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Boule (crystal)

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Crystallization
Fundamentals
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Monocrystalline silicon boule

A boule is a single-crystal ingot produced by synthetic means.[1]

A boule of silicon is the starting material for most of the integrated circuits used today. In the semiconductor industry synthetic boules can be made by a number of methods, such as the Bridgman technique[2] and the Czochralski process, which result in a cylindrical rod of material.

In the Czochralski process a seed crystal is required to create a larger crystal, or ingot. This seed crystal is dipped into the pure molten silicon and slowly extracted. The molten silicon grows on the seed crystal in a crystalline fashion. As the seed is extracted the silicon solidifies and eventually a large, cylindrical boule is produced.[3]

A semiconductor crystal boule is normally cut into circular wafers using an inside hole diamond saw or diamond wire saw, and each wafer is lapped and polished to provide substrates suitable for the fabrication of semiconductor devices on its surface.[4]

The process is also used to create sapphires, which are used for substrates in the production of blue and white LEDs, optical windows in special applications and as the protective covers for watches.[5]

A popular method used to create sapphire boules is the Heat Exchanger Method (HEM), in which aluminum oxide is placed in a molybdenum crucible and heated until melting at 2200°C. It allows for very large crystals over 30 cm wide to be produced. The process takes place in a vacuum. A sapphire seed crystal sits at the bottom of the crucible and is kept from melting by heat exchange (cooling) with helium gas or liquid helium in which is shielded from the vacuum by a cold finger heat exchanger. The furnace is kept at a temperature just above melting, but the heat exchanger is at a temperature just below melting. Then the heat exchanger temperature is lowered to start crystalization, and then the aluminum oxide is cooled over a period of at least 72 hours to 17 days to crystalize it into sapphire. The crucibles are single use, the process is similar to the Bridgman technique and the Stöber methods for crystal growth,[6][7] and was used for iPhone screens.[8][9][10][11] The crystal grows upward from the bottom of the crucible.[12]

Another method is the Edge-defined Film-fed Growth (EFG) method, very similar to the Czochralski method but the material passes through a die before cooling, which shapes the crystal. The crystal does not rotate.[13] Chemical Vapor Deposition (CVD), gradient furnace[14] or vertical bridgman processes can be used for sapphire crystal growth.[15]

The temperature gradient method uses a furnace in which a crucible containing material is located. On the bottom of the crucible, a seed crystal is placed. The material is melted and then kept molten for hours until the temperature stabilizes. The furnace is operated in a vacuum and when the temperature reaches 1400°c at the bottom of the crucible, argon gas is injected. Crystallization then starts by cooling the molten material at 1.3 to 3 kelvin per hour, with a linear temperature gradient across the height of the furnace making the material crystallize from the bottom up. The gradient is produced by cooling parts of the furnace with water.[16] The temperature gradient can also be created by having several heaters along the height of the furnace, dividing the heaters into temperature zones and changing the temperature of the zones.[17]

Large crystals 50cm across, of water soluble material such as Monopotassium phosphate (KDP) can be made by dissolving the KDP in hot water and salt, creating a growth solution, placing a seed crystal in the solution and then cooling the solution, done in a holden-type crystallizer, in what is known as solution growth.[18][19][20]

References

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  1. ^ "Bulk Growth of Silicon Carbide". Fundamentals of Silicon Carbide Technology. 2014. pp. 39–74. doi:10.1002/9781118313534.ch3. ISBN 978-1-118-31352-7.
  2. ^ Li, Hongjun; Xu, Jun (2010). "Crystal Growth of Laser Host Fluorides and Oxides". Springer Handbook of Crystal Growth. pp. 479–507. doi:10.1007/978-3-540-74761-1_15. ISBN 978-3-540-74182-4.
  3. ^ Rea, Samuel N. (1978). "Continuous Czochralski Process Development". Retrieved March 1, 2017.
  4. ^ BOSE (2013). IC Fabrication Technology. McGraw Hill Education (India) Pvt Ltd. p. 53. ISBN 978-1-259-02958-5.
  5. ^ Colinge, Jean-Pierre (2004). "Soi Materials". Silicon-on-Insulator Technology: Materials to VLSI. pp. 9–68. doi:10.1007/978-1-4419-9106-5_2. ISBN 978-1-4613-4795-8.
  6. ^ Tatartchenko, V. A. (2010). "Sapphire Crystal Growth and Applications". Bulk Crystal Growth of Electronic, Optical & Optoelectronic Materials. pp. 299–338. doi:10.1002/9780470012086.ch10. ISBN 978-0-470-01208-6.
  7. ^ Feigelson, Robert S. (2015). "Crystal Growth through the Ages". Handbook of Crystal Growth. pp. 1–83. doi:10.1016/B978-0-444-56369-9.00001-0. ISBN 978-0-444-56369-9.
  8. ^ Wakabayashi, Daisuke (19 November 2014). "Inside Apple's Broken Sapphire Factory". Wall Street Journal.
  9. ^ Sapphire Screen: The Making of a Scratch-Proof Smartphone Display on YouTube
  10. ^ Pishchik, Valerian; Lytvynov, Leonid A.; Dobrovinskaya, Elena R. (2009). Sapphire. Bibcode:2009smma.book.....P. doi:10.1007/978-0-387-85695-7. ISBN 978-0-387-85694-0.[page needed]
  11. ^ Khattak, Chandra P.; Schmid, Frederick (May 2001). "Growth of the world's largest sapphire crystals". Journal of Crystal Growth. 225 (2–4): 572–579. Bibcode:2001JCrGr.225..572K. doi:10.1016/S0022-0248(01)00955-1.
  12. ^ Feigelson, Robert S. (2015). "Crystal Growth through the Ages". Handbook of Crystal Growth. pp. 1–83. doi:10.1016/B978-0-444-56369-9.00001-0. ISBN 978-0-444-56369-9.
  13. ^ Pishchik, Valerian; Lytvynov, Leonid A.; Dobrovinskaya, Elena R. (2009). Sapphire. Bibcode:2009smma.book.....P. doi:10.1007/978-0-387-85695-7. ISBN 978-0-387-85694-0.[page needed]
  14. ^ Schmid, F.; Viechnicki, D. (September 1970). "Growth of Sapphire Disks from the Melt by a Gradient Furnace Technique". Journal of the American Ceramic Society. 53 (9): 528–529. doi:10.1111/J.1151-2916.1970.TB16009.X.
  15. ^ Khattak, Chandra P.; Schmid, Frederick (May 2001). "Growth of the world's largest sapphire crystals". Journal of Crystal Growth. 225 (2–4): 572–579. Bibcode:2001JCrGr.225..572K. doi:10.1016/S0022-0248(01)00955-1.
  16. ^ Li, Hongjun; Xu, Jun (2010). "Crystal Growth of Laser Host Fluorides and Oxides". Springer Handbook of Crystal Growth. pp. 479–507. doi:10.1007/978-3-540-74761-1_15. ISBN 978-3-540-74182-4.
  17. ^ Lan, Y. C.; Chen, X. L.; Crimp, M. A.; Cao, Y. G.; Xu, Y. P.; Xu, T.; Lu, K. Q. (May 2005). "Single crystal growth of gallium nitride in supercritical ammonia". Physica Status Solidi (C). 2 (7): 2066–2069. Bibcode:2005PSSCR...2.2066L. doi:10.1002/pssc.200461557.
  18. ^ Atherton, L.; Burnham, A.; Combs, R.; Couture, S.; De Yoreo, J.; Hawley-Fedder, R.; Montesant, R.; Robey, H.; Runkel, M.; Staggs, M.; Wegner, P.; Yan, M.; Zaitseva, N. (1999). Producing KDP and DKDP crystals for the NIF laser (Report). doi:10.2172/14145.
  19. ^ Zaitseva, N.; Carman, L. (January 2001). "Rapid growth of KDP-type crystals". Progress in Crystal Growth and Characterization of Materials. 43 (1): 1–118. Bibcode:2001PCGCM..43....1Z. doi:10.1016/S0960-8974(01)00004-3.
  20. ^ Zaitseva, N.P.; Dehaven, M.R.; Vital, R.L.; Carman, M.L.; Spears, R.; Montgomery, K.; Atherton, L.J.; De Yoreo, J.J. (1996). "Rapid Growth of Large-Scale (20-50cm) KDP Crystals". Nonlinear Optics: Materials, Fundamentals and Applications. pp. NPD.5. doi:10.1364/NLO.1996.NPD.5. OSTI 492018.
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