A method for forming an image sensor is provided to prevent the generation of flicker noises by restraining nitrogen from penetrating into a gate insulating layer using an enhanced dual gate insulating layer structure with a second gate insulating layer formed under nitrogen gas atmosphere. A first region and a second region are defined on a semiconductor substrate(1). A first gate electrode layer(9) is formed on the first region of the substrate via a first gate insulating layer(7) and a second gate electrode layer is formed on the second region of the substrate via a second gate insulating layer(15). A first gate pattern and a second gate pattern are formed within the first and the second regions by performing selectively a patterning process on the resultant structure. The second gate insulating layer is formed under nitrogen atmosphere.
