Hassan, Mothana A. (2015) Surface Profile Measurement Using Spatially Dispersed Short Coherence Interferometry. Doctoral thesis, University of Huddersfield.

Modern manufacturing processes require better quality control of the manufactured products at a faster rate, for achieving good throughput. This is increasing the need for process-oriented precision metrology capable of providing faster inspection and yielding valuable feedback to the manufacturing system for quality control of the manufactured products. Over the past twenty years optical sensors have emerged as a preferred method for the measurement applications in manufacturing automation, owing to some inherent advantages, such as high speed, high resolution, non-contact operation, and low cost. Improved online optical sensors for surface measurement would enable incorporating measuring systems into production processes and machines, improving the production performance and the quality of products, which is highly desired in many high/ultraprecision manufacturing applications.
This thesis presents a novel spatially dispersed short coherence interferometry (SDSCI) sensor system for online surface measurement applications on the nanoscale. The SDSCI sensor system uses a low-cost broadband super-luminescent diode (SLD) with an emission bandwidth of 25 nm. Two measurement methods, phase shifting interferometry and Fourier transform for surface profile measurements, have been investigated in this study. The metrology sensor system incorporated the Michelson interferometer configuration with an optical probe in the measurement arm, while the reference arm had a mirror with a piezo-electric transducer. The technique involves surface scanning by spatially dispersing the broadband light using a reflective grating and a scan lens, and recording the resulting interferogram by using a high-speed spectrometer.
The first measurement method involved investigations of implementation of phase shifting interferometry and the Carré algorithm for phase retrieval from the measured phase-shifted interferograms for profile measurements. Standard diamond turned multi-stepped and NPL artefact samples with 550-nm and 100-nm-high steps, respectively, were measured and confirmed the capability of the measurement sensor. The measurement speed of this technique was limited by the spectrometer speed and by the piezo-electric transducer movement. The optimised system has a measurement time of 1s. The second method was then investigated based on the Fourier transform profilometry technique for further increasing the measurement speed of the sensor device, as it required a single-shot interferogram, alleviating the need of any phase shifting. With increased measurement speed, this technique further reduced the problem of environmental noise inherent to all interferometer-based systems. Similar artefacts were measured by using this technique for evaluating its applicability for surface profile measurements. Once the sensor system was optimised and calibrated, the resulting open-space system could be further miniaturised into a compact sensor system by using optical fibres with a remote probe connected via a fibre link for use in embedded metrology applications. This method will be very beneficial in online inspection of samples in rollto-roll manufacturing processes, where the measurand is constantly moving. An example of such a measurement challenge is detection of defects on vapour barrier films formed by depositing an aluminium oxide layer several tens of nanometres thick on a flexible polymer substrate. Effective detection and characterisation of defects in this layer requires a single-shot approach with nanometre-scale vertical resolution.

Mothana_Amaar_Hassan__Thesis.pdf - Accepted Version
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