Factors Influencing Oxidation

A number of factors affect the oxidation rate, including temperature, pressure, crystal orientation, oxygen source (oxygen or water) and impurity doping.

1. Temperature: Oxide growth rate is very sensitive to temperature, because the oxygen diffusion rate in silicon dioxide is exponentially related with temperature, D µ exp(-E_a/kt). Here D is the diffusion coefficient, E_a is the activation energy, k=2.38X10^-23 J/K is the Boltzmann constant and T is the temperature. Increasing temperature can significantly increase both B and B/A, and the oxide growth.[1]

2. Oxygen source: Oxide growth rate is also related to the oxygen source. Dry oxidation with O₂ has a lower oxide growth rate than wet oxidation with H₂O. This is because the diffusion rate of the oxygen molecule O₂ in silicon dioxide is lower than that of hydroxide HO generated from the dissociation of H₂O molecules at high temperature. For example, with <100> silicon at 1000°C, the wet oxide layer grows to »2.2 mm after 20 hours, whereas the dry oxide layer grows to only 0.34mm. Therefore the wet oxidation process is preferred to grow thick oxide layers such as masking oxide and field oxide.[3]

3. Crystal orientation: Oxide growth rate is also influenced by the orientation of the single-crystal silicon. Normally, <111> orientation silicon has a higher oxide growth rate than <100> orientation silicon. This is because the <111> silicon surface has higher silicon atom density than that of the <100> silicon surface. Thus <111> can provide more silicon atoms to react with oxygen and form a thicker silicon dioxide layer.[2]

4. Dopant Concentration: In general, heavily doped silicon is oxidized faster than lightly doped silicon. During oxidation, boron in the silicon tends to be drawn up to the silicon dioxide and causes depletion of the boron concentration at the silicon-silicon dioxide interface. N-type dopants such as phosphorous, arsenic, and antimony have the opposite effect. While oxide grows into the silicon, these dopants are driven deeper into the silicon and the n-type dopant concentration in the silicon-silicon dioxide interface can be significantly higher than its original value. Also the addition of HCl can increase oxidation by about 10%.[2]

5. Pressure can be used to control oxide growth rate. High pressure can increase oxidation rates. Low-pressure decreases oxidation rate and is being investigated for growing very thin oxide required for VLSI.[1]