Dispersed reference interferometry for on-machine metrology

The reliance of emerging engineering and scientific applications on nanometre scale surfaces has led to the requirement for embedded metrology instrumentation for on-machine measurement in precision and ultra-precision manufacturing processes. In-situ measurement provides reduction in waste, cost and production cycle time of manufactured components however the presence of environmental noise and difficulties of integration make embedded metrology challenging.

In this thesis a review of on-machine methods of optical metrology is provided and commercially available instruments are summarised. The requirement for highly miniaturised methods of distance and surface topography measurement for operation within volume limited manufacturing environments is presented and the necessity for smaller, faster probes with increased range, resolution and robustness outlined. The potential for dispersed reference interferometry (DRI) to exceed the capability of existing single-point remote fibre probing instruments is then introduced.

DRI as a technique for single-point distance and surface topography measurement is first evaluated in a bulk optics configuration using a short coherence light source and chromatic dispersion within the reference arm. The resulting spectral interferograms have symmetry positions which are directly related to absolute surface position to yield measurement with a 279 nm axial resolution over a 285 μm axial range.

Improvement of this resolution results from introduction of template matching, a signal processing technique which correlates a measured interferogram with a set of pre-calculated template interferograms resulting in a relative method of measurement with an axial resolution of 0.6 nm across the 285 μm axial range. Combination of this high resolution, relative measurement method with low resolution but absolute position data is then explored to improve the robustness of DRI to discontinuous and structured surfaces. Determination of high resolution wraparound order using low resolution absolute data is demonstrated over a 30 μm range, with extension to the full range of DRI expected as future improvements increase the measurement rate.

Next a DRI topology is introduced which makes use of miniature common-path probes linked by fibre to a remote interrogation interferometer allowing reduction in size of the on-machine metrology apparatus. Modifications to the DRI are described to enable this common-path remote fibre probing, along with changes to the light source, spectrometer and dispersive element which allow extension of the range of DRI to 800 μm while maintaining nanometre axial resolution.

Finally, a further work section offers insights into methods to improve miniaturisation of DRI probes as well as providing discussion of methods of hardware and signal processing optimisation to augment the instrument measurement rate.