Development of a Scalable Two-Additive Electrodeposition System for Bottom-up Copper Filling of Sub-Millimeter through Silicon Vias
Authors: Menk Lyle Alexander , Schmitt Rebecca Pauline , Han Sang M , Jordan Matthew , Sadler Corrie , Hollowell Andrew E 
Published: May 2020

A two-additive electrodeposition system for Cu feature filling of sub-millimeter scale through silicon vias (TSVs) is discussed. This 1.0 mol/L CuSO4 – 0.5 mol/L H2SO4 acid sulfate electrolyte contains a polyether surfactant and halide additive which together promote a bottom-up Cu growth profile through positive feedback between localized disruption of the adsorbed polyether-halide layer and metal deposition.1 The functionality of this system relies on a tightly controlled additive concentration gradient down the length of the feature at the onset of metal deposition to allow bottom-up Cu growth to propagate. Careful consideration of convective contributions is also made to ensure an appropriate boundary layer thickness is maintained for non-linear bottom-up filling, where localized active and passive zone bifurcation of rapid electrodeposition is present. Initial experiments demonstrate the efficacy of potentiostatic control in determining the applied current window by which galvanostatic control can ultimately be achieved for a sub-millimeter feature geometry. Nominally void-free bottom-up deposition of multiple recessed feature geometries is presented.

One advantage of the two-additive bottom-up electrodeposition system is that it has been demonstrated across multiple length scales and with multiple materials,2-9 making it suitable for heterogeneous integration (HI) of a wide array of devices which can require unique feature geometries and interconnect materials. Depending on the application, the two-additive bottom-up electrodeposition system can replace the more traditional three or four-additive system in damascene processing10 which can yield superconformal deposition profiles in TSVs as explained by the curvature enhanced adsorbate coverage (CEAC) mechanism.11 The CEAC mechanism relies upon high curvature areas and is thus most appropriate for micro and nanoscale features which exhibit proximity of locally high curvature areas as deposition proceeds due to area change affects. Void-free superconformal deposition is more difficult to repeatably achieve on larger, mesoscale features. Thus, a bottom-up deposition technique that relies upon the non-curvature dependent S-shaped negative differential resistance (S-NDR) between just two constituent additives to fill conductively seeded features is desirable.

The issue of scalability is of significant interest after developing an electrodeposition process at the die level. While this two-additive system has been successfully implemented using potentiostatic control, industrial full wafer plating tools rely exclusively on galvanostatic control as they are not equipped with reference electrodes. In addition, they combine wafer rotation and forced fluid flow to maintain a controlled convective contribution across the wafer. So, to demonstrate scalability of this two-additive chemistry, the effect of the field surface area, feature geometry, and sample rotation rate on the deposition profile at a given applied potential or current is investigated. Altering the feature quantity and field area for a given deposition experiment can provide a means to estimate the effective current density required to fill features at the same rate across different sample sizes. By altering the sample rotation rate in the otherwise quiescent electrolyte, the impact of the transport phenomena of the constituent surfactant and halide additives are assessed. Cross-sectional images along with computed tomography (CT) produced images are produced to obtain Cu fill profiles across features arrays to demonstrate TSV uniformity.

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Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.