JP6822098B2 - How to adjust ablation - Google Patents

Description

The present invention relates to a method for adjusting ablation.

Patent Document 1 describes a plasma light source that generates extreme ultraviolet light. This plasma light source generates plasma by supplying steam of a plasma medium between the center electrode and a plurality of external electrodes in a state where a voltage is applied between the center electrode and the external electrode.

Japanese Unexamined Patent Publication No. 2013-254693

In the plasma light source described in Patent Document 1, high-temperature and high-density plasma is generated by converging the plasma generated between the center electrode and each external electrode. Ablation gas (steam of plasma medium) for generating plasma is generated by irradiating a solid or liquid plasma medium with laser light to supply energy and ablating (evaporating) the plasma medium. In such a plasma light source, if the distribution of ablation gas between the center electrode and each external electrode is uneven, the timing of plasma generation between the center electrode and each external electrode may be different. is there. As a result, the plasma cannot be converged at the same time, and the output of the plasma light source may decrease.

An object of the present invention is to provide a method for adjusting ablation that can make the timing of plasma generation uniform between the center electrode and each external electrode.

The method for adjusting ablation according to one aspect of the present invention has a first center electrode and a plurality of first external electrodes arranged so as to be separated from the first center electrode, and generates plasma that emits extreme ultraviolet light. In a plasma light source including a first coaxial electrode for making the electrode and a plasma medium for emitting an ablation gas for generating plasma by being irradiated with a laser beam, the first center electrode and the respective first external electrodes are provided. An ablation gas is supplied between the first center electrode and each of the first external electrodes by irradiating the plasma medium with a pulse of laser light in a voltage application step of applying a voltage between them, and the first center electrode and each of them are used. In the plasma generation step of generating plasma between the first external electrode and the measurement step of measuring the current flowing between the first center electrode and each of the first external electrodes in the plasma generation step, and in the measurement step. By changing the irradiation condition of the laser beam to the plasma medium when the difference between the rising times of the plurality of currents measured and the threshold is larger than the difference between the rising times of the plurality of currents and the threshold. It is seen including an adjustment step of adjusting the distribution of ablation gas and, in the adjustment step adjusts the distribution of ablation gas by changing the angle of the irradiated surface of the plasma medium that is irradiated with the laser light.

In this ablation adjustment method, when the difference in the rise time of the current flowing between the first center electrode and each first external electrode is larger than the threshold value by the adjustment step of adjusting the distribution of the ablation gas, the difference with respect to the plasma medium is obtained. Change the irradiation conditions of the laser beam. The rise time of each current, that is, the timing at which plasma is generated, changes depending on the concentration of the ablation gas. Therefore, by adjusting the distribution of the ablation gas, it is possible to make the timing of plasma generation uniform between the first center electrode and each of the first external electrodes. Further, the ablation gas is emitted centering on the direction perpendicular to the irradiation surface of the plasma medium. According to the above method, the direction in which the ablation gas is discharged can be changed, so that the distribution of the ablation gas between the first center electrode and each of the first external electrodes can be adjusted. When the second coaxial electrode is provided, it is possible to adjust the distribution of the ablation gas between the second center electrode and each of the second external electrodes.

In some embodiments, the plasma light source is arranged on the axis so as to face the first coaxial electrode, with a second center electrode and a plurality of second external electrodes arranged apart from the second center electrode. A second coaxial electrode that generates plasma that emits extreme ultraviolet light is further provided, and in the voltage application step, between the first center electrode and each of the first external electrodes, and with the second center electrode. A voltage is applied between each of the second external electrodes, and in the plasma generation step, between the first center electrode and each of the first external electrodes, and between the second center electrode and each of the second external electrodes. An ablation gas is supplied between them to generate plasma between the first center electrode and each of the first external electrodes, and between the second center electrode and each of the second external electrodes. The current flowing between the 1 center electrode and each of the first external electrodes and between the second center electrode and each of the second external electrodes may be measured. According to this method, the difference and the threshold value of the rise times of a plurality of currents flowing between the first center electrode and each of the first external electrodes and between the second center electrode and each of the second external electrodes. Is compared, and the distribution of the ablation gas is adjusted. Therefore, it is possible to make the timing of plasma generation uniform between the first coaxial electrode and the second coaxial electrode.

In some embodiments, the plasma light source further comprises a light reflecting section that is arranged on the optical path of the laser beam to reflect the laser beam, and in the adjustment step, ablation is performed by changing the irradiation position of the laser beam by the light reflecting section. The distribution of the gas may be adjusted. According to this method, the position where the ablation gas is discharged can be changed, so that the distribution of the ablation gas between the first center electrode and each of the first external electrodes can be adjusted. When the second coaxial electrode is provided, it is possible to adjust the distribution of the ablation gas between the second center electrode and each of the second external electrodes.

In some embodiments, the adjustment step may adjust the distribution of the ablation gas by varying the intensity of the laser beam. According to this method, since the energy density of the laser beam is changed, the amount of ablation gas generated can be changed. Therefore, it is possible to adjust the distribution of the ablation gas between the first center electrode and each first external electrode. When the second coaxial electrode is provided, it is possible to adjust the distribution of the ablation gas between the second center electrode and each of the second external electrodes.

According to the present invention, there is provided a method for adjusting ablation that can make the timing of plasma generation uniform between the center electrode and each external electrode.

It is a figure which shows typically an example of the plasma light source which can carry out the ablation adjustment method which concerns on one Embodiment of this invention. It is a figure which shows the structure of the coaxial electrode shown in FIG. 1 and an optical path control part. It is a figure which shows the change of the irradiation position of a laser beam in a light reflection part. It is a figure for demonstrating operation of a plasma light source. It is a flow figure which shows the adjustment method of ablation which concerns on one Embodiment of this invention. It is a figure which shows an example of each current measured before the adjustment process. It is a figure which shows an example of each current measured after the adjustment process. It is a figure which shows roughly a part of the plasma light source which can carry out the ablation adjustment method which concerns on 1st modification. It is a figure which shows roughly a part of the plasma light source which can carry out the ablation adjustment method which concerns on 2nd modification.

Hereinafter, various embodiments will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals, and duplicate description will be omitted.

With reference to FIG. 1, a plasma light source 1 capable of implementing the ablation adjustment method according to the embodiment of the present invention will be described. The plasma light source 1 shown in FIG. 1 employs a so-called opposed plasma focus method. The plasma focus method has a first coaxial shape having a first center electrode 11A (center electrode 11) and a plurality of first external electrodes 12A (external electrodes 12) arranged so as to be separated from the first center electrode 11A. A second coaxial electrode having an electrode 10A, a second center electrode 11B (center electrode 11), and a second external electrode 12B (external electrode 12) arranged so as to be separated from the second center electrode 11B. It is equipped with 10B. When the opposed plasma focus type plasma light source 1 operates, first, a high voltage is applied between the first center electrode 11A and the first external electrode 12A and between the second center electrode 11B and the second external electrode 12B. Is applied. Then, by inputting some kind of trigger, a ring-shaped initial plasma is generated between the first center electrode 11A and the first external electrode 12A and between the second center electrode 11B and the second external electrode 12B, respectively. Let me. The initial plasma converges at the tips of the first center electrode 11A and the second center electrode 11B to reach high temperature and high density states.

In the plasma light source 1 that employs the opposed plasma focus method, the first coaxial electrode 10A and the second coaxial electrode 10B as the plasma source are arranged so as to face each other. Then, by colliding the plasmas that have reached the tips of the respective center electrodes 11 (first center electrode 11A and second center electrode 11B) with each other, plasma in a high temperature and high density state is formed. Extreme ultraviolet light is generated from this high-temperature and high-density plasma.

One of the methods for generating the initial plasma is a method for ablating (evaporating) the plasma medium with a laser beam. In this method, a solid or liquid plasma medium is irradiated with a laser beam to evaporate the plasma medium and generate an ablation gas (medium vapor). An electric discharge is generated between the first center electrode 11A and the first external electrode 12A and between the second center electrode 11B and the second external electrode 12B via this ablation gas. Further, the plasma medium may be solid, liquid, or gas, and is selected according to the wavelength of light to be generated. In the opposed plasma focus method, liquid or solid lithium, tin, xenon or the like may be used as the plasma medium.

The plasma light source 1 is applied to, for example, an exposure apparatus for manufacturing a semiconductor element. The plasma light source 1 is configured to be capable of generating, 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 comprises a pair of coaxial electrodes 10 (first coaxial electrode 10A and second coaxial electrode 10B) that generate plasma, a voltage applying device 20 that causes a potential difference between the coaxial electrodes 10, and an ablation gas. The ablation gas forming unit 30 to be formed and the plasma medium supply unit 41 for supplying the plasma medium are provided.

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 symmetrically with respect to the virtual central surface P. A certain distance (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 (not shown) is connected to the exhaust pipe 3. The inside of the chamber 2 is maintained at a predetermined degree of vacuum. The chamber 2 is grounded. Each coaxial electrode 10 includes a center electrode 11, a plurality of external electrodes 12, and an insulator 13 that insulates the center electrode 11 and the plurality of external electrodes 12.

The center electrode 11 is a rod-shaped conductor extending along the axis A. In other words, in FIG. 1, the center electrode 11 of one coaxial electrode 10 extends toward the other coaxial electrode 10. The center electrode 11 is formed of a refractory metal such as tungsten (W) or molybdenum (Mo) that is not easily damaged by high temperature plasma. The axis A of the center electrode 11 is orthogonal to the above-mentioned central surface P. The end surface of the center electrode 11 facing the central surface P is, for example, hemispherical. The side surface of the center electrode 11 is, for example, conical.

The external electrode 12 of one coaxial electrode 10 is a rod-shaped conductor extending toward the other coaxial electrode 10. The external electrode 12 may 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 of the center electrode 11 so that the distance from the side surface of the conical center electrode 11 to the external electrode 12 is always constant. The external electrode 12 is formed of a refractory metal such as tungsten (W) or molybdenum (Mo) that is not easily damaged by high temperature plasma. The end surface of the external electrode 12 facing the central surface P may be a curved surface or a flat surface. In the other coaxial electrode 10, the center electrode 11 and the plurality of external electrodes 12 are configured in the same manner as the one coaxial electrode 10 described above.

As shown in FIG. 2, the external electrode 12 is arranged around the center electrode 11. 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 symmetrical) in the circumferential direction of the axis A. Each of the pair of coaxial electrodes 10 has six external electrodes 12. The six external electrodes 12 are arranged at intervals of 60 ° with respect to the axis A. The number of external electrodes 12 is not limited to 6, and may be appropriately set according to the size and shape of the center electrode 11 and the external electrode 12, the spacing between them, and the like. By arranging the plurality of external electrodes 12 around the center electrode 11, an initial discharge (for example, creepage discharge) is generated between the center electrode 11 and the external electrode 12. This initial discharge leads to a planar discharge 6 (see FIG. 4).

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

The voltage applying device 20 creates a potential difference by applying a discharge voltage having the same polarity or the opposite polarity to the coaxial electrode 10. The voltage application device 20 includes two high-voltage power supplies (HV Charging Devices) 21 and 22. The output side of the first high-voltage power supply 21 is connected to the center electrode 11 of the first coaxial electrode 10A. The common side of the first high-voltage power supply 21 is connected to the external electrode 12 corresponding to the center electrode 11. The first high voltage power supply 21 applies a positive discharge voltage higher than that of 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 second coaxial electrode 10B. The common side of the second high-voltage power supply 22 is connected to the external electrode 12 corresponding to the center electrode 11. The second high voltage power supply 22 applies a positive discharge voltage higher than that of the external electrode 12 corresponding to the center electrode 11. 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. The common side of any high-voltage power supply may be grounded. In the following description, the first high voltage power supply 21 is simply referred to as a power supply 21. The second high voltage power supply 22 is simply referred to as a power supply 22.

A line inductively coupled using a Rogowski coil or the like may be provided on the common side of the power supplies 21 and 22. These lines allow the current through the center electrode 11 (ie, all discharge currents) to 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. The energy storage circuit 26 includes a plurality of capacitors 26a that individually connect between the center electrode 11 and the external electrode 12. The capacitor 26a is connected to the output side and the common side of the power supplies 21 and 22. By providing a capacitor 26a for storing discharge energy for each external electrode 12, discharge can occur in all the external electrodes 12. That is, even if there is a slight deviation in the discharge generation timing, it is possible to prevent a large amount of discharge energy from being consumed by the first discharge. By providing the energy storage circuit 26, an ideal planar discharge 6 generated over the entire circumference of the center electrode 11 can be obtained in the coaxial electrode 10.

The plasma light source 1 further includes a discharge current blocking circuit 28 that prevents the discharge current from returning to the voltage applying device 20. The discharge current blocking circuit 28 includes an inductor 28a that connects the external electrode 12 and the voltage applying device 20 (specifically, the common side of the power supplies 21 and 22). Since the inductor 28a 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 thereof. As a result, it is possible to prevent the discharge energy stored in the capacitor 26a from being supplied to the external electrode 12 other than the external electrode 12 directly connected to the capacitor 26a. As a result, it is possible to prevent the distribution of discharge generation in the circumferential direction of the center electrode 11 from being biased.

The operation of the voltage application device 20 described above will be described. First, electric charges are stored (charged) in advance in the capacitor 26a by the power supplies 21 and 22. Then, the steam of the plasma medium M is supplied between the center electrode 11 and the external electrode 12, so that a current flows from the positive electrode side to the negative electrode side of the capacitor 26a. As for this current, a current corresponding to the amount of electric charge stored in the capacitor 26a flows in a pulsed manner according to the time constant of the electric circuit. That is, the electric charge flows 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 26a. While this current is flowing, the planar discharge 6 moves to the tip of the center electrode 11 and converges as a single plasma at the tip to emit light.

The ablation gas forming unit 30 has a laser device 31 that emits the laser light L, and provides the laser light L to the plasma medium supply unit 41. For example, the ablation gas forming unit 30 collects the laser beam L having a beam diameter of 30 mm and irradiates the plasma medium M (see FIG. 3). On the irradiation surface Ma (see FIG. 3) of the plasma medium M irradiated with the laser beam L, a part of the plasma medium M is emitted as vapor (ablation gas) of the plasma medium M by ablation. The ablation gas contains a neutral gas or ions. Further, the laser device 31 generates an initial discharge of plasma (that is, planar discharge 6: see part (b) of FIG. 7). The laser device 31 is, for example, a YAG laser, and outputs a fundamental wave or a double wave of the fundamental wave as a pulsed laser beam L for ablation.

The ablation gas forming unit 30 further includes an optical path control unit 32. As shown in FIG. 2, the optical path control unit 32 provides the laser light L to the plasma medium supply unit 41 by dividing the laser light L and changing the direction of the optical path of the laser light L. In the present embodiment, the plasma medium supply unit 41 is provided on the outer peripheral surface of the center electrode 11. The optical path control unit 32 includes a beam splitter 34 and a mirror 35 (light reflection unit). The beam splitter 34 is arranged on the optical path of the laser beam L emitted from the laser apparatus 31. The beam splitter 34 transmits a part of the incident laser beam L and reflects the rest in a predetermined direction corresponding to the incident direction. That is, the beam splitter 34 has an optical path parallel to the optical path direction of the incident laser beam L and an optical path in a direction corresponding to the incident angle of the laser beam L with respect to the optical path direction of the incident laser beam L. Form. Further, the mirror 35 reflects all of the incident laser light L in the direction corresponding to the incident direction. That is, the mirror 35 forms an optical path inclined by 90 ° with respect to the optical path direction of the incident laser beam L.

The arrangement of the beam splitter 34 and the mirror 35 shown in FIG. 2 is an example, and a necessary number of beam splitters 34 and mirrors 35 are used according to the positional relationship between the laser device 31 and the plasma medium supply unit 41. , It may be arranged so that a desired optical path is formed.

In addition to the beam splitter 34 and the mirror 35, the optical path control unit 32 further includes an attitude control unit 36 that controls the attitude of the mirror 35. Specifically, the attitude control unit 36 is a device that rotates the mirror 35 around a predetermined axis A1 and controls the angle of the mirror 35 with respect to the laser beam L incident on the mirror 35. As shown in FIG. 3, when the incident direction of the laser beam L is constant and the mirror 35 is rotated clockwise by a minute angle, the incident angle of the laser beam L on the mirror 35 changes. Then, the direction of the laser beam L reflected by the mirror 35 changes according to the incident angle of the laser beam L. In this case, the irradiation position of the laser beam L changes. By changing the angle of the mirror 35 in this way, the reflection direction of the laser beam L changes. When the reflection direction of the laser light L changes, the irradiation position of the laser light L on the plasma medium supply unit 41 changes.

At the time of irradiation with the laser beam L, the discharge voltage by the voltage applying device 20 has already been applied to the center electrode 11 and the external electrode 12 of the coaxial electrode 10. 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 (see FIG. 4).

The location where the discharge is generated may be limited to the irradiation region of the laser beam L and its vicinity. Therefore, it is preferable that a plurality of laser beams L are irradiated at the same time at intervals along the circumferential direction of the axis A, and the number of the laser beams L is at least two. This is based on the experimental result that the region of the induced discharge had an opening angle of 180 degrees or more with respect to the axis of the center electrode 11. Considering this result, it is desirable to irradiate the laser beam L at a position rotationally symmetric with respect to the center electrode 11 as the number of irradiation points decreases. Simultaneous irradiation of a plurality of laser beams L can be easily achieved by forming a plurality of optical paths having matching optical path lengths using optical elements such as a beam splitter and a mirror.

The plasma medium supply unit 41 supplies the plasma medium M used for generating plasma light. The plasma medium M can be selected depending on the required wavelength of ultraviolet light. For example, when ultraviolet light of 13.5 nm is required, lithium (Li), xenon (Xe), tin (Sn) or the like is used as the plasma medium M. When ultraviolet light of 6.7 nm is required, at least one of gadolinium (Gd), terbium (Tb) and the like is used as the plasma medium M.

Subsequently, the operation of the plasma light source will be described with reference to FIG. The part (a) in FIG. 4 is the state when the laser beam L is irradiated, the part (b) in FIG. 4 is the state when the planar discharge 6 is generated, and the portion (c) in FIG. 4 is the moving state of the planar discharge 6. The part (d) in FIG. 4 is the state in which the planar discharge 6 reaches the vicinity of the tip of the electrode, the part (e) in FIG. 4 is the state in the initial confinement of the plasma 7, and the part (f) in FIG. Shows the state of high temperature and high density plasma 7.

As shown in FIG. 4A, when the plasma medium M of the plasma medium supply unit 41 is irradiated with the laser light L while the discharge voltage is applied, an ablation gas is generated, and immediately after that, the center electrode 11 And a discharge is generated between the external electrodes 12. A discharge is generated between each of the plurality of external electrodes 12 and the center electrode 11. As a result, as shown in FIG. 4B, a planar discharge 6 distributed over the entire circumference of the center electrode 11 is obtained.

As shown in FIG. 4C, the planar discharge 6 moves in the direction of being discharged from the electrode (direction toward the central surface P) by the self-magnetic field. The shape of the planar discharge 6 at this time is substantially annular when viewed from the axis A.

Here, since the plasma light source 1 includes an energy storage circuit 26, the probability of occurrence of the planar discharge 6 is increased by the cooperation between the energy storage circuit 26 and the plurality of external electrodes 12. The plurality of external electrodes 12 arranged discontinuously at intervals facilitate the formation of the planar discharge 6 as compared with the case where a continuous tubular (cylindrical) external electrode is adopted. Is advantageous.

After that, as shown in FIG. 4D, 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 end face 11a. Due to this current transfer, the plasma containing lithium (Li) that has moved along with the pair of planar discharges 6 converges to a high density and high temperature.

Since this phenomenon proceeds at the coaxial electrodes 10 sandwiching the central surface 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 the intermediate position (that is, the position of the central surface P) where the coaxial electrodes 10 face each other under pressure from both directions along the axis A, and is a single plasma containing the plasma medium M as a component. 7 is formed.

As shown in FIG. 4 (e), even after the plasma 7 is formed, the current continues to flow through the planar discharge 6, surrounds the plasma 7 as a whole, and holds the plasma 7 near the middle of the center electrode 11. To do.

While the planar discharge 6 is generated, the density and temperature of the plasma 7 are increased, and the ions containing lithium (Li) are ionized. As a result, as shown in FIG. 4 (f), plasma light 9 including extreme ultraviolet light is emitted from plasma 7. In this state, the plasma light 9 can be generated for a long time by continuing the current flowing through the planar discharge 6.

Here, a method of adjusting ablation according to an embodiment of the present invention will be described with reference to FIG. This ablation adjustment method is performed in the trial run stage before the start of operation of the plasma light source 1. As shown in FIG. 5, in this ablation adjustment method, the plasma light source 1 is first started up (step S1). In step S1, preparations for generating plasma are made, such as creating a vacuum inside the chamber 2. Next, a voltage is applied between the center electrode 11 and each of the external electrodes 12 (step S2, voltage application step). In step S2, the voltage application device 20 applies a voltage to each of the first coaxial electrode 10A and the second coaxial electrode 10B.

Subsequently, plasma (initial plasma) is generated between the center electrode and each external electrode (step S3, plasma generation step). In step S3, the plasma medium M is irradiated with the laser beam L in pulses to ablate the plasma medium M. As a result, an ablation gas is supplied between the center electrode 11 and the external electrode 12, and plasma (initial plasma) is temporarily generated between the center electrode 11 and the external electrode 12. When plasma is generated in this way, a current flows between the center electrode 11 and the external electrode 12.

Next, in step S3, the current flowing between the center electrode 11 and each of the external electrodes 12 is measured (step S4, measurement step). The plurality of currents can be measured, for example, by providing an inductively coupled line to each of the external electrodes 12 using a Rogowski coil or the like and detecting the current flowing through this line with an oscilloscope or the like.

Next, the difference between the rise times of the plurality of currents measured in step S4 and the threshold value are compared (step S5, comparison step). Here, the "rise time" of the current means the time from the irradiation of the laser beam L until the current exceeds a certain value. The "constant value" can be set to, for example, about 1/10 to 1/20 of the peak value of the current. Further, the "difference in rise time" of the plurality of currents can be, for example, the difference between the latest rise time and the earliest rise time of the measured rise times of the plurality of currents. The threshold value can be, for example, 1/10 or less of the time (plasma arrival time) until the plasma (initial plasma) generated in step S3 moves to the tip of the center electrode 11. The time until the plasma moves to the tip of the center electrode 11 depends on the amount of current and the length of the center electrode 11. When the plasma light source 1 is a opposed plasma focus type having a first coaxial electrode 10A and a second coaxial electrode 10B as in the present embodiment, the first center electrode 11A and the first external electrode 12A are combined. The "difference in rise time" can be obtained from the total of 12 currents flowing between the second center electrode 11B and the second external electrode 12B. Further, in each of the first coaxial electrode 10A and the second coaxial electrode 10B, the "difference in rise time" may be obtained from a total of six currents.

In step S5, when the difference between the rise times of the plurality of currents is smaller than the threshold value, the plasma medium M is continuously irradiated with the laser beam L, and the operation of the plasma light source 1 is started (step S6). On the contrary, in step S5, when the difference between the rise times of the plurality of currents is larger than the threshold value, the distribution of the ablation gas is adjusted (step S7, adjustment step).

In this adjustment step, the distribution of the ablation gas is adjusted by changing the irradiation conditions of the laser beam L to the plasma medium M. More specifically, the attitude control unit 36 changes the angle of the mirror 35 (see FIG. 3) to change the irradiation position of the laser beam L with respect to the plasma medium M. As a result, the position where the ablation gas is discharged can be changed. For example, since the rise time of the current is expected to be slow in the portion where the concentration of the ablation gas is low, it is possible to adjust so that the laser beam L is irradiated near the external electrode 12 having a slow rise time of the current. The irradiation position of the laser beam L may be changed by moving the mirror 35 on the optical path of the laser beam L without changing the angle of the mirror 35.

By adjusting the distribution of the ablation gas in this way, it is possible to reduce the difference in the rise time of a plurality of currents. FIG. 6 is a diagram showing an example of each current measured before the adjustment step (step S7). Further, FIG. 7 is a diagram showing an example of each current measured after the adjustment step (step S7). Note that, in FIGS. 6 and 7, only four currents are shown for the sake of simplicity. As shown in FIGS. 6 and 7, by adjusting the distribution of the ablation gas in step S7, it is possible to reduce the difference between the rise times of the four currents.

After adjusting the distribution of the ablation gas in step S7, steps S3 to S5 are executed again. In this way, steps S7, step S3, step S4, and step S5 are repeatedly executed until the difference between the rise times of the plurality of currents becomes smaller than the threshold value. As a result, in step S6, which is a step of starting operation, the timing of plasma generation between the center electrode 11 and each of the external electrodes 12 can be made uniform, so that high-temperature and high-density plasma 7 can be generated. Can be done.

As described above, in the ablation adjusting method according to the embodiment of the present invention, the first center electrode 11A and the respective first external electrodes 12A are connected by the step S7 (adjusting step) for adjusting the distribution of the ablation gas. When the difference between the rise times of the plurality of currents flowing between them is larger than the threshold value, the irradiation conditions of the laser beam L to the plasma medium M are changed. The rise time of each current, that is, the timing at which plasma is generated, changes depending on the concentration of the ablation gas. Therefore, by adjusting the distribution of the ablation gas, it is possible to make the timing of plasma generation uniform between the first center electrode 11A and the respective first external electrodes 12A.

Further, in step S2 (voltage application step), a voltage is applied between the first center electrode 11A and each of the first external electrodes 12A, and between the second center electrode 11B and each of the second external electrodes 12B. To do. In step S3 (plasma generation step), ablation gas is supplied between the first center electrode 11A and each of the first external electrodes 12A, and between the second center electrode 11B and each of the second external electrodes 12B. , Plasma is generated between the first center electrode 11A and each of the first external electrodes 12A, and between the second center electrode 11B and each of the second external electrodes 12B. In step S4 (measurement step), the currents flowing between the first center electrode 11A and each of the first external electrodes 12A and between the second center electrode 11B and each of the second external electrodes 12B are measured. .. As a result, the difference and the threshold value of the rise time of a plurality of currents flowing between the first center electrode 11A and each of the first external electrodes 12A and between the second center electrode 11B and each of the second external electrodes 12B. The distribution of the ablation gas is adjusted by comparing with. Therefore, it is possible to make the timing of plasma generation uniform between the first coaxial electrode 10A and the second coaxial electrode 10B.

The plasma light source 1 includes a mirror 35 that is arranged on the optical path of the laser light L and reflects the laser light L. In step S7 (adjustment step), the ablation gas is changed by changing the irradiation position of the laser light L by the mirror 35. Adjust the distribution of. As a result, the position where the ablation gas is discharged can be changed, so that the distribution of the ablation gas between the first center electrode 11A and the respective first external electrodes 12A can be adjusted. Further, it is possible to adjust the distribution of the ablation gas between the second center electrode 11B and each of the second external electrodes 12B.

Next, a first modification of the ablation adjustment method will be described. FIG. 8 is a diagram schematically showing a part of a plasma light source in which the ablation adjustment method according to the first modification can be implemented. In the plasma light source 1, one plasma medium supply unit 41 is provided on the outer peripheral surface of each center electrode 11, whereas as shown in FIG. 8, a method for adjusting ablation according to the first modification is used. The feasible plasma light source 1B has two plasma medium supply units 41 for one coaxial electrode 10, and the plasma medium supply unit 41 is provided outside the coaxial electrode 10. 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. 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 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 on a virtual circumference line centered on the axis A. Further, the plasma medium supply unit 41 is arranged between the tip of the center electrode 11 and the insulator 13 in the direction of the axis A. The pair of plasma medium supply units 41 are configured to be rotatable about the axis A41, respectively.

When adjusting the distribution of the ablation gas in the plasma light source 1B having the above configuration, the angle of the plasma medium supply unit 41 is changed in step S7. As a result, the angle of the irradiation surface Ma of the plasma medium M to which the laser beam L is irradiated is changed. The ablation gas is emitted centering on the direction perpendicular to the irradiation surface of the plasma medium. According to this method, the direction in which the ablation gas is discharged can be changed, so that the distribution of the ablation gas between the center electrode 11 and each of the external electrodes 12 can be adjusted. That is, the "irradiation condition" in this modification is the angle of the irradiation surface Ma of the plasma medium M.

Next, a second modification of the ablation adjustment method will be described. FIG. 9 is a diagram schematically showing a part of a plasma light source capable of carrying out the ablation adjustment method according to the second modification. As shown in FIG. 9, the ablation gas forming unit 30 of the plasma light source 1C capable of carrying out the ablation adjusting method according to the second modification has an energy control unit 33.

The energy control unit 33 can adjust the energy supplied to the plasma medium M by changing the irradiation conditions of the laser beam L to the plasma medium M. As shown in FIG. 9, the energy control unit 33 has a focus control unit 37. The focus control unit 37 is arranged between the optical path control unit 32 and the plasma medium supply unit 41, and can adjust the condensing position of the laser beam L with respect to the plasma medium M. The focus control unit 37 has a lens 37a and a lens drive unit 37b. The lens 37a is arranged on the optical path between the mirror 35 of the optical path control unit 32 and the plasma medium supply unit 41, and converges the laser beam L to the focal position FP. The focal length of the lens 37a is, for example, 300 mm. The lens driving unit 37b reciprocates the lens 37a along the direction of the optical axis A2 of the lens 37a. The energy control unit 33 may adjust the energy supplied to the plasma medium M by controlling the output of the laser device that emits the laser beam L.

When adjusting the distribution of the ablation gas in the plasma light source 1B having the above configuration, the focusing position of the laser beam L is changed in step S7. As a result, the intensity of the laser beam L is changed and the distribution of the ablation gas is adjusted. According to this method, since the energy density of the laser beam L irradiated to the plasma medium M changes, the amount of ablation gas generated can be changed. More specifically, when the focal position FP of the laser beam L is on the irradiation surface Ma of the plasma medium M, the energy density of the laser beam L is the highest. Therefore, the focal position FP of the laser beam L is set to the plasma medium M. The concentration of the ablation gas can be reduced by shifting it from the irradiation surface Ma, that is, by defocusing. Therefore, it is possible to adjust the distribution of the ablation gas between the center electrode 11 and each of the external electrodes 12. That is, the "irradiation condition" in this modification is the energy supplied to the plasma medium M.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be adopted. For example, in the above embodiment, a plurality of currents (12 currents) flowing between the center electrode 11 and each of the external electrodes 12 are measured with respect to the pair of coaxial electrodes 10 to adjust the distribution of the ablation gas. However, for only one of the coaxial electrodes 10, a plurality of currents (six currents) flowing between the center electrode 11 and the respective external electrodes 12 were measured, and the distribution of the ablation gas was adjusted. May be good.

Further, in the above embodiment, as the irradiation conditions in step S7, when the irradiation position of the laser beam L is changed, when the angle of the irradiation surface Ma of the plasma medium M is changed, the intensity of the laser beam L is changed. Although each case has been described, the distribution of the ablation gas may be adjusted by combining these cases. For example, when the plasma light source 1 further includes an energy control unit 33, the distribution of the ablation gas may be adjusted by changing the irradiation position of the laser beam L and the intensity of the laser beam L in step S7. ..

1, 1B, 1C Plasma light source 2 Chamber 3 Exhaust pipe 6 Planar discharge 7 Plasma 9 Plasma light 10 Coaxial electrode 10A 1st coaxial electrode 10B 2nd coaxial electrode 11 Center electrode 11A 1st center electrode 11B 2nd center electrode 11a End face 11b Side surface 12 External electrode 12A First external electrode 12B Second external electrode 13 Insulator 20 Voltage application device 21, 22 High-voltage power supply (power supply)
26 Energy storage circuit 26a Capacitor 28 Discharge current blocking circuit 28a Inductor 30 Ablation gas forming unit 31 Laser device 32 Optical path control unit 33 Energy control unit 34 Beam splitter 35 Mirror (light reflecting unit)
36 Attitude control unit 37 Focus control unit 37a Lens 37b Lens drive unit 41 Plasma medium supply unit FP Focus position L Laser light M Plasma medium Ma Irradiation surface P Central surface

Claims (4)

A first coaxial electrode having a first center electrode and a plurality of first external electrodes arranged so as to be separated from the first center electrode and generating plasma that emits extreme ultraviolet light, and laser light are irradiated. In a plasma light source including a plasma medium that emits an ablation gas for generating the plasma by being generated.
A voltage application step of applying a voltage between the first center electrode and each of the first external electrodes,
By irradiating the plasma medium with the laser beam in a pulse, the ablation gas is supplied between the first center electrode and each of the first external electrodes, and the first center electrode and each of the first external electrodes are supplied. A plasma generation step of generating the plasma between the electrodes and
In the plasma generation step, a measurement step of measuring the current flowing between the first center electrode and each of the first external electrodes, and a measurement step.
A comparison step of comparing the difference between the rise times of a plurality of currents measured in the measurement step and the threshold value, and a comparison step.
If the difference between the rise time of a plurality of said current is greater than the threshold value, by changing the irradiation conditions of the laser beam with respect to the plasma medium, viewed including the adjusting step, the adjusting the distribution of the ablation gases,
In the adjustment step, an ablation adjustment method for adjusting the distribution of the ablation gas by changing the angle of the irradiation surface of the plasma medium irradiated with the laser beam . The plasma light source has a plurality of second external electrodes arranged so as to face the first coaxial electrode on the axis and separated from the second center electrode. A second coaxial electrode that generates the plasma that emits the extreme ultraviolet light is further provided.
In the voltage application step, a voltage is applied between the first center electrode and each of the first external electrodes, and between the second center electrode and each of the second external electrodes.
In the plasma generation step, the ablation gas is supplied between the first center electrode and each of the first external electrodes, and between the second center electrode and each of the second external electrodes. The plasma is generated between the first center electrode and each of the first external electrodes, and between the second center electrode and each of the second external electrodes.
The measurement step is claimed to measure the current flowing between the first center electrode and each of the first external electrodes and between the second center electrode and each of the second external electrodes. The method for adjusting ablation according to 1. The plasma light source further includes a light reflecting portion that is arranged on the optical path of the laser light and reflects the laser light.
The method for adjusting ablation according to claim 1 or 2, wherein in the adjusting step, the distribution of the ablation gas is adjusted by changing the irradiation position of the laser beam by the light reflecting portion. The method for adjusting ablation according to any one of claims 1 to 3 , wherein in the adjusting step, the distribution of the ablation gas is adjusted by changing the intensity of the laser beam.

Images