., an operational amplifier), is mounted on base portion  415  of housing  410 , for example by an adhesive bond. 
         [0051]      FIG. 7E  shows a next intermediate product where one or more wire bonds  440  are applied to provide connections between the amplifier and/or acoustic transducer(s) of semiconductor die  220  and/or pads  435  of lead frame  510 . 
         [0052]      FIG. 7F  shows a next intermediate product where lid  550  has been applied to housing  410  and lead frame  510 . 
         [0053]    Although not specifically shown in  FIGS. 7A-F , in a step somewhere in the manufacturing process terminal leads  430  are disconnected from a supporting lead frame structure. 
         [0054]      FIG. 8  shows a side view of the final packaged acoustic device  400 . 
         [0055]      FIG. 9  shows a bottom view of the final packaged acoustic device  400 . 
         [0056]    While example embodiments are disclosed herein, one of ordinary skill in the art appreciates that many variations that are in accordance with the present teachings are possible that remain within the scope of the appended claims. The embodiments therefore are not to be restricted except within the scope of the appended claims.