The ISONICS Company began to deliver isotopically pure Silicon-28
to the AMD microprocessor producer. The news did not prove to be absolutely
unexpected because ISONICS has been delivering isotopically pure materials
for a long time. A year ago there was information concerning delivery of
trial isotopically pure silicon for AMD. What turned out to be new, it is the
isotope number, Si-28. It allows the thermal conductivity to be increased by
fifty per cent. The thermal conductivity is a parameter rather critical in
chip production because the most effective heat-sink cooling is needed.
Otherwise a chip can be damaged because of localized overheating.
Why avoiding of dirt admixtures can increase the thermal conductivity to such
a great degree? The natural silicon is known to contain about 92 per cent
of 28Si isotope. Silicon is a semiconductor, so electronic conductivity does
not greatly exceed that of phonons (when heat is transmitted because of atom
vibrations in lattice points). The less is the mass of an atom, the greater its
moving ability, which leads to increased phonon conductivity. The atoms turn out
to vibrate in a strict resonance if a lattice has been constructed from an
isotopically pure material. In this case the heat sink is better what results in
significant increase in conductivity. The conductivity of 28Si is 40 per cent
as high as that of natural silicon at room temperatures, and 60 per cent more at
100C (W.S.Capinski et al. // Appl. Phis. Lett. 1997, v. 71.). ISONICS informs
that the maximum value turns out to be near 30K but now one is more interested in
room temperatures.
The price of microprocessors produced with pure 28Si is an order of magnitude
as high as that of ordinary costs:
28Si epi wafers - $0.10 to $1.0
28Si bulk wafers - $2 to $25.
The cost is not very critical compared with the effectivity because few
processors are produced from one wafer.
The producers are localized in the towns of Sarov, Novoural'sk (Sverdlovsk-44),
Seversk (Tomsk-7), Zelenogorsk (Krasnoyarsk-25), and Angarsk. Technology
for isotope separation in Russia is considered to be the best in the world.
Earlier, however, the whole country was working to meet military requirements and
silicon was not supposed to be a material of significance. An approach to
control isotopically pure materials has been elaborated in Russia using a laser
mass-spectroscopy. The approach allows the main nuclide abundance to be determined
within 0.1 per cent, and admixture abundances to be estimated to the accuracy of 6 - 10
per cent.
Very pure materials are of great effectivity in many high technologies. For example,
114Cd when applied to He - Cd lasers results in their UV output which some times exceeds that of
ordinary lasers and increases their durability.
ZnO is a necessary admixture for a primary reactor contour heat carrier working
on boiling water to avoid dangerous radionuclides (58Co and 60Co) washing-out.
Using Zn with 64Zn isotope (<1 per cent) suppresses formation of 65Zn,
a dangerous radionuclide with a great half-decay period. It is an example of using
isotopically depleted materials which allow radiation effects to be reduced
significantly.
58Ni reflects cool neutrons well. When in admixture, it suppresses subsidiary
nuclear reactions. 12C and 28Si are of importance for future microelectronics.
They have thermal conductivity 50 per cent as high as natural very pure
analogues at room temperatures. So their applications appear to decrease chip
element sizes significantly and make them as small as 0.18 microns
(J. of Atomic Energy Soc. of Jap. 1993, v. 35, N 11). What is nice, conversion
to 28Si application does not require any changes in technological process.
Moreover, in the Institute of Molecular Physics belonging to Kurchatov
Institute, 70Ge isotope has been obtained using centrifugal technology of
isotope separation. A few hundred grams of this isotope were enriched up
to 99.99 per cent. The material was subjected to chemical purification
using repeated floating-zone method in Lowrence Berkeley National Laboratory
(USA) and Ge-monocrystals were grown up that seemed to be the most perfect
synthetic crystals of non-monoisotopic elements.
Actually almost all materials used in electronic devices may be replaced with
their isotopically pure variations. Because of that, devices can appear
with absolutely new and unknown (or inaccessible) so far features. The
most interesting is the fact that these materials with earlier unattainable
properties are now being produced at Russian plants.
Translated by Nataliya Lipunova
Review by Kirill Krylov from "Terra & Comp".
