MATERIAL AND METHOD FOR THE PREPARATION AND PATTERNING OF COLOR FILTERS HAVING ELECTRICALLY CONDUCTING PROPERTIES
Original Publication Date: 2000-Mar-01
Included in the Prior Art Database: 2003-Jun-12
We describe a method and materials possessing novel advantages for the fabrication of color filter arrays. A major application of such filters is in liquid crystal panels where pixels are grouped together into red green and blue transmitting or reflecting groups. Another applica- tion is to the filter arrays in color ccd array photo detectors. Liquid crystal display panels contain an array of red, green and blue color filters applied on top of the liquid crystal display pixels. These filters are commonly fabricated by a process involving a dyed photo active material which is applied as a blanket layer and is patterned by normal photolithographic methods. In a typical process, the red green and blue color filters are applied to the upper plate of the liquid crystal panel in an array matching in dimensions the pixel array by patterning photoresist materials containing dyes. Thus to apply the red filters, the plate is coated with a layer of red dyed photosensitive material, this layer is then taken through a process cycle consisting of at least an exposure step through a mask deline- ating the areas to receive the filters, a chemical dissolution and thermal curing step. This whole process is repeated to prepare the green filters and repeated again for the blue filters. Such filters are typically of the order of one micrometer thick and are not electrically con- ducting. It will be appreciated that such a process requires careful alignment between the placement of the red, green and blue color filters and the pixels themselves, that is, there is no automatic registration. Described here is an alternate method of fabricating color filter arrays using an electrodeposition process. This method has the advantage of allowing automatic registration of filters with few processing steps. The material employed is of a polyaniline type that pos- sesses the properties of electrical conductivity and high optical absorption. The absorption occurs in color bands that may be selected by the incorporation of suitable dye entities. Figure I shows a curve of optical transmission for a polyaniline sample with for comparison a similar curve for a standard color filter as used in a liquid crystal display. Since the filter layers are electrically conducting they may be placed directly upon the pixel electrodes if desired, without affecting the voltage applied to the liquid crystal layer. The elec- trical conductivity may be adjusted within the range 1 femto Siemens/cmsq, to 1 kilo Siemens/cmsq. It is a feature of this material that it may be deposited out of solution by a process similar to electroplating. The polyaniline based materials described here allow the fab- rication of color filters right onto the pixels themselves by applying a voltage between the pixels to be colored and a suitable solution. Such voltages are of a magnitude easily realized by the pixel circuitry, namely of the order of one or several volts.