Polarized micro-Raman spectroscopy for studying stresses in as-grown and tensile-tested diamond films

The determination of the stress/strain level in diamond films was carried out here by polarized Raman spectroscopy. For as-grown polycrystalline films, a classical shift and splitting of the Raman spectra was evidenced. A polarized Raman study in two directions allowed confirming the isotropic biaxial nature of the stresses which are principally from thermal origin. In case of stresses measured after a tensile test, the polarized Raman study permitted us to evidence the anisotropic nature of the biaxial stresses. These stresses were unambiguously determined by the averaging method of Anastassakis. Very high compressive values were obtained in the direction perpendicular to the tensile one while the stresses from thermal origin were just over-compensated in the tensile direction. These high anisotropic stresses resulting from the initial thermal stresses and the plastic deformation of the substrate after the tensile test explain the surprising positive Raman shifts of the splitted diamond bands after such a tensile test. While these high anisotropic stresses are determined with a fairly good accuracy, the average Raman spectrum of the film could not be modelled by the averaging procedure of Anastassakis. The influence of the numerous possible orientation of each grain relatively to the stress directions was shown experimentally, especially in perpendicular direction to the tensile one, the direction of high compressive stresses. This local crystallographic influence however does not hamper studying the local stresses, as shown near the edge of a diamond film which had partially peeled off after a tensile test. Polarized micro-Raman spectroscopy is therefore of particular interest for evaluating the stress variations in such regions and then for inferring information about the adhesion of the films after mechanical tests.