Rapid quantitative phase imaging using phase retrieval for optical metrology of phase-shifting masks

摘要

The accuracy of a wave front generated by a lithography phase-shifting mask is essential for the performance of a lithography system. The main task of mask inspection is therefore to detect and quantify phase distortions caused by defects on the mask. There are three different classical ways to get the phase information generated by an object: First, interferometric techniques such as Linnik- or Mach-Zehnder-interferometry, second the method of spatial filtering, as is done in quantitative Zernike phase contrast microscopy, and third the use of phase-retrieval. The first two methods need a highly adapted set-up and are difficult to adjust. Phase retrieval, however, just needs a set of intensity images captured under different focus conditions to reconstruct the phase information. Therefore, this method is ideally suited for fast inspection in an industrial environment. We present phase measurements on MoSiN phase masks reconstructed on the basis of the transport of intensity equation. The effect of low-pass filtering inherent in this method is quantitatively investigated by numerical simulations done with our microscope simulation tool MicroSim. By systematic variation of imaging conditions as well as object-parameters we give insight to the applicability and the limits of the method. Comparison to Mach-Zehnder-interferometry allows the evaluation of the method also for partially coherent illumination conditions.