Model for surface diffusion of aluminum–(1.5%) copper during sputter deposition

Abstract

Surface diffusion plays a critical role in improving the step coverage of sputter deposited aluminum–copper (Al–Cu) films, which are widely used in the microelectronics industry. Unfortunately, values of surface diffusivity as a function of temperature have not been published for aluminum copper films commonly used. We present a model for surface diffusion during sputter deposition of Al–Cu films and show that semiquantitative agreement with experimental Al–(1.5%)Cu film profiles can be obtained. This modeling and experimental work is a step toward developing a method to estimate diffusivity values using films deposited in process equipment, which would prove useful in process design. Al–(1.5%)Cu films were deposited at 303, 423, 523, and 623 K, into ‘‘infinite’’ trenches which have a variety of initial aspect ratios. No substrate bias was applied in order to minimize resputtering of deposited material. Surface diffusivity as a function of temperature was estimated by comparing experimental film profiles with profiles simulated using evolve, a physically based low pressure deposition process simulator. The equations which govern the deposition process and the required constitutive models for both curvature driven surface diffusion and deposition kinetics are discussed. Assuming a surface-free energy of 1100 erg/cm2, the expression for the surface diffusivity is D=6×10−4 exp(−5800/T) cm2/s with temperature in Kelvin. Our model predicts that step coverage increases with decreasing feature size, if all other deposition parameters are held constant, which is consistent with experiment.