PrBa2 (Cu0.8Al0.2) 3O7 Thin Film Surface Smoothing by ArCluster Ion Beam Bombardment
X. M. Wang, M. J. Jin, Q. Y. Chen, J. R. Liu, and W. K. Chu
Texas Center for Superconductivity and Advanced Materials and Department of Physics, University of Houston,
Houston, Texas 77204
Abstract. Al doped PrBa2Cu 3O7 epitaxial thin film is an excellent insulating material for oxide superconducting junction
application. The surface of such films grown at high temperature is usually very rough and requires smoothing during device
fabrication. A gas cluster ion beam is a good tool for smoothing the surface down to the sub-nanometer level. In this paper,
PrBa2(Cu0.8Al0.2)3O7 thin films of different thickness grown by Pulsed Laser Deposition (PLD) were irradiated with a 20 keV Arcluster ion beam. The average size of the Ar-clusters was about 3000 atoms per cluster. A film thickness varying from 48nm to
920 nm with corresponding initial roughness of 2 nm to 10 nm was studied. The roughness was gradually reduced with increasing
cluster ion dosage, reaching an ultimate of 0.7 nm. For a sample of average sputtering yield, surface damage and the annealing
effect after smoothing were also studied.
transfer and is relatively easy to operate with good
reproducibility. Ablation was carried out in a vacuum
chamber with an O2 environment (partial pressure:
190mTorr) using a KrF excimer laser with a
wavelength of 248nm operating at 5 HZ repetition rate
and 250-300 mJ laser beam energy. The energy density
of the laser at the target was 1-1.5 J/cm2 and the basic
pressure of the deposition chamber was 1×10-5 Torr. A
sintered
two-inch
diameter
stoichiometric
PrBa2(Cu0.8Al0.2)3O7 target was mounted on a ratable
holder, while the films were deposited at 750 oC and
annealed at 450 oC in an O2 environment for 1hour. The
(00l) oriented LaAlO3 substrates were cleaned in a
methanol and acetone ultrasonic bath for 15minutes,
then immediately placed
I. INTRODUCTION
Cluster ion beam technology is anticipate to be an
advanced tool for surface modification in order to
process the nanometer size surface structure. Several
papers that have reported cluster ion irradiation had
unique characteristics surface modification such as very
shallow implantation [1], high yield sputtering [2],
surface smoothing [3-4], surface cleaning [5] and thin
film formation [6-7]. A molecular dynamic simulation
was also done to understand the fundamental effect of
various cluster ions on a solid surface [8-10]. Surface
smoothing with cluster ion beam is one of the most
promising applications. As gas cluster ion beam
technique becomes better known, it has found more
applications in the industry. In recent years, we have
been dedicated to the surface modification of high
temperature superconducting thin films. Since smooth
thin film could not be directly obtained by Pulsed Laser
Deposition (PLD) because of the extensive particle
formation on the film surface, we used Ar cluster ion
beam processing at low energy to smooth out the rough
surface. Here, we present the results on Al-doped
PrBa2Cu3O7 thin film surface smoothed at a high
sputtering rate. An effective smoothing with a very thin
damage layer has been obtained.
Table 1. The intensity of Ar cluster ion current from gas
cluster ion beam system (nA). The Size of the cluster ion is
estimated to be 3000 Ar atoms.
II. EXPERIMENTAL
A. Thin Film Deposition
Pulsed Laser Deposition (PLD) is a very attractive
method for fabricating multi-component oxide thin film
and multi-layer film because it allows stoichometric
Extraction Energy (keV)
Ion Current (nA)
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15
450
20
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30
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CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
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in a vacuum. The position of the substrate was located
at the end of the visible plasma. Deposition of the films
and their electrical properties were described elsewhere
[11].
B. Ar Cluster Ion Beam System
Formation of cluster ions relies on the throttling of a
gas stream. In short, a neutral argon cluster beam is
formed by supersonic expansion of gas under high
pressure through a nozzle. A skimmer with a small
aperture produces a collimated beam into a highvacuum chamber. Neutral argon clusters are ionized by
bombardment of electrons coming from a W filament.
Ionized clusters containing 3000 atoms per cluster are
mass selected by a quadrapole and accelerated at an
energy ranging from 10keV up to 30 keV. The sample
is mounted on a target holder behind the X -Y beam
electrostatic scanner. Descriptions of such a gas cluster
beam system can be found in literature [12]
(a)
C. Measurement of Thickness and Surface
Damage
The crystallinity and homogeneity of composition and
thickness of the deposited thin films were analyzed by
Rutherford
Backscattering
Spectroscopy
(RBS)/Channeling using 2MeV He ions with the
detector positioned 70mm away from the target at an
angle of 165° relative to the beam direction.
RBS/channeling is also applied to study the Ar cluster
ions induced damage on thin film.
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(b)
film thickness: 600nm
film thickness: 48nm
Surface Rougness (nm)
8
Figure 2. 10x10 µm2 AFM images of Al-doped PBCO thin
Film with thickness of 200nm (a) before irradiation with
surface Ra=4.2nm and (b) after 2.5x1015 ions/cm2 Ar cluster
ions irradiation, Ra=1.1nm
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III. RESULT AND DISCUSSION
4
For an ion cluster with 3000 atoms/ion at 10-30 keV,
the equivalent energy per constituent atom is 3.33 10eV. A cluster ion impacts the surface at very low
equivalent energy but with extremely high particle
density. Table 1 shows the intensity of Ar cluster ion
beam extraction after adjusting the operational
parameter. Although the ion current is not very high, it
is sufficient for various fundamental studies. The
Milliampere level is now available from commercial
2
0
0
2
4
6
8
15
10
12
14
2
Dosage (x10 clusters/cm )
Figure 1. Shows the irradiation dosage dependence of
surface roughness of the Al-doped PBCO.
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equipment [13]. Al-doped PBCO thin films of 48, 200,
600 and 920 nm thickness were deposited on (00l)
LaAlO3 substrates by PLD. All the films were shiny as
observed by the naked eye. There are only (00l) peaks
in X-ray 2θ scans and rocking curves of the (005) peaks
are in the range of 0.3~0.5o, indicating reasonable caxis orientation of the films. The surface roughness of
the thin films of different thickness varies from 2 to 10
nm. The surface roughness of samples was observed by
Atomic Force Microscopy (AFM). Here, we selected
some of them to describe the roughness change before
and after cluster ion bombardment. The original
roughness for the thicker film (thickness: 600nm) in a
10 µm × 10 µm area was 9nm, characteristic of many
particulates on the surface. It was irradiated by Ar
cluster ions with 20 keV energy for doses varying from
1.25x1015 to 1.25x1016 ions/cm2 in normal incident.
After the Ar cluster beam bombardment, the surface
roughness decreased significantly with increasing
dosage, reaching a saturated value (~0.7nm) at 7.5 ×
1015 ions/cm2. For thinner films with original roughness
of 2.4nm, such saturation is reached at ~ 2.5 × 1015
ions/cm2. Fig.1 shows the change in surface roughness
with increasing dosage. We also observed that the
smoothing rate of surface roughness is much lower for
thinner films than that for thicker films. To illustrate the
Al-doped PBCO thin film morphology change before
and after ion cluster beam bombardment, two typical
AFM images of surface were given in Fig 2. For
200nm films, the scanned area is 10µm x 10µm, and the
full scale in Z-axis is 50nm in height. It is very clear
that there are islands or mesas on the non-irradiated
surface and the surface smoothness of the film
improved dramatically after cluster ion beam
bombardment.
The high sputtering yield played an important role in
the surface smoothing process. The removed thickness
was measured by RBS/Channeling. For instance, for
35000
25000
Counts
Xmin=17.97%
5000
0
320
Removed thickness(nm)
100
80
60
40
20
0
7.0
360
380
400
420
440
the 180nm thin film, 38nm can be removed after cluster
ion bombardment at 5x1015 ions/cm2 and energy of
20keV, while 98nm is removed at 6.25x1015 ions/cm2
and 153 nm at 7.5x1015 ions/cm2. Fig 3. shows the
sputtering rate with respect to dosages. From these
experimental results, the removed thickness of the thin
film is proportional to cluster ion dosage. Since the
thickness of some thin films is only a few nanometer
thick, it is difficult to characterize using a routine
method. With this trend line, we can determine the
remainder thickness after sputtering for very thin film.
The slope of Fig 3 gives the sputtering rate to be 26.2
unit cell of PBCO per Ar cluster.
One undesirable effect during the cluster ion beam
bombardment is the formation of a damaged layer.
Atoms are displaced near the surface. A damaged
region remains on the surface. The samples were
measured by RBS/channeling technique at 2 MeV He1+
and a 165° scattering angle. As shown in Fig 4,
thickness of the damaged layer increased with cluster
ion dosage. From a previous report, the damaged
YBCO thin film can be recovered by annealing in
oxygen ambience [3] or preferably etched away.
Unfortunately, it is not a desirable process for Al doped
PBCO in our experiment. After annealing at 750°C
under oxygen pressure and for another 450 oC for
1hour, which is the same as that used in the film
deposition process, the value of χmin decreased just a
little, from 18.0% to 17.2%. We believe that accurate
controls of annealing temperature and time are the most
important factors in recovering irradiation-induced
damage.
120
15
340
Figure 4. Aligned RBS/Channeling spectra of Al doped
PBCO thin film bombarded by Ar cluster ions with different
dosage at 20 keV. Surface damage and dechanneling can be
seen from the channeled spectra.
140
6.5
Xmin=12.9%
Xmin=10.9%
Channel
160
6.0
15000
10000
180
5.5
Random
20000
200
5.0
Before Irradiation
15
2
20keV,5x10 Ar clusters/cm
15
20keV,6.5x10 Ar clusters/cm
Random
30000
7.5
2
A to m ic D o sa g e (x 1 0 /c m )
Figure 3. The removed thickness of sputtering as a function
of the incidence ions dosage for 20keV Ar 3000 atoms/cluster
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REFERENCES
IV. CONCLUSION
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Modification of Materials, in press
The PrBa2 (Cu0.8Al0.2)3O7 epitaxial thin films grown
by PLD were smoothed using Ar cluster ion beam
bombardment. The original surface was rougher for
thicker films. The surface roughness decreased with
increasing the ion dose down to 0.7 nm, then saturated
during irradiation. The high sputtering yield and the
dramatic smoothing effect of the PrBa2(Cu0.8Al0.2)3O7
thin film indicate that this process is a promising
method for the HTS multilayer junction fabrications.
Although, one needs to carefully consider the damaged
layer formation by Ar cluster ion bombardment.
ACKNOWLEDGMENTS
This project was supported by The State of Texas
through the Texas Center for Superconductivity and
Advanced Materials.
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