X-Ray Mask Technology Utilizing an Optical Stepper

An X-ray mask with half of a 64-Mbit dRAM chip whose minimum feature size was 0.3 µm wide was developed using a W absorber on a SiN membrane. To precisely control the W film stress, Ar+ ions were implanted into the W film prepared by chemical vapor deposition (CVD). To evaluate mask-to-mask overlay accuracy except that due to electron-beam-writing error, pattern placement measurement marks were delineated by optical exposure. It was found that the overlay accuracy between two masks with different pattern densities could be 0.08 µm (3σ) which is very nearly the limit of measurement accuracy.