Stress reduction of Mo/Si multilayer structures

Due to the stringent surface figure requirements for the multilayer-coated optics in an extreme ultraviolet (EUV) projection lithography system make it desirable to minimize deformation due to the multilayer structure stress. However, the stress must be reduced or compensated without reducing the EUV reflectivity, since the reflectivity has a strong impact on the throughput of the EUV lithography tool. In this work, we evaluate techniques for stress reduction and compensation as applied to multilayer Mo/Si structures. The experimental research of multilayer Mo/Si-structures by the methods of X-ray reflectometry and electron microscopy has shown that reflectivity of Mo/Si-structures largely depends on the parameter γ=dMo/d and in the spectral region of 13–13.7 nm reaches the maximum value at γ≈0.42–0.44. The measured film stress for Mo/Si films with EUV reflectances near 66–69% at 13.3 nm is approximately −250 MPa (compressive). Variation of the Mo-to-Si ratio can be used to reduce the stress to near-zero levels but at a large loss in EUV reflectance (≈54–57%). Technologically it is easiest to use for a buffer layer a multilayer structure made of the same materials (Mo and Si) as the top, reflecting, structure, but with a different parameter γ≈0.8. The stress in the structures containing antistress buffer sublayers was σ=−6 MPa. The reflectivity values for these structures are quite close, and still the reflection coefficient of the Mo/Si structure deposited on a substrate with an antistress Mo/Si buffer coating was smaller by 0.5–0.7% in absolute values smaller.