Abstract:
The object of an arrangement and a method for generating extreme ultraviolet radiation by an electrically operated gas discharge is to improve the adjustment of the layer thickness and, in particular, to prevent an uncontrolled accumulation of the metal layer to be applied to the rotary electrodes during pauses in the pulse operation for generating radiation when, e.g., liquid flows through these rotary electrodes for efficient cooling. In this connection, the rotating speed of the rotary electrodes can be increased in particular until there is always a freshly coated surface region of the electrodes in the discharge area at repetition frequencies of several kilohertz. An edge area to be coated on at least one electrode has at least one receiving area which extends in a closed circumference along the electrode edge on the electrode surface and which is formed so as to be wetting for the molten metal. A coating nozzle for regenerative application of the molten metal is directed to this receiving area and has a shutoff valve connected to a valve regulating device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority of German Application No. 10 2007 004 440.4, filed Jan. 25, 2007, the complete disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    a) Field of the Invention 
         [0003]    The invention is directed to an arrangement for generating extreme ultraviolet radiation by means of an electrically operated gas discharge, containing a discharge chamber which has a discharge area for a gas discharge for forming a radiation-emitting plasma, a first disk-shaped electrode and a second disk-shaped electrode, at least one of which electrodes is rotatably mounted and has an edge area to be coated by a molten metal, an energy beam source for supplying a pre-ionization beam, and a discharge circuit connected to the electrodes for generating high-voltage pulses. 
         [0004]    The invention is further directed to a method for generating extreme ultraviolet radiation by means of an electrically operated gas discharge for forming a radiation-emitting plasma from pre-ionized emitter material in which at least one rotatably mounted, disk-shaped electrode of a pair of electrodes provided for the gas discharge is coated in the edge area by a molten metal. 
         [0005]    b) Description of the Related Art 
         [0006]    Studies of a large number of electrode shapes for gas discharge sources such as, e.g., Z-pinch electrodes, hollow-cathode electrodes or plasma focus electrodes have shown that the life of electrodes constructed in these ways is insufficient for EUV projection lithography. 
         [0007]    In contrast, rotary electrodes, as they are called, have turned out to be a very promising solution for appreciably increasing the life of gas discharge sources. One advantage is that these electrodes, which are disk-shaped in particular, can be cooled better. Another advantage consists in that inevitable electrode erosion can be prevented from shortening life by a constant renewal of the electrode surface. 
         [0008]    A device previously known from WO 2005/025280 A2 uses rotating electrodes which are immersed in a vessel containing molten metal, e.g., tin, for regenerative application of a molten metal. The metal applied to the electrode surface is evaporated by laser radiation at the location where the two electrodes are closest together, whereupon the vapor is ignited by a gas discharge to form a plasma. The cooling of the electrodes is carried out by the metal baths. 
         [0009]    The solution proposed in WO 2005/025280 has the following disadvantages: Because of the immersion process, the rotating speed of the electrodes is limited and is not sufficient for the required output specification of an EUV source. Owing to insufficient rotating speed, subsequent arrival of unconsumed electrode portions in the discharge area is too slow, which causes instabilities in the plasma generation. The rotating speed should be designed in such a way that the electrodes continue to rotate between two successive discharge pulses by an amount that is greater than the radius of the region of influence of the preceding discharge pulse on the electrode surface. 
         [0010]    Because of the short dwell period of the electrodes in the molten metal, cooling the electrodes by means of the melt is insufficient for the required high output specifications. However, an additional cooling of the electrodes, for example, by a throughflow of water, would allow the temperature of the electrode surface to fall below the melting temperature of the metal applied by means of the molten baths during the prolonged pauses in the pulse operation provided for radiation generation which are common in exposure processes in semiconductor fabrication. This would result in a heavy, uncontrolled accumulation of the metal layer on the electrodes. Rapidly switching the additional cooling off and on would lead to a temperature gradient between the electrode surface and the interior of the electrode. Since this temperature gradient balances out when the additional cooling is switched off, an impermissibly high heating of the coolant can occur so that any gas bubbles that might possibly occur form a thermally insulating layer which prevents efficient cooling. Further, it is difficult to adjust the layer thickness of the applied material. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0011]    Therefore, it is the primary object of the invention to facilitate adjustment of the layer thickness and, in particular, to prevent an uncontrolled accumulation of the metal layer to be applied to the rotary electrodes during pauses in the pulse operation for generating radiation when, e.g., liquid flows through these rotary electrodes for efficient cooling. In this connection, the rotating speed of the rotary electrodes can be increased in particular until there is always a freshly coated surface region of the electrodes in the discharge area at repetition frequencies of several kilohertz. 
         [0012]    This object is met in an arrangement for generating extreme ultraviolet radiation by means of an electrically operated gas discharge of the type mentioned above in that the edge area to be coated has at least one receiving area, which extends in a closed circumference along the electrode edge on the electrode surface and which is formed so as to be wetting for the molten metal, and a coating nozzle for regenerative application of the molten metal having a shutoff valve connected to a valve regulating device is directed to this receiving area. 
         [0013]    Particularly advisable, advantageous constructions and further developments of the arrangement according to the invention are indicated in the dependent claims. 
         [0014]    The valve regulating device is preferably connected to a temperature measuring device for measuring the surface temperature of the electrodes. 
         [0015]    The disk-shaped electrodes are outfitted with a permanently operating cooling device. The coolant to be used can have an operating temperature below the melting temperature of a material provided for the molten metal. For example, cooling channels through which a liquid flows and which can also have temperature regulating means can be provided in the disk-shaped electrodes for cooling purposes. 
         [0016]    The coating nozzle can be directed to the electrode surface in an area of the electrode which is located opposite the discharge area and which is provided for applying the molten metal. 
         [0017]    In another advantageous further development of the invention, the electrodes are constructed as circular disks, are rigidly connected to one another at a mutual distance and are supported so as to be rotatable around a common axis of rotation which coincides with their center axes of symmetry. Each of the electrodes contains, on electrode surfaces facing one another, the at least one receiving area which is formed so as to be wetting for the molten metal and to which a coating nozzle is directed. 
         [0018]    In order to prevent electrical short-circuiting, it is advantageous when a disk-shaped insulating body which penetrates into the intermediate space between the two electrodes is provided in the electrode area to which the molten metal is to be applied. In this construction, the coating nozzles which are directed to the electrode surfaces of the two electrodes can be guided through the disk-shaped insulating body from opposite sides. 
         [0019]    The arrangement according to the invention can be further developed in a particularly advantageous manner in that the coating nozzle comprises two microstructured plates which lie one on top of the other, and a portion of a first plate is perforated by a hole structure, the second plate being outfitted with a membrane which lies opposite to the hole structure and which is flexible toward the hole structure. A closure element for the hole structure which can be pressed against the hole structure by actuating means acting at the flexible membrane is arranged on the flexible membrane so that the flow of molten metal can be interrupted. Accordingly, a movement away from the hole structure allows the molten metal to resume flowing. The two plates enclose a channel into which the hole structure opens and which is guided out of the first plate as a nozzle outlet. 
         [0020]    The hole structure can also serve as a filter for larger particles in order to prevent clogging of the coating nozzle in that the hole structure has hole diameters that are smaller than the diameter of the nozzle outlet. Further, the coating nozzle can be constructed so as to be heatable by means of a current-carrying resistor which is arranged on the surface of at least one of the plates. 
         [0021]    A pre-ionization of the emitter material is advantageous for igniting the plasma, particularly the evaporation of a droplet of advantageous emitter material that is injected between the electrodes. For this purpose, on the one hand, an injection device is directed to the discharge area and supplies a series of individual volumes of an emitter material, which is used to generate radiation, at a repetition frequency corresponding to the frequency of the gas discharge and by limiting the amount of the individual volumes so that the emitter material which is injected into the discharge area at a distance from the electrodes is entirely in the gaseous phase after the discharge. On the other hand, the pre-ionization beam supplied by the energy beam source is directed synchronous to the frequency of the gas discharge to a location for plasma generation in the discharge area at a distance from the electrodes at which the individual volumes arrive and are successively ionized by the pre-ionization beam. 
         [0022]    Alternatively, the ignition of the plasma can also be initiated in that the regeneratively applied molten metal is emitter material serving for the generation of radiation and the pre-ionization beam supplied by the energy beam source is directed to the emitter material synchronous to the frequency of the gas discharge in the discharge area. 
         [0023]    Because of the discharge process in which a plasma radiating in the EUV range is formed, a portion of the layer applied to the electrode surface in the region of influence of the plasma is evaporated or is expelled as molten material. This amounts to several 10 −7  to several 10 −−6  grams per pulse. This loss of mass is compensated by the steady supply of molten metal so that a constant protective layer remains on the electrode surface even under discharge conditions with repetition frequencies of several kilohertz. 
         [0024]    The inventive application of molten metal is also particularly advantageous because the contact between the rotary electrodes and the discharge circuit can have a particularly low inductance owing to a horizontal arrangement of the two rotary electrodes. 
         [0025]    Therefore, in another construction of the invention, the electrodes are in electrical contact with contact elements which are oriented coaxial to the axis of rotation and which are immersed in ring-shaped, electrically separated molten metal baths which are electrically separated from one another and which communicate with a discharge circuit of the high-voltage supply. 
         [0026]    In another construction, the electric contacting can also be carried out by means of the coating nozzle and the liquid jet. 
         [0027]    The above-stated object is further met according to the invention by a method for generating extreme ultraviolet radiation of the type mentioned above in that the regenerative coating of the edge area is controlled during the rotation depending on the electrode surface temperature. 
         [0028]    According to the method, the coating is interrupted when the temperature drops below a limit temperature lying above the melting temperature of a material provided for the molten metal and is continued when the temperature rises above the limit temperature. 
         [0029]    In a particularly advantageous manner, the electrodes are cooled during coating by a coolant which has an operating temperature below the melting temperature of the material provided for the molten metal. Further, the cooling can be regulated. 
         [0030]    The invention will be described more fully in the following with reference to the schematic drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    In the drawings: 
           [0032]      FIG. 1  shows the principle according to the invention for applying a defined thin layer of a molten metal along a track on a rotating electrode surface; 
           [0033]      FIG. 2  shows an arrangement for applying a molten metal to oppositely located, liquid-cooled electrode surfaces of two electrodes that are rigidly connected to one another and mounted so as to be rotatable around a common axis; 
           [0034]      FIG. 3  shows the isothermal curve inside an electrode during pulse operation; 
           [0035]      FIG. 4  shows the isothermal curve inside an electrode during a pause in the pulse operation; 
           [0036]      FIG. 5  shows the time temperature curve on the electrode surface depending on the operating state of the radiation source; 
           [0037]      FIG. 6  is a sectional view showing an arrangement of a controllable coating nozzle between two electrodes; 
           [0038]      FIG. 7  shows a perspective view of a coating nozzle; 
           [0039]      FIG. 8  shows a first construction of a radiation source with a rotary electrode arrangement according to the invention; and 
           [0040]      FIG. 9  shows a second construction of a radiation source with a rotary electrode arrangement according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    In  FIG. 1 , which serves to illustrate the principle of the invention, a disk-shaped electrode  1  is rigidly connected to a rotatable shaft  2  in such a way that the center axis of symmetry of the electrode coincides with the axis of rotation R-R. A circumferential edge track on the electrode surface serves as a receiving area  3  for a molten metal, e.g., tin or a tin alloy, and is formed so as to be wetting for this material. Surfaces for the edge track having a wetting action can be, e.g., copper, chromium, nickel or gold. However, a structural steel, heat-treated molybdenum or other electrically conductive materials are also suitable. 
         [0042]    The rest of the electrode surface, or at least a portion of the electrode surface adjoining the receiving area, should not be wetting for the material to be applied because application of the molten metal to these areas is not wanted. Suitable non-wetting surfaces can comprise, e.g., PTFE, stainless steel, glass, or ceramic. 
         [0043]    A coating nozzle  4  of a liquid generator, not shown, is directed to the receiving area  3  to apply the molten metal as a liquid jet  5  to the receiving area  3  in a regenerative manner during the rotation of the electrode  1 . Due to the fact that the applied liquid metal is propelled to the edge of the electrode by the centrifugal force, it is necessary to provide a spray guard  6  to prevent detaching molten metal from spreading in an uncontrolled, unlimited manner. 
         [0044]    An energy beam, e.g., a laser beam, which serves as a pre-ionization beam  7  is directed in a discharge area  8  to an injected droplet of advantageous emitter material in order to evaporate the latter. 
         [0045]    The adjustment of a defined layer thickness for the metal to be applied within a range between 1 μm and 20 μm requires an electrode surface temperature above the melting temperature of the material to be applied. A temperature measuring device  9 , for example, a pyrometer, carries out the measurement of the electrode surface temperature. A valve regulating device  10  connected to the temperature measuring device  9  ensures by means of a shutoff valve  11  that the supply of material and, therefore, the regenerative coating of the receiving area  3 , is interrupted at a limit temperature that is still above the melting temperature of the material to be applied. However, when the electrode surface temperature increases again above the limit temperature, the shutoff valve  11  in the material feed is opened again proceeding from the valve regulating device  11  and the coating process is continued. 
         [0046]    In the construction shown in  FIG. 2 , a first and a second disk-shaped electrode  1 ,  12  are rigidly connected at a distance from one another to the rotatably mounted shaft  2  in such a way that the center axes of symmetry of the electrodes  1 ,  12  coincide with the axis of rotation (R-R) of the shaft  2 . Each of the electrodes  1 ,  12  contains, on surfaces that face one another, a receiving area  3 ,  13  which is formed as an edge track and which has a wetting action for the molten metal, a coating nozzle  4 ,  14  being directed to these receiving areas  3 ,  13 . The receiving areas  3 ,  13  are arranged on the electrode surfaces in such a way that they are located opposite one another. 
         [0047]    A disk-shaped insulating body  16 , particularly an electrically insulating ceramic plate which is immersed in the intermediate space between the two electrodes  1 ,  12  in an area of the electrode provided for applying the molten metal is provided for preventing electric short-circuiting between the electrodes  1 ,  12  due to the liquid jets  5 ,  15  of molten metal. As is shown in  FIG. 2 , the two coating nozzles  4 ,  14  are guided through the electrically insulating ceramic plate from opposite sides. One coating nozzle  4  acts in the direction of the force of gravity and the other coating nozzle  14  acts counter to the direction of the force of gravity. 
         [0048]    The disk-shaped electrodes  1 ,  12  are penetrated by cooling channels  17 ,  18  through which a cooling liquid flows. Because cooling of this kind is relatively sluggish and therefore cannot be regulated quickly, it may happen during relatively short pauses in pulse operation that the temperature of the electrode surface drops below the melting temperature of the material to be applied. Therefore, as is described with reference to  FIG. 1 , the material feed is regulated depending on the electrode surface temperature and is interrupted by shutoff valves  11 ,  19  particularly when it falls below a limit temperature. 
         [0049]    The curve of the isotherms  20  which is shown in  FIG. 3  reflects a strong temperature gradient which results between electrode surfaces and the cooling channels during an ongoing pulse operation at maximum output. At a given temperature of the electrode surface of, e.g., around 500° C. at which the material applied to the edge area is liquid and at a cooling water temperature of, e.g., around 80° C., the regenerative rotational coating takes place. 
         [0050]    On the other hand, if the temperature gradient flattens out during a pause in the pulse operation, the temperature of the electrode surface at about 120° C. lies below the melting temperature of the coating material. The temperature of the cooling water has fallen to approximately 40° C. The rotational coating is interrupted according to the invention ( FIG. 4 ). 
         [0051]      FIG. 5  shows the time-temperature curve on the electrode surface during period t pulse  of the pulse operation for the pulsed generation of radiation and during a period t pause  in which the pulse operation is adjusted and during which, accordingly, no radiation is generated. When after a sharp rise in temperature at the start of the pulse operation the temperature exceeds a limit temperature T limit  above the melting temperature T melt  of the material to be applied, the rotational coating is switched on for a period T coat . Depending on the length of the pulse operation, an equilibrium temperature T equilibrium  can occur until the temperature drops at the end of the pulse operation and, therefore, at the end of the pulsed generation of radiation. The rotational coating continues to be carried out until the temperature falls below the limit temperature T limit . This results in the formation of a sacrificial layer which can be consumed at the start of the next pulse operation for as long as the electrode temperature remains below the limit temperature T limit  for the rotational coating and the coating nozzles  4 ,  14  are switched off. 
         [0052]    A coating nozzle carrying out the coating function according to  FIG. 2  must have a flat structural shape in order to be able to penetrate into the gap between the disk-shaped electrodes. Further, a coating nozzle of this kind must be heatable to ensure that the molten metal remains liquid. 
         [0053]    A coating nozzle according to  FIG. 6  which is manufactured using silicon layer technology and which contains an integrated shutoff valve comprises two silicon plates  22 ,  23 , which are preferably anodically bonded, and is oriented with respect to its position to the edge area of an electrode, in this instance electrode  12 , by holding elements  24 ,  25 . The silicon plates  22 ,  23  are formed by established methods of silicon structuring, corresponding to the nozzle function to be carried out by them, as microstructured components. Openings in the form of a hole structure  26  with hole diameters which are preferably smaller than the diameter of a nozzle outlet  27  are incorporated in the silicon plate  22  which, in this instance, lies on top. A channel  28  that is fashioned in the silicon plate  22  leads to the nozzle outlet  27  and communicates with a recess  29  in the other silicon plate  23  into which the hole structure  26  opens. The hole structure  26  can advantageously form a filter for larger particles to prevent clogging of the nozzle structure. 
         [0054]    A flexible membrane  30  which is arranged opposite the hole structure  26  and has a die-like closure element  31  that can be moved against the hole structure  26  by the bending of the membrane  30  is incorporated in the bottom silicon plate  23  referring to the drawing. Accordingly, by means of actuating means  32  accommodated in the holding element  25 , the closure element  31  can be pressed against the hole structure  26  so that, if necessary, the supply of liquid coating material  33 , a supply channel  34  being incorporated in the holding element  24  for this purpose, can be interrupted (shown in dashes). When the force of the actuating means  32  is withdrawn, the closure element  31  disengages from the hole structure  26  so that the flow of coating material  33  can resume. 
         [0055]    By integrating the shutoff valve in the coating nozzle, the dead volume can be advantageously minimized in such a way that afterrunning of coating material or a delay in switching on can be prevented to a great extent, which is important particularly for fast switching cycles. 
         [0056]    Finally, the coating nozzle  21  can be constructed so as to be heatable by a current-carrying resistor  35  ( FIG. 7 ) arranged on the surface so that the molten metal does not solidify inside the coating nozzle  21 . The current-voltage characteristic of the layer-type resistor  35  can be used simultaneously as a temperature measurement signal for regulating the temperature of the coating nozzle  21 . 
         [0057]    The radiation source shown in  FIG. 8  comprises a rotary-electrode arrangement according to  FIG. 2  in a discharge chamber  38  that can be evacuated by means of vacuum pumps  36 ,  37 . Electric feeds to the electrodes  1 ,  12  are preferably formed by ring-shaped, electrically separated melt baths  39 ,  40  of molten metal, e.g., tin or other low-melting metal baths such as, e.g., gallium, in which the electrodes  1 ,  12  are immersed by contact elements  41 ,  42 . The contact elements  41 ,  42  are either formed of a plurality of individual contacts (contact element  41 ) which are arranged along a circular ring on one electrode  12  and guided through openings  43  in the other electrode  1  so as to be electrically insulated, or they are formed as a closed cylindrical ring (contact element  42 ). Suitable partial covers of the metal baths  39 ,  40  in the form of inwardly turned-down outer walls  44 ,  45  prevent the pressed out molten metal from exiting from the vessels for the melt baths  39 ,  40 . 
         [0058]    Since an arrangement of the type mentioned above requires horizontally placed disk-shaped electrodes  1 ,  12  or a vertically directed axis of rotation R-R, a technique for applying a molten metal such as that provided by the invention is particularly advantageous because, contrary to what was previously known, the molten metal can be applied to the electrodes  1 ,  12  against the force of gravity. 
         [0059]    By means of the rotary-electrode arrangement according to the invention, current pulses can be supplied to the electrodes  1 ,  12  without wear and, above all, with low inductance. Further, to this end, there is an electrical connection leading out of the discharge chamber  38  from the melt baths  39 ,  40  to capacitor elements  48 ,  49  via vacuum feedthroughs  46  to  47 . The capacitor elements  48 ,  49  are part of a discharge circuit which, by generating high-voltage pulses at a repetition rate between 1 Hz and 20 kHz and with a sufficient pulse size, ensures that a discharge is ignited in the discharge area  8  which is filled with a discharge gas and that a high current density is generated which heats pre-ionized emitter material so that radiation of a desired wavelength (EUV radiation) is emitted by an occurring plasma  50 . 
         [0060]    After passing through a debris protection device  51 , the emitted radiation arrives at collector optics  52  which direct the radiation to a beam outlet opening  53  in the discharge chamber  38 . An intermediate focus ZF which is located in or in the vicinity of the beam outlet opening  53  is generated by the formation of the plasma  50  by means of the collector optics  52  and serves as an interface to exposure optics in a semiconductor exposure installation for which the radiation source, preferably formed for the EUV radiation range, can be provided. 
         [0061]    In a particularly advantageous manner, the ignition of the plasma  50  can be initiated by evaporation of a droplet of advantageous emitter material injected between the electrodes  1 ,  12 . An advantageous emitter material of this kind can be xenon, tin, a tin alloy, a tin solution, or lithium. As was already shown in  FIG. 1 , the pre-ionization beam  7  which is directed to an injected droplet in the discharge area  8  synchronous to the frequency of the gas discharge serves to pre-ionize the emitter material. Therefore, in another construction according to  FIG. 9 , the emitter material is introduced into the discharge area  8  in the form of individual volumes  54 , particularly at a location in the discharge area  8  provided at a distance from the electrodes  1 ,  12  at which the plasma generation is carried out. The individual volumes  54  are preferably supplied as a continuous stream of droplets in dense, i.e., solid or liquid, form through an injection device  55  directed to the discharge area  8  at a repetition frequency corresponding to the frequency of the gas discharge. The pulsed pre-ionization beam  7 , preferably a laser beam of a laser radiation source, which is provided by an energy radiation source  56 , is directed to the location of the plasma generation in the discharge area  8  synchronous to the frequency of the gas discharge in order to evaporate the droplet-shaped individual volumes  54 . 
         [0062]    When the molten metal applied to the electrodes  1 ,  12  for purposes of regeneration comprises emitter material, the energy beam  7  for the pre-ionization of the emitter material can also be directed thereto synchronous with the frequency of the gas discharge, specifically either to only one electrode  1  or  12  or to both electrodes  1 ,  12  simultaneously, or alternately to one and then the other electrode  1  or  12 . 
         [0063]    While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention.