Stress Changes and Stability of Sputter-Deposited Mo/B4C Multilayer Films for Extreme Ultraviolet Mirrors

Abstract

Periodic Mo/B4C multilayer films (MLs) for extreme ultraviolet mirrors were deposited by an rf-enhanced plasma magnetron sputtering system with changing the sputtering Ar pressure and layer thickness ratio. Internal stresses in the entire film were measured by Newton's rings method. Microscopic internal stresses in Mo crystallites in Mo/B4C MLs and in Mo single layer film were also determined by wide-angle X-ray diffraction from which lattice spacing in Mo (110) planes was measured. The internal stresses changed from a compressive value of about 1.1 GPa to a tensile value of about 0.6 GPa with increasing Ar pressure from 1.3 to 6.0 mTorr. The transition point of changing the stress from compressive to tensile shifted to lower Ar pressure with increasing the Mo layer thickness ratio. From atomic force microscope observation, the surface roughness of Mo/B4C multilayer films was found to increase monotonically from 0.11 to 0.23 nm (rms) with increasing Ar pressure. We examined the effect of the storage conditions of the films under vacuum, in low-humidity air and in air saturated with water vapor. Deterioration of film quality in Mo/B4C MLs was observed within a few weeks in the samples stored in open air with high humidity.