RCA Standard Cleaning Method

In the manufacturing process of semiconductor devices, the cleanliness of the wafer surface is one of the key factors determining device performance, reliability, and yield.

In 1965, Kern, Puotinen, and others pioneered a standardized wet-chemical cleaning process at RCA Laboratories in Princeton, New Jersey, which later became known as the "RCA Standard Cleaning Method" after the lab's name. The introduction of this method marked a pivotal shift in wafer cleaning—from an empirically driven practice toward a systematic, theoretically grounded, and scientifically rigorous approach. The RCA cleaning method employs a series of sequentially applied chemical solutions designed to effectively remove various types of contaminants from the wafer surface, including organic residues, particles, metallic impurities, and natural oxide films.

1. The RCA standard cleaning method primarily consists of four core cleaning solutions:

SPM (Sulfuric Acid and Hydrogen Peroxide Mixture): Used to remove heavy organic pollutants and certain metals.

DHF (Dilute Hydrofluoric Acid): Used to remove the natural oxide film and any metal adhering to it.

APM (a mixture of ammonia and hydrogen peroxide, also known as SC-1): primarily used for removing particulate contaminants.

HPM (a mixture of hydrochloric acid and hydrogen peroxide, also known as SC-2): Used to remove alkali metal and transition metal ions.

Since its introduction, the RCA standard cleaning method has rapidly become the globally recognized benchmark for semiconductor industry cleaning processes—thanks to its outstanding cleaning performance and excellent process repeatability—and has laid the foundation for decades of subsequent advancements in wet-process cleaning technology. This article aims to provide a comprehensive overview and review of the RCA standard cleaning method, covering its historical origins, chemical principles, standardized process procedures, and technological evolution.

2. Principle and Process Flow of the RCA Standard Cleaning Method

The standard RCA cleaning process typically involves combining the aforementioned cleaning solutions in a specific sequence to achieve synergistic and highly effective results. A typical two-step procedure is as follows: SPM → DHF → APM (SC-1) → HPM (SC-2). Each step has its own unique chemical action and targets specific contaminants.

• SPM Cleaning (H₂SO₄/H₂O₂)

SPM is typically mixed in a ratio of H₂SO₄ : H₂O₂ = 4:1 and operated at high temperatures ranging from 120 to 150°C.

Mechanism of action: Concentrated sulfuric acid (H₂SO₄) exhibits strong dehydrating and oxidizing properties, while hydrogen peroxide (H₂O₂) decomposes under high-temperature acidic conditions to release oxygen, further enhancing its oxidizing capacity. This mixed solution effectively oxidizes and breaks down organic contaminants—such as photoresist and resins—into CO₂ and H₂O, which then volatilize and are removed. At the same time, it can also oxidize certain metals, like Cu and Ag, into their ionic forms, dissolving them completely into the solution.

Important note: In cases of extremely severe organic contamination, incomplete oxidation may lead to carbonization of the organic material, forming hard carbon residue that is even more difficult to remove.

• DHF Cleaning (Dilute Hydrofluoric Acid)

DHF typically uses a 0.5% to 5% aqueous solution of hydrofluoric acid (HF), carried out at room temperature between 20 and 25°C.

Mechanism of action: HF can rapidly dissolve the natural oxide layer (SiO₂) on the silicon surface. Its core function can be attributed to two key points:

Remove metals: Many metallic impurities (such as Fe, Zn, Ni, Al) can adsorb onto or become embedded within the natural oxide film. When the oxide film is dissolved by HF, these metals are simultaneously released into the solution and removed.

Surface Passivation: After HF removes the oxide layer, it reacts with the silicon on the surface, terminating the Si-H bonds and creating a hydrophobic surface. This Si-H bond layer effectively inhibits the immediate regrowth of the oxide film during the cleaning process, thereby maintaining surface stability.

Advantage: HF exhibits extremely high selectivity toward SiO₂, enabling it to remove the oxide layer while maintaining a very low etching rate on the underlying silicon substrate.

• APM Cleaning (SC-1, NH₄OH/H₂O₂/H₂O)

APM, commonly known as SC-1, typically uses a ratio of NH₄OH : H₂O₂ : H₂O = 1 : 1 : 5 to 1 : 2 : 7, with an operating temperature range of 30–80°C.

Mechanism of action: This is a complex process involving oxidation and complexation corrosion.

Oxidation: H₂O₂ continuously forms a thin, dense chemical oxide layer (~1 nm) on the silicon surface under alkaline conditions.

Corrosion: NH₄OH can slightly corrode the newly formed oxide layer (SiO₂).

Particle Removal: The aforementioned micro-cycling process of "oxidation-corrosion" (occurring approximately 0.1 nm per cycle) continuously "pry[s]" particles adhering to the wafer surface, loosening them gradually. Meanwhile, thanks to the hydrophilic nature of the oxide layer, the cleaning solution can fully penetrate the gap between the particles and the silicon wafer. Combined with physical assistance methods like Megasonic waves, these particles are ultimately carried away into the solution. This is the core mechanism behind APM's particle removal capability.

Challenge: The mild isotropic etching of silicon by NH₄OH may lead to an increase in surface micro-roughness. Meanwhile, if process control is inadequate, metal ions in the solution could redeposit onto the wafer surface.

• HPM Cleaning (SC-2, HCl/H₂O₂/H₂O)

HPM, commonly known as SC-2, typically uses a ratio of HCl : H₂O₂ : H₂O = 1 : 1 : 6 to 1 : 2 : 8, with an operating temperature range of 65–85°C.

Mechanism of action:

Complex dissolution: HCl can provide high-concentration Cl⁻ ions, which form soluble chloride complexes with numerous metal cations (such as Na⁺, Fe²⁺/³⁺, Mg²⁺, Ca²⁺, Al³⁺), thereby dissolving these metals and stabilizing them in the cleaning solution.

Surface Passivation: The presence of H₂O₂ ensures that the silicon wafer surface remains oxidized, forming a protective film that prevents direct corrosion of the silicon substrate by HCl, while simultaneously "trapping" metallic impurities within the oxide layer, making them easier to dissolve with HCl.

Feature: HPM is particularly effective in removing residual alkali and transition metal ions that may remain after the APM step.

The success of the RCA standard cleaning method lies in its ability to achieve step-by-step, highly effective removal of various contaminants through a clever combination of chemicals. Its cleaning performance is significantly influenced by factors such as temperature, concentration, duration, and the type of contaminant involved.

Kexinwei Company – Leading the Innovation in Semiconductor Cleaning Technology

For more technical details, please consult our technical advisor: 13861996325!

▲ Technical Consulting ▲ Follow Kexinwei's WeChat Official Account