Roughness measurement and ion-beam polishing of super-smooth optical surfaces of fused quartz and optical ceramics N. I. Chkhalo,1,* S. A. Churin,1 A. E. Pestov,1,2 N.N. Salashchenko,1 Yu. A. Vainer,1 and M. V. Zorina1 1
Institute for Physics of Microstructures of the Russian Academy of Sciences, GSP-105, 603950 Nizhny Novgorod, Russia 2 Nizhny Novgorod N.I. Lobachevskii State University, av. Gagarina, 23, 603950 Nizhny Novgorod, Russia *[email protected]
Abstract: The main problems and the approach used by the authors for roughness metrology of super-smooth surfaces designed for diffractionquality X-ray mirrors are discussed. The limitations of white light interferometry and the adequacy of the method of atomic force microscopy for surface roughness measurements in a wide range of spatial frequencies are shown and the results of the studies of the effect of etching by argon and xenon ions on the surface roughness of fused quartz and optical ceramics, Zerodur, ULE and Sitall, are given. Substrates of fused quartz and ULE with the roughness, satisfying the requirements of diffraction-quality optics intended for working in the spectral range below 10 nm, are made. ©2014 Optical Society of America OCIS codes: (240.5770) Roughness; (220.5450) Polishing; (120.4800) Optical standards and testing; (340.7480) X-rays, soft x-rays, extreme ultraviolet (EUV).
References and links 1. 2. 3.
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Y. Platonov, J. Rodriguez, M. Kriese, E. Gullikson, T. Harada, T. Watanabe, and H. Kinoshita, “Multilayers for next generation EUVL at 6.Xnm,” Proc. SPIE 8076, 80760N (2011). M. M. Barysheva, A. E. Pestov, N. N. Salashchenko, M. N. Toropov, and N. I. Chkhalo, “Precision imaging multilayer optics for soft X-rays and extreme ultraviolet,” Phys.- Usp. 55(7), 681–699 (2012). S. S. Andreev, M. M. Barysheva, N. I. Chkhalo, S. A. Gusev, A. E. Pestov, V. N. Polkovnikov, N. N. Salashchenko, L. A. Shmaenok, Yu. A. Vainer, and S. Yu. Zuev, “Multilayered mirrors based on La/B4C(B9C) for x-ray range near anomalous dispersion of boron (l near 6.7 nm),” Nucl. Instrum. Methods Phys. Res. Sect. A 603(1–2), 80–82 (2009). C. Wagner and N. Harned, “EUV lithography: lithography gets extreme,” Nat. Photonics 4(1), 24–26 (2010). J. Kirz, C. Jacobsen, and M. Howells, “Soft X-ray microscopes and their biological applications,” Q. Rev. Biophys. 28(1), 33–130 (1995). I. Artyukov, Y. Bugayev, O. Devizenko, E. Gullikson, V. Kondratenko, and A. Vinogradov, “X-ray Schwarzschild objective for the carbon window (λ~45 nm),” Opt. Lett. 34(19), 2930–2932 (2009). U. Dinger, F. Eisert, H. Lasser, M. Mayer, A. Seifert, G. Seitz, S. Stacklies, F.-J. Stickel, and M. Weiser, “Mirror substrates for EUV lithography: progress in metrology and optical fabrication technology,” Proc. SPIE 4146, 35– 46 (2000). C. Gwyn, “White Paper on Extreme ultraviolet Lithography,” in Proceedings of EUV LLC, Livermore (1998). E. M. Gullikson, S. Baker, J. E. Bjorkholm, J. Bokor, K. A. Goldberg, J. E. M. Goldsmith, C. Montcalm, P. Naulleau, E. A. Spiller, D. G. Stearns, J. S. Taylor, and J. H. Underwood, “Scattering and flare of 10X projection cameras,” Proc. SPIE 3676, 717–723 (1999). D. G. Stearns, D. P. Gaines, D. W. Sweeney, and E. M. Gullikson, “Nonspecular x-ray scattering in a multilayercoated imaging system,” J. Appl. Phys. 84(2), 1003–1028 (1998). S. Schröder, T. Feigl, A. Duparré, and A. Tünnermann, “EUV reflectance and scattering of Mo/Si multilayers on differently polished substrates,” Opt. Express 15(21), 13997–14012 (2007). S. K. Sinha, E. B. Sirota, S. Garoff, and H. B. Stanley, “X-ray and neutron scattering from rough surfaces,” Phys. Rev. B Condens. Matter 38(4), 2297–2311 (1988). V. Holy and T. Baumbach, “Nonspecular x-ray reflection from rough multilayers,” Phys. Rev. B Condens. Matter 49(15), 10668–10676 (1994). D. K. G. de Boer, “X-ray reflection by rough surfaces,” Phys. Rev. B 51(8), 5297–5305 (1995).
#214116 - $15.00 USD Received 16 Jun 2014; revised 28 Jul 2014; accepted 28 Jul 2014; published 12 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020094 | OPTICS EXPRESS 20094
15. I. V. Kozhevnikov and M. V. Pyatakhin, “Use of DWBA and perturbation theory in X-ray control of the surface roughness,” J X-ray Sci. Technol. 8, 253–275 (2000). 16. J. E. Griffith and D. A. Grigg, “Dimensional metrology with scanning probe microscopes,” J. Appl. Phys. 74(9), R83–R109 (1993). 17. R. Blunt, “White light interferometry – a production worthy technique for measuring surface roughness on semiconductor wafers,” in CEMANTECH Conference, Vancouver, Canada (2006). pp. 59–62. 18. V. E. Asadchikov, I. V. Kozhevnikov, Yu. S. Krivonosov, R. Mercier, T. H. Metzger, C. Morawe, and E. Ziegler, “Application of X-ray scattering technique to the study of supersmooth surfaces,” Nucl. Instrum. Methods Phys. Res. A 530(3), 575–595 (2004). 19. V. V. Azarova, V. G. Dmitriev, Yu. N. Lokhov, and K. N. Malitskii, “Measuring the roughness of high-precision quartz substrates and laser mirrors by angle-resolved scattering,” J. Opt. Technol. 69(2), 125–129 (2002). 20. M. Barysheva, Yu. A. Vainer, B. A. Gribkov, M. V. Zorina, A. E. Pestov, D. N. Rogachev, N. N. Salashenko, and N. I. Chkhalo, “Particulars of studying the roughness of substrates for multilayer X-ray optics using smallangle X-ray reflectometry, atomic-force, and interference microscopy,” Bull. Russ. Acad. Sci., Physics 75(1), 67–72 (2011). 21. M. M. Barysheva, B. A. Gribkov, Yu. A. Vainer, M. V. Zorina, A. E. Pestov, Yu. Ya. Platonov, D. N. Rogachev, N. N. Salashchenko, and N. I. Chkhalo, “Problem of roughness detection for supersmooth surfaces,” Proc. SPIE 8076, 80760M (2011). 22. M. M. Barysheva, B. A. Gribkov, M. V. Zorina, N. N. Salashchenko, and N. I. Chkhalo, “On the problems of the application of atomic-force microscopes for studying the surface roughness of elements for imaging optics,” J. Surf. Invest. X-ray. Synchrotron and Neutron Technol. 7(4), 797–801 (2013). 23. K. Otaki, K. Ota, I. Nishiyama, T. Yamamoto, Y. Fukuda, and S. Okazaki, “Development of the point diffraction interferometer for extreme ultraviolet lithography: Design, fabrication, and evaluation,” J. Vac. Sci. Technol. B 20(6), 2449–2458 (2002). 24. N. N. Salaschenko, M. N. Toropov, and N. I. Chkhalo, “Physical limitations of measurement accuracy of the diffraction reference wave interferometers,” Bull. Russ. Acad. Sci., Physics 74(1), 53–56 (2010). 25. P. P. Naulleau, K. A. Goldberg, S. H. Lee, C. Chang, D. Attwood, and J. Bokor, “Extreme-ultraviolet phaseshifting point-diffraction interferometer: a wave-front metrology tool with subangstrom reference-wave accuracy,” Appl. Opt. 38(35), 7252–7263 (1999). 26. N. I. Chkhalo, A. Yu. Klimov, V. V. Rogov, N. N. Salashchenko, and M. N. Toropov, “A source of a reference spherical wave based on a single mode optical fiber with a narrowed exit aperture,” Rev. Sci. Instrum. 79(3), 033107 (2008). 27. V. E. Asadchikov, A. Duparré, S. Jakobs, A. Yu. Karabekov, I. V. Kozhevnikov, and Y. S. Krivonosov, “Comparative study of the roughness of optical surfaces and thin films by use of X-ray scattering and atomic force microscopy,” Appl. Opt. 38(4), 684–691 (1999). 28. M. M. Barysheva, N. I. Chkhalo, A. E. Pestov, N. N. Salashchenko, M. N. Toropov, and M. V. Zorina, “Mirrors with a sub-nanometer surface shape accuracy,” in Fundamentals of Picoscience, K. D. Sattler, ed. (CRC Press, 2013) pp. 595–615. 29. B. L. Henke, E. M. Gullikson, and J. C. Davis, “X-ray interactions: photoabsorption, scattering, transmission, and reflection at E=50-30000 eV, Z=1-92,” Data Nucl. Data Tables Vol. 54(2), 181–342 (1993). 30. E. Ziegler, L. Peverini, N. Vaxelaire, A. Cordon-Rodriguez, A. Rommeveaux, I. V. Kozhevnikov, and J. Susini, “Evolution of surface roughness in silicon X-ray mirrors exposed to a low-energy ion beam,” NIM A 616(2–3), 188–192 (2010). 31. N. I. Chkhalo, M. M. Barysheva, A. E. Pestov, N. N. Salashchenko, and M. N. Toropov, “Manufacturing and characterization the diffraction quality normal incidence optics for the XEUV range,” Proc. SPIE 8076, 80760P (2011). 32. I. G. Zabrodin, B. A. Zakalov, I. A. Kas’kov, A. E. Pestov, N. N. Salashchenko, and N. I. Chkhalo, “Device for the precise shape correction of optical surfaces by ion-beam and reactive plasma etching,” J. Surf. Invest. X-ray, Synchrotron and Neutron Techn. 7(5), 913–915 (2013).
1. Introduction In connection with the progress in the technology of the deposition of normal incidence multilayer mirrors [1,2], there is now a real possibility of transferring conventional optical methods to extreme ultraviolet (EUV) and soft X-ray (SXR) ranges. Among the most important optics applications actively discussed in recent years are nanolithography at a wavelength of 13.5 nm, as well as beyond extreme ultraviolet (BEUV) lithography at 6.7 nm [3,4]. Very promising is the X-ray microscopy of biological objects in the areas of “water” and “carbon” transparency windows, at 2.3–5 nm wavelengths [5,6]. To ensure diffractionquality imaging, when the spatial resolution is determined only by the radiation wavelength and the numerical aperture of the lens, substrates with atomically smooth surfaces are required.
#214116 - $15.00 USD Received 16 Jun 2014; revised 28 Jul 2014; accepted 28 Jul 2014; published 12 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020094 | OPTICS EXPRESS 20095
With the impact on the image, the roughness is divided into low frequency at a lateral dimension range of 1 mm – 1 m, middle frequency at 1 µm – 1 mm and high frequency at 1 nm – 1 μm [7]. High-frequency roughness leads to radiation scattering at angles greater than the width of the Bragg reflection peak of the multilayer mirror and to a sharp loss in reflectivity of the mirrors. Since interference gain of scattered waves does not occur, the contrast of the image is not affected, leading only to a loss of light. Roughness in the lowfrequency range, with lateral dimensions in the range of 1 mm – 1 m, does not affect the luminance and contrast of the image, and leads to a distortion of the image as a whole. For example, rings are shown as ellipses. The greatest effect on the spatial resolution of the mirror is roughness of the middlefrequency range. For EUV lithography at 13.5 nm, the root mean-square (r.m.s.) roughness should not exceed σ ≤ 0.2 nm [8–10]. The scaling factor at the transition to the other wavelength λ can be written as (σ/λ)2 [11]. Accordingly, reducing the operating wavelength requires at least a proportional decrease in the roughness. For example, for a wavelength shorter than 6 nm the roughness should not exceed 0.1 nm. Solving the problem of manufacturing super-smooth optical surfaces involves two main tasks. The first is an audit of the capabilities of the traditional and the development of new methods for roughness measurements with sub-angstrom sensitivity. The second is improving the traditional and developing new methods of polishing. It is clear that the solution to the problem of manufacturing super-smooth low-scattering substrates is of interest not only for EUV and SXR, but also for other optical ranges. In this paper, we report on the latest results of the research undertaken by the authors into manufacturing substrates satisfying the requirements for diffraction-quality optics in the spectral range shorter than 10 nm. Two main aspects of the problem of creating substrates are considered: the first is related to the development of adequate methods for measuring the roughness and the second is the development of advanced polishing methods. With respect to metrology, we propose an approach in which reliable data are only those that are confirmed by the “first-principle” method of diffuse X-ray scattering (DXRS) [12–15]. To improve the process of obtaining super-smooth substrates for multilayer mirrors, we studied the impact of ion-beam etching on the surface roughness of fused silica and optical ceramics: Zerodur, ULE and Sitall, which have abnormally low coefficients of thermal expansion and, therefore, are of greatest interest for ultra-high spatial resolution systems. 2. Roughness measurement Traditionally, for roughness measurements, atomic force microscopy (AFM) and white light interferometry (WLI) [16,17] have been used. In a number of papers, good agreement between AFM and WLI measurement results [7,9,18,19] is observed. In some papers, in contrast, the contradictory nature of the WLI data is marked. In [20,21] the problems which arise when these methods are applied to the study of super-smooth optical surfaces and the principal causes of these contradictions are analysed. In particular, both AFM and WLI cannot be attributed to the “first-principle” methods. For example, the results of AFM measurements are strongly affected by the geometric dimensions of the probe, the surface under study, electrification and contamination, and the non-linearity of the piezo-scanner, which is most pronounced on “large” frames [22]. Another source of uncertainty in AFM measurements is sample homogeneity and the fact that only a very small sample area is investigated (ergodic hypothesis). In the WLI method, the surface profile is measured directly due to the interference of light reflected from the surface under study and a reference. Since the coherence length in WLI is small, the maximum contrast of the interference pattern is achieved with strict equality of the optical paths. Alignment of the paths occurs due to displacement of the lens mounted on a piezo-ceramic. An obvious source of error in this method is the non-linearity of the piezoelectric ceramic and/or the capacitor position sensor, if present, which means that both the
#214116 - $15.00 USD Received 16 Jun 2014; revised 28 Jul 2014; accepted 28 Jul 2014; published 12 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020094 | OPTICS EXPRESS 20096
inclination of the sample with respect to the reference and the surface profile of the measurement results are influenced. The other error sources are the roughness of the reference surface and the errors that occur when light passes through the optical elements. The importance of the latter was demonstrated by interferometry with a diffraction reference wave which was used to study the shape of the surfaces and the aberrations of the optical systems [23,24]. Therefore, it is essential for this type of instrument to have a so-called transfer function of the optical path using a “mythical” reference. Therefore, in practice, the adequacy of the AFM and WLI must be confirmed by the “first-principle” method, as it was, for instance, in the case of point diffraction interferometry, used for precision measurements of low frequency roughness, and when aberrations of the reference wave were measured in a simple Young’s experiment on two-source interference [25,26]. Moreover, taking into account variable external factors, for example, the samples under study having very different reliefs, slopes with respect to the reference, mechanical vibration, and so on, such inspections must be carried out regularly. Ideally, for each surface the measurements obtained using different methods should be compared. Since the physics of scattering X-ray radiation by the rough surfaces is well known, the diffuse scattering of X-rays is the most reliable “first-principle” method for roughness measurements [12–15]. In our work we apply an approach based on the perturbation theory (for details, see [27]), which makes possible to recover the power spectral density (PSD) function of the surface directly from the experimental data without a priori assumptions concerning its topography. This is possible due to a linear relationship between the scattering indicatrix and the PSD function. For example, if radiation with the wavelength λ falls at grazing angle θ0, see Fig. 1(a), on a surface with correlation length a, the widths of the scattering indicatrix Φ(θ,φ) in perpendicular directions θ and φ can be estimated as δθ ~λ/παsinθ0 and δφ ~λ/πα. For X-ray radiation and grazing incidence, for isotropic or weakly anisotropic surfaces, this means that δφ
Institute for Physics of Microstructures of the Russian Academy of Sciences, GSP-105, 603950 Nizhny Novgorod, Russia 2 Nizhny Novgorod N.I. Lobachevskii State University, av. Gagarina, 23, 603950 Nizhny Novgorod, Russia *[email protected]
Abstract: The main problems and the approach used by the authors for roughness metrology of super-smooth surfaces designed for diffractionquality X-ray mirrors are discussed. The limitations of white light interferometry and the adequacy of the method of atomic force microscopy for surface roughness measurements in a wide range of spatial frequencies are shown and the results of the studies of the effect of etching by argon and xenon ions on the surface roughness of fused quartz and optical ceramics, Zerodur, ULE and Sitall, are given. Substrates of fused quartz and ULE with the roughness, satisfying the requirements of diffraction-quality optics intended for working in the spectral range below 10 nm, are made. ©2014 Optical Society of America OCIS codes: (240.5770) Roughness; (220.5450) Polishing; (120.4800) Optical standards and testing; (340.7480) X-rays, soft x-rays, extreme ultraviolet (EUV).
References and links 1. 2. 3.
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Y. Platonov, J. Rodriguez, M. Kriese, E. Gullikson, T. Harada, T. Watanabe, and H. Kinoshita, “Multilayers for next generation EUVL at 6.Xnm,” Proc. SPIE 8076, 80760N (2011). M. M. Barysheva, A. E. Pestov, N. N. Salashchenko, M. N. Toropov, and N. I. Chkhalo, “Precision imaging multilayer optics for soft X-rays and extreme ultraviolet,” Phys.- Usp. 55(7), 681–699 (2012). S. S. Andreev, M. M. Barysheva, N. I. Chkhalo, S. A. Gusev, A. E. Pestov, V. N. Polkovnikov, N. N. Salashchenko, L. A. Shmaenok, Yu. A. Vainer, and S. Yu. Zuev, “Multilayered mirrors based on La/B4C(B9C) for x-ray range near anomalous dispersion of boron (l near 6.7 nm),” Nucl. Instrum. Methods Phys. Res. Sect. A 603(1–2), 80–82 (2009). C. Wagner and N. Harned, “EUV lithography: lithography gets extreme,” Nat. Photonics 4(1), 24–26 (2010). J. Kirz, C. Jacobsen, and M. Howells, “Soft X-ray microscopes and their biological applications,” Q. Rev. Biophys. 28(1), 33–130 (1995). I. Artyukov, Y. Bugayev, O. Devizenko, E. Gullikson, V. Kondratenko, and A. Vinogradov, “X-ray Schwarzschild objective for the carbon window (λ~45 nm),” Opt. Lett. 34(19), 2930–2932 (2009). U. Dinger, F. Eisert, H. Lasser, M. Mayer, A. Seifert, G. Seitz, S. Stacklies, F.-J. Stickel, and M. Weiser, “Mirror substrates for EUV lithography: progress in metrology and optical fabrication technology,” Proc. SPIE 4146, 35– 46 (2000). C. Gwyn, “White Paper on Extreme ultraviolet Lithography,” in Proceedings of EUV LLC, Livermore (1998). E. M. Gullikson, S. Baker, J. E. Bjorkholm, J. Bokor, K. A. Goldberg, J. E. M. Goldsmith, C. Montcalm, P. Naulleau, E. A. Spiller, D. G. Stearns, J. S. Taylor, and J. H. Underwood, “Scattering and flare of 10X projection cameras,” Proc. SPIE 3676, 717–723 (1999). D. G. Stearns, D. P. Gaines, D. W. Sweeney, and E. M. Gullikson, “Nonspecular x-ray scattering in a multilayercoated imaging system,” J. Appl. Phys. 84(2), 1003–1028 (1998). S. Schröder, T. Feigl, A. Duparré, and A. Tünnermann, “EUV reflectance and scattering of Mo/Si multilayers on differently polished substrates,” Opt. Express 15(21), 13997–14012 (2007). S. K. Sinha, E. B. Sirota, S. Garoff, and H. B. Stanley, “X-ray and neutron scattering from rough surfaces,” Phys. Rev. B Condens. Matter 38(4), 2297–2311 (1988). V. Holy and T. Baumbach, “Nonspecular x-ray reflection from rough multilayers,” Phys. Rev. B Condens. Matter 49(15), 10668–10676 (1994). D. K. G. de Boer, “X-ray reflection by rough surfaces,” Phys. Rev. B 51(8), 5297–5305 (1995).
#214116 - $15.00 USD Received 16 Jun 2014; revised 28 Jul 2014; accepted 28 Jul 2014; published 12 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020094 | OPTICS EXPRESS 20094
15. I. V. Kozhevnikov and M. V. Pyatakhin, “Use of DWBA and perturbation theory in X-ray control of the surface roughness,” J X-ray Sci. Technol. 8, 253–275 (2000). 16. J. E. Griffith and D. A. Grigg, “Dimensional metrology with scanning probe microscopes,” J. Appl. Phys. 74(9), R83–R109 (1993). 17. R. Blunt, “White light interferometry – a production worthy technique for measuring surface roughness on semiconductor wafers,” in CEMANTECH Conference, Vancouver, Canada (2006). pp. 59–62. 18. V. E. Asadchikov, I. V. Kozhevnikov, Yu. S. Krivonosov, R. Mercier, T. H. Metzger, C. Morawe, and E. Ziegler, “Application of X-ray scattering technique to the study of supersmooth surfaces,” Nucl. Instrum. Methods Phys. Res. A 530(3), 575–595 (2004). 19. V. V. Azarova, V. G. Dmitriev, Yu. N. Lokhov, and K. N. Malitskii, “Measuring the roughness of high-precision quartz substrates and laser mirrors by angle-resolved scattering,” J. Opt. Technol. 69(2), 125–129 (2002). 20. M. Barysheva, Yu. A. Vainer, B. A. Gribkov, M. V. Zorina, A. E. Pestov, D. N. Rogachev, N. N. Salashenko, and N. I. Chkhalo, “Particulars of studying the roughness of substrates for multilayer X-ray optics using smallangle X-ray reflectometry, atomic-force, and interference microscopy,” Bull. Russ. Acad. Sci., Physics 75(1), 67–72 (2011). 21. M. M. Barysheva, B. A. Gribkov, Yu. A. Vainer, M. V. Zorina, A. E. Pestov, Yu. Ya. Platonov, D. N. Rogachev, N. N. Salashchenko, and N. I. Chkhalo, “Problem of roughness detection for supersmooth surfaces,” Proc. SPIE 8076, 80760M (2011). 22. M. M. Barysheva, B. A. Gribkov, M. V. Zorina, N. N. Salashchenko, and N. I. Chkhalo, “On the problems of the application of atomic-force microscopes for studying the surface roughness of elements for imaging optics,” J. Surf. Invest. X-ray. Synchrotron and Neutron Technol. 7(4), 797–801 (2013). 23. K. Otaki, K. Ota, I. Nishiyama, T. Yamamoto, Y. Fukuda, and S. Okazaki, “Development of the point diffraction interferometer for extreme ultraviolet lithography: Design, fabrication, and evaluation,” J. Vac. Sci. Technol. B 20(6), 2449–2458 (2002). 24. N. N. Salaschenko, M. N. Toropov, and N. I. Chkhalo, “Physical limitations of measurement accuracy of the diffraction reference wave interferometers,” Bull. Russ. Acad. Sci., Physics 74(1), 53–56 (2010). 25. P. P. Naulleau, K. A. Goldberg, S. H. Lee, C. Chang, D. Attwood, and J. Bokor, “Extreme-ultraviolet phaseshifting point-diffraction interferometer: a wave-front metrology tool with subangstrom reference-wave accuracy,” Appl. Opt. 38(35), 7252–7263 (1999). 26. N. I. Chkhalo, A. Yu. Klimov, V. V. Rogov, N. N. Salashchenko, and M. N. Toropov, “A source of a reference spherical wave based on a single mode optical fiber with a narrowed exit aperture,” Rev. Sci. Instrum. 79(3), 033107 (2008). 27. V. E. Asadchikov, A. Duparré, S. Jakobs, A. Yu. Karabekov, I. V. Kozhevnikov, and Y. S. Krivonosov, “Comparative study of the roughness of optical surfaces and thin films by use of X-ray scattering and atomic force microscopy,” Appl. Opt. 38(4), 684–691 (1999). 28. M. M. Barysheva, N. I. Chkhalo, A. E. Pestov, N. N. Salashchenko, M. N. Toropov, and M. V. Zorina, “Mirrors with a sub-nanometer surface shape accuracy,” in Fundamentals of Picoscience, K. D. Sattler, ed. (CRC Press, 2013) pp. 595–615. 29. B. L. Henke, E. M. Gullikson, and J. C. Davis, “X-ray interactions: photoabsorption, scattering, transmission, and reflection at E=50-30000 eV, Z=1-92,” Data Nucl. Data Tables Vol. 54(2), 181–342 (1993). 30. E. Ziegler, L. Peverini, N. Vaxelaire, A. Cordon-Rodriguez, A. Rommeveaux, I. V. Kozhevnikov, and J. Susini, “Evolution of surface roughness in silicon X-ray mirrors exposed to a low-energy ion beam,” NIM A 616(2–3), 188–192 (2010). 31. N. I. Chkhalo, M. M. Barysheva, A. E. Pestov, N. N. Salashchenko, and M. N. Toropov, “Manufacturing and characterization the diffraction quality normal incidence optics for the XEUV range,” Proc. SPIE 8076, 80760P (2011). 32. I. G. Zabrodin, B. A. Zakalov, I. A. Kas’kov, A. E. Pestov, N. N. Salashchenko, and N. I. Chkhalo, “Device for the precise shape correction of optical surfaces by ion-beam and reactive plasma etching,” J. Surf. Invest. X-ray, Synchrotron and Neutron Techn. 7(5), 913–915 (2013).
1. Introduction In connection with the progress in the technology of the deposition of normal incidence multilayer mirrors [1,2], there is now a real possibility of transferring conventional optical methods to extreme ultraviolet (EUV) and soft X-ray (SXR) ranges. Among the most important optics applications actively discussed in recent years are nanolithography at a wavelength of 13.5 nm, as well as beyond extreme ultraviolet (BEUV) lithography at 6.7 nm [3,4]. Very promising is the X-ray microscopy of biological objects in the areas of “water” and “carbon” transparency windows, at 2.3–5 nm wavelengths [5,6]. To ensure diffractionquality imaging, when the spatial resolution is determined only by the radiation wavelength and the numerical aperture of the lens, substrates with atomically smooth surfaces are required.
#214116 - $15.00 USD Received 16 Jun 2014; revised 28 Jul 2014; accepted 28 Jul 2014; published 12 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020094 | OPTICS EXPRESS 20095
With the impact on the image, the roughness is divided into low frequency at a lateral dimension range of 1 mm – 1 m, middle frequency at 1 µm – 1 mm and high frequency at 1 nm – 1 μm [7]. High-frequency roughness leads to radiation scattering at angles greater than the width of the Bragg reflection peak of the multilayer mirror and to a sharp loss in reflectivity of the mirrors. Since interference gain of scattered waves does not occur, the contrast of the image is not affected, leading only to a loss of light. Roughness in the lowfrequency range, with lateral dimensions in the range of 1 mm – 1 m, does not affect the luminance and contrast of the image, and leads to a distortion of the image as a whole. For example, rings are shown as ellipses. The greatest effect on the spatial resolution of the mirror is roughness of the middlefrequency range. For EUV lithography at 13.5 nm, the root mean-square (r.m.s.) roughness should not exceed σ ≤ 0.2 nm [8–10]. The scaling factor at the transition to the other wavelength λ can be written as (σ/λ)2 [11]. Accordingly, reducing the operating wavelength requires at least a proportional decrease in the roughness. For example, for a wavelength shorter than 6 nm the roughness should not exceed 0.1 nm. Solving the problem of manufacturing super-smooth optical surfaces involves two main tasks. The first is an audit of the capabilities of the traditional and the development of new methods for roughness measurements with sub-angstrom sensitivity. The second is improving the traditional and developing new methods of polishing. It is clear that the solution to the problem of manufacturing super-smooth low-scattering substrates is of interest not only for EUV and SXR, but also for other optical ranges. In this paper, we report on the latest results of the research undertaken by the authors into manufacturing substrates satisfying the requirements for diffraction-quality optics in the spectral range shorter than 10 nm. Two main aspects of the problem of creating substrates are considered: the first is related to the development of adequate methods for measuring the roughness and the second is the development of advanced polishing methods. With respect to metrology, we propose an approach in which reliable data are only those that are confirmed by the “first-principle” method of diffuse X-ray scattering (DXRS) [12–15]. To improve the process of obtaining super-smooth substrates for multilayer mirrors, we studied the impact of ion-beam etching on the surface roughness of fused silica and optical ceramics: Zerodur, ULE and Sitall, which have abnormally low coefficients of thermal expansion and, therefore, are of greatest interest for ultra-high spatial resolution systems. 2. Roughness measurement Traditionally, for roughness measurements, atomic force microscopy (AFM) and white light interferometry (WLI) [16,17] have been used. In a number of papers, good agreement between AFM and WLI measurement results [7,9,18,19] is observed. In some papers, in contrast, the contradictory nature of the WLI data is marked. In [20,21] the problems which arise when these methods are applied to the study of super-smooth optical surfaces and the principal causes of these contradictions are analysed. In particular, both AFM and WLI cannot be attributed to the “first-principle” methods. For example, the results of AFM measurements are strongly affected by the geometric dimensions of the probe, the surface under study, electrification and contamination, and the non-linearity of the piezo-scanner, which is most pronounced on “large” frames [22]. Another source of uncertainty in AFM measurements is sample homogeneity and the fact that only a very small sample area is investigated (ergodic hypothesis). In the WLI method, the surface profile is measured directly due to the interference of light reflected from the surface under study and a reference. Since the coherence length in WLI is small, the maximum contrast of the interference pattern is achieved with strict equality of the optical paths. Alignment of the paths occurs due to displacement of the lens mounted on a piezo-ceramic. An obvious source of error in this method is the non-linearity of the piezoelectric ceramic and/or the capacitor position sensor, if present, which means that both the
#214116 - $15.00 USD Received 16 Jun 2014; revised 28 Jul 2014; accepted 28 Jul 2014; published 12 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020094 | OPTICS EXPRESS 20096
inclination of the sample with respect to the reference and the surface profile of the measurement results are influenced. The other error sources are the roughness of the reference surface and the errors that occur when light passes through the optical elements. The importance of the latter was demonstrated by interferometry with a diffraction reference wave which was used to study the shape of the surfaces and the aberrations of the optical systems [23,24]. Therefore, it is essential for this type of instrument to have a so-called transfer function of the optical path using a “mythical” reference. Therefore, in practice, the adequacy of the AFM and WLI must be confirmed by the “first-principle” method, as it was, for instance, in the case of point diffraction interferometry, used for precision measurements of low frequency roughness, and when aberrations of the reference wave were measured in a simple Young’s experiment on two-source interference [25,26]. Moreover, taking into account variable external factors, for example, the samples under study having very different reliefs, slopes with respect to the reference, mechanical vibration, and so on, such inspections must be carried out regularly. Ideally, for each surface the measurements obtained using different methods should be compared. Since the physics of scattering X-ray radiation by the rough surfaces is well known, the diffuse scattering of X-rays is the most reliable “first-principle” method for roughness measurements [12–15]. In our work we apply an approach based on the perturbation theory (for details, see [27]), which makes possible to recover the power spectral density (PSD) function of the surface directly from the experimental data without a priori assumptions concerning its topography. This is possible due to a linear relationship between the scattering indicatrix and the PSD function. For example, if radiation with the wavelength λ falls at grazing angle θ0, see Fig. 1(a), on a surface with correlation length a, the widths of the scattering indicatrix Φ(θ,φ) in perpendicular directions θ and φ can be estimated as δθ ~λ/παsinθ0 and δφ ~λ/πα. For X-ray radiation and grazing incidence, for isotropic or weakly anisotropic surfaces, this means that δφ
