Wet Oxidation

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1. Mechanism of oxidation process

In an oxidation process of silicon that usually takes place at very high temperature (thermal oxidation), silicon (Si) reacts with either water vapor (H20) or oxygen (O2) to form silicon dioxide,SiO2 on the silicon surface. The reaction is represented by following equations:

Dry oxidation: Si + O2 → SiO2
Wet oxidation: Si + 2H2O → SiO2 + 2H2

The oxidation process can be implemented through diffusion of either water or an oxygen species through the oxide already formed up which then reacts with the silicon at the Si-SiO2 interface. At high temperature, thermal energy makes oxygen molecule moving much faster, which can drive them to diffuse across an existing oxide layer and react with Si to form SiO2. As the oxidation continues, the interface moves into the silicon and a new, clean silicon surface is produced.

The thicker of the film, the lower of the growth rate when the temperature is higher, oxygen molecules moving faster and hence, the quicker the oxide film grown. The oxide film quality is better than that grown when the temperature is low.

2. Steps of wet oxidation

1. There are two nitrogen sources in an oxidation system, one for process application, with higher purity, and another with lower purity for the chamber purge. 2. Nitrogen is used because it is a stable gas and will not react with silicon even at 900oc. 3. For wet oxidation, high purity oxygen gas is used to oxidize silicon. 4. Since the quartz tube starts to sag when temperature is above 1150oC, the dry oxidation process usually operates at about 900oC. 5. When the silicon dioxide is forming on the single-crystal silicon surface, there is an abrupt change at the silicon-silicon dioxide interface. Post oxidation annealing in N2/H2 ambient is a common technique to reduce the interface state charge. 6. During the system idle time, the furnace is always kept at a high temperature,...
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