Micro-Raman Investigation of Residual Stresses in Machined Semiconductors.

Residual stress has been measured in machined silicon and germanium with high spatial resolution using micro-Raman spectroscopy. The technique is a direct non -destructive probe which provides a spatial resolution of 1 mum. The Raman microprobe has been extended to incorporate high axial resolution (i.e. depth into the sample) by measuring Raman spectra with several probe wavelengths. Determinations of the depth dependence of the average in-plane residual stresses in the samples may therefore be made. The use of various probe wavelengths, by virtue of their differing characteristic penetration depths, makes determinations of the depth dependence of the stress fields possible. This technique of Differential Absorption Profiling has been used to explore the effects of various machine parameters on the residual stress fields in machined germanium. These studies have shown that the residual stress field in machined brittle semiconductors is characterized by a surface layer of compression with a region of tension beneath. The transition from compressive to tensile stress is indicative of the transition from plastic to elastic deformation in the sample. The validity of the technique to accurately determine residual stress profiles is demonstrated by mapping the well understood stress fields around Vicker's hardness indents. The use of various polarization states has allowed the determination of stress components to be performed in the vicinity of these indents. The technique has also been used to characterize the transition from ductile to brittle material removal in a series of interrupted test cuts in machined germanium. These investigations have found that fractured regions of the machined surface possess proportionately higher tensile stresses that occur at shallower depths than do the unfractured regions. The extension of micro-Raman depth profiling to transparent materials using confocal optical techniques is also presented.