Abstract:
The present invention relates to optical devices. In one embodiment, a display apparatus includes a display medium, a transparent substrate, a non-transparent substrate. The display medium is disposed between the first and second substrates and an adhesive coupling material couples the substrates together. The adhesive material is disposed proximate to a channel, which is in at least one of the substrates.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to silicon processing where multiple layers of material are joined together with an adhesive material such as in Liquid Crystal on Silicon (LCoS) displays. The invention may be used to produce high quality static as well as dynamic real time color field micro images on an active pixel matrix.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional flat-panel displays use electroluminescent materials or liquid crystals in conjunction with incident light to produce high quality images in products such as digital wristwatches, calculators, panel meters, thermometers, and industrial products. Liquid crystals (LCs) are liquids in which the molecules can be arranged to possess an orientational order, which in turn causes macroscopic optical properties such as birefringence to appear in the material. A liquid crystal display is made by disposing the liquid crystal material in a layer between two closely spaced, treated, substrates. This ordered structure can be deformed by application of an electric (or magnetic) field, and this deformation results in a change in the optical properties of the layer. By a suitable choice of external polarization control components, such as polarizers and retarders, the change in optical properties of the LC can result in a change in the amount of light transmitted or reflected by the LC display device. One approach for developing high quality liquid crystal displays (LCDs), also referred to as liquid crystal spatial light modulators (SLMs), utilizes an active-matrix approach where transistors, sometimes thin film transistors (TFTs), are operationally co-located with a matrix of LCD pixels. The active-matrix approach allows pixels to be independently addressed, which essentially eliminates cross-talk, and allows for other display quality improvements over passive matrix displays such as increased speed, and increased number of gray-scales.  
           [0003]    Very small displays are sometimes termed micro-displays. Both reflective and transmissive micro-displays can be made, and they are used in applications such as camera viewfinders, display glasses and projector systems. Reflective micro displays are usually based on single-crystal silicon integrated circuit substrates with a reflective aluminum pixel forming a pixel mirror. Because it is reflective, the pixel mirror can be fabricated over the pixel transistors and addressing lines. This results in an aperture ratio (reflective area/absorptive area) that is much larger than equivalent transmissive displays. Aperture ratios for reflective displays can be greater than 90%. Because of the large aperture ratio and the high quality silicon transistors, the resolution of a reflective micro display can be very high within a viewing area that is quite small. For example, a QVGA (320×200) pixel display with a 12 μm pixel pitch has an active area of 3.84 mm×2.40 mm.  
           [0004]    There are several different liquid crystal technologies that can be used in reflective micro displays. These include nematic liquid crystals, ferroelectric liquid crystal (FLC), and polymer disbursed liquid crystal (PDLC). Reflective micro displays are designed to be made as small as possible, because as the size increases the cost increases and yield decreases.  
           [0005]    For further background in this area, see Douglas J. McKnight, et al., 256×256  Liquid-Crystal-on-Silicon Spatial Light Modulator , Applied Optics Vol. 33 No. 14 at 2775-2784 (May 10, 1994); and Douglas J. McKnight et al.,  Development of a Spatial Light Modulator: A Randomly Addressed Liquid-Crystal-Over-Nmos Array , Applied Optics Vol. 28 No. 22 (November 1989)  
           [0006]    Liquid Crystal on Silicon (LCoS) displays are fabricated using a process that includes applying a sheet of glass over a silicon wafer (or chip). Typically the glass is glued to the underlying silicon wafer with an adhesive such as Mitsui XN-651, Mitsui XN-21, or World-Rock 780. The adhesive that is used forms a “gasket” around the active area of the display and is required both for mechanical strength and to keep the liquid crystal in place. It is either dispensed onto one of the substrates (usually the silicon) with a dispensing machine such as a Camalot, or is printed onto one of the substrates using a screen printing process. Subsequent to the dispensing process, the two substrates are mated together. Force is applied to squeeze the substrates together until the gap between them is at the desired size. Spacers that are dispersed between the silicon and the glass typically control the gap dimension.  
           [0007]    It is desirable to control the amount of gasket material deposited on the substrate since the gasket material expands laterally as the substrate and top sheet are squeezed together. FIG. 1 shows a cross sectional view of part of a prior art assembly.  
           [0008]    With reference to FIG. 1, adhesive  16  is contained between substrate  12  and top sheet  14 . Lateral expansion can result in gasket width  20  reaching approximately five hundred microns, when display cell gap  18  is closed to approximately 2 microns. This result is undesirable since the cost of the display is related to the number of displays that can be placed on a single silicon wafer. For small displays, the area that has to be set aside for the gasket is significant. If the total display chip area is 7 mm by 7 mm, then this gasket occupies about 20% of the silicon area.  
           [0009]    Often it is difficult to dispense a very small amount of adhesive without incurring problems such as the presence of unwanted breaches in the gasket. A breach in the gasket (other than the designed-in fill-port) results in a failed display, with resulting cost increases and reliability problems.  
           [0010]    The problem of lateral gasket expansion worsens, as the cell gap is made smaller. For example, if a display were constructed with a cell gap of 1 micron, then the same gasket would spread out to approximately 1 mm in width. This situation presents several problems. First, the size of gasket occupies a very large fraction of the silicon area. Second, it requires more pressure to squeeze the gasket material out into such a thin, wide, strip. Third, the choice of some gap sizes may limit the use of conductive “crossover” material, which may be used to connect a transparent conductive layer on the glass to specific regions of metal on the silicon chip. If gap is too thin then the conductive crossover material can result in non-uniformity in the cell gap because the size of the conductive crossover material could prevent formation of the desired cell gap between the substrates at the crossover location(s). Fourth, some adhesives that are otherwise good for these applications contain a particulate “filler” material, that can limit the achievable cell gap to one which is larger than desired.  
           [0011]    Prior art methods of applying adhesive gaskets result in large gasket width/gap ratios. What is needed in the art is a way of reducing lateral expansion of adhesive as the substrates are pressed together to form a finished gap there between.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention relates to methods and apparatuses directed to optical devices. In one embodiment, a display apparatus includes a display medium, a transparent substrate, a non-transparent substrate. The display medium is disposed between the first and second substrates and an adhesive coupling material couples the substrates together. The adhesive material is disposed proximate to a channel, which is in at least one of the substrates.  
           [0013]    An optical apparatus, in one embodiment, includes_a non-transparent substrate; a transparent substrate; an adhesive material disposed on at least one of the transparent substrate and the non-transparent substrate; and a channel, formed in at least one of the transparent substrate and the non-transparent substrate, to receive a flow of the adhesive material.  
           [0014]    A semiconductor method, in one embodiment, includes_applying a channel resist mask to at least one of a transparent substrate and a non-transparent substrate; and applying a dielectric-etch to form a channel, in at least one of the transparent substrate and the non-transparent substrate, to receive a flow of adhesive material.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 shows a cross sectional view of a prior art adhesive layer after assembly of the substrate with the top sheet.  
         [0016]    [0016]FIG. 2 shows a. top view of a typical Liquid Crystal On Silicon (LCoS) display die.  
         [0017]    [0017]FIG. 3 illustrates a cross-sectional view of the typical LCoS display shown in FIG. 2 cut along section A-A.  
         [0018]    [0018]FIG. 3 a  shows several variations on the shape of the channel.  
         [0019]    [0019]FIG. 3 b  depicts one of the many possible shapes the adhesive may assume within the channel.  
         [0020]    [0020]FIG. 4 illustrates a combined isometric and cross-sectional view of an LCoS display.  
     
    
     DETAILED DESCRIPTION  
       [0021]    A top view of a typical LCoS display die is shown in FIG. 2. LCoS display device  200  is actually one of a plurality of such devices, which would be manufactured on a single silicon wafer. The present invention will be discussed with respect to a single LCoS display device. However, those of skill in the art will recognize that the invention is applicable in any application directed to limiting the lateral expansion of an adhesive/gasket material used in manufacturing optical devices with semiconductors. Additionally, the invention finds application to optical devices that are not manufactured with semiconductors.  
         [0022]    With respect to semiconductor application, the present invention is employed on the active side of the silicon chip. The optical device may be similar to that described and shown in FIG. 7 of U.S. Pat. No. 5,426,526, which is hereby incorporated herein by reference. Reflective electrode  68  is on the active side of the silicon chip in FIG. 7 of U.S. Pat No. 5,426,526 as opposed to silicon substrate  60 , which is the inactive side of the semiconductor. It will be appreciated that the “active” side of the semiconductor substrate is the side, which is doped to create the active devices (e.g. field effect transistors (FETs), etc.) as opposed to the backside of the semiconductor substrate or inactive side. It is typically true that the active side of the semiconductor substrate is also the side which interacts with light to create an optical device. Other examples of optical devices employing semiconductors are found in U.S. Pat. No. 6,046,716 which is hereby incorporated herein by reference.  
         [0023]    Other hybrid technologies can be fabricated on semiconductor chips, and although they don&#39;t have the exact same requirements as LCoS devices, they can also benefit from improved adhesive ring control. These include organic light emitting diode on silicon displays, silicon-based micro-mechanical displays, and CCD or photodiode sensor arrays for cameras. Furthermore, these technologies that are primarily associated with displays can be used for applications other than displays. Both liquid crystal on silicon devices, and silicon based micro-mechanical devices can be used for optical applications such as laser beam steering.  
         [0024]    With respect to application of the present invention to optical devices that do not employ particular use of semiconductors, the present invention is useful in, for example, reflective liquid crystal light valves such as those described in U.S. Pat. No. 4,019,807 and U.S. Pat. No. 4,378,955. Other examples would be passive matrix type Organic Light Emitting Diode (OLED) displays and passive matrix LCDs.  
         [0025]    With reference to FIG. 2, pixel array  210  is configured as shown and surrounded by shield  208 . Exterior to shield  208  is adhesive channel  206 . In one embodiment, of the present invention, adhesive channel  206  extends around the periphery of shield  208  and contains the active display area interior to adhesive channel  206 . Crossover locations  202  are placed exterior to, or inside, adhesive channel  206 . There may be one or more bond pads  204  located exterior to adhesive channel  206 . A typical LCoS display may have thirty bond pads.  
         [0026]    [0026]FIG. 3 illustrates a cross-sectional view of the typical LCoS display die shown in FIG. 2 cut along section A-A. The problem of large gasket width/gap ratio is solved by forming a “channel” around the peripheral area of the display on the silicon chip. With reference to FIG. 3, adhesive  16  is deposited proximate to adhesive channel  206 . Proximate, as used herein, means either in the adhesive channel or near the adhesive channel. Channel cross-section  300  illustrates the LCoS display after top sheet  14  has been pressed against substrate  12 . Top sheet  14  can be alternatively referred to as a substrate. No limitation is implied by the use of the term top sheet. Top sheet is used simply for clarity and convenience. LC material  304  is contained between substrate  12  and top sheet  14 . In one embodiment of the present invention, adhesive channel  16  provides a reservoir of extra volume for adhesive  16  to flow into during assembly. Thus, mitigating the problems of excessive adhesive spread and large press pressures described above.  
         [0027]    In one embodiment, adhesive  16 , deposited proximate to adhesive channel  206 , will spread, uniformly at first, during assembly (since adhesive  16  would flow through a uniform gap dimension) until adhesive  16  reaches adhesive channel  206 , at which time adhesive  16  will be disposed to flow into the channel rather than away from the channel. Predisposition of the flow of adhesive  16  results from the larger gap presented to adhesive  16 , by adhesive channel  206 , as opposed to cell gap  316 . The frictional force generated by the flow of adhesive  16  is inversely related to gap thickness. For example, the larger gap presented by adhesive channel  16 , interacts with adhesive  16  to generate a lower frictional force (to oppose flow), as adhesive  16  expands during assembly of top sheet  14  with substrate  12 . Thus, adhesive  16  flows into adhesive channel  206  rather than spreading widely as shown in FIG. 1.  
         [0028]    Adhesive channel  206  is not limited to the rectangular cross section shown in FIG. 3. Adhesive channel  206  may be formed into any shape that presents an increase in gap thickness relative to the gap thickness presented to the adhesive in the region where the gasket width is to be contained. When the term “channel” is used in the context of the present invention, the channel need not be closed or symmetric about a geometric axis. The channel need only create a difference in gap dimensions, as previously discussed, when substrate  12  is moved proximate to top sheet  14 , as shown in FIG. 3.  
         [0029]    [0029]FIG. 3 a  illustrates several typical “channels” according to the use of the term channel herein. With reference to FIG. 3 a , open channel  350   a  is shown within substrate  12 . The term “open” indicates that there is no side opposing side  374 . The desired geometry, the difference in gaps, is achieved since channel gap  370   a  is larger than small gap  360 . A variation on open channel  350   a  is created with “double” open channel  350   b . Both substrate  12  and substrate  13  are non-planar such that a channel is formed in each substrate, hence the use of the term double. The desired difference in gaps is obtained and it is readily evident that channel gap  370   b  is larger than small gap  360 .  
         [0030]    The channel gap need not be a constant dimension but may be a variable dimension. Closed variable channel  350   c  illustrates one such geometry. The term “closed” indicates that the channel has sides opposing each other, such as side  376  and side  378 . Variable channel gap  372   c  varies from small to large to small as the channel is traversed from side  376  to side  378 . This channel could be termed closed and is also symmetric about a vertical axis centered on the intersection of side  376  and  378 . Closed variable channel  350   c  provides the desired relationship where variable channel gap  372   c  is greater than small gap  360 .  
         [0031]    Alternatively, the variable channel could be open as illustrated with variable open channel  350   d . Variable open channel gap  372   d  follows a curve, which provides for an increasing channel gap as the distance away from small gap  360  increases.  
         [0032]    In another embodiment of the present invention, the adhesive may be initially deposited within the channel. With reference to FIG. 3 b , unassembled view  380  illustrates substrate  12  and substrate  13  with adhesive  382  contained within adhesive channel  384 . Assembly of the substrates proceeds by moving substrate  12  and substrate  13  together. One method would be to move substrate  13  in the direction indicated by arrow  381 . Assembled view  390  illustrates the position of substrate  12  and substrate  13  after the substrates have been moved close enough to create the desired small gap  392 . During assembly, adhesive  382  may change shape to a shape similar to that depicted by adhesive  398 , resulting in gasket width  399 . Alternatively, more adhesive could have been deposited in adhesive channel  384  than that shown in unassembled view  380  such that upon assembly, adhesive  398  filled adhesive channel  384  and flowed into small gap  392 . Such a condition would effectively produce a gasket width roughly equivalent to channel width  387 . Adhesive  398  may assume a variety of shapes in adhesive channel  384  and may flow into small gap  392 , small gap  393  or a combination of each of these gaps depending on the initial location of adhesive  398  deposited proximate to adhesive channel  384  and the amount of adhesive deposited therein. Channel depth  386  and channel width  387  can be varied without departing from the teaching of the invention. The channel should be properly sized so as not to occupy too much silicon area, and not be so small that adhesive gasket expansion is too large. A channel depth of 1.4 μm and a channel width 300 μm has been used to create a gasket width of approximately 200 μm in one application of the present invention.  
         [0033]    The channel illustrated in FIG. 3 is created in a typical 3 layer metal CMOS silicon substrate with inter-metal dielectric  310  disposed between metal layer  314  and metal layer  308  and inter-metal dielectric  310  being disposed between metal layer  308  and the metal layer used for pixel array  210 . Inter-metal dielectric  310  and inter-metal dielectric  312  may be made of the same or different material depending on the particular design of substrate  12 . However, the following concepts and procedures apply to all typical semiconductor processes. Modifications described herein were all made to standard silicon processes, such as resist coat and lithography masking steps, fluorine based plasma dielectric plasma etching, and chlorine based plasma metal etching. In one embodiment, of the present invention, a mask is applied so that chlorine based plasma etching may be used to etch through shield  208 , thus exposing inter-metal dielectric  310 . A channel resist mask may be applied leaving the adhesive channel  206  and crossover location  202  exposed. A dielectric-etch is applied to create the channel to a specified depth. Etching may be terminated prior to reaching metal layer  308 . Termination of etching ensures that some amount of inter-metal dielectric  310  is left over metal layer  308 , under adhesive channel  206 . Following channel resist removal, passivation dielectric  306  may be applied to substrate  12 . A bond pad/crossover mask is applied that exposes crossover location  202  and bond pad  204  areas. Dielectric-etch is applied to remove any dielectric above bond pad  204  and above crossover location  202 . The etching time is governed by the removal of dielectric in the crossover area. The time required for complete etching in the crossover area provides excessive etching time for dielectric above bond pad  204 . However, this does not present a problem since bond pad  204  metal is removed very slowly in a dielectric plasma etch process.  
         [0034]    Alternatively, according to another embodiment of the present invention, channel etching could be allowed to remove all of inter-metal dielectric  310  over metal layer  308 , directly beneath adhesive channel  206 , resulting in metal layer  308  being exposed at crossover location  202  and at adhesive channel  206 . Bond pad  204  metal would also be exposed following this dielectric-etch process.  
         [0035]    Passivation dielectric  306  is then applied. Passivation dielectric  306  may be removed from bond pad  204  and crossover location  202 . The second approach may be easier to process, if metal layer  308  exists under adhesive channel  206 . It will be noted by those of skill in the art that this alternative method of forming adhesive channel  206  can be employed even if no metal layer existed under adhesive channel  206 .  
         [0036]    Furthermore, the same processes can be used to increase gap  318  in the region of crossover location  202 . Thus allowing use of conductive cross over material  302  which may require gap  318  to be larger than cell gap  316 .  
         [0037]    [0037]FIG. 4 illustrates a combined isometric and cross-sectional view of a LCoS display die. With reference to FIG. 4, isometric cross-section  400  illustrates an assembled LCoS display. Conductive crossover material  302  is deposited in crossover location  202  subsequent to the final processing that leaves metal layer  308  exposed. Conductive crossover material  302  is typically made of nickel particles or gold-coated plastic spheres. Conductive layer  15  applied to the underside of top sheet  14  is typically made of Indium Tin Oxide (ITO). Conductive crossover material  302  enables electrical continuity between metal layer  308  and conductive layer  15 . It is important that the processing used on substrate  15  prior to assembly of the LCoS display leave metal layer  308  clean, in crossover location  202 , so that good electrical contact can be made by conductive crossover material  302 .  
         [0038]    In the foregoing detailed description, the apparatuses and methods of the present invention have been described with reference to specific exemplary embodiments. However, it will be evident that various modifications and changes may be made without departing from the broader scope and spirit of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.