news  review Super smooth sapphire growth


PLANSEE says its new tungsten crucibles make growing sapphire in a crucible much easier. The firm says their very smooth surface gives the crucibles a longer service life and reduces costs.


Ultra-pure sapphire is used in the production of LEDs. The Kyropoulos method, which involves melting a sapphire crystal in a heat-resistant crucible made from molybdenum or tungsten, is in widespread use. One particularly critical stage during this production process is when the sapphire crystal is extracted from the crucible.


The easier it is to release the sapphire from the crucible, the greater the yield and quality of the sapphire. If the surface of the crucible is too rough, the sapphire cannot be released or can only be partially released from the wall of the crucible. This results in cracks and faults in the crystal. Not only that, the crucible itself is also damaged in the process and has to be reworked or replaced earlier than planned. A fully automated manufacturing process coupled with more than 90 years of experience in the field of powder metallurgy allow Plansee to exercise precise control over the material properties of refractory metals such as tungsten and molybdenum.


In order to reduce the costs of sapphire growth, the materials experts developed a


become seriously deformed or even break.


special pressed-sintered tungsten crucible with an ultra-smooth surface. With its surface roughness of less than 0.8 µm, sapphire growth is now a smooth operation: The sapphire can be extracted from the crucible without difficulty and without damaging the surface of the crucible.


For sapphire manufacturers, this results in less complex and expensive reworking of the surface of the crucible. Plansee says the cycles run smoothly and deliver high- quality ingots. What’s more, the smooth surface is less susceptible to corrosion caused by the aggressive melted sapphire. This increases the service life of the reusable tungsten crucibles.


Plansee’s tungsten crucibles also score well in terms of density. You may wonder why is this important. Crucibles with a low density and a large number of porous areas can shrink under the high process temperatures. This means that they become denser in use and can thus


GaN Systems beats own development targets


GaN SYSTEMS has closed a Series B financing round with existing investors Chrysalix Energy Venture Capital and RockPort Capital, two cleantech venture capital firms. This funding comes six months after the closing of GaN Systems’ Series A round.


“This significant financing round is a stamp of approval from our existing investors which validates our engineering excellence,” says Girvan Patterson, CEO of GaN Systems. “It enables us now to accelerate commercialisation of the gallium nitride power devices that we have developed on our breakthrough island technology platform.


Mike Sherman, Managing Director at Chrysalix, adds, “Since the Company’s Series A round, the market momentum for


GaN as a credible alternative to silicon for power devices has grown significantly, with devices becoming mainstream faster than we anticipated. This means commercial reality is closer and more tangible than expected.


GaN Systems’ gallium nitride transistors are targeted at the next generation of


highly efficient power conversion applications, the fastest growing segment of the $14 billion-a-year power devices market. This includes high growth applications such as solar inverters, server farms, and hybrid vehicles. Beyond the inherent advantages of GaN in efficiency, speed and temperature, GaN Systems’ Cool Switching technology enables extremely compact, low cost designs. It also provides a seamless transition to foundry-independent GaN-on-silicon fabrication for flexible access to high volume production resources.


“With these advantages, GaN Systems is uniquely positioned to drive broad adoption of gallium nitride as a superior alternative to conventional silicon power devices,” remarks Daniel Hullah, Partner at RockPort Capital.


July 2012 www.compoundsemiconductor.net 11


The company has fine tuned its sintering process for the crucibles. Sintering is performed at very high temperatures and a uniform temperature distribution. This gives the tungsten crucibles a very high material density of 93 % coupled with an extremely homogeneous density distribution. Even under high temperatures and rapid changes of temperature, the crucible is guaranteed to retain its shape.


So what about purity? Contaminants in the plant components can be transferred to the sapphire in a way that is detrimental to its quality. Iron, titanium and chromium are particularly critical in this respect.


To ensure that only flawless sapphire comes out of the crucible, Plansee has adopted an ultra-clean process. Everything from the metal powder to the finished crucibles is manufactured in-house. This allows every step to be carefully monitored, and the resulting crucibles have a purity of over 99.97 %.


Nowadays, crucibles large enough for 35 kg up to 100 kg of sapphire are commonplace. And yet the industry is already considering a new generation for up to 200 kg of sapphire. Plansee says its extremely large sintering plants and processing machines mean that it is already geared up to take this step.


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