With the continuous advancement of integrated circuit manufacturing technology and the reduction of device critical dimensions to nanoscale levels, high-precision overlay measurement has become a core challenge in photolithography technology. As the overlay label size gradually reduces and complex processing techniques are introduced, the aperture shape in angle-resolved scatterometer used for diffraction-based overlay (DBO) evolves from traditional annular apertures to BMW apertures and then to arch apertures, thus adapting to the constantly changing measurement needs. An optical characteristic model is constructed based on the principle of the angle-resolved scatterometer. On this basis, a simulation method for diffraction signals with different aperture shapes is proposed. The characteristics of the annular aperture, BMW aperture, and arch aperture are analyzed through theoretical analysis and simulation experiments. The performance parameter curves of overlay labels with different structural characteristics under different aperture conditions are calculated and compared. The applicable scenarios of each aperture are summarized, which provides a reference for aperture selection under various overlay label structures.

To analyze and compare the characteristics of different apertures, we propose a diffraction signal simulation method. Specifically, the shape of the converging light spot corresponding to different apertures is calculated through the Fourier transform. By sampling the pupil, we calculate the relationship between the wave vector components of the diffraction signal generated by incident light illuminating the label at various positions along the x, y, and z directions. This allows for the diffraction signal distribution at the back focal plane of the objective lens to be obtained. By combining rigorous coupled-wave analysis (RCWA), the intensity of diffraction signals at specific positions can be further calculated to determine the overlay performance parameters under different measurement conditions. The flowchart for diffraction signal distribution calculation is shown in Fig. 2. Using the proposed simulation method, the converging spot and diffraction signal distribution are simulated for different apertures, and the characteristics of each aperture are analyzed. The overlay performance parameters of conventional stacked overlay labels (Fig. 8) and multi-stacked overlay labels (Fig. 13) are simulated and analyzed under different aperture shapes and incident light parameters. Suitable scenarios for each aperture shape are provided based on the simulation results.

The simulated converging spots and diffraction signal distributions corresponding to different apertures are shown in Figs. 5 to 7. The simulation results reveal that there is an overlap between the 0th-order and ±1st-order diffracted light signals of the annular aperture. The BMW aperture can achieve the separation of the 0th-order and ±1st-order diffraction signals. The arch aperture similarly offers the advantage of separating the 0th-order and ±1st-order diffraction signals, while the transparent area is the area with good uniformity of the diffraction signal in the BMW aperture. The curves of the performance parameters of conventional stacked overlay labels as a function of the incident light wavelength are shown in Figs. 10 to 12. The curves corresponding to the annular and BMW apertures are similar, with a maximum value higher than that of the arch aperture. However, there is an overlap between the 0th-order and ±1st-order diffraction signals of the annular aperture, which is not conducive to signal processing. It is recommended that the BMW aperture be prioritized for the overlay measurement of conventional stacked overlay labels. The curves of the performance parameters of multi-stacked overlay labels concerning the wavelength of incident light are shown in Figs. 14 to 16. Under most measurement conditions, the peak value of the curve of the arch aperture is higher than that of the other two types of apertures. It is recommended that the arch aperture be prioritized to achieve better measurement robustness and signal-to-noise ratio.

Based on the principle of the angle-resolved scatterometer, we propose a diffraction signal simulation method for different apertures. Using this method, the characteristics of the annular aperture, BMW aperture, and arch aperture are analyzed. Through practical simulation cases, suitable scenarios for each aperture type are presented. The research concludes that for conventional stacked overlay labels, the performance parameter curves corresponding to annular and BMW apertures are similar, with a maximum value higher than that of the arch aperture. Considering the diffraction signal characteristics of apertures, it is recommended to prioritize using BMW apertures for overlay measurements. For multi-stacked overlay labels in the preparation of 3D devices, it is recommended to prioritize using arch aperture to achieve better measurement results. The simulation method and analysis results provide theoretical support and application references for the analysis and selection of aperture characteristics in angle-resolved scatterometer.