Residual stress measurement in thin carbon films by Raman spectroscopy and nanoindentation

Abstract

The reliability of hermetic carbon-coated optical fibers is affected by residual stresses in the coating created during the fiber draw process. Thermally induced residual stresses are caused by differences in the coefficient of thermal expansion (CTE) of the coating and the optical fiber. This mismatch creates shear stresses at the interface that can delaminate the film. This work presents and validates a surface residual-stress measurement technique using Raman spectroscopy. Since select Raman peaks for carbon films exhibit a wavenumber shift in proportion to the magnitude of residual stress, Raman spectra can be correlated to a theoretical model to obtain its residual stress. The model, validated with nanoindentation, shows an equi-biaxial stress field through the depth of the film. Nanoindentation also provides an accurate measure of residual stress in thin films with unknown material properties. The approach presented in this study is a non-destructive and non-intrusive method for measuring residual surface stress in thin films, and is ideal for small curved-surface specimens such as carbon-coated optical fibers.