Patent Document

[0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/982,382 filed Apr. 22, 2014, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to microphones and in particular to MEMS microphones. 
       BACKGROUND 
       [0003]    A MEMS device (micro-electro-mechanical system) is a miniaturized device which integrates mechanical and electrical functions in a chip or die of semiconductor material, for example of silicon. MEMS devices offer many advantages including miniaturization, and low manufacturing costs. These advantages are realized using micro-manufacturing lithographic techniques. The final assembled device is typically made of the silicon die wherein the MEMS device is integrated and, optionally, of integrated circuits for specific applications mounted on a substrate using conventional assembling processes. 
         [0004]    A cover or cap, fixed to the substrate is typically formed by overmolding the MEMS device and any other devices mounted on the substrate, forming a casing which protects the MEMS device against external physical stresses. 
         [0005]    Many types of sensors can be produced in a MEMS platform. One such sensor is a microphone. Many applications incorporate MEMS microphones including cellular telephones, laptops, etc. The attraction of MEMS microphones is the ability to provide a lightweight and small form factor microphone. Because of the low manufacturing costs and small footprint, MEMS microphones are increasingly replacing electric condenser microphones (ECMs). Additionally, a MEMS microphone has an inherent advantage of low power consumption (160 μA), which is approximately ⅓ of that of the ECM. For mobile phone and laptop applications with limited power storing capacity, the power savings available using MEMS microphones is significant. 
         [0006]    One limitation of known MEMS microphones, however, is their susceptibility to damage from moisture. For example, in order to function properly, the sensor portion of the MEMS microphone must be configured to react to sound waves. By providing access for sound waves, however, an access is also provided for moisture. 
         [0007]    What is needed, therefore, is MEMS microphone which provides protection to the sensor area against moisture. A need also exists for a MEMS microphone which provides a defined front volume that is compatible with epoxy overmold processes such as are common to standard semiconductor packages 
       SUMMARY 
       [0008]    In one embodiment a micro-electro-mechanical system (MEMS) microphone package includes a multiple layer substrate, an upper acoustic port formed through a plurality of upper layers of the multiple layer substrate and exposing an upper surface of a membrane portion, a lower acoustic port formed through a plurality of lower layers of the multiple layer substrate and exposing a lower surface of the membrane portion, a ring trench formed through at least one of the plurality of upper layers and exposing a metal ring, a MEMS die located above the ring trench, a copper pillar ring extending between the metal ring on the substrate and the MEMS die, and a solder pillar ring positioned on a first surface of the copper pillar ring, the copper pillar ring and solder pillar ring attaching the MEMS die to the metal ring located on the substrate. 
         [0009]    In one or more embodiments the membrane portion comprises a water resistant membrane portion. 
         [0010]    In one or more embodiments the first surface is a lower surface of the copper pillar ring. 
         [0011]    In one or more embodiments a MEMS microphone package includes a back cavity defined within the MEMS die and enclosed on an upper end portion by a cap positioned on an upper surface of the MEMS die. 
         [0012]    In one or more embodiments the plurality of upper layers includes a first upper metal layer and an upper solder mask on an upper surface of the first upper metal layer, and a plastic overmold is positioned on an upper surface of the upper solder mask encasing the MEMS die. 
         [0013]    In one or more embodiments the plastic overmold extends within the ring trench but does not pass beyond the copper seal ring connecting the MEMs die and substrate. 
         [0014]    In one or more embodiments a MEMS microphone package includes at least one well extending through the upper solder mask, and at least one input/output post extending downwardly from the MEMS die into that well, the input/output post including an upper copper post portion soldered within the corresponding well using a solder post portion positioned on a lower surface of the upper copper post portion. 
         [0015]    In one or more embodiments the plurality of upper layers comprises a middle Pre-preg organic layer, and a lower metal layer, the middle Pre-preg organic layer is located between the upper metal layer and the lower metal layer, a respective contact pad formed in the upper metal layer is located within each the described wells, and the corresponding input/output post is soldered to the respective contact pad located in the well. 
         [0016]    In one or more embodiments a MEMS microphone package includes a metal acoustic seal ring positioned in the lower acoustic port. 
         [0017]    In one or more embodiments a method of forming a MEMS microphone package includes forming an upper acoustic port through a plurality of upper layers of a multiple layer substrate to expose an upper surface of a membrane portion, forming a lower acoustic port through a plurality of lower layers of the multiple layer substrate to expose a lower surface of the membrane portion, forming a ring trench through at least one of the plurality of upper Pre-preg layers to expose a metal ring, positioning a MEMS die above the ring trench, and attaching the MEMS die to the metal ring using a copper pillar ring extending between the metal ring and the MEMS die, and using a solder pillar ring positioned on a first surface of the copper pillar ring. 
         [0018]    In one or more embodiments forming the upper acoustic port includes forming the upper acoustic port through the plurality of upper layers of the multiple layer substrate to expose an upper surface of a water resistant membrane portion. 
         [0019]    In one or more embodiments the method of forming a MEMS microphone package includes inserting the solder pillar ring into the ring trench, and reflowing the solder pillar ring. 
         [0020]    In one or more embodiments the method of forming a MEMS microphone package includes defining a back cavity within the MEMS die, and enclosing an upper end portion of the back cavity with a cap. 
         [0021]    In one or more embodiments the method of forming a MEMS microphone package includes providing the multiple layer substrate with a first upper metal layer and an upper solder mask on an upper surface of the first upper metal layer, and forming a plastic overmold on an upper surface of the upper solder mask thereby encasing the MEMS die. 
         [0022]    In one or more embodiments forming the plastic overmold includes flowing the plastic overmold within the ring trench up to but not beyond the copper seal ring connecting the MEMs device to the substrate. 
         [0023]    In one or more embodiments the method of forming a MEMS microphone package includes forming at least one well through the upper solder mask, and positioning at least one input/output post extending downwardly from the MEMS die into the described well, the described input/output post including an upper copper post portion and a lower solder post portion, and reflowing the lower solder post portion within the described well. 
         [0024]    In one or more embodiments the method of forming a MEMS microphone package includes forming at least one pad within the upper metal layer, wherein forming the at least one well includes exposing the at least one pad. 
         [0025]    In one or more embodiments providing the multiple layer substrate further includes providing a multiple layer substrate with a middle Pre-preg organic layer, and a lower metal layer, the middle Pre-preg organic layer located between the upper metal layer and the lower metal layer. 
         [0026]    In one or more embodiments forming a lower acoustic port includes exposing a metal acoustic seal ring positioned on a lower pre-preg organic layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  depicts a side cross-sectional view of a MEMS microphone assembly incorporating an acoustically transparent water resistant membrane and a copper pillar ring which functions as an overmold dam and defines a microphone front volume; 
           [0028]      FIG. 2  depicts a bottom perspective view of the MEMS die of  FIG. 1  with the cap removed and showing the copper pillar ring; 
           [0029]      FIG. 3  depicts a side plan view of the MEMS die of  FIG. 2 ; 
           [0030]      FIG. 4  depicts a bottom plan view of the MEMS die of  FIG. 2 ; 
           [0031]      FIG. 5  depicts a top perspective view of the MEMS die of  FIG. 2 ; 
           [0032]      FIG. 6  depicts a bottom plan view of the substrate of  FIG. 1 ; 
           [0033]      FIG. 7  depicts a top plan view of the substrate of  FIG. 1 ; and 
           [0034]      FIG. 8  depicts a side cross-sectional view of a substrate which can be used with the MEMS microphone assembly of  FIG. 1 . 
       
    
    
     DESCRIPTION 
       [0035]    For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written description. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the present disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one of ordinary skill in the art to which this disclosure pertains. 
         [0036]      FIG. 1  depicts a MEMS microphone assembly  100 . The MEMS microphone assembly  100  includes a MEMS die  102  supported on a water resistant substrate  104  and encased in a plastic overmold  106 . A cap  108  is positioned over a back-cavity  110  of the MEMS die  102 . The MEMS die  102  further includes a downwardly extending copper pillar ring  112  shown in additional detail in  FIGS. 2-5 . At a lower distal end of the copper pillar ring  112  a solder pillar ring  114  is provided on a lower surface of the copper pillar ring  112 . In some embodiments, the solder pillar ring is positioned at an upper surface of the copper pillar ring. The solder pillar ring  114  is used to attach the MEMS die  102  to the substrate  104  as discussed in further detail below. 
         [0037]    The MEMS die  102  further includes four I/O posts  116 ,  118 ,  120 , and  122 . Each of the I/O posts  116 ,  118 ,  120 , and  122  includes an upper copper post  126 ,  128 ,  130 , and  132  and a lower solder post  136 ,  138 ,  140 , and  142 . The I/O posts  116 ,  118 ,  120 , and  122  provide for electrical connection points between the MEMS die  102  and the substrate  104 . In different embodiments, more or fewer I/O posts are included. 
         [0038]    Returning to  FIG. 1 , the substrate  104  is a multiple layer substrate. The bottom layer of the substrate  104  is a solder mask layer  150 . A metal layer  152  is located on an upper surface of the solder mask layer  150 . A pre-impregnated (Pre-preg) organic layer  154  is located between the metal layer  152  and another metal layer  156 . Each of the lower layers  150 ,  152 ,  154 , and  156  are perforated to form a lower acoustic port  158  which terminates at a water resistant membrane  160 . The opening in the solder mask layer  150  and the metal layer  152  are larger than the openings in the other layers  154  and  156  to allow for a metal acoustic seal ring (pad)  162  positioned on the lower surface of the Pre-preg organic layer  154  as further shown in  FIG. 6 . Also shown in  FIG. 6  are four package I/O pads  164 . Typically, the acoustic seal ring  162  and the I/O pads  164  are formed from the metal layer  152 . 
         [0039]    Returning to  FIG. 1 , above the water resistant membrane  160 , a pair of metal layers  166  and  168  sandwich an upper Pre-preg organic layer  170 . An upper solder mask layer  172  is located on the upper surface of the metal layer  168 . Each of the upper layers  172 ,  166 ,  168  and  170  are perforated to form an upper acoustic port  174  which terminates at the water resistant membrane  160 . The upper acoustic port  174  is aligned with the lower acoustic port  158 . 
         [0040]    With reference to  FIG. 7 , four wells  180 ,  192 ,  184 , and  186  are formed through the upper solder mask layer  172  to expose portions of I/O pads  188 ,  190 ,  192 , and  194  formed in the metal layer  168 . A ring trench  196  is also formed through the upper solder mask layer  172  and the metal layer  168  to expose a ring of the Pre-preg organic layer  170  and form a metal ring  171 . 
         [0041]    Fabrication of the MEMS microphone assembly  100  is substantially accomplished using any desired fabrication processes. For example, electrical connections for the I/O pads  164  and the I/O pads  188 ,  190 ,  192 , and  194  can be provided within the substrate  104  by the drilling of vias that are then filled with conductive epoxy or lined and plated using standard PCB via plating techniques. Likewise, the layers of the substrate  104  are sandwiched using any desired process. In at least one embodiment, however, the substrate  104  includes the water resistant membrane  160 . The water resistant membrane  160  is an acoustically transparent breathable material which restricts the passage of moisture therethrough. In one embodiment, the water resistant membrane  160  is formed from Polytetrafluoroethylene (PTFE). Accordingly, fluid is hindered from moving into the MEMS die  102  through the acoustic ports  158  and  174 . 
         [0042]    One difference from known devices is the formation of the metal ring  171  using the ring trench  196  through the upper solder mask layer  172  and the metal layer  168 . The ring  171  is configured to attach to the copper pillar ring  112  using the solder pillar ring  114  as depicted in  FIG. 1  during a flip-chip assembly process. The copper pillar ring  112  and solder pillar ring  114  are formed, for example, using a modification of a process described in U.S. Pat. No. 6,681,982 which issued on Jan. 27, 2004. The modified &#39;982 patent process is used in one embodiment to form the copper pillar ring  112  with a 80 μm height while the solder pillar ring  114  has a 20 μm height. The copper pillar ring  112  and the solder pillar ring  114  both have a 75 μm diameter. The I/O posts  116 ,  118 ,  120 , and  122  are each formed in like manner to the copper pillar ring  112  and solder pillar ring  114 . 
         [0043]    Accordingly, when the MEMS die  102  is positioned on the substrate  104  during the flip-chip process, the lower solder posts  136 ,  138 ,  140 , and  142  are positioned through the respective wells  180 ,  182 ,  184 , and  186  and into contact with the respective I/O pad  188 ,  190 ,  192 , and  194 . Additionally, the solder pillar ring  114  is positioned through the ring trench  196  and into contact with the ring  171 . The lower solder posts  136 ,  138 ,  140 , and  142  are then connected to the respective I/O pad  188 ,  190 ,  192 , and  194  using a reflow process. The solder pillar ring  114  is also connected to the ring  171  during the reflow process. The copper pillar ring  112  thus forms the microphone front volume in the finished assembly. 
         [0044]    The cap  108  is then positioned on the upper end of the MEMS die  102  (see  FIG. 1 ) and the plastic overmold is overmolded onto the MEMS die  102  and the upper solder mask layer  172 . In some embodiments, the ring trench  196  is sufficiently larger than the copper pillar ring  112  so as to allow some of the plastic overmold  106  to flow into the well to provide increased bonding as depicted in  FIG. 1 . 
         [0045]    The MEMS microphone assembly  100  thus provides a water proof seal between the MEMS die and the substrate, which may further include a water proof membrane integrated into a substrate such that an acoustic port that is water proof/resistant may be implemented at the package level. This is achieved by bonding two substrate cores that have pre-drilled acoustic port holes over a water proof breathable membrane. In some embodiments, the water proof breathable membrane is provided in the form of an insert. 
         [0046]    By way of example,  FIG. 8  depicts a substrate  198  which is used in the MEMS microphone assembly  100  in place of the substrate  104 . The substrate  198  is a multiple layer substrate. The bottom layer of the substrate  198  is a solder mask layer  200 . A metal layer  202  is located on an upper surface of the solder mask layer  200 . A Pre-preg organic layer  204  is located between the metal layer  202  and another metal layer  206 . Each of the layers  200 ,  202 ,  204 , and  206  are perforated to form a lower acoustic port  208  which terminates at a water resistant membrane  210 . The opening in the solder mask layer  200  and the metal layer  202  are larger than the openings in the other layers  204  and  206  to allow for a metal acoustic seal ring (pad)  212  located on the lower surface of the Pre-preg organic layer  204 . 
         [0047]    Above the water resistant membrane  210 , a pair of metal layers  216  and  218  sandwich an upper Pre-preg organic layer  220 . An upper solder mask layer  222  is located on the upper surface of the metal layer  218 . Each of the layers  216 ,  218 ,  220  and  222  are perforated to form an upper acoustic port  224  which terminates at the water resistant membrane  210 . The upper acoustic port  224  is aligned with the lower acoustic port  208 . A ring trench  226  is formed through the upper solder mask layer  222  to expose a metal ring  227 , and wells, vias, and I/O ports (not shown) are also provided. The substrate  198  is thus substantially the same as the substrate  104 , and is assembled to the MEMS die  102  in like manner as the substrate  104 . The main difference is that instead of a membrane layer, the water resistant membrane  210  is formed as an insert within a substrate core  228 . 
         [0048]    Both lower substrates allow for the provision of electrical connection between the top and bottom substrates in any desired manner such as by laser or mechanical drilling of vias that are then filled with conductive epoxy or lined and plated using standard PCB via plating techniques. The MEMS microphone assembly  100  further incorporates a copper pillar ring plated onto the MEMs die forms. This ring forms a barrier for subsequent processes when the die has been bonded to the substrate/chip carrier. Specifically, the ring forms an epoxy barrier during overmold processing of the microphone assembly. 
         [0049]    The MEMS microphone assembly  100  thus provides a new semiconductor package form factor that incorporates wafer proof/resistant characteristics and allows for overmolded package construction. The package design in some embodiments further addresses a need for integrating a water-proof membrane into the porthole of a MEMs microphone package to prevent moisture damage. Additionally, the MEMS microphone assembly  100  provides a method for using a copper pillar seal ring to create a defined front volume that is compatible with epoxy overmold processes that are common to standard semiconductor packages. 
         [0050]    While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.

Technology Category: 5