descum

**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.