As there appears to be discussion of furnace flows during idle and actual
process conditions, I'll throw in the numbers of how we are currently
running things here at Stanford.
We run 100 mm wafers in oxidation tubes that have an ID of 155 mm
(Cross-sectional area ~189 cm2). The exhaust of these tubes (when the boat
loader is in, of course) is a 1 cm hole in the side of the tube facing the
scavenger (cross-sectional area ~0.8 cm2).
Our typical flows during normal operations include:
1. Dry oxidation: O2 flow of 2.5 slm.
2. Anneal: "N2" flow of 2.5 slm. Note: we actually have Ar plumbed to our
main N2 lines in these furnaces. Because the MFCs are calibrated for N2, an
"indicated" flow of 2.5 slm N2 is actually a real flow of 1.42 * 2.5 = 3.55
slm of Argon.
3. Wet oxiditaion: O2 flow of 1.0 slm. H2 flow of 1.5 slm. Note that this
is a total "input" flow of 2.5 slm. However, assuming that the O2 and H2
react completely to form steam (which we hope it does ....) what we should
actually have is a flow of 1.5 slm of water vapor and an unreacted O2 flow
of 0.25 slm for a total flow of 1.75 slm (and a steam partial pressure of
4. Ramp up: we typically ramp up with a "N2" flow of 2.4 slm (actually 1.42X
of Ar) and 100 sccm O2 to have enough oxygen present during rampup to avoid
pitting, but low enough oxygen to not do too much oxidizing.
5. Boat loading/unloading: Programs typically increase the "N2" flow from
2.5 slm to 5.0 slm (i.e., 7.1 slm of Ar) while the wafers are being
pushed/pulled/loaded/unloaded to help more rapidly expell room air from the
tube. Given the diameter of the open tube and the flows involved, I believe
that it is safe to assume that this flow is INSUFFICIENT to keep room air out
when the tube is open.
6. Idle conditions: we idle with a flow of 2.5 slm flow of "N2" and don't
back it down from normal flow conditions.
Related to the discussion of minimum idle flows to prevent contamination
from room air, I believe that it is useful to consider some of the numbers
that are often quoted related to air velocities at the face of a fume hood
so that gases don't actually escape from the fume hood. I am most familiar
with a face velocity of approximately 100 ft/min (3000 cm/min) to prevent
fumes from escaping which, presumably, is a conservative number. This would
imply that a flow of 3 slm would be required to keep room air out of the
tube if the cross-sectional area of the exhaust port were 1 cm2 assuming
that the exhaust gas had cooled back down to room temperature. It would
also seem that a higher temperature of the exhaust gas would increase this
velocity and make it less likely to get unwanted room air in the furnace
when actually in operation. Of course, these same numbers indicate that it
would be virtually impossible to increase the flows sufficiently to keep
room air out when the wafers are being loaded/unloaded.
Well, those are the numbers from Stanford ...
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