GB2353034A - Reduction of noxious substances - Google Patents

Abstract

A process for the removal of noxious substances from a gas stream comprises introducing the gas stream and an inorganic hydride (as a reducing agent) into a heated chamber. Preferably, the gas stream contains at least one of Cl2, F2, AlCl3, NF3, ClF3, SiCl4, C3F8, and C4F10. The inorganic hydride is typically one or more of silane, diborane, phosphine and ammonia. The chamber may be heated to a temperature of between 350°C to 900°C.

Description

2353034 REMOVAL OF NOXIOUS SUBSTANCES FROM GAS STREAMS This invention

relates to the removal of noxious substances from exhaust gas streams, in particular the removal of such substances from gas streams emanating from semiconductor processing chambers by means of combustion.

Many substances used in semiconductor device manufacturing, and which are extracted from a chamber in which such manufacturing takes place, are toxic and/or environmentally harmful and must therefore be scrubbed from the exhaust gas,stream before its release in to the atmosphere. The substances include halide containing species, including chlorine and fluorine containing species, chloro-fluoro gases and the like.

A number of different types of wet or dry chemical scrubbing reactors have been proposed and numerous are commercially employed in the semiconductor industry.

For example, in our Patent Specification No. WO 89/11905 there is disclosed a dry chemical reactor sold by our BOC Edwards Division comprising a heated packed tube of granular substances through which the exhaust stream is directed including in particular a first stage of silicon (with an optical addition of copper when the exhaust stream contains nitrogen trifluoride in particular) and a second stage of calcium oxide commonly in the form of lime. Such a reactor - The BOC Edwards "GRC" (Trade Mark) reactor - has met with considerable commercial success for the scrubbing of such toxic substances.

It is also known from European Patent Specification No. 0 694 735 in the name of Alzeta Corporation, that noxious substances of the type in question can be removed from exhaust streams by combustion.

There is described in this prior Specification a process for the combustive destruction of noxious substances which comprises injecting an exhaust gas and added fuel gas in to a combustion zone that is laterally surrounded by the exit surface of a foraminous gas burner, simultaneously supplying fuel gas and oxygen or air to the burner to effect combustion at the exit surface, the amount of the fuel gas supplied to the foraminous gas burner being on a BTU basis, greater than that of the added fuel gas, and the amount of the air being in excess of the stoichiometric requirement of all the combustibles entering the combustion zone, and discharging the remitting combustion product stream from the combustion zone.

A central feature of the prior combustive process is the critical need to supply the fuel gas admixed with the exhaust gas stream in to the combustion zone of the burner.

A great advantage of the prior combustive scrubbing process described above is that it inherently limits the maximum temperature that can be attained in the combustion chamber and thereby suppresses the formation of NOx gas by-products which otherwise may be formed.

Such premixing of the fuel gas and exhaust gas stream allows for a much greater and efficient scrubbing of the perfluorocarbon hexafluoroethane (C2F6). However, the relatively low maximum temperature may become a limiting factor in the destruction of the most stable perfluorocarbon gases, in particular tetrafluoromethane (CF4).

In our European Specification 0 802 370, we disclosed that an addition of oxygen, as well as fuel gas in accordance with European Patent Specification No. 0 694 735, to the exhaust gas stream prior to the introduction of the gas stream in to a foraminous gas burner generally allows for a more efficient combustion of perfluorocarbon gases including tetrafluoromethane (CF4).

More recent developments in the semiconductor industry require the abatement of chlorine trifluoride (CIF3) and of fluorine gas (F2) from different gas exhaust streams, Whereas CIF3 can be reacted in the type of combustion process described above to form generally non-toxic species, in- order to optimise the cleanliness of the exhaust both oxygen and hydrocarbon fuel need to be injected as described in European Patent Application Nos. 0 694 735 and 0 802 370, especially the latter.

Even in cases where the combustion processes described above can scrub the species in question, there are difficulties in the processes if a supply of oxygen or air is not readily available for injection in to the inlet, or is interrupted, or if the use of oxygen or air is undesirable in view, for example, of the different types of species present in the exhaust stream. In view of the very reactive nature of the species present in semiconductor exhaust streams generally, a lack of an adequate supply of oxygen/air may result in an increased level of un-burnt hydrocarbons arising from the fuel gas introduced with the incident toxic gas stream in to the combustion chamber, thereby causing an undesirably dirty emission.

For example, tests have shown that, for an exhaust stream containing CIF3 and/or chlorine gas (C12), its introduction in to a combustion chamber is a process of the type described above, in insufficient oxygen or air inject flows, resulted in a high incidence of un-burnt hydrocarbons, including benzene, and the presence of the very toxic chloromethane (CH3CI) was detected.

There is therefore a need for an improved abatement process in which exhaust gas streams can be scrubbed of noxious species without the need for oxygen/air to be present. The invention generally satisfies this need.

In accordance with the invention, there is provided a process for the combustive abatement of noxious substances from a gas stream, which comprises introducing the gas stream and an inorganic hydride (as reducing agent) in to a heated chamber.

The invention is primarily directed to the scrubbing of halogen containing species, especially chlorine and fluorine containing species including C12, F2, BC13, AIC13, NF3, CIF3, NOCI, SiC14, C3 F8 and C4 Flo.

The inorganic hydricle is preferably one or more of silane (SiH4), diborane (1321-16), N2H4, phosphine (PH3) and ammonia (NH3). The hydride may be hydrogen gas itself but is more difficult and more dangerous to handle and contain.

Ammonia is most preferred, being relatively low cost, can readily be stored in liquid form and generally produces only'gaseous by-products.

The chamber is preferably heated to a temperature of from 350 to 9000C, more preferably to a temperature of at least 6000C. In most preferred embodiments, the chamber temperature is from 800 to 9000C, for example about 8500C.

Semiconductor exhaust gas streams in particular are generally diluted with an inert gas, commonly nitrogen.

The toxic gas inlet stream is caused to be brought in to contact with the inorganic hydride at or near the introduction of the inlet gas stream in to the reaction chamber. This is achieved for instance by:

a) having the inorganic hydride introduced via a lance, preferably in the form of a co-axial inlet nozzle, positioned at the centre of the mouth of the inlet nozzle, or b) via a concentric tube surrounding the inlet line, or C) via a pattern of holes in the lid of the chamber surrounding the inlet toxic gas line. Preferably the lance is displaced a distance away from its point of entry in to the combustion chamber.

The purpose of such positioning of these inlet devices is to cause the hydride gas to contact the incoming toxic gas and intermix before the gases are diluted by the contents of the reactor chamber. The reactor chamber serves the purposes of:

a) to heat the resultant gas mixture to cause reactions to occur, and b) to cause heated oxygen containing gases to remove any trace of unreacted inorganic halide. Preferably, the combustion chamber is a radiant porous ceramic burner or, alternatively, a heated tube furnace, advantageously electronically heated.

Reaction times between the noxious species being scrubbed and the inorganic halides are fast and a residence time in the chamber of up to a few seconds is usually sufficient. Ideally, a residence time of between 0. 1 and 2 seconds is satisfactory.

The size of the chamber relative to its inlet and outlet sizes can be adjusted as necessary to allow for a suitable resident time together with adjustment of other parameters including the pressure of the exhaust gas stream.

In the semiconductor including in particular, the exhaust gas stream is evacuated from a semiconductor device manufacturing plant by means of a vacuum pump system which pump will provide the flow of the exhaust stream from the plant and to the combustion chamber used in the invention.

It may be necessary or appropriate to use supplementary means, for example wet scrubber or condensers, in order to collect the by products of the combustive abatement of the invention, depending on the specific species being scrubbed.

Experiments were conducted showing the use of processes of the invention.

In a first experiment, an insulated combustion chamber in the form of a hollow metal cylinder having a first end plate defining a chamber inlet and a second end plate defining a chamber outlet was heated to a temperature of 8500C. The chamber outlet was connected to a water scrubber in which the abated exhaust stream was bubbled through a volume of water.

An exhaust gas stream comprising 3-litres of fluorine gas diluted in a 47 litres/minute nitrogen flow was passed in to the heated chamber, the stream being chosen to replicate a typical exhaust gas stream evacuated from a semiconductor device processing plant by means of a vacuum pump system.

On adding 1.5 litres/minute of ammonia gas via a lance in to the exhaust stream at the chamber inlet it was observed that the exhaust stream exiting the water scrubber contained only about 2 parts per million (ppm) of fluorine. It was also observed by use of a mass spectrometer that no OF2 could be detected at the chamber outlet, indicating that the fluorine had been reacted before the exhaust stream was incident as the water scrubber stage.

In a second experiment, an incident gas stream containing chlorine, BC12, AIC13, and SiC14, was taken as representative of a gas stream from a metal etch system used in the semiconductor business. The incident gases were passed into a heated chamber in the form of a tube, initially with air alone present. Analysis of the duct down stream of wet scrubber indicated a high presence of chlorine, as would be predicted down-stream of an un-closed wet scrubber, chlorine having only very limited solubility in water.

To the flow as described above, ammonia gas was added in excess. Subsequent down stream analysis determined an absence of chlorine or other toxic substance indicating that the chlorine had been converted into a soluble substance.

When the air stream was removed and nitrogen substituted the results continued to indicate complete removal of the chlorine species, reducing substantially the possibility of formation of chlorinated dioxins and furans.

Claims (6)

1 A process for the combustive abatement of noxious substances from a gas stream, which comprises introducing the gas stream and an inorganic hydride (as reducing agent) in to a heated chamber.

2. A process according to Claim 1 in which the gas stream contains one or more Of C12, F2, BC13, AIC13, NF3, CIF3, NOCI, SiC14, C3 F8 and C4 Flo.

3. A process according to Claim 1 or Claim 2 in which the inorganic hydride is one or more of silane, diborane, phosphine and ammonia.

4. A process according to any preceding claim in which the chamber is heated to a temperature of from 3500C to 9000C.

S. A process according to Claim 4 in which the chamber is heated to a temperature of at least 6000C.

6. A process according to Claim 4 or Claim 5 in which the chamber is heated to a temperature of 8000C to 9000C.