Semiconductor Wet Cleaning and Etching Processes

I. RCA Standard Cleaning

The RCA cleaning method is a classic wet‑etching sequence that primarily comprises two steps:

1. SC1 Cleaning Solution

Composition: Ammonia solution, hydrogen peroxide, and deionized water in a ratio of NH₄OH : H₂O₂ : H₂O = 1 : 2 : 10 A proportional mixture.

Process conditions: The temperature is typically maintained at 50 ± 3 °C. In practice, megasonic cleaning is often employed, utilizing high-frequency acoustic waves to generate cavitation and acoustic streaming, thereby more effectively removing sub-micron particles.

Function: Removes particulate contamination from the silicon wafer surface, eliminates trace organic residues, and abates certain light-metal contaminants. The underlying mechanism involves mild, uniform etching of the silicon surface by ammonium hydroxide, followed by oxidation of the surface with hydrogen peroxide; by adjusting the surface potential, particles are detached through electrostatic repulsion.

2. SC2 Cleaning Solution

Composition: hydrochloric acid, hydrogen peroxide, and deionized water in a ratio of HCl : H₂O₂ : H₂O = 1 : 2 : 5 A proportional mixture.

Process conditions: The temperature is also 50 ± 3 °C.

Function: Removal of metallic contaminants. Hydrochloric acid forms soluble chloride complexes with metal ions, effectively eliminating impurities such as alkali metals and transition metals.

After RCA cleaning, a thin layer of silicon dioxide is sometimes formed via ozone‑water treatment or thermal oxidation to obtain a clean and chemically stable surface—commonly referred to as the SCROD process—which is often used as a pre‑treatment prior to gate oxide growth.

II. SPM Cleaning

SPM (sulfuric acid/hydrogen peroxide mixture) is used to remove stubborn organic contaminants.

Composition: Sulfuric acid and hydrogen peroxide in a ratio of H₂SO₄ : H₂O₂ = 5 : 1 A proportional mixture.

Process conditions: Conducted at a high temperature of 130 ± 5 °C.

Function: Removes photoresist and other complex organic contaminants. High-temperature concentrated sulfuric acid provides strong oxidizing and dehydrating properties, effectively breaking down organic substances.

As an alternative or complementary process to SPM, ozone‑water cleaning (O₃ water) can also be employed under conditions of room temperature to 50 °C and an ozone concentration of 5–20 ppm; its advantages include the absence of high temperatures and no residual sulfate ions.

III. Silica Etching: DHF and BHF/BOE

This process sequence is used for the controlled etching of silicon dioxide dielectric layers.

1. DHF (Dilute Hydrofluoric Acid)

Typical mix ratio: by volume HF (49%) : H₂O = 1 : 100 or 1 : 10 。

Process conditions: Typically 25 ± 1 °C.

Function: Etches thermally grown silicon dioxide, removing the native oxide layer from the silicon surface. The reaction equation is: SiO₂ + 6HF → H₂SiF₆ + 2H₂O After removing the native oxide layer, the silicon surface exhibits hydrophobicity.

Applying a dilute HF treatment in the final step of the cleaning process (the HF‑last process) yields a hydrogen‑terminated, hydrophobic surface and suppresses the regrowth of native oxide, making it well suited for downstream processes with stringent surface‑quality requirements, such as pre‑epitaxial‑growth preparation.

2. BHF / BOE (Buffered Hydrofluoric Acid)

Composition: A mixed solution of hydrofluoric acid and ammonium fluoride. Typical ratio: NH₄F : HF = 10 : 1 (Commonly used).

Process conditions: Temperature is typically 25 or 26.5 ± 1 °C.

Function and Principle: Achieves uniform, stable silicon dioxide etching. The buffering action of ammonium fluoride maintains the solution’s… HF₂⁻ Ion concentration ensures a stable etching rate, mitigating process‑to‑process variability caused by fluctuations in HF concentration; meanwhile, a stable pH value prevents erosion of the photoresist mask.

IV. H₃PO₄ Cleaning: Selective Etching of Silicon Nitride

Hot phosphoric acid is used for the selective removal of silicon nitride layers.

Process conditions: Treat with 86% phosphoric acid at a high temperature of 160 ± 5 °C.

Function: Uniformly etches silicon nitride with an extremely low etch rate for silicon dioxide, providing a very high selectivity ratio between silicon nitride and silicon oxide. Commonly used to selectively remove silicon nitride mask or stop layers on top of silicon oxide layers.

V. Solvent Cleaning

Solvent cleaning is used for organic contaminants that cannot be removed by water-based solutions.

Composition: Typically contains hydroxylamine and a complexing agent, and often employs IPA (isopropyl alcohol) or NMP (N-methylpyrrolidone) as co-solvents to enhance cleaning efficiency.

Process conditions: Treat at 75 ± 5 °C for approximately 20 minutes.

Function: Specifically removes polymeric residues and stubborn photoresist formed after dry etching and ion implantation.

Precautions: This process may slightly etch metal films such as aluminum and copper; its impact on the metal layers must be taken into account in integrated manufacturing processes.

VI. General Rinsing and Drying

After each chemical cleaning or etching step, the sample must be rinsed with high-purity deionized water (DIW) to remove residual chemicals and prevent cross-contamination. Rinsing is typically performed at room temperature or at a warm temperature (25–60 °C), using overflow, spray, or immersion methods. This is followed by drying, with common techniques including spin‑drying, IPA vapor drying, or Marangoni drying, to avoid water‑spot formation.

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