Method and apparatus for power supply synchronization

Abstract

A method is disclosed for controlling power supplied to an EUV light source that includes receiving, by a controller, a trigger signal configured to initiate a plurality of pulses from a light source, and synchronizing, by the controller, a power control signal with the trigger signal. The power control signal is configured to control a supply of power to one or more light amplifiers. A controller for a light source and a light source are also disclosed.

Description

METHOD AND APPARATUS FOR POWER SUPPLY SYNCHRONIZATIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims pnority to US Application No. 63 / 613,920, filed December 22,2023, titled METHOD AND APPARATUS FOR POWER SUPPLY SYNCHRONIZATION, which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure relates to a method and an apparatus for controlling the power supply to a light amplifier in a light source, for example, synchronizing a power supply to a trigger signal in a laser system.BACKGROUND

[0003] 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). In that instance, a patterning device, which can be a mask or a reticle, can 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., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer ofthe pattern is typically via imaging onto a layer of radiationsensitive material (photoresist or simply “resist”) provided on the substrate. Light commonly used to in the transfer process includes Extreme Ultraviolent (EUV) light.

[0004] In some applications, EUV light is generated using fuel droplets, and a plurality of light seed pulses can be amplified using radio frequency (RF) power. In some cases, a fuel droplet is exposed to an amplified pre-pulse light pulse, transforming the fuel droplet into plasma. The plasma is then exposed to an amplified main light pulse, which stimulates the emission of EUV light that is then collected and used for lithography. Although the position of the fuel droplet can be measured and used to trigger one or more processes within the light source, the supply of RF power to the pump laser is independent of the triggering caused by the fuel droplet. Instead, the supply of RF power relates to a power control signal for adjusting an active interval cycle and the power control signal is generated from a control module. Because of this, the power control signal in some EUV light sources is free from synchronization to the triggering (also called as “free running”) and is shown as a rectangular wave with a frequency of, for example, from about 95 kHz to about 110 kHz.

[0005] During lithographic processes, a dose control routine can be implemented in some EUV light sources so that the pulses of the pump laser are moved “off droplet” or delayed in time, causing a “missed pulse” wherein light pulses are wasted by missing the fuel droplet. However, due to the free running nature of the power control signal, the light amplifier still receives RF power and dischargesthe plurality of pulses, so that missed pulses lead to a waste of RF power in the light amplifier. In addition, because the light source still discharges amplified light pulses, missed pulses can lead to contamination in which the fuel droplet is exposed to a portion of the main pulse without being transformed into a target by the pre-pulse. The result causes partial evaporation of the tin that then deposits onto the interior of the light source.SUMMARY

[0006] Accordingly, there exists a need for synchronizing the power supplied into the light amplifier of the EUV light source with a trigger signal based on the fuel droplet to reduce wasted power and contamination due to missed pulses.

[0007] In some aspects, a method can comprise receiving, by a controller, a trigger signal configured to initiate a plurality of pulses from a light source, and synchronizing, by the controller, a power control signal with the trigger signal. The power control signal can be configured to control a supply of power to one or more light amplifiers.

[0008] In some aspects, a controller for a light source can comprise a controller comprising a processor communicatively coupled to a memory. The memory can be configured to store instructions that, when executed, cause the processor to receive a trigger signal configured to initiate a plurality of pulses from a light source, and synchronize a power control signal with the trigger signal. The power control signal can be configured to control a supply of power to the light one or more light amplifiers.

[0009] In some aspects, a light source for a lithographic apparatus can comprise a power supply module configured to provide radio frequency (RF) power to one or more light amplifiers and a controller. The controller can be configured to receive a trigger signal from a metrology device configured to measure a position of a droplet configured to provide a target for the light source, and, in response to receiving the trigger signal, synchronize a power control signal with the trigger signal. The power control signal can be a control signal configured to control supply of the RF power to the one or more light amplifiers to initiate a plurality of pulses from the light source.BRIEF DESCRIPTION OF THE DRAWINGS / FIGURES

[0010] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with this written description, further serve to explain the principles of the present disclosure and to enable a person skilled in the relevant art(s) to make and use embodiments described herein.

[0011] FIG. 1A shows a reflective lithographic apparatus, according to some aspects.

[0012] FIG. IB shows a transmissive lithographic apparatus, according to some aspects.

[0013] FIG 2 shows more details of a reflective lithographic apparatus, according to some aspects.

[0014] FIG. 3 shows a lithographic cell, according to some aspects.

[0015] FIG. 4 shows a schematic representation of a light source of a lithographic apparatus, according to some aspects.

[0016] FIG. 5 A shows a schematic representation of the position of a tin droplet and a plasma target during normal operation of an EUV light source, according to some aspects.

[0017] FIG. 5B shows a schematic representation of the position of a tin droplet during a missed pulse operation of an EUV light source, according to some aspects.

[0018] FIG 6 illustrates the timing of a trigger signal, RF power, and a main pulse in a free run mode of an EUV light source, according to some aspects.

[0019] FIG. 7A illustrates the timing of a trigger signal and a power control signal in a trigger- synchronized mode, according to some aspects.

[0020] FIG. 7B illustrates the timing of RF power and a main pulse in a trigger-synchronized mode, according to some aspects.

[0021] FIG. 8A illustrates a delay of a power control signal from a trigger signal in a trigger- synchronized mode, according to some aspects.

[0022] FIG. 8B illustrates a delay of RF power and a main pulse from a trigger signal in a trigger-synchronized mode, according to some aspects.

[0023] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements. Additionally, generally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.DETAILED DESCRIPTION

[0024] This specification discloses one or more embodiments that incorporate the features of the present disclosure. The disclosed embodiment(s) are provided as examples. The scope of the present disclosure is not limited to the disclosed embodiment(s). Claimed features are defined by the claims appended hereto.

[0025] The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0026] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, may be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0027] The terms “substantially,” “about,” “approximately,” or the like may be used herein to indicate a value of a quantity that may vary or be found to be within a range of values, based on a particular technology. Based on the particular technology, the terms may indicate a value of a given quantity that is within, for example, 1-20% of the value (e.g., ±1%, ±5% ±10%, ±15%, or ±20% of the value).

[0028] FIGS. 1A and IB show a lithographic apparatus 100 and a lithographic apparatus 100’, respectively, in which aspects of the present disclosure can be implemented. Lithographic apparatus 100 and lithographic apparatus 100’ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus 100 and 100’ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100’, the patterning device MA and the projection system PS are transmissive.

[0029] The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B.

[0030] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus 100 and 100’, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0031] The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0032] The patterning device MA can be transmissive (as in lithographic apparatus 100’ ofFIG IB) or reflective (as in lithographic apparatus 100 of FIG. 1A). Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase shift, or attenuated phase shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B, which is reflected by a matrix of small mirrors.

[0033] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0034] Lithographic apparatus 100 and / or lithographic apparatus 100’ can be of atype having two (dual stage) or more substrate tables WT (and / or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

[0035] In some aspects, the lithographic apparatus can also be of a type wherein at least a portion of the substrate can be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid. For example, a liquid can be located between the projection system and the substrate during exposure.

[0036] Referring to FIGS. 1A and IB, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus 100, 100’ can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part ofthe lithographic apparatus 100 or 100’, and the radiation beam B passes fromthe source SO to the illuminator IL with the aid of a beam delivery system BD (in FIG. IB) including, for example, suitable directing mirrors and / or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, 100’, for example, when the source SO is a mercury lamp. A radiation system can comprise the source SO, the illuminator IL, and / or the beam delivery system BD.

[0037] The illuminator IL can include an adjuster AD (in FIG. IB) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as “outer” and “inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (in FIG. IB), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

[0038] Referring to FIG. 1A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IF 1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0039] Referring to FIG. IB, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at the mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

[0040] The projection system PS projects an image of the mask pattern MP, where the image is formed by diffracted beams produced from the mark pattern MP by radiation from the intensity distribution, onto a photoresist layer coated on the substrate W. For example, the mask pattern MP can include an array of lines and spaces. A diffraction of radiation at the array and different from zeroth order diffraction generates diverted diffracted beams with a change of direction in a directionperpendicular to the lines. Undiffracted beams (i.e., so-called zeroth order diffracted beams) traverse the pattern without any change in propagation direction. The zeroth order diffracted beams traverse an upper lens or upper lens group of the projection system PS, upstream of the pupil conjugate PPU of the projection system PS, to reach the pupil conjugate PPU. The portion of the intensity distribution in the plane of the pupil conjugate PPU and associated with the zeroth order diffracted beams is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD, for example, is disposed at or substantially at a plane that includes the pupil conjugate PPU of the projection system PS.

[0041] The projection system PS is arranged to capture (e.g., using a lens or lens group L) the zeroth order diffracted beams, first order diffracted beams, and / or higher order diffracted beams (not shown). In some aspects, dipole illumination for imaging line patterns extending in a direction perpendicular to a line can be used to utilize the resolution enhancement effect of dipole illumination. For example, first-order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the wafer W to create an image of the line pattern MP at highest possible resolution and process window (i.e., usable depth of focus in combination with tolerable exposure dose deviations). In some aspects, astigmatism aberration can be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some aspects, astigmatism aberration can be reduced by blocking the zeroth order beams in the pupil conjugate PPU of the projection system associated with radiation poles in opposite quadrants. This is described in more detail in US 7,511,799 B2, issued Mar. 31, 2009, which is incorporated by reference herein in its entirety.

[0042] With the aid of the second positioner PW and position sensor IFD (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown in FIG. IB) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0043] In general, movement of the mask table MT can be realized with the aid of a long- stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks Pl, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (known as scribe -lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

[0044] Mask table MT and patterning device MA can be in a vacuum chamber V, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, an out-of-vacuum robot can be used for various transportation operations, similar to the invacuum robot IVR. Both the in-vacuum and out-of-vacuum robots can be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

[0045] The lithographic apparatus 100 and 100’ can be used in at least one of the following modes:1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that a different target portion C can be exposed.2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT can be determined by the (de-) magnification and image reversal characteristics of the projection system PS.3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated as needed after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

[0046] Combinations and / or variations on the described modes of use or entirely different modes of use can also be employed.

[0047] In some aspects, lithographic apparatus 100 includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0048] In some aspects, lithographic apparatus 100’ includes a deep ultraviolet (DUV) source, which is configured to generate a beam of DUV radiation for DUV lithography. In general, the DUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the DUV radiation beam of the DUV source.

[0049] FIG. 2 shows the lithographic apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in anenclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 can be formed by a discharge produced plasma source. In some aspects, a plasma of excited tin (Sn) (e.g., excited via a laser) is provided to produce EUV radiation.

[0050] The radiation emitted by the EUV radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap 230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure.

[0051] The collector chamber 212 can include a radiation collector CO, which can be a so- called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral fdter 240 to be focused in a virtual source point INTF. The virtual source point INTF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus INTF is located at or near an opening 219 in the enclosing structure 220. The virtual source point INTF is an image of the EUV radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infra-red (IR) radiation.

[0052] Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by the wafer stage or substrate table WT.

[0053] More elements than shown can generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in the FIG. 2, for example there can be one to six additional reflective elements present in the projection system PS than shown in FIG. 2.

[0054] Collector optic CO, as illustrated in FIG. 2, is depicted as a nested collector with grazing incidence reflectors 253, 254, and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254, and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.Example Lithographic Cell

[0055] FIG 3 shows a lithographic cell 300, also sometimes referred to as a lithocell or cluster, according to some aspects. Lithographic apparatus 100 or 100’ can form part of lithographic cell 300. Lithographic cell 300 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. Conventionally, these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input / output ports I / Ol, I / O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus 100 or 100’. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.Example Light Source

[0056] FIG 4 shows a schematic representation of a light source 400 according to some aspects. Light source 400 can be used to implement source SO. According to some aspects, light source 400 comprises a plurality of modules. In some aspects, light source 400 can comprise a controller 410 connected to a light seed control module 412, a power supply module 414, and a metrology module 420. In some aspects, light seed control module 412 can control one or more modulators in light seed module 413 to generate light for light seed pulses such as pre-pulse 441 and main pulse 442. In some aspects, power supply module 414 can control the supply of RF power into light amplifier 415 based on instructions from controller 410. In some embodiments, power supply module 414 adjusts duty cycles and energy of RF power based on a dose a light source can generate from the pulses. In some embodiments, light amplifier 415 includes one or more gain medium units or stages of amplifiers. In some embodiments, power supply module 414 controls the RF power into the gain medium units or stage of amplifiers separately or jointly. For simplicity, only one light amplifier 415 is shown in FIG. 4. However, it should be understood that light source 400 can comprise a plurality of light amplifiers. For example, in some aspects, light source 400 can comprise a separate light amplifier for each of light seed pulse among a plurality of light seed pulses.

[0057] According to some aspects, operation of light source 400 can begin with the dropping of a fuel droplet 401. In some aspects, fuel droplet 401 can be configured to travel along the fuel travel direction 408 indicated by a dotted line into fuel collector 450. During the travel of fuel droplet 401, fuel droplet 401 can be configured in some aspects to be exposed to amplified pre-pulse 445, which is configured to transform fuel droplet 401 into a target 403. Target 403 can continue to travel substantially along fuel travel direction 408 to be exposed in some aspects to amplified main pulse 446, causing atoms within target 403 to transition to an excited energy state . As the excited atoms in target 403 decay from the excited energy state, target 403 can in some aspects emit EUV light, which can be collected and transmitted to a lithographic apparatus.

[0058] According to some aspects, as fuel droplet 401 descends along fuel travel direction 408 towards fuel collector 450, metrology module 420 measures a position of fuel droplet 401. In some aspects, metrology module 420 can be configured to measure a position of fuel droplet 401 by imaging light 432 from fuel droplet 401. In some aspects, metrology module 420 can be, for example, a camera or a photo detectorthat images fuel droplet 401. When fuel droplet 401 reaches a predetermined position along fuel travel direction 408 as detected by metrology module 420, the power supply synchronization is initiated in response to metrology module 420 transmitting a trigger signal 433 to controller 410.

[0059] In response to receiving trigger signal 433, controller 410 can, in some aspects, be configured to transmit a light seed control signal 435 to light seed module 412. According to some aspects, controller 410 can also control power supply module 414 by a power control signal 436.

[0060] According to some aspects, upon receiving light seed control signal 435, light seed control module 412 can send one or more light seeding signals 437 to light seed module 413. In some aspects, light seed control signal 435 from controller 410 can be configured to set a timing that is used by light seed control module 412 to control modulators in light seed module 413 to generate a plurality of light seed pulses using light seeding signals 437.

[0061] In some aspects, light seeding signals 437 can cause one or more modulators of light seed module 413 to generate and block light for seeding into power amplifier 415. In some aspects, light seed module 413 can comprise a plurality of modulators. For example, light seed module 413 can comprise one or more modulators configured to generate light for light seed pre-pulse 441 and one or more separate modulators configured to generate light for light seed main pulse 442. In some embodiments, one or more modulators of light seed module 413 are configured to block or stop at least one of light seed pre-pulse 441 and light seed main pulse 442 based on instructions from light seed control module 412. In some embodiments, light seed control module 412 enables, disables, and adjusts timings of the modulators to stop the pulses entering light amplifiers 415. In some embodiments, at least one laser in light seed module 413 operates when the pulses are inhibited from being amplified. According to some aspects, these modulators include at least one of an electro -optical modulator (EOM), which is configured to adjust a duration and / or a shape of the pulse, and an acousto-optic modulator (AOM), which is configured to adjust a frequency and / or a propagation direction or energy of the pulse. In some embodiments, the control of light seed control module 412 to the operations of the modulators of light seed module 413 can be synchronized to trigger signal 433.

[0062] According to some aspects, controller 410 can control power supply module 414 by a power control signal 436. In some aspects, power control signal 436 is configured to adjust parameters of pulses, such as a duty cycle and an energy. For example, power control signal 436 is a pulse width modulation signal. In some aspects, power supply module 414 operates in a “trigger synchronized” mode, wherein power control signal 436 is provided to power supply module 414 based on a timing arising from trigger signal 433. For example, when a required dose is lower than a pre-determined requirement, power supply module 414 can purposefully advance or delay a set timing to miss one or aplurality of pulses and controller 410 also initiate control, via power supply module 414, to inhibit light amplifier 415.

[0063] According to some aspects, power supply module 414 provides RF power into light amplifier 415 based on power control signal 438. Using the RF power provided by power supply module 414, power amplifier 415 can be configured to receive and amplify light seed pre-pulse 441 and transmit amplified pre-pulse 445. Similarly, power amplifier 415 can be configured to receive and amplify light seed main pulse 442 using the RF power provided by power supply module 414, and to transmit amplified main pulse 446.

[0064] FIG. 5A shows a schematic representation of a fuel droplet 501 and atarget 503 during normal operation of an EUV light source, such as light source 400 (in FIG. 4), according to some aspects. In some aspects, fuel droplet 501 can be released to travel downwards, where the x-axis indicates the direction of travel of fuel droplet 501. A metrology device, such as metrology module, can measure the position of fuel droplet 501 along the x-axis. Based on a measurement of the position of fuel droplet 501, the metrology device can send a trigger signal to a controller, as discussed above.

[0065] According to some aspects, pulses of light from a light amplifier can be divided into“on droplet” pulses and “off droplet” pulses (also called “missed pulses”). For example, during an “on droplet” pulse, shown in FIG. 5 A, fuel droplet 501 can descend to a first position 512 indicated by a dotted line. In some aspects, at first position 512, fuel droplet 501 can be exposed to pre-pulse 545, such as amplified pre-pulse, which can transform fuel droplet 501 into target 503. In some aspects, pre-pulse 545 can cause fuel droplet 501 to undergo one or more phase transitions. In some aspects, pre-pulse 545 can cause fuel droplet 501 to transition from a solid or liquid into a vapor or plasma. In at least one aspect, fuel droplet 501 comprises liquid tin that transitions, by exposure to pre-pulse 545, into plasma.

[0066] According to some aspects, target 503 can continue to descend substantially along the x-axis to be exposed to a main pulse 546 at a second position 514, indicated by a dotted line. In some aspects, by being exposed to main pulse 546, atoms within target 503 can transition to a higher energy level. Upon decaying from this higher energy level, the atoms of target 503 can emit EUV light that can be collected and focused for use in EUV photolithography.

[0067] FIG. 5B shows a schematic representation of a fuel droplet 505 during an “off droplet” or “missed pulse” of an EUV light source (e.g., light source 400 shown in FIG. 4), according to some aspects. During a missed pulse, aposition or timing of the plurality of pulses may be changed or delayed so that the position of fuel droplet 505 does not coincide with the vertical first position 512 shown by the dotted line during the firing of a pre-pulse 555, such as an amplified pre-pulse. As an example, during a missed pulse, fuel droplet 505 can be at a higher position 510, shown by a dotted line. Thus, during a missed pulse, fuel droplet 505 is not transformed into a target. Instead, fuel droplet 505 continues to descend through the vacuum chamber as unexposed fuel droplet 507. In some aspects, unexposed fuel droplet 507 can fall into a region to be partially exposed to a main pulse 556, such as an amplified main pulse. According to some aspects, the exposure of unexposed fuel droplet 507 tomain pulse 556 can cause portions of unexposed fuel droplet 507 to evaporate without being transformed into plasma. In some aspects, this evaporated fuel can condense on the internal walls of a vacuum chamber that houses the fuel droplet, such as in the tin contamination discussed above.

[0068] FIG. 6 shows the timing of a trigger signal (graph 601, where the y-axis is measured in volts), the RF power into the power amplifier of a light source (graph 603, where the y-axis is measured in relative units), and a main pulse from the light amplifier (graph 605, where the y-axis is measured in megawatts) in a “free run” mode of the EUV light source, according to some aspects. For simplicity, the timing of a pre-pulse light pulse is not shown. According to some aspects, in such a free run mode, main pulse 605 is synchronized to trigger signal 601, and thus a delay between trigger signal 601 and main pulse 605 is fixed. However, RF power 603 is not synchronized to trigger signal 601 in free run mode, and instead is supplied, for example, from a control module at a fixed frequency that is independent of trigger signal 601.

[0069] According to some aspects, in free run mode, as shown by RF power 603, a maximum in the RF power supplied to the light amplifiers may or may not correspond with main pulse 605. This occurs because, in free run mode, the power control signal is independent of the trigger signal, and is supplied at an interval that is not necessarily consistent with that of the trigger signal. Thus, in some aspects, due to the mismatch between a maximum in the RF power 603 and main pulse 605 in a free run mode, the energy and the maximum power of the main pulse 605 may be reduced.

[0070] In addition, according to some aspects, because RF power 603 is not synchronized to trigger signal 601, the power supply module will continue to supply RF power into the power amplifiers of the light source, even in the case of a “missed pulse.” That is, when the timing of a light seed module is changed or delayed to create a missed pulse, as described above, the power supply module is independent of this change, and continues to supply RF power 603 into the light amplifier. As a result, the light amplifier can still fire amplified light pulses, as described above, thereby wasting power and causing tin contamination.

[0071] FIG. 7A shows the timing of a trigger signal (graph 701, where the y-axis is measured in volts) and a power control signal (graph 703, where the y-axis is measured in volts) in a synchronized mode according to some aspects. In addition, FIG. 7B shows the timing of RF power into the light amplifier (graph 705, where the y-axis is measured in relative units) and the main pulse (graph 707, where the y-axis is measured in millijoules, mJ) in a synchronized mode according to some aspects. The x-axis of each graph shows the same time period. For illustrative purposes, the periods when the pulses are “on droplet” (“ON droplet”) and “off droplet” (“OFF”, corresponding to missed pulses) are labeled. Again, the timing of a pre-pulse laser pulse is not shown for simplicity. In synchronized mode, according to some aspects, power control signal 703 and RF power 705 are synchronized with trigger signal 701. In some aspects, the receipt of a first signal in the trigger signal 701 starts a “clock” that controls the power control signal 703, and thus RF power 705, as well as main pulse 707.

[0072] According to some aspects, a first signal in power control signal 703 is delayed from a first signal in trigger signal 701, so that each subsequent signal in power control signal 703 synchronizes with a received signal in trigger signal 701. In some aspects, an independent parameter, such as a predetermined delay timing, causes at most one missing pulse (“the first pulse” in some instances.), resulting in a small waste. However, in comparison with the energy saving from the proposed synchronization, such waste is negligible. Nonetheless, this synchronization ensures that each subsequent droplet (each later signal in trigger signal 701) coincides with a respective signal in power control signal 703. In some aspects, such synchronization can ensure that each signal in main pulse 707 occurs after a corresponding signal in power control signal 703 (e.g., compare the timing in 707 to that of 703). In addition, in some aspects, such synchronization ensures that RF power 705 reaches a maximum before main pulse 707.

[0073] According to some aspects, the output of light from the light source can be inhibited during missed pulse(s). The output of light from the light source can, in some aspects, be inhibited by either inhibiting power supply by the power supply module or by inhibiting light emission from the light seed module. According to some aspects, both power supply by the power supply module and light emission from the light seed module can be inhibited to thereby realize significant energy savings and substantially reduce, or even eliminate, tin contamination in the light source.

[0074] According to some aspects, inhibiting power supply by the power supply module comprises inhibiting the power supply control signal. For example, as shown in the “OFF” periods of FIGS. 7A and 7B, due to the synchronization of power control signal 703 with trigger signal 701, power control signal 703 can be inhibited for missed pulse(s) (the “OFF” periods). Thus, in some aspects, no signal in power control signal 703 is sent from the controller to the power supply module during a missed pulse. As a result, no RF power is generated by the power supply module, and thus the light amplifier does not receive any RF power to amplify light pulses received from the light seed module. In some aspects, such unamplified light pulses can lead to wasted electrical power. As an example, normal firing of an EUV light source in a lithographic apparatus may consume, for example, 500 kW, while merely maintaining the light source (without firing) may consume approximately half of that power. Thus, in some aspects, unamplified light pulses according to this example can waste, for example, approximately 250 kW of energy for each missed pulse. In addition, although such light pulses are not amplified, they can nonetheless have enough power to cause some amount of tin contamination.

[0075] In some aspects, although the power control signal may still be transmitted from the controller to the power supply module, actual delivery of RF power from the power supply module to the light amplifier can be inhibited. For example, power supply module can still receive a power control signal and generate RF power. However, in some aspects, during missed pulse(s), the generated RF power is not delivered to the light amplifier.

[0076] By inhibiting the supply of RF power to the light amplifier in the above ways, main pulse 707 does not fire during missed pulses. In some aspects, inhibiting the main pulse in either of theabove ways can suppress or avoid the problem of tin contamination, as discussed above. However, in some aspects, by synchronizing the power control signal to the trigger signal and inhibiting the power supply control during missed pulse(s), energy costs are reduced because RF power is not generated during missed pulse(s). According to the example above, it can be possible to realize from about 10% to about 20% energy savings in the operation of an EUV light source.

[0077] According to some aspects, power conservation is achieved by inhibiting the power control signal even where “OFF” periods, as discussed above, do not occur. For example, in some applications requiring high radiation doses, a light source and power amplifiers are operated in a continuous manner such that no “OFF” period occurs. As shown in FIG. 7B, by synchronizing the power control signal to the trigger signal, the energy of main pulse 707 increases slightly with each pulse of main pulse 707 (see the arrow labeled 708, showing the increase in the energy for each pulse of main pulse 707). The energy increases because more power is provided into the power amplifier(s) with each power control signal pulse than is extracted from the power amplifier(s) with each pulse. The result is a build-up of stored energy in the power amplifier(s).

[0078] In some aspects, the above building-up of energy in the power amplifier(s) helps to inhibit the power control signal without substantially suppressing the energy of the main pulse. That is, in some aspects, firing a main pulse while inhibiting the power control signal allows the system to extract some of the built-up energy from the power amplifier(s). In some aspects, up to 20% of pulses of the power control signal are inhibited while providing a pulse of main pulse 707 having an acceptable laser energy. Accordingly, comparing with other approaches, power conservation is achieved in the operation of the lithographic apparatus by inhibiting one or more pulses of the power control signal while operating the light source in a continuous pulse mode without any “OFF” periods.

[0079] According to some aspects, another option for inhibiting the output of light from the light source during missed pulse(s) is to inhibit the emission of light seed pulses by the light seed module. For example, the light seed control module can inhibit the output of light from the light source by mistiming or disabling one or more of modulators, such as the EOM and / or the AOM. In some aspects, a light seed control module can inhibit the output of light from the light source by mistiming or disabling only a first modulator or only a second modulator. In some aspects, the output of light from the light source can be inhibited by mistiming or disabling only the second modulator so that the light amplifier does not receive a main pulse light seed pulse from the light seed module.

[0080] According to some aspects, mistiming a modulator can comprise adding a delay to a light seeding signal that is sent to the modulator, so that the modulator does not function at the proper time during the missed pulse. In some aspects, disabling a modulator can comprise not sending a light seeding signal to the respective modulator, so that the modulator does not emit light seed pulse(s) during the missed pulse(s). In some aspects, disabling a modulator can comprise sending a disable signal or an off signal to the modulator as a light seeding signal, so that the modulator does not emit seed light during missed pulse(s). By inhibiting the emission of light seed pulses by the light seed module, in someaspects, it can be possible to eliminate tin contamination since no light is delivered to the light amplifier or fuel droplet during a missed pulse.

[0081] FIG. 8A shows a trigger signal (graph 801, where the y-axis is measured in volts) and a power control signal (graph 803, where the y-axis is measured in volts), according to some aspects. In addition, FIG. 8B shows RF power supplied into the power amplifier of the light source (graph 805, where the y-axis is measured in relative units) and a main pulse (graph 807, where the y-axis is measured in megawatts), according to some aspects. The x-axis of each graph in FIG. 8 shows the same time period. A pre-pulse light pulse is not shown for simplicity. In synchronized mode, according to some aspects, power control signal 803, and thus RF power 805, is synchronized with trigger signal 801. According to some aspects, because power control signal 803 is synchronized to trigger signal 801, a delay period T (shown by the double-ended arrows in 803) can be provided between trigger signal 801 and power control signal 803. Note that, in some aspects, no delay is provided after the first signal in trigger signal 801, to ensure that the light amplifier receives full RF power for the first signal in main pulse 807. According to some aspects, power control signal 803 can then be delayed by time period T so that each main pulse 807 occurs only after RF power 805 in the light amplifier is at a maximum.

[0082] In some aspects, a length of the delay T can be configured so that the sum of the length of the delay and the length of power control signal 803 is less than a time between a preceding signal in trigger signal 801 and the following signal in trigger signal 801, shown by the double arrow in 801.

[0083] According to some aspects, a “gain,” as used herein, relates to an increase in output energy between a light seed pulse and an amplified light pulse arising from RF power supplied to the light amplifier. In other words, the term “gain” or “RF gain” herein refers to an increase in output power of an amplified light pulse compared to the input power of the corresponding light seed pulse to the light amplifier.

[0084] According to some aspects, it may be desirable that the light seed module outputs light seed pulses having a power less than a maximum power of the light source. In some aspects, an output power of the light source can still approximate the maximum power of the light source by increasing the RF gain. As an example, in some aspects, the output power of the light seed module can be adjusted to be approximately 70% of the maximum power of the light source. In such cases, the RF gain can be adjusted so that the output power of the amplified light pulses is approximately equal to the maximum power of the light source, and such a condition can be set as a gain of 1.0. Thus, in some aspects, the gain can range from 0 (resulting in an output power of the amplified light pulses being 70% of the maximum power of the light source) to 1.0 (resulting in an output power of the amplified light pulses being 100% of the maximum power of the light source).

[0085] According to some aspects, the energy and the maximum power of the EUV light source can depend on the adjusted power of the light seed module and the RF gain in the light amplifier. In some aspects, in using a trigger-synchronized mode according to aspects herein, it is possible to realize an overall increase in the energy and the maximum power output by the EUV light source. Insome aspects, such a trigger-synchronized mode can result, for example, in an increase of approximately 1-2% in the energy and the maximum power of the EUV light source. However, this increase in the energy and the maximum power of the EUV light source can depend on the RF gain of the light amplifier. In some aspects, this increase in the energy and the maximum power of the EUV light source can be at a maximum value of approximately 1-2% when the RF gain of the light amplifier is approximately zero. This increased energy and maximum power may decrease as the gain increases, so that for a gain 1.0, the increased energy and maximum power are essentially gone. That is, in some aspects, the energy and the maximum power output by the EUV light source may be essentially the same in a trigger-synchronized mode as in a free run mode for a gain of 1.0. However, in some aspects, operating the EUV light source in a trigger-synchronized mode at a gain less than 1.0 can realize an increase in the energy and the maximum power output by the EUV light source compared to operating the EUV light source at the same gain in a free run mode.

[0086] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present disclosure is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0087] It is to be appreciated that the Detailed Description section, and not the Summary andAbstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way.

[0088] The present disclosure has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0089] While specific embodiments of the disclosure have been described above, it will be appreciated that embodiments of the present disclosure may be practiced otherwise than as described. The descriptions are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the disclosure as described without departing from the scope of the claims set out below.

[0090] The foregoing description of the specific embodiments will so fully reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and / or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein.

[0091] The breadth and scope of the protected subject matter should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

CLAIMS1. A method comprising : receiving, by a controller, a trigger signal configured to initiate a plurality of pulses from a light source; and synchronizing, by the controller, a power control signal with the trigger signal, the power control signal being configured to control a supply of power to one or more light amplifiers2. The method of claim 1, wherein the plurality of pulses comprises a pre-pulse and a main pulse, and the method further comprises: exposing a fuel droplet to the pre -pulse to transform the fuel droplet into a target, wherein the trigger signal is received in response to a measurement of a position of the fuel droplet; and exposing the target to the main pulse to cause the target to emit EUV radiation.

3. The method of claim 1, wherein in synchronizing the power control signal with the trigger signal: the power control signal is configured to control the supply of power to one or more light amplifiers.

4. The method of claim 1, wherein in synchronizing the power control signal with the trigger signal: the power control signal controls the supply of radio frequency (RF) power to the light source, and a light seeding signal is synchronized with the trigger signal, the light seeding signal being configured to control one or more modulators of a light seed module of the light source.

5. The method of claim 1, wherein the plurality of pulses comprises a missed pulse having a timing such that the missed pulse is configured to miss a fuel droplet, and the method further comprises: inhibiting an output of light from the light source for the missed pulse.

6. The method of claim 5, wherein inhibiting the output of light from the light source for the missed pulse comprises inhibiting performing pulse width modulation control of a radio frequency (RF) power supply for the missed pulse.

7. The method of claim 5, wherein inhibiting the output of light from the light source for the missed pulse comprises inhibiting RF power to the one or more light amplifiers.

8. The method of claim 5, wherein inhibiting the output of light from the light source for the missed pulse comprises mistiming or disabling one or more modulators in a light seed module of the light source.

9. The method of claim 1, wherein synchronizing the power control signal with the trigger signal comprises providing a delay between the trigger signal and the power control signal.

10. The method of claim 9, wherein a sum of a length of the delay and a length of the power control signal is less than a time between the trigger signal and a next trigger signal.

11. The method of claim 9, wherein a gain of the power provided by the light amplifier is controlled to have a value less than 1.0.

12. The method of claim 1, wherein the plurality of pulses are provided in a continuous manner, and the method further comprises: inhibiting performing pulse width modulation control of a radio frequency (RF) power supply for one or more pulses of the plurality of pulses.

13. A controller for a light source, the controller comprising a processor communicatively coupled to a memory, the memory being configured to store instructions that, when executed, cause the processor to: receive a trigger signal configured to initiate a plurality of pulses from a light source; and synchronize a power control signal with the trigger signal, the power control signal being configured to control a supply of power to the light source.

14. The controller of claim 13, wherein the processor is further configured to: inhibit an output of light from the light source for a missed pulse, the missed pulse being a pulse configured to miss a droplet that is configured to provide a target for the light source .

15. The controller of claim 13, wherein the plurality of pulses are provided in a continuous manner, and the processor is further configured to: inhibit performing pulse width modulation control of a radio frequency (RF) power supply for one or more pulses of the plurality of pulses.

16. The controller of claim 13, wherein the power control signal is further configured to: block one or more of the plurality of pulses traveling to a light amplifier.

17. A light source for a lithographic apparatus, the light source comprising: a power supply module configured to provide radio frequency (RF) power to one or more light amplifiers; and a controller configured to: receive a trigger signal from a metrology device configured to measure a position of a fuel droplet configured to provide a target for the light source; and in response to receiving the trigger signal, synchronize a power control signal with the trigger signal, the power control signal being a control signal configured to control supply of the RF power to the one or more light amplifiers to initiate a plurality of pulses from the light source.

18. The light source of claim 17, wherein the controller is further configured to inhibit an output of light from the light source for a missed pulse among the plurality of pulses, the missed pulse having a timing such that the missed pulse is configured to miss a droplet.

19. The light source of claim 17, further comprising a light seed module, wherein the light seed module comprises: a first modulator configured to provide a first light seed to the one or more light amplifiers to generate a pre-pulse of the plurality of pulses; and a second modulator configured to provide a second light seed to the one or more light amplifiers to generate a main pulse of the plurality of pulses.

20. The light source of claim 19, wherein the controller is configured to inhibit an output of light from the light source by mistiming or disabling the second modulator.

21. The light source of claim 19, wherein the first modulator is configured to adjust at least one of a duration and a shape of at least one of the pre-pulse and the main pulse.

22. The light source of claim 19, wherein the first modulator is configured to adjust at least one of a frequency and a propagation direction of one or more of the plurality of pulses.

23. The light source of claim 19, wherein the plurality of pulses are provided in a continuous manner, and the controller is further configured to: inhibit performing pulse width modulation control of a radio frequency (RF) power supply for one or more pulses of the plurality of pulses.

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