Irreversible Tensile Stress Development in PECVD Silicon Nitride Films

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Irreversible Tensile Stress Development in PECVD Silicon Nitride Films Michael P. Hughey and Robert F. Cook Department of Chemical Engineering and Materials Science University of Minnesota, Minneapolis, MN 55455 USA ABSTRACT 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. INTRODUCTION PECVD films are ubiquitous in microelectronic and micro-electromechanical devices. Fabrication of such devices involves many thermal cycles for the deposition or annealing of each layer of material deposited, requiring a complete understanding of the thermo-mechanical properties of PECVD films to optimize their mechanical reliability. It is known that irreversible tensile stress can develop in PECVD films on heating due to the evolution of incorporated hydrogen, but no theoretical and few empirical models have been suggested to quantitatively predict this process [1,2]. Conversely, the hysteretic stress responses exhibited by metal films on thermal cycling have been successfully modeled by creep deformation processes [3]. The goal of this work is to establish a framework for the quantitative modeling of irreversible tensile stress development in PECVD films that is comparable to the state-of-the-art models for metal films. Silicon nitride is chosen as a representative PECVD material. To develop a model to predict irreversible tensile stress development of a PECVD film subjected to an arbitrary thermal history, film stress is measured during multiple anneals to study the effects of temperature and time on stress change kinetics. In particular, very long time anneals are used to evaluate any possible equilibrium behavior and to track the changing kinetic behavior with time. Spectroscopic techniques are used to measure the incorporated hydrogen concentration, and the kinetics of hydrogen evolution are compared to those of irreversible stress change. EXPERIMENT PECVD silicon nitride films were deposited in