Matteo,
The primary difference between hotplate and oven bake of materials has to do
with thermal transfer modes.
For an oven, thermal transfer is primarily accomplished through radiative and
convective means (especially in a convection oven). Radiative is typically a
rather slow method, unless there is a significant difference in temperature
between the radiating units and the object to be heated. Convective heat
transfer is also rather slow, as air itself is not a tremendously good heat
conductor, and heating is accomplished by flowing high temperature air as close
to the surface as possible. This becomes difficult, since the fluid flow
properties (viscosity) of the air prevent rapid movement close to a non-moving
object.
What this boils down to say is, oven baking is rather slow.
Hot plate bakes, by contrast, have conduction as the primary mode of heat
transfer. Silicon is a fairly good heat conductor, and so use of a hotplate
speeds up the bake rapidly. For thin films (<5 µm), a standard rule of thumb is
that for every 30 minutes of oven bake, only one minute of hotplate bake need be
performed.
When thicker films are being discussed, two other concerns need to be taken into
account.
First, with thick films, the layer does not instantaneously reach the same
conditions at the same time. In the case of an oven bake, the top portion of
the polymer layer will be the first to reach baking temperature, which will
cause the solvent in that region to evaporate, and the resist to densify. This
has two effects. One is that the thermal transfer to the lower layers slows
down, since polymer layers are typically resistive to heat transfer. The second
is that this densified layer becomes a diffusion barrier, preventing the easy
transfer of solvent from the film to the surrounding atmosphere.
On a hot plate, since the heating is from the bottom, the solvent will be able
to successively rise from the lower layers to the upper. However, this bottom
heating points to the primary concern with hotplate baking of thick films. If a
thick-film wafer is heated too rapidly, the solvent in the polymer closest to
the surface of the wafer will flash to vapor. Since the polymer above this
flashed solvent is too thick for easy diffusion, this will cause bubbles to form
in the film.
The easy solution to this is to effectively combine the oven and hotplate bake
concepts in the initial stages of the bake, by using programmable proximity
pins. By suspending the substrate a short distance above the surface of the
hotplate, and gradually moving it into contact, a slower heat-up is accomplished
(although still from the bottom, avoiding the diffusion barrier effect). The
effect would be similar to performing an initial bake at a lower temperature.
However, it is still possible to achieve the full bake parameters required in a
reduced time, since after the initial heat-up, the wafer can still be brought
into contact with the plate, without the need to wait for the hotplate
temperature to ramp up and stabilize (or ramp down again prior to the processing
of the next wafer)..
Best Regards,
Chad Brubaker
EV Group invent * innovate * implement
Technology - Tel: 480.727.9635, Fax: 480.727.9700 e-mail:
[email protected], www.EVGroup.com
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-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Matteo Dainese
Sent: Thursday, November 11, 2004 8:01 AM
To: [email protected]
Subject: [mems-talk] oven/hot plate bake
Hello all,
I would like to increase my awareness
when I'm processing with the different
polymers I'm using. I have this small question:
what is the difference between
curing/soft baking/hard baking a photoresist,
or a polymer in general, on a hotplate or
in a normal oven (same temperature)?
Is there any reason why I should choose either
one or the other?
I would be happy if any of you can give an opinion.
Thank you
Matteo Dainese
Graduate student
KTH - Royal Institute of Technology
Stockholm
Sweden
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