Laboratory silicon wafer cleaning is a critical step in semiconductor processing, aimed at removing organic matter, inorganic contaminants, particles, and oxide layers from the wafer surface to ensure cleanliness and surface activity. The following are common laboratory silicon wafer cleaning steps, mainly based on the RCA cleaning process (Radio Corporation of America standard) and megasonic-assisted cleaning technology.
I. Pre-Cleaning Laboratory Silicon Wafer Preparation
Equipment and Materials:
- Cleaning machine (with megasonic, spray, and heating functions)
- Cleaning tanks (made of chemically resistant materials such as PFA or quartz)
- Deionized water (DI Water, resistivity ≥18 MΩ·cm)
Cleaning Agents:
- SC-1 solution (NH₄OH:H₂O₂:DI Water = 1:1:5, for removing organic contaminants and particles)
- SC-2 solution (HCl:H₂O₂:DI Water = 1:1:6, for removing metal ions and oxides)
- DHF solution (dilute hydrofluoric acid, for removing native oxide layer)
- Nitrogen gun or IPA (isopropanol) drying equipment
Accessories:
- Clean tweezers, carrier baskets, glove boxes (to avoid secondary contamination)
Wafer Preparation:
Place wafers in a clean carrier basket, avoiding direct hand contact (use gloves or tweezers).
If visible stains are present on the wafer surface, pre-wipe with acetone or ethanol (followed by thorough cleaning).
II. Standard Laboratory Silicon Wafer Cleaning Steps
Step 1: Pre-cleaning (Removing Organic Matter)
Purpose: Remove photoresist, grease, and other organic contaminants.
Method:
- Place wafers in a megasonic cleaner with acetone or ethanol, clean ultrasonically for 5–10 minutes.
- Rinse thoroughly with DI water to remove residual solvent.
Note: Adjust megasonic frequency to wafer size (e.g., 40 kHz for 6-inch wafers).
Step 2: SC-1 Cleaning (Alkaline Peroxide Cleaning)
Purpose: Remove organic contaminants, particles, and light metal ions.
Formula: NH₄OH:H₂O₂:DI Water = 1:1:5 (by volume), temperature 70–80°C.
Procedure:
- Immerse wafers in SC-1 solution, activate megasonics and heat to 70–80°C, maintain for 10–15 minutes.
- Rinse thoroughly with DI water until solution pH is near neutral (around 7–8).
Function:
- NH₄OH provides alkaline environment to decompose organics.
- H₂O₂ oxidizes organics and sterilizes.
- Megasonics enhance particle detachment.
Step 3: SC-2 Cleaning (Acidic Peroxide Cleaning)
Purpose: Remove metal ions (e.g., Na⁺, Ca²⁺) and oxides.
Formula: HCl:H₂O₂:DI Water = 1:1:6 (by volume), temperature 70–80°C.
Procedure:
- Immerse wafers in SC-2 solution, activate megasonics and heat to 70–80°C, maintain for 10–15 minutes.
- Rinse thoroughly with DI water until pH is near neutral (around 6–7).
Note: HCl corrodes aluminum components; if wafers contain aluminum, adjust formula (e.g., replace NH₄OH with TMAH).
Step 4: DHF Cleaning (Removing Native Oxide Layer)
Purpose: Remove natural SiO₂ on wafer surface to form hydrogen-terminated surface.
Formula: DHF solution (HF:DI Water = 1:50 by volume), room temperature.
Procedure:
- Immerse wafers in DHF solution for 1–2 minutes.
- Immediately rinse with large amounts of DI water to stop the reaction.
Note: HF is highly corrosive; control time to avoid over-etching.
Step 5: Final Rinse and Drying
Rinse:
- Ultrasonically clean wafers in DI water for 5 minutes to remove remaining contaminants.
Drying:
Use nitrogen gun or IPA dehydration followed by drying (to prevent water stains).
Optional Methods:
- Nitrogen blow-drying: High-purity nitrogen gas for small batches.
- IPA dehydration: Use IPA to displace water, then blow dry (reduces watermarking).
- Hot air drying: Temperature ≤100°C to avoid heat damage.
III. Special Laboratory Silicon Wafer Cleaning Methods
Stubborn Photoresist Removal
Steps:
- Ultrasonic clean with acetone for 10 minutes.
- SC-1 → SC-2 → DHF cleaning (same as standard process).
Note: Acetone is volatile; use in fume hood.
Particle Contamination Control
Enhancements:
- Use megasonics (40–80 kHz) during SC-1/SC-2 cleaning.
- Two-stage DI rinsing (first at room temperature, then at elevated temperature) to remove thermal particles.
Metal Contamination Cleaning
Special Formulas:
- HMK solution (H₂SO₄:H₂O₂:DI Water, for heavy metal removal)
- Aqua regia (HCl:HNO₃:DI Water, for stubborn metal contamination; use with caution)
IV. Precautions
Cleaning Order:
Remove organics (SC-1) → remove metals/oxides (SC-2) → remove oxide layer (DHF).
Do not reverse the order (e.g., acid first may degrade organic structure and reduce effectiveness).
Safety:
Wear protective gear (gloves, goggles), take extra precautions with HF.
Handle waste separately (neutralize acids/bases, collect HF in dedicated containers).
Cleanliness Verification:
Post-cleaning wafer should pass:
- Optical microscopy: Check for particle residue.
- Ellipsometry: Measure oxide thickness (should be <1 nm).
- Contact angle test: Verify hydrophobicity (after HF, contact angle should approach 90°).
Summary:
The core of laboratory wafer cleaning is the staged removal of specific contaminants using megasonics, temperature control, and chemical optimization. A typical process:
Pre-clean (organics) → SC-1 (organics + particles) → SC-2 (metals + oxides) → DHF (oxide layer) → Drying.
Formulas and steps may be adjusted based on wafer application (e.g., omit DHF before oxidation or add acid treatment for lithography prep).
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