Optical topography measurement of steeply-sloped surfaces beyond the specular numerical aperture limit

Thomas, Matthew; Su, Rong; de Groot, Peter ; Leach, Richard

Abstract

Engineered functional surfaces often feature varying slopes on macro- and micro-scales. When surfaces are mirror-like, the highest surface slope that can be measured by a far-field 3D imaging optical surface measuring instrument is the arcsine of the numerical aperture (NA) of the objective lens, i.e. the acceptance angle of the lens. However, progress in instrument design has allowed for measurement of non-specular surfaces with slopes steeper than this “traditional” NA limit. Nonetheless, there is currently a lack of understanding about the instrument response to surfaces with steep slopes beyond this limit. It is unclear over what surface spatial frequencies we can expect to accurately report fine surface-feature details. Here we present results demonstrating the capability of a commercial coherence scanning interferometer for measuring surface topography of a roughened flat and a blazed grating with tilt angles greater than the NA slope limit. We show that the surface form, i.e. the tilted plane, can be measured correctly. But, while surface texture information that can appear useful is also obtained, tilting significantly influences the measurement accuracy of micro-scale texture, and for asymmetric gratings, can depend on the tilting direction. A simplified surface scattering model suggests that the loss of scattered power captured by the instrument and a low signal-to-noise ratio causes the reduction of measurement accuracy. However, a rigorous three-dimensional instrument model is needed for a full understanding; we will develop this in our future work.

Citation

Thomas, M., Su, R., de Groot, P., & Leach, R. (2020). Optical topography measurement of steeply-sloped surfaces beyond the specular numerical aperture limit. Proceedings of SPIE, 11352, Article 1135207. https://doi.org/10.1117/12.2554568

• Journal Article Type: Conference Paper
• Conference Name: Optics and Photonics for Advanced Dimensional Metrology
• Conference Location: Online Only, France
• Acceptance Date: Mar 4, 2020
• Online Publication Date: Apr 1, 2020
• Publication Date: Apr 1, 2020
• Deposit Date: Apr 22, 2020
• Publicly Available Date: Apr 22, 2020
• Journal: Proceedings of SPIE - The International Society for Optical Engineering
• Print ISSN: 0277-786X
• Electronic ISSN: 1996-756X
• Publisher: Society of Photo-optical Instrumentation Engineers
• Peer Reviewed: Peer Reviewed
• Volume: 11352
• Article Number: 1135207
• DOI: https://doi.org/10.1117/12.2554568
• Public URL: https://nottingham-repository.worktribe.com/output/4326049
• Publisher URL: https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11352/2554568/Optical-topography-measurement-of-steeply-sloped-surfaces-beyond-the-specular/10.1117/12.2554568.short
• Additional Information: Copyright 2020. Society of Photo‑Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this publication for a fee or for commercial purposes, and modification of the contents of the publication are prohibited.