US 20070195439 A1
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.
1. A micromirror apparatus, comprising:
a device layer having a recess;
a multilayer thin-film dielectric reflector coupled to and structurally supported by said device layer on the opposite side of said device layer from said recess;
a stress compensator seated in said recess, said stress compensator operable to resist device layer bending moments resulting from intrinsic and thermal mismatch stresses between said multilayer thin-film dielectric reflector and said device layer.
2. The apparatus according to
3. The apparatus according to
a flexure extending from said device layer.
4. The apparatus according to
5. The apparatus according to
a mechanical support connected to said flexure, said flexure enabling movement of said device layer relative to said mechanical support.
6. The apparatus according to
a support column coupled to said mechanical support;
a supporting substrate coupled to said support column, said supporting substrate spaced adjacent to said stress compensator.
7. The apparatus according to
8. The apparatus according to
9. The apparatus according to
10. The apparatus according to
11. The apparatus according to
12. A micromirror apparatus, comprising:
a device layer;
first and second multilayer thin-film dielectric reflectors carried on opposite sides of said device layer, said second multilayer thin-film dielectric reflector seated in said device layer and having a common linear thermal expansion coefficient with said first multilayer thin-film dielectric reflector to reduce warping of said device layer in response to intrinsic and thermal mismatch stresses between said first multilayer thin-film dielectric reflector and said device layer.
13. The apparatus according to
14. The apparatus according to
a flexure support connected by said flexure to said device layer, said flexure enabling movement of said device layer relative to said flexure support.
15. The apparatus according to
a support substrate coupled to said flexure support, said support substrate comprising active control circuitry to provide actuation of said first multilayer thin-film dielectric reflector.
16. The apparatus according to
a support substrate coupled to said flexure support, said support substrate comprising active control circuitry to provide electrostatic actuation of said first multilayer thin-film dielectric reflector.
17. The apparatus according to
18. A micromirror array, comprising:
a plurality of micromirror structures, each structure comprising:
a device layer;
a multilayer thin-film dielectric reflector coupled to and structurally supported by said device layer;
a stress compensator seated in the opposite side of said device layer from said multilayer thin-film dielectric reflector, said stress compensator operable to resist device layer bending moments resulting from intrinsic and thermal mismatch stresses between said micromirror and said substrate; and
respective flexures extending from said device layers.
19. The array of
respective supports connected to said flexures, said flexures enabling movement of said device layers relative to said supports.
20. The array of
respective electrodes positioned to actuate movements of corresponding device layers in response to applied voltages between said electrodes and their respective device layers.
21. The array of
22. The array of
a base substrate which provides support for each of said electrodes.
1. Field of the Invention
This invention relates to micromirrors, and more particularly, micromirrors used in micro electrical mechanical systems (MEMS) devices.
2. Description of the Related Art
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.
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.
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)
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.
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.
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.
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.
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.
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.
In one embodiment of the invention illustrated in
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
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.
Preferably, the stress compensator and reflector layers 120, 110 would have identical lateral dimensions, permitting identical 3D layer structures to be used for stress balancing. Differences between the lateral dimensions of the two may be required by the specific micromirror device design or process used, and in these cases slight differences in compensator thickness or fabrication process parameters may be used to accomplish a substantially similar intrinsic stress as that generated by the reflector layer. Similarly, for the thermal expansion stresses, the stress compensator layer 120 may accomplish the same function if composed of approximately the same volume fraction of the component materials as the reflector layer but may require different thicknesses to achieve the same thermal expansion stress as a function of temperature. For example, although illustrated in
Also, although labeled as a “layer”, if the reflector layer 110 is a thin film reflector, the stress compensator layer 120 would preferably be formed from multiple layers of dielectric materials similar to the reflector layer 110 to accomplish a comparable structure, material distribution and total thickness as exists with the reflector layer 110.
An electrode 125 sits on the base substrate 130 and is spaced apart from and in complementary opposition to the device layer 105. The electrode and device layer 105 function as electrically isolated counterelectrodes. Upon application of a voltage differential between them, the micromirror assembly 100 will mechanically deflect in an essentially vertical fashion with respect to the electrode 125. Upon removal of the voltage differential, the micromirror assembly 110/105/120 returns to its resting position by virtue of release of elastic energy stored in the compliant flexures. In the preferred embodiment, the mechanical deflection of the micromirror is accomplished using electrostatic actuation, although alternate actuation methods, such as thermal, piezoelectric, electromagnetic, Lorentz force, or others may be used without limitation within the scope of the invention. While the specific example described above is for a vertical-motion (piston) device, the present invention can apply equally well to a micromirror designed for one-axis tilt, two-axis tilt, or combined piston-tilt operation without limitation.
The plan view of
In a preferred embodiment, the mechanical deflection of the micromirror is accomplished using electrostatic actuation, although alternate actuation methods, such as thermal, piezoelectric, electromagnetic, Lorentz force, or others may be used without limitation within the scope of the invention.
A substrate electrode 305 is formed preferably on, or in, the base substrate 130 by conductive thin films, such as metals, or by suitably doping the substrate material to sufficiently low resistivity. In one embodiment, an insulating layer 310 sits on the substrate electrode layer 305 (not shown in
While the specific embodiments described in
The use of multiple layers of dielectric thin films to create transmissive or reflective optical devices such as that preferably used by the reflective surface 110 is well known, and described in references such as: “Thin-Film Optical Filters, Third Edition”, by H. Angus Macleod. IoP, 2001, or “Optical Interference Coatings,” by Norbert Kaiser and H. K. Pulker, Editors. Springer, 2003.
Selection of the materials, thicknesses, and stacking sequences of these layers, provides great design flexibility in tailoring the optical response characteristics (for example transmission or reflection as a function of wavelength) of the device. One well-known multilayer dielectric stack configuration uses alternating layers of material with high optical index of refraction and low index of refraction, each at a thickness of one-quarter wavelength optical thickness at the desired operating wavelength. This layer structure will create a high optical reflectance at the design wavelength. In this structure, a quarter-wave optical thickness of high index material is denoted H and a quarterwave optical thickness of low index material is denoted L. The multilayer device structure of this device, referred to as a quarterwave stack, may be described by the notation:
Another type of optical thin film device is the gradient index, or rugate, filter. In these devices, sinusoidal variations in optical index of refraction as a function of thickness are created in the thin film structure. Fabrication processes such as controlled co-deposition of high-and low-index material or sequential deposition of digital approximations can be used to accomplish the desired index profiles. These devices are characterized by high levels of reflection over a narrow wavelength range, and are described in:
The level of reflectance of the rugate filter at the design wavelength will depend in part on the number of periods of the sinusoidal variation in index with thickness.
In both examples (quarterwave stack and rugate filter), the level of reflectance at the design wavelength will depend on the number of periodic cycles of index of refraction. Thus, to achieve higher levels of optical reflectivity one must use larger numbers of dielectric film layers, which results in greater total thicknesses for the thin film stack.
The supporting substrate 130 has been shown as a purely mechanical element in the embodiment described above. In an alternate embodiment, the supporting substrate may contain active electronic circuitry used to provide the electrical drive signals needed to actuate the individual mirror elements. For the case of electrostatically-actuated micromirrors, the circuitry would provide a varying voltage signal to control mirror deflection. In these embodiments, the materials and processes used for the micromirror-to-substrate bonding step would be selected to maintain compatibility with the restrictions of the circuit wafer.
While various implementations of the application have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention.