Abstract:
A method of applying to a display substrate colour elements and addressing busbars in a defined alignment relative to each other includes: forming said colour elements and said busbars on a surface of a transfer carrier; 10 adhering said colour elements and said busbars to said display substrate; and removing said transfer carrier.

Description:
BACKGROUND TO THE INVENTION  
       [0001]     The present invention relates to a method of applying to a display substrate colour elements and addressing busbars in a defined alignment relative to each other. The colour elements may comprise a colour element matrix of the substrate.  
         [0002]     The term ‘colour element’ is used herein to refer to a display component from which coloured light is emitted, transmitted, reflected or scattered when the display is suitably activated. The term includes light filters which absorb certain wavelengths of light, and other components such as wavelength-sensitive reflectors or scatterers, and fluorescent or phosphorescent materials. Commonly, the colour elements are light filters which are part of a colour filter matrix. Displays employing phosphorescent colour elements are described in U.S. Pat. No. 4,830,469 and U.S. Pat. No. 5,608,554.  
         [0003]     The colour element matrix is one of the most expensive components in a lightvalve-type backlit display, for example a liquid crystal polarisation switch mode display. The colour element must be physically close to the electro-optic switching layer to avoid colour parallax, and must be aligned with at least the ‘column’ electrode patterning in the case of RGB colour stripes. Difficulties in achieving this alignment add to manufacturing costs.  
         [0004]     A known production process involves patterning the colour matrix onto the final display substrate, planarising the matrix, and then forming the display cell. While this minimises the distance between the electro-optic switch and the colour filter element, it is very expensive and requires multiple lithographic steps.  
         [0005]     A method of forming electrode patterns for a passively-addressed alphanumeric liquid crystal display (LCD) is described in U.S. Pat. No. 3,902,790. The method involves providing gold-plated stripes for busbars and other conductive elements which are in areas where characters are not displayed, to provide highly conductive paths between display characters. Methods of forming colour filters for LCDs by ink jet printing in pre-defined channels are described in JP 2003035814, JP 11142641, U.S. Pat. No. 5,552,192 and U.S. Pat. No. 5,576,070. Busbars are also used to address other types of displays, for example active matrix LCDs, in which operation of each pixel is controlled by a corresponding thin film transistor (TFT).  
       SUMMARY OF THE INVENTION  
       [0006]     According to an aspect of the present invention there is provided a method of applying to a display substrate colour elements and addressing busbars in a defined alignment relative to each other, the method comprising: 
        forming said colour elements and said busbars on a surface of a transfer carrier;     adhering said colour elements and said busbars to said display substrate; and     removing said transfer carrier.        
 
         [0010]     In a preferred embodiment, the colour elements are light filters. The invention will for convenience be described with reference to light filters unless the context requires otherwise, but it is to be understood that the invention is not limited to this embodiment.  
         [0011]     By forming light filters that absorb visible light to produce colour, the method may provide a colour filter matrix which is aligned with the addressing busbars. The method is suitable for accurately aligning the colour filter matrix to the pixel matrix on a large area display. The display substrate may be glass or a plastics material. Additionally, or alternatively, the light filters may absorb UV light, which will enable them to be used in the formation of transparent electrode tracks in self-alignment with the busbars, as will be described in more detail later.  
         [0012]     In another embodiment of the invention, photoluminescent or other optically modifying materials can be used in place of materials which only filter the incident illumination. Displays of this type are described in “Photoluminescent LCDs (PL-LCDs) Using Phosphors”, W. A. Crossland, I. D. Springle and A. B. Davey, Society Information Display Proceedings 1997, p837 and “Light-Efficient Liquid Crystal Displays Using Photoluminescent Color Filters”, S. W. Njo et al, Society Information Display Processings 2000, p343. It is noted in U.S. Pat. No. 4,830,469 (Breddels et al.), that there is significant advantage in placing the patterned photoluminescent material in close proximity to the light valve to avoid the requirement for a collimated backlight. In this invention a practical means of constructing such a display is presented.  
         [0013]     The method is of particular application to the manufacture of substrates for passive-addressed x/y matrix structures which are elongate parallel lines or strips, and the invention will be illustrated with reference to this application. However the busbars could also take other shapes and forms of addressing metal structures. For example, the busbars may be used as addressing structures for active matrix LCDs and may form the TFT devices and crossovers for the addressing matrix.  
         [0014]     The surface of the transfer carrier is preferably planar, and this planarity defines the final surface quality of the colour element matrix/busbar combination. By using a carrier with a highly planar surface, the invention may provide a final, highly planar surface to the colour element matrix or matrix/busbar combination without the need for a separate planarising operation. An advantage of using a carrier with a planar surface is that the surface quality of the display substrate onto which this is transferred does not have to be very good. If a polariser is laminated on the inner surface of the substrate then birefringence of the substrate becomes unimportant and a substrate with uncontrolled birefringence can be used.  
         [0015]     The busbars and colour filters are transferred by adhesive onto the final display substrate. The alignment of the busbars and colour filters relative to each other on the transfer carrier is preserved on the display substrate. Before the transfer step there is the opportunity to deposit one or more optical films, for example polarisers or compensation retarders, which are also transferred and end up on the inside of the display. A polariser may be of conventional construction, adhered between the substrate surface and the colour filter matrix, or it may be a coatable polariser which may be coated on the colour filter matrix or the substrate surface. The term “optical film” is used herein to denote a film which modifies at least one property of light incident thereon.  
         [0016]     According to another aspect of the invention there is provided a method of applying to a display substrate light-filters and addressing busbars in a defined alignment relative to each other, the method comprising the steps of: 
        (a) forming a series of translucent dielectric structures on a planar surface of a carrier, each structure comprising a colour element-receiving surface region and a raised levee, adjacent dielectric structures being spaced apart to define a trench therebetween;     (b) forming said busbars by at least partially filling each of said trenches with an electrically conductive material;     (c) depositing a colour-element material on each of said colour element-receiving surface regions to form a series of colour elements;     (d) affixing said colour elements and levees to a translucent display substrate by means of a translucent adhesive material; and     (e) removing said carrier.        
 
         [0022]     The dielectric structures may be formed on the carrier by any suitable means, for example embossing, micromoulding, laser ablation or photolithography. In a preferred embodiment the dielectric material is optically transparent and is formed by UV micromoulding, as taught in WO 96/34971, the content of which is incorporated herein by reference.  
         [0023]     The colour element material may be a colour filter material, and the colour element may be a colour filter.  
         [0024]     An embodiment of the invention uses the same dielectric structures to define both the position of the busbars and the formation of defined channels into which the colour element material can be deposited. It is preferred that the element-receiving surface is generally flat but suitably roughened to help the applied colour element material wet out and key in. A preferred method of depositing the colour element material is by inkjet deposition, for example drop-on-demand inkjet printing.  
         [0025]     Once the busbars and colour elements are transferred to the final substrate a transparent conducting material (eg, PEDOT or ITO) is applied and, if required, patterned, using a serial (eg, laser ablation) aligned technique, or by using the colour filter/busbar construction as a shadowing/alignment system.  
         [0026]     A further embodiment of the invention relates to the deposition of transparent conducting material into the defined channels prior to the deposition of the colour filter material using a technique described in GB 0423134.6. In this embodiment there is a further advantage in using the same raised structures to define the electrode patterning directly by using a simple uniform coating technique such as gravure or slot coating for the transparent conductor.  
         [0027]     A combination of elements and busbars may also be used to provide transparent electrode structures in alignment with the busbars even where there is no colour element matrix. By using other element materials instead of colour elements, for example UV-absorbing filters, with UV-transmitting dielectric structures, the electrode structures may be patterned in the same manner as when colour elements are present.  
         [0028]     The trenches and levees will typically be linear structures that will extend across the length of the substrate. Any desired spacing may be used, for example they may be 50 to 200 μm apart, notably about 100 μm apart, and they may be many metres in length. Although the colour element-receiving regions are preferably roughened to promote wetting of the surface by the colour element material, the tops of the levees are preferably smooth or otherwise surface-treated to discourage wetting and flow of one colour material into an adjacent channel.  
         [0029]     Other aspects and benefits of the invention will appear in the following specification, drawings and claims.  
         [0030]     The invention will now be further described, by way of example only, with reference to the following drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]     FIGS.  1  to  9  illustrate stages in the manufacture of a display substrate having colour filters, busbars and electrode tracks in a predetermined alignment, in accordance with an embodiment of the present invention;  
         [0032]     FIGS.  10  to  13  illustrate stages in a method of manufacture in accordance with an alternative embodiment of the present invention;  
         [0033]     FIGS.  14  to  17  illustrate stages in a method of manufacturing a device substrate in accordance with a further alternative embodiment of the present invention;  
         [0034]     FIGS.  18  to  21  illustrate stages in a method of manufacturing a device substrate in accordance with still further alternative embodiment of the invention;  
         [0035]      FIGS. 22 and 23  are schematic sectional and plan views respectively of a display device incorporating a display substrate manufactured in accordance with an embodiment of the invention;  
         [0036]      FIG. 24  is a perspective view corresponding to  FIG. 3 , illustrating another alternative embodiment of the present invention;  
         [0037]      FIGS. 25 and 26  illustrate the formation of patterned transparent electrode structures in accordance with further alternative embodiments of the invention;  
         [0038]      FIGS. 27 and 28  illustrate respectively the formation of an emissive colour matrix and a display employing the matrix in accordance with further embodiments of the invention; and  
         [0039]      FIGS. 29 and 30  are flow charts illustrating steps in carrying out methods according to embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0040]     In the drawings, different parts have been enlarged or reduced to aid illustration of the invention. The drawings are therefore not to scale.  
         [0041]     A transfer carrier  1  for use in the invention is shown in  FIG. 1 . The carrier  1  comprises a base film  2  on which is coated a planar, conductive layer  3 . The carrier  1  may be rigid or flexible. In this example, the base film  2  comprises 150 μm thick PET and the conductive layer  3  is copper metal of about 1 μm thickness. In this example, the conductive copper layer  3  has an exposed surface that is optically flat and which has been passivated by immersion in 0.1 N potassium dichromate solution for 5 minutes, rinsed with deionised water and air dried.  
         [0042]     Referring now to  FIGS. 2-5  and  29 , according to an embodiment of the present invention, the first general step, designated as block  33 , is to form colour elements (in this embodiment, light filters) and busbars on the transfer carrier substrate. A multiple-level, controlled-roughness pattern of dielectric structures  4  is formed on the exposed surface of the conductive layer  3  of the carrier  1  ( FIG. 2 ). The dielectric material is optically transparent and in this example is formed by micromoulding as taught in WO 96/35971. The dielectric structures  4  are separated from each other by a series of parallel trenches  5 , in which the busbars will be formed. Each structure  4  comprises a rough, planar area  6  and a raised levee  7 . The planar areas (filter-receiving surfaces)  6  will accept the colour filter layers and the levees  7  will separate the colour filters. The trenches and levees are substantially linear structures which will run across the length or width of the substrate to which they are transferred. They are typically about 100 μm apart and up to many metres in length. The rough planar surfaces  6  will permit spreading of an applied ink jet coating and may optionally be treated to promote wetting. The levees  7  are smooth and may optionally be treated to further discourage wetting and flow of one colour material into an adjacent planar area.  
         [0043]     Referring now to  FIG. 3 , conductive material  8  is formed in the trenches  5 . The conductive material is preferably a metal and, in this example, is formed by additive electroforming. It is preferred that the conductor  3  forms the cathode of an electrolytic cell with a nickel anode and standard nickel sulphamate-based electrolyte. Plating may be carried out by DC, with pulsed or biased AC current being used to fill in the trenches completely. Other known electroplating or electroless plating techniques may be employed. Suitable metals include nickel, copper and gold.  
         [0044]     The resulting metallised structure is coated with colour filter material ( FIGS. 4 and 5 ). In this example the material is deposited by ink jet printing in the colour-receiving planar areas  6  to produce red  9 , green  10 , and blue  11  colour filter triads. Other colour combinations may optionally be used. Alternatively, for an embodiment which will be described later, the filters  9 ,  10 ,  11  may be UV absorbing but substantially transmit all wavelengths of visible light. In a preferred embodiment, the colour filter material is a dyed UV-curable resin (Brewer Science, Inc PDC). Examples of suitable inkjet nozzles include thermal and piezo nozzles, although other depositing means and techniques may be used. The alignment of droplets is not critical because the filter material is allowed to spread out across the planar regions  6  and is constrained by the levees  7  from flowing into the adjacent channels. The filter material  9 ,  10 ,  11  may be cured after coating, for example by UV exposure and/or thermal treatment.  
         [0045]     Referring now to  FIGS. 6 and 29 , the next general step, designated as block  34 , is to adhere the light filters (colour elements) and busbars to a display substrate. After curing of the colour elements  9 ,  10 ,  11 , the resulting structure is then treated with a transfer adhesive  12 , and the final display substrate  13  is laminated and the adhesive  12  is cured ( FIG. 6 ). In a preferred embodiment the transfer adhesive  12  is a UV-curable material such as NOA81 (Norland Optical Products) but may be thermal- or moisture-cured. The display substrate  13  is preferably a plastics material, for example PEN (DuPont Teijin Teonex Q65), PES (Sumitomo Bakelite) or polyArylate (Ferrania SpA-Arylite), but could comprise glass, preferably a UV-translucent glass.  
         [0046]     The carrier  1  is then removed ( FIG. 29 , block  35 ), in this example by peeling away of the transfer carrier, leaving the colour element/busbar laminate shown in  FIG. 7 .  
         [0047]     Another embodiment of the invention is illustrated with reference to  FIGS. 1-7  and  FIG. 30 . In a first general step, designated as block  36 , a series of translucent dielectric structures  4  are formed on a planar surface of a carrier  1 , each dielectric structure  4  comprising a colour element-receiving surface region  6  (in this example, a filter-receiving surface region  6 ) and a raised levee  7 , adjacent dielectric structures  4  being spaced apart to define a trench  5  therebetween ( FIG. 2 ). In the next general step, designated as block  37 , busbars  8  are formed by at least partially filling the trenches  5  with electrically conductive material ( FIG. 3 ). In the next general step, designated as block  38 , a light-filter (colour element) material is deposited on each colour element-receiving surface region  6  to form a series of light filters  9 ,  10 ,  11  ( FIGS. 4 and 5 ). In the next general step, designated as block  39 , the light filters  9 ,  10 ,  11  and levees  7  are affixed to a translucent display substrate  13  by means of a translucent adhesive material  12  ( FIG. 6 ). In the next step, designated as block  40 , the carrier  1  is removed ( FIG. 7 ). In the illustrated embodiments, further optional process steps are carried out as described below.  
         [0048]     To form electrodes, a transparent conductor  14  is deposited onto the released surface of the laminate structure, as illustrated in  FIG. 8 . The conductor  14  may comprise indium oxide, tin oxide, indium tin oxide (ITO) or the like, but is preferably an organic conductor such as PEDOT:PSS (Bayer Baytron P).  
         [0049]     The transparent conductor  14  is then selectively etched or patterned to provide transparent electrodes  17 . In the present embodiment, illustrated in  FIG. 9 , the conductor  14  is photopatterned by illuminating the laminate from the reverse (substrate  13 ) side. The colour filters  8 ,  10 ,  11  are at least partially opaque to UV, whilst the substrate  13 , transfer adhesive  12  and dielectric  4  are not. Consequently, substantially the only UV transparent areas are the raised levees  7 . This method has the additional advantage in that any faults in the deposition of the filter material resulting in a hole in that layer will result in the transparent electrode at that location being removed and consequently no electro-optical switch in that area. In the preferred embodiment, PEDOT:PSS is bleached directly by the incident UV light to form the electrode structures  17 . Alternatively, standard photoresists and etching may be employed, as will be described later.  
         [0050]     The resulting display substrate has colour filters, busbars and transparent electrodes in a predetermined alignment. It may be incorporated in a display, for example a liquid crystal display, using fabrication techniques well known per se to those skilled in the art of display manufacture.  
         [0051]     Referring now to FIGS.  10  to  13 , a modification of the process in illustrated in which a coatable polariser layer  15   a  is applied on top of the colour filter laminate structure shown in  FIG. 5 . After curing of the coatable polariser layer  15   a , the resulting structure is adhered to a display substrate  13  using a transfer adhesive  12 , followed by removal of the carrier  1 , application of a transparent conductor  14 , and formation of transparent electrodes  17  in a manner as previously described. A suitable coatable polariser material is sold by Optiva, Inc. Coatable polarisers are described in Bobrov, Y., Cobb, C., Lazarev, P., Bos, P., Bryand, D., Wonderly, H. “Lyotropic Thin Film Polarisers”,  Society for Information Display, Int. Symp. Digest of Technical Papers , Long Beach, Calif. May 16-18, 2000, Vol. XXXI, 1102-1107.  
         [0052]     The process illustrated with respect to FIGS.  14  to  17  is similar to that illustrated with respect to FIGS.  6  to  9 , with the difference that the colour filter/busbar structure shown in  FIG. 5  is adhered to an optical film  15 , in this example a polariser, which is in turn adhered to the display substrate  13 . Methods for adhering conventional polarisers to display substrates will be well known to those skilled in the art of LCD manufacture. Other optical films  15 , such as a compensation retarder, may also optionally be laminated inside the display without affecting the planarity and performance of the electro-optic layer interface.  
         [0053]     An alternative method of forming electrode tracks  17  from the transparent conductor layer  14  shown in  FIG. 16  is illustrated in FIGS.  18  to  22 . A positive photoresist material  16  (Shipley 1805) is coated on the transparent conductor  14  ( FIG. 18 ). UV illumination through the substrate  13  transmits UV light through the levees  7  ( FIG. 19 ) thereby curing the resist  16  in regions corresponding to the levees  7 . The resist  16  is developed (Shipley Microposit Developer) to remove the exposed material ( FIG. 20 ), and the transparent conductor  14  is then wet or dry etched (for example by sodium hypochlorite solution) to produce electrode tracks  17  ( FIG. 21 ). Finally, the resist  16  is removed to leave the final substrate with electrode tracks as shown in  FIG. 17 . The resist  16  may be removed by means of standard solvents or a commercial resist stripper, for example acetone.  
         [0054]     For a display substrate in which colour filters are not required, busbars may be aligned with transparent electrode structures formed thereon using the techniques described above, but using UV-absorbing filters  9 ,  10  and  11  that absorb little or no visible light.  
         [0055]     Turning now to  FIGS. 22 and 23 , an example of an electro-optic display device using a substrate manufactured in accordance with an aspect of the present invention is described. The device is a liquid crystal display (LCD) in this example, but the substrates may be employed in other types of display device. The device comprises a first display substrate  13  and a second display substrate  18 , each of which is provided with an adhered polariser  15  in a manner known per se. The structure shown in  FIG. 17  is provided with an alignment layer  19  for inducing a desired local uniform alignment in molecules of a liquid crystal material  27 . The polariser  15  on the second substrate  18  is affixed to a UV-blocking layer  21  by a layer of adhesive  15 . The UV-blocking layer  21  has been used to form electrode tracks  17  on busbars in the layer shown schematically as  20 . The layer  20  contains busbars, dielectric structures, and UV-filters (not shown). The lower electrode structures  17  are also provided with an alignment layer  19 . Any desired alignment layers  19  known to those skilled in the art may be used, for example rubbed polyimide. Depending on the type of display mode, the two alignment layers may induce the same type of alignment (for example planar, tilted planar, or homeotropic) or different types. Where both alignment layers  19  produce a planar or tilted planar alignment, the direction of alignment may be the same or different. For example, in a twisted nematic display, both alignment layers may induce planar alignment, with the orientation of the alignments being perpendicular.  
         [0056]     The display is provided with a peripheral seal  25  to retain the liquid crystal material  27 . In the example illustrated in  FIG. 23 , a plurality of busbars  22  form row-addressing electrodes and a plurality of busbars  23  form column-addressing electrodes. Pixels  26  are defined at locations where row and column electrodes overlap, and characters or other indicia may be displayed in regions where a sufficient voltage is applied across appropriate pixels, thereby modifying the optical behaviour of the liquid crystal in the region of the pixels so that there is a visible difference when the display is viewed between the polarisers  15 .  
         [0057]     Other features known per se may optionally be included in the display by conventional means. Examples include backlights and one or more antiglare layers.  
         [0058]     Each busbar  8  need not be in the middle of its associated electrode track  17 , but may be located at any desired contact line on the track. In  FIG. 24 , part of a transfer carrier is shown, in which the busbars  8  are formed adjacent to the levees  7  on the planar conductive surface  3 . Subsequent UV exposure of a transparent conductor through the levees  7  will result in the busbars being aligned at the sides of corresponding transparent electrode tracks.  
         [0059]     A further alternative method for forming the transparent conductors in alignment with the colour elements and busbar structures is shown in  FIGS. 25 and 26 . The carrier  1  is initially processed as shown in FIGS.  1  to  3 .  FIG. 25  shows the deposition of a dilute solution-based transparent conductor  30  (e.g. PEDOT:PSS Baytron P) by known means (e.g. gravure or slot coating).  FIG. 26  shows the processed carrier after the removal of the solvent and any required thermal or radiative baking of the material to form discrete thin transparent conducting areas  17 . The surface interaction between the transparent conducting material and the patterned dielectric layer is such that the transparent conductor does not form a continuous layer over the levee structure and consequently is patterned by that means. Subsequent steps for the deposition of the colour filter or other materials continue as described above and shown in FIGS.  4  to  7 .  
         [0060]     A further alternative means of forming an emissive colour matrix is shown in  FIGS. 27 and 28 , wherein a photoluminescent optical layer and a suitable wavelength backlight is used instead of a visible wavelength backlight and optical colour filters. The carrier  1  is initially processed as described above and shown in FIGS.  1  to  3 . Preferably the transparent electrode is deposited and patterned as above and shown in  FIGS. 25 and 26 . If a polarisation state-modifying lightvalve LC effect is to be used a coatable polariser  15   a  is deposited into the channels and aligned by any suitable means, including alignment structures on the surface of the transparent resin defining the channels as shown in  FIG. 27 .  FIG. 28  shows the deposition of photoluminescent materials for example by inkjet printing to form the patterned emitters of red, green and blue  9 ,  10  and  11  respectively into channels into which a transparent conductor  17  and polariser  15   a  have already been deposited. Prior or further materials may also be deposited to enhance the optical efficiency of the effect (e.g. colour filters upon the photoluminescent layer to reduce reflections of ambient light, or reflective materials underneath the photoluminescent layer to reduce back-scattered radiation). The carrier and final display substrate are laminated and transferred as described above resulting in the completed substrate  13 .  
         [0061]      FIG. 29  shows an electro-optic display device using a substrate manufactured in accordance with this aspect of the present invention. A second display substrate  14  is prepared and in this embodiment an LC lightvalve is effected by means of alignment layers  19 , and liquid crystal layer  27  with suitable electro-optic cell spacing and construction by methods known per se. In this example a backlight  31  emitting ultra-violet light is polarised by means of a polariser  15 . UV light which passes the second polariser  15   a  and falls on the photoluminescent layer  9 ,  10  or  11  causes the emission of narrowband visible light  32  (i.e. red, green or blue respectively) in a diffuse manner which is desirable for wide angle viewing and optically efficiency.  
         [0062]     The articles “a” and “an” when used herein denote “at least one” where the context permits.  
         [0063]     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable combination.  
         [0064]     It is to be recognized that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the scope of the present invention.  
         [0065]     The disclosures in United Kingdom patent application number 0406310.3, from which this application claims priority, are incorporated herein by reference.  
         [0066]     The disclosures in United Kingdom patent application number 0423134.6, from which this application claims priority, are incorporated herein by reference.