Backside illuminated image sensor with three-dimensional transistor structure and forming method thereof

A method for forming a backside illuminated image sensor with a three-dimensional transistor structure, wherein forming a gate of the three-dimensional transistor structure comprises:

• forming a source follower transistor and/or a reset transistor with a three-dimensional transistor structure, wherein the source follower transistor and/or the reset transistor correspond to a protruding structure;
• forming an insulating sidewall around the protruding structure, forming a groove between the insulating sidewall and a channel region of a transistor corresponding to the protruding structure, and forming a gate of the transistor in the groove, wherein the gate of the transistor is isolated by the insulating sidewall;
• forming a floating diffusion region in the protruding structure which is a portion of a semiconductor substrate;
• and
• forming a sidewall of the floating diffusion region during the formation of the insulating sidewall, wherein the floating diffusion region and a gate of a transfer transistor are isolated from each other through the sidewall of the floating diffusion region, to reduce parasitic capacitance.