A method of forming a semiconductor structure includes depositing a mask layer over a substrate. The method includes etching the substrate to define a first opening. The method includes depositing a sacrificial material in the first opening. The method includes depositing a dielectric liner along sidewalls of the first opening, wherein a bottom surface of the dielectric liner contacts the sacrificial material. The method includes removing the sacrificial material. The method includes etching the substrate to enlarge the first opening to define a second opening. The second opening includes a first portion extending a first depth from the dielectric material in a first direction perpendicular to a top surface of the substrate, and a second portion extending in a second direction, parallel to the top surface of the substrate. The method includes removing the dielectric liner. The method includes filling the second opening with a dielectric material.