Patent Document

FIELD OF THE INVENTION 
     The present invention relates to generally to MEMS devices, and more particularly, to a MEMS microphone. 
     BACKGROUND 
     Most commercially available MEMS microphones or silicon microphones are formed by two chips, an application specific integrated circuit (ASIC) chip and a MEMS chip attached to a substrate. These chips are generally enclosed by a conductive cover or lid. An acoustic input can be provided from an opening on a top surface of the microphone or from an opening on the substrate. Typically, in commercial applications where the acoustic input is from the top, an acoustic back cavity is formed mainly by a volume under the MEMS chip and the substrate. By contrast, in commercial applications where the acoustic input is from the bottom, the acoustic cavity is typically formed by the volume enclosed by the substrate and the cover. 
     It is desirable to provide improvements to MEMS microphones which allow them to be more easily manufactured at a lower cost. The improvement to the MEMS microphone must be easily implemented, cost effective and adaptable to existing manufacturing processes. 
     The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     A MEMS device is disclosed. The MEMS device comprises a first plate with a first surface and a second surface; and an anchor attached to a first substrate. The MEMS device further includes a second plate with a third surface and a fourth surface attached to the first plate. A linkage connects the anchor to the first plate, wherein the first plate and second plate are displaced in the presence of an acoustic pressure differential between the first and second surfaces of the first plate. The first plate, second plate, linkage, and anchor are all situated in an enclosure formed by the first substrate and a second substrate, wherein one of the first and second substrates contains a through opening to expose the first surface of the first plate to the environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. One of ordinary skill in the art readily recognizes that the particular embodiments illustrated in the figures are merely exemplary, and are not intended to limit the scope of the present invention. 
         FIGS. 1A and 1B  show different embodiments of the top view of the device layer of a torsional microphone. 
         FIG. 2A  shows the cross section of the torsional microphone with integrated back cavity along  2 A- 2 A in  FIG. 1A . 
         FIG. 2B  shows the cross section of the torsional microphone with integrated back cavity along  2 B- 2 B in  FIG. 1B . 
         FIGS. 3A and 3B  show the operation of the torsional microphone using a symbolic representation for the linkage with torsional compliance 
         FIG. 4  shows an embodiment of the top view of a device layer of a piston microphone. 
         FIG. 5  shows the cross section of the piston microphone with integrated back cavity along  5 - 5  in  FIG. 4 . 
         FIGS. 6A and 6B  show the operation of a piston microphone using a symbolic representation for the linkage with bending compliance. 
         FIG. 7  shows alternative manufacturing options for a torsional microphone. 
         FIG. 8  shows alternative manufacturing options for a piston microphone. 
         FIGS. 9A ,  9 B and  9 C show packaging schemes for the current invention. 
         FIG. 10  shows an example of integration of MEMS microphone with other MEMS device. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to MEMS devices, and more particularly, to a MEMS acoustic sensor such as a microphone. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     In the described embodiments Micro-Electro-Mechanical Systems (MEMS) refers to a class of structures or devices fabricated using semiconductor-like processes and exhibiting mechanical characteristics such as the ability to move or deform. MEMS devices often, but not always, interact with electrical signals. MEMS devices include but are not limited to gyroscopes, accelerometers, magnetometers, pressure sensors, microphones, and radio-frequency components. Silicon wafers containing MEMS structures are referred to as MEMS wafers. 
     In the described embodiments, the MEMS device may refer to a semiconductor device implemented as a micro-electro-mechanical system. The MEMS structure may refer to any feature that may be part of a larger MEMS device. The semiconductor layer with the mechanically active MEMS structure is referred to as the device layer. An engineered silicon-on-insulator (ESOI) wafer may refer to a SOI wafer with cavities beneath the silicon device layer or substrate. A handle wafer typically refers to a thicker substrate used as a carrier for the thinner silicon device substrate in a silicon-on-insulator wafer. A handle substrate and a handle wafer can be interchanged. 
     In the described embodiments, a cavity may refer to an opening or recession in a substrate wafer and an enclosure may refer to a fully enclosed space. A post may be a vertical structure in the cavity of the MEMS device for mechanical support. A standoff is a vertical structure providing electrical contact. 
     In the described embodiments, a back cavity may refer to a partially enclosed cavity equalized to ambient pressure via Pressure Equalization Channels (PEC). In some embodiments, a back cavity is also referred to as a back chamber. A back cavity formed within the CMOS-MEMS device can be referred to as an integrated back cavity. Pressure equalization channels, also referred to as venting or leakage channels/paths, are acoustic channels for low frequency or static pressure equalization of a back cavity to ambient pressure. 
     In the described embodiments, a rigid structure within a MEMS device that moves when subject to force may be referred to as a plate. Although rigid plates are preferred for the described embodiments, semi rigid plates or deformable membranes could replace rigid plates. Plates may comprise of silicon, silicon containing materials (e.g. poly-silicon, silicon oxide, silicon nitride), metals and materials that are used in semiconductor processes (e.g. aluminum nitride, germanium). A back plate may be a solid or perforated plate comprising at least one electrode. The electrode can be comprised of semiconductor process compatible conductive materials (e.g. poly-silicon, silicon, aluminum, copper, nickel, titanium, chromium, gold). The electrodes may have insulating films on one or more surfaces. 
     In the described embodiments, perforations refer to acoustic openings for reducing air damping in moving plates. An acoustic port may be an opening for sensing the acoustic pressure. An acoustic barrier may be a structure that prevents acoustic pressure from reaching certain portions of the device. Linkage is a structure that provides electrical conductivity and compliant attachment to a substrate through an anchor. Extended acoustic gap can be created by step etching of the post and creating a partial post overlap over the PEC. In-plane bump stops limit range of movement in the plane of the plate if the plates move more than desired (e.g. under a mechanical shock). Similarly rotational bump stop are extensions of the plate to limit the displacement normal to the plane due to out-of-plane rotation. 
     In the described embodiments, structures (plates) of MEMS device and electrodes formed on CMOS substrate form sensor capacitors. Sensor capacitors are electrically biased for detection of change of capacitance due to acoustic pressure. 
     To describe the features of the present invention in more detail, refer now to the following description in conjunction with the accompanying figures. 
       FIGS. 1A and 1B  show different embodiments of top views of device layers  100 A and  100 B of torsional microphone.  FIGS. 1A and 1B  illustrates a first plate  140 ,  142  that senses acoustic pressure on its first surface, and a second plate  150  with perforations  160  and a linkage  250 ,  252  attached to an anchor  240 ,  242 . In an embodiment the first plate  140 ,  142  and second plate  150  are rigid. The difference between  FIGS. 1A and 1B  are the locations of linkages  250 ,  252 . A different embodiment may include combination of linkages  250  and  252  resulting in four linkages, adding a central cutout portion to  FIGS. 1A and 1B . The first plate  140 ,  142  is partially surrounded by a pressure equalization channel (PEC)  230 ,  232 , and the device layer  100 A,  100 B is surrounded by a seal  260  to ensure that the only acoustical input to the device will be via an acoustic port  190  (in  FIGS. 2A and 2B . 
     When a force is applied (acoustic pressure variation) on the first surface of first plate  140 ,  142 , the first plate  140 ,  142  is rotationally displaced around an axis passing through linkages  250 ,  252 , hence the second plate  150  is displaced in an opposite direction (rotational displacement around the same axis). The linkages  250 ,  252  form torsional restoring forces acting against movement and will bring the plates to their initial position once externally applied acoustic force is zero. Undesired in plane movements can be limited by introducing in plane bump stops  340  at locations where undesired movement/rotation has a high amplitude, e.g. furthest away from linkages  250 ,  252 . The in plane bump stops  340  can be defined and manufactured on the second plate  150  or the device layer  100 A,  100 B or the first plate  140 ,  142  or any combination of these. 
     In an embodiment, protruding tabs that form rotational bump stops  350  are provided to limit the rotation of the first  140 , 142  and second plates  150 . By proper design the rotational bump stops  350  may eliminate need for reduction or turning off the potential difference between first and second plates  140 ,  142  and  150 , and the electrode  170  shown in  FIGS. 2A and 2B  for recovery from a tip-in or out of range condition. 
       FIGS. 2A and 2B  show the cross section of the torsional microphone  200 A and  200 B with integrated back cavity  130  along  2 A- 2 A and  2 B- 2 B in  FIGS. 1A  and B respectively. In an embodiment, integrated back cavity  130  is formed by a fusion bond  220  between the second substrate  120  and the device layer  100 A and  100 B which is further bonded to the first substrate  110  by conductive alloy (eutectic) bond  200  by processes as described in a commonly owned U.S. Pat. No. 7,442,570, entitled, “Method of Fabrication of a Al/Ge Bonding in a Wafer Packing Environment and a Product Produced Therefrom”, which is incorporated herein by reference. 
     Static pressure in the back cavity  130  is equalized by ambient pressure via air flow through the PEC  230  and  232 . Ideally, PEC  230  and  232 , provide high resistance to air flow in the frequency range of interest (e.g. 100 Hz and above), and low resistance at lower frequencies down to static pressure changes. Linkages  250  are attached to standoffs  180  both mechanically and electrically. The standoffs  180  in an embodiment are lithographically defined protruding members of device layer that are mechanically and electrically connected to top conductive layers of the first substrate  110  via alloy or eutectic bonding. The device layer  100 A and  100 B in an embodiment is lithographically patterned to form the first plate  140 , a second plate  150 , with perforations  160 , PEC  230 , 232  and an acoustic seal  260 , around the active device. 
     The second plate  150  with perforations  160  forms a first electrode and is electrically connected to an integrated circuit (IC) manufactured on the first substrate  110 , while a second electrode  170  is disposed on the first substrate  110 . Second electrode  170  is aligned with the first electrode or second plate  150 . A first surface of second plate  150  and the second electrode  170  form a variable capacitor whose value changes due to pressure being applied on a first surface of first plate  140 . 142 . In an embodiment, additional material such as silicon nitride or silicon oxide is deposited on the second electrode  170 . The additional material can be lithographically patterned to form bump stops  270  to reduce stiction force by reducing the contact area in the undesired event that first and/or second plate  140 , 142  and  150  come into contact with first substrate  110 . 
       FIGS. 3A and 3B  illustrate the conceptual design describing the operation of the torsional microphone of  FIG. 2A  or  2 B with a symbolic anchor  183 , and a symbolic torsional linkage,  253 . 
     Referring now to  FIG. 3A , the acoustic port  193  is a channel in the first substrate  110  that allows acoustic pressure to reach the first surface of the first plate  143 . Under an applied acoustic pressure, the first plate  143  rotates slightly either clockwise or counter-clockwise depending on polarity of acoustic pressure. In  FIG. 3B , the case where the first plate  143  rotates in a clockwise direction around a rotation axis that coincides with linkage like structure  253  is depicted. 
     Rotational movement coupled to the perforated second plate  153  results in a reduced gap between first surface of the second plate  153  and a second electrode  173 , hence the capacitance defined by these two surfaces increases. An IC manufactured on the first substrate  110  is electrically connected to both the second plate  153  and second electrode  173  detects the change in capacitance proportional to the acoustic pressure. 
       FIG. 4  shows a top view of device layer  400  of a piston microphone with rigid first plate  144  that senses acoustic pressure on its first surface, a rigid second plate  154  with perforations  164 , and linkages  254  attached to an anchor  244 . The number of linkages  254  shown in the device is four, but the number of linkages could be any number and that would be within the spirit and scope of the present invention. Undesired in plane movements can be limited by introducing in plane bump stops  344  at locations where undesired movement/rotation has a high amplitude, e.g., furthest away from the linkages  254 . The in plane bump stops  344  can be defined on the second plate  154  or the device layer  104  or the first plate  144 , or any combination thereof. 
       FIG. 5  shows the cross section of the piston microphone  500 , with integrated back cavity  134  along  5 - 5  in  FIG. 4 . In an embodiment, the device layer  104  is device layer  400  in  FIG. 4 . The integrated back cavity  134  is formed by a fusion (oxide) bond  224  between a second substrate  124  and the device layer  104  which further is bonded to the first substrate  114  by a conductive alloy (eutectic) bond  204  by processes as described in a commonly owned U.S. Pat. No. 7,442,570, entitled, “Method of Fabrication of a Al/Ge Bonding in a Wafer Packing Environment and a Product Produced Therefrom”, which is incorporated herein by reference. Static pressure in the back cavity  134  is equalized by ambient pressure via air flow through the PEC  234 . Linkages  254  are attached to the standoffs  184  both mechanically and electrically. 
     Acoustic barriers  364  may be introduced wherever suitable for required low frequency response enhancement. 
     The first plate  144  is partially surrounded by a PEC  234 . The entire structure is surrounded by a seal  264  to ensure that the only acoustical input to a cavity  134  is via acoustic port  194 . When an acoustic force is applied on the first surface of first plate  144 , the first plate  144  is displaced up or down depending on polarity of pressure. The second plate  154  is displaced in the same direction as the first plate  144 . Both plates  144  and  154  are attached to the anchors  244  via the linkages  254 , which apply an opposite restoring force to first and second plates  144  and  154 . When the acoustic force is reduced to zero, the restoring force brings first and second plates  144  and  154  to their original operating position. 
     The standoffs  184  are lithographically defined protruding members of the device layer that are mechanically and electrically connected to the first substrate  114  via alloy (eutectic) bonding to a top metal layer of the first substrate  114 . The device layer  104  is lithographically patterned to form the first plate  144 , second plate  154  and plate with perforations  164 , the PEC  234  and an acoustic seal around the active device. The second plate  154  forms a first electrode and is electrically connected to an integrated circuit (IC) manufactured on the first substrate  114 , while a second electrode  174  manufactured on the first substrate  114  is designed to be aligned with first electrode  174 . A first (bottom) surface of the second plate  154  and the second electrode  174  forms a variable capacitor whose value depends on the pressure applied on the first surface of the first plate  144 . The second electrode  174  in an embodiment is buried under a stack of silicon nitride and silicon dioxide which further can be lithographically patterned to form bump stops  274  to reduce stiction force by reducing contact area in the undesired event that first and/or second plates  144  and  154  come into contact with the first substrate  114 . 
       FIGS. 6A and 6B  illustrate the conceptual designs showing the operation of a piston microphone of  FIG. 5 . The linkages  254  in  FIG. 5  are now represented by symbolic springs  256  and support the first plate  146 , second plate  156  the acoustic port  196  is a channel in a first substrate  116  for acoustic pressure to reach the first surface of the first plate  146 . Under an applied acoustic pressure the first plate  146  slightly moves up or down depending on polarity of sound pressure. In  FIG. 6B , the case where the first plate  146  moves up is depicted. This upward movement of first plate  146  is coupled to a second plate  156  with perforations  166 , which in turn results in increased gap between the first surface of the second plate  156  and the second electrode  176 ; hence the capacitance defined by these two surfaces decreases. An IC manufactured on the first substrate  116  is electrically connected to both of the electrodes  156  and  176 ; hence it is used to detect the change in capacitance, which is proportional to the acoustic pressure. 
       FIG. 7  shows alternative manufacturing options for a torsional microphone  700 . In one alternative scheme, the posts  210  can be made wider to overlap over a PEC  230 , while forming a shallow recess step to form a well-controlled and shallow extended PEC  280  for improving the low frequency response of the microphone. The depth of the channel is controllable as well as the length to provide a means to properly design a pressure equalization channel for proper frequency response. Similarly defining a partial overlap of the second substrate  120  over the outer periphery of the second plate  150  creates a bump stop  310  which limits out of plane, upward movement of the first and second plates  140  and  150 . By proper design of the bump stop  310  the potential risk of the first plate  140  touching the first substrate  110  can be reduced significantly. Similarly, proper design of the length of an extended PEC  300  over outer edge (furthest away from the rotation axis) of the first plate will limit the rotational movement of the first and second plates  140  and  150  and may be used for significantly reducing the potential risk of first or second plates  140 ,  150  touching the first substrate  110 . Limiting out of plane movement improves device reliability, especially against stiction, vibrations and shocks. 
     In another embodiment, the first and second plates  140  and  150  can be thinned down selectively so as to have a thicker portion and a thinner portion, creating a stepped device layer  290 , for increasing resonant frequency of the device and reducing acoustic resistance of the perforations  160 . In an embodiment, linkage  250  can have the same thickness as the thicker portion of first plate  140  or second plate  150 . In another embodiment, linkage  250  can be same thickness as the thinner portion of first plate  140  or second plate  150 . In another embodiment, linkage  250  can be of any thickness independent of the first and second plates. By proper design of the step profile of the first and second plates  140  and  150 , first and second plates can be manufactured to be stiff enough to perform as microphone plates. 
     In another embodiment, back plate  330  with perforations  320  is provided to serve as a rigid electrode on the first substrate covering acoustic port  190 , which faces the first surface side of the first plate  140 . In an embodiment, the rigid back plate  330  can partially or completely cover the acoustic port  190 . By proper design of a plate  330  with perforations  320 , acoustic pressure input through acoustic port  190  will reach the first surface of the first plate  140  without noticeable attenuation, while the parallel plate capacitance formed by this backplate  330  and the first plate  140  will increase the electronic sense capacitance. 
     Under the influence of acoustic input, the capacitance between the backplate  330  and first plate  140  will change in the opposite phase to the capacitance formed between the second plate  150  and the second electrode  170 . The phase difference between sense capacitances enables differential sensing. An additional benefit of the differential structure is the possibility of recovering from a stiction. In the event that either the first plate  140  or the second plate  150  comes into contact with the first substrate  110  and gets stuck, an electrical bias can be applied between the plate that is not in contact with the first substrate  110  and corresponding electrode (second electrode  170  or the backplate  330 ) for recovering from stiction. It is also possible to sense the tilting of plates and dynamically adjust bias applied across the plates to ensure that they do not come into contact with the first substrate  110 . 
       FIG. 8  shows alternative manufacturing embodiment for the piston microphone. In one embodiment, the posts  214  can be made wider to overlap over a PEC  234 , while forming a shallow recess step to form a well-controlled and shallow extended PEC  284 , in order to improve low frequency response of the microphone. In a similar way, a partial overlap of bump stop  314  of the second substrate  124  over the outer periphery of the second plate  154  limits out of plane (upward) movement of the first and second plates  144 ,  154 . Limiting of out of plane movement improves device reliability, especially to vibrations and shocks. 
     In another alternative scheme, the first and second plates  144 ,  154  can be thinned down selectively, creating a stepped device layer  294  to increase resonant frequency of the structure and to reduce acoustic resistance of perforations. 
     In another embodiment, backplate  334  with perforations  324  is provided to serve as an electrode on the first substrate covering acoustic port  194 , which faces the first surface side of the first plate  144 . In an embodiment, the rigid back plate  334  can partially or completely cover the acoustic port. By proper design of a plate  334  with perforations  324 , acoustic input (sound pressure) through the opening (acoustic port  194 ) will reach the first surface of the first plate  144  without noticeable attenuation, while the parallel plate capacitance formed by this backplate  334  and the first plate  144  will increase the electronic sense capacitance. 
     Under the influence of acoustic input, this capacitance will change in the same phase as the capacitance formed between the second plate  154  and the second electrode  174 . Hence the total sense capacitance will increase. 
       FIGS. 9A ,  9 B, and  9 C show packaging schemes for that can be applied to any of the described embodiments of a microphone.  FIG. 9A  illustrates a capped package  900 A with integrated device  914 . Back cavity  916  is self-contained in the integrated device  914 .  FIG. 9B  shows a molded package  900 B where a plastic or similar encapsulating material  924  is molded or formed over the integrated device  922 .  FIG. 9C  illustrates a capped package  900 C that forms an extended back cavity  927  via an acoustic port  926  opened on top surface of integrated device  918 . 
       FIG. 10  shows an embodiment which integrates a MEMS microphone  370  with one or more other MEMS devices  380  on the first and second substrates. Other MEMS devices include but are not limited to the gyroscope, accelerometer, pressure sensor and compass. MEMS microphone  370  can be a piston microphone or a torsional microphone as described in  FIGS. 1 ,  2 ,  4 ,  5 ,  7 , and  8 . 
     Both torsional and piston designs of microphone provide improvements over conventional designs. The integrated back cavity where the enclosure is defined by the first and second substrates and integrated electronics from the CMOS-MEMS construction enables a significantly smaller package footprint than in conventional two-chip solutions. The integrated back cavity also relieves packaging considerations where the MEMS die and package together form the back cavity. 
     The torsional design inherently is expected to be less sensitive to accelerations during operation compared to similar dimensioned or larger microphones. Piston design, in terms of electronic pickup and movement of plates, is similar to existing MEMS and condenser microphones, but unlike the others is based on movement of solid plates, not diaphragms. Also, unlike other designs, pressure sensing area and electrode area can be adjusted separately, giving extra flexibility on design at a cost of area/mass. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.

Technology Category: b