Inhibition mechanism of pyridine functional group of nicotinic acid. – Fig. 2 represents the structure of 3-pyridinecarboxylic acid (nicotinic acid) and the schematic illustration of its suggested inhibition mechanism. As shown in Fig. 2a, nicotinic acid has the formula C6H5NO2 and belongs to a group of monocarboxaylic derivatives of pyridine. Since the nitrogen atom of the pyridine functional group has electron pairs, they can form complexes by using σ-bonding (i.e., coordinate covalent bonding) with transition metal ions from oxidized metal surfaces such as Cu, Ir, W, Co, and Ru [20]. Such complexes mentioned above form a passivation layer on the metal film surface, preventing corrosion reactions by preventing water adsorption. However, the density of the inhibitor layer through the metal complex differs depending on the type of metal film. As shown in Fig. 2b, the electron configurations of metal ions in oxidized Cu and Ru films are Cu+ : (Ar)3d10 and Ru4+ : (Kr)4d4, respectively. As a result, there is a difference in terms of π-back bonding which participates in the delocalized π-electrons present in planar cyclic hydrocarbon molecules of the pyridine ring structure and filled d-orbital of metal ions (Fig. 2b) [21]. In the case of Cu film, the d-orbital of Cu+ ion derived from Cu2O is fully occupied. Therefore, the π-electron of pyridine ring structure can form π-backbonding with the Cu+ ion better than the Ru4+ ion because of the insufficient outermost electrons of the Ru4+ ion. Consequently, the affinity between nicotinic acid and Cu film should be more robust than its affinity with Ru film, resulting in a dense inhibitor layer formation on the surface of Cu oxide (Fig. 2c). In comparison, only a sparse inhibitor layer is formed on the Ru oxide film (Fig. 2d).
Interfacial interaction behavior analysis between Cu/Ru films and nicotinic acid inhibitor – For analysis of interfacial interaction between Cu/Ru films and nicotinic acid, the contact angle measurement and X-ray photoelectron spectroscopy (XPS) are evaluated. The contact angle measurements are a valuable tool for measuring the hydrophobicity of thin films. In general, the nitrogen atom from the pyridine functional group of nicotinic acid forms a complex by creating a σ-bond between its electron pair and metal ions from oxidized metal films. It forms an adsorption layer and makes a hydrophobic surface [22]. The contact angle values of Cu and Ru films as a function of nicotinic acid concentrations were shown in Fig. 3a and 3b. The contact angle values of Cu and Ru immersed in a slurry without nicotinic acid were 28.7˚ and 58.4˚, respectively. The difference in the contact angle value means that the intrinsic surface property of Ru film is more hydrophobic than the Cu film. With the addition of nicotinic acid, the contact angle values of Cu and Ru films increased to 41.9˚ and 62.1˚, respectively. The increase of contact angle values (i.e., increased hydrophobicity) of both Cu and Ru films with nicotinic acid-treated is commonly attributed to a specific orientation of adsorbed nicotinic acid molecules hydrophobic pyridine group to form a protective hydrophobic layer. The change in contact angle value with and without nicotinic acid was 13.2˚ for Cu and 3.7˚ for Ru, and the amount of contact angle change is large for Cu film. It means that nicotinic acid adsorbed more onto the Cu film than the Ru film.
The XPS analysis was utilized to characterize the chemical composition changes of Cu and Ru films after being immersed for 10 min in different slurries (Fig. 3c-f). Figure 3c and Fig. 3d show the exemplary spectra of Cu film; Cu 2p and Cu O1s, respectively. In Fig. 3c, the metallic Cu binding energy for 2p3/2 is located at 932.2eV. The Cu binding energies for CuO and Cu2O are found at 933.8eV and 932.0eV, respectively. These results are consistent with the other research reports [23–26]. The Cu and Cu2O peak intensities were significantly decreased according to the concentration of the nicotinic acid. At the addition of the 0.03M nicotinic acid, there was no significant change in Cu 2p peak intensity. However, it was confirmed that when the concentration was increased to 0.05M, the Cu2p peak intensity was overall lowered. The decrease in Cu 2p peak intensity is that nicotinic acid adsorbed onto the Cu2O, forming a dense inhibitor layer.
Meanwhile, the binding energy peaks for CuO and Cu2O in the Cu O1s spectra are detected at 534.0eV and 529.7eV, respectively. The Cu O1s peak from Fig. 3d also shows that the Cu2O peak rapidly decreases with the nicotinic acid addition. This suggests that the pyridine functional group of nicotinic acid adsorbs well to Cu2O (Cu+ state, fully occupied d-orbital) rather than CuO (Cu2+ state, partially occupied d-orbital).
Figure 3e and 3f show the fine spectra of Ru film; Ru 3d and Ru O1s, respectively. The metallic Ru binding energies for 3d5/2 and 3d3/2 are located at 280.0eV and 284.4eV [27, 28]. Ru binding energy for RuO2 is found to be 280.8eV and 285.0eV. Because RuO2∙H2O is a metallic oxide with a partially filled conduction band, the core-hole coupling may occur on this surface [28]. Thus, RuO3 is considered present with the bulk phase of RuO2, and the binding energy peaks at 282.3eV and 286.5eV, respectively. Meanwhile, the binding energy peaks for RuO2 and RuO3 appear at 529.2eV and 530.7eV in Ru O1s spectra [29, 30]. No change with nicotinic acid concentration was observed in the case of both Ru 3d (Fig. 3e) and Ru O1s (Fig. 3f) XPS results. In other words, the above results mean that nicotinic acid was barely adsorbed on the Ru film. These results correspond with the contact angle measurements.
Electrochemical interaction evaluation between Cu/Ru films and nicotinic acid inhibitor. – For analysis on electrochemical properties, the potentiodynamic plots and Nyquist plots are evaluated. The corrosion potentials (Ecorr) and corrosion currents (Icorr) of the Cu and Ru films under the different nicotinic acid concentrations are noted in Fig. 4a-c and Table 1. As shown in Fig. 4a-c, the Tafel curves of Cu and Ru films were evaluated through potentiodynamic polarization measurements. The potential difference between Cu and Ru films decreased from 0.49V to 0.09V as inhibitor concentrations increased from 0M to 0.05M. The galvanic corrosion occurs in the heterojunction of Cu/Ru films because two metal films have different potentials. In Cu film, oxidation reactions that donate electrons occur (as an anodic site), while reduction reactions occur in the Ru film that accepts electrons (as a cathodic site). Therefore, by controlling the potential of both films by reducing the gap of potential difference, the redox reaction can be suppressed, resulting in galvanic corrosion prevention. The experimental results in Fig. 4c show that the potential difference between Cu and Ru is significantly reduced to 0.09V, indicating the galvanic corrosion between Cu and Ru films was suppressed. On the other hand, the Cu film potential change is 0.63V (from − 0.27V to 0.36V), more significant than that of Ru film (0.23V, from 0.22V to 0.45V). In both cases, adsorption of nicotinic acid tended to form an inhibitor passivation layer leading to the potential value increase. Still, the Ecorr value change of Cu film was more extensive than that of Ru film. This phenomenon is due to the denser layer formation on the Cu film, consistent with the adsorption affinity trends of nicotinic acid mentioned above. EIS (Electrochemical impedance spectroscopy) was performed to evaluate the barrier protection properties of nicotinic acid to both Cu and Ru films. The impedance data were fitted using the electrical equivalent circuits with three resistances and two constant phase elements (CPE) shown in Fig. S1. Rs represents the solution resistance, and Rf is the film resistance. R1 includes the Rct (charge transfer resistance), Rd (diffusion layer resistance), and Ra (accumulation resistance) at the metal/solution interface [31, 32]. The CPE1 and CPE2 represent the film capacitance and electric double-layer capacitance, respectively. From Fig. S1, the Rp (polarization resistance) values, representing the corrosion inhibition effect characteristics, consisted of Rf and R1. Therefore, obtaining a high Rp value indicates an improved inhibition effect. In Table 2, the Rp value change is more considerable for Cu film because nicotinic acid adsorbed more onto Cu film. That is consistent with the adsorption behaviors and potentiodynamic polarization measurements described earlier.
Table 2
Polarization resistance (Rp) and inhibition efficiency (η) of metal films according to inhibitor concentration.
|
Cu film
|
Ru film
|
Solution system
|
Rp [Ω∙cm2]
|
η [%]
|
Rp [Ω∙cm2]
|
η [%]
|
None
|
5704.2
|
-
|
69518
|
-
|
Nicotinic acid 0.03M
|
13830
|
58.75
|
74011
|
6.07
|
Nicotinic acid 0.05M
|
49897
|
88.57
|
78549
|
11.50
|
Nyquist plots as a function of nicotinic acid concentrations for Cu and Ru films at pH 10 are shown in Fig. 4d and Fig. 4e, respectively. As nicotinic acid concentrations increase, the real impedance difference at lower and higher frequencies for both films was increased, leading to an increase in Rp value (Table 2). That is owing to the formation of the inhibitor protection layer on each film surface. Detailed impedance parameters and inhibition efficiencies (η%) are listed in Table 2. The inhibition efficiency could be calculated from the polarization resistance values as follows:
$${\eta }\left[\text{\%}\right]= \frac{{R}_{p}-{R}_{p}^{0}}{{R}_{p}}\times 100$$
(where, \({R}_{p}^{0}\) is the polarization resistance without nicotinic acid.) The increase in the inhibition effect (i.e., increasing Rp value) due to nicotinic acid adsorption is more dramatic in Cu film (Fig. 4d and Fig. S2) comparing with Ru film (Fig. 4e and Fig. S3).
Effect of pyridine functional group on Cu and Ru removal rate and surface roughness. – Fig. 5a and Table 3 represent the removal rates of Cu and Ru films under the different inhibitor conditions and their removal selectivity at pH 10 by the CMP process. Before the CMP process, the colloidal stability of each slurry is observed by zeta potential analysis (Fig. S4) and large particle counter evaluation (Fig. S5), which shows no harmful effects on the colloidal stability as CMP slurries. Without nicotinic acid as an inhibitor, the initial removal rates of Cu and Ru film were 95.98Å/30s and 24.85Å/30s, respectively, with a Cu to Ru selectivity of 3.86. However, as nicotinic acid content increased, the removal rate of Cu film decreased steeply from 95.98Å/30s to 26.23Å/30s. In contrast, the removal rate of Ru film was maintained at a constant value within the margin of error range regardless of the nicotinic acid concentration (Ultimately optimized with the Cu to Ru selectivity of 1: 1). The affinity between nicotinic acid and Ru film is relatively smaller than that of Cu film, confirmed by XPS and contact angle results above. Therefore, the amount of nicotinic acid adsorbed on Ru film is comparatively weak and insignificant. This result is consistent with the small potential change (ΔEcorr: 0.23V) observed by the potentiodynamic polarization measurements. In a sub-5nm logic semiconductor device using a ruthenium barrier structure, the Cu to Ru selectivity requirement to achieve a completely flat surface is 1:1 for the Ru barrier CMP [12, 19]. That is to minimize defects such as dishing, erosion and protrusion. Therefore, using nicotinic acid as an inhibitor with an affinity difference between Cu and Ru films prevents galvanic corrosion and controls the Cu to Ru selectivity simultaneously.
Table 3
Results of the removal rate and selectivity between Cu/Ru films according to inhibitor concentration.
|
Cu removal rate
[Å/30s]
|
Ru removal rate
[Å/30s]
|
Selectivity
[Cu/Ru]
|
None
|
95.98
|
24.85
|
3.86
|
Nicotinic acid 0.03M
|
41.33
|
24.52
|
1.69
|
Nicotinic acid 0.05M
|
26.23
|
25.0
|
1.05
|
Additionally, as shown in Fig. 5b, the RMS (Root-mean-square) value of surface roughness (Rq) was estimated using AFM (Atomic Force Microscope) for each sample of Cu and Ru. Since Ru is a chemically more inert material compare to Cu, the Rq value of Ru is smaller than that of Cu in all cases regardless of the nicotinic acid concentration. Meanwhile, as the concentration of nicotinic acid increased, the Rq value of Cu gradually decreased. This indicates that a smooth surface with improved roughness was obtained because the dissolution rate of Cu film was suppressed by forming a dense inhibiting layer. On the other hand, the improvement of Ru roughness is much smaller than that of the Cu film due to the suggested sparse inhibiting layer formation on Ru film.