MULTISCALE SIMULATIONS OF THE RF DIODE
SPUTTERING OF COPPER
H. N. G. WADLEY, W. ZOU, X. W. ZHOU, J. F.GROVES
Intelligent Processing of Materials Laboratory
Department of Materials science and Engineering
University of Virginia
Charlottesville, VA 22903
S. DESA, R. KOSUT, E. ABRAHAMSON, S. GHOSAL, A. KOZAK
SC Solutions
Santa Clara, CA 95054
D. X. WANG
Nonvolatile Electronics, Inc.
Eden Prairie, MN 55344-3617
ABSTRACT
The morphology and microstructure of RF diode sputter deposited materials is a
complicated function of many parameters of the reactor operating conditions. Using a
combination of computational fluid dynamics (CFD), RF plasma, molecular dynamics (MD)
sputter, and direct simulation Monte Carlo (DSMC) transport models, a multiscale approach has
been used to analyze the RF diode sputtering of copper. The CFD model predicts the velocity
and pressure distribution of the working gas flows in the deposition chamber. The plasma model
uses these CFD results to compute ion energies and fluxes at the target and substrate. The MD
model of sputtering is used to determine the initial energy distribution of sputtered atoms and
reflected neutral working gas atoms and both of their angular distributions. A DSMC transport
model then deduces the target atom deposition efficiency, the spatial distribution of the film
thickness, the target and reflected neutral atoms energy and impact angle distributions given
reactor operating input conditions such as background pressure, temperature, gas type, together
with the reactor geometry. These results can then be used in atomistic growth models to begin a
systematic evaluation of surface morphology, nanoscale structure, and defects dependences
upon the reactor design and its operating conditions.
INTRODUCTION
Physical vapor deposition processes are being used to produce increasingly complex devices
whose performance is critically dependent upon atomic scale features of their structure. A good
example is giant magnetoresistive (GMR) metal multilayers (e.g., NiFe/Cu/NiFe) which exhibit
large drops in their electrical resistance when a magnetic field is applied [1-3]. These materials
can be used for making new magnetic field sensors [4-6], read heads for disk drives, and magnetic
random access memories (MRAM) [5,6]. GMR-based MRAM has many potentially attractive
features, such as non-volatility, radiation hardness, low power consumption, high memory densities (comparable to those of dynamic random access memory), and high access speed. The key
issue for this technology is to design an economical deposition process that can produce thermally
stable GMR multilayers with high GMR ratios (defined as the maximum resistance change
divided by the resistance at magnetic saturation) at low magnetic field. Both theoretical and
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Mat. Res. Soc. Symp. Proc. Vol. 538 0 1999 Materials Research Society
experimental work indicated that the best GMR properties are achieved when the atomic scale
interfacial roughness and interlayer chemical mixing are both minimized. Molecular dynamics
simulation of GMR multilayer deposition identified that adatom incident energies in the range
between 0.1 to 5.0 eV are needed to minimize both interfacial roughness and intermixing [7].
Sputter deposition methods can result in incident adatom energies in the range of 0.1 to 20 eV, and
have been widely explored for GMR multilayer deposition [8-23]. However, because the experimental atomic scale characterization is difficult, and the dimensions of the processing parameter
space is large, the experimental search for an optimized sputter deposition process for synthesis of
GMR multilayers has been prolonged. A multiscale reactor simulation tool relating the morphology and microstructure of multilayers to the processing conditions of a diode sputter deposition
system has been developed to help the optimization of the process.
DIODE SPUTTER DEPOSITION
simulations
1Atomistic
Vacuum chamber at pressure P
Water cooling
Working gas
(Ar or Xe)
Ftion
of
vapor
deposited films have shown that film
structure is a function of the key deposi-
conditions including substrate temAm pat
ttered
-perature,
deposition rate, adatom incident
RF
energy, and incident angle [7,24]. These
a
ep a ,
S ....
power
key deposition conditions are in turn controlled by many process parameters. To
supply
Ar+
illustrate, a diode sputter deposition sysSPatered
tem is schematically shown in Figure 1.
I
l
An inert gas plasma is initiated and maintained between target and substrate by an
RF power. The inert gas ions created in the
plasma are accelerated to the metal target
under the bias voltage. The high energy
bombardment of inert gas ions on the tarpumps
Vacuum
get surface results in the sputtering of
Figure 1. Schematic of A Diode Sputter System
metal atoms from the target. These sputtered atoms are then transported to the
substrate and deposited on the substrate surface. Generally, the system geometry and the RF
power determine the densities, the energies, and the angles of the atoms emitted from the target.
Scattering with the lower energy working gas modifies these quantities during the transportation
of the sputtered metal atoms to the substrate. The final distributions of these quantities depend on
the target-substrate distance, the working gas temperature, and pressure. Some inert gas ions are
neutralized after their bombardment of the target and can be reflected toward the substrate. These
reflected neutral particles can also modify the morphology and the structure of a growth film. To
develop a better understanding of the process and to be able to predict the uniformity, morphology, and structure of thin films, it is essential to model the deposition efficiency (defined by the
ratio of the deposited material at the substrate to the sputtered materials at the target), the distributions of density, energy, and angle of depositing atoms, as well as inert ions at the substrate, all as
a function of reactor scale pressure, temperature, system geometry, power, etc.
324
EXPERIMENTS
Vapor deposited copper films were grown on silicon wafer using a Randex Model 2400-6J
diode sputter deposition system at fixed Ar pressure and target-substrate distance but with
different plasma power. The surface morphology was characterized using a Pico SPM MS 300
atomic force microscope. The morphology of two typical samples are shown in Figure 2.
Pressure = 20 mTorr, target-substrate distance = 1.5 in, Thickness = 2000 A
Power = 50 W
Power = 350 W
Figure 2. Atomic Force Microscope of Sputter Deposited Copper Films
Figure 2 indicates that an increase of the power from 50 to 350 W dramatically increases
the grain size and reduces the surface roughness.
SIMULATION METHODOLOGY
The multiscale reactor model is illustrated in
Figure 3. A CFD finite element model was used to
calculate the velocity and pressure distribution of
inert gas flow in the chamber; a steady-state plasma
model was used to simulate the density and energy
of Ar ions striking the target and substrate; a
molecular dynamics (MD) model was used to
determine the sputtering yield of target by the inert
ions, the energy and angular distribution of the
sputtered atoms at the target; and a Direct
Simulation Monte Carlo (DSMC) model was used
to trace the change of density, energy, and angle of
the sputtered atoms as they transported through the
low-pressure inert gas to the deposition substrate.
The individual models for gas flow, plasma
discharge, sputtering, and atom transport were then
325
Model
Inputs Model
Ram
r
350eWvsrM
drama
model
Outputs
ly
i
Pa
....
W mm
*
MW
Figure 3. Reactor Scale Integrated Model
integrated to create a detailed, steady-state, input-output model capable of predicting incident
energy, incident angle, deposition-rate, and uniformity as a function of the process input
variables: power, pressure, gas temperature, and electrode spacing. These results can in turn
account for the morphology and microstructure of vapor deposited films [7,24].
RESULTS
The multiscale model was used to explore the effects of power during deposition of copper
by argon ion sputtering at fixed pressure of 20 mTorr and fixed target-substrate distance of 1.5
inches. The energy and current density for argon ions impacting the target as a function of
power are given in Figures 4 and 5. The deposition rate as a function of power is shown in
Figure 6. The energy and current density of depositing fluxes at the substrate as a function of
power are drawn in Figures 7 and 8. It can be seen that increasing the power from 50 to 350 W
increases the average ion energy from 0 to 600 eV, and the ion current density from 0 to 7x10 19
atoms/m 2 . Higher ion energy at the target leads to higher energy sputtered atoms and a higher
sputtering yield [25]. As a result, increase of power not only increases the deposition rate, Figure
6, but also increases the incident energy of the atoms deposited at the substrate [26], Figure 7.
600
1 .
500 St400
(D
.
.
.
.
Pressure=2OmTorr
Target-substrate
.
*
distance=1.5 inches
.
12
.
E o--agtsusrt
distance=1.5inches
N
-
Pressure=2OmTorr
Target-substrate
10
9
101
17.50x
- -62
- 6.25
"o
8 --
21
5.00
6-
300 -~
3.5
o200
4St
(C
C
100
.9
0
0
- - 2.50
2
1.25
0
Input power (W)
.
3.7
'
00'
50 100 150 200 250 300 350 400
E
E
'
i
i
i
0.00
50 1 100
1 2000O250 300 350 400
Input power (W)
Figure 4. Effect of Power on Inert
Ion Energy at Target
Figure 5. Effect of Power on Inert
Ion Current Density at Target
It can be seen from Figure 6 that the deposition rate is almost linearly related to the power.
The simulated results are in good agreement with experimental measurements. The results
shown in Figure 7 indicate that during RF diode sputter deposition at a pressure of 20 mTorr and
a working distance of 1.5 inches, the scattering with the background working gas almost
completely thermalize the metal flux. A net increase of power of 350 W results in only a 0.04
eV increase in the depositing energy of copper. This is unlikely to significantly change the thin
film morphology and cannot account for the results shown in Figure 2. Interestingly, Figure 8
indicates that the density of the reflected inert gas atoms at the substrate is comparable to the
density of the depositing flux. On the other hand, Figure 7 reveals that increasing input power
significantly increases the energy of the reflected inert flux to 100 eV range. Molecular
dynamics simulations indicated that 100 eV inert atom bombardment on the substrate can cause
a significant transient local heating that results in surface atom athermal diffusion and a
326
flattening of a growth surface. It appears that the neutral flux is a powerful means for modifying
the surface morphology. However, it may be undesirable for growing thin metal multilayers
used for GMR devices because it induces interlayer mixing. The results obtained by the
integrated model allow us to use atomistic simulations [7,24] to quantify the microstructure and
morphology of deposited films as a function of reactor scale processing parameters.
Fd)(3
E
40oo
Cuatoms
St
Pressure=2rnTorr
'
160
NIR
.......
te
It-subsra•
CD
distance=t.5 nches
0.12
--
120
ot
30
CC
ca
0
0
2C
00
Ar ions
80
Pressure=2OmTorr
40
Simidati
E5
0.
0
'L
0.08 --
10
E
0"'"
E rpeiment
0.04
o
0
Target-substrate
distance=1.5 inches
.2
n nn
00
50
100
150
200
250
300
350
400
0
50
100
Input power (W)
150
200
250
300
350
-0
400
Input power (W)
Figure 6. Effect of Power on Deposition Rate
Figure 7. Effect of Power on
Flux Energies at Substrate
CONCLUSIONS
1.6
A multiscale reactor model has been
developed to simulate the effects of processing
conditions (power, pressure, temperature, targetsubstrate distance, etc.) on the deposition rate, the
density and the energy of depositing atoms, and
reflected inert atoms. These results can be in turn
used in atomistic growth models to simulate
surface morphology and microstructure of
deposited films. Our results indicate that:
1. Increasing the RF power increases the inert
gas ion density and energy at the target.
CS 1.2
Cu
0
o
cc
0.8
-
9
-
Ar ions
U)
0.4
Pressure=2OmTorr
.o
cc
0:
0
50
Target-substrate
distance=1.5 inches
100
150
200
250
300
350
40)0
Input power (W)
Figure 8. Effect of Power on Flux
Current Densities at Substrate
2. Increasing power linearly increases the metal
flux density at the substrate and therefore the
deposition rate, but causes little change in metal incident energy at pressure of 20 mTorr.
3. The flux of reflected inert gas neutral flux is comparable to that of the depositing flux. The
neutral energy increases rapidly with power.
4. The energetic bombardment of the substrate by reflected neutral Ar is the apparent origin of
the smoother surfaces observed after higher power deposition. However, this may lead to
multilayer intermixing of metal like those of interest for GMR devices.
327
ACKNOWLEDGEMENTS
We are grateful to the Defence Advanced Research Projects Agency (A. Tsao, Program
Manager) and the National Aeronautics and Space Administration for support of this work
through NASA grants NAGW 1692 and NAG- 1-1964.
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