Patent Application
20050078379
The present invention is related generally to display systems, and, more particularly, to micromirror-based display systems.
The ratio of luminance between the brightest white in a display system to the darkest black that can be produced is called the contrast ratio. In a micromirror based display system, higher contrast ratios can be achieved when there is a larger difference in angle between light reflected from the mirrors in their ON state and light reflected from the mirror in the OFF state. One way to increase the difference in reflected angles is by having the micromirrors be electrostatically deflected in opposite directions (to ON and OFF states) from a non-deflected state.
Micromirror based display systems that have only a single addressing electrode per micromirror often deflect the micromirror to an ON state, but leave the micromirror undeflected in the OFF state (or deflect minimally the micromirror in the OFF state). This results in a decrease in the difference in angle of the light reflected in the ON state vs. the OFF state, thus decreasing contrast ratio. Therefore, what is needed for such single addressing electrode systems, is higher contrast ratio.
In view of the foregoing, the present invention provides a display system employing a spatial light modulator having an array of deflectable reflective micromirrors with improved contrast ratio. The objects and advantages of the present invention will be obvious, and in part appear hereafter and are accomplished by the present invention. Such objects of the invention are achieved in the features of the independent claims attached hereto. Preferred embodiments are characterized in the dependent claims.
On aspect of the invention is improving the contrast ratio of a micromirror based display system by providing the display system with an array of micromirrors each of which can be switched between an ON and OFF state angle by one addressing electrode, wherein the difference between the ON and OFF state angle is from 12° degrees to 22° degrees such that the contrast ratio of the display system can be 1200:1 or higher.
Another aspect of the invention is improving the contrast ratio of a micromirror based display system by providing the display system with an array of micromirrors, each of which can be switched between an ON and OFF state angle by one addressing electrode, and wherein the ON state angle is 14° or higher.
While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which:
a schematically illustrates a cross-section view of a micromirror at an OFF state in the spatial light modulator of
b schematically illustrates a cross-section view of a micromirror at another OFF state in the spatial light modulator of
c schematically illustrates a cross-section view of a micromirror at an ON state in the spatial light modulator of
a illustrates a bottom view of another exemplar micromirror;
b shows a bottom view of the hinge support in the micromirror of
a and
c is a top views of a portion of another exemplar micromirror array in a spatial light modulator;
d illustrates the incident angle of the illumination light to the reflecting surface of the mirror plate;
a illustrates a top view of a portion of an array of micromirrors usable in the spatial light modulator;
b illustrates a top view of a portion of an array of addressing electrodes and circuitry for addressing the micromirrors of the micromirror array in
Turning to the drawings, an exemplary micromirror based display system is illustrated in
Illumination system 101 further comprises light source 102, which can be an arc lamp, lightpipe 104 that can be any suitable integrator of light or light beam shape changer, and color filter 106, which can be a color wheel. The filter in this particular example is positioned after light pipe 104 at the propagation path of the illumination light. In another example, the color filter can be positioned between the light source and light pipe 104, which is not shown in the figure.
An exemplary rear-projection system having a reduced dimension is set forth in U.S. patent application, publication number US20040114230 to Peterson et al, filed Dec. 5, 2003, the subject matter being incorporated herein by reference. More complex display systems are also applicable, especially those having more than one spatial light modulator for color images, such as the display system in
Referring to
In operation, incident white light 174 from light source 102 enters into TIR 176a and is directed towards spatial light modulator 186, which is designated for modulating the blue light component of the incident white light. At the dichroic surface 198a, the green light component of the totally internally reflected light from TIR surface 205a is separated therefrom and reflected towards spatial light modulator 182, which is designated for modulating green light. As seen, the separated green light may experience TIR by TIR surface 205b in order to illuminate spatial light modulator 182 at a desired angle. This can be accomplished by arranging the incident angle of the separated green light onto TIR surface 205b larger than the critical TIR angle of TIR surface 205b. The rest of the light components, other than the green light, of the reflected light from the TIR surface 205a pass through dichroic surface 198a and are reflected at dichroic surface 198b. Because dichroic surface 198b is designated for reflecting red light component, the red light component of the incident light onto dichroic surface 198b is thus separated and reflected onto spatial light modulator 184, which is designated for modulating red light. Finally, the blue component of the white incident light (white light 174) reaches spatial light modulator 186 and is modulated thereby. By collaborating operations of the three spatial light modulators, red, green and blue lights can be properly modulated. The modulated red, green and blue lights are recollected and delivered onto display target 114 through optic elements, such as projection lens 202, if necessary.
The spatial light modulator, in general, comprises an array of a hundred thousands or more, or a million or more of micromirrors, the total number of which determines the resolution of the displayed images. For example, the micromirror array of the spatial light modulator may have 1024×768, 1280×720, 1400×1050, 1600×1200, 1920×1080, or even larger number of micromirrors. In other applications, the micromirror array may have fewer micromirrors.
For achieving a high resolution and optical efficiency while maintaining the same dimension of the micromirror array so as to be compatible with commonly used light source, such as arc lamps which often has small arc sizes (e.g. 1.0 mm or shorter), the micromirror array preferably has certain dimensions as set forth in U.S. patent application Ser. No. 10/627,303 filed Jul. 29, 2003, the subject matter being incorporated herein by reference. For example, the micromirror array preferably has a pitch size of 10.16 micrometers or less or from 4.38 to 10.16 micrometers with the pitch defined as the distance between adjacent micromirrors in the micromirror array. The gap between adjacent micromirrors is preferably 0.5 micrometers or less, or from 0.15 to 0.5 micrometer.
In this particular example, spatial light modulator 110 comprises an array of micromirrors (e.g. micromirrors 214 and 216) that are individually addressable and deflectable. The micromirrors are formed on substrate 210, which is a substrate transmissive to visible light, such as glass or quartz. For deflecting the micromirrors, an array of addressing electrodes (e.g. addressing electrode 220) is formed on semiconductor substrate 132, which is placed proximate to the micromirror array such that the mirror plates of the micromirrors can be deflected in response to electrostatic forces derived from electrostatic fields that are established between the mirror plates and addressing electrodes. In accordance with an embodiment of the invention, each mirror plate of a micromirror is associated with a single addressing electrode that is connected to a voltage output of a circuitry (e.g. a memory cell) storing a voltage signal, the magnitude of which changes according to image data of the desired images. The image data, such as bitplane data, can be produced according to a pulse-width-modulation algorithm. In another embodiment, each mirror plate can be associated with another electrode, on which a bias voltage (e.g. a constant electrical voltage) can be applied during the operation.
In response to the electrostatic force, the mirror plates of the micromirrors rotate to either an ON state or an OFF state. In the ON state, the mirror plate (e.g. the mirror plate of micromirror 216) reflects the incident light into the projection lens-resulting in a bright pixel in the display target, and in the OFF state, the mirror plate (e.g. the mirror plate of micromirror 214) reflects the incident light away from the projections lens-resulting in a dark pixel in the display target. Specifically, the mirror plates rotate to the ON state due to the electrostatic forces applied thereon, while rotations of the mirror plates to the OFF state from the ON state can be accomplished through internal restoration forces established during the deformation of the hinges to which the mirror plates attached. Alternatively, rotation of the mirror plates to the OFF state can be accomplished through application of an electrostatic voltage on a substrate, such at substrate 210 on which the micromirrors are formed.
In the above example, the micromirrors are formed on light transmissive substrate 130 separate from substrate 132 on which the addressing electrodes are formed. Alternatively, the micromirrors and the addressing electrodes can be formed on the same substrate, preferably a semiconductor wafer, such as semiconductor substrate 132. In another embodiment of the invention, the micromirror substrate can be formed on a transfer substrate that is light transmissive. Specifically, the micromirror plate can be formed on the transfer substrate and then the micromirror substrate along with the transfer substrate is attached to another substrate such as a light transmissive substrate followed by removal of the transfer substrate and patterning of the micromirror substrate to form the micromirror.
According to the invention, the display system has a contrast ratio that is 1000:1 or higher, such as 1200:1 or higher, 1500:1 or even 2000:1 or higher. To achieve such contrast ratios, among other things, the micromirrors are made such that the reflected light from the mirror plates at the ON and OFF states are separated as far away as possible, or at least not mixed. Degradation of the contrast ratio due to mixture of the reflected light from the mirror plates at the ON and OFF state can thus be avoided or suppressed.
For example, when the mirror plates can only be deflected at small angles for the ON state, the angle between the vertical direction along which the projection lens is disposed and the reflected light from the mirror plate at the OFF state (e.g. the non-deflected state) is small. Part of the reflected light from the mirror plate at the OFF state may travel through the projection lens and mix with the reflected light from the mirror plate at the ON state. This situation is deteriorated when the incident light has finite size and opens a solid angle toward the micromirrors. Accordingly, the desired dark pixels will not be as dark as they should be. Accordingly, the contrast ratio of the produced images is poor.
To solve this problem, the present invention provides a micro-mirror that can be deflected at large angles such that the reflected light from the non-deflected and deflected states are safely separated, even if only one addressing electrode is provided per micromirror. An exemplary micro-mirror according to an embodiment of the invention is illustrated in
Referring to
The mirror plate is attached to the deformable hinge such that the mirror plate extends a greater distance from the hinge on one side of the hinge than on the other side of the hinge. For example, the distance Lb measured from the left end of the mirror plate to the hinge is less than the distance La measured from the right end of the mirror plate to the hinge. This asymmetric configuration enables the mirror plate rotating relative to the substrate by the hinge at much larger angles. Reflected light from different operational states that are defined in accordance with rotational positions (angles) of the mirror plate can thus be well separated. In this way, the contrast ratio of a displayed image by the micromirror can be improved.
In operation, binary operation states, such as the ON and OFF states, are defined for the micromirror according to the rotation status of the mirror plate and the relative position of the target (e.g. a screen or a viewer's eyes). For example, the OFF state can be the non-deflected resting state as the mirror plate is positioned parallel to the substrate as shown in
The ON state can be defined as the shorter portion (e.g. Lb) of the mirror plate being rotated toward the substrate and making an angle θ with the substrate as shown in
In accordance with an embodiment of the invention, the ON state angle can be from 14° degrees to 16° degrees, while the OFF state angle is from −1° degrees to −6° degrees. The difference between the ON and OFF state angles is from 12° to 18° degrees, or from 16° to 22° degrees, or from 22° to 23° degrees.
In a projection system, such as those illustrated in
For improving the contrast ratio by enhancing the reflectivity of the mirror plate to the incident illumination light to be modulated, the mirror plate has a reflecting surface with the reflectivity to the incident illumination of 85% or higher, such as 95% or higher, or 97% or higher. This can be achieved by coating the reflecting surface with a proper light reflecting material, which is preferably a suitable metallic material, such as aluminum, silver, or gold. Moreover, the surfaces of the light transmissive substrate, such as substrate 210 in
For securing a uniform reflection of the incident light without introducing undesired light scattering from the reflecting surface of the mirror plate, it is preferred that the reflecting surface is substantially perfectly smooth. Specifically, the standard deviation of the surface roughness is preferably 0.1% or less, such as 0.01% or less.
Although it is advantageous to have a smooth reflecting surface of the mirror plate, the mirror plate, especially the reflecting surface preferably has limited negative curvature so as to improve the contrast ratio. Specifically, the reflecting surface is flat or curved with its edges towards the addressing electrode associated therewith, and the center of the reflecting surface away from the addressing electrode. The curvature is preferably less than 2.5 mm−1, and more preferably less than 1 mm−1.
The high rotation angle design can be implemented in a variety of micromirrors, such as a micromirror whose top view is illustrated in
Referring to
The large rotation angle can also be implemented in many other micromirrors, such as that shown in
Referring to
In the above example as shown in
When mapping individual micromirrors, such as the micromirrors in
In order to reduce the light scattering introduced by the openings of gaps without generating undesired diffraction patterns, the micromirrors of the spatial light modulator are arranged with selected gap sizes, which will be discussed with reference to
Referring to
The gap determines the fill factor of the micromirror array device, wherein the fill factor is defined as the ratio of the total area of the mirror plates of the micromirrors to the area of the micromirror array. For example, the fill factor can be calculated by: the area of a micromirror plate of the micromirror divided by the pitch squared, provided that the mirror plates of the micromirrors are identical and the pitch size is uniform over the entire micromirror array. In an embodiment of the invention, the fill factor of the micromirror array device is 85% or higher, and more preferably, 90% or higher.
According to an embodiment of the invention, the gap between the adjacent micromirrors in the micromirror array can be 0.5 micrometers or less, and in other embodiments the gap is from 0.1 to 0.45 micrometer, or from 0.25 to 0.35 micrometers. The pitch can be from 4.38 to 10.6 microns; or from 5 to 9 microns; or from 6 to 8 microns, as set forth in U.S. patent applications Ser. No. 10/627,302 and Ser. No. 10/627,155, both to Huibers and filed Jul. 24, 2003, the subject matter of each being incorporated herein by reference. The pitch of the pixel subset may also have other values, such as values higher than 10.6 microns.
The contrast ratio can be further improved by positioning the posts of each micromirror along the sides, instead of the opposite comers of the mirror plate, as shown in
In another way of describing this arrangement of the invention, the centers of the micromirrors in each row (the micromirrors disposed along the X-direction) are connected with an imaginary line, and the imaginary lines for all rows form a set of parallel imaginary lines along the X-direction. The centers of the micromirrors in each column (the micromirrors disposed along the Y-direction) are connected with another imaginary line, and all such imaginary lines form sets of parallel imaginary lines along the Y-direction. The imaginary lines along the X-direction are orthogonal to the imaginary lines along the Y-direction, and all imaginary lines form an orthogonal grid. Each one of the four predominant edges of each micromirror, however, is not parallel to an imaginary line of the grid.
In accordance with another embodiment of the invention, spatial light modulator 300, the micromirror array is rectangular in shape as illustrated in
Angle θ relates to the ratio of the width of the mirror plate to the width of the post. For example, assuming both of the post and mirror plate are square, angle θ can then be written as: ctgθ=(m/p−1), wherein m is the width of the mirror plate and p is the width of the post. Corresponding to the preferred range of angle θ from 2° to 30°, the ratio of m/p ranges from 0.7 to 27.6.
In operation, incident light beam 324 has an angle φ relative to the micromirror array plane, and the angle between the projection of the incident light on the micromirror array plane and the edge (e.g. edge 210) of the micromirror array is represented by ω, as shown in the figure. The incident angle φ is preferably from 50° to 70° degrees. Angle ω relative to edge 320 of the micromirror array is preferably from 50° to 65° degrees.
c illustrates another exemplary micromirror array that can be used for the spatial light modulator in the display systems, as those shown in
As a way of example, angle α can be from 5° to 30° degrees, such as from 10° degrees to 20° degrees, preferably around 12° degrees. Angle φ can be from 20° to 50° degrees, preferably around 33° degrees. Angle β is preferably determined upon the ON state angle θON of the mirror plate in operation, which is preferably equal to (90°−2×θON). For example, when θON is 16° degrees, angle β is preferably 58° degrees.
According to another embodiment of the invention, a micromirror array device having an array of addressing electrodes and an array of micromirrors is disclosed, wherein the mirror plates of the micromirrors in the array form a plurality of pockets, in which posts can be formed, and the pockets are covered by the extended areas of the addressing electrodes when viewed from the top of the micromirror array device, as shown in
Referring to
Referring to
For further improving the contrast ratio by reducing the undesired light scattering, the mirror plate may have tapered side edges, as shown in
In the above examples, each micromirror comprises two posts for supporting the micromirrors on the substrate. The posts, however, may also introduce undesired light scattering, which in turn reduces the contrast ratio. In accordance with another embodiment of the invention, only selected micromirrors of a spatial light modulator are provided with posts, as set forth in U.S. patent application “A micromirror array device and a method for making the same” to Patel, attorney docket number P136-US, which is filed along with the current application, the subject matter being incorporated herein by reference.
In addition to reducing the undesired light scattering using the methods as discussed above, other features may also be implemented in the micromirrors and/or the micromirror array. For example, a light blocking film can be formed on either one or both of the substrates (e.g. the substrate having the micromirrors formed thereon and the substrate having the addressing electrodes formed thereon) in a form of a continuous film, or strips, a grid or a frame or any combination thereof. When taking a shape of a grid, the light blocking film is formed such that the micromirrors (or the addressing electrodes) are located within the nets of the grid. When in a form of strip, the light blocking strips are preferably disposed in spaces between adjacent micromirrors (or addressing electrodes). The light blocking film may also be disposed on the addressing electrodes, the edges of the mirror plates, and/or the surfaces of the posts exposed to the incident light, as set forth in U.S. patent applications Ser. No. 10/305,631, Ser. No. 10/305,536, Ser. No. 10/305,507, Ser. No. 10/305,509, both filed Nov. 26, 2002, the subject matter of each being incorporated herein by reference.
In practical applications, the micromirror arrays are generally packaged for protection, as described in U.S. patent applications Ser. No. 10/443,318 filed May 22, 2003, and Ser. No. 10/852,981 filed May 24, 2004, the subject matter of each being incorporated herein by reference. The micromirror packages are often packaged between a package substrate having a supporting surface to which the micromirror array device is attached, and a package cover, which is transmissive to visible light and is hermetically bonded to the package substrate. A light blocking film can also be deposited on the package cover, preferably in the form of a frame surrounding the micromirror array, as set forth in U.S. patent application “Light Blocking Layer in MEMS Package”, attorney docket number P177-US, filed along with the present application. In particular, package cover can be slanted so as to improve the transmission of the incident light, as set forth in U.S. patent Ser. No. 10/343,307, filed Jan. 29, 2003, the subject matter being incorporated herein by reference.
It will be appreciated by those skilled in the art that a micromirror-based display system with improved contrast ratio has been described herein. In view of the many possible embodiments to which the principles of this invention may be applied, however, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of invention. For example, those of skill in the art will recognize that the illustrated embodiments can be modified in arrangement and detail without departing from the spirit of the invention. Therefore, the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof.
This is a continuation-in-part of a pending U.S. patent application Ser. No. 10/366,296 filed Feb. 12, 2003, the subject matter being incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10366296 | Feb 2003 | US |
| Child | 10969502 | Oct 2004 | US |
