Metrology & Failure Analysis
Florian Buchholz, International Solar Energy Research Center (ISC Konstanz)
Introduction
Surface contamination during processing of silicon semiconductor products plays a crucial role in terms of the quality of the final product. Although the IC industry has conducted extensive research in this field, not much work has been done in the field of silicon photovoltaics. However, with profit margins decreasing and a shakeout among cell producers to be expected,[1] interest in process analytics on the trace contamination level for process optimization is increasing.
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Alain C. Diebold, Empire Innovation Professor of Nanoscale ScienceCollege of Nanoscale Science and Engineering
Metrology and a PV Roadmap
The roadmapping experience of the semiconductor industry provides a useful guide for roadmapping photovoltaic technology. In the International Technology Roadmap for Semiconductors (ITRS), the metrology requirements for research, development and manufacturing arise from the research and development needs of the key process areas such as lithography, transistor and capacitor (front-end processes), and on-chip interconnect. The ITRS provides the driving force for guiding academic and national lab research and equipment-supplier R&D for all areas including metrology.
Dean Levi, National Center for Photovoltaics; National Renewable Energy
The focus of this section is on advanced characterization for PV using non-contact techniques. As the PV industry continues its maturation process, there is a relentless push for lower cost and higher device efficiency. New device architectures and new manufacturing processes are constantly being introduced. Greater demands are placed on quality control and quality assurance. Manufacturers depend on rapid, insightful and accurate characterization techniques to understand and optimize new device elements and manufacturing processes. The following two articles present two valuable new approaches to PV metrology.
Marshall Wilson et al., Semilab SDI LLC
Advanced characterization for PV is a complex process that must address bulk defects, interfaces, passivation and degradation phenomena. It requires not only appropriate measurement techniques, but also a coupling of measurements with treatments altering defect/interface activity. Preferably, the metrology should be non-contact and cost-effective. In this paper, we review developments in wafer-scale characterization capability for silicon PV originally presented in references 1-5. We additionally describe a multi-function metrology platform. Example applications are given that illustrate the importance of sequenced measurements for 1) monitoring of the light-induced degradation in PV wafers and solar cells; 2) correlation between interface trap density and surface recombination and the role of surface barrier; and 3) monitoring of the field-effect potential emitter passivation.
Biancamaria Maniscalco, Loughborough University
Surface metrology measurements play an important role in the development and quality assurance of photovoltaic cells and modules. A university PV laboratory is working with a leading metrology provider, Taylor Hobson, to explore the capabilities and applications of coherence correlation interferometry, including its state-of-the-art ability to accurately measure the thickness of semi-transparent thin films.
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Jean Randhahn, Dr. Randhahn Solar Engineering
In current thin film solar manufacturing, module quality and process conditions are primarily determined by physical metrics and the final electrical metrics of the product, the solar module. For established well-known manufacturing processes such as optical disc, this is feasible, as the effects and causes of defects have been thoroughly studied and documented. For TF PV production, however, this is not the case, which leads to several shortcomings in process control and production optimization. This article shows one optimization approach and some of its pitfalls and proposes a quality indicator.
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Alain C. Diebold, College of Nanoscale Science and Engineering, University at Albany
The Opportunity for Manufacturing Control-Based Cost Reduction
Amid the effort to reduce the costs associated with solar energy, there is a golden opportunity to import some of the established principles of manufacturing control from the silicon semiconductor community.
W. McMillan, T. Trupke, J. Weber - BT Imaging Pty Ltd
In current solar cell manufacturing, incoming wafer quality is primarily determined by physical metrics (such as dimension, thickness, cracks, chips) and by limited electrical parameters (such as the bulk resistivity). Spatially averaged values or line scans of the surface limited effective minority carrier lifetime are also sometimes measured. While physical metrics are important to ensure wafers are handled correctly by automated equipment and to minimize breakage rate, they do not qualify a wafer in terms of the ultimately important metric – solar cell efficiency entitlement.
Surface-limited effective lifetime provides only very limited information on wafer quality. This is due to two reasons: 1) the strong influence of surface recombination on the measured data; and 2) being an area-averaged measurement, it is influenced by features that do not correlate strongly with cell performance.
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Dean Levi, National Center for Photovoltaics; National Renewable Energy Laboratory
In this issue’s Metrology, Test & Failure Analysis section, W. McMillan and colleagues at BT Imaging, in collaboration with colleagues from Solarworld, DEL Solar and REC, provide an insightful look into the challenges and opportunities for the use of photoluminescence imaging for quality control and process optimization in multi-crystalline silicon solar cell manufacturing. Photoluminescence imaging, or PLI, is a new and potentially powerful metrology technique that is under development for application in PV manufacturing. BTI and several other companies have developed PLI equipment for use in the PV industry.
Manuel J. Romero, National Renewable Energy Laboratory
Abstract
Electroluminescence (EL) imaging is finding application in the diagnostics of electrical breakdown in silicon solar cells. The luminescence emitted from localized junction breakdown under reverse bias is consistent with bremsstrahlung radiation and is therefore easily differentiated from the luminescence under forward bias. In this contribution, the author reports on near-field scanning optical microscopy (NSOM) measurements as an approach to resolve at the microscale the processes involved in the junction breakdown.
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