Abstract:
An apparatus for transducing between an acoustical signal and an electrical signal, comprises a backplate assembly comprising a charged layer and a conductive layer and a diaphragm assembly positioned at a predetermined distance from the backplate assembly. The diaphragm assembly comprising a support structure and a diaphragm, the diaphragm vibrates in response to an acoustical signal, is monolithically formed on the support structure, wherein the support structure and the diaphragm are composed of a common material having a thermomechanical property. The apparatus further comprises a spacer, a printed circuit board (PCB), and a housing. The spacer is formed between the backplate assembly and the diaphragm assembly, collectively constituting a motor portion. The motor portion and the PCB are disposed in the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This patent claims benefit under 35 U.S.C. §119 (e) to U.S. Provisional Application No. 60/895,798, filed Mar. 20, 2007 and entitled Silicon Electret Microphone and Manufacturing Method Thereof, the disclosure of which is hereby incorporated herein for all purposes. 
     
    
     BACKGROUND 
       [0002]    Conventional electret condenser microphones utilize metalized Mylar film stretched across and adhesively attached to a metal ring to serve as a diaphragm. The tension in this ring/film assembly is a major factor in determining the sensitivity of the microphone. Temperature and humidity changes affect the ring/film assembly and the adhesive that is used to attach the film to the ring is subject to creep. This leads to instability over time and environment changes. This is particularly a problem when using matched pairs as they tend to drift apart in performance over time. A need exists for a microphone having a ring/film assembly that expands equally as temperature changes and does not require any adhesive for the ring/film assembly attachment. Further, the film is less sensitive to humidity, thereby yielding a more stable performance over a period of time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
           [0004]      FIG. 1  is an exploded view illustrating a microphone assembly embodying the teachings of the present invention; 
           [0005]      FIG. 2  is a perspective view of the microphone assembly of  FIG. 1  embodying the teachings of the present invention; 
           [0006]      FIG. 3  is an enlarged partial view of a motor portion of the microphone assembly shown in  FIG. 1  embodying the teachings of the present invention; 
           [0007]      FIGS. 4A-4D  are cross-sectional views of a diaphragm assembly embodying the teachings of the present invention; 
           [0008]      FIG. 5  is a sectional view illustrating the diaphragm assembly of  FIG. 4D  embodying the teachings of the present invention; 
           [0009]      FIG. 6  is an enlarged sectional view of the diaphragm assembly shown in  FIG. 5  embodying the teachings of the present invention; and 
           [0010]      FIG. 7  is a top view of a wafer for forming a plurality of diaphragm assemblies embodying the teachings of the present invention. 
       
    
    
       [0011]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. 
       DETAILED DESCRIPTION 
       [0012]    While the present disclosure is susceptible to various modifications and alternative forms, certain embodiments are shown by way of example in the drawings and these embodiments will be described in detail herein. It will be understood, however, that this disclosure is not intended to limit the invention to the particular forms described, but to the contrary, the invention is intended to cover all modifications, alternatives, and equivalents falling within the spirit and scope of the invention defined by the appended claims. 
         [0013]      FIG. 1  illustrates an exploded view of a transducer  100  that can be used in virtually any type of listening devices such as earphones, headphones, Bluetooth wireless headsets, insert earphone, UWB wireless headsets, hearing aids, or the like. The hearing aids may be a behind-the-ear (BTE), in-the-ear (ITE), in-the-canal (ITC), completely-in-the-canal (CIC), combined BTE/ITE, combined BTE/ITC, combined BTE/CIC, or the like. Other types of listening device are possible. The transducer  100  may be a receiver, a speaker, a microphone, a combined receiver and microphone, dual microphones, depending on the desired applications. In the embodiment shown, the transducer  100  is a microphone. The microphone  100  comprises a housing having a top housing  106  and a bottom housing  104  attached together by any known techniques. The microphone  100  further comprises a diaphragm assembly  110 , a spacer  116 , and a backplate assembly  118 , collectively constituting a motor portion  140 . More details about the formation of the motor portion will follow. 
         [0014]    At least one port  108  is formed on the bottom housing  104  by any known technique to allow acoustic waves to enter and interact with the motor portion  140  disposed within the housings  104 ,  106 . An electronic device (not shown) mounted to a printed circuit board (PCB)  120  is disposed within the housings  104 ,  106 . The electronic device may be an integrated circuit (IC) die, a capacitor, a resistor, an inductor, or other passive device, depending on the desired applications. It will be understood that one or more dies and electronic components may be included. In one embodiment, the device is a hybrid circuit. The hybrid circuit includes an impedance buffer circuit (not shown) such as, for example, a source-follower field effect transistor (FET) IC. The PCB  120  may include three connecting wires  126 ,  128 ,  130  that provide a ground, a power supply input, and an output for the processed electrical signal corresponding to a sound that is transduced by the motor portion of the microphone  100 . As shown, a connecting wire  124  located on the motor portion  140  is electrically coupled to the PCB  120  via the connecting wire  126 . When the PCB  120  and the motor portion  140  are placed in final or closed position within the bottom housing  104 , the top and bottom housings  104 ,  106  are fixedly attached together locking the internal components in position. A flex circuit assembly  122  is then mounted to the top surface of the housing  102  for providing an electrical connection to the components within the listening devices (not shown). The flex circuit assembly  122  comprises a plurality of terminals  136  that provides a ground terminal, an output terminal, and a power terminal. A plurality of solder pads  138  on a flex circuit  134  of the assembly  122  are electrically connected to the terminals  136 . The wires  126 ,  128 ,  130  of the PCB  120  extend through an opening  132  formed on the top housing  106  are electrically connected to the terminals  136  of the assembly  122 . 
         [0015]      FIG. 2  illustrates a perspective view of a microphone  100  embodying the teachings of the present invention. A diaphragm assembly  110  (as shown in  FIG. 1 ) as part of a motor portion  140  is disposed within a housing  102 . A flex circuit assembly  122  is fixedly attached to the top surface of the housing  102 . A housing  102  comprises a first housing  104  and a second housing  106  attached to the first housing  104  by known technique. While the housing  102  has a cylindrical shape, it will be understood that any housing shape or configuration suitable for any desirable applications may be suffice, including a roughly square shape, a rectangular shape or any other desired geometry and size. The housing  102  may be manufactured from a variety of materials such as, for example, stainless steel, alternating layers of conductive and non-conductive materials (e.g. metal particle-coated plastics), or the like. 
         [0016]      FIG. 3  illustrates a motor portion  140  disposed within a bottom housing  104  of a microphone  100  as depicted in  FIG. 1 . The motor portion  140  comprises a diaphragm assembly  110 , a backplate assembly  118 , and a spacer  116 . The spacer  116  having a thickness is placed between the diaphragm assembly  110  and the backplate assembly  118 . The spacer  116  is in the form of an annular ring shape and corresponds to the internal configuration of the bottom housing  104 . It may typically be manufactured of an electrically insulating material such as polyethylene terephthalate (PET), polyimide, plastic, or the like. Other types of material are possible. Alternatively, the spacer may chosen from a set of metal like materials such as nickel or stainless steel. 
         [0017]    The backplate assembly  118  in the form of a disc shape having a central portion  142 , at least one relief section  144 , three are illustrated in  FIG. 1  and at least one protrusion  146 , three are illustrated in  FIG. 1 , is mounted on the spacer  116 . It will be understood that the backplate assembly  118  may take any form of shape or configuration suitable for any desirable applications may be suffice, including a roughly square shape, a disc shape, a rectangular shape or any other desired geometry and size with or without a backplate support and correspond to the configuration of the spacer  116 . The backplate assembly  118  includes a conductive layer  118   a  and a charged layer  118   b . The charged layer  118   b  may be chosen from a set of materials that are thermo-plastic materials with good charge storage characteristics, good chemical resistance, and high temperature stability. In one embodiment, the charged layer  118   b  may be a fluorinated ethylene propylene material commonly available under the trade name TEFLON, or any similar materials. Other types of material are possible. The conductive layer  118   a  is made of an electrically conductive material such as a stainless steel, gold, metal particle-coated polymer, or the like for transmitting signals from the charged layer  118   b . Other types of material are possible. An optional polymer layer (not shown) may be attached to the conductive layer  118   a  by any known technique. 
         [0018]    The diaphragm assembly  110  includes a support structure  112  and a diaphragm  114 . More details about the formation of the diaphragm assembly will be discussed in greater detail therein. As shown in  FIG. 3 , the charged layer  118   b  of the backplate assembly  118  is directly exposed to the diaphragm  114  of the diaphragm assembly  110  and is separated from the diaphragm assembly by the spacer  116 . The bottom surface of the support structure  112  is held in contact with the inner wall of the bottom housing  104 . The diaphragm  114  of the diaphragm assembly  110  is typically exposed to an acoustic port  108  which is separated from the diaphragm  114  by the support structure  112 . An optional damping element (not shown) may be attached to the bottom housing  104  to prevent debris from entering the bottom housing  104  through the port  108  that may damage the motor portion  140 . 
         [0019]      FIGS. 4A-4D  illustrate one example of fabricating a diaphragm assembly  110  used in a microphone  100 . Silicon on insulator (SOI) wafers  200 , commonly available under the trade designation UNIBOND from Shin-Etsu Handotai Co., Ltd, or of any similar materials are provided. Other types of material are possible without departing the scope of the invention. The wafers  200  comprise a first layer of single crystal silicon  202  having a uniformed thickness of about 1 μm, an intermediate layer  206 , and a handle wafer  204 . The intermediate layer  206  is typically a silicon dioxide film; however, other film materials are possible. As shown in  FIG. 4B , the first silicon layer  202  and the film  206  are etched on the perimeter using Reactive Ion Etching (RIE) (not shown). Other types of etching are possible. Alternatively, the perimeter of the film  206  may be etched in the final step of fabricating the diaphragm assembly  110  without departing the scope of the invention. A portion of the first layer  202  is selectively etched to form a pierce hole  148 . More than one pierce hole may be possible for desired applications. A portion of the intermediate layer  206  exposed to the surrounding via the pierce hole  148  is then etched. The first layer  202  forms a diaphragm  114  of the diaphragm assembly  110 . 
         [0020]    Now, referring to  FIG. 4C , the handle wafer  204  is then back-etched to form a support structure  112  of the diaphragm assembly  110 . The etching process may be, for example, deep reactive ion etched (DRIE). Other types of etching are possible. A second surface of the film  206  exposed to the environment is then removed using any common etch solution (not shown), thereby releasing the diaphragm  114  as shown in  FIG. 4D . As mentioned earlier, the perimeter of the film  206  and the second surface of the film  206  exposed to the environment are etched in a single process after the support structure  112  is formed. 
         [0021]      FIGS. 5-6  illustrate a diaphragm assembly  110  for a microphone  100 . The diaphragm assembly  110  comprises a support structure  112  and a diaphragm  114 . The support structure  112  in the form of an annular ring shape and corresponding to the internal configuration of the housing comprises an outer diameter of about 1.0 mm to 3.0 mm, such as approximately 3.0 mm, approximately 2.5 mm, approximately 2.2 mm, approximately 2.0 mm, approximately 1.5 mm, or approximately 1.0 mm. The support structure  112  has an inner diameter of about 0.5 mm to 2.0 mm, such as approximately 2.0 mm, approximately 1.8 mm, approximately 1.5 mm, approximately 1.0 mm, or approximately 0.5 mm. The thickness of the support structure is about 80 μm to 200 μm, such as approximately 200 μm, approximately 150 μm, approximately 125 μm, approximately 100 μm, or approximately 80 μm. The diaphragm  114  in the form of a disk is held in contact with the support structure  112  by the intermediate oxide layer  206  (See  FIG. 4D ). The thickness of the diaphragm  114  is about 0.5 μm to 2.0 μm, such as approximately 2.0 μm, approximately 1.5 μm, approximately 1.0 μm, or approximately 0.5 μm. It will be understood that the size of the support structure  112  and the thickness of the diaphragm  114  correspond to the configuration of the microphone, depending on the desired applications. A pierce hole  148  having a diameter of smaller than 75 μm, such as smaller than 60 μm, such as smaller than 50 μm, such as smaller than 40 μm, such as smaller than 30 μm, such as smaller than 20 μm, such as smaller than 15 μm, such as smaller than 13 μm, such as smaller than 10 μm, such as smaller than 8 μm, is formed on the diaphragm  114 , as depicted in  FIG. 6 . The pierce hole  148  is used to control the low frequency roll-off of the microphone  100 . One advantage of the silicon diaphragm assembly is that, no adhesive is required since the diaphragm and the support structure is fabricated in a single process. The silicon diaphragm assembly expands uniformly as temperature changes because the ring and diaphragm are composed of identical material. Furthermore the diaphragm does not respond to changes in humidity. Because the diaphragm assembly is a monolithic silicon structure, matching of pairs of transducers can be guaranteed for longer periods of time. 
         [0022]      FIG. 7  illustrates a top view of a wafer  300  for forming a plurality of diaphragm assemblies. The wafer  300  is then mounted on a dicing tape (not shown) and subsequently diced along a dicing street  302  to produce a plurality of diaphragm assemblies  110 . The dicing may be realized by using a saw, a laser, or by scribing and breaking. Other examples of dicing processes are possible. 
         [0023]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention.