Abstract:
A test circuit and method provide testing of a capacitive type microsensor. The method includes applying a first signal having a first voltage potential to an input of a microsensor during a non-test operating mode. The method also includes applying a second voltage signal having a second voltage potential different than the first voltage potential during a test mode. The second voltage potential induces a net differential electrostatic force in the microsensor. The method further includes the steps of monitoring an output signal of the microsensor, comparing the output signal to an expected value when the microsensor is in the test mode, and determining if the microsensor is functioning properly as a function of the comparison.

Description:
TECHNICAL FIELD  
         [0001]    The present invention generally relates to microsensors and, more particularly, relates to the testing of microfabricated capacitive type microsensors.  
         BACKGROUND OF THE INVENTION  
         [0002]    Microsensors are miniaturized sensing devices that are increasingly being employed for sensing dynamic motion such as acceleration and rate of change of position. Accelerometer microsensors measure the second derivative of displacement with respect to time and include linear and angular accelerometer microsensors. Linear and angular accelerometers are frequently employed to generate an output signal (e.g., voltage) proportional to the sensed acceleration for use in vehicle controls systems. For example, the sensed output from a linear accelerometer microsensor may be used to control safety-related devices on an automotive vehicle, such as front and side impact air bags, or may be employed for vehicle dynamics control and suspension control applications. The sensed output from an angular accelerometer microsensor may be employed to determine a potential vehicle rollover event, to control various automotive control devices, and to control disc drive read/write head assemblies.  
           [0003]    Many microsensors are capacitive type sensing devices that employ a capacitive coupling between a fixed plate and a movable plate that is movable in response to the sensed motion. One example of a linear accelerometer microsensor is disclosed in application Ser. No. 10/059,010, filed on Jan. 31, 2002, entitled “MICROFABRICATED LINEAR ACCELEROMETER,” which is hereby incorporated herein by reference. An example of an angular accelerometer microsensor is disclosed in U.S. Pat. No. 6,393,914, assigned to the assignee of the present invention, which is hereby incorporated herein by reference. Other examples of angular accelerometer microsensors are disclosed in commonly assigned U.S. application Ser. No. 10/085,536, filed on Feb. 28, 2002, entitled “BALANCED ANGULAR ACCELEROMETER,” and application Ser. No. 10/085,933, filed on Feb. 28, 2002, entitled “ANGULAR ACCELEROMETER HAVING BALANCED INERTIA MASS,” both of which are also hereby incorporated herein by reference.  
           [0004]    The aforementioned microsensors are generally fabricated by employing micro-electro-mechanical (MEM) fabrication techniques, such as etching and micromachining processes. Following manufacture of the microsensor, the microsensor is typically tested to determine if the microsensor functions properly and to determine the need for any scale factor calibration. In the past, mictosensors were typically tested by employing expensive hardware including a mechanical shaker designed to physically shake the microsensor under test to apply a predetermined motion (e.g., acceleration) to the microsensor. In response to applying the predetermined motion, the microsensor output is monitored and compared to an expected value. The deviation between the expected and measured values is processed to determine any error. The error may be used to determine if the microsensor is faulty and/or to trim integrated circuitry to adjust the scale factor calibration of the microsensor, prior to using the microsensor. While the mechanical shaker testing approach offers the ability to test the microsensor immediately following manufacture, the test procedure generally cannot be easily implemented once the microsensor is employed in an application. Additionally, the mechanical hardware of the shaker is generally expensive.  
           [0005]    It is therefore desirable to provide for a low cost, easy to implement test circuit for testing the functioning of a microsensor. It is further desirable to provide for a reliable self-test circuit for testing the microsensor that allows for testing while the microsensor is implemented in its intended application.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the teachings of the present invention, a test circuit and method are provided for testing a capacitive type microsensor. According to one aspect of the present invention, the method includes the steps of applying a first signal having a first voltage potential to an input of a microsensor during a non-test operating mode. The method also includes applying a second voltage signal having a second voltage potential different than the first voltage potential during a test mode. The second voltage potential induces a change in electrostatic force in the microsensor as compared to the first voltage potential. The method further includes the steps of measuring an output signal of the microsensor, comparing the output signal to an expected value when the microsensor is in the test mode, and determining if the microsensor is functioning properly as a function of the comparison.  
           [0007]    According to another aspect of the present invention, the test circuit includes a voltage selection circuitry for selecting one of a first voltage signal and a second voltage signal. The second voltage signal has a voltage potential offset from the first voltage signal. The test circuit has output circuitry for applying the first voltage signal to an input of a microsensor during a non-test mode of operation, and applying the second voltage signal to the input of the microsensor during a test mode. The test circuit also includes an input for receiving an output signal generated by the microsensor. A comparator compares the output signal of the microsensor to an expected value during the test mode. The test circuit further has a controller for determining whether the microsensor functions properly based on the comparison.  
           [0008]    The test circuit and method of the present invention induces an electrostatic force on the microsensor during the test mode and monitors the output signal to determine whether the microsensor is functioning properly. The test circuit and method may be employed following manufacture of a microsensor, and may be further employed following implementation of the microsensor in an intended application. Thus, the microsensor may be routinely tested to check if it is functioning properly.  
           [0009]    These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1 is a top view of a linear accelerometer microsensor formed on a substrate;  
         [0012]    [0012]FIG. 2 is a cross-sectional view of the linear accelerometer microsensor taken through lines II-II of FIG. 1;  
         [0013]    [0013]FIG. 3 is an enlarged view of section III of FIG. 1;  
         [0014]    [0014]FIG. 4 is an enlarged view of central section IV of FIG. 1;  
         [0015]    [0015]FIG. 5 is a block/circuit diagram illustrating processing circuitry including self-test circuitry coupled to the microsensor;  
         [0016]    [0016]FIG. 6 is a circuit diagram of the self-test circuitry for testing the microsensor;  
         [0017]    [0017]FIG. 7 is a circuit diagram further illustrating the self-test circuitry for testing the microsensor;  
         [0018]    [0018]FIG. 8 is a flow diagram illustrating a method for testing a microsensor according to the present invention; and  
         [0019]    [0019]FIG. 9 is a graph illustrating the application of input voltages to the microsensor during both a non-test mode and a test mode. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    The present invention is directed to an electronic self-test circuit and method for testing the operation of a mechanical capacitive type microsensor. The self-test circuit and method of the present invention are described herein for use with one example of a linear accelerometer microsensor  10 , generally shown in FIGS. 1-4 and also disclosed in U.S. application Ser. No. 10/059,010, filed Jan. 31, 2002. It should be appreciated that the test circuit and method of the present invention may be used to test any of a variety of microsensors having a capacitive-type sensing arrangement and, thus, is not limited to the linear accelerometer disclosed herein. The test circuit and method is applicable to any of a number of microsensors including, but not limited to linear accelerometers, angular accelerometers, linear rate sensors, and angular rate sensors. Examples of angular accelerometer microsensors are described in U.S. Pat. No. 6,393,914 and U.S. application Ser. Nos. 10/085,536 and 10/085,933, both filed on Feb. 28, 2002.  
         [0021]    Referring to FIGS. 1 and 2, a linear accelerometer microsensor  10  is illustrated for sensing linear acceleration along a designated sensing axis, shown configured as the X-axis. The linear accelerometer microsensor  10  senses linear acceleration along the sensing X-axis, while preventing the sensing of linear off-axis accelerations along other axes, such as the Y-axis and Z-axis, and rotational cross-axis accelerations. The linear accelerometer microsensor  10  is a micromachined accelerometer having an inertial mass and supporting structure.  
         [0022]    The microsensor  10  is fabricated on a single-crystal silicon substrate  60  using a trench etching process, such as DRIE and bond-etchback process. The etching process may include etching out a pattern from a doped material suspended over a cavity  34  to form a conductive pattern that is partially suspended over the cavity  34 . One example of an etching process that may be used to form the microsensor  10  is disclosed in commonly assigned U.S. Pat. No. 6,428,713, issued on Aug. 6, 2002, and entitled “MEMS SENSOR STRUCTURE AND MICROFABRICATION PROCESS THEREFOR,” which is hereby incorporated herein by reference. While the microsensor  10 , as described herein, is fabricated on a single-crystal silicon substrate using a trench etching process, it should be appreciated that the microsensor  10  could be fabricated using other known fabrication techniques, such as: an etch and undercut process; a deposition, pattern, and etch process; and an etch and release process.  
         [0023]    The linear accelerometer microsensor  10  includes an inertial mass  12 , generally formed in the shape of an annular ring, suspended over cavity  34 , and a stationary central member  15  trench etched from the mass  12  and fixedly attached to the underlying substrate  60  via oxide layer  64  and centered in the annular ring-shaped mass  12 . The inertial mass  12  has a plurality of rigid comb-like conductive fingers  14  extending outward from the outer peripheral edge of the annular ring to serve as movable capacitive plates. The conductive fingers  14  are formed along an axis (e.g., Y-axis) perpendicular to the sensing axis (e.g., X-axis). The inertial mass  12  with comb-like conductive fingers  14 , is a movable mass that is suspended over a cavity by support arms which are formed to allow inertial mass  12  to move linearly about the sensing X-axis when subjected to a linear acceleration along the sensing X-axis of the linear accelerometer  10 . For purposes of discussion herein, the X-axis and Y-axis are defined as shown oriented in FIG. 1, and the Z-axis is defined as shown in FIG. 2.  
         [0024]    The linear accelerometer microsensor  10  has a main central portion having a substantially elliptical shaped ring, with the conductive fingers  14  extending outward from the central portion and perpendicular to the sensing X-axis. Accordingly, the plurality of conductive finger  14  are arranged extending along the Y-axis. The length of the conductive fingers  14  may vary as shown such that longer conductive fingers  14  are formed at the narrower part of the central portion of mass  12 , as compared to the wider part of the central portion of mass  12 , to achieve a substantially round-shape for the overall configuration of the inertial mass  12  and conductive fingers  14 .  
         [0025]    The inertial mass  12  is shown generally suspended above cavity  34  via a support assembly including four support arms (i.e., tethers)  16 A- 16 D. The four support arms  16 A- 16 D are spaced apart from one another so as to support four corresponding quadrants of the inertial mass  12 . While four support arms  16 A- 16 D are shown and described herein, any number of a plurality of support arms may be employed.  
         [0026]    A central member  15  is fixed to the underlying substrate and is located substantially in the center region of the inertial mass  12 . The central member  15  is connected to rigid members  19  extending on opposite sides along the X-axis, with each of the support arms  16 A- 16 D extending along the Y-axis from the outer end of one of the rigid members  19 . Thus, support arms  16 A- 16 D are formed as extensions from the rigid members  19  which, in turn, are formed as extensions from the central member  15 . The central member  15  and rigid members  19  are substantially fixed with respect to the substrate and are generally inflexible to acceleration. The support arms  16 A- 16 D are flexible beams that act as springs which are compliant to bending along the sensing X-axis, but are relatively stiff to bending in the direction of the Z-axis which extends perpendicular to a plane formed by the X-axis and Y-axis. Additionally, the extension of the support arms  16 A- 16 D along the Y-axis further prevents movement along the Y-axis. The support arms  16 A- 16 D may have a thickness (depth) in the range of three to two hundred micrometers and a width in the range of one to twenty micrometers. According to one example, support arms  16 A- 16 D may have a thickness of approximately thirty micrometers as compared to a width of approximately ten micrometers to provide a sufficient aspect ratio of thickness-to-width to allow for flexibility along the X-axis and stiffness in the Z-axis.  
         [0027]    A pair of parallel slots (trenches)  17  are etched in the inertial mass  12  to form each of the support arms  16 A- 16 D. The slots  17  extend through the entire depth of the inertial mass  12  and, in effect, results in slots  17  formed on opposite sides of each support arm. The slots  17  form air gaps which allow the support arms  16 A- 16 D to be connected at a location radially outward from the inner edge, thereby providing for an increased effective overall length and greater flexibility of the support arms  16 A- 16 D. The four support arms  16 A- 16 D thereby substantially suspend the inertial mass  12  above cavity  34 , and allow linear movement of the inertial mass along the X-axis when subjected to linear acceleration along the X-axis. By employing four support arms  16 A- 16 D, the entire structure is stiff with respect to linear accelerations along the Y-axis, yet the inertial mass  12  is free to move along the X-axis within the constraints of the support arms  16 A- 16 D.  
         [0028]    Fixed to a thick oxide insulation layer  64  on top of substrate  60  are four fixed electrodes  20 A- 20 D, each having a plurality of fixed capacitive plates  24  interdisposed between adjacent movable capacitive plates  14 , to form four banks of variable capacitors. The first fixed electrode  20 A has a clocked input line  22 A for receiving a clocked signal CLKPB  26 , such as a square wave signal. The plurality of fixed capacitive plates  24  provided with the first fixed electrode  20 A are interdisposed between adjacent movable capacitive plates  14  of inertial mass  12  for approximately one-quarter rotation (i.e., a ninety degree window) of inertial mass  12 , to provide a first bank of capacitors. The second fixed electrode  20 B likewise has a plurality of fixed comb-like capacitive plates  24  interdisposed between adjacent movable capacitive plates  14  of inertial mass  12  for approximately one-quarter of its rotation to provide a second bank of capacitors. The second fixed electrode  20 B has a clocked input  22 B for receiving a clocked signal CLKP  28 , such as a square wave signal. The third fixed electrode  20 C also includes a plurality of fixed comb-like capacitive plates  24  for approximately one-quarter of movable capacitive plates  14  of inertial mass  12 , to provide a third bank of capacitors, and likewise receives clocked signal CLKPB  26  via input line  22 C. The fourth fixed electrode  20 D has a plurality of fixed capacitive plates  24  for approximately the remaining one-quarter of the movable capacitive plates  14  of inertial mass  12 , to provide a fourth bank of capacitors, and receives clocked signal CLKP  28  via input line  22 D. It should be appreciated that the number of fixed electrodes can be increased to multiplies of four, as represented by equation 4×N, where N=1, 2, 3, 4, etc., which may advantageously provide for good matching and cross-axis rejections.  
         [0029]    Each of the fixed electrodes  20 A- 20 D are formed near the outer perimeter of the inertial mass  12  extending through an angular rotation of approximately ninety degrees (90°). Adjacent fixed electrodes  20 A- 20 D are dielectrically isolated from one another via isolators  18 . Each isolator  18  has one or more slots that serve to provide a dielectric air gap. The fixed electrodes  20 A- 20 D and corresponding plurality of fixed capacitive plates  24  are fixed in place supported on top of insulation layer  64  and substrate  60 . Accordingly, the inertial mass  12  and its rigid outer peripheral capacitive plates  14  are able to move relative to fixed capacitive plates  24  in response to a linear acceleration experienced along the sensing X-axis.  
         [0030]    The inertial mass  12  and movable capacitive plates  14  are electrically conductive and are electrically connected via an output line  30  to output pad  32  for providing an output charge V 0 . The output charge V 0  is processed to generate a voltage which has a voltage level indicative of the linear displacement of the inertial mass  12  relative to the fixed electrodes  20 A- 20 D due to linear acceleration about the sensing X-axis. Accordingly, by measuring the output charge V 0  at output pad  32 , the linear accelerometer microsensor  10  provides an indication of the linear acceleration experienced along the sensing X-axis.  
         [0031]    With particular reference to the cross section shown in FIG. 2, the linear accelerometer microsensor  10  includes substrate  60  which serves as the underlying support. Substrate  60  may include a silicon or silicon-based substrate having the thick oxide insulation layer  64  formed on the top surface, and a bottom oxide insulation layer  62  formed on the bottom surface. The substrate  60  may include silicon, or alternate materials such as glass or stainless steel. The substrate  60  and thick oxide insulation layer  64  are configured to provide a cavity  34  below the inertial mass  12 . Additionally, substrate  60  and oxide layer  64  form a central pedestal  36  below the fixed central member  15  for purposes of fixing the central member  15  in place relative to the substrate  60 . Central pedestal  36  also provides structural support during the fabrication process.  
         [0032]    Formed above the substrate  60  and on top of insulation layer  64  is an EPI layer  66  made of conductive material, such as silicon. EPI layer  66  is made of a conductive material and is etched to form various components including the inertial mass  12 , central member  15 , isolation trenches  80 , air gaps  13  and  25 , and other elements that support or isolate conductive signal paths. Trenches  80  and air gaps  13  and  25  provide physical and electrical isolation between adjacent elements. The EPI layer  66  may have a thickness in the range of 3 to 200 micrometers, and more particularly of approximately 30 micrometers. With the main exception of the inertial mass  12 , central member  15 , and fingers of movable and stationary plates, the EPI layer  66  further includes a field passivation layer  68  disposed on the top surface thereof. The conductive signal paths of electrodes  20 A- 20 D, lines  22 A- 22 D, and data line  30  are formed on top of the conductive EPI layer  66  and partially on top of dielectric field passivation layer  68  to provide signal transmission paths. In addition, a metal passivation layer  90  is formed over each of these signal paths.  
         [0033]    Prior to the etching process, the central pedestal  36  provides structural support for the EPI layer  66  to allow the central mass  15  to be fixedly provided on top thereof. By providing a central pedestal  36 , the structural integrity of the linear accelerometer microsensor  10  is enhanced during the fabrication process. After the etching process, the central pedestal  36  supports the central member  15  which, in turn, supports the inertial mass  12  via rigid members  19  and support arms  16 A- 16 D. By supporting the EPI layer  66  in the central region during the manufacturing process, the maximum stress experienced is greatly reduced.  
         [0034]    Referring to FIG. 3, a portion of the linear accelerometer microsensor  10  is further illustrated in greater detail. Data line  30  extends within a pair of parallel radial slots  31  extending along the Y-axis formed through the entire depth of inertial mass  12  to provide an electrical path between the central member  15  and the output pad  32 . The slots  31  provide dielectric isolation between the data line  30  and inertial mass  12 , as well as between adjacent fixed electrodes  20 A and  20 B, while allowing the inertial mass  12  and capacitive plates  14  to move along the X-axis, within limits. Trenches  80  isolate the fixed electrodes from the outer surrounding elements. The fixed capacitive plates  24  are interdisposed between adjacent movable capacitive plates  14  and separated one from another via air gap  25 . The air gap  25  between capacitive plates  14  and  24  allows for movable capacitive plates  14  to move relative to the fixed capacitive plates  24 . Each of the movable capacitive plates  14  has a very small mass as compared to the inertial mass  12 , and are rigid to prevent movement relative to the inertial mass  12 . Additionally, the movable and fixed capacitive plates  14  and  24 , respectively, each has a thickness equal to the thickness of the EPI layer  66 . Because total change of capacitance is proportional to thickness of the capacitive plates  14  and  24 , the signal-to-noise ratio is enhanced with enlarged thickness.  
         [0035]    The air gap  25  between capacitive plates  14  and  24  is greater on one side of plate  14  as compared to the opposite side. For example, with respect to the bank of capacitors formed by fixed electrode  20 B, the width W L  of air gap  25  between capacitive plates  14  and  24  is approximately twice the width W s . The air gap  25  between adjacent pairs of capacitive plates  14  and  24  is configured substantially the same for each of the fixed capacitive plates connected to the fixed electrode. However, for adjacent fixed electrodes  20 A and  20 B, the orientation of the conductive plates  14  and  24  is switched in that the larger air gap with W L  and smaller gap width W s  of air gap  25  is on the opposite side as compared to the adjacent fixed electrode. For example, the fixed capacitive plates  24  on fixed electrode  20 A are separated from movable capacitive plates  14  by an air gap  25  of width W L  twice as wide on the left side of capacitive plates  14  as the width W s  on the right side of capacitive plates  14 , while fixed electrode  20 B is configured with a larger air gap width W L  on the right side of plate  14  as compared to its left side. Additionally, motion stop beads (not shown) can be formed on either or both of the movable and fixed capacitive plates  14  and  24 , respectively, for limiting the relative movement between capacitive plates  14  and  24 , in the event excessive linear acceleration is experienced along the X-axis.  
         [0036]    The linear accelerometer microsensor  10  shown and described herein has four banks of variable capacitors formed by capacitive plates  14  and  24  arranged in four quadrants. The capacitive plates  14  and  24  associated with the first fixed electrode  20 A in quadrant  1  are a mirror image of the capacitive plates  14  and  24  associated with the fixed electrode  20 B in quadrant  2 . Likewise, the arrangement of the capacitive plates  14  and  24  associated with fixed electrode  20 C in quadrant  3  is a mirror image of the capacitive plates  14  and  24  associated with fixed electrode  20 D in quadrant  4 . The capacitive plates  24  associated with fixed electrodes  20 A and  20 C in quadrants  1  and  3  have a certain positive-to-negative orientation with respect to capacitive plates  14 . In contrast, the positive-to-negative orientation between capacitive plates  14  and  24  for the fixed electrodes  20 B and  20 D in quadrants  2  and  4  are arranged oppositely with respect to the Y-axis. By alternating the orientation of the plurality of four banks of capacitors in four quadrants as disclosed, the linear accelerometer microsensor  10  essentially nulls out any residual rotational cross-axis sensitivities and linear off-axis sensitivities, and allows for linear acceleration to be sensed about the sensing X-axis. Further, by employing a plurality of fixed capacitive plates  24  commonly connected to fixed electrodes  20 A- 20 D a reduced number of signal input and output lines may be achieved.  
         [0037]    Referring to FIG. 4, an enlarged central portion of the linear accelerometer microsensor  10  is illustrated therein in greater detail. The central member  15  is shown separated from inertial mass  12  via air gap  13 . Rigid members  19  extend on opposite sides and likewise are separated from the inertial mass  12  via air gap  13 . At the outer ends of each of rigid members  19  are the vertically (Y-axis) disposed support arms  16 A- 16 D, each of which extends perpendicular to the sensing X-axis. Each of support arms  16 A- 16 D is formed by cutting out air gaps  13  from inertial mass  12 . Each of support arms  16 A- 16 D is formed of a continuous conductive signal line which, in addition to physically supporting the inertial mass  12 , also transmits electrical signals. Support arms  16 A- 16 D are formed by etching to remove material to form the bordering slots  17 . Support arms  16 A- 16 D flex within slots  17  to allow linear movement of the inertial mass  12  along the sensing X-axis relative to the central member  15  and rigid members  19 . Accordingly, support arms  16 A- 16 D provide rigid support along the Y-axis and the Z-axis, while allowing for linear motion due to acceleration about the X-axis.  
         [0038]    Referring to FIG. 5, a signal processing integrated circuit (IC)  100  is shown for providing the signals applied to the microsensor  10  and processing the signals generated by the microsensor  10 . The signal processing IC  100  includes self-test circuitry  110  for performing diagnostic testing of the microsensor  10 . A controller  102  is also shown for performing a self-test routine (method) according to the present invention. The fixed electrodes  20 A- 20 D are generally shown receiving clocked signal CLKPB at input pad  26  and clocked signal CLKP at input pad  28 . Clocked signals CLKPB and CLKP may be rectangular, e.g., square, wave-generated signals that have alternating high and low voltage levels of either V 1  and V 3  or V 2  and V 4  depending on whether the microsensor  10  is operating in the test mode or non-test mode. Clocked signal CLKPB is one hundred eighty degrees (180°) out of phase, i.e., inverse, as compared to clocked signal CLKP and therefore provides an opposite phase rectangular waveform. According to one example, the voltage potentials V 1  and V 3  applied to clocked signal CLKP or signal CLKPB is set at 5.0 volts and 1.0 volt, and the voltage potentials V 2  and V 4  applied to clocked signal CLKPB or signal CLKP is set at 4.5 volts and 0.5 volts.  
         [0039]    The signal processing integrated circuitry  100  includes a summer  30 , a charge-to-voltage converter and demodulator  33 , a summer  34 , an offset trim  36 , a continuous offset drift trim  38 , an output driver and gain trim  40 , and the self-test circuitry  110 . The summer  30  receives the output charge V 0  on output pad  32  and a charge V 02  received from the summation of the capacitors, represented herein as CT, when a voltage source V S  is applied thereto. Charge V 02  contains noise present in the sensed signal, and summer  30  subtracts the noise from the output charge V 0 . The output of summer  30  is applied to the charge-to-voltage converter and demodulator  33  which converts the processed charge to a voltage signal. The voltage signal is then input to summer  34  which receives a signal from an offset trim  36  and a signal from a continuous offset drift trim  38 . The offset trim  36  provides a signal which compensates for bias errors. The continuous offset drift trim  38  provides a signal which compensates for bias drift, particularly due to temperature variations. Accordingly, summer  34  sums the trimmed signals with the voltage output so as to compensate for bias errors. The bias compensated voltage is then applied to the output driver and gain trim  40  which rescales the voltage to within a desired range and slope and produces the output signal V OUT .  
         [0040]    The output signal V OUT  is processed by controller  102  which generally includes a microprocessor  104  and memory  106 . The controller  102  may include a commercially available microprocessor  104  capable of processing the output signal V OUT  and performing a self-test routine  200  as described herein. The memory  106  includes non-volatile memory storing the self-test routine  200 . While a microprocessor-based controller  102  is shown and described herein, it should be appreciated that the self-test circuit and method for testing a microsensor according to the present invention may be implemented in analog or digital circuitry. In one embodiment, the self-test circuitry  110  receives a control signal from controller  102  to initiate the self-test of the microsensor  10 . Alternately, the self-test method may be initiated via analog circuitry or via other input controls, such as a manual input.  
         [0041]    The self-test circuitry  110  controls the clocked voltage signals CLKP and CLKPB applied to the microsensor input pads  28  and  26 , depending on whether the microsensor  10  is operating in a non-test mode or a test mode. In the non-test mode, the clocked voltage signals CLKPB and CLKP applied to input pads  28  and  26 , respectively, are set at predetermined high and low voltage potentials V 2  and V 4 . Upon initiating the self-test mode, the clocked voltage signals CLKP and CLKPB applied to input pads  28  and  26 , respectively, are changed to high and low voltage potentials V 1  and V 3 , which are offset in amplitude from voltages V 2  and V 4  by a predetermined voltage offset. By changing the voltage potential of the input signals, such as increasing (or decreasing) the voltage potential applied to the input pads  28  and  26  by a predetermined offset voltage, an electrostatic force is generated in the microsensor  10 . The electrostatic force generated in the microsensor  10  causes the microsensor  10  to generate an output signal which is then compared to an expected value. If the measured microsensor output signal deviates from the expected value in excess of a predetermined amount, the microsensor  10  is determined to be out of calibration and/or faulty, and, hence an error flag is set.  
         [0042]    In the non-test mode of operation, the microsensor  10  provides a measurement of the sensed motion (e.g., linear acceleration). In measuring linear acceleration, the inertial mass  12 , when subjected to a linear acceleration about the sensing axis, moves relative to the fixed electrodes  20 A- 20 D and within the restraining limits of the support arms  16 A- 16 D. If the inertial mass  12  moves linearly and in a positive direction along the sensing axis, the opposing banks of variable capacitors formed by fixed electrodes  20 A and  20 C in quadrants  1  and  3  increase in capacitance, while the opposing banks of variable capacitors formed by electrodes  20 B and  20 D in quadrants  2  and  4  decrease in value, or vice versa. The change in capacitance provides a charge output signal V 0  that is indicative of the linear acceleration experienced. Since the support arms  16 A- 16 D are integrally formed within slots  17  in the inertial mass  12  and attached to the central member  15 , susceptibility to damage by external shock is thus reduced.  
         [0043]    Referring to FIG. 6, the test circuit  110  is shown including integrated circuitry configured to control initiation of the test mode, to provide the voltage potentials V 1  through V 4 , and to apply the selected voltage potentials as clocked signals CLKP and CLKPB to inputs on the microsensor. The test circuitry  110  includes an input  112  for receiving signal VSTCAP, and input  114  for receiving signal VSTL. Signal VSTL indicates whether the microsensor is to operate in either the test mode or the non-test mode. According to one example, VSTL is set to a logic one when the microsensor is in the test mode, and a logic zero when in the non-test mode. Alternatively, the logic of the VSTL signal may be reversed. The input signal VSTCAP controls which quadrants of the microsensor are to be tested with VSTCAP. In one state (e.g., logic one state), voltage potentials V 1  and V 3  are applied to the capacitive plates in quadrants  2  and  4  during the test mode, with VSTCAP in the other state (e.g., logic zero state), the voltage potentials V 1  and V 3  are applied to the capacitive plates in quadrants  1  and  3  during the test mode.  
         [0044]    Test circuitry  110  also includes input  116  for receiving a clock signal, such as a square wave signal. The CLK clock signal includes a clock frequency which is used to select the frequency of the alternating voltages applied as signals CLKPB and CLKP. The test circuitry  110  further includes a resistor divider network  140  including four resistors coupled in series between a voltage supply V DD  and ground. The voltage supply, according to one example, is +5 volts. The resistors R 1  through R 4  are selected to provide the voltage potentials V 1  through V 4  at corresponding nodes  130  through  136 . According to the example shown and described herein, voltage V 1  at node  136  is set to 5.0 volts, voltage V 2  at node  134  is set to 4.5 volts, voltage V 3  at node  132  is set to 1.0 volts, and voltage V 4  at node  130  is set to a voltage of 0.5 volts. The test circuitry  110  produces clocked signals CLKP and CLKPB at outputs  128  and  126 , respectively, which in turn are applied to the inputs  28  and  26  of the microsensor. The clocked signals CLKP and CLKPB have a frequency set by clock signal CLK and high and low voltage potential amplitudes set by the selected pairs of voltages V 1  and V 3  or V 2  and V 4 , depending on the selected mode of operation.  
         [0045]    Referring to FIG. 7, the self-test circuit  110  is further shown in a simplified form coupled to exemplary equivalent capacitors CS 1  and CS 2  which generally represent the equivalent electrical circuit of the above-described microsensor. Capacitor CS 1  represents the equivalent sum total of capacitance between electrode  20 A and movable plates  14 , and the capacitance between electrode  20 C and movable plates  14 , while capacitor CS 2  represents the equivalent sum total of capacitance between electrode  20 B and movable plates  14 , and the capacitance between the electrode  20 D and movable plates  14 . An operational amplifier A V  and capacitor C FB  represent an element of the charge-to-voltage amplifier network. Voltage V REF  is a reference voltage that defines the virtual ground of the amplifier A V . The voltages V 1  through V 4  generated by the test circuitry  110  are stable direct current (DC) reference voltages that are applied as inputs to switches S 1  through S 8 . The clock signal and its inverse CLKB define the state of the switches S 1  through S 8 , and thus set the frequency of the clocked signals CLKP and CLKPB. Signals ST and STB are complimentary signals controlling the state of switches S 9  through S 12  that are used to connect the input terminals of equivalent capacitors CS 1  and CS 2  to the voltage potentials V 1  through V 4 . According to an alternate embodiment, the switches S 9  through S 12  can be eliminated by employing a logic AND gate operation of the clock signals CLK and CLKB with the signals ST and STB.  
         [0046]    During the normal non-test mode of operation of the microsensor, the switches S 9  through S 10  are turned off while switches S 11  through S 12  are turned on. Clock signals CLK and CLKB are non-overlapped square wave signals, which periodically turn on and off the switches S 1  through S 8  resulting in square wave signals with high and low voltage amplitude swings from either V 1  to V 3  or V 2  to V 4  at the same frequency of the clock signal CLK and CLKB. When voltage is applied to the inputs of the microsensor, an attractive electrostatic force F e  is generated between the fixed and movable plates defining equivalent capacitance CS 1  and CS 2 . The electrostatic force F e  can be defined by the following equation. 
           F   e   =A ·ε·[( V   2   −V   REF ) 2 +( V   4   −V   REF ) 2 ]/[2·( G−x   1 ) 2 ] 
         [0047]    In the above equation, A represents the cross-sectional area of the capacitive plates of the equivalent capacitors CS 1  and CS 2 , ε represents the composite permitivity of the medium between the capacitor plates, and G−x 1  is the gap-width change between the capacitor plates of the microsensor. The equation describing the displacement of the movable plate with respect to the fixed plate is F e +(m×a)−Kx 1 =0, where K is the spring constant of the movable capacitive plate, and m×a is the internal force in the x-direction (for a linear accelerometer microsensor). Assuming no input acceleration, the above equation can be solved to find an expression for the gap-width between capacitive plates.  
         [0048]    To initiate the self-test, the high and low voltages V 2  and V 4  are switched out and high and low voltages V 1  and V 3  are switched in as signals CLKP and CLKPB. This, in effect, results in a change in the overall offset voltage applied to the microsensor which, in the current example shown, is an increase of about 0.5 volts. The electrostatic force generated during the self-test mode is represented by F S  shown in the following equation. 
           F   S   =A ·ε·[( V   1   −V   REF ) 2 +( V   3   −V   REF ) 2 ]/[2·( G−x   2 ) 2 ] 
         [0049]    G−x 2  is the gap-width change between the capacitor plates of the microsensor under test. During the test mode, there is a net force change across the gap which is proportional to the ΔF=F e   −F   S . By increasing the high and low voltages of clocked signals CLKP and CLKPB by an offset voltage, a net force change occurs between the fixed and movable capacitive plates. This change in net force results in a change in the output signal V OUT  generated by the microsensor. The output signal generated by the microsensor is then compared to an expected value to determine if the microsensor is faulty and/or needs to be calibrated.  
         [0050]    Referring to FIG. 8, a self-test routine  200  is illustrated for testing a microsensor according to the present invention. The self-test routine  200  begins at step  202  and reads the input signal VSTL in step  204 . In decision step  206 , the self-test routine  200  determines if the self-test has been requested, which occurs when the input signal VSTL is set equal to the binary one value. If no self-test has been requested, self-test routine  200  proceeds to step  208  to apply high and low voltage potentials V 2  and V 4  as the reference voltage for clock levels to all sensor signals CLKP and CLKPB. The microsensor is then operated in its normal non-test mode, during which the sensor output V OUT  is measured in step  210 , and the determined acceleration value is processed in step  212  before returning in step  214 .  
         [0051]    If the self-test mode has been requested, self-test routine  200  proceeds to decision step  216  to determine whether the self-test sign VSTLCAP signal is positive. If the self-test sign (VSTCAP) is positive, routine  200  proceeds to apply high and low voltage potentials V 1  and V 3  as the clocked signal CLKP input to the capacitor plates in quadrants  1  and  3  in step  218 . At the same time high and low voltage potentials V 2  and V 4  are applied as the clocked signal CLKPB input to the capacitor plates in quadrants  2  and  4 . If the self-test sign (VSTCAP) is negative, routine  200  applies high and low voltage potentials V 1  and V 3  as the clocked CLKPB signal input to the capacitor plates in quadrants  2  and  4  in step  220 . At the same time, high and low voltage potentials V 2  and V 4  are applied as the clocked signal CLKP input to the capacitor plates in quadrants  1  and  3 . Accordingly, the capacitive plates in quadrants  1  and  3  are tested separate from the capacitive plates in quadrants  2  and  4 . The self-test causes both negative and positive mechanical movement along the sensing axis of the microsensor.  
         [0052]    Once the signal inputs for clocked signals CLKP and CLKPB are selected in either of steps  218  or  220 , the self-test routine  200  proceeds to step  222  to generate an electrostatic force F e , and then to measure the sensor output V OUT  in step  224 . The sensor output V OUT  is compared to an expected value in step  226 . Routine  200  determines whether the comparison is within an allowable tolerance in decision step  226 . If the comparison is not within an allowable tolerance, self-test routine  200  sets an error flag in  230 , before ending the test in step  232 . It should be appreciated that by setting an error flag, the self-test routine  200  provides an indication that the microsensor is faulty and/or requires calibration.  
         [0053]    Referring to FIG. 9, one example of the clocked signals CLKP and CLKPB applied to the microsensor both during a non-test mode and a test mode are illustrated. The signal  250  and one hundred eighty degrees (180°) out of phase version illustrate the clocked signals CLKP and CLKPB to the microsensor during the non-test mode of operation during which the voltage potential applied to the microsensor is an alternating wavefonn which alternates between high and low voltage potentials V 2  and V 4 . During the test-mode of operation, represented by signal  260 , the clocked signals CLKP or CLKPB applied to the microsensor have alternating high and low voltage potentials V 1  and V 3 . Accordingly, the test circuit method of the present invention advantageously changes the high and low voltage potentials applied to the microsensor by an offset voltage, shown herein as 0.5 volts according to one example, so as to induce an electrostatic force in the microsensor, which generates an output in the microsensor that is then compared to an expected value to determine if the microsensor is functioning properly. It should further be appreciated that the self-test may be performed following manufacture of the microsensor, or may be performed at any time thereafter, including intermittent tests performed following use of the microsensor for its intended application.  
         [0054]    It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.