Patent Publication Number: US-7218742-B2

Title: Condenser microphone assembly

Description:
This application is a continuation of common-owned, U.S. application Ser. No. 09/745,179 (“Condenser Microphone Assembly”) filed on Dec. 20, 2000 now U.S. Pat. No. 6,741,709, naming Kelly Q. Kay and Mark W. Gilbert as inventors, the entire disclosure of which is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to microphones, and more particularly to condenser microphone assemblies, such as a backplate with integral spacer made From semiconductor components. 
   BACKGROUND OF THE INVENTION 
   Condenser or capacitance microphones are widely used in the audio, electronics and instrumentation industries. Condenser microphones include a flexible diaphragm or membrane and a rigid backplate that may contain one or more openings. Sound waves cause the diaphragm to move, resulting in a pressure variation between the membrane and the backplate. This pressure variation results in a difference in the charge between the diaphragm, and the difference in charge is converted to an electrical signal that corresponds to the sound wave. As is known in the art, conventional diaphragms may be constructed From metal films or metallized polymer films. 
   For a variety of applications, it is desirable to manufacture small, high quality condenser microphones. As is known in the art, openings in the backplate may be created by drilling or punching holes. Controlling the precise size and location of such holes, which can be critical, becomes more difficult as the holes become smaller. 
   As is also known in the art, entire condenser microphones, including diaphragms, can be formed on silicon substrates through MicroElectroMechanical Systems (MEMS) fabrication methods, which is the formation of mechanical components based on silicon integrated circuit manufacturing processes. For example, U.S. Pat. No. 5,889,872 discloses a capacitive microphone formed with semiconductor processing techniques. A diaphragm is formed as part of the fabrication by applying a polysilicon layer on a silicon nitride layer. The polysilicon layer is patterned or etched to form a diaphragm. 
   U.S. Pat. No. 5,870,482 explains challenges associated with maintaining highly compliant and precisely positioned diaphragms fabricated from a silicon wafer. That patent discloses an alternative solid state condenser microphone with a semiconductor support structure. 
   U.S. Pat. No. 6,075,867 discloses a micromechanical microphone with multiple diaphragms. To address problems of humidity, dust and dirt, the microphone includes two sealing membranes on either side of a transducer. However, an environmental membrane in front of a sensing transducer may affect audio characteristics, such as signal to noise ratio, frequency response, and sensitivity. 
   The formation of complete condenser microphones through MEMS processing is extremely difficult and expensive. Moreover, condenser microphones constructed entirely from MEMS processing often exhibit inferior audio and reliability characteristics. 
   SUMMARY OF THE INVENTION 
   The present invention solves many of the aforementioned problems by a microphone assembly comprising a housing, a semiconductor backplate mounted in the housing and a flexible diaphragm located above the backplate. The semiconductor spacer is integrally formed with the backplate and intermediate the backplate and the diaphragm. The backplate and spacer is not integrally formed with the diaphragm, the diaphragm frame, or the housing. 
   The diaphragm is stretched over and adhesively affixed to the diaphragm frame. The diaphragm frame maintains tension in the diaphragm. The diaphragm is comprised of a metal film or metallized polymer film, and the diaphragm is both a protective environmental barrier and a sensing electrode of a capacitive electroacoustic transducer. The housing may be made of metal, and the backplate made of silicon. The spacer may further comprise an electrically insulating layer, such as silicon dioxide or a fluoropolymer. 
   The backplate includes a top portion, a bottom portion, and a side portion and a plurality of openings extending from the top portion of the backplate to the bottom portion of the backplate. In one embodiment, the plurality of openings are located along the side portion of the backplate and are radially outward of the spacer. The backplate may be circular, rectangular or another desirable shape. The spacer may consist of an annular wall, a series of arcuate walls, a series of arcuate extensions or a rectangular wall. 
   The housing comprises an upper lip, and the diaphragm frame comprises a metal ring positioned against the upper lip. The assembly may further comprise a metal contact on the bottom portion of the backplate. Furthermore, the invention may include a spring positioned between the backplate and a lower portion of the housing. 
   In addition, the invention may comprise a transistor coupled to the housing or the backplate. The microphone assembly may also comprise an application specific integrated circuit (ASIC) coupled to the backplate, and the ASIC may include a transistor. 
   These as well as other novel advantages, details, embodiments, features and objects of the present invention will be apparent to those skilled in the art from following the detailed description of the invention, the attached claims and accompanying drawings, listed herein, which are useful in explaining the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following text and drawings, wherein similar reference numerals denote similar elements throughout the several views thereof, the present invention is explained with reference to illustrative embodiments, in which: 
       FIG. 1  is a perspective view of a first embodiment of a microphone assembly made in accordance with the present invention; 
       FIG. 2  is a perspective view of a portion of the microphone assembly made in accordance with the present invention 
       FIG. 3  is a plan view of a first embodiment of a backplate made in accordance with the present invention; 
       FIG. 4  is a plan view of a second embodiment of a backplate made in accordance with the present invention; 
       FIG. 5  is a plan view of a third embodiment of a backplate made in accordance with the present invention; 
       FIG. 5A  is an enlargement of the area shown by the region  104  in  FIG. 5 ; and 
       FIG. 6  is a plan view of a fourth embodiment of a backplate made in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1 and 2 , in a preferred embodiment, the present invention includes a membrane or diaphragm  10  that is separated from a backplate  12 . The diaphragm  10  is flexible and is exposed to the air. A protective grille (not shown) may be mounted above the diaphragm  10 . The diaphragm  10  is made of a known material for constructing microphone diaphragms, such as metal film or metallized polymer film. 
   The backplate  12  is rigid or fixed. Integrally formed with the backplate  12  are spacers, shown for example at  14  in  FIG. 1 and 15  in  FIG. 2 . The diaphragm  10  is separated from the backplate  12  by a narrow air gap  13  (shown only in  FIG. 2 ) defined by the spacers  14 ,  15 . The backplate  12  and spacer  14  are fabricated, for example, from semiconductor material, such as silicon, by batch processing techniques. Referring to  FIG. 1 , a top region  28  of the spacer  14  includes a layer of electrically insulating material, such as silicon dioxide or a fluoropolymer, such as TEFLON. Similarly, referring to  FIG. 2 , a top region  30  of the spacer  15  includes a similar insulating layer. The spacer may take the form of many shapes, such as a wall or a ridge. 
   The membrane  10  and the backplate  12  form a capacitor, also known as a condenser. When a sound wave hits the membrane  10 , the membrane moves, (causing a variation in height of the air gap  13  between the membrane  10  and the backplate  12 . This gap variation results in a change in the capacitance of the condenser formed by the membrane  10  and the backplate  12 . If a fixed or controlled charge Q is maintained on the capacitor, a voltage will be formed across the capacitor that will then vary proportionally to the change in the height of the air gap  13 . 
   The diaphragm  10  is stretched over a diaphragm frame  16  and glued or adhesively affixed to the diaphragm frame  16 . The diaphragm frame  16  maintains tension in the diaphragm  16 . The diaphragm frame  16  is positioned between the spacer  14  and an upper edge  18  of a housing  20 . The housing  20  is a known housing not manufactured from batch processing techniques, and is preferably made of metal, not silicon. The housing  20  serves as an electrical ground. 
   The backplate  12  may include openings or holes indicated by arrows  22 ,  24  and  26 . These openings allow air to pass from the area above the backplate  12  to the area below the backplate  12 . 
   The backplate  12  shown in  FIG. 1  is rectangular or square. The backplate is situated in the housing  20  by a nest  32 . An opening  34  between the backplate  12  and the nest  32  also allows air to pass from the area above the backplate  12  to the area below the backplate  12 . In one embodiment, materials. such as metal, could be −9 selectively deposited in the circular portion indicated by the numeral  40 . 
   Referring to  FIG. 2 , a spring  42  is used to mechanically bias the backplate  12  against a bottom portion  44  of the housing  20 , which is a PC board. The spring  42  causes the spacer  15  of the backplate  12  to be pushed into the diaphragm  10  and the diaphragm frame or ring  16 , which consequently press against the upper edge or lip  18  of the housing  20 . In this manner, the diaphragm is coupled to the spacer  15 . Thus, together, the spring  42 , the diaphragm frame  16 , the upper lip  18  of the housing  20 , the housing  20  and the PC board  44  cooperate to secure the diaphragm  10  against the insulating layer  30  of the spacer  15 . The diaphragm  10  is not integrally formed with the spacer  15 . 
   The microphone assembly preferably employs a single diaphragm  10  that serves as both a protective environmental barrier and a sensing electrode of a capacitive electroacoustic transducer. In contrast, prior art systems of silicon fabricated condenser microphones employ either no protective environmental barrier or more than one diaphragm or membrane, one of which serves as an environmental barrier and one of which does not. 
   A variety of shapes and configurations may be used for the diaphragm  10  and backplate  12 . For example in  FIG. 1  the diaphragm frame  16  is round and in the form of an annular ring and the backplate  12  is square. One skilled in the art will −10 appreciate that the diaphragm frame  16  and backplate  12  could include other shapes depending on the shape of the housing  20  and the other components of the invention. 
   Because the diaphragm  10  is not fabricated or processed as part of the backplate  12 , the diaphragm is free from stress associate with fabricating and mounting the backplate  12 . In addition, the tension on the diaphragm  10  is independent of the internal stresses in the backplate  12 . As is recognized in the art, these uncontrolled internal stresses are a common undesirable consequence of semiconductor fabrication processing. Thus, the diaphragm  10  is free floating relative to stress parallel to the face of the backplate  12  or the face of the diaphragm  10 . By mounting the diaphragm  10  on a suitable diaphragm frame  16  that is independent from the backplate  12  and spacer  15 , the tensile stress of the diaphragm  10  is free from influences from the packaging and the backplate. 
     FIGS. 3–6  illustrate alternative embodiments with different arrangements of the spacers and holes on a backplate. As would be appreciated by one of ordinary skill in the art, the location, number and size of holes affects the audio characteristics of the microphone. MEMS will allow improved control of the hole size and placement, which will enhance the ability to control frequency response and sensitivity. 
   Referring to  FIG. 3 , holes  80  may be located radially inward of spacers  82 . Spacers  82  may be small circular protrusions. 
   Alternatively,  FIG. 4  shows holes  90  and notches  92  along a side of a backplate  95  that allow air to pass from above to below the backplate.  FIG. 4  also shows an annular spacer wall  94 . 
     FIG. 5  shows a backplate with no holes radially inward of a series of arcuate spacer portions  100 . Instead, air passes from above the backplate to below the backplate via openings  102 . Arrows  106 ,  108  and no in  FIG. 5A , which is an enlargement of the area  104  in  FIG. 5 , depict the flow of air from the top of a backplate  112  to the underside of the backplate  112 .  FIG. 6  further illustrates a rectangular or square backplate  130  with a square or rectangular spacer wall and grid or holes, one of which is shown by  134 . As will be appreciated by one of ordinary skill in the art, the spacers may also be or arcuate portions of a wall sufficient to support the diaphragm  10  and diaphragm frame  16 . 
   Referring again to  FIG. 2 , the backplate  12  is externally biased at output  140  with a voltage bias. The backplate could be externally biased with direct current (DC) voltage or a radio frequency (RF) bias. In one embodiment, a transistor or FET (not shown) is mounted to the PC board  44  within the area defined by the PC board  44  and the housing  20 . The FET could also be located outside the housing  20  or directly on the bottom of the backplate  12 . Generally, locating the FET closer to the −12 backplate should improve noise characteristics of the invention. The unit could also be biased by an electret, for example, a charged or polarized layer on the backplate  12  (not shown). 
   The underside of the backplate  12  may include contact regions  142 , which are preferably metal, that can be deposited by chemical vapor deposition (CVD) techniques. The spring  42  may provide an electrical contact from the contact region  142  to the region  140 . 
   Referring again to  FIG. 1 , an integrated circuit (IC) or application specific integrated circuit (ASIC)  180  could be mounted beneath the PC board (not shown). The ASIC could contain a transistor, such as a FET. The ASIC could also include a preamplifier to increase the electrical output of the microphone and/or modify the response of the microphone. 
   The ASIC could also include an analog to digital converter (AID). The purpose of the AID is to convert the analog output of the microphone, or microphone preamplifier, to a digital signal that can either be used as a direct digital output from the microphone, or a feed to digital signal processing (DSP) circuitry. The purpose of the DSP is to modify the output of the microphone after an AID. The output can either be a digital or analog or both. Specific applications can include equalization, signal compression, frequency dependent signal compression, and self-calibration. 
   A voltage step up circuit could also be used to allow a readily available compact battery source (e.g. a 9 v battery) to provide an elevated voltage (e.g. 200 v) for externally DC biasing a condenser. 
   Another embodiment of the invention would include a radio frequency (RF) biasing circuit to provide a bias voltage that oscillates with an RF wavelength. A further purpose for such a circuit is to allow the microphone to output a RF modulated signal for wireless transmission. 
   Thus, different backplates and different ASIC circuits that could be combined in the housing  20  would permit a variety of potential operations and functions of the microphone. 
   In the foregoing specification, the present invention has been described with reference to specific exemplary embodiments thereof. Although the invention has been described in terms of a preferred embodiment, those skilled in the art will recognize that various modifications, embodiments or variations of the invention can be practiced within the spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, therefore, to be regarded in an illustrated rather than restrictive sense. Accordingly, it is not intended that the invention be limited except as may be necessary in view of the appended claims.