Error compensation of a heterodyne grating interferometer based on redundancy characteristics

We address the issue where the measurement accuracy of the grating interferometer system in the lithography machine wafer stage is difficult to achieve the precision of theoretical models due to various interferences. A stepwise compensation method for the grating interferometer is studied, and a compensation method for the readhead displacement inconsistency error is proposed using the redundant measurement axes of the grating interferometer. On this basis, further compensation methods for self-gain and nonorthogonal errors are introduced. Through comparative experiments between laser and grating interferometers, the maximum X- and Y-axis errors were reduced from 30 to 1 μm after compensating for the inconsistency error, improving the measurement accuracy by an order of magnitude. Self-gain and nonorthogonal error compensation further improved the X- and Y-axis measurement accuracy from ±15 μm to 10 nm. We provide guidance and experimental validation for establishing an error compensation system for wafer stage grating interferometers.