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Wider Adoption of Optical Sensors for Control of Part Surfaces and Geometries Planned

Surface topography significantly influences the functionality of manufactured components; about 10 % of manufactured parts fail due to surface effects, while surface roughness affects measurement performance in lithography, additive manufacturing processes and biological samples. High-resolution, contactless measuring systems, such as optical microscopes and optical distance sensors, could speed up quality control processes, and support digitalization concepts such as industry 4.0, but are infrequently used in industry due to complex interactions between devices and surfaces.

The EURAMET project on comparison measurement study #1242 on areal roughness measurements by optical microscopes showed that roughness measurements depend on choices of measurement principles applied, and measured parameters depend on instrument setups. Yet, end-users are offered little guidance on instrument selection, meaning tactile measurements are more usually performed. These non-optical methods, while accurate, can be destructive, slow, costly, and harder to integrate into production lines.

The EUROMET project will enable accurate 3D roughness and dimensional measurements using optical 3D microscopy and distance sensors, and produce guidance on instrumentation selection.

Data evaluation and uncertainty estimation methods will be developed and procedures devised to guide selection of instrumentation. Industrial take-up of the developed technologies and advice will be promoted to measurement supply chains, standards organizations and end-users via good practice guides, publications, and training courses.

The developed metrology will encourage wider adoption of optical sensors for faster, more accurate non-destructive control of manufactured component surfaces and geometries. The inherent time efficiency and cost advantages could then accelerate product development and production processes, and so help improve Europe’s industrial competitiveness.

Overall Objectives

The overall objective of the project is to enable traceable areal roughness and dimensional measurements using optical 3D microscopy and optical distance sensors, with special emphasis on giving guidance for selection of most suitable instrumentation for a particular purpose. The specific objectives are:

To determine suitable surface texture parameters and, in some cases, dimensional properties of different types of samples: (i) available well-known roughness standards (profile and areal, coarse to superfine roughness (Sa from several µm down to several nm) (ii) typical technical surfaces made by turning, milling, grinding, polishing, lapping or spark-erosion and roughness samples produced by new manufacturing technologies (e.g. FIB, lithography, and additive manufacturing); (iii) spheres with different surface characteristics and (iv) solid roughness samples from biology and medicine.

To characterize the measurement capabilities of 3D optical microscopy, AMI interferometric nanoscopy and optical distance sensors, including (i) measurable local slope distribution, (ii) bandwidth by power spectral density (PSD) (iii) measurement noise, (iv) influence of the bidirectional reflectance distribution function (v) topography fidelity and structural resolution and (vi) length error for distance measurements. Additionally, to investigate the influence of (i) measurement principle, (ii) hardware setup, (iii) feature geometries (e.g. amplitude, spatial frequency, slope distribution, curvature) and (iv) software on areal roughness and dimensional measurements.

To develop numerical models to predict the sensor response for any complex surface geometry, and to allow such models to be used for systematic error analysis based on analytical models or computation models using rigors 3D Maxwell solvers for solving the light matter interaction (e.g. Fourier modal methods, Rigorous coupled-wave analysis (RCWA) methods, finite element (FE), or boundary element (BE) methods). This will include the development of approaches for the correlation between roughness and dimensional parameters and the PSD, topography fidelity and slope distribution. Additionally, to evaluate the performance of a systematic error analysis and error correction.

To develop and validate procedures for the selection of appropriate instrumentation for a given measurand. The target uncertainties are 5 nm (10 % deviation for 50 nm < Sq < 100 nm) for optical roughness measurements and 100 nm for optical dimensional measurements. This will include the development of methods for data evaluation and simplified uncertainty estimation.

To facilitate the take up of the technology, measurement infrastructure and good practice guides developed in the project by the measurement supply chain, standards developing organizations (e.g. ISO TC 213 WG10 and WG16) and end users (in the fields of optical, semiconductor, automotive and mechanical engineering).

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