Patent Publication Number: US-2023164899-A1

Title: Extreme ultraviolet light source apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase Application under 35 U.S.C. 371 of International Application No. PCT/JP2021/009330, filed on Mar. 9, 2021, which claims priority to Japanese Patent Application No. 2020-086261, filed on May 15, 2020. The entire disclosures of the above applications are expressly incorporated by reference herein. 
    
    
     BACKGROUND 
     Technical Field 
     The present invention relates to extreme ultraviolet light source apparatuses. 
     Related Art 
     Recently, due to increasing miniaturization and integration of semiconductor integrated circuits, shortening of a wavelength of an exposure light source has been promoted. As a next-generation light source for exposing semiconductors, an extreme ultraviolet light source apparatus (hereinafter also referred to as an “EUV light source apparatus”) that emits extreme ultraviolet light (hereinafter also referred to as “EUV (extreme ultraviolet) light”) having a wavelength of 13.5 nm has been developed. 
     There are several known methods for generating EUV light (EUV radiation) in an EUV light source apparatus. One method is to generate a high-temperature plasma by heating and exciting an extreme ultraviolet light radiation subject (hereinafter, also referred to as “EUV radiation subject”), and to generate EUV light emitted from the high-temperature plasma. 
     EUV light source apparatuses that employ such a method are classified into an LPP (Laser Produced Plasma) method and a DPP (Discharge Produced Plasma) method, depending on the generation scheme. 
     An EUV light source apparatus of the DPP type applies a high voltage to a gap between electrodes in which a discharge gas containing EUV radiation subject (plasma raw material in a gas phase) is supplied, to generate a high-density and high-temperature plasma by discharge, and utilizes extreme ultraviolet light radiated therefrom. As a specific DPP method as disclosed, for example, in JP-A-2017-103120, a method is proposed in which a liquid high-temperature plasma raw material (for example, Sn (tin)) is supplied to surfaces of electrodes that generate a discharge, an energy beam such as a laser beam is directed to the raw material to vaporize the raw material, and a high-temperature plasma is generated by the discharge. Such an approach may be referred to as LDP (Laser Assisted Discharge Plasma). 
     In the technology disclosed in JP-A-2017-103120, a pair of disk-shaped rotating electrodes are used to reduce a thermal load on the electrodes. The electrodes are arranged so that their peripheral edges are close to each other. Pulsed power for discharge is applied to the rotating electrodes at a fixed period (e.g., 4 kHz or higher), so that a plasma that emits extreme ultraviolet light is generated at the fixed period. 
     To generate plasma in an energy efficient manner (i.e., to reduce loss of energy needed for plasma generation), it is desirable to make the rise of the discharge current between both the rotating electrodes faster. 
     Accordingly, it is an object of the present invention to provide an extreme ultraviolet light source apparatus that can enable faster rising of discharge current. 
     SUMMARY 
     In accordance with the present invention, there is provided an extreme ultraviolet light source apparatus, including a disk-shaped cathode; a first rotational shaft made of a metal to which the cathode is connected; a disc-shaped anode disposed apart from the cathode; a second rotational shaft made of a metal to which the anode is connected; a first tin supplier configured to coat the cathode with liquid-phase tin; a second tin supplier configured to coat the anode with liquid-phase tin; an energy beam irradiation device configured to irradiate the tin on the cathode with an energy beam to vaporize the tin and generate gas-phase tin in a gap between the cathode and the anode; an electric power supply configured to supply electric power to the cathode and anode to cause a discharge between the cathode and anode for generating a plasma in the gap between the cathode and anode, the plasma emitting extreme ultraviolet light; a first support structure made of a metal supporting the cathode; and a second support structure made of a metal supporting the anode. The first support structure and the second support structure are not in contact with each other. The first support structure has a first support wall portion and a first tubular portion protruding from the first support wall portion and surrounding the first rotational shaft, and the first tubular portion is disposed between the first support wall portion and the cathode. The second support structure has a second support wall portion and a second tubular portion protruding from the second support wall portion and surrounding the second rotational shaft, and the second tubular portion is disposed between the second support wall portion and the anode. The first support wall portion and the second support wall portion overlap each other. The first support wall portion has a through-hole through which the second tubular portion is inserted, or the second support wall portion has a through-hole through which the first tubular portion is inserted. 
     In this aspect, the rise of the discharge current that flows between the cathode and anode can be made faster. This reduces loss of the energy needed for plasma generation and increases the luminance of the extreme ultraviolet light generated. The reason for faster occurrence of the rise of the discharge current is not certain, but can be estimated as follows: The first and second support structures made of metal that respectively support the cathode and the anode constitute a discharge circuit that causes a discharge between the cathode and the anode. In this aspect, the first support wall portion of the first support structure and the second support wall portion of the second support structure overlap each other, and a tubular portion of a support structure is inserted into a through-hole in the support wall portion of the other support structure. This allows the first and second support structures made of metal, in particular the first and second support wall portions to be disposed extremely close to each other. In this way, two objects in which current flows in opposite directions are placed very close to each other, so that the magnetomotive forces of the first support structure and the second support structure cancel each other out. Accordingly, it is estimated that the inductance of the entire discharge circuit can be reduced and that the rise of the discharge current can be made faster. 
     Preferably, the first support structure has outer wall portions protruding from the first support wall portion and facing outer surfaces of the second support wall portion. 
     In this case, the magnetomotive forces of the first and second support structures further cancel each other out since adjacent portions of the first and second support structures are increased. 
     Preferably, the second support structure has outer wall portions protruding from the second support wall portion and facing outer surfaces of the first support wall portion. 
     In this case, the magnetomotive forces of the first and second support structures further cancel each other out since adjacent portions of the first and second support structures are increased. 
     Preferably, insulators are disposed between the first support structure and the second support structure. 
     In this case, the first and second support structures are insulated from each other and the spacing therebetween is properly maintained. 
     Preferably, the extreme ultraviolet light source apparatus further includes a support base member made of a metal for supporting the first support structure and the second support structure, and at least one of a gap between the support base member and the first support structure and a gap between the support base member and the second support structure is closed with a capacitor. 
     In this case, the capacitor is arranged around the discharge circuit, the inductance of the entire discharge circuit can be further reduced. 
     EFFECT OF THE INVENTION 
     In accordance with an aspect of the present invention, the rise of discharge current can be made faster. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view of an extreme ultraviolet light source apparatus according to an embodiment of the present invention, showing a cross section of a first support structure that supports a cathode and a second support structure that supports an anode; 
         FIG.  2    is a perspective view showing the cathode, the first support structure, the anode, and the second support structure of the extreme ultraviolet light source apparatus; 
         FIG.  3    is a perspective view of the cathode and first support structure; 
         FIG.  4    is a perspective view of the anode and second support structure; 
         FIG.  5    is a schematic view of an extreme ultraviolet light source apparatus according to a comparison example; 
         FIG.  6    is a graph showing a current waveform during discharge that occurs between the rotating electrodes in the extreme ultraviolet light source apparatus according to the embodiment; 
         FIG.  7    is a graph showing a current waveform during discharge that occurs between the rotating electrodes in the extreme ultraviolet light source apparatus according to the comparison example; 
         FIG.  8    is a schematic view of an extreme ultraviolet light source apparatus according to another embodiment of the present invention, showing a cross section of a first support structure that supports a cathode and a second support structure that supports an anode; 
         FIG.  9    is a schematic view of an extreme ultraviolet light source apparatus according to another embodiment of the present invention, showing a cross section of a first support structure that supports a cathode and a second support structure that supports an anode; and 
         FIG.  10    is a schematic view of an extreme ultraviolet light source apparatus according to another embodiment of the present invention, showing a cross section of a first support structure that supports a cathode and a second support structure that supports an anode. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, with reference to the accompanying drawings, various embodiments according to the present invention will be described. It is of note that the drawings are not necessarily to scale, and certain features may be exaggerated or omitted. 
     An extreme ultraviolet light source apparatus (EUV light source apparatus)  1  is an apparatus that is configured to emit extreme ultraviolet light (EUV light) having a wavelength of, for example, 13.5 nm, and that can be used as a light source apparatus of a lithography apparatus for manufacturing semiconductor devices or as a light source apparatus for an apparatus for inspecting masks used for lithography. 
     The EUV light source apparatus  1  according to the embodiment is an EUV light source apparatus of the DPP type. More specifically, the EUV light source apparatus uses an energy beam such as a laser beam to irradiate a plasma raw material in a liquid phase applied to the surfaces of the pair of electrodes to vaporize the plasma raw material, and an electric power source to supply electric power to the electrodes to cause a discharge and generate a high-temperature plasma between the electrodes. EUV light is emitted from the plasma. 
     As shown in  FIGS.  1  and  2   , the EUV light source apparatus  1  has a pair of discharge electrodes  21   a  and  21   b.  The discharge electrodes  21   a  and  21   b  are each discs of the same shape and the same size; the discharge electrode  21   a  is used as a cathode, whereas the discharge electrode  21   b  is used as an anode. The discharge electrodes  21   a  and  21   b  are formed of, for example, a high melting point metal, such as tungsten, molybdenum, or tantalum. 
     The discharge electrodes  21   a  and  21   b  are disposed at positions spaced apart from each other, but the peripheral portions of the discharge electrodes  21   a  and  21   b  are close to each other. At a position in which the peripheral portion of the cathode  21   a  and the peripheral portion of the anode  21   b  are closest, a discharge occurs in the gap between the cathode  21   a  and the anode  21   b,  and a high-temperature plasma is generated. Hereinafter, the gap between the cathode  21   a  and the anode  21   b  at the position in which the peripheral portion of the cathode  21   a  and the peripheral portion of the anode  21   b  are closest is referred to as a “discharge region D”. 
     The cathode  21   a  is connected to the rotational shaft (first rotational shaft)  23   a  of the motor  22   a  and rotates about the axis of the cathode  21   a.  The anode  21   b  is connected to the rotational shaft (second rotational shaft)  23   b  of the motor  22   b  and rotates about the axis of the anode  21   b.  Thus, the discharge electrodes  21   a  and  21   b  are respectively driven by separate motors  22   a  and  22   b.  The rotational shafts  23   a  and  23   b  of the motors  22   a  and  22   b  are made of metal. 
     The rotation of each of the motors  22   a  and  22   b  is controlled by a controller  15 . 
     The EUV light source apparatus  1  includes a tin reservoir  26   a  for storing liquid-phase tin  25   a,  which is a plasma raw material, and a tin reservoir  26   b  for storing liquid-phase tin  25   b.  Heated liquid-phase tin  25   a  and  25   b  is supplied to the tin reservoirs  26   a,    26   b.    
     A lower part of the cathode  21   a  is immersed in tin  25  in the tin reservoir  26   a,  and a lower part of anode  21   b  is immersed in tin  25   b  in the tin reservoir  26   b.  As a result, liquid-phase tin (Sn), which is the plasma raw material, adheres to the discharge electrodes  21   a  and  21   b.  As the discharge electrodes  21   a  and  21   b  rotate, the liquid-phase tin  25   a  and  25   b  is transported to the discharge region D at which the high-temperature plasma is to be generated. 
     Accordingly, the tin reservoir  26   a  alone or the combination of the tin reservoir  26   a  and the motor  22   a  serves as a first tin supplier for coating the cathode  21   a  with the plasma raw material in a liquid phase. Similarly, the tin reservoir  26   b  alone or the combination of the tin reservoir  26   b  and the motor  22   b  serves as a second tin supplier for coating the anode  21   b  with the plasma raw material in a liquid phase. However, the material supplier is not limited to the type of this embodiment, and may be of other types. 
     The EUV light source apparatus  1  includes a laser (energy beam irradiation device)  28  that irradiates tin  25   a  coated on the cathode  21   a  with an energy beam to vaporize tin  25   a.  The laser  28  is, for example, Nd:YVO 4  laser (Neodymium-doped Yttrium Orthovanadate laser), and emits an infrared laser beam L. However, the energy beam irradiation device may be another device that emits a beam other than a laser beam capable of vaporizing tin  25   a.    
     Illumination timing of the laser beam by the laser  28  is controlled by the controller  15 . 
     The infrared laser beam L emitted from the laser  28  is directed to a movable mirror  31 . Between the laser  28  and the movable mirror  31 , typically a light-collecting means is disposed. The light-collecting means includes, for example, a condensing lens  29 . 
     The infrared laser beam L is reflected by the movable mirror  31  and illuminates the outer peripheral surface of the cathode  21   a  in the vicinity of the discharge region D. 
     The infrared laser beam L reflected by the movable mirror  31  travels from the side of the anode  21   b  toward the cathode  21   a,  passes near the outer peripheral surface of the anode  21   b,  and then reaches the outer peripheral surface of the cathode  21   a.  The anode  21   b  is retracted behind the cathode  21   a  toward the upper side of  FIG.  1    so as not to interfere with the progress of the infrared laser beam L. 
     The liquid phase tin  25   a  on the outer peripheral surface of the cathode  21   a  in the vicinity of the discharge region D is vaporized by irradiation with the infrared laser beam L, and a gas-phase plasma raw material is generated in the discharge region D. 
     To generate a high-temperature plasma in the discharge region D (to transform the plasma raw material in a gas phase to a plasma), a pulse power supply (electric power supply)  35  supplies electric power to the cathode  21   a  and the anode  21   b,  to cause a discharge between the cathode  21   a  and the anode  21   b.  The pulse power supply  35  periodically supplies pulsed electric power to the discharge electrodes  21   a  and  21   b.    
     In this embodiment, the two electric supply lines extending from the pulse power supply  35  are connected to the tin reservoirs  26   a  and  26   b,  respectively. The tin reservoirs  26   a  and  26   b  are made of an electric conductive material, and tin  25   a  and  25   b  inside the tin reservoirs  26   a  and  26   b  is an electric conductive material, i.e., tin. The discharge electrodes  21   a  and  21   b  are immersed in tin  25   a  and  25   b  inside the tin reservoirs  26   a  and  26   b.  Thus, when the pulse power supply  35  supplies pulsed electric power to the tin reservoirs  26   a  and  26   b,  the pulsed electric power is consequently supplied to the discharge electrodes  21   a  and  21   b.    
     When discharge occurs between the cathode  21   a  and the anode  21   b,  the plasma raw material in a gas phase in the discharge region D is heated and excited by a large current, so that a high-temperature plasma is generated. In addition, due to the high heat generated, tin  25   b  on the outer peripheral surface of the anode  21   b  in the vicinity of the discharge region D is also converted into plasma. 
     EUV light E is emitted from the high temperature plasma. EUV light E is used in other optical apparatus (a lithography apparatus or an inspection apparatus for masks). 
     The pulse power supply  35  applies pulsed power to the discharge electrodes  21   a  and  21   b  at a fixed period (e.g., 4 kHz or higher), so that a plasma that emits the EUV light E is generated at the fixed period. 
     Next, support structures that support the discharge electrodes  21   a  and  21   b  and the rotational shafts  23   a  and  23   b  will be described in detail. 
     The EUV light source apparatus  1  has a first support structure  40  made of a metal that supports the cathode  21   a  and a second support structure  50  made of a metal that supports the anode  21   b.  The first support structure  40  and the second support structure  50  are not in contact with each other and are insulated by a plurality of insulators  60 , which are interposed therebetween. 
     As shown in  FIGS.  1  to  3   , the first support structure  40  includes a first support wall portion  41  and a first tubular portion  42  that protrudes from the first support wall portion  41  and surrounds the first rotational shaft  23   a.  The first tubular portion  42  is disposed between the first support wall portion  41  and the cathode  21   a.    
     In this embodiment, the first support wall portion  41  is a rectangular flat plate. However, the contour of the first support wall portion  41  does not need to be rectangular. In this embodiment, the first tubular portion  42  has the shape of a hollow square pipe. However, the first tubular portion  42  may have a shape of a cylinder or other tubular shape. In this embodiment, the first tubular portion  42  completely surrounds the rotational shaft  23   a,  but the first tubular portion  42  may have a hole or notch formed therein. 
     At the end of the first tubular portion  42  opposite to the first support wall portion  41 , an end wall  43  is provided on which a bearing B 1  is mounted. The bearing B 1  supports the rotational shaft  23   a  to which the cathode  21   a  is fixed. Although not shown, sealing members are provided around the bearing B 1  to keep the discharge region D in vacuum. The end wall  43  is also used as a wall of the tin reservoir  26   a.    
     As shown in  FIGS.  1 ,  2 , and  4   , the second support structure  50  includes a second support wall portion  51  and a second tubular portion  52  that protrudes from the second support wall portion  51  and surrounds the rotational shaft  23   b.  The second tubular portion  52  is disposed between the second support wall portion  51  and the anode  21   b.    
     In this embodiment, the second support wall portion  51  is a rectangular flat plate. However, the contour of the second support wall portion  51  does not need to be rectangular. In this embodiment, the second tubular portion  52  has the shape of a hollow square pipe. However, the second tubular portion  52  may have a shape of a cylinder or other tubular shape. In this embodiment, the second tubular portion  52  completely surrounds the rotational shaft  23   b,  but the second tubular portion  52  may have a hole or notch formed therein. 
     At the end of the second tubular portion  52  opposite to the second support wall portion  51 , an end wall  53  is provided on which a bearing B 3  is mounted. The bearing B 3  supports the rotational shaft  23   b  to which the anode  21   b  is fixed. Although not shown, sealing members are provided around the bearing B 3  to keep the discharge region D in vacuum. The end wall  53  is also used as a wall of the tin reservoir  26   b.    
     The first support wall portion  41  and the second support wall portion  51  overlap each other. In this embodiment, the second support wall portion  51  is positioned closer to the discharge electrodes  21   a  and  21   b  than the first support wall portion  41 , and a through-hole  55  is formed in the second support wall portion  51  through which the first tubular portion  42  is inserted. In this embodiment, the first tubular portion  42  and the second tubular portion  52  are arranged approximately parallel to each other, and the rotational shafts  23   a  and  23   b  are also arranged approximately parallel to each other. 
     The rotational shaft  23   a  of the motor  22   a  fixed to the cathode  21   a  is supported by the bearing B 1  located in the end wall  43  of the first tubular portion  42  of the first support structure  40  and another bearing B 2  located in the first support wall portion  41  of the first support structure  40 . 
     The rotational shaft  23   b  of the motor  22   b  fixed to the anode  21   b  is supported by the bearing B 3  located in the end wall  53  of the second tubular portion  52  of the second support structure  50  and another bearing B 4  located in the first support wall portion  41  of the first support structure  40 . The second support wall portion  51  of the second support structure  50  has a through-hole  56  through which the rotational shaft  23   b  passes. Instead of or in addition to the bearing B 4  disposed in the first support wall portion  41 , another bearing may be disposed in the through-hole  56  for supporting the rotational shaft  23   b.    
     In this embodiment, the second support structure  50  has two outer wall portions  57  that protrude from the second support wall portion  51  and face outer surfaces of the first support wall portion  41 . The two outer wall portions  57  are disposed on two sides of the second support wall portion  51  and face the two side surfaces of the first support wall portion  41 . However, the outer wall portions  57  may be disposed on the top and bottom of the second support wall portion  51  and may face the top and bottom surfaces of the first support wall portion  41 . The outer wall portions  57  may surround the first support wall portion  41  all around. 
     The EUV light source apparatus  1  further has a support base member  65  made of a metal for supporting the first support structure  40  and the second support structure  50 . A gap between the support base member  65  and the first support structure  40  and a gap between the support base member  65  and the second support structure  50  are closed with capacitors  66 . 
     The support base member  65  is part of the chamber (housing) of the EUV light source apparatus  1 . The chamber is formed of a rigid material such as a metal. The discharge electrodes  21   a  and  21   b,  the tin reservoirs  26   a  and  26   b,  and the tubular portions  42  and  52  are located within the chamber. The interior of the chamber  11  is evacuated to reduce the attenuation of the EUV light E. As described above, sealing members (not shown) are provided around the bearings B 1  and B 3  to maintain the discharge region D in vacuum. 
     The inventors conducted an experiment to investigate the rise of discharge current in the EUV light source apparatus  1  according to the embodiment. 
       FIG.  5    is a schematic view of an EUV light source device used as a comparison example. In the comparison example shown in  FIG.  5   , the rotational shaft  23   a  fixed to the cathode  21   a  is surrounded by the first tubular portion  42  made of a metal, and the rotational shaft  23   b  fixed to the anode  21   b  is surrounded by the second tubular portion  52  made of a metal. However, in this comparison example, there is no equivalent corresponding to the first support wall portion  41 , and there is no equivalent corresponding to the second support wall portion  51 . The rotational shaft  23   a  of the motor  22   a  fixed to the cathode  21   a  is supported by the bearing B 1  located in one end wall  43  of the first tubular portion  42  and another bearing B 2  located in the other end wall of the first tubular portion  42 . The rotational shaft  23   b  of the motor  22   b  fixed to the anode  21   b  is supported by the bearing B 3  located in one end wall  53  of the second tubular portion  52  and another bearing B 4  located in the other end wall of the second tubular portion  52 . The first tubular section  42  and the second tubular section  52  are supported by the support base member  65 . The first and second tubular portions  42  and  52  and the support base member  65  are not in contact with each other and are insulated by multiple insulators  60 . 
       FIG.  6    is a graph showing a current waveform during the discharge that occurs between the discharge electrodes  21   a  and  21   b  and showing change in radiant energy of the EUV light E in the EUV light source apparatus  1  according to the embodiment.  FIG.  7    is a graph showing a current waveform during the discharge that occurs between the discharge electrodes  21   a  and  21   b  and showing change in radiant energy of the EUV light E in the EUV light source apparatus  1  according to the comparison example shown in  FIG.  5   . 
     As is clear from  FIGS.  6  and  7   , the rise of the discharge current that flowed between the cathode  21   a  and the anode  21   b  can be faster in the embodiment than in the comparison example. As a result, there is less loss of energy needed for plasma generation in the embodiment, so that the luminance of the generated EUV light E can be enhanced. 
     The reason for faster occurrence of the rise of the discharge current by the embodiment is not certain, but can be estimated as follows: The first and second support structures  40  and  50  made of metal that respectively support the cathode  21   a  and the anode  21   b  constitute a discharge circuit that causes a discharge between the cathode  21   a  and the anode  21   b.  The rotational shafts  23   a  and  23   b,  to which the discharge electrodes  21   a  and  21   b  are connected, and the bearings B 1  to B 4 , which are in contact with the rotational shafts  23   a  and  23   b,  are also made of metal and constitute the discharge circuit. 
     The first support wall portion  41  of the first support structure  40  and the second support wall portion  51  of the second support structure  50  overlap each other, and the first tubular portion  42  of the first support structure  40  is inserted into the through-hole  55  of the second support wall portion  51  of the second support structure  50 . This allows that the first support structure  40  and the second support structure  50  made of metal, in particular the first support wall portion  41  and the second support wall portion  51  to be disposed extremely close to each other. In this way, two objects in which current flows in opposite directions are placed very close to each other, so that the magnetomotive forces of the first support structure  40  and the second support structure  50  cancel each other out. Accordingly, it is estimated that the inductance of the entire discharge circuit can be reduced and that the rise of the discharge current can be made faster. 
     In addition, in this embodiment, the second support structure  50  has outer wall portions  57  that protrude from the second support wall portion  51  and that face outer surfaces of the first support wall portion  41 . Thus, adjacent portions of the first support structure  40  and the second support structure  50  are increased, so that the magnetomotive forces of the first support structure  40  and the second support structure  50  further cancel each other out, and the inductance of the entire discharge circuit is further reduced. 
     In this embodiment, the insulators  60  are interposed between the first support structure  40  and the second support structure  50 . Thus, the first support structure  40  and the second support structure  50  are insulated from each other and the spacing therebetween is properly maintained. 
     In addition, in this embodiment, the first support structure  40  and the second support structure  50  are supported by the metal support base member  65 , and a gap between the support base member  65  and the first support structure  40  and a gap between the support base member  65  and the second support structure  50  are closed with capacitors  66 . In this way, since the capacitors  66  are arranged around the discharge circuit, the inductance of the entire discharge circuit can be further reduced. 
       FIG.  8    shows an EUV light source apparatus  1 A according to another embodiment of the present invention. In the EUV light source apparatus  1 A, contrary to the EUV light source apparatus  1 , the first support wall portion  41  is located closer to the discharge electrodes  21   a  and  21   b  than the second support wall portion  51 , and a through-hole  45  is formed in the first support wall portion  41  for insertion of the second tubular portion  52 . 
     In this embodiment, again, the first support wall portion  41  of the first support structure  40  and the second support structure  50  overlap each other, and the second tubular portion  52  of the second support structure  50  is inserted into the through-hole  45  of the first support wall portion  41  of the first support structure  40 . This allows the first support structure  40  and the second support structure  50  made of metal, in particular the first support wall portion  41  and the second support wall portion  51  to be disposed extremely close to each other. It is estimated that in this way, two objects in which current flows in opposite directions are placed very close to each other, so that the magnetomotive forces of the first support structure  40  and the second support structure  50  cancel each other out. 
     In addition, in this embodiment, the second support structure  50  has outer wall portions  57  that protrude from the second support wall portion  51  and that face outer surfaces of the first support wall portion  41 . Thus, adjacent portions of the first support structure  40  and the second support structure  50  are increased, so that the magnetomotive forces of the first support structure  40  and the second support structure  50  further cancel each other out, and the inductance of the entire discharge circuit is further reduced. 
     In this embodiment, the rotational shaft  23   a  of the motor  22   a  fixed to the cathode  21   a  is supported by the bearing B 1  located in the end wall  43  of the first tubular portion  42  of the first support structure  40  and another bearing B 2  located in the second support wall portion  51  of the second support structure  50 . The first support wall portion  41  of the first support structure  40  has a through-hole  46  through which the rotational shaft  23   a  passes. Instead of or in addition to the bearing B 2  disposed in the second support wall portion  51 , another bearing may be disposed in the through-hole  46  for supporting the rotational shaft  23   a.    
     The rotational shaft  23   b  of the motor  22   b  fixed to the anode  21   b  is supported by the bearing B 3  located in the end wall  53  of the second tubular portion  52  of the second support structure  50  and another bearing B 4  located in the second support wall portion  51  of the second support structure  50 . 
       FIG.  9    shows an EUV light source apparatus  1 B according to another embodiment of the present invention. In the EUV light source apparatus  1 B, the first support structure  40  has two outer wall portions  47  that protrude from the first support wall portion  41  and that face outer surfaces of the second support wall portion  51 . The two outer wall portions  47  are disposed on two sides of the first support wall portion  41  and face the two side surfaces of the second support wall portion  51 . However, the outer wall portions  57  may be disposed on the top and bottom of the first support wall portion  41  and may face the top and bottom surfaces of the second support wall portion  51 . The outer wall portions  47  may surround the second support wall portion  51  all around. 
       FIG.  10    shows an EUV light source apparatus  1 C according to another embodiment of the present invention. In the EUV light source apparatus  1 C, to facilitate irradiation of the outer peripheral surface of the cathode  21   a  with the infrared laser beam L, the axes of the discharge electrodes  21   a  and  21   b  are not parallel. The spacing between the rotational shafts  23   a  and  23   b  is narrower on the side of the motors and wider on the side of the electrodes. Therefore, the first tubular portion  42  is inclined with respect to the first support wall portion  41 , and the second tubular portion  52  is inclined with respect to the second support wall portion  51 . 
     Other Modifications 
     The present invention has been shown and described with reference to preferred embodiments thereof. However, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the claims. Such variations, alterations, and modifications are intended to be surrounded in the scope of the present invention. 
     For example, the above embodiments may be combined as long as they are not contradictory.