Abstract:
A MEMS microphone. The MEMS microphone includes a membrane, a spring, and a first layer having a backplate, and a first OTS structure. The spring has a first end coupled to the membrane, and a second end mounted to a support. The first OTS structure is released from the backplate and coupled to a structure other than the backplate, and is configured to stop movement of the membrane in a first direction after the membrane has moved a predetermined distance.

Description:
RELATED APPLICATION 
     The present application claims the benefit of previous filed U.S. Provisional Patent application No. 61/506,832, filed on Jul. 12, 2011, the entire content of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present invention relates to a type of vertical overtravel stop for a MEMS microphone which does not incorporate the substrate and requires no dedicated insulation layer or special electrical measures to avoid electric shorts during an overtravel event. 
     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 overtravel stops (OTSs) in both directions. While an OTS towards the backplate (i.e., when the membrane is moving towards the backplate) is relatively easy to realize, the opposite direction (i.e., OTS towards the substrate, when the membrane is moving towards the substrate) either requires another dedicated layer or (typically) uses the substrate as the OTS. 
       FIG. 1  illustrates a typical capacitive MEMS microphone  100 . The microphone  100  includes a backplate  105 , a membrane  110 , and a substrate  115 . The membrane  110  is coupled to the backplate  105  at point  120  (the membrane  110  is insulated from the backplate  105  as they are at different electrical potentials). Sound waves passing through the backplate  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 backplate  105 , shorting the membrane  110  to the backplate  105 , overtravel stops (OTSs)  135  are provided at both ends of the membrane  110 . Each OTS  135  is sometimes referred to as “an OTS toward the backplate.” In addition, the substrate  115  itself provides a second OTS (“an OTS toward the substrate”). 
     During microphone operation, a high bias voltage (e.g., 1 to 40 V) is typically applied between the membrane  110  and the backplate  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 backplate  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 backplate  105 . 
     Creating the OTS towards the substrate 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. 2A  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 substrate  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. 
       FIG. 2B  shows another solution to this technical problem. A two-step backside trench  215  is used. This results in sufficient accuracy, but doubles the cost of this processing step. 
     Overlapping of the membrane  110  and the substrate  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 substrate  115  to a minimum. 
     SUMMARY 
     In one embodiment, the invention provides a MEMS microphone. The MEMS microphone includes a membrane, a spring, and a first layer having a backplate, and a first OTS structure. The spring has a first end coupled to the membrane, and a second end mounted to a support. The first OTS structure is released from the backplate and coupled to a structure other than the backplate, and is configured to stop movement of the membrane in a first direction after the membrane has moved a predetermined distance. 
     In another embodiment the invention provides a method of limiting the movement of a membrane. The method includes coupling the membrane to a spring, coupling the spring to a rigid structure, releasing a first OTS structure from a backplate, and coupling the first OTS structure to a structure other than the backplate. The first OTS structure prevents the membrane from moving more than a first distance in a first direction. 
     In another embodiment the invention provides a MEMS device. The MEMS device includes a moveable structure, a plurality of springs, and a first layer having a rigid structure, a first OTS structure, and a second OTS structure. Each spring has a first end coupled to the moveable structure, and a second end mounted to a support. The first OTS structure is released from the rigid structure and coupled to the moveable structure. The first OTS structure is configured to stop movement of the moveable structure away from the rigid structure after the moveable structure has moved a predetermined distance. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cut-away view of a prior-art MEMS microphone. 
       FIGS.  2 Aa and  2 B are cut-away views of a prior-art MEMS microphone showing variations of a backside trench forming an overtravel stop. 
         FIGS. 3A and 3B  are top views of a prior-art suspended membrane. 
         FIG. 4  is a top view of a suspended membrane incorporating an embodiment of the invention. 
         FIG. 5  is a view of an embodiment of OTS structures in relation to a membrane and spring. 
         FIGS. 6A and 6B  are cutaway side views of the OTS structures, membrane, and spring of  FIG. 5 . 
         FIG. 7  is an enlarged view of the OTS structures of  FIGS. 5 ,  6 A, and  6 B. 
     
    
    
     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. 
     The invention allows the elimination of insulation on the OTS bumps. This reduces processing and costs of producing the MEMS microphone. The invention also addresses the OTS towards the substrate issues, removing the need for excess overlap and/or the use of a two-step backside trench. The result is a microphone that is more sensitive (due to reduced or eliminated parasitic capacitance from overlapping of the membrane and substrate) and less expensive (due to reduced processing tolerances and costs). 
       FIG. 3A  shows a prior-art MEMS spring suspended membrane  300 . The membrane  300  is supported by a plurality of springs  305 . The springs  305  are mounted to supports. In some constructions, the supports are part of the backplate layer. In other constructions, the supports are part of the substrate.  FIG. 3B  shows the overlap of the substrate. Circles  325  and  330  represent variations in a backside trench due to processing tolerances. In order to ensure that the backside trench provides an OTS, the backside trench overlaps at least the springs  305  (circle  325 ) and, depending on processing, the membrane  300  (circle  330 ). 
       FIG. 4  shows a portion of a MEMS microphone. The microphone includes a spring suspended membrane  400  (i.e., a moveable structure) incorporating the invention. Similar to the prior-art membrane  300  shown in  FIGS. 3A and 3B , the membrane  400  is supported by a plurality of springs  405 . The springs  405  have a first end  408  mounted to a rigid structure via an insulation pad  410 . In the construction shown the springs  405  are mounted to a backplate  515  (i.e., a rigid structure). In other constructions, the springs  405  can be mounted to a substrate (i.e., a rigid structure). The springs  405  also have a second end  413  connected to the membrane  400 . A plurality of OTS structures  415  (which are part of a backplate layer) provide OTS toward the substrate and toward the backplate. In the construction shown, there are four springs  405  each mounted to the backplate via the insulation pads  410 . Circles  420  and  425  represent variations in a backside trench in the substrate due to processing tolerances. Because the OTS structures  415  provide the OTS toward the substrate, the backside trench is outside the springs  405  and membrane  400 . Therefore, there is little or no parasitic capacitance between the substrate and the membrane  400 , and tolerances can be looser when the backside trench does not function as the OTS. 
       FIGS. 5 ,  6 A,  6 B, and  7  are more detailed views of a portion of the MEMS microphone. As shown in the figures, the OTS structures  415  include an OTS toward the substrate structure  500  and an OTS toward the backplate structure  505 . Both structures  500  and  505  are part of the backplate layer  510 , and are released from the backplate  515  (released refers to a process that disconnects the structures  500  and  505  from the backplate  515 ).  FIG. 6A  is a side view showing the OTS toward the substrate structure  500 , and  FIG. 6B  is a side view showing the OTS toward the backplate structure  505 . In other constructions, only the OTS toward the substrate structure  500  or the OTS toward the backplate structure  505  are used. 
     In the construction shown, the backplate  515  is adhered to the insulation pad  410  which is also adhered to the spring  405 . The OTS structures  500  and  505  each include a mounting pad  550  and an OTS bump  555 . The mounting pad  550  and the OTS bump  555  are formed during processing of the backplate layer. As shown in  FIG. 7 , the mounting pad  550  has a first height  560  and the OTS bump  555  has a second height  565 . The second height  565  is less than the first height  560 , the difference in heights defining the distance the membrane  400  can move (e.g., a predetermined distance). The predetermined distance can be the same or different for the first and second OTS structures  500  and  505 . Thus, the predetermined distance can be a first distance for the first OTS structure  500  and a second distance for the second OTS structure  505 . In other constructions, an OTS bump is not provided. In such a construction, the OTS structure itself stops further movement of the membrane once the membrane has traveled a predetermined distance. 
     Referring back to  FIGS. 6A and 6B , in the OTS toward the backplate structure  505 , the mounting pad  550  is adhered to the spring  405 , and the OTS bump  555  is positioned above the membrane  400 . As the membrane  400  moves toward the backplate  515 , the membrane  400  contacts the OTS bump  555  before the membrane  400  can contact the backplate  515 . This prevents the membrane  400  from coming into contact with the backplate  515  and shorting out. Because the OTS toward the backplate structure  505  is released from the backplate  515 , and is mounted to the spring  405 , which is at the same electrical potential as the membrane  400 , the OTS bump  555  does not need to be insulated, and there are no electrical consequences when the membrane  400  comes into contact with the OTS bump  555 . In other constructions, the OTS structures are mounted to other structures (e.g., the substrate), rather than the springs or membrane. 
     In the OTS toward the substrate structure  500 , the mounting pad  550  is adhered to the membrane  400 , and the OTS bump  555  is positioned above the spring  405 . As the membrane  400  moves away from the backplate  515 , the membrane  400  pulls the OTS toward the substrate structure  500  down with it. When the membrane  400  has traveled a maximum desired distance, the OTS bump  555  comes into contact with the spring  405  stopping further movement of the membrane  400  away from the backplate  515 . This prevents the membrane  400  from moving too far. Again, because the OTS toward the substrate structure  500  is released from the backplate  515 , and is mounted to the membrane  400 , which is at the same electrical potential as the spring  405 , the OTS bump  555  does not need to be insulated, and there are no electrical effects when the OTS bump  555  comes into contact with the spring  405 . 
     The construction shown uses layers that already exist in a MEMS microphone: a membrane layer, a backplate layer, a via layer (for electrical or mechanical) contacts, and a layer forming the OTS bumps. The OTSs in both directions are fully symmetrical, and use the same basic layout. It is not required that both sides of the OTS structure are on an electrically same node. However, putting both sides of the OTS structure on the electrically same node results in:
         No electrostatic forces at the OTS which otherwise could keep the membrane stuck to the backplate (electrostatic stiction). If the overtravel generating force disappears, the membrane will immediately release and go back to operating mode.   No insulation layers required to have a safe design.   Touching of the OTS will not overload the electronics because neither the capacitance nor resistance or leaks changes during touch.       

     In addition, the invention applies to MEMS designs which attach a released/insulated part of a stationary layer (e.g., the backplate  515  in the above example) to a movable structure (e.g., the membrane  400  in the above example) to realize any functionally relevant structure. The OTS towards the backplate  515  also acts as a gap defining spacer or post. Thus, when a microphone is operated under conditions which pull the membrane  400 , by a high electrostatic force, the posts prevent the membrane  400  from moving too far during regular operation. 
     Various features and advantages of the invention are set forth in the following claims.