Patent GB-1527259-A (GB)
Title: Method and apparatus for manufacturing a colour cathode ray tube
IPC: H01J9/227
Filed: 1975-12-18
Abstract: 1527259 Making shadow masks ZENITH RADIO CORP 18 Dec 1975 [23 Dec 1974] 51807/75 Heading H1D [Also in Division B3] In manufacturing a shadow mask colour tube, both the mask and the phosphor screen are prepared by photoexposure through re. spective stencils, all of which are prepared from a single master stencil, whereby any resulting mask may be associated with any resulting faceplate screen in the final tube assembly. The method is described for a tube which has a slotted shadow mask, the slots being regularly interrupted by strengthening bridges 101, Fig. 15; a phosphor strip screen, the strips being continuous across the faceplate, narrower than the incident electron beams defined by the mask slots, and separated by the strips of a black matrix grill of light absorbing material; and the mask being corner supported and provided with strengthening ribs, channels and skirts (Fig. 3, not shown), such that it is relatively stiff with respect to its major and minor axes, but is capable of flexure about its diagonals. The initial master pattern for the mask and screen manufacture comprises a photosensitive layer on which has been recorded a pattern corresponding to the slotted mask, but with the slots of reduced width equal to that of the phosphor strips on the completed faceplate. The pattern is subsequently transferred via a flexible recording medium to a prime master blank, and from there to five intermediate, and hence to five working, master stencils, one each of the latter being employed in making the mask and the black matrix grill, and the other three in producing the three colour phosphor strips, all the master pattern blanks having the curvature of the completed mask or faceplate, as appropriate; being of thin, flexible glass coated with an opaque layer of iron oxide or chromium and provided with support and edge structures similar to those of the completed shadow mask, to ensure comparable deformation behaviour upon mounting in the photoexposure apparatus. The intermediate and the working master stencils are prepared from the prime master stencil using lighthouse photoexposure methods, with correction lens, and a graded filter, if required, for producing a gradation of slot and strip size. In making the intermediate mask stencil the light source is moved perpendicular to the slots during exposure by an amount sufficient to produce slots of the width required in the completed mask. In making the intermediate black matrix grill and phosphor pattern stencils, the mask bridges are eliminated by use of a line light source and the penumbra effect, and the intermediate stencil is spaced from the prime master by a distance equal to that between the mask and the screen in the completed tube. In all other exposure operations, the master stencil and the blank being exposed are mounted as close as possible while avoiding actual contact, e.g. at a spacing of 0À025 inch. The production of the working master stencils from the corresponding intermediate stencils requires the use of exposing radiation which converges upon, rather than diverges from, the centre of deflection of the relevant electron beam in the working tube, and a suitable lighthouse for this is described (Fig. 50, not shown). The final mask blank is shaped before photoexposure through the working mast stencil, and is of steel having a nickel, aperture-defining, layer on its gun side, the nickel layer being etched first, and the steel thereafter being etched through with an etchant to which the nickel is resistant (Figs. 61 to 65, not shown). In producing the black matrix grill and phosphor stripes on the faceplate, it is sufficiently accurate to illuminate the corresponding working stencils by a light source located on the faceplate axis. Reference is made to the application of the general method to a tube having a phosphor dot rather than a phosphor stripe screen.