Surface Smoothing of Compound Semiconductor Substrates
with Gas Cluster Ion Beams
S. Houzumi, N.Toyoda, I.Yamada
Laboratory of Advanced Science and Technology for Industry, Himeji Institute of Technology, 3-1-2 Kouto,
Kamigori, Ako-gun, Hyogo, 678-1205 Japan
Abstract. Surface smoothing by gas cluster ion beams (GCIB) was studied for compound semiconductor
such as GaN and SiC. Average cluster size of Ar cluster ions was 2000atoms/cluster measured by time of flight (TOF).
Since the total acceleration energy was 20keV, the energy per atom was 10eV/atom. This low-energy characteristic of
gas cluster ion beams is desirable for compound semiconductors. GCIB irradiation was employed to remove the
scratches of the mechanically polished SiC surface. After irradiation at acceleration energy of 15keV, the
scratches was completry removed. The GaN film with initial average roughness of 4nm was also smoothed to that of
1.4nm by Ar cluster ion beams. Furthermore SiC substrates were irradiated with SF6 cluster ions. The sputtering yield of
SiC with SF6 cluster ions was enhanced almost 3 times than that with Ar cluster ions.
sputtering phenomena is developed by lateral
sputtering effects [3]. This effect involves high
sputtering yields [4] and strong surface smoothing
effects. For example, surface smoothing with Ar
cluster ion beams have been successfully demonstrated
for CVD diamond films [5]. Also, dense energy
deposition near surface is occurred in the case of gas
cluster ion bombardment. When reactive gas cluster
ions are employed, enhancements of chemical
reactions are remarkable.
In this study, Ar and SF6 cluster ion beams were
irradiated to GaN and SiC surface and the surface
smoothing and etching effects were studied.
INTRODUCTION
Compound semiconductors such as GaN and SiC
are used for power device, opt-electronic device, light
emitting diode (LED) [1] and high-speed
semiconductors. The developments of these substrates
are also enthusiastically carried out, however, grown
surface is rough and it has to be polished by
mechanical polishing. But these mechanical polishing
easily induces scratches and subsurface damages in
compound semiconductor surfaces which are not easy
to recover. So, new surface smoothing method is
withed that it changes into mechanical polishing.
Gas Cluster Ion Beam (GCIB) is one of a candidate
to realize surface smoothing without these scratches.
Surface smoothing with gas cluster ion beams, which
have been developed at Kyoto university. Cluster is
consist of a few thousands atoms, and the energy per
constituent atom of cluster is total energy divided by
its cluster size. Therefore very low-energy (several
eV/atom) ion beam [2] can be easily realized by
employing large cluster ions. As compound
semiconductor is very sensitive to damages by
energetic ions, it is desirable to employ these lowenergy ion beams.
When substrates surface is irradiated with GCIB,
EXPERIMENT
A schematic of the gas cluster ion beams apparatus
for surface smoothing is shown in figure.1.
This system has four vacuum chambers; source,
differential pumping, ionizing and target chambers.
Source chamber has a nozzle and neutral clusters are
formed by supersonic expansions of high-pressure Ar
and SF6 gas. To improve the vacuum pressure in
ionizing chamber, differential pumping chamber was
located between source and ionizing chamber. In
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
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ionizing chamber, there are ionizer, accelerator and
beam scanner. Cluster ion current was measured with a
Faraday cup in the target chamber.
He gas mixture was 95% in SF6, SF6 neutral beam
intensity shows maximum intensity.
Mass [amu]
50000
100000
150000
200000
2atm
Intensity [a.u]
4atm
FIGURE1. A schematic of the gas cluster ion beams
apparatus for surface smoothing
To measure the cluster size distribution of ion beam
used for surface smoothing in this system, simple time
of flight (TOF) measurements were performed. By
using the scanner installed in the ionizing chamber, the
actual size of the cluster ions under the irradiation
condition can be measured. Pulse bias (~3kV) with
10µsec duration time was supplied on the beam
scanner and ion-bunch was formed with a 1mm slit
located at 5cm downstream of the scanner. Then flight
times of these ions were measured from ion current at
a Faraday cup located at 50cm from the scanner and
stored in a digital oscilloscope. Cluster size
distribution was obtained from an average of 256
repetition of time of flight measurements.
Figure.2 shows cluster size distribution of Ar cluster
ion beam with various gas inlet pressures (1-6atom).
The acceleration energy was 20keV. The ionization
energy of electron and ionization current were 100eV
and 60mA, respectively. As the TOF system used here
was very simple, mass resolution of TOF was quite
poor. However, rough cluster size distribution can be
extracted from these spectra and it was enough to
ascertain the existence of cluster ions.
From figure 2, the peak position and size
distribution shifted to larger sizes with increasing the
inlet gas pressure. When source gas pressure was 6
atm, the TOF mass spectrum was a maximum at
around a size of 3000 atoms/cluster (atomic mass
120,000). As we found that the increase of ionization
current caused increase of fraction of monomer ions,
the ionization current was fixed at 60mA throughout
this study.
Ar gas is easy to form cluster beams at room
temperature, since intermolecular force of Ar is strong.
On the other hand, SF6 is difficult to form intense
cluster beams from pure SF6 gas because SF6 molecule
has many vibration modes. To produce strong SF6
cluster beams, Helium gas was mixed in SF6 gas. The
role of He is to remove heat during the expansion and
He itself is not incorporated in SF6 cluster beam. When
6atm
1atm
1000
2000
3000
4000
5000
Cluster size [atoms/cluster]
FIGURE2. TOF spectra of Ar cluster size at different
source gas pressures. Cluster size ranges from 400 to 5500
atoms/cluster.
GaN and SiC were used as compound
semiconductor target. GaN was deposited by hydride
vapor phase epitaxy method (HVPE) [6]. Ar cluster
ion beams irradiated to GaN films at normal incidence
with acceleration energy of 20 keV and ion dose of
2×1016ions/cm2. After irradiation, the surface
morphology of the GaN was observed with atomic
force microscope (AFM, JSPM-4200, JEOL). The
scanning area was 1µm×1µm. Also 3C-SiC wafers
were irradiated with Ar and SF6 cluster ion beams at
normal incidence. The acceleration energy and ion
dose were 15keV and 7×1016 ions/cm2, respectively.
After irradiation, surface morphology of the SiC was
also observed with AFM (Scanning area was
8µm×8µm).
When a reactive gas such as SF6 is used as a cluster
source gas, enhancement of sputtering yields due to
chemical reactions with target is expected. The
sputtering yield was obtained from an etching depth
measured with a contact depth profiler. The irradiated
area was covered with a screen as a mask.
To confirm the ion dose dependence of the etching
depth and average roughness of SiC with Ar and SF6
cluster ion beams, SiC wafers were irradiated with Ar
and SF6 cluster ion beams. The acceleration energy
was 20keV and the ion dose ranged from 5×1014
ions/cm2 to 2×1016 ions/cm2.
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acceleration energy was 20keV at normal incidence.
The ion dose ranged from 5×1014 to 2×1016 ions/cm2
RESULTS AND DISCUSSIONS
Figure.3 shows AFM images of GaN surface before
and after irradiation with Ar cluster ion beams. Before
irradiation, GaN surface was quite rough due to its
columnar growth with average roughness of 4.0nm.
After irradiation of 20keV Ar cluster ion beams, the
average roughness was reduced to 1.4nm. It was
almost one third of the initial value. Also no large
grains were observed after irradiation.
(a) Before irradiation, Ra=7.4nm
(a) Before irradiation, Ra=4nm
(b) After irradiation, Ra=0.9nm
FIGURE4. AFM images of SiC surface before and after
irradiation with Ar cluster ion beams. The acceleration
energy was 15keV and the ion dose was 7×1016 ions/cm2
600 Total energy:20keV
SiC
(b) After irradiation, Ra=1.4nm
FIGURE3. AFM images of GaN surface before and after
irradiation with Ar cluster ion beams. The acceleration
energy was 20keV and the ion dose was 2×1016ions/cm2
SF6 cluster
Etching Depth [nm]
500
3C-SiC wafers were also irradiated with Ar cluster
ion beams in order to remove the scratches induced by
mechanical polishing. The initial surface of SiC is
shown in Figure.4 (a). Before irradiation, there were
many scratches on the surface. The average roughness
of the SiC substrate was 7.4nm with scanning area of
8µm square. Figure.4 (b) shows the AFM images of
SiC wafer after Ar cluster ion beams irradiation. When
the SiC were irradiated with Ar cluster ion beam at the
energy of 15 keV, the average roughness was reduced
to 0.9nm and there was no scratches on its surface. As
it is very difficult to remove these scratches with
mechanical polishing, this scratch removal is very
promising for surface finishing of compound
semiconductor wafers.
Figure 5 shows the ion dose dependence of etching
depth of SiC with Ar and SF6 cluster ion beams. The
400
300
200
Ar cluster
100
0
0.0
0.5
1.0
1.5
16
2.0
2
Dose [ ×10 ions/cm ]
FIGURE5. Ion dose dependence of the etching depth the
SiC with Ar and SF6 cluster ion beams, the acceleration
energy was 20keV, the ion dose ranged from 5×1014
ions/cm2 to 2×1016 ions/cm2
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CONCLUSIONS
20
Average roughness [nm]
Total energy:20keV
SiC
In this paper, surface smoothing effects of Ar and
SF6 cluster ion beams for compound materials such as
GaN and 3C-SiC were studied. In both cases, the
surface roughness monotonically decreased with
increasing the ion dose by 20keV Ar cluster ion beams.
Scratches on SiC wafer induced by mechanical
polishing were completely removed after irradiation.
The sputtering yield of SiC was enhanced with SF6
cluster ions and the etching rate was almost three times
higher than that of Ar cluster ions. However, it
requires thicker SiC layers to realize the surface with
the same roughness as that processed with Ar cluster
ions. Ar and SF6 cluster ion offers various smoothing
and etching methods and these are promising to apply
for surface polishing of compound semiconductor
wafers.
15
SF6 cluster
10
5
0
Ar cluster
0
100
200
300
400
500
Etching Depth[nm]
FIGURE6. Etching depth dependence of average roughness
of SiC surface with Ar and SF6 cluster ions, the acceleration
energy was 20keV, the ion dose ranged from 2×1015
ions/cm2 to 5×1016 ions/cm2
REFERENCES
and the average cluster size was 2000atoms/ion. From
Figure 5, the etching depth increased monotonically
with increasing ion doses. When irradiation dose of Ar
cluster ion was 2×1016 ions/cm2, etching depth was
about 190nm. On the other hand, when SF6 cluster ion
beams were irradiated with the same ion dose,
sputtering depth of SiC was about 620nm. It was
almost three times higher than that of Ar cluster ion
beams. Since SF6 is a reactive gas, enhancement of
sputtering yields due to chemical reactions with SiC
was significant. These enhancements of sputtering
yields with SF6 were also observed in Si and W.
Figure 6 shows etching depth dependence of
average roughness of SiC surface after etching
experiment with Ar and SF6 cluster ions shown in
figure 5. In the case of Ar irradiation, the average
roughness of SiC decreased monotonically with
increasing the etching depth from the initial value of
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times thicker SiC than Ar at the same ion dose. It
means that SF6 cluster ion requires thicker SiC layers
to smooth the surface, and the smoothing effect is
weaker than that of Ar cluster ion beams. From these
results,
surface
smoothing
of
compound
semiconductor was demonstrated with Ar and SF6
cluster ion beams, and it is candidate for surface
finishing process of these substrates.
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