Metrology & Failure Analysis
Andrew Findlay, Semilab SDI LLC
Unified lifetime metrology utilizes the quality of decay control technique enabling parameter-free, self-consistent determination of the two lifetimes most frequently used in silicon solar cell manufacturing: the excess carrier decay lifetime, teff.d; and the quasi-steady-state effective lifetime, teff. This technique opens new possibilities for parameter-free monitoring and wafer mapping of the factors controlling cell efficiency. Map-ping of passivated wafers discloses the common presence of “weak spots” in PV cells due to passivation defects such as areas with high emitter saturation current J0, high surface recombination, and field-effect degradation spots. Elimina-tion of the weak PV spots will benefit the cell efficiency and manufacturing yield.
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Greg Horner, Tau Science Corporation
Flash Quantum Efficiency (FlashQE®) is a new solar cell metrology technique that measures the full-spectrum external quantum efficiency, reflectance, internal quantum efficiency and short circuit current in one second.[1,2] The method uses a unique full-spectrum light engine to measure, in real time, the cell performance as a function of wavelength. Mapping throughput of >2400 sites per hour is possible on cells and modules, with a signal-to-noise ratio that is significantly higher than conventional QE.
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Alain Diebold, Empire Innovation Professor of Nanoscale Science, College of Nanoscale Science and Engineering – University at Albany, SUNY
Data Correlation
Correlating the physical characteristics of a transistor with electrical performance is critical, both for development and for manufacturing at high yield and performance. The same can be said for photovoltaic devices. One example is the strong correlation between high recombination lifetimes in Cu(In, Ga)Se2 solar cells and high efficiency.[1]
Mike Conroy, Taylor Hobson
As the interest in PV devices increases, improving the efficiency of the cells becomes increasingly important. Using metrology to study the surface structure and dimensions of critical features of the cells is an important part of this optimization. This is true for both development and production. Modern interferometry is coming to the fore as the ideal approach to metrology for these systems because it is fast, non-contact and gives full 3D information about the surface and features. A recent advance in measurement of thin films has enhanced the usefulness of the technique.[6]
Kristopher Davis, U.S. Photovoltaic Manufacturing Consortium (PVMC)
The U.S. Photovoltaic Manufacturing Consortium (PVMC) is a partnership led by SEMATECH and the College of Nanoscale Science and Engineering (CNSE) of the University at Albany. PVMC is an industry-led consortium for cooperative R&D among industry, university and government partners to accelerate the development, commercialization and manufacturing of solar photovoltaic (PV) systems.
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Dean Levi, Principal Scientist, National Center for Photovoltaics; National Renewable Energy Laboratory
One of the biggest challenges in developing metrology for the PV industry is determining the critical issues that need to be addressed through advanced metrology. While each individual PV manufacturer is keenly aware of the problems in their own production process, they are typically loath to discuss these problems in an open forum lest they reveal their weaknesses to their competitors. Hence the challenge for PV researchers and metrology equipment manufacturers is often “What is the question we need to answer?” Unlike the famous game show “Jeopardy,” we are usually not provided with either the answer or the question.
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.
