National Physical Laboratory

Nanometre and sub-nanometre displacement measurement

Optical interferometry

Multi axis traceable displacement measurement is becoming increasingly important in nanotechnology and nanometrology. Optical interferometry is the most widely used tool for displacement at the nanometre level.

NPL has developed several interferometers and fringe counting systems for high accuracy displacement measurement for both applications within NPL and externally. For example, in the Technology Strategy Board project 'High resolution low noise floor nanopositioning system for testing of the next generation of hard disc drive read write heads', NPL is developing high resolution, low noise optical interferometer systems for the verification of the performance of nanopositioning stages developed by Queensgate Instruments that will be used within this project for hard disc drive testing.

X-ray interferometry

An x-ray interferometer can be thought of as a ruler or translation stage where the graduations or steps are equal to the spacing between planes in a crystal from which x-rays have been diffracted, in this case the silicon (220) planes with a spacing of 192 pm. The x-ray interferometer provides a route to traceability at the nanometre and sub-nanometre level without the need for interpolation, as in the case of optical interferometry.

X-ray interferometry

NPL led an EU-funded project with the Physikalisch-Technische Bundesanstalt (PTB) and Istituto Nazionale di Ricerca Metrologica (INRIM) to develop a Combined Optical and X-ray Interferometer (COXI) for the sub-nanometre calibration of displacement measuring transducers. The x-ray interferometer facility was then further developed within the EMRP project NANOTRACE and successfully used to measure picometre non-linearity in optical interferometers.

NPL is currently participating in several projects to develop x-ray interferometry and use it for nanometre and sub-nanometre metrology.


EMRP 3D Nano

Within this project, NPL and PTB are combining the techniques of x-ray interferometry, optical interferometry and atomic force microscopy for the measurement of non-linearities in grating structures used to support Scatterometry.

For more information, please visit the project website

EMRP project Sub Nano

The x-ray interferometer is being developed to support true sub-nanometre metrology by applying quadrature fringe counting techniques, usually associated with optical interferometry to the x-ray interferometer where the fringe spacing is 192 pm as opposed to 158 nm. Work is also being undertaken to verify the performance of capacitance sensors.

For more information, please visit the project website

Research team

  • Andrew Yacoot
  • Herve Dongmo
  • Edward Heaps
  • Kieran Best
  • Andrew Lancaster
  • John Mountford

Recent publications


For more information, please contact Andrew Yacoot

Last Updated: 25 Sep 2017
Created: 23 Jul 2013


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