ClearlySapphire.com


 

Clearly Brilliant, Clearly Powerful, Clearly Sapphire

Growth

Although it is typically associated today with high-tech applications such as LEDs and spacecraft windows, the production of industrial sapphire dates back to the late 19th century, when A. V. L.  Verneuil developed a flame fusion process to produce ruby and sapphire.  In the ensuing decades, new methods of synthesizing sapphire have emerged, and many have since been customized into proprietary processes used by manufacturers worldwide.

 

To produce a sapphire crystal “boule,” all of these processes uses commercially available aluminum oxide (Al2O3) as starting raw material.  This material is melted in furnaces that operate up to 4000° F, which is moderately above the aluminum oxide’s melting point.  The size and shape of the resulting sapphire crystal or ingot depends on the method employed and the intended uses of the resulting sapphire.

The following are the generally recognized core crystal growth technologies from which all other processes have evolved.

 

Verneuil Method

 

Kyropoulous Method

 

Heat Exchanger Method (HEM)

 

Czochralski Method (CZ)

 

Edge-Defined Film-Fed Growth Method (EFG)

 

Verneuil

Industrial crystal production started in Paris with Verneuil,  who in 1902 used the the flame-fusion growth process named after him to develop single crystals of ruby and sapphire with melting points above 2000◦ C (3632◦ F).


The physics involved in the Verneuil process, virtually unchanged from its original design, limit the size and shape of items that can be produced.  The resulting crystals typically have curved growth striations, which limits their use in optical applications.  The primary use for Verneuil-grown sapphire and ruby today is still for synthetic sapphire and ruby gemstones, watch jewels and watch windows.


Today there remains significant demand for sapphire grown using the Verneuil method because it is still the least expensive way to make sapphire adequate for many applications.


Czochralski Method (CZ)

The Czochralski (CZ) method of crystal growth was discovered in 1916 by Jan Czochralski – and was the fortunate result of an accident and insightful observation.  One evening, Czochralski left a crucible with molten tin on his desk and began writing notes.  Instead of dipping his pen in its inkwell, he mistakenly dipped it in the crucible and quickly pulled it out.  A thin thread of metal hung from the tip of the pen.  Czochralski observed that the crystallized wire was in fact a single crystal, and the process was developed.


CZ remains essentially unchanged today.  Seed material is loaded into a crucible inside a custom growth chamber, and all gases inside the chamber are evacuated.  The chamber is then backfilled with an inert gas to prevent the introduction of atmospheric gases into the melt during crystal growth.  The material is melted, and a thin seed of sapphire with precise orientation is dipped into the melt. The seed crystal is withdrawn at a controlled rate, and crystal and crucible are rotated in opposite directions.  The process is repeated, and crystal layers are added with each cycle until the target size and shape are realized.


The growth process can last up to eight weeks, and requires careful, continuous power and monitoring.

The resulting sapphire has good optical qualities, and is used widely in lasers, infrared and ultraviolet windows, transparent electronic substrates, high-temperature process windows, and other optical applications.


Kyropoulous Method

The Czochralski method poduced material in thin crystal filaments, and Verneuil boules had basic dimensional limitations.  So in 1926, Spyro Kyropulos developed a process for direct crystallization of the melt by decreasing the boule's temperature while still in the crucible.  Kyropoulos introduced his technique as a way of producing large single crystals that were free of cracks and damage due to restricted containment.


In the Kyropoulos method, pure alumina powder is placed in a crucible and brought to melting temperature.  The sapphire crystal is formed deep under the surface of the molten alumina ‘melt.'  As it solidifies, it takes on the cylindrical shape of the crucible. 

Thermal gradient controls the process so that only the crystal layer at the solid-liquid interface remains molten, and as the seed crystal is slowly drawn back out of the the added crystal layers increase the size of the boule.


This growth technique is ideal for materials with low thermal conductivity and a high degree of thermal expansion, the combination of which can make crystal material vulnerable to various imperfections unless grown and cooled in a low-stress environment.


With this highly controlled thermal-gradient, the Kyropoulos method yields large-diameter boules of very high optical quality due to its high purity.  The resulting boules can be cut to any crystallographic orientation or plane. Kyropolous grown sapphire is ideal for many optical applications including electronics (substrates, IR detectors, fiber-optic lenses), optics (windows, missile domes, lenses, probes, lasers), and manufacturing.

 

Heat Exchanger Method (HEM)

The Heat Exchanger Method (HEM) for growing large sapphire boules, often called the inverted or modified Kyropoulos method, w as invented by Fred Schmid and Dennis Viechnicki at the Army Materials Research Lab in Watertown, MA in 1967.


In the modern implementation of HEM, a sapphire seed crystal is placed at the bottom of a crucible which is then loaded with pure alumina crackle, a byproduct of the Verneuil process.  The furnace is evacuated and heated to melt the crackle while keeping the seed just below its melting point by passing helium gas through the heat exchanger beneath the center of the crucible. 


Heat and vacuum help purify the alumina by vaporizing impurities.  After partial melting of the seed, helium flow is increased to cool the seed and initiate crystallization of alumina onto the seed.  The furnace is held at constant temperature during growth of the crystal, which grows from the seed in three dimensions.  Slight changes in the thermal flow affect the shape of the growing crystal.


When crystallization is complete, the furnace temperature and the gas flow are decreased and the crystal boule slowly anneals.  The long slow cooling process results in exceptional crystal quality.  Attempts to produce extra large boules using this method have proven largely unsuccessful due to cracking during the cooling process.


Edge-Defined Film-Fed Growth Method (EFG)

In 1965, technician Harold Labelle was enlisted by Tyco Industries in Waltham, MA to develop a process for growing sapphire fibers as reinforcement for metal-matrix compounds.  


Through his experience in Tyco's semiconductor crystal growth experiments he’d gained experience in controlling the solid-liquid interface – a key concept of crystal growth. 


Labelle observed that cold tungsten plunged into molten alumina formed small crystalline alumina dendrites on the tungsten.  He introduced temperature control to the process to increase the size of the resulting crystals.


Melting the alumina in a troth inside a vacuum, Labelle inserted a tungsten wire into the melt. When it was carefully pulled out, a sapphire crystal grew on the wire.  The resulting process was essentially a Czochralski technique with the benefit of crystal shape control.


Eventually, dies were added with orifices of the size and shape of the desired crystal.  In 1967 LaBelle used his method to produce the first as-grown sapphire tubes.


Today, the EFG crystal growing technique remains as simple as the process Labelle developed.   Alumina is melted in a crucible, and the melt 'wets' the surface of die and moves up by capillary attraction. A sapphire seed of specific crystallinity is dipped into the melt on top of the die and drawn out –solidifying into sapphire in the shape of the die – typically a tube, rod or ribbon.  The process also allows for tight control over crystal orientation.


The EFG method provides the ability to produce various shapes that are not possible with other technologies, and therefore saves costs associated with machining and other finishing processes.  The primary drawback is the time and cost associated with producing the dies to create shaped crystals. 


EFG-produced crystal material is typically of low to medium optical quality, and can be precisely produced in different crystallographic orientations (A, C, random). It is most often used for mechanical, industrial and lesser-grade optical applications.