Effects of Novel Inhibitor on Galvanic Corrosion of Copper and Cobalt and Particle Removal (2024)

In the electrochemical experiments, CHI660E electrochemical workstation was used to measure the open circuit potential and dynamic potential polarization curve. The open circuit potential-time (OCP-t) curve and dynamic potential polarization curve (Tafel) of the working electrode were measured respectively. The scanning range of the OCP-t is from −1V to 1V, and the scanning time is 600s. The scanning range of Tafel curve was OCP ± 0.3V, and the scanning rate was 0.01V•s⁻¹. Copper-cobalt corrosion test was carried out with alkaline cleaning solution as electrolyte.

During the cleaning experiment, in order to evaluate the particle removal efficiency, the prepared slurry with silica abrasive particles (2.5wt%, 100nm) was span on copper surface.

Then the copper samples were cleaned by brush-type cleaning equipment (G&P, 412S, Korea) with the cleaning solution for 1min and flow rate for 200mL/min, then rinsed with DIW followed by N₂ dry. The morphology of original and cleaned copper surface was characterized by scanning electron microscope (SEM). On this basis, the optimal ratio of FA/O II/CBT solution was selected to inhibit the galvanic corrosion and minimize the residue of silica particles on the copper surface. The content of copper surface oxide and the residue of inhibitor before and after cleaning were investigated by X-ray photoelectron spectrometer (XPS, PHi 250Xi, ESCA System).

FA/O II chelating agent is a multi-hydroxyl and multi-amine organic molecule with 13 chelating rings, 12 oxygen atoms, 4 amines and 16hydroxyl groups, abbreviated as R(NH₂)₄, which the coordination atoms are oxygen and nitrogen. Under alkaline conditions, stable and soluble copper amines complex ions can be formed with CuO and Cu(OH)_x, making some silica particles detachable from the surface of copper wire with the exfoliation of oxides.^9,10

The addition of FA/O II chelating agent can keep the cleaning solution in alkaline environment and reduce the chemical corrosion of copper wire caused by acidic cleaning solution. The research of Liu Yang has showed that the FA/O II chelating agent could remove silica particles residual effectively and decrease surface roughness of copper.¹¹ Firstly, the effect of FA/O II chelating agent with different concentration on galvanic corrosion was studied. The concentrations of FA/O II chelating agents were selected as 100,150,200,250ppm, and the pH value is shown in Figure1. With the increase of FA/O II chelating agent concentration from 100ppm to 250ppm, the pH value of the solution increased from 9.52 to 9.96.

The Tafel curves of Cu and Co with different concentrations of FA/O II chelating agent are shown in Figure2. The corrosion potential of Cu and Co decreased with the increase of chelating agent. In the solution, CuO and Cu₂O on surface of copper could further react with H₂O and eventually convert to copper and cuprous hydroxyl groups.¹² Cu(OH)₂ and CuOH are weakly ionized during post cleaning process (e.g. Equations1, 2). In the post-CMP cleaning process, FA/O II chelating agent will react with ionized copper ions to form stable soluble complexes (e.g. Equations3, 4).¹³ As the TableI shows the potential difference of copper and cobalt with different concentrations of FA/O II. With the increase of the concentration of FA/O II chelating agent, the complex reaction of copper ions was accelerated and the formation of soluble complex was promoted. Therefore, the dissolution rate of copper in solution was increased. E_corr,Cu decreased from −0.270V to −0.322V, I_{corr, Cu} increased from 0.2408 μA·cm⁻² to 0.4825 μA·cm⁻².

Fig.2b shows that the alkalinity of solution accelerated the dissolution of cobalt, E_corr,Co further decreased from −0.556V to −0.624V, I_corr,Co increased from 0.3034μA•cm⁻² to 0.3509 μA•cm⁻². The reaction of Equations5–8 occurred on the surface of cobalt.

Due to the difference in the magnitude of corrosion potential reduction, at the chelating agent concentration of 150ppm, the Cu-Co corrosion potential difference was the lowest, at 0.285V. It shows that the inhibition effect of single chelating agent on galvanic corrosion of Cu/Co is not obvious, so it is necessary to add inhibitor to the basis of FA/O II.

The chemical structure of 1-H carboxyl benzotriazole (CBT) is shown in the Figure3. CBT is a derivative of benzotriazole, which can form polycentric adsorption and greatly facilitate the adsorption process.

The concentrations of CBT were selected as 1.25, 2.5, 5, 7.5ppm on the basis of 150ppm FA/O II chelating agent, the Tafel curves of Cu and Co with different concentrations of CBT are shown in Figure4. Compared to the electrochemistry of sole chelating agent in the previous section, CBT has obvious inhibitory effect on both copper and cobalt. However, the corrosion potential of copper has little change with the increase of the inhibitor.

The potential difference of copper and cobalt with different concentrations of CBT are shown in TableII. As the concentration of inhibitor increases, the passivation film becomes more compact, so the corrosion potential of copper increases slightly. The corrosion potential of copper increased from −0.179V to −0.158V, the corrosion current of copper decreased from −0.0995 μA·cm⁻² to −0.0714 μA·cm⁻².

For cobalt, the concentration of inhibitor has a greater impact on the corrosion potential. When the concentration of the inhibitor changes from 1.25ppm to 7.5ppm, the self-corrosion potential of cobalt decreases significantly with the increase of the concentration of the inhibitor, E_corr,Co increased from −0.268v to −0.209v, and I_corr,Co decreased from 0.1301 μA·cm⁻² to 0.0739 μA·cm⁻². It Indicated that cobalt would react with the inhibitor first and form a passivation film on the cobalt surface.

The OCP of copper and cobalt with different concentrations of CBT is shown in Figure5. The results show that the adsorption relaxation time of cobalt is longer than that of copper. From the phenomenological point of view, the anion adsorption rate of cobalt is relatively slow.¹⁴ Copper has a comparatively higher affinity for CBT⁻ adsorption, which can maintain the Cu-CBT film on the surface of copper even when the surface coverage of CBT⁻ is relatively low. Therefore, low concentration of CBT has a strong inhibitory effect on copper, and the inhibitory effect gradually increases with the increase of concentration. For cobalt, due to the long adsorption time, when the same low concentration of 1.25ppm CBT was used for copper, the inhibition of cobalt is inferior to that of copper. However, the corrosion potential changed obviously with the increase of inhibitor concentration. When the inhibitor concentration was 2.5ppm, the copper and cobalt corrosion potential difference was the smallest, 0.042V.

The effect of different concentrations of CBT on silica particles removal based on 150ppm FA/O II was shown in Figure6. Figure6a shows the surface of copper before cleaning, and Figures6b–6e show the surface of copper after cleaning with cleaning solutions of 150ppm FA/O II and 1.25, 2.5, 5, 7.5ppm CBT respectively. The results show that the number of silica particles has significantly reduced.

This is because the copper surface are oxidized to CuO and Cu₂O, which may cause the silica to be chemically adsorbed to the oxide layer through the oxygen-bridging bond. Large amounts of silica remained on the copper surface. As the Figure7 shows, In order to remove silica particles from copper surface, the chemical bond between silica and copper oxide is bound to be broken.¹⁵ With the addition of FA/O II chelating agent, the oxidation products on the copper surface dissolved and silica particles were released. As the corrosion inhibitor of copper, CBT can form Cu-CBT passivation layer to protect the continuous oxidation of the underlying copper. The cleaned copper oxide surface was replaced by Cu-CBT passivation layer, and silica particles were removed from the copper surface.

Figure8 shows the removal efficiency of surface particles with different concentrations of CBT. With the increase of concentration, the number of residual silica particles on the copper surface decreased first and then increased, so the removal efficiency of cleaning solution increased first and then decreased. When the CBT concentration was 2.5ppm, the maximum removal efficiency was 95.9%. As the CBT concentration continues to increase, the particle removal efficiency of copper surface decreased from 92.8% to 85.6%. This is because the higher the concentration of CBT, the faster the formation rate of Cu-CBT passivation layer is. If the formation rate of Cu-CBT passivation layer is higher than the dissolution rate of copper oxide, CBT can also adsorb on the surface of copper oxide and inhibit further corrosion of oxide layer. Therefore, adding higher concentration of CBT into the cleaning solution, the silica particles will remain on the surface of copper oxide.¹⁶

The relationship between structures and inhibitor activities of CBT was investigated with density functional theory (DFT) at the B3LYP/6-31G (d,p) level,¹⁷ basis set implemented in the GAUSSIAN 03 program package.¹⁸ Quantum chemical parameters, such as highest occupied and lowest unoccupied molecular orbital energy levels, energy gap, ionization potential, atomic charges and electron-density distribution were calculated at the same level. Reactive behavior of CBT was investigated in terms of electron-negativity, electron-donating ability. The local reactivity was studied with the Fukui indices approach to predict both the reaction center and electrophilic or nucleophilic behavior.^8,19 Fukui et al. proposed the concept of the importance of the highest occupied and lowest unoccupied molecular orbital energy levels for determining their reactivity of chemical substances.²⁰ The charge density curves of CBT characteristic molecular orbitals are shown in the Figure9.

Organic substances with a higher energy level of HOMO, i.e. a less negative value, easily donate electrons from HOMO to an empty orbital of appropriate acceptors and LUMO denotes the ability of the molecule to accept electrons.²¹ The HOMO is distributed throughout the entire molecule of CBT, which means that the preferred site for electrophilic attack would be the nitrogen of the triazole ring and the oxygen of the carboxyl group.

The following electronic parameters of CBT and BTA are presented in TableIII: the frontier orbital energy, E_HOMO, E_LUMO and their energy difference (ΔE), dipole moment (μ), electronegativity (χ) and vertical ionization energy (I_vert), which would be related to the inhibition of properties.

According to DFT- Koopmans' theorem, the ionization potential I can be approximated as the negative value of the highest occupied molecular orbital (HOMO) energy,

The negative value of the lowest unoccupied molecular orbital (LUMO) energy is similarly related to the electron affinity A,

The proper vertical ionization potential I_vert was evaluated as the energy difference between the neutral species and the positive ion at the neutral equilibrium geometry,

Natural bond orbital (NBO) analysis was performed to evaluate as the electron-density distributions. The electron density plays an important role in calculating the chemical reactivity parameters. The global reactivity refers to electronegativity, χ, which is identified in the finite difference approximation as the negative of the chemical potential μ,

Lower gap values are associated with effective inhibition. The smaller the energy gap, the better the inhibition effect of inhibitors. As shown in the TableIII, as a derivative of BTA, the energy gap of CBT is lower than that of BTA. The E_HOMO of CBT is higher, and CBT adsorbs tightly on metal surface. I_vert and E_HOMO are closely related, and they all support the same order of the inhibition effects. Electron negativity is used to indicate the ability of atoms to attract electrons.²² CBT has larger electron negativity, which is easy to attract electrons in metals and the electron transfer ratio between metal and CBT is larger.

The atomic charge and molecular electrostatic potential (MEP) of CBT, which obtained from the analysis of natural bond orbitals are shown in the Figure10.

The atomic charge reflects the electron charge of the atom, while the electrostatic potential of the molecule reflects the electron density distribution inside the molecule. It indicated that all the nitrogen atoms possess an excess of negative charge, which suggested that there are more nucleophilic centers in their interactions between these molecules and metal surface.

The local reactivity has been analyzed by means of Fukui indices, which are an indication of the reactive centers within the molecules.²³ Condensed Fukui indexes means that the Fukui indexes shrink to an atom. In this way, each atom has an accurate value, which can be used to quantitatively compare the Fukui indexes at different locations. For electrophilic attack and nucleophilic attack, based on finite difference approximation, the condensed Fukui functions fi⁻ and fi⁺ are given and expressed by Equation13.

Where qⁱ_N, qⁱ_N-1 and qⁱ_N+1 are the atomic charges on the ith atom in the neutral, cationic and anionic species, respectively.

The local reactivity behavior was analyzed by means of Fukui indices which are presented in Figure11. The atomic sites of molecule, which possess the largest condensed Fukui indices and favor the higher reactivity. Thus, the molecular sites with the maximum value of fi⁺ are the preferred sites to which the inhibitor molecule will donate charge when attacked by an electrophilic reagent. On the other hand, a large value of fi⁻ is assigned to the sites where the inhibitor molecule will receive charge when attacked by a nucleophilic reagent. The O₁₂ is the most favorable site on the CBT for an electrophilic attack. The very similar values of the fi⁻ indexes for the nitrogen atoms N₁₇ and O₁₁ indicated that both centers are equivalent when it undergoes an electrophilic attack. In nucleophilic attack, N₁₇ is the main reaction site of CBT, which participates in the electron transfer from metal to inhibitor molecule to a large extent. Turning to nucleophilic attack, N₁₇ is the main reaction site of CBT, which participates substantially in electron transfer from metal to inhibitor molecules. Due to the position of the donor in the ring, the strong interaction between N₁₇ on the imidazole ring, O₁₂ on the carboxyl group with copper, the CBT is adsorbed on the surface of copper, providing effective protection to prevent the corrosion of copper.

The XPS analysis of copper surface is shown in Figure12. Sample A and B were untreated copper surface and the copper surface soaked by CBT respectively, sample C was cleaned with FA/O II after contaminated by silica sol, sample D was cleaned with FA/O II and CBT after contaminated by silica sol.

For sample A, the peak area of Cu(OH)₂was much larger than that of CuO/Cu₂O, indicating that the main form of copper in the sample A was Cu(OH)₂, but a small amount of CuO/ Cu₂O were also present.²⁴ For sample B, after immersed in inhibitor CBT, the peak of Cu(OH)₂ decreased and Cu₂O increased, indicating that the content of Cu(OH)₂ reduced and Cu₂O increased. This is because that the CBT is adsorbed on the surface of copper oxides to prevent further oxidation. For sample C, the content of Cu₂O decreased and CuO/Cu(OH)₂ disappeared on the copper surface cleaned with FA/O II chelating agent, indicating that the combined action of mechanical force of PVA brush and FA/O II can remove the oxide layer of copper.

For sample D, compared with sample C cleaned with FA/O II chelating agent only, the content of Cu₂O appeared and copper increased, indicating that CBT adsorbed on the surface of copper to inhibit the further oxidation of copper.

The spectra of N element on the copper surface are shown in Figure13. Compared with sample A, the increase of the peak in sample B indicated the adsorption of CBT on the Cu surface. For sample C, The nitrogen peak on the copper surface cleaned with FA/O II did not change significantly compared with that of untreated copper surface. For sample D, the peak of N element had little change compared with the original copper sample, which indicated that the residue of CBT on the copper surface cleaned with FA/O II and CBT is not much.

The XPS spectra of Cu_2p peaks on copper surface are shown in Figure14. The spectra show two main peaks at the binding energies of 953eV and 933eV in addition to satellite peaks at the binding energies of 940–945eV, 961–964eV and 934eV which are characteristic of cupric compounds especially CuO (red region) and Cu(OH)₂ (green region).²⁵

It can be seen from the XPS spectrum of sample A, CuO and Cu(OH)₂ peaks appeared simultaneously on the copper surface. For the XPS spectrum of sample B, after immersed in inhibitor CBT, the 934.6eV next to the main peak of 933eV indicated the adsorption of CBT (The position indicated by the arrow). According to the XPS spectrum of sample C, CuO/Cu(OH)₂ disappeared on the copper surface cleaned with FA/O II chelating agent, this is the same conclusion as that of sample C in Figure12. For sample D, there was a slight increase of the peak of 934.6eV, which compared with that of copper sample B cleaned with FA/O II only, indicating that the residue of CBT on the copper surface cleaned by FA/O II with CBT is not much.