Maintaining Sidewall Verticality >85° in Ultra-Thick Copper (>6oz) Etching

1. Etchant System Optimization

• High-selectivity etchant formulation
  Use H₂SO₄-H₂O₂ (3:1 ratio) with 0.5–1.0 g/L benzotriazole (BTA) to reduce side etching by 40–50% compared to FeCl₃ (side etch ratio <0.15).

• Temperature & concentration control

  • Temperature: 50±2°C;

  • H₂O₂ concentration: 8–12% with auto-dosing (etch rate 25–30 μm/min).

2. Process Parameter Tuning

• Spray etching parameters

  • Pressure: 2.5–3.0 bar, 30° nozzle angle;

  • Coverage: 200–300% (reciprocating spray);

  • Conveyor speed: 0.8–1.2 m/min.

• Multi-stage etching

  1. Rough etch (0–140 μm): 35 μm/min, 80° sidewall;

  2. Fine etch (140–200 μm): 25 μm/min, 85°;

  3. Final etch (200–210 μm): Pulsed spray (3:1 on/off), 87°±1°.

3. Mask Technology & Design Compensation

• High-adhesion dry film
  LDI films (e.g., DuPont RISTON® FX-930, 30–40 μm) with etch factor ≥5:1.

• OPC (OptICal Proximity Correction)
  ±5 μm linewidth compensation for features ≤100 μm.

4. Equipment Innovation & Monitoring

• Vacuum-assisted etching
  50–100 mbar environment improves sidewall verticality by 15%.

• In-line metrology
  Laser confocal microscopy (Keyence VK-X3000) provides real-time feedback (±0.5° accuracy).

5. Post-Etching Treatments

• Microetch planarization
  H₂SO₄-Na₂S₂O₈ solution (Ra<0.2 μm).

• Plasma cleaning
  O₂ plasma (500 W, 120s) removes residues.

6. Validation Metrics

7. Challenges & Solutions

• Challenge 1: Etchant aging increases undercut
  Solution: ORP sensor with auto-H₂O₂ dosing (ORP≥800 mV).

• Challenge 2: Incomplete deep trench etching
  Solution: Ultrasonic assistance (40 kHz, 200 W) for 10:1 aspect ratio.

• Challenge 3: Grain boundary attack
  Solution: 0.2% thiourea inhibitor (grain size <20 μm via EBSD).