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Topic*: 6. Solidification, crystal growth
MODEL EXPERIMENTS FOR HEATER CONCEPTS
IN CZOCHRALSKI CRYSTAL GROWTH PROCESSES
J. Pal*, A. Enders-Seidlitz, and K. Dadzis
Leibniz-Institut für Kristallzüchtung (IKZ), Max-Born-Straße 2, 12489 Berlin, Germany
*E-mail: josef.pal@ikz-berlin.de
Key words: multiphysics, crystal growth, Czochralski method, model experiment, induction
heater
Crystalline materials are produced in complex high-temperature processes involving a
large variety of physical phenomena from heat transfer to fluid dynamics. A new research group
"Model experiments" has been established at the IKZ recently. Within the NEMOCRYS (Next
Generation Multiphysical Models for Crystal Growth Processes) project funded by a Starting
Grant from the European Research Council (ERC), we are developing a new generation of
multiphysical models for crystal growth processes. This shall be achieved by a series of unique
crystal growth setups (model experiments) for materials with low working temperatures,
relaxed vacuum-sealing requirements and hence by easy experimental access for various in-situ
measurement techniques. Model experiments for crystal growth are of growing interest [1-6]
and allow one to investigate selected physical phenomena extracted from the complex real
crystal growth process. However, in previous studies, mostly a very limited number of physical
aspects have been addressed in such experiments (e.g., only melt flow).
The Czochralski (CZ) growth technique is widely applied in crystal growth, both with
induction heating and graphite resistance heaters. Each of these types of heater exhibits varying
energy efficiency, electromagnetic forces, temperature (in)stability in the melt, diameter
(in)stability during the growth process, relevant heat flows, as well as in-situ sensor access to
the process. In this study, we investigate the advantages and disadvantages of both heating
concepts.
Figure 1: Picture of the experimental setup with sensors and induction heater (left); mounting of the
graphite heater (center) with surrounding thermal insulation (right).
The model experiment setup (see Figure 1) consists of a vacuum furnace equipped with
an optionally rotatable crucible support surrounded by a heater. The crucible is made of graphite
and filled with tin (TSn,m = 232 °C) – the model material used for this experimental study. A thin
tin rod is used as seeding crystal. The setup is equipped with sensors of several types. Both
contact (thermocouples, resistance PT type) and contactless temperature sensors (pyrometer,
infrared camera) are used to measure and record the temperatures at various positions. The
complete growing process is recorded by common and high-speed optical cameras for
documentation purposes. Additionally, heater current is measured continuously.
First experiments with the presented setup applied the induction heater, and variations of
pulling velocity, crystal rotation, and pressure condition (furnace is evacuated or its door is
open with normal air pressure) were investigated. Figure 2 shows an optical and an infrared
image recorded during the process, and the diagram at the right side displays a typical
temperature recording. The very prominent orange-colored temperature was recorded by a
thermocouple placed very near to the induction coil; it exemplarily demonstrates a case where
the sensor positioning has to be optimized. We will further discuss the influence of the different
heater types on both the measurement setup and the growth process.
Figure 2: Pictures recorded during the process (left with the optical and center with the infrared
camera); temperature diagram of sensors inside the melt and the crucible wall (right).
In a next step, the obtained in-situ measurement data will be applied for validation of
numerical simulation of the process. For this purpose, a new thermal model of the Czochralski
process using the open source software Elmer FEM is currently being developed by the
NEMOCRYS project group. One aim will be to validate the convective cooling boundary
conditions for solid surfaces and the crystal diameter calculation. Furthermore, the presented
model experiments may be employed to optimize the techniques for in-situ observation as well
as to build a new basis for model benchmarking or for big data application in crystal growth.
Acknowledgements. This project has received funding from the European Research Council
(ERC) under the European Union’s Horizon 2020 research and innovation programme (grant
agreement No 851768).
References
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[3]
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