Dark-field optics inspect thin-film solar

To drive down the cost per watt of PV (photovoltaic) solar panels, some industry observers say that substrate material must shift from silicon to glass, using thin-film technology. Dark Field Technologies, a supplier of glass and thin-film scanners, recently developed an inspection and metrology tool for thin-film PV deposited on glass, based on its NxtGen system technology, said CEO Timothy Potts.

Thin-film PV processes place three layers of coating on the glass substrate, Potts explained. After each layer is deposited, some is scribed away, forming lines that isolate the panel’s conductive areas. Each of the scribes, known as P1, P2, and P3, is 40 to 50 microns wide. A scribe’s width, pitch, and offsets must be precisely controlled, and coatings must be precisely and thoroughly removed, to maximize yield and produce panels with maximum conversion efficiencies.

“Our challenge was that no technology could do 100% on-line metrology and inspection of thin-film solar panels being coated and scribed, at real-time production line speeds of 100 to 150 feet per minute, and on glass, which is not perfectly flat and bounces during its transport on the conveyor,” said Potts.

The first scribe, P1, creates hundreds of lines on the conductive oxide or molybdenum layer of coating. The panel’s active layers are deposited during the second and third coatings. The P2 scribe, which is performed on the second layer, must be inspected to ensure it is complete, and also to ensure that offsets between P1 and P2 are within certain limits. For example, center-line to center-line distances can’t vary by more than 2 to 5 microns. Typically, a third, metal coating is deposited. After the third scribe, P3, is applied, its width and offsets to P2 and P1 must be measured.

Dark Field tried building thin-film inspection systems with bright-field optics, said Potts. These systems used matrix cameras, which sample less than 0.05% of thin-film scribes, and provide only scribe width and offset data, not defects or residual coating information. But the system proved too slow for on-line production use. Images from a production line tend to go out of focus due to small changes in panel flatness or bounce, and autofocusing systems do not work at production speeds. “Instead, we came up with the idea of using dark-field laser optics and high-speed line-scan cameras for the inspection and metrology of on-line scribing,” he said. “In an optical dark field, you see and image not the scratch but the scattering effect it has on rays of light. For the same resolution, you can provide several times the data you get in bright-field optics.” This system became the NxtGen Scribe 100.

To create the optics for the NxtGen Scribe 100, the company used Hi Resolution Macro Line Scan enlarging lenses from Schneider Optics and Piranha 2 linescan cameras from Teledyne Dalsa. “At high resolution, any kind of optical aberration has a multiplier effect,” said Potts. “The Schneider lenses gave us the minimum optical aberration, since they are so precise across the entire field of view.”

Potts and his team also looked for the right kind of lasers to solve the speckle problem, inherent in all lasers. “If you blow up a laser image, you see bright and dark spots in it,” he said. “These give an unusable signal when you try to do high-resolution work with lasers and cameras to create enhanced images of the scribes. So, we had to develop proprietary solid-state lasers.” Potts said the NxtGen Scribe 100 is the first production metrology and inspection system to combine cameras with lasers.

After generating enhanced images, the NxtGen Scribe 100 system uses high-speed signal-processing hardware from Matrox and uses software analysis for defect detection and measurement. “We had to develop the processing hardware and software needed to get very precise measurement accuracy,” said Potts. “We had to write some clever algorithms to perform image processing at these high speeds.”

The resulting system provides detailed information about each scribe, including width, pitch, offset, standard deviation, percentage missing, and spacing violations, and it measures the amount of remaining residual coating. The system is accurate within 2 microns, and a large depth of field ensures the system stays in focus on warped panels and even when the glass bounces, said Potts.