Is Sapphire Malleable?

 

The word “malleable” comes from the Latin malleus meaning hammer or mallet.  It refers to the property—most often associated with metals—of being susceptible to plastic deformation (particularly at room temperature).  In other words, something that is ductile and can be physically hammered into a new shape.  The most common malleable materials are metals with relatively low melting points and high crystalline symmetry such as gold, which can be pounded into sheets thinner than paper.  We tend to think of ceramic materials as being distinctly non-malleable.  They deform elastically until they reach their elastic limit at which point they fail by brittle fracture.  Sapphire is no exception – or is it?

A few years ago I was setting up an experimental growth run of a sapphire crystal by the EFG (Edge-defined, Film-fed Growth) method.  I was using a long sapphire seed about 10 mm in diameter (itself growFig 1n by EFG).  The first figure shows the seed hanging in the hot zone which is mostly graphite.  The die structure has some sapphire “crackle” placed on top of it as an indicator that the right growth temperature has been reached (on heating up it melts).  I had intentionally suspended the seed from a flexible mount to ensure that it hung vertically.  The problem was that the surface tension of sapphire is so strong that, unless the die assembly and seed are perfectly symmetrical and perfectly aligned, surface tension will pull the crystal to one side until gravity pulls it back to center, at which point it overshoots and surface tension pulls it to the other side.  This continues until you grow a zigzag crystal—which by 3 o’clock in the morning more closely resembles the cloven hoof of Satan.  You can see this in tZig-zag xtalhe second photo where I have graphically indicated the original seed.

On the next growth attempt the same behavior began so I decided to force the seed down hard on the die.  The next thing I knew was that the seed had disappeared from view.  (As you can see from the first photo, the sight window does not provide a very wide angle of view.)  At first I assumed that I had simply broken the seed, except that I couldn’t see any pieces of seed crystal lying on or near the die.  I was totally flummoxed by this until I cooled the system down to room temperature and opened the chamber.  Then I saw that I had actually bent the sapphire seed!  Since the seed is hanging in the afterheater section of the furnace at a temperature above 1900 C, we can conclude that, at a sufficiently high temperature, sapphire can indeed be plastically deformed.  (The dotted lines show the extent of the bending in the seed.)

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Once I redesigned the seed holder assembly I was able to grow the sapphire plates with no problem.  The next picture shows the early stages of growth of a typical plate produced by this system.  The fully grown plates can be seen on other pages of my website and the last photo shows a typical end use application—windows for the infrared sensor pod on the F-22 Raptor.  Sapphire’s extreme hardness combined with its wide rangeof transparency make it an ideal material for this application.

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The Birth of the Sapphire POS Terminal Window

It is a well-known fact that a thin layer of sapphire forms the top surface of almost all point-of-sale (POS) scanner windows.  I had the privilege of making the prototype for this component around 1978 when I worked for Allied Chemical in their synthetic crystal products division in Charlotte, North Carolina.
Among other oxide crystals that we were growing at the time was r-plane sapphire for SOS substrates.  We grew nominally 3 inch wide ribbons by the EFG process that had been licensed from Tyco’s Saphikon division.  We cut and polished 3” wafers from these ribbons for the SOS market.

We were approached by IBM who was finding that their glass windows on the POS terminals were scratching fairly quickly after being installed.  Imagine a supermarket where canned vegetables and six-packs of beer were dragged across the window on a continuous basis.  A typical glass window would have to be replaced every month at least.  IBM asked us if we could bond a sapphire layer to glass so that they could test fire their laser through it and see if there was no distortion in the beam so that it could read the bar code on the products.

A typical SOS wafer was polished only on one side, so I asked my guys to polish a few wafers on both sides.  I then took them and a piece of ordinary window glass that I bought at a hardware store.  I cleaned them both with window cleaner and then mixed up some 5 minute epoxy.  I smeared the epoxy on the glass, put the wafer on it and put it in a hydraulic press with a large rubber stopper on each side to spread out the force.  I cranked up the pressure as high as I dared to squeeze as much of the epoxy from between the glass and the sapphire wafer.

After the epoxy had enough time to set up, I removed the pieces from the press, cleaned away the excess epoxy from the glass with a razor blade and sent the piece off to IBM for testing.  The results were positive, and the sapphire POS window was born.  I still have the original (pictured here) which I have been using as a coaster for a ceramic coffee mug for about 35 years.  The glass around the sapphire is scratched, but the sapphire itself is nearly as pristine as the day that I first made the sample.  If you look at it obliquely, some faint scratches are visible.  But, as sapphire is the second hardest material in nature after diamond, it should remain virtually unscratched for all time.

After I came to Santa Rosa I had a friend who was a store manager for Safeway, the big supermarket chain.  Apparently they were well aware of the availability of the sapphire windows, but were put off by the price.  I got him to explain to his management that, although the sapphire windows were more expensive, they were a one-time purchase, as opposed to replacing the glass windows on a monthly basis.  Soon after, Safeway switched over to the sapphire POS windows which can now be found virtually everywhere that counter level scanners are used.

 

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Who will train the next generation of crystal growers?

Crystal Technology—the synthetic crystal and epilayer production including crystal machining and the required design and construction of equipment—forms the foundation of the modern electronics revolution and underpins all aspects of global commerce, communication, energy production and medical technology.  It is fundamental to the advances in electricity production, transport and storage that will be critical to future improvements in energy technologies necessary to reduce the impact of energy generation on global climate change.

At the invitation of Dr. Hans Scheel (www.hans-scheel.ch), a group of experienced technologists in this field from Europe, Asia and the United States, convened for a meeting for a week in Poulithra, Arcadia, Greece in May 2012 to discuss the need for the requisite education and training of future engineers and scientists who will form the first generation of crystal technologists specifically trained with the basic knowledge of crystal materials technology (CMT) enabling the CMT engineers to consult and collaborate with leading specialists of related scientific fields.  I was honored to be included among the invitees.

The goal of the meeting was to generate a White Paper outlining the need for this training regimen as well as to include the recommended training courses on both the undergraduate and graduate levels. This White Paper will be circulated to leaders of industry, government and academia with the goal of promoting this specialized training in universities and engineering schools.

Hans and his wife Regula are residents of Switzerland, but maintain a winter home in Poulithra.  He had organized an international workshop on crystal growth technology for a number of years, and he used the residual funds from those workshops to pay for the accommodations, food and touring expenses of those who participated in the meeting in Greece.  Scientists from the U.S., Britain, France, Germany, Russia and Japan, many accompanied by their wives, attended this meeting.  The meeting was held at the Smyros Resort on the Aegean shore.

Between technical sessions, presentations and discussions, Hans and Regula organized tours of historical archeological sites including Olympia, Epidaurus, Mycenae and Athens.  (A ladies program which ran in parallel with the technical sessions enabled those wives in attendance to see additional sites of interest).

My wife, Fran, and I returned to Greece in March of this year and, I, working with Dr. Scheel, completed the final stages of editing the White Paper which can be found online in pdf format:  https://files.secureserver.net/0fieEZc7s9vynL.  If a password is requested use “sapphire”, all lower case.

Is C-Axis Czochralski the Wave of the Future for Sapphire?

The Czochralski process has been the dominant method of growing sapphire crystals for about 50 years ever since it displaced the Verneuil process.  It produces crystals of high quality at a reasonably low price.  In the past decade there has been an explosive growth in the consumption of sapphire substrates primarily driven by the demands of the GaN blue LED market.  This application requires crystals oriented such that the substrate surface is the c-plane, a basal plane of the hexagonal structure.  Sapphire crystals form a strong facet on the c-plane, and growth in that direction generally results in crystals with high defect densities, particularly dislocations and low angle grain boundaries.  To overcome this drawback, the usual methodology is to grow the crystal in the a-direction and then core drill rods perpendicularly which are then sliced into c-plane substrates.  For all crystal growth techniques commonly employed for sapphire (Czochralski, Kyropoulos, HEM), this approach suffers from poor material utilization.  Although this has generally been viewed as an acceptable trade-off in the manufacturing process as long as 2” substrates were the dominant market, as substrate diameters have increased towards 6” and 8”, this compromise is no longer seen as a viable alternative because of the low material utilization and the high energy consumption of the growth process.  Recently, interest in the Czochralski process has been rekindled because of its greater material utilization when crystals are grown in the c-axis direction.  This has been abetted by developments in GaN epitaxy which overcome the issues related to substrate quality.  Growth of large diameter c-axis sapphire crystals by the Czochralski process is not without its own issues.  Scaling to large diameter crucibles requires an abandonment of the traditional approach of using induction heated iridium crucibles and operating in an unsealed growth chamber.  Now the manufacture of large refractory metal crucibles combined with the thermodynamic interactions of the metal crucible, insulation (generally graphite) and oxide (sapphire) become important considerations in the successful implementation of the process.

I have already seen x-ray topographs of 6” CZ wafers cut from c-axis crystals.  Although the quality does not compare to a-axis material, it is highly likely that this will have little impact on the GaN epilayer and the subsequent LED yield and performance.  I expect to see increased activity in this area in the future.