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SCHOTT LITHOTEC manufactures calcium fluoride crystals for use in wafer stepper optics. The Erlangen Crystal Growth Laboratory is developing special furnaces in cooperation with SCHOTT for growing the material.
Prof. Georg Mueller, Dr. Jochen Friedrich, Friedrich Alexander University, Erlangen-Nuremberg, Germany

“Jewels” for Innovations

In the form of jewels, monocrystals have fascinated people for centuries. New methods of analysis are now making it possible to grow them for demanding applications, for example in lithography.

In addition to their beauty, monocrystals are now of extremely great significance for technical reasons. Many revolutionary new products developed in the past 50 years would not have been possible without monocrystals. They are absolutely essential for a whole range of modern technologies. Monocrystal semiconductors play an important role in microelectronics and information technology. In optics, monocrystals are incorporated, for example, in highly developed optical systems, in the form of windows and lenses. Instruments equipped with laser crystals are facilitating the development of new types of treatment methods in medicine. In modern metrology and materials processing, laser tools have now become standard.

The enormous importance of monocrystals is reflected in the production figures. The world annual production of large monocrystals rose from 5,000 tonnes in 1979 to around 20,000 in 1999.

Defect engineering, a modern solution strategy

State-of-the-art: Crystal growth facility for the manufacture of 300-millimeter diameter silicon monocrystals at Wacker Siltronic AG.
Researchers and developers are using what is known as defect engineering to help them grow larger crystals, and at the same time improve their quality in the microscopic or even to the atomistic range. This is achieved by a precisely controlled choice of growing conditions. The formation of defects is, in the majority of cases, coupled directly to the temperature field during growth. The solution strategy, therefore, takes two routes. On the one hand the crystal growth process is carefully analyzed to discover the relationship between the important growth conditions and then to draw up a fault model with this information. On the other hand a process model is needed that places the growth parameters (i.e. those elements of the growth process that can be manipulated directly) in relation to the growth conditions – mainly the temperature field. This is achieved by the combined use of experimental analysis and computer simulation.

Outstanding results

The Erlangen Crystal Growth Laboratory is a worldwide-acknowledged competence center for crystal growth. It has both the necessary experimental infrastructure and also powerful simulation tools to develop and apply innovative methods. Its successes are impressive. The Crystal Growth Laboratory played a crucial role in the development of the VGF process for the industrial production of GaAs and InP monocrystals and has grown crystals of this type with extremely low defect densities.

In cooperation with SCHOTT LITHOTEC AG the Crystal Growth Laboratory has developed special furnaces for growing calcium fluoride (CaF2) crystals that are used as lens material for lithography. SCHOTT LITHOTEC’s production facilities are now the largest in the world. In addition the Crystal Growth Laboratory has developed and built a new prototype furnace with which it will be possible to gain valuable knowledge for growing CaF2 crystals.