WO1993011065A1

WO1993011065A1 - Improvements in halogen generators

Info

Publication number: WO1993011065A1
Application number: PCT/GB1992/002152
Authority: WO
Inventors: Andrew John Kearsley; Thomas Neville Walton Salkeld; Alan Howard Ferguson; Gerard Sean Mcgrady
Original assignee: Oxford Lasers Limited
Priority date: 1991-11-23
Filing date: 1992-11-20
Publication date: 1993-06-10
Also published as: GB9124948D0

Abstract

An improved design of halogen generator in which gas entering the generator is pre-heated to prevent cooling of the halogen source in the generator and gas flow through the generator exhibits improved uniformity, to give a more consistent performance from the generator.

Description

Title: Improvements in Halogen Generators Halogen generators based on the heating of metal halides or the heating of zeolites containing a halogen compound, have been known for some time. See, for example, US Patent No. 3,989,808 Asprey. This invention relates to the use of halogen and hydrogen halide generators (herein referred to as 'halogen generators') , which can be used as solid-vapour feed sources for use, for example, with excimer lasers. Henceforth, the metal halide or zeolite, is referred to as a 'source' .

Figure 1 of the accompanying drawings shows an excimer laser system in which the gas from a laser 1 is circulated through a cryogenic trap 2, this trap being of the kind disclosed in UK Patent No. 2,126,327. The gas passes through a halogen generator 3 and then back into the laser 1. A simple halogen cell is limited in that the gas flow through it cools the halogen source inside and causes the internal temperature to be non-uniform. A high flow of gas through the generator 3 is potentially important during the filling process. If the laser is first filled with the inert gas mixture and the halogen then added to the mixture by circulating the gas through the generator 3, a high gas flow rate is necessary in order to build up the appropriate halogen partial pressure in the laser in a reasonably short time.

Figure 2 of the accompanying drawings shows a simple halogen cell 4 comprising a container 5 which is filled with halogen source 6. A heater 7 is used to heat the cell walls to about 250°C and the walls in turn heat the source 6. Gas is passed through the container 5 from an inlet 8 to an outlet 9 and, because the incoming gas stream is approximately at room temperature, its passage through the source removes heat, resulting in the source nearer the incoming gas being cooler than the source around the periphery near the cell walls, as indicated by the approximate isotherms 10. As a result of the gas flow, the temperature of the source is lower than the temperature of the cell walls and is non-uniform. In turn, the desired halogen partial pressure is not achieved and this pressure varies with the gas flow rate and changes with time as the various regions of the source are depleted of halogen. A further problem is that the metal halide and zeolite are powdery materials which can be carried out of the generator by the gas flow; this dust can contaminate the laser and cause malfunctioning of valves. Furthermore, the dust can be toxic, corrosive and oxidising, and its exposure during laser servicing would constitute a health hazard. Figure 3 of the accompanying drawings illustrates a further problem caused by the flow of gas through the halogen cell being non-uniform across the whole cross-section of the cell. In the arrangement of Figure 3 gas flows from an inlet 11 to an outlet 12 through the source 13, the cell being heated by a surrounding heater 14. The gas flow rate is highest along the centre of the cell in a line between the inlet 11 and the outlet 12 along the axis of the cell and the speed of gas flow is lower radially outward from this central axis as indicated by the flow paths 15 of approximate egual speed. As a result of the higher flow through the central region of the cell this region is depleted of halogen before the radially outward areas and these areas contribute little to the halogen output. This is highly undesirable because once the central region is substantially depleted the gas stream will no longer pick up adequate halogen in passing through the cell. A third problem is that the cell must be removed from the gas circuit for recharging or replacement. Halogens are highly toxic and must not be allowed to escape during this process; nor may moisture be admitted as this can contaminate the system with hydrolysis products. A fourth problem is that, although the cell can be heated rapidly using a powerful heater, cooling will be slow unless some form of cooler is included on the cell. Without this forced cooling temperature control will be poor and the cell will not be able to be shut down quickly in an emergency. A fifth problem is that the hot gas leaving the cell will heat the pipework, any attached valves and the laser itself. This is undesirable as the heat can damage components, impair laser performance and may also accelerate attack by fluorine. According to the present invention there is provided a halogen generator in which the gas is pre-heated to approximately the cell temperature before being admitted to the cell. Pre-heating the gas eliminates the cooling of the source and ensures accurate temperature control and hence halogen pressure control.

Preferably, the gas is pre-heated by the use of a heat exchanger. Preferably, the cell is fitted with halogen resistant filters to prevent dust being blown out of the cell into the external circuit.

Preferably also, the cell is designed to allow uniform flow through the cell by inclusion of open chambers on the 'gas in' and 'gas out' ends of the cell, and by making the gas flow paths equal for all parts of the salt. Preferably, the cell is connected into the primary gas circuit with appropriate valves and piping to allow the system to be pumped clean of halogen and be removed from the system when necessary and to be backfilled with inert gas to prevent air and moisture being carried into the equipment.

In one form the cell can be provided with a cooling mechanism which will allow rapid cooldown in an emergency. Also, the gas exhaust is preferably fitted with a cooler to cool the gas before its passage into the rest of the equipment. The cooler may be in the form of the heat exchanger used to pre-heat the gas entering the cell. Alternatively, or additionally, it may be cooled using a separate cooler.

Embodiments of the invention will now be described, by way of example only, with reference to Figures 4 to 8 of the accompanying drawings in which, Figure 4 is a block diagram of a gas system for a laser, Figures 5, 6 and 7 are diagrams of halogen cells having different features, and,

Figure 8 is a pipework diagram of a system for connecting a halogen generator to a laser which allows replacement of the generator.

Figure 4 shows a laser gas system having a heat exchanger 20, a gas heater 21 and a halogen generator cell 22. The gas is passed through the heat exchanger 20 so that the hot gas 23 leaving the cell 22 transfers heat to incoming cold gas 24. This has the effect of raising the inlet gas temperature to close to the cell temperature and cooling the outlet gas to close to room temperature. If necessary the inlet gas can be further heated by passing the gas through the heater 21 to ensure a very good match between the gas and the cell temperatures. The outlet gas can be further cooled using a cooler 25.

The heater 21 can be used without the heat exchanger 20; however, higher powered heater 21 and cooler 25 will be needed.

In practice the gas heater can be incorporated into the cell design as shown in Figures 5 and 6. In Figure 5 the inlet gas pipe 31 is wrapped around the halogen cell 32 and is attached in good thermal contact with the cell 32. A single heating element 33 can then be used to heat both the inlet gas and the cell.

In Figure 6 a thick-walled cell 41 is used and the gas is passed through a series of linked holes in the walls. A heating element 43 is used to heat the whole assembly. The gas is heated by passage through the linked holes before entering the inner portion of the cell containing the source. Also shown in Figure 6 are the open chambers 44 which allow the gas stream to fan out and pass uniformly through the filters 45 used to contain the source and prevent dust being carried out of the cell.

If the cell of Figures 5 and 6 were mounted as shown, the source would settle under gravity leaving a gap above it which would permit passage of the gas through the cell without contact with the source. In practice the cell would be mounted in a position normal to the position shown in these Figures to preclude such an occurence. Shown in Figure 7 are the cell and heater 51 fitted with a cooling pipe 52 through which a coolant (air, water or oil) may be passed when it is necessary to shut down the generator rapidly. The cooling pipe is placed inside the thermal insulation 53 in good thermal contact with the cell.

Shown in Figure 8 is a suitable connection scheme for allowing pumping and purging of the gas lines when connecting / disconnecting the cell assembly. The generator 61 is connected to the laser and circulator (or other apparatus) 62 via pipelines and valves 63, 64, 65 and 67. Valves 63 and 64 are simple shut-off valves to isolate the generator for transport. Valves 65 and 67 are three- way valves to allow both isolation of the laser 62 and connection to dry gas 68 and vacuum 69. Valves 66 and 72 are three-way valves to allow selection of either dry gas (e.g. Neon), or of vacuum.

The operational sequence is as follows:- The starting condition assumed is that the generator is connected to the laser.

1) The generator is switched off and allowed to cool. Valves 66 and 72 are shut (neither vacuum nor gas selected) . Valves 65 and 67 are set to isolate the laser 62 and connect the pipes 70 and 71 to the gas / vacuum feed.

2) Valves 66 and 72 are used to select 'vacuum' and pump out the generator 61. Valve 72 is then used to select 'gas' and dry gas is allowed to flow through the pipework and the generator to purge the system of any residual halogen.

3) Valve 66 is then set to 'gas' and the generator filled with dry gas. Valves 64 and 63 are then closed and the pipes 70 and 71 disconnected allowing the generator to be removed. If a new generator cell is to be connected immediately valves 66 and 72 are left open to prevent air entering the pipework 70 and 71 while the new generator cell is being connected. Otherwise these valves and valves 65 and 67 are sealed. When the new generator is connected valves 65, 66, 67 and 72 are used to select 'vacuum'. This pumps out any air contamination and will remove moisture from the pipes 70 and 71 and from the exposed parts of valves 64 and 63.

4) Valves 63 and 64 are opened to pump gas out of the new generator 61. Valves 66^' and 72 may then be used to fill the generator with gas (assuming it is the same gas as is in the laser 62) . Valves 66 and 72 are then shut. The generator is then heated to operating temperature and valves 65 and 67 set to select flow from the laser, through the generator, and back into the laser. In this way the laser can be operated continuously while the generator is being charged without any contamination or degradation of the gas mixture.

In all cases the cell should preferably be constructed from a material with high thermal conductivity (for example, copper) . If additional chemical resistance is required a protective coating can be applied (for example, nickel) . Whilst the above has been described for halogen generators the technique can be applied equally to other halogen generators (for example, hydrogen chloride generators as described in patent application no. PCT/GB91/00799) .

In addition, while the example of excimer laser gas has been used the technique can be applied to any gas stream to which addition of halogen is required.

Claims

CLAIMS A halogen generator comprising a container, a means of heating the container, and a halogen source from which the halogen or hydrogen halide is to be evolved, adapted to be disposed in said container; together with a means of preheating the gas flowing through the generator to approximately the temperature of the cell before being admitted to the cell. A halogen generator as claimed in claim 1, wherein said preheating is performed using a heat exchanger. A halogen generator as claimed in claim 2, wherein said heat exchanger is a contra-flow heat exchanger. A halogen generator as claimed in any one of the preceding claims, wherein a separate heater is provided to preheat the gas. A halogen generator as claimed in any one of the preceding claims, wherein the container is fitted with halogen-resistant filters. A halogen generator as claimed in claim 5, wherein said filters are constructed from monel, stainless steel, or nickel. A halogen generator as claimed in any one of the preceding claims, wherein said container includes open chambers comprising 'gas in' and 'gas out' ends of the the container. A halogen container as claimed in any one of the preceding claims, wherein said container is constructed to ensure that the gas flow paths are approximately equal from all parts of the cell. A halogen generator as claimed in any one of the preceding claims, which is provided with a cooling mechanism allowing rapid cooldown of the halogen generator. A halogen generator as claimed in any one of the preceding claims, wherein the gas exhaust is fitted with a cooler. A halogen generator as claimed in claim 10, wherein the cooler is in the form of a heat exchanger. A halogen generator as claimed in claim 11, wherein the gas is heated by arranging the incoming gas stream to be in intimate contact with the said heater and with said container prior to entering said container. A rare gas halide excimer laser system comprising a laser head, a pump, a gas purification system and a closed circulation loop including a halogen generator having piping to allow said halogen generator to be purged, back-filled with inert gas and / or removed from the primary gas circuit. A halogen generator as claimed in any one of the preceding claims wherein said halogen generator is mounted vertically.