Various embodiments of the present disclosure are directed towards an image sensor having a substrate including a plurality of sidewalls that define a plurality of protrusions along a first side of the substrate. The substrate has a first index of refraction. A photodetector is disposed within the substrate and underlying the plurality of protrusions. A plurality of micro-lenses overlying the first side of the substrate. The micro-lenses have a second index of refraction that is less than the first index of refraction. The micro-lenses are respectively disposed laterally between and directly contact an adjacent pair of protrusions in the plurality of protrusions. Further, the micro-lenses respectively comprise a convex upper surface.