Lithographic technology is one of the core technologies for large scale integrated circuit manufacturing. The projection lens is the core component of the lithographic system, and its imaging quality determines the lithographic resolution and critical dimension. Wavefront aberration is an important parameter to evaluate the imaging quality of lithographic projection lens. A high-precision wavefront aberration measurement device is necessary for the installation and adjustment of lithographic projection lens. Phase extraction is an important step in wavefront aberration measurement using double-grating Ronchi phase-shift lateral shearing interferometry, which directly affects the final measurement accuracy. There is parasitic interference of multi-level high diffraction orders in the double-grating Ronchi lateral shearing interference field. The traditional phase extraction algorithms cannot eliminate the impact of high diffraction orders, which seriously reduces the accuracy of phase extraction. The high-precision shear phase extraction algorithm can improve the accuracy of wavefront aberration measurement of projection lens. Eliminating the impact of high diffraction orders is very important to improve the measurement accuracy of Ronchi lateral shearing interferometry. In this paper, a high-precision shear phase extraction algorithm is proposed based on the double-grating Ronchi lateral shearing interferometry.

In this paper, based on the double-grating Ronchi lateral shearing interferometry for the projection lens wavefront aberration measurement technology and system, the shear phase extraction algorithm is studied. The shear phase between +1 and -1 diffraction orders is calculated directly through the double-grating Ronchi shearing interferograms, and the impact of all high diffraction orders in the double-grating Ronchi shearing interference field is eliminated to improve the measurement accuracy of Ronchi shearing interferometry. In this paper, the phase extraction error of the Ronchi lateral shearing interferometry is simulated firstly, and then the sensitivity of the error of the grating period, the accuracy of the positioning stage, and the vibration error are simulated and analyzed. A double-grating Ronchi lateral shearing interferometry system with a shear ratio of 0.058 is used to carry out the verification experiments, and the measurement results are compared with the dual-fiber point diffraction interferometer, which further verifies the effectiveness of the shear phase extraction algorithm.

In order to verify the effectiveness of the proposed high-precision shear phase extraction algorithm, simulation is carried out for the system with a shear ratio of 0.048. In the simulation, the Z₄ Zernike polynomial aberration with a coefficient of 0.1 is used as the nominal wavefront to be measured. The 10-step and 13-step phase extraction algorithms proposed by Wu et al. and the proposed high-precision shear phase extraction algorithm are simulated, respectively. It can be seen that there is obvious impact of high diffraction orders in the phase extraction error of the 10-step and 13-step phase extraction algorithms [Fig. 6(b), (d)]. The phase extraction error of the proposed high-precision shear phase extraction algorithm is in the order of 10^-16 when 31 steps of phase-shift are used [Fig. 6(f)], and only high-frequency residuals exist in the results, indicating that the proposed high-precision shear phase extraction algorithm can effectively eliminate the impact of high diffraction orders. Simulation results show that with the relative error of the grating period less than 1% (Fig. 7), the introduced RMS shear phase extraction relative error increases as the shear ratio decreases; the accuracy of the positioning stage is better than 0.1% (Fig. 8), and the lower the positioning accuracy of the stage, the greater the relative error of root mean square (RMS) shear phase extraction; the normalized vibration frequency is greater than 5 (Fig. 9), and the larger the ratio of the vibration frequency to the natural frequency of the mechanical structure of the experimental platform, the smaller the relative error of RMS shear phase extraction. The experiment is carried out on double-grating Ronchi shearing interferometry system with a shear ratio of 0.058, and compared with the measurement result of dual-fiber point diffraction interferometer, both of the results have the same wavefront aberration distribution, which further verifies the effectiveness of the shear phase extraction algorithm (Fig. 14, Fig. 16).

Based on the high diffraction orders model contained in the overlapping area between ±1st orders of the double-grating Ronchi lateral shearing interferometry, the shear phase extraction algorithm is studied, and a high-precision shear phase extraction algorithm is proposed. This algorithm can eliminate all high diffraction orders in the overlapping area between ±1st orders, thereby improving the accuracy of the shear phase extraction. The proposed algorithm is compared with the current 10-step and 13-step Ronchi shearing interferometry phase extraction algorithms. The simulation results show that the error of the proposed algorithm is in the order of 10^-16, and only high-frequency residuals exist in the shear phase extraction errors and are almost negligible. The error simulation and analysis of the proposed shear phase extraction algorithm show that the relative error of the grating period needs to be less than 1%, the positioning accuracy of the stage needs to be better than 0.1%, and the normalized vibration frequency needs to be greater than 5. The proposed algorithm is verified by experiments and compared with the measurement result of dual-fiber point diffraction interferometer. The measurement results of the two methods have the same wavefront aberration distribution, which further verifies the effectiveness of the proposed high-precision shear phase extraction algorithm.