Abstract:
A Micro-Electro-Mechanical System closed-loop (MEMS) inertial device having a vertical comb drive that exhibits improved performance under vibration. The device includes one or more stator tines extending from a housing into a cavity formed by the housing. One or more rotor tines extend from a proof mass located in the cavity. The proof mass is joined to the housing by flexures which allow movement in the vertical direction. The rotor tines have a first length value in the direction of movement and the stator tines have a second length value in the direction of movement. The second length value is greater than the first length value. Also, the stator tines include two electrically separated portions. The lesser length of the rotor tines relative to the stator tines causes the attractive force between the rotor tines and either the upper or lower half of the stator tines to be relatively independent of rotor vertical position. This, in turn, produces better accelerometer accuracy in vibration environments.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     High performance Micro-Electro-Mechanical Systems (MEMS) inertial instruments (accelerometers and gyros) require closed-loop operation. Good performance under vibration requires that the magnitude of the electrostatic feedback force be highly insensitive to the position of the instrument&#39;s inertial mass relative to it&#39;s null position. Because of their high force dependency on position, the typical electrostatic vertical comb drive as employed in MEMS actuators cannot be applied to precision MEMS inertial instruments.  
         [0002]      FIG. 1  illustrates an instrument that performs acceptably under static accelerations. However, if vibration were superimposed on a static acceleration, a large error in the time-average measured acceleration would result. This error is called vibration rectification, whose nature and sources are well known by those in the field. One major source of vibration rectification error in closed-loop instruments is the proof mass positional-dependence of the feedback force. When the rotor and stator teeth are substantially aligned (same height), the electrostatic force between the rotors and the upper or lower half of the stators changes greatly with a small vertical movement of the rotor teeth. As these rotor teeth move up and down with applied external vibration, the time average differential voltage that results is different from the voltage that would occur with only the static acceleration input. This is an accelerometer error. The 2 nd  order portion of the force-position dependency is the main source of the error, but 1 st  and higher than 2 nd  order components can also contribute. The contribution to the error from the various order components depends on the details of the design. Regardless, the need is for a design which significantly reduces this position dependency, especially a 2 nd  order dependency.  
         [0003]     Therefore, there exists a need for an electrostatic MEMS drive, which has very low position sensitivity.  
       SUMMARY  
       [0004]     The present invention provides a Micro-Electro-Mechanical System (MEMS) device having a vertical comb drive that exhibits an improved position-independence force by promoting a substantially uniform rate of capacitance change with respect to change in position. The device includes one or more stator tines extending from a housing into a cavity formed by the housing. One or more rotor tines extend from a proof mass located in the cavity. The two groups of tines are interleaved so that each rotor tine fits between two stator tines, and visa-versa. Each rotor tine is positioned at a capacitive distance from two stator tines (except at the end of the line). When an inertial force is experienced, the instrument&#39;s proof mass tends to move in a direction approximately orthogonal to a normal vector between the corresponding tines. The rotor tines have a first length value in the direction of movement and the stator tines have a second length value in the direction of movement. The second length value is greater than the first length value. Also, the stator tines include two electrically separated portions.  
         [0005]     The electrically separated portions in the rotor tines are separated by an oxide, or other insulating layer, into approximately equal halves. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:  
         [0007]      FIG. 1  illustrates a cross-sectional view of slice of a comb structure used in the prior art;  
         [0008]      FIG. 2  illustrates a block diagram of a sensor system formed in accordance with an embodiment of the present invention;  
         [0009]      FIG. 3  illustrates a cross-sectional view of slice of a comb structure used in a component of the system shown in  FIG. 1 ;  
         [0010]      FIGS. 4-18  illustrate cross-sectional slice views of steps in a fabrication process for creating a vertical comb structure in accordance with an embodiment of the present invention;  
         [0011]      FIGS. 19-29  illustrate cross-sectional slice views of steps in an alternate fabrication process for creating a vertical comb structure in accordance with an alternate embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]      FIG. 2  illustrates a sensor system  38  for improved vibration rectification performance. In this example, the system  38  includes a Micro-Electro-Mechanical Systems (MEMS) device  40  that is an accelerometer for sensing forces due to acceleration along the vertical or Z-axis. All figures and description refer to an accelerometer, but this invention also applies to gyros.  
         [0013]     The MEMS device  40  includes a proof mass  42  having a plurality of rotor tines  44  (or combed fingers or teeth, all used interchangeably). The proof mass  42  is located within a housing  48  and is joined to this housing by flexures which allow relatively free motion of the proof mass in the vertical direction but are generally stiff in other directions. Extending from sections of the housing  48  and interleaved with the rotor tines  44  are stator tines  46 . The stator tines  46  are electrically coupled to a servo/controller device  60 . The device  60  senses vertical relative motion between the proof mass and supporting frame by a method and means not discussed here. Typically this sense, or pick-off, would operate capacitively. Once the device  60  senses a proof mass movement, the device  60  sends a signal to the stator tines  46  in order to force the rotor tines  44  back to a null position. The signal (e.g., voltage value) that is sent to the stator tines  46  for forcing the rotor tines  44  back to null position is used to calculate the amount of acceleration that the proof mass  42  is experiencing.  
         [0014]      FIG. 2  illustrates a side view of a slice of the device  40  of  FIG. 1 . The rotor and stator tines  44  and  46  as well as the remainder of the proof mass (not shown) are located within a cavity  50  formed by the housing  48 . The housing  48  includes first and second wafers  62  and  64  and a sidewall structure  66 . The rotor tines  44  are shorter in a vertical direction than the stator tines  46 . Also, each of the tines  44  and  46  includes an insulating layer  68  that separates the tines  44  and  46  into two electrically separate halves. The insulating layer  68  acts as an electrical insulator between the silicon (Si) halves. In one embodiment, each of the halves of the stator tines  46  are electrically coupled to the servo/controller device  60 . Although the oxide layers  68  separates the halves of the rotor tines  44 , a device is applied to the rotor tines  44  in order to electrically couple both of the halves. This electrically coupling device is located internal to the tines  44  or external to the tines  44 .  
         [0015]     If a sudden steady-state upwards acceleration is applied to the frame and stator tines  46 , the proof mass, with rotor tines  44 , will initially lag behind. This positional lag is sensed by elements of the device not discussed here, and the action of the device  60  causes the appropriate differential voltage to be applied between the upper half of the stator tine  46  and the rotor tine  44 , returning the proof mass to its null position (assuming the controller device  60  has an integrator function).  
         [0016]     From elementary electrostatics theory, it is know that the magnitude of the attractive force experienced by the rotor tines  44  of the proof mass  42  is proportional to a rate of change of capacitance experienced between the rotor and stator tines  44  and  46  with vertical motion, and to the square of the differential voltage. The action of the controller device  60  is to determine the voltage required to exactly counter-balance the inertial force applied to the proof mass. The differential voltage required is approximately proportional to the square-root of the applied acceleration, and thus can be used to determine the acceleration. With downwards acceleration, the differential voltage is applied between the rotors tines  44  and the lower half of the stators tines  46 .  
         [0017]     The problem discussed above is overcome by arranging for the rotor tines  44  to be vertically offset from the stator tines  46  at both their tops and bottoms, as shown in  FIG. 2 . Analytically, the force-position relation for any given tine geometry can be determined by performing a series of capacitance calculations (via a finite element method) as the rotor tines  44  are moved vertically about their null position. The force at any position is proportional to the first derivative of the capacitance at that position. The rectification error that results from a force that varies with position depends on many details of the device, as well as the input acceleration, and can be analytically calculated using system simulation software (Simulink, for example).  
         [0018]      FIGS. 4-18  illustrate side cut-away views of an example fabrication of the device  40  shown in  FIGS. 2 and 3 . As shown in  FIGS. 4 and 5 , the fabrication process begins by using two silicon-on-insulator (SOI) wafers  70  and  71 . Each of the wafers  70 ,  71  include a handle layer (silicon-Si)  72  and  82 , an oxide layer  74  and  80 , and a device layer  76  and  78  that separates the oxide layer  74  and  80  from the handle layer  72  and  82  from the device layers  76  and  56 . In this first step, an oxide layer  84  is grown or deposited on the device layer  78 .  
         [0019]     As shown in  FIG. 6 , the exposed surface of the device layer  76  of the first SOI wafer  70  is bonded to the oxide layer  84  of the second SOI wafer  71  preferably using a fusion bonding technique. The handle layer  72  is then removed, thus producing a double SOI wafer.  
         [0020]     As shown in  FIG. 7 , a photoresist mask pattern  88  is applied over the oxide layer  74 . Then, as shown in  FIG. 8 , the unprotected oxide of the oxide layer  74  is etched and then the photoresist pattern  88  is removed.  
         [0021]     As shown in  FIG. 9 , a Si deep reactive ion etch (DRIE) is performed on the device layer  76  to a pre-defined depth. As shown in  FIG. 10 , the remaining oxide in the oxide layer  74  is removed and a first cap wafer  90  is bonded to the device layer  76 . This bonding is a Si—Si bond using a fusion bond process or can be performed using a eutetic bond. The cap wafer  90  has been previously etched in order that posts created in the DRIE process shown in  FIG. 9  do not come in contact with the surface of the cap wafer  90 .  
         [0022]     As shown in  FIG. 11 , the handle layer  82  of the second SOI wafer  71  is removed by a wet chemical etch (KOH, TMAH and EDP) or a DRIE, or mechanical grinding followed by a wet or dry etch. The oxide layer  80  is removed and a nitride (Si3N 4 ) layer  94  is deposited on the surface of the device layer  78 . Next, as shown in  FIG. 12 , the nitride layer  94  is etched according to an applied pattern of photoresist  98 . As shown in  FIG. 13 , the photoresist  98  is removed and an oxide layer  100  is grown on the surface of the device layer  78  in the locations where the nitride no longer exists.  
         [0023]     As shown in  FIG. 14 , a photoresist pattern  104  is applied and then exposed portions of the oxide layer  100  and the nitride layer  94  are etched. The oxide layer  100  and the nitride layer  94  can be etched simultaneously suing a plasma etch. As shown in  FIG. 15 , the unprotected portion of the device layer  78  is then etched using a DRIE all the way to the oxide layer  84 . Then, the exposed oxide layer  84  is etched to the surface of the device layer  76 .  
         [0024]     As shown in  FIG. 16 , the photoresist  104  is removed and then the remaining nitride  94  is removed by hot phosphoric acid. Next, as shown in  FIG. 17 , a DRIE etch is performed in order to remove a portion of the device layer  78  previously covered by the nitride  94  and the remaining exposed portion of the device layer  76 . As shown in  FIG. 18 , the oxide layer  100  is etched and a second cap wafer  110  that is similar to the first cap wafer  90  is fusion bonded to the exposed surface of the device layer  78 , thereby producing rotor tines that are smaller than stator tines in a combed drive structure.  
         [0025]      FIGS. 19-29  illustrate side cross-sectional slice views of an alternate fabrication process that begins after the step shown in  FIG. 5 , except that a layer of nitride  120  is deposited on the surface of the device layer  76  instead of an oxide layer. A photoresist mask  122  is then applied over the nitride layer  120 . Next, as shown in  FIG. 19 , a nitride etch is performed and the photoresist mask  122  is removed.  
         [0026]     As shown in  FIG. 21 , an oxide layer  130  is applied to the exposed surface of the device layer  76  and then a photoresist pattern  132  is applied over portions of the oxide layer  130  and nitride layer  120 .  
         [0027]     Next, as shown in  FIG. 22 , an oxide and nitride etch are performed down to the surface of the device layer  76 , then a DRIE of the exposed device layer  76  is performed down to the oxide layer  84 , another oxide etch is performed to remove the oxide layer  84 , and finally, a second DRIE is performed to etch a portion of the device layer  78 .  
         [0028]     As shown in  FIG. 23 , the photoresist pattern  132  is removed. Then, the remaining nitride layer  120  is etched (for example, using hot phosphoric acid which only removes nitride).  
         [0029]     Next, as shown in  FIG. 24 , another DRIE is performed thus etching the remaining exposed portion of the device layer  78  down to the surface of the oxide layer  80  and a portion of the device layer  76  that was previously protected by the just removed nitride layer  120 . As shown in  FIG. 25 , the cap wafer  90  is bonded to the device layer  76  after removal of the oxide layer  120 .  
         [0030]     As shown in  FIG. 26 , the handle layer  82  is removed, thereby exposing the oxide layer  80 . A photoresist pattern  138  is applied over portions of the oxide layer  80 . Next, as shown in  FIG. 27 , the portion of the oxide layer  80  that is exposed is etched. As shown in  FIG. 28 , a DRIE is performed on the exposed area of device layer  78 . Finally, as shown in  FIG. 29 , the photoresist pattern  138  is removed as well as the oxide layer  80 . Then, the cap wafer  110  is bonded to the device layer  78 .  
         [0031]     During fabrication electrical leads are attached to the portions of the stators and any other components of the system.  
         [0032]     While the preferred embodiment of the invention has been illustrated and as noted above, many changes can be made without departing from the spirit of the invention. Accordingly, the scope of the invention is not limited by the of the preferred embodiment. Instead, the invention should be determined reference to the claims that follow.