Calibration of system errors in lateral shearing interferometer for EUV-wavefront metrology

Abstract

A new systematic error calibration method in lateral shearing interferometry (LSI) is proposed for extreme ultraviolet lithography. This method is used to remove the most significant errors: geometric optical path difference (OPD) and detector tilt error. The difference fronts of 0th and +/- 1st order diffracted waves are used to reconstruct wavefront. The Zernike coefficients of the reconstructed wavefront are used to calculate the distance among different diffracted light converging points (d). The difference front of 0th and +1st order diffracted waves is mirrored and added to the difference front of 0th and -1st order diffracted waves. The sum is used to calculate detector tilt angle. The geometric OPD and detector-tilt induced systematic errors are removed based on the calculated d and detector tilt angle. Simulations show that the root-mean-square (RMS) value of the residual systematic error is smaller than 0.1nm. The proposed method can be used to accurately measure the aberration of EUV optics with large numerical aperture (NA 0.5) in LSI.