Submicron nickel deposition on silicon from an electrolytic solution
controlled by near-field optics
H. Diesinger, A. Bsiesy,
a)
and R. He
´
rino
Laboratoire de Spectrome
´
trie Physique, Universite
´
J. Fourier Grenoble 1-CNRS UMR 5588, BP 87,
F-38402 Saint Martin d’He
`
res Cedex, France
͑Received 27 April 2001; accepted for publication 27 July 2001͒
The application of a near-field optical device to the electrochemical deposition of submicron nickel
dots on silicon is demonstrated. The silicon–electrolyte junction behaves like a Schottky diode
where the electrolyte plays the role of the metal. The junction is reverse biased so that only a
negligible dark current is flowing across the junction. The optical tip of the near-field device is used
as a local lightsource to control a photocurrent on a submicron scale, which allows one to create
submicron objects of nickel by locally triggering the electrochemical reduction of nickel ions. The
effect of the lateral diffusion of the photogenerated carriers on the form of the deposited nickel dots
is described by a two-dimensional carrier diffusion model. © 2001 American Institute of Physics.
͓DOI: 10.1063/1.1405136͔
Since the pioneering work of Dagata et al.
1
who have
demonstrated the possibility of using a scanning tunneling
microscope to induce the nano-oxidation of hydrogen-
passivated silicon surface, other scanning probe microscopy
͑SPM͒ techniques have been widely used to obtain high reso-
lution modification of silicon as well as other semiconductor
surfaces. Atomic force microscopy
2,3
and more recently near-
field scanning optical microscopy
4
have thus been widely
used for this purpose. Generally, these works aim to develop
a SPM nanolithography process on semiconductors by using
the so-formed oxide layer as an effective mask for pattern
transfer onto the substrate. However, although oxidation of
the semiconductor surface can be obtained, other technologi-
cal processes like direct etching or metal deposition are be-
yond the scope of these techniques. Another attractive SPM
technique called scanning electrochemical microscopy has
been proposed by Mandler and Bard who have demonstrated
metal
5
and semiconductor
6
etching as well as metal deposi-
tion on various substrates.
7
In this technique, an ultramicro-
electrode ͑UME͒ tip whose motion is controlled by piezo-
electric elements is scanned over the substrate surface. Both
the UME and the surface are immerged in an electrolyte
containing redox species. The faradaic current that flows be-
tween the UME and the substrate promotes the desired elec-
trochemical reaction at the substrate surface. However, this
technique yields patterns with a lateral resolution in the
range of a few microns ͑Ref. 6͒, at best. This is partly due to
a technological limitation since the minimum size of the
UME that can be achieved is in the range of a few microns.
Furthermore, there is a more fundamental limitation related
to the free diffusion of chemical species in the electrolyte
between the surface and the UME which tends to proceed via
hemispherical diffusion pattern.
In a recent article, we have demonstrated a technique
using near-field scanning optical microscopy to perform pho-
tocurrent mapping on silicon surfaces
8
contacted by an elec-
trolyte and evidenced lateral resolution as good as 100 nm.
In the present communication, we extend our technique to
the deposition of metal patterns with submicron lateral size
and we study the factors that govern the lateral dimensions.
The experimental setup used in the present study has
been described in our previous work on photocurrent map-
ping ͑see Ref. 8͒. The main difference is the nature of the
electrolyte which is, in this study, an aequeous solution con-
taining 0.5 M boric acid and 2 M NiSO
4
. Boron-doped
p-type substrate having a resistivity of 5 to 8 ⍀cm has been
used in all experiments. The substrate ͑working electrode͒ is
reversely ͑negatively͒ biased versus a pseudoreference elec-
trode which is a platinum wire dipped into the solution.
Figure 1 shows the current–voltage behavior of the pho-
tocurrent measured through a lock-in amplifier. The thresh-
old voltage is about Ϫ0.8 V/Pt, followed by a saturation
area. The deposition has been performed at Ϫ1.1 V/Pt, which
is well in the saturation area. The cathodic current results
from electron consumption by electroactive species in the
solution. If Ni
2ϩ
ions are involved, nickel deposition takes
place according to Ni
2ϩ
ϩ2e
Ϫ
→Ni.
Figure 2 is a three-dimensional topographic view of a
dot that has been deposited in 5 min at a photocurrent of 12
nA. The deposition has been performed in a stationary re-
gime, i.e., without scanning the tip. The topography has been
acquired in situ immediately after the deposition. The dot has
a diameter of about 350 nm and a height of about 25 nm. Its
top is extremely flat and it features a well-defined sharp bor-
der. Two more deposits have been performed for different
amounts of exchanged electric charge. In the first one, a pho-
tocurrent of 10 nA has been applied for 10 min, in the second
one, a photocurrent of 12 nA has been flowing for 3 min.
Figure 3 shows for all three deposits the volume estimated
from topography as a function of the consumed charge
amount. The solid line is a linear adjustment forced through
zero which yields a photocurrent efficiency of about 4.8
ϫ10
Ϫ3
%. It shows that the quantity of deposited matter is
a͒
Author to whom correspondence should be addressed; electronic mail:
ahmad.bsiesy@ujf-grenoble.fr
JOURNAL OF APPLIED PHYSICS VOLUME 90, NUMBER 9 1 NOVEMBER 2001
48620021-8979/2001/90(9)/4862/3/$18.00 © 2001 American Institute of Physics