The present disclosure relates to a BSI image sensor with improved DTI structures, and an associated method of formation. In some embodiments, the BSI image sensor comprises a plurality of image sensing elements disposed within a substrate corresponding to a plurality of pixel regions. A deep trench isolation (DTI) grid is disposed between adjacent image sensing elements and extending from an upper surface of the substrate to positions within the substrate. The DTI grid comprises air-gaps disposed under the upper surface of the substrate, the air-gaps having lower portions surrounded by a first dielectric layer and some upper portions sealed by a second dielectric layer.