Abstract:
A condenser microphone having a flexure hinge diaphragm and a method of manufacturing the same are provided. The method includes the steps of: forming a lower silicon layer and a first insulating layer; forming an upper silicon layer on the first insulating layer; forming sound holes by patterning the upper silicon layer; forming a second insulating layer and a conductive layer on the upper silicon layer; forming a passivation layer on the conductive layer; forming a sacrificial layer on the passivation layer; depositing a diaphragm on the sacrificial layer, and forming air holes passing through the diaphragm; forming electrode pads on the passivation layer and a region of the diaphragm; and etching the layers to form an air gap between the diaphragm and the upper silicon layer. Consequently, a manufacturing process may improve the sensitivity and reduce the size of the condenser microphone.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of co-pending U.S. Application Ser. No. 11/875,996, filed Oct. 22, 2007. This application also claims priority to and the benefit of Korean Patent Application Nos. 2006-122736, filed Dec. 6, 2006, and 2007-54259, filed Jun. 4, 2007, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a condenser microphone and a method of manufacturing the same, and more particularly, to a micromini condenser microphone having a flexure hinge diaphragm and a method of manufacturing the same. 
     This work was supported by the IT R&amp;D program of Ministry of Information and Communication/Institute for Information Technology Advancement [2006-S-006-01, Components/Module technology for Ubiquitous Terminals.] 
     2. Discussion of Related Art 
     Generally, a condenser microphone uses a principle in which a change in capacitance caused by vibration of a diaphragm due to external vibration sound pressure is output into an electrical signal, which can be applied to a microphone, a telephone, a mobile phone and a video tape recorder. 
       FIG. 1A  is a cross-sectional view of a conventional condenser microphone having a disk-shaped diaphragm, and  FIG. 1B  is a cross-sectional view of a conventional condenser microphone having a pleated diaphragm. 
     Referring to  FIGS. 1A and 1B , the conventional condenser microphone includes a silicon wafer  11 , a back plate  12  formed on the silicon wafer  11 , and a diaphragm  14  disposed on the back plate  12  with an air gap  13  interposed therebetween. A plurality of sound holes  12   a  passing through the back plate  12  and in communication with the air gap  13  are formed, and an insulating layer  16  is formed between the back plate  12  and the diaphragms  14  and  15 . 
     The diaphragm  14  illustrated in  FIG. 1A  has a disk-shape, and the diaphragm  15  illustrated in  FIG. 1B  has a pleated structure. Generally, the flexible diaphragms  14  and  15  may be formed to be easily vibrated by minor external vibration and to improve the sensitivity of a microphone, and thus a conventional diaphragm may be formed in a disk-shape or pleated structure to obtain mechanical flexibility. 
     However, the condenser microphone having the above-described structure may need an energy higher than a certain level to sufficiently vibrate the diaphragm, so the pleated diaphragm  15  illustrated in  FIG. 1B  may be formed rather than the disk-shaped diaphragm  14  illustrated in  FIG. 1A , thereby enhancing flexibility of the diaphragm. However, sufficient sound pressure has to be input to vibrate the diaphragms of these condenser microphones. 
     Moreover, the conventional condenser microphones having the conventional structure described above have poor performance in a low frequency range when scaled-down to 1 mm or less using a semiconductor MEMS process. Also, general frequency response characteristics of the condenser microphone exhibit high sensitivity in a low frequency range when the area of the diaphragm is large, and low sensitivity in a high frequency range when the area of the diaphragm is small. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a condenser microphone having a flexure hinge diaphragm and a method of manufacturing the same. 
     The present invention is also directed to a condenser microphone covering an audible frequency range and exhibiting very high sensitivity using a flexure hinge diaphragm and a method of manufacturing the same. 
     One aspect of the present invention provides a method of manufacturing a condenser microphone, including the steps of: forming a lower silicon layer and a first insulating layer; forming an upper silicon layer to be used as a back plate on the first insulating layer; forming a plurality of sound holes by patterning the upper silicon layer; forming a second insulating layer on the upper silicon layer; forming a conductive layer on the upper silicon layer having the sound holes, and forming a passivation layer on the conductive layer; forming a sacrificial layer on the passivation layer; depositing a diaphragm on the sacrificial layer, and forming a plurality of air holes passing through the diaphragm; forming electrode pads on the passivation layer and a region of the diaphragm; and etching the sacrificial layer, the passivation layer, the conductive layer, the upper silicon layer, the first insulating layer and the lower silicon layer to form an air gap between the diaphragm and the upper silicon layer. 
     The method may use an SOI wafer formed of the lower silicon layer, the first insulating layer and the upper silicon layer. The sound holes may be formed by a deep reactive ion etching (DRIE) process. Forming the second insulating layer may include: depositing a second insulating layer on the upper silicon layer having the sound holes by chemical vapor deposition (CVD); and patterning the second insulating layer formed in the sound hole region to remain on an edge of the upper silicon layer by photolithography. 
     Forming the sacrificial layer may include spin-coating a planarization material to planarize an uneven region created by the sound holes, after depositing the sacrificial layer. The planarization material may include silicon on glass (SOG). The thickness of the sacrificial layer may be changed by controlling the number of spin-coatings, thereby controlling the height of the air gap formed between the diaphragm and the back plate. The diaphragm may be formed of at least one of silicon nitride, polyimide and polysilicon, and a metallic material. Forming the air holes in the diaphragm may be performed by etching. 
     Etching the sacrificial layer, the passivation layer, the conductive layer, the upper silicon layer, the first insulating layer and the lower silicon layer may include: etching the passivation layer, the conductive layer, the upper silicon layer, the first insulating layer and the lower silicon layer by the DRIE process; and etching the sacrificial layer by a wet etching process. To prevent deformation of the diaphragm during etching of the sacrificial layer, the method may further include: coating a photoresist layer on the diaphragm before etching the sacrificial layer; and removing the photoresist layer after etching the sacrificial layer. 
     Another aspect of the present invention provides a condenser microphone, including: a first insulating layer formed on a lower silicon layer; a back plate formed on the first insulating layer and having a plurality of sound holes passing through the back plate; a second insulating layer formed on an edge of the back plate such that the sound holes are not plugged; and a diaphragm including a contact region in contact with the second insulating layer, a vibration region forming an air gap with the back plate by upwardly projecting from the contact region, and a plurality of air holes passing through the vibration region. 
     The air holes may be in communication with the air gap and the sound holes. The back plate may be formed of a silicon layer. The diaphragm may be formed in a single layer or a multi-layer using at least one of silicon nitride, polyimide and polysilicon, and a metallic material. The metallic 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1A  is a cross-sectional view of a conventional structure of a condenser microphone having a disk-shaped diaphragm, and  FIG. 1B  is a cross-sectional view of a conventional structure of a condenser microphone having a pleated diaphragm; 
         FIG. 2A  is a partial perspective view of a structure of a condenser microphone having a flexure hinge diaphragm according to the present invention, and  FIG. 2B  is a cross-sectional view of the structure of the condenser microphone having the flexure hinge diaphragm according to the present invention; 
         FIGS. 3A to 3H  sequentially illustrate a manufacturing process of the condenser microphone of  FIG. 2B ; and 
         FIG. 4A  illustrates flexibility of a conventional disk-shaped diaphragm, and  FIG. 4B  illustrates flexibility of a flexure hinge diaphragm according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, the present invention will be described in detail with reference to drawings illustrating exemplary embodiments of the present invention. 
       FIG. 2A  is a partial perspective view of a structure of a condenser microphone having a flexure hinge diaphragm according to the present invention, and  FIG. 2B  is a cross-sectional view of the structure of the condenser microphone having the flexure hinge diaphragm according to the present invention. For convenience of description, sectional lines for some elements such as a sound hole and an air hole will be omitted. 
     Referring to  FIGS. 2A and 2B , a condenser microphone  20  according to the present invention includes a silicon on insulator (SOI) wafer  21  including a lower silicon layer  21   a , a first insulating layer  21   b  and an upper silicon layer  22  used as a back plate (hereinafter, referred to as “a back plate  22 ”), a second insulating layer  23  formed along an edge of the back plate  22 , and a diaphragm  25  formed over the back plate  22 . 
     The diaphragm  25  includes a contact region  25   b  in contact with the second insulating layer  23  and a vibration region  25   a  upwardly projecting from the contact region  25   b . An air gap  24  is formed between the vibration region  25   a  of the diaphragm  25  and the back plate  22 , and a plurality of air holes  25   c  in communication with the air gap  24  and passing through the diaphragm  25  are formed in the vibration region  25   a  of the diaphragm  25 . A plurality of sound holes  22   a  passing through the back plate  22  and in communication with the air gap  24  are formed in the back plate  22 . Condenser microphones having various frequency characteristics can be manufactured depending on the size and number of the air holes  25   c  and the number, size and distribution of the sound holes  22   a.    
     A method of manufacturing the condenser microphone having the above-described structure will now be described in detail with reference to  FIGS. 3A to 3H .  FIGS. 3A to 3H  sequentially illustrate a manufacturing process of the condenser microphone of  FIG. 2B . 
     Referring to  FIG. 3A , to manufacture the condenser microphone according to the present invention, an SOI wafer  21  is first prepared. The SOI wafer  21  is composed of a lower silicon layer  21   a , a first insulating layer  21  and an upper silicon layer  22  used as a back plate (hereinafter, referred to as “a back plate  22 ”). 
     Referring to  FIG. 3B , the back plate  22  is patterned to form sound holes  22   a  in the back plate  22 . Here, deep reactive ion etching (DRIE) equipment is used. Then, an insulating layer  23  is formed on the patterned back plate  22 . The insulating layer  23  is deposited by chemical vapor deposition (CVD). 
     Referring to  FIG. 3C , after forming the insulating layer  23 , the insulating layer  23  is patterned to remain only on an outer region of the back plate  22  in which the sound holes  22   a  are not formed. Here, the insulating layer  23  is patterned by photolithography. 
     After that, referring to  FIG. 3D , a conductive layer  31  is formed on the patterned insulating layer  23  and back plate  22 . In this embodiment, the conductive layer  31  may be formed of a metal such as Al, Au or TiW by implanting charges into its surface. The conductive layer  31  is used as a lower electrode layer for applying an electrode of the back plate  22  to the condenser microphone. A passivation layer  32  protecting the conductive layer  31  is formed on the conductive layer  31 . 
     After that, referring to  FIG. 3E , a sacrificial layer  33  is formed on the passivation layer  32 . The sacrificial layer  33  formed on the passivation layer  32  is formed to cover the region having the sound holes  22   a , and to expose edges of the passivation layer  32 . The sacrificial layer  33  is formed of a material having an excellent etch selectivity with respect to the passivation layer  32  since it will be etched in the final step. The sacrificial layer  33  may be formed of one of various polymers such as silicon oxide, photoresist and polyimide, or metal materials such as Al. Also, in order to planarize the uneven sacrificial layer  33  formed in the sound hole region  22   a , silicon on glass (SOG) may be employed. However, when the sacrificial layer  33  is formed of, for example, photoresist which cannot be processed at a high temperature, dry film-resist (DFR) may be employed. The planarization material for the sacrificial layer  33  may be coated several times by spin coating. A thickness of the sacrificial layer  33  may depend on the number of spin-coatings of the planarization material, thereby controlling the height of the air gap  24  formed between a diaphragm  25  and the back plate  22  during the vibration of the diaphragm  25 . A sufficient space in which the diaphragm  25  and the back plate  22  are not in contact with each other may be created by controlling the height of the air gap  24  (refer to  FIG. 3H ). 
     Referring to  FIG. 3F , the diaphragm  25  surrounding the sacrificial layer  33  is formed over the sacrificial layer  33 . The diaphragm  25  has a contact region  25   b  in contact with the passivation layer  32  and a vibration region  25   a  formed along the sacrificial layer  33 . The diaphragm  25  is formed of metal and silicon nitride. In the present invention, the diaphragm  25  is formed of two layers of metal and silicon nitride. Meanwhile, the diaphragm  25  may include various materials such as silicon nitride, polyimide, polysilicon, etc., and metals such as Al, Ag, TiW and Cu. After the diaphragm  25  is formed on the sacrificial layer  33 , a plurality of air holes  25   c  passing through the vibration region  25   a  of the diaphragm  25  are formed. The diaphragm  25  has an elastic deformable hinge structure having flexibility. The air holes  25   c  may have a hole shape and a slotted shape which is radially formed from centers of the vibration region  25   a.    
     Referring to  FIG. 3G , electrode pads  34   a  and  34   b  including positive and negative electrodes are formed. The electrode pad  34   a  is formed on the passivation layer  32  to be electrically connected with the conductive layer  31 , and the electrode pad  34   b  is formed to be electrically connected with the diaphragm  25 . To form the electrode pads  34   a  and  34   b , a part of the contact region  25   b  between the passivation layer  32  and the diaphragm  25  is etched, and then a conductive material having a small surface resistance such as Au or Ag is deposited thereon and patterned. 
     Referring to  FIG. 3H , after forming the electrode pads  34   a  and  34   b , the lower silicon layer  21   a , the first insulating layer  21   b , the conductive layer  31 , the passivation layer  32  and the sacrificial layer  33  are etched. The lower silicon layer  21   a , the first insulating layer  21   b , the conductive layer  31  and the passivation layer  32  are etched by a DRIE process, and the sacrificial layer  33  is removed by a wet etching process. As the lower silicon layer  21   a , the first insulating layer  21   b  and the conductive layer  31  are removed, a plurality of sound holes  22   a  are formed in the upper silicon layer used as the back plate  22 , and as the sacrificial layer  33  is removed, an air gap  24  in communication with the air holes  25   c  and the sound holes  22   a  is formed. Forming the air gap  24  further includes applying photoresist on the diaphragm  25  to prevent deformation of the diaphragm  25  that can occur in the removal of the sacrificial layer  33 , and removing the photoresist applied on the diaphragm  25  using a dry etching process after the removal of the sacrificial layer  33 . 
     The condenser microphone  20  manufactured by the above-described process may variously change frequency characteristics and sensitivity by controlling the thickness of the diaphragm  25  or the diameter, width and thickness of the vibration region  25   a , the length and number of the air holes  25   c , or the number, size and distribution of the sound holes  22   a  formed in the back plate  22 . When the flexure hinge diaphragm  25  manufactured in the above-described process is used, the condenser microphone is more flexible than that using the conventional disk-shaped or pleated diaphragm, so it may be more sensitively vibrated due to external sound pressure which is input to the microphone, and increase its output voltage. 
       FIG. 4A  illustrates flexibility of a conventional disk-shaped diaphragm, and  FIG. 4B  illustrates flexibility of a flexure hinge diaphragm according to the present invention. 
     Referring to  FIG. 4A , when the conventional disk-shaped diaphragm is used, a displacement (d max ) is 0.7314E-4 μm/Pa, and referring to  FIG. 4B , when the diaphragm in the present invention is used, a displacement (d max ) is 0.01826 μm/Pa. These are results obtained under the same conditions, e.g., the thickness and material of the diaphragm, the number of the sound holes, applied voltage, etc., which show that the diaphragm of the present invention has a vibration range (d) 250 times larger than the conventional diaphragm. When the conventional condenser microphone is reduced to a certain size or less (i.e., 1 mm or less), its sensitivity is decreased and its performance is poor in a low frequency range. However, even when the condenser microphone including the flexure hinge diaphragm according to the present invention is manufactured to a size of 1 mm or less, it has very high sensitivity so that it may cover all audio frequency ranges. 
     According to the above-described structure, the present invention may include a flexure hinge diaphragm having a plurality of air holes, thereby being more sensitively vibrated by external sound pressure which is input to the microphone and increasing output voltage. 
     Also, even when the diaphragm formed by the above-described manufacturing process has a small size, it may have very high sensitivity, and thus may cover all audio frequency ranges. A condenser microphone of the present invention employs a silicon wafer, so it may be integrated with a driving circuit of a CMOS transistor and also applied to mobile devices such as mobile phones, PDAs and PMPs. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.