GB2201418A - Preparation of metal alkyls - Google Patents

Abstract

A process for preparing a pure Group III metal alkyl such as a trimethyl aluminium, gallium or indium, the process comprising the steps of dissolving (1) the low purity metal alkyl in a suitable solvent, adding (2) a nitrogen donor ligand and forming a metal alkyl adduct, crystallising (3) the adduct from the solution, dissociating (4) the adduct by heating under vacuum to a temperature below the sublimation temperature of the adduct, then cooling (5) the released material to obtain the required metal alkyl in purified form. The pure metal alkyls are used in the manufacture of semiconductors.

Description

PREPARATION OF METAL ALKYLS This invention relates to the preparation of metal alkyls. It relates particularly to the preparation of Group III metal alkyls such as trimethyl aluminium, trimethyl gallium and trimethyl indium.

The abovementioned trimethyl compounds have become of considerable importance since they can be used together withhydrides of Group V elements (arsine or phosphine) for the growth of epitaxial layers of compound semiconductors (gallium arsenide, indium phosphide, aluminium arsenide, and their ternary and quaternary alloys). These semiconductor materials are of major importance in the electronics industry.

An essential prerequisite is that the metal alkyls must be of very high purity and this requirement presents a major challenge to the organometallic chemist. Recently, purification methods have been reported which involve the formation of adducts between the metal alkyl and a Lewis Base such that the base and the adduct are essentially non-volatile, so that volatile impurities can be removed at room temperature, but which dissociate into the free metal alkyl on heating in vacuo Some examples of methods which make use of phosphorus donor ligands are disclosed in International Patent Publication No. WO 85/04405.

The present invention makes use of an alternative donor ligand composition which has been found to have advantages in cost and in simplicity in preparation of the required alkyl.

According to the invention, there is provided a process for preparing a pure Group III metal alkyl such as a trimethyl aluminium, gallium or indium, the process comprising the steps of dissolving the low purity metal alkyl in a suitable solvent, adding a nitrogen donor ligand and forming a metal alkyl adduct, crystallising the adduct from the solution, dissociating the adduct by heating under vacuum to a temperature below the sublimation temperature of the adduct, then cooling the released material to obtain the required metal alkyl in purified form.

The choice of nitrogen donor ligand is an important one because not all nitrogen containing bases will be able to dissociate all of the resulting metal alkyl adduct at a temperature which is below the sublimation temperature of the adduct.

One particularly suitable nitrogen donor ligand is 4,4' methylenebis (N,-dimethylaniline), herein termed MBDA.

Preferably, the crystallisation step is followed by a recrystallisation of the adduct material from a quantity of clean solvent. The crystals produced at each stage may also be washed with clean solvent to remove traces of the mother liquor.

The crystals may be dried under vacuum prior to a step of heating the crystals to remove volatile impurities.

The solvent used may be benzene.

The invention also comprises a purified metal alkyl compound when prepared by the abovementioned process. The metal alkyl compound may be a trimethyl compound of aluminium, gallium or indium.

By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawing, in which: Figure 1 is a block diagram of the stages in the process, and Figure 2 and 3 are the chemical structures respectively of the MBDA base and the metal alkyl base adduct.

As depicted in Figure 1, the process of the invention comprises the steps of dissolving 1 the low purity metal alkyl in a suitable solvent, adding 2 the selected nitrogen donor ligand, crystallising 3 the resulting adduct from the solution, dissociating 4 the adduct by heating under vacuum to a temperature below the sublimation temperature of the adduct, then cooling 6 the released material to obtain the required metal alkyl in purified form.

The preparation of the adduct was carried out in standard glass flasks and schlenks using all greaseless taps etc.

Figure 2 shows the chemical structure of the preferred nitrogen donor ligand material, 4,4'-methylenebis (N,N-dimethylaniline) which in this specification is termed MBDA. Figure 3 is the chemical structure of the corresponding metal adduct, where M= indium, gallium or aluminium.

In general, the recrystallisation and washing of the adduct material after its formation is effected by recrystallisation from petroleum ether (40/60 C) as small white needles on cooling to a temperature of minus 30 C. The solution can be decanted from these crystals and the crystals washed with petroleum ether cooled to minus 30or, before drying in vacuo. The crystals can then be warmed up to about 60 C in vacuo to remove all volatile impnrities.

This step is able to be effected without significant loss of any trimethyl indium, gallium or aluminium.

Further details of the process will be given in the more detailed descriptions which follow. These deal respectively with the preparation of trimethyl indium, gallium and aluminium.

PREPARATION OF METHYLINDIUM MBDA Methyllithium, in a volume of 550ml of a 1.5 molar solution in diethylether (that is, 0.83 moles MeLi) was added to a stirred suspension of anhydrous indium chloride (InC13, 58.7g, 0.265mol) in diethylether (50ml) under reflux conditions. The addition took place over a period of two hours and the mixture was then refluxed for a further two hours. The mixture was next vacuum distilled by heating the reaction vessel at temperatures of up to 150 C for five hours and a distillate of methylindium and diethylether was collected as a clear colourless solution in a cold trap cooled to -196 C.

From the resulting distillate, excess diethylether was removed by a distillation at atmospheric pressure, thus affording the ether adduct Me3in.OEt2 To this etherate, a solution of MBDA (33.lg, 0.130 mol) in benzene (l5Oml) was added and the remaining diethylether was removed by fractional distillation through a vigreux column at atmospheric pressure.

The-benzene was then removed by distillation in vacuo to afford a viscous pale yellow oil of the adduct. Petroleum ether (40/60or) (600ml) was added to the oil and this then formed a clear almost colourless solution. A cooling of this solution to -30 C afforded small white needle-shaped crystals of the product, these were washed with cold (-300C) petroleum ether (lOOml) before drying in vacuo. The yield was 67.2g, 0.117 mol, 90%. The crystals were heated at 60 C in vacuo for two hours to remove the final traces of solvent and other volatile impurities.

LIBERATION OF ME'mYLTNDTUM FROM METlfllLlNDTUM MBDA The methylindium MBDA (66.7g, 0.116 mol) was heated in vacuo (about 10-2 Torr) at temperatures rising from 80 C to 1300C for four hours. The resulting methylindium (36.0g, 0.225 mol, 97) was collected in an adjoining flask acting as a cold trap at -1960C.

PREPARATION OF METHYLGALLIUM MBDA Trimethylgallium (28.0g, 0.244 mol) in petroleum ether (50ml) was added to a suspension of MBDA (29.0g, 0.114 mol) in petroleum ether (150ml). The base dissolved on shaking to give a clear colourless solution. Cooling of the solution to -300C afforded white platelet crystals of the product, which were washed with cold (-30 C) petroleum ether (100ml) before drying in vacuo. The yield was 53.9g, 0.111 mol, 98%. The crystals were heated at 700C in vacuo for three hours to remove the final traces of solvent and other volatile impurities.

lBERAT10N OF METHYLGALLlUM FROM METHYLGALLIUM MBDA The methylgallium MBDA (53.9g, 0.111 mol) was heated Ln vacuo at temperatures rising from 800C to 1300C for five hours. The liberated methylgallium (22.3g,0.194 mol, 87%) was collected in a adjoining flask acting as a cold trap at -1960C.

PREPARATION OF METHYLALUMINIUM MBDA Trimethylaluminium (34.4g, 0.477 mol) in petroleum ether (50ml) was added to a suspension of MBDA (60.6g, 0.238 mol) in a petroleum ether (500ml)/toluene (50ml) solvent mixture. The base dissolved on shaking to give a clear colourless solution. Cooling of the solution to -300C afforded white needle-shaped crystals of the product which were washed with petroleum ether (100ml) before drying in vacuo. The yield was 92.0g, 0.231 mol, 97%. The crystals were heated to l200C in vacuo for two hours to remove the last traces of solvents and other volatile impurities.

LIBER AT10N' OF METHYLALUMINIUM FROM METHYLALUMINIUM MBDA The methylaluminium MBDA (91.lg, 0.229 mol) was heated in vacuo at temperatures rising from 1400C to 2200C for twelve hours.

The resulting liberated methylaluminium (21.1g, 0.293 mol, 64%) was collected in an adjoining flask acting as a cold trap at -196 C.

Although the residue in the flask was shown to still contain considerable quantities of methylaluminium, by 1H nuclear magnetic resonance spectroscopy, this residue could not be liberated at 220 C and any beating above this temperature was found to lead to problems associated with the sublimation of the free base material.

The use of the MBDA nitrogen donor ligand material (which is sometimes also called Arnold's Base) has been found to have several advantages over the phosphorus donor ligands that have been already mentioned. These advantages include the following: 1. The MBDA can be used for the purification of all Group Ill trimethyls.

2. The MBDA does not coordinate to dimethyl zinc or dimethyl cadmium so these compounds can be easily removed.

3. The MBDA base is inexpensive to use (the current price may be about 100 per kilogram).

4. Very high yields can be obtained for the process steps of forming the adduct and for dissociating the adduct.

5. The low molecular weight of the MBDA base means that it carries a large amount of metal trialkyl per kilogram of base material.

6. The adducts are very easy to prepare.

7. Only small amounts of benzene, a possible carcinogen material, are required in the synthesis of the adduct.

8. The MBDA base does not contain Group VI donor atoms.

9. The vapour pressure of trimethyl indium over the adduct is up to 8mmHg even at a temperature below the melting point of the adduct.

10. Vapour pressures of trimethyl indium up to 80 mmHg can be obtained below 1500C.

11. The adduct is only mildly air-sensitive.

The foregoing descriptions of embodiments of the invention have been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims.

For instance, it is not essential that the nitrogen donor ligand material should be MBDA and possible alternative nitrogen donor ligands include 3,3'-bipyridyl and N,N,N',N'-tetramethyl-l,4- phenylenediamine (TMPDA). Further suitable alternative ligand materials will also be known to the organometallic chemist.

Claims (6)

1. A process for preparing a pure Group m metal alkyl such as a trimethyl aluminium, gallium or indium, the process comprising the steps of dissolving the low purity metal alkyl in a suitable solvent, adding a nitrogen donor ligand and forming a metal alkyl adduct,crystallising the adduct from the solution, dissociating the adduct by heating under vacuum to a temperature below the sublimation temperature of the adduct, then cooling the released material to obtain the required metal alkyl in purified form.

2. A process as claimed in Claim 1, in which the nitrogen donor ligand is 4,4'-methylenebis (N,N-dimethylaniline), herein termed MBDA.

3. A process as claimed in Claim 1 or 2, in which the crystallisation step is followed by a recrystallisation from clean solvent material.

4. A process as claimed in any one of Claims 1 to 3, in which prior to the dissociation step, the crystals are dried under vacuum to remove volatile impurities.

5. A process for preparing a pure Group m metal alkyl substantially as hereinbefore described with reference to the accompanying drawing.

6. A high purity Group III metal alkyl materiai when prepared by a process as claimed in any one of Claims 1 to 5.

6. A high purity Group III metal alkyl material when prepared by a process as claimed in any one of Claims 1 to 5.

Amendments to the claims have been filed as follows CLAIMS 1. A process for preparing a pure Group III metal alkyl such as a trimethyl aluminium, gallium or indium, the process comprising the steps of dissolving the low purity metal alkyl in a suitable solvent, adding a nitrogen donor ligand containing two or more nitrogen atoms and forming a metal alkyl adduct, crystallising the adduct from the solution, dissociating the adduct by heating under vacuum to a temperature below the sublimation temperature of the adduct, then cooling the released material to obtain the required metal alkyl in purified form.

2. A process as claimed in Claim 1, in which the nitrogen donor ligand is 4,4'-methylenebis (N,N-dimethylaniline), herein termed MBDA.

3. A process as claimed in Claim 1 or 2, in which the crystallisation step is followed by a recrystallisation from clean solvent material.

4. A process as claimed in any one of Claims 1 to 3, in which prior to the dissociation step, the crystals are dried under vacuum to remove volatile impurities.

5. A process for preparing a pure Group III metal alkyl substantially as hereinbefore described with reference to the accompanying drawing.