Deep trench isolation for pixels in image sensors

A method, comprising:

• forming a doped layer in a substrate;
• etching the substrate to form first and second trenches;
• depositing a polymer layer in the first and second trenches;
• increasing a depth of the first and second trenches until the depth is greater than a distance between the doped layer and a top surface of the substrate, wherein increasing the depth comprises etching the polymer layer and a portion of the substrate in the first and second trenches;
• and
• forming a recess at an intersection of the first and second trenches while increasing the depth of the first and second trenches, wherein an edge of the recess is above a bottom surface of the first and second trenches.

A structure, comprising:

• a doped layer on a substrate;
• first, second, third, and fourth radiation sensing regions on the substrate, wherein lower portions of the first, second, third, and fourth radiation sensing regions are surrounded by the doped layer;
• first and second trenches crossing each other and separating the first, second, third, and fourth radiation sensing regions, wherein the first and second trenches extend vertically to a top surface of the doped layer;
• and
• a dielectric layer filling the first and second trenches and having a portion protruding into the doped layer, wherein the portion is at an intersection of the first and second trenches.