If you're relatively new at this, the variations between coats 2,3,
and 4 don't seem that bad. Before you try and figure out what's going
"wrong," make sure you've first figured out what your typical
coefficient of variation is for a single layer coating.

Also note that the thickness of the second+ layers *shouldn't* be the
same as that of the first layer, barring some coincidences. The
wetting properties of your Si wafer (plus whatever adhesion promoter
you're using) will be different from a layer of resist. The surface
roughness of the resist is higher, and its contact angle will be
different, both of which will affect the film thickness as a function
of RPMs. So, if you really want layer one to be the same as layer two,
first generate a curve for first layer thicknesses (as dispensed by
you) and then generate a curve for second layer thickness and match
appropriately.

- Kevin

On Fri, Jul 20, 2012 at 8:27 AM, João Tiago dos Santos Fernandes
 wrote:
> Hello everyone,
>
> I have been trying to do a multi-layer approach to obtain a thick-film of AZ
4562 (35-40 µm) on Si but I stumbled into something rather odd.
> After each coating I scratched the film to expose the Si and measured the
thickness of the film in a profile-meter. The values I obtained were:
>
> - Coating 1: 7,86 µm
> - Coating 2: 19,50 µm (∆ = 11,64 µm)
> - Coating 3: 32,65 µm (∆ = 13,15 µm)
> - Coating 4: 45,63 µm (∆ = 12,98 µm)
>
> I use the same protocol for each coating: spin-coat at 500 rpm (10 s, 100
rpm/s) + 2000 rpm (43 s, 500 rpm/s) followed by a 5 min bake at 100 ºC.
>
> My question is: is there any obvious reason for this variation in thickness
for each coating (e.g. why the first coating is 7,86 µm thick and the second one
is 11,64 µm)? And how could I obtain uniform coating thicknesses?
>
> Thank you in advance for your time and help.
>
> - João Tiago
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