New Products for Synchrotron Application Based on Novel ... · The measurement data are suitably transformed for the next iterative lapping step. The treatment of the optical surface

New Products For Synchrotron Application Based On Novel Surface Processing Developments

Andreas Seifert

Carl Zeiss Laser Optics GmbH, Carl-Zeiss-Strafie 22, 73447 Oberkochen, Germany

Abstract. Preserving high beam quality of synchrotron light necessitates the use of aspherical optics. Lack of suitable processes as well as high costs for complex optical components made it often impossible bringing aspheres into play for synchrotron application. Meticulous mathematical analysis of processing tools and measurement results leads to improvement of all process steps within a complex process chain. Novel complex surface geometries are accessible, one-dimensional curved aspheres as e.g. elliptic cylinders are no longer exotics, on the contrary these components are established as standard products even for high-end performance requirements. Innovative surface processing developments and suitable process chains make also two-dimensional curved aspheres as e.g. ellipsoids or paraboloids available. Current status and results will be presented.

Keywords: Surface processing, Process chains, Two-dimensional curved aspheres, Local machining, PSD analysis PACS: 41.50.th, 41.85.-p, 41.85.Ct, 42.70.-a, 81.05.-t, 07.85.Qe

INTRODUCTION

Machining of spherical or flat surfaces is a self-stabilizing process, and overall tools will match to the optical surface at any position and for any rotational state. This makes polishing of spheres and flats successful, and high-end performance of optical surface quality can be achieved easily up to about 1 A microroughness or 0.1 arcsec for the slope error without using local fine correction approaches. In many cases even cylindrical and toroidal surfaces can be machined with overall tools up to a certain quality level by similar self-stabilizing processes. But the achievable tolerances are limited, mainly driven by the radii or ratio of radii. Also the absolute size of the optical surfaces restricts the surface quality to be achieved by large tools.

In order to maintain the beam quality of synchrotron light, it is often necessary using aspherical optics, especially for preserving the high quality of new generation X-ray sources. According to the mismatch between large tools and aspherical surfaces machining by small tools can be a proper solution. Complex aspheres make it necessary to study and develop not only suitable tools and corresponding optical processes, but also new mathematical methods for data analysis and for the characterization and control of the local machining process.

PRINCIPAL PROCESS CHAIN FOR ASPHERES

Modern 5-axes computer controlled grinding machines with ultrasonic spindles make nearly arbitrarily shaped substrates and surface geometries accessible. If necessary, further etching steps will be applied, to get rid of residual sub-surface damage and to reduce material stress.

Lapping Of Aspherical Surfaces

Subsequent lapping procedures for aspheres are often realized by Computer Controlled Lapping (CCL) with small subaperture tools of optimized size according to the aspherical behaviour and current status. The lapping procedure has to fulfill several purposes: Reducing of surface and subsurface defects, coarse figuring, precision adjusting of aspheric parameters (e.g. ellipsoidal semi-axes) and high precision adjusting of optical axes with respect to outer dimensions. Last two mentioned points are a result of a very stable interplay between surface processing and

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metrology. A precision computer controlled 3d coordinate measuring machine (Carl Zeiss UPMC 850 Carat) provides the surface data, measured absolutely with respect to the outer dimensions with an accuracy of 0.5 - 1 urn The measurement data are suitably transformed for the next iterative lapping step. The treatment of the optical surface is done by a 6-axes computer controlled robot. Figure 1 shows the coordinate measuring machine, measuring a large mandrel for space application (Constellation-X [1]), and a robot while tooling an aspheric surface.

FIGURE 1. a) 3d-coordinate measuring machine UPMC 850 Carat; b) 6-axes robot tooling an aspherical surface

Polishing And Fine Correction Of Aspherical Surfaces

Polishing of aspherical optics is a demanding iterative procedure [2] with completely different phases: coarse figuring, complete elimination of surface defects, fine correction, reduction of ripples and smoothing. Polishing tools with totally different size, shape and behaviour have to be used, and completely diverse machine kinematics respectively lead to individual work or removal functions. Use of 6-axes technology (CCP: Computer Controlled Polishing) is necessary for several process steps (Fig. lb). Another powerful tool leading to high performance is non-contact tooling by ion beam figuring (IBF) [3]. This technology is also implemented within a complex iterative process chain. Fig. 2a shows the iterative polishing sequence for aspherical surfaces. The ideal process is the straight forward path, but for strong curved aspheres and especially surfaces with strongly variating curvature, several different loops have to be made for a stable convergence with respect to all specified tolerances.

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Metrology And Data Analysis

Besides stable control of optical processes, metrology is the second important pillar of the process chain. In order to qualify all surface tolerances for spatial sampling wavelenghts from some nanometers up to the clear aperture, several measurement tools are necessary: Coordinate measuring devices and interferometers for large and medium scaling, microinterferometry and atomic force microscopy for different small sampling regions.

Figure 2b exemplarily shows the characterization of optical surfaces by completely different metrological methods. All measurements are combined or overlapped within one double logarithmic power spectrum (PSD: Power Spectral Density). The behaviour of the power spectrum characterizes the optical surface quality and gives information about the polishing or lapping processes. According to Parsevals theorem, the rms (root mean square) figure error or roughness values can be integrated for arbitrarily chosen spatial sampling.

MATHEMATICAL ANALYSIS OF PROCESSES

Fourier analyses of measured surface data [4] lead to an understanding of previous process steps. With other words, every tool and corresponding machine kinematics can leave its signature on the surface which may often not be seen obviously but nevertheless may deteriorate the performance. PSD analysis can help unveiling such affection. Fig. 3a shows an example with periodic surface errors occurring as sharp peaks, according to different process steps. A suitable choice of subsequent steps and polishing tools will reduce these error structures, and the process can converge. Work functions with dimensions of the substrate size down to about 1 mm are simulated, optimized and applied for fine correction and surface smoothing. Some characteristic small work functions are shown in Fig. 3b.

1D-PSD

FIGURE 3. a) PSD after different process steps; b) Examples for workfunctions for different tools and machine kinematics

EXAMPLES

Developments and analyses of process steps led to a significant improvement of optical quality and make new aspherical designs accessible. Fig. 4 shows results from two different types of aspheres: a) PSD analysis for an off-axis paraboloid for Naval Research Laboratory (NRL). It is one of four grating substrates for the Joint Astrophysical Plasmadynamic Experiment (J-PEX) high-resolution EUV spectrometer [5]. Holographic patterning has been done by Carl Zeiss, too. This impressive example shows the excellent control of surface processing for all spatial regions: The measurement data fit accurately over about 13 power decades, b) Off-axis ellipsoid (length 510 mm) for the VUV-FEL at DESY. This example shows that high optical quality is even available for ellipsoidal mirrors.

An example for a high accurate off-axis elliptic cylinder is given in Fig. 5. The end-finishing of the optical performance was supported by BESSY measurements by means of the Nano Optics Measurement machine NOM.

SUMMARY

Mathematical analysis of single process steps leads to improvements of optical process chains and control. Stable iterative process chains could be settled even for strong curved 2d-aspheres. The convergence of the process depends on the curvature of the optical surface and on the local variation of the curvature. It could be shown that slope error values below 0.3 arcsec are achievable, better performance should be accessible, too, but suitable metrology has to

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be developed. Finishing of ld-aspheres (e.g. elliptic or parabolic cylinders) is mainly limited by metrology. The optical process chain is an established procedure even for high end performance mirrors, slope errors below 0.1 arcsec are accessible.

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ACKNOWLEDGMENTS

Special thanks for support, contributions and great cooperation to: K. Tiedtke, U. Hahn (DESY, Hamburg), R. Sankari (University of Oulu), R. Nyholm (MAX-lab, Lund), T. Zeschke, F. Siewert (BESSY, Berlin), R.G. Cruddace, M.P. Kowalski, F.B. Berendse (NRL, Washington) and H. Lasser (Carl Zeiss Laser Optics, Oberkochen).

REFERENCES

W. Egle, A. Matthes, G. Willma, A. Ilg, A. Schmidt, A. Seifert, "Figuring, polishing, metrology, and performance-analyses of Wolter type 1 forming mandrels for the Constellation-X mirror development program", Proc. SPIE 5488, 2004, pp. 351-360 U. Dinger, F. Eisert, H. Lasser, M. Mayer, A. Seifert, G Seitz, S. Stacklies, FJ. Stickel, M. Weiser, "Mirror substrates for EUV lithography: progress in metrology and optical fabrication technology", Proc. SPIE 4146, 2000, pp. 35-46 H. Handschuh, J. Froschke, M. Jlilich, M. Mayer, M. Weiser, G Seitz, "Extreme ultraviolet lithography at Carl Zeiss: Manufacturing and metrology of aspheric surfaces with angstrom accuracy", /. of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1999, Vol. 17, Issue 6, pp. 2975-2977 H. Glatzel, D. Pauschinger, HJ. Frasch, H. Gross, "Optical performance prediction of thin-walled Wolter type 1 mirror shells for X-rays in the 1 to 8 keV energy range", Proc. SPIE 1742, 1992, pp. 245-255 M.P. Kowalski, F.B. Berendse, T.W. Barbee Jr., W.R. Hunter, K.F. Heidemann, R. Lenke, A. Seifert, R.G Cruddace, "The Joint Astrophysical Plasmadynamic Experiment (J-PEX) high-resolution EUV spectrometer: Diffraction grating efficiency", to be published in Proc. SPIE, 2006

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