Disclosed herein is a method that includes: forming a composite layer, the composite layer comprising first and second insulative materials and a first polysilicon layer that is between the first and second insulative materials, forming a hole in the composite layer, the hole penetrating through the composite layer to define respective edge portions of the first and second insulative materials and the first polysilicon layer, and converting the edge portion of the first polysilicon layer into third insulative material so that the third insulative material is between the respective edges of the first and second insulative materials.