Acceleration computing process in wavelength scanning interferometry

The optical interferometry has been widely explored for surface measurement due to the advantages of non-contact and high accuracy interrogation. Eventually, some interferometers are used to measure both rough and smooth surfaces such as white light interferometry and wavelength scanning interferometry (WSI). The WSI can be used to measure large discontinuous surface profiles without the phase ambiguity problems. However, the WSI usually needs to capture hundreds of interferograms at different wavelength in order to evaluate the surface finish for a sample. The evaluating process for this large amount of data needs long processing time if CPUs traditional programming is used. This paper presents a parallel programming model to achieve the data parallelism for accelerating the computing analysis of the captured data. This parallel programming is based on CUDATM C program structure that developed by NVIDIA. Additionally, this paper explains the mathematical algorithm that has been used for evaluating the surface profiles. The computing time and accuracy obtained from CUDA program, using GeForce GTX 280 graphics processing unit (GPU), were compared to those obtained from sequential execution Matlab program, using Intel® Core™2 Duo CPU. The results of measuring a step height sample shows that the parallel programming capability of the GPU can highly accelerate the floating point calculation throughput compared to multicore CPU

Implementation of wavelength scanning interferometry for R2R flexible PV barrier films

Roll to Roll manufacture of nano-scale thinf ilm layers faces the challenge of micro/nano-scale defects appearing in the films. Atomic Layer Deposition (ALD) coatings of aluminium oxide, Al2O3 are used as barrier layers for photovoltaic (PV) solar modules where the primary function of the barrier layer is to prevent the water vapour ingress to the PV cells. Barrier layer defects have been shown to have negative impact on the performance of the barrier layers. Poor barriers cause module degradation resulting in reduced PV efficiency and lifespan. In order to ensure the quality of manufacture of the barriers, defects should be detected during the barrier production process and the information used to optimise the production process. This paper introduces, as part of EU funded NanoMend project, a full solution for inspection of entire surface regions of Al2O3 barrier films across large area substrates. The solution principle is based on implementing an opto-mechanical in-process inspection system to measure the significant defects using a wavelength scanning interferometer (WSI) embedded within the film-rewinder stage and integrated with the substrate translation and kinematic stages. The opto-mechanical system allows full surface measurement over full substrate widths of approximately 0.5m. The system provides an auto-focus for the WSI with an accuracy and repeatability better than 6 µm at optimum optical alignment conditions. The system is combined with a porous air-bearing conveyor used to hold the film web at fixed height within the focal depth of WSI objective lens and with height variation of 3 µm is also presented as a case study to highlight the system capability

Surface Profile Height Measurement Using Optical Interferometry Method

Interferometer is one of the methods that are used for measuring surface profile with high accuracy reach to the order of 1/1000 of the wavelength of the light. The accuracy in a given interferomter depends on many factors such as the light source properties and environmental mechanical vibration. This poster illustrates a method to increase the accuracy of Linnik interferometer for nano-scale measurements

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

Wavelength Scanning Interferometery for large area roll to roll metrology applications in photovoltaic manufacturing environment

The wavelength scanning interferometer is currently being applied as a core metrology technology as part of the EU project NanoMend - Nanoscale Defect Detection, Cleaning and Repair for Large Area Substrates ̴500 mm width. NanoMend Project aims to develop technologies that are able to detect and correct micro and nano-scale defects in roll-to-roll produced films in order to improve product performance, yield and lifetime

In-line metrology of functional surfaces with a focus on defect assessment on large area Roll to Roll substrates

This paper reports on the recent work carried out as part of the initial stages of the EU funded NanoMend project. The project seeks to develop integrated process inspection, cleaning, repair for nano-scale thin films on large area substrates. Flexible photovoltaic (PV) films based on CIGS (Copper Indium Gallium Selenide CuInxGa(1-x)Se2) have been reported to have light energy conversion efficiencies as high as 19%. CIGS based multi-layer flexible devices are fabricated on polymer film by the repeated deposition, and patterning, of thin layer materials using roll-to-roll processes (R2R), where the whole film is approximately 3μm thick prior to final encapsulation. The resultant films are lightweight and easily adaptable to building integration. Current wide scale implementation however is hampered by long term degradation of efficiency due to water ingress to the CIGS modules causing electrical shorts and efficiency drops. The present work reports on the use of areal surface metrology to correlate defect morphology with water vapour transmission rate (WVTR) through the protective barrier coatings

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

Wavelength Scanning Interferometry for PV Production In-line Metrology

Flexible PV modules are manufactured using roll to roll (R2R) technology. These modules require a flexible barrier material to prevent water vapor ingress

Vertical axis non-linearities in wavelength scanning interferometry

The uncertainty of measurements made on an areal surface topography instrument is directly influenced by its metrological characteristics. In this work, the vertical axis deviation from linearity of a wavelength scanning interferometer is evaluated. The vertical axis non-linearities are caused by the spectral leakage resulting from the Fourier transform algorithm for phase slope estimation. These non-linearities are simulated and the results are compared with experimental measurements. In order to reduce the observed non-linearities, a modification of the algorithm is proposed. The application of a Hamming window and the exclusion of edge points in the extracted phase are shown to increase the accuracy over the whole instrument range

Verification of an in Process Optical System based on High Resolution Interferometry for Detecting Flexible PV Barrier Films Defects

The relationship between surface morphology, defects density and water vapor permeability of aluminium oxide (Al2O3) barrier coating on polyethylene naphthalate (PEN) films, used particularly to prevent water vapor and/or oxygen ingress into flexible photovoltaic (PV) modules has to date been studied in the laboratory. However; detecting defects off-line for flexible PVs is difficult and time consuming, and as such devices are manufactured by R2R methods this procedure can often result in large quantities of barrier films being manufactured before defects are detected. In addition, the quality requirements and line speed are continuously increasing and off-line methods are not efficient to operate within these requirements. Hence, it is desirable to make use of non-contact optical based in-line inspection systems during PV manufacturing processes. Nevertheless, implementing highly accurate in-line (optical) measurement system in the PV production environment can be challenging, as the requirements on positioning and stability are demanding. This research paper reports on the deployment of new in-line interferometric optical technique based on wavelength scanning interferometry (WSI), for detecting PV barriers defects. The instrument has built-in environmental vibration compensation, providing areal measurement at high speed of less than a second per field of view. The technique is being deployed on a demonstrator system at a Roll2Roll production facility as shown in Figure 1. The results show the capability of the WSI to be used as a quality assurance tool in PV production lines, where the results compare favourably with off-line metrology techniques