Method of producing perforated glass plate

A perforated glass plate for use in gas discharge display devices is produced by providing an etch-resistant coating in a select pattern on surfaces of a glass plate so that areas to be perforated are coating-free, subjecting the so-coated glass plate to a gaseous hydrofluoric acid, obtained from concentrated hydrofluoric acid, for a period of time sufficient to etch fine sized perforations, for example holes, through the plate and rinsing the etched plates.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to perforated glass plates as well as to a method and 
apparatus for producing such plates and somewhat more particularly the 
invention relates to glass plates having fine sized perforations therein 
so as to be useful in gas display discharge devices, such as plasma 
display devices, particularly in such devices which have a high picture 
point density. 
2. Prior Art 
German Offenlegungsscrift No. 2,412,869 (which corresponds to U.S. Pat. No. 
3,956,667 and which is incorporated herein by reference) describes a 
luminescent gas discharge display device. In order to produce a plasma 
display with this type of device, a so-called control plate is necessary 
for controlling the individual picture points. This control plate divides 
the interior space of such display devices into two realms, a plasma realm 
and a post-acceleration realm. Generally, the control plate comprises a 
perforated glass plate having a matrix-type array of metal tracks or 
conductor paths surrounding such perforations. In such a matrix array, 
electrodes or conductor path facing the plasma realm extend horizontally 
(line or row tracks) while the electrodes facing the post-acceleration 
realm extend vertically (column tracks). With such an electrode 
arrangement, each picture point in the display device can be individually 
controlled. In this manner, electrons present in the plasma realm can be 
controllable moved via a circuit through the perforated glass plate in the 
post-accelaration realm and onto the luminescent screen. 
The number of perforations or holes in glass plate of the above-described 
type is determined by a given utility and, in plasma display devices, is 
determined by the number of picture points desired. For example, with 
present-day television line scanning patterns on a picture tube having a 
diagonal dimension of 67 cm, the control array comprises a horizontal grid 
of 0.32 mm and a vertical grid of 0.64 mm and determines the number of 
holes in a control plate. Since such holes or perforations must be 
provided by etching, control limitation in the manufacture of plates 
containing such holes results, since a specific relationship between the 
thickness of a glass plate, the hole size and the spacing of the holes 
must be accurately attained. In addition, since etching occurs after the 
desired conductor paths or tracks have been positioned on the plate 
surfaces, a danger exists that such tracks will either insufficiently 
adhere to the glass surfaces or be completely separated therefrom during 
the etching process because of the undercutting or underetching (lateral 
etching) of the glass area beneath the tracks. Accordingly, control 
problems exists in etching plates having predetermined geometric patterns 
and a solution is required which allows such etching and satisfies all of 
the required conditions. 
SUMMARY OF THE INVENTION 
The invention provides a perforated glass plate, a gas discharge display 
device having such glass plate therein, a process for producing such 
perforated glass plate and an apparatus for practicing the process for 
producing such plate, whereby perforated glass plates having a high hole 
density are reliably and simply attained and which have an exceptionally 
good etching factor. 
In accordance with the principles of the invention, the method embodiment 
of the invention comprises etching a glass plate coated with an 
etch-resistant material in a pattern corresponding to a desired hole 
pattern in sealable container (etching chamber) with a gaseous 
hydrofluoric acid so that the etch-resist coated areas are not etched 
while the uncovered areas are etched to provided the desired holes. This 
process is substantially advantageous over prior art immersion etching or 
spray etching processes because a substantially more favorable etching 
factor, which is defined as the ratio of etched depth dimension to etched 
lateral dimension, is attained. In contrast to known immersion or spray 
etching processes, the gas etch process of the invention provides a 
relation between etched depth dimension and etched lateral dimension which 
is more favorable by at least a factor of two. 
In certain method embodiments of the invention, when a metal etch-resist 
coating is utilized, a protective gas, such as hydrogen or nitrogen, may 
be provided within the etching chamber to further protect the metal 
etch-resist. The embodiment where copper comprises the metal etch-resist, 
an adhesive layer composed of titanium may be provided between the surface 
of the glass plate and the etch-resist coating so as to further improve 
the resultant etching factor. In certain method embodiments of the 
invention, a target mark may be provided on surfaces of the glass plates 
to be etch and left uncoated with the etch-resist material so that 
progress of the etching process can be readily observed. 
In accordance with the principles of the invention, the apparatus 
embodiment of the invention comprises a gas-sealable and at least 
partially transparent hollow container having an etching chamber therein 
for containing a gaseous hydrofluoric acid and a means for supporting at 
least one glass plate within such chamber. A source of concentrated 
hydrofluoric acid may be provided in communication with such chamber so 
that only gaseous hydrofluoric acid contacts the glass plate being etched. 
In some embodiments of this apparatus, a gas-circulating means may be 
positioned within the etching chamber and a washing means, such as a 
sluic, may be provided in communication with the etching chamber. In some 
embodiments of this apparatus, a source of concentrated hydrofluoric acid 
may be included a controllable heating means for heating the concentrated 
acid and such source may comprise a separate container in communication 
with the etching chamber. In some embodiments of this apparatus, the 
source of concentrated hydrofluoric acid may include a spray means for 
spraying such acid within the source so as to increase and attain a 
constant vapor pressure in the etching chamber while avoiding spraying of 
liquid acid on glass plates therein. 
In accordance with the principles of the invention, the perforated glass 
plates of the invention are incorporated in a gas discharge or plasma 
display device as actual control plates or as spreaders between a control 
plate and luminescent screen thereof. In certain embodiments of such 
display devices, a plurality of such perforated glass plates are 
positioned, one on top of the other, to fill-out the space between the 
control plate and luminescent screen of the display device. In some 
embodiments of such display devices, the perforated glass plates have a 
titanium layer on all surfaces thereof except those which contact the 
control plate of the device.

DESRIPTION OF THE PREFERRED EMBODIMENT 
The invention provides a perforated glass plate having a relatively high 
perforation density, with such perforations having a relatively high 
etching factor, a method of producing such perforated plates, an apparatus 
for practicing such method and gas discharge display devices incorporating 
such perforated plates therein. 
In accordance with the principles of the invention, a glass plate of any 
select size is positioned in a gas-sealable container having an etching 
chamber therein so that only gaseous hydrofluoric acid contacts select 
areas of such glass plate, which correspond to those areas whereat 
perforations are desired. The glass plate may be coated with a select 
pattern of a layer of an etch-resist material, such as a metal or the 
like, to facilitate perforating select areas of such plate. A material 
advantage of such gas phase etching process is that the resultant 
perforations have an etching factor which is significantly higher and 
substantially more favorable than that obtained via prior-art immersion 
etching or spraying etching (i.e., where the glass plate is immersed in a 
liquid etchant or sprayed with a liquid etchant). An "etching factor", as 
utilized herein, is defined as the ratio between an etching depth 
dimension and an etched lateral dimension in a given perforation or group 
of perforations. 
In contrast to known immersion etching or spray etching (as defined above), 
the relation of an etched depth dimension to an etched lateral dimension 
obtained in the practice of the invention is more favorable by a factor of 
at least 2. If for example, with known liquid etching process, a control 
plate having a hole grid dimension of 0.32 mm or 0.64 mm respectively, and 
an individual hole size of about 0.2 mm.times.0.5 mm can be produced from 
a glass pate which is 0.1 mm thick, then with the gas etching process of 
the invention, the identical hole grid dimension and hole sizes can be 
produced from a glass pate which is 0.2 mm thick. With the novel etching 
process of the invention, perforated plates are produced which are 
substantially stronger and can readily brace or support control plates in 
the display devices, as required. 
In instances where a metal, for example copper, is utilized as an 
etch-resistant material to define a select hole pattern on a glass plate, 
it is preferrable to apply an adhesive layer of titanium between the glass 
surface and the copper layer in order to further reduce the amount of 
undercutting (i.e., lateral etching) that may occurs beneath the 
etch-resist coated areas. During etching of holes in glass plates, 
hydrogen is generated by the reaction of hydrofluoric acid and titanium 
and generally adheres to the vertical glass surfaces of the resultant 
holes and displaces hydrofluoric acid therefrom so as to reduce the amount 
of lateral etching. In contrast, during immersion etching or spray etching 
with a liquid etchant, this effect does not occur because the etching 
reaction products are removed too quickly to build-up a barrier against 
the attaching hydrofluoric acid. 
With the gas etching process of the invention, there is no longer any 
substantially danger of completely etching away glass areas which are to 
remain, (i.e., no danger of destroying land areas between the individual 
holes). Further, with the gas etching process of the invention, a more 
uniform etching reliably takes place so that large-surface plates can be 
uniformal etched. Further, such gas etching process can be practice with 
substantially simpler apparatus, in contrast to immersion or spray etching 
processes and requires less washing or rinse solution. In comparison to 
conventional immersion etching processes, problems caused by 
contaminations do not occur with the gas etching process of the invention 
and the danger of breakage is significantly reduced. 
The apparatus for practicing the gas etching process of the invention is 
relatively simple and comprises a closeable hollow container having at 
least one completely or partially transparent wall providing a view of an 
etching chamber therein and means for supporting one or more glass plates 
within such reaction chamber. Such container is in communication with a 
source of concentrated hydrofluoric acid so that only gaseous hydrofluoric 
acid contacts glass plates within the etching chamber. With this type of 
device, etchant losses via removal of the work-pieces (i.e. glass plates), 
which invariably occur in immersion and spray etching processes, are 
substantially completely avoided. Further, the etch-resist material, which 
may be a metal, a photo-resist, etc., is more constant or effective with a 
vapor or gas phase etchant. Yet further, the progress of the etching 
process can be readily observed via the fully or partially transparent 
wall. 
In an exemplary embodiment of the invention, the abovedescribed apparatus 
may be provided with a washing means, such as a sluice, such that etched 
plates can be immediately washed or rinsed after the etching process 
without removal from the etching apparatus. With such embodiment, an 
endangerment of the surrounding area by gaseous hydrofluoric acid is 
avoided. In another exemplary embodiment, a gas-circulating means, such as 
a fan, may be provided within the etching chamber to homogenize or 
otherwise mix the gas therein. In this manner, more gaseous etchant is 
always present than is used up by the etchant process and there is always 
an excess of gaseous hydrofluoric acid available. 
In order to accelerate the gas etching process, the concentrated 
hydrofluoric acid may be heated. Apparatus embodiments which include this 
feature can process a larger number of work-pieces in a single etch 
process and allow a higher density of glass plates to be positioned within 
the etch chamber. 
In a preferred embodiment of the invention, the concentrated hydrofluoric 
acid is stored in a separate container which is in communication with the 
etching chamber so that gaseous hydrofluoric acid can be controllably 
delivered to the etching chamber. This embodiment enables an easier 
working procedure since the etching chamber can be isolated from the 
source of hydrofluoric acid and the gaseous hydrofluoric acid can be 
removed from the etching chamber before removal of the etched glass plates 
therefrom. Further, this embodiment enables removal of gaseous reaction 
products from the etching chamber so that no dilution of the hydrofluoric 
gas occurs. 
In another preferred embodiment, the concentrated hydrofluoric acid is 
sprayed or atomized within its container or source so as to increase and 
attain a relatively constant vapor pressure of gaseous hydrofluoric acid 
in the etching chamber without, however, spraying liquid hydrofluoric acid 
onto the glass plates being etched. This embodiment allow a particularly 
economical process sequence to be attained and allows removal of any 
gaseous reaction products from the reaction chamber in order to prevent 
dilution of the hydrofluoric gas therein. 
In certain embodiments of the invention, means are provided for removing 
reaction products from the etched glass plates. In producing fine size 
holes, solid reaction products may remain within the etched holes and such 
must be removed, for example, by intense rinsing with plain water, a 
mixture of water and a surfaceactive agent, sulfuric acid or someother 
suitable washing liquid, with or without auxiliary means, such as a 
supersonic energy producing means. 
The process of the invention may, of course, be utilized to etch other 
defined structures, in glass plates for example a bar pattern, if desired. 
Referring now to FIGS. 1 and 2, wherein like reference numerals refer to 
like elements or portions, a glass plate 1 is shown having an etch-resist 
coating 2 on opposite surfaces thereof. FIG. 1 illustrates a glass plate 
after a wet-etching process and, as can be seen, the relation between the 
etched depth dimension (schematically indicated via arrow 3) and the 
etched lateral dimension (schematically indicated by arrow 4) is quite 
unfavorable so that such plate is relatively weak and relatively small 
glass areas remain between the respective holes. FIG. 2, in contrast, 
illustrates a glass plate after it has been gas etched in accordance with 
the principles of the invention and, as can be seen, such plate exhibits a 
very favorable etching factor. Further, as can be seen from FIG. 1, the 
resist coating 2 has been undercut to such a degree that it lifts off and 
no longer adheres to the glass surface. 
With the foregoing general discussion in mind, there is now presented 
detailed exemplary examples that will illustrate to those skilled in the 
art the manner in which the invention is carried out. However, these 
examples are not to be construed as limiting the scope of the invention in 
any way. 
EXAMPLE I 
A glass plate of 0.2 mm thickness is metallized on both sides thereof 
(i.e., on the major surfaces thereof) with a 30 nm thick layer of titanium 
and a 300 nm thick layer of copper. A layer of a photo-resist is then 
provided on the copper surfaces so as to correspond to the negative image 
of a desired track or conductor array via conventional technology. The 
metal surfaces not covered with photo-resist are galvantically reinforced 
(i.e., electro-plated) with a 5 .mu.m thick layer of copper and a 1 .mu.m 
thick layer of nickel. Thereafter, the photo-resist layer is 
photographically removed (i.e., exposed and washed with a suitable 
solvent) at those regions where holes are to be etched and the underlying 
areas of metal (copper and titanium) are etched away to uncover the glass 
surface at those regions as well as at another select region of the glass 
plate so that a target or test mark can be placed on the so-covered 
surface for monitoring the subsequent etching process. The electrode grid 
pattern applied via the photo-resist is 0.32 mm.times.0.64 mm, with a hole 
size of about 0.16 mm.times.0.4 mm. The so-prepared glass plate is then 
positioned within a gas-sealable container having an etching chamber 
therein and at least one transparent wall. The etching chamber includes a 
support tray for receiving and supporting the glass plate in such a manner 
that the opposite surfaces thereof, i.e., the surfaces to be etched, are 
not covered or masked by the support surfaces. A gaseous hydrofluoric acid 
obtained from a source containing 75% hydrofluoric acid at atmospheric 
pressure is fed into the etching chamber so as to contact the glass plate 
therein. The etching process takes about 15 to 20 minutes, depending upon 
the desired degree of etching. During the etching process, the 
hydrofluoric acid gas pressure is maintained as constant as possible by a 
gas-circulating means and a heating means. During the etching reaction, 
the progress thereof is monitored via the target mark viewed through the 
transparent etching chamber wall. After the etching is completed, the 
etched glass plate is briefly rinsed. Etching reaction produces, such as 
calcium fluoride, alkali hexafluorosilicates, etc., remain in at least 
some of the etched holes and are practically rinsed away with water having 
a surface-active or wetting agent therein, or may be dissolved away with 
sulfuric acid. Thereafter, the remaining photo-resist layer is removed and 
the metal layer (titanium and copper) remaining between the galvantically 
reinforced tracks or conductor paths is removed by etching so as to 
produce an insulating area between the respective tracks. The resultant 
metallic tracks have adequate adhesion to the underlying glass surface and 
the resulting perforated plate is useful as a control plate in a plasma 
display device. 
EXAMPLE 2 
A plurality of glass plates, each having a thickness of 0.3 mm, are 
metalized on both sides thereof with a 100 nm thick layer of titanium and 
a 1000 nm thick layer of copper. Thereafter, via conventional photo-resist 
technology, an electrode array having grid dimensions of 0.32 mm and 0.64 
mm respectfully as well as surfaces areas of about 0.1 mm.times.0.3 mm are 
etched free of the metal (copper and titanium) cladding on each plate. The 
so-prepared glass plates are then brought into contact for about 25 
minutes with a gas phase of a 75% hydrofluoric acid maintained at room 
temperature and atmospheric pressure. After etching and thoroughly 
rinsing, the remaining photo-resist layer is dissolved and the remaining 
copper layer is etched away. Any solid etching reaction products remaining 
in the holes etched through the glass plate are readily removed in about 
one minute via water with the aid of supersonic energy. The resultant 
glass plates may be utilized as spreaders or spacing members between a 
control plate and a luminescent screen in a plasma display device. 
Accordingly, it is not necessary to etch away the remaining titanium layer 
on all such plates. The titanium layer need only be removed from the 
perforated plate which actually comes into direct contact with the control 
plate of a display device. With the selected etching time of 25 minutes, 
the holes in the glass plates in the foregoing process are clearly larger 
than 0.1 mm.times.0.3 mm (the size of the uncovered glass surface areas 
which were etched), however, the perforated glass support structure that 
remains is of adequate strength so that a plurality of such glass plates 
may be positioned one on top of the other to fill-out the space between a 
control plate and a luminescent screen in a plasma display devices. Of 
course, the glass plate which comes into direct contact with the control 
plate has it remaining titanium layer removed therefrom. 
The foregoing is considered as illustrative only of the principles of the 
invention. Further, since numerous modifications and changes will readily 
occur to those skilled in the art, it is not desired to limit the 
invention to the exact construction and operation shown and described, and 
accordingly, all suitable modifications and equivalence may be resored to, 
falling within the scope of the invention as claimed.