Czochralski method

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Crystallization
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The Czochralski method, also Czochralski technique or Czochralski process, is a method of crystal growth used to obtain single crystals of semiconductors (e.g. silicon, germanium and gallium arsenide), metals (e.g. palladium, platinum, silver, gold), salts and synthetic gemstones. The method is named after Polish scientist Jan Czochralski, [1] who invented the method in 1915 while investigating the crystallization rates of metals. [2] He made this discovery by accident: instead of dipping his pen into his inkwell, he dipped it in molten tin, and drew a tin filament, which later proved to be a single crystal. [3] The method is still used in over 90 percent of all electronics in the world that use semiconductors. [4]

Contents

The most important application may be the growth of large cylindrical ingots, or boules, of single crystal silicon used in the electronics industry to make semiconductor devices like integrated circuits. Other semiconductors, such as gallium arsenide, can also be grown by this method, although lower defect densities in this case can be obtained using variants of the Bridgman–Stockbarger method.

The method is not limited to production of metal or metalloid crystals. For example, it is used to manufacture very high-purity crystals of salts, including material with controlled isotopic composition, for use in particle physics experiments, with tight controls (part per billion measurements) on confounding metal ions and water absorbed during manufacture. [5]

Application

Monocrystalline silicon (mono-Si) grown by the Czochralski method is often referred to as monocrystalline Czochralski silicon (Cz-Si). It is the basic material in the production of integrated circuits used in computers, TVs, mobile phones and all types of electronic equipment and semiconductor devices. [6] Monocrystalline silicon is also used in large quantities by the photovoltaic industry for the production of conventional mono-Si solar cells. The almost perfect crystal structure yields the highest light-to-electricity conversion efficiency for silicon.

Production of Czochralski silicon

Crystal of Czochralski-grown silicon Monokristalines Silizium fur die Waferherstellung.jpg
Crystal of Czochralski-grown silicon

High-purity, semiconductor-grade silicon (only a few parts per million of impurities) is melted in a crucible at 1,425 °C (2,597 °F; 1,698 K), usually made of quartz. Dopant impurity atoms such as boron or phosphorus can be added to the molten silicon in precise amounts to dope the silicon, thus changing it into p-type or n-type silicon, with different electronic properties. A precisely oriented rod-mounted seed crystal is dipped into the molten silicon. The seed crystal's rod is slowly pulled upwards and rotated simultaneously. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt. Occurrence of unwanted instabilities in the melt can be avoided by investigating and visualizing the temperature and velocity fields during the crystal growth process. [7] This process is normally performed in an inert atmosphere, such as argon, in an inert chamber, such as quartz.

Crystal sizes

Silicon crystal being grown by the Czochralski method at Raytheon, 1956. The induction heating coil is visible, and the end of the crystal is just emerging from the melt. The technician is measuring the temperature with an optical pyrometer. The crystals produced by this early apparatus, used in an early Si plant, were only one inch in diameter. Silicon grown by Czochralski process 1956.jpg
Silicon crystal being grown by the Czochralski method at Raytheon, 1956. The induction heating coil is visible, and the end of the crystal is just emerging from the melt. The technician is measuring the temperature with an optical pyrometer. The crystals produced by this early apparatus, used in an early Si plant, were only one inch in diameter.

Due to efficiencies of scale, the semiconductor industry often uses wafers with standardized dimensions, or common wafer specifications. Early on, boules were small, a few centimeters wide. With advanced technology, high-end device manufacturers use 200 mm and 300 mm diameter wafers. Width is controlled by precise control of temperature, speeds of rotation, and the speed at which the seed holder is withdrawn. The crystal ingots from which wafers are sliced can be up to 2 metres in length, weighing several hundred kilograms. Larger wafers allow improvements in manufacturing efficiency, as more chips can be fabricated on each wafer, with lower relative loss, so there has been a steady drive to increase silicon wafer sizes. The next step up, 450 mm, was scheduled for introduction in 2018. [8] Silicon wafers are typically about 0.2–0.75 mm thick, and can be polished to great flatness for making integrated circuits or textured for making solar cells.

Incorporating impurities

A puller rod with seed crystal for growing single-crystal silicon by the Czochralski method Silicon seed crystal puller rod.jpg
A puller rod with seed crystal for growing single-crystal silicon by the Czochralski method
Crucibles used in Czochralski method Czochralski method crucibles.jpg
Crucibles used in Czochralski method
Crucible after being used Czochralski method used crucible 1.jpg
Crucible after being used

When silicon is grown by the Czochralski method, the melt is contained in a silica (quartz) crucible. During growth, the walls of the crucible dissolve into the melt and Czochralski silicon therefore contains oxygen at a typical concentration of 1018
 cm−3
. Oxygen impurities can have beneficial or detrimental effects. Carefully chosen annealing conditions can give rise to the formation of oxygen precipitates. These have the effect of trapping unwanted transition metal impurities in a process known as gettering, improving the purity of surrounding silicon. However, formation of oxygen precipitates at unintended locations can also destroy electrical structures. Additionally, oxygen impurities can improve the mechanical strength of silicon wafers by immobilising any dislocations which may be introduced during device processing. It was experimentally shown in the 1990s that the high oxygen concentration is also beneficial for the radiation hardness of silicon particle detectors used in harsh radiation environment (such as CERN's LHC/HL-LHC projects). [9] [10] Therefore, radiation detectors made of Czochralski- and magnetic Czochralski-silicon are considered to be promising candidates for many future high-energy physics experiments. [11] [12] It has also been shown that the presence of oxygen in silicon increases impurity trapping during post-implantation annealing processes. [13]

However, oxygen impurities can react with boron in an illuminated environment, such as that experienced by solar cells. This results in the formation of an electrically active boron–oxygen complex that detracts from cell performance. Module output drops by approximately 3% during the first few hours of light exposure. [14]

Mathematical form

Impurity concentration in the final solid is given by where C and C0 are (respectively) the initial and final concentration, V and V0 the initial and final volume, and k the segregation coefficient associated with impurities at the melting phase transition. This follows from the fact that impurities are removed from the melt when an infinitesimal volume dV freezes. [15]

See also

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References

  1. ↑ Paweł Tomaszewski, "Jan Czochralski i jego metoda. Jan Czochralski and his method" (in Polish and English), Oficyna Wydawnicza ATUT, Wrocław–Kcynia 2003, ISBN   83-89247-27-5
  2. ↑ J. Czochralski (1918). "Ein neues Verfahren zur Messung der Kristallisationsgeschwindigkeit der Metalle [A new method for the measurement of the crystallization rate of metals]". Zeitschrift für Physikalische Chemie. 92: 219–221. doi:10.1515/zpch-1918-9212.
  3. ↑ Nishinaga, Tatau (2015). Handbook of Crystal Growth: Fundamentals (Second ed.). Amsterdam, the Netherlands: Elsevier B.V. p. 21. ISBN   978-0-444-56369-9.
  4. ↑ Stuart Dowell. "Scientist who laid the foundations for Silicon Valley honoured at long last". thefirstnews.com. Archived from the original on 13 July 2023. Retrieved 3 May 2023.
  5. ↑ Son, JK (2020-05-14). "Growth and development of pure Li2MoO4 crystals for rare event experiment at CUP". Journal of Instrumentation. 15 (7): C07035. arXiv: 2005.06797 . Bibcode:2020JInst..15C7035S. doi:10.1088/1748-0221/15/07/C07035. S2CID   218630318.
  6. ↑ "Czochralski Crystal Growth Method". 30 January 2003. Archived from the original on 2016-08-17.
  7. ↑ Aleksic, Jalena; Zielke, Paul; Szymczyk, Janusz A.; et al. (2002). "Temperature and Flow Visualization in a Simulation of the Czochralski Process Using Temperature-Sensitive Liquid Crystals". Ann. N.Y. Acad. Sci. 972 (1): 158–163. Bibcode:2002NYASA.972..158A. doi:10.1111/j.1749-6632.2002.tb04567.x. PMID   12496012. S2CID   2212684.
  8. ↑ Manners, David (2013-12-30). "Doubts over 450mm and EUV". Electronics Weekly. Retrieved 2014-01-09.
  9. ↑ 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. Bibcode:1992ITNS...39.1730L. doi:10.1109/23.211360.
  10. ↑ Lindström, G; Ahmed, M; Albergo, S; Allport, P; Anderson, D; Andricek, L; Angarano, M.M; Augelli, V; Bacchetta, N; Bartalini, P; Bates, R; Biggeri, U; Bilei, G.M; Bisello, D; Boemi, D; Borchi, E; Botila, T; Brodbeck, T.J; Bruzzi, M; Budzynski, T; Burger, P; Campabadal, F; Casse, G; Catacchini, E; Chilingarov, A; Ciampolini, P; Cindro, V; Costa, M.J; Creanza, D; et al. (2001). "Radiation hard silicon detectors—developments by the RD48 (ROSE) collaboration". Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 466 (2): 308. Bibcode:2001NIMPA.466..308L. doi:10.1016/S0168-9002(01)00560-5. hdl: 11568/67464 .
  11. ↑ CERN RD50 Status Report 2004, CERN-LHCC-2004-031 and LHCC-RD-005 and cited literature therein
  12. ↑ Harkonen, J; Tuovinen, E; Luukka, P; Tuominen, E; Li, Z; Ivanov, A; Verbitskaya, E; Eremin, V; Pirojenko, A; Riihimaki, I.; Virtanen, A. (2005). "Particle detectors made of high-resistivity Czochralski silicon". Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 541 (1–2): 202–207. Bibcode:2005NIMPA.541..202H. CiteSeerX   10.1.1.506.2366 . doi:10.1016/j.nima.2005.01.057.
  13. ↑ Custer, J. S.; Polman, A.; Van Pinxteren, H. M. (1994). "Erbium in crystal silicon: Segregation and trapping during solid phase epitaxy of amorphous silicon". Journal of Applied Physics. 75 (6): 2809. Bibcode:1994JAP....75.2809C. doi:10.1063/1.356173.
  14. ↑ Eikelboom, J.A.; Jansen, M.J. (2000), "Characterisation of PV modules of new generations; results of tests and simulations" (PDF), Report ECN-C-00-067, 18
  15. ↑ James D. Plummer, Michael D. Deal, and Peter B. Griffin, Silicon VLSI Technology, Prentice Hall, 2000, ISBN   0-13-085037-3 pp. 126–27
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