Abstract

It is now generally recognized that the source of preference for soft x-ray projection lithography is a high repetition-rate (1200 Hz) laser plasma source¹. The design of this source is usually envisaged as comprising a compact high repetition-rate laser, preferably a solid-state laser, diode-pumped, and a renewable target system capable of operating for prolonged periods of time. For a production-line facility, this would imply uninterrupted shot cycles of > 3 x 10⁶ shots. In addition, current cost scenarios of the complete irradiation system suggest that the unit shot cost must be in the vicinity of $ 10^-6 per shot². 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 nm soft x-ray generation, suggest that these types of targets will be unsuitable³. 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. It is thus our viewpoint that considerable progress must be made in the design of efficient soft x-ray-emitting laser plasmas for them to satisfy the needs of projection lithography.

© 1993 Optical Society of America