Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Physical deformation
Reexamination Certificate
2001-01-18
2003-04-01
Everhart, Caridad (Department: 2825)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Physical deformation
C257S417000
Reexamination Certificate
active
06541831
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to micromirrors and to arrays of such micromirrors, and more particularly to a low voltage, single crystal silicon micromirror assembly having a high fill factor, and to methods of fabricating such micromirrors and arrays.
BACKGROUND OF THE INVENTION
Significant applications of microelectromechanical structures (MEMS) arise from the fact that the fabrication process for such devices allows individual microactuators to be organized into massive arrays which cooperatively perform a macroscopic function. A prominent example of the application of MEMS to such arrays, in which interconnected microactuators are used, are digital projection displays based on digital micromirror devices (DMD). This technology has been used in several applications, including fiber optic crossbar switches, wave front correctors used in adaptive optics and free space communication, optical beam steering, and variable optical gratings. The DMD technology is based on a released polysilicon thin film and metal micromachining process, and on flexible dielectric membrane fabrication using KOH wet silicon chemistry.
Mirror surfaces can be produced on such flexible dielectric membranes, and these have several advantages over conventional piezoelectrically-actuated mirrors. Fabrication is made considerably easier, because no discrete assembly is required, and the actuators for the mirrors can be integrated on the same chip. This process utilizes existing semiconductor fabrication technology to take advantage of batch fabrication processes so that the final cost is low. Performance is enhanced because the system is operated at low voltage and low power. Because the process permits a high actuator density, high spatial resolution is available, and the arrays are lightweight so that high frequency operation is possible.
The biggest drawback of the deformable mirrors produced by this thin film technology is a residual stress in the deposited thin films as well as a stress interaction between the substrate films and reflective mirror films. A polysilicon mirror deformation of 50 nm after the release of the structure has been reported. This produces deviations from the flatness required for a mirror surface, and although it is relatively small compared to the thickness of the mirror and the overall motion of the mirror that can be produced by actuators, such deviations represent significant fractions of the wave length of visible light and, therefore, adversely affect the performance of the mirror. Because of this residual stress problem, the rectangular size of such a mirror is limited to about 200 micrometers or less on each side. To relieve some of this stress, the mirror surface is often fabricated with apertures, which also help in the release process, but the resulting surface of the thin film can be optically rough, although post-deposition treatments can minimize this surface roughness.
Another difficulty with such thin film devices is that they are normally driven by capacitive forces, with the result that motion in the vertical, or z-direction, is based on parallel plate forces. As a result, the deflection of the mirror is not controllable if the motion exceeds about one third of the initial parallel plate gap.
Prior mirror actuators normally include a spring system for motion in response to signals supplied to the capacitor plates, and the design of such spring systems plays an important role in lowering the operating voltages and minimizing actuator area. A variety of spring designs have been developed, some using flexible torsional hinges as the spring system, with the hinges being hidden underneath the mirror structure. Deformable micromirror arrays composed of a flexible polysilicon membrane supported by an underlying array of electrostatic parallel plate actuators can provide extremely high fill factors, in the range of 90%. However, where the thin membrane acts as the spring system for the mirror structure, up to 150 V may be required to achieve a center deflection of 1 &mgr;m, due to the typical geometry of the membrane. Further, the deflection of the membrane may not be uniform, resulting in nonuniform vertical motion across the mirror surface. Tests of arrays using folded flexures attached to the moving mirrors produced an overall effective spring constant in the z-direction that was less than that of the membrane system so that a considerably lower operating voltage was achieved. However, both the flexures and the mirror had to be fabricated in the same layer so that any area taken up by the flexures would directly reduce the area of the array covered by the movable mirror surfaces. Thus, a low fill factor of only about 40% was achieved with this design.
SUMMARY OF THE INVENTION
The foregoing difficulties are overcome, in accordance with the present invention, by a micron-scale, single crystal silicon (SCS) micromirror assembly including a mirror platform having an optical surface, which is optically flat and smooth, free of residual stress, and which is highly reflective after the deposition of a thin metal layer. The assembly also includes a high aspect ratio MEMS actuator structure which supports the mirror platform and produces enhanced manipulation of the optical surfaces.
In accordance with a preferred form of the invention, a suspended, or released, drive actuator is fabricated in one surface of a double polished wafer, with the drive actuator supporting, and being precisely aligned with a corresponding micromirror platform structure fabricated in the opposite surface of the wafer. The polished wafer surface in which the mirror platform is fabricated provides an optically flat mirror surface for receiving a reflective coating such as a thin metal layer, or multiple layer thin films. The mirror assembly is fabricated in the wafer by a suitable process, such as the Single Crystal Reactive Etch And Metallization process (SCREAM) process described and illustrated in U.S. Pat. No. 5,426,070 to Shaw et al, issued Jun. 20, 1995, and is released from the wafer by a through wafer etch. The actuator is connected to the back of the corresponding mirror platform by rigid mounting posts to transfer the motion of the actuator to the mirror.
Micromirror assemblies may be fabricated in arrays of any desired size, utilizing known fabrication techniques, with the individual actuators being operable through individually addressable electrical connections. Routine silicon patterning can be done on the optically flat, SCS mirror surfaces for various optical applications, and scaling up of the arrays may be done.
The actuator which supports the mirror platform preferably utilizes an asymmetric comb finger design having interdigitated stationary and movable fingers having high aspect ratios. Vertical motion of the micromirror assembly with respect to the surface of the substrate is produced by applying a net electric field between adjacent fingers. By providing comb finger electrodes having different heights the range of motion limitations of other parallel plate actuator configurations are avoided.
The MEMS single crystal silicon micromirror platform provided by the present invention has sufficient thickness and rigidity to permit fabrication of features such as optical gratings on the platform and the mirror assembly has an excellent structural rigidity, to provide a uniform motion across the optical surface of each mirror upon operation of its corresponding actuator. By providing an array of mirror platforms on one side of a wafer and providing the corresponding actuators, contact pads, address metal lines and related structural features on the other side of the wafer, a high fill factor can be attained for the mirror array; that is, up to about 90% of the array surface is mirrored, with the remaining portion being taken up by the spaces between adjacent mirrors.
A MEMS micromirror assembly in accordance with the present invention is fabricated in a single crystal silicon (SCS) substrate or wafer using, in one embodiment, a two-mask process. Th
Lee Seung B.
MacDonald Noel C.
Cornell Research Foundation Inc.
Everhart Caridad
Jones Tullar & Cooper P.C.
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