ENERGETIC INERT GAS ATOM IMPACT EFFECTS
DURING ION BEAM MULTILAYER DEPOSITION
X. W. ZHOU, W. ZOU, H. N. G. WADLEY
Department of Materials Science and Engineering
University of Virginia, Charlottesville, VA 22903
ABSTRACT
New magnetic field sensors and non-volatile magnetic random access memories can be
built from giant magnetoresistive multilayers with nanoscale thickness. The performance of these
devices is enhanced by decreasing the atomic scale interfacial roughness and interlayer mixing of
the multilayers. During ion beam sputtering, inert gas neutrals with energies between 50 and 200
eV impact the growth surface. A molecular dynamics method has been used to study the effects
of these impacts on the surface roughness and interlayer mixing of model nickel/copper
multilayers. The results indicate that impacts with energy above 50 eV cause mixing due to the
exchange of Ni atoms with Cu atoms in an underlying Cu crystal. The extent of the mixing
increases with impact energy, but decreases as the number of the Ni monolayers above the Cu
crystal increases. While Xe and Ar impacts have a similar mixing effect at low energies, heavier
Xe ions/neutrals induce more significant mixing at high energies.
INTRODUCTION
Many nanostructured materials exhibit technologically interesting properties that are not
possessed by their bulk constituents. For instance, if a -20A thick conductive layer is sandwiched
between -10-100 A thick ferromagnetic layers, the metallic multilayer stack can exhibit giant
magnetoresistance (GMR) manifested as a large (5-50%) change in electrical resistance upon the
application of an external magnetic field [1]. GMR materials have been used to build the read
heads for hard drives that can greatly increase the storage capacity of computers [2]. They are also
being investigated for a new class of low cost, nonvolatile magnetic random access memories [2].
These applications require materials with a high magnetoresistance. This can be achieved if the
multilayers have flat interfaces with minimal chemical mixing between the layers [3]. Experimental data indicate that deposition at low temperatures is required to minimize thermally activated
interlayer diffusion [4]. A recent molecular dynamics (MD) simulation of the deposition of the
model Ni/Cu/Ni multilayers [5,6] showed that a moderate (1 eV) adatom energy can be used to
flatten the interfaces of the multilayers grown under low temperature conditions without causing
extensive interfacial mixing. This may account for recent observations that hyperthermal metal
atoms created by sputtering processes such as RF diode (or magnetron) sputtering and ion beam
sputtering produce better GMR multilayers than their thermal energy (e.g., those used in molecular beam epitaxy) counterparts [7]. MD simulation also revealed that very high incident energies
could promote interlayer mixing by an atomic exchange mechanism at the Ni on Cu interface
[5,6]. This is consistent with other observations that magnetron sputtering gives rise to the best
GMR multilayers under intermediate energy deposition conditions [8]. These observations have
led to a proposal that further reductions of both interfacial roughness and interlayer mixing could
be obtained by a modulated adatom energy deposition, in which the first half of a new layer was
deposited with a thermalized flux and the remainder with a hyperthermal flux [5,6].
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Mat. Res. Soc. Symp. Proc. Vol. 616 ©2000 Materials Research Society
However, other fluxes are incident upon a growth surface during metal sputtering depositions. For ion beam sputter deposition, energetic ions generated in the ion beam gun are used for
the target sputtering. These ions resulted in a reflected neutral flux that can reach the substrate
with energies in excess of 100 eV [9]. This may potentially cause significant atomic exchange and
related mixing, especially on the Ni on Cu surface [10]. To improve growth of GMR materials, a
better understanding of the effects of these high energy particle impacts needs to be developed.
Here, a molecular dynamics simulation method is used to investigate the effects of high energy
xenon and argon neutral impacts on model (111) Ni on Cu surfaces.
COMPUTATIONAL METHODS
Since the embedded atom method (EAM) potential for the Cu-Ni system [11] captures the
local environment dependence of the potential and realistically describes the energetics near surfaces and interfaces, it was used to define the interactions between Cu-Cu atoms, Ni-Ni atoms,
and Cu-Ni atoms. A universal potential that was fitted to a significant amount of surface bombardment experimental data [12] was employed to calculate the interactions between the xenon/
argon particles and the surface metal atoms. Cu crystals containing 84(112) planes in the x direction, 11 (111) planes in the y direction, and 48 (110) planes in the z direction were created by
assigning the atoms to the equilibrium lattice sites (see Figure 1). 1 to 7 Ni (111) planes were then
placed on top of the surface based upon an epitaxial lattice extension. Inert particles with a given
kinetic energy were injected from random locations to impact the top (y) surface at a normal
angle. Periodic boundary conditions were applied in the x and z directions to extend the crystal
dimensions. The bottom two Cu planes were fixed at their equilibrium positions to prevent the
shift of the crystal. A thermostat algorithm [13] was used to maintain a fixed substrate temperature of 300 K. The surface evolution during impact were then simulated by tracking the positions
of atoms and inert neutrals using Newton's equations of motion.
RESULTS
Impacts on the surface composed of one Ni monolayer above a Cu crystal were first simulated at different impact energies between 50 and 200 eV. Representative atomic surfaces after 10
xenon impacts at xenon energies of 50 eV and 200 eV are shown in Figures 1(a) and 1(b) respectively. In Figure 1,dark and light spheres are used to distinguish Ni and Cu atoms. Figure I indicates that xenon impacts caused atomic exchanges between Ni atoms in the perfect Ni layer and
underlying Cu atoms even at the low energy of 50 eV. The degree of this impact induced mixing
increased with the impact energy. In addition, the impacts caused pits and surface roughness by
ejecting in-plane atoms onto the top of the surface. The extent of this type of surface roughness
also increased with the impact energy.
The effect of impact induced mixing should depend on the distance between the surface and
the interface. Impacts on the Ni on Cu surfaces that contained more Ni monolayers were hence
explored. Figure 2 shows the atomic structure of a surface containing 7 Ni monolayers above a Cu
crystal after 10 Xe impacts at an ion energy of 200 eV. To examine mixing at the Ni-Cu interface,
some of the surface layers are displaced in the vertical (y) direction. It can be seen that no mixing
occurred during ion impact when the Ni on Cu interface was 7 Ni monolayers below the surface.
104
Y[111] )ArorXe
#
LýA
( Normal incidence
Ni
(a) Exe = 50 eV
CL
[112]
J
ore impacts
100tototooo'**,*
0#
00 #0
to #
00
t
#
to00 W#
ot
oft *0
to00 V
000 0i
#
# V*to
V#
'00 Wo
0#
to 0,0
0#V*
it
#v 0
to
04 0000V##0
to 0
Or*
to to W#
__# _##
Fig. 1.The structure of the Ni monolayer above a Cu crystal after 10 Xe ion impacts at an ion
energy of (a) 50 eV and (b) 200 eV.
To more clearly explore the impact effect trend, a mixing parameter was defined as the average number of exchanged Cu atoms per impact. The mixing parameter obtained at different
impact energies and impact species is plotted in Figure 3 as a function of the number of Ni monolayers above the Cu crystal. Clearly, the mixing parameter rapidly decreased as the number of Ni
monolayers increased. Otherwise, the mixing parameter increased with the impact energy. At the
high energy of 200 eV, Xe impacts generally caused more significant mixing than Ar impacts.
However, this difference became much smaller at 100 eV, and diminished at 50 eV.
105
y [111]
Exe = 200 eV
Aror Xe
Normal incidence
/zA1[[X
110] Before impacts[11r]
Fig. 2. The structure of 7 Ni monolayer/Cu crystal after 10 Xe ion impacts at an ion
energy of 200 eV.
To quantify the impact induced surface pitting and roughening, the number of atoms ejected
onto the top of the surface per impact was also calculated at different impact energies and impact
species. The results of this calculation are plotted in Figure 4 as a function of the number of Ni
monolayers above the Cu crystal. It can be seen from Figure 4 that the pitting and the roughening
induced by impacts are insensitive to the number of the Ni monolayers. While no big difference is
observed for the Ar and Xe impacts, increases in the impact energy increased the roughness of an
originally flat Ni surface. At high deposition temperatures or low deposition rates, the density of
these pits and roughness regions may be reduced due to thermally activated diffusion.
106
DISCUSSION
C 5
.2_
pis
0n
E
0 4
Xe
Ar+
Xe+
Cu
C.)
3
E
x
2
X
-
Ar+
le+
r
Z
0
..0
1
E
z=
0
1
2
3
4
5
6
7
Number of Ni monolayers above Cu
Fig. 3. Mixing as a function of Ni monolayers
.2 20
0
161
J
SI
I
I
Symbols
o 200 eV X
* 200 eV A
'5100 eV X 8+÷
100 eVA r+
0)
12
50 eV Xe
* 50 eV Arl
ca 8
4
-
materials
Nanostructured
such as GMR multilayers contain different material layers,
each with very thin thickness
(in the scale of several atomic
layers). These materials are
often required to have very low
interfacial roughness and interlayer chemical mixing. Because
low temperature must be used
to deposit these nonequilibrium
materials and prevent thermal
diffusion induced mixing, flat
surfaces (and interfaces) cannot be achieved under thermal
energy conditions. Hyperthermal energy sputtering deposition methods such as RF diode
or ion beam sputtering deposition techniques need to be used
to promote energetic impact
induced surfaces flattening.
However, inert gas particles
with energies between 50 and
200 eV are likely to impact the
substrate during either the RF
diode or the ion beam sputter
deposition processes. The calculations here indicated that
0
these impacts can cause signifi7
1
2
3
4
5
6
cant interlayer mixing in the
Number of Ni monolayers above Cu
multilayers.
Experiments have shown
Fig. 4. Surface roughness as a function of Ni monollayers.
that the insertion of 1-2 monolayer of Co between the Nis1Fejg/Cu interface can significantly improve the GMR properties of
the Nis1FeI 9/Cu/Ni81Fe, 9 multilayers [141. This may be helpful to growth because mixing between
the Co and Nis1Fej9 alloy is not expected to affect the magnetic properties of the layer, while the
impact induced mixing between the thermodynamically immiscible Co and Cu can be subsequently reduced by annealing. Nevertheless, the recognition of this high energy impact effect
emphasize the need to reduce the high energy particle flux during a further exploration of growth
methods for GMR technology. A number of ways can be used to do this. For the ion beam sputter
deposition process, this may involve refining the position of the substrate to avoid the high flux
reflected neutral zone, or switching the working gas from Ar to Xe because the density and energy
of reflected Xe neutrals are both much lower than those of reflected Ar [9].
0ý
107
CONCLUSIONS
Molecular dynamics simulations of high energy inert gas impacts with Ni on Cu surface
indicated that:
* 50-200 eV impacts caused significant mixing between surface Ni and the underlying Cu.
The degree of the mixing increased with the impact energy. At high impact energies, Xe
impacts caused more mixing than Ar impacts, but the difference became smaller at low
impact energies.
0 The degree of mixing rapidly decreased as the number of Ni monolayers on top of the Cu
crystal increased. It is hence important to reduce the probability of high energy impacts at
the substrate surface during deposition of the first few monolayers of Ni on to a predeposited Cu surface.
* High energy impacts also induced pits and roughness on an originally flat surface. The
extent of the roughness increased with the impact energy, but was insensitive to impacting
species and the number of the Ni monolayers above Cu.
ACKNOWLEDGEMENTS
We are grateful to the Defense Advanced Research Projects Agency (A. Tsao and S. Wolf,
Program Managers) and the National Aeronautics and Space Administration for support of this
work through NASA grants Nos. NAGW 1692 and NAG- 1-1964.
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