Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-10-11
2004-11-30
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S291000
Reexamination Certificate
active
06825968
ABSTRACT:
FIELD OF THE INVENTION
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 electromechanical systems (MEMS) in the form of micromirror device arrays for use in optical display, adaptive optics and/or switching applications. Optionally, the invention also comprises individual or isolated micromirror elements.
BACKGROUND
Generally, 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.
In display systems, 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.
The unique properties of current and future 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. Micromirror-based display systems now offer compact, high resolution and high brightness alternatives to other existing technology.
Presently, such systems are further characterized by: 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); and 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.
Some of this progression is published on the world wide web site of Texas Instruments. Further review and technical details as may be employed (including in the present invention) are presented in
MEMS and MOEMS Technology and Applications
, by P. Rai-Choudhury, 169-208 (SPIE Press, 2000).
Despite such advances in design, several aspects of known micromirror devices may be further improved. First, 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. In addition, less complicated manufacturing procedures, including a process requiring fewer masks or steps for production of micromirror devices would be desirable.
Still further, performance aspects of existing micromirror devices can be improved. One such aspect concerns increasing the percentage of light return from the micromirrors. Another involves the angular displacement that can be realized in deflecting a given mirror. The overall deflection ability or total angular resolution can be particularly important in terms of optical switching applications as well as in the contrast ratio of image production.
Yet another performance aspect in which improvement is possible concerns power consumption. 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.
Yet another avenue for 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
Bocking Frank P.
Dang Hung Xuan
Exajoule LLC
Tra Tuyen
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