Irreversible Tensile Stress Development in PECVD Silicon Nitride Films
The thermo-mechanical behavior of plasma-enhanced chemical vapor deposited (PECVD) silicon nitride films are investigated during thermal cycling and annealing. It is well known that PECVD films have a large amount of incorporated hydrogen that evolves on heating. This reduction in hydrogen is shown to be directly responsible, via constrained volume decrease, for irreversible increases in tensile stress. It is demonstrated that no stress equilibrium is attained during very long time anneals. The thermal cycling behavior of PECVD films can be modeled by chemical reaction theory, with the irreversible development of film stress a mechanical consequence. The model assumes first-order reaction kinetics of Si-H and N-H bonds, which react to form molecular hydrogen and reformed network bonds. The activation energy of reaction is not single-valued, indicative of the strong influence that the local bonding environment has on bond energies. If the incorporated hydrogen reactant pairs are assumed to be normally distributed with activation energy, irreversible stress development is well modeled, and the mean activation energy ranges from 2.44 to 2.93 eV for 150 to 300 °C deposited films.