Debris-free, droplet laser plasma sources in the EUV and soft X-ray ranges

Abstract

Laser plasma short wavelength sources are finding increasing applications in science and advanced technologies. Our previous work with a 100 kHz water droplet system generated 13 nm and 116 nm line emission from a Li-like oxygen plasma produced by 10 ns duration, 10 Hz Nd:YAG laser pulses at /spl sim/10/sup 12/ W/cm/sup 2/. We now present a detailed quantitative study of this source, performed with a 100 Hz laser, that characterizes the radiation efficiency and the long-term operation. The results show that the droplet laser plasma source comes close to satisfying all the near-term needs of EUV lithography. In particular we demonstrate an overall conversion efficiency of laser light to 13 nm emission within the required spectral bandwidth in excess of 0.6% comparable to any other existing source at this wavelength. In addition we have performed an exhaustive examination of the long-term effects of plasma emissions on the reflectivity of multilayer mirrors, exposed to this source. This included detailed surface science studies of the multilayer mirrors after EUV radiation doses approaching those anticipated in a first generation exposure tool. This study has lead to the development of novel techniques that extend debris-free operation of this source by over an order of magnitude to close to that needed for long-term, continuous use. In a separate study we have developed an adaptation of the droplet source for the generation of soft X-rays. Plasmas created from these targets by laser intensities range will provide a debris-free source across a rich range of wavelengths. Moreover, the use of a mass-limited target permits a detailed quantitative understanding of the radiation and kinetic energetics of these plasmas to be made. We will report X-ray spectroscopic data, and discuss new applications of these new debris-free X-ray sources.