Anisotropic Copper Electrodeposition on Microelectrodes

Abstract

Copper electrodeposition is critical to electronics manufacturing, ranging across lengthscales from nanometer damascene plating to 100s of micrometers for through-silicon-vias in 3D interconnect packaging and even millimeter-scale in printed circuit board manufacturing. Essential to this process is formation of void-free copper structures in high aspect ratio cavities, which is achieved with various additives that enable superconformal or bottom-up electrodeposition. When the additive package consists of only a suppressor (polyether) and chloride, the superconformal phenomenon is driven by S-shaped negative-differential resistance (S-NDR), which promotes spatial bifurcation of the electrode surface into active and passive depositing regions. All S-NDR systems involve kinetic suppression, polyether-chloride adsorption in this case, and kinetic activation, potential-driven disruption of the polyether-chloride adlayer. The term S-NDR is derived from the s-shape the i-V curve expresses when cyclic voltammetry (CV) is post-experimentally corrected for ohmic resistance, resulting in a system that expresses a multiplicity of reaction rates (current) for a given driving force (potential).Electroanalytical measurements using microelectrodes (UMEs) offer an avenue for exploring S-NDR systems without significant iR-drop in solution. In other reaction systems, the reduced electrode dimension of UMEs has shown the ability to frustrate active-passive bifurcation, resulting in a homogeneously reacting electrode surface. Nonetheless, potential-controlled measurements such as cyclic voltammetry restrict S-NDR behavior from fully expressing itself, as the potential is the control parameter throughout the experiment. Current-controlled techniques, however, allow the thermodynamic state (i.e., potential) to dynamically shift as the polyether-chloride suppressing layer is formed and disrupted. In this work, current-controlled measurements in the form of linear galvanodynamic sweeps (GDS) are utilized to explore S-NDR behavior and electrode bifurcation during copper electrodeposition. GDS and CV measurements on UMEs are compared across a range of additive concentrations and polyether types. Influence of the operating mode on pattern formation is evaluated by in-situ optical microscopy on UMEs ranging from 10 μm to 25 μm in diameter. Spatial bifurcation during GDS measurements is further exploited to grow anisotropic copper deposits.