PROCESS AND APPARATUS FOR THE ISOLATION OF PURE, OR SUBSTANTIALLY PURE, ORGANOMETALLIC COMPOUNDS

Abstract: A process and apparatus to enable the continuous isolation of an organometallic compound, such as trimethylindium form a liquid feedstock. The liquid feedstock is delivered to a distillation column (2) having two heating zones (6, 8) to effect dissociation of the feedstock thereby liberating the organometallic compound which is collected as a vapour form the top (4) of the column.

Description: PROCESS AND APPARATUS FOR THE ISOLATION OF URE, OR SUBSTANTIA LLY P U RE , ORGANOMETALLIC COMPOUNDS DESCRIPTION The present invention relates to an improved process and apparatus for the isolation of pure, or substantially pure, organometallic compounds, in particular organo compounds of Group 3 a metals, especially Trimethylindium. Organometallic compounds, such as Trimethylindium, Trimethylgallium and Triethylgallium are commonly employed as metal sources in the fields of epitaxial semiconductor growth and/or processing, vapour or plasma etching, plasma deposition or thin film deposition, for example Metalorganic Chemical Vapour Deposition (MOCND). The compounds are generally isolated using a batch process wherein the reactants are fed into a vessel and the product collected under the correct experimental conditions. Organometallic compounds are usually isolated from a liquid feedstock, which is formed in the production process, comprising the group 3 a compound and a suitable ligand, for example tetraglyme. Thermal dissociation of this feedstock liberates the desired product under mild conditions and leaves the less volatile components in the main vessel. This process provides compounds that are sufficiently pure but requires the use of large reaction vessels to produce sufficient yield of product. The production rate of the compound is also limited due to the necessity to charge the main vessel, collect the product and then clean out the vessel before commencing the process again. Cleaning equipment between runs may also lead to increased contamination of the prodμct. It is an aim of the present invention to provide an improved process and apparatus for the isolation of pure, or substantially pure, organometallic compounds, in particular the isolation of Trimethylindium, which overcome the abovementioned drawbacks. Accordingly, a first aspect of the present invention provides a continuous process for the isolation of an organometallic compound comprising the steps of delivering a liquid feedstock containing the desired organometallic compound to a heated reaction centre, allowing thermal dissociation of the feedstock to occur, effecting vapourisation of the organometallic compound and collecting the isolated organometallic compound. A second aspect of the present invention provides an apparatus for the continuous isolation of a substantially pure organometallic compound, the apparatus comprising a delivery conduit for input of a liquid feedstock, a distillation column and at least one outlet, the distillation column having at least one controllable heat source such as to cause thermal dissociation of the feedstock and vapourisation of the product thereby enabling continuous collection of the vaporised product through the outlet. The process of the present invention may be used to isolate any organometallic compound from a feedstock wherein the product is volatile and the by-products are less volatile. The process requires the feedstock to be fed into a distillation column, preferably into the centre of the column. An outlet is preferably provided at or near the top of the column for the collection of the isolated product and a further outlet is preferably provided at or near the base of the column for removal of waste products. A collection vessel, connected to the first outlet, is preferably provided for collection of the product. Preferably, a second or further collection vessel, connected to the second outlet, is provided for collection of waste products. The first collection vessel is preferably cooled prior to commencing the process. More preferably, the vessel is cooled to at least -15°C, more preferably at least -20°C. The equipment is maintained at specified temperatures to control the rate of removal of the product from the top of the column. The continuous delivery of the feedstock at a predetermined rate into the column, together with the maintenance of a specified temperature and pressure differential in the column enables a continuous production of the pure organometallic compound to be achieved. The rate of addition of the feedstock may be controlled by means of appropriate flow controllers, preferably liquid mass flow controllers. The process is preferably carried out under vacuum, preferably at a pressure of at least 1 Torr (133.32 Nm "2 ), more preferably at least 2 Torr (266.64 Nm '2 ). The continuous process relies on an equilibrium being set up in the column such that the product goes up the column and waste products move down. A number of factors effect the maintenance of this equilibrium, such as temperature of the column, the system pressure, the addition rate of the feedstock, and the removal rate of material from the column. The conditions employed will vary according to the feedstock composition and the organometallic compound being collected. Preferably the temperature and/or pressure will remain fixed with the main control being the addition rate of the feedstock. Preferably the distillation column for use i the process and apparatus of the present invention is equipped with two heating zones, a first heating zone to dissociate the liquid feedstock and liberate the desired organometallic compound and a second heating zone to ensure the organometallic remains in the vapour phase and the other components are condensed. The first zone is preferably at and/or below the centre of the column and the second zone is preferably above the centre. The temperatures of the respective heating zones will depend upon the feedstock being used and the organometallic compound to be isolated. Any suitable distillation column may be used for carrying out the process of the present invention, such as packed or plate distillation columns. The column should be equipped with sufficient plates in the decomposition section to ensure maximum removal of product from the feedstock and sufficient plates in the rectification section to ensure pure product leaves the top of the column. The reaction centre is preferably heated prior to commencing delivery of the feedstock in order to achieve efficient product separation and a steady state. Collection of product may be commenced immediately on addition of the feedstock but preferably is commenced after a constant temperature in the rectification column has been achieved. Preferably, collection of product is halted if a rise in temperature in the rectification column is observed indicating less volatile component boil up. The process of the present invention is particularly suitable for the preparation of Trimethylindium in a continuous fashion. Preferably, the feedstock is the ad uct trimethylindium.tetraglyme. The ratio of trimethylindium.tetraglyme is preferably in the range 2:1 to 3:1. The lower heating zone is preferably heated to 100°C - 140°C, more preferably 120°C to 130°C to achieve cracking of the adduct and to vapouαrise the trimethylindium. The upper heating zone is preferably heated to 30°C to 60°C, more preferably at least 40°C to prevent condensation of trimethylindium as it passes up the column. The present invention will now be further illustrated by means of the following Examples which describe the continuous preparation of Trimethylindium using the process and apparatus of the present invention and with reference to the accompanying drawing which is a schematic diagram of one embodiment of the present invention. Example 1 A main distillation column 2 was packed and placed under inert atmosphere using a series of vacuum/nitrogen cycles passed through the upper outlet 4 at the top of the column. A lower heated zone 6 was raised in temperature to 120°C and a upper heated zone 8 raised to 60°C. The initial product collection vessel (not shown) was cooled to -20°C. A vacuum was then established in the system through the collection vessel of approximately lTorr (133.32 Nm- 2 ). Once a steady state had been achieved, dosing of the feedstock through inlet 10 into the centre 12 of the column was begun at a rate of 7ml/min. The feedstock was an adduct of Trimethylindium (TMI) and Tetraglyme in a ratio 2.25:1. Solid TMI was observed to be collected in the collection vessel, via the outlet 4, after several minutes once a liquid equilibrium had been established in the column. After 3 hours, approximately 1.25 litres of feedstock had been added to the column with 500g of solid TMI product collected from the top of the column. The waste product, a reduced ratio TMI.Tetraglyme adduct, was removed from the bottom of the column into a separate vessel (not shown), via a lower outlet 14. As the 1:1 TMI. Tetraglyme adduct is thermally stable and thus will not liberate further TMI the process efficiency was calculated on the free TMI present in the feedstock and proved to be 84% Example 2 The apparatus used in Example 1 above was employed without cleaning. Hence, the column was primed with feedstock/waste products from the start. The lower heated zone 6 was again raised in temperature to 120°C but the upper heated zone 8 was raised to 40°C. All other procedures were the same except a vacuum of approximately 2.4 Torr (319.968Nm "2 ) was established in the system through the collection vessel and dosing of the TMI.Tetraglyme feedstock was begun at a rate of 4ml/min in a ratio 2.3:1. Sohd TMI was observed to be collected almost immediately as the liquid equilibrium in the column was established. After 6.5 hours approximately 1.5 litres of feedstock had been added to the column with 45 Og of sohd TMI product collected from the top of the column. The waste product, a reduced ratio TMI. Tetraglyme adduct, was removed from the bottom of the column. As the 1:1 TMI.Tetraglyme adduct is thermally stable and thus will not liberate further TMI the process efficiency was calculated on the free TMI present in the feedstock and proved to be 70%. Example 3 The process described in Example 1 above was repeated. The lower heated zone was again raised in temperature to 120°C but the upper heated zone was raised to 40°C. A vacuum of approximately 2Torr (266.64 Nm "2 ) was established and once a steady state had been achieved, dosing of the TMI.Tetraglyme feedstock (in a ratio 2.3: 1) was begun at a rate of 2ml/min. Solid TMI was observed to be collected after several minutes once a liquid equilibrium had been established in the column. After 8 hours approximately 1 litre of feedstock had been added to the column with 390g of solid TMI product collected from the top of the column. The waste product, a reduced ratio TMLTetraglyme adduct, was removed from the bottom of the column. As the 1:1 TMLTetraglyme adduct is thermally stable and thus will not liberate further TMI the process efficiency was calculated on the free TMI present in the feedstock and proved to be 96%. The process and apparatus of the present invention enable the continuous isolation of an organometallic compound to be achieved with sufficient yields of product. This allows the ready scale up of the process without the need for huge plant vessels and additional processing steps. The continuous process allows the use of much smaller vessels, for example 10L holding vessels as opposed to 200L vessels, to produce a similar rate of production with associated safety benefits. Additionally the process avoids the need for frequent dismantling and cleaning of the equipment thereby reducing potential contamination of the product and again increasing production rates and improving safety ratings. The process and apparatus are applicable to both solid and liquid products using a suitable liquid feedstock.

Claims: CLAIMS 1. A continuous process for the isolation of an organometallic compound comprising the steps of delivering a liquid feedstock containing the desired organometallic compound to a heated reaction centre, allowing thermal dissociation of the feedstock to occur, effecting vapourisation of the organometallic compound and collecting the isolated organometallic compound. 2. A continuous process as claimed in claim 1, wherein the isolated product is volatile and waste products produced from the dissociation are less volatile. 3. A continuous process as claimed in claim 1 or 2 wherein the liquid feedstock is delivered to the centre of a distillation column. 4. A continuous process as claimed in claim 3 wherein the dissociated organometallic compound is collected as a vapour at or near the top of the column. 5. A continuous process as claimed in any one of claims 3 to 4, wherein waste products are collected at or near the base of the column. 6. A continuous process as claimed in any one of the preceding claims wherein the isolated product is collected in a separate collection vessel that is cooled to at least -15°C. 7. A continuous process as claimed in claim 6 wherein the selection vessel is cooled to - 20°C. 8. A continuous process acclaimed in any one of claims 3 to 7wherein the process is maintained at specified temperatures to control the rate of removal of the product from the column. 9. A continuous process as claimed in any one of claims 3 to 8 wherein continuous delivery of the feedstock at a predetermined rate into the column, together with the maintenance of a specified temperature and pressure differential in the column enables a continuous production of isolated organometallic compound to be achieved. 10. A continuous process as claimed in any one of the preceding claims wherein the dissociation and isolation is carried out under vacuum. 11. A continuous process as claimed in claim 10 wherein the dissociation and isolation is carried out a pressure of at least 1 Torr (133.32 Nm "2 ). 12. A continuous process as claimed in any one of claims 9 to 11 wherein the temperature and/or pressure is kept fixed and the main control for the isolation of the product is the addition rate of the feedstock. 13. A continuous process as claimed in any one of the preceding claims wherein the feedstock is heated in a first heating zone to dissociate the feedstock and liberate the organometallic compound and the organometallic compound passes through a second heating zone to keep it in the vapour phase. 14. A continuous process as claimed in claim 13 wherein the first heating zone is kept at a higher temperature than the second heating zone. 15. A continuous process as claimed in any one, of the preceding claims wherein the reaction centre is heated prior to commencing delivery of the feedstock. 16. A continuous process -as claimed in any one of claims 3 to 15 wherein collection of the product is commenced after a constant temperature has been achieved in the column. 17. A continuous process as claimed in any one of the preceding claims for the preparation of trimethylindium. 18. A continuous process as claimed in claim 17 wherein the feedstock is the adduct trim.ethylindium.tetraglyme. 19. A continuous process as claimed in claim 18 wherein the ratio of trimethylindium.tetraglyme is in the range 2:1 to 3:1. 20. A continuous process as claimed in any one of claims 17 to 19 wherein the lower heating zone is maintained at a temperature of between 100 to 140°C and the upper heatmg zone is maintained at a temperature of between 30°C to 60°C. 21. An apparatus for the continuous isolation of a substantially pure organometallic compound, the apparatus comprising a delivery conduit for input of liquid feedstock, a distillation column and at least one outlet, the distillation column having at least one controllable heat source such as to cause thermal dissociation of the feedstock and vapourisation of the product thereby enabling continuous collection of the vapourised product through the outlet. 22. An apparatus as claimed in claim 21 wherein the outlet is provided at or near the top of the column for collection of the isolated product and a further outlet is provided at or near the base of the column for removal of waste products. 23. An apparatus as claimed in claims 21 or 22 wherein a collection vessel is connected to the outlet for collection of the product. 24. An apparatus as claimed in claim 23 wherein the collection vessel is cooled to at least - 15°C. 25. An apparatus as claimed in any one of claims 21 to 23 wherein the column is provided with at least two heating zones, a first zone to dissociate the liquid feedstock and liberate the desired organometallic compound and second zone to prevent condensation of the vapourised product. 26. An apparatus as claimed in claim 25 wherein the first heating zone is at and Or below the centre of the column and the second heating zone is above the centre of the column. 27. An apparatus as claimed in any one of claims 21 to 25 wherein liquid mass flow controllers are provided to control the rate of addition of the feedstock to the column. 28. An apparatus as claimed in any one of claims 21 to 27 for the isolation of trimethylindium. 29. An apparatus as claimed in claim 28 wherein the feedstock is tiimethylindium.tetraglvme. 30. An apparatus as claimed in claim 28 or claim 29 wherein the first heating zone is maintained at a temperature in the range 100°C - 140°C and the second heating zone is maintained at a temperature in the range of 30°C to 60°C. 31. An apparatus substantially as hereinbefore described and with reference to the accompanying drawing.

Inventors: JACOBS PHILIIP REEVE, ODEDRA RAJESH, RAVETZ MEGAN SARAH, WILLIAMS GRAHAM, JACOBS, PHILIIP, REEVE, ODEDRA, RAJESH, RAVETZ, MEGAN, SARAH, WILLIAMS, GRAHAM
Applicants: EPICHEM LTD, JACOBS PHILIIP REEVE, ODEDRA RAJESH, RAVETZ MEGAN SARAH, WILLIAMS GRAHAM, EPICHEM LIMITED, JACOBS, PHILIIP, REEVE, ODEDRA, RAJESH, RAVETZ, MEGAN, SARAH, WILLIAMS, GRAHAM
Application year: 2001
Main classification: B01D3
IPC codes: B01D3, C07F5