Abstract:
A micro-mirror includes stiffer end sections for limiting curvature, and thin middle sections forming ground electrodes and a hinge. Spacers arc provided beneath the thin middle sections of the micro-mirror for supporting hot electrodes, which attract the ground electrodes for rotating the micro-mirror about a tilt axis. The spacers enable the gap between the hot electrode and the micro-mirror to be designed separately from the thickness of the micro-mirror, and the gap between the ends of the micro-mirror and the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Patent Application No. 61/289,473 filed, Dec. 23 2009, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a micro-mirror device, and in particular to a MEMS micro-mirror device with an electrode spacer enabling the gap between hot and ground electrodes to be independent of any other stiffening structures. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventional micro-electro-mechanical (MEMs) micro-mirror devices  1 , illustrated in  FIG. 1 , comprise a mirror platform  2 , suspended above a substrate  3  via hinges  4  and cap  5 . One or more pedestals  7 , extend upwardly from the substrate  3  in the middle or on each side of the mirror platform  2  for supporting ends of the hinges  4 . A reflective coating  6  (typically metallic) is disposed on the top surface of the mirror platform  2  for redirecting optical signals in dependence upon the tilt angle of the mirror platform  2  relative to the substrate  3 . Residual stresses in the reflective coating  6  introduces a mirror curvature in the mirror platform  2 , which adversely affects optical performance. Moreover, any change in the curvature of the mirror platform  2 , e.g. due to metal stress relaxation, affects reliability. Therefore, stiffening the mirror platform  2  is highly desirable to control stress-induced mirror curvature. 
         [0004]    Typically, as illustrated in  FIG. 1 , the mirror platform  2 , the hinge  4  and the cap  5  have a uniform thickness, whereby the aforementioned structures can be formed, e.g. etched, in a single etching step, and mounted on the raised pedestal  7  extending from the substrate  3 . Hot electrodes  8  are positioned beneath each side of the mirror platform  2  for selectively attracting the underside of the mirror platform  2 , which act as ground electrodes, for tilting the mirror platform  2 , as desired, e.g. for switching optical signals. The tolerance of the thickness of the mirror platform/hinge affects the stiffness of the hinge  4 , and also affects the electrode gap  9  between the hot electrode  8  and ground electrode, i.e. the underside of the mirror platform  2 . The tolerance of the electrode gap  9  determines the variation of the driving torque generated for a given voltage applied to the hot electrode  8 . The advantage of the design shown in  FIG. 1  is a self-compensating effect from the etch depth, whereby an increase in hinge stiffness due to a smaller etch depth, i.e. a thicker hinge  4 , is compensated by an increase in the electrostatic force from a smaller electrode gap  9 , as a result of the smaller etch depth. The etch depth compensation reduces the voltage variation of the device due to etch depth tolerance. Unfortunately, there are several drawbacks to the conventional structure, which include: 
         [0005]    1) A limited scope to improve the stiffness of the mirror platform  2 ; i.e. since the mirror platform  2  has a uniform rectangular shape, only the thickness of the mirror platform  2  can he adjusted to improve the mass moment of inertia (MOT) thereof, and the mechanical resonance thereof. 
         [0006]    2) The process considerations for the width of the hinge  4  puts an upper limit on the thickness of the mirror platform  2 , i.e. the stiffness of the mirror platform  2 , since the hinge  4  has the same thickness as the mirror platform  2 . 
         [0007]    3) The mirror swing space for tilt is determined by the electrode gap, thereby constraining the electrode design by the swing space requirement and vice versa. 
         [0008]    An object of the present invention is to overcome the shortcomings of the prior art by providing a micro-mirror structure that provides additional mirror stiffness, while maintaining the compensation of hinge stiffness by electrode gap, by means of an electrode spacer. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, the present invention relates to a micro-mirror device comprising: 
         [0010]    a substrate defining a lower level; 
         [0011]    a first spacer extending upwardly from the substrate forming an upper level; 
         [0012]    an anchor post extending upwardly from the substrate or the spacer; 
         [0013]    a tilting platform pivotally connected to the anchor post above the substrate via a hinge defining a tilting axis, the tilting platform including a first thin section above the upper level of the substrate, and a stiffer section at each end thereof including stiffening ribs in a lower surface thereof above the lower level of the substrate defining a swing space therebetween; and 
         [0014]    a first hot electrode on the first spacer below the first thin section defining an electrode gap for pivoting the tilting platform about the tilting axis; and 
         [0015]    whereby tolerance variations in hinge thickness are at least partially compensated for by a corresponding variation in electrode gap; and 
         [0016]    whereby the swing space is independent of the electrode gap 
         [0017]    Another aspect of the present invention relates to a method of manufacturing a micro-mirror device comprising: 
         [0018]    a) providing a substrate defining a lower level; 
         [0019]    b) providing a first spacer on the substrate defining an upper level; 
         [0020]    c) providing an anchor post extending from the first spacer or the substrate; 
         [0021]    d) providing a first hot tilt electrodes on the first spacer; 
         [0022]    e) providing a tilting mirror platform with a thinner middle section including a hinge and a ground electrode, and stiffer end sections with a reflective coating on at least one of the end sections; and 
         [0023]    f) mounting the tilting mirror platform with ground electrode over the hot tilt electrode, and the stiffer end sections over the lower level. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0025]      FIG. 1  is a cross-sectional view of a conventional micro-mirror device; 
           [0026]      FIG. 2  is a cross-sectional view of a micro-mirror device in accordance with the present invention; 
           [0027]      FIG. 3  is a cross-sectional view of another embodiment of a micro-mirror device in accordance with the present invention; 
           [0028]      FIG. 4  is a cross-sectional view of another embodiment of a micro-mirror device in accordance with the present invention; 
           [0029]      FIG. 5  is a cross-sectional view of another embodiment of a micro-mirror device in accordance with the present invention; 
           [0030]      FIG. 6  is an isometric view of another embodiment of a micro-mirror device in accordance with the present invention; 
           [0031]      FIG. 7  is a cross-sectional view of the embodiment of  FIG. 6 ; 
           [0032]      FIG. 8  is an isometric view of the mirror platform of the embodiment of  FIGS. 6 and 7 ; and 
           [0033]      FIG. 9  is an isometric view of the substrate wafer of the embodiment of  FIGS. 6 and 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    With reference to  FIGS. 2 and 3 , a micro-mirror device  11  in accordance with the present invention includes a mirror wafer  12  and a substrate wafer  13 . The mirror wafer  12  includes a mirror platform  14 , a hinge  15  and a pedestal cap  16 . One end of the hinge  15  extends from the mirror platform  14 , and the other end of the hinge  15  extends from the pedestal cap  16 . The pedestal cap  16  can be a single element disposed at the intersection of the longitudinal and lateral central axes of the mirror platform  14  or a pair of elements disposed on opposite sides of the mirror platform  14  along the lateral or tilt axis of the mirror platform  14 . Stiffening bulkheads  17  and  18  are provided on the underside of each side of the mirror platform  14  to increase the stiffness thereof, and therefore decrease the curvature thereof. The bulkheads  17  and  18  can be laterally extending structures, longitudinally extending ribs, or a combination of both, e.g. longitudinally extending ribs extending from laterally extending structures. 
         [0035]    In a first embodiment, the mirror wafer  12  is formed in a single-step backside etching process. The etching step forms the pattern of bulkheads  17  and  18  and the pedestal cap  16 , along with defining the thickness of the main section of the mirror platform  14 . The pattern of the bulkheads  17  and  18  on the underside of the mirror platform  14  provides the required stiffness, and at the same time removes most of the material for mass reduction. A final top-side etch step is conducted to pattern and etch the hinge  15 , ideally after the pedestal cap  16  is mounted on the substrate wafer  13 . 
         [0036]    One or more hot electrodes  19  are disposed on the substrate wafer  13  below the bulkheads  17  for selectively attracting the underside of the bulkheads  17 , which act as ground electrodes, for tilting the mirror platform  14 , as desired, e.g. for switching optical signals. A single hot electrode  19  can provide limited angular control; however, two or more hot electrodes  19  are preferred to ensure better control and a greater range of angular motion. Ideally, a reflective surface  20  is disposed, e.g. coated, on an upper surface of the mirrored platform  14  on one or both sides of the hinge  15  for reflecting optical signals. Each of the hot electrodes  19  are electrically connected to an adjustable voltage source (not shown) for generating the required amount of voltage to tilt the mirror platform  14  relative to the substrate wafer  13  producing a desired angular position. One or two pedestals  22  extend from the substrate wafer  13  for supporting the one or two pedestal caps  16 . 
         [0037]    In this case the electrode gap  21  and the thickness of the hinge  15  are independently formed by etching the mirror wafer in two steps: a first backside etch to define the pedestal cap  16  and the bulkheads  17  and  18 , and then a second topside etch to define the hinge  15 . Any change in the driving torque due to a variation in the electrode gap  21  is decoupled from the stiffness of the hinge  15 , and therefore lacks the compensation effect of the hinge stiffness by the electrode gap due to an etch depth tolerance. However, the pedestal  22  does provide a stiffened mirror beam structure. It will be shown later how a spacer design allows a stiffened beam with a compensated electrode gap. 
         [0038]    In an alternate embodiment, illustrated in  FIG. 3 , a micro-mirror  31  with several similar elements to micro-minor  11  includes a mirror wafer  22  is formed in a two-step backside etch process. In a first step a pedestal cap  26  is patterned and formed, providing complete support for the mirror wafer  22  above the substrate wafer  23 , without need of the pedestal  22 . The thickness of the bulkheads  17  and  18  are also thereby defined providing the electrode gap  21  distance. In a second step, the bulkheads  17  and  18  are patterned and etched, providing the stiffness requirements of the mirror platform  24 . The depth of the second step etch also dictates the thickness of the rest of the mirror platform  24  and the hinge  15 , although an additional step can be added to adjust the thickness of either the mirror platform  24  or the hinge  15  independent of each other. A final topside etching step is provided to define the hinge  15  extending form the pedestal cap  26 . 
         [0039]      FIG. 4  illustrates a preferred embodiment of the present invention, in which a micro-mirror device  41  a includes a mirror wafer  42  and a substrate wafer  43 . The mirror wafer  42  includes a mirror platform  44 , a hinge  45  and a pedestal cap  46 . One end of the hinge  45  extends from the mirror platform  44 , and the other end of the hinge  45  extends from the pedestal cap  46 . The pedestal cap  46  can be a single element disposed at the intersection of the longitudinal and lateral central axes of the mirror platform  44  or a pair of elements disposed on opposite sides of the mirror platform  44  along the lateral or tilt axis of the mirror platform  44 . 
         [0040]    Stiffening bulkheads  47  and  48  are provided on the underside of each side of the mirror platform  44  to increase the stiffness thereof, and therefore decrease the curvature thereof. The bulkheads  47  and  48  can be laterally extending structures, longitudinally extending ribs, or a combination of both, e.g. longitudinally extending ribs extending from laterally extending structures. In a first embodiment, the mirror wafer  42  is formed in a single step backside etching process. The backside etching step forms the bulkheads  47  and  48  and the pedestal cap  46 , along with defining the thickness of the main section of the mirror platform  44 , and the hinge  45 . The bulkheads  47  and  48  provide an improved stiffness/mass ratio as described earlier. An additional step can be added to adjust the thickness of the hinge  45  or the mirror platform  44 , if the thickness of the hinge  45  is required to be different than the thickness of the mirror platform  44 . A final topside etching step is conducted to define the hinge  45  separate from the pedestal cap  46 . 
         [0041]    The substrate wafer  43  is provided with a spacer or riser  55   a  extending upwardly from the main substrate  43  providing a raised section defining an upper level with a step down to the main substrate  43  at a lower level. The spacer  55   a  is formed in the substrate wafer  43 , e.g. by an etching process.  FIG. 5  illustrates an identical micro-mirror  41   b,  in which a spacer  55   b  is manufactured separately and bonded to the substrate wafer  43 . Hot electrodes  49  are disposed on the spacer  55   a  and  55   b  below a relatively thin section of the mirror platform  44  for selectively attracting the underside of the mirror platform  44 , which act as ground electrodes, for tilting the mirror platform  44 , as desired, e.g. for switching optical signals. A single hot electrode  49  can provide limited angular control; however, two or more hot electrodes  49  are preferred to ensure better control and a greater range of angular motion. The thin section of the mirror platform  44  is formed above the spacers  55   a  and  55   b,  which forms the electrode gap  51 . The hinges  45  are fabricated in the thin region, thereby preserving the compensation effect described earlier, at the same time achieving a stiffened mirror design. The stiffening bulkheads  47  and  48  are disposed above the main substrate  43  beyond the spacers  55   a  and  55   b  providing a swing gap (between the bulkheads  47  and  48  and the substrate wafer  43 ) independent of the electrode gap  51 , and enabling the mirror platform  44  to rotate to a much larger rotation angle before hitting the lower main substrate wafer  43 . 
         [0042]    Ideally, a reflective surface  50  is disposed, e.g. coated, on an upper surface of the mirrored platform  44  for reflecting optical signals on one or both sides of the hinge  45 . A structurally stiffer section of the mirror platform  44  is provided by means of the bulkheads  47  and  48 , without penalizing mirror inertia; ideally the reflective coating  50  is provided only on the stiffer section of the mirror platform  44 . Each of the hot electrodes  49  arc electrically connected to an adjustable voltage source (not shown) for generating the required amount of voltage to tilt the mirror platform  44  relative to the substrate wafer  43  producing a desired angular position. The spacers  55   a  and  55   b  also support the pedestal cap  46 , which is bonded thereon. 
         [0043]    In this case the electrode gap  51  and the thickness of the hinge  45  are formed by the mirror etch process to preserve the aforementioned compensation effect, the spacer  55   a  or  55   b  enables the formation of the stiffening bulk heads  47  and  48  in the mirror platform  44 , which is then free to swing to the desired angle through the swing gap independent of the spacer  55   a  or  55   b.  Moreover, the swing space of the mirror platform  44 , i.e. between the ends of the mirror platform  44  and the substrate  43 , is decoupled from the electrode gap  51 , enabling flexibility in optimizing the electrode gap  51 . 
         [0044]    Accordingly, the thickness of the spacer  55   a  or  55   b  may be relatively large, e.g. 50 μm or more, therefore aerodynamic crosstalk due to pressure gradient created by the motion of the minor platform  44  squeezing the air film underneath is greatly alleviated by the larger swing gap. 
         [0045]    A preferred embodiment of the present invention is depicted in  FIGS. 6 to 9 , which focuses on a single mirror device  61  in what would normally be an array or parallel micro-mirrors. The micro-mirror device  61  includes a mirror wafer  62  and a substrate wafer  63 . The mirror wafer  62  includes an elongate rectangular mirror platform  64 , a square pedestal cap  66  disposed at the intersection of the lateral and longitudinal central axes of the mirror platform  64 , and a hinge  65  extending between the pedestal cap  66  and the mirror platform  64  along the lateral central axis of the mirror platform  64 . However, the hinge  65  can also extend out from the side of the mirror platform  64  to pedestal caps disposed on opposite sides of the mirror platform  64 . Stiffening bulkheads  67  and  68  are provided on the underside of each side of the mirror platform  63  to increase the stiffness thereof, and therefore decrease the curvature thereof. The illustrated bulkheads  67  are laterally extending structures, while the illustrated bulkheads  68  are longitudinally extending ribs extending from the laterally extending structures  67  to the outer free ends of the mirror platform  64  parallel to the longitudinal axis thereof. 
         [0046]    In a preferred embodiment, see  FIG. 8 , the mirror wafer  62  is formed in a single- step backside etching process. The etching step forms the bulkheads  67  and ribs  68  and the pedestal cap  66 , along with defining the thickness of the main section of the mirror platform  64 . A final topside etching step involves patterning and then etching the hinge  65  and around the pedestal cap  66 . 
         [0047]    The substrate wafer  63 , see  FIG. 9 , is provided with a first spacer or riser  75   a  defining an upper level for a left hot electrode  76   a,  a second spacer or riser  75   b  extending to the tipper level for a right hot electrode  76   b,  and a central spacer  75   c  for supporting the pedestal cap  66  extending upwardly from the base substrate wafer  63  providing raised sections with a step down to the lower level or the main substrate wafer  63 . The single central spacer  75   c  can be replaced by a pair of spacers at either side of the mirror platform  64 , as hereinbefore discussed. A single hot electrode  76   a  can provide limited angular control; however, two or more hot electrodes  76   a  and  76   b  arc preferred to ensure better control and a greater range of angular motion of the mirror platform  64 . Each of the electrode spacers  75   a  and  75   b  may comprise a conductive material, e.g. Silicon, in which case there, is no need for a separate electrode layer. Alternatively, all the spacers  75   a,    75   b  and  75   c  may comprise an insulator material, e.g. glass, in which case, the three separate spacers  75   a,    75   b  and  75   c  may be substituted by a single spacer with patterned thin metal electrodes, with separate left and right hot electrode deposited thereon, as in  FIGS. 5 and 6 . The single spacer can be formed in the substrate wafer  63 , e.g. by an etching process, while the individual spacers  75   a,    75   b  and  75   c  are typically manufactured separately and bonded to the substrate wafer  63 . Hot electrodes  76   a  and  76   b  are disposed on the spacer  75   a  and  75   b  below a relatively thin section of the mirror platform  64 , between the bulkheads  67  and the pedestal cap  66 , for selectively attracting the underside of the mirror platform  64 , which act as ground electrodes, for tilting the mirror platform  64 , as desired, e.g. for switching optical signals. The thin section of the mirror platform  64  is formed above the spacers  75   a  and  75   b,  which forms the electrode gap  71 . The hinged  65  are fabricated in the thin region, thereby preserving the aforementioned compensation effect. 
         [0048]    Ideally, a reflective surface  70  is disposed, e.g. coated, on an upper surface of the mirrored platform  64  on one or both sides of the hinge  65  for reflecting optical signals. A structurally stiffer section of the mirror platform  64  is provided by means of the bulkheads  67  and ribs  68 , without penalizing mirror inertia, and ideally the reflective coating  70  is provided only on the stiffer section of the mirror platform  64  to minimize curvature thereof. Each of the hot electrodes  76   a  and  76   b  are electrically connected to separate adjustable voltage sources (not shown) for generating the required amount of voltage to tilt the mirror platform  64  relative to the substrate wafer  63  producing a desired angular position. The spacer  75   c  supports the pedestal cap  66 , which is bonded thereon. 
         [0049]    In this case the electrode gap  71  and the thickness of the hinge  65  are formed by the single backside mirror etch, hence preserving the compensation effect, at the same time providing stiffened mirror design. The mirror swing space is increased by using the spacer  75   a,  and thereby decouples the swing space from the electrode gap  71  providing design degree of freedom for the electrode  69 . 
         [0050]    Design of the bulkheads  67  and ribs  68  involves a compromise between mirror inertia and mirror stiffness. As an example, a 30 μm thick mirror wafer  62  with a 20 μm etch to define the bulkheads  67  and ribs  68 , and including 100 μm long bulkheads  67 , and 500 μm long ribs  68  provides approximately three times the improvement in mirror stiffness, as compared to a uniformly thick 15 um mirror platform with equivalent mass moment of inertia. 
         [0051]    An apparent solution to increase the bending stiffness without increasing mirror inertia excessively is to add ribs  68  underneath the mirror platform  64 . The approach involves a relatively thin mirror base and relatively thick ribs  68  as depicted in  FIG. 8 . The hinges  65  may be fabricated in the thin region. In one implementation, the electrode gap  71  may be formed by etching the substrate wafer  63 , and the ribbed mirror  64  is formed by etching the mirror wafer  62 ; in this case the electrode gap/hinge height compensation is totally lacking ( FIG. 2   a ). Alternatively, the mirror  64  may be etched twice to provide both electrode gap and thin regions; in this case the electrode gap/hinge height compensation is mostly lacking ( FIG. 2   b ), also suffering from complexity of a two-mask mirror process.