native oxide removal

**Native oxide removal** is a critical wafer surface preparation step that **strips the thin layer of silicon dioxide (SiO₂)** that naturally forms on exposed silicon surfaces when they come in contact with air or water. This native oxide — typically **1–2 nm thick** — must be removed before key process steps to ensure proper interface quality. **Why Native Oxide Forms** - Silicon is highly reactive with oxygen. Even at room temperature, exposure to air or DI water causes a thin SiO₂ layer to grow on bare silicon within minutes. - This oxide is amorphous, non-stoichiometric, and often contains trapped contaminants — making it unsuitable as a controlled dielectric. **Why It Must Be Removed** - **Pre-Epitaxy**: Native oxide between the substrate and epitaxial layer creates **crystal defects** and prevents proper single-crystal growth. - **Pre-Gate Oxide**: Native oxide beneath a thermally grown gate oxide degrades **dielectric integrity**, increasing leakage and reducing reliability. - **Pre-Contact/Silicide**: Native oxide in contact openings creates **high resistance** interfaces, increasing contact resistance and degrading device performance. - **Pre-Deposition**: Native oxide can affect **film adhesion** and nucleation behavior for deposited films. **Removal Methods** - **HF Dip (Wet)**: The most common method. Dilute hydrofluoric acid (typically **1:100 HF:H₂O** or 0.5% HF) dissolves SiO₂ selectively without attacking silicon. Leaves a **hydrogen-terminated** surface that is temporarily passivated against re-oxidation. - **Vapor HF**: Gas-phase HF removes native oxide without immersing the wafer in liquid — useful for delicate structures or when liquid processing is undesirable. - **In-Situ Thermal Desorption**: Heat the wafer to **~850°C in vacuum** or hydrogen ambient. The native oxide decomposes and desorbs as SiO gas. Used in epitaxy chambers. - **Plasma-Based**: Hydrogen or argon plasma can reduce or sputter away native oxide at lower temperatures. - **Chemical Oxide Removal (COR)**: Uses NH₃/HF gas mixtures at low temperature to selectively remove SiO₂ by forming a volatile reaction product. **Timing Sensitivity** - After HF dip, native oxide **regrows within minutes** in air. The wafer must be processed quickly — typical queue time limits are **30 minutes to 2 hours** between HF clean and the next process step. - This "queue time" management is a major fab logistics challenge. Native oxide removal is one of the **most frequently performed** and **most time-sensitive** steps in semiconductor manufacturing — getting it right directly impacts device yield and performance.