Position measurement method and device

A position measurement method, configured to measure a position of a stage, wherein the stage comprises a first mirror and a second mirror, a first laser interferometer and a second laser interferometer face the first mirror along a first axis, a third laser interferometer and a fourth laser interferometer face the second mirror along a second axis perpendicular to the first axis, and the position measurement method comprising:

• performing a calibration operation, comprising:
• moving the stage along the second axis;
• measuring, by the first laser interferometer and the second laser interferometer, N sets of first values corresponding to N consecutive sections of the first mirror along the second axis as the stage moves, wherein an Mth set of first values comprises a first distance between the first mirror and the first laser interferometer and a second distance between the first mirror and the second laser interferometer;
• measuring, by the third laser interferometer and the fourth laser interferometer, N sets of second values as the N sets of first values are measured, wherein an Mth set of second values comprises a third distance between a second mirror and the third laser interferometer and a fourth distance between the second mirror and the fourth laser interferometer;
• obtaining a first flatness profile of the first mirror along the second axis according to the N sets of first values and the N sets of the second values, wherein N and M are a positive integer, and M is less or equal to N;
• and
• generating a mapping table according to the first flatness profile;
• and
• measuring and calibrating a position of the stage according to the mapping table so as to control movement of the stage during an inspection operation.