| 摘要 |
Two-colored illumination light emitted from first and second laser beam sources illuminates a reticle mark and a wafer mark. Diffraction light from the reticle mark and the wafer mark is received by two photoelectric detection elements, respectively. The one photoelectric element receives single-colored diffraction light from light of the first light source through a color filter to generate a reticle beat signal. The other photoelectric element receives two-colored light to generate a wafer beat signal. A phase difference between the reticle beat signal and the wafer beat signal when shutting off the second laser beam source is aligned with a phase difference between the two signals produced when turning on the second laser beam source and decreasing the power of the first laser light source.
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