AZ P4620 resist is the conventional novolak-based resist which has the
glass transition temperature (Tg) around 125C. Tg is for novolak resin.
Besides solvent diffusion, it's more because the PGMEA solvent in this
resist has a boiling temperature around 145C, so hard bake 120C helps to
drive the solvent out but not all. Ideally, the resist will not reflow if
Tg is above the solvent boiling point, but not for this type of resist.
Other ways to hard bake without resist reflow include deep UV cure of the
resist surface to form a hard outer shell, then hard bake on hotplate up
to 200C to completely drive out the solvent and cross-link the novolak.
But you can't use stripper or acetone to remove the resist after process.
You need O2 plasma to etch it completely.

Isaac

On Tue, 14 Mar 2006, Brubaker Chad wrote:

> The reason for this distortion is that the glass-transition temperature
> for the resin is in the ~105 - 110C range.  Hard baking at 120C is
> significantly above this temperature.
>
> The reason why a hard-bake temperature is designated at a temperature
> above the glass transition temperature is that, especially for thick
> films, the diffusion rate for the solvent in a solid film is extremely
> low, so that removal of the last few percentage points of solvent
> becomes virtually impossible.
>
> By heating above the glass transition temperature, the resist becomes
> semi-liquid, which greatly improves the diffusion rate of the solvent.
> This provides the ultimate purpose for the hard bake - full removal of
> solvent from the polymer.
>
> Best Regards,
> Chad Brubaker
>
>
> -----Original Message-----
> From:  Rupesh Sawant
> Sent: Tuesday, March 14, 2006 10:01 AM
> To: General MEMS discussion
> Subject: [mems-talk] Post bake for AZ P4620 photoresist
>
> In my experiments, I tried a post bake procedure for a 20 u thick AZ
> P4620 photoresist spun on a glass slide. i observed that the features
> were destroyed after the post bake. the post bake was done for 5 min
> at 120 deg. C. According to the literature, post bake is an optional
> step for thick photoresist layer and the features may distort due to
> "Reflow" if the hard bake is done. Can someone tell me a clear  reason
> for this distortion for thick photoresist.