ATOMIC STRUCTURE OF FLOAT POLISHED SURFACES OF CALCIUM FLUORIDE SINGLE CRYSTALS FOR ULTRA VIOLET APPLICATIONS Naoyuki Ohnishi*, Yoshiharu Namba, Shinji Yoshida, Tomohiro Yoshida and Yasuhiro Kanda Chubu University, Kasugai, Aichi 487-8501, Japan
Kunio Yoshida Osaka Institute of Technology, Osaka 535-8585, Japan Abstract Calcium fluoride (111) single crystals for ultra violet applications have been finished to flat by the float polishing process with 7nm-diameter silicon dioxide powder, pure water and a tin lap. The flatness of 32nm P-V on 90-mm-diameter samples that had a surface roughness of 0.72 nm Ry, and 0.077 nm rms were obtained. High-resolution transmission electron microscopy study of the cross-sectioned crystal surface showed that the float-polished surface had (111) plane with small atomic steps, and no subsurface defect. The polished surface roughness depends on the mismatch between the sample surface and the (111) plane. This result was also verified by scanning probe microscope observation. Key Words: Calcium fluoride, Ultra-precision polishing, Atomic surface structure, Transmission electron microscopy 1. Introduction High purity calcium fluoride (CaF2) single crystal is the only candidate for materials of micro-lithography optics using excimer lasers of Deep Ultra Violet (DUV) and Vacuum Ultra Violet (VUV) region. The requirements for the CaF2 optical components surface are to have a high precision figure and an ultra-smooth surface, even though the CaF2 crystal is difficult to polish because it is soft and has a relatively high thermal expansion coefficient. The major problem of commercially polishing CaF2 crystals is producing surface sleeks or fine scratches which introduce subsurface damage and scattering. In order to increase the laser-induced damage threshold, the polished surface must be extremely smooth and without subsurface damage caused by finishing. Float polishing has been used to polish various single crystals and optical glasses to an ultra-smooth finish without a deformed subsurface layer. [1-3] From previous report [4], the smoothest surface of CaF2 single crystals is expected to be achieved by float polishing, even though the material is fairly soft. In this study, we performed the float polishing of CaF2 single crystals to obtain DUV lithography-grade surfaces, and an analysis of the finished surface structure mainly focused on detailed atomic structures of the polished surfaces by means of high-resolution transmission electron microscopy (TEM) . 2. Experimental Procedure High purity, DUV-grade CaF2 single crystals were sawed into 100-mm-diameter, 40-mm-thick discs and 10 mm x 10 mm x 5 mm rectangular plates. The large flat area of the samples was aligned parallel to the (111) plane that is used for optical applications. The samples were mounted on a stainless steel holder and precisely ground by the ultra-precision surface grinder using an SD3000-75-B wheel. The samples were float polished on a tin lap with 7-nm-diameter SiO2 powder in a water solution on the ultra-precision float polishing machine [3]. The polishing fluid was a mixture of pure water and 3 wt% SiO2 powder, and was controlled within a temperature range of 0.01 K. Polishing pressure was only applied by the weight of the sample and sample holder. The flatness of a 100-mm-diameter, float-polished CaF2 sample was measured with a phase measuring interferometer system. All machined or etched surfaces were observed under a Nomarski differential interference contrast microscope. Surface microtopography of polished samples was measured with a scanning probe microscope as well as with two three-dimensional optical surface profilers. In order to investigate the microstructure and nanostructure of both the surface and subsurface of the polished CaF2 crystals, two transmission electron microscopes (TEM) were used to observe cross sections of crystals. For the TEM specimen preparation, the float-polished crystal samples were sectioned oriented normal to the polished surface. The samples were then ground and polished with a water slurry of ~ 50-nm-diameter Al2O3 powder. They were finally ion polished with a 3 keV argon ion beam oriented at 15 deg to the sample surface. The TEM observations were made using either the microscope with a 200 kV accelerating *corresponding author:
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voltage or the other with a 400 kV accelerating voltage. The theoretical resolutions of the 200 kV and 400 kV accelerating voltages are 0.23 nm and 0.17 nm, respectively. 3. Results and Discussion Figure 1 shows the flatness of a float-polished 100-mm-diameter, 40-mm-thick CaF2 sample, as measured with a laser interferometer on a 90-mm-diameter area. After 80 min of float polishing, we obtained a flatness of 31.9 nm p-v and 6.03 nm rms. This is excellent flatness for such a high thermal expansion material. Figure 2 shows the surface roughness of a float-polished CaF2 sample measured with an optical profiler. The surface roughness values are 0.120 nm Ra, 1.25 nm Ry, and 0.151 nm rms in the measurement area of 250 x 250 µm2.
Figure 1: Surface flatness of a float-polished CaF2 sample measured on a 90-mm-diameter area.
Figure 2: Surface roughness of a float-polished surface as measured on a 250 x 250 µm area with an optical profiler.
Figures 3(a) and 3(b) are cross-sectional TEM images of CaF2 single crystal surfaces after the ultra-precision grinding and the float polishing, respectively. The images were obtained by using the 200 kV accelerating voltage TEM. In Figure 3(a), the image clearly shows subsurface damage due to the grinding process. The thickness of damaged layer observed in the TEM image is of about 200-300 nm. The image contrast also shows that the damaged area consists of a severely deformed upper layer with fine strain contrast, and an elastic strain field with broad contrast beneath the upper layer. This can be a typical example of subsurface damage structure of brittle ionic crystals induced by the machining. However, the thickness of damaged layer in this case is much smaller than those predicted for ordinary grinding processes. After float polishing process following the ultra-precision grinding, the subsurface damage was completely removed as shown in Figure 2(b). In this image, none of strain field contrast is observed, indicating that the float-polished CaF2 has dislocation-free subsurface structure. This is consistent with the result obtained by Nomarski differential interference contrast microscope observations of surface etch-pits.
Figure 3: Low magnification cross-sectional TEM images of CaF2 single crystal samples after ultra-precision grinding (a), and float polishing (b). Figures 4(a) and 4(b) show high-magnification TEM images of a float-polished CaF2 crystal surface obtained with the 400 kV accelerating voltage TEM. Although the float-polished crystal showed a damage-free subsurface structure, the roughness of the surface imaged in the TEM was not always excellent compared with the ideal roughness expected from the theoretical analysis [5]. Specifically, the TEM images often indicated a characteristic “ridged” shape surface roughness on the profiles, depending on the specimens, as shown in Figure 4(a). Figure 4(b) is a high-resolution TEM image of a part of the region in Figure 4(a). It shows the (111) crystal lattice fringes with a spacing of 0.32 nm. It should be noted that the profiles of terraced part of the surface are aligned exactly parallel to the (111) lattice fringes, as marked on the images.
Figure 4: TEM images of a float-polished CaF2 surface, indicating the (111) lattice fringes. Figure 5 is an atomic structure model of the ridged shape surface derived from the observed TEM image and the atomic arrangement of the CaF2-type crystal structure. The surface is composed of the flat (111) plane terraces and the nanometer height steps in between. This implies that the chemical-mechanical reactions during the float polishing process preferentially remove surface atoms of the CaF2 crystal along the (111) planes. This also suggests that the roughness of the surface mainly arises from the mismatch between the macroscopic surface orientation and the (111) crystal planes. In fact, the average orientation of the specimen surface in Figure 4 is mismatched by about 5 deg from the (111) plane. This causes a relatively large surface roughness. The above results suggest that an atomically smooth CaF2 surface can be obtained by the precise alignment of the crystal surface to the (111) planes. Figures 6(a) and 6(b) show the roughness of float-polished surfaces as measured on a 1 x 1 µm areas with an SPM. Before the samples were float polished, they were precisely oriented with the (111) lattice plane. The misfit angles θ of the surface from the (111) plane are 0.18 deg and 0.02 deg, respectively, for the specimens of Figures 6(a) and 6(b). Both the SPM images clearly show that the surface has the characteristic ridged shape with a step height of a few nanometers. This is quite consistent with the results of TEM observations. Further, the smallest roughness for 1 x 1 mm area, which is of 0.059 nm Ra, 0.077 nm rms, and 0.72 nm Ry, was obtained, as shown in Figure 6(b).
Figure 5: An atomic structure model of a floatpolished CaF2 surface.
Figure 6: Surface roughness of float-polished surfaces near the (111) planes, as measured on 1 x 1 µm areas with an SPM.
Figure 7 is a high-resolution TEM image of a float-polished CaF2 surface obtained with the 400 kV TEM. Before starting the ultra-precision grinding prior to the float polishing process, the sample was precisely aligned and carefully sawed so that the mismatch angle of the crystal surface from the (111) plane become as small as possible. The mismatch angle roughly estimated by the SPM measurement was of the order of 1/100 deg, which makes the interval between the surface steps over several hundred nanometers. The image clearly demonstrates that an atomically smooth surface with a p-v roughness smaller than the (111) single crystal lattice spacing (i.e., 0.32 nm) can be achieved. The image also proves that no subsurface crystal defects, shown as a deformation of the lattice fringes, are introduced by the machining. This TEM result suggests that it is possible to obtain an atomically perfect, flat surface on a CaF2 crystal by using the float polishing technique. From above-mentioned results, in addition to float polishing, the nanoaligning technology necessary to obtain a sample exactly oriented to the (111) plane is essential for making atomically-flat, real surfaces.
Figure 7: High resolution TEM image of a float-polished CaF2 crystal surface of which the orientation precisely aligned parallel to the (111) plane. 4. Conclusions High purity DUV grade calcium fluoride (111) single crystals have been float polished in order to obtain the highest quality surfaces. The finished surfaces were observed with a Nomarski differential interference contrast microscope, optical profilers, a scanning probe microscope and transmission electron microscopes. 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. 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 CaF2 single crystal sample. 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. 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 µm area with an SPM. Acknowledgements Part of this work was supported by the Grants-in-Aid for Scientific Research (B) Nos. 11450063, 12450063, 14350077 and 15360075 of the Japan Society for the Promotion of Science, and the "Nanotechnology Support Project" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] Y. Namba and H. Tsuwa, “Ultra-Fine Finishing of Sapphire Single Crystal”, Annals of the CIRP, 26/1 (1977) p.325-329. [2] J. M. Bennett, J. J. Shaffer, Y. Shibano, and Y. Namba, “Float Polishing of Optical Materials”, Appl. Opt., 26/4 (1987) p.696-703. [3] Y. Namba, H. Tsuwa, and R. Wada, “Ultra-Precision Float Polishing Machine”, Annals of the CIRP, 36/1 (1987) p.211-214. [4] Y. Namba, N. Ohnishi, K. Harada, and K. Yoshida, “Float Polishing of Calcium Fluoride Single Crystals for Ultra Violet Applications” Proc. 17th Annl. Mtg. of ASPE, (2002) p.450-453. [5] Y. Namba, J. Yu, J. M. Bennett, and K. Yamashita, “Modeling and Measurements of Atomic Surface Roughness”, Appl. Opt., 39/16 (2000) p.2705-2718.
