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  Thermal and intrinsic stress compensated micromirror apparatus and methodUSPTO Application #: 20070195439Title: Thermal and intrinsic stress compensated micromirror apparatus and method Abstract: A micromirror apparatus includes a device layer having a recess, a multilayer thin-film dielectric reflector coupled to and structurally supported by the device layer on the opposite side of the device layer from said recess, and a stress compensator seated in the recess, with the stress compensator operable to resist device layer bending moments resulting from intrinsic and thermal mismatch stresses between the multilayer thin-film dielectric reflector and the device layer. (end of abstract)
Agent: Koppel, Patrick & Heybl - Thousands Oaks, CA, US Inventors: Jeffrey F. DeNatale, Philip A. Stupar, Chialun Tsai, Robert L. Borwick USPTO Applicaton #: 20070195439 - Class: 359871000 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20070195439. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention relates to micromirrors, and more particularly, micromirrors used in micro electrical mechanical systems (MEMS) devices. [0003] 2. Description of the Related Art [0004] Micromirrors are used in a variety of consumer and industrial devices, including wavefront correction arrays, digital projection displays and fiber optic switching. For example, micromirrors in digital light processing (DLP) televisions are used to turn light to the projection screen on and off at the pixel level to form a projected image. In fiber optic switches, micromirrors are used to steer light from one fiber to another for reconfigurable signal routing. In wavefront-correction arrays, micromirrors are translated relative to one another to correct for wavefront distortion in a propagating optical wave. [0005] In general, it is desirable to have a micromirror reflect light with high efficiency and high fidelity. This imposes two common and desirable design characteristics on the micromirrors used in such applications: high reflectivity at the operating wavelength and high optical figure, otherwise known as mirror flatness. To achieve high reflectivity, reflective metal films are often deposited onto the microfabricated MEMS mirror. Unfortunately, intrinsic stress associated with the thin film deposition and thermal stresses arising from differences in coefficients of thermal expansion may compromise the mirror flatness for such micromirror assemblies. For example, some micromirrors incorporate deposited metal layers on a mechanical support microfabricated from materials such as polysilicon or single crystal silicon. Intrinsic stresses created during deposition and subsequent coalescence of the metal layers may result in deformation of the mirror structure. Thermal stresses introduced by differential expansion of the reflective and support layers, respectively, when introduced to environmental heating and cooling, may similarly result in mirror deformation. The problem is exacerbated as thinner structural supports are used for the mirror surface to accomplish quicker micromirror response. [0006] A number of solutions exist for addressing the intrinsic and thermal mismatch stresses in micromirror assemblies that may lead to loss of mirror flatness. To minimize thermally induced distortion, constraints on the operating temperature of the device may be imposed. This adds considerable system-level complexity and associated cost. Similarly, the deformation induced by the thin film layer stresses may be reduced by measures such as reducing the thickness of the reflective metal film, reducing the lateral size of the micromirror itself to reduce the bending moment caused by the stress, or by tailoring the stresses in the metal layers used for the micromirror surface to achieve a stress-neutral state. In another solution, a double-layered metallization is used to deposit the same metallization in exactly the same thickness onto both the top and bottom surface of the mirror support, so that the metallization-induced stresses are balanced. (See U.S. Pat. No. 6,618,184). In yet another solution, a stress-balancing layer is formed on a side of the mirror support opposite to that of the light reflective optical layer, with the stress-balancing layer being the same material or a different material as the light reflective optical layer. (See U.S. Pat. No. 6,639,724) [0007] Unfortunately, for some micromirror applications, such as high-intensity projectors or those subject to illumination by moderate-to high-energy lasers, the thin metal reflective layers may not have sufficient optical durability. The ability to use thicker metal reflective layers would improve the robustness and reliability of the micromirrors relative to those using thin metal layers. The thicker metal layers would, however, impose greater stress-induced deformation to the mirror relative to the thin layers. Similarly, micromirrors used in these high-intensity applications would benefit from the lower energy absorption (higher reflectivity) provided by non-metallic, multilayer thin-film dielectric mirrors. These multilayer dielectric reflectors may be quite thick, however, and may similarly exacerbate the stress-induced deformation of the micromirror. In those applications, reducing the thickness of the micromirror surface to reduce stress-induced deflection of the entire assembly is not possible without degrading the mirror's performance in the wavelength band of interest. Also, further reduction in reflecting area of the micromirror to reduce warping introduces manufacturing challenges for the typically thick, multi-layer dielectric mirrors. [0008] A need exists, therefore, for a structure and method to reduce the deformation of micromirrors incorporating thick or complex optical coatings such as dielectric reflectors induced by intrinsic and thermal stresses without requiring a reduction in reflecting area of such micromirrors. SUMMARY OF THE INVENTION [0009] A micromirror apparatus is disclosed for use in micro electrical mechanical (MEMS) devices. It has a device layer having a recess, a multilayer thin-film dielectric reflector coupled to and structurally supported by the device layer on the opposite side of the device layer from said recess, and a stress compensator seated in the recess, with the stress compensator operable to resist device layer bending moments resulting from intrinsic and thermal mismatch stresses between the multilayer thin-film dielectric reflector and the device layer. [0010] A micromirror apparatus is also disclosed that has two multilayer thin-film dielectric reflectors carried on opposite sides of the device layer with the second reflector seated in the device layer. Each of the reflectors shares a common linear thermal expansion coefficient to reduce warping of the device layer in response to intrinsic and thermal mismatch stresses between the first reflector and the device layer. BRIEF DESCRIPTION OF THE DRAWINGS [0011] The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. Like reference numerals designate corresponding parts throughout the different views. [0012] FIG. 1 is a perspective view of a stress-compensated micromirror in accordance with one embodiment of the invention; [0013] FIG. 2 is a top plan view of the embodiment of the invention illustrated in FIG. 1. [0014] FIG. 3 is a cross-section view of the embodiment of the invention shown in FIG. 2 along the line 3-3. [0015] FIGS. 4A-4E are cross-sectional views illustrating various stages of fabrication of the embodiment of the invention illustrated in FIG. 3; [0016] FIG. 5 is a perspective view of an array of stress-compensated micromirrors in accordance with an embodiment of the invention; and DETAILED DESCRIPTION OF THE INVENTION [0017] A micromirror device is described that compensates for intrinsic and thermal mismatch stresses without resorting to disadvantageous reduction in the mirror's reflecting area or reflecting surface thickness. A stress compensator is seated in the support of a multilayer thin-film dielectric reflector (a "device layer") on a side opposite to that of the reflector. Mismatch stresses created between the stress compensator and the device layer are approximately equal to those mismatch stresses created between the multilayer thin-film dielectric reflector and the device layer, creating opposite bending moments and resulting in improved micromirror flatness both as-fabricated and during subsequent thermal environmental changes. [0018] In one embodiment of the invention illustrated in FIG. 1, a micromirror assembly 100 has a device layer 105 providing structural support for a highly reflective micromirror reflective layer 110. The device layer 105 is preferably single-crystal silicon (Si), but can be any of a variety of materials used for the structural layers in a MEMS device, such as polysilicon, metals, or dielectric thin films, to allow fabrication of the micromirror assembly in standard surface or bulk micromachining processes. The micromirror reflector layer 110 is preferably a thin film reflector built up from multiple layers of dielectric materials. Highly reflective multilayer interference coatings comprised of dielectric thin films are well known, with the materials and thicknesses of the layers selected to achieve particular optical performance characteristics. Other materials and approaches can similarly be adopted to achieve the highly reflective surface. These may include thin or thick metal layers (individually or in combination, such as Au, Ag, Au/Ag) or metal-containing compounds (such as hydrides, nitrides, silicides, carbides, etc.) The device layer 105 provides a substantially planar structural support for the micromirror reflective layer 110. [0019] Flexures 115 are connected to the device layer 105 and ultimately to a rigid base substrate 130 (connection not shown) (otherwise referred to as a "support substrate") to enable mechanical movement of the micromirror relative to the base substrate 130. These compliant flexures are typically designed to achieve particular mechanical characteristics, such as mechanical stiffness and resonant frequencies, commonly dictated by the application and other elements of the complete micromirror assembly. For the embodiment of a micromirror device illustrated in FIG. 1, which uses single crystal Si as the structural support for the reflective layer 100, the flexures 115 are formed from the Si device layer 105, and may further be thinned in an etching step to provide greater mechanical compliance than what would otherwise exist without such thinning. The flexures 115 allow elastic coupling of the micromirror assembly 100 to a fixed flexure support at a distal end 117 of the flexures 115. The fixed flexure support is preferably defined by a continuation of the etched device layer 105 (see 205, below) and flexures 115 and is itself coupled to the base substrate 130 (connection not shown). [0020] A stress compensator, preferably a stress compensator layer 120, is formed on a side opposite to the reflector layer 110 and is formed partially seated in the device layer 105 to reduce the height of that portion of stress compensator layer 120 extending from the surface of the device layer 105. The material of the stress compensator layer 120 is preferably substantially similar to the material of reflector layer 110 so that a stress induced in the device layer 105 (which would also introduce a bending moment in such layer 105) as a result of intrinsic and/or thermal mismatch stresses between the reflector layer 110 and device layer 105 is opposed by approximately equal intrinsic and/or thermal mismatch stresses between the stress compensator layer 120 and device layer 105. More particularly, the reflector layer 110 and stress compensator layer 120 preferably have equal linear thermal expansion coefficients to accomplish the function of balanced thermal mismatch stresses. Similarly, the intrinsic stress associated with the stress compensator layer 120 should be substantially similar to that of the reflector layer. Continue reading... Full patent description for Thermal and intrinsic stress compensated micromirror apparatus and method Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Thermal and intrinsic stress compensated micromirror apparatus and method patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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