Larger than specification Raith Circles on Photonic Crystals

[James Conway]

Ofer:

In my thinking about your problem with Larger than specification Raith 
Circles in your ZEP-520-12 exposures, I think I have come up with 
several reasons for the results you showed to me last week!  I would 
like you to consider the following factors and make some exposure test 
over the holiday period.

Check your single pixel line dose and the area step parameter.  If your 
SPL line dose is too high you will get much larger SPL dimensions.  That 
coupled with a very small area step which would exacerbate this effect 
and in my thinking could lead you to the results you showed me. (Raith 
circles with diameters nearly 50% larger than specified in GDS file.)

Please do an exposure test on your substrates and expose at DF 0.7, 
0.75, 0.8 of the  SPL dose you determine to just clear a SPL with a 
matrix of area steps on the order of 20, 30, and 35 nm. This should 
likely get you back into the right size specification for your Photonic 
Crystal patterns.  My ZEP-520 optimized dose for SPL's is 90 - 120 uC/cm 
on Si substrates at 10 kV using the 20 um aperture

With the pitches you are using (~ 500 - 500 nm?) you will experience 
some contribution for exposure proximity effects and to adjust for this 
you would need to reduce your exposure dose while evaluating CD 
dimension, in this case the diameter of your Photonic Crystals.

The only other significant factor could be if there exist a decided 
offset in the backscatter response of your quartz substrates.  In the 
past I would have increased my dose slightly when writing on my 
substrates but in your case it is better to carefully evaluate and 
determine your actual 'dose to clear' on your substrates.  You may find 
that the 10 kV 'dose to clear' is in the 30 - 35 uC/cm^2 regime instead 
of the normal 40 uC/cm^2 we typically employ for Silicon substrates.  
(reference ZEP -520 resist Thickness ~240 nm.)

I have not had a chance to get any feedback from RAITH in this 
concern..  I do hope to speak with Michael Kahl and Joe Klingfus with 
RAITH after the  Thanksgiving holiday on this and a number of 
application issues and questions.

Thank you for your support!

James Conway
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