I've recently been looking at diffusion constants in gas mixes and I
think the strongest effect is due to a significant increase in the
diffusion constant with the addition of He. This is important since the
reactive species must diffuse through a stagnate surface layer of gas.
Here are some numbers from a CVD process around atmosphere: CH4 in CH4
Diff. Coef. = 3.2436 cm^2/s; CH4 (20%) in He = 9.6614 cm^2/s. I think
this is likely to apply even at the low pressures used in RIE.
-Mike
>>> [email protected] 04/05/02 06:02AM >>>
Dear Peng,
A possible explanation for your observation of increased etch rate
with
higher He addition to SF6 etch gas could be, that the He increases the
concentration of free radicals (eigher ionized or non-ionized). Since
SF6 alone doesn't etch anything at all, the etch rate is primarily
controlled by the number (and kind) of reactive species in the process
chamber at a time and the bias voltage at the substrate. The He is
easily ionized and thus can enhance ionization of SF6 itself.
Best regards,
Joern Koblitz
******************************
Joern Koblitz
microFAB Bremen GmbH - the MEMS Wafer Foundry
Universitaetsallee 5
D-28359 Bremen, Germany
Phone: +49 421 24 100 0
Fax: +49 421 24 100 99
eMail: [email protected]
website: www.microfab.de
******************************
-----Original Message-----
From: Peng Yao [mailto:[email protected]]
Posted At: Thursday, March 28, 2002 9:48 PM
Posted To: MEMS-talk
Conversation: [mems-talk] Helium's role in RIE
Subject: [mems-talk] Helium's role in RIE
Hi all,
I am using SF6+He to etch silicon. I thought He will be helpful to get
a
smother surface. I fixed all the other parameters like pressure,
power,
etching time... and only change the flow rate of SF6 and He. The
strange
thing is that the etching speed of SF6:He=10:20 was larger than that
of
SF6:He=20:20. (They were etched in same machine and at same day, one
after the other) And also I found that if I fix the gas flow of SF6
and
only change the persentage of He, the etching rate will increase with
increasing the persentage of He. All these seem to indicate that He
can
not only cool the photoresist, but also will help the etching. Does
anyone
else has similar experience.
And I am not sure what kind effect of He to side wall, will it make
the
process more isotropic or more anisotropic?
Thanks a lot!
Peng Yao
DOEs lab
Electrical Engineering Dept.
Univeristy of delaware
Newark D.E 19716
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