Problem with PMMA exposure/lift off

James Conway jwc at snf.stanford.edu
Thu Apr 10 11:01:16 PDT 2008


Hello Arash Hazeghi and Others:

Thanks for the images.

In the first two images you are below the dose to clear for the finer 
lines.  What is evident is you can see the 1 um sub-sub-field butting as 
these field overlap slightly, and at the 40 um sub-field you can see an 
apparent stitch break between the write fields.  It is  normal practice 
to increase dose slightly to eliminate these effects and both of those 
exposure artifacts will develop out.  If your resist stack height is 
above 400 nm you may wish to increase development time and agitation as 
well. On the 300 nm 5% 495K PMMA I routinely develop fro 30 seconds, 
perform a quick high Magnification Microscope inspection, and then if I 
see these artifacts perform additional development in ten second 
intervals until the features are cleared.

As the EBEAM tool slowly accumulates hydrocarbons in the lower sections 
of the column, specifically the final aperture in the objective lens and 
in the area around the blanking aperture, the astigmatism in the lens 
increases as does some charge up in the apertures when blanked; 
resulting in shifts of the ebeam spots placement as it writes these 
sub-fields.  Within a very small range these fields can be adjusted -- 
but at some point we may need to perform a PM on the column to return 
the system to a normal state.  I am currently evaluating these effects 
and have just two evenings ago refocused and changes stigmation slightly 
to the column settings.  In my exposure test I reviewed last evening I 
reported a reduction in the magnitude of the stitching issues 
particularly for the 40 um sub-fields.

IN the last two images it is plainly evident that there is an adhesion 
issue in your metal lift off films.  You may wish to reconsider your 
initial substrate clean up method to ensure you are starting out clean 
and without any small source residues on your wafers surface.  A simple 
TAPE test will assist you in evaluating thin film adhesion.

All Users are advised to be sure to run the $DEF CF CL write field 
calibration routine for all of their exposures.  Users working on any 
charging substrates such as SOI, or substrates with oxides or nitrides 
are recommended to coat their samples with a charge compensation coating 
such as ESPACER or a thin metal film to reduce charging effects on your 
EBL writes.

Thank you for your support!

James Conway



Arash Hazeghi wrote:
> Hi all,
> I have been trying liftoff on my samples recently but it looks like 
> either I am doing something wrong or there is an exposure problem with 
> eBeam. I am trying to resolve 150nm lines on  300nm 5% 950-K PMMA 
> (standard coat procedure). I have tried area exposure dose from 
> 350uC/cm2 to 430uC/cm2. metal is 80A Pd+200A Pt. lift off was in 
> acetone with syringing. looking under the optical microscope some 
> higher dose exposures seem to have come out except for there are cuts 
> in the thin lines. In other samples looks like the exposed areas were 
> also lifted off. I have attached several pictures, exposure does is 
> indicated in each filename. Some users have already suggested using 
> higher doses which I am trying now, but I'd be happy to hear if there 
> are any other comments.
>
> Thank you,
> Arash
>
>
>
>  
>
> --------------------------------------------------------------------------
> Arash Hazeghi
>
> PhD Candidate
> Stanford Center for Integrated Systems,
> 420 Via Palou Mall, CIS-X 300
> tel: 650-725-0418
> mobile: 650-353-1866
> http://www.stanford.edu/~ahazeghi <http://www.stanford.edu/%7Eahazeghi>
>
>
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