Crystal Growth Techniques

Crystal Growth Techniques

Ron Graham

October 31, 2006

Agenda

•Summarize current techniques•Discuss advantages/disadvantages•Propose hybrid method

Basic Methods•Czochralski (CZ)•Bridgman (and variations)•Various floating zone methods•EB drip melting•Strain annealing•Other methods

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

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

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

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

Bridgman Technique•Vertical or horizontal•Uses a crucible•Requires seed crystal•Directional solidification•Precise temperature gradient required

Bridgman Technique

Furnace tube

Seed Crystal

MoltenPull

Heater Polycrystal

Moltenzone

Pull Crystal

Seed

Bridgman Technique

Carefully controlledtemperature gradientrequired.

Temperature TM

solid-liquid interface

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

Floating Zone Techniques

•Electron beam floating zone (EBFZ)•RF floating zone

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

EB Floating Zone Melt stock (anode)

W filament cathode

Liquid metal Focusing electrodes

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

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

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

Floating Zone RF

Melt

RF Coil

Single Crystal

Offset

MeltStock

RF Coil

MeltStock

Seed

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

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

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

EB Vertical Drip Melting

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

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

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

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

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

EB Floating Zone on TubeTubular melt stock

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.

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.