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Gas Mixtures for Excimer Lasers

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Gas Mixtures for Excimer Lasers Europaisches Patentamt 19 European Patent Office Office europeen des brevets © Publication number : 0 565 242 A1 EUROPEAN PATENT APPLICATION (2j) Application number : 93301920.0 (£) Int. CI.5: H01S 3/225 (22) Date of filing : 12.03.93 (30) Priority : 04.04.92 GB 9207762 (72) Inventor : Gabzdyl, Jacek Tadeusz 9 Broadacres Guildford, Surrey GU3 3AZ (GB) (43) Date of publication of application 13.10.93 Bulletin 93/41 @ Representative : Gough, Peter c/o THE BOC GROUP PLC Patent Department @ Designated Contracting States : Chertsey Road BE DE FR GB IT NL Windlesham Surrey GU20 6HJ (GB) @ Applicant : BOC LIMITED Chertsey Road Windlesham, Surrey GU20 6HJ (GB) (54) Gas mixtures for excimer lasers. (57) A rare gas-halide excimer laser uses a buffer gas consisting of from 5 to 50% by volume FIG.1 HELIUM /NEON helium the balance :Xe:Hcl:H2 being neon. 133.3 100.0 .«- -J- Q- 66.7 33.3 0.0 0.0 10.0 20.0 30.0 40,0 HELIUM C0NC.(%] CM CM IO CO IO LU Jouve, 18, rue Saint-Denis, 75001 PARIS 1 EPO 55 242 A1 2 The present invention relates to rare gas-halide At the heart of the present invention is the unex- excimer lasers and in particularto gas mixtures for ex- pected effect which a buffer gas comprising a mixture cimer lasers. of helium and neon provides in that it allows for an in- Excimer lasers are molecular lasers which have creased laser power at a constant gas pressure and a metastable excited state. The lasing medium is a 5 excitation voltage. gas which usually consists of active components such The graph shown in Figure 1 illustrates the re- as a halogen/rare gas combination in an inert buffer sults used when operating an XeCI excimer laser us- such helium or neon. Heretofore the buffer gas has ing a gas mixture of Xe/HCI/H2 with a buffer gas com- usually been either pure neon or pure helium. This prising from 0 to 40% helium with the remainder neon. known choice of neon or helium gives the user the op- 10 The graph shows that the buffer gas mixture using tion between a high cost gas with high output power 20% helium can yield substantially the same output or a low cost gas with a lower laser power output. For power as pure neon which is of considerable benefit example, the use of the relatively cheaper helium as to the user at it will significantly reduce his gas cost the buffer gas typically yields only 60 to 70% of the since the costs of neon is considerably more than the power achieved with a pure neon buffer gas. 15 cost of helium. It is known from European Patent Publication No. The graph shown in Figure 2 illustrates the re- 0430411 for a gas mixture for an excimer laser to con- sults achieved with a KrF excimer laser which was op- tain about 99% of one or more of helium and neon. erated using a buffer gas ranging from 0 to 60% hel- However, this document does not give details of the ium with the remainder neon. The graph shows that percentage of helium or neon in the inert buffer. Uni- 20 from 1% up to 50% helium gives an increase in laser ted States Patent No. 4674099 describes a typical output power when compared with pure neon. In this lasing gas mixture including a diluent rare gas such as instance increased laser power output of up to 30% helium and/or neon in concentrations in the order of has been demonstrated with a buffer mixture of 30% 95%. Again, this document does not give details of helium and 70% neon. any helium/neon mixture. 25 Figure 3 shows that by using a helium/neon mix- For the avoidance of doubt the expression "Buf- ture buffer gas the operating pressure at which max- fer Gas" used throughout this specification is intend- imum power can be achieved is reduced. This graph ed to embrace the inert gas(es) which represent the shows that the power output for a KrF excimer laser balance of the lasing gas mixture excluding the active initially filled with neon to a pressure of 1400mb with lasing constituents. 30 a subsequent addition of (a) neon and (b) helium up Laser gases are expensive and efforts are con- to a total pressure of 2800mb it can be seen that the tinuously being made to reduce costs in this area. maximum power output from the laser can be achieve It is an aim of the present invention to provide a at a lower total gas pressure using a helium/neon buf- buffer gas which is relatively inexpensive when com- fer gas mixture. pared to substantially pure neon and can provide an 35 This is significant in view of the fact that lasing increased laser power at a constant gas pressure and gas costs are a major cost in the operation of lasers. excitation voltage. It will be apparent from the results obtained and According to the present invention, in a rare gas- illustrated in the Figures 1 to 3 that replacing a pure halide excimer laser a gas mixture comprises a rare gas, such as helium or neon, with a buffer gas mixture gas, a halogen or halogen donor gas and a buffer gas 40 of helium and neon produces unexpectedly beneficial consisting of a mixture of from 5 to 50% by volume of results, such as:- helium the balance being neon. 1) An increase in the maximum output power of In a preferred embodiment, the buffer gas may the excimer laser can be provided over the use comprise 5 to 30% by volume of helium the balance or as compared to the use of pure neon or helium. being neon. 45 This is a positive deviation and completely unex- Embodiments of the invention will now be descri- pected on the usual law of gas mixing. bed by way of example reference being made to the 2) Reducing the cost of the lasing gas mixture Figures of the accompanying drawings in which:- over the use of, for example, pure neon. Figure 1 is a graph which illustrates the effects of 3) The use of a buffer gas mixture as described a helium/neon buffer gas mixture on laser output so above can enable to laser to be operated at lower power using a XeCI excimer laser; pressures to achieve maximum power output giv- Figure 2 is a graph illustrating the effect of a hel- ing a considerable saving in the gas volumes re- ium/ neon buffer gas mixture on laser output pow- quired to fill the laser cavity. er using a KrF excimer laser; and 4) The advantage referred to in 3) above also Figure 3 is a graph illustrating that by using a hel- 55 makes the expectation of the operation of the las- ium/neon mixture buffer gas the operating pres- er safer as lower operating pressures mean less sure at which maximum power can be achieved stress on components such as the output window is reduced when using a KrF excimer laser. of the laser through which the laser beam pass- 2 EP 0 565 242 A1 es. The exact composition of the buffer mixture for optimum results will, of course, depend on the design features of any particular excimer laser. Claims 1. In a rare gas-halide excimer laser a gas mixture 10 comprising a rare gas, a halogen or halogen do- nor gas and a buffer gas consisting of a mixture of from 5 to 50% by volume of helium the balance being neon. 15 2. A gas mixture as claimed in Claim 1, in which the buffer gas comprises from 5 to 30% by volume of helium the balance being neon. 3. A gas mixture as claimed in Claim 1 or 2, in which 20 the halogen is fluorine. 4. A gas mixture as claimed in Claim 1 or 2, in which the halogen donor gas is hydrogen chloride. 25 5. A gas mixture as claimed in any one of Claims 1 to 4, in which the rare gas is krypton. 6. A gas mixture as claimed in any one of Claims 1 to 5, in which the rare gas is xenon. 30 35 40 45 50 55 3 EP 0 565 242 A1 FIG.1 HELIUM /NEON :Xe:Hd:H2 133.3 100.0 4- £ 66.7 ZD o UJ ^ 33.3 0.0 0.0 10.0 20.0 30.0 40.0 HELIUM C0NC.(%) HELIUM/NEON FIG.2 :Kr.-F2 133.3 100.0 0s Q- 66.7 i— ZD o cc ^ 33.3 0.0 0.0 15.0 30.0 45.0 60.0 HELIUM C0NC.(%) 4 I 1 TO ^ Ol QJ 1 o o o o CO o ="= d o IO cj .. _ E LU cc I o 00 cs) LU EP 0 565 242 A1 Patent piuniDer European EUROPEAN SEARCH REPORT Application Office EP 93 30 1920 DOCUMENTS CONSIDERED TO BE RELEVANT Citation of document with indication, where appropriate, Keievant L,i^Aaairii.Aiiifr<i \jr inc. Category of relevant passages to claim APPLICATION (Int. CI.5 ) \PPLIED PHYSICS LETTERS -6 H0153/Z2b /ol. 42, no. 10, May 1983, NEW YORK US aages 860 - 861 E.ARMANDILLO ET AL. 'IMPROVED LASING 3ERF0RMANCE OF KrCl EXCIMER LASER' * the whole document * IEEE JOURNAL OF QUANTUM ELECTRONICS, [-6 /ol. 26, no. 9, September 1990, NEW YORK JS aages 1569 - 1573 ?. J. PETERS ET AL. 'EFFICIENT XeF (C— A) .ASER EXCITED BY A COAXIAL ELECTRON BEAM VT INTERMEDIATE PUMPING RATES' * page 1571, right column, paragraph 3; figures 6,7 * D,A EP-A-0 430 411 (BRITISH AEROSPACE) 1-6 * claims 1-6 * D,A JS-A-4 674 099 (R.E.TURNER) L-6 1 P.V.I , ruux) * claims 1-9 * SEARCHED (Int.
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