Cleaning High Aspect Ratio Through-Holes Using Supercritical CO₂ Fluid

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:

• Low suRFace tension: <5 mN/m, enabling penetration into micron-scale channels;

• Tunable solvation: Adjustable via pressure/temperature for polar/nonpolar contaminants;

• Eco-friendliness: CO₂ is recyclable with no toxic residues.

2. Key Process Steps

(1) Pretreatment & Contaminant Analysis

• Contaminants: Photoresist residues, metal particles (Cu, Sn), flux (rosin, organic acids);

• Pretreatment:

  • Mechanical: Ultrasonication (40 kHz) or micro-sandblasting for large particles;

  • Chemical: Low-concentration solvents (e.g., acetone) for loose contaminants.

(2) scCO₂ System Design

• Equipment:

  • High-pressure reactor (≥30 MPa), CO₂ pumps, heaters, co-solvent injectors;

  • Flow control: CO₂ at 1–5 L/min for dynamic coverage.

• Parameters:

  • Temperature: 35–60°C (balances solvation and safety);

  • Pressure: 10–25 MPa (higher pressure enhances solvation);

  • Time: 10–30 min (adjusted for via depth);

  • Co-solvents: 1–5% polar solvents (e.g., ethanol) for polar contaminants.

(3) Dynamic Circulation Cleaning

• Counterflow circulation: CO₂ injected from via bottom to enhance penetration;

• Pulsed pressure: ±2 MPa fluctuations to induce turbulence.

(4) Post-Processing & CO₂ Recovery

• Depressurization: Gradual pressure reduction to precipitate contaminants (captured by 0.1 μm Filters);

• CO₂ recycling: >95% recovery via condensation.

3. Cleaning Validation

• Surface analysis:

  • SEM/EDS for residue coverage (<0.1% target);

  • White-light interferometry for via wall roughness (Ra<0.5 μm).

• Functional tests:

  • Post-plating via resistance (±5% uniformity);

  • No delamination/cracks after thermal shock (-55–125°C, 1000 cycles).

4. Challenges & Solutions

• Challenge 1: Incomplete cleaning at via bottom:

  • Solution: Multi-stage nozzles + ultrasonic assistance (20–40 kHz).

• Challenge 2: Nanoparticle residues:

  • Solution: Surfactants (e.g., perfluoropolyether, 0.1–0.5 wt%) to encapsulate particles.

• Challenge 3: High equipment cost:

  • Solution: Modular design + high-strength alloys (e.g., Inconel 718).

5. Applications & Economics

• HDI boards: AR=20:1 via cleaning improves plating yield from 85% to 98%;

• 3D TSV: Reduces metal contamination in Si vias (resistivity ≤2 μΩ·cm);

• Cost savings: 70% lower wastewater costs vs. wet cleaning (H₂SO₄-H₂O₂).