The Fundamentals and Reaction Mechanism of Wet Cleaning

In the semiconductor manufacturing process, contaminants on the wafer surface—including particles, organic substances, metal ions, and natural oxide films—can severely affect device performance and reliability. Wet cleaning uses chemical solutions to regulate surface conditions at the nanoscale, not only removing contaminants but also providing an ideal “starting point” for subsequent process steps such as thin-film deposition, photolithography, and etching.

I. The Necessity of Wet Cleaning and Classification of Contaminants

The reason why wet cleaning has become the mainstream method lies in its excellent selectivity and penetrative ability. Water-based solutions, combined with reactions such as oxidation, complexation, and acid-base interactions, can efficiently remove various types of contaminants and precisely control surface hydrophilicity/hydrophobicity, bonding states, and charge distribution within a nanoscale roughness range.

Pollutants are mainly divided into four categories:

• Particulate contaminants: Lead to increased defect density and circuit open circuits;
• Organic contamination: Causes dielectric breakdown and leakage current;
• Metal ion contamination (such as Fe, Cu, Ni): reduces the breakdown voltage of the pn junction and induces junction leakage current.
• Natural oxide film: Increases contact resistance and affects the adhesion of the thin film.

II. The Core Chemical Reagents in the RCA Cleaning System

1. SPM (sulfuric acid + hydrogen peroxide): Strong oxidation for removing organic substances

SPM decomposes organic substances such as photoresists, polymers, and oil contaminants through strong oxidation. Concentrated sulfuric acid provides a highly polar environment, while hydrogen peroxide generates reactive oxygen species, enabling the “cutting-oxidation-dissolution” of organic materials. However, its drawbacks include the potential for sulfur residues and high wastewater treatment costs. To enhance environmental friendliness, ozone water can be used in place of hydrogen peroxide (SOM process).

2. APM (Ammonia + Hydrogen Peroxide): Gentle Etching and Particle Removal

APM removes particulate contaminants through an oxidation-dissolution cycle. Hydrogen peroxide forms an ultra-thin oxide layer on the silicon surface, while aqueous ammonia dissolves this oxide layer and carries away the embedded particles, simultaneously rendering the surface hydrophilic. It is important to carefully control the etching degree to prevent surface roughening and re-adsorption of metal ions.

3. HPM (hydrochloric acid + hydrogen peroxide): Complexation and removal of metal ions

HPM removes metallic contaminants through an oxidation-complexation mechanism. Hydrogen peroxide oxidizes metals into soluble forms, while chloride ions form stable complexes that prevent the re-deposition of metals. It is often used in conjunction with APM and combined with multi-stage rinsing to reduce the risk of Cl⁻ contamination.

IV. Logic and Optimization of the Cleaning Sequence

The typical RCA cleaning sequence is: APM (SC-1) → HPM (SC-2) → HF cleaning → rinsing and drying. This sequence follows the following logic:

• First, remove organic matter and particulates to prevent them from obscuring metal contaminants.
• Remove metal ions again to prevent the metal from re-adsorbing onto the surface after HF has opened it.
• Finally, the natural oxide film is removed using HF to create a hydrogen-terminated hydrophobic surface.
• Rinse and dry immediately to prevent contaminants from re-adhering.

V. HF Cleaning: The Critical Control Step in the Final Stage

HF is used to remove the natural oxide film and is categorized as follows:

• DHF (Dilute Hydrofluoric Acid): Quickly removes thin oxide films, but highly activates the surface, making it prone to adsorption of metal ions. Therefore, it must be followed by subsequent cleaning and rinsing with high-purity water.
• BHF (Buffered Hydrofluoric Acid): Adding an NH₄F buffer stabilizes the etching rate and is suitable for controlled film removal. However, there is a risk of crystal blockage, so strict management of both the chemical solution and equipment is required.

VI. Rinsing and Drying Processes

1. Rinsing not only removes residual pharmaceutical solutions but also dilutes contaminants below the threshold concentration at which they can irreversibly deposit. Common rinsing methods include:

Quick Drain Rinse (QDR)

• Final rinse (FR), ensuring that ionic contamination meets the standard by monitoring resistivity;
• Overflow rinsing (OR), suitable for removing fluorides and colloids after HF treatment.

2. During the drying process, it is necessary to prevent the formation of water marks. Common methods include:

• Spin Dry
• Marangoni drying, which uses IPA vapor to reduce surface tension and ensure uniform removal of the water film.

7. The Role of Interface Chemistry in Cleaning Processes

The hydrophilicity or hydrophobicity of silicon surfaces can be regulated through chemical treatment:

• After HF treatment: Si-H termination is formed, making the surface hydrophobic and favorable for metallization processes.
• After APM treatment: Si-OH terminations are formed, making the surface hydrophilic and facilitating rinsing, yet it is prone to adsorbing contaminants.

Surface charge (zeta potential) influences the behavior of colloidal particles. By adjusting the pH and ionic strength, electrostatic repulsion between particles and surfaces can be enhanced, thereby improving cleaning efficiency. Megasonic cleaning utilizes a high-frequency acoustic field to generate microscale shear forces, reducing the adhesion energy of particles and facilitating their detachment from surfaces.

8. Process Window Reference

The following are typical parameter ranges for wet-etching processes (for reference only; actual parameters should be adjusted based on process nodes and material systems):

9. Conclusion

The essence of wet cleaning is not merely to remove contaminants; rather, it involves systematically leveraging chemical and physical interactions to optimize the wafer surface into a state that is most favorable for subsequent processing steps. Its success hinges on the scientifically sound design of the cleaning sequence, the highly effective synergy among chemical agents, the meticulous control of rinsing and drying processes, and a deep understanding of interfacial chemistry. Only by treating each stage as an integral part of a cohesive whole can we ensure that the cleaning process plays a critical role in improving yield and guaranteeing device performance.

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