Method and apparatus for measuring positions on the surface of a flat object

A method of measuring a distance between first and second measuring points on a surface of a flat object mounted, along a reference plane, on a stage movable parallel to said reference plane, said method comprising the steps of:

• (a) moving said stage to detect coordinate positions of each of said first and second measuring points in said reference plane and thereby determine a distance between said first and second measuring points;
• (b) moving said stage to detect coordinate positions in said reference plane and an amount of deviation from said reference position of each of a plurality of points adjacent to said first measuring point and thereby determine an incline .theta.1 of the surface of said object from said reference plane in the vicinity of said first measuring point;
• (c) moving said stage to detect coordinate positions in said reference plane and an amount of vertical deviation from said reference plane of each of a plurality of positions adjacent to said second measuring point and thereby determine an incline .theta.2 of the surface of said object from said reference plane in the vicinity of said second measuring point;
• and
• (d) correcting the distance between said first and second measuring points in accordance with said inclines .theta.1 and .theta.2.

An apparatus for measuring a position of a measuring point on a surface of a flat object comprising:

• (a) supporting means adapted to mount thereon said object along a reference plane and movable parallel to said reference plane;
• (b) means for detecting a position of said supporting means to detect coordinate positions of each of said measuring point and a plurality of points adjacent to said measuring point in accordance with a position of said supporting means;
• (c) means including an optical system having an optical axis perpendicular to said reference plane for detecting an amount of deviation of each of said adjacent points from said reference plane when each of said adjacent points is at a position of intersection with said optical axis;
• (d) means for determining an incline .theta.1 of a surface of said object with respect to said reference surface in the vicinity of said measuring point in accordance with said coordinate positions and amounts of deviation of said plurality of adjacent points;
• and
• (e) means for correcting the coordinate positions of said measuring point in accordance with said determined incline .theta.1.