The core of wet photoresist stripping lies in the specific chemical reaction between solvents and photoresist. Depending on the type of photoresist (positive/negative), three primary dissolution methods are employed:

1. Alkaline Stripping Solution

Suitable for positive photoresist, this method leverages alkaline solutions (e.g., tetramethylammonium hydroxide, TMAH) to trigger hydrolysis reactions with ester and ether bonds in the photoresist. These reactions break down the polymer chains into smaller, water-soluble molecules.

Typical conditions: 5%–10% TMAH concentration, 60–80°C temperature, and 5–15 minutes processing time.

Advantages/Drawbacks: Causes minimal damage to silicon substrates but requires strict control of sodium/potassium ion contamination (to avoid compromising downstream processes).

2. Acidic Stripping Solution

Used primarily for negative photoresist, this approach employs a sulfuric acid-hydrogen peroxide mixture (SPM) (volume ratio 7:2:1, as noted). The strongly oxidizing species—bisulfate ions (HSO₄⁻) and hydroxyl radicals (OH·)—oxidize the organic matrix of the photoresist, converting it into water-soluble byproducts.

Performance: Fast stripping speed (~0.3–0.5 μm/min).

Limitations: May corrode underlying metal layers (e.g., aluminum interconnects) if not optimized.

3. Solvent-Based Stripping Solution

This mild method uses organic solvents (e.g., acetone, dichloromethane) to penetrate the photoresist network and induce swelling. As the solvent diffuses, the photoresist softens and dissolves.

Advantages: Low temperature (<50°C), ideal for heat-sensitive substrates (e.g., flexible polymers or low-k dielectrics).

Drawbacks: Slow stripping speed (~0.1–0.2 μm/min) and high volatility of solvents (requiring robust fume extraction and waste management).

This breakdown highlights how solvent chemistry is tailored to photoresist type—balancing speed, substrate compatibility, and process control in wet stripping operations.