Phase Feedback Active Stabilisation Of A Multiplexed Fibre Interferometer

Current optical technologies for surface measurement such as white light scanning interferometry (WLSI) can produce sub-nanometric resolution areal height information. However, the sensitivity of such devices to external vibration coupled with their bulky nature requires that any workpiece be removed from the manufacturing line in order for measurement to be taken place. Clearly, a production line mountable, vibration stabilised device would provide benefits in manufacturing throughput and help reduce scrap rates. The multiplexed fibre interferometer (MFI) aims to provide this capability for high precision surfaces with submicron form deviation

A single-shot line-scanning spatially dispersed short coherence interferometer using Fourier transform profilometry

Single-shot inspection at nanoscale resolution is a problematic challenge for providing on-line inspection of manufacturing techniques such as roll-to-roll processes where the measurand is constantly moving. An example of such a measurement challenge is defect detection on vapor barrier films formed by depositing an aluminum 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. This paper describes a line-scanning interferometer where a short coherence light source having a 25 nm linewidth source is spatially dispersed across the measurand thus encoding spatial position along a profile by wavelength. Phase shift interferometry (PSI) can be used to decode phase and thus height information, but requires multiple image captures. In order to realise single-shot measurement which is more suitable for online applications, a Fourier transform profilometry (FTP) approach is necessary. This paper explores the implementation of the FTP approach and presents a comparison of the measurement capability of FTP with the previously reported PSI method

High resolution point-sensing using template matching to extract phase from spectral interferograms

Dispersed reference interferometry (DRI) is a variant of spectral interferometry and has potential to improve on existing commercial single point measurement techniques such as chromatic confocal (CC) sensors by improving dynamic range. The DRI has previously been demonstrated with a resolution of only 250 nm over a range of 300 µm. However, because DRI is an interferometric technique, phase information is inherent in the generated spectral interferograms and nanometre resolution could be achieved if that information can be extracted efficiently from a single interferogram. This talk describes a method of phase calculation using template matching which is a technique commonly used in image processing. Template matching is used to extract high resolution phase information from an experimental DRI apparatus. Spectral interferogram templates, representing axial measurement positions are generated using a simple simulation of the optical apparatus. These templates are cross-correlated against a spectral interferogram generated from the DRI apparatus. The peak of the resulting correlogram indicates the closest matching template interferogram and allows the inference of a measured position with high resolution. The template matching phase extraction method is evaluated in terms of linearity, resolution and operating range. The computational requirements and avenues for optimisation in this area are also considered

Burg algorithm for enhancing measurement performance in wavelength scanning interferometry

Wavelength scanning interferometry (WSI) is a technique for measuring surface topography that is capable of resolving step discontinuities and does not require any mechanical movement of the apparatus or measurand, allowing measurement times to be reduced substantially in comparison to related techniques. The axial (height) resolution and measurement range in WSI depends in part on the algorithm used to evaluate the spectral interferograms. Previously reported Fourier transform based methods have a number of limitations which is in part due to the short data lengths obtained. This paper compares the performance auto-regressive model based techniques for frequency estimation in WSI. Specifically, the Burg method is compared with established Fourier transform based approaches using both simulation and experimental data taken from a WSI measurement of a step-height sample

Investigations into a multiplexed fibre interferometer for on-line, nanoscale, surface metrology

Current trends in technology are leading to a need for ever smaller and more complex featured surfaces. The techniques for manufacturing these surfaces are varied but are tied together by one limitation; the lack of useable, on-line metrology instrumentation. Current metrology methods require the removal of a workpiece for characterisation which leads to machining down-time, more intensive labour and generally presents a bottle neck for throughput. In order to establish a new method for on-line metrology at the nanoscale investigation are made into the use of optical fibre interferometry to realise a compact probe that is robust to environmental disturbance. Wavelength tuning is combined with a dispersive element to provide a moveable optical stylus that sweeps the surface. The phase variation caused by the surface topography is then analysed using phase shifting interferometry. A second interferometer is wavelength multiplexed into the optical circuit in order to track the inherent instability of the optical fibre. This is then countered using a closed loop control to servo the path lengths mechanically which additionally counters external vibration on the measurand. The overall stability is found to be limited by polarisation state evolution however. A second method is then investigated and a rapid phase shifting technique is employed in conjunction with an electro-optic phase modulator to overcome the polarisation state evolution. Closed loop servo control is realised with no mechanical movement and a step height artefact is measured. The measurement result shows good correlation with a measurement taken with a commercial white light interferometer

A computerised data handling procedure for defect detection and analysis for large area substrates manufactured by roll-to-roll process

The development of optical on-line/in-process surface inspection and characterisation systems for flexible roll to roll (R2R) thin film barriers used for photo-voltaic (PV) modules is a core research goal for the EU funded NanoMend project. Micro and nano scale defects in the ALD (atomic layer deposition) Al2O3 barrier coating produced by R2R techniques can affect the PV module efficiency and lifespan. The presence of defects has been shown to have a clear correlation with the water-vapour-transmission-rate (WVTR). Hence, in order to improve the PV cell performance and lifespan the barrier film layer must prevent water vapour ingress. One of the main challenges for the application of in process metrology is how to assess large and multiple measurement data sets obtained from an in process optical instrument. Measuring the surface topography over large area substrates (approximately 500 mm substrate width) with a limited field-of-view (FOV) of the optical instrument will produce hundreds/thousands of measurement files. Assessing each file individually to find and analyse defects manually is time consuming and impractical. This paper reports the basis of a computerised solution to assess these files by monitoring and extracting areal surface topography parameters. Comparing parameter values to an experimentally determined threshold value, obtained from extensive lab-based measurement of Al2O3 ALD coated films, can indicate the existence of the defects within a given FOV. This process can be repeated automatically for chosen parameters and the existence of defects can be indicated for the entire set of measurement files spontaneously without interaction from the inspector. A running defect log and defect statistics associated with the captured set of data files can be generated. This paper outlines the implementation of the auto-defect logging using advanced areal parameters, and its application in a proof of concept system at the Center for Process Innovation (UK) is discussed

An integrated opto-mechanical measurement system for in-process defect measurement on a roll-to-roll process

This paper reports on the recent work carried out to develop and implement a high precision on-line optical measurement system with the aim of providing defect detection and characterisation for ALD coated vapour barrier films produced by a roll-to-roll process. This proof-of-concept system is designed to detect and measure pre-existing defects on the film and define their size, location, form and density. The aim is to be able to detect defects in a thin film Al2O3 layer that are critical to vapour barrier performance, and eventually provide valuable process control information. Such an inspection system must be fast in order to evaluate large areas involved (500 mm width foil) at high magnifications. In addition the flexibility of the foil introduces challenges in terms of dealing with surface deviation away from an ideal plane and vibrations. Our solution is a wavelength scanning interferometer (WSI) combined with two kinematic stages, vertical (for auto-focus) and a traverse stage to provide full coverage of the foil. A porous air-bearing conveyor system is used to hold the foil at a fixed height and improve the flatness of the film relative to the measurement plane. This paper describes the principle and design of the inspection system

Comparative study between online and offline defect assessment methods for roll to roll flexible PV modules

Flexible PV modules are manufactured using roll to roll (R2R) technology. These modules require a flexible barrier material to prevent water vapor ingress. The basic methodology to produce such barriers is to combine oxide layer (e.g. AlOx) coating with suitable polymer substrates. However, micro and nano scale defects can appear at any stage of the coating process thus affecting the module efficiency and lifespan. To guarantee the barrier properties, surface inspection needs to be carried out during the manufacturing process. In the present study a stabilised in-line detection system based on wavelength scanning interferometry (WSI) has accurately and reliably captured defects down to 3um in lateral dimension. The main aim of this research paper is to compare results of coherence correlation interferometry (CCI) as an offline technique with WSI results for detecting PV barrier film defects. The results show the capability of the WSI to be implemented in the manufacturing environment for defect detection in roll to roll manufacture

Investigations into a multiplexed fibre interferometer for on-line, nanoscale, surface metrology

Current trends in technology are leading to a need for ever smaller and more complex featured surfaces. The techniques for manufacturing these surfaces are varied but are tied together by one limitation; the lack of useable, on-line metrology instrumentation. Current metrology methods require the removal of a workpiece for characterisation which leads to machining down-time, more intensive labour and generally presents a bottle neck for throughput. In order to establish a new method for on-line metrology at the nanoscale investigation are made into the use of optical fibre interferometry to realise a compact probe that is robust to environmental disturbance. Wavelength tuning is combined with a dispersive element to provide a moveable optical stylus that sweeps the surface. The phase variation caused by the surface topography is then analysed using phase shifting interferometry. A second interferometer is wavelength multiplexed into the optical circuit in order to track the inherent instability of the optical fibre. This is then countered using a closed loop control to servo the path lengths mechanically which additionally counters external vibration on the measurand. The overall stability is found to be limited by polarisation state evolution however. A second method is then investigated and a rapid phase shifting technique is employed in conjunction with an electro-optic phase modulator to overcome the polarisation state evolution. Closed loop servo control is realised with no mechanical movement and a step height artefact is measured. The measurement result shows good correlation with a measurement taken with a commercial white light interferometer.EThOS - Electronic Theses Online ServiceGBUnited Kingdo