Abstract:
A MEMS microphone. The MEMS microphone includes a back plate, a membrane, a support structure, a substrate, and an overtravel stop. The membrane is coupled to the back plate. The support structure includes a support structure opening and a first side of the support structure is coupled to a second side of the back plate. The substrate includes a substrate opening and a first side of the substrate is coupled to a second side of the support structure. The overtravel stop limits a movement of the membrane away from the back plate and includes at least one of an overtravel stop structure coupled to the substrate, an overtravel stop structure formed as part of a carrier chip, and an overtravel stop structure formed as part of the support structure in the support structure opening.

Description:
BACKGROUND 
     The present invention relates to a method improving the mechanical stability of a microelectromechanical (“MEMS”) microphone by limiting the movement of a membrane away from a back plate using a type of overtravel stop (“OTS”) which is not coupled to the membrane. 
     Capacitive MEMS microphones are mechanically extremely sensitive devices. They need to operate in a very high dynamic range of 60-80 db ( 1/1000- 1/10000). To create a membrane which is sensitive enough to detect the lowest pressures (˜1 mPa), it must be very compliant to pressure changes. At the same time, the membrane must withstand pressures in the range of several 10s of Pascals without being destroyed. This is typically achieved by clamping the membrane between OTSs in both directions. While an OTS towards the back plate (i.e., when the membrane is moving towards the back plate) is relatively easy to realize, the opposite direction (i.e., OTS towards the support structure, when the membrane is moving towards the support structure) either requires another dedicated layer or (typically) uses the support structure as the OTS. 
       FIG. 1  illustrates a typical capacitive MEMS microphone  100 . The MEMS microphone  100  includes a back plate  105 , a membrane  110 , and a support structure  115 . The membrane  110  is coupled to the back plate  105  at point  120  (the membrane  110  is insulated from the back plate  105  as they are at different electrical potentials). Sound waves passing through the back plate  105  cause the membrane  110  to vibrate up (in the direction of arrow  125 ) and down (in the direction of arrow  130 ). To prevent the membrane  110  from traveling too far toward the back plate  105 , shorting the membrane  110  to the back plate  105 , OTSs  135  are provided at both ends of the membrane  110 . Each OTS  135  is sometimes referred to as “an OTS toward the back plate.” In addition, the support structure  115  itself provides a second OTS (“an OTS toward the support structure”). 
     During microphone operation, a high bias voltage (e.g., 1 to 40 V) is typically applied between the membrane  110  and the back plate  105 . To avoid a short and potential destruction of the electronics, or the MEMS structure itself, series resistors or insulating layers on top of the OTS bumps are required. The use of series resistors requires careful design of the electronics, and the use of insulating layers increases the complexity/cost of the device significantly and may even be impossible due to process constraints. In addition, an insulating layer on top of the bumps is not an ideal solution as long as the membrane and the OTS bump are on different electrical potentials. In this case, electrostatic forces can decrease the pull-in voltage and/or provide sufficient force to keep the membrane  110  stuck to the back plate  105  after contact due to overload. Additional circuitry may be required to detect this and switch off the bias voltage to allow the membrane  110  to release from the back plate  105 . 
     Creating the OTS towards the support structure is especially difficult. Due to processing tolerances during the backside processing, which typically incorporates a high rate trench, accommodations must be made to compensate for possible misalignment.  FIG. 2  shows how the trench can vary from the frontside  200  to the backside  205 . To accommodate for the typical misalignment  210  between the frontside  200  and the backside  205 , the membrane  110  and the support structure  115  have a large, e.g., several microns, overlap. Additionally, the variation of the backside trench leads to a large variation at the deep end of the trench, and adds to the overall tolerances (several tens of microns). The accuracy of the backside trench can be improved at the cost of processing time. Longer processing increases the device&#39;s cost. 
     Overlapping of the membrane  110  and the support structure  115  results in a significant and varying parasitic capacitance which directly influences the final sensitivity of the sensor element. Accordingly, it is important to keep the overlap of the membrane  110  and the support structure  115  to a minimum. 
     SUMMARY 
     In one embodiment, the invention provides a MEMS microphone. The MEMS microphone includes a back plate that has a first side and a second side, a membrane that has a first side and a second side, a support structure that has a first side and a second side, a substrate that has a first side and a second side, and an overtravel stop. The membrane is coupled to the back plate. The support structure includes a support structure opening and the first side of the support structure is coupled to the second side of the back plate. The substrate includes a substrate opening and the first side of the substrate is coupled to the second side of the support structure. The overtravel stop limits a movement of the membrane away from the back plate and includes at least one of an overtravel stop structure coupled to the substrate, an overtravel stop structure formed as part of a carrier chip, and an overtravel stop structure formed as part of the support structure in the support structure opening. The overtravel stop structure has a first side and a second side. 
     In another embodiment the invention provides a method of providing mechanical stability to a MEMS microphone. The MEMS microphone includes a back plate that has a first side and a second side, a membrane that has a first side and a second side, a support structure that has a first side and a second side, a substrate that has a first side and a second side, and an overtravel stop. The method includes coupling the membrane to the back plate, coupling the first side of a support structure to the second side of the back plate, coupling the support structure to the substrate, and coupling the overtravel stop to the MEMS microphone. The support structure includes a support structure opening. The substrate includes a substrate opening. The overtravel stop limits the movement of the membrane away from the back plate and includes at least one of an overtravel stop structure coupled to the substrate, an overtravel stop structure formed as part of a carrier chip, and an overtravel stop structure formed as part of the support structure in the support structure opening. The overtravel stop structure has a first side and a second side. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-section side view of a prior-art MEMS microphone. 
         FIG. 2  illustrates a cross-section side view of a prior-art MEMS microphone showing variations of a backside trench forming an OTS. 
         FIG. 3A  illustrates a cross-section side view of a MEMS microphone, according to a first embodiment of the invention. 
         FIG. 3B  illustrates a cross-section bottom view of an OTS structure and a support structure, according to a first embodiment of the invention. 
         FIG. 4A  illustrates a cross-section side view of a MEMS microphone, according to a second embodiment of the invention. 
         FIG. 4B  illustrates a cross-section top view of a support structure, a base structure, and an OTS structure, according to a second embodiment of the invention. 
         FIG. 4C  illustrates a cross-section side view of a MEMS wafer and an OTS carrier wafer, according to a third embodiment of the invention. 
         FIG. 5A  illustrates a cross-section side view of a MEMS microphone, according to a forth embodiment of the invention. 
         FIG. 5B  illustrates a cross-section bottom view of a support structure, according to a forth embodiment of the invention. 
         FIG. 5C  illustrates a cross-section side view of a MEMS wafer and an OTS carrier wafer, according to a fifth embodiment of the invention. 
         FIG. 6A  illustrates a cross-section side view of a MEMS microphone, according to a sixth embodiment of the invention. 
         FIG. 6B  illustrates a cross-section bottom view of a support structure that includes an OTS structure, according to a sixth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 3A  illustrates a cross-section side view of a MEMS microphone  300 , according to a first embodiment of the invention. The MEMS microphone  300  includes a MEMS chip  302 , a substrate  305  having a first side  307 , and an OTS structure  310  having a first side  312  and a second side  313 . The MEMS chip  302  includes a back plate  315  having a first side  317  and a second side  318 , a membrane  320  having a first side  322  and a second side  323 , and a support structure  325  having a first side  327  and a second side  328 . The first side of the membrane  322  is coupled to the second side of the back plate  318 . The first side of the support structure  327  is also coupled to the second side of the back plate  318 . The first side of the substrate  307  is coupled to the second side of the support structure  328 . The substrate  305  includes a substrate opening  330 . The second side of the OTS structure  313  is coupled to the substrate  305  and is located in the substrate opening  330 . The first side of the OTS structure  312  is positioned a predetermined distance away from the second side of the membrane  323  and prevents the membrane  320  from traveling beyond the predetermined distance in a direction that is away from the back plate  315 . 
       FIG. 3B  illustrates a cross-section bottom view of the OTS structure  310  and the support structure  325 , according to the first embodiment of the invention. For illustrative purposes, the substrate  305 , the back plate  315 , and the membrane  320  are not included in  FIG. 3B . In this embodiment, an outer shape of the support structure  335  is a square and an inner shape of the support structure  340  is a hollow cylinder. In other embodiments, the outer shape of the support structure  335  and the inner shape of the support structure  340  may be different shapes. In this embodiment, the OTS structure  310  is a hollow pillar. In other embodiments, the OTS structure  310  may be different shapes. 
       FIG. 4A  illustrates a cross-section side view of a MEMS microphone  400 , according to a second embodiment of the invention. The MEMS microphone  400  includes a MEMS chip  402 , a substrate  405  having a first side  407 , and an OTS carrier chip  410 . The MEMS chip  402  includes a back plate  415  having a first side  417  and a second side  418 , a membrane  420  having a first side  422  and a second side  423 , and a support structure  425  having a first side  427  and a second side  428 . The substrate  405  includes a substrate opening  430 . The OTS carrier chip  410  includes a base structure  435  having a first side  437  and a second side  438 , and an OTS structure  440  having a first side  442  and a second side  443 . The first side of the membrane  422  is coupled to the second side of the back plate  418 . The first side of the support structure  427  is also coupled to the second side of the back plate  418 . The first side of the base structure  437  is coupled to the second side of the support structure  428  and is positioned so that the OTS structure  440  is located inside the hollow area of the support structure  425 . The first side of the substrate  407  is coupled to the second side of the base structure  438 . The first side of the OTS structure  442  is positioned a predetermined distance away from the second side of the membrane  423  and prevents the membrane  420  from traveling beyond the predetermined distance in a direction that is away from the back plate  415 . The OTS structure  440  includes an acoustic opening  445 . The acoustic opening  445  allow sound waves to travel between the first side  442  and the second side  443  of the OTS structure  440  and impact the second side of the membrane  423 . 
       FIG. 4B  illustrates a cross-section top view of the support structure  425 , the base structure  435 , and the OTS structure  440 , according to the second embodiment of the invention. For illustrative purposes, the substrate  405 , the back plate  415 , and the membrane  420  are not included in  FIG. 4B . In this embodiment, the outer shape of the support structure  450  is a square and the inner shape of the support structure  455  is a hollow cylinder. In other embodiments, the outer shape of the support structure  450  and inner shape of the support structure  455  may be different shapes. In this embodiment, the OTS structure  440  is a hollow pillar. In other embodiments, the OTS structure  440  may be different shapes. 
       FIG. 4C  illustrates a cross-section side view of a MEMS wafer  460  and an OTS carrier wafer  465 , according to a third embodiment of the invention. The MEMS wafer  460  includes a plurality of MEMS chips  402 . The OTS carrier wafer  465  includes a plurality of OTS carrier chips  410 . The MEMS wafer  460  and the OTS carrier wafer  465  are bonded together to form a plurality of MEMS microphones  400 . This bonding is a MEMS process. 
       FIG. 5A  illustrates a cross-section side view of a MEMS microphone  500 , according to a fourth embodiment of the invention. The MEMS microphone  500  includes a MEMS chip  502 , a substrate  505  having a first side  507 , and an OTS carrier chip  510 . The MEMS chip  502  includes a back plate  515  having a first side  517  and a second side  518 , a membrane  520  having a first side  522  and a second side  523 , and a support structure  525  having a first side  527  and a second side  528 . The substrate  505  includes a substrate opening  530 . The OTS carrier chip  510  includes a base structure  535  having a first side  537  and a second side  538  and an OTS structure  540  having a first side  542  and a second side  543 . The first side of the back plate  517  is coupled to the second side of the membrane  523 . The first side of the support structure  527  is coupled to the second side of the back plate  518 . The first side of the substrate  507  is coupled to the second side of the support structure  528 . The first side of the back plate  513  is coupled to the second side of the base structure  538 . The second side of the OTS structure  543  is positioned a predetermined distance away from the first side of the membrane  522  and prevents the membrane  520  from traveling beyond the predetermined distance in a direction that is away from the back plate  515 . The OTS structure  540  includes a plurality of acoustic openings  545 . The plurality of acoustic openings  545  allow sound waves to travel between the first side  542  and the second side  543  of the OTS structure  540  and impact the first side of the membrane  522 . 
       FIG. 5B  illustrates a cross-section bottom view of the support structure  525 , according to the fourth embodiment of the invention. For illustrative purposes, the substrate  505 , the OTS carrier chip  510 , the back plate  515 , and the membrane  520  are not included in  FIG. 5B . In this embodiment, an outer shape of the support structure  550  is a square and an inner shape of the support structure  555  is a hollow cylinder. In other embodiments, the outer shape of the support structure  550  and the inner shape of the support structure  555  may be different shapes. 
       FIG. 5C  illustrates a cross-section side view of a MEMS wafer  560  and an OTS carrier wafer  565 , according to a fifth embodiment of the invention. The MEMS wafer  560  includes a plurality of MEMS chips  502 . The OTS carrier wafer  565  includes a plurality of OTS carrier chips  510 . The MEMS wafer  560  and the OTS carrier wafer  565  are bonded together to form a plurality of MEMS microphones  500 . This bonding is a MEMS process. 
       FIG. 6A  illustrates a cross-section side view of a MEMS microphone  600 , according to a sixth embodiment of the invention. The MEMS microphone  600  includes a MEMS chip  602 , a substrate  605  having a first side  607 , and an OTS structure  610  having a first side  612 . The MEMS chip  602  includes a back plate  615  having a first side  617  and a second side  618 , a membrane  620  having a first side  622  and a second side  623 , and a support structure  625  having a first side  627  and a second side  628 . The first side of the membrane  622  is coupled to the second side of the back plate  618 . The first side of the support structure  627  is also coupled to the second side of the back plate  618 . The first side of the substrate  607  is coupled to the second side of the support structure  628 . The substrate  605  includes a substrate opening  630 . The OTS structure  610  is part of the support structure  625 . The OTS structure  610  is generated by using a pattern during the formation of a backside trench of the support structure  625 . The first side of the OTS structure  612  is positioned a predetermined distance away from the second side of the membrane  623  and prevents the membrane  620  from traveling beyond the predetermined distance in a direction that is away from the back plate  615 . 
       FIG. 6B  illustrates a cross-section bottom view of the support structure  625  that includes the OTS structure  610 , according to the sixth embodiment of the invention. For illustrative purposes, the substrate  605 , the back plate  615 , and the membrane  620  are not included in  FIG. 6B . In this embodiment, an outer shape of the support structure  635  is a square and an inner shape of the support structure  640  is a hollow cylinder. In other embodiments, the outer shape of the support structure  635  and the inner shape of the support structure  640  may be different shapes. The inner shape of the support structure  640  includes the OTS structure  610 . The OTS structure  610  bisects the inner shape of the support structure  640 . The pattern of the OTS structure  610  illustrated in  FIG. 6B  is an example of one possible pattern, used in this embodiment of the invention. It is to be understood that different patterns may be used in other embodiments of the invention. 
     The specific layouts, component, and manufacturing techniques described above are exemplary and are capable of different implementations. As used above, the term “OTS carrier” may refer to or include silicon, stamped metal, and liquid injected molded plastic. 
     Thus, the invention provides among other things, a MEMS microphone and a method of providing mechanical stability to the MEMS microphone with an OTS structure. Various features and advantages of the invention are set forth in the following claims.