Chapter III. Decomposition of SPS
3.1. Decomposition of SPS
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the performances of plating solutions, leading to defects in either trench filling or the subsequent chemical-mechanical planarization (CMP) process. However, the detail failure mechanism including the roles of breakdown products has not been reported yet.
Therefore, in this chapter, the influences of breakdown products from SPS decomposition are examined in detail. The byproducts from SPS decomposition are characterized using 1H-NMR, which is further used to analyze the changes in the concentrations of active and inactive organics in the plating bath. Then, the impacts of byproducts are analyzed.
Experiments were performed to confirm the differences in filling performance as a function of aging time. Figures 3.1(a)–(c) show that the plating solution had good filling performance up to a aging time of 60 min. Voids appeared after 90 min [Figure 3.1(d)], accompanied by changes in filling profiles from superconformal to conformal [Figure 3.1(f)] because of SPS decomposition during electrodeposition, as reported by Wang et al.
and Koh et al. who observed that the voids were formed during via filling when the charge passing through the circuit exceeded 15 Ah/L and 25 Ah/L, respectively.66,70 These values are significantly higher than those obtained through our experiments (0.93 Ah/L).
This difference can possibly be ascribed to processing factors such as the sizes of
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features, the presence of other chemical components, or the replenishment of additives using the CVS method.
Figure 3.2 shows the CV results of the fresh and aged solutions. The presence of fresh SPS resulted in typical hysteresis, which is associated with the competitive and the potential dependent adsorption behavior of SPS and PEG–PPG on the Cu surface.11,12,54,55
As the degradation time increased, the current density in the forward scan decreased slightly, whereas that in the backward scan declined significantly after initial preservation until 90 min. Both the decrease of forward current density and disappearance of hysteresis indicated the decrease of active accelerating compounds in the plating solution.
The hysteresis curve finally disappeared after 180 min, at which point the filling profile completely changed from superconformal to conformal [Figure 3.1(f)].
1H-NMR analyses were carried out to identify the chemical components of the plating solutions aged for 0, 30, 60, 90, and 180 min. Figure 3.3 shows that SPS initially had three characteristic peaks at 2.18 ppm, 2.89 ppm, and 3.06 ppm with an area ratio of 1:1:1 (denoted as S1, S2, and S3, respectively). It was noticeable that the relative intensity of the 2.89 ppm triplet (S2) decreased as the aging time increased. This decrease was associated with the dissociation of the SPS disulfide bond on the Cu surface. This reaction resulted in the formation of two MPSs, whose peaks appeared at 2.03 ppm (M1),
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2.48 ppm (M2), 3.05 ppm (M3). Another byproduct from the decomposition of the disulfide bond was the PDS observed at 2.18 ppm (P1) and 3.06 ppm (P2) with the area ratio of 1:2. The formation of MPS and PDS are already well known as byproducts of SPS decomposition during Cu electrodeposition. Lee et al. and Wang et al. observed MPS and PDS in aged plating solutions by means of mass spectroscopy and each suggested a mechanism of SPS decomposition using density functional theory.64,66 They proposed that PDS was formed from the oxidation reaction of SPS catalyzed by the metallic Cu surface. Koh et al. observed that oxygen purging into the plating solution accelerated the decomposition of SPS.69
Before quantitative analysis of each additive, the 1H-NMR spectrums of the power and standard samples (additives after pretreatment from non-electrolyzed bath) of SPS- related compounds were obtained in order to check the stability of each additive during pretreatment. As shown in Figure 3.4 (a) and (b), SPS and PDS were stable throughout the pretreatment. However, most of MPS changed to SPS during the pretreatment step by the oxidative dimerization reactions. No evidence on the oxidation of MPS to PDS was observed. The results of Figure 3.4 (a) and (b) imply that some portion of MPS formed by the decomposition of SPS was converted back to SPS during the pretreatment step.
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Therefore, the amounts of SPS and MPS in the samples subjected to NMR were different with the actual amounts of them right after the aging step.
The concentrations of active accelerating compounds (SPS+1/2MPS), PDS, and overall compound (SPS+1/2MPS+1/2PDS) were obtained by comparing the relative area of each peak with 10 ppm saccharin, as shown in Figure 3.5. The concentrations of active accelerating compounds were expressed based on the following considerations. First, two MPS molecules could be reconverted to one SPS molecule by an oxidative dimerization reaction with the reduction of cupric ions. It was essential step of the acceleration because it aided the reduction of Cu2+ to Cu+, the rate determining step in Cu electrodeposition. Furthermore, the individual determination of MPS and SPS concentrations had little meaning due to the interconversion reaction of MPS to SPS during the pretreatment step (Figure 3.4 (a), (b)). As shown in Figure 3.5, as the aging time increased, the concentration of active accelerating compounds steadily decreased whereas the PDS concentration increased. The concentration of overall compounds (SPS+1/2MPS+1/2PDS) steadily decreased due to the incorporation of additives into the growing Cu films. The total incorporated amount of SPS and its by-products was 14.7 μM after 180 min of aging test in our conditions. A possible mechanism of SPS decomposition is illustrated in Figure 3.6. One significant reaction is the dissociation of
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the SPS disulfide bond on the Cu surface, producing two MPS molecules. Meanwhile, SPS could be sequentially oxidized through a reaction with O2 and H2O, leaving PDS and MPS as the byproducts. PDS seemed to be most stable, and it remained in the plating solution without any further chemical reactions.
Considering the initial and final product presented in Figure 3.3, the net reactions of SPS decomposition can be expressed by following equation:
SPS2- + O2 + H2O→PDS2- + MPS- + H+ (oxidation by molecular oxygen) (Eq. 3.1) SPS2- + 6H2O → 2PDS2- + 10e- + 12H+ (anodic-oxidation) (Eq. 3.2)
SPS2- + oxidant (dissolved O2, anode)→2PDS2- (overall) (Eq. 3.3)
Since either the decrease in active species (SPS) or the increase in breakdown products (MPS, PDS) can deteriorate filling capability, the effects of SPS, MPS, and PDS will be discussed in the next chapter.
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Figure 3.1. Filling performance of plating solutions aged for (a) 0 min, (b) 30 min, (c) 60 min, (d) 90 min, (e) 120 min, and (f) 180 min. 360 μM PEG–PPG was added before the experiments.
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Figure 3.2. CV voltammograms of plating solutions aged for 0, 30, 60, 90, 120, and 180 min. 360 μM PEG–PPG was added before the experiments.
‐0.6 ‐0.5 ‐0.4 ‐0.3 ‐0.2 ‐0.1 0.0
0
‐30
‐60
‐90
‐120
‐150
‐180
Current density (mA/cm2 )
Potential (V vs. Ag/AgCl)
Aging time:
0 min (0 C/L) 30min (840 C/L) 60 min (1680 C/L) 90 min (2520 C/L) 120 min (3360 C/L) 180 min (5040 C/L)
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Figure 3.3. (a) 1H-NMR spectrums of plating solutions aged for 0, 30, 60, 90, 180 min.
Black (S1~S3), blue (M1~M3), and green (P1, P2) arrows indicate the peaks of SPS, MPS, and PDS, respectively. 1H-NMR spectrums of (b) power and (c) standard samples were also displayed.
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Figure 3.4. 1H-NMR spectrums of (a) power and (b) standard samples.
3.2 2.8 2.4 2.0
0.973 0.959
0.989 0.902
0.511 1.000
1.000
SPS
MPS
Chemical shift (ppm)
PDS 1.000
3.2 2.8 2.4 2.0
1.060 0.890
1.000
0.562 0.078 0.846 0.918
1.000 1.000
PDS MPS
Chemical shift (ppm)
SPS
(a) (b)
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Figure 3.5. Concentrations of active accelerating compounds, PDS and overall compounds in the plating solution aged for 0, 30, 60, 90, and 180 min. The active accelerator concentrations were calculated from the sum of SPS and one-half of MPS concentrations.
‐30 0 30 60 90 120 150 180 210
‐10 0 10 20 30 40 50 60
Additive concentration (M)
Aging time (min)
Active accelerating compounds (SPS+1/2MPS) PDS
Overall compounds (SPS+1/2MPSA+1/2PDS)