Improving Contrast Ratio in Color Filters for AMLCD Devices

The widespread use of pigmented color filters in the AMLCD industry has focused attention on their deficiencies, which include marginal film transmittance, significant depolarization and generally poor imaging characteristics for the corresponding imaging resists. These limitations result in diminished color filter contrast and curtailed brightness for the AMLCD device. Incremental color filter contrast improvements have been generated through use of smaller pigment particles which reduce, but do not eliminate, light scattering. As standard pigment synthetic and grind processes give only small color contrast gains, at a cost of protracted pigment manufacture time, another route to more significant gains in color filter contrast is probably necessary. The work herein describes our attempt at defining one viable approach to real improvements in the key color filter responses of reduced depolarization and improved color contrast. Our approach employs colorants prepared from colloidal suspensions of nanosized, mono-dispersed, dense silica spheres to which appropriate red, green and blue dyes are sorbed. Subsequent dispersion of each colorant nanocomposite in carrier polymer and photoimaging chemistry yields the corresponding negative-tone red, green and blue imageable photoresist. The synthesis and use of monodispersed colorant colloids has been described by Giesche and Matijevic for this and other related areas, but their utility is limited to applications not addressing the more advanced color filter requirements mentioned above. To those ends, our work focused on the synthesis, formulation and optical testing of nanocomposite colorant particles ranging from 10–30 nm in size, using dye chemistry which bridges compatibility between both silica nanosphere and carrier polymer, resulting in clear, highly transparent color films. Two independent optical tests comparing the color contrast on nanocomposite red films with coatings of similar transmittance prepared from reference red pigment dispersion provided rationalization that light scattering is significantly reduced in the nanocomposite film, as evidenced by the 3–10 fold increase in the color contrast ratios.