Oxidation and Adhesion on the Quasi-Crystalline AlPdMn Surface
Studied by Nanolithography
J.Smith1,2*, G.Torricelli2, F. Marchi1,3, P.Budau1,4, F. Comin2, J. Chevrier1,2,3
1. LEPES-CNRS – BP 166, F-38042 Grenoble cedex 9, France
2. ESRF - BP 220, F-38043 Grenoble Cedex, France
3. Université Joseph Fourier, Grenoble, France
4. University of Bucharest, Faculty of Physics, Magurele, MG-11, Romania
*To whom correspondence should be addressed:
Department of Chemistry and Chemical Biology
610 Taylor Rd.
Piscataway, NJ 08854-8087
Phone: (732) 445-4351 Fax: (732) 445-5006, email: jasmith@soemail.rutgers.edu
1
Abstract:
We present the novel application of a relatively new technique (nanolithography) to the
study of quasicrystalline surface oxidation. The 5-fold surface of an AlPdMn alloy was
oxidized using a metallized AFM tip. The electrochemical nature of this process was
confirmed by investigating the influence of humidity and polarity of the applied voltage
on the quasicrystalline oxide. Oxides of different thickness and adhesive properties were
created by altering the applied voltage and the humidity during the lithographic process.
The technique can be used in an exhaustive study of properties of the various types of
oxides that form on the AlPdMn surface and the preliminary results of one such study are
reported.
KEYWORDS Atomic force microscopy, Adhesion, Oxidation, Surface chemical reaction
2
1. Introduction:
Since the discovery of quasicrystals in 1982 [1], the focus of their study has
evolved from attempts to clarify the nature of this solid state and its surfaces [2] to
attempts to reveal several of the more remarkable quasicrystalline properties of direct
practical interest. The extremely low adhesive properties, high hardness, good corrosion
resistance and low coefficients of friction [3-8] of quasicrystalline materials make them
particularly attractive as wear preventative coatings. Such coatings have found
application in turbine blades, cookware, razor blades, pistons and cylinders [9].
While quasicrystalline oxidation has been studied by several groups[10-13], very
few have studied the adhesive properties of the oxides that form on quasicrystalline
surfaces[14-16] and none have done so on the nm scale. As such alloys oxidize in all but
the most chemically inert or reducing environments, properties of the surface oxide (i.e.
structure, thickness, chemical composition) determine the adhesive properties of the
surface. The goal of the present work is to use the relatively new technique of AFMassisted lithography in order to study the adhesive properties of the various oxides that
form on the AlPdMn quasicrystalline surface.
Both Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy
(STM) assisted lithography have been demonstrated on Si[17-19].
The process is
explained in detail elsewhere [20]. AFM lithography is an electrochemical process
characterized by the following brief description. A metallized tip is held close to the
sample surface for a few seconds and a large negative bias is applied to the tip with
respect to the sample.
If the environment is sufficiently humid and the tip-sample
3
distance sufficiently small, a water meniscus will form between the tip and sample. The
electric field created by the tip bias ionizes the meniscus and oxidizes the sample surface
exactly where the meniscus comes into contact with it.
This process is strongly
dependent upon three factors: (1) the formation of the water meniscus between the tip
and sample surface (i.e. the humidity and tip-sample distance), (2) the creation of a field
of sufficient strength to ionize the water meniscus and (3) the duration of the application
of that field. This process has been well characterized when performed on Si [17-19].
However, it has not been studied on any quasicrystalline surface to date.
The native oxide of AlPdMn is Al2O3 and studies of the adhesive properties of the
oxidized surface have been performed [9].
It has been shown that the coefficient of
dynamic friction of the 5-fold surface of AlPdMn decreases by 50% with the growth of a
very thin (<10 Å) oxide layer. This suggests that the coefficient of dynamic friction of
the oxide may decrease with thickness.
2. Experiment
Initial Sample Preparation
The purpose of the initial sample preparation was to create a well-defined surface
for AFM lithography and the adhesion studies. The procedure outlined in this section
was performed only one time prior to all AFM work (i.e. it was not repeated prior to each
experiment).
The quasicrystalline sample involved in this work was Al70.3Pd20.6Mn 9.1. Its
5-fold surface was polished (in ambient air) to 0.25 μm using standard SiC and diamond
polishing papers.
This process yielded a shiny, mirror-like surface, whereas the
4
unpolished surface is rough, dull and relatively non-reflective.
It should be noted,
however, that even the polished surface is considerably less shiny than those resulting
from cleaving the quasicrystal under Ultra High Vacuum (UHV) conditions
The surface treatment was performed in a (10-10 Torr) vacuum and consisted of
cycles of Ar ion bombardment (20 minutes, 1keV < Ein < 2keV) and anneal (3 hours) at
600ºC. The cycles of bombardment and anneal were continued until a 5-fold LEED
pattern was obtained. Subsequently, the sample was removed from the chamber and
allowed to oxidize in ambient air (density of water vapor of approximately 7x10 -6 g/cm2)
for several hours.
AFM Studies
The AFM studies were performed in ambient air and in flowing gaseous nitrogen
at room temperature using the Digital Instrument NANOSCOPE 3100 system. The
humidity of the experimental environment was measured using a high sensitivity fast
response thermometer-hygrometer [21].
Relative adhesive properties of the various oxides were obtained using the AFM
tip as the probe in a nm-scale probe tack experiment. This is done by obtaining the
“Force curve” [22,23] (i.e. the deflection of the cantilever vs. the tip-sample separation)
as the AFM tip approaches, contacts and then withdraws from the sample surface. The
tip deflection is linearly related to the force of tip-sample interaction through the
cantilever spring constant. Then the force of adhesion, which is the force needed to
disengage the tip from contact with the surface, can be directly extracted from such a
force curve. The force of adhesion is usually a few nN and is related to the irreversible
5
work of adhesion. All adhesion measurements presented were performed with a single
non-metallized SiC tip. Spring constants mentioned in this work are those supplied by
the manufacturer. Normal deflection spring constants and tip radii are 0.60 N/m and 20
to 50 nm. The results presented here were all obtained using the same tip in as dry an
environment as possible (h = relative humidity ~ 19%, corresponding to a density of
water vapor of approximately 3x10-6 g/cm2). This was done in order to minimize the
effect of adsorbed moisture on the measurement.
After the lithography and surface analysis were performed, the chemistry and
depth of the native oxide on the surface were analyzed.
Using Scanning Electron
Microscopy in conjunction with depth profile micro-Auger spectroscopy, we measured
the thickness of the native oxide layer on our sample to be approximately 2 nm.
The
composition of the native oxide of AlPdMn was seen to be essentially Al2O3, which is
consistent with the literature [24].
3. Results:
A. Lithographic Process
The electrochemical nature of the lithographic process needed to be confirmed as
this study marked the first attempt to apply AFM-assisted lithography to quasicrystalline
materials in general, and to the AlPdMn system in particular.
In other words, it could
not be concluded a priori that any observed protrusions created on the surface were
indeed oxide. These could easily have been caused by other phenomena due to the high
magnitude (on order of 109 V/m) of the field created between the tip and sample. For
6
example, this field might cause damage and subsequent deposition of some part of the
AFM tip (e.g. the metallic outer layer) or direct surface damage.
Either such
phenomenon would result in bumps on the surface that would be indistinguishable from
electrochemically created oxide using only AFM.
The volume of electrochemically created oxide, however, should strongly depend
on the ambient humidity while tip deposition or surface damage should not. Therefore,
the dependence of the results of the lithographic process on humidity was verified and
taken and as confirmation of the electrochemical formation of surface oxide. Results
from this experiment appear in Figure #1. Twelve patches of oxide, or oxide dots, are
shown.
Each dot was created by tapping mode lithography while using the same
metallized AFM tip. The dots were formed in succession, starting with the bottom left
and finishing with the top right, as indicated by the large white arrow in the figure.
During the deposition, the tip was held for 20 s at a constant bias of –9V and within
several nm of the grounded sample surface. The partial pressure of nitrogen gas was
controlled in order change the relative humidity of the experimental environment during
the creation of each of the dots. The map on the right hand side of the figure gives the
humidity during the creation of each of the dots.
The dots in the lower left were created at maximum humidity (~45%) which
corresponds to ~10-5 g/cm3 of water vapor.
The humidity was then decreased
continuously (from lower left to upper right in the image) until two dots were made at the
minimum (~19%). These dots are indicated in the figure. In the progression, one can
easily see that dot volume decreases substantially as the humidity decreases in the tested
range 45-19%. This corresponds to a decrease in the density of water vapor in the
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experimental environment from 10-5 to 3x10-6 g/cm3. Finally, the humidity was again
increased to maximum (~45%, ~10-5 g/cm3 of water vapor) in order to create the two dots
in the upper right portion of the image. The volume of these last two dots is nearly
identical to that of the dots created under conditions of maximum humidity at the
beginning of the experiment (lower left of image). This confirms that the observed
decrease in dot volume was due to changes in humidity as opposed to a gradual
degradation in the quality of the tip or to an effect of surface structure on the lithographic
process.
Unexpectedly, the shape and width of these dots in the plane of the surface does
not appear to change with humidity. During the lithographic process, the dot width
would be expected to decrease with humidity as the volume of the water meniscus
between the tip and surface decreases. The absence of this effect in the image is perhaps
due to the impact of the tip shape. As the tip diameter is comparable to the lateral
dimensions of the dots, the images of the dots in Figure #1 result from a convolution of
both the physical shape of the dots and the physical shape of the tip. Then, only the dot
dimension perpendicular to the surface could be considered to be measured accurately by
AFM. This dimension is referred to as the relative “height” of an oxide dot or line. It
was measured by taking the difference between the height of the feature in the
topographic AFM image and the average height of the adjacent surface over 2 square nm.
By this measure, the decrease in relative humidity from 47 % to 24 % causes a decrease
in dot height from 25.1 nm to 4.1 nm (Figure #1). In other words, a decrease in the
humidity from its maximum to half this value results in an 84 % decrease in dot height.
8
An electrochemical process, as outlined in the introduction, would require a
negative tip bias. Therefore, the effect of a change in polarity of the tip bias on the
process was also investigated. A bias of +9V was applied to the tip as it was held within
a few nm’s of the surface for 20 s. The mark created by this process on the surface was
then compared to that which was left by repeating the process on a neighboring region
with a negative tip bias (–9V). While positive bias ‘lithography’ did induce changes on
the surface (possibly due to deposition of some part of the tip), the difference between
these changes and those observed for negative bias lithography was dramatic. While the
former process created a form on surface less than a few nm’s in height and width, the
latter created an oxide dot more than 20 nm tall and approximately 20 nm wide.
The facts that this process is shown to require the presence of water and a
negatively biased tip are consistent with the process of tip –assisted oxidation. They are
wholly inconsistent with tip deposition or field-induced surface damage.
B. Lithographic Parameters
The effect of change in tip bias on lithography was investigated. For this series of
experiments, lithography was performed in contact mode with W2C coated tips. Five
oxide lines were drawn under high humidity (~35%) conditions, each with a different tip
bias ( –10, -8, -6, -4 and –2V ). This experiment was repeated 10 times. It was found
that lithospeed in the formation of lines needed to be between 10-30nm/s. Below 10
nm/s, the shape of the oxide pattern was clearly non-linear, while 30 nm/s was too fast to
create substantial amounts of lithographic oxide on the surface.
Topographic images were then taken and used to deduce the relative heights of
the lines. The results from a representative attempt appear in Figure #2. The figure
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shows an obvious correlation between the magnitude of the applied bias and line height.
In order to make the comparison quantitative, relative oxide line height vs. applied tip
voltage is plotted in Figure #3 as averaged over the ten trials. The results clearly show a
strong and possibly linear dependence of lithographic oxide height on the magnitude of
the applied voltage. Indeed, the figure shows that changing the voltage from - 4 to - 10 V
results in a change in line height from 6.1 ± 2.2 to 27.2 ± 7.0 nm. Thus, a six volt
increase in the magnitude of the applied voltage results in an increase in dot height which
is on order of hundreds of percent.
Note that the results in Figure #3 show that the threshold voltage for lithography
on AlPdMn has been determined to lie within –2 and –4V. A similar lithographic study
on CVD Al metal, performed under the same conditions and with the same equipment,
showed a threshold also within the range of -2 and -4 V. The threshold is the same order
of magnitude when measured on oxidized silicon [25]. These results are consistent with
the hypothesis that the lithographic threshold depends primarily on the water meniscus
formed between the tip and sample and, so, is relatively substrate independent.
C. Adhesion Results
Adhesion measurements show a definite difference in the characteristics of the
lithographic oxide from those of the native oxide. In total, 360 adhesion measurements
were performed on 12 different lithographic oxides and 240 adhesion measurements on
10 different areas of native oxide. Two representative force curves appear in Figure #4
and the force of adhesion is indicated in the figure. Note that the scales in Figures 4a,b
are identical.
Both force curves were obtained using the same tip under identical
conditions. It is evident from Figure #4 that the measure of adhesive force presented for
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the native oxide is nearly twice that for the lithographic oxide. On average, the AFM tip
adhesion to the lithographic oxide surface is only 68% as strong as the adhesion
measured on the native oxide. This difference was observed for the entire series of
measurements.
4. Discussion:
We note that the variance of the measurement of the force of adhesion on the
native oxide was 21% over 240 measurements. On the lithographic oxide, the variance in
this measurement was closer to 29% over 360 separate measurements. To date, we have
not measured how the chemical composition of the oxide varies laterally on the surface.
If this is substantial, it is one possible source of this error.
While chemical
inhomogeneity in the lithographic and native oxides remains an issue for further study,
we may say with confidence that it will not affect the major conclusion we draw from the
results presented here, i.e. that the adhesive properties of the lithographic oxide are
significantly weaker than those for the native oxide.
Previous studies by Dubois et al have shown [26,27] that simply increasing the
thickness of the amorphous, native oxide surface layer (below 12 nm) substantially
decreases the adhesive properties of quascrystalline surfaces. The results presented here
are consistent with those of Dubois et al. A detailed understanding of the chemical
composition and structure of the oxide produced by nanolithography would help to
clarify whether our results are fully explained by their conclusion, i.e. that this effect is
due to the influence of the electronic structure of the underlying quasicrystal. While we
11
have not yet succeeded in this rather difficult experiment, we can discuss the results
obtained in the context of the literature.
All studies to date have shown that oxidizing the surface of the AlPdMn
quasicrystal under a variety of conditions yields a surface layer that is nearly chemically
identical to the native oxide of aluminum metal, i.e. Al2O3 [14-16,26,27]. Then, the
simplest plausible description of the lithographic oxide is to consider it is an amorphous
layer of Al2O3. However, we do expect the precise composition and structure of the
oxide to depend somewhat on the fabrication process, as has been observed in the case of
the oxidation of Si (100) by nanolithography [28]. Indeed, our experimental observations
indicate an oxidation mechanism that depends upon experimental details such as the
precise level of humidity, the tip quality as well as parameters such as the applied voltage
and normal force. To a first approximation, however, it is reasonable to consider the
oxide surface in contact with the AFM tip as the same as the surface of the native oxide
layer.
In this case, our results show that the thickness of the oxide layer is inversely
related to the adhesive properties of the oxide (Figure #4). That this difference cannot be
ascribed to changes in the lateral dimension of the oxide (i.e. changes from that of the
surface covering native oxide with its effectively infinite lateral extent to the finite
lithographic oxide), can be assumed because the lateral dimension of the lithographic
oxide (300-500nm) is greater than the radius of the tip (20-50nm) by at least an order of
magnitude. Then, even the lithographic oxide has a large lateral dimension with respect
to the tip. The nm scale changes in oxide thickness (Figures #1-3) are known be of the
scale over which substantial variations in Van der Waals interactions occur in any
12
materials system. As the extraordinary adhesive properties of quasicrystals are suspected
to lie in their peculiar Van der Waals interactions [26,27], our results show that
nanolithography provides a practical method to probe changes in adhesive properties
quasicrystalline oxides in detail.
5.Conclusions and Future Directions:
The primary result of this study is to show the first successful use of AFMassisted lithography to create various oxides on the quasicrystalline AlPdMn surface. We
have proved the electrochemical nature of this process by demonstrating the effect of
humidity and the polarity of applied voltage on it.
We also have determined the
“lithographic range” (i.e. the range necessary to perform lithography on this surface) of
for several key parameters. These include the humidity (23 – 47%), tip bias: -(4 - 11V)
and lithospeed in the formation of lines (10-30nm/s). We have studied effect of changing
several of the lithographic parameters on oxide thickness. First, we determined that a
decrease in humidity to half its maximum value decreases oxide height by 84 %. Next,
we observed that the increase in the magnitude of the tip bias by 6V (from -4V)
introduces an increase in oxide height that is on order of hundreds of percent.
We have also demonstrated the utility of this technique in the study of the
adhesive properties of quasicrystals. Our results show that the adhesion of the thicker
lithographic oxide to a SiC probe is 32% weaker than that measured if the same
experiment is performed on the native oxide of AlPdMn. This result along with the size
scale of the difference in the thickness of these layers are consistent with the hypothesis
13
Van der Waals interactions play a key role in the peculiarity of the adhesive properties of
these surfaces.
While it is difficult to extend this analysis further without a better
knowledge of the oxide layer, this raises an issue that merits future study. Further, the
work shows that it would also be useful to measure the adhesive properties of the
quasicrystalline surface in the absence of oxidation, i.e. in UHV just subsequent to the
surface preparation described in the introduction.
In such an experiment, adhesive
properties can be quantified via the non-contact interaction between a tip and the surface
at distances varied from a few nanometers up to 500nm. The analysis of such results,
using the Lifschitz formula for the non retarded Van der Waals interaction at distance
shorter than the plasma length, will lead to accurately quantifying the adhesive force
between the tip and the sample. This work is currently in progress.
Finally, these results suggest that it may be possible to diminish the already
phenomenally weak adhesive properties of AlPdMn by increasing the thickness of its
oxide. If such a procedure is developed it would have a very significant impact in the use
of quasicrystalline materials as wear-resistant coatings.
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Figure Captions
Figure #1 a) AFM tapping mode topographic image of oxide dots made on AlPdMn .
Large white arrow indicates the direction of tip motion in the formation of dots. The dots
imaged in a) are labeled in b) by the humidity as they were created.
Figure #2: Oxide Lines created by lithography: a) Topographic image in contact mode
showing four oxide lines, each labeled by the lithographic voltage used to create them.
The green dotted line marks the section of a) used to create the plot in b). b) Section
(averaged) showing decrease in height of oxide lines in a) with decrease in magnitude of
applied voltage. Lines are labeled by the applied voltage used to create them.
Figure #3: Oxide line height as a function of applied voltage on surface of AlPdMn.
Data is averaged over 10 trials.
Figure #4: Force curves obtained on a) the native oxide b) the lithographic oxide using a
SiC in a dry (19%, 3x10-6 g/cm2) environment. Tip deflection is measured which can be
related to tip-sample force, plotted on the y-axis, through the spring constant of the tip.
The x axis is the lateral displacement of the tip from sample where “0.0 nm” denotes
contact. The measure of the force of adhesion is labeled in the figure.
16
3.0
Last Dot
Formed
Dust
Particles
45%
45%
3.0
19%
18%
Formed under
conditions of
lowest humidity
2.0
μm
2.0
μm
23%
27%
25%
1.0
1.0
26%
28%
47 %
First Dot Formed (highest humidity)
0.0
1.0
a)
μm
2.0
0.0
3.0
43%
0.0
1.0
b)
35%
μm
0.0
2.0
3.0
Figure #1
15.0
-10 V
-6 V
-8 V
-4 V
μm
30.0
μm
15.0
5.0
0.0
10.0
0.0
5.0
μm
a)
10.0
-10V
-8V
-6V
-4V
0.0
15.0
b)
Figure #2
17
Height of Oxide Line (nm)
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
Magnitude of Voltage Applied to the Tip (V)
Figure #3
0.0
-5.0
Measure of
adhesive force
-10.0
Force (nN)
Force (nN)
0.0
Measure of
adhesive force
-5.0
-10.0
a)
0.0
75.0
150.0
Lateral Displacement (nm)
0.0
b)
150.0
75.0
Lateral Displacement (nm)
Figure #4
18
19