My guess is that your bond is not good, especially in the center of the
wafer because the resist "glue" can't ever dry in the center.

We have had success using backing wafers which have a shallow (50 um
deep) pattern of radial spokes and annular rings (looks like a dart
board  pattern) etched into the surface. The spokes extend to the edge
of the wafer and are a few mm wide. We oxidize these wafers so they
don't get etched after punchthrough. We bond the wafer to be etched to
the backing wafer by spinning the backing wafer with 1-2 um resist
(Shipley 510LA) at 2000 rpm. The wafers are gently pressed together and
baked on a hotplate for 30 minutes at 110 C. The channels provide a
path for the gas which evolves during resist drying to escape. After
etching, the channels provide a path for acetone to enter, speeding up
the release.

We have also had success with Aremco Crystalbond 509 (clear) wax which
dissolves in acetone.

Jim

[product/brand names offered for example only, no endorsement implied]
On Apr 22, 2004, at 11:08 AM, neo man wrote:

> Hello everyone,
>
> I am looking to etch THROUGH a standard thickness wafer (about
> 500micron).
> As a result, I am using a secondary support wafer (also 500micron
> thickness)
>
> I have tried bonding the wafers prior to etch (using 7 um of SPR220-7
> photoresist),
> but I am not getting good heat transfer and my resist is buring very
> quickly...
> (I am using 7-10 micron SPR220-7 as a mask as well)
>
> If I am just etching one wafer (ie -- no secondary support wafer) I
> get decent
> results with the same process. (but obviously I am stopping before
> going all the
> way through the wafer....)
>
> I have tried reducing chiller temp, but this didn't help much. I am
> using 30-40W platen
> during the etch and 20W platen during the passivation. I think we have
> a 300W generator, so not sure how much lower I can go.
>
> I am thinking of switching to an oxide hardmask.
>
> 1 - Does anyone have an idea of how drastically this will affect the
> etch results. ?
> 2 - How should I compensate ? (in order to get similar results as
> using PR mask)
>
> My wafers have large features mostly and about 25-30% exposed silicon.
>
> Current process is approximately:
> etch - 450sccm SF6, 20sccm O2 ---> 2200Wsource, 40W platen (6 seconds)
> passivation - 200sccm C4F8 ----> 1600Wsource, 20W platen (3 seconds)
>
> Thanks in advance for any help.
>
> neo man
> [email protected]
>
>
>
>
> ---------------------------------
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>
-Jim Beall
303-497-5989