I think a guaranteed solution would be going to Photolithography. Good
photolithography could give 1um feature, and it looks to me using e-beam
(which typically could generate 100nm feature) is an overkill. If you have
stepper in your fab, then that could easily give 1um critical dimension.
If we have to stick to e-beam lithography, I guess we need increase the
thickness of the lower layer of the bi-layer. The advantage of the bi-layer
process is that the undercutting of the lower layer naturally break the
continuity of the deposition. If the thickness of the deposition is thicker
than the lower layer, we are totally relying on the negative tone of the top
PMMA. However I guess the deposition is so long that the metal may scatter
away the edge of the top PMMA and make it a positive tone. In this way the
metal layer becomes conformal and cause the failure.

Jie

On Fri, Jan 16, 2009 at 5:03 AM, James Paul Grant wrote:

> Thank you to all who have replied so far to my questions
>
> I shall state a few other relevant points:
>
> 1. The Al is e-beam evaporated at a base pressure of ~ 1x10-7 mbar.
> Sputtering deposition tools are also available.
> 2. Deposition rate is 0.3 nm/s
> 3. The bi-layer resist thickness is 1.3 um while the metal thickness is
> 1um. Now I know one should always have a resist thickness at least two time
> greater than the metal thickness however I can add the caveat that one of
> the groups here at Glasgow Uni have successfully lifted off 1.2 microns Au
> using the same resist thickness (1.3 microns) - and they say their process
> is very repeatable! The problem my colleague has is he is already using the
> thickest e-beam resist he has available. He could probably sneak another 200
> nm from spinning the resist at lower rpm which may of course aid lift-off.
> 4. I told him to try sonication which he has done for around 10 minutes. No
> joy.
> 5. All processing was completed within 2 days (i.e. substrate clean, resist
> spinning, bake, exposure, development, ashing, HF dip, Al evaporation)
> 6. He has not attempted to deposit thin Al
>
> I had a look at his sample for him and the metal in large areas has lifted
> off however in the fine feature areas (~1 um feature size) the metal has not
> even begun to lift-off.
>
> His next step will be to deposit 500 nm instead of 1um. I hope lift-off is
> more successful this time.
>
> Thanks for all your helpful comments,
>
> James

*  Zou Jie (Jay)
*  Department of Physics
*  University of Florida
*  Tel: +1-352-846-8018
*  Email: zoujiepku@gmail.com
*  Homepage: http://plaza.ufl.edu/zoujie/