A method of forming a Σ-shaped trench is disclosed. The method includes: providing a silicon substrate; and performing a plasma etching process to form a Σ-shaped trench in the silicon substrate. The plasma etching process includes: etching the silicon substrate using a first plasma etching gas including a sulphur-containing fluoride; and etching the silicon substrate using a second plasma etching gas including a sulphur-containing fluoride and a polymer gas. A method of forming a semiconductor device is also disclosed.
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