Tuning surface morphologies of ion-assisted diamondlike carbon film on the nanometer scale

Article abstract of DOI:10.1063/1.1473827

The surface morphologies of ion-assisted diamondlike carbon films were tuned on the nanometer scale. The graphitization with increasing temperature induced different dominant smoothening mechanisms which were responsible for the observed morphological cha

Full text of DOI:10.1063/1.1473827

Tuning surface morphologies of ion-assisted diamondlike carbon film on the
nanometer scale
X. D. Zhu, H. Naramoto, Y. Xu, K. Narumi, and K. Miyashita
Citation: The Journal of Chemical Physics 116, 10458 (2002); doi: 10.1063/1.1473827
View online: http://dx.doi.org/10.1063/1.1473827
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 116, NUMBER 23
15 JUNE 2002
Tuning surface morphologies of ion-assisted diamondlike carbon film
on the nanometer scale
X. D. Zhu
Takasaki-Branch, Advanced Science Research Center, JAERI, 1233 Watanuki, Takasaki, Gunma 370-1292,
Japan, and Department of Modern Physics, University of Science and Technology of China, Hefei,
Anhui 230026, People’s Republic of China
H. Naramoto, Y. Xu, and K. Narumi
Takasaki-Branch, Advanced Science Research Center, JAERI, 1233 Watanuki, Takasaki,
Gunma 370-1292, Japan
K. Miyashita
Gunma Prefecture Industrial Technology Research Laboratory, 190 Toriba, Maebashi,
Gunma 371-0845, Japan
͑
Received 20 November 2001; accepted 7 March 2002͒
We report the unstable surface feature in nanometer-scale of diamondlike carbon ͑DLC͒ films
ϩ
deposited through C60 evaporation with simultaneous bombardment of 1.5 keV Ne ions. The
periodical ripples, commonly appearing in postprocessing of the deposited films, form directly at
550 °C and 700 °C, which is qualitatively consistent with the theoretical model based on sputtering
yield variation with surface curvature. A dramatic transition from ripple surface to mounding
roughening occurs at 400 °C. The graphitization with increasing temperature induces the different
dominant smoothening mechanisms, which is responsible for the morphological change observed.
Further, the calculations of height–height correlation function show that the roughness exponents
are around 0.8 at 200 °C and 400 °C, implying self-affinity of roughened surfaces. This study
exhibits a potential of ion beam assisted deposition to tune DLC morphologies by controlling the
deposition parameters to drive the competition between ion erosion and film deposition. © 2002
American Institute of Physics. ͓DOI: 10.1063/1.1473827͔
In thin film deposition, important physical properties of
the films deposited are linked tightly to surface morpholo-
gies. Control of film microstructure is a critical issue in
achieving best performance of film. Great effort has been
paid to pattern a surface on the nanometer scale via the con-
trol of self-assembly in growth process or postprocessing.1
It is shown that ion sputtering has been an effective approach
on the formation of nonequilibrium surface. A periodical pat-
topographes. As a functional material, DLC possesses excel-
lent physical and chemical properties. In its applications as
protective coatings, electron emission devices as well as op-
tical films, etc., one of challenges is to control surface mor-
phologies effectively. It is of interest to investigate surface
topograph in nanometer scale of DLC films and understand
the dynamic response during evolution of surface morphol-
ogy. This can shed important insight into the interaction of
the various surface processes, which is essential to atomic-
level control of surface morphology. By changing substrate
temperature, a morphological transition from periodical
ripples to mounding coarsening is realized during deposition.
We expected that this research might provide a possibility of
tuning nonequilibrium structure of the DLC films.
–5
3
4
tern, ripple, formed in metal, semiconductor, and amor-
5
phous materials after off-normal sputtering. The common
denominator for ripple formation is postdeposition sputter-
ing.
Combining both deposition and sputtering, ion beam as-
sisted deposition ͑IBAD͒ exhibits distinctive advantages for
film growth. In IBAD, a series of thermal spikes can be
created on the paths of the ion trajectories. The requirements
of a chemical reaction of ions and source species can be
satisfied energywise and timewise. The reactant products
condensate on the substrate surface to form films, which is
physical vapor deposition. Simultaneously, ion beam bom-
bardment during deposition strongly influences growing sur-
face topographies, which differs from film postdeposition
sputtering. In the latter case, incident ion beam faces only a
solid surface, while in the former case the solid surface ion
beam bombards is also exposed to a flux of vapor atoms.
In this letter, we take advantage of IBAD to fabricate
directly diamondlike carbon ͑DLC͒ films with pronounced
Our samples were grown on Si ͑111͒ wafers by using ion
beam assisted deposition. Here we give a brief description.
Ϫ6
The background pressure in the chamber was Ͻ2ϫ10 Pa.
C60 powder with the purity of 99.99% was heated resistively
to 390 °C to produce C60 vapor as carbon source. Growing
ϩ
film was bombarded simultaneously by Ne ion. The angle
ϩ
of incident Ne ion beam is 60° from the substrate normal.
ϩ
During deposition, the Ne ion energy was kept at 1.5 keV,
and also the emission of the filament in the ion gun was fixed
Ϫ4
at 20 mA. Under the constant working pressure of 6ϫ10
Pa in the chamber, the fluence of incident ion is therefore
invariant, we only changed the substrate temperature during
deposition that can be controlled from room temperature to
0
021-9606/2002/116(23)/10458/4/$19.00
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10458
© 2002 American Institute of Physics
1

J. Chem. Phys., Vol. 116, No. 23, 15 June 2002
Surface morphology of diamondlike carbon film
10459
FIG. 1. Raman spectra of DLC films at different temperatures, 200 °C,
4
00 °C, 550 °C, and 700 °C. Raman features present significant changes
with increasing the substrate temperatures.
8
00 °C. After deposition, the deposited films were character-
2
FIG. 2. 500ϫ500 nm AFM scans of DLC films. The sample was deposited
at 200 °C, 400 °C, 550 °C, and 700°C, respectively. The growing surfaces
exhibit periodical ripples oriented to projection of incident ion beam at
5
4
ized by Micro-Raman spectroscopy using the 514 nm line of
Ar ion laser and atomic force microscopy ͑JSPM-4200͒.
Figure 1 depicts Raman spectra of the amorphous carbon
50 °C and 700 °C. A dramatic change in surface morphology takes place at
00 °C. Visual irregular growth mounds are observed at 200 °C and 400 °C.
films deposited at four different substrate temperatures under
ϩ
1.5 keV Ne ion bombardments. Graphite has two signifi-
cant Raman lines, so-called G peak and D peak. The major
features of Raman spectra of amorphous carbon films can be
derived from corresponding features in the spectrum of
graphite. The Raman spectrum at 200 °C presents a strong
broad band centered about 1537 cmϪ1. This means typical
structures of DLC film. With increasing the substrate tem-
perature to 400 °C, however, separated G and D lines appear
slightly. For amorphous carbon, the D and G lines tend to be
separated, distinct peaks are indicative of the more graphite
material. The presence of the slight separated D and G peaks
at 400 °C indicate the graphitization in the growing film.
This phenomenon becomes more pronounced as the substrate
temperature is higher than 400 °C. For DLC film growth by
energetic particle methods, it is generally accepted that the
phenomenon is not yet well understood experimentally and
theoretically now. This kind of unstable surface commonly
appears after postsputtering of the films. Different from our
case, Habenicht et al. found ripples at room temperature as
ϩ
͑0001͒ graphite surfaces were eroded by a 5 keV Xe ion
7
beam. We believed that this should be correlated to the vari-
ous smoothing mechanisms taken place under ion bombard-
ment as discussed later.
Bradley and Harper first proposed a theory ͑BH model͒
for this kind of periodical surface instability induced by ion
5
bombardment. BH model predicts that ripple topography
appears as an amorphous solid is bombarded by off-normal
incidental ion beam. The ripple orientation depends on the
projection of the ion beam. This theory is based on the fact
that the power deposition that causes sputtering is maxi-
mized below the surface. The BH model only considers ther-
mal diffusion at the higher temperature range and provides a
qualitative fit to the experimental data. The ripple formation
at high temperature in our case is in qualitative agreement
with BH model that the ripple is a consequence of a
curvature-dependent sputtering yield.
3
mechanism promoting sp bonding over the normally more
2
stable trigonal planar (sp ) structure due to local compres-
sive stress for amorphous carbon is related to subplantation
6
of low energy. At high substrate temperature, the enhanced
mobility of atoms may cause the relaxation of compressive
2
3
stress, thus suppress the conversion of sp to sp bonding.
Higher substrate temperature further induces an increase and
2
order of sp -bonded clusters, i.e., graphitization.
Because of underestimating the diffusion term at lower
temperature, BH model does not fit experimental data at low
temperature. On the basis of BH model, the ripple theory has
been developed by some authors, the common scenario for
ripple formation is that the surface instability is caused by
the competition between roughening termed as negative sur-
face tension and smoothing described by positive surface
tension. Besides thermal-surface diffusion considered in BH
model, collision induced atomic drift and viscous-flow relax-
ation were also suggested to aid smoothing.4 We believe
that the change of surface structures with temperature may
respond to diverse dominant smoothening mechanisms in-
volved in the DLC deposition. The fact shown in Fig. 1
should be noted that the Raman features of DLC films vary
significantly with the substrate temperature. The pronounced
separated D and G peaks appear in the Raman spectra at
550 °C and 700 °C, demonstrating distinct graphitization in
We employed atomic force microscopy to examine the
surface morphologies of the specimens. Figure 2 shows AFM
images of DLC films at different substrate temperatures,
200 °C, 400 °C, 550 °C, and 700 °C. The interesting finding
is that a periodical ripple forms directly in the deposited film
at 700 °C. The wave vectors are parallel to the component of
the ion beam in the surface plane. The dominant feature for
the DLC film at 550 °C is still similar to that at 700 °C,
showing ripple structure. However, a dramatic change in sur-
face topograph occurs at the substrate temperature of 400 °C,
visual irregular growth mounds appear in the film. As the
temperature is decreased further at 200 °C, the film maintains
this kind of mounding coarsening. The periodical ripples do
not appear at relatively low temperature instead of growth
mounds.
,8
Ripple formation reveals an unstable growth mode. This
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10460
J. Chem. Phys., Vol. 116, No. 23, 15 June 2002
Zhu et al.
0
Ͻ␣Ͻ1 is indicative of the texture of self-affine surface.9,12
In addition, as shown in Fig. 1 the slight separation of the
D peak and G peak is observed at 400 °C. With increasing
growth temperature, sp2 bonded atoms increases in the
films, leading to more roughening. This should be contrib-
uted to the difference of the roughness exponents shown in
Fig. 3.
One model determined by a nonlinear growth equation
describes an intermediate range diffusion with the scaling
exponents ␣ϭ2/3 and ␤ϭ1/5.1
0,11
The ‘‘unstable growth’’
FIG. 3. The height–height correlation function based on the data from the
AFM images at 200 °C and 400 °C in Fig. 2. The roughness exponents are
determined to be about 0.8.
described by a linear equation yields scaling exponents of
ϭ1 and ␤ϭ1/4, suggesting a local diffusion corresponding
␣
2,10
to mounded morphologies.
The physical nature for this
unstable growth is often ascribed to the ‘‘Ehrilich–
Schwoebel’’ barrier. In our case, the roughness exponent ␣ is
closer to 2/3, far from the value predicted by the linear
growth equation. This is not entirely surprising, because the
the deposited films. In such high temperature, thermal diffu-
sion is primary smoothing mechanism, while ion irradiation
induced viscous flow is weakened compared with the case at
relatively low temperature. Because of curvature-dependent
sputtering effect, ripple structures form during deposition. At
low temperature, however, the samples exhibit the evident
amorphous feature. Viscous flow relaxation induced by ion
irradiation enhances in the amorphous surface. The surface
morphology acquired during deposition at last relies on the
‘‘Ehrilich–Schwoebel’’ barrier is not known to be present in
amorphous system due to the lack of well- defined steps. We
think the process might be better described by nonlinear dif-
fusion. This deduction is supported by the work of Mayer
et al. recently.13 They determined the roughness exponent
␣
ϭ0.8 in the growth of vapor-deposited amorphous
Zr Al Cu films. This value is close to that of our ex-
6
5
7.5
27.5
microscopic dynamic of roughening and smoothening. The8
periments. In order to understand the growth process, they
have carried out a numerical simulation of a Monte Carlo
large values of viscous relaxation hinder ripple formation.
Thus, the growth mounds appear in the film as the tempera-
ture is below 550 °C. It can be concluded that the transition
from growth mounds to ripple formations with substrate tem-
peratures should be ascribed to the different dominant
smoothening mechanisms, where the relaxation level of ion
irradiation induced viscous-flow varies with the substrate
temperature due to graphitization.
Further, the scaling analysis of surface is carried out
based on height–height correlation function in order to ob-
tain better insight into the mounding roughening. The scaling
2
2
model and added a nonlinear term C/2ٌ (ٌh) to the con-
tinuum growth equation that includes surface diffusion and
the curvature dependent deposition process. The numerical
simulation is quantitative agreement of the experimental re-
sults.
In summary, we have shown that ion beam assisted
deposition exhibits a potential to adjust surface morphology
in nanometer scale for DLC film. By controlling deposition
parameters to drive the competition between smoothing and
roughness, we can tune mounding coarsening to ripple struc-
ture. The transition is due to the change of ion irradiation
induced viscous-flow relaxation caused by graphitization
during DLC deposition. The ripple formations are in qualita-
tively agreement with BH model as a consequence of
curvature-dependent sputtering yield. The vectors of ripples
are parallel to projection of incident ion beam. At relatively
low temperature 200 °C and 400 °C, the roughness exponent
␣ is determined to be around 0.8Ϯ0.04, suggesting self-
affine growths.
theory has been applied for nonequilibrium processes in film
growth or postsputtering,9
–11
a dynamic scaling hypothesis
where the growth surface is assumed to have both short-
range space scaling and long-range time scaling is expected.
According to this scaling theory, the rms roughness ␴,
i.e., the standard deviation of the surface height h(r,t) can
2
be expressed in the form, ␴ (r,t)ϭG(r,t)ϭ
͗
͓h(r,t)
2
2
2␣
Ϫh(0,t)͔
͘
, which reduces to ␴ (r,t)ϭG(r,t)ϰr
for
2
2␤
small r with tϭconstant, and to ␴ (r,t)ϰt as r→ϱ, where
G(r) is the height–height correlation function, r is the length
scale over which the roughness is measured, ␣ and ␤ are the
roughness and dynamic scaling exponents, respectively, and
z is ␣/␤. The exponents ␣ and ␤ depend on the special
growth models, their experimental determination and com-
parison with theory can be valuable in identifying the domi-
nating underlying kinetic process. The roughness exponent ␣
is determined from the fit to the linear part of the log–log
plot of G(r) vs r. AFM images in Fig. 2 is further analyzed,
log–log plots of G(r) as a function of lateral distance r for
the DLC films at substrate temperatures of 200 °C and
Zhu appreciates the support by the STA Exchange Sci-
entist Program in Japan.
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1