WO2019147648A1

WO2019147648A1 - Methods for the purification of diborane

Info

Publication number: WO2019147648A1
Application number: PCT/US2019/014718
Authority: WO
Inventors: Ce Ma; Atul Athalye; Johnny Chen; Carl Jackson
Original assignee: Linde Aktiengesellschaft
Priority date: 2018-01-26
Filing date: 2019-01-23
Publication date: 2019-08-01
Also published as: TWI821241B; TW201934474A

Abstract

A method for separating higher boranes from a mixture continuing diborane, higher boranes and a balance gas is disclosed. The higher boranes are typically tetraborane, pentaborane and decaborane. In one embodiment, the diborane is separated using at least one cold trap. Alternatively, the higher boranes are separated by a liquid filtration unit before forming a gaseous diborane mixture. In another embodiment, the higher boranes are separated from the mixture by a distillation column.

Description

METHODS FOR THE PURIFICATION OF DIBORANE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001 ] This application claims priority form US Provisional Patent Application Serial Number 62/622,199, filed on January 26, 2018.

BACKGROUND OF THE INVENTION

[0002] Diborane is an important gas source for p-type dopant Boron in silicon and silicon-germanium semiconductor crystalline layers by either in-situ doped epitaxy processes or plasma/ion implantation processes. Diborane is also used in making doped silicon dielectrics (borosilicate glass). Another application for diborane is as a reducing agent in tungsten atomic layer deposition (ALD).

[0003] However, at room temperatures, diborane will slowly decompose into higher boranes (BxHy where x>2 and y>6) and hydrogen. Typical higher boranes are tetraborane B₄HIO, pentaborane B5H9 and decaborane B10H14 that are detectable by GC-based methods such as GC-MS and GC-DID and FTIR analytical techniques. Due to the instability of diboranes (B2H6), the level of higher boranes can continue to increase without controls in high pressure gas or gas mixtures, particularly during transportation, storage, and usage in cylinder products. The decomposition problem becomes worse with higher diborane concentration in gas mixtures (e.g. 30%), higher cylinder pressures, and non-hydrogen balance gases such as argon and nitrogen. Higher boranes may be harmful in semiconductor manufacture processes. In in-situ doped embedded silicon-germanium (eSiGe) source and drain deposition of leading edge PMOS FinFET transistors, higher borane molecules may incorporate in the eSiGe layers and generate crystalline defects.

[0004] Shipping pure BaHe and diborane mixtures with constant cooling can be a solution to extending the product shelf life. However, it may be difficult to meet under international shipping requirements and local storage requirements and conditions.

[0005] Producing B2H6 in the point-of-use may be another solution to use the product before increased level of higher borane impurities occurs. However, it is difficult or inefficient to produce high purity BaHe in current synthesis methods using boron halides and metal hydride reactants.

[0006] There is still a need to purify diborane and its mixtures by removing higher boranes for providing local semiconductor customers with fresh and high purity diborane products.

[0007] In the present invention, a purification method is presented that is suitable for purifying diborane and its mixture gases, locally or on-site. Therefore, the stated problems are minimized. The local purification is close to the customer bases within the same country or region. The location of purification can be either at regional ESG production plants or at customer sites as on-demand or always on diborane mixture supply systems or subsystems.

SUMMARY OF THE INVENTION

[0008] The methods of purification in the present invention are disclosed in three different embodiments. The first is a gas-phase diborane mixture cold trap. The second embodiment facilitates liquid-phase diborane and hydrogen gas separation and higher boranes removal using liquid filtration techniques. The third embodiment uses gas/liquid phase of cryogenic distillation.

[0009] Accordingly, these three embodiments can be summarized as follows.

The first embodiment is a method of removing higher borane impurities from a mixture containing higher boranes, diborane, and balance gas such as hydrogen, comprising feeding the gas mixture to a cold trap by methods described herein, but maintaining the blending ratio of diborane to the balance gas.

[00010] The second embodiment is a method of removing higher boranes from a mixture containing higher boranes, diborane, and balance gas, comprising feeding the mixture to a diborane liquefaction unit followed by a liquid filtration unit by methods described herein, with purified liquid diborane produced in the separation process that can be used for later blending with a balance gas.

[00011] The third embodiment is a method of removing higher boranes from a mixture containing higher boranes, diborane, and balance gas, comprising feeding the mixture to a distillation column by methods described herein.

[00012] In the first embodiment, there is disclosed a method for removing higher boranes from a mixture containing diborane, higher boranes and balance gas, comprising the steps of feeding the gas mixture to at least one cold trap wherein the higher boranes are frozen onto walls of the cold trap; recovering the diborane and balance gas mixture without substantially liquefying the diborane, thereby

maintaining the ratio of diborane to balance gas; feeding a regeneration gas to the at least one cold trap after the recovery of the diborane and the balance gas, thereby regenerating the at least one cold trap; and venting the higher boranes and regeneration gas as waste.

[00013] In the second embodiment, there is disclosed a method for removing higher boranes from a mixture containing diborane, higher boranes, and a balance gas, comprising the steps of feeding the gas mixture to a cold trap liquid separator to condense the majority of the diborane and higher boranes; rejecting the balance gas from the cold trap liquid separator; filtering the liquid in the cold trap to remove higher boranes in the form of solid (ice) particles; recovering purified diborane liquid from the cold trap liquid separator; feeding the recovered purified diborane liquid to a heat exchanger, wherein the purified liquid diborane forms purified gaseous diborane; and blending a balance gas selected from the group consisting of hydrogen, argon and nitrogen with the purified gas diborane, thereby forming a gaseous mixture with the purified gaseous diborane.

[00014] In a further embodiment, there is disclosed a method for removing higher boranes from a mixture containing diborane, higher boranes, and a balance gas, comprising the steps of feeding the mixture to a distillation column, wherein a gaseous mixture of purified diborane and hydrogen is recovered from the top of the distillation column and a liquid waste containing diborane and a majority of the higher boranes is discarded from the bottom of the distillation column as waste; feeding the recovered gaseous mixture to a cold trap, wherein purified liquid diborane is formed; feeding the purified liquid diborane to a heat exchanger thereby forming gaseous diborane; and recovering the gaseous diborane from the heat exchanger for further blending with a balance gas.

[00015] In the first embodiment, the diborane gas mixture would maintain the original mixing ratio with limited removal of the higher boranes. This method would work with low concentration diborane mixtures (e.g., 1 % to 5% diborane in hydrogen). This method would be suitable for example at a customer on-site diborane mixture supply system or subsystem.

[00016] Advantages of the first embodiment including staying above diborane condensation temperatures thereby to maintain the original diborane mixing ratio with the balance gas. Lower temperatures will be better at condensing the higher boranes while still producing diborane mixtures with higher borane impurity B₄HIO as high as the 10s of parts per million of the gas mixture. [00017] !n the second embodiment, liquid-phase diborane is produced. This mixture will have a lower concentration of higher boranes than that which is achieved in the first embodiment. This method can be employed to produce a mixture with a selected balance gas such as hydrogen, argon or nitrogen and can be produced locally with respect to an end user’s sites.

[00018] The second embodiment provides advantages such as at the cold temperatures, diborane is liquefied while the hydrogen stays in the gas phase. The purified diborane can be kept for future mixing with the appropriate balance gas or remixed with pure hydrogen streams. The chiller temperature can be as low as - 135°C. The colder temperatures can be also applied before diborane reached its triple point at -164.9°C when a cryogenic gas is used as the cooling agent.

[00019] The third embodiment uses cryogenic distillation to purify the diborane feed stream mixture. This embodiment can produce a higher purity diborane or slightly reduced purity of diborane from the feed stream mixture depending upon the desired results and operating conditions.

[00020] The third embodiment also provides removal of higher boranes with a small diborane liquid flow. The recovered diborane liquid may be collected in a cold trap for future mixing with selected balance gases. Ten theoretical stages can be employed and still achieve contaminant levels of B4H10 as low as 1 part per billion.

[00021] The second and third embodiments can both be employed with a full range of diborane mixtures and are not limited to lower diborane mixture ratios as used in the first embodiment. Although both embodiments can be employed in regional purification plants or at customer sites, they are most efficient when used in a supplier’s regional purification plants. [00022] In all of these embodiments, analytical equipment can be incorporated to measure the purified gas stream higher borane concentrations. The methods of analysis are typically FTIR and GC-based methods such as GC-MS and GC-DID for higher boranes. Measurements for adjusting mixing ratios can also be performed using binary gas analyzers.

[00023] The higher boranes can be selected from the group consisting of tetraborane, pentaborane and decaborane.

[00024] The diborane in the mixture will typically be less than 5% by mole or volume, typically 1 or 2% by mole or volume and below for the methods of the first embodiment. There is no limit to the diborane concentration in the second and third embodiments.

[00025] For 100% diborane feed, B4H10 is typically present at around 200 parts per million per supplier’s information. Different feed mixtures will have B4H10

concentrations that are diluted by balance gas molar fractions. However, thermal decomposition is a kinetic process with chain reactions that form higher and more stable borane hydride polymers. Therefore, the measurable higher boranes such as tetra-, penta- and decaborane are reaction intermediates. Concentration as such may then be different from case to case. The goals of the purification as embodied in the present invention is to achieve single digit parts per million, sub parts per million and parts per billion levels of higher boranes in embodiments, 1 , 2 and 3 respectively.

[00026] The purified diborane will typically be in the form of a diborane gas mixture or a pure diborane liquid form. [00027] The end user can determine the diborane concentration and select a balance gas based upon the pure diborane liquid. The balance gas accordingly can be selected from the group of hydrogen, nitrogen and argon.

[00028] The concentration of higher boranes in the purified diborane will range from 1 ppb to 0.1 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

[00029] Figure 1 is a graph showing vapor pressure versus temperature for diborane and higher boranes (B4H I 0, B5H9, B10H14).

[00030] Figure 2 is a graph showing estimated subliming pressure curves for higher boranes of B4H10 and B5H9.

[00031] Figure 3 is a schematic of a polishing cold trap bed to remove higher boranes in the first embodiment.

[00032] Figure 4 is a graph comparing the maximum percentage of diborane in feed mixtures versus the temperature of the trap in the first embodiment.

[00033] Figure 5 is schematic of a purification system using filtration to remove higher boranes from a diborane containing mixture in the second embodiment.

[00034] Figure 6 is a graph of B4H10 solid vapor pressures as functions of the latent heat of fusion and temperatures.

[00035] Figure 7 is a schematic of a gas/liquid phase cryogenic distillation according to the invention in the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[00036] Figure 1 is a graph showing the vapor pressure of diborane and various higher boranes versus the temperature. The curves are for liquid phase materials only. Notice that B4H10 is in solid/vapor phase below -120.8°C. B4H10 is the critical higher borane impurity since it is most volatile among all higher boranes. Diborane will be in a vapor-liquid mixture when the diborane partial pressure is over the vapor pressure line. Hydrogen stays in the gas phase.

[00037] Figure 2 is a graph of vapor pressure curves for gas/liquid and gas/solid phases of diborane and higher boranes. The Yaws handbook with respect to vapor pressure curves are provided. The heat of vaporization is extracted and the estimated heat of fusion for solids is added. The estimated solid sublimating vapor pressures are thus shown in Figure 2.

[00038] Figure 3 is a schematic 10 of a purification process using a cold trap in a first embodiment of the invention. A full gas phase diborane mixture of higher boranes, diborane and a balance gas are fed through a feed line 12 and split into two lines 14 and 16. The feed is typically a lower percentage of diborane, roughly 1 to 5% mixture. This allows for colder temperatures to be employed for more effective removal of the higher boranes in the mixture. The split feed gas is fed through valves V1 and V2 and lines 18 and 20 respectively to polishing cold trap beds A and B.

[00039] The cold trap beds are typically structured heat exchange parts such as bundled tubes with flowing coolant with maximized heat exchange surface with thermal conductive metallic materials. In the gas contact space around the heat exchanger parts, adsorbent materials such as zeolites or metal-organic-frameworks are added to increase the higher borane removal capacity of the cold trap bed.

Lines 22 and 24 extend from the bottom of each bed A and B that are controlled by valves V3 and V4 allowing for recovery of the purified gas mix through line 26.

[00040] Preferably in the multi-bed system, one bed is in production mode while the other bed is in regeneration mode. In this embodiment, a hydrogen regeneration gas is fed to a line 28 and flow controlled by a valve V5 or V6. When a bed is in production mode, the corresponding valve V5 or V6 is closed while the valve V6 or V5 to the other bed is open allowing for the flow of hydrogen thereto through lines 30 and 34 or 32 and 36 respectively. This regeneration will remove higher boranes. When the other bed has reached a predetermined limit of higher boranes, the flow is reversed and the producing bed is now regenerated and the bed previously regenerated begins producing the diborane. An advantage to this cycle is that continuous or semi-continuous operations can be achieved. The regeneration process can be at room or elevated temperatures.

[00041] In order to keep the original diborane mixing ratio, the diborane must be kept in the gas phase without condensation by setting lower temperature limits. Therefore, without substantially liquefying the diborane, the original diborane gas mixture ratios are maintained. Colder temperatures are typically preferred to condense higher boranes. Due to this trade-off, the higher borane concentration could be as high as 10s ppm levels of purified diborane mixture gas streams. As shown in Figure 4, an estimate of target bed (trap temperature) versus maximum percentage of diborane in the mixture. Also on the right axis is the amount in parts per million of B4H10 versus the trap temperature. Higher trap temperatures can result in both larger amounts of diborane in the feed mixture as well as higher amounts of B4H10. So at -110°C a 5 percent diborane mixture will have an upper limit of B4H10 in the purified mixture of 26 ppm. For 2 percent diborane mixture, the trap temperature is -123°C, and the B4H10 concentration can be as high as 4.4 ppm. The total pressure is 6 bara in these examples.

[00042] The second embodiment of the invention is shown schematically in Figure 5. In this embodiment, filtration is employed to assist in separating a mixture of diborane, higher boranes and a balance gas. In this schematic 40 a 30% feed of diborane in hydrogen 42 is fed to the cold trap liquid separator C. A second feed of ultra-high purity hydrogen 44 (optional) may be also fed into the cold trap liquid separator C as a purge gas. The condensed liquid is fed through the lower portion of C into a filtration unit C1 , which could be SS 315L sintered metal filters or polytetrafluoroethylene (PTFE) membrane filters. These filters will remove higher borane solid particles 48 resulting in pure diborane liquid in D.

[00043] The pure diborane liquid is then fed 50 into a heat exchanger E where it will be warmed to form a gas of purified gaseous diborane. Recycled hydrogen can be fed 52 from the cold trap liquid separator or alternatively a fresh ultra-high purity hydrogen or argon or nitrogen can be fed 54 into the heat exchanger C, all to mix with the pure gaseous diborane. This mixture can be fed to a gas mixing chamber F which can deliver through line 62 the express purity diborane per the end user’s desired concentration. Lines 56 and 58 are continuations of ultra-high purity balance gas 54 and recycled hydrogen line 52 respectively. For higher purity operations, for example, recycled hydrogen is discarded as it would contain an impurity. Fresh ultra-high purity balance gas is preferred, such as a new UHP hydrogen gas for heat exchange and final mixing.

[00044] A purification simulation with reuse of the recycled hydrogen from a feed was performed. At -135°C and 50 psig, the most critical higher borane B₄H₁₀ is about 1 ppm or less. If new ultra-high purity makeup gas is used for mixing, the impurity level should be at least 10 times lower in estimation or about 0.1 ppm levels of B4H10.

[00045] In this simulation, a borane mixture and hydrogen from a cooler are fed into a cold trap liquid separator. The hydrogen is removed from the top of a liquid separator and fed to a heater. The bottoms from the liquid separator is liquids that are fed to a filtration unit where solid higher boranes are removed from the bottom of the filtration unit. Hydrogen is also fed into the filtration unit from a second cooler. The pure diborane liquid is fed from the filtration unit to a heater to form a gas. The hydrogen, or fresh hydrogen feed, and diborane can be fed as gases to a gas mixer where a predetermined concentration of diborane can be produced and delivered.

[00046] Alternatively, metal organic frameworks (MOF) can be employed as the filtration unit. The MOF is used to selectively adsorb diborane molecules so that wider operational temperature range window can be achieved.

[00047] Figure 6 is a graph showing B4H10 solid vapor pressures as functions of the latent heat of fusion and temperatures the cold trap performance may be estimated from this graph. For example, performance with a chiller at -135°C will have an upper limit of B4H10 approximately 1 ppm at 50 psig when reuse recycled hydrogen. In certain embodiments, purer mixtures may require nitrogen rather than hydrogen.

[00048] Figure 7 is a schematic showing the third embodiment of the invention. In this embodiment, a gas/liquid phase cryogenic distillation method is employed. By using a cryogenic distillation method, higher temperatures can be employed where the vapor/liquid phases are well defined. Although more complicated equipment is required in distillation, separation of the higher boranes only needs 10 or so theoretical plates to achieve parts per billion levels in the vapor phase for the product diborane and hydrogen mixture. Since diborane concentration will be reduced slightly for the removal of heavier high boranes impurities, a new flow of ultra-high purity hydrogen is used to meet different dilution level requirement adjustments.

[00049] The loss of diborane to liquid waste is about 1 to 10% of the amount of diborane in the feed, and typically ranges from 2 to 5%. Since ultra-high purity (i.e., part per billion levels of high boranes) diborane gas mixture is produced in light product stream, it is suitable for customer on-site purification as a subsystem to feed in-situ blending system for ultra-high purity diluted diborane gas mixtures.

[00050] Alternatively, liquefied, ultra-high purity diborane is collected in a cold trap with hydrogen separation which can then be mixed with hydrogen, argon or nitrogen to be collected as a diborane mixture.

[00051] In the schematic 70 of Figure 7, a feed of 30% diborane in hydrogen is fed 72 to a distillation column having approximately 10 stages The liquid waste of higher boranes is collected from the bottom 74 of the distillation column G and a purified diborane and hydrogen gas mixture is collected from its top 76. This purified diborane and hydrogen mixture is either collected for end use or is fed 78 into a cold liquid trap H which produces pure diborane liquid. The pure diborane liquid may be fed 80 to a heat exchanger and mixing unit I with or without ultra-high purity hydrogen, argon or nitrogen 82 to produce either a diborane gas mixture or a pure diborane gas 84 as determined by the desired end user.

[00052] A purification simulation with reuse of the recycled hydrogen from the feed was performed. At -90.4°C and 30 psig, the most critical higher borane B4H10 is about 1 part per billion in a vapor pure output gas stream. Liquid diborane can be collected from a liquid tank after being separated from a hydrogen balance gas. In this simulation, a distillation column is in fluid communication with a unit for receiving the higher boranes waste from the bottom of the column as well as directing the purified diborane and hydrogen mixture to a cold trap system which separates the hydrogen from a pure diborane liquid, and recovers the gaseous hydrogen and liquid diborane.

[00053] While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.

Claims

Having thus described the invention, what we claim is;

1. A method for separating higher boranes from a mixture containing diborane and higher boranes and a balance gas comprising the steps of feeding the mixture to at least one cold trap wherein the higher boranes are frozen onto walls of the at least one cold trap; recovering the diborane and balance gas mixture without substantially liquefying the diborane; feeding a regeneration gas to the at least one cold trap after the recovery of the diborane and the balance gas, thereby regenerating the at least one cold trap, and venting the higher boranes and regeneration gas as waste.

2. The method as claimed in claim 1 wherein the at least one cold trap is two cold traps.

3. The method as claimed in claim 1 wherein the regeneration gas is hydrogen.

4. The method as claimed in claim 1 wherein the concentration of the higher boranes in the mixture is up to 50 parts per million in the feed and 1 parts per million or less in purified product mixture.

5. The method as claimed in claim 1 wherein the diborane is present in the mixture in amounts from 1 % to 5%.

6. The method as claimed in claim 1 wherein the concentration of diborane or the mixing ratio remain unchanged.

7. The method as claimed in claim 1 wherein the higher boranes are selected from the group consisting of tetraborane, pentaborane and decaborane.

8. A method for separating higher boranes from a mixture containing diborane, higher boranes, and a balance gas comprising the steps of feeding the gas mixture to a cold trap liquid separator to condense the diborane and the higher boranes; rejecting the balance gas from the cold trap liquid separator; filtering condensed liquid in the cold trap liquid separator to remove the higher boranes in the form of solid particles;

recovering purified diborane liquid from the cold trap liquid separator; feeding the recovered purified diborane liquid to a heat exchanger, wherein the purified liquid diborane forms purified gaseous diborane; and blending a balance gas selected from the group consisting of hydrogen, argon and nitrogen with the purified gaseous diborane, thereby forming a gaseous mixture with the purified gaseous diborane

9. The method as claimed in claim 8 wherein the recovered diborane is pure diborane from diborane feed streams that contain diborane from 1% to 100% in mixtures.

10. The method as claimed in claim 8 wherein the diborane is present in an amount of about 30% by volume in the mixture

11. The method as claimed in claim 8 wherein the hydrogen fed to the heat exchanger can be recycled hydrogen from the cold trap liquid separator.

12. The method as claimed in claim 8 wherein the hydrogen fed to the cold trap liquid separator is a fresh ultra-high purity hydrogen.

13. The method as claimed in claim 8 wherein the liquid mixture is fed through a filtration unit before being fed to the heat exchanger.

14. The method as claimed in claim 8 wherein the filtration unit contains a sintered metal filter or a polytetrafluoroethylene membrane filter.

15. The method as claimed in claim 8 wherein the filtration unit is a metal organic framework.

16. The method as claimed in claim 8 wherein an end user can determine the concentration of pure diborane recovered from the gas mixing chamber.

17. The method as claimed in claim 8 further comprising producing liquid phase diborane.

18. The method as claimed in claim 8 wherein the concentration of higher boranes in purified diborane is less than 0.1 ppm.

19. The method as claimed in claim 18 wherein the concentration of higher boranes is less than 1 ppm.

20. A method for separating higher boranes from a mixture containing diborane, higher boranes, and a balance gas comprising the steps of feeding the mixture to a distillation column, wherein a gaseous mixture of purified diborane and hydrogen is recovered from the top of the distillation column and a liquid waste containing diborane and a majority of the higher boranes is discarded from the bottom of the distillation column; feeding the recovered gaseous mixture to a cold trap, wherein purified liquid diborane is formed; feeding the purified liquid diborane to a heat exchanger thereby forming gaseous diborane; and recovering the gaseous diborane from the heat exchanger for further blending with a balance gas.

21. The method as claimed in claim 20 further comprising feeding a gas selected from the group consisting of hydrogen, argon and nitrogen to the heat exchanger, wherein the gaseous diborane will mix with the gas selected from the group consisting of hydrogen, argon and nitrogen.

22. The method as claimed in claim 18 wherein the distillation column has about 5 to 50 stages.

23. The method as claimed in claim 18 wherein the waste liquid comprises higher boranes

24. The method as claimed in claim 20 wherein the recovered diborane is pure diborane from diborane feed streams that contain diborane from 1% to 100% in mixtures

25. The method as claimed in claim 20 wherein the diborane is present in an amount of about 30% by volume in the mixture.

26. The method as claimed in claim 20 wherein the purified diborane is in a diborane gas mixture or pure diborane liquid form,

27. The method as claimed in claim 20 wherein an end user can determine the diborane concentration and select a balance gas from pure diborane liquid.

28. The method as claimed in claim 27 wherein the balance gas is selected from the group consisting of hydrogen, nitrogen and argon.

29. The method as claimed in claim 20 wherein the concentration of higher boranes in the purified diborane ranges from 1 ppb to 0.1 ppm.