Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
  • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
  • G02B26/0816—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements
  • G02B26/0833—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD
  • G02B26/0841—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD the reflecting element being moved or deformed by electrostatic means

Definitions

• the present invention generally relates to the field of spatial light modulators that can form optical images by the modulation of incident light.
• the invention may involve micro electro-mechanical systems (MEMS) in the form of micromirror device arrays for use in optical display, adaptive optics and/or switching applications.
• MEMS micro electro-mechanical systems
• the invention also comprises individual or isolated micromirror elements.
• MEMS devices are small structures, typically fabricated on a semiconductor wafer using processing techniques including optical lithography, metal sputtering, plasma oxide deposition, and plasma etching developed for the fabrication of integrated circuits.
• Micromirror devices are a type of MEMS device.
• Other types of MEMS devices include accelerometers, pressure and flow sensors, fuel injectors, inkjet ports, and gears and motors - to name a few.
• Micromirror devices have already met with a great deal of commercial success.
• Micromirror devices are primarily used in optical display systems.
• the large demand for micromirror-based display systems is a result of the superior image quality the systems can provide.
• Commercial and home-theater segments drive this facet of market demand.
• Other market segments are characterized by cost concerns more than image quality concerns. Since these devices are produced in bulk on semiconductor wafers, they take advantage of the same wafer processing economies of scale that characterize the semiconductor industry, thus making the sale of these devices competitive at all price points.
• the micromirror device is a light modulator that often uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, many micromirror devices are operated in a digital bistable mode of operation.
• micromirror-based display systems will allow them to capture market share for applications including theatre and conference room projectors, institutional projectors, home theater, standard television and high definition displays from various lesser-quality solutions including liquid crystal display (LCD) and cathode ray tube (CRT) type systems.
• LCD liquid crystal display
• CRT cathode ray tube
• all-digital display mirror control is completely digital except for the possible A/D conversion necessary at the source
• progressive display removing interlace display artifacts such as flicker - sometimes necessitating an interlace to progressive scan conversion
• fixed display resolution the number of mirrors on the device defines the mirror array resolution; combined with the 1 : 1 aspect ratio of the on-screen pixels, the fixed ratio presently requires re-sampling of various input video formats to fit onto the micromirror array
• digital color creation spectral characteristics of color filters and lamp(s) are coupled to digital color processing in the system
• digital display transfer characteristics micromirror device displays exhibit a linear relationship between the gray scale value used to modulate the mirrors and the corresponding light intensity, thus a "de-gamma" process is performed as part of the video processing prior to display).
• MEMS display devices have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane that was electrostatically attracted to an underlying address electrode. When address voltage was applied, the membrane would dimple toward the address electrode. Schlieren optics was used to illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. The images formed by Schlieren systems were very dim and had low contrast ratios, making them unsuitable for most image display applications.
• Later generation micromirror devices used flaps or cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. These devices typically used a single metal layer to form the reflective layer of the device. This single metal layer bent downward over the length of the flap or cantilever when attracted by the underlying address electrode, creating a curved surface. Incident light was scattered by this surface thereby lowering the contrast ratio of images formed with flap or cantilever beam devices.
• Devices utilizing a mirror supported by adjacent torsion bar sections were then developed to improve the image contrast ratio by concentrating the deformation on a relatively small portion of the reflecting surface.
• These devices used a thin metal layer to form a torsion bar, which is often referred to as the hinge, and a thicker metal layer to form a rigid member.
• the thicker member typically has a mirror-like surface.
• the rigid mirror remains flat while the torsion hinges deform, minimizing the amount of light scattered by the device and improving its contrast ratio. Though improved, the support structure of these devices was in the optical path, and therefore contributed to an unacceptable amount of scattered light.
• the more successful micromirror configurations have incorporated a "hidden-hinge" or concealed torsion/flexure member(s) to further improve the image contrast ratio by using an elevated mirror to block most of the light from reaching the device support structures. Because the mirror support structures that allow it to rotate are underneath the mirror instead of around the perimeter of the mirror, more of the surface area of the device is available to reflect light corresponding to the pixel image. Since much of the light striking a concealed-flexure micromirror device reaches an active pixel surface and is either used to form an image pixel or reflected away from the image to a light trap, the contrast ratio of such a device is much higher than the contrast ratio of other known devices.
• micromirror devices may be further improved.
• general considerations of manufacturability, which play directly into cost may be improved. For instance, increasing the yield of devices (in the form of pixels that pass functional criteria) from a given processed wafer offers both improvement in product quality and cost savings.
• less complicated manufacturing procedures, including a process requiring fewer masks or steps for production of micromirror devices would be desirable.
• micromirror devices currently in production for SVGA applications include over half a million active mirrors, SXGA applications require over one point three million active mirrors. Since powering so many elements has a cumulative effect, addressing power consumption issues will be of increasing importance in the future as the number of pixels employed in image creation continues to increase.
• micromirror device improvement lies in continued miniaturization of the devices. In terms of performance, this can improve power consumption since, smaller distances between parts and lower mass parts will improve energy consumption and increase display system resolution by providing a micromirror device with greater mirror density given overall package size constraints. In terms of manufacturing, continued miniaturization of mirror elements can offer a greater number of micromirror systems for a wafer of a given size.
• aspects of the present invention offer improvement in terms of one or more of the considerations noted above.
• certain features may be offered in one variation of the invention, but not another.
• features offered by aspects of the present invention represent a departure from structural approaches represented by the Texas Instruments DMDTM.
• the inventive features represent an altogether distinct evolutionary branch of "hidden-hinge” or concealed-flexure micromirror device development, rather than mere sequential refinement of features as may be noted in the development of the Texas Instruments DMDTM element described in detail below.
• the divergent approaches marked by aspects of the present invention offer a competitive edge to the present invention to benefit consumers in any of a number of ways.
• Micromirror array devices generally comprise a superstructure disposed over a substructure including addressing features.
• Features of the superstructure set upon and above the substrate include electrodes, hinges, micromirrors, support members or portions thereof. Support member pairs are provided to hold a mirror/micromirror above the hinge and the electrode features used to actuate it.
• One major aspect of the invention involves supporting each micromirror element above its respective hinge portions at or along the sides of the mirror.
• the hinge is then supported above the substrate by one or more features set toward the pixel center with respect to the mirror supports.
• the micromirrors are preferably operated in a bistable fashion, rotating about an axis formed by each hinge or hinge portion.
• mirrors While the supports between the hinge and mirror portions are placed opposite each other, their location along each micromirror may vary. Preferred placement locations include opposite corners or sides of the mirrors. Generally, mirrors will have a polygonal plan in which the shapes are closely-packed (e.g., triangles, hexagons, and quadrilaterals such as squares, rectangles, trapezoids, parallelograms, and rhombi).
• Another major aspect of the invention involves arranging the electrodes used to actuate the mirrors so that they progressively attract mirror portions. This may be accomplished using electrode portions that step down from a higher level close to a mirror's hinge to a lower outer level. With a portion of each electrode being closer to said mirror closer to a center of said mirror and another portion of each electrode being farther from said mirror further from said mirror center both sequential attraction of a mirror by said electrode portions and clearance for allowing adequate mirror tilt is possible.
• Electrodes thus configured may be supported in any manner, whether provided by a unitary structure or separately built-up portions.
• a plurality of discrete planar levels (as few as two) may be provided, or even one or more angled electrode surfaces. Electrode variations in which high multiples of levels or stages are employed will model a true angular surface. Such angular and even curved surfaces are within the scope of the invention.
• Still another major aspect of the invention involves employing "open" support structures to separate a given element from the feature to which it is secured.
• an open support it is meant that the structure does not have a closed periphery as do known support structures, e.g., as in the Texas Instruments DMDTM columnar support posts formed within "vias" having a substantially square cross section.
• Support members according to the invention may be single-sided or multi- sided/faceted. They may support a given structure in a cantilevered mamier or at points across from each other. Mirror and hinge elements are preferably supported along opposite sides, i.e., by placing supports across from each other. Electrodes are preferably configured to include a cantilever section. Other variations within the scope of the invention are, however, possible.
• Laying down material over a stepped or angled sacrificial material during manufacture may produce cantilever-style electrodes. When the sacrificial material is removed the structures remains.
• at least two column or via-type support precursors are provided with a spanning portion there between. Then, any sacrificial material employed as a depositing surface for the spanning portion is removed along with portions of the column not providing support for the spanning portion, thus creating one or more "open" supports at the side(s) of the segment of material that provides a spanning segment, section or portion therebetween.
• the present invention includes any of the above improvements described either individually, or in combination.
• Systems employing micromirror devices including the improved superstructure form aspects of the invention, as does methodology associated with the use and manufacture of apparatus according to the present invention.
• FIGs. 1-8H represent information known in the art, in which FIGs. 6 and 8A-8H represent aspects of a known micromirror device. The features shown in the other figures may be used in the present invention.
• Figures 9A-15H show features particular to the present invention.
• Figures 11 A and 1 IB compare micromirror devices according the present invention against the device shown in the referenced figures. Certain aspects of the figures diagrammatically represent the present invention, while others are indicative of preferred relations. Regardless, variation of the invention from what is shown in the figures is contemplated.
• Figures 1A and IB are side views illustrating bi-stable micromirror operation.
• Figure 2 is a perspective-combined view illustrating the projection of three pixels utilizing a portion of a micromirror device display system.
• Figure 3 is a perspective view illustrating grayscale image production for a single line of mirrors in a micromirror device utilizing pulse width modulation (PWM).
• PWM pulse width modulation
• Figure 4 is a perspective view of an exemplary color micromirror projection system.
• Figure 5A is a perspective view of a micromirror device based projector
• figure 5B is a perspective view of a micromirror device based projection television
• Figure 6 is an exploded perspective view of a DMDTM element.
• Figure 7 is a circuit diagram showing a manner of addressing a micromirror device array.
• Figures 8A-8H are perspective views showing the micromirror elements of FIG. 6 at various stages of production.
• Figure 9A shows a perspective view of a micromirror element according to the present invention
• figure 9B shows the element in FIG. 9A without a mirror
• figure 9C shows the element of FIG. 9A from the side.
• Figures 9A'-9C show the same views of another variation of the invention employing a single-stage electrode, with an alternate mirror support approach.
• Figures 9A"-9C" show the same views of a further variation of the present invention that employs a hexagonal mirror.
• Figures 10A-10G are perspective views showing the micromirror element(s) of FIGs.
• Figure 11 A is a top view comparing the DMDTM of FIG. 6 with the micromirror device of FIG. 8; figure 1 IB is a perspective view of arrays of elements as shown in FIG. 11 A.
• Figures 12A-12C show different mirror support configurations according to the present invention.
• Figures 13 A and 13B show optional manners of producing support portions with and without a base, respectively.
• Figures 14A-14C show different mirror configurations in an intermediate stage of production.
• Figures 15A-15H are side views of various electrode configurations employing a variety of levels, shapes and support approaches. DETAILED DESCRIPTION
• FIGs. 1 A and IB bistable operation of a micromechanical light modulator 2 is shown.
• the device comprises a mirror portion 4, a hinge portion 6 and electrode portions 8 set upon a substrate 10.
• FIG. 1 A the mirror is shown rotated or flexed about a hinge portion 6 in a clockwise direction from a horizontal position.
• the hinge is configured to provide a mechanical restoring force in returning from mirror rotation.
• Mirror rotation occurs as a result of electrostatic attraction between at least the mirror portion 4 and an electrode portion 8 of the device located above a substrate 10 which carries each of the elements.
• IB shows the mirror deflected to a second minimum potential energy state opposite a second electrode. Operation of a micromirror device mirror between two such full-angle states represents what is referred to as "bistable" operation. Such operation is employed in a digital mode.
• Micromirror devices may also be operated in analog mode. Sometimes referred to as
• beam steering this operation involves charging address electrode(s) to a voltage corresponding to the desired deflection of the mirror.
• Light striking the micromirror device is reflected by the mirror at an angle determined by the deflection of the mirror.
• a ray of light reflected by an individual mirror is directed to fall outside the aperture of a projection lens, partially within the aperture, or completely within the aperture of the lens, depending on the voltage applied to the address electrode(s).
• the reflected light is focused by the lens system onto an image plane. Each individual mirror pixel corresponds to a pixel on the image plane. As the ray of reflected light is moved from completely within the aperture to completely outside the aperture, the image location corresponding to the mirror dims, creating continuous brightness levels.
• micromirror devices especially those produced according to the present invention lend themselves to directing light from one path to another to optically connect and disconnect pathways as desired.
• FIG. 2 illustrates an approach to producing images in a digital mode of micromirror device operation.
• Incident light from a light source 12 striking a mirror 2 rotated toward the light source is reflected to pass through a lens 14 and be displayed as a corresponding bright pixel 16 on a screen or the like (turned upward relative to the other components shown for ease of viewing).
• mirrors rotated away from the light source reflect light away from the projection lens into a light trap 18 leaving a corresponding dark pixel 20 at the projection image surface.
• Mirrors rotated to produce a bright pixel may be regarded as "on,” while those positioned to leave a pixel dark may be regarded as "off.”
• FIG. 3 illustrates a manner in which intermediate pixel brightness may be obtained.
• Digital mode micromirrors employ pulse width modulation techniques to rapidly rotate a mirror on and off to vary the quantity of light reaching the image plane.
• the human eye integrates the light pulses and the brain perceives a flicker-free intermediate brightness level.
• FIG. 3 an active row of micromechanical light modulator elements 2 are depicted, forming a portion of a larger array 22.
• Directional markers 24 indicate the location of corresponding pixels within a projected pixel row 26 opposite a lens 14.
• a full-intensity bright pixel 16 is displayed by constant application of light rays 28.
• a dark pixel 20 is provided by leaving the corresponding reflective element 2 "off so that essentially no light reaches the projection target.
• Pixels of intermediate intensity 30 are provided by application of intermediate lighted intervals by turning "on” and "off the corresponding micromirror element 2.
• FIG. 4 shows a digital projection subsystem 32 in which the digital operation principle(s) discussed above are applied to project a cogent image on a screen 34.
• the subsystem includes a light source 12 and a projection lens 14 as well as a board or module 36 including a processor 38, memory 40 and micromirror array 42 comprising light modulating elements 2.
• the micromirror device shown is "packaged" in that the MEMS portion micromirror array 22 element of the device is set within a housing 44 sealed by a window 46.
• Full-color images are generated by sequentially forming three single-color images. This process in concert with the former discussion of analog or digital methods of grayscaling gives many levels of shading of each color. The viewer perceives a single, full color image from the sum of the three single-color grayscaled images.
• a color wheel may continue to be utilized with a plurality of micromirror devices in conjunction with a color separating prism (not shown). Still further, a plurality of micromirror devices may be provided and used in conjunction with a light source, no color wheel, but with color filtering prisms.
• optics may vary. Providing additional light sources and/or additional micromirror arrays allows for image creation through superposition offering the potential for greater brightness and resolution. Simply providing dedicated light sources for a single micromirror array may improve brightness as well.
• One limitation to current micromirror device implemented solutions involves brightness levels. Since there is a practical limit to the brightness of a single source, one solution to this malady is to utilize multiple light sources. Factors of greater cost/system complexity will typically be weighed in determining whether to implement these improvements in a given system.
• FIGs. 5A and 5B depict a projector 54.
• the projector shown is suitable for the typical consumer home-theater.
• Other devices that may incorporate systems according to the present invention may be suitable for larger venues (i.e., staging events and cinema presentations), being configured for high light output and waste heat generation.
• the second figure depicts a projection television 56.
• the television pictured is a rear projection system, though other styles (e.g., front projection) may be employed.
• such systems may be specifically designed for or designed around micromirror devices according to the present invention.
• a packaged "light engine” according to the present invention could be substituted into existing systems (with or without further modification or substituting the entire module 36) to upgrade performance.
• FIG. 6 shows a single mirror element 2 of an array in an exploded perspective view.
• the bottom level is a semiconductor substrate
• FIG. 7 The manner in which such circuitry is addressed (whether as provided in the referenced micromirror devices or those according to the present invention) is illustrated in FIG. 7.
• addressing architecture is shown that incorporates N addressing inputs 62 for every 2N rows and 1 data input 64 for every 16 columns.
• Such substrate material in various configurations, with a passivation layer including vias to provide connectivity at selected locations/spacing is commercially available.
• the physical alignment of superstructure components above the address circuitry is such that, upon selection, address voltage is applied to the electrodes of the device.
• the bias voltage discussed above is applied to the mirror by way of intermediate structures connected to a bias/reset bus 66 provided upon substrate 58.
• Hinge supports 68 are set above the bias bus, and supported above bus 66 by substantially square, columnar via-based supports 70. (The final alignment of these components and others is indicated in dashed lines.)
• the support posts are produced by deposition within a hole provided within a sacrificial layer of material in an intermediate stage of device production. Accordingly, they are not solid, but rather hollow until the solid base portion 72, with a closed outer wall or periphery.
• the hinge supports are attached to hinge segments or portions 74 which are in-turn attached to a yoke 76. The corners of the yoke are provided with spring tips 78.
• the spring tips provide bumpers to cushion or moderate contact between the yoke and bias bus upon full mirror actuation, rather than having to precisely control voltages or rely on other interfering contact. While potentially useful, it is contemplated that micromirror devices according to the present invention may or may not make use such features.
• micromirror element 2 includes a mirror 80.
• the mirror is connected to the yoke by way of a via-type support 70 like those provided for the hinge supports, leaving a hole 118 in the mirror face.
• a via-type support 70 like those provided for the hinge supports, leaving a hole 118 in the mirror face.
• a voltage is applied to the electrodes 82 and 84 that electrostatically attract both the mirror and yoke, respectively.
• the electrodes are set at two levels.
• the higher-up outer electrode portions 82 are electrically connected to the lower electrode portions 84 by way of another connecting columnar via 70.
• This combination of elements is placed in electrical contact with the addressing circuitry by a filled-in via 86 in the base of each electrode portion 84.
• the upper electrodes are positioned to attract the mirror, whereas the lower electrodes are positioned to attract the yoke.
• FIGs. 8A-8H The stages shown are indicative of action taken after intermediate masking steps between material deposition (sacrificial material or structural material) and sacrificial material removal. To most clearly portray the structure being produced, the perspective view shown takes the device across the sectional line shown in FIG. 6 and tilts the structure.
• bus 66 and a lower electrode 84 are shown, formed by a conductive material. These are provided by material deposited over substrate 10, with the overlaid material strategically etched away. The raised portions will have been covered by a protection layer, configured using a first mask 88 (diagrammatically pictured).
• the substrate comprises the addressing circuitry covered by a passivation layer, the layer having holes strategically placed to provide access vias to the underlying circuitry. The vias are filled-in to provide electrical connections 86 between the substrate and electrodes as noted above with respect to FIG. 6.
• FIG. 8B shows a layer of sacrificial material 90, deposited over the structure in FIG. 8 A.
• Nia column holes 92 are provided, again by selectively etching the material in connection with a second mask 94.
• FIG. 8C another layer of conductive material 96 suitable for use in producing hinge sections 74 and spring tips 78 is laid-down. Following this, a third mask 98 is employed in setting a protective layer such as an oxide (not shown) over the regions of layer 96 serving as hinge precursors 100, and spring tip precursors 79. [076] In FIG. 8D, another layer of conductive material 102 is deposited thereon. A fourth mask 104 is utilized to form a protective layer (not shown) over the regions of layer 102 serving as hinge support precursors 106, a beam or yoke precursor 108 and upper electrode precursor(s) 110.
• a protective layer such as an oxide (not shown)
• FIG. 8D another layer of conductive material 102 is deposited thereon.
• a fourth mask 104 is utilized to form a protective layer (not shown) over the regions of layer 102 serving as hinge support precursors 106, a beam or yoke precursor 108 and upper electrode precursor(s) 110.
• Both the hinge metal layer 96 and yoke/electrode metal layer 102 fill via holes 92, providing columnar support portions 70.
• the portions of the material layers not protected during processes involving the third and fourth masks are selectively etched as shown in FIG. 8E to define hinge supports 68, hinges 74, yoke 76 and upper electrode portions 82.
• FIG. 8F shows the micromirror device in another intermediate stage of production with another layer of sacrificial material 112. This layer is deposited over the structure in FIG. 8E. It includes a via column hole 96, patterned utilizing a fifth mask 114. When a mirror material layer 116 is deposited over sacrificial layer 112 as shown in FIG. 8G, via hole 96 is partially filled in, providing support column 70, but leaving a hole or opening 118 in what is to become the "face" of the mirror element.
• FIG. 8H shows the micromirror element 2 as completed, with all sacrificial material removed to release the structure.
• Patent No.6,028,690 to Carter, et al entitled “Reduced Micromirror Mirror Gaps for Improved Contrast Ratio”
• U.S. Patent No.6,323,982 to Hornbeck entitled “Yield Superstructure for Digital Micromirror Device”
• U.S. Patent No. 6,337,760 to Huibers entitled: "Encapsulated Multi-Directional Light Beam Steering Device”
• U.S. Patent No. 6,6,348,907 to Wood entitled “Display Apparatus with Digital Micromirror Device”
• U.S. Patent No. 6,356,378 to Huibers entitled “Double Substrate Reflective Spatial Light Modulator”
• micromirror devices according to the present invention may be produced and/or operated according to the same details or otherwise.
• FIG. 9A shows a preferred micromirror element 124 per the invention.
• the variation of the invention shown includes each of the optional features that may be employed, though not all such features need be provided in a given product.
• FIG. 9B shows the micromirror device 124 in FIG. 9A minus its mirror.
• FIG. 9C shows the same from the side.
• a first group concerns supporting a mirror portion 126 at its sides; a second group concerns providing electrodes 128 adapted for sequential attraction of the mirror; and a third group concerns supporting various components including the mirror, electrode portions and/or hinge portions 130 with open support structures.
• the mirror shown in FIG. 9A has an uninterrupted “face” in that its reflective surface is unbroken as compared to device 2 of FIGs. 6 and 8. While the “potential face” or “prospective face” of the mirror (indicated by solid and dashed lines together) may be somewhat larger than the actual face of the mirror (the area indicated by solid lines alone), "dim” or “dead” space 132 resulting, generally, in light scattering may be reduced. As described below, such space may be minimized or even eliminated according to an aspect of the present invention.
• hinge portions 130 may comprise individual segments, or may be part of a unitary structure.
• the hinge defined is attached to substrate 136 by a bridge-type support 138.
• the support is preferably open underneath the hinge center 140, which is attached to a spanning segment 142 between vertical support segments 144.
• Feet 146 may additionally be provided to stabilize the support structure.
• Yet another option is to produce support segments 144 at an angle relative to the surface of the substrate (i.e., having both vertical and horizontal components).
• support 134 may be set at an angle with respect to the substrate. Yet, it is more preferable that support(s) be provided orthogonally as shown. A base 148 of each support 134 may directly connect each hinge portion 130. However, it may be preferred that an intermediate layer or nub 150 of material (e.g., serving as a bonding interface) is employed.
• the device is configured so that the hinge is set some distance (as little as about 0.1 micron, or less) above the surface of substrate 136 and mirror 126 is set some distance (as little as about 0.1 micron, or less) above the hinges (as little as about 0.2 micron, or less, above the surface of substrate 136). Avoidance of a yoke allows creation of very low profile micromirror devices by the invention that are still able to attain high deflection angles (typically about +/- 10 deg., even upwards of about +/- 15 deg., to about +/- 20 deg. or more).
• mirror/micromirror devices according to the present invention may be advantageously manufactured on a larger scale (even using MEMS techniques) - possibly utilizing other actuation techniques, including electromagnetic, electromechanical, thermo-mechanical or piezo-based approaches - especially for non-projection technology.
• Electrodes 128 may be configured with a plurality of portions 152 and 154 (or more) at different levels. Whether provided in a series of steps by continuous members (as shown with a support portion 156 between each stage 152/154), by steps formed with discrete members or a continuous angled member, the electrodes are configured so that portions further from the center or point of rotation of the mirror are at a lower level.
• the electrode configuration shown with higher portions closer to the center and lower portions more distant provides clearance for the mirror as it is tilted at an angle. Furthermore, the configuration provides for sequential attraction of mirror 126.
• the upper electrode portion is the first to exert significant attractive electrostatic force on the mirror (in light of the inverse squared relationship between electrostatic attraction and distance between objects).
• the influence of the electrode lower portion(s) increase. Further aiding attraction of the mirror to its full angular displacement is the increased mechanical advantage or lever arm offered at more remote regions of the mirror interacting with lower electrode portion 152.
• FIGs. 10A-10G The manner in which a micromirror device 124 according to the present invention may be produced is illustrated in FIGs. 10A-10G.
• the process steps employed will vary depending on which inventive features are actually employed in a given variation of the invention. But again, a most preferred approach is shown.
• a sacrificial layer of material 158 is set upon substrate 136. It is patterned with a first mask 210 to define openings 160 and a substrate-level portion 162 upon etching.
• a hinge metal layer 164 is deposited over the entire surface including a portion of the sacrificial layer.
• a second mask 166 is utilized in defining a passivation layer (not shown) over the region(s) of layer 164 serving as a hinge precursor region 168.
• Metal layer 164 fills in via 206 provided in substrate 136 to form a connection 208 between underlying address circuitry beneath an oxide layer of the substrate.
• the same approach to addressing and substrate construction may be employed as described above, or another manner of electrical control of device superstructure produced may be utilized. This holds true with respect to connectivity between the device elements as well as the configuration of substrate 136.
• a thicker layer of conductive material 170 is deposited over the hinge material. This layer builds-up the electrodes 128 and further fills openings 160, defining a support precursor region 172 for hinge portions 130. Layer 170 also further fills in via 206 and connecting structure 208. A third mask 174 is employed to define a protective layer (not shown) over the region of layer 170 serving as electrode precursor(s) 176.
• FIG. 10D layers 164 and 170 are shown selectively etched to reveal hinge 130, support spanner 142, and electrode portions 152 and 154. As shown in FIG. 10E, these structures are then covered by another sacrificial layer 178.
• a fourth mask 180 is used to pattern sacrificial layer 178 to form support precursor regions 182 upon etching the sacrificial layer.
• FIG. 10F shows sacrificial layer 178 as it is selectively etched, and then coated with a layer 184 of conductive material suitable to serve as a mirror (or a substrate that may be subsequently coated with a highly reflective metal or dielectric material).
• a fifth mask 186 is used in order to define a passivation layer over mirror precursor regions 188 to be retained, but not the adjacent borders 190, which are removed to form spaces between adjacent micromirrors 126.
• FIG. 10G shows a micromirror element 124 according to aspects of the invention after all sacrificial materials have been removed.
• the mirror is supported at or along its opposite sides or edges by supports attached to a hinge, which is in turn supported above the device substrate.
• the support members may be characterized as being "open” in nature. Progressive or dual-stage electrodes are shown as well.
• micromirror device produced according to the methodology described merely requires 5 masks - i.e., as constructed on a pre-fabricated substrate.
• Texas Instruments DMDTM is produced using 6 masks under the same conditions.
• the methodology according to the present invention is highly advantageous from both fabrication cost and device yield standpoints.
• a micromirror device according to the present can be produced with the same pixel dimensions as known devices. In doing so, a device according to the present invention will offer a performance benefits at least in terms of light return. Reasons for this advance are discussed below.
• micromirror devices according to certain aspects of the present invention may be made smaller than the referenced devices by between about 25% and about 65% or more (i.e., devices according to the present invention may be about 75% to about 35% of the size of known devices) due the absence of a yoke layer in order to allow for a smaller sacrificial layer gap - while still employing a plurality of electrode levels.
• mirrors elements employed in the present invention can be made smaller than
• DMD-sized mirrors that have roughly a 19 micron diameter.
• Mirrors/pixel elements according to the present invention may advantageously be produced at less than about 10 microns in diameter.
• diameter what is meant is the distance across any long axis that may be defined; stated otherwise, the diameter will correspond to that of any circle in which the structure can be circumscribed.
• mirrors used in the present invention may be as small as 6 microns in diameter in view of present manufacturing techniques.
• a mirror so-sized may represent a 69% reduction in diameter from known DMDTM mirror size (i.e., the inventive mirror element will be about 31% the diameter of known mirrors).
• the inventive mirror element will be about 31% the diameter of known mirrors.
• FIGs. 9A'-9C show components of a device 124 according to the present invention constructed using a single-level set of electrodes 128.
• the configuration shown may be produced using a modified version of the five-mask process described above.
• the differences in production methodology will be readily apparent to one with skill in the art. Generally, it will be preferred to maximize the size of the electrodes given space constraints and in view of clearance considerations as in other variations of the invention.
• FIGs. 9A'-9C show components of a device 124 constructed using another means or approach to mirror support.
• the support configuration shown may also be produced in connection with a modified version of the five-mask process described above, wherein differences in production methodology will be readily apparent to one with skill in the art.
• columnar supports or posts 212 are utilized which may be created by filling in vias produced in sacrificial material. Such an approach may be desired from a manufacturing perspective, or possibly other reasons depending on the circumstances.
• each of the pair of supports is positioned opposite one another and across the body of mirror 126.
• Supports 212 are shown to have a wall 214 at the edge of mirror 126 (each may have four walls or more or may define curved surfaces - depending on the original via shape that is filled-in to create the structure).
• the supports may be inset from the side/comer or edge of a mirror (depending on the style of micromirror device chosen) to which they are closest.
• supports 212 it may be preferred to position supports 212 in such a way as to maximize hinge or torsion member length in view of the mirror style/format selected (i.e., square with comer support positions, hexagonal with comer supported positions, hexagonal with side support positions, etc.).
• the base of each support or an intermediate structure
• supports 212 will generally be positioned outside of the hinge support member 138 or members.
• FIGs. 9A"-9C provide details of a hexagonal-shaped mirror supported at opposite comer positions. Its construction and appearance closely resemble the micromirror elements 124 shown in FIGs. 9A-9C.
• the hexagonal mirror format offers certain advantages in use. For one, they can be closely packed in a manner like a honeycomb, where sequential rows (or columns) overlap. Such overlap provides the ability in image creation to mimic higher resolution output where there is overlap. The principles of such operation are well documented and may be understood in reference to U.S. Patent No. 6,232,936 to Gove, et al, entitled “DMD Architecture to Improve Horizontal Resolution". Further potential advantages associated with the mirror format shown in FIGs. 9A"-9C" are presented below.
• FIGs. 9A-9C and 9A"-9C another immediately apparent distinction between the Texas Instruments device and those shown in the reference figures concerns what may be regarded as “dead” or “dim” space that is substantially non-reflective or poorly reflective relative to the mirror face(s).
• a large central hole 118 is present in mirror face 80 of the former structures. As shown in FIG. 2, this actually results in a central dark or missing region in each pixel image.
• each mirror 126 in FIGs. 9A and 9A" is inviolate at the center. Any dim or dead space 132 associated with the prospective mirror face only involves the space above support base portions 148.
• FIG. 12A show mirror sections 192 from above, the base 148 of each support member and wall portions 194 defining vertical sections(s) in connection with square mirrors.
• FIG. 12B shows configurations advantageously employed with hexagonal mirrors as indicated by identical reference numerals.
• base 148 may even be altogether eliminated, especially in mirror side-mount configurations.
• a hexagonal mirror is portrayed in which support wall(s) 134 attach directly to the underlying structure without the addition of an extended base portion 148. Supports 134 are depicted in broken line because (as apparent in FIG. 9 A) some thickness of the wall resides below the surface of mirror 126 as viewed from above.
• FIGs. 13 A and 13B The manner in which producing support regions with no base is depicted in FIGs. 13 A and 13B.
• a support precursor 196 is shown in FIG. 13 A. It is etched-out as indicated by dashed lines 198 in accordance with the discussion above, removing region 200.
• the resulting, separated stractures include support 134 and base 148 regions, with mirror regions 126 above.
• the support precursor region is so small that removal of region 200 leaves no discrete base(s) 148, but only base surfaces 202 (attached to underlying structure).
• FIG. 9A' offers advantages relative to the Texas Instruments approach that includes a large, central hole 118 in each pixel.
• the dead or dim zones associated with mirror holes 216 as provided in mirror faces according to the present invention are spread apart from each other and of a combined area that is less than the Texas Instruments column support. Also, it is believed that this delocalization of such space will make its effects less apparent to a viewer. Decentralization of dim or dead space in the pixel may further diminish the ability of a viewer to pick-out the features upon close inspection.
• each micromirror element is surrounded by a border 188.
• This gap or border provides clearance for the mirrors as they tilt back and forth in an array. In the active regions of any micromirror array, this dead space cannot be eliminated. It can, however, be reduced by providing lower- profile micromirror assemblies.
• Highly-elevated mirrors as in the Texas Instruments DMDTM that are set above a yoke 76 and greatly separated from the underlying hinge and/or substrate require more lateral space in which to accomplish such angular deflection as desired than lower profile structures as may be achieved with the present invention.
• the ability to produce low- profile micromirror devices according to aspects of the present invention enables reducing overall gap or border space to less than in known micromirror devices, where gap space is believed to represent about 11.4% of the area in the active array region.
• gap size may be more significant than increasing use of prospective mirror face. For example, where shorter supports 134 are provided (or via hole 118 is more filled-in), partial light return can be expected. In which case, the zones are more "dim” than "dead” as to reflection.
• array 22 comprising Texas Instruments micromirror devices as described is not capable of producing the resolution of array 191 using micromirror devices as may be produced according to the present invention.
• array 191 packs 100 light modulator elements as compared to 36 in array 22. The result of this difference nearly triples of the number of pixels that may be projected.
• Provision of such a dramatically increased number of mirrors may, however, require certain accommodations.
• mirrors in a DLPTM system are controlled by loading data into the memory cell below the mirror, a data stream configured to actuate a lesser number of mirrors with different addressing will typically not be suitable for running another array.
• Accommodation for such differences as presented may be provided by means of hardware/software.
• Equipment exists that can take a given input signal at a particular resolution and either up- or down-convert the signal to a resolution that is compatible with the device at hand.
• mirror precursor regions are provided. These are patterned in such a way as to provide for supports.
• Mirror precursor region sections 206 are shown for three different mirror types in FIGs. 14A-14C. Dashed lines are presented to indicate the location where individual mirror elements 126 will reside upon separation. The solid lines indicate pits or holes 132, portions of the edges will form support sections 134 (and possibly portions of the bottom forming bases 148 as well). What may be observed is that spaces 132 reside partly in the spaces 188 to be provided between each mirror element. This positioning, in effect, allows certain "theft" of space in producing the support structures.
• this configuration accommodates the longest hinge length for the smallest pixel area. Especially where very small mirrors/pixels are concerned, longer hinge length can be very useful. Since for a given hinge cross-section, stiffness decreases and overall torsional displacement capability increases with length, it will be possible to achieve relatively larger mirror deflection using such a design. Additionally (or alternately), the additional hinge length available allows for producing the smallest pixel size possible - at least with respect to such other mirror and connector configurations shown and discussed herein. [0117] With the hexagonal mirrors using comer mounting points a larger relative mirror area versus hinge length can be achieved. Such a configuration provides for generating greater electrostatic forces. According, reduced voltages may be applied to deflect each mirror. Reducing voltages allows a beneficial reduction in overall device power requirements.
• this configuration accommodates a longer mirror axis perpendicular to the hinge and mirror area versus hinge length.
• the increased lever-arm offered by the overhanging mirror portion at the comer of the mirror (as compared to the hex mirror/hinge configurations where opposite edges are parallel to the hinge) may offer greater electrostatic attraction, especially toward the extremes of mirror actuation where restoring forces from the hinge are greatest. As such, this may offer relative advantages in power consumption and/or maximum mirror deflection.
• FIGs. 15A-15H present side view of various potential electrode configurations. Each figure shows an electrode including a plurality of levels. In the variation in FIG. 15A two levels 152 and 154 are shown. Progressively more levels 218 are shown in FIGs. 15B-15D.
• FIG. 15E a continuum of levels is presented in the form of a substantially uniform or angled electrode 204. Whereas the continuum of levels in FIG. 15E provides a simply angled surface, in FIG. 15F, an electrode with a measure of curvature is provided.
• a curved section 220 may be especially useful in tailoring electrostatic attractions between an electrode and mirror (or electrode and any intermediate structure such as a yoke as in the Texas Instruments design) in order to match or otherwise account for nonlinearities in restoring force provided by flexure members. Yet, it should be appreciated that a simpler set of stepped electrode surfaces (varied in size, height, orientation/plan, etc.) may be so-tuned . Using curvature (as opposed to steps in the alternative) and the curve shown are merely exemplary. However configured, curved and angled electrode formats may be produced utilizing advanced photolithography techniques (e.g., grayscale masking) known to those with skill in the art.
• advanced photolithography techniques e.g., grayscale masking
• FIGs. 15G and 15H provide examples of such approaches.
• level steps 222 are provided, optionally supported by a column 224 with a central via 228, a cantilever design 226, or any combination of electrode designs described herein.
• level steps 222 and angled steps 230 are provided. Any such electrodes may be addressed individually or electrically interconnected.
• Such structures may be provided by the technique(s) described above or otherwise.
• one method involves deposition of multiple layers that build up the tiers.
• stepped electrodes can also be created using an individual mask per tier. Each mask allows selective etching to define the separate tiers of the whole electrode.
• the Sandia developed SUMMiTTM technique involves a combination of these and other techniques.
• Determining optimal curvature (and plan view), angle or electrode level(s) - relative to substrate 136 may be determined using known empirical and/or statistical modeling or analysis techniques.
• the design of such aspects of the invention may account for relationship between desired hinge/torsion bar deflection and associated stresses, together with electrostatic attractions.
• Certain configurations may be contemplated that have electrostatic actuation advantages for given mirror and/or deflection characteristics.
• Electrode shapes, in any of three dimensions, may be determined via mathematical models accounting for theoretical attractions and/or computer simulation or otherwise. For bistable operation, the electrode shapes and nature of the models may be relatively simple. Where the intent is to provide micromirror devices suited for control analog or beam steering techniques, more complex relationships between mirror angular displacement, related forcing and electrode attraction may be required.
• the height or relative spacing of selected items may impact the size and/or orientation of components such as the electrode regions.
• electrode shape and height may require customization to avoid interference in meeting desired deflection ranges of the micromirror.
• micromirror device configurations and related systems can be made utilizing the various optional features disclosed herein. These variations each present certain respective advantages as suitable for a given application. Some of these advantages and applications have been described merely by way of example. Such discussion is not intended to limit the scope of the present invention. Indeed, certain variations of the invention covered hereby may not even present such advantages presented above by way of example. Further, the invention may comprise, individually, micromirror devices or element as described herein, just as it may encompass arrays of such stractures. The applicability may depend on the intended use, many of which (but not all possible uses) have been mentioned.
• micromirror may be applicable to mirror structures upwards of 1 mm in diameter/length/width. Such larger stractures may find applications outside the field of known projector or monitors.
• devices made according to the present invention may be employed not only in the context discussed referring to displays and image projection. Further applications may involve optical switching, adaptive optics, communications, light-shaping, photocopiers, micro-displays (such as used in mobile electronics), etc.

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• 2003
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