The thermal oxide provides the following:
  1. passivates and protects the surface of Si
  2. dielectric material for MOS technology
  3. mask for diffusion & ion implantation
Theory of oxide growth
    SiO2 can be grown from both dry oxygen and water vapor.  The chemical reactions are as follows:
Si + O2 -- SiO2    dry oxygen
Si + 2H20 -- SiO2 + 2H2    wet oxygen (steam)

During the growth of SiO2 some of the silicon is consumed.  This amount is given by:

XsiNsi = XoxNox Nox : density of oxide (#/cm3) Nsi : density of silicon

Xsi : thickness of Si consumed

Xox : thickness SiO2 grown

 Xsi = 0.46Xox The Growth Kinetics

Also
 
J = kNi    ks interfacial reaction rate constant

combine the above two equations.

J = DNo / (Xo + D/ ks)

remember J = #/cm2 sec        J/M = growth rate, M = # of molecules

dXo/dt = J/M = (DNo/M) / (Xo + D/ ks)

Solution for bc conditions    Xo (t=0) = Xi
where
                    Xi2 + AXi = Bt
or
                    t = Xo2 /B + Xo /(B/A) - t

A=2D/k            B=2DNo/M        t = Xi/B + Xi (B/A)

where Xi is the initial thickness

For small value of t

Xo(t) = (B/A)(t + t ) (t + t ) << A2/4B

For long times of t

Xo = (Bt)½ t >> t

Water vapor has a much higher solubility than oxygen is SiO2 which accounts for the much higher oxide rate in a wet atmosphere.

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