“NEXCERA”, ZERO THERMAL EXPANSION CERAMICS FOR …aspe.net/publications/annual_2010/poster/12optics/3090.pdfApplications of Low Thermal Expansion Ceramics to Ultra Precision Machine

“NEXCERA”, ZERO THERMAL EXPANSION CERAMICS FOR OPTICAL APPLICATIONS

Jun Sugawara1, Hiroto Unno1, Nobumasa Kosugi1, Yuji Kuma1 and Kozo Abe1

1Ceramics Division, Krosaki Harima Corporation,

Kitakyushu City 806-8586, Japan

INTRODUCTION NEXCERA, a cordierite (2MgO-2Al2O3-5SiO2) based polycrystalline ceramic has been developed as a cutting-edge material with both an extremely low thermal expansion coefficient of < 0.03 ppm/K and superior mechanical properties. NEXCERA differs fundamentally from a conventional low thermal expansion glass (LTEG) in consisting of a kind of the crystal and slight amorphous phases along grain boundaries. Accordingly, NEXCERA has high dimensional stability for long-term passage and temperature changes as compared to LTEG [1,2]. Taking advantage of these properties, NEXCERA is used as calibration tools and primary standards in the precision metrology field requiring high accuracy and reliability, in addition to structural components of steppers for LSI lithography [3]. In the previous paper, the advantage of NEXCERA as an optical reflecting mirror was discussed through the manufacture of ultra light-weight boxed structures [4]. The present work has been conducted to evaluate the results of surface finishes for actual mirrors. Special emphasis has been placed on the effect of improving ceramic material on the smooth surface achieved by the polishing. CERAMICS PROCESSING FOR MIRRORS AND EXPERIMENTAL PROCEDURE Two kinds of mixed powders of cordierite and sintering additives according to ceramic compositions were ball-milled together with

organic binder and water. The slurries thus obtained were dried to granulated powders, and then isostatically pressed into chalk-like blocks. The compressed blocks were machined into desired shapes such as disk-shaped top-plates and ribbed bodies of 401 mm in diameter and 83 mm in height, by making use of a machining center equipped with cutting tools. The machined bodies were once dewaxed at 350 °C in air, and then sintered at 1360 °C in an Ar-gas flow atmosphere. The sintered bodies were ground using diamond wheels, and then some bodies were bounded to form boxed structures of 340 mm in diameter by the diffusion method, as shown in Fig. 1 and the previous paper [4]. After that, the hollow bodies and some specimens were ground, lapped and finally polished to obtain fine finishes for mirror surfaces. The finished roughness was measured by Zygo New View 7300. The flatness when the mirror was setting vertically was measured by Zygo GPI-XPD 12 inch. The flatness when the mirror was supported horizontally on three points around the perimeter was also measured by Japanese ultimate flatness interferometer developed by national institute of advanced industrial science and technology (AIST), as given in Fig. 2 [5]. To confirm structural properties, finite element method (FEM) calculation was carried out under aforementioned condition.

FIGURE 1. Bonding process of the mirrors. (a) top-plate, (b) hexagonal rib arranged body and (c) boxed

mirror.

(a) Top-plate (b) Hexagonal rib arranged body (c) Boxed mirror

Diffusion Bonding

+

FIGURE 2. Japanese ultimate flatness interferometer developed by AIST [5]. RESULTS AND DISCUSSIONS The productivity for smooth surfaces of NEXCERA and improved material Table 1 compares two kinds of low thermal expansion materials such as original NEXCERA and improved material for polishing in terms of various characteristics. There is practically no difference between two materials in mechanical and thermal properties. TABLE 1. The comparison of characteristics of NEXCERA and improved material

NEXCERA Improved Material

Bulk Density g/cm3 2.50 2.55 Young’s Modulus GPa 130 140 Poisson’s Ratio - 0.30 0.31 Flexural Strength MPa 210 230 Fracture Toughness MPa1/2 1.2 1.2 Hardness HV GPa 8.0 8.1 Thermal Expansion Coefficient

10-6/K (23°C) < 0.03 < 0.03

Thermal Conductivity

W/m K (23°C) 3.7 4.2

Specific Heat J/g K 0.83 0.78 The disk specimens of 150 mm in diameter and 30 mm in thickness were polished with diamond slurry and/or ceria slurry to get insights into the productivity for smooth surfaces of two materials. Figure 3 illustrates the surface roughness of two materials, which were polished with diamond slurry and ceria slurry. For NEXCERA, some dimples of around 100 nm in depth are recognized on the surface therefore, an averaged-roughness (Ra) is adversely affected, as given in Fig. 3 (a). For improved

material, the mirror surface achieved by the polishing turns to be extremely FIGURE 3. The Bird’s eye view of surface roughness of (a) NEXCERA and (b) improved material, which were polished with diamond slurry and ceria slurry. FIGURE 4. The Bird’s eye view of surface roughness of (a) NEXCERA and (b) improved material, which were polished with ceria slurry.

(a) NEXCERA / Ra 0.67 nm, PV 132 nm

(b) Improved material / Ra 0.36 nm, PV 32 nm

(a) NEXCERA / Ra 2.40 nm, PV 261 nm

(b) Improved material / Ra 0.65 nm, PV 34 nm

material, the mirror surface achieved by the polishing turns to be extremely smooth with Ra of 0.36 nm, as shown in Fig. 3 (b). Figure 4 shows the surface roughness of two materials, which were polished with only ceria slurry, being a severe condition which exhibits a high chemical action. The difference of two materials in Ra is noticeable compared to the polishing with diamond slurry and ceria slurry. It indicates that improved material has a more homogeneous mechanochemical polishing rate from a microstructural point of view, as compared with original NEXCERA. Thus, improved material is thought to be more applicable for a reflecting mirror requiring high accuracy. Ultra light-weight boxed mirrors The ultra light-weight mirrors were manufactured using MEXCERA and improved material. Light weight can be attained by making top-plates and ribbed bodies bond together to form boxed structure by means of the diffusion method shown in Fig. 1 and the previous paper [4]. The specifications of the boxed mirrors are shown in table 2. To confirm structural properties, the mirrors with two types of ribbed body such as concentric and hexagonal rib arrangement were fabricated as shown in table 3. TABLE 2. Specifications of light-weight mirrors.

Diameter mm 340 Height mm 70 Rib Thickness mm 3 Rib Depth mm 55 Accuracy Diameter Region mm 300 Thickness on Mirror Side mm 10 or 7 Weight kg 5.4, or 4.8 Apparent Density g/cm3 0.85, or 0.76

TABLE 3. The details of manufactured mirrors and measurement results of flatness when the mirrors are setting vertically. The letters attached in the table represent as follows; Con.: concentric, Hex.: hexagonal, Nex.: NEXCERA, and Imp.: improved material.

Mirror Number A B C D Material Nex. Nex. Nex. Imp.Cell Arrangement Con. Hex. Hex. Hex.Thickness on Mirror Side mm 10 10 7 10

Flatness nm 55 48 94 57

The mirrors with both rib arrangements using NEXCERA and improved material are accurately finished to less than λ/10, as shown in table 3 and Fig. 5. However, mirror number C is finished to only around 100 nm in flatness yet, due to the thin thickness on mirror side. For all mirrors, no surface irregularity along the rib arrangement is recognized. Thus, the rib arrangements have no effect on flatness of mirrors for the conditions. However, the surface shape is not a uniform convex or concave over the entire surface, and local convex and concave are still found. Therefore, there is still room for improvement in the polishing technique. FIGURE 5. The bird’s-eye vies of flatness measured by GPI of (a) mirror number A, (b) B and (c) D. The flatness when the mirrors are supported horizontally on three points of 166 mm in radius is given in Fig. 6. The flatness change shows that the mirror changes its shape with the dead weight. Dead weight deformations of the mirrors

(a) A / Flatness 55 nm

(b) B / Flatness 48 nm

(c) D / Flatness 57 nm

are thought to be from 30 to 40 nm, and independent of rib arrangements, almost corresponding to FEM results shown in previous paper [4]. FIGURE 6. The bird’s-eye vies of flatness measured by Japanese ultimate flatness interferometer of (a) mirror number A, (b) B and (c) D.

CONCLUSIONS The fine polishing finishes improved material to an extremely smooth surface with Ra of 0.36 nm. The improved material has high productivity for smooth surfaces as compared with original NEXCERA. The light-weight boxed mirrors of 340 mm in diameter can be polished to a precise flatness of less than λ/10, without adverse effect of rib arrangements and ceramic materials. Dead weight deformations when the mirrors are supported on three points around the perimeter are found to be quite small and approximately corresponding to the results of FEM calculations. As a consequence of this work, it is expected that NEXCERA will be applied to the mirror using in the field of ultra precision metrology and outer space. REFERENCES [1] F.Bayer-Helms, H. Darnedde and G. Exner

Metrologia. 1985; 21: 49-57. [2] R. Schodel and Bonsch, Interferometric

measurements of thermal expansion, length stability and compressibility of glass ceramics. Proc. of the 3rd euspen International Conference. 2002; 691-694.

[3] J. Sugawara, H. Unno and N. Kosugi, Applications of Low Thermal Expansion Ceramics to Ultra Precision Machine Tool and Precision Metrology Field. HEAT technical committee Open symposium No6 JSAT 2008.

[4] H. Unno, J. Sugawara, N. Kosugi, Y. Kuma, K. Abe and T. Aoki, “NEXCERA”, Zero Thermal Expansion Ceramics for Ultra Precision Applications. Proc. of the 10th euspen International Conference. 2010; 1: 160-163.

[5] T. Takatsuji, N. Ueki, K. Hibino, S. Osawa and T. Kurosawa, Japanese Ultimate Flatness Interferometer (FUJI) and its preliminary experiment. Proc. SPIE. 2003; 4401: 83.

(b) B / Flatness 45 nm

(c) D / Flatness 80 nm

(a) A / Flatness 36 nm