JP6663823B2 - Plasma light source - Google Patents

Description

  The present invention relates to a plasma light source that generates extreme ultraviolet light.

  As a technique in this field, a plasma light source described in Patent Literature 1 is known. The plasma light source generates a discharge by supplying vapor of a plasma medium between the center electrode and the external electrode in a state where a potential difference is generated between the center electrode and the external electrode. In Patent Literature 1, the plasma medium serving as the raw material of the vapor is disposed on the side surface of the center electrode.

JP 2013-254693 A

  The vapor of the plasma medium is generated by evaporating from the plasma medium by irradiating the solid or liquid plasma medium with the laser light. The coaxial electrode including the center electrode and the external electrode functions as a plasma source that generates plasma. The vapor of the plasma medium is supplied between the center electrode and the external electrode, and the generated initial plasma moves to the tip of the center electrode by electromagnetic force and converges to a high temperature and high density state.

  In the conventional technique described in Patent Document 1, a facing-type plasma focus method is applied, and a pair of coaxial electrodes are arranged to face each other in the axial direction of a center electrode. In Patent Literature 1, extreme ultraviolet light is generated by colliding plasma that has reached the tip of a center electrode from both sides to make the plasma have a higher temperature and a higher density. In such a conventional technique, when a difference occurs in the timing at which the vapor of the plasma medium is supplied to the pair of coaxial electrodes, a difference occurs in the time when the initial plasma reaches the tip of the electrode. May be reduced.

  SUMMARY OF THE INVENTION It is an object of the present invention to provide a plasma light source capable of reducing the difference in arrival time of initial plasma moving from both sides in the axial direction and colliding, thereby increasing the amount of emitted extreme ultraviolet light.

  The plasma light source of the present invention is a plasma light source including a pair of coaxial electrodes opposed to each other with a central plane therebetween, wherein the coaxial electrode has a center electrode extending in a direction orthogonal to the central plane, A plurality of external electrodes spaced apart in the circumferential direction of the imaginary circle centered on the axis and extending toward the center plane, and the plasma light source connects the external electrodes on the center plane and adjacent in the circumferential direction. A plasma medium holding unit that is disposed outside the virtual line and holds the plasma medium; a laser irradiation unit that irradiates the laser to the plasma medium held by the plasma medium holding unit; and a plasma medium holding unit that holds the plasma medium in the radial direction of the virtual circle. A restriction unit disposed between the unit and the external electrode, the restriction unit restricting the direction in which the medium vapor evaporated in the plasma medium holding unit diffuses.

  In this plasma light source, the diffusion direction of the medium vapor evaporated in the plasma medium holding unit is restricted by the restriction unit, so that the medium vapor is suitably supplied to the discharge start position (desired position). Further, in this plasma light source, since the medium holding portion is arranged on the center plane, the plasma medium is held at a distance equal to a pair of coaxial electrodes opposed to each other across the center plane. . Thereby, the time difference until the medium vapor evaporated from the plasma medium is supplied to the pair of coaxial electrodes can be reduced. Therefore, in the plasma light source, it is possible to suppress a shift in the arrival time until the initial plasma generated by the pair of coaxial electrodes reaches the center plane. Further, in this plasma light source, since the medium holding portion is disposed outside a virtual line connecting external electrodes adjacent in the circumferential direction, the distance between the medium holding portion and the center electrode in the radial direction of the virtual circle is reduced. Can be secured. Thus, the medium vapor evaporated from the plasma medium is easily diffused uniformly in the circumferential direction of the center electrode before being supplied between the center electrode and the external electrode. Therefore, it is easy to make the circumferential distribution of the ring-shaped initial plasma formed around the center electrode uniform. In this plasma light source, in addition to suppressing the deviation of the arrival time of the initial plasma to the center plane, the plasma is also obtained by converging the ring-shaped initial plasma uniformized in the circumferential direction to the tip of the center electrode. Temperature and density are promoted, and the amount of emitted extreme ultraviolet light can be increased.

  The restricting portion has a pair of through-holes through which the medium vapor passes.The restricting portion includes a shielding portion disposed around the through-hole to shield the medium vapor. It may be configured to be symmetrical with respect to the surface. According to this configuration, the medium vapor can be supplied to the pair of opposing coaxial electrodes with increased symmetry, and it is easy to simultaneously generate a ring-shaped initial plasma that is uniform in the circumferential direction. The generated initial plasma subsequently proceeds at the same time, the final plasma converges at the same time, and the temperature and density of the plasma are promoted. Further, according to this configuration, since the medium vapor is shielded by the shielding portion, diffusion of the medium vapor to a position other than the discharge start position is prevented. Thus, contamination due to the medium vapor can be suppressed.

  Further, the pair of through holes may be rectangular slits extending in a direction intersecting the axis. Thereby, the diffusion of the medium vapor can be restricted in the direction in which the axis extends, and the diffusion of the medium vapor can be ensured in the direction intersecting the axis. As a result, the distribution of the initial plasma in the circumferential direction can be easily made uniform.

  Further, the opening width of the portion on the center surface side of the through hole may be narrower than the opening width of the portion on the side opposite to the center surface of the through hole. According to this configuration, it is possible to supply the medium vapor having a relatively low concentration to the tip side from the discharge start position in the direction in which the axis extends. Thereby, the medium vapor can be further supplied until the initial plasma generated at the discharge start position moves to the tip side of the center electrode, and the density of the plasma can be increased.

  ADVANTAGE OF THE INVENTION According to this invention, the shift | offset | difference of the arrival time of the initial plasma which moves from both sides of an axial direction and collides can be reduced, and the light emission amount of extreme ultraviolet light can be increased.

FIG. 1 is a diagram illustrating a configuration of a plasma light source according to an embodiment of the present invention. FIG. 2 is a diagram illustrating an end surface of a coaxial electrode in a virtual plane in FIG. 1 and an arrangement of a plasma medium supply unit viewed from an axial direction. It is a side view which shows a pair of coaxial electrodes arrange | positioned facing each other across the center plane, and the plasma medium supply part arrange | positioned on the center plane. FIG. 4A is a diagram illustrating a restriction unit that covers the plasma medium supply unit from the axis A side. FIG. 4B is a diagram illustrating a restriction unit according to a first modification. FIG. 4C is a diagram illustrating a restriction unit according to a second modification. FIG. 4D is a diagram illustrating a restriction unit according to a third modification. FIG. 4 is a diagram illustrating a current path formed when the plasma light source operates. FIG. 6A is a cross-sectional view illustrating a restriction unit according to a fourth modification. FIG. 6B is a cross-sectional view illustrating a restriction unit according to a fifth modification.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In each of the drawings, the same or corresponding portions have the same reference characters allotted, and overlapping description will be omitted.

  The plasma light source 1 shown in FIG. 1 is applied to, for example, an exposure apparatus for manufacturing a semiconductor device. The plasma light source 1 is configured to generate, for example, extreme ultraviolet light (EUV light) having a wavelength of 13.5 nm. The plasma light source 1 enables photolithography to form a fine pattern by generating EUV light.

  The plasma light source 1 employs a so-called opposed plasma focus method. The plasma light source 1 includes a pair of coaxial electrodes 10 for generating plasma, a voltage applying device 20 (voltage applying unit) for generating a potential difference between the coaxial electrodes 10, and a pair of medium vapor forming units 30 for forming medium vapor. , A plasma medium supply unit 41 that holds the plasma medium 43.

  The pair of coaxial electrodes 10 are housed in the chamber 2 and are arranged so as to face each other on the axis A. The pair of coaxial electrodes 10 are arranged plane-symmetrically with respect to a virtual center plane (symmetric plane) P. A fixed space (space) is provided between the pair of coaxial electrodes 10. The chamber 2 is provided with one or more exhaust pipes 3, and a vacuum pump or the like (not shown) is connected to the exhaust pipes 3. The inside of the chamber 2 is maintained at a predetermined degree of vacuum. The chamber 2 is grounded.

  The coaxial electrode 10 includes one center electrode 11, a plurality of external electrodes 12, and one insulator 13. The center electrode 11 is a rod-shaped conductor extending along the axis A. The center electrodes 11 of the pair of coaxial electrodes 10 are arranged to face each other on the axis A. The tip portions 11a of the center electrode 11 are arranged to face each other with the center plane P interposed therebetween (see FIG. 3).

  The center electrode 11 is desirably made of a metal having high resistance to high temperatures. The center electrode 11 is made of a high melting point metal such as tungsten (W) or molybdenum (Mo). The axis A of the center electrode 11 is orthogonal to the center plane P described above. The front end 11a of the center electrode 11 facing the center plane P has, for example, a hemispherical shape. The side surface 11b of the center electrode 11 has, for example, a conical shape.

  The external electrode 12 is a rod-shaped conductor extending toward the opposed coaxial electrode 10. The external electrode 12 may be configured to extend in a direction inclined with respect to the axis A. For example, the external electrode 12 may extend in a direction parallel to the side surface 11b so that the distance from the side surface 11b of the conical center electrode 11 to the external electrode 12 is always constant. It is desirable that the external electrode 12 be made of a metal having high resistance to high temperatures. The external electrode 12 is made of a high melting point metal such as tungsten (W) or molybdenum (Mo). The end surface of the external electrode 12 facing the center plane P may be a curved surface or a flat surface.

  The external electrode 12 is arranged around the center electrode 11 as shown in FIG. The external electrode 12 has a predetermined distance from the center electrode 11. The plurality of external electrodes 12 are arranged at equal intervals (that is, rotationally symmetric) in the circumferential direction of the virtual circle F about the axis A. The coaxial electrode 10 has six external electrodes 12. The six external electrodes 12 are arranged every 60 ° with respect to the axis A. Note that the number of the external electrodes 12 is not limited to six, and can be set as appropriate according to the size and shape of the center electrode 11 and the external electrode 12, the distance between them, and the like. By arranging the plurality of external electrodes 12 around the center electrode 11, an initial discharge (for example, a creeping discharge or an arc discharge) is generated between the center electrode 11 and the external electrode 12. This initial discharge leads to a planar discharge 6 (see FIG. 5).

  The insulator 13 shown in FIG. 1 is, for example, a disk-shaped ceramic plate. The insulator 13 supports the base of the center electrode 11 and the base of the external electrode 12, and defines the space between them. Insulator 13 electrically insulates center electrode 11 and external electrode 12.

  The voltage applying device 20 generates a potential difference by applying a discharge voltage of the same polarity or opposite polarity to the coaxial electrode 10. The voltage applying device 20 includes, for example, two high voltage power supplies (HV Charging Devices) 21 and 22. The number of high-voltage power supplies may be one, or three or more. The output side of the first high-voltage power supply 21 is connected to the center electrode 11 of one coaxial electrode 10 (for example, the left side in the figure). The first high-voltage power supply 21 applies a positive discharge voltage higher than the external electrode 12 corresponding to the center electrode 11. The first high-voltage power supply 21 may apply a negative discharge voltage lower than that of the external electrode 12 corresponding to the center electrode 11. The output side of the second high-voltage power supply 22 is connected to the center electrode 11 of the other (for example, the right side in the drawing) coaxial electrode 10. The second high-voltage power supply 22 applies a higher positive discharge voltage than the external electrode 12 corresponding to the center electrode 11. In addition, the second high-voltage power supply 22 may apply a negative discharge voltage lower than that of the external electrode 12 corresponding to the center electrode 11. Any of the external electrodes 12 may be grounded. In the following description, the first high-voltage power supply 21 is simply referred to as the power supply 21. Similarly, the second high-voltage power supply 22 is simply referred to as a power supply 22.

  For example, a line inductively coupled using a Rogowski coil or the like may be provided on the common side (return side) of the power supplies 21 and 22. With these lines, the current passing through the center electrode 11 (ie, all discharge currents) can be observed with an oscilloscope.

  The plasma light source 1 further includes an energy storage circuit 26 that stores the discharge voltage from the voltage application device 20 as discharge energy for each external electrode 12. Energy storage circuit 26 includes a plurality of capacitors C that individually connect between center electrode 11 and external electrode 12. The capacitor C is connected to the output side and the common side of the power supplies 21 and 22. By providing the capacitor C for storing the discharge energy for each external electrode 12, discharge can occur in all the external electrodes 12. That is, it is possible to prevent the discharge distribution in the circumferential direction of the center electrode 11 from being biased, so that a large amount of discharge energy is unevenly consumed. By providing the energy storage circuit 26, an ideal planar discharge 6 generated over the entire circumference of the center electrode 11 in the coaxial electrode 10 can be obtained.

  The plasma light source 1 further includes a discharge current blocking circuit 28 that blocks the discharge current from returning to the voltage application device 20. The discharge current blocking circuit 28 includes an inductor L that connects between the external electrode 12 and the voltage application device 20 (specifically, the common side of the power supplies 21 and 22). Since the inductor L has a sufficiently high impedance with respect to the discharge current, the discharge current via the center electrode 11 and the external electrode 12 can be returned to the energy storage circuit 26, which is the source of the discharge current. This prevents the discharge energy stored in the capacitor C from being supplied to the external electrodes 12 other than the external electrode 12 directly connected to the capacitor C. As a result, it is possible to prevent the occurrence distribution of the discharge in the circumferential direction of the center electrode 11 from being biased.

  The medium vapor forming unit 30 includes a laser device (laser irradiation unit) 31. The medium vapor forming section 30 includes a beam splitter (half mirror) and a mirror (not shown). The laser light 32 emitted from the laser device 31 is applied to the plasma medium supply unit 41 via a beam splitter and a mirror. By the irradiation of the laser beam 32, ablation of the plasma medium 43 occurs, and vapor (medium vapor V) of the plasma medium 43 is generated. The laser device 31 is, for example, a YAG laser, and outputs a fundamental wave or a second harmonic of the fundamental wave as a short pulse laser beam for performing ablation.

  The laser device 31 emits a laser beam 32. The laser light 32 is split into two laser lights 32 a, 32 a by an optical element such as a beam splitter or a mirror, and is applied to the plasma medium supply unit 41. At the surface (irradiation surface) of the plasma medium 43 irradiated with the laser beams 32a, 32a, a part of the plasma medium 43 is emitted as medium vapor V by ablation. Here, the medium vapor V contains a neutral gas or ions.

  At the time of irradiation with the laser beam 32, a discharge voltage has already been applied to the center electrode 11 and the external electrode 12 of the coaxial electrode 10 by the voltage application device 20. Therefore, when the above-mentioned ablation occurs, a discharge is induced between the center electrode 11 and the external electrode 12. Further, this discharge forms a planar discharge 6.

  There is a possibility that the location where the discharge occurs is limited to the supply region of the medium vapor V and its vicinity. Therefore, it is preferable to irradiate a plurality of laser beams 32 simultaneously at intervals along the circumferential direction of the axis A, and the number is at least two. This is based on experimental results in which the area of the induced discharge has an opening angle of 180 degrees or more from the axis of the center electrode 11 as a base point. In consideration of this result, it is desirable to irradiate the laser beam 32a to a rotationally symmetric position with respect to the center electrode 11 as the number of irradiation locations is smaller. Note that simultaneous irradiation of a plurality of laser beams 32 can be easily achieved by forming a plurality of optical paths having the same optical path length using optical elements such as a beam splitter and a mirror.

  As shown in FIGS. 1 to 3, the plasma medium supply unit 41 holds a plasma medium 43 used for generating plasma light. The plasma medium supply unit 41 includes a solid or liquid plasma medium 43 and a holding unit (plasma medium holding unit) 42 that holds the plasma medium 43. The plasma medium 43 can be selected according to the required light wavelength. For example, when 13.5 nm ultraviolet light is required, the plasma medium 43 is made of lithium (Li), xenon (Xe), tin (Sn), or the like. When 6.7 nm ultraviolet light is required, at least one of gadolinium (Gd) and terbium (Tb) is used as the plasma medium 43. The plasma medium 43 may be any of a solid, a liquid, and a gas, and is selected according to the wavelength of light to be generated. In the plasma light source 1, liquid or solid lithium can be used as the plasma medium 43.

  The plasma light source 1 includes a common plasma medium supply unit 41 for the pair of coaxial electrodes 10. The plasma light source 1 includes a pair of plasma medium supply units 41 facing each other across the axis A on the central plane P. As shown in FIGS. 1 and 3, the pair of plasma medium supply units 41 are arranged at equal distances from the distal ends 11 a of the pair of opposed center electrodes 11 in the direction in which the axis A extends. Note that the number of plasma medium supply units 41 is not limited to two, and can be appropriately set according to the size and shape of the coaxial electrode 10.

  As shown in FIG. 2, the pair of plasma medium supply units 41 are arranged around the coaxial electrode 10. Specifically, the pair of plasma medium supply units 41 are arranged around the axis A at an interval of 180 degrees. In other words, the pair of plasma medium supply units 41 are arranged point-symmetrically with respect to the axis A. Further, the pair of plasma medium supply units 41 are arranged at positions where the distances from the axis A are equal to each other. Further, the pair of plasma medium supply units 41 are arranged at equal intervals in the circumferential direction of the virtual circle F about the axis A.

  The plasma medium supply unit 41 is not in physical contact with the center electrode 11 and the external electrode 12. As shown in FIG. 1, a virtual plane P2 perpendicular to the axis A is defined, and the end face of the coaxial electrode 10 in the virtual plane P is shown in FIG. FIG. 2 also illustrates the arrangement of the plasma medium supply unit 41 when the center plane P is viewed from the outside in the direction in which the axis A extends.

  Then, a virtual line B2 passing through the axis B of the external electrode 12 adjacent in the circumferential direction of the virtual circle F is defined. Then, in the coaxial electrode 10, six virtual lines B2 are defined, and a hexagonal region BS surrounded by these six virtual lines B2 is defined. The center electrode 11 is arranged at the center of this area BS. On the other hand, the plasma medium supply unit 41 is arranged outside this area BS. In short, the plasma medium supply unit 41 is arranged outside the closed area BS including all the external electrodes 12 and surrounding the center electrode 11. Also, it can be said that the plasma medium supply unit 41 and the center electrode 11 are arranged with the virtual line B2 interposed therebetween.

  Here, the plasma medium supply unit 41 includes a restriction unit 51 that restricts the direction in which the medium vapor V evaporated in the holding unit 42 diffuses. The restricting portion 51 is disposed between the holding portion 42 and the coaxial electrode 10 in a direction perpendicular to the axis A (radial direction of the virtual circle). In FIG. 2, illustration of the restriction unit 51 is omitted.

  The restricting portion 51 includes a restricting plate 52 having a plate shape. The thickness direction of the limiting plate 52 is arranged in a direction along the central plane P. The limiting plate 52 is arranged to be orthogonal to the center plane P. The limiting plate 52 is formed to have a rectangular shape, as shown in FIG. The longitudinal direction of the limiting plate 52 is arranged along the direction in which the axis A extends.

  The restriction plate 52 is provided with a pair of slits (through holes) 53. The pair of slits 53 are formed symmetrically with respect to the center plane P. In the slit 53, a side 53a extending in a direction intersecting with the axis A has a rectangular shape longer than a side 53b arranged in a direction along the axis A. The size and position of the pair of slits 53 are set such that the medium vapor V passing through the slits 53 can reach a desired position (discharge start position H).

Specifically, as shown in FIG. 3, the laser irradiation position 43a in the plasma medium 43, on the straight line L _V connecting the discharge start position H, the slits 53 are disposed. The size of the opening of the slit 53 is determined in consideration of a range in which the medium vapor V is to be distributed. By increasing the length of the slit 53 in the direction along the axis A, the range in which the medium vapor V is distributed can be expanded in the direction of the axis A. By increasing the length of the slit 53 in the direction intersecting the axis A, the range in which the medium vapor V is distributed can be widened in the direction intersecting the axis A.

  In addition, the closer the limiting plate 52 is to the plasma medium 43, the smaller the size of the opening of the slit 53 can be set and the smaller the area of the limiting plate 52 can be.

  Further, the restricting portion 51 may have a configuration including a side wall (not shown) arranged so as to surround the periphery of the restricting plate 52. The side walls are arranged to extend from the four sides of the limiting plate 52 to the plasma medium supply unit 41 side. Such a side wall can prevent the diffusion of the medium vapor V evaporated in the plasma medium supply unit 41 to the side. Here, the “side” refers to a direction along the surface of the plasma medium 43. The material of the limiting plate 52 and the side wall may be a material that can transmit laser light or a material that does not transmit laser light. The limiting plate 52 and the side wall only need to be able to suppress the diffusion of the medium vapor V.

Further, as shown in FIG. 3, in the direction in which the axis A extends, the distance L ₁ between the discharge start position H and the central plane P is, for example, 15 mm. The discharge start position H is a position where the medium vapor V is supplied and the discharge is started to generate initial plasma. The discharge start position H is set so that the initial plasma is accelerated to obtain a desired energy. By increasing the distance L _1, it is possible to increase the distance the initial plasma is accelerated.

The irradiation surface of the plasma medium 43 (the laser irradiation position 43a), when the distance L ₂ between the axis A is short, it is possible to reduce the size of the chamber 2. Distance when L ₂ is long, it is possible to secure a distance to coaxial electrodes 10, becomes easy to uniformly distribute the medium vapor V in the circumferential direction of the center electrode.

  Next, the operation of the plasma light source 1 will be described.

  First, the inside of the chamber 2 is maintained at a temperature and pressure suitable for generating plasma (initial plasma). A discharge voltage is applied to the coaxial electrode 10 before the discharge by the voltage application device 20. The voltage application device 20 accumulates (charges) electric charges in the capacitor C in advance by the power supplies 21 and 22 and applies a discharge voltage to the coaxial electrode 10.

  In a state where the discharge voltage is applied, the plasma medium 43 of the plasma medium supply unit 41 is irradiated with the laser beam 32a. Thereby, the plasma medium 43 evaporates at the laser irradiation position 43a on the surface of the plasma medium 43, and medium vapor V is generated. The medium vapor V diffuses in a direction normal to the surface of the plasma medium 43. For example, it is assumed that the light is diffused along the cosine law with the normal as the center.

Part of the medium vapor V evaporated from the plasma medium 43 is shielded by a limiting plate (shielding part) 52. Thereby, the direction in which the medium vapor V diffuses is restricted. Some of the remaining medium vapor V passes through the pair of slits 53, and travels along the straight line L _V, traveling toward the pair coaxial electrodes 10. The medium vapor V moves toward the set discharge start position H.

  Then, when the medium vapor V reaches the vicinity of the discharge start position H and the concentration of the medium vapor V between the center electrode 11 and the external electrode 12 increases, as shown in FIG. A current path K is formed, and a current flows from the positive electrode side of the capacitor C to the negative electrode side. As this current, a current corresponding to the amount of charge stored in the capacitor C flows in a pulsed manner according to the time constant of the electric circuit. That is, the charge flows in the current path K in the order of the center electrode 11, the planar discharge 6, and the external electrode 12, and finally returns to the negative electrode side of the capacitor C. In this way, the discharge is started at the discharge start position H, and a ring-shaped (planar) planar discharge 6 along the circumferential direction of the center electrode 11 is generated.

  At this time, the distance between the plasma medium 43 and the pair of coaxial electrodes 10 is equal, and the medium vapor V is generated from the common plasma medium 43 and moves toward the discharge start position H. Similarly, the concentration of the medium vapor V similarly increases, and the planar discharge 6 is simultaneously generated in the pair of coaxial electrodes 10.

  The planar discharge 6 moves toward the tip of the center electrode 11 in the direction of the axis A by a self-magnetic field (electromagnetic force) while discharging between the center electrode 11 and the external electrode 12. In other words, the initial plasma generated by the pair of coaxial electrodes 10 proceeds toward the central plane P from both sides.

  Further, since the plasma light source 1 includes the energy storage circuit 26, the probability of occurrence of the planar discharge 6 is increased by cooperation of the energy storage circuit 26 and the plurality of external electrodes 12. The plurality of external electrodes 12 that are discontinuously arranged at intervals make it easier to form the planar discharge 6 than when a continuous tubular (cylindrical) external electrode is employed. Is advantageous.

  Then, the plasma accompanying the planar discharge 6 reaches the tip of the coaxial electrode 10. When the planar discharge 6 reaches the tip of the center electrode 11, the starting point of the discharge current shifts from the side surface 11b of the center electrode 11 to the tip 11a. Due to the transfer of the current, the plasma that has moved along with the pair of planar discharges 6 converges to a high density and a high temperature.

  Since this phenomenon proceeds with the coaxial electrode 10 sandwiching the central plane P, the initial plasma is pushed out from one coaxial electrode 10 toward the other coaxial electrode 10. As a result, the initial plasma moves to an intermediate position where the coaxial electrode 10 faces (that is, the position of the center plane P) under pressure from both directions along the axis A, and a single plasma having a plasma medium as a component is generated. It is formed. Then, even after the plasma is formed, the current continues to flow through the planar discharge 6, surrounding the plasma as a whole and holding the plasma near the center of the pair of center electrodes 11.

  While the planar discharge 6 is occurring, the density and temperature of the plasma increase, and ionization of ions proceeds. As a result, plasma light including light corresponding to the plasma medium is emitted. In this state, when the current continues to flow through the planar discharge 6, plasma light can be generated for a long time.

  In the plasma light source 1, the diffusion direction of the medium vapor V evaporated in the holding unit 42 is restricted by the restriction plate 52, and is suitably supplied to the discharge start position H. Further, in this plasma light source 1, since the holding portion 42 is disposed on the center plane P, the plasma medium is positioned so as to be at the same distance from the pair of coaxial electrodes 10 facing each other with the center plane P interposed therebetween. 43 is retained. Thereby, the time difference until the medium vapor V evaporated from the plasma medium 43 is supplied to the pair of coaxial electrodes 10 can be reduced. Therefore, in the plasma light source 1, a shift in arrival time until the initial plasma generated by the pair of coaxial electrodes 10 reaches the center plane P is suppressed.

  Further, in this plasma light source 1, the distance between the holding portion 42 and the center electrode 11 can be made longer than in the conventional case. Thereby, the medium vapor V evaporated from the plasma medium 43 tends to be uniformly diffused in the circumferential direction of the center electrode 11 before being supplied between the center electrode 11 and the external electrode 12 at the discharge start position H. . Therefore, the circumferential distribution of the ring-shaped initial plasma formed around the center electrode 11 is made uniform. As a result, the ring-shaped initial plasma uniformed in the circumferential direction is converged on the tip 11a of the center electrode 11, so that the temperature and density of the plasma can be increased, and the amount of emitted extreme ultraviolet light can be reduced. Can be increased.

  Next, with reference to FIGS. 4B to 4D, limiting plates 52B to 52D according to modifications will be described. Limiting plates 52B to 52D are different from limiting plate 52 in that the shapes of through holes 56 to 58 are different. In the through holes 56 to 58, the opening width (length in the direction intersecting with the axis A) is smaller at a portion closer to the center plane P than at a portion farther from the center plane P.

  In the through hole 56 of the limiting plate 52B of the first modification shown in FIG. 4B, an extended portion 60 having a narrower opening width than the slit portion 59 is formed closer to the center plane P. The extended portion 60 has a rectangular shape in plan view (when viewed from the thickness direction).

  In the through hole 57 of the limiting plate 52C of the second modified example, an extended portion 61 including a portion having a narrower opening width than the slit portion 59 is formed closer to the center plane P. The extended portion 61 forms a triangle in plan view.

  In the through hole 58 of the limiting plate 52D of the third modified example, an extended portion 62 including a portion having a smaller opening width than the slit portion 59 is formed closer to the center plane P. The extended portion 62 has a semicircular shape (semielliptical shape) in plan view.

  As described above, the opening width of the extended portions 60 to 62 on the center plane P side of the through holes 56 to 58 is larger than the opening width of the slit portion 59 arranged on the side opposite to the center plane P of the through holes 56 to 58. When it is narrow, the medium vapor V having a lower concentration than the discharge start position H can be supplied to the tip end 11a side from the discharge start position H in the direction in which the axis A extends. Thereby, the medium vapor V can be further supplied until the initial plasma generated at the discharge start position H moves to the front end portion 11a of the center electrode 11, and the density of the plasma can be increased. As a result, the energy of the plasma can be increased, and the amount of emitted extreme ultraviolet light can be increased.

  Further, the limiting plate is not limited to a plate formed in a flat plate shape, and may be bent or curved as shown in FIG. The limiting plate 52E according to the fourth modification shown in FIG. 6A is formed to be bent to form a mountain shape. The limiting plate 52F according to the fifth modified example shown in FIG. 6B is formed so as to be curved to form an arc.

  The present invention is not limited to the embodiments described above, and various modifications as described below are possible without departing from the gist of the present invention.

  In the above embodiment, the restricting portion 51 is configured to include the restricting plate 52. However, the restricting portion 51 is not limited to including the restricting plate 52. For example, the restricting portion 51 may have a through-hole formed in the block body.

  Further, the limiting portion 51 has a configuration in which a through hole is formed in a plate-shaped member. However, a plurality of plate-shaped members may be combined to limit the direction in which the medium vapor V is diffused. Good. Further, in the above embodiment, a pair of through-holes is formed in the restriction portion 51, but a configuration in which a plurality of pairs of through-holes are formed with the center plane P interposed therebetween may be used. Further, the through-hole is not limited to the one including the slit 53, and may have another shape such as a circle.

  The phrase “the plasma medium holding unit is arranged on the center plane P” means that the plasma medium held by the plasma medium holding unit is held on the center plane P, and the laser irradiation position 43a is positioned on the center plane P. Including being on P.

  Further, at the discharge start position H, a protruding portion that protrudes from the side surface of the center electrode 11 toward the external electrode 12 may be formed. Similarly, at the discharge start position H, a configuration in which a protruding portion protruding from the side surface of the external electrode 12 toward the center electrode 11 may be formed.

DESCRIPTION OF SYMBOLS 1 Plasma light source 2 Chamber 3 Exhaust pipe 6 Planar discharge (initial plasma)
Reference Signs List 10 coaxial electrode 11 center electrode 11a tip of center electrode 11b side surface of center electrode 12 external electrode 13 insulator 20 voltage applying device 21 first high-voltage power supply 22 second high-voltage power supply 30 medium vapor forming unit 31 laser device (laser irradiation unit) )
32, 32a Laser light 41 Plasma medium supply unit 42 Holding unit (plasma medium holding unit)
43 Plasma Medium 43a Laser Irradiation Position 51 Limiting Section 52, 52B, 52C, 52D, 52E, 52F Limiting Plate (Shielding Section)
53 slit (through hole)
56-58 Through-hole 59 Slit part 60-62 Expansion part A axis (axis of center electrode)
Laser at a distance Lv linear (plasma medium between the irradiation surface and the axis A of the distance L ₂ plasma medium with B axis B2 virtual line BS region F imaginary circle H discharge starting position K current path L ₁ discharging start position H and the central plane P A straight line connecting the irradiation position and the discharge start position H)
P center plane (symmetric plane)
P2 Virtual plane V Medium vapor

Claims (4)

A plasma light source including a pair of coaxial electrodes opposed to each other with a central surface therebetween,
The coaxial electrode, a central electrode extending in a direction orthogonal to the central plane,
A plurality of external electrodes extending in the circumferential direction of an imaginary circle centered on the axis of the center electrode and extending toward the central plane,
The plasma light source is on the central plane, and is disposed outside a virtual line connecting the external electrodes adjacent in the circumferential direction, and a plasma medium holding unit that holds a plasma medium.
A laser irradiation unit that irradiates a laser to the plasma medium held by the plasma medium holding unit,
A limiting portion disposed between the plasma medium holding portion and the external electrode in a radial direction of the virtual circle, and configured to limit a direction in which the medium vapor evaporated in the plasma medium holding portion is diffused. The restriction portion has a pair of through holes through which the medium vapor passes,
The restriction unit includes a shielding unit that is disposed around the through hole and shields the medium vapor.
The plasma light source according to claim 1, wherein the pair of through holes are formed symmetrically with respect to the center plane.   The plasma light source according to claim 2, wherein the pair of through holes are rectangular slits extending in a direction intersecting the axis.   3. The plasma light source according to claim 2, wherein an opening width of a portion of the through hole on the central surface side is smaller than an opening width of a portion of the through hole on a side opposite to the central surface.

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