Descum is a brief, low-power plasma treatment applied to a wafer after photoresist development to remove thin residual resist films (scum) that remain in areas that should be completely clear. It cleans up the pattern without significantly affecting the intended resist features.

What Resist Scum Is

• After resist development, the developer should completely dissolve resist in exposed areas (positive tone) or unexposed areas (negative tone).
• In practice, a very thin layer of residual resist (typically 1–5 nm) often remains on the "cleared" surface. This scum can be caused by:
• Insufficient development time.
• Resist footing at the resist-substrate interface.
• Redeposition of dissolved resist material.
• Under-exposure in some regions.

Why Descum Is Needed

• Etch Blocking: Even a thin scum layer can mask the underlying material from etch, causing pattern transfer failures such as incomplete etch or micro-masking.
• Contact Resistance: In contact or via layers, scum at the bottom of openings creates high-resistance interfaces.
• Adhesion: Scum can interfere with adhesion of subsequently deposited films.
• Yield: Consistent descum removes a variable source of pattern transfer error.

Descum Process

• Oxygen Plasma: The most common descum chemistry. O₂ plasma oxidizes and volatilizes organic resist material as CO₂ and H₂O. Typical conditions: low power (50–200 W), low pressure (50–200 mTorr), 10–30 seconds.
• Short Duration: The key is to remove only the thin scum layer without significantly etching the intended resist features or underlying materials.
• Low Power: Gentle plasma conditions minimize ion bombardment damage to the wafer surface.
• End-Point: Usually time-controlled rather than endpoint-detected, since the scum is too thin for reliable endpoint monitoring.

Impact on CD

• CD Trim Effect: Descum simultaneously trims (narrows) resist features slightly, since the plasma attacks all resist surfaces. This effect must be accounted for in the CD budget.
• Typical CD Loss: 2–10 nm of resist width lost during a standard descum. Process engineers account for this by adjusting the target CD at lithography.

Descum is a standard, almost universal step in the lithography-to-etch handoff — it ensures clean pattern transfer by removing the thin resist residues that development alone cannot completely clear.