| 摘要 |
Before a mask (R) is loaded in a mask chamber (15) filled with a specific gas containing impurities whose concentration is less than a first concentration (e.g., 1 PPb) and having a characteristic that the gas absorbs a little the exposure light, the mask (R) is temporally loaded in a preliminary chamber (R1), and the gas in the preliminary chamber (R1) is replaced with a specific gas containing oxygen whose concentration is one (e.g., 10 ppb) higher than the first concentration by means of a gas replacing mechanism (23, 24). Therefore when the mask (R) for exposure is loaded in the mask chamber thereafter, external impurities (including absorbing gases) hardly mix into the optical path inside the mask chamber. When a wafer (W) is replaced, gas replacement in the preliminary chamber (W1) is similarly carried out.
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