Patent Number: 
Section: description

FIG. 1 shows a plasma focus discharge source 210 according to a first embodiment of the present invention. The plasma focus discharge source 210 comprises a generally cylindrical cathode 211 surrounding an elongate anode 212 with an annular space therebetween. A voltage source 214 applies a high voltage between the anode and cathode sufficient to cause ionization of the gas in the annular space so that a discharge current I begins to flow radially from anode to cathode. The discharge current I generates a circular magnetic field B in the annular space between anode and cathode. The ions of the discharge current are driven by their interaction with the magnetic field B along the anode 212, as indicated by arrows 216. The anode 212 is shorter than the cathode 211 and has a hollow tip so that the plasma is driven over the end of the anode 212 and converges to form a very hot plasma in pinch volume 218. According to the invention, the plasma is formed in a driver gas which fills the annular space between anode 212 and cathode 211 between each discharge. The driver gas is chosen according its magneto-hydrodynamic properties to effectively form a conducting medium, guiding the current from anode to cathode, and, induced by the thus generated magnetic field, comprising the enclosed volume around and onto the axis. To provide EUV radiation of the desired wavelength, a working (primary) substance, e.g. gas, vapor, clusters or liquid, is provided into the enclosed volume and is heated by the converging plasma to emit EUV radiation. The working substance is chosen for its efficiency in emitting EUV radiation at the desired wavelength, e.g. about 9 to 16 nm, preferably 11 or 13 nm, and may be Li, Xe or water. The working substance is preferably emitted into the region of the pinch volume 218 of the converging plasma as a jet, e.g. a cluster jet or a droplet-like jet, appropriately timed to the discharge voltage that is derived from an appropriately pulsed source 214. The working substance can be supplied from a source 215 via a bore 213 in the anode 212 to form a jet 217 in the hollow tip of the anode 212. The source 215 comprises a reservoir of the working substance as well as necessary pumps, valves, etc to control the jet. A second embodiment of the invention, which may be the same as the first embodiment of the invention save as described below, comprises a so-called Z-pinch plasma discharge source. The Z-pinch plasma discharge source 220 is shown in FIG. 2. It comprises an annular cathode 221 and annular anode 222 provided at opposite ends of a cylindrical chamber 223 having insulating walls. A quantity of driver (secondary) gas is injected from source 225 through an annular opening close to the outer wall of the cylindrical chamber 223 and pre-ionized. Voltage source 224 then applies a high voltage between anode 222 and cathode 221 causing a cylindrical discharge starting on the insulating walls of the chamber 223 which generates an azimuthal magnetic field. The magnetic field causes the discharge to contract into a thin axial thread, or pinch volume, 229 at high pressure and temperature. Ceramic plug 226 defines the aperture through which the extreme ultraviolet radiation to form projection beam PB is emitted. To enhance the emission of EUV, according to the invention, a working substance is jetted into the region of the pinch volume 229 in chamber 223 from source 227 at an appropriate time to be entrained with and compressed by the plasma discharge. As in the first embodiment, the driver gas can be chosen for its effectiveness in generating a high-temperature plasma and the working substance for its efficiency in emitting EUV radiation of the desired wavelength. A third embodiment, which may be the same as the first embodiment, save as described below, comprises a capillary discharge plasma source. FIG. 3 shows the capillary discharge source 230, which has a cathode 231 and anode 232 forming the end plates of a small chamber 233. The anode 232 has a small central through-hole aligned with a narrow capillary 236 formed in an insulator 235 which covers the side of the anode 232 which faces the cathode 231 and the side walls of the chamber 233. A discharge will be formed in the capillary 236, which, as in the previous embodiments, will compress on the axis of the capillary into a pinch volume to create a highly-ionised, high-density plasma having a high temperature. The emission aperture is defined by aperture plate 237. According to the invention, a working (primary) substance is jetted into capillary 236 from source 238. As in the previous embodiments, a driver gas can be chosen for its effectiveness in generating a high-temperature plasma and the working substance for its efficiency in emitting EUV radiation of the desired wavelength. In the third embodiment, and also the first and second embodiments, the driver gas can be injected into the chamber for each discharge (shot) of the source. The working and driver gasses can be ejected, e.g., by a two part annular nozzle, as will be described in the seventh and eight embodiment. This provides for a decreased divergence of the jet of working fluid ejected and for a shielding gas around the pinch volume to increase the efficiency of the source. The primary jet nozzle preferably provides for a supersonic jet to have a sharply-peaked density distribution of the working gas on-axis of the ejection from the jet nozzle. FIG. 4 shows a fourth embodiment of a radiation source according to the invention, which is a variant of the first embodiment described above. The Figure shows the configuration of anode 110 and cathode 120, which are kept separated by an electrical insulator 130 and which are connected to a capacitor bank 140. A central part of the radiation source has cylindrical symmetry around central axis A. FIG. 8 further shows an annular cathode aperture 121 and an annular cathode cavity 122 around central axis A. A driver gas or vapor is supplied to cavity 122 via an inlet 125 so as to provide a low pressure within the cavity. In the present embodiment, argon (Ar) is taken as the driver gas, but basically any gas, such as for instance helium (He), neon (Ne) and hydrogen (H2), is suitable. Hydrogen may be specially preferred since it shows a low absorption of radiation in the EUV range. The driver gas inside cavity 122 is used as a source of electrons to start a discharge between anode and cathode. The cathode cavity 122 surrounds a (primary) working gas or vapor source 160, which ejects a working gas or vapor in the anode-cathode gap in a region around central axis A. The working gas or vapor is chosen for its spectral emission characteristics as a plasma. The present embodiment uses lithium (Li) for its very strong emission line at approximately 13.5 nm. Xenon (Xe) may also be used, which has a broad emission spectrum in the XUV (and EUV) region of the electromagnetic radiation spectrum. The Li source 160 shown comprises a heater 161 below a container 162 containing solid lithium. Vaporized Li reaches the anode-cathode gap through a supersonic (Laval) nozzle 163, however other types of nozzle may also be used. A trigger electrode 150 is inserted in cathode cavity 122. Electrode 150 is connected to appropriate electrical circuitry (not shown in FIG. 8) for applying a voltage pulse to the electrode to start the discharge described below. Initially, the radiation source is close to auto-triggering. A voltage pulse applied to trigger electrode 150 causes a disturbance of the electrical field within cathode cavity 122, which will cause triggering of the hollow cathode and the formation of a breakdown channel and subsequently a discharge between cathode 120 and anode 110. An initial discharge may take place at low initial pressure (p less than 0.5 Torr) and high voltage (V less than 10 kV) conditions, for which the electron mean free path is large compared to the dimension of the anode-cathode gap, so that Townsend ionization is ineffective. Those conditions are characterized by a large electrical field strength over gas or vapor density ratio, E/N. This stage shows rather equally spaced equipotential lines having a fixed potential difference. The ionization growth is initially dominated by events inside the hollow cathode that operates at considerable lower E/N, resulting in a smaller mean free path for the electrons. Electrons e from hollow cathode 120, and derived from a driver gas or vapor within cavity 122, are injected into the anode-cathode gap, a virtual anode being created with ongoing ionization, which virtual anode propagates from anode 110 towards hollow cathode 120, bringing the full anode potential close to the cathode. The electric field inside the hollow cavity 122 of cathode 120 is now significantly enhanced. In the next phase, the ionization continues, leading to a rapid development of a region with high ion density inside the hollow cathode, immediately behind the cathode aperture 121. Finally, injection of an intense beam of electrons 126 from this region into the anode-cathode gap, forms the final breakdown channel. The configuration provides for a uniform pre-ionization and breakdown in the discharge volume. When a working gas or vapor has been ejected from source 160 and a discharge has been initiated, a partially ionized, low-density and relatively cold plasma of the working gas or vapor is created in the anode-cathode gap above aperture 121. An electrical current will be flowing within the plasma from cathode 120 to anode 110, which current will induce an azimuthal magnetic field, having magnetic field strength H, around the radiation source. The azimuthal magnetic field causes the partially ionized plasma above cathode aperture 121 to compress toward central axis A. Dynamic compression of the plasma will take place, because the pressure of the azimuthal magnetic field is much larger than the thermal plasma pressure: H2/8xcfx80 greater than  greater than nkT, in which n represents plasma particle density, k the Boltzmann constant and T the absolute temperature of the plasma. Electrical energy stored in capacitor bank 140 connected to anode 110 and cathode 120 will most efficiently be converted into energy of the kinetic implosion during the full time of the plasma compression. A homogeneously filled pinch volume with a high spatial stability is created. At the final stage of plasma compression, i.e. plasma stagnation in the pinch volume on central axis A, the kinetic energy of the plasma is converted into thermal energy of the plasma and finally into electromagnetic radiation having a very large contribution in the XUV range. Radiation emitted from a collapsed plasma will pass through an opening 111 in the anode 110 into a vacuum chamber 170 that is evacuated through opening 171 in a wall of the vacuum chamber. Plasma and debris particles may also escape through opening 111. A flywheel shutter 180 is present to block these particles when no XUV radiation pulse is emitted for preventing them to reach any optical elements in the radiation path of the XUV radiation to the projection system PL. FIG. 5 depicts a fifth embodiment of the invention, which is a variation of the fourth embodiment and further shields the aperture region of cathode 120 from plasma collapse at central axis A. Both anode 110 and cathode 120 have a xe2x80x9chat-likexe2x80x9d structure. Annular cathode cavity 122 and aperture 121 are located at the bottom side of the hat. A partially ionized, low-density and relatively cold plasma created by a discharge at aperture 121 will compress upwards and xe2x80x9caround the cornerxe2x80x9d towards central axis A. Further, the positions of anode 110 and cathode 120 have been interchanged. Cathode 120 is located on the outside of the configuration and comprises aperture 123 for passing XUV radiation to vacuum chamber 170. However, the density of the working gas or vapor, also Li vapor in the present embodiment, may be too low at annular aperture 121 of cathode 120 for creating a discharge and a plasma. In embodiment 6, the radiation source is configured so as to yield a sufficiently high pressure of the driver gas or vapor, Ar in the present embodiment, within the anode-cathode gap in the region at the annular aperture 121 for creating a discharge in the driver gas. The resulting plasma of the driver gas will start to compress towards central axis A and at some point encounter a sufficiently high pressure of the working gas or vapor to create a plasma of the working gas or vapor, which will then further compress until stagnation into a pinch volume on central axis A. The plasma of the driver gas or vapor may even first have to go xe2x80x9caround the cornerxe2x80x9d to reach a sufficiently high pressure of the working gas or vapor. A radiation source according to a sixth embodiment of the invention is shown schematically in FIG. 6 and 7 and comprises primary and secondary jet nozzles 10 and 20 and a supply of primary and secondary gases 11, 21 to the primary and secondary jet nozzles, respectively. In this embodiment, both jet nozzles are pulsed jet nozzles, in which both supply lines 11, 21 comprise valves which are opened at certain instants in time to supply a pulse of primary and secondary gases to the respective jet nozzles. FIG. 6 shows a longitudinal section through the jet nozzle source for the primary and secondary gases. FIG. 7 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 off 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. Further, the use of continuous nozzles is also possible. 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. Ejecting a pulse of secondary gas 25 as well 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 secondary gas results in a less divergent beam if 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 sixth 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 provide worse confinement of xenon as a primary gas with respect to a heavier secondary gas due to the smaller momentum transfer 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 a less divergent, confined or an approximate parallel outflow of the working (primary) gas from the primary jet nozzle 10 may be obtained to receive the ejected working gas in a rather confined region at the pinch volume, which is preferably located at some distance from the nozzle source outlet to not produce debris from the jet nozzle by interaction of the plasma with the nozzle. A continued ejection of secondary fluid from the annular secondary jet nozzle will provide for a gas shield around the compressed high-temperature plasma in the pinch volume to block or slow down and neutralize any fast particulates that will be emitted from the hot plasma. Parts of the source, and possibly also optical elements comprised in the illuminator of a lithographic projection apparatus, are thus protected from damage by such fast particulates or from deposition of those particulates. Further, the flushing gas shield of secondary gas also provides for an environment around the pinch volume, which is highly transparent for the generated XUV radiation when an appropriate secondary fluid is chosen. Heavy (metal) particles, for instance, eroded from the electrodes or primary Xenon (working) gas that might be present around the high-temperature plasma in the pinch volume would cause a large absorption of the XUV radiation generated. FIG. 8 schematically shows a front view of a nozzle source used in a variant of the radiation source according to the sixth embodiment of the invention. The variant differs from the basic arrangement of the sixth 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, which may be convenient in some applications. 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 opposite sides of or all around the outlet of the primary jet nozzle may also be envisaged. FIG. 9 schematically depicts a lithographic projection apparatus 1 in which the radiation sources according to the invention may be used. The apparatus comprises: a radiation system LA, IL for supplying a projection beam PB of EUV radiation; a first object table (mask table) MT provided with a first object (mask) holder for holding a mask MA (e.g. a reticle), and connected to first positioning means PM for accurately positioning the mask with respect to item PL; a second object table (substrate table) WT provided with a second object (substrate) holder for holding a substrate W (e.g. a resist-coated silicon wafer), and connected to second positioning means PW for accurately positioning the substrate with respect to item PL; a projection system (xe2x80x9clensxe2x80x9d) PL (e.g. a refractive, catadioptric or reflective system) for imaging an irradiated portion of the mask MA onto a target portion C (die) of the substrate W. As here depicted, the apparatus is of a reflective type (i.e. has a reflective mask). However, in general, it may also be of a transmissive type, for example. The radiation system comprises a source LA which may be any of the radiation sources described above and which produces a beam of extreme ultraviolet (EUV) radiation. This beam is passed along various optical components included in illumination system (xe2x80x9clensxe2x80x9d) IL so that the resultant beam PB is collected in such a way as to give illumination of the desired shape and intensity distribution at the entrance pupil of the projection system and the mask. The beam PB subsequently impinges upon the mask MA which is held in the mask holder on the mask table MT. Having been selectively reflected by the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target area C of the substrate W. With the aid of the interferometric displacement measuring means IF and positioning means PW, 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. Similarly, the positioning means PM and interferometric displacement measuring means IF can be used to accurately position the mask MA with respect to the path of the beam PB. In general, movement of the object tables MT, WT will be realized with the aid of a long stroke module (course positioning) and a short stroke module (fine positioning), which are not explicitly depicted in FIG. 9. The depicted apparatus can be used in two different modes: 1. In step mode, the mask table MT is kept essentially stationary, 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 beam PB; 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 Y 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 lens PL (typically, M=xc2xc or ⅕). In this manner, a relatively large target area C can be exposed, without having to compromise on resolution.