ald process

**Atomic Layer Deposition (ALD)** — depositing ultra-thin films one atomic layer at a time through self-limiting sequential chemical reactions, providing angstrom-level thickness control. **Process Cycle** 1. **Pulse A**: First precursor adsorbs on surface (self-limiting — only one monolayer sticks) 2. **Purge**: Remove excess precursor and byproducts 3. **Pulse B**: Second precursor reacts with adsorbed layer, forming one atomic layer of film 4. **Purge**: Remove excess 5. Repeat cycles for desired thickness (~1 angstrom per cycle) **Key Properties** - **Self-limiting**: Film thickness determined by number of cycles, not time or flow - **Conformality**: Perfect step coverage in high-aspect-ratio features (>100:1) - **Uniformity**: Excellent across 300mm wafer - **Thickness control**: Sub-angstrom precision **Applications in CMOS** - High-k gate dielectric (HfO2): 1-2nm precision critical - Metal gate work function layers - Spacers and liners in FinFET/GAA - Barrier layers in advanced interconnects **Trade-off**: ALD is slow (~1 A/cycle, ~1 sec/cycle) compared to CVD, so it's used only where atomic precision is essential. **ALD** is indispensable at advanced nodes — you cannot build a 3nm transistor without it.