Spatial emission characteristics of EUV plasma sources

摘要

The successful implementation of EUV lithography systems strongly relies both on the efficiency of the employed optical components and the precise control of the relevant source parameters. Metrology tools for comprehensive characterization of EUV radiation and the related optics are developed at Laser-Laboratorium Gottingen, utilizing a laser-based source for the generation of 13nm radiation. The EUV plasma is produced by focusing a Nd:YAG laser into a pulsed xenon or oxygen gas jet. The alternate use of these two target gases accomplishes either an intense broad-band (Xe) or a less intense narrow-band line emission (O_2) at 13nm. Different types of nozzles were tested in order to optimize the emitted radiation with respect to maximum EUV intensities, small source dimensions, pulse-to-pulse fluctuations and positional stabilities. The investigation of these crucial source parameters was performed with a specially designed EUV pinhole camera, utilizing evaluation algorithms developed for standardized laser beam characterization (cf. Fig. 1). In addition, a rotatable pinhole camera was developed which allows spatially and angular resolved monitoring of the soft X-ray emission characteristics. With the help of this camera a strong angular dependence of the EUV intensity was found, indicating reabsorption of the EUV radiation in the surrounding gas. The results were compared with Rayleigh scattering measurements for visualization of the target gas density.