Abstract:
An acoustic apparatus includes a back plate, a diaphragm, and at least one pillar. The diaphragm and the back plate are disposed in spaced relation to each other. At least one pillar is configured to at least temporarily connect the back plate and the diaphragm across the distance. The diaphragm stiffness is increased as compared to a diaphragm stiffness in absence of the pillar. The at least one pillar provides a clamped boundary condition when the diaphragm is electrically biased and the clamped boundary is provided at locations where the diaphragm is supported by the at least one pillar.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/873,816, filed Oct. 2, 2015, now U.S. Pat. No. 9,743,191, which claims the benefit of and priority to U.S. Provisional Application No. 62/063,183, filed Oct. 13, 2014, both which are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to acoustic devices and, more specifically, to MEMS microphones. 
       BACKGROUND 
       [0003]    Different types of acoustic devices have been used through the years. One type of device is a microphone. In a microelectromechanical system (MEMS) microphone, a MEMS die includes a diaphragm and a back plate. The MEMS die is supported by a base and enclosed by a housing (e.g., a cup or cover with walls). A port may extend through the base (for a bottom port device) or through the top of the housing (for a top port device) or through the side of the housing (for a side port device). In any case, sound energy traverses through the port, deforms the diaphragm and creates a changing electrical capacitance between the diaphragm and the back-plate, which creates an electrical signal. Microphones are deployed in various types of devices such as personal computers, cellular phones and tablets. 
         [0004]    One type of a MEMS microphone utilizes a free plate diaphragm. The biased free plate diaphragm typically sits on support posts located along the periphery of the diaphragm. The support posts restrain the movement of the diaphragm. Free plate diaphragms tend to have a high mechanical compliance. Consequently, designs that utilize free plate diaphragms may suffer from high total harmonic distortion (THD) levels, particularly when operating at high sound pressure levels (SPLs). 
         [0005]    All of these problems have resulted in some user dissatisfaction with previous approaches. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
           [0007]      FIG. 1  comprises a perspective cut-away drawing of a portion of a microphone apparatus according to various embodiments; 
           [0008]      FIG. 2  comprises a perspective cut-away drawing of a portion of a microphone apparatus taken along line A-A in  FIG. 1  according to various embodiments; 
           [0009]      FIG. 3  comprises a top view of the microphone apparatus of  FIGS. 1 and 2  according to various embodiments; 
           [0010]      FIG. 4  comprises a side cutaway view of the center part of the apparatus of  FIG. 3  along line B-B according to various embodiments; 
           [0011]      FIGS. 5A-B  comprises a graph showing some of the aspects of the operation of the microphone of  FIG. 1-4  according to various embodiments. 
           [0012]      FIG. 6  comprises a top view of the microphone apparatus of  FIGS. 1 and 2  demonstrating an embodiment with non-circular diaphragm and multiple pillars according to various embodiments; 
           [0013]      FIG. 7  comprises a perspective cut-away drawing of a portion of another example of a microphone apparatus taken along line A-A in  FIG. 1  according to various embodiments. 
       
    
    
       [0014]    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 
       [0015]    In the present approaches, a microelectromechanical system (MEMS) apparatus with a center clamped diaphragm is provided. Such devices provide greater linearity and lower THD compared to previous free plate approaches. More specifically and in some aspects, a central pillar connects the diaphragm center of one or more diaphragms to the back plate center. The central pillar advantageously approximates a clamped boundary condition at the diaphragm center thereby increasing diaphragm stiffness. In some embodiments, the central pillar also provides an electrical connection to the diaphragm thereby eliminating the need for a separate diaphragm runner that is used (and typically required) in previous approaches. In some embodiments, the pillar may be located at an offset with respect to the diaphragm center. 
         [0016]    In other aspects and when the diaphragm is biased, the diaphragm is tensioned as it is pulled against the posts by the electrostatic field established by the bias. Additionally, certain regions of the diaphragm assume a doubly-curved shape upon bias. One or both of the tensioning and the doubly-curved shape result in increased stiffness of the diaphragm and improved linearity of operation such that the relationship between the input signal of the microphone and the output signal of the microphone has very low nonlinearity. 
         [0017]    Referring now to  FIG. 1-4 , a microphone apparatus  100  is described. A MEMS device  102  includes a first motor  104  (including a first diaphragm  106  and a first back plate  108 ) and a second motor  110  (including a second diaphragm and a second back plate both not shown). It will be appreciated that the detailed description herein relates only to the first motor, but that this description applies equally to the second motor. 
         [0018]    Referring now especially to  FIG. 1 , the MEMS device  102  is disposed on a base  120 . Also disposed on the base  120  and coupled to the MEMS device  102  is an application specific integrated circuit (ASIC)  122 . Port  124  extends through the base  120  and allows sound energy to be received by the motors in the MEMS device  102 . A cover  128  is disposed on top of the base  120 . It will be appreciated that this is a bottom port device, but it will be understood that ports could alternatively extend through the cover  128  and the device would become a top port device or a side port device depending on port location. 
         [0019]    In operation, sound energy is received by the two motors  104  and  110  in the MEMS device  102  via ports  124 . The motors  104  and  110  in the MEMS device  120  convert the sound energy into electrical signals. The electrical signals are then processed by the ASIC  122 . The processing may include, for example, attenuation or amplification to mention two examples. Other examples are possible. The processed signals are then transmitted to pads (not shown) on the base  120 , which couple to customer devices. For example, the apparatus  100  may be incorporated into a cellular phone, personal computer, or tablet and the customer devices may be devices or circuits associated with the cellular phone, personal computer, tablet, or other device. 
         [0020]    Turning now to a description of the central pillar arrangement, it will be appreciated that this discussion is with respect to the first motor  104 . However, it will be appreciated that the structure of the arrangement of the second motor  110  may be identical to the description of the first motor  104 . 
         [0021]    Referring now especially to  FIG. 2 ,  FIG. 3  and  FIG. 4 , the first motor  104  includes a central pillar  112  that connects the back plate  108  to the diaphragm  106 . Typically, the back plate  108  consists of an electrically conductive back plate electrode  109 , and one or more structural materials. The diaphragm  106  and the back plate electrode  109  form an electrical capacitor. Posts  114  constrain the movement of the diaphragm  106  at a periphery of the diaphragm  106 . In one example, the posts  114  are constructed of silicon nitride and approximately 6 posts are utilized. This number is significantly less than previous approaches that utilize a free-plate diaphragm.  FIG. 3  shows a top-view layout schematic of a MEMS die with two motors. The diaphragms  302  are attached to the pillar  301 . Each motor has six posts  303 . The star-like shape  304  represents the back-plate electrode. The back-plate electrodes  304  and the diaphragms  302  form the working capacitance of the MEMS. The star-shaped electrode  304  maximizes the working capacitance of the MEMS and provides improved signal-to-noise ratio compared to circular or donut shaped electrodes. Other construction materials and numbers of posts and pillars may also be used. Some embodiments may have one or more pillars and no posts. Some examples may have one or more pillars and one or more posts. In some embodiments, the back-plate electrode may not be star-shaped. A side-view cross-section along the line BB in  FIG. 3  is shown in  FIG. 4 . Referring now to  FIG. 4 , the central pillar  112  is described in detail. The central pillar  112  includes a silicon nitride layer  440  and polysilicon layer  446 . Polysilicon layer  448  forms the diaphragm  106 . In this embodiment, the polysilicon and silicon nitride deposition steps that form the pillar also form the back-plate. Consequently, the central pillar is, in this example, formed integrally with the back plate  108  and is physically connected to the diaphragm  106 . However, it will be understood that in other embodiments the central pillar can be formed only with the diaphragm material, only with the back plate material, or that all three elements are formed separately. Together, these elements form a central pillar having a hollow area  456 . It will be appreciated that this is one example of the configuration of a central pillar and that other examples are possible. In this example, the pillar is axisymmetric about the central axis  449 . In other embodiments, the pillar need not be axisymmetric. In certain embodiments, the pillar may be solid or it may have a cage-like structure formed with multiple segments. In this example, a sharp angle  450  exists at the pillar-diaphragm interface. In other embodiments, the pillar-diaphragm junction and/or the pillar-back plate junction may be chamfered and/or filleted. Chamfering and/or filleting are expected to make the structure robust, so that it can better withstand airburst events. 
         [0022]    So configured, the central pillar  112  advantageously approximates a clamped boundary condition at the center of the diaphragm  106  thereby increasing diaphragm stiffness. The central pillar  112  also provides an electrical connection to the diaphragm  106  thereby eliminating the need for a separate diaphragm runner that was used in previous approaches to implement electrical connection to the diaphragm. However, in other embodiments, the pillar may be used for providing clamped boundary condition only, and electrical connection to the diaphragm may be implemented by other approaches. 
         [0023]    In yet another example, the unbiased diaphragm may not be physically attached to the pillar as shown in  FIG. 7 ; a bias applied between the diaphragm and the back-plate may be used to pull the diaphragm against the pillar, thereby approximating a clamped boundary condition in the diaphragm-pillar contact region. 
         [0024]    When an electrical bias is applied between the diaphragm and the back plate electrode, the diaphragm is tensioned due to an electrostatic force. Additionally, certain regions of the diaphragm assume a doubly-curved shape upon bias. One or both of the tensioning and the doubly curved shape result in increased stiffness of the diaphragm and improved linearity of operation such that a nearly linear relationship exists between the input signal of the microphone and the output signal of the microphone. 
         [0025]    Referring now to  FIGS. 5A-B , various graphs showing some of the aspects of the operation of the microphone, is described. The graph  5 A shows a diaphragm  502  when unbiased (no electrical bias applied between the diaphragm  106  and the back plate electrode  109 ). It can be seen that the diaphragm  502  is domed shaped. The graph in  FIG. 5B  shows deflection of the diaphragm  502 , around peripheral posts when biased. The contact point between the diaphragm  502  and the posts are labeled  504 . The diaphragm  502  is clamped by the center pillar  506 .  FIG. 5B  depicts the diaphragm shape when an electrical bias is applied between the diaphragm  106  and the back plate electrode  109 . As mentioned, a stiffer diaphragm is provided by the approaches provided herein. When an electrical bias is applied between the diaphragm  106  and the back plate electrode  109 , the diaphragm is tensioned and doubled curved. In  FIG. 5B , the double curves are indicated by the arrows labeled  508  and  510 . Instead of a single maximum deflection point, the present approaches provide a maximum deflection region around a donut-like region  512  (that is present between the center pillar and the peripheral posts and is shaped by the curves indicated by arrows  508  and  510 ). This resultant configuration compensates for all or much of the sensitivity lost due to increased stiffness of the diaphragm. 
         [0026]    As has also been mentioned, the central clamp can also be used as an electrical connection to the diaphragm and this helps with improved miniaturization. 
         [0027]    The pillar may not be located at the center of the diaphragm. Moreover, there may be multiple pillars within a single motor.  FIG. 6  comprises a top view of the microphone apparatus of  FIGS. 1 and 2  demonstrating an example of an apparatus with a non-circular diaphragm  602  and multiple pillars  601 . In this example, there are ten posts  603 , three pillars  601 , and the non-circular diaphragm  602  maximizes MEMS die area utilization, thereby improving signal-to-noise ratio per unit die area. 
         [0028]    Embodiments that utilize a capacitive transduction mechanism have been described, however transduction modes such as piezoresistive, piezoelectric, and electromagnetic transduction are also possible. Other modes of transduction are also possible. 
         [0029]    Referring now to  FIG. 7 , another example of a motor structure is described. The example of  FIG. 7  is similar to the example of  FIG. 2  and like-numbered elements in  FIG. 2  correspond to like numbered elements in  FIG. 7 . In the example of  FIG. 7 , the first motor  704  includes a central pillar  712  that connects the back plate  708  to the diaphragm  706 . However, in contrast to  FIG. 2  in the example of  FIG. 7  the central pillar  712  is formed separately and is not permanently connected to diaphragm  706 . The back plate  708  consists of an electrically conductive back plate electrode  709 , and one or more structural materials. The diaphragm  706  and the back plate electrode  709  form an electrical capacitor. Posts  714  constrain the movement of the diaphragm  706  at a periphery of the diaphragm  706 . In one example, the posts  714  are constructed of silicon nitride and approximately 6 posts are utilized. Other examples are possible. 
         [0030]    It will be appreciated that in some aspects with the central pillar arrangements described herein, the central pillar can be offset from a central axis. In other aspects, multiple pillars can be used as shown in  FIG. 6 . 
         [0031]    Preferred embodiments are described herein, including the best mode known to the inventors. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the appended claims.