| Work Detail |
Scientists in China have investigated the fracture strength of commercial G12 monocrystalline wafers via the 4-point bending test and have found that wafer thickness, the position of the silicon wafer in the silicon brick, and the bending test direction are key parameters to develop future strategies aimed at reducing fractures.
Researchers at Shandong University in China have investigated the fracture strength of commercial 210 mm x 210 mm monocrystalline silicon G12 wafers used for solar cell production and have found that several strategies may be adopted to produce large-size and ultra-thin silicon wafers while reducing fracture probability during the sawing process and the post-processing.
“We investigated the effect of wafer thickness on the fracture strength and the effect of the position of the silicon wafer in the silicon brick on the fracture strength of the silicon wafer,” the researchs lead author, Yufei Gao, told pv magazine. “Four-point-bending (PB) tests were conducted on three thicknesses of G12 mono-Si wafers, and the load-displacement curves during the testing process were recorded.”
This kind of testing is used to assess the flexural strength of a material and its tendency to crack under the bending load. The material is usually placed on two supporting pins and centrally charged at two pressure lines with a testing piston.
For their analysis, the researchers used the finite element method (FEM), which is a numerical technique used to perform finite element analysis (FEA) of any given physical phenomenon, and the Weibull function, which is the most widely used in analyzing silicon wafer fracture.
“The basic assumption of the Weibull function is the weakest link assumption: the survival probability of a sample is the product of the survival probabilities of each volume element within the sample,” the scientists explained. “Therefore, the fracture strength of silicon wafers is determined by the weakest defect.”
They analyzed three different wafer thicknesses of 130 µm, 140 µm, and 150 µm and took into account the position of the silicon wafer in the silicon brick and the bending test direction.
The analysis showed that the surface roughness of the wafers decreases with the increase in the usage time of the saw wire. It also demonstrated that the curvature of the saw marks on the wafer surface grows with the increase of the usage time of the saw wire.
Furthermore, the team found that the distribution of the fracture probability density curve is more concentrated in the front wafers, while it is more dispersed in the rear wafers. It also ascertained that fracture characteristics of the middle wafers and the rear wafers are similar.
They also found that the characteristic fracture strength of bending in the perpendicular direction to the saw marks is two or three times that of bending in the parallel directions to the saw marks.
“The relationship between the saw wire usage time, the surface roughness, and the saw mark characteristics with silicon wafer fracture strength has been revealed, which provides the improvement direction for improving the strength of large-size ultra-thin silicon wafers and reducing the fracture probability in the production process,” the academics said.
Their findings can be found in the paper “Fracture strength analysis of large-size and thin photovoltaic monocrystalline silicon wafers,” published in Engineering Fracture Mechanics. |
