Abstract: A silicon substrate is etched by dipping it in a NH.sub.4 F solution while charging it with a potential more negative than an open-circuit potential. The NH.sub.4 F solution preferably has NH.sub.4 F concentration of 10M or less. The potential applied to the silicon substrate is controlled within the range of from the open-circuit potential to a more negative potential by -1.5 V vs. SCE. Since the etched silicon substrate has flatness in atomic order, it is suitable for the precise fabrications to manufacture high-density ir high-functional semiconductor devices.