Wafer Cleaning: RCA and the SC1/SC2 System
The RCA standard wet process uses SC1 for particles/organics and SC2 for ionic metals, aided by megasonics to lift yield.
1The two-step RCA clean
SC1 (APM: NH₄OH/H₂O₂/H₂O, ~1:1:5) at 60-80°C removes particles and organics via oxidation-dissolution and slight surface etch; SC2 (HPM: HCl/H₂O₂/H₂O, ~1:1:6) at 60-80°C dissolves ionic metals and passivates the surface. Each step needs a full DI rinse (≥18.2 MΩ·cm).
2Physical assist: megasonic & ultrasonic
For sub-micron particles, chemistry alone is not enough; megasonics (0.8-1.7 MHz) adds acoustic streaming and cavitation to lift particles. Lower frequency raises cavitation but also damage risk; fine geometry (≤0.5µm) prefers high-frequency megasonic with controlled power.
3Bath life & safety
SC1/SC2 are two-part systems (alkali or acid + oxidizer); mixing is exothermic, so mix fresh and slowly. Activity decays over time — typical bath life is 4-8 h. Wrong mixing order or overtemperature can cause a violent reaction.