Abstract:
A MEMS device having a fixed element and a movable element wherein one or the other of the fixed element and the movable element has at least one radially-extended stop or overdeflection limiter. A fixed overlayer plate forms an aperture. The aperture is sized to minimize vignetting and may be beveled on the margin. Overdeflection limitation occurs during deflection before the movable element can impinge on an underlying electrode. The overdeflection limiter may be conveniently placed adjacent a gimbaled hinge.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
         [0001]    NOT APPLICABLE  
         STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    NOT APPLICABLE  
         REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.  
         [0003]    NOT APPLICABLE  
         BACKGROUND OF THE INVENTION  
         [0004]    This invention relates to electro-mechanical components such as micro electrical mechanical system (MEMS) devices or MEMS arrays and particularly to electrostatically-actuated electro-mechanical components wherein undesired deflection of movable elements due to nonlinear attractive effect may be controlled, preventing device failure. Components constructed according to the invention are MEMS mirror arrays or other micromachined elements.  
           [0005]    Conventional MEMS array structures comprise Silicon on Insulator (SOI) array structures on which is fabricated an integrated electrode array. One of the problems encountered is device failure due to uncontrolled electrostatic deflection of the movable element of the device resulting in contact with the underlying substrate during operation due to overvoltage drive or inertial ringing of the device. Failure mechanisms include voltage breakdown, mechanical stiction or welding, and mechanical deformation of hinge elements. Anyone one of these failures can render a cell useless and severely hamper the usefulness of an array.  
           [0006]    In a commonly-owned co-pending patent application Ser. No. 10/128,368 filed Apr. 23, 2002, the structure of a MEMS device is disclosed which employs a form of a stop based on the present invention. The co-pending application is not prior art to the present invention.  
           [0007]    In U.S. Pat. No. 6,315,423 claiming a priority date of Jul. 13, 1999, one form of mechanical stop is disclosed for a MEMS device which has high angles of displacement with highly compliant hinges. The travel stops therein are for overswing and x-axis shock protection, since the hinge design therein allows for large desired rotational motions as well as undesired multi-axis translational motions. Hence the travel stops therein are not one-dimensional stops.  
           [0008]    A solution is needed that limits the maximum deflection of the MEMS device due to inherent electrostatic parallel plate instability without compromising device performance or manufacturability.  
         SUMMARY OF THE INVENTION  
         [0009]    According to the invention, a MEMS device is provided having a fixed element and a movable element wherein one of the fixed element and the movable element has at least one radially-extended stop or overdeflection limiter. A fixed overlayer plate forms an aperture. The aperture is sized to minimize vignetting and may be beveled on the margin. Overdeflection limitation occurs during deflection before the movable element can impinge on an underlying electrode. The overdeflection limiter may be conveniently placed adjacent a gimbaled hinge.  
           [0010]    The mechanical stop may be a plate mounted to the top of a conventional MEMS structure or integrated into a three layer MEMS structure or two two-layer MEMS structures combined. The invention will be better understood by reference to the following detailed description in connection with the accompanying illustrations. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a perspective view in cutaway of a first embodiment according to the invention of a double-gimbaled mirror with a top mounted overhang stop layer.  
         [0012]    [0012]FIG. 2 is a top view of a double gimbaled mirror with mechanical stop tabs extending from the mirror.  
         [0013]    [0013]FIG. 3 is a side cross-sectional view of a single array element with a flipped SOI overhang mounted on a standoff.  
         [0014]    [0014]FIG. 4 is a side cross-sectional view of a single array element with an overhang formed in a single process step as a bevel.  
         [0015]    [0015]FIG. 5 is a perspective view of the element of FIG. 4.  
         [0016]    [0016]FIG. 6 is a side cross-sectional view of a single array element with an overhang of multiple bonded wafers.  
         [0017]    [0017]FIG. 7 is a perspective view in cutaway of a first embodiment according to the invention of a double-gimbaled mirror with a top mounted overhang stop layer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring to FIG. 1 there is shown a perspective view in cutaway of a first embodiment of a single array element  10  of a MEMS array according to the invention. A mirror  12  is mounted on a first pair of hinges (FIG. 2)  14 ,  16  to a gimbal  18 . The gimbal  18  is connected by a second pair of hinges  20 ,  22  having one degree of freedom to a frame  24 , more particularly to a cantilever  26  of the frame  24  wherein an oxide layer  25  (FIG. 3) is provided for bonding and etch stop during manufacturing. The oxide layer  24  is typically on the order of only one micron thick. The nature of the frame construction is immaterial so long as it provides support. For example, surface micromachine processes and bulk micromachine processes can be employed to construct a frame without departing from the spirit and scope of the invention. According to the invention, an overhang  28  is provided which serves as a mechanical stop against rotation of the mirror  12 . The overhang  28  is typically mounted above the mirror  12  (i.e., outside the electrode cavity) as part of a cap  30 . In FIG. 1, tab extensions  32 ,  34  extend from the mirror  12  adjacent the second hinge pair  20 ,  22 . Referring to FIG. 7, an alternative and less desirable embodiment employs tab extensions  32 A,  34 A which extend from the overhang  28  to encounter the edge of the mirror  12 . While the tab extension allow for reduced mirror mass, they potentially vignette the beam to result in signal loss.  
         [0019]    The frame  24  spaces the mirror from electrodes  36 - 39  mounted on a substrate  40 . A dielectric  42  spaces the frame  24  from the substrate  40 , and a plurality of standoffs  44 , of which only one is shown in FIGS.  1  or  7 ) separate the cap  30  from the frame. Depending on the design of over-rotation sensors, the standoffs  44  can be either insulative or conductive. Insulative standoffs are useful if there is a circuit to be formed that senses contact between the mirror and the overhang indicating overdeflection. Conductive standoffs make it easier to construct a conductive cap  30 , thus maintaining the mirror  12  and the cap  30  at equal potential.  
         [0020]    [0020]FIG. 3 illustrates in cross section a view of FIG. 1 or  2  along sight line  3 - 3  through tabs  32 ,  34 , gimbal  18  and cantilever  26 . The overhang  28  is an aperture plate mounted on standoffs  44 , which in this embodiment is a separately-formed layer formed on the cap  30 . The cap  30  and overhang  28  are typically SOI structures (Silicon on Insulator where two different silicon layers are typically bonded together but separated by an oxide) which are “flipped” after fabrication and mounted by an appropriate adhesive such as an epoxy to standoffs  44 , which could be cylinders, beads or the like. Alternatively, the top layer of the mirror  12  on its frame  24  could be etched to match a complementary cross etch in the mating face of the overhang  28  to assure accurate seating when beads are used as spacers. A KOH etch technique could be used wherein the crystal structures of the respective SOI chips are cross-ways to one another.  
         [0021]    [0021]FIG. 4 is a side cross-sectional view of a single array element along sight line  4 - 4  with a typical deep KOH etch along exposed crystal planes to form an aperture  30 A. The aperture  30 A (FIG. 5) is a rectangle with exposed facets  52 - 56 , etc. It can be attached in the same manner as the structure of FIG. 3 with an appropriate adhesive to standoffs  44 .  
         [0022]    Flipped SOI has advantanges over conventional KOH fabrication of an aperture, since it permits relatively loose manufacturing tolerances of lithography, thus allowing the tabs  32 ,  34  to be reliably shorter. It should be noted, however, that polishing and spacing are critical to the required tolerances.  
         [0023]    [0023]FIG. 6 is a side cross-sectional view of a single array element with multiple bonds of SOI, so-called triple-stacked SOI. A dielectric  42  separates electrodes  38 ,  39  from the frame  40 , and an oxide layer  25  is disposed between the frame  40  and the layer forming the mirror  12 . A further oxide layer  27  is disposed between the layer of the mirror  12  and a further silicon cap  30 , which in this case is step etched (before bonding) to form a shelf  31 . The cap  30  is bonded to the layer of the mirror  12  at the oxide layer  27  after the oxidation and release step of the manufacturing process. This structure was disclosed but not claimed in a co-pending patent application which is not prior art. No extra assembly is required, since the triple SOI manufacturing step is an integrated process. However, it may be difficult to maintain isolation of mechanical stop layer from the stand-off layer due to the thin oxide.  
         [0024]    The invention has been explained with reference to specific embodiments. Other embodiments will be evident to those of skill in the art. It is therefore not intended that this invention be limited, except as indicated by the appended claims.