Editors' Choice—Simulation of Copper Electrodeposition in Through-Hole Vias

Abstract

Copper electrodeposition processes for filling metallized through-hole (TH) and through-silicon vias (TSV) depend on spatially selective breakdown of a co-adsorbed polyether-chloride adlayer within the recessed surface features. In this work, a co-adsorption-dependent suppression model that has previously captured experimental observations of localized Cu deposition in TSV is used to explore filling of TH features. Simulations of potentiodynamic and galvanostatic TH filling are presented. An appropriate applied potential or current localizes deposition to the middle of the TH. Subsequent deposition proceeds most rapidly in the radial direction leading to sidewall impingement at the via center creating two blind vias. The growth front then evolves primarily toward the two via openings to completely fill the TH in a manner analogous to TSV filling. Applied potentials, or currents, that are overly reducing result in metal ion depletion within the via and void formation. Simulations in larger TH features (i.e., diameter = 85 μm instead of 10 μm) indicate that lateral diffusional gradients within the via can lead to fluctuations between active and passive deposition along the metal/electrolyte interface.

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“…By COMSOL simulation, Moffat et al [44] . reported that the power lines at the mouth of THs were denser than that at the interior wall.…”

“…By COMSOL simulation, Moffat et al [44] reported that the power lines at the mouth of THs were denser than that at the interior wall. This infers that the electro-reduction potential at the mouth will be more negative than that at the middle of THs.…”

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“…2,43,66,67 For copper electroplating, the difference between active and passive deposition can be amplified to great effect by adding hydrophobic secondary structure to the polyether suppressor, e.g., poloxamers, poloxamines, etc. 2,4,68 Given a modest concentration of either the polyether or halide, critical behavior manifests during suppression breakdown when transport and/or kinetic effects limit the ability of the passive state to reform. For highly conductive electrolytes, the breakdown to fully activated deposition can be particularly sharp and not easily controlled by potentiostatic regulation.…”

“…For higher aspect ratio and larger-scale TSV and PCB features with dimensions similar to or greater than those of the hydrodynamic boundary layer, transport and the kinetics of adsorption, desorption, and/or consumption on advancing interfaces play an important role in feature filling. This is particularly true for a subset of accelerator-free additive systems that display critical behavior where positive feedback, from the disruption of the suppressor phase by metal deposition, results in the system jumping from the passive to the active state over a narrow potential window. ,,, For copper electroplating, the difference between active and passive deposition can be amplified to great effect by adding hydrophobic secondary structure to the polyether suppressor, e.g., poloxamers, poloxamines, etc. ,, Given a modest concentration of either the polyether or halide, critical behavior manifests during suppression breakdown when transport and/or kinetic effects limit the ability of the passive state to reform. For highly conductive electrolytes, the breakdown to fully activated deposition can be particularly sharp and not easily controlled by potentiostatic regulation. , Significantly, the presence of substantial ohmic resistance will attenuate the criticality whereby, for a fixed applied potential, the driving force is distributed dynamically between the interface potential, which determines the rate of metal deposition and additive adsorption, and the ohmic losses in the electrolyte.…”

“…Suppressor formation and breakdown may occur via a number of mechanisms. The simplest involves adsorption subject to transport constraint for dilute additive concentrations balanced by additive incorporation into the growing deposit at a rate that scales with the local coverage and metal deposition rate. ,,,, A more sophisticated approach explicitly recognizes the role of halide coadsorption in forming the suppressor phase. ,,,− Beyond additive consumption mechanisms, another avenue for generating positive feedback during suppressor disruption involves solubilization of the local polymer ligaments by the waters of hydration released during the reduction of Cu 2+ at defect sites in the blocking polymer layer. ,,, Accelerated metal deposition further disturbs the polymer–halide layer, disrupting the hydrophobic character of the static Cl – surface. ,,, Analytically, this effect is captured by adding a negative growth velocity dependence to the adsorption kinetics coupled with a simple desorption term to support an equilibrium coverage in the absence of metal deposition. The net result is that a recessed surface segment, once in motion, will prefer to stay in motion.…”