Dear Martyn,

I had a look at the powerpoint presentation you quoted.  It seems to
be a pretty good overview of etching, however the limitation that you
can't do wet etching for features smaller than 3 microns is wrong.

In general, wet etching is not anisotropic:  material in all
directions will be etched, not just directionally from the surface.
This can be modified by the crystal orientations of your material, but
for your polycrystalline silver or aluminum, it should be rather
isotropic.  This means that the material under your photoresist will
be attacked at approximately the same rate as your exposed regions.

Thus the limitation of size depends on the aspect ratio of the wires
you want.  If the metal layer is very thin and much wider than it is
thin, then wet etching should work fine, down to a very small size.
If, on the other hand, you want wires to be thick and narrow you will
have difficulty doing it using wet etching.

Best of luck,

Rob Knobel
--
Robert Knobel
Assistant Professor, Dept. of Physics, Queen's University
Kingston, Ontario, Canada K7L 3N6
http://www.physics.queensu.ca/~knobel

On 9/19/06, Martyn Gadsdon  wrote:
> Dear All,
>
> I am trying to make a nanowire grating using a simple process. I am spinning
> a layer of photoresist on top of a ~50nm layer of silver or aluminium. The
> photoresist is then exposed and developed to have a sinusiodal profile, of
> between 200 and 300nm pitch, where the troughs of the profile expose the
> metal underneath. I then aim to etch the exposed areas of metal using
> typical chemistries such as H3PO4:HAc:HNO3:H2O to leave an array of parallel
> metallic wires of the same pitch. However, I seem to be having problems and
> never seem to be able to etch the exposed areas.
>
> I have looked into this on the web and came across a short presentation.
>
> The link is:
>
> ccms.ntu.edu.tw/~chihiwu/ch09%20rev3.ppt
>
> and on the slide discussing the disadvantages of wet etching they state:
>
> "Can't pattern sub-3micro-m feature"
>
> I have worked out they are at the National Taiwan University, but I can not
> find the actual author's name or email address to enquire directly to them,
> so I thought I'd try to see if anyone else can confirm their statement.
>
> Can anyone tell me if this is correct and may be the reason why I cannot wet
> etch my 200-300nm profiles, and if so perhaps provide a reference I can look
> up and quote.