Overview

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.

Need

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.

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

1. 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.
2. 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.
3. 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.
4. 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.
5. 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

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 such as new developed areal roughness standards used in EURAMET comparison #1242, or fabricated by FIB, or new lithography methods in advanced manufacturing. Beyond the height parameters, the spatial wavelength range, the local slopes and curvatures and the BRDF of the artefacts will be determined.

In order to characterise measurement capabilities of 3D optical microscopy and optical distance sensors, 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.

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, 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 four typical optical instruments, CM (confocal microscope), CSI (coherence scanning interferometry), FV (focus variation microscope) and ODS (optical distance sensors), 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.

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.

Results

Determination of suitable surface texture parameters of different types of samples

In the first nine months of this project, different types of samples including spheres, chirp structures, RS-M and RS-N, ARS samples, typical surfaces fabricated by conventional and new advanced manufacturing methods and typical samples from biotechnology and medicine have been selected or developed and characterised. The surface texture and dimensional parameters have been characterised using AFM or tactile instruments and BRDFs of samples have been measured using a gonioreflectometer.

Relevant surface texture and dimensional parameters characterized by AFM, tactile instruments or gonioreflectometer will be further collected. From the list of available samples, the most suitable samples have been selected for the comparison measurements within 4 groups of optical instruments, i.e., CSI, CM, FV and ODS. These include calibration standards (optical flats, spheres, chirp structures, rectangular gratings, ball bars and micro-contours), BRDF samples, and relevant industrial samples from stakeholders. Circulation plans for measurements have also been developed.

Characterisation of the measurement capabilities of 3D optical microscopy and optical distance sensors

The project has started to characterise optical microscopes and optical distance sensors in dependence on the workpiece characteristics. Samples have been developed and selected (e.g. spheres, sinusoidal chirp standards, rectangular resolution standards, BRDF samples and relevant industrial samples from stakeholders) to characterise the maximum measurable slope, topographic spatial resolution/structural resolution and topography fidelity of the instruments.

Technical protocols describing the measurement procedures of the selected samples using the 4 different groups of optical instruments have been developed and the circulation among the project partners has started. The AMI nanosope from TUC has been enhanced to an interferometric nanoscope in the initial phase of the project 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

To compare the scattering models, a clear understanding of the validity conditions of each model is required. Different approximate models based on Kirchhoff approximation are being developed by project partners. The comparison of the results is under development. For more complex geometries, rigorous scattering models  will be applied. A BEM (Boundary Element Method) model, rigorous FEM computations and a software package using BEM are being developed by project partners which can simulate large area 3D geometrical surfaces.

Universal theoretical platforms are now under development, including a virtual CSI based on the foil model of the surface and linear theory of 3D imaging, a virtual CSI combining 3D scattering models and Fourier optics modelling of CSI instrument transfer characteristics and a computationally efficient version of an Elementary Fourier Optics (EFO) model for rapid calculation of the response as a function of surface spatial frequency, for estimation of the instrument transfer function and for evaluating the measurement uncertainty of measurement as a function of the spatial frequency content of an arbitrary smooth surface.

Development and validation of procedures for the selection of appropriate instrumentation for a given measurand

In the first nine months of this project two stitching methods (stage coordinate-based and feature-based cross correlation methods) were investigated, and different stitching modes (manual mode, manufacturer mode and computer-based advanced mode) were compared. The impact of stitching parameters (different overlaps, orientations along fast/slow scan axis, pre-processing parameters) on roughness parameters and dimensional measurements was observed for different samples by using different confocal microscopes. Moreover, to ensure the accuracy of a calibration routine of the CSI, the flatness deviation of the CSI was evaluated by comparing measurements of the CSI at one field of view with the measurement conducted with a confocal point sensor by using a topographic Siemens star (ASG) material measure.

Impact

In the first nine months of the project, a stakeholder committee has been formed (e.g. ZEISS, Sensofar, Precitec, DigitalSurf, Ametek, WaveOptics)  and the first stakeholder meeting was held virtually in January 2022 to update stakeholders on project progress.

To promote the uptake of project results and to share insights generated throughout the project, results were shared broadly with scientific and industrial end-users. Three papers reporting project results have been published in peer-reviewed journals (e.g. Optical Engineering, Metrology) and one article was published in conference proceedings (Applied Optical Metrology). Three presentations have been given at conferences and the project was presented at an international workshop (Wiley Analytical Science, SPIE Optical Engineering, SPIE Photonics Europe, VirtMet) . In addition, the project has been presented at national and international standardization committees (e.g. DIN, ISO, METSTA, AENOR, VDI/VDE) and at EURAMET TC-L.

Impact on industrial and other user communities

The methods, metrology and Good Practice Guides developed within the framework of this project will lead to a better reliability 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 in the global 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.at

stakeholder meetings, and be advertised through workshop, symposia or special sessions of international or European conferences.

In the first nine months of the project, the partners have begun to interact with various scientific, metrological and industrial networks in order to present the preliminary results.

Impact on relevant standards

The project will make significant contributions to national and international standards. The consortium has participated in several committee meetings such as ISO/TC 213/WG 16 Areal and profile surface texture,METSTA TC K290, AENOR UNE CTN82/SC Dimensional Metrology. DIN NA 062-08-02 AA Test Methods, VDI/VDE-GMA Technical Committee 3.41 Surface Measurement Technology in Micro- and Nanometer range and VDI/VDE-GMA Technical Committee 3.31/DIN/NA 152-03.02.12 GUA KMT Coordinate Metrology.

As the project progresses metrological outputs in the fields of surface metrology will be presented to ISO/TC 213/WG16 in order to advance the development of corresponding standards of 25178-70x series and to be transferred to the national level such as DIN NA 152-03-03 AA, 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 ISO 10360-13. The guideline for the application of ISO 10360-8 will be further 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 as well as 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 in 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.