One of the key steps in materials processing is the achievement of a special and controlled micro/nanostructure, in order to satisfy specific and sometimes conflicting properties. This is particularly true for materials and devices containing different phases, like composites and microelectronic structures. Since the interaction between the processing parameters of the different phases makes optimisation complex, phase and structure analysis at the micro/nanometric scale is mandatory. This is particularly important to control the reactions and the sign/level of the residual mechanical stress arising after processing from the thermal expansion mismatch, with or without relaxation through the interfacial region. The fact that the wavenumbers of Raman bands are dependent on the elastic strain in solids and that the exciting laser beam can be focused with an optical microscope to a spot only a few microns or less in diameter explains why micro-Raman spectroscopy has been widely applied in this field. It covers a wide range of materials such as semiconductors, 1,2 diamond films, 3 corrosion scales, 4,5 abradable films, 6 polymer, 7,8 ceramic 9,10 and metal 10,11 matrix-composites. Ceramics have been studied for a long time 12,13 using micro Raman microscopy and a general survey of its application in this field can be found in Ref. 14. Since Raman scattering normally involves visible radiation, the spatial distribution in a given heterogeneous material can be obtained by isolating one Raman line, characteristic of a phase, in the spectra. This older direct-imaging technique can now be replaced by the seriesimaging technique. After step-by-step scanning over the surface of the sample with a finely focused laser beam Raman spectra fitting schemes allow the construction of various maps showing, for instance, the bandwidth variation or the stress-induced wavenumber shift and make a better understanding of the materials possible.
Interest in the effect of mechanical stress/strain on Raman spectra was initially purely academic. Monocrystalline samples of semi-conductors with rather simple structures and spectra were studied. 15 After the first reports on the application of Raman spectroscopy to simple semi-conductor devices, 16 this technique rapidly found application in semi-conductor research. However, a precise knowledge of the Raman spectra of the phases was necessary and detailed studies of semi-conductors were still needed. At the same time stress analysis of basic oxides (e.g. sapphire single crystals and corresponding ceramics) was investigated using various other techniques, e.g. the fluorescence of Cr 3C impurities. 7,17 In many oxides, chromium traces or rare earth ions could be used as a probe, but the electronic levels involved in the fluorescent transition were more or less screened by outer electrons and the fluorescence signal did not exhibit a sufficient stress/strain dependence. This method is discussed in the paper of G. Pezzotti (Kyoto Institute of Technology) and applied to the analysis of the fracture of semi-brittle ceramics (Al 2 O 3 , Si 3 N 4 basedceramics) in order to establish the link between macroscopic fracture data and microscopic fracture mechanisms.
The sensitivity of spectroscopic methods for stress/strain analysis is an important problem. The Raman cross-section depends on the materials analysed (through the transition polarisability tensor, which depends on the atomic number, the chemical bonding, . . ., and through the vibration mode considered and the geometry of observation, . . .) and on the excitation wavelength (any (pre)resonance phenomenon is a function of the electronic absorption).