Abstract:
A micro-display has a device chip with a transparent layer overlying one or more micro-electromechanical system devices. A transparent cover overlies the transparent layer. An index-of-refraction-matching medium is interposed between the transparent layer and the transparent cover. An index of refraction of the index-of-refraction-matching medium is substantially equal to an index of refraction of the transparent layer and the transparent cover.

Description:
BACKGROUND 
   Digital projectors often include micro-displays that include arrays of pixels (e.g., 1280×1024, etc.) Each pixel usually includes a micro-electromechanical system (MEMS) device, such as a micro-mirror, liquid crystal on silicon (LcoS) device, interference-based modulator, etc. A micro-display is used with a light source and projection lens of the digital projector. The micro-display receives light from the light source. When the pixels of the micro-display are ON, the pixels direct the light to the projection lens. When the pixels are OFF, they do not direct the light toward the light source, e.g., they may direct the light from the light source away from the projection lens, absorb the light, etc. The projection lens images and magnifies the micro-display. 
   Many micro-displays include a transparent, e.g., glass, cover for packaging, e.g., sealing and/or protecting, the pixels. However, when the pixels are OFF, the transparent cover can reflect some light to the projection lens. This degrades the “Black/White Contrast ratio” that is often defined as the ratio of the light imaged by the projection lens when all of the pixels in the micro-display are ON to the light imaged by the projection lens when all of the pixels are OFF and is a measure of the blackness of the projector&#39;s black state. 
   The pixels of many micro-displays are formed on a semiconductor substrate using semiconductor processing methods, and the transparent cover is adhered to a layer, e.g., an oxide layer, formed on the substrate overlying the pixels. However, the interface between the layer and the cover may have a substantially different index of refraction than either the layer or the cover, e.g., due to air gaps etc. This causes some of the light that reaches the cover to be reflected by the cover to the projection lens, which acts to reduce the contrast ratio. 
   SUMMARY 
   One embodiment of the invention provides a micro-display that has a device chip with a transparent layer overlying one or more micro-electromechanical system devices. A transparent cover overlies the transparent layer. An index-of-refraction-matching medium is interposed between the transparent layer and the transparent cover. An index of refraction of the index-of-refraction-matching medium is substantially equal to an index of refraction of the transparent layer and the transparent cover. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view illustrating a micro-display, according to an embodiment of the invention. 
       FIGS. 2-5  are cross-sectional views of a plurality of micro-displays, during various stages of fabrication, according to another embodiment of the invention. 
       FIG. 6  illustrates a micro-display, according to another embodiment of the invention. 
       FIG. 7  illustrates a micro-display, according to an embodiment of the invention. 
       FIG. 8  is a cross-sectional view illustrating a micro-display, according to another embodiment of the invention. 
       FIGS. 9-11  illustrate micro-displays in use, according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   In the following detailed description of the present embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that process, electrical or mechanical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof. 
     FIG. 1  is a cross-sectional view illustrating a micro-display  100 , e.g., as a portion of a digital projector, according to an embodiment of the invention. For one embodiment, micro-display  100  functions as a light modulator of the digital projector. Micro-display  100  includes a device die (or device chip)  102  and a transparent cover  104 , e.g., a glass cover having a low coefficient of expansion, with an index-of-refraction-matching medium  106  interposed between device chip  102  and transparent cover  104 . Index-of-refraction-matching medium  106  acts to reduce reflections from transparent cover  104  by substantially matching the index of refraction at the interface between transparent cover  104  and device chip  102  to transparent cover  104  and device chip  102 . For one embodiment, index-of-refraction-matching medium  106  is an elastomer, such as silicone, Polydimethylsiloxane (PDMS), etc. Polyimide is also suitable. For other embodiments, index-of-refraction-matching medium  106  may be a gel, and adhesive that cures into a solid, or a liquid. 
   Device chip  102  includes an array of pixels  108  (e.g., 1280×1024 pixels, etc.) formed on a semiconductor substrate  110 , e.g., of silicon or the like. For one embodiment, each pixel  108  is a MEMS device, such as a micro-mirror, liquid crystal on silicon (LcoS) device, interference-based modulator, etc. Specifically, for another embodiment the MEMS device includes a micro-mirror  112  supported by flexures  114  so that a gap  116  separates the micro-mirror  112  from an electrode  118 . For one embodiment, electrodes  118  are electrically connected to contacts  120  formed in substrate  110 . A gap  122  separates micro-mirror  112  from a partially reflective layer  124 , e.g., a tantalum aluminum (TaAl) layer, formed on the underside of a transparent layer  126 , e.g., of TEOS (tetraethylorthosilicate) oxide, silicon oxide, etc., formed on substrate  110 . Transparent layer  126  acts to reinforce and protect partially reflective layer  124 . For one embodiment, transparent layer  126  includes sidewalls  128  located on either side of the pixel array that extend from substrate  110 . Therefore, transparent layer  126  contains (or encloses) the pixel array. 
   Transparent cover  104  is adhered to a seal ring  130 , e.g., by plasma-enhanced bonding, laser-assisted welding, gluing, or the like, that surrounds transparent layer  126  and protrudes from substrate  110 . For one embodiment, seal ring  130  is of the same material as transparent layer  126 . A gap  132  is formed between seal ring  130  and sidewalls  128  of transparent layer  126 . Index-of-refraction-matching medium  106  is interposed between transparent layer  126  and transparent cover  104 . Transparent cover  104  and seal ring  130 , for some embodiments, hermetically seal micro-display  100 . For another embodiment, transparent cover  104  contains a desiccant  134 . 
     FIGS. 2-5  are cross-sectional views of a plurality of micro-displays, such as micro-displays  100 , during various stages of fabrication, according to another embodiment of the invention.  FIG. 2  shows a transparent cover  204  for covering a plurality of device chips, according to another embodiment of the invention. Specifically, a transparent layer  206 , e.g., of TEOS (tetraethylorthosilicate) oxide, silicon oxide, etc. is formed on one side of transparent cover  204 , e.g., by chemical vapor deposition. Slots  208  are formed in transparent cover  204  and desiccant  134  is disposed therein. Recesses  210  are also formed in transparent cover  204 .  FIG. 3  illustrates a plurality of device chips  102 , e.g., formed as a single device wafer  302 , according to another embodiment of the invention. Device wafer  302  can be fabricated using various semiconductor processing methods that are known to those skilled in the art. 
   For one embodiment, the transparent layers  126  and seal rings  130  of device wafer  302  and transparent layer  206  of transparent cover  204  are treated with plasma in preparation for plasma bonding transparent cover  204  to device wafer  302 . Then, index-of-refraction-matching medium  106  is disposed on transparent layers  126 , e.g., using a syringe etc. For one embodiment, index-of-refraction-matching medium  106  has substantially the same index of refraction as transparent layer  206  of transparent cover  204 . Note that transparent cover  204 , transparent layer  206  of transparent cover  204 , and transparent layers  126  have substantially the same indices of refraction. For some embodiments, thermal or ultra-violet processes may be used to partially cure and/or stabilize index-of-refraction-matching medium  106  to provide for chemical inertness and for mechanical stability. 
   For one embodiment, transparent layer  206  includes stand-offs  220  for mating with seal rings  130 . This creates a gap between transparent layer  206  and transparent layer  126  for the index-of-refraction-matching medium  106  when transparent cover  204  is adhered to seal rings  130 . Alternatively, increasing the height of seal rings  130 , as indicated by the dashed regions  320  in  FIG. 3 , can create this gap. For another embodiment, transparent layer  206  includes stand-offs  222 , as shown in  FIG. 2 , for mating with portions of transparent layer  126 . Stand-offs  222  contain the index-of-refraction-matching medium  106  within the gap. Alternatively, adding stand-offs  322  to portions of transparent layer  126 , as shown in  FIG. 3 , contains the index-of-refraction-matching medium  106  within the gap. For some embodiments, the index-of-refraction-matching medium  106  may be left in a liquid state and is held in place by surface tension. 
   Transparent cover  204  is adhered to seal rings  130  of device wafer  302  in  FIG. 4 . Portions of transparent cover  204  at recesses  210  that are aligned with contacts  120  are removed, e.g., by sawing, in  FIG. 5 . The recesses act to prevent the saw blade from damaging the contacts. Substrate  110  is cut at line  500  of  FIG. 5  to form a plurality of micro-displays  100 . 
     FIG. 6  illustrates a micro-display  600 , e.g., as a portion of a digital projector, according to another embodiment of the invention. Micro display  600  includes a device chip  602 , such as described for device chip  102  of  FIG. 1 , and a lens  604  adhered to device chip  602  that serves as a transparent cover of micro-display  600  and that may be index matched to device chip  602 , e.g., using an index-matching medium as described above. For one embodiment, lens  604  is plano-convex and has a curved surface  606 . For one embodiment, a mirror  608  is located between the curved surface  606  and device chip  602 . For another embodiment, mirror  608  may be formed integrally with lens  604 . 
   During operation, light  610  from a light source  612  of the projector is directed along an illumination path  614  of the projector to curved surface  606 . Curved surface  606  refracts the light through lens  604  and onto device chip  602 . When the pixels of device chip  602  are ON, they reflect light  610  back through lens  604  to curved surface  606 . Curved surface  606  refracts light  610  onto a mirror  620  of the projector. Mirror  620  reflects light  610  onto mirror  608  that reflects light  610  onto a projection path  622  of the projector and ultimately to a projection screen. When the pixels of device chip  602  are OFF, they do not reflect light  610  back through lens  600  and ultimately onto the projection screen and thus produce a “black” state. 
   Regardless of whether the pixels are on or off, a portion  624  of light  610  is reflected by curved surface  606  before light  610  reaches the pixels. The curvature of the surface is selected so that the reflected light portion  624  is directed away from mirror  620  and thus is not directed onto projection path  622  and ultimately onto the projection screen. If the reflected light portion  624  is not directed away from mirror  620 , it follows the path of light  610  when the pixels are on and eventually reaches the projection screen. This is especially detrimental when the pixels are OFF because it produces a lighter “black” state and thus reduces the contrast ratio. 
     FIG. 7  illustrates a micro-display  700 , e.g., as a portion of a digital projector, according to another embodiment of the invention. Micro display  700  includes a device chip  702 , such as described for device chip  102  of  FIG. 1 , and a lens  704  adhered to device chip  702  that serves as a transparent cover of micro-display  700  and that may be index matched to device chip  702 , e.g., using an index-matching medium as described above. Lens  704  has a curved surface  706 . 
   During operation, light  710  from a light source  712  of the projector is directed along an illumination path  714  through a refraction system  708 , e.g., a series of lenses, of the projector that refracts light  710  onto curved surface  706 . Curved surface  706  refracts the light through lens  704  and onto device chip  702 . When the pixels of device chip  702  are ON, they reflect light  710  back through lens  704  to curved surface  706 . Curved surface  706  refracts light  710  to refraction system  708  that refracts light  710  onto a projection path  722  of the projector and ultimately to a projection screen. Regardless of whether the pixels are on or off, a portion  724  of light  710  is reflected by curved surface  706  before light  710  reaches the pixels. Reflected light portion  724  is reflected so that a substantial portion of light portion  724  does not reach projection path  722  and ultimately the projection screen. 
   For one embodiment, an axis  750  substantially bisects device chip  702  and lens  704  into two substantially symmetric halves. For another embodiment, the bisecting axes of device chip  702  and lens  704  are substantially parallel, but are offset. Moreover, for another embodiment, a prism or a sub-wavelength plate, e.g., a quarter wavelength plate, may replace lens  704 . 
     FIG. 8  is a cross-sectional view illustrating a micro-display  800 , e.g., as a portion of a digital projector, according to another embodiment of the invention. For one embodiment, micro-display  800  functions as a light modulator of the digital projector. Micro-display  800  includes a device chip  802  and a transparent cover  804 , e.g., a glass cover having a low coefficient of expansion. For one embodiment, an anti reflective coating  806  may be applied to one or both sides of transparent cover  804  For another embodiment, transparent cover  804  may be a lens or a prism. 
   Device chip  802  includes an array of pixels  808  formed on a semiconductor substrate  810 , e.g., of silicon or the like. For one embodiment, each pixel  808  is a MEMS device, such as a micro-mirror, liquid crystal on silicon (LcoS) device, interference-based modulator, etc. Specifically, for another embodiment, the MEMS device includes a micro-mirror  812  supported by flexures  814  so that a gap  816  separates the micro-mirror  812  from an electrode  818 . A gap  822  separates micro-mirror  812  from a partially reflective layer  824 , e.g., a tantalum aluminum (TaAl) layer, formed on the underside of a transparent layer  826 , e.g., of TEOS (tetraethylorthosilicate) oxide, silicon oxide, etc., formed on substrate  810 . Transparent layer  826  acts to reinforce and protect partially reflective layer  824 . 
   Transparent cover  804  is adhered to a seal ring  830 , e.g., by plasma bonding, gluing, or the like, that surrounds transparent layer  826  and protrudes from substrate  810  to form an enclosure  850  that encloses pixels  808 , partially reflective layer  824 , and transparent layer  826 . For some embodiments, enclosure  850  is hermetically sealed by transparent cover and may contain a desiccant. A sub-wavelength grating  860  is formed in an upper surface of transparent layer  826 , e.g., by patterning and etching. For one embodiment, grating  860  forms a quarter wavelength plate (or polarization retarder). 
     FIGS. 9-11  illustrate micro-displays  800  in use as portions of a projector, according to another embodiment of the present invention. Specifically,  FIG. 9  illustrates light being delivered to micro-displays  800 ;  FIG. 10  illustrates micro-displays  800  in an OFF or an ON state, and light being reflected off transparent cover  804  of each of micro-displays  800 ; and  FIG. 11  illustrates micro-displays  800  in an ON state. 
   For one embodiment, micro-display  800   1  receives light of one color, and micro-display  800   2  receives light of another color. For example, micro-display  800   1  may receive red light, and micro-display  800   2  may receive blue and green light. 
   For another embodiment, the projector is an on-axis projector, meaning that a polarization recovery system  910  receives randomly polarized light  912  from a light source  915  along a illumination path  916  of the projector and polarizes the randomly polarized light to a first polarization, e.g., to linearly (or plane) polarized at a first polarization, as indicated by an arrow  918 . For another embodiment, the light at the first polarization subsequently passes through a color select filter  920  that separates the light into color components  912   1  and  912   2  of the light  912  to be respectively received at micro-displays  800   1  and  800   2 . For one embodiment, color component  912   1  has the first polarization, as indicated by arrow  918 , and color component  912   2  has a second polarization, e.g., linearly (or plane) polarized at a second polarization, that for one embodiment, is rotated by 90 degrees from the first polarization, as indicated by dot  922 . For another embodiment, color components  912   1  and  912   2  are respectively passed by a polarizing beam splitter  930  to micro-display  800   1  and reflected by polarizing beam splitter  930  to micro-display  800   2 . 
     FIG. 10  shows that transparent cover  804  reflects color components  912   1  and  912   2  at their incoming polarizations, i.e., respectively at the first and second polarizations when micro-displays  800  are OFF or ON. Therefore, polarizing beam splitter  930  can pass the reflected light color component  912   1  onto the illumination path  916  toward light source  915  and reflect the reflected light color component  912   2  onto the illumination path  916  toward light source  915 . This prevents the reflected color components  912   1  and  912   2  from being directed onto a projection path  934  and reaching a projection lens  935  of the projector when micro-displays  800  are OFF or ON. 
   It should be noted that the portions of color components  912   1  and  912   2  that are not reflected by transparent covers  804  pass into micro-displays  800  and are not reflected back through transparent covers  804  by pixels  808  when micro-displays  800  are OFF, as is the case when micro-displays  800  are ON. Rather, for some embodiments, pixels  808  direct the light away from transparent covers  804 . 
     FIG. 11  shows that when micro-displays  800  are ON, color components  912   1  and  912   2  pass through transparent covers  804  and through grating (or quarter wavelength plate)  860  and are reflected back through quarter wavelength plate  860  by pixels  808 . That is, color components  912   1  and  912   2  make two passes through the quarter wavelength plate  860  of their respective micro-displays  800 . The two passes through quarter wavelength plate  860  rotates the polarization of color component  912   1  from the first polarization, indicated by arrow  918 , to the second polarization, indicated by dot  922 , and the polarization of color component  912   2  from the second polarization to the first polarization. As shown in  FIG. 11 , changing the polarization of color component  912   1  enables polarizing beam splitter  930  to reflect it onto projection path  934  so that it reaches projection lens  935 , and changing the polarization of color component  912   2  enables polarizing beam splitter  930  to pass it to projection lens  935 . It will be appreciated that the transparent covers  804  reflect the respective color components  912   1  and  912   2  regardless of whether the respective micro-displays  800  are on or off. 
   CONCLUSION 
   Although specific embodiments have been illustrated and described herein it is manifestly intended that this invention be limited only by the following claims and equivalents thereof.