1. Raw Material Extraction
The manufacturing process begins with extracting raw silicon, the second most abundant element in the Earth's crust. Silicon is primarily derived from quartz (SiO₂) through the following chemical reaction in a submerged electric arc furnace:
SiO₂ (solid) + C (solid) → Si (liquid) + CO₂ (gas)At high temperatures, the carbon (usually in the form of coke) reduces silica into metallurgical-grade silicon (MGS), which has a purity of about 98-99%.
2. Purification of Silicon: Chemical Vapor Deposition (CVD)
For semiconductor applications, the purity of silicon must be increased to 99.9999% (often referred to as “six nines” or 6N purity). The Siemens process is used to purify the metallurgical-grade silicon into electronic-grade silicon (EGS). This involves reacting silicon with hydrogen chloride to form trichlorosilane (HSiCl₃), which is then distilled to remove impurities. The purified trichlorosilane is decomposed in a chemical vapor deposition (CVD) reactor, resulting in high-purity polycrystalline silicon:
HSiCl₃ (gas) → Si (solid) + HCl (gas)3. Crystal Growth: Czochralski Method
Once purified, the next step is growing a single-crystal silicon ingot using the Czochralski (CZ) method. In this process:
- High-purity silicon is melted in a quartz crucible.
- A small seed crystal of silicon, with the desired crystal orientation, is dipped into the molten silicon.
- The seed crystal is slowly withdrawn while rotating, allowing molten silicon to solidify in a single-crystal structure as it cools.
Czochralski Crystal Growth Process
- Melt high-purity silicon in a crucible.
- Insert seed crystal into the melt.
- Slowly withdraw the seed crystal while rotating.
- Form a cylindrical ingot of single-crystal silicon.
The cylindrical silicon ingot typically has a diameter of 150 mm to 300 mm, depending on the application.
4. Ingot Shaping and Slicing
The ingot is then subjected to several machining processes:
- Grinding and Shaping: The ingot is ground to a precise diameter and its surface is smoothed. Flat edges, called "flats" or "notches," are added to indicate the crystal orientation and electrical properties.
- Slicing: A diamond wire saw is used to slice the ingot into thin wafers, typically 200 µm to 750 µm thick. This is one of the most critical steps in the wafer manufacturing process, as it determines the thickness uniformity of the wafer.
5. Wafer Surface Treatment and Polishing
The freshly sliced wafers have a rough surface and contain damage from the slicing process. To rectify this, the wafers undergo several steps of surface treatment:
- Edge Rounding: The sharp edges of the wafer are rounded to prevent chipping during handling.
- Etching: The wafer is chemically etched to remove the damaged surface layer and to prepare a uniform surface.
- Polishing: The wafer is polished using a slurry of fine abrasive particles. This results in a mirror-like finish that is flat at the atomic level.
Polishing is typically done in two stages:
- Mechanical Polishing: Abrasives are used to smoothen the wafer.
- Chemical Mechanical Polishing (CMP): A combination of chemicals and abrasives are used to achieve the final ultra-flat surface.
6. Cleaning and Inspection
After polishing, the wafers are thoroughly cleaned using deionized water and chemical solutions. This removes any remaining particles, organic residues, or metallic contamination. A series of inspections are performed to ensure wafer quality:
- Flatness Inspection: Ensures that the wafer is uniformly flat.
- Surface Roughness Inspection: Confirms that the wafer has the desired smoothness.
- Defect Detection: Inspects the wafer for defects like cracks, pits, or contamination.
Silicon Wafer Cleaning and Inspection Process
- Deionized water wash.
- Chemical treatment.
- Rinse and dry.
- Optical inspection for defects.
7. Doping Process (Optional)
In some cases, wafers are doped with specific impurities to modify their electrical properties. Doping introduces controlled amounts of elements like phosphorus, boron, or arsenic to create n-type or p-type semiconductors. This step occurs before the wafers are sent for IC fabrication.
8. Packaging and Shipment
Once the wafers pass inspection, they are packaged in clean, anti-static containers to prevent contamination and shipped to semiconductor fabrication facilities (fabs). At the fab, these wafers will undergo processes like photolithography, etching, and doping to produce integrated circuits and chips.
Conclusion
The process of manufacturing silicon wafers is complex and requires precision at every stage, from crystal growth to final polishing and inspection. Silicon wafers are the foundation upon which the modern electronics industry is built, making the quality of the wafers critical for the performance of semiconductor devices. By mastering the manufacturing process, industries can ensure high yields and reliable semiconductor products.