fin formation
**Fin Formation** is the **process of defining the vertical silicon "fin" that forms the transistor channel in FinFET technology** — the tall, narrow silicon ridge that the gate wraps around on three sides, enabling superior channel control at sub-28nm.
**Why Fins?**
- Planar MOSFET: Gate controls channel from one side only → poor short-channel control at < 28nm.
- FinFET (Intel 22nm, 2011; TSMC 16nm, 2015): Gate wraps around three sides of fin → much better electrostatic control.
- Fin width Wfin controls Vt and SCE: Wfin < Lg/2 required for adequate channel control.
**Fin Patterning Process**
**Lithography + Etch (Old Method, > 22nm)**:
- Coat resist → expose → etch Si directly.
- Limited by litho pitch: Min fin pitch ~80nm at 193nm ArF immersion.
**SADP (Self-Aligned Double Patterning)**:
- Standard for sub-22nm fins.
- Core film patterned at 2× target pitch → spacers formed → core removed → spacer = fin mask.
- Fin pitch halved vs. litho pitch (e.g., 42nm pitch from 80nm litho pitch).
**SAQP (Self-Aligned Quadruple Patterning)**:
- Second SADP iteration halves pitch again → 21nm fin pitch (TSMC 10nm, 7nm).
**Fin Etch**
- Hard mask (SiN or TiN) defined by SADP → fin etch through Si → deep trench.
- Si RIE: HBr/Cl2/O2 plasma, selectivity to SiO2/SiN > 50:1.
- Fin height: 100–200nm above STI. Taller fins = more current per pitch but harder process.
**Fin Reveal (STI Recess)**
- After fin etch + STI fill + STI CMP: Fins buried in oxide.
- Dilute HF wet etch recess STI oxide to expose fin of desired height.
- Fin height ± 2nm control required for Vt uniformity.
**Fin CD and Profile Control**
- Fin width at gate oxide interface: 5–8nm at 7nm node.
- Profile: Slightly tapered (not perfectly vertical) due to etch loading.
- LER (Line Edge Roughness): < 1nm required — contributor to Vt variation.
Fin formation is **the most critical process module in FinFET manufacturing** — fin width, height, profile, and roughness control directly determine transistor performance uniformity and Vt variation across the die.