At 77K, the temperature of liquid nitrogen, the enhancement amounts to a factor of 2.4, while at 20K, where the thermal conductivity has its maximum, it is close to a factor of six. The suppression of this scattering in isotopically enriched crystals results in an enhancement of the thermal conductivity which can be very strong. Most elements contain considerable fractions of many different stable isotopes. While originally used predominantly for basic research, isotopically modified materials are now entering applications. Already at present, but even more so in near-future microelectronics applications with still larger integration densities and switching speeds, thermal problems severely limit the chip performance.
Appropriate measures are thus necessary to prevent destruction.
Of particular interest is the fact that the maximum effect occurs at a temperature at which the thermal expansion of silicon nearly vanishes. Festk rperforschung Heisenbergstr. 1 D-70569 Stuttgart, Germany Tel.