Abstract:
A micro electro mechanical system (MEMS) microphone includes a base; a MEMS die disposed on the base; and a cover coupled to the base and enclosing the MEMS die. The MEMS die includes and diaphragm and back plate and posts extend from a first periphery of the back plate. The diaphragm is free to move within a boundary created by the posts. A front volume is formed on a first side of the diaphragm and a back volume is formed on a second side of the diaphragm between the diaphragm and the cover. A plurality of openings extend through the diaphragm about an outer periphery of the diaphragm, the openings being effective to mitigate noise.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This patent claims benefit under 35 U.S.C. §119 (e) to U.S. Provisional Application No. 62/032,829 entitled “Electrostatic Microphone with reduced acoustic noise” filed Aug. 4, 2014, the content of which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to microphones and, more specifically to diaphragms in these microphones. 
       BACKGROUND OF THE INVENTION 
       [0003]    Various types of microphones and receivers have been used through the years. In these devices, different electrical components are housed together within a housing or assembly. Other types of acoustic devices may include other types of components. These devices may be used in hearing instruments such as hearing aids, personal audio headsets, or in other electronic devices such as cellular phones and computers. 
         [0004]    One type of microphone is a micro electro mechanical system (MEMS) microphone. The MEMS microphone uses a MEMS die that supports a diaphragm and a back plate. When the diaphragm deforms/moves due to changing sound pressure, the electrical potential between the microphone and the back plate changes to produce an electrical signal that is representative of the incident sound pressure. The diaphragm typically divides the microphone into a front volume and a back volume. 
         [0005]    Some microphones use free plate diaphragm. A free plate diaphragm is typically disposed between the back plate and the substrate. The free plate diaphragm is not constrained at its boundary and consequently is free to move. As the free plate diaphragm deforms/moves in the presence of sound pressure, air flow leakage occurs between the front volume and the back volume. The portion of the diaphragm overlapping the substrate, is typically very close to the substrate. Up and down motion of the overlapping region of the diaphragm results in squeeze film damping. As a consequence of damping, unwanted and undesirable noise is produced. 
         [0006]    This damping is a limiting factor to achievable microphone signal-to-noise ratio. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
           [0008]      FIG. 1  comprises a diagram of a MEMS microphone according to various embodiments of the present invention; 
           [0009]      FIG. 2  comprises a side cutaway view of portions of a MEMS microphone according to various embodiments of the present invention; 
           [0010]      FIG. 3  comprises a top view of the MEMS microphone of  FIG. 2  according to various embodiments of the present invention; 
           [0011]      FIG. 4  comprises a view of a portion of the MEMS microphone of  FIG. 2  and  FIG. 3  according to various embodiments of the present invention; 
           [0012]      FIG. 5  comprises a side cutaway view of portions of a MEMS microphone according to various embodiments of the present invention; 
           [0013]      FIG. 6  comprises a top view of the MEMS microphone of  FIG. 2  according to various embodiments of the present invention; 
           [0014]      FIG. 7  comprises a view of a portion of the MEMS microphone of  FIG. 2  and  FIG. 3  according to various embodiments of the present invention; 
           [0015]      FIG. 8  comprises a graph show aspects of the operation of the microphones described herein according to various embodiments of the present invention; 
           [0016]      FIG. 9  comprises a perspective drawing of a portion of a microphone apparatus according to various embodiments of the present invention; 
           [0017]      FIG. 10  comprises a perspective cut-away drawing of a portion of a microphone apparatus taken along line A-A in  FIG. 9  according to various embodiments of the present invention. 
       
    
    
       [0018]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. 
       DETAILED DESCRIPTION 
       [0019]    While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail a preferred embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiment illustrated. 
         [0020]    In the approaches described herein, a MEMS microphone having a free plate diaphragm and with improved operating performance is provided. In one aspect, holes or openings may be provided around an outer periphery of the diaphragm in order to mitigate noise. In another aspect, a flow-constrainer (or other resistive element) is provided or disposed around the outer periphery of the diaphragm (or around portions of the outer periphery of the diaphragm) in order to reduce air flow into the back volume of the microphone. In other examples, both holes and a flow-constrainer are provided. In yet another example, a combination of vent holes and a flow-constrainer (or other resistive element) may be implemented. The approaches provided herein are easy and cost effective to implement and result in better microphone performance and user satisfaction with the microphone. 
         [0021]    It will be appreciated that the examples presented in this disclosure have been exemplified using MEMS microphones. However, the approaches described herein are general and widely applicable to various microphone architectures and are in no way limited to MEMS microphones. 
         [0022]    Referring now to  FIG. 1 , one example of a MEMS microphone  100  is described. The microphone  100  includes a MEMS device  102  (including a MEMS die or substrate  104 , a back plate  106 , and a diaphragm  108 ), a base  109 , a lid or cover  110 , an integrated circuit (IC)  111  (that performs various processing functions on the received signal), and a port  112 . In the example shown in  FIG. 1 , the microphone  100  is a bottom port device. That is, the port  112  extends through the base  109  (rather than the lid  110 ). Alternatively, the microphone may be a top port device where the port  112  extends through the cover  110 . In another aspect, the microphone  100  may be a MEMS-on-lid microphone where the port  112  extends through the lid and the MEMS device  102  is disposed on the lid  110 . 
         [0023]    The diaphragm  108  is a free plate diaphragm that is not secured about its outer periphery. In one aspect, holes or openings may be provided around an outer periphery of the diaphragm  108  in order to mitigate noise. In another aspect, a flow-constrainer (or other resistive element) is provided around the outer periphery of the diaphragm  108  (or around portions of the outer periphery of the diaphragm  108 ) in order to reduce air flow into the back volume of the microphone  100 . In other examples, both holes and a flow-constrainer are provided. 
         [0024]    Referring now to  FIG. 2 ,  FIG. 3 , and  FIG. 4 , one example of a MEMS microphone  200  is described. The microphone  200  includes a MEMS die  202  a back plate  204  and a diaphragm  206 . Posts  208  extended from the back plate  204 . A back volume  205  and front volume  207  exists. 
         [0025]    In one aspect, capacitive detection may be used to detect diaphragm motion/deformation. In this embodiment, a bias voltage is typically applied between the diaphragm  206  and the back plate  204 . The capacitance between the back plate and the diaphragm varies about quiescent value when sound energy is received by the microphone  200 . Consequently, sound energy is converted into an electrical signal and the electrical signal represents the sound energy that is received. In another example, an electret is used to establish a bias between the back plate and the diaphragm. Besides capacitive detection, transduction may be achieved by other mechanisms as well. An incomplete list of transduction mechanisms include piezoresistive, piezoelectric, magnetostrictive, and optical mechanisms for detecting the movement/deformation of the active component of the microphone. Other examples are possible. 
         [0026]    The diaphragm  206  is free to move within the boundaries of the post and the space where it is disposed. Other structures may also be used to restrain the diaphragm  206 , but the diaphragm  206  is not restrained about the entirety of its outer periphery. In one aspect, the free-plate diaphragm  206  is restrained in a small region of the diaphragm periphery. In this region and in one example, an approximately 10 microns wide “runner” connects the diaphragm to the MEMS substrate. Without any restriction, the diaphragm position may be somewhat unpredictable and hard to control. 
         [0027]    Holes or openings  210  extend through the diaphragm  206  in order to mitigate noise. In one example, the holes or openings  210  are approximately 5 microns wide. Other dimensions are possible. 
         [0028]    As shown, air flows in the direction indicated by the arrows labeled  212 . The holes  210  around the periphery of the diaphragm  206  reduce, for example, squeeze film damping or any aerodynamic damping between the diaphragm  206  and the MEMS die  202  thereby mitigating noise and improving system performance. 
         [0029]    Referring now to  FIG. 5 ,  FIG. 6 , and  FIG. 7 , another example of a MEMS microphone  500  is described. The microphone  500  includes a MEMS die  502 , a back plate  504 , and a diaphragm  506 . Posts  508  extend from the back plate  504 . 
         [0030]    The diaphragm  506  and the back plate  504  operate to create an electrical potential. As the diaphragm  506  moves in the present of sound, an electrical potential is created and changes between the back plate  504  and the diaphragm  506 . Consequently, sound energy is converted into an electrical signal and the electrical signal represents the sound energy. 
         [0031]    The diaphragm  506  is free to move within the boundaries of posts  508  and the space where it is disposed. Other restraining structures may also be used to restrain movement of the diaphragm  506 . A back volume  505  and a front volume  507  exist and are separated by the diaphragm  506 . 
         [0032]    Holes or openings  510  extend through the diaphragm  506  and operate to mitigate noise. In one example, the holes or openings  210  are approximately 5 microns wide. Other dimensions are possible. As shown, air flows in the direction indicated by the arrows labeled  512 . The holes  510  around the periphery of the diaphragm  506  reduce squeeze film damping between the diaphragm  506  and the MEMS die  502  thereby reducing noise. 
         [0033]    A flow-constrainer  514  is disposed about the periphery of the diaphragm  506 . The flow-constrainer  506  may be an integrally formed part of the back plate  504 , an integrally formed part of the diaphragm  506 , or a separate element that is connected to either the diaphragm  506  or the back plate  504  or the MEMS substrate  515 . The flow-constrainer  514  limits air leakage into the back volume of the microphone  500 . The flow-constrainer  514  may be a full (complete) ring or may comprise multiple segments. 
         [0034]    The holes  510  and the flow-constrainer  514  are two structures whose use, dimensions, and structure advantageously allow a designer to control damping noise and leakage into the back volume. By controlling both the holes  510  (e.g., size and number) and the dimensions of the flow-constrainer  514 , optimum system performance can be achieved. 
         [0035]    Referring now to  FIG. 8 , one example of a graph showing some of the advantages of the present approaches is descried. As shown, the graph represents values of frequency on the x-axis and represents values for the response of the microphone on the y-axis. 
         [0036]    A first curve  802  represents the response for a microphone that does not use periphery holes or flow-constrainer. The response has a low frequency response value  803  (LR 01 ). 
         [0037]    A second curve  804  represents the response for a microphone that uses periphery holes in the diaphragm as has been described herein. This response has a higher value for the low frequency response  805  (LR 02 ). 
         [0038]    A third curve  806  represents the response for a microphone that uses only a flow-constrainer. This response has a lower value for the low frequency response  807  (LR 03 ) as compared to the low frequency response value  803 . 
         [0039]    A fourth curve  808  represents the response for a microphone that uses both periphery holes and a flow-constrainer. This response has a low frequency response  809  (LR 04 ). The low frequency response  809  (LR 04 ) lies between the low frequency response (LR 01 )  803  (where no periphery holes or flow-constrainer are used) and the low frequency response (LR 02 )  805  (where only periphery holes are used). 
         [0040]    Advantageously, it can be appreciated that the low frequency response regions (and frequency values) are fine tuned. Thus, a designer can design a microphone that achieves optimum performance. 
         [0041]    It will be appreciated that the present approaches are described with respect to a bottom port microphone (that is, a microphone with a port extending through the base). However, the present approaches are widely applicable to various port configurations. A partial list includes top port devices (e.g., microphones where the sound port extends through the lid or cover); MEMS on lid devices (where the MEM die is secured to the lid or cover and the post extends through the lid or cover); side-port devices etc. (where the port is located on the lid wall adjacent to the base). 
         [0042]    Referring now to  FIG. 9  and  FIG. 10 , a microphone  900  includes cover  928 , MEMS device  902 , ASIC  922 , substrate  920 , port  924 . The MEMS device  902  includes two MEMS motors  904  and  910  each with a diaphragm  906  and back plate  908 . The diaphragm  906  is attached to a pillar  912 . 
         [0043]    In other examples, the diaphragm  906  may not be physically attached to the pillar  912 . In the example shown in  FIGS. 9 and 10 , there are posts  914  near the periphery of the motor. When electrical bias is applied between the diaphragm  906  and a back plate electrode  909 , the diaphragm  906  engages with the posts  914 . In other examples, there may be no posts at the motor periphery. 
         [0044]    Similar to the examples of  FIG. 2  and  FIG. 5 , holes or openings  918  are incorporated in the diaphragm of the example of  FIG. 9  and  FIG. 10  to act as damping countermeasure. These openings  918  couple the front volume and the back volume of the microphone. 
         [0045]    Similar to the example of  FIG. 5 , flow-constrainer (or other resistive element) may be incorporated in the embodiment described in the example of  FIG. 9  and  FIG. 10 . 
         [0046]    It will be appreciated that the examples herein, the whole diaphragm or portions of the diaphragm may be rigid, and the MEMS output may be generated by rigid body motion. 
         [0047]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention.