| 摘要 |
PROBLEM TO BE SOLVED: To execute a stable and accurate focus control action by setting a prescribed threshold value with respect to the output of a photodetector, calculating the width of a light beam reflected on a photosensitive substrate and deciding whether the threshold value is changed or not. SOLUTION: The first reflected light consisting of the first light beam reflected on a mask 5 and the second reflected light consisting of the second light beam reflected on the photosensitive substrate 6 are introduced to the light receiving surface 13A of a detector 13 in parallel. Thereafter, first and second pulse width corresponding to the outputted first or the second reflected light are detected and an interval between a first and a second pulses is calculated. When at least one of the width of the first and the second pulses is out of a prescribed allowable range with respect to a previously set reference value, a slice level is lowered to a base level near to a zero level. Then, the driving mechanism of a substrate stage 4 is controlled based on the obtained interval between the first and the second pulses so that the photosensitive surface 6A of the substrate 6 is set at the focusing position of a projection optical system 7. |
