Nanoscale properties of copper metallization: real-time stress measurements during electrodeposition, in-situ AFM imaging and ductility test development

Copper was electrodeposited on a polycrystalline gold substrate both in the presence and absence of electrochemical bath additives. Stress evolution was monitored in situ using a cantilever beam method and surface morphology was observed using in situ atomic force microscopy (AFM). Measurements of stress and relaxation behaviour provided adequate data-sets to generate reliable and reproducible plots of local stress in the film. This was possible because (i) the magnitude of stress relaxation upon resumption of deposition was small relative to the stress generated during deposition and (ii) stress upon resuming deposition eventually recovered to its pre-interrupted value. In-situ stress measurements showed that tensile stress peaked during deposition and eventually reached a relatively constant (plateau) value above a certain thickness (typically > 50 nm). The measured values of plateau stress increased with overpotential, ranging from 25 to 45 MPa for -125 to -200 mV. In-situ AFM imaging showed nucleation of individual copper islands which grew in size and eventually coalesced forming grain boundaries. Coalescence and grain boundary formation coincided with an increase in tensile stress. Surface features smoothened during interruption of deposition and recovered to a roughened configuration upon resumption of deposition. This behaviour coincided with a recovery in stress and this has been attributed to reversible movement of adatoms between the surface and grain boundaries. Chloride ion significantly reduced tensile stress and produced rougher deposits than without additives. As roughness increased, stress decreased as was observed also for additive-free deposition. PEG alone had no significant effect on stress. However, PEG significantly enhanced stress reduction in the presence of chloride. A flat copper surface with scattered large pyramidal structures was produced. Three prototypes of ductility testers were developed, the latest of which employed an optical-lever method for determining the displacement of a freestanding nanofilm under load. Several freestanding film structures were fabricated. A commercial AFM microscope was used to generate the load-displacement response of RF-MEMS switches which showed good agreement with FEM simulations.