I have asked this question of several STS engineers. Here are some answers.

STS does not recommend aluminum or other metals being exposed in their
DRIE tools as the metal will be sputtered by SFx+ ions.
This results in several problems:

1. The sputtered metal can land elsewhere on the wafer, causing micromasking
(grass).

2. The sputtered metal can coat the cylindrical ceramic liner in the top of
the chamber.
Outside of this liner is the single-turn coil that generates the plasma.
When a thin layer of metal coats the liner, electric fields cannot penetrate
it as well,
resulting in less plasma generation and hence lower etch rate.
The liner can be mechanically cleaned by bead blasting or perhaps just
scrubbing.
Buying a new one costs on the order of $10k.

Some STS users have, however, used metal masks without a problem.
STS does not have an explanation for this, and recommends playing it safe.

        --Kirt Williams         Agilent Technologies

> >
> >I know that with Alcatel DRIE engines we can use Al as mask
> can we do it
> >with STS multiplex ICP too ?