epi growth

**Epitaxy** — growing a crystalline thin film on a crystalline substrate where the film's crystal structure aligns perfectly with the substrate, enabling precise material engineering. **Types** - **Homoepitaxy**: Same material (Si on Si). Used for high-quality device layers - **Heteroepitaxy**: Different material (SiGe on Si, GaN on sapphire). Enables bandgap and strain engineering - **Selective Epitaxy (SEG)**: Growth only on exposed silicon, not on oxide/nitride. Used for raised S/D **Methods** - **CVD Epitaxy**: Most common for Si, SiGe. Precursors: SiH4, SiH2Cl2, GeH4. Temperature: 500-900C - **MBE (Molecular Beam Epitaxy)**: Ultra-precise, layer-by-layer growth in ultra-high vacuum. Used for III-V devices and research - **MOCVD**: For III-V compounds (GaN, GaAs). Used for LEDs and power devices **Applications in CMOS** - **SiGe S/D**: Compressive stress for PMOS mobility boost (since 90nm node) - **Raised S/D**: Reduce contact resistance in FinFET/GAA - **Si/SiGe Superlattice**: Alternating layers for GAA nanosheet transistors - **Channel SiGe**: Higher hole mobility channel material **Epitaxy** is foundational for modern transistor engineering — every FinFET and GAA device relies on epitaxial layers.