Image sensor and manufacturing method thereof

A method for manufacturing an image sensor, comprising:

• A:
• forming a second-conductive-type region in a first-conductive-type semiconductor substrate to form a photo diode, wherein the second-conductive-type region serves as a region for collecting photo-induced carriers, and forming a first-conductive-type isolating layer above the photo diode in the first-conductive-type semiconductor substrate;
• B:
• forming a second-conductive-type lightly-doped region and a first-conductive-type lightly-doped region under the second-conductive-type lightly-doped region, wherein the second-conductive-type lightly-doped region is isolated from the second-conductive-type region by the first-conductive-type lightly-doped region, wherein the second-conductive-type lightly-doped region and the first-conductive-type lightly-doped region have a self-alignment feature, and the second-conductive-type lightly-doped region and the first-conductive-type lightly-doped region do not contact the second-conductive-type region and the first-conductive-type isolating layer;
• C:
• forming a gate structure of a transfer transistor on a surface of the first-conductive-type semiconductor substrate by a deposition process and a patterning process, wherein the gate structure covers at least a portion of the second-conductive-type lightly-doped region;
• and
• D:
• forming a floating diffusion region which is second-conductive-type heavily-doped by a doping process, the floating diffusion region contacting with the second-conductive-type lightly-doped region,
• wherein in the step B, photoresist is coated on the surface of the first-conductive-type semiconductor substrate, a photolithography process is performed on the photoresist using a first mask plate to expose and develop to form a patterned photoresist, an etching process is performed by taking the patterned photoresist as a mask to form a groove on the surface of the first-conductive-type semiconductor substrate, a doping process is performed to a first portion of the first-conductive-type semiconductor substrate which corresponds to the groove to form the first-conductive-type lightly-doped region, an inner side of the groove is etched to enlarge the groove, and a doping process is performed to a second portion of the first-conductive-type semiconductor substrate which corresponds to the enlarged groove to form the second-conductive-type lightly-doped region.