Development of x-ray mask fabrication process using w-sputtering

Abstract

Properties of DC magnetron sputter-deposited W and WNx absorber films were investigated for x-ray mask applications. Low stress film (5 x 10⁸ dyne/cm² tensile stress) is difficult to obtain with pure Ar gas as the film stress changes from highly compressive to highly tensile with pressure change. The variation of stress with pressure is significantly reduced with N₂/Ar mixture gas, and a reasonable tensile stress and stress stability were obtained with 5% N₂ at 3.5 mTorr. Film density decreases with increasing sputtering pressure and N₂/Ar + N₂ ratio. XRD patterns for films deposited at 3.5 mTorr show crystalline α-W structure for Ar sputtered film but amorphous structure for 5% N₂-sputtered film. Surface smoothness is very good at 5% N₂ but further increase of N₂/Ar+N₂ ratio results in a surface roughening and this is also confirmed by TEM analysis. At this sputtering condition (5% N₂, 3.5 mTorr), film stress stability during air-exposure and annealing was also superior, suggesting a optimum process condition for W-based absorber films.