Abstract:
A rotational sensor for measuring rotational acceleration is disclosed. The rotational sensor comprises a sense substrate; at least two proof masses, and a set of two transducers. Each of the at least two proof masses is anchored to the sense substrate via at least one flexure and electrically isolated from each other; and the at least two proof masses are capable of rotating in-plane about a Z-axis relative to the sense substrate, wherein the Z-axis is normal to the substrate. Each of the transducers can sense rotation of each proof mass with respect to the sense substrate in response to a rotation of the rotational sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Under 35 U.S.C. 120, this application is a continuation application and claims priority to U.S. application Ser. No. 13/096,732 filed Apr. 28, 2011, which is a continuation of U.S. Pat. No. 7,934,423, issued May 3, 2011, all of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to accelerometers and more specifically to multi axis accelerometers that sense angular (rotational) accelerations. 
     BACKGROUND OF THE INVENTION 
     Angular or rotational accelerometers are used to measure rotational acceleration about a specific axis. Rotational accelerometers have many applications such as vehicle rollover event prevention, rotational vibration suppression for hard disk drives, airbag deployment and so on. With the advances in MEMS technology various rotational accelerometers that can be fabricated using silicon micromachining techniques have been proposed in U.S. Pat. No. 5,251,484, “Rotational accelerometer,” Oct. 12, 1993; U.S. Pat. No. 6,718,826, “Balanced angular accelerometer,” Apr. 13, 2004; U.S. Pat. No. 5,872,313, Temperature-compensated surface micromachined angular rate sensor,” Feb. 16, 1999; U.S. Pat. No. 6,257,062, “Angular accelerometer,” Jul. 10, 2001. In these applications surface micromachining used to fabricate the moving proof masses. Surface micromachining imposes limits on the structures. For example, the proof mass thickness is limited to the thickness of the deposited films. Surface micromachining also suffers for the stiction problem as a result of sacrificial etching and wet release processes. Therefore proof masses fabricated using this method requires additional supports around the perimeter of the proof mass to reduce stiction and to increase the stability. This results in relatively more complicated devices and stringent requirements for the fabrication of additional springs that would not disturb the operation of the rotational accelerometer. On the other hand bulk micromachining overcomes most of the problems associated with the surface micromachining. U.S. Pat. No. 7,077,007, “Deep reactive ion etching process and microelectromechanical devices formed thereby,” Jul. 18, 1006 describes DRIE etching for bulk micromachined angular accelerometers. 
     The sensing methods used in MEMS accelerometer vary. Capacitive sensors provide high performance as well as low cost. Because of these features it became the method of choice for most of the consumer market applications. But to be able to obtain high sensitivity and low noise floor the parasitic capacitances need to be reduced or eliminated. This can be achieved by integrating MEMS and electronics. The accelerometers described in the above-identified patents are not integrated with the detection electronics. In a typical system, the detection electronics needs to be connected to the MEMS substrate through wire bonding. Accordingly, this system suffers from increased parasitics and is susceptible to noise and coupling of unwanted signals. 
     Therefore, there is a need for rotational accelerometers that are fabricated using bulk micromachining methods and integrated with electronics. There is also need for multi-axis accelerometers that are insensitive to linear accelerations. The present invention addresses such needs. 
     SUMMARY OF THE INVENTION 
     A rotational sensor for measuring rotational acceleration is disclosed. The rotational sensor comprises a sense substrate; at least two proof masses, and a set of two transducers. Each of the at least two proof masses is anchored to the sense substrate via at least one flexure and electrically isolated from each other; and the at least two proof masses are capable of rotating in-plane about a Z-axis relative to the sense substrate, wherein the Z-axis is normal to the substrate. Each of the transducers can sense rotation of each proof mass with respect to the sense substrate in response to a rotation of the rotational sensor. 
     Two structures or more can be used per axis to enable full bridge measurements to further reduce the susceptibility to power supply changes, cross axis coupling and the complexity of the sense electronics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows top view of a portion of a MEMS assembly according to an embodiment. 
         FIG. 1B  shows the cross section AA′ of the angular accelerometer in  FIG. 1A . 
         FIG. 1C  shows an angular accelerometer comprising two proof masses. 
         FIG. 1D  shows the detection electronics for the accelerometer shown in  FIG. 1C . 
         FIG. 2A  illustrates an angular accelerometer composed of two proof masses. 
         FIG. 2B  shows an alternative arrangement of proof masses shown in  FIG. 2A . 
         FIG. 3A  illustrates multi-axis accelerometer (X and Z rotational accelerometer, Z linear accelerometer). 
         FIG. 3B  illustrates the cross section of the accelerometer shown in  FIG. 3A  and the proof mass deflection as a function of input rotational X and linear Z acceleration. 
         FIG. 3C  illustrates one proof mass that is sensitive to rotational X and Z accelerations but insensitive to linear accelerations. 
         FIG. 4A  illustrates four axis accelerometer (X, Y and Z rotational, Z linear accelerometer). 
         FIG. 4B  illustrates an alternative arrangement of proof masses shown in  FIG. 4A . 
         FIG. 4C  shows the detection electronics for the Z-axis angular accelerometers shown in  FIGS. 4A and 4B . 
         FIG. 5  illustrates one variation of flexures. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to motion sensing devices and more specifically to angular accelerometers utilized in integrated circuits. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
     A method and system in accordance with the present invention relates to the accelerometers that are fabricated using silicon micromachining methods that have been described in U.S. Pat. No. 7,104,129, entitled “Vertically Integrated MEMS Structure with Electronics in a Hermetically Sealed Cavity,”, issued Sep. 12, 2006, and assigned to the assignee of the present application; and U.S. Pat. No. 7,247,246, entitled “Vertical Integration of a MEMS Structure with Electronics in a Hermetically Sealed Cavity,” issued Jul. 24, 2007, and assigned to the assignee of the present application, both of which are incorporated by reference in their entirety herein. The assembly approach (Nasiri fabrication process) described in the said patents provides a cost effective means to simultaneously protect the movable sensing element and to integrate the low noise electronics. The electronic circuitry is fabricated on a dedicated electronics silicon substrate. The MEMS assembly then bonded on the electronic or sense substrate using a metal bonding technique using a low temperature process that does not damage or compromise the electronic circuitry. A plurality of transducers is assembled in this manner at the wafer level where hundreds to thousands are produced simultaneously. A small size form factor is achieved by the vertical integration of the sensing element with its sensing circuit. Other patents that are relevant for accelerometer fabrication are: U.S. Pat. No. 6,939,473 “Method of making an X-Y axis dual-mass tuning fork gyroscope with vertically integrated electronics and wafer-scale hermetic packaging”; U.S. Pat. No. 7,258,011 “Multiple axis accelerometer”; and U.S. Pat. No. 7,250,353 “Method and system of releasing a MEMS structure” assigned to the assignee of the present application. 
       FIGS. 1A and 1B  show a rotational accelerometer  100  and the cross section AA of the accelerator  100 , respectively. As is seen, a proof mass  102  is attached to the cover plate  104  at a single anchor  106  through flexural springs  108 . The anchor  106  is at the center of the proof mass  102 . The proof mass  102  is movable. It is constraint to rotate along Z axis which is perpendicular to the proof mass  102 . The springs  108  can be made in any shape to adjust the spring constant. The anchor  106  is attached to the cover plate  104  utilizing a bonding process such as fusion bonding. The anchor  106  is also connected to the sense substrate  118  through an electrical connection  112 . 
     The electrical connection  112  can be made under the anchor  106  as described in published U.S. Published Application No. 2006/0208326, entitled “Method of fabrication of al/ge bonding in a wafer packaging environment and a product produced therefrom” which is also assigned to the assignee of the present application. The method described in that published patent application allows making mechanical and electrical connections on the same anchor. The single anchoring of the proof mass  102  reduces the stresses that may be induced by the package warpage. The sense electrodes  114   a  and  114   b  are coupled to sensor substrate  118  and do not move with respect to proof mass. When the proof mass  102  is subjected to an angular acceleration about the axis (Z-axis) perpendicular to the plane, the forces acting on the proof mass  102  rotates it about the anchor  106 . The rotation of the proof mass  102  is sensed capacitively. The moving electrodes  116  extending from the proof mass  102  form capacitors with the sense electrodes  114 . 
     The sense electrodes  114   a - 114   b  are bonded to the cover plate  104  and electrically connected to the sense substrate  118 . There are two sense electrodes  114   a - 114   b  per each moving electrode  116  on the proof mass  102  forming two capacitors. The value of one of these capacitors increases, whereas the value of other one decreases as the proof mass  102  rotates. The capacitors are labeled C CW    120  and C CCW    122  as shown in  FIG. 1A . 
     C CW    120  increases if the proof mass  102  rotates in clock wise direction and C CCW    122  increases if the proof mass  102  rotates in the counter-clockwise direction about the axis perpendicular to the sense substrate. C CW    120  and C CCW    122  allow for the differential detection of the proof mass  102  rotation and hence provide an indication of the angular acceleration. 
     Referring to  FIG. 1A , the accelerometer  100  has features to provide reliable operation. For example, it has motion stoppers  124   a - 124   b  about Z rotation to restrict the motion for excessive acceleration. In the directions that are out of plane, its stiffness is high enough to provide mechanical stability. The accelerometer  100  has also self test electrodes  126  to actuate the proof mass  102  for test purposes. 
     As before mentioned,  FIG. 1B  shows the cross section of the angular accelerometer shown in  FIG. 1A . Other MEMS devices such as accelerometers and gyros have been disclosed previously by the assignee of the present application (U.S. Pat. No. 7,258,011, “Multiple axis accelerometer”; U.S. Pat. No. 6,939,473, “Method of making an X-Y axis dual-mass tuning fork gyroscope with vertically integrated electronics and wafer-scale hermetic packaging”; U.S. Pat. No. 6,892,575, “X-Y axis dual-mass tuning fork gyroscope with vertically integrated electronics and wafer-scale hermetic packaging”). A similar fabrication platform described in these patents may also be used for the angular accelerometers shown in  FIG. 1B . 
     The fabrication process starts with the manufacturing of the cover plate  104 . First alignment marks are patterned on top of the cap or cover wafer. These marks will be later used to align the cover wafer to the sense substrate. Then the cover plate  104  is oxidized preferably using thermal oxidation to form an oxide layer. The preferable thickness of the oxide is between 0.5 and 1 micron. The oxide is patterned using lithographic methods to define the cavities in the cover plate  104 . 
     The cavity depth can be further increased by etching the exposed silicon surfaces in the cover plate  104 . But, if the structures in the actuator layer are not supposed to move more than the oxide thickness in vertical direction or there are no difficulties associated with having a cover in the close proximity of the moving parts, the silicon etch step may be skipped. 
     Then, the cover plate  104  is cleaned and bonded to another low total thickness variation wafer. The second wafer will form an actuator layer after thinning it down to preferably 40 microns. The actuator layer includes the proof mass  102  the sense electrodes  114   a  and  114   b  and the flexure springs  108  any other structures such as self test electrodes and over travel stoppers. The next step in the process is the formation of the stand offs. An etch, such as an KOH etch, is suitable for this step. The height of the stand offs determine the vertical separation between actuator layer and the sense substrate  118 . If there are electrodes on the sense substrate  118 , this gap also determines the sensing capacitor gaps. Then, a germanium (Ge) layer is deposited and patterned. In the next step, elements of the rotational accelerometer are defined lithographically and etched using DRIE in the actuator layer. In the final step, the actuator layer is bonded a sense substrate using eutectic bonding. 
     Accordingly, as is seen in  FIG. 1B , the active areas of the sense substrate  118  include regions that will make electrical contact with an actuator layer where the angular accelerometer  100  is defined, as well as circuitry  125  for sensing output signals from the angular accelerometer  100 . Such circuitry  125  is preferably conventional CMOS circuitry. The top layer  127  of metal deposited in the conventional CMOS process is suitable for use as a bond metal. This upper layer  127  of metal defines bond pads for the connections to the sense electrodes  114   a  and  114   b  and the proof masses  102 . One can also put electrodes on the top layer  127  to measure out of plane motion of the accelerometer in the case X and Y angular accelerometer. 
     The connections to the proof masses  102  and sense electrodes  114   a  and  114   b  can be routed in the lower CMOS metals  129 ,  131  and  133  where metals can cross over each other in different layers. This allows for complicated routing schemes to be utilized for connecting the MEMS device to the active electronics. Another advantage of having the sense substrate  118  in the close proximity of the angular accelerometer is that the connections between the MEMS device and sense electronics can be made very short. This reduces the parasitic coupling to ground, cross coupling between the wires and EMI coupling. 
     The above-described fabrication process produces hermetically sealed sensors for example utilizing sealing rings  135 . The sense substrate  118  is preferably attached to the actuator layer via a metal-to-metal bond, which can be made hermetic. Likewise, the actuator layer is preferably attached to cover plate  104  by a fusion bond, which can also be made hermetic. As a result, the entire assembly of sense substrate  118 , actuator layer and cover plate  104  can provide a hermetic barrier between angular accelerometer elements and an ambient environment. The pressure in the cavity can be adjusted during the eutectic bonding process. This allows the quality factor of the angular accelerometer to be controlled for better noise performance and dynamic response. 
     The above-described fabrication process also allows combining various inertial measurement devices on the same substrate. The angular accelerometers described in this patent can be easily integrated with linear accelerometers as well as low cost gyroscopes. 
     One can use two of the structures shown in  FIG. 1A  to provide four changing capacitances as shown in  FIG. 1C . Note that, in this case the proof masses are electrically isolated. This allows for the detection of a capacitance change utilizing a full bridge configuration in so doing common mode signals are eliminated and allows using simpler electronics can be utilized. The capacitance change of the accelerometers described above can be detected by various circuits. 
     An example of circuitry for detecting the capacitance change due to rotational acceleration is shown in  FIG. 1D  where a full bridge configuration is utilized. As is seen, AC voltages  201   a ,  201   b , which are 180 degree out of phase with respect to each other are applied to the proof masses  202   a  and  202   b . The output voltage is detected off the sense electrodes utilizing an operational amplifier  204 . When there is no acceleration, the bridge is in balanced and the output voltage is zero. Angular acceleration of the proof masses  202   a  and  202   b  disturbs the balance and gives rise to an AC voltage at the operational amplifier  204  output which amplitude is proportional to the acceleration. The operational amplifier  204  output later can be demodulated to obtain a signal directly proportional to the acceleration. 
     In this embodiment, a full bridge circuit is described but one of ordinary skill in the art readily recognizes other means of capacitive detection such as pseudo bridge, half bridge can also be employed. Alternatively, one can also drive the sense electrodes and monitor the proof mass motion and by observing the output voltages of the op-amp. 
     In an alternative configuration to obtain full bridge configuration, instead of using the full circular proof mass of  FIG. 1A , one can use only half of the proof mass  302 - 304 . This reduces the area usage as shown in  FIG. 2A  at the expense of sensitivity. Each proof mass  302 - 304  is connected to a single anchor point by three or more flexures. In this case C CW1    306  and C CW2    308  increase with the clock wise rotation of the proof mass  302  and  304  whereas C CCW1    310  and C CCW2    312  decrease. The change in the capacitance can be detected in a full bridge configuration as shown in  FIG. 1D .  FIG. 2B  shows an alternative placement for the proof masses of  FIG. 2A . 
     In another configuration as shown in  FIG. 3A , the flexures can be configured such that the proof masses  402 ′ and  404 ′ become sensitive to rotation about another axis (X) in addition to the first rotational axis which is Z in this case. The Z-rotation detection method is same as the scheme described in  FIG. 2 . However, attaching the proof masses  402  and  404  with flexures along the edge of the half circle makes them sensitive to the rotations about the axis parallel to that edge. For Z axis rotation, the flexures simply flex allowing the rotation of the proof mass  402  and  404 . For X axis rotation, the flexures make a torsional motion. The out of plane motion of the proof masses  402  and  404  can be measured by parallel plate capacitors between the proof masses  402  and  404  and the sense substrate. 
     In  FIG. 3A , the capacitance between the proof mass  402  and the substrate is C PM1    414  and the other capacitor is C PM2    416  which is between the proof mass  404  and sense substrate. The rotational acceleration about X moves one of the proof masses away from the substrate and the other one closer to the substrate. This increases the capacitance C PM2    416 ′ and reduces the C PM1    414 ′ according to  FIG. 3B . These two capacitors can be used for differential detection of the rotation. Again, by replicating the structure shown in  FIG. 3A , one can obtain four capacitances for X axis rotation to implement full bridge detection. However, for Z rotation, one structure as shown in  FIG. 3A  is enough to implement full bridge configuration, but two structure configuration also improves Z sensitivity. In addition to rotational accelerations, these accelerometers can be used to measure linear acceleration along Z direction as shown in  FIG. 3B . In this case, the sum of the C PM1    414 ′ and C PM2    416 ′ needs to be detected, rather than the difference of them which is the case for measuring rotational acceleration about the X axis. 
     Alternatively, one can use the structure shown in  FIG. 3C  for X and Z rotation where X direction is in plane and parallel to the flexure  612  and Z direction is perpendicular to the lateral plane. In this structure, the two proof masses ( 402 ,  404 ) of  FIG. 3A  are combined to form a single proof mass  610 . The proof mass  610  is constraint to rotate about the anchor  618  and about the flexure  612 . Electrodes  602  and  604  are sensitive to rotations about Z. The electrodes  606  and  608  which are between the sense substrate and the proof mass  610  are sensitive to rotations about X. However, for this structure linear acceleration along Z direction will not result in any capacitance change on C PMP  and C PMN  therefore this accelerometer is insensitive to linear acceleration along Z. Full bridge configuration will require two of these structures. 
       FIG. 4A  shows a three axis rotational accelerometer. There are four proof masses  702 ,  704 ,  706  and  708 . The Z rotation is detected through C CW1    710 , C CCW1    712 , C CW2    722 , C CCW2    724 , C CW3    726 , C CCW3    728 , C CW4    730 , and C CCW4    732  capacitors. One can easily construct full bridge configuration for this case as shown in  FIG. 4C . Basically, proof mass  702  and  706  are connected in parallel likewise proof mass  704  and  708 . The rotations about X and Y are sensed through capacitors C PM2    716 , C PM4    720  and, C PM1    714 , C PM3    718  respectively. When there is rotation about positive X direction, C PM2    716  increases and C PM4    720  decreases. Since the rotation axis is through the centers of C PM1    714  and C PM3    718  these capacitors do not change. Similarly, for Y axis rotation only C PM1    714  and C PM3    718  change, but C PM2    716  and C PM4    720  remain the same. The accelerometer shown in  FIG. 4A  also sensitive to Z axis linear accelerations. This acceleration can be detected by sensing the sum of C PM1    714 , C PM2    716 , C PM3    718  and C PM4    720 . 
     In an alternative configuration shown in  FIG. 4C , the orientation of the proof masses are changed. Placing the X and Y axis rotation detection sensors away from the center of the device increases the sensitivity. 
       FIG. 5  shows an example of a proof mass  800  which includes a different type of flexure  802 . One can use folded springs to tailor the spring constant. A folded spring can be connected to the proof mass at the center as shown in  FIG. 5 . This configuration allows obtaining small spring constants for increased sensitivity in small areas. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.