Corrosion Inhibitors in Copper CMP Slurry: BTA Mechanism, Galvanic Risks & Alternatives

1. Why Corrosion Inhibitors Are Essential in Copper CMP

In copper CMP, the oxidizer (H₂O₂) must be present throughout the polishing process to maintain the chemical softening of the copper surface that makes mechanical removal efficient. But the oxidizer cannot distinguish between copper that should be removed (the overburden) and copper that should be preserved (the recessed copper in completed damascene features). Without a mechanism to protect recessed copper, the oxidizer would dissolve it at the same rate as the overburden — destroying the interconnect geometry before planarization is even complete.

The static etch rate (SER) is the quantitative measure of this uncontrolled dissolution: the rate at which copper is removed by the slurry in the complete absence of mechanical pad contact. A copper CMP slurry with SER of 10 nm/min and a Step 2 process time of 120 seconds would dissolve 20 nm of copper from every exposed copper surface — including the recessed trench features — causing catastrophic dishing.

Corrosion inhibitors suppress SER by forming a protective film on the copper surface that physically blocks the oxidizer from accessing the metallic copper below. This film must be stable in the absence of mechanical disruption (protecting recessed features) but readily disrupted at the mechanical pad-asperity contact points where material removal is intended (enabling protrusion removal). This combination of properties — “stable unless mechanically disturbed” — is what makes BTA ideally suited for copper CMP.

2. BTA: Molecular Structure & Surface Chemistry

Benzotriazole (C₆H₅N₃, 1H-benzotriazole) is a bicyclic aromatic compound consisting of a benzene ring fused to a 1,2,3-triazole ring. Three nitrogen atoms in the triazole ring give BTA its copper-binding capacity: the lone pair electrons on N1 and N3 coordinate with copper surface atoms, while the aromatic π-system provides additional stabilization through hydrophobic interaction with adjacent BTA molecules on the surface.

BTA interacts with the copper surface through two distinct mechanisms depending on the copper oxidation state:

• On Cu⁰ (metallic copper): BTA adsorbs through physisorption and weak chemisorption, forming a reversible molecular monolayer. This adsorption alone provides limited corrosion protection — it is the secondary reaction with Cu⁺ that produces the robust passivation film.
• On oxidized copper (Cu⁺/Cu²⁺ ions): BTA reacts with Cu⁺ ions (produced by H₂O₂ oxidation of the copper surface) to form an insoluble, polymeric coordination compound: [Cu(I)(BTA)]ₙ. This polymer is a one-dimensional chain complex where each Cu⁺ center is bridged by two BTA molecules through N1–Cu–N3 coordination bonds, forming a film that is thermodynamically stable, mechanically robust, and essentially insoluble in the pH 4–9 range.

The Cu(I)-BTA polymer film grows to a thickness of approximately 2–5 nm under typical CMP conditions — thin enough to be mechanically disrupted at moderate pad-asperity contact pressures (0.5–3 psi) but thick enough to provide effective corrosion suppression on undisturbed copper surfaces. The film’s hydrophobic outer surface (from the aromatic BTA rings) further reduces the diffusion rate of H₂O₂ to the underlying copper.

3. The Cu-BTA Passivation Layer: Properties & Function

The Cu(I)-BTA passivation layer has several key physical and chemical properties that make it uniquely suited to copper CMP:

• Insolubility: The solubility product of Cu(I)-BTA polymer is extremely low (K_sp ≈ 10⁻²⁰ M²), making the film thermodynamically stable against dissolution in all CMP-relevant pH ranges. The film does not simply dissolve and re-form — it requires mechanical disruption to be removed.
• Thermal stability: Cu(I)-BTA is stable up to at least 60°C, well above typical CMP pad interface temperatures (40–55°C). This ensures consistent passivation behavior across the range of thermal conditions encountered during polishing.
• Mechanical compliance: The polymer film is sufficiently compliant that it does not significantly alter the friction coefficient at the pad-wafer interface at the BTA concentrations used in production, making the friction-based endpoint signal reliable.
• Self-repairing: After mechanical disruption at a pad-asperity contact event, BTA from the slurry solution rapidly re-adsorbs and reforms the passivation layer on the newly exposed copper surface within milliseconds — faster than the next asperity contact event at typical pad rotation speeds. This self-repair mechanism is what allows BTA to provide consistent protection across thousands of contact-disrupt-reform cycles during a polishing run.

4. BTA Concentration Optimization

BTA concentration is the most sensitive and consequential tuning parameter in copper CMP slurry formulation. The relationship between BTA concentration and the key performance metrics of dishing, removal rate, and SER is highly non-linear:

The optimal BTA concentration for any specific application depends on: the target dynamic removal rate (set by throughput requirements), the target SER (set by the dishing specification), the oxidizer concentration (higher H₂O₂ requires higher BTA to maintain the same SER), and the polishing time (shorter processes can tolerate slightly higher SER because less chemical exposure time means less cumulative dishing).

In JEEZ’s formulation optimization process, BTA and H₂O₂ concentrations are always co-optimized through a two-dimensional DoE that maps the SER vs. removal rate response surface simultaneously — identifying the Pareto-optimal formulation point that minimizes SER without sacrificing throughput below the customer’s acceptance criterion.

5. Galvanic Corrosion at Cu/Barrier Interfaces

When two dissimilar metals are in electrical contact in an electrolyte, a galvanic cell is established: the metal with the lower standard reduction potential (more negative E°) acts as the anode and is preferentially oxidized, while the metal with the higher E° is cathodically protected. In copper CMP, the copper/barrier metal interface creates exactly this situation when the barrier is partially exposed during Step 2 polishing.

Standard electrode potentials (vs. SHE in aqueous solutions):

• Cu²⁺/Cu⁰: E° = +0.34 V (copper is relatively noble)
• Ta⁵⁺/Ta⁰: E° = −0.60 V (tantalum is less noble than Cu — but Ta’s native oxide provides kinetic protection)
• Co²⁺/Co⁰: E° = −0.28 V (cobalt is less noble than Cu — and lacks Ta’s oxide kinetics protection)
• Ru²⁺/Ru⁰: E° ≈ +0.45 V (ruthenium is slightly more noble than Cu — inverted galvanic pair)

For Ta and TaN barriers, the large potential difference with copper would predict severe galvanic corrosion of Ta in the electrolyte — but Ta’s native Ta₂O₅ oxide film forms spontaneously and kinetically passivates the surface against aqueous attack, making galvanic corrosion of Ta minimal in practice. BTA provides effective copper protection at the Cu/Ta interface.

For Cobalt (Co) liners, the situation is more problematic. Co lacks the spontaneous, kinetically protective oxide that shields Ta, making it susceptible to galvanic attack in the oxidizing, slightly acidic environment of standard Step 2 slurry. At the Cu/Co interface, Co acts as the anode and is preferentially dissolved — potentially pitting the liner before the copper is fully cleared. Standard BTA has minimal inhibition efficiency on Co surfaces, requiring alternative inhibitor systems that provide Co-specific passivation.

For Ruthenium (Ru) liners, the galvanic polarity is inverted — Ru (E° ≈ +0.45 V) is slightly more noble than copper. This means copper is the anode at the Cu/Ru interface and is preferentially dissolved — potentially causing enhanced dishing at the copper/Ru liner edge. JEEZ’s Ru-compatible CuS formulations address this through careful pH and oxidizer concentration control that limits the galvanic current density at the Cu/Ru interface to levels consistent with acceptable dishing.

6. BTA-Free Alternatives for 2026

As of July 2026, several regulatory and environmental programs are scrutinizing BTA and its environmental behavior. BTA is not classified as a PFAS compound, but its aquatic toxicity and persistence have drawn attention in the context of increasingly strict industrial wastewater discharge standards in several manufacturing regions. This has motivated a growing number of fabs — particularly in Europe and some Asian markets — to seek BTA-free copper CMP slurry alternatives.

Mercaptobenzimidazole (MBI)

2-Mercaptobenzimidazole (MBI, C₇H₆N₂S) is the most technically mature BTA alternative in copper CMP slurry. MBI forms a protective film on copper through both N–Cu and S–Cu coordination bonds, providing stronger adsorption than BTA on a molar basis and slightly lower SER at equivalent concentrations. Its aquatic toxicity profile is considered more acceptable than BTA in some regulatory frameworks.

The challenge with MBI is its tendency to precipitate as a sparingly soluble Cu(II)-MBI complex in high-oxidizer environments, potentially contributing to particle counts in the slurry. Formulation with MBI requires careful pH control (optimal range pH 5.5–7.5) and oxidizer concentration moderation to prevent precipitation.

Azole Derivatives

JEEZ’s CuG BTA-free series uses a proprietary azole-derivative inhibitor blend that provides equivalent passivation efficiency to BTA at the same mass concentration, with improved Co/Ru surface compatibility and a more favorable environmental persistence profile. Patterned wafer validation of CuG formulations against JEEZ CuS BTA-containing baselines shows equivalent dishing and erosion performance within measurement uncertainty at both 28 nm and 7 nm-class process nodes.

Imidazole and Triazole Derivatives

Various substituted triazoles (e.g., 3-amino-1,2,4-triazole, 5-methyl-1H-benzotriazole) and imidazole derivatives have been evaluated in the academic and patent literature as BTA alternatives. The general trend is that electron-donating substituents on the benzene ring of benzotriazole improve inhibition efficiency and reduce the optimal concentration needed, while electron-withdrawing substituents reduce inhibition efficiency. No single alternative has yet achieved the broad production qualification and supply chain maturity that BTA enjoys, but 2026 represents the most active period of BTA-alternative qualification since copper CMP was introduced commercially.

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7. Frequently Asked Questions