Patent Abstract:
A micro-electro-mechanical-system (MEMS) micromirror for use in high fill factor arrays which includes at least one stationary body and a movable body. The movable body has opposed ends and is secured to the stationary body at each of the opposed ends by a resilient primary axis pivot. A mirror support is supported by and movable with the movable body. The mirror support has a first unfettered side and a second unfettered side. A primary axis actuator is provided including a fixed portion connected to the stationary body, and a movable portion corrected to the movable body. The movable portion is adapted to move away from the fixed portion in response to an electrical potential difference between the fixed portion and the movable portion, such that the movable body rotates about the primary axis resilient pivot. A mirror is supported by the mirror support.

Full Description:
TECHNICAL FIELD 
     The present invention is directed towards a MEMS (Micro-Electro-Mechanical-system) micromirror with one or two dimensional rotation and micromirror arrays with a high fill factor. 
     BACKGROUND 
     The MEMS (Micro-Electro-Mechanical Systems) mirrors and mirror arrays have wide applications in the light process and fiber optic networks such as in optical cross-connect switches, attenuators, wavelength blocker, dynamic gain equalizer, configurable grating and tunable filter etc. The MEMS mirror arrays with high fill factors and two axes rotation have particular importance in the wavelength division multiplexing systems. Tile fill factor is generally defined as the ratio of the active area to the total area in an array. The high fill factor improves optical channel shape and reduces the optical loss in the system. A micromirror with two axes of rotation can provide switching of the optical beam among the channels while avoiding undesirable optical transient cross-talk during switching, and achieving variable optical attenuations. 
     There are a number of actuation methods for the MEMS micromirror array such as electromagnetic actuation disclosed in U.S. Pat. No. 6,760,145 B1, thermal actuation disclosed in U.S. Pat. No. 7,091,057 B2, and electrostatic actuation disclosed in U.S. Pat. No. 7,095,546 B2. Electrostatic actuation is favored due to its low power consumption and relative simple structure and small footprint. 
     Existing micromirrors with electrostatic actuation fall into two categories: vertical combdrive type micromirrors and parallel plate type micromirrors. The drawback for conventional vertical combdrive type micromirrors is its failing to form the high fill factor arrays due to its typical gimbaled and framed structure. Since it is difficult to reduce the gap between adjacent micromirrors, it is hard to form a mirror array with high fill factor. One of these kinds of MEMS micromirrors was disclosed in U.S. Pat. No. 6,822,776 B2. 
     It is much easier to form high fill factor mirror arrays based on the parallel plate type of actuators. The majority of existing high fill factor micromirror array designs use parallel plate type electrostatic actuators, such as those taught in U.S. Pat. Nos. 7,095,546, 6,934,439, 6,694,073, 6,781,744, 6,778,728, 7,209,274 and 7,053,981. The advantage of using a parallel plate electrostatic actuator is that no typical gimbaled structure or frame is required for the design. As such, the gap between the mirrors can be very small to form a high fill factor mirror array. However, there are several disadvantages for the micromirror array using parallel plate electrostatic actuators. 
     First of all, the pull-in effect of parallel plate type electrostatic actuator of micromirror limits the controllable tilting angle range under the certain actuation voltage. When an actuation voltage is applied between the 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 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 increased, and the tilting of the hinged mirror is over one third of the initial gap between tie fixed electrode and the mirror, the electrostatic force between the electrode and the mirror surpasses the mechanical restoring force of the hinges, such that the hinged mirror will snap and physically contact die fixed electrode. Thus, the usable and controllable tilting range of the mirror is limited to only one third of the gap between the mirror and fixed electrode. Furthermore, even within the small controllable titling range, the parallel plate electrostatic actuator won&#39;t provide linear actuation. In other word, the mirror tilting angle is not linear with the actuation voltage. 
     Secondly, a high actuation voltage creates issues with respect to electrical charging, tilting angle drifting, and cross-talking between adjacent mirrors. In order to have a larger controllable titling angle for the mirror, the gap between the fixed electrode and the mirror has to be increased. This increased gap results in a higher actuation voltage. Often, several hundreds of volts have to be used to obtain a couple of degrees of mirror titling. Such higher driving voltage causes electrical charging on the dielectrical materials of the mirror device, which will in turn cause the undesired tilting angle drifting of the mirror. Also it is very difficult to shield the electrical field from one mirror actuation electrode to interfere the performance of the adjacent mirrors. 
     Thirdly, there is a mechanical coupling between the two axes rotation for the micromirror. In the micromirrors disclosed in U.S. Pat. Nos. 7,095,546, 6,934,439, 6,694,073, 6,781,744, 6,778,728, 7,209,274 and 7,053,981, the actuation about one axis rotation will cause the move of the mirror about the other axis. This coupling of two axes rotation makes operation control of the device complicated, and unreliable. 
     Fourthly, squeezed air between the movable mirrors and the fixed electrodes during tilting will lead to interference among adjacent mirrors. Since the space between the movable mirror and fixed electrode is very small, and the gap between the adjacent mirrors is also small to obtain a high fill factor, the fast titling/switching of one mirror will cause the air film between its mirror and fixed electrode to be either compressed or decompressed. As such, the air flow will be formed. The air flow resulting from the switching of one mirror will therefore interfere with the adjacent mirrors, and cause them to tilt. Furthermore, the air damping from the squeezed air film will effectively lower the switching speed of the mirror. 
     Lastly, the microfabrication process is costly and complex, especially for making complex actuation electrodes and electrical wirings of two dimensional rotation mirrors. 
     SUMMARY 
     There is provided a micro-electro-mechanical-system (MEMS) micromirror for use in high fill factor arrays which includes at least one stationary body and a movable body. The movable body has opposed ends and is secured to the stationary body at each of the opposed ends by a resilient primary axis pivot. A mirror support is supported by and movable with the movable body. The mirror support has a first unfettered side and a second unfettered side. A primary axis actuator is provided including a fixed portion connected to the stationary body, and a movable portion connected to the movable body. The movable portion is adapted to move away from the fixed portion in response to an electrical potential difference between the fixed portion and the movable portion, such that the movable body rotates about the primary axis resilient pivot. A mirror is supported by the mirror support. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features 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 be in any way limiting, wherein: 
         FIG. 1  is a perspective view of a prior art micromirror using a conventional parallel plate type electrostatic actuator. 
         FIG. 2  is a perspective view of a micromirror using electrostatic vertical combdrive actuators. 
         FIG. 3  is a perspective view of a design variation of the electrostatic vertical combdrive actuator. 
         FIG. 4  is a perspective view of another design variation of the electrostatic vertical combdrive actuator. 
         FIG. 5  is a perspective view of a Silicon On Insulator (SOI) wafer. 
         FIG. 6  is a perspective view of the SOI wafer of  FIG. 5  after being trenched by deep reactive ion etching (DRIE). 
         FIG. 7   a  is a perspective view of the SOI wafer after trench refilling. 
         FIG. 7   b  is an enlarged perspective view of detail A of the trench refilling depicted in  FIG. 7   a.    
         FIG. 8  is a perspective view of the SOI wafer after partial DRIE silicon etching. 
         FIG. 9  is a perspective view of a carrier wafer. 
         FIG. 10  is a perspective view of a silicon carrier wafer after deep cavity silicon etching and shallow silicon etching to form supporting structures. 
         FIG. 11  is a perspective view of a silicon carrier wafer after through hole silicon etching and shallow silicon etching to form supporting structures. 
         FIG. 12  is a perspective view of a glass carrier wafer after through hole machining and shallow glass etching to form supporting structures. 
         FIG. 13   a  is a perspective view of a glass carrier wafer after shallow glass etching on top of the supports and forming metal electrical interconnection wires. 
         FIG. 13   b  is an enlarged perspective view of detail B of the metal contact tab on the top of glass supporter depicted in  FIG. 13   a.    
         FIG. 14  is a perspective view of the SOI wafer in  FIG. 8  bonded with a carrier wafer. 
         FIG. 15  is a perspective view of an etched away the handle wafer of SOI wafer. 
         FIG. 16   a  is a perspective view of the buried oxide layer of the SOI wafer that has been partially etched. 
         FIG. 16   b  is an enlarged perspective view of detail C of the partially etched buried oxide layer depicted in  FIG. 16   a.    
         FIG. 17   a  is a perspective view of a fully etched pattern of the buried oxide layer of the SOI wafer. 
         FIG. 17   b  is an enlarged perspective view of detail D of the patterned buried oxide depicted in  FIG. 17   a.    
         FIG. 18   a  is a perspective view of the SOI wafer after through device silicon layer DRIE silicon etching. 
         FIG. 18   b  is an enlarged perspective view of detail E of the comb fingers after through device layer DRIE etching depicted in  FIG. 18   a.    
         FIG. 19   a  is a perspective view of the SOI wafer of  FIG. 18  after DRIE silicon etching to form the lower comb fingers. 
         FIGS. 19   b ,  19   c  and  19   d  are enlarged perspective views of details F, G and H, respectively, of the comb fingers depicted in  FIG. 19   a.    
         FIG. 20   a  is an exploded view of a micromirror, vertical combdrive actuators and carrier glass wafer with metal interconnection wires. 
         FIG. 20   b  is an exploded backside view of a micromirror, vertical combdrive actuators and carrier glass wafer with metal interconnection wires. 
         FIG. 21  is a perspective view of a single row micromirror array configuration. 
         FIG. 22  is a perspective view of another single row micromirror array configuration. 
         FIG. 23  is a perspective view of a double row micromirror array configuration. 
         FIG. 24   a  is a perspective view of a micromirror with the tilting detection feature for primary rotation. 
         FIG. 24   b  is an enlarged perspective view of detail I of the tilting detection feature for primary rotation in  FIG. 24   a.    
         FIG. 25  is a perspective view of the two dimensional rotation mirror using metal electrodes on the carrier wafer for secondary rotation. 
         FIG. 26  is a perspective view of a low voltage driven, one dimensional rotation micro mirror. 
         FIG. 27  is a perspective view of a low voltage driven, one dimensional rotation micromirror array. 
         FIG. 28   a  is a perspective view of taper shape hinge. 
         FIG. 28   b  is a perspective view of double beam shape hinge. 
     
    
    
     DETAILED DESCRIPTION 
     While the device described below is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail, preferred embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principles of the device and is not intended to limit the broad aspects of the device 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. 
     A micromirror with parallel plate electrostatic actuators known in the art is shown in  FIG. 1 . The mirror  13  is coated with reflective materials such as a metal film, and is supported by two hinges  12  and  16  which are connected to the anchors  10  and  11 . Two fixed actuation electrodes  14  and  15  are located below mirror  13 . Mirror  13 , hinges  12  and  16  and anchors  10  and  11  can be made of doped conductive silicon. When the actuation voltage is applied between mirror  13  and electrode  14 , the resulting electrostatic force will pull mirror  13  towards electrode  14  and cause the deformation of hinges  12  and  16 . Mien the electrostatic force is balanced with the mechanical restoring force of the deformed hinges, mirror  13  will stabilize at a tilting angle. The disadvantages of the parallel plate electrostatic actuator, such as the pull-in effect, tilting angle drifting and cross-talking between adjacent mirrors, squeezed air damping, etc., yield poor performance for high fill factor micromirror and micromirror arrays, as well as very complex, low production yield and expensive manufacturing methods. 
     To overcome these problems and to obtain micromirrors and micromirror arrays with a high fill factor, a vertical electrostatic combdrive actuator design may be used. One example of such a vertical electrostatic combdrive is shown in  FIG. 2 . The advantages of the vertical combdrive over the parallel plate electrostatic actuator include higher actuation force density, better actuation linearity, and no pull-in effects. The micromirror design and the fabrication method for the micromirror array as discussed below reduce the effect of tilting angle drifting and cross-talking between adjacent mirrors as well as squeezed air damping. The mirror  21  is fixed to the anchors  17  and  18  by hinges  26  and  27 . The fixed electrodes  19  and  20  have fixed sets of upper comb fingers  22  and  24 . The movable sets of lower comb fingers  23  and  25  are on the outside edges of mirror  21 . When an actuation voltage is only applied between moveable lower fingers  23  and fixed upper comb fingers  24 , the resulting electrostatic force will pull mirror  21  counterclockwise around hinge  26  and  27 , and cause hinges  26  and  27  to deform. Mirror  21  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 only applied between fixed upper comb fingers  22  and movable lower comb fingers  25 , mirror  21  will rotate around the hinge  26  and  27  clockwise instead. The micromirror shown uses different upper and lower finger designs to achieve the direction control of the mirror rotation. This vertical combdrive design is such that when mirror  21  is electrically grounded, an applied voltage on either of the two fixed electrodes  19  and  20  will independently rotate the mirror and cause it to tilt into two different directions, either clockwise or counterclockwise. 
     Referring to  FIG. 3 , the vertical combdrive design shown has different upper and lower comb finger arrangements. The mirror  21  has upper comb fingers  29  and lower fingers  23  on its outside edge. Fixed electrode  19  has fixed lower comb fingers  28 , while fixed electrode  20  has upper comb fingers  24 . If an electrical potential is applied on both fixed electrodes  19  and  20  when mirror  21  and associated upper fingers  29  and lower fingers  23  are electrically grounded, the mirror will rotate counterclockwise. The combdrives on both sides of the mirror  21  will thus work together to actuate the mirror in the same direction. The advantages of such design are a reduction in the actuation voltage for certain mirror tilting angles, and that the resulting unbalanced force on the hinges is eliminated, which can cause the up or down piston translation movement of the mirror besides the desired mirror rotation. 
     The vertical combdrive designs shown in  FIGS. 2 and 3  have upper comb fingers, which are formed by removing the materials from its lower part. Another comb finger arrangement option is shown in  FIG. 4 , where the material is kept on the lower part of the finger. This design simplifies the fabrication process, but it increases the driving voltage to achieve the same mirror titling angle compared with the comb finger designs shown in  FIGS. 2 and 3 . 
     It will be understood that a micromirror or a micromirror array may use any of the three comb finger arrangements shown in  FIG. 2 ,  FIG. 3  and  FIG. 4  for its vertical combdrive actuators. In the following description of the microfabrication method for micromirrors and micromirror arrays, only comb finger designs shown in  FIG. 2  and  FIG. 3  will be included since the comb finger design shown in  FIG. 4  does not require all the steps in the process to make the vertical combdrive actuator shown in  FIG. 2  and  FIG. 3 . Furthermore, the following process description is one example of a microfabrication method designed for the micromirror and the high fill factor micromirror array. Those skilled in the art will recognize that there are other alternative microfabrication methods, and that the description below is a representative fabrication method for the micromirror and micromirror array. Furthermore, only the major process steps for fabricating the mirror and micromirror array will be described. 
     The micromirror and micromirror array and corresponding vertical combdrive actuators are made from a single crystal silicon device layer of Silicon On Insulator (SOI) wafer shown in  FIG. 5 . The thinner single crystal silicon device layer  32  is bonded to the handle silicon wafer  34  with Buried Oxide (BOX) layer  33 . The SOI wafer can be purchased commercially from SOI wafer vendors, or can be made using some well known method such as fusion bonding and etching back process. The single crystal device silicon should be heavily doped to have good electrical conductivity. 
     Since, in some applications, each micromirror needs to be controlled to rotate two dimensionally, and the actuators for primary and secondary rotation are made on the same conductive single crystal device layer, the primary actuator should be electrically isolated from the secondary actuator in order to have independent actuation. A trenching and dielectric material refilling method is used in the microfabrication method described below. 
     The SOI wafer shown in  FIG. 5  may have a thin nitride film grown on both the top and bottom sides using Low Pressure Chemical Vapor Deposition (LPCVD). Lithography is then used to define the specific shape for the trench on silicon device layer of the SOI wafer. It is very important to have good electrical isolation while maintaining strong mechanical strength when designing the trench shape. After Reactive Ion Etching (RIE) of silicon nitride layer  35  as shown in  FIG. 6  and Deep Reactive Ion Etching (DRIE) of silicon device layer  32 , a hollow trench  36  of several microns wide is formed on the single crystal device layer  32 . The BOX layer  33  is used as a DRIE etching stop layer. Simple photoresist or other materials can be used as the RIE and DRIE etching mask. A single trench or multiple parallel trenches should be formed in order to have good electrical isolation and mechanical strength. A single trenching  36  is shown in  FIG. 6  after stripping RIE and DRIE etch masking material. 
     The processed SOI wafer is then sent to the thermal oxidation furnace for wet oxidation. Since the SOI wafer is covered with LPCVD nitride on the top and bottom sides, the oxidation will only occur on the sidewalls of the trench. The oxide formed from both sidewalls of the trench will meet each other to close the trench after a certain period of wet thermal oxidation. The LPCVD nitride thin films will prevent the growth of thermal oxide on both top and bottom sides of the SOI wafer, and are etched away using RIE shown in  FIG. 7   a . The trench  36  filled with wet thermal oxide  37  is shown in  FIG. 7   b.    
     There are many other options to do the trench etching and dielectric material refilling. For example, instead of filling the trench with oxide alone, polysilicon can be used to fill most of the trench after initial thin thermal oxide growth. The choice of the trench etching and refilling depends on the requirements of the micromirror and process cost. 
     After trench forming and dielectric material refilling, another photolithography process is performed on the device silicon layer for the subsequent partial silicon DRIE. The DRIE etching depth may be, for example, around half the thickness of the device silicon layer  32 . The partial silicon DRIE step is used for multiple purposes. In  FIG. 8 , silicon DRIE is used in region  54  and  55  to remove the lower part of the silicon to form upper vertical comb fingers. Silicon DRIE may also be used in regions  51   a ,  51   b ,  52   a  and  52   b  in order to thin down the hinge heights to make them more flexible, which reduces the required actuation voltage. The silicon DRIE in regions  53   a  and  53   b  are on the backside the micromirror surface. As a result, the micromirror mass is reduced, and its resonant frequency is increased, without sacrificing the mirror structure strength and mirror flatness. 
     A handle or carrier wafer  39  shown in  FIG. 9  could be Pyrex glass or regular silicon wafer. After forming some supporting structures to support the anchors of vertical electrostatic combdrive actuators, and a large cavity under micromirrors to reduce or eliminate the squeezed air damping or squeezed air flow as discussed before, wafer  39  will be bonded to the SOI wafer produced above by using either fusion bonding, anodic bonding or other bonding techniques. 
     In  FIG. 10 , if the wafer  39  is a regular silicon wafer, the lithographies and silicon DRIEs will be used to form the supports  40  and  41  by shallow silicon etching in region  42 , and to form a deep cavity under the micromirror by deeper silicon etching in region  43 . This may also be done by simply etching through the carrier wafer  39  to leave a hole under the micromirror as shown in  FIG. 11 . Other etching methods can also be used for this etching, such as wet silicon anisotropic etching in KOH (Potassium hydroxide) or TMAH (Tetramethylammonium hydroxide), etc. After silicon etching and before bonding with SOI wafer in the inert gases such as forming gases, a very thin thermal oxide will grow on the carrier silicon wafer in order for single crystal silicon device layer  32  of SOI wafer to be electrically isolated from the carrier silicon wafer  39 . 
     If Pyrex glass is chosen as the carrier wafer, the isotropic glass etching in HF (Hydrofluoric acid) will form the supports  45  and  46  by shallow glass etching in area  47  as shown in  FIG. 12 . While the through hole under micromirror on the glass carrier wafer is not shown in  FIGS. 12 and 13 , it can be made using wet HF etching, sand blasting, or laser micromaching. 
     When electrical interconnections are required on the glass carrier wafer  39 , very shallow glass etching is done in region  49  on the tops of supports  45  and  46  as shown in  FIG. 13   a . The shallow glass etching depth is little deeper than the deposited metal wire  50  thicknesses. As such, the device silicon layer of the SOI wafer will be intimately bonded to the supports  45  and  46  while still having electrical connection with the metal wires through the metal tab  48 . A detailed view of glass support  46  with metal tab  48  and metal wire is shown in  FIG. 13   b . On the rest of bottom area of the shallow glass etched cavity, besides the metal electrical connections wires  50 , the electrical ground metal film should be coated and patterned to prevent the glass charging issues. 
     The SOI wafer is bonded to the etched regular silicon carrier wafer or etched glass carrier wafer using fusing boding or anodic boding respectively. In the following process description, the glass carrier wafer with electrical wires and metal tabs will be used. The SOI wafer is bonded to the glass carries wafer using anodic bonding as shown in  FIG. 14 . If the glass carrier wafer has through etched holes, the bonded wafer set should be protected on tie glass side during the removal of the SOI handle wafer  34  in the subsequent KOH or TMAH bath to prevent the etchant from attacking the device silicon layer  32  through the through holes on the glass carrier wafer  56 . The protection method can be as simple as using a wafer protection holder which seals the whole glass wafer and only exposes the handle wafer  34  of the SOI wafer in the KOH or TMAH bath. If the glass carrier wafer  56  only has deep etching cavity under micromirror, there is no need to have extra protect since the glass wafer itself will provide good protection to the single crystal silicon device layer  32  during KOH or TMAH silicon etching of the SOI handle wafer. The buried oxide layer  33  of the SOI wafer is the etching stop layer for KOH or TMAH silicon etching. The bonded wafer after SOI handle wafer removal is shown in  FIG. 15 , with a carrier wafer  56 . 
     The self alignment process is applied to achieve the equal gap between adjacent movable and fixed comb fingers. Any unequal gap between adjacent movable and fixed comb fingers will cause asymmetry of electrostatic force, which in turn results in the malfunction of the vertical combdrive such as sidewall snapping movement of movable comb fingers. The BOX layer  33  shown in  FIG. 16   a  is kept and used as silicon DRIE masking materials for the self alignment process. After photolithography, the partial RIE etching of BOX layer  33  is done to prepare to make the lower comb fingers. The BOX layer oxide partial etching regions  57  are shown in  FIG. 16   a , while the partial RIE oxide etching cavity  58  is shown in  FIG. 16   b.    
     A subsequent photolithography is done after the partial BOX oxide etching, where a layer of photoresist is coated on the wafer to be patterned. After this lithography, an oxide RIE is performed to pattern hinges  65  and  67 , anchor  64 , primary vertical combdrive actuator  59 , and secondary vertical combdrive actuator  63  shown in  FIG. 17   a . In the detailed view of  FIG. 17   b , the upper comb fingers have the masking oxide layer  61  with the same thickness as the original buried oxide layer  33 , while the lower comb fingers have the masking oxide layer  60  which is only a portion of the thickness of the original buried oxide layer  33 . The exposed single crystal device silicon is indicated by  62 . 
     Following up the photolithography, silicon DRIE is used to etch through the single crystal device silicon as show in  FIG. 18   a . After this DRIE, an oxide RIE is utilized to etch away any remaining oxide on the lower fingers  72  as shown in  FIG. 18   b , while the upper comb fingers  71  still have some remaining oxide  61   
     The last silicon DRIE is used to etch away the silicon on the top half of the lower comb fingers  72  to form its final shape, while the upper comb finger  71  is still protected by the remaining oxide  61 .  FIG. 19   a  shows the final shape of the micromirror and its vertical combdrive actuators after final oxide RIE removal, and  FIGS. 19   b ,  19   c  and  19   d  show the details of upper and lower comb fingers of the primary and secondary actuators. 
     There are many microfabrication methods to deposit and pattern the reflective metal film on the mirror surface and bonding metal film on the anchors/bonding pads  75 ,  78 ,  79  and  80 . One simple method is to perform metallization using sputtering or E-Beam evaporation with a shallow mask. Another method is to perform the metallization first and then pattern the metal layer before the self alignment process, and then use the patterned photoresist to protect the metal layer during subsequent self alignment process. In the end, the remaining metal protecting photoresist is removed using oxygen plasma. 
     The design of micromirror and its corresponding vertical combdrive is shown in  FIG. 19   a . When the electrical potential is applied to the anchor  78 , which may also serve as an electrical wire bonding pad depending on its size in the specific micromirror and micromirror arrays designs, mirror  72  with its movable comb fingers of the secondary vertical combdrive actuators will be at the same electrical potential as anchor  78 , since mirror  72  is electrically connected with anchor  78  through its electrically conductive silicon hinge  73 , electrical conductive silicon frame  83  of the secondary rotation actuator, electrical conductive silicon frame  82  of the primary rotation actuator and electrically conductive silicon hinge  77 , The movable comb fingers of the primary rotation actuator  76  and  84  are also electrically connected with anchor  78  through the electrical conductive silicon frame  82 . Usually, the anchor  78  is chosen to connect to the electrical ground by metal wire bonded to the metal film on its top, or through contacting the metal tab  92  of metal interconnection wire  90  on the glass etching cavity as shown in  FIG. 20   a.    
     The primary actuators  76  and  94  are responsible for the primary rotation of the mirror  72 . If all the movable comb fingers on the silicon frame  82  are designed to be all upper fingers or all lower fingers while the fixed comb fingers on fixtures  79  and  80  are designed to be all lower or upper fingers, then the mirror can be rotated either clockwise or counterclockwise until a certain tilting angle is reached. Fixtures  79  and  80  are anchors that connect the etched wafer to the carrier wafer along with anchors  64  in  FIG. 17   a  or anchors  75  and  78  that act as electrodes in  FIG. 19   a . As will be apparent, each of these anchors may perform a different function in addition. For example, fixtures  79  and  80  are used to support their fixed comb fingers. If an actuation voltage is only applied between the fixture  79  and anchor  78 , the electrostatic force is created in primary combdrive  76  between the fixture  79  and the silicon frame  82 . The corresponding torque resulting from the electrostatic force will rotate the mirror  72 , for example clockwise, until a certain tilting angle is reached. On the other hand, if an actuation voltage is only applied between the fixture  80  and anchor  78 , the electrostatic force is created in primary combdrive  84  between the fixture  80  and the silicon frame  82 . The corresponding torque resulting from the electrostatic force will rotate the mirror  72 , for example, counterclockwise, until a certain tilting angle is reached. The comb finger arrangement of actuator  76  and  84  can also be designed so that both actuators work together to actuate the mirror in a single direction. 
     The anchor  75 , silicon hinge  85 , silicon connector  81  and the fixed comb fingers of the secondary vertical combdrive actuator on the silicon connector  81  are electrically isolated from other parts of the micromirror structure by the oxide filled trench  86 . The movable comb fingers of the secondary actuator  74  are electrically connected to the anchor  78 , which is usually an electrical ground. The actuation voltage for secondary rotation could be directly applied to the metal film on top of the anchor  75  shown in  FIG. 19   a , or through the metal tab  93  of metal interconnection wire  91  on the glass etching cavity, as shown in  FIG. 20   a . Once the actuation voltage is applied to anchor  75  shown in  FIG. 19   a  and  FIG. 20   a , the mirror  72  will have secondary tilting due to the electrostatic force created in secondary actuator  74 . Detailed views of comb finger designs for actuators  76 ,  84  and  74  are shown in  FIGS. 19   b ,  19   c  and  19   d .  FIG. 20   b  shows the backside view of the silicon mirror and its actuators. A partially etched silicon cavity  94  ( FIG. 20   b ) is on the backside of the mirror to reduce the mirror mass and increase its response speed of the actuation. The strengthen silicon bar  95  is used to maintain the stiffness and strength of the mirror. 
     The structure design described above of primary and secondary actuators eliminates any mechanical coupling between primary rotation and secondary rotation. If the anchor  78  is connected to the electrical ground, the primary and secondary rotations of the mirror will be independently controlled by the amount of voltage on fixture  79  and/or  80 , and anchor  75 . As depicted, the arrangement of the micromirror is such that micromirror is away from the actuators and stands on one end of the micromirror device. There is no gimbal structure around the mirror. This arrangement provides excellent flexibility to form high fill factor micromirror arrays. Two examples of single row high fill factor arrays are shown in  FIGS. 21 and 22 . 
     If the micromirror width is small, in order to prevent the interference or crosstalk of primary actuators of adjacent two micromirrors, the configuration of the micromirror array shown in  FIG. 21  should be used, where the actuators of adjacent two micromirrors are located on the different side of the micromirror. If the micromirror width is large, and the gap or space between adjacent primary rotation actuators is large enough to avoid any electrical actuation interference, the configuration of the micromirror array shown in  FIG. 22  should be used, where all the actuators are located on the same side of the micromirrors.  FIG. 23  shows a double row configuration for a micromirror array. 
     For some applications, the actual primary rotation is very important, and its rotation has to be electrically monitored. The device may then be designed to provide sensing structures to detect electrically the primary rotation of the micromirror. For example, the device may be designed to utilize a portion of vertical comb fingers of the primary actuators as the sensing structures, as shown in  FIG. 24 . The fixtures  101  and  102  are electrically isolated and mechanically separated from fixtures  79  and  80  respectively. The movable and fixed comb fingers in region  103  and  104  no longer act as electrostatic actuators; instead, they form variable electrical capacitors when the micromirror is rotated by the primary actuators. The primary rotation can be detected by measuring the capacitance in portion  103  or  104 , or by measuring the differential capacitance changes between the portion  103  and  104 . 
     Instead of trenching and dielectrical material refilling process to create the electrically isolated secondary actuator, the metal electrodes  301  and  302  can be formed on the etched cavity surface of the carrier wafer. The electrode  301  is underneath the mirror  304  and forming a parallel plate electrostatic actuator, while electrode  302  is underneath the paddle  303  and forming another parallel plate electrostatic actuator. These two actuators can be utilized to replace the secondary vertical combdrive actuator to control the secondary rotation of the mirror. 
     Based on the same design and microfabrication method, low voltage driven, high fill factor and one directional rotation micromirror array can also be achieved. The micromirror  106  is supported be two hinges  118  and  119  which are connected to the anchor  112  and  113  ( FIG. 26 ). All the vertical combdrive actuators  114 ,  115 ,  116  and  117  are working together to actuate the micromirror  106  towards the same direction. This approach will significantly reduce the actuation voltage while the quicker repose of the micromirror is still maintained. This micromirror configuration is especially useful for such devices as wavelength blocker, VOA (Variable Optical Attenuator) and VOA array, configurable grating etc. with low driving voltage such as less than 5 Volts. One of the array configurations using this kind of micromirror is shown in  FIG. 27 . 
     The micromirror can have a variety of hinge shape designs. A V shape hinge is shown in some of the drawings such as  FIG. 19   a , however, other hinge designs are also used such as a taper shape hinge shown in  FIG. 28   a , or a double beam hinge shown in  FIG. 28   b . The V shape hinge, taper shape hinge and double beam hinge design provide very good rotation stability in terms of preventing side snapping of vertical combdrive actuator due to the process imperfection. Imperfection of microfabrication processes is one of major reasons to cause side snapping of the combdrive actuators. 
     The teachings in this document provide Micro-Electro-Mechanical-Systems (MEMS) micromirrors for high fill factor arrays. The electrostatic vertical combdrive may be used for the actuation of the mirror. The vertical combdrive has good linearity for the actuation with no pull-in effect. The vertical combdrive actuator provides large tilting angle at low actuation voltage. It eliminates mirror tilting drifting due to electrical charging, interference of adjacent mirrors, and squeezed air damping. The actuation of primary and secondary rotation is totally decoupled, which results in the simpler electrical control system. Further more, the micromirror may be designed to have a tilting sensing feature to monitor the position of the mirror along primary rotation axis. This may be done by electrically isolating one combdrive actuator, and connecting it to a detector to measure the capacitance. The capacitance between the fixed and movable fingers will change as they move relative to each other such that the capacitance give an indication of the rotation between the fixed and movable fingers. If the direction of movement is known, the position of the mirror can then be determined. 
     The present teachings offers micromirror array with higher than 95% of fill factor at lower than 100 volts of actuation voltage. The design and fabrication method of micromirrors and micromirror arrays could be used for optical switch especially optical multi wavelength selective switch, optical scanner, and optical variable attenuator, optical tunable filter, optical display device, compact spectrometer, and wavelength blocker etc. 
     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 device as hereinafter defined in the Claims.

Technology Classification (CPC): 1