Image sensor with dual trench isolation structure

In some embodiments, the present disclosure relates to an image sensor. The image sensor comprises a substrate. A photodetector is in the substrate and includes a semiconductor guard ring extending into a first side of the substrate. A shallow trench isolation (STI) structure extends into the first side of the substrate. An outer isolation structure extends into a second side of the substrate, opposite the first side of the substrate, to the STI structure. The STI structure and the outer isolation structure laterally surround the photodetector. An inner isolation structure extends into the second side of the substrate and overlies the photodetector. The inner isolation structure is vertically separated from the photodetector by the substrate. Further, the outer isolation structure laterally surrounds the inner isolation structure.