Deep trench isolation structure and methods for fabrication thereof

A structure, comprising:

• a deep trench isolation (DTI) structure extending a depth into a substrate, wherein the DTI structure separates photodiode doping regions, and the DTI structure comprises:
• a filling material defining an air gap therein;
• and
• a high-K dielectric layer enclosing a sidewall and a bottom of the filling material;
• and
• an isolation structure, comprising:
• a first isolation layer disposed between and in contact with the high-K dielectric layer and the sidewall and the bottom of the filling material;
• and
• a second isolation layer disposed on the first isolation layer, a portion of the second isolation layer being extended from and in contact with a top of the filling material.

A method for forming an image sensor, comprising:

• forming a plurality of photodiode doping regions in and on a front side of a substrate;
• forming deep isolation trenches from a back side of the substrate, wherein the deep isolation trenches separate the plurality of photodiode doping regions;
• depositing a hole accumulation layer on sidewalls of the deep isolation trenches;
• depositing a first isolation layer on the hole accumulation layer, wherein the first isolation layer has a first thickness;
• depositing a first filling material on the first isolation layer in the deep isolation trenches, wherein the first filling material encloses an air gap therein;
• and
• depositing a second isolation layer on the first isolation layer and exposed surfaces of the first filling material, wherein the second isolation layer has a second thickness greater than the first thickness.