| 摘要 |
PURPOSE:To protect a controlled space from application of a pressure exceeding an allowable range by a method wherein a 1st maintaining means which maintains the pressure at about a certain value within a predetermined range when the pressure is judged to exceed the predetermined value and a 2nd maintaining means which maintains the pressure nearly equal to the atmospheric pressure when the operation of a control means is interrupted are provided. CONSTITUTION:A control means which controls the pressure of an enclosed space 3 provided in a light path between a mask R and a photosensitive substrate W in accordance with conditions of application of a lighting beam to a projection optical system or environmental conditions, a 1st maintaining means which maintains the pressure at about a certain value within a predetermined range when the pressure is judged to exceed the predetermined value while the operation of the control means is normal and a 2nd maintaining means which maintains the pressure nearly equal to the atmospheric pressure when the normal operation of the control means is interrupted are provided. With this constitution, a pressure exceeding the allowable range is not applied to the enclosed space and damages of lens elements and so forth constituting the enclosed space can be avoided.
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