Laser focusing module

Abstract

A laser focusing system (330) for an EUV radiation source is described, the laser focusing system comprising: a first curved reflector (330.1), the first curved reflector being configured to receive a laser beam from a beam delivery system and to generate a first reflected laser beam (316); and a second curved reflector (330.2), the second curved reflector being configured to receive the first reflected laser beam (316) and to generate a second reflected laser beam (317), wherein the laser focusing system (330) is configured to focus the second reflected laser beam (317) onto a target location (370) in a container (350) of the EUV radiation source (360).

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 19167261.7 filed on April 4, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The invention relates to a laser focusing system which can be applied in an EUV radiation source. The invention also relates to an EUV radiation source comprising a laser focusing system. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate (usually on a target portion of the substrate). For example, a lithographic apparatus can be used in the manufacture of integrated circuits (ICs). In this case, a pattern forming device, alternatively referred to as a mask or reticle, can be used to produce a circuit pattern to be formed on a single layer of the IC. The pattern can be transferred to a target portion (e.g., including a portion of a die, one or more dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is usually performed by imaging the pattern onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are continuously patterned.

[0005] Photolithography is widely regarded as one of the key steps in manufacturing ICs and other devices and / or structures. However, as the size of features manufactured using photolithography becomes smaller and smaller, photolithography is becoming a more critical factor in allowing the manufacture of miniature ICs or other devices and / or structures.

[0006] A theoretical estimate of the limit of pattern printing can be given by the Rayleigh law for resolution, as shown in equation (1):

[0007]

[0008] Where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process-dependent adjustment factor also known as the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. From equation (1), it is known that a reduction in the minimum printable size of a feature can be achieved in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA, or by reducing the value of k1.

[0009] In order to shorten the exposure wavelength and thus reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength in the range of 5 to 20 nm, for example in the range of 13 to 14 nm. It has further been proposed that EUV radiation having a wavelength less than 10 nm, for example in the range of 5 to 10 nm, such as 6.7 mm or 6.8 mm, could be used. Possible sources include laser produced plasma (LPP) sources, but other types of sources are also possible.

[0010] An example of current progress in the development of LPP sources for EUV lithography is described in the paper "High power LPP EUV source system development status" by Benjamin Szu Min Lin, David Brandt, Nigel Farrar (SPIE Digital Library Reference DOI: 10.1117 / 12.839488) in Volume 7520 of SPIE Proceedings, Lithography Asia 2009, December 2009. In a lithographic apparatus, the source apparatus will typically be contained within its own vacuum housing, with a small exit aperture provided to couple the EUV radiation beam into the optical system where the radiation is to be used.

[0011] To be useful in high-resolution patterning for lithography, the EUV radiation beam must be conditioned to obtain desired parameters, such as uniformity of intensity and angular distribution, when it reaches the mask. Examples of irradiation systems are described in U.S. Patent Application Publication Nos. US 2005 / 0274897A1 (Carl Zeiss / ASML) and US 2011 / 0063598A (Carl Zeiss). Example systems include a "fly's eye" illuminator that converts the highly non-uniform intensity distribution of the EUV light source into a more uniform and controllable source.

[0012] To generate EUV radiation in an LPP radiation source, a laser or laser system is used to irradiate a target, such as Sn droplets. In particular, such an LPP radiation source may include one or more lasers for irradiating the target with one or more pre-pulses and a main pulse to convert the target and generate the EUV radiation.

[0013] Typically, such lasers or laser systems include a laser focusing system configured to focus a laser beam (e.g., a pre-pulse laser beam or a main pulse laser beam) onto a target location within the container of the LPP radiation source or EUV source. Known arrangements of such laser focusing systems can be quite bulky and can produce a focused laser beam that does not have the desired optical properties for irradiating the target and thereby effectively converting the target into EUV radiation. Summary of the invention

[0014] Aspects of embodiments of the present invention are directed to providing alternative laser focusing systems for use in EUV radiation sources.

[0015] According to one aspect of the present invention, there is provided a laser focusing system for an EUV radiation source, the laser focusing system comprising:

[0016] - a first curved reflector configured to receive the laser beam from the beam delivery system and to produce a first reflected laser beam;

[0017] - a second curved reflector configured to receive the first reflected laser beam and generate a second reflected laser beam,

[0018] Wherein, the laser focusing system is configured to focus the second reflected laser beam onto a target location in the container of the EUV radiation source.

[0019] According to another aspect of the present invention, a laser source is provided, comprising the laser focusing system according to the present invention.

[0020] According to yet another aspect of the present invention, there is provided an EUV radiation source, comprising the laser source according to the present invention.

[0021] According to a further aspect of the present invention, there is provided a lithographic apparatus comprising an EUV radiation source according to the present invention.

[0022] These aspects of the present invention and its various optional features and implementations will be understood by those skilled in the art from the following description of examples. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference numerals indicate corresponding parts, and in which:

[0024] Figure 1 schematically depicts a lithography system according to an embodiment of the present invention;

[0025] Figure 2 Depicted Figure 1 A more detailed view of the device;

[0026] Figure 3 A laser system is described, comprising a laser focusing system according to the invention;

[0027] Figure 4 A laser focusing system known in the art is depicted;

[0028] Figure 5 depicts a laser focusing system according to an embodiment of the present invention;

[0029] Figure 6 A fuel target for use in an LPP radiation source and a laser beam configured to direct toward the fuel target are described.

[0030] Figure 7 A laser focusing system according to the invention is described, wherein a first control strategy is implemented;

[0031] Figure 8 A laser focusing system according to the invention is described, wherein a second control strategy is implemented;

[0032] Fig. 9 A laser focusing system according to the invention is described, wherein a third control strategy is implemented.

[0033] Figures 10 to 12 Three possible embodiments of the laser focusing system according to the invention are depicted. DETAILED DESCRIPTION

[0034] Figure 1 A lithographic system 100 according to an embodiment of the present invention is schematically depicted, the lithographic system comprising a lithographic apparatus and an EUV radiation source configured to generate EUV radiation (eg, an EUV radiation beam). In the embodiment shown in the figure, the EUV radiation source comprises a source collector module SO. In the embodiment shown in the figure, the lithography scanning device includes: an illumination system (illuminator) IL, which is configured to adjust a radiation beam B (e.g., EUV radiation); a support structure (e.g., a mask table) MT, which is configured to support a pattern forming device (e.g., a mask or a mask plate) MA and is connected to a first positioner PM configured to accurately position the pattern forming device; and a substrate table (e.g., a wafer stage) WT, which is configured to hold a substrate (e.g., a wafer coated with a resist) W and is connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g., a reflective projection system) PS, which is configured to project a pattern imparted to the radiation beam B by the pattern forming device MA onto a target portion C (e.g., including one or more tube cores) of the substrate W.

[0035] The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping or controlling radiation.

[0036] The manner in which the support structure MT holds the patterning device MA depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as, for example, whether the patterning device is held in a vacuum environment. The support structure may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be, for example, a frame or table that may be fixed or moved as required. The support structure may ensure that the patterning device is in a desired position, for example relative to a projection system.

[0037] The term "patterning device" should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section, such as creating a pattern in a target portion of a substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device (such as an integrated circuit) created in the target portion.

[0038] The pattern forming device can be transmissive or reflective. Examples of pattern forming devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in photolithography and include mask types such as binary masks, alternating phase shift masks, and attenuated phase shift masks, 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 to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam reflected by the mirror matrix.

[0039] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, depending on the exposure radiation used or other factors such as the use of a vacuum. It may be desirable to use a vacuum for EUV radiation, as other gases may absorb too much radiation. Therefore, with the aid of a vacuum wall and a vacuum pump, a vacuum environment may be provided for the entire beam path.

[0040] As depicted here, the apparatus is reflective (eg, employing a reflective mask).

[0041] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and / or two or more mask tables). In such a "multi-stage" machine, the extra tables may be used in parallel, or preparatory steps may be performed on one or more tables while one or more other tables are being used for exposure.

[0042] Reference Figure 1 , the illuminator IL receives the extreme ultraviolet radiation beam from the source collector module SO of the EUV radiation source. Methods for generating EUV light include but are not necessarily limited to converting a material into a plasma state, the material having at least one element having one or more emission lines in the EUV range, such as xenon, lithium or tin. In one such method, which is generally referred to as laser produced plasma ("LPP"), the desired plasma can be generated by irradiating a fuel, such as droplets, beams or clusters of a material having the desired emission line element, with a laser beam. The source collector module SO can be a laser (in Figure 1 The laser and the EUV radiation source may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0043] In this case, the laser is not considered to form part of the lithography system, and the radiation beam is delivered from the laser to the source collector module by means of a beam delivery system including, for example, suitable directional mirrors and / or a beam expander. In other cases, the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator (commonly referred to as a DPP source).

[0044] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Typically, at least the outer radial extent and / or the inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator may be adjusted. Furthermore, the illuminator IL may comprise various other components, such as a facet field and a pupil mirror arrangement. The illuminator may be used to adjust the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

[0045] A radiation beam B is incident on a patterning device (e.g., mask) MA held on a support structure (e.g., mask table) MT and is patterned by the patterning device. After having been reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through a projection system PS which focuses the beam B onto a target portion C of a substrate W. With the aid of a second positioner PW and a position sensor PS2 (e.g., an interferometer, a linear encoder, or a capacitive sensor), the substrate table WT can be accurately moved, for example, so as to position a different target portion C in the path of the radiation beam B. Similarly, a first positioner PM and a further position sensor PS1 can be used to accurately position the patterning device (e.g., mask) MA relative to the path of the radiation beam B. The patterning device (e.g., mask) MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

[0046] The depicted device can be used in at least one of the following modes:

[0047] 1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT remain substantially stationary while an entire pattern imparted to the radiation beam is projected at one time onto a target portion C (i.e. one static exposure). The substrate table WT is then moved in the X and / or Y direction so that a different target portion C can be exposed.

[0048] 2. In scan mode, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The speed and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0049] 3. In another mode, the support structure (e.g. mask table) MT is kept substantially stationary so as to hold the programmable patterning device, and the substrate table WT is moved or scanned whilst a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, a pulsed radiation source is typically employed, and the programmable patterning device is updated as required after each movement of the substrate table WT or between successive radiation pulses during a scan. This mode of operation can readily be applied to maskless lithography using a programmable patterning device, such as a programmable mirror array of the type described above.

[0050] Combinations and / or variations of the above modes of use or entirely different modes of use may also be employed.The embodiments to be described involve scanning, as in modes 2 and 3 just mentioned.

[0051] Although the use of lithographic equipment in IC manufacturing may be specifically mentioned herein, it should be understood that the lithographic equipment described herein may have other applications, such as manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc. It should be understood by those skilled in the art that in the context of these alternative applications, any use of the term "wafer" or "die" in the present disclosure may be considered to be synonymous with the more general term "substrate" or "target portion", respectively. The substrates mentioned herein may be processed in, for example, a coating and developing system or track (a tool that typically applies a resist layer to a substrate and develops the exposed resist), a measurement tool, and / or an inspection tool before or after exposure. Where applicable, the disclosure herein may be applied to such substrate processing tools and other substrate processing tools. In addition, the substrate may be processed more than once, for example, to produce a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0052] Figure 2 The system 100 is shown in more detail, the system comprising an EUV radiation source (including a source collector module SO) and a lithography scanning device (including an illumination system IL), and a projection system PS. The source collector module SO of the EUV radiation source is constructed and arranged so that a vacuum environment can be maintained in the enclosure 220 of the source collector module SO. The systems IL and PS are also contained in their own vacuum environments. The EUV radiation emitting plasma 210 can be formed by a laser-generated LPP plasma source. The function of the source collector module SO is to transmit the EUV radiation beam 20 from the plasma 210 so that it is focused on a virtual source point. The virtual source point is usually called an intermediate focus (IF), and the source collector module is arranged so that the intermediate focus IF is located at or near an aperture 221 in the enclosure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[0053] From the aperture 221 at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example comprises a faceted field mirror arrangement 22 and a faceted pupil mirror arrangement 24. These arrangements form a so-called "fly's eye" illuminator arranged to provide a desired angular distribution of the radiation beam 21 at the patterning device MA, and a desired uniformity of radiation intensity at the patterning device MA. When the beam 21 is reflected at the patterning device MA, it is held by the support structure (mask table) MT to form a patterned beam 26, and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by a wafer stage or substrate table WT.

[0054] Each system IL and PS is arranged in its own vacuum or near-vacuum environment, defined by an enclosure similar to enclosure 220. There may typically be more elements in illumination system IL and projection system PS than shown. Furthermore, there may be more mirrors than those shown in the figures. For example, in addition to Figure 2 In addition to those reflective elements shown in , there may be one to six additional reflective elements in the illumination system IL and / or the projection system PS. For example, the above-mentioned US patent application publication shows three additional elements in the illumination system.

[0055] Considering the source collector module SO in more detail, a laser energy source including a laser 223 is arranged to deposit laser energy 224 into a fuel such as xenon (Xe), tin (Sn) or lithium (Li), thereby producing a highly ionized plasma 210 with an electron temperature of several tens of eV. Using other fuel materials, such as Tb and Gd, higher energy EVU radiation can be produced. The high energy radiation produced during the deexcitation and recombination / recombination of these ions is emitted from the plasma, collected by a near normal incidence collector CO and focused on an aperture 221. The plasma 210 and the aperture 221 are located at the first and second focal points of the collector or collector mirror CO, respectively.

[0056] To deliver the fuel (e.g., liquid tin), a droplet generator 226 is disposed within the enclosure 220, the droplet generator being arranged to emit a high frequency droplet stream 228 toward a desired location of the plasma 210. In operation, laser energy 224 is delivered synchronously with the operation of the droplet generator 226 to deliver a pulse of radiation to convert each fuel droplet into a plasma 210. The delivery frequency of the droplets may be several thousand hertz, such as 50 kHz. In practice, the laser energy 224 is delivered in at least two pulses: a pre-pulse with limited energy is delivered to the droplets before the droplets reach the plasma site in order to vaporize the fuel material into a small cloud, and then a main pulse of laser energy 224 is delivered to the desired location of the cloud to generate the plasma 210. A trap or well 230 is provided on the opposite side of the enclosure 220 to capture fuel that has not been converted to plasma for any reason.

[0057] Many additional components in the source collector module and the lithographic apparatus are present in a typical apparatus, although not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example, to prevent the deposition of fuel material from damaging or compromising the performance of the collector or collector reflector CO and other optical devices. In addition, one or more spectral purity filters will be included in the source collector module SO and / or the irradiation system IL. These filters are used to eliminate radiation of as many unwanted wavelengths as possible, except for the desired UV radiation wavelength, generated by the laser and / or plasma 210. The spectral purity filter can be located near the virtual source point or at any point between the collector and the virtual source point. The filter can be placed at other locations in the radiation path, for example, downstream of the virtual source point IF. Multiple filters can be deployed. Those skilled in the art are familiar with the need for these measures and the ways in which these measures can be implemented, and no other details are required for the purposes of this disclosure.

[0058] For more details, refer to Figure 2 The laser 223 of FIG. 1 is a laser 223, which in this embodiment is of the MOPA (Master Oscillator Power Amplifier) ​​type. This consists of a "master" laser or "seed" laser, labeled MO in the figure, followed by a power amplifier (PA). A beam delivery system 240 is provided to deliver laser energy 224 to module SO. In practice, the pre-pulse element of the laser energy will be delivered by a separate laser, not shown separately in the figure. The laser 223, the fuel source (i.e., droplet generator) 226, and other components may be controlled, for example, by a source control module 242.

[0059] As will be known to those skilled in the art, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components and the radiation beams 20, 21, 26. At each portion of the apparatus, a local reference coordinate system of the X, Y and Z axes may be defined. The Z axis is substantially coincident with the directional optical axis O at a given point in the system and is generally perpendicular to the plane of the patterning device (reticle) MA and to the plane of the substrate W. In the source collector module, the X axis is substantially coincident with the direction of the fuel flow (228, described below), while the Y axis is orthogonal to the fuel flow, as shown in FIG. Figure 2 On the other hand, near the support structure MT holding the mask MA, the X-axis is generally transverse to the scanning direction aligned with the Y-axis. Figure 2 In this region, the X-axis points out of the page, again as marked. These nomenclatures or designations are conventional in the art and will be used herein for convenience. In principle, any reference frame may be chosen to describe the device and its behavior.

[0060] Referring to the illumination system in more detail, the faceted field mirror device 22 comprises an array of a plurality of discrete facets such that the EUV radiation beam 20 is divided into a plurality of sub-beams, one of which is marked 260 in the figure. Each sub-beam is directed towards a single facet on the faceted pupil mirror device 24. The facets of the pupil mirror device 24 are arranged to direct their discrete sub-beams onto a target which is a slit-like region of the pattern forming device MA. The division into sub-beams 260 and the combination into a single beam 21 are designed to produce a very uniform illumination over the slit region when the illumination arriving from the source collector module is very non-uniform in its angular distribution. As is well known, the facets of the devices 22 and / or 24 may be steerable and / or shieldable in order to implement different illumination modes.

[0061] The conditioned EUV radiation beam 21 is delivered to the patterning device MA via a conditioning and masking module 262. This module comprises a masking unit, also called a reticle mask (REMA), which may have movable blades or vanes defining the extent of an illumination slit in the X and Y directions. Typically, the illumination slit applied in EUV type lithography apparatus may be curved.

[0062] REMA may also be preceded by an illumination uniformity correction module (UNICOM).

[0063] To expose a target portion C on the substrate W, a radiation pulse is generated on the substrate table WT and the mask table MT performs a synchronized motion 266, 268 to scan the pattern on the patterning device MA through the illumination slit.

[0064] Examples of illumination systems that include REMA and UNICOM functionality are described in US Patent Application Publication Nos. 2005 / 0274897A1 and 2011 / 0063598A.

[0065] A number of measures are applied in the source controller 242. Such measures include monitoring to ensure that the virtual source point IF is aligned with the aperture 221 at the exit of the source collector module SO. In systems based on LPP sources, control of alignment is usually achieved by controlling the position of the plasma 210 rather than by moving the collector optics CO. The collector optics, exit aperture 221 and illuminator IL are accurately aligned during the setup process so that the aperture 221 is located at the second focus of the collector optics. However, the exact position of the virtual source point IF formed by the EUV radiation at the exit of the source optics depends on the exact position of the plasma 210 relative to the first focus of the collector optics. In order to fix this position accurately enough to maintain adequate alignment, active monitoring and control are usually required.

[0066] To this end, the source control module (controller) 242 in this example controls the location of the plasma 210 (source of EUV radiation) by controlling the injection of fuel and also, for example, the timing of the excitation pulses from the laser. In a typical example, the excitation pulses of laser radiation 224 are sent at a rate of 50kHz (period 20μs) and in bursts lasting anywhere from (say) 20ms to 20 seconds. The duration of each main laser pulse may be about 1μs, and the resulting EUV radiation pulse may last about 2μs. By appropriate control, the EUV radiation beam is maintained accurately focused by the collector CO on the aperture 221. If this is not achieved, all or part of the beam will impinge on the surrounding material of the enclosed structure.

[0067] The source control module 242 is supplied with monitoring data from one or more sensor arrays (not shown) which provide a first feedback path related to information about the location of the plasma. The sensors may be of various types, for example, as described in the above-mentioned U.S. Patent Application Publication No. 2005 / 0274897A1. The sensors may be located at more than one position along the radiation beam path. For example only, they may be located around and / or behind the field mirror device 22. The sensor signals just described may be used to control the optical systems of the illuminator IL and the projection system PS. They may also be used via a feedback path to assist the control module 242 of the source collector module SO to adjust the intensity and position of the EUV plasma source 210. For example, the sensor signals may be processed to determine the observed location of the virtual source IF, and this is extrapolated to indirectly determine the location of the EUV source. If the virtual source location drifts, a correction is applied by the control module 242 to re-center / center the beam in the aperture 221 as indicated by the sensor signals.

[0068] Rather than relying entirely on signals from the illuminator sensors, additional sensors and feedback paths may typically be provided in the source collector module SO to provide more rapid, direct and / or self-contained, i.e., self-sustaining, control of the radiation source. Such sensors may include one or more cameras, for example, to monitor the location of the plasma. In this way, the beam 20 is maintained in the aperture 221, damage to equipment is avoided, and efficient use of the radiation is maintained.

[0069] References Figure 1 and Figure 2 The EUV radiation system described will typically include a laser or laser system ( Figure 1The laser system is configured to deliver laser energy in at least two pulses: a pre-pulse with limited energy is delivered to the droplets before the droplets reach the plasma site in order to vaporize the fuel material into a small cloud. Then, a main pulse of laser energy is delivered to the desired site of the cloud to generate the plasma. The plasma thus generated emits output radiation (e.g., EUV radiation) that is collected using a radiation collector disposed in the source collector module.

[0070] Typically, for example when a CO2 laser is used to provide the laser beam for fuel excitation, the laser system and the EUV radiation source may be separate entities.

[0071] In this arrangement, the laser system may, for example, include a laser source configured to generate one or more laser beams, an optional laser beam delivery system configured to direct or guide the generated laser beam from the laser toward the EUV radiation source, and a laser focusing system configured to focus the laser beam onto a target (e.g., a tin droplet) within a container of the EUV radiation source.

[0072] The present invention provides a laser focusing system which can be applied to such a laser system.

[0073] Figure 3 A laser system 300 is schematically shown, comprising a laser or laser source 310 configured to generate a laser beam 315. Such a laser or laser source 310 may, for example, include a seed laser and one or more power amplifiers for generating the laser beam 315. The laser system 300 also includes a beam delivery system 320 configured to direct or guide the laser beam 315 toward a desired location. In the embodiment shown, the laser system 300 also includes a laser focusing system 330 according to an embodiment of the present invention. It should be noted that the laser focusing system 330 may also be used to direct or guide the laser beam toward a desired location, as will be explained in more detail below.

[0074] In the embodiment shown in the figure, the laser focusing system 330 includes a first curved reflector 330.1, which is configured to receive the laser beam 315 from the beam delivery system 320 and generate a first reflected laser beam 316. The laser focusing system 330 also includes a second curved reflector 330.2, which is configured to receive the first reflected laser beam 316 and generate a second reflected laser beam 317. In the embodiment shown in the figure, the laser focusing system 330 is configured to focus the second reflected laser beam 317 onto a target site 340. Figure 3 Schematically, a target site 340 is shown disposed within a container or enclosure 350 of an EUV radiation source or source collector module 360. Such an enclosure 350 may thus be used in conjunction with Figure 2 The enclosure 220 of the source collector module is shown for comparison.

[0075] and Figure 2 In contrast to the arrangement shown, the laser focusing system 330 according to an embodiment of the invention is arranged such that the second reflected laser beam 317 has an optical axis 370 that is substantially parallel to a horizontal plane. In an embodiment of the invention, the laser focusing system is thus configured such that the laser beam 317 focused on the droplet target in the EUV radiation source has an optical axis that is substantially horizontal, i.e. substantially parallel to the indicated Y direction. Substantially horizontal within the meaning of the invention refers to an angle less than or equal to 25 degrees, preferably less than 20 degrees, relative to a horizontal plane. The application of a laser focusing system such as the system 330 enables the second reflected laser beam 317 to be arranged substantially parallel to the laser beam 315 as received from the beam delivery system. Within the meaning of the invention, the two laser beams may be configured to be substantially parallel when the difference in orientation between the two laser beams is less than 25 degrees, preferably less than 20 degrees. In the case where the laser beam 315 has a substantially horizontal orientation, the optical axis of the laser beam 317 focused on the droplet target is thus at an angle greater than 65 degrees relative to the vertical axis, i.e. the indicated Z axis. As will be appreciated by those skilled in the art, the choice of the angle of the optical axis of the laser beam 317 focused on the target also affects the optical axis of the downstream generated EUV light. Figure 2 As will be apparent from the arrangement shown, the optical axis O of the generated EUV radiation beam will generally be in the same direction as the optical axis 224 of the laser beam focused on the target droplet. Figure 3 As shown, the laser beam focused on the droplet target (typically a fuel target) will be reflected by a collector mirror (e.g., Figure 3 The reflector 380 shown in Figure 2 The target is irradiated by a central opening in a reflector (CO) shown in FIG. 1 , which focuses the generated EUV radiation substantially along the same optical axis as the laser beam focused on the fuel target. Thus, the laser focusing system according to the present invention is capable of generating an EUV radiation beam along a substantially horizontal axis, whereas in known EUV radiation source designs used in lithographic apparatus, the optical axis O of the EUV radiation beam, for example, is applied to Figure 2The angle of the EUV radiation beam of the illumination system IL shown is at a relatively large angle relative to the horizontal plane. The inventors have observed that the use of relatively large angles adversely affects the design options of the downstream illumination system IL and projection system PS. It has been envisaged that the use of relatively small angles of the generated EUV radiation beam enables the design of an optical system downstream of the EUV radiation source with fewer components. As shown in FIG. Figure 3 Alternatively to the arrangement shown, whereby the second reflected laser beam 317 has an optical axis 370 substantially parallel to a horizontal plane, the laser focusing system may be configured such that the angle between the optical axis 370 of the second reflected laser beam 317 and the horizontal plane is between 50 and 75 degrees.

[0076] It may further be noted that the laser focusing system according to the invention also leads to an improved optical design compared to typical or known designs of laser focusing systems as applied in EUV radiation sources. Figure 3 It can be seen that the laser focusing system according to the present invention uses a curved reflector 330.2 as the final optical element to guide the laser beam 317 to the fuel target or toward the target site 340. In the known laser focusing system, a flat reflector is usually used as the final optical element. Figure 4 An example of such a known arrangement is schematically shown. Figure 4 Schematically shown, the laser focusing system 400 comprises a curved reflector 400.1 configured to receive a laser beam 401 and reflect the laser beam 401 towards a second curved reflector 400.2, which reflects the laser beam towards a third flat reflector 400.3, which focuses the laser beam towards a focal point 402 along a substantially vertical optical axis 403. The inventors have observed that known designs may suffer from one or more of the following drawbacks: the third flat reflector 400.3 may cause a significant contribution to system aberrations, such as astigmatism, due to deformation of the reflector. Since the angle of incidence on the flat third reflector depends on the site of incidence, delays may also be caused. In addition, since the flat third reflector is relatively close to the focal point 402, and thus the plasma generation site, and its orientation is close to horizontal, it may be more susceptible to contamination or degradation, such as degradation of the coating or optical surface quality.

[0077] According to the present invention, a laser focusing system is provided in which these disadvantages are eliminated or mitigated. In an embodiment of the present invention, the final reflector 330.2 that focuses the laser beam 317 onto a target or target portion is therefore a curved reflector rather than a flat reflector. Examples of such reflectors that can be applied to the present invention include spherical reflectors, paraboloid concentrators (paraboloid) or parabolic reflectors, elliptical reflectors, axisymmetric reflectors, or free-form reflectors. According to the present invention, the penultimate reflector 330.1 along the optical path of the laser beam focused on the target portion is also a curved reflector. In an embodiment, the reflector is configured to diverge the received laser beam, for example, a laser beam received from a laser source or a beam delivery system. By applying a diverging reflector that diverges the received laser beam as the penultimate reflector, the laser beam 317 focused on the target portion can be configured to have a relatively large numerical aperture (NA) while being relatively far away from the target portion 340. Or, in other words, if the reflector 330.1 would not be a diverging reflector but a flat reflector, the cross-section of the beam 316 striking the second reflector 330.2 would be smaller and in order to obtain a focused laser beam 317 with a relatively large NA, the reflector 330.2 would have to be relatively close to the target site 340. Having the second reflector 330.2 at a relatively large distance from the target site 340 provides the advantage of reducing the risk of contamination of the reflector, such as caused by the focused laser beam 317 striking the fuel target. As such, a sequence having a diverging penultimate reflector 330.1 followed by a converging reflector 330.2 (which reflects the focused laser beam towards the target site) can be considered to provide a significant advantage over prior art laser focusing systems. Further advantages can be obtained by providing a focused laser beam towards the target site along a substantially horizontal optical axis.

[0078] In an embodiment of the present invention, the first curved reflector and / or the second curved reflector of the laser focusing system are configured to receive the laser beam at a relatively small angle of incidence. In an embodiment, the angle of incidence of the laser beam onto the first curved reflector, the second curved reflector or both is less than or equal to 30 degrees, preferably less than or equal to 25 degrees. By doing so, a more accurate wavefront of the laser beam irradiated on the droplet target or the fuel target can be achieved. To achieve this, the vertical positions of the reflectors should be relatively close to each other. In other words, when the vertical positions of the curved reflectors 330.1 and 330.2 are substantially different, it may be difficult to achieve the desired low angle of incidence. Typically, the vertical position of the laser beam 315 received from the laser source or beam delivery system is lower than the desired vertical position of the focused laser beam 317 focused on the target portion 340. In addition, for practical reasons, it may be cumbersome or undesirable to lift the laser source or laser beam delivery system to a desired height in order to cause the laser beam to reach a desired vertical position to achieve a relatively small angle of incidence of the curved mirror of the laser focusing system. To avoid this, embodiments of the present invention also include one or more additional mirrors configured to redirect a laser beam (e.g., laser beam 315 received from the beam delivery system) toward the first curved mirror. Since the one or more additional mirrors are configured to redirect or reposition the laser beam so that it reaches an appropriate location and orientation relative to the first curved mirror, the one or more additional mirrors may also be considered as part of the beam delivery system.

[0079] In an embodiment of the present invention, the number of additional mirrors configured to redirect the laser beam towards the first curved mirror is less than five.

[0080] In an embodiment of the invention, the one or more additional reflectors may include or form a periscope or telescope system. Such a system may, for example, be configured to translate in a vertical direction a laser beam 315 as provided by the laser source or the beam delivery system. Thus, such a periscope system as applied in an embodiment of the invention enables translation in a vertical direction of a laser beam 315 as received by the laser focusing system.

[0081] Figure 5A laser focusing system 430 according to an embodiment of the present invention is schematically shown, the laser focusing system comprising a periscope system including a pair of reflectors 410.1, 410.2 configured to reflect an incident laser beam 315 (e.g., a laser beam received from a laser source or a laser beam delivery system) such that a laser beam 318 reflected off a second reflector 410.2 of the periscope system propagates in a substantially horizontal direction at an elevated vertical position. In the embodiment shown, the incident laser beam 315 is elevated, for example, by a distance D in a vertical direction. By elevating the laser beam 315 as received from the laser source or beam delivery system, the angle of incidence of the laser beam at the curved reflectors 330.1 and 330.2 can be kept relatively small. This can be understood when considering that in the absence of the flat reflectors 410.1 and 410.2, the reflector 330.1 would have to be located at the location of the reflector 410.1 to receive the laser beam 315.

[0082] In the embodiment shown in the figure, the periscope system 410.1, 410.2 includes two plane mirrors 410.1, 410.2. Alternatively, one or more mirrors of the periscope system 410.1, 410.2 can also be curved mirrors. By applying one or more curved mirrors in the periscope system, the characteristics of the incident laser beam 315 can be adjusted. In particular, the size of the incident laser beam 315 can be controlled by using one or more curved mirrors in the periscope system. In addition, by using one or more curved mirrors in the periscope system, the outgoing laser beam 318 of the periscope system can be transformed into a divergent or convergent laser beam.

[0083] As will be understood by a skilled person, the periscope system applied in the present invention may also include more than two reflectors, for example three or four.

[0084] In an embodiment of the present invention, the laser focusing further comprises a control unit, which is configured to control the position of the target portion 340, that is, the position of the focus of the second reflected laser beam 317 in the EUV container 360, such as Figure 3 This control of the location of the focal point of the laser beam 317 helps ensure that the laser beam 317 hits the fuel target, such as a tin droplet.

[0085] Figure 6 The trajectory of a fuel target as it might appear in an EUV container is schematically shown. Figure 6A fuel target source 510, also referred to above as a droplet generator 226, is schematically shown, which is configured to generate a stream of fuel targets, such as tin droplets. The generated stream of fuel targets is configured to propagate along an axis 520. The axis 520 may, for example, span across a collector reflector (such as Figure 2 The reflector CO shown Figure 3 Axis 520 is considered to define the z direction of a coordinate system as applied in EUV containers. Figure 6 In FIG. 5 , a dashed line 550 represents an xy plane within the EUV container, and the xy plane 550 includes a focal point 530. The focal point 530 shown in the figure can be regarded as corresponding to Figure 5 A target location 340 is shown, i.e., the location at which the laser beam 317 should be focused in order to achieve effective conversion of the fuel target to EUV light. In the embodiment as shown, the focus 530 can, for example, be located at the origin of a defined xyz coordinate system. Due to defects in the fuel target source 510 and / or the applied fuel, the actual trajectory 522 of the fuel target may deviate slightly from the ideal or desired trajectory 520. Therefore, the laser beam 317 as provided by the laser focusing system may have to be focused at a slightly different location 530' in order to effectively convert the fuel target to EUV light. In order to predict possible deviations from the trajectory of the fuel target, it is desirable to control the position of the target location, i.e., the location at which the laser beam output by the laser focusing system is focused.

[0086] In order to control the position of the target site, an embodiment of the laser focusing system according to the present invention further includes a control system 540. The control unit 540 may be embodied as a controller, a microcontroller, a computer, etc. The control unit 540 may include one or more input terminals 540.1 for receiving one or more input signals 542 and one or more output terminals 540.2 for outputting one or more output signals 544 for controlling the laser focusing system.

[0087] In an embodiment of the present invention, the control unit 540 as applied may be configured to receive a target location signal representing the location of a fuel target to be struck by the laser beam 317 of the laser focusing system as an input signal 542. Such a target location signal may be generated, for example, by a detector configured to detect the location of a target such as a fuel target. Figure 6, such a detector 570 is schematically shown. In the embodiment shown in the figure, the detector 570 is configured to detect a target 532, such as a liquid fuel target, when the target 532 crosses a plane 560. In an embodiment, the detector may include a plurality of detectors or one or more detector arrays configured to determine the x, y position of the target 532 when the target 532 crosses the plane 560. The detector 570 may also be configured to output a target position signal representing the x, y position of the target. In the embodiment shown in the figure, the plane 560 is substantially parallel to the plane 550 and is arranged between the fuel target source 510 and the plane 550 including the focus 530 of the collector reflector. In this way, the detector 570 will detect the fuel target 532 before the fuel target crosses the plane 550. During the time when the target moves from the position 532 to the position 530', the control unit 540 may derive the coordinates of the position 530', i.e., the position where the fuel target will cross the plane 550, based on the target position signal as received. Based on the determined coordinates of the location of the fuel target, the control unit 540 can then determine one or more output signals 544 (also called control signals), which are used to control the laser focusing system so that the focus of the laser beam 317 roughly coincides with the location 530' of the fuel target.

[0088] In an embodiment, the one or more output signals 544 may be applied, for example, to control the position, orientation and / or shape of one or more optical elements of the laser focusing system, thereby controlling the position of the focus of the laser beam (e.g., laser beam 317) output by the laser focusing system. In such an embodiment, the one or more output signals may be applied, for example, to control the position, orientation and / or shape of one or more reflectors as applied in the laser focusing system according to the present invention. By controlling the position, orientation and / or shape of one or more reflectors as applied in the laser focusing system, the position of the focus of the second reflected laser beam 317 can be controlled with one or more degrees of freedom.

[0089] In an embodiment, the one or more optical elements of the laser focusing system may be mounted to one or more frames. In such an embodiment, the position, orientation and / or shape of one or more reflectors as applied may be controlled by controlling the position of one or more frames. In an embodiment with at least two optical elements of the laser focusing system, the at least two optical elements may be mounted to a common frame such that their position, orientation and / or shape may be controlled synchronously and / or simultaneously.

[0090] According to the present invention, various control strategies may be implemented to ensure that the second reflected laser beam 317 as output by the laser focusing system is focused on the target site when the fuel target reaches the site.

[0091] The strategy may include controlling the position of the focus within the EUV vessel with one or more degrees of freedom by controlling the position, orientation or shape of at least one of the first periscope mirror, the second periscope mirror, the first curved mirror or the second curved mirror or any combination thereof. The control strategy as applied may also include controlling the timing of the laser beam or laser pulses, thereby controlling where along the trajectory of the fuel target is irradiated.

[0092] As a first example, the control unit is configured to control the position of a focus within the EUV container with three degrees of freedom by controlling the position, orientation or shape of one of the first periscope mirror, the second periscope mirror, the first curved mirror or the second curved mirror or any combination thereof. Figure 7 Such an example is schematically illustrated. Figure 7 A laser focusing system 530 according to an embodiment of the present invention is schematically shown, the laser focusing system 530 comprising a first periscope reflector 410.1, a second periscope reflector 410.2, a first curved reflector 330.1 and a second curved reflector 330.2. The laser focusing system 530 can be advantageously applied to focus the laser beam 317 to a focal point 620, which corresponds, for example, to the focal point of a collector reflector of an EUV radiation source. In the example shown in the figure, it is assumed that the Z axis coincides with the optical axis of the second reflected laser beam 317 as output by the laser focusing system 530, the Y axis is as shown in the figure, and it is assumed that the X axis is perpendicular to the YZ plane. Figure 6 Compared with the coordinate system of Figure 7 The x-axis shown may be, for example, Figure 6The Z axis shown in the figure coincides or is parallel. In the embodiment shown in the figure, the second curved reflector 330.2 is configured to be able to shift along the Z axis, the Y axis and the X axis. This shift can be achieved, for example, by an actuator assembly 610. Such an actuator assembly can, for example, include one or more actuators for shifting the reflector 330.2 along the indicated axis. As will be understood by those skilled in the art, the shift of the reflector 330.2 in the Z direction, the Y direction or the X direction will also result in the shift of the focus 620 of the second reflected laser beam 317. In this way, by controlling the position of the second curved reflector 330.2 with three degrees of freedom, the focus 620 can be adjusted according to the expected target location, that is, the location where the fuel target is expected to be when the laser pulse is supplied. When such an embodiment is used, the control unit of the laser focusing system can, for example, be configured to determine where the fuel target will be at a certain moment (for example, the moment when the laser is fired). In such an embodiment, the control unit (e.g. control unit 540) can thus determine the trajectory of the fuel target based on the target location signal as received, and use this trajectory to control the laser focusing system to focus the laser beam 317 to the location where the target will be when the laser is fired. In such an embodiment, it is thus assumed that the timing or firing of the laser is fixed. Based on the known moment when the laser will be fired and the known trajectory of the target, the control unit determines where the target will be at the known firing moment. In an alternative embodiment, the firing of the laser, in particular the timing of the firing of the laser, can be considered as a variable. In such an embodiment, it is sufficient to control the focus position with only two degrees of freedom. Reference Figure 6 It can be noted that the control unit 540 can be configured to determine when a specific target that has been detected while passing through the plane 560 will reach or cross the plane 550. Based on this, the control unit can be configured to control the time of emitting the laser beam 317 so that the laser is emitted when the fuel target reaches or crosses the plane 550. In this embodiment, there is no need to control the focus along the Z axis by controlling the orientation or position of the laser beam. Figure 6 Thus, in an embodiment of the invention, the control unit of the laser focusing system is configured to control the position of the laser focus with two degrees of freedom combined with controlled timing of the laser beam. In such an embodiment, Figure 7 The actuator assembly 610 shown schematically can be configured to control the position of the focus of the laser beam 317 in the YZ plane as indicated in the figure in combination with the control of the timing of the laser beam, so as to ensure that the fuel target is irradiated by the laser beam 317 at a desired location (for example, as close as possible to the focus of the collector mirror of the EUV radiation source using the laser focusing system).

[0093] In such Figure 7 In the illustrated embodiment, the actuator assembly 610 is configured to control the position of the second curved mirror 330.2 with three or two degrees of freedom in order to control the position of the focus 620 of the laser beam 317. A similar effect may be obtained by controlling the position of one or more other mirrors as applied in the laser focusing system according to the present invention.

[0094] In a second example of a control strategy as may be applied in a laser focusing system according to the invention, the required focus control is distributed over a plurality of mirrors. Figure 8 Such an embodiment is schematically shown. Figure 8 The laser focusing system 630 according to an embodiment of the present invention is schematically shown, and the laser focusing system 630 includes a first periscope reflector 410.1, a second periscope reflector 410.2, a first curved reflector 330.1 and a second curved reflector 330.2. Figure 7 . In the embodiment shown in the figure, the laser focusing system 630 further comprises a first actuator assembly 610, which is configured to position or displace the second curved reflector 330.2 in the Z direction, i.e., in the direction of the optical axis of the second reflected laser beam 317. The laser focusing system 630 further comprises a second actuator assembly 720, which is configured to rotate or tilt the second periscope reflector 410.2 around the Y axis and the X axis. By means of the first actuator assembly 710 and the second actuator assembly 720, the laser beam propagating through the laser focusing system is thus controlled with three degrees of freedom. By doing so, the focus 620 of the second reflected laser beam 317 is controlled with three degrees of freedom X, Y, Z by controlling the position of the second curved reflector 330.2 in the Z direction and tilting the second periscope reflector 410.2 around the X axis and the Y axis. Z, Rx and Ry (Rx and Ry refer to rotation or tilting around the X axis and the Y axis). By doing so, the focus 620 of the second reflected laser beam 317 can also be controlled with three degrees of freedom. Such a laser focusing system 630 may thus advantageously be applied to control the position of the laser focus 620 within an EUV radiation source.

[0095] In a similar manner as discussed above, the control strategy as illustrated by the second example may also be combined with control of the timing of the emission of the laser beam. By doing so, control of position or orientation with only two degrees of freedom may be sufficient to obtain effective control of the laser focus, i.e., control whereby the second reflected laser beam 317 is configured to focus on a target portion of the fuel target.

[0096] Fig. 9A third example of a control strategy as may be applied in an embodiment of a laser focusing system according to the invention is schematically illustrated. Fig. 9 A laser focusing system 930 according to an embodiment of the present invention is schematically shown, and the laser focusing system 930 includes a first periscope reflector 410.1, a second periscope reflector 510.2, a first curved reflector 330.1 and a second curved reflector 330.2. In the embodiment shown in the figure, the second periscope reflector 510.2 is a curved reflector, not a flat reflector. The second periscope reflector 510.2 can be a concave reflector or a convex reflector. In the embodiment shown in the figure, the laser focusing system 930 also includes an actuator assembly 800, which is configured to control the position and orientation of the second periscope reflector 510.2. Specifically, the actuator assembly 800 is configured to position or shift the reflector 510.2 along the optical axis 510.3 of the second periscope reflector 510.2. By doing so, the position of the focus 620 of the laser beam 317 along the Z axis can be modified or adjusted. The actuator assembly 800 may further be configured to rotate or tilt the second periscope reflector 510.2 about the Y-axis and the X-axis (e.g., in a manner similar to the actuator assembly 720 of the second example). By doing so, control of the position of the focus 620 of the laser beam 317 with three degrees of freedom is again obtained. In a similar manner as discussed above, the control strategy as illustrated by the third example may also be combined with control of the timing of the emission of the laser beam. By doing so, control of the position or orientation with only two degrees of freedom may be sufficient to obtain effective control of the laser focus, i.e., control whereby the second reflected laser beam 317 is configured to focus on the target portion of the fuel target.

[0097] As a fourth example of how to use the laser focusing system according to the present invention to control the focus position, the use of one or more deformable mirrors can be mentioned. By using a deformable mirror, the position of the focus 620 of the second reflected laser beam 317 can also be controlled. For example, by adjusting the curvature of such a deformable mirror, the focus position 620 can be shifted along the optical axis of the laser beam 317.

[0098] In an embodiment, the various control strategies described above may be advantageously combined.

[0099] In an embodiment of the present invention, control of the position of the focus of the second reflected laser beam 317 in a specific degree of freedom or direction is established by combined control of two or more optical components (e.g., mirrors) of the laser focusing system. As will be appreciated by those skilled in the art, various different optical components (e.g., mirrors) used in the laser focusing system may have different characteristics, such as different weights, different resonant frequencies, etc. Therefore, the control performance or ability regarding the accuracy or resolution that can be obtained may be different for different mirrors or optical components applied. In addition, the available or obtainable range may be different for different optical components, in which respect, the range refers to the available or possible displacement range of the focus under certain degrees of freedom.

[0100] In view of these characteristics, it may be suboptimal to use only one component to control the position of the focus with a particular degree of freedom.For example, it may be advantageous to control the position of the focus with a particular degree of freedom by using two or more optical components to control the degree of freedom in combination.

[0101] With reference to the examples described above, it can be pointed out that if Figures 7 to 9 As indicated, the positioning of the focus of the laser beam 317 in the Z direction can be determined by the translation of the second curved reflector 330.2, or by the translation of the second periscope reflector 410.2 or the displacement or deformation of the curved periscope reflector 510.2. Instead of using only one of the options to shift the focus of the laser beam 317 in the Z direction, it may be advantageous to achieve the desired focus shift (e.g., based on the target site signal) by a combined displacement or deformation of at least two components, for example. By doing so, the requirement for controlling the position of the focus with a particular degree of freedom can be achieved by the combined efforts of at least two components.

[0102] As an example, control of the position of the focus of the laser beam 317 in the Z direction may be obtained, for example, by controlling the displacement of a first reflector of the laser focusing system and controlling the displacement of a second reflector of the laser focusing system. In such an embodiment, the displacement of the first reflector may, for example, enable a displacement of the focus position in the Z direction within a relatively large range, while the displacement of the second reflector may, for example, enable a displacement of the focus position in the Z direction within a relatively small range but with a higher degree of accuracy. In such an embodiment, the displacement of the first reflector may result in a rough positioning of the focus in the Z direction within a relatively large range, while the displacement of the second reflector may thus result in a fine positioning of the focus in the Z direction within a relatively small range. Using such a combined effort of two or more optical components, improved performance may be achieved and / or control requirements of one or more optical components may be relaxed.

[0103] In such an embodiment according to the present invention (i.e., an embodiment whereby the position of the focus is controlled with a specific degree of freedom through a combination of at least two components), the control unit of the laser focusing system may be configured to determine a first control signal for controlling a first optical component of the at least two components and a second control signal for controlling a second optical component of the at least two components based on the target site signal as received. In such an embodiment, the first control signal may be configured to cause the first optical component to shift, rotate or deform so as to achieve a first shift of the focus with a specific degree of freedom, and the second control signal may be configured to cause the second optical component to shift, rotate or deform so as to achieve a second shift of the focus with a specific degree of freedom; the combination of the first shift and the second shift results in the focus being shifted to a desired position.

[0104] To achieve such a combined control effort, whereby two or more optical components (such as mirrors) are applied to achieve a combined goal, such as achieving a specific focus position with specific degrees of freedom, various control strategies may be applied.

[0105] In an embodiment, the control unit is configured to control the position of the focus with one degree of freedom by controlling a single mirror with one or more degrees of freedom. Alternatively, the control unit may be configured to control the position of the focus with one degree of freedom by controlling multiple mirrors with one or more degrees of freedom. In general, the control unit as applied in the embodiments of the laser focusing system according to the present invention may be configured to control the position of the focus with N degrees of freedom by controlling one or more mirrors with M degrees of freedom, where M=N, M and N are non-zero integers. Alternatively, the control unit may be configured to control the focus with N degrees of freedom by controlling one or more mirrors with M degrees of freedom, where M≠N.

[0106] In an embodiment of the present invention, a set of reflectors as applied, i.e. a first curved reflector, a second curved reflector and optionally one or more additional reflectors, comprises at least one long-stroke reflector and at least one short-stroke reflector. Within the meaning of the present invention, a long-stroke reflector refers to a reflector that can be displaced within a relatively large range, and a short-stroke reflector refers to a reflector that can be displaced within a relatively short range. Thus, in one embodiment of the present invention, the control unit of the laser focusing system can be configured to control the position and / or orientation of at least one long-stroke reflector within a relatively large range, and to control the position and / or orientation of at least one long-stroke reflector within a relatively short range.

[0107] In such an embodiment, the control unit of the laser focusing system may, for example, include a low bandwidth controller for controlling at least one long stroke mirror and a high bandwidth controller for controlling at least one short stroke mirror. The low bandwidth controller may, for example, be configured to apply a set and forget control of the long stroke mirror. In such an embodiment, the low bandwidth control may be used to desaturate the high bandwidth control, i.e., reduce the saturation.

[0108] In an embodiment, at least one long-stroke mirror may be controlled with a bandwidth of less than 5 Hz. In an embodiment, at least one short-stroke mirror may be controlled with a bandwidth of more than 0,1 Hz.

[0109] In order to displace or deform the one or more mirrors of the laser focusing system according to the present invention, various types of actuators may be applied. Examples of these actuators include, but are not limited to, electromagnetic actuators, such as Lorentz actuators or piezoelectric actuators. In general, any linear or rotary actuator may be suitable or adapted to be applied to control the position, orientation or shape of a mirror as applied in the laser focusing system according to the present invention.

[0110] Figures 5 to 9 Various aspects and embodiments of a laser focusing system according to the present invention are illustrated. As mentioned above, the laser focusing system employs curved mirrors as the final and penultimate mirrors to direct the laser beam to a focal point or target location 340, 530, 620. Various embodiments of the laser focusing system according to the present invention include additional mirrors upstream of the final and penultimate curved mirrors.

[0111] It can be pointed out that there are many options for the position and orientation in space of the different reflectors as applied. In this regard, it can be pointed out that although Figures 5 to 9 The laser beams shown in FIG. 5 are drawn in a single plane, but this is not necessarily the case.

[0112] therefore, Figures 10 to 12 Three possible embodiments of the laser focusing system according to the invention are schematically illustrated, in which not all laser beams are arranged in the same plane. These arrangements may also be referred to as out-of-plane arrangements.

[0113] Fig.10Schematically shown is the path of laser beam 1000 which is received by first mirror 1010 and then travels along mirrors 1020, 1030, 1040 towards focal point FP. In the embodiment shown, mirrors 1040 and 1030 may correspond to, for example, final mirror 330.2 and penultimate mirror 330.1, respectively, as described above.

[0114] In the embodiment as shown, reflectors 1010 and 1020 may correspond to, for example, a reflector or periscope system, such as reflectors 410.1 and 410.2 described above.

[0115] In such Fig.10 In the embodiment shown, it can be observed that the laser beam 1000 received by the first reflector 1010 and the laser beams 1000.1 and 1000.2 directed to the reflectors 1020 and 1030, respectively, are arranged in a plane, namely the front plane of the cube 1050. In the embodiment shown in the figure, the laser beams 1000.2, 1000.3 and 1000.4 are also arranged in the plane indicated by the reference numeral 1060. In the embodiment shown in the figure, the laser beams 1000, 1000.1 and 1000.2 are thus arranged in a first plane, while the laser beams 1000.2, 1000.3 and 1000.4 are arranged in a different second plane. It can be seen that in the embodiment shown in the figure, the first plane is perpendicular to the second plane. It can be pointed out that this is not necessarily the case.

[0116] Fig.11 An embodiment of the laser focusing system according to the invention is schematically shown, whereby the first plane, i.e. the front plane of the cube 1050 comprising the laser beams 1000, 1000.1 and 1000.2, is not perpendicular to the second plane 1070 comprising the laser beams 1000.2, 1000.3 and 1000.4. Fig.10 Compared to the schematically shown arrangement, it can also be noted that laser beam 1000 . 1 is no longer perpendicular to laser beam 1000 .

[0117] exist Fig.10 and Fig.11 In the embodiment of , laser beams 1000.2, 1000.3 and 1000.4 are arranged in the same plane (plane 1060 or plane 1070). It can be pointed out that alternative out-of-plane arrangements can be designed that do not require each plane to contain three of laser beams 1000, 1000.1, 1000.2, 1000.3 and 1000.4.

[0118] Fig.12An alternative embodiment of a laser focusing system according to the invention is schematically shown, whereby the laser beams 1000.2, 1000.3 and 1000.4 are not arranged in the same plane. In the embodiment shown, the laser beams 1000, 1000.1 and 1000.2 are arranged in the front plane of the cube 1050, while the laser beams 1000.3 and 1000.4 are arranged in the plane 1060. Fig.10 In contrast to the arrangement of , laser beam 1000.2 is not arranged in the same plane as laser beams 1000.3 and 1000.4.

[0119] It may be noted that, from an optical point of view, it may be preferred that the laser beam reaching the penultimate mirror, the laser beam reaching the final mirror and the laser beam emitted by the final mirror are located in the same plane.

[0120] Despite Figures 10 to 12 The mirrors 1010 and 1020 shown may be referred to as mirrors of a periscope system, but it may be noted that the present invention may also be practiced without the use of a periscope system.

[0121] In this regard, it may be noted that a single mirror configured to receive a laser beam, such as laser beam 1000, and configured to redirect laser beam 1000 toward penultimate curved mirror 1030 may be used to replace the single mirror configured to receive a laser beam, such as laser beam 1000, and to redirect laser beam 1000 toward penultimate curved mirror 1030. Figures 10 to 12 Reflectors 1010 and 1020 are shown.

[0122] Generally speaking, the laser focusing system according to the present invention may thus comprise, in addition to the final curved reflector and the penultimate curved reflector, one or more additional reflectors for redirecting the laser beam towards the first curved reflector (i.e. the penultimate curved reflector). In this arrangement, a plane may be defined (the plane comprising the laser beam received by the most downstream reflector of the one or more additional reflectors and the laser beam reflected by the most downstream reflector towards the penultimate curved reflector). Figures 10 to 12 In the embodiment of the present invention, the reflector 1020 can be used as the most downstream reflector of the one or more additional reflectors.

[0123] With regard to the use of the one or more additional mirrors, the out-of-plane arrangement of the laser focusing system may also be defined as an arrangement whereby the laser beam as reflected by the most downstream mirror (e.g., mirror 1020) toward the penultimate curved mirror (e.g., mirror 1030), the laser beam reflected by the penultimate curved mirror, and the laser beam reflected by the penultimate curved mirror (e.g., mirror 1040) are not arranged in the same plane. In this arrangement, the laser beam redirected toward the first curved mirror, the first reflected laser beam, and the second reflected laser beam are thus arranged in substantially different planes.

[0124] The laser focusing system according to the invention may advantageously be applied in a laser source according to the invention.Such a laser source may for example comprise a seed laser, one or more power amplifiers and optionally a beam delivery system.

[0125] The laser source according to the invention may advantageously be applied in an LPP radiation source, for example a radiation source for generating EUV radiation suitable for use in an EUV lithography apparatus.

[0126] Although specific embodiments of the present invention have been described above, it should be understood that the present invention can be practiced in a manner other than that described. The behavior of the device can be largely defined by a computer program containing one or more machine-readable instruction sequences for implementing certain steps of the method disclosed above, or a data storage medium (e.g., a semiconductor memory, a magnetic disk, or an optical disk) having such a computer program stored therein. The above description is intended to be exemplary and not restrictive. Thus, those skilled in the art will appreciate that the described invention may be modified without departing from the scope of the various aspects set forth below.

[0127] 1. A laser focusing system for an EUV radiation source, the laser focusing system comprising:

[0128] - a first curved reflector configured to receive a laser beam from a beam delivery system and to produce a first reflected laser beam;

[0129] - a second curved reflector configured to receive the first reflected laser beam and generate a second reflected laser beam,

[0130] The laser focusing system is configured to focus the second reflected laser beam onto a target location in the container of the EUV radiation source.

[0131] 2. The laser focusing system according to aspect 1, wherein the angle between the optical axis of the second reflected laser beam and the horizontal plane is less than 25 degrees, preferably less than 20 degrees.

[0132] 3. The laser focusing system according to aspect 1, wherein an angle between an optical axis of the second reflected laser beam and a horizontal plane is between 50 degrees and 75 degrees.

[0133] 4. The laser focusing system of aspects 1, 2 or 3, wherein the first curved mirror is configured to receive the laser beam at an incident angle of less than 30 degrees.

[0134] 5. The laser focusing system of aspects 1, 2 or 3, wherein the first curved mirror is configured to receive the laser beam at an incident angle between 30 degrees and 45 degrees.

[0135] 6. The laser focusing system according to any one of aspects 1 to 5, wherein the second curved mirror is configured to receive the laser beam at an incident angle of less than 30 degrees.

[0136] 7. The laser focusing system of any one of aspects 1 to 5, wherein the second curved mirror is configured to receive the laser beam at an incident angle between 30 degrees and 45 degrees.

[0137] 8. The laser focusing system according to any one of the preceding aspects, wherein the first curved reflector comprises a parabolic, spherical, ellipsoidal, axisymmetric or free-form reflective surface.

[0138] 9. The laser focusing system according to any one of the preceding aspects, wherein the second curved reflector comprises an elliptical, parabolic, spherical, axisymmetric or free-form reflective surface.

[0139] 10. The laser focusing system according to any of the preceding aspects, further comprising one or more additional mirrors configured to redirect the laser beam towards the first curved mirror.

[0140] 11. The laser focusing system of clause 10, wherein the number of additional mirrors configured to redirect the laser beam toward the first curved mirror is less than five.

[0141] 12. The laser focusing system of clause 10, wherein one or more of the additional mirrors are configured to overcome a difference in orientation between an incident laser beam and the laser beam desired by the first curved mirror.

[0142] 13. The laser focusing system of clause 10, wherein the one or more additional mirrors comprise a periscope system.

[0143] 14. The laser focusing system of aspect 13, wherein the periscope system is configured to overcome differences in location between the received laser beam and the redirected laser beam.

[0144] 15. The laser focusing system of aspect 13 or 14, wherein the periscope system is configured to translate the laser beam in a substantially vertical direction.

[0145] 16. A laser focusing system according to any one of Aspects 13 to 15, wherein the periscope system includes a first periscope reflector, the first periscope reflector is used to receive the laser beam and reflect the laser beam toward a second periscope reflector, and the second periscope is configured to reflect the laser beam toward the first curved reflector.

[0146] 17. A laser focusing system according to any one of clauses 10 to 16, wherein one or more of the additional mirrors is curved.

[0147] 18. The laser focusing system of clause 17, wherein the one or more additional reflectors of a curved surface comprise an ellipsoidal, parabolic, spherical, axisymmetric, or free-form reflective surface.

[0148] 19. The laser focusing system of aspect 17 or 18, wherein the one or more additional mirrors of a curved surface form a telescope system configured to change the diameter of the laser beam.

[0149] 20. The laser focusing system of aspect 18 or 19, wherein the one or more additional mirrors of a curved surface form a telescope system configured to change the divergence of the laser beam.

[0150] 21. The laser focusing system of any one of aspects 10 to 20, wherein the laser beam redirected toward the first curved reflector, the first reflected laser beam, and the second reflected laser beam are arranged in substantially the same plane.

[0151] 22. The laser focusing system according to any one of aspects 10 to 20, wherein the laser beam redirected toward the first curved mirror, the first reflected laser beam and the second reflected laser beam are arranged in substantially different planes.

[0152] 23. A laser focusing system according to any one of aspects 10 to 20, wherein the laser beam received by the one or more additional reflectors and the laser beam redirected toward the first curved reflector define a first plane, and wherein the first reflected laser beam and the second reflected laser beam define a second plane that is different from the first plane.

[0153] 24. The laser focusing system of clause 23, wherein the first plane is substantially perpendicular to the second plane.

[0154] 25. The laser focusing system of clause 23, wherein the second plane is not perpendicular to the first plane.

[0155] 26. The laser focusing system of aspect 23 or 25, wherein the second plane is parallel to the first plane.

[0156] 27. The laser focusing system of clause 22, 23 or 24, wherein the laser beam redirected toward the first curved mirror is arranged in both the first plane and the second plane.

[0157] 28. The laser focusing system of clause 22, 23 or 24, wherein the laser beam redirected toward the first curved mirror is not arranged in the second plane.

[0158] 29. The laser focusing system according to any one of the preceding aspects, wherein the laser beam as received by the first curved mirror, the first reflected laser beam and the second reflected laser beam are not arranged in the same plane.

[0159] 30. The laser focusing system according to any one of the preceding clauses, further comprising a control unit configured to control a position of a focus of the second reflected laser beam within the EUV container.

[0160] 31. A laser focusing system according to aspect 30, wherein the control unit is configured to receive a target site signal representing a site of a target of the second reflected laser beam, and wherein the control unit is configured to control the position of the focus of the second reflected laser beam based on the target site signal.

[0161] 32. The laser focusing system of clause 31, wherein the target location signal comprises a target location trajectory representing the location of the target as a function of time.

[0162] 33. The laser focusing system of aspect 31, wherein the control unit is configured to control the timing of the laser beam based on the target position trajectory.

[0163] 34. The laser focusing system of clause 33, wherein the control unit is configured to control the position of the focal point within the EUV vessel in a plane substantially perpendicular to the target position trajectory.

[0164] 35. The laser focusing system according to aspect 34, wherein the control unit is configured to control the timing of the laser beam so as to focus the laser beam on the target when the target reaches or crosses the plane.

[0165] 36. The laser focusing system of clause 35, wherein the plane comprises a focal point of a collector mirror of the EUV radiation source.

[0166] 37. A laser focusing system according to any one of Aspects 30 to 36, wherein the control unit is configured to control the position of the focus within the EUV container with three degrees of freedom by controlling the position, orientation or shape of at least one of the first curved mirror, or the second curved mirror, or one or more of the additional mirrors.

[0167] 38. A laser focusing system according to aspect 37, wherein the control unit is configured to control the position and / or orientation of the second curved mirror with one, two or three degrees of freedom.

[0168] 39. A laser focusing system according to aspect 38, wherein the control unit is configured to control the position and / or orientation of the first curved mirror with one, two or three degrees of freedom.

[0169] 40. The laser focusing system of aspect 39, wherein the first curved mirror and the second curved mirror are mounted to a frame.

[0170] 41. A laser focusing system according to aspect 40, wherein the control unit is configured to control the position and / or orientation of the frame in one, two or three degrees of freedom.

[0171] 42. A laser focusing system according to any one of aspects 38 to 41, wherein the first curved mirror, the second curved mirror and the one or more additional mirrors include at least one long-stroke mirror and at least one short-stroke mirror.

[0172] 43. A laser focusing system according to aspect 42, wherein the control unit is configured to control the position and / or orientation of the at least one long-stroke reflector within a relatively large range, and to control the position and / or orientation of the at least one short-stroke reflector within a relatively short range.

[0173] 44. The laser focusing system of aspect 43, wherein the at least one long-stroke mirror comprises a set-and-forget function.

[0174] 45. The laser focusing system of aspect 44, wherein the at least one long-stroke mirror comprises a bandwidth of less than 5 Hz.

[0175] 46. ​​A laser focusing system according to aspect 43, wherein the at least one short-stroke mirror comprises a bandwidth greater than 0, Hz.

[0176] 47. A laser focusing system according to any of the preceding aspects referring to aspect 31, wherein the control unit is configured to control the position of the focus with one degree of freedom by controlling a single mirror with one or more degrees of freedom.

[0177] 48. A laser focusing system according to any of the preceding aspects referring to aspect 31, wherein the control unit is configured to control the position of the focus with one degree of freedom by controlling a plurality of mirrors with one or more degrees of freedom.

[0178] 49. A laser focusing system according to any of the preceding aspects of cited aspect 31, wherein the control unit is configured to control the position of the focus with N degrees of freedom by controlling one or more mirrors with M degrees of freedom, where M=N.

[0179] 50. A laser focusing system according to any of the preceding aspects of reference aspect 31, wherein the control unit is configured to control the position of the focus with N degrees of freedom by controlling one or more mirrors with M degrees of freedom, wherein M≠N.

[0180] 51. The laser focusing system according to any one of aspects 31 to 50, further comprising a detector configured to detect a location of the target, the detector being configured to output the target location signal.

[0181] 52. A laser focusing system according to any one of clauses 31 to 51, wherein the control unit is configured to control the position of the focus within the EUV container in a plane substantially perpendicular to the trajectory of the target with two degrees of freedom.

[0182] 53. A laser focusing system according to any of the preceding aspects, wherein the laser beam has an optical axis substantially parallel to the horizontal plane.

[0183] 54. The laser focusing system of aspect 16, wherein the first periscope reflector and / or the second periscope reflector are substantially plane mirrors.

[0184] 55. The laser focusing system of aspect 16, wherein the first periscope reflector and / or the second periscope reflector comprises a curved reflector.

[0185] 56. The laser focusing system of aspect 16, wherein the first periscope reflector and / or the second periscope reflector are substantially plane mirrors.

[0186] 57. A laser source comprising a laser focusing system according to any one of the preceding aspects.

[0187] 58. An EUV radiation source comprising the laser source according to clause 57.

[0188] 59. A lithographic apparatus comprising an EUV radiation source according to clause 58.

Claims

1. A laser focusing system for an EUV radiation source, the laser focusing system comprising: - a first curved reflector configured to receive a laser beam from a beam delivery system and to produce a first reflected laser beam; - a second curved reflector configured to receive the first reflected laser beam and generate a second reflected laser beam, wherein the laser focusing system is configured to focus the second reflected laser beam to a target location in a container of the EUV radiation source, wherein the container comprises a collector mirror, wherein an angle between an optical axis of the second reflected laser beam and a horizontal axis is less than 25 degrees, wherein the horizontal axis is defined by the optical axis of the collector reflector, wherein the laser beam is received by one or more additional mirrors, the one or more mirrors redirecting the laser beam toward the first curved mirror; and The one or more additional mirrors are configured to overcome a position difference between the received laser beam and the redirected laser beam and translate the laser beam in a substantially vertical direction.

2. The laser focusing system of claim 1, wherein the first curved mirror is configured to receive a laser beam having an incident angle of less than 30 degrees, and wherein the second curved mirror is configured to receive a laser beam having an incident angle of less than 30 degrees.

3. The laser focusing system of any one of the preceding claims, wherein the one or more additional mirrors comprise a periscope system configured to overcome differences in location between the received laser beam and the redirected laser beam, and Wherein the periscope system is configured to translate the laser beam in a substantially vertical direction.

4. The laser focusing system of claim 3, wherein the periscope system comprises a first periscope reflector for receiving the laser beam and reflecting the laser beam toward a second periscope reflector, the second periscope being configured to reflect the laser beam toward the first curved reflector.

5. The laser focusing system according to any one of claims 1 to 4, wherein the one or more additional mirrors of a curved surface form a telescope system, which is configured to change the diameter of the laser beam and / or change the divergence of the laser beam.

6. The laser focusing system according to claim 3 or 4, wherein: The periscope system and the first and second curved reflectors are arranged such that the laser beam redirected toward the first curved reflector, the first reflected laser beam, and the second reflected laser beam are arranged in substantially the same plane.

7. The laser focusing system according to claim 3 or 4, wherein: The periscope system and the first and second curved reflectors are arranged such that the laser beam redirected toward the first curved reflector, the first reflected laser beam, and the second reflected laser beam are arranged in substantially different planes.

8. The laser focusing system of any one of claims 1, 3 and 4, wherein the laser beam received by the one or more additional reflectors and the laser beam redirected toward the first curved reflector define a first plane, and wherein the first reflected laser beam and the second reflected laser beam define a second plane that is different from the first plane.

9. The laser focusing system of claim 8, wherein the first plane is substantially perpendicular to the second plane.

10. The laser focusing system of claim 8, wherein the second plane is not perpendicular to the first plane.

11. The laser focusing system according to claim 8 or 10, wherein the second plane is parallel to the first plane.

12. The laser focusing system of claim 8 or 9, wherein the laser beam redirected toward the first curved mirror is arranged in both the first plane and the second plane.

13. The laser focusing system according to claim 8 or 9, wherein: The laser beam redirected toward the first curved mirror is not arranged in the second plane.

14. The laser focusing system of claim 1, wherein an angle between an optical axis of the second reflected laser beam and a horizontal axis is less than 20 degrees.

15. A laser source comprising a laser focusing system according to any one of the preceding claims.

16. An EUV radiation source comprising the laser source according to claim 15.

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