水中レーザピーニングによって Si 中に形成された欠陥組織の透過電子顕微鏡観察

抄録

  Surfaces of {100} Si wafers were laser-peened in water by a Q-switch YAG laser with an energy density ψ_o ranging from 1 to 10 GW/cm². To start with, morphology of the ablated surfaces was analyzed by a 3-dimensional optical microscope. When the energy density ψ_o is higher than 5 GW/cm², macroscopic cracking did not take place. Therefore, on three samples irradiated with ψ_o=2, 3 and 5 GW/cm², defect structures in the sub-surface layers were examined by transmission electron microscopy comprehensively. When ψ_o=2 GW/cm², the ablated surface was quite smooth and no extensive damage was introduced in the sub-surface region. However, close inspection showed that a subsurface layer about 200 nm thick contained a considerable density of small bubbles and a small number of dislocations running vertically towards the ablated surface. When ψ_o=3 GW/cm², the sub-surface damaged layer became more profound with a much higher density of small bubbles and dislocations. On top of this, a considerable density of much larger bubbles were formed, on the inside-wall of which quite a high density of fine crystalline particles were attached. It is concluded that these bubble-containing layer must have been melted on laser irradiation. The bubbles must have been vapor Si formed in the liquid Si, which condensed on the inner wall on cooling. The vertical dislocations are misfit dislocation formed on solidification of the molten Si. However, in the matrix of Si underneath dislocations were rarely observed. This indicates that that region of Si that remained crystalline during the laser irradiation did not receive a stress strong enough to induce dislocations even at a high temperature just below the melting point. When ψ_o=5 GW/cm², underneath the bubble-containing layer a high density of dislocations were introduced. However, most of these dislocations appeared different from the ordinary 1/2〈110〉{111} dislocations. Electron diffraction showed no evidence of the high-pressure phases.