Abstract:
A micro-electro-mechanical system (MEMS) microphone and a forming method therefore. The MEMS microphone comprises: a first substrate, the first substrate is provided with a first bonding face, the first substrate comprises an MEMS microphone component and a first conductive bonding structure arranged on the first bonding face, a second substrate, the second substrate is provided with a second bonding face, the second bonding substrate comprises a circuit and a second conductive bonding structure arranged on the second bonding face; the first substrate and the second substrate are oppositely fitted together via the first conductive bonding structure and the second conductive bonding structure. Embodiments of the present invention have a simple packaging technique and a compact size; the MEMS microphone packaging structure formed has a great performance on signal-to-noise ratio, and a great anti-interference capability

Description:
[0001]    This application claims the priority to Chinese Patent Application No. 201110061561.9, entitled “MEMS MICROPHONE AND FORMING METHOD THEREFORE”, filed with the Chinese Patent Office on Mar. 15, 2011, which is hereby incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of micro-electro-mechanical system process, and particularly to a Micro-Electro-Mechanical Systems (MEMS) microphone and a forming method therefore. 
       BACKGROUND OF THE INVENTION 
       [0003]    A Micro-Electro-Mechanical System (MEMS) microphone formed by a micro electronic mechanical system process has become one of the best candidates for replacing an Electret Condenser Microphone (ECM) with organic film, due to its characteristics of miniaturization and lightweight of the MEMS microphone. 
         [0004]    The MEMS microphone is a miniature microphone made by etching a pressure sensing diaphragm on a semiconductor using micro-electro-mechanical system process, and is widely used in mobile phone, headset, notebook computer, video camera and car. Driven by the need of MEMS microphone&#39;s compatibility with Complementary Metal Oxide Semiconductor (CMOS) and further reducing the size of the MEMS microphone, package structure for MEMS microphone has attracted many development activities in recent years. Many companies invest a lot of money and scientific manpower to study the package structure for MEMS microphone. However, a method adopted by all these companies is that a CMOS circuit and a MEMS microphone are manufactured separately, then the CMOS circuit and MEMS microphone are attached on a substrate, and finally, the CMOS circuit is coupled to the MEMS microphone by wire-bonding technology. 
         [0005]    A package structure for the MEMS microphone is given in a paper of “Silicon microphone development and application”, Sensors and Actuators A, 2007, 133: 283-287. Referring to  FIG. 1 , the package structure includes a package substrate  100 , which has an opening in communication with the package substrate  100 , with the opening being adapted to pass through an acoustic signal; a MEMS microphone  110  and a CMOS circuit  120  adapted to control the MEMS microphone  110  which are located respectively on the package substrate  100 ; a wire  140  electrically coupling the MEMS microphone  110  to the CMOS circuit  120 ; and a package frame  130  by which the package substrate  100 , the MEMS microphone  110  and the CMOS circuit  120  are covered. The CMOS circuit  120  and the MEMS microphone  110  are manufactured separately, and then are packaged on the package substrate 100  by the wire-bonding technology. The package structure for the MEMS microphone packaged on the package substrate  100  by wire-bonding technology has a large size, and needs an additional package frame  130 , which has a complex manufacturing and packaging process, a large size, and high cost. 
       SUMMARY OF THE INVENTION 
       [0006]    A problem to be solved in an embodiment of the present invention is to provide a MEMS microphone and a forming method therefore, which have a simple manufacturing and packaging process, a small size, good signal-to-noise ratio performance, and high interference immunity. 
         [0007]    To solve the problem described above, a MEMS microphone is provided according to an embodiment of the present invention, and the MEMS microphone includes: 
         [0008]    a first base-structure having a first bonding surface and including a MEMS microphone component and a first conductive bonding structure having the first bonding surface; and 
         [0009]    a second base-structure having a second bonding surface and including a circuit and a second conductive bonding structure, with the second conductive bonding structure having the second bonding surface; 
         [0010]    wherein the first base-structure is bonded correspondingly to the second base-structure via the first conductive bonding structure and the second conductive bonding structure. 
         [0011]    Alternatively, the MEMS microphone component may include a sensitive diaphragm and a fixed electrode corresponding to the sensitive diaphragm. 
         [0012]    Alternatively, the MEMS microphone may further include: a first structure located in the first base-structure, wherein the first structure is a first opening or a first air cavity. 
         [0013]    Alternatively, the sensitive diaphragm or the fixed electrode may be exposed through the first opening. 
         [0014]    Alternatively, the MEMS microphone may further include: a second structure located in the second base-structure, wherein the second structure is a second opening or a second air cavity. 
         [0015]    Alternatively, the sensitive diaphragm or the fixed electrode may be exposed through the second opening. 
         [0016]    Alternatively, the first structure may be in communication with the second structure. 
         [0017]    Alternatively, the first conductive bonding structure may include a first top-layer electrode and a bonding layer located on a surface of the first top-layer electrode. 
         [0018]    Alternatively, the first top-layer electrode is located in the same layer as the fixed electrode or the sensitive diaphragm, and is made from the same material as the fixed electrode or the sensitive diaphragm. 
         [0019]    Alternatively, the MEMS microphone may further include: a bonding pad located on the first bonding surface and a third opening through which the bonding pad located on the first bonding surface is exposed. 
         [0020]    Alternatively, the MEMS microphone may further include: a bonding pad located on the second bonding surface and a fourth opening through which the bonding pad located on the second bonding surface is exposed. 
         [0021]    Alternatively, the MEMS microphone component may further include a travel stopper which is adapted to prevent stiction between the sensitive diaphragm and the fixed electrode. 
         [0022]    Alternatively, the sensitive diaphragm may be made from polycrystalline silicon material. 
         [0023]    A method for forming a MEMS microphone is further provided according to the embodiment of the present invention, and the method includes: 
         [0024]    providing a first base-structure, which has a first bonding surface and includes a MEMS microphone component and a first conductive bonding structure having the first bonding surface; 
         [0025]    providing a second base-structure, which has a second bonding surface and includes a circuit and a second conductive bonding structure having the second bonding surface; and 
         [0026]    bonding correspondingly the first base-structure to the second base-structure via the first conductive bonding structure and the second conductive bonding structure. 
         [0027]    Alternatively, the MEMS microphone component may include a sensitive diaphragm and a fixed electrode corresponding to the sensitive diaphragm. 
         [0028]    Alternatively, the method for forming the MEMS microphone may further include: 
         [0029]    forming a first structure located in the first base-structure, wherein the first structure is a first opening or a first air cavity. 
         [0030]    Alternatively, the method for forming the MEMS microphone may further include: forming a second structure located in the second base-structure, wherein the second structure is a second opening or a second air cavity. 
         [0031]    Alternatively, the method for forming the MEMS microphone may further include: communicating the first structure with the second structure. 
         [0032]    Alternatively, the method for forming the MEMS microphone may further include: forming a bonding pad on the first bonding surface of the first base-structure; and forming a third opening through which the bonding pad located on the first bonding surface is exposed. 
         [0033]    Alternatively, the method for forming the MEMS microphone may further include: forming a bonding pad on the second bonding surface of the second base-structure; and forming a fourth opening through which the bonding pad located on the second bonding surface is exposed. 
         [0034]    Alternatively, the first conductive bonding structure may include a first top-layer electrode and a bonding layer located on a surface of the first top-layer electrode. 
         [0035]    Alternatively, the MEMS microphone component may include a sensitive diaphragm and a fixed electrode corresponding to the sensitive diaphragm, and the first top-layer electrode is formed in the same process step as the fixed electrode or the sensitive diaphragm. 
         [0036]    The embodiment of the present invention has the following advantages as compared with the prior art. In the MEMS microphone and the forming method therefore according to the embodiment of the present invention, the first base-structure is bonded to the second base-structure via the first conductive bonding structure and the second conductive bonding structure, specifically, the second base-structure within which a circuit is formed is bonded correspondingly to the first base-structure within which a MEMS microphone is formed via the first conductive bonding structure and the second conductive bonding structure. The MEMS microphone formed in the embodiment of the present invention has a small size and high performance. The MEMS microphone and the forming method therefore according to the embodiment of the present invention has a simple manufacturing and packaging process, a small size, good signal-to-noise ratio performance, and high interference immunity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  is a schematic diagram of a package structure for a MEMS microphone in the prior art; 
           [0038]      FIG. 2  is a schematic flowchart of a method for forming a MEMS microphone according to an embodiment of the present invention; 
           [0039]      FIG. 3  is a schematic flowchart of a method for forming a MEMS microphone according to a first embodiment of the present invention; 
           [0040]      FIG. 4  is a schematic flowchart of a method for forming a MEMS microphone according to a second embodiment of the present invention; 
           [0041]      FIG. 5  to  FIG. 16  are process diagrams of a method for forming the MEMS microphone according to the second embodiment of the present invention; 
           [0042]      FIG. 17  is a schematic flowchart of a method for forming a MEMS microphone according to a third embodiment of the present invention; 
           [0043]      FIG. 18  is a schematic flowchart of a method for forming a MEMS microphone according to a fourth embodiment of the present invention; 
           [0044]      FIG. 19  is a schematic flowchart of a method for forming a MEMS microphone according to a fifth embodiment of the present invention; 
           [0045]      FIG. 20  to  FIG. 22  are process diagrams of a method for forming the MEMS microphone according to the fifth embodiment of the present invention; 
           [0046]      FIG. 23  is a schematic flowchart of a method for forming a MEMS microphone according to a sixth embodiment of the present invention; 
           [0047]      FIG. 24  to  FIG. 27  are process diagrams of a method for forming the MEMS microphone according to the sixth embodiment of the present invention; 
           [0048]      FIG. 28  is a schematic diagram of a MEMS microphone according to a seventh embodiment of the present invention; and 
           [0049]      FIG. 29  is a schematic diagram of a MEMS microphone according to an eighth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    It has been found by the inventor of the present invention that the existing MEMS microphone is formed in such a method that a CMOS circuit and a MEMS microphone are manufactured separately and then are packaged on a package substrate by wire-bonding technology, which has a complex manufacturing and packaging process, a large size, and high cost. 
         [0051]    To this end, a preferred method for forming a MEMS microphone is proposed by the inventor of the present invention, referring to  FIG. 2 , and the method includes the following steps S 11  to S 13 . 
         [0052]    Step S 11 , providing a first base-structure, which has a first bonding surface and includes a MEMS microphone component and a first conductive bonding structure, with the first conductive bonding structure having the first bonding surface. 
         [0053]    Step S 12 , providing a second base-structure, which has a second bonding surface and includes a circuit and a second conductive bonding structure, with the second conductive bonding structure having the second bonding surface. 
         [0054]    Step S 13 , bonding correspondingly the first base-structure to the second base-structure via the first conductive bonding structure and the second conductive bonding structure. 
         [0055]    A MEMS microphone formed by the forming method described above includes: 
         [0056]    a first base-structure having a first bonding surface and including a MEMS microphone component and a first conductive bonding structure having the first bonding surface; and 
         [0057]    a second base-structure having a second bonding surface and including a circuit and a second conductive bonding structure having the second bonding surface; 
         [0058]    wherein the first base-structure is bonded correspondingly to the second base-structure via the first conductive bonding structure and the second conductive bonding structure. 
         [0059]    Specifically, the MEMS microphone component includes a sensitive diaphragm and a fixed electrode corresponding to the sensitive diaphragm, and further includes a travel stopper which is adapted to prevent the stiction between the sensitive diaphragm and the fixed electrode. It should also be noted that the MEMS microphone component in the present invention is not limited to the MEMS microphone component exemplified in this embodiment. The existing MEMS microphone components, such as a microphone having a sensitive diaphragm and a fixed electrode corresponding to the sensitive diaphragm, and a microphone having a travel stopper will all fall within the scope of protection of the technical solution of the present invention. 
         [0060]    The MEMS microphone described above further includes: a first structure located in the first base-structure, wherein the first structure includes a first opening or a first air cavity. The sensitive diaphragm or the fixed electrode is exposed through the first opening. 
         [0061]    The MEMS microphone further includes: a second structure located in the second base-structure, wherein the second structure includes a second opening or a second air cavity. The sensitive diaphragm or the fixed electrode is exposed through the second opening. 
         [0062]    In order to illuminate the spirit and essential of the present invention better, the embodiment of the present invention will be further described in detail in conjunction with the drawings and the embodiments below. 
       First Embodiment 
       [0063]    A preferred method for forming a MEMS microphone is proposed by the inventor of the present invention, referring to  FIG. 3 , and the method includes the following steps S 101  to S 106 . 
         [0064]    Step S 101 , providing a first substrate having a first surface and a second surface opposite to the first surface; forming a sensitive diaphragm and a dielectric layer by which the sensitive diaphragm is covered being formed on the first surface of the first substrate; forming a first top-layer electrode and a fixed electrode which is corresponding to the sensitive diaphragm on a surface of the dielectric layer, and forming the fixed electrode having a plurality of through holes running through the fixed electrode. 
         [0065]    Step S 102 , providing a second substrate within which a circuit is formed, with the second substrate having a third surface and a fourth surface, and forming a second top-layer electrode, corresponding to the first top-layer electrode, on the third surface of the second substrate. 
         [0066]    Step S 103 , bonding the top-layer electrode to the second-layer electrode. 
         [0067]    Step S 104 , removing a part of the first substrate from the second surface to form a first opening. 
         [0068]    Step S 105 , removing a part of the second substrate to form a second opening. 
         [0069]    Step S 106 , removing the dielectric layer corresponding to the sensitive diaphragm, so that a variable capacitor is formed between the sensitive diaphragm and the fixed electrode, and the capacitance of the variable capacitor changes under the stimulus of an acoustic signal. 
         [0070]    Alternatively, after step S 106  is carried out, the first opening may be further communicated with the second opening to form an air cavity. 
         [0071]    It should be noted that in the MEMS microphone according to this embodiment, the first opening may be not communicated with the second opening. Whether or not the first opening is communicated with the second opening in the MEMS microphone according to this embodiment can be selected by those skilled in the art depending on the actual need. It should be noted specifically here that the scope of protection of the present invention should not be limited excessively. 
         [0072]    Alternatively, the method for forming the MEMS microphone according to the first embodiment further includes a step of forming a travel stopper which is adapted to prevent stiction between the sensitive diaphragm and the fixed electrode. 
         [0073]    Specifically, the first top-layer electrode may be a single layer structure or a multilayer stack structure, and the second top-layer electrode may be a single layer structure or a multilayer stack structure. 
         [0074]    In an embodiment of the present invention, the first top-layer electrode may be a conductor, such as aluminum, gold, silicon, germanium, copper, or an alloy of one or more thereof The second top-layer electrode may be a conductor, such as aluminum, gold, silicon, germanium, copper, or an alloy of one or more thereof The first base-structure within which a MEMS microphone is formed is bonded correspondingly to the second base-structure having a circuit via the first top-layer electrode and the second top-layer electrode, specifically via eutectic bonding of the first top-layer electrode made from a metal or an alloy and the second top-layer electrode made from a metal or an alloy. Specifically, the fist base-structure includes the first substrate and the sensitive diaphragm and the fixed electrode formed on the first substrate. The second base-structure includes the second substrate having a circuit and the second top-layer electrode. The first top-layer electrode is a first conductive bonding structure, and the second top-layer electrode is a second conductive bonding structure. 
         [0075]    In another embodiment of the present invention, the first top-layer electrode includes a polycrystalline silicon electrode layer formed on the surface of the dielectric layer and an adhesion layer located on a surface of the polycrystalline silicon electrode layer. The adhesion layer is a conductor, such as aluminum, gold, silicon, germanium, copper, or an alloy of one or more thereof The second top-layer electrode is a single layer structure or a multilayer stack structure, and the second top-layer electrode is made from a conductive material. The second substrate having the circuit is bonded correspondingly to the first substrate within which the MEMS microphone is formed via the adhesion layer of the first top-layer electrode. 
         [0076]    In another embodiment of the present invention, the first top-layer electrode may be a single layer structure or a multilayer stack structure, and the first top-layer electrode is made from a conductive material. The second top-layer electrode includes an electrode layer formed on the third surface of the second substrate and an adhesion layer located on a surface of the electrode layer. The second substrate having the circuit is bonded correspondingly to the first substrate within which the MEMS microphone is formed via the adhesion layer of the second top-layer electrode. 
         [0077]    In the MEMS microphone and the forming method therefore according to the embodiment of the present invention, the first base-structure is bonded to the second base-structure via the first top-layer electrode and the second top-layer electrode. Specifically, the second base-structure having the circuit is bonded correspondingly to the first base-structure having the MEMS microphone via the first top-layer electrode and the second top-layer electrode, and the first top-layer electrode corresponds to the second top-layer electrode. The MEMS microphone formed in the embodiment of the present invention has a small size and high performance. The MEMS microphone and the forming method therefore according to the embodiment of the present invention has a simple manufacturing and packaging process, a small size, good signal-to-noise ratio performance, and high interference immunity. 
         [0078]    A MEMS microphone formed by the method for forming a MEMS microphone described above includes: a first substrate; a dielectric layer located on a surface of the first substrate; a first top-layer electrode located on a surface of the dielectric layer; a second top-layer electrode located on a surface of the first top-layer electrode; a second top-layer electrode; a second substrate which has a circuit formed within; an air cavity running through the first substrate and located within the second substrate; a sensitive diaphragm located within the air cavity; and a fixed electrode located within the air cavity and corresponding to the sensitive diaphragm; with a plurality of through holes running through the fixed electrode are formed. 
         [0079]    Specifically, the first top-layer electrode is a single layer structure or a multilayer stack structure. In the case where the first top-layer electrode is a single layer structure, the first top-layer electrode is made from a conductor material, such as aluminum, gold, silicon, germanium, copper, or an alloy of one or more thereof 
         [0080]    In the case where the first top-layer electrode is a multilayer stack structure, the first top-layer electrode includes a polycrystalline silicon electrode layer formed on the surface of the dielectric layer and an adhesion layer located on a surface of the polycrystalline silicon electrode layer. 
         [0081]    The second top-layer electrode is a single layer structure or a multilayer stack structure. In the case where the second top-layer electrode is a single layer structure, the second top-layer electrode is made from a conductor material, such as aluminum, gold, silicon, germanium, copper, or an alloy of one or more thereof 
         [0082]    In the case where the second top-layer electrode is a multilayer stack structure, the second top-layer electrode includes an electrode layer formed on the third surface of the second substrate and an adhesion layer located on a surface of the electrode layer. 
         [0083]    The MEMS microphone further includes a plurality of interconnects, which are located on the first substrate and located in the same layer as the sensitive diaphragm. 
         [0084]    The MEMS microphone further includes a conductive plug located within the dielectric layer and electrically coupled to the interconnect. The conductive plug is made from the same material as the fixed electrode, which is polycrystalline silicon. 
         [0085]    It should also be noted that in the case where the first top-layer electrode is a multilayer stack structure, the polycrystalline silicon electrode layer of the first top-layer electrode is located in the same layer as the fixed electrode, and is made from the same material as the fixed electrode. 
         [0086]    The MEMS microphone according to the embodiment of the present invention has a small size, high performance, good signal-to-noise ratio performance, and high interference immunity. 
       Second Embodiment 
       [0087]    The method for forming the MEMS microphone of the present invention will be described in detail in conjunction with the second embodiment below.  FIG. 4  is a schematic flowchart of a method for forming a MEMS microphone according to a second embodiment of the present invention, and the method includes the following steps S 201  to S 212 . 
         [0088]    Step S 201 , providing a first substrate having a first surface and a second surface opposite to the first surface. 
         [0089]    Step S 202 , forming, on the first surface of the first substrate, a sensitive diaphragm and a plurality of interconnects. 
         [0090]    Step S 203 , forming a dielectric layer by which the sensitive diaphragm and the plurality of interconnects are covered. 
         [0091]    Step S 204 , forming a plurality of first through holes within the dielectric layer, with the first through holes being corresponding to the interconnect. 
         [0092]    Step S 205 , forming a polycrystalline silicon layer on a surface of the dielectric layer, with the polycrystalline silicon layer filling the first through hole. 
         [0093]    Step S 206 , etching the polycrystalline silicon layer to form a fixed electrode corresponding to the sensitive diaphragm and a top-layer electrode corresponding to the first through hole, with the fixed electrode having a second through hole running through the fixed electrode. 
         [0094]    Step S 207 , forming a bonding layer on a surface of the top-layer electrode to form a first top-layer electrode. 
         [0095]    Step S 208 , providing a second substrate within which a circuit is formed, with the second substrate having a third surface and a fourth surface, a second top-layer electrode being formed on the third surface of the second substrate, and the second top-layer electrode being corresponding to the first top-layer electrode. 
         [0096]    Step S 209 , aligning the second top-layer electrode with the first top-layer electrode, and bonding the second top-layer electrode to the first top-layer electrode via the bonding layer. 
         [0097]    Step S 210 , removing a part of the first substrate from the second surface to form a first opening. 
         [0098]    Step S 211 , removing a part of the second substrate from the fourth surface to form a second opening. 
         [0099]    Step S 212 , removing the dielectric layer corresponding to the first opening from the first opening and/or the second opening, until the first opening becomes in communication with the second opening, so as to form an air cavity. 
         [0100]      FIG. 5  to  FIG. 16  are process diagrams of a method for forming the MEMS microphone according to the second embodiment of the present invention. 
         [0101]    Referring to  FIG. 5 , by carrying out step S 201 , a first substrate  200  is provided. The first substrate  200  has a first surface I and a second surface II opposite to the first surface I. 
         [0102]    The first substrate  200  may be made from a semiconductor material. For example, the first substrate  200  may be made from a single crystal semiconductor material such as single crystal silicon or single crystal silicon-germanium (e.g., a group II-VI compound semiconductor and a group III-V compound semiconductor). And the first substrate  200  may also be an amorphous substrate and a polycrystalline. 
         [0103]    As an embodiment of the present invention, the first substrate  200  is a single crystal silicon substrate  202  having an upper surface on which an insulating layer  201  is formed. The first surface I of the first substrate  200  is the upper surface of the insulating layer  201 , and the second surface II of the second substrate  200  is a lower surface of the single crystal silicon substrate  202 . The insulating layer  201  is adapted to isolate a sensitive diaphragm from a plurality of interconnects formed in the subsequent step. 
         [0104]    The insulating layer  201  may be made from silicon oxide, silicon nitride or silicon oxynitride. The process for forming the insulating layer  201  is a deposition process or a thermal oxidation process. In this embodiment, the insulating layer  201  may be made from silicon oxide material, and the insulating layer  201  is formed by performing oxidation on the first surface of the single crystal silicon substrate  202  by a thermal oxidation process. The thickness and the material of the insulating layer  201  can be selected by those skilled in the art depending on the MEMS microphone to be formed. It should be noted specifically here that the scope of protection of the present invention should not be limited excessively. 
         [0105]    Referring to  FIG. 6 , by carrying out step S 202 , a sensitive diaphragm  210  and a plurality of interconnects  211  are formed on the first surface I of the first substrate  200 . 
         [0106]    The sensitive diaphragm  210  is adapted to form a capacitor with a fixed electrode to be formed later. The sensitive diaphragm  210  may vibrate under the stimulus of an acoustic signal to convert the acoustic signal into an electrical signal. The sensitive diaphragm  210  is made from low stress polycrystalline silicon material, and the shape of the sensitive diaphragm  210  may be square, circular or other shapes. A suitable shape of the sensitive diaphragm  210  can be selected by those skilled in the art depending on the MEMS microphone to be formed. It should be noted specifically here that the scope of protection of the present invention should not be limited excessively. It should also be noted here that the sensitive diaphragm  210  is made from low stress polycrystalline silicon material, so that the size and the production cost of the MEMS microphone using the sensitive diaphragm  210  can be further reduced. 
         [0107]    The interconnect  211  is adapted to electrically couple the sensitive diaphragm  210  of the MEMS microphone. The interconnect  211  is made from a conductive material. The position where the interconnect  211  is formed, the number and the shape of the interconnct  211  may be determined depending on the specific MEMS microphone and can be selected by those skilled in the art depending on the MEMS microphone to be formed. It should be noted specifically here that the scope of protection of the present invention should not be limited excessively. 
         [0108]    It should also be noted that in this embodiment, the interconnect  211  may be made from the same material as that of the sensitive diaphragm  210 , which is low stress polycrystalline silicon. Thus the interconnect  211  may be formed in the same deposition process and the same etching process as the sensitive diaphragm  210 , so as to eliminate some process steps. 
         [0109]    The specific step of forming the interconnect  211  and the sensitive diaphragm  210  includes: depositing a low stress polycrystalline silicon thin film (not shown) on the first surface I of the first substrate  200  by a chemical vapor deposition process; forming a photoresist layer (not shown) on a surface of the low stress polycrystalline silicon thin film; exposing and developing the photoresist layer using a mask corresponding to the interconnect  211  and the sensitive diaphragm  210  so as to form a photoresist pattern; and removing the low stress polycrystalline silicon thin film by a plasma etching process by using the photoresist pattern as a mask, until the first substrate  200  is exposed, so as to form the interconnect  211  and the sensitive diaphragm  210 . In this embodiment, the interconnect and the sensitive diaphragm are formed by etching the same polycrystalline silicon thin film, thus the interconnect is located in the same layer as the sensitive diaphragm. 
         [0110]    In the case where the material of the interconnect  211  is different from that of the sensitive diaphragm  210 , a forming method in which the interconnect  211  is formed firstly and then the sensitive diaphragm  210  is formed, or another forming method in which the sensitive diaphragm  210  is formed firstly and then the interconnect  211  is formed, may be adopted, which will not be described in detail here. 
         [0111]    It should also be noted that in order to improve the conductivity characteristic of the interconnect  211  and the sensitive diaphragm  210  and reduce the stress on the sensitive diaphragm  210 , the low stress polycrystalline silicon thin film may be doped when or after the low stress polycrystalline silicon thin film is formed so as to reduce the resistance of the interconnect  211  and the sensitive diaphragm  210 , and the low stress polycrystalline silicon thin film may be annealed so as to reduce the stress on the sensitive diaphragm  210 . An ion implantation process or an in situ deposition and doping process may be used as the doping process, and a rapid annealing or a tube furnace annealing may be used as the annealing process. 
         [0112]    Referring to  FIG. 7 , by carrying out step S 203 , a dielectric layer  220  is formed, by which the sensitive diaphragm  210  and the plurality of interconnects  211  are covered. 
         [0113]    The dielectric layer  220  is made from a material which has a selective etching characteristic with respect to the sensitive diaphragm  210  and the interconnect  211 . Specifically, the dielectric layer  220  is made from silicon oxide. 
         [0114]    The dielectric layer  220  is adapted to form an air cavity within the dielectric layer in the subsequent step, that is, a portion of the dielectric layer  220  corresponding to the air cavity will serve as a sacrifice layer which will be removed in the subsequent step. The remaining portion of the dielectric layer  220  is adapted to electrically isolate the interconnect  211  from a conductive electrode to be formed later. 
         [0115]    The process for forming the dielectric layer  220  is a deposition process, and is preferably a chemical vapor deposition. 
         [0116]    Referring to  FIG. 8 , by carrying out step S 204 , a plurality of first through holes  211  are formed within the dielectric layer  220 , with the first through holes  221  being corresponding to the interconnect  211 . 
         [0117]    A conductive plug is formed after the first through hole  221  is filled with a conductive material. The conductive plug is adapted to electrically couple the interconnect  211  to an electrode corresponding to the interconnect  211  and located in other layers. 
         [0118]    The specific step of forming the first through hole  221  includes: forming a photoresist layer (not shown) on a surface of the dielectric layer  220 ; exposing and developing the photoresist layer using a mask corresponding to the through hole  221  so as to form a photoresist pattern; etching the dielectric layer  220  by using the photoresist pattern as a mask to form the first through hole  221 , wherein the etching process may be wet etching or dry etching; and removing the photoresist pattern after the first through hole  221  is formed, wherein the removing process may be an ashing process. 
         [0119]    Referring to  FIG. 9 , by carrying out step S 205 , a polycrystalline silicon layer  230  is formed on a surface of the dielectric layer  220 , with the polycrystalline silicon layer  203  filling the first through hole  221 . 
         [0120]    A conductive plug  223  is formed by filling the first through hole  221 . The polycrystalline silicon layer  230  formed on the surface of the dielectric layer  220  is adapted to form a fixed electrode and a first top-layer electrode in the subsequent step. Thereby the conductive plug  223  can be formed in the same deposition process as the polycrystalline silicon layer  230 , so as to eliminate some process steps. 
         [0121]    It should be noted that in this embodiment, the conductive plug  223  is formed in the same deposition process as the polycrystalline silicon layer  230 . Thereby the conductive plug  223  is made from the same material as that of the first top-layer electrode and the fixed electrode to be formed later, which is polycrystalline silicon. 
         [0122]    In other embodiments, the first through hole  221  may also be filled with a conductive material, and then a conductive material layer is formed on the surface of the dielectric layer  220 . A suitable process can be selected by those skilled in the art depending on the specific process requirement. It should be noted specifically here that the scope of protection of the present invention should not be limited excessively. 
         [0123]    Referring to  FIG. 10 , by carrying out step S 206 , the polycrystalline silicon layer  230  is etched to form a fixed electrode  232  corresponding to the sensitive diaphragm  210  and a top-layer electrode  234  corresponding to the first through hole  221 , with the fixed electrode  232  having a second through hole  233  running through the fixed electrode  232 . 
         [0124]    The specific step of forming the fixed electrode  232  and top-layer electrode  234  includes: forming a photoresist layer on a surface of the polycrystalline silicon layer  230 ; exposing and developing the photoresist layer using a mask corresponding to the fixed electrode  232  and the top-layer electrode  234  so as to form a photoresist pattern; and etching the polycrystalline silicon layer  230  by using the photoresist pattern as a mask, so as to form the fixed electrode  232  and the top-layer electrode  234 , with a plurality of second through holes  233  running through the fixed electrode  232  being formed within the fixed electrode  232 , and the top-layer electrode  234  being electrically coupled to the conductive plug  223 ; and removing the photoresist pattern. It should be noted that the top-layer electrode  234  and the fixed electrode  232  are formed by etching the same polycrystalline silicon layer  230 , and thereby the top-layer electrode  234  is located in the same layer as the fixed electrode  232 . 
         [0125]    The fixed electrode  232  is adapted to form a capacitor with the sensitive diaphragm  210  formed previously, and convert the acoustic signal sensed by the capacitor into an electrical signal. 
         [0126]    The second through hole  233  running through the fixed electrode  232  is formed in the fixed electrodes  232 . The second through hole  233  is adapted to pass through an acoustic signal, so that the acoustic signal is able to pass through the fixed electrode  232  without being isolated, therefore the acoustic signal can be sensed by the sensitive diaphragm  210 . 
         [0127]    Referring to  FIG. 11 , by carrying out step S 207 , a bonding layer  235  is formed on a surface of the top-layer electrode  234  to from a first top-layer electrode. 
         [0128]    The bonding layer  235  is adapted to bond a first substrate  200  to a second substrate. 
         [0129]    The bonding layer  235  is made from a conductive bonding material, such as aluminum, gold, silicon, germanium, copper, or an alloy of one or more thereof 
         [0130]    The bonding layer  235  is formed on the surface of the top-layer electrode  234  by electron beam evaporation, sputtering or plating process, depending on the material of the bonding layer  235 . 
         [0131]    The bonding layer  235  and the top-layer electrode  234  constitute the first top-layer electrode in this embodiment. 
         [0132]    Referring to  FIG. 12 , by carrying out step S 208 , a second substrate  300  is provided, within which a circuit or a wiring is formed, with the second substrate  300  having a third surface III and a fourth surface IV, a second top-layer electrode  301  being formed on the third surface III of the second substrate  300 , and the second top-layer electrode  301  being corresponding to the first top-layer electrode  234 . 
         [0133]    The second substrate  300  may include a semiconductor material. For example, the second substrate  300  may include a single crystal semiconductor material such as single crystal silicon or single crystal silicon-germanium. 
         [0134]    The circuit (not shown) formed within the second substrate  300  functions to drive the MEMS microphone and process a signal output by the MEMS microphone. That is, when the MEMS microphone receives an acoustic signal, the acoustic signal can be sensed by the sensitive diaphragm  210  and transmitted to the circuit via the interconnect electrically coupled to the sensitive diaphragm  210  or the fixed electrode  232 , and processed by the circuit. 
         [0135]    The circuit may be a CMOS circuit. The forming process for the circuit is a standard forming process for a CMOS circuit, which will not be described in detail here. A suitable CMOS circuit can be selected by those skilled in the art depending on a design and parameter of the MEMS microphone. Practically, the circuit may also be other circuits which are formed within the second substrate  300 , drive the MEMS microphone, and process the signal output by the MEMS microphone. It should be noted specifically here that the scope of protection of the present invention should not be limited excessively. 
         [0136]    A second top-layer electrode  301  is formed on the third surface III of the second substrate  300 . The second top-layer electrode  301  is coupled to the CMOS circuit via the conductive plug located within the second substrate  300 . It should also be noted that part of the second top-layer electrode  301  is corresponding to the first top-layer electrode, and is adapted to bond to the first top-layer electrode, so as to bond correspondingly the first substrate to the second substrate and form an electrical connection structure. 
         [0137]    The forming process for the second top-layer electrode  301  may be an existing semiconductor process. The specific step of forming the second top-layer electrode  301  include: forming a conductive material layer on the third surface III of the second substrate  300 ; and removing a redundant portion of the conductive material layer by lithography process. 
         [0138]    Referring to  FIG. 13 , by carrying out step S 209 , the second top-layer electrode  301  is aligned with the first top-layer electrode, and the second top-layer electrode  301  is bonded to the first top-layer electrode via the bonding layer  235 . 
         [0139]    It is known from the previous steps that the bonding layer  235  is made from a conductive bonding material. Taking the bonding layer  235  is made from gold-tin alloy as an example, the second top-layer electrode  301  is aligned with the first top-layer electrode, and an annealing process is carried out, so that the second top-layer electrode  301  is bonded to the first top-layer electrode. 
         [0140]    In this embodiment, the first top-layer electrode corresponds to the second top-layer electrode  301 . In this step, the first substrate is bonded to the second substrate, and the CMOS circuit is electrically coupled to the MEMS microphone via the second top-layer electrode  301  and the first top-layer electrode, and no additional wire-bonding process is needed. 
         [0141]    Referring to  FIG. 14 , by carrying out step S 210 , a part of the first substrate  200  is removed from the second surface II to form a first opening  240 . 
         [0142]    The first opening  240  is adapted to be an acoustic signal transmission port of the MEMS microphone, via which an acoustic signal is passed through to the sensitive diaphragm  210  of the MEMS microphone. 
         [0143]    The process for forming the first opening  240  is an etching process, and specifically may be wet etching or dry etching. 
         [0144]    The depth of the removed portion of the first substrate  200  can be set by those skilled in the art depending on the specific process requirement. The sensitive diaphragm  210  may be exposed through the first opening, that is, the first opening runs through the first substrate  200 . The running through the substrate  200  of the first opening  210  has a simple process, and no additional step of removing the remaining first substrate  200  is needed later. 
         [0145]    In this embodiment, the insulating layer  201  is exposed by the first opening  240  formed in step S 210 , that is, the first opening  240  runs through the single crystal silicon substrate  202 . The first opening  240  formed in this embodiment does not run through the first substrate  200 , and the insulating layer  201  remains. The reason for this is that the etching process for the first opening  240  is generally a fast etching process, and the etching rate is high, and if the sensitive diaphragm  210  is exposed when the etching is carried out, the sensitive diaphragm  210  tends to be damaged due to the excessively fast rate, thereby resulting in the lowered performance of the MEMS microphone. In this embodiment, the sensitive diaphragm  210  may be protected by the insulating layer  201 , so that the sensitive diaphragm  210  will not be damaged during the etching process. The insulating layer  201  is removed in a subsequent etching step having a strong controllability on etching. Thereby both the efficiency of the process step and the process will be improved. 
         [0146]    The specific step of forming the first opening includes: forming, on the second surface II, a photoresist pattern corresponding to the first opening  240 ; and etching the first substrate  200  by using the photoresist pattern as a mask, until the insulating layer  201  is exposed, so as to form the first opening  240 . 
         [0147]    Referring to  FIG. 15 , by carrying out step S 210 , a part of the second substrate  300  is removed from the fourth surface IV to form a second opening  310 . 
         [0148]    The second opening  310  is adapted to form an acoustic signal transmission passage. 
         [0149]    The forming process for the second opening  310  is an etching process, and specifically may be wet etching or dry etching. 
         [0150]    In this embodiment, the specific step of forming the second opening  310  includes: forming, on the fourth surface IV, a photoresist pattern corresponding to the second opening  310 ; etching the second substrate  300  by using the photoresist pattern as a mask, until the fixed electrode  232  is exposed entirely, so as to form the second opening  310 . 
         [0151]    Referring to  FIG. 16 , by carrying out step S 212 , the dielectric layer  220  corresponding to the first opening  240  is removed from the first opening  240  and/or the second opening  310 , until the first opening  240  becomes in communication with the second opening  310 , so as to form an air cavity  320 . 
         [0152]    It should be noted that the insulating layer  201  by which the sensitive diaphragm  210  is protected may be removed while the dielectric layer  220  is removed. The sensitive diaphragm  210  will not be damaged because this removing process has a higher selectivity. 
         [0153]    The air cavity  320  is adapted to provide a space for the sensitive diaphragm  210 , so that the sensitive diaphragm  210  is able to vibrate within the air cavity  320 . 
         [0154]    Referring to  FIG. 16 , the MEMS microphone formed adopting the package structure of the MEMS microphone described above includes: 
         [0155]    a first substrate  200 ; a plurality of interconnects  211  formed on the first surface I of the first substrate  200 ; a dielectric layer  220  by which the plurality of interconnects  211  are covered; a conductive plug  223  located within the dielectric layer  220  and electrically coupled to the interconnect  211 ; a first top-layer electrode  234  located on the surface of the dielectric layer  220  and electrically coupled to the conductive plug  223 ; a bonding layer  235  located on a surface of the first top-layer electrode  234 ; a second top-layer electrode  301  located on a surface of the bonding layer  235  and corresponding to the first top-layer electrode  234 ; a second substrate  300  within which a circuit is formed, located on a surface of the second top-layer electrode  301 ; an air cavity  320  running through the first substrate  200  and the second substrate  300 ; a sensitive diaphragm  210  located within the air cavity  320  and located in the same layer as the interconnect  211 ; and a fixed electrode  232  corresponding to the sensitive diaphragm  210  and located in the same layer as the first top-layer electrode  234  located within the air cavity  320 ; with a second through hole  233  miming through the fixed electrode  232 . 
         [0156]    The first substrate  200  may include a semiconductor material. For example, the first substrate  200  may include a single crystal semiconductor material such as single crystal silicon or single crystal silicon-germanium. And the first substrate  200  may include an amorphous substrate and a polycrystalline, such as a glass substrate. 
         [0157]    The sensitive diaphragm  210  is made from low stress polycrystalline silicon material. The interconnect  211  is adapted to electrically couple the sensitive diaphragm  210  of the MEMS microphone. The interconnect  211  is made from a conductive material. 
         [0158]    The first top-layer electrode  234 , the conductive plug  223 , and the fixed electrode  232  are made from polycrystalline silicon material. 
         [0159]    The second substrate  300  may include a semiconductor material. For example, the second substrate  300  may include a single crystal semiconductor material such as single crystal silicon or single crystal silicon-germanium. 
         [0160]    A circuit or a wiring (not shown) is formed within the second substrate  300 . The circuit functions to drive the MEMS microphone. That is, when the MEMS microphone receives an acoustic signal, the acoustic signal can be sensed by the sensitive diaphragm  210  and transmitted to the circuit via the interconnect electrically coupled to the sensitive diaphragm  210  or the fixed electrode  232 , and processed by the circuit. The circuit may be a CMOS circuit. 
         [0161]    It should also be noted that the MEMS microphone formed in the second embodiment may further include a travel stopper (not shown) which is adapted to prevent the stiction between the sensitive diaphragm and the fixed electrode. As for the forming method for the travel stopper, reference may be made to the existing method for forming the travel stopper of the MEMS microphone, which will not be described in detail here. 
         [0162]    In the MEMS microphone and the forming method therefore provided in the present invention, the second substrate having a circuit is packaged correspondingly to the first substrate on which the MEMS microphone is formed via the first top-layer electrode  234  and the second top-layer electrode  301 . Specifically, the first top-layer electrode  234  is corresponding to the second top-layer electrode  301 . The MEMS microphone formed in the present invention has a small size. The MEMS microphone and the forming method therefore according to this embodiment has a simple manufacturing and packaging process, a small size, good signal-to-noise ratio performance, and high interference immunity. 
       Third Embodiment 
       [0163]    The method for forming the MEMS microphone of the present invention will be described in detail in conjunction with the third embodiment below.  FIG. 17  is a schematic flowchart of a method for forming a MEMS microphone according to a third embodiment of the present invention, and the method includes the following steps S 301  to S 312 . 
         [0164]    Step S 301 , providing a first substrate having a first surface and a second surface opposite to the first surface. 
         [0165]    Step S 302 , forming, on the first surface of the first substrate, a sensitive diaphragm and a plurality of interconnects. 
         [0166]    Step S 303 , forming a dielectric layer by which the sensitive diaphragm and the plurality of interconnects are covered. 
         [0167]    Step S 304 , forming a plurality of first through holes within the dielectric layer, with the first through holes being corresponding to the interconnect. 
         [0168]    Step S 305 , forming a polycrystalline silicon layer on a surface of the dielectric layer, with the polycrystalline silicon layer filling the first through hole. 
         [0169]    Step S 306 , etching the polycrystalline silicon layer to form a fixed electrode corresponding to the sensitive diaphragm and a top-layer electrode corresponding to the first through hole, with the fixed electrode having a second through hole running through the fixed electrode. 
         [0170]    Step S 307 , providing a second substrate within which a circuit is formed, with the second substrate having a third surface and a fourth surface, and a top-layer electrode being formed on the third surface of the second substrate. 
         [0171]    Step S 308 , forming a bonding layer on a surface of the top-layer electrode to from a second top-layer electrode, with the second top-layer electrode being corresponding to the first top-layer electrode. 
         [0172]    Step S 309 , aligning the second top-layer electrode with the first top-layer electrode, and bonding the second top-layer electrode to the first top-layer electrode via the bonding layer. 
         [0173]    Step S 310 , removing a part of the first substrate from the second surface to form a first opening. 
         [0174]    Step S 311 , removing a part of the second substrate from the fourth surface to form a second opening. 
         [0175]    Step S 312 , removing the dielectric layer corresponding to the first opening from the first opening and/or the second opening, until the first opening becomes in communication with the second opening, so as to form an air cavity. 
         [0176]    As for the forming process for the specific step of this embodiment, reference may be made to the specific forming process of the second embodiment. It should be noted that the difference between this embodiment and the first embodiment is that the bonding layer is formed on the top-layer electrode on the second substrate, and the top-layer electrode on the second substrate and the bonding layer located on the top-layer electrode constitute the second top-layer electrode. A first substrate is packaged correspondingly to a second substrate via the second top-layer electrode corresponding to the first top-layer electrode. The MEMS microphone and the forming method therefore according to this embodiment has a simple manufacturing and packaging process, a small size, good signal-to-noise ratio performance, and high interference immunity. 
       Fourth Embodiment 
       [0177]    The method for forming the MEMS microphone of the present invention will be described in detail in conjunction with the fourth embodiment below.  FIG. 18  is a schematic flowchart of a method for forming a MEMS microphone according to a fourth embodiment of the present invention, and the method includes the following steps S 401  to S 412 . 
         [0178]    Step S 401 , providing a first substrate having a first surface and a second surface opposite to the first surface. 
         [0179]    Step S 402 , forming, on the first surface of the first substrate, a sensitive diaphragm and a plurality of interconnects. 
         [0180]    Step S 403 , forming a dielectric layer by which the sensitive diaphragm and the plurality of interconnects are covered. 
         [0181]    Step S 404 , forming a plurality of first through holes within the dielectric layer, with the first through holes being corresponding to the interconnect. 
         [0182]    Step S 405 , forming a polycrystalline silicon layer on a surface of the dielectric layer, with the polycrystalline silicon filling the first through hole. 
         [0183]    Step S 406 , etching the polycrystalline silicon layer to form a fixed electrode corresponding to the sensitive diaphragm, with the fixed electrode having a second through hole running through the fixed electrode. 
         [0184]    Step S 407 , forming, on the surface of the dielectric layer, a first metal top-layer electrode which corresponding to the first through hole. 
         [0185]    Step S 408 , providing a second substrate within which a circuit is formed, with the second substrate having a third surface and a fourth surface, and a second top-layer electrode being formed on the third surface of the second substrate. 
         [0186]    Step S 409 , aligning the second top-layer electrode with the first top-layer electrode, and bonding the second top-layer electrode to the first top-layer electrode. 
         [0187]    Step S 410 , removing a part of the first substrate from the second surface to form a first opening. 
         [0188]    Step S 411 , removing a part of the second substrate from the fourth surface to form a second opening. 
         [0189]    Step S 412 , removing the dielectric layer corresponding to the first opening from the first opening and/or the second opening, until the first opening becomes in communication with the second opening, so as to form an air cavity. 
         [0190]    As for step S 401  to step S 405 , reference may be made to step S 201  to step S 205  in the second embodiment. As for step S 406 , reference may be made to step S 206  which is a corresponding step of forming the fixed electrode and the second through hole. 
         [0191]    After that, step S 407  is carried out, in which a first top-layer electrode made from metal and corresponding to the first through hole is formed. 
         [0192]    In this embodiment, the first top-layer electrode made from metal is not formed in the same deposition process or the same etching process as the fixed electrode. Instead, the first top-layer electrode made from metal and corresponding to the first through hole is formed on the surface of the dielectric layer by an additional physical vapor deposition process. The material of first top-layer electrode made from metal is a conductor, such as aluminum, gold, silicon, germanium, copper, or an alloy of one or more thereof 
         [0193]    After that, Step S 408  is carried out. A second substrate is provided, within which a circuit is formed. The second substrate has a third surface and a fourth surface, and a second top-layer electrode being formed on the third surface of the second substrate. 
         [0194]    The second substrate may include a semiconductor material. For example, the second substrate may include a single crystal semiconductor material such as single crystal silicon or single crystal silicon-germanium. 
         [0195]    The circuit (not shown) formed within the second substrate functions to drive the MEMS microphone and process a signal output by the MEMS microphone. That is, when the MEMS microphone receives an acoustic signal, the acoustic signal can be sensed by the sensitive diaphragm  210 , and transmitted to the circuit via the interconnect electrically coupled to the sensitive diaphragm  210  or the fixed electrode  232 , and processed by the circuit. 
         [0196]    The circuit may be a CMOS circuit. The forming process for the circuit is a standard forming process for a CMOS circuit, which will not be described in detail here. A suitable CMOS circuit can be selected by those skilled in the art depending on the design and parameter of the MEMS microphone. Practically, the circuit may also be other circuits which are formed within the second substrate, drive the MEMS microphone, and process the signal output by the MEMS microphone. It should be noted specifically here that the scope of protection of the present invention should not be limited excessively. 
         [0197]    A second top-layer electrode is formed on the third surface of the second substrate, and the second top-layer electrode is coupled to the CMOS circuit via the conductive plug located within the second substrate. It should also be noted that the second top-layer electrode corresponds to the first top-layer electrode, and is adapted to bond to the first top-layer electrode, so as to bond the first substrate to the second substrate and form an electrical connection structure. 
         [0198]    In this embodiment, the second top-layer electrode may be made from a conductor, such as aluminum, gold, silicon, germanium, copper, or an alloy of one or more thereof The forming process for the second top-layer electrode is a physical vapor deposition process. Thereby the second top-layer electrode is bonded to the first top-layer electrode, and no additional bonding layer is needed. 
         [0199]    After that, step S 409  is carried out, in which the second top-layer electrode made from metal is aligned with and bonded to the first top-layer electrode. 
         [0200]    It is known from the description of the previous steps that the second top-layer electrode made from metal and the first top-layer electrode may be made from a conductor, such as aluminum, gold, silicon, germanium, copper, or an alloy of one or more thereof The second top-layer electrode is aligned with the first top-layer electrode, and an annealing process is carried out, so that the second top-layer electrode is bonded to the first top-layer electrode. 
         [0201]    As for step S 410  to step S 412 , reference may be made to step S 210  to step S 212  in the first embodiment. 
         [0202]    In the MEMS microphone and the forming method therefore provided in the present invention, the second substrate having a circuit is packaged correspondingly to the first substrate within which the MEMS microphone is formed via the first top-layer electrode  234  and the second top-layer electrode  301 . Specifically, the first top-layer electrode  234  corresponds to the second top-layer electrode  301 . The MEMS microphone formed in the present invention has a small size. The MEMS microphone and the forming method therefore according to this embodiment has a simple manufacturing and packaging process, a small size, good signal-to-noise ratio performance, and high interference immunity. 
       Fifth Embodiment 
       [0203]    The method for forming the MEMS microphone of the present invention will be described in detail in conjunction with the fifth embodiment below.  FIG. 19  is a schematic flowchart of a method for forming a MEMS microphone according to a fifth embodiment of the present invention, and the method includes the following steps S 501  to S 513 . 
         [0204]    Step S 501 , providing a first substrate having a first surface and a second surface opposite to the first surface. 
         [0205]    Step S 502 , forming, on the first surface of the first substrate, a sensitive diaphragm and a plurality of interconnects. 
         [0206]    Step S 503 , forming a dielectric layer by which the sensitive diaphragm and the plurality of interconnects are covered. 
         [0207]    Step S 504 , forming a plurality of first through holes within the dielectric layer, with the first through holes being corresponding to the interconnect. 
         [0208]    Step S 505 , forming a polycrystalline silicon layer on a surface of the dielectric layer, with the polycrystalline silicon layer filling the first through hole. 
         [0209]    Step S 506 , etching the polycrystalline silicon layer to form a fixed electrode corresponding to the sensitive diaphragm and a top-layer electrode corresponding to the first through hole, with the fixed electrode having a second through hole running through the fixed electrode. 
         [0210]    Step S 507 , forming a bonding pad on the surface of the dielectric layer. 
         [0211]    Step S 508 , forming a bonding layer on the surface of the dielectric layer to form a first top-layer electrode. 
         [0212]    Step S 509 , providing a second substrate within which a circuit is formed, with the second substrate having a third surface and a fourth surface, a second top-layer electrode is formed on the third surface of the second substrate, and the second top-layer electrode corresponds to the first top-layer electrode. 
         [0213]    Step S 510 , aligning the second top-layer electrode with the first top-layer electrode, and bonding the second top-layer electrode to the first top-layer electrode via the bonding layer. 
         [0214]    Step S 511 , removing a part of the first substrate from the second surface to form a first opening through which the sensitive diaphragm is exposed. 
         [0215]    Step S 512 , removing a part of the second substrate from the fourth surface to form a second opening corresponding to the first opening and a fourth opening through which the bonding pad is exposed. 
         [0216]    Step S 513 , removing the dielectric layer corresponding to the first opening from the first opening and/or the second opening, until the first opening becomes in communication with the second opening, so as to form an air cavity. 
         [0217]    As for step S 501  to step S 506 , reference may be made to step S 201  to step  206  and  FIG. 5  to  FIG. 10  in the second embodiment. 
         [0218]    Referring to  FIG. 20 , by carrying out step S 507 , a bonding pad  236  is formed on the surface of the dielectric layer  220 . 
         [0219]    Specifically, the bonding pad is adapted to provide an electrical connection platform for the MEMS microphone. And the bonding pad is generally made from metal, for the reason that the bonding pad  236  has a larger area and need to bear a certain pressure from wire bonding. The forming process for the bonding pad  236  may be such a process in which a metal layer (not shown) is deposited by a physical vapor deposition process, and the metal layer is photoresist patterned and etched to form the bonding pad  236 . The specific step of forming the bonding pad  236  can be determined by those skilled in the art depending on the specific requirement of the MEMS microphone product, and reference may be made to the existing step of forming the bonding pad. 
         [0220]    After the bonding pad  236  is formed, step S 508  to step S 511  are carried out, and reference may be made to corresponding step S 207  to step S 210  and  FIG. 11  to  FIG. 16  in the second embodiment. 
         [0221]    After step S 511  is carried out, step S 512  is carried out. Referring to  FIG. 21 , a part of the second substrate  300  is removed from the fourth surface IV to form a second opening  310  corresponding to the first opening  240  and a fourth opening  311  through which the bonding pad  236  is exposed. 
         [0222]    Specifically: a photoresist pattern is formed on the fourth surface IV, with the photoresist pattern being corresponding to the second opening  310  and the fourth opening  311 ; and the second substrate  300  is etched by using the photoresist pattern as a mask, with the second opening  310  being formed in the same etching process as the fourth opening  311 . 
         [0223]    It should be noted that because the second opening  310  has the same depth as the fourth opening  311  due to a design characteristic of the MEMS microphone in this embodiment, the second opening  310  can be formed in the same etching process as the fourth opening  311 . Thereby some process steps will be eliminated and production cost will be saved. 
         [0224]    Referring to  FIG. 22 , by carrying out step S 513 , the dielectric layer  220  corresponding to the first opening  240  is removed from the first opening  240  and/or the second opening  310 , until the first opening  240  becomes in communication with the second opening  310 , so as to form an air cavity  320 . 
         [0225]    As for the specific forming step, reference may be made to the corresponding step in the second embodiment, which will not be described in detail here. 
         [0226]    Referring to  FIG. 22 , the MEMS microphone formed by the method for forming the MEMS microphone according to the fifth embodiment includes: 
         [0227]    a first substrate  200 ; a plurality of interconnects  211  formed on the first surface I of the first substrate  200 ; a dielectric layer  220  by which the plurality of interconnects  211  are covered; a conductive plug  223  located within the dielectric layer  220  and electrically coupled to the interconnect  211 ; a first top-layer electrode  234  located on a surface of the dielectric layer  220  and electrically coupled to the conductive plug  223 ; a bonding pad  236  located on the surface of the dielectric layer  220 ; a bonding layer  235  located on a surface of the first top-layer electrode  234 ; a second top-layer electrode  301  located on a surface of the bonding layer  235  and corresponding to the first top-layer electrode  234 ; a second substrate  300 , within which a circuit is formed, located on a surface of the second top-layer electrode  301 ; an air cavity  320  running through the first substrate  200  and the second substrate  300 ; a sensitive diaphragm  210  located within the air cavity  320  and located in the same layer as the interconnect  211 ; a fixed electrode  232  located within the air cavity  320  and corresponding to the sensitive diaphragm  210  and located in the same layer as the first top-layer electrode  234 ; a second through hole  233  miming through the fixed electrode  232 ; and a fourth opening  311  through which the bonding pad  236  is exposed. 
         [0228]    In the MEMS microphone formed in the fifth embodiment of the present invention, there is provided with the bonding pad  236  formed on the surface of the dielectric layer  220 , and the bonding pad  236  is exposed through the fourth opening  311 . And no additional step of forming the exposed bonding pad is needed in the subsequent packaging process, thereby some packaging process steps will be eliminated and production cost will be saved. 
       Sixth Embodiment 
       [0229]    The method for forming the MEMS microphone of the present invention will be described in detail in conjunction with the sixth embodiment below.  FIG. 23  is a schematic flowchart of a method for forming a MEMS microphone according to a sixth embodiment of the present invention, and the method includes the following steps S 601  to S 611 . 
         [0230]    Step S 601 , providing a first substrate having a first surface and a second surface opposite to the first surface. 
         [0231]    Step S 602 , forming, on the first surface of the first substrate, a sensitive diaphragm and a plurality of interconnects. 
         [0232]    Step S 603 , forming a dielectric layer by which the sensitive diaphragm and the plurality of interconnects are covered. 
         [0233]    Step S 604 , forming a plurality of first through holes within the dielectric layer, with the first through holes being corresponding to the interconnect. 
         [0234]    Step S 605 , forming a polycrystalline silicon layer on a surface of the dielectric layer, with the polycrystalline silicon layer filling the first through hole. 
         [0235]    Step S 606 , etching the polycrystalline silicon layer to form a fixed electrode corresponding to the sensitive diaphragm and a top-layer electrode corresponding to the first through hole, with the fixed electrode having a second through hole running through the fixed electrode. 
         [0236]    Step S 607 , forming a bonding layer on a surface of the top-layer electrode to form a first top-layer electrode. 
         [0237]    Step S 608 , providing a second substrate within which a circuit is formed, with the second substrate having a third surface and a fourth surface, and a second top-layer electrode and a bonding pad being formed on the third surface of the second substrate. 
         [0238]    Step S 609 , aligning the second top-layer electrode with the first top-layer electrode, and bonding the second top-layer electrode to the first top-layer electrode via the bonding layer. 
         [0239]    Step S 610 , removing a part of the second substrate from the fourth surface to form a second opening through which the fixed electrode is exposed. 
         [0240]    Step S 611 , removing a part of the first substrate from the second surface to form a first opening which is in communication with the second opening and a third opening through which the bonding pad is exposed. 
         [0241]    Specifically, as for step S 601  to step S 07 , reference may be made to step S 201  to step  207  and  FIG. 5  to  FIG. 11  in the second embodiment. 
         [0242]    After that, step S 608  is carried out. Referring to  FIG. 24 , a second substrate  300  is provided, within which a circuit (not shown) is formed, with the second substrate  300  having a third surface III and a fourth surface IV, and a second top-layer electrode  301  and a bonding pad  302  being formed on the third surface III of the second substrate  300 . 
         [0243]    The second substrate  300  may include a semiconductor material. For example, the second substrate  300  may include a single crystal semiconductor material such as single crystal silicon or single crystal silicon-germanium. 
         [0244]    The circuit (not shown) formed within the second substrate  300  functions to drive the MEMS microphone and process a signal output by the MEMS microphone. That is, when the MEMS microphone receives an acoustic signal, the acoustic signal can be sensed by the sensitive diaphragm  210  and transmitted to the circuit via the interconnect electrically coupled to the sensitive diaphragm  210  or the fixed electrode  232 , and processed by the circuit. 
         [0245]    The circuit may be a CMOS circuit. The forming process for the circuit is a standard forming process for a CMOS circuit, which will not be described in detail here. A suitable CMOS circuit can be selected by those skilled in the art depending on the design and parameter of the MEMS microphone. Practically, the circuit may also be other circuits which are formed within the second substrate, drive the MEMS microphone, and process the signal output by the MEMS microphone. It should be noted specifically here that the scope of protection of the present invention should not be limited excessively. 
         [0246]    A second top-layer electrode  301  is formed on the third surface III of the second substrate  300 , which is coupled to the CMOS circuit via the conductive plug located within the second substrate. It should also be noted that the second top-layer electrode corresponds to the first top-layer electrode, and is adapted to bond to the first top-layer electrode, so as to bond the first substrate to the second substrate and form an electrical connection structure. 
         [0247]    The forming process for the second top-layer electrode  301  is an existing semiconductor process. The specific forming step include: forming a conductive material layer on the third surface III of the second substrate  300 ; and removing a redundant portion of the conductive material layer by lithography process so as to form the second top-layer electrode  301 . 
         [0248]    It should be noted that a bonding pad of the MEMS microphone is also formed in step S 608  in this embodiment as compared with the second embodiment. 
         [0249]    Specifically, the bonding pad  302  is adapted to provide an electrical connection platform for the MEMS microphone. And the bonding pad  302  is generally made from metal for the reason that the bonding pad  302  has a larger area and need to bear a certain pressure from wire bonding. The forming process for the bonding pad  302  may be such a process in which a metal layer (not shown) is deposited by physical vapor deposition process, and the metal layer is photoresist patterned and etched to form the bonding pad  302 . The specific step of forming the bonding pad  302  can be determined by those skilled in the art depending on the specific requirement of the MEMS microphone, and reference may be made to the existing step of forming the bonding pad. 
         [0250]    Referring to  FIG. 25 , by carrying out step S 609 , the second top-layer electrode  301  is aligned with the first top-layer electrode  234 , and the second top-layer electrode  301  is bonded to the first top-layer electrode  234  via the bonding layer  235 . 
         [0251]    The bonding layer  235  is made from a conductive bonding material. Specifically, the bonding layer  235  is formed on a surface of the second top-layer electrode  301  or a surface of the first top-layer electrode  234 . And the second top-layer electrode  301  is bonded to the first top-layer electrode  234 . 
         [0252]    Referring to  FIG. 26 , by carrying out step S 610 , a part of the second substrate  300  is removed from the fourth surface IV to form a second opening  310  through which the fixed electrode is exposed. 
         [0253]    Specifically, a photoresist layer (not shown) on the fourth surface IV is formed; the photoresist layer is exposed and developed so as to form a photoresist pattern corresponding to the second opening  310  within the photoresist layer; the second substrate  300  is etched by using the photoresist pattern as a mask, until the second opening  310  is formed, through which the fixed electrode is exposed. 
         [0254]    Referring to  FIG. 27 , by carrying out step S 611 , a part of the first substrate  200  is removed from the second surface II to form a first opening  410  which is in communication with the second opening  310 , and a third opening  411  through which the bonding pad is exposed. 
         [0255]    Specifically, a photoresist layer (not shown) on the second surface II is formed; the photoresist layer is exposed and developed so as to form, within the photoresist layer, a photoresist pattern corresponding to the first opening  410  and the third opening  411 ; and the first substrate  200  is etched by using the photoresist pattern as a mask, until the first opening  410  and the third opening  411  are formed, with the second opening  310  being in communication with the first opening  410 , and the bonding pad  302  being exposed through the third opening  411 . 
         [0256]    It should be noted that because the first opening  410  has the same depth as the third opening  411  depending on a design of the MEMS microphone in this embodiment, the first opening  410  can be formed in the same etching process as the third opening  411 . Thereby some process steps will be eliminated and production cost will be saved. 
         [0257]    Referring to  FIG. 27 , the MEMS microphone formed by the method for forming the MEMS microphone according to the sixth embodiment includes: 
         [0258]    a first substrate  200 ; a plurality of interconnects  211  formed on the first surface I of the first substrate  200 ; a dielectric layer  220  by which the plurality of interconnects  211  are covered; a conductive plug  223  located within the dielectric layer  220  and electrically coupled to the interconnect  211 ; a first top-layer electrode  234  located on a surface of the dielectric layer  220  and electrically coupled to the conductive plug  223 ; a bonding layer  235  located on a surface of the first top-layer electrode  234 ; a second top-layer electrode  301  located on a surface of the bonding layer  235  and corresponding to the first top-layer electrode  234 ; a bonding pad  302 ; a second substrate  300 , within which a circuit is formed, located on a surface of the second top-layer electrode  301  and the bonding pad  302 ; an air cavity running through the first substrate  200  and the second substrate  300 , with the air cavity being comprised of the first opening  410  and the second opening  310 ; a sensitive diaphragm  210  located within the air cavity and located in the same layer as the interconnect  211 ; a fixed electrode  232  located within the air cavity  320 , corresponding to the sensitive diaphragm  210  and located in the same layer as the first top-layer electrode  234 ; a second through hole  233  running through the fixed electrode  232 ; and a third opening  411  formed within the first substrate  200  through which the bonding pad is exposed. 
         [0259]    The MEMS microphone formed in the sixth embodiment of the present invention has the bonding pad formed on the surface of the second substrate  300 , and the bonding pad is exposed through the third opening  411 . And no additional step of forming the exposed bonding pad is needed in the subsequent packaging process, thereby some packaging process steps will be eliminated and production cost will be saved. 
       Seventh Embodiment 
       [0260]    Referring to  FIG. 28 , the MEMS microphone formed in the seventh embodiment of the present invention includes: a first substrate  200 ; a plurality of interconnects  211  formed on the first surface I of the first substrate  200 ; a dielectric layer  220  by which the plurality of interconnects  211  are covered; a conductive plug  223  located within the dielectric layer  220  and electrically coupled to the interconnect  211 ; a first top-layer electrode  234  located on a surface of the dielectric layer  220  and electrically coupled to the conductive plug  223 ; a bonding layer  235  located on a surface of the first top-layer electrode  234 ; a second top-layer electrode  301  located on a surface of the bonding layer  235  and corresponding to the first top-layer electrode  234 ; a second substrate  300 , within which a circuit is formed, located on a surface of the second top-layer electrode  301 ; an air cavity  320  running through the first substrate  200  and the second substrate  300 ; a fixed electrode  232  located within the air cavity  320 , and located in the same layer as the interconnect  211 ; a second through hole  233  running through the fixed electrode  232 ; and a sensitive diaphragm  210  located within the air cavity  320 , corresponding to the fixed electrode  232  and located in the same layer as the first top-layer electrode  234 . 
         [0261]    The MEMS microphone formed in the seventh embodiment of the present invention has the same components as the MEMS microphone formed in the second embodiment, except that the positions of the fixed electrode  232  and the sensitive diaphragm  210  are exchanged. As for the forming method in this embodiment, reference may also be made correspondingly to the forming method in the second embodiment. However, steps of forming the fixed electrode  232  and forming the sensitive diaphragm  210  should be adjusted accordingly. 
         [0262]    It should also be noted that as for the second embodiment, the third embodiment, the fourth embodiment, the fifth embodiment and the sixth embodiment, reference may also be made to the MEMS microphone formed in the seventh embodiment. In respective embodiments, the position of the fixed electrode  232  and the sensitive diaphragm  210  may be adjusted similarly without affecting the scope of the second embodiment, the third embodiment, the fourth embodiment, the fifth embodiment and the sixth embodiment. 
       Eighth Embodiment 
       [0263]    Referring to  FIG. 29 , the MEMS microphone formed in the eighth embodiment of the present invention includes: a first substrate  500 ; a plurality of interconnects  511  and a sensitive diaphragm  510  formed on the first surface I of the first substrate  500 ; a dielectric layer  520  by which the plurality of interconnects  511  and a part of the sensitive diaphragm  510  are covered; a conductive plug  523  located within the dielectric layer  520  and electrically coupled to the interconnect  511 ; a first top-layer electrode  534  located on a surface of the dielectric layer  520  and electrically coupled to the conductive plug  523 ; a bonding layer  535  located on a surface of the first top-layer electrode  534 ; a second top-layer electrode  601  located on a surface of the bonding layer  535  and corresponding to the first top-layer electrode  534 ; a second substrate  600 , within which a circuit is formed, located on a surface of the second top-layer electrode  601 ; an air cavity  620 , through which the sensitive diaphragm  510  is exposed, running through the second substrate  600 ; a fixed electrode  532  located within the air cavity  620 , corresponding to the sensitive diaphragm  510  and located in the same layer as the first top-layer electrode  534 ; a second through hole  533  running through the fixed electrode  532 ; and a first opening  540  located within the first substrate  500 , through which the sensitive diaphragm  510  is exposed. 
         [0264]    It should also be noted that as for the second embodiment, the third embodiment, the fourth embodiment, the fifth embodiment, the sixth embodiment and the seventh embodiment, reference may also be made to the MEMS microphone formed in the eighth embodiment. In respective embodiments, the design and the position of the sensitive diaphragm may be adjusted similarly without affecting the scope of the second embodiment, the third embodiment, the fourth embodiment, the fifth embodiment, the sixth embodiment and the seventh embodiment. 
         [0265]    Although preferred embodiments of the present invention have been disclosed above, they are not intended to limit the present invention. Any possible variations and modifications can be made to the technical solution of the present invention by those skilled in the art according to the above-disclosed methods and technical contents without departing from the spirit and scope of the present invention. Therefore, any content which does not deviate from the technical solution of the present invention, and any modifications and equivalent variations made to the above embodiments according to the technical essence of the present invention will fall within the scope of protection of technical solution of the present invention.