Image Number 4 for United States Patent #6323945.
Two coma aberration automatic measuring marks M1 and M2 of a firstorder diffraction grating are each composed of a plurality of elongated isosceles triangle patterns which are so arranged that the axis of symmetry passing on the center of each elongated isosceles triangle is parallel to one another, that a half P1, P2, and P3 of the elongated isosceles triangle patterns have the widths thereof which extend in a direction opposite to that of the remaining half P4, P5 and P6 of the elongated isosceles triangle patterns, and the elongated isosceles triangle patterns are located separately from one another, in a direction perpendicular to the axis of symmetry passing on the center of each elongated isosceles triangle, and at a pitch diffracting a measuring coherent light. The two firstorder diffraction gratings M1 and M2 are located separately from each other in a direction of the axis of symmetry passing on the center of the elongated isosceles triangle, in such a manner that the elongated isosceles triangle patterns included in the two firstorder diffraction gratings are in symmetry to each other, in connection with a line positioned between the two firstorder diffraction gratings and which is perpendicular to the axis of symmetry passing on the center of the elongated isosceles triangle. The two firstorder diffraction gratings are scanned by the measuring coherent light, and a relative distance R1 between diffraction lights generated by the two firstorder diffraction gratings, is measured and compared with a distance R between the two firstorder diffraction gratings.
