Date Visited: October 9, 1991
Report Author: M. Thompson
Toppan is an independent company with a 1991 revenue of $54 million working in publishing, packaging, photo fabrication, and electronic precision components. They have sophisticated CAD/CAM and produce photomasks, PWB, lead frames, shadow masks, color filters for LCD, and image sensors. For example, they have produced X-ray masks, 62-layer PWB, 336-pin lead frames, and TAB tapes.
Toppan is the world's largest producer of color filters for CCD, LCD, and image sensors. At the Shiga Plant, dyed gelatin and pigment dispersed filters are produced, defined with an accuracy of plus/minus 5 microns.
Dyeing is the most popular method, with gelatin as the binder. However, this method has some problems. Gelatin is not strong enough, and colors fade because the heat resistance (180 degrees centigrade) and high-light resistance are insufficient. Acrylic binders have superior heat resistance (260 degrees centigrade) and better high-light resistance. Dye filters produce better selectivity than pigment-dispersed filters. Toppan also produces multilayer thin-film dichroic filters, but they are expensive.
There are four principal methods for producing filters: dyeing pigment colorant, printing, electric deposit, and dichroic. For LCDs the dye-based gelatin has been the predominant method, but the pigment-dispersed filters are expanding because, compared with the gelatin filters, pigment-dispersed filters prove to be superior in light resistance for OA, TV, automobile and aircraft applications.
The production cost is similarly high for dye and pigment filters. The pigment-based filter is spun onto the substrate. For the pigment-based filter, the material cost is higher than for the dye filter, but the production method is easier. There is only one supplier, Fuji-Hunt, for pigment-based filter material, which requires other vendors to reduce material cost. Roller coating, when perfected, could reduce the amount of pigment filter material dramatically. A proximity aligner is used to pattern the filters. Low-reflectivity chrome is used for the black matrix and is patterned with a stepper. An overcoat is deposited on the gelatin filters (but sometimes not for pigment filters) before the ITO is deposited. Improvement in the transmission of pigment-dispersed filters is under investigation but no dramatic improvement is expected in the short term. If the pigment is made smaller, the transmission increases, but generally speaking the light resistance has the tendency to gradually decrease.
Electro depositing production of color filters can only be used with certain filter layouts (like stripe) that have a continuous path from one side of the display to the other. Printing technologies offer the possibility of fabricating low-cost filters. The main issue with printing is the topographic profile of the filter and the maximum size of the substrate. A comparison of the dye, printing, and pigment methods is shown in Table Toppan.1.
In summary, the main emphasis is on reducing cost of filters.