Journal of the European Optical Society - Rapid publications, Vol 5 (2010)

Scanning deflectometric form measurement avoiding path-dependent angle measurement errors

M. Schulz, G. Ehret, A. Fitzenreiter


For the highly accurate topography measurement of nearly flat optical surfaces, scanning deflectometric methods are capable of achieving nanometer accuracy. In these systems, an autocollimator is typically used as the deflectometric sensor and a pentaprism is applied for the scanning process. When ultimate accuracy is desired, a drawback of these systems is that the autocollimator output signal often depends slightly on the optical path length, resulting in topography errors during scanning. Here, we present a new deflectometric method which separates the angle measurement from the scanning process and, thereby, avoids possible errors due to different optical path lengths. In contrast to conventional deflectometry, the new technique achieves an almost exact autocollimation by appropriately tilting the specimen during scanning. The tilt angle necessary to achieve autocollimation complies with the deflectometric angle determined in conventional deflectometry. The tilt angle is measured with an additional autocollimator at a fixed distance without errors due to different optical path lengths. The separation of angle measurement and the scanning process enable both tasks to be optimized independently. This opens up new possibilities of reducing lateral resolution by facilitating smaller apertures and of assessing topographies with larger curvatures. The concept was tested successfully by a demonstrator setup. The first measurements on a test specimen agree with results obtained with the established Extended Shear Angle Difference (ESAD) technique at the one nanometer level.

© The Authors. All rights reserved. [DOI: 10.2971/jeos.2010.10026]

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I. Weingärtner, and M. Schulz, "Novel scanning technique for ultraprecise measurement of slope and topography of flats, spheres and aspheres and complex surfaces" Proc. SPIE 3739, 274-282 (1999).

I. Weingärtner, M. Schulz, and C. Elster, "Novel scanning technique for ultra-precise measurement of topography" Proc. SPIE 3782, 306-317 (1999).

C. Elster, and I. Weingärtner, "Solution to the shearing problem" Appl. Opt. 38, 5024-5031 (1999).

R. Geckeler, and I. Weingärtner, "Sub-nm topography measurement by deflectometry: Flatness standard and wafer nanotopography" Proc. SPIE 4779, 1-12 (2002).

G. Ehret, M. Schulz, M. Stavridis, and C. Elster, "A new flatness reference measurement system based on deflectometry and difference deflectometry" in Fringe 2009: 6th International Workshop on Advanced Optical Metrology, W. Osten, M. Kujawinska, eds., 318-323 (Springer-Verlag, Berlin, 2009).

G. Ehret, M. Schulz, M. Baier, and A. Fitzenreiter, "A new optical flatness reference measurement system" Proc. DGaO 110, P22 (2009).

M. Schulz, G. Ehret, M. Stavridis, and C. Elster, "Concept, design and capability analysis of the new deflectometric flatness reference at PTB" Nucl. Instrum. Meth. A 616, 134-139 (2010).

F. Siewert, T. Noll, T. Schlegel, T. Zeschke, and H. Lammert, "The nanometer optical component measuring machine: a new sub-nm topography measuring device for x-ray optics at BESSY" Proc. AIP 705, 847-850 (2004).

H. Lammert, T. Noll, T. Schlegel, F. Siewert, and T. Zeschke, "Optisches Messverfahren und Präzisionsmessmaschine zur Ermittlung von Idealformabweichungen technisch polierter Oberflächen" Patent No. DE 103 03 659 (2005).

F. Siewert, H. Lammert, and T. Zeschke, "The nanometer optical component measuring machine" in Modern Developments in XRay and Neutron Optics, A. Erko, M. Idir, T. Krist, A. G. Michette, eds., 193-200 (Springer-Verlag, Berlin, 2008).

S. G. Alcock, K. J. S. Sawhney, S. Scott, U. Pedersen, R. Walton, F. Siewert, T. Zeschke, F. Senf, T. Noll, and H. Lammert, "The Diamond-NOM: A non-contact profiler capable of characterizing optical figure error with sub-nanometre repeatability" Nucl. Instrum. Meth. A 616, 224-228 (2009).

P. Z. Takacs, S. Qian, and J. Colbert, "Design of a long-trace surface profiler" Proc. SPIE 749, 59-64 (1987).

S. Qian, P. Z. Takacs, and W. Jark, "The penta-prism LTP: A long-trace-profiler with stationary optical head and moving penta prisma" Rev. Sci. Instrum. 66, 2562-2569 (1995).

R. D. Geckeler, and A. Just, "Distance-dependent influences on angle metrology with autocollimators in deflectometry" Proc. SPIE 7077, 70770B (2008).

F. Siewert, J. Buchheim, and T. Zeschke, "Characterization and calibration of 2nd generation slope measuring profiler" Nucl. Instrum. Meth. A 616, 119-127 (2010).