Wet Clean Systems

Wet Clean Chemistry

Wet cleaning removes particles, metals, and organic contaminants between process steps. A wafer is cleaned 100+ times during fabrication. Standard cleaning sequences:

• SC-1 (APM): NH₄OH:H₂O₂:H₂O — removes particles and organic contamination by etching a thin oxide layer and lifting particles off. The workhorse clean.
• SC-2 (HPM): HCl:H₂O₂:H₂O — removes metallic contamination (Fe, Al, Mg) by dissolving metals as soluble chloride complexes.
• HF dip: Dilute HF (0.5–1%) — removes native oxide and some embedded metals. Also used for oxide etch.
• SPM (Piranha): H₂SO₄:H₂O₂ — aggressive organic removal for heavy contamination (e.g., post-implant resist stripping).

Megasonic and Post-CMP Cleans

Two specialised wet processes deserve their own attention because they protect yield in very different ways.

Megasonic cleaning

• Acoustic transducers excite the cleaning chemistry at ~0.8–2 MHz (well above the 20–60 kHz of legacy ultrasonics)
• Acoustic streaming and microbubble cavitation lift sub-100 nm particles off the wafer surface
• The shorter wavelength is gentle enough to spare 10–20 nm features — ultrasonics would shatter them

Post-CMP clean (PCMP)

After Cu CMP the wafer carries slurry residue, polishing-pad debris, and dissolved copper. A typical PCMP sequence on a single-wafer tool:

1. Brush scrub with PVA brushes in alkaline (NH₄OH) chemistry — bulk slurry removal
2. Dilute organic complexer (e.g., citric acid, BTA) — chelates and lifts Cu residue without etching the lines
3. Megasonic rinse in DI water — removes sub-100 nm particles
4. IPA / Marangoni dry — leaves a defect-free, watermark-free surface