New mechanism of plasma induced damage on CMOS image sensor: Analysis and process optimization

A new plasma induced damage mechanism on CMOS image sensor is analyzed. An increase of the mean pixel dark current is observed after the plasma etch of a cavity on the pixel area. The degradation increases non-linearly when the dielectric layers between the photodiode and the plasma become thinner. This can be explained by a photo generation phenomenon in the dielectric nitride layer, induced by the plasma UV, and assisted by the wafer surface charge. This mechanism leads to the creation of a positive fixed charge on the pixel surface, which can next deplete the top P layer of the pinned photodiode. The temperature dependence of the dark current again demonstrates that the degradation is mainly due to a diode surface generation effect. Process and pixel architecture optimization are finally proposed.