Wafer cleaning process

As the feature sizes of semiconductor devices continue to shrink into the nanoscale, the cleanliness of the wafer surface is exerting an increasingly significant impact on device performance and manufacturing yield.

The core objective of wafer cleaning is to efficiently remove chemical impurities and particulate contaminants without damaging the substrate or its surface structure. Studies have shown that the yield of silicon wafers is inversely proportional to the defect density—i.e., the level of cleanliness and the number of particles—during processing. For advanced-node devices, removing sub‑micron particles is particularly challenging, as strong electrostatic forces exist between the particles and the wafer substrate. Consequently, the development of efficient, low‑damage wafer‑cleaning technologies has become a critical and ongoing research focus in the semiconductor manufacturing industry.

I. Purpose of Wafer Cleaning

Wafer cleaning must meet the following core requirements:

• Effective removal of contaminants: eliminates particulates, metal ions, organic residues, and natural oxide layers.
• Preserve surface integrity: Avoid introducing defects such as surface roughness, pitting, or localized corrosion during the cleaning process.
• Improving yield: Reducing defect density to increase the final yield of integrated circuit manufacturing.

As device feature sizes continue to shrink, the ability to remove smaller particles has become a key metric for evaluating the performance of cleaning processes.

II. Wafer Cleaning Technology

1. Traditional RCA cleaning process

The RCA cleaning process was developed by the Radio Corporation of America and primarily involves two standard cleaning solutions:

• SC-1 (Standard Cleaning 1): Composed of ammonium hydroxide, hydrogen peroxide, and deionized water (NH₄OH : H₂O₂ : H₂O), it is primarily used to remove particulates and certain organic contaminants. Its mechanism of action relies on the oxidizing power of hydrogen peroxide and the mild etching effect of ammonium hydroxide, which detach particles from the wafer surface and keep them in suspension.
• SC-2 (Standard Cleaning 2): Composed of hydrochloric acid, hydrogen peroxide, and deionized water (HCl : H₂O₂ : H₂O), it is used to remove metallic ion contaminants. Chloride ions can form soluble complexes with metal ions, thereby removing them from the wafer surface.

Although RCA cleaning remains the dominant approach, this process suffers from high chemical consumption, costly wastewater treatment, and the potential for surface roughness caused by high-temperature operation, thereby driving the development of numerous subsequent optimization techniques.

2. Emerging Cleaning Technologies

• Ozone cleaning
• Ozone (O₃) dissolved in deionized water forms ozonated water, which can effectively oxidize and degrade organic contaminants while significantly reducing the consumption of chemical reagents—such as ammonia and hydrogen peroxide—and the associated wastewater‑treatment costs compared with conventional SC‑1. Ozone cleaning at ambient or low temperatures delivers comparable cleaning performance to SC‑1, with reduced damage to the wafer surface, making it particularly well suited for advanced processes that are sensitive to surface roughness.
• Megasonic cleaning
• Megasonic cleaning leverages high-frequency acoustic waves—typically in the 0.8–2 MHz range—to induce cavitation and microjetting in the cleaning bath, thereby enhancing the removal of sub‑micron particles with chemical agents. Compared with low‑frequency ultrasound, megasonic waves generate smaller cavitation bubbles with a more uniform energy distribution, significantly reducing the risk of damage to delicate structures such as gates and fins. Today, megasonic cleaning has become the dominant technology for particle removal at advanced process nodes, including 7 nm and below.
• Supercritical CO₂ cleaning
• Supercritical carbon dioxide (scCO₂) combines the properties of both gases and liquids, with a surface tension close to zero, enabling it to penetrate high aspect‑ratio structures—such as FinFET fins and 3D NAND channel vias—without causing damage and effectively dissolving nonpolar organic residues. When used in combination with small amounts of cosolvents like acetone or isopropyl alcohol, it can completely eliminate watermarks and pattern collapse during the drying process, making it particularly well suited for critical cleaning steps in advanced memory and logic devices.
• Dry cleaning
• Dry‑process cleaning primarily includes plasma cleaning and vapor‑phase cleaning. For example, a vapor‑phase mixture of HF/H₂O or HF/CH₃OH can remove the native oxide layer under vacuum conditions without leaving water‑based stains or residues. Plasma cleaning employs reactive free radicals—such as oxygen radicals and hydrogen radicals—to react with organic contaminants, forming volatile byproducts, and is well suited for single‑wafer processing equipment. By circumventing the liquid surface‑tension issues inherent in wet‑process techniques, dry‑process cleaning represents a key direction for the future development of cleaning technologies for three‑dimensional devices.

III. Typical Steps in Wafer Cleaning

IV. Latest Trends in Process Development

The current development trends in wafer cleaning processes primarily include:

• Single-chip cleaning: replaces conventional tank-based cleaning, reducing cross-contamination and enhancing process control accuracy and uniformity.
• Functional water cleaning: such as electrolyzed ionized water, hydrogen‑rich water, and carbonated water, reduces chemical consumption and lowers environmental impact.
• Low-temperature cleaning: Cleaning is performed at temperatures below 50 °C, making it suitable for novel heat-sensitive materials such as low‑dielectric‑constant dielectrics and organic materials.
• In-situ drying techniques—such as Marangoni drying or supercritical drying—are employed to prevent water‑streaking and pattern collapse, making them particularly well suited for structures with high aspect ratios.

V. Conclusion

Wafer cleaning is an indispensable core step in semiconductor manufacturing. While the traditional RCA cleaning process has maintained a foundational chemical regime over many years, it has continually evolved by integrating emerging technologies such as ozone cleaning, megasonic cleaning, supercritical CO₂ cleaning, and dry‑process cleaning, thereby meeting the stringent demands of advanced nodes for ultra‑high cleanliness, minimal damage, and exceptional efficiency. Looking ahead, wafer‑cleaning processes will continue to advance toward greater environmental sustainability, enhanced precision, reduced costs, and higher yield rates, supporting the industry’s ongoing drive to shrink device feature sizes.

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