Mechanical and thermal properties of silicon nitride films deposited by different plasma process type have been studied. The initial mechanical stress, thickness and hydrogen content have been evaluated respectively by wafer curvature measurements, ellipsometry and Fourier Transform Infra Red spectrometry. These nitrides presented as-deposited stress values ranging from compressive to tensile. High temperature Rapid Thermal Anneal (RTA) at 1100°C or longer thermal treatments at medium temperature, from 700°C to 850°C were carried out on these materials. The evolution of their properties along the different anneals have been measured and compared to the behaviour of high temperature thermal nitride. One can observe that these stoechiometric plasma nitrides have shifted to an equilibrium tensile stress around 1100-1200 Mpa when submitted to the RTA, independently of their initial stress values. Results are interpreted in terms of H desorption and Si-N bond formation. Chemical reaction Si2-N-H + 2 N-H → 2 Si-N + NH3 appears to be the best candidate to figure out the phenomena.