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 i t desirable to minimize deformation due to the multilayer structure stress. However, the stress must be reduced or compensated without reducing the EU V reflectivity. since the reflectivity has a strong impact on the throughpu t of the EUV lithography tool. In this work. we evaluate techniques for str ess reduction and compensation as applied to multilayer Mo/Si structures. T he experimental research of multilayer Mo/Si-structures by the methods of X -ray reflectometry and electron microscopy has shown that reflectivity of M o/Si-structures largely depends on the parameter gamma = d(Mo)/d and in the spectral region or 13-13.7 nm reaches the maximum value at gamma approxima te to 0.42-0.44. The measured film stress for Mo/Si films with EUV reflecta nces near 66-69% at 13.3 nm is approximately -250 MPa (compressive). Variat ion of the Mo-to-Si ratio call be used to reduce the Stress to near-zero le vels but at a large loss in EUV reflectance (approximate to 54-57%). Techno logically it is easiest to use for a buffer layer a multilayer structure ma de of the same materials (Mo and Si) as the top, reflecting, structure, but with a different parameter gamma approximate to 0.8. The stress in the str uctures containing antistress buffer sublayers was sigma = -6 MPa. The refl ectivity values for these structures are quite close, and still the reflect ion coefficient or the Mo/Si structure deposited on a substrate with an ant istress Mo/Si buffer coating was smaller by 0.5 0.7% in absolute values sma ller. (C) 2001 Elsevier Science B.V. All rights reserved.