Detection of Accelerator Breakdown Products in Copper Plating Baths

The mercaptopropylsulfonic acid (MPS) breakdown product of the bis(sodiumsulfopropyl)disulfide (SPS) additive used in acid copper plating baths accelerates copper electrodeposition and can be detected by cyclic voltammetric stripping (CVS) analysis. In the presence of oxygen, MPS decomposes rapidly in acid copper sulfate baths so that the CVS stripping peak area (Ar) decreases on successive cycles. The slope of a plot of Ar vs. log of the CVS cycle number (or time) provides a measure of the initial MPS concentration.

INTRODUCTION & BACKGROUND

Acid copper sulfate baths are employed in the “Damascene” process (1) to electrodeposit Cu within fine trenches and vias in dielectric material on semiconductor chips. Two organic additives are required to provide bottom-up filling of the Damascene features. The “suppressor” additive, which is typically high-molecular-weight polyalkene glycol (e.g., PEG), adsorbs strongly on the Cu cathode surface, in the presence of chloride ion, to form a film that sharply increases the overpotential for Cu deposition. The “anti-suppressor” or “accelerator” additive counters the suppressive effect of the suppressor to provide the accelerated deposition within trenches and vias needed for bottom up filling.

Close organic additive control needed for Damascene plating is provided by Cyclic Voltammetric Stripping (CVS) analysis, which involves alternate plating and stripping of Cu at a Pt rotating disk electrode. The additives are detected from the effect that they exert on the electrodeposition rate measured via the Cu stripping peak area (Ar). The accelerator concentration is typically determined by the linear approximation technique (LAT) or modified linear approximation technique (MLAT) described by Bratin (2). During Damascene Cu plating, however, additive species break down into breakdown products that may interfere with the electrodeposition process. These breakdown products need to be controlled to ensure that high quality Damascene deposits are obtained. A method for detecting suppressor breakdown products was described in our earlier publications (3,4).

This paper describes a CVS method for detecting breakdown products of accelerator additives that are widely used for Damascene copper plating. Results are presented for the 3-mercaptopropylsulfonic acid (MPS) species, which is a breakdown product of the bis(sodiumsulfopropyl)disulfide (SPS) additive (5).

EXPERIMENTAL DETAILS

CVS measurements were made using a Qualilab QL-10® plating bath analyzer (ECI Technology, Inc.) with a polyethylene beaker cell containing 50 mL of solution (open to the atmosphere). For some experiments to verify that oxygen plays a role in MSA decomposition, the cell was partially sealed and deaerated via nitrogen bubbling (stopped during the CVS measurements). The supporting electrolyte contained 40 g/L Cu (added as CuSO₄ . 5 H₂O), 10 g/L H₂SO₄, 50 ppm chloride ion, and 2.0 g/L of 5000 molecular weight (MW) polyethylene glycol (Aldrich). The SPS and MPS materials were purchased from Raschig Chemical (Germany). The working electrode was a Pt rotating disk (4 mm diameter, 2500 rpm). Unless otherwise noted, the potential was scanned at 100 mV/s between a positive limit of +1.575 V and a negative limit of either -0.225 and -0.325 V vs. SSCE-M (standard silver-silver chloride electrode modified by replacing the solution with a saturated AgCl solution also containing 0.1 M KCl and 10 volume% sulfuric acid). The counter electrode was usually a stainless steel rod (6 mm diameter). During CVS measurements, the solution temperature was controlled at 24°C within +0.5°C. Specimens of MPS and SPS were injected into the cell at the positive limit in the CVS cycle. The effects of the commercial Viaform™ (Enthone Inc.) and Ultrafill™ (Shipley, Inc.) additives (at normal concentrations) were also investigated.

RESULTS & DISCUSSION

MPS Analysis Method

Figure 1 shows that Ar measured on the first CVS cycle after addition of MPS to the acid copper electrolyte varies monotonically with the MPS concentration. However, a simple Ar measurement cannot be used for MPS analysis since organic additives and other species present in plating baths also affect Ar values.

Figure 2 shows that Ar remains constant for acid copper baths containing only SPS, but decreases monotonically with potential cycling in the presence of the MPS breakdown product. Both compounds tend to accelerate the copper deposition rate but the accelerating effect of MPS is stronger and much more time-dependent. After addition of the Viaform™ and Ultrafill™ accelerator additives at the normal concentrations, the Ar values were also constant (not shown) but were smaller (1.5 and 1.4, respectively) than the value of 2.2 observed for SPS at 1.0 ppm concentration. For the potential scan rate and limits used, a CVS cycle corresponded to 38 seconds and copper deposition occurred over a time frame of about 6 seconds. Since the MPS and SPS specimens were injected at the positive potential limit, the copper deposition rate measurement for cycle number 1 began after about 16 seconds and ended at about 22 seconds (onset for copper deposition is about 0.0 V vs SSCE-M). It is clear from these data that MPS decomposes rapidly when its concentration is high, and much more slowly as its concentration decreases.

Figures 3 and 4 illustrate the effects of delaying CVS cycling (after addition of the MPS sample) and interrupting CVS cycling.

Note that a relatively negative potential scan limit (-0.325 V) was used to enhance measurement sensitivity. It is evident that Ar continues to decrease unabated even when no voltage is applied to the working and counter electrodes, indicating that MPS decomposes chemically in the bath. When the beginning of the curve for a 3-minute delay was shifted to the 5th CVS cycle (corresponding to 3.2 minutes), the 3-minute delay curve practically coincided with the curve for which cycling had been interrupted for 2 cycles (Fig. 4). Potential cycling appears to actually slow MPS decomposition, possibly because of SPS formation.

Figure 5 illustrates that an exposed copper counter electrode tends to increase the rate of MPS decomposition compared to a stainless steel electrode (or a copper electrode partitioned from the electrolyte via a double-junction glass frit). This effect is relatively small and may result from adsorption of MPS on the relatively large copper counter electrode.

Figure 6 shows that the decrease in Ar after MPS additions is exponential since a plot of Ar vs. Log (CVS cycle number) is linear.

The theoretical basis for this empirical relationship would be difficult to ascertain from the present data since the measured copper deposition rate is a composite for a range of potentials, and both electrochemical and chemical processes may be involved in the decomposition process. Nonetheless, as shown in Fig. 7, the slope of such plots provides a measure of the initial MPS concentration.

Figure 10 illustrates the effect of negative potential scan limit on plots of Ar vs. Log (CVS cycle number). Linear plots are obtained in all cases, although the slopes vary. Figure 11 shows the dependence of the slope on the negative potential limit. Obviously, the negative potential limit must be held constant for the MPS analysis.

Previous work by Healy et al. (6) has shown that SPS and MPS undergo complicated chemical and electrochemical reactions in acid copper plating baths. The SPS species, also known as 4,5-dithiaoctane-1,8-disulphonic acid, is slowly oxidized chemically in the bath but only in the presence of copper metal, although the oxidation rate is accelerated in the presence of oxygen. A complex involving Cu(I) and MPS, e.g., Cu(I)SCH₂CH₂CH₂SO₃H, apparently plays a key role as an intermediate in electrochemical reduction of SPS and oxidation of both SPS and MPS in acid copper baths. Nonetheless, Healy et al. conclude that oxidation of MPS via this intermediate does not lead to regeneration of the SPS species. However, Moffat et al. (5) provide convincing evidence that decomposition of MPS eventually results in formation of SPS in acid copper baths.

Figure 12 shows that the CVS stripping peak area (Ar) for a 1.0 ppm MPS acid copper electrolyte decays (after about two days) to a constant value corresponding to that for a 1.0 ppm SPS electrolyte.

Since the concentration of the two species based on weight was the same (1.0 ppm), the molar concentration of the MPS electrolyte was initially double that of the SPS electrolyte. The equivalent Ar values observed for the aged MPS electrolyte and the fresh SPS electrolyte indicate that the MPS dimerized to SPS, resulting in the same molar concentration of SPS in both solutions. Thus, our results support the conclusion of Moffat et al. (5) that MPS dimerizes to form SPS in acid copper baths. Our results also indicate that this process is reversible (under some conditions) since the initial Ar value for the MPS solution aged for one day was somewhat greater than the final Ar value from the previous day (fresh MPS solution).

Oxygen also plays a role in the decomposition of MPS in acid copper baths since partial deaeration of the solution was found to significantly reduce the decomposition rate (slow the decrease in Ar value with time). On the other hand, stirring of the solution had little effect, indicating that mass transport is not an important factor. Likewise, removal of the Pt rotating disk electrode from the solution had no effect. Future studies will determine the effects of acidity and copper ion concentration. The goal of this work is to provide metrology that helps tool manufacturers, chemical suppliers, and users to better control acid copper plating processes.

CONCLUSIONS

The mercaptopropylsulfonic acid (MPS) breakdown product of the bis(sodiumsulfopropyl)disulfide (SPS) additive used in acid copper plating baths can be detected by cyclic voltammetric stripping (CVS) analysis. Decomposition of MPS in acid copper baths apparently involves dimerization to SPS, which is accelerated in the presence of oxygen.

Authored By

M. Pavlov, E. Shalyt, P. Bratin and D. M. Tench
ECI Technology, Inc.
60 Gordon Drive, Totowa, NJ 07512

REFERENCES