The thickness of multi-crystalline silicon (mc-Si) wafers used for photovoltaic applications has been predicted to halve in the next ten years to just ~100μm. With silicon's intrinsic brittleness, fracture of wafers during sawing and solar cell processing is becoming an important issue.
Our research aims to understand (and ideally prevent) the origin of fracture in mc-Si wafers. The current focus is on effects pertaining to SiC and Si3N4 precipitates, which are found near the top of cast mc-Si ingots. The project exploits novel methods of testing materials at the micron scale. These use a combination of focussed ion beam machining (to produce specimens) and atomic force microscopy / nanoindentation (to test them). The image on the right (top) shows a "microcantilever" fabricated across the interface of a SiC precipitate (right of image) and bulk silicon. Loading the end of the cantilever allows the fracture toughness of the interface to be determined.
Further experiments are performed using high-resolution electron backscatter diffraction (EBSD) strain mapping to understand strains around these particles. An example of a strain map around a Si3N4 inclusion is shown on the right (bottom). This work is in collaboration with Prof. Angus Wilkinson at the University of Oxford, and Dr Ben Britton at Imperial College, London.