PURPOSE: To etch the <100> face of a GaAs substrate with right angles by employing a solution composed of 29% NH3, 31% H2O2 and water which are selectively mixed together under a room temperature.

CONSTITUTION: An SiO2 layer 12 is formed on the mirror surface of a GaAs substrate 1 by sputtering and a resist mask 13 is applied and the SiO2 layer 12 is etched with buffer fluoric acid. Then, if the <100> face of the GaAs substrate 1 is etched with a solution composed of 29% NH3, 31% H2O2 and water which are mixed together with selective volume ratios of 10W50%, 10W50% and 40W80% respectively, a right angle shape can be obtained on the surface in accordance with the face orientation. The etched quantity is proportional to a time. With this constitution, the GaAs substrate surface can be etched with right angles with a relatively high etching rate, easy control and a high efficiency.