Fabrication of x-ray masks for giga-bit DRAM by using a SiC membrane and W-Ti absorber

Advanced x-ray masks for giga-bit DRAM require an optimization of the mask fabrication process and a stress-free absorber. In this article, we will describe the performance of the x-ray masks fabricated by a membrane process where back-etch is performed prior to electron beam writing. With other techniques such as stress control by step-annealing and electron beam (EB) multiple writing, mask-to-mask overlay was reduced to less than 40 nm. In spite of multiple EB writing, the pattern size accuracies of 0.14 mu m critical dimension were about 10% and 20% for the line and two-dimensional patterns, respectively. For further improvement in the uniformity of absorber stress and resultant higher placement accuracy, a space variant annealing method is proposed, where the nonuniformity of the absorber stress is corrected by spatially variant temperature distribution during the annealing. Under this method, the uniformity of the amorphous W-Ti absorber improved to about 10 MPa in a 50 mm diam region. (C) 1996 American Vacuum Society.