Abstract:
A method for reduces the sticking of proof masses in micro-electromechanical systems (MEMS) devices to sense plates in the MEMS device due to acceleration forces to which the MEMS device is subjected. The method includes determining a beginning of acceleration events which would cause proof masses to contact sense plates, reducing sense bias voltages to the sense plates, determining an end of the acceleration event, and increasing sense bias voltages to their former levels.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to micro-electromechanical systems (MEMS), and more specifically, to avoidance of stick-down within MEMS devices due to forces generated during operation of the MEMS devices. 
     Micro-electromechanical systems (MEMS) integrate electrical and mechanical components on the same substrate, for example, a silicon substrate, using microfabrication technologies. The electrical components are fabricated using integrated circuit processes, while the mechanical components are fabricated using micromachining processes that are compatible with the integrated circuit processes. This combination makes it possible to fabricate an entire system on a chip using standard manufacturing processes. 
     One common application of MEMS devices is in the design and manufacture of sensor devices. The mechanical portion of the sensor device provides the sensing capability, while the electrical portion of the sensor device processes the information received from the mechanical portion. One example of a MEMS device is a gyroscope. Some inertial measurement units (IMUs) incorporate one or more MEMS gyroscopes MEMS gyroscopes. 
     One known type of MEMS gyroscope uses vibrating elements to sense angular rate through the detection of a Coriolis acceleration. The vibrating elements are put into oscillatory motion in a drive plane, which is parallel to the substrate. Once the vibrating elements are put in motion, the gyroscope is capable of detecting angular rates induced by the substrate being rotated about an input axis. Coriolis acceleration occurs in a sense plane, which is perpendicular to both the drive plane and the input plane. The Coriolis acceleration produces a Coriolis motion having an amplitude proportional to the angular rate of the substrate. 
     However, due to external acceleration forces and electrostatic forces within the MEMS device, the vibrating elements sometimes become stuck to sense plates which are mounted on the substrate, affecting operation of the MEMS device. Such phenomena are sometimes referred to as “stick-down”. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, a method for reducing the sticking of proof masses to sense plates in micro-electromechanical systems (MEMS) devices is provided. The method comprises determining a beginning of acceleration events, which may cause proof masses to contact sense plates, reducing sense bias voltages to the sense plates during the event, determining an end of the acceleration event, and increasing sense bias voltages to the pre-reduced levels. 
     In another aspect, a micro-electromechanical systems (MEMS) device is provided which comprises a substrate, and a plurality of sense plates, a plurality of motor drive combs, and a plurality of motor pickoff combs, all attached to the substrate. The MEMS device further comprises a plurality of proof masses each suspended above one of the sense plates and between one of the motor drive combs and one of the motor pickoff combs, and a control circuit configured to control a sense bias voltage applied to the sense plates based upon acceleration forces applied to the MEMS device. 
     In still another aspect, a control circuit for reducing stick-down within a micro-electromechanical systems (MEMS) device is provided. The control circuit comprises a processor configured to control sense bias voltages applied to the sense plates based upon acceleration forces applied to the MEMS device. 
     In yet another aspect, a method for suppressing stick-down of proof masses to sense plates in micro-electromechanical systems (MEMS) devices is provided. The method comprises adjusting a sense bias voltage applied to the sense plates based upon acceleration forces applied the MEMS device. 
     In a further aspect, a micro-electromechanical systems (MEMS) gyroscope is provided which comprises a substrate, and a plurality of sense plates, a plurality of motor drive combs, and a plurality of motor pickoff combs all attached to the substrate. The gyroscope also comprises a plurality of proof masses each suspended above one of the sense plates and between one of the motor drive combs and one of the motor pickoff combs. The gyroscope also comprises a control circuit configured to reduce or eliminate stick-down between the proof masses and the sense plates. The circuit is configured to adjust a sense bias voltage applied to the sense plates based upon an amount of acceleration applied to the MEMS device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a MEMS device during normal operation. 
     FIG. 2 is an illustration of a MEMS device exhibiting stick-down of a proof mass. 
     FIG. 3 is an illustration of a micro-electromechanical system (MEMS) device which utilizes control circuitry to avoid stick-down of proof masses. 
     FIG. 4 is a flowchart illustrating a stick-down reduction method utilizing the MEMS device of FIG.  3 . 
     FIG. 5 is a chart illustrating acceleration over time for a MEMS device and a sense bias voltage over time for the MEMS device as controlled utilizing the circuitry of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a side plan view of a known exemplary micro-electromechanical system (MEMS) device  10 , specifically a MEMS gyroscope. MEMS device  10  is formed on a substrate  12  and includes at least one proof mass  14 ,  15  suspended above a respective sense plate  16 ,  17  by a plurality of suspensions (shown in FIG.  3 ). Proof masses  14 ,  15  are fabricated from any mass suitable for use in a MEMS device. In one embodiment, proof mass  14  is a plate of silicon. Other materials compatible with micro-machining techniques may also be utilized. While FIG. 1 shows two proof masses  14 ,  15 , MEMS devices utilizing less than or greater than two proof masses  14 ,  15  may also be utilized. 
     MEMS device  10  also includes motor drive combs  18 ,  19  and motor pickoff combs  20 ,  21  which correspond to respective proof masses  14 ,  15 . As shown, proof mass  14  is suspended substantially between motor drive comb  18  and motor pickoff comb  20 , and proof mass  15  is suspended substantially between motor drive comb  19  and motor pickoff comb  21 . Although not shown in FIG. 1, proof masses  14 ,  15  are caused to oscillate between their respective motor drive comb  18 ,  19  and motor pickoff comb  20 ,  21  due to a motor drive signal applied to motor drive combs  18 ,  19 . A bias voltage is applied to sense plates  16 ,  17  and a changing position of proof masses  14 ,  15  is detected, as the movement causes a change in capacitance between proof mass  14  and sense plate  16  and between proof mass  15  and sense plate  17 . 
     FIG. 2 illustrates a side plan view of MEMS device  10  (also shown in FIG. 1) while it is subjected to a large rotational acceleration, as shown by input rate arrow  30 . As proof masses  14 ,  15  are suspended with flexible suspensions (shown in FIG.  3 ), an excessive linear acceleration force causes proof mass  15  to contact sense plate  17 . In one embodiment, sense plates  16 ,  17  are at a potential of an applied bias voltage, and proof masses  14 ,  15  are at a neutral potential. As proof mass  15  approaches or makes physical contact with sense plate  17 , electrostatic force caused by the difference in potential causes proof mass  15  to stick to sense plate  17 . This condition is generally referred to as “stick-down”, and as described above, is contact between a proof mass and a sense plate in the MEMS device caused by at least one of acceleration forces applied to the MEMS device and a voltage difference between the proof mass and the sense plate. As proof mass  15  is stuck to its sense plate  17 , it cannot oscillate properly and operation of device  10  is adversely affected. 
     FIG. 3 is a top plan view of MEMS device  10 , and circuitry which reduces or eliminates stick-down of proof masses  14 ,  15 . Device  10  includes substrate  12  (shown in FIGS.  1  and  2 ), sense plates  16 ,  17 , and proof masses  14 ,  15 . In the embodiment shown, device  10  further includes suspensions  40  for supporting proof masses  14 ,  15 , and at least one cross beam  42  connected to suspensions  40 . Crossbeam  42  is affixed to substrate  12  at anchor  44 , which also provides support of crossbeams  42 . In an alternative configuration, suspensions  40  are individually and directly connected to substrate  12  at anchor points  46 , and crossbeam  16  is not utilized. In one embodiment, anchors  44  are formed as part of substrate  12 . While two anchors  44  are depicted in FIG. 3, any number of anchors  44  can be utilized. Anchors  44  are positioned along a respective cross beam  42  in any manner that provides support for proof masses  14 ,  15 . While four suspensions  40  are depicted in FIG. 3 for suspending cach of proof masses  14 ,  15 , any number of suspensions  40  which provide adequate support for proof masses  14 ,  15  may be utilized. Suspensions  40  are, in one embodiment, beams micro-machined from a silicon wafer. Suspensions  40  also act as springs allowing proof masses  14 ,  15  to move within a drive plane (X-axis) and a sense plane (Y-axis). 
     Proof mass  14  is located substantially between motor drive comb  18  and motor pickoff comb  20 . Proof mass  15  is located substantially between motor drive comb  19  and motor pickoff comb  21 . As known in the art, proof masses  14 ,  15  include a plurality of comb-like electrodes  48 . A portion of electrodes  48  extend towards motor drive combs  18 ,  19  and a portion of electrodes  48  extend towards motor pickoff combs  20 ,  21 . While, in the illustrated embodiment, proof masses  14 ,  15  have ten electrodes  48 , it is known to utilize proof masses incorporating different numbers of electrodes. 
     Motor drive comb  18  includes a plurality of comb-like electrodes  50  extending towards proof mass  14 . Motor drive comb  19  includes a plurality of comb-like electrodes  50  extending towards proof mass  15 . While motor drive combs  18 ,  19  are shown as having four electrodes  50 , the number of electrodes  50  on motor drive combs  18 ,  19  typically is determined by the number of electrodes  48  on their respective proof mass  14 ,  15 . Motor drive combs  18 ,  19  are typically connected to a motor drive circuit  52 . Motor drive comb  18  and motor drive comb  19  are driven at potentials that are opposite from one another, in one embodiment through use of an inverter circuit  54 . Electrodes  48  and electrodes  50  are interdigitated as they extend from proof mass  14  and motor drive comb  18 , and from proof mass  15  and motor drive comb  19 , and form capacitors. 
     Motor drive circuit  52  applying signals to motor drive combs  18 ,  19  causes respective proof masses  14 ,  15  to oscillate at substantially a tuning fork frequency along the drive plane (X-axis) by using the capacitors formed by the plurality of interdigitated comb-like electrodes  48 ,  50  of proof mass  14  and motor drive comb  18  and of proof mass  15  and motor drive comb  19 . MEMS device  10  has two closely spaced modes of oscillation. One of the modes, sometimes referred to as a motor mode, is driven by an electrostatic force, at a resonant frequency of device  10  to produce a relatively large amplitude of oscillation. When a rotational force is applied to device  10 , a Coriolis force is generated which is proportional to the velocity of proof masses  14 ,  15  in the motor mode. The Coriolis force drives a second mode of oscillation of device  10 , sometimes referred to as a sense mode. One or more electrodes are provided to detect oscillations in the sense mode, as described below, utilizing capacitance. A DC and/or an AC sense bias voltage  56  is applied to sense plates  16 ,  17 , which are sometimes referred to as sense electrodes, so that a motion of proof masses  14 ,  15  in the sense mode produces an output current. 
     Motor pickoff comb  20  includes a plurality of comb-like electrodes  58  extending toward proof mass  14  and motor pickoff comb  21  includes a plurality of comb-like electrodes  58  extending toward proof mass  15 . While motor pickoff combs  20 ,  21  are depicted as having four electrodes  58 , the number of electrodes  58  extending from motor pickoff combs  20 ,  21  is typically determined by the number of electrodes  48  on proof masses  14 ,  15 . Motor pickoff combs  20 ,  21  are sometimes referred to as sense combs. Electrodes  48  and electrodes  58  are interdigitated as they extend from proof masses  14 ,  15  and motor pickoff combs  20 ,  21  and form capacitors. The capacitors allow MEMS device  10  to sense motion in the drive plane (X-axis). As shown in FIG. 3, motor pickoff combs  20 ,  21  are typically connected to a DC bias voltage, for example, comb  21  connected to a positive bias voltage source  60 , and comb  20  connected to a negative bias voltage source  62 . Voltage source  62  supplies substantially the same voltage as source  60 , but at an opposite polarity. 
     Sense plate  16  is parallel to proof mass  14  and forms a capacitor. Sense plate  17  is parallel to proof mass  15  and forms a capacitor. If an angular rate (i.e. an aircraft turning) is applied to MEMS gyroscope  10  along an input plane (Z-axis) while proof masses  14 ,  15  are oscillating along the drive plane (X-axis), a Coriolis force is detected in the sense plane (Y-axis). The capacitance is used to sense motion in the sense plane (Y-axis). An output of MEMS gyroscope  10  typically is a signal proportional to the change in capacitance caused by the motion. Sense plates  16 ,  17  are typically connected to sense electronics, not shown in FIG.  1 . Sense electronics detect changes in capacitance as proof masses  14 ,  15  move toward and/or away from their respective sense plates  16 ,  17  and the respective motor drive combs  18 ,  19  and motor pickoff combs  20 ,  21 . 
     In one embodiment, proof mass  14  and proof mass  15  oscillate mechanically out-of-phase with one another and such oscillation is generally referred to as a differential mode of oscillation. For example, as proof mass  14  moves towards motor drive comb  18 , proof mass  15  moves in an opposite direction towards motor drive comb  19 . However, since suspensions  40  acts as springs for proof masses  14 ,  15 , other movements of proof masses  14  and  15  can exist. Specifically, while operating during high acceleration events, movements of proof masses  14 ,  15  of MEMS device  10  can exceed operational limits and therefore become stuck to sense bias plates  16 ,  17  resulting in the above described stick-down of proof masses. 
     Proof mass  14  is electrically attracted to biased sense plate  16  and proof mass  15  is electrically attracted to biased sense plates  17 . When the displacement of one or more of proof masses  14 ,  15  exceeds a limit, for example, through acceleration of MEMS device  10 , proof mass  14  can contact biased sense plate  16 , or proof mass  15  can contact biased sense plates  17  and become “stuck down”, due to the difference in potential between sense plates  16 ,  17  and proof masses  14 ,  15 . FIG. 3 further illustrates a control circuit  70  for reducing or eliminating stick-down. Circuit  70  operates by reducing sense bias voltages  60 ,  62  on sense plates  16  in anticipation of and during high acceleration events to which MEMS device  10  is subjected. By reducing sense bias voltages  60  and  62 , potential differences between proof mass  14  and sense plate  16  and proof mass  15  and sense plate  17  are reduced to a level which also reduces a propensity for stick-down. 
     However, changing sense bias voltages  60 ,  62  does not alter the normal mode of motor axis motion. Since sense bias voltages  60 ,  62  can be changed quickly by an event driven process, through control circuit  70 , stick-down is reduced while MEMS device  10  continues to operate properly. Control circuit  70  also allows MEMS device  10  to quickly resume the previous operating condition (i.e. return sense bias voltages to former levels) after a high acceleration event. 
     In one embodiment, control circuit  70  includes a mission processor  72  and associated memory  74  of an Inertial Measurement Unit (IMU)  76 . Mission processor  72  is programmed to command a change in sense bias voltages  60 ,  62  to a safe level prior to a high acceleration event, for example, high acceleration shock caused by steering mechanisms in a missile. After the high acceleration shock is completed, mission processor  72  is programmed to command a change in sense bias voltages  60 ,  62  back to normal operating levels. In the embodiment shown, control circuit  70  controls sense bias voltages through two methods. In the first, memory  74  for mission processor  72  is pre-programmed with acceleration events that will occur, for example, in the course of a programmed flight plan, and mission processor  72  causes sense bias voltages  60 ,  62  to be adjusted in accordance with the flight plan. In the second, control circuit  70 , through mission processor  72  monitors inputs  78  from acceleration sensors (not shown), and when high acceleration amounts are detected, mission processor  72  reduces sense bias voltages  60 ,  62  to a level that avoids stick-down of proof masses  14 ,  15 . 
     FIG. 4 is a flowchart  100  which illustrates at least one embodiment of the methods performed by control circuit  70  (shown in FIG.  3 ). First, high acceleration events are determined  102 , either through pre-programming or through acceleration sensor inputs  78  (shown in FIG. 3) as described above. Next, sense bias voltages  60 ,  62  (shown in FIG. 3) are reduced  104 , based on an amount of acceleration sensed or pre-programmed. An end to the high acceleration event is determined  106 , again, either through sensors or pre-programming, and sense bias voltage  60 ,  62  are increased  108  to their former levels. 
     FIG. 5 is a graph  120  of acceleration over time as it affects sense bias voltages  60 ,  62  (shown in FIG.  3 ), specifically a missile flight path, both pre-programmed flight and unexpected accelerations experienced during flight. At launch  122 , acceleration forces are extreme, and sense bias voltages are held at or near zero. As the launch is completed acceleration forces return to near zero, and the sense bias voltages are increased to their normal operating levels. A high acceleration event  124 , for example, steering mechanism engagement, and programmed maneuvers  126  cause acceleration forces to increase, and sense bias voltages are reduced accordingly through control circuit  70 , based upon an amount, and timing, of the acceleration forces. A sensed change in acceleration  128 , although shown as small compared to other acceleration forces, nonetheless causes control circuit  70  to provide an appropriate change to the sense bias voltages. 
     Operation of MEMS device  10  is at least partially based upon sense bias voltages applied to sense bias plates  16 ,  17  (shown in FIGS. 1,  2 , and  3 ) and proper operation (movement) of proof masses  14 ,  15 . As described above, acceleration forces experienced by a MEMS device  10 , electrical potential differences between proof mass  14  and sense bias plate  16 , and electrical potential differences between proof mass  15  and sense bias plate  17 , can combine to cause proof masses to become stuck-down to sense bias plates. Utilization of control circuit  70  provides compensation to sense bias voltages  60 ,  62 , which allow proof masses  14 ,  15  to move during periods of acceleration, but not remain stuck-down to sense bias plates  16 ,  17  after the acceleration forces have dissipated. 
     The above described embodiments are utilized to compensate operational characteristics of MEMS devices. While FIGS. 1,  2 , and  3  illustrate MEMS device  10  as an in-plane tuning fork gyroscope, other MEMS vibratory devices that use Coriolis acceleration to detect rotation, such as an angular rate sensing gyroscope, may benefit from the use of the circuits herein described. In addition, such circuitry can be incorporated into other MEMS devices, including, but not limited to, accelerometers, inertial measurement units, resonators, pressure sensors, and temperature sensors. While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.