The present disclosure describes a semiconductor structure and a method for forming the same. The semiconductor structure can include a substrate, a first vertical structure and a second vertical structure formed over the substrate, and an isolation structure between the first and second vertical structures. The isolation structure can include a center region and footing regions formed on opposite sides of the center region. Each of the footing regions can be tapered towards the center region from a first end of the each footing region to a second end of the each footing region.