Mo/Si multilayer films exhibit the highest measured reflectivity in the extreme ultraviolet (EUV) region, and their combination with an EUV light source enables EUV lithography. In practical applications of EUV light sources, Mo/Si multilayer mirrors are always curved and have large diameters. The angle of the incident light constantly changes along the curved surface. To match the multilayer peak reflectivity with the angle of the incident light, the period thickness of the Mo/Si multilayer films must be distributed in a transverse gradient along the surface to ensure high EUV reflectivity. Simultaneously, given that the mirror is close to the light source, the multilayer films must operate in an environment with a high thermal load. Higher temperatures can increase the formation of silicide at the multilayer interface, causing the optical performance to decrease. Therefore, Mo/Si multilayer films for EUV light sources also have high thermal stability requirements. To address these issues, we use a shadow mask to correct the periodic thicknesses of the multilayer films at different positions on a curved substrate with a diameter of 300 mm. Carbon is selected as the diffusion barrier material to investigate the influence of C-barrier layer on the thermal stability of Mo/Si multilayer.

In this study, two sets of Mo/Si multilayer films are deposited via direct current(DC) magnetron sputtering onto super-polished silicon wafers, and the thickness control and thermal stability of the Mo/Si multilayer films are investigated separately. For the study of thickness control, the target period thickness is from 6.96 nm to 7.31 nm, and the ratio of Mo layer thickness to period thickness is approximately 0.40. As the multilayer thickness on a large curved substrate cannot be measured directly, we prepare a substituted substrate to estimate the multilayer thickness at selected points on the surface. The shadow mask technique is used to adjust the periodic thickness of the multilayer films at different positions on the entire mirror. C is selected as the barrier material for the thermal stability study. Mo/Si, Mo/Si/C, and Mo/C/Si/C multilayer films are annealed at 300 ℃ for 2 h. By observing the X-ray reflectivity (XRR) and EUV reflectivity before and after annealing, the effect of the C barrier layer on the thermal stability of Mo/Si multilayer films is investigated.

In the study involving film thickness control, the XRR measurement results show that the samples at different positions exhibit similar layer structures (Fig.4). Atom force microscope (AFM) tests are performed on Mo/Si multilayer films deposited at four different positions on the entire mirror. The surface roughness values of the four samples are 0.128, 0.123, 0.124, and 0.118 nm. The morphologies of the four samples are similar (Fig.5). Using the shadow mask, the deviation of the period thickness on the 300-mm diameter curved substrate is controlled within ±0.45% of the expected period thickness (Fig.6).

In the study involving thermal stability, after annealing at 300 ℃ for 2h, the period thickness of Mo/Si multilayer films changes from 6.99 nm to 6.69 nm, the period thickness of Mo/Si/C multilayer films changes from 6.96 nm to 6.91 nm, and the period thickness of Mo/C/Si/C multilayer films is almost same before and after the annealing, which changes from 6.97 nm to 7.00 nm. The C barrier layer can effectively mitigate the interdiffusion at the interface of the Mo and Si layers, which improves the thermal stability of the multilayer films (Fig.7). The EUV reflectivity of Mo/Si multilayer films decreases from 64.4% to 55.4% after annealing at 300 ℃ for 2 h, and the central wavelength has a shift of 0.51 nm. The EUV reflectivity of the Mo/Si/C multilayer films decreases from 66.4% to 59.6% after annealing, and the center wavelength shifts by 0.11 nm. The reflectivity of Mo/C/Si/C multilayer films decreases by 1.8% after annealing, and central wavelength shifts by 0.02 nm (Fig.8 and Table 2). Furthermore, the EUV reflectivity results show that the C barrier inserted at both interfaces of the Mo/Si multilayer films can significantly improve thermal stability.

Initially, graded Mo/Si multilayer films are deposited on a large-diameter curved substrate using a shadow mask. Compared with the designed period thickness, the deviation in the period thickness at different positions on the entire substrate is controlled within ±0.45%. The layer structure and surface roughness of the Mo/Si multilayer films are almost identical at different positions. This study provides useful guidance for the fabrication of large curved multilayer mirrors for EUV light sources. Next, the thermal stability of the Mo/Si multilayer films is investigated after inserting a C-barrier layer. The results show that the thermal stability of the Mo/C/Si/C multilayer films is optimal, and that of the Mo/Si multilayer films is the worst. The Mo/C/Si/C multilayer films exhibit only 1.8% reflectivity loss after annealing at 300 ℃ for 2 h, and the center wavelength and bandwidth do not change. The higher reflectivity and multistability of Mo/Si multilayer films for EUV sources are currently under investigation.