LOW ENERGY ION ASSISTED VAPOR DEPOSITION
X. W. ZHOU AND H. N. G. WADLEY
Department of Materials Science and Engineering, University of Virginia,
Charlottesville, VA 22903
ABSTRACT
The performance of multilayered thin film materials often depends sensitively upon the
(physical) roughness and degree of (chemical) mixing at interfaces. Irradiation of a growth
surface with an assisting ion beam is often used to modify surface roughness. Molecular
dynamics has been used to explore the use of low energy (less than 20 eV) Xe+ and Ar+ assisted
deposition of model Ni/Cu/Ni multilayers to control both physical roughness and chemical
mixing. The study indicated that under normal ion incidence condition, ion energies as low as 3
eV could effectively flatten the relatively weakly bonded copper surface (and therefore the nickel
on copper interface). Higher ion energies (at least 10 eV) were required to flatten the more
strongly bonded nickel surface. Chemical intermixing by an exchange mechanism between a
surface atom and an underlying atom in an already deposited layer depended upon the binding
energy of the already deposited layer. As a result, significant chemical mixing occurred as 9 eV
(and above) ions impacted with nickel atoms on an already deposited copper surface. At a given
ion incident energy, (the heavier) Xe÷ ions resulted in less roughness but more mixing. A
modulated ion assistance strategy in which no assisting ion beam was used while depositing the
first few monolayers of each new metal layer was found to successfully reduce both interfacial
roughness and interlayer mixing.
INTRODUCTION
Multilayer structures often possess properties not possessed by either of their constituents.
For instance, multilayers containing ferromagnetic thin films (e.g., permalloy or cobalt) separated
by a thin conductor spacer (such as copper) can exhibit giant magnetoresistance (GMR) [1,2]. The
read heads of all computer disk drives manufactured today now use these GMR materials to
increase storage capacity [3). GMR materials are also being explored for the development of a
new class of low cost, nonvolatile magnetic random access memories [3,4]. To achieve a high
magnetoresistance with small magnetic fields, the interfaces between adjacent layers must be both
flat and exhibit minimal intermixing of the layers. Various physical vapor deposition (PVD) processes have used in attempts to accomplish this. These studies indicate that GMR multilayers
need to be deposited at low temperatures to minimize the thermally activated interlayer diffusion
[5]. However, low temperature deposition promotes rough surfaces and interfaces. Sputtering processes can be used to reduce this roughness presumably because of the energetic impact of the
adatoms or working gas ions with the surface. Magnetron sputtering experiments have indicated
that best GMR properties occur for intermediate energy deposition conditions [6]. A recent
molecular dynamics approach has investigated the effects of the incident metal atom energy upon
the interfacial roughness and intermixing of model Ni/Cu/Ni multilayers [7,8]. The results indicated that high incident energies successfully reduced the surface (and subsequent interface)
roughness, but promoted intermixing by an exchange mechanism. The simulations led to the recognition that a modulated metal atom energy deposition strategy, in which the first half of a new
layer was deposited with a thermalized flux and the remainder with a hyperthermal flux, could
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Mat. Res. Soc. Symp. Proc. Vol. 585 © 2000 Materials Research Society
significantly reduce both interfacial roughness and intermixing. These results also indicated that
because adatoms are thermalized in molecular beam epitaxy (MBE), sputtering or ion beam deposition are preferred processes for synthesizing high performance GMR multilayers [9].
An ion beam deposition (IBD) process is shown in Figure 1. A high energy
Ie O.
Substrate
(0.1-2 keV) inert ion beam from the priV.
.assist
\
lo'
mary ion beam gun causes the sputtering
of the metal target. The average energy of
basin
the sputtered atoms increases with incident ion energy. Because ion beam deposition avoids the use of a plasma, it can be
Sputtered metal
target atoms
operated at very low pressure. This minimizes the energy loss of the sputtered
on beam
Kr or Xe
atoms by background gas collisions during their transportation to the substrate.
Hence, the average energy of the deposi..
.
Background
tion atoms can be directly controlled by
pressure: Pe
the ion acceleration voltage of the pri\ 3 ...
mary ion beam gun. The IBD method has
Figure 1. Ion beam assisted ion beam deposition.
resulted in high quality GMR films [10].
The IBD approach also allows a secondary assisting ion beam to be directed at the subtrate. In this ion assisted ion beam (IBAD)
approach, the impact of the hyperthermal assisting ions with a growth surface promotes surface
atom diffusion and significantly affect surface roughness. Its effects upon interfacial mixing during multilayer deposition are much less clear. Similar energetic ion impacts with growth surface
can also occur during rf diode and magnetron sputter deposition. Here molecular dynamics simulations are used to investigate the effects of these assisting ions. The growth of a model Ni/Cu/Ni
multilayer in the [I ll] direction has been modeled.
COMPUTATIONAL METHODS
An embedded atom method (EAM) potential for binary Cu-Ni alloys was used to calculate
the interactions between Cu-Cu atoms, Ni-Ni atoms, and Cu-Ni atoms [11]. The EAM captures
the local environment dependence of the potential, and hence realistically describes the energetics
near defective crystal regions such as the growth surface and interfaces. A pairwise two-body universal potential [12] was employed to define the interactions between the inert ions and the surface metal atoms. The universal potential is a suitable potential for this work because it was well
fitted to a vast amount of experimental data for low energy ion impacts with metal surfaces. Computational crystals were created by assigning the positions of atoms based on the lattice sites. Periodic boundary conditions were applied in the x- and z- directions (e.g., Figure 2). Ion assistance
and surface growth were simulated by injecting inert ions and metal atoms from random locations
towards the top (y) surface. Normal incident angle (0 = 00) was used for both assisting ions and
deposition atoms. The incident energy, Ei, and incident angle, 0, were introduced by assigning an
appropriate velocity vector to each particle. A thermostat algorithm [13] was used to ensure that
all simulations were conducted at a fixed substrate temperature T = 300 K. The evolution of atom-
222
istic structures were determined by solving for the trajectories of both lattice atoms and vapor
phase particles using Newton's equations of motion.
RESULTS
Ni/Cu/Ni multilayer deposition was simulated for various assisting ion energies using a low
metal atom deposition energy of 0.1 eV and an ion / metal flux ratio of 2. The resultant atomic
configurations are shown as a function of ion energy in Figure 2. Here, dark and light spheres represent nickel and copper atoms respectively. High interfacial roughness can be seen when no ion
assistance was used, Figure 2(a). The roughness of the copper on nickel interface was higher than
that of nickel on copper interface. The mixing of copper in the nickel layer near the nickel on copper interface was also much more significant than the mixing of nickel in the copper layer near the
copper on nickel interface. The interfacial roughness and the intermixing were both significantly
reduced at an ion energy of 0.5 eV, Figure 2(b). As the ion energy was increased from 0.5 to 3 eV,
the roughness of the copper on nickel interface continued to decrease, but more intermixing was
observed at the nickel on copper interface, Figure 2(c).
Ertil=0
,1e
Ion / metal = 2
T
(a) No ion bombardment
Sy[•1111
.
.......
300K
(b) Exo = 0,5eV
(c) Ex, = 3,0eV
'Cu on Ni
Interfolace
20A Na
Nion Cu
20A Cu
interface
M'4substtale
/ZIIIOX
(112)
Figure 2. Multilayer structure as a function of assisting Xe÷ ion energy.
The effects shown in Figure 2 are the result of ion interactions with a continuously evolving
complex surface morphology. To explore detailed mechanisms, ion impacts with a prescribed
(model) rough surface was investigated without synchronized deposition. The initial surface configuration is shown in Figure 3(a). The effects of Xe÷ ion impacts are summarized in Figures 3(b)
- 3(e). It can seen from Figure 3(b) that even a Xe÷ ion energy as low as 3 eV could fully flatten
the copper islands on a nickel crystal. However, the same 3 eV Xe÷ ions could not flatten the
nickel island on a copper surface, Figure 3(c). Figure 3(d) indicates that increasing the Xe÷ energy
to 12 eV did flatten the nickel on copper surface, but this caused extensive Ni-Cu mixing. When
nickel islands were formed on a more strongly bonded nickel (as opposed to copper) crystal surface, then 12 eV Xe÷ impacts could flatten the nickel islands without causing the exchange
between the island nickel and the underlying nickel, Figure 3(e). Clearly, the nickel on copper surface is the only surface that cannot be satisfactorily synthesized with ion assistance because the
ion energy necessary for flattening the nickel islands are sufficiently high to cause nickel mixing
with the underlying copper.
223
Ex,&= variable y [
S~
(b) After 3 eV Xe+ impacts
(a, Before Xe+ impacts
CL (or Ni)
Cu (or t
T = 300K
F - 0.5 ionsiA2
_"M _
IF)
JI
x [112]
-elz [1 101
Cu
t
(e) After 12 eV Xe+ impacts
(d) After 12 eV Xe+ impacts
(C) After 3 eV Xe+ impacts
Ni
Ni
Ni
Figure 3. Xe+ ion impact effects on different surfaces.
For the atomic configurations shown in Figure 3, the surface roughness can be quantified by
the fraction of atoms remaining above the first island layer, and the degree of mixing can be measured by the mixing probability. The surface roughness and the mixing probability after 1500
(Xe+ and Ar+) ion impacts are plotted in Figures 4(a) and 4(b) as a function of ion energy for the
nickel on copper surface. Figure 4 indicates that flattening and mixing were less for the lighter
Ar+ ion impacts, but the general trend was similar.
1.0CL
0.8
04
I
• 0.4-//
Irio
.*Ar ion impacts
ion impacts
0.*Ar
0.5
Mixing
Surface roughness macs
'
"-
-Xe ion impacts
Xe ion impacts
0
-0.3
)-0.
E00
Lto
-0.4
0
4
0.2
0.00
o
0.0
-
_o ___0.0_........_-____-__....
0
3
6
9
12
15
0
3
6
" '
9
12
15
Ion energy E,,, (eV)
Ion energy Ei., (eV)
Figure 4. Roughness and mixing of nickel on copper surface as a function of ion energy.
MECHANISMS OF ION IMPACT EFFECTS
To understand the mechanism of these assisting ion effects, individual ion impacts were
simulated. Figures 5(a) and 5(b) show three ion impacts with a copper and a nickel surface respectively. On the right of the copper island shown in Figure 5(a), a 4 eV Xe÷ bombarded a cluster of
224
three copper atoms near the edge of the island. Because the binding energy of copper atoms is
low, this ion impact caused one copper atom to jump to a lower surface. The ion impact therefore
promoted step flow growth and the flattening of the copper surface (or the nickel on copper interface). At the center of the copper island, a 3 eV Xe÷ impacted a single nickel atom. At the front of
the island, a 0.1 eV Xe÷ ion collided with a single nickel atom attached to the edge of the island.
During both impacts, the weakly bonded underlying copper lattice was penetrated, resulting in the
exchange of the nickel atom with a copper atom.
(a) Xe+ impact on copper
(b) Xe+ impact on nickel
(1) t = 0.0 ps
(1) t = 0.0 ps
(3 eV)(12 eV)(8 eV)
(0 1 eV)(3 eV)
CuIatom
Nao1o
Figure 5. Mechanisms of
41
[11o1
iimpact effects.
Figure 5(b) shows that even a 8 eV Xea ion could not cause the flattening of the three nickel
atom cluster. Because nickel has a higher cohesive energy than copper, ion impact induced atomic
jumps required significantly higher ion energies, and the nickel surface (or the copper on nickel
interface) tended to remain rougher than the copper surface. Figure 5(b) also shows that atomic
exchange between a copper atom and an underlying nickel atom did not occur during a 12 eV ion
impact at the center of the nickel island or a 3 eV ion impact at the edge of the island. This is
because the strongly bonded nickel lattice could not be penetrated at low ion energies. As a result,
the nickel on copper interface is more chemically intermixed than the copper on nickel interface.
Mixing was promoted at low ion energy near ledge sites. Reducing the ledge density during
growth may therefore also reduce mixing.
DISCUSSION
Low energy ion assistance cannot reduce the nickel on copper surface roughness without
causing mixing. An alternative approach is to use a modulated ion assistance in which the first
few monolayers of a new material are deposited without ion assistance and the remainder of that
material is deposited with ion assistance. To explore this idea, modulated ion assisted depostion
was simulated as a function of ion energy for a low (metal) deposition energy of 0.1 eV and an ion
metal flux ratio of 3. Figure 6 shows that the multilayer interfaces can be flattened without inducing mixing by modulated ion assistance scheme.
225
03=0'
~
CONCLUSIONS
I
Xe / metal = 3
T = 300K
-
(a) Ex.= 0.5eV
Yl'111
I
/
20A Ni
'20A Cu
(b) Ex, = 5.OeV
Molecular dynamics simulations of ion assisted
deposition of multilayers indicated that:
1. 3 eV ion impacts can flatten a Cu surface.
Higher ion energies (> 10 eV) are required to
V-Xe on
X0 Off 2.
Xe on
Xe off
Xe on
Xe off
/Z (1i0l
flatten a nickel surface.
Copper mixing in the nickel layer near the
nickel on copper interface is much more significant than nickel mixing in the copper layer near
the copper on nickel interface. Unlike other surfaces, ion assistance cannot fully flatten the
nickel on copper surface without causing mixing.
3. A modulated ion assistance can result in a flat
nickel on copper surface without inducing mixing.
ACKNOWLEDGEMENTS
Figure 6. Effects of ion modulation.
We are grateful to the Defence 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 NAGW 1692 and NAG- I-1964.
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