Hi Alexandre,

Thank you for the clarification.  First of all, using an image-reversal
technique can be pretty tricky.  Certainly there are easier ways using negative
resists to effect a lift-off process, but of course you need a mask with the
opposite tone.  Good choices for this would be AZ nLOF 2000 series, or a bilayer
system such as MicroChem's LOR or Brewer Science ProfLift.

Regardless, the first thing I would say is that you should check your processing
conditions to make sure they are optimal, particularly the pre-exposure bake,
which is the step that makes the negative profile possible.

Usually when I have had problems with a lift-off process it is because the
resist either got way too hot, or the material being deposited conformed to the
profile of the resist resulting in an essentially solid coat of deposited
material.

How directional is your deposition of the Si?  Most vapor processes are pretty
conformal which makes lift-off difficult or impossible.  Most lift-off processes
that I am aware of use very directional deposition techniques such as sputtering
or e-beam evaporation.

As to the bubbles, I am sort of at a loss.  The resist should be fully cross-
linked, especially after the dehydration step.  You might try a higher
temperature, say 120° to promote better dehydration and densification of the
resist though.  Is it possible that there are pinholes in the deposited material
and that solvent is getting through the pinhole removing the resist
preferentially?

Hope some of this may help you!

Best Regards,
Dustin Warren, PE

EV Group
invent * innovate * implement
Application Engineer - Direct: +1 (480) 305 2447, Main: +1 (480) 305 2400 Fax:
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E-Mail: D.Warren@EVGroup.com, Web: www.EVGroup.com


-----Original Message-----
From: Alexandre BOE [mailto:alexandre.boe@uclouvain.be]
Sent: Tuesday, April 28, 2009 12:54 AM
To: General MEMS discussion
Subject: Re: [mems-talk] Photoresist and silane

Hi,

For the bubbles, using a microscope we can see something like bubbles in
the resist used for the lift-off process.

Yes you are right, by "etching the resist", I was thinking of lift-off
the silane layer.

The silane is injected at 80 °C during about 3h (I am mot sure),  but
befor injection, a dehydratation is done at 100 °C under vacuum.

Thank you for this first answer.

Regards,

Alexandre