Blackened silicon is etched silicon. It is not deposition. I have seen this
all the time when I HF-etched my PolyMUMPS devices. I strongly suspect your
device layer is galvanic attacked since you have Al also on the device
layer. A little more information can be found on PolyMUMPS design handbook
from MEMSCAP's website (then search for galvanic in that pdf file).

Jie

2010/4/28 Sven TS Holmström 

> 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.
>
> best,
>
> /Sven Holmström, Koc University, Istanbul, Optical Microsystems

*  Zou Jie (Jay)
*  Department of Physics
*  University of Florida
*  Tel: +1-352-846-8018
*  Email: zoujiepku@gmail.com
*  Homepage: http://plaza.ufl.edu/zoujie/