Mechanisms for forming image-sensor device with epitaxial isolation feature

An image-sensor device, comprising:

• a substrate having a front surface and a back surface, the substrate including a pixel region and a logic region;
• a logic transistor disposed in or over the logic region and being surrounded by a logic isolation feature;
• a radiation-sensing region disposed in the pixel region and being surrounded by a pixel isolation feature;
• and
• a doped region extending continuously under the pixel region and the logic region, wherein the doped region is in direct contact with a bottom surface of the epitaxial pixel isolation feature and is spaced apart from a bottom surface of the logic isolation feature.

An image-sensor device, comprising:

• a substrate having a front surface and a back surface, the substrate including a pixel region and a logic region;
• an anti-reflective layer contacting the back surface of the substrate;
• a doped region of a single conductivity type located proximate to the back surface of the substrate and extending continuously under the pixel region and the logic region;
• a logic isolation feature surrounding the logic region;
• and
• a pixel isolation feature surrounding the pixel region;
• wherein a bottom surface of the pixel isolation feature is in direct contact with a top surface the doped region and a bottom surface of the logic isolation feature is spaced apart from the top surface of the doped region;
• and
• wherein a bottom surface of the doped region is in direct contact with the anti-reflective layer.