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  • Solar inspection mixes sound with imaging

Photonics Spectra
Apr 2010
Anne L. Fischer, Contributing Editor, anne.fischer@photonics.com

Solar manufacturers have a few technologies at their disposal that can detect cracks in cells and modules. In an industry where energy efficiency is the name of the game, manufacturers want to reduce or eliminate cracks to improve yield, reduce waste and avoid putting faulty modules into solar panels. Defects are detected with various types of imaging (see “Camera makers find a niche in the solar market,” page 43, May 2009), but some companies prefer to begin with resonance ultrasonic vibration (RUV) technology.


Resonance ultrasonic vibration is used to inspect solar cells on the assembly line. Photo courtesy of RUV Systems.


The process known as RUV was invented and developed by Dr. Sergei Ostapenko, who conducted his research at the University of South Florida in Tampa. Initially, his projects focused on measuring stress in silicon wafers for microelectronics. But with a personal interest in photovoltaics (PVs) and an involvement in numerous PV projects at the university, his research moved quickly in that direction.

His team found that resonance vibration modes in wafers could be measured while they were running along the production line. The group also developed data analysis software; both technologies became the basis for the launch of Ultrasonic Technologies Inc., a startup based in Tampa, Fla.

The way it works, according to Ostapenko, is basically akin to tapping on several identical new wine glasses. A glass with a crack in it will sound different from an undamaged one. “Ultrasonic testing does the same thing with silicon wafers and solar cells, but at a frequency range above what you can hear with your ear, which ultimately increases crack detection sensitivity and accuracy,” Ostapenko explained. On the production line, RUV also allows fast real-time crack inspection, matching throughput of solar cell wafers and modules at a few seconds per unit.

Optical testing an option

Yield and quality are directly related to finding and rejecting unstable wafers or cells while they are still on the assembly line, manufacturers find. This can be achieved with RUV, so the technique can eliminate the need for optical testing. However, if discarding defective units is not the sole goal and locating the crack is essential, optical testing off-line can perform this function.

The main difference between RUV and optical methods is that RUV is “listening” to the cracks rather than seeing them. Manufacturers can use both techniques first by listening to the cells and then, as a backup, using infrared or electroluminescent (EL) inspection on any wafers or cells that are rejected or questionable.

A clear strength of optical testing, said Ostapenko, is its ability to get a visualization of the full wafer. However, the drawbacks of using optical testing alone include its inability to see tiny, sealed cracks, or to distinguish between a surface scratch and a true crack. Furthermore, he added, EL imaging can be applied only to finished cells and cannot be used on wafers, which don’t yet have electrical contacts to which the bias voltage needed for EL would be applied. Similarly, infrared transmission imaging can be used exclusively on bare wafers and cannot be applied to processed material with a nontransparent back contact. The RUV method, on the other hand, is applicable to the entire silicon cell production process, from wafers to finished cells.

When RUV is used in conjunction with optical inspection, the two technologies complement and reinforce one another. Rimas Systems BV is a Dutch manufacturer of solar module manufacturing equipment. At RUV Systems, a company partially owned by Rimas, RUV technology is being implemented in inline production equipment for module, cell and wafer manufacturing. RUV Systems, in cooperation with Ostapenko, also studies RUV in conjunction with optical imaging.

According to Michel van Dooren, plant manager at RUV Systems, RUV technology is “the most reliable for detecting cracks in an in-line situation.” He added that optical imaging is best for analyzing the rejects off-line, where there is more time to make the image. Using the two technologies in conjunction helps to reject faulty product from the assembly line and then validate the problem with an image.


GLOSSARY
camera
A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum. These sensors are transducers...
optical testing
Refers to a variety of methods and tools used to determine the surface contour and performance of optical components and systems.
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