Download lecture6_Thermaloxidation

Thermal oxidation of Si:
SiO2 – good electrical isolator
Barrier to dopants
Fick’s first law of diffusion:
The particle flow per unit area J (particle flux) is directly
proportional to the concentration gradient of the particles
(N concentration of particles)
D = diffusion coefficient
Neg. sign = particles are moving from more concentrated to
less concentrated area
Assume: J is constant throughout the SiO2 volume – but is
still a function of time.
x = distance from surface of wafer
No = concentration at the surface
Ni = concentration at the interface between Si and SiO2
At Si-SiO2 interface
assume that the oxidation rate is proportional to the
concentration of the oxidant:
ks = rate constant for the reaction at the Si-SiO2 inerface
The rate of change of thickness of the oxide layer with time
is given by the oxidizing flux J divided by the number of
molecules of the oxidizing species that are incorporated
into a unit volume (M)
For thin layer time t is small so that
B/A = linear (growth) rate constant
Thin film limitation is caused by the reaction time in SiO2
B = parabolic rate constant