2PFC Image Sensors: Better Image Quality at Lower Cost

A modification to the standard Bayer CFA and photodiode structure for CMOS image sensors is proposed, which we call 2PFC, meaning “Two Pixel, Full Color”. The blue and red filters of the Bayer pattern are replaced by magenta filters. Under each magenta filter are two stacked, pinned photodiodes; the diode nearest the surface absorbs mostly blue light and the deeper diode absorbs mostly red light. The magenta filter absorbs green light, improving color separation between the resulting blue and red diodes. The dopant implant defining the bottom of the red-absorbing region can be made the same as the green diodes, simplifying the fabrication. Since the spatial resolution for the red, green, and blue channels are identical, color aliasing is greatly reduced. Luminance resolution can also be improved, the thinner diodes lead to higher well capacity with resulting better dynamic range, and fabrication costs can be similar to or less than standard Bayer CMOS imagers. Also, the geometry of the layout lends itself naturally to frequency-based demosaicing.