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

高功率深紫外激光器有望成为光刻、激光微加工和激光聚变的光源。高纯度、深紫外级氟化钙（CaF_2）单晶材料是此类光学器件的唯一候选材料。尽管CaF_2晶体由于其柔软性和较高的热膨胀系数而难以抛光，但对CaF_2晶体表面的要求是具有高精度的图形和超光滑的表面。为了提高激光诱导损伤阈值，抛光表面必须非常光滑，并且没有由精加工引起的亚表面损伤。在这项研究中，高纯度氟化钙（111）单晶已浮动抛光，以获得最高质量的表面。浮法抛光是一种获得最光滑表面而不引入亚表面损伤的方法。用Nomarski微分干涉对比显微镜、两个光学轮廓仪、扫描探针显微镜和两个透射电子显微镜观察完成的表面。从本研究项目的结果中可以得出以下结论.光学抛光后，经过80 min的浮动抛光，在90 mm直径的样品上获得了31.9 nm p-v和6.03 nm rms的平坦度.高分辨透射电子显微镜、扫描探针显微镜和Nomarski微分干涉差显微镜观察表明，浮法抛光CaF_2单晶样品中没有亚表面损伤.浮法抛光表面由具有小原子台阶的完美（111）晶格形成，因此抛光表面粗糙度取决于样品表面与（111）面之间的失配。通过浮动抛光工艺获得0.059 nm Ra、0.077 nm rms和0.72 nm Ry的表面粗糙度，如使用SPM在1 x 1 □m面积上测量的。

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.