Metal Removal Mechanism of SC-2 Cleaning Solution

1. Introduction

In semiconductor manufacturing, metallic contamination on the silicon wafer surface can severely degrade device performance and yield. SC‑2 (Standard Cleaning Solution No. 2), a critical step in the RCA cleaning process, effectively removes metal ions through the synergistic action of HCl and H₂O₂. This paper systematically analyzes the mechanism of SC‑2 from three perspectives: reaction kinetics, oxide layer characteristics, and surface‑state control.

2. The Triple Synergistic Mechanism of Metal Removal

2.1 Oxidation

In SC‑2, H₂O₂ decomposes under strongly acidic conditions (pH < 1), generating highly reactive ·OH radicals and O₂. The ·OH radicals can oxidize low-valent metal ions (such as Fe²⁺) to their high-valent forms (such as Fe³⁺). High-valent metal ions typically exhibit greater water solubility and a stronger tendency to form complexes, thereby facilitating their desorption from the silica surface into the solution.

2.2 Complexation Effect

by HCl The abundant Cl⁻ ions provided form stable chloride complexes with various heavy metal ions, for example:

• Fe³⁺ + 4Cl⁻ → [FeCl₄]⁻
• Cu²⁺ + 4Cl⁻ → [CuCl₄]²⁻

The solubility of such complexes is significantly higher than that of the corresponding metal hydroxides or oxides, markedly enhancing the metal’s capacity to remain dissolved in the cleaning solution and preventing its redeposition on the silicon surface.

2.3 Acid Inhibition Effect

The solution maintains a strongly acidic pH (pH < 1), which serves a dual purpose: on the one hand, it suppresses the hydrolysis of metal ions to form hydroxide precipitates; on the other hand, it prevents dissolved metal complexes from desorbing and re‑adsorbing onto the silicon wafer surface due to localized pH increases. Together, these three mechanisms ensure that SC‑2 achieves a metal removal efficiency exceeding 99.99%.

2.4 Corrosion Control of Silicon Substrates

SC-2 exhibits extremely mild chemical etching of monocrystalline silicon, with silicon loss per cleaning cycle remaining below 0.1 nm. This characteristic ensures its safe use in the fabrication of high‑precision devices, without compromising critical dimension control.

3. Formation and Characteristics of the Surface Oxide Layer

3.1 Formation of the Chemical Oxide Layer

After SC-2 treatment, the silicon surface is oxidized, forming a chemical oxide layer. The main characteristics of this oxide layer are as follows:

• Thickness range: 0.6 ~ 1.5 nm
• Chemical composition: Rich in Si–OH groups.
• Structural characteristics: porous, unlike the dense thermally oxidized SiO₂.

Due to its porous structure, this oxide layer does not exhibit superior dielectric or passivation properties and is typically employed as a sacrificial or interfacial layer.

3.2 Post-Processing Options

Depending on the process requirements, the chemically oxidized layer formed by SC-2 can be subjected to one of two treatment strategies:

• Hydrogen-terminated surface route: The oxide layer is removed in a subsequent HF etching step, yielding a hydrophobic Si–H‑terminated surface. This surface exhibits an extremely low propensity for re‑adsorption of particles and metals, making it an ideal condition for achieving ultra‑clean surfaces.
• Retain as a transition layer: If thermal oxidation or chemical vapor deposition (CVD) is performed directly afterward, the chemically oxidized layer left by SC‑2 can serve as a protective barrier, mitigating the adverse effects caused by non-uniform thickness of the native oxide.

4. Conclusion

SC-2 cleaning solution, through the synergistic effects of oxidation, complexation, and acid inhibition, efficiently removes metallic contaminants with an efficiency exceeding 99.99%, while exhibiting minimal etching of silicon substrates—less than 0.1 nm per pass. Following treatment, a chemically oxidized layer of controllable thickness (0.6–1.5 nm) forms on the silicon surface; this layer can be either retained or removed using HF, as required by the process, to obtain a hydrogen-terminated surface. These mechanisms provide a theoretical foundation for optimizing wet‑chemical cleaning processes in semiconductor manufacturing.

For more technical details, please contact our technical consultant at 13861996325!

▲Technical Consultation ▲Follow the Kexinwei WeChat Official Account