Patent Publication Number: US-8982440-B2

Title: Microelectromechanical system with balanced center of mass

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. Non-Provisional application Ser. No. 13/106,709 filed on May 12, 2011, entitled “MICRO ELECTROMECHANICAL SYSTEM WITH A CENTER OF MASS BALANCED BY A MIRROR SUBSTRATE,” now U.S. Pat. No. 8,705,159 B2, the entire contents of which are hereby incorporated by reference in its entirety for all purposes. 
     TECHNICAL FIELD 
     Embodiments of the invention relate to the field of microelectromechanical systems (MEMS) and, in particular, to MEM electrostatic actuators. 
     BACKGROUND 
     Many MEMS include a released structure which has a high aspect ratio (AR) member in which a longitudinal length of the member is at least five times larger than a transverse length of the member or a member spaced apart from another structure by a gap defining a space with a high AR. High AR members and/or associated gaps are useful for providing large capacitances. In the case of an accelerometer, a high capacitance structure translates into greater device sensitivity. In the case of an electrostatic motor, a high capacitance enables a high electrostatic force between the released structure and a surrounding drive electrode. A high force allows for released structure to be actuated over a large distance or angle at a lower applied voltage, for improved electrostatic motor performance. Even for MEMS implementations which do not need a large actuation angle, a high electrostatic force allows flexures to be mechanically stiffer to increase the resonant frequency of the released structure and overall reliability of the device in an operating environment. 
     Another consideration in many MEMS is fill factor, which for a micromirror array implementation is a ratio of the active refracting area to the total contiguous area occupied by the lens array. To maximize the fill factor, it is beneficial to suspend the high aspect ratio member with the longest dimension oriented perpendicularly to a surface of the mirror, as is described for actuator members in commonly assigned U.S. Pat. No. 6,753,638. 
     Whatever the MEMS application and however a member may be oriented, it is challenging to ensure the center of mass of the released structure is best positioned with respect to the structure&#39;s center of rotation (i.e., fulcrum). For example, where a released structure has uniform density, and the center of mass is the same as the centroid of the structure&#39;s shape, a high aspect ratio member will often cause the centroid to be offset from a plane containing the released structure&#39;s center of rotation. 
       FIG. 1  is a perspective view illustrating a schematic of a released structure  100 . The member  111  has a high AR, such as a beam or a blade, with a longitudinal length  150  that is significantly longer than at least one of the transverse lengths  140  and  130 . The high AR member  111  is coupled to a substrate (not depicted) via a flexure which is the center of rotation for the released structure  100  within the X-Y plane ( 192 ,  191 ) only while other flexures may further allow motion within the Z-direction (into/out of the page)  193 . Because the high AR member  111  has a finite mass, the center of mass (CM) is located within the high AR member  111 , a distance  138  below the center of rotation  135  (e.g., 50-100 μm). With a center of mass offset from the center of rotation, the released structure generally forms a pendulum subject to motion in response to external forces (e.g., vibration). Such motion is undesirable as it corresponds to noise during operation of the MEMS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention are illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which: 
         FIG. 1  illustrates a perspective view of a MEMS released structure with a conventional high AR member; 
         FIG. 2A  illustrates a perspective view of a MEMS released structure with a member in accordance with an embodiment of the present invention; 
         FIGS. 2B ,  2 C and  2 D illustrate plan views of an end face of a member in accordance with embodiments of the present invention; 
         FIG. 3  illustrates a perspective view of a micromirror electrostatic actuator with members in accordance with an embodiment of the present invention; 
         FIG. 4  illustrates a plan view of a micromirror electrostatic actuator in accordance with an embodiment of the present invention; 
         FIGS. 5A ,  5 B,  5 C,  5 D,  5 E, and  5 F illustrate cross-sectional views of a micromirror electrostatic actuator in stages of fabrication, in accordance with an embodiment of the present invention; 
         FIGS. 6A ,  6 B,  6 C illustrate cross-sectional views of a micromirror electrostatic actuator in stages of fabrication, in accordance with an embodiment of the present invention; 
         FIG. 7  illustrate a plan view of a plurality of micromirror electrostatic actuators in accordance with an embodiment of the present invention; 
         FIGS. 8A and 8B  illustrate isometric views of a mirror substrate in stages of fabrication, in accordance with an embodiment of the present invention; 
         FIG. 8C  illustrates a cross-sectional view of a wafer bonded mirror substrate assembled from a first and second substrate, in accordance with an embodiment; and 
         FIGS. 8D-8F  illustrate cross-sectional views of a micromirror electrostatic actuator including a mirror substrate disposed on a base substrate in stages of fabrication, in accordance with an embodiment of the present invention. 
     
    
    
     SUMMARY OF THE DESCRIPTION 
     MEMS and fabrication techniques for positioning the center of mass of released structures in MEMS are provided. In an embodiment, a released structure includes a member with a recess formed into an end face of its free end. 
     In an embodiment, the recess is fabricated concurrently with an etching of sidewalls defining a longitudinal length of the member. In an embodiment, a released structure includes a plurality of members, with the longitudinal lengths of the members being of differing lengths. In an embodiment, members with differing longitudinal lengths are fabricated via multiple patterning of a masking layer. In another embodiment, members with differing longitudinal lengths are fabricated via embedding a patterned masking layer within a material stack from which member is formed. 
     In an embodiment, mass of a released member disposed below a plane of a flexure is balanced by mass of a second substrate affixed to the released member. In an embodiment, a second substrate is affixed to a member partially released from a first substrate and a through hole formed in the second substrate is accessed to complete release of the member. In an embodiment, a second substrate is affixed to a plurality of partially released members and a single etch process is utilized to thin the second substrate, form the through hole, complete release of the member and singulate the second substrate into a plurality of mechanically independent structures. As described herein, one or more of the structures and techniques provided are utilized to balance masses of a released structure and thereby improve performance (e.g., reduce noise) in a MEMS. 
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known features, such as specific fabrication techniques, are not described in detail in order to not unnecessarily obscure the present invention. Reference throughout this specification to “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. Also, it is to be understood that the various exemplary embodiments shown in the Figures are merely illustrative representations and are not necessarily drawn to scale. 
     The terms “coupled” and “connected,” along with their derivatives, may be used herein to describe structural relationships between components. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” my be used to indicate that two or more elements are in either direct or indirect (with other intervening elements between them) physical or electrical contact with each other, and/or that the two or more elements co-operate or interact with each other (e.g., as in a cause an effect relationship). 
     The terms “over,” “under,” “between,” and “on” as used herein refer to a relative position of one material layer with respect to other material layers. As such, for example, one layer disposed over or under another layer may be directly in contact with the other layer or may have one or more intervening layers. Moreover, one layer disposed between two layers may be directly in contact with the two layers or may have one or more intervening layers. In contrast, a first layer “on” a second layer is in contact with that second layer. Additionally, the relative position of one layer with respect to other layers is provided assuming operations are performed relative to a substrate without consideration of the absolute orientation of the substrate. 
       FIG. 2A  is a perspective view illustrating a schematic of a released structure  200 . Depending on the embodiment, the released structure  200  is a deep etched structure itself having a high AR or is adjacent to a deep etched gap defining a space having a high AR between the released structure  200  and another structure. The released structure  200  is displaceable relative to a mechanically supportive substrate (not depicted). In the exemplary embodiment illustrated, the released structure  200  includes a member  211  with a high AR, such as a beam or a blade, with a longitudinal length  250  that is significantly longer than at least one of the transverse lengths  240  and  230 . In embodiments, the longitudinal length  250  is at least five times, and may be tens or even hundreds of times, longer than the shortest of the transverse lengths  240  and  230 . The member  211  is otherwise a rigid object having any one of various shapes. For example, the member  211  may be a polyhedron as illustrated in  FIG. 2A . 
     The transverse lengths  230  and  240  define the area of an end face  262  of a free end of the member  211 . In the illustrated embodiment, the end face  262  undergoes maximum displacement relative to the substrate during operation of MEMS incorporating the released structure  200 . The member  211  is coupled to a substrate (not depicted) via a flexure  254 . The flexure  254  may be, for example, a torsional flexure which is the center of rotation for the released structure  100  within the X-Y plane ( 292 ,  291 ). In alternative embodiments, a flexure may further allow motion within the Z-direction (into/out of the page)  193 , depending on the specific MEMS design. 
     In an embodiment, the member  211  comprises a recess  276  disposed in the end face  262 . The recess  276  has a longitudinal axis  277  substantially parallel to a longitudinal axis  212  of the member  211 . The recess  276  is dimensioned to have a transverse area smaller than the area of the end face  262  such that a sidewall  214  defining the longitudinal length of the member  211  is independent of the longitudinal recess length  251  (i.e., not reduced by the recess  276 ). It is beneficial to maintain the continuous sidewall surface area so capacitance of the member  211  is independent of the recess geometry and depth. The recess  276  thereby eliminates mass from the free end of the member  211  without reducing the sidewall surface area of the member  211  to advantageously move the center of mass from a first location (CM 1 ), more distal from the center of rotation  254 , to a second location (CM 2 ), more proximate to the center of rotation  254 . It is more beneficial to remove mass at the free end of the member  211  than elsewhere in the released structure  200  because mass at the free end is the greatest distance from center of rotation  254  and therefore has the greatest mechanical lever. Depending on dimensions of the end face  262  and fabrication capabilities, the recess  276  may have a longitudinal length  251  equal anywhere from 10% to 100% of the member&#39;s longitudinal length  250 . To provide a significant mass reduction, the longitudinal length  251  is preferably at least 25% of the longitudinal length  250 . 
       FIGS. 2B ,  2 C and  2 D further illustrate plan views of the end face  262  in accordance with embodiments of the present invention. As shown in  FIG. 2B , a single recess  276  has transverse dimensions  231  and  241  less than the member&#39;s transverse dimensions  230  and  240 . In further embodiments, a plurality of recesses  276  is formed in the end face  262  of the member  211 . For example, in  FIG. 2B , recesses  276 A,  276 B, and  276 C are formed with transverse dimensions  232  and  242  sufficient to provide mechanical reinforcement to the member  211 . The number of recesses  276  as well as their transverse dimensions  231 ,  241  may be optimized based on the particular AR of the member  211 , the longitudinal depth  251 , and the mechanical properties of the material from which the member  211  is formed.  FIG. 2C , for example, illustrates a larger number of recesses,  276 A- 276 H, with reduced transverse dimensions  233  and  243 . The shape and arrangement of the recess(es)  276  within the area of the end face  262  may also be tailored following known mechanical design considerations (e.g., hexagonal packing of recesses, etc.) to permit maximum mass reduction while also imparting the member  211  with a desired mechanical strength sufficient to avoid unwanted actuation. 
     In a particular embodiment, the released structure  200  may be implemented as an electrostatic actuator for a micromirror.  FIG. 3  illustrates a perspective view of an exemplary micromirror electrostatic actuator  300  with members  320 ,  321 ,  325  and  326 , in accordance with an embodiment of the present invention. Actuator  300  includes a stage  340  and a frame  335 . Either or both of the stage  340  and frame  335  may have a reflective top surface  345 , such as a mirror.  FIG. 2A  shows the stage  340  parallel to the frame  335 , but the stage  320  is pivotally coupled to the frame  335  using flexures  353  and  354  on diametrically opposed sides of stage  340  so that the stage  340  may pivot (D.sub.1) about a first axis. Flexures  353  and  354  suspend stage  340  in a cavity formed by frame  335  such that stage  340  is free to pivot around a rotational axis formed by flexures  353  and  354 . Similarly, frame  335  is pivotally coupled to an outer mesa (not shown) using flexures  351  and  352  on diametrically opposed sides of frame  335 . The outer mesa may be a stationary substrate, or alternatively, may also move relative to yet another reference substrate. Flexures  351  and  352  suspend frame  335  in a cavity formed by the outer frame such that frame  335  is free to pivot (D.sub.2) around a rotational axis formed by flexures  351  and  352 . Flexures  351  and  352  are orthogonal to flexures  353  and  354 , thereby enabling a reflective element coupled to stage  340  to be pivoted in two dimensions (e.g., rolled and pitched). 
     Stage  340  and frame  335  each have one or more members coupled to and extending from them. For example, members  320  and  321  are coupled to stage  340  and members  325  and  326  are coupled to frame  335 . Members  320 ,  321  extend in a direction perpendicular to the undersurface of stage  340  and members  225 , 226  extend in a direction perpendicular to the undersurface of frame  335 . An electric potential applied between members  320  and  325  may cause an attraction between the members. Because member  320  is coupled to stage  340 , an attraction between members  320  and  325  causes stage  340  to pivot about the rotational axis formed by flexures  353  and  354 . A similar principal may be applied to pivot the frame  335  about flexures  351 ,  352 . A more complete description of the operation of the electrostatic actuator  300  may be found in U.S. Pat. No. 6,753,638. 
     In an embodiment, one or more of the high AR members coupled to an electrostatic actuator which are displaced during operation of the actuator have one or more recesses formed in an end face most distal from respective pivot points. Members which are not released and therefore not displaceable during operation need not have such recesses. The recesses formed in the members may be in any of the forms illustrated in  FIGS. 2A-2D  and described elsewhere herein. For example, as depicted in  FIG. 3 , recesses  374 ,  375 ,  376 , and  377  are formed with longitudinal axis substantially parallel to longitudinal axis the members  320 ,  321 ,  325  and  326  with surface areas between the various members  320 ,  321 ,  325  and  326  independent of the longitudinal lengths of the recesses. 
       FIG. 4  illustrates a plan view of the undersurface of the micromirror electrostatic actuator  500 , in accordance with an embodiment of the present invention. In the illustrated embodiment, members  612  and  622  are coupled to stage  340  and AR members  613   a ,  613   b ,  623 A, and  623 B are coupled to frame  335  on opposite ends of member  612 . Members  621  and  611  are coupled on the opposite end of frame  35 , with AR members  620   a ,  620   b , and  610 A,  610 B coupled to frame  340  on opposite ends of members  621  and  611 , respectively. Additional members  615  are coupled to the undersurface of stage  340  for mechanical stiffening and for reducing top surface distortions. Members  615  may further reduce etch depth variations across the device (e.g., as a result of etch loading, etc.). 
     In an embodiment, for a MEMS including a plurality of high aspect ratio members displaceable relative to a substrate, at least one member of plurality has a longitudinal length that is different than that of another member. Reducing the longitudinal length of certain members advantageously moves a released structure&#39;s center of mass in much the same manner as described for the recess  376 . Selectively reducing the AR of only certain members therefore permits tuning of the mass center position while still offering the advantage of high electrostatic force via the longer AR members. 
     In one embodiment where a single released structure includes a plurality of high aspect ratio members at least one of the plurality has a reduced AR than another of the plurality. Such embodiments are useful where certain members, need not provide as much (or any) electrostatic function. For example, it is advantageous for stiffening members  615  to have an AR independent of other members which have electrostatic function. Such embodiments are also useful for situations where the released structure does not need to function isometrically (e.g., stroke length is a function of axis or direction along a particular axis) and therefore electrostatic properties may vary between different dimensions of the released member. As a further illustration, in the micromirror electrostatic actuator  500  the frame  335  may need to pivot about the flexures  351 , 352  by smaller amount the stage  340  is pivot about the flexures  353 ,  354 . As such, in  FIG. 4  one or more of members  611 ,  621 ,  610 A,  610 B,  620 A and  620 B may be made shorter (i.e., with a lower AR) than are one or more of members  612 ,  613 A,  613 B,  622 ,  623 A and  623 B. As another example, where the frame  335  may need to pivot about one axis (e.g., the flexures  351 , 352 ) by a differing amounts in opposite directions, one or more of members  610 A and  620 A may be may be made shorter (i.e., with a lower AR) than are one or more of members  610 B an  620 B. 
     While the difference in longitudinal length between members may vary greatly depending on design, the difference is to be significantly greater than any differences passively induced by manufacturing non-unifomities. In one embodiment for example, the difference in longitudinal length between members is at least 10% of the longer member&#39;s longitudinal length, with the magnitude of difference depending on the different force/stroke distance required and the desired centroid position for the released structure (e.g., a low AR member may be provided to have a AR larger than dictated by force/stroke requirements alone to better position the centroid for vibration insensitivity). It should also be appreciated that either or both the highest AR members and members of reduced longitudinal length may further include a recess in an end face of the member, as previously described. Thus, various combinations of recessing and selective AR reductions can be utilized to position the released structure&#39;s centroid (center of mass). 
     A number of techniques may be used to fabricate the micromirror electrostatic actuator  500 . Actuator  500  may be fabricated on a wafer-level using semiconductor fabrication techniques. Certain exemplary fabrication embodiments are discussed with reference to  FIGS. 5A-5F  providing cross-sectional views along cross-section line  401  shown in  FIG. 4 . 
     For certain embodiments, frame  335  and/or stage  340  may be formed from a substrate, for example, of silicon.  FIG. 5A  shows a silicon wafer  501  with a exemplary thickness in the range of 200-800 micrometers (μm). The silicon wafer  501  has a topside (or device side or simply a top)  506  and a backside or bottom  507 . On the top  506  is a dielectric layer  503  and isolation trenches  1120  of a material such as silicon dioxide, and formed with conventional trench isolation techniques. Interconnect metallization  910  makes contact to the silicon substrate  501  to provide actuator control. For one embodiment, the interconnect metallization  910  is aluminum and may be patterned using wet or dry etching or liftoff techniques known in the art. A reflective layer  513  may also be deposited to provide a reflective mirror surface tuned to provide high mirror reflectivity at the optical wavelengths of interest (e.g., for fiber optical communication). Exemplary materials for the reflective layer  513  include a single metal layer, such as aluminum, a stack of metal layers, such as Cr/Pt/Au, or Bragg-type structures of materials known in the art. 
     Standard front-to-back alignment is used to lithographically pattern a masking layer on the wafer backside  507 . The member pattern is exposed and etched into a masking layer  504 . The masking layer  504  may be a dielectric, for example comprised of a thermally grown silicon oxide, a chemical vapor deposited silicon dioxide, or combination thereof. A lithography pattern is transferred in the masking layer  504  by reactive ion etching, as illustrated in  FIG. 5B . 
     For embodiments which are to include released structure members with end faces having one or more recesses, the masking layer  504  is to be patterned with one or more openings within a mask area of an end face  376 A or  367 B. For further embodiments which include released structure members of differing longitudinal lengths, the masking layer may be etched a first time with a first pattern to modify the masking layer  504  into regions with a first thickness  504 A and regions with a second, reduced, thickness  504 B. Then, as shown in  FIG. 5B , the masking layer  504  is etch through with a second pattern to define a mask of an end face  376 B for a longitudinally shorter member in the first region of the substrate  501  and a mask of an end face  376 A for a longitudinally longer member in a second region of the substrate  501 . It should also be noted the first and second patterning may be facilitated if the masking layer  504  is a stack of materials, such as silicon nitride and silicon dioxide, etc, which provides for etch selectivity enabling control of the reduced thickness  504 B. For embodiments where all released structure members are to have the same length, the first patterning depicted in  FIG. 5A  may be skipped with only the second patterning illustrated in  FIG. 5B  needed to define mask patterns for the released structure members. 
     As further shown in  FIG. 5B , prior to forming the released structure members, the processing of the substrate topside  506  is completed. A protection layer  515  may be applied on the metal surfaces  911  and  913  to protect the metallization  510 ,  513  during subsequent processing. Mirror structures including frame  335 , stage  340 , and flexures are defined using one or more etches that define trenches  516  separating the structural elements. The various etches may self-aligned and proceed through the interconnect metallization  510 , dielectric  503 , and into the substrate  501 . 
     As shown in  FIG. 5C , backside silicon etching transfers the blade pattern into the substrate  501  to obtain the members  610 A,  620 A and  615  (which fall on the cross-section line  401  in  FIG. 4 ). The etching is performed using deep silicon etching at high selectivity to oxide, for example using the techniques reported in U.S. Pat. No. 5,501,893 which is now commonly used in the industry. The deep silicon etching achieves near vertical profiles in the members  610 A,  620 A and  615 , which may have a transverse dimension (width) of nominally 5-20 μm and have a longest longitudinal length  691  in excess of 300 um. The etch is timed so that the etch front  519  approaches or just reaches the bottom of the isolation joints  1120  or the structure trenches  516 , yet does not punch through to the topside surface  506 . An etch stop layer, such as a buried layer of a silicon-on-insulator (SOI) substrate may also be utilized to stop the etch at the appropriate thickness. Multiple members are etched simultaneously across the mirror element and across the mirror array. It should also be noted that depending on the aspect ratio of the recesses in the members  610 A,  620 A and  615 , the etch front  519  may be tapered, as depicted, or flat. 
     As illustrated in  FIG. 5C , for embodiments with recesses  376  formed in the members  610 A,  615  and  620 A, the deep anisotropic etch utilized to form the sidewalls of the members  610 A,  615  and  620 A concurrently forms the recesses  376 . Generally, with the members having end face areas corresponding minimum transverse dimensions for the deep etch (e.g., 10 μm for a 300 μm longitudinal length), the recesses patterned in the masking layer  504  represent subminimum dimensions (e.g., 5 μm) for which an etch front will not proceed at the same rate as the etch front  519 . Depending on the recess dimensions and the capabilities of the deep anisotropic etch utilized to form the members, etching of the recesses  376  is self-limited by aspect ratio (e.g., etch stop) to a depth that is less than the sidewall lengths, thereby limiting the longitudinal length (depth) of the recesses  376  to be somewhat less than the longest longitudinal length  691 . The recesses  376  may further have tapered profiles along their longitudinal lengths. Through characterization of the deep anisotropic etch, the recesses  376  may dimensioned with the masking layer  504  to achieve a depth desired for moving the centroid a predetermined amount. 
     As further illustrated in  FIG. 5C , for embodiments with members of differing length, the differing thicknesses  504 A and  504 B of the masking layer  504  are such that are reduced masking layer thickness  504 B is entirely consumed by the etching (i.e., etch breakthrough) prior to termination of the timed etch so that the silicon substrate  501  where a shorter member  615  is to be formed is partially consumed and the longitudinal length  690  of the shorter member thereby reduced relative to longest longitudinal length  691  which had the benefit of the greater masking layer thickness  504 A. In particular embodiments, the reduced masking layer thickness  504 B is of a thickness (e.g., 0.5 μm) so that it is entirely consumed in sufficient time prior to termination of the backside etch for at least 10% of the longest longitudinal length  691  to be consumed (i.e., member  615  has a longitudinal length  690  that is 10% shorter than the longitudinal length  691  of member  610 A). 
       FIGS. 6A ,  6 B,  6 C illustrate cross-sectional views of the micromirror electrostatic actuator  500 , in accordance with an embodiment. The stages depicted in  FIG. 6A-6C  may be performed in alternative to the stages depicted in  FIGS. 5A-5C  to provide a plurality of members of differing length. In  FIG. 6A , a first substrate  601  is bonded to a second substrate  602 , for example with a known compression bonding technique. At least one of the substrates  601 ,  602  includes a patterned masking layer  603 , which for example may be comprised of silicon dioxide or another material which offers selectivity to the material of substrates  601  and  602 . As bonded, the substrates  601 ,  602  and patterned masking layer  603  form a member material stack with the masking layer  603  embedded below the thicknesses of substrate material. 
     The patterned masking layer  603  provides for a second material of greater thickness in a first substrate region than in a second substrate region. One or both sides of the bonded substrate  604  may be thinned (e.g., with buff grinding, chemical mechanical planarization, etc.) to dispose the patterned masking layer  603  at distances from the opposite surfaces of the bonded substrate that are appropriate to form members having the desired longest and shortest longitudinal lengths disposed the desired distance from topside features. For example, the thickness of the bonded substrate  604  may be thinned to be in the range described for the substrate  501 . In a further embodiment, the substrate  601  is thinned to have a thickness approximately equal to the desired difference in longitudinal lengths of the members. 
     As shown in  FIG. 6B , the bonded substrate  604  is then patterned substantially as described for  FIG. 5A  to form topside features  616 , which include any of those described elsewhere herein for  FIG. 5A . Similarly, a masking layer  504  is patterned to define masks for protecting end faces of the members to be formed. The masking layer  5045  may be formed and patterned substantially as previously described in the context of  FIG. 5B  with the exception that only a single patterning is performed. 
     As shown in  FIG. 6C , deep anisotropic etching of the bonded substrate  604  is performed substantially as described for the substrate  501  illustrated in  FIG. 5C . As shown, masking layer  504  defines the members  610 A and  620 A to have longitudinal lengths  691  while the material stack of member  615  is etched away until stopped by the embedded masking layer  603  prior to termination of the deep anisotropic etch. Upon the deep anisotropic etch exposing the embedded masking layer  603 , the member  615  is formed as the deep etch continues and the etch front  519  passes through a reduced thickness of the embedded masking layer  603 . As further shown in  FIG. 6C , each of the masking layer  504  and the embedded masking layer  603  may be patterned to further provide for recesses  376  in any of the members, regardless of member aspect ratio. 
     Following the embodiment illustrated in  FIGS. 6A-6C , the longer members  610 A and  620 A may incorporate features indicative of a bonded substrate interface at a longitudinal a distance from the end face approximately equal to the difference in longitudinal lengths between the longer members  610 A and  620 A and a shorter member  615 . For example, as illustrated in  FIG. 6C , the longer member includes a second material  603 B where the bonding process depicted in  FIG. 6A  entails an unpatterned bonding layer, such as silicon dioxide, on one of the substrates  601  or  602  while the other of the substrates has the patterned masking layer  603 . Through silicon via technology known in the art may then be used to make contact below  603 B for application of a control voltage to a completed device. 
     Fabrication of the micromirror electrostatic actuator  500  progresses to  FIG. 5D  following either  FIG. 5C  or  FIG. 6C . As shown in  FIG. 5D , a base wafer  521  is bonded to the substrate  501  to protect the members after their subsequent release. Any bonding technique known in the art may be employed. For one embodiment, the bonding is accomplished through the use of a frit glass material  522  that is heated to its flow temperature and then cooled. Alternative embodiments include eutectic solder systems. 
     As shown in  FIG. 5E , structure release is accomplished on the substrate topside  506 , for example using a dry substrate etchant, such as SF 6  plasma, to puncture through the trenches  516  from the substrate topside  506 , forming through holes  526  and suspending the release element comprising the members  610 A and  620  of the frame  335  and member  615  of the stage  340 . 
     In an embodiment, a second substrate is affixed to a top side of a MEMS structure formed from a first substrate. Assembly of multiple substrates into a final released structure may advantageously provide a greater MEMS fill factor for array devices. For example, for an array of micromirror actuators  500  on a single base substrate, fill factor may be increased from 25-35% to 75-99% by attaching to the stage  340  a larger reflective surface fashioned from the second substrate. For many MEMS structures however, an addition of a second substrate destabilizes the released structure because the mass addition shifts the center of mass far from the plane containing the flexure (e.g., the MEMS structure becomes top heavy with the center of mass far above the flexure plane). However, as was illustrated in  FIG. 1 , for released structures comprising members with significant mass below a flexure plane, addition of a second substrate can render the center of mass balanced at the flexure. Hence in particularly advantageous embodiments, the addition of a second substrate to the first substrate is performed in conjunction with one or more of the member mass reduction techniques described elsewhere herein to realize, a balanced released structure simultaneously with a high fill factor. 
     Returning to the exemplary implementation of the micromirror electrostatic actuator  500  illustrated in  FIG. 4 , a mirror  710 A is disposed over the stage  340  with an undersurface of the mirror  710 A affixed to the top surface of the stage  340  at an attachment point  705 . The attachment point  705  may utilize any material in the art known to be suitable for attaching one substrate to another substrate or to a package. For example, in an embodiment, the attachment point  705  utilizes an epoxy. In another embodiment, the attachment point  705  utilizes solder (e.g., microball technology or jetting) or compression. Either solder or epoxy enables wafer-level assembly technology for attachment of the mirror  710 A to the stage  340 , as described elsewhere herein. 
     As further illustrated in cross-section in  FIG. 5F , the mirror  710 A includes a reflective layer  707  which may comprise any of the materials previously described for the reflective layer  513 . It should also be noted that for embodiments which employ the mirror  710 A, the reflective layer  513  may be either retained (e.g., as a matter of convenience as an established portion of device fabrication) or it may be replaced with alternate non-reflective materials since the reflective function of the reflective layer  513  is replaced by the mirror  710 A. 
     As illustrated in both  FIGS. 4 and 5F , the mirror  710 A may have lateral dimensions such than the reflective layer  707  has a reflective area which is anywhere from 1.5 to 5 times, or more, greater than a top surface area of the stage  340  as limited by the mass, mechanical stiffness, and mechanical clearance of previously formed device structures etc. of the mirror  710 A. In particularly advantageous embodiments, the reflective layer  707  has a reflective area which is between 2 and 3 times greater than a top surface area of the stage  340 . Depending on the mass of the mirror  710 A, and the mass of the high aspect ratio member(s) attached to the stage  340 , the center of mass of the MEMS may be positioned anywhere between the undersurface of the mirror  710 A and the undersurface of the stage  340 , as depicted in  FIG. 5F . 
       FIG. 7  illustrate a plan view of a plurality of micromirror electrostatic actuators  500  in accordance with an embodiment of the present invention. As illustrated, each of the arrayed electrostatic actuators  500  include a stage  340 A,  340 B, and  340 C coupled to a mirrors  710 A,  710 B, and  710 C at the attachment points  705 A,  705 B,  705 C respectively. The arrayed actuators  500  may include any number of actuators  500  in an M×N grid, for example to form a non-blocking optical cross-point switch. In one such embodiment, the actuator array  700  includes  384  actuators sharing a single base substrate with each of the mirrors movable independent of each other as controlled by electrostatic forces generated by members coupled to undersurfaces of the stages  340 . In one advantageous embodiment, each of the mirrors  710 A,  710 B and  710 C are singulated from a single mirror substrate  710  which is affixed to the base substrate at the multiple attachment points (e.g., one or more attachment points  705  per actuator) using a wafer-level assembly process. 
     Attachment of a second substrate over a base substrate complicates MEMS structure release. A certain amount of attachment force is to be expected, for example in a ball grid array (BGA) type attachment process force is applied during solder reflow. The attachment force however may be detrimental to fragile released structures; therefore it would be advantageous to perform structure release after attachment of the second substrate. Post-attachment release however may be complicated by the presence of the second substrate. For example, many release techniques, like those described for  FIG. 5E  need unfettered access to the topside of the base substrate to form the minimally dimensioned through holes  526 . 
     As such, in one embodiment, a two stage structure release is performed. In a first phase, structure release is performed as described for  FIG. 5E  and in a subsequent second phase, a through hole opening is formed through the second substrate to remove anchors bridging the free space gaps between the stage and the base substrate formed by the first release phase. For example, referring to  FIG. 4 , the mirror  710 A includes through holes disposed over the anchors  711  and  712  bridging gaps  526  between the released structure (e.g., stage  340  and/or frame  335 ) and the reference substrate (that which the released structure is displaceable from). The anchors  711  and  712  are structural supports left behind by the first release phase which are dimensioned to provide sufficient support to the otherwise suspended structure during attachment of the mirror substrate. The through holes in the mirror substrate are then utilized for localized access to the structural supports following attachment of mirror substrate, thereby permitting the structural supports  711 ,  712  to be etched away leaving the completely released structure suspended from only the flexures. 
       FIGS. 8A and 8B  illustrate isometric views of a mirror substrate in stages of fabrication, in accordance with an embodiment of the present invention. Beginning with a first substrate  801 , the reflective layer  707  is formed which is ultimately to become the optically reflective top surface of the mirror substrates  710 A,  710 B, and  710 C. The first substrate may be any suitable material, and in one embodiment is silicon having a thickness of 200-300 μm, for example. A protection layer, such as silicon dioxide or the like, may further be disposed over reflective layer  707  and either or both the protection layer and reflective layer  707  is etched to define a pattern including openings  811  and  812  where through holes are to be subsequently formed through the mirror substrate. The reflective layer  707 , or protection layer thereon, is to be of a bondable material. 
     In the exemplary embodiment illustrated in  FIG. 8A , a backside  887  of the first substrate  801  is further patterned, for example with a deep silicon etch, to form a recess  888  aligned with an outer perimeter of the mirror substrates  710 A,  710 B and  710 C. Such a recess may be timed or stopped on the reflective layer  707  or isolation layer disposed between the reflective layer  707  and the first substrate  801 . As described further in herein in reference to  FIG. 8C , the recess  888  is to further reduce the thickness of the first substrate  801  at a periphery of the mirror substrate  710  for greater mechanical clearance between the mirror substrate  710  and a base substrate. 
     As illustrated in  FIG. 8B , the first substrate  801  is to be bonded (e.g., compression bonded) to a second substrate  802 . The second substrate may be any suitable material, for example the same material and nominal thickness as the first substrate  801  (e.g., silicon). A bonding layer  806  may be disposed on a side of the first substrate  801 . The bonding layer  806  may be unpatterned and is to be a bondable material, such as gold, for example. 
       FIG. 8C  illustrates a cross-sectional view along the line  808  from  FIG. 8A  enlarged to illustrate only the exemplary mirror  710 B (with other regions of the substrate similarly treated). The first and second substrates  801 ,  802  are bonded together so that the reflective layer  707  is disposed between thicknesses of the first and second substrates  801 ,  802  using any bonding technique known in the art, depending on the interface materials chosen. The bonded substrates  801 ,  802  are then thinned in preparation for one or the other of the first and second substrates  801 ,  802  to be attached to a base substrate. For example, as further illustrated in  FIG. 8C , the first substrate  801  is thinned, removing the excess substrate thickness  801 B, to form the undersurface of the mirror substrate  710 B. The recess  888  previously formed in the backside  887  the first substrate then defines a reduced thickness at the periphery of the mirror For this exemplary embodiment, thinning of the first substrate  801  determines the approximate final thickness of the mirror substrate and therefore the ultimate mass of the mirror substrate. As an example, the first substrate  801  is thinned to less than 50 μm. Alternatively, the second substrate  802  may be thinned and attached to a base substrate. 
       FIGS. 8D-8F  illustrate cross-sectional views of a micromirror electrostatic actuator along the line  402  illustrated in  FIG. 4  which corresponds to the line  808  in  FIG. 8C . As shown in  FIG. 8D , a surface of the mirror substrate  710 B is affixed to a base substrate  850  with the attachment point  705  coupling the thinned substrate  801  to the top surface of the stage  340  (e.g., reflective layer  513 ). The recess  888 , if present, reduces the thickness of the substrate  801  under at least a portion of the reflective layer  707 . For one embodiment, attachment of the mirror substrate  710 B is performed to every like actuator in parallel (e.g., at wafer-level). Alternatively, a die-level attachment may be performed. During a wafer-level attachment process, the mirror substrate  710 B is aligned (along with the mirror substrates  710 A and  710 C) to the base substrate  850  so that the pattern openings  811  are aligned to be disposed over the anchor locations  711 . At the time of attachment, the through holes  526  are present in the base substrate  850 , having been formed in the first release phase while through holes are not yet present in mirror substrate  710 . 
     As shown in  FIG. 8E , the mirror substrate  710 B is then further thinned to expose the openings  811  and expose either the embedded reflective layer  707  or the bonding layer  806 . Continued etching of the mirror substrate further etches the pattern openings  811  through the thinned substrate  801  to expose the anchors  711 . In one embodiment where the substrates  801 ,  802  are silicon, an anisotropic silicon etch such as the deep anisotropic silicon etch described for formation of the members or the like is utilized to both etch away the bulk of substrate  802  and form an anisotropic through hole into the thinned substrate  801 . 
     As illustrated in  FIG. 8F , anchors  711  are then etched way where exposed by the through holes  813  to complete the second structure release phase. In one exemplary embodiment where the anchors  711  are silicon, the same anisotropic silicon etch utilized to form the through holes  813  further removes the anchors  711 . The etch of the anchors  811  may be timed to also remove the protection layer (if present) and thereby expose the reflective layer  707 . Just as the pattern openings  811  are etched through to remove the anchors  711 , the bulk of the mirror substrate  710 B is similarly etched away to singulate the plurality of mirrors  710 A,  710 B,  710 C and arrive at the assembled released structure including an array of actuators, each actuator attached to a mirror substrate, as illustrated in plan view in  FIG. 7 , and two different cross-sectional planes  401  and  402  in  FIGS. 5F and 8F , respectively. 
     The above description of illustrative embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific implementations of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.