Simulation
2.28e+016
2.28e+018
Ar+ Absorption [1/m²/s]
F r a u n h o F e r I n s t I t u t e F o r s u r F a c e e n g I n e e r I n g a n d t h I n F I l m s I s t
Simulation in tHin Film tECHnoloGYProgressive product requirements based on thin film coatings
call for improvements in productivity and precision of underly-
ing coating processes. The increasing size and complexity
of such processes, however, hamper development by purely
empirical means. Thus, process simulation becomes a necessity
here as it can provide insights into process dynamics and
relevant parameters to augment process understanding and
promote further innovations. Simulation – if reliable – can
substitute the experimental evaluation in process and facility
development thus saving time and costs for corresponding
experimental set-ups and measurements.
Feasible simulation approaches
Vapor phase deposition in high vacuum chambers is currently
the most established technology for high-precision thin film
coatings on large areas; prominent examples of processes
are thermal vacuum evaporation, magnetron sputtering
and plasma-enhanced chemical vapor deposition. Such
processes are characterized by rarefied flow conditions with
high Knudsen numbers and low-temperature plasmas. Thus,
widespread computational fluid dynamic (CFD) simulation
based on continuum mechanics can not be used here. For
example gyrokinetic plasmas, i. e. magnetron discharges, are
not described properly if assuming uniform velocity states
per grid cell. Instead, non-equilibrium statistical mechanics as
specified by the Boltzmann Transport Equation (BTE) must be
applied. However, in contrast to continuum CFD, up to now
there are only few BTE solvers commercially available.
The DSMC and PIC-MC method
Suitable algorithms for solving the BTE in the above men-
tioned parameter regime are the statistically motivated Direct
Simulation Monte Carlo (DSMC) and Particle-in-Cell Monte
Carlo (PIC-MC) method. Here, the BTE is solved by means
of representative macro-particles in a computational grid.
Changes in particle states are calculated at discrete time steps
as a function of border and particle interactions. The statistical
noise which incorporates such a representative mapping is
reduced by averaging the particle states over multiple time
steps and particles per cell. The averaged particle states
thus supply a spatially and temporally resolved image of the
macroscopic state variables in a gas and plasma. Whereas
DSMC addresses neutral gas flow dynamics only, PIC-MC
also includes field solvers and algorithms to simulate charged
particle kinetics. Hence, PIC-MC is an extension of the DSMC
method.
Products and services we offer
� Licensing of DSMC software for rarefied gas flow simulation
� Licensing of PIC-MC software for gas discharge simulation
� Modeling of deposition profile on moving 3D substrates
� Modeling of dust particle behaviour
� Optimization of vacuum coating processes and facilities
� Consulting and training on modelling thin film deposition processes
1 DC dual magnetron
discharge simulated with
PIC-MC.
1
DSMC SIMULATION
The DSMC method for rarefied gas flow simulation is particu-
larly suitable for pressure regimes well below 100 Pascal, i. e.
when continuum CFD approaches become inapplicable.
Exemplary application I: Thermal evaporation sources
optimization
Thermal evaporation is an important alternative to plasma
assisted deposition processes, particularly when substrates or
precursors are sensitive to damage by high-energy particles.
Vaporization of large areas with high uniformity is challenging,
though. For efficient development of evaporation sources it
is essential to have a model-predictive simulation tool which
takes the interaction between the evaporated species and the
background gas into account. However, modeling of evapora-
tion is often performed by superposition of multiple sources
with cosine emission profiles via ray-tracing even in recent
literature; in such models collective effects are switched off
by definition. With DSMC simulation results are more reliable
provided that all relevant reaction channels, i. e. interactions
between evaporated species and background gas, are incor-
porated via corresponding cross sections. Two-dimensional
DSMC simulations have been carried out to investigate the
collective effects mentioned above. Thus, they could be
considered – and eventually put to use – in the development
process of line sources for thermal evaporation.
Exemplary application II: Sputter simulation for shaper
optimization
Optical high-precision filters are important core elements in
many special applications, for example spectroscopic instru-
ments in space technology, high-performance optical fibers,
UV photolithography or optical high-precision analysis. With
the ongoing specialization of coating products, the number
of required layers increases leading to ever higher demands
made on homogeneity and reproducibility. DSMC simulation
of gas dynamics and particle transport in sputtering processes
has been carried out in order to optimize processes and gain
a better understanding of them. In this way the influence of
the geometry of installations and masks on the coating profile
could be identified and quantified. This allows for shaper
optimization on a purely virtual basis which could be validated
successfully in subsequent experiments.
Measured and simulated deposition rate.
50080
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90
95
100
105
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115
120
550 600 650 700
Rela
tive
rate
var
iatio
n [%
]
Radial position [mm]
Without shaper:
With shaper (2nd version):
SimulationMeasurement
SimulationMeasurement
Numerically optimized shaper: Measurement
inward outward2 Simulated pressure profile
in aluminum vaporization
from four linear sources at a
vapor pressure of 1 mbar.
3 Simulated absorption
profile of a dual magnetron
sputter set-up.
2 3
PIC-MC SIMULATION
The PIC-MC simulation was optimized for gas discharges in a
low-pressure and low-temperature regime generally, and for
plasma-based deposition processes specifically. The simulation
provides an effective analytic and development tool for
corresponding processes like magnetron sputtering or plasma
activated chemical vapor deposition.
Exemplary application I: Optimization of magnetron targets
Local plasma density aggregations, notably occurring in
the curved sections of linear magnetron targets, are a main
factor in limiting service life. Ways of dealing with such
erosion anomalies are primarily focused on magnetic field
amendments. This is also the case when the target racetrack
is widened which is first and foremost determined by electron
confinement in the magnetic field. PIC-MC simulation
together with BEM magnetic field computation should
provide all relevant information to improve magnetron targets
accordingly. To this end, various magnetron configurations
were computed with BEM and it was analyzed whether the
magnetic field shape fulfills basic requirements. In a subse-
quent step, 3D PIC-MC simulations were performed with the
corresponding magnetic field and geometric set-up mapped
into the simulation domain. From the stationary state of the
PIC-MC simulation, the ion current onto the target surface
was extracted. These data predominantly allow for conclusions
about the empiric erosion profile for the corresponding
magnetron layout. Thus, PIC-MC simulation could be used to
validate recent optimization and lead on further optimization
steps. With this approach large-scale magnetron targets have
been re-designed and implemented successfully, i. e. showing
homogeneous erosion across the whole target racetrack.
Exemplary application II: Simulation of RF-superimposed
DC sputtering
Magnetron sputtering is especially suited for industrial
up-scaling towards large-area and high-throughput processes
with high precision and reproducibility. Thus, it is an attractive
method within semiconductor technology e. g. for fabrication
of metal contact structures. However, high-energy charged
particles in magnetron discharges may lead to severe plasma
damage within the coated devices. The flux and energy
distribution of ions depend in a complicated manner on
the coater geometry, the magnetic configuration of the
target and on process parameters such as total pressure and
electric excitation frequency. In order to investigate these
dependencies a practical magnetron sputtering set-up for
Ag contact structures has been mapped into the PIC-MC
simulation environment. The deposition process is carried
out by RF-superimposed DC sputtering of Ag targets in pure
Argon. Since these PIC-MC simulations are computationally
quite intensive, they have been performed on GRID resources,
which are part of the German GRID initiative (D-GRID).
According to an experimental variation study, the ion energy
distribution functions resulting from different set-points of RF
fraction, total pressure, power and magnetic configuration are
determined in a series of PIC-MC simulation runs. The experi-
mentally observed ion damage could be related to features of
the simulated ion energy distribution.
4 Planar magnetron
discharge simulation.
5 Cylindrical magnetron
discharge simulation.
4 5
6 Magnetron sputtering
chamber and corresponding
finite element mesh model.
Features Implementation Remarks
Linux OS – all flavors (Debian, SUSE, etc.)
GNU g++ compilation
No Windows OS support in near future
Parallel computing Domain decom-position and MPI communication
Capable to run on large clusters equipped with grid schedulers
Electric field solver(plasma simulation)
Taylor extrapolati-on method (TEX) – iterative solving with SOR
Quasi-stationary solution and thus not feasible for microwave plasma simulation
Magnetic field solver(plasma simulation)
Boundary element method (BEM) – direct solving with Gaussian elimina-tion
Permanent magne-tic fields only, i. e. magnetrons and DC coils
Finite element mesh modeling
Cut-plane compu-tation for triangular elements in a regu-lar grid
Use of open-source grid generator GMSH
External CAD file import Import into open-source grid genera-tor GMSH
GMSH supports IGES and STEP file formats
Scaling functionality Inhomogeneous grid spacing; par-ticle splitting and merging
Automated refinement will be implemented in the medium term
Cross section data base Fit functions from literature and expe-rimental data
Reliable data hard to come by for some complex molecules, e. g. Alq3, HMDSO, CH4 , etc.
DSMC / PIC-MC features.Features of the DSMC / PIC-MC simulation software
The DSMC / PIC-MC simulation software from Fraunhofer IST
stands out on account of its efficient parallelization scheme.
All runtime constituents, including the electric and magnetic
field solvers, are massive parallel implementations. This allows
workload and memory consumption for gas flow and gas
discharge simulations to be distributed over any number of
networked CPU cores, with networked computing clusters
also being supported. An automated distribution method for
capacity utilization of all CPU cores ensures optimal parallel
computing efficiency. Additional scaling functionalities such
as inhomogeneous cell spacing and particle splitting / merging
schemes make it possible to handle workload variations over
several orders of magnitudes – with adequate performance.
This allows for more demanding simulation set-ups, e. g.
magnetron discharge simulations bearing three-dimensional,
industrial-sized recipient models. A further special feature of the
DSMC / PIC-MC simulation software is the geometric modeling
approach where finite element mesh models are mapped
onto a regular computational grid. Hence, decent contour
approximation with finite elements is provided without losing
the computational performance of a regular grid. For geometric
modeling any commercial or open-source CAD system (e. g.
GMSH, Solid Edge, Autodesk) can be used if common output
formats like STEP or IGES are supported. Parameter input files
are generated automatically from geometry and finite element
mesh models. These parameter files can be adapted to the
individual problem with little effort. Automated consistency
checks simplify the usability by catching input errors, such as, in-
sufficient grid resolution or time step width, right from the start.
The DSMC / PIC-MC simulation software has been extensively
tested over several years and is continuously extended to ensure
reliability, efficiency and usability.
6
2.28e+016
2.28e+018
Ar+ Absorption [1/m²/s]
ContaCtFraunhofer Institute for Surface
Engineering and Thin Films IST
Bienroder Weg 54 E
38108 Braunschweig
Dr. Andreas Pflug
Group Manager Simulation
Phone +49 531 2155-629
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