Ed, some questions. 1) Yes but it is not totally necessary.  The heat
combined with vacuum dehydrates the surface totally.  More heat more
dehydration, more vacuum more dehydration.  One word of caution, vacuum
causes vacuum freezing and as the water vaporizes it cools the wafer.
Pre-heating the nitrogen and using vacuum hot nitrogen purges stops
vacuum freezing.  Only one real theory.  HMDS is NHS(CH3)3Si(CH3)3.  The
NH reacts with Hydrogen giving NH3.  This is why you always smell
ammonia when you open a bottle of HMDS.  This leaves behind a monolayer
of Si(CH3).  An original molecule that is part organic and part
inorganic.  The inorganic Si clings to the wafer replacing the H of the
Hydroxyl ion and the organic Methyl CH3 sticks up in the air for the
organic resist to cling to.

                    2) No,

                    3) Yes but, if you use a small amount of heat and a
vacuum you will boil away the solvent better.  Boiling occurs when the
internal vapor pressure equals the external atmospheric pressure.
Heating increases the internal vapor pressure and creating a vacuum
reduces the external pressure.  Bill Moffat

-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Edward Sebesta
Sent: Friday, August 14, 2009 4:30 AM
To: 'General MEMS discussion'
Subject: Re: [mems-talk] Adhesion promoter for LiNbO3?

Some questions.

1. When you do the vapor prime, do you use heat? You need to have heat
so you don't leave a film of excess HMDS molecules. There are two
theories out there how HMDS works, but you need to bake the wafer for
either theory.

2. In the vacuum bake you run a risk of the surface of the resist
"crusting." That is you desolvate the top layer and that film blocks the
further loss of solvent from the bulk of the film.

3. Finally without heat, you will have low internal diffusion of
solvents from the bulk and I would think that your vacuum baking might
have to go for a long time, too long for a practical process.

4. I don't understand why you think softbaking would cause a charge
build up. A hot plate shouldn't build up charge and a largely
atmospheric oven bake shouldn't build up charge.


Ed