A quick tutorial.  SiO2 has a water contact angle of 22 degrees.  This
is hydrophilic.  0 is extremely hydrophilic and 180 is extremely
hydrophobic.  Teflon for example is 120 degrees.  HMDS reacts with
Hydrogen and if it sees a moist surface it reacts with the Hydrogen in
the water and the hydrogen in Hydroxyl ions.  Water is easy to remove
and hydroxyl ions are difficult to remove.  This moist surface will
break down rapidly because the water will lift off with the reacted
HMDS. The Hydroxyl ions will not.  If you vacuum vapor prime the vacuum
removes all the moisture leaving only the hydroxyl ions for the
reaction.  Using this 1 minute of HMDS prime gives 45 degrees, 2 minutes
in the 50's, 3 minutes in the high 50's.  4 minutes in the 60's, 5
minutes close to 75 degrees.  The aim is to balance the surface tension
to the liquid surface tension of the resist being used.  Most positive
resists have a molecular weight close to 100,000 and this 75 degrees
matches the liquid surface tension associated with this resist.  If you
are applying a positive resist as the mask and removing it with an
organic remover you will remove the HMDS monolayer.  Let me know more
about your resist layer and process and I will try for more help.

    Bill Moffat

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Andrew
Sent: Monday, August 17, 2009 5:01 AM
To: [email protected]
Subject: [mems-talk] HDMS treatment of SU8 mold used for PDMS thin
filmfabrication

Hi Bill,

How do you deposit the heptadecafluoride? I am looking for a surface
that remains hydrophobic after patterning. For example: deposit SiO2,
functionalise SiO2 to remain hydrophobic, pattern and etch SiO2 +
hydrophobic layer and  remove mask. The surface must remain hydrophobic
after processing.

Thanks,

Andrew