Abstract:
The present invention relates to a silicon based MEMS microphone, comprising a silicon substrate and an acoustic sensing part supported on the silicon substrate, wherein a mesh-structured back hole is formed in the substrate and aligned with the acoustic sensing part, the mesh-structured back hole includes a plurality of mesh beams which are interconnected with each other and supported on the side wall of the mesh-structure back hole, the plurality of mesh beams and the side wall define a plurality of mesh holes which all have a tapered profile and merge into one hole in the vicinity of the acoustic sensing part at the top side of the silicon substrate. The mesh-structured back hole can help to streamline the air pressure pulse caused, for example, in a drop test and thus reduce the impact on the acoustic sensing part of the microphone, and also serve as a protection filter to prevent alien substances such as particles entering the microphone.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of microphone technology, and more specifically, to a silicon based MEMS microphone. 
       BACKGROUND 
       [0002]    Silicon based MEMS microphones, also known as acoustic transducers, have been in research and development for many years. The silicon based MEMS microphones may be widely used in many applications, such as cell phones, tablet PCs, cameras, hearing aids, smart toys and surveillance devices due to their potential advantages in miniaturization, performances, reliability, environmental endurance, costs and mass production capability. 
         [0003]    In general, a silicon based MEMS microphone consists of a fixed perforated backplate and a highly compliant diaphragm with an air gap formed in between. The perforated backplate and the compliant diaphragm, forming a variable air-gap condenser, are typically formed on a single silicon substrate, with one of which being directly exposed to the outside through a back hole formed in the silicon substrate. 
         [0004]    Patent application No. WO 02/15636 discloses an acoustic transducer, which has a diaphragm positioned between a cover member (equivalent to the said backplate) and a substrate. The diaphragm, made of low stress polysilicon, is directly above a back hole formed in the substrate and can be laterally movable within a plane parallel to the planar surface of the cover member. The floating diaphragm is free to move in its own plane, and thus can release its intrinsic stress, resulting very consistent mechanical compliance. 
         [0005]    Patent document PCT/DE97/02740 discloses a miniaturized microphone, in which an SOI substrate is used for formation of the microphone and related CMOS circuits. Specifically, the silicon layer of the SOI substrate is used to form the backplate of the microphone which is directly above a back hole formed in the SOI substrate, and a subsequently deposited polysilicon thin film, which is above the backplate with an air gap in between and is exposed to the outside through the opening in the backplate and the back hole in the SOI substrate, serves to be the diaphragm of the microphone. 
         [0006]    When a silicon microphone is packaged, it is usually mounted on a printed circuit board (PCB) with the back hole formed in the substrate of the microphone aligned with an acoustic port formed on the PCB board, so that an external acoustic wave can easily reach and vibrate the diaphragm of the microphone. For example,  FIG. 1  shows a cross-sectional view of an exemplary structure of a conventional silicon based MEMS microphone package. As shown in  FIG. 1 , in the conventional MEMS microphone package, a MEMS microphone  10 ′ and other integrated circuits  20  are mounted on a PCB board  30  and enclosed by a cover  40 , wherein a back hole  140 ′ formed in the substrate of the MEMS microphone  10 ′ is aligned with an acoustic port  35  formed on the PCB board  30 . An external acoustic wave, as shown by the arrows in  FIG. 1 , travels through the acoustic port  35  on the PCB board  30  and the back hole  140 ′ in the substrate of the microphone  10 ′ to vibrate the diaphragm of the microphone  10 ′. 
         [0007]    However, as can be seen from the above description, there exist problems with either the stand-alone conventional MEMS microphones or the conventional MEMS microphone package. With such a structure having a widely opened back hole in the substrate of the conventional microphones and/or acoustic port on the PCB board, the diaphragm of the conventional microphones is easily damaged due to the air pressure pulse caused, for example, in a drop test, and on the other hand, alien substances such as particles are easily trapped inside the microphone through the back hole and/or the acoustic port. 
       SUMMARY 
       [0008]    In order to solve the above problems, the present invention provides a silicon based MEMS microphone with a mesh-structured back hole, which may help to streamline the air pressure pulse caused, for example, in a drop test and thus reduce the impact on the acoustic sensing part of the microphone, and also serve as a protection filter to prevent alien substances such as particles entering the microphone. 
         [0009]    In one aspect of the present invention, there is provided a silicon based MEMS microphone, comprising a silicon substrate and an acoustic sensing part supported on the silicon substrate, wherein a mesh-structured back hole is formed in the substrate and aligned with the acoustic sensing part; the mesh-structured back hole includes a plurality of mesh beams which are interconnected with each other and supported on the side wall of the mesh-structure back hole; the plurality of mesh beams and the side wall define a plurality of mesh holes which all have a tapered profile and merge into one hole in the vicinity of the acoustic sensing part at the top side of the silicon substrate. 
         [0010]    Preferably, the mesh-structured back hole may be formed from the bottom side of the silicon substrate using a controlled deep reactive ion etching process. 
         [0011]    Preferably, the plurality of mesh holes of the mesh-structured back hole may be uniformly and/or symmetrically distributed across the mesh-structured back hole. 
         [0012]    Preferably, the mesh-structured back hole may be provided with a circular shape, a square shape, a rectangular shape or other polygonal shape. 
         [0013]    The acoustic sensing part of the said microphone may include at least a compliant diaphragm, a perforated backplate, and an air gap formed between the diaphragm and the backplate, wherein the diaphragm and the backplate are used to form electrode plates of a variable condenser. 
         [0014]    In one example, said compliant diaphragm may be formed with a part of a silicon device layer stacked on the silicon substrate with an oxide layer sandwiched in between, said perforated backplate may be located above the diaphragm and formed with CMOS passivation layers with a metal layer imbedded therein which serves as an electrode plate of the backplate, and the acoustic sensing part of the said microphone may further include an interconnection column provided between the center of the diaphragm and the center of the backplate for mechanically suspending and electrically wiring out the diaphragm using CMOS metal interconnection method, and the periphery of the diaphragm is free to vibrate. In this case, preferably, the mesh-structured back hole may include a central mesh beam extending from the bottom side to the top side of the silicon substrate and attaching to the center of the diaphragm as a supporting anchor. 
         [0015]    In another example, said compliant diaphragm may be formed with a part of a silicon device layer stacked on the silicon substrate with an oxide layer sandwiched in between, said perforated backplate may be located above the diaphragm and formed with CMOS passivation layers with a metal layer imbedded therein which serves as an electrode plate of the backplate, and the acoustic sensing part of the said microphone may further include an interconnection column provided between the edge of diaphragm and the edge of the backplate for electrically wiring out the diaphragm using CMOS metal interconnection method, and the periphery of the diaphragm is fixed. 
         [0016]    In another aspect of the present invention, there is provided with a microphone system, comprising any silicon based MEMS microphone described above and a CMOS circuitry which are integrated on a single chip. 
         [0017]    In still another aspect of the present invention, there is provided with a microphone package, comprising a PCB board; any silicon based MEMS microphone described above, mounted on the PCB board; and a cover, enclosing the microphone, wherein an acoustic port may be formed on any of the PCB board and the cover, and aligned with the mesh-structured back hole of the microphone. 
         [0018]    While various embodiments have been discussed in the summary above, it should be appreciated that not necessarily all embodiments include the same features and some of the features described above are not necessary but can be desirable in some embodiments. Numerous additional features, embodiments and benefits are discussed in the detailed description which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The objectives and features of the present invention will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which: 
           [0020]      FIG. 1  is a cross-sectional view showing an exemplary structure of a conventional silicon based MEMS microphone package; 
           [0021]      FIG. 2  is a cross-sectional view showing the structure of the silicon based MEMS microphone according to the first embodiment of the present invention; 
           [0022]      FIG. 3  is a plan view showing an exemplary pattern of the mesh-structured back hole formed in the silicon substrate of the microphone of  FIG. 2  when viewed from the bottom side of the silicon substrate; 
           [0023]      FIG. 4  is a perspective view, showing a simplified structure of the mesh-structured back hole of  FIG. 3  when viewed from the bottom side of the silicon substrate; 
           [0024]      FIG. 5  is a perspective view, showing a simplified structure of the mesh-structured back hole of  FIG. 3  when viewed from the top side of the silicon substrate; 
           [0025]      FIG. 6  is a cross-sectional view showing the structure of the silicon based MEMS microphone according to the second embodiment of the present invention; 
           [0026]      FIG. 7  is a plan view showing an exemplary pattern of the mesh-structured back hole formed in the silicon substrate of the microphone of  FIG. 6  when viewed from the bottom side of the silicon substrate; and 
           [0027]      FIG. 8  is a cross-sectional view showing an exemplary structure of a silicon based MEMS microphone package according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Various aspects of the claimed subject matter are now described with reference to the drawings, wherein the illustrations in the drawings are schematic and not to scale, and like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more aspects. It may be evident, however, that such aspect(s) may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more aspects. 
         [0029]    Generally speaking, a silicon based MEMS microphone, which specifically means an acoustic sensing element herein, may comprise a silicon substrate and an acoustic sensing part supported on the silicon substrate. Specifically, the acoustic sensing part of the said microphone may include at least a compliant diaphragm, a perforated backplate, and an air gap formed between the diaphragm and the backplate, wherein the diaphragm and the backplate are used to form electrode plates of a variable condenser. The inventive concepts of the present invention are as follows: a mesh-structured back hole is formed in the substrate and aligned with the acoustic sensing part of the microphone so that an external acoustic wave traveling through the mesh-structured back hole can be streamlined to have less impact on the acoustic sensing part of the microphone, and alien substances such as particles can be prevented from entering the microphone through the mesh-structured back hole, In more detail, the mesh-structured back hole may include a plurality of mesh beams, which are interconnected with each other and supported either directly or indirectly on the side wall of the mesh-structure back hole. In the meantime, the plurality of mesh beams and the side wall define a plurality of mesh holes which all have a tapered profile and merge into one hole in the vicinity of the acoustic sensing part at the top side of the silicon substrate. 
         [0030]    Hereinafter, embodiments of the present invention will be described in details with reference to the accompanying drawings to explain the structure of the microphone described above. 
       The First Embodiment 
       [0031]      FIG. 2  is a cross-sectional view showing the structure of the silicon based MEMS microphone according to the first embodiment of the present invention. A MEMS microphone may receive an acoustic signal and transform the received acoustic signal into an electrical signal for the subsequent processing and output. As shown in  FIG. 2 , the silicon based MEMS microphone  10  according to the first embodiment of the present invention may include a silicon substrate  100  and an acoustic sensing part  11  supported on the silicon substrate  100  with an isolating oxide layer  120  sandwiched in between. The acoustic sensing part  11  of the microphone  10  may include at least: a conductive and compliant diaphragm  200 , a perforated backplate  400 , and an air gap  150 . The diaphragm  200  is formed with a part of a silicon device layer such as the top-silicon film on a silicon-on-insulator (SOI) wafer or with a polycrystalline silicon (Poly-Si) membrane through a deposition process. The perforated backplate  400  is located above the diaphragm  200 , and formed with CMOS passivation layers with a metal layer  400   b  imbedded therein which serves as an electrode plate of the backplate  400 . The air gap  150  is formed between the diaphragm  200  and the backplate  400 . The conductive and compliant diaphragm  200  serves as a vibration membrane which vibrates in response to an external acoustic wave reaching the diaphragm  200  from the outside, as well as an electrode. The backplate  400  provides another electrode of the acoustic sensing part  11 , and has a plurality of through holes  430  formed thereon, which are used for air ventilation so as to reduce air damping that the diaphragm  200  will encounter when starts vibrating. Therefore, the diaphragm  200  is used as an electrode plate to form a variable condenser  1000  with the electrode plate of the backplate  400 . 
         [0032]    In the silicon based MEMS microphone  10  as shown in  FIG. 2 , the acoustic sensing part  11  of the microphone  10  may further include an interconnection column  600  provided between the center of the diaphragm  200  and the center of the backplate  400  for mechanically suspending and electrically wiring out the diaphragm  200  using CMOS metal interconnection method, and the periphery of the diaphragm  200  is free to vibrate. An example of the acoustic sensing part  11  as above is described in details in the international application No. PCT/CN2010/075514, the related contents of which are incorporated herein by reference. 
         [0033]    Furthermore, in the silicon based MEMS microphone  10  according to the first embodiment of present invention, as shown in  FIG. 2 , a mesh-structured back hole  140  is formed in the substrate  100  and aligned with the acoustic sensing part  11  of the microphone  10  so that an external acoustic wave traveling through the mesh-structured back hole  140  can be streamlined to have less impact on the acoustic sensing part  11  of the microphone  10 , and alien substances such as particles can be prevented from entering the microphone through the mesh-structured back hole  140 . 
         [0034]      FIG. 3  is a plan view showing an exemplary pattern of the mesh-structured back hole formed in the silicon substrate of the microphone of  FIG. 2  when viewed from the bottom side of the silicon substrate.  FIG. 4  and  FIG. 5  are perspective views, showing a simplified structure of the mesh-structured back hole of  FIG. 3  when viewed from the bottom side and the top side of the silicon substrate, respectively. As shown in  FIG. 2-5 , the mesh-structured back hole  140  may include a plurality of mesh beams  141 , which are interconnected with each other and supported either directly or indirectly on the side wall  142  of the mesh-structure back hole  140 . In the meantime, the plurality of mesh beams  141  and the side wall  142  define a plurality of mesh holes  143  which all have a tapered profile and merge into one hole in the vicinity of the acoustic sensing part  11  at the top side of the silicon substrate  100 . 
         [0035]    The mesh-structured back hole  140  may be formed from the bottom side of the silicon substrate using a controlled deep reactive ion etching process. In the controlled deep reactive ion etching process, the plurality of mesh holes  143  can be formed with a tapered profile as shown in  FIG. 2-5  by adjusting the etching conditions such as gas compositions, gas flow rates, pressure, supply power and so on. As a matter of fact, both etching process and depositing process coexist in a deep reactive ion etching process, and the etching process and the depositing process can alternately dominate the deep reactive ion etching process under varying etching conditions and thus alternately produce an etching effect and a depositing effect on the side walls of the mesh holes. In a deposition dominated process, polymers can be deposited on the side walls of the mesh holes  143  to protect them from further etching. Therefore, a carefully controlled deep reactive ion etching process may produce mesh holes  143  with a tapered profile and the mesh holes  143  finally merge into one hole at the top side of the silicon substrate. 
         [0036]    Since the mesh holes  143  merge into one hole in the vicinity of the acoustic sensing part  11  of the microphone  10 , there is enough room for the diaphragm  200  to vibrate. In this embodiment, however, the mesh-structured back hole  140  may include a central mesh beam  141   c  extending from the bottom side to the top side of the silicon substrate  100  and attaching to the center of the diaphragm  200  as a supporting anchor. The central mesh beam  141   c  will not affect the vibration of the center-constrained diaphragm  200  in this embodiment, but can both provide additional support to the acoustic sensing part  11  and enhance the strength of the mesh beams. Furthermore, the plurality of mesh holes of the mesh-structured back hole are uniformly or symmetrically distributed across the mesh-structured back hole, so that an acoustic wave can be uniformly streamlined to ensure a balanced vibration of the diaphragm  200 . 
       The Second Embodiment 
       [0037]      FIG. 6  is a cross-sectional view showing the structure of the silicon based MEMS microphone according to the second embodiment of the present invention, and  FIG. 7  is a plan view showing an exemplary pattern of the mesh-structured back hole formed in the silicon substrate of the microphone of  FIG. 6  when viewed from the bottom side of the silicon substrate. 
         [0038]    Comparing  FIG. 6  with  FIG. 2 , the acoustic sensing part of the microphone according to the second embodiment of the present invention is distinguished from that according to the first embodiment in that, in the second embodiment, the acoustic sensing part  11  of the microphone  10  may further include an interconnection column  600  provided between the edge of diaphragm  200  and the edge of the backplate  400  for electrically wiring out the diaphragm using CMOS metal interconnection method, and the periphery of the diaphragm  200  is fixed. An example of the acoustic sensing part  11  as above is described in details in the international application No. PCT/CN2010/075514, the related contents of which are incorporated herein by reference. 
         [0039]    Comparing  FIG. 7  with  FIG. 3 , the mesh-structured back hole  140  according to the second embodiment of the present invention is distinguished from that according to the first embodiment in that, in the second embodiment, the mesh beams  141  are all formed with their thickness (or height) less than that of the silicon substrate  100 , so that there is enough room for the diaphragm  200  in the microphone  10  according to the second embodiment to vibrate. 
         [0040]    Although two kinds of mesh beam patterns are described with reference to the first and the second embodiment, other uniform and/or symmetrical mesh beam patterns are possible, which will not be described in details. 
         [0041]    It should be noted that the mesh-structured back hole may be provided with a circular shape, a square shape, a rectangular shape or other polygonal shape. 
         [0042]    Furthermore, any silicon based MEMS microphone according to the present invention can be integrated with a CMOS circuitry on a single chip to form a microphone system. 
         [0043]    Hereinafter, a microphone package according to the present invention will be briefly described with reference to  FIG. 8 . 
         [0044]      FIG. 8  is a cross-sectional view showing an exemplary structure of a silicon based MEMS microphone package according to the present invention. As shown in  FIG. 8 , a microphone package according to the present invention comprises a PCB board provided with an acoustic port thereon, a silicon based MEMS microphone according to the present invention, and a cover. 
         [0045]    Specifically, in a silicon based MEMS microphone package according to the present invention, as shown in  FIG. 8 , a silicon based MEMS microphone  10  according to the present invention and other integrated circuits  20  are mounted on a PCB board  30  and enclosed by a cover  40 , wherein a mesh-structured back hole  140  according to the present invention formed in the substrate  100  of the MEMS microphone  10  is aligned with an acoustic port  35  formed on the PCB board  30 . An external acoustic wave, as shown by the arrows in  FIG. 8 , travels through the acoustic port  35  on the PCB board  30  and the mesh-structured back hole  140  in the substrate  100  of the microphone  10  to vibrate the diaphragm  200  of the microphone  10 . 
         [0046]    It should be noted that the acoustic port  35  may be formed on the cover, and aligned with the mesh-structured back hole of the microphone in the silicon based MEMS microphone package according to the present invention. 
         [0047]    The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.