Embedded light shield structure for CMOS image sensor

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

• forming a plurality of photodetectors within a substrate;
• forming a buffer layer over a back-side surface of the substrate;
• forming a light shield structure within the buffer layer, wherein the buffer layer directly contacts opposing sidewalls and a bottom surface of the light shield structure;
• depositing a composite grid layer over the buffer layer and the light shield structure;
• and
• performing a patterning process on the composite grid layer to form a grid structure over the buffer layer, wherein the grid structure comprises a first grid segment laterally offset from a second grid segment, wherein the light shield structure continuously laterally extends from under the first grid segment to under the second grid segment, wherein the light shield structure comprises protrusions and a lower body, and wherein the protrusions continuously vertically extend from the lower body to contact a bottom surface of the grid structure.

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

• forming a plurality of photodetectors within a substrate;
• forming an interconnect structure along a front-side surface of the substrate;
• patterning the substrate to form an isolation trench extending into a back-side surface of the substrate, wherein the isolation trench comprises a first trench segment laterally offset a second trench segment, wherein a first photodetector of the plurality of photodetectors is spaced laterally between the first and second trench segments;
• forming an isolation structure in the isolation trench, wherein the isolation structure comprises a buffer layer extending vertically above the back-side surface;
• forming an opening in the buffer layer;
• depositing a light shield layer over the buffer layer and within the opening, wherein the light shield layer continuously laterally extends from over the first trench segment to over the second trench segment;
• performing a planarization process on the light shield layer to define a light shield structure, wherein a top surface of the buffer layer is co-planar with a top surface of the light shield structure;
• and
• forming a grid structure over the buffer layer, wherein the light shield structure is disposed vertically between the back-side surface and the grid structure.