I. Types of Etchants and Their Mechanisms of Action

1. Acidic Etchants

Core Components: Hydrofluoric acid (HF), nitric acid (HNO₃), phosphoric acid (H₃PO₄), etc.

Application Scenarios:

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  Silicon-based Materials: The mixed solution of HF and nitric acid (e.g., HNA system) achieves isotropic etching via synergistic oxidation-dissolution, suitable for silicon through-vias (TSV) and shallow trench isolation (STI).

  
  

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  Dielectric Layers: Buffered HF (BHF) solution (HF:NH₄F = 6:1) selectively etches SiO₂ (rate: 100 nm/min) by stabilizing fluoride ion concentration, with a selectivity ratio of 100:1 for Si₃N₄.

  Key Parameters: HF concentration must be controlled at 5%–49%, and temperature fluctuation should be within ±1℃ to avoid side reactions (e.g., toxic NO₂ generation from HF and nitric acid).

  
  

2. Alkaline Etchants

Core Components: Potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH).

Application Scenarios:

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  Anisotropic Etching of Silicon: A KOH solution (20–40 wt%) has an etching rate of 1–2 μm/min on the (100) plane of silicon at 85℃, and forms a 54.7° sidewall on the (111) plane (due to lattice differences), used in MEMS device manufacturing.

  
  

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  Etching of Aluminum Layers: The phosphoric-nitric-acetic acid mixture (75:5:5) generates Al₂O₃ via oxidation, and phosphoric acid dissolves the oxide to achieve micron-level linewidth control.

  
  

3. Redox Etchants

Core Components: Hydrogen peroxide (H₂O₂) and acid/alkali composite systems.

Application Scenarios:

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  Metal Etching: The mixed solution of FeCl₃ and H₂O₂ oxidizes copper to Cu²⁺ (dissolved in acidic environments), used for copper interconnect layer etching.

  
  

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  Silicon Nitride Etching: Phosphoric acid (85%) catalyzes water decomposition at 180℃ to selectively remove silicon nitride (rate: 50 Å/min) with a selectivity ratio >10:1 for SiO₂.

  
  

II. Key Points of Precision Mixing Process

1. Concentration Control Technology

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  Dynamic Compensation System: Based on the Arrhenius equation, real-time temperature monitoring via PT100 sensors adjusts mixing rates to compensate for reaction rate changes (e.g., HF volatilization-induced concentration fluctuations at high temperatures).

  
  

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  Gradient Mixing Technology: Multi-stage pumping is used for high-viscosity components (e.g., concentrated sulfuric acid) to avoid excessive local concentrations that trigger side reactions (e.g., NOₓ gas generation from sulfuric and nitric acids).

  
  

2. Equipment Selection and Process Optimization

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  Corrosion-resistant Pipelines: Perfluoroalkoxy (PFA) pipelines (surface roughness Ra ≤ 0.8 μm) resist HF corrosion and reduce concentration deviations from surface adsorption.

  
  

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  Mass Flow Meters: Coriolis flow meters (accuracy ±0.5%) enable dynamic volume proportioning to ensure a stable HF:NH₄F ratio (6:1 ± 0.1) in BHF buffer.

  
  

III. Key Points of Process Control

1. Selectivity Control

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  Addition of Complexing Agents: Ethylene glycol (EG) binds to hydroxyl groups on SiO₂ surfaces to inhibit excessive silicon corrosion, increasing the SiO₂/Si selectivity ratio to 300:1.

  
  

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  Mask Protection: Utilize differences in photoresist stability in alkaline environments; the high selectivity of TMAH for silicon enables 0.1 μm linewidth pattern transfer.

  
  

2. Contamination Control

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  Metal Ion Filtration: A two-stage purification system (0.1 μm → 0.01 μm filters) controls Fe/Cu ion concentrations at <1 ppb to prevent device failure.

  
  

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  Particle Monitoring: An online laser particle sizer (detection limit: 0.5 μm) provides real-time cleanliness feedback and triggers automatic filtration.

  
  

3. Lifetime Management

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  Arrhenius Model Prediction: A mathematical model for solution shelf life (error ±5%) is built based on temperature-rate relationships to warn of replacement cycles.

  
  

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  Performance Decay Compensation: Online refractive index monitoring (resolution ±0.0001 RIU) back-calculates concentration changes to dynamically adjust replenishment.

  
  

Summary

The research and development of semiconductor etchants and process optimization are the core drivers for advancing process scaling. From traditional wet processes to intelligent mixing systems, technological iterations focus on improving selectivity, precision control, and environmental compliance. In the future, the deep integration of nanomaterials and AI will break physical limits further, providing solutions for sub-3nm nodes.