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Dimensional micro and nanometrology using areal topography data produced by 3D surface metrology instruments

Final Report Summary - METROSURF (Dimensional micro and nanometrology using areal topography data produced by 3D surface metrology instruments)

The measurement of micrometric and sub-micrometric surface formations has always been a difficult, but rewarding challenge. Many engineering disciplines heavily rely on quantitative knowledge about surface features that have been generated as a consequence of a manufacturing process, or as a consequence of functional life of a product. Tribologists, for example, are interested in quantification of micro scratches, cavities and other formations generated, e.g. during contact between surfaces; researchers involved in the development of advanced manufacturing processes (e.g. additive manufacturing, and high-precision, unconventional manufacturing) are interested in investigating the physics underlying the fabrication process, and rely on the inspection of the formations left on the surface as a fingerprint of the process; developers involved in the fabrication of micro and sub-micrometric sized products and components (for example, in the fabrication of MEMS and microfluidics devices), need to be able to measure really small functional features for quality control; precision engineers, finally, are interested in measuring very small surface formations which may affect the ultimate performance of their products, such as for example, the micro-scale regularity of optical components such as aspheric or Fresnel lenses.

The lack of appropriate measurement solutions for micro and sub-micrometric surface features has always been a significant bottleneck, and common practice in inspection at such small scales has typically consisted in the use of optical or electron microscopy, which are suitable to provide an overview of what is happening on a surface, but do not reliably provide quantitative measures of size and shape. Without any solution to obtain reliable, quantitative measurement data, wherever possible engineers have relied on functional testing which, while still detecting faulty products, in general does not provide enough information to understand where things went wrong, and how the product, or fabrication process can be improved.

The recent introduction of a whole new series of instruments for quantitative measurement of surface topography at micro and sub-micrometric scales has opened new perspectives. Three-dimensional microscopes/profilometers, mostly based on optical technologies such as focus-variation, confocal laser scanning, and coherence scanning interferometry (but recently including also non-optical techniques such as computed tomography) now allow the acquisition of entire portions of surfaces and the reconstruction of digital models that accurately depict their topographic formations. This is revolutionizing the entire discipline of surface metrology. However, having such new data available is generally not enough to solve the inspection problems described above. Surface topography data analysis has historically been approached through the computation of summary descriptors of “roughness”, the most famous being the arithmetic mean roughness Ra (a measure of the average height error of a profile dataset with respect to a best-fit mean line). Such conventional approaches were developed to operate on profile data and have recently been adapted to operate on areal datasets. The potential of having three-dimensional digital reconstructions of surface topography hasn’t been fully exploited yet, and when it comes to the measurement of the shape and size of local surface formations, no formalized approach is available, and users are left with do-it-yourself solutions such as point-to-point measurement, or manual tracing of contours on top of the digital models.

The METROSURF project has originated from the need to bridge this gap and finally exploit the untapped potential of three-dimensional digital topography models, by developing a complete framework of mathematical methods and software for digital data analysis, dedicated to the automated identification, extraction and quantitative characterisation of localized surface topography formations (surface features).
To foster the development of truly applicable solutions, during its execution the project has addressed a broad range of industrially-relevant scenarios, covering multiple applications in tribological testing, micro-product development, and investigation of advanced manufacturing processes. Many applications have been originated thanks to the numerous collaborations with researchers at the host institution (the University of Nottingham), and with other universities in UK, Europe and worldwide. Some of the test cases have also come from collaborations established with commercial, international companies, some operating in the development of products and manufacturing processes, others in the development of measurement instruments and data analysis software. Some examples amongst the selected applications are listed in the following:
- inspection of high aspect-ratio micro-holes (laser drilling, abrasive waterjet machining)
- inspection of laser-milled micro-pockets (laser milling)
- inspection of micro-channels for microfluidics (soft lithography)
- inspection of conductor micro-ridges (material jetting)
- investigation of light-entrapment microstructures for solar cells (nano-photopolymerisation)
- characterisation of micro-embossing patterns (laser and forming)
- feature-based investigation of additive, polymeric surfaces (selective laser sintering)
- feature-based investigation of additive, metal surfaces (selective laser melting)
- quantitative assessment of wear regions in turbine blades (tribology testing)
- characterisation of size and volume of worn region in fretting wear test (tribology testing)
- characterisation of material jetting artefacts for antibacterial applications (material jetting)
- characterisation of electrolyte jet machined artefact grooves (non-conventional machining)

Starting from the analysis and resolution of the measurement problems originated by the test cases, METROSURF has led to the development of a series of generalized methods and procedures, applicable to a wider array of test cases. Original methods developed within the METROSURF project include (see also list of publications):
- new methods and software for topography segmentation and feature identification (i.e. solutions for the automated partitioning of topography datasets into relevant regions and for the extraction of the relevant topographic formations)
- new methods and software for dimensional and geometrical quantification of complex topographic formations (shape descriptors for surface features)

Moreover, a deeper insight on how surface measurement technologies operate, what measurement errors they generate and how such errors propagate through the dimensional and geometric characterisation of selected surface features, has been gained, which also has led to additional published results (see list of publications).

In addition to providing benefits to the wider scientific community through dissemination via numerous journal papers and conference presentations, METROSURF has contributed solving a wide array of scientific and technical problems within other people’s research, through its contribution to the resolution of the selected test cases. With METROSURF, new awareness has been generated regarding the importance of solving problems of dimensional and geometric measurement of micrometric and sub-micrometric surface formations in a wide array of engineering disciplines. Many new scientific collaborations have been established, so that the project will continue beyond the termination of the funded period.