TARGET MATERIAL, HIGH-BRIGHTNESS EUV SOURCE AND METHOD FOR GENERATING EUV RADIATION

Abstract

The invention relates to plasma source comprising a target material to produce the plasma emitting EUV radiation. The target material comprises a Li-based alloy with at least one further element selected from the group consisting of Ag, Au, Bi, Ba, Sr. The alloy is configured to increase the density of the target material by more than three times compared to the density of Li. As a result, compared with the Li target, the velocity of the droplet fraction of debris particles may be sharply reduced, which makes it possible to control the direction of its exit from the plasma due to the high velocity of the target. The plasma source is preferably a laser-produced plasma light source with a fast rotating target (at least 100 m/s). The target material may allow the creation of compact low-debris EUV light sources with high spectral brightness designed for a wide range of applications.

Claims

1.-19. (canceled)

20. A plasma source of EUV radiation, wherein the plasma source is configured to produce a plasma as either a laser produced plasma or a laser-initiated discharge produced plasma, the plasma source comprising a target material to produce the plasma, wherein the target material comprises a lithium, Li, -based composition with at least one further element, wherein the composition is an alloy and wherein the at least one further element is selected from the group consisting of Ag, Au, Bi, Ba, Sr, wherein a type and an amount of the at least one further element in the composition is configured to increase a density of the target material by more than three times compared to a density of Li.

21. The plasma light source according to claim 20, wherein the composition comprises an eutectic alloy.

22. The plasma light source according to claim 20, wherein an atomic percentage of Li in the target material is in a range from 60% to 90%.

23. The plasma light source according to claim 20, wherein an atomic percentage of the at least one further element in the target material is in a range from 10% to 40%, preferably from 15% to 30%, wherein preferably, a sum of an atomic percentage of Li and of the at least one further element in the target material amount to about 100%.

24. The plasma source according to claim 20, wherein the plasma source is configured such that a speed of a droplet fraction of debris particles ejected from the plasma of the target material is less, preferably multiple times less, more preferably about an order of magnitude less, compared to a speed of a droplet fraction of debris particles ejected from a plasma of a lithium target.

25. The plasma source according to claim 20, in which a speed of a target (3) including the target material is greater or equal to an average speed of a droplet fraction of debris particles ejected from the plasma.

26. A laser produced plasma EUV source comprising: a vacuum chamber (1), a rotating target assembly (2) configured to supply a target (3) into an interaction zone (4) with a laser beam (5) focused onto the target (3), wherein the target (3) is a layer of molten target material on a surface of an annular groove implemented in the rotating target assembly (2) with a target surface facing a rotation axis (6) of the rotating target assembly (2), wherein the laser produced plasma EUV source is configured to pass an EUV radiation beam (8) exiting the interaction zone (4), and means for debris mitigation (10, 11, 12, 13, 14, 15), wherein the laser produced plasma EUV source comprises a plasma source according to claim 1, and a linear velocity of the target is preferably not less than 100 m/s.

27. The laser produced plasma EUV source according to claim 26, wherein a spectral purity filter is installed in a way of the EUV radiation beam.

28. The laser produced plasma EUV source according to claim 26, further comprising a spectral purity filter selected from a group comprising: a reflective filter in a form of a multilayer Mo/Si mirror (9), foil containing zirconium or beryllium.

29. The laser produced plasma EUV source according to claim 26, wherein the means of debris mitigation is provided by one or more debris mitigation techniques comprising: a protective gas flow, a magnetic mitigation, a foil trap, a debris shield, a membrane mostly transparent for EUV radiation.

30. A method for generating extreme ultraviolet, EUV, radiation, comprising: generating a radiation beam having a wavelength in an EUV range by means of a plasma source according to claim 20.

31. The method according to claim 30, further comprising: forming under an action of a centrifugal force a target (3) as a layer of the target material in a molten state on a surface of an annular groove that is implemented in a rotating target assembly (2) with a target surface facing a rotation axis (6) of the rotating target assembly (2); irradiating the target (3) by a focused laser beam (5); generating a laser produced plasma in an interaction zone (4); and outputting an EUV radiation beam (8) through means for debris mitigation (10, 11, 12, 13, 14, 15), wherein the target (3) is preferably rotated at a high linear velocity, not less than 100 m/s.

32. The method according to claim 31, wherein a centrifugal acceleration of the target (3) is at least 10000 g, where g is an acceleration of gravity.

33. The method according to claim 31, wherein a spatial distribution of a debris ejection rate from the interaction zone (4) is calculated and directions of a passage of both focused laser beam (5) and the EUV radiation beam (8) are selected in spatial regions with minimal debris ejection rates.

34. The method according to claim 31, wherein the debris mitigation is provided by one or more debris mitigation techniques comprising: a protective gas flow (13), a magnetic mitigation (14), a foil trap, a debris shield (12), a membrane mostly transparent for EUV radiation (15) with a transparency of more than 60%.

35. The method according to claim 31, wherein in the EUV radiation beam (8) a spectral filtering of narrow-band radiation is provided at a transition of ionized Li.sup.2+ with a wavelength of 13.5 nm.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0047] An exemplary implementation of the invention is illustrated by the drawings, in which:

[0048] FIG. 1 is a schematic diagram of a high brightness LPP EUV light source with fast rotating target in accordance with an embodiment,

[0049] FIG. 2 is a simplified schematic of a high brightness LPP EUV light source in accordance with the embodiment,

[0050] FIG. 3A and FIG. 3Bshow diagrams that illustrate the results of calculating the spatial distribution of droplet ejection from the interaction zone for target materials of Li and composition with atomic fractions of Li (80%) and Ag (20%),

[0051] FIG. 4A and FIG. 4B show SEM images, demonstrating achievement of debris mitigation effect by replacing the Li target material with the composition 80% Li+20% Ag in the high brightness LPP EUV light source,

[0052] FIG. 5A and FIG. 5B show spectra of the EUV radiation source for the target material of Li and the composition of 80% Li+20% Ag,

[0053] FIG. 6 shows a spectral reflectivity curve of the Mo/Si mirror used as a reflective filter,

[0054] FIG. 7A and FIG. 7B show radiation spectra after reflection from a Mo/Si mirror for a target material of Li and a composition of 80% Li+20% Ag,

[0055] FIG. 8A, FIG. 8B, FIG. 8C and FIG. 8D show simplified schematics of the EUV light source in accordance with the embodiments of the invention.

[0056] In the drawings, the matching elements of the device have the same reference numbers.

[0057] These drawings do not cover and, moreover, do not limit the entire scope of the options for implementing this technical solution, but represent only illustrative material of a particular case of its implementation.

DETAILED DESCRIPTION OF THE INVENTION

[0058] According to an exemplary embodiment of the invention illustrated in FIG. 1, the high-brightness source of short-wavelength radiation contains a vacuum chamber 1 with a rotating target assembly 2, which supplies a target 3 to the interaction zone 4 where the target 3 interacts with a focused laser beam 5. A part of the rotating target assembly 2 is made in the form of a disk fixed to the rotation shaft. Said disk has a peripheral portion in the form of an annular barrier with the annular groove facing the rotation axis 6. The target 3 is a layer of molten metal formed by centrifugal force on the surface of the annular groove of the rotating target assembly 2. The target 3 may employ the target material in any of the configurations disclosed herein.

[0059] Material of target is melted and maintained in a given optimal temperature range using a heating system 7.

[0060] The annular groove configuration prevents material of the target 3 from being ejected in the radial direction and in both directions along the rotation axis 6, if the target material volume does not exceed the groove's volume.

[0061] In the interaction zone 4, under the action of a focused laser beam 5, a pulsed high-temperature plasma of the target material is generated. The plasma generates radiation in the EUV spectral range. Generated EUV radiation leaves the interaction zone 4 in the form of a divergent beam of EUV radiation 8.

[0062] In the embodiment of the invention, the beam of EUV radiation is directed at the optical collector 9 in the form of a Mo/Si mirror with a maximum reflection at a wavelength of 13.5 nm. In other cases, a collector based on glazing incidence mirrors or optical elements in the form of Fresnel zone plates can be used.

[0063] On the paths of the focused laser beam 5 and the beam of EUV radiation 8, there are means for debris mitigation, provided by one or more techniques comprising: [0064] casings 10, 11 surrounding the beams 5, 8 of laser and EUV radiation; [0065] protective gas flows directed through gas inlets 13 in the casings 10, 11 to mitigate the vapor fraction of debris particles in the path of laser and EUV radiation beams; [0066] debris shield 12, separated from the rotating target assembly 2 by slit gaps, having only two small holes (for the input of a focused laser beam and for the output of a beam of EUV radiation), through which debris particles can leave the target assembly; [0067] foil trap (not shown), highly transparent for plasma radiation which is essentially a system of plates oriented in radial directions with respect to the plasma, providing a sufficiently effective trapping of neutral atoms and clusters of the target material; [0068] a magnetic field preferably generated by permanent magnets to mitigate the charged fraction of debris particles; and [0069] a preferably replaceable membrane 15 essentially transparent for short-wavelength radiation and impermeable for debris and gas.

[0070] The membrane installed on the path of the EUV radiation beam is preferably made of a material belonging to a group which includes carbon nanotubes (CNT), Ti, Al, Si, ZrSi, BN.

[0071] Similar means for debris mitigation are placed in the propagation path of the focused laser beam 5.

[0072] FIG. 2 shows a simplified diagram of the above-described EUV light source, for which computational modeling of spatial distribution of debris ejection from the interaction zone 4 performed using the RZLINE code which is created for applications in the field of radiation hydrodynamics of dense hot plasma. The code uses mathematical models based on years of experimental and theoretical work as, for example, it is known from the publication K. Koshelev, V. Ivanov, V. Medvedev, et al., Return-to-zero line code modeling of distributed tin targets for laser-produced plasma sources of extreme ultraviolet radiation, Journal of Micro/Nanolithography, MEMS, and MOEMS Vol. 11, Issue 2 (May 2012). The code allows modeling the interaction of laser radiation with gases, liquids and solid surfaces with subsequent generation of plasma, as well as interaction with the plasma itself.

[0073] In FIG. 3 for Li and Li/Ag target materials are shown maps of the spatial distribution of ejection rates of debris particles (particles of all fractions of all velocities are taken into account) in experimental coordinates, in which is the angle to rotation axis, is the azimuthal angle lying in the plane of the figure. The origin of coordinates is in the interaction zone. Typical directions in the interaction zone are as follows: [0074] Iparallel to the rotation axis: =0, any, [0075] IIalong target velocity: =90, =0, [0076] IIInormal to the target surface: =90, =90, [0077] IVagainst target velocity: =90, =180.

[0078] Spatial distributions of the debris ejection rate shown in FIG. 3 were calculated in nm/(month-W) as a specific growth rate of the film thickness of deposited debris particles on a surface of the exposed sample located at a distance of 40 cm from the interaction zone per unit of laser power with the EUV source operating 24/7.

[0079] This distribution was obtained for typical values of the light source parameters: the laser radiation wavelength is 1-2 m, the laser pulse energy is several mJ at pulse duration of several ns, the focal spot diameter is several tens of m and the target linear velocity is 200 m/s. Apart from the fast target rotation no other debris mitigation techniques were used.

[0080] As illustrated in FIG. 3, the mass of debris particles is mainly concentrated in the sector along the direction of the target velocity limited by the azimuth angles of 0-80 and by the polar angles of 0-90. Maximum debris ejection rate along the direction of the target rotation is 107 nm/(month-W).

[0081] In FIG. 3 ovals are used to indicate the spatial regions selected in accordance with embodiments with minimal levels of debris ejection rates, in which the cones of laser and EUV radiation beams are located.

[0082] Calculation results for target materials from Li, FIG. 3A, and compositions of 80% Li and 20% Ag (in atomic fractions), FIG. 3B, show the following. The increase in the density of the target material (in this case by 4.7 times compared to lithium) makes it possible to sharply (by more than an order of magnitude) reduce the ejection rate of debris particles into the spatial regions of laser and EUV beams.

[0083] The achievement of this positive effect of the invention is confirmed experimentally using the source schematically shown in FIG. 1, in which the test sample was placed instead of the membrane 15.

[0084] FIG. 4 shows scanning electron microscope (SEM) images of the witness samples obtained for target materials from Li, FIG. 4A, and compositions of 80% Li and 20% Ag, FIG. 4B. Frame size of SEM images is 125 m. Tests were carried out under the following conditions: [0085] linear velocity of the target is 150 m/s, centrifugal acceleration is 23000 g, [0086] laser beam energy is 3.3 mJ/pulse on the target, focal spot size120 m at 1/e2 intensity level, pulse repetition rate25 kHz, [0087] exposure time of the witness samples is 2.5 hours (FIG. 4A) and 10 hours (FIG. 4B).
It can be seen that an increase in the density of the target material in accordance with the invention leads to a sharp debris mitigation effect.

[0088] Based on the tests performed, it was shown that relatively large droplets larger than 300 nm, which can penetrate through the CNT membrane, are completely suppressed. Thus, the replacement of the test sample with a replaceable membrane, in particular made of carbon nanotubes, allows for ultra-high purity of a high-brightness monochromatic radiation source at 13.5 nm, made in accordance with the present invention.

[0089] FIG. 5A and FIG. 5B show spectra of the EUV light source for the target material of Li and the composition of 80% Li+20% Ag, respectively. Spectra shown in FIG. 5 are measured using the EUV light source schematically shown in FIG. 1 for radiation coming directly from the plasma (without reflection from the mirror 9 and without the use of a membrane 15). It can be seen that the emission intensity of Li2+ line at 13.5 nm practically does not change when replacing the target material from Li with a composition of 80% Li+20% Ag. At the same time, due to the presence of Ag ions in the plasma, an emitting band with a peak at a wavelength of 17 nm appears which does not overlap with the emission line of Li2+ at 13.5 nm, which can be easily filtered out, for example, using a Mo/Si mirror 9, FIG. 1.

[0090] FIG. 6 shows the spectral reflectivity curve of the Mo/Si mirror with a reflection band into which the radiation from the Ag plasma does not fall. This determines the possibility of using a multilayer Mo/Si mirror as a reflective filter in accordance with the present invention, not limited only to this option. Similarly, other types of filters can be used, for example, made from foils containing zirconium or beryllium.

[0091] As can be seen from FIG. 7A and FIG. 7B, spectra of the EUV light source after reflection from the Mo/Si mirror practically coincide for the target materials of Li and the composition of 80% Li+20% Ag.

[0092] The radiation behind the multilayer mirror is monochromatic with a bandwidth of /1300. This value was estimated from measured bandwidths (a spectrometer with a spectral resolution / up to 500 was used) of spectral lines Li2+ (1s4p) at 10.8 nm, Li2+ (1s3p) at 11.39 nm and Li2+ (1s2p) at 13.5 nm, FIG. 5A.

[0093] In the EUV light source, the maximum conversion efficiency CE13.5 of laser radiation energy into in band EUV radiation (13.50.135 nm) in 2 sr spatial angle was 2% for both of these Li-based materials of target.

[0094] In general, there is a set of parameters, which includes the size of the laser spot, the energy and duration of the laser pulse, which provide, along with a high CE, a minimum amount of debris.

[0095] The method for generating radiation using a EUV light source, schematically shown in FIG. 1, FIG. 2 and FIG. 3 is implemented as described below.

[0096] Under the action of centrifugal force, the target is created in the form of a layer of molten metal on the surface of an annular groove of a rotating target assembly 2, the surface facing the rotation axis 6. The target 3 is irradiated by a pulsed focused laser beam 5 resulting in the formation of plasma in the interaction zone 4. The output beam of EUV radiation 8 is generated passing into an optical collector 9 through the means for debris mitigation 11, 12, 13, 14, 15.

[0097] The rotation of the target is carried out with a high linear velocity, not less than 100 m/s. In accordance with the embodiments disclosed herein, a target material containing a lithium composition with at least one further element is used, which makes it possible to increase multiple times, e.g. more than three times, the density of the material of target compared to the density of Li and thereby dramatically reduce the ejection rate of droplet fraction of debris particles. The droplet fraction of debris particles, due to the fast target rotation, acquires a significant tangential component of the velocity comparable to the velocity of droplet flyout, which, in accordance with the invention, is sharply reduced by increasing the density of the Li-containing target material. As a result, the velocity vector of the droplet fraction of debris particles is directed away from the beams 5, 8 of laser and EUV radiation, the propagation paths for which are chosen in spatial regions with minimal levels of debris ejection rate.

[0098] Ag or Au is preferably chosen as a further element of the target material, which makes it possible, even with its small, e.g. 20%, atomic fraction, to increase the density of the target material many times (about 5 and 8 times, respectively) compared to the density of Li. In preferred embodiments of the invention, the material of target is a eutectic alloy in which the atomic fraction of Li is in the range from 60% to 90%, which ensures the uniformity of the target material and its relatively low melting point.

[0099] Bi, Ba, Sr can also be used as a further element of the target material.

[0100] In the EUV radiation beam 8, spectral filtering of narrowband line emission of Li2+ at 13.5 nm is carried out, for example, using a Mo/Si mirror 9, which plays the role of a reflective filter. Foils, in particular, containing zirconium or beryllium, can also be used as a filter.

[0101] When generating EUV radiation, means for debris mitigation 11, 12, 13, 14, 15 are used, comprising protective gas flow, magnets, a foil trap, a membrane 15 largely transparent at 13.5 nm, and a debris shield 12 installed outside the regions of propagation of laser and EUV radiation beams 5, 8.

[0102] When using fast target rotation with high target velocity of more than 100 m/s and centrifugal acceleration of at least 10000 g, the surface of the target 4 is round-cylindrical. As a result, a stable shape of the target surface 3 in the interaction zone 4 is ensured and long-term stability of the EUV radiation source is achieved due to continuous circulation and updating of the target material in the interaction zone and restoration of the shape of the target surface after the next laser pulses.

[0103] In general, the target material made in accordance with the present invention is applicable to a wide range of EUV light sources of various types known from the prior art, both based on laser produced plasma and discharge produced plasma.

[0104] For example, FIG. 8 shows embodiments of the EUV light sources using a target material made in accordance with embodiments of the present invention.

[0105] In FIG. 8A the LPP EUV light source has the target 3 in the form of liquid metal jet circulating at high speed through the interaction zone 4 along a closed loop 16 with a nozzle 17 and a high-pressure pump 18 for transferring molten target material. A debris shield 12 with a temperature exceeding the melting point of target material is installed around the jet. The jet can be continuous or consist of individual target droplets following each other at high speed. Other parts of the device in this embodiment are the same as in the above embodiments (FIG. 1, FIG. 2), have the same item numbers in FIG. 8, and their detailed description is omitted here and below.

[0106] In another embodiment, shown in FIG. 8B, two high-speed liquid metal jets are used as electrodes 19 connected to a pulsed power source 20 to produce a laser-initiated discharge plasma 21 in vapors of the target material of target 3.

[0107] In another embodiment, shown in FIG. 8C, the LPP EUV light source is shown in which the target material of the target 3 is delivered into interaction zone 4 by a fast rotating disk 22 with a casing 23, in which there are holes for the input of laser beam 5, and exit of EUV radiation beam 8. Outside the interaction zone 4, the disk 22 is wetted with the molten target material 24.

[0108] Similarly, in the embodiment illustrated by FIG. 8D, two fast rotating disks 22 wetted with molten target material are used as electrodes 19 to produce a laser-initiated discharge produced plasma 21 in vapors of the target material of target 3.

[0109] Thus, the present invention may make it possible to create monochromatic EUV light sources at a wavelength of 13.5 nm characterized by high spectral brightness and average power, long service life and ease of use.

INDUSTRIAL APPLICATION

[0110] The proposed devices are intended for a number of applications, including microscopy, materials science, X-ray diagnostics of materials, biomedical and medical diagnostics, inspection of nano- and microstructures and lithography, including actinic control of lithographic EUV masks.