Metal Ion Contamination Control in Semiconductor Wet Cleaning Processes

1. Introduction

The wet cleaning process in semiconductor manufacturing is used to remove metal ions, particles, and organic contaminants from the surface of wafers. Residual metal ions can severely compromise device performance, leading to issues such as leakage current, electromigration, or corrosion; hence, controlling their concentration is absolutely critical.

2. Metal Ion Concentration Specifications

2.1 Permissible Concentration Range

• Common metal ions (Such as Fe, Cu, Ni, Cr, etc.): Concentrations must be below 1 × 10¹⁰ atoms/cm².
• Alkali metal ions (Such as Na, K, Ca, etc.): Due to their tendency to induce electromigration and corrosion, stricter requirements apply—typically below 1 × 10⁹ atoms/cm².

2.2 Detection Method

• Atomic Absorption Spectroscopy (AAS) With Inductively Coupled Plasma Mass Spectrometry (ICP-MS) : Used for quantitative analysis of surface metal content.
• Surface Energy Spectroscopy : For instance, scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) can detect metallic residues in microscopic areas.

3. Cleaning Process Control of Metal Ions

3.1 SC-2 Cleaning Solution (HCl/H₂O₂/DI Water)

• Temperature : Maintain temperatures between room temperature and 60°C, avoiding high temperatures that could cause metal corrosion.
• Time : 5–15 minutes, balancing cleaning efficiency with the risk of substrate damage.
• HCl concentration : 0.5%–5%; excessively high levels may corrode aluminum/copper interconnect structures.
• Mechanism of action :HCl removes metal ions through chelation, while H₂O₂ provides oxidizing power, promoting metal dissolution and organic matter degradation.

3.2 Other Key Processes

• DHF Cleaning : Maintain the HF concentration between 0.5% and 2% to prevent the redeposition of metal ions.
• DI Water Rinse : The resistivity must be higher than 18.2 MΩ·cm to prevent secondary contamination.

4. Sources of Pollution and Preventive Measures

4.1 Major Pollution Sources

• Trace metal impurities in chemical reagents;
• Metal leaching from equipment materials (such as stainless steel);
• Environmental particulate deposition and operational tool contamination.

4.2 Preventive Measures

Using electronic-grade, high-purity chemicals;

• The equipment is made from corrosion-resistant materials such as PFA and PTFE;
• The chemical liquid was treated with a 0.1-μm filter;
• Regularly clean the equipment with DI water to prevent crystal formation.
• The cleanroom environment maintains Class 1000 or higher standards.

5. Process Validation and Monitoring

5.1 Online Monitoring

• Electrical Conductivity Detection: Real-time monitoring of ion concentration changes in the cleaning solution;
• Dissolved hydrogen control: Prevents metal oxidation and precipitation.

5.2 Offline Spot Check

• Wafer surface sampling: Cleaning effectiveness verified via RCA testing;
• Waste liquid analysis: Regularly test metal content to assess the lifespan of the cleaning solution.

6. Handling Special Cases

• Stubborn Metal Contamination : Chelating agents (such as citric acid, EDTA) can be introduced, or ultrasonic waves can be used to enhance cleaning efficiency;
• Aluminum/Copper Interconnect Structure Cleaning : Avoid cleaning with strong acids; instead, prioritize mild organic acids or ionic cleaning solutions.

7. Conclusion

Controlling metal ions in semiconductor wet cleaning processes requires maintaining levels within the range of 10⁹–10¹⁰ atoms/cm². This can be achieved by optimizing the cleaning solution formulation, process parameters, equipment materials, and environmental conditions, combined with both online and offline monitoring techniques, thereby effectively ensuring the stability and reliability of the cleaning process. If超标 (exceedance) issues arise, they can be addressed by adjusting chelating agents, upgrading the filtration system, or switching to more corrosion-resistant materials such as PFA.

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