US2371758A - Method of etching - Google Patents

Description

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Patented Mar. 20, 1945 METHOD OF ETCHING Lester M. Hicks, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application May 18, 1943, Serial No. 487,541

3 Claims.

This invention relates to an improved method of etching and particularly to the removal of the ash or insoluble deposit formed by hydrofluoric acid. While of general application in acid polishing of glass surfaces, it is of use wherever a clear etched surface is desired. One instance where in practice it has been found particularly useful is in the making of reticles for optical instruments.

A method in general use in making reticles is to coat the glass with wax, engrave a line through the wax, etch the glass with hydrofluoric acid, remove the wax, rub the surface with a suitable filling material, and wipe off the plate, leaving the etched line filled.

This technique frequently results in scratches on the surface of the glass, or, if the wax is not a good resistant to the hydrofluoric acid, there will be burned places on the glass. It is also necessary to etch quite deeply for the filling materials to hold well, and such deeply etched lines do not give a good focusing image.

Also the so-called "ash or waste material produced by etching is left in the etched line, with the result that the filling material does not adhere well and tends to fall out with time and use.

My improved method includes only a light etch followed by a cleaning operation and yields a very fine line suitable for accurate optical work.

The glass is, as before, coated with wax or other suitable engraving material and a line or design engraved through the coated layer. The surface of the glass may also be very lightly engraved in the same or a separate operation, the depth being of the order of a few wave lengths.

The wax-coated glass is, after etching and 7 cleaning as hereinafter described, coated with a thin layer of metal as by sputtering or evaporation, preferably the latter. The thin, deposited metallic film adheres to the wax and directly to the glass where it has been engraved. The layer is so thin that it is not a coherent sheet and the removal of the wax would carry with it only the metal adherent to the wax immediately under it. The wax is then entirely removed with the metallic layer, but leaving the metal adherent to the glass where lines had been engraved through the wax, producing a very fine reticle with lines having clean edges.

The wax may be removed by heating or by a solvent but preferably I place the object in a heated liquid bath which is a solvent for the coating material but not for the metal layer. This combination of heat and solvent acts more rapidly than the application of heat alone or the use of a cold solvent, although these are entirely operative.

While the particular composition used is not of great importance, I propose to use wax or equivalent layers of the types already in common use. It may, for instance, be a mixture of gilsonite asphaltum, gum mastic and cooked beeswax, with wIiichftri-chloro ethylene could be used as a solvent.

The etching and cleaning operation takes place before the plating or coating operation. The reticles are then slightly etched with hydrofluoric acid. For instance, a bath of hydrofluoric acid of 50-60% strength may be applied at -100 F. temperature for 1-2 seconds, or by more dilute acid solutions for a longer period.

In order to removethe "ash formed by the etching bath, the surface is treated with a dilute solution of hydrochloric or nitric acid, 'A solution which has been found satisfactory contains 20 cc. of either acid per liter of water. The etched plate is submitted to this bath at 70 F. for a period of two minutes with agitation. The time, temperature, and dilution given are not at all critical, and the acid may be used at much greater concentration for a correspondingly shorter time. It is followed by a rinse of water.

A clean glass surface is needed for good adherence of the metal. Sulfuric acid has been used for cleaning after etching, but it will precipitate barium sulfate if used on glass containing barium.

l lydrofluoric acid followed by nitric or hydrochloric acid or a mixture of them, or the mixture gijfljydrofiuoric acid with either acid or both may be used for the purpose of clarifying ground Having thus described my invention, what I claim is:

1. The method of removing the waste products resulting acid that comprises treating the glass with a bath including nitric acid.

. LESTER M. HICKS.