Omnivision's patent application US20140210028 "Color filter including clear pixel and hard mask" by Gang Chen, Duli Mao, Hsin-Chih Tai, and Howard Rhodes proposes hard mask process to improve CFA etching accuracy:
"With pixel size scaling down, design rules for color filters should follow the trend and allow the color filters to be scaled down as well, but due to specific processing features of the color filters the scaling is challenging and painful. Introducing new materials is a common approach for improving color filter scaling, but light transmission, process controllability, yield, delivery, and overall optical performance could be sacrificed."
"Clear filter 106 and color filters 108, 110 and 112 are formed on surface 103 of optional planarization layer 104. A first hard mask layer 114 and a second hard mask layer 116 are formed over clear filter 105 and color filter material 107, while only second hard mask layer 116 is formed over color filter material 109. In the illustrated embodiment no hard mask layer is formed over color filter material 111, but in other embodiments an additional clear layer can be formed over filter material 111 and second hard mask layer 116. Hard mask layers 114 and 116 are clear (i.e., substantially colorless) and optically transparent in a range of wavelengths that includes at least the wavelengths of the color filters. In the illustrated embodiment, one or both of hard mask layers 114 and 116 can be made of the same clear material as clear filter 106. In one embodiment hard mask layers 114 and 116 are oxide layers, but in other embodiments other materials can be used provided they meet optical performance and manufacturing requirements. By using oxides or other materials with good light transmission properties, the “old” low cost, mature, optimized color filter materials can extend their application to pixels of any size, the only limit being patterning resolution, which is smaller than the shortest wavelength of the visible light."