PURPOSE:To automatically and precisely detect a final point of photo etching by deciding a point that the intensity of light with a specific wave length decreases to the value of predetermined ratio from a maximum intensity level at photo etching as a final point wherein the above is applied to a formed substance produced by the reaction of a treated substance and photo etching gas. CONSTITUTION:An Si substrate formed Al is exposed in plasma generated by adding high frequency to CCl4 of about 0.1-0.01torr and photo etching is applied to Al. At that time, the discharge of an Al atom occurs in reactive formed gas simultaneously with the start of photo etching and output voltage will suddenly increase in a short period by converting light having a specific wave length of 396mum into an electric signal by a photo diode through an interface filter and the output voltage maintains at the maximum output Vmax during Al photo etching and suddenly drops at the time of finishing Al photo etching to reach 1/k (k is a constant). A final point of photo etching will precisely be detected and uniform photo etching will be performed by feeding the output at the diode to an offset circuit 10 through a terminal 9 for amplification 11 and then by comparing 14 the output through a peak holder 12 and a divider 13.
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