Abstract:
A plasma pinch extreme ultraviolet source using lithium vapor requires surrounding surfaces that are heated or cooled in order to evaporate the desired quantity of lithium, typically setting the vapor pressure of lithium at a pressure of a few torr. Two distinct surfaces within the whole set are designated as the electrodes that emit and receive the high current of the plasma pinch. A method is described whereby the temperature of these designated electrode surfaces is manipulated in order to condense lithium and provide a liquid metal protective layer to absorb both plasma and extreme ultraviolet heat thereby controlling electrode erosion. A further method is described that provides a protective flow of liquid lithium exactly on the axis of the pair of discharge electrodes.

Description:
FIELD OF INVENTION 
     This invention relates to plasma X-ray sources and, more particularly, to sources of soft X-ray or extreme ultraviolet photons. 
     BACKGROUND OF THE INVENTION 
     In the case of an extreme ultraviolet light source at 13.5 nm produced by a lithium plasma, it is difficult to design the electrode for least heat flux because the EUV light can carry even more heat than the plasma particles, and its propagation is not hindered by protection via for example an applied magnetic field. This combined flux of particles and EUV light can raise the electrode surface temperature transiently, raising the danger that the melting point of the electrode material could be approached, followed by flow of surface material and deformation of the electrode shape. 
     The general principle of surface protection via liquid metal from concentrated electric discharge heat is long established, having been the subject of Swiss Patent 301203, titled “Ignitron”, awarded to Westinghouse in 1954. In that work the liquid metal, mercury, was delivered to the surface area containing the discharge location via porosity of the underlying substrate, which typically was a porous matrix of sintered tungsten or molybdenum. That approach allowed the ignitron to operate on the first pulse from cold, independent of device orientation, and protected the substrate from plasma erosion during a long operating life. 
     Subsequently, in U.S. Pat. No. 7,518,300 awarded to Philips, a discharge between electrodes, at least one of which is constructed from a matrix material or a carrier material, is claimed to reduce electrode erosion by providing charge carriers via evaporation and ionization of a sacrificial substrate material disposed within or upon the matrix or carrier material. The evaporated material may be partially ionized by a laser or other means to provide carriers for the discharge, thereby localizing the discharge and at the same time protecting the underlying carrier material. 
     SUMMARY OF THE INVENTION 
     In accordance with embodiments of the invention, electrode melting can be avoided by the production at an electrode tip of a layer of condensed liquid lithium, between pulses, in sufficient depth to absorb the heat of a pulse via evaporation, keeping the underlying electrode tip cool. In the present invention we describe a plasma extreme ultraviolet source and a method of operation of the source in which surface protection is provided by a thin liquid metal surface layer, in our case comprising lithium. In our approach sufficient electric discharge may be established independently via the magnetically assisted electrode configuration that is the subject of Patent Publication No. US 2012/0146510. This type of self-initiating discharge does not rely upon the generation of charge carriers that arise from the sacrificial substrate. We only require the protective lithium layer to absorb plasma heat and EUV radiation emitted from a plasma that has already been compressed in a plasma pinch that utilizes charge carriers generated elsewhere. 
     The capacity for a surface film of liquid lithium to absorb discharge or EUV heat is substantial. The latent heat of evaporation of lithium is 147 kJ/mole. Considering the molar volume of approximately 13.6=cm 3  (at the lithium melting point of 180 C), a 1 Joule pulse of energy may be absorbed through the evaporation of as little as 9×10 −5  cm 3  of lithium. Pulses of 1 J incident at 1 kHz may be absorbed via the evaporation of a lithium flow as small as 0.1 cm 3  sec −1 . It is an object of the present invention to create the required protective flow of lithium at or close to the tip of an electrode within an EUV source. 
     In the present invention a device and method are described for replenishment of that protective lithium layer via differential heating or cooling of the elements that comprise the wide-angle heat pipe discharge configuration of the subject type of EUV source. In particular, the two designated electrode elements may be cooled relative to the balance of the lithium return surface elements so that lithium condensation at or near to the electrode tips is enhanced, in order to create a protective lithium layer at the electrode tips. A further apparatus and method is described for production of a flow of protective liquid lithium that is exactly centered on the axis of the discharge, including the possibility of a lithium pump. 
     In accordance with a first aspect of the invention, there is provided a lithium plasma source of 13.5 nm light within a wide-angle buffer gas heat pipe containing two designated electrode structures and a plurality of heated cone-shaped structures wherein all the structures are initially heated to create a working lithium vapor density, then during repetitively pulsed operation the electrode structures are differentially cooled with respect to the cone-shaped structures so that lithium preferentially condenses on them to form a protective liquid layer between pulses. 
     In accordance with a second aspect of the invention, a plasma source comprises two electrode structures; two or more heated structures; a gas source configured to supply a working gas, wherein the electrode structures and the heated structures are heated during initial operation to form a working gas vapor density in a central region between the electrode structures and the heated structures; a pulse source configured to pulse the electrode structures during pulsed operation to form a plasma in the central region; and a cooling structure configured to cool the electrode structures with respect to the heated structures to form a liquid layer of the working gas on the electrode structures between pulses of the pulsed operation. 
     In accordance with a third aspect of the invention, a method for forming a plasma comprises establishing a working gas vapor density in a central region between two electrode structures and two or more heated structures, by heating of the electrode structures and the heated structures; pulsing the electrode structures during pulsed operation to form a plasma in the central region; and cooling the electrode structures with respect to the heated structures to form a liquid layer of the working gas on the electrode structures between pulses of the pulsed operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a structure for the production of an electric discharge in lithium vapor. 
         FIG. 2  shows an embodiment of the present invention with electrode cooling applied to the electrode tips 
         FIG. 3  shows an embodiment of the present invention with electrode cooling applied to regions adjacent to the electrode tips 
         FIG. 4  shows a detail of an embodiment of the present invention like the one shown in  FIG. 3 , but with internal channels for lithium to flow to the electrode tips 
         FIG. 5  shows a detail of an embodiment of the present invention like the one shown in  FIG. 3 , but with a cone-shaped internal channel for lithium to flow to an electrode tip 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention relate to refinements that may be appropriate to EUV sources based upon prior U.S. Pat. No. 7,479,646 “Extreme Ultraviolet Source with Wide Angle Vapor Containment and Reflux”. The present invention may also be relevant to the protection of electrodes in a wide-angle vapor containment device that employs radio frequency heaters as described in U.S. patent application Ser. No. 13/326,043, “Induction Heated Buffer Gas Heat Pipe for use in an Extreme Ultraviolet Source,” filed Dec. 14, 2011. The present invention also may be appropriate to EUV sources based upon Patent Publication No. US 2012/0146510. These documents are incorporated herein to the maximum extent allowable by law. 
     A configuration for the production and maintenance of lithium vapor is shown in  FIG. 1 . With reference to that figure, which has a vertical axis of rotational symmetry  80 , there are four separate structures  10 ,  20 ,  30 ,  40  which each are radio-frequency heated via internal coils  11 ,  21 ,  31 ,  41  respectively in order to set the temperature of all the most central surfaces with a view to establishment of a certain lithium density. There may be more than four structures in general. Two of the structures, in the case of  FIG. 1  those labeled  10  and  40  are designated as the electrodes, shown attached by leads  88  to an external pulsed electrical supply  85 . When an electric current of the order of 10 kA to 20 kA is pulsed between electrodes  10  and  40  through lithium vapor, a compact  20  electron volt plasma  60  is formed that emits 13.5 nm radiation from hydrogen-like lithium ions with very high efficiency. Particles, such as electrons and lithium ions, leave location  60  and may strike the electrode tips  50  of the designated electrodes, causing heating, and in some circumstances sputter erosion. In addition to the particle flux on to electrode tips  50 , there is extreme ultraviolet (EUV) light emitted from plasma location  60 . 
     A first embodiment of the present invention is shown in  FIG. 2 . With reference to that figure, quasi-toroidal structures  10 ,  20 ,  30 ,  40  are shown in cross section in the plane that also contains rotational symmetry axis  80 . Within each structure are radio frequency heater coils  11 ,  21 ,  31 ,  41  that heat by inducing currents on the inside surface of the hollow structures. These heating currents must flow to the innermost point of the structures in order to complete their current return loop, a flow pattern that is dictated by the toroidal topology of the structures. The application of radio frequency power to each of coils  11 ,  21 ,  31 ,  41  creates a hot central lithium vapor region as shown in  FIG. 2  due to evaporation of lithium held on the surfaces  90  of the structures. As long as radio frequency power is being applied at the appropriate level, lithium that migrates outward condenses on the cooler outer parts of surfaces  90 , flows inward again via capillary action, and is re-evaporated near the center. The outer boundary of the lithium vapor region is stabilized by a helium buffer while lithium displaces helium in the central region. Without differential temperature control, the electrode tips  50  are at approximately the same temperature as the inner parts of all surfaces  90 , and thus lithium also evaporates from electrode tips  50  and cannot form a protective surface layer on them. Once the desired lithium density has been achieved, current pulses are applied from generator  85  that is connected to the electrodes via conductors  88 . The current pulses induce hot, dense lithium plasma  60  to form at the central location on each pulse. This plasma emits EUV radiation in beams  70  that exit the device and are utilized elsewhere. 
     Once in pulsed operation, in order to prevent plasma heat and EUV radiation from overheating electrode tips  50 , potentially causing melting, a regime of electrode cooling is then instituted whereby heat-conducting elements  95  remove heat from electrode tips  50  and themselves are cooled by contact with ducts  101  cooled by the passage of fluid  103  (this part of the device does not necessarily have rotational symmetry about axis  80 ). Elements  95  can operate via straight thermal conduction, phase-change cooling, or forced convection cooling. The phase-change method includes heat pipe cooling. At the same time as cooling is initiated via elements  95 , heating via radio frequency coils  11  and  41  may be reduced to zero, or an appropriate low level. The rate of cooling via elements  95 , and/or the radio frequency power to coils  11 ,  41 , is/are controlled so that the temperature of electrode tips  50  rides below that of the center of the other cone-shaped heating surfaces powered (in this example) by radio frequency coils  21  and  31 . This state allows preferential condensation on electrode tips  50  of a new layer of lithium between each pulse, thereby giving thermal protection to the electrode surfaces. 
     A second embodiment of the invention is illustrated in  FIG. 3 . In this embodiment the two designated electrodes of approximately conical shape are labeled  100  and they are connected by leads  88  to pulsed power supply  85 . The temperature of electrode tip regions  50  is set prior to the commencement of discharge pulsing by the rate of heating applied at heater structures  105 . Heat flow, indicated by arrows in  FIG. 3 , is from heater structures  105  to the rear of electrode tips  50 , then via thin-walled cone structures  115  to cooling channels  110  around the base of each electrode  100 . Prior to pulsed discharge operation a working lithium vapor density is established via wide-angle heat pipe recirculation, established via the radio frequency heating of structures  90  and heating within the electrodes by heater elements  105 . Once pulsed operation has begun, the rate of heating applied to heaters  105  is reduced, so as to compensate for discharge heating of electrode tips  50 , and reduce the temperature of the electrode tips and their surrounds to the value at which lithium condensation is substantial. The principal condensation region  130  is close to the inner termination of cone walls  115 . Lithium that has condensed at position  130  is able to migrate over the surface by capillary action toward electrode tips  50 . This migration may be assisted by the presence of radial grooves running from region  130  toward electrode tip  50 . 
     A third embodiment of the invention is illustrated in  FIG. 4  which shows a detailed view of the electrode tip region of an equipment that is overall similar to  FIG. 3 . In  FIG. 4  there is implied a vertical axis of rotational symmetry  200 . In this embodiment the structures bear the same labeling as in  FIG. 3 , but additional features are incorporated that feed liquid lithium via an internal channel or channels toward the exact tip  50  of each electrode. Channels  140  initially run from principal condensation region  130 , converging at inner location  150 . Lithium is then able to flow through single central channel  160  and emerge through the center of electrode tip  50 . In this way, liquid lithium is delivered at precisely the most effective position for its task of evaporative electrode protection. As an additional aid to the return flow of lithium from condensation region  130  there may be radial grooves  170  that extend as far inward as the entry location to channels  140 . 
     A fourth embodiment of the invention is illustrated in  FIG. 5  which shows a detailed view of the electrode tip region of an equipment that is overall similar to  FIG. 3 . In  FIG. 5  there is implied a vertical axis of rotational symmetry  200 . In this embodiment the structures bear the same labeling as in  FIG. 3 , but additional features are incorporated that feed liquid lithium via an internal conical sheet channel  180  toward the exact tip  50  of each electrode. By “conical sheet channel” is intended an approximately cone-shaped hollow region that is of thickness small compared to its length. The flow exits through passage  205  connecting the apex of conical channel  180  with the tip region  50 . The temperature of the electrode  100  surface is controlled so that lithium condensation is strongest in region  130 . Condensed liquid lithium is drawn by capillary action up radial grooves  170  toward a plurality of entry holes  190 , or a plurality of connecting narrow slots, at the top of each radial groove. Liquid lithium gains access via entry holes  190  to channel  180 . In this way, liquid lithium is delivered at precisely the most effective position for its task of evaporative electrode protection. Such lithium flows may apply to either or both of the two electrodes in the plasma device. 
     Various pumping means may be deployed in order to move larger quantities of liquid lithium toward electrode tips  50  than available by capillary action alone. The pumping means have to be deployed at a location within, or in contact with, the lithium passageway between condensation regions  130  and electrode tips  50 . Many different designs of pump are possible, but the principle of inserting a pump within this range of locations is the substance of one or more of the following claims. 
     Further realizations of this invention will be apparent to those skilled in the art. Having thus described several aspects of at least one embodiment of this invention it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.