Post-Apply Bake (PAB) — also called soft bake or pre-bake — is the thermal treatment performed immediately after coating the photoresist onto the wafer, before exposure. Its primary purpose is to evaporate residual solvent from the resist film and improve film quality.

Why PAB Is Needed

• After spin-coating, the resist film still contains 5–15% residual solvent. This solvent must be removed because:
• Excess solvent changes the resist's optical and chemical properties, affecting exposure sensitivity.
• Solvent in the film can cause adhesion problems and contaminate the exposure tool.
• Resist film thickness and uniformity are affected by solvent content.

What PAB Does

• Solvent Evaporation: The primary function — reduces residual solvent to typically 1–3% of the film.
• Film Densification: Drives the resist polymer chains closer together, creating a denser, more uniform film.
• Adhesion Improvement: Thermal treatment improves resist-to-substrate adhesion by enabling better molecular interaction with the wafer surface or adhesion promoter (HMDS).
• Stress Relaxation: Relieves mechanical stresses introduced during spin-coating.

Typical PAB Conditions

• Temperature: 90–110°C for most CARs. Must stay well below the PAG activation temperature to avoid premature acid generation.
• Time: 60–90 seconds on a hotplate (the standard method in semiconductor fabs).
• Equipment: Proximity hotplate (wafer hovers ~100 µm above the plate surface via proximity pins) for uniform heating and controlled cooling.

Critical Parameters

• Temperature Uniformity: The hotplate must maintain ±0.1°C uniformity across the wafer — temperature variations directly translate to film thickness and sensitivity variations.
• Bake Time Control: Consistent bake time ensures reproducible solvent content — even small variations affect CD.
• Cool-Down: After PAB, the wafer is placed on a chill plate (23°C) to stop the bake process and bring the wafer to a defined temperature for the next step.

PAB vs. Other Bakes

• PAB (Post-Apply Bake): After coating, before exposure. Removes solvent.
• PEB (Post-Exposure Bake): After exposure, before development. Drives acid-catalyzed reactions in CARs.
• Hard Bake: After development. Cross-links resist for etch resistance.

PAB is a seemingly simple but critical step — small variations in bake temperature or time can propagate through exposure and development, causing measurable CD shifts in the final pattern.