Effects of Accumulation of Impurities in Target Materials forSputtering
Takahiro Kenmotsu 1, and Motoi Wada 212Introduction

Sputtering yields have been measured for many ion-target
combinations so far [1][2]. Monte Carlo simulation codes [3][4]have been developed to estimate ion-solid interactions, and arein good agreement with experimental data on sputtering yields[1][2]. Several empirical formulae which predict the energy
dependence of sputtering yields for any ion-target combinations
were proposed [2][5]. The Yamamura-Tawara semi-empiricalformula [2], hereafter we abbreviate to Y-T formula, are applied
to estimate sputtering yields in the present work.
Experimental works on the sputtering yields of carbon due toxenon ions have shown significant yields below the thresholdenergy predicted by the empirical formula at normal incidence[6-12]. The predicted threshold energy is 160.84 eV for thexenon–carbon combination in the Y-T formula [2]. In this paper
we report the results of sputtering yields of carbon under thexenon ion bombardment calculated with a Monte Carlo code
ACAT [4] to find the reason to explain the discrepancy between
the experimental results and the predictions from the Y-T
formula. Atoms of Xe are retained in graphite to make atomic
xenon fraction as high as 14% after the low energy Xe plasma
sputtering experiment [10]. We will correlate the amount of theincident particles retained into a target material and the resultant
sputtering yields from graphite materials, under a continuous
bombardment of Xe ions.
2

Calculation model

2.1
Monte Carlo simulation code ACAT
A Monte Carlo simulation code ACAT has been used tocalculate sputtering yields of carbon from xenon containing
carbon materials due to xenon bombardment in the present work.
Since the ACAT code has been described in detail elsewhere [9],the main features of the code are briefly outlined here. The code
numerically calculates trajectories of atoms colliding in an
amorphous target based on the binary collisions approximation
with the target atom positions assigned by a Monte Carlo
method. The target atoms are thus, randomly distributed in each
unit cubic cell, the lattice constant of which is calculated from
R0 = N-1/3, where N is the atom number density of the target
material. Target and retained atoms are assigned in each unit cellin accordance with the atomic fractions. The target and retained
atoms are distributed uniformly over the target material. The
lattice constant of the target material is determined from thedensity composed of target and retained atoms.

Eth 1  5.7M1 M 2 
,
M1  M 2 ,(2)US
where M1 and M2 are the masses of the projectile and the target
atom, respectively. Us is the surface binding energy of the target
material. We take the heat of sublimation as the surface binding
energy of the target material. The surface binding energy forcarbon targets is 7.37 eV.  is the energy transfer factor in anelastic collision.

3Results

3.1
Effect of accumulation of Xe for carbon sputtering
Shown in Fig.1 are the sputtering yields of carbon in puregraphite bombarded by xenon ions calculated with ACAT
together with the experimental data and the curve obtained fromY-T formula. The yields from ACAT agree well with the Y-Tformula, but the ACAT results differ from the experimental data.
The sputtering yields of carbon have been confirmed present
even below the threshold energy predicted by eq. (1), which is160.84 eV. The threshold energy obtained from ACAT result issimilar to the threshold energy predicted by eq. (1), and issubstantially larger than that obtained by experiments.
Considering the present case where the mass of the projectile islarger than that of the target atom, no mechanism to reduce thethreshold energy is predicted from the collision theory for purecarbon target [13]. The dominant process of sputtering in thecase of M1  M 2 (Mechanism I) is shown in Fig. 2 (a) in thecollision theory [13].
110

Rosenberg and Wehner [6]Deltschew et al. [7]
Gruber et al. [8]Funaki et al. [9]Doener et al. [10]Williams et al. [11]Kolasinski et al. [12]Yamamura and Tawara [2]ACAT (pure carbon)
Theoretical threshold : 160.84 eV

0Sputtering yield (atoms/ion)

1

Faculty of Life and Medical Sciences, Doshisha University, Kyoto 610-0321, Japan
Graduate School of Science and Engineering, Doshisha University, Kyoto 610-0321, Japan10-1

10Xe+-2

C at 0o10-3

10-4

102.2
Threshold energy for sputtering
The threshold energy of sputtering, Eth, in the Y-T formula
are given byEth 6.7
,M1  M 2 ,(1)US110210Incident Energy (eV)310Fig.1 Experimental sputtering yields of carbon bombarded byxenon ions. Also shown in the figure are the results calculatedwith the Y-H formula and ACAT.

Figure 3 shows the results of ACAT with 14% xenon atoms
retained in graphite together with the experimental data. TheACAT results are in good agreement with the experiments. The
reason why the Y-T formula differs from the experimental datais that the model does not take into account the effect ofaccumulation of xenon in graphite. Considering the sputtering ofcarbon from graphite containing 14% xenon atoms, atomic
collisions in the target consists of collisions between carboncarbon, carbon-xenon and xenon-xenon. The threshold energy of
sputtered atoms is determined by the process of minimum
energy loss in the target material. In the xenon retaining system,
the primary recoil carbon atoms produced by the incoming
xenon ions can be sputtered out through the collisions with only
xenon atoms in the target (Mechanism II in Fig. 2 (b)). Thus, theaccumulation of xenon in graphite may lead to the reduction of
the threshold energy for sputtering of carbon due to the xenon
ion bombardment. Considering the projectiles are carbon and thetarget atoms are xenon, the threshold energy calculated by eq.
(2) is only 36.5 eV, and this threshold energy is in good
agreement with the experimental results.
xenoncarbon(b) Mechanism II(a) Mechanism IFig. 2 Schematic representations of mechanism I and II.
(a) pure carbon model. (b) recoil carbon collision withretained Xe atoms.

Sputtering yield (atoms/ion)

10101

Rosenberg and Wehner [6]Deltschew et al. [7]
Gruber et al. [8]Funaki et al. [9]Doener et al. [10]Williams et al. [11]Kolasinski et al. [12]ACAT (pure carbon)
ACAT (14% xenon)

010-1

10-2

10-3

10-4

10Xe+

1

C at 0o210Incident Energy (eV)310Fig.3 Experimental sputtering yields of carbon bombarded
by xenon ions. Also Shown in the figure are the results
calculated with ACAT with pure carbon and ACAT withgraphite contained 14% xenon atoms.
4Conclusion

The sputtering yields of carbon have been measured duringxenon ion bombardment under the threshold energy predicted by
the Y-T formula at normal incidence. We have confirmed thepresence of the sputtering yields of carbon below the thresholdenergy by calculating the collision cascade in the xenon
retaining carbon target with a Monte Carlo code ACAT. The
yields of carbon calculated with ACAT are in good agreementwith the experimental data. In other word, the retained xenon
atoms in graphite enhance the sputtering yield of carbon below
the threshold energy. This reduction in the threshold energy cantake place in the case where the mass of the projectile is larger
compared with that of the target atom.

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