Yes, I have seen this, but not the entire underside of the sample,
just along the edges. If you are using carbon containing gases - CF4,
CHF3.. the C-F polymer deposition will cause deposition. It is more or
less isotropic, but the removal only occurs on horizontal surfaces,
and you get a net accumulation along other surfaces. The deposition is
taking place under the wafer by gases diffusing in.   Try a different
grease, or try making it more even and thinner so there is less chance
of gases penetrating it.
These C-F polymers cannot be removed by O2 plasma. You might be able
to sputter clean it. They are hard to remove.


On Fri, Apr 24, 2009 at 11:41 AM, James Paul Grant
 wrote:
> Hello all,
>
> Some colleagues of mine have been doing some ICP silicon etching and have
> come across a major issue.
>
> They use small substrates which have to be attached to a 4 inch silicon
> carrier wafer. The following structure is used (from top to bottom):
>
> Patterned AZ4562 photomask
> Silicon substrate (this will be etched)
> Cool grease used to provide thermal contact between carrier wafer and
> substrate
> AZ4562 ~ 6 um thick spun onto Carrier Wafer
> 4 inch Carrier wafer
>
> They have found that after the etch the back of the silicon substrate (i.e.
> side which is not etched) has some carbon looking deposits which cannot be
> removed in O2 plasma or solvent. Does anyone have any idea where this carbon
> is coming from?
>
> I myself do a lot of ICP etching and have not seen this before so am at a
> loss to explain it.
>
> Many thanks!
>
> --
> Dr. James Paul Grant
> Postdoctoral Research Associate
> Microsystems Technology Group
> 76 Oakfield Avenue Room 3
> University of Glasgow
> Glasgow
> Scotland
> G12 8LS
>
> Telephone: +44(0)141 330 3374
>