| 摘要 |
The apparatus includes a reticle stage for carrying a reticle and a wafer stage having an X-stage for carrying a wafer. The reticle is moved in -X-direction by means of the reticle stage while the wafer is moved in +X-direction by means of the X-stage for scanning, so that the image of the pattern on the reticle is serially transferred and printed onto the wafer. A laser interferometer supplies i) to a velocity calculation unit, measured values representing the X-coordinate of the reticle stage as periodically measured with a predetermined period and ii) to a frequency divider, a sampling timing signal indicating each time the measured value has become definite. The frequency divider outputs a calculation trigger signal. The velocity calculation unit, in synchronism with the calculation trigger signal, reads (or samples) the measured values representing the X-coordinate of the reticle stage and performs a calculation in which the difference between two successively sampled measured values is divided by the period of the calculation trigger signal, so as to derive the velocity of the reticle stage with accuracy. The velocity of the X-stage is derived in a similar manner.
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