Depth profiling of residual stress along interrupted test cuts in machined germanium crystals

Abstract

Residual stress profiles in machined germanium have been determined by measuring the longitudinal optical phonon spectra using micro-Raman spectroscopy. Lateral spatial resolution of 1 μm results from this direct nondestructive technique. By measuring the Raman spectra with several probe wavelengths, axial resolution of 10 nm is obtained through differential absorption profiling (DAP). The stress field in these machined brittle semiconductors is characterized by a surface layer under compression, an abrupt compressive-to-tensile transition, and a deeper-lying region of tension which ultimately relaxes in the unstressed bulk. The abrupt transition is indicative of a change from plastic to elastic deformation. The DAP technique has been applied to interrupted test cuts in diamond-turned germanium to reveal that fractured regions of the machined surface possess higher tensile stress that occurs at shallower depths than unfractured regions. A qualitative mechanism of the machining process consistent with these residual stress data is presented.