Hello Daniel,

Thank you for the advise.

I am trying to produce a photonic crystal with two molecule basis (means pillars
of semiconductor plus holes all enveloped in SU-8). Computer simulations show
very exotic properties, but our paper was rejected on the basis that it is
nearly impossible to produce it in practice. Still there is way to do it and I
only want to prove it is possible so the structure have 300um feature. The real
one need 2.7um for 7um light source.

It should be two step process. First create one set of holes with SU-8 around
them and empty trenches between them. Then fill the holes with high dielectric
semiconductor. Clean the trenches from the semiconductor. Then pour more SU-8 on
the wafer and expose it for second time to create the second set of holes.

You are absolutely right about the scattered light. The imprinter had been
optimized for stamp imprinting and I was carried away with the need to learn
very fast how to work with SU-8 (stamp works perfectly).

Another problem is that I lack UV filter for the light below 350 um.

I see the yellowish pattern on ordinary light. Nearly all of the processes are
in a clean box. Only transportation to and from imprinter expose the samples to
the environment but I use improvised aluminum foil box to protect it.

Best regards

Dimitar


-----Original Message-----
From: [email protected] on behalf of
Daniel Figura
Sent: Wed 7/9/2014 4:40 AM
To: 'General MEMS discussion'
Subject: Re: [mems-talk] Unremoved SU-8 after exposure

Hello Dimitar,

You forgot to share with us the details on what patterns you are exposing?
Lines, dots, spaces, islands? Are these dense structures or isolated? What
it the minimum feature size you are exposing? Related to this is the
question of thickness of the "washers" you use to maintain the proximity.
Can you at least theoretically resolve the features you intend in the
proximity you use?

As many had suggested, I would also look first into the exposure system.
>From the description of your "imprinter" I would guess that it is flood
exposure system used to UV cure material (like NIL imprinting resist or UV
curing epoxies etc.) Though it might be well suited for such purposes, it
might not work for optical lithography at all. Try to calculate the angle at
which the light from one corner of the lamp strikes the opposite corner of
the substrate. In very simple approximation, if the lamp diameter is same as
the substrate (100 mm) and you are exposing in 200 mm distance, the light
from one edge of lamp will strike the opposite edge of the wafer under angle
of 26.6 deg, which means 15.5 deg in SU8 (assuming index of 1.68). Under
this angle the light will travel in the 50 um thick resist 13 um sideways
off from the pattern on the mask. If you use Si wafer, most of the light
will reflected back under the same angle and crossing more resist sideways
(26 um). Under these assumption 52 um line on mask will be completely cross
linked and cannot be resolved. The situation will get only worse if your
lamp size is larger or is you exposure in proximity.

In order to make out of your lamp a collimated light source, you would need
to buy a front lens (larger than substrate), place it in considerable
distance from lamp (such that your lamp starts to resemble the point
source). The focal length of the lens and the distance of lens need to be
the same. Then you need still enough light from your lamp to enter the front
lens at such distance, which might be an issue, so you would need to design
some condensing optics as well. All the light which does not enter the front
lens need to be absorbed in some kind of lamp house. All in all, I bet it
will be easier and more cost effective to find an access to proper aligner
or light source of optical lithography.

Out of the curiosity - how can you tell that the exposed patterns became
yellowish in yellow light? Or do you perform your process in white light
clean room?


With best regards,

Daniel



Daniel Figura

smartfabgroupT Company

process consulting . data processing . fab software

Phone: +421 944 45 26 86

E-mail: [email protected], Web: www.smartfabgroup.com



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-----Original Message-----
From: [email protected]
[mailto:[email protected]] On
Behalf Of Dimitar Dimitrov
Sent: Tuesday, July 08, 2014 19:26
To: General MEMS discussion
Subject: Re: [mems-talk] Unremoved SU-8 after exposure

Thank you,

The imprinter is kind of strange. The lamp is at the bottom. 20 cm above is
a vacuum chamber. The bottom of the chamber is quartz 1/2 inches width. I
put the mask over the quartz. The Si wafer is upside down so SU-8 is facing
the lamp. Two washers are separating the mask with wafer.

Do you think that the quartz is scattering the light and the distance
between the mask and film is enough to allow it to expose everything?

I never thought of this.


-----Original Message-----
From: [email protected] on behalf of
Andrew Sarangan
Sent: Mon 7/7/2014 7:21 PM
To: General MEMS discussion
Subject: Re: [mems-talk] Unremoved SU-8 after exposure

My guess is you are getting exposure due to scattered light from the
back side of the wafer (since you are using pyrex).  SU8 is very
transparent to UV. I use a second wafer on the back side painted with
a mat black paint.

On Mon, Jul 7, 2014 at 1:25 PM, Dimitar Dimitrov
 wrote:
>
> Hello,
>
> I am graduate student in Brooklyn College and I am trying to reinvent the
whole MEMS procedure as I do not have any professional help. My science
adviser is from another field.
>
> I am having difficulties with the developing of SU-8 2025. Here is what I
am doing:
>
> I am spin - coating 4 inch Si or Pyrex wafer to 50 - 60 um thin film
> Pre - baking at 96 degrees for 15 min
> Sometimes instead I am putting the wafer in the vacuum chamber of my UV
imprinter. The vacuum is taking away the solvent and clear most of the
bubbles.
> UV exposure with a hand made Imprinter: Xenon lamp 22kV at 10 microsec.
pulses. Three 30 seconds exposures with couple of seconds pause.
> UV lamp creates heat so the wafer is let for 2 - 3 min to relax.
> Post bake of 15 min at 95 degrees.
>
> Developing by tenderly shaking the wafer in a vessel with Propylene glycol
monomethyl ether forever.
>
> The result is that I clearly can see my pattern as exposed SU-8 is
yelowish but the unexposed parts are not removed and form thin film with the
exposed part.
> Less exposure leads to removing all of the polymer.
> (This is happening even without any mask but the pattern is formed by
metal structure that divides 4 inch wafer into several smaller regions.)
>
> Can anybody figure out what is my mistake?
>
> Thank you
> Dimitar Dimitrov
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