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A clearer view of crystal growing

September 20, 2002

As technology progresses toward ever finer microelectronic structures, the wavelength of the ultraviolet radiation used to depict them has to be shortened. The emerging generation of photolithographic processes works at 157 nanometers - a wavelength at which glass and even quartz glass are not transparent enough. The preferred material here is extremely pure, low-defect, single-crystal calcium fluoride (mineralogically: fluorite or fluorspar). In the form of lenses or prisms, it can concentrate and deflect UV at down to about 130 nanometers. Its refractive behavior should be as uniform as possible, so as not to lessen the quality of chip structures.

Anyone who has ever tried to grow large, perfect crystals from aqueous salt solutions foresees: The difficulties multiply when one flawless mono crystal has to be bred from the molten substance at temperatures of over 1400 °C. To determine the ideal process conditions and optimize the production plant, the Device Technology Department of the Fraunhofer Institute for Integrated Circuits IIS developed the computer program CrysVUn. With the help of this program, since 1998, the Institute's industrial partner Schott Lithotec has advanced to become the world's leading manufacturer of calcium fluoride crystals.

"The crucial factor is to know the temperature distribution during the crystal growth process," emphasizes Georg Müller, who heads the crystallography laboratory. "As in glassmaking, the molten mass has to cool down in a controlled manner, to prevent areas of thermal tension. Like streaks or reams in glass, such defects reduce optical quality." In order to obtain a better understanding of the interactions between process conditions and material properties, the team built a crystal-growing apparatus which they used to investigate such factors as the effect of temperature distribution on the final quality of the single crystal. This experimental work enabled them to produce an improved computational model, which provides results that deviate by only one percent from the experimental data. Müller explains why: "The old standard model used an inaccurate description of heat transfer by infrared radiation inside the calcium fluoride. The improved model now delivers more precise information: It is even capable of predicting the form of the phase boundary between the melt and the growing solid crystal, which is a highly influential factor on crystal quality." The impressive reward for their efforts is a completely transparent, colorless crystal cylinder, measuring 15 centimeters across and in length.

Fraunhofer-Gesellschaft

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