Abstract:
A microelectromechanical system (MEMS) microphone assembly includes a base and a cover. The cover is coupled to the base and together with the base defines a cavity. The base forms a recess and the recess has dimensions and a shape so as to hold a MEMS die. The MEMS die includes a diaphragm and back plate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent claims benefit under 35 U.S.C. §119 (e) to U.S. Provisional Application No. 61805993 entitled “MEMS Apparatus with Increased Back Volume” filed Mar. 28, 2013, the content of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     This application relates to MicroElectroMechanical System (MEMS) devices and, more specifically, to MEMS microphones with improved performance characteristics. 
     BACKGROUND OF THE INVENTION 
     MicroElectroMechanical System (MEMS) devices include microphones and speakers to mention two examples. In the case of a MEMS microphone, sound energy enters through a sound port and vibrates a diaphragm and this action creates a corresponding change in electrical potential (voltage) between the diaphragm and a back plate disposed near the diaphragm. This voltage represents the sound energy that has been received. Typically, the voltage is then transmitted to an integrated circuit (e.g., such as an application specific integrated circuit (ASIC)). Further processing of the signal may be performed on the integrated circuit. For instance, amplification or filtering functions may be performed at the voltage signal at the integrated circuit. MEMS microphones can be used with various customer electronic devices such as those associated with cellular phones or personal computers to mention two examples. 
     It is typically desired to have the microphone have as great a sensitivity as possible. The greater the sensitivity, the better the performance of the microphone. As is well known, the back volume affects the sensitivity and, generally speaking, the greater the back volume, the greater the sensitivity of the microphone. 
     As mentioned, MEMS microphones typically have a MEMS device and an integrated circuit (e.g., an application specific integrated circuit (ASIC)) disposed in an assembly. These components, although needed for operation of the microphone, also take up back volume and therefore to some extent limit the amount of sensitivity that can be provided for a particular MEMS device. 
     Because of these shortcomings, previous approaches have not adequately addressed the above-mentioned problems and user dissatisfaction with these previous approaches has increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
         FIG. 1  comprises a perspective view of a microphone assembly according to various embodiments of the present invention; 
         FIG. 2  is a side cut-away view of a microphone assembly  1  with a ceramic substrate with a recess in the substrate along line A-A of  FIG. 1  according to various embodiments of the present invention; 
         FIG. 3  is a side cut-away view of a microphone assembly with a ceramic substrate with a recess in the substrate along line A-A of  FIG. 1  according to various embodiments of the present invention; 
         FIG. 4  is side cut-away view of a microphone assembly with a ceramic substrate with a recess in the substrate along line A-A of  FIG. 1  according to various embodiments of the present invention; 
         FIG. 5  is a side cut-away view of a microphone assembly with a layered printed circuit board as a substrate along line A-A of  FIG. 1  according to various embodiments of the present invention; 
         FIG. 6  is a side cut-away view of a microphone assembly with a layered printed circuit board as a substrate along line A-A of  FIG. 1  according to various embodiments of the present invention; 
         FIG. 7  is a side cut-away view of a microphone assembly with a layered printed circuit board as a substrate along line A-A of  FIG. 1  according to various embodiments of the present invention; and 
         FIG. 8  is a graph showing the beneficial effects with regards to sensitivity by using the approaches described herein according to various embodiments of the present invention. 
     
    
    
     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 
     MicroElectroMechanical System (MEMS) devices (e.g., MEMS microphones) are provided that have increased back volumes, and therefore improved performances as compared to previous MEMS devices. In one aspect, a recess is created in a substrate of a bottom port MEMS microphone. A MEMS device is disposed in the substrate. Disposing the MEMS device in the recess in the substrate increases the back volume of the MEMS microphone. Increasing the back volume improves the performance characteristics of the MEMS microphone. For example, an increased sensitivity is achieved. By “sensitivity” and as used herein, it is meant the transduction efficiency of the MEMS microphone. For a capacitive sensing MEMS microphone, this would be the electric potential per unit pressure. It will be understood that in the examples described herein a MEMS device is disposed in the recess. Alternatively, a MEMS device plus an integrated circuit (e.g., an application specific integrated circuit (ASIC)) can be disposed in the recess. Further, only the integrated circuit (e.g., an ASIC) may be disposed in the recess. Still further, any component on the substrate (e.g., a printed circuit board (PCB)) can be disposed in the recess together with the MEMS device or by itself to increase the back volume. 
     Referring now to  FIG. 1 ,  FIG. 2 ,  FIG. 3 , and  FIG. 4  an example MEMS device (e.g., a MEMS microphone) having a ceramic substrate is described. Each of these examples includes a substrate  102 , a recess  104  disposed in the substrate  102 , a MEMS device  106  in the recess  104 , an integrated circuit  108 , and wire bonds  111  (between the MEMS device  106  and the integrated circuit  108 ). A lid or cover  110  covers the MEMS device  106  and the integrated circuit  108 , and attaches to the substrate  102 . A port  109  extends through the substrate  102 . 
     The MEMS device  106  includes a diagram  112  and a back plate  114 . A back volume  116  is formed between the lid  110  and the substrate  102 . A front volume  115  is formed in the port  109 . Pads (not shown) on the bottom of the substrate may communicate with the integrated circuit  108  using conductive vias (not shown). The pads may also couple to customer electronic devices such as those used in personal computers or cellular phones. Other examples of customer electronic devices are possible. 
     In each of the examples of  FIG. 1 ,  FIG. 2 , and  FIG. 3 , the recess  104  is of a different depth. In  FIG. 2 , the recess  104  is shallow, for example, 25 microns in depth. In  FIG. 3 , the recess is deeper (e.g., 250 microns in depth) and the top of the MEMS device  106  is approximately flush with a surface  120  of the substrate  102 . 
     The recesses described herein generally are box shaped having a flat bottom surface. However, it will be appreciated that the recesses may assume other shapes and configurations as well. 
     In the example of  FIG. 4 , the surface  120  is lower than the surface  122 , but not low as the surface  124  creating a stepped substrate  102 . The exact dimensions and depths of the recess  104  are selected according to factors such as how easily the bonds  110  can be attached. Other factors may also be considered and other configurations for the substrate  102  can also be used. 
     In one example of the operation of the MEMS microphone assembly  100 , sound energy enters through the sound port  109  and vibrates the diaphragm  112  and this action creates a corresponding change in electrical potential (voltage) between the diaphragm  112  and the back plate  114  disposed near the diaphragm  112 . This voltage represents the sound energy that has been received. Typically, the voltage is then transmitted to the integrated circuit  108 . Further processing of the signal may be performed at the integrated circuit  108 . For instance, amplification or filtering functions may be performed at the voltage signal at the integrated circuit  108 . 
     Advantageously, the approaches described increase the back volume since, in part, the MEM device  104  is not taking up back volume space. This results increased sensitivity for the microphone. 
     It will be understood that in the examples described herein a MEMS device is disposed in the recess. Alternatively, a MEMS device plus an integrated circuit (e.g., an application specific integrated circuit (ASIC)) can be disposed in the recess. Further, only the integrated circuit (e.g., an ASIC) may be disposed in the recess. Still further, any component on the substrate (e.g., a printed circuit board (PCB)) can be disposed in the recess together with the MEMS device or by itself to increase the back volume. 
     Referring now to  FIG. 1  (which is common to all examples described herein),  FIG. 5 ,  FIG. 6 , and  FIG. 7 , examples of MEMS microphones with layered substrates are described. Each of these examples, includes a substrate  502 , a recess  504  dispersed in the substrate  502 , a MEMS device  506  disposed in the recess  504 , an integrated circuit  508 , wire bonds  511  (between the MEMS device  506  and the integrated circuit  508 ). A lid  510  covers the MEMS device  506  and the integrated circuit  508  and attaches to the substrate  502 . 
     In  FIG. 5 , the recess  504  is shallow (e.g., 25 microns in depth). In  FIG. 6 , the recess  504  is deeper (e.g., 250 microns in depth) and the top of the MEMS device  506  is approximately flush with a surface  520  of the substrate  502 . A port  509  extends through the substrate  502 . 
     In the example of  FIG. 7 , the surface  520  is lower than the surface  522 , but not low as the surface  524  creating a stepped substrate  502 . The exact dimensions and depths of the recess are selected according to factors such as how easily the bonds  510  can be attached. Other factors can also be considered. Other configurations for the substrate  502  can also be used. 
     Advantageously, the approaches described increase the back volume since, in part, the MEM device  504  is not taking up back volume space. This results increase sensitivity for the microphone assembly  500 . 
     The substrate  502  include a first metal layer  540 , a second metal layer  542 , a third metal layer  544 , a first insulative layer  546 , and a second insulative layer  548 . Conduits  550  couple the metal layers together. Pads  552  couple the MEMS device  506  and integrated circuit  508  to the metal layers. Pads on the exterior of the device couple external customer electronic devices to the assembly  500 . It will be appreciated that the exact dimensions, shape, and composition of each layer described above can vary. 
     In one example of the operation of the MEMS microphone assembly  500 , sound energy enters through the sound port  509  and vibrates the diaphragm  512  and this action creates a corresponding change in electrical potential (voltage) between the diaphragm  512  and the back plate  514  disposed near the diaphragm  512 . This voltage represents the sound energy that has been received. Typically, the voltage is then transmitted to the integrated circuit  508 . Further processing of the signal may be performed at the integrated circuit  508 . For instance, amplification or filtering functions may be performed at the voltage signal at the integrated circuit  508 . 
     It will be understood that in the examples described herein a MEMS device is disposed in the recess. Alternatively, a MEMS device plus an integrated circuit (e.g., an application specific integrated circuit (ASIC)) can be disposed in the recess. Further, only the integrated circuit (e.g., an ASIC) may be disposed in the recess. Still further, any component on the substrate (e.g., a printed circuit board (PCB)) can be disposed in the recess together with the MEMS device or by itself to increase the back volume. 
     Referring now to  FIG. 8 , one example of the beneficial effects of the present approaches is described. The x-axis represents the back volume of a MEMS microphone while the y-axis represents sensitivity of the MEMS microphone. It can be seen that as the back volume increases, the sensitivity of the microphone also increases, yielding a device with improved performance characteristics. For example, at point  802  in the curve, the back volume is relatively small (e.g., 1.5 mm 3 ). As the back volume increases, for instance to a point  804 , the sensitivity also increases to a larger volume. In one example, the back volume changes by approximately 2 or 2.5 mm 3  and the subsequent change in sensitivity is from approximately 0.5 to 2 dB. 
     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.