He O-ring in PlasmaQuest

jim kruger jimkruger at yahoo.com
Tue Nov 23 09:41:03 PST 2004

What is the policy on wafer chuck O-ring material (and
hence chuck temperature limit) in PlasmaQuest?   I
assume 120 C is the limit for silicone O-rings and 220
or 250 C for the Kalrez.  I am interested in high
temperature operation and I believe Hatice Altug is as
well.  I use the chiller cooling up to 90 C  (chiller
at 80 C) but switch to the N2 gas cooling for higher

I see that the O-ring material was changed back to
silicone last night; I encourage use of the higher
temperature Kalrez O-ring if it is feasible.  What
will the policy be? 

I see that the new O-ring last night appears to have
been overheated already.  This would be consistent
with a blocked or restricted He line to the wafer
chuck.  On Sunday, 11/21/04, I noticed (and sent a
comment) that the He flow at 10 Torr had dropped back
to a more normal 6 sccm but that with no wafer, the
flow was only 8 sccm.  This seems wrong to me since 
16 or even 25 sccm He flow at 10 Torr was recently
reported.  I was using the chiller at 80C with the
chuck at 90 C.

   I will check “no wafer” flow as part of “system
normal” checks on start up and I encourage others to
do the same. 

 The issue is that the wafer backside is usually
monitored and servoed at some distance from the chuck,
connected by long, thin tubing to the chuck.  Any
substantial leakage will result in a pressure drop in
the tubing such that the actual pressure cooling the
back of wafer might be too low.  An estimate of the
pressure drop can be had by noting the He flow for “no
wafer”, when the pressure at the end of the tubing is
essentially zero.  When a wafer is in place, if the
flow past the wafer is only 10 or 20% of this maximum
flow, then the pressure at the back of the wafer must
be close to the measured 10 Torr and so adequate for
cooling.  If the flow past the wafer is 50% or
greater, the pressure at the back of the wafer is too
low for good thermal contact so at high power, the
wafer can overheat and stick to the O-ring, not to
mention the probable shift in the etch behavior.

I am happy to discuss this with anyone who might have
questions.  I would like to understand the He circuit
so would be glad to participate with Elmer or Cesar 
(or Jim McVittie) in looking into this problem.

 (jimkruger at snf.stanford.edu or jimkruger at yahoo.com)

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