Wafer Oxidation (Step 1)
Oxidation is a fundamental front-end process step where a thin layer of silicon dioxide (SiO2) is thermally grown on the wafer surface. This oxide serves multiple roles: gate dielectric, field isolation, surface passivation, and as a protective barrier during later processing steps. Oxidation is performed at high temperatures in dedicated furnaces or through rapid thermal processing (RTP) systems.
Process Overview
- Purpose: Create a uniform insulating layer of SiO2 on silicon wafers.
- Methods: Dry oxidation (O2 ambient) for high-quality thin oxides; wet oxidation (H2O ambient) for faster growth and thicker oxides.
- Applications: Gate oxide layers, shallow trench isolation (STI), masking layers for ion implantation.
- Iteration: Performed multiple times at different stages of wafer fabrication depending on device design.
Dry vs Wet Oxidation — Comparison
| Attribute | Dry Oxidation (O2) | Wet Oxidation (H2O/Steam) |
|---|---|---|
| Growth Rate | Slow | Fast (˜5–10× dry) |
| Typical Thickness Range | Ultra-thin to thin oxides (˜1–20 nm) | Thicker oxides (˜20–500+ nm) |
| Oxide Quality | Highest density, low defect, superior interface | Lower density, more hydrogen incorporation |
| Temperature (Typical) | ˜900–1150 °C | ˜900–1100 °C |
| Best-Fit Applications | Gate-quality thin oxides, pad ox, precise isolation interfaces | Field oxides, LOCOS/STI liners, sacrificial oxides, power devices |
| Throughput | Lower | Higher |
| Process Control | Excellent thickness control at thin films | Good for thick films; less ideal for sub-10 nm |
| Thermal Budget Impact | High (longer time for target thickness) | High (but faster growth reduces time) |
| Typical Equipment | Vertical/batch furnaces, RTP oxidation (short, thin) | Vertical/batch furnaces with steam/H2/O2 injection |
| Pros | Best electrical quality; tight interface control | High throughput; efficient thick oxide growth |
| Cons | Slow growth; cycle-time penalty for thick ox | Lower film density; not ideal for ultra-thin gate ox |
Major Equipment Vendors
- ASM International: Rapid thermal oxidation and furnace systems.
- Tokyo Electron Limited (TEL): Vertical furnace systems for batch oxidation.
- Kokusai Electric: Batch thermal processing tools for dry/wet oxidation.
- Applied Materials: RTP and oxidation systems for advanced nodes.
Process Consumables
- High-purity oxygen (O2) gas for dry oxidation.
- High-purity water vapor (steam) for wet oxidation.
- Quartz furnace tubes and boats resistant to high temperatures.
Representative Oxidation Vendors
- ASM International (Netherlands): Leading supplier of rapid thermal oxidation (RTO) and furnace systems for advanced logic nodes.
- Tokyo Electron Limited (TEL, Japan): Provides vertical furnace systems widely used for batch oxidation in 200 mm and 300 mm fabs.
- Kokusai Electric (Japan): Specializes in batch oxidation, diffusion, and LPCVD systems, with a strong presence in Asia.
- Applied Materials (U.S.): Supplies oxidation-capable RTP tools integrated into front-end process flows.
Representative Consumables & Gas Suppliers
- Air Liquide (France): Global supplier of high-purity O2, H2, and process gases for wet and dry oxidation environments.
- Linde (Germany): Provides ultrapure oxygen and hydrogen, along with delivery and monitoring systems.
- Messer Group (Germany): Specialty gases including high-purity steam sources for wet oxidation.
- High-Purity Quartz (various suppliers): Used in furnace tubes, boats, and wafer carriers resistant to >1000 °C operation.
Recycling & Efficiency Considerations
- Oxidation furnaces consume significant amounts of O2 and/or steam; recycling is less feasible compared to UPW but efficiency is achieved through batch processing.
- Low-pressure oxidation systems improve gas utilization efficiency and uniformity.
- Some fabs employ thermal recovery and waste-gas treatment systems to lower energy and emissions footprint.
Cleanroom & Environment
- Process carried out inside Class 1–10 cleanroom environments to minimize contamination.
- Dedicated furnace areas separated from photolithography and metrology zones.
- Batch processing (25 wafers per furnace tube) common for cost efficiency.
Advantages & Constraints
- Advantages: High-quality, uniform, defect-free SiO2 layers; excellent dielectric properties.
- Constraints: High thermal budget; diffusion of dopants during oxidation; limited scalability to sub-1 nm gate dielectrics (leading to high-? metal gate adoption).
Market Outlook
While advanced CMOS processes have largely replaced thick SiO2 gate oxides with high-?/metal gate stacks, thermal oxidation remains essential for isolation structures, surface passivation, and specialty devices. Demand for advanced oxidation systems continues in both logic and power semiconductor fabs.