Deep-UV liquid immersion mastering of high density optical discs

A water immersion technique has been developed to increase the numerical aperture (NA) of a 257 nm optical disc mastering system from 0.9 to 1.2. The developed system is capable of performing point exposures on a rotating disc at scanning velocities of more than 5 m/s. Gas inclusion in the immersion liquid, either from the photo-resist or through the dynamic movement of the substrate could be avoided. Furthermore, unwanted mechanical disturbances were sufficiently suppressed. Contamination of the photo-resist was avoided through careful handling of the water that was applied as immersion liquid. The resulting resolution enhancement was demonstrated in grooves. In 25 nm Novolac type resist, 70 nm wide grooves were written with liquid immersion, compared to 100 nm wide grooves at NA = 0.9. Liquid immersion was also successfully applied to write the master substrate from which a ROM disc can be made through injection molding. ROM discs for the recently launched Blu-ray Disc system have been successfully written with a density corresponding to 25 GB in a single layer of a 12 cm disc.