COLOR LIQUID CRYSTAL DISPLAY AND METHOD FOR MANUFACTURING IT

Abstract: A color liquid crystal display having chromatic pixels and achromatic pixels, comprises: color filters (12, 14, 16) formed at locations corre­ sponding to said chromatic pixels (13, 15, 17) and having a light transmitting area for predetermined wavelength; and translucent filters (18) formed at locations corresponding to said achromatic pixels (19) and near an outer periphery of locations corresponding to said chromatic pixels so as to be overlapped with said color filters.

Description: This invention relates to a color liquid crystal display which has color filters and which can display colors, and relates to a method for manufacturing the color liquid crystal display. Generally, a liquid crystal display has a structure in which transparent electrodes are formed on opposite surfaces of opposed glass substrates so as to constitute a number of pixels and in which a liquid crystal composition is interposed between these electrodes. When it is intended to provide a color display using such a liquid crystal display, it is well-known to form colour filters of red (R), green (G) and blue (B) at locations of corresponding pixels respectively on the glass substrate so that one display element consists of three pixels of R, G, and B. Published Unexamined Japanese Patent Application (PUPA) 60-263122 (263122/85) discloses a colour display panel which has achromatic pixels (colourless pixels) such as black or grey in addition to chromatic pixels (colour pixels) of red, green and blue. Published Unexamined Japanese Patent Application (PUPA) 56-140324 (140324/81) discloses a method used to form a light shielding layer by laminating red, green and blue colour filters on their peripheral areas. Japanese Unexamined Patent Application kokai No. 60-61 724 titled "Liquid Crystal Display Panel" describes an LCD having a red, green and blue filter formed as a mosaic on a transparent substrate. A substrate is obtained by forming a transparent electrode on a colour filter on which red, green, blue and a transparent area are placed in the shape of a square having a cross. An active matrix substrate obtained by placing a thin film transistor and a transparent electrode in the shape of a matrix on the transparent substrate is used as the other substrate. Because the light shielding layer formed by laminating the colour filters as shown in the above-mentioned PUPA 56-140324 has a substantial thickness so that difference in level between the light shielding layer and the colour filters increases, a thickness of the liquid crystal layer cannot be made thinner than the difference in level. In addition, in order to arrange the liquid crystal in such a manner that its thickness can be freely set, it is necessary to laminate a substantially thick transparent material over the colour filters. In addition, the above-mentioned PUPA 60-263122 only discloses achromatic pixels, and does not refer to the filters and the light shielding layer. It is one object of this invention to provide a colour liquid crystal display which not only has a wide reproduction range of colours, but also has high contrast. It is another object of this invention to provide a method for manufacturing such a colour liquid crystal display. The present invention provides a colour liquid crystal display with chromatic and achromatic pixels comprising: a liquid crystal cell having first and second transparent substrates arranged substantially parallel to each other; on the inward facing surface of the first substrate a filter layer of substantially constant thickness comprising a mosaic of colour filters and areas of transparent material, the filter layer being covered by a common electrode; on the inward facing surface of the second substrate a plurality of thin film transistors and pixel electrodes whereby each pixel electrode is located to correspond to one of the colour filters or one of the areas of transparent material; a liquid crystal material interposed between the first and second substrates; and is characterised in that translucent filters are disposed between each of the areas of transparent material and the first substrate and are partly overlapped by the surrounding colour filters, such that the thickness of the combination of the transparent material and translucent filter is substantially the same as that of the colour filter, and the translucent filters have an optical density of 0.3 to 1.5 so as to provide a predetermined, substantially equal attenuation to all wavelengths of the visible spectrum. The present invention also provides a method for manufacturing a colour liquid crystal display comprising the steps of: forming material constituting translucent filters on a substrate, the translucent filters having an optical density of 0.3 to 1.5 so as to provide a predetermined, substantially equal attenuation to all wavelengths of the visible spectrum; removing the material constituting the translucent filters at locations corresponding to pixels on which colour filters are to be formed on the substrate; forming material constituting the colour filters at colour filter locations corresponding to the pixels and at locations which are near the colour filter locations so that the colour filters overlap the translucent material; and forming a common electrode over the translucent and colour filters. In accordance with this invention, colors can be produced by combining light which has passed through the color filters with light which has passed through the translucent filters. Therefore, many colors can be displayed. Further, the light shielding layer is formed by overlapping the translucent filters with the color filters. So, high contrast is obtained and the light shielding layer can be thinned. The details of embodiments of this invention will be described in connection with the accompanying drawings. Figure 1 shows a front cross sectional view of one embodiment a color liquid crystal display according to this invention. The color liquid crystal display shown in Figure 1 has a basic structure in which transparent electrodes 3 and 6 are formed on opposite surfaces of opposed glass substrates 1 and 2 to constitute a number of pixels, and TN (twisted-nematic) liquid crystal 5 is interposed between these electrodes 3 and 6. On the glass substrate 2, circuits for pixels each of which has a pixel electrode 6, which is transparent electrode, and an amorphous thin film transistor (a-SiTFT, not shown) are vertically and horizontally positioned all over a screen to constitute a screen with a number of vertically and horizontally positioned pixels. Gates and sources of TFTs are connected by row and column to gate electrode wirings 7 and source electrode wirings (not shown) that extend vertically and horizontally, respectively. Then, orientation film 8 is formed to cover them. Formed on the glass substrate 1 is a filter layer 9 over which a protective film 10 and a common electrode 11, which is the transparent electrode 3, consisted of ITO (indium-tin-oxide) are laminated. Figure 2 is a plane view of the glass substrate 1 illustrating a structure of the filter layer 9 in more detail. In the color liquid crystal display, one display element is formed by positioning four pixels, that is, a red pixel 13 with a square red filter 12, a green pixel with a square green filter 14, a blue pixel 17 with a square blue filter 16 and a pixel 19 with a translucent (gray) filter 18, at the upper left quadrant, the upper right quadrant, the lower right quadrant and the lower left quadrant of a square respectively. The structure of filter layer 9 of the red, green and blue pixels 13, 15 and 17 with the red, green and blue filters 12, 14 and 16, respectively, that is, the chromatic pixels is such that only the filters 12, 14 and 16 are formed at locations which are slightly inside (2 - 5 µm) the locations corresponding to the pixel electrodes 6 formed on the glass substrate 2, and the translucent filter 18 and each of the color filters 12, 14 and 16 are laminated at other locations, that is, outside the above-mentioned locations which are slightly inside the locations corresponding to the pixel electrodes 6 formed on the glass substrate 2. The filter layer 9 of the achromatic pixel 19 with the translucent filter 18 is formed by laminating the translucent filter 18 and a transparent material 20. The red, green and blue filters 12, 14 and 16 consist of gelatin dyed in corresponding colors by, for example, gelatin dyeing method, the translucent filter 18 consists of Cr (chromium), and the transparent material 20 consists of non-colored gelatin. Spectrum transmission rate characteristics of the filters are set as shown in Figure 3. Here, the average transmissivity of the red, green and blue filters 12, 14 and 16 in a range of wavelength from 400 nm to 650 nm is 10%, 7% and 8%, respectively. The transmissivity of the translucent filter 18 in this range is 39.8% (optical density O.D. = 0.4). Multicoloring can be realized by setting the transmissivity of the translucent filter 18 at this value. That is, if the transmissivity of the translucent filter 18 is too low, the colors of the red, green or blue filter 12, 14 or 16, or their combination cannot be made lighter by the translucent filter 18. For example, red cannot be made pink by combining the red filter 12 and the translucent filter 18, but the color remains substantially red as it is. On the other hand, if the transmissivity of the translucent filter 18 is too high, the colors of the red, green or blue filter 12, 14 or 16, or their combination is substantially the color of a light source (not shown) by combination with the translucent filter 18. For example, red cannot be made pink by combining the red filter 12 with the translucent filter 18, but the color becomes substantially the same as the color of the light source as it is. According to an evaluation test on visibility for images by the human eye, it has been confirmed that color can be added by the translucent filter 18 as described above to the filters 12, 14 and 16 of other colors if the optical density of the translucent filter 18 is in a range of 0.3 to 1.5. In addition, the following conclusion has been obtained. Figure 4 is a chromaticity diagram at an opening rate of 50% in a first case where only the filters 12, 14 and 16 are provided, in a second case where the translucent filter 18 whose optical density O.D. is 1.0 is provided as well as the filters 12, 14 and 16, and in a third case where the translucent filter 18 whose optical density O.D. is 0.4 is provided as well as the filters 12, 14 and 16. Figure 4 shows a chromaticity triangle of only red, green and blue, and a chromaticity triangle when white light passed through the translucent filters 18 with different optical density O.D. is added. Here, the opening rate indicates a ratio of area of a pixel where light should be passed. As can be seen from the drawing, although, when white light is added to red, green and blue, each coordinate approximates the color temperature of the light source (7,000°K in this embodiment), its rate can be controlled by the optical density O.D. of the translucent filter 18. The condition where difference of colors can be recognized by difference of lightness, which is one of three elements of color, was sought from evaluation of a plurality of subjects. Then, in this embodiment, it has been found that addition of color at the optical density O.D. = 0.4 is excellent in visibility. For example, in the case where a color is added to blue, if white light of O.D. = 1.0 is added, blue cannot be distinguished from light blue, and there is a high possibility that they will be mistaken for each other. On the other hand, if O.D. = 0.4 is added, blue, light blue and white (7000°K) can surely be identified, and there is no possibility of mis-identification. If, in this embodiment, the liquid crystal is used as an optical shutter, it is possible to display total 16 colors which consist of six colors on the RGB triangle [red, yellow, green, bluish green, blue and pink (purplish pink)], six colors on the RGBW triangle (the above colors with lighter hues), black, black + W (light black), white (red + green + blue), and white + W (intensified white). Thus, when the opening rate is 50%, the optical density O.D. of 0.4 is optimum. If the opening rate becomes larger than 50%, there is no trouble in practical use even if the optical density O.D. is made larger than 0.4. The thickness of the filter layer 9 should be in a range of 0.5 to 2.0 µm. This is because, although the density of color depends generally on the thickness of the filter layer 9, if it is thinner than 0.5 µm, desired spectrum transmission characteristics for R, G and B cannot be obtained, while, if it is thicker than 2.0 µm, the filter layer 9 becomes dark according to the state in which light passes through the liquid crystal layer, and thus the contrast is lowered. Variation of the thickness of the filter layer 9 should be as small as possible. As shown in Figure 5 (a), relation between the thickness d of the liquid crystal layer and the transmissivity of the light which passes through the liquid crystal in a state where planes of polarization of polarizing plates are parallel with each other and voltage is not applied on the liquid crystal layer (off state) depends on wavelength of light, that is, the color of each filter. Figure 5 (b) shows integration of relation between the thickness of the liquid crystal layer and the transmissivity of light passing through the liquid crystal in the above state. The graph in Figure 5 (a) will be described in more detail. The anisotropy of refractive index Δn of the liquid crystal through which light passes is 0.1325, and B represents characteristics of blue light with wavelength of 4,460 angstroms, G represents characteristics of green light with wavelength of 5,460 angstroms, R represents characteristics of red light with wavelength of 6,110 angstroms. Thickness d (µm) of the liquid crystal layer where light transmissivity T becomes 0% is 2.92, 6.52 ... for B, 3.57, 7.98 ... for G, and 3.99, 8.93 ... for R. In this embodiment, the thickness of the liquid crystal layer is set to d = 8.98 µm which is one of the points corresponding to the trough the graph in Figure 5 (b). If the transparent material 20 is not used in this embodiment, the difference between the thickness of the liquid crystal layer at the locations of the filters 12, 14 and 16 of corresponding colors and the thickness of the liquid crystal layer at the locations of the translucent filters 18 becomes about 1.5 µm, so that, as can be clearly seen from Figure 5, if the thickness is set so as not to allow transmission of light when no voltage is applied, light leaking from the translucent area will significantly reduce the contrast. Therefore, in this embodiment, the transparent material 20 is laminated on the translucent filter 18 which is a relatively thin film, so that the variation of the thickness between the color filter 12, 14 and 16 and the translucent filter 18 will be 0.1 µm. Experiments have revealed that variations of the thickness up to 0.5 µm do not cause trouble in the practical use. The thickness of the liquid crystal layer will be further discussed. The following is the reason why, in this embodiment, the thickness is not set at, for example, d = 3.51 µm, which is also one of points corresponding to troughs of the graph in Figure 5 (b). It is usually difficult to manufacture the color filters 12, 14 and 16 of the identical thickness, then variation is produced in the thickness. In Figure 5 (a), the light transmissivity of R, G and B near d = 3.51 µm varies greater than that near d = 8.98 µm. Therefore, because usually the thickness of the color filters 12, 14 and 16 varies, the difference of light transmissivity between the color filters 12, 14 and 16 becomes smaller when the thickness of the liquid crystal layer is set at 8.98 µm rather than at 3.51 µm, and the variation of the contrast becomes smaller. On the other hand, because the response speed of the liquid crystal becomes lower as the thickness of the liquid crystal layer is increased, in the case of this embodiment, the range of the thickness allowable for the liquid crystal is 11 µm or less and it may be best to set the thickness of the liquid crystal layer near d = 8.98 µm. However, it may be set at d = 3.51 µm if the thickness of the film can be accurately controlled. In addition, the red, green and blue pixels 13, 15 and 17 or the chromatic pixels other than the achromatic pixel 19 corresponding to translucent filter 18 have the light shielding layers 21 ... which, as described above, are constituted by laminating the translucent filter 18 and the color filters 12, 14 and 16 at the outside of locations which are slightly inside the locations corresponding to the pixel electrodes 6 formed on the glass substrate 2 of the filter layers 9 for the pixels 13, 17 and 17 having the color filters 12, 14 and 16, respectively. The light shielding layer 21 constituted in such a manner has a transmissivity of 1% or less because of the color filters 12, 14 and 16, and the translucent filter 18 with the above-mentioned transmissivity. This eliminates light leakage from adjacent pixels or that between pixels, and provides a high contrast display. Next, a method for manufacturing a glass substrate 1 of the color liquid crystal display will be described. First, Cr is sputtered on the glass substrate 1 so that a thickness of Cr becomes 300 angstroms (optical density O.D. = 0.4). Then, as shown in Figure 6, for the pixels 13, 15 and 17 at which the red, green and blue filters 12, 14 and 16 should be formed, Cr is removed by photolithography from locations which are slightly (2 - 5 µm) inside the locations corresponding to the pixel electrodes 6 formed on the glass substrate 2. At this time, Cr is not removed from the pixels 19 at which the translucent filters 18 are to be formed. In addition, for the pixels at which the red, green and blue filters 12, 14 and 16 should be formed, color filters of corresponding colors are formed on the glass substrate 1 and residual Cr as the red, green and blue filters 12, 14 and 16 by, for example, a gelatin dyeing method. On the other hand, for the pixels 19 at which the translucent filters 18 should be formed, gelatin without dyeing process is formed on Cr as the transparent material 20. Then, the protective layer 10 and the common electrodes 11 are laminated. In the manufacturing method as described above, the step for forming the light shielding layer 21 can be performed simultaneously with the step for forming other filters so that the number of steps can be reduced. Although, in the embodiment described above, the light shielding layer 21 is constituted by laminating Cr and the filters 12, 14 and 16 of corresponding colors respectively, it may be arranged so that, for example, as shown in Figure 7, the light shielding layer 21 is constituted by laminating Cr and the filters of all colors, for example, in this case, by laminating Cr and red filter 12, and green filter 14, and blue filter 16. In this case, the optical density O.D. of the light shielding layer 21 becomes 3.65, which provides higher contrast. In addition, only one photomask is needed in manufacturing. Furthermore, the variation of the thickness of the shielding layer 21 becomes small. In the above embodiments Cr is used as the translucent filter 18, other metal such as Mo, Ti, Ta, or Ni may be used instead of Cr. It is desirable for the thickness of the translucent filter 18 to be sufficiently less than that of the filters 12, 14 and 16, in view of the difference of the level when the peripheries of the filters 12, 14 and 16 overlap with the translucent filter 18. Although the above embodiment provides the filters on the side of the common electrodes, it may be possible to provide the filters on the side of the pixel electrodes. Although one display unit is of a square consisting of squares of four colors in the above embodiments, the invention does not restrict its shape or its number. For example, one display unit may consist of two pixels, each pixel may be a rectangular, or a pixel pitch may be shifted by one half. As described, according to the color liquid crystal display of the invention, a multicolor display can be attained by combination with the translucent filter, high contrast can be obtained by the light shielding layer in which the color filters are laminated on the translucent filter, and the light shielding layer can be made thinner. In addition, according to the manufacturing method of the invention, a color liquid crystal display that can display multiple colors in such high contract can be efficiently manufactured. Brief Description of the Drawings : Figure 1 is a front cross sectional view of one embodiment of a color liquid crystal display according to this invention. Figure 2 is a partial plane view of Figure 1. Figure 3 is a diagram illustrating spectrum transmission characteristics of each filter. Figure 4 is a chromaticity diagram for illustrating optimum values of the optical density of a translucent filter. Figures 5 (a) and (b) are diagrams illustrating relations between the thickness of a liquid crystal and transmissivity of light passing through the liquid crystal. Figure 6 is a partial plane view of a color liquid crystal display for illustrating a manufacturing method according to an embodiment of the invention. Figure 7 is a partial plane view of another embodiment of a color liquid crystal display according to this invention. 1, 2 glass substrate 5 liquid crystal composition 6 pixel electrode 9 filter layer 12 red filter 14 green filter 16 blue filter 18 gray filter 20 transparent material 21 light shielding layer

Claims: A colour liquid crystal display with chromatic and achromatic pixels comprising: a liquid crystal cell having first and second transparent substrates (1, 2) arranged substantially parallel to each other; on the inward facing surface of the first substrate (1) a filter layer (9) of substantially constant thickness comprising a mosaic of colour filters (12, 14, 16) and areas of transparent material (20), the filter layer being covered by a common electrode (3); on the inward facing surface of the second substrate (2) a plurality of thin film transistors and pixel electrodes (6) whereby each pixel electrode is located to correspond to one of the colour filters or one of the areas of transparent material; a liquid crystal material (5) interposed between the first and second substrate, characterised in that translucent filters are disposed between each of the areas of transparent material (20) and the first substrate (1) and are partly overlapped by the surrounding colour filters (12, 14, 18), such that the thickness of the combination of the transparent material and translucent filter is substantially the same as that of the colour filter; and the translucent filters (18) have an optical density of 0.3 to 1.5 so as to provide a predetermined, substantially equal attenuation to all wavelengths of the visible spectrum. A colour liquid crystal display as claimed in claim 1 wherein the mosaic of colour filters includes a red filter, a green filter and a blue filter. A colour liquid crystal display as claimed in any one of claims 1 or 2 wherein the translucent filter is thinner than any of the colour filters. A colour liquid crystal display as claimed in any of the preceding claims wherein the thickness of the filter layer does not vary more than 0.5µm. A colour liquid crystal display as claimed in any one of the preceding claims wherein the thickness of the filter layer is between 0.5µm and 2.0µm. A colour liquid crystal display as claimed in any one of the preceding claims wherein the translucent filters are made of chromium. A method for manufacturing a colour liquid crystal display comprising the steps of: forming material constituting translucent filters on a substrate, the translucent filters having an optical density of 0.3 to 1.5 so as to provide a predetermined, substantially equal attenuation to all wavelengths of the visible spectrum; removing the material constituting the translucent filters at locations corresponding to pixels on which colour filters are to be formed on the substrate; forming material constituting the colour filters at colour filter locations corresponding to the pixels and at locations which are near the colour filter locations so that the colour filters overlap the translucent material; and forming a common electrode over the translucent and colour filters. A method as claimed in claim 7 wherein the material constituting the translucent filters is chromium.

Inventors: KAJIMURA, MOTOJI, MATINO, HARUHIRO, OANA, YASUHISA, UEKI, TOSHIHIRO
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: G02F1
IPC codes: G02F1, G09F9, G02B5