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Production sequence of Si-spheres and interferometrical determination of the sphere volume

Precision Engineering

Division 5

News from the Annual Report 2017

Fundamentals of Metrology

Precision angle metrology with autocollimators is affected substantially by the refractive index of air and by air pressure in particular. When calibrations obtained at different locations are compared, pressure differences due to weather must be considered. PTB has developed strategies for correcting measuring results and for the uncertainty evaluation.

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Für die Neudefinition des Kilogramm ist eine genaue Volumenbestimmung der 28Si-Kugeln erforderlich. Durch eine Modellierung des optischen Aufbaus und eine Simulation der Messabfolge konnten die Einflüsse von Fehlstellungen der Optiken sowie von realen Oberflächen berechnet werden. Dadurch war es möglich, diese Unsicherheiten erstmals genauer zu bestimmen und so die Gesamtunsicherheit deutlich zu reduzieren.

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A new double-path heterodyne interferometer with spatially separated input beams showed periodic deviation with an amplitude of 0.63 nm caused by multi-reflections. The nonlinearities were characterized in a measurement with a non-constant moving speed by means of amplitude spectra for spline-interpolated data using the average speed. In this way the requirement of a constant speed, which is difficult to realize practically, is replaced by the need of a constant data acquisition rate.

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Metrology for Economy

The contact resonance method, known from atomic force microscopy, was successfully tested on PTB's profile scanner with 5 mm long piezoresistive micro-probes. Using a polycarbonate reference sample whose indentation modulus was determined by nanoindenation, the modulus of elasticity of a SU-8 polymer sample was successfully determined. In the future, not only roughness measurements but also Young’s modulus measurements in microbores <100 μm in diameter will be possible.

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Within the scope of an MNPQ project a novel calibration technique for areal inspection of threads has been developed. This includes a special strategy for tactile probing as well as a sophisticated evaluation algorithm by means of holistic best-fit. The novel measurement concept offers a holistic calibration of threads on a universal coordinate measuring machine with newly defined measurands. This knowledge will now be transferred to standardisation bodies and calibration services.

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Datasets have been developed for a round robin of the German Calibration Service to compare the algorithms employed to evaluate roughness parameters RSm, Rc. These parameters are important for functional surfaces. The implementation of the most recent version of the RSm/Rc algorithm in PTB’s roughness reference software RPTB has been finished and verified. Furthermore, a filtration algorithm optimized for highly stressed surfaces has been implemented in RPTB.

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In the project CT-ÜW, which PTB and Volume Graphics GmbH, Heidelberg, worked on, a task-specific interim check procedure was developed and implemented for industrial CT measurements. The developed procedure does not only allow to compare measurement results with predefined tolerances, but provides additionally statements about the applicability of analyses and thus the reliability of achieved CT results.

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The precise measuring of the thickness of foils or wafers in the range some micrometers until about one millimeter is required for different industrial applications. Increasing inquiry from the industry made a reconstruction and modernization of the measuring facility necessary. Focus of the modernization lay out the simplification and automation of the measurement what leads essentially to a cost reduction, at last also for the customer.

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The PTB organised a successful national intercomparison on thread measurement for the first time since 2002. In cooperation with the Technical Committee “Length” by the German Calibration Service (DKD) 20 DAkkS laboratories participated using their accredited and not accredited measurement procedures. They conducted the comparative measurement on 4 thread measurement standards to assure the quality of their results. 

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In a cooperation with the Technical University of Braunschweig in the framework of the Laboratory for Emerging Nanometrology (LENA), the mechanical properties of silicon and gallium nitride micro- and nanopillars were measured by nanoindentation. A difficulty resulted from the small diameter of the pillars of <1.6 µm which moreover, should result in an additional compressibility of the pillars during the indentation. This effect has been experimentally confirmed for the first time.

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In der PTB steht jetzt ein neues Referenzmesssystem für die rückführbare Messung von großen Bauteilen mit Abmessungen bis zu 2 m × 2 m × 10 m bereit. Eingesetzt wird das System derzeit für die messtechnische Prüfung von Messanlagen für Rohholz in der Forstwirtschaft und holzverarbeitenden Industrie. Es stellt mit genau vermessenen Abschnitten von Baumstämmen die metrologische Rückführung dieser Messgeräte in der Holzwirtschaft sicher.

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Camshafts are of major influence for the performance of combustion engines. For the assessment of the cam form quality it is necessary to compare the measured and the intentional design profile. For this purpose, the design profile is reconstructed by a B-spline based function and is transformed to the measured data. This procedure allows to evaluate deviations from the cam profile and therefrom derived cam parameters.

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The implementation of a position-dependent correction of the homodyne yaw-interferometer of the nanometer comparator enables the PTB to calibrate straightness deviations with sub-nanometre uncertainty. This capability was used to analyse the limitations of straightness interferometers, especially the influence of the divergence angle. For a displacement of 250 mm it could be shown that an enlargement of the angle of divergence reduces the uncertainty.

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In collaboration with the Advanced Mask Technology Center (AMTC), a new EUV photomask standard has been developed. Different parameters such as the structure width (CD), CD uniformity (CDU), pitch, height, sidewall angle, line edge/width roughness (LER/WR) etc. were calibrated accurately and traceably using 3D/CD-AFM and HR-TEM techniques. A measurement uncertainty for CD of 2.5 nm was obtained. The standard serves as a “golden standard” for several CD tools of the AMTC.

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During the CTE measurements at low temperatures in the ultra-precision interferometer it was necessary to include a daily warm-up phase to avoid the condensation of CO2. This was caused by the contamination of the contact gas which condensed on the sample. By means of an activated carbon absorber around the sample the condensation of CO2 and, therefore, the warm-up phase can be prevented. Thus, the multi-week duration of the measurements could be reduced by more than one third.

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The German association of professional surveyors DVW e.V. (Deutscher Verein für Vermessungswesen – Gesellschaft für Geodäsie, Geoinformation und Landmanagement e.V.) has adopted two good practice guides developed by the EMRP joint research project "SIB60 Surveying: Metrology for long distance surveying" which had been coordinated by PTB.

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Within a national research project PTB developed the worldwide largest involute gear ring standard. It embodies three different internal and external gears on a ring with a diameter of about 2 m. A national intercomparison on twelve different measurement machines has been successfully conducted and evaluated in relation to PTB’s calibration values.

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Within the scope of the EMPIR project "Strength-ABLE", a picoindenter was developed based on a microelectromechanical system (MEMS). The system is designed to bridge the measurement gap between scanning force microscopes (AFM) and nanoindentation devices. It offers indentation forces up to 400 μN and reaches indentation depths up to 9 μm at a force resolution of 9 nN. Commercially available AFM cantilevers with diamond tips can be used as indenters.

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Metrology for Society

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International Affairs

PTB has completed the first key comparison using an autocollimator as a standard in which 25 participants are involved from all over the world. The comparison enabled us to compare the participant’s calibration capabilities. In addition, new knowledge on the influence of environmental changes on the angle metrology with autocollimators could be obtained.

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