Some embodiments relate to a CMOS image sensor disposed on a substrate. A plurality of pixel regions comprising a plurality of photodiodes, respectively, are configured to receive radiation that enters a back-side of the substrate. A boundary deep trench isolation (BDTI) structure is disposed at boundary regions of the pixel regions, and includes a first set of BDTI segments extending in a first direction and a second set of BDTI segments extending in a second direction perpendicular to the first direction to laterally surround the photodiode. The BDTI structure comprises a first material. A pixel deep trench isolation (PDTI) structure is disposed within the BDTI structure and overlies the photodiode. The PDTI structure comprises a second material that differs from the first material, and includes a first PDTI segment extending in the first direction such that the first PDTI segment is surrounded by the BDTI structure.