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When crack front makes waves

In collaboration with researchers from DRT and Soitec, we have deciphered the mechanism behind the defects observed on the surface of silicon wafers during the manufacture of SOI substrates.

Published on 10 July 2019
Silicon-on-Insulator (SOI) substrates used for the manufacture of high performance / low power electronic components include a monocrystalline silicon film obtained by fracture of a bulk silicon wafer. To precisely control this fracture plane, a thin layer of tiny cavities is created at the desired depth by implanting hydrogen ions into the wafer (SmartCutTM). The surface thus revealed by the fracture has an alternation of rough and smooth textures. However, these inhomogeneities may impact the performances of the circuit that will be engraved on this surface.

In collaboration with researchers from the DRT and Soitec (a world leader in the production of innovative semiconductor materials) we have deciphered the mechanism behind the patterns observed on the surface of silicon wafers when manufacturing SOI substrates.
On the wafer images taken after fracturation, wave-like patterns looking like the wake of a boat put us on the sound waves track (Figure). We therefore measured the speed of propagation of the crack generated by the cleavage (a few km/s) using an infrared optical device and analysed the acoustic waves emitted with piezoelectric sensors. These self-emitted acoustic waves propagate from one side of the fracture front to the other and are reflected at the end of the silicon wafer, which behaves like a filter or a resonator. The system selects only certain acoustic (flexural) waves whose velocities are in phase with that of the fracture. These bending waves produce periodic deformations of the silicon wafer assembly, which deflects the fracture from its nominal trajectory, resulting in observed variations in roughness. A simple model makes it possible to check that the periodicity of the patterns agrees with what is experimentally observed.
This discovery will allow further improvements in this process. More widely, it will help to understand the dynamics of the brittle fracture of materials.

Wave patterns observed on SOI wafer images after fracturing.
Smart Cut™ is a generic thin film transfer process used for the manufacture of silicon-on-insulator substrates on an industrial scale invented by Michel Bruel at CEA-Leti. The implantation of light ions in an oxidized silicon substrate leads to the formation of a weakened zone buried within the crystal.

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