Etching Alumina: Key Gases Used in Semiconductor Industry

Etching Gases and Mechanism of Alumina (Al₂O₃) in Semiconductor Processes

In semiconductor manufacturing, alumina (Al₂O₃), due to its high dielectric constant, excellent chemical stability, and insulating properties, is widely used in key structures such as gate dielectrics, isolation layers, and passivation layers (e.g., charge trap layers in 3D NAND, isolation oxides in FinFETs). Alumina etching must achieve high precision (nanometer-scale control), high anisotropy (vertical sidewalls), and high selectivity (avoiding damage to the substrate or adjacent materials). The choice of etching gases and their mechanisms are as follows:

I. Mainstream Etching Gases: Synergistic Effect of Cl₂ and BCl₃

Alumina etching typically uses a mixture of Cl₂ and BCl₃, where both complement each other for efficient etching.

Chlorine (Cl₂): Primary Etchant
Cl₂ is the core reactive gas for alumina etching, based on chemical reaction mechanisms. In a plasma environment, Cl₂ molecules ionize into Cl⁺, Cl radicals, and other active species. These chlorine species react with surface Al atoms in alumina, forming volatile aluminum chlorides (e.g., AlCl₃). The simplified reaction:

• AlCl₃ has a low boiling point (~180 °C), allowing it to be pumped away in vacuum, thus removing alumina.

Limitations of Cl₂ alone: reaction rate is relatively slow, etch directionality is poor (prone to lateral etching), requiring other gases for optimization.

Boron Trichloride (BCl₃): Enhancing Anisotropy and Etch Efficiency
BCl₃ addresses Cl₂’s shortcomings through multiple roles:

• Ion bombardment energy: In plasma, BCl₃ ionizes into BCl₂⁺, BCl⁺, etc. Heavy ions are accelerated by the electric field, vertically bombarding the substrate surface. This sputtering enhances anisotropy (suppressing lateral etching, ensuring vertical sidewalls).
• Promoting chlorine reactions: B atoms from BCl₃ decomposition bond with O atoms in alumina, forming B₂O₃. This weakens Al–O bonds, making Al more reactive with Cl to form AlCl₃, thus increasing etch rate.
• Suppressing side reactions and protecting sidewalls: B₂O₃ or boron oxychlorides (e.g., BClO) form a thin passivation film on sidewalls, reducing lateral etching and preventing excessive chlorine attack on the substrate (Si, silicides), thereby improving selectivity.

II. Why Not Use Boron Tetrachloride (BCl₄)?

BCl₄ is rarely used in practice:

• Poor chemical stability: Not a stable compound, it readily decomposes in plasma into BCl₃ and Cl radicals. Its behavior is similar to BCl₃ but more expensive and harder to control.
• Low reaction efficiency: Despite higher chlorine content, the strong B–Cl bonds in BCl₄ make active Cl release less efficient than the Cl₂ + BCl₃ system, potentially reducing etch rate.

III. Key Process Parameter Controls

To achieve precise etching, optimization of parameters is essential:

• Gas ratio: Typically Cl₂:BCl₃ flow ratio is 2:1 to 5:1. Excess Cl₂ reduces anisotropy, while excess BCl₃ lowers etch rate.
• Plasma power: RF power controls ion energy. High power enhances ion bombardment and anisotropy but risks substrate damage; low power improves selectivity.
• Pressure and temperature: Low pressure (1–10 mTorr) extends ion mean free path, improving directionality. Moderate heating (80–150 °C) promotes AlCl₃ volatilization, preventing residue buildup that clogs etch channels.

IV. Application Scenarios and Advantages

The Cl₂ + BCl₃ system is particularly suited for:

• 3D NAND: Patterning alumina charge trap layers, requiring precise depth control to ensure storage performance.
• FinFETs: Etching alumina isolation layers, protecting fins from damage and ensuring leakage requirements are met.

Core Advantages:

• Moderate etch rate (10–50 nm/min)
• High anisotropy ratio (>10:1)
• High selectivity to Si and SiO₂ (>10:1)
  This meets advanced process demands for nanometer-scale precision.

Conclusion

Etching alumina with a Cl₂ + BCl₃ gas mixture relies on the synergy of chemical reactions and physical sputtering, balancing etch rate, anisotropy, and selectivity. It is an indispensable key process in advanced semiconductor manufacturing.

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