19 May 2010
From ensuring wafer quality to assessing quantum efficiency, solar cell inspection
increases yield, improves processing, and enhances performance.
Forget about the days when an estimated global capacity of 25 MW was something to boast about; today, a single manufacturer can run multiple 25 MW production lines, routinely turning out 20,000 to 30,000 cells per day. Of course, fabrication is only half the battle. A product can't succeed unless it performs. That makes solar cell inspection, whether for quality assurance or process control, absolutely essential.
Solar cells must be inspected from start to finish
(Image courtesy of KLA-Tencor)
As crystalline silicon wafers are sliced ever thinner, breakage during the fabrication process becomes a more significant drag on yield. Even a small flaw can propagate during handling, turning a $3 wafer into expensive scrap. Manufacturers need fast, reliable techniques for crack detection, from bare wafers to finished cells.
Resonance ultrasonic vibration (RUV) technology provides one solution. A silicon wafer of a specific size and thickness has a characteristic set of resonance vibrations in the ultrasound frequency range. When a crack or flaw compromises the integrity of the wafer, it distorts resonance frequency curve characteristics like amplitude, peak position, and bandwidth (see figure 2). In RUV systems, resonance vibrations generated in the wafer by an external transducer are captured by a contact ultrasonic probe. A proprietary algorithm analyzes the data for distortions, to indentify flawed wafers/cell so that an automatic handling system can remove them from the line.
Resonance ultrasound vibration systems (top) detect flawed wafers using the fact that
cracks shift the ultrasound resonance curve of a material (bottom).
(Image courtesy of Ultrasonic Technologies, Inc.)
The system currently boasts an inspection speed of 2 s per wafer and can detect cracks as small as 1 mm, says Sergei Ostapenko, president of Ultrasonic Technologies Inc. With multiple systems deployed in production lines worldwide, the technology has moved well beyond the development stage. Even better, it is scalable.
The primary drawback to RUV is that it is a high volume pass/fail inspection rather than a mapping process. It does not report the exact location of a flaw. Then again, in a fab where the throughput of PV lines is below 2 s per wafer, a pass/fail solution is probably the only practical option. For process control data or to salvage a smaller cell from the flawed wafer, manufacturers can turn to techniques like as optical imaging or scanning acoustic microscopy.
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