For a commercial system usually a barrel shaped reactor is
used.
Any process, epitaxy for example, is a process where the
adjustment of the different parameters, we have to optimize
the parameters.
Basically the system is operating at a particular
(a). Temperature and,
(b) Pressure.
What is the best temperature ? what is the best pressure?
All depends on your particular requirements.
For epitaxy,, higher growth temperature is good in certain
ways.
The higher the temperature, the higher will be the surface
mobility of the atoms. So, the atoms will have sufficient time
to come and deposit on the substrate surface, align
themselves in a particular direction, maintaining the crystal
orientation of the substrate. In result, the crystal quality will be
better.
But, higher temperature, will cause of
(i). thermal stress and
(ii). the more is the chance of intermixing of the dopants.
As we know, that he thickness of the boundary
layer is inversely proportional to “ v”
if the velocity of the gas flow is increased, the boundary layer
thickness will decrease. Lesser boundary layer problem. But for a
higher flow velocity, more consumption of gas, more
consumption of reactant materials and more cost in terms of
money.
By reducing the pressure, minimized convection effects, a better
temperature stability, lesser the boundary layer problem. This
technique is known as low pressure chemical vapour deposition
(LPCVD), pressure is in the range of 20 to 200 torr.
The term chemical vapour deposition and vapour phase epitaxy
are used almost indiscriminately.
The vapour phase epitaxy (VPE) is nothing but a chemical vapour
deposition (CVD) process
In chemical vapour deposition, we do not specify whether the layer
is going to be crystalline or amorphous.
But in vapour phase epitaxy, always it is a crystalline layer.
All VPE is a subset of CVD.
Now, let us coming to the particular problem of silicon epitaxy.
In this, silicon tetrachloride or chlorosilane are reduced by using
hydrogen.
SiCl4 +H2 = SiCl2 + 2HCl
2SiCl2 + Si → 2Si + SiCl4
This SiCl2 is active reactant species and
SiCl2 +H2 = Si + 2HCl
This reaction will happen only in the presence of sample surface and
thus it is known as surface catalysed reaction.
As the concentration of SiCl4 increases, a reversible reaction will
take place as
SiCl4 +2H2 ↔ Si + 4HCl
As the concentration of silicon tetrachloride is increased, the
growth rate of silicon is going to increase but only up to a certain
point, beyond this instead of growth , etching will start.
And with the increase of the growth rate (greater than
2µm/minute), the epitaxial layer is no longer going to be single
crystal but moving towards polycrystalline growth.
Silicon tetrachloride is the most stable material and it is also
relatively insensitive to trace amount of oxygen present in the
reactor chamber. That is why, silicon tetra chloride was widely
used.
But if we use SiHCl3 or SiH2Cl2 in place of SiCl4 then it offers two
advantages - highest efficiency, also the formation of SiCl2 at a
comparatively lower temperature.
All of them will have HCl as a by product, which is a very
dangerous gas. Therefore, all silicon epitaxial reactors must have
excellent venting system.
The other way for Si epitaxy is the pyrolytic decomposition of
Silane in non-reversible.
SiH4 →Si + 2H2
Advantages :-
1, no HCl.
2, reaction takes place at a relatively lower temperature ie 500 0C,
But disadvantage is to get very abrupt layer (the morphology will
be poorer) and it is not surface catalysed therefore reaction can
take place anywhere in the reactor and sensitive to the presence
of oxygen.
Another concern during epitaxy is that there should be a provision
for doping in the epitaxial layer.
So, how does one incorporate the dopants into the epitaxial
layer?
For doping , hydrides of the dopants (boron, phosphorus and
arsenic ) may be used.
They will decompose and introduce the dopants into the silicon.
One important consideration during epitaxy is that the surface
must be absolutely clean, totally oxide free and damage free
surface. If there is a damage present on the surface, that damage
will travel in the epitaxial layer also. If there is a stacking fault on
the surface, that will travel inside the epitaxial layer.
If patches of oxide on the surface, there will be stresses
generated in the epitaxial layer, because of this might result in
dislocations.
In-situ cleaning and etching of wafer surface can make it
perfectly damage free and oxide free, is performed normally in
two steps.
1. flushed in dry hydrogen at about 12000C, it will reduce any
oxide that might have been present on the sample
2. Anhydrous hydrogen chloride (1 to 5 mole fraction of
hydrogen chloride is used.) diluted with hydrogen is passed
and the etching proceeds ie Silicon tetrachloride plus two
hydrogen gives rise to silicon and four HCl.
Etching rate is 0.5 micron to 2 microns per minute.
Excessive hydrogen chloride or Chlorine will damage the surface
and will be cause of halogen pitting.
Instead of hydrogen chloride, we can also use SF6 for the same .
2SF6 + 4Si = SiS2 + 3 SiF4
