Abstract:
A micro-electromechanical systems (MEMS) device includes a substrate comprising at least one anchor, a proof mass having first and second deceleration extensions extending therefrom, a motor drive comb, a motor sense comb, a plurality of suspensions configured to suspend the proof mass over the substrate and between the motor drive comb and the motor sense comb. The suspensions are anchored to the substrate. A body is attached to the substrate. At least one deceleration beam extends from a first side of said body. The at least one deceleration beam is configured to engage at least one of the first and second deceleration extensions and slow or stop the proof mass before the proof mass contacts the motor drive comb and the motor sense comb.

Description:
BACKGROUND OF THE INVENTION 
     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 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. 
     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 along a motor (X) axis, which is parallel to the substrate, in a resonant mode of vibration referred to as a motor mode. 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 (Z) axis, which is perpendicular to the substrate. Coriolis acceleration occurs along a sense (Y) axis, which is also perpendicular to both the X and Z axes, causing oscillatory motion along the Y-axis, in a resonant mode referred to as a sense mode. The amplitude of oscillation of the sense mode is proportional to the angular rate of the substrate. However, the vibrating elements are sometimes acted upon by external forces. As an example, aircraft or other flight platforms sometimes make high gravitational force maneuvers. The forces can cause proof masses within the MEMS device, for example, a MEMS gyroscope, to contact a motor drive, a motor pickoff or a sense plate, sometimes at such a high rate of speed, that damage can occur to one or more of the above-listed components. Such contact is undesirable and effects performance of the MEMS device. 
     SUMMARY OF THE INVENTION 
     In an embodiment, a micro-electromechanical systems (MEMS) device includes a substrate comprising at least one anchor, a proof mass having first and second deceleration extensions extending therefrom, a motor drive comb, a motor sense comb, a plurality of suspensions configured to suspend the proof mass over the substrate and between the motor drive comb and the motor sense comb. The suspensions are anchored to the substrate. A body is attached to the substrate. At least one deceleration beam extends from a first side of said body. The at least one deceleration beam is configured to engage at least one of the first and second deceleration extensions and slow or stop the proof mass before the proof mass contacts the motor drive comb and the motor sense comb. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
         FIG. 1  is an illustration of micro-electromechanical system (MEMS) device which incorporates deceleration stops extending from the proof masses and a support structure. 
         FIG. 2  is an illustration showing an enlarged view of deceleration-stopping structure that can be implemented in the device shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the invention may utilize structure and/or concepts described in commonly owned U.S. Pat. No. 6,865,944, which is herein incorporated by reference in its entirety. 
       FIG. 1  illustrates a plan view of a micro-electromechanical system (MEMS) device  10 , for example, a gyroscope. MEMS device  10  is formed on a substrate (not shown) and includes at least one proof mass  12 , a plurality of suspensions  14  for supporting proof masses  12 , and at least one cross beam  16  connected to suspensions  14 . In an alternative configuration, suspensions  14  are individually and directly connected to the substrate. MEMS device  10  also includes motor drive combs  18 , motor pickoff combs  20 , and sense plates  22 , which correspond to individual proof masses  12 . 
     Proof masses  12  are fabricated from any mass suitable for use in a MEMS device. In one embodiment, proof mass  12  is a plate of silicon. Other materials compatible with micro-machining techniques may also be utilized. While  FIG. 1  shows two proof masses  12 , MEMS devices utilizing fewer or greater than two proof masses may also be utilized. 
     Proof masses  12  are located substantially between motor drive comb  18  and motor pickoff comb  20 . Proof masses  12  include a plurality of comb-like electrodes  26 . A portion of electrodes  26  extends towards motor drive comb  18  and a portion of electrodes  26  extends towards motor pickoff comb  20 . While, in the illustrated embodiment, proof masses  12  have thirty-four electrodes  26 , it is known to utilize proof masses incorporating different numbers of electrodes. In other embodiments of MEM devices (not shown), motor drive comb and motor pickoff comb may be located next to one another. 
     Proof masses  12 , in the embodiment shown, are supported above a respective sense plate  22  by suspensions  14 . While four suspensions  14  are depicted for suspending each proof mass  12 , any number of suspensions  14  which properly support proof masses  12  may be utilized. Suspensions  14  are, in one embodiment, beams micro-machined from a silicon wafer. Suspensions  14  also act as springs allowing proof masses  12  to move within a drive plane (X-axis) and a sense plane (Y-axis), as shown in  FIG. 1 . 
     Motor drive combs  18  include a plurality of comb-like electrodes  28  extending towards a respective proof mass  12 . While motor drive combs  18  are shown as having eighteen electrodes  28 , the number of electrodes  28  on motor drive combs  18  typically is determined by the number of electrodes  26  on the respective proof mass  12 . Motor drive combs are typically connected to drive electronics (not shown in  FIG. 1 ). Electrodes  26  and electrodes  28  are interdigitated as they extend from respective proof masses  12  and motor drive combs  18  and form capacitors which are utilized to generate motion in the drive plane (X-axis). 
     Motor pickoff combs  20  also include a plurality of comb-like electrodes  30  extending towards a respective proof mass  12 . While motor pickoff combs  20  are depicted as having eighteen electrodes  30 , the number of electrodes  30  extending from motor pickoff combs  20  is typically determined by the number of electrodes  26  on a respective proof mass  12 . Motor pickoff combs  20  are sometimes referred to as sense combs. Electrodes  26  and electrodes  30  are interdigitated as they extend from respective proof masses  12  and motor pickoff combs  20  and form capacitors which are utilized to sense motion in the drive plane (X-axis). 
     Sense plates  22  are parallel with their respective proof mass  12  and form a capacitor. If an angular rate (i.e. an aircraft turning) is applied to MEMS device  10  operating as a gyroscope along an input vector (Z-axis) while the at least one proof mass  12  is oscillating along the drive plane (X-axis), a Coriolis acceleration 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 device  10  typically is a signal proportional to the change in capacitance caused by the motion. Sense plates  22  are typically connected to sense electronics, not shown in  FIG. 1 . Sense electronics detect changes in capacitance as proof masses  12  move toward and/or away from their respective sense plates  22  and the respective motor drive combs  18  and motor pickoff combs  20 . 
     Motor pickoff combs  20  are typically connected to a bias voltage (not shown) used in sensing motion of proof masses  12 . Motor drive combs  18  are typically connected to drive electronics (not shown). The drive electronics cause the respective proof mass  12  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  26 ,  28  of proof mass  12  and motor drive comb  18 . 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 produced which is proportional to the velocity of proof mass  12  in the motor mode. The Coriolis force drives proof masse  12  in a sense mode direction at a frequency of the motor 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 bias voltage is applied to sense electrodes, so that a motion of proof masses  12  in the sense mode produces an output current. 
     In certain operating environments, MEMS devices, for example, gyroscopes are subjected to extreme shock and vibration exposure, but also have to be mechanically sensitive enough to measure minute angular velocities and linear accelerations. Such forces may cause extensions  26  of proof masses  12  to forcefully come into contact with one or more of motor drive comb  18 , its extensions  28 , motor pickoff comb  20 , and its extensions  30 . In addition to a possibility that one or more of extensions  26 ,  28 , and  30  could be broken off or otherwise damaged, electrostatic forces might cause proof mass  12  to remain in physical contact with the component of device  10  the proof mass  12  has contacted. Other forces may cause the main body of proof mass  12  to come into contact with sense plate  22 . Again, the electrostatic forces may cause proof mass  12  to remain in contact with sense plate  22 . 
     MEMS device  10  is also configured with a plurality of deceleration stops  50  which reduce or alleviate the above described operational problems caused by excessive external mechanical forces. Device  10  utilizes deceleration stops  50  to provide the external force protection. Proof masses  12  are further identified as a left proof mass  54  and a right proof mass  56 . The terms “left” and “right” as used herein are for illustrative purposes with respect to the Figures only to describe operation of deceleration stops  50 , and do not imply any type of structural limitations of MEMS device  10 . Left proof mass  54  and right proof mass  56  are supported above the substrate, as described above, by suspensions  14 . While suspensions  14  suspend proof masses  54  and  56  above a substrate (not shown) onto which a sense plate (not shown) is typically mounted, suspensions  14  also allow proof masses  54  and  56  to vibrate upon application of a bias voltage. As proof masses  54  and  56  vibrate, extensions  26  move back and forth between extensions  28  of motor drive combs  18  and extensions  30  of motor pickoff combs  20 , causing capacitance changes which can be quantified. 
       FIG. 2  is an illustration that details a deceleration stop  50 , which operates to prevent left proof mass  54  from contacting motor drive comb  18  and motor sense comb  20 . While a single deceleration stop  50  is illustrated and described with respect to left proof mass  54 , it is to be understood that the description applies to deceleration stops utilized in conjunction with any proof mass (including right proof mass  56 ) and that multiple deceleration stops  50  can be associated with any individual proof mass (as shown in  FIG. 1 ). 
     Deceleration stop  50  includes a body  60  which, in one embodiment, is located between cross beam  16  and proof mass  54 , and is attached to crossbeam  16  through an anchoring extension  62 . In one embodiment, body  60  is attached to the substrate and provides an anchoring function for the MEMS device. As illustrated in  FIG. 2 , and in an embodiment, the suspensions  14  may have a convoluted or serpentine configuration so as to provide the suspensions a spring-like functionality. 
     In addition, a plurality of deceleration beams  64  extend from body  60  towards proof mass  54 . As illustrated in  FIG. 2 , and in an embodiment, one or more of the beams  64  may include a plurality of tines  70 ,  72  of uniform or varying width and separated from one another by respective gaps  74 . 
     At least one deceleration extension  66  located in between deceleration beams  64  extends from proof mass  54 . As illustrated in  FIG. 2 , and in an embodiment, the extension  66  may include a plurality of tines  76 ,  78 ,  80  of uniform or varying width and separated from one another by respective gaps similar to gap  74 . 
     As further illustrated in  FIG. 2 , and in an embodiment, an additional plurality of deceleration beams  82  extend from the body  60 , and an additional deceleration extension  84  extends from the crossbeam  16 . The beams  82  and extension  84  may be tined in the manner described above with reference to deceleration beams  64  and deceleration extension  66 . 
     Deceleration beams  64 ,  82 , deceleration extensions  66 ,  84 , and serpentine suspensions  14  allow proof mass  54  to move freely under normal motion conditions, but serve to decelerate proof mass  54  when the motion of proof mass  54  exceeds a certain limit. In one embodiment, deceleration beams  64 ,  82  are positioned closer to extensions  66 ,  84  than proof mass  54  is to combs  28  and  30 . As shown in  FIGS. 1 and 2 , deceleration beams  64 ,  82  and deceleration extensions  66 ,  84 , in one embodiment, are elongated rectangular structures. 
     Specifically, when a motion of proof mass  54  causes one or more of deceleration extensions  66 ,  84  to engage one or more of deceleration beams  64 ,  82 , due to an external force, one or more of deceleration beams  64 ,  82  and deceleration extensions  66 ,  84  bend, decelerating proof mass  54  such that when proof mass  54  contacts a fixed stop, an impact is significantly reduced or eliminated. The deceleration of proof mass  54  due to deceleration stops  50  prevents damage to the interdigitating members of proof mass  54 , motor drive comb  18 , and motor pickoff comb  20 . Moreover, the tined configuration of the beams  64 ,  82  and/or extensions  66 ,  84  enable deceleration of the proof mass  54  to occur in successive stages. That is, for example, upon sudden movement of the proof mass  54  to the right in  FIG. 2 , tine  80  and tine  70  may engage one another and bend prior to tine  78  and tine  72  bending and, perhaps, obviate the need for tine  78  and tine  72  to bend. 
     While a preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.