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Views: 657 Author: Ruqinba Publish Time: 2026-07-28 Origin: Site
In the process of semiconductor device manufacturing, chemical cleaning refers to the removal of various harmful impurities or oil stains adsorbed on the surfaces of semiconductors, metal materials, and tools.
The process utilizes various chemical reagents and organic solvents to react with or dissolve contaminants. This is often accompanied by physical measures such as ultrasonication, heating, or vacuuming to achieve the desorption of impurities. Finally, the surfaces are rinsed with large amounts of high-purity hot and cold deionized water to obtain a pristine finish.
Chemical cleaning is a critical step in every process experiment. The quality of the cleaning directly affects experimental results; improper treatment can lead to failed or poor-quality outcomes. Understanding the principles of chemical cleaning is vital for successful process engineering.
Semiconductors are extremely sensitive to impurities. Even trace amounts (one part per million or less) can drastically alter their physical properties. We utilize this sensitivity through controlled doping to create various semiconductor devices. However, this same sensitivity makes unintended contamination a major hurdle. Chemical reagents, production tools, and even rinse water can become sources of contamination. Even a clean silicon wafer will attract impurities if exposed to air for an extended period. Chemical cleaning exists to eliminate these contaminants and maintain surface integrity.
Chemical cleaning primarily encompasses three areas:
Wafer Surface Cleaning: Cleaning the silicon substrate itself.
Metal Material Cleaning: Cleaning metals used in evaporation, such as tungsten wires for electrodes, molybdenum sheets for pads, aluminum alloys for sources, and chromium for mask plates.
Tool and Vessel Cleaning: Cleaning metal tweezers, quartz tubes, glassware, graphite molds, and plastic/rubber products.
Molecular contaminants typically adsorbed on silicon surfaces include natural or synthetic oils, resins, and waxes. These are often introduced during substrate preparation steps like cutting, grinding, and polishing. Additionally, oils from operators' fingers, photoresists, and organic solvent residues fall into this category.
Molecular adsorption is characterized by physical attraction, often maintained by Van der Waals forces or electrostatic pull. Since oils and resins are generally non-polar, they interact with the residual forces of unbonded silicon atoms on the surface. These forces are relatively weak and diminish quickly as distance increases (typically effective within 2–3 Å). While these impurities are easy to remove physically, they are mostly water-insoluble. When adsorbed, they create a hydrophobic surface that prevents cleaning solutions (acids or bases) from making effective contact with the wafer, hindering the chemical cleaning process.
Common ionic impurities include K+,Na+,Ga2+,Mg2+,Fe2+,H+,(OH)−, F−,Cl−,S2−, and(CO3)2−. These originate from air, tools, equipment, chemical reagents, low-purity deionized water, exhaled breath, and perspiration.
Ionic adsorption falls under chemical adsorption. These ions bond with the silicon surface through chemical bonds. The distance between the impurity ion and surface atoms is so small that they effectively become part of the silicon structure. Depending on their nature, they can act as electron traps (acceptors) or hole traps (donors), severely impacting electrical performance. Due to the strength of chemical bonds, removing ionic impurities is significantly more difficult than removing molecular ones.
Atomic contaminants primarily refer to metal atoms such as gold, silver, copper, iron, and nickel. These are often introduced via acidic etching solutions, where metal ions are reduced to atoms through displacement reactions and adsorbed onto the surface.
Atomic adsorption is the strongest and most difficult to remove. Since heavy metals like gold and platinum do not react with standard acids or bases, specialized reagents like Aqua Regia must be used. These reagents form soluble complexes with the metal atoms, which can then be rinsed away with high-purity deionized water.
Based on the analysis above, molecular impurities are the easiest to remove but can "mask" ionic and atomic impurities. Therefore, they must be cleared first.
The general cleaning sequence is:
Degreasing → Deionization → Deatomization → Deionized Water Rinse
While this is the standard logic, every experiment requires a flexible approach. Because the surface condition varies before each test, the choice of chemicals and the focus of the cleaning must be adjusted based on specific requirements and desired results.
Metal materials and tools used in experiments are major sources of contamination. To ensure wafer cleanliness, metals and vessels must be treated carefully. The general principle is to remove grease first, followed by etching or cleaning with acids, bases, or Aqua Regia, followed by a final high-purity deionized water rinse.
Semiconductor experiments frequently involve hazardous chemicals and gases, including flammable, explosive, toxic, and corrosive substances. Proper handling is essential to prevent accidents. In the event of an emergency, stay calm and take immediate corrective measures.
Common solvents include toluene, acetone, ethanol, methyl ethyl ketone, trichloroethylene, and carbon tetrachloride.
Flammability: Most are highly flammable. Store in cool areas away from ignition sources.
Heating: Never heat directly on an electric stove; use a water bath.
Fire Suppression: Use wet cloths or sand for small fires. For larger fires, use CO2 or foam extinguishers. Do not use water or water-based acid-base extinguishers.
Toxicity: These solvents are volatile and toxic. Always operate within a fume hood and ensure strong ventilation.
Common reagents include strong acids (sulfuric, nitric, hydrochloric, hydrofluoric, and Aqua Regia) and strong bases (sodium hydroxide, potassium hydroxide). These are highly corrosive to human tissue and clothing.
Ensure high-quality ventilation in areas with acid/base vapors.
Wear appropriate protective clothing (rubber gloves, masks, etc.).
Strict Prohibition: Never use your mouth to draw liquid into a pipette; use a rubber bulb.
Handle bottles with care to prevent tipping or breakage. Point bottle openings away from people when unsealing.
Dilution Rule: Always slowly pour acid into water; never pour water into concentrated acid.
Neutralize concentrated acids or bases by diluting them in water first before adding the neutralizing agent.
Rinse containers thoroughly after use. In case of chemical burns, rinse with massive amounts of water immediately. If splashed in eyes, rinse with tap water and seek immediate medical attention.
Gases are supplied via cylinders or pipelines. Mixed gases can be flammable or explosive.
Identification: Cylinders are color-coded and labeled for identification.
Pressure: New cylinders have high pressure (approx. 150 kg/cm ²).
Storage: Keep cylinders out of direct sunlight in summer. Keep away from flammable materials and open flames.
Separation: Reactive gases (e.g., hydrogen and oxygen) must be stored in separate rooms to prevent accidental mixing.
Residual Pressure: Never deplete a cylinder completely; always leave a small amount of residual gas.
Contamination: Keep cylinder valves and wrenches free of grease and oil.
Hydrogen Safety: Before use, purge the system with inert gas and check for leaks. Flashback arrestors must be installed.
To prevent poisoning when using toxic reagents:
All operations must be performed in a fume hood. Residual liquids must be disposed of at designated safety sites.
Operators must wear masks and gloves. Avoid skin contact or ingestion.
Wash gloves thoroughly with water before removing them after the experiment.
