Role of oxygen and cuprous ions in copper electroplating

Liu Yang, Wouter Marchal, Aleksandar Radisic, Johan Deconinck, Philippe Vereecken

Research output: Chapter in Book/Report/Conference proceedingMeeting abstract (Book)

Abstract

Cu electroplating has been used for interconnect and more recently 3-D stacking in integrated circuits. Although the reaction mechanisms in additive-free CuSO4 solutions seems straightforward at first sight, the roles of inorganic species such as Cu+ and dissolved oxygen still require a closer look. The Rotating Ring-Disk Electrode (RRDE) can be employed to detect the Cu+ intermediate during the Cu plating process. It has been proven a very useful tool for such a study. The effects of various species on Cu plating, under different conditions, were studied using RRDE in [1]. It was also shown that the addition of Cu2O into the Cu plating bath leads to increased Cu plating rate [2]. While Cu+ is supposed to be directly or indirectly responsible for such acceleration, the underlying mechanism is not yet clear.
By maintaining a very low O2 concentration through purging with N2 gas for an extended time, Cu+ generated from the comproportion reaction accumulates and leads to a bulk concentration of Cu+. This provides an opportunity to study the Cu plating rate with additional bulk Cu+ but without O2. Fig.1 shows rotating-ring-disk voltammograms (RRDV) for different bath compositions. For both Fig.1a and Fig.1b, the bulk Cu+ concentration increases significantly at very low O2 concentration, as indicated by the increased background ring current. However, there is no significant change of disk current. A careful check of the disk currents in both figures will even show slight decreases for lower O2 and higher Cu+ concentrations. Fig.2 shows the steady-state measurement, it is shown clearly that, with Cu+ accumulated in the bulk at very low O2 concentration, the disk current is lower compared with the ambient case. Therefore, Cu+ itself is not the cause of increased Cu plating rate. Instead, the formation of Cu(I) oxide, or other forms of Cu-O bounding on the Cu surface in the presence of dissolved O2, is likely responsible for the faster Cu plating rate at ambient condition. It is also consistent with the increased plating rate observed in [2], as Cu2O, not Cu+ ion, was added to the plating bath. Since Cu2O is thermodynamically unstable in an acidic electrolyte, it probably exists as active intermediates with a short lifespan. The Cu plating rate is higher around these active intermediates. At ambient condition, these active intermediates are formed at greater number due to the dissolved O2. With the dissolved O2 been removed through purging with N2 gas, the formation of Cu2O is reduced and the plating rate is lower.
In the Cyclic RRDV curves shown in reported RRDE studies [1, 2], as well as the curves in Fig.1 and Fig.2, a small plateau is always present in the ring current curve around the Open Circuit Potential (OCP). Although this plateau is typically ignored, it cannot be predicted by a simple Butler-Volmer equation. Yokoi [1] attributed this plateau to the surface diffusion of Cu adatom to the lattice site. Fig. 3 shows the ring current in two consecutive CV scans. While the plateau is evident in the 1st scan, it is no longer visible in the 2nd, supporting that this plateau is due to adsorbed species, such as HSO4- or SO42-. The same phenomenon was observed for various bath concentrations. Before the CV scan, the surface is saturated with adsorbed species which act as a barrier for the incorporation of Cu adatoms into the bulk Cu lattice. As a result, the surface activity shifts to a lower value when the potential goes slightly negative from OCP, leaving a plateau on the ring current curve. As the potential goes more negative and Cu deposition happens at a high rate, the adsorption is disrupted. During the 2nd scan the surface is not yet re-covered. Therefore, no plateau is observed in the 2nd scan. Various organic additives are used as suppressor or accelerator in Cu plating. These additives adsorb stronger on the Cu surface. Therefore, the width and shape of the ring current plateau are also affected by these additives [3].
Original languageEnglish
Title of host publication22nd Materials for advanced metallization conference
Publication statusPublished - 10 Mar 2013
EventUnknown -
Duration: 10 Mar 2013 → …

Publication series

Name22nd Materials for advanced metallization conference

Conference

ConferenceUnknown
Period10/03/13 → …

Keywords

  • oxygen
  • copper electroplating

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