Photos: KLA-Tencor Corporation
Industry & Suppliers
KLA-Tencor’s ICOS PVI-6 inspection modules provide high speed, automated, optical inline inspection of monocrystalline and polycrystalline solar wafers and cells.
Take control Process control: Important tools to reduce the balance of system costs in PV manufacturing are the use of advanced optical defect inspection systems and surface metrology systems, in combination with a comprehensive process control methodology, explains Pieter Vandewalle of KLATencor Corporation.
Government incentives are not sustainable in the long run, because they are subject to policy shifts that can potentially cause disruptions in the deployment process. Therefore the PV industry must drive down the overall cost of solar energy deployment. Key factors to reduce the balance of system costs in PV manufacturing include the use of technologies and processes enabling the production of high-efficiency cells; a much higher degree of automation; comprehensive tightly-integrated process control; and high-volume yield improvements. The average price per kilowatt-hour of electricity from traditional generation is projected to continue to rise over the next decade, while the price for solar generated electricity is projected to fall significantly. As the curves approach convergence, “grid parity” will be achieved, with timeframes varying by region. Solar cell manufacturers need to lay the foundation today in order to respond to the impending upswing in market demand over the next few years. Deployment of comprehensive process control methodologies will be a key enabling factor not only in manufacturers’ ability to profitably meet the solar energy industry’s need for high volumes and consistent quality at ever-lower prices, but also to enable production of higherefficiency cells. Increasing productivity through improvements to manufacturing processes that result in greater yields is a key contributor to lower costs. Experience has shown that for every onepercent increase in yield, there is a corresponding five-percent cost reduction. The ability to drive up production volumes while maintaining quality will lead directly to a lower cost per device. For example, doubling the production output will typically result in a 22 percent decrease in the overall unit cost.
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Evidence from research at leading laboratories and academic institutions indicate that there are a range of significant opportunities for improving the manufacturing processes used in solar cell production. However, the advanced lab-proven techniques that enable greater efficiencies have not yet been implemented on a widespread basis in real-world production environments. Other improvements in manufacturing technologies, such as reducing wafer thickness or Kerf loss during sawing processes, result in efficiency gains that offer significant cost reduction. For example, a one-percent improvement in efficiency can deliver up to ten-percent cell manufacturing cost savings. In addition, a 20 micron (μ) reduction in wafer thickness (from 200μ to 180μ) can provide a seven-percent cost saving. It can be expected that significant cost reductions can be achieved by deploying comprehensive process control solutions within volume production photovoltaic manufacturing environments to leverage the disciplines and methods that have already been modeled in the labs. Process control modeling initiatives at institutions such as University of Maine, University of South Florida, Georgia Tech, Stuttgart University and Stanford, as well as corporate labs such as the laboratories of Boeing, RCA, Sharp and United Solar, have demonstrated the ability to achieve dramatically higher yields and efficiencies.
Deploying process control Bare Wafer Test: Wafers come into the line (upper left in the graphic on page 85) and undergo dimensional measurement and checks for thickness variation, surface defects, microcracks, saw marks, chipping, etc. Texture inspection: Wafers are weighed and reflectivity is checked offline. Sheet resistance is measured and offline sam-
06 / 2011 | www.pv-magazine.com
Graphic: Solarpraxis AG/Harald SchĂźtt
Industry & Suppliers
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ple testing is performed on edge isolation parameters. Color management, ARC inspection: After cells move through this process, camera systems check for color uniformity, thickness, homogeneity, etc. Metallization process management, print inspection: this process involves front-side steps (silver) and back-side steps (silver and aluminum paste). The problem areas are paste defects, blisters, busbar defects, finger knots and finger interrupts. Cell classification and sorting: the finished cells are checked for micro-cracks. Sorting and rating involve separation into bins depending on parameters.
Elements of process management Traditionally, quality control (QC) is the inspection process that is focused on “good vs. bad� and “go vs. no-go� decisions. The primary objectives of quality control are 1) error & scrap detection, 2) sorting out bad quality, and 3) checking process limits. Process control takes the management of production to the next level by incorporating the ability to “stop the line� in order to avoid waste and further errors. The primary objectives include 1) real-time measuring of production parameters, 2) warning and alarm triggering, and 3) control of the process. Process improvement focuses on “identifying root causes� and “correcting processes� to raise the overall consistency and yield for the production system. Key objectives include 1) detecting issues, 2) identifying sources of errors, and 3) taking corrective actions to eliminate error sources. Classification and sorting are focused on rating and categorizing cells according to specific parameters. Key objectives include 1) definition of material or cell quality, and 2) categorizing and sorting according to results. Classification and sorting functions can be an important element for optimizing overall yields and revenues in industry segments such as solar cells, where a range of acceptable devices can be marketed according to performance parameters.
06 / 2011 | www.pv-magazine.com
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Specific KLA-Tencor systems are optimized to meet the full range of requirements in the solar fab. The ICOS PVI-6 inspection platform is optimized for high-speed, high-accuracy inline functions while the P16+ Profiler is optimized to deAdvertisement
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Analysis of defect count Line 1
Line 2
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Source: KLA-Tencor Corporation
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liver the special capabilities and extra accuracy for sampling functions. The ICOS PVI-6 system is an in-line inspection and metrology system, optimized for a combination of high speed and accuracy in order to accommodate the requirements for real-time inline inspection in PV manufacturing lines. Specially configured versions of the PVI-6 are deployed for bare wafer inspection, ARC coating inspection, print/paste inspection during metallization, and final cell classification inspection. The key features of the PVI-6 inspection platforms include: high speed and accuracy; adaptability for all the process steps; easy integration into production lines; standardized module hardware across all applications; easy-to-use Advertisement
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GUI allowing quick recipe set-up; extensive reporting and data processing tools; and data integration into Plant Information Network. The PVI inline inspection stations are also optimized to drive overall process management goals through direct feedback of measurement data into process control and process improvement, continuous quality control and process optimization, and accurate classification of final product. KLA-Tencor’s P-16+ and (P-16 OF+ open frame) surface profilers provide complete, detailed two- and three-dimensional analysis of surface topography for a variety of surfaces. The P-16+ uses a diamond stylus for advanced profiling applications combined with intuitive menus to capture measurement data such as step height, surface roughness, microwaviness, and overall substrate form, with excellent vertical resolution, precision and reliability, thereby providing highly accurate and adaptable solutions for offline sampling.
Centralized process management As solar cell manufacturing ramps up to meet market demands, next-generation high volume solar fabs will need to coordinate the operation of hundreds of production tools within multiple lines. There is no way that a piecemeal approach of individually controlling the tools can achieve overall process management goals. A comprehensive approach with integrated, centralized process management is required. KLA-Tencor’s Central Module Manager (CMM) software enables one central location to monitor the status of all modules, inspections and classifications, as well as storing recipes, results and classifications. This allows an efficient and timely update and deployment of recipes as well as tracking of trends/ charts and communication using industry-standard manufacturing execution systems (MES).
Real-time process control scenarios The following examples illustrate some of the ways in which integrated process control solutions can lead to real-world efficiency and yield improvements, which will directly result in lower unit costs. Analyzing yields by production line: The first two boxes in the chart above show an analysis of surface defects after the coating step – consisting of a comparison of one week’s production for two different lines (150,000 wafers each). Defect counts on Line 1 show more defects and a clear profile pattern that can then be addressed. By capturing and looking at a historical
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06 / 2011 | www.pv-magazine.com
Graphic: Solarpraxis AG/Harald Schütt
Industry & Suppliers
Industry & Suppliers
Validation of the structural integrity of bare wafers for cracks, foreign material inclusions, and pitting. Pictured above is a non-penetrating crack.
Inspection after the coating process. PVI-6’s advanced surface inspection can detect contamination, wafer stain and finger prints.
sample of defects, manufacturers gain a much clearer picture of the process than could ever be achieved by looking at individual defects. As patterns become more apparent, corrective actions can be taken, leading to significant yield increases.
Efficiency Managing cell efficiency: The third and the fourth boxes in the chart on page 86 provide an analysis that looks at efficiency for a print screen cell over a period of time and identifies patterns in print finger-width. While the trend for an extended period of time shows little variation around the mean, honing in on the specific time segments, such as shift changeover points, shows a pattern of variation that is useful to track. For example, this illustration shows a higher print finger width when the print screen is new (at the beginning of a shift) but after eight hours, the finger width becomes smaller. The availability of such information enables process engineers to optimize the print screen replacement cycles to maintain the best balance between maximizing cell efficiency and production costs. Comparing wafer suppliers: Deploying comprehensive process control systems at the bare wafer entry point also allows manufacturers to spot differences between wafer suppliers. Identifying differences in wafer quality can account for as much as 0.5 percent difference in cell efficiency. Managing the inputs entering the production line is a critical element to both improving efficiency and optimizing the outputs from the line.Comparing cell efficiency for multiple production lines: Process control analysis can also be used effectively to compare results from multiple production lines. By
Enhanced solar cell inspection, including antireflective coating (ARC) and print inspection, as well as end-of-line cell classification.
identifying the differences between lines, process engineers can hone in on “best practices� and use the information to raise the efficiency of all lines. Comprehensive deployment of advanced process control methods and disciplines within cell manufacturing environments can deliver significant benefits, including improved production with higher yields and greater cell efficiency, faster ramp-up to meet volume demands and drive down cost per watt and faster development cycles to improve processes and adapt to new requirements. Deployment of a comprehensive process control methodology accelerates time-to-market, improves competitiveness and contributes to greater profitability. ◆ Advertisement
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THE AUTHOR
Pieter Vandewalle is Senior Director of Marketing, Growth and Emerging Markets Group, ICOS Division, at KLA-Tencor. KLA-Tencor Corporation offers products, software, analysis, services and expertise that are used by every major semiconductor manufacturer in the world.
06 / 2011 | www.pv-magazine.com
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