Patent Number: 
Section: description

The fog generation device A according to the invention that is shown diagrammatically in FIG. 1, and part of which is shown diagrammatically in FIG. 2 comprises a reservoir 2 that is designed to contain a liquid 4 that will be used to generate a dense fog of micrometric and submicrometric droplets. The device A also comprises means of pressurization of the liquid 4 contained in the reservoir 2. These pressurization means are symbolized by the arrow 6 in FIG. 1 and, in the example shown, are designed to send a pressurized gas into the reservoir. This pressurized gas, derived from means not shown, is an inert gas for example such as air, nitrogen or argon. The pressure in this gas that is applied to the liquid is equal to 5xc3x97105 Pa to 107 Pa. The pressurized gas is directed to the upper part of reservoir 2 through a pipe 8. The device A conform with the invention also comprises a nozzle 10 that leads to the bottom of the reservoir 2 through a pipe 12. For example, this nozzle is made of metal, ceramic or quartz. FIGS. 1 and 2 show the nozzle 10 oriented vertically, but any other orientation would be possible depending on the needs, for example it could be oriented horizontally. A hole 16 is formed in the lower part 14 of the nozzle 10, which may have a cylindrical, conical or exponential shape. The diameter of the upper end 18 of this hole 16 is between 20 xcexcm and 1 mm. This diameter is called the xe2x80x9cnozzle diameterxe2x80x9d. The nozzle 10 opens up into a vacuum chamber 20. This vacuum chamber 20 is provided with pumping means 22 capable of creating a pressure equal to about 10xe2x88x922 Pa. The liquid, for example water, that is inlet into nozzle 10 is thus expelled violently through the hole 16 in this nozzle into the vacuum chamber 20 and forms a high density mist 23, or a dense fog of micrometric and submicrometric liquid particles in this vacuum chamber. The diameters of these droplets are of the order of 10 xcexcm to 30 xcexcm. The mist 23 is strongly confined along the centre line X of the nozzle which is also the centre line of the hole 16 in this nozzle. In the device according to the invention shown in FIGS. 1 and 2, no cooling of the nozzle 10 is necessary to obtain the dense and highly directional fog 23. However, it is preferable to provide nozzle heating means 24. In the example shown in FIG. 2, these heating means 24 comprise a heating strip 26 that is wound in a circular groove 28 that has its axis along the centre line X and that is formed on the lower part 14 of the nozzle 10. Means not shown are provided to create an electrical current circulating in this heating strip 26. This heating of the nozzle 10 can improve the uniformity of the liquid droplets formed, apart from compensation of heat absorbed by the liquid during formation of the dense fog. In particular, this better uniformity improves the interaction efficiency between a laser beam and the droplets in the particular dense fog application that will be considered later and is related to the generation of EUV radiation using this type of interaction. For example, heating means 24 may be used to heat the nozzle 10 to temperatures equal to or less than 300xc2x0 C. The size of the liquid droplets contained in the dense fog 23 depends on the temperature of the nozzle 10 and the geometric parameters of this nozzle (particularly the shape of the hole 16 in the nozzle). These parameters must be optimised as a function of the properties of the liquid used, for example the viscosity, the vapour pressure and the boiling point of this liquid. If water is used, it is preferable to use a reservoir 2 and a pipe 12 made of aluminium or stainless steel or coated with Teflon (Trade mark) on the inside, in order to prevent internal corrosion of this reservoir and this pipe. A dense continuous fog of liquid droplets, or a pulsed fog, can be formed. For example, this type of pulsed fog can be formed by providing a piston 30 inside the nozzle 10, the end of the piston 30 being pointed and designed to periodically close off the hole 16, and means symbolised by the arrow 32 in FIG. 2, that can be made by an expert in the subject and designed to make the piston 30 oscillate along the centre line X of the nozzle. For example, these oscillation means may be electromagnetic or piezoelectric. For example, the pulse frequency of piston 30 may be of the order of 20 Hz, but it may be increased to 1 kHz using techniques known to an expert in the subject. The fog 23 of micrometric or submicrometric droplets is very similar to a jet of very large aggregates, but has a much more pronounced directivity. In the case of water, the half divergence angle a (FIG. 2) of the fog 23 is of the order of 1xc2x0. For an application of the invention to the generation of EUV radiation, it then becomes possible to make the excitation laser beam interact at a distance D from the nozzle 10 and thus prevent erosion of this nozzle by the plasma resulting from the interaction between the liquid (water) and the laser beam. This distance D is shown in FIG. 2. It is the distance between the lower end 14 of the nozzle 10 and the centre line Y of the laser beam focused on the dense fog 23. The distance D may be adjusted between 2 mm and 10 mm. Furthermore, the liquid droplets injected into the vacuum chamber 20 may pass almost entirely through the laser beam focusing area, thus considerably reducing the quantity of material that does not participate in the generation of EUV radiation. The strong confinement of the fog 23 of water droplets also enables the use of cryogenic type pumping means 22. Note that these pumping means communicate with the inside of the vacuum chamber 20 through an opening located facing the lower end 14 of the nozzle 10 and through which the centre line X (geometric) passes. Cryogenic pumping means are inexpensive and very efficient, and do not generate any vibrations. The ice formed can then be eliminated from the vacuum chamber 20 by various methods, for example using a device (not shown) consisting of an airlock in order to obtain uninterrupted operation of the device. We will now consider an application of the dense fog 23 of liquid droplets to the generation of an EUV radiation. This fog 23 is excited using a laser irradiation of the type described in document [1]. For example a nanosecond laser 34 of the Nd:YAG or excimer type is used with a pulse duration between 0.1 nanosecond and 100 nanoseconds and an energy per pulse greater than 10 mJ. The beam 36 supplied by the laser 34 is focused using a lens 38 or a mirror, onto the fog 23 in order to obtain a laser illumination on this fog between 1010 W/cm2 and 1014 W/cm2. Note that in the example shown, the laser beam 36 is brought into the vacuum chamber 20 through a port 40 transparent to this laser beam and mounted on a wall of the vacuum chamber. In FIG. 1, the intense EUV radiation emitted by the liquid droplets is symbolised by the arrows 42 oriented in all directions. However, the largest quantity of EUV light is produced by the plasma half-sphere facing the laser beam. One or several ports (not shown) are provided on one or several walls of the chamber 20 to retrieve EUV radiation and to use it. For the application of EUV radiation to nanolithography, it is preferable if the wave length of the EUV radiation emitted by the fog 23 is within the range of between 10 and 14 nanometers, which is the optimum range of wave lengths of reflective optics provided for this nanolithography. For example, such wave lengths can be obtained using water as the liquid to generate radiation of the 05+1s22pxe2x88x921s24d oxygen transition (located at 13 nm) Note that water has the advantage that it is inexpensive. Unlike xenon, it does not require any recycling device. However, it is possible to use other liquids, liquid mixes or solutions, in order to optimise the generation of EUV radiation between 10 and 14 nanometers or close to another wave length if this becomes necessary. The dense fog of micrometric of submicrometric liquid droplets obtained in a vacuum according to the invention has the advantages of a jet of xenon aggregates (fog-laser coupling causing absorption rates close to 1 and no solid debris that could degrade the EUV radiation treatment optics). But the device used to generate this dense fog is simpler and more reliable than the device used to generate xenon aggregates. In principle, it would be possible to produce a jet of water aggregates according to document [1], but the technique for production of this jet would be complex since the water would have to be gasified and it would have to be kept in the gaseous state until adiabatic expansion occurs through a nozzle. Furthermore, cryogenic cooling of the nozzle as in the case of xenon would have to be used to produce large aggregates. The production of sufficiently large water aggregates using this technique would be extremely difficult. FIG. 3 very diagrammatically illustrates the use of EUV radiation obtained with a device according to the invention for nanolithography. The nanolithography apparatus diagrammatically illustrated in this FIG. 3 comprises a device 44 for the generation of EUV radiation of the same type as the EUV radiation source described with reference to FIG. 1. The nanolithography apparatus according to FIG. 3 also comprises a support 46 for the semi conducting substrate 48 to be treated and that is covered with a photoresist layer 50 that will be insolated according to a determined pattern. The apparatus also comprises: a mask 52 including this pattern in enlarged form, optics 54 designed to format EUV radiation reference 43 output from device 44, and carry this radiation 43 to the mask 52 that then supplies an image of the pattern in enlarged form, and optics 56 designed to reduce this enlarged image and project the reduced image onto the photo resist layer 50. The support 46, the mask 52 and optics 54 and 56 are located in a vacuum chamber not shown which, for simplification reasons, is preferably the vacuum chamber in which the EUV insolation radiation 43 is created. The following documents are referred to in this description: [1] Kubiak et al., xe2x80x9cCluster beam targets for laser plasma extreme ultraviolet and soft X-ray sourcesxe2x80x9d, U.S. Pat. No. 5,577,092 A, [2] Richardson et al., xe2x80x9cWater laser plasma X-ray point sourcesxe2x80x9d, U.S. Pat. No. 5,577,091 A [3] Hertz et al., xe2x80x9cMethod and apparatus for generating X-ray or EUV radiationxe2x80x9d international application PCT/SE97/00697 Apr. 25, 1997 [4] Matsui et al., xe2x80x9cLaser plasma X-ray source, and semiconductor lithography apparatus and method using the samexe2x80x9d, EP 0858249 A [5] McPherson et al., xe2x80x9cMultiphoton-induced X-ray emission and amplification from clustersxe2x80x9d, Applied Physics B57, 667-347 (1993) [6] McPherson et al., xe2x80x9cMultiphoton-induced X-ray emission from Kr Clusters on M-Shell (xe2x88x92100xcex94) and L-Shell (xcx9c6xcex94) transitionsxe2x80x9d Physical review letters vol. 72 No. 12 (1994), p.1810 to 1813 [7] Kubiak et al., xe2x80x9cScale-up of a cluster jet laser plasma source for extreme ultraviolet Lithographyxe2x80x9d, SPIE Conference on Emerging Lithographic Technologies III, March 1999, P. 669 to 678.