Dear Sven,

  I exclude the polymer layer C4F8.
I think it might be the photoresist that remained after one of the earlier
step you had performed before HF etching. The remained PR has been become
black on the front side.
It could be not stripped well,
 Maybe the PR was burned during DRIE?

Best Regards.

 Karolina


---------- Wiadomość przekazana dalej ----------
From: "Sven TS Holmström" 
To: General MEMS discussion 
Date: Wed, 28 Apr 2010 15:57:20 +0200
Subject: [mems-talk] Problems with HF vapor phase etching
Hi MEMS-talkers,

I'm using Idonus' HF vapor phase etching apparatus to etch the buried
oxide in DRIE-etched SOI-wafers.

The problem is a blackening of the silicon. When the blackened part
are scratched a still polished surface seems to be revealed, so it
would seem to be a question of deposition, rather than a
transformation of the silicon. .

Which silicon is attacked is very specific. The polished silicon on
the backside of the wafer is not attacked (I have loaded the wafers
backwards to check this). The exposed silicon from the handle layer is
also not attacked. When performed at a high enough temperature 400 nm
thick Aluminum on the front side is not attacked in any way (at low
temperatures the HF will condense and etch Aluminum as aqueous HF
would). Only the polished front side silicon is attacked.

Because of these facts it seems clear that the darkening of the
silicon is due to earlier processing, but I can't figure out how. Has
anyone here seen something similar while performing HF vapor etch?

Further information:

When the wafers are introduced into the HF vapor the device layer is
patterned with DRIE and part of the silicon surface is covered with
400nm Aluminum.  Materials used in different steps before the HF
vapour phase etching are: resist (AZ1512HS and AZ9260), parylene and
the polymer used for cyclic protection in the DRIE Bosch-process.