Since it is the trend in the industry to make smaller and cheaper electronic devices and provide higher resolution for performance, it has become necessary to develop new photoimageable materials to manufacture such devices. Photo-patterning technologies offer uniform finer lines and space resolution when compared to traditional screen-printing methods. A photo-patterning method, such as DuPont's FODEL® printing system, utilizes a photoimageable organic medium as found in U.S. Pat. No. 4,912,019, U.S. Pat. No. 4,925,771, and U.S. Pat. No. 5,049,480, whereby the substrate is first completely covered (printed, sprayed, coated or laminated) with the photoimageable thick film composition and dried. An image of the pattern is generated by exposure of the photoimageable thick film composition with actinic radiation through a photomask bearing a pattern. The exposed substrate is then developed. The unexposed portion of the pattern is washed away leaving the photoimaged thick film composition on the substrate that is then fired to remove organic materials and sinter inorganic materials. Such a photo-patterning method demonstrates line resolution of about 30 microns depending on the substrate smoothness, inorganic particle size distribution, exposure and development variables. It has been proven that such a technique is useful in the manufacture of flat panel display applications, such as plasma display panels.
Resolution and brightness of the images in the AC plasma display panel (PDP) device depend on electrode width, interconnecting conductor pitch and transparency of the dielectric layer. It is difficult to obtain fine line and space resolution for the formation of the electrodes and interconnecting conductor patterns when these materials are applied by conventional patterning techniques such as screen printing, sputtering or chemical etching methods. Moreover, to improve the display contrast, it is essential to decrease the reflection of external light from the electrodes and conductors arranged on the front glass substrate. This reflection decrease can be most easily accomplished by making the electrodes and conductors black as viewed through the front plate of the display.
Composition(s), such as that described in U.S. Pat. No. 6,342,322 to Kakinuma, disclose “a photosensitive composition, comprising in combination (a) a carboxyl group-containing photosensitive polymer . . . (b) a diluent, (c) a photopolymerization initiator; (d) an inorganic powder, and (e) a stabilizer . . . ”. The photosensitive composition for use in a black matrix additionally contains a black pigment. Where the paste is required to have a black color tone, a black pigment which is formed of a metal oxide containing as a main component thereof one or more members selected from among Fe, Co, Cu, Cr, Mn, and Al may be additionally incorporated. These black pigments disclosed in Kakinuma may be conductive as in their pure state, but often decompose partially in the low softening point glass of the black conductive, resulting in a film which in its fired form is essentially nonconductive. In a two-layer structure (for example where the black conductive layer is adjacent to the patterned conductive tin oxide, SnO2, or indium tin oxide, ITO, layer on the front panel, and a more conductive layer, typically make from finely divided Ag particles, is formed on top of the black conductive layer), enhanced conductivity in the black conductive layer improves electron flow from the bus electrodes to the patterned conductive SnO2 or ITO layer.
Other black pigments, notably graphite and/or carbon black disclosed in Kakinuma degrade during firing in air and result in losses in both conductivity and the ability to provide contrast in the black conductive layer.
The present invention is directed to a photoimageable thick film composition containing photocrosslinkable polymers for use in photo-patterning methods. The composition(s) of the present invention are particularly useful in flat panel display applications, such as PDP devices, fabricated by using photoimageable thick film compositions for use in photo-patterning methods wherein a black electrode is present between the substrate and a conductor electrode arrangement, as described in U.S. Pat. Nos. 5,851,732 and 6,075,319. In particular, the photoimageable composition(s) of the present invention utilize metal oxide(s) of two or more rare earth metals, wherein such metal oxides have metallic or semi-metallic conductivity.