The present disclosure relates to a CMOS image sensor having a photodiode surrounded by a back-side deep trench isolation (BDTI) structure, and an associated method of formation. In some embodiments, a plurality of pixel regions is disposed within a substrate and respectively comprising a photodiode. A back-side deep trench isolation (BDTI) structure is disposed between adjacent pixel regions, extending from a back-side of the substrate to a position within the substrate. The BDTI structure comprises a doped layer lining a sidewall surface of a deep trench and a dielectric fill layer filling a remaining space of the deep trench. By forming the disclosed BDTI structure that functions as a doped well and an isolation structure, the implantation processes from a front-side of the substrate is simplified, and thus the exposure resolution, the full well capacity of the photodiode, and the pinned voltage is improved.