Concave and convex micromirrors and methods of making the same

A method comprising providing a first substrate and forming a first sacrificial layer over the first substrate, the first sacrificial layer comprising a curved surface portion, and forming a curved micromirror by depositing a reflective material over at the curved surface portion.

FIELD OF THE INVENTION

This invention relates to micromirrors, methods of making the same and products using the same, and more particularly to micromirrors, digital micromirror devices, projection systems and methods of making the same.

BACKGROUND OF THE INVENTION

New advancements in projection systems utilize an optical semiconductor known as a digital micromirror device. A digital micrometer device chip may be the world's most sophisticated light switch. It contains an array from 750,000 to 1.3 million pivotally mounted microscopic mirrors. Each mirror many measure less than ⅕ of the width of a human hair and corresponds to one pixel in a projected image. The digital micromirror device chip can be combined with a digital video or graphic signal, a light source, and a projector lens so that the micromirrors reflect an all-digital image onto a screen or onto another surface.

Although there are variety of digital micromirror device configurations, typically micromirror are mounted on tiny hinges that enable each mirror to be tilted either toward the light source (on) in a projector system to reflect the light or away from the light source (off) creating a darker pixel on the projection surface. A bitstream of image code entering the semiconductor directs each mirror to switch on or off several times per second. When the mirror is switched on more frequently than off the mirror reflects a light gray pixel. When the mirror is switched off more frequently than on the mirror reflects a darker gray pixel. Some projection systems can deflect pixels enough to generate 1024 shades of gray to convert the video or graphic signal entering the digital micromirror device into a highly detailed grayscale image. In some systems, light generated by a lamp passes through a color wheel as it travels to the surface of the digital micromirror device panel. The color wheel filters to light into red, green and blue. A single chip digital micromirror vice projector systems can create at least 16.7 million colors. When a prism is used to divide a light source into red, green and blue light and three digital micromirror device chips are utilized, more than 35 trillion colors can be produced. The on and off states of each micromirror are coordinated with the three basic building blocks of color: red, green and blue to produce a wide variety of colors.

A variety of digital micromirror devices (DMD) are known.FIG. 1illustrates one embodiment of a prior art DMD that may be used in the present invention with the substitution of a unique mirror structure according to the present invention. As shown inFIG. 1, a DMD10may include a semiconductor device12such as a CMOS memory device that includes circuitry13that is used to activate an electrode(s) in response to a video or graphic signal. A first layer14is formed over the semiconductor device12and may include a yoke address electrode16, and vias18formed therein down to the circuitry13on the semiconductor device12, and a bias-reset bus20. A second layer22is formed over the first layer14and may include a yoke24torsion hinge26and mirror address electrodes28. A micromirror32is formed over the second layer22and positioned so that the micromirror32may be deflected diagonally when one of the electrodes28is activate by the semiconductor device12. The micromirror32includes a reflective layer typically including aluminum. The DMD10shown inFIG. 1while being an excellent engineering accomplishment is very complex, costly to manufacture and has low manufacturing yield.

FIG. 2illustrates a prior art projector system300that includes an array of micromirrors302, typically formed on a semiconductor chip. The array of micromirror302may be attached to a printed circuit board304or similar substrate that include additional microelectronic devices306,308to perform video processing of video or graphic signal and scaling of the image to be projected. A bright light source310is provided and a first optical lens312may be present and positioned to direct light from the source310through a color wheel314. The color wheel314includes transparent sections with different color filters such as red, green and blue filters. Additional color filters and clear sections may be provided on the color wheel314. Light emitted from (or passing through) the color wheel314may be focused by a second optic lens316onto the array of micromirrors302so that each micromirror is operated to selectively reflect (or not) the light projected thereon. Light reflected from the array of micromirrors302may be focus by a third optic lens318onto a wall or screen320.

A variety of different micromirror configurations are known to provide pivotal movement of the micromirrors. Huibers et al U.S. Pat. No. 6,396,619 discloses a deflectable spatial light modulator including a mirror plate that is substantially ridge and may be made up of a laminate having layers of silicon nitride and aluminum. In one embodiment, the mirror laminate may include a layer of aluminum sandwiched by two layers of silicon nitride. In other embodiments, include only a layer of aluminum and a layer of silicon nitride is provided. Multi-layer arrangements with multiple layers of aluminum and/or silicon nitride are disclosed. The reference states that other materials besides aluminum (such as conductive and reflective metals) could be used. Other materials besides silicon nitride, such as silicon dioxide are also disclosed. The reference discloses that the silicon nitride layer may be 1400 A thick and that the aluminum layer may be 700 A thick. Disclosed also is one or more dielectric films, that act as a reflective coating, may be deposited on the mirror laminate to improve reflectivity.

The present invention provides alternatives to and improvements over the micromirror, DMD and projection systems of the prior art.

SUMMARY OF THE INVENTION

A method comprising providing a first substrate and forming a first sacrificial layer over the first substrate, the first sacrificial layer comprising a curved surface portion, and forming a curved micromirror by depositing a reflective material over at the curved surface portion.

A product comprising a micromirror assembly comprising a micromirror comprising a reflective layer comprising a first face for reflecting light and an opposite back face, the first face having one of a generally convex shape and a generally concave shape

These and other embodiments of the present invention will become apparent from the following brief description of the drawings, detailed description of the preferred embodiments, and appended claims and drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 3illustrates a first subassembly40for a second type of DMD. The subassembly40may include a transparent layer42that may be any transparent material including, but not limited to, glass. A hinge44is formed on the transparent layer44and a micromirror32is secured thereto for pivotal movement with respect to the hinge44and the transparent layer42.

FIG. 4illustrates the first subassembly40including a plurality of micromirrors32each connected by a hinge44to the transparent layer42. All of the component and subassemblies of the various DMD devices can be made by semiconductor or MEM micro processing techniques known to those skilled in the art.

FIG. 5illustrates a second subassembly46of the second type of DMD and may include a semiconductor device12such as, but not limited to, a CMOS memory device. A plurality of electrodes48, one for each micromirror32are formed over the semiconductor device12for communication with the circuitry (not shown) contained therein so that the electrode48may be selectively activated in response to a video or graphic signal.

FIG. 6illustrates a DMD structure10that may be utilized by the present invention with the substitution of a unique micromirror according to the present invention. The DMD ofFIG. 6includes the first subassembly40flipped over and overlying the second subassembly46so the micromirrors32of the first subassembly40face and are closest to the electrodes48of the second subassembly46. Post50are provided and positioned so that the micromirrors32are spaced a distance from the electrodes48and so that micromirror32is free to be defected or pivotally moved by the activation of an associated electrode48. The first and second subassemblies40and46may be formed from a single silicon substrate with additional deposited on top and a sacrificial layer(s) remove to provide the space between the micromirrors32and the electrode40. As illustrated inFIG. 5, when light is director on to the micromirrors32, an electrode48associated with for each micromirror32may be activated causing the micromirror to pivotally move about the hinge44. As a result, the light will be reflected or not depending on whether the electrode48associated with the micromirror32has been activated or not. As described above, depending on how fast and often a particular micromirror32is deflected by the electrode48, the image projected by the micromirror32(pixel) will appear light or dark on the projection screen or other surface. The micromirror32shown inFIGS. 3-6is flat. A micromirror according to the present invention, including concave and convex micromirrors, may be substituted for the flat micromirrors in the subassemblies40and46ofFIGS. 3-6.

FIG. 7Aillustrates one embodiment of the invention including forming a first sacrificial layer10over a first substrate12. The first substrate12may be a semiconductor wafer, ceramic, plastic, fiberglass board, flexible board, or any other substrate useful in making microelectronic devices known to those skilled in the art. An electrode48may be provided on the first substrate12.FIG. 7Billustrates another embodiment of the invention, including forming a second sacrificial layer400over the first sacrificial layer110. The second sacrificial layer400has an upper surface406that is substantially convex in shape. In one embodiment, the first sacrificial layer110includes silicon, for example amorphous silicon, and the second sacrificial layer400is formed by growing field oxide from the amorphous silicon. The field oxide may be grown by exposing the amorphous silicon to oxygen in the form of dry oxygen gas, or steam. Field oxide growth is a well known process that is very controllable. The process parameters for growing the field oxide may be controlled to ensure that the upper surface406is substantially convex in shape. However, it is not necessary for the profile of the upper surface406to be symmetrical to be within the scope of the invention. Thereafter a micromirror32may be formed over the upper surface406of the field oxide400. The micromirror32may be a laminate of several layers including a reflective layer100that has a reflective surface (upper surface) that is substantially convex in shape. As shown inFIG. 7C, a third sacrificial layer112, such as amorphous silicon, may be formed over the second sacrificial layer400and the micromirror32. Post50may be provided in the first and second sacrificial layers110,112. A transparent layer42, such a glass, may be formed over the third sacrificial layer112. A hinge44may be provided pivotally connecting the micromirror32to the transparent layer42. Of course, the hinge44may be formed to pivotally connect the micromirror32to the first substrate12. As shown inFIG. 7D, the first, second and third sacrificial layers may be removed to provide a micromirror32with a convex reflective surface pivotally connected to one of the transparent layer42or the first substrate12. The amorphous silicon may be removed, for example, by etching with XeF2 gas, and the field oxide (silicon dioxide) may be removed by etching with CHF3/O2 gas mixture.

The curved micromirror32may be a laminate of several layers including a reflective layer that includes a light reflecting material such as, but not limited to, at least one of aluminum or silver. In one embodiment, the reflective layer may be 2000-4000 A (angstroms) thick, and preferably 2400-3000 A thick, and most preferably 2500 A thick. In one embodiment, the reflective layer includes aluminum, silicon and copper. In another embodiment, the reflective layer includes 98.5 weight percent aluminum, 1 weight percent silicon and 0.5 weight percent copper. The reflective layer may be formed by any method known to those skilled in the art, including screen printing, chemical vapor deposition, by securing a foil to a first protective layer (such as silicon nitride), but preferably is formed by sputtering a reflective material onto the first protective layer or onto another surface from which the reflective layer100can be removed.

As shown inFIG. 8A, another embodiment of the invention including forming a first sacrificial layer110over a first substrate12. Again, the first substrate12may be a semiconductor wafer, ceramic, plastic, fiberglass board, flexible board, or any other substrate useful in making microelectronic devices known to those skilled in the art. An electrode48may be provided on the first substrate12.FIG. 8Billustrates another embodiment of the invention including forming a second sacrificial layer400over the first sacrificial layer110. In one embodiment, the first sacrificial layer110includes silicon, for example amorphous silicon, and the second sacrificial layer400is formed by growing field oxide from the amorphous silicon. The field oxide may be grown by exposing the amorphous silicon to oxygen in the form of dry oxygen gas, or steam. Field oxide growth is a well known process that is very controllable. The growth of the field oxide consumes a portion of the underlying silicon providing a silicon/field oxide interface. The process parameters for growing the field oxide may be controlled to ensure that the upper surface408of the first sacrificial layer110, at the silicon/field oxide interface, is substantially concave in shape. However, it is not necessary for the profile of the upper surface408to be symmetrical to be within the scope of the invention. Thereafter, as shown inFIG. 8C, the second sacrificial layer400is removed and a micromirror32may be formed over the upper substantially concave surface408of the first sacrificial layer110. The micromirror32may be a laminate of several layers including a reflective layer100(upper surface) that has a reflective surface that is substantially convex in shape. As shown inFIG. 8D, a third sacrificial layer112, such as amorphous silicon, may be formed over the first sacrificial layer110and the micromirror32. Post50may be provided in the first and second sacrificial layers110,112. A transparent layer42, such a glass, may be formed over the third sacrificial layer112. A hinge44may be provided pivotally connecting the micromirror32to the transparent layer42. Of course, optionally the hinge may be formed to pivotally connect the micromirror32to the first substrate12. As shown inFIG. 8E, the first and third sacrificial layers may be removed to provide a micromirror with a convex reflective surface pivotally connected to one of the transparent layer42or the first substrate12. Again, the amorphous silicon may be removed by, for example etching with XeF2 gas and the field oxide (silicon dioxide) may be removed by etching with CHF3/O2 gas mixture.

When the terms “overlying”, “overlie”, “over” and the like terms are used herein regarding the position of one component of the invention with respect to another component of the invention, such shall mean that the first component may be in direct contact with the second component or that additional components such as under bump metallurgies, seed layers and the like may be interposed between the first component and the second component.