The wafer is placed is the best contact with the chuck as possible.
The chuck is often aluminum, which has a high thermal conductivity.
The problems are that (a) the contact may not be perfect due to wafer
curvature, and
(b) there are always small gaps when two solids are placed in contact due to
surface roughness
(see how the back side of a wafer is rougher?). Solid to solid contact is
only a small percentage of the total area.
Helium fills any space between the wafer and the chuck, improving heat
conduction.
    --Kirt Williams

----- Original Message -----
From: "Yuzhu Li" 
To: 
Sent: Sunday, September 19, 2004 8:09 AM
Subject: [mems-talk] Why He backside cooling?


> 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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