Microelectrode Studies of the Critical Breakdown and Spatial Bifurcation in Additive Derived S-NDR Copper Electrodeposition
Authors: Braun Trevor M , Moffat Thomas P. 
Published: February 2020

Copper electrodeposition is widely used in integrated circuit manufacturing, ranging from damascene plating of sub-100 nanometer features to deposition of micron-scale through-silicon-via (TSV) interconnects used for 3D-chip stacking.1 Electrodeposition of void-free copper structures in high aspect ratio cavities is achieved with various suppressing, accelerating, and leveling organic additives; enabling superconformal, bottom-up filling of these recessed features. Recently, the use of a single suppressing organic additive has been demonstrated and modeled in a variety of different metal systems (Cu, Ni, Co, Au) for TSV-scale feature filling.2-5 In these systems, bottom-up fill is derived from an S-shaped negative-differential-resistance (S-NDR), generating active plating regions at the bottom of the TSVs and passive regions along the walls and surface. This active/passive bifurcation is driven by adsorption/desorption kinetics of the suppressing polymer and chloride complexes, mass-transport of the chemical species, and the TSV geometry. Additional studies of S-NDR systems demonstrate that in the absence of any electrode geometric constraints the bifurcation occurs in random Turing-like patterns.2 In other reaction systems, such bifurcating active-passive reactions are found to be dependent on the electrode size such that for reduced electrode dimensions the ability to bifurcate is frustrated and a homogeneous surface reactions occurs across the electrode.6

In this work, microelectrodes are used to probe the dependence of bath composition on the critical breakdown behavior in single organic additive, acid copper sulfate electrolytes. The well-defined, high mass-transport rates and negligible IR-drop inherent in microelectrodes provides new insight into the chemical behavior of copper S-NDR systems at length scales relevant to industrial applications. Moreover, voltammetry at microelectrodes with dimensions less than the critical dimensions of the spatial instability in an S-NDR system provides more precise kinetic relationships; a result enabled by the homogeneity of the reaction across the electrode surface. However, one challenge of electrodeposition on microelectrodes is significant shape and area change during electrodeposition. Thus, our cyclic voltammetry and galvanodynamic linear sweeps are designed to ensure copper deposition is less than 10% of the microelectrode diameter, maintaining the disk-shaped geometry.

Microelectrode experiments that vary chloride concentration demonstrate a clear dependence of the critical breakdown potential (a more negative shift) with increased chloride concentration. This dependence is far more apparent at microelectrodes (12.5 & 25 μm diameter), where an increase in chloride concentration from 10 μM to 100 μM shifts the breakdown potential by 80 mV more negative. At macroscopic electrodes (0.5 cm diameter), the breakdown potential shifts by only 10-20 mV. This observation and its interplay with transport and the polymer-chloride adlayer is currently being further studied in more detail. Similar experiments varying the supporting electrolyte concentration (sulfuric acid in this case) show little dependence on the breakdown potential and overall shape in the cyclic voltammetry hysteresis, validating that microelectrodes indeed have negligible solution IR contributions. In addition to individual microelectrode voltammetry, microelectrode arrays are used to explore the effect of electrode size on the active-passive bifurcation and subsequent pattern formation.

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