Classic RCA Cleaning Technology

1. Introduction: The History and Status of RCA Cleaning

In 1970, Werner Kern at RCA Laboratories developed the RCA cleaning process, which laid the foundation for modern wafer cleaning. Over the past half-century, despite the semiconductor industry’s dramatic transition from micrometer‑scale to nanometer‑scale technologies, the core RCA cleaning recipes—SC‑1 and SC‑2—have remained standard equipment in fabs worldwide. Their enduring success stems from each formulation’s specialized, virtually irreplaceable cleaning capabilities.

2. SC-1: The “Bottom-Removal Demolition Squad” for Particles

SC-1 cleaning solution is prepared by mixing aqueous ammonia, hydrogen peroxide, and ultrapure water in an approximate ratio of 1:1:5, and is used after heating to 65–80°C. It is specifically designed to remove particulate contamination. Its mechanism of action is quite ingenious: rather than directly impacting the particles, it employs a “bottom‑up” removal strategy:

• Oxidation and Micro‑Dissolution: Hydrogen peroxide continuously oxidizes the silicon surface, forming a thin layer of silicon dioxide. Simultaneously, ammonia solution micro‑dissolves this oxide layer at a rate of 0.2–0.5 nanometers per minute. As a result, the contact interfaces where particles are embedded within the oxide layer become “hollowed out,” causing the particles to loosen and detach naturally.
• Electrostatic repulsion: The SC-1 solution is alkaline, and under these conditions, the silicon surface as well as particles of silicon, oxides, nitrides, and other materials all acquire a negative charge. Based on the principle of like charges repelling, detached particles are electrostatically “repelled,” effectively preventing secondary adsorption.

3. SC-2: The “Specialized Catcher” of Metal Ions

SC-1 is ineffective against metal ion contamination, because under alkaline conditions, metals such as iron and aluminum form hydroxide precipitates that can adhere to the wafer surface. To address this issue, the SC-2 formulation employs a mixture of hydrochloric acid and hydrogen peroxide (approximately 1:1:6), specifically designed to tackle metal contamination.

• Oxidation and Complexation: Hydrogen peroxide oxidizes metallic impurities to their ionic forms, after which chloride ions in hydrochloric acid promptly form complexing reactions with the metal ions, yielding water‑soluble complexes (such as iron–chloride complexes and aluminum–chloride complexes).
• Dissolution and removal: These soluble complexes dissolve in the cleaning solution and are thoroughly rinsed away during the water‑washing step.

4. DHF: Hydrofluoric Acid for “Removing Residue” from Wafers

Dilute hydrofluoric acid (DHF) is a highly potent chemical in semiconductor fabrication facilities, whose primary function is to completely remove the native oxide layer from the wafer surface. This oxide layer forms spontaneously in air and adsorbs various impurities.

The chemical reaction equation for DHF is: SiO₂ + 6HF → H₂SiF₆ + 2H₂O. By dissolving the oxide layer, adsorbed impurities are also removed. In certain processes, a “HF‑last” step—where dilute HF is used as the final treatment—can form a hydrophobic “silicon–hydrogen” termination on the silicon surface, facilitating subsequent drying and preventing water‑mark residues.

5. Conclusion

The RCA cleaning technology framework, with its well-defined division of labor and sequential integration of SC‑1 (particle removal), SC‑2 (metallic contaminant removal), and DHF (oxide layer removal), has established an efficient and reliable wafer‑level cleaning process. This system, grounded in publicly available chemical knowledge, remains a cornerstone of semiconductor manufacturing to this day.

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