A number of factors affect the
oxidation rate, including temperature, pressure, crystal orientation, oxygen
source (oxygen or water) and impurity doping.
Oxide growth rate is very sensitive to temperature, because the oxygen
diffusion rate in silicon dioxide is exponentially related with
temperature, D µ
exp(-Ea/kt). Here D is the diffusion coefficient, Ea
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.
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
the wet oxide layer grows to »2.2
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.
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.
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
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.