WO2018206244A1 - Source de plasma produit par laser - Google Patents

Source de plasma produit par laser Download PDF

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Publication number
WO2018206244A1
WO2018206244A1 PCT/EP2018/059775 EP2018059775W WO2018206244A1 WO 2018206244 A1 WO2018206244 A1 WO 2018206244A1 EP 2018059775 W EP2018059775 W EP 2018059775W WO 2018206244 A1 WO2018206244 A1 WO 2018206244A1
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WO
WIPO (PCT)
Prior art keywords
pulses
source
seed laser
laser module
seed
Prior art date
Application number
PCT/EP2018/059775
Other languages
English (en)
Inventor
Gerardus Hubertus Petrus Maria Swinkels
Ramon Mark HOFSTRA
Johannes Hubertus Josephina Moors
Paul William Cleary
Bert Pieter VAN DRIEËNHUIZEN
Original Assignee
Asml Netherlands B.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Asml Netherlands B.V. filed Critical Asml Netherlands B.V.
Priority to JP2019558698A priority Critical patent/JP7241027B2/ja
Priority to CN201880030698.8A priority patent/CN110612482B/zh
Publication of WO2018206244A1 publication Critical patent/WO2018206244A1/fr

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
    • H01S3/2308—Amplifier arrangements, e.g. MOPA
    • H01S3/2316—Cascaded amplifiers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0064—Anti-reflection devices, e.g. optical isolaters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0071—Beam steering, e.g. whereby a mirror outside the cavity is present to change the beam direction

Definitions

  • the present invention relates to seed lasers for laser-produced plasma sources, e.g. for lithographic apparatus or metrology apparatus.
  • a lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate.
  • a lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs).
  • a patterning device which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC.
  • This pattern can be transferred onto a target portion (e.g. including part of a die, one die, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
  • a single substrate will contain a network of adjacent target portions that are successively patterned.
  • EUV Extreme ultraviolet
  • soft x-rays is generally defined to be electromagnetic radiation having wavelengths of between 10 and 120 nanometers (nm) with shorter wavelengths expected to be used in the future.
  • EUV lithography is currently generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features, for example, sub-32 nm features, in substrates such as silicon wafers.
  • the most commercially successful method to produce EUV light is to create a plasma from a material that has one or more elements, e.g. xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc. , with one or more emission line(s) in the EUV range.
  • a target material such as a droplet, stream or cluster of material having the desired line- emitting element, with a laser beam at an irradiation site.
  • the line-emitting element may be in pure form or alloy form, for example, an alloy that is a liquid at desired temperatures, or may be mixed or dispersed with another material such as a liquid.
  • each droplet in a droplet stream is irradiated by a separate laser pulse to form a plasma from each droplet.
  • each droplet is sequentially illuminated by more than one light pulse.
  • each droplet may be exposed to a so-called “pre-pulse” to heat, expand, gasify, vaporize, and/or ionize the target material and/or generate a weak plasma, followed by a so-called “main pulse” to generate a strong plasma and convert most or all of the pre-pulse affected material into plasma and thereby produce an EUV light emission.
  • pre-pulse heat, expand, gasify, vaporize, and/or ionize the target material and/or generate a weak plasma
  • main pulse to generate a strong plasma and convert most or all of the pre-pulse affected material into plasma and thereby produce an EUV light emission.
  • EUV output power in an LPP system generally scales with the drive laser power that irradiates the target material
  • seed laser relatively low-power oscillator
  • the use of a large amplifier allows for the use of a low power, stable seed laser while still providing the relatively high power pulses used in the LPP process.
  • US patent application publication US 2014/0203194, incorporated herein by reference, describes a laser source for an LPP system in which the main pulse used to create the plasma is preceded by a "pedestal".
  • the pedestal starts approximately 400 ns before the main pulse and has a beam intensity rising to from 1 to 10 % of the intensity of the peak of the main pulse.
  • the presence of the pedestal is found to increase the EUV output power of the LPP source.
  • the pedestal is controlled using an optical shutter (e.g. comprising a Pockels cell and two polarizers) and/or a saturable absorber. Given the high pulse repetition rates that are desirable to achieve a high power source, control of the pedestal energy is difficult.
  • the present invention aims to provide improved laser-produced plasma sources, e.g. for use with lithographic device manufacturing processes and/or metrology apparatus.
  • the invention in a first aspect provides a seed laser module for a laser produced plasma source, the seed laser module comprising: a pulsed laser source configured to emit source radiation pulses at a pulse repetition rate; a sub-system configured to provide an electric signal; and an electro-optic modulator coupled to the sub-system and configured to receive the source radiation pulses and to emit shaped radiation pulses under control of the electric signal; wherein the electric signal comprises gating pulses at the pulse repetition rate in phase with the source radiation pulses and one or more secondary pulses between successive ones of the gating pulses.
  • the invention in a second aspect provides a seed laser module for a laser produced plasma source, the seed laser module comprising: a pulsed laser source configured to emit source radiation pulses at a pulse repetition rate; a sub-system configured to provide an electric signal; an electro-optic modulator coupled to the sub-system and including an electro-optic crystal configured to receive the source radiation pulses and to emit shaped radiation pulses under control of the electric signal; and an acoustic device configured to apply an acoustic signal to the electro-optic crystal.
  • the invention in a third aspect provides a drive laser device for a laser produced plasma source, the drive laser device comprising: a seed laser module as described above; and an amplifier configured to amplify the shaped radiation pulses to form drive radiation pulses.
  • the invention in a fourth aspect provides a laser produced plasma source comprising: a drive laser device as described above; a target material delivery system configured to deliver target material to a target location to be irradiated by the drive radiation pulses to form a plasma; and a radiation
  • collector configured to collect radiation emitted by the plasma.
  • FIGURE 1 is a simplified schematic view of some of the components of one embodiment of an LPP EUV light source
  • FIGURE 2 is a simplified schematic view of some of the components of a seed laser module that may be used in an LPP EUV system;
  • FIGURE 3 is a simplified schematic view of some of the components of another seed laser module that may be used in an LPP EUV system;
  • FIGURE 4 is a graph of the intensity of a main pulse versus time, showing in an enlarged inset the pedestal portion;
  • FIGURE 5 is a simplified schematic view of some of the components of a seed laser module having two electro-optic devices according to an embodiment of the invention
  • FIGURE 6 is a diagram showing timing of pulses in an embodiment of the invention
  • FIGURE 7 is a simplified schematic view of some of the components of another seed laser module having two electro-optic devices according to an embodiment of the invention
  • FIGURE 8 is a simplified schematic view of some of the components of another seed laser module having one electro-optic device according to an embodiment of the invention.
  • FIGURE 9 is a simplified schematic of the seed laser module of FIGURE 8 in an OFF configuration.
  • FIGURE 10 is a simplified schematic view of some of the components of another seed laser module having two electro-optic devices according to an embodiment of the invention.
  • FIGURE 1 is a simplified schematic view of some of the components of one embodiment of an LPP EUV light source 10.
  • the EUV light source 10 includes a laser source 12 for generating a beam of laser pulses and delivering the beam along one or more beam paths from the laser source 12 and into a plasma chamber 14 to illuminate a respective target, such as a droplet, at an irradiation site 16.
  • the EUV light source 10 may also include a target material delivery system 26 that, for example, delivers droplets of a target material into the interior of plasma chamber 14 to the irradiation site 16, where the droplets will interact with one or more laser pulses to ultimately produce plasma and generate an EUV emission.
  • a target material delivery system 26 that, for example, delivers droplets of a target material into the interior of plasma chamber 14 to the irradiation site 16, where the droplets will interact with one or more laser pulses to ultimately produce plasma and generate an EUV emission.
  • the target material is an EUV emitting element that may include, but is not necessarily limited to, a material that includes tin, lithium, xenon or combinations thereof.
  • the target material may be in the form of liquid droplets, a mist and/or solid particles contained within liquid droplets.
  • the element tin may be presented as a target material as pure tin, as a tin compound, such as SnBr4, SnBr2, SnELt, as a tin alloy, e.g. tin-gallium alloys, tin-indium alloys, or tin- indium-gallium alloys, or a combination thereof.
  • the target material may be presented to the irradiation site 16 at various temperatures including room temperature or near room temperature (e.g. tin alloys or SnBrt), at a temperature above room temperature (e.g. pure tin), or at temperatures below room temperature (e.g. SnELt).
  • these compounds may be relatively volatile, such as SnBr4.
  • Similar alloys and compounds of EUV emitting elements other than tin, and the relative advantages of such materials and those described above will be apparent to those of skill in the art.
  • the EUV light source 10 may also include an optical element 18 such as a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid (i.e. an ellipse rotated about its major axis), such that the optical element 18 has a first focus within or near the irradiation site 16 and a second focus at a so-called intermediate region 20, where the EUV light may be output from the EUV light source 10 and input to a device utilizing EUV light such as an integrated circuit lithography tool, also referred to as a "scanner" (not shown).
  • the optical element 18 is formed with an aperture to allow the laser light pulses generated by the laser source 12 to pass through and reach the irradiation site 16.
  • optical element 18 should have an appropriate surface for collecting the EUV light and directing it to the intermediate region 20 for subsequent delivery to the device utilizing the EUV light.
  • optical element 18 might have a graded multi-layer coating with alternating layers of molybdenum and silicon, and in some cases, one or more high temperature diffusion barrier layers, smoothing layers, capping layers and/or etch stop layers.
  • optical element 18 may alternatively be a parabola rotated about its major axis or may be configured to deliver a beam having a ring-shaped cross section to an intermediate location.
  • optical element 18 may utilize coatings and layers other than or in addition to those described herein. Those of skill in the art will be able to select an appropriate shape and composition for optical element 18 in a particular situation.
  • the EUV light source 10 may include a focusing unit 22 which includes one or more optical elements for focusing the laser beam to a focal spot at the irradiation site 16.
  • EUV light source 10 may also include a beam conditioning unit 24, having one or more optical elements, between the laser source 12 and the focusing unit 22, for expanding, steering and/or shaping the laser beam, and/or shaping the laser pulses.
  • a beam conditioning unit 24 having one or more optical elements, between the laser source 12 and the focusing unit 22, for expanding, steering and/or shaping the laser beam, and/or shaping the laser pulses.
  • Various focusing units and beam conditioning units are known in the art, and may be appropriately selected by those of skill in the art.
  • laser source 12 comprises one or more seed lasers and one or more amplifiers.
  • the seed laser generates laser pulses, which are then amplified to become the laser beam that irradiates the target material at irradiation site 16 to form a plasma that produces the EUV emission.
  • seed lasers may be used to generate the pre-pulse and the main pulse.
  • a conventional dual-chamber transverse-flow laser source in what has traditionally been known as a "master oscillator power amplifier" ("MOP A") configuration may be used.
  • the power amplifier may comprise a fast axial flow laser.
  • a single laser source may produce both the pre-pulse and the main pulse.
  • separate seed lasers may be used to produce the pre-pulse (PP) and the main pulse, in what is commonly known as a
  • One type of seed laser commonly used in some embodiments of EUV systems is a CO 2 laser, while other embodiments may use a YAG (yttrium-aluminum-garnet) laser. Where there are two seed lasers, they may be of different types; however, for example, a YAG laser will need an amplifier or amplifier chain that are different from those used with a CO2 laser.
  • a YAG laser will need an amplifier or amplifier chain that are different from those used with a CO2 laser.
  • One of skill in the art will recognize that there are other types of lasers than CO 2 and YAG lasers, and other configurations than MOPA and MOPA+PP lasers, and will be able to determine which types and configurations of lasers will be suitable for the desired application.
  • an EUV energy detector 28 detects the amount of EUV power generated in the plasma chamber 14.
  • the EUV energy detector 28 comprises a sensor within the plasma chamber 14, e.g. an EUV side sensor positioned at 90° with respect to the laser beam, or a sensor within the scanner measuring energy passed through intermediate focus 20.
  • EUV energy detectors comprise, e.g., photodiodes and are generally known to those skilled in the art. As is familiar to those skilled in the art, by integrating the EUV power signal provided by the EUV energy detector 28 over the time span that the droplet is irradiated, the EUV energy generated from the impact of the droplet and the laser pulse is calculated.
  • An EUV controller 29 is configured to determine an intensity of a next laser pulse based on the amount of EUV generated by one or more previous pulse.
  • the EUV controller obtains, via the EUV energy detector 28, measurements of amounts of EUV generated from previous pulses.
  • the EUV controller 29 determines, using an algorithm described below, a target intensity of a subsequent laser pulse.
  • the target intensity is based on a determined stability of the plasma persisting between laser pulses in the plasma chamber 14. The more stable the plasma is, the higher the intensity of the subsequent laser pulse can be, up to a known limit. If the plasma is less stable or unstable, the EUV controller 29 can reduce the intensity of the subsequent laser pulse.
  • the EUV controller 29 can be implemented in a variety of ways known to those skilled in the art including, but not limited to, as a computing device having a processor with access to a memory capable of storing executable instructions for performing the functions of the described modules.
  • the computing device can include one or more input and output components, including components for communicating with other computing devices via a network (e.g. the Internet) or other form of communication.
  • the EUV controller 29 comprises one or more modules embodied in computing logic or executable code such as software.
  • a pulse actuator (not shown) actuates the laser source 12 to fire the laser pulse at the irradiation site 16.
  • Actuators can be electrical, mechanical, and/or optical components and are generally known to those skilled in the art.
  • the laser source 12 may comprise a seed laser module and one or more amplifier stages.
  • An example of a seed laser module 30 is illustrated in FIGURE 2.
  • Seed laser module 30 includes two seed lasers, a pre-pulse seed laser 32 and a main pulse seed laser 34.
  • the target material may be irradiated first by one or more pulses from the pre-pulse seed laser 32 and then by one or more pulses from the main pulse seed laser 34.
  • Seed laser module 30 is shown as having a "folded" arrangement rather than arranging the components in a straight line. In practice, such an arrangement is typical in order to limit the size of the module.
  • the beams produced by the laser pulses of pre-pulse seed laser 32 and main pulse seed laser 34 are directed onto desired beam paths by a plurality of optical components 36.
  • optical components 36 may be such elements as lenses, filters, prisms, mirrors, gratings or any other elements which may be used to direct the beam in a desired direction. In some cases, optical components 36 may perform other functions as well, such as altering the polarization of the passing beam.
  • the seed lasers contain optical components, such as the output coupler, polarizer, rear mirror, grating, acousto-optical modulation (AOM) switches, etc.
  • the seed lasers 32 and 34 contain a Q-switch AOM used in giant pulse formation and to produce a pulsed output beam.
  • the Q-switch AOM controls the timing of the release of the pulse from the seed laser.
  • the beams from each seed laser are first passed through electro-optic modulators 38 (EOM).
  • EOMs 38 are used with the seed lasers as pulse shaping units to trim the pulses generated by the seed lasers to pulses having shorter duration as well as faster rise- and /or fall-time.
  • a shorter pulse duration and relatively fast rise and/or fall-time may increase EUV output and light source efficiency because of a short interaction time between the pulse and a target, and because unneeded portions of the pulse do not deplete amplifier gain.
  • two separate pulse shaping units (EOMs 38) are shown, alternatively a common pulse shaping unit may be used to trim both pre-pulse and main pulse seeds.
  • the EUV controller 29 can control the intensity of the laser pulse by adjusting the timing of the EOM 38 relative to a Q-switch AOM within the seed laser.
  • the timing is adjusted by changing the delay of the Q-switch AOM trigger or EOM trigger relative to laser firing trigger which is based on when the droplet arrived at a detection line laser in the plasma chamber.
  • the EUV controller 29 can control the intensity of the laser pulse by changing the voltage applied to the EOM crystal of the EOM 38.
  • One material that may be used for such a crystal is cadmium telluride (CdTe); there are other materials used in EOMs as well.
  • CdTe cadmium telluride
  • HV high voltage
  • the high voltage HV is adjusted in proportion to the desired intensity. As such, to generate a laser pulse with a higher intensity, the high voltage HV applied to the crystal is increased and, to generate a laser pulse with a lower intensity, the high voltage HV applied to the crystal is decreased.
  • the beams from the seed lasers are then passed through acousto- optic modulators (AOMs) 40 and 42.
  • AOMs acousto- optic modulators
  • the AOMs 40 and 42 act as “switches” or “shutters,” which operate to divert any reflections of the laser pulses from the target material from reaching delicate parts of the system, such as the EOM(s) and the seed lasers.
  • the AOMs 40 and 42 thus prevent any reflections from causing damage to delicate parts.
  • the beams from each seed laser pass through two AOMs; however, in some embodiments, the beams from each seed laser may be passed through only a single AOM on each path.
  • the AOMs 40 and 42 can be manipulated.
  • the AOMs 40 and 42 are opened and closed by applying radio frequency (RF) power to a transducer bonded to a crystal within the AOM.
  • RF radio frequency
  • the intensity of the laser pulse is proportional to the amount of RF power applied.
  • more RF power is applied and to reduce the intensity of the laser pulse, less RF power is applied.
  • the two beams are "combined" by beam combiner 44. Since the pulses from each seed laser are generated at different times, this really means that the two temporally separated beams are placed on a common beam path 46 for further processing and use.
  • the beam from one of the seed lasers passes through a beam delay unit 48 such as is known in the art.
  • the beam is directed through a pre-amplifier 50 and then through a beam expander 52.
  • the beam passes through a thin film polarizer 54, and is then directed onward by optical component 56, which again is an element which directs the beam to the next stage in the LPP EUV system and may perform other functions as well.
  • optical component 56 the beam typically passes to one or more optical amplifiers and other components, as is known in the art.
  • a seed laser may be a CO2 laser having a sealed filling gas including CO 2 at sub-atmospheric pressure, for example, 0.05 to 0.2 atmospheres, and pumped by a radio-frequency discharge.
  • a grating may be used to help define the optical cavity of the seed laser, and the grating may be rotated to tune the seed laser to a selected rotational line.
  • FIGURE 3 is a simplified block diagram of one embodiment of a seed pulse generation system 60.
  • the seed pulse generation system 60 Like the seed laser module 30, the seed pulse generation system 60 generates seed pulses, shapes the seed pulses, and amplifies the seed pulses.
  • the seed pulse generation system 60 includes two pre-amplifiers 74 and 84 instead of the one pre-amplifier 50 of seed laser module 30 of FIGURE 2.
  • the addition of a second pre-amplifier, and the additional gain provided by the second pre-amplifier can result in a higher likelihood that power amplifiers positioned beyond the seed pulse generation system 60 will self-lase, inducing modulation of forward laser pulses and gain- stripping the pre-amplifiers 74 and 84 in the seed pulse generation system 60.
  • the seed pulse generation system 60 of FIGURE 3 includes additional isolation stages positioned between the elements of the seed laser module 30 of FIGURE 2 to prevent reflected light from reaching a seed laser as well as a second pre- amplifier.
  • the isolation stages of the seed pulse generation system 60 can be added to, or implemented within, the seed laser module 30 of FIGURE 2.
  • the seed laser 62 is depicted as a single unit, it produces a beam as described in connection with the pre-pulse seed laser 32 and the main pulse seed laser 34 of FIGURE 2.
  • the seed pulse generation system 60 may include more than one seed laser 62.
  • the EOM 64 shapes the pulses as described in connection with the EOM 38 of FIGURE 2 above.
  • a first isolation stage 66 is positioned between the EOM 64 and the first preamplifier 74.
  • the first isolation stage 66 comprises a first AOM 68, a delay device 70, and a second AOM 72; the delay device 70 again has a beam folding optical arrangement.
  • the first isolation stage 66 like the AOMs 40 and 42 and the delay line 41 of FIGURE 2, operates to divert any reflections of the laser pulses from the target material from reaching the seed laser 62.
  • the isolation stage 66 provides improved isolation from amplified pulses that have passed through a first pre-amplifier 74.
  • the seed pulses are passed through two or more pre-amplifiers, rather than just one pre-amplifier, as shown in FIGURE 2.
  • the seed pulses can be amplified in stages, which has a number of benefits.
  • the use of separate amplifiers having smaller individual gains prevents self-lasing of the amplifiers.
  • Another benefit following from the use of isolation stages with multiple pre-amplifiers is that reflected light can be diverted mid-amplification, before the gain is so high that even 1% of the reflected light is still powerful enough to damage the seed laser 62 after 99% of the reflected light is diverted.
  • the first pre-amplifier 74 is followed by a second isolation stage 76 which comprises a first AOM 78, a delay device 80, and a second AOM 82.
  • the second isolation stage 76 is able to divert reflected light originating in other parts of the LPP EUV system than the first isolation stage. Since the second pre-amplifier 84 follows the second isolation stage 76 for a pulse traveling to the irradiation site, all of the reflected light that reaches the second isolation stage 76 will have also been amplified by the second pre-amplifier 84.
  • a further isolation stage may follow the second preamplifier 84 before the beam is directed to still further elements of the LPP EUV generation system.
  • Such a further isolation stage can divert reflected light arriving from further components in the LPP EUV system before the reflected light is amplified by the second pre-amplifier 84.
  • FIGURE 4 is a graph of radiation power at the point of irradiation of the target material vs time.
  • the main pulse has a peak at a time index, in this example, of 1 ⁇ which a peak power of 15 MW.
  • the pedestal Pd which is only visible in the enlarged inset, has a duration of up to about 400 ns and a power of about 0.03 MW (i.e. less than 1% of the peak power of the main pulse).
  • the profile of the pedestal may be a slow rise (or ramp up) in power, perhaps with a plateau immediately before the peak of the main pulse.
  • the exact duration, profile and power level of the pedestal that results in optimum EUV output can be determined empirically, and may vary according to the type of target material, and or the manner of delivery of the target material to the target point.
  • the optimum parameters for the pedestal may vary over time and between source modules.
  • the present invention aims to provide an improve approach to controlling the pedestal.
  • pedestal control can be achieved with the addition of little or no additional hardware, in particular without additional elements on the beam path.
  • FIGURE 5 depicts in greater detail an electro-optic modulator 38 for the main pulse.
  • Seed laser 34 outputs mostly polarized radiation, e.g. horizontally polarized radiation.
  • First polarizer 381 is used to clean-up the polarization of the seed laser and reject any radiation of the unwanted polarization state, e.g. vertical polarization.
  • the horizontally polarized radiation is then incident on first electro-optic crystal 382.
  • Electro-optic crystal 382 is a birefringent crystal provided with electrodes and configured so that, if a suitable voltage is applied by high voltage source 386, the polarization state of the incident radiation is rotated by 90°. Therefore incident radiation that is horizontally polarized is converted to and emitted as vertically polarized radiation.
  • Second polarizer 383 is oriented at 90° to first polarizer 381 and so passes the radiation if a voltage is applied to first electro-optic crystal 382 and blocks the radiation if not.
  • Second electro-optic crystal 384 is similar to first electro-optic crystal 382 and likewise is configured to rotate the polarization state of radiation passing through it only if a suitable voltage is applied by high voltage source 386.
  • Third polarizer 385 is oriented at 90° to second polarizer 383 and so passes the radiation if a voltage is applied to second electro-optic crystal 382 and blocks the radiation if not.
  • First, second and third polarizers 381, 383, 385 may be thin-film polarizers.
  • the timing and shape of the gating pulses applied to the electro-optic crystals determine the timing and shape of the pulses output by the seed laser module and subsequently amplified.
  • Seed laser 34 may be a pulsed laser and the gating electric pulses are therefore synchronized to the radiation pulses output by seed laser 34.
  • the present inventors have determined that an effective way of creating a pedestal is to allow a controlled leak of radiation through electro-optic modulator 38.
  • a controlled leak of radiation from the seed laser 34 through the electro-optic modulator 38 is achieved by applying additional voltage pulses, referred to herein as secondary pulses, to the crystal(s) of the electro-optic modulator in between the gating pulses GP. This is shown in FIGURE 6.
  • the top graph of FIGURE 6 illustrates the radiation power output by the seed laser 34 as a function of time.
  • the second graph illustrates the conventional gating pulses GP which are used to control the shape of the radiation pulse passed to the amplifying stages of the drive laser.
  • the third line, labelled A illustrates a first option according to an embodiment of the invention.
  • the frequency of the pulses output by the high voltage source is doubled so that a secondary pulse SP is applied to the electro-optical crystal in between the gating pulses GP.
  • Frequency doubling the high voltage pulse can be achieved very easily, e.g. by doubling the frequency of a timing signal supplied to the high voltage source. It is also possible to increase the frequency of the gating pulses by a larger multiple, e.g. 3 or 4, to increase the pedestal.
  • the secondary pulses create or increase a residual electrical charge on surfaces of the electro-optical crystals 382, 384 which causes sufficient leakage of radiation at the beginning of the seed pulse to create the pedestal.
  • the frequency of the secondary pulses and of the gating pulses is high, e.g. 50 kHz, so that there is insufficient time for all charge on the electro-optic crystal as generated by the secondary pulses to drain away after the secondary pulses and before the next gating pulse.
  • a second option according to the first embodiment of the invention is depicted in the fourth graph of FIGURE 6, labelled B.
  • the amplitude A and/or duration d of the secondary pulse SP are controlled to control the amount of residual charge on the electro-optic crystal and hence control the size and/or shape of the pedestal. It is also possible to control the timing of the secondary pulse SP relative to the gating pulses GP.
  • a third option according to the first embodiment of the invention is depicted in the fifth graph of FIGURE 6, labelled C.
  • the multiple secondary pulses SP1, SP2 are provided between each successive pair of gating pulses GP.
  • the amplitudes Al, A2 and/or durations dl, d2 of the secondary pulses SP1, SP2 are controlled to control the amount of residual charge applied to the electro-optic crystal and hence control the size and/or shape of the pedestal. It is also possible to control the timings of the secondary pulses SP1, SP2 relative to the gating pulses GP. More than two secondary pulses can be provided per gating pulse cycle.
  • the high voltage source 387 is configured to apply a DC bias voltage to the electro-optic crystals 382, 384 in addition to the gating pulses.
  • the DC bias voltage can be less than about 20% of the peak voltage of the gating pulses. For example if the peak voltage of the gating pulse is about 5,000 V or more then the DC bias voltage can be less than about 1,000 V.
  • the DC bias at a specific one of the electro-optic crystals brings about an electric field in the electro-optic crystal that in turn results in an electric charge distribution eventually affecting the pedestal. Accordingly, the DC bias voltage is controlled to optimize the output of EUV radiation.
  • FIGURES 8 and 9 show an electro-optic modulator using a single electro-optic crystal in the ON state and in the OFF state, respectively. Parts of the third embodiment that are the same as those of the earlier embodiments are indicated with like reference numerals and not described further for the sake of brevity.
  • first polarizer 381 selects the horizontal polarization state from the output of seed laser 34 and passes it to the electro-optic crystal 382.
  • a sufficient voltage is applied to the electro-optic crystal 382, e.g. by a gating pulse or a secondary pulse, the polarization state of the radiation is rotated to be vertical and the radiation therefore passes the second polarizer 383, which passes the vertical polarization state.
  • the electro-optic crystal 382 does not rotate the polarization state of the radiation and so it is blocked by the second polarizer 383.
  • a controlled amount of leakage of radiation sufficient to create the pedestal can be caused by a residual charge on the electro-optic crystal 382.
  • the residual charge can be caused by the application of secondary pulses as in the first embodiment or a bias voltage as in the second embodiment.
  • a certain amount of radiation may leak past the second polarizer 383, which might not be 100% selective, e.g. through misalignment. Such leakage will contribute to formation of the pedestal and is taken in to account in determining the controlled leakage created by application of residual charge to the electro-optic crystal.
  • FIGURE 10 A fourth embodiment is depicted in FIGURE 10. Parts of the fourth embodiment that are the same as those of the earlier embodiments are indicated with like reference numerals and not described further for the sake of brevity.
  • an actuator 388 is provided to rotate one or more of the polarizers, e.g. second polarizer 383. By selectively rotating polarizer 383, a controlled leak of radiation even when the electro-optic crystals 382, 384 are not energized, can be achieved. A similar effect can be achieved by rotating the first and third polarizers together.
  • An actuator allows for real-time control of the pedestal however in some cases this might not be necessary and a mechanical adjuster can be used instead.
  • the controlled leak of radiation can create a pedestal as desired.
  • an electric field in an electro-optic crystal is created by applying a mechanical force to the electro-optical crystal.
  • an electro-optic crystal is typically also piezo-electric.
  • the mechanical force can be applied by, e.g., a piezo-electric actuator or another form of actuator.
  • the mechanical force has a frequency selected to cause a resonant vibration in the electro-optic crystal.
  • control of the radiation leak to form the pedestal can be performed in a feedback mode or a feed forward mode or a combination thereof.
  • a feed back control of the radiation leak is desirably performed on the basis of a measurement of useful EUV radiation power in the source module or lithographic apparatus.
  • a laser produced plasma source comprising a seed laser module as specified above, that further comprising a sensor configured to provide a sensor signal indicative of a property of radiation of a predetermined wavelength collected by the radiation collector, wherein the sub-system is configured to provide the electric signal under control of the sensor signal so as to be able to control the seed laser module to adjust the intensity of a pedestal portion of the shaped radiation pulses in response to measurements from the sensor.
  • the sensor signal may be used to control the acoustic device that determines the residual electric charge on the electro-optic crystal of the electro-optic modulator.
  • the sensor signal may be used to control the adjuster configured to rotate one or more of the polarizers of the electro-optic modulator.
  • An embodiment may include a computer program containing one or more sequences of machine-readable instructions configured to instruct various apparatus as depicted in FIGURE 1 to perform measurement and optimization steps and to control a subsequent exposure process as described above.
  • This computer program may be executed, for example, within the laser control unit LACU or the supervisory control system SCS of FIGURE 1 or a combination of both.
  • a data storage medium e.g. semiconductor memory, magnetic or optical disk having such a computer program stored therein.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • X-Ray Techniques (AREA)
  • Exposure And Positioning Against Photoresist Photosensitive Materials (AREA)

Abstract

L'invention concerne un module laser germe pour une source de plasma produit par laser, le module laser germe comprenant : une source de laser pulsée configurée pour émettre des impulsions de rayonnement source à une fréquence de répétition d'impulsions ; un sous-système configuré pour fournir un signal électrique ; un modulateur électro-optique accouplé au sous-système et configuré pour recevoir les impulsions de rayonnement source et pour émettre des impulsions de rayonnement mises en forme sous la commande du signal électrique ; et le signal électrique comprenant des impulsions de déclenchement à la fréquence de répétition d'impulsions en phase avec les impulsions de rayonnement source et une ou plusieurs impulsions secondaires entre des impulsions de déclenchement successives.
PCT/EP2018/059775 2017-05-10 2018-04-17 Source de plasma produit par laser WO2018206244A1 (fr)

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NL2003192A1 (nl) * 2008-07-30 2010-02-02 Asml Netherlands Bv Alignment of collector device in lithographic apparatus.
NL2004837A (en) * 2009-07-09 2011-01-10 Asml Netherlands Bv Radiation system and lithographic apparatus.
JP2013229553A (ja) * 2012-03-30 2013-11-07 Gigaphoton Inc レーザ装置及び極端紫外光生成装置
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US20140203194A1 (en) 2011-10-05 2014-07-24 Gigaphoton Inc. System and method for generating extreme ultra violet light
US20160007434A1 (en) * 2014-07-07 2016-01-07 Asml Netherlands B.V. Extreme ultraviolet light source

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WO2020081734A1 (fr) * 2018-10-18 2020-04-23 Asml Netherlands B.V. Commande de modulateur optique

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CN110612482A (zh) 2019-12-24
JP7241027B2 (ja) 2023-03-16

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