Czochralski crystal growth

Introduction

Method of crystallization worked out by Czochralski(CZ) in 1916 proves to be very useful for

crystallization of many intermetallics. Czochralski invented this simple method for crystal

growth during the experiments measuring the crystallization velocity of metals. The idea of this

method was based on pulling of fibers of different metals from their melts. The obtained in such

way metallic wires proved to be single crystals. The results of the experiments Czochralski

published in Zeitschrift fr Physikalische Chemie in 1918. This new technique allowed him to

obtain the good quality single crystals of pure metals like Sn, Pb, and Zn grown in air. Later this

method was adopted for crystal growth of semiconductors, salts, and many oxide crystals

(LaAlO3, YAG, .and GGG etc) for electronic and other applications.

Crystal growth process

In CZ-growth, a silica crucible (SiO2) is filled with undoped electronic grade polysilicon. The

dopant is introduced by adding pieces of doped silicon (for low doping concentration) or

elemental dopants P, B, Sb or As (for high doping concentration). The crucible is heated in

vacuum to ca. 1420 C to melt the silicon (Figure 1). A single-crystalline seed of known crystal

orientation is dipped into the silicon melt. The silicon solidifies into a crystal structure

determined by the seed crystal. A thin neck is quickly drawn to suppress the defects that develop

because of a large temperature difference between the seed and the melt, and then the pulling

rate is lowered. Both the ingot and the crucible are rotated (in opposite directions); ingot rotation

is ca. 20 rpm and crucible rotation about 10 rpm. The ingot diameter is determined by the ingot

pull rate. The pulling rate is limited by heat conduction away from the crystallization interface,

and therefore large-diameter ingots have lower pulling rates. While a 100mm diameter ingot can

be pulled at 1.4 mm/min, the 200mm ingot pull rate is 0.8 mm/min. In order to grow low

vacancy concentration crystals, pulling rates as low as 0.35 mm/min are employed. Typical

pulling time is 30 h, not including heating and cooling, which add another 30 h to the process,

for 200mm ingots. The ingot length is determined by the yield strength of silicon neck and

crucible size. The thin neck is not a perfect material as it has defects arising from thermal shock,

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and torsional forces are also acting on it. Silicon yield strength is significantly lower at high

temperatures, but 300mm ingots can weigh up to 300 kg. Not all EGS can be utilized: ca. 10% of

the original polysilicon remains in the crucible. The crucibles cannot be reused; they are

extremely expensive disposable objects.

Crystal purification

There is an inevitable contamination of the growing crystal from the materials that are essential

to the growth set-up: the silica crucible is slightly dissolved during the crystal growth process,

and therefore oxygen is always present in CZ-silicon in concentrations of 5 to 20 ppma

(according to ASTM standard F121-83). Some of the Oxygen evaporates as SiO gas (silicon

monoxide) and is transported around the vacuum vessel. Electronic grade silicon (EGS) is

Figure 1: Czochralski crystal pulling: silicon (melting point 14140C) solidifies as it is pulled

up. Pulling speed (~ mm/min), ingot rotation speed (20 rpm) and crucible counter rotation

speed (10 rpm) together determine the ingot diameter.

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extremely pure, for instance, boron, phosphorous and iron levels can be as low as 0.01 to 0.02

ppb. However, the crucible is a source of impurities, and for boron, sodium and aluminium, it is

the crucible and not the EGS that determines the ingot purity. If synthetic silica is used for the

crucibles, much higher purity CZ-ingots can be pulled.

Conclusion

In the early days of the technology, the boules were quite thin, only a few inches wide. However,

the crystal growers have had a lot of practice, and nowadays they can make nice, fat 300mm (12-

inch) wide boules. The thickness is controlled by precise control of the temperature, the speeds

of rotation and how fast the seed holder is withdrawn. Widths of 400mm (16 inches) are

expected in the next several years. This is one reason for the rapidly decreasing cost of chips that

we have enjoyed over the years, because more LSI chips can be created from a single wafer with

the same number of fabrication process steps.

The electrical characteristics of the silicon are controlled by adding stuff like phosphorus or

boron to the silicon before it is melted. The stuff added is called dopant and the process is called

doping. This method is also used with semiconductor materials other than silicon, such as

gallium arsenide.

As a necessary step in the production of large-scale integrated circuit chips, the Czochralski

method is a basic technique in the making of computers, TVs, cell phones and the advanced

electronic equipment of all kinds that shape modern life as we know it at the beginning of the

21st Century.

Reference

1. Talik, E. and Oboz, M., Czochralski method for crystal growth of reactive intermetallics Acta Phys. Polon. A, Vol. 124 No. 2, (2013).

2. Li, Z.; Kraner, H.W.; Verbitskaya, E.; Eremin, V.; Ivanov, A.; Rattaggi, M.; Rancoita, P.G.; Rubinelli, F.A.; Fonash, S.J. et al. (1992). "Investigation of the oxygen-vacancy

(A-center) defect complex profile in neutron irradiated high resistivity silicon

junction particle detectors". IEEE Transactions on Nuclear Science 39 (6): 1730.

3. Franssila, S., Introduction to Microfabrication John Wiley & Sons Ltd, 2004.

4. http://www.bbc.co.uk/

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