crystal growth techniques
TRANSCRIPT
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Crystal Growth Techniques
Ron Graham
October 31, 2006
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Agenda
•Summarize current techniques•Discuss advantages/disadvantages•Propose hybrid method
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Basic Methods•Czochralski (CZ)•Bridgman (and variations)•Various floating zone methods•EB drip melting•Strain annealing•Other methods
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Czochralski
•Czochralski (CZ) typically used for Si•Can grow boules to 300 mm with
400 mm being introduced•Uses seed crystal•“Pulls” boule out of the melt
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Czochralski Puller
View-port
Melt
Seed
Crystal
Heaters
•Resistance or RF heating•Melt contained in quartz or
Si3N4 crucible•Chamber under Argon•Si melts 1421°C
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Czochralski•Growth speed is 1–2 mm/min•Crucible introduces oxygen contamination•Feed material form is unconstrained•Axial resistivity uniformity is poor•Heat up/cool down times are long•Materials of construction are issue Nb Tm = 2477°C•Ingot weight can reach 400 kg
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Czochralski•Modification is a “Tri-arc” furnace•Melting accomplished by 3 arcs•Rotating, water-cooled Cu crucible•Melt conducted under vacuum•Reportedly can melt to 3000°C
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Bridgman Technique•Vertical or horizontal•Uses a crucible•Requires seed crystal•Directional solidification•Precise temperature gradient required
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Bridgman Technique
Furnace tube
Seed Crystal
MoltenPull
Heater Polycrystal
Moltenzone
Pull Crystal
Seed
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Bridgman Technique
Carefully controlledtemperature gradientrequired.
Temperature TM
solid-liquid interface
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Bridgman Technique
•Growth rates of about 1 mm/hr•Crucibles usually used one time•Used for small Nb crystals 10 x 40–60 mm•Requires only tip of seed to be molten•Can reach 200 mm for Si and GaAs crystals
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Floating Zone Techniques
•Electron beam floating zone (EBFZ)•RF floating zone
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EB Floating Zone•Actual experience with refractory alloys
including Nb, Ta, Mo, Re, and W•Vacuum melting chamber, annular EB gun•Crystal rotator and translator•No crucible•0.5–50 mm/min growth rates
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EB Floating Zone Melt stock (anode)
W filament cathode
Liquid metal Focusing electrodes
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EB Floating Zone
•Zone refining is added benefit•Diameters up to 110 mm reported for Nb•Diameters limited by surface tension/runout•EB heating penetration limited•Does not seem practical for 300 mm
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EB Floating ZoneO <0.03
C <0.3
N <50
H <0.1
Si <0.03
Al <0.03
K <0.03
Ca <0.3
Na <0.03
P <0.03
S <0.1
Impurity concentrationof Nb as reported by Giebovsky and Semenov
ppm
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EB Floating Zone•Modified pedestal technique reported for Nb•Used annular EB gun•Nb feedstock is rotating pedestal•Melt top of pedestal and touch seed to it•Pull non-rotating seed up off the pedestal•1.5 x 30 - 50 mm length•After Naramota and Kamada
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Floating Zone RF
Melt
RF Coil
Single Crystal
Offset
MeltStock
RF Coil
MeltStock
Seed
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Floating Zone RF•No practical advantage over EB heating•Diameter of Xtal can be made larger by off-
setting bottom pull rod from melt stock•Requires multiple passes to achieve crystal•Molten zone stability critical
•Surface tension•Cohesion•Levitation
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EB Vertical Drip Melting•Well known technology•Can readily make large-grain ingots to 400 mm•Rotating melt-stock, vertically oriented above
water-cooled copper crucible•Multiple EB guns at 30° axis to melt stock•Bottom withdrawal of ingot•Excellent refining and purification
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EB Vertical Drip Melting•“Single grain” (with surrounding equiaxed grains)
demonstrated on small diameter•Large grains 150 x 220 mm possible•Not a “robust” process at this time•Limited by perturbations such as thermal gradients,
vibrations, fluid flow, nucleation off crucible wall
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EB Vertical Drip Melting
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EB Vertical Drip Melting•A reminder of how refractory
metals solidify•These are the nuclei for new grains•Dendrites are easily disturbed and
broken off•If they don’t re-dissolve they form
new grains•There can only be one dendrite in
a single crystal
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Single Crystal Turbine Blades
MoltenMetal
RadiationHeating
RadiationCooling
SingleCrystalSelector
Columnar GrainSeed CrystalWater Cooled
Chill
Ceramic Mold
•Uses columnar seed grain•Single crystal selector (pigtail)•Ceramic mold maintained at ~Tm
•Directional solidification from chill to top of blade
•Side entry gate/runner•15 Kg is considered a large pour
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Strain Annealing•Relies on principal of critical grain growth•Low strains = low dislocation density•Insufficient nucleation sites for new grains•Strain to ~ 3–5%, anneal•Results in large grains•Single grains to 5 mm2
•Impractical for our purposes
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Other methods•Epitaxial growth - thin film only, very slow growth
rate•Variations of Bridgman technique using IR heat
lamps (so called image or mirror furnaces)•Levitation melting
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One Proposal•EBFZ on tubular melt stock•May be able to produce a single crystal tube•Thin wall contains molten zone•Surface tension may be able to support molten
metal column•Benefits of zone refining•Tube could be hydroformed to cavity shape
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EB Floating Zone on TubeTubular melt stock
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References1. Handbook of Semiconductor Silicon Technology, W.C. O’Mara, R. B. Herring, L. P.
Hunt, Noyes Publications, Norwich, NY, (1980).2. Moment, R. L., J. Nucl. Mater. 20, (1966), pp 341.3. Schulze, K. K. “Preparation and Characterization of Ultra-High Purity Niobium”, JOM,
May, 1981, pp 33–4.4. Giebovsky, V.G., Semenov, V.N., “Growing Single Crystals of High-Purity Refractory
Metals by Electron-Beam Zone Melting”, High Temp. Materials and Processes, V. 14, No. 2, (1995) pp. 121–130.
5. Yudin, I.A., Elotin, A.V., “Usage of EB Floating Zone Melting for Production of Rhenium Alloys Wire”, Rhenium and Rhenium Alloys, ed. By B. D. Bryskin, TMS, (1997), pp. 805 808.
6. Liu, J., Zee, R.H. “Growth of molybdenum-based alloy single crystals using electron beam zone melting, J. of Crystal Growth, 163 (1996) pp. 259–265.
7. Naramoto, H., Kamada, K., “Growth of Niobium Single Crystals by a Pedestal Method”, J. of Crystal Growth, 24/25, (1974), pp. 531-536.
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References8. Chen, H. et. Al., “Growth of lead molybdate crystals by vertical Bridgman method”,
Bull. Mater. Sci, Vol. 28, No. 6, Indian Academy of Sciences, (2005), pp. 555-560.9. Singh, J., Electronic and Optoelectronic Properties of Semiconductor Structures,
Cambridge University Press, 0521182379X, Chapter 1, Structural Properties of Semiconductors, Cambridge, UK, (2003).
10. Lawley, A., “Crystal Growing”, Vacuum Metallurgy, ed. By O. Winkler, R. Bakish, Elsevier Publishing Co., Amsterdam, (1971), pp 633-642.
11. Yang, X.L., Lee, P.D., D’Souza, N., “Stray Grain Formation in the Seed Region of Single-Crystal Turbine Blades”, JOM, (May, 2005), pp. 40-44.
12. Ford, T., “Single Crystal Blades”, Aircraft Engr. & Aerospace Tech., V. 69, No. 6, (1997), pp. 564-566.
13. M. Gell, D. N. Duhl, and A. F. Giamel, “The Development of Single Crystal SuperalloyTurbine Blades”, Superalloys 1980: Proceedings of the Fourth International Symposium on Superalloys, edited by J. K. Tien, AIME/ASM, Metals Park, Ohio, 1980, pp 205-214.