We present a model for compressive stress generation during thin film growth in which the driving force is an increase in the surface chemical potential caused by the deposition of atoms from the vapor. The increase in surface chemical potential induces atoms to flow into the grain boundary, creating a compressive stress in the film. We develop kinetic equations to describe the stress evolution and dependence on growth parameters. The model is used to explain measurements of relaxation when growth is terminated and the dependence of the steady-state stress on growth rate.

Real-time measurements of stress evolution during the deposition of VolmerWeber thin films reveal a complex interplay between mechanisms for stress generation and stress relaxation. We observed a generic stress evolution from compressive to tensile, then back to compressive stress as the film thickened, in amorphous and polycrystalline Ge and Si, as well as in polycrystall;ne Ag, Al, and Ti. Direct measurements of stress relaxation during growth interrupts demonstrate that the generic behavior can occur even in the absence of stress relaxation. When relaxation did occur, the mechanism depended sensitively on whether the film was continuous or discontinuous, on the process conditions, and on the fildsubstrate interracial strength.For Ag films, interracial shear dominated the early relaxation behnavior, whereas this mechanism was negligible in Al films due to the much stronger bonding at the A1/SiOz interface. For amorphous Ge, selective relaxation of tensile stress was observed only at elevated temperatures, consistent with surface-diffusion-based mechanisms. In "all the films studied here, stress relaxation was suppressed after the films became continuous...

The evolution of stress in gallium nitride films on sapphire has been measured in real time during metalorganic chemical vapor deposition. In spite of the 16% compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050 °C. Furthermore, in situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling.

A simple model is presented that predicts the kinetics of tensile stress evolution during the deposition of thin films that grow by the Volmer–Weber mechanism. The generation of a tensile stress was attributed to the impingement and coalescence of growing islands, while concurrent stress relaxation was assumed to occur via a microstructure-dependent diffusive mechanism. To model the process of island coalescence, finite element methods were employed and yielded average tensile stresses more consistent with experimental observations than those predicted using previously reported analytical models. A computer simulation was developed that models the process of film growth as the continuous nucleation of isolated islands, which grow at a constant rate to impinge and coalesce to form a continuous polycrystalline film. By incorporating the finite element results for stress generation and a microstructure-dependent stress relaxation model, the simulation qualitatively reproduced the complex temperature-dependent trends observed from in situ measurements of stress evolution during the deposition of Ag thin films. The agreement includes simulation of the decreasing stress relaxation rate observed during deposition at increasing temperatures.

During deposition, many thin films go through a range of stress states, changing from compressive to tensile and back again. In addition, the stress depends strongly on the processing and material parameters. We have developed a simple analytical model to describe the stress evolution in terms of a kinetic competition between different mechanisms of stress generation and relaxation at the triple junction where the surface and grain boundary intersect. The model describes how the steady state stress scales with the dimensionless parameter D/LR where D is the diffusivity, R is the growth rate, and L is the grain size. It also explains the transition from tensile to compressive stress as the microstructure evolves from isolated islands to a continuous film. We compare calculations from the model with measurements of the stress dependence on grain size and growth rate in the steady state regime and of the evolution of stress with thickness for different temperatures.