Hi

The reason for Helium back-side injection is to improve the thermal contact
between the wafer and the electrode.  Two solid surfaces will only be in
three point contact, so this will not give much cooling at all, especially
in a vacuum.  Effectively, the only way that the substrate will lose heat is
by radiation.  At normal plasma etch temperatures, this is very inefficient.


If a thin layer of helium is injected between the wafer and the electrode,
this will provide a much better heat path than the three points.  A thin
(few microns) layer of Helium is quite good at conducting heat.

Many systems do not use stainless steel as electrodes either, for two
reasons.  These can sputter, giving iron contamination in the chamber (not
good!) and also have poor thermal conductivity, resulting in hot-spots on
the electrode.  Often, if the electrode temperature is not going over about
350 degrees C, then an aluminium alloy will be used.  This gives a much more
uniform temperature across the electrode face (and hence on the wafer).

Martin

> Message: 1
> Date: Sun, 19 Sep 2004 11:09:57 -0400 (EDT)
> From: Yuzhu Li 
> Subject: [mems-talk] Why He backside cooling?
> To: [email protected]
> Message-ID: 
> Content-Type: TEXT/PLAIN; charset=US-ASCII
>
> Hi, I have a naive question about plasma etcher: it is very
> importan to
> cool the wafer during etching, then why not put wafer on the
> stainless steel
> subtrate directly, why use He gas cooling? since steel have
> much higher
> thermal conductivity than He gas.
>
>

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