Patent Publication Number: US-6905614-B1

Title: Pattern-transfer process for forming micro-electro-mechanical structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of and claims priority under 35 U.S.C. §119(e) from U.S. patent application Ser. No. 10/032,198 entitled “Multi-Axis Micro-Electro-Mechanical Actuator,” by Vlad J. Novotny and Yee-Chung Fu, filed on Dec. 20, 2001, which is a continuation-in-part of U.S. patent application Ser. No. 09/865,981 entitled “Optical Cross Connect Switching Array System With Optical Feedback,” by Vlad J. Novotny, filed on May 24, 2001, now U.S. Pat. No. 6,483,962, which claims benefit of U.S. patent application Ser. No. 60/206,744, entitled “Optical Cross Connect Switching Array Systems With Optical Feedback Control,” by Vlad J. Novotny, filed May 24, 2000, and Ser. No. 60/241,269, filed Oct. 17, 2000. This application additionally relates to U.S. patent application Ser. No. 10/027,882 entitled “Deep-Well Lithography Process For Forming Micro-Electro-Mechanical Structures,” by Vlad J. Novotny, Dec. 21, 2001. Each of the above-identified documents is incorporated herein by reference. 
    
    
     BACKGROUND 
     As the result of continuous advances in technology, particularly in the area of networking, such as the Internet, there is an increasing demand for communications bandwidth. For example, the transmission of images or video over the Internet, the transfer of large amounts of data in transaction processing, or videoconferencing implemented over a public telephone network typically require the high speed transmission of large amounts of data. As applications such as these become more prevalent, the demand for communications bandwidth will only increase. 
     Optical fiber is a transmission medium that is well suited to meet this increasing demand. Optical fiber has an inherent bandwidth much greater than metal-based conductors, such as twisted-pair or coaxial cable; and protocols such as Synchronous Optical Networking (SONET) have been developed for the transmission of data over optical fibers. 
     Optical fiber is used to form optical networks that carry data, voice, and video using multiple wavelengths of light in parallel. Light is routed through the network from its originating location to its final destination. Since optical networks do not generally have a single continuous optical fiber path from every source to every destination, the light is switched as it travels through the optical network. Previously, this switching was accomplished using optical-electrical-optical (“OEO”) systems, where a light signal was converted to an electrical signal, switched electrically, and then output optically. Because in OEO systems the signal must be converted from optical to electrical, switched, then converted back to optical, OEO systems are relatively large, complex, and expensive. More seriously, the OEO systems are slower than purely optical systems, and consequently introduce undesirable bottlenecks. 
     Much effort is being expended on the development of all-optical cross-connect switching systems, some of which employ arrays of electrostatically, electromagnetically, piezoelectrically, or thermally actuated mirrors. Digitally controlled mirrors with on and off states can be used to switch between small numbers of ports while analog controlled mirrors can be implemented with a small or a large number of ports. Analog controlled mirrors require bi-axial actuation; unfortunately, most electrostatic actuators used to position these mirrors suffer from relatively low torque, and consequentially require relatively high supply voltages to produce sufficient motion. The lack of torque also renders electrostatic actuators very sensitive to vibrations. There is therefore a need for a bi-axial actuator that operates at lower voltages and is relatively insensitive to vibration. 
     SUMMARY 
     The invention is directed to Micro-Electro-Mechanical Systems (MEMS) actuators that employ electrostatic comb electrodes to position mirrors along multiple axes. In one embodiment, an actuator assembly includes an actuator support, typically a silicon wafer, supporting one or more fixed comb-shaped electrodes, each with a plurality of teeth. A frame flexibly connected to the actuator support includes complementary sets of movable comb electrodes, the teeth of which are arranged interdigitally with the teeth of the fixed combs. The frame can be tilted with respect to the actuator support along a first fulcrum axis by applying a potential difference between the fixed and movable combs. 
     Each actuator assembly also includes an actuated member flexibly connected to the frame. In the depicted embodiment, the actuated member is a mirror mount. In other embodiments, the actuated member may support e.g. a filter, a lens, a grating, or a prism. 
     The actuated member and the frame include electrically isolated, interdigitated, comb electrodes. The actuated member can be moved relative to the frame along a second fulcrum axis by applying a potential between the comb on the frame and the comb on the actuated member. The actuated member can also be moved translationally by applying a potential between interdigitated combs. 
     In one embodiment, the hinges are made using the same conductive layers as the combs. The process used to form the hinges may differ from the process used to form the combs. Such processes allow the stiffness of the hinges to be adjusted independently. For example, the hinges may be made thinner to reduce the amount of torque required to move the actuated member. In another embodiment, serpentine hinges are employed to provide still greater flexibility. 
     A number of novel process sequences can be employed to manufacture MEMS actuators in accordance with the invention. In one such process, referred to herein as a “wafer bonding” process, one device layer on a Silicon-On-Insulator (SOI) or Spin-On-Glass (SOG) wafer is patterned to include the combs, hinges, etc., of the MEMS actuator(s) being formed. This patterned layer is then oxide- or glass-bonded to an intrinsic anchor wafer. A via etching is then performed on the other side of the intrinsic anchor wafer to electrically connect the devices to the driving circuitry. The other side of the original SOI or SOG wafer is then ground, polished, patterned, and etched as another device layer. Up to four different thicknesses are defined in these lithographic processes. 
     In another process, referred to herein as a “pattern transfer” process, one device layer is patterned to include features similar to the combs, hinges, etc., of the MEMS actuators being formed. The resulting pattern is then “transferred” to the surface of a second material layer by etching the top surface of the first material layer—including the raised portions and the valleys defined between the raised portions—until the second layer is exposed between the raised portions. 
     A third process that can be used to form MEMS actuators in accordance with the invention, referred to herein as “deep-well lithography,” differs from conventional lithography in that the surface being patterned is not the uppermost surface. The focal plane of the photolithography equipment is offset from the uppermost surface as appropriate to account for the depth of the well in which the pattern is to be formed. 
     Both the pattern-transfer process and deep-well lithography advantageously reduce the number of process steps required to produce MEMS actuators in accordance with the invention, and can additionally be used to form structures other than MEMS actuators. 
     This summary does not limit the invention, which is instead defined by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1A and 1B , respectively, are plan views of the upper and lower portions of a two-axis, Micro-Electro-Mechanical System (MEMS) actuator in accordance with one embodiment of the present invention. 
         FIGS. 2 through 32  depict a wafer-bonding process sequence in accordance with an embodiment of the invention. 
         FIGS. 33A and 33B  are plan views depicting the top and bottom halves  3300  and  3310  of an actuator in accordance with another embodiment of the invention. 
         FIGS. 34-49  depict an alternate fabrication process, referred to here as the “pattern transfer” process, that can be used to fabricate MEMS actuators in accordance with the invention. 
         FIGS. 50-65  depict an alternate fabrication process, referred to here as “deep-well lithography,” that can be used to fabricate MEMS actuators in accordance with the invention. 
         FIGS. 66A and 66B  depict an optical switch  6600  in accordance with one embodiment of the invention. 
         FIGS. 67A and 67B  depict a packaging concept for MEMS actuators in accordance with one embodiment of the invention. 
         FIGS. 68A and 68B , respectively, are plan views of a top half  6800  and a bottom half  6805  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. 
         FIGS. 69A and 69B , respectively, are plan views of a top half  6900  and a bottom half  6905  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. 
         FIGS. 70A and 70B , respectively, are plan views of a top half  7000  and a bottom half  7005  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. 
         FIGS. 71A and 71B , respectively, are plan views of a top half  7100  and a bottom half  7105  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. 
         FIGS. 72A and 72B , respectively, are plan views of a top half  7200  and a bottom half  7205  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. 
         FIGS. 73A and 73B , respectively, are plan views of a top half  7300  and a bottom half  7305  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A and 1B , respectively, are plan views of the top (T) and bottom (B) portions of a multi-axis, Micro-ElectroMechanical Systems (MEMS) actuator in accordance with one embodiment of the present invention. Bottom half  100  includes a pair of fixed combs  107  and  109  and a pair of electrodes  117  ard  118  attached firmly to the underlying substrate (not shown). Each of fixed combs  107  and  109  includes a respective plurality of teeth  106 B and  108 B that extend in the direction depicted as horizontal in  FIGS. 1A and 1B . Fixed combs  107  and  109  are electrically isolated from one another so that disparate voltages can be applied thereto. The “B” in numerical designations  106 B and  108 B indicate that teeth  106 B and  108 B are associated with bottom half  100 . The remainder of this application follows this convention. 
     Bottom half  100  includes frame portions  111 B and  112 B, which also function as frame combs. Frame portions  111 B and  112 B each support a plurality of frame teeth  113 B and  115 B, respectively, which extend in a direction perpendicular to the fixed teeth of combs  107  and  109 . Frame portions  111 B and  112 B connect to respective electrodes  117  and  118  via a pair of hinge portions  119 B. Frame portions  111 B and  112 B, including teeth  113 B and  115 B, are disposed above the underlying substrate so frame portions  111 B and  112 B can pivot along a fulcrum axis FA 1  defined along hinges  119 B. 
     Turning to  FIG. 1A , top half  105  is bonded over bottom half  100  with an electrically insulating layer (detailed below) sandwiched in between. Top half  105  includes a frame  111 T (“T” is for “top”) bonded to frame portions  111 B and  112 B of FIG.  1 B. Frame portion  111 T includes a plurality of movable combs  106 T and  108 T, each including a plurality of comb teeth extending in the horizontal direction of  FIGS. 1A and 1B . Movable comb teeth  106 T and  108 T are rigidly connected to frame portion  111 T, and are arranged above fixed combs  106 B and  108 B such that the fixed and movable teeth are interdigitated from a perspective perpendicular to a plane defined by the horizontal and vertical axes depicted in  FIGS. 1A and 1B  (i.e., normal to the page). 
     Top half  105  includes an actuated member  123 , in this case a mirror surface, connected to frame portion  111 T via a pair of hinges  125 . Hinges  125  allow member  123  to pivot along a second fulcrum access FA 2  perpendicular to the fulcrum access FA 1  defined by hinges  119 B. Member  123  additionally includes a collection of combs  113 T and  115 T, each of which includes a plurality of teeth extending over and in parallel with respective teeth  113 B and  115 B of bottom half  100 . The teeth in combs  113 T (and  115 T) and teeth  113 B (and teeth  115 B) are arranged interdigitally from a perspective perpendicular to a plane defined by the vertical and horizontal axes of  FIGS. 1A and 1B . 
     The lower counterparts to hinge portions  119 T, depicted in  FIG. 1B  as hinge portions  119 B, electrically connect electrodes  117  and  118  to respective frame-portions  111 B (and teeth  113 B) and  112 B (and teeth  115 B) so that voltage may be applied to combs  113 B and  111 B via electrodes  117  and  118 , respectively. Returning to  FIG. 1A , hinge portions  119 T and hinges  125  electrically connect combs  113 T,  115 T, frame portion  111 T, and combs  106 T and  108 T to the surrounding silicon  130 . 
     In one embodiment, member  123  is actuated in one direction along FA 2  axis (say positive direction) by holding silicon  130  (i.e. teeth  106 T,  108 T,  113 T and  115 T) at ground potential and also teeth  115 B,  106 B and  108 B at ground potential while adjusting the voltage levels applied to teeth  113 B. Electrical leads that run along hinge portions  119 B connect teeth  113 B and  115 B to the respective electrodes  117  and  118 . To move member  123  in the negative direction, ground potential is kept again at all top teeth, i.e.  106 T,  108 T,  113 T and  115 T, and at  108 B,  108 B and  113 B, while a desired voltage is applied to teeth  115 B. To rotate frame  111 T along FA 1  axis in one direction, all top teeth and bottom teeth  113 B,  115 B, and  108 B are at ground potential and teeth  106 B have voltage applied to them; to rotate frame  111 T along FA 1  in the other direction, all top teeth and bottom teeth  113 B,  115 B, and  106 B are at ground potential and teeth  108 B have voltage applied to them. To rotate member  123  along both FA 1  and FA 2  axes, different voltages are applied to  106 B and  113 B (or  115 B) or to  108 B and  113 B (or  115 B) at the same time. Member  123  may also be moved in a direction normal to the fulcrum axes by applying a potential difference between the combs of top half  105  and bottom half  100 . Member  123  may therefore be positioned in three dimensions. 
     Bottom frame portions  111 B and  112 B are bonded to top frame portion  111 T during the process sequence described below. The resulting frame can be rotated along the axis FA 1  defined by hinges  119 B and  119 T by applying a voltage difference between teeth  106 B and ground or between teeth  108 B and ground. Combs  106 T and  108 T are termed “movable” because they move relative to stationary combs  106 B and  108 B. Similarly, actuated member  123  can be rotated along axis FA 2  by applying a voltage difference between the silicon  130  and either electrode  117  or electrode  118 . 
       FIGS. 2 through 32  depict a process sequence in accordance with an embodiment of the invention. The process sequence can be employed to fabricate an actuator of the type depicted in  FIGS. 1A and 1B .  FIGS. 2 through 32  depict the device in cross section, with the resulting structure appearing similar to the device of  FIGS. 1A and 1B  cut along line A-A′. 
       FIG. 2  depicts an SOI or SOG wafer  200  that includes a layer of handle silicon  205  connected to a 20-100 micron thick device silicon layer  210  via a 1-2 micron thick silicon dioxide or glass layer  215 . As depicted in  FIG. 3 , the exposed surfaces of silicon layers  205  and  210  are coated with silicon dioxide mask layers  300  and  305 . The resulting structure is then masked using a photoresist layer  400  ( FIG. 4 ) to define a set of alignment marks  405 . Alignment marks  500  are then etched in oxide layer  300  and the photoresist layer  400  is removed to produce the structure of FIG.  5 . Silicon layers  205  and  210  are both doped, either n-type or p-type, and have a resistivity of about 5 to 100 ohms-cm in one embodiment. 
     Next, a layer of photoresist is patterned over oxide layer  305  to create a mask  600  used to define each element of bottom half  100  ( FIG. 1B ) except for hinge portions  119 B. The exposed portions of oxide layer  305  are then subjected to a dry silicon-dioxide etch, leaving an oxide mask  700  of the pattern defined by mask  600 . Mask  600  is then removed, leaving the structure of FIG.  7 . 
     A photoresist layer  800  is patterned over oxide mask  700  and over those portions of device silicon  210  that are to become hinge portions  119 B (FIG.  8 ). The resulting structure is then subjected to a silicon reactive-ion etch (RIE) to remove a desired depth of device layer  210  in the exposed regions (FIG.  9 ). The mask used in this etch step includes two sub-masks: oxide mask  700  and the pattern photoresist layer  800 . The etch depth is related to the final thickness of hinge portions  119 B. The photoresist mask  800  is then removed, exposing oxide mask  700  and the portions of device layer  210  that will become hinge portions  119 B (FIG.  10 ). 
     A second silicon RIE removes the remaining unmasked silicon of layer  210  down to oxide or glass layer  215 , which acts as an etch-stop layer (FIG.  11 ). Portions  1100  of silicon layer  210  that will later become hinge portions  119 B are left adhered to oxide layer  215  because, as shown in  FIG. 10 , portions  1100  entered the etch step thicker than the surrounding exposed portions of silicon layer  210 . The hinges undergo this fabrication sequence to make them thinner, and consequently more flexible, than the surrounding device features. When it is desired to keep hinges of the same thickness as the teeth, the steps of  FIGS. 8-10  are skipped. 
     In an optional step, a refractory coating  1200  is applied through a shadow mask to an exposed portion of oxide layer  215  to balance the stress imposed by a reflective layer applied opposite coating  1200  on layer  205  in a later step. The resulting structure is depicted in FIG.  12 . 
     The process of fabricating the actuator support begins, as shown in  FIG. 13 , with an intrinsic silicon wafer  1300  coated with a layer of silicon dioxide (or glass)  1302 . Layer  1302  is conventionally masked using a layer of photoresist  1400  to define electrical contacts to silicon layer  1300  and a plurality of alignment marks (FIG.  14 ). Layer  1302  is shown in  FIG. 15  to include an area  1500  in which will be formed a via and a number of openings  1502 . 
     In  FIGS. 16 and 17 , a photoresist layer  1600  is patterned over silicon  1300  to define an area  1700  in which approximately 100 microns of silicon is etched away from silicon layer  1300  using RIE. The resulting structure, including area  1700  and alignment marks  1502 , is depicted in FIG.  17 . 
     In the next step, the structure of  FIG. 17  is brought into contact with the structure of  FIG. 12 , out of which will be formed bottom and top halves  100  and  105  (FIG.  18 ). The two portions are aligned using the respective alignment marks  400  and  1502  and then fused together using a heat treatment. In an embodiment in which mask  700  is silicon dioxide, the structure is heated to approximately 1,000 to 1,100 degrees Celsius. In an embodiment in which mask  700  is sol gel glass, the structure is heated to between 200 and 400 degrees Celsius. The lower process temperatures employed when glass is used for layers  215  and mask  700  minimize stresses associated with thermal expansion in the multi-layer structures. 
     Referring now to  FIG. 19 , the top surface of silicon layer  205  is ground, lapped, and polished to a mirror finish. The resulting thinned silicon layer  205  is approximately 20 to 100 microns thick. Next (FIG.  20 ), a photoresist mask  2000  is applied to oxide layer  1302  using the same mask used to define the pattern of oxide layer  1302  in  FIG. 14. A  combined mask of photoresist ( 2000 ) and oxide ( 1302 ) is used for very deep silicon etching. A subsequent silicon RIE step removes some of layer  1300  in the vicinity  1500  to expose a portion of oxide mask  700 , producing the structure of FIG.  21 . The RIE used to form the structure of  FIG. 21  is adjusted so that the sidewalls of opening  2100  are not normal to the surface. Alternatively, wet etching of silicon can be used to produce sloping wall vias. In this case, silicon nitride mask is preferable to silicon dioxide mask. This leaves alignment marks  2102  in layer  1300  but prevents those marks from extending far into layer  1300 . 
     In the next step, oxide layer  1302  and the portion of oxide layer  700  exposed during the previous step are removed using an oxide dry-etch process. In the resulting structure, illustrated in  FIG. 22 , the underside of layer  210  is exposed to allow a subsequently formed via to make electrical contact to the portion of layer  210  that will become the body of comb  109  of FIG.  1 A. Similar vias make contact to electrode  117 , electrode  118 , and comb  107 , though these are not shown in this cross section. 
     Continuing to  FIG. 23 , another oxide layer  2300  is formed on the top surface of silicon layer  205  using either chemical vapor deposition or sputter deposition. The pads (not shown) and vias, one of which is depicted in  FIG. 24 , are then metalized using a conventional metalization process that employs a shadow mask. Via  2400  contacts the underside of silicon layer  210  at a portion that will become the body of comb  109  ( FIG. 1B ) of the bottom half of the actuator under fabrication. 
     Most of the features of bottom layer  100  of  FIG. 1B  have been defined at this stage in the process sequence. The process of patterning the structures required to form top half  105  begins with a photoresist mask  2500  depicted in FIG.  25 . The upper surface of oxide layer  2300  is dry etched through mask  2500  to expose the underlying silicon layer  205 . The resulting structure is depicted in FIG.  26 . 
     Turning to  FIG. 27 , a photoresist layer  2700  is applied over each feature of the oxide mask patterned in layer  2300 , and additionally over those portions of silicon layer  205  that will form hinges  125  and hinge portions  119 T. Those portions can be identified in  FIG. 27  as the portions of photoresist layer  2700  deposited directly on the surface of silicon layer  205 . The top surface of the resulting structure is then subjected to a silicon RIE process that removes a desired thickness of the exposed portions of silicon layer  205 . This etch step defines the thickness of the upper half of hinges  125  and hinge portions  119 T, the portions depicted in upper half  105  of FIG.  1 A. The resulting structure is depicted in FIG.  28 . 
     Patterned mask layer  2700  is then removed (FIG.  29 ). Another RIE then removes the remaining silicon in the thinned portions of silicon layer  205 . As shown in  FIG. 30 , those portions of silicon layer  205  protected from the first RIE step of  FIG. 28 , being thicker than the other etched portions of layer  205 , leave features  3000  to form the upper portion of hinges  125  and hinge portions  119 T. Like structures  1100 , which form the bottom half of the hinges, structures  3000  are formed thinner than adjacent elements to adjust hinge flexibility. The resulting structure is subjected to a silicon-dioxide etch to remove oxide layer  2300  and those portions of oxide layer  215  that connect adjacent elements depicted in the cross section of  FIG. 30 , thereby producing the structure of FIG.  31 . 
     Alternatively, oxide layer  2300  is removed through a shadow mask that allows oxide etching over the whole surface except the portion that will become actuated member  123 . In the resulting embodiment, the actuated member is coated with an oxide layer on both principal surfaces to minimize mirror distortion. Finally, a reflective surface (a mirror)  3200  is added to silicon layer  205 . In this case, mirror  3200  is formed by depositing first chromium and then gold onto layer  205  through a shadow mask. The completed structure, illustrated in  FIG. 32 , is annotated using the numbers introduced in  FIGS. 1A and 1B  to identify the actuator structures shown in the cross section in FIG.  32 . One feature not shown in  FIGS. 1A and 1B  is the actuator support formed from silicon layer  1300 . 
     As is apparent from  FIG. 32 , teeth  106 T ( 115 T) and the underlying teeth  106 B ( 115 B) appear interdigitated from a perspective normal to mirror  3200 , but not from a perspective normal to the cross section of FIG.  32 . However, the teeth can be drawn toward one another, and therefore actually interdigitated, by applying a sufficient voltage between the upper and lower teeth. The ability to interdigitate the opposing teeth minimizes the clearance, increases the efficiency, and reduces the voltage required to produce a desired deflection angle. 
     The cross section of  FIG. 32  differs slightly from what would be obtained along line A-A′ of  FIGS. 1A and 1B . For example, the number of comb teeth differs, and the layers and patterns are not to scale. Such variations are commonly used to simplify the description of the process, as is well understood by those of skill in the art. In an actual embodiment, combs  113 ,  115 ,  106 , and  108  might have 10-100 teeth, for example, and the teeth might be 5-20 microns wide and 200-500 microns long. 
       FIGS. 33A and 33B  are plan views depicting the respective top and bottom halves  3300  and  3305  of an actuator in accordance with another embodiment of the invention. The actuator depicted in  FIGS. 33A and 33B  is functionally similar to the one depicted in  FIGS. 1A and 1B . However, the structure of  FIGS. 33A and 33B  employs a different comb configuration, as is obvious from the plan views, and also includes more flexible serpentine hinges. The serpentine hinges can be made the same thickness as other elements (e.g., the comb teeth), or can be made thinner using the process shown in connection with  FIGS. 2 through 32 . 
     The distance from the tip of teeth  3306 ,  3308 ,  3313  and  3315  to their rotational axes are longer than in the embodiment of  FIGS. 1A and 1B . Therefore, the torque generated by the same voltage difference is increased. Mirror teeth  3313 T and  3315 T with variant teeth length are attached to the mirror directly and to surrounding silicon  3330  via hinges  3325 , frame  3311 T, and hinge portions  3319 T. Variable teeth length is important for linearization of voltage response and damping of resonances. Frame teeth  3313 B and  3315 B are arranged interdigitally with mirror teeth  3313 T and  3315 T and connected to electrodes  3317  and  3318  independently through hinge portions  3319 B. A voltage difference can be applied between  3313 T and  3313 B or between  3315 T and  3315 B to rotate the mirror with respect to the frame in the axis defined by hinges  3325 . The frame teeth  3306 T and  3308 T are also arranged interdigitally with the stationary comb teeth  3306 B and  3308 B to rotate the mirror/frame with respect to the axis defined by the hinges formed of top and bottom hinge portions  3319 T/B. Two separated frame portions  3316  are designed to increase the frame rigidity without increasing the electrostatic coupling between different sets of teeth,  3306  and  3313 . Also important, the actuator is designed so that the reflective surface  3323  is as great a percentage of the total actuator area (including the exposed portions of the actuator support) as practical, which is over 25% in the depicted embodiment. 
       FIGS. 34-49  depict an alternate fabrication process, referred to here as the “pattern transfer” process, that can be used to fabricate MEMS actuators in accordance with the invention.  FIGS. 34-49  depict the device in cross section, with the resulting structure appearing similar to the device of  FIGS. 1A and 1B  cut along line A-A′. 
       FIG. 34  depicts a wafer  3400  that includes a layer of handle silicon  3405  connected to a 20-100 micron thick device silicon layer  3410  via a 1-2 micron thick layer  3415  of silicon dioxide or spin-on glass. As depicted in  FIG. 35 , the exposed surfaces of silicon layers  3405  and  3410  are coated with silicon dioxide mask layers  3500  and  3505 . Silicon layers  3405  and  3410  are both doped, either n-type or p-type, and have a resistivity of about 5 to 100 ohms-cm in one embodiment. 
     A layer of photoresist is patterned over each of respective oxide layers  3500  and  3505  to create a pair of masks  3600  and  3610  (FIG.  36 ). The exposed portions of oxide layers  3500  and  3505  are then subjected to a dry silicon-dioxide each, leaving oxide masks  3700  and  3710 . Masks  3600  and  3610  are then removed, and the upper surface of the resulting structure is subjected to another photolithographic patterning step that forms the hinge patterns  3805 , leaving the structure of FIG.  38 . Optionally, layer  3505  can be a metal film, such as aluminum, chromium, or titanium, and mask  3610  can be formed of oxide. In such embodiments, the metal layer is etched using metal etches rather than oxide etches. 
     Next, a silicon RIE removes a desired depth of device layer  3410 , leaving the structure of FIG.  39 . Mask  3800  is then removed, leaving the structure of  FIG. 40. A  second silicon RIE then removes the remaining unmasked silicon of layer  3410  down to oxide layer  3415 , which acts as an etch-stop layer (FIG.  41 ). Portions  4100  of silicon layer  3410  that will later become hinges  125  and hinge portions  119 T are left adhered to oxide layer  3415 . The hinges undergo this fabrication sequence to make them thinner, and consequently more flexible, than surrounding device features. When it is desired to keep the hinges of the same thickness as the teeth, the steps of  FIGS. 38-42  are skipped. 
     In the next step, a photoresist layer  4200  is patterned over the oxide mask  3710 , with the addition of the portion  4205  that masks what will become hinge portion  119 B (FIG.  42 ). Next, the lower surface of silicon layer  3405  is subjected to a silicon RIE that removes a desired thickness of the exposed portions of silicon layer  3405 . This etch step defines the thickness of hinge portions  119 B of FIG.  1 B. The resulting structure is depicted in FIG.  43 . The photomask is then removed, leaving the structure of  FIG. 44 , which includes a raised element  4400 . 
     Another silicon RIE then removes a second desired thickness of the exposed portions of silicon layer  3405 . This step defines the thickness of what will become the comb teeth of bottom portion  100  of the actuator of  FIGS. 1A and 1B .  FIG. 45  depicts the resulting structure. 
     Another photoresist mask  4600  is added by spray coating to protect portions of oxide mask  3710  (FIG.  46 ); the exposed portions of oxide mask  3710  are then removed using a dry silicon-dioxide etch step.  FIG. 47  depicts the resulting structure. The remaining silicon in the thinned portions of silicon layer  3405  is then removed using another RIE, with oxide layer  3415  acting as an etch-stop layer. As shown in  FIG. 48 , those portions of silicon layer  3405  protected from previous RIE steps, being thicker than the other etched portions of layer  3405 , leave features  4800  and  4805 . Features  4800  and  4805  will form the bottom combs ( 115 B and  106 B) and hinge portion  119 B, respectively, of FIG.  1 B. 
     Finally, the structure of  FIG. 48  is subjected to a silicon-dioxide etch to remove oxide layers  3700  and  3710 , and to remove those portions of oxide layer  3415  that connect adjacent elements depicted in the cross section of FIG.  48 . Though not shown, a reflective surface is subsequently added to silicon layer  3410 . The completed actuator, illustrated in  FIG. 49 , is annotated using some of the numbers introduced in  FIGS. 1A and 1B  to identify the actuator structures shown in the cross section. As with the previous example of  FIG. 32 , the cross section of  FIG. 32  differs slightly from what would be obtained along line A-A′ of  FIGS. 1A and 1B . What remains of silicon layer  3405  forms the actuator support. 
     As noted above, the process of  FIGS. 34-49  is referred to as a “pattern transfer” process. The name “pattern transfer” refers to the steps by which a pattern is formed on one surface and transferred to another. Such a pattern transfer is shown, for example, in  FIGS. 41-48 . In  FIGS. 41-47 , the bottom surface of silicon layer  3405  (a first material layer) is patterned to include features similar to the combs, hinges, etc., of bottom half  100  of the MEMS actuator of  FIGS. 1A and 1B . This pattern is then “transferred” to the bottom surface of a second material layer, oxide layer  3415  (FIG.  48 ), by etching silicon layer  3405  until oxide layer  3415  is exposed between elements of the pattern. The original elements of the pattern, shown in e.g.  FIG. 47 , are wholly or partially consumed in the etch process that culminates in the structure of FIG.  49 . 
       FIGS. 50-65  depict an alternate fabrication process, referred to herein as “deep-well lithography,” that can be used to fabricate MEMS actuators in accordance with the invention.  FIGS. 50-65  depict the device in cross section, with the resulting structure appearing similar to the device of  FIGS. 1A and 1B  cut along line A-A′. 
       FIG. 50  depicts a wafer  5000  that includes a layer of handle silicon  5005  covered with a silicon-dioxide layer  5010 , a device silicon layer  5015 , a second silicon-dioxide layer  5020 , and a second device silicon layer  5025 . Device silicon layers  5015  and  5025  are each about 20-100 microns thick; oxide layers  5010  and  5020  are each between one and two microns thick. Silicon layers  5005 ,  5015 , and  5025  are doped, either n-type or p-type, and have a resistivity of about 5 to 100 ohms-cm in one embodiment. 
     As depicted in  FIG. 51 , the exposed surfaces of silicon layers  5005  and  5025  are coated with respective silicon dioxide mask layers  5105  and  5100 . Next, a layer of photoresist is patterned over each of respective oxide layers  5100  and  5105  to create a pair of masks  5200  and  5210  (FIG.  52 ). The exposed portions of oxide layers  5100  and  5105  are then subjected to a dry silicon-dioxide etch, leaving oxide masks  5300  and  5310 . Masks  5200  and  5210  are then removed, and another photoresist layer  5400  is patterned over the oxide mask  5300  with the additional patterns  5405  that are to become hinges  125  and hinge portions  119 T (FIG.  54 ). 
     Next, the upper surface of the resulting structure is subjected to a silicon RIE to remove a desired thickness of device layer  5025 , leaving the structure of FIG.  55 . The photoresist layer  5400  is then removed, leaving the structure of FIG.  56 . 
     A second silicon RIE removes the remaining unmasked silicon of layer  5025  down to oxide layer  5020 , which acts as an etch-stop layer (FIG.  57 ). Portions  5700  of silicon layer  5025  that will later become hinges  125  and hinge portions  119 T are left adhered to oxide layer  5020 . The hinges undergo this fabrication sequence to make them thinner, and consequently more flexible, than the surrounding device features. As in the previous examples, several of the foregoing steps can be eliminated if the hinges need not be thinner than surrounding device features. 
     In the next step, another RIE removes the unmasked portion of silicon layer  5005  down to oxide layer  5010 , which acts as an etch-stop layer (FIG.  58 ). Turning to  FIG. 59 , a photoresist mask  5900  is then applied by spray coating to oxide layer  5010  before the lower surface of silicon layer  5010  is subjected to a dry silicon-dioxide etch process that removes exposed portions of oxide layer  5010  to form a mask  6000  (FIG.  60 ). Photoresist mask  5900  is then removed, and another photoresist layer  6100  is patterned over the oxide mask  6000 , with the addition of the portion  6105  that will become hinge portion  119 B. The resulting structure is depicted in FIG.  61 . 
     Next, as shown in  FIG. 62 , a desired thickness of the exposed portions of silicon layer  5015  is etched away using an RIE. This etch step defines the thickness of hinge portions  119 B of FIG.  1 B. The resulting structure, after removing photoresist  6100  (FIG.  63 ), includes an element  6300 . 
     Another RIE removes the remaining silicon in the thinned portions of silicon layer  5015 , with oxide layer  5020  acting as an etch-stop layer. As shown in  FIG. 64 , the portion of silicon layer  5015  protected from previous reactive-ion etching, being thicker than the other etched portions of layer  5015 , leaves feature  6400  that will form hinge portion  119 B of FIG.  1 B. Once again, several of the foregoing steps can be eliminated if the hinges need not be thinner than surrounding device features. 
     Finally, the structure of  FIG. 64  is subjected to a silicon-dioxide etch to remove oxide layers  5300  and  5310 , and to remove those portions of oxide layer  5020  that connect adjacent elements depicted in the cross section of FIG.  65 . Though not shown, a reflective surface is then added to silicon layer  3410 . The completed actuator is annotated using some of the numbers introduced in  FIGS. 1A and 1B  to identify the actuator structures shown in the cross section. As with the previous example of  FIG. 32 , the cross section of  FIG. 65  differs slightly from what would be obtained along line A-A′ of  FIGS. 1A and 1B . What remains of silicon layer  5005  provides the actuator support. 
     As noted above, the process of  FIGS. 50-65  is referred to as “deep-well lithography.” The name refers to the steps by which a pattern is formed upon a surface that is below the uppermost surface of the structure being fabricated (i.e., in a well). Such a process is shown, for example, in  FIGS. 59-64 , during which silicon layer  5015  is patterned to form features of bottom half  100  of the MEMS actuator of  FIGS. 1A and 1B . 
     Deep-well lithography differs from conventional lithography in that the surface being patterned is not the uppermost surface. The focal plane of the photolithography equipment must therefore be offset as appropriate to account for the depth of the well in which the pattern is to be formed. To form mask  5900  of  FIG. 59 , for example, the photolithography equipment is first focused on the top surface of oxide layer  5310  to define the well within which mask  5900  will be formed. The focal plane of the photolithography equipment is then adjusted to account for the combined thickness of silicon layer  5005  and oxide layer  5310  so that the exposure pattern is focused on the portion of mask layer  5900  in contact with oxide layer  5010 . The offset can take into account the thickness of a material layer of uniform composition, or a material layer made up of two or more sub-layers (e.g., oxide layer  5310  and silicon layer  5005 ). 
       FIGS. 66A and 66B  depict an optical switch  6600  in accordance with one embodiment of the invention. Switch  6600  includes a nine-by-nine mirror array  6605  hermetically sealed within a package  6610 . Package  6610  protects the very fragile mirror array  6605  from physical and chemical hazards (e.g., dust and condensation), which can easily damage sensitive MEMS structures or interfere with device operation. Package  6610  is preferably assembled in an inert, low humidity environment. 
     Within package  6610 , array  6605  is mounted on an integrated circuit  6615  that includes the requisite circuitry for controlling array  6605 . Array  6615  is, in turn, mounted on a ceramic substrate  6620 . Package  6610  is sealed using a window  6625 , both primary surfaces of which include non-reflective coatings. A heat sink  6630  affixed to substrate  6620  dissipates heat generated by circuit  6615 . A collection of feed-through pins  6635  conveys external signals, including power and ground, to circuit  6615 . 
     Array  6605  includes 81 mirrors, and each mirror requires a number of electrical contacts. Other implementations will have more or fewer mirrors, and consequently require more or fewer electrical contacts. As the number of contacts increases, wirebond pad pitch limitations make it increasingly difficult to convey a sufficient number of control signals between circuit  6615  and array  6605 . “Flip-chip” technology is used in some embodiments to solve this problem. For more information about flip-chip technology, see “Flip Chip Challenges,” by Steve Bezuk, Applied Technology Development and Flip Chip, Kyocera America, Inc., which was first published in HDI Magazine, February 2000, and is incorporated herein by reference. 
       FIG. 67A  depicts an application-specific integrated circuit (ASIC)  6700  that includes a collection of contact bumps  6705 .  FIG. 67B  shows ASIC  6700  in cross-section along line A-A′ of  FIG. 67A , and additionally shows a portion of a MEMS actuator  6710  with electrical contacts (vias)  6715  positioned over and in contact with bumps  6705 . Bumps  6705  can be conductive bonding material, such as solder or conductive epoxy; alternatively, bumps  6705  can be replaced with an anisotropic conductive film, provided MEMS actuator  6710  is sufficiently robust to withstand the compressive force required to make effective electrical contact through such material. 
       FIGS. 68A and 68B , respectively, are plan views of a top half  6800  and a bottom half  6805  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. Top half  6800  includes a frame  6810  supporting an actuated member  6815 . Frame  6810  includes a number of curved, moveable combs  6820  interdigitated with a corresponding number of fixed combs  6825 . The actuator of  FIGS. 68A and 6813  is similar to the actuator in  FIGS. 1A and 1B , but additionally affords the ability to rotate member  6815  in the X-Y plane (FIG.  68 B). The actuator of  FIGS. 68A and 68B  can be fabricated using any of the process sequences described above. 
       FIGS. 69A and 69B , respectively, are plan views of a top half  6900  and a bottom half  6905  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. Top half  6900  includes a frame  6910  supporting an actuated member  6915 . Frame  6910  includes a number of moveable combs  6920  interdigitated with a corresponding number of fixed combs  6925 . The actuator of  FIGS. 69A and 69B  is similar to the actuator in  FIGS. 1A and 1B , but affords the ability to translate member  6915  linearly along the X and Z axes ( FIG. 69B ) and rotationally around the X and Y axes. The actuator of  FIGS. 69A and 69B  can be fabricated using any of the process sequences described above. 
       FIGS. 70A and 70B , respectively, are plan views of a top half  7000  and a bottom half  7005  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. The actuator of  FIGS. 70A and 70B  is similar to the actuator of  FIGS. 1A and 1B , but includes non-perpendicular fulcrum axes FA 1  and FA 2 . The actuator of  FIGS. 70A and 70B  can be fabricated using any of the process sequences described above. 
       FIGS. 71A and 71B , respectively, are plan views of a too half  7100  and a bottom half  7105  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. Top half  7100  includes four sets of combs  7110  interdigitated with four separate fixed combs  7115  on bottom half  7105 . An actuated member  7120  suspended by four bending, serpentine hinges  7125  can pivot along either of two fulcrum axes FA 1  and FA 2 , or can be moved vertically along the Z axis normal to the plane defined by the two fulcrum axes. Advantageously, the actuators described in connection with FIGS.  71 A/B, can be fabricated using fewer process steps than other embodiments described herein. The simplified process sequence is similar to the process described in connection with  FIGS. 2-32 , but eliminates the need for the steps described in connection with  FIGS. 8-10  and  14 - 17 . Also advantageous, this embodiment eliminates the need to align two patterned wafers before bonding; instead, an unpatterned wafer is bonded to a patterned wafer. The structures disclosed below and described in connection with FIGS.  72 A/B and  73 A/B afford the same advantages. 
       FIGS. 72A and 72B , respectively, are plan views of a top half  7200  and a bottom half  7205  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. Top half  7200  includes three sets of combs  7210  interdigitated with three separate fixed combs  7215  on bottom half  7205 . By supplying different voltages on selected ones of fixed combs  7215 , an actuated member  7220  can be tilted in an X-Y plane and can be moved along a Z axis normal to the X-Y plane. 
       FIGS. 73A and 73B , respectively, are plan views of a top half  7300  and a bottom half  7305  of a multi-axis MEMS actuator in accordance with another embodiment of the invention. Top half  7300  includes a frame  7310  supporting an actuated member  7315 . Frame  7310  includes a number of frame teeth  7320  interdigitated with corresponding fixed teeth  7325  on bottom half  7305 ; likewise, member  7315  includes a number of member teeth  7330  interdigitated with corresponding fixed teeth of combs  7335  on bottom half  7305 . The actuator of  FIGS. 73A and 73B  is similar to the actuator in  FIGS. 1A and 1B , and affords the ability to rotate member  7315  along a first rotational axis defined by torsional hinges  7340 , a second rotational axis defined by torsional serpentine hinges  7345 , and translationally along the Z axis normal to the two rotational axes. 
     The foregoing embodiments include springs that lie in substantially the same plane as the actuated member. It is also possible to attach an actuated member to a member support using one or more flexible elements extending from the bottom of the actuated member. In the case of a mirror, such a structure might be similar to a table on one or more flexible legs. The table surface (the mirror) would be movable in at least two dimensions. Such a structure could be fabricated using e.g. LIGA micromachining technology (“LIGA” is an acronym from German words for lithography, electroplating, and molding). 
     For additional information relating to MEMS actuators in general, and optical cross-connect switches in particular, see the following U.S. patent applications, each of which is incorporated by reference;
         1. Ser. No. 09/880,456, entitled, “Optical Cross Connect Switching Array System With Electrical And Optical Position Sensitive Detection,” by Vlad Novotny, filed Jun. 12, 2001; and   2. Ser. No. 09/981,628, entitled “Micro-Opto-Electro-Mechanical Switching System,” by Vlad J. Novotny and Parvinder Dhillon, filed on Oct. 15, 2001.       

     While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example, each fulcrum axis may be provided along an edge of the actuated member and the number of combs may be different. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.