The other thing that maybe happening is that Al is attacked by OH ions,
just think about how many of those are in water and how many more are in
hot water. If it is  DI water than that is your issue, DI water attacks
metal very quickly as it does not want to be DI so it looks for metal
ions to leach off the surface.

Rick Morrison
Senior Member Technical  Staff
Draper Laboratory
555 Technology Square
Cambridge, MA  02139

617-258-3420

-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of David Casale
Sent: Friday, February 19, 2010 2:27 PM
To: General MEMS discussion
Subject: Re: [mems-talk] Al thin film dissolved inside hot water

Hi Shifeng,

This is something easy to forget, but diffusion is length^2/time, and at
really short length scales and moderate temperatures, diffusion can be
pretty "big". I'd guess that it is diffusing off of the surface and into
solution, but I haven't tried to look at any data to verify that. Try
lower temperature water, and see if it takes longer to dissolve, and you
problem will be pretty obvious.

As an example, mild temperatures can fully diffuse adhesion layers into
top layers with thin films (say 150 angstroms of Cr under 2000 angstroms
of Au), and it is a very complicated process to prevent this diffusion.
At say 300C and a few hours of soak time, on glass, Cr will diffuse
completely into Au if the length scales are small enough and grain
structure right. Easy experiment to perform if you do it on glass too,
just look through the back side at the adhesion layer of different
color, no fancy equipment.

I have some sources for this as well if you decide this is your problem,
or want to look more into it.

Sincerely,
David Casale