| 摘要 |
PURPOSE: To prevent a drop in performance and suppress damages to the optical elements of a projection aligner by two-photon absorption by returning a laser beam onto the same optical axis, which was once divided into two laser beams with different passages having a different length. CONSTITUTION: A laser beam L1 transmitted through a first partial reflecting mirror 11 and transmitted through a second partial reflecting mirror 12, and a laser beam L2 reflected by the first partial reflecting mirror 11, reflected by total reflection mirrors 13 and 14, and also reflected by the second partial reflecting mirror 12, will have a difference in a beam passage equal to the sum of a distance from the first partial reflecting mirror 11 to the total reflection mirror 13 and a distance from the total reflecting mirror 14 to the second partial reflecting mirror 12. Because of this, if the mirrors are arranged in such a manner that the relation of L1/c>Δt is valid where c is the velocity of a beam andΔt is the pulse width of the laser beam, then two pulses returned to the same optical axis by the partial reflecting mirror 12 have been deviated timewise. By doing this, the total energy of the pulse is divided timewise and damages to the optical elements can be restricted.
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