Key Engineering Materials Vol. 613

Abstract: At least four units of the International System of Units (SI) are on the way to a new definition. Especially for the unit of mass, the kilogram, a rigorous change is considered. Instead of the current definition, a 1kg-artifact in form of a Pt-Ir-cylinder, the intended formulation relates the unit of mass to a fundamental constant. In detail this requires in a first step a measurement of the chosen fundamental constant with contemporary lowest uncertainty and best reproducibility. The constant will then be fixed to that value. As an example the metre is related to the fixed constant speed of light.For the kg there are considered two ways: one is a watt balance, which determines the mass in units of the Planck constant, h. While at present the watt balances show a heterogeneous appearance, the second class of experiment the determination of the Avogadro constant, N_A, which measures the mass in terms of the number of elementary entities has reached a considerable level of uncertainty and reproducibility. The fundament of the new determination of the Avogadro constant is a highly enriched ²⁸Si crystal. The different working groups of the Avogadro team determine molar mass and lattice parameter of the crystal, and mass and volume of two precision spheres made from different positions, but of the same crystal. All measurements are carried out for both spheres and all measurement quantities are determined at least from two independent working groups, usually of different countries.

Abstract: For the aimed redefinition of the SI-unit kilogram the international Avogadro group is to measure the amount of substance of a silicon sphere with a relative uncertainty of better than 2 x 10^-8. Up to now 3.0 x 10^-8 is reached, mainly caused by the 2.7 x 10^-8 uncertainty of the volume determination. Which itself is essentially affected by the uncertainty of wave front distortion caused by optical fabrication errors and possible misalignment. To gain more information on how the wave front distortion influences the result a new interferometer with improved optics and rather large changes in the etalon is built up. A comparison of the measurements of one sphere taken with two different interferometers gives direct access to this information. Additionally a ray tracing software is being developed to simulate the alignment and measurement processes. Following the alignment steps the resulting positioning uncertainties of each element can be simulated and elements which contribute mainly to the uncertainty can be identified. This gives evidence where an improvement of the alignment techniques is most effective and necessary. With the additional use of Monte Carlo method based calculations the influence on the measurement results can be determined.

Abstract: The General Conference on Weights and Measures (CGPM) discusses the improvements of a possible revision of the International System of Units (SI). For the new definition of the kilogram apart from an artifact of Platinum-Iridium a suitable fundamental constant seems to be found, to which the kg could be related. Although the Planck constant, h, is being considered for the new definition, its value can currently be determined with less uncertainty from the value of the Avogadro constant, N_A. As well the determination of the Avogadro constant is suitable as a primary method for the subsequent realization of the kilogram. The international Avogadro group has reached so far a relative measurement uncertainty of 3×10^-8, mainly limited by the interferometric measurement of the volume of the ²⁸Si-spheres, used to count the atoms. The dominant influence on the measurement uncertainty is a contribution which subsumes wavefront aberrations due to surface deviations and irregularities of the spheres polished from our partner at CSIRO, Australia. A new multi-step machining process, developed and realized at PTB, reduces considerably the surface contamination and creates spheres with surface properties which exceed the standards in matters of form deviation and surface roughness. The manufacturing process incorporates highly reproducible multi-step grinding and polishing steps. The surfaces are almost free of scratches and show average roughness values below 0.3 nm. The form shows some regular, long wavelength errors below 30 nm in amplitude, collocated conform to crystal orientation.

Abstract: Key comparison (KC) measurements are the backbone of validating the calibration and measurement capability (CMC) of national metrology institutes (NMIs) against each other. Recommendations providing procedures for the preparation and execution of KCs as well as supplementary comparisons are given. They are justified by mathematical reasoning and by aspects of practical use. Arguments and a formal description of calculating the key comparison reference value (KCRV) by its weighted mean based on the expanded measurement uncertainty expressed by a confidence level of 95 % will be presented. Moreover, it will be explained why the calculation of the normalized error relates to the weighted mean only and why its absolute value for the expanded measurement uncertainty shall be below 1 to pass the equivalence criterion (E_n) of comparability. A deeper discussion is given on how reference values and measurement results shall be treated if few individual measurement results differ significantly from the others. Finally, it will be explained how the proposed style of the graphical presentation of measurement results allows a clear visual interpretation of the comparison. Besides the formal description, numerical examples and diagrams are presented.

Abstract: A dedicated experimental setup has been designed and built up for determining the reproducibility of a 1-DOF moving-scale measurement system. A 3-DOF moving-scale system would enable the measurement of the position of a workpiece table moving in three perpendicular directions with minimized Abbe-offset. This allows normal machine tools and CMMs to match or even surpass the accuracy of ultra-precision machines. An uncertainty budget indicates that the 1-DOF moving-scale system can obtain a measurement uncertainty of 18 nm. Thermal drift is the largest contribution to the measurement uncertainty and should be accurately determined.

Abstract: To improve the measurement accuracy of two-dimensional (2D) precision working stages in the semiconductor manufacturing industry, an error separation and dynamic compensation method based on a planar grating is proposed for a 2D coplanar displacement measurement system. In the system, a planar grating of HEIDENHAIN PP281R is fixed in the center of precision working stage to build coplanar structure and reduce Abbe errors. And the system errors including grating period carved error, temperature effect, vibration effect and mount effect are modeled, calculated and separated. Then the separated errors are dynamically compensated from the corresponding displacement measurement results through data-processing software. Analysis results show that the proposed error separation and dynamic compensation method based on planar grating can improve the measurement accuracy of the coplanar displacement measurement system effectively.

Abstract: This paper presents capacitive sensor interface with very good stability, small thermal drift and high resolution. The measurement principle is based on a charge amplification and multi-slope conversion which utilizes stable resistors as reference components. Five capacitance ranges from 5 pF to 100 pF can be selected. Based on previous works, a so-called "zoom-in" technique is implemented for sensors with small variation of their capacitance, thus significantly extending the signal-to-noise ratio. The conversion time can be less than 100 μs, the temperature stability with offset cancellation is 2 ppm/K, and the dynamic range using "zoom-in" is 18.5 bits. The 1/f noise corner of the converter with noise floor of 0.1 fF is 20 mHz which indicates very good long-term stability. These characteristics make the interface suitable solution for wide range of industrial and scientific applications, including positioning systems and transfer of length standards with sub-nanometer accuracy.

Abstract: The heterodyne laser interferometer is widely applied in ultra-precision displacement measurement, but its accuracy is seriously restricted by the optical nonlinearity which arises from the optical mixing in the reference and measurement arms. In an ideal heterodyne laser interferometer, the beam from the laser source consists of two orthogonally linear-polarized components with slightly different optical frequencies and the two components can be completely separated by the polarizing optics, one traverses in the reference arm, the other traverses in the measurement arm, both of them are in the form of a pure optical frequency. However, in a real heterodyne laser interferometer, due to the imperfect laser polarization, the optics defect and the misalignment, the two components of the laser beam cant be perfectly separated, therefore both of the reference arm and the measurement arm contain a portion of the two laser components, which leads to an optical mixing in the two arms of the heterodyne interferometer and causes the cyclic nonlinearity of several to tens of nanometers.

Abstract: Cross-gratings or two-dimensional gratings have found applications in precision displacement measurement for nanoscale machine tools or mask-wafer alignment in photo-lithography. The orthogonality of displacement measurement or alignment in two perpendicular directions is assured by the orthogonality of the grating grooves. Once the grooves are not exactly normal to each other, it should be calibrated before measurement or alignment to guarantee the accuracy. Scanning probe microscopy (SPM) and optical diffractometry (OD) are commonly used for highly accurate calibration of cross-grating orthogonality. In this paper, two methods based on diffraction are proposed for middle accuracy measurement at relative higher speed. At first, we briefly explain the configuration of each method. The calibration results of cross-gratings are shown. We also discuss the results of uncertainty analysis and the cause of main uncertainty sources.