Abstract:
A technique using a new diaphragm structure and support design is provided herein for microphones or structure designs for pressure sensing. The structure includes a set of capacitive structures. The capacitive structure has a combination of a diaphragm structure, a back plate structure and a surrounding micro-structure for fixing the diaphragm. After the diaphragm structure has deformed due to a pressure load, a gap between the back plate and the diaphragm is changed accordingly, and variation occurs in the capacitance value between the two parallel plates. By using the principle of the effect of capacitance value variation, the capacitive sensor causes the capacitance value to vary with the change in the sound, thus accomplishing the object of measuring.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present invention claims the benefit of U.S. provisional patent application, Ser. No. 60/815,374, filed on Jun. 20, 2006. This application also claims the priority of Taiwan application serial no. 95149965, filed Dec. 29, 2006. All disclosure of the U.S. and Taiwan application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a miniature acoustic transducer. More particularly, the present invention relates to a miniature acoustic transducer having a structure with a low spring constant.  
         [0004]     2. Description of Related Art  
         [0005]     The acoustic transducer, produced by a capacitive microphone chip integrated through silicon micro-manufacturing technique and integrated circuit (IC) processing technique, has the advantages of a light mass, a small volume and a good signal quality etc. In applications of national home appliances products, as the demand for handsets has expanded increasingly and the requirements for sound quality have enhanced increasingly, and the markets and techniques for hearing aids have started to flourish as well, capacitive microphone chip has gradually become a mainstream of microphone chips. From the perspective of the market, it is anticipated that the North American market of microphone chips will reach the level of 500 millions in the year of 2004, and will grow stably by 20% annually towards the market from 2004 to 2009, according to the sections about mobile handsets in the market trend reports by Digitimes Corp. Application of microphones on handsets becomes the mainstream of the present market.  
         [0006]     Because integrated circuit processes using silicon as the base material are cheap and frequently employed in electronic products, and their application field continues to expand outward, more applications will be fabricated through processes using silicon as the base material combined with the CMOS process to directly integrate reader circuits onto a chip in the future. Additionally, since Taiwan has become the globally largest contract manufacturer for semiconductors, with a contract manufacture share of about 60-70% in the current market, mass production and acceleration of its commercialization process are expected in the future. Therefore, in order to keep away and differentiate in terms of the microphone layout from element designs by various primary factories, it is necessary to acquire novel designs and seize the first chance in manufacturing in the first place, so as to obtain the superiority in the microphone element market and the capability to share the occupancy.  
         [0007]     Presently, the application of microphone element structures in mass production is limited to a few types of structures, because manufactories producing micro electro-mechanical systems (MEMS) microphones are currently only a few manufactories, such as Knowles Corp., Infineon Corp. or Sonion Corp., and most of the package processes on the market are still based on the designs developed by Knowles.  
         [0008]     Referring to FIGS.  1  to  3 , a microphone structure design by Knowles Corp. is shown. An acoustic transducer  10  includes a conductive diaphragm  12  and a perforated member  40 , which are supported by a base  30  and separated by an air gap  20 . An air gap  22 , extremely thin, is present between the conductive diaphragm  12  and the base  30 , to enable the diaphragm  12  to move up and down freely and decouple the diaphragm  12  from the base  30 . A number of indentations  13  are formed beneath the diaphragm  12 , for obviating adsorption phenomena between the diaphragm  12  and the base  30 .  
         [0009]     The lateral movement of the diaphragm  12  is restricted by the support portion  41  of the member  40 , which may serve as a suitable enabling space between the diaphragm  12  and the member  40 . Such support portion  41  may be constructed of a ring or a number of bumps. If the support portion  41  is constructed of a ring, a tense sound-sealed space would be formed when the diaphragm  12  rests against the support portion  41 , and as a result, the acoustic transducer would have a well-controlled low frequency roll-off. A dielectric layer  31  is provided between the diaphragm  12  and the base  30 . A conducting electrode  42  is fixed beneath the nonconductive member  40 . The member  40  has several holes  21 , and the diaphragm  12  also has several holes for creating a passageway  14  for sound flow with the holes  21  in the member  40 .  
         [0010]     The microphone structure design by Knowles Corp. is mainly a finger structure design directed to a back plate for increasing the strength of the back plate so as to reduce the resistance of the back plate. The diaphragm utilizes a design approach of decreasing the residual stress, and employs a common circular diaphragm design. The diaphragm provides only a simple support, and although its structure can avoid the problem of residual stress and a high natural frequency response, the effective deformation amount and the compliance of its design are still inadequate.  
         [0011]     Referring to  FIG. 4 , another microphone structure design by Knowles Corp. is shown. This structure is essentially the same as that of FIGS.  1  to  3 , with the only difference that the diaphragm  12  is connected to the base  30  via several spring structures  11  in order to decrease the intrinsic stress of the diaphragm and the stress generated from the base  30  or the packaged device.  
         [0012]     Traditional microphone element designs utilize a simple and fixed diaphragm design. Although there are design approaches for increasing the diaphragm compliance, such as the finger structure shown in  FIG. 5  in which a diaphragm  510  has a finger structure, or the corrugated structure shown in  FIG. 6  in which a diaphragm  610  has a corrugated structure, most designs have disadvantages. Though the diaphragm of finger diaphragm design is soft and sensitive, it has a low resonant frequency response and is prone to fracture. Though the diaphragm of corrugated diaphragm design can effectively reduce the influence of the residual stress to enable large diaphragm compliance, it has a complicated process and is difficult to be processed, and the increase in the compliance is limited.  
       SUMMARY OF THE INVENTION  
       [0013]     Accordingly, the present invention is directed to provide a structure for increasing the diaphragm compliance and creating a low spring constant through a new structure design, to enable an acoustic transducer to further have the performance of high compliance and deformation amount.  
         [0014]     An acoustic transducer provided by the present invention includes a capacitive sound pressure sensing element, which includes two or more parallel plates with conductive material thereon to constitute a capacitor, with acoustic holes formed on at least one of the parallel plates, and a spring structure provided on at least one of the other parallel plates.  
         [0015]     The structural composition of a miniature acoustic transducer provided by the present invention includes a substrate and a back plate and diaphragm formed thereon. The back plate has multiple acoustic holes, and a surface of the diaphragm has one or more indentations. The indentations contact the back plate to form a support structure. The other surface of the diaphragm has a cut bridge structure. When a sound pressure is transmitted to the diaphragm, the bridge structure would deform because of the support of the indentations. The deformation amount of displacement of the diaphragm is thus increased, whereby the electric field distribution of the capacitor is between the diaphragm and the back plate. When a sound pressure causes the diaphragm to deform and the bridge structure to displace, the resulting variation magnitude in the capacitance serves as the principle of the sensing.  
         [0016]     In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, preferred embodiments accompanied with figures are described in detail below.  
         [0017]     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0019]     FIGS.  1  to  3  are the structure designs of a conventional microphone.  
         [0020]      FIG. 4  is the structure design of another conventional microphone.  
         [0021]      FIG. 5  is a diaphragm design with finger structure in a conventional microphone design.  
         [0022]      FIG. 6  is a diaphragm design with corrugated structure in a conventional microphone design.  
         [0023]      FIGS. 7A and 7B  are cross-sectional schematics illustrating an acoustic transducer with a bridge spring according to a preferred embodiment of the present invention.  
         [0024]      FIGS. 8 and 9  are perspective and cross-sectional views illustrating an acoustic transducer with a bridge spring according to a preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF EMBODIMENTS  
       [0025]     The present invention provides an acoustic transducer, in which a bridge-like spring structure is constructed by fabricating a special structural pattern on a pressure sensing diaphragm in combination with indentations on the diaphragm as supports, utilizing the conception of a spring structure, so that the performance of the acoustic transducer is improved. The present acoustic transducer follows a principle that in order to effectively increase the compliance with a simple structure, the design pattern for the diaphragm may be changed and a structural effect similar to that of a spring may be produced via a support structure to increase the diaphragm compliance. The present invention provides a structure of a miniature acoustic transducer, which is useful in, for example, a miniature microphone element or any electronic device requiring sounds to be converted into signals, such as a handset or a miniature microphone, or any electronic device that detects variations in the air pressure and converts the variations into signals.  
         [0026]     The structural composition of a miniature acoustic transducer provided by the present invention includes a capacitive sound pressure-sensing element. This capacitive sound pressure-sensing element includes two or more parallel plates with conductive materials thereon to compose a capacitor, wherein acoustic holes are formed on at least one of the parallel plates and a spring structure is constructed on at least one of the other parallel plates.  
         [0027]     A miniature acoustic transducer provided by the present invention may be applied to devices such as pressure sensors, acceleration sensors or ultrasonic sensors.  
         [0028]     In one embodiment, the structural composition of a miniature acoustic transducer provided by the present invention includes a substrate and a back plate and diaphragm on the substrate. The back plate has multiple acoustic holes, and the surface of the diaphragm has one or more indentations. The indentations contact the back plate to form a support structure. The surface of the diaphragm described above has a cut bridge structure. When a sound pressure is transmitted to the diaphragm, the bridge structure would deform because of the support from the indentations. Thus the deformation amount of displacement of the diaphragm is thus increased, whereby the electric field distribution of the capacitor is between the diaphragm and the back plate. When a sound pressure causes the diaphragm to deform and the bridge structure to displace, the resulting variation magnitude in the capacitance serves as the principle of the sensing.  
         [0029]     The diaphragm is a deformable diaphragm sensor unit, for example, of a pattern design having one or more special bridges or beams. Additionally, the surface of the diaphragm has a single or more indentations for supporting the diaphragm. The indentations under each bridge or beam structure create a set of spring-like effect, so that multiple sets of structures with spring-like effect, referred to as bridge spring or beam spring, exist on the diaphragm.  
         [0030]     When the air pressure is transferred to the diaphragm, the diaphragm would deform. The indentations on the lower surface of the diaphragm create a contact support with the back plate. The bridges or beams on the diaphragm deform considerably because of the supporting force from the indentations. At this time, the diaphragm plate deforms accordingly with up and down displacement, which increases the deformation and displacement amount between the two plates and thus indirectly increases the value of the capacitance variation between the plates. Such a design significantly increases the diaphragm compliance. The capacitance variation between the diaphragm and the back plate in the microphone will be sent out as measured signals via the conductive design.  
         [0031]     The aforementioned deformable diaphragm sensor unit and the back plate structure may be comprised of one or more materials, including carbon-based polymers, silicon, silicon nitride, polycrystalline silicon, amorphous silicon, silicon dioxide, silicon carbide, germanium, gallium, arsenide, carbon, titanium, gold, iron, copper, chromium, tungsten, aluminum, platinum, nickel, tantalum, or the alloys thereof etc.  
         [0032]     The present invention provides an acoustic transducer with a bridge spring or beam spring structure, and the construction of the acoustic transducer in one embodiment is shown in  FIGS. 7A and 7B . Refer also to  FIG. 8 , which illustrates a perspective testing schematic of the acoustic transducer with a bridge spring structure provided by the present invention, which is described altogether hereafter. A structure of two parallel plates is formed on a substrate  700 . One is a back plate structure  710  and the other is a sensing diaphragm  730 , as  820  in  FIG. 8 . The back plate structure  710 , as  810  in  FIG. 8  is separated from the diaphragm  730 , as  820  in  FIG. 8 , by an insulating layer  720 , such as a layer of silicon oxide. The back plate structure  710  has multiple acoustic holes  712 , as  812  in  FIG. 8 . The diaphragm  730 , as  820  in  FIG. 8 , is a deformable diaphragm sensor unit, such as of a pattern design having a special bridge or beam structure.  
         [0033]     A single or a plurality of bridge or beams is formed on the surface of the diaphragm  730 , as  820  in  FIG. 8 . For example, as shown in  FIG. 7A , a position  722  in the insulating layer  720  is combined with the base  736  of the diaphragm  730 , as  820  in  FIG. 8 . The base  736  extends outward to form a diaphragm beam structure  732 , and a structure of a single or a plurality of indentations  734  is formed on a side of the diaphragm beam structure  732  opposite to the back plate structure  710 , for supporting the diaphragm beam structure  732 . Of course, as described above, a part of the diaphragm beam structure  732 , as the structure designated by  830  in  FIG. 8 , may be a bridge or a beam structure, which is described below with a bridge structure  830 . The bridge structure  830  creates a set of spring-like effect with its indentations  734 .  
         [0034]     In the acoustic transducer provided by the present invention, one or more sets of structures with spring-like effect, referred to herein as bridge or beam springs, are disposed on the diaphragm  730 . When the air pressure is transferred to the diaphragm  730 , the diaphragm  730  would deform. The indentations  734  on the lower surface of the beam structure  732  create a contact support with the back plate  710 . The bridge structure  830  on the diaphragm  730  deforms considerably because of the supporting force from the indentations  734 . At this time, the diaphragm  730  deforms accordingly with up and down displacement, which increases the deformation and displacement amount between the two plates, i.e. the back plate structure  710  and the diaphragm  730 , and thus indirectly increases the value of the capacitance variation between the two plates. With the conductive material disposed on the diaphragm  730  and the whole layer of a conductive layer  714  applied on the substrate  700 , the capacitance variations are sensed and measured. In the aforementioned conductive design, the two plates, i.e. the back plate structure  710  and the diaphragm  730  may alternatively be comprised of conductive materials and constitute two parallel electrodes of a capacitor. The above design would significantly increase the diaphragm compliance. The capacitance variation between the diaphragm and the back plate in the microphone will be sent out via such a conductive design.  
         [0035]     Referring to  FIG. 8 , a structure of two parallel plates, including a back plate structure  810  and a diaphragm  820 , is formed on a substrate. The back plate structure  810  has multiple acoustic holes  812 . The surface of the diaphragm  820  has four bridge spring structures  830 , though the amount may be adjusted depending on design requirements. The bridge structure  830  includes two beams  832  and  834 , and a central portion  836  of the bridge with indentations  734  below. The indentations  734  are formed on a side of the bridge structure  830  opposite to the back plate structure  810 . The indentations  734  on the lower surface of the bridge spring structure  830  create a contact support with the back plate structure  810 , to make the bridge structure  830  create a set of spring-like effect with its indentations  734 . That is to say, the bridge structures  830  on the diaphragm  820  deform considerably because of the supporting force from the indentations  734 . At this time, the diaphragm  820  deforms accordingly with up and down displacement, which increases the deformation and displacement amount between the two plates, i.e. the back plate structure  810  and the diaphragm  820 , and thus indirectly increases the value of the capacitance variation between the two plates.  FIG. 9  illustrates a bridge structure of the acoustic transducer of  FIG. 8  in which the back plate structure  810  has the structure of multiple acoustic holes  812 .  
         [0036]     The present disclosure provides a structure for increasing diaphragm compliance and creating a low spring constant through a new structure design, enabling an acoustic transducer, such as a microphone element, to further have the performance of high compliance and deformation amount.  
         [0037]     Although the present invention has been disclosed as above with preferred embodiments, the present invention is no limited thereto. It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.