CMOS image sensor and preparation method thereof

The invention provides a CMOS image sensor and a preparation method thereof, and the method comprises the steps: providing a substrate, forming a photodiode region and a floating diffusion region in the substrate, and forming a first well region in the photodiode region; respectively forming a second well region and a third well region in the photodiode region; forming a gate oxide layer and a gate electrode; forming a pinning layer on the surface of the third well region; carrying out nitrogen element doping on the surface of the substrate in the photodiode region; and executing a thermal annealing process. Nitrogen element doping is performed on the surface of the substrate, the thermal annealing process is performed to activate doped ions in the semiconductor structure, and the nitrogen element is promoted to fully react with the silicon element in the substrate to form a silicon-nitrogen bond with higher bonding energy. In the subsequent plasma process, the silicon-nitrogen bond is not easy to break under