Jie,

This is true however I'd suggest your findings are more likely a result
of the exposure tool rather than a property of the resist itself. Here
at Glasgow Uni our resist profiles are similar to what you say but this
is because of our exposure tool.

A discussion of positive versus negative photoresists would take a
while. I'll just say that in choosing a particular type of resist for an
application one should consider things like:

Area to be patterned

Will the mask be used for an additive or subtractive process?

If used for a subtractive process the etch resistance of the mask is
important (many e-beam resists, such as PMMA, suffer in this aspect).

Cost (the high end Chemcially amplified negative e-beam resists are very
expensive, especially for a research institute)

Resolution required

I don't like advertising other universities however the Stanford SNF
website is an excellent resource.

http://snf.stanford.edu/Process/Process.html

You will find details of photo and e-beam resists they commonly use.

This gives a good idea of what is out there.

Cheers

James


Jie Zou wrote:
> One thing that I came across was the sidewall profile of the edge.
> This is critical for the lift-off.
>
> Positive resists gave out positive tone profile which might generate a
> lot of troubles in lift-off. If you try to lift-off a thick film, the
> deposited film could be conformal on the positive tone sidewall and
> the lift-off will fail. By contrast, negative resist usually provides
> a negative tone profile (you can imagine this as an undercut on the
> edges). So the deposited film is naturally discontinuous on the edge.
> The lift-off is easy then.
>
> Jie

--
Dr. James Paul Grant
Postdoctoral Research Associate
Microsystems Technology Group
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University of Glasgow
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