Exploiting the Spatial Homogeneity of Adlayer Breakdown on Microelectrodes to Develop a Kinetic Model for S-NDR Copper Electrodeposition
Authors: Braun Trevor M , Moffat Thomas P. 
Published: February 2020

Copper electrodeposition is a major component in integrated circuit manufacturing, ranging across lengthscales from nanoscale damascene plating to 100s of microns for thorugh-silicon-vias used in 3D interconnect packaging.1 Critical to this process is electrodeposition of void-free copper structures in high aspect ratio cavities, which is achieved with various additives that enable superconformal bottom-up filling. This physicochemical phenomenon is driven by an S-shaped negative-differential resistance (S-NDR), which promotes spatial bifurcation of the electrode surface into active and passive deposition regions. 2 Electrode bifurcation is dependent on additive adsorption/desorption kinetics, chemical species transport, and electrode geometry. Prior studies have demonstrated that the active/passive bifurcation is dependent on electrode size and that reduced electrode dimensions will permit a homogeneous reaction across the electrode.3

In this work, microelectrodes are used to experimentally probe the critical breakdown behavior in acid copper-sulfate electrolytes in the presence of micromolar concentrations of chloride and a single polymeric additive. In-situ optical microscopy indicates that the entirety of the microelectrode surface (either 12.5 or 25 μm in diameter) is either active or passive during cyclic voltammetry measurements, permitting current to be normalized to the geometric electrode area. Experimental electrolyte parameter sweeps cover chloride concentration, polymer concentration, copper concentration, and polymer adsorption kinetics (i.e. polymer type). These measurements are compared to static macroscopic electrodes and rotating disk electrodes at dimensions known to exhibit active/passive bifurcation. Experimental measurements are fit to finite element method computations using a two-additive model tracking the adsorption and rate-dependent desorption for chloride and polymer species. This work is an extension of previous computational models for describing S-NDR behavior in single additive systems.4-5 Kinetic constants for copper electrodeposition as well as adsorption and desorption dependent rate constants for chloride and polymer are fit to experimental cyclic voltammograms. Electrochemical kinetic constructs using microelectrodes and a 2-additive adsorption/consumption model provide a more accurate representation of critical breakdown phenomenon in S-NDR electrochemical systems.

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