Patent Publication Number: US-10759659-B2

Title: Stress isolation platform for MEMS devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a continuation, claiming the benefit under 35 U.S.C. § 120, of U.S. patent application Ser. No. 14/502,475, filed Sep. 30, 2014, and entitled “STRESS ISOLATION PLATFORM FOR MEMS DEVICES,” which is hereby incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to MEMS devices, and more particularly to structures for MEMS devices. 
     BACKGROUND ART 
     It is known in the prior art to package a micromachined (or “MEMS”) device in a cavity package. Cavity packages are attractive for MEMS devices because they include an internal “cavity” that encloses the MEMS device without physically contacting or restraining a moveable portion of the MEMS device. The cavity area mainly protects the MEMS device from external stresses originating from thermal, torque and pressure loads. Although cavity packages are significantly reliable, they suffer from high cost. 
     Overmold packaging, while common for packaging non-micromachined integrated circuits, has presented challenges to MEMS packaging. The process of encapsulating a MEMS device may involve physical and thermal shock to the MEMS device. In addition, the overmold material properties widely change with temperature. In the case of a silicon-based MEMS device encapsulated in plastic overmold, this includes both the plastic&#39;s stiffness and thermal expansion coefficient, which are largely different from the corresponding properties of silicon. As a result, thermal stresses in the package due to the wide operational temperature, which may range for example from 175 C to −40 C, create large stresses that physically propagate through the structures in the MEMS sensor, and may cause performance problems, such as large offset drift of these sensors over the temperature. 
     SUMMARY OF THE EMBODIMENTS 
     In a first embodiment of the invention there is provided a MEMS device have a MEMS platform suspended within a substrate layer to define a stress-relief gap between the plat form and the substrate. The stress-relief gap provides a barrier against the transmission of mechanical stress from the substrate to the platform. 
     For example, in one embodiment, the MEMS device includes a substrate layer having a substrate. Suspended within the substrate layer is a MEMS platform. The MEMS platform may be the same material as the substrate layer (e.g., a semiconductor such as silicon). For example, the MEMS platform may be etched from the substrate layer. 
     The MEMS platform is suspended in such a way as to define a stress-relief gap between the MEMS platform and the substrate. The substrate may contain at least one bridge (i.e., one or more bridges) spanning the stress-relief gap and configured to rigidly suspend the MEMS platform within the substrate. The bridges may be the same material as the substrate layer (e.g., a semiconductor such as silicon). For example, a bridge may be etched from the substrate layer. Alternately, or in addition, in some embodiments, the MEMS device may include at least one pillar (i.e., one or more pillars) spanning the stress-relief gap and configured to rigidly suspend the MEMS platform within the substrate. 
     The MEMS device also includes a MEMS device layer that includes a MEMS structure movably suspended from the MEMS platform by at least one flexure. The MEMS device also includes at least one flexible electrical conductor electrically coupled to the MEMS structure on the MEMS platform. The at least one flexible electrical conductor spans the stress-relief gap between the substrate and the MEMS platform, and is configured to carry an electrical signal across the stress-relief gap. 
     In some embodiments, the MEMS device layer has a peripheral region circumscribing the stress-relief gap, and a top cap coupled to the peripheral region such that the MEMS platform is disposed between the top cap and the bottom cap, and defining a top void between the top cap and the MEMS platform. Some embodiments also have a bottom cap coupled to the substrate and defining a bottom void between the bottom cap and the MEMS platform. 
     In another embodiment, a MEMS device has a substrate and a MEMS platform suspended within the substrate and defining a stress-relief gap circumscribing the MEMS platform in every direction. The device also includes a MEMS structure on the MEMS platform, the MEMS structure. The MEMS structure has a member movably suspended from the MEMS platform by at least one flexure. 
     The MEMS platform is rigidly suspended within the substrate by a suspension means, and an electrical conductor means spans the stress-relief gap, and is configured to carry an electrical signal across the stress-relief gap. For example, the suspension means may include one or more pedestals extending between the bottom cap and the MEMS platform. The electrical conductor means may include at least one conductive jumper spanning the stress-relief gap. 
     In some embodiments the substrate has a peripheral region circumscribing the stress-relief gap, and a bottom cap coupled to the peripheral region and defining a bottom void between the bottom cap and the MEMS platform, as well as a top cap coupled to the substrate such that the MEMS platform is disposed between the top cap and the bottom cap, and defining a top void between the top cap and the MEMS platform. 
     In one embodiment, a MEMS device does not include bridges at all. For example, such a MEMS device includes a substrate; a MEMS platform within the substrate layer and defining a bridge-free stress-relief gap between the MEMS platform and the substrate; a MEMS structure on the MEMS platform, the MEMS structure having a member movably suspended from the MEMS platform by at least one flexure; at least one flexible electrical conductive electrically coupled to a MEMS structure on the MEMS platform, and spanning the stress-relief gap between the substrate and the MEMS platform, the flexible electrical conductor configured to carry an electrical signal across the stress-relief gap; a bottom cap coupled to the substrate and defining a bottom void between the bottom cap and the MEMS platform; a top cap coupled to the substrate such that the MEMS platform is disposed between the top cap and the bottom cap, and defining a top void between the top cap and the MEMS platform; and at least one pillar physically coupled to the bottom cap and the MEMS platform, and configured to support the MEMS platform without bridges spanning the stress-relief gap. 
     In various embodiments, the bridges may take any of a variety of configurations, such as a Z-shaped bridge, an L-shaped bridge, or a U-shaped bridge, for example. For example, a bridge may include a first segment extending from the substrate in the direction of the MEMS platform, a second segment extending from, and disposed at an angle to, the first segment, and a third segment extending from, and disposed at an angle to, the second segment, and coupled to the MEMS platform. 
     In some embodiments, the electrical conductor may be a jumper. In some embodiments, the electrical conductor is on, or part of, at least one bridge. 
     An embodiment of a method of fabricating a MEMS device includes providing a substrate and fabricating a stress-relief gap through the substrate and defining a MEMS platform, the gap circumscribing the MEMS platform in every direction, as well as fabricating a MEMS structure on the MEMS platform. 
     In some embodiments, the MEMS structure is fabricated before the fabrication of the stress-relief gap. Indeed, in some embodiments, the MEMS structure is fabricated before the fabrication of the stress-relief gap, and the fabrication of the MEMS structure includes fabricating a MEMS structure, immobilizing the MEMS structure relative to the substrate, etching a stress-relief gap through the substrate and defining a MEMS platform surrounding the MEMS structure; and releasing the MEMS structure. In other embodiments, the MEMS structure is fabricated after the fabrication of the stress-relief gap. 
     In some embodiments, providing a substrate includes providing a substrate having a first side and a second side, and the process of fabricating a stress-relief gap and fabricating a MEMS structure includes etching a trench extending into the first side of the substrate and extending partially through the substrate, the trench outlining a MEMS platform, and filling the trench with sacrificial trench material, fabricating the MEMS structure on the MEMS platform grinding the second side of the substrate to expose the sacrificial trench material; and removing the sacrificial trench material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of embodiments will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which: 
         FIGS. 1A and 1B  schematically illustrate a MEMS accelerometer according to one embodiment; 
         FIG. 2A - FIG. 2B  schematically illustrate a Z-axis MEMS device according to one embodiment; 
         FIG. 3A - FIG. 3T  schematically illustrate features of embodiments of MEMS devices; 
         FIG. 4  schematically illustrates a MEMS platform according to one embodiment; 
         FIG. 5  schematically illustrates an encapsulated MEMS product according to one embodiment; 
         FIG. 6A  is a flow chart of an embodiment of a method of fabricating a MEMS product; 
         FIG. 6B  is a flow chart of another embodiment of a method of fabricating a MEMS product; 
         FIGS. 7A-7H  schematically illustrate features of a MEMS device at various stages of fabrication according to the method of  FIG. 6A ; 
         FIGS. 8A-8P  schematically illustrates a MEMS device at various stages of fabrication according to the method of  FIG. 6B ; 
         FIG. 9  is a graph schematically illustrating die stress among several packaging technologies. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Various embodiments provide solutions to minimize the effect of package stresses into the MEMS sensors packaged in overmold (e.g., plastic) packages. Any of a variety of stress-relief structures may be included in a wafer or silicon-on-insulator substrates to block or divert compressive or tensile stresses within the substrate, with the result that distortion of MEMS structures is reduced and the accuracy of the MEMS structures is increased in comparison to prior art MEMS device. A variety of such MEMS devices are detailed below. 
     To mitigate such distortions, various embodiments include a stress-relief gap which serves to intercept, block or divert stresses within the substrate of the MEMS device. 
     Definitions. As used in this description and the accompanying claims, the following terms shall have the meanings indicated, unless the context otherwise requires: 
     A “void” is a three-dimensional space substantially free of solid material. A “void” may be spanned by a bridge or other structure, provided that the bridge or other structure does not provide a linear path by which a mechanical stress could cross the void. 
     A “stress-relief gap” is a space or void between two parts of a device. 
     A “bridge-free stress-relief gap” is a space or void between two parts of a device, but which does not contain or include a bridge as described below. 
     A “MEMS device” is device having a first member, and a second member that is moveable with respect to the first member, for example in reaction to an external force or external stimulus. The second member may be referred to as a “MEMS structure.” One examples of a MEMS device is an micromachined accelerometer as known in the art, having a support structure and a beam movably suspended from the support structure by one or more flexures, such that beam is configured to move relative to the support structure in response to an acceleration applied to the support structure. Another example of a MEMS device is a micromachined gyroscope as known in the art, having a support structure and a beam movably suspended from the support structure by one or more flexures, and one or more beam drivers configured to apply electrostatic force to the beam so as to cause the beam to oscillate in a first direction relative to the support structure, such that beam is configured to move in a second direction due to Coriolis forces in response to a rotation applied to the support structure. 
     An “integrated circuit” is a circuit that includes active semiconductor devices, such as transistors for example. 
     Some MEMS devices include structures suspended above a substrate. For example,  FIGS. 1A and 1B  schematically illustrate a portion of a micromachined accelerometer  100 , in which a proof mass (or “beam”)  101  is suspended by flexures  109  and anchors  108  above a substrate  102 . The anchors  108  are fixedly coupled to the substrate  102 . Flexures  109  are flexible and allow the beam  101  to move relative to the substrate  102  in response to an acceleration applied to the substrate  102 . The flexures  109  and anchors  108  may be known as “beam support structures.”  FIG. 1B  schematically illustrates a plan view of the accelerometer  100 , while  FIG. 1A  schematically illustrates a perspective view of a portion  150  of accelerometer  100 . 
     When the accelerometer  100  is not subject to an acceleration, the beam  101  remains suspended above the substrate  102  in a position that may be known as its “nominal” position, and does not move relative to the substrate  102 . However, when the substrate  102  is subjected to acceleration, for example in the +X direction, the inertia of the beam  101  causes a displacement of the beam  101  relative to the substrate  102 . Under acceleration, flexures  109  change their shape and length to allow the beam  101  to move relative to the anchors  108 . 
     A finger  103 , on the beam  101  forms a variable capacitor across gap  107  with a counterpart finger  104 , and a separate variable capacitor with fixed finger  106 . Finger  106  is coupled to the substrate  102 , and finger  104  is suspended from finger anchor  110 , which is coupled to the substrate  102 . The capacitance of each variable capacitor varies when the beam  101  moves relative to the substrate  102 . The variable capacitance can be electronically processed to produce an electrical signal representing the displacement of the beam  101 , which in the case of accelerometer  100  correlates to the applied acceleration. 
     The gap  107  is typically quite small. For example, in the accelerometer  100 , the gap  107  may be on the order of one micron to a few microns. As such, any distortion in the proof mass  101  or substrate  102 , for example due to thermal stress within substrate  102 , may also cause a change in the gap  107 . Such a change may appear as a DC offset in the displacement signal. 
     Some MEMS devices are fabricated on or from a Silicon-on-Insulator (or “SOI”) wafer, such as the SOI wafer  201  in  FIG. 2A . A typical SOI wafer  201  has a base layer, sometimes known as a “handle layer”  211 . The handle layer  211  may be silicon, for example. The SOI wafer  201  also has a top layer, sometimes known as a “device” layer  213 , coupled to one side of the handle layer  211  by an insulator layer  212 . The device layer  213  may be doped or un-doped silicon, for example, and may be thinner than the handle layer  211 . The insulator layer  212  may be an oxide (e.g., a “buried” oxide, or “box”), and is sandwiched (e.g., laminated) between the handle layer  211  and the device layer  213 . 
     To further illustrate such devices,  FIG. 2A  and  FIG. 2B  schematically illustrate embodiments of MEMS devices  250 ,  270 . It should be noted that various embodiments are sometimes described herein using words of orientation such as “top,” “bottom,” or “side.” These and similar terms are merely employed for convenience and typically refer to the perspective of the drawings. For example, the substrate  201  is below the moveable mass  251  from the perspective of  FIG. 2A . However, the substrate  201  may be in some other orientation relative to the moveable mass  251  depending on the orientation of the MEMS device  250 . Thus, in the present discussion, perspective is based on the orientation of the drawings. 
     As with many MEMS devices, devices  250  and  270  each have a moveable mass suspended above a substrate. In some MEMS devices, the moveable mass  251  is formed from single crystal silicon (e.g., a part of the device layer  213 ), while in other MEMS devices the moveable mass  251  is formed from deposited polysilicon. For example, in the device  250  schematically illustrated in  FIG. 2A , the moveable mass  251  is fabricated above the SOI substrate  201 . As another example, the device  270  schematically illustrated in  FIG. 2B  includes a proof mass  271  in the device layer  213  of an SOI wafer  201 . 
     In  FIG. 2A , an accelerometer  250  has a substrate  201 , one portion of which supports a moveable mass  251  suspended by flexures  209  so that the mass  251  is moveable relative to the substrate  201 . The mass  251  and flexures  209  are separated from the substrate  201  by a gap  208 , and together form a variable capacitor across gap  208 . 
     In operation, the motion of the moveable mass  251  relative to the substrate  201  causes the flexures  209  to change shape, thus allowing variation in the gap  208  fingers on the moveable mass  251  (e.g., such as finger  103  in  FIG. 1A , for example) and fingers fixedly attached to the substrate  201  (e.g., such as finger  104  in  FIG. 1A , for example), to produce a changing capacitance. 
     In the embodiment shown in  FIG. 2B , the substrate  201  includes the device layer  213 , a bottom wafer  211  and a buried oxide layer (or “box layer”)  212  of an SOI wafer. One or more terminal  221  may electrically couple features of the MEMS device, such as moveable mass  251  for example, to circuitry on the MEMS device, or external circuitry. 
     The capacitors on a MEMS device generally have gaps of very small dimensions between their fingers (or plates), and the capacitance of such capacitors is generally very sensitive to even small variations in the gaps between their fingers. Such sensitivity is, in large part, responsible for the ability of the sensor to sense motion of a moveable mass. Similarly, the motion characteristics of a flexure (e.g.,  209 ) may also depend on small dimensions of its features. Consequently, if the gaps between the fingers (or plates) or dimensions of a flexure, a gap spanned by a flexure, change for reasons other than responding to the motions they are intended to detect or measure, such changes can adversely effect the sensitive and usefulness of the MEMS sensor. For example, if the substrate  201  in  FIG. 2A  or  FIG. 2B  extends its length along the X axis, the gap spanned by flexures  209  may also expand, causing the flexures  209  to expand, and become less flexible. In such a circumstance, the flexibility of the flexures may decrease, with the result that the motion of the moveable mass  251  may be damped, making the device less sensitive. Similarly, if such an expansion changes a gap between fingers of a variable capacitor, the sensitivity of that capacitor, or its ability to apply an electrostatic force to an opposing member, may be reduced, again rendering the device less sensitive. Also, such a gap between the fingers of a variable capacitor may cause an undesirable voltage offset between the fingers, also rendering the device less sensitive. 
     An embodiment of a capped MEMS sensor  300  having a substrate  311  with a stress-relief gap is schematically illustrated in  FIG. 3A  and  FIG. 3B . In this illustrative embodiment, the substrate  311  defines a substrate plane  391 , and includes a MEMS platform  310  supporting a MEMS device  301 . In this embodiment, the MEMS device  301  is a cantilevered accelerometer having a cantilevered beam  302  suspended above an electrode  304  by a flexure base  303 . However, in other embodiments, the MEMS device  301  could be any MEMS device, such as accelerometer  100 , or a switch or a gyroscope, to name but a few examples. 
     When the sensor  300  is not under acceleration, the beam  302  and electrode  304  define a nominal gap  305  between them. In response to acceleration in the positive or negative Z-axis, however, the force of the acceleration causes a change in the gap  305  between the electrode  304  and the cantilevered beam  302 . The magnitude of the acceleration may be determined by the amount of change in the gap  305 , and the direction of the acceleration may be determined by whether the gap  305  widens (acceleration in the −Z direction) or narrows (acceleration in the +Z direction), both of which may be detected and measured in ways known in the art, for example by integrated circuits  350  on the sensor  300  or off the sensor  300  on a separate circuit chip. Stresses in the sensor, for example residual stresses from fabrication or encapsulation of a sensor, may negatively impact the accuracy of such measurements, however. 
     To mitigate the impact of stresses, the MEMS device  301  is disposed on a stress-isolated MEMS platform  310  suspended within and from substrate  311  by bridges  330 . Alternately, or in addition, the MEMS platform may be suspended within the substrate  311  by one or more pillars  736 . In this embodiment, the MEMS device  301  is formed as part of a MEMS device layer  307 , and the MEMS layer  307  is coupled to the substrate  311  by one or more intermediate layers  306 . The MEMS layer  307  defines a MEMS layer plane  397 , parallel to the substrate layer plane  391 . 
     A stress-relief gap  340 , having a variable width  345 , separates the MEMS platform  310  from the substrate  311  and from cap  321  and cap  322 . The cap  322  may be coupled to the substrate  311  by adhesion layer  325 . More particularly, as schematically illustrated in  FIG. 3A  and  FIG. 3B , the stress-relief gap  340  includes lateral gap  341 , top gap (or “void”)  342 , and bottom gap (or “void”)  343 . The lateral gap  341  may have a nominal width  345  of about 20 to 100 micrometers, for example, and the bottom gap  353  may have a width of about several microns. The cap  321  forms a cavity  342 , which has a width sufficient to allow the moveable MEMS structure (e.g., beam  302  in the embodiment of  FIG. 3A ) to move, and may be determined according to ways known in the art. 
     As schematically illustrated in  FIG. 3A  and  FIG. 3B , the stress-relief gap  340  completely surrounds the MEMS platform  310  in every direction (i.e., in the X-axis, in the Y-axis, and in the Z-axis). The stress-relief gap may be filled with a gas, such as air or nitrogen, for example, or may be a vacuum. The MEMS platform  310  may be thought of as an island surrounded by the stress-relief gap  340 . The stress-relief gap  340  prevents physical stresses, e.g., from substrate  311 , from reaching the MEMS platform  310  because such physical stresses cannot jump across the stress-relief gap  340 . 
     The bridges  330  are flexible and configured to flex in response to stresses within, or propagating within, the substrate  311 . For example, a physical stress propagating in the X axis in substrate  311  would, in the absence of lateral gap  341 , propagate through to a portion of the substrate beneath the MEMS device  301  (i.e., to a point on the MEMS platform  310  in  FIG. 3A  and  FIG. 3B ). However, because the stress-relief gap  340  does not provide a physical path through which such stress may propagate, the stress is prevented from propagating from the substrate  311  to the MEMS platform  310 , thus mitigating that amount of stress that reaches the MEMS device  301 . 
     Several features of the bridges  330  are noteworthy. First, the bridges  330  are flexible so as to allow the stress-relief gap to expand and contract in response to stresses in the substrate  311 , and yet sufficiently rigid to suspend the MEMS support structure  310  from the substrate  311 . For example, if a stress in the substrate  311  would tend to propagate towards the MEMS platform  310 , the lateral gap  341  responds by narrowing or widening. More particularly, the stress causes the face  313  of the substrate  311  to move towards (or away from) the MEMS platform  310 , thus narrowing (or widening) the lateral gap  341 . As the lateral gap  341  narrows, the bridges  330  change shape to absorb this motion, thus allowing the face  313  to move (e.g., expand) relative to the MEMS platform  310  without forcing or causing the MEMS platform  310  to move, or at least mitigating any such induced motion of MEMS platform  310 . 
     Second, the bridges  330  are not so flexible so as to substantially bend, change shape of otherwise respond in reaction to an acceleration applied to the substrate  311 . In other words, while an accelerometer beam (e.g., beam  101 ) is suspended by flexures (e.g., flexures  109 ), and those flexures are pliable such that the flexures allow the accelerometer&#39;s beam to move fluidly in response to an applied acceleration, the bridges  330  are less flexible so as to rigidly suspend the MEMS platform  310  from the substrate  311 . In other words, the MEMS platform  301  is suspended from the substrate  311  such that when the MEMS platform  310  moves in response to stresses, it does so in a way that is not detected by the MEMS device  301 , and forces that cause a second member of a MEMS device  301  to move relative to a first member do not cause motion of the MEMS platform  310  relative to the substrate  311 . 
     In general, the flexures in a MEMS device, and the jumpers and the bridges disclosed herein, may be characterized by their resonant frequency. For example, the flexures of an accelerometer (e.g., flexures  109  in  FIG. 1 ) may be about 5 kHz, while the resonant frequency of a bridge  330  may be about 200 kHz or higher (i.e., a response ratio of 40:1). Depending on the application and the stress tolerance of the MEMS device, the response ratio may be more or less than 40:1, and may, for example, be 25:1; 30:1; 35:1; 45:1; 50:1; or 55:1, to name but a few examples. The ratio of the resonance frequency of a MEMS device flexure  109  to resonant frequency of a bridge  330  may be specified by the designer of the MEMS device, depending for example on the application for the MEMS device. The jumpers  352  are flexible and pliable. Because the bridges  330  suspend the MEMS platform  310 , the resonant frequencies of the jumpers  352  have a negligible impact on the motion of the MEMS platform  310 . 
     An embodiment of a bridge  330  is schematically illustrated in  FIG. 3C  and  FIG. 3D . In  FIG. 3C , an embodiment of a bridge  330  includes a first leg  331  and a second leg  332  that, together, have a length configured to span the stress-relief gap  340  (e.g., across lateral gap  341 , for example). The first leg  331  and the second leg  332  are coupled by a crossbeam  333 . The crossbeam  333  in this embodiment is approximately 20 micrometers wide and 300 micrometers in length. 
     In the embodiment of  FIG. 3C , the crossbeam  333  meets the first leg  331  and the second leg  331  at right angles, respectively, although in other embodiments the angles may be other than right angles. Under stress, such as when the lateral gap  341  is narrowed (i.e., the gap  345  is reduced relative to the gap  345  in  FIG. 3C ), the bridge  330  changes shape to absorb the change in the lateral gap  341 , for example as schematically illustrated in  FIG. 3D . In the embodiment of  FIG. 3D , the angles between the crossbeam  333  and the legs  331  and  332 , respectively, has change and is less than 90 degrees. As such, the bridge  330  of  FIG. 3C  and  FIG. 3D  may be referred to as a “Z” bridge. Consequently, physical stress from the substrate does cross the stress-relief gap  340 . However, the crossbeam  333  is very stiff along its length, and is substantially incompressible. The stiffness of the crossbeam  333  helps resist motion of the MEMS platform  310  in the direction of the length of the crossbeam  333 , and therefore resists compression or narrowing of a the lateral gap  341  in the direction of the length of the crossbeam  333 . 
     An alternate embodiment of a bridge  330  is schematically illustrated in  FIG. 3E , and may be known as a “U-shaped” bridge or a “top-hat” bridge. The U-shaped bridge  330  includes two legs  331  and  332 , which may define a line between the substrate  311  and the MEMS platform  310 . The U-shaped bridge  330  also includes a first crossbeam  333 A coupled to leg  331  and a second crossbeam  333 B coupled to leg  332 , and a crossbeam connector  335  coupled between the first crossbeam  333 A and second crossbeam  333 B distal from the legs  331  and  332 . 
     Generally, the bridges  330 , and the legs  331 ,  332  and crossbeams  333 ,  333 A and  333 B are fabricated from the material of the substrate  311  and are therefore in the substrate layer plane, and have a thickness  337  that is the same as the thickness  315  of the substrate, although that is not a limitation of various embodiments, and the thickness  337  could be greater or less than the thickness  315  of the substrate. Each crossbeam  333  (also  333 A and  333 B) also has a length  338  (as schematically illustrated in  FIG. 3F , (although only crossbeam  333  is schematically illustrated in  FIG. 3F ) that is, generally, longer than the length  338  of legs  331  and  332  as schematically illustrated in  FIG. 3G  (although only leg  331  is schematically illustrated in  FIG. 3G ). In some embodiments, the length  339  of a crossbeam is 2 or more times the length of the leg  331  or leg  332 , and in some embodiments the length of the crossbeam may by 3, 4, 5, 6, or 10 times or more the length of the leg  331  or leg  332 . Generally, the longer the length  338  of the crossbeam, the greater the flexibility of the bridge  330 . 
     The width  336  of crossbeams  333 ,  33 A,  333 B and the width  334  legs  331 ,  332 , as schematically illustrated in  FIG. 3H  for example, may be 20 micrometers, for example. 
     In some embodiments, a bridge  330  may include an electrical conductor (such as electrical conductor  355  as schematically illustrated in  FIGS. 3F and 3G , for example) coupled to the MEMS device  301  and configured to conduct electrical signals across the stress-relief gap  340 . As such, the conductor  355  provides and electrical connection as part of the substrate layer  311 . 
       FIG. 3I  schematically illustrates a substrate  311  and identifies a peripheral region  360  on the surface  319  of the substrate  311 . The peripheral region  360  circumscribes the MEMS platform  310  and the stress-relief gap  340 , and defines an area on the surface  319  of the substrate  311  suited for bonding of a cap, as described below, because, for example, the surface  319  within the peripheral region  360  if free of obstructions and gaps  340 , so that cap bonded to the surface  319  within the peripheral region  360  may make a robust and preferably hermetic seal with the substrate  311 . If the substrate  311  includes integrated circuitry  350 , that integrated circuitry  350  may be disposed within the inner periphery  361  of the peripheral area  360 , or disposed external to the inner periphery  361  of the peripheral area  360  as schematically illustrated in  FIG. 3I  for example. The surface  319  of the substrate  311  external to the peripheral area may be referred to as a shoulder  365 , and may provide a space for bond pads  354  and/or circuits  350 , for example. In an alternate embodiment, the MEMS device layer  307  may include such a peripheral region  362 , and a cap may be coupled to the peripheral region  362  on the MEMS device layer  307 , for example as schematically illustrated in  FIG. 3A . 
       FIG. 3I  also schematically illustrates an optional jumper peninsula  370  extending into the stress-relief gap  340  from the substrate  311 , thereby decreasing the width  346  of the gap  340 . This narrower portion of the gap  340  provides a span for jumpers  352  that is shorter than the nominal width  345 , thus reducing the length of the jumpers  352 . A shorter length  359  for jumpers  352  results in less physical stress on the jumpers  352  as the stress-relief gap  340  changes. 
     An alternate embodiment of a jumper  352  is schematically illustrated in  FIG. 3L , in which two such jumpers are shown. Each jumper  352  has two end regions  381  and  382  at opposite ends, and a mid-section  383  between the end regions  381  and  382 . The mid-section  383  optionally includes one or more jumper apertures  386  that pass all the way through the jumper  352 , to provide a passage by which etchant can flow through the jumper  352  for purpose of etching a substrate or other material at the opposite side of the jumper  352 . In the embodiment of  FIG. 3L , the jumper  352  includes a flexure  385 , disposed between the end region  381  and the mid-section  383 . The flexure  385  allows the jumper  352  to change its length  359  in response, for example, to stress-induced displacement of a MEMS platform  310 . In the embodiment of  FIG. 3L , the flexure  385  has a serpentine shape, but in other embodiment may have a variety of other shapes, such as the the shape of closed box flexure  109  in  FIG. 1B  for example. 
       FIG. 4  schematically illustrates a MEMS platform  310  coupled to a substrate  311  by a eight bridges  330 . As shown in  FIG. 4 , the crossbeam  333  meets the legs  331 ,  332  at an angle of less than 90 degrees, for example because the device  300  is under physical stress, or because the bridges are fabricated to have such an angle even in the absence of such stress (e.g., in their nominal positions). 
     Some embodiments also include electrically-conductive gap-spanning jumpers  352  that span the stress-relief gap to provide power, ground and signal connections to the MEMS device  301  on the MEMS platform  310 . For example, as schematically illustrated in  FIG. 3B , the sensor  300  includes one or more jumpers  352  that electrically couple to conductor or bus  351 , which is electrically coupled to integrated circuit  350 .  FIG. 3J  and  FIG. 3K  schematically illustrate an embodiment of a jumper  352  having a length  359  and a width  358 , and a thickness  357 . In some embodiments, the jumper  352  is conductive, and in some embodiments the jumper  352  includes a conductive layer  355 . Generally, the thickness  357  of a jumper  352  is substantially less than the thickness  337  of a bridge  330  and substrate  311 . The length  359  of the jumper  352  is sufficient to span the stress-relief gap  340  at the point where the jumper  352  crosses the gap  340 . In an illustrative embodiment, a jumper  352  has a length  359  of 100 microns, a width  358  of 16 microns, and a thickness  357  of 8 microns. 
     In some embodiments, bridges  330  couple between the MEMS platform  310  and the inside corner  380  of the surrounding substrate  311 , as schematically illustrated in  FIG. 3M ,  FIG. 3N ,  FIG. 3O  and  FIG. 3P , for example.  FIG. 3M  schematically illustrates two bridges  330 , each having a shape as schematically illustrated in  FIG. 3C . Each bridge couples to the surrounding substrate  311  at or near the inside corner  380 , and couples to the MEMS platform  310  at a point distal from the corner  380 . The embodiment of  FIG. 3O  schematically illustrates two bridges  330  as in  FIG. 3M , and includes an “L” shaped stress-relief notch  3303  inside of the corner formed by the two bridges  330 . 
       FIG. 3N  schematically illustrate two bridges  330 , each having an “L” shape, in which one end couples to the surrounding substrate  311  at or near the inside corner  380 , and the other end couples to the MEMS platform  310  at a point distal from the corner  380 . 
       FIG. 3P  schematically illustrates a single curved, or “hook shaped” bridge  330 , having a straight end  3301  coupled to the MEMS platform  310 , and a curved end  3302  coupled the surrounding substrate  311 . 
     Some embodiments include one or more “U”-shaped bridges, wherein the open-end of the “U” is coupled to the surrounding substrate  311 , and the closed-end of the “U”  3304  is coupled to the MEMS platform  310 , and schematically illustrated in  FIG. 3Q  for example. In the embodiment of  FIG. 3Q , one of the open-ends  3305  of the “U”-shaped bridge is coupled to the surrounding substrate  311  at or near the corner  380 . 
     In some embodiments, bridges  330  couple nearer the center of MEMS platform  310 , and have one or more jumpers  352  coupled between the MEMS platform  310  and the surrounding substrate  311  and disposed between the bridges  330  and the corners  380 , as schematically illustrated in  FIG. 3R , for example. In  FIG. 3R , one end of the each bridge  330  is coupled to the surrounding substrate  311  at or near a point that is across from the center point of an opposing edge of the MEMS platform  310 , while distal end of each bridge  330  couples to the MEMS platform  310  at points distal from such center point. In this configuration, the ends of the each bridge  330  that couples to the substrate  380  are very close to each other therefore the strain transferred from substrate  380  to the bridges are minimal. One or more jumpers  352  span the gap between the MEMS platform  310  and the surrounding substrate  311  at points between the bridges  310  and the inside corner  380 . In some embodiments, the bridges are disposed symmetrically about X and Y axis to mitigate stress mismatch. 
     An alternate embodiment of a MEMS device is schematically illustrated in  FIG. 3S  and  FIG. 3T , and includes an accelerometer  100  on the MEMS platform  310 . As shown in  FIG. 3S , and  FIG. 3T , in order for a physical stress from substrate  311  to reach beam  101 , the stress would have to cross bridge  330  and flexure  109 . As such, the bridge  330  and the flexure  109  may be described as being arranged or disposed in series with one another. In some embodiments, such as in  FIGS. 3S, and 3T , the bridge  330  and flexure  109  are not in the same plane (i.e., bridge  330  is in a different X-Y plane than flexure  109 ), so the bridge  330  and flexure  109  are not along a linear path, and any stress traveling from the substrate  311  to the beam  101  could not do so by traveling in a straight line from substrate  311  to beam  101 . In this configuration, as in the other embodiments described herein, the stress would induce a physical distortion in, and thus be absorbed by, the bridge  330 , so little if any of the physical stress would reach flexure  109 . 
     Some embodiments also include one or more conductors or busses  351  that electrically coupled the integrated circuit  350  to bond pads  354 . Alternately, or in addition, some embodiments include one or more jumpers  352  and conductors  351  that electrically couple to one or more bond pads  354 . The jumpers  352  are short and are as flexible as, or more flexible than, the bridges  330 , so as not to impede the relative motion of the substrate  311  and MEMS platform  310 . 
       FIG. 5  schematically illustrates an encapsulated MEMS sensor  500 . For illustrative purposes, sensor  500  includes a capped MEMS device schematically illustrated as device  300 , but alternate embodiments could include any of a variety of MEMS devices, such as any of the embodiments described herein. 
     The encapsulated sensor  500  includes a lead frame  501  having a paddle  502  and leads  503 . Each of the leads  503  is electrically isolated from the paddle  502 . 
     The MEMS device  300  is physically coupled to the paddle portion  502  of the lead frame  501 , and is electrically coupled to the leads  503  by one or more wirebonds  509 . The integrated device  300 , paddle  502 , one or more wirebonds  509 , and a portion of each lead  503  are encapsulated in encapsulant  507 . The packaged sensor  300  may be mounted to a substrate  508  by leads  503  extending to the outside of the encapsulant  507 . 
     The material properties of the encapsulant (i.e., mold compound for example, as known in the art) such as Young&#39;s modulus and coefficient of thermal expansion (CTE) vary largely with the temperature. The Young&#39;s modulus of the mold compound changes more than 2 orders of magnitude over the temperature of −50 C to 150 C. Specifically, it changes from being a stiff material (E=24 GPa) in low temperatures (−50 C to 25 C) to a soft material (E=0.8 GPa) above 100 C. Also, its CTE changes more than three times over this temperature range (from 12e-6/C to 38e-6/C) and it is greatly higher than the CTE of Silicon (2e-6/C). As a result, large thermal stresses are generated and transferred to the MEMS sensor  300 . This creates large sensitivity drift in the MEMS sensor over the temperature range of 175 C to −40 C. This issue is more pronounced in the MEMS sensors based on the capacitive transductions and it is desirable to address this issue for the sensors having capacitive gaps of less than or equal to 1 micrometer. 
     An embodiment of a method  600  of fabricating a chip or wafer with a stress-relief trench is illustrated in  FIG. 6A , and partial views of a device  799  at various stages of fabrication are schematically illustrated in  FIGS. 7A-7F . The views in  FIGS. 7A-7F  show only one end of a MEMS device  799 . Although method  600  and  FIGS. 7A-7F  schematically illustrate fabrication of MEMS device  799  on a silicon wafer  700 , a person of ordinary skill in the art would be able to adapt the steps of method  600  to the fabrication of a MEMS device using an SOI wafer, for example by fabricating a MEMS island (e.g., MEMS island  740 ) as with the wafer  700  below. 
     The method  600  begins at step  601  with the provision of a wafer  700 , which may be similar to wafer  311 , and may be a silicon wafer as known in the art, for example. The wafer may have a thickness  707  of 780 microns, for example. In  FIG. 7A , only a portion of the wafer  700  is shown. More specifically, the portion schematically illustrated in  FIG. 7A  is only a part of a MEMS device, and the wafer  700  includes many other identical MEMS devices. 
     Next, at step  602 , the method forms an unreleased MEMS structures. As known in the art, a MEMS device includes at least one member (e.g., a beam or proof mass) that is movable with respect to another feature (e.g., a substrate or a fixed finger). In fabricating MEMS devices, however, it is known to fabricate the moveable member that is, at an intermediate stage of fabrication, not yet movable with respect to the substrate. Such a MEMS structure may be described as “immobile” or “immobilized.” 
     For example, in  FIG. 7A , after forming other features, such as polysilicon layer  745 , and inter-poly nitride  750 , step  602  includes depositing a layer  710  of oxide, and patterning the oxide layer  710 , by methods known in the art, to form oxide region  713 . The method then deposits polysilicon beam layer  770 , from which MEMS structures  771  and  772 , are fabricated. MEMS structures  771  and  772  are in physical and electrical contact with polysilicon layer  745  and inter-poly nitride layer  750 , respectively, to provide electrical communication between the MEMS structures  771  and  772  and terminals or circuits. At this point in the process, the MEMS structures  771  and  772  are not movable with respect to the wafer  770  because they are secured by oxide region  713 . In this way, additional processing may be performed while the MEMS structures  771  and  772  are immobilized. 
     Step  602  also includes patterning the beam layer  770  to leave a polysilicon span  779 , to form a jumper that spans a gap, such as jumper  352  in  FIG. 3B  for example. Such a span  779  is schematically illustrated in  FIG. 7B , for example. 
     The method  600  next forms the gap  781 , at step  603 , as schematically illustrated in  FIG. 7C  and  FIG. 7D . To etch the gap  781 , the layer of oxide  710 , which is approximately 2 microns thick, is patterned to provide an opening  719  in the desired location of the gap  781 . The opening  719  in the oxide  710  may have a width  704  of about 20 microns, for example. Then the gap  781  may be etched by, for example, deep reactive-ion etching (“DRIE”) through the opening  719 . The gap  781  will have a width  709  approximately equal to the width  704  of the opening  719 . 
     The gap  781  defines the island  740  (which will ultimately be a MEMS platform, e.g., platform  310 ). The process of forming the gap  781  does not, however, remove all of the substrate within the area of the gap  781 . Rather, portions of the substrate material are left in place to form bridges, such as bridges  330  in  FIG. 3B , for example. 
     Optionally, at step  604 , the wafer  700  is flipped and the backside  703  of the wafer  700  is ground. The grinding at step  605  reduces the thickness  707  of the wafer  700 . 
     Next, step  605  bonds a backside cap  730  to back surface  706  of the wafer  700 , as schematically illustrated in  FIG. 7E . The cap  730  may be silicon, and may be bonded to the wafer  730  by a silicon-silicon fusion bond, or by a metal-metal bond, such as an Au—Au bond or a GE-AL bond, as known in the art. 
     The backside cap  730  may be hermetically sealed to the back surface  706  around the periphery  732  of the backside cap  730 . The periphery  732  of the backside cap  730  circumscribes the island  740 , so that the island  740  will be physically isolated from physical stress from the periphery  732  of the backside cap  730 . For example, if thermal stress causes the backside cap  730  to expand in along the X axis, that stress will not directly couple to the island  740 . 
     The backside cap  730  defines a backside cavity  735  (corresponding to cavity  353  in  FIG. 3A , for example) between the back surface  706  and an inside surface  731  of the backside cap  730 , to further physically isolate the island  740  from stresses of the backside cap  730 . In some embodiments, there is no physical coupling directly between the cap  730  and the island  740 . In other embodiments, one or more optional pillars  736  may extend between the inner surface  731  of the backside cap  730  and back surface  706  of the wafer  700 , to provide support for the island  740  from the backside cap and contribute to defining and maintaining the backside cavity  735 . In some embodiments, the pillar or pillars  736  may be an integral part of the backside cap  730 . 
     At step  606 , the method  600  removes the oxide at oxide region  713 , by methods and processes known in the art. Removal of the top oxide layer  713  releases the MEMS structure (e.g.,  771 ,  772 ) so that those structures are subsequently moveable with respect to the wafer  700 . 
     A top cap  790  is bonded to the wafer  700  at step  607 , to seal the device. The top cap  790  does not have physical contact with the island  740 , but is coupled to the wafer  700  so as to circumscribe the gap  781 . 
     The top cap  790  may be electrically conductive, and may be silicon for example, and may be bonded to the substrate  700  through metal-to-metal bonding. In some embodiments, the cap  790  may be a part of a cap wafer of similar dimensions to the wafer  700 . Such a cap wafer may have many cap sections, each cap section corresponding to a device on the wafer  700  to be capped. The outer periphery  791  of the top cap  790  may be thinner (e.g., in the Z axis as schematically illustrated in  FIG. 7F ) than other portions of the cap  790  to facilitate dicing, or grinding to open. 
     In the embodiment of  FIG. 7F , the top cap  790  is bonded to the wall  775 . The top cap  790  may be bonded to the wafer  770  or wall  775  in a variety of ways known in the art. In  FIG. 7F , for example, the top cap  790  is bonded to the wall  775  by an eutectic metal bond  792 , such as an AL-GE bond for example. 
     Finally, as step  608 , the wafer and its caps are diced to into individual, capped MEMS devices. 
     An alternate embodiment of a method  650  of fabricating a chip or wafer with a stress-relief trench is illustrated in  FIG. 6B , and partial views of a device  799  at various stages of fabrication are schematically illustrated in  FIGS. 8A-8P . The views in  FIGS. 8A-8P  show only one end of a MEMS device  799 . Although method  650  and  FIGS. 8A-8P  schematically illustrate fabrication of MEMS device  799  on a silicon wafer  700 , a person of ordinary skill in the art would be able to adapt the steps of method  650  to the fabrication of a MEMS device using an SOI wafer, for example by fabricating a MEMS island (e.g., MEMS island  740 ) as with the wafer  700  below. 
     The method  650  begins at step  651  with the provision of a wafer  700 , which may be similar to wafer  311 , and may be a silicon wafer as known in the art, for example. The wafer may have a thickness  707  of 780 microns, for example. In  FIG. 8A , only a portion of the wafer  700  is shown. More specifically, the portion schematically illustrated in  FIG. 8A  is only a part of a MEMS device, and the wafer  700  includes many other identical MEMS devices. 
     At step  652 , a trench  705  is etched into the top surface  701  of the wafer  700 . In some embodiments, the trench  705  has a depth  708  more than half of the thickness of the wafer  700 , but less than the total thickness  707  of the wafer  700 . For example, in the embodiment of  FIG. 8A , the wafer  700  has a thickness  707  of about 780 microns, and the depth  708  of the trench  705  is approximately 500 microns. The trench  705  circumscribes a portion of the wafer  700  to form a MEMS island  740 , as illustrated for example in the way that stress-relief gap  340  circumscribes MEMS platform  310  in  FIG. 3B . The trench  705  is not continuous, in that some portions of the wafer  700  are left to span between the island  740  and the remaining portions of the wafer  700 . Such remaining portions form bridges, such as bridges  330  in  FIG. 3B , for example. 
     To etch the trench  705 , a layer of oxide  710  approximately 2 microns thick may be grown on the top surface  702  of the wafer  700 , and then patterned to provide an opening  719  in the desired location of the trench  705 . The opening  719  in the oxide  710  may have a width  704  of about 20 microns, for example. Then the trench  705  may be etched by, for example, deep reactive-ion etching (“DRIE”) through the opening  719 . The trench  705  will have a width  709  approximately equal to the width  704  of the opening  719 . 
     At step  653 , additional oxide  711  is grown on the walls  715  of the trench  705  to line the trench  705 , as schematically illustrated in  FIG. 8B . The additional oxide  711  may be about 2 microns thick, and may effectively merge with the initial oxide layer  710  to become a contiguous part of oxide layer  710 . As used herein, to “line” the trench  705  means to apply a thin layer of oxide to the walls  715  of the trench, but without filling the trench  705 . In other words, the lined trench  705  is still a trench in the wafer  700  having a width  704  of approximately 16 microns. 
     Next, a layer of sacrificial polysilicon  720  is deposited on the oxide layer  710  at step  654 , to cover the wafer  700  and fill the trench  705 . As schematically illustrated in  FIG. 8C  the layer of sacrificial polysilicon  720  may be approximately 10 microns thick. The sacrificial polysilicon  720  is then etched at step  655  to remove the sacrificial polysilicon  720  over the top side  701  of the wafer, but leave the sacrificial polysilicon  720  filling the trench  705 , as schematically illustrated in  FIG. 8D . That body of remaining sacrificial polysilicon  720  filling the trench  705  may be referred to as the “plug”  721 . 
     Next the process  650  turns to the backside  703  of the wafer  700 . At step  655 , the wafer  700  is flipped and the backside  703  of the wafer  700  is ground. The grinding at step  655  reduces the thickness  707  of the wafer  700  to expose the sacrificial polysilicon  720  in the trench  705  at the backside of the wafer  700 , as schematically illustrated in  FIG. 8D . In the present embodiment, since the wafer  700  is approximately 780 microns thick, and the trench  705  is approximately 500 microns deep, extending from the topside  701  of the wafer, the grind at step  655  removes at least 280 microns of wafer (i.e., 780 microns initial thickness-280 microns removed by grinding=500 microns residual wafer thickness) to expose the sacrificial polysilicon  720 . The grinding  655  leaves an exposed back surface  706  of the wafer  700 , except where the polysilicon  720  and trench-lining oxide  711  are exposed. 
     Next, step  656  bonds a backside cap  730  to back surface  706  of the wafer  700 . The cap  730  may be silicon, and may be bonded to the wafer  730  by a silicon-silicon fusion bond, or by a metal-metal bond, such as an Au—Au bond or a GE-AL bond, as known in the art. 
     The backside cap  730  may be hermetically sealed to the back surface  706  around the periphery  732  of the backside cap  730 . The periphery  732  of the backside cap  730  circumscribes the island  740 , so that the island  740  will be physically isolated from physical stress from the periphery  732  of the backside cap  730 , after the sacrificial polysilicon  720  is removed (described below). For example, if thermal stress causes the backside cap  730  to expand in along the X axis, that stress will not directly couple to the island  740 . 
     The backside cap  730  defines a backside cavity  735  (corresponding to cavity  353  in  FIG. 3A , for example) between the back surface  706  and an inside surface  731  of the backside cap  730 , to further physically isolate the island  740  from stresses of the backside cap  730 . In some embodiments, there is no physical coupling directly between the cap  730  and the island  740 . In other embodiments, one or more optional pillars  736  may extend between the inner surface  731  of the backside cap  730  and back surface  706  of the wafer  700 , to provide support for the backside cap and contribute to defining and maintaining the backside cavity  735 . In some embodiments, the pillar or pillars  736  may be an integral part of the backside cap  730 . With the sensor substrate  700  supported by the pillars  736 , the bridges  330  could be omitted or removed. 
     Returning to the top side of the wafer, conductive routing traces  745  are deposited on the oxide layer  710  at step  657 . Such traces  745  are schematically illustrated in  FIG. 8E , and may be polysilicon or metal, to name but a few examples. 
     An additional layer of oxide  712  is added at step  658 , to cover the traces  745 , as schematically illustrated in  FIG. 8F . The additional layer of oxide  712  also covers the top of the polysilicon plug  721 , and integrates with and becomes a part of polysilicon layer  710 . The additional layer of oxide  712  may protect the traces  745  during subsequent fabrication steps. 
     At step  659 , some of the oxide  710  (which includes some of the oxide  712 ) is removed to expose some of the top side  701  of the wafer  700  around the trench  705 , as schematically illustrated in  FIG. 8G . As such, the oxide  711  lining the trench  705  is no longer integrated with the oxide layer  710  and additional oxide layer  712 . Next, at step  660 , a layer of nitride  750  is added to cover the oxide layer  710  and at least a portion of the exposed top surface  701  of the wafer  700 , as schematically illustrated in  FIG. 8H . The nitride layer  750  is approximately 1200 angstroms thick, and seals and passivates the oxide  710 . 
     Another, second, layer of polysilicon  755  is added at step  661 , to further protect the oxide  710  and the inter-poly  750 . An embodiment of the second polysilicon layer  755  is schematically illustrated in  FIG. 8I . The second polysilicon layer  755  may be conductive and may also provide electrical interconnections between features of the device. 
     A top layer of oxide  713 , as schematically illustrated in  FIG. 8J , is added at step  662 , and covers the second polysilicon layer  755 , any exposed portions of the inter-poly nitride layer  750 , as well as any exposed portions of the top surface  701  of the wafer  700 , and the polysilicon plug  721 . The top layer of oxide  713  may integrate with the trench-lining oxide  711 . However, the top oxide layer  713  does not integrate with the initial oxide layer  710 , or the later oxide layer  712 , because those layers are now isolated by one or more of the inter-poly nitride layer  750  and the second polysilicon layer  755 . 
     At step  662 , one or more anchor passages  761 ,  762  and  763 , are etched through the top oxide layer  713  to an underlying polysilicon layer, such as portions of polysilicon layers  745  and  755 , as schematically illustrated in  FIG. 8K . The anchor passages  761 ,  762  and  763  provide access for physical and electrical connections between to those portions of polysilicon layers  745  and  755  and MEMS structures described below. 
     Next, at step  663 , a beam layer  770  of polysilicon is deposited on the top oxide layer  713 , as schematically illustrated in  FIG. 8L . The beam layer  770  may be 8 to 16 microns thick, and initially covers the top oxide layer  713 , and fills the passages  761 ,  762 , and  763 , to couple to the polysilicon layers  745  and  755 . The beam layer is patterned to form MEMS structures (e.g.,  771 ,  772 ) and an outer wall  775  that circumscribes the trench  705 . 
     To form a jumper that spans a gap, such as jumper  352  in  FIG. 3B  for example, some embodiments pattern the beam layer  770  to leave a polysilicon span  779  across the trench  705 , as schematically illustrated in  FIG. 8P , for example. The polysilicon span  779  will remain after the polysilicon plug  721 , and the polysilicon in the release aperture  765 , and the portion of the top oxide layer  713  are removed, as described below. The polysilicon span  779  will be protected and will not be etched away during the subsequent polysilicon plug  721  etching steps. The jumper  352  formed from the polysilicon span  779  may electrically couple to a MEMS structure (e.g.,  771 ) or to trace  745  for example. 
     The polysilicon plug  721  is removed at step  664  by masking all other portions of polysilicon on top oxide layer  713 , for example with a photo resist, and etching an aperture  765  through the oxide layer  710  to expose the polysilicon plug  721 . The polysilicon plug is then etched away, by application of xenon di-fluoride, for example. The etching removes the polysilicon plug  721 , leaving the gap  781  between the island  740  and the body of the wafer  700 , as schematically illustrated in  FIG. 8M . At this point, the island  340  is suspended from, and moveable with respect to, the rest of the wafer  700 , and may be described as a MEMS platform, as MEMS platform  310  in  FIG. 3A . 
     The MEMS structures  771 ,  772  and outer wall  775  are not etched by this step. After step  664 , the masking layer is removed at step  665 . 
     At step  666 , the trench-lining oxide  711  is removed, as well as the top oxide layer  713 , by methods and processes known in the art, such as vapor HF etching. Removal of the top oxide layer  713  releases the MEMS structure (e.g.,  771 ,  772 ) so that those structures are subsequently moveable with respect to the wafer  700 . 
     A top cap  790  is bonded to the wafer  700  at step  667 , to seal the device. The top cap  790  does not have physical contact with the island  740 , but is coupled to the wafer  700  so as to circumscribe the gap  781 , as described in connection with peripheral region  360 , above. 
     The top cap  790  may be electrically conductive, and may be metal or silicon for example. In some embodiments, the cap  790  may be a part of a cap wafer of similar dimensions to the wafer  700 . Such a cap wafer may have many cap sections, each cap section corresponding to a device on the wafer  700  to be capped. The outer periphery  791  of the top cap  790  may be thinner (e.g., in the Z axis as schematically illustrated in  FIG. 8N ) than other portions of the cap  790  to facilitate dicing. 
     In the embodiment of  FIG. 8N , the top cap  790  is bonded to the wall  775 . The top cap  790  may be bonded to the wafer  770  or wall  775  in a variety of ways known in the art. In  FIG. 8N , for example, the top cap  790  is bonded to the wall  775  by a eutectic metal bond  792 , such as an AL-GE bond for example. 
     Finally, as step  668 , the wafer and its caps are diced to into individual, capped MEMS devices. Such a device  799  is schematically illustrated in  FIG. 8O , for example. 
     In an alternate embodiment, the MEMS platform  310  is suspended within the substrate  311  without bridges  330 , but by one or more pillars  736 . Such an embodiment may be schematically illustrated by  FIG. 8D  for example. Such an embodiment may be fabricated according to the process  650  of  FIG. 6A , except that the etching of the trench (e.g., at step  652 ) does not form bridges  330 . 
     In some embodiments, the pillar  736  may be a cluster of two or more pillars disposed near the center  737  of MEMS platform  310 , as schematically illustrated in  FIG. 7G  and  FIG. 7H , for example. In an embodiment having a cluster of pillars  736 , the distance  738  between the pillars may be small, such as less than five percent of the length of the shortest edge of the MEMS platform  310 . In such an embodiment, thermal expansion of the MEMS platform  310  or cap  730  will have a reduced effect on the movement of the pillars  736  in the cluster because the distance between them is relatively small, and so the opportunity for expansion (e.g., along the X axis) is correspondingly small. 
     The inventors have discovered that MEMS products having a stress-isolated MEMS platform, as described above, show a marked improvement in die stress, as shown in the graph  900  in  FIG. 9 , in which the horizontal axis describes process technologies and the vertical axis is logarithmic, and shows die stress in MPa over a temperature range of 165 degrees centigrade. 
     Points along the upper curve  910  indicate die stress in a MEMS product encapsulated in an overmold package. Point  911  shows a die stress of almost 100 MPa for a MEMS device in an overmold package. Point  912  shows a die stress of somewhat less than 100 MPa for a similar MEMS device having 100 micrometer-deep stress-relief trench, where a trench—unlike a stress-relief gap  340  described herein—does not extend all the way through the substrate. Point  913  shows a die stress of slightly more than one MPa for a MEMS product having a stress-relief gap  340 , such as those described above for example. As shown, the stress-relief gap provides substantially more relief from die stress than the prior art technologies. 
     Points along lower curve  920  indicate die stress in a MEMS product in a cavity package. Point  921  shows a die stress of slightly more than 1 MPa for a MEMS device in a cavity package. Point  922  shows a die stress of somewhat less than 1 for a similar MEMS device having 100 micrometer-deep stress-relief trench, where a trench—unlike a stress-relief gap  340  described herein—does not extend all the way through the substrate. Point  923  shows a die stress of about 0.01 MPa for a MEMS product having a stress-relief gap  340 , such as those described above for example. As shown, the stress-relief gap provides substantially more relief from die stress than the prior art technologies. 
     Various embodiments of the present invention may be characterized by the potential claims listed in the paragraphs following this paragraph (and before the actual claims provided at the end of this application). These potential claims form a part of the written description of this application. Accordingly, subject matter of the following potential claims may be presented as actual claims in later proceedings involving this application or any application claiming priority based on this application. Inclusion of such potential claims should not be construed to mean that the actual claims do not cover the subject matter of the potential claims. Thus, a decision to not present these potential claims in later proceedings should not be construed as a donation of the subject matter to the public. 
     Without limitation, potential subject matter that may be claimed (prefaced with the letter “P” so as to avoid confusion with the actual claims presented below) includes: 
     P1. A MEMS device comprising: 
     a substrate layer comprising a substrate; 
     a MEMS platform within the substrate layer and defining a stress-relief gap between the MEMS platform and the substrate; 
     a MEMS structure movably suspended from the MEMS platform by at least one flexure, the at least one flexure having a flexure resonant frequency; 
     a plurality of bridges disposed between the substrate and the MEMS platform and spanning the stress-relief gap between the substrate and the MEMS platform, each of the plurality of bridges having a bridge resonant frequency, the flexure resonant frequency having a ratio to the bridge resonant frequency of at least 25:1 such that the bridges are configured to rigidly suspend the MEMS platform from the substrate; and 
     at least one flexible electrically conductive jumper electrically coupled to the MEMS structure on the MEMS platform, and spanning the stress-relief gap between the substrate and the MEMS platform, the flexible electrically conductive jumper configured to carry an electrical signal across the stress-relief gap. 
     P10. A MEMS device comprising: 
     a substrate layer defining a substrate layer plane, and comprising a substrate and a MEMS platform within the substrate layer, the MEMS platform defining a stress-relief gap within the substrate layer between the MEMS platform and the substrate; 
     a MEMS layer defining a MEMS layer plane parallel to, but offset from, the substrate layer plane, the MEMS layer comprising a MEMS structure movably suspended from the MEMS platform by at least one flexure, the at least one flexure in the MEMS layer plane, the at least one flexure having a flexure resonant frequency; 
     a plurality of bridges within the substrate layer plane and disposed between the substrate and the MEMS platform and spanning the stress-relief gap between the substrate and the MEMS platform, each of the plurality of bridges having a bridge resonant frequency, the flexure resonant frequency having a ratio to the bridge resonant frequency of at least 25:1 such that the bridges are configured to rigidly suspend the MEMS platform from the substrate; and 
     at least one flexible electrically conductive jumper electrically coupled to the MEMS structure on the MEMS platform, and spanning the stress-relief gap between the substrate and the MEMS platform, the flexible electrically conductive jumper configured to carry an electrical signal across the stress-relief gap. 
     P30. A MEMS device comprising: 
     a substrate layer comprising a substrate; 
     a MEMS platform within the substrate layer and defining a stress-relief gap between the MEMS platform and the substrate; 
     a MEMS structure movably suspended from the MEMS platform by at least one flexure; 
     at least one flexible electrically conductive jumper electrically coupled to the MEMS structure on the MEMS platform, and spanning the stress-relief gap between the substrate and the MEMS platform, the flexible electrically conductive jumper configured to carry an electrical signal across the stress-relief gap; 
     a bottom cap coupled to the substrate and defining a bottom void between the bottom cap and the MEMS base; 
     a top cap coupled to the substrate such that the MEMS base is disposed between the top cap and the bottom cap, and defining a top void between the top cap and the MEMS base; and 
     at least one pillar spanning a bottom stress-relief gap between the bottom cap and the MEMS platform. 
     The embodiments of the invention described above are intended to be merely exemplary; numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.