For crystal growth of semiconductor materials a short-term temperature stability of 0.1 C at 1500 C is one of the essential parameters to be addressed for achieving high-quality crystals. Hence, for temperature monitoring and control with high precision in a floating zone furnace two sets of thermo-sensors, type B thermocouples and optical fibre thermometers, have been implemented and successfully operated in the furnace for more than 2000 h. The optical fibre thermometers consist of an optical system made of sapphire (two fibres plus a prism in between for deflection) and transmit the infra-red radiation of the heater to the outside of the hot core of the furnace for pyrometric temperature measurement. A dedicated control algorithm has been set up which controlled the power settings to the individual heaters. Both sensor types showed no degradation after this period and yielded a short-term stability at 1200 C of 0.05 C (optical fibre thermometers), respectively 0.08 C (thermocouples)

Breuer, D.

Croell, A.

Hess, A.

Sauermann, H.

Stenzel, Ch.

NASA Technical Reports Server

HIGH-PRECISION TEMPERATURE CONTROL OF A CRYSTAL GROWTH 
FURNACE AT 1500°C 
 
Ch. Stenzel1, A. Hess2, A. Cröll2, D. Bräuer3, H. Sauermann3 
1Astrium GmbH, D-88090 Immenstaad 
2Kristallographie, Institut für Geowissenschaften, Universität Freiburg, D- 79104 Freiburg 
3Institut für Automatisierungstechnik, TU Bergakademie Freiberg, D- 09596  Freiberg 
 
Keywords: Temperature control, Crystal growth, High-temperature furnace 
 
 
Abstract 
 
For crystal growth of semiconductor materials a short-term temperature stability of 0.1°C at 
1500°C is one of the essential parameters to be addressed for achieving high-quality crystals.  
 
Hence, for temperature monitoring and control with high precision in a floating zone furnace two 
sets of thermo-sensors, type B thermocouples and optical fibre thermometers, have been 
implemented and successfully operated in the furnace for more than 2000 h. The optical fibre 
thermometers consist of an optical system made of sapphire (two fibres plus a prism in between 
for deflection) and transmit the infra-red radiation of the heater to the outside of the hot core of 
the furnace for pyrometric temperature measurement. 
 
A dedicated control algorithm has been set up which controlled the power settings to the 
individual heaters. Both sensor types showed no degradation after this period and yielded a short-
term stability at 1200°C of 0.05°C (optical fibre thermometers), respectively 0.08°C 
(thermocouples). 
170
https://ntrs.nasa.gov/search.jsp?R=20120015601 2019-08-29T18:02:23+00:00Z
Introduction 
 
For float zone crystal growth experiments onboard the International Space Station (ISS) the 
implementation of a dedicated heater insert for the Materials Science Laboratory (MSL) is 
envisaged - the Float Zone Furnace with Magnetic Field (FMF) [1]. One of the essential 
parameters to control inhomogeneities in the crystal growth of semiconductor materials is the 
short-term temperature stability (in the range of several minutes) of the furnace [2]. To set up a 
high-precision temperature control system requires an appropriate combination of fast actuators 
(heaters) and sensors as well as the implementation of a sophisticated temperature control 
system. This furnace is especially designed to provide enhanced short-term temperature stability 
in addition to the ability of adjusting the temperature gradient at the solid-liquid interface over a 
wide range. The specification on the short-term stability of the furnace was set to T < 0.05°C. In 
a laboratory model of this furnace the above mentioned features have been addressed in the 
design and the performance has been verified in an experimental test programme. 
 
In the following sections of this paper the detailed design of the respective subsystems of the 
FMF furnace to achieve a short-temperature stability  - the heating system, the thermometry and 
the control system - will be introduced, the results of the performance tests reported, and a 
summary given. 
 
 
Heating system 
 
In order to provide a zone profile at 1500°C with a variable gradient at the solid/liquid interface, 
the FMF is laid out as a furnace with 7 individually controlled heating zones. In the area of the 
central zone the melt zone is established. Figure 1 shows a conceptual sketch of the FMF.  
 
The single heaters consist of encapsulated graphite heaters where a meandered graphite layer 
(50 µm thick) is deposited onto a pBN substrate. The heating layer is covered by a second pBN 
CVD layer to protect it against mechanical, electrical, and chemical injuries. The resulting 
thickness of wall of less than 2 mm provides a rather low thermal mass enabling fast heating rates 
and temperature changes. Due to the excellent material compatibility between the different 
materials and similar values for thermal expansion this type of heater can bear up a power density 
of more than 30 W/cm2 leading to a compact design for high-temperature applications and also to 
long operation times of more than 2000 h. Figure 2 shows a photograph of the central heater of 
the FMF. 
 
171
 
 
Figure 1. Conceptual sketch of the FMF exhibiting the inner structure of the furnace: The 
furnace provides a symmetrical thermal profile with seven individual heating 
zones. Six zones are placed symmetrically at both sides of the central heater (on 
each side a cooling zone, a support, and a plateau heater is placed). 
 
 
 
 
 
Figure 2. Encapsulated graphite heater for the central zone of the FMF: The thermal load of 
1000 W provides a maximum temperature of 1500°C in this area 
 
 
To improve the radial homogeneity of the temperature profile in the FMF tubular thermal 
diffusors for each zone consisting of a graphite tube have been employed; thereby the heaters are 
placed directly around the diffusors tubes.  
172
Thermometry 
 
For temperature measurement and control each zone is equipped with two thermocouples. 
Additionally, one optical sensor is employed in the three inner zones of the furnace. The 
temperature control system of the furnace could thereby use signals of both sensor types as input.   
 
Thermocouples 
 
The application of sheathed thermocouples represents the standard technique for temperature 
measurement and control in furnaces. The achieved stability amounts to about 0.1 K [3]. For 
temperature above 1200°C only type C (W-Re) or type B (Pt-Rh) thermocouples can be applied. 
Because of the better drift behaviour type B thermocouples with the following specifications 
have been selected: 
 Type B (Pt-PtRh) 
 Insulation material: MgO 
 Sheath Pt (: 1,0 mm) 
 Protection tube Ta (: 1,6 x 0,1 mm) 
 
The thermocouples were implemented into axial borings in the diffusors. In order to prevent a 
degradation of the Pt sheath due to a reaction with the graphite diffusor they were inserted into 
protection tubes of Ta. The compatibility of this protection tube with a graphite environment has 
been tested in advance for several hours at 1600°C in vacuum (1 x 10-5 mbar). No drifts or 
degradation have been observed. 
 
Optical Sensors 
 
According to Planck's law higher temperature resolution can be obtained by utilising optical fibre 
thermometry with sapphire fibres at elevated temperatures [4, 5]. These sensors consist of an 
optical waveguide to transmit the IR radiation emitted from a small spot of the object to be 
measured out of the hot region to a photo detector which converts the radiation signal into a 
temperature reading following Planck's law. For application in high-temperature facilities these 
waveguide are made of sapphire. Using straight fibres in a 4-zone laboratory furnace a short-term 
stability of 0.015°C has been achieved [6]. 
Due to constructional boundary conditions of the MSL facility the sensors may not be routed in a 
straight line within the furnace, one 90° bend of the sensors has to be included. For the FMF 
optical sensors this is realised by the Optical Deflection System (ODS) which is built up by 
straight sapphire waveguides. The path of rays is being deflected by a sapphire prism which is 
embedded in a support structure out of ceramic. To enhance transmission by focussing a sapphire 
lens is implemented behind the prism. Similarly to the thermocouple design the sapphire fibres 
are integrated into protection tubes out of alumina. The distance to the heater surface amounts to 
2 mm. The final transmission for the ODS was measured to amount to 40%. Three out of 7 
heating zones of the FMF have been equipped with an ODS as illustrated in Fig. 3.  
 
173
 
 
Figure 3. Example of an ODS sensor: The short arm is directed to the hot side, the long arm is 
routing the IR signal out of the hot core; at its end a conventional quartz fibre leads 
the signal to a Si photo diode located outside the furnace. 
 
 
 
Temperature Control Algorithm 
 
The developed high-precision temperature control system is based on a holistic view of the 
furnace, reflecting sensors, heaters, software, and control algorithm. Since the individual zones 
are not adiabatically separated from each other, an initial temperature difference between the 
zones causes thermal fluxes which affect the temperatures of the zones again. Hence, there exists 
a strong thermal coupling between zones and the furnace is to be regarded as a multiple-input 
multiple-output (MIMO) plant. In order to develop an optimised feedback controller the thermal 
properties of each zone as well as the couplings have to be known exactly. In a first step a 
theoretical furnace model based on conduction and radiation between the zones is established, 
and then the unknown parameters are experimentally determined by process identification. This 
means measuring the dynamic temperature changes as response to well-defined power signals 
(Pseudo Random Binary Signal). These semi-empirical data represent the input for the parameter 
estimation algorithm which finds optimised model parameters. The obtained model is basis for 
feedback controller synthesis [7]. The final controller consists of a separate, standard PID 
(Proportional-Integral-Differential)-algorithm for each zone with pre-filter, pilot control, 
decoupling and anti-reset-wind-up. This structure and the V-canonical plant are depicted in 
Fig. 4. A V-canonical structure means that the loop interactions are regarded as feedback 
couplings, hence modifying the input signal. 
174
  
Figure 4. Structure of the feedback controller and the V-canonical MIMO plant as example 
for two zones: Thereby the power setting to zone I depend on the temperature of 
this zone and of its neighbouring zones via the thermal coupling which have to be 
experimentally determined. 
 
Tests 
 
At 1200°C the short-term stability has been investigated for both type of sensors control 
(thermocouples or ODS). The central heaters were set to 1200°C and the cooling zone to 1100°C 
to establish a temperature gradient of 50 K/cm. The test was performed over a time period of 18 h 
(ODS) and 20 h (TC). In the central zone temperature fluctuations of T = 0.05°C could be 
achieved for control with the ODS sensors, and T = 0.051°C for control with thermocouples as 
shown in Fig. 5. In the cooling zones even better values of T = 0.026°C (ODS) and T = 
0.05°C (TC) could be obtained. 
 
 
 
 
Figure 5: Measured short-term temperature stability for control with ODS sensors (left) 
and with thermocouples (right) 
Pilot control
Pre-
filter
PID-
control
Pilot control
Pre-
filter
PID-
control
Static de-
coupling
Static de-
coupling
-
-
-
-+
+
Saturation
Saturation
-
-
Thermal 
coupling
Thermal 
coupling
Zone i
Zone i+1
Plant
175
 
 
Conclusion 
 
Temperature control within a multi-zone furnace for semiconductor crystal growth has been 
realised with thermocouples and optical sensors as input signals for the control algorithm. With 
both systems a superior short-term stability of less than T < 0.05°C could be achieved in a 
multi-zone high-temperature furnace. No significant performance difference between the two 
types of sensors could be detected in the investigated temperature range. However, for 
environments with strong EMC perturbations the optical sensors reveal some advantage. 
 
 
 
References 
 
[1] Hess, A., Cröll, A., Stenzel, C., Bräuer, D., Sauermann, H., Uhlig, V., Floating Zone 
Furnace with Rotating Magnetic Field. Poster Presentation DGKK 2010 
[2] Müller, G., Convection and Inhomogeneities in Crystal Growth from the Melt, in: 
Crystal Grwoth, Properties and Applications, Vol. 3, III-V Semiconductors, Springer-
Verlag, Berlin, Heidelberg, New York, 1980 
[3] Sauermann, H., Schneider, K., Janda, O., Gröll, L.,Bretthauer, G., Linn, H., Das 
Gradient-Freeze Verfahren und seine regelungstechnische Umsetzung. 39. IWK der TU 
Ilmenau, 1994, p. 809-817 
[4] Schietinger, C.: Temperature measurements using optical fibers, in: Thermal 
Measurements in Electronics Coolings, ed. K. Azar CRC Press, 1997, Boca Raton 
[5] Dils, R. R., J. Appl. Phys, 54, 1983, p. 1198 
[6] H. Sauermann, Ch. Stenzel, S. Keesmann, B. Bonduelle, Cryst. Res. Technol. 36, 2001, 
p. 1329-1343 
[7] Hermann, K., Rehkopf, A., Sauermann, H., Betrachtungen zur Identifikation einer 
Gradienz-Freeze-Anlage zur Einkristallzüchtung, Automatisierungstechnik, Vol. 54 
2006, p. 566-573 
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High-Precision Temperature Control of a Crystal Growth Furnace at 1,500 C

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