Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Reexamination Certificate
2002-08-07
2004-10-05
Wong, Edna (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
Reexamination Certificate
active
06800827
ABSTRACT:
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2001-249048, filed Aug. 20, 2001, the entire contents of this application are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to a method for laser isotope separation and enrichment of silicon on the basis of infrared multiple-photon dissociation of silicon halides, particularly to a method in which the selectivity for and yield of isotope to be separated and enriched are remarkably improved by irradiating silicon halides synchronously with two or more infrared pulsed laser beams at different wavelengths.
Non-radioactive stable isotopes exist for many elements and their use is being expanded since there is no fear of potential risk for radiation exposure.
Natural silicon consists of isotopes of mass numbers 28, 29 and 30 in the abundance ratios of 92.2% (
28
Si), 4.7% (
29
Si) and 3.1% (
30
Si).
Separated and enriched stable silicon isotopes have already been used in various fields and their applications in future have also been proposed. Namely,
28
Si and
30
Si have been used as tracers in studies on the effect of silicon fertilizers on rice and
30
Si has also been used in the production of novel nuclides. The other isotope
29
Si has been used in the ion implantation processes for semiconductor fabrication. In the past few years, the single crystal of high-purity
28
Si has been demonstrated to show higher thermal conductivity than that of natural silicon and semiconductor chips using it are considered to permit faster heat dissipation; therefore, the single crystal of high-purity
28
Si is drawing much attention as a promising candidate that contributes to further increase in the packaging density and operating speed of integrated circuits. As a potential application of single crystal silicon that is totally free of
29
Si having a nuclear spin, namely, the single crystal of high-purity
28
Si, the idea of the quantum computer using the magnetic properties of dopant phosphorus has been proposed and met with growing interest.
Thus, silicon isotopes have a greater possibility for large-scale use in various fields in the near future; however, in order to meet the demand for expanded use of silicon isotopes, it is essential to develop a technique capable of high-efficient isotope separation and enrichment.
Separation and Enrichment of Silicon Isotopes by Methods other than the Laser-Assisted Technology
An electromagnetic mass separator has long been used to separate silicon isotopes. In this method, however, the yield cannot be increased to a practical level by using even a much larger separator and the desired isotopes cannot be supplied in large enough quantities at low cost.
In Russia, gigantic centrifugal separators originally developed as a component of military nuclear facilities have been diverted to the purpose of silicon isotope separation; however, such gigantic centrifugal separators require huge initial investment and cannot be operated by the private sector without touching the sensitive issue of military secret.
Separation and Enrichment of Silicon Isotopes by the Laser-Assisted Technology
In order to separate and enrich silicon isotopes, the laser-assisted method has been proposed that is based on a carbon dioxide laser induced isotopically selective infrared multiple-photon dissociation of Si
2
F
6
(Japanese Patent H2-56133, U.S. Pat. No. 4,824,537, EU Patent 0190758, Japanese Patent H5-80245, and S. Arai, H. Kaetsu and S. Isomura, Appl. Phys., B32, 199 (1991)). On the pages that follow, the laser-assisted method has been described in details and then their several problems have been pointed out.
[Infrared Multiple-Photon Dissociation of Si
2
F
6
]
When molecules are irradiated with intense laser beam in a region within their infrared absorption bands, the individual molecules may sometimes be dissociated by absorbing as many as several tens of photons. This phenomenon is called infrared multiple-photon dissociation.
FIG. 1
shows an infrared absorption spectrum for a silicon fluoride Si
2
F
6
and emission lines from a carbon dioxide laser. As shown, Si
2
F
6
has an infrared absorption band in the emitting region of the carbon dioxide laser due to the stretching of the Si—F bond. If one of the emission lines in the 10R- or 10P-branch of the carbon dioxide laser is selected and Si
2
F
6
is irradiated with its intense pulsed laser light (hv) the individual molecules of Si
2
F
6
absorb a large number of photons to become excited in a highly vibrational state and are eventually decomposed into SiF
2
and SiF
4
. This is what is commonly called the infrared multiple-photon dissociation of Si
2
F
6
and described by the following scheme, where n refers to the number of laser photons absorbed by the Si
2
F
6
molecule:
Si
2
F
6
+nhv→SiF
2
+SiF
4
One of the two decomposition products, SiF
4
is a stable gas molecule whereas SiF
2
is an unstable molecule which undergoes cascade polymerization to produce a white solid substance having the composition (SiF
2
)
m
in accordance with the following scheme, where m refers to the number of the SiF
2
molecules being polymerized:
mSiF
2
→(SiF
2
)
m
Studies on pyrolysis have shown that the process of Si
2
F
6
decomposition into SiF
2
and SiF
4
requires energy of about 190 kJ/mol. This value means that the molecule of Si
2
F
6
that has undergone infrared multiple-photon dissociation must have absorbed at least 17 or more photons from the carbon dioxide laser.
[Absorption Spectrum and Isotope Shift]
The infrared absorption spectrum of Si
2
F
6
shown in
FIG. 1
was measured with an infrared spectrophotometer. In the ordinary measurement using the spectrophotometer, the intensity of incident light is low and an absorption spectrum corresponds to the process in which a molecule of interest absorbs one photon. The spectrum corresponding to this single-photon absorption is hereunder referred to as the ordinary infrared absorption spectrum.
In the ordinary infrared absorption spectrum of Si
2
F
6
, the peak of the absorption band due to the stretching of the Si—F bond is located at 990 cm
−1
for
28
Si—F, at 982 cm
−1
for
29
Si—F and at 974 cm
−1
for
30
Si—F. An effect generally called the isotope shift is often observed between absorption bands for different isotopes. Since the peaks of the absorption bands for
28
Si—F,
29
Si—F and
30
Si—F in
FIG. 1
are located so close to each other that adjacent bands overlaps, and, in addition, the amounts of
29
Si—F and
30
Si—F are much smaller than that of
28
Si—F, it is difficult to distinguish between the peak positions except for
28
Si—F.
Each Si
2
F
6
molecule absorbs at least 17 photons and undergoes infrared multiple-photon dissociation. As is well known, the multiple-photon absorption spectrum which corresponds to the process of molecular absorption of many photons differs from the ordinary single-photon absorption spectrum shown in FIG.
1
and the peak positions of the absorption bands shift toward a longer wavelength side or toward a smaller wave number side, and the widths of the absorption bands become broader. The occurrence of isotope shifts comparable to those in the single-photon absorption spectrum is also predicted for the multiple-photon absorption spectrum and, in fact, has been demonstrated in experiments.
[Principle of Laser Isotope Separation and Enrichment]
Noting the difference in infrared multiple-photon absorption that is observed between molecules containing different isotopes, one can irradiate such molecules with laser beam at an appropriate wavelength and thereby perform selective multiple-photon excitation of a molecule containing an isotope of interest so as to decompose it. Consequently, the isotope of interest is enriched in either the decomposition product or the yet to be decomposed feed. This is the principle of isotope separation and enrichment on the basis of infrared multiple-photon dissociation.
[Current Status of La
Arai Shigeyoshi
Hashimoto Masashi
Ishii Takeshi
Ohba Hironori
Ohya Akio
Banner & Witcoff , Ltd.
Japan Atomic Energy Research Institute
Wong Edna
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