Image sensor comprising polysilicon gate electrode and nitride hard mask

An image sensor includes a semiconductor substrate, a gate dielectric layer, a gate electrode, a protection oxide film, and a nitride hard mask. The gate dielectric layer is over the semiconductor substrate. The gate electrode is over the gate dielectric layer. An entirety of a first portion of the gate dielectric layer directly under the gate electrode is of uniform thickness. The protection oxide film is in contact with a top surface of the gate electrode. The gate dielectric layer extends beyond a sidewall of the protection oxide film. The nitride hard mask is in contact with a top surface of the protection oxide film.