Etching method using a hardened PVA stencil

The method comprises producing on the surface to be etched a stencil of a light-hardened PVA. The stencil is produced by light hardening selected portions of a film or coating consisting essentially of dichromate-sensitized PVA. The PVA is about 85% to 97% hydrolyzed and has a molecular weight of 75,000 to 100,000. After light hardening, the nonhardened portions of the film are removed, thereby producing the stencil. The stencil is baked to improve its etch resistance, the surface is etched through the stencil, and then the stencil is removed from the surface.

BACKGROUND OF THE INVENTION 
This invention relates to a novel method of etching a metal body and, 
particularly, to an etching method that includes producing on the surface 
of the body an etch-resistant stencil of light-hardened poly(vinyl 
alcohol-acetate). The stencil is easily removable with the aid of a dilute 
aqueous alkali solution after the etching step is completed. Poly(vinyl 
alcohol-acetate) is also designated herein as PVA. 
The preparation of apertured masks by photoexposure and etching has been 
described previously, for example, in U.S. Pat. No. 4,061,529 issued Dec. 
6, 1977 to A. Goldman et al. In a typical process, light-sensitive 
coatings of sensitized protein materials are applied to both major 
surfaces of a thin metal sheet, such as a sheet of a cold-rolled steel or 
of a copper-nickel alloy. The coatings are exposed to light images, as by 
contact-printing exposure, to harden (render less soluble) the exposed 
portions of the coatings in an aqueous solvent. The exposed coatings are 
developed by removing only the more-soluble portions of the coatings, 
thereby producing a stencil on each surface of the sheet. Then, the 
stencils are baked to make them more resistant to an etchant, usually an 
aqueous ferric chloride-hydrochloric acid solution. Then, the sheet with 
the stencils thereon is selectively etched as desired, after which the 
stencils are removed from the sheet. 
In present commercial practice, the light-sensitive coatings are a 
dichromate-sensitized casein; such as the coatings disclosed in the 
above-cited Goldman et al. patent. Etch resistance is developed by baking 
the coatings in air at about 260.degree. C. to 350.degree. C. After 
etching with a ferric chloride-hydrochloric acid solution, the stencils 
are removed by applying thereto a hot aqueous solution of sodium 
hydroxide. 
It is desirable to provide alternative light-sensitive coatings, 
particularly light-sensitized synthetic materials in place of the 
light-sensitized protein materials that are presently used. In addition, 
it is desirable to reduce the cost of materials and manufacture of the 
etched product. U.S. Pat. No. 4,208,242 issued June 17, 1980 to P. 
Zampiello suggests using films of dichromate-sensitized poly(vinyl 
alcohol-acetate) as the light-sensitive coating for making apertured masks 
in steel sheets. The PVA employed in this film is 98.5% to 100% hydrolyzed 
and has a molecular weight of about 14,000. The film, after developing, is 
baked at about 250.degree. C. to 300.degree. C. for about 2 to 3 minutes 
to improve the acid resistance thereof. This produces very adherent 
stencils which require a special treatment to remove them. That treatment 
requires, after etching the sheet, baking the stencils at about 
250.degree. C. to 300.degree. C. until the stencils are carmelized. Then, 
the carmelized stencils can be removed with a hot aqueous sodium hydroxide 
solution. 
SUMMARY OF THE INVENTION 
The novel method follows the prior methods except in the following 
important respects. The novel method employs coatings of 
dichromate-sensitized PVA in which the PVA is about 87% to 97% hydrolyzed 
and has a molecular weight of about 75,000 to 100,000. As a result, the 
stencils can be baked in a lower temperature range of about 200.degree. C. 
to 240.degree. C. prior to etching in order to improve the acid resistance 
thereof. In addition, after the etching step, the stencils do not require 
baking or carmelizing or any other special treatment in order to remove 
the stencils effectively. Finally, more dilute aqueous solutions of alkali 
can be used to remove the stencils from the metal body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a plan view of an etched apertured mask blank 21 as it emerges 
from an etching machine. The mask blank 21 (which is to be used in a 
color-television picture tube) is in a metal sheet 23 comprising a 
succession of such mask blanks 21a, 21 and 21b which are etched through at 
the margins 25 thereof except at convenient points (not indicated) 
sufficient to hold the mask blank 21 in place in the sheet 23. The mask 
blank 21 is comprised of an apertured central portion 27 defined by the 
broken line 28; and a skirt or peripheral portion 29 which is not 
apertured, although in some embodiments it may be etched partly through. 
This application is particularly concerned with the etch-resistant stencil 
used for etching the apertures in the apertured central portion 27. The 
apertures may be round and arranged in a hexagonal, diamond-shaped or 
other array. Or, the apertures may be rectangular slits arranged in 
vertical rows; for example, 6-mil by 30-mil slits on 30-mil centers. The 
apertures may be of other shapes and arrangements. In any of the 
embodiments, the aperture width may be uniform across the mask or may be 
graded in width or diameter from the center to the edge of the array as is 
known in the art. 
The mask blank 21 is etched into a regular-carbon or a low-carbon 
cold-rolled-steel sheet about 4 to 10 mils in thickness. The etching also 
may be conducted in sheets of other materials, such as invar alloy, or 
copper-nickel alloy. The sheet 23 is passed through the various operations 
including cleaning the sheet, producing etch-resistant stencils on the 
sheet, etching the sheet to produce the apertures and to define the mask 
blanks, and then stripping the stencils from the sheet. Subsequently, the 
mask blanks 21 are separated from the sheet 23. The mask blanks 21 are 
then heat treated (annealed), roller leveled, formed on a press, and then 
blackened as is known in the art, to produce masks suitable for assembly 
into a color-television picture tube. 
FIGS. 2 through 5 illustrate the novel method by a sequence of steps that 
may be used in making an aperture in the central portion 27 of a hexagonal 
array of apertures in a 6-mil-thick strip of low-carbon cold-rolled steel, 
as shown in FIG. 1. The sheet 23 is coated on both major surfaces with one 
of the liquid coating compositions set forth below. The coatings are dried 
in air, producing light-sensitive coatings 31 and 33 of 
dichromate-sensitized PVA, as shown in FIG. 2. After the coatings have 
dried, the coated strip is positioned in a chase between two light-opaque 
master patterns; one master pattern 35 for the coating 31 on the one major 
surface of the sheet 23; and the other master pattern 37 for the other 
coating 33 on the other major surface of the sheet 23, as shown in FIG. 3. 
The light-opaque patterns physically contact the coatings 31 and 33. The 
one master pattern 35 has a circular shape about 5 mils in outside 
diameter. The other master pattern 37 has a circular shape about 16 mils 
in diameter. Center lines of the one and the other master patterns are 
coincident, but may be offset from one another if desired. 
With the patterns 35 and 37 positioned as shown in FIG. 3, the coatings 31 
and 33 on each of the surfaces of the sheet 23 are now exposed to 
hardening radiation, as from a carbon-arc source, or xenon radiation lamp, 
which radiation passes through the glass plates 39 and 41 incident on the 
coatings 31 and 33 except where the one and the other master patterns 35 
and 37 shadow the coatings. When the coatings are suitably exposed, the 
exposure is stopped, and the master patterns are removed. 
The coatings 31 and 33 are now developed as by flushing with water or other 
solvent to remove the unexposed, more-soluble, shadowed portions of the 
coatings 31 and 33. As shown in FIG. 4, after development, the sheet 23 
carries on its one major surface a stencil comprising a coating 31 having 
a first circular opening 43 therein and, on its other major surface, a 
stencil comprising a coating 33 having a second circular opening 45 
therein. The stencil coatings 31 and 33 with the openings 43 and 45 
therein are now baked in air at about 200.degree. C. to 240.degree. C. to 
develop better etch-resistance in the coatings. 
The sheet 23 with the etch-resistant stencils thereon is now selectively 
etched from both sides thereof in a single step or in successive steps to 
produce the desired aperture. FIG. 5 shows the stencil-coated sheet 23 at 
the end of etching. The etching is conducted in the usual manner employing 
a ferric-chloride hydrochloric-acid liquid etchant. Controlled amounts of 
chlorine gas are fed into the etchant to maintain its etching strength. 
After the etching has been completed, the coated sheet 23 is washed with 
water to remove any residual etchant. Then, the etch-resistant stencils 31 
and 33 are removed from the sheet 23, as by spraying thereon an aqueous 
solution of sodium hydroxide maintained at temperatures of about 
50.degree. C. to 80.degree. C. After removing the stencils, the sheet 23 
is washed in water and dried. 
In a comparative study, the liquid coating compositions shown in the TABLE 
were used to compare the novel method (Group B) with methods (Group A) 
that use different PVAs and therefore require additional steps and/or 
expense in order to be practical. The PVAs used in compositions 1 through 
5 are marketed under the trade name VINOL by Air Products & Chemicals, 
Inc., Allentown Pa. The PVA used in composition 6 is marketed under the 
trade name GELVATOL by Monsanto Plastic & Resin Co., Indian Orchard, Mass. 
To prepare the coating compositions, the deionized water is heated to about 
50.degree. C. to 60.degree. C., then the PVA is added in a steady stream 
with vigorous agitation of the solution. After all of the PVA has been 
added and dissolved, the solution is cooled to room temperature and then 
the viscosity is measured. Just before usage, the sodium dichromate is 
added, which lowers the pH of the solution to about the indicated pH. 
In the coating composition used in the novel method, several factors are 
important: 
1. Any PVA that is 85% to 97% hydrolyzed and has a molecular weight of 
about 75,000 to 100,000 may be used. These are synthetic polymeric 
materials that are available under various trade names. A stencil made of 
a PVA with a higher percent hydrolysis requires a higher baking 
temperature prior to etching and requires in addition a carmelizing baking 
after the etching step to aid in the removal of the stencil. Higher 
molecular weights do not compensate for the disadvantages that result from 
the higher percent hydrolysis as shown by compositions 1, 2 and 3 of Test 
Group A. However, it has been noticed that the desired advantages can be 
realized if the PVA used has a molecular weight in the specified range. 
The PVAs used in the novel method are less hydrolyzed and have higher 
molecular weight than the PVAs disclosed in the Zampiello patent, op. cit. 
2. Among sodium, potassium and ammonium dichromate photosensitizers, sodium 
dichromate is the preferred alkali dichromate because it imparts the 
greater sensitivity to the coatings. The photosensitizer comprises about 
0.03 (3%) to 0.15 (15%) of the weight of the PVA present, and determines 
the pH of the coating composition. Less than 3% dichromate requires too 
long an exposure to be practical. More than 15% dichromate results in poor 
adhesion of the photoexposed portions of the stencil. The photoexposure 
times of the coatings are equivalent to those used for 
dichromate-sensitized casein coatings. 
3. No surfactant is used in the coating compositions because its presence 
reduces the adherence of the photoexposed coatings of the baked stencils. 
4. No borax or other alkalizing material is used in the coating 
compositions because it gels the composition and generally makes it more 
difficult to coat the composition. 
5. Coating thicknesses on the metal sheet are important parameters in mask 
manufacturing. If the coating is too thin, under 40 microinches, the 
etching will be nonuniform and cause mask defects. If the coating is too 
thick, over 200 microinches, the coating will require longer exposure 
times and there may be mechanical problems, pumpdown problems and poor 
mask uniformity. 
6. The preferred bake-in temperature for imparting etch resistance to a 
developed PVA coating is in the range of 200.degree. C. to 240.degree. C. 
PVA coatings baked at these lower temperatures are most efficiently 
removed by caustic wash after etching. Generally, the higher the 
temperature of the etchant used, the higher should be the baking 
temperature for the developed coating. The lower bake-in temperatures used 
in the novel method can result in savings in energy and cost as compared 
with prior methods which require higher bake-in temperatures. 
7. After etching, the hardened stencils may be removed from the metal sheet 
with hot (50.degree. C. to 80.degree. C.) dilute (1.0 molar) sodium 
hydroxide solution. A light brushing permits the removal of the stencil in 
about 1 to 5 minutes. Without brushing, but using a spray of the solution, 
3 to 10 minutes may be required to remove the stencils. Any alkali 
solution may be used, and the selection of temperature, concentration and 
time of application may be optimized by a relatively few trials by persons 
of ordinary skill in the art. The novel method does not require the step 
of carmelizing the stencils prior to applying the alkali solution, 
resulting in a savings in energy and cost as compared to the method 
disclosed in Zampiello, op. cit. which requires carmelizing. In fact, 
after the etching step, baking of the stencils is limited to temperatures 
of less than 200.degree. C. in the novel method. 
TABLE 
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Test Group 
Composition 
A B 
No. 1 2 3 4 5 6 
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PVA - V125 V165 V350 V425 V523 20/60 
Vendor No. 
% Hydro- 
lyzed 99.6 99.6 98.4 95.5 88.0 86.5 
Mole- 78,000 125,000 125,000 
78,000 
78,000 
96,000 
cular Wt. 
Viscosity 
28-32 55-65 55-65 26-30 21-25 21-25 
Wt. % 4.0 4.0 4.0 4.0 4.0 4.0 
Solids 
Sodium 
Dichromate 
Wt. % 0.4 0.4 0.4 0.4 0.4 0.4 
Solids 
Deionized 
Water 
Wt. % 95.6 95.6 95.6 95.6 95.6 95.6 
pH of &lt;6.0 &lt;6.0 &lt;6.0 &lt;6.0 &lt;6.0 &lt;6.0 
Composition 
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