Distortion inspecting method for projection optical system

A distortion inspection method for a projection optical system for exposing images of measurement patterns onto a photosensitive substrate via a projection optical system to be inspected, and inspecting distortion characteristics of said projection optical system from states of the exposed images of the measurement patterns, comprising:

• the first step of exposing an image on a mask, on which pairs of first main and sub measurement marks aligned at an interval .DELTA.T1 in a first measurement direction, and pairs of second main and sub measurement marks aligned at an interval .DELTA.T2 in a second measurement direction are formed at positions corresponding to a plurality of points at which distortion amounts are to be inspected in a projection view field of said projection optical system, onto said photosensitive substrate via said projection optical system;
• the second step of exposing the image on said mask onto said photosensitive substrate via said projection optical system after said mask and said photosensitive substrate are moved in the first measurement direction relative to each other by an amount .DELTA.a determined by the interval .DELTA.T1 and an interval between adjacent mark pairs, and exposing the image on said mask onto said photosensitive substrate via said projection optical system after said mask and said photosensitive substrate are moved in the second measurement direction relative to each other by an amount .DELTA.b determined by the interval .DELTA.T2 and the interval between adjacent mark pairs;
• the third step of measuring a relative displacement, in the first measurement direction, between the first main and sub measurement mark images, and a relative displacement, in the second measurement direction, between the second main and sub measurement mark images at each of the positions, corresponding to the plurality of points at which the distortion amounts are to be inspected of said projection optical system, on said photosensitive substrate;
• the fourth step of exposing the first measurement mark images onto said photosensitive substrate via said projection optical system after said photosensitive substrate is moved by an amount corresponding to an exposure field size of said projection optical system in the first measurement direction, and exposing the second main measurement mark images onto said photosensitive substrate via said projection optical system after said photosensitive substrate is moved by the amount corresponding to the exposure field size of said projection optical system in the second measurement direction;
• the fifth step of exposing the first sub measurement mark images near the first main measurement mark images exposed in the fourth step, and exposing the second sub measurement mark images near the second main measurement mark images exposed in the fourth step;
• and
• the sixth step of calculating stepping errors of said photosensitive substrate in the first and second measurement directions on the basis of intervals between the first and second main measurement mark images exposed in the fourth step, and the first and second sub measurement mark images exposed in the fifth step,
• wherein a distortion amount of said projection optical system is obtained by accumulating the relative displacements measured in the third step while correcting the stepping errors of the photosensitive substrate calculated in the sixth step.