This invention relates to a novel method for precision etching of a succession of articles from a moving metal strip.
The method is particularly useful for preparing flat apertured masks which are subsequently formed and installed in color television picture tubes.
Precision etching is employed to produce articles with complex arrays of apertures therein, where the sizes and shapes of the apertures are to be held within very narrow size tolerances. An apertured mask, which is an important part of a shadow-mask-type picture tube used in color television receivers, is one such article. The production of flat apertured masks by photoexposure and precision etching has been described previously. In a typical process, light-sensitive coatings are applied to both major surfaces of a continuous strip of thin sheet metal. In one practice, both major surfaces of a cold-rolled-steel strip about 0.15 mm (6 mils) thick and about 550 mm (22 inches) wide are coated with a dichromate-sensitized casein composition. The light-sensitive coatings are exposed to a succession of actinic light images, as by contact-printing exposure, to render the exposed portions thereof less soluble in water. The exposed coatings are developed to remove the more-soluble unexposed portions, thereby producing a succession of stencils on each surface of the strip, and then baked to render the retained, less-soluble, exposed portions etch resistant. Then, the strip with the etch-resistant stencils thereon is advanced through an etching station where it is selectively etched from both surfaces with an etching solution that is sprayed on the strip. The strip continues to advance beyond the etching station through successive stations where the strip is rinsed, the stencils are removed, the strip is dried and the light transmissions through the masks are monitored. The flat masks produced by the foregoing method have an array of apertures therein, which apertures are usually round holes or rectangular slits, but may be of any desired shape. Round apertures are typically about 0.30 to 0.38 mm (12 to 15 mils) in diameter, and rectangular apertures are typically about 0.13 to 0.20 mm (5 to 8 mils) wide by about 0.76 to 1.27 mm (30 to 50 mils) high. The flat mask is formed to a desired shape and detachably mounted in the faceplate panel of the tube. The formed and mounted mask is then used as an optical master for photographically depositing one or more screen structures of the tube. The mask is also used to shadow the scanning electron beams during the operation of the picture tube.
The sizes and shapes of the apertures are critical towards reliably and reproducibly implementing these functions. Many factors affect the sizes of the apertures in the mask. Some important process variables relating to the photoexposure and etching steps that are now carefully controlled are (1) temperature of the etching solution, (2) density of the etching solution, (3) pressure applied to spray the etching solution, (4) thickness of the stencils, (5) baking temperature of the stencils, (6) size of the apertures in the developed stencils, and (7) conditions for photoexposing light-sensitive coating. Even though these many process controls are applied, there is still a need to reduce the variation in aperture sizes in the etched masks.
We have found that: (a) the prior photoexposure-and-etching process produces mask apertures whose sizes, and therefore light transmissions of the masks, are very dependent on small changes in the thickness of the metal strip, and (b) the ordinary thickness range of the metal strip received from the metal supplier can produce variations in aperture sizes which are greater than can be tolerated by the user of the mask. For example, 0.025 mm (0.1-mil) change in the thickness of a 0.150 mm (6-mil)-thick steel strip can cause a change of 0.3% in the light transmission of the etched mask, or about one third of the allowable etching tolerance. Steel strip, as received from the supplier, may have a thickness variation of .+-.0.0125 mm (.+-.0.5 mil), which would, if uncompensated for, produce masks with a greater than allowable variation in transmission.
Where these wide variations in strip thickness exist, the masks must be more thoroughly inspected and a substantial proportion of the etched masks must be discarded as being out of tolerance. Frequently, masks selected for retention are classified into one of several groups according to light transmission so that wider ranges of aperture sizes can be tolerated through other compensations made later in the manufacturing processes.