Six European National Measurement Institutes (NMIs) have joined forces within the European Metrology Research Programme funded project NANOTRACE to develop the next generation of optical interferometers having a target uncertainty of 10 pm. These are needed for NMIs to provide improved traceable dimensional metrology that can be disseminated to the wider nanotechnology community, thereby supporting the growth in nanotechnology. Several approaches were followed in order to develop the interferometers. This paper briefly describes the different interferometers developed by the various partners and presents the results of a comparison of performance of the optical interferometers using an x-ray interferometer to generate traceable reference displacements.
In this work we have studied the feasibility of DNA origami nanostructures as dimensional calibration standards for atomic force microscopes (AFMs) at the nanometre scale. The stability of the structures and repeatability of the measurement have been studied, and the applicability for calibration is discussed. A cross-like Seeman tile (ST) was selected for the studies and it was found suitable for repeatable calibration of AFMs. The height of the first height step of the ST was 2.0 nm. Expanded standard uncertainty (k = 2) of the measurement Uc was 0.2 nm. The width of the ST was 88 nm and width of its arm was 28 nm with Uc = 3 nm. In addition, prepared dry samples were found out to be stable at least for 12 months.
We present a laser-based system to measure the refractive index of air over a long path length. In optical distance measurements, it is essential to know the refractive index of air with high accuracy. Commonly, the refractive index of air is calculated from the properties of the ambient air using either Ciddor or Edlén equations, where the dominant uncertainty component is in most cases the air temperature. The method developed in this work utilizes direct absorption spectroscopy of oxygen to measure the average temperature of air and of water vapor to measure relative humidity. The method allows measurement of temperature and humidity over the same beam path as in optical distance measurement, providing spatially well-matching data. Indoor and outdoor measurements demonstrate the effectiveness of the method. In particular, we demonstrate an effective compensation of the refractive index of air in an interferometric length measurement at a time-variant and spatially nonhomogeneous temperature over a long time period. Further, we were able to demonstrate 7 mK RMS noise over a 67 m path length using a 120 s sample time. To our knowledge, this is the best temperature precision reported for a spectroscopic temperature measurement.
Laser diffractometer constructed at MIKES for calibration of pitches of 1D and 2D gratings is based on the Littrow configuration. The grating is rotated by a rotary table. In order to obtain optimal beam quality a fibre-coupled frequency-doubled stabilized Nd:YAG laser is used as a light source and a CCD without internal reflections is used to detect the diffracted beam position. The angle corresponding to the Littrow null position is calculated using a linear fit to equidistantly spaced angles around the null position, which reduces the effect of nonlinearity of the angle encoder scale. Pitch is calculated as a weighted average of the diffraction orders. Novel use of an uncalibrated 1D grating applied to classical error separation methods for angle calibration by full-circle subdivision is described. Diffraction angle measurements are repeated in many sample holder alignments rotated relative to the absolute angle scale. The sequences overlap so that the full circle of the angle scale is covered with multiple evenly distributed sequences, needed for circle-subdivision metrology. The average grating pitch over all sequences is used to separate angle scale errors and a correction table for the rotary table is calculated. The method was verified using a calibrated polygon and an autocollimator. Uncertainty estimates for the grating pitch calibration are given, e.g. the standard uncertainty of 9 pm for 2 µm grating.
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