Hi everyone

This question came up in another thread, but I have started a new one so
that more people will see the info.

It is recommended that deep silicon etchers (ICP systems running deep RIE
processes using SF6) should not be used for etching oxides.

The reason for this is not fully understood, but the recommendation is based
on experience.  Running oxide etches in deep silicon process chambers has
been found to affect the process repeatability, to the point that it is not
possible to get back to the original process performance.  Etch rate,
profile and uniformity are all affected.  Some systems appear to be more
sensitive to this than others.  The most critical part of the chamber is the
ICP tube.  Some success has been seen in swapping ICP tubes, keeping one for
deep silicon work and a second tube for everything else.  This only works if
it is reasonably easy to swap the tubes (and you have/are a competent
technician).

The proposed mechanism is that fluorocarbon polymer deposited during the
oxide etching affects the surface of the ICP tube, possibly even penetrating
open porosity in the ceramic.  Extended cleaning does not remove all of this
polymer (and is not good for aluminium parts in the chamber).  This affects
the amount of oxygen present on the surface of the tube, which shifts the
free radical concentration gradients within the chamber.

This may be complete rubbish, but no-one has produced any experimental data
to indicate whether this is true or not.

This does have implications for MEMS fabrication processes.  Sometimes,
oxide is used as a mask for deep silicon etching.  In general, it is better
not to proceed directly from etching the mask into the deep etch process in
the same chamber (but there are exceptions to this).  Again, if MEMS
structures are being fabricated on SOI wafers and it is necessary to remove
the buried oxide to release structures, this too should be done in a
different system.

If anyone reading this has experiences, either to back up this advice, or
showing opposite effects, please post them, so we all can benefit from the
discussion.

Martin Walker B.Sc.(Tech.) M.Sc.
Tactical Marketing Engineer
Oxford Instruments Plasma Technology
North End, Yatton, Bristol, BS49 4AP UK
T.+44 (0)1934 837031  F.+44 (0)1934 837001
E. 
W. 


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