The purity requirement of nitrogen (N₂) used for purging after wafer wet cleaning is critical, as it directly affects wafer surface cleanliness, drying effectiveness, and subsequent process yield. The key analysis is as follows:
01
Core Requirements of Wafer Wet Cleaning
Typical purity: ≥99.999% (5N); some advanced processes (e.g., EUV lithography) may require 99.9999% (6N).
Key impurity controls:
- Oxygen (O₂): <1 ppm (to prevent oxidation of residual contaminants or impact on metal layers).
- Water vapor (H₂O): Dew point ≤ -70°C (to avoid water stains leading to particle agglomeration or corrosion).
- Particulates: <0.1 μm (achieved via filters, e.g., 0.003 μm filter element).
- Other gases: Residual inert gases like argon (Ar) and helium (He) must be below ppb level.
02
Technical Basis
RCA Cleaning Standard: After SC1/SC2 wet cleaning, nitrogen purge is a standard step for rapid drying and forming an inert protective atmosphere.
SEMI Standards:
- F47-1101: Specifies total particle count in nitrogen ≤3.5×10⁴/m³ (≥0.1 μm).
- C79-13: Moisture content must be ≤10 ppb (equivalent to dew point ≤ -70°C).
Industry Practice:
Advanced processes (e.g., 3nm and below) demand stricter nitrogen purity. Some manufacturers adopt on-site nitrogen generation systems (PSA or membrane separation technology) to ensure real-time high-purity nitrogen supply.
03
Key Impact Factors
Consequences of insufficient purity:
- Oxidation risk: Residual O₂ may oxidize metal layers (e.g., Al, Cu), affecting electrical performance.
- Water stain residue: Incomplete H₂O removal may cause particle adhesion or chemical corrosion (e.g., NaCl solution residue).
- Particle contamination: Particles in nitrogen may recontaminate wafer surfaces, reducing yield.
Purge Process Optimization:
- Flow rate and angle: Use laminar or cyclone purge (flow rate 20–30 m/s) to ensure uniform drying of wafer surface.
- Temperature control: Preheat nitrogen to 30–50°C to accelerate moisture evaporation.
- Environmental isolation: Maintain chamber cleanliness during purging (Class 10 or better) to prevent external contamination.
The nitrogen purge purity requirement after wafer wet cleaning is typically above 99.999%, with strict control over O₂, H₂O, and particulate levels. Actual processes must integrate SEMI standards, process node requirements, and equipment capabilities (e.g., filter precision, gas supply systems) in design to ensure a residue-free wafer surface and meet the yield demands of critical steps such as lithography and etching.
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