Morphological Evolution of Bistable Copper Electrodeposition during Potentiodynamic and Galvanodynamic Measurements on Microelectrodes

Abstract

Copper electrodeposition from an electrolyte containing a polyether suppressor and chloride exhibits bistable behavior characteristic of s-shaped negative differential (S-NDR) critical systems. The polyether and chloride form a co-adsorbed adlayer on the electrode that inhibits copper electrodeposition by sterically blocking cupric ion from accessing the interface. At sufficiently negative overpotentials, the adlayer is disrupted and copper electrodeposition occurs freely. In cyclic voltammetry (CV) measurements this behavior manifests as large hysteretic windows. The current response is negligible during the forward sweep until potential-dependent breakdown of the suppressor-halide adlayer occurs. The return sweep exhibits elevated current levels until the blocking layer is reformed at potentials 100s of millivolts more positive than the breakdown potential. When operating at a potential within the hysteretic window, and in an electrochemical configuration with sufficient ohmic resistance, copper electrodeposition exhibits bistable, passive – active deposition on the electrode surface. On macroscale planar electrodes this manifests as random Turing patterns whereas on topographically engineered electrodes, common in electronics manufacturing applications, deposition is localized to the most recessed regions of the interface and progresses in a bottom-up manner.The inversion associated with NDR behavior is only apparent in CV measurements after post-experimental correction for the iR-drop in the electrochemical cell. Thus, linear or cyclic galvanodynamic sweeps (GDS) offer an alternative measurement method to capture the overpotential inversion associated with critical systems. Electroanalytical measurements on microelectrodes further offer an avenue for exploring systems without significant iR-drop in solution. In this work, CV and GDS measurements on microdisk electrodes are used to explore the influence of additive (suppressor and chloride) concentration on critical behavior during copper electrodeposition. Further, in-situ optical microscopy during electroanalytical measurements is used to correlate characteristics of global potential/current responses to observations of copper growth dynamics. Continuum simulations capture key characteristics, such as hysteresis and overpotential inversion, of CV and GDS measurements as well as the different deposit growth profiles observed for potentiodynamic versus galvanodynamic operating modes.