The present invention is a circular groove having a diameter smaller than the diameter of the detection port 240 on the surface facing the inside of the reaction chamber 210 of the detection port 240 is smaller than the diameter of the detection port 240 and has a suitable width and depth. An optical fiber cable 250 is connected to a center of a surface of the detection port 240 in which the groove 300 is formed to the outside of the reaction chamber 210 so that the surface 300 of the detection port 240 is formed. In this way, the dispersion of the polymer produced on the surface of the detection port is achieved, thereby providing the effect of preventing the occurrence of the endpoint detection error and lengthening the cleaning cycle of the reaction chamber.
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