Steps for cleaning silicon wafers

Wafer cleaning is a crucial process in semiconductor manufacturing, aiming to remove surface contaminants (particles, organic matter, metal ions, native oxide layers, etc.) to ensure the quality and yield of subsequent processes (such as lithography, oxidation, diffusion, and thin film deposition).

I. Initial Cleaning/Organic Cleaning

Purpose: To remove grease, wax, rosin, photoresist residue, organic solvent residue, and other organic contaminants from the wafer surface.

Methods:

1. Organic solvent immersion/ultrasonic: Commonly used solvents include Acetone, Isopropyl Alcohol (IPA), N-Methyl-2-pyrrolidone (NMP), or other specialized stripping solutions. Ultrasonic agitation significantly improves cleaning efficiency.

2. Specialized photoresist remover: For stubborn photoresist, strong specialized stripping solutions may be used.

II. Chemical Cleaning (Wet Etching/Cleaning)

This is the core step for removing inorganic contaminants (metals, oxides). Commonly used systems include:

1. Acid Cleaning:

Purpose: To remove metallic impurities (such as Fe, Cu, Zn, Al), some organic matter, and thin native oxide layers.

Common solutions:

①Diluted HF (DHF): HF: H₂O = 1:50 to 1:100. Main function: Selectively removes the silicon dioxide (SiO₂) layer (native oxide layer) from the silicon surface, etching silicon itself very slowly. Important note: After HF treatment, the wafer surface becomes hydrophobic.

②Hydrochloric acid/hydrogen peroxide mixture (HPM or SC-2): HCl : H₂O₂ : H₂O = 1 : 1 : 5 to 1 : 2 : 8 (usually heated to 65-85°C). Main function: Effectively removes alkali metal ions (such as Na⁺, K⁺) and metal ions such as aluminum, magnesium, and iron. Hydrogen peroxide (H₂O₂) oxidizes metals, and hydrochloric acid (HCl) dissolves them into soluble chlorides.

③Sulfuric acid/hydrogen peroxide mixture (SPM or Piranha): H₂SO₄ : H₂O₂ = 2:1 to 4:1 (usually heated >100°C). Main function: Effectively removes stubborn organic matter, photoresist residue, and some metals. It produces a violent reaction and requires extremely careful operation. Note: Due to its high risk, environmental impact, and potential introduction of sulfate residue, its use in modern processes is gradually decreasing, with a preference for alternative solutions.

Operation: Immersion or spraying, at a specific temperature and time. Key: After each acid cleaning step, thorough rinsing with a large amount of ultrapure water (UPW) is necessary to avoid cross-contamination of solutions.

2. Alkaline/Oxidant Cleaning:

Purpose: To remove particles, light organic matter, and grow a thin, uniform hydrophilic chemical oxide layer (SiO₂) on the silicon surface.

Common solutions:

①Ammonium hydroxide/hydrogen peroxide mixture (APM or SC-1): NH₄OH : H₂O₂ : H₂O = 1 : 1 : 5 to 1 : 2 : 7 (usually heated to 65-85°C). Main function: Core particle removal mechanism. Ammonium hydroxide (NH₄OH) slightly etches silicon, and hydrogen peroxide (H₂O₂) oxidizes the silicon surface and stabilizes the solution. Particles are removed through the "sacrificial layer" effect of repeated growth/dissolution of the oxide layer. The effect on removing some metals (such as Cu) is limited, and it may cause "Metal Induced Precipitation" (MIC) problems.

②Dilute KOH/TMAH: (Correction: The original text mentioned "beryllium hydroxide," which is highly toxic and rarely used, and has been eliminated.) Sometimes used for specific etching or cleaning purposes, but the concentration and temperature must be strictly controlled to avoid excessive etching of silicon.

Operation: Immersion or spraying, at a specific temperature and time. Key: Thorough rinsing. The surface is hydrophilic after APM cleaning.

3. RCA Standard Cleaning Method:

Purpose: To provide a standardized and efficient wet chemical cleaning process for comprehensive removal of various contaminants.

Typical sequence (adjustable according to the process):

SPM (or substitute): Effectively removes organic matter/heavy organic matter.

DHF: Removes the oxide layer or native oxide layer that may form after SPM, exposing a fresh silicon surface (hydrophobic).

APM (SC-1): Primarily removes particles and light organic matter, and makes the surface hydrophilic.

DHF (optional): Removes the oxide layer grown in the APM step again.

HPM (SC-2): Removes metal ions (especially alkali metals and transition metals).

III. Physical Cleaning/Mechanically Assisted Cleaning

Purpose: To assist in removing particles, especially larger or strongly adhering particles.

Common methods:

1. Brush Scrubbing/PVA Roller Scrubbing: Using ultrapure water or chemical solution with a soft brush head (nylon, PVA - polyvinyl alcohol) to mechanically scrub the surface. It is an effective method for removing submicron particles. Brush pressure and speed must be strictly controlled to prevent scratching.

2. Megasonic Cleaning: (Increasing importance) Using high-frequency sound waves (usually 0.8-1 MHz) to generate acoustic cavitation in the cleaning solution. It is gentler and more concentrated in energy than ultrasonic waves, effectively removing submicron particles with extremely low risk of damage to the wafer. It is often used with APM (SC-1) or other chemical solutions to significantly improve particle removal efficiency (PRE).

3. High-Pressure Jet Spray: Uses high-pressure (tens to hundreds of Bar) ultrapure water or chemical solution to spray the wafer surface. Effective in removing specific types of large particles or patterned wafer sidewall residues. Requires precise control of pressure, angle, distance, and fluid properties to avoid damage and electrostatic accumulation (antistatic agents can be added).

IV. Special Treatments and Advanced Technologies

1. Ozonated DI Water Cleaning: Utilizing the strong oxidizing properties of ozone (O₃) dissolved in UPW to remove organic matter and some metals. It is an environmentally friendly and low-cost alternative or supplementary solution, often used before APM or as a final rinse.

2. Electrolyzed Ionized Water/Functional Water Cleaning: Uses specially electrolyzed acidic or alkaline ionized water for cleaning. It has a specific oxidation-reduction potential and is effective for certain contaminants with minimal residue.

3. Surface Activation/Passivation Treatment: After cleaning, specific treatments are sometimes performed (such as brief DHF immersion followed by rapid drying, or annealing under a specific atmosphere) to control the surface state (hydrophobic/hydrophilic) and reduce the growth rate of the natural oxide layer before subsequent processes.

4. Single Wafer Processing: As technology nodes shrink and uniformity requirements increase, more and more key cleaning steps are performed in single wafer equipment, enabling more precise chemical distribution, temperature control, mechanical action, and drying, reducing the risk of cross-contamination.

Key Principles and Precautions

Cleanliness: All chemicals, water (must be ultrapure deionized water UPW), gases, environment, and equipment must meet semiconductor-grade cleanliness standards.

Cross-Contamination Control: Strictly separate the processing areas and tools for different chemicals to avoid cross-contamination. Thorough rinsing is critical.

Process Control: Parameters such as temperature, time, concentration, flow rate, megasonic/ultrasonic power must be precisely controlled and continuously monitored.

Safety and Environmental Protection: Many cleaning chemicals (HF, H₂SO₄, H₂O₂, O₃, etc.) are highly corrosive, toxic, or explosive. Operations must strictly adhere to safety regulations. Waste liquid treatment must comply with environmental regulations. Promoting green chemistry (such as reducing SPM usage and finding alternatives) is a trend.

Damage Control: Physical cleaning methods (brushing, megasonics, high-pressure jetting) require optimized parameters to avoid scratches, breakage, or material loss on the wafer surface or edges.

Drying Technology: Effective drying (SRD, Marangoni, IPA Vapor, etc.) is crucial to prevent water mark defects.

 

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