Screening of Silicon Wafers Used in Photovoltaics

Stage: Production
While preventative measures that minimize wafer breakage in the semiconductor industry exist, they specify certain criteria for wafer preparation, handling, and processing that cannot be adopted without a significant increase in solar-cell manufacturing costs for the solar-cell industry. Moreover, the yield loss due to wafer breakage has a considerable influence on the economies of producing solar cells. Because the revenue loss associated with wafer breakage increases as the cell fabrication progresses, fabrication-line exclusion of those wafers that may break during cell processing is desirable. Identifying the sources of wafer breakage, understanding the wafer breakage mechanisms, and developing methods of detecting and separating these wafers that are susceptible to breakage is of value, especially at early stages of solar cell fabrication. A method and apparatus are disclosed to enable the detection and removal of compromised wafers early in the production stream.


The screening system may consist of a conveyor belt that carries numerous wafers and exposes them in succession to an optical source, such as a tungsten-halogen lamp. A wafer’s narrow, uneven illumination from the optical source causes a pre-determined thermal stress in a wafer such that wafers compromised by micro-cracks may break and then be removed from the production stream. The energy generated by the optical source is used to induce the maximum thermal stress that a wafer would experience during manufacturing. If a wafer survives this stress test, then it likely would survive the cell fabrication process without breakage.


Traditional wafer-screening methods like three-point testing may suffer from relatively complex and slow implementation schemes and from inaccurate results gained from tests performed over local areas of the wafers. As an alternative to the application of mechanical stress, wafer cracks may be analyzed with thermal imaging. However, small cracks can be difficult to detect with the imaging method. Moreover, because thermal imaging provides sufficiently indirect data about the wafers’ propensity to break from detected micro-cracks, selection of uncompromised wafers can be difficult with this crack-detection method. The disclosed screening method and apparatus provide an efficient and easy way to detect and remove compromised wafers from the production stream before unnecessary labor, materials, and energy are invested.


To learn more about Screening of Silicon Wafers Used in Photovoltaics, please contact Bill Hadley at:


Bill.Hadley@nrel.gov


ROI 04-07.

Patent: US 8,006,566 B2

Applications and Industries

  • Solar cell manufacturing
  • Solar module manufacturing
  • Wafer manufacturing
  • Solar equipment manufacturing

Benefits

  • A superior screening method and apparatus to both detect and remove compromised solar-cell wafers from the production stream
  • Screening of compromised wafers before unnecessary labor, materials, and energy are invested
  • Enhanced micro-crack detection efficiency and reliability over mechanical and imaging detection methods.

Attachments

US8006566.pdf

Sep 25, 2019