Abstract:
An accelerometer. A silicon wafer is etched to form a fixed portion, a movable portion, and a resilient coupling between, the fixed and movable portions generally arranged in the plane of the wafer, the mass of the movable portion being concentrated on one side of the resilient coupling. One of the fixed and moveable portions of the silicon structure includes a first electrode. The other of the fixed and moveable portions includes a second electrode oriented parallel to the axis of acceleration, and an electrically-conductive layer electrically connected as a third electrode coplanar and mechanically coupled with the second electrode. The second and third electrodes are arranged in capacitive opposition to the first electrode, the capacitance between the first electrode and third electrode increasing as the movable portion moves in a direction along the axis of acceleration relative to the fixed portion and decreasing as the movable portion moves in an opposite direction. A resilient coupling retains the first and third electrodes in capacitive opposition to each other across a capacitance gap while allowing motion of the first electrode relative to the second and third electrodes in response to acceleration along an axis of acceleration perpendicular to the plane of the wafer, and resiliently restores the first electrode to an equilibrium position when the acceleration ceases. The second electrode is in opposition to a majority of the surface area of the first electrode when the electrodes are in the equilibrium position. Capacitance between the first and third electrodes is measured to obtain a measurement of acceleration along the axis.

Description:
BACKGROUND  
         [0001]    The invention relates to accelerometers.  
           [0002]    Accelerometers are devices that measure acceleration, or changes in a rate of motion. When an elevator starts or stops, several portions of the human body can detect the change in motion and report the change to the brain. Similarly, known accelerometers use different mechanical and electrical techniques to detect changes in motion, and to report the changes to processors. Accelerometers are used in navigational systems, automatic seat belt and air bag triggers, and many other applications.  
           [0003]    In known techniques for manufacturing semiconductors, a single crystal of silicon is grown, and then photographic and lithographic techniques are used to etch away unwanted parts of the silicon, and to introduce doping atoms into the silicon to change the electrical properties of the silicon. It is also known to deposit other materials onto the silicon—for example, thin layers of metal may be deposited onto the silicon to serve as conducting wires between different portions of a circuit. The underlying silicon serves as a structural base to provide mechanical support for the metal, while the metal provides the electrical conductivity.  
         SUMMARY  
         [0004]    In general, in a first aspect, the invention features an accelerometer. The accelerometer includes a fixed structure, a movable structure, and a resilient coupling. The fixed and movable structures generally lie in a plane. The fixed structure bears a fixed electrode, and the movable structure bears a movable electrode. The resilient coupling is designed to retain the fixed and movable structures in capacitive opposition to each other across a capacitance gap while allowing motion of the movable electrode relative to the fixed electrode in response to acceleration along an axis of acceleration perpendicular to the plane, and to resiliently restore the two electrodes to an equilibrium position when the acceleration ceases. Electronics and/or software is designed to translate a measurement of capacitance between the fixed and movable electrodes into a measurement of the acceleration along the axis.  
           [0005]    In general, in a second aspect, the invention features an accelerometer. The accelerometer includes a fixed portion, a movable portion, and a resilient coupling. The fixed and movable portions generally lie in a plane. The resilient coupling is designed to allow motion of the movable portion relative to the fixed portion in response to acceleration along an axis of acceleration perpendicular to the plane and to resiliently restore the two portions to an equilibrium position when the acceleration ceases. One of the fixed and moveable portions of the silicon structure is electrically connected as a first electrode. The other of the fixed and moveable portions bears an electrically-conductive layer electrically connected as a second electrode. The first and second electrodes are arranged in capacitive opposition to each other. Electronics and/or software are designed to translate a measurement of capacitance between the first and second electrodes into a measurement of acceleration along the axis.  
           [0006]    In general, in a third aspect, the invention features an accelerometer. A silicon wafer is etched to form a fixed portion, a movable portion, and a resilient coupling between. The fixed and movable portions generally lie in a plane. The resilient coupling is designed to allow motion of movable portion relative to the fixed portion perpendicular to the wafer in response to acceleration perpendicular to the wafer and to resiliently restore the two portions to an equilibrium position when the acceleration ceases. The mass of the movable portion is concentrated on one side of the resilient coupling. The fixed and moveable portions each bear an electrode, the electrodes being arranged in capacitive opposition. Electronics and/or software are designed to translate a measurement of capacitance between the first and second electrodes into a measurement of acceleration perpendicular to the wafer.  
           [0007]    In general, in a fourth aspect, the invention features an accelerometer. A first electrode is oriented parallel to an axis of acceleration. A second electrode is oriented parallel to the axis of acceleration. A third electrode is coplanar with the second electrode. The second and third electrodes are arranged in capacitive opposition to the first electrode. A resilient coupling is designed to allow motion of the first electrode relative to the second and third electrodes along the axis of acceleration in response to acceleration and to resiliently restore the first electrode to an equilibrium position when the acceleration ceases. The second electrode is in opposition to a majority of the surface area of the first electrode when the electrodes are in the equilibrium position. Electronics and/or software are designed to translate a measurement of capacitance between the first and third electrodes into a measurement of acceleration along the axis.  
           [0008]    Embodiments of the invention may include one or more of the following features. The fixed structure, movable structure and resilient coupling may be integrally formed primarily by etching a silicon wafer. The fixed structure and movable structure may be formed at least primarily of high aspect ratio beams. The third electrode may be connected to a ground potential. The first electrode may be formed as a high-aspect-ratio beam with a larger cross-sectional dimension oriented parallel to the axis of acceleration. Various structures of the movable and fixed structures may be electrically isolated from each other by isolation joints formed within the silicon wafer. Various structures etched from the wafer may be released from an underlying substrate of the silicon wafer. The electronics and/or software may measure differential capacitance between at least two pairs of electrodes, and translate the measured differential capacitance into an expression of acceleration. A capacitance between the fixed and movable electrode may be at a maximum when the movable structure is displaced from the equilibrium position. The resilient coupling may be a torsional flexure. The fixed and movable electrodes may be arranged in first and second regions, such that (a) motion in a direction of the movable structure results in increased capacitance between electrodes in the first region and decreased capacitance in the second region; and (b) motion in an opposite direction of the movable structure results in decreased capacitance between electrodes in the first region and increased capacitance in the second region. The axis of acceleration may be perpendicular to the wafer. The metal electrode may be formed as a layer deposited on the silicon of the movable portion. The movable portion may include a stop designed to engage a floor of the fixed portion to limit excess motion. The second silicon electrode may be in opposition to a majority of the surface area of the first silicon electrode when the electrodes are in the equilibrium position.  
           [0009]    The above advantages and features are of representative embodiments only. It should be understood that they are not to be considered limitations on the invention as defined by the claims. Additional features and advantages of the invention will become apparent in the following description, from the drawings, and from the claims. 
       
    
    
     DESCRIPTION OF THE DRAWING  
       [0010]    [0010]FIG. 1 a  is a perspective view, partially cut away, of an accelerometer.  
         [0011]    [0011]FIGS. 1 b,    1   c,    1   d,    1   e,    1   f  and  1   g  are end views of an accelerometer.  
         [0012]    [0012]FIG. 2 is a plan view of an accelerometer.  
         [0013]    [0013]FIGS. 3 and 4 are plan views of details of an accelerometer.  
         [0014]    [0014]FIG. 5 a  is a plan view of an accelerometer.  
         [0015]    [0015]FIGS. 5 b  and  5   c  are details of FIG. 5 a.    
         [0016]    [0016]FIGS. 6 a,    6   b,    6   c,    6   d,    6   e,    6   f  and  6   g  are diagrammatic views of steps in fabricating an accelerometer.  
         [0017]    [0017]FIG. 7 is a plan view of an accelerometer. 
     
    
     DESCRIPTION  
     I. Overview  
       [0018]    Referring to FIGS. 1 a - 1   g,  accelerometer  100  may be etched as a solid state structure, for instance, out of a block of silicon. Accelerometer  100  may include a movable portion  300  and a fixed portion  400 , each including a plurality of electrodes  112 ,  114 ,  116 ,  118 ,  122 ,  128  generally formed as high-aspect-ratio beams or capacitor plates, each oriented in planes perpendicular to the silicon wafer and parallel to the z-axis  130  of the wafer along which acceleration is to be measured. Electrodes  112 ,  114 ,  122  of fixed portion  400  may be interdigitated between electrodes  116 ,  118 ,  128  of movable portion  300 , with capacitance gaps  142 ,  148  between. Resilient mounting  120  holds movable portion  300  in an equilibrium rest position relative to fixed portion  400  (as shown in FIGS. 1 d  and  1   e ), and allows motion of movable portion  300  relative to fixed portion  400  along z-axis  130  in response to acceleration (as shown in FIGS. 1 a,    1   b,    1   c,    1   f  and  1   g ). As movable electrodes  116 ,  118 ,  128  move relative to the fixed electrodes  112 ,  114 ,  122 , some of the opposed pairs of electrodes come into more-direct opposition to each other (that is, as the surface area of movable electrode  116  comes to be more directly opposed to fixed electrode  112 ,  122 , and movable electrode  118 ,  128  comes to be more directly opposed to fixed electrode  114 ), and the capacitance increases. Other pairs of opposed electrodes move out of opposition to each other, and the capacitance between these pairs decreases. These changes in capacitance can be measured, giving a measure of the displacement of movable portion  300  relative to fixed portion  400 , and thus a measure of the acceleration imposed on the accelerometer  100  as a whole.  
         [0019]    At least some of the capacitor fingers may be divided into two separate conductors (e.g.,  112 ,  122  and  118 ,  128 ), with an insulating layer  132 ,  138  separating the two conductors of a single finger. For instance, electrodes  112 ,  114 ,  116 ,  118  may be formed in silicon layers (typically doped to improve conductivity), and electrodes  122 ,  128  may be formed in a metal layer laid atop the silicon structural members. Capacitance  142 ,  148  may be measured between pairs of electrodes ( 122 ,  116  and  114 ,  128 ) that are not directly opposed to each other when the accelerometer is in its equilibrium state. Capacitance  142 ,  148  may be measured between electrodes of different materials—for instance, capacitance may be measured between metal electrodes  122 ,  128  and silicon electrodes  114 ,  116 . Some electrodes  112 ,  118  may be grounded  152 ,  158 , or otherwise electrically connected to consume field lines  154 ,  156 , to reduce the total capacitance  142 ,  148  between electrodes  114 ,  116 ,  122 ,  128 .  
         [0020]    To a first-order approximation, the capacitance  142 ,  148  between metal electrodes  122 ,  128  and their opposed silicon electrodes  114 ,  116  is at a maximum when the metal electrode  122 ,  128  is most nearly centered on the face of the opposing silicon electrode  114 ,  116 , because of the shapes of the fringing fields around the electrodes. (Because of the field lines that terminate in the grounded electrodes  112 ,  118 , the maximal capacitance is actually achieved when the metal electrode  122 ,  128  is somewhat below the mid-point of the opposing silicon electrode  114 ,  116 .) Thus, capacitance  142  between electrodes  116  and  122  increases as movable portion  300  moves up within fixed portion  400  (the motion depicted in moving from FIG. 1 d  to FIG. 1 b ), and capacitance  148  between electrodes  114  and  128  decreases as metal electrode  128  moves out from between electrodes  114  (the motion depicted in moving from FIG. 1 e  to FIG. 1 c ). Similarly, capacitance  142  between electrodes  116  and  122  falls as movable portion  300 , carrying silicon electrode  116 , moves down between electrodes  122  (the motion depicted in moving from FIG. 1 d  to FIG. 1 f ), and the capacitance  148  between electrodes  114  and  128  increases (the motion depicted in moving from FIG. 1 e  to FIG. 1 g ).  
         [0021]    Electronics may measure differential capacitance. For instance, because the  114 -to- 128  capacitance  148  and the  116 -to- 122  capacitance  142  change in opposite directions with motion, and electronics (discussed in section II.E, infra) may measure the  114 -to- 128  capacitance  148  less the  116 -to- 122  capacitance  142 . That difference will generally reflect the degree of deflection of movable portion  300 . From that difference, electronics may determine the amount of acceleration imposed on accelerometer  100 .  
         [0022]    Isolation joint  160  electrically isolates electrodes  116  on the left half of FIG. 1 a  (and FIGS. 1 b,    1   d,  and  1   f ) from the electrodes  118 ,  128  on the right half of FIG. 1 a  (and FIGS. 1 c,    1   e  and  1   g ).  
         [0023]    Resilient mounting  120  may be a torsional flexure, or a beam that acts in torsion, that provides for movement of movable portion  300  through torsional rotation (arrows  162 ). Various portions of resilient mounting  120  may also serve as conductors to drive electrodes  112 ,  114 ,  116 ,  118 ,  122 ,  128 .  
       II. Structure  
       [0024]    A. Major Structural Elements  
         [0025]    Referring to FIG. 2, accelerometer  100  may include a center backbone  204 , movable outer frame  210 , electrodes  112 ,  114 ,  116 ,  118  mounted to center backbone  204  and outer frame  210 , and torsional flexure  120 . Center backbone  204  and fixed electrodes  112 ,  114  may be anchored to the silicon wafer to form fixed portion  400 . Resilient torsional flexure  120  may be anchored to the wafer at anchor point  206  and may be otherwise released from the floor of the wafer (by undercutting, as described below in connection with FIGS. 6 c  and  6   g ). Outer frame  210  may carry movable electrodes  116 ,  118 , to form movable portion  300 . Outer frame  210  and movable electrodes  116 ,  118  are released from the floor of the wafer, so that motion tends to be greatest at the right-most end  212  of movable portion  300 . Center backbone  204 , outer frame  210  and torsional flexure  120  may all be formed from truss-structured silicon members. The walls of torsional flexure  120  may be thinner than the walls of center backbone  204  or outer frame  210  in order to increase flexibility of torsional flexure  120 .  
         [0026]    Anchor point  206  may be relatively small, to provide strain relief, or to allow movable portion  300  and fixed portion  400  to curl together in a common mode of deformation when the manufacturing process or temperature variations cause curling or bending. In other embodiments, anchor point  206  may extend farther along the edge of torsional flexure  120 , to provide rotational stability for movable portion  300  about z-axis  130  (FIG. 1 a ). Torsional flexure  120  may be compliant, primarily in torsion, permitting rotation of movable portion  300  about axis x′. The rotation angle may typically be less than 0.04 degrees; therefore, the motion of movable portion  300  may be predominantly along z-axis  130  and proportional to the distance from the torsional flexure  120 . Maximum deflection of movable portion  300  relative to fixed portion  400  along the z-axis  130  perpendicular to the wafer may be on the order of tens to hundreds of nanometers. Off-axis motion (within the plane of the wafer) may be confined to an order of magnitude less, by designing appropriate stiffening truss structures into movable portion  300  and fixed portion  400 .  
         [0027]    The overall size of accelerometer  100  may be about 1 mm×1.5 mm. The mass of the movable structure may be about 10 −8  kg. The inertial moment of movable portion  300  around torsional flexure  120  may be about 5 to 6×10 −8  kg m 2 .  
         [0028]    The resonant frequency of movable portion  300  within fixed portion  400 , moving in the z-axis direction  130 , may be about 1.3 to 1.4 kHz, and is desirably about 1 kHz for sensing accelerations in the 1 to 10 g range. For higher g accelerometers or higher frequency response, higher resonances are generally preferred and can be accommodated through stiffer torsional flexures  120 . Movable portion  300  may move relative to fixed portion  400  in other vibrational modes, for instance, rotating around the z-axis  130 , or rotating side-to-side around the y-axis (where the left half of FIG. 1 a rotates up and the right half down, for instance). In an ideal accelerometer, movable portion  300  would only move only in the z-axis direction, giving a finite resonance frequency for resonance in the z-axis direction, and would resist motion in all other directions, giving an infinite resonance frequency for all other vibrational modes. Thus, higher resonant frequencies are generally preferable for these off-axis motions, and resonant frequencies of between 5 and 10 kHz may be achieved in actual devices. For a given amount of acceleration along z-axis  130 , fixed portion  400  may deflect by about 1/29 of the amount of deflection of movable portion  300 . Generally, smaller ratios of deflection are better than larger, to the degree permitted by other engineering constraints.  
         [0029]    Referring again to FIG. 1 a,  typically, the silicon structures may be 20 to 40 microns high (dimension  240 ). The silicon elements may typically be 2 microns wide (dimension  242 ).  
         [0030]    Referring to FIGS. 3 and 4, movable portion  300  and fixed portion  400  may include repeated patterns of silicon and metal structures. Two important patterns are shown in FIG. 3 and FIG. 4. The primary structure of movable portion  300  and fixed portion  400  may be formed of silicon beams  112 ,  114 ,  116 ,  118 ,  332 ,  338 ,  362 ,  364 ,  432 ,  438 . These beams may be etched from a single-crystal silicon wafer. Before the silicon is etched, the top surface of the silicon may be oxidized to form an insulating layer of silicon dioxide of 0.5 to 1 microns, and metal may be laid on top of the silicon in the pattern shown in stipple. When the silicon wafer is etched to form the physical structure, metal overlaying the etched portions of the silicon may be removed as well, so that the metal remains only atop the silicon beams. Isolation joints  160 ,  360 ,  336 ,  436 ,  462  electrically isolate different portions of the silicon structure from each other. Vias  324 ,  334 ,  337 ,  423 ,  434 ,  437  connect the metal layer through the insulating oxide to the underlying silicon.  
         [0031]    B. Movable Portion  
         [0032]    Four voltage potentials (which will be designated potentials  310 ,  312 ,  314  and  316 ) may be applied to the various components. Capacitances (including changes in capacitance, differential capacitance, or changes in differential capacitance) between pairs of these potentials may then be measured to determine acceleration.  
         [0033]    Referring to FIG. 3, in conjunction with the left half of FIG. 1 a,  and FIGS. 1 b,    1   d  and  1   f,  potential  316  may be applied to silicon electrode  116 . Electrode  116  may be electrically contiguous with silicon beam  322 . Beam  322  may be electrically connected through via  324  to metal  326 . Metal  326 ,  328  may connect drive and measurement electronics (see discussion in section II.E, infra) to apply potential  316  to silicon electrode  116 . Because silicon electrode  112  and metal electrode  122  are part of fixed portion  400 , detailed discussion will be deferred until section II.C, infra, and discussion of FIG. 4. The gap between fingers  112 ,  116  and  114 ,  118  may be about 3 microns.  
         [0034]    Referring to FIG. 3, in conjunction with the right half of FIG. 1 a,  and FIGS. 1 c,    1   e  and  1   g,  ground potential  310  may be applied to silicon electrode  118 . Electrode  118  may be electrically contiguous with silicon beam  332 . Silicon beam may connect through via  334  to metal  335 , which crosses isolation joint  336 , and reconnects to silicon beam  338  through via  337 . (Vias  334 ,  337 , isolation joint  336  and metal  335  may not be required by electrical considerations; via  334  and isolation joint  336  may serve to improve the match between the thermal expansion of leg  332  and the thermal expansion of leg  322 .) Drive and measurement electronics may apply ground potential  310  to silicon beam  338 . Metal electrode  128  may be connected through metal  342  to drive and measurement electronics, which may drive metal electrode  128  at potential  312 . Because silicon electrode  114  may be part of fixed portion  400 , detailed discussion will be deferred until section II.C and discussion of FIG. 4.  
         [0035]    Isolation joints  160 ,  360  may electrically isolate portions of the silicon from each other. For instance, isolation joints  160  may isolate electrodes  116  (electrical potential  316 ) on the left half of FIG. 3 from electrodes  118  (ground potential  310 ) on the right half of FIG. 3. Isolation joints  360  may isolate electrodes  116  (electrical potential  316 ) from silicon beams  338  (ground potential  310 ). Isolation joints  160 ,  360  may be formed as follows. Slits or trenches may be etched into the wafer, in the locations that become isolation joints  160 ,  360 . During the same thermal oxidation process that forms the oxide layer on top of the entire wafer to insulate metal layer  122 ,  128  from the underlying silicon, silicon dioxide may be grow on the wafer to fill in the trenches. This growth may cause the two opposing faces of silicon dioxide to fuse to each other. Further, the growth of silicon dioxide around the circular ends of the trenches may provide a connection across the two sides of the isolation joint. Together, the fusing of opposing faces and growth across the ends of the trenches may provide sufficient structural integrity to provide mechanical support for electrodes  116 ,  118  on silicon beams  362 ,  364 .  
         [0036]    Metal laid across the tops of isolation joints  160 ,  360  is electrically insulated from the silicon on both sides of the isolation joint, but is electronically continuous across the top of the isolation joints.  
         [0037]    Conducting vias  324 ,  334 ,  337 ,  423 ,  434 ,  437 , etc. may be formed in the conventional manner. In FIG. 3, they are shown as slightly wider beam region than the silicon beam regions immediately adjacent. Beams may be widened where vias are placed in order to keep the vias interior to the beam geometry.  
         [0038]    C. Fixed Portion  
         [0039]    Referring to FIG. 4, in conjunction with the right half of FIG. 1 a,  and FIGS. 1 c,    1   e  and  1   g,  potential  314  may be applied to silicon electrode  114 . Electrode  114  may be connected through beam  422  through via  423  to metal  424 , which may run over isolation joint  462  out to the edges of the device. Drive and measurement electronics may apply potential  314  to metal  424 .  
         [0040]    Referring to FIG. 4, in conjunction with the left half of FIG. 1 a,  and FIGS. 1 b,    1   d  and  1   f,  ground potential  310  may be applied to silicon electrode  112 . Electrode  112  may be electrically contiguous with silicon beam  432 . Beam  432  may connect through via  434  to metal  435 . Metal  435  may cross isolation joint  436 , to via  437 , which may in turn connect metal  435  to silicon beam  438 . Drive and measurement electronics may apply ground potential  310  to beam  438 . (Vias  434 ,  437 , isolation joint  436  and metal  435  may not be required by electrical considerations; via  434  and isolation joint  436  may improve the match between thermal expansion of leg  432  and thermal expansion of leg  422 .) Metal electrode  122  may be electrically connected to metal  424  across isolation joints  460 . Potential  314  may be applied to metal  424  as discussed above.  
         [0041]    Isolation joint  460  separates silicon electrode  112  from silicon electrode  114 .  
         [0042]    D. Mechanical Stops  
         [0043]    Referring to FIGS. 5 a  and  5   b,  recall that movable portion  300  may be held by anchor  206  about 10 microns above a “floor” of the silicon substrate that remains after the etching process. Downward motion of movable portion  300  may be contained when the right edge  212  of frame  210  reaches this floor. Excessive upward movement of movable portion  300  may be contained by a stop  510  that extends in the opposite direction from the capacitive fingers of movable portion  300 , so that as movable portion  300  moves up, stop  510  moves down until it makes contact with the substrate floor. The length of stop  510  may be anywhere from one-fourth as long as the distance from anchor  206  to right edge  212  (allowing movable portion to move up four times as far as it can move down), to essentially the same length as the distance from anchor  206  to edge  212  (confining both ranges of motion roughly equally). It may be desirable that stop  510  have a low moment of inertia around anchor  206 , to reduce the attenuation of response of movable portion  300  in response to acceleration.  
         [0044]    Stop  510  may end with fingers  512  that are interdigitated with fingers  514  mounted on fixed portion  400 . Similarly, stops  520  mounted on movable portion  300  may be interdigitated with fingers  522  mounted on fixed portion  400 . Stops  512 ,  520  constrain rotation and translation of movable portion  300  in the plane of the wafer.  
         [0045]    E. Drive and Measurement Electronics  
         [0046]    An accelerometer as described above may have a sensitivity in the range of 10 to 15 fF/g (femtofarads per g of acceleration).  
         [0047]    As discussed in section  1 , supra, accelerometer  100  may use a differential capacitor approach. In a differential capacitor arrangement, for an acceleration in one direction, capacitance increases between one pair of electrodes, and capacitance decreases between the other pair. For acceleration in the opposite direction, the changes in capacitance are reversed. Thus, the difference between the capacitances indicates the amount of acceleration. An ASIC (application-specific integrated circuit) converts the capacitance difference into a voltage that represents acceleration.  
         [0048]    In one design, the ASIC places equal but opposite square wave voltages across the two capacitors and integrates the difference of the capacitor currents. The output of the integrator will be a voltage that is proportional to the difference in capacitance. This voltage is then amplified and low pass filtered to give the desired sensitivity and frequency response. A programmable voltage can be added or subtracted from this signal to provide for an offset adjustment. Additionally the gain of the capacitance-to-voltage conversion can be programmed to account for sensor performance distributions and different sensor designs.  
         [0049]    Just as an electrical generator can function as a motor if the proper electrical current is applied to the generator outputs, so voltages can be applied to conductors  310 ,  312 ,  314  and  316  of accelerometer, to cause movable portion  300  to move relative to fixed portion  400 . By altering the carrier signals used to sense the capacitance difference it is possible to implement a self-test mechanism. In self-test mode, electrical signals  310 ,  312 ,  314  and  316  are driven so that movable portion  300  is displaced, to verify that the movable structure  300  can move and that the appropriate capacitance change results. An electrical force is always generated by voltages such as the carrier signals for sensing. However, under normal operation the carrier signals are balanced and no net force arises. By altering the carrier signals such that the RMS voltages are not the same on the two sides of the differential capacitor used for sensing, a net force results. The net force causes a relative motion between the fixed portion  400  and movable portion  300 . This is a standard self-test method used in most commercial accelerometer ASIC&#39;s.  
         [0050]    ASIC&#39;s operating under this principle, as well as other techniques for translating a capacitance change into a voltage representing acceleration, are available from a number of universities and companies, including Kionix, Inc., Bosch GmbH, and MicroSensors, Inc. of Costa Mea, Calif.  
       III. Fabrication  
       [0051]    The overall silicon structure may be manufactured using silicon fabrication technologies available from Kionix, Inc. of Ithaca, N.Y. This is a mature process that is well suited to mass production. The Kionix process is an all-dry process, and lithography steps are carried out on planar surfaces.  
         [0052]    Referring to FIGS. 6 a - 6   g,  accelerometer  100  may be fabricated using a plasma micromachining process. One such plasma micromachining process may use four masks and industry-standard silicon wafers. The first mask may define trenches that are etched into the silicon to form isolation joints. As shown in FIG. 6 a,  these trenches may be filled with silicon dioxide  612 . Using the second mask, vias  620  may be defined and opened in field. Implants  622  may be made and aluminum  624  may be deposited. As shown in FIG. 6 b,  using the third mask, metal  624  may be patterned to break electrical connections where necessary. The fourth and final mask may be used to define the structural beams. The profile of the structural beams may etched into the silicon using a production ICP silicon etcher, for example, a PlasmaTherm VLR 770 with ICP Bosch Etch &amp; ICP Oxide Etch Chambers, resulting in the structures shown in FIG. 6 e.  The sidewalls may be passivated  630  with a deposited layer of silicon dioxide. The oxide on the trench bottoms that surround the beams may be cleared using an anisotropic silicon dioxide etch, while the sidewall passivation  630  remains, yielding the configuration of FIG. 6 f.    
         [0053]    Finally, as shown in FIGS. 6 c  and  6   g,  the silicon may be etched isotropically to release  640  the beams  642  from the substrate  644 . (Beam  642  may be any one of electrode fingers  116 ,  118 , beams  332 ,  338 , torsional flexure  120 , or any other portion of movable structure  300 . In some embodiments, fixed structure  400  may also be released from substrate  644 , and the truss structure of f) “Tails”  646  of oxide may extend below the silicon of beams  642 . These tails  646  may provide added stability in thermal expansion, because tails  646  may counter-balance any bending moment imparted by thermal expansion or contraction of the oxide  648  at the tops of the beams. In embodiments in which the release etch is carried out as a dry-etch process, stiction between adjacent structures or between structures and the substrate floor may be reduced to negligible levels, or so as to be non-existent.  
         [0054]    Referring again to FIGS. 3 and 4, in conjunction with FIGS. 6 c  and  6   g,  in some embodiments some of the fingers  112 ,  114 ,  116 ,  118  may be omitted, to make easer the step of FIGS. 6 c  and  6   g  in which beams  114 ,  118 ,  332 ,  338 ,  642  are released from the substrate. On the other hand, preserving all of the electrode fingers increases the sensitivity of accelerometer  100 .  
         [0055]    Typical beams  642  generated by the plasma micromachining process are 2 μm wide, 10 to 30 μm tall, and separated from the substrate by 15 μm. Structures that are larger than 10 μm wide generally do not release from substrate  644  during the isotropic etch. Such wide structures may provide the points where the movable or fixed structures anchor to the silicon substrate.  
         [0056]    To form large structures on the order of millimeters, typically the beams are laid out in an open cellular structure, as shown in FIG. 2. Such layouts aid in achieving higher oscillation Q&#39;s (a high Q-factor oscillator is one that oscillates consistently at the same frequency, a low Q oscillator may resonate a different frequencies depending on the impulse applied). A high oscillation Q may in turn improve precision in accelerometer  100 . Since the structures formed from the plasma micromachining process are predominantly composed of stress-free, single-crystal crystal silicon, which is a well-characterized and reproducible material, the performance of the structures may be predictable and reproducible.  
         [0057]    Processes for forming accelerometer  100 , and isolation joints  160 ,  360 , are discussed in U.S. Pat. No. 6,239,473, Adams et al., Trench Isolation Process for Microelectromechanical Devices, U.S. Pat. No. 5,719,073, Adams et al., Microstructures and Single-mask, Single-crystal Process for Fabrication Thereof, U.S. Pat. No. 5,846,849, Microstructure and Single mask, Single-crystal Process for Fabrication Thereof, U.S. Pat. No. 6,051,866, and S. G. Adams et. al., “Single-Crystal Silicon Gyroscope with Decoupled Drive and Sense,” in Micromachined Devices and Components V, Patrick J. French, Eric Peeters, Editors, Proceedings of SPIE Vol. 3876, 74-83(1999), K. A. Shaw, Z. L. Zhang, and N. C. Macdonald, “SCREAM I: A single mask, single-crystal silicon process for microelectromechanical structures,” Sensors and Actuators A, vol. 40, pp. 63-70, 1994, and Z. L. Zhang, N. C. MacDonald, “A rie process for submicron, silicon electromechanical structures,” J. Micromech. Microeng., v2, pp. 31-38, 1992, all of which are incorporated herein by reference.  
       IV. Alternative Embodiments  
       [0058]    In another embodiment, electrodes  112 ,  114 ,  116 ,  118  may be formed out of a thick polysilicon layer deposited on a silicon substrate wafer, with the silicon substrate serving only as a structural substrate or as shielding, and not playing an active role in sensing.  
         [0059]    In another embodiment, electrodes  11 ,  114 ,  116 ,  122 ,  128  may be formed in multiple SOI (silicon-on-insulator) layers laid on the silicon substrate wafer.  
         [0060]    In another embodiment, electrodes  112 ,  114 ,  116 ,  118 ,  122 ,  128  may be formed in multiple metal layers laid on the silicon, with the silicon serving only as a structural substrate or as shielding, and not playing an active role in sensing.  
         [0061]    Referring to FIG. 7, torsional flexure  120  may be reconfigured to reduce motion in directions other than the z-axis  130  (up and down out of the paper) while preserving compliance for z-axis motion. For instance, torsional flexure  120  may configured in a more triangular shape, to maintain good torsional compliance - and freedom of movement of movable portion  300  along the z-axis—while maintaining good stiffness in other directions and resistance to other vibrational modes.  
         [0062]    Referring again to FIG. 7, additional structures  710 ,  712 ,  714  may be incorporated to maintain a relatively uniform density of structure, to improve the uniformity with which the fabrication steps operate. In alternative embodiments, the spacing between some elements may be increased, to ease the release step (see FIGS. 6 c  and  6   g ).  
         [0063]    The placement of electrodes  112 ,  114 ,  116 ,  118  may be arranged to reduce temperature-sensitive curvature of the device that may result from fabrication.  
         [0064]    The internal truss structure of movable portion  300  and fixed portion  400  may be configured to improve rigidity.  
         [0065]    In the embodiment shown in FIG. 2, movable portion  300  is arranged as a “diving board,” cantilevered toward a single side of anchor point  206 . In other embodiments, movable electrodes  116 ,  118  may be arranged as a “teeter totter,” arranged bilaterally about a central resilient mounting. In other cases, electrodes  112 ,  116  may be formed on one side, and electrodes  114 ,  118  on the other. In other cases, movable electrodes  116 ,  118  may be arranged on one side of anchor point  206 , and a dummy mass may extend from the other side of anchor point  206 . The two sides of the teeter totter will generally have different rotational moments about the resilient mounting, so that acceleration will induce rotation.  
         [0066]    It should be understood that all dimensions, electrical values, geometrical aspects, fabrication technologies, etc. describe only some example embodiments as they may be preferred in 2000-01. As new fabrication technologies emerge, these values may change.  
         [0067]    For the convenience of the reader, the above description has focused on a representative sample of all possible embodiments, a sample that teaches the principles of the invention and conveys the best mode contemplated for carrying it out. The description has not attempted to exhaustively enumerate all possible variations. Further undescribed alternative embodiments are possible. It will be appreciated that many of those undescribed embodiments are within the literal scope of the following claims, and others are equivalent.