Abstract:
MEMS mass-spring-damper systems (including MEMS gyroscopes and accelerometers) using an out-of-plane (or vertical) suspension scheme, wherein the suspensions are normal to the proof mass, are disclosed. Such out-of-plane suspension scheme helps such MEMS mass-spring-damper systems achieve inertial grade performance. Methods of fabricating out-of-plane suspensions in MEMS mass-spring-damper systems (including MEMS gyroscopes and accelerometers) are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application No. 61/181,565 filed on May 27, 2009, the subject matter of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to microelectromechnical (MEMS) mass-spring-damper (MSD) systems generally. More specifically, the present invention relates to MEMS MSD systems, including MEMS gyroscopes and accelerometers, that are inertial grade and/or that use an out-of-plane (or vertical) suspension scheme and a method for fabricating such MEMS MSD systems. 
     BACKGROUND OF THE INVENTION 
     MEMS sensors, such as MEMS gyroscopes and accelerometers, are known in the art. In the past few decades, such sensors have drawn great interest. MEMS technology is attractive because, among other reasons, it enables efficient packaging, minimizes sensor area, and significantly reduces power consumption. Further, more specifically, MEMS sensors can be easily integrated with driving and sensing electronics (CMOS-compatible), such that everything can be packaged on the same chip. 
     Prior art MEMS sensors typically operate in the rate grade. In other words, generally speaking, such MEMS sensors have a rate resolution greater than 0.1°/hr 1/2 , and require 100 μg for the resolution of detection. Rate grade sensors are useful in certain applications, such as in airbag deployment systems, vehicle stabilization systems, and navigation systems in the automotive industry. But in applications where greater sensor sensitivity is required, rate grade sensors may not be suitable. For example, in applications in the space industry (such as Picosatellites and planetary landers), inertial grade sensors should be used. Inertial grade sensors generally have a rate resolution less than 0.001°/hr 1/2 , and may require fewer than 4 μg for the resolution of detection. 
     Prior art MEMS sensors may typically operate in the rate grade due to the configuration of suspensions in the sensor. Typically, such MEMS sensor use in-plane (or horizontal) suspensions (which may be attributable, at least in part, to the fact that such configuration makes it easier and more cost-effective to fabricate such MEMS sensors). The use of in-plane suspensions, however, makes it difficult to obtain inertial grade operation. This may be due to a number of reasons. For example, with such configuration, the suspensions and the proof mass are geometrically coupled to one another. In other words, the dimensions of the suspensions cannot be modified without affecting the geometry of the proof mass. Such configuration also limits the proof mass area fill factor of the sensor (or, in other words, the ratio between the area occupied by the proof mass and the total area of the sensor). This may in turn limit the potential size of the proof mass. Reducing the size of the proof mass may result in, among other things, a degraded Brownian noise floor, an increase in the minimum detectable angular rate, and a worsening of output signal sensitivity to input angular rate, as well as a decease in signal-to-noise ratio (SNR). Further, in such arrangement, out-of-plane deflection may be suppressed, which may, in certain instances, detrimentally affect performance. 
     The use of out-of-plane suspensions in MEMS sensors, however, significantly improves sensor performance, enabling MEMS sensors to achieve inertial grade operation. Such configuration may do so for a number of reasons. For example, the configuration decouples the suspensions from the proof mass, allowing the dimensions of the suspensions to be optimized without affecting the space available for the proof mass, and, further, significantly improves the proof mass area fill factor of the sensor, as well as the volume fill factor. Such configuration permits a larger proof mass and reduces the resonance frequency and Brownian noise floor, as well as improves the mechanical quality factor, the output signal sensitivity to input angular rate, and SNR. 
     For the aforementioned reasons and others, there is a need in the art for MEMS sensors (including MEMS gyroscopes and accelerometers) that are inertial grade and/or that use out-of-plane (or vertical) suspensions, as well a method for fabricating such MEMS sensors. 
     SUMMARY OF THE INVENTION 
     Novel MEMS MSD systems, which use out-of-plane suspensions, are presented. In some embodiments, such MEMS MSD systems may be inertial grade. 
     An embodiment of a MEMS gyroscope of the present invention is also presented, and may be comprised of a shared proof mass, one or more anchors, one or more movable combs, and a plurality of suspensions, wherein said suspensions are out-of-plane with said shared proof mass. In some embodiments, such MEMS gyroscope may be inertial grade. 
     An embodiment of a MEMS accelerometer of the present invention is also presented, and may be comprised of a proof mass, one or more anchors, and a plurality of suspensions, wherein said suspensions are out-of-plane with said proof mass. In some embodiments, such MEMS accelerometer may be inertial grade. 
     A manufacturing process for fabricating out-of-plane suspensions in MEMS MSD systems is also presented. In an embodiment of such fabrication process, the first two sides of an out-of-plane suspension may be realized by etching from a top surface of a substrate, and the other two sides of said out-of-plane suspension may be realized by etching from a bottom surface of said substrate. Embodiments of fabrication processes for embodiments of a MEMS gyroscope and a MEMS accelerometer of the present invention are also presented, as applications of the aforementioned manufacturing process for fabricating out-of-plane suspensions. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1   a  is an angled, top view of an embodiment of a MEMS gyroscope of the present invention. 
         FIG. 1   b  is an angled, top view of another embodiment of a MEMS gyroscope of the present invention. 
         FIGS. 1   c  and  1   d  illustrate how driving and/or sensing electronics may be connected to a MEMS gyroscope of the present invention. 
         FIG. 2   a  is an angled, top view of an embodiment of a MEMS accelerometer of the present invention. 
         FIG. 2   b  is a front view of an embodiment of a MEMS accelerometer of the present invention. 
         FIG. 2   c  is a front view of another embodiment of a MEMS accelerometer of the present invention. 
         FIG. 2   d  illustrates how driving and/or sensing electronics may be connected to a MEMS accelerometer of the present invention. 
         FIG. 3   a  is an example of a potential quad-mass sensing scheme using prior art dual-mass gyroscopes. 
         FIG. 3   b  illustrates how prior art dual-mass gyroscopes cannot be “stacked” next to one another in a quad-mass sensing scheme. 
         FIGS. 3   c  and  3   d  are two embodiments of a quad-mass sensing scheme of the present invention, in which a quad-mass gyroscope is linearly driven and linearly sensed. 
         FIGS. 3   e  and  3   f  are two embodiments of a quad-mass sensing scheme of the present invention, in which a quad-mass gyroscope is linearly driven but non-linearly sensed. 
         FIG. 3   g  illustrates the sensing directionality for the quad-mass sensing schemes in  FIGS. 3   c  through  3   f.    
         FIGS. 4   a  through  4   b  illustrate steps that may be used to realize vertical suspensions in a MEMS MSD system. 
         FIGS. 5   a  through  5   l  illustrate cross-sections of various steps in an embodiment of the manufacturing process for an embodiment of a MEMS gyroscope of the present invention. 
         FIGS. 6   a  through  6   l  illustrate cross-sections of various steps in an embodiment of the manufacturing process for an embodiment of a MEMS accelerometer of the present invention. 
         FIGS. 7   a  and  7   b  are scanning electronic microscope (SEM) images of an embodiment of a MEMS gyroscope of the present invention.  FIG. 7   c  is a SEM image of an embodiment of a MEMS gyroscope of the present invention that illustrates the openings that may be used to short circuit certain components in said gyroscope. 
         FIG. 8  is a SEM image of an embodiment of a MEMS accelerometer of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Gyroscope 
       FIG. 1   a  illustrates an embodiment of a MEMS gyroscope of the present invention. The gyroscope  10  may comprise, among other things, a shared proof mass  20 ; first, second, third, and fourth anchors  30   a ,  30   b ,  30   c ,  30   d ; first and second drive combs  40   a ,  40   b ; first and second sense combs  50   a ,  50   b ; and a plurality of suspensions  60 . 
     The shared proof mass  20  may be located at the center of said gyroscope  10 , and may have first, second, third, and fourth edges and first, second, third, and fourth corners. In some embodiments, the shared proof mass  20  may be square-shaped. The shared proof mass  20  (as well as the other components of the gyroscope  10 ) may be made of any dielectric substance. In some embodiments, the shared proof mass  20  may be comprised of crystalline Silicon (Si). Also in some embodiments, there may be a layer of oxide dividing the shared proof mass  20  into an upper and a lower mass. During operation of the gyroscope  10 , the shared proof mass  20  may vibrate. Where there is a rotation of the gyroscope  10 , the shared proof mass  20  may experience a “secondary vibration,” or vibrate in an orthogonal direction. Such secondary vibrations may be used to determine the angular velocity (and thus angular displacement) of the object or device to which the gyroscope  10  is affixed or connected. 
     Each anchor  30   a - d  of the gyroscope  10  may lie parallel with an edge of said shared proof mass  20 , such that, for example, the first anchor  30   a  lies parallel with the first edge of the shared proof mass  20 . Each anchor  30   a - d  may have a first corner and a second corner. In some embodiments, the anchors  30   a - d  may have the same range of length and thickness as the shared proof mass  20 , and further may each have a width ranging from 200 μm to 400 μm. Varying the dimensions of said anchors should generally not affect the performance of the gyroscope  10 . In some embodiments, the anchors  30   a - d  may be made of crystalline Si. The anchors  30   a - d  may be used to fix, or “anchor,” the gyroscope  10  and/or the components thereof to the substrate on which the gyroscope  10  rests. 
     Suspensions  60  may extend out-of-plane, or vertically or upward, from the first and second corners of the anchors  30   a - b . Suspensions  60  may similarly extend out-of-plane from the first, second, third, and fourth corners of the shared proof mass  20 . Thus, in some embodiments, said shared proof mass  20  may rest below the suspensions  60 , and, in certain of such embodiments, in plane with the anchors  30   a - b . Also in some embodiments, said suspensions  60  may have a cross-section ranging from 5×5 μm 2  to 100×100 μm 2 , and, in certain of such embodiments, have a cross-section of 10×70 μm 2 . Also in some embodiments, said suspensions  60  may have a length ranging from 150 μm to 600 μm, and, in certain of such embodiments, have a length of 250 μm. The cross-sectional dimensions and the length of the suspensions  60  may affect the stiffness constant of said suspensions  60 , which may affect such sensor&#39;s resonant frequency, support losses, quality factor, and/or noise level, as well as rotation rate. Generally speaking, increasing the size of a suspension&#39;s  60  cross-section increases its stiffness, and increasing a suspension&#39;s  60  length decreases its stiffness. The cross-section and length of the suspensions  60  should be designed so as to minimize suspension  60  stiffness. In some embodiments, the suspensions  60  may be made of crystalline Si. 
     Said suspensions  60  may provide support for the movable combs. In some embodiments, said movable combs may be comprised of first and second drive combs  40   a - b  and first and second sense combs  50   a - b . The first and second drive combs  40   a - b  may have first, second, third, and fourth corners. Similarly, the first and second sense combs  50   a - b  may have first, second, third, and fourth corners. Said suspensions  60 , extending from said anchors  30   a - d  and said shared proof mass  20 , may connect with said corners of said combs  40   a - b ,  50   a - b , with said combs  40   a - b ,  50   a - b  resting on top of said suspensions  60 . 
     In some embodiments, the drive combs  40   a - b  and sense combs  50   a - b  may have the same dimensions as one another, and the gyroscope  10  may be a three-fold-symmetric gyroscope (3FSG). In other words, the gyroscope  10  may have three geometrical symmetries: about the center line, parallel to the X-axis; about the center line, parallel to the Y-axis; and about the diagonal of the gyroscope  10 . Such symmetry aids in matching the driving and sense modes of said gyroscope  10 . The drive combs  40   a - b  may be used for the actuation of the spring-mass-damper system (for example, in the X-direction). When a rotational rate is applied (for example, in the Z-direction), the sense combs  50   a - b  may be used to sense the Coriolis force in the cross-product direction (for example, in the Y-direction). In some embodiments, the combs  40   a - b ,  50   a - b  may be made of crystalline Si. 
     The total proof mass of the gyroscope  10  may be the shared proof mass  20  plus two combs  40   a - b ,  50   a - b . In some embodiments, the total drive proof mass may be the shared proof mass  20  plus the first and second drive combs  40   a - b , and, similarly, the total sense proof mass may be the shared proof mass  20  plus the first and second sense combs  50   a - b . The total proof mass may have a length and width ranging from 100 μm to 3 mm and a thickness ranging from 10 μm to 300 μm. In certain embodiments, the total proof mass may be 1200 μm×1200 μm×200 μm, and have a proof mass area fill factor of 73.4%. Also in some embodiments, the total proof mass may have a weight ranging from 30 μg to 3 mg. In certain embodiments, the two combs  40   a - b ,  50   a - b  may comprise less than 10% of the area and weight of the total proof mass. 
       FIG. 1   b  illustrates another embodiment of a MEMS gyroscope  10 ′ of the present invention, specifically a gimbaled MEMS gyroscope  10 ′. In this embodiment, four anchors  30   a′ - 30   d′  are used to fix, or “anchor,” said gyroscope  10 ′ to the substrate on which the gyroscope  10 ′ rests. Outer suspensions  60   a′  may then extend out-of-plane (or upwardly therefrom) to support an outer proof mass (or gimbal)  20   a′ . Inner suspensions  60   b′  may then extend out-of-plane from said outer proof mass  20   a′ . Resting on top of said inner suspensions  60   b′  may be an inner proof mass  20   b′ . As can be seen in  FIG. 1   b , in this embodiment, the proof masses  20   a′ - b′  may rest on top of, instead of being suspended below (as in  FIG. 1   a ), their respective suspensions  60   a′ - b′ . In this embodiment, the outer proof mass  20   a′  may vibrate in the drive mode (X-axis), while the inner proof mass  20   b′  may vibrate in the sense mode (Y-axis). Such configuration may decouple sensed motion in two orthogonal directions. 
     Accelerometer 
       FIG. 2   a  illustrates an embodiment of a MEMS accelerometer of the present invention. The accelerometer  110  may comprise, among other things, a proof mass  120 ; first, second, third, and fourth anchors  130   a ,  130   b ,  130   c ,  130   d ; and a plurality of suspensions  160 . 
     The proof mass  120  may be located at the center of said accelerometer  110 , and may have first, second, third, and fourth corners. In some embodiments, there may be a layer of oxide dividing the proof mass  120  into an upper mass  122   a  and a lower mass  122   b . Also in some embodiments, the upper mass  122   a  and lower mass  122   b  may have a length and width ranging from 100 μm to 3 mm, as well as a thickness ranging from 10 μm to 300 μm. Also in some embodiments, the proof mass  120  may have a weight ranging from 30 μg to 3 μg. As with the gyroscope, increasing the weight of the proof mass  120  may improve the sensitivity of the accelerometer  110 , while increasing the dimensions of the proof mass  120  may detrimentally affect performance. The proof mass  120  (as well as the other components of the accelerometer  110 ) may be made of any dielectric substance. In some embodiments, the proof mass  120  may be made of crystalline Si. Where the object or device to which the accelerometer  110  is affixed or connected to moves, the proof mass  120  will vibrate or be displaced. Such vibration or displacement may be used to determine the angular acceleration of such object or device. 
     Each anchor  130   a - d  of the accelerometer  110  may lie at or around a corner of the proof mass  120 , such that, for example, the first anchor  130   a  lies at or around the first corner of the proof mass  120 . Each anchor  130   a - d  may have a first corner. Varying the dimensions of said anchors  130   a - d  should generally not affect the performance of the accelerometer  110 . In some embodiments, the anchors  130   a - d  may be made of crystalline Si. The anchors  130   a - d  may be used to fix, or “anchor,” the accelerometer  110  and/or the components thereof to the substrate on which the accelerometer  110  rests. 
     Suspensions  160  may extend out-of-plane, or vertically or upward, from the first corners of the anchors  130   a - d , and may connect with the first, second, third, and fourth corners of the proof mass  120 . Thus, in some embodiments, the proof mass  120  may rest on top of said suspensions  160 . Such configuration is consistent with the function of an accelerometer, which need only sense in a single mode. In some embodiments, said suspensions  160  may have a cross-section ranging from 5×5 μm 2  to 100×100 μm 2 . Also in some embodiments, said suspensions  160  may have a length ranging from 250 μm to 600 μm. The cross-sectional dimensions and length of the suspensions  160  may affect the stiffness constant of said suspensions  160 , as such dimensions and length similarly affect the gyroscope  10  (as described above), and, more specifically, may affect the resolution in acceleration. In some embodiments, the suspensions  160  may be made of crystalline Si. 
       FIGS. 2   b  and  2   c  illustrate front views of two embodiments of a MEMS accelerometer of the present invention.  FIG. 2   b , like  FIG. 2   a , illustrates an embodiment where the proof mass  120  rests on the suspensions  160 .  FIG. 2   c  illustrates another embodiment, where the suspensions  160  are connected directly to the upper mass  122   a , and where the lower mass  122   b  is smaller than, and thus “hangs” below, said upper mass  122   a . In some embodiments, said lower mass  122   b  may be 100 μm to 200 μm shorter in length than said upper mass  122   a . Such configuration may be achieved during the selective etching in the fabrication process described below. As can be seen by a comparison of  FIGS. 2   b  and  2   c , the configuration in  FIG. 2   c  allows for an increased suspension  160  length.  FIG. 2   c  further demonstrates the geometrical decoupling of the suspensions  160  and the proof mass  120 , a benefit of the present invention. 
     Quad-Mass MEMS Gyroscopes and Other Sensors 
     The use of out-of-plane (or vertical) suspensions provides a further benefit with respect to multi-mass sensors. By way of example, multi-mass gyroscopes may be implemented by situating two prior art dual-mass gyroscopes  210   a ,  210   b  next to each other, facing in opposite directions, as can be seen in  FIG. 3   a . (The anchors are labeled as  212 , and the in-plane suspensions are labeled as  214 .) There are numerous drawbacks to such arrangement. For example, this arrangement doubles the overall area that the sensors (or gyroscopes) occupy. Further, such prior art dual-mass gyroscopes  210   a - b  must be wired to one another to share sensory information. Such drawbacks cannot be alleviated using such prior art dual-mass gyroscopes because, as can be seen from  FIG. 3   b , such gyroscopes  210   a - b  cannot be “stacked” next to one another due to the presence of the in-plane (or horizontal) suspensions  214 , which would interfere with each other. 
     As can be from  FIGS. 3   c  through  3   f , by contrast, the use of out-of-plane suspensions permits four sensors (including, without limitation, MEMS gyroscopes and accelerometers) of the present invention to be placed one next to the other. By way of example, in a quad-mass gyroscope of the present invention, four gyroscopes  220   a ,  220   b ,  220   c ,  220   d  of the present invention may be placed one next to the other to create a dual-differential sensing scheme. The gyroscopes  220   a - d  may share sense combs and/or drive combs (and thus electrical sense and/or drive signal circuitry). With respect to  FIGS. 3   c  and  3   d , such quad-mass gyroscopes  220  are linearly driven and linearly sensed. With respect to  FIGS. 3   e  and  3   f , such quad-mass gyroscopes  220  are linearly driven but non-linearly sensed. Locations of where first (driving), third (sensing), and, where applicable, fourth (sensing) signals may be found are labeled on  250   a ,  250   c , and  250   d . (A second (driving) signal is not shown, but may be connected to the gyroscopes  220   a - d  to provide a DC polarization voltage, as well other driving signals.) As can be seen by comparing  FIGS. 3   c  and  3   d  to  FIGS. 3   e  and  3   f , the embodiments in  FIGS. 3   e  and  3   f  use parallel plate capacitors with respect to the sense combs, whereas the embodiments in  FIGS. 3   c  and  3   d  use comb-drive capacitors. Parallel plate capacitors may provide more sensing capacitance and capacitance changes due to displacement. 
     As should be obvious to one skilled in the art, the out-of-plane (or vertical) suspension scheme presented herein is not limited to use in respect of gyroscopes, accelerometers, and quad-mass gyroscopes, but rather can be used in other MEMS sensory systems. Specifically, such out-of-plane suspension scheme can be used in any MEMS MSD system (including, by way of example and without limitation, radio frequency MEMS resonators and MEMS-based mechanical filters), as MSD systems use a mass attached to a suspension to detect and/or determine sensory information. 
     Method of Fabrication 
     The MEMS MSD systems of the present invention, including MEMS gyroscopes and accelerometers, can be fabricated using various types of micromachining. By way of example, various type of bulk micromachining may be used, including, without limitation, deep reactive ion etching (DRIE), LIGA, and electroforming. Bulk micromachining provides a number of advantages over surface micromachining. For example, bulk micromachining allows for a sensor with a larger proof mass and improved capacitance. Also by way of example, with surface micromachining, the lateral (in-wafer-plane) dimensions are generally tighter than those allowed by bulk micromachining techniques, in part due to the inherent mechanical stresses and stress gradient of surface micromachined structural layers. 
     As previously described, the MEMS MSD systems of the present invention use out-of-plane suspensions. The following steps represent an embodiment of the manufacturing process that may be used to fabricate such out-of-plane suspensions in such MEMS MSD systems. First, a dielectric substrate  500  may be etched from the top surface  502  to form the first two sides of an out-of-plane suspension  506   a - b , as can be seen in  FIG. 4   a . In some embodiments, said first two sides of said out-of-plane suspension  506   a - b  may be patterned using DRIE. Second, said dielectric substrate  500  may be etched from the bottom surface  504  to form the other two sides of said out-of-plane suspension  506   c - d , as can be seen in  FIG. 4   b . In some embodiments, said other two sides of said out-of-plane suspension  506   c - d  may also be patterned using DRIE. The cross-section and length of said suspension may be dictated by the desired performance characteristics of the applicable MEMS MSD system. 
     Methods of fabricating an embodiment of a MEMS gyroscope of the present invention and an embodiment of a MEMS accelerometer of the present invention will now be presented, as applications of the fabrication process described in the preceding paragraph. Although the following methods are presented in a specific sequence, other sequences may be used and certain steps omitted or added. It should be noted that the shapes of any etchings, and the dimensions of such shapes, as well as the shapes and depths of any deposited metal, will be dictated by the desired dimensions and shapes of the sensor and the components thereof, as will be obvious to one having ordinary skill in the art. 
     MEMS Gyroscope 
     As shown in  FIG. 5   a , a substrate  510 , such as silicon on insulator (SOI) substrate, having a top surface  512  and a bottom surface  514  may be provided. In some embodiments, said substrate  510  may, starting from the top surface  512 , have the following layers: a first layer of oxide  520  having a thickness of 4 μm; a first layer of silicon  522  having a thickness of 100 μm; a second layer of oxide  524  having a thickness of 4 μm; a second layer of silicon  526  having a thickness of 570 μm; and a third layer of oxide  528  having a thickness of 4 μm. Said oxide layers  520 ,  524 ,  528  may each act as a sacrificial layer during etching. 
     As shown in  FIGS. 5   b  and  5   c , the top surface  512  of the substrate  510  may be selectively etched to define the shared proof mass, the movable combs, and the out-of-plane suspensions, as well the openings  530  that may be used to short circuit portions of certain gyroscope components that are separated by the second layer of oxide  524 . Generally speaking, said openings  530  should be narrow enough to allow for a conformal coating of a conductive metal and to permit a trench that may be etched therethrough to close completely when said metal is deposited. By depositing a conductive material in the trenches that may be etched in said openings  530 , said portions of said certain gyroscope components may be “short circuited” and thus electrically connected to one another. Contact pads for the shared proof mass may also be etched in these steps. Signals sensed by the shared proof mass through the out-of-plane suspensions and down to the sensor&#39;s anchors may be transmitted to such contact pads. In some embodiments, the etching may be to a depth of 130 μm to 140 μm, thereby etching through said first layer of oxide  520 , said first layer of silicon  522 , said second layer of oxide  524 , and a portion of said second layer of silicon  526 . 
     As shown in  FIG. 5   d , the top surface  512  of the substrate  510  may be further etched to remove the remainder of the first oxide layer  520 . This will prepare the substrate  510  for the deposition of the conductive metal as described in the next paragraph. 
     As shown in  FIG. 5   e , in this step, a conductive metal may be deposited by various techniques (including, without limitation, sputtering, platting, and pulse laser deposition) on the top surface  512  of the substrate  510 . In some embodiments, a conformal coating of Silicon Germanium (SiGe) may be deposited, using low pressure chemical vapor deposition (LPCVD), on said top surface  512  and in the areas etched in the preceding two steps (e.g., the openings  530 ). Such conductive metal may short-circuit the total proof mass. In this case, short circuiting the total proof mass may be done through the movable combs, which, in this embodiment, are the parts that are connected to the suspensions affixed to the anchors. Such short-circuit may be necessary so that the signal from the movable combs may be passed through the anchors by way of the out-of-plane suspensions. 
     As shown in  FIG. 5   f , a thin layer of oxide may be selectively deposited on the top surface  512  of the substrate  510 , over the conductive metal deposited in the previous step. In some embodiments, Silicon Dioxide (SiO 2 ) may be selectively deposited using LPCVD. This may be used to define, among other things, the fixed electrodes. The fixed electrodes are fixed comb-drive electrodes that may face the movable comb-drive electrodes of the drive and sense modes. Said fixed electrodes represent the other terminal of the linear comb-drive capacitance (whether for the drive and sense modes of the gyroscope or for the sense mode of the accelerometer). They may be used to interface the fabricated gyroscope to the drive and sense circuitry (i.e., the CMOS). In some embodiments, the thickness of the oxide may range sub-μm to 10 μm, depending the permissible parasitic capacitance. 
     As shown in  FIG. 5   g , the areas between said fixed electrodes  550  and the gyroscope may be selectively etched. In some embodiments, said areas may be etched using DRIE through to the second layer of oxide  524 . 
     As shown in  FIG. 5   h , the layer of oxide remaining on the top surface  512  of the substrate  510  may then be etched, in some embodiments using DRIE, so as to remove the remainder of such oxide. Such etching may complete the fabrication of the top surface  512  of the substrate  510 , realizing the fixed electrodes  550 , the shared proof mass  20 , and the drive and sense combs  40   a - b ,  50   a - b . (As can be seen in this  FIG. 5   h , there is a “short circuit” in the middle of the movable combs, connecting the portions of said combs that are below and above the second layer of oxide  524 .) 
     As shown in  FIGS. 5   i  and  5   j , the bottom surface  514  of the substrate  510  may be selectively etched, in some embodiments using DRIE, to define the outer boundaries  532  of the gyroscope. Such etching may be to a depth of 50 μm. 
     As shown in  FIG. 5   k , the bottom surface  514  of the substrate  510  may be further selectively etched to remove selected portions of the third layer of oxide, in order to prepare the bottom surface  514  of the substrate  510  for further etching. 
     As shown in  FIG. 5   l , the bottom surface  514  of the substrate  510  may be selectively etched to finalize the gyroscope. At the outer boundaries, said bottom surface  514  may be etched up to the second layer of oxide  524 . The bottom surface may be etched to a depth of 250 μm to 500 μm to realize the out-of-plane suspensions  60 . In some embodiments, DRIE may be used to perform such etching. Such etching may, among other things, reduce parasitic capacitance. 
     MEMS Accelerometer 
     As shown in  FIG. 6   a , a substrate  610 , such as silicon on insulator (SOI) substrate, having a top surface  612  and a bottom surface  614  may be provided. In some embodiments, said substrate  610  may, starting from the top surface  612 , have the following layers: a first layer of oxide  620  having a thickness of 4 μm; a first layer of silicon  622  having a thickness of 100 μm; a second layer of oxide  624  having a thickness of 4 μm; a second layer of silicon  626  having a thickness of 570 μm; and a third layer of oxide  628  having a thickness of 4 μm. Said oxide layers  620 ,  624 ,  628  may each act as a sacrificial layer during etching. 
     As shown in  FIG. 6   b , the first layer of oxide  620  may be selectively etched to define the openings  630  for the trenches that will be used to “short circuit” the proof mass, as well as to define the out-of-plane suspensions. Further, as shown in  FIG. 6   c , the openings  630  and out-of-plane suspensions may be etched through to a depth of 130 μm to 140 μm. In some embodiments, said etching may be performed using DRIE. 
     As shown in  FIG. 6   d , the top surface  612  of the substrate  610  may be further etched to remove the remainder of the first oxide layer  620 . In some embodiments, said etching may be performed using DRIE. This will prepare the substrate  610  for the deposition of the conductive metal as described in the next paragraph. 
     As shown in  FIG. 6   e , in this step, a conductive metal may be deposited by various techniques (including, without limitation, sputtering, platting, and pulse laser deposition) on the top surface  612  of the substrate  610 . In some embodiments, a conformal coating of Silicon Germanium (SiGe) may be deposited, using low pressure chemical vapor deposition (LPCVD), on said top surface  612  and in the openings  630  and out-of-plane suspensions. As with the gyroscope, the conductive metal deposited in the openings may short-circuit parts of the proof mass (i.e., parts below and above the second oxide layer). 
     As shown in  FIG. 6   f , a thin layer of oxide may be selectively deposited on the top surface  612  of the substrate  610 . In some embodiments, Silicon Dioxide (SiO 2 ) may be selectively deposited using LPCVD. This may be used to define, among other things, the isolated electrodes. The isolated electrodes carry the necessary electrical signals for sensing purposes. In some embodiments, the thickness of the oxide may range from sub-μm to 10 μm, depending on the maximum permissible parasitic capacitance. 
     As shown in  FIG. 6   g , the areas between the isolated electrodes  650  and the accelerometer may be selectively etched. In some embodiments, said areas may be etched using DRIE through to the second layer of oxide  624 . 
     As shown in  FIG. 6   h , the layer of oxide remaining on the top surface  612  of the substrate  610  may then be etched, in some embodiments using DRIE, so as to remove the remainder of such oxide. Such etching may complete the fabrication of the top surface  612  of the substrate  610 , realizing the isolated electrodes  650  and the proof mass  120 . 
     As shown in  FIGS. 6   i  and  6   j , the bottom surface  614  of the substrate  610  may be selectively etched, in some embodiments using DRIE, to define the outer boundaries  632  of the accelerometer. Such etching may be to a depth of 50 μm. 
     As shown in  FIG. 6   k , the bottom surface  614  of the substrate  610  may be further selectively etched to remove selected portions of the third layer of oxide  628 , in order to prepare the bottom surface  614  of the substrate  610  for further etching. 
     As shown in  FIG. 6   l , the bottom surface  614  of the substrate  610  may be selectively etched to finalize the accelerometer. At the outer boundaries, said bottom surface  614  may be etched up to the second layer of oxide  624 . Around the out-of-plane suspensions, the bottom surface  614  may be etched to a depth of 250 μm to 550 μm to realize said suspensions  160 . In some embodiments, DRIE may be used to perform such etching. Such etching may, among other things, reduce parasitic capacitance. 
     Test Results 
     MEMS Gyroscope 
     An embodiment of a MEMS gyroscope of the present invention was tested against a prior art MEMS gyroscope, as such prior art gyroscope is described in the following publication: A. Sharm, F. Zaman, B. Amini, F. Ayazi, “ A High - Q In - Plane SOI Tuning Fork Gyroscope ,” IEEE, 2004, pp. 467-470. Such gyroscopes had the same sensor area, 2 mm 2 , and wafer thickness, 675 μm. The key difference between such gyroscopes was that the prior art MEMS gyroscope used in-plane suspensions, whereas the MEMS gyroscope of the present invention used out-of-plane suspensions. With respect to the MEMS gyroscope of the present invention, such tests were performed using COMSOL Multiphysics v3.5. The performance results of the prior art gyroscope were obtained from the aforementioned publication. A comparison of the performance of each gyroscope can be seen in the following Table 1. 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                   
                 Embodiment of the 
               
             
          
           
               
                   
                 Performance Measure 
                 Prior Art Design 
                 Present Invention 
               
               
                   
               
             
          
           
               
                 1. 
                 Dimensions of the Total 
                 570 * 570 * 40 μm 3   
                 1200 * 1200 * 200 μm 3   
               
               
                   
                 Proof Mass and Proof Mass 
                 PMAFF~17% 
                 PMAFF~73.4% 
               
               
                   
                 Area Fill Factor (PMAFF)  
                 PMVFF~1% 
                 PMVFF~22% 
               
               
                   
                 and Proof Mass Volume  
                   
                 (The effective mass is around 
               
               
                   
                 Fill Factor (PMVFF) 
                   
                 22 times larger due to area and 
               
               
                   
                   
                   
                 thickness expansion inherent 
               
               
                   
                   
                   
                 with the novel gyroscope 
               
               
                   
                   
                   
                 architecture.) 
               
               
                 2. 
                 Total Proof Mass (M e ) 
                 0.03 mg 
                 0.67 mg 
               
               
                   
                   
                   
                 (The resulting mass is in the 
               
               
                   
                   
                   
                 order of 1 mg.) 
               
               
                 3. 
                 Resonance Frequency (F r )  
                 17.4 KHz 
                 3.7 KHz 
               
               
                   
                 for the Same Stiffness  
                   
                 (The frequency is still high in 
               
               
                   
                 and Support Losses 
                   
                 this embodiment because of 
               
               
                   
                   
                   
                 the Poly-Silicon material 
               
               
                   
                   
                   
                 properties.) 
               
               
                 4. 
                 Quality Factors for Drive and  
                 81,000 and  
                 380,000 and 300,000 
               
               
                   
                 Sense Modes (Q d  and Q s ) 
                 64,000 
                 (These will be limited or 
               
               
                   
                   
                   
                 clipped by the Q values due to 
               
               
                   
                   
                   
                 finite support losses and 
               
               
                   
                   
                   
                 thermo-elastic damping.) 
               
               
                 5.  
                 Theoretical Mechanical Noise 
                 0.3°/hr 
                 0.014°/hr 
               
               
                   
                 Equivalent Angular Rate (MNEΩ) 
                   
                 (The noise floor is reduced by 
               
               
                   
                   
                   
                 a factor of 22, and is deeply in 
               
               
                   
                   
                   
                 the inertial grade range.) 
               
               
                 6. 
                 Drive Mode Amplitude (X d ) for the  
                 1 μm 
                 4.69 μm 
               
               
                   
                 Same Drive Voltage 
                   
                   
               
               
                 7. 
                 Sense Mode Amplitude (X s ) 
                 1 nm 
                 103 nm 
               
               
                   
                   
                   
                 (The Coriolis displacement 
               
               
                   
                   
                   
                 sensed at the output is more 
               
               
                   
                   
                   
                 than two orders of magnitude 
               
               
                   
                   
                   
                 more.) 
               
               
                 8. 
                 Drive and Sense Capacitances  
                 0.16 pF 
                 0.34 pF 
               
               
                   
                 (C d  and C s ) 
                   
                 (The capacitance is increased 
               
               
                   
                   
                   
                 due to improving the PMAFF, 
               
               
                   
                   
                   
                 but the active or electrical 
               
               
                   
                   
                   
                 thickness is the same.) 
               
               
                 9. 
                 Parasitic or Coupling Sustaining  
                 100 * 570 μm 2   
                 100 * 100 μm 2   
               
               
                   
                 Area for the Same SOI Oxide 
                   
                 (The parasitic capacitance is 
               
               
                   
                 Thickness 
                   
                 reduced by a factor of 5.7 as a 
               
               
                   
                   
                   
                 result of the suggested support 
               
               
                   
                   
                   
                 for the fixed combs.) 
               
               
                 10. 
                 Electrical Output Sensitivity (S e ) 
                 1.25 mV/°/s 
                 125 mV/°/s 
               
               
                 11. 
                 Signal to Noise Ratio (SNR) 
                 4.17 mV/°/hr 
                 9,166 mV/°/hr 
               
               
                   
                   
                   
                 (The SNR is improved by 
               
               
                   
                   
                   
                 2200 times or more than three 
               
               
                   
                   
                   
                 orders of magnitude.) 
               
               
                   
               
             
          
         
       
     
     As can be seen in Table 1, with the MEMS gyroscope of the present invention, the total proof mass size is increased by more than an order of magnitude (i.e., ten times) in the same overall device area (i.e., 2 mm 2 ). The quality factor, drive amplitude, and resonance frequency are improved by a factor of 4.7. The dominating Brownian noise floor is lowered by a factor of 22, i.e., more than an order of magnitude. Furthermore, the sensed Coriolis displacement, output signal, and sensor sensitivity are improved by a factor of 103. Finally, the SNR is improved by more than three orders of magnitude. 
     Table 2 shows the resonance frequency and coupling percentage for various embodiments of a MEMS gyroscope of the present invention. Such simulation results were obtained using COMSOL Multiphysics v3.5. As can be seen from this table, the decoupling ratio of such various embodiments of a MEMS gyroscope of the present invention are in the same range as prior art MEMS gyroscopes (i.e., MEMS gyroscopes using in-plane suspensions). Thus, the MEMS gyroscopes of the present invention can provide improved performance (as shown in Table 1) without detrimentally affecting other performance measures. 
                                         TABLE 2                   Shared Proof Mass   800 μm   1000 μm   1000 μm   1000 μm   1800 μm       Side Length                           Shared Proof Mass   100 μm    100 μm    100 μm    100 μm    100 μm       Thickness                           Width of the Flying    100 μm    100 μm    100 μm    100 μm    100 μm       Comb                           Portion                           Suspension    10 * 70 μm 2     10 * 70 μm 2     10 * 50 μm 2     10 * 50 μm 2     10 * 70 μm 2         Cross-Section                           Dimensions                           Suspension    300 μm    400 μm    400 μm    300 μm    400 μm       Length                           Resonance   20.1 KHz   7.8 KHz   8.4 KHz   11.9 KHz   4.9 KHz       Frequency                           Coupling   2.3%   2.0%   1.7%   1.6%   1.4%       Percentage                    
MEMS Accelerometer
 
     Embodiments of a MEMS accelerometer of the present invention were tested against a prior art MEMS accelerometer, as such prior art accelerometer is described in the following publication: B. V. Amini and F. Ayazi, “ Micro - Gravity Capacitive Silicon - On - Insulator Accelerometers ,” Journal of Micromechanics, Vol. 15, No. 11, October 2005, pp. 2113-2120. The key difference between the accelerometers was that the prior art MEMS accelerometer used in-plane suspensions, whereas the MEMS accelerometers of the present invention obviously used out-of-plane suspensions. With respect to the MEMS accelerometers of the present invention, such tests were performed using COMSOL Multiphysics v3.5. The performance of the results of the prior art gyroscope were obtained from the aforementioned publication. A comparison of the performance of each accelerometer can be seen in the following Table 3. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Embodiment of the 
                 Embodiment of the 
               
               
                   
                   
                 Present Invention  
                 Present Invention 
               
               
                 Performance Measure  
                 Prior Art Design 
                 With Reduced Area 
                 With Same Area 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Area of the Proof Mass (mm 2 ) 
                 12 
                 1.65 
                 12 
               
               
                 Proof Mass (mg) 
                 1.7 
                 0.98 
                 7.1 
               
               
                 Resonance Frequency (Hz) 
                 2000 
                 670 
                 250 
               
               
                 Brownian Noise Floor (μg Hz −1/2 ) 
                 0.7 
                 0.842 
                 0.19 
               
               
                 Static Sensitivity (pF g −1 ) 
                 &gt;0.2 
                 &gt;0.07 
                 &gt;0.98 
               
               
                   
               
             
          
         
       
     
     As can be seen from Table 3, the MEMS accelerometer of the present invention can provide the same or similar performance as the prior art design, but with less than 15% of the proof mass area. The resulting resonance frequency is 66% less. Moreover, with the present invention, the Brownian noise floor will remain sub-μg. Furthermore, when the MEMS accelerometer of the present invention is designed to use the same device area as the prior art design, performance is significantly improved. First, the proof mass becomes rather large (approximately 7 mg). Further, the resonance frequency is decreased to 250 Hz, which is less than 13% of the resonance frequency of the prior art design. In addition, the Brownian noise floor is deeply in the inertial grade range. Finally, with the present invention, the SNR is improved by more than an order of magnitude. 
     Table 4 shows the resonance frequency and coupling percentage for various embodiments of a MEMS accelerometer of the present invention. Such simulation results were obtained using COMSOL Multiphysics v3.5. This table shows the range of resonant frequencies that be achieved using the present invention with a small proof mass. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
             
             
               
                 Upper Mass Dimensions (μm) 
                 1500 * 1500 * 100 
                 1500 * 1500 * 100 
                 1500 * 1500 * 100 
                 1500 * 1500 * 100 
               
               
                 Lower Mass Dimensions (μm) 
                 1500 * 1500 * 100 
                 1500 * 1500 * 100 
                 1500 * 1500 * 100 
                 1440 * 1440 * 100 
               
               
                 Suspension Cross- 
                 10 * 30 
                 10 * 30 
                 10 * 30 
                 10 * 30 
               
               
                 section Dimensions (μm) 
                   
                   
                   
                   
               
               
                 Suspension Length (μm) 
                 250 
                 300 
                 350 
                 450 
               
               
                 Resonance Frequency (Hz) 
                 5389 
                 4121 
                 3288 
                 2316