chemistry of additives in damascene copper plating, The

IBM Journal of Research and Development, Jan 2005 by Vereecken, P M, Binstead, R A, Deligianni, H, Andricacos, P C

Acceleration by cuprous ion complexes

Effect of Cu^sup ^ on copper deposition overpotential

A technique well suited to study the formation of cuprous intermediates is the rotating ring-disk electrode technique [27-29]. A fraction of the cuprous intermediates (determined by the collection efficiency N^sub 0^ of the ringdisk electrode) formed at the Cu disk are swept to a Pt ring, where they are oxidized quantitatively to Cu when the ring is set at a potential >0.75 V (SHE) [>0.1 V vs. the Hg/Hg^sub 2^SO^sub 4^/K^sub 2^SO^sub 4,sat^ saturated mercury sulfate electrode (SMSE)]. Figure 3 shows a current-potential curve for a rotating Cu-disk-Pt-ring electrode. The disk potential was scanned from the open-circuit state to -0.5 V (SMSE) and back; the ring was held at 0.25 V (SMSE). At the open-circuit potential (zero current at disk), a ring current was measured because of the dissolution of copper at the disk (comproportionation reaction {1}). After undergoing an initial small peak, the ring current decreased as the copper deposition current increased and reached its lower limit around -0.5 V, when the copper deposition current was about 20 mA/cm^sup 2^. In the reverse scan, chemical dissolution of copper through the comproportionation reaction {1} started again around -0.45 V (copper deposition current of 10 mA/cm^sup 2^) and reached a plateau when the open-circuit potential was reached. Even though these partial currents for cuprous formation were small and negligible with respect to the copper deposition reaction, the observation indicates that cuprous species are present near the surface at current densities typically used in the damascene process (2-15 mA/cm^sup 2^). In the potential region, where cuprous ions are detected at the ring, a current peak was seen in the forward (negative) potential scan at the copper disk. In the reverse scan (from a cuprous-depleted surface), a peak was no longer observed.2 The excess copper deposition current in the forward scan (difference between forward and reverse current) is also plotted as a function of potential in Figure 3. It appears that the excess current varied with the ring current or was dependent on the near-surface concentration of cuprous ions.

Figure 4 shows the ring current associated with cuprous ion formation during plating [part (a)], together with the copper potential during galvanostatic (constant-current) deposition [part (b)]. For applied current densities smaller than 5 mA/cm^sup 2^, a relatively large concentration of cuprous ions was present at all times during deposition. For copper deposition current densities between 5 and 20 mA/cm^sup 2^, an initial current plateau was observed at the ring, which then relaxed to a steady-state value after about 40 s for 5 mA/cm to 2 s for 20 mA/cm^sup 2^. At even higher applied current densities, the ring current immediately dropped to its low steady-state value. The measured potential in Figure 4(b) shows a transient which tracks the corresponding ring current very well. (Note that the observed correlation is between the logarithm of the ring current and the potential, in accordance with the Ncrnst equation.) The deposition potential was more positive when a higher ring current was measured or when the cuprous ion surface concentration was high. The deposition potential became constant when the cuprous ion formation reached steady state, and the shortest transients were seen for the highest applied copper deposition currents. The peak observed in the forward scan of the cyclic voltammogram (dynamic measurement) in Figure 3 follows directly from the time dependence of the potential and cuprous ion surface concentration of Figure 4. In the forward scan, the non-steady-state condition led to higher current densities corresponding to the more positive potentials and the cuprous-rich surface at small times during the galvanostatic experiments; the reverse scan represented the more negative steady-state potentials and the lower steady-state cuprous ion surface concentration conditions.