Generally, green, red, and blue phosphors are regularly arranged in a dotted or striped pattern in the inner surface of a color CRT, and electron beams irradiate onto a target dot or stripe to make the corresponding phosphor luminesce.
However, the electron beams do not irradiate onto a target dot or stripe exactly but an adjacent phosphor dot or stripe of another color may also luminesce, which lowers color purity. Also, a fluorescent layer may luminesce due to electrons scattering at the inner wall of a panel or around shadow mask apertures, which causes lowering of contrast.
To avoid such lowering of color purity and contrast, generally a non-luminescent absorbent such as graphite is filled in a space between the respective phosphor dots or stripes to form a black matrix.
A black matrix is largely classified into a negative black matrix and a positive black matrix according to the size of a phosphor dot or stripe and the size of an electron beam irradiating onto a dot or stripe.
According to the negative black matrix, the irradiated beam is made larger than the phosphor dot or stripe, a landing margin is excellent, and contrast and luminance can be improved. However, if the black matrix size between the red, green, and blue phosphors cannot be adjusted uniformly, a poor quality screen, e.g. a color blot, is liable to be generated.
The negative black matrix is generally fabricated in the following manner.
First, a negative photoresist forming composition comprising a base resin, a binder, a dispersant, and a hardening agent are applied to the inner surface of a panel of a CRT and dried to form a photoresist layer. Then, the photoresist layer is exposed to light and developed so that an exposed portion where phosphor patterns are to be formed is hardened by a photo-reaction, to thereby form a photoresist pattern having a strong adherence to an underlying layer (here, the inner surface of the CRT panel) and being not developed by water.
Subsequently, graphite is applied to the photoresist pattern to form a black matrix layer. Then, etching and developing processes are sequentially performed to remove the exposed portion of the photoresist layer and the black matrix material layer on the exposed layer, thereby completing a black matrix pattern.
As described above, the negative black matrix is, however, formed through a long and complicated process line, and requires an etching step in which materials detrimental to the environment are used.
Also, manufacturing cost is too high to use Ti-containing black pigment composition which can form a black matrix having excellent blackness.
On the contrary, according to a positive black matrix, the irradiated beam is made smaller than the phosphor dot or stripe. Although an excellent landing margin is not easily obtained, the positive black matrix is advantageous in being easily fabricated without an etching step. However, since a photoresist forming material suitable for this method is not in practical use, the positive black matrix is not widely used.
SUMMARY OF THE INVENTION
To solve the above problems, it is an objective of the present invention to provide a method for forming a positive black matrix having excellent blackness and an improved landing margin.
Accordingly, to achieve the above objective, there is provided a method for forming a black matrix comprising the steps of:
coating a photoresist forming composition containing a compound represented by the formula (1) on the inner surface of a panel and drying the same to form a photoresist layer;
exposing the photoresist layer;
coating a black pigment solution for forming a black matrix on the photoresist layer and drying the same to form a black pigment layer; and
developing the resultant structure; ##STR2## wherein, X is a halogen atom and l:m:n is 1.0-4.5:70.0-98.0:1.0-20.0.
Particularly, the main chain of the compound represented by the formula (1) has a degree of polymerization of 300 to 4000. Also, the photoresist forming composition further comprises a surfactant of 0.5 to 5.0% by weight with respect to the compound represented by the formula (1). Here, if the content of the surfactant is not less than the above range, a sufficient surface activity does not occur, whereas, if the content of the sufactant is over the above range, surface activity is not enhanced further and the surplus amount of surfactant is not removed during subsequent processes.
Preferable examples of the surfactant include polyoxyethylene-polyoxypropylene copolymer (for example, PES manufactured by BASF AG.), fluorocarbon-denatured polyacrylate (for exampled, EFKA7070 manufactured by EFKA) or the like.
Also, the black pigment composition for forming the black matrix is not specifically restricted if it can be used in the art of the present invention, and preferably, it can be selected from graphite and Ti-containing black pigment composition. Particularly, the Ti-containing black pigment composition is more preferably because it can fabricate a black matrix having more excellent blackness than the conventional black matrix forming composition containing graphite.
The compound represented by the formula (1) which is a main component of the photoresist forming composition according to the present invention, is a copolymer of vinyl alcohol-vinyl acetate acetalated by 4-[2-(4-formyl phenyl)ethenyl]-1-methylpyridinium salt, which is nontoxic and excellent in storage stability and thermal stability. Also, whereas the photoresist forming composition used for forming the negative black matrix includes a base resin and a photosensitive agent causing a cross linkage reaction with the base resin, the photoresist forming composition according to the present invention does not require a separate photosensitive agent since the compound represented by the formula (1) itself has a function as a photosensitive agent.
According to the present invention, the positive photoresist forming composition containing the compound represented by the formula (1) is coated on the inner surface of the CRT panel to form a photoresist layer and then exposed using a shadow mask so that the exposed portion is softened to have a structure deprived of adhesiveness with the underlying layer (the inner surface of the CRT panel).
Subsequently, the black matrix forming composition is coated on the photoresist layer to form a black matrix material layer and developed with water, so that the exposed portion of the photoresist layer and the black matrix material layer formed thereon are easily developed and removed to thus form a black matrix pattern on the upper layer of the non-exposed portion, thereby providing a phosphor forming portion.
In other words, since the method of the present invention does not require an etching process, the process is simplified and is free from environmental contamination caused by using an etching solution.
Also, the photosensitivity of the compound represented by the formula (1) is so excellent that the profile of the formed pattern becomes better, thereby improving a landing margin, compared to the conventional positive black matrix.
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