Hi Yingtao,
I guess you are using contact mode for baking SU8, which will have the issue
that hotplate and wafer can never perfectly contact. In this way, no matter
what the hotplate accuracy is, the temperature on the wafer coated with SU8 can
never be uniform.
For thick SU8 baking, I think you should use non-contact mode. The wafer should
be supported with pins (normally 3), and leave a well controlled gap between
the wafer and the hotplate. Radiation will make a uniform temperature
disstribution over the whole wafer.
I'm not sure what the hotplate model is, we are using the hotplates on GAMMA
60, which work in this way.
Hope it will help you.
All the best,
WANG, Yingnan
> From: y.tian@bham.ac.uk
> To: mems-talk@memsnet.org
> Date: Tue, 3 Jul 2012 16:29:52 +0100
> Subject: Re: [mems-talk] Ultrathick SU8 photoresist processing
>
> Hi Yingnan,
>
> I think we have a pretty good hotplate. I measured the temperature and it does
have variation about 0.5-1 C. Could you suggest some better hotplate model or
brands which I can compare/
>
> Thanks,
> Yingtao
>
> -----Original Message-----
> From: mems-talk-bounces+y.tian=bham.ac.uk@memsnet.org [mailto:mems-talk-
bounces+y.tian=bham.ac.uk@memsnet.org] On Behalf Of WANG Yingnan
> Sent: 02 July 2012 17:09
> To: General MEMS discussion
> Subject: Re: [mems-talk] Ultrathick SU8 photoresist processing
>
> Hi Yingtao,
> I believe the thickness variation results from the baking method. The
temperature distribution varies across the surface, which in turn affects the PR
thickness, greatly.
> What you need is a good hot plate, and don't use contact mode for baking.
> Wish you good luck.
> Wang Yingnan
>
> Sent from my iPhone
>
> On 2 Jul, 2012, at 9:12 PM, Yingtao Tian wrote:
>
> > Hi Bill,
> >
> > As suggested by Gareth, I usually leave the wafer resting for few hours
before soft bake. Of course, I would cover the wafer with a glass dish during
the resting period. Would the air turbulence during spinning coating be
critical? Or, the metal cage setting can be applied during the soft bake as
well?
> >
> > Thanks,
> > Yingtao
> >
> > -----Original Message-----
> > From: mems-talk-bounces+y.tian=bham.ac.uk@memsnet.org
> > [mailto:mems-talk-bounces+y.tian=bham.ac.uk@memsnet.org] On Behalf Of
> > Bill Moffat
> > Sent: 28 June 2012 15:28
> > To: General MEMS discussion
> > Subject: Re: [mems-talk] Ultrathick SU8 photoresist processing
> >
> > They made a metal cage that held the resist coated mask face up and a
> > blank mask about 1/4" away from it. This trapped air between the
> > masks and there was little if any turbulence, during spin. This was
> > particularly important in the case of mask blanks. Because the masks
> > were square the corners were particularly affected by turbulence. One
> > French company tried to patent the technique years ago. Bill
> >
> > -----Original Message-----
> > From: mems-talk-bounces+bmoffat=yieldengineering.com@memsnet.org
> > [mailto:mems-talk-bounces+bmoffat=yieldengineering.com@memsnet.org] On
> > Behalf Of Yingtao Tian
> > Sent: Thursday, June 28, 2012 4:42 AM
> > To: General MEMS discussion
> > Subject: Re: [mems-talk] Ultrathick SU8 photoresist processing
> >
> > Hi Bill,
> >
> > Thanks for the suggestion. Did you mean they used to put the second blank
mask during soft bake process and will the two masks remain parallel to each
other ? I tried to put a petri dish set up with a tweezer on one side, but it
does help very much. Could you suggest some literature to read more details
please?
> >
> > Thanks a lot!
> >
> > Regards,
> > Yingtao
> >
> > -----Original Message-----
> > From: mems-talk-bounces+y.tian=bham.ac.uk@memsnet.org
> > [mailto:mems-talk-bounces+y.tian=bham.ac.uk@memsnet.org] On Behalf Of
> > Bill Moffat
> > Sent: 27 June 2012 20:54
> > To: General MEMS discussion
> > Subject: Re: [mems-talk] Ultrathick SU8 photoresist processing
> >
> > Air turbulence can be the answer. When engineers were experimenting
> > with super thick resist on photo masks they used to put a second blank
> > mask a short distance above the mask being coated. This protected the
> > resist surface from air turbulence. You may want to use a variation
> > of this. Bill Moffat
> >
> > -----Original Message-----
> > From: mems-talk-bounces+bmoffat=yieldengineering.com@memsnet.org
> > [mailto:mems-talk-bounces+bmoffat=yieldengineering.com@memsnet.org] On
> > Behalf Of Yingtao Tian
> > Sent: Wednesday, June 27, 2012 9:36 AM
> > To: mems-talk@memsnet.org
> > Subject: [mems-talk] Ultrathick SU8 photoresist processing
> >
> > Dear All,
> >
> > I am trying to make very thick SU8 photoresist on silicon wafer, however, I
found that, even the resist was spun coated, the surface will not be flat. I
tried to put over 200 um thick resist but the thickness can vary between 180 -
220 across the whole wafer. I have tried to adjust the levelness of the hotplate
by using an inclinometer and it was almost perfectly levelled. Somebody
suggested that the air turbulence in the cleanroom does matter for such very
thick resist. But, I have no evidence to doubt it.
> >
> > Anybody has ideas?
> >
> > Thanks,
> > Yingtao
> > _______________________________________________
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> provider of MEMS and Nanotechnology design and fabrication services.
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