Hi Rafael,

Al will alloy with refractory metals (W, Mo, Ta) over time when heated.  If you
look at your filament, I bet it now looks dull grey rather than shiny (like a
pure Al coating would appear).  I had similar problems and contacted the Kurt
Lesker company for advice; they recommended two possibilities to me:

1. Replace the W spring every few runs.  Can get expensive over time.

2. Use a boron nitride crucible with an outer coil heater, or a coated box
heater.  Will require higher operating currents and impose a higher thermal load
on your substrate.

The company sent me a useful technical brief on evaporating Al; if you want, I
can forward it to you directly.

Good luck,
Mike

On Mar 22, 2011, at 4:06, Rafael García Valverde wrote:

> Hi everybody,
>
> We need some advice, critics or suggestion.
>
> We are trying to thermally evaporate Al layers on glass substrates. We are
> using Tungsten filaments in spring form and holding the Al filaments inside
> the Tungsten spring. Our vacuum arrives to 10^(-6) mbar and the DC current
> is gradually raise to 18-20A (the tungsten filament becomes incandescent),
> we keep the DC current until we don't see any Al inside the tungsten spring..
> Apperently everything is right and the first days we achieved some
> acceptable layers, but after a few days the Al layers are extremelly thin
> (almost invisible).
>
> Maybe the chamber is dirty? Any possible treatment for cleaning it?