Download Efficient production of monocrystalline semiconductors

Projektinfo 01/2015
Detailed information on energy research
Efficient production of mono­
crystalline semiconductors
New procedure simultaneously crystallises up to nine gallium
arsenide single crystals for electronics and photovoltaics
Modern communication technologies such as mobile
phones, PCs and transmitters operate thanks to high-quality,
high-purity crystals. The electronic circuits and microprocessors in the devices are based on monocrystalline semiconductors made from silicon, germanium or gallium arsenide.
In a cost- and energy-intensive process, the semiconductor
material is pulled from the melt in the form of monocrystalline
ingots. A Freiberg-based semiconductor manufacturer has now
developed a new process that enables up to nine crystals to be
grown simultaneously in parallel. This reduces the specific
energy consumption for the production by two thirds. The
new technological principle can be applied to growing other
semiconductor substrates.
This research project
is funded by the
Federal Ministry for Economic Affairs
and Energy (BMWi)
Semiconductor single crystals such as silicon, germanium or gallium arsenide (GaAs)
are among the purest materials of our time. In such single crystals, the atoms are
almost perfectly aligned in a grid, whereby only about every millionth is a foreign
atom. The additional incorporation of foreign atoms (doping) specifically compensates
for this and produces the desired semiconductor properties.
In addition to silicon, which dominates semiconductor technology, gallium arsenide has become a basic material for microelectronics. Its high electron mobility
and direct band transition make the material highly suitable for high-frequency
transmission applications and optoelectronics. GaAs can be used, for example, to
produce amplifiers for mobile phones and laptops in order to enable wireless
communication – or for solar cells, light emitting diodes and lasers to produce
infrared, red and orange light.
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BINE-Projektinfo 01/2015
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Riegel vor Sanierung
Verbrauch
Riegel 1 WB
Riegel 2 WB
Riegel 3 WB
Bedarf
Einsparung
Verbrauch/Verbrauch
view
port
Einsparung Bedarf/Verbrauch
ambient gas
water cooled pressure vessel
seed
melt
encapsulant (boron oxide)
crucible (boron nitride)
heater system
crystal
encapsulant (boron oxide)
seed
heat insulation
melt
Fig. 1 View into the growth crucible for
growing 9 x 100 mm crystals
Fig. 2 Schematic representation of the crystallisation process.
Left: LEC (Liquid Encapsulated Czochralski), right: VGF (Vertical Gradient Freeze)
Crystal growth processes are becoming more efficient
The technology for growing single crystals has developed
rapidly during the last 70 years. Initially, the crystals only
achieved a weight of just a few grams and a diameter of
just a few centimetres. Today, silicon single crystals can
be produced that weigh several hundred kilograms and
are up to 45 cm in diameter. Most semiconductor crystals
are grown from the melt at temperatures between around
900 °C and 1,500 °C. The processes are very energy-intensive.
The “Liquid Encapsulated Czochralski” process (LEC) was
developed in the 1960s to grow gallium arsenide single
crystals. The decomposition pressure at the melting point
means that the growth is only possible under an encapsulant. The crystal is pulled at a speed of around 7 mm/h
from the crucible. This process takes two to three days,
and during this time the melt must be maintained at
temperatures of up to 1,300 °C.
At the turn of the millennium, “Vertical Gradient Freeze”
(VGF) technology became prevalent. This method enables
crystals to be produced with greater perfection. It has
the disadvantage, however, that the crystals only harden
about half as quickly as the LEC method. This increases
the energy requirement per process (Fig. 2).
In order to achieve a crystallographic structure that is as
undisturbed and defect-free as possible, the temperatures
must remain as constant as possible during the growth
process. In addition, there should be no melt-back phenomena during crystallisation.
Parallel growth of several crystals saves energy
The Freiberg-based developers had already tested a new
multicrystal growing process for GaAs at the turn of the
millennium, in which several single crystals can be grown
in a crystal puller. The idea of parallelising slow processes was adapted here for the first time for growing semiconductor single crystals with diameters between 150
and 200 mm. With this so-called “Hutofen” (hat furnace)
method, the crucibles are placed between two horizontal
hot plates that are supported by lateral heaters (Fig. 3).
FCM further developed this method under productionoriented conditions in the EcoCrys project, in which the
modules and the crystal growth software were tailored
to fivefold parallel operation. This enabled the specific
energy requirement for manufacturing GaAs crystals and
substrates to be substantially reduced from 0.2 to 0.05 kWh
per square centimetre of wafer surface. Today it is now
even significantly lower than the faster LEC method.
Recipient
Insulation
Upper heater
Crucible with
seed, melt
and crystal
Side heater
Crucible support
Lower heater
Fig. 3 Comparison of the standard VGF process with the VGF method
using panel heaters and several crucibles
The developers gradually enlarged the structure of the inner crystallisation
zone from 3 to 5, and then up to 9 crystals. In contrast to a single crystal
puller, temperature differences occur between the puller‘s central and peripheral areas that affect the crystal formation process. To compensate for
this, FCM changed the heat supply: for more than three crucibles the energy
is mainly supplied from the top and extracted from the bottom, which optimises the control of the temperature field. The developers also simultaneously succeeded in increasing the solidification rate, yield and the weight
of each single crystal up to 18 kg, so that the total weight currently amounts
to 90 kg.
The crystal puller requires only slightly more energy (approx. 10 %) for growing
three, five or nine crystals than it does for growing one crystal. The changed
growing structure and the modified process times have improved the energy
efficiency by a factor of 2.9.
Dr Berndt Weinert, R&D Manager at FCM: “The method has proved itself in
practice and the production has almost completely moved over to using it.
An estimated 4,000 MWh were saved in 2014.”
The investment and operating costs, dimensions and energy and cooling water
consumption are only slightly higher than with conventional crystal pullers.
The new method makes it possible to retrofit existing machines or to use
new, more effective ones. The investigations showed that the proprietary
crystal growth puller can also be used in principle for growing other substances. Using indium phosphide (InP) and gallium phosphide (GaP), it has
been experimentally shown that the furnace principle can also be used for
Rezipient
BINE-Projektinfo 01/2015
+
6 µm Epi
InGaP 1.89eV
p-dotierte Schicht
GaAs 1.42eV
InGaAsNSb 1eV
SYNTHESIS
GaAs- oder
Ge-Substrat
Gallium /
Arsenic
High-pressure
synthesis of GaAs
laseraktiver
Bereich
(Quantentopf)
CRYSTAL
MECHANICAL
FINAL WAFERING
GROWTH
WAFERING
n-dotierte Schicht
Crystal
grindingSubstrat
Annealing
LEC/VGF crystal
growth
Customer
view port
Packaging /
Certification
Wire sawing
How crystals and diamonds grow
crystal
melt
Edge
rounding
Cleaning crucible
(boron nitride)
Etching /
Cleaning
encapsulant
Polishing
(boron oxide)
Fig. 4 Production chain from the GaAs synthesis to the wafer,
which provides the basis for manufacturing components.
+
6um Epi
InGaP 1.89ev
p-cladding
GaAs 1.42eV
InGaAsNSb 1eV
GaAs or Ge
substrate
Whether diamond, rock crystal or semiconductor crystal,
extreme conditions such as heat and high pressure
are usually required to ensure that monocrystals are
formed.
As with rock crystal, which is formed during the cooling
of silicate-rich
melts, semiconductor single crystals are
ambient
gas the melt. The high melting temperature
also grown from
and the slow crystal growth make the production very
energy-intensive.
seed
The production of artificial diamonds is based on the
creation of natural diamonds during prehistoric times
at a depth of more than 130 kilometres below ground
under high temperatures and pressure. At about
2,000 °C and 8,000 bar, graphite is deposited on a
diamond seed crystal. Within days a diamond grows
that is almost indistinguishable from ones that have
grown naturally.
encapsulant
(boron oxide)
crucible
(boron nitride)
melt
crystal
seed
active zone
(quantum
wells)
n-cladding
substrate
Fig. 5 Schematic showing the structure of GaAs multi-junction
solar cells (left) and lasers (right)
Fig. 6 GaAs crystals produced in the EcoCrys crystal
materials with a high vapour pressure of more than 20 bar. Based on the
results with gallium antimonide (GaSb), the researchers conclude that it is
also suitable for different substances at atmospheric pressure.
High-performance components made of GaAs wafers
operate as power amplifiers and switches, for example
in mobile phones, Wi-Fi-enabled devices as well as in
communications and data networks.
Because of its electrical properties, GaAs is also used in
many optoelectronic devices such as solar cells, lasers
and LEDs (Fig. 5).
In multi-junction solar cells, GaAs helps to optimally convert the incident radiation into electrical energy. At the
end of 2014, a four-junction solar cell achieved a record
efficiency of 46 per cent. GaAs cells are very temperature-resistant and resistant to UV radiation.
Further improving the crystallisation and process chain
In order to improve the VGF method further, FCM and the Institute for Crystal
Growth (IKZ) want to launch a follow-up project in which they will investigate
how the flow and temperature field of the melt can be better controlled. Here
additionally created travelling magnetic fields or vibration can be used to
improve the crystallisation conditions by means of targeted forced convection. The IKZ had already shown that this influences the growth front with the
crystallisation of semiconductor materials. Compared with the conventional
method, a downward-oriented travelling magnetic field minimises the local
thermal stresses and improves the crystal quality.
The researchers also want to illuminate the entire process chain for producing
the GaAs substrate in terms of possible savings, including for the raw, auxiliary and operating materials.
GaAs in microelectronics, photovoltaics and laser technology
The GaAs single crystals are processed into wafers. To adjust the electrical
properties, the material is doped with carbon, silicon, tellurium or zinc –
this results in an n-type or p-type semiconductor. Ordered crystal growth
also enables additional functional layers to be applied to the extremely lowimpurity wafer surfaces.
puller with a diameter of 150 mm
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The right semiconductor for every task
The market constantly demands even cheaper and more efficient semiconductor
materials. Researchers are working in many projects on improving the manufacturing
processes and on developing new materials and functions.
The semiconductor material gallium nitride (GaN) is very versatile: it is used as white
and blue LEDs in energy saving lamps, as blue laser diodes for data storage, as transistors in mobile communications and in power converters for solar and electric vehicles.
The production of large GaN single crystals is very expensive and standard growing
methods cannot be used because of the high melting point above 2,500 °C. Today GaN
components are still predominantly based on a very thin GaN layer, which is deposited
using the so-called MOVPE (Metal-Organic Vapour Phase Epitaxy) process on a carrier
substrate made of sapphire, silicon or silicon carbide. The HVPE (Hydride Vapour Phase
Epitaxy) method, on the other hand, enables thick GaN layers and freestanding GaN
single crystals to be produced. Using a newly developed HVPE crystal puller, researchers
from FCM and the Fraunhofer Technology Centre for Semiconductor Materials THM are
lowering the material consumption and production costs.
Synthetic diamond as a semiconductor material for nanoscale computers?
Diamonds, GaAs and GaN have more in common than just their energy-intensive
production. With their unique properties, these crystals can complement one another
very well: as a highly thermally conductive insulator, diamond can be additionally
doped with foreign atoms to create a semiconductor which, in conjunction with other
semiconductors, is forming a new generation of heat-resistant processors, amplifiers
and sensors. Unlike conventional silicon chips, such diamond wafers function without
cooling and withstand temperatures up to 1,000 °C.
A diamond in which boron atoms have been implanted is not only semi-conductive but
also has very good thermoelastic and mechanical properties. Several EU-funded projects
are aimed at developing quantum technologies based on synthetic diamond: applications
include quantum computers, data storage as well as magnetic and electrical sensor
technology. Another goal is to create composites made of diamond and GaN that function under extreme conditions such as high temperatures and in strong electric fields.
Application areas include electronic high-current and high-frequency components such
as power transistors.
Imprint
Project organisation
Federal Ministry for Economic Affairs and
Energy (BMWi)
11019 Berlin
Germany
Project Management Jülich
Forschungszentrum Jülich GmbH
Dr Gordon Kaußen
52425 Jülich
Germany
Project number
0327437A
ISSN
0937-8367
Publisher
FIZ Karlsruhe · Leibniz Institute
for Information Infrastructure GmbH
Hermann-von-Helmholtz-Platz 1
76344 Eggenstein-Leopoldshafen
Germany
Author
Gerhard Hirn
Copyright
Cover image and all other images:
Freiberger Compound Materials GmbH
Text and illustrations from this publication
can only be used if permission has been
granted by the BINE editorial team. We
would be delighted to hear from you.
Project participants
>> Project management: Project management: Freiberger Compound Materials GmbH, Freiberg,
Germany, Dr Berndt Weinert, [email protected], www.fcm-germany.com
>> Basic research: Institute for Crystal Growth Berlin (IKZ), Dr Christiane Frank-Rotsch,
[email protected], www.ikz-berlin.de
Links and literature
>> F raunhofer Technology Center for Semiconductor Materials THM
www.thm.fraunhofer.de Kristallzüchtung von GaN
>> F raunhofer Institute for Applied Solid State Physics IAF
www.iaf.fraunhofer.de/en / Anwendungsforschung Synthetischer Diamant
> > Element Six: Products from synthetic diamond
>> www.e6.com
> > EU Project: Materials for robust gallium nitride (MORGaN) www.morganproject.eu
>> W
einert, B.; Eichler, S.; Kretzer, U. u. a.: Entwicklung eines energieeffizienten
Kristallisationsverfahrens für Halbleiter. Abschlussbericht. Förderkennzeichen 0327437A.
Freiberger Compound Materials GmbH, Freiberg (Hrsg.). 2013. 39 S.
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