If the chamber is dirty, you would notice some kind of darkness in the film.
The actual level of vacuum and it's stability are also important.
You may check for the physical inspection of the quality of the film and also
the  thickness measurement.



2011/3/22 Rafael García Valverde 

> 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?
>
> Thanks in advance.
>
> Regards
>
> Rafael García Valverde, PhD