Abstract: A method for surface treating of thin substrates such as semiconductor wafers, wherein a semiconductor wafer formed with relatively deep but transversely minute trenches on its surface is horizontally placed on a spinner in a chamber with its trenched surface directed up, ultraviolet light is then emitted onto the surface of the wafer to dissolve impurities sticking in the trenches, and thereafter etchant is spouted from a nozzle to the trenched surface of the wafer being spinned about a vertical axis at a high speed. Next, the inside of the chamber is subjected to a lower pressure than atmospheric pressure, and atmospheric pressure is restored after the lapse of a predetermined time. Thus a series of these steps of etchant supplying, pressure reducing, and pressure recovering are carried out until the complete entrance of the etchant into the interior surfaces of the trenches is effected, so as to even or smooth the interior surfaces and, finally the wafer is treated with rinsing, heating and drying steps.