Patent Publication Number: US-2023136105-A1

Title: Vertical mechanical stops to prevent large out-of-plane displacements of a micro-mirror and methods of manufacture

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 63/263,373, filed Nov. 1, 2021, entitled VERTICAL MECHANICAL STOPS TO PREVENT LARGE OUT-OF-PLANE DISPLACEMENTS OF A MICRO-MIRROR which application is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND 
     Technical Field: This disclosure relates to microelectromechanical systems (MEMS) mirror arrays and methods of manufacturing the MEMS mirror arrays that reduces and/or constrains out-of-plane displacements caused by shock. 
     Background: A MEMS (microelectromechanical systems) device is a micro-sized mechanical structure having electrical circuitry and is fabricated using conventional integrated circuit (IC) fabrication methods. One type of MEMS device is a microscopic gimbaled mirror device. A gimbaled mirror device includes a mirror component, which is suspended off a substrate, and is able to pivot about a gimbal due to electrostatic actuation. Electrostatic actuation creates an electric field that causes the mirror component to pivot. By allowing the mirror component to pivot, the mirror component is capable of having an angular range of motion in which the mirror component can redirect light beams to varying positions. 
     An optical switch is a switching device that couples light beams from an input fiber to an output fiber. Typically, the light beams from an input fiber are collimated and directed toward a desired location such as an output fiber. A movable mirror (e.g., a gimbaled mirror) in a switch mirror array redirects the light beams to desired locations. 
     Inside the optical switch, the mirrors in the array may need to rotate  10  —  20  degrees or more to direct the light beams to the desired locations. The mirrors need sufficient space above and below to allow these rotations. However, having this much space above and below the mirrors allows the mirrors to also move linearly in the vertical (out-of-plane) direction. The mirror flexures are designed to prevent this undesired linear, out-of-plane motion under normal operating conditions. When the optical switch is being handled during shipment or installation, for example, the optical switch could be dropped or impacted with sufficient force to cause the mirrors to undergo large out-of-plane deflections. This may damage mirrors in the switch array resulting in them being inoperable. 
     What is needed are MEMS mirror arrays and methods of manufacturing the MEMS mirror arrays that reduces and/or constrains out-of-plane displacements caused by shock. 
     SUMMARY 
     Disclosed are MEMS mirror arrays and methods of manufacturing the MEMS arrays that constrains out-of-plane displacement caused by shock which reduces the likelihood of damage. Also disclosed are MEMS mirror arrays and methods of manufacturing the arrays that constrains out-of-plane displacements while still allowing large angular rotations needed for optical switching. 
     One aspect of the disclosure provides a mirror array. The mirror array includes a lid, a base, and a movable mirror between the lid and the base. The movable mirror includes a stationary frame including a cavity, a movable frame in the cavity, and a central stage in the cavity. The mirror array also includes a first protrusion on the base. The first protrusion overlaps with the central stage in a first direction. 
     Implementations of the disclosure may include one or more of the following optional features. The central stage can include a bottom portion facing the first protrusion. Additionally, the first protrusion can be spaced apart from the bottom portion of the central stage by a predetermined distance. The predetermined distance between the first protrusion and the bottom portion of the central stage is between 3 μm and 15 μm. 
     Optionally, the mirror array further may include a second protrusion on the lid and a third protrusion on the lid. The second protrusion and the third protrusion can be configured to extend towards the base. The mirror array can also include a first stationary frame flexure and a second stationary frame flexure. The first stationary frame flexure and the second stationary frame flexure suspend the movable frame from the stationary frame. The second protrusion can be configured to overlap with the first stationary frame flexure in the first direction, and the third protrusion can be also configured to overlap with the second stationary frame flexure in the first direction. The second protrusion can also be positioned apart from the first stationary frame flexure by a predetermined distance. The predetermined distance between the second protrusion and the first stationary frame flexure is between 3 μm and 15 μm. The third protrusion can also be positioned apart from the second stationary frame flexure by a predetermined distance. The predetermined distance between the third protrusion and the secondary stationary frame flexure is also between 3 μm and 15 μm. The second protrusion and the third protrusion can be positioned so that the second protrusion and the third protrusion are non-overlapped with the central stage in the first direction. 
     The mirror array is further configurable to include a fourth protrusion on the base and a fifth protrusion on the base. The first protrusion can be positioned between the fourth protrusion and the fifth protrusion. Additionally, the fourth protrusion can be configured to support a first support member, while the fifth protrusion is configured to support a second support member. The central stage can also include a bottom portion extending towards the first protrusion, wherein the bottom portion is positioned between the first support member and the second support member. 
     Another aspect of the disclosure provides a mirror array. The mirror array includes a movable mirror and a lid wafer covering the movable mirror. The movable mirror also includes a stationary frame including a cavity, a movable frame in the cavity, and a central stage in the cavity. The movable frame can be suspended from the stationary frame by a first stationary frame flexure and a second stationary frame flexure. The mirror array can also include a first protrusion on the lid wafer. The first protrusion is extended towards the movable mirror. 
     Implementations of the disclosure may include one or more of the following optional features. The first protrusion can overlap with the first stationary frame flexure in a first direction. A gap can be provided between the first protrusion and the first stationary frame flexure that is between 3 μm and 15 μm. The mirror array can further include a second protrusion on the lid wafer. The second protrusion can be positioned to overlap with the second stationary frame flexure in the first direction. A gap can be provided between the second protrusion and the second stationary frame flexure that is between 3 μm and 15 μm. 
     The mirror array is further configurable to include a base wafer and a third protrusion on the base wafer. The third protrusion can also overlap with the central stage in the first direction. The third protrusion can also be spaced apart from a bottom portion of the central stage by a predetermined distance, such as 3 μm and 15 μm. 
     Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosed embodiments, as claimed. 
     INCORPORATION BY REFERENCE 
     All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference. 
     U.S. Pat. No. 5,501,893 A issued Mar. 26, 1996 to Laermer et al.; 
     U.S. Pat. No. 5,635,739 A issued Jun. 3, 1997 to Grieff et al.; 
     U.S. Pat. No. 5,696,619 A issued Dec. 9, 1997 to Knipe et al.; 
     U.S Pat. No. 6,430,333 B1 issued Aug. 6, 2002 to Little et al.; 
     U.S Pat. No. 6,664,706 B1 issued Dec. 16, 2003 to Hung et al.; 
     U.S Pat. No. 6,914,711B2 issued Jul. 5, 2005 to Novotny et al.; 
     U.S. Pat. No. 7,092,141 B2 issued Aug. 15, 2006 to Kim et al.; 
     U.S. Pat. No. 7,261,826 B2 issued Aug. 28, 2007 to Adams et al.; 
     U.S. Pat. No. 7,330,297 B2 issued Feb. 12, 2008 to Noh et al. and 
     BEHIN, et al., Magnetically Actuated Micromirrors for FiberOptic Switching, 1998, Dec. 31. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which: 
         FIG.  1    illustrates a MEMS mirror array including a plurality of movable mirrors; 
         FIG.  2    illustrates the wafer after a release etch separates portions of the structure and after the attachment of the lid wafer; 
         FIGS.  3 A-B  illustrates a maximum out-of-plane deflection of a top surface vs. rotation angles ( FIG.  3 A ) and a minimum out-of-plane deflection of a bottom surface vs. rotation angles ( FIG.  3 B ); 
         FIG.  4    illustrates a partial cross-section of a mirror array; 
         FIG.  5    illustrates a partial cross-section of a mirror array with mechanical stops; 
         FIG.  6    illustrates a movable mirror with a base mechanical stop location identified; 
         FIGS.  7 A-H  illustrates a process for fabricating a base wafer; 
         FIG.  8    illustrates a partial cross-section of a mirror array with a base wafer and a lid wafer; 
         FIG.  9    illustrates a movable mirror with an indication of locations for mechanical stops; and 
         FIGS.  10 A-I  illustrates a process for fabricating the lid wafer. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    illustrates a partial MEMS mirror array  10  including a plurality of MEMS mirrors  100  (also referred as actuators). At each end of a stage or frame, actuator  100  uses a single movable blade, with two corresponding fixed blades as an actuation mechanism structure to enable rotation. Actuator  100  uses two such actuation mechanism structures per stage and two such actuation mechanism structures per frame. A plurality of blades are provided. A first blade  112  is coupled to stage  102  (central stage  220  in  FIGS.  2 ,  4 ,  5 , and  8   ) and is flanked on either side by a pair of first flanking blades  114 ,  114 ′ which are coupled to moveable frame  104  (moveable frame  227  in  FIGS.  2 ,  4 ,  5 , and  8   ) on opposite ends of first blade  112 . Stage  102  is pivotally coupled to moveable frame  104  such that first blade  112  is configured to move relative to first flanking blades  114 ,  114 ′. When a potential difference is applied between first blade  112  and one of the first flanking blades  114 ,  114 ′, an attraction is generated between the blades causing stage  102  to pivot. For example, first blade  112  may be held at a ground potential while an active voltage is applied to either of the first flanking blades  114 ,  114 ′. The application of an active voltage to first flanking blade  114 , for example, will attract the first blade  112 , thereby causing stage  102  to rotate or pivot in a corresponding direction. Similarly, the application of an active voltage to first flanking blade  114 ′ will attract first blade  112  and cause stage  102  to rotate or pivot in an opposite direction to that resulting from the attraction to first flanking blades  114 . 
     A second blade  116  is coupled on end of stage  102  opposite the location of the first blade  112 , with a pair of second flanking blades  118 ,  118 ′ coupled to moveable frame  104  on opposite ends of second blade  116 . Second blade  116  moves relative to second flanking blades  118 ,  118 ′. In order to provide the desired motion of stage  102  and to resist unwanted rotations, actuation voltages are applied concurrently with respect to first blade  112  and second blade  116 . For example, the range of motion for the stage  102  is between +15 degrees and −15 degrees, approximately. When the potential difference is applied between the second blade  116  and one of second flanking blades  118 ,  118 ′, an attraction is generated between the blades resulting in the rotation of stage  102  in a manner similar to that discussed above with respect to the first blade  112 . The use of actuation mechanisms in tandem on each end of stage  102  minimizes or reduces undesired twisting of the stage  102  to provide for more uniform rotation. 
     A similar actuation mechanism structure may be used for rotation of moveable frame  104 . For example, a first side blade  122  is coupled to moveable frame  104  and first side flanking blades  124 ,  124 ′are coupled to stationary frame  140  (stationary frame  214  in  FIGS.  2 ,  4 ,  5 , and  8   ) on opposite ends of first side blade  122 . 
     Moveable frame  104  is pivotally coupled to the stationary frame  140  such that first side blade  122  is configured to move relative to first side flanking blades  124 ,  124 ′. When a potential difference is applied between the first side blade  122  and one of the first side flanking blades  124 ,  124 ′, an attraction is generated between the blades causing moveable frame  104  to pivot in a manner similar to that discussed above in relation to stage  102 . As shown, the moveable frame  104  is suspended from the stationary frame  140  by mirror flexure  152  (e.g., spring, first stationary frame flexure) and a second mirror flexure  154  (e.g., spring, second stationary frame flexure). 
     Second side blade  126  is coupled on the opposite end of moveable frame  104 , with second side flanking blades  128 ,  128 ′ coupled to stationary frame  140  on opposite ends of second side blade  126 . Second side blade  126  moves relative to second side flanking blades  128 ,  128 ′. When the potential difference is applied between second side blade  126  and one of second side flanking blades  128 ,  128 ′, an attraction is generated between the blades facilitating the rotation of moveable frame  104 . The use of actuation mechanisms in tandem on each end of moveable frame  104  minimizes or reduces undesired twisting of the frame to provide for more uniform rotation. For example, the range of motion for the movable frame  104  is between +20 degrees and −20 degrees, approximately. 
     Alternatively, a stage  102  or moveable frame  104  may only have an actuation mechanism structure on a single end. For another embodiment, actuator  100  may have other actuation mechanism structures without departing from the scope of the disclosure. 
     For one embodiment, a plurality of elongated members  130  can be provided (e.g., elongated member  130 ) which are coupled to the undersurface of stage  102  to stiffen the stage  102  and minimize or reduce top surface distortions. In addition, the elongated members  130  on stage  102  may be used to remove etch depth variations across the device. Elongated member  130  may be constructed similar to that of blades discussed herein.  FIG.  1    illustrates seven elongated members  130  where six of the elongated members have substantially the same length and are positioned off-center on the stage  102 , and the seventh elongated member  130  has a shorter length and is positioned centrally on the stage  102 . For example, the seventh elongated member  130  is approximately 10% to 75% shorter than other elongated members  130  (that are positioned “off-center”). 
     For one embodiment, actuator  100  may be fabricated on a wafer level using semiconductor fabrication techniques, as discussed below. For such an embodiment, stationary frame  140  may be formed from a substrate, for example, constructed from silicon. Where all blades are directly driven by different control voltages, actuator  100  may use four voltages, plus a ground. With this arrangement, the number of conductive paths on a substrate quickly becomes very large as multiple actuators are combined to form an array. The low voltages required by the blade actuators discussed herein may allow for control circuitry to be fabricated into the substrate so that only control signals need be routed, rather than separate lines for each blade. This results in a significant reduction in lead count. Lower voltages may also reduce the necessity for spacing between leads to avoid arcing and cross-talk. 
       FIG.  2    illustrates a partial cross-section of the mirror array  10  illustrated in  FIG.  1    along the lines  2 - 2  which illustrates a final structure release on the wafer topside using dry etching, which punctures through trenches  226  to suspend movable elements of the mirror  213  and the frame  227  (also referred as movable frame). In addition, the release etch promotes electrical isolation by separating, for example, the silicon of the frame  227  from the silicon of central stage  220  and stationary frame  214 . The vias  209  serve to connect the regions of silicon to the metal interconnects  211 . To completely seal the mirrors  213  from the outside environment, a lid wafer  230  is bonded to the device wafer  220 ′, preferably through the frit glass seal  231 . The lid wafer  230  is typically glass to allow incoming light to be transmitted with low loss in the mirror cavity  232 , reflect off of the surface of the mirror surface, and transmit out of the mirror cavity  232 . 
     The trenches  210  are filled with a dielectric material, which for one embodiment is silicon dioxide. The filled trenches  210  provide the electrical isolation between blades after the mirror is released. A dielectric layer  203  also remains on the surface of the device wafer  220 ′ and is planarized after the fill process to ease subsequent lithographic patterning and eliminate surface discontinuities. Structure release is accomplished at the upper surface (topside) of the device wafer  220 ′ using dry etching, which punctures through a plurality of trenches  226  to suspend the movable elements of the mirror  213  and the frame  227 . Support webbing  234  (also referred as support member) is also provided. As shown, the bottom portion of the central stage  220  is between the support webbings  234 . A base wafer  212  is bonded to the device wafer  220 ′ to protect the blades after release. A hermetic seal  204  can surround the entire mirror array. The hermetic seal  204  can be formed by the frit material between the base wafer  212  and the device wafer  220 ′. 
       FIGS.  3 A-B  illustrates a maximum out-of-plane deflection of a top surface (of mirror  213 ) vs. rotation angles  302  and a minimum out-of-plane deflection of a bottom surface (of the central stage  220 ) vs. rotation angles  304 . The maximum out-of-plane deflection in  FIG.  3 A  reflects frame degrees from 0-20 on a y-axis and mirror degrees of 0-14 in an x-axis. The values range from 0 μm to 85 μm, with the lowest values being closest to the area around 0 frame degrees and 0 mirror degrees and the high values being from about 19-20 frame degrees and 0-14 mirror degrees. The minimum out-of-plane deflection of a bottom surface vs. rotation angles in  FIG.  3 B  reflects frame degrees from 0-20 on a y-axis and mirror degrees of 0-14 in an x-axis. The values range from −60 μm to 0 μm, with the highest values being closest to the area around 0 frame degrees and 0 mirror degrees and the lowest values being from about 19-20 frame degrees and 0-14 mirror degrees. 
       FIG.  4    illustrates a partial cross-section of a mirror array  400  with support anchors  430 . As shown, the base wafer  212  includes the support anchors  430 . The support anchors  430  (also referred as protrusions or base protrusions) are positioned below the support webbings  234 . Accordingly, each of the support anchors  430  of the base wafer  212  overlaps with a corresponding support webbing  234  in a first direction (e.g., vertical direction). As a result, the support anchors  430  support the support webbings  234 . As shown, the support anchors  430  also support the device wafer  220 ′. The base wafer  212  is bonded to the support anchors  430  via thermal compression, eutectic, or fusion bonding. As a result, the base wafer  212  is bonded to the device wafer  220 ′. 
       FIG.  5    illustrates a partial cross-section of a mirror array  500  with mechanical stops  502  (also referred as protrusions or base protrusions) on a base wafer  700  (e.g., silicon wafer, substrate). As shown, each of the mechanical stops  502  overlaps with a corresponding central stage  220 /mirror  213  in the first direction. As shown, each of the central stages  220  has a bottom portion facing a corresponding mechanical stop  502 . A gap  704  is provided between the bottom of the device wafer  220 ′ (base or bottom portion of the central stage  220 ) and the top surface of the mechanical stop  502 . As shown, the mechanical stop  502  is between the support anchors  430 . 
       FIG.  6    illustrates a central stage  102  (central stage  220  in  FIGS.  2 ,  4 ,  5 , and  8   ), which supports a mirror  213 , with a location  610  for the mechanical stop  502  of  FIG.  5   . For example, the mechanical stops  502  can be located directly under the center of each mirror  213 /central stage  220 . The gap  704  between the bottom of the center loading structure of the mirror  213  (base or bottom portion of the central stage  220 ) and the mechanical stop  502  can be between 3 μm and 15 μm. 
       FIGS.  7 A-H  illustrates a process for fabricating a base wafer  700 . A base wafer  700  is shown in  FIG.  7 A . A hard mask layer  702  is deposited on the base wafer  700  as shown in  FIG.  7 B . The hard mask layer  702  can be silicon dioxide, silicon nitride, aluminum, or another material that can serve as a mask for deep reactive ion etching of silicon. The hard mask layer  702  can also serve as the bonding material later in the process. Turning to  FIG.  7 C , hard mask layer  702  is patterned to allow for the etching of the cavity below the mirror  213 . The patterning results in the hard mask layer  702  having spaces  706  between a first end of the silicon wafer  14  and a second end of the silicon wafer  16 . The spaces  706  expose top surfaces  710  of the base wafer  700 . The patterning is done using standard photolithography and etching methods. 
     A coating of photoresist material  720  (also referred as photoresist layer) is deposited on the hard mask layer  702  and the base wafer  700  as shown in  FIG.  7 D . The photoresist material  720  is patterned as shown in  FIG.  7 E  to define the locations of the mechanical stop  502  in  FIG.  5   . In  FIG.  7 F , deep reactive ion etching is used to partially etch into the base wafer  700 . As a result, the deep reactive ion etching etches exposed surfaces  714  of base wafer  700 . The depth of this etch can be determined by the final desired depth of the cavity minus the gap desired between each of the mechanical stops  502  and the bottom of the center loading structure on a corresponding mirror  213 . For example, the final desired depth of the cavity is equal or greater than 55 μm and less than the thickness of the base wafer  700 . The photoresist material  720  is stripped from the silicon wafer  700  as shown in  FIG.  7 G  to start forming the gaps  704 . Deep reactive ion etching is then used to complete the etching of the cavity to a final desired depth as shown in  FIG.  7 H . As shown in  FIG.  7 H , during the deep reactive ion etching process, top surfaces of the mechanical stops  502  are also etched. Accordingly, the height for each of the mechanical stops  502  is adjusted to a final desired height. As a result, the gap  704  (e.g., gap between 3 μm and 15 μm) is formed between each of the mechanical stops  502  and the bottom of the center loading structure on the corresponding mirror  213  (base or bottom portion of the central stages  220 ). Un-etched portions of the base wafer  700  covered by the hard mask layer  702  are the support anchors  430 . The hard mask layer  702  can remain on the base wafer  700  or be removed, depending on the bonding technique. 
       FIG.  8    illustrates a partial cross-section of a mirror array  800  with a base wafer  700  and a lid wafer  802  (e.g., glass wafer). Mechanical stops  502  prevent a large downward out-of-plane deflection of the mirrors  213  and are fabricated on the base wafer  700  as described above. In addition, mechanical stops  1012  (also referred as protrusions or lid protrusions) to prevent large upward out-of-plane deflections are fabricated on the lid wafer  802 . 
       FIG.  9    illustrates an indication of locations  910  for mechanical stops  1012 . The mechanical stops  1012  can be located directly above the center of each mirror&#39;s flexures  152 ,  154  (also referred as stationary frame flexures) as shown in  FIG.  9   . In other words, each of the mechanical stops  1012  may overlap with a corresponding mirror&#39;s flexure  152 ,  154  in the first direction. Special care should be taken in the location of these mechanical stops  1012  to ensure that the mechanical stops  1012  will not block the light reflected from the mirrors  213  (e.g., mechanical stops  1012  non-overlapping with the mirrors  213 /central stages  220 ). A gap between the top of the mirror flexures  152 ,  154  and the mechanical stop  1012  can be between 3 μm−15 μm. 
     As shown in  FIGS.  8  and  9   , one aspect of the disclosure provides the mirror array  800 . As shown, the mirror array  800  includes a lid (lid wafer  802 ), a base (base wafer  700 ), and a movable mirror  100  between the lid (lid wafer  802 ) and the base (base wafer  700 ). The movable mirror  100  includes a stationary frame  214  including a cavity  232 , a movable frame  227  in the cavity  232 , and a central stage  220  in the cavity  232 . As shown, the mirror array  800  also includes a first protrusion (mechanical stop  502 ) on the base (base wafer  700 ). The first protrusion (mechanical stop  502 ) overlaps with the central stage  220  in a first direction (e.g., vertical direction). As shown, the protrusion (mechanical stop  502 ) is formed from the base (base wafer  700 ). However, the first protrusion (mechanical stop  502 ) may be formed from a separate layer on the base (base wafer  700 ). As show, the central stage  220  includes a bottom portion facing the first protrusion (mechanical stop  502 ). The first protrusion (mechanical stop  502 ) is spaced apart from the bottom portion of the central stage  220  by a predetermined distance (e.g., distance between 3 μm and 15 μm). 
     As shown, the mirror array  800  also includes a second protrusion (mechanical stop  1012 ) on the lid (lid wafer  802 ), and a third protrusion (mechanical stop  1012 ) on the lid (lid wafer  802 ). The second protrusion (mechanical stop  1012 ) and the third protrusion (mechanical stop  1012 ) extend towards the base (base wafer  700 ). As shown, the mirror array  800  also includes a first stationary frame flexure (mirror&#39;s flexure  152 ) and a second stationary frame flexure (mirror&#39;s flexure  154 ). The first stationary frame flexure (mirror&#39;s flexure  152 ) and the second stationary frame flexure (mirror&#39;s flexure  154 ) suspend the movable frame  227  from the stationary frame  214 . As shown, the second protrusion (mechanical stop  1012 ) overlaps with the first stationary frame flexure (mirror&#39;s flexure  152 ) in the first direction, and the third protrusion (mechanical stop  1012 ) overlaps with the second stationary frame flexure (mirror&#39;s flexure  154 ) in the first direction. As shown, the second protrusion (mechanical stop  1012 ) is apart from the first stationary frame flexure (mirror&#39;s flexure  152 ) by a predetermined distance (e.g., distance between 3 μm and 15 μm). As shown, the third protrusion (mechanical stop  1012 ) is apart from the second stationary frame flexure (mirror&#39;s flexure  154 ) by a predetermined distance (e.g., distance between 3 μm and 15 μm). As shown, the second protrusion (mechanical stop  1012 ) and the third protrusion (mechanical stop  1012 ) are non-overlapped with the central stage  220  in the first direction. 
     As shown, the mirror array  800  also includes a fourth protrusion (support anchor  430 ) and a fifth protrusion (support anchor  430 ) on the base (base wafer  700 ). As shown, the fourth protrusion (support anchor  430 ) and the fifth protrusion (support anchor  430 ) are formed from the base (base wafer  700 ). However, the fourth protrusion (support anchor  430 ) and the fifth protrusion (support anchor  430 ) may be formed from a (separate) layer on the base (base wafer  700 ). As shown, the first protrusion (support anchor  430 ) is between the fourth protrusion (support anchor  430 ) and the fifth protrusion (support anchor  430 ). As shown, the fourth protrusion (support anchor  430 ) is configured to support a first support member (support webbing  234 ), and the fifth protrusion (support anchor  430 ) is configured to support a second support member (support webbing  234 ). As shown, the central stage  220  includes a bottom portion extending towards the first protrusion (mechanical stop  502 ). As shown, the bottom portion of the central stage  220  is between the first support member (support webbing  234 ) and the second support member (support webbing  234 ). 
       FIGS.  10 A-I  illustrates a manufacturing process for fabricating the lid wafer  802  (also referred as lid). A glass wafer  1004  (lid wafer  802  in  FIG.  8   ) as shown in  FIG.  10 A  is provided. A silicon wafer  1002  (also referred as silicon substrate) is then fusion bonded to the glass wafer  1004  as shown in  FIG.  10 B . A hard mask material  1006  (also referred as hard mask layer) is then deposited on the silicon wafer  1002 . This material can be silicon dioxide, silicon nitride, aluminum, or another material that can serve as a mask for deep reactive ion etching of silicon. The material can also serve as the bonding material later in the process. 
       FIG.  10 D  illustrates the hard mask material  1006  patterned to allow for the etching of a cavity below the movable mirrors  100 . The patterning is done using standard photolithography and etching methods. Turning to  FIG.  10 E , a coating of photoresist  1008  (also referred as photoresist layer) is deposited on the silicon wafer  1002  and the hard mask material  1006 . The photoresist  1008  is patterned to define the locations of the mechanical stop  1012  as shown in  FIG.  10 F . Deep reactive ion etching is used to partially etch into the silicon wafer  1002  as shown in  FIG.  10 G . The depth of this etch can be determined by thickness of the silicon wafer  1002  minus the gap desired between each of the mechanical stops  1012  and the top of a corresponding mirror flexure  152 ,  154 . As discussed above, for example, the desired gap distance is between 3 μm and 15 μm, approximately. The photoresist  1008  is stripped from the silicon wafer  1002  in  FIG.  10 H . Deep reactive ion etching is then used once again, as shown in  FIG.  10 I , to complete the etching of the cavity to the final desired depth which clears the silicon and exposes the glass wafer surface  1110 . As shown in  FIG.  10 I , during the deep reactive ion etching process, top surfaces of the mechanical stops  1012  are also etched. Accordingly, the height for each of the mechanical stops  1012  is adjusted to a final desired height. At a result, the gap is formed between each of the mechanical stops  1012  and the corresponding minor flexure  152 ,  154  as shown in  FIGS.  8  and  9   . The hard mask material  1006  can remain on the silicon wafer  1002  or be removed, depending on the bonding technique. As shown in  FIG.  10 I , the entire structure is lid wafer  802 . 
     As shown in  FIGS.  7 A-H ,  8 ,  9 , and  10 A- 10 I, another aspect of the disclosure provides a fabrication method of the minor array  800 . In particular, the disclosure provides a fabrication method of the lid (lid wafer  802 ) as well as a fabrication method of a base (base wafer  700 ). As shown in  FIG.  8   , the minor array  800  includes a plurality of movable minors  100  that are spaced apart from each other. As discussed, each of the plurality of movable mirrors  100  may include a stationary frame  214  including a cavity  232 , a movable frame  227  in the cavity  232 , and a central stage  220  in the cavity  232 . As shown, the movable frame  227  is suspended from the stationary frame  214  by a first stationary frame flexure  152  and a second stationary frame flexure  154 . As shown in  FIGS.  8 ,  9 , and  10 A- 10 I , the lid (lid wafer  802 ) may be formed separately from the plurality of movable mirrors  100  and the base (base wafer  700 ). After forming the lid (lid wafer  802 ) as shown in  FIGS.  10 A- 10 I , the lid (lid wafer  802 ) can be placed on the plurality of movable mirrors  100 . In some circumstances, the plurality of movable mirrors  100  is placed under the lid (lid wafer  802 ). As shown, when the lid (lid wafer  802 ) is covering the plurality of movable mirrors  100 , each of the protrusions (mechanical stops  1012 ) on the lid (lid wafer  802 ) overlaps with one of the stationary frame flexures  152 ,  154  in a first direction (e.g., vertical direction). As shown in  FIGS.  7 A- 7 H,  8 ,  9 , and  10 A- 10 I , the base (base wafer  700 ) may be formed separately from the plurality of movable mirrors  100  and the lid (lid wafer  802 ). After forming the base (base wafer  700 ) as shown in  FIGS.  7 A- 7 H , the base (base wafer  700 ) can be placed under the plurality of movable mirrors  100 . In some circumstances, the plurality of movable mirrors  100  is placed on the base (base wafer  700 ). As shown, the base (base wafer  700 ) includes a plurality of protrusions (mechanical stops  502 , support anchors  430 ). As shown, when the base (base wafer  700 ) is supporting the plurality of movable mirrors  100 , some of protrusions (mechanical stops  502 ) on the base (base wafer  700 ) overlap with central stages  220  in the first direction and some of the protrusions (support anchors  430 ) on the base (base wafer  700 ) overlap with support webbings  234  of the plurality of movable mirrors in the first direction. In a broad view, as shown in  FIG.  8   , the mirror array  800  can be fabricated by bonding the base (base wafer  700 ) to the device wafer  220 ′ (including the plurality of movable mirrors  100 ) and bonding the device wafer  220 ′ to the lid (lid wafer  802 ). 
     While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.