A method includes forming an opening extending from a top surface of a silicon substrate into the silicon substrate to a predetermined depth. The method further includes forming an insulation structure on the silicon substrate along the sidewalls and the bottom of the opening and forming a conductive layer on the insulation structure to fill the opening. A first interface between the insulation structure and the silicon substrate has an interface roughness with a peak-to-valley height less than 5 nm, and a second interface between the insulation structure and the conductive layer has an interface roughness with a peak-to-valley height less than 5 nm.