Method of inspecting flatness of substrate

A method of inspecting flatness of a substrate, comprising:

• providing the substrate having a surface;
• letting an optical dimension measuring device select N first inspecting points (Xi, Yi) from the surface along a first straight line in a X direction;
• letting the optical dimension measuring device measure a plurality of heights Zi of the N first inspecting points;
• by using a formula:
• D=Σi=1N−1√{square root over ((Xi−1−Xi)2+(Yi−1−Yi)2+(Zi−1−Zi)2)}, calculating a first measurement length D;
• by using a formula:
• F=(D−S)/S, calculating a first flatness index F, wherein S is a horizontal distance between 1st first inspecting point and N-th first inspecting point along the first straight line;
• and
• scrapping the substrate having the first flatness index F larger than a first threshold, determining that the substrate is unqualified.