METHOD AND APPARATUS FOR APPLYING A COATING

Abstract: The invention relates to a method of applying a coating to a display window (38) of a color display device, said display window (38) being rotated about a rotary shaft (32) during said coating process. The method is characterized in that, during the application of the coating, the display window (38) is moved in a plane transverse to the rotary shaft (32). Preferably, the coating comprises a phosphor layer and/or a color filter layer. Preferably, three phosphor layers are provided and the display window is moved ina plane transverse to the rotary shaft (32) during the provision of the second and, in particular, the third phosphor layer. Preferably, the display window (38) is moved, during the provision of the coating, in such a manner that, during the rotation, the axis of symmetry of the display window (38) performs a translatory movement (40) or an eccentric movement about a further rotary shaft in a plane transverse to the rotary shaft (32). The rotational speed of the furter rotary shaft preferably ranges between 1 and 5 rpm. An apparatus for applying the phosphor pattern is described.

Description: Method and apparatus for applying a coating. The invention relates to a method of applying a coating to a display window of a color display device, in which method the display window is rotated about a rotary shaft during the provision of the coating. The invention also relates to an apparatus for applying a coating to a display window of a color display device, which apparatus comprises a display -window holder for making the display window rotate about a rotary shaft. Display devices for displaying color images include, inter alia, cathode ray tubes (CRTs) and plasma display panels (PDPs). Said CRTs and PDPs are used, inter alia, as television receivers and computer monitors. A color display device of the type mentioned in the opening paragraph is known. Said known color display device comprises a phosphor pattern containing sub- patterns of phosphor regions luminescing in red, green and blue (hereinafter also referred to as "red", "green" and "blue" phosphors) and it often further comprises a so-called black- matrix layer. A black-matrix layer is a black layer having apertures or a system of black stripes on the substrate and (partly) between the phosphor regions of which the phosphor pattern is composed. Said black-matrix layer improves the contrast of the image displayed. The black-matrix layer comprises apertures which are sometimes provided with colored layers (also referred to as color filter layers) on which a phosphor region of a corresponding color is deposited. The color filter layer absorbs incident light of wavelengths which differ from the wavelengths of the light emitted by the relevant phosphor. This leads to a reduction of the diffuse reflection of the incident light and to an improved contrast of the image displayed. In addition, the color filter layer (for example a "red" layer) can absorb a part of the radiation emitted by the "red" phosphor, namely the part having wavelengths outside the red portion of the visible spectrum. By virtue thereof, the color point of the red phosphor is improved. The color display device may comprise a color filter layer for each of the phosphor layers (red, green and blue). For clarity, it is observed that "red", "blue" and "green" color filter regions demonstrate a relatively high transmission for, respectively, red, blue and green light. The color indication for the color filter layers relates to the transmission properties of the filters, not to their color. Phosphor layers and color filter layers are applied as coatings to the display window of a color display device. Such coatings are generally applied by means of a so-called "flow coating process" in which a suspension comprising the desired (phosphor) color is applied to a (rotating) display window, whereby the entire display window is moistened, and subsequently the phosphor layer or color filter layer is dried. In the patent application, this coating process is briefly referred to as the application of a coating (phosphor layer or color filter layer). After the flow coating process, the dried coating is exposed to light (lithographic step), developed and subsequently dried again. In general, this process is repeated for each individual color filter layer and/or each individual phosphor layer. After assembly of the color display device, these coatings are situated on the inside of the display window of the color display device. The known method has a number of shortcomings, in particular the fact that the coating is not provided uniformly on the display window of the color display device, which adversely affects the brightness or brightness distribution of the image displayed. Therefore, it is an object of the invention to provide a method and an apparatus for manufacturing a color display device of the type mentioned in the opening paragraph, in which the above-mentioned problem is reduced. The invention further aims at providing an apparatus for carrying out said method. To achieve this, the method in accordance with the invention is characterized in that during the provision of the coating, the display window is moved in a plane transverse to the rotary shaft. The apparatus in accordance with the invention is further characterized in that said apparatus is provided with means for making the display window move, during the rotation of the display window, in a plane transverse to the rotary shaft. The inventors have recognized that the layer thickness of structures already present, such as a black-matrix layer and/or one or more (patterned) color filter layers and/or one or more (patterned) phosphor layers gives rise to the formation of so-called (line) defects. In the case of color filter layers applied to a display window of the color display device on which a black-matrix layer is already present, such a (line) defect is referred to as the north/south line. Also in the case of the phosphor pattern of the known television receivers (TVT), which is applied to a display window which has already been provided with a black-matrix layer and/or (a) color filter layer(s) and/or (an) earlier phosphor layer(s), such a (line) defect is referred to as the north/ south line. In the case of the (hexagonal) phosphor pattern of the known computer monitors (CMT), such (line) defects are generally referred to as the 60° -cross. A substantial reduction of the rotational speed does lead to a reduction of said defects, but it also implies an undesirable increase of the cost price of the product. A reduction, for example, of the layer thickness of existing phosphor structures or color filter layers during the provision of these structures does lead to a reduction in line defects but, in the case of phosphor structures, it also causes a decrease in luminance, which is governed by the phosphor-layer thickness, and, in the case of color filter layers, it additionally causes a reduction of the effective action of the color filter layer (reduced increase in contrast). However, a decrease in brightness and/or brightness distribution of the color display device is undesirable. Both the above-mentioned defects in the (patterned) coating or coatings (north/ south line and 60° -cross) are non-uniformities in the phosphor layers and/or in the color filter layers. These non-uniformities are caused by a difference in weight between lines which pass through the center of rotation of the display window and lines which are parallel thereto during the application of the coating (the phosphor layers and/or the patterned color filter layers), the display window being rotated about a rotary shaft. In the known method, this center of rotation coincides with the center (generally also the center of mass) of the display window. These line defects extend throughout the display window and their width is generally no more than twice the so-called pitch of the display window. During operation of the color display device, these (line) defects cause a difference in brightness (luminance) or brightness distribution, so that these structures are visible (in a disturbing manner) in the image displayed. Such defects may additionally give rise to defects during the manufacture of color display devices, which is undesirable. The factors involved in the development of said non-uniformity in the above-mentioned coatings are: the topography of the suspension, the evaporation of the suspension, the centrifugal flow of the suspension and the change in surface tension and viscosity in the suspension. During the evaporation of the suspension, substantial lateral concentration differences occur which can be attributed to the fact that, during the application of one of the color filter layers or one of the phosphor layers, the thickness of an existing black-matrix layer or the thickness of existing (patterned) color filter layers or the thickness of an underlying phosphor topography may amount to tens of percents of the height of the suspension layer of the coating to be provided. Besides, concentration and/or temperature gradients cause gradients in the surface tension. These gradients along the (liquid-gas) interface cause a lateral mass transfer or convection, which is also referred to as Marangoni convection. During the application of one of said coatings (by means of flow coating), said Marangoni convection is influenced by the centrifugal flow. The above-mentioned effects lead to a phosphor pattern or one or more (patterned) color filter layers having a non- homogeneous layer distribution. The north/south line is caused by differences in lateral material transfer (Marangoni convection) in the suspension transverse to already existing lines. For example, a hexagonal phosphor pattern (CMT) comprises so-called troughs in the topography, which originate from the center of rotation and which are formed by as yet empty phosphor-dot positions in a hexagonal phosphor pattern which is structured already with one or two colors. At angles of 60°, these empty dot positions lie on a straight line, without phosphor dots of the first or second phosphor layer being present on this line. Also in this case, the centrifugal suspension flow transverse to these troughs brings about a non- uniform concentration-averaging in these troughs around the center of the display window. This results in line defects in the hexagonal pattern, the forming mechanism being analogous to that of the north/ south line. The inventors have recognized that the formation of the (line) defects occurs, in particular, in the drying operation of the flow coating process in which the coating is formed. The formation of these defects is spread over a plurality of lines in the display window by changing the center of rotation of the display window (during this drying operation) in accordance with the inventive method. A movement of the display window in a plane transverse to the rotary shaft is to be taken to mean in this application that the center of the display window moves during rotation and that the movement of the center of the display window has at least a motion component in a plane transverse to the rotary shaft. By virtue of the measure in accordance with the invention, visible line defects in an image to be displayed during operation by the color display device are reduced or precluded. An embodiment of the method in accordance with the invention is characterized in that the coating comprises a phosphor layer for emitting, during operation, light of a specific color. Preferably, the display window is provided with three phosphor layers and, during the provision of the third phosphor layer, or during the provision of the second and the third phosphor layer, the display window is moved in a plane transverse to the rotary shaft. In a color display device, the three (primary) phosphor layers are successively applied and provided with a pattern. The phosphor pattern already present during the provision of the second phosphor layer and, in particular, during the provision of the third phosphor layer, causes said line defects in the phosphor pattern (north/south line and 60°-cross). Such undesirable effects (line defects) occur, in particular, if at least one of the phosphor layers of the phosphor pattern has already been provided on the display window. The undesirable effect notably occurs during the application of the third phosphor layer if the first and the second phosphor layer have already been applied to the display window. Another embodiment of the method in accordance with the invention is characterized in that the coating comprises a color filter layer. A preferred embodiment of the method in accordance with the invention is characterized in that, during the provision of the coating, the display window is moved about a further rotary shaft in such a manner that the axis of symmetry of the display window moves eccentrically relative to the rotary shaft. As a result of this eccentric rotation during the provision of the second or third phosphor layer, the center of mass (center of rotation) of the display window describes a so-called Lissajous figure during the flow coating process. Preferably, the direction of rotation of the further axis is at least substantially parallel to the direction of rotation of the (main) rotation axis. A further advantage of a (semi-)eccentric rotation of the display window is that, an irregularity in the applied layer in the center of the display window is precluded if, after the provision (pouring out) of the suspension on the display window, such a rotation is also carried out during the distribution of the suspension on the display window. Preferably at the rotational speed of the further rotary shaft ranges between 0.1 and 25 revolutions per minute (rpm). If the rotational speed of the further rotary shaft is too low (rotational speed < 0.1 rpm) or too high (rotational speed > 25 rpm) the non-uniformities remain. Preferably at a rotational speed of the further rotary shaft in the range between 1 and 5 rpm, the formation of non-uniformities is (at least substantially) precluded, so that a visible line defect in the color display device is avoided. A preferred embodiment of the method in accordance with the invention is characterized in that, during the provision of the coating, the display window is moved such that, during rotation, the axis of symmetry of the display window performs a translatory movement in a plane transverse to the rotary shaft. Such a (straight) translation of the center of rotation during the flow coating process, which is in fact a so-called semi-eccentric rotation of the display window, leads to a substantial reduction of the non-uniformity of the phosphor pattern and/or of the color filter layers on the display window. An advantage of such a motion mechanism is that it can be driven by the original display-window rotation. A preferred embodiment of an apparatus for applying the coating in accordance with the invention is characterized in that, in the absence of rotation of the display window, the axis of symmetry of the display window is at a distance of d ! (d, > 0) from the rotary shaft, and, during rotation, the axis of symmetry is at a distance of d 2 from the rotary shaft, where d 2 > d,, and in that the means comprise restoring means which, after the rotation has ended, take the axis of symmetry back to, at least substantially, the distance of d,. Preferably, the distance of d 2 is governed by the rotational speed. The eccentrically arranged center of mass causes the display device to be flung away, as it were, by the rotational movement. The translation speed can preferably be controlled by a spring which ensures that the display window returns to its original display window position (relative to the rotary shaft). In an alternative embodiment, a damper-spring combination is used which does not only influence the restoring force but also the degree of displacement (the difference between d 2 and d,) during the rotation. The use of the method or apparatus in accordance with the invention leads to a reduction of the non-uniformities in the phosphor pattern and/or in the (patterned) color filter layers and, in addition, it enables thicker coatings to be used, which has a further favorable effect on the brightness and brightness distribution of the color display device. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Figs. 1A and IB show a cut-away view of a display device comprising a cathode ray tube; Figs. 2 A and 2B show two (mutually perpendicular) side views of a display window holder which, in accordance with the invention, during rotation, performs a translatory movement relative to the rotary shaft, and Figs. 3 A and 3B show a plan view and a side view, respectively, of a display window holder which, in accordance with the invention, during rotation, rotates eccentrically relative to the rotary shaft. The Figures are purely schematic and not drawn to scale. In particular for clarity, some dimensions are exaggerated strongly. In the Figures, like reference numerals refer to like parts, whenever possible. Fig. 1A schematically shows a cut-away view of a display device comprising a cathode ray tube (CRT) 1 with a glass vacuum envelope 2 which includes a display window 3, a cone portion 4 and a neck 5. Said neck accommodates an electron gun 6 for generating three electron beams. These electron beams are focused on a phosphor layer 7 on the inside of the display window 3. The electron beam(s) is or are deflected across the display window 3 in two mutually perpendicular directions by means of a deflection coil system 8. On their way to the display window 3, the electron beam(s) pass(es) through a shadow mask (not shown in Fig. 1A) which is arranged in front of the display window and which comprises a thin plate having apertures. The three electron beams pass through the apertures of the shadow mask at a small angle with respect to each other and, consequently, each electron beam impinges only on phosphor elements of one color. Fig. IB is a cross-sectional view of a detail of Fig. 1A, in which the phosphor layer 7 on the inside of the display window 3 comprises a regular pattern of (electro)luminescent picture lines or phosphor dots 19R, 19G, 19B. The phosphor dots 19R, 19G, 19B each include a suitable phosphor of the proper color: red 19R, green 19G and blue 19B. Figs. 2 A and 2B schematically show two (mutually perpendicular) side views of a display window holder 31 which, in accordance with the invention, during rotation, perform a translatory movement relative to the rotary shaft. The display window holder is mounted on a rotary shaft 32 which, during the flow coating process, rotates about an axis 30. In this example, the display window holder 31 comprises a first plate-shaped part 33 and a second plate-shaped part 37 which are connected to each other by means of connecting parts 35, 36 in such a manner that the second plate-shaped part 37 can move in a direction transverse to the rotary shaft. The display window holder 37 is provided with a display window 38 on which a phosphor pattern is formed; the center of mass of the display window holder and the display window is referenced c m in Fig. 2B. In this example, said connecting parts comprise two so-called dovetail-shaped connecting parts 35, 36, which are shown separately for clarity of the drawing. If the dovetail-shaped connecting parts 35 and 36 are fitted into one another in the customary manner, they can perform a (straight) translatory movement (indicated by means of the two-sided arrow 40 in Fig. 2B) relative to each other in the y-direction of Fig. 2B. This translatory movement allows the two plate-shaped parts 33 and 37 to move relative to each other. If the center of mass c m does not coincide with the axis 30 of the rotary shaft 32 (in Fig. 2B, the center of mass is at a distance of d] from the axis 30) the center of mass will move further away from the axis 30 during the rotation of the rotary shaft, the distance between the axis 30 and the center of mass becoming d 2 (not shown in Fig. 2B), where d 2 > d,. The distance d 2 is preferably dependent on the rotational speed. The eccentrically arranged center of mass c m causes the display window 38 to be flung away as it were by the rotational movement. The center of the display window (in other words, the display window as a whole) moves during the rotation, and said movement comprises a component in a plane transverse to the rotary shaft 32. Preferably, the translatory speed can be controlled by means of a spring (not shown in Fig. 2), which ensures that the display window 38 returns to its original display window position (relative to the rotary shaft). Alternatively, a combination of a damper and a spring can be used, which does not only influence the restoring force but also the degree of displacement (d 2 -d,) during rotation. In a further alternative embodiment, the displacement of the connecting parts 35 and 36 relative to each other is controlled by a motor which enables the position of the center of mass c m relative to the axis 30 to be set in a desired manner. If the direction of the translatory movement of the display window is parallel to the direction of a line structure already present, the undesirable line defect in the layer to be applied is not suppressed. Therefore, the direction of the translatory movement is preferably selected such that said direction has at least a component which extends at right angles to the direction of (a) line defect(s) already present. Figs. 3A and 3B schematically show a plan view and a side view, respectively, of a display window holder 51 which, in accordance with the invention, during rotation, performs an eccentric rotation movement relative to the rotary shaft. The display window holder is mounted on a rotary shaft 52, also referred to as main shaft, which rotates about an axis 50 during the flow coating process. In this example, the display window holder 51 comprises a first plate-shaped part 53 and a second plate-shaped part 57 which are interconnected by means of a further rotary shaft 52', also referred to as auxiliary shaft, in such a manner that the second plate-shaped part 57 can rotate about an axis 50' which is generally parallel to the axis of rotation 50. A display window 58 which is provided with a phosphor pattern is mounted on the display window holder 57. In this embodiment, the center of mass of the second plate-shaped part 57 and of the display window 58 coincides with the axis 50'. Thus, the center of the display window is situated substantially on the axis 50 '.To preclude that the suspension is flung away, a wall 60 is provided around the display window holder. In this example, the display window rotates about two spaced apart rotary shafts, i.e. the main shaft 52 and the auxiliary shaft 52'. The further (auxiliary) shaft 52' is driven by an additional motor (not shown in Fig. 3) which is arranged on the (main) shaft 52. When both shafts 52, 52' are rotated at different speeds, the center of rotation of the display window 58 is displaced continually (see Fig. 3A), and the center of rotation of the display screen performs a so-called Lissajous movement. Consequently, during rotation, the center of the display window is subject to a movement, which movement extends in a plane transverse to the axis 50' , and hence comprises a component in a plane transverse to axis 50'. In an experiment with a standard TVT display window glass, the sum of the rotational speeds of the shaft 52 and the further shaft 52' during the drying operation was approximately 120 revolutions per minute (rpm). If eccentric rotation is dispensed with, the known, undesirable north/south line is formed. At rotational speeds of the further shaft 52' of > 25 rpm, the known north south line is formed. In the range between 0.1 and 25 rpm, a reduction of the line defect is observed; particularly at rotational speeds of the auxiliary shaft 52' in the range between 1 and 5 rpm no line defects are visible. At rotational speeds below approximately 0.1 rpm, the line defect becomes visible again. A suitable choice of the rotational speed of the further rotary shaft is determined as a function of the rotational speed of the main shaft. Preferably, the rotational-speed ratio of the rotary shaft to the further rotary shaft ranges from 1200 to 5, preferably from 120 to 24. At a rotational speed of the main shaft of, for example, 120 rpm and a rotational speed of the auxiliary shaft of, for example, 2 rpm, the rotational-speed ratio of the main shaft to the auxiliary shaft is: 120/2 = 60. Preferably, the eccentricity of the further rotary shaft relative to the (main) shaft is of the order of 10 x the pitch of the display window, for example an eccentricity value of 3 mm. A (relatively) high eccentric rotational speed is not effective because the centrifugal flow is too slow to bring about a substantial concentration-averaging during one half eccentric revolution. In this case, the line-defect forming process is not disturbed by the eccentric movement, so that a north/ south line remains in the center of the display window. If the eccentric rotational speed is (relatively) too low, the center of rotation of the display window changes insufficiently during the formation period of the line defect. The optimum eccentric rotational speed is found to range between 1 and 5 rotations per minute. Experiments have further shown that a good concentration-averaging (that is, no Marangoni convection) does not only lead to the absence of a north/south line but also to the formation of a flat phosphor profile. The phosphor lines near the edges of the display window are almost as high as in the center of the north/south line. It will be obvious that within the scope of the invention many variations are possible to those skilled in the art. In general, the invention relates to a method of applying a coating to a display window of a color display device, said display window being rotated about a rotary shaft during said coating process. The method is characterized in that, during the application of the coating, the display window is moved in a plane transverse to the rotary shaft. Preferably, the coating comprises a phosphor layer and/or a color filter layer. Preferably, three phosphor layers are provided and the display window is moved in a plane transverse to the rotary shaft during the provision of the second and, in particular, the third phosphor layer. Preferably, the display window is moved, during the provision of the coating, in such a manner that, during the rotation, the axis of symmetry of the display window performs a translatory movement or an eccentric movement about a further rotary shaft in a plane transverse to the rotary shaft. The rotational speed of the further rotary shaft preferably ranges between 1 and 5 rpm. An apparatus for applying the phosphor pattern is described.

Claims: Claims: 1. A method of applying a coating to a display window of a color display device, in which method the display window is rotated about a rotary shaft during the provision of the coating, characterized in that during the provision of the coating, the display window is moved in a plane transverse to the rotary shaft. 2. A method as claimed in claim 1, characterized in that the coating comprises a phosphor layer for emitting, during operation, light of a specific color. 3. A method as claimed in claim 2, characterized in that the display window is provided with three phosphor layers and, during the provision of the third phosphor layer, or during the provision of the second and the third phosphor layer, the display window is moved in a plane transverse to the rotary shaft. 4. A method as claimed in claim 1, characterized in that the coating comprises a color filter layer. 5. A method as claimed in claim 1, characterized in that, during the provision of the coating, the display window is moved such that, during rotation, the axis of symmetry of the display window performs a translatory movement in a plane transverse to the rotary shaft. 6. A method as claimed in claim 1, characterized in that, during the provision of the coating, the display window is moved about a further rotary shaft in such a manner that the axis of symmetry of the display window moves eccentrically relative to the rotary shaft. 7. A method as claimed in claim 6, characterized in that the rotational-speed ratio of the rotary shaft to the further rotary shaft ranges from 1200 to 5, preferably from 120 to 24. 8. An apparatus for applying a coating to a display window of a color display device, which apparatus comprises a display window holder for making the display window rotate about a rotary shaft, characterized in that the apparatus is provided with means for making the display window move, during the rotation of the display window, in a plane transverse to the rotary shaft. 9. An apparatus as claimed in claim 8, characterized in that the means cause the axis of symmetry of the display window to perform, during the rotation, a translatory movement in a plane transverse to the rotary shaft.

Inventors: BEERENS MARINUS NICOLAAS MARIA, GEHRING FREDERIC CHRISTIAAN, THEELEN GERARDUS WILHELMUS MAR, VAN MARREWIJK RICHARD NICOLAAS, BEERENS, MARINUS, NICOLAAS, MARIA, GEHRING, FREDERIC, CHRISTIAAN, THEELEN, GERARDUS, WILHELMUS, MARIA, VAN MARREWIJK, RICHARD, NICOLAAS
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: H01J9
IPC codes: H01J9