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Monday, 20 April 2020

New optical method for rapid and non-destructive dimensional metrology

The high production of smart surfaces with nanoscale features requires fast and precise quality control technologies. A new method for this aim is described in Scientific Reports. Former ICN2 postdoctoral researcher at the Phononic and Photonic Nanostructures Group Guy Whitworth is the first author of this work, which uses diffraction to determine the critical dimensions of the studied object. The research was integrated in the FLEXPOL EU project, which seeks to develop a pilot line for the production of a cost-effective antimicrobial adhesive film for its use in hospitals.

Smart surfaces with nano- and micro- scale features find application in a wide variety of sectors in modern society: electronics, security, photonics, micro-fluidics… High-throughput nanoimprint lithography (NIL) technologies enable high-volume production of such devices and surfaces. For quality control purposes, inspection technologies must be compatible with these high-speed patterning technologies, i.e., fast and suitable for large area critical dimension measurements. As a consequence, there is an ever-growing need for rapid and non-destructive dimensional metrology techniques to keep pace with the speed of production.

This is what former postdoctoral researcher at the ICN2 Dr Guy Whitworth has presented, together with Dr Achille Francone, Dr Nikolaos Kehagias and ICREA Prof. Dr Clivia M. Sotomayor Torres, Group Leader of the Phononic and Photonic Nanostructures Group. They have proposed a new real-time optical scatterometry technique, diffractometry, applicable at the mesoscale when optical inspection produces multiple orders of diffraction. The results can be read at Scientific Reports.

For complex structural determination, optical scatterometry has become a widely used technique. Scatterometry techniques study scattered light from a surface as a function of a variable such as angle-of-incidence or wavelength. This surface scattering response is then compared to a library of simulated data to fit the measured response to a computational prediction.

The proposed technique works on an analogous principle: the optical diffraction pattern can be used to determine the critical dimensions of the studied object comparing an imaged diffraction pattern to a theoretically computed library. In this technique, the diffraction efficiency is used as a function of diffraction order as opposed to wavelength or angle-of-incidence.

The researchers have validated the method by inspecting silicon gratings with a variety of structural parameters. They have compared these measurements with FIB, SEM and scanning stylus profilometry, showing that diffractometry detects structural and surface defects with extremely high sensitivity, making it easy to identify high fidelity areas and those with a higher degree of defectivity. They have also measured thermally imprinted structures at different imprinting temperatures to demonstrate the method is suitable for in-line quality control in nanoimprint lithography.

Further to the presented work, the experts have explored the possibility to integrate real-time diffractometry in-line to a UV-assisted roll-to-roll nanoimprint, laboratory scale tool. Even though it is promising, technical issues of stability during the roll-to-roll NIL process have yet to be solved. They expect diffractometry to become an important quality control inspection technique for high-volume microstructure production giving critical dimension accuracy down to ~10 nm.

This research was integrated in the FLEXPOL EU project, which aims to develop a pilot line for the production of a cost-effective antimicrobial adhesive film for its use in hospitals. The idea of the project is to apply these films to large areas such as walls and floors and thus extensively minimize contamination with microbes. This is achieved by specially developed nanostructures and polymer materials containing antimicrobial oil blends. If researchers manage to reduce the size of the characteristics of these nanostructures and perform their corresponding metrological analysis, this technology could be a remarkable candidate to prevent hospital infections and, thus, be better prepared for situations like the one we are experiencing these days with the COVID-19 crisis.

Article reference:

Whitworth, G.L., Francone, A., Sotomayor-Torres, C.M. et al. Real-time Optical Dimensional Metrology via Diffractometry for Nanofabrication. Sci Rep 10, 5371 (2020). https://doi.org/10.1038/s41598-020-61975-3