Lenses in solar, flat-panel inspection

Two lens types commonly used in FPD and solar-cell inspection are large-format and telecentric.

By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 2/1/2011 12:00:00 AM

Manufacturing steps for semiconductors, PCBs, FPDs, and solar wafers and cells are very similar, said Jason Baechler, manager of sales and marketing, machine-vision division, for Moritex USA. "For example, similar optics can be used when inspecting incoming glass for either solar-cell or flat-panel display manufacturing, even though the glass coatings may differ," he said. "Within these four industries, most applications can be divided into two main optics usage types: alignment and orientation versus inspection."

Common to all is the need to align targets on different layers or materials for inspection, said Baechler. "Alignment is one of the most common applications for telecentric lenses, since they are better than other lens types at measuring objects' true sizes and determining their true locations," he said. "In alignment, you're not usually zooming in to see the target, so fixed-magnification lenses are often used. But in inspection, zoom lenses give a lot of flexibility: You can zoom in to see specific pins, pads, or other features. Fixed-magnification lenses are also common here, especially when you want higher resolution and speed."

The requirements for lens performance also tend to be fairly similar in all of these areas, said Nicholas James, product line manager for Edmund Optics. Solar-wafer, solar-cell, and flat-panel inspection form one cluster, however, because they all require a larger scale. This, in turn, demands a larger-format lens, "since the defects are larger and small defects aren't quite as critical; the whole cell still works even if there are lower-efficiency areas," he said. "Semiconductors and PCBs make a different cluster, with a higher risk proposition: One small defect can make the entire wafer or board fail, so you need higher precision."

Solar cells are larger than semiconductors and must be inspected at multiple levels, so no single camera can give them the resolution they require, said Stuart Singer, VP of Schneider Optics. "For flat-panel or solar-cell inspection, [working distance] is a couple of inches to almost a foot," he said. Since operators must inspect these glass panels several times during the manufacturing process, a different lens is required for each step. Consequently, customers generally use several cameras and stitch the images together. "But what they really want is bigger sensors with more pixels, and only one lens, to eliminate stitching images," he said. "That means one-to-one imaging, or maybe a 3X to 5X enlargement."

Two lens types commonly used in FPD and solar-cell inspection are large-format and telecentric. "Telecentric lenses can image soldering and all the thousands of energy transfer lines across each cell," said James. "Large-format lenses, often large-format linescan imagers, are used for high-precision objects, as well as for components with sizes that aren't critical, but that must be checked for placement or presence/absence."

A telecentric lens must be larger than its field of view, said Baechler. Telecentric lenses are used for inspecting very small as well as relatively large areas, such as FPDs or solar panels, where high measurement accuracy is required. "We've developed macro lenses that are similar in size to our compact telecentric lenses, but they cost considerably less due to the limited number of optical components and lower complexity," he said.

A large-format lens with a linescan sensor is especially useful for imaging large areas in solar panels or FPDs, said Craig Fitzgerald, Navitar's VP of product and business development. "Then, when you see a defect, you go offline using a high-resolution, high-magnification, area-scan lens to look at the actual circuitry."

Both large-format and linescan lenses are used in high-speed applications, said Baechler. Large-format lenses can capture large surface areas, such as glass sheets on an FPD or solar-cell production line. "Since that line may be moving, manufacturers will often choose a large-format linescan lens," he said. Linescan lenses are less common in semiconductor manufacturing.

Although the lenses in linescan and area-scan cameras tend to be similar, their format is different. "Linescan cameras generally have longer arrays," said James. "Area-scan lenses typically max out around the 30-mm mark, usually in 1/2-, 2/3-, and 1-in. formats." Above that size, the price rises steeply. Most users would prefer two cameras with smaller fields of view than a single large area-scan camera because of price and availability concerns. "If the field of view is wide and the object for inspection is on a moving conveyor belt, a fixed camera combining a linescan sensor with a lens, or multiples of them placed side by side, can inspect the field of view all at once," he said.

It's less feasible to use four or five area-scan cameras for one job, since they would have to move continually, said James. This could make them break down sooner, or it could cause other mechanical problems. "If you are looking at a flat-panel display or a big solar cell, you'd still need multiple linescan sensors side by side, but there wouldn't be enough room for area-scan cameras: You'd need too many," he said. "If you could use standard fixed-focal-length cameras, that would be even better, but their field of view is too large for the defect size you need to image."

Lens requirements for solar-wafer and solar-cell inspection vary depending on the technology and its intended use, said Baechler. "For example, the cost of manufacturing PV solar cells continues to be fairly high because it is still an emerging industry, although the efficiency of the cells is not very high," he said. Since PV solar post-cell production is relatively low-tech, manufacturers aren't as likely to use complex optics. But manufacturing in some solar technologies, like advanced scribed cell modules, is more complex than manufacturing a standard monocrystalline panel, he said.