Abstract:
A micro-electro-mechanical system (MEMS) micro mirror and a method of making the same. The micro mirror includes a body having a mirror support, opposed anchor s and flexible hinges which connect the mirror support to the anchor s. The mirror support has opposed comb edges with comb fingers. Electrodes, which have comb fingers to interact with the comb fingers of the mirror support, are spaced from the comb edges. The comb fingers along each of the comb edges of the mirror support surface are positioned on different horizontal planes from and the comb fingers on the electrodes so as to maximize electrostatic actuation.

Description:
FIELD 
   The present invention relates to a micro mirror for use in micro-electro-mechanical systems. 
   BACKGROUND 
   MEMS (Micro-Electro-Mechanical systems) mirrors have wide applications in fiber optic networks, such as optical cross connect switches, optical attenuators, optical tunable filter etc. The most mature MEMS product in optical telecommunication industry is MEMS Variable Optical attenuator (VOA). 
   There are a number of MEMS VOAs disclosed in US patents. A micro shutter type MEMS VOA is disclosed in U.S. Pat. Nos. 6,275,320B1, 6,459,845B1, 6,751,395B1, 6,780,185B2, 6,816,295B2, 6,876,810B2, 6,901,204B2, 6,954,579B2, 6,980,727B1, 6,996,306B2 and 7,224,097B2. These VOAs use a micro shutter to partially block a light beam in order to achieve optical attenuation. These micro shutters are actuated by such means as electro-thermal actuation or electrostatic actuation. Micro shutter type of MEMS VOAs has difficulties, such as optical component alignment and hermetical packaging. 
   Micromirror type MEMS VOAs have advantages of simple packaging. The optical attenuation is realized by the tilting micro mirror, which redirects the light beam. The commercially available lens and TO metal cans can be readily available for low cost packaging of micro mirror type VOAs. As such, most of the commercial available MEMS VOAs use a tilting micro mirror. U.S. Pat. Nos. 6,628,856B1, 6,838,738B1, 6,915,061 6,963,679 and 7,224,097B2 disclose MEMS micro mirrors. These micro mirrors use electrostatic actuation. The electrostatic actuation is favored for micro mirror due to its low power consumption and relative small footprint. 
   In the disclosed prior arts, micro mirrors with electrostatic actuation fall into vertical combdrive type and parallel plate type. U.S. Pat. No. 6,838,738B1 disclosed vertical combdrive actuated micro mirror, it has several drawbacks of device design and fabrication. First of all, the design of the taller and shorter fingers using the same layer of material have some initial overlapping areas, which will have effect to against the actuation. The electrical field in this initial overlapping area has opposite contribution to the mirror actuation. Secondly, the micro mirror is required to have a certain minimum thickness to maintain its mechanical strength to overcome the residual stress of the reflective metal film on its top surface as well as environment vibration during its operation etc. Thinner than 20 microns of material will cause undesirable higher radius of curvature (ROC) of the micro mirror. If 20 microns of thick material is used to make taller and shorter fingers, it is very difficult to have good photolithography in its process step  840  since higher topography created in the previous step (step  830 ). Even if the photolithography can be managed, then the finer finger gap has to be sacrificed, which in turn results in higher actuation voltage. Thirdly, one metal coating is used in U.S. Pat. No. 6,838,738B1 for both reflective metal film on the mirror surface and metal film on the bonding pads for wire bonding. The requirements for both metal films are quite different. The requirements for the reflective metal film on the mirror surface are higher reflectivity within the light wavelength interested and low residual stress. Usually this metal film is very thin for easy residual stress control. On the other hand, the requirements for the bonding metal film on the bonding pads are thicker metal film for easy wire bonding and good electrical conductivity. Usually this bonding metal film is thicker and stressful. One metal coating process in U.S. Pat. No. 6,838,738B1 will cause either higher ROC of the micro mirror, poor reflectivity and/or poor wiring bonding. Fourthly, U.S. Pat. No. 6,838,738B1 disclosed the wet structure release processing step  890 , which will cause stiction of the macrostructure such as fingers. Stiction will lead to defective devices. Last but not least, due to the existing and unavoidable process defects, the vertical combdrive actuator has tendency to rotate side ways so that the electrical shorting will occur from contact of fixed and movable fingers. Such electrical shorting can permanently destroy the device. There is no indication in U.S. Pat. No. 6,838,738B1 as to how to prevent undesirable side way rotation. 
   Compared with vertical combdrive actuator, parallel plate electrostatic actuators have following several disadvantages in all the prior arts. First of all, the pull-in effect of parallel plate electrostatic actuator of micro mirror limits the controllable tilting angle range under the certain actuation voltage. When actuation voltage is applied between fixed electrode and the movable hinged mirror, the resulting attractive electrostatic force will pull the mirror towards the fixed electrode to create tilting of hinged mirror. Initially, the mechanical restoring force from deformed hinge will balance the electrostatic force to keep the mirror in the controllable position. But when the actuation voltage is further increasing, and the tilting of the hinged mirror is over one third of the initial gap between the fixed electrode and the mirror, the electrostatic force between the electrode and the mirror surpasses the mechanical restoring force of the hinge, the hinged mirror will snap and physically contact to the fixed electrode. The usable and controllable tilting range of the mirror is very limited, only one third of the gap between the mirror and fixed electrode. Secondly, within the small controllable titling range, parallel plate electrostatic actuator won&#39;t provide linear actuation. In other word, the mirror tilting angle is not linear with the actuation voltage. Thirdly, higher actuation voltage causes issues of electrical charging, tilting angle drifting. In order to have larger controllable titling angle of the mirror, the gap between the fixed electrode and mirror has to be increased. Increased gap results in the higher actuation voltage. Higher driving voltage causes electrical charging on the dielectrical materials of the micro mirror device, which will in turn cause the undesired tilting angle drifting of the mirror. Fourthly, squeezed air between movable mirror and fixed electrode during tilting will lead into air damping. Since the space between the movable mirror and fixed electrode is very small, the fast titling/switching of the micro mirror will cause the air between its mirror and electrode either compressed or decompressed. As such, the air damping from the squeezed air will effectively lower the tilting/switching speed of the mirror. Lastly, the micro fabrication process is costly and complex, especially for making complex actuation electrodes and electrical wirings of the micro mirrors. 
   SUMMARY 
   According one aspect there is provided a micro-electro-mechanical system (MEMS) micro mirror. The micro mirror includes a body having a mirror support portion, opposed anchor portions and flexible hinge portions which connect the mirror support portion to the anchor portions. The mirror support portion has a mirror support surface, a first comb edge, a second comb edge opposed to the first comb edge, and comb fingers extending outwardly from each of the first comb edge and the second comb edge. A first fixed electrode is spaced from the first comb edge of the mirror support portion and has comb fingers extending outwardly toward the mirror support portion to interact with the comb fingers on the first comb edge and interlace upon movement of the mirror support portion of the body in a first direction about the flexible hinges. Prior to being energized, the comb fingers along the first comb edge are positioned on one horizontal plane and the comb fingers of the first fixed electrode being on another horizontal plane. A second fixed electrode is spaced from the second comb edge of the mirror support portion and having comb fingers extending outwardly toward the mirror support portion to interact with the comb fingers on the second comb edge and interlace upon movement of the mirror support portion of the body in a second direction about the flexible hinges. Prior to being energized, the comb fingers along the second comb edge are positioned on one horizontal plane and the comb fingers of the second fixed electrode being on another horizontal plane. 
   According to another aspect there is provided a method of making a micro mirror consisting of a body having a mirror support portion, opposed anchor portions and flexible hinge portions which connect the mirror support portion to the anchor portions, in which combs on the mirror support portion interact with combs on the anchor portions. A first step involves using photolithography and partial silicon etching to form an upper portion of the mirror support portion, an upper portion of the opposed anchor portions, an upper portion of the hinge portions and a pair of upper combs in a silicon wafer. A second step involves bonding the partially etched silicon wafer to a carrier wafer. A third step involves using photolithography and partial silicon etching to form a lower portion of the mirror support portion, a lower portion of the opposed anchor portions, a lower portion of the hinge portions arid a pair of lower combs in the silicon wafer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein: 
       FIG. 1 , labeled as PRIOR ART, is a perspective view of a micro mirror using a parallel plate type of electrostatic actuator. 
       FIG. 2  is a perspective view of a micro mirror using electrostatic vertical combdrive actuators with two directional rotations. 
       FIG. 3  is a perspective view of a micromirror using electrostatic vertical combdrive actuators with one directional rotation. 
       FIG. 4  is a perspective view of a Silicon On Insulator (SOI) wafer. 
       FIG. 5  is a perspective view of Silicon On Insulator (SOI) wafer after Deep Reactive Ion Etching (DRIE). 
       FIG. 6  is a perspective view of a carrier wafer. 
       FIG. 7  is a perspective view of a silicon carrier wafer after silicon etching to form a supporting structure and cavity. 
       FIG. 8  is a perspective view of a glass carrier wafer after glass etching to form supporting structure and cavity. 
       FIG. 9  is a perspective view of SOI wafer bonded with carrier wafer. 
       FIG. 10  is a perspective view of bonded wafer after etched away handle wafer of SOI. 
       FIG. 11  is a perspective view of partial etching of buried oxide of SOI. 
       FIG. 12   a  is a perspective view of full pattern of buried oxide of SOI. 
       FIG. 12   b  is a detailed perspective view of full pattern of buried oxide of SOI in  FIG. 12   a.    
       FIG. 13  is a perspective view of buried oxide of SOI after etching away the oxide in the mirror location. 
       FIG. 14  is a perspective view of the bonded wafer after deposition and patterning of low stress and thin reflective metal film on the top of the mirror. 
       FIG. 15  is a perspective view of buried oxide of SOI after etching away the oxide in the areas of bonding pads and electrical connection. 
       FIG. 16  is a perspective view of the bonded wafer after deposition and patterning of thicker metal film on the top of the bonding pads and electrical connection area. 
       FIG. 17   a  is a perspective view of the bonded wafer after deposition and patterning of thicker photoresist on the top of the metal films. 
       FIG. 17   b  is a detailed perspective view of the bonded wafer illustrated in  FIG. 17   a.    
       FIG. 18   a  is a perspective view of the bonded wafer after DRIE etching through device silicon of SOI wafer. 
       FIG. 18   b  is a detailed perspective view of the bonded wafer illustrated in  FIG. 18   a.    
       FIG. 19   a  is a perspective view of the bonded wafer after RIE (Reactive Ion Etching) etching away the buried oxide on top of the lower comb fingers. 
       FIG. 19   b  is a detailed perspective view of the bonded wafer illustrated in  FIG. 19   a.    
       FIG. 20   a  is a perspective view of the bonded wafer after DRIE etching to form the lower comb fingers. 
       FIG. 20   b  is a detailed perspective view of the bonded wafer illustrated in  FIG. 20   a.    
       FIG. 21   a  is a perspective view of the final micro mirror device after etching away remaining buried oxide and photoresist. 
       FIG. 21   b  is a detailed perspective view of the final micro mirror device illustrated in  FIG. 21   a.    
       FIG. 22   a  is a perspective view of a taper shape hinge configuration. 
       FIG. 22   b  is a perspective view of a double beam hinge configuration. 
   

   DETAILED DESCRIPTION 
   The preferred embodiment, a MEMS micro mirror generally identified by reference numeral  21 , will now be described with reference to  FIG. 1 through 22   b.    
   While this invention is susceptible of embodiments in many different forms, there is shown in the drawing and will herein be described in detail, preferred embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated. The figures are not necessarily drawn to scale and relative sizes of various elements in the structures may be different than in an actual device. 
   One of prior art of micromirrors with parallel plate actuators is shown in  FIG. 1 . The mirror  21  coated with reflective materials such as metal film are supported by two hinges  12  and  16  which are connected to the anchors  10  and  18 . Two fixed actuation electrodes  14  and  15  are located below the mirror  21 . The mirror, hinges and anchors can be made of heavily doped electrical conductive silicon. When the actuation voltage is applied between mirror  21  and electrode  14 , the resulting electrostatic force will pull the mirror  21  towards electrode  14  and cause the deformation of the hinges. When the electrostatic force is balanced with the mechanical restoring force of the deformed hinges, the mirror  21  will stabilize. The previously described disadvantages of the parallel plate electrostatic actuator such as pull-in effect, tilting angle drifting and squeezed air damping, lead to poor performance of the micro mirror. In addition, the parallel plate electrostatic actuator uses a very complex, low yield and expensive manufacturing method. 
   A vertical electrostatic combdrive is shown in  FIG. 2 . The advantages of the vertical combdrive over the parallel plate electrostatic actuator are higher actuation force density, better actuation linearity, no pull-in effects. The micromirror design and fabrication of the micromirror in the present invention eliminate tilting angle drifting and squeezed air damping. The micromirror  21  is fixed to the anchors  17  and  18  through hinge  26  and  27 . The fixed electrodes  19  and  20  have fixed upper comb finger  22  and  24 . The movable lower comb finger  23  and  25  are on the outside edges of the mirror  12 . When an actuation voltage is applied between the moveable finger  23  and fixed comb finger  24 , the resulting electrostatic force will pull the mirror  21  anticlockwise around the hinge  26  and  27 , and causes the hinge  26  and  27  to deform. The mirror will reach a stable position when the electrostatic force is balanced with the resulting mechanical restoring force of the deformed hinges  26  and  27 . If the actuation voltage is applied between fixed upper comb finger  22  and movable comb finger  25 , the mirror  21  will rotate around the hinge  26  and  27  clockwise. 
   The present invention uses different upper and lower finger designs to achieve the direction control of the mirror rotation. The vertical combdrive design shown in  FIG. 2  has two fixed electrodes  19  and  20 , if the micromirror  21  is electrically grounded, the applied voltage on any of fixed electrodes  19  and  20  can independently rotate the mirror tilt into two different directions, either clockwise or anticlockwise. 
   The vertical combdrive design shown in  FIG. 3  has different upper and lower comb finger arrangement. The mirror  21  has upper comb fingers  29  and lower fingers  23  on its outside edges. The fixed electrode  19  has fixed lower comb fingers  28 , while fixed electrode  20  has fixed upper comb fingers  24 . When the mirror  21  and associated upper fingers  29  and lower fingers  23  are electrical grounded, if an electrically potential is applied on fixed electrodes  19  and  20  at the same time, the mirror will rotate anticlockwise. The combdrives on both side edges of the mirror  21  will work together to actuate the mirror in the same direction. The advantages of such design are the reduction of the actuation voltage for certain mirror tilting angle, and eliminating the resulting unbalanced force on the hinges, which can cause the up or down piston movement of the mirror besides desired mirror rotation. 
   The following process description gives the microfabrication method and design of micromirror. Although there are many other alternative microfabrication methods, we only give the representative fabrication method for the micromirror and vertical combdrive structure. The micromirror and vertical combdrive actuator structure designs will remain the same in the present invention. Only the major process steps for fabricating the micromirror device will be described. 
   The micromirror and corresponding vertical combdrive actuators are made of the single crystal device silicon of Silicon On Insulator (SOI) wafer shown in  FIG. 4 . Relative thinner single crystal device silicon layer  32  is bonded to handle silicon wafer  34  with Buried Oxide (BOX)  33 . This starting material SOI wafer can be directly purchased from SOI wafer vendors, or some well known method such as fusion bonding and etching back process can be used to make such material. The single crystal device silicon should be heavily doped to have good electrical conductivity. The reason for using single crystal silicon for micromirror is its residual stress free and excellence mechanical material properties as well as optical quality surface finish. 
   A photolithography process is performed on the single crystal silicon layer for the subsequent partial silicon Deep Reactive Ion Etching (DRIE). The DRIE etch depth can be around half thickness of the single crystal silicon, for example. The purposes of partial silicon DRIE are multiple. In  FIG. 5 , silicon DRIE in region  35  is to remove the part of silicon materials to form upper vertical comb fingers, while the silicon DRIE in regions  36  and  37  are on the backside the micromirror and actuation arm respectively. The partial silicon etching reduces the masses of the micromirror and actuation supporting arms and increases micromirror&#39;s resonant frequency without sacrificing the mirror and actuation supporting arm structure strength and mirror flatness etc. The partial silicon DRIE etch can also be used to thin down the hinge heights to make them more flexible, therefore less actuation voltage is required. 
   A handle or carrier wafer  38  shown in  FIG. 6  could be Pyrex glass or regular silicon wafer. After forming some supporting structures to support the anchor of vertical combdrive actuators, and a deep cavity under micromirrors to reduce or eliminate the squeezed air damping, the carrier wafer  38  will be bonded to the SOI wafer by using either fusion bonding, anodic bonding or other bonding techniques. 
   In  FIG. 7 , if the carrier wafer  38  is a regular silicon wafer, the lithographies and silicon DRIE will be conducted to form the supporter  40  and the a deep cavity  39  under the micromirror by DRIE, or simply etching through the carrier wafer to leave a hole under the micromirror. Other etching methods can also be applied for this etching such as wet silicon anisotropic etching in potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH) etc. After silicon etching and before fusion bonding with SOI wafer, a very thin thermal oxide  41  will grow on the carrier silicon wafer in order for single crystal silicon device layer of SOI to have electrical isolation with the carrier silicon wafer. 
   If Pyrex glass is chosen as the carrier wafer, the isotropic glass etching in hydrofluoric acid (HF) will form the supporters  42  and a deep cavity  43  by glass etching in  FIG. 8 . Through hole under micromirror on the glass carrier wafer can be formed to further reduce the air damping. The through hole can be made using wet HF etching, simply sand blasting or laser micromaching from back side of the glass carrier wafer. 
   Referring to  FIG. 7  and  FIG. 8 , for some applications such as optical VOA (Variable Optical Attenuator), certain squeeze air damping is required to reduce the stabilizing time of the mirror during switching mirror positions; also the low driving voltage has high priority. For such applications, a thin metal film, identified by reference numerals  73  and  74  is deposited and patterned on the bottom of shallower cavities  39  and  43 . A part  73  of the patterned metal film is forming the bottom driving electrode to pull the micromirror downwards, similar to the design in  FIG. 1 . This extra actuation force will help vertical combdrives to further reduce the total actuation voltage of the micromirror. The other part  74  of patterned metal film, which is often electrically connected to the micromirror, is used to shield exposed glass or thin thermal oxide  41  facing the micro mirror. Otherwise, accumulated charging on these dielectrical materials can cause undesirable titling angle drifting. 
   The SOI wafer is bonded to the Pyrex glass carrier wafer  45  using anodic bonding shown in  FIG. 9 . If the Pyrex glass carrier wafer has through etched holes under the location of the mirror, this wafer should be protected on the glass side during the SOI handle wafer removal in the KOH or TMAH bath to prevent the enchant from attacking the device silicon of SOI wafer through the hole on the glass carrier wafer. The protection method can be as simple as using wafer protection holder which seals the whole glass wafer and only expose the handle wafer  34  of the SOI wafer in the KOH or TMAH bath. If the glass carrier wafer only has deep etching cavity under micromirror, there is no need to have extra protect since the glass wafer  45  itself will provide good protection to single crystal silicon device layer during KOH or TMAH silicon etching. The buried oxide layer  33  of SOI wafer is used as the etching stop layer for KOH or TMAH silicon etching. The bonded wafer after SOI handle wafer removal is shown in  FIG. 10 . 
   It is very critical to have actuation stability of vertical combdrive actuator. The self alignment process is applied to achieve the equal gap between adjacent comb fingers. Any unequal gap between adjacent comb fingers will cause asymmetry of electrostatic force, which in turn results in the malfunction of the vertical combdrive such as sideway snapping movement of movable comb fingers. 
   The Buried Oxide layer  33  shown in  FIG. 11  is kept and used as silicon DRIE etching masking materials for the self alignment process. After photolithograph, the partial RIE (Reactive Ion Etch) etching of buried oxide layer is done to make preparation for making etching masking layer of lower comb fingers. The oxide partial etching region  47  is shown in  FIG. 11 . 
   A subsequent photolithograph is done after the partial oxide etching; a layer of photoresist is coated on the wafer for the patterning. Since the total thickness of buried oxide layer is only a couple of micron, there is no high topography issue for photolithography, and high photolithography resolution can be maintained. After this photolithography, an oxide RIE is performed to have buried oxide patterns of hinges  51   a  and  51   b , hinge anchors  56   a  and  56   b , primary vertical combdrive actuators  52   a ,  52   b ,  52   c  and  52   d , and monitoring vertical combdrive finger bank  53   a  and  53   b  for the detection of the mirror position, arms  54   a  and  54   b  for the primary vertical combdrive actuator, bonding pads  49   a ,  49   b ,  50   a  and  50   b  as well as mechanical stops  55   a  and  55   b . The mechanical stops  55   a  and  55   b  are used for preventing the undesirable over displacement under the certain environments such as accidental shock ( FIG. 12   a ). In the detailed view of  FIG. 12   b , the upper comb finger has full thickness of buried oxide  33  as DRIE etching mask layer, while the lower comb finger has only partial thickness of the buried oxide  33  as DRIE etching mask layer. 
   The tapered shape of the supporting arms has advantage of reducing the undesirable side way micromirror rotation. Also the location of the hinges are located far away from the micromirror, the purpose is also to suppress the undesirable side way micromirror rotation. 
   In order to have the good reflectivity of the mirror surface, a reflective metal film such as gold film is deposited on the mirror surface. The low stress metal film is required since high metal film stress can cause the undesirable higher Radius of Curvature (ROC) of the mirror. Usually a very thin layer of metal film with low residual stress is applied.  FIG. 13  shows that buried oxide  33  in the mirror region  60  is etched away using standard photolithographic patterning and etching process.  FIG. 14  shows that a very thin layer of metal film  61  with low residual stress is applied on the top of silicon mirror. 
   The thicker metal film is required on the bonding pads and areas for electrical connections. This thicker metal film with low electrical resistance could have some residual film stress since the bonding pads and electrical connection areas are not very sensitive to the residual thin film stress.  FIG. 15  shows that buried oxide  33  in the bonding pads and electrical connection areas are etched away using standard photolithographic patterning and etching process.  FIG. 16  shows that a thicker layer of metal film  62  with some residual stress is applied on the top of bonding pads  49   a ,  49   b ,  50   a ,  50   b  and electrical connection area  63 . 
   Before DRIE releasing the micromirror and forming lower and upper comb fingers, all the metal films on the micro mirror, bonding pads and electrical connection area should be protected from strong plasma etching during DRIE etching. The very thick layer of photoresist  64  is coated and patterned on the wafer using standard lithography process ( FIG. 17   a ). The higher resolution of photolithography is not required since patterned photoresist is only used for the etching protection. In the detailed view in  FIG. 17   b , the thin metal film  61  and thicker metal film  62  are under the thicker photoresist  64 . The comb fingers with buried oxide on the tops have no thick photoresist protection. 
   The silicon DRIE etching is used to etch through the single crystal device silicon  34  as show in  FIG. 18 . The micro mirror  72  is released to be free. In the detailed view in  FIG. 18   b , the upper comb finger  70  and arms  54   a  have thicker buried oxide left on the tops, while lower comb fingers  71  have thinner buried oxide left on the top. Again the hinges  51   a  and  51   b , anchors  56   a  and  56   b  and mechanical stops  55   a  and  55   b  have thicker buried oxide left on the tops. 
   A subsequent oxide RIE is utilized to etch away any remaining oxide on the lower finger  71  ( FIG. 19 ), while the upper comb fingers  70 , arms  54 , hinges  51 , anchors  56 , mechanical stops  55  still have some remaining buried oxide ( FIG. 19   b ) left on the tops. 
   The last silicon DRIE etching is used to etch away the silicon on the lower comb finger to form its final shape, while the upper comb finger  70  is protected by the remaining oxide  33 . The oxide Reactive Ion Etching (RIE) process is to etch away any remaining buried oxide on the upper comb fingers, arms, hinges and anchors. Oxygen plasma or equivalent photoresist ashing process is conducted to remove all the thick photoresist. The final shape of micromirror and its corresponding vertical combdrive actuator and mirror position detector are shown in  FIG. 21 .  FIG. 21   b  shows the final shape of upper and lower comb fingers. 
   The bonding pads  49   a  and  49   b  are electrically connected to the micromirror  72  through mechanical stops  55   a  and  55   b , anchors  56   a  and  56   b , V shape hinges  51   a  and  51   b  as well as supporting arms  54   a  and  54   b . All the comb fingers connected to the supporting arms  54   a  and  54   b  in the actuators ( 52   a ,  52   b  and  53   c  and  52   d ) and position detectors ( 53   a  and  53   b ) are movable and in the same electrical potential with the mirror  72 . 
   As mentioned before, the vertical combdrive could have different design variations shown in  FIG. 2  and  FIG. 3 , so the mirror can be actuated in one direction or two directions. The design configuration of the vertical combdrives in  FIG. 21  is same as design configuration shown in  FIG. 3 . In  FIG. 21 , all the fixed combdrive fingers in  52   a ,  52   c  and  53   a  are upper fingers, while all the fixed combdrive fingers in  52   b ,  52   d  and  53   b  are lower fingers. Again, all the movable combdrive fingers in  52   a ,  52   c  and  53   a  are lower fingers, while all the fixed combdrive fingers in  52   b ,  52   d  and  53   b  are upper fingers. 
   When the bonding pad  49   a  or  49   b  is electrically grounded, and an electrical potential is applied on the metal layer  63 , the mirror  72  will be actuated by  52   a ,  52   b ,  52   c  and  52   d  towards the same rotation at the same time. The mirror  72  will tilt in the direction indicated by the arrow in  FIG. 21 . Since all the vertical combdrive actuators  52   a ,  52   b ,  52   c  and  52   d  are working together to actuate the micromirror  72  at the same direction. This design approach will significantly reduce the actuation voltage while the quicker repose of the micromirror is still maintained. This micromirror design configuration is especially useful for Variable Optical Attenuator (VOA) with low driving voltage such as less than 5 volts. 
   For some applications, it is required to electrically monitor the actual mirror rotation. The present invention provides sensing structures to detect electrically the rotation of the micromirror. Present invention utilizes vertical combdrive  53   a  and  53   b , which are electrically isolated and mechanically separated from actuators  52   a ,  52   b ,  52   c  and  52   d . The movable and fixed comb fingers in  53   a  and  53   b  are no longer forming electrostatic actuators, instead; they are forming variable electrical capacitors when the micromirror is rotated by the actuators  52   a ,  52   b ,  52   c  and  52   d . When the mirror  72  is actuated by actuator  52   a ,  52   b ,  52   c  and  52   d , the relative position between the fixed and movable fingers in the vertical combdrive  53   a  and  53   b  are changed. This position change results in the capacitance change between the bonding pads  50   a  (and/or  50   b ) and bonding pads  49   a  or  49   b.    
   The hinge design also is very important to the actuation stability of vertical combdrive actuator. The hinges provide not only the flexures to support the mirror and allow the mirror rotate in the expected direction, but also suppress any undesirable side way movement of the micromirror. The micromirror in the present invention can have verities of hinge shape designs to meet such needs. V shape hinge is already presented in the previous description and process. Other hinge designs are also used in the present invention such the taper shape hinge and double beam hinge ( FIG. 22 ). The V shape hinge, taper shape hinge and double beam hinge design provide very good stability in terms of preventing side snapping or side way instability of vertical combdrive actuator due to the process imperfection. Imperfection of microfabrication processes is one of major reasons to cause side way snapping of the combdrive actuators. 
   In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. 
   It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the claims.