Stress engineering using low oxygen background pressures during Volmer–Weber growth of polycrystalline nickel films

Abstract

A robust strategy for controlling the level of residual stress in polycrystalline films remains elusive, owing to the complex coevolution of the surface, microstructure, and intrinsic stress during Volmer–Weber film growth. Recent improvements in the understanding of stress evolution mechanisms have led to the possibility of engineering the intrinsic stress through the control of thin film growth conditions. Here, the authors demonstrate stress engineering during deposition of polycrystalline Ni films through control of the oxygen partial pressure. The physical mechanisms of stress management during codeposition of nickel and oxygen are investigated using in situ stress measurements and ex situ structural and chemical characterizations. The intrinsic stress in Ni films is affected by grain growth during deposition (which causes a tensile stress) and by Ni adatom trapping at grain boundaries and oxygen incorporation in the Ni lattice (which cause a compressive stress). The authors show direct evidence that a small amount of oxygen suppresses grain growth during deposition. They suggest that the presence of chemisorbed oxygen limits surface diffusion of Ni adatoms, thereby limiting adatom trapping at grain boundaries. The presence of oxygen therefore affects the mechanisms for development of both tensile and compressive stresses, providing a direct method for engineering the residual stress in as-deposited Ni films. Finally, the authors demonstrate a process for evaporative deposition of “zero” stress Ni films by introducing a very low level of background impurities, with the resultant films containing only 1.2 at. % oxygen.