Angular and energy distributions of secondary ions in the sputtering of gold by swift Au(n) clusters.
S. Bouneau, S. Della-Negra, D. Jacquet, Y. Le Beyec, M. Pautrat
Institut de Physique Nucleaire Orsay, IN2P3/CNRS, F-91406 Orsay, France
Angular and energy distributions of negative secondary ions Aun (n = 2 - 7) emitted from gold targets by gold cluster
projectiles Au4 and Au9 at 200 keV per atom have been measured. A 256-anodes channel plate based detector for
simultaneous detection in time of flight measurements was used. It is shown that angular distributions are
rotationally symmetric with respect to the surface normal .They depend on the ejection energy. The energy spectra of
the different emitted ions have similar shapes with almost the same mean energy. Energy spectra are satisfactorily
reproduced by a spike-like model.
transfer and emission processes. Others MD
simulations [5] were also developed but could not
be compared for the moment to the set of
experimental data with large and swift cluster
projectiles.
We present here experimental results of a first
series of measurements on energy and angular
distributions of secondary ions (SI), Au-, Au2- ,
Au3-, Au5- and Au7-, emitted from the same type of
gold targets bombarded by Au4 and Au9 cluster
projectiles at ∼200 keV/atom where the total
sputtering yield is close to maximum. It is shown in
these multi-parameter event by event experiments
that angles and energies of emission are correlated
since angular distributions depend on kinetic
energies. Although secondary atomic ions and
cluster ions may not be fully representative of the
total sputtering processes the measurements with
well identified SI may emphasized the importance
of the emitted secondary particle characteristics.
Very large secondary ion and neutral sputtering
yields have been recently measured in the
bombardment of solid targets by fast cluster
projectiles as Aun (n = 1-13) at energy per atom
between 20 keV and several MeV. A broad range
of projectile energy has thus been investigated in
the past years [1, 2]. As many as 2.104 Ag atoms
are for example ejected on the average upon impact
of a single Au13 cluster at 120 keV/atom on a silver
target. The emission yield is very high and its
variation with energy shows that the highest yield
value occurs at a bombarding energy much lower
than the maximum nuclear stopping energy.
Theoretical predictions by any sputtering models
and/or molecular dynamic simulation are still
uncertain at cluster projectile energy of several tens
of keV/atom and higher. Additional measurements
as angular distributions of emitted particles as well
as kinetic energy distributions could provide
valuable information on the nature of the
mechanisms involved in the emission processes.
Results on angular distributions at cluster energies
of 10-30 keV/atom were already obtained with
polycrystalline gold targets and Au, Au2, Au3
projectiles [3]. The sputtered distributions were
found more isotropic for cluster bombardment than
for atomic bombardment and this qualitatively
indicates that a thermal spike mechanism could be
involved in the total sputtering induced by clusters
(emission energy and size of emitted species were
not considered in this case). An interesting
molecular dynamic study [4] was also performed
with Aun at 16 keV/atom striking gold targets
which demonstrates the role of various parameters
as the nature of the emitted species (monomer or
cluster) and the temporal evolution of the energy
I.
Experimental
Beams of gold clusters were accelerated by the
Orsay tandem accelerator which is equipped with a
cluster liquid metal ion source in the high voltage
terminal [6, 7]. Gold targets (thickness of about
1000 nm) were prepared by vapor deposition on
thick stainless steel foils, and bombarded at an
angle of 45° with respect to the normal to the
surface. The beam size was defined by horizontal
and vertical slits which were mechanically adjusted
to an aperture of 300 x 300 microns. The rate was
about 100 projectiles/s. To measure the energy and
angular distributions of secondary ions a linear
time of flight technique -using two acceleration
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
767
2.54 mm and there are 256 independent electronic
channels which provide 256 time of flight mass
spectra with the same start signal issued from the
impact of the projectile on the gold target (electron
signal). In this experiment all negative ions emitted
from the gold surface can be in principle identified.
One has first focused on the emission of the
negative metal gold cluster ions Au-, Au2-, Au3-,
Au5-, Au7The detector plane is parallel to the target plane.
The initial radial velocity of a secondary ion
ejected from the surface is obtained by the
simultaneous measurement of its position of impact
on to the detector surface and its flight time t. x and
y being the coordinates of the impact with respect
to the detector center (defined as the intersection of
the normal to the target center with the detector
grids in front of the target- and a multi-impact
position sensitive stop detector were used. Figure 1
shows the geometrical arrangement of target and
stop detector. The grids are not represented in the
schematic drawing. The precise determination of SI
time of flights gives access to their initial axial
velocity Vax. The effect of the initial axial velocity
on time of flight peak shapes has been enlarged in
TOF measurements by applying a low acceleration
voltage (1 keV) between the target and the first
grid. The corresponding mass lines in TOF spectra
are thus considerably broadened, and axial and
radial energies can therefore be precisely
determined. The electron line always present in the
negative SI mass spectrum was used to determine
the zero origin of the time scale. Based on the
accurate distance measurements and voltage values
a computer program was used to calculate ion mass
peak origins (zero axial energy) in the TOF spectra.
Uncertainties of time zero were estimated to be
about 1ns. The second grid voltage was set at 9 kV
in order to provide efficient detection and
collection of SI. In this respect the distance
between the target plane and the detector combined
with the acceleration voltages are critical
parameters for determining the detector angular
acceptance and the maximum radial energy values
that can be measured.
x 2+ y
. From the axial
t
and radial velocity measurement of a given mass
one obtains the angle of emission defined as
tan(θ) = Vr and the total energy Et = Er + Eax.
Vax
The pixel size of the multi-impact detector is not
small enough to determine directly and accurately
the radial energy. The experimental measured
quantities for any ion impact are the time t and the
pixel coordinates position Xp and Yp. To built the
total axial and radial energy distributions one must
take into account the encoding time bin, ∆t = 0.5
ns, and the finite size of the pixel (∆X, ∆Y).
Assuming an uniform distribution of impact within
the pixel and time bin, the analyse procedure
consists in choosing randomly for each SI impact,
values of t, x, y in the intervals [t, t + ∆t], [Xp, Xp +
∆X], [Yp, Yp + ∆Y] [10].
2
surface), Vr is written as
Y’
Y
X
Vr
Vt
X’
Vy
φ Vx
θ
0’
Vax
0
Z
II.
ET
RG
TA NE
A
PL
Results and Discussion
OR
ΒΕΑΜ
CT
TE
DE ANE
PL
A. Radial energy distribution
Experimentally the cluster beam hits the target
center in an horizontal plane perpendicular to the
target, at an angle of 45° with respect to the normal
to the target. After impact the secondary cluster
ions are emitted in all directions and detected by
the multi-anode detector. A simple way to examine
if there is or not a favoured direction of emission is
Figure 1. Schematic drawing of the experimental
arrangement.
The detector used in this work is a 256 anodes
micro channel plate detector which is described
elsewhere [8, 9]. The size of one pixel is 2.54mm x
768
Since the measurements with the multi-anode
detector are made event by event the data can be
processed off line and several types of correlations
can be analysed. For example fig. 3 shows the 3D
representation of the distributions Et, θ, dN(θ, Et)
for Au−2 emitted by 200 keV/atom Au9. The
different colours represent the number of counts.
The geometrical acceptance of the detector in the
present experimental condition are shown in the
plane (E, θ) by the counting limits. It can be seen
that only angular emission between 10 and 70
degrees is fully detected for total kinetic energies
between a few eV and about 20 eV. The maximum
number of counts is around 40 degrees.
dN/dEr (a. u.)
to measure the radial energy distributions dN
dEr
observed in the left and right part of the detector
surface, defined by each side of the O’Y’ direction
in fig. 1. The radial energies are declared positive
in the O’X’ direction and negative in the other
direction. For Au−2 cluster ions sputtered by Au9 at
200 keV/atom, fig. 2 presents three distributions of
dN relative to three total energy windows. The
dEr
distributions in both parts of the detector look
exactly the same, and therefore it can be concluded
that there is no favoured direction of emission. In
addition, the distributions in the upper and the
lower part of the detector surface (with respect to
the O’X’ direction) were also found similar. These
data indicate that there is a rotational symmetry in
the emission normal to the target surface.
Furthermore the incident direction of the beam
does not play a role in the SI angular distribution of
emission. The same behaviour was observed for
larger emitted clusters and for Au4 projectile as
well. In other words there is no memory of the
primary beam direction and this is in favour of
models where the energy flux in the solid is close
to isotropy.
0.8
0.6
0.4
0.2
0
0.3
Eradial ≤ 16 eV
200
50
N
Total emission energy in eV
60
100
0
100
40
0
Em
iss 50
ion
an
20
gle 0
Θ
30
rgy
40
on
si
mis
ene
E
20
10
−
Au2 : 3 eV < Et < 5 eV
0
0
20
40
60
emission angle Θ in o
80
100
Figure 3. 3D distribution of total energy of Au2ions as a function of the emission angle θ.
−
Au2 : 5 eV < Etot < 10 eV
0.2
0.1
As the secondary emission is symmetric with
respect to the normal to the target, one can extract
from these data the distribution :
1 .
dN = dN x
dΩ
dθ
2πsinθ
Fig. 4 shows two dN distributions of Au−2 ion
dΩ
corresponding to the energy windows 3eV<Et<5eV
and 11eV<Et<15eV. It is usual to fit the
experimental sputtering angular distribution
through a (cosθ)n dependence over the whole range
of kinetic energies. The n exponent is predicted to
be between 1 and 2 in the linear collision cascade
0
−
Au2 : 10 eV < Et < 15 eV
0.15
0.1
0.05
0
−20−15−10 −5 0 5 10 15 20
Er (eV)
Figure 2. Radial energy distribution of Au2- ions
corresponding to three total energy windows with
Au9 projectile.
769
observed for 20° < θ < 40° – and cluster projectiles
of Au9 at 200 keV/atom. In the linear collision
cascade models the high energy tails of the energy
E
where
spectra falls off approximately as
(E+ Ub)n
E is the total energy and Ub the surface binding
energy and n between 2 and 3 according to mass
and regime of incident energies. In order to
estimate the shape of such a distribution in the
present case a tentative fit of the Au−2 spectrum by
this expression (with n = 2.4) is represented by the
line in fig. 5(b). Above 20 eV there is an important
disagreement between the calculated curve and the
experimental points. This is not surprising since
linear collision cascades are unexpected with Au9
projectiles at 200 keV/atom. To account for the
rapid decrease of the energy spectrum a least
squared fit with an expression including a “spike
volume” temperature T :
E
exp - ( E+ Ub)
dN/dE ∝
kT
n
(E+ Ub)
has been applied to the case of Au9 projectiles.
Although the validity of keeping Ub constant is
questionable for large size emitted clusters the
sublimation energy Ub value was kept constant at
3.8 eV (sublimation energy for gold). The curve
fitting the Au2- energy spectrum is shown in fig.
5(a). Values of n and kT obtained from the fit at
angles of emission ranging from 20 to 40 degrees
are around n = 1.8 and kT ~ 19 eV. The kT values
increase with the size of ejected ions ( Au−2 to Au7− )
from 19 eV to 55 eV. The energy spectra (with Au9
and Au4 projectiles) show that the secondary ion
energy depends on angles of emission. For example
small angles at θ < 40° are clearly associated to a
larger temperature spike than for θ > 50°. This was
already deduced from the angular distribution
studies. Furthermore it is observed that dN/dEt
values in the high energy tail region of the energy
spectra are larger with Au9 projectiles than with
Au4 projectiles. This also indicates as expected that
spike temperatures increase when the number of
cluster constituents increases.
theory. The larger the n value the more narrow the
angular distribution (i.e more peaked in the normal
direction to the surface). In fig. 4 it is shown that
the exponent increases with the ejection energy.
This slight increase with Et (from n = 1.8 to n =
2.4) is also observed for Au4 projectiles. The low
energy components of the angular distribution
seems to follow a more isotropic emission while
the high energy component would be more peaked
in the normal direction. A tentative explanation
could be that low energy clusters would originate
mainly from the surface and the high energies SI
which are more peaked would originate from
deeper layers at the early time of the ejection
process.
dN/dΩ
Au2- 3 ≤ Etot ≤ 5eV
dN/dΩ ∝ cos Θ
1.8
Emission angle Θ
dN/dΩ
Au2- 11 ≤ Etot ≤ 15eV
dN/dΩ ∝ cos Θ
2.4
Emission angle Θ
Figure 4. Angular distributions of Au2- ions for two
total energy windows. The exponent of the cosnθ fit
increases with emission energy.
B. Total energy distribution
As mentioned earlier total energy distributions
cannot be measured at all angles (see fig. 3). It is
therefore non-sense to show total energy
distribution for 0° < θ < 90°. All measured energy
distributions from Au−2 to Au7− are somewhat
similar with almost the same average energy. As an
example, fig. 5 presents the dN energy
dEt
distributions of secondary cluster ions Au−2
770
(a)
dN/dE
1000
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U b = 3.78 eV
n = 1.74 +/- 0.06
kTspike = 18.9 +/- 1.5
100
Au 9 @ 1.8 MeV
10
0
20
40
60
Energy (eV)
1000
(b)
dN/dE
U b = 3.78 eV
n = 2.4
100
Au 9 @ 1.8 MeV
10
0
20
40
60
Energy (eV)
Figure 5. Comparison of experimental energy
distribution (Au2-) with calculation.
(a) fit with the spike expression. Parameters are
shown in the figure.
E
.
(b) tentative fit with the expression
(E+ Ub)2.4
A large amount of information is collected in these
multi-parameter experiments which are not easy to
analyse in the framework of the existing models. A
“spike like” model could be appropriate since the
emission is normal to the surface. A similar
conclusion can be deduced from results on total
angular distributions (neutrals + total ions)
measured in recent experiments using Gold target
and gold cluster projectiles as well [11].
A relativity large amount of data has now been
accumulated with gold clusters bombarding gold
targets over a large range of energy. More complete
results on energy and angular distributions are
being obtained with secondary ions. It would
certainly be very informative to perform MD
simulations to test various hypothesis and
parameters (in comparison with experimental data)
in order to better understand the sputtering by large
massive and energetic clusters.
771