Pixel isolation structure for image sensor

An image sensor comprising:

• a semiconductor substrate, a bulk region of the semiconductor substrate having a first doping type, a photodetector region of the semiconductor substrate having a second doping type different than the first doping type, a floating region of the semiconductor substrate having the second doping type and extending from a frontside of the semiconductor substrate toward a backside of the semiconductor substrate;
• a transfer gate along the frontside of the semiconductor substrate, the transfer gate being over and between the photodetector region and the floating region;
• a deep trench isolation (DTI) structure extending into the semiconductor substrate from the backside of the semiconductor substrate toward the frontside and surrounding the photodetector region, the DTI structure having a first upper surface at a first depth from the backside of the semiconductor substrate along a first wall of the DTI structure, the DTI structure having a second upper surface at a second depth from the backside of the semiconductor substrate along an intersection of the DTI structure where the first wall and a second wall of the DTI structure intersect, the second depth being different than the first depth;
• and
• a shallow trench isolation (STI) structure extending into the semiconductor substrate from the frontside of the semiconductor substrate toward the backside and over the DTI structure at the intersection of the DTI structure, the STI structure having a first lower surface at a third depth from the backside of the semiconductor substrate, the STI structure having a second lower surface on the second upper surface of the DTI structure, the third depth being less than the second depth so a lower portion of the STI structure has a vertical overlap with a first upper portion of the DTI structure.

A method for forming an image sensor, the method comprising:

• forming a photodetector region and a floating region in a semiconductor substrate along a frontside of the semiconductor substrate, a bulk region of the semiconductor substrate having a first doping type, the photodetector region and the floating region having a second doping type different than the first doping type;
• forming a transfer gate along the frontside of the semiconductor substrate, the transfer gate being over and between the photodetector region and the floating region;
• etching the frontside of the semiconductor substrate to form a first opening in the semiconductor substrate;
• depositing a first dielectric in the first opening to form a shallow trench isolation (STI) structure in the first opening;
• etching a backside of the semiconductor substrate and the STI structure to form a trench in the semiconductor substrate and the STI structure and surrounding the photodetector region;
• and
• depositing a second dielectric in the trench to form a deep trench isolation (DTI) structure in the trench.