To remain competitive, European manufacturers strive to make constant improvements in their manufacturing processes. The surface topography of a component part can have a profound effect on the function of the part. This is true across a wide range of industries (such as precision engineering, automotive and medical). It is estimated that surface effects cause 10 % of manufactured parts to fail which has financial implications. Optical measuring systems are widespread in surface and coordinate metrology as they are fast, with high resolution, and contactless (aspects that are essential for the factory of the future). Unfortunately, optical measurements are not often used in industry as they are not traceable. This is due to the complexity of the interaction between the object’s surface and the measuring system. This project aims to improve the traceability of 3D roughness and dimensional measurements using optical 3D microscopy and optical distance sensors. Data evaluation, and uncertainty estimation methods will be developed, and be made accessible to industry by good practice guides, publications, and training courses.
For European industry to remain competitive improvements of inline measurements in manufacturing processes across a wide range of industries (such as precision engineering, automotive and medical) are required. Optical microscopes and optical distance sensors have become indispensable in manufacturing processes. The possibility to integrate these into production lines enables fast and non-destructive measurements, and fast data evaluation accelerates the evolution of Industry 4.0. The European dimensional metrology market generated revenues of $1.11 billion in 2017 and is estimated to continue growing by 6 % (CAGR, Compound Annual Growth Rate) until 2022.
EURAMET comparison measurement #1242 on areal roughness measurements by optical microscopes, has revealed discrepancies of roughness values, e.g. Sa (arithmetical mean height) and Sq (Root mean square height), which strongly depend on the measurement principles. For example, the deviation of Sq can reach 85 % for Sq ~ 50 – 60 nm and 60 % for Sq ~ 100 nm. Moreover, the measured parameters also depend on the optical instrument’s setup, the chosen analysis strategy, and the feature geometries of the surfaces, i.e. amplitudes, spatial frequencies, slopes, and curvatures. Each of these is critical to understand if a given instrument can reliably, accurately and traceably measure a certain roughness. Yet, end users (e.g. precision engineering and automotive manufacturers) have little guidance on the selection of a suitable instrument (and align the settings) to accurately and traceably measure roughness. As a result, often tactile measurements are carried out. These are traceable but much more time consuming and costly.
Dimensional measurements with optical distance sensors are also strongly influenced by the surface and geometrical characteristics of the measured object (in particular roughness, form and slope). This results in a significant deviation in the determined geometrical measurands, and it limits traceable optical measurements of dimensions. This influence is not considered in acceptance and reverification testing according to ISO 10360. Only optical cooperative reference standards are used for the tests. Therefore, the specifications in the data sheets are of limited value for the selection of the appropriate sensor for a specific measurement task.
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 characterise 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 rigours 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 organisations (e.g. ISO TC 213 WG10 and WG16) and end users (in the fields of optical, semiconductor, automotive and mechanical engineering).
Progress beyond the state of the art and results
Determination of suitable surface texture parameters of different types of samples
In recent projects like EURAMET comparison #1242 only basic roughness parameters are evaluated and there is no systematic approach for the classification or the surface texture parameter determination of the utilised samples.
The project will for the first time develop a systematic classification of different types of samples, beyond conventional roughness artefacts and conventional artefacts for testing of coordinate measuring systems:
- new developed areal roughness standards used in EURAMET comparison #1242,
- typical technical surfaces made by conventional and new advanced manufacturing methods like FIB, lithography, and additive manufacturing,
- spherical and aspherical samples with different material and roughness, and workpiece like standards,
- and typical samples from biotechnology and medicine.
Beyond the spatial wavelength range, the local slopes and curvatures and the BRDF of the artefacts will also be determined.
Characterisation of the measurement capabilities of 3D optical microscopy and optical distance sensors
The state of the art of testing procedures of optical microscopes and optical distance sensors rely on the application of artefacts featuring cooperative surfaces. Standards like ISO 10360-8 do not take workpiece characteristics into account.
The characteristics of optical microscopes and optical distance sensors in dependence on the workpiece characteristics will be investigated using appropriate reference standards. Sinusoidal chirp standards developed at PTB and at TUK, rectangular resolution standards from SiMetrics and spheres will be used to characterise the topographic spatial resolution, topography fidelity, structural resolution and maximum measurable slope. The most suitable material measure for topography fidelity investigation will be recommended. The project also aims to improve lateral resolution of optical instrumentation by exploring the suitability of novel nanoscopic techniques based on the AMI method for surface-topography measurements.
Development of numerical models
Approximate and rigorous methods will be used to model light scattering and diffraction by a surface. The conditions for validity for these models will be defined in this project as this has not been achieved by now.
Universal theoretical platforms will be developed, which allows prediction of an optical instrument response, from any complex surface geometry, and to allow such platforms to be used in uncertainty evaluation.
Virtual instruments of some typical optical instruments such as CM, CSI and FV and ODS will be developed and verified by comparison with real instruments. Systematics deviations, correlation between lateral resolution (topography fidelity, structural resolution), PSD and roughness and dimensional parameters will be investigated using the virtual instrument models.
Development and validation of procedures for the selection of appropriate instrumentation for a given measurand
The project will go beyond the state of the art by delivering Good Practice Guides with validated procedures for the selection of appropriate instrumentation for measuring roughness and dimension. These guides will cover four different measuring principles, setups, data analysis including stitching and simplified uncertainty estimation.
Impact on industrial and other user communities
The metrology developed within the framework of this project will lead to benefits for the fast and non‑destructive control of surface quality and geometry for the optical, semiconductor, automotive industries, precision mechanical engineering, advanced manufacturing industry (including workpieces fabricated with new manufacturing technologies such as additive manufacturing, laser manufacturing and FIB), medical industries, biotechnology, metrology service providers, precision investment casting, and consequently will accelerate the evolution of industry 4.0 and the Key Enabling Technologies (KETs).
The manufacturers of optical measuring instruments will strengthen their position on the world market and secure jobs. The speed and non-destructive working principle of optical measuring methods will have a huge advantage, which will be exploited and strengthened.
The needs of industrial users will be clarified and fed into the project through the Stakeholder Committee (SC) and the feedback from the SC will be collected for the development of good practice guides. Corresponding guidelines for industrial users and end-users from various disciplines, on which instrument setup should be used for traceable measurements within a specified uncertainty range, will be available on the project website and as publications. The technologies developed during the project will be transferred to industry and cross‑disciplinary end-users through training courses/workshops, collaborations, and case studies
Impact on the metrology and scientific communities
Metrology impact: A broad range of end users will profit from the project outcomes with the ability to select the appropriate sensor and settings to obtain traceable measurement results. This will reduce measurement uncertainties and costs. In addition, calibration laboratories, NMIs, research institutes and universities will also benefit from the results due to the possibility of offering services for traceable 3D roughness and dimensional measurements as well as traceable reference materials.
Scientific impact: This project provides a significant step forward in traceable dimensional and roughness measurements with optical measuring systems. This will lead to a deeper understanding of the relation between topography features, sensor characteristics and measurement results and reduce uncertainty. The developed measurement methods will be presented at the international conferences and/or submitted to open access peer-reviewed journals during the project. Good practice guides will be made available on the project website and institutes’ newsletters, the output of the project will be discussed by stakeholder meetings, and be advertised through workshop, symposia or special sessions of international or European conferences.
Impact on relevant standards
The project will make significant contributions to national and international standards. The metrological outputs of this project in the fields of surface metrology will be presented to standardisation committee ISO/TC 213/WG 16 “Areal and profile surface texture” especially for the further development of corresponding standards of 25178-70x series and be transferred to the national level such as DIN NA 152-03-03 AA: Surfaces, UNM 10 GPS, METSTA TC K290, UNI CT047 GL Superfici. Guidelines for calibration of CSI and CM will be further developed or revised in the German VDI/VDE-GMA Technical Committee 3.41 "Surface Measurement Technology in Micro- and Nanometer range". In the field of dimensional metrology, the results will be presented to ISO/TC 213/WG 10 “Coordinate Metrology” for the further development of the standards ISO 10360-8 and the upcoming ISO 10360-13. A guideline for the application of ISO 10360-8 will be developed in the German Technical Committee VDI/VDE-GMA 3.31 / DIN/NA 152-03-02-12 GUA KMT “Coordinate Measuring Machines”. In addition, the results of this project will be disseminated through discussions and presentations at meetings of EURAMET TC Length committee. Feedback on the presented results will be sought.
Longer-term economic, social and environmental impacts
Improved optical measurements will lead to an increased use in industry and this will save time and costs and improve the competitiveness of European industry. Providing the European manufacturing industry with a strong tool such as traceable optical measurements, will help the industry to keep their production in Europe and to reduce the amount of outsourcing to, for example Asia. The research will accelerate the evolution of industry 4.0 and the Key Enabling Technologies (KETs) by fast and traceable measurements enabling non‑destructive 100 % control of manufactured surfaces and geometries. Moreover, it will support innovative designs because of better measurement capabilities for geometrical and surface roughness specifications of components with smaller tolerances.
Any European enterprises which require measurements of complex parts will benefit from the project results as their manufacturing costs will be reduced and their market penetration will increase. The manufacturers of optical measuring instruments will strengthen their position on the world market and secure jobs on a sustained basis. Reliable precision parts further result in improved long-lasting technical solutions.
The increased use of optical sensors will speed up product development and production processes, and it will reduce the energy used in the manufacture of physical prototypes. The use of traceable optical measurements in quality assurance and quality control will result in the earlier detection of scrap parts with an increased possibility to reuse materials and components as well as reducing waste.