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Chemical Cleaning and Safety

In semiconductor device process experiments, chemical cleaning refers to the removal of various harmful impurities or oil stains adsorbed on the surfaces of semiconductors, metal materials, and utensils. The cleaning method utilizes various chemical reagents and organic solvents to react chemically and dissolve impurities and oil stains adsorbed on the surface of the cleaned object, or accompanied by physical measures such as ultrasound, heating, vacuuming, etc., to desorb (or desorb) impurities from the surface of the cleaned object, and then rinse with large quantities of high-purity cold and hot deionized water to obtain clean surfaces.

1.1 Importance of Chemical Cleaning

Chemical cleaning is a crucial aspect of every experiment in the process. The quality of chemical cleaning significantly impacts the experimental results. Improper treatment may result in poor or even unusable experimental outcomes. Therefore, understanding the role and principles of chemical cleaning is of vital importance for conducting process experiments effectively. It is well known that one of the important characteristics of semiconductors is their sensitivity to impurities. Even trace amounts of impurities, in the order of parts per million or even lower, can affect the physical properties of semiconductors. This characteristic is utilized in the fabrication of various semiconductor devices through doping methods. However, it also brings challenges and difficulties to semiconductor device process experiments. The chemical reagents, production tools, and cleaning water used may all become sources of harmful impurity contamination. Even semiconductor chips, if exposed to the air for a prolonged period, can introduce significant impurity contamination. Chemical cleaning aims to eliminate harmful impurity contamination and maintain the cleanliness of silicon wafer surfaces.

1.2 Scope of Chemical Cleaning

Chemical cleaning mainly includes three aspects of cleaning: first, cleaning of silicon wafer surfaces; second, cleaning of metal materials used (such as tungsten wire for evaporation electrodes, molybdenum sheets for evaporation boat plates, aluminum alloys for evaporation sources, chromium for chrome plates, etc.); and third, cleaning of tools and utensils used (such as metal tweezers, quartz tubes, glass containers, graphite molds, plastic and rubber products, etc.).

1.3 Types of Impurities Adsorbed on Silicon Wafer Surfaces

(1) Adsorption of Molecular Impurities

Typical impurities adsorbed in molecular form on silicon wafer surfaces include natural or synthetic oils, resins, and oils. Impurities introduced during substrate preparation processes such as cutting, grinding, and polishing belong to this type. Additionally, residues of operator's fingerprints, photoresists, and organic solvents also fall into this category.

The characteristic of molecular impurity adsorption is that their contact with the silicon wafer surface is usually maintained by electrostatic forces, representing a physical adsorption phenomenon. Since molecules of natural or synthetic oils, resins, and oils generally exist as non-polar molecules, they are attracted to the residual forces on the silicon surface due to unbalanced silicon atoms. The binding force is similar to the van der Waals force between molecules in a molecular crystal structure. This attractive force is relatively weak and diminishes rapidly with increasing molecular distance. Therefore, thoroughly removing these molecular impurities is relatively easy. Another important characteristic of molecular impurities is that they are mostly organic compounds insoluble in water. When adsorbed on the silicon wafer surface, they render the surface hydrophobic, hindering effective contact between deionized water or acid-base solutions and the silicon wafer surface, making it impossible to perform effective chemical cleaning.

(2) Adsorption of Ionic Impurities

Impurities adsorbed on silicon wafer surfaces in ionic form typically include K+, Na+, Ga2+, Mg2+, Fe2+, H+, (OH)-, F-, Cl-, S2-, (CO3)2-, etc. These impurities have the widest range of sources and can come from various aspects such as air, utensils and equipment, chemical reagents, low-purity deionized water, tap water, gases exhaled by operators' nose and mouth, sweat, etc.

Adsorption of ionic impurities mostly belongs to the category of chemical adsorption. The main characteristic is the chemical bond between impurity ions and the silicon wafer surface, with these impurity ions reaching an equilibrium distance with the surface atoms of the silicon wafer, so much so that these impurity ions are considered part of the silicon wafer as a whole. Depending on the properties of chemically adsorbed impurities, some may serve as trapping centers for lattice free electrons, acting as acceptors; others may serve as trapping centers for free holes, acting as donors. Due to the strong chemical adsorption force, removal of these impurity ions is much more difficult compared to molecular impurities.

(3) Adsorption of Atomic Impurities

The adsorption of impurities in atomic form on the surface of silicon wafers, forming contamination, mainly refers to metal atoms such as gold, silver, copper, iron, nickel, and so on. These metal atoms generally originate from acidic etchants, where metal ions are reduced to atoms through displacement reactions and then adsorbed on the surface of silicon wafers.

The adsorption force of atomic impurities is the strongest, making them relatively difficult to remove. Moreover, heavy metal atoms such as gold and platinum do not readily undergo chemical reactions with common acidic or alkaline solutions. Therefore, chemical reagents such as aqua regia must be used to form complexes with them, dissolving them in the reagent before being rinsed away with high-purity deionized water.

1.4 General Procedure for Silicon Wafer Cleaning

From the analysis above, molecular impurities are relatively easy to remove. The presence of such impurities has a masking effect on the removal of ionic and atomic impurities. Therefore, when performing chemical cleaning of silicon wafers, they should be cleaned thoroughly first.

Impurities adsorbed in ionic and atomic forms belong to chemisorbed impurities, and their adsorption forces are relatively strong. In general, the amount of atomic adsorbed impurities is small, and they do not undergo chemical reactions with acids or bases, requiring the use of chemical reagents such as aqua regia or acidic hydrogen peroxide for dissolution. Aqua regia and acidic hydrogen peroxide can also dissolve ionic impurities. Therefore, in chemical cleaning, acidic or alkaline solutions or alkaline hydrogen peroxide are typically used first to remove ionic adsorbed impurities, followed by aqua regia or acidic hydrogen peroxide to remove any remaining ionic and atomic impurities. Finally, the wafer is rinsed clean with high-purity deionized water.

In summary, the general procedure for cleaning silicon wafers is: degreasing → deionization → deatomization → rinsing with deionized water.

Before each experiment, silicon wafers must undergo chemical cleaning. However, since the surface condition of silicon wafers varies before each cleaning, the specific chemical reagents used and the focus of cleaning may vary. The cleaning process is flexible and must be tailored based on specific conditions, considering the actual effectiveness of the cleaning to determine the specific methods and steps for each cleaning operation.

1.5 Cleaning and Handling of Common Metals and Utensils

The metals and utensils used in experiments are also one of the main reasons for silicon wafer contamination. To ensure the cleanliness of the silicon wafer surface, meticulous cleaning and handling of metals and utensils must be done. The general principle of cleaning metals and utensils is to first remove grease substances, then clean with acid, alkali, or detergent, and finally rinse thoroughly with high-purity deionized water.

Safety Knowledge

Various chemical reagents and gases are frequently used in experiments, some of which are flammable, explosive, toxic, or corrosive acids and bases. Mishandling can lead to accidents. First, accidents should be avoided; however, in case of an emergency, it's important not to panic and take appropriate measures promptly to troubleshoot.

2.1 Safe Use of Organic Solvents

Common organic solvents include toluene, acetone, ethanol, butanone, trichloroethylene, carbon tetrachloride, and others. Most of these organic solvents are flammable and can ignite when exposed to high temperatures or flames. They should be stored in a cool place away from ignition sources. When heating is required, it's advisable to use a water bath for heating. In case of fire, it can be extinguished with a damp cloth or fine sand. It's best to use carbon dioxide, carbon tetrachloride fire extinguishers, or foam fire extinguishers rather than water or water-containing acid-base fire extinguishers.

These organic solvents are volatile and have varying degrees of toxicity. Therefore, when using them, it's important to operate in a fume hood with enhanced ventilation.

2.2 Safe Use of Acids and Bases

Sulfuric acid, nitric acid, hydrochloric acid, hydrofluoric acid, aqua regia, sodium hydroxide, potassium hydroxide, and other strong acid solutions or strong alkaline solutions are commonly used in experiments. They have strong corrosive effects on the human body and clothing, so safety precautions must be taken.

(1) Good ventilation and exhaust equipment should be installed in places where acid and alkali vapors are present.

(2) Operators should wear appropriate protective clothing (such as rubber gloves and masks).

(3) It's strictly prohibited to suck acid or alkali liquids into pipettes with the mouth; instead, pipettes with rubber bulbs should be used.

(4) Handle with care during transportation to prevent tilting or rupture of acid or alkali bottles. When opening, the bottle mouth should be directed away from people.

(5) When diluting sulfuric acid, it must be slowly poured into water, not vice versa.

(6) When neutralizing concentrated acids or bases, they must be diluted with water first, and then neutralized with acid or alkali solutions.

(7) Containers that have held strong acids or bases should be rinsed thoroughly with water several times before general cleaning.

In case of acid or alkali burns, flush with plenty of water. If acid or alkali splashes into the eyes, rinse immediately with plenty of tap water. Regardless of the severity of the burns or splashes, seek medical attention immediately after flushing.

2.3 Safe Use of Gases

Gases are generally stored and transported in steel cylinders or through gas pipelines. Some gas mixtures can easily cause combustion or explosions. To facilitate the identification of the gases in cylinders, different colors and markings are used to indicate the type of gas contained. In addition, the name of the gas is also labeled. The gases in cylinders usually have high pressure. When storing and using them, safety precautions should be taken.

(1) In summer, do not place gas cylinders in places exposed to sunlight. Flammable items should not be placed near the gas cylinder storage area, and open flames should not be used.

(2) Gas cylinders containing gases that can cause combustion or explosion when in contact with each other (such as hydrogen and oxygen cylinders) must be stored separately in individual rooms to avoid accidents caused by leakage.

(3) A certain amount of residual gas should be left in the gas cylinder after use.

(4) The nozzle and trolley of the gas cylinder should not be contaminated with grease.

(5) When using hydrogen, inert gas should be used to purge the air from the equipment, and the system should be checked for leaks before reinstalling the ignition device.

2.4 Safe Use of Toxic Substances

When using toxic chemical reagents and compounds in experiments, precautions should be taken to prevent poisoning. The principles of safe use are as follows:

(1) Operations should be carried out in a fume hood, and residues or liquids should be disposed of properly after use.

(2) Operators should wear masks and gloves and avoid direct contact with the skin or ingestion.

(3) Gloves must be rinsed thoroughly with water after use before removal.

These translations can help ensure safety and proper handling during experiments.

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