Abstract

At the present time, it appears that the most likely source to be used for soft x-ray projection lithography is a high repetitionrate (about 1200 Hz) laser plasma source,¹ The design of this kind of sources is usually envisaged as comprising a compact high repetition-rate laser and a renewable target system capable of operating for prolonged periods of time. Current cost scenarios of the complete irradiation system suggest that the unit shot cost must be in the vicinity of $10~^e per shot.² These stringent requirements press the boundaries of current technologies. Moreover, recent studies of the particulate matter ablated from solid and tape targets under laser irradiation conditions similar to those deemed optimum for 13 ran soft x-ray generation, suggest that these types of targets will be unsuitable.³ The levels of neutral clusters of particulate matter and of high velocity ions are many orders of magnitude above those that can be tolerated in an environment requiring the long-term preservation of expensive, high-reflecting multilayer collecting optics. We introduce a new concept of a high repetition-rate soft x-ray laser plasma source for projection lithography , that is, the mass- limited cryogenic target, A mass-limited cryogenic gas target that emits strong line emission as a laser plasma target of a high repetition rate x-ray source for a production line soft x-ray lithography system will circumvent the principal problems associated with current configurations.⁴ It will remove the primary threat to expensive state-of- the-art high reflectivity x-ray mirrors used to focus the soft x-ray emission onto the reflective mask. A limited mass cryogenic target system could also, in principle, function continuously at a cost commensurate with the required unit shots costs.

© 1994 Optical Society of America