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.