JP2018124440A - Plasma light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an intensity decrease of extreme UV light by stably supplying an appropriate amount of plasma medium in a plasma light source according to a counter-facing plasma focusing method.SOLUTION: A plasma light source has a central electrode 12 extending on a single axial line and a plurality of external electrodes 14 and is provided with a pair of coaxial electrodes 10, 10 oppositely arranged with a symmetrical plane 1 sandwiched. One or both of the plurality of external electrodes 14 and the central electrode 12 include a projection part 15 projected toward the other thereof. The projection part 15 is positioned at a position more closer to the symmetrical plane 1 as the projection part is located further from an irradiation point P.SELECTED DRAWING: Figure 5

Description

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

  In order to further miniaturize semiconductor devices, it is essential to shorten the wavelength of an exposure light source in photolithography. In recent years, extreme ultraviolet light has attracted attention as light for that purpose. Extreme ultraviolet light is obtained from high-temperature, high-density plasma, and there are various sources of such plasma (in other words, light sources using plasma, hereinafter referred to as plasma light sources). From an industrial point of view, it is desirable that the plasma light source can be miniaturized, and as a candidate for this, a plasma light source of a discharge generated plasma (DPP) system or a plasma of a laser generated plasma (LPP) system is a candidate. A light source is known. Note that any of the extreme ultraviolet light emitted from these plasma light sources is pulsed light.

  In photolithography, control of the exposure time is extremely important. For that purpose, it is necessary not only to ensure sufficient intensity and luminance of extreme ultraviolet light, but also to obtain them stably. Moreover, since the emission time of extreme ultraviolet light is as short as about several μs or less, it is necessary to repeat the generation of plasma (that is, emission of extreme ultraviolet light) at high speed.

  A plasma light source related to the above is disclosed in Patent Document 1. The plasma light source of the same document is a plasma light source that employs a plasma focus method, which is a kind of DPP method, and is disposed opposite to each other with respect to a symmetry plane to generate a plasma that emits extreme ultraviolet light and confine the plasma. A coaxial electrode and a voltage applying device that applies a discharge voltage to each coaxial electrode are provided. Each coaxial electrode has a rod-shaped center electrode and a plurality of external electrodes arranged at a certain distance from the center electrode and in the circumferential direction of the center electrode.

  In the plasma light source of Patent Document 1, a pulsed voltage is further applied in a state where a high voltage is applied between the center electrode and the external electrode, or laser ablation is performed at any part of the coaxial electrode. Induces an initial discharge between both electrodes to generate plasma (initial plasma). The initial discharge is formed in an annular shape centering on the center electrode, and moves toward the tip of the center electrode by electromagnetic force while promoting the generation and growth of plasma. Furthermore, the plasma of each coaxial electrode receives electric energy, and is fused, confined, and converged between the coaxial electrodes, thereby achieving high temperature and high density. As a result, light including extreme ultraviolet light is emitted.

JP2013-089634A

  In the plasma light source of Patent Document 1, the initial discharge generated in each coaxial electrode grows into an annular plasma (initial plasma) as described above, and then fuses with each other between the coaxial electrodes. It becomes density. In this series of processes, the temperature and density of the plasma confined between the coaxial electrodes depends on the amount of plasma that has reached between the coaxial electrodes in a short period of time. That is, paying attention to the timing at which each part in the circumferential direction of the annular plasma reaches the tip of the coaxial electrode, the larger the timing deviation of each part, the lower the temperature and density of the plasma in the final form. Becomes unstable. In other words, the greater the timing shift of each part, the lower the intensity of the extreme ultraviolet light emitted from the plasma, making it unstable.

  Therefore, the present invention stably supplies an appropriate amount of plasma medium in a plasma light source including a pair of coaxial electrodes that are arranged opposite to each other with respect to a plane of symmetry and generate plasma that emits extreme ultraviolet light and confine the plasma. It is possible to reduce the intensity of extreme ultraviolet light.

  One embodiment of the present invention is a plasma light source, which includes a central electrode extending on a single axis and a plurality of external electrodes arranged so as to surround the outer periphery of the central electrode, and is disposed to face each other with a plane of symmetry interposed therebetween. A pair of coaxial electrodes for generating plasma that emits extreme ultraviolet light and confining the plasma; a holding unit for holding the medium of the plasma; and a voltage applying device for applying a discharge voltage to each of the coaxial electrodes And a laser device that irradiates an irradiation point in the holding unit with laser light for performing ablation of the medium, and one or both of the plurality of external electrodes and the central electrode are the plurality of external electrodes And a protrusion protruding toward the other of the center electrodes, and the protrusion is located closer to the symmetry plane as the distance from the irradiation point increases. The gist.

  For one protrusion, the total time of the average movement time of the medium from the irradiation point to the protrusion and the average movement time of the initial discharge of the medium from the protrusion to the tip of the external electrode was assumed. In this case, the protrusions may be displaced from each other along the axis so that the total times for the corresponding external electrodes are equal to each other.

  According to the present invention, an appropriate amount of plasma medium can be stably provided in a plasma light source including a pair of coaxial electrodes that are opposed to each other with respect to a plane of symmetry and generate plasma that emits extreme ultraviolet light and confine the plasma. It can be supplied, and a decrease in the intensity of extreme ultraviolet light can be suppressed.

It is a schematic block diagram (sectional drawing) of the plasma light source which concerns on embodiment of this invention. It is a figure which shows the electric system of the plasma light source which concerns on embodiment of this invention. It is a figure which shows the III-III cross section of FIG. It is a figure which shows the protrusion which concerns on embodiment of this invention. FIG. 4 is a side view of the center electrode and the external electrode shown in FIG. 3. It is a figure for demonstrating the positional relationship of the protrusion which concerns on embodiment of this invention, (a) is a front view of the external electrode and center electrode containing a protrusion, (b) is the external shown to (a) It is a side view of an electrode and a center electrode. It is sectional drawing which shows the holding | maintenance part which concerns on embodiment of this invention, and its periphery. It is an image which shows an example of the time-dependent change of initial stage discharge. It is an image which shows an example of the time-dependent change of initial stage discharge. It is a figure for demonstrating generation | occurrence | production of initial stage discharge, and the state immediately after that. It is a figure which shows the movement of the plasma medium with respect to two external electrodes from which the position of a protrusion differs, and the movement of initial stage discharge (initial stage plasma). It is a figure which shows the modification of the center electrode and external electrode which concern on embodiment of this invention. It is a figure which shows the modification of the protrusion which concerns on embodiment of this invention.

  Hereinafter, a plasma light source according to an embodiment of the present invention will be described with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the common part in each figure, and the overlapping description is abbreviate | omitted.

  FIG. 1 is a schematic configuration diagram (sectional view) showing a plasma light source according to the present embodiment. FIG. 2 is a diagram showing an electrical system of the plasma light source. FIG. 3 is a view showing a cross section taken along the line III-III in FIG. As shown in these drawings, the plasma light source of the present embodiment includes a pair of coaxial electrodes 10 and 10, a reservoir 20 provided individually for each coaxial electrode 10, a voltage application device 30, and a laser device. 40. In FIG. 1, the right coaxial electrode 10 has the same configuration as the left coaxial electrode 10, and thus detailed illustration is omitted.

  The pair of coaxial electrodes 10 and 10 are installed at positions symmetrical to each other with respect to the symmetry plane 1 in a vacuum chamber (not shown). That is, this plasma light source employs a counter-type plasma focus system. Specifically, it is installed at a constant interval with the symmetry plane 1 in between, and the front end sides (sides on which the planar discharge 2b is emitted) face each other. The coaxial electrodes 10 and 10 receive the laser beam 42 from the laser device 40, emit a plasma medium (hereinafter referred to as plasma medium) 6, and generate an initial discharge 2a that ionizes the plasma medium 6 (see FIG. 10). Further, the coaxial electrodes 10 and 10 grow the initial discharge 2a into a planar discharge 2b, generate a plasma 3 therebetween, and confine it. The plasma 3 confined between the coaxial electrodes 10 and 10 is heated by receiving electric energy from the coaxial electrodes 10 and 10 and emits plasma light 8 including extreme ultraviolet light. The sheet discharge is a sheet discharge current that spreads two-dimensionally and is also called a current sheet or a plasma sheet.

  The plasma medium 6 of the present embodiment is a low melting point metal (low melting point alloy) that can be supplied from the reservoir 20 to a holding unit 18 (described later), and its composition is selected according to the required wavelength of ultraviolet light. For example, when 13.5 nm ultraviolet light is required, Li (lithium) or Sn (tin) is included, and when 3-4 nm ultraviolet light is required, Bi (bismuth) is included.

  Each coaxial electrode 10 includes a center electrode 12, a plurality of external electrodes 14 provided so as to surround the outer periphery of the center electrode 12, and an insulator 16. As shown in FIG. 1 and FIG. 3, the center electrode 12 has a single axis ZZ common to the coaxial electrodes 10 as a central axis (hereinafter, this axis is referred to as the central axis Z). It is a rod-shaped conductor extending on Z. For convenience of explanation, the axes orthogonal to the central axis Z and orthogonal to each other are referred to as an X axis and a Y axis (see FIG. 3). The X axis is an axis that passes through the center between two external electrodes 14 that are adjacent to each other. In the present embodiment, the X axis coincides with the optical path of the laser light 42 including the irradiation point P.

  The center electrode 12 has the front-end | tip part 12a which faces the symmetry plane 1, and the side surface 12b formed around the central axis Z, and a diameter is 5 mm, for example. Note that a holding portion 18 (described later) for the plasma medium 6 is provided on the side surface 12b. The center electrode 12 is formed using a material having heat resistance. Such a material is, for example, a refractory metal such as W (tungsten) or Mo (molybdenum).

  The distal end portion 12 a has a hemispherical curved surface facing the symmetry plane 1. However, the shape of the surface facing the symmetry plane 1 is not limited to a curved surface, and may be a simple plane. Moreover, you may provide the recessed part (not shown) recessed along the central axis Z. As shown in FIG.

  As shown in FIG. 1, the external electrode 14 is a rod-shaped conductor extending in parallel with the central axis Z of the central electrode 12, and has a diameter of 3 mm, for example. As shown in FIG. 3, the external electrode 14 is disposed at every angle θ along the circumferential direction of the center electrode 12. In other words, each external electrode 14 is disposed in parallel with the center electrode 12 and surrounds the periphery of the center electrode 12. In the example shown in FIG. 3, six external electrodes 14 are arranged around the center electrode 12 every 60 °.

  It is desirable that the external electrodes 14 be installed at equiangular intervals around the center electrode 12. For example, it is desirable that each external electrode 14 be installed at a rotationally symmetric position with respect to the center electrode 12 from the viewpoint of processing and assembly, or from the ease of forming a planar discharge 2b (described later). However, the present invention is not limited to such an arrangement. Further, the number of external electrodes 14 is not limited to six as shown in FIG. 3, and is appropriately set according to the size and shape of the center electrode 12 and the external electrode 14, the distance between them, and the like.

  FIG. 4 is a diagram illustrating an example of the protrusion 15 according to the present embodiment. As shown in this figure, the external electrode 14 has a protrusion 15 on the side surface 14 a facing the center electrode 12. The protrusion 15 protrudes from the side surface 14 a toward the irradiation point P of the laser beam 42 on the holding unit 18 or from the side surface 14 a toward the center electrode 12. Further, the protrusion 15 is formed so as to include at least one apex G (see FIG. 3) closest to the irradiation point P of the laser beam 42 in the holding unit 18. The shape of the protrusion 15 is not particularly limited as long as this condition is satisfied. The protrusion 15 may be formed in an annular shape along the circumferential direction of the external electrode 14 as shown in FIG. 4, or as at least one cone or pyramid with the apex as the apex G directed to the irradiation point P It may be formed.

  The protrusion 15 protrudes toward the center electrode 12 from other portions of the external electrode 14. Accordingly, the electric field intensity around the protrusion 15 is larger than the other portions of the external electrode 14 due to the electric field concentration. The increase in the electric field strength promotes the induction of the initial discharge 2a (see FIG. 10) of the planar discharge 2b (see FIG. 10). Therefore, in this embodiment, the discharge path including the protrusion 15 is formed with priority. That is, the protrusion 15 defines the initial position of the initial discharge 2 a generated between the center electrode 12 and the external electrode 14 in the circumferential direction and the axial direction of the center electrode 12.

  FIG. 5 is a side view of the center electrode 12 and the external electrode 14 shown in FIG. As shown in this figure, the positions of the protrusions 15 are shifted from each other along the center electrode 12. As will be described later, the protrusion 15 is located closer to the symmetry plane 1 (see FIG. 1) as the distance from the irradiation point P increases. In other words, the protrusion 15 is located at a position farther from the symmetry plane 1 as it is closer to the irradiation point P. This point will be described later.

  The external electrode 14 is formed using a heat-resistant conductive material, like the center electrode 12. Further, the end face of the external electrode 14 facing the symmetry plane 1 may be either a curved surface or a flat surface.

  The insulator 16 is formed using, for example, ceramic, supports the base portions of the center electrode 12 and the external electrode 14, defines the distance between them, and electrically insulates between them. The insulator 16 is formed in a disk shape, for example, and has a structure such as a hole or a groove that supports the center electrode 12 and the external electrode 14.

  FIG. 7 is a cross-sectional view showing the holding portion 18 and its surroundings. The holding unit 18 holds the plasma medium 6 so that the plasma medium 6 is exposed in the space between the center electrode 12 and the external electrode 14. For example, as shown in FIG. 7, the holding portion 18 is provided on the side surface 12 b of the center electrode 12. The holding | maintenance part 18 is formed in the strip | belt shape including the irradiation point P over the perimeter of the side surface 12b. The outer surface of the holding portion 18 is located at the same height as the side surface 12 b of the center electrode 12. The holding part 18 is made of a porous body, and can accumulate the melted plasma medium 6 and allow it to exude to the outside. The porous body is made of, for example, the same material as the center electrode 12. Note that the holding unit 18 may be configured by the plasma medium 6. In this case, the holding part 18 does not need to be a porous body, and the plasma medium 6 remains in a molten state or a solid state. For example, when the plasma medium 6 is in a solid state, it is installed (embedded) on the side surface 12b of the center electrode 12, and the reservoir 20 described later is omitted. In this case, not only a low melting point metal such as lithium (Li) but also gadolinium (Gd) or terbium (Tb) emitting ultraviolet light of 6.7 nm can be used as the plasma medium 6.

  The reservoir 20 is individually provided for each coaxial electrode 10. As shown in FIG. 1, the reservoir 20 supports the base of the center electrode 12, and stores the plasma medium 6 in a space 20a formed therein. The space 20 a communicates with the holding unit 18 through the flow path 12 c of the center electrode 12. The reservoir 20 is equipped with a heater 22. The heater 22 is composed of, for example, a heat medium (oil) circulation heater or an electrothermal heater, and melts the plasma medium 6 in the space 20a and maintains the temperature of the center electrode 12 at a temperature at which the plasma medium 6 melts. To do. Therefore, even when the plasma medium 6 flows out to the holding unit 18 through the flow path 12c, the holding unit 18 can hold the plasma medium 6 in a molten state.

  Next, an electrical system in the plasma light source of this embodiment will be described. As shown in FIG. 2, the plasma light source includes a voltage applying device 30 connected to each coaxial electrode 10. The voltage application device 30 applies a discharge voltage of the same polarity or a reverse polarity to each coaxial electrode 10.

  The voltage application device 30 includes a high voltage power supply 32. The output side of the high voltage power supply 32 is connected to the center electrode 12 of the coaxial electrode 10, and the common side of the high voltage power supply 32 is connected to the external electrode 14 corresponding to the center electrode 12. The high voltage power supply 32 applies a discharge voltage (for example, 5 kV) between the center electrode 12 and the external electrode 14. The polarity of the discharge voltage may be either positive or negative with respect to the external electrode 14. Further, as shown in FIG. 2, the common side of the high-voltage power supply 32 may be grounded. Similarly, a vacuum chamber (not shown) for housing the coaxial electrode 10 may be grounded.

  As described above, a plurality of external electrodes 14 are provided around each center electrode 12. In order to obtain an ideal discharge, it is necessary to generate a discharge between all the external electrodes 14 and the center electrode 12. Moreover, it is desirable that these discharges are spatially distributed at equal intervals around the center electrode 12. However, the discharge energy supplied from the high-voltage power supply 32 tends to be preferentially consumed with respect to the first generated discharge. In this case, it is difficult to generate a plurality of discharges at different locations substantially simultaneously.

  Therefore, the voltage application device 30 of this embodiment includes an energy storage circuit 34 that stores the discharge energy of the discharge voltage for each external electrode 14. For example, as shown in FIG. 2, the energy storage circuit 34 includes a plurality of capacitors C that individually connect the center electrode 12 and each external electrode 14. Each capacitor C has a capacitance that allows a discharge current of about 10 kA to flow at the peak of discharge, and is connected to each output side and each common side of the high-voltage power supply 32.

  Thus, by providing the capacitor C for storing discharge energy for each external electrode 14, it is possible to generate discharge in all the external electrodes 14. That is, it is possible to prevent the discharge energy from being excessively consumed by the first generated discharge, and to generate the planar discharge 2b over the entire circumference of the center electrode 12.

  Furthermore, the voltage application device 30 of this embodiment may include a discharge current blocking circuit 36 that blocks the discharge current from returning. For example, as shown in FIG. 2, the discharge current blocking circuit 36 includes an inductor L that connects each external electrode 14 and the voltage application device 30 (specifically, the common side of the high-voltage power supply 32). Since the inductor L has a sufficiently high impedance with respect to the discharge current, the discharge current that has passed through the center electrode 12 and the external electrode 14 can be returned to the energy storage circuit 34 that is the generation source thereof. That is, in order to prevent the discharge energy accumulated in each capacitor C from being supplied to the external electrode 14 other than the external electrode 14 directly connected to the capacitor C, the distribution of discharge in the circumferential direction of the center electrode 12 is biased. Can be prevented.

  As described above, the plasma light source of this embodiment includes the laser device 40. The laser device 40 irradiates the central electrode 12 of each coaxial electrode 10 with the laser light 42 to release the medium of the plasma 3 and cooperates with the voltage application device 30 to perform an initial discharge of the plasma 3 (initial plasma). ) 2a is generated. The laser device 40 is, for example, a YAG laser, and outputs a fundamental wave or a double wave thereof as a short pulse laser beam 42 in order to perform ablation. The laser beam 42 is branched by an optical element such as a half mirror, and is irradiated to the holding unit 18 of each center electrode 12. For example, the laser beam 42 travels toward the central axis Z between two adjacent external electrodes 14 and is irradiated to the irradiation point P of the holding unit 18. In the holding unit 18 irradiated with the laser beam 42, the plasma medium 6 is emitted as neutral particles or ions by ablation of the laser beam 42.

  On the other hand, at the time of irradiation with the laser light 42, the discharge voltage by the voltage application device 30 has already been applied between the center electrode 12 and the external electrode 14 of each coaxial electrode 10. Therefore, when ablation occurs, an initial discharge 2a between the center electrode 12 and each external electrode 14 is induced.

  Depending on the shapes of the center electrode 12 and the external electrode 14, the distance between the two, etc., a plurality of laser beams 42 may be irradiated simultaneously at intervals along the circumferential direction of the central axis Z. In the present embodiment, for example, the irradiation points P of the laser light 42 are set at two places with the central axis Z interposed therebetween. This makes it easy to form the initial discharge 2a in an annular shape.

  This is because, in the coaxial electrode in which the external electrode is arranged on the same circle with the center electrode as the center, the generation region of the initial discharge 2a has an opening angle of 180 degrees or more with the central axis Z of the center electrode 12 as the base point. Based on experimental results. An example of the experimental results is shown in FIGS. 8 (a) to 8 (c). 8 (a) to 8 (c) are measured with a high-speed camera having four CCDs, and show the change over time in the discharge distribution between the center electrode and the external electrode to which a discharge voltage of 5 kV is applied. It is an image. 8A shows a state after 100 ns from the first discharge, FIG. 8B shows a state after 300 ns from the first discharge, and FIG. 8C shows a state after 500 ns from the first discharge. Is shown. White parts in the figure indicate that plasma is generated and light is emitted. The accumulation time (exposure time) in each image is 100 ns. In order to induce the first discharge, laser ablation is performed on one place of the insulator between the center electrode and the external electrode. However, the occurrence of such discharge and its change with time can be similarly obtained by laser ablation with respect to the holding portion 18 shown in FIG.

  As can be seen from the time changes shown in FIGS. 8A to 8C, the discharge induced by ablation occurs even though the laser beam irradiation point is only one point. It can be confirmed that each of the light has been enlarged approximately 90 degrees clockwise and counterclockwise from the light irradiation point. As a result, it can be confirmed that a sufficient discharge is generated between each of the four external electrodes in the center and the right side of the photograph and the center electrode. That is, the discharge is not only between the external electrode and the center electrode closest to the place where the phenomenon that induces the discharge (laser ablation in FIGS. It also occurs between the external electrode located at the center and the center electrode. That is, by disposing a plurality of external electrodes to which discharge energy is applied around the center electrode, it is possible to expand the entire discharge without localizing the discharge in the circumferential direction of the center electrode.

  FIG. 9 shows the change over time of the discharge after laser ablation is performed on two portions of the insulator sandwiching the center electrode, and corresponds to FIG. 8B. As shown in this figure, the initial discharge grows into a circularly distributed discharge (planar discharge 2b described later). In consideration of this result, it is desirable to irradiate the laser beam 42 at a rotationally symmetric position with respect to the center electrode 12 as the number of irradiated portions is smaller. In addition, simultaneous irradiation of a plurality of laser beams can be easily achieved by forming a plurality of optical paths having the same optical path length using an optical element such as a half mirror.

  Next, the arrangement of the protrusions 15 will be described in detail. 6A and 6B are diagrams for explaining the positional relationship of the protrusions 15 according to this embodiment, and FIG. 6A is an external electrode 14 including the protrusions 15 and a center electrode. FIG. 6B is a side view of the external electrode 14 and the center electrode 12 shown in FIG. As shown in FIG. 6A, the optical path of the laser light 42 is assumed to be located on the X axis. Therefore, the irradiation point P is also located on the X axis. Further, 14A is assigned to the external electrode 14 closest to the X axis, and 14B is assigned to the external electrode 14 that is adjacent to the external electrode 14A in the clockwise direction. Similarly, reference numeral 15A is assigned to the protrusion 15 of the external electrode 14A, and reference numeral 15B is assigned to the protrusion 15 of the external electrode 14B. Further, in the protrusion 15A and the protrusion 15B, the symbols G and GB are given to the apex G closest to the irradiation point P, respectively. Further, it is assumed that the external electrode 14A and the external electrode 14B are located at 30 ° and 90 ° from the X axis with the central axis Z as the center.

  As shown in FIG. 6A, when the distance from the irradiation point P to the apex G is a converted distance R projected on a plane orthogonal to the central axis Z (for example, the symmetry plane 1), the protrusion of each external electrode 14 The conversion distance R with respect to 15 is expressed by the following formula 1.

... (Formula 1)
Here, R ₀ is the center electrode 12 and the distance between the centers of the external electrodes 14, r _c is the radius of the center electrode 12, r _s is protruding from the radial (center of the external electrode 14 when forming the protrusion 15 in the annular 15). For example, the radius _{r c} is 3.5 mm, if the radius _{r s} is 1.5 mm, the distance from the side surface 12b of the center electrode 12 to the projection 15 of 3.5 mm, the distance _{R 0} is a 8.5 mm. In this case, the conversion distance R _A from the irradiation point P to the top GA shown in FIG. 6A and the conversion distance R _B from the irradiation point P to the top GB are 4.24 mm and 7.69 mm, respectively.

  By the way, the initial discharge 2a is generated when the plasma medium 6 reaches the protrusion 15, and thereafter proceeds toward the symmetry plane 1. The speed (average speed) Va at which the plasma medium 6 moves in space (in a vacuum) and the speed (average speed) Vp at which the initial discharge 2a (subsequent planar discharge 2b) moves are the external electrode 14 with respect to the center electrode 12. It does not depend on the position of Therefore, when the protrusion 15A and the protrusion 15B are located in a plane including the irradiation point P and orthogonal to the central axis Z, the plasma medium 6 emitted from the holding portion 18 first reaches the top GA, Then, it reaches the top GB. On the other hand, the distance from the top GB to the handling surface 1 and the distance from the top GA to the handling surface 1 are the same. Therefore, in the above-described case, there is a difference in the timing of occurrence of discharge between the initial discharge 2a between the center electrode 12 and the external electrode 14A and the initial discharge 2a between the center electrode 12 and the external electrode 14B. It means that there is a positional deviation along the traveling direction at the same time when the initial discharge moves. This positional shift causes a time shift of the plasma medium 6 input to the plasma 3 confined between the coaxial electrodes 10 and 10, and becomes a factor that hinders high density and high temperature of the plasma 3. .

  Therefore, in the present embodiment, the time from the timing at which the irradiation point P is irradiated with the laser light 42 to the timing at which the initial discharge 2a reaches the symmetry plane 1 is constant so as to be constant with respect to each external electrode 14. The positions of the portions 15 are shifted from each other along the axial direction (central axis Z) of the center electrode 12. Specifically, the protrusion 15 is located closer to the symmetry plane 1 as it is farther from the irradiation point P. Hereinafter, this positional relationship will be described.

  For convenience of explanation, it is assumed that the protrusion 15B and the irradiation point P shown in FIG. 6A are located in the same plane orthogonal to the central axis Z. On the other hand, the protrusion 15A is located at a different distance from the protrusion 15B. When a distance from a plane including the protrusion 15B and the irradiation point P (hereinafter referred to as a reference surface) to the protrusion 15A is d, the distance d is expressed by the following Expression 2.

... (Formula 2)
Here, Va is the speed at which the plasma medium 6 moves in the space, Vp is the speed at which the initial discharge 2a (the subsequent planar discharge 2b) moves, R _A is the converted distance from the irradiation point P to the top GA, and R _B Is a conversion distance from the irradiation point P to the top GB. The speed ^{Va 1.0 × 10 4 m / s} , the speed Vp and ^{4.0 × 10 4 m / s,} 4.24mm the converted distances _{R A,} when a 7.69mm the converted distance _{R B,} Formula 2 The distance d is 8.99 mm and −4.89 mm. A negative value in this calculation result indicates a position farther from the symmetry plane 1 than the reference plane.

If top GB is positioned in the reference plane, the actual distance from the irradiation point P to the top GB is also 7.69mm and converted distance R _B. On the other hand, the actual distance from the irradiation point P to the top GA is 9.94 mm when the distance d is 8.99 mm, and 6.47 mm when the distance d is −4.89 mm. As can be seen from this tendency, the protrusion 15 is located closer to the symmetry plane 1 as the distance from the irradiation point P increases. Depending on the value of the distance d, a part of the side surface of the external electrode 14 may be closer to the irradiation point P than the protrusion 15. In this case, the part may be covered with an insulator to prevent (suppress) the induction of discharge to a portion other than the protrusion 15.

  Next, the operation of the plasma light source of this embodiment will be described. FIG. 10 is a diagram for explaining the occurrence of the initial discharge 2a and the state immediately after that. FIG. 11 is a diagram showing the movement of the plasma medium 6 and the movement of the initial discharge (initial plasma) 2a with respect to the two external electrodes 14A and 14B shown in FIGS. 6 (a) and 6 (b).

  As described above, in the plasma light source of the present embodiment, a pair of coaxial electrodes 10 are provided in a vacuum chamber (not shown). The pair of coaxial electrodes 10 are disposed to face each other with the symmetry plane 1 in between. On the other hand, the inside of the vacuum chamber is maintained at a temperature and pressure suitable for generating the plasma 3. Further, a discharge voltage having the same polarity or opposite polarity is applied to each coaxial electrode 10 before discharge by the voltage application device 30.

  In a state where a discharge voltage is applied to each coaxial electrode 10, the laser beam 42 is simultaneously irradiated onto the holding portion 18 of each coaxial electrode 10 at time t <b> 0. In each coaxial electrode 10, the plasma medium 6 is released in a large amount as neutral gas or ions by this irradiation, and proceeds toward the external electrodes 14 </ b> A and 14 </ b> B around the irradiation point P. On the other hand, the velocity Va of the plasma medium 6 is constant. Accordingly, the plasma medium 6 first reaches the protrusion 15B of the external electrode 14B at time t1. When the plasma medium 6 reaches the protrusion 15B, an initial discharge 2a is generated between the protrusion 15B and the center electrode 12. One possible reason for this is that the particle density of the plasma medium 6 increases to such an extent that a discharge path is formed between the external electrode 14 and the center electrode 12. The initial discharge 2a generated between the center electrode 12 and the protrusion 15B proceeds toward the symmetry plane 1.

  When the initial discharge 2a traveling between the external electrode 14B and the center electrode 12 reaches the plane where the protrusion 15A is located and perpendicular to the central axis Z (time t2), the plasma medium 6 is applied to the protrusion 15A. To reach. Immediately thereafter, an initial discharge 2 a is generated between the center electrode 12 and the protrusion 15 </ b> A, and this initial discharge 2 a proceeds toward the symmetry plane 1. Accordingly, the initial discharge 2a between the center electrode 12 and the external electrode 14A and the initial discharge 2a between the center electrode 12 and the external electrode 14B proceed toward the symmetry plane 1 in a state of being located in the same plane, At the time t3, the tips of the external electrodes 14A and 14B are reached, and at the time t4, they reach between the coaxial electrodes 10 and 10.

  Since all such transient phenomena occur on the external electrode 14, the initial discharge 2a is a planar shape distributed over the entire circumference of the center electrode 12 while taking in the plasma medium 6 released by ablation. It can also be said that it has grown into a discharge 2b. The planar discharge 2b moves in the direction discharged from the coaxial electrode 10 by the self magnetic field (that is, the direction toward the symmetry plane 1). The planar discharge 2b at this time is distributed in a substantially annular shape when viewed from the central axis Z.

  When the sheet discharge 2b reaches the tip of the coaxial electrode 10, the starting point of the discharge current of the sheet discharge 2b shifts from the circumferential side surface of the center electrode 12 to the tip 12a. In other words, the discharge current flows intensively from the tip 12a. Due to this current concentration, the current density around the tip 12a rapidly increases, and the plasma medium 6 around the tip 12a sandwiched between the pair of planar discharges 2b becomes high density and high temperature.

  Further, since this phenomenon proceeds at each coaxial electrode 10 across the symmetry plane 1, the plasma medium 6 is pushed out from one coaxial electrode 10 toward the other coaxial electrode 10. As a result, the plasma medium 6 receives electromagnetic pressure from both directions along the central axis Z and moves to an intermediate position where the coaxial electrodes 10 face each other (that is, the symmetry plane 1 of the center electrode 12). A single plasma 3 having the component as a component is formed.

  While the sheet discharge 2b is generated, the current of each sheet discharge 2b is concentrated on the tip 12a of each center electrode 12. Therefore, electromagnetic pressure is applied to the plasma 3 around the tip 12a, and the density and temperature of the plasma 3 increase. That is, ionization of the plasma medium 6 proceeds. As a result, plasma light 8 including extreme ultraviolet light is emitted from the plasma 3. In this state, the voltage application device 30 continues to supply electric energy to the plasma 3. With this energy supply, the plasma light 8 can be generated for a long time.

  In these series of processes, the control of the generation timing of the initial discharge 2a is very important for improving the intensity of extreme ultraviolet light. As described above, the initial discharge 2a is generated between each external electrode 14 and the center electrode 12, and each initial discharge 2a expands in the circumferential direction and grows into an annular planar discharge 2b. As soon as each initial discharge 2a is generated, it proceeds toward the tip 12a of the center electrode 12. Therefore, if each initial discharge 2a occurs at different timings, each position in the circumferential direction of the sheet discharge 2b is shifted back and forth along the traveling direction according to this time difference. For example, in the coaxial electrode shown in FIG. 7, the displacement of the initial discharge position in the traveling direction is confirmed. From this displacement, about 200 ns between the earliest initial discharge and the earliest initial discharge. It is estimated that there is a difference in the occurrence timing.

  The traveling speed of the planar discharge 2b does not depend on the circumferential position of the planar discharge 2b. Accordingly, the deviation in the timing of occurrence of each initial discharge 2a is the deviation in the timing at which the respective positions in the circumferential direction of the planar discharge 2b reach between the coaxial electrodes 10 and 10 as they are. On the other hand, extreme ultraviolet light having a desired wavelength is obtained when the plasma 3 is grown at a desired high temperature and high density. In other words, it is obtained when the valence distribution of the plasma medium in the plasma 3 reaches a desired value. Further, the growth of the plasma 3 requires a time of about several tens to several hundreds ns. According to the present embodiment, the above-described shift in arrival timing in the planar discharge 2b is reduced as much as possible, and the intensity of extreme ultraviolet light is improved.

  By the way, it is estimated that the emission angle dependency on the average velocity of the particles (neutral particles and ions) of the plasma medium 6 is small. That is, it is considered that the plasma medium 6 travels at substantially the same average speed toward each protrusion 15 (top G).

  Therefore, in the present embodiment, the protrusions 15 (the top portions G) of the external electrodes 14 are arranged so as to be shifted from each other as shown in FIG. That is, for one protrusion 15, the average movement time of the plasma medium 6 from the irradiation point P to the protrusion 15 and the average movement time of the initial discharge of the plasma medium 6 from the protrusion 15 to the tip of the external electrode 14. When the total time is assumed, the plurality of protrusions 15 are shifted from each other along the central axis Z so that the total time for the corresponding external electrodes 14 is equal to each other. Accordingly, a shift in timing at which the initial discharge 2a generated between each external electrode 14 and the center electrode 12 reaches between the coaxial electrodes 10 and 10 is suppressed. For example, as compared with the case where the protrusions 15 of the external electrodes 14 are arranged on the same plane as the irradiation point P of the center electrode 12, the timing deviation reaching between the coaxial electrodes 10 and 10 is reduced.

  Since the difference in timing at which the circumferential positions of the planar discharge 2b reach between the coaxial electrodes 10 and 10 is small, the shift in the supply timing of the plasma medium 6 from each position in the circumferential direction is also small. Accordingly, since the amount of the plasma medium 6 in the same growth process between the coaxial electrodes 10 and 10 increases, the density of the plasma 3 is improved and the intensity of extreme ultraviolet light is also improved.

  Further, the protrusion 15 (the top G) of each external electrode 14 is arranged around the irradiation point P as a reference. However, each protrusion 15 is farther from the irradiation point P than when the same protrusion is arranged on the same plane as the irradiation point P. However, each protrusion 15 can preferentially generate the initial discharge 2a because of its shape. That is, the plasma medium 6 can be stably supplied between the coaxial electrodes 10 and 10.

  Therefore, according to the present embodiment, an appropriate amount of the plasma medium 6 can be stably supplied, and a decrease in the intensity of extreme ultraviolet light can be suppressed.

  A modification of the center electrode 12 and the external electrode 14 will be described. FIG. 12 is a view showing a modification of the center electrode 12 and the external electrode 14 according to the above-described embodiment. As shown in this figure, the diameter of the center electrode 12 may become smaller toward the symmetry plane 1. For example, the center electrode 12 may be formed in a substantially conical shape having the apex 12a at the apex angle. In this case, the portion of the external electrode 14 that is closest to the center electrode 12 in the plane orthogonal to the center axis Z may be formed so that the distance from the center axis Z becomes shorter as it goes toward the symmetry plane 1. Good. For example, when the external electrode 14 is formed in a rod shape, the external electrode 14 is inclined with respect to the central axis Z so as to approach the central axis Z as it approaches the symmetry plane 1. In the example shown in FIG. 12, the distance between the center electrode 12 and the external electrode 14 is constant. However, this distance may be smaller as it approaches the symmetry plane 1.

  As shown in FIG. 13, the protrusion 15 may be provided on the side surface 12 b of the center electrode 12. In this case, each protrusion 15 is formed in a conical shape, for example. The protrusion 15 is provided at a position closest to the corresponding external electrode 14 in the circumferential direction of the center electrode 12, and protrudes toward the corresponding external electrode 14. That is, each protrusion 15 is provided so as to generate a discharge with the corresponding external electrode 14, and is scattered on the side surface 12 b of the center electrode 12. Note that the positions of the protrusions 15 are shifted from each other along the axial direction (center axis Z) of the center electrode 12, which is the same as when the protrusions 15 are provided on the external electrode 14.

  The protrusion 15 may be provided on both the center electrode 12 and the external electrode 14. In this case, each protrusion 15 in the center electrode 12 and the external electrode 14 is formed in a conical shape, for example. Further, the projection 15 on the center electrode 12 side and the corresponding projection 15 on the external electrode 14 side are provided on each electrode so that the respective top portions G face each other. That is, these protrusions 15 form a “pair”. The initial discharge 2a is likely to occur between the pair of protrusions 15 and 15. Therefore, it is easier to control the location where the initial discharge 2a occurs than when the protrusion 15 is provided only on one of the center electrode 12 and the external electrode 14.

  A modification of the holding unit 18 will be described. The holding unit 18 may be provided only around the irradiation point P of the laser beam 42. In this case, a counterbore hole (bottomed hole) matching the shape of the holding portion 18 is formed in the side surface 12b of the center electrode 12, and the holding portion 18 is installed in the hole. As compared with the case where the holding portion 18 is formed over the entire circumference of the side surface 12b, the installation of the holding portion 18 is simplified.

  The holding part 18 may be installed outside the coaxial electrode 10. “Outside” in this case means, for example, a space outside the region where the center of each external electrode 14 surrounds the center electrode 12. The holding unit 18 is configured, for example, as a container (not shown) that holds the plasma medium 6 or the plasma medium 6 itself, and between the two adjacent external electrodes 14 and between the external electrode 14 and the center electrode 12. A plasma medium 6 is supplied. The supply points of the plasma medium 6 to the coaxial electrode 10 are desirably distributed symmetrically with respect to the center electrode 12. Accordingly, it is desirable that a plurality of holding portions 18 are provided around the coaxial electrode 10 and positioned at a point-symmetrical or rotationally symmetric position around the central electrode 12. However, the installation location of the holding part 18 is not limited to these. In any case, the surface of the plasma medium 6 including the irradiation point P of the laser beam 42 faces the space between the external electrode 14 and the center electrode 12 or the center electrode 12. Further, the position of each protrusion 15 is set with reference to a plane orthogonal to the center electrode 12 (center axis Z) and including the irradiation point P.

  The present invention is not limited to the above-described embodiment, but is shown by the description of the scope of claims, and further includes all modifications within the meaning and scope equivalent to the description of the scope of claims.

DESCRIPTION OF SYMBOLS 1 ... Symmetry surface, 2a ... Initial discharge, 2b ... Planar discharge, 3 ... Plasma, 6 ... Plasma medium, 8 ... Plasma light, 10 ... Coaxial electrode, 12 ... Center electrode, 12a ... Tip part, 12b ... Side surface, 12c ... channel, 14 ... external electrode, 14A ... external electrode, 14B ... external electrode, 15 ... projection, 15A ... projection, 15B ... projection, 16 ... insulator, 18 ... holding part, 20 ... reservoir, 20a ... Space, 22 ... Heater, 30 ... Voltage application device, 32 ... High-voltage power supply, 34 ... Energy storage circuit, 36 ... Discharge current blocking circuit, 40 ... Laser device, 42 ... Laser light, P ... Irradiation point, G ... Top, GA ... top, GB ... top

Claims (2)

A center electrode extending on a single axis and a plurality of external electrodes arranged so as to surround the outer periphery of the center electrode are arranged opposite to each other across a plane of symmetry, and generates plasma that emits extreme ultraviolet light A pair of coaxial electrodes for confining the plasma;
A holding unit for holding the plasma medium;
A voltage applying device for applying a discharge voltage to each of the coaxial electrodes;
A laser device that irradiates an irradiation point in the holding unit with laser light for performing ablation of the medium;
One or both of the plurality of external electrodes and the center electrode includes a protrusion protruding toward the other of the plurality of external electrodes and the center electrode,
The plasma light source, wherein the protrusion is located closer to the symmetry plane as the distance from the irradiation point increases. For one protrusion, the total time of the average movement time of the medium from the irradiation point to the protrusion and the average movement time of the initial discharge of the medium from the protrusion to the tip of the external electrode was assumed. If
2. The plasma light source according to claim 1, wherein the protrusions are offset from each other along the axis so that the total time with respect to the corresponding external electrode is equal to each other.