The Effect of Different Additives in the in-Situ Stress of Thin Co Films

Abstract

Fe, Co and Ni (and their alloys) are widely used due to their interesting ferromagnetic, chemical, and physical properties. Electrodeposited thin films tend to develop sizable mechanical stresses as a result of the nucleation and growth process. Often, these stresses can approach or exceed the yield stress of the bulk material and can lead to loss of adhesion and the generation of bulk and surface defects. Models have been developed that treat steady-state stress as a dynamic competition between tensile and compressive stress generation mechanisms that are largely governed by atomic mobility, microstructure, and deposition rate. Although additives are often used to mitigate residual stress during thin film growth, the exact mechanisms by which these additives influence residual stress is an active area of research. In this talk, we examine the growth stress associated with electrodeposition of Co thin films onto (111)-textured Au cantilever electrodes from 0.5 M Na2SO4 containing 0.1 M CoSO4 and 0.5 M H3BO3. The stress was measured with an optical cantilever curvature technique that can be used during deposition to determine the real-time stress evolution. Deposition was also examined using an electrochemical quartz crystal microbalance (EQCM) in order to determine the current efficiency as a function of both overpotential and deposition time. We have measured the average biaxial film stress as a function of overpotential and observe a significant increase in tensile stress as the deposition potential is made more negative. These results are discussed in the context of stress generating models that appear in the literature. The growth stress was also examined from 0.5 M Na2SO4 + 0.1 M CoSO4 electrolyte with controlled additions of glycine and boric acid. Each additive was studied under a specific potential window, as determined by the observed influence the additives have on Co deposition. Glycine acts as a buffer, maintaining a stable H+ concentration. The buffering product glycinate (Gly-) complexes with Co2+, thereby shifting the deposition window. Baths with boric acid have an average current efficiency between 40 and 90%, produce deposits with high tensile stress which undergo little postdeposition relaxation. Current efficiency from glycine baths range between 20 and 70%. Deposits generally have lower tensile stress and exhibit additional post-deposition stress relaxation.