Plasma etching method

A plasma etching method for forming a trench having a side wall of smooth shape while simultaneously satisfying the requirement of the shape of the trench and the requirement of the aspect ratio. A silicon substrate is placed on a lower electrode (120). An etching gas is supplied through a gas introducing port. Exhaustion through an exhaust port (150) is performed. High-frequency power is fed to an upper electrode (110) and the lower electrode (120) from the high-frequency power supplies (130a, 130b) so as to change the etching gas into a plasma by an ICP method and to produce active species. Thus, the silicon substrate is etched. The etching gas is a mixture gas composed mainly of SF6 gas and additionally O2 gas and He gas.