Fabrication of distortion free x-ray masks using low stress tungsten

X-ray lithography is an emerging technology for future integrated circuit fabrication for linewidth of O.5J.1.mor below. There are two main criterion of applying x-ray lithography in production: x-ray source and mask technology. At this time, x-ray mask fabrication presents the greatest challenge. The two main elements of a x-ray mask are the membrane and absorber. Ideally a good x-ray mask should consist of high mechanical strength, high optical and high x-ray transmission membrane, together with a stress free absorber. This thesis will address the problems associated with the membrane and absorber technology for x-ray masks. Particularly, I will focus on the technology to deposit low stress tungsten that is compatible with minimum feature size down to O.lJ.1.mand below. The main difficulties in applying tungsten absorber are the high intrinsic stress in sputtered tungsten films, and the ability to control the stress down to 5xl07 dynes/cm2. This low stress film is required to minimize the distortion in a x-ray mask below one part per million. Methodologies used to deposit low stress tungsten films will be presented. Sputtered tungsten films were also characterized for their thermal and radiation stability. Measurement techniques and issues regarding the stress induced distortion (both in-plane distortion and out-of-plane distortion) were also discussed. An "in-situ tt stress measurement technique, by monitoring the resonant frequency of the membrane, was developed. Experimental results correlating the measured membrane frequency response and the final measured film stress will be presented. Finally a comparison of various membrane materials and membrane characterization techniques will also be presented and discussed. Thesis Supervisor: Henry I. Smith Title: Professor of Electrical Engineering and Computer Science