wet oxidation

Wet oxidation grows silicon dioxide by exposing silicon wafers to water vapor (H₂O) or a steam/oxygen mixture at 800-1100°C, producing oxide 5-10× faster than dry oxidation—used for thick field oxide, isolation oxide, and applications where growth rate matters more than ultimate oxide quality. Reaction: Si + 2H₂O → SiO₂ + 2H₂ at the Si/SiO₂ interface. Water molecules diffuse through the oxide faster than O₂ due to their smaller molecular size and higher solubility in SiO₂, resulting in significantly higher growth rates. Steam generation methods: (1) external torch (H₂ and O₂ burn in an external torch to generate steam, which flows into the process tube—the pyrogenic method; most common), (2) bubbler system (carrier gas bubbles through heated DI water to create water vapor—simpler but less pure), (3) in-situ steam generation (ISSG—H₂ and O₂ introduced directly into the furnace tube at low pressure where they react on the wafer surface; produces thin, high-quality oxides with growth rates between dry and traditional wet). Growth rates: at 1000°C, wet oxidation grows approximately 100-500nm/hour (compared to 5-10nm/hour for dry oxidation). At 1100°C, rates exceed 1μm/hour for thick oxide growth. Oxide quality: wet oxides have lower density than dry oxides, higher hydrogen content (Si-OH bonds), slightly lower breakdown voltage (8-10 MV/cm vs. 10-12 MV/cm for dry), and higher fixed charge density. These are acceptable for non-critical applications. Applications: (1) field oxide / LOCOS isolation (thick oxide 300-600nm for device isolation—speed is essential), (2) STI liner oxide (thin oxide lining shallow trenches before fill), (3) hard mask oxide (thick oxide for etch masking), (4) passivation oxide (surface protection layers). The Deal-Grove model applies with different rate constants—higher linear and parabolic rate constants for H₂O compared to O₂ oxidation.