SC-2 Cleaning Solution

  In semiconductor wafer fabrication, the control of surface contaminants directly impacts device performance and reliability. The wet‑cleaning process proposed by RCA in 1970 remains the industry standard to this day. It consists of two steps: SC‑1, an alkaline cleaning solution primarily used to remove particles and organic residues, and SC‑2, an acidic cleaning solution designed to eliminate metallic contaminants. When used in combination, these steps address the vast majority of surface contamination types on wafers. Notably, SC‑2 is often referred to as the “final arbiter” of metallic contamination on wafers; without it, achieving high device yield would be impossible.

I. Definition and Formulation of SC-2 Cleaning Solution

1. Basic Definitions

SC-2 stands for “Standard Clean 2” and is one of the core steps in the RCA standard cleaning process. It is an oxidizing acidic solution with an extremely low pH, strong oxidizing power, and excellent complexing ability, specifically formulated to remove metallic contaminants.

2. Standard Formulation and Process Conditions

The standard volume ratio of SC-2 is: HCl : H₂O₂ : H₂O = 1 : 1 : 6 , the operating temperature is typically maintained within the range of 65–85°C.

II. Core Functions and Mechanisms

1. Types of Metal Contaminants to Be Removed

SC-2 primarily removes the following two categories of metallic impurities:

• Alkali metal ions: Na⁺, K⁺, etc.
• Heavy metal impurities: Fe, Cu, Zn, etc.

The presence of these metallic contaminants can lead to degradation of device electrical performance, with copper (Cu) contamination being particularly critical—Cu serves as a deep-level trap impurity in CMOS devices, and even residual concentrations at the ppb level can significantly increase leakage current.

2. Chemical Reaction Mechanism

The cleaning action of SC-2 is based on a two-step synergistic process:

• Oxidation: H₂O₂ in solution provides a strong oxidizing environment, oxidizing elemental metals or low-valent metal ions to their higher oxidation states.
• Complexation dissolution: HCl Provided Cl⁻ It forms soluble chloride complexes with high-valence metal ions, thereby dissolving and removing the metals from the wafer surface.

3. Additional Protective Effect

While removing metallic contaminants, SC-2 forms a chemically oxidized layer approximately 1–2 nm thick on the silicon surface. This oxide layer serves as a clean protective barrier for subsequent processes, helping to maintain the stability of the surface condition.

4. Roles in the RCA Process

When used in conjunction with SC-1, SC-2 forms a complete RCA cleaning process:

III. Conclusion

SC‑2 cleaning solution is an indispensable component of semiconductor wet‑etching processes. Its standard formulation (1:1:6) and operating temperature range of 65–85°C ensure highly effective removal of metallic contaminants. By leveraging both oxidation and complexation mechanisms, SC‑2 thoroughly eliminates harmful metal impurities such as Na⁺, K⁺, Fe, Cu, and Zn, while simultaneously forming a thin oxide layer on the silicon surface to protect the substrate. When used in conjunction with SC‑1, the RCA cleaning sequence provides a fundamental guarantee of high yield and reliability for semiconductor devices.

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