Patent Publication Number: US-2012027235-A1

Title: Mems capacitive microphone

Description:
FIELD OF THE INVENTION 
     The present invention relates to an MEMS capacitive microphone, particularly to an MEMS capacitive microphone whose diaphragm has high flatness and low residual stress. 
     BACKGROUND OF THE INVENTION 
     The current tendency is toward fabricating slim, compact, lightweight and high-performance electronic products, including microphones. A microphone is used to receive sound and convert acoustic signals into electric signals. Microphones are extensively used in daily-life appliances, such as telephones, mobiles phones, recording pens, etc. For a capacitive microphone, variation of sound forces the diaphragm to deform correspondingly in a type of acoustic waves. The deformation of the diaphragm induces capacitance variation. The variation of sounds can thus be obtained via detecting the voltage difference caused by capacitance variation. 
     Distinct from the conventional electret condenser microphones (ECM), mechanical and electronic elements of MEMS (Micro Electro-Mechanical Systems) microphones can be integrated on a semiconductor material by the IC (Integrated Circuit) technology to fabricate a miniaturized microphone. Now, MEMS microphones have become the mainstream of miniaturized microphones. MEMS microphones have advantages of compactness, lightweightness and low power consumption. Further, MEMS microphones can be fabricated with a surface-mount method, can bear a higher reflow temperature, can be easily integrated with a CMOS process and other audio electronic devices, and are more likely to resist radio frequency (RF) and electromagnetic interference (EMI). 
     Refer to  FIG. 1  for a diagram schematically showing the structure of a conventional MEMS microphone. The conventional MEMS microphone  1  comprises a back plate  2 , a flexible diaphragm  3  and a spacer  4 . The spacer  4  is interposed between the back plate  2  and the flexible diaphragm  3 , whereby the spacer  4  supports the rim of the flexible diaphragm  3  and insulates the flexible diaphragm  3  from the back plate  2 . Thus, the back plate  2  and the flexible diaphragm  3  are parallel to each other and respectively form a lower electrode and an upper electrode of a parallel capacitor plate. The back plate  2  has a plurality of air holes  5  which are corresponding to the flexible diaphragm  3  penetrating the back plate  2  and intercommunicating with a back chamber  7  formed on a silicon substrate  6 . 
     Applying voltage to the back plate  2  and flexible diaphragm  3  makes them respectively carry opposite charges and form a capacitor structure. A capacitance equation correlates to a parallel electrode plate is C=εA/d, wherein ε is the dielectric constant, A is the overlapped area of the two electrode plates, and d is the gap between the two capacitor plates. According to the equation, variation of the gap between the two capacitor plates will change the capacitance. When an acoustic wave causes the flexible diaphragm  3  to vibrate and deform, the gap between the back plate  2  and the flexible diaphragm  3  varies. Thus, the capacitance also varies to be converted into electric signals and output. The disturbed or compressed air between the flexible diaphragm  3  and the back plate  2  is released to the back chamber  7  via the air holes  5  lest drastic pressure damage the flexible diaphragm  3  and the back plate  2 . 
     Refer to  FIG. 2  for a diagram schematically showing the package structure of a conventional MEMS microphone. The conventional MEMS microphone  1  is installed on a baseplate  8  and packaged inside a holding space formed by a metallic cover  9 . The flexible diaphragm  3  and the back plate  2  are respectively electrically connected with a conversion chip  10 . The conversion chip  10  converts the variation of the capacitance between the back plate  2  and the flexible diaphragm  3  into electric signals to be output. 
     In the conventional MEMS microphones, sound pressure induces the deformation of the flexible diaphragm and changes the gap between the flexible diaphragm and the back plate, whereby the capacitance is varied. However, the flexible diaphragm is fabricated with a film-deposition method at a very high temperature. As different materials respectively have different thermal expansion coefficients, the diaphragm would accumulate tensile or compressive stress with different levels. Residual stress on the diaphragm will cause the warping or buckles of the diaphragm and lower the precision of detection. Moreover, due to the sensitivity of a microphone is inversely proportional to the residual stress of the diaphragm, higher residual stress results in low sensitivity. 
     An U.S. Pat. No. 5,490,220 entitled “Solid State Condenser and Microphone Devices” proposes a suspended diaphragm without the constant boundary, wherein a cantilever is used to support the diaphragm, such that the diaphragm is suspended to release stress caused by temperature effect. Another U.S. Pat. No. 5,870,482 entitled “Miniature Silicon Condenser Microphone” designs a large plate diaphragm with only one side fastened. An U.S. Pat. No. 7,023,066 entitled “Silicon Microphone” proposes a special structure design of the rim of the diaphragm to solve the problem of residual stress, such as provides tangential supporting springs along the rim of the diaphragm. No matter whether the cantilever or the tangential supporting spring is used to overcome the problem of residual stress, the design and fabrication process thereof are complicated and hard to completely overcome the problem of residual stress. 
     SUMMARY OF THE INVENTION 
     One objective of the present invention is to provide an MEMS (Micro Electro-Mechanical Systems) capacitive microphone, whose diaphragm is easy to fabricate, favorable to release stress, and has high flatness, whereby is solved the conventional problem of thermal residual stress. 
     To achieve the abovementioned objective, the present invention proposes an MEMS capacitive microphone, which uses a supporting portion to support the center of a diaphragm to provide sufficient space to release stress, and which comprises a base, a back plate, an anchor member and a diaphragm. The back plate is arranged on the base and has a plurality of air holes. The base has a back chamber interconnecting with the air holes. The anchor member is arranged on the base and includes a supporting portion. The supporting portion supports the center of the diaphragm to make the diaphragm parallel to the back plate. Thereby, stress on the diaphragm is released outwards from the supporting portion. 
     The MEMS capacitive microphone of the present invention is characterized in that the diaphragm is supported in the center thereof. Thus, stress of the diaphragm is released outwards from the center thereof. Thereby is overcome the problem of deformation, buckles or fractures of the diaphragm caused by stress. Below, the embodiments are described in detail to demonstrate the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments are described in cooperation with the following drawings. 
         FIG. 1  is a diagram schematically showing the structure of a conventional MEMS microphone; 
         FIG. 2  is a diagram schematically showing the package structure of a conventional MEMS microphone; 
         FIG. 3A  is a perspective view of an MEMS capacitive microphone according to one embodiment of the present invention; 
         FIG. 3B  is a perspective sectional view of an MEMS capacitive microphone according to one embodiment of the present invention; 
         FIG. 4  is a diagram schematically showing the operation of an MEMS capacitive microphone according to one embodiment of the present invention; 
         FIG. 5A  is a perspective view of an MEMS capacitive microphone according to another embodiment of the present invention; 
         FIG. 5B  is a perspective sectional view of an MEMS capacitive microphone according to another embodiment of the present invention; 
         FIG. 6  is a diagram schematically showing the operation of an MEMS capacitive microphone according to another embodiment of the present invention; 
         FIGS. 7A-7I  are sectional views schematically showing the process of fabricating an MEMS capacitive microphone according to another embodiment of the present invention; and 
         FIG. 8  is a diagram showing the result of a test under different frequencies of an MEMS capacitive microphone according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention proposes an MEMS capacitive microphone, which uses a supporting portion to support the center of a diaphragm to favorably release residual stress. The technical contents of the present invention are described in detail in cooperation with the drawings below. 
     Refer to  FIG. 3A  and  FIG. 3B . In one embodiment of the present invention, the MEMS capacitive microphone  20  comprises a base  21 , a diaphragm  22 , an anchor member  23  and a back plate  24 . The back plate  24  is arranged on the base  21  and has a plurality of air holes  25  penetrating the back plate  24 . The base  21  has a back chamber  26  corresponding to the back plate  24 , such that the air holes  25  interconnect with the back chamber  26 . The anchor member  23  is arranged on the base  21  and straddles the back chamber  26 . The anchor member  23  further comprises a supporting portion  27 . The supporting portion  27  supports the center of the diaphragm  22  to make the diaphragm  22  parallel to the back plate  24 . 
     It should be particularly mentioned that residual stress on the diaphragm  22  is normally released radially from the center toward the rim. Therefore, supporting the diaphragm  22  in the central portion thereof favors releasing residual stress of the diaphragm  22 . Considering stability of the diaphragm  22 , the abovementioned central portion can be the geometric center (gravitational center) of the diaphragm  22  or on the symmetric axis of the diaphragm  22 . In one embodiment, the supporting portion  27  supports the center of a circular diaphragm  22 . For convenience of description, the present invention will use this embodiment as the exemplification thereinafter. However, the present invention is not limited by this embodiment. Besides, the supporting portion  27  can a post fixedly installed on the anchor member  23  in one embodiment. 
     In one embodiment, the diaphragm  22  is a flexible diaphragm. The geometric center of the diaphragm  22  is supported by the supporting portion  27  to form a static end. The rim of the diaphragm  22  thus forms a free end vibrated or deformed by sound waves. Such a design makes residual stress of the diaphragm  22  released from the static end toward the free end and prevents the diaphragm  22  from buckling or deforming. 
     In one embodiment, the base  21  is a silicon substrate having a circular back chamber  26  formed therein. The anchor member  23  is formed in a cross shape. The terminals of the cross-shape anchor member  23  are fixed to the rim of the back chamber  26 . The back plate  24  is fixedly arranged on one side of the back chamber  26  of the base  21 , has a plurality of air holes  25 , and has a holding space reserved for the anchor member  23 . The diaphragm  22  is arranged above the back plate  24  and parallel to the back plate  24 , whereby is formed a parallel capacitor plate. Refer to  FIG. 4 . When positive and negative voltages are respectively applied to the diaphragm  22  and the back plate  24 , the diaphragm  22  and the back plate  24  are oppositely charged to form a parallel capacitor plate capacitor. When sound waves act on the diaphragm  22 , the free end of the diaphragm  22  vibrates and deforms. Thus is changed the capacitance between the diaphragm  22  and the back plate  24 . Via computation and analysis of an external circuit, the acoustic signals are converted into electric signals to be output. Meanwhile, the air, which is disturbed by the vibration of the diaphragm  22 , is discharged from the air holes  25  of the back plate  24  to the back chamber  26 . 
     Refer to  FIG. 4  again. In one embodiment, the MEMS capacitive microphone  20  further comprises at least one insulation element  28  arranged between the diaphragm  22  and the back plate  24 . The insulation elements  28  may be arranged on one side of the diaphragm  22  facing the back plate  24  or arranged on one side of the back plate  24  facing the diaphragm  22 . In  FIG. 4 , two insulation elements  28  are respectively arranged on two sides of the back plate  24 . When too much acoustic pressure causes too great deformation of the diaphragm  22 , the insulation elements  28  can provide cushion effect and function as electric separation of the diaphragm  22  from the back plate  24  lest the electric contact of the diaphragm  22  and the back plate  24  damage the microphone. 
     The design that the supporting portion  27  supports the geometric center of the diaphragm  22  also can be applied to a rigid diaphragm. Refer to  FIG. 5A  and  FIG. 5B  for respectively a perspective view and a sectional view schematically showing an MEMS capacitive microphone according to a second embodiment of the present invention. The MEMS capacitive microphone  30  of the present invention comprises a base  31 , a rigid diaphragm  32 , an elastic element  33  and a back plate  34 . The back plate  34  is arranged on the base  31 . The back plate  34  has a plurality of air holes  35  penetrating the back plate  34 . The base  31  has a back chamber  36  corresponding to the back plate  34 , and the air holes  35  interconnect with the back chamber  36 . The rigid diaphragm  32  is fixed on the elastic element  33  and parallel to the back plate  34 . The back plate  34  forms a static end relative to the rigid diaphragm  32 . The rigid diaphragm  32  may be moved by the elasticity of the elastic element  33  and forms a movable end relative to the back plate  34 . Thus, when a sound wave acts on the rigid diaphragm  32 , the rigid diaphragm  32  is moved relative to the back plate  34 , but always parallel to a normal of the back plate  34 , i.e. the z axis. According to the abovementioned equation of a parallel capacitor plate, the capacitance variation between the rigid diaphragm  32  and the back plate  34  can be rewritten to ΔC=εA/(d−Δx), wherein d is the original gap between the back plate  34  and the rigid diaphragm  32  before acted by acoustic pressure, and Δx is the displacement of the rigid diaphragm  32  acted by acoustic pressure. Comparing with a conventional flexible diaphragm that the gap between the back plate  34  and each point of the flexible diaphragm has different displacement, the capacitance variation only correlates with Δx in the present invention. Therefore, the present invention can provide a greater capacitance variation output and enhance the sensitivity of a microphone. 
     Refer to  FIG. 5B . In one embodiment, the base  31  may be a silicon substrate with a circular back chamber  36  formed thereon. The elastic element  33  is formed in a cross shape, and the four ends thereof are fixed to the rim of the back chamber  36  of the base  31 . The rigid diaphragm  32  is formed in a circular shape and fixedly anchored to the intersection of the cross-shape elastic element  33  by a supporting element  37 . Thus, the rigid diaphragm  32  is parallel to the plane of the elastic element  33 . The supporting element  37  has one end relative to the elastic element  33  is fixed to the center of the circular rigid diaphragm  32 . The supporting element  37  can maintain physical balance of the rigid diaphragm  32  and facilitate release of the thermal stress generated in a thermal fabrication process. 
     The back plate  34  is fixedly installed on one side of the back chamber  36  of the base  31  and has a plurality of air holes  35  formed thereon, but a holding space of the back plate  34  is reserved for receiving the elastic element  33 . The rigid diaphragm  32  is arranged above the back plate  34  and parallel to the back plate  34 , whereby is formed a parallel capacitor plate. Refer to  FIG. 6 . In operation, the positive and negative voltages are respectively applied to the rigid diagram  32  and the back plate  34 , whereby the rigid diagram  32  and the back plate  34  respectively carry positive charges and negative charges and form a parallel-plate capacitance. When a sound wave acts on one surface of the rigid diaphragm  32 , the acoustic pressure is transmitted to the elastic element  33  to be deformed. Thus, the rigid diaphragm  32  is moved toward the back plate  34  along the Z axis, and the capacitance between the rigid diaphragm  32  and the back plate  34  is changed. By means of analysis and computation of the capacitance variation by an external circuit, sound signals are converted into electric signals to be output. 
     Refer to  FIG. 6  again. In one embodiment, the MEMS capacitive microphone  30  of the present invention further comprises at least one insulation element  38  arranged between the rigid diaphragm  32  and the back plate  34 . The insulation element  38  may be arranged on one surface of the rigid diaphragm  32  facing the back plate  34  or arranged on one surface of the back plate  34  facing the rigid diaphragm  32 . In  FIG. 6 , two insulation elements  38  are respectively arranged on two ends of the back plate  34 . When too much acoustic pressure causes too great displacement of the rigid diaphragm  32  toward the back plate  34 , the insulation elements  38  can provide cushion effect and function as electric separation of the rigid diaphragm  32  from the back plate  34  lest electric contact of the rigid diaphragm  32  and the back plate  34  damage the microphone. 
     In one embodiment, the rigid diaphragm  32  includes a plurality of reinforcing members (not shown in the drawings), such as reinforcing ribs arranged on one side of the rigid diaphragm  32  to enhance the structural strength of the rigid diaphragm  32  and maintain the rigidity of the rigid diaphragm  32 . 
     In one embodiment, the back plate  34  includes a plurality of reinforcing members  39 , such as reinforcing ribs arranged on one side of the back plate  34  back on the rigid diaphragm  32  to enhance the structural strength of the back plate  34  and maintain the rigidity of the back plate  34 . 
     For convenient illustration, the parts having different functions are separately defined hereinbefore. However, it should be noted that the abovementioned parts can be fabricated independently and then assembled together, or fabricated directly with an MEMS or semiconductor process, such as the etching, photolithographing, and refilling technologies. For example, an MEMS capacitive microphone can be fabricated with a MOSBE platform, which was disclosed in “The Molded Surface-micromachining and Bulk Etching Release (MOSBE) Fabrication Platform on (111) Si for MOEMS” (Journal of Micromechanics and Microengineering, vol. 15, pp. 260-265) in 2005. Thus, it is not repeated herein. 
     Refer to  FIGS. 7A-7I  for sectional views schematically showing the process of fabricating the MEMS capacitive microphone  30  according to one embodiment of the present invention, wherein the sectional views are taken along Line K-K′ in  FIG. 5A , and electric wiring processes of different elements are omitted if the omission does not affect the implementation and understanding of the present invention. Firstly, prepare a substrate for fabricating the base  31 , such as a silicon substrate  40 , as shown in  FIG. 7A . Next, define the installation position of the back plate  34  on the silicon substrate  40 , and fabricate trenches  41  for forming the reinforcing members  39  on the silicon substrate  40  via an etching method, as shown in  FIG. 7B . Next, deposit a poly-silicon layer  42  on the silicon substrate  40  to refill the trenches  41  and form the reinforcing members  39  of the back plate  34 , as shown in  FIG. 7C . Next, define the positions of the elastic element  33  and the air holes and define the area of the back plate  34  via etching the poly-silicon layer  42 , as shown in  FIG. 7D . The reinforcing members  39  can maintain the flatness and rigidity of the back plate  34 . The elasticity of the elastic element  33  can be adjusted via varying the thickness of the poly-silicon layer or selecting the material thereof. 
     Next, form the insulation elements  38  on the back plate  34 , as shown in  FIG. 7E . In one embodiment, the insulation elements  38  are made of silicon nitride (Si 3 N 4 ). Next, form an intermediary layer  43  above the back plate  34 , and define the position for forming the supporting element  37  on the intermediary layer  43 , wherein the position for forming the supporting element  37  is above the elastic element  33 , as shown in  FIG. 7F . In one embodiment, the intermediary layer  43  is made of silicon dioxide (SiO 2 ). Next, deposit a poly-silicon layer  44  on the intermediary layer  43  for forming the rigid diaphragm  32  and the supporting element  37 , as shown in  FIG. 7G . Next, etch the bottom of the silicon substrate  40  to form the back chamber  36 , as shown in  FIG. 7H . Then, remove the intermediary layer  43  via etching such that the rigid diaphragm  32  is supported by the supporting element  37  on the elastic element  33  and parallel to the back plate  34 , as shown in  FIG. 7I . 
     Refer to  FIG. 8  for a diagram showing the result of a test under different frequencies of an MEMS capacitive microphone according to one embodiment of the present invention, wherein the MEMS capacitive microphone  30  is electrically connected with a capacitance readout IC (MS3110) and placed in a semi-anechoic chamber to receive signals from a loudspeaker. When the sound level is below 94 dB, the MEMS capacitive microphone  30  can sense a frequency of a sound of 10-20000 Hz. The MEMS capacitive microphone  30  has a sensitivity of about 12.63 mV/Pa or −37.97 dB/Pa. The MEMS capacitive microphone  30  has advantages of high sensitivity, compactness and low cost. Further, the rigid diaphragm  32  of the MEMS capacitive microphone  30  is less likely to have residual stress and thus has higher sensitivity in comparison with the conventional flexible diaphragm. 
     It should be explained that “rigid” of the rigid diaphragm  32  is not purely defined by the hardness thereof but related to capacitive sensing principle thereof. As described above, the rigid diaphragm  32  means that the diaphragm is incorporated with the elastic element  33  to change the capacitance between the rigid diaphragm  32  and the back plate  34  due to the elasticity or deformation of the elastic element  33  but not the deformation of the diaphragm itself. Further, the realizations of the elastic element  33  are not limited to those in abovementioned embodiments. 
     In the MEMS capacitive microphone of the present invention, the diaphragm is supported in the geometric center thereof to make the residual stress of the diaphragm released outward from the center. Thereby is overcome the problem of deformation or fracture of the diaphragm caused by the residual stress generated by high temperature processes. 
     The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the technical contents of the specification or drawings is to be also included within the scope of the present invention.