Chemical Mechanical Polished Hybrid Bonding Copper: Advanced Planarization Strategies And Integration Challenges

Fundamental Mechanisms Of Chemical Mechanical Polishing For Copper In Hybrid Bonding Applications

Chemical mechanical polishing of copper in hybrid bonding contexts operates through synergistic chemical oxidation and mechanical abrasion mechanisms. The chemical component typically involves oxidizing agents such as hydrogen peroxide (H₂O₂) or ferric nitrate that continuously form a copper oxide (CuO or Cu₂O) passivation layer on the copper surface 3. This oxide layer exhibits distinct mechanical properties—lower hardness and higher brittleness compared to metallic copper—facilitating preferential removal by abrasive particles (commonly silica, alumina, or ceria) suspended in the CMP slurry 3. The mechanical component simultaneously abrades elevated copper features while the chemical component dissolves abraded material and passivates recessed areas, minimizing over-etching and dishing 4. In hybrid bonding scenarios, this dual-action mechanism must be precisely balanced: excessive oxidation leads to increased static etch rates and copper loss in recessed bond pads, whereas insufficient oxidation results in inadequate removal rates and poor planarization 8. Recent formulations incorporate dual chelators and corrosion inhibitors (e.g., benzotriazole derivatives, tetrazole-triazole combinations) to achieve high copper removal rates (>200 nm/min) while maintaining low static etch rates (<5 Å/min), critical for preserving copper pad integrity during the overpolish step required in hybrid bonding 919.

The second mechanism, relevant when no protective oxide forms, involves direct chemical attack and dissolution of copper by complexing agents (e.g., glycine, oxalic acid, amino polycarboxylic acids) in the slurry, with mechanical action enhancing dissolution kinetics by exposing fresh surface area, elevating local temperature via frictional heating, and thinning the diffusion boundary layer 716. For hybrid bonding copper CMP, the first mechanism (oxidation-passivation-abrasion) is predominantly employed due to superior control over selectivity between copper and barrier materials (TaN, Ta) and between copper and dielectric layers (SiO₂, low-k materials) 112.

Copper Dishing Mitigation And Surface Topography Control In Hybrid Bonding

Copper dishing—the depression of copper bond pad surfaces below the surrounding dielectric—is the most critical defect in hybrid bonding CMP, as it directly impairs Cu-Cu contact formation and bonding yield 110. Dishing severity scales with bond pad size: larger pads (>50 μm diameter) exhibit dishing depths exceeding 20–50 nm after conventional CMP, sufficient to prevent effective bonding 1. Two primary factors drive dishing during the overpolish phase: continued mechanical aggression as the polishing pad conforms around features and maintains contact with recessed copper lines, and ongoing chemical etching due to prolonged slurry exposure 10.

To mitigate dishing in hybrid bonding applications, a multi-step CMP strategy is employed 114. The first step (bulk copper removal) uses aggressive slurries with high abrasive loading (0.5–2 wt% silica or alumina), high oxidizer concentration (1–3 wt% H₂O₂), and high down-force (3–5 psi) to rapidly remove 1–2 μm of electroplated copper overburden, stopping when ~500–1000 Å of copper remains 14. The second step (copper clearing and barrier stop) employs a less aggressive slurry with lower abrasive content (0.1–0.5 wt%), reduced oxidizer (0.6–1.5 wt% H₂O₂), lower down-force (1–2 psi), and enhanced passivation additives (50–200 ppm benzotriazole or branched-alkylphenol-substituted benzotriazole) to selectively remove residual copper while stopping on the barrier layer with minimal dishing 48. A critical innovation for hybrid bonding involves a third step: selective barrier removal followed by a non-metal or barrier CMP process that converts the dished copper surface into a convex profile 1. This is achieved by partially removing the oxide dielectric (e.g., 10–30 nm recess) to protrude the copper pad above the dielectric plane, then applying a copper passivation slurry (pH 9–11, low abrasive, high inhibitor concentration) that preferentially polishes the protruding copper edges, creating a slight convex curvature (5–15 nm protrusion at pad center) 1. During subsequent bonding, this convex copper surface on one wafer mates with the dished concave on the mating wafer, compensating for topography mismatch and ensuring intimate Cu-Cu contact across the entire bond pad area 1.

Quantitative control of dishing requires real-time monitoring of removal rates and endpoint detection. Optical interferometry and eddy current sensors are integrated into CMP tools to measure copper thickness and detect barrier layer exposure with <10 Å precision 12. Slurry formulations are optimized for high copper-to-barrier selectivity (>50:1) and high copper-to-oxide selectivity (>100:1) to minimize dielectric erosion, which would otherwise compromise oxide-oxide fusion bonding 1217.

Advanced CMP Slurry Formulations For Hybrid Bonding Copper: Composition And Performance Optimization

State-of-the-art CMP slurries for hybrid bonding copper integrate multiple functional components to achieve the stringent requirements of high removal rate, low dishing, low defectivity, and excellent surface passivation 568. A representative formulation comprises:

• Abrasive particles (0.01–1 wt%): Colloidal silica (20–100 nm diameter) or fumed alumina (50–150 nm) provide mechanical abrasion. Low-solids slurries (<0.5 wt%) are preferred for hybrid bonding to reduce scratching and particle-induced defects 56. Negatively charged polymer-treated alpha-alumina has been reported to enhance selectivity and reduce copper redeposition 11.

• Oxidizing agents (0.6–3 wt%): Hydrogen peroxide (H₂O₂) is the primary oxidizer, generating Cu²⁺ ions and forming CuO. Dual oxidizer systems (e.g., H₂O₂ combined with periodate or persulfate) are employed in advanced nodes to sustain high removal rates (>300 nm/min) while maintaining low static etch rates (<3 Å/min) 89.

• Copper-complexing agents (0.1–1 wt%): Amino polycarboxylic acids (e.g., ethylenediaminetetraacetic acid, EDTA), glycine, oxalic acid, or citric acid chelate dissolved Cu²⁺ ions, preventing redeposition and facilitating transport away from the surface 56. Dual chelator systems (e.g., glycine + citric acid) provide synergistic effects, enhancing removal rate uniformity across wafer 89.

• Copper-passivating agents (10–1000 ppm): Benzotriazole (BTA), 5-methyl-1H-benzotriazole, or branched-alkylphenol-substituted benzotriazole adsorb onto copper surfaces via nitrogen lone-pair coordination, forming protective monolayers that inhibit corrosion and reduce static etch rates 245. For hybrid bonding, passivating agents bearing an acidic OH group and an additional oxygen substituent in a 1,6 relationship to the acidic OH group (e.g., certain hydroxybenzotriazole derivatives) have demonstrated superior performance, achieving static etch rates <2 Å/min while maintaining removal rates >250 nm/min 5. Tetrazole-triazole combinations (weight ratio 0.06–0.18) are optimized for copper wiring CMP, providing balanced passivation and removal 19.

• Polyelectrolytes and surfactants (0.01–0.5 wt%): Anionic, cationic, or amphoteric polymers (e.g., polyacrylic acid, polymethacrylic acid, copolymers of poly(ethylene glycol) methyl ether (meth)acrylate and 1-vinylimidazole) modify slurry rheology, stabilize abrasive dispersion, and reduce particle agglomeration 618. Amphiphilic non-ionic surfactants enhance wetting and reduce hydrophobic defects 11. Polyelectrolytes also suppress copper debris accumulation on polishing pads, a major source of wafer defects 6.

• pH adjusting agents: Slurries are typically formulated at pH 6–11. Alkaline pH (9–11) favors oxide formation and passivation, reducing dishing and corrosion, whereas neutral pH (6–8) provides higher removal rates but requires careful passivation control 28. Ammonium hydroxide, potassium hydroxide, or amine-based buffers are used for pH adjustment 11.

• Corrosion inhibitors and antioxidants: Calcium or magnesium ions (10–100 ppm) and organic antioxidants (e.g., ascorbic acid derivatives) prevent galvanic corrosion at Cu-barrier interfaces and minimize copper oxide deposition on wafer surfaces during post-CMP rinsing 11.

Performance benchmarks for hybrid bonding copper CMP slurries include: copper removal rate >200 nm/min, barrier (TaN) removal rate <20 nm/min (selectivity >10:1), oxide removal rate <5 nm/min (selectivity >40:1), static etch rate on copper <5 Å/min, dishing <15 nm for 50 μm pads, surface roughness (Ra) <0.3 nm, and defect density <0.1 defects/cm² 8912.

Process Integration: Multi-Step CMP Sequences And Endpoint Strategies For Hybrid Bonding

Hybrid bonding copper CMP employs a three- to four-step process sequence to sequentially remove copper overburden, clear residual copper, remove barrier material, and prepare the bonding surface 11217:

Step 1: Copper Bulk Removal
Objective: Remove 1–2 μm electroplated copper overburden at high rate (>400 nm/min) with minimal dishing initiation. Slurry: High-solids (1–2 wt% silica), high oxidizer (2–3 wt% H₂O₂), pH 9–10, down-force 4–5 psi, platen speed 100–120 rpm. Endpoint: Optical interferometry detects <500 Å remaining copper thickness 14.

Step 2: Copper Clearing And Soft Landing On Barrier
Objective: Remove residual copper (500–1000 Å) and stop on barrier layer (TaN, Ta) with <10 nm dishing. Slurry: Low-solids (0.3–0.5 wt% silica), moderate oxidizer (1–1.5 wt% H₂O₂), high passivation (100–200 ppm BTA), pH 9–10, down-force 2–3 psi, platen speed 80–100 rpm. Endpoint: Eddy current sensor detects barrier exposure; overpolish time 10–20 s to ensure complete copper clearing 112.

Step 3: Selective Barrier Removal
Objective: Remove barrier layer (20–50 nm TaN/Ta) with high selectivity to copper (<5:1 barrier:copper removal rate) and oxide (<10:1 barrier:oxide removal rate). Slurry: Silica or alumina abrasive (0.5–1 wt%), oxidizer-free or low oxidizer (<0.5 wt% H₂O₂), barrier-selective etchant (e.g., fluoride-based or peroxide-amine complexes), pH 3–5 or 10–11 depending on chemistry, down-force 2–3 psi. Endpoint: Optical or capacitance-based detection of dielectric exposure 1217.

Step 4: Copper Passivation And Convex Profile Formation (Hybrid Bonding-Specific)
Objective: Convert dished copper surface to slight convex profile (5–15 nm protrusion) and passivate copper to prevent oxidation prior to bonding. Process: Partially recess oxide dielectric by 10–30 nm using dilute HF or CMP with oxide-selective slurry, then apply copper passivation slurry (low abrasive <0.1 wt%, high inhibitor 200–500 ppm BTA, pH 10–11, down-force 1–2 psi, 30–60 s) to polish protruding copper edges into convex shape 1. Post-CMP cleaning with dilute citric acid or EDTA solution removes residual slurry particles and passivates copper surface 1.

Step 5 (Optional): Final Oxide Touch-Up Polish
Objective: Ensure co-planarity of copper and oxide surfaces within ±2 nm for optimal hybrid bonding. Slurry: Oxide-selective CMP slurry (ceria-based, pH 10–11), down-force 1 psi, 20–30 s 12.

Endpoint detection strategies are critical: optical interferometry monitors copper thickness in real-time (±5 Å precision), eddy current sensors detect barrier layer exposure (±10 Å precision), and motor current or acoustic emission sensors provide secondary confirmation of material transitions 12. Advanced process control algorithms adjust slurry flow rate, down-force, and platen speed dynamically to compensate for within-wafer and wafer-to-wafer variations, achieving <3% removal rate uniformity (1σ) 8.

Defect Mechanisms And Mitigation Strategies In Hybrid Bonding Copper CMP

Defect formation during copper CMP for hybrid bonding arises from multiple sources: mechanical scratching by agglomerated abrasive particles or pad debris, chemical corrosion (pitting, galvanic attack at Cu-barrier interfaces), copper redeposition (dendrite formation, nanoparticle adhesion), organic residue from slurry additives, and dielectric erosion 61015. Each defect type requires targeted mitigation:

Scratching And Particle-Induced Defects

Scratching results from large abrasive agglomerates (>500 nm), pad debris, or hard particles (e.g., ceria, alumina) embedded in the pad. Mitigation strategies include: (1) using low-solids slurries (<0.5 wt%) with narrow particle size distribution (coefficient of variation <20%) 56, (2) employing soft polishing pads (Shore A hardness 50–60) with optimized groove patterns to enhance slurry transport and debris removal 10, (3) implementing in-situ pad conditioning with diamond disks to maintain pad surface roughness and prevent debris accumulation 10, and (4) post-CMP brush scrubbing with dilute surfactant solutions (0.1–0.5 wt% non-ionic surfactant, pH 10) followed by megasonic cleaning (0.8–1.2 MHz, 5–10 W/cm²) to remove adhered particles 6.

Copper Corrosion And Pitting

Corrosion occurs when passivation is insufficient, particularly during post-CMP rinsing and drying. Pitting is exacerbated by chloride contamination (>10 ppm Cl⁻ in slurry or rinse water) and galvanic coupling between copper and barrier metals 11. Mitigation: (1) maintain high passivation agent concentration (>100 ppm BTA) throughout CMP and post-CMP rinse 25, (2) use deionized water (resistivity >15 MΩ·cm, Cl⁻ <1 ppb) for rinsing 11, (3) apply corrosion inhibitor rinse (e.g., 50