Which one is gets fragmented more easily on surface at heated surface? Also I want to know is there any energy transfer to surface by breaking the SF6 and CF4 bonds occurs and to what extent?
The question can be best explained by first knowing the mechanisms of fragmentation:
by Si surface
CF4 (Freon 14)
CF4 does not directly react with Si; rather it follows the path:
CF4+e----CF3 (free radical) +F (free radical) +e
Both the free radicals adsorb on Si.
Now CF3 and F react to form CF4 which desorbs
The presence of carbon on the surface reduces the Fluorine availability to etch Si. C will react with F and reduce its amount. C remains to form C-F polymers (C2F4/C2F6 /C4F10) which, in turn, inhibits etching.Of couse, high F/C favour etching.
Again, SF6 does not directly react with Si; rather it follows the path:
SF6+e----F +SFx (x in between1 and 5) +e .
SFx (x in between1 and 5) +Si--- SiFx (x in between1 and4) .
Or
SiFx (x in between 1 and 4) +F----- SiFx+1.
The S forms S-F polymers like SF4/SF2/S2F2/S2F10.
The end point being SiF4 which is desorbed .It is strongly hygroscopic.So it is to be water flushed and following reaction takes place.
SiF4+2H2O—SiO2+ SiF6^-2+2H^+ +2HF.
The first step in both the dissociations is the formation of the respective free radicals
and their adsorption onSi. So, overall energy change will be the total change in energies involved in the formation of free radical (CF3 or SF5) from (CF4 or SF6) and their respective adsorption energies on Si. Now their respective adsorption energies do not differ much and so are assumed to have the same values.
So, enery change in CF4:
Enthaly of formation of CF4 (g) - Enthaly of formation of CF3 (g; free radical)
=930.0-(-470.7) = 459.5 kJ/ mol
Again, enery change in SF6:
Enthaly of formation of SF6 (g) - Enthaly of formation of SF5 (g; free radical)
=1220.5-(-912.0) =308.5
As the energy absorbed is more in CF4, the SF6 will get easily fragmented.
[II] Ideally, there should not be any energy transfer to the surface.
[III]Note that it is assumed that the major step is the formation of SF5 free radical.
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