Abstract:
A microelectromechanical system (MEMS) device with a mechanism layer having a first part and a second part, and at least one cover for sealing the mechanism layer. The inner surface of at least one of the covers is structured such that a protruding structure is present on the inner surface of the cover and wherein the protruding structure mechanically causes the first part to be deflected out of a plane associated with the second part.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Standard microelectromechanical systems (MEMS) processing techniques create structures that are symmetric in the z axis (out of the wafer&#39;s surface) but can vary in the x and y axes (in the plane of the wafer&#39;s surface). This leads to devices which can only move in the x/y plane. Presently, creating asymmetry in the z-axis can be performed by deflecting with stiction plates or by selective thinning. Deflecting with stiction plates leads to devices which are sensitive to z motion, but is not easily implemented for multiple z-offsets in both directions and also requires more steps and additional processing layers, thereby costing more money. Selective thinning is performed by thinning one set of teeth in the Z-direction, but this requires an extra mask and additional etches, and it is also rather inaccurate.  
         [0002]     Thus, there exists a need for methods to easily form z-offsets in MEMS devices.  
       BRIEF SUMMARY OF THE INVENTION  
       [0003]     A microelectromechanical system (MEMS) device with a mechanism layer having a first part and a second part, and at least one cover for sealing the mechanism layer. The inner surface of at least one of the covers is structured such that a protruding structure is present on the inner surface of the cover and wherein the protruding structure mechanically causes the first part to be deflected out of a plane associated with the second part. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIGS. 1A, 1B , and  1 C illustrate a cross-sectional side view before assembly, a cross-sectional side view after assembly, and a cross-sectional top view of a microelectromechanical system (MEMS) comb structure device in accordance with one embodiment of the invention;  
         [0005]      FIGS. 2A, 2B , and  2 C illustrate a cross-sectional side view before assembly, a cross-sectional side view after assembly, and a cross-sectional top view of an alternative embodiment of the invention; and  
         [0006]      FIG. 3  illustrates a cross-sectional top view of an additional embodiment of the invention.  
         [0007]      FIG. 4  illustrates a schematic view of a system including one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0008]      FIGS. 1A, 1B  and  1 C illustrate a side view before assembly, a side view after assembly and a top view of a microelectromechanical system (MEMS) comb structure device  30  formed in accordance with one embodiment of the invention.  FIGS. 1A and 1B  show that the device  30  has a top cover  4  and a bottom cover  5  enclosing a mechanism layer  32  that includes a first side  10 , a second side  12  directly opposite the first side  10 , a movable part  14 , a first fixed part  16 , a second fixed part  18 , and flexures  20 . Flexures may also be referred to as torsional flexures or as hinges.  FIG. 1A  illustrates a cross-sectional side view of the comb structure device  30  shown in  FIG. 1B  before the top cover  4  and the bottom cover  5  have been attached to the device  30 . For purposes of  FIGS. 1B and 1C , the positive z direction is defined to run from the bottom cover  5  to the top cover  4  such that it is orthogonal to the outer surfaces of both of the covers and orthogonal to the mechanism layer. The top cover  4  has a structure  6  protruding from its inner surface that causes the second fixed part  18  to be mechanically deflected in the negative z direction and away from the plane associated with the movable part  14  when the top cover  4  is attached to the first side  10  and the second side  12 . Bottom cover  5  has a structure  8  protruding from its inner surface that causes the first fixed part  16  to be mechanically deflected in the positive z direction and away from the plane associated with the movable part  14  when the bottom cover  5  is attached to the first side  10  and the second side  12 .  
         [0009]     Cross-sectional top view  FIG. 1C  shows that the mechanism layer also has a third side  40  and a fourth side  42  so that the movable part  14 , the first fixed part  16 , and the second fixed part  18  are surrounded on four sides by the first side  10 , the second side  12 , the third side  40 , and the fourth side  42 . The movable part  14  is held in place by hinges  24  and  26  attached to the third side  40  and the fourth side  42  which allow the movable part  14  to rotate about the hinges  24  and  26  but keep the movable part relatively fixed with respect to translational movement in the x/y plane.  FIG. 1C  also illustrates that the movable part  14  is formed such that a series of comb electrodes protrude towards the first fixed part  16  and the second fixed part  18 . The first fixed part  16  and the second fixed part  18  include a series of comb electrodes protruding from the side facing the movable part  14 . The comb electrodes of the first fixed part  16  and the second fixed part  18  are interleaved with the comb electrodes protruding from the sides of the movable part  14 . In another embodiment, a non-sealed device may be formed without using the first side  10 , the second side  12 , the third side  40 , and the fourth side  42 . An alternative embodiment based on the non-sealed device could also be formed, where flexures  20  are temporary structures that are put in a dicing space between each comb structure device  30 , and removed in a final configuration. In some embodiments, structure  6  will be bonded to the second fixed part  18  and structure  8  will be bonded to the first fixed part  16 .  
         [0010]      FIGS. 2A, 2B , and  2 C illustrate a cross-sectional side view before assembly, a cross-sectional side view after assembly, and a cross-sectional top view of an alternative embodiment of the invention.  FIGS. 2A and 2B  show that a device  80  has a top cover  100  and a bottom cover  102  enclosing a mechanism layer  120  that includes a first side  106 , a second side  108  directly opposite the first side  106 , a movable part  110 , a fixed part  112 , and flexure  20 .  FIG. 2A  illustrates a cross-sectional side view of the comb structure device  80  shown in  FIG. 2B  before the top cover  100  and the bottom cover  102  have been attached to the device  80 . For purposes of  FIGS. 2B and 2C , the positive z direction is defined to run from the bottom cover  102  to the top cover  100  such that it is orthogonal to the outer surfaces of both of the covers and the mechanism layer  120 . The top cover  100  has a structure  104  protruding from its inner surface that causes the fixed part  112  to be mechanically deflected in the negative z direction and away from the plane associated with the movable part  110  when the top cover  100  is attached to the first side  106  and the second side  108 . Bottom cover  102  is attached to the first side  106  and the second side  108 .  
         [0011]     Cross-sectional top view  FIG. 2C  shows that the mechanism layer also has a third side  130  and a fourth side  132  so that the movable part  110  and the fixed part  112  are surrounded on four sides by the first side  106 , the second side  108 , the third side  130 , and the fourth side  132 . The movable part  110  is held in place by hinges  134  and  136  attached to the third side  130  and the fourth side  132  which allow the movable part  110  to rotate about the hinges but keep the movable part relatively fixed with respect to translational movement in the x-y plane.  FIG. 2C  also illustrates that the movable part  110  is formed such that a series of comb electrodes protrude on the side facing the interior of the device. The fixed part  112  is also shown to each have a series of comb electrodes protruding from the side facing the movable part  110 . The comb electrodes of the fixed part  112  are interleaved with the comb electrodes protruding from the side of the movable part  110 .  
         [0012]      FIG. 3  illustrates a cross-sectional top view of a device  150  that is an additional embodiment of the invention. In this embodiment, more than two parts are deflected. Three fixed parts  152  are deflected up and three fixed parts  154  are deflected down relative to a central comb part  156 .  
         [0013]      FIG. 4  illustrates a schematic view of a system  190  including one embodiment of the present invention. A comb structure accelerometer  200  such as that described in  FIGS. 1B and 1C  in signal communication with rebalance electronics  202 . The rebalance electronics  202  rebalances the comb structure accelerometer  200 . Sense electronics  204 , receives signals from the comb structure accelerometer  200  and produces a relevant output signal  206  to be used in further processing or storage. The signal  206  can be fed back into the rebalance electronics  202 , if closed loop operation is desired.  
         [0014]     The structures  6 ,  8 , and  104  protruding from the inner surfaces of the covers  4 ,  5 , and  100  and the covers  4 ,  5 , and  100  themselves may be formed of a monolithic material such as silicon or pyrex, for example, or the structures  6 ,  8 , and  104  may be attached or deposited on the surface of each cover in alternative embodiments. If structures  6 ,  8 , or  104  are attached or deposited on the surface of covers  4 ,  5 , or  100 , structures  6 ,  8 , or  104  may be made of the same material such as silicon or pyrex, for example, or a different material such as a metal, for example, as covers  4 ,  5 , and  100 . Also, for example, the structures  6 ,  8 , and  104  protruding from the inner surfaces of the covers  4 ,  5 , and  100  could be used to deflect the movable parts  14  and  110  of the devices  30  and  80  instead of or in addition to deflecting the fixed parts  16 ,  18 , and  112 .  
         [0015]     While the 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. Additionally, the MEMS device itself may be a sensor or an actuator acting as a sense mechanism or a drive mechanism. 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.