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: [email protected] [mailto:mems-talk-
[email protected]] 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: [email protected] [mailto:mems-
[email protected]] 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: [email protected] [mailto:mems-talk-
[email protected]] 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: [email protected] [mailto:mems-
[email protected]] On Behalf Of Yingtao Tian
> Sent: Wednesday, June 27, 2012 9:36 AM
> To: [email protected]
> 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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