A method of forming a semiconductor structure is provided. In this method, a semiconductor substrate is provided. A SoC layer is formed on the semiconductor substrate. A hard mask layer is formed over the SoC layer. The hard mask layer is patterned to expose a portion of the SoC layer. At least one opening is formed on the portion of the SoC layer using an ALE operation, thereby enabling the remaining portion of the SoC layer adjacent to the at least one opening to have a re-entrant angle included between a sidewall of the SoC layer and a bottom of the SoC layer.