I believe that you are in fact encountering the situation that Norbert has
described below.  We typically use several hot plate ovens to process SU-8
wafers on our automated systems.  In this manner each hot plate is vented after
a few minutes of bake time.  This prevents excess solvent from building up
inside the hot plate oven and potentially condensing and dripping.

Each of our hotplates have greater than 5 cm of clearance to the lid and are
greater than 200 mm in diameter.

I hope this helps.

- Garrett

EV Group
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Applications Engineer - Direct: +1 (480) 305 2443,
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-----Original Message-----
From: 郑瑞麟(Ruilin Zheng) [mailto:zhengruilin@gmail.com]
Sent: Monday, July 07, 2008 5:57 PM
To: General MEMS discussion
Subject: Re: [mems-talk] Islands found during SU-8 soft-bake

Hello, everyone

the hot plate is of about 20cm in diameter, with a metal lid on top, and the
lid roof is about 3cm  from the hot plate.
Do you think it is too close for the lid to hot plate?


On Mon, Jul 7, 2008 at 5:13 PM, Nodes Norbert  wrote:

> There could be also a different reason for these islands.
> What does the bake module look like?
> Is the volume between wafer and bake module cover very low?
> It can happen that the solvent condenses inside the bake module at cold
> spots and create droplets. Then, these solvent droplets can drip down on the
> wafer and cause the islands.
> It seems the bake module can handle the solvent fume volume with 2" wafers.
> But with 4" wafers, there is too much of it.
>
> First approach to solve this increasing the bake module's exhaust rate.
>
> Best regards,
> Norbert Nodes