Progress of Color Resist Development at TVS
More than 30 years have passed since we started to develop RGB color materials (color resist) for color filters. In 1983, we started to develop a positive type polyimide resist. In 1990, our product was adopted in the production line of liquid crystal displays, and since then we have lead the development of the mainstream negative-type resists.
Diagram 1. History of Color Resist Development
In the 1990s, as the demand for large-sized TV increased, liquid crystal display manufacturers have tried to enlarge the size of the glass substrate (G) in each generation. In such competition of development, bigger glass substrates are advantageous in increasing the non-defective rate and reducing the manufacturing cost. Therefore, the Toyo Ink Group has endeavored to improve the required performances of the color resist (coated film, sensitivity and coating process). Since 2001, we have been providing color resist for large substrates. Starting with the production of color resist for G5 substrates in Taiwan, we now provide products for up to G10 substrates, contributing to the improvement of performance and to the mass production of liquid crystal displays. Also, for the improvement of product functionality, we have developed high contrast type color resist, high brightness type color resist, dye type color resist, and contributed to the colorization of all types of FPDs.
Diagram 2. Transition of glass substrate size
Behind-the-scene Episodes of Development
In the early 1980s, one of our clients brought up the joint development of color resist. This was the beginning of the TVS color resist project. After years of research, the project went full-scale in 1988. Later in the year 2000, the Moriyama factory was newly built to accommodate to the production increase of liquid crystal TVs and IT devices. However, this whole project was started off by three employees, one veteran and two fresh recruits, who were gathered one day to a small R&D section in a factory located in Itabashi, Tokyo.
When the project started, there was only one staff member who had knowledge on color filters and we encountered many difficulties during the early years. At that time, the manufacturing method of color filters was under development as well, so we tested several manufacturing methods other than the eventually adopted pigment resist method such as the dyeing method, the PI etching method, the electrodeposition method and the printing method. At first, color filters manufactured by the pigment resist method showed a low contrast range compared to those made by the dyeing method because the relatively large pigment particles caused light diffusion. This is where we came up with the idea of pulverizing the pigment into fine small particles to realize a color resist with high contrast.
Table 1. Manufacturing Method of Color Filters in the 1980
There were also many difficulties in trying to fix the color resist onto the glass substrate. In the early stage of development, we coated the resist with polyvinyl alcohol, an oxygen blocking film, before exposing it to ultraviolet rays. This took a considerable amount of time and money, so there was a demand for omission of this coating process. However, exposing color resist in an atmospheric environment was unthinkable at that time. Fixation failed again and again, and the color resist could not withstand the developing process. But after many trials and errors, we succeeded in fixing the color resist on the glass substrate by optimizing the type and mixing ratio of the photo initiator. This technology is also inherited in our current color resist.
In the 1990s, production of color resists increased dramatically, accommodated by the enlargement and production increase of liquid crystal displays (LCDs). Business profit increased by nearly 10 times in one decade, and in the year 2000 we set up the Moriyama factory to increase our production capacity. Today, the color resist business has become one of the major enterprises sustaining the Toyo Ink Group. During this period, electronics manufacturers in Japan were all putting effort in developing new LCD products. At TVS, new color filters were being developed every year, and the demand for better color filters (color resist) was becoming higher and higher.
The most emphasized property was a high contrast. In order to achieve a high contrast, it is crucial to reduce the diffusion of light by decreasing the size of the pigment particles. However, small pigment particles are difficult to disperse, and also destabilizes the color resist. Therefore, in order to realize a brighter and clearer LCD, we started to develop new dispersion technologies and new pigments to transmit light more efficiently. Through our research, we have been pursuing a color resist that can satisfy the many demands for higher quality.
Later, the domestic demand for LCDs settled down, and the needs from neighboring Asian countries such as Taiwan, Korea and China increased. As the factories for large-sized glass substrates were set up in these areas, the manufacturing stronghold moved overseas. Instead, a new domestic demand for on-chip color resist is beginning to grow. Being installed into multi-purpose and multi-functional products such as smart phones and digital cameras, the further improvement of on-chip color resist is required as well.