Semiconductor Cleaning: Processes, Methods, and Principles

I. Introduction

Since the invention of the four basic processes of ion implantation, diffusion, epitaxial growth, and lithography in the 1950s, semiconductor manufacturing processes have continued to develop. Chips are extremely susceptible to particle and metal contamination during production, leading to failures such as short circuits or open circuits. Therefore, in addition to strictly preventing external contamination throughout the entire production process, wet or dry cleaning is required before and after key manufacturing steps such as high-temperature diffusion and ion implantation. Cleaning processes utilize chemical solutions or gases to remove particles, metal ions, and organic impurities remaining on the wafer surface, ensuring a clean wafer surface and maintaining good electrical performance.

II. Classification of Contaminants

The integrated circuit (IC) manufacturing process requires the use of a variety of organic and inorganic compounds. Although manufacturing is primarily carried out in a cleanroom environment, factors such as human intervention can still lead to wafer contamination. According to their morphology, contaminants are mainly divided into four categories: particles, organic matter, metallic contaminants, and oxides.

2.1 Particles

Main sources: Polymer residue, photoresist residue, etching byproducts.

Impact: Particles adhering to the silicon wafer surface interfere with the subsequent process of pattern formation (geometric features) and device electrical performance.

Removal principle: The adhesion of particles to the surface is mainly based on van der Waals forces. The main removal method is to "undercut" the bottom of the particles through physical or chemical means, reducing the contact area between the particles and the silicon surface, and finally removing them.

2.2 Organic Contaminants

Main sources: Human skin oils, cleanroom air, machine lubricating oil, organosilicon vacuum grease, photoresist, cleaning solvent residue, etc.

Impact: Forms an organic film on the wafer surface, hindering the effective contact of the cleaning solution and cleaning the surface.

Processing strategy: Removing organic matter is usually the first step in the entire cleaning process.

2.3 Metallic Contaminants

Source: Metal interconnect processes (such as Al-Si, Cu). Involves lithography, etching to form contact holes, and evaporation, sputtering, or chemical vapor deposition (CVD) to form metal interconnects. Subsequent chemical mechanical polishing (CMP) processes may also introduce metal contamination.

Impact: Metal ions can lead to degradation of device electrical characteristics (such as junction leakage, reduced gate oxide integrity).

Necessity: Special cleaning steps must be taken to remove metallic contaminants.

2.4 Oxides

Natural oxide layer: Silicon is easily oxidized to form in an environment containing oxygen and water.

Chemical oxide layer: Formed on the silicon surface after cleaning with a strong oxidizing solution (such as APM, HPM containing H₂O₂).

Process oxide layer: Silicon nitride (SiN), silicon dioxide (SiO₂), and other dielectric layers deposited by CVD processes, which need to be selectively removed in specific steps.

Impact: Surface oxides (especially natural and chemical oxide layers) can seriously affect the quality of the gate oxide layer and interface characteristics.

Necessity: Surface oxides must be removed in key steps (such as before gate oxidation).

III. Classification of Cleaning Methods

Cleaning technologies are mainly divided into two categories: wet cleaning and dry cleaning.

3.1 Wet Cleaning

Wet cleaning uses liquid chemical reagents and deionized water (DIW) to remove particles, organic debris, and metal ion contamination from the wafer surface through oxidation, etching, and dissolution. The main methods include: RCA cleaning, diluted chemical cleaning, IMEC cleaning, and single-wafer cleaning.

3.1.1 RCA Cleaning

Background: Developed by Radio Corporation of America (RCA) in 1965, it has become the basis for many subsequent cleaning processes and is still widely used or derived today.

Principle: Combining solvents, acids, surfactants, and water, it removes contaminants through spraying, cleaning, oxidation, etching, and dissolution, while minimizing damage to the wafer surface characteristics. Each chemical step must be thoroughly rinsed with a large amount of deionized water.

Core solutions:

APM (Standard Clean 1, SC1): NH₄OH : H₂O₂ : H₂O = 1 : 1 : 5, temperature 75~80°C.

Function: Oxidizes and slightly etches to remove particles; removes light organic matter and some metal contamination.

Side effects: May increase surface roughness.

HPM (Standard Clean 2, SC2): HCl : H₂O₂ : H₂O = 1 : 1 : 6, temperature 75~80°C.

Function: HCl dissolves alkali metal ions and hydroxides of Al, Fe, Mg, etc.; Cl⁻ complexes with residual metal ions to form soluble complexes, removing metal contaminants.

SPM (Piranha): H₂SO₄ : H₂O₂ = 4 : 1, temperature 100~130°C.

Function: Strongly removes organic contaminants. H₂SO₄ dehydrates and carbonizes organic matter, and H₂O₂ oxidizes the carbonized product into CO/CO₂ gas and escapes.

DHF (Dilute HF): HF : H₂O = 1 : 50, temperature 20~25°C.

Function: Removes oxides (natural oxide layer, chemical oxide layer); reduces surface metal contamination to a certain extent. After removing the oxide layer, Si-H bonds are formed, making the surface hydrophobic.

Optimization: RCA cleaning often incorporates Megasonics technology, significantly reducing chemical and DIW consumption, shortening etching time, reducing the impact of isotropic wet etching, and extending cleaning solution life.

3.1.2 Diluted Chemical Cleaning

Objective: To significantly reduce chemical and DIW usage by diluting standard RCA solutions.

Results:

Diluted APM (e.g., 1:1:50): Effectively removes particles and hydrocarbons.

Diluted HPM (e.g., 1:1:60) or Diluted HCl (1:100): Comparable to traditional HPM in removing metallic contaminants.

Particle Control Advantages: A significant advantage is the inhibition of particle deposition on the wafer surface at low HCl concentrations.

Principle: The zeta potential of silicon (Si) and silicon dioxide (SiO₂) are equal at pH ≈ 2-2.5 (isoelectric point). At pH > 2.5, the silicon surface is negatively charged; at pH < 2.5, it is positively charged.

When the solution pH > 2.5, particles are usually negatively charged, having the same charge as the silicon surface, creating electrostatic repulsion (shielding) and preventing particle deposition.

When the solution pH < 2 (silicon surface is positively charged, particles may be negatively charged), the repulsive force disappears, and particles are easily deposited.

By controlling the HCl concentration in diluted HPM/HCl (maintaining pH > 2.5), particle deposition during etching can be effectively avoided.

Economic Benefits:

Total chemical consumption is reduced by approximately 14%.

Diluted APM/HPM/DHF combined with Megasonics can lower solution temperature and optimize cleaning time, extending bath life.

Using hot UPW instead of cold UPW rinsing can save 75-80% of UPW usage.

Low flow rate and precisely controlled cleaning time requirements mean that diluted chemical cleaning itself also saves a significant amount of rinse water.

3.1.3 IMEC Clean

Concept: Based on the success of diluted chemicals, the Interuniversity Microelectronics Centre (IMEC) in Belgium proposed a simplified process, combining ozone (O₃) and diluted chemicals to further save chemicals and DIW.

Steps:

Organic Removal and Pre-oxidation:

Objective: To eliminate organic matter and form a thin chemical oxide layer (beneficial for subsequent particle removal).

Method: Traditionally, sulfuric acid (SPM) was used, but for environmental reasons, ozonated deionized water (DI-O₃) is now used. This avoids the use of sulfuric acid and the subsequent complex rinsing steps.

Challenge: Complete removal of HMDS (hexamethyldisilazane, a commonly used adhesion promoter) is difficult. Ozone concentration and solution temperature must be strictly controlled to balance reaction rate (high temperature accelerates reaction but reduces ozone solubility) and removal efficiency.

Oxide Layer and Particle Removal and Metal Inhibition:

Method: Using a diluted HF/HCl mixed solution.

Function: Removes the oxide layer and particles formed in the first step; simultaneously inhibits the deposition of metal ions (such as Cu²⁺, Ag⁺) on the silicon surface.

Principle: Metals such as Cu and Ag are easily deposited in pure HF (electrochemical process). Adding HCl, low concentrations of Cl⁻ may catalyze copper deposition, but high concentrations of Cl⁻ will form soluble cuprous chloride complexes (CuCl₂⁻), preventing copper redeposition. The optimized HF/HCl ratio effectively prevents metal plating and extends solution life.

Surface Hydrophilization:

Objective: To obtain a hydrophilic surface, avoiding watermarks or drying stains after drying, and preventing the readsorption of metallic contaminants.

Method: Usually uses a diluted HCl/O₃ solution. The HNO₃ concentration can be increased during the final rinse to reduce the risk of calcium (Ca) contamination.

3.1.4 Single Wafer Cleaning

Driving Force: With the continuous shrinking of critical dimensions (CD) and the introduction of new materials, surface treatment in front-end-of-line (FEOL) processes has become extremely critical. CD shrinkage narrows the cleaning process window, making it difficult to balance high cleaning efficiency and low surface/structural damage. Traditional batch wet cleaning (such as tank cleaning) is gradually becoming inadequate.

Advantages: Single wafer cleaning (usually using a rotating spray method) can significantly reduce the risk of cross-contamination between wafers, improve product yield, and potentially reduce costs (by precisely controlling chemical usage).

Typical Process:

Core Steps (Room Temperature): Spray cleaning using DI-O₃ / DHF solution in a cycle.

DHF: Etches silicon oxide, removing particles and metallic contaminants.

DI-O₃: Forms silicon oxide on the surface.

By precisely controlling short spray/reaction cycles, satisfactory cleaning results can be achieved and cross-contamination avoided.

Final Rinse: Using deionized water (DIW) or ozonated deionized water (DI-O₃).

Drying: Commonly uses isopropyl alcohol (IPA) mixed with a large amount of nitrogen (N₂) for spin drying to avoid watermarks.

3.2 Dry Cleaning

Dry cleaning uses chemical vapor deposition techniques to remove impurities from the wafer surface without involving liquids. The main technologies include thermal oxidation and plasma cleaning.

Principle: Introduce hot chemical gases or activated gases in plasma into the reaction chamber. The reactive gases chemically react with contaminants on the wafer surface, generating volatile products that are subsequently removed by a vacuum system.

Main Methods:

Thermal oxidation: such as annealing in a furnace (often in an oxygen-containing environment). In-situ argon (Ar) sputtering cleaning is often performed before sputtering deposition.

Plasma cleaning: Using methods such as laser, microwave, and thermal ionization to excite reactive gases (such as O₂, H₂, N₂, CF₄, Ar) into highly active plasma (containing ions, electrons, and free radicals). Reactive particles react with surface molecules to generate volatile products that are removed.

Advantages:

1. No liquid waste is generated.

2. Selective or local area processing can be achieved.

3. The anisotropy of plasma etching is conducive to the formation of fine patterns.

Limitations:

1. It is difficult to selectively react only with metal impurities without affecting the silicon substrate.

2. Volatile compounds of different metal contaminants have different vapor pressures. Under specific temperature and time conditions, it is difficult to completely remove all metal contaminants (e.g., complete removal of Fe, Cu, Al, Zn, Ni requires specific conditions, Ca can be volatilized at low temperatures using Cl-containing chemicals).

Conclusion: Dry cleaning cannot completely replace wet cleaning. In actual production, a strategy combining dry cleaning and wet cleaning is often adopted.

IV. Summary

Cleaning is one of the most frequently used processes in semiconductor chip manufacturing. The quality of cleaning has a decisive impact on the yield, device performance, and reliability of IC processes. However, improper handling of the cleaning process can lead to serious environmental burdens, and extensive recycling consumes huge amounts of chemicals and deionized water.

To address these challenges, the industry has developed and applied various technologies:

Diluted chemical cleaning and IMEC cleaning significantly reduce the consumption of chemicals and DIW in traditional wet cleaning.

Dry cleaning avoids waste liquid problems and provides unique processing capabilities.

Combined dry/wet cleaning has become an effective strategy for meeting high-demand cleaning tasks.

With the continuous shrinking of process nodes, increasing integration (e.g., removal of finer particles), the introduction of new materials, and the development of three-dimensional structures, semiconductor cleaning technology faces increasingly severe challenges (such as cleaning uniformity, selectivity, material compatibility, and micro-damage control). Continuous research is dedicated to developing more efficient and environmentally friendly cleaning technologies. For example, the effective synergy of megahertz acoustic energy and chemical cleaning has been proven to enhance the removal capability of submicron and even nanometer-level particles. In the future, higher-precision semiconductor manufacturing will continue to drive innovation and development in cleaning technology.

 

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