THE PROVISION OF COLOR ELEMENTS ON SUBSTRATES BY MEANS OF A SCREEN-PRINTING OR STENCIL-PRINTING METHOD

Abstract: The invention relates to the provision of a paste on a substrate to manufacture color elements in accordance with a pattern, such as the provision of phosphor elements on display windows (for example of a CRT or a plasma display) or color-filter elements on substrates for display devices (for example LCDs). The dots can be provided on a substrate into a high or a low matrix, or on a substrate without a matrix. In particular, use can be made of special stencils which enable different layers to be successively printed on a substrate without a matrix, with previously printed layers hardly exerting an influence on the layers to be printed next.

Description: The provision of color elements on substrates by means of a screen-printing or stencil- printing method. The invention relates to the provision of a paste on a substrate to manufacture color elements in accordance with a pattern, such as the provision of phosphor elements on display windows or color-filter elements on substrates for display devices (for example LCDs). In general, the display window of a CRT (cathode ray tube) is provided with phosphor dots by means of a lithographic process. This has the disadvantage that it requires many production steps, expensive masks and expensive illumination equipment. Hitherto, this is the only usable method for printing the bidirectionally curved window of most conventional CRTs. The invention is based on the recognition that for printing the screen of a unidirectionally curved window or of the window of a flat display (for example a plasma display) use can advantageously be made of a screen- printing method or a stencil- printing method. Screen printing and stencil printing are used, inter alia, for PCBs, LCDs, multilayer capacitors and resistors. In the case of flat or unidirectionally curved displays, the duration and the cost of the process can be reduced, in particular, if this technique is used to print phosphors of three different colors in the apertures of a matrix. A matrix is a grid of compartments which are surrounded by raised edges. Examples of matrices are: a black matrix on (TTO-coated) glass (in a CRT) a glass matrix (in an embodiment of a plasma display). The screen-printing process is carried out as follows: a screen comprising woven, clamped gauze, which is made, for example, of stainless steel, polyester, nylon or another synthetic material, with a partly removed photopolymer layer, the removed portion forming the desired pattern, is provided on the substrate (the surface to be printed). During a screen printing step, a squeegee is moved over the screen, thereby a paste (for example a mixture of a phosphor and a binder) is spread. With the squeegee a pressure is exerted on the screen so that the screen engages the substrate, and the hydrodynamic pressure causes a specific quantity of paste to enter the apertures in the screen. When the squeegee recedes, the screen comes away from the substrate and the paste is transferred from the screen to said substrate. The stencil- printing process takes place in the same manner, with this difference that a stencil is used instead of a screen- printing screen. A stencil is a metal foil in which holes are provided at the locations where the paste is to pass through it. The result of the printing process is governed by three sub-processes: spreading the paste on the screen or stencil filling the apertures in the screen or stencil - discharging the paste from the screen or stencil onto the substrate. In printing phosphors on the display window of a color display tube, three different layers must be printed which do not overlap. The reason for this being that each pixel consists of three dots which each produce a different color when they are excited. The entire screen must be covered with a pattern of small phosphor dots luminescing in three different colors. If this process is carried out by means of the screen-printing technique, the following problems are encountered: dots luminescing in different colors may overlap. (If during printing of the second and/or third layer, the screen is not properly positioned, the screen deformation is too high or the reproducibility of the deformation too low.) - overlap of dots luminescing in different colors causes parts of the surface to remain unprinted. the dots may smear. (If a subsequent screen is provided on the already existing printed layer.) the dots may have the wrong thickness. (When the squeegee is moved over the screen it may penetrate into the apertures and, thus, influence the layer thickness.) a layer to be printed may be influenced by the preceding layer. (This can be attributed to the fact that due to said preceding layer the printing surface is no longer flat.) - previously printed layers influence the shape and the thickness of the dots in the subsequent layer. Within the scope of the invention various solutions are possible: screen printing + another printing method: dosing of paste by means of a screen-printing screen into a matrix having (high) walls, which is made by means of another method (for example a photolithographically produced black matrix for a CRT or a glass matrix for a specific type of plasma display. In this manner, dots having an at least slightly concave shape are formed, stencil printing + another printing method: dosing of a paste by means of a stencil into a matrix having (high) walls. stencil printing by means of special stencils (smart stencil printing): dosing of a paste into a matrix having (low) walls or onto a substrate without a matrix, and utilizing a stencil having additional cavities. In this manner, dots having an at least slightly convex shape are formed. The advantage of printing into a matrix having (high) walls is that a special stencil is redundant. The use of an ordinary screen printing screen or stencil is sufficient. The accuracy of the pattern is determined by the process with which the matrix is printed. The dimensions of the apertures in the screen or stencil may be smaller than the compartments of the matrix, so that small deformations of the screen or the stencil do not affect the printing result. In a preferred embodiment of the invention use is made of special stencils for printing phosphor dots onto display screens. For this purpose, use is made of solid stencils comprising capillary apertures at the location of the desired pattern. The lower side of the stencil is provided with cavities which can accommodate the layers already printed. Apart from the capillaries, the stencils are solid structures, so that they are much more stable than a screen- printing screen. By virtue of the cavities at the lower side of the special stencil, which can accommodate the layers which have already been printed, the substrate can be qualified as flat in a subsequent printing step. The flow resistance of the capillaries is a function of the diameter, length and shape of said capillaries. Thus, the quantity of paste passing through said capillaries during a printing step can be adjusted. The special stencils can also be used for applications other than the provision of phosphors. By virtue of the above-mentioned characteristics, the stencils are very suitable for each application in which a plurality of layers, possibly of special thicknesses, are to be provided. In summary, special stencils have, inter alia, the following advantages: - a higher resolution (finer patterns are possible) variable layer thickness is possible (also in a printing step) a plurality of layers can be provided on one substrate. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 is a schematic, cross-sectional view of a part of a customary screen-printing screen from which the photopolymer layer is partly removed. Fig. 2 schematically shows the screen-printing process in which a squeegee is moved over the screen to spread a paste, where after lifting the screen paste remains on the substrate at the location of the apertures in the screen. Fig. 3 schematically shows how patterns of phosphor dots luminescing in different colors are successively printed by means of a special stencil. Fig. 4 is a schematic, cross-sectional view, respectively, of a part of a special stencil having two layers and a special stencil having three layers. Fig. 5 A shows a substantially convex dot and a substantially concave dot if printing takes place into a matrix having high walls. Fig. 5B shows a convex dot if printing takes place into a matrix having low walls or if a matrix is absent. Fig. 5C shows a flat dot which is ideal for specific applications. Fig. 6 is a schematic sectional view of a part of an embodiment of a plasma display. Fig. 1 is a schematic, cross-sectional view of a part of an ordinary screen- printing screen, in which the threads of the woven screen and the resist material are referenced (1) and (2), respectively. Fig. 2 A shows the position of the squeegee (3) on the screen (4). Said squeegee (3) spreads the paste (5), thereby exerting a pressure on the screen (4) so that said screen engages the substrate (6). Reference numeral (7) indicates the location of an aperture in the screen (4). (The squeegee may have different shapes. In this case, the shape of the squeegee is schematically shown.) Fig. 2B shows that after the squeegee (3) distributing the paste (5) has passed over the aperture (7), a quantity of paste (8) is provided on the substrate (4) at the location of said aperture (7). In Fig. 3A, the squeegee (3) spreads green phosphor paste (9) on the special stencil, which results in the formation of dots on the substrate as the paste is passed through the apertures (16). Fig. 3B shows how a special stencil is arranged on the substrate after the provision of the green phosphor dots (10), said printed green dots (10) being accommodated by cavities (17) of the stencil. Subsequently, the red paste (11) is passed through the apertures (16). Fig. 3C shows how the green dots (10) and the red dots (12) are accommodated by the cavities (17) of the stencil. Next, the blue paste (13) is pressed through the apertures (16), so that the blue dots (14) are printed on the substrate (6). The sequence in which the different colors are printed is not defined. The above-mentioned sequence serves only as an example. Fig. 3D shows the substrate on which 3 colors are printed. In the case of customary phosphor patterns for display screens, three printing steps are carried out and the stencil(s) employed for printing the three colors are equal. Of course, it is also possible to successively print more or fewer than three layers, and the stencils for the various printing steps do not have to be identical. In addition, the height dimension of the various cavities may vary within a stencil and from cavity to cavity. Fig. 4A shows a special stencil (18) having two layers. For example, the upper layer may be made of metal and the lower layer may be made of a flexible material. Fig. 4B shows a special screen (19) having three layers. The upper layer is provided with cavities (20). Figs. 5A, 5B and 5C show possible shapes of dots. The dot shape is an important aspect with regard to the brightness and homogeneity of the display screen. The eventual shape of the phosphor dots is attained during the drying process. However, for some phosphors and properties of the paste, the shape after printing and before drying does have an influence on the shape after drying. The viscosity and formulation of the paste have an influence on the properties during drying. The paste, the screen and the drying conditions influence the density and shape of the dots after drying. The viscosity of the paste, the screen resistance and the geometry of the pattern have an influence on the quantity of paste being dosed. During the firing process, which takes place after drying, the binder is removed from the paste, so that almost only the phosphor particles are left. If printing takes place in a matrix having edges which are higher than the height of the desired dots, then the dots having three different colors can be successively provided without an intermediate drying step, provided that the matrix compartments are not filled excessively. Depending on the properties of the paste and on the screen or stencil, the dots can be printed in any shape ranging from convex to concave. Under certain conditions, the profile changes from convex to concave during drying. The printing of a color into a number of the compartments normally does not lead to contamination of the other compartments. To minimize the variation in brightness within dots and over the entire screen, preferably, the original flat shape (Fig. 5C) of the dots is maintained, inter alia in CRTs, instead of allowing the dots to spread so as to form a bump having a convex shape. For example, in the case of a plasma display a concave shape is generally desired. Dots printed into a matrix have less spread. The height dimension of the spots is larger, which can probably be attributed to said reduction in spread. If the viscosity of the paste used is not too high and the matrix compartments are not filled excessively, a concave dot shape is attained since the paste sinks downwards yet sticks to the side walls as a result of adhesion. If the glass is provided with an indium-tin oxide coating, this coating does not adversely affect the quality of the dots. By subjecting a previously printed layer of dots to a short drying process, for example, for 1 minute or more in a hot-air oven, running of said layer is precluded. Consequently, printing can take place into a matrix having low walls or the matrix can be omitted. If the cavities of the stencil are larger than the dots, a drying period is superfluous. The cavities of the stencil may have a hexagonal shape if printing is carried out into a matrix having hexagonal compartments or if a hexagonal shape of the dots is desired. The shape and the material of the squeegee used also have an effect on the shape of the dots. Apart from other things, the angle which the squeegee and the screen or the stencil make with each other, and the influence of said angle on the pressure in the paste and hence on the quantity of paste pressed through the holes, are the most important factors in this respect. If the compartments of a stencil are filled only partly, for example, adhesion to the walls may lead to a concave shape which also remains after the removal of the stencil. There are various types of squeegees (in terms of shape and material) which, depending on the conditions, have different properties. A small angle between the squeegee and the screen generally leads to a better result, as does a material having a greater hardness. Fig. 6 is a schematic, cross-sectional view of a part of an embodiment of a plasma display. The compartments (21) of the glass matrix are surrounded by walls (23) and their inner surfaces are coated with a phosphor (22). Reference numeral (24) indicates the window through which the light emitted by excited phosphors exits. There are various embodiments of the special stencils. For example: a stencil which is entirely made of metal a stencil having a metal upper layer in which the apertures are formed through which the paste is supplied, and a lower layer of a flexible material, for example a polymer, in which the cavities for accommodating preceding layers are provided a metal stencil whose bottom side is covered, either completely or partly, with a flexible material (at least the parts contacting the substrate are preferably covered with said flexible material.) any one of the above embodiments, with reservoirs being situated above the apertures provided with capillaries (Fig. 4B). An advantage of the first embodiment is that said stencil is less subject to deformation than other stencils. An advantage of the second and third embodiments is that the flexibility of the coating material enables a better sealing to be attained, so that the risk of running is reduced. Advantages of the last embodiment are: thinner layers can be printed, using relatively thick stencils which cause less pattern deformation than thin stencils. by virtue of the reservoirs, the squeegee cannot penetrate into the capillaries, so that the printing thickness cannot be influenced in this manner. the quantity of paste discharged from the stencil can be more accurately controlled. Consequently, the invention enables a plurality of layers to be successively provided, while the degree of deformation is substantially reduced, and the thickness of the layers can be more accurately controlled. In summary, the invention relates to the provision of a paste on a substrate to manufacture color elements in accordance with a pattern, such as the provision of phosphor elements on display windows (for example of a CRT or a plasma display) or color-filter elements on substrates for display devices (for example LCDs). The dots can be provided on a substrate into a high or a low matrix, or on a substrate without a matrix. In particular, use can be made of special stencils which enable different layers to be successively printed on a substrate without a matrix, with previously printed layers hardly exerting an influence on the layers to be printed next.

Claims: CLAIMS: 1. The provision of a paste on a substrate to manufacture color elements in accordance with a pattern, characterized in that the paste is provided by means of a screen printing technique or a stencil-printing technique. 2. The provision of a paste on a substrate to manufacture color elements in accordance with a pattern, characterized in that the paste is provided by means of a stencil- printing technique in which use is made of a stencil which is made of a solid material, in which the pattern to be printed is built up of capillaries. 3. A method as claimed in Claim 1 or 2, characterized in that printing is performed on a substrate without a matrix. 4. A method as claimed in Claim 1 or 2, characterized in that printing is performed into a matrix. 5. A method as claimed in Claim 2, characterized in that a stencil is used in which cavities are provided on the side of the screen facing the substrate, and which cavities can accommodate previously printed layers. 6. A method as claimed in Claim 2, 3, 4 or 5, characterized in that a stencil is used in which reservoirs are situated above the capillaries. 7. A method as claimed in Claim 1, 2, 3, 4, 5 or 6, characterized in that a stencil is used which is entirely made of metal. 8. A method as claimed in Claim 5 or 6, characterized in that use is made of a stencil whose upper part is made of metal and whose lower part is made of a flexible material. 9. A method as claimed in Claim 5 or 6, characterized in that the side of the stencil facing the substrate is covered at least partly with a flexible material. 10. A method as claimed in any one of the preceding Claims, characterized in that the method is used to provide phosphor elements on display windows by means of printing. 11. A method as claimed in any one of the preceding Claims, characterized in that the method is used to provide a substrate with color-filter elements by means of printing.

Inventors: HENRIETTE PEDRO FRANCISCO, TEEUWEN GERARD HARRY LEON, VAN DEN BOGERT MARCEL JACOB, VISSER-BARTELDS JANTJE, HENRIETTE, PEDRO, FRANCISCO, TEEUWEN, GERARD, HARRY, LEON, VAN DEN BOGERT, MARCEL, JACOB, VISSER-BARTELDS, JANTJE
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H01J9
IPC codes: B41C1, B41F15, B41F17, B41M1, B41N1, G02F1, H01J9, H05K3