Use of ion implantation to eliminate stress-induced distortion in x-ray masks

Abstract

One of the most important questions with regard to using x-ray lithography for integrated circuit manufacturing is whether distortion in the x-ray mask, caused by intrinsic stress in the absorber material, can be made sufficiently small for all types of patterns. Pattern features should deviate from their assigned in-plane positions by no more than a fraction of the minimum linewidth. Tungsten (W) is a more desirable absorber than gold because of its thermal stability and its closer match to the thermal expansion coefficient of Si. However, it is difficult to reproducibly deposit W films with zero stress. We describe a compensation technique for eliminating distortion, regardless of the initial absorber stress. Experiments were done on 0.3–0.4-μm-thick W patterned on 1-μm-thick Si and Si3N4 membranes, suitable for x-ray masks. Distortions caused by tensile stresses ∼109 dyns/cm2 can be eliminated by Si implantation at doses ∼1016 cm−2. Measurements of deflections at an edge were made using a Linnik interferometer, and from this the stress was calculated. Si ions were implanted to a projected range of 10 nm in the W at doses in the range 1015–1016 cm−2. Distortion correction takes place because the implantation produces compressive stress near the upper surface, resulting in a torque that balances a torque of opposite sign at the absorber–membrane interface due to tensile stress in the W. We present a theory for stress compensation together with an empirical prescription for eliminating stress-induced distortion. We also report crystallographic analysis of the W and describe a method for getting α-phase W, in either tensile or compressive stress, which appears to be highly stable.