Pramod,

What you are seeing is likely local arching within the plasma.  Local
arching can create feedback that causes the plasma to "flicker" as you
say.  This is usually caused by organic contamination that is outgassing
or even sputtering during the etch cycle.  Higher power levels will also
exacerbate the problem as will moisture in the chamber.  Tuning the
current or plasma density is key to a robust process.  I am curious if
you would see "sparkles in the plasma at lower power levels, say 125 -
175W.  As you know the [F+] radical is very reactive and can be prone to
external reactions.  What I have found to be of help in the past is a
thorough polymeric residue clean of, not just the chamber, but the
electrodes as well, followed by a bake out under HiVac to drive off
moisture.

Good Luck!
Mike

Michael W. Stan
Sr. Technical Sales Representative
MicroChem Corporation
1254 Chestnut St.
Newton, MA 02464
(617) 965-5511 ext. 237
(508) 335-2911 (mobile)
mstan@microchem.com


-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of PRAMOD GUPTA
Sent: Monday, November 12, 2007 6:39 PM
To: General MEMS discussion
Subject: [mems-talk] Reasons for Plasma Flickering in Oxford Plasmalab
80Plus RIE system

Hi

  Does any one know what could be the reasons for Plasma Flickering in
Oxford Plasmalab 80 Plus RIE system?. I am trying to etch polymers with
SF6/O2/CHF3 gas chemistry at 30mTorr, 200W. The recipe was working
perfect earlier. Pure O2 plasma, even at 30mTorr, does not flicker at
all. It starts flickering when you mix O2 with SF6 and/or CHF3. What
could be the reasons?