Patent Number: 
Section: description

Embodiment 1 FIG. 1 schematically depicts a lithographic projection apparatus according to the invention. The apparatus comprises: a radiation system LA, Ex, IN, CO for supplying a projection beam PB of extreme ultraviolet radiation (e.g. with a wavelength of about 10 nm); a mask table MT provided with a mask holder for holding a mask MA (e.g. a reticle); a substrate table WT provided with a substrate holder for holding a substrate W (e.g. a resist-coated silicon wafer); and a projection system PL for imaging an irradiated portion of the mask MA onto a target portion C (die) of the substrate W. The radiation system comprises a radiation source LA which produces a beam of radiation. This beam is passed along various optical components,xe2x80x94e.g. beam shaping optics Ex, an integrator IN and a condenser COxe2x80x94so that the resultant beam PB is substantially collimated and uniformly intense throughout its cross-section. The beam PB subsequently intercepts the mask MA which is held in a mask holder on a mask table MT. Having passed through the mask MA, the beam PB passes through the projection system PL, which focuses the beam PB onto a target area C of the substrate W. With the aid of the interferometric displacement and measuring means IF, the substrate table WT can be moved accurately, e.g. so as to position different target areas C in the path of the beam PB. The condenser CO and the projection system PL are schematically shown as refractive components in FIG. 1. In practice, however, they will generally comprise reflective components for a beam of extreme ultraviolet radiation. The components shown in FIG. 1 should only be considered as a schematic representation. The depicted apparatus can be used in two different modes: 1. In step mode, the mask table MT is fixed, and an entire mask image is projected in one go (i.e. a single xe2x80x9cflashxe2x80x9d) onto a target area C. The substrate table WT is then shifted in the x and/or y directions so that a different target area C can be irradiated by the (stationary) beam PB; and 2. In scan mode, essentially the same scenario applies, except that a given target area C is not exposed in a single xe2x80x9cflashxe2x80x9d. Instead, the mask table MT is movable in a given direction (the so-called xe2x80x9cscan directionxe2x80x9d, e.g. the x direction) with a speed v, so that the projection beam PB is caused to scan over a mask image; concurrently, the substrate table WT is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the projection system PL (typically, M=xc2xc or ⅕). In this manner, a relatively large target area C can be exposed, without having to compromise on resolution. FIG. 2 schematically shows a radiation source LA of the radiation system, comprising primary and secondary jet nozzles 10 and 20 and a supply of primary and secondary gases 11 and 21 to the primary and secondary jet nozzles, respectively. Both jet nozzles are in the embodiment shown pulsed jet nozzles, in which both supply lines comprise valves which are opened at certain instants in time to supply a pulse of primary and secondary gases to the respective jet nozzles. The outflow of the primary and secondary gasses is designated by the reference numerals 15 and 25, respectively, in FIG. 2 FIG. 2 further shows a laser 30 for supplying a laser beam 31, which in general will be a pulsed laser beam. The laser beam 31 is directed to and focussed in the outflow of the primary gas from its jet nozzle 10. The frequency of the light in the laser beam and the intensity of the laser beam are chosen such that a plasma will be created in the region in which the laser beam 31 crosses the outflow of the primary gas 15. In the plasma electrons are detached from the atoms of the primary gas. After some time the electrons and nuclei will recombine under the emission of electromagnetic radiation which may have a large contribution in the extreme ultraviolet range of the radiation spectrum. The electromagnetic radiation is collected and redirected by optical elements, such as a condenser system, and not shown in FIG. 2. FIG. 4 shows a longitudinal section through the jet nozzle source for the primary and secondary gases. FIG. 6 shows a front view of the nozzle source. The primary and secondary jet nozzles are arranged co-axial, the secondary jet nozzle 20 enclosing the primary jet nozzle 10. The primary jet nozzle 10 has a circular outlet 13 and the secondary nozzle 20 has a annular outlet 23. Plungers 12 and 22 are arranged in the supply of the primary and secondary gases 11 and 21, respectively, and may be independently operated to close of their respective supply by abutting against a tapered end of the supply. In this way valves are obtained for opening and closing the respective supplies to yield a pulsed outflow of the primary and secondary gasses. However, pulsed nozzles may be obtained in various other configurations. The plungers 12, 22 are operated by means which are not shown in the drawings. When a pulse of primary gas and no pulse of secondary gas is ejected from the nozzle source, the outflow of the primary gas 15 from the jet nozzle outlet 13 will be strongly divergent. Also ejecting a pulse of secondary gas 25 results in a less divergent or even parallel or convergent outflow of the primary gas 15. An optimum outflow of the primary gas for the radiation source can be reached by varying one or more of several parameters. One of these parameters is the supply rate of secondary gas to the secondary jet nozzle 20 with respect to the supply rate of primary gas to the primary jet nozzle 10. Another parameter is the timing of the pulse of secondary gas with respect to the timing of the pulse of primary gas. It appears that an appropriately delayed pulse of primary gas with respect to the pulse of secundary gas results in a less divergent beam in case the secondary gas is a lighter gas than the primary gas as compared to a non-delayed pulse at the same flow rates of primary and secondary gasses. Other relevant parameters are the backing pressures of the gases in the nozzle source and the jet geometry. The optimum parameters will depend on the gases or liquids used and on the specific geometry of the primary and secondary jet nozzles. The primary gas of the first embodiment of the radiation source comprises krypton or xenon, which may be supplied pure or in a mixture with other (inert) gases. A xenon plasma, for instance, has been shown to emit a large contribution of extreme ultraviolet radiation. In an alternative embodiment water droplets or cryogenic liquids, such as liquid xenon, in a carrier gas may be used as a primary liquid. The secondary gas may be selected from the group comprising helium, neon, argon, krypton, methane, silane and hydrogen. In the preferred embodiment the secondary gas is hydrogen, because hydrogen hardly absorbs extreme ultraviolet radiation. Since hydrogen has favorable absorption characteristics with respect to extreme ultraviolet radiation, a very large outflow of hydrogen from the secondary nozzle can be employed, resulting in a high local density in the outflow. A lighter secondary gas is expected to yield a worse confinement of xenon as a primary gas with respect to a heavier secondary gas due to a smaller transfer of momentum in a collision. The much larger outflow and higher pressure of hydrogen which can be employed in the radiation source according to the invention overcompensates for the smaller mass of hydrogen with respect other secondary gasses, due to the considerably larger local pressures which can be tolerated. With the above jet nozzles an approximate parallel outflow of the primary gas from the primary jet nozzle 10 may be obtained. The laser beam 31 of the laser 30 for creating a plasma in the primary gas is crossed with and focussed in the outflow of primary gas at a distance from the nozzle outlet which is sufficient not to produce debris from the jet nozzle by interaction of the plasma with the nozzle. Embodiment 2 FIG. 5 schematically shows another configuration for the nozzle source of the radiation source according to a second embodiment of the invention. The second embodiment differs from the first in that it comprises a continuous nozzle source for a continuous outflow of primary and secondary gasses from the respective outlets 13 and 23 of the primary and secondary jet nozzles 10 and 20. An embodiment comprising a continuous jet nozzle for the primary jet nozzle and a pulsed nozzle for the secondary jet nozzle, or vice versa, may also be envisaged. Embodiment 3 FIG. 7 schematically shows a front view of a nozzle source of a radiation source according to a third embodiment of the invention. The third embodiment differs from the first and second embodiment in that the secondary nozzle is positioned at one side of the primary nozzle. The figure shows the outlets 13 and 23 of the primary and secondary jet nozzles, respectively. The divergence of the outflow from the primary nozzle may for this embodiment only controlled at this one side. Shielding of a plasma created in the primary gas will also only be present at this one side. An embodiment in which the secondary jet nozzle partly encloses the primary jet nozzle, or having, for instance, outlets of the secondary jet nozzle on two opposite sides of the outlet of the primary jet nozzle may also be envisaged. Embodiment 4 FIG. 3 schematically shows another embodiment of the radiation source according to the invention. The fourth embodiment differs from the first, second and third embodiment in that the plasma in the outflow of the primary gas from the nozzle is created by an electrical discharge in the primary gas ejected from the primary nozzle 10. The discharge is generated in between electrodes 40 which are connected to a high voltage source 41. However, other means for creating a plasma in the outflow of the primary gas may also be employed. Whilst specific embodiments of the invention are disclosed above it will be appreciated that the invention may be practiced other than described. The description is not intended to limit the invention.