Color Filter Array - Manufacture of The CFA

Manufacture of The CFA

Diazonaphthoquinone (DNQ)-novolac photoresist is one material used as the carrier for making color filters from color dyes. There is some interference between the dyes and the ultraviolet light needed to properly expose the polymer, though solutions have been found for this problem. Color photoresists sometimes used include those with chemical monikers CMCR101R, CMCR101G, CMCR101B, CMCR106R, CMCR106G, and CMCR106B.

A few sources discuss other specific chemical substances, attending optical properties, and optimal manufacturing processes of color filter arrays.

For instance, Nakamura said that materials for on-chip color filter arrays fall into two categories: pigment and dye. Pigment based CFAs have become the dominant option because they offer higher heat resistance and light resistance compared to dye based CFAs. In either case, thicknesses ranging up to 1 micrometre are readily available.

Theuwissen says "Previously, the color filter was fabricated on a separate glass plate and glued to the CCD (Ishikawa 1981), but nowadays, all single-chip color cameras are provided with an imager which has a color filter on-chip processed (Dillon, 1978) and not as a hybrid." He provides a bibliography focusing on the number, types, aliasing effects, moire patterns, and spatial frequencies of the absorptive filters.

Some sources indicate that the CFA can be manufactured separately and affixed after the sensor has been manufactured, while other sensors have the CFA manufactured directly on the surface of the imager. Theuwissen makes no mention of the materials utilized in CFA manufacture.

At least one early example of an on-chip design utilized gelatin filters (Aoki et al., 1982). The gelatin is sectionalized, via photoliothography, and subsequently dyed. Aoki reveals that a CYWG arrangement was used, with the G filter being an overlap of the Y and C filters.

Filter materials are manufacturer specific. Adams et al. state "Several factors influence the CFA's design. First, the individual CFA filters are usually layers of transmissive (absorptive) organic or pigment dyes. Ensuring that the dyes have the right mechanical properties—such as ease of application, durability, and resistance to humidity and other atmospheric stresses—is a challenging task. This makes it difficult, at best, to fine-tune the spectral responsivities.".

Given that the CFAs are deposited on the image sensor surface at the BEOL (back end of line, the later stages of the integrated circuit manufacturing line), where a low-temperature regime must be rigidly observed (due to the low melting temperature of the aluminum metalized "wires" and the substrate mobility of the dopants implanted within the bulk silicon), organics would be preferred over glass. On the other hand, some CVD silicon oxide processes are low temperature processes.

Ocean Optics has indicated that their patented dichroic filter CFA process (alternating thin films of ZnS and Cryolite) can be applied to spectroscopic CCDs. Gersteltec sells photoresists that possesses color filter properties.