SOI etching question

Jim McVittie mcvittie at
Tue Oct 22 18:35:12 PDT 2002


I am not convinced that it is totally a charging problem. In particlar, there is
a chemistry change occurring when you hit the bottom oxide layer. There is no
more Si at the very bottom to use up the free F atoms coming from the ions and
generated from the ions hitting the CFx polymer at the bottom. I think it is
likely that part of the problem is excess free F attacking of the sidewall at
the bottom.

The charging issue in that once the Si is gone off the bottom the neutralizing
current path into the Si is gone and the exposed oxide charges up. The voltage
that builds up is large enough to deflect the lowest energy ions to the sidewall
where they help erode the sidewall polymer. The reason for charging to occur is
that the electrons coming from the plasma are not as directional as the ions are
when they get to the wafer surface. This results in not as many electrons making
it down to bottom of structures, thus there is a charge imbalance and charging
occurs. One way that low frequency (50 KHz range) modulation of the RF bias to
the wafer affects charging is that it changes the distribution of ion energies
such that there is a increase in low energy ions. The trejectory of lower energy
ions are more strongly effected by local charging with the result that their
increase relative to the higher energu ions tends to reduce charging voltages
and thus charging effects.

A year or so Aaron Partridge installed a low frequency modification to our tool.
See Aaron about it use.

    Jim McVittie

Masa Rao wrote:

> Hello Fellow STS Users,
>     Have a quick question about etching down to the BOX layer in SOI.  Have
> noticed notching at the device layer / BOX interface and have read some
> papers from STS's website saying that this is a charging effect that can be
> alleviated by running a lower frequency RF plasma when you get close to the
> BOX layer. Heard from Nancy that the STS is equipped for this but she didn't
> know how to use it.  Was wondering if any of you knew how to use it.  Does
> it require any kind of special recipes?  If so, does anyone happen to have a
> few?  Any and all help you guys could offer would be greatly appreciated.
> Thanks!
> Masa

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