Optical Characterization of Nanostructured Surfaces

Publication: ResearchPh.D. thesis – Annual report year: 2016


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Micro- and nanostructured surfaces are interesting due to the unique properties they add to the bulk material. One example is structural colors, where the interaction between surface structures and visible light produce bright color effects without the use of paints or dyes. Several research groups are investigating the manufacturing of these structures using established high-volume polymer fabrication methods, such as injection molding and roll-to-roll manufacturing. These methods are interesting as they can ease the industrial uptake of nanopatterning technology. However, for a successful industrial implementation, a range of complementing characterization methods is needed to perform high-speed quality control of the nanostructures.
This thesis concerns the development of a new method for fast in-line characterization of periodic nanostructures. The focus is on optical scatterometry, which uses inverse modeling to evaluate the dimensions of subwavelength gratings, by correlating the reflected light measured from the structures with a database of simulations. A new method is developed and termed color scatterometry, since compared to typical spectroscopic scatterometry, which evaluates the full reflection spectrum; the new method only evaluates the color of the reflected light using a standard RGB color camera. Color scatterometry provides the combined advantages of spectroscopic scatterometry, which provides fast evaluations, and imaging scatterometry that provides an overview image from which small regions can be analyzed independently. With color scatterometry, a single exposure with the camera is sufficient to evaluate the grating profile for thousands of individual regions spanning a millimeter-sized area. The accuracy of color scatterometry is evaluated on injection molded polymer line gratings, with trapezoidal profiles approximately ~200 nm high and with periods between 600 nm and 5000 nm. The heights and filling factors are determined with an accuracy of ~8 %, while the sidewall slopes have larger uncertainties due to a lower influence on the reflected light.
The thesis also evaluates the use of angular scatterometry for characterization of nanoscale surface roughness. This study is motivated by the need for highly polished surfaces for the production of master molds in injection molding and roll-to-roll manufacturing. Three characterization instruments are compared: a confocal optical profiler, a high-resolution laboratory scatterometer, and a simple commercial scatterometer designed for in-machine measurements. The study is focused on characterizing the commercial scatterometer, to support the implementation of in-situ roughness evaluation during polishing processes. We present an algorithm for expanding the length scale of evaluated surface structures, and a method for converting the standard output parameter, “Aq”, to the more widely used root-mean-square roughness parameter (Rq). The study also includes a detailed analysis of the range of spatial surface wavelengths correctly evaluated by each instrument, and a small study of the implications if the sample surface is covered with an interface layer, e.g. a thin liquid film.
For roughness evaluation on hard-to-reach surfaces, the thesis includes a study of surface replication using the thermosetting polymer PDMS.
Original languageEnglish
PublisherDTU Nanotech
Number of pages168
StatePublished - 2016
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