High contrast photoresist for interference lithography
On 1/15/2013 9:00 PM, Andrew Sarangan wrote:
> Having done interference lithography for more years than I'd like to admit,
> I can offer some of these suggestions:
> - what presents itself as sloped sidewalls, under-developed or underexposed
> can often be entirely due to poor resist/substrate interface. Lines that
> are fully developed with straight walls can often float away during
> development, making diagnostics very difficult. I've had much better
> success since switching to vapor priming the substrates (assuming silicon).
I tend to see scalloped resist profile, almost like an inverted
cycloid. With long enough development
time I am left with the cusps of the inverted cycloids. I don't think
the resist is
underexposed as I am working at an exposure dose where the development
As I understand it, this should correspond to the regime where the DNQ
has been fully
broken down, giving the highest ratio between development rate between
unexposed and exposed
Currently, I am spin coating an 80/20 primer. I have a standing offer
to vapor prime some wafers
with Bill Moffatt's company, so I should probably pursue that.
> - I have had better success with i-line resists than with g-line, even as
> far away as 488nm (Ar ion laser). The sensitivity will be low, so you have
> to increase the dose.
Interesting. I wouldn't have considered trying that. Currently, I
don't have any i-line resists.
> - You can get even better sidewall results with an anhydrous ammonia image
Yes, I'm working on a proposal to acquire an oven capable of carrying
out this process.
> - as for resistance to fluorine plasma etching, it depends on what
> substrate you are trying to etch. Most resists are more sensitive to ion
> energy than to the fluorine radicals. Reducing the ion energy and
> increasing the plasma density would give better results, although it will
> produce some undercut. An ICP would be useful here to control these
Thank you for the information. We have been able to obtain anisotropic
etching so far.
We can also implement hard bake (which hasn't been necessary so far) to
> - For even greater etch resistance, I have used negative-acting chemically
> amplified resist in the deep-UV (266nm).
We are currently limited in our operating wavelength. It looks like
those DUV resists don't absorb
at our wavelength. If we have selectivity problems we have some ideas
to get around that.
> - As for dilution, I am surprised to hear the difficulty getting consistent
> dilutions. I have found the variations due to spin & bake steps to be more
> significant than the dilution step. If you use a sensitive weighing scale,
> and make large batches, that should not be much different than the premixed
This was probably due to working in small batches and running into snags
related to mixing vs.
Thank you for your advice!
Justin M. Hannigan, Ph.D.
Process Development Engineer
875 Wilson St, Unit C
Eugene, OR 97402
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