1. Properties and Advantages of Supercritical CO₂
Supercritical CO₂ (scCO₂) combines gas-like diffusivity (≈10⁻⁷ m²/s) and liquid-like solvation power (solubility parameter≈0.5–2.5 (cal/cm³)¹/²) above its critical point (31.1°C, 7.38 MPa). Key features:
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Low surface tension: <5 mN/m, enabling penetration into micron-scale channels;
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Tunable solvation: Adjustable via pressure/temperature for polar/nonpolar contaminants;
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Eco-friendliness: CO₂ is recyclable with no toxic residues.
2. Key Process Steps
(1) Pretreatment & Contaminant Analysis
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Contaminants: Photoresist residues, metal particles (Cu, Sn), flux (rosin, organic acids);
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Pretreatment:
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Mechanical: Ultrasonication (40 kHz) or micro-sandblasting for large particles;
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Chemical: Low-concentration solvents (e.g., acetone) for loose contaminants.
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(2) scCO₂ System Design
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Equipment:
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High-pressure reactor (≥30 MPa), CO₂ pumps, heaters, co-solvent injectors;
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Flow control: CO₂ at 1–5 L/min for dynamic coverage.
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Parameters:
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Temperature: 35–60°C (balances solvation and safety);
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Pressure: 10–25 MPa (higher pressure enhances solvation);
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Time: 10–30 min (adjusted for via depth);
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Co-solvents: 1–5% polar solvents (e.g., ethanol) for polar contaminants.
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(3) Dynamic Circulation Cleaning
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Counterflow circulation: CO₂ injected from via bottom to enhance penetration;
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Pulsed pressure: ±2 MPa fluctuations to induce turbulence.
(4) Post-Processing & CO₂ Recovery
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Depressurization: Gradual pressure reduction to precipitate contaminants (captured by 0.1 μm filters);
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CO₂ recycling: >95% recovery via condensation.
3. Cleaning Validation
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Surface analysis:
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SEM/EDS for residue coverage (<0.1% target);
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White-light interferometry for via wall roughness (Ra<0.5 μm).
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Functional tests:
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Post-plating via resistance (±5% uniformity);
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No delamination/cracks after thermal shock (-55–125°C, 1000 cycles).
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4. Challenges & Solutions
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Challenge 1: Incomplete cleaning at via bottom:
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Solution: Multi-stage nozzles + ultrasonic assistance (20–40 kHz).
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Challenge 2: Nanoparticle residues:
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Solution: Surfactants (e.g., perfluoropolyether, 0.1–0.5 wt%) to encapsulate particles.
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Challenge 3: High equipment cost:
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Solution: Modular design + high-strength alloys (e.g., Inconel 718).
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5. Applications & Economics
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HDI boards: AR=20:1 via cleaning improves plating yield from 85% to 98%;
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3D TSV: Reduces metal contamination in Si vias (resistivity ≤2 μΩ·cm);
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Cost savings: 70% lower wastewater costs vs. wet cleaning (H₂SO₄-H₂O₂).