Wet stripping is not a single soaking step but a systematic process comprising pre-treatment, stripping reaction, multi-step rinsing, drying, and inspection. Each stage is interconnected and interdependent; deviations in parameters at any step may lead to issues like photoresist residue, surface corrosion, or structural damage.

(I) Process Pre-Treatment: Foundational Step for Ensuring Reaction Uniformity

The core objective of pre-treatment is to remove contaminants from the wafer surface, eliminate barriers to solution contact, and ensure uniform stripping. This stage involves two key operations:First, wafer surface cleaning employs megasonic cleaning technology. High-frequency acoustic waves generate micro-bubbles that collapse to produce impact forces, removing particulate contaminants (e.g., resist slag from lithography, environmental dust). The cleaning medium is typically ultrapure water with trace non-ionic surfactants to enhance decontamination. Cleaning time is controlled at 30–60 seconds to avoid resist swelling or deformation from prolonged exposure.Second, drying treatment involves placing the cleaned wafer in a hot air drying oven at 80–100°C for 3–5 minutes to thoroughly remove surface moisture. Residual moisture would locally dilute the stripping solution, causing uneven stripping—particularly prone to residue accumulation at wafer edges.

(II) Core Stripping: Key Stage for Precise Reaction Control

Based on photoresist type and process stage, the core stripping stage includes three mainstream paths, each with distinct parameter controls:

1.Organic Stripping: Primarily used for unexposed negative photoresist or some positive photoresist. Common solvents include N-methylpyrrolidone (NMP) and dimethyl sulfoxide (DMSO). Process parameters: temperature 60–80°C (balances dissolution efficiency and solvent evaporation); soaking time 5–10 minutes (adjusted by resist thickness); mechanical stirring (50–100 rpm) to enhance solution flow and prevent redeposition of dissolved residue. Advantage: no corrosion to metal layers, suitable for wafers with aluminum/copper wiring.

2.Alkaline Stripping: Mainstream method for positive photoresist. Uses ammonium hydroxide (NH₄OH)-water mixtures (1–5% concentration) or specialized alkaline strippers (e.g., hydroxylamine-containing compounds). Alkaline substances break carboxyl groups in photoresist, degrading it into water-soluble fragments. Parameters: temperature 40–60°C, soaking time 3–8 minutes, nitrogen bubbling for uniform reaction. For highly crosslinked exposed resists, small amounts of oxidizers (e.g., hydrogen peroxide) are added to enhance degradation.

3.Acidic Oxidation Stripping: Used for stubborn photoresist after ion implantation (which carbonizes and crosslinks the resist). Typical solution: sulfuric acid (H₂SO₄)-hydrogen peroxide (H₂O₂) mixture (volume ratio 3:1–5:1), known as "piranha solution." This strongly oxidizing solution converts carbonized resist to CO₂ and H₂O. Parameters: temperature 100–130°C, reaction time 2–5 minutes. Due to extreme corrosivity, quartz reaction tanks and exhaust gas treatment systems (for SO₂, O₃, etc.) are required.

(III) Multi-Step Rinsing: Safeguard Against Residues

After stripping, the wafer surface retains residual chemicals, degraded resist byproducts, etc. Incomplete removal impairs subsequent film deposition or metal wiring conductivity. Rinsing uses a "gradient rinsing + megasonic enhancement" approach:

•Initial Deionized Water Rinse: Immerse the wafer in deionized water for 1–2 minutes to preliminarily flush away bulk stripping solution.

•Megasonic Cleaning: Apply high-frequency megasonic waves (800–1200 kHz) in deionized water for 30–60 seconds to remove micro-residues and adsorbates via vibration.

•Precision Spray Rinsing: Use ultrapure water (resistivity ≥18.2 MΩ·cm) with spray pressure 0.1–0.3 MPa for thorough surface cleaning.

•Ion-Exchanged Water Soaking: Soak for 1–2 minutes to further reduce ionic contamination.

(IV) Drying Treatment: Final Step to Prevent Secondary Contamination

Drying requires rapid, watermark-free, and particle-free results. Common methods combine centrifugal and hot air drying: First, centrifuge the wafer at 1000–1500 rpm for 30–60 seconds to remove most moisture. Then, dry in a hot air circulation oven at 60–80°C, humidity ≤30%, for 5–10 minutes. For high-precision wafers, isopropyl alcohol (IPA) vapor drying is used, leveraging IPA’s hydrophilicity and volatility for watermark-free drying and enhanced cleanliness.

(V) Quality Inspection: Key Node for Closed-Loop Control

Inspection focuses on three core metrics:

1.Residue Detection: Use optical microscopy (500–1000x magnification) or atomic force microscopy (AFM) to ensure no visible residue or slag.

2.Surface Roughness: Measure Ra value via AFM; requirement: Ra ≤0.5 nm to avoid surface damage.

3.Ionic Contamination: Use an ion chromatograph to measure metal ions (e.g., Na⁺, K⁺, Cu²⁺); limit: ≤1×10¹⁰ atoms/cm² per ion.

If standards are unmet, the wafer returns to repeat the stripping-rinsing process with adjusted parameters.