| 摘要 |
PURPOSE:To reduce the interference between laser beams from a plurality of measured points when the laser beams from a plurality of the measured point are superposed and improve alignment accuracy by determining the differences between the light path lengths from a light source to a detector through the measured points within a specific range. CONSTITUTION:The resonator length of a laser oscillator 10 is denoted by L, the refractive index of a medium in the resonator is denoted by (n), the coherent length of a laser beam which is applied to a plurality of measured points is denoted by (l) and the differences between lengths of the light paths from the laser oscillator to a detector through a plurality of the measured points are denoted by DELTAx. Then the optical length nL of the laser oscillator 10 and the light path differences DELTAx between a plurality of the measured points are so determined as to conform to an inequality, 2nLXN+l<DELTAx<2nLX(N +1)-l, (N=0, 1, 2, 3...). By selecting the light path differences DELTAx properly like this, the interference between the reflected lights from both of the alignment marks 17 and 19 can be reduced to the extent almost neglisible. |
