In situ aberration measurement method using a phase-shift ring mask

An in situ aberration measurement method using a phase-shift ring mask is proposed for lithographic projection lenses. Two dimensional (2D) phase-shift rings are designed as the measurement mask. A linear model between the aerial image intensity distribution and the aberrations is built by principal component analysis and multivariate linear regression analyses. Compared with the AMAI-PCA method, in which a binary mask and through-focus aerial images at are used for aberration extraction, the aerial images of the phase-shift ring mask contain more useful information. This provides the possibility to eliminate the crosstalk between different kinds of aberrations. Therefore, the accuracy of aberration measurement is improved. Simulations with the lithography simulator Dr. LiTHO showed that the accuracy is improved by 15% and 5 more Zernike aberrations can be measured compared with AMAI-PCA. Moreover, the speed of aberration measurement is improved because less aerial images are required using the new 2D mask.