Surface Smoothing and etching by gas cluster ion beam
J.H. Song and W.K. Choi
Thin Film Technology Research Center, Korea Institute of Science and Technology, Cheongryang P.O.
Box 131, Seoul 136-791, Korea
Abstract. Ar and CO2 gas cluster ion beam with a few nm size were generated by an adiabatic expansion through
Laval nozzle. The existence and the mean size distribution of the cluster were analyzed by time-of-flight
measurement. Crater induced by Ar cluster ion beam and crown-like hillock by CO2 cluster ion impact on Si(100)
were observed by an atomic force microcopy. CO2 cluster ion was irradiated on Si at 25 kV with the variations of ion
dose from 1010 to 1013 cluster ions(CI)/cm2, at the flux of 109/cm2 s. Through this isolated cluster ion impact, the
interaction mechanism between cluster ion with solid surface was suggested to be made of three steps: surface
embossment, surface sputtering and smoothing, and surface etching. Another surface smoothing and etching
experiment using CO2 cluster ion beam were carried out over ITO/glass and Cr-masked Si3N4 thin film surfaces at 25
kV.
I. Introduction
II. Experimental
Recently gas cluster ion beam having a few
nm size has been interesting since this
nanoparticle ion beam known to be exclusively
prominent in atomic-scale surface smoothing and
useful in hard material etching like CVD diamond
and SiC, due to large sputtering yield and lateral
momentum transfer [1]. Besides improving the
performance of TMR magnetic multilayer NiFe
[2] and microwave resistance of high Tc cuprate
superconducting YBaCuO [3] films through
surface smoothing, its application for nano
secondary ion mass spectroscopy (n-SIMS) is
quite predictable in future. Related to
semiconductor technology, new cluster ion beam
source with high current and broad beam size is
highly demanded for high speed and large area
surface smoothing for CMP process. Also due to
its capability of delivering large kinetic energy
without much damage and negligible surface
charging, it emerged as new candidate over new
low energy ion implantation technique for
fabricating shallow junction in VLSI [4]. In view
of fundamental cluster ion-solid interaction, there
is still controversy over the mechanism of
formation of hillock or crater when the cluster ion
beam is irradiated on solid surface [5-7].
In this article, surface interaction with
cluster ion beam using Ar and CO2 cluster was
investigated and surface smoothing and etching
results are presented when CO2 cluster ion beam
was irradiated on Si, ITO, and Si3N4.
Figure 1 illustrated the schematic diagram
of 150 kV cluster ion accelerator consisting of
cluster
source,
acceleration,
and
main
experimental chamber. Ar and CO2 cluster was
generated by an adiabatic expansion through a
Laval nozzle with the diameter of 0.1 mm throat
at 4.5 and 5 bar, respectively. Expanded cluster
was by skimmer and then ionized by electron
impact. The cluster ion beam could be accelerated
up to 150 kV and irradiated on the sample surface.
Other experimental set up can be found in detail
elsewhere [8,9]. From the time-of-flight
measurement, the mean size of cluster distribution
was about 1000 for Ar and 750 molecules for CO2
at room temperature. At 25 kV and the very low
cluster ion flux, Ar and CO2 cluster were
impacted on Si(100) surface and the formation of
cluster ion induced structure was examined. For
the study of evolution of surface morphology,
CO2 cluster ion was irradiated on Si(100) surface,
in which native oxide layer was removed by
dilute HF solution, and the ion dose was varied in
the range of 1010 - 1013 cluster ions(CI)/cm2. In
addition, CO2 cluster ion beam was irradiated on
commercial ITO surfaces and Cr-masked Si3N4
thin film grown by plasma enhanced chemical
vapor deposition (PECVD) on Si
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
745
Expansion
chamber
Skimmer
Source
chamber
V
Reflectron
Ionizer
Process
chamber
Acceleration
tube
Scanner
Permanent
magnet
Nozzle
Einzel
Lens
MBP
TMP
TMP
TMP
Intensity (arb. units)
Fig. 1 Schematic diagram of a 150 kV cluster
the point of ballistic collision. As shown in Fig.
3(a), the shape of the crater is not symmetry and
the sputtered particles are not isotropically
redeposited. On the other hand, in case of CO2
cluster ion impact, outgrown hillocks are induced
with the a few ten’s nm diameter and a few height.
Among them, huge size of crater shaped structure
with size of about 1 µm and a few tens nm height
is rarely found as shown in Fig. 3(b).
Previously, observation of crater formation
from the ion bombardment of 125 keV Bi+ and
250 keV Bi+ was reported by Merkle and
Jager.[10] Beuhler and Friedman also presented
that holes with a diameter of 6 nm formed in a 9.5
nm thick Pt-C film as (H2O)50 cluster ions at 250
keV.[11,12] Recently, Yamada et al, reported that
an isolated Ar cluster ion impact induced not
hillocks but craters, and which were observed
using a scanning tunneling microscope (STM) on
Au/sapphire [7] and HOPG (highly oriented
pyrolitic graphite) surface [13].
Gspann reported that impacting on Si
surfaces at the acceleration voltage of 100 kV,
supersonic cluster ion beam induced hillocks with
nm height instead of surface craters through
atomic force microscope [5]. This unexpected
result was explained in terms of the rebounce of
elastic target materials for generated shock waves.
According to the calculation [14], a hemispherical
crater and two or three-layered shock waves were
once created after the impact, but the created
crater was immediately filled up with the
fluidized hot carbon material due to elastic
recovery before the arrival of reflected
shockwaves.
From the results in this experiment, the
difference of the cluster ion induced structure
results from the different chemical reactivity. In
case of Ar cluster ion impact, the cluster ion
Ar, p=4.5 bar
CO 2, p=5.0 bar
0
200
400
600
800
Flight time ( µ s)
Fig. 2 Time-of-flight spectra of Ar (P=4.5 bar) and
CO2 cluster (P=5 bar) at room temperature.
III. Results and discussion
1. Isolated Cluster Impact
induced Structure
In order to investigate the cluster ion-solid
interaction, isolated impact at the very low flux
was carried out over Si surfaces. It is very
interesting that two different kinds structures are
formed, crater or hillocks at the low dose of Ar
and CO2 cluster ion impact. In case of Ar cluster
ion impact, crater is formed as it is expected from
746
(a)
Fig. 4 AFM image of Si surface irradiated at the dose
of 5x1014 CI/cm2 at 25 kV.
resolution NEXAFS and TEM [15].
As the variation of ion dose, the Si surface
roughness (not shown here) is increased from 0.4
nm for bare Si wafer cleaned by dilute HF to 1.2
nm after cluster ion impact 5x1011 cluster ions
(CI)/cm2 by the increase of the number of induced
hillocks. And it was saturated 1.22 nm after ion
dose 5x1012 CI/cm2. Based upon these results of
isolated cluster ion impact, it was already
suggested [16] that cluster ion-solid interaction
evolves with subsequent three-step processes. It
was described into surface embossment, surface
sputtering and smoothing, and surface etching.
Firstly surface embossment happens at the
beginning stage of low ion dose by the formation
of protruding hillocks. And then secondly, surface
sputtering begins over critical ion dose at which
the area of induced hillock is equal to unirradiated
area. Simultaneously, sputtered atoms from the
hillocks migrate and fill the valleys, called surface
smoothing. Lastly, modified surface region by
cluster ion impact will be easily removed into the
vacuum and in consequence so-called surface
etching occurs.
Figure 4 shows the Si surface irradiated at
the dose of 5x1014 CI/cm2 at 25 kV and was
etched as deep as 6 nm and the surface roughness
becomes 0.7 nm. Compared to unirradiated area,
it shows very flat surfaces.
(b)
(c)
Fig. 3 AFM images of isolated (a), (b) Ar cluster
and (c) CO2 ion impacted Si surface.
collides with the substrate and Si atoms are
sputtered and it coincides with the model of
macroscopic ballistic collision. However, when
CO2 cluster ion is impinged into the Si surface, a
chemical reaction occurs and some silicon oxide
species are easily composed in locally high
pressure and very high temperature environment.
This Si-O bonding is larger than Si-Si bonding
and therefore the compound should be outgrown
for the existence. Later a chemical analysis of the
hillocks will be ready for the identification of
chemical composition and structure through high
2. Cluster Ion Irradiation On
ITO and Si3N4
Figure 5 illustrates AFM images for bare and
irradiated ITO surfaces by CO2 monomer and
cluster ions. On the scanned ITO area in Fig. 5(a)
747
σrms=0.16 nm
σrms=1.1 nm
(a)
(b)
Fig. 6 AFM image of cluster ion irriadiated Si3N4
surfaces
from the ITO surfaces. After the cluster ion
bombardment at the dosage of 5x1014/cm2, the
irradiated surface becomes smoother σrms =0.94
nm than the bare ITO surface and the ITO surface
irradiated by monomer ions as shown in Fig. 5(c).
In order to extend cluster ion beam in MEMS
technology, one of the widely used thin film of
strain-released Si3N4 was irradiated by cluster ion
beam. Si3N4 thin film was deposited on Si by
PECVD and Cr mask was deposited for the
fabrication of some MEMS structure. As shown in
Fig. 6, irradiated area was etched and the surface
roughness became atomically flat as much as 0.16
nm, which is exceptionally smooth compared to 1
nm of as-deposited Si3N4.
(c)
Fig. 5 AFM images of (a) bare ITO, (b) monomer ion
irradiated ITO, and (c) cluster ion irradiated ITO
about 10 hillocks with 15 nm in height and a few
hundreds nm in width are observable. The root
mean square roughness σrms of the bare ITO
surface is 1.31 nm. Fig. 5(b) illustrates the ITO
surface irradiated by the monomer ions formed at
the inlet pressure of 1 bar and with the dosage of
1.5x1014/cm2. In this case, the density of the
hillocks on the surface is not much changed.
However, the shape of the hillocks turns into a
spike-like one from a stalagmite-like one.
Moreover, the surface roughness slightly
increases up to σrms =1.6 nm.
On the other hand, when cluster ions are
irradiated, some different features are observed
IV. Conclusions
Ar and CO2 cluster ion beam was generated by an
adiabatic expansion through a Laval nozzle at
room temperature and irradiated onto Si, oxide
and nitride surface at 25-50 kV. In case of an
isolated impact, two different kinds of induced
structures were observed, hillocks and crater.
When large CO2 cluster ion was impinged into Si,
crown like denting crater was found at top of the
748
and I. Yamada, Chem. Phys. 54, 76 (1998).
14. Y. Yamaguchi and J. Gspann, Eur. Phys.
J.D.16, 103 (2001).
15. L.P. Allen, Z. Insepov, D.B. Fenner, C.
Santeufemio, W. Brooks, K.S. Jones, and I.
Yamada, J. Appl. Phys. 92, 3671 (2002).
16. J.H. Song, D.K. Choi, and W.K. Choi, Nucl.
Instru. Meth. B, 196, 268 (2002).
hillock. This difference feature of the cluster ion
impact is expected from the difference in both
size and chemical reactivity. After prolonged
isolated cluster ion impact on clean Si surface, it
is observed that cluster ion-solid interaction is
phenomenologically evolved in three steps:
surface embossment, surface sputtering and
smoothing, and surface etching. When cluster ion
beam was irradiated onto ITO surface where
hillocks were preexisted, it shows different
sputtering phenomena from monomer ion
irradiation. From the cluster ion irradiation on Si,
ITO, and Si3N4, all irradiated surfaces became
very smooth and it was proved that cluster ion
beam is very effective in surface smoothing and
nano etching.
Acknowledgement
This work is partially supported by the Tera-level
Nano Device (TND) National Program.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
I. Yamada, Eur. Phys. J.D9, 55 (1999).
J.A. Greeger, D.B. Fenner, J. Hautala, L.P.
Allen, V. DiFilippo, N. Toyoda, I. Yamada, J.
Matsuo, E. Minami, H. Katsumata, Surf.
Coat. Tech. 133/134, 274 (2000).
D. Fathy, O.W. Holland, R. Liu, J. Wosik,
and W.K. Chu, Mater. Lett. 44, 248 (2000)
I. Yamada, J. Matsuo, and N. Toyoda, 13th
International Conference on Ion Beam
Modification of Materials (Kobe, Japan,
2002)
Gspann, J., Nucl. Instru. Meth. B112, 86
(1996).
J.H. Song and W.K. Choi, Nucl. Instru.
Meth. B190, 792 (2002).
D. Takeukuchi, K. Fukushima, J. Matsuo, I.
Yamada, Nucl. Instru. Meth. B 153, 264
(1999).
J.H. Song, S.N. Kwon, D.K. Choi, and W.K.
Choi, Nucl. Instru. Meth. B179, 568 (2001).
J. H. Song, D.K. Choi, and W.K. Choi,
Current Appl. Phys. 1, 521 (2001)
K.L. Merkle and W. Jager, Philo. Mag. A44,
741 (1981)
R. Beuhler and L. Friedman, Chem. Rev. 86,
521 (1986)
M.W. Matthew, R. Beuhler, M. Ledbetter,
and L. Friedman, J. Phys. Chem. 90, 3152
(1986)
D. Takeuchi, T. Seki, T. Aoki, J. Matsuo,
749