Wafer Cleaning Technology in Semiconductor Manufacturing

Wafer Cleaning Technology in Semiconductor Manufacturing

Wafer cleaning is a critical step throughout the entire semiconductor manufacturing process and one of the key factors that directly affects device performance and production yield. During chip fabrication, even the slightest contamination can degrade device characteristics or cause complete failure. As a result, cleaning processes are applied before and after nearly every manufacturing step to remove surface contaminants and ensure wafer cleanliness. Cleaning is also the most frequent operation in semiconductor production, accounting for roughly 30% of all process steps.

With the continuous scaling of very-large-scale integration (VLSI), process nodes have advanced to 28 nm, 14 nm, and beyond, driving higher device density, narrower linewidths, and increasingly complex process flows. Advanced nodes are significantly more sensitive to contamination, while smaller feature sizes make cleaning more difficult. Consequently, the number of cleaning steps continues to rise, and cleaning has become more complex, more critical, and more challenging. For example, a 90 nm chip typically requires about 90 cleaning steps, whereas a 20 nm chip requires around 215 cleaning steps. As manufacturing progresses to 14 nm, 10 nm, and smaller nodes, the number of cleaning operations will keep increasing.

In essence, wafer cleaning refers to processes that use chemical treatments, gases, or physical methods to remove impurities from the wafer surface. Contaminants such as particles, metals, organic residues, and native oxides can all adversely affect device performance, reliability, and yield. Cleaning serves as the “bridge” between consecutive fabrication steps—for example, before deposition and lithography, or after etching, CMP (chemical mechanical polishing), and ion implantation. Broadly, wafer cleaning can be divided into wet cleaning and dry cleaning.

Wet Cleaning

Wet cleaning uses chemical solvents or deionized water (DIW) to clean wafers. Two main approaches are applied:

• Immersion method: wafers are submerged in tanks filled with solvents or DIW. This is the most widely used method, especially for mature technology nodes.

• Spray method: solvents or DIW are sprayed onto rotating wafers to remove impurities. While immersion allows batch processing of multiple wafers, spray cleaning handles only one wafer per chamber but provides better control, making it increasingly common in advanced nodes.

Dry Cleaning

As the name suggests, dry cleaning avoids solvents or DIW, instead using gases or plasma to remove contaminants. With the push toward advanced nodes, dry cleaning is gaining importance due to its high precision and effectiveness against organics, nitrides, and oxides. However, it requires higher equipment investment, more complex operation, and stricter process control. Another advantage is that dry cleaning reduces the large volumes of wastewater generated by wet methods.

Common Wet Cleaning Techniques

1. DIW (Deionized Water) Cleaning

DIW is the most widely used cleaning agent in wet cleaning. Unlike untreated water, DIW contains almost no conductive ions, preventing corrosion, electrochemical reactions, or device degradation. DIW is mainly used in two ways:

1. Direct wafer surface cleaning – Typically performed in single-wafer mode with rollers, brushes, or spray nozzles during wafer rotation. A challenge is electrostatic charge buildup, which may induce defects. To mitigate this, CO₂ (and sometimes NH₃) is dissolved into DIW to improve conductivity without contaminating the wafer.

2. Rinsing after chemical cleaning – DIW removes residual cleaning solutions that might otherwise corrode the wafer or degrade device performance if left on the surface.

2. HF (Hydrofluoric Acid) Cleaning

HF is the most effective chemical for removing native oxide layers (SiO₂) on silicon wafers and is second only to DIW in importance. It also dissolves attached metals and suppresses re-oxidation. However, HF etching can roughen wafer surfaces and undesirably attack certain metals. To address these issues, improved methods dilute HF, add oxidizers, surfactants, or complexing agents to enhance selectivity and reduce contamination.

3. SC1 Cleaning (Standard Clean 1: NH₄OH + H₂O₂ + H₂O)

SC1 is a cost-effective and highly efficient method for removing organic residues, particles, and some metals. The mechanism combines the oxidizing action of H₂O₂ and the dissolving effect of NH₄OH. It also repels particles via electrostatic forces, and ultrasonic/megasonic assistance further improves efficiency. However, SC1 can roughen wafer surfaces, requiring careful optimization of chemical ratios, surface tension control (via surfactants), and chelating agents to suppress metal redeposition.

4. SC2 Cleaning (Standard Clean 2: HCl + H₂O₂ + H₂O)

SC2 complements SC1 by removing metallic contaminants. Its strong complexation ability converts oxidized metals into soluble salts or complexes, which are rinsed away. While SC1 is effective for organics and particulates, SC2 is particularly valuable for preventing metal adsorption and ensuring low metallic contamination.

5. O₃ (Ozone) Cleaning

Ozone cleaning is mainly used for removing organic matter and disinfecting DIW. O₃ acts as a strong oxidant, but can cause re-deposition, so it is often combined with HF. Temperature optimization is critical since O₃ solubility in water decreases at higher temperatures. Unlike chlorine-based disinfectants (unacceptable in semiconductor fabs), O₃ decomposes into oxygen without contaminating DIW systems.

6. Organic Solvent Cleaning

In certain specialized processes, organic solvents are used where standard cleaning methods are insufficient or unsuitable (e.g., when oxide formation must be avoided).

Conclusion

Wafer cleaning is the most frequently repeated step in semiconductor manufacturing and directly impacts yield and device reliability. With the move toward larger wafers and smaller device geometries, requirements for wafer surface cleanliness, chemical state, roughness, and oxide thickness are becoming increasingly stringent.

This article reviewed both mature and advanced wafer cleaning technologies, including DIW, HF, SC1, SC2, O₃, and organic solvent methods, along with their mechanisms, advantages, and limitations. From both economic and environmental perspectives, continuous improvements in wafer cleaning technology are essential to meet the demands of advanced semiconductor manufacturing.