Julie,

Sound like hillocks. It forms when heating evaporated aluminum because of
the TCE differences. The aluminum expands gets detached from the surface
causing the small bumps that remain when the sample is cooled.

Shay

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Julie Verstraeten
Sent: Friday, September 26, 2008 9:07 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] "cavity" formation after annealing of evaporated
aluminium on Si

Hi everyone,

We currently have some problems with the deposition of aluminium layers.
Maybe some of you already experienced that phenomenon:

Some bump-like structures appear all over the sample after the annealing of
the evaporated Al layer (see second SEM picture). These structures seem to
be empty inside however this could be the effect of the cleave.

The test samples are bare silicon cleaned with solvents (opticlear, acetone
and IPA, 5min each with ultra-sound). They are then immersed in a solution
of 8% HF: DI to etch the native oxide (10-15sec until hydrophobicity) just
before evaporation. The aluminium is then evaporated (e-beam) at a rate of
0.3nm/sec (250nm).

The samples surface seems very clean before the evaporation. After
evaporation, the Al surface shows little protuberances (~100nm, see
first SEM picture).

The 450=B0C annealing, during 30min, reveals the cavern-like structures
(~500nm-1=B5m).  The annealing was performed under N2 atmosphere and forming
gas (N2 80%, H2 20%); both lead to the same results.

We don=92t know if this comes from
- a cleaning problem (traces of water or solvent at the surface before
evaporation),
- some problem during evaporation  (contamination, non uniform deposition,
stresses) amplified during annealing,
- some contamination in the oven.


Does anyone have any advice or suggestions?

Aluminium on Si after evaporation (no annealing)

Evaporated Aluminium on Si after 30min annealing at 450=B0C

Thank you,

Julie