Micro-Raman analysis of residual stresses and phase transformations in crystalline silicon under microindentation

Abstract

Vickers microindentations obtained with loads between 0.05 N and 2 N were performed on crystalline (100) silicon. The residual stress field and the different structural states induced by loading were studied by mapping the indented zones by their micro-Raman response. A Raman signature of amorphous silicon is found in the center of the impression. The energy of the Γ₂₅ zone center phonon is found to vary from 522 cm⁻¹ when probing the silicon at a distance of 80 µm from the center of the indentation up to 527 cm⁻¹ when probing the pileup region of the impression. When probing cracked zones in the vicinity of the pileup region, wave numbers as high as 536 cm⁻¹ are measured. The stress components induced by a point indentation (1 N) have been calculated from analytical expressions given in the literature. For an average conversion factor of 3.2 cm⁻¹/GPa, the residual local stresses after unloading are found of the same order of magnitude or even larger than the calculated stresses that are generated during loading. A tentative explanation is proposed. Finally a systematic laser-induced thermal treatment of the central area and of the pileup region of indentations was performed. It is shown that the amorphous silicon in the center can partly recrystallize but that the residual stress state in the pileup region cannot be completely relaxed by local laser heating.