Materials Processing with Focused EUV/Soft-X-ray Pulses

This section discusses the focusing of EUV/soft x-ray pulses obtained by HHG with an elliptical focusing mirror. The setup for focusing HH pulses is shown in Fig. 15B.1. The HH pulses obtained by irradiating a rare gas with a femtosecond laser are separated from the fundamental laser pulse by a metal filter and are incident on an ellipsoidal focusing mirror. The pulses are focused on the sample material by using one side of a cylindrical ellipsoidal mirror. A knife-edge scanner is used to measure the focal spot size of the beam, and the focal position is determined. Figure 15B.2 shows the measured focusing profile as a function of the beam propagation direction. The size at the condensing point is 1.48 μm (horizontal)x0.79 mm (vertical), and the Rayleigh length is very short, about 10 μm, so it is necessary to precisely align the sample to perform processing at the focal point. The surface of the ellipsoidal mirror is made of nickel and is very precisely polished. The incident angle of the mirror in this experiment is 70 mrad, and the working distance is 20 mm. It should be noted that the FEL beam can also be focused in this way. The mirror parameters are almost the same for the FEL; the beam spot on the mirror surface becomes very large due to the grazing-incidence optical system, and even for a FEL with high intensity, the fluence on the mirror surface is low enough to be comparable to the damage threshold fluence of the metals, as mentioned above. Also, it can be expected that the damage threshold fluence of the mirror will be further improved because the energy deposition to the surface is reduced by reflecting the incident beam.