Novel effects on the fracture strength of poly-crystalline silicon wafers

Abstract

Nowadays, silicon wafers are widely used in a number of areas, in particular in the semiconducting and photovoltaic industries. In solar cells, large-grained poly-crystalline silicon wafers have gained popularity due to their low-cost manufacturing which involves wire-cutting of ingots. However, this process introduces surface defects, which, added to the brittleness of silicon, lead to fracture in manufacturing, transportation, and exploitation. The silicon wafers in solar cells are subjected to harsh working environment when used on house and car roofs, since wind blasting and car racing introduce vibrations and the changing seasonal and daily temperature introduce thermal stresses. All this affects the resistance of silicon wafers and ultimately solar cells, to fracture. Based on the above, this thesis aims to determine the fracture strength of commercially available large-grained poly-crystalline silicon wafers, as well as their fatigue resistance.

On this project, a combination of experimental, analytical and numerical approaches has been applied to resolve the issue of fracture under quasi-static and cyclic loadings. The flexural strength has been elucidated for both raw and polished wafers by means of ball-on-ring flexural tests, and the range of fatigue stresses has been ascertained. Mechanical and thermal fatigue tests have been conducted on initially indented specimens by means of Vickers indentation, to simulate surface defects. The mechanical fatigue has been carried out by ball-on-ring test setting with a BOSE equipment of 3kN loading cell, and the number of cycles at crack initiation has been recorded. Optical microscopy and SEM have been employed to reveal the crack propagation and damage mechanism. Some thermal fatigue tests have also been conducted, to assess the resistance of silicon wafers to changes in the thermal environment. Based on the experimental work and a comparison with the available reports in the literature, this thesis draws conclusions on the difference between the fracture strength and fatigue resistance of single-crystal and poly-crystalline silicon wafers under flexural loading conditions. Recommendations are also given on possible future studies on fatigue of poly-crystalline silicon wafers.