Method of producing a substrate plate for a liquid crystal cell with black matrix areas

A method of producing a substrate plate for a liquid crystal cell with black matrix areas in which blackened polymer material is already deposited before the structured areas of photosensitive resist overlying the structured electrode areas are removed. Rather, these structured areas of photosensitive resist are removed only after the blackened polymer material has been hardened. This processing sequence has the advantage that no special alignment processes are needed in order to arrange the black matrix areas accurately between the electrode areas. This alignment is obtained autonomously by virtue of the fact that the electrode areas, even after they have been structured with the help of the photosensitive resist areas, are still covered by these selfsame photosensitive resist areas, thereby ensuring in an ideal manner that only the areas between the electrode areas will be filled with black matrix material and this in such a way as to make them butt directly against the electrode areas, so that the maximum possible surface will be covered by black matrix material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of producing a substrate plate 
for a liquid crystal cell with black matrix areas. Such black matrix areas 
are indeed preferably arranged in places where the substrate plate is free 
of electrode areas. 
2. Description of the Prior Art 
Black matrix areas have hitherto been applied by means of one of two known 
methods. One of these methods is a photolithographic method, while the 
other is a printing method. 
Both methods commence by producing structured electrode areas. To this end 
a photosensitive resist is first applied to a segmental plate provided with 
an electrode layer; 
this photosensitive resist is then exposed to light through a mask; 
the exposed photosensitive resist is developed and 
the electrode layer is then etched away at the point where the 
photosensitive resist has been removed. 
In both the known methods the next processing step consists of removing the 
remaining photosensitive resist. In the photolithographic method this is 
followed by depositing a layer of blackened polymer material on the entire 
substrate surface with the electrode areas and then applying a 
photosensitive resist to this blackened layer. Subsequently, a mask is 
carefully and accurately aligned with respect to the electrode areas to 
ensure that, following exposure, the photosensitive resist can be 
developed in such a way as to permit the removal of those parts of the 
blackened layer that are underlain by the electrode areas. Blackened 
material will therefore remain only between the electrode areas. 
In the printing method, on the other hand, the aforementioned removal of 
the remaining photosensitive resist is followed by carefully and 
accurately aligning a printing device with respect to the electrode areas, 
this being done in such a way that the subsequent printing will deposit 
blackened polymer material only in the interstices between electrode 
areas. The printed material is then hardened. 
It is quite obvious that in both the known methods considerable costs are 
incurred in order to ensure accurate alignment of the exposure mask and/or 
the printing device with respect to the electrode areas. 
SUMMARY OF THE INVENTION 
The method according to the invention differs from the known methods by 
virtue of the fact that the blackened polymer material is applied prior to 
the removal of the parts of the photosensitive resist layer that are 
underlain by areas of the electrode layer that have not been etched away. 
The following steps are performed after the partial etching away of the 
electrode layer: 
a blackened polymer material is deposited on the entire substrate surface 
with the electrode areas and the photosensitive resist areas; 
the blackened polymer material is hardened, and 
the still remaining areas of photosensitive resist are removed, together 
with the blackened polymer material deposited on them, so that such 
blackened polymer material will remain only in the areas between the 
electrode areas. 
This processing sequence is entirely devoid of adjustment or aligning 
steps, because the residual photosensitive resist areas are utilized for 
the purpose of ensuring that black matrix material can be applied only 
between the electrode areas. As compared with a traditional processing 
sequence for producing a substrate plate without black matrix areas, 
therefore, one only needs the additional steps of depositing the blackened 
polymer material and then hardening the deposited material. Over and above 
this simplification of the processing as compared with other methods of 
producing black matrix areas, the method according to the invention also 
has the advantage of ensuring with absolute certainty that the black 
matrix material not only covers the non-electrode areas, but also that it 
is every where in direct contact with the electrode areas, so that the 
maximum possible surface area is covered by black matrix material. 
With a view to ensuring that the blackened polymer material can be readily 
removed in the areas where the photosensitive resist has been removed, it 
will be of advantage, depending on the particular polymer material that is 
being used, to employ only relatively slight prehardening of the polymer 
material, at about 130.degree. C. for example, and to obtain the desired 
final degree of hardening only by means of post-hardening after the 
removal of the photosensitive resist and the areas of polymer material 
deposited on it. 
Given substrate plates for liquid crystal cells where the liquid crystal 
layer is very thin, it may be of advantage to smooth the level differences 
between the electrode areas and the black matrix areas by means of an 
equalization layer. All the commercially available equalization layer 
materials are suitable for this purpose. 
Carbon is preferably used for blackening polymer materials. A suitable 
material, for example, is described in DE 38 12 030 A1. The polymer 
material referred to in this specification is a resin that is hardened by 
means of drying. Polyimide is another suitable polymer material. This 
material, once again, can be hardened by after-baking. But it is also 
possible to use photosensitive resins or imides or other polymers that are 
hardened by exposure to UV radiation. 
It is very advantageous if, following the etching away of the electrode 
layer in specified areas, hollows can be etched into the substrate in 
these areas. This is associated with two advantages. The first of these is 
that it becomes possible to produce blackened areas of greater thickness 
and therefore with an improved impression of blackness. The second 
advantage is that the absolute shrinkage of the blackened layer during 
hardening becomes greater, which ensures that the blackened areas will 
either no longer project above the level of the electrode layer or project 
only very slightly above it.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The starting point of the method described herein below is a substrate 
plate 10 made of glass and carrying an electrode layer 11 of ITO. A 
typical substrate plate 10 has a thickness of 1 to 2 mm, while the 
electrode layer 11 will have a thickness of about 0.1 mm. A photosensitive 
resist layer 12 is then applied in the customary manner, by spincoating 
for example, to the entire substrate plate 10 with the electrode layer 11, 
as shown in FIG. 2. This layer is then exposed to light through a mask 13. 
The embodiment here considered employs a photosensitive resist that 
becomes soluble in alkalis after exposure. If the photosensitive resist is 
now treated with a slightly alkaline solution, the exposed parts are 
removed, so that only isolated photosensitive resist areas 12b remain 
behind. These may be elongated strips, for example, though in FIG. 3 they 
are shown only in section. The subsequent processing step makes use of a 
solvent for the electrode layer 11. In the embodiment here considered, for 
example, dilute hydrochloric acid was sprayed on at 55.degree. C. 
This caused the electrode areas not covered by the photosensitive resist 
areas 12b to become etched away, so that only the electrode areas 11b 
below the photosensitive resist areas 12b remained behind. 
All the processing steps up to this point have been exactly as in the 
traditional methods. While all the known traditional methods, quite 
irrespective of whether they are used for producing substrate plates with 
black matrix areas or plates without such areas, next proceed to remove 
the photosensitive resist areas 12b, the method according to the invention 
employs a different procedure. The next processing step, the result of 
which is illustrated in FIG. 5, consists of using hydrofluoric acid to 
etch the substrate in the areas where the electrode layer 11 has been 
removed, thereby producing the cavities 16. Using 4% hydrofluoric acid at 
room temperature, cavities having a depth of several microns can be 
produced by means of an etching process lasting a few minutes. Cavities 
having a depth of 4 mm were produced in the embodiment here described. 
A blackened polymer material 14 is then applied to the entire substrate 
surface with the electrode areas 11b and the photosensitive resist areas 
12b. The result of this processing step is illustrated by FIG. 6. "Black 
Matrix DARC" supplied by Messrs. Brewer Science, a US company, was the 
material used in the embodiment under consideration. A concentrated 
solution of this polymer was poured onto the substrate plate and the 
excess material removed with a squeegee. It was then dried and 
subsequently prehardened at 130.degree. C. next the residual 
photosensitive resist areas 12b, together with the overlying black matrix 
material 14, were removed with a dilute alkaline solution. Consequently, 
on the substrate plate 10 there only remained the electrode areas 11b and 
the black matrix areas 14b situated between them, all as shown in FIG. 7. 
It should be noted that the black matrix areas 14b in FIG. 7 do not project 
substantially above the electrode areas 11b. This characteristic can be 
preserved if due account is taken of the following. After the drying of 
the blackened material and the removal of the photosensitive resist areas 
12b, the height to which the black matrix areas 14b project above the 
electrode areas 11b will be the same as the original height of the 
photosensitive resist areas 12b above the electrode areas 11b, i.e. 1 mm 
in the case of the embodiment under consideration. The total thickness of 
the black matrix area will then be of the order of 5.1 mm, which 
corresponds to the sum of the depth of the etched cavities 16 (4 mm), the 
thickness of the electrode areas 11b (0.1 mm) and the thickness of the 
photosensitive resist areas 12b (1 mm). But the black material shrinks 
very considerably during drying and hardening, about 20% in the embodiment 
under consideration, so that on completion the surface of the blackened 
areas will to all intents and purposes come to lie in the same plane as 
the surface of the electrode areas 11b. 
When producing the cavities 16, care should be taken to make their depth 
such that the overall depth of blackened material will be sufficiently 
great to obtain an absorption of incident light coming from outside that 
will be as close as possible to 100%. As regards the material used in the 
present embodiment, this calls for a layer thickness of the order 4-5 mm. 
Smaller layer thicknesses will be sufficient when carbon particles are 
used. The depth of the cavities is therefore determined primarily by the 
desired absorption result. When the depth of the cavities and the 
thickness of the photosensitive resist layer are known, the shrinkage 
behaviour of the blackened material is preferably set in such a way that, 
following hardening, the surface of the blackened material will come to 
lie substantially in the same plane as the surface of the electrode areas 
11b. The shrinkage behaviour can be determined, above all, by adjusting 
the quantity of the solvent in which the blackened material is dissolved. 
However, if the shrinkage behaviour cannot be set in such a way as to 
cause the surface of the black matrix areas to shrink down to the plane of 
the surfaces of the electrode areas 11b, this constitutes little or no 
prejudice to the proper functioning of a liquid crystal cell. 
At this point, attention should also be drawn to the fact that FIGS. 1 to 8 
are not true to scale. Considering a liquid crystal cell for use as a 
television display, the black matrix areas 14b will have a width of 25 mm 
and a maximum thickness of about 1.1 mm, which corresponds to the sum of 
the thickness of the electrode areas (about 0.1 mm) and the thickness of 
the photosensitive resist areas prior to shrinkage (about 1 mm). The black 
matrix areas 14b have the form of elongated strips, which are arranged at 
intervals of about 25 mm centre to centre. If for the purposes of ready 
visualization the distance of 1 mm is replaced by 1 m, this would mean 
that on a level glass surface extending for several kilometers in all 
directions there would be arranged strip-shaped electrode areas having a 
height of 10 cm and a width of 225 m, each pair of such areas being 
separated by an intervening black matrix area rising to a height of 1.1 m 
and having a width of 25 m. 
An orientation layer can be applied directly to the product as shown in 
FIG. 7, if necessary also a sequence consisting of a barrier layer made of 
SiO2 and an orientation layer. The orientation layer is then rubbed in the 
usual manner with a velvet roll. The example of the preceding paragraph 
shows that this rubbing is not unduly prejudiced by any black matrix areas 
that may still project above the general level. But if there is to be 
certainty that the rubbing will be practically unhindered by projecting 
black matrix areas 14b, the structure as shown in FIG. 7 is provided with 
a traditional equalization layer 15 (FIG. 8) prior to the application of 
an orientation layer (not shown) or a layer sequence of the type described 
above. As material for the equalization or planarization layer one may 
use, among others, "Selektilux HTRB-200" supplied by the Merck Company. 
Before applying any other layers to the structure as shown in FIG. 7, 
however, the black matrix material is post-hardened for one hour at 
250.degree. C. 
Attention should be drawn to the fact that any known material and any known 
deposition method can be used--each for its own particular purpose--in the 
processing sequence described above. What is new are not the materials 
used or the methods for depositing these materials, but rather the 
processing sequence itself, namely the fact that the blackened polymer 
material is applied before the structure photo sensitive resist areas 12b 
are removed from the structured electrode areas 11b. The etching of 
cavities is not essential in order to obtain the simplified processing 
sequence for applying the blackened electrode areas. Rather, it 
constitutes an additional measure that will prove appropriate whenever 
particularly high absorption by the blackened areas is to be assured. But 
the etching of the cavities enjoys the same advantage as the application 
of the blackened layer, namely the fact that the photosensitive resist 
layers 12b, which are in any case already present, can be used as mask for 
structuring the electrode layer 11. With a view to ensuring that the 
etchant will etch only the areas to which the blackened polymer material 
is to be applied, the etchant must either be applied locally or the 
remaining substrate areas must be coated in such a way as to prevent them 
from being attacked. In the latter case the whole of the substrate plate 
can be dipped into the etching solution, for example a dilute solution of 
hydrofluoric acid.