Hello Malar,
What is the feature size you are trying to create on the bottom of the
trench?
Which type of mask aligner are you using?
If the features are not too small (<20 um) and you are hands-on person, you
might try to do the following:
In the lamp house of the most mask aligners, there is element to make the
light more uniform, usually fly-eye or similar kind of the lenses, which is
located in the focal point of the front lens. Now this element is quite
large - several cm and that is for several reasons - to reduce the heat on
fly-eye, make the light distribution more uniform and sometimes to create so
called "off-axis" illumination which reduces the diffraction and improves
the resolution in close proximities. This is exactly what you do not want
when you are exposing in large proximity (260 um) because all off-axis light
will blur the image too much. So you have to reduce the size of the fly-eye
by making an aperture there. How big the aperture needs to be you can find
out experimentally or by simulation if you have access to such software.
I was once involved in the project where similar thing was required, and
that time I have used the aluminum foil to make the aperture - I cut out the
aperture with my wedding ring :) as it was close to the dimension predicted
by simulation and there was no better tool in the clean room available to do
it. Stack it to the fly-eye by folding and edges of the foil and voila - the
machine was ready. I do not recall exactly what the resolution and depth of
the was (20 um L/S in 250 um depth or 10 um L/S in 250 um depth) but we were
able to achieve it, both in simulation and also physically. I used the
software called Layout LAB from GenISys to predict the size of the aperture
~ 1.5 cm or something like that.
Do not expect sharp profile of the resist or anything like that. Be careful
when sticking the aperture - the fly-eye and the holder can get quite hot.
And try to position the aperture as close as possible into the center of the
fly-eye as this is the whole point to use only the on-axis light. And of
course, you will have to increase the exposure time a lot to keep the same
energy.
Now if this would not be enough, you can always try to do more funny thing
like etch the recesses into the photo mask so you can physically get your
mask closer to the bottom of the trenches. That could be a nice experiment
to do and maybe even to publish, so if you would need help with that let me
know.
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 Malar C
Sent: Friday, May 09, 2014 12:49
To: General MEMS discussion
Subject: Re: [mems-talk] Comb Structure Lithography
Thank you for the reply
I am using contact lithography with minimum feature size of 5 um. I won't
be able to use focus offset to focus at the bottom of the pit.
Malar
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