Hello,

First, sorry for my poor English !

I'm working on p-type (5-10 ohm.cm) silicon on insulator substrate.
The Si top layer is 15 nm thick.
My purpose is to realise Si nanowires.

I use an Atomic Force Microscope to realise the lithography on H passivated
surface (HF treatement) which leads to a SiO2 mask (for example : height 1.5
nm / width : 20 nm).
Afterwards I use a KOH etching (1 mol.l-1 20 °C 1 min) which gives very good
and reproductible results to obtain the Si wires on insulator.

The problem comes when I try to realise nanowires between metallic contacts.
I have made metallisation using PMMA resist and ebeam lithography.
The contacts are 2 nm Ti, 20 nm Au ; and they are separated by 2 µm.

The sample is cleaned using first aceton and isopropyl alcool and  after
sulfochromic solution during 10 minutes and DI Water rinsing (30 s).
 The H passivation is then realise with 5% HF during 20 s (which seems not
to etch the metal) followed by DI water rinsing (30 s)
So I  make the AFM lithography between the contacts.

Afterwards I etch the sample using KOH (1mol.l-1 20 °C). In about 1 minute
the top layer of Si is etched everywhere on the SOI sample EXCEPT between
the metallic contacts either I have realised AFM lithography or not. So I
can't obtain the nanowires !!!

Either if I etch the sample longer, the Si top layer remains between the
metallic contacts. I have tried ultrasonic agitation but it does not change
anything. Sometimes the results are a bit better and the Si is partly etched
between the metallic contacts but I don't know the reason why.

So if you have any idea to improve the KOH etching I will be very grateful.

Best regards

Bernard Legrand

PS : please answer by e-mail !

legrand@isen.iemn.univ-lille1.fr

Ph D student, IEMN (Lille) France