Optical isolation structure for reducing crosstalk between pixels and fabrication method thereof

A semiconductor device, comprising:

• a semiconductor substrate;
• a plurality of optical isolation structures disposed in the semiconductor substrate to define a plurality of pixel regions, wherein each of the optical isolation structures comprises:
• a deep trench filling material in the semiconductor substrate;
• an epitaxial layer surrounding a lower portion of a sidewall of the deep trench filling material, wherein the epitaxial layer is conductive;
• and
• a dielectric layer in the semiconductor substrate and surrounding an upper portion of the sidewall of the deep trench filling material, wherein the epitaxial layer has a first portion extending along a bottom surface of the deep trench filling material and a second portion extending from the first portion along the sidewall of the deep trench filling material to a bottom surface of the dielectric layer but not beyond the bottom surface of the dielectric layer, and the bottom surface of the dielectric layer extends from a sidewall of the semiconductor substrate at least to an outer sidewall of the epitaxial layer without extending to an inner sidewall of the epitaxial layer;
• a plurality of transfer gates disposed in the pixel regions;
• and
• a contact coupled to one of the pixel regions between one of the transfer gates and one of the optical isolation structures.

A semiconductor device, comprising:

• a semiconductor substrate having a stepped sidewall that has an upper portion and a lower portion below and misaligned with the upper portion;
• a deep trench filling material in the semiconductor substrate, wherein the deep trench filling material comprises an upper portion and a lower portion adjoining the upper portion, and the deep trench filling material is defined by a single piece of dielectric material that is continuous throughout;
• an epitaxial layer surrounding the lower portion of the deep trench filling material, wherein the epitaxial layer is conductive;
• and
• a plurality of dielectric layers surrounding the upper portion of the deep trench filling material, wherein bottom surfaces of the dielectric layers are lower than a top surface of the semiconductor substrate, and the deep trench filling material has a sidewall in contact with the epitaxial layer and the dielectric layers, the upper portion of the semiconductor substrate is in contact with one of the dielectric layers, and the lower portion of the semiconductor substrate is in contact with the epitaxial layer.