I've had this exact same thing happen myself.  I never did figure out the
problem but the comment regarding baking the first layer too long sounds
plausible.
On Jul 3, 2012 8:52 AM, "Yingtao Tian"  wrote:

> Hi Darren,
>
> Could you remind how thick is the first layer and second layer? Also, when
> you say 'the resist flow away from the edges into the centre', did you mean
> the resist at the edge shows poor wettability and completely flows off the
> first layer surface? Or, did you mean the resist at the edge just flows
> back a bit and the edge is thinner than the centre? If it is the latter
> case, I think that's normal situation for thick SU8 due to the high surface
> tension.
>
> But, it would be strange to see the SU8 to 'ball up'. I would suspect the
> first layer was baked too long.
>
> Yingtao
>
> -----Original Message-----
> From: [email protected] [mailto:
> [email protected]] On Behalf Of Darren
> Alvares
> Sent: 03 July 2012 03:07
> To: [email protected]
> Subject: [mems-talk] Su-8 2075 Process
>
>  Hi All,
>
> SU-8 is a pretty well studied and commonly used resist and it seems in
> most instances I can achieve the types of results I want. However, recently
> I have encountered a few problems with multilayer structures. The main
> issue I think stems from the uniformity of the thickness. However, I could
> be wrong on this.
>
> I am after making a freestanding su-8 mould for use in pdms moulding. The
> su-8 is applied to a silicon wafer.  I would like to have a thin first
> layer followed by my patterned top layer. The first layer seems to work ok.
> I use a high spin speed of 4000RPM and a long softbake (gradual ramp up and
> down). The exposure is done in steps to avoid any over heating and a
> shortened prebake is done after. The problem occurs when I spin on the
> second layer. During the softbake (which is designed to be around 2hrs) the
> resist flows away from the edges into the center. This is not an issue I
> think that stems from the hotplate incline as that would cause the the
> resist to flow to one edge.
>
> The only explainations I had were
>
>   *   The edge effect of the first layer means it is thicker on the
> outside then on the inside which means that the second layer when heated
> flows inward. I tried increasing the spin speed of layer 1 but this still
> happened.
>   *   The surface tension of the SU-8 uncured is very high causing it to
> ball up.
>
> I had a search online for multilayer papers and found quite a few but none
> mentioned this issue. Anyone experience this or heard of it in the
> literature? Would be great to get your thoughts.
>
>
> Regards,
>
> Darren Alvares
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