Wafer Resist Strip (Step )
Resist strip, also known as photoresist removal, is the final stage of a lithography cycle. After etching, implantation, or deposition, the remaining photoresist is no longer needed and must be completely removed before the next process step. Incomplete removal can cause contamination, yield loss, or defects in subsequent layers, making resist strip critical for maintaining wafer quality.
Process Overview
- Purpose: Eliminate residual photoresist and organic contamination after pattern transfer.
- Techniques: Wet stripping (solvent or oxidizing solutions) and dry stripping (plasma ashing).
- Iteration: Performed after every lithography cycle — repeated dozens of times per wafer.
- Integration: Often paired with post-etch cleaning or surface conditioning.
Wet vs Dry Resist Stripping
| Method | Process | Advantages | Constraints |
|---|---|---|---|
| Wet Strip | Uses chemical solvents (NMP, DMSO, EKC) or oxidizers (H2SO4/H2O2) to dissolve resist. | Effective bulk removal; lower tool cost. | Chemical waste handling; less effective for advanced resists; potential substrate attack. |
| Dry Strip (Plasma Ashing) | O2 plasma oxidizes and volatilizes resist residues in a vacuum chamber. | High uniformity, residue-free, compatible with nanoscale nodes. | Requires vacuum systems; generates plasma-induced damage if not controlled. |
Major Equipment Vendors
- Lam Research (U.S.): Plasma ashing and dry strip systems integrated with etch platforms.
- Tokyo Electron Limited (TEL, Japan): Dry strip and cleaning systems for advanced nodes.
- SCREEN Semiconductor Solutions (Japan): Wet cleaning and strip systems, particularly for legacy processes.
- Applied Materials (U.S.): Integrated strip and clean solutions in cluster tools.
Process Consumables
- Solvents: NMP (N-Methyl-2-pyrrolidone), DMSO, and commercial EKC formulations.
- Oxidizers: Sulfuric acid + hydrogen peroxide mixtures (SPM) for legacy wet strip.
- Process Gases: O2, CF4, or forming gas in dry strip plasma ashing.
- Ultrapure Water (UPW): Final rinse and wafer drying after wet strip processes.
Cleanroom & Environment
- Performed in Class 1–10 cleanroom bays, often integrated with etch and clean modules.
- Plasma tools require RF generators, vacuum pumps, and exhaust abatement for volatile byproducts.
- Wet strip chemicals are hazardous and require closed-loop chemical handling and recycling systems.
Advantages & Constraints
- Advantages: Ensures clean wafer surfaces for the next process; enables reliable multi-layer integration.
- Constraints: Environmental and safety challenges with solvents; plasma-induced damage risks; cost of chemical waste management.
Market Outlook
Dry plasma strip dominates at advanced nodes due to superior precision and compatibility with fragile device structures. Wet strip remains common for legacy, MEMS, and compound semiconductor fabs. Market growth is tied to lithography intensity — every new node increases the number of resist strip cycles per wafer.