The impact of saw mark direction on the fracture strength of thin (120 µm) monocrystalline silicon wafers for photovoltaic cells

Abstract

Using a multi-diamond-wire saw, we cut monocrystalline silicon bricks into thin (120 µm) wafers, on which we observed saw marks and elongated pits with surface cracks. To address the fracture issues, we performed three-line bending tests with the load applied in the parallel and perpendicular direction to the saw mark direction. Under parallel loading, pits and accompanying cracks resulted in lower fracture strength than under perpendicular loading and the wafers were clearly separated into two groups: lower-strength wafers from the fresh-wire side and higher-strength wafers from the worn-wire side. Under perpendicular loading, the pits and surface cracks had less effect, resulting in higher strength. Using Raman spectroscopy, we confirmed that the wafers from the fresh-wire side had a higher fraction of slicing damage on the surface and subsurface region than those from the worn-wire side. This damage resulted in lower-strength wafers in the parallel loading test.

The impact of damage etching on fracture strength of diamond wire sawn monocrystalline silicon wafers for photovoltaics use

“…There has been several testing methods proposed to measure fracture strength of whole wafers. 7,8,[15][16][17] In the present study, we employ a simple uniaxial three-line bending test, where loads are applied on the middle of the wafer. In our bending test, a maximum peak stress is generated on the wafer surface just under a loading bar and minimum negligible stress on both wafer edges parallel to the loading bar.…”

“…In our bending test, a maximum peak stress is generated on the wafer surface just under a loading bar and minimum negligible stress on both wafer edges parallel to the loading bar. 8,16) The bending test was performed using three-line bending tester (Shimadzu Autograph AG-X plus) to measure fracture strength of silicon wafers. The schematic of bending setup was shown in Figs.…”

“…2) In diamond multi-wire (DMW) sawing, material cutting occurs in a ductile or plastic mode of two body (diamond abrasives and silicon) lapping process, allowing to reduce the kerf and subsurface damage removal. 7,8) In addition, several benefits of DMW sawing were reported, [6][7][8][9][10][11][12] such as being suitable for cutting thin wafers, easier recycling of silicon kerfs, higher production yield, less consumable cost and being environmentally friendly. However, there are still some issues in as-sawn wafers cut by the DMW sawing.…”

“…Saw marks and pits are formed on wafer surfaces in the wire motion direction and these cause asymmetry of fracture strength depending on the bending direction (parallel and perpendicular to the saw marks). 8) The wafers have lower fracture strength in parallel bending and higher fracture strength in perpendicular bending to the saw marks. 8) Therefore, we need to find the detailed mechanism of the asymmetry in fracture strength that appears in DMW-sawn wafers.…”

“…8) The wafers have lower fracture strength in parallel bending and higher fracture strength in perpendicular bending to the saw marks. 8) Therefore, we need to find the detailed mechanism of the asymmetry in fracture strength that appears in DMW-sawn wafers.…”