RAMAN SPECTROSCOPY

Raman spectroscopy is often used to study and/or measure the local mechanical stresses in materials, as well as their grain size and phase properties. Modern µ-Raman microscopes have a focused laser with a spot size of only about 2 µm which means that very small volumes of material (e.g., micropillars) can be easily analysed. A typical experimental setup is shown where a µ-Raman system is combined with the Alemnis Standard Assembly (ASA) either in-situ or ex-situ, depending on the objectives.

Detail of a typical µ-Raman spectroscopy setup with the laser focused on a micropillar which is mounted for testing in the Alemnis Standard Assembly (ASA).

The Raman spot is focused on the micropillar to be tested and then the applied load, displacement and the µ-Raman spectra can all be recorded simultaneously during the test cycle. Typically, the Raman shift caused by compressive stresses during micropillar compression are evident and can help to understand the transition from elastic to plastic properties. However, it should be taken into account that different volumes of material are probed by load cell measurements versus Raman microscopy: with the load cell, the entire volume of the micropillar contributes to the deformation response, whereas Raman microscopy only measures a relatively small volume close to the surface (spot diameter ̴2 µm for a depth of ̴250 nm).

In addition, the microstructure and the deformation mechanisms of the micropillars (phase transformations and/or crack initiation and propagation arising during the test) can be further investigated by preparing Transmission Electron Microscope (TEM) lamellae by Focused Ion Beam (FIB) milling.

Selected References

Ghisleni R, Liu J, Raghavan R, Brodard P, Lugstein A, Wasmer K, et al. In situ micro-Raman com-pression: characterization of plasticity and fracture in GaAs, Philosophical Magazine 91 (2011) 1286-1292
Wasmer K, Wermelinger T, Bidiville A, Spolenak R, Michler J., In situ compression tests on micron-sized silicon pillars by Raman microscopy – Stress measurements and deformation analysis, Journal of Materials Research 23 (2008) 3040-3047