Rapid Characterization of Additives for Bottom-up Fill in Electroless Copper Plating
Authors: Zeszut Ronald , Landau Uziel 
Published: February 2020

Electroless plating, unlike electroplating, enables the application of metallic coatings without the need of providing electronic pathway to the plated region. As such, electroless plating is used to metalize non-conductive substrates and electrically isolated features. In semiconductor device metallization, electroless copper deposition is particularly advantageous for metalizing extremely small (nm scale) features because the process does not require a conductive seed layer. However, void-free metallization of vias and trenches typically requires bottom-up plating of the patterned feature, corresponding to high plating rate at the feature bottom and inhibited plating on the feature sidewalls, rim, and the top flat substrate. This is enabled in electrolytic plating by a special additives mixture that is not effective in the electroless system. In the absence of molecular level mechanistic understanding of the interaction between the additives and the plating process, the search for more effective additives is typically conducted empirically. However, the conventional injection technique,1,2 that provides rapid screening of electroplating additives by measuring the voltage transient following additives injection, is not applicable to the electroless system. Consequently, testing of additives for bottom-up fill in electroless systems entails actual plating of features and their microscopic examination, requiring significant effort and time. Further complication arises due to the fact that the additives containing electroless system is significantly more sensitive to transport and agitation than its electrolytic counterpart.
Presented here is a novel approach providing simple and rapid screening of additives applicable to electroless systems. The technique is based on electroless plating of a flat (featureless) rotating disk electrode. Two electroless plating experiments are conducted in the same tested electrolyte: one, with the disk rotating at a high speed, and the second, at a low rotation rate, measuring in both experiments the electroless plating rate from the deposit amount. As illustrated schematically in Fig. 1, we look for an additives mixture that provides a low plating rate at the high rotation speed (which corresponds to the high transport rates and thin boundary layer prevailing on the flat substrate and feature rim), and a high plating rate at the low rotation speed (simulating the thick boundary layer at the bottom of the feature).
The high rotation speed, ω1, which relates to the mass transport boundary layer thickness, δ1 through the Levich equation3 [Eq 1], is selected such that it corresponds to the relatively high transport rates prevailing over the flat substrate. In equation 1, D and ν are the additive’s diffusivity and the electrolyte kinematic viscosity, respectively. The low rotation rate, ω2, is selected to simulate the hindered transport of the additive to the feature bottom, corresponding the significantly larger mass transport boundary layer, δ2.
The equality on the left-hand side of equation 2, can be derived following the analysis of Adolf and Landau4, where L and R are the feature’s depth and width, respectively, Γ is the additive surface saturation concentration, and Cb is its bulk concentration. ω2is selected from the equality on the right-most side of equation 2.
As shown in Figure 2, in the absence of additives, the electroless plating rate increases with rotation rate, as copper transport is enhanced. This system is not expected to provide bottom-up fill, since the deposition rate will be higher on the via rim and the flat top substrate. However, utilizing 1.5 ppm mercaptopropanesulfonic acid (MPS) as an inhibiting additive, leads to the desired significantly lower plating rates at higher rotation speeds, as compared to those at lower rotation speeds. This is most likely due to the enhanced transport towards the substrate of the low concentration inhibitor at the higher rotation rates. Consequently, MPS is a promising additive for promoting bottom-up electroless plating of recessed features. Unfortunately, MPS by itself yields a dark and nonuniform deposit, so polypropylene glycol and dipyridyl were added to generate a bright and uniform deposit. Additional analysis and further experimental details are provided in the presentation.

Acknowledgements

Atotech GMBH is acknowledged for funding this study and for providing helpful input.

References

R. Akolkar and U. Landau, J. Electrochem. Soc., 151, C702 (2004).

Lindsay Boehme and Uziel Landau, J. Appl. Electrochem., 46(1), 39-46 (2016).

V. G. Levich, “Physicochemical Hydrodynamics”, Prentice-Hall, 1962.

James Adolf and Uziel Landau, J. Electrochem. Soc., 158 (8) 1-8 (2011).

Figure 1