Reduction in x-ray mask distortion using amorphous WNx absorber stress compensated with ion bombardment

Abstract

In order to reduce distortion and to obtain good pattern‐placement accuracy in x‐ray masks, a stress‐compensation technique in x‐ray absorber layers has been developed using ion bombardment. The absorber material selected is amorphous WN_x (x=0.1) deposited with rf sputtering because its compressive stress of 10¹⁰ dyn/cm² can be completely compensated with the present technique, although it is effective in all the metals attempted with both compressive and tensile stresses. It is shown that ion bombardment at an elevated temperature enables precise stress control together with excellent thermal stability in stress. In‐plane distortion caused by opening of 3×3‐mm² patterns in the WN_x absorber was measured in masks with 2‐μm‐thick SiN_x supporting membranes. The 0.4‐μm‐thick WN_x layers used were stress compensated by bombardment with 400‐keV N⁺ ions at 300 °C to a dose of 4 to 6×10¹⁴ ions/cm². The distortion has been reduced to less than 5×10⁻⁶ relative to the pattern size. The mechanism of the stress compensation is also discussed.