Solar Metrology Gets Serious
Suppliers of optical inspection systems are improving central data management capabilities and tool uptime. The ramp up of solar cell production volume is spurring development of better inspection and metrology tools.
Paula Doe, Contributing Editor, SEMI, San Jose -- PV Society, 6/12/2009
The ramp up of solar cell production volume is spurring development of better inspection and metrology tools, and better ways to use existing semiconductor metrology tools, to improve efficiencies and yields.
Suppliers of inline optical inspection systems are redesigning their systems for high-volume production environments. Solar cell makers are using profilers from the semiconductor industry to better control surface roughness and trace lines, and are starting to look at a range of other metrology systems as well. In addition, thin-film solar tool suppliers have developed some sophisticated inline metrology tools of their own.
New optical inspection systems from KLA-Tencor's ICOS division aim to improve yields and reduce costs. The print inspection module more accurately detects when defects start to increase enough to require stopping the line to change the screen, thus improving uptimes. The end-of-line classification module more accurately distinguishes between high-performance cells and less valuable ones, potentially reducing a customer's classification overkill from perhaps 2% to 0.5%. "At 1200 cells an hour, that can quickly add up to $1M in extra revenues," said marketing director Pieter Vandewalle.
Users can now also optimize these yields by testing out parameters for print inspection or end-of-line classification with a new offline simulation tool. And improved software now allows the inspection modules — up to 50-100 vision modules per plant as facilities reach 500 MW annual capacity — to be managed in one central system, so recipes can be uploaded to all the modules at once, and performance analyzed across tools and lines. This central inspection module management system can of course then be linked to the factory automation system.
"We applied KLA's disciplined product development system to determine where to really add value," said Jeff Donnelly, KLA-Tencor group VP of growth and emerging markets, of this first new generation of products since KLA-Tencor acquired the company last May. ICOS's early engagement in the solar industry — starting more than seven years ago — and its high-speed vision inspection technology from the semiconductor packaging sector has allowed it to take a leading position in the solar cell inspection market, according to the company.
Other suppliers of optical inspection systems are on the same path toward improving central data management capabilities and tool uptime. GP Solar (Konstanz, Germany), a solar process technology consulting firm that used to provide its customers with ICOS systems, introduced its own line of vision inspection tools for crystalline cell lines last fall. International sales manager Willi Huber said the product has taken off fast, with sales of 140 units in the months since introduction. Automation companies have also now qualified the tools for inclusion in their lines and shipped first units to customers, he said.
GP Solar's improved software now runs on a server-based system, so the vast volume of data from a high-volume plant can be collected and managed on one central workstation, Huber said. The company devised an automatic calibration system, which uses a highly accurate, laser-cut reference calibration plate, so the systems calibrate to the same standards even in different buildings and different sites, whatever the different reflections in these particular environments. The same recipes can thus be easily downloaded to all systems across the company. The company also added a new camera holder that can be adjusted more quickly and accurately, to cut adjustment time down to 10 minutes, significantly improving uptime. Finally, GP Solar designed a new illumination unit that gets better homogeneity from its LED light sources by a specially designed dome, and better lifetime from the LEDs by extremely short flash cycle times that keep it operating in the right zones.
Advanced high-volume cell production lines typically inspect incoming wafers optically for cracks and chips, and run an assortment of automated non-optical measurements to check thickness, resistivity and mechanical properties. A series of eight or so separate inline optical inspection nodes monitors results after each process step on each line, checking texturing, coating, color, and printed lines and contacts, at 2000-3000 wph. Finally, the efficiency of the finished cells is measured under a flash of sun-like light, and the cells are sorted by performance.
Profilers help improve efficiency
PV producers are also improving the efficiency of their crystalline solar cells by using profilers from the semiconductor industry.
Key applications for stylus profilers are process development for surface roughness, and near-line sampling to track quality of surface texture and printed silver traces, said Geoff Anderson, senior product manager for surface profilers at Veeco Instruments. Tight control of the silver print lines allows companies to maintain the optimal usage of silver for best efficiency at lowest cost.
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| An optical profiler clearly images saw marks to monitor saw health and wafer stress. (Source: Veeco) |
KLA-Tencor offers a smaller, more economical tabletop stylus profiler optimized for the solar industry with simpler software and less automation than its tools for the semiconductor industry. "We'd already developed the technology for the semiconductor market," Donnelly said. "You wouldn't get this kind of investment otherwise. The solar industry can re-use it very effectively."
Solar cell makers are also using the more detailed 3-D images made possible with optical profilers to monitor and characterize the etch process to optimize surface roughness. Small improvements in the etch process for surface texturing can mean big improvements in wafer efficiency for a direct impact on profits, but it's not easy to tell why some cells work so much better than others that measure as the same roughness by other metrology methods. The detailed 3-D analysis of different roughness parameters better reveals which aspects of the feature topology correlate with key differences in efficiency. Anderson reports that one customer found that a change in the area of the faces of the pyramids on its surface texturing correlated with a 0.3% improvement in efficiency, which adds up to a big improvement in the electricity that can be produced from 8000 or 9000 wafers a day.
There's also interest in 3-D optical profiling to monitor incoming wafers, to track the saw marks indicating mechanical stress that can cause breakage. The 3-D image makes it easy to calculate diagnostics than can monitor saw wear and predict optimal replacement intervals. "The saw marks just pop out," Anderson said, noting the company is now marketing the tool to wafer and saw makers as well.
In the thin-film world, CIGS makers are also using 3-D optical profiling for process development, especially to control scribing, and deal with things like laser redeposition near the edges and the effects of laser beam degradation that may impact efficiencies.
Solar cell makers typically use home brew tools of their own as well, to measure quantum efficiency, devising systems to shine different wavelengths of light through the cell to see how it works across the spectrum. There is also interest in measuring photoluminescence hitting the cell with a laser of a particular wavelength and looking at the wavelength of the emitted photons, to learn about efficiency. Producers are looking as well at electroluminescence, running a current through the cell and imaging it with an infrared camera to look for hot and dead spots.
On the thin-film side, metrology remains the least standardized part of the turnkey line, with multiple custom options, and requiring integration of different protocols to get the data out and into a system where it's usable for analysis and yield management. "It's where we see the most requests for changes from customers," said Jim Cushing, product manager director for Applied Materials' SunFab line. "Different customers want different kinds of metrology, more or less optical, here or there."
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| Applied Materials is applying metrology to its turnkey thin-film solar line. |
Applied Materials has developed its own inline metrology system for its turnkey thin-film line to measures resistivity to find and fix shunts midway through the line. A bed-of-nails arrangement of pins sends a high current through the cell to locate shorts, and then quickly zaps away those it finds. The line also includes a solar simulator at the end that checks the efficiency by flashing the modules and taking an I-V measurement. Customers also use an offline tool, sold separately, that Applied developed with Carl Zeiss to extract the transmissivity of single films on smooth glass, without the complications of roughness.
Customers also have the option to include optical inspection at multiple places along the line, to check for chips, cracks and scratches in the glass. The technology is based on that used in the glass industry, but developed to handle larger substrates, tighter specs, and process recipes for deciding if a defect will affect the integrity of the module so it must be scrapped, or if it will reduce efficiency so it is worth continuing processing but binning to a lower performance category. There is also the option of adding spectrophotometry in several places along the line, to check the transmission and reflectance of the film stack to extrapolate performance. Most users also do offline measurement of quantum efficiency by some system.
Applied Materials is also investing in developing ways to use the electrical data already collected in finding the shunts partway through the line as a window back into the upfront process, for better characterization beyond the film and the glass, according to product metrology manager Kashif Maqsood. "Thin-film photovoltaic is a mix between the LCD industry and the glass industry, but the glass industry doesn't track the glass," he said. "We've put energy into tracking the glass." Cushing added that analysis and yield management efforts are still hampered by having to integrate lots of different protocols, noting, "The challenge for us is to start to get some standardization."
The situation is even more challenging with more exotic thin films that need a wider variety of exotic metrology tools. "CIGS needs a lot of metrology that exists only in the academic world — everything is a science experiment. But all these tools have to be set up to work in a factory," Veeco's Anderson said. "The world would be a lot easier if everyone all used the same system to exchange data between the data source and the host."
Leveraging semiconductor metrology
Though new high-volume automated solar cell plants are starting to add centralized management of inspection data, use of metrology and inspection systems, like most everything else in PV production, varies widely across the industry. Suppliers note that the big wafer-based cell facilities in Europe inspect for quality at each step, while some lines in China use no metrology at all. Engineers from the semiconductor industry tend to want lots of metrology, while those from the traditional solar industry want much less.
But automated high-volume production with tight process control is emerging as key to producing the highest-efficiency cells at lowest cost to get to grid parity. "It's only automation of the whole process, with a standardized system tied together with software, that will get yields up, and volumes up, and allow replication," KLA's Donnelly argued. "Solar can leverage the huge investments made by the IC industry there to get costs down." Still, he noted, "It's still the Wild West — it's still up in the air where the value in the chain will come from."
