Inspection, metrology solar tools evolve
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 2/1/2010 2:00:00 AM
Although thin-film and crystalline-silicon PV (photovoltaic) solar wafers and cells share some similar defects, they require different manufacturing and inspection techniques. Crystalline-silicon solar cells are based on wafers, so software tools used for machine vision, inspection, metrology, and yield analysis of regular semiconductor wafers are being applied to high-volume manufacturing of silicon solar wafers and cells. Thin-film solar cells, on the other hand, are created with substrate materials such as glass or stainless steel, and thus require different inspection techniques.
The main differences between semiconductor and crystalline-silicon PV solar-cell manufacturing are volumes and cost, according to Mike Plisinski, VP and GM of Rudolph Technologies’ Data Analysis and Review business unit. “A large, first-tier semi fab has about 100,000 wafer starts a month, versus a first-tier crystalline-silicon PV fab with about 500,000 starts a day,” he said. In the semiconductor world, manufacturers depend on metrology and analysis data after most process steps. Unlike a high-value semiconductor wafer, “a PV cell or wafer is worth maybe a couple of dollars,” said Plisinski. So, manufacturers do not need as much data after each step to ensure wafer quality.
The current state of wafer-based solar-cell manufacturing resembles semiconductor wafer manufacturing in the early 1970s, said John Petry, manager of marketing for vision software at Cognex. At that time, traceability was first introduced and individual wafers were still handled by technicians. Similarly, in most solar-wafer production, people still handle wafers, geometries are coarser, and precise registration is not nearly as important.
But Petry said Cognex is seeing changes in crystalline-silicon solar-cell manufacturing, which is beginning to employ procedures used in today’s 300-mm semiconductor wafer fabs, where wafers are tracked throughout production and handling is completely automated. Petry noted that solar-cell manufacturing techniques such as multilayer screen printing and thinner solder fingers on wafers need highly precise alignment, and thus must be automated.
The primary uses for machine vision in solar-wafer production are checking for edge chips and sorting and grading by color, said Petry, fairly simple applications for today’s vision systems. “The other commonly requested test today is to detect nonpenetrating microcracks,” he said. “Unfortunately, no one’s yet found an efficient in-line solution because the necessary imaging techniques—thermal sensors, for instance—are fairly slow. Given a good image, the visual inspection task isn’t actually that hard.” At present, manufacturers are solving the problem by handling the wafers more gently throughout production.
The need for machine vision in crystalline-silicon solar-wafer inspection is growing for a couple of reasons, said Petry. In the industry’s early years, manufacturing processes were simple and fabs could sell wafers as fast as they made them, but since the recent downturn, fabs are competing more on quality.
“Manufacturing processes are becoming better understood, so fabs are better able to correlate wafer appearance to efficiency,” he said. “Newer processes are also becoming more demanding, so you’ll need more wafer inspection, for example, to confirm the alignment in a multilayer screen-printing step.”
For simpler tasks such as wafer handling, said Petry, solar-cell manufacturers use vision software tools such as Cognex’ VisionPro Solar Toolbox, which includes preconfigured tools for standard wafer alignment, edge and print inspection, and color checking. For key OEMs, Cognex also produces custom solar-cell inspection tools. “For example, we have high-speed inspection software for screen-print registration that includes optical distortion correction for high-resolution linescan cameras,” he said. (See “Machine vision in solar-cell fabrication,” below.)
Cognex is working with wafer foundries on new algorithms to detect dislocations in polycrystalline wafers, since inconsistent crystal formation can lead to less efficient electrical conduction, said Petry. The company is also developing more complex pattern-recognition algorithms. “Foundries are just now beginning to understand the relationship between these pattern characterizations and wafer performance,” he said. “But we expect that eventually these algorithms will also be useful for incoming inspection at some solar-cell fabs to ensure good-quality wafers from the foundry.”
The need for solar metrology tools
Increased competition is driving manufacturers of both crystalline-silicon and thin-film PV solar cells to look for improvements through more intelligent use of data to make their lines more productive, said Rudolph’s Plisinski. “In addition to differentiating on efficiency and in cost per watt, they are now trying to differentiate on product quality,” he said. “With the long, 20- to 30-year warranties on PV panels, they need complete traceability.”
Plisinski sees two types of yield- and quality-improvement issues in PV cell manufacturing: those at the factory-management level and those at the equipment level. Most manufacturers have not used data-management systems for monitoring factory-wide processes, due to limitations in existing software. At the equipment level, wafer breakage caused by manufacturing tools is still a big problem. Because of lack of traceability at the tool level, manufacturers can’t put pressure on vendors to address the problem. “One use for Rudolph’s Discover Solar fab-management software is to add that traceability,” he said.
A big factor in PV cell efficiency is the quality of incoming materials, according to Plisinski. “You might have 10,000 samples in a batch, and some or all of that batch could be bad because of a material problem from one supplier,” he said. That problem could cause a drop in overall PV cell efficiency that triggers process engineers to spend time looking for process or equipment problems. “Customers require systems that can identify these issues instantly and that allow manufacturers to trace these problems back to the supplier,” Plisinski explained.
The much larger volumes in silicon PV fabrication vs. semiconductor fabrication strains databases that were designed for fewer samples, and also strains data-collection and yield-management systems, said Plisinski, “so we had to design a yield-management system specifically for [the solar] industry.” To create its Discover Solar fab-management system, Rudolph re-engineered its Discover analysis and data-management software for IC manufacturing to accommodate those differences.
“First, we redesigned the underlying database structures so they could handle and display such huge volumes of samples quickly,” he said. “Next, we modified the analysis engine. In PV solar manufacturing, there’s not much metrology data available. Although thin-film panel manufacturers have some useful metrologies, a lot of analysis is still done by examining electrical test results and tool and sensor data. We needed domain-specific algorithms that would automate a process engineer’s basic analysis to dramatically improve the time required to identify and resolve problems.”
As PV manufacturers bring new technologies into pilot production and then strive to ramp quickly to high-volume production, they will continue to tighten process windows, said Plisinski. “We see customers looking to further improve line performance by using run-to-run control technologies to compensate for the variability of tools and materials over time,” he said. “We also see PV manufacturers pushing process equipment vendors to make a greater amount of process data available to the factory to enable predictive metrology.”
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Fig. 1 As measured by a percentage of total photovoltaic solar-panel watts, the proportion of solar cells based on thin-film technology is increasing and will double by 2013. |
Thin-film technology grows
“While crystalline wafers constitute the vast bulk of solar-cell surface area manufactured today, many feel that the long-term trend is in thin-film, since they believe it will cost less and will be more flexible, both as a substrate and in the areas where it can be used,” said Petry of Cognex. According to a recent report from market research firm iSuppli, the proportion of solar cells created with thin-film technology, as measured by a percentage of total solar panel watts, is growing quickly (Fig. 1).
PV solar-cell manufacturing can be divided into three generations, said Darin Cerny, marketing manager at Cognex. “The first involves processing of mono- or multicrystalline wafers,” he said. “The second is thin-film deposition on glass or on a flexible substrate such as stainless steel.” In thin-film solar-cell fabrication, inspection is currently being performed on incoming substrates and thin-film coatings.
Although the third generation is still in development, most third-generation materials will be polymer-based. “The goal is lower-cost yet higher-complexity materials that will produce even higher efficiencies than either first- or second-generation solar cells,” Cerny said.
In wafer-based solar-cell fabrication, manufacturers have already learned much of what it is they want to measure, said Cerny. “But in thin-film solar, manufacturers are just starting to figure this out. They have some idea of what the defects are and what problems they cause. Now, they must concentrate on reliably and consistently finding these defects so that they can either adjust their processes to prevent them from occurring or decide whether to continue processing less-efficient material.”
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