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Industry & Suppliers

06 / 2011 | www.pv-magazine.com

KLA-Tencor’s ICOS PVI-6 inspection modules provide high speed, auto-mated, optical inline inspection of monocrystalline and polycrystalline solar wafers and cells.

Take controlProcess 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 KLA-Tencor Corporation.

Government incentives are not sustainable in the long run, be-cause they are subject to policy shifts that can potentially cause disruptions in the deployment process. Therefore the PV indus-try must drive down the overall cost of solar energy deploy-ment. 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 pro-cess control; and high-volume yield improvements.

The average price per kilowatt-hour of electricity from tra-ditional 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 conver-gence, “grid parity” will be achieved, with timeframes vary-ing by region.

Solar cell manufacturers need to lay the foundation today in order to respond to the impending upswing in market de-mand 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 en-ergy industry’s need for high volumes and consistent quality at ever-lower prices, but also to enable production of higher-efficiency cells.

Increasing productivity through improvements to manufac-turing processes that result in greater yields is a key contrib-utor to lower costs. Experience has shown that for every one-percent increase in yield, there is a corresponding five-percent cost reduction.

The ability to drive up production volumes while maintain-ing quality will lead directly to a lower cost per device. For ex-ample, doubling the production output will typically result in a 22 percent decrease in the overall unit cost.

Evidence from research at leading laboratories and academic institutions indicate that there are a range of significant oppor-tunities for improving the manufacturing processes used in solar cell production. However, the advanced lab-proven tech-niques that enable greater efficiencies have not yet been imple-mented on a widespread basis in real-world production envi-ronments.

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 de-liver up to ten-percent cell manufacturing cost savings. In ad-dition, 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 envi-ronments to leverage the disciplines and methods that have al-ready been modeled in the labs. Process control modeling ini-tiatives at institutions such as University of Maine, University of South Florida, Georgia Tech, Stuttgart University and Stan-ford, 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, micro-cracks, saw marks, chipping, etc.

Texture inspection: Wafers are weighed and reflectivity is checked offline. Sheet resistance is measured and offline sam-

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Texture inspection Sheet resistance Lifetime Edge isolation test ARC control

Electrical test Cell classification (FS and BS) Print-inspection (FS and BS)

Chemical etching and texturization POCl3 diffusion P-glass removal Silicon nitride ARC

Classification Co-firing Backside printing Frontside printing

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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 (sil-ver 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 ob-jectives include 1) real-time measuring of production param-eters, 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) de-tecting issues, 2) identifying sources of errors, and 3) taking corrective actions to eliminate error sources.

Classification and sorting are focused on rating and catego-rizing cells according to specific parameters. Key objectives in-clude 1) definition of material or cell quality, and 2) categoriz-ing and sorting according to results.

Classification and sorting functions can be an important el-ement for optimizing overall yields and revenues in industry segments such as solar cells, where a range of acceptable de-vices can be marketed according to performance parameters.

Specific KLA-Tencor systems are optimized to meet the full range of requirements in the solar fab. The ICOS PVI-6 in-spection platform is optimized for high-speed, high-accuracy inline functions while the P16+ Profiler is optimized to de-

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Analysis of defect count

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Line 1 Source: KLA-Tencor CorporationLine 2

y: Cell position y; x: Cell position x y: Cell position y; x: Cell position x y: Print finger width; x: Time y: Print finger width; x: Efficiency

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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 metrol-ogy system, optimized for a combination of high speed and ac-curacy in order to accommodate the requirements for real-time inline inspection in PV manufacturing lines. Specially config-ured versions of the PVI-6 are deployed for bare wafer inspec-tion, ARC coating inspection, print/paste inspection during metallization, and final cell classification inspection.

The key features of the PVI-6 inspection platforms in-clude: high speed and accuracy; adaptability for all the pro-cess steps; easy integration into production lines; standard-ized module hardware across all applications; easy-to-use

GUI allowing quick recipe set-up; extensive reporting and data processing tools; and data integration into Plant Infor-mation 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 improve-ment, continuous quality control and process optimization, and accurate classification of final product.

KLA-Tencor’s P-16+ and (P-16 OF+ open frame) surface pro-filers 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 ap-plications combined with intuitive menus to capture mea-surement data such as step height, surface roughness, micro-waviness, 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 individu-ally controlling the tools can achieve overall process manage-ment goals. A comprehensive approach with integrated, cen-tralized process management is required.

KLA-Tencor’s Central Module Manager (CMM) software enables one central location to monitor the status of all mod-ules, 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 manufac-turing 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 ef-ficiency 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 produc-tion 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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sample of defects, manufacturers gain a much clearer picture of the process than could ever be achieved by looking at indi-vidual 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 seg-ments, such as shift changeover points, shows a pattern of vari-ation 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 produc-tion costs.

Comparing wafer suppliers: Deploying comprehensive pro-cess 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 crit-ical 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 ef-fectively to compare results from multiple production lines. By

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 environ-ments can deliver significant benefits, including improved pro-duction 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 method-ology accelerates time-to-market, improves competitiveness and contributes to greater profitability. ◆

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 manufac-turer in the world.

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 ad-vanced surface inspection can detect contamina-tion, wafer stain and finger prints.

Enhanced solar cell inspection, including anti-reflective coating (ARC) and print inspection, as well as end-of-line cell classification.

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