Source Drain Recess

Source/Drain Recess and Epitaxy is the process of etching a recess into the source and drain regions and refilling with a strained epitaxial layer — engineering channel stress to enhance transistor drive current in advanced CMOS nodes.

Why S/D Epitaxy?

• Strained channel: Deformed Si crystal lattice → altered band structure → higher carrier mobility.
• PMOS: Compressive strain → higher hole mobility (50–100% improvement).
• NMOS: Tensile strain → higher electron mobility.
• S/D epitaxy injects stress directly adjacent to the channel — most effective stress location.

PMOS: SiGe S/D Stressor

• SiGe has ~4% larger lattice constant than Si.
• Epitaxially grown SiGe in S/D tries to maintain Si lattice spacing → compressively strained SiGe.
• Compressive SiGe squeezes channel laterally → compressive channel stress → boosts hole mobility.
• Typical: Si0.6Ge0.4 (40% Ge) → ~1 GPa compressive stress in channel.
• First deployed: Intel 90nm (2003), now universal.

NMOS: SiC or SiP S/D Stressor

• Si:C (carbon in Si) has smaller lattice constant → tensile stress in channel.
• Or n-SiP (Si:P with high P concentration) grown selectively in NMOS S/D.
• Less common than SiGe — tensile stress in NMOS also achieved via SMT and SiN capping.

Process Steps

1. Recess Etch: Dry etch (Cl2/HBr) + selective wet etch to create sigma-shape (diamond) recess.
- Sigma-shape (anisotropic Si etch along <111> planes) maximizes stress transfer to channel.
- Depth: 30–80nm below gate level.
2. Pre-clean: Remove native oxide, contaminants (dilute HF).
3. Selective Epi: CVD SiGe (DCS + GeH4 + HCl) — grows only on Si, not on dielectrics.
4. In-Situ Doping: Boron (PMOS) or phosphorus (NMOS) incorporated during epi growth.
- Boron: B2H6 during growth → p+ contact region.
- High boron: 1–2 × 10²¹ cm⁻³ for low contact resistance.

FinFET SiGe

• Fin recess: More complex — recess must not undercut gate spacer.
• Higher Ge% at leading edge: Intel 14nm → 35% Ge; TSMC 7nm → 45–55% Ge.

S/D epitaxy with stressor materials is the backbone of PMOS performance from 90nm to current-generation FinFET and GAAFET — without SiGe stressors, PMOS performance would lag NMOS by 3x rather than the near-equal drive currents achieved in modern CMOS.