Residual stress in diamond films: origins and modelling

The evolution of stress in diamond layers is investigated both experimental ly and theoretically in order to develop a comprehensive view of the format ion of residual stress. A compressive stress maximum associated with grain coalescence, a decreasing stress with increasing layer thickness and strong stress inhomogeneities at the level of the grain size are observed by in s itu macro- and ex situ micro-Raman spectroscopy in diamond films grown on s ilicon (001) substrates. For most diamond deposits on silicon, neglecting w afer bending for the calculation of thermal stress turns out to be an inapp ropriate approximation, but even the exact modelling of the thermal stress by means of plate theory and finite element calculations only explains a mi nor part of the observed stress. Detailed finite element calculations revea l that the average thermal stress, and the thermal stress distribution, are largely modified by temperature gradients during deposition, and by film m orphology. Tensile stresses can form due to temperature gradients and surfa ce roughness relaxes an essential part of the thermal stress. The expected average stresses are calculated for common cases. Stress measurements using micro-Raman spectroscopy confirm these predictions obtained from modelling . The microstructure, in particular coherency strains, surface energy effec ts and disclinations, can contribute substantially to the observed compress ive stress maximum at small layer thickness. During grain coalescence, the formation of disclinations can be energetically more favourable than small angle grain boundaries. The related stress fields are estimated to be of th e order of several GPa. The formation of large local compressive stresses d uring grain coalescence is confirmed by micro-Raman spectroscopy. At small layer thicknesses, the evolution of stress is dominated by the microstructu re and morphology, whereas at higher thicknesses the thermal stress, includ ing bending effects and temperature inhomogeneities during deposition, is m ore important. (C) 1999 Elsevier Science S.A. All rights reserved.