Plasma generation and characterization in the extreme ultraviolet spectral range

摘要

At Laser-Laboratorium Goettingen different types of laser-plasma EUV sources based on gas and cluster targets were tested to optimize the spatially resolved EUV radiation with respect to maximum EUV intensities, small source diameters, and pointing stability. The EUV radiation is generated by focusing a Q-switched Nd:YAG laser at 1064nm into a pulsed gas puff target. By the use of different target gases, broad-band as well as narrow-band EUV radiation is obtained, respectively. The influence of the laser and target gas parameters on the plasma shape and EUV intensity was investigated by the help of specially designed EUV pinhole cameras, utilizing evaluation algorithms developed for standardized laser beam characterization. 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. The results were compared with fluorescence and Rayleigh measurements for visualization of the target gas jet. To explain these results a theoretical model was developed, including the reabsorption of the EUV radiation in the surrounding target gas. In addition, an EUV-sensitive Hartmann sensor was utilized to characterize the wavefront of 13nm radiation before and after reflection from Mo/Si multilayer mirrors.