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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95

100

105

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115

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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.

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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

andreas.pflug@ist.fraunhofer.de

Phone +49 531 2155-629

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