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

Precision Engineering

Division 5

Im Zuge der Revision des SI Einheitensystems, die am 20. Mai 2019 in Kraft trat, hat das beratende Komitee für die „Länge“ (CCL) der internationalen Meterkonvention vorgeschlagen, den Gitterparameter des Siliziumkristalls als Basis für die sekundäre Darstellung des Meters im Nanometerbereich zu nutzen. In der hochauflösenden Mikroskopie sind damit Pikometerunsicherheiten für Messungen von Stufenhöhen bis 10 nm erreichbar.

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Ein DKD-Ringvergleich zur Bestimmung von Rauheitskenngrößen an Raunormalen und Tiefeneinstellnormalen sowie der verwendeten Auswertesoftware zwischen DAkkS akkreditierten Laboratorien und der PTB wurde erfolgreich durchgeführt. Alle akkreditierten Laboratorien konnten ihre Kompetenz im akkreditierten und teilweise auch im nichtakkreditierten Servicebereich nachweisen.

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Die Herstellung additiv gefertigter Werkstücke erweitert signifikant die Möglichkeiten des Wissenschaftlichen Gerätebaus, Kundenanforderungen an Design und Funktionsumfang um-zusetzen. Das Laser-Pulverbettverfahren mit den Grundwerk-stoffen Polyamid, Edelstahl, Aluminiumlegierung und Titanlegierung erlaubt u.a. die Integration von innenliegenden Strukturen wie Kühlkanälen, strömungsbeeinflussenden Strukturen (Diffusoren) oder Sensoren.

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With the increasing use of atomic force microscopes for the mechanical characterization of nanomaterials, the need to determine the contact area of the cantilever tip increases. A novel stylus tip testing standard, developed at PTB with rectangular grooves and edge radii in the nm range, has now been used for the first time to determine the tip shape of an AFM tip. The measurements prove that the 3d tip shape of an AFM diamond tip can be successfully measured.

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FEM simulations were used to identify the most important influencing parameters on the non-linearity of capacitive displacement sensors. Geometric parameters, the surface roughness and the local distribution of surface potentials can also influence the non-linearity of the displacement measurement. For the validation of the simulations, capacitive sensors with different properties were manufactured and an experimental setup for measuring the non-linearity of these sensors was put into operation.

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The metrological large range atomic force microscopy (Met. LR-AFM) has been further extended for reference metrology for quantitative areal surface measuring 3D-microscopy. The application includes the traceable and accurate calibrations of ISO 25178 series of standards, as well as the development of a novel circular chirp sample. The research progress characterized by a reduced noise level of 0.13 nm and an increased measurement speed of 1 mm/s has received the “best paper award” of the International Symposium on Measurement Technology and Intelligent Instruments (ISMTII) 2019.

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Since implementing a high-accuracy coordinate measuring machine extensive investigations were carried out to estimate the achievable measurement uncertainty. In cooperation with the manufacturer of the machine measurements were realised at one- and two-dimensional geometrical standards like parallel gauge blocks and step gauges as well as sphere and hole plates. These investigations contribute to decreasing the calibration uncertainty during transfer of the length unit to industry.

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The virtual coordinate measuring machine is a powerful software tool to determine the task-specific measurement uncertainty using Monte-Carlo simulations. Following a substantial update and extension of the physical models and their thorough verification, the first DAkkS-accredited calibration laboratories were reviewed. In preparation of the audits, the assessors were trained by the PTB and introduced into the details of the digital twin of a coordinate measuring machine.

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For dimensional quality assessment with computed tomography both fast measurements and adequately low measurement deviations are required. In cooperation with the manufacturer of CT machines YXLON GmbH a new measurement method was developed, where less X-ray projections are required compared to an average CT scan, and the evaluation is done without tomographic reconstruction. With this procedure only those dimensions of the workpiece are acquired, which are relevant for the posed measurement task. In this way, the measurement time is significantly reduced and the measurement deviations arising due to errors in the reconstruction process are minimized. 

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Medicine is placing high expectations on implants and auxiliary materials that are manufactured specifically for each individual patient. This can be done by means of additive manufacturing methods (3D printing), which allows nearly any complex shape to be produced. Before starting to use such implants widely, the medical sector and certification bodies demand proof of the fact that their high quality is matched and will remain stable over time. Within the scope of a European project, MetAMMI, the basis for the quality control of medical implants and auxiliary means produced by additive manufacturing was provided.

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An "Absolute Multiline System" was integrated into the Large Coordinate Measuring Machine (large CMM) in the "WIND" Competence Centre (CCW) within the framework of a project funded by the BMWi. The commercial system allows the precise absolute length measurement of distances over several meters by means of a laser beam coupled in via optical fiber and a reflector. The laser source with multiple measuring channels is located in PTB's computer center and uses the existing fiber network infrastructure to distribute the measuring signals to different measuring facilities. At the large CMM, the machine geometry is planned to be automatically monitored at regular intervals by 20 measuring lines.

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The operation of a refractometer cell in a pressure range beyond the usual atmospheric pressure fluctuations is necessary if, for example, the compressibility of a material measure is to be investigated. Due to the associated pressure-induced optical path length changes, a correction has been derived in order to take this effect into account in the result. For interferometric length measurements on gauge blocks, this correction can be up to several nanometers.

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The EMPIR research project 18SIB01 ‘Large-scale dimensional measurements for geodesy - GeoMetre’ was launched on June 1, 2019. In this, novel refractivity-compensating length measurement technologies will be applied to surveying problems in satellite geodesy. One of the targets is to improve the traceability of the International Terrestrial Reference Frame (ITRF) to the SI definition of the metre. The ITRF is used, e.g., as reference for globally connected observations such as of the sea level. 

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The book released under this name from the publisher IOPScience arose with a major contribution from PTB authors. Based on the representation of a length according to the definition of the meter in the International System of Units and its realization, this comprehensive book covers the basic concepts of length metrology, sophisticated methods to reach smallest measurement uncertainties in length measurements and describes innovative interferometer concepts: https://iopscience.iop.org/book/978-0-7503-1578-4.

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Light sources are typically assumed to be monochromatic when these are used for length measurements by interferometry. From the calibrated frequency of the light source, the (vacuum-) wavelength can be calculated on a relative level of 10−12. To our surprise, while implementing a new I2-stabilized Nd:YAG laser, a relative error in the order of up to 10−7 was observed in length measurements by interferometry. Ultimately, a certain amount of ‘parasitic light’, originating from an unwanted mode of the laser’s resonator was found as the source of this error. 

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A new stabilization method of Nd:YAG lasers featuring a wavelength of 532 nm and output powers of up to 100 mW was implemented within the EMPIR research project GeoMetre. The solid-state laser was locked onto a commercially-available optical frequency comb. The use of fiber-based couplers ensures applications outside the laboratory environment. A key element in this development is an FPGA-based signal processing method, which provides narrowband filtering to increase the signal-to-noise ratio.

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As part of the establishment of the defining constants and as a contribution to the first international key comparison for the realization of the kg, the number of silicon spheres to be measured has increased strongly. At the same time, validation of the results is required in order to achieve the extremely low measurement uncertainty. Therefore, two natural silicon spheres were measured in two sphere interferometers - the consensus of the corresponding volumes was below 6x10-9. On this basis, first measurements on 28Si spheres were started in sphere interferometer 2.

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As part of the Avogadro project, a complex manufacturing chain was developed to produce silicon balls for redefining the kg. To date, PTB is the first and only institution in the world capable of producing suitable spheres for this experiment with form deviations around 30 nm, average roughness values below 0.3 nm, damage-free surface without contamination and a very uniform oxide layer. In addition, natural silicon balls are also used as mass standards. The method for manufacturing such balls has been successfully transferred to the German industry

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