Improved color filter process for CCD and CMOS imagers

Abstract

Several different methods exist for converting monochromatic solid-state imagers into color. One of the first techniques was to rotate a color filter wheel in front of the sensor for each exposure. More recently, high-definition cameras align three different sensors and separate the color signals using dichroic filters mounted on a prism. Yet another method, known as "on-chip" color filters, applies a pattern of colored elements directly to the surface of the sensor. This type of filter is commonly produced by depositing successive layers and transferring a dye through a patterned photoresist. At Polaroid Corporation, color filters are produced in a unique, single-step photolithographic process by incorporating a dye directly into the resist prior to coating. The patterned photoresist and dye combination is then baked to stabilize the filter element against the next coating of colored photoresist. These color filters can often be sensitive to the high temperature bakes as well as other heat treatments found in the packaging process. Often the resist will reflow and discolor, thus reducing the resolution, sensitivity and production yield of the device. We are proposing a solution for eliminating or reducing the hard bakes between color by treating the filter elements with a silylating compound capable of cross-linking the photoresist by incorporating silicon into the polymeric chain. This process eliminates three baking steps and reduces the risk of reflowing and yellowing the resist as well as degrading the dye. Furthermore, the filters are sufficiently stabilized by silylation such that they can survive the high temperatures required for producing microlenses and completing the packaging process. A preferred silylation compound is hexamethylcyclotrisilazane (HMCTS). This same silylation compound can also be used to promote adhesion of the dyed photoresist layers and to replace a separate treatment by other commonly used silylation compounds such as hexamethyldisilazane (HMDS). This results in a process which is more robust and has a higher yield.