OW1B.3.pdf Classical Optics 2014 © 2014 OSA Measuring High-Slope and Super-Smooth Optics with High-Dynamic-Range Coherence Scanning Interferometry Martin F. Fay, Xavier Colonna de Lega, and Peter de Groot Zygo Corporation, Laurel Brook Lane, Middlefield, Connecticut, USA 06455 mfay@zygo.com Abstract: Advances in the implementation of coherence scanning interferometry have dramatically extended the range of application for this well-known technique. New data acquisition and data processing methods significantly improve dynamic range, enabling measurements of steeply-sloped surfaces usually considered beyond the reach of high-NA objectives. Hybrid data acquisition incorporating sinusoidally-modulated phase shifting reduces signal-to-noise to the 0.1 nm/√Hz level, extending the technique to super-polished surfaces. OCIS codes: 120.3180 Interferometry; 120.3940 Metrology; 120.6660; Surface measurements, roughness 1. Introduction Optical components present a number of metrology challenges, including reflectances ranging from mirror-like to anti-reflective, slopes from normal to near-vertical, and surface roughness from multi-μm to sub-Å. For optical metrology, these collectively correspond to signal strengths spanning several orders of magnitude. Coherence scanning interferometry (CSI) provides non-contact areal topography maps with typical single- measurement topography repeatability on smooth, high-reflectivity surfaces of less than a nm [1, 2]. Recent advances in the technology both significantly improve the baseline sensitivity of CSI and enable high-dynamic- range operation. High-dynamic-range CSI can measure previously-inaccessible high slopes and sub-Å roughness, capabilities well-suited for measuring optical components. 2. Measurement and analysis A modern commercial CSI microscope was used to measure a variety of challenging optical components [3]. The following sections each highlight a different technique to boost dynamic range: dynamic noise reduction (DNR) for detection of weak signals; high dynamic range (HDR) for parts with wide reflectance ranges; and CSI combined with sinusoidally-modulated phase shifting interferometry (PSI) for measurement of low-departure super-smooth surfaces. For these experiments, the CSI data acquisition rate is 0.14 seconds per micron scanned. All surface plots shown represent raw height data without smoothing, masking or interpolation of missing data points. 3. Dynamic noise reduction (DNR) Fig. 1. Measurements of (a) polymer micro-lens using 3X DNR; and (b) retroreflector array using 4X DNR. For an ideal measurement setup the highest observed slope   is accommodated within the specular limit of the objective NA, satisfying     . However, in practice a lower-NA objective may be prescribed by