WO2000048941A1

WO2000048941A1 - Enrichment of silicon or germanium isotopes

Info

Publication number: WO2000048941A1
Application number: PCT/US2000/003016
Authority: WO
Inventors: Teimuraz Abesadze; William E. Saunders; Marvin Y. Wachs; Dennis K. Manning
Original assignee: Eagle-Picher Industries, Inc.
Priority date: 1999-02-16
Filing date: 2000-02-03
Publication date: 2000-08-24
Also published as: AU3483700A

Abstract

Isotopes of either germanium or silicon are separated by a chemical exchange reaction. Preferably the compound is a hydro halide or halide composition and the donor can be a wide variety of compounds such as a lower molecular weight alcohol. In operation, for example, the silicon tetrafluoride is introduced through (14) into column (12). This will pass upwardly through stripping section (18) and outlet (34) into recombiner (36). Here it mixes with clean, purified methanol pumped through line (92). When equilibrium in the column is established, a fraction of the Si-28 enriched silicon tetrafluoride will be collected through outlet (24).

Description

ENRICHMENT OF SILICON OR GERMANIUM ISOTOPES

Silicon and germanium are both used extensively in

semiconductor applications. Both of these elements are made up of

multiple isotopes. Silicon, for example, includes ²⁸Si, ²⁹Si and ³⁰Si. By far

the most abundant isotope is the ²⁸Si making up over 92% of natural

silicon.

Germanium has five different isotopes, ⁷⁰Ge, ⁷²Ge, ⁷³Ge,

⁷⁴Ge and ⁷⁶Ge. Although ⁷⁴Ge is predominant forming about 36% of

natural germanium. ⁷⁰Ge and ⁷²Ge form 20% and 27% of natural

germanium, respectively.

In certain applications, it is desirable to utilize isotopically

enriched germanium or silicon. For example, Arai et al., U.S. Patent

4,824,537 indicates that enriched silicon isotopes can be used in

electronic devices. Ma U.S. Patents 5,442,191 and 5,144,409 both

disclose the use of isotopically enriched semiconductor materials. To date very little work has been done with such isotopically

enriched silicon and germanium. The primary reason for this is the

inability to efficiently separate the isotopes. The Arai '537 patent

discloses one technique utilizing a carbon dioxide laser. The yields of

laser separation are very low and the process itself is not actually

commercially viable.

Distillation has also been used to separate silicon isotopes.

(See Mills, Thomas R., Silicon Isotope Separation by Distillation of

Silicontetrafluoride. Separation Screen and Technology, 25(3), pp. 335-

345, 1990). However, because of the very low separation constant this

process is simply not commercially viable.

Chemical exchange reactions have been used to separate

isotopes of elements with lower atomic weights such as carbon, nitrogen,

oxygen and boron. However, silicon and germanium have such high

atomic weights that they were not considered suitable for chemical

exchange reactions.

Summary of the Invention

The present invention is premised upon the realization that

silicon as well as germanium isotopes can be separated by chemical

exchange reactions. In particular, silicon or germanium halides or hydro halides undergo chemical exchange reactions with complexes formed by

these compounds and Lewis base ligands. This separation mechanism

can be used to efficiently separate ²⁸Si from ²⁹Si and ³⁰Si or ²⁹Si from ³⁰Si.

Likewise these chemical reactions will permit separation of ⁷⁰Ge from the

⁷²Ge, ⁷³Ge, ⁷⁴Ge and ⁷⁶Ge and subsequent separation of the remaining

germanium isotopes from each other. In particular, low molecular weight

alcohols including methanol, ethanol and isopropanol are effective

ligands as well as carbon.

By utilizing chemical exchange reactions according to the

present invention, the cost of isolating silicon or germanium isotopes can

be reduced by a factor of between 10 and 100.

The present invention will be further appreciated in light of

the following detailed descriptions and drawings in which:

Brief Description of Drawing

The Figure is a diagramatic view of an isotopic separation

chemical exchange process for use in the present invention.

Detailed Description

For use in the present invention, the germanium and silicon

compounds useful in the present invention should have one of the following general formulas: GeH_nX₄._n or SiH_nX₄._n wherein X represents a

halide and _n equals an integer from 0 to 4. In the present invention, the

preferred halide is fluorine and preferably _n is 0 or 1 for either a

germanium or silicon compound.

The isotopic separation reaction for silicon and germanium

utilized in the present invention may be written in the following general

form:

^*AH_nX₄._n ^{+ y}AH_nX₄._nL = ^yAH_nX₄._n + ^xAH_nX₄._nL

In this reaction ^x and ^y represent isotopic species of silicon

or germanium, A represents those elements and L is a complexing agent.

Si or Ge compounds will be reacted with a complex during

the isotope separation process to form an isotopically enriched complex

which is recovered. Ligands which form complexes with those

compounds, can vary widely. Generally the ligands are volatile organic

solvents, particularly lower alkyl alcohols, such as methanol, ethanol, and

propanol. Polyethylene glycol, crown ethers and alkyl amines also can

be used as a solvents in this chemical exchange process. It is preferable

that the complexing agent be liquid at separation temperature, but this is

not critical. It may be in the gas phase. Further the present invention can

utilize solid ligands such as activated carbon, silica gel or molecular

sieves. Since Si and Ge have 3 and 5 isotopes respectively, the

chemical exchange process will utilize 3 and 10 such reactions.

The general process and apparatus for enriching either

silicon or germanium isotopes is depicted in Fig. 1. This will be discussed

with respect to silicon and specifically silicon tetrafluoride. However, the

same process can be used with other silicon precursors as well as

germanium precursors.

Preparation of the complex may take place either inside or

outside the column. Correspondingly circulation of the complexing agent

may be arranged externally or internally of the column.

Selection of the complex circulation scheme greatly

depends on the thermal properties of the complex and ligand itself. The

following description utilizes external ligand circulation and complex

formation inside the column. Again this describes the separation of

silicon isotopes using a compound such as silicon tetrafluoride. The

same scheme works in the separation of germanium isotopes using, for

example, germanium tetrafluoride.

As shown in the figure, the column 12 used for the

separation of the present invention generally includes a complex inlet 14

which leads into the stripping section 16 of the column 12. The heavier

isotope is then collected through outlet 22 wherein the lighter isotope is collected through outlet 24. A further midsection outlet 26 is provided to

collect the intermediate weight isotope. This is optional and its location

can be varied depending upon the particular separation ligand,

temperature, flow rates and the like.

Below the stripping section 16 is an outlet line 28 which

leads to a low temperature stripper 38. Likewise, there is an upper outlet

34 above the enrichment section 18 which is directed to a recombiner 36.

Complex formed in the recombiner flows downwardly in the

column and undergoes a chemical exchange reaction with the Silicon

compound penetrating upwardly. As a result of this reaction the complex

is enriched with Si-28 and depleted with Si-29 and Si-30, while gas will

have just the opposite shift in isotopic abundance. Complex collected

through outlet 28 is directed to the low temperature stripper 38 and then

reboiler 46, where the primary separation of the ligand and silicon

compound takes place. A high level of complex dissociation and stripping

of silicon compound from the complexing agent is achieved in the high

temperature stripper 62 and reboiler 66. Silicon compound from reboilers

66 and 46, after passing corresponding strippers enter column through

inlet 52.

The outlet 74 from the high temperature reboiler 66 directs

ligand to a purification apparatus 76. Although this may vary widely depending upon the particular ligand chosen as well as operating

conditions, the purification apparatus shown includes a purification still 78

connected to a reboiler 80 and to a condensing unit 84. A pump 86 in

outlet line 82 forces ligand from condensing unit 84 through a chiller 88

and in turn through ligand feedline 92 to the recombiner 36. In the

recombiner 36 the ligand is combined with isotopically depleted silicon

gas and directed back downwardly through line 96 into the enrichment

section of the column 12.

In operation, for example with silicon tetrafluoride utilizing

methanol as a ligand, the silicon tetrafluoride is introduced through line

14 into column 12. This will pass upwardly through stripping section 18

and outlet 34 into recombiner 36. Here it mixes with clean, purified

methanol pumped through line 92. The methanol should contain less

than 100 ppm of Silicon tetrafluoride to prevent short-circuiting of the

isotope separation process. In the recombiner SiF₄-Methanol complex is

formed and cooled below 0°C. The complex is introduced into the

column and flows downward. At normal conditions the molar ratio of

silicon tetrafluoride/methanol complex would be 1 to 4.

When equilibrium in the column is established, a fraction of

the Si-28 enriched silicon tetrafluoride, approximately 1 % or less of the

material passing in the line 52, will be collected through outlet 24. On the other end of the column about the same amount of Si-28 depleted SiF₄

is collected through the outlet 22. Additional SiF₄ is introduced through

line 14 to compensate for the collected SiF₄.

The height of the column 12 will vary depending upon which

silicon compound and complexing agent are used, the reflux ratio and

other reaction conditions. For the silicon tetrafluoride/methanol system

with a reflux ratio of about 150-200, a column of 120 feet is sufficient to

obtain 99.9% enriched Si-28 product.

Further the separation coefficient, alpha, will vary widely

depending on the temperature and type of the solvent. Accordingly, as

is well known to those skilled in the art, the overall conditions of the

separation column can be varied to optimize the separation efficiency.

The separation coefficient can be determined by bubbling

the silicon or germanium compound, for example silicon tetrafluoride, into

a cooled jacketed reactor through the ligand, either solid or liquid, until it

is saturated. After the complex/silicon tetrafluoride reaches equilibrium,

a gas sample is taken. The amount of silicon tetrafluoride in the liquid or

solid complex is much greater than in the gas phase. Therefore, one can

assume the silicon isotope distribution in the complex is close to natural

and is shifted alpha times in the gas phase. Thus, the measurement of

isotopic ratio of the gas sample provides direct data on the enrichment coefficient alpha for each complexing agent. This was evaluated using

silicon tetrafluoride with respect to the preferred ligands for use in the

present invention with the following results.

METHANOL

Natural Samples

#1 #2 #3 #4

Si-28 92.256 92.250 92.251 92.251

Si-29 4.650 4.652 4.651 4.651

Si-30 3.094 3.099 3.098 3.098

Average

Si -28 92.252

Si -29 4.651

Si -30 3.097

Enriched Samples

Si-28 92.094 92.079 92.116

Si-29 4.745 4.751 4.735

Si-30 3.161 3.169 3.149

Average

Si -28 92.096

Si -29 4.744

Si -30 3.160

Separation Coefficient

α²⁸ = 1.022 +/-0.003: (0.9786) reverse isotopic effect α²⁹ = 1.021 +/-0.002: α³⁰ = 1.021 +/-0.004. ETHANOL

Enriched Samples

Si-28 92.171

Si-29 4.687

Si-30 3.143

Separation Coefficient

α²⁸ = 1.0116 +/-0.0005: (0.9885) reverse isotopic effect ²⁹ = 1.0081 +/-0.0002: α³⁰ = 1.015 +/-0.001.

ISOPROPANOL

Enriched Samples

#1 #2 Average

Si-28 92.229 92.146 92.188

Si-29 4.663 4.712 4.688

Si-30 3.108 3.142 3.125

Separation Coefficient

α²⁸ = 1.009 +/-0.006: (0.9911 ) reverse isotopic effect α²⁹ = 1.008 +/-0.005: ³⁰ = 1.009 +/-0.004

ACTIVATED CARBON BEADS (ACB)

Enriched Samples

Si-28 92.232 92.237 92.221

Si-29 4.679 4.680 4.693

Si-30 3.089 3.083 3.141 Average

Si-28 92.230

Si-29 4.684

Si-30 3.104

Separation Coefficient

α²⁸ = 1.003 +/-0.001 : (0.9969) reverse isotopic effect α²⁹ = 1.007 +/-0.003: α³⁰ = 1.002 +/-0.014.

This data indicates the separation technique of the present

invention using the silicon halide in combination with the selected ligands

effectively separates the respective isotopes. As it is known for some

other isotope separation processes, SiF₄ shows so-called reverse isotopic

effect, i.e., the heavier isotope is collected from the top of the column

whereas the lighter isotope concentrates in the lower portion of the

column.

This data further indicates the present separation technique

will effectively separate isotopes of either silicon or germanium and is

more efficient than separation techniques such as distillation or laser

separation. This has been a description of the present invention along

with the preferred method of practicing the invention. However, the

invention itself should only be defined by the appended claims wherein

we claim:

Claims

1. A method of isotopically enriching a compound having the

following general formula:

AH_nX₄._n

wherein A represent Si or Ge

n is an integer from 0 to 4

X represents a halide ion.

comprising subjecting said compound to a chemical

exchange reaction with a donor compound to form an isotopically

enriched complex and separating said enriched complex from depleted

compound.

2. The method claimed in claim 1 wherein said donor

compound is selected from the group consisting of methanol, ethanol,

proponal and activated carbon.

3. The method claimed in claim 1 wherein A represents silicon.

4. The method claimed in claim 3 wherein X represents halid

5. The method claimed in claim 4 wherein said compound is

SiF₄.

6. The method claimed in claim 1 wherein A represents

germanium.

7. The method claimed in claim 6 wherein X represents a

halide and _n represents 0.

8. The method claimed in claim 7 wherein said compound is

GeF₄.

9. The method claimed in claim 1 where said compound is

GeF₄.

10. The method claimed in claim 1 wherein said donor

compound is selected from group consisting of C₁-C₄ alkyl alcohols,

polyethylene glycols, ethers, molecular sieves, silica gel and activated

carbon.