Abstract:
The claim invention is directed at a MEMS microphone die fabricated using CMOS-based technologies. In particular, the claims are directed at various aspects of a diaphragm for a MEMS microphone die which is fabricated as stacked metallic layers separated by vias using CMOS fabrication technologies.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application 61/871,957, filed on Aug. 30, 2013, and Patent Cooperation Treaty Application PCT/US14/53235, filed on Aug. 28, 2014. These applications are hereby incorporated by reference in their entireties for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In the 1960s, practitioners in the field of microelectronics first developed techniques for fabricating tiny mechanical structures using a series of steps involving the depositing of layers of materials onto the surface of a silicon wafer substrate, followed by selectively etching away parts of the deposited materials. By the 1980s, the industry began moving toward silicon-based surface micromachining using polysilicon as the mechanical layer. However, although polysilicon has proven a useful building block in fabricating microelectromechanical systems (MEMS) because of its mechanical, electrical, and thermal properties, fabrication techniques used for polysilicon-based MEMS do not work well with fabrication techniques used for complementary metal-oxide semiconductor (CMOS) technology. As such, in the prior art, the circuitry for controlling the MEMS traditionally was fabricated on a separate die. While there has been some success in integrating CMOS and polysilicon fabrication on a single die, these hybrid CMOS-polysilicon devices have proven less than ideal because of long design times and complex fabrication requirements. 
         [0003]    More recently, practitioners have attempted to fabricate MEMS structures using standard CMOS materials rather than the materials traditionally used in polysilicon-based MEMS structures. In standard CMOS fabrication, transistors are formed on the surface of a silicon wafer and electrical pathways are built above the transistors by repeatedly depositing and selectively removing layers of metallic and dielectric material. In an integrated CMOS/MEMS die, at the same time as the CMOS circuits are being interconnected on one part of the wafer, patterned layers of metallic and dielectric materials on another part of the wafer can form complex MEMS structures. Once all of the layers have been built up, the MEMS structure is “released”—that is, the sacrificial dielectric material around the MEMS structures is removed using an etchant such as vHF (vapor hydrofluoric acid), leaving the mechanical components of the MEMS structure free to move. Other sacrificial etchants can be used such as a wet “pad etch,” plasma or RIE dry etching, or a combination of any of these. Certain sacrificial etchants attack the silicon nitride passivation. Polyimide, included in some CMOS processes on top of the passivation layer can mitigate the attack on the silicon nitride. 
         [0004]    This simplifies the design and manufacturing since there is no need for the use of special procedures and materials to accommodate the disparate requirements of fabricating a hybrid CMOS-polysilicon die. However, as a structural building block, the metallic layers used in CMOS lack the stiffness required for use as structural MEMS components, and moreover, the thin metallic layers tend to curve after release. While it is possible to address these problems by building structures composed of stacked layers of metal having with metal vias connecting each metallic layer, many other problems remain unresolved. 
         [0005]    First, while a multi-layer metallic MEMS structure may be rigid, in some instances the rigidity of a MEMS structure should be anisotropic (that is, rigid in one axis of movement and flexible in another axis of movement). For example, many MEMS structures use springs to control movement; using multiple layers of metal for a spring structure may create the extra stiffness that prevents the spring from curving, but the stiffness in the x-, y-, and z-axes may limit the structure&#39;s effectiveness as a spring. 
         [0006]    Second, many types of MEMS require an airtight chamber after release, so either a cap wafer must be installed or else holes must be created in the top layer to allow the etchant to reach the dielectric material. In the former case, attaching a cap wafer requires non-standard CMOS processing and cost, makes access to the bonding pads more challenging, and adds height to the die. In the latter case, in order to seal the holes after the etching step, metal or other materials must be deposited, which risks inadvertent introduction of the sealing material into the interior of the chamber, potentially affecting the movement of the mechanical components. 
         [0007]    Third, in order to remove the dielectric material, the vHF (or other sacrificial etchants) must come into physical contact with the material. For a narrow stacked structure, the vHF can readily remove the dielectric material. However, for a wide plate structure (for example, a microphone back plate), the vHF may take considerable time to reach the interior of the plate, and this may result in removal of more dielectric material than desired from other parts of the MEMS structure. 
         [0008]    Fourth, for a wide plate structure, even after removal of the dielectric material between the metallic layers, the plate may have significant mass. This can lead to lower resonant frequencies, which can negatively impact the frequency response of the microphone. 
         [0009]    Fifth, as noted above, single layers of metal are relatively weak. Where an unreinforced top metallic layer covers a sealed chamber containing the MEMS structure, the top layer may bow inward because of the vacuum within the chamber. Adding space between the MEMS structure and the top layer may keep the top layer from interfering with the MEMS structure, but the additional space increases the height of the die. 
         [0010]    Sixth, when the surfaces of mechanical components of a MEMS structure come into contact with one another, adhesive surface forces, commonly known as “stiction,” can cause the surfaces to become stuck to one another, compromising the mechanical functions of the device. 
         [0011]    Therefore, there is an unmet need for structures and methods that address the known problems in fabricating integrated CMOS/MEMS dice. 
       SUMMARY OF THE INVENTION 
       [0012]    In one embodiment of the present invention, the etchant is introduced into the interior of the die through a hole in the bottom of the wafer rather than introducing the etchant from the top side of the wafer. After completion of the etching step a sealing wafer, for example, silicon or glass, can be attached to the bottom of the wafer. This is simpler and less costly than adding a patterned cap wafer to the top of the wafer or taking the precautions necessary to prevent sealing material from entering the MEMS chamber through the holes in the top surface. Further, sealing the bottom of the wafer leaves the bonding pads on the top surface unaffected. Still further, the sealing wafer can be lapped after applying to thin the overall structure thickness. 
         [0013]    In another embodiment of the present invention, a plate is made of multiple alternating layers of metal and dielectric material, with metal vias between the metallic layers. At least one of the metallic layers has a plurality of openings, such that when the etchant is introduced, it removes the dielectric material through the openings and quickly reaches and removes the dielectric material between the metallic layers. The resulting structure is easier to fabricate since the etchant reaches all of the dielectric material more quickly. Further, in comparison to a multilayer plate having continuous metallic layers, the inventive plate is nearly as stiff but significantly lower mass. 
         [0014]    In another embodiment of the present invention, where the top metallic layer covers a sealed chamber containing the MEMS structure, structural supports running between the wafer and the top metallic layer provide support for the top metallic layer. These structural supports, which can be stand-alone pillars or they can be a part of the fixed portion(s) of the MEMS structure itself, provide support to the top metallic layer that might otherwise bow inward because of a vacuum within the chamber. 
         [0015]    In another embodiment of the present invention, multiple alternating layers of metal and dielectric material, with metal vias between the layers of metal, make up a spring for a piston-type MEMS microphone diagram. The spring is much taller than it is wide, so that after the removal of the dielectric material between the layers, the spring is much stiffer in the vertical direction than in the horizontal direction; as such, in comparison to a diaphragm supported by an isotropic spring, the diaphragm supported by the inventive spring has roughly 50% more change in capacitance for a given acoustic signal. 
         [0016]    In another embodiment of the present invention, multiple alternating layers of metal and dielectric material, with metal vias between the layers of metal, make up a piston-type MEMS microphone diagram. On one side of the diaphragm, the top metallic layer of the diaphragm is offset from a metallic layer of the adjacent support structure, such that when the diaphragm moves downward, the metallic layer of the diaphragm will come into contact with the metallic layer of the support structure, preventing further downward movement of the diaphragm. On another side of the diaphragm, the bottom metallic layer of the diaphragm is offset from a metallic layer of the adjacent support structure, such that when the diaphragm moves upward, the metallic layer of the diaphragm will come into contact with the metallic layer of the support structure, preventing further upward movement of the diaphragm. 
         [0017]    In another embodiment of the present invention, some rows of vias may be formed without a layer of metal above them, looking effectively like stalagmites of a cave. Similarly, some rows of vias may be formed without a layer of metal below them, looking effectively like stalactites of a cave. When a moving component and a support structure component are offset with respect to one another, similarly to the previous embodiment, movement will be limited when a stalactite via comes into contact with a metallic layer below it, or when a stalagmite via comes into contact with a metallic layer above it. Or in another configuration, movement will be limited when a stalactite comes into contact with a stalagmite directly below it. Eliminating one or both metallic layers allows for a different range of movement of the device than in the previous embodiment where movement was stopped by metallic layer to metallic layer contact. Further, eliminating one or both metallic layers reduces the weight of the device. Further, since the contact area is only as wide as the vias rather than the entire metallic layer, the chance of stiction between the two components is greatly reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is an angled view of a three-layer spring structure. 
           [0019]      FIG. 2  is an angled view of a five-layer spring structure. 
           [0020]      FIG. 3  is cross section view of a vacuum-sealed die before release. 
           [0021]      FIG. 4  is a cross section view of a vacuum-sealed die after release. 
           [0022]      FIG. 5  is a cross section view of a portion of a rigid capacitive sensor plate. 
           [0023]      FIG. 6  is an angled view of a rigid capacitive sensor plate used as a diaphragm in a piston-type capacitive microphone. 
           [0024]      FIG. 7  is a cross section view of mechanic stops built into a movable MEMS structure (at rest). 
           [0025]      FIG. 8  is a cross section view of mechanical stops built into a movable MEMS structure (extended to the upward stop point). 
           [0026]      FIG. 9  is a cross section view of mechanical stops built into a movable MEMS structure (extended to the downward stop point). 
           [0027]      FIG. 10  is a cross section view of mechanical stops built from vias and a metallic layer (at rest). 
           [0028]      FIG. 11  is a cross section view of mechanical stops built from vias and a metallic layer (extended to the stop point). 
           [0029]      FIG. 12  is a cross section view of mechanical stops built from opposing vias (extended to the stop point). 
           [0030]      FIG. 13  is a cross section view of mechanical stops built without the use of offset metallic layers. 
           [0031]      FIG. 14  is a cross section view of a structural support pillar comprising a single via series. 
           [0032]      FIG. 15  is a cross section view of a structural support pillar comprising a plurality of metallic layers and a plurality of vias. 
           [0033]      FIG. 16  is a cross section view of a structural support pillar integrated into a MEMS structure. 
           [0034]      FIG. 17  is an angled view of the diaphragm of an exemplar MEMS microphone die fabricated using the inventive structures and methods. 
           [0035]      FIG. 18  is a second angled view of the diaphragm of an exemplar MEMS microphone die fabricated using the inventive structures and methods. 
           [0036]      FIG. 19  is an angled view of an exemplar MEMS microphone die fabricated using the inventive structures and methods. 
           [0037]      FIG. 20  is an angled view of an exemplar MEMS resonator die fabricated using the inventive structures and methods. 
           [0038]      FIG. 21  is a second angled view of an exemplar MEMS resonator die fabricated using the inventive structures and methods. 
           [0039]      FIG. 22  is an angled view of an exemplar MEMS pressure sensor die fabricated using the inventive structures and methods. 
           [0040]      FIG. 23  is a second angled view of an exemplar MEMS pressure sensor die fabricated using the inventive structures and methods. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    The following sections set forth numerous specific embodiments taking advantage of various aspects of the invention. These are not intended to be an exhaustive collection of every embodiment of the invention, as embodiments of the invention can be combined in a multiplicity of ways without departing from the principles of the invention. 
       General Fabrication Techniques 
       [0042]    The embodiments disclosed can be fabricated using standard sub-micron CMOS fabrication techniques known to one of skill in the art, for example: 
         [0043]    1. On the portions of a silicon wafer substrate intended to be populated by transistors, build the transistors using standard CMOS techniques. The portions of the wafer for the MEMS structures remain untouched, leaving the field oxide in this area. 
         [0044]    2. Deposit a layer of SiO 2  over the entire wafer. 
         [0045]    3. Apply a patterned mask onto the SiO 2  layer with openings for the electrical vias needed for the transistor interconnects and for the vias needed for the structure intermetal supports for the MEMS structure. 
         [0046]    4. Etch the SiO 2  layer using reactive ion etching (RIE). 
         [0047]    5. Fill the vias with tungsten using physical vapor deposition (PVD). 
         [0048]    6. Planarize the layer using chemical-mechanical polishing (CMP). 
         [0049]    7. Deposit an adhesion layer of Ti using sputtering. 
         [0050]    8. Deposit a barrier layer of TiN using sputtering. 
         [0051]    9. Deposit a metallic layer of Al/Cu alloy (1% Cu) using sputtering. 
         [0052]    10. Apply a patterned mask onto the metallic layer to create interconnects for electrical pathways and for the MEMS structures. 
         [0053]    11. Etch the metallic layer using RIE. 
         [0054]    12. Repeat steps 2-11 for as many metallic layers as required. 
         [0055]    13. Deposit a passivation layer of Si 3 N 4 , and pattern and dry etch openings in the passivation layer as needed. 
         [0056]    14. Optionally, add a polyimide layer on top of the passivation and pattern openings as needed. 
         [0057]    15. Optionally, create one or more openings through the silicon wafer beneath the MEMS structure. 
         [0058]    16. Introduce vHF (or other etchant) through the openings of the passivation layer and/or silicon wafer to etch the SiO 2  portions of the MEMS structures. (The length of exposure to the vHF required to release the MEMS structures will vary according to the concentration of the vHF, the temperature and pressure, and the amount of SiO 2  to be removed.) 
         [0059]    17. Dice the silicon wafer. 
         [0060]    The dimensions of the various components can vary according to application requirements. For example, the metallic layers can range in thickness from approximately 0.5 μm to 1.0 μm, and each layer needn&#39;t be the same thickness as the other layers. The vias can range in from approximately 0.2 μm to 0.5 μm and be spaced apart from one another between approximately 0.5 μm to 5.0 μm, and the vias needn&#39;t be uniform in size or pitch. The vias on any given layer could be lined up in rows and columns or they could be offset from one another; the vias of one layer could be directly above the vias of the layer below or they could be offset from the vias of the layer below. The thickness of the SiO 2  between metallic layers can range from approximately 0.80 μm to 1.0 μm, and each layer of SiO 2  between metallic layers needn&#39;t be the same thickness as other layers of SiO 2 . 
         [0061]    Further, other materials common to CMOS fabrication may be used. Metals other than the Al/Cu (1%) alloy, such as copper or Al/Cu alloys of different proportions, may be used for the metallic layers. Dielectrics other than SiO 2 , such as polymers, may be used for the intermetal layers and would likely require use of a different release etchant. A material other than silicon may be used for the wafer substrate, provided that it is otherwise compatible with the CMOS fabrication process. 
         [0062]    Further, during the release step, in addition to controlling the depth of the etching through time, temperature, and pressure, the structure could include physical barriers that block the further penetration of the etchant. 
         [0063]    Further, the foregoing list of steps can be altered to meet the requirements for the use of specific fabrication equipment, the fabrication requirements of the non-MEMS components of the die, and the fabrication requirements of specific MEMS structures. The following sections describe examples of additional fabrication requirements for specific MEMS structures. 
       Anisotropic MEMS Spring Structure 
       [0064]    In a preferred embodiment of MEMS spring structure  1000 , shown in  FIG. 1 , each of the metallic layers  1001 ,  1002 , and  1003  are approximately 1.0 μm wide and approximately 0.555 μm thick, and are composed of aluminum. In between metallic layers  1001 ,  1002 , and  1003  are intermetal layers  1004  and  1005 , which are approximately 1 μm wide and 0.850 μm thick. Vias  1006  are approximately 0.26 μm square, are spaced approximately at 1.0 μm intervals, and are composed of tungsten. 
         [0065]    Spring structure  1000  is fabricated using standard sub-micron CMOS fabrications techniques, for example, as disclosed above under “General Fabrication Techniques.” 
         [0066]    The following table compares spring structure  1000  to a solid metal structure of the same dimensions: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Structure 1000 
                 Comparable Solid Beam 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Moment of Inertia (Z) 
                 2.234 
                 3.175 
               
               
                 Moment of Inertia (Y) 
                 0.139 
                 0.280 
               
               
                 Ratio of Z to Y Stiffness 
                 16.1:1 
                 11.3:1 
               
               
                   
               
             
          
         
       
     
         [0067]      FIG. 2  shows spring structure  1007 , comparable to spring structure  1000  except that spring structure  1007  consists of two additional metallic layers  1008  and  1009  and two additional intermetal layers  1010  and  1011 . The following table compares spring structure  1007  to a solid metal structure of the same dimensions: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Structure 1006 
                 Comparable Solid Beam 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Moment of Inertia (Z) 
                 11.027 
                 19.621 
               
               
                 Moment of Inertia (Y) 
                  0.231 
                  0.514 
               
               
                 Ratio of Z to Y Stiffness 
                 47.7:1 
                 38.1:1 
               
               
                   
               
             
          
         
       
     
         [0068]    Depending on the purpose of the spring structure in the MEMS device, the length of the metallic layers may vary. For example, when used to support a piston-style diaphragm in a MEMS microphone die, it may be approximately 100 μm, but when used for other applications, such as an accelerometer or valve, its length would differ according to the configuration of the device and the mass of the moving component. Likewise, number of metallic layers and/or the width of the spring can be changed to increase or decrease the stiffness of the spring as needed for the purpose of the spring in the MEMS device. Generally, the stiffness of the spring will vary with the third power of the length (inversely), linearly with the width, and with the third power of the height. 
       Vacuum Sealed MEMS Die 
       [0069]    In a preferred embodiment of vacuum sealed MEMS die  2000 , shown in cross-section before release in  FIG. 3  and after release and capping in  FIG. 4 , layers of metal and unreleased dielectric material making up an unreleased MEMS structure  2001  residing in chamber  2002 . MEMS structure  2001  could be, for example, an accelerometer, resonator, gyroscope, or other structure. Prior to release, layers of dielectric material  2003  fill the empty space in chamber  2002 . Support structure  2004 , which may be made of layers of metal and dielectric materials, surrounds chamber  2002 , and support structure  2004  may have other features and purposes that are not relevant for describing this embodiment. Structures  2001  and  2004  and dielectric material  2003  all sit above wafer  2005 . Metallic layer  2006 , composed of a 1.0 μm-thick layer of aluminum, has been deposited on top of support structure  2004  and chamber  2002 . Passivation layer  2007 , composed of Si 3 N 4 , has been deposited on top of metallic layer  2006 . An opening  2008  runs through wafer  2005  into chamber  2002 . 
         [0070]    After fabricating unreleased structure  2001  in MEMS die  2000  an etchant is introduced into chamber  2002  through opening  2008 . The etchant removes the dielectric material  2003  in chamber  2002 , including any exposed dielectric material in now-released MEMS structure  2001   a  and in support structure  2004 . The extent of etching of the dielectric in support structure  2004  is controlled by etch time. As shown  FIG. 4 , after release, a silicon sealing wafer  2009  has been bonded to the bottom of wafer  2005 . 
         [0071]    Vacuum sealed MEMS device  2000  is fabricated using the standard sub-micron CMOS fabrications techniques, for example, as disclosed above under “General Fabrication Techniques,” with the following change: 
         [0072]    17. In a vacuum, attach a silicon sealing wafer to the bottom of the die wafer using techniques such as electrostatic bonding, eutectic bonding, or glass frit. 
         [0073]    18. Reduce the thickness of the sealing wafer to approximately 100 μm, using techniques such as grinding, lapping, polishing, chemical-mechanical polishing (CMP), or combinations of these techniques. 
         [0074]    19. Dice the silicon wafer. 
       Lightweight-but-Rigid Capacitive Sensor Plates 
       [0075]    With the lightweight-but-rigid capacitive sensor plate  3000  partially shown in  FIG. 5 , each of the metallic layers  3001  and  3002  are approximately 0.5 μm thick, and are preferably composed of an aluminum/copper alloy. In between metallic layers  3001  and  3002  is intermetal layer  3003 , which is approximately 0.850 μm thick and typically composed of silicon oxide. Tungsten vias  3004  are approximately 0.26 μm square, are spaced approximately at 1.0 μm intervals, and are between metallic layers  3001  and  3002 . As shown in  FIG. 6 , individual metallic layer  3001  is a solid hexagon approximately 600 μm wide, while individual metallic layer  3002  is similarly shaped and sized but is latticed, having equilateral triangular openings  3005 , approximately 10 μm in size and spaced throughout. 
         [0076]    Sensor plate  3000  is fabricated using the standard sub-micron CMOS fabrications techniques, for example, as disclosed above under “General Fabrication Techniques. 
         [0077]    As suggested by  FIG. 6  sensor plate  3000  is ideal for use as a diaphragm in a piston-type capacitive microphone when connected by springs  3006  to support structure  3007 . As it includes metallic layers  3001  and  3002 , no additional conductive material must be deposited for it to act as one of the capacitive plates. Further, because it has metallic layers  3001  and  3002  which are connected by vias  3004 , it will effectively function as a solid component, and yet, because during release intermetal layer  3003  is removed through triangular openings  3005 , it is significantly lighter and has higher resonant frequencies than a solid component. 
         [0078]    The shape and size of the plate may be varied according to the application for the plate. For example, when used as a back plate of a capacitive sensor, it may be rectangular and extend into the walls of a supporting structure surrounding the sensor structure. Further, when used as a back plate of a capacitive sensor, metallic layer  3001  could be perforated to be acoustically transparent; alternatively, openings  3005  could extend through metallic layer  3001 . Further, the shape of the openings  3005  in metallic layers  3001  and/or  3002  could be any regular or irregular polygon, circle, or oval, the shape of the plate could be any regular or irregular polygon, circle, or oval, and the plate could include additional metallic layers. 
       Mechanical Stops 
       [0079]    In the preferred embodiment of mechanical stops  4000   a  and  4000   b  of capacitive sensor diaphragm  4001 , shown in  FIG. 7 , the edges of each side of bottom metallic layer  4002  of diaphragm  4001  are slightly offset (approximately 10 μm) from the edges of each side of top metallic layer  4003  in an alternating pattern around the hexagonally-shaped sensor diaphragm  4001 . That is, on three sides, the edges of metallic layer  4002  extend beyond metallic layer  4003 , and on the other three sides, the edges of metallic layer  4003  extend beyond metallic layer  4002 . Metallic layers  4002  and  4003  are approximately 0.5 μm thick, and are composed of an aluminum/copper alloy. In between metallic layers  4002  and  4003  is intermetal layer (not shown, removed during release etch), which is approximately 0.850 μm thick. A plurality of tungsten vias  4005 , approximately 0.26 μm square, are spaced approximately at 1.0 μm intervals between metallic layers  4002  and  4003 . 
         [0080]    In a pattern opposite that of the edges of metallic layers  4002  and  4003  of sensor diaphragm  4001 , support structure  4006  includes at least two metallic layers  4007  and  4008  with offset edges adjacent to the offset edges of metallic layers  4002  and  4003 . That is, on three sides, the edges of metallic layer  4007  extend beyond metallic layer  4008 , and on the other three sides, the edges of metallic layer  4008  extend beyond metallic layer  4007 , such that the edges of metallic layers  4007  and  4008  act as mechanical stops that prevent excessive movement of sensor diaphragm  4001 . 
         [0081]    Referring now to  FIG. 8 , when pressure moves sensor diaphragm  4001  upward, the top of metallic layer  4002  comes into contact with the bottom of metallic layer  4007  to create a mechanical stop  4000   a , stopping further upward movement of sensor diaphragm  4001 . As shown in  FIG. 9 , when pressure moves sensor diaphragm  4001  downward, the bottom of metallic layer  4003  comes into contact with the top of metallic layer  4008  to create a mechanical stop  4000   b , stopping further downward movement of sensor diaphragm  4001 . 
         [0082]    A sensor with mechanical stops  4000   a  and  4000   b  can be fabricated using the standard sub-micron CMOS fabrications techniques, for example, as disclosed above under “General Fabrication Techniques.” 
         [0083]    In another preferred embodiment, metallic layer  4003   b  of cantilever  4009 , shown in  FIG. 10 , includes a row of vias  4005   a  extending downward from metallic layer  4003   b , but metallic layer  4002   b  does not extend to the bottom of vias  4005   a , such that vias  4005   a  resemble stalactites in a cave. All metallic layers are 0.5 μm thick, and are composed of an aluminum/copper alloy. In between metallic layers is an intermetal layer (not shown, removed during release etch), which is approximately 0.850 μm thick. All vias are approximately 0.26 μm square and are spaced approximately at 1.0 μm intervals between metallic layers. 
         [0084]    As shown in  FIG. 11 , when cantilever  4009  bends downward towards component  4010 , its movement is limited when vias  4005   a  come into physical contact with metallic layer  4002   a  on component  4010 . In a variation on this embodiment, shown in  FIG. 12 , rows of vias  4005   a  extend downward from metallic layer  4003   b , while rows of vias  4005   b  extend upward from metallic layer  4002   a . When cantilever  4009  bends downward towards component  4010 , its movement is limited when vias  4005   a  come into physical contact with vias  4005   b.    
         [0085]    In another preferred embodiment, shown in  FIG. 13 , upward movement of moveable component  4011  will be limited when the top metallic layer of component  4011  comes into contact with the mechanical stops of metallic layer  4013 . Likewise, downward movement of component  4011  will be limited when the bottom metallic layer comes into contact with the mechanical stops of metallic layer  4014 . In this configuration, the edges of the top and bottom metallic layers of component  4011  need not be offset from one another. 
         [0086]    A sensor with mechanical stops is fabricated in part using the standard sub-micron CMOS fabrications techniques, for example, as disclosed above under “General Fabrication Techniques.” However, standard CMOS fabrication “rules” would not normally allow vias without metallic layers above and below, and so the rules would need to be overridden during fabrication (there is nothing that physically prohibits fabricating such vias). 
         [0087]    While the embodiments of  FIG. 7  through  FIG. 12  depict the use of the inventive mechanical stops in the context of a piston-type capacitive sensors and cantilevers, similar mechanical stops could be used to limit the movement of other mechanical components within a MEMS structure. By way of example and not limitation, the stops of any of these embodiments could be used to limit the motion of diaphragms, springs, plates, cantilevers, valves, mirrors, micro-grippers, and so forth. 
       Structural Supports for a MEMS Device 
       [0088]    In a first preferred embodiment of a structural support for a MEMS die  5001 , shown in  FIG. 14 , a support structure  5002 , approximately 0.26 μm square and composed of patches of metallic layers with a single column of aligned vias tungsten, resides in chamber  5003 , and is formed between device wafer  5004  and metallic layer  5005 . Chamber  5003  extends between die wafer  5004  and metallic layer  5005 . A MEMS structure  5006  (shown in outline), also resides within the chamber. 
         [0089]    In a second preferred embodiment of structural support for a MEMS die  5011 , shown in  FIG. 15 , a support pillar  5012 , composed of alternating metallic and intermetal layers (not shown, removed during release etch), with metal vias between the metallic layers, resides in a chamber  5013 , and is formed between die wafer  5014  and metallic layer  5015 . Chamber  5013  extends between die wafer  5014  and metallic layer  5015 . The metallic layers of pillar  5012  are between approximately 1 μm and 5 μm square and approximately 0.555 μm thick, and are composed of aluminum. The intermetal layers of pillar  5012  are approximately 0.850 μm thick. The vias of pillar  5012  are approximately 0.26 μm square, are spaced approximately at 1.0 μm intervals, and are composed of tungsten. The number of vias between each metallic layer may be varied to achieve the necessary strength of the pillar. A MEMS structure  5016  (shown in outline), also resides within the chamber. 
         [0090]    In a third preferred embodiment of structural support for a MEMS die  5021 , shown in  FIG. 16 , a support pillar  5022 , composed of alternating metallic and intermetal layers (not shown, removed during release etch), with metal vias between the metallic layers, resides in a chamber  5023 , and is formed between a fixed portion of MEMS structure  5026  (shown in outline) and metallic layer  5015 . Chamber  5023  extends between die wafer  5024  and metallic layer  5025 . The metallic layers of pillar  5022  are approximately 1 μm and 5 μm square and 0.5 μm thick, and are composed of aluminum. The intermetal layers of pillar  5022  are approximately 0.850 μm thick. The vias of pillar  5022  are approximately 0.26 μm square and composed of tungsten. 
         [0091]    Support via  5002 , pillar  5012 , and pillar  5022  are fabricated using the standard sub-micron CMOS fabrications techniques, for example, as disclosed above under “General Fabrication Techniques.” The specific shapes, locations, and number of supports  5002 ,  5012 , and  5022  can be varied according to the shape, location, and purpose of the MEMS structures  5006 ,  5016 , and  5026 . 
       Exemplar Application—Capacitive Microphone 
       [0092]      FIG. 17 ,  FIG. 18 , and  FIG. 19  show views of an embodiment of a MEMS capacitive microphone die  6000  fabricated using some of the inventive methods and structures. Hexagonal diaphragm  6001  has been built with a solid metallic layer, a lattice metallic layer, and a plurality of metal vias between the two metallic layers. Springs  6002 ,  6003 , and  6004  attach diaphragm  6001  to a support structure  6005  which surrounds diaphragm  6001 . Springs  6002 ,  6003 , and  6004 , built with three metallic layers each, have a width to height ratio of approximately 1.0:3.6. Diaphragm  6001  and support structure  6005  include pressure stops  6006  and  6007 . Back plate  6008  has been built with two lattice metallic layers, with a plurality of metal vias between the two layers. Guard electrode  6009 , in between diaphragm  6001  and back plate  6008 , is driven by the CMOS circuit to minimize stray coupling capacitance existing in the support structure between the diaphragm and back plate. Pads  6010  and  6011  provide the electrical connection between the die and external circuitry. Area  6012  (the portion of the die not occupied by the MEMS structure) contains CMOS circuitry supporting the operation of the microphone (for example, voltage control, amplifiers, A/D converters, and the like). 
         [0093]    In operation, as sound waves strike diaphragm  6001 , diaphragm  6001  moves up and down like a piston within the structure  6005 , changing the capacitance between diaphragm  6001  and back plate  6008 . Springs  6002 ,  6003 , and  6004  act to restore the position of diaphragm  6001  in between wave fronts. Pressure stops  6006  and  6007  limit the movement of diaphragm  6001  in response to excess pressure or physical shock. 
         [0094]    In this embodiment, back plate  6008  is positioned above substrate  6013 , with diaphragm  6001  positioned above back plate  6008 . Alternatively, microphone die  6000  could have been fabricated such that diaphragm  6001  is positioned above substrate  6013 , with back plate  6008  positioned above diaphragm  6001 . In either embodiment, the sound waves would strike diaphragm  6001  either from the top or from the bottom, depending on how microphone die  6000  is mounted in the microphone package. Various configurations for mounting microphone die  6000  in a package are disclosed, for example, in U.S. Pat. No. 8,121,331, which is incorporated by reference in its entirety. 
       Exemplar Application—Resonator 
       [0095]      FIG. 20  and  FIG. 21  show an embodiment of a MEMS resonator die  7000  fabricated using some of the inventive methods and structures. Fixed combs  7001  and moving combs  7002  have been built with five metallic layers and a plurality of metal vias between each layer. Fixed combs  7001  extend into the surrounding structure  7003 . Moving combs  7002  are attached to springs  7004 , which in turn are attached to anchors  7005 . Anchors/pillars  7005 , incorporated into the fixed portions of the MEMS structure, have been built from metallic layers with a plurality of vias between each layer; anchors/pillars  7005  are fixed in place by connecting them to wafer  7006  on the bottom and metallic layer  7007  on the top; passivation layer  7008  covers the top of the die. Release etch access holes (not shown) in wafer  7006  have been covered with sealing wafer  7009 , creating a vacuum in the chamber formed by wafer  7006 , metallic layer  7007 , and surrounding structure  7003 . 
         [0096]    In operation, when an alternating current is applied to the resonator, the fingers of moving combs  7002  move between the fingers of fixed combs  7001 , the resonant frequency of which determines an impedance minimum between the two elements. Although there is a vacuum in the chamber, anchors/pillars  7005  prevent metallic layer  7007  from bowing and potentially interfering with the movement of moving combs  7002 . As such, there is no need for extra space in the chamber to account for bowing, and resonator  7000  will be thinner than prior art resonators. Additionally, metallic layer  7007  will act as a shield to protect the resonator from electromagnetic interference. 
       Exemplar Application—Fluid Pressure Sensor 
       [0097]      FIG. 22  and  FIG. 23  show an embodiment of a MEMS fluid pressure sensor die  8000 . Back plate  8001  has been built from three latticed metallic layers with a plurality of metal vias between each layer. Diaphragm  8002  is built from a top metallic layer above back plate  8001 , and a passivation layer  8003  composed of Si 3 N 4  is formed on top diaphragm  8002 . 
         [0098]    As can be seen in  FIG. 23 , an outer portion of diaphragm  8002  includes a second metallic layer  8002   a . Metallic layer  8002   a  adds firmness to diaphragm  8002 , and can be varied in size to change the sensitivity of the sensor. This makes the compliance of the diaphragm less sensitive to the release etch process and its attack on the dielectric of the support structure surrounding the diaphragm. 
         [0099]    In operation, as sensor die  8000  is exposed to pressure exerted by fluids or gases, diaphragm  8002  bows in proportion to the amount of pressure, changing the capacitance between diaphragm  8002  and back plate  8001 . CMOS circuitry (not shown) in die  8000  detects the change in capacitance and converts it to a usable external signal. Further, as diaphragm  8002  is composed of a metallic layer, it also functions as a low resistance EMI shield to protect the die from electromagnetic interference. 
         [0100]    The embodiment of  FIG. 22  and  FIG. 23  functions an absolute pressure sensor. During the release step, etchant enters through release hole  8004 , and after creating release, hole  8004  is covered using sealing wafer  8005 , creating a vacuum within the die. As an alternative embodiment, sensor die  8000  could be built without sealing wafer  8005 , thus functioning as a differential pressure sensor.