Nanostructure analysis and laser damage mechanism of ultra-precision polished surface of calcium fluoride single crystals for high-power DUV applications.

点击翻译按钮获取中文摘要

High power, Deep Ultra Violet (DUV) lasers are expected to be light sources for optical lithography, laser microprocessing, and laser fusion. High purity, DUV-grade calcium fluoride (CaF_2) single crystal material is the only candidate for such optics. The requirements for the CaF_2 surface are to have a high precision figure and an ultra-smooth surface, even though the CaF_2 crystal is difficult to polish because it is soft and has a relatively high thermal expansion coefficient. In order to increase the laser induced damage threshold, the polished surface must be extremely smooth and without subsurface damage caused by finishing. In this research, high purity calcium fluoride (111) single crystals have been float polished in order to obtain the highest quality surfaces. Float polishing is a means of obtaining the smoothest surfaces without introducing subsurface damage. The finished surfaces were observed with a Nomarski differential interference contrast microscope, two optical profilers, a scanning probe microscope and two transmission electron microscopes. The following conclusions can be drawn from the results of this research project.1. A flatness of 31.9 nm p -v and 6.03 nm rms were obtained on 90-mm-diameter area samples by 80 min of float polishing after optical polishing.2. It is clear from high resolution transmission electron microscope images, scanning probe microscope and Nomarski differential interference contrast microscope observations that there is no subsurface damage in the float-polished CaF_2 single crystal sample.3. The float-polished surface is formed from a perfect (111) lattice with small atomic steps, so that the polished surface roughness depends on the mismatch between the sample surface and the (111) planes.4. A surface roughness of 0.059 nm Ra, 0.077 nm rms, and 0.72 nm Ry was obtained by the float polishing process, as measured on a 1 x 1 □m area with an SPM.