In a world where oil production is declining, and where both nuclear energy plant and spent-fuel storage safety have proven to be inadequate, there is increased pressure on solar power generation to fill the gap. In response to the increased demands for energy, the photovoltaic manufacturing industry has focused on two primary objectives: driving down the cost of solar panels and increasing panel efficiency.
That first objective is being addressed by casting silicon into blocks, which has reduced the input costs for cell fabrication. There are significant costs, however, resulting from blindly cutting the blocks into cells. Silicon-based CCD or CMOS sensors are not able to inspect the blocks, as silicon detectors do not absorb wavelengths beyond 1140 nm and thus cannot image the longer wavelengths where the bulk silicon becomes transparent enough to see the defects.
This article will illustrate how shortwave infrared (SWIR) imaging arrays uniquely find voids and inclusions in solid blocks before they can damage diamond wire saws or result in bad or inefficient cells.
The Inspection Process
For a long time, silicon solar-cell raw materials were supplied by the infrastructure created from the integrated circuit business. In 2008, the solar industry consumed about half of available supply, and spot prices soared as a result. The greater demand brought both an increase in the production of high-purity crystalline silicon using traditional growing methods, but also the development of other methods of forming more affordable solar-grade (SoG) silicon. The metallurgists recognized that cast metallurgical-grade silicon (MG Si) could be purified from 98 percent pure to the higher level needed by solar cell production, which is pure to 6 nines or 99.9999 percent. As a result, silicon began to be cast in huge vats, cutting raw material costs to the wafer processors by several times, and greatly increasing production capacity.
In order to cut the 680 x 680 mm (26.8") ingots into blocks of 156 x 156 mm (6.1 inches) and then into thin wafers, diamond wire saws must be used. If there is a carbon deposit in one of the silicon blocks, the saw could be damaged or dulled. In addition, if an air bubble is captured in a block, significant production time is wasted, the lifetime of the saw is reduced, product yield falls, and costs are driven up. Therefore, it is vital to identify bad blocks before cutting.
Silicon detectors are not usable for wavelengths beyond ~1140 nm (where the silicon becomes transparent), but shortwave infrared detectors and imaging arrays made from indium gallium arsenide (InGaAs) see through the silicon, even solid blocks. Figure 1 shows the 1140 nm peak wavelength of silicon electroluminescence (red curve), which marks the end of the material’s absorbance region (green curve). Beyond this wavelength, silicon becomes transparent, as shown by the green curve, since the fall of absorbance implies that transmission is rising. The bandgap of InGaAs occurs at the longer wavelength of 1680 nm (blue curve), and therefore has absorbance to that wavelength, making it usable to image through the now transparent silicon. The low dark-current and high uniformity of modern InGaAs, developed at Sensors Unlimited – Goodrich ISR Systems (Princeton, NJ), makes it a valuable material for detection and imaging throughout the shortwave infrared wavelength band.
The video below demonstrates light transmission from behind the silicon block imaged with an InGaAs camera. The cracks and material inhomogeneities of this scrap block are clearly visible; a computer can easily map the location of the defects. Take note of the almost complete blocking of light transmission on the right end of the block. The lack of light identifies the top of the original casting, as carbon floats to the top of the molten silicon.
The three photos in Figure 3 show how InGaAs detectors can image through the entire width of a silicon block before cutting it into wafers. The block in the image measures 156 x 156 x 254 mm (6.1 x 6.1 x 10"). It is completely solid and weighs 40 pounds. Despite that, the InGaAs camera is able to resolve detail in the small Air Force resolution chart taped to the back of the block. The illumination comes from an incandescent flashlight positioned to project collimated light, creating a shadow pattern imaged by the camera through the block.
In this image, one side of the block (the side facing the camera) is polished. Unpolished, the light rays would be scattered in many directions, obscuring the detail. With careful management of illumination and imaging, however, and by the use of InGaAs linescan cameras, coordinated with line illumination sources to map the defects within the block, the image processing software tells the diamond saws which parts to cut into wafers and which parts to recycle.
InGaAs detectors are also used to inspect blocks and wafers to gauge material quality. Using photoluminescence (PL), short-wavelength laser power irradiates the silicon and causes bright glow at the bandgap wavelength. Producers map good solar cell material with this technique. However, certain defects resulting from dislocation within the crystalline structure can actively quench the photovoltaic effect of the solar cell. The photoluminescence process will also reveal their locations by weakly emitting in the 1300 to 1600 nm range. As a result of Goodrich process improvements that reduce dark current and system-read noise in its imaging arrays, this very weak glow is now visible to their InGaAs cameras running at video frame rates.
Figure 4, for example, shows several areas of defect locations in a finished wafer. As PL is a contact-less form of inspection, it evaluates raw silicon blocks and freshly cut wafers for problems before proceeding with expensive processing. This is a clear advantage over electroluminescent testing of completed solar cells, though that remains a powerful inspection tool for monitoring product quality on the cell fabrication line, and for panel assemblers.
Beyond the solar industry, PL, at SWIR wavelengths, captures from outside of living small animals the weak glow from nano-tube structures tagged to tumor cells inside their internal organs. Other imaging applications that have been addressed with InGaAs technology include inspection of thin-film and triple-junction photovoltaic cells with both PL and EL techniques, hot hollow glassware inspection with direct imaging, and NIR spectroscopic-based sorting of agricultural, pharmaceutical, and recyclable products.
Improved InGaAs-SWIR imaging has also benefited biomedical imaging with optical coherence tomography (OCT). This method detects disease in the eye, arteries, or organs, either with spectral-domain or full-field approaches. Currently developing is the use of SWIR transillumination to detect tooth decay and demineralization of tooth enamel. The sensor technology has also benefited military and homeland defense in areas such as covert surveillance, laser spotting, and hyperspectral imaging for camouflage and IED detection. Even the art world benefits from this technology; infrared reflectography techniques image paintings, looking under the paint layers of an artwork to see the artist’s original intent or detect counterfeits, or to see through browned varnish to see potential detail to be recovered in the restoration process.
This article was written by Doug Malchow, Manager, Business Development, Industrial Products for Sensors Unlimited-Goodrich ISR Systems. For more information, visit http://info.hotims.com/34461-142 .