Subsurface solar-cell characterization
Equipment makers are applying dual-beam systems to the material characterization and inspection needs of PV solar cells.
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 12/1/2009 2:00:00 AM
Inspection methods such as x-ray, laser, and photoluminescence can scan photovoltaic solar cells and wafers for surface defects. But engineers at firms that manufacture solar cells also must detect and localize subsurface defects such as microcracks in substrates, crystalline dislocations, and micro-shunts, said Vinh Van Ngo, MEMS/Solar/LED business development manager for FEI. In addition, the engineers need to simultaneously locate and measure submicron features for detailed structural and process analyses. Because dual-beam systems—a SEM (scanning-electron microscope) combined with a FIB (focused ion beam)—can find subsurface and submicron defects, equipment makers such as FEI are applying them to the material characterization and inspection needs of PV (photovoltaic) solar cells, said Ngo.
“Regular, unenhanced optical imaging doesn’t penetrate through the substrate, while x-ray imaging techniques are limited in the amount of contrast and resolution they provide and thus in the amount of information you can get about subsurface defects,” said Ngo. Optical techniques, including laser and photoluminescence, are primarily surface inspection techniques with limited resolution, and therefore cannot see critical thin-film or crystalline structures at dimensions below 1 micron.
SEMs, however, are well known in both R&D labs and production facilities for their nanometer-level imaging ability, as well as for their wide imaging range and material analytical capabilities. FIB technology is a long-established technique for milling, shaping, and modifying to reveal subsurface detail or to construct material structures at the micron and nanometer scales.
Ngo said that FEI is combining SEM and FIB technologies to address both thin-film and crystalline silicon PV solar cells on the manufacturing floor. The bulk of PV solar-cell manufacturing capacity is crystalline silicon-based, in either polycrystalline or monocrystalline forms.
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Subsurface, submicron defects in crystalline PV solar cells include voids at contacts (Fig. 1), micro-shunts, cracks, and dislocations (crystalline wafer imperfections). Thin-film defects include voids at contacts, as well as delamination, micro-shunts, and dislocations (primarily lattice mismatches).
In R&D, SEMs have traditionally been used for imaging and metrology of surface features as small as 10 nm, and, in conjunction with FIBs, of subsurface features. FIBs have also proven useful for sample preparation for higher-resolution analysis using STEM (scanning-transmission-electron microscope) technology and for material composition analysis using energy-dispersive x-ray spectroscopy, according to Ngo. “Tool sets that address PV-specific challenges can be adapted from the ones commonly used for nanotechnology and semiconductors,” he said. The ability to methodically and accurately locate defects that impact performance efficiencies, and therefore the cost per watt, of PV cells has previously been available only in a laboratory setting.
Scientists and engineers at R&D centers and service labs are already using the dual-beam SEM/FIB combination for solar-cell inspection, characterization, or metrology. “The challenge is to demonstrate the technologies’ scalability, familiarize people with [the technologies], and make them easier to use,” said Ngo. “Traditionally, the tools used by R&D staff require a highly trained operator. That’s OK in a laboratory, but a production-support environment requires instruments that are faster and easier to use. We need to create automated tools that simplify and streamline applications for PV production support engineers, scientists, and developers who are not already familiar with SEMs and FIBs.”
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These users need a semiautomated PV tool that uses a SEM column to scan PV samples or wafers sampled from the production line (Fig. 2) and generate charge carriers within them to detect defects, said Ngo. The corresponding scan profile of measurements of EBIC (electron-beam-induced current) provides target site locations for detailed examination or further micro-analysis using a FIB on the same system. This technique provides the subsurface data needed to evaluate or verify the PV production process’s quality and parameters, said Ngo.
In PV, this type of dedicated dual-beam is still limited by throughput, approximately 3 to 5 hr per 6-in. wafer, so it is not practical as an in-line technique. Instead, it must operate in parallel with the production process to provide near-production sampling, said Ngo.
“We are confident that these technologies can ultimately provide the speed and depth of data needed for near-line production support. These capabilities will play a critical role in silicon-based and thin-film-based PV manufacturers’ efforts to achieve cost parity with other energy sources,” he said.
Changes are also coming in solar-cell technology, such as new substrate and thin-film materials for higher device efficiency. These will necessitate better definition of defects and features of interest, as well as better sampling requirements for production and R&D, and new metrics, according to Ngo. He added, “These are all part of a new global PV standards roadmap effort by the Department of Energy and SEMI’s newly founded PV Group initiative.”
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