Machine vision aids solar-cell inspection
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 6/1/2009 2:00:00 AM
As manufacturers of solar cells face pressure to drive down costs, they are turning to higher-volume automated manufacturing, accompanied by high-speed inspection to increase yields and improve the quality of their products.
Inspection of photovoltaic solar wafers and cells is required throughout the solar-cell production process. For example, KLA-Tencor equipment inspects at several steps in the solar-cell production line, from incoming wafers to finished cells, said Jeff Donnelly, group VP of the growth and emerging markets group for KLA-Tencor. "We examine incoming bare wafers to determine their dimensions and inspect them for various defects, such as stains and microcracks," he said. "We also conduct in-line cell inspection at different process steps for excursion monitoring and to enable immediate interventions in case there are production problems, such as a defective print screen. Finally, our systems are used in the cell classifiers for final sorting and binning of completed cells."
The two basic kinds of photovoltaic solar-cell technologies are crystalline silicon and thin-film photovoltaic. About 86% of the industry's photovoltaic solar-cell capacity is based on crystalline silicon, in either polycrystalline or monocrystalline forms, said Donnelly. "This technology creates solar cells on top of silicon substrates, not unlike a semiconductor manufacturing process," he said. The finished active devices are then placed in a panel.
Efficiencies of silicon-based photovoltaic solar cells, typically 15 to 20% for polycrystalline and 18 to 25% for monocrystalline, are higher than efficiencies of thin-film photovoltaic cells, Donnelly said. "Developers of thin-film technologies are still using different types of substrate materials, such as plastic, glass, or very thin stainless steel, and are therefore inventing their own process equipment." Although some believe that thin film has cost advantages, KLA-Tencor has only just started to look at that technology because volumes have been so low, he said.
By leveraging industry knowledge and the technology for dealing with silicon, manufacturers can produce photovoltaic solar cells in high volumes with automated manufacturing and handling equipment. For example, KLA-Tencor's P-6 stylus surface profiler is commonly used offline in surface topology measurements, including measurements of solar cells. In March, the company's ICOS division debuted the PVI-6 system, which performs in-line optical inspection of photovoltaic solar wafers and cells, said Donnelly. The PVI-6 is a family of inspection modules combined with software, including analytical tools that increase the overall yield of the solar-cell production process.
Since polycrystalline solar cells are semiconductor devices, they have inspection and metrology requirements similar to those of non-solar semiconductors, and the basic inspection technologies are similar, said Donnelly. In both, the challenge is to combine high resolution with high throughput. "Typical semiconductor manufacturing throughput is 100 to 200 wafers per hour, but in solar cells, it's 2000 to 3000 wafers per hour," he said. "On the other hand, the typical scale of semiconductor inspection is in nanometers to microns, whereas for solar inspection it's in microns to millimeters. In both cases, the data sets are huge and require fairly sophisticated data processing."
In general, the solar-cell industry is moving from the research stage into the production stage, and aiming at lower costs and maximized yields, said Dave Cochrane, Dalsa's director of product management. "The market is tending toward larger [solar] panel sizes, produced for a lower cost, and with somewhat less power per square inch," he said.
In March, Dalsa announced its entrance into solar inspection with a new 22-Mpixel camera and the application of existing capabilities, such as its TDI (time delay integration) technology. "We are taking the standard technologies used in wafer inspection and other applications such as flat-panel display inspection, and tailoring them a little for the needs of solar-cell inspection," said Philip Colet, Dalsa's VP of sales and marketing. "Right now, the degree of customization is somewhat small, but as we learn more, that will increase. The main differences are in the areas of speed, sensitivity, and spectral response."
Since a solar panel can measure up to 2x2 m, the inspection equipment has to scan that surface very quickly to be efficient, said Colet. "For example, our area-scan Pantera 22-Mpixel camera takes fewer snapshots to look for gross surface defects because of its very large array," he said. "For higher volumes and higher speeds, our TDI line-scan camera is used for large, thin-film solar cells."
For More Information:
Moresco, Justin, "Solar Market: Dip in 2009, Rise by 2011," Red Herring, February 18, 2009. www.redherring.com/Home/25858.
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