Advances in the reduction and compensation of film stress in high-reflectance multilayer coatings for extreme-ultraviolet lithography

Abstract

Due to the stringent surface figure requirements for the multilayer-coated optics in an extreme UV (EUV) projection lithography system, it is desirable to minimize deformation due to the multilayer film stress. However, the stress must be reduced or compensated without reducing EUV reflectivity, since the reflectivity has a strong impact on the throughput of a EUV lithography tool. In this work we identify and evaluate several leading techniques for stress reduction and compensation as applied to Mo/Si and Mo/Be multilayer films. The measured film stress for Mo/Si films with EUV reflectances near 67.4 percent nm is approximately -420 MPa, while it is approximately +330 MPa for Mo/Be films with EUV reflectances near 69.4 percent at 11.4 nm. Varying the Mo-to-Si ratio can be used to reduce the stress to near zero levels, but at a large loss in EUV reflectance. The technique of varying the base pressure yielded a 10 percent decrease in stress with a 2 percent decrease in reflectance for our multilayers. Post-deposition annealing was performed and it was observed that while the cost in reflectance is relatively high to bring the stress to near zero levels, the stress can be reduced by 75 percent with only a 1.3 percent drop in reflectivity at annealing temperatures near 200 degrees C. A study of annealing during Mo/Si deposition was also performed; however, no practical advantage was observed by heating during deposition. A new non-thermal buffer-layer technique was developed to compensate for the effects of stress. Using this technique with amorphous silicon and Mo/Be buffer-layers it was possible to obtain Mo/Be and Mo/Si multilayer films with near zero net film stress and less than a 1 percent loss in reflectivity. For example a Mo/Be film with 68.7 percent reflectivity at 11.4 nm and a Mo/Si film with 66.5 percent reflectivity at 13.3 nm were produced with net stress values less than 30 MPa.