Abstract:
A spatial light modulator having a substrate holding an array of deflectable (e.g. mirror) elements. The deflectable elements are deflectably coupled to the substrate via corresponding hinges, each hinge being disposed on a side of the deflectable element opposite to the side on which the substrate is disposed. By placing the hinge in this way the fill factor of the array is improved. The hinge can be provided flush against the deflectable element, or it can be provided with a gap between the deflectable element and the hinge. The hinge can be attached via one or more posts or walls connecting to the substrate, and with a flexible or deformable portion that is substantially or entirely hidden from view when viewed through the substrate (e.g. a glass substrate). In one embodiment, the hinge is connected to the undersides of both the substrate and the deflectable element, and connects towards a center part of the deflectable element. In this way, a longer hinge is provided thus reducing strain on any one part of the hinge. Advantages of the present invention include: (1) increased fill factor as the torsion hinge is hidden behind the reflective plate; (2) increased contrast due to fewer scattering optical surfaces exposed, and due to a greater ability to control their angle and geometry; and (3) increased geometric flexibility to optimize electro-mechanical performance and robustness with respect to manufacturing.

Description:
RELATED CASES 
     The instant application is a continuation-in-part of U.S. patent application Ser. No. 09/437,586, (now U.S. Pat. No. 6,172,797) filed on Nov. 9, 1999, entitled “A Double Substrate Reflective Spatial Light Modulator with Self-Limiting Micro-Mechanical Elements,” by Huibers, which is a continuation of U.S. patent application Ser. No. 09/160,361 (now U.S. Pat. No. 6,046,840) filed on Sep. 24, 1998, of the same title, the subject matter of each being incorporated herein by reference. This application is also related to provisional application No. 60/178,903, filed Jan. 28, 2000, entitled “Structure and Process for Spatial Light Modulator: Designs Using Two Sacrificial Layers,” by Huibers, the subject matter of which is also incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains generally to the field of micro-electromechanical systems (MEMS). More specifically, the present invention pertains to the field of MEMS spatial light modulators, and systems, such as display projection systems, printing systems, and light beam switching systems that utilize MEMS spatial light modulators. 
     BACKGROUND OF THE INVENTION 
     Spatial light modulators (SLMs) are transducers that modulate an incident beam of light in a spatial pattern that corresponds to an optical or electrical input. The incident light beam may be modulated in phase, intensity, polarization, or direction. This modulation may be accomplished through the use of a variety of materials exhibiting magneto-optic, electro-optic, or elastic properties. SLMs have many applications, including optical information processing, display systems, and electrostatic printing. 
     An early SLM designed for use in a projection display system is described by Nathanson, U.S. Pat. No. 3,746,911. The individual pixels of the SLM are addressed via a scanning electron beam as in a conventional direct-view cathode ray tube (CRT). Instead of exciting a phosphor, the electron beam charges deflectable reflective elements arrayed on a quartz faceplate. Elements that are charged bend towards the faceplate due to electrostatic forces. Bent and unbent elements reflect parallel incident light beams in different directions. Light reflected from unbent elements is blocked with a set of Schlieren stops, while light from bent elements is allowed to pass through projection optics and form an image on a screen. Another electron-beam-addressed SLM is the Eidophor, described in E. Baumann, “The Fischer large-screen projection system (Eidophor)” 20 J.SMPTE 351 (1953). In that system, the active optical element is an oil film, which is periodically dimpled by the electron beam so as to diffract incident light. A disadvantage of the Eidophor system is that the oil film is polymerized by constant electron bombardment and oil vapors result in a short cathode lifetime. A disadvantage of both of these systems is their the use of bulky and expensive vacuum tubes. 
     A SLM in which movable elements are addressed via electrical circuitry on a silicon substrate is described in K. Peterson, “Micromechanical Light Modulator Array Fabricated on Silicon” 31 Appl. Phys. Let. 521 (1977). This SLM contains a 16 by 1 array of cantilever mirrors above a silicon substrate. The mirrors are made of silicon dioxide and have a reflective metal coating. The space below the mirrors is created by etching away silicon via a KOH etch. The mirrors are deflected by electrostatic attraction: a voltage bias is applied between the reflective elements and the substrate and generates an electrostatic force. A similar SLM incorporating a two-dimensional array is described by Hartstein and Peterson, U.S. Pat. No. 4,229,732. Although the switching voltage of this SLM is lowered by connecting the deflectable mirror elements at only one corner, the device has low light efficiency due to the small fractional active area. In addition, diffraction from the addressing circuitry lowers the contrast ratio (modulation depth) of the display. 
     Another SLM design is the Grating Light Value (GLV) described by Bloom, et. al., U.S. Pat. No. 5,311,360. The GLV&#39;s deflectable mechanical elements are reflective flat beams or ribbons. Light reflects from both the ribbons and the substrate. If the distance between surface of the reflective ribbons and the reflective substrate is one-half of a wavelength, light reflected from the two surfaces adds constructively and the device acts like a mirror. If this distance is one-quarter of a wavelength, light directly reflected from the two surfaces will interfere destructively and the device will act as a diffraction grating, sending light into diffracted orders. Construction of the GLV differs substantially from the DMD. Instead of using active semiconductor circuitry at each pixel location, the approach in the &#39;360 patent relies on an inherent electromechanical bistability to implement a passive addressing scheme. The bistability exists because the mechanical force required for deflection is roughly linear, whereas the electrostatic force obeys an inverse square law. As a voltage bias is applied, the ribbons deflect. When the ribbons are deflected past a certain point, the restoring mechanical force can no longer balance the electrostatic force and the ribbons snap to the substrate. The voltage must be lowered substantially below the snapping voltage in order for the ribbons to return to their undeflected position. This latching action allows driver circuitry to be placed off-chip or only at the periphery. Thus addressing circuitry does not occupy the optically active part of the array. In addition, ceramic films of high mechanical quality, such as LPCVD (low pressure chemical vapor deposition) silicon nitride, can be used to form the ribbons. However, there are several difficulties with the GLV. One problem is stiction: since the underside of the deflected ribbons contacts the substrate with a large surface area, the ribbons tend to stick to the substrate. Another problem is that a passive addressing scheme might not be able to provide high frame rates (the rate at which the entire SLM field is updated). In addition, with a passive addressing scheme, the ribbons deflect slightly even when off. This reduces the achievable contrast ratio. Also, even though the device is substantially planar, light is scattered, as in the DMD, from areas between the pixels, further reducing the contrast ratio. 
     Another diffraction-based SLM is the Microdisplay, described in P. Alvelda, “High-Efficiency Color Microdisplays” 307 SID 95 Digest. That SLM uses a liquid crystal layer on top of electrodes arrayed in a grating pattern. Pixels can be turned on and off by applying appropriate voltages to alternating electrodes. The device is actively addressed and potentially has a better contrast ratio than the GLV. However, the device, being based on the birefringence of liquid crystals, requires polarized light, reducing its optical efficiency. Furthermore, the response time of liquid crystals is slow. Thus, to achieve color, three devices—one dedicated for each of the primary colors—must be used in parallel. This arrangement leads to expensive optical systems. 
     A silicon-based micro-mechanical SLM with a large fractional optically active area is the Digital Mirror Device (DMD), developed by Texas Instruments and described by Hornbeck, U.S. Pat. No. 5,216,537 and other references. One of the implementations includes a square aluminum plate suspended via torsion hinges above addressing electrodes. A second aluminum plate is built on top of the first and is used as mirror. Although increasing manufacturing complexity, the double plate aluminum structure is required to provide a reasonably flat mirror surface and cover the underlying circuitry and hinge mechanism. This is essential in order to achieve an acceptable contrast ratio. The entire aluminum structure is released via oxygen plasma etching of a polymer sacrificial layer. Aluminum can be deposited at low temperatures, avoiding damage to the underlying CMOS addressing circuitry. However, the hinges attaching the mirrors to the substrate are also made of aluminum, which is very susceptible to fatigue and plastic deformation. 
     Therefore, what is needed is a spatial light modulator that has a high resolution, a high fill factor and a high contrast ratio. What is further needed is a spatial light modulator that does not require polarized light, hence is optically efficient, and that is mechanically robust. 
     SUMMARY OF THE DISCLOSURE 
     Accordingly, the present invention provides a spatial light modulator that has a higher resolution and an increased fill factor. The present invention also provides a spatial light modulator that has an increased contrast ratio. The present invention further provides a spatial light modulator that operates in the absence of polarized light and that has improved electromechanical performance and robustness with respect to manufacturing. 
     According to one embodiment of the present invention, the spatial light modulator has an optically transmissive substrate and a semiconductor substrate. An array of reflective elements are suspended from underneath the optically transmissive substrate, and are positioned directly across from the semiconductor substrate. The semiconductor substrate includes an array of electrodes and electronic circuitry for selectively deflecting individual reflective elements by electrostatic force. In operation, as individual reflective element deflects, light beams that are incident to and reflected back through the optically transmissive substrate are spatially modulated. 
     In accordance with one embodiment, each reflective element has a front surface that faces the optically transmissive substrate and a back surface that faces the semiconductor substrate. Each reflective element is deflectably attached to the optically transmissive substrate by means of a mirror support structure. The mirror support structure includes one or more contact points that are attached (directly or indirectly) to the optically transmissive substrate. The mirror support structure also includes a torsion hinge that extends across the back surface of the reflective element, attaching thereto at one or more places. 
     The mirror support structure of one embodiment is reinforced with deflection stoppers configured for resisting deflection of the reflective element beyond a pre-determined tilt angle. Specifically, the deflection stoppers are configured such that, when the reflective element is deflected to the pre-determined tilt angle, the reflective element can come into contact with the deflection stoppers. In addition, one end of the reflective element will come into contact with the optically transmissive substrate. In this way, mechanical robustness of the mirror support structure is significantly improved. Moreover, contrast of the spatial light modulator is increased due to a greater ability to control the tilt angle of the reflective elements. The reflective element of the present embodiment may also include bump(s) positioned along a substrate-touching edge such that the area of contact between the reflective element and the substrate is reduced. 
     In furtherance of the present invention, one embodiment of the mirror support structure includes an attraction electrode that is attached to the back surface of the reflective element. When a voltage bias is applied between the attraction electrode and a corresponding actuating electrode on the semiconductor substrate, the attraction electrode will be pulled towards the actuating electrode, causing the reflective element to deflect. In one embodiment, the mirror support structure and the attraction electrode are composed of a same conductive laminate. Therefore, the reflective element needs not be conductive (though the reflective element, in another embodiment, can be conductive and act as the electrode). Consequently, mechanical and reflective properties of the reflective element can be optimized without regard to conductivity. Fabrication flexibility is also increased because the present embodiment does not require a metal coating step after sacrificial silicon layers are removed. 
     Embodiments of the present invention include the above and further include a spatial light modulator fabrication process. In one embodiment, the process includes the steps of: (a) depositing a sacrificial (e.g. silicon) layer on an optically transmissive substrate; (b) depositing a reflective laminate on the sacrificial layer; (c) pattern-etching the reflective laminate to define a reflective element; (d) depositing another sacrificial (e.g. silicon) layer; (e) pattern-etching the second sacrificial layer to expose a portion of the reflective element; (f) etching a pattern of holes through the sacrificial layers such that subsequent layers can be attached to the optically transmissive substrate via the holes; (g) depositing a hinge-electrode laminate layer on the second sacrificial layer and on the exposed portion of the reflective element; (h) pattern-etching the hinge-electrode laminate to define a hinge-electrode that is attached to the optically transmissive substrate through the holes and that is attached to the exposed portion of the reflective element; (i) etching the first sacrificial layer and the second sacrificial layer to release the reflective element; (j) forming addressing circuitry and electrodes on a semiconductor substrate; and (k) aligning and joining the optically transmissive substrate and the semiconductor substrate. 
     In cross section, the spatial light modulator has an optically transmissive substrate, a first gap below the optically transmissive substrate, a deflectable element below the first gap, a second gap below the deflectable element, a hinge below the second gap, a third gap below the hinge, and a second (e.g. circuit) substrate below the third gap. The hinge is substantially entirely blocked from view by the deflectable element (when viewing through the optically transmissive substrate). As such, the hinge is disposed on a side of the deflectable element opposite to that of the optically transmissive substrate. The hinge is connected to the bottom surface of the deflectable element (not on the edges of the deflectable element in most cases). Posts or walls can be provided which extend from the hinge to the optically transmissive substrate. The hinge can extend across the middle of the deflectable element, with the same area of deflectable element on either side (or the hinge could divide the deflectable element in other ways, e.g. ⅓ on one side and ⅔ on the other). With some deflectable element extending on either side of the hinge, movement of one side of the deflectable element in one direction results in movement of the other side of the deflectable element in the other direction. 
     The hinge can also be provided flush against the deflectable element (though still with the deflectable element between the hinge and the optically transmissive substrate). Preferably, however, the hinge is connected to a center portion of the deflectable element so as to allow for an elongated hinge (thus reducing flexing, torqueing and/or stress to any one part of the hinge). The deflectable element can be provided with a laminate support structure which can comprise multiple layers of dielectric material. Also, the deflectable element can comprise a layer which is both reflective and conductive (e.g. a metal layer such as gold or aluminum) or separate reflective and conductive layers. The deflectable element and hinge can be formed by LPCVD deposition, whereas the circuit substrate utilized for actuating the deflectable element can be formed using standard VLSI/CMOS processes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 illustrates a deflectable micro-mirror structure that may be incorporated as part of a spatial light modulator in accordance with one embodiment of the present invention. 
     FIG. 2 illustrates an exploded view of the deflectable micro-mirror structure of FIG.  1 . 
     FIGS. 3A and 3B illustrate a deflection stopping mechanism of the micro-mirror structure of FIG. 1 in accordance with one embodiment of the present invention. 
     FIG. 4 is a flow diagram illustrating steps of a fabrication process for producing micro-mirror structures according to one embodiment of the present invention. 
     FIG. 5A illustrates an isometric perspective view of a portion of a substrate after a mirror patterning step of the fabrication process of FIG.  4 . 
     FIG. 5B illustrates an isometric perspective view of a portion of a substrate after an etching step of the fabrication process of FIG.  4 . 
     FIG. 5C illustrates an isometric perspective view of a portion of a substrate after another etching step of the fabrication process of FIG.  4 . 
     FIG. 6 illustrates an isometric perspective view of a small section of an exemplary mirror array that includes a micro-mirror structure of FIG. 1 according to one embodiment of the present invention. 
     FIG. 7A illustrates an isometric perspective view of a deflectable micro-mirror structure in accordance with another embodiment of the present invention. 
     FIG. 7B illustrates a side view of the deflectable micro-mirror structure of FIG.  7 A. 
     FIG. 8 illustrates an isometric perspective view of a small section of an exemplary mirror array that includes a micro-mirror structure of FIG. 7A according to one embodiment of the present invention. 
     FIG. 9 illustrates an isometric perspective view of a deflectable micro-mirror structure in accordance with yet another embodiment of the present invention. 
     FIG. 10 illustrates an isometric perspective view of a small section of an exemplary mirror array that includes a micro-mirror structure of FIG. 9 according to one embodiment of the present invention. 
     FIG. 11 illustrates an isometric perspective view of a deflectable micro-mirror structure in accordance with yet another embodiment of the present invention. 
     FIG. 12 illustrates an embodiment of the invention where deflection stopping mechanisms are provided separate from the post and hinge assembly. 
     FIG. 13 illustrates an embodiment similar to that shown in FIG. 12, with leaf hinges instead of torsion hinges. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, 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 structures and devices are not described in detail in order to avoid obscuring aspects of the present invention. 
     Accordingly, the present invention provides a SLM structure that has an improved fill factor (e.g., ratio between reflective areas and non-reflective areas). The present invention also provides a deflectable micro-mirror structure that does not require the mirror plate to be conductive. The present invention further provides a micro-mirror structure that is mechanically robust and easy to manufacture. These and other advantages of the present invention would become more apparent in the description below. 
     FIG. 1 illustrates a deflectable micro-mirror structure  100  in accordance with one embodiment of the present invention. FIG. 2 illustrates an exploded view of the deflectable micro-mirror structure  100 . It should be appreciated that micro-mirror structure  100  is part of a mirror array. However, for simplicity, other micro-mirror structures of the array are not shown FIG.  1 . It should be appreciated that the number of mirror structures within a mirror array may be very large. For example, in a typical SLM implementation having 1024×768 pixels, the mirror array may have more than seven hundred fifty thousand micro-mirror structures. Additionally, it should be noted that the semiconductor substrate containing electronic circuitry for actuating the micro-mirror structure  100  is not illustrated in FIGS. 1 and 2 to avoid obscuring aspects of the present embodiment. 
     In the embodiment as shown in FIGS. 1 and 2, a mirror plate  120  of micro-mirror structure  100  is suspended above, and deflectably coupled to, optically transmissive substrate  110  by means of a mirror support structure  130 . Mirror plate  120  has a reflective front surface that faces the optically transmissive substrate  110 , and a back surface that faces the semiconductor substrate. In one embodiment, mirror plate  120  is substantially rigid and may be made up of a laminate having layers of silicon nitride and aluminum. 
     With reference still to FIGS. 1 and 2, mirror support structure  130  includes two hinge supports  136  and  138  attached to the optically transmissive substrate  110 . The mirror support structure  130  also includes a torsion hinge  134  that extends across and attaches to the back surface of mirror plate  120 . Also attached to the back surface of mirror plate  120  is an electrode  132 . In the embodiment as illustrated, electrode  132  is electrically conductive and is connected to torsion hinge  134  via a support  137 . Support  137 , as shown, is shorter than hinge supports  136  and  138  such that mirror plate  120  is spaced apart from substrate  110  when undeflected. 
     Also illustrated in FIGS. 1 and 2 are deflection stoppers  131  and  133  that are configured for resisting deflection of mirror plate  120  beyond a pre-determined tilt angle. Particularly, when mirror plate  120  is deflected to the pre-determined tilt angle, part of mirror plate  120  will come into contact with deflection stoppers  131  and  133 . In addition, in the present embodiment, micro-mirror structure  100  is configured such that, when the deflecting mirror plate  120  comes into contact with deflection stoppers  131  and  133 , the mirror plate  120  will also come into contact with optically transmissive substrate  110 . In the illustrated embodiment, mirror plate  120  includes bumps  122  and  124  positioned along the substrate-touching edge such that the area of contact between mirror plate  120  and substrate  110  is reduced. 
     FIGS. 3A and 3B illustrate a deflection stopping mechanism of micro-mirror structure  100  in accordance with one embodiment of the present invention. FIG. 3A illustrates a side-view of micro-mirror structure  100  when mirror plate  120  is undeflected. FIG. 3B illustrates a side-view of micro-mirror structure  100  when mirror plate  120  is deflected to a pre-determined tilt angle, θ. Also illustrated are semiconductor substrate  310  and one or more attraction electrodes  320  for deflecting mirror plate  120 . 
     As shown in FIG. 3A, when undeflected, mirror plate  120  is supported above optically transmissive substrate  110 . However, in FIG. 3B, when a voltage bias between attraction electrode  320  and electrode  130  is applied, an electrostatic force F is generated, causing mirror plate  120  to deflect towards semiconductor substrate  310 . Deflection of mirror plate  120  is stopped at a pre-determined tilt angle, or deflection angle, θ. As illustrated in FIG. 3B, mirror plate  120  comes into contact with deflection stoppers  131 / 133 , and bumps  122 / 124  come into contact with substrate  110  at tilt angle θ. 
     In one embodiment, the tilt angle θ at which bumps  122 / 124  come into contact with substrate  110  is approximately 15°. However, it should be appreciated that the tilt angle θ is dependent on the geometry and dimensions of the micro-mirror structure and that many other tilt angles are within the scope of the present invention. Further, it should be noted that the angle at which mirror plate  120  comes into contact with deflection stoppers  131 / 133  may be slightly different from the angle at which bumps  122 / 124  come into contact with substrate  110 . For instance, in another embodiment of the present invention, deflection stoppers may be used as a safeguard against excessive stretching of the torsion hinge. Therefore, in that embodiment, the angle at which the mirror plate comes into contact with the deflection stoppers may be slightly larger than the angle at which the mirror plate comes into contact with the substrate. Or, only the deflection stoppers could be used to stop the movement of the mirror plate. Therefore, it can be seen that preferably there are two stopping mechanisms (e.g. the stop against the substrate, and the stop against the deflection stopper). The deflectable element can abut against both stopping mechanisms at the same time, or one of the stopping mechanisms can be a “back-up” stopping mechanism, and be constructed to stop the deflectable element at an angle of deflection greater than the main stopping mechanism (e.g. in the case where the main mechanism fails, the deflectable element changes shape over time, etc.). 
     A small section of an exemplary mirror array  600  according to one embodiment of the present invention is shown in FIG.  6 . As illustrated, because the torsion hinges are attached to the back surface of the mirror plates, the fill factor of the array  600  is very high. Almost the entire surface of the optically transmissive substrate  610  can be covered with reflective surfaces. 
     FIG. 4 is a flow diagram illustrating steps of a fabrication process  400  for producing micro-mirror structures according to one embodiment of the present invention. In the present embodiment, micro-mirror structures (e.g., structure  100 ) are formed on top of an optically transmissive substrate (e.g., substrate  110 ), which is made from glass or other materials that can withstand subsequent processing temperatures. 
     At shown in FIG. 4, at step  410 , a first sacrificial layer (of e.g. silicon) is deposited on the optically transmissive substrate. In the present embodiment, the first sacrificial layer of silicon is approximately 5000A to 8000A (or even over 20,000A) thick. Other sacrificial material (e.g. polymers) other than silicon could be used. 
     At step  420 , mirror laminate is deposited on the first sacrificial layer. In one embodiment, the mirror laminate includes a layer of aluminum sandwiched by two layers of silicon nitride. In other embodiments, the mirror laminate may include only a layer of aluminum and a layer of silicon nitride. Or, a multi-layer arrangement with multiple layers of aluminum and/or silicon nitride could be used. Other materials besides aluminum (such as other conductive and reflective metals) could be used. And, other materials besides silicon nitride are envisioned (e.g. silicon dioxide). In a typical implementation, each silicon nitride layer is approximately 1400A thick, and the aluminum layer is approximately 700A thick. Further, to enhance the reflectivity of the mirror laminate, one or more dielectric films that act as a reflective coating may be deposited on the mirror laminate. 
     At step  430 , the mirror laminate is patterned to define a mirror plate. An isometric perspective view of a portion of a substrate after step  430  is illustrated in FIG.  5 A. Particularly, an optically transmissive substrate  511 , a sacrificial layer  512  and a mirror plate  513  are shown. It should also be noted that, in a typical SLM implementation in accordance with the present invention, an entire array of micro-mirrors are fabricated at the same time. For simplicity, other mirror plates that are formed on the substrate  511  are not illustrated. 
     With reference again to FIG. 4, at step  440 , a second sacrificial silicon layer is deposited on top of the first sacrificial silicon layer and the mirror plate. In one embodiment, the second sacrificial silicon layer is approximately 2500-5000A thick. 
     Then, at step  450 , the sacrificial silicon layers are pattern etched to expose a portion of the mirror plate and to create two holes that allow subsequent layers to be attached to the optically transmissive substrate. An isometric perspective view of a portion of substrate  511  after the etching step  450  is illustrated in FIG.  5 B. In particular, optically transmissive substrate  511 , sacrificial layer  512 , mirror plate  513 , and another sacrificial layer  514  that is patterned to expose a portion of the mirror  513  are shown. Also illustrated are two holes  516  and  518  that are also pattern-etched into the sacrificial layers  512  and  514  such that subsequent layers can be deposited through holes  516  and  518  onto optically transmissive substrate  511 . 
     In the embodiment as illustrated in FIG. 5B, a substantial portion of the mirror plate  513  is exposed such that subsequent layers can be attached thereon. However, it should be noted that it is optional to expose a substantial portion of the mirror plate  513 . In another embodiment, several openings may be pattern etched through the second sacrificial layer such that subsequent layers can be attached to the mirror plate via the openings. 
     At step  460 , a hinge-electrode laminate is deposited on top of the second sacrificial layer. Particularly, in one embodiment, the hinge-electrode laminate covers the exposed portion of the mirror plate and portions of the optically transmissive substrate through the holes formed at step  450 . In the present embodiment, the hinge-electrode laminate includes a 500A layer of silicon nitride and a 500A layer of aluminum. Other metals may be substituted for the aluminum, such as titanium or titanium nitride. Other laminate materials that can function as conductors and have good mechanical properties may also be used. Other metals, which are both conductive and reflective, could be formed. Or, a conductive layer could be made out of metal, and a separate metal or non-metal reflective layer or layers (e.g. two layers with different indices of refraction) could be formed. 
     At step  470 , the hinge-electrode laminate is etched to define a mirror support structure. An isometric perspective view of a portion of substrate  511  after the etching step  470  is illustrated in FIG.  5 C. Optically transmissive substrate  511 , sacrificial layer  512 , mirror plate  513  (not exposed), another sacrificial layer  514 , and mirror support structure  515  are shown in FIG.  5 C. Mirror support structure  515  as shown has an electrode portion that is attached to the mirror plate  513 , and a mirror support structure that is attached to the optically transmissive substrate  511 . 
     At step  480 , the sacrificial silicon layers are etched away to release the mirror plate. The resulting micro-mirror structure is similar to micro-structure  100 , and is ready to be sandwiched with a semiconductor substrate having electrodes and electronic circuitry therein to form a light valve device. The process for forming the semiconductor substrates for actuation of the micro-mirror structure is described in U.S. Pat. No. 5,835,256 and co-pending application 09/160,361, which are incorporated by reference, and is therefore not discussed herein to avoid obscuring aspects of the present invention. 
     FIG. 7A illustrates an isometric perspective view of a deflectable micro-mirror structure  700  in accordance with another embodiment of the present invention. FIG. 7B illustrates a side view of the deflectable micro-mirror structure  700 . Deflectable micro-mirror structure  700  can be fabricated by a process similar to process  400 . It should be noted that micro-mirror structure  700  is typically fabricated as part of a mirror array that may have many mirrors. For simplicity, other mirror structures of the mirror array are shown in FIGS. 7A and 7B. The mirror array may be made up of a large number of micro-mirror structures. A small section of an exemplary mirror array  800  according to the present embodiment is shown in FIG.  8 . 
     In the present embodiment, a mirror plate  720  is suspended above, and deflectably coupled to an optically transmissive substrate  710  by means of a mirror support structure. As shown in FIG. 7A, the mirror support structure includes a torsion hinge  734  that extends diagonally across two corners of mirror plate  720 . Further, torsion hinge  734  is connected to electrodes  732   a  and  732   b  that are symmetrical about the torsion hinge  734 . Mirror plate  720  is attached to electrodes  732   a  and  732   b.    
     With reference to FIG. 7B, electrodes  732   a  and  732   b  are aligned with electrodes  750   a  and  750   b , respectively, of semiconductor substrate  740 . According to the present embodiment, electrodes  732   a  and  732   b  are held at a constant voltage. In order to deflect mirror plate  720 , a voltage bias can be applied to electrode  750   a  to pull the electrode  732   a  towards the electrode  750   a , or a voltage bias can be applied to electrode  750   b  for pulling the electrode  732   b  towards the electrode  750   b.    
     Deflection of the mirror plate  720  is stopped when one corner of the mirror plate  720  comes into contact with the optically transmissive substrate  710 . In addition, the mirror plate  720  can come into contact with the torsion hinge  734 . Thus, in the embodiment as shown in FIGS. 7A and 7B, the micro-mirror structure  700  is configured for resisting the deflection of the mirror plate  720  beyond a certain pre-determined tilt angle. 
     FIG. 9 illustrates an isometric perspective view of a deflectable micro-mirror structure  900  in accordance with yet another embodiment of the present invention. Micro-mirror structure  900  is typically fabricated as part of a mirror array having a large number of mirrors. A small section of an exemplary micro-mirror array  1000  according to one embodiment is shown in FIG.  10 . 
     With reference to FIG. 9, a mirror plate  920  is suspended above, and deflectably coupled to, optically transmissive substrate  910  by means of a mirror support structure  930 . Mirror support structure  930  includes a torsion hinge  934  that extends across two corners of mirror plate  920 . Torsion hinge  934 , as shown, is attached to mirror plate  920  by means of support  937 . Also illustrated in FIG. 9 is an electrode  932  that is electrically connected to and is co-planar with torsion hinge  934 . Electrode  932  is attached to mirror plate  920  by means of supports  936 . 
     FIG. 11 illustrates a deflectable micro-mirror structure  1100  in accordance with yet another embodiment of the present invention. In the embodiment as shown in FIG. 11, a mirror plate  1120  of micro-mirror structure  1100  is suspended above, and deflectably coupled to, optically transmissive substrate  1110  by means of a mirror support structure  1130 . Mirror plate  1120  has a reflective front surface that faces optically transmissive substrate  1110 , and a back surface that faces the actuating circuitry substrate. In one embodiment, mirror plate  1120  is substantially rigid and may be made up of a laminate having layers of silicon nitride and aluminum. 
     With reference still to FIG. 11, mirror support structure  1130  includes two hinge supports  1136  and  1138  attached to the optically transmissive substrate  1110 . The mirror support structure  1130  also includes a torsion hinge  1134  that extends across and attaches to the back surface of mirror plate  1120  by means of support  1141 . Also attached to the back surface of mirror plate  1120  is an electrode  1132 . In the embodiment as illustrated, electrode  1132  is co-planar with torsion hinge  1134 , and is attached to mirror plate  1120  by means of supports  1140 . In the present embodiment, supports  1140  and  1141  are formed by first pattern etching a sacrificial layer of silicon to create holes that reach down to the mirror plate  1120 , and then depositing a hinge-electrode laminate over the holes. Electrode  1142  also includes openings  1132  for facilitating the removal of sacrificial materials that are beneath the electrode  1132  during the fabrication process. 
     Also illustrated in FIG. 11 are deflection stoppers  1131  and  1133  that are configured for resisting deflection of mirror plate  1120  beyond a pre-determined tilt angle. Particularly, when mirror plate  1120  is deflected to a pre-determined tilt angle, part of mirror plate  1120  can come into contact with deflection stoppers  1131  and  1133 . In addition, in the present embodiment, micro-mirror structure  1100  can be configured such that, when mirror plate  1120  comes into contact with deflection stoppers  1131  and  1133 , one edge of mirror plate  1120  will come into contact with optically transmissive substrate  1110 . In the illustrated embodiment, mirror plate  1120  includes bump  1122  positioned along the substrate-touching edge such that the area of contact between mirror plate  1120  and substrate  1110  is reduced, thus reducing contact forces. 
     In the embodiments shown, there are dual stopping mechanisms whereby the deflectable element is stopped by two different types or abutments. In one example, the deflectable element abuts against the optically transmissive substrate at one or more locations. In addition, the deflectable element abuts against a post and hinge assembly (whether before, after, or at the same time as abutting against the optically a transmissive substrate). However, it should be noted that the two stopping mechanisms need not be provided together. A single stopping mechanism, where a portion of the hinge and post assembly stops the pivoting of the deflectable element, can be sufficient. Or, a post and lip assembly separate from the hinge can alone stop the deflectable element, or be used as a backup stop to (or together with) the abutment of the deflectable element against the optically transmissive substrate. 
     As can be seen in FIG. 12, deflectable element  1202  can be pivotably held on optically transmissive substrate  1200  by means of posts  1204 ,  1205  and hinges  1206 ,  1207 . As can be seen in FIG. 12, deflection stoppers  1210 ,  1212  are not provided as part of the post and hinge assembly, but rather are separately provided spaced apart from the posts and hinges. The deflection stoppers are made of posts  1215 ,  1217  and corresponding lips or protrusions  1216 ,  1218 . In operation, a first portion  1222  of deflectable element  1202  pivots towards the optically transmissive substrate  1200  as a second portion  1220  pivots away from the optically transmissive substrate (due to electrostatic attraction of electrode  1225  to an opposing electrode on a circuit substrate (not shown). The deflectable element can be constructed to abut against deflection stoppers  1210 ,  1212  before, after, or at the same time as the first portion of the deflectable element comes into contact with the optically transmissive substrate (or the deflection stoppers alone may be used to stop the deflection of the deflectable element). FIG. 13 is a similar embodiment to that illustrated in FIG. 12 (with like numerals identifying like structural elements), except that leaf hinges  1300 ,  1303  take the place of the torsion hinges in FIG.  12 . 
     The present invention is also able to achieve a controlled tilt of each mirror in the array. This control is made possible by the stopping mechanisms for each mirror as mentioned above. The plurality of stopping mechanisms can be constructed so as to abut the corresponding mirror at the same time. Or, a back-up stopping mechanism could be provided in the event that the mirror is not sufficiently stopped by the primary mechanism. In this way, the degree of tilt of each mirror is more accurately controlled, thus resulting in long term accurate resolution and contrast ratio. 
     Accordingly, the mirror support structure is reinforced with deflection stoppers configured for resisting deflection of the reflective element beyond a pre-determined tilt angle. The deflection stoppers can be configured such that, when the reflective element is deflected to the pre-determined tilt angle, the reflective element can come into contact with the deflection stoppers. In addition, one end of the reflective element can come into contact with the optically transmissive substrate. In this way, mechanical robustness of the mirror support structure is significantly improved. Moreover, contrast of the spatial light modulator is increased due to a greater ability to control the tilt angle of the reflective elements. The reflective element of the present embodiment may also include bump(s) positioned along a substrate-touching edge such that the area of contact between the reflective element and the substrate is reduced. 
     In one embodiment, the mirror support structure and the attraction electrode are composed of a same conductive laminate. Therefore, the reflective element needs not be conductive (though the reflective element, in another embodiment, can be conductive and act as the electrode). Consequently, mechanical and reflective properties of the reflective element can be optimized without regard to conductivity. Also, the stopping mechanism can be disposed on a side of the reflective element opposite to that of the substrate, and the support structure preferably comprises hinges and posts, the posts extending past the reflective element to connect directly or indirectly to the substrate, and each hinge extending from the posts and connecting to the reflective element. 
     The primary and secondary stopping mechanisms can be constructed to stop movement of the deflectable element at different angles of deflection of the deflectable element, and may be constructed in different planes relative to the deflectable element. One of the primary and secondary stopping mechanisms preferably comprises a portion or extension of the deflectable element which abuts against the first substrate during deflection of the deflectable element, and the other of the primary and secondary stopping mechanisms preferably comprises support structure connected to the first substrate which is disposed on a side of the deflectable element opposite to the side on which the first substrate is disposed, the deflectable element adapted to abut against the support structure when the deflectable element is deflected. The secondary stopping mechanism preferably comprises a portion of the hinge, the hinge portion constructed so as to abut against the deflectable element when the deflectable element is deflected, and a gap is disposed between the first substrate and the deflectable element, and a second gap is disposed between the deflectable element and one of the primary and secondary stopping mechanisms. Also, one of the primary and secondary stopping mechanisms comprises a post or wall connected at one end to the first substrate and having a second end with a protrusion which is adapted to abut against the deflectable element when the deflectable element pivots up to a predetermined angle, whereas the secondary stopping mechanism is constructed to avoid abutment against the deflectable element unless the primary stopping mechanism fails. 
     In one embodiment of this aspect of the invention, a spatial light modulator is provided comprising a first substrate, a deflectable element pivotably held on the first substrate, a post or wall extending from the first substrate and having a lip or protrusion which extends past a portion of the deflectable element such that when the deflectable element pivots, a portion of the deflectable element abuts against the lip or protrusion so as to stop the movement of the deflectable element. In another embodiment, a spatial light modulator comprises in cross section, an optically transmissive substrate, a first gap disposed below the optically transmissive substrate, a pivotable mirror disposed below the first gap, a second gap disposed below the mirror, and a deflection stopper disposed below the second gap. Also provided is a connector which connects the pivotable mirror with the optically transmissive substrate, the connector preferably comprising the deflection stopper. The deflection stopper may be part of a hinge and post assembly for pivotably holding the mirror to the optically transmissive substrate, or, the hinge and post assembly can be spaced apart from the deflection stopper with the hinge disposed below the second gap. The deflection stopper can comprise a protrusion which extends below the second gap and a wall or post which connects to the optically transmissive substrate. 
     It should be appreciated that the present invention has been described with specific references. However, it should be noted that specific references within the present disclosure should not be construed to limit the scope of the present invention. Rather, the scope of the present invention should be construed according to the below claims.