Role of humidity in local anodic oxidation: A study of water condensation
and electric field distribution
Miroslav Bartošík, David Škoda, Ondřej Tomanec, Radek Kalousek, Pavel Jánský, Jakub Zlámal, Jiří Spousta,
Petr Dub, and Tomáš Šikola
Faculty of Mechanical Engineering, Institute of Physical Engineering, Brno University of Technology, Technická 2, 616 69 Brno,
Czech Republic
Received 21 October 2008; published 7 May 2009
This paper deals with the analysis of the influence of humidity on the process of local anodic oxidation
carried out by atomic force microscope AFM on GaAs 100 surfaces. Recent experiments have shown that
the height and half width of oxide nanolines do not increase monotonously with relative humidity, but for
lower relative humidities 50% the lines comparable in size to those prepared at 90% were obtained.
However, their height and width along the lines revealed significant variations. To better understand these
phenomena, the AFM force-distance spectroscopy measurements together with computer simulations of an
electric-field distribution and water bridge formation between the tip and the substrate at different relative
humidities were carried out. Our experiments on AFM force-distance spectroscopy have not proved an en-
hanced water condensation between the tip and the surface at lower humidities. However, the simulations of
the electric field in the vicinity of the tip at the early stages of the oxidation process at low relative humidities
showed an increase in the average intensity in the oxide layer promoting the diffusion of oxidizing species
toward the substrate and, hence, the formation of oxide lines under these conditions. Finally, our simulations on
water bridge variations along the tip track showed that at lower humidities there are higher relative standard
deviations in the size of the water bridge while the tip is being moved along the surface. This indicates why the
oxide lines showed a bigger variability in size.
DOI: 10.1103/PhysRevB.79.195406 PACS numbers: 81.16.Pr, 81.16.Nd
I. INTRODUCTION
Local formation of oxides on solid surfaces in the close
vicinity of the tip of an AFM probe has become a favorite
inexpensive nanotechnology method used in many laborato-
ries worldwide. Despite the relative experimental simplicity
of this technique called generally local anodic oxidation
LAO, its principles have not been understood in detail yet.
The LAO is generally described by the Cabrerra-Mott
1
mechanism of oxidation adapted to the configuration with an
electric field between the tip and the sample
2
providing en-
hanced diffusion of charged particles in the sample.
3,4
Most
of the papers on LAO deals with empirical or semiempirical
data related to particular materials. For instance, the influ-
ence of operational parameters on local oxidation, such as
tip-sample voltage, tip writing speed, and tip load force, has
been already studied using contact and noncontact modes of
atomic force microscope AFM.
5,6
LAO depends on an ionic current between the tip and the
sample surface going through a water bridge between the tip
and the surface. The size and shape of this bridge depends on
the amount of the water condensed both on the surface and
between the tip and the surface. Generally, the amount of this
water increases with humidity and, hence, it has been widely
accepted that the dimensions of oxide elements produced by
LAO should grow with humidity as well. This presumption
has been reported by several research teams for various dis-
tinct surfaces but just few exact and detailed humidity de-
pendences have been published yet; e.g., for n-Si100,
7
p-Si001,
8
and p-GaAs100 substrates for relative humidity
from 50–80 %.
9
In our previous work,
10
the role of humidity in fabrication
of oxide nanostructures at GaAs 100 surfaces by LAO was
investigated for a wider relative humidity interval
35–90 %. Contrary to the expectations, the height and the
half width of oxide nanolines grew monotonously with rela-
tive humidity just in the relative humidity interval starting
from 60% up. However, for lower relative humidities
50%, the lines were surprisingly comparable in size to
those prepared at 90%. Nevertheless, unlikely to the lines at
higher humidities their height and width along the lines re-
vealed a significant variability indicating unstable conditions
for the oxidation process at lower humidity.
A similar nonstandard behavior-independence of oxide
height for low relative humidities under 50% and linear de-
pendence for higher humidities has been observed by another
group as well.
11
This paper deals with such a phenomenon, in
a more detail, and looks for some possible key factors e.g.,
water condensation, surface roughness, and electric-field dis-
tribution responsible for this more complex behavior.
II. EXPERIMENTAL DETAILS AND PRINCIPLES
An atomic force microscope AutoProbe CP-R, Veeco
operated in the contact mode was used both in nanolithogra-
phy, microscopic, and spectroscopic experiments. The probes
for nanolithography were triangular silicon cantilevers Mi-
cromasch with a W
2
C coated tip having the average force
constant 0.35 N/m and tip apex radius 35 nm, according to
the producer. The real tip apex radii found by scanning elec-
tron microscope SEMFEI were in the range from 50 to
108 nm. The surface imaging and force-distance measure-
ments were performed using rectangular phosphorus-doped
silicon cantilevers Veeco with the average force constant
PHYSICAL REVIEW B 79, 195406 2009
1098-0121/2009/7919/1954066 ©2009 The American Physical Society 195406-1