Patent Publication Number: US-2013242244-A1

Title: Methods and apparatus for electrically connecting a substrate and electrically conductive glass

Description:
FIELD OF THE INVENTION 
     Embodiments of the present invention are generally related to the field of liquid crystal displays, and, more particularly, to the optical performance of such displays. 
     BACKGROUND 
     A liquid crystal (LC) display cell can have an electrically conductive glass layer over a liquid crystal layer which can be supported by a silicon backplane substrate. The LC display cell can be die-attached to a printed circuit board to produce an LC panel. The printed circuit board can be used to make electrical connections to the cell for power and data purposes. A conventional LC panel can have one or more electrical connections directly between the electrically conductive glass and the printed circuit board. Power can be supplied from the printed circuit board to the conductive glass through a conductive adhesive pillar without passing through the silicon substrate. These electrical connections can be made from a conductive adhesive which can be formed into one or more pillars to connect a conductive layer of the conductive glass to a conductive trace on the circuit board. 
     A diagrammatic elevational view of a conventional LC panel is shown in  FIG. 1  and is generally designated using reference number  10 . Panel  10  can have a display cell  12  die-attached to a printed circuit board  14 , such as FR4. Display cell  12  can include an electrically conductive glass layer  16 , a liquid crystal layer  18  and a silicon backplane substrate  20 . Other layers can be included, but are not shown in this simplified example for purposes of clarity. The electrically conductive glass can have an overhang such that the glass overhangs the LC and silicon substrate layers. A pillar  24  of conductive adhesive can be formed between the printed circuit board and overhang of the conductive glass to electrically connect the printed circuit board to the conductive glass. In this arrangement, the pillar does not contact the LC or silicon backplane substrate layers, but instead extends directly from the printed circuit board to the conductive glass. 
     During operation, the display cell applies electrical field signals across the liquid crystal layer between pixel electrodes of the silicon backplane substrate and the electrically conductive glass to change a characteristic of the liquid crystal to modulate light for creating an image. If the electrical connection through the pillar is broken, then the display cell is unable to create the electrical fields and the display cell becomes non-functional. 
     The pillar can be formed after display cell  12  is die-attached to the printed circuit board using carefully controlled dispense methods and custom made dispensing equipment. 
     It is recognized that the pillar can be a source of failure in the LC panel. Since the pillar is required to span at least the thickness of the silicon substrate and the LC layer, the pillar can be on the order of 0.7 mm thick. The thickness of the pillar can exceed the recommended maximum thickness of the conductive adhesive used to form the posts. As a result of the required thickness, the pillar can be subject to handling related mechanical failure. The pillar can also be subject to failure caused by adverse environmental conditions, such as high temperature and high humidity. 
     The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic side view of certain layers of a conventional liquid crystal panel. 
         FIG. 2  is a diagrammatic side view of an embodiment of a liquid crystal panel with a silicon substrate and having an electrical connection according to the present disclosure. 
         FIG. 3  is a diagrammatic top view of an embodiment of a liquid crystal panel having an electrical connection according to the present disclosure. 
         FIG. 4  is a diagrammatic top view of another embodiment of a liquid crystal panel having an electrical connection according to the present disclosure. 
         FIG. 5  is a diagrammatic top view of yet another embodiment of a liquid crystal panel having an electrical connection according to the present disclosure. 
         FIG. 6  is a diagrammatic side view of an embodiment of a liquid crystal panel with a glass substrate having an electrical connection according to the present disclosure. 
         FIG. 7  is a diagrammatic top view of an embodiment of the liquid crystal panel of  FIG. 6 . 
         FIG. 8  is a flow diagram illustrating an embodiment of a method involving the application of electrical connection according to the present disclosure. 
         FIG. 9  is a flow diagram illustrating another embodiment of a method involving the application of an electrical connection according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable one of ordinary skill in the art to make and use embodiments of the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles taught herein may be applied to other embodiments. Thus, embodiments of the present invention are not intended to be limited to the embodiments shown, but are to be accorded the widest scope consistent with the principles and features described herein including modifications and equivalents, as defined within the scope of the appended claims. It is noted that the drawings are not to scale and are diagrammatic in nature in a way that is thought to best illustrate features of interest. Descriptive terminology may be adopted for purposes of enhancing the reader&#39;s understanding, with respect to the various views provided in the figures, and is in no way intended as being limiting. 
     Attention is now directed to the remaining figures wherein like reference numbers may refer to like components throughout the various views.  FIG. 2  is a diagrammatic representation of an embodiment of a liquid crystal on silicon (LCOS) panel in a side view, generally indicated by reference number  100 . LCOS panel  100  includes a display cell  102  which is die-attached to a printed circuit board  104 . Display cell  102  can be a laminate which includes an electrically conductive glass  106 , a liquid crystal (LC) layer  108  and a silicon backplane substrate  110 . The display cell can include bond pads  112  which can be used to electrically connect the display cell to electrical traces  114  of the printed circuit board using wire bonds  116 . Electrically conductive glass  106  can include a glass layer  118  and a transparent electrically conductive layer  120 , such as Indium-Tin-Oxide (ITO) on the side facing the LC layer. Other suitable embodiments of electrically conductive glass can be used, as is known to a person of ordinary skill in the art. The display cell can include layers other than those shown in  FIG. 2 , such as for example alignment layers. In addition, each of the layers discussed can be made up of one or more different areas and sub-layers that form an entire layer but which are not individually shown for purposes of clarity. 
     Turning now to  FIG. 3  in conjunction with  FIG. 2 , LCOS panel  100  includes a carbon black conductor  124  made from a carbon black doped adhesive. Carbon black conductor  124  is electrically conductive and can span relatively small gaps as discussed below. The carbon black conductor is positioned between silicon substrate  110  and electrically conductive glass  106  to electrically connect a contact area  126  of the silicon substrate to transparent electrically conductive layer  120  of the electrically conductive glass. Contact area  126  can be formed on the silicon substrate and can be electrically connected one or more bond pads  112 , wire bond  116  and electrical trace  114  to receive a signal source for powering the electrically conductive glass. The contact area, for example, can be formed of metal using conventional semiconductor manufacturing processes, and can be connected to the signal source using the silicon substrate, the wire bonds and/or the circuit board traces or other electrical connections formed using conventional manufacturing techniques. The contact area can be large or small as long as sufficient electrical conductivity is provided to sufficiently pass the signals to the carbon black conductor. 
     Referring now to  FIG. 3  which is a diagrammatic top view, LC layer  108  includes an LC reservoir  132  containing a liquid crystal material  134 . The LC layer can have a liquid crystal perimeter seal  130  which contains the liquid crystal material in the LC reservoir. During the assembly of the display cell, the liquid crystal perimeter seal can be formed on either the electrically conductive glass or the silicon substrate. The liquid crystal perimeter seal can be made from an adhesive and can be applied using a syringe needle or can be printed using offset printing, an ink jet printer, or other suitable printing or application methods. The perimeter seal can bond the electrically conductive glass to the silicon substrate to create laminated display cell  102 . The perimeter seal, electrically conductive glass and silicon substrate form the boundaries of LC reservoir  132 . 
     The liquid crystal layer can have spacers  136  which can be located in the perimeter seal and/or in the reservoir to maintain a gap  138  ( FIG. 2 ) between the electrically conductive glass and the silicon substrate. The spacers can be particles of silica or polymer or another material having a specific dimension that is substantially the same as gap  138 . During manufacture, the perimeter seal can be formed with an opening  140  so that the reservoir can be filled with liquid crystal material  134  after the perimeter seal has cured. After the reservoir is filled through the opening, the reservoir can be sealed with a plug  142  which can be formed using an adhesive such as the adhesive used for the perimeter seal. 
     In the embodiment shown in  FIGS. 2 and 3 , carbon black conductor  124  can be positioned external to perimeter seal  130  of the LC layer to bridge gap  138  to electrically connect the conductive layer  120  of the electrically conductive glass to contact area  126 . The carbon black conductor can be applied to contact area  126  before or after the formation of the perimeter seal and the carbon black conductor can electrically connect to the electrically conductive glass when the display cell is assembled into the laminate. In an embodiment, when the display cell is assembled, gap  138  and carbon black conductor  124  can be one micron or less since spacers  136  can be one micron or less. One or more spacers can also be included in the carbon black conductor to maintain the distance between the electrically conductive glass and the substrate. In another embodiment, the carbon black conductor can have a thickness of less than approximately 3.5 microns and can still produce acceptable results for conducting electrical signals. 
     Carbon black conductor  124  can be made, by way of non-limiting example, using a mixture of carbon black and adhesive. The carbon black can be a high purity carbon black that is 99.9% carbon black particles having an average particle size of approximately 0.042 micron, such as is produced by Alpha Aesar Company, Ward Hill, Mass., Stock number 39724. The carbon black can be mixed with a UV curing acrylic adhesive, epoxy adhesive or other optical adhesive. The carbon black adhesive can be produced by mixing the carbon black by weight with the adhesive. A range of about 2% to about 10% by weight of carbon black to adhesive can be used, with about 5% by weight having good conductivity and workability. When too much carbon black is used in the mixture, the viscosity becomes excessive and the mixture is difficult to work with. When too little carbon black is used in the mixture, the mixture does not exhibit a high enough conductivity. A workable mixture can have a gel like consistency which can be formed to hold a shape to allow time for curing. In one embodiment, an overall resistance of under approximately 500 Ohms for the combination of the electrically conductive glass and the carbon black connector can be sufficient for operation of LCOS panel  100 . 
     The carbon black conductor can be applied to the substrate using an application process that is used for forming the perimeter seal. Because of this, the application of the carbon black conductor does not require special dispensing methods or custom made dispensing equipment. The formation of the carbon black conductor can be accomplished using typical manufacturing processes and the thickness of the carbon black conductor can fall within the thickness ranges specified by adhesive manufacturers. Since carbon black conductor  124  only has to extend across gap  138  between the electrically conductive glass and the silicon substrate, which is relatively small in comparison to the pillar discussed above, mechanical stresses on the carbon black conductor can be reduced relative to a conventional pillar. While the pillar type structure can be made from a conventional conductive adhesive, these conventional conductive adhesives can have particles that are too large to be used between the electrically conductive layer and the substrate. Other conventional conductive adhesives can include silver or gold nano-particles which can be mixed with adhesives in percentages by weight that are greater than 40% to achieve usable conductivity. In addition to the high cost of using precious metal particles in these adhesives, such high concentrations of particles can result in high viscosities which can create difficulties when working with these conventional conductive adhesives. 
     In an embodiment shown in  FIG. 4 , a carbon black conductor  150  can be formed in the shape of a line and can be positioned external to perimeter seal  130 . Line-shaped carbon black conductor  150  can be electrically connected to a substrate  154  which can have one or more contact areas  156 . Carbon black conductor  150  can electrically connect between an electrically conductive layer of electrically conductive glass  158  and the contact areas of substrate  154 . Line-shaped carbon black conductor  150  can have a relatively lower resistance than the substantially dot shaped carbon black conductor  124  shown in  FIGS. 2 and 3  since carbon black conductor  150  can dispose more conductive material between the electrically conductive glass and the substrate and more material in contact with both the electrically conductive glass and the substrate. 
     In an embodiment shown in  FIG. 5 , the carbon black adhesive can be used to form a carbon black conductor perimeter seal  160 . A silicon substrate  162  can have one or more contact areas  164  positioned to electrically connect to electrically conductive glass  106  through perimeter seal  160 . By using the carbon black conductor perimeter seal, more conductive material can be placed between the electrically conductive glass and the substrate to provide a relatively lower overall resistance. In addition, by using the perimeter seal conductor, an area on the substrate does not have to be used for externally placing the carbon black conductor. Using the carbon black material for the perimeter seal conductor can be integrated into the manufacture of the LCOS panel without having to introduce additional printing steps. Spacers  136  can be mixed into the carbon black material and can serve to maintain the gap between the electrically conductive glass and the substrate. 
     Turning now to  FIG. 6 , a diagrammatic side view of an LC panel  180  is shown. LC panel  180  includes a glass substrate  182 , an LC layer  184  and an electrically conductive glass  186 . Glass substrate  182  can have an electrically conductive layer  188  and electrically conductive glass  186  can have an electrically conductive layer  190 . LC panel  180  can selectively modulate light passing through the device using the LC layer under the control of the electrically conductive layers  188  and  190 . LC panel can be utilized in a flat panel display or can be a polarization rotator or other type of LC panel which uses an LC layer to modulate light passing through the panel. 
     Referring now to  FIG. 7  in conjunction with  FIG. 6 , a carbon black conductor  192  can electrically connect electrically conductive layer  190  to a contact area  194  of glass substrate  182 . Contact area  194  can be electrically isolated from the remainder of conductive layer  188 . The carbon black conductor  192  can be applied to the contact area so that contact area  194  of conductive layer  188  is electrically connected to the electrically conductive layer  190  of the electrically conductive glass when substrate  182 , LC layer  184  and electrically conductive glass  186  are laminated together with a perimeter seal  196 . An electrical signal wire  198  can be soldered to contact area  194  and an electrical signal wire  200  can be soldered to conductive layer  188  electrically isolated from the signal wire  198 . Perimeter seal  196  can provide a perimeter of an LC reservoir  202  which can be filled with a liquid crystal material  204  and a plug  206  can contain the LC material in the reservoir. 
     The LC layer of LC panel  180  can be approximately  1  micron or less in thickness depending on the dimensions of spacers used to maintain a gap  208  between glass substrate  182  and electrically conductive layer  190 . The carbon black conductor can be positioned externally to the perimeter seal or can be used for the perimeter seal. While LC panel  180  only shows a single electrical connection for each of glass substrate  182  and electrically conductive glass  186 , multiple electrical connections can be made to either the substrate or the electrically conductive glass. For example, electrically conductive layer  188  of the glass substrate can include an array of pixels, each of which can have a separate electrical connection. The carbon black conductor allows all of the wires to be soldered onto the glass substrate which can make manufacturing in volume more efficient, especially in the case where electrically conductive layer  188  has been patterned into multiple pixels. 
     LC panels having an electrically conductive glass, such as represented by  FIGS. 2 and 6 , typically include a polyimide (PI) alignment layer between the LC material and the electrically conductive layer of the electrically conductive glass. This PI layer does not interfere with the electrical connection between the carbon black conductor and the electrically conductive layer of the glass even if the PI layer is not removed. 
     Turning now to  FIG. 8 , a flow diagram illustrating an embodiment of a method involving the application of the carbon black conductor is generally indicated by reference number  220 . Method  220  begins at a start  222  and proceeds to  224  where a carbon black substance or other suitable electrically conductive material is mixed with an adhesive to produce a carbon black adhesive. Spacers can also be mixed with the carbon black adhesive. Method  220  then proceeds to  226  where an LC perimeter seal is printed onto a substrate. Method  220  then proceeds to  228  where a carbon black conductor of carbon black adhesive is printed onto the substrate at a position to electrically connect the carbon black conductor to the substrate. Method  220  then proceeds to  230  where the substrate and an electrically conductive glass are laminated together with the LC perimeter seal. Method  220  then proceeds to  232  where the LC perimeter seal and the carbon black conductor are hardened by curing. Method  220  then proceeds to  234  where the method ends. 
     Turning now to  FIG. 9 , a flow diagram illustrating another embodiment of a method involving the application of the carbon black conductor is generally indicated by reference number  240 . Method  240  begins at start  242  and proceeds to  244  where a carbon black substance or other suitable electrically conductive material is mixed with an adhesive and spacers to produce a carbon black adhesive. Method  240  then proceeds to  246  where a carbon black conductor LC perimeter seal is printed with the carbon black adhesive onto a substrate at a position to electrically connect the carbon black conductor to the substrate. Method  240  then proceeds to  248  where the substrate and an electrically conductive glass are laminated together with the carbon black conductor LC perimeter seal. Method  240  then proceeds to  250  where the perimeter seal is cured. Method  240  then proceeds to  252  where the method ends. 
     The foregoing descriptions of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or forms disclosed, and other modifications and variations may be possible in light of the above teachings wherein those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof.