Boron-Doped SiGe (B:SiGe) for PMOS Source/Drain is the in-situ doped epitaxial material grown in the source/drain regions of PMOS transistors that simultaneously provides compressive channel strain for hole mobility enhancement and heavy boron doping for low contact resistance — where the germanium concentration (25-60 at%), boron doping level (1-5 × 10²⁰/cm³), and epitaxial layer geometry are precisely engineered to maximize PMOS drive current while maintaining crystal quality and avoiding relaxation defects.
Why B:SiGe for PMOS
- Silicon channel: Hole mobility is ~2.5× lower than electron mobility → PMOS is inherently slower.
- Compressive strain: SiGe has larger lattice than Si → compressed channel → splits valence band → 40-60% mobility boost.
- Higher Ge%: More strain → more mobility gain, but risk of relaxation defects.
- In-situ boron: Eliminates S/D implant step → junction abruptness → lower resistance.
B:SiGe S/D Process Flow
1. S/D recess etch: Remove Si from S/D regions (typically 30-60nm deep). 2. Pre-epitaxy clean: HF + H₂ bake → remove native oxide from recess. 3. SiGe nucleation: Thin undoped SiGe buffer → smooth interface. 4. B:SiGe growth: Main stressor layer with target Ge% and B doping. 5. Optional Si cap: Thin Si layer for silicide contact formation.
Ge Content and Strain
| Ge Content | Lattice Mismatch | Channel Strain | Mobility Gain | Risk |
|---|---|---|---|---|
| 25% | 1.0% | Moderate | ~25% | Low |
| 35% | 1.4% | High | ~40% | Medium |
| 45% | 1.8% | Very high | ~55% | Higher |
| 60% | 2.5% | Maximum | ~70% | Relaxation risk |
Boron Doping
- Target: 1-5 × 10²⁰ /cm³ (extremely high → metallic-like conductivity).
- In-situ: B₂H₆ or BCl₃ co-flowed during epitaxial growth → incorporated during crystal formation.
- Advantages over implant: No implant damage, atomically abrupt junction, no need for activation anneal.
- Challenge: High B concentration depresses growth rate → recipe adjustment needed.
- B segregation: B tends to segregate to surface → graded doping profile.
Epitaxy Challenges
| Challenge | Cause | Mitigation |
|---|---|---|
| Relaxation | Exceeding critical thickness at high Ge% | Multi-step Ge grading |
| Dislocations | Lattice mismatch strain relief | Optimize recess geometry |
| Ge non-uniformity | Gas depletion, loading effects | Multi-zone gas delivery |
| Faceting | Crystal-orientation-dependent growth | Temperature/pressure tuning |
| Boron out-diffusion | Later thermal steps diffuse B | Minimize thermal budget |
| Pattern-dependent growth | Dense vs. isolated features grow differently | Dummy pattern insertion |
FinFET/GAA Specific Considerations
- FinFET: S/D epi grows from narrow fin → diamond-shaped cross-section.
- Merged fins: Adjacent fins' epi merges → larger contact area → lower resistance.
- GAA nanosheet: Epi wraps around multiple sheets → complex 3D growth.
- Higher Ge at top: Graded Ge profile → more strain closer to channel.
Boron-doped SiGe source/drain epitaxy is the single most impactful PMOS performance enhancement in modern CMOS technology — by combining strain engineering (Ge content), doping engineering (in-situ B), and geometric optimization (recess depth and shape) in one process step, B:SiGe S/D delivers the 40-60% PMOS mobility improvement that closes the gap with NMOS performance and enables the balanced circuit speeds required for competitive logic products at every node from 22nm through 2nm and beyond.
Explore 500+ Semiconductor & AI Topics
From EUV lithography to CUDA optimization — search the full knowledge base or chat with our AI assistant.