Slow sweep rate voltammetric analysis of the Cu/Cu~II! deposition reaction is shown to be an effective tool for examining aging effects associated with thiol and disulfide additives that are widely employed as brighteners. Sulfonate-terminated short chain thiols are spontaneously oxidized by Cu~II! to form disulfide molecules with the conversion being complete within a few hours of electrolyte preparation. An additional aging effect occurs during electrolysis in conventional unseparated electrochemical cells. At the anode, the disulfide is reduced by Cu~I! forming thiolate complexes which subsequently affect the copper deposition reaction occurring at the cathode. The latter effect may be avoided by using a cation selective membrane to isolate the anode compartment. The application of electrodeposition in state of the art manufacturing of microelectronic devices together with advances in analytical methods has revitalized scientific investigations into the role of organic additives in electroplating. Of particular interest is the combined use of rate accelerating and inhibiting species for copper ‘‘superfilling,’’ or ‘‘bottom-up’’ filling, of submicrometer features in dual damascene processing. 1 Sulfonate-terminated alkanethiols or disulfides are representative of a class of accelerators which are usually present at micromolar concentrations in bright plating baths. 2,3 These species adsorb on the copper surface as either thiolates or disulfides and, when combined with a sulfonate end group, disrupt the inhibiting function of the polyether-halide-Cu ~I! layer. 3,4 The superconformal growth mode that arises from this competition is well described by the curvature enhanced accelerator coverage mechanism ~CEAC! whereby surface area decrease of an advancing surface of concave curvature results in enrichment of the more strongly bound surface species. 5-10 In the thiol/disulfide-polyetherhalide system, it is the accelerating thiol or disulfide species that are more strongly chemisorbed; they thus become concentrated during deposition on concave surfaces of trenches and vias leading to bottom-up filling. The overriding importance of this geometrical effect in superfilling was recently demonstrated by first derivitizing a patterned electrode with submonolayer quantities of thiolate or disulfide followed by electrodeposition of copper from an electrolyte containing only the polyether-halide inhibitor precursors as additives. 11 Feature filling proceeded in a manner analogous to that observed when the thiol or disulfide were present in the copper plating solution. This indicates that homogeneous thiol/disulfide chemistry has little to do with the superconformal feature filling process per se. Nevertheless, evidence of accelerator aging effects associated with homogeneous chemistry, beyond simple consumption, have been widely noted with an emphasis on the interactions occurring between copper, Cu~I! ,C u~II!, thiol/disulfide, oxygen, and related products. 12-15 From a practical perspective these reactions appear to significantly hamper process control. 12-15 A previous study of the instability of thiols and disulfides in copper plating indicates that Cu~II! slowly oxidizes thiols while Cu~I! stimulates the decomposition of disulfides. 12 The report was based on examination of the oxidation behavior of these compounds at glassy carbon electrodes in combination with colorimetric studies which were generally performed in electrolytes containing high accelerator concentrations ~up to 1 mmol/L!. 12 The high concentrations were used to provide a strong analytical signal although plating operations usually employ accelerator concentrations in the 5 mmol/L range. It was anticipated that the effect of concentration might be simply reflected in the kinetics of the stated decomposition reactions. From a technological perspective, aging effects are usually dealt with by using a ‘‘bleed and feed’’ scheme whereby new additives are continually added to the electrolyte while used electrolyte is drained to maintain the cell volume. In this paper, the sensitivity of the kinetics of the copper deposition reaction to accelerator chemistry will be exploited in order to examine accelerator aging under conditions directly relevant to the superfilling process. It is shown that these effects can be understood in terms of conversion between disulfides and thiols and vice-versa. Furthermore, it is demonstrated that significant improvements in process stability can be obtained by using a cation selective membrane to separate the anode and cathode compartments. Experimental Slow sweep cyclic voltammetry was used to examine the aging effects associated with thiol and disulfide-based accelerators. For all the experiments described herein, the base electrolyte was 0.24
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Jul 1, 2006
Benno Weigmann + 5
Open Access
