Note: Descriptions are shown in the official language in which they were submitted.
RCA 68,334
105;~673
This invention relates to a novel method for
producing an apertured work piece, particularly a shadow
mask for a color television picture tube.
A shadow-mask-type color television picture tube
includes a color-selection electrode closely spaced from a
viewing-screen structure The electrode is in the form of
an apertured mask which shadows portions of the viewing
screen from the electron beams during the operation of the
tube. To reduce scattering of beam electrons off the sides
of the shadow-mask apertures during electron-beam scanning,
; the apertures are tapered from the screen side towards the
electron-beam source. For practical reasons, the narrowest
part of each aperture is a "knife edge" which is located a
short distance below the mask surface. This short distance
is referred to as the "step height" of the knife edge.
. United States Patent Nos. 2,750,524 and 3,679,500
describe methods for producing a shadow-mask with apertures
having a small step height. Both methods, however, involve
two separate etching steps and two separate resist-coating
steps.
In the novel method of the invention, an array
of tapered apertures is produced in a metal sheet by coating
the opposite major surfaces of the sheet with etch-resistant
patterns, one pattern comprising larger open areas surrounded
by etch-resistant material, and the other pattern comprising
similarly-shaped, but smaller open areas registered with the
.
larger areas. Each of the smaller open areas has therewith-
in a still-smaller solid area of etch-resistant material.
; Preferably, in at least one dimension, the still-smaller
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solid area is at least 1.0 mil wide and at least 2.0 mils
smaller than the smaller open area. Both sides of the
coated sheet are etched simultaneously until the desired
tapered apertures are produced therein. Then, the etching
is stopped, and the etch-resistant patterns~are removed
from both major surfaces
By employing the still-smaller solid areas of
etch-resistant material within the smaller open areas,
the step height can be reduced, and the uniformity of the
knife edge can be improved, relative to prior-art methods.
The openings produced by the combination of patterns permits
etching to occur from both surfaces of the sheet, but
aontrollably limits the etching from the surface carrying
the pattern comprising the smaller open areas. The novel
l 15 method requires only a single coating step and a single
- etching step to achieve what is achieved in two steps with
the above-cited prior-art methods.
In the drawings, FIGURE l is a plan view of a
metal sheet produced according to the novel method.
FIGURES 2 through 6 are sectional views through
one aperture of the metal sheet, illustrating the steps of
an example of the novel method;
FIGURE 7 is a superimposed plan view of etch-
resistant patterns for producing circular apertures
accord1ng to an example of the invention;
FIGURE 8 is a superimposed plan view of etch-
resistant patterns for producing slit apertures according
to another example of the invention; and
FIGURE 9 is a superimposed plan view of etch-
3 resistant patterns for producing slit apertures according
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l to still another example of the invention.
FIGURE 1 shows a plan view of an etched apertured
mask blank 21 as it emerges from an etching machine used
in an example of the novel method. The mask blank 21
(which here is to be used in a color televlsion picture
tube) is in a metal sheet 23 comprising a succession of
such mask blanks 21a, 21 and 21b. The mask blanks 21a,
21 and 21b are etched through at the margins 25 thereof,
except at convenient points (not indicated) sufficient to
hold the blan~s 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, although not apertured, may be etched partly through.
The apertures may be circular holes arranged in a hexagonal,
diamond-shaped or other array; rectangular slits arranged
in vertical rows, for example, 6-mil by 30-mil slits on
30-mil centers; or of other shapes and arrangements. Also,
the widths of the apertures may be uniform or graaed across
~- the array, as is known in the art.
The mask blank 21 is etched into a regular-carbon
" or low-carbon cold-rolled-steel sheet about 40 to 10 mils
in thickness. The etching may also be conducted in sheets
of other materlals, such as invar alloy or a copper-nickel
alloy. The sheet 23 is unwound from a first roll thereof,
passed through the various operations including cleaning,
coating~ drying, exposing, developing, etching, washing
:.:
and drying (as described below), then rewound on a second
roll. Subsequently, the second roll is unwound and the
mask blanks 21a, 21 and 21b are stripped or torn from the
; sheet 23. The blanks are then heat treated (annealed),
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I roller leveled, formed on a press, and then blackened, as
is known in the art, to produce masks suitable for assembly
into a picture tube.
FIGURES 2 through 6 show a sequence of steps that
may be used in making one of a hexagonal array of circular
apertures in the central portion 27 of a 6-mil^thick sheet
23 of cold-rolled steel (as shown in FIGURE 1). The
sheet 23 is coated on both major surfaces with suitable
light-sensitive coatings 31 and 33 of etch-resistant
materials, e.g., dichromate-sensitized fish glue, as shown
in FIGURE 2. After the coatings have dried, the coated
sheet is positioned in a chase (such as is shown in United
States Patent No. 3,751,250) between two light-opaque
master patterns,one master pat~ern 35 for the coating 31
on 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 FIGURE 3. The light-
opaque patterns may comprise chromium or nickel metal
coated layers on the inner surfaces of glass plates 39 and
41, so that the patterns 35 and 37 are physlcally against ;`
the coatings 31 and 33 respectively. The one master
pattern 35 is an annular ring of about 5 mils outside
diameter and 3 mils inside diameter. The other master
pattern 37 is a disc or solid circle about 16 mils in
diameter. Genter lines of the two master patterns are
~-~ coincident, but may be offset from one another if desired.
As shown in FIGURE 3, the coatings 31 and 33 are
next exposed to hardening radiation ~shown by the arrows
above and below the glass plates 39 and 41), as from a
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1 carbon-arc source, which radiation passes through the glass
plates 39 and 41 to be incident on the coatings 31 and 33.
The radiation insolubilizes the coatings 31 and 33 except
where the 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 are then developed as by flushing
with water or other aqueous solvent to remove the unexposed,
shadowed portions of the coatings 31 and 33. As shown in
FIGURE 4, after development the sheet 23 carries, on its
one major surface, an etch-resistant coating having an
annular opening 43 therein and, on its other major surface,
an etch-resistant coating 33 having a circular or disc-
s shaped opening 45 therein.
The sheet 23 with the etch-resistant coatings
thereon is now etched in a single step to produce the
desired tapered aperture. FIGURES 5 and 6 show the coated
` sheet 23 at an early stage (FIGURE 5) and then at the end
of etching ~FIGURE 6). The etching is conducted in the
usual manner by employing a ferric chloride-hydrochloric
,~ acid liquid etchant. At the initial stage shown in FIGURE
Y, 5, the etchant dissolves a small amount of the surfaces of
, the sheet 23 in the uncoated areas thereof. FIGURE 5 also
shows, by dotted lines, various subsequent etching surfaces
that the etchant is believed to advance to.
The use of an annular opening 43 instead of a
disc-shaped opening on the one major surface severely
restricts the effective etching from that surface, thereby
imparting only a small step height 47, as shown in FIGURE
6. If the annular opening 43 were replaced with a disc-
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shapeld opening, the step height would be substantially
grc~ter. The coatings 31 and 33 on thc major surfaces of
the sheet 23 are removed after the etching has been completed,
whereafter the work piece is ready for further processing.
FIGURE 7 shows, in plan view and superimposed
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upon one another, the master patterns in the working plates.
The significant dimensions of the annular opening of the one
master pattern 35 are the inside diameter 53; the outside
diameter 55; and the width S9, which is one half the difference
between the outside and inside diameters. For practical
reasons, the inside diameter 53 should be about 1 to 8 mils,
and the outside diameter 55 should be about 3 to 10 mils.
Preferably, the difference between the outside and inside
diameters is at least 2 mils, so that the width 59 of the
annulus is at least 1 mil. The significant dimension for
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the circular opening of the other master pattern 37 is the
; dia~eter 57 which, for practical reasons is about 12 to 20
mils.
Where the apertures are graded in size from the
center to edge of the apertured portion 27, the diameters
` of the annular openings are graded. Typically, the outside
diameter of the annular opening 35 may grade from about 9.5
mils at the center of the mask to about 7.5 mils at the
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~ edge of the apertured portion 27 of the mask. The inner
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diameter of the annular opening 53 may also be graded, but
the widths 59 are preferably at least 1 mil with present
` etch-resistant patterns. (As the width 59 decreases, the
step height decreases to a minimum of about 1 mil and then
increases.) In this example, the diameter 57 of the
circular area on the reverse side is about 16 mils, but
this dimension is not critical and ma~ be between about 12
and 20 mils. ~here the apertures are graded in size, the
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1 larger circular areas may or may not be graded in size.A typical center-to-center aperture spacing is about 25
mils.
The novel method may be empl~yed also to produce
rectangular slit apertures, as illustrated by the super-
imposed master patterns shown in FIGURE 8. The one master
pattern 35' ~solid lines) and the other master pattern 37'
~dotted line) are shown with rectangles having rounded
corners. For one example, the outside width 65 of the one
master pattern 55', is about 5 mils and the outside length
is about 30 mils; the inside width 63 is about 2 mils
and the inside length is about 27 mils. Tn other examples,
the outside width may vary from 3 to 20 mils and the
inside width 63 may vary from 1 to 10 mils. However, the
difference between the outside and inside widths is
, . .
` preferably at least 2 mils, so the width of the annulus
h;,: 69, 61 is at least 1 mil. The other master patterns 37'
may vary between 12 and 24 mils in width and between 20 and
50 mils in length; however, each of the length and width
dimensions of the other master pattern 37' should be larger
than the corresponding dimension of the one master pattern
35'
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FIGURE 9 shows another application of the novel
~ method for producing an array of rectangular slits. The
`. 25 slit aperture master patterns shown in FIGURE 9 differ from
-~, those of FIGURE 8 in that, for the one master pattern 35"
of the former, the length of the still-smaller solid area
is equal to the length of the smaller open area. For
practical reasons, the annular spacing 61 of the one master
pattern of FIGURE 8 may be omitted, with substantially
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l equivalent results to those obtained with the master
pa1:terns shown. Thus, in the example of FIGURE 9, only
one, i.e., the horizontal, dimension of the one master
pattern requires that the still-smaller solid area be at
~; 5 least 2 mils smaller than the smaller open area.
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