METHOD FOR PRODUCING PHOSPHOR SCREENS, AND COLOUR CATHODE RAY TUBES INCORPORATING SAME

Abstract: Adherence of the phosphor screen to the face panel of a colour cathode ray tube is improved by placing a UV-reflective filter on the inside of the face panel prior to photolithographic forming of the screen, in order to reflect transmitted UV light back onto those areas from which the light emerged, thereby effectively increasing the exposure dosage of those areas.

Description: Method for producing phosphor screens, and colour cathode ray tubes incorporating same This invention relates to phosphor screens for colour cathode ray tube (CRT's), and more particularly relates to a method for increasing the adherence of such screens to the face panels of the tubes, and to tubes incorporating such screens. In producing colour CRT's for colour television and allied display applications, it is customary to form the phosphor screen photolithographically by forming three interlaced patterns of phosphor elements, one for each of the primary colours red, blue and green. This is accomplished by successively exposing and developing three photoresist layers, each containing a different colour phosphor, using a single photomask and a single light source. For each exposure, the light source is moved, resulting in three different beam landing areas for each aperture of the photomask. See, for example, U.S. patent 4,070,596. In forming such phosphor screens, it is known that too little light during exposure results in incomplete polymerization of the photoresist in the phosphor layer, and consequent poor adhesion of the phosphor elements to the face plate of the tube. As beam landing areas become smaller to accommodate the finer pitch screens of present interest for high resolution displays such as computer displays and high definition television displays, it becomes increasingly difficult to produce screens having adequate adhesion. Increasing the intensity of the light source to improve adhesion often results in the unintentional enlargement of the phosphor elements due to spontaneous polymerization beyond the exposed areas. In U.S. patent 3,953,621, adhesion of colour CRT phosphor screens is improved by increasing the exposure dosage without increasing the intensity of light from the source. This is accomplished by providing a mirror to reflect light transmitted through the phosphor-photoresist layer and the face panel back onto the layer. However, the reflected light tends to scatter beyond the beam landing areas from which it emerged, resulting in relatively little additional exposure in these areas, as well as the undesired exposure of adjacent areas, causing a condition known as "poor wash". Poor wash occurs because the adjacent areas become insolubilized by the unintentional exposure, and thus cannot be removed by development. The residual phosphor contaminates these areas and consequently leads to degradation of colour purity of the resultant display. In addition, the light which is reflected back to the desired beam landing areas can be of reduced intensity due to losses such as reflection at the panel surfaces and absorption by the panel itself. The absorption loss can be especially significant, since the transmission of the panel is often intentionally reduced (e.g., from 85% to 52% or even 31 %) in order to increase display contrast. Accordingly, it is an object of the invention to improve the adherence of the phosphor screen to the face panel of colour CRT's. It is another object of the invention to improve such adherence without increasing the intensity of the light source used to photolithographically form the screen. It is another object of the invention to improve such adherence without scattering light beyond the intended beam landing areas. It is another object of the invention to improve adherence to enable the production of fine pitch screens. It is another object of the invention to improve adherence without passing light through the face panel. In accordance with the invention, there is provided a method for producing a phosphor screen for a colour cathode ray tube, the method comprising photolithographically disposing an array of phosphor elements of at least two alternating colours on the interior surface of the tube's face panel, the array being formed by passing light through an adjacent aperture mask to expose portions of photosensitive phosphor layers on the face panel corresponding to the aperture array, and developing the layers to remove the unexposed portions, characterized in that prior to formation of the phosphor layers, a UV-reflective filter is formed on the interior surface of the face panel. This results therein that during exposure, UV light is reflected back onto those portions of the layers from which such light was transmitted. Hereby, the exposure dosage of the layers is effectively increased. In accordance with another aspect of the invention, there is provided a colour CRT incorporating a UV-reflective filter layer on the inner surface of the tube's face panel, under the phosphor screen. A preferred embodiment of the invention is characterized in that the phosphor screen comprise at least one UV-excitable phosphor, that is, a phosphor which emits visible light upon excitation by UV radiation. Thus, any UV radiation emitted by the phosphors in the screen upon excitation by the tube's electron beam(s), and reflected back onto the screen by the UV-reflective filter, is available to excite additional visible light emissions from the UV-excitable phosphor(s). This visible light is substantially transmitted by the UV filter, resulting in increased light output, and because UV-excited emissions generally persist longer than electron excited emissions, increased persistence of the tube. In accordance with a further preferred embodiment of the invention, the tube is also provided with a UV-reflective layer behind the screen, to reflect UV radiation emitted in this direction. Such layer is preferably reflective of visible light as well as UN light. An evaporated metal layer, such as the evaporated aluminum layer normally present on the back side of the screen, is suitable for this purpose. In such an arrangement, the relatively thin (for example, 18-22 microns) screen is effectively sandwiched between the front and back UV-reflective layers, so that substantially all UV radiation emitted by the screen is reflected back onto the screen, regardless of the direction in which it is emitted. The invention will now be described in terms of a a limited number of embodiments, as elucidated by the drawing in which: Fig 1 A is a perspective view, partly cut away, of one embodiment of a colour CRT of the invention, including a UV-reflective filter and a phosphor screen. Fig. IB is a cross-section view showing the mask-panel assembly of a colour cathode ray tube positioned on a light exposure apparatus, with a reflecting surface positioned above the assembly; Figs. 2(A) through (L) are diagrams representing the steps of the photolithographic process used to produce phosphor screens according to a preferred embodiment of the invention; Fig. 3 is a diagram representing ray traces through a mask aperture in the apparatus of Fig. 1; Fig. 4 is a sectional enlargement illustrating a portion of the panel and associated reflective surface of Fig. 1; Fig. 5 is a cross-section view similar to that of Fig. 1, in which a UV- reflective surface has replaced the reflective surface; Fig. 6 is a further sectional enlargement similar to that of Fig. 4, showing one phosphor element in association with a UV-reflective surface which has replaced the reflective surface of Fig. 1; and Fig. 7 is a plot of percent transmission (%T) versus wavelength (nm) of a UV-reflective filter suitable for use in the method of the invention. Currently, most of the colour CRTs for colour television employ phosphor screens composed of an array of vertically oriented alternating red, blue and green stripes of phosphor material. The stripes are all formed photolithographically through a single aperture mask having vertically elongated slot-shaped apertures oriented in vertical rows. In the photolithographic process employed, an aqueous photoresist material, such as polyvinyl alcohol sensitized with a dichromate, which becomes insoluble in water upon exposure to light, is exposed through the mask, and then developed by washing with water to remove the unexposed portions and leave the exposed pattern. By employing an elongated light source having a length several times that of a single aperture, the shadows cast by the bridges of mask material between the vertically adjacent apertures are almost completely eliminated, resulting in a pattern of continuous vertical stripes. In addition, by making multiple exposures, a single aperture row can result in multiple stripes. Fig. IA is a perspective view, partly broken away, of a colour display tube of the "in-line" type. The tube 25 is composed of a glass envelope 110, consisting of a display window 11, a cone 130 and a neck 140. Three electron guns 150, 160, and 170 situated in one plane in the neck 140, generate three electron beams 18, 19 and 20. These electron beams enclose a small angle with each other, the so-called colour selection angle, and pass through apertures 21 in a shadow mask or colour selection electrode 13 which is adjacent to, but spaced from, the inside surface of the display window 120. A cathodoluminescent display screen 130, which consists of a large number of triplets of red, green and blue light-emitting stripe-shaped elements R, G, B, is present on the inside of the display window or face panel 11. The convergence of the electron beams 18, 19, 20 should be such that their centre axes coincide at the mask 22. The vertical rows of apertures 21 in the mask are parallel to the direction of elongation of the phosphor stripes. For each aperture 21 in the mask 13, there is an associated triplet of phosphor elements. Since the electron beams enclose a small angle with each other, the electron beam 20, when the tube is properly adjusted for colour purity, impinges only on the red phosphor elements R. The electron beam 19 impinges only on the green phosphor elements G and the electron beam 18 impinges only the blue phosphor elements B. Referring now to Fig. IB, there is illustrated a face panel 11 of a colour cathode ray tube having an aperture mask 13 positioned adjacent to the face panel 11 by means not shown. An opaque matrix 15, disposed on the interior surface of the viewing area of the face panel 11, defines windows corresponding to the apertures of the mask 13. A coating 17 of a negative photoresist material and particles of an associated phosphor is disposed over matrix 15 in preparation for the formation of one set of pattern elements comprising the patterned screen structure. As shown, the mask panel assembly 19 is positioned on an optical exposure apparatus 21 including light source 23 for exposing the coating 17 through the apertures in the mask over the window areas of the matrix. As illustrated in Fig. 3, movement of the light source to three different locations, indicating by the three ray traces 250, 270 and 290, results in three different stripes 170, 171, and 172, through a single aperture row 40a in mask 40. Positioned above the panel 11 is a substrate member 25 having surface 27 facing the panel, which is contoured to correspond with the exterior contour 29 of the panel. Disposed on the surface 27 is a layer 31 of a light reflective medium. In U.S. patent 3,953,621, this layer 31 is preferably continuous and may be formed, for example, by vapour deposition of a reflective material such as aluminum, silver, or rhenium. Substrate movement means 33 enables positioning of the reflective medium against the exterior surface of the panel 11 prior to exposure and removal therefrom after exposure, such movement being necessitated to facilitate placement and removal of the panels for exposure. While vertical movement is shown, other forms such as angular movement, eg., a side oriented hinge, may also be appropriate. As is known, colour screens for colour CRTs can be made either with or without a light absorbing matrix surrounding the phosphor elements. Such a matrix is generally used to improve contrast and/or brightness of the image display. Referring now to Fig. 2, a cross-sectional portion of the screen is depicted during the various steps of a preferred embodiment of the photolithographic process in which prior to the formation of the phosphor array, a light-absorbing matrix is first formed by successively exposing a single photoresist layer 60 to a source of actinic radiation from three different locations through the mask, to result in insolubilized portions 60a and 60b, 61a and 61b, and 62a and 62a (Figs. 2(a), 2(b) and 2(c)). The exposed resist is then developed to remove the unexposed portions and leave an array of photoresist elements corresponding to the contemplated phosphor pattern array (Fig. 2(d)). Next, a light-absorbing layer 70 is disposed over the array, (Fig. 2(e)), and the composite layer is developed to remove the photoresist array and overlying light-absorbing layer, leaving a matrix 71 defining an array of windows corresponding to the contemplated phosphor pattern array (Fig. 2(f)). Because the exposed resist is insoluble in water, a special developer is required for this step, such as hydrogen peroxide or potassium periodate, as is known. Next, phosphor layers are formed over the windows as follows. First, a layer of a red phosphor and photoresist 72 is disposed over the matrix layer 71 and exposed (Fig. 2(g)), and developed to result in red elements 72a and 72b (Fig. 2(h)). This procedure is then repeated for the blue and green phosphors (Figs. 2(i) through (1)) to result in the phosphor array having alternating red (72a and b), blue (73a and b), and green (74a and b) stripes. Fig. 4 shows an enlarged portion of the panel 11, the associated matrix 15 and coating 17, and the contiguous reflective medium 31. Pattern elements 37 of the screen structure are being exposed in coating 17. Pattern elements 39 and 41 respectively, have been previously disposed between respective window areas of matrix 15. The third pattern areas 37 are receiving rays 47 of light that have traversed the mask apertures, not shown, to effect desired polymerization of the photoresist. A portion of the rays 47 traverse the phosphor and associated coating, while others are randomly scattered from points p within the coating. Those rays which traverse the panel 11, are reflected back by reflective medium 31, thereby producing reflected rays 47'. Depending upon the angle of incidence of rays 47 on medium 31, the reflected rays 47' strike areas 37 to enhance the exposure of those areas, or may land on matrix 15 or on adjacent areas 39 and 41, leading to a condition known as "poor wash". That is, during development, portions of these adjacent areas remain to contaminate the other phosphor colours, leading to a degradation in colour purity of the resultant display image. Referring now to Fig. 5, there is shown a mask-panel assembly similar to that of Fig. 1, except that in accordance with the invention, reflective surface 31 has been replaced by UV-reflective filter layer 50, on the inner surface of face panel 11, under the matrix 15 and coating 17. Referring now to Fig. 6, an enlargement of the cross-section of Fig. 5 showing one area 37 and portions of adjacent areas 39 and 41 of coating 17, and the UV- reflective filter 50, which is located directly on the inner surface of face panel 11. Due to the fact that the reflective surface 50 is also in contact with layer 17, most of the scattered UV rays are returned to the area 37, while some will be absorbed by the adjacent matrix 15. This condition has the beneficial result of enhancing exposure of area 37, thus improving the adherence of the phosphor element to the surface of the face panel 11. In addition, this condition prevents the spurious landing of scattered UV rays on adjacent areas 39 and 41, thus reducing the occurrence of poor wash. An added advantage of placing filter 50 on the inner surface of the face panel is that absorption and reflection losses due to the panel are avoided, further enhancing exposure of area 37. Such enhanced exposure also makes possible finer pitch screens, due to the improved adherence. In one embodiment of the invention, a simple UV-reflective filter comprises alternating layers of high and low refractive index materials, for example, TiO_ as the high index layer and SiOj as the low index layer. Techniques for designing and forming such filters are well known and are described, for example, in Thin-Film Optical Filters, by H.A. MacLeod, MacMillan, N.Y., 1985, Adam Hilger, Ltd. A typical filter, also known as a high pass filter, would have 22 layers in the pattern design, beginning at the inner surface of the display panel 0.125H, 0.25L, 0.25H, (0.25L,0.25H)*N, 0.25L, 0.25H, 0.25L, where H is TiO 2 , L is SiO 2 and N is a whole number, preferably 8 and the numerical coefficients indicate optical thickness, nd, where n is the refractive index and d is the physical thickness of the layer. A calculated transmission vs. wavelength characteristic of such a filter is shown in Fig. 7. As may be seen, such a filter would be substantially transmissive in the visible region of the spectrum, i.e., above 400 nm, and substantially reflective in the UV region, below 400 nm. As a consequence the filter has no or hardly any negative effect on the intensity of the light emitted by the colour tube, but does reflect appreciable amounts of the UN-light thus increasing the exposure dosage. Actual filters may show a small amount of absorption. A typical method of forming such a filter is by vapour deposition, although other techniques are also possible. It is known that the high intensity mercury lamp typically used to expose the phosphor/photoresist mixture in layer 17 has significant emissions in the UV portion of the spectrum, and that the ammonium dichromate sensitizer typically used in the photoresist absorbs significant amounts of this UV radiation. See, for example, L. Grimm et al., J. Hectrochem. Soc., Vol. 130, No. 8, p. 1768, Aug. 1983. Typically the most important emission occurs around 365 nm. As can be seen in figure 7 for this wavelength the reflection coefficient R, which is approximately equal to 100-T, is substantial, in this example approximately 80% -90%. Thus, when a UV reflective filter is formed on the inner surface of the face panel of the CRT prior to formation of the phosphor layers as described above, UV light which passes from the mercury lamp through the phosphor/photoresist layers during exposure, is substantially reflected by the filter back into the layer, where it can enhance exposure of the photoresist, leading to increased adherence of the phosphor/photoresist layers, without the necessity of increasing the intensity of the mercury lamp. The invention has necessarily been described in terms of a limited number of embodiments. However, other embodiments and variations of embodiments will be apparent to those skilled in the art, and these are intended to be encompassed within the scope of the appended claims. For example, other sources which emit UN, such as mercury xenon, as well as other photoresists which absorb UV radiation, such as diazo-sensitized photoresists, may be used. In operation, the electron beam(s) excite the phosphor particles in the screen to emit visible light, which passes through the UV-filter and the face-panel to the viewer. Any UV radiation which is emitted (in addition to visible radiation) by the phosphors as a result of stimulation by the electron beam, is reflected back to the screen. This reflected UV-radiation stimulates additional visible light emissions. A preferred embodiments of a cathode ray tube in accordance with the invention is characterized in that the phosphor screen comprise at least one UV-excitable phosphor, that is, a phosphor which emits visible light upon excitation by UV radiation. Thus, any UV radiation emitted by the phosphors in the screen upon excitation by the tube's electron beam(s), and reflected back onto the screen by the UV-reflective filter, is available to excite additional visible light emissions from the UV-excitable phosphor(s). This visible light is substantially transmitted by the UV filter, resulting in increased light output, and because UV-excited emissions generally persist longer than electron excited emissions, increased persistence of the tube. UV-excitable phosphors are known. See for example, An Introduction to the Luminescence of Solids, Humboldt Leverenz, Dover Publications 1968, pp 254-255; 262- 265. In particular, the blue emitting ZnS:Ag phosphor used in colour television picture tubes is known to luminesce in the visible region of the spectrum upon excitation by UV radiation. See curve 1 on page 254 of Leverenz. In addition, the persistence of visible light stimulated by UN is generally greater than that stimulated by electron beams. See, for example, Fig. 71 on page 264 of Leverenz. As is also known, such a blue-emitting or other UV excitable phosphor can be used in a blend with other phosphors, e.g., the blue emitting ZnS:Ag can be blended with a yellow-emitting halophosphate, to achieve white emission. In addition, such a blue-emitting or other UV excitable phosphor may be used in a layered screen structure, such as that found in the so-called penetron tube, in which the layers can be selectively excited by varying the energy, and thus the depth of penetration, of the electron beam(s). In either case, the UV-stimulation of visible light will result in enhanced light output and/or increased perceived persistence of the tube. Other examples of UV-excitable phosphors useful in the invention include: Manganese activated Zinc Silicate (Zn 2 Si0 4 :Mn NBS 1028); Manganese activated Calcium Silicate (CaSi0 3 :Pb;Mn NBS 1029); and Zns:Ag, ZnS:Cu,Y 2 θ 2 S:Eu, Y 2 0 2 S:Eu;Sm, YV0 4 :Eu, YV0 4 :Eu;Bi. Examples of UV emitting phosphors to be used with the UV-excitable phosphors include: Cerium activated Calcium Magnesium Silicate (P16 phosphor; emission spectrum has a max at 382nm); Cerium activated Yttrium Silicate (P47 phosphor; emission max at about 398nm); Titanium activated Zinc Silicate (emission max at 400nm); Silver and Nickel activated Zinc Sulfide (emission max at 400 nm). The color CRT depicted in the drawings is a color CRT having a shadow mask. The present invention is not restricted to the shown examples, but also relates to other types of color CRT's having a phosphor screen, such as e.g. index color CRT's and flat panel display CRT's including field emission displays and plasma displays.

Claims: Claims: 1. A method for producing a phosphor screen for a colour cathode ray tube, the method comprising photolithographically disposing an array of phosphor elements of at least two alternating colours on the interior surface of the tube's face panel, the array being formed by passing light through an adjacent aperture mask to expose portions of photosensitive phosphor layers on the face panel corresponding to the aperture array, and developing the layers to remove the unexposed portions, characterized in that prior to exposure, a UV-reflective filter is formed on the interior surface of the face panel to reflect transmitted UN light back onto those portions of the layers from which such light was transmitted. 2. A colour cathode ray tube, comprising a face panel and a phosphor screen on the face panel, characterized in that a UV-reflective filter layer is present on the interior surface of the face panel, and the phosphor screen is present on the filter layer. 3. The colour cathode ray tube as claimed in claim 2, characterized in that the UV-reflective filter comprises alternating layers of high and low refractive index material. 4. The colour cathode ray tube as claimed in claim 3, characterized in that the layers are in the pattern, beginning at the inner surface of the face panel: 0.125H, 0.25L, 0.25H (0.25L, 0.25H)*n, 025L, 0.25H, 0.25L, wherein n is a whole number. 5. The colour cathode ray tube as claimed in any of the claims 2 to 4, characterized in that the phosphor screen comprises at least one phosphor which emits visible light upon excitation by UN radiation, whereby upon electron excitation, the UV light emitted by the phosphor is reflected back onto the phosphor screen. 6. The colour cathode ray tube as claimed in claim 5, characterized in that the UV-excitable phosphor is a blue emitting phosphor. 7. The colour cathode ray tube as claimed in claim 6, characterized in that the phosphor is ZnS:Ag. 8. The cathode ray tube as claimed in any of the claims 5 to 7, characterized in that the phosphor screen is a penetron screen comprising at least two separate overlying layers of different phosphors. 9. The colour cathode ray tube as claimed in any of the claims 5 to 7, characterized in that the phosphor screen comprises a blend of two or more phosphors. 10. The colour cathode ray tube as claimed in any of the claims 5 to 9, characterized in that a UV-reflective layer is located on the phosphor screen. 11. The colour cathode ray tube as claimed in claim 10, characterized in that the UV-reflective layer also substantially reflects visible light. 12. The colour cathode ray tube as claimed in claim 5, characterized in that the UV-excitable phosphor is selected from the group consisting of CaSiO 3 :Pb,Mn; ZnS:Ag; ZnS:Cu; Y 2 O 2 S:Eu; Y 2 O 2 S:Eu,Sm; YNO 4 :Eu; and YNO 4 :Eu,Bi. 13. The colour cathode ray tube as claimed in claim 5, characterized in that the phosphor screen includes at least one UV-emitting phosphor selected from the group consisting of Cerium activated Calcium, Magnesium Silicate; Cerium activated Yttrium Silicate; Titanium activated Zinc Silicate; and Silver and Nickel activated Zinc Sulfide.

Inventors: DONFRIO ROBERT, DONFRIO, ROBERT
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H01J9
IPC codes: C09K11, H01J9, H01J29