Abstract:
A hearing aid microphone module housing all the electronic components needed for a functional hearing aid other than the battery and receiver is described which uses flip-chip technology to couple a JFET buffer to the components. The buffer is disposed on a PCB which defines a back volume of the housing.

Description:
RELATED APPLICATION 
     This application is related to U.S. Provisional Application Ser. No. 60/115,011, filed on Jan. 7, 1999, U.S. Provisional Application Ser. No. 60/134,896, filed May 19, 1999 and U.S. Provisional Application Ser. No. 60/157,872, filed Oct. 6, 1999, and U.S. Patent Application entitled “Hearing Aid with Large Diaphragm Microphone Element Including a Printed Circuit Board”, filed this date, the contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The performance of a hearing aid depends, among other things, upon the design of the microphone assembly which includes the microphone transducer, sound port, and a housing containing the signal processing electronics. The microphone transducer is typically a variable capacitor or electret type microphone formed of a charged diaphragm forming one plate of the capacitor and a backplate forming the other terminal. Sound impinging on the diaphragm varies the capacitance and produces a voltage signal proportional to the sound waves which is picked off the backplate and coupled to signal processing circuits where it is amplified in an amplifier and electrically processed to, inter alia, reduce noise content. The processed signal is then coupled to a receiver and converted back to sound waves to aid the user. 
     Conventional in the ear (ITE) or in the canal (ITC), hearing aids must of necessity be of relatively small size. Therefore, such aids have been fabricated with accessible replaceable batteries which are accessed via a faceplate door on the hearing aid enclosure. These size and battery requirements cause the microphone assembly and also the diaphragm to be relatively small in size in relation to the size of the hearing aid faceplate. The small diaphragm size lowers the quality of the transducer function. 
     An electret microphone for hearing aids typically uses a Junction Field Effect Transistor (JFET) buffer to convert the voltage signal from the high impedance transducer source to a low impedance source. This impedance conversion typically requires a difficult connection to be made to a high quality and hence, expensive substrate on a Printed Circuit Board (PCB) containing the signal processing components, so as to avoid compromising the input impedance of an amplifier on the substrate. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a microphone assembly for a hearing aid comprising a metal housing with a front wall with sound openings and a side wall extending longitudinally away from the front wall. Within the housing is an electret type microphone or transducer having a diaphragm electrode and a backplate electrode. External sound entering through the openings are converted into an electrical voltage signal which is coupled from the backplate to a Junction Field Effect Transistor (JFET) buffer device. The buffered signal is then coupled to an amplifier and signal processing components within the housing. 
     In one embodiment of the invention, the JFET device is a flip-chip component with four active terminals. Drain, source, bias and gate terminals are provided. The gate terminal is located on a side of the flip-chip proximal to and adjacent the backplate. The other terminals are connected to respective traces on a PCB. All the signal processing circuits needed to provide a functional hearing aid are contained on the PCB. The PCB also provides an acoustic seal to a back volume of the microphone and contains an electromagnetic interference (EMI) ground shield in the form of a ground plane of conductive material extending across the side wall of the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same or similar parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     A more detailed understanding of the invention may be had from the following description of preferred embodiments, given by way of example and to be understood in conjunction with the accompanying drawing, wherein: 
     FIG. 1 is a schematic side view of a first embodiment of the invention in which a microphone assembly contains a JFET buffer with source/drain flip-chip pads and a backside gate fastened to a microphone backplate. 
     FIG. 2 is an exploded view of the assembly of FIG.  1 . 
     FIG. 3 is an enlarged schematic detail of the JFET buffer portion of FIG. 2 prior to assembly. 
     FIG. 4 is a detail as in FIG. 3 after assembly. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the apparatus and method of the invention, an electret microphone for hearing aids uses a JFET buffer to convert the signal from the backplate, i.e., a high impedance source (the microphone) to a low impedance source. This impedance conversion results in a higher level loaded output signal level to the hearing aid amplifier than would be produced from the condenser microphone element itself without a buffer. A JFET gate contact to the backplate of the microphone&#39;s condenser must somehow be made. A direct connection from a small pad on the JFET to the microphone backplate is difficult to do and the use of an intermediate wire bond pad requires that the pad be mounted on ceramic, which complicates assembly. If the JFET gate connection is on the PCB substrate, the substrate must have high resistivity to not compromise the input impedance of the amplifier. A ceramic (alumina) substrate has such properties. The electrical connections for the JFET can be wire bonded from the microphone element onto a ceramic substrate. However, wire bonds are normally formed with a loop from pads on the JFET to extra bonding pads on the ceramic substrate, a practice that requires extra space vertically and horizontally and produces stray capacitance to ground and other circuit nodes which reduce sensitivity and introduce noise. Other disadvantages of a ceramic substrate itself are that it is relatively costly for use in a disposable hearing aid application. It also has a high dielectric constant which makes stray capacitance even higher. 
     In accordance with the embodiment shown in FIGS. 1-4, flip chip technology is used to minimize the physical size and lead lengths required to connect die bond pads of a JFET  10  to reduce the lead length between the electret microphone backplate  12  and the JFET. The result is a lower noise and higher sensitivity connection than could be made by longer paths formed by conventional wiring. The JFET backside gate  14  is connected to the backplate  12  by conductive epoxy  20 . This keeps the connection to the JFET off the PCB substrate  18  so that a lower cost substrate such as a glass-epoxy printed circuit board (e.g., FR 4 ) may be used. Since the JFET gate  14  does not contact the substrate  18  and then connect to the microphone backplate  12  (rather the JFET is connected to the backplate directly), the stray capacitance should be lower and, hence, sensitivity should be higher. 
     FIG. 1 is a sectional view of this embodiment of the hearing aid microphone module or assembly  100  and FIG. 2 is an exploded view of the assembly  100 . Assembly  100  contains all the electronic components other than the battery and a receiver necessary for a functional hearing aid. A circular metallic cover  40  is provided with a large diameter opening  52  for passage of sound from a faceplate (not shown) of a hearing aid enclosure in which the assembly  100  is adapted to be disposed proximally adjacent thereto. Sound impinges on large circular diaphragm  54  supported and attached to circular frame  42  and underlying spacer  44  which prevents the diaphragm  54  from contacting backplate  12 . Backplate  12 , in turn, is supported at its edges by an insulative bushing, such as, polytetrafluorethylene (PTFE) and is disposed over PCB  16  and acoustically and electrically sealed to cover  40  by a conductive cement, such as, epoxy. This partial assembly is then retained by snap ring  48  in cover  40 . The electrical component PCB  50  is then attached to the cover  40  to complete the assembly. 
     FIGS. 3 and 4 show details of the flip-chip JFET connections including the gate to backplate connection  14  using conductive epoxy  20 . FIG. 3 is an exploded view before assembly, while FIG. 4 shows the JFET after assembly with the PCB  16  and the backplate  12 . The metallization  22  on the top of the JFET die  10  is the gate connection, which is a very high impedance point. The solder bumps  24  on the bottom are the low impedance connections such as the drain and source connections. In this embodiment of the invention, four solder bumps: Drain, Source, Bias, and one dummy solder bump that is a No-Connect (NC) are provided. (NC is not connected to any part of the JFET circuit.) The underfill material  28  provides mechanical support. 
     This embodiment of the invention produces the following advantages: 
     a. A flip-chip JFET  10  with no gate contact made to the PCB, allows use of low cost FR 4  or other such materials instead of ceramic for the PCB substrate. 
     b. By controlling the depth of the front chamber  30  in the microphone assembly so that the spacing from the backplate to the PCB substrate is small enough, a single blob of conductive (epoxy) cement  20  is sufficient to bridge the gap, eliminating the need for wire bonds. 
     c. Stray capacitance from the gate to PCB substrate is reduced because of this gate isolation, resulting in decreased signal loss and decreased noise pickup. 
     d. The use of four solder balls on JFET provides better mechanical support and alignment during assembly. (Solder bumps on Drain, Source, Bias, and NC solder bumps  752 ). 
     EQUIVALENTS 
     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form, modification, variation and details may be made therein without departing from the scope of the invention as defined by the appended claims.