A new plasma etching technique using microwave discharge is presented. Silicon wafers are etched by the discharge in a (CF4+O2) gas mixture. Fine patterns with dimensions of 1 µm are etched up to 1 µm in depth without undercutting at a pressure of 5×10-4 Torr with an Al mask having 0.08 µm thickness. Etching is thought to be carried out by chemical reactions. With this technique, the etching rate becomes maximum (2.6×10-2 µm/min) when the mixing ratio γ is 20%. Symbol γ is the partial pressure of O2 divided by the total pressure. The etched depth is proportional to the etching time. This technique is suitable for etching fine patterns of semiconductor devices.