Abstract:
Described are methods for fabricating Micro-Electro-Mechanical Systems (MEMS) actuators with hidden combs and hinges. The ability to hide the combs renders the actuators useful in digital micro-mirror devices. Comb actuators provide increased torque, which facilitates the use of stiffer, less fragile hinge structures. Also important, comb actuators do not require mechanical stops to define stable states, and thus avoid problems associated with physical contact. The actuators are infinitely variable through a range of angles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This U.S. patent is a continuation of application Ser. No. 11/100,953 entitled “Methods for Fabricating Spatial Light Modulators with Hidden Comb Actuators,” by Vlad Novotny and Paren Shah, filed Apr. 6, 2005. Application Ser. No. 11/100,953 is a continuation of application Ser. No. 10/394,835 entitled “Spatial Light Modulator with Hidden Comb Actuator,” also by Vlad Novotny and Paren Shah, filed Mar. 22, 2003. Each of the above-referenced patents and applications is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Spatial Light Modulators (SLMs) have found numerous applications in the areas of optical information processing, projection displays, video and graphics monitors, televisions, and electrophotographic printing. SLMs are devices that modulate incident light in a spatial pattern to form an image corresponding to an electrical or optical input. The incident light may be modulated in its phase, intensity, polarization, or direction. The light modulation may be achieved with a variety of materials exhibiting various reflective, refractive, diffractive, electro-optic or magneto-optic effects, or with materials that modulate light by surface deformation. 
         [0003]    An SLM typically includes an area or linear array of addressable picture elements (pixels). Using well-known algorithms, source pixel data (e.g., data representing an image) is formatted by an associated control circuit and loaded into the pixel array using any of a number of well-known addressing schemes, typically addressing all pixels in parallel. 
         [0004]    One type of SLM, referred to herein as a micro-mirror array, is a monolithic integrated circuit with an array of movable micro-mirrors fabricated over the requisite address, control and drive circuitry. Micro-mirrors are normally bistable, switching between two stable positions in response to digital control signals. Each mirror in a given array forms one pixel, wherein a source of light directed upon the mirror array will be reflected in one of two directions depending upon the selected one of the two stable mirror positions. In an “on” mirror position, incident light to a given mirror is reflected to a projector lens and focused on a display screen or a photosensitive element of a printer; in an “off” mirror position, light directed on the mirror is deflected to a light absorber outside of the numerical aperture of the projecting lens. 
         [0005]    When the micro-mirror array is used in a display, the projector lens magnifies the modulated light from the pixel mirrors onto a display screen. Gray scale of the pixels forming the image is achieved by pulse-width modulation, as described in U.S. Pat. No. 5,278,652, entitled “DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System,” which is incorporated herein by reference. 
         [0006]    For more detailed discussions of conventional micro-mirror devices, see the following U.S. patents, each of which is incorporated herein by reference:
       1. U.S. Pat. No. 5,535,047 to Hornbeck, entitled “Active Yoke Hidden Hinge Digital Micro-mirror Device;   2. U.S. Pat. No. 5,079,544 to DeMond, et al, entitled “Standard Independent Digitized Video System”; and   3. U.S. Pat. No. 5,105,369 to Nelson, entitled “Printing System Exposure Module Alignment Method and Apparatus of Manufacture.”       
 
         [0010]    The evolution and variations of the micro-mirror devices can be appreciated through a reading of several issued patents. The “first generation” micro-mirror based spatial light modulators were implemented with analog control of electrostatically driven mirrors using parallel-plate configurations. That is, an electrostatic force was created between the mirror and the underlying address electrode to induce deflection thereof. The deflection of these mirrors can be variable and operate in the analog mode, and may comprise a leaf-spring or cantilevered beam, as disclosed in the following U.S. patents, each of which is incorporated herein by reference:
       1. U.S. Pat. No. 4,662,746 to Hornbeck, entitled “Spatial Light Modulator and Method”;   2. U.S. Pat. No. 4,710,732 to Hornbeck, entitled “Spatial Light Modulator and Method”;   3. U.S. Pat. No. 4,956,619 to Hornbeck, entitled “Spatial Light Modulator”; and   4. U.S. Pat. No. 5,172,262 to Hornbeck, entitled “Spatial Light Modulator and Method.”       
 
         [0015]    This first generation micro-mirror can also be embodied as a digital or bistable device. The mirror is supported by a torsion hinge and axially rotated one of two directions 10 degrees, until the mirror tip lands upon a mechanical stop, or “landing pad.” Such an embodiment is disclosed in U.S. Pat. No. 5,061,049 to Hornbeck entitled “Spatial Light Modulator and Method,” which is incorporated herein by reference. To limit the static friction (stiction) force between the mirror tips and the landing pads, the landing pads may be passivated by an oriented monolayer formed upon the landing pad. This monolayer decreases the stiction forces and prevents the mirror from sticking to the electrode. This technique is disclosed in U.S. Pat. No. 5,331,454 to Hornbeck, entitled “Low Reset Voltage Process for DMD,” and also incorporated herein by reference. 
         [0016]    A “second generation” of micro-mirror device is embodied in U.S. Pat. No. 5,083,857 entitled “Multi-Level Deformable Mirror Device,” and U.S. Pat. No. 5,583,688 entitled “Multi-level Digital Micro-mirror Device,” both of which are incorporated herein by reference. In this second generation device, the mirror is elevated above a “yoke,” this yoke being suspended over the addressing circuitry by a pair of torsion hinges. An electrostatic force is generated between the elevated mirror and an elevated electrode, again with parallel-plate actuator configuration. When rotated, it is the yoke that comes into contact with a landing electrode: the mirror tips never come into contact with any structure. The shorter moment arm of the yoke, being about 50% of the mirror, allows energy to be more efficiently coupled into the mirror by reset pulses due to the fact that the mirror tip is free to move. Applying resonant reset pulses to the mirror to help free the pivoting structure from the landing electrode is disclosed in U.S. Pat. No. 5,096,279, entitled “Spatial Light Modulator and Method,” and U.S. Pat. No. 5,233,456 entitled “Resonant Mirror and Method of Manufacture,” both of which are incorporated herein by reference. However, some of the address torque generated between the mirror and the elevated address electrode is sacrificed compared to the first generation devices because the yoke slightly diminishes the surface area of the address electrode. 
         [0017]    Despite the aforementioned advances, parallel-plate electrostatic devices generate very low deflection torque and require very low stiffness suspension hinges. Consequently, conventional micro-mirrors are relatively fragile and difficult to fabricate, and may therefore suffer from low yield and increased manufacturing expense. Also, while various process techniques have been developed to ameliorate the stiction problem, the repeated physical contact between the moveable and fixed surfaces still reduces device reliability and lifetime. There is therefore a need for methods and actuators that significantly increase driving torque, eliminate or reduce effects of stiction, improve production yield, reduce micro-mirror production cost, and increase micro-mirror reliability. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0018]      FIG. 1  depicts a Micro-Electro-Mechanical Systems (MEMS) actuator  100  in accordance with one embodiment of the invention. 
           [0019]      FIG. 2  depicts a mirror array  200  made up of sixteen (4×4) actuators  100  formed integrally on a single substrate  116 . 
           [0020]      FIG. 3A  is a top view of actuator  100  of  FIG. 1 . 
           [0021]      FIG. 3B  is a cross-sectional diagram of actuator  100  taken along line A-A′ of  FIG. 3A . 
           [0022]      FIGS. 3C and 3D  are side views of actuator  100  from a perspective parallel to the long dimension of hinge  115 . 
           [0023]      FIG. 3E  depicts the relationship between electrostatic torque T and applied voltage V for a comb actuator (curve  301 ) and a parallel-plate actuator (curve  302 ) of the type employed in the above-referenced Hornbeck patents. 
           [0024]      FIG. 3F  includes a curve  305  illustrating the relationship between the deflection angle θ of rotational comb actuator  100  and the voltage V applied between the movable and fixed combs. 
           [0025]      FIG. 3G  includes a curve  310  illustrating the relationship between the deflection angle θ of a parallel-plate actuator (e.g., of the type described in the above-cited Hornbeck patents) and the voltage applied between the movable and fixed plates. 
           [0026]      FIGS. 4A through 4Y  depict a process of fabricating an actuator similar to actuator  100  of  FIGS. 1 ,  2 ,  3 A, and  3 B. 
           [0027]      FIG. 5  depicts an actuator  500  in accordance with another embodiment. 
           [0028]      FIG. 6A  is a top view of actuator  500  of  FIG. 5 . The actuated member  177  is removed to expose the underlying features. 
           [0029]      FIGS. 6B and 6C  are cross-sectional views of actuator  500  taken along lines A-A′ and B-B′, respectively, including actuated member  177 . 
           [0030]      FIGS. 7A and 7B  are cross-sectional views of actuator  500  taken along line B-B′ of  FIG. 6A  with voltage applied between the movable and fixed combs to induce translational motion. 
           [0031]      FIG. 8  depicts a mirror array  800  in accordance with one embodiment incorporated into an otherwise conventional projection-display system  805 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  depicts a Micro-Electro-Mechanical Systems (MEMS) actuator  100  in accordance with one embodiment of the invention. Actuator  100  employs hidden comb electrostatic actuators that produce much greater torque than the parallel-plate electrostatic actuators of the above-referenced Hornbeck patents. Greater torque facilitates the use of stiffer, less fragile hinge structures. Also important, comb actuators adapted for use with the invention have a more stable response curve than parallel-plate actuators, and consequently afford greater position control. These and other advantages, and the means of achieving them, are detailed below. 
         [0033]    Actuator  100  is broadly divided into a fixed portion  105  and a movable portion  110 , the two of which are interconnected via a tortional hinge  115 . Fixed portion  105  includes a pair of fixed combs  107  and  109  disposed over a respective pair of addressing electrodes  111  and  113 , which are in turn disposed over a substrate  116  and through an insulating layer  117 . 
         [0034]    Substrate  116  is, in an embodiment formed using a monolithic fabrication process, a wafer with an application-specific integrated circuit (ASIC) that incorporates the control, driving, and addressing electronics for actuator  100 . Actuator  100  is formed on top of substrate  116 , e.g. in the manner described below in connection with  FIGS. 4A-4Y . The electronics can be implemented using any of a number of conventional device fabrication processes, including those commonly used to form Complementary Metal Oxide Semiconductor (CMOS) circuits. In embodiments formed using hybrid fabrication processes, actuators  100  and the ASIC electronics are formed separately, on different substrates, and later bonded together using any of a number of conventional bonding techniques, such as those commonly employed in “flip-chip” technologies. In such embodiments, the substrate upon which actuator  100  is formed, the so-called “handle” substrate, can be on top of actuator  100  from the perspective of  FIG. 1A  during fabrication and later removed after bonding to an ASIC wafer. The handle substrate can be e.g. silicon, glass, or some other sacrificial substrate. 
         [0035]    Each of fixed combs  107  and  109  includes a respective plurality of teeth  120  and  121  that extend in the direction perpendicular to a fulcrum axis  125  defined along hinge  115 . Fixed combs  107  and  109  are electrically isolated from one another so that disparate voltage levels can be applied thereto. Fixed combs  120  and  121  are all of a conductive material, such as highly doped polysilicon or polysilicon-germanium or metals or metal alloys, and are electrically connected to respective electrodes  111  and  113 . 
         [0036]    Movable portion  110  includes a pair of movable combs  130  and  135  connected to hinge  115  via a bridge  140 . Moveable combs  130  and  135 , bridge  140 , and hinge  115  are all of a conductive material, such as doped polysilicon or polysilicon-germanium, metals, or metal alloys, and are electrically connected to a pair of contact pads  150  via a pair of conductive hinge posts  155 . Teeth  160  and  165  of respective movable combs  130  and  135  are interdigitated from a perspective normal to a first plane  170  extending through the fixed combs and a second plane  175  extending through the movable combs. 
         [0037]    An actuated member  177  covers the top surface of movable combs  130  and  135  and bridge  140 . It is formed either by a single metallic layer such as gold or aluminum or by two layers  128  and  129 . Layer  128  can be made from polysilicon or polysilicon-germanium while layer  129  can be made from highly reflective metal such as gold or aluminum or a metal alloy. In a typical embodiment, actuated member  177  is one of an array of mirrors used to form a spatial light modulator. Top portion  110  is tilted in one direction along fulcrum axis  125  (e.g., a positive direction) by holding movable combs  130  and  135  at ground potential while adjusting the voltage level applied to teeth  120  of fixed comb  107  to a level between e.g. zero and three volts or zero and five Volts. Applying a potential difference between combs  130  and  107  creates an electrostatic attraction that draws combs  130  and  107  together. With sufficient applied voltage, the teeth of the respective combs  130  and  107  interdigitate. To tilt top portion  110  in the opposite (e.g., negative) direction, movable combs  130  and  135  are held again at ground potential while adjusting the voltage level applied to teeth  121  of fixed comb  109 . Movable combs  130  and  135  can both be moved, to a small extent, in a direction normal to planes  170  and  175 , by applying the same potential to both fixed combs  107  and  109 , thereby causing hinge  115  to flex toward substrate  116 . 
         [0038]      FIG. 2  depicts a mirror array  200  made up of sixteen (4×4) actuators  100  formed integrally on a single substrate  116 . Because actuated members  177  obscure the underlying actuators when viewed from a perspective normal to the mirror surfaces (the top surfaces of actuated members  177 ), three of actuated members  177  are removed to expose various underlying structures. 
         [0039]    In array  200 , the mirror surfaces are the active areas, and should be closely spaced. The mirror surfaces obscure the hidden comb actuators, allowing the combined active mirror surfaces to account for more than 85% of the total array surface, where the total array surface is the active mirror surface combined with interstitial spaces  210 . In some embodiments, the active mirror surfaces account for more than 90% of the total array surface. Though not shown, the mirror surfaces may be of other shapes, preferably those that can be positioned close to one another without excessive interstitial spacing. Possible shapes include rectangles, hexagons, and triangles. Also important, actuator  100  and other embodiments of the invention do not include the conspicuous hole in the center of conventional micro-mirror arrays of the type described in the above-referenced U.S. Pat. No. 5,535,047. The elimination of these holes advantageously increases the active array surface. 
         [0040]      FIG. 3A  is a top view of actuator  100  of  FIG. 1 : actuated member  177  is removed to show the spatial relationship between the movable and fixed combs.  FIG. 3B  is a cross-sectional diagram of actuator  100  taken along line A-A′ of  FIG. 3A .  FIGS. 3C and 3D  are side views of actuator  100  from a perspective parallel to the long dimension of hinge  115 . 
         [0041]    In  FIG. 3C , a potential difference applied between fixed teeth  120  and movable teeth  160  tilts the surface of actuated member  177  θ degrees to the left, where θ is e.g. about ten degrees; in  FIG. 3D , a potential difference applied between fixed teeth  121  and movable teeth  165  tilts the surface of actuated member  177  θ degrees to the right. (In  FIGS. 3C and 3D , β refers to the angle of incidence, which is typically about 10 degrees.) 
         [0042]      FIG. 3E  depicts the relationship between electrostatic torque T and applied voltage V for a comb actuator (curve  301 ) and a parallel-plate actuator (curve  302 ) of the type employed in the above-referenced Hornbeck patents. The torque provided by the rotational comb actuators employed in various embodiments of the invention rises sharply with applied voltage and then saturates asymptotically with rotation as the movable and fixed teeth interdigitate. In contrast, the torque provided by rotational parallel-plate actuators rises with applied voltage and does not saturate. This characteristic produces a natural instability in rotational parallel-plate actuators. Due to this instability, the movable portion moves suddenly toward the fixed portion to collide with a physical stop.  FIG. 3E  additionally illustrates that, given the same applied voltage V 0 , the torque T c  of the comb actuators is much greater than the torque T p  of the parallel-plate actuator. 
         [0043]      FIG. 3F  includes a curve  305  illustrating the relationship between the deflection angle θ of actuated member  177  and the voltage V applied between the movable and fixed combs. Because the torque levels off as the teeth interdigitate, the deflection angle θ approaches an asymptote. The deflection angle θ of actuator  100  is infinitely variable through the range of curve  305 . The asymptotic nature of the response is beneficial for operating actuator  100  in a bistable mode: for example, an “on” or “off” state can be defined using an applied voltage Vs between the movable comb and one of the fixed combs. Stable states can be defined over the range of deflection angles using controlled voltage levels. 
         [0044]      FIG. 3G  includes a trace  310  illustrating the relationship between the deflection angle θ of a rotational actuator (e.g., of the type described in the above-cited Hornbeck patents) and the voltage applied between the movable and fixed plates. The electrostatic torque T increases approximately quadratically with applied voltage, while the mechanical opposition to torque offered by the hinge increases linearly with deflection angle. As a consequence, the electrostatic torque overcomes the hinge at an angle θu, which typically represents about one third of the initial gap between parallel plates. Upon reaching the unstable angle θu, the movable portion of the parallel-plate actuator “snaps” to a stable state θs defined by a physical stop, or “landing pad.” 
         [0045]    The comb actuators employed in embodiments of the invention offer significant advantages over parallel-plate actuators. For example, the greater torque provided by the comb actuator means that, for comparable deflection angles, comb actuators can employ suspensions with much higher stiffness as compared with parallel-plate actuators. Consequently, fabrication yield, resonance frequencies, response times, insensitivity to vibration and shock, and device reliability are significantly improved. Moreover, the stiffer hinges can be made from materials that resist the fatigue other materials suffer due to repeated flexing, which may improve the useable life of actuators in accordance with the invention. Many variations in hinge dimension and shape (e.g., serpentine) can be used to reduce or otherwise alter hinge stiffness, if desired. 
         [0046]    Comb actuator  100  does not require mechanical stops because the deflection angle is a stable function of the applied voltage and the spring constant of hinge  115 , particularly when the deflection angle is in an area of the response curve (e.g., curve  305  of  FIG. 3E ) at which deflection angle is only very weakly affected by small variations in applied voltage. The ability to operate without mechanical stops is a significant advantage over conventional micro-mirrors that use landing pads to position mirrors in “on” and “off” states and that seek to ameliorate the stiction problem using e.g. landing pads that employ special materials that reduce adhesion and spring arrangements and driving waveforms that overcome stiction. 
         [0047]    Landing pads, such as those passivated by an oriented monolayer, can be included in embodiments of the invention, but are not required. Landing pads are not necessary because the comb actuator has a natural stopping point that depends upon the applied voltage (e.g., voltage Vs of  FIG. 3F ). However, if landing pads are desired, the higher torque and stiffer hinges of the comb actuator advantageously provide greater torque for overcoming stiction forces. 
         [0048]    For bistable operation, the applied voltage V can be selected to produce just two stable states, e.g. such that deflection angle θ at which the driving electrostatic torque equals the restoring torque of hinge  115  corresponds to a desired “on” or “off” state. The number of operational states need not be defined by stops, but can instead be defined using any number of allowed signal combinations applied between the fixed and movable combs. For example, actuator  100  can have the two operational states of  FIGS. 3C and 3D  by limiting the number of signal combinations to the two that produce the depicted “on” and “off” states. In general, actuator  100  can employ N signal combinations to produce N states. 
         [0049]    Returning to  FIG. 1 , hinge  115  extends diagonally across actuator  100 , but might be oriented differently, for example along one edge or across the middle of actuator  100  in parallel with an edge. However, extending hinge  115  diagonally enables a longer and therefore more flexible hinge, and supports the use of teeth of varying length. Configured as shown, the longer teeth begin to interdigitate before the shorter teeth as voltage is applied, with more teeth coming into play as the torque required to twist hinge  115  increases. 
         [0050]    Returning to  FIG. 3E , torque generated between a single moving tooth and two corresponding fixed teeth has three overlapping regions. In the first region, torque increases relatively slowly with applied voltage until the deflection angle at which the movable tooth is lightly interdigitated with corresponding fixed teeth. The torque increases rapidly in the second region with significant interdigitation. In the third region, the torque asymptotically saturates as the interdigitation is completed. In rotational comb actuators that employ teeth of different lengths, these three regions of torque generation occur at different voltages for teeth of different lengths, so the overall actuator responds somewhat linearly to the applied driving voltage. The effect is to produce a more linear actuator response than a similar rotational comb actuator in which all teeth are of similar length. Also desirable, comb actuators with teeth of various lengths exhibit more damping than otherwise similar actuators in which all the teeth are of equal length. 
         [0051]      FIGS. 4A through 4Y  depict a process of fabricating an actuator similar to actuator  100  of  FIGS. 1 ,  2 ,  3 A, and  3 B, like-numbered elements being the same or similar; this process sequence develops along line A-A′ of  FIG. 3A , culminating with a cross section similar to that of  FIG. 3B , like-numbered element being the same or similar. 
         [0052]    The process begins ( FIG. 4A ) with substrate  116 , an ASIC in this example. Substrate  116  includes a number of conductive traces  402 , shown as rectangles, connected to the requisite drive electronics (not shown) within substrate  116 . Conductive vias  403  extend up from traces  402  to the surface of substrate  116 . 
         [0053]    As depicted in  FIG. 4B , the exposed surface of substrate  116  is coated with a silicon nitride layer, with or without an underlying silicon dioxide layer, to produce an insulating layer  404 . The resulting structure is then masked using a photoresist layer  406 , which is patterned to define contact areas  408  ( FIG. 4C ) within which electrodes  111 ,  113 , and  150  will make contact to vias  403 . (The cross-section of  FIGS. 4A-4Y  does not intersect electrode  111 ). Insulating single or double layer  404  is then etched to expose contact areas  408 , leaving the structure of  FIG. 4D . 
         [0054]    Next, a layer of metal  410  is deposited using a conventional metalization process, resulting in the structure of  FIG. 4E . Metal layer  410  is then patterned with photoresist  412  ( FIG. 4F ) to define electrodes  111 ,  113 , and  150 . The exposed portions of metal layer  410  are then etched and photoresist mask  412  is removed, leaving electrodes  111  (not shown),  113 , and  150  ( FIG. 4G ). Metal layer  410  makes contact to underlying vias  403  to communicate with underlying traces  402 . The patterned layer  404  becomes insulating layer  117 , which acts as an etch stop when removing sacrificial material at later stages of fabrication. 
         [0055]    The process sequence depicted in  FIGS. 4H through 4K  defines fixed combs  107  and  109  and hinge posts  155 . Referring first to  FIG. 4H , a layer of highly doped polysilicon  414  is deposited to an appropriate thickness for the height (i.e., normal to plane  170  of  FIG. 1 ) of fixed combs  107  and  109 , two microns in this example. A photoresist mask  416  ( FIG. 4I ) then defines fixed combs  107  and  109 , hinge post  115 , and an alignment pattern (not shown) for alignment of the later-formed movable teeth. 
         [0056]    Layer  414  and the other conductive layers can be formed of materials other than polysilicon. For example, polysilicon-germanium alloys can be deposited and annealed at lower temperatures, potentially allowing for simpler and less expensive ASIC metallization processes. Another alternative is to use metal or metal alloy instead of polysilicon, also allowing lower temperature processing; however, degradation of the mechanical properties of a hinge would occur due to the sensitivity of metals and metal alloys to mechanical fatigue compared with that of polysilicon or single-crystal silicon. 
         [0057]    A silicon deep reactive-ion etch (RIE) removes unmasked portions of polysilicon layer  414 , leaving walls that can be close to normal with respect to the surface of film  414  and with a good aspect ratio. The photoresist mask  416  is then removed, leaving conductive posts  155  and the fixed comb teeth  121  shown in the cross-section of  FIG. 4J . The whole wafer is then coated with a sacrificial material  418 , such as silicon dioxide. For subsequent higher temperature processing, silicon dioxide or another inorganic dielectric is used, while for low temperature processing, photoresist can be used as a sacrificial layer. The resulting structure is then planarized, e.g. by chemical mechanical polishing, to produce the cross section of  FIG. 4K . 
         [0058]    The planarization process removes the topography from the oxide, polysilicon, etc. A suitable method of oxide polishing employs a slurry that consists of a silica-based colloidal suspension in a dilute alkaline solution (a pH of 10-11). The alkaline process hydrolyzes the oxide surface, weakening silicon-oxide bonds. This chemical erosion combines with mechanical erosion to selectively remove relatively high surface features. 
         [0059]    The process sequence of  FIGS. 4H through 4J  is repeated with a different mask sequence to form hinge  115 . (In other embodiments, the bottom portions of movable combs  130  and  135 , the top portions of fixed combs  107  and  109 , or both, are formed at the same time.) First, as depicted in  FIG. 4L , a second layer of highly doped polysilicon  420  is deposited to a depth appropriate for the thickness of hinge  115 , 0.5 microns in this example. Hinge  115 , bridge  140 , and, if desired, the bottom 0.5 microns of movable combs  130  and  135 , the top 0.5 microns of fixed combs  107  and  109 , or both, are patterned on layer  420  with a photoresist mask  422  ( FIG. 4M ). The exposed polysilicon is then etched away, using an RIE, before removing photoresist mask  422 . The resulting structure is depicted in  FIG. 4N , in which the cross-section includes a portion of hinge  115 . 
         [0060]    Next, the process sequence of  FIGS. 4H through 4K  is repeated with different masks to form movable combs  130  and  135  and bridge  140 . First, as depicted in  FIG. 4O , a third layer of highly doped polysilicon  426  is deposited to a depth appropriate for the thickness of movable combs  130  and  135 , 1.5 microns in this example. Windows (not shown) are then opened in layer  426  to expose the alignment features in layer  414 . Movable combs  130  and  135  and bridge  140  are patterned on layer  426  with a photoresist mask  428  ( FIG. 4P ). 
         [0061]    The exposed polysilicon is then etched away, using an RIE, before removing the photoresist mask. The resulting structure, including portions of bridge  140  and movable teeth  165 , is depicted in  FIG. 4Q . The structure is then coated with a sacrificial material  430  and planarized ( FIG. 4R ) in the manner discussed above in connection with  FIG. 4K . 
         [0062]      FIG. 4S  depicts the first step in forming actuated member  177 . First, a fourth highly doped polysilicon layer  432  is deposited, to a depth of approximately 0.5 microns in this embodiment. The resulting structure is annealed at about 1,000 to 1,100 degrees Centigrade. Next, layer  432  is smoothed to a mirror finish using chemical mechanical polishing techniques commonly applied to polysilicon (see for e.g. A. A. Yaseen, et al, J. Electrochem. Soc. 144, 237-242, 1997). In one embodiment, this polishing step leaves a surface  434  (FIG.  4 T) having an approximate RMS roughness of less then 0.5 nm. If the initial surface finish of layer  432  is adequate, the polishing step can be skipped. The resulting polished polysilicon layer  432  is slightly thinned (by approximately 10% of the initial thickness). A reflective layer  436  is then formed over surface  434  ( FIG. 4U ). Layer  436  can be a single or compound layer, and is formed in one embodiment by depositing first a chromium adhesion layer and then a reflective gold or aluminum layer. 
         [0063]    The sequence of  FIGS. 4A through 4U  depicts the formation of a single actuator  100 . However, arrays of such actuators will normally be formed together, as discussed above in connection with  FIG. 2 , for example.  FIGS. 4V through 4X  and the associated discussion illustrate how individual mirrors are separated in accordance with a multiple-mirror embodiment. 
         [0064]    First, a photoresist layer  440 , formed over the total array surface, is patterned to define the mirror surfaces ( FIG. 4V ). The exposed portions of reflective layer  436  are then removed, leaving metal layer  436  patterned as an array of mirrors ( FIG. 4W ). What remains of metal layer  436  then masks the underlying polysilicon layer  432  during a dry RIE process that removes portions of layer  432  to separate the actuated members of the array ( FIG. 4X ). Finally, a silicon-dioxide dielectric etch, using wet or vapor hydrofluoric acid, for example, removes the remaining material of sacrificial layers  430  and  418 ; nitride insulating layer  117  acts as an etch stop. The wet structure is then carefully rinsed and dried. A suitable drying process is described in “Supercritical Carbon Dioxide Solvent Extraction From Surface-Micromachined Micromechanical Structures,” by C. W. Dyck, et al. (SPIE Micromachining and Microfabrication, October 1996), which is incorporated herein by reference. The resulting structure, depicted in  FIG. 4Y , is similar to that of  FIG. 3B , like-numbered elements being the same or similar. 
         [0065]    When monolithic fabrication with actuators built directly on top of driving electronics is used, polysilicon annealing is performed after all metallization steps, except mirror coating, and the interconnects provided for the metallization and driving electronics are of materials, such as tungsten, that exhibit high melting temperatures. When polysilicon-germanium, metal, or metal alloys are used for structural members of the actuators, annealing temperatures are lower and conventional metallization of CMOS and vias with aluminum or copper is possible. 
         [0066]      FIG. 5  depicts an actuator  500  in accordance with another embodiment. Actuator  500  is in many ways similar to actuator  100  of  FIG. 1 , like-numbered elements being the same or similar. Actuator  500  differs from actuator  100 , however, in that actuator  500  employs a translational comb drive in place of the rotational comb drive of actuator  100 . The hinge and relating support members are adapted, in this embodiment, to convert from translational to rotational motion. Some elements (e.g., address electrodes) are omitted from  FIG. 5  for ease of illustration. 
         [0067]    Actuator  500  includes a pair of fixed combs  505  and  510  mounted on substrate  116 , each comb including a number of fixed teeth  515 . Actuator  500  also includes a movable comb  520  having two sets of movable teeth  525  that interdigitate with fixed teeth  515  (comb  520  might be considered two combs connected back-to-back). Fixed combs  505  and  510  electrically connect to respective addressing electrodes (not shown) and movable comb  520  connects to another electrode (also not shown) via conductive hinges  522  and anchors  523  so that a potential can be applied between the fixed and movable combs. When applied, such potentials cause movable comb  520  to move translationally in the plane of teeth  515  and in a direction perpendicular to the fulcrum axis  530  of hinge  115 . 
         [0068]    Movable comb  520  connects to hinge  115  via a pair of hinges  535  and a vertical bridge  540  that together convert the translational movement of movable comb  520  into a twisting movement of hinge  115 . (This aspect of actuator  500  is depicted more clearly in  FIGS. 7A and 7B .) Actuated member  177  connects to hinge  115  via a second bridge  545 , so that actuated member  177  tilts as hinge  115  twists in response to the movement of comb  520 . This embodiment simplifies the important task of aligning the fixed and movable teeth because the coplanar fixed and movable combs can be defined using the same mask but requires one additional layer to be built compared with the rotational actuator. 
         [0069]    Many variations in hinge dimension and shape (e.g., coil or serpentine) can be used to reduce stiffness if desired. Moreover, additional process steps can be employed to alter the thickness of hinges  522  and  535  relative to comb  520 . Hinges  522  and  535  are sufficiently stiff in a direction parallel to fulcrum axis  530  to prevent movable comb  520  from contacting either of fixed combs  505  or  510 . 
         [0070]      FIG. 6A  is a top view of actuator  500  of  FIG. 5 : actuated member  177  is removed to expose the underlying features.  FIGS. 6B and 6C  are cross-sectional views of actuator  500  taken along lines A-A′ and B-B′, respectively, including actuated member  177 . As evident in  FIGS. 6B and 6C , anchors  523  hold movable comb  520  and associated hinges  522  above substrate  116  so that comb  520  moves freely, without rubbing against substrate  116 . The space is created e.g. using a sacrificial oxide layer in the manner discussed above in connection with  FIGS. 4A-4Z . 
         [0071]      FIGS. 7A and 7B  are cross-sectional views of actuator  500  taken along line B-B of  FIG. 6A  with voltage applied between the movable and fixed combs to induce translational motion. In  FIG. 7A , translating movable comb  520  to the right tilts the surface of actuated member  177  θ degrees to the left, where θ is typically about ten degrees; in  FIG. 7B , translating movable comb  520  to the left tilts the surface of actuated member  177  θ degrees to the right. The spacing between the bottom of bridge  540  and the surface of substrate  116  is sufficient to prevent contact between the two. In other embodiments, the bottom of bridge  540  may be modified to provide a mechanical stop. 
         [0072]      FIG. 8  depicts a mirror array  800  in accordance with one embodiment incorporated into an otherwise conventional projection-display system  805 . The display system includes a lamp  810  focusing white light through a color wheel  815  onto mirror array  800 . Mirror array  800  selectively reflects portions of the resulting colored light onto a display surface  820  via a projection lens  825 . 
         [0073]    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:
       a. 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   b. Ser. No. 09/981,628, entitled “Micro-Opto-Electro-Mechanical Switching System,” by Vlad J. Novotny et al., filed Oct. 15, 2001.       
 
         [0076]    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, actuators in accordance with embodiments of the invention can be used as optical switches in fiber-optical systems, and the mirrors can be replaced by other light-modulating surfaces, such as refractive lenses, diffraction gratings or thin-film stacks and materials can differ from polysilicon. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.