Backside Illuminated Global Shutter Cmos Image Sensors

We report on 2and 4 megapixel backside illuminated image sensors with a 5.5 μm global shutter pixel manufactured in 0.18 μm CMOS. Global shutter pixels require an in-pixel storage node next to the photodiode. This storage node should not be light sensitive. This is specified by parasitic light sensitivity, which is the ratio of the amount of light detected by the storage node divided by the amount of light detected by the photodiode. In many global shutter pixels, the signal is stored on a junction, either the floating diffusion [1], a combination of floating diffusion and other capacitance [2] or a dedicated buried diode implantation [3]. In either case, the in-pixel storage node is shielded from light by a metal shield. Parasitic light sensitivity is worse for longer wavelengths and typically around 1/500 to 1/1000 for a frontside illuminated sensor. For a backside illuminated sensor parasitic light sensitivity is considered to deteriorate considerably due to the lack of a light shield on top of the storage node. The approach taken for this backside illuminated sensor avoids storage of the pixel signal on a diffusion. This guarantees a good parasitic light sensitivity, even for a backside illuminated device.