EUV radiation source and position-controllable mirror

Abstract

An EUV radiation source comprising a laser configured to emit a laser beam and an optical system configured to control the position of the laser beam so that the laser beam is incident on a fuel droplet, wherein the optical system includes an optical path containing a position-controllable mirror having a control bandwidth of at least 500 Hz.

Description

Technical Field

[0001] (Cross - reference to related applications)

[0001] This application claims priority to European Application No. 23163835.4 filed on March 23, 2023, which is incorporated herein by reference in its entirety.

[0002]

[0002] The present invention relates to an EUV radiation source and a position - controllable mirror.

Background Art

[0003]

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can project, for example, the pattern of a patterning device (e.g., a mask) onto a layer of radiation - sensitive material (resist) provided on a substrate.

[0004]

[0004] To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features that can be formed on the substrate. To form smaller features on a substrate than those formed by a lithographic apparatus using radiation having a wavelength of, for example, 193 nm, a lithographic apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 - 20 nm, for example 6.7 nm or 13.5 nm, can be used.

[0005]

[0005] A lithographic apparatus, in combination with an EUV radiation source, may be referred to as a lithographic system.

[0006]

[0006] An EUV radiation source can be a laser - produced plasma (LPP) system. In an LPP system, a laser is used to deposit laser energy into a fuel material. The deposition of laser energy into the fuel material creates a plasma. EUV radiation is emitted from the plasma upon de - excitation and recombination of electrons with ions of the plasma.

[0007]

[0007] The LPP system may use two laser beams, a main pulsed laser beam and a pre-pulsed laser beam, to generate EUV radiation from the fuel material. The pre-pulsed laser beam provides some energy to droplets of the fuel material, causing them to expand. The fuel material can also change shape, for example, from spherical to disk-like. The main pulsed laser beam is then incident on the fuel material to generate an EUV-emitting plasma.

[0008]

[0008] A potential problem in an EUV radiation source is the misalignment of the pre-pulsed laser beam with respect to the droplets of fuel material. This results in a significant reduction in the amount of EUV radiation provided by the droplets of fuel material. [Overview of the project]

[0009]

[0009] The object of the present invention is to provide an EUV radiation source and a position-controllable mirror that overcome the drawbacks associated with the prior art.

[0010]

[0010] According to a first aspect of the present disclosure, an EUV radiation source is provided, comprising a laser configured to emit a laser beam and an optical system configured to control the position of the laser beam so that the laser beam is incident on a fuel droplet, the optical system comprising an optical path including a position-controllable mirror having a control bandwidth of at least 500 Hz.

[0011]

[0011] The position-controllable mirror has a bandwidth of at least 500 Hz, so that the laser beam can be guided more precisely onto the fuel droplet. This increases the amount of EUV radiation provided by the fuel droplet.

[0012]

[0012] According to a second aspect of the present invention, a position-controllable mirror for use in an EUV radiation source is provided, the position-controllable mirror having a control bandwidth of at least 500 Hz.

[0013]

[0013] The position-controllable mirror may have a control bandwidth of at least 1 kHz.

[0014]

[0014] The EUV radiation source may further include a controller configured to control the tilt of a position-controllable mirror, the controller configured to control the position-controllable mirror to at least partially compensate for laser beam position errors caused by an operable optical device in the optical path having a lower control bandwidth than the position-controllable mirror. The controller may also be configured to control both the tip and the tilt of the position-controllable mirror.

[0015]

[0015] The position-controllable mirror may be equipped with multiple optical encoders.

[0016]

[0016] The optical encoder may include an encoder scale mounted on an arm extending radially from the encoder support.

[0017]

[0017] The optical encoder may include an encoder scale provided on the side wall of the mirror substrate of the position-controllable mirror.

[0018]

[0018] The encoder scale may be convex.

[0019]

[0019] The encoder scale may have a radius of curvature that roughly corresponds to the radial distance from the encoder scale to the central axis of the position-controllable mirror.

[0020]

[0020] The position-controllable mirror may also include a hybrid reluctance actuator formed from an actuator yoke and a stator, each of which has multiple radially extending arms.

[0021]

[0021] At least one of the actuator yoke and the stator may be formed from a soft magnetic composite material.

[0022]

[0022] The stator may be formed from a soft magnetic composite material. The stator may further include pillars extending perpendicularly from stator arms extending in the radial direction. The pillars may be integrally formed with the arms extending in the radial direction.

[0023]

[0023] At least one of the actuator yoke and the stator may be formed from a laminated steel material.

[0024]

[0024] The lamination of the arms extending in the radial direction of the stator may be perpendicular to the central axis of the position controllable mirror.

[0025]

[0025] The stator may further include pillars extending perpendicularly from the arms extending in the radial direction. The lamination of the pillars may be parallel to the central axis of the position controllable mirror.

[0026]

[0026] The pillars may extend through openings provided in the stator arms extending in the radial direction.

[0027]

[0027] The position controllable mirror may further include a clamp configured to clamp together the actuator yoke and a radially projecting portion protruding from the stem of the mirror.

[0028]

[0028] The clamp may also clamp an encoder support together with the actuator yoke and the radially projecting portion protruding from the stem of the mirror.

[0029]

[0029] The mirror may include a substrate having a reflective surface, and the mirror substrate may include at least one of one or more hollow volumes, one or more undercuts, and a variable wall thickness.

[0030]

[0030] The position controllable mirror may further include a flexible stem connecting the mirror to the base.

[0031]

[0031] The flexible stem may have a reduced diameter portion.

[0032]

[0032] The center of gravity of the movable part of the position-controllable mirror may be located at the reduced diameter portion of the bendable stem.

[0033]

[0033] The center of gravity of the movable part of the position-controllable mirror may correspond to the elastic center of the movable part of the position-controllable mirror.

[0034]

[0034] The position-controllable mirror may further include a flexure located below the mirror and above the actuator yoke.

[0035]

[0035] The flexure may have multiple connecting parts between the inner part and the outer part, and the connecting parts include bent parts.

[0036]

[0036] The flexure may have multiple connecting parts between the inner part and the outer part, and the connecting parts are U-shaped.

[0037]

[0037] The position-controllable mirror may be configured for use in a vacuum environment.

[0038]

[0038] The mirror body of the position-controllable mirror may be made of a ceramic material.

[0039]

[0039] The position-controllable mirror may include an adjustable end stopper.

[0040]

[0040] The position-controllable mirror may include a heat dump.

[0041]

[0041] The position-controllable mirror may further include an encoder head and a locking system, the locking system comprising a leaf spring and an actuator. The actuator may be configured to engage the leaf spring with the encoder head holder. The leaf spring may be located in a plane substantially parallel to the central axis of the position-controllable mirror.

[0042]

[0042] The encoder head holder may further include a plate configured to be sandwiched between a leaf spring and the housing wall of a position-controllable mirror.

[0043]

[0043] The actuator may include a chamfered block that is bolted and screw-engaged, and the chamfered block is located within an opening in the housing wall.

[0044]

[0044] According to a third aspect of the present invention, a method is provided for guiding a laser beam onto a fuel target in an EUV radiation source, the method comprising controlling the position of the laser beam using a position-controllable mirror having a control bandwidth of at least 500 Hz.

[0045]

[0045] The position-controllable mirror has a bandwidth of at least 500 Hz, so that the laser beam can be guided more precisely onto the fuel droplet. This increases the amount of EUV radiation provided by the fuel droplet.

[0046]

[0046] The position-controllable mirror may be in the optical path together with at least one operable optical device. The position-controllable mirror may be tilted to at least partially compensate for laser beam position errors caused by a lower control bandwidth of the operable optical device compared to the control bandwidth of the position-controllable mirror.

[0047]

[0047] A third aspect of the present invention may include features of the first and second aspects of the present invention.

[0048]

[0048] Features of different embodiments of the present invention may be combined. [Brief explanation of the drawing]

[0049]

[0049] Next, embodiments of the present invention will be described as merely examples with reference to the attached schematic diagrams.

[0050] [Figure 1] A lithography system comprising a lithography apparatus and an EUV radiation source according to one embodiment of the present disclosure is illustrated. [Figure 2] A schematic diagram shows the optical system that forms part of the EUV radiation source. [Figure 3] A partially exploded perspective view of a position-controllable mirror according to one embodiment of the present disclosure is shown. [Figure 4] The position-controllable mirror shown in Figure 3 is illustrated in a cross-sectional perspective view. [Figure 5] The stator of a position-controllable mirror is shown in the diagram. [Figure 6] The movable part of the position-controllable mirror is shown in cross-section. [Figure 7] The flexure of a position-controllable mirror is illustrated. [Figure 8] The encoder head and locking system are shown in a perspective view. [Figures 9A-9C] The diagram illustrates the adjustment of the height of the encoder head 600 and how to lock the encoder head in a predetermined position. [Modes for carrying out the invention]

[0051]

[0050] Figure 1 shows a lithography system comprising a radiation source SO and a lithography apparatus LA according to one embodiment of the present disclosure. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0052]

[0051] The illumination system IL is configured to adjust the EUV radiation beam B before it is incident on the patterning device MA. For this purpose, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. Together, the faceted field mirror device 10 and the faceted pupil mirror device 11 give the EUV radiation beam B a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and the faceted pupil mirror device 11.

[0053]

[0052] After being adjusted in this manner, the EUV radiation beam B interacts with the patterning device MA. This interaction results in the generation of a patterned EUV radiation beam B'. The projection system PS is configured to project its patterned EUV radiation beam B' onto the substrate W. For this purpose, the projection system PS may include a number of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thereby forming an image with features smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 can be applied. Although the projection system PS is shown in Figure 1 with only two mirrors 13, 14, the projection system PS may include various numbers of mirrors (e.g., six or eight mirrors).

[0054]

[0053] The substrate W may include a previously formed pattern. In that case, the lithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the previously formed pattern on the substrate W.

[0055]

[0054] A small amount of gas (e.g., hydrogen) at a relative vacuum, i.e., a pressure significantly below atmospheric pressure, may be provided within the radiation source SO, the illumination system IL, and / or the projection system PS.

[0056]

[0055] The radiation source SO shown in Figure 1 is of a type sometimes referred to as, for example, a laser-generated plasma (LPP) source. Laser system 1 may include a pre-pulse laser system. The pre-pulse laser system may include, for example, an Nd:YAG laser (or another laser configured to provide a laser beam having a wavelength of about 1 micron). Laser system 1 may also include a main-pulse laser system. The main-pulse laser system may include, for example, a CO2 laser (or another laser configured to provide a laser beam having a wavelength of about 10 microns). Embodiments of this disclosure relate to a pre-pulse laser system.

[0057]

[0056] The laser system 1 is arranged to deposit energy into a fuel, such as tin (Sn), supplied from a fuel ejector 3, via a pre-pulsed laser beam and a main laser beam (illustrated as a single laser beam 2). In the following description, tin will be mentioned, but any suitable fuel may be used. The fuel may be in the form of a liquid, for example, or it may be a metal or alloy, for example. The fuel ejector 3 may have a nozzle configured to guide tin, for example, in the form of droplets, along a trajectory toward the plasma-forming region 4. The laser beam 2 is incident on the tin in the plasma-forming region 4. The pre-pulsed laser beam provides some energy to the tin, causing it to expand. The tin can also change shape, for example, from spherical to disk-like. The main-pulsed laser beam is then incident on the tin.

[0058]

[0057] The deposition of laser energy onto tin creates a tin plasma 7 in the plasma formation region 4. During the de-excitation and recombination of electrons by plasma ions, radiation including EUV radiation is emitted from the plasma 7.

[0059]

[0058] EUV radiation from the plasma is collected and focused by the collector 5. The collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes more commonly referred to as a normal incidence radiation collector). The collector 5 may have a multilayer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration with two foci. As will be discussed below, the first of the foci may be in the plasma-forming region 4, and the second of the foci may be in an intermediate focus 6.

[0060]

[0059] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 can be delivered from the laser system 1 to the radiation source SO with the help of a beam delivery system (not shown) equipped with, for example, a suitable guide mirror and / or beam expander, and / or other optical components. The laser system 1, the radiation source SO, and the beam delivery system can together be considered as a radiation system.

[0061]

[0060] The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at the intermediate focus 6, forming an image of the plasma in the plasma-forming region 4 at the intermediate focus 6. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is positioned such that the intermediate focus 6 is located at or near the aperture 8 of the closed structure 9 of the radiation source SO.

[0062]

[0061] Figure 1 shows the radiation source SO as a laser-generated plasma (LPP) source, but EUV radiation can be generated using any suitable radiation source such as a discharge-generated plasma (DPP) source or a free electron laser (FEL).

[0063]

[0062] Figure 1 also shows an optical system 100 which may form part of one embodiment of the present disclosure. The optical system 100 may be configured to control the position of the prepulse laser beam 2 so that the prepulse laser beam 2 is incident on a fuel droplet. The optical system 100 includes a position-controllable mirror according to one embodiment of the present disclosure.

[0064]

[0063] Figure 2 illustrates an example of the optical system 100. The optical system includes a position-controllable mirror according to one embodiment of the present disclosure. The optical system 100 may be referred to as a metrology and control system 100.

[0065]

[0064] The metrology and control system 100 includes an optical pickup 405. The optical pickup 405 is configured to measure a forward beam 410 (laser beam) guided toward a target location 420 and a return beam 415 reflected from the target location 420. The forward beam 410 may be a pre-pulsed laser beam 2, and the target location may be a plasma formation location (see Figure 1). The laser beam 410 travels along an optical path comprising a plurality of optical devices, at least one of which is a position-controllable mirror according to one embodiment of the present disclosure.

[0066]

[0065] The metrology and control system 100 comprises a plurality of operable optical devices 425, 430, 465, 475, and 501 which can be configured to guide and focus the forward beam 410 to the target location 420 and to align the measurement plane of the optical pickup 405 with the target location 420, as will be described in more detail below. Specifically, the operable optical devices 425, 430, 465, 475, and 501 are positioned before and after the optical pickup 405 in the path of the forward beam 410, and the operable optical devices are controlled in response to the measurement of the forward beam 410 and the return beam 415 by the optical pickup 405.

[0067]

[0066] An exemplary metrology and control system 100 includes a prepulse laser 495 configured to provide a forward beam 410. The prepulse laser 495 may be used to optimize the mass density and / or distribution of fuel in the EUV radiation source before interacting with the subsequent main pulse.

[0068]

[0067] The exemplary metrology and control system 100 shown in Figure 2 includes a first operable optical device 425. The first operable optical device 425 is positioned in front of the optical pickup 405 in the path of the forward beam 410. The first operable optical device 425 is controlled by a first operable optical device controller 480.

[0069]

[0068] The first operable optical device 425 can be configured to control the wavefront curvature of the forward beam 410. In this example, the first operable optical device 425 is configured to control the diameter of the forward beam 410, either additionally or alternatively.

[0070]

[0069] Accordingly, the first operable optical device controller 480 is shown in Figure 2 as the "beam space d, c controller," where "d" refers to the diameter of the forward beam 410 and "c" refers to the curvature of the wavefront of the forward beam 410.

[0071]

[0070] The first operable optical device 425 may comprise a plurality of devices, such as one or more lenses. The position and / or characteristics of the first operable optical device 425 may be configurable, for example, operable. That is, the position and / or characteristics of one or more of the plurality of devices forming the first operable optical device 425 may be configurable.

[0072]

[0071] In one example, the first operable optical device 425 may include one or more lenses, the position of which can be adjusted. That is, the first operable optical device 425 may include one or more position-controllable lenses. An actuator, such as a servo motor, may be configured to control the position of one or more lenses.

[0073]

[0072] An exemplary metrology and control system 100 includes a second operable optical device 430. The second operable optical device 430 is positioned in the path of the forward beam 410 before the optical pickup 405. The second operable optical device 430 is positioned in the path of the forward beam 410 after the first operable optical device 425. The second operable optical device 430 is controlled by a second operable optical device controller 485.

[0074]

[0073] The second operable optical device 430 is a mirror, for example, a reflective surface suitable for reflecting radiation having the wavelength of the forward beam 410.

[0075]

[0074] In this example, the second operable optical device 430 is a position-controllable mirror. That is, the tilt of the second operable optical device 430 can be adjusted by an actuator such as a servo motor. The second operable optical device 430 can be tilted in two directions.

[0076]

[0075] When in use, the second operable optical device 430 may be configured to compensate for the offset of the forward beam 410 with respect to the optical axis of the second operable optical device 430. That is, the operation of the second operable optical device 430 for controlling the offset of the forward beam 410 may be controlled in response to a measurement of the forward beam 410 by the optical pickup 405, where the measurement of the forward beam 410 and the return beam 415 by the optical pickup 405 may include a measurement of the wavefront and / or position of the forward beam 410 and the return beam 415.

[0077]

[0076] Accordingly, the second operable optical device controller 485 is shown in Figure 2 as the "beam position XY controller," where the controller 185 is configured to adjust the position of the forward beam 410 in the XY plane to compensate for the offset of the forward beam 410.

[0078]

[0077] An exemplary metrology and control system 100 includes a third operable optical device 501. The third operable optical device 501 is positioned in front of the optical pickup 405 in the path of the forward beam 410. The third operable optical device 501 is controlled by a third operable optical device controller 490.

[0079]

[0078] The third operable optical device 501 is a mirror for reflecting the forward beam 410. In this example, the third operable optical device 501 is a position-controllable mirror. That is, the tilt of the third operable optical device 501 can be adjusted. The tilt can be adjusted in two directions. The position-controllable mirror 501 may be according to one embodiment of the present disclosure. The position-controllable mirror 501 will be described further below. Two or more of the operable optical devices may be position-controllable mirrors according to one embodiment of the present invention.

[0080]

[0079] When in use, the position-controllable mirror 501 may be configured to compensate for the tilt of the forward beam 410 with respect to the optical axis. The operation of the position-controllable mirror 501 to compensate for the effect of the tilt of the forward beam 410 may be controlled in response to measurements of the forward beam 410 and the return beam 415 by the optical pickup 405.

[0081]

[0080] The second operable optical device 430 and / or the third operable optical device 501 may be a position-controllable mirror according to one embodiment of the present disclosure.

[0082]

[0081] The third operable optical device controller 490 is shown in Figure 2 as the "beam tilt Rx, Ry controller," which configures the position-controllable mirror 501 to compensate for the tilt of the forward beam 410.

[0083]

[0082] Although Figure 2 shows only two position-controllable mirrors 430, 501 in front of the optical pickup 405 in the path of the forward beam 410, this is for illustrative purposes only, and it will be understood that more position-controllable mirrors may be provided in other examples. One or more of the position-controllable mirrors may be according to one embodiment of the present disclosure.

[0084]

[0083] The optical pickup 405 includes a first sensor 440 for measuring the forward beam 410, a second sensor 445 for measuring the return beam 415, a beam splitting device 450 for directing a portion of the forward beam 410 onto the first sensor 440, and a reflective surface for directing the return beam 415 onto the second sensor 445. The optical pickup 405 also includes a first focusing device 455 for focusing the forward beam 410 onto the first sensor 440 and a second focusing device 460 for focusing the return beam 415 onto the second sensor 455.

[0085]

[0084] The first sensor 440, the second sensor 445, the beam splitting device 450, the reflecting surface, the first focusing device 455, and the second focusing device 460 may be conventional, and therefore, for the sake of brevity, they will not be described in detail here.

[0086]

[0085] The exemplary metrology and control system 100 also includes a fourth operable optical device 465. The fourth operable optical device 465 is located in the path of the forward beam 410 after the optical pickup 405. The fourth operable optical device 465 is located in the path of the return beam 415.

[0087]

[0086] A fourth operable optical device 465 is a mirror for reflecting the forward beam 410 and the return beam 415. In this example, the fourth operable optical device 465 is a position-controllable mirror (and may be a mirror according to one embodiment of the present disclosure).

[0088]

[0087] Thus, the fourth operable optical device 465 can be effectively used in conjunction with the second operable optical device 430 and the third operable optical device 501 to steer the forward beam 410.

[0089]

[0088] The fourth operable optical device 465 can also be configured to center the return beam 415 on the second sensor 445 of the optical pickup 405. By centering the return beam 415 on the second sensor 445, the range required by the second sensor 445 can be minimized, and the second sensor 445 can operate near the center of the available range and within a region of the sensor that exhibits a relatively high linear response.

[0090]

[0089] Further optical devices 470 in the paths of the forward beam 410 and the return beam 415 are also shown, and for illustrative purposes, these further optical devices 470 are mirrors (and may be mirrors according to one embodiment of the present disclosure).

[0091]

[0090] The exemplary metrology and control system 100 also includes a fifth operable optical device 475. The fifth operable optical device 475 is located after the optical pickup 405 in the path of the forward beam 410. The fifth operable optical device 475 is located in the path of the return beam 415.

[0092]

[0091] Although only two operable devices 465, 475 (e.g., position-controllable mirrors or lenses) are shown after the optical pickup 405 in the paths of the forward beam 410 and the return beam 415, this is for illustrative purposes only, and it will be understood that one or more additional position-controllable mirrors or lenses may be provided in other examples. One or more of the position-controllable mirrors may be according to one embodiment of the present disclosure.

[0093]

[0092] The fifth operable optical device 475 is positioned after the optical pickup 405 in the path of the forward beam 410 and the return beam 415. In this example, the fifth operable optical device 475 is positioned after the fourth operable optical device 465 in the path of the forward beam 410.

[0094]

[0093] An exemplary fifth operable optical device 475 includes an operable lens. When in use, the fifth operable optical device 475 may be operated to focus the forward beam 410 onto the fuel at the target location 410, thereby ensuring that the measuring plane of the optical pickup 405 is also aligned with the target location 410.

[0095]

[0094] The optical focal length of the fifth operable optical device 475 is configured to match that of the first focusing device 455 and the second focusing device 460 of the optical pickup 405. Thus, by operating the fifth operable optical device 475 to focus the forward beam 410 onto the fuel at the target location 410, for example, by ensuring that the target location 410 is just within the rear focal plane, both the first focusing device 455 and the second focusing device 460, which match the focal length of the forward beam 410, can provide an indication that the focal location of the forward beam 410 is just at the target location 420.

[0096]

[0095] The fourth and fifth operable optical devices 465, 475 are controlled by the fourth operable optical device controller 499. The fourth operable optical device controller 499 is shown in Figure 2 as the "laser-to-droplet X, Y, Z controller" because the fourth controller 499 can operate the fourth and fifth operable optical devices 465, 475 to control the steering and positioning of the focal point of the forward beam 410 in the x, y, and z directions.

[0097]

[0096] That is, the fourth and fifth operable optical devices 465, 475 can control the focal position of the forward beam 410 in the x, y, and z directions to ensure that the forward beam 410 is accurately injected into the fuel at the target location to optimize the EUV plasma generation process in the EUV radiation source. By controlling the focal position of the forward beam 410 in this way, the fourth and fifth operable optical devices 465, 475 also ensure that the return beam 415 reflected by the fuel at the target location 420 can be guided toward the center of the second sensor 445.

[0098]

[0097] The fourth operable optical device 465 may have a lower control bandwidth than the third operable optical device 501. Control errors resulting from this lower control bandwidth may be transmitted from the fourth controller 499 to the third controller 490. The third controller 490 may move the third operable optical device 501 to compensate for the lower control bandwidth of the fourth operable optical device 465.

[0099]

[0098] Generally, a controller may be configured to control a position-controllable mirror according to one embodiment of the present disclosure in order to at least partially compensate for laser beam position errors caused by an operable optical device having a lower control bandwidth than the position-controllable mirror. In this context, laser beam position error can be interpreted as the deviation of the laser beam from a desired position where the laser beam is incident on a fuel droplet.

[0100]

[0099] Figure 3 shows a partially exploded perspective view of a position-controllable mirror 501 according to one embodiment of the present disclosure. Figure 4 shows a cross-sectional view of the movable portion 544 of the position-controllable mirror. The position-controllable mirror 501 has a central axis A.

[0101] [000100] The position-controllable mirror includes a reflective surface 502 provided on a substrate 504. The substrate may be formed from a ceramic material such as silicon-coated silicon carbide, or from a metal such as aluminum. The substrate may be, for example, diamond-filled silicon carbide. The reflective surface 502 may be formed as a multilayer structure (e.g., a hafnia / silica multilayer structure). A stem 505 extends downward from the substrate 504. The stem 505 may be formed integrally with the substrate 504. A radial projection 506 extends from the stem 505. The radial projection 506 may be formed integrally with the stem 505.

[0102] [000101] The substrate 504, stem 505, and radial projection 506 may be integrally formed (i.e., formed from a single piece of material). For ease of reference, the substrate 504, stem 505, and radial projection 506 may be referred to as the mirror body 503. The mirror body 503 may be formed from a ceramic material such as silicon-coated silicon carbide, or from a metal such as aluminum.

[0103] [000102] A clamp plate 508 is positioned between the substrate 504 and the radial projection 506. The clamp plate 508 may be formed from two parts that fit together from both sides of the stem. The clamp plate 508 may be formed from a metal, such as aluminum or titanium. The clamp plate 508 presses against the upper surface of the radial projection 506. There is a gap between the stem 505 and the clamp plate 508. This configuration allows for thermal expansion of the stem 505 and may also allow for thermal expansion of the radial projection 506. Advantageously, applying the clamp plate 508 does not deform the substrate 504 or the reflective surface 502.

[0104] [000103] Below the radial projection 506, an encoder support 510 is provided. The encoder support 510 comprises a body 512 from which encoder arms 514 extend. The encoder support 510 may be formed from titanium (or other metal or ceramic). The encoder arms 514 extend radially outward. An encoder scale 516 is provided on the outward-oriented surface of each encoder arm 514. The encoder scale 516 comprises a piece of glass fixed to the encoder arm 514 (for example, using adhesive). A scale is engraved on the glass, for example, using a laser. Other encoder scales may be used, as will be further described below.

[0105] [000104] Below the encoder support 510 is an actuator yoke 518. The actuator yoke comprises a body and four actuator arms 522. The actuator yoke is formed from a ferromagnetic material (e.g., soft magnetic composite or cobalt ferrite). The actuator arms 522 extend radially outward. In the illustrated embodiment, the actuator arms are oriented at 45° with respect to the encoder arm 514. However, in other embodiments, the actuator arms 522 may have different angles with respect to the encoder arm 514.

[0106]

[0105] A second clamp plate 524 is located below the actuator yoke 518. The second clamp plate 524 may be made of a metal, such as aluminum or titanium. A bolt 526 extends through the second clamp plate 524, the actuator yoke 518, the encoder support 510, and the radial projection 506. The bolt 526 also extends through the flexure 546 (which is omitted from Figure 3 but shown in Figure 4). The bolt 526 is housed within the first clamp plate 508. The bolt 526 is used to pull the first and second clamp plates 508, 524 toward each other, thereby pulling various parts of the position controllable mirror 501 toward each other. This is advantageous as it provides improved robustness to the movable part 544 of the position controllable mirror 501. For example, the robustness of the actuator yoke 518 affects the precision and bandwidth of the movement of the position controllable mirror 501. The clamps formed by the clamp plates 508 and 524 are advantageous because they increase the rigidity of the actuator yoke 518 (compared to the case where no clamps were present). Generally, the improved robustness provided by the clamps may allow the position-controllable mirror 501 to move at higher frequencies (e.g., above 1 kHz).

[0107]

[0106] In the illustrated embodiment, eight bolts 526 are provided. However, a different number of bolts may be used.

[0108]

[0107] Below the actuator yoke 518 is the stator 530. The stator 530 comprises four stator arms 532 extending radially outward. From each stator arm 532, a pillar 534 extends upward. An electric coil 536 is provided around each pillar 534. For ease of explanation, only one electric coil 536 is illustrated. The electric coil is connected by wire 538 to a current source which may form part of a controller (not shown). From the center of the stator 530, a central pillar 540 extends upward. A permanent magnet 541 is provided on the pillar 540. The actuator 518, stator 530, permanent magnet 541, and coil 536 may comprise a hybrid reluctance actuator.

[0109] [000108] A flexible stem 542 supports the movable portion 544 of the position-controllable mirror 501. The flexible stem may be made of a metal, such as steel or titanium. The flexible stem 542 is coaxial with the central axis A of the position-controllable mirror 501. The upper end of the flexible stem 542 is housed by an encoder support 510. The flexible stem 542 extends through the actuator yoke 518, through the permanent magnet 541, and through the central pillar 540 of the stator 540. The flexible stem 542 is housed and fixed within a base (not shown) located below the stator 530. The flexible stem 542 has some flexibility, which allows the movable portion 544 of the position-controllable mirror 501 to tilt.

[0110] [000109] A flexure 546 is located between the radial projection 506 and the encoder support 510. The flexure 546 will be described further below.

[0111] [000110] When no current is supplied to the electric coil 536 during use, the position-controllable mirror 501 is in a neutral orientation. Neutral orientation may be, for example, a state in which the reflective surface 502 is perpendicular to the central axis A. The pillars 534 allow the conduction of a magnetic field from the permanent magnet 541, and each pillar attracts its respective actuator arm 522 with the same attractive force. When current is supplied to the electric coil 536 of the pillar 534, the current generates an additional magnetic field. The additional magnetic field can enhance the magnetic field supplied by the pillar 534, thereby increasing the overall attractive magnetic field, in which case the actuator arm 522 moves toward its pillar. The additional magnetic field may be opposite to the magnetic field supplied by the pillar 534, thereby reducing the overall attractive magnetic field, in which case the actuator arm 522 moves toward its pillar. By supplying different currents to different coils 536, any desired orientation and magnitude of the inclination of the reflective surface 502 can be obtained.

[0112] [000111] The controller 490 (see Figure 2) controls the current supplied to each electrical coil 536 of each stator pillar 534. The controller 490 uses this current to control the tilt of the position-controllable mirror 501. In this way, the controller 490 moves the position-controllable mirror 501 to a desired orientation.

[0113] [000112] As further described above, the position-controllable mirror may be configured to reflect a laser beam having a wavelength of about 1 micron (e.g., 1.033 to 1.064 microns). The movable part 544 of the position-controllable mirror 501 may have a mass of, for example, up to 100 g. The movable part 544 of the position-controllable mirror 501 may have a mass of, for example, 30 g or more. This mass is relatively low compared to the mass of a mirror used to reflect a laser beam having a longer wavelength, such as 10 microns. This is because the reflective surface 502 of the position-controllable mirror 501 has a relatively high reflectivity (e.g., greater than 99.8%) and as a result does not experience some heating due to laser radiation absorption. In contrast, a mirror configured to reflect laser radiation of about 10 microns has a significantly lower reflectivity (e.g., about 98%) and as a result absorbs a significant amount of laser radiation. Such a mirror requires active cooling (e.g., water cooling) to avoid overheating of the mirror. The active cooling component adds considerable weight to the mirror. The position-controllable mirror 501 has a relatively low mass because it does not involve active cooling (e.g., water cooling).

[0114] [000113] Conventional prepulse laser systems use position-controllable mirrors with relatively high mass. In embodiments of the present disclosure, the movable portion of the position-controllable mirror 501 has a mass of up to 100 g. The movable portion 544 of the position-controllable mirror 501 may have a mass of, for example, 30 g or more. Because the position-controllable mirror 501 has a relatively low mass, it enables better dynamic performance compared to mirrors with higher mass. The bandwidth over which the position-controllable mirror 501 can be controlled is higher than that of mirrors with higher mass. The control bandwidth of the position-controllable mirror 501 in embodiments of the present disclosure may be, for example, 500 Hz or higher, or for example, 1 kHz or higher.

[0115] [000114] A control bandwidth of 1 kHz or more is particularly advantageous because it enables feedback control to compensate for plasma disturbances in the position of fuel droplets within the EUV radiation source. Plasma disturbances occur when plasma generated from a fuel droplet affects the trajectory of the next fuel droplet. This disturbance of the fuel droplet position due to plasma occurs in the frequency range of 200-300 Hz. When the position-controllable mirror 501 has a bandwidth of at least 1 kHz, this makes it possible to control the position of the pre-pulsed laser beam to compensate for the movement of fuel droplets caused by plasma disturbances. As a result, the laser beam pre-pulse is incident on the fuel droplet with greater precision. This increases the efficiency of EUV radiation generation by the EUV radiation source. Generally, the bandwidth of the position-controllable mirror 501 can be at least twice as large as the frequency of the plasma disturbance. The bandwidth of the position-controllable mirror 501 may be at least three times as large as the frequency of the plasma disturbance. A bandwidth of at least 500 Hz is sufficient to compensate for most plasma disturbances, thereby providing an improvement over conventional systems. To ensure that all plasma disturbances can be compensated for, a bandwidth of at least 1 kHz would be preferable.

[0116] [000115] The position-controllable mirror 501 may have, for example, a tilt range of at least 10 mrad. The position-controllable mirror 501 may have, for example, a tilt range of up to 20 mrad. This is a relatively small range, but this range may be sufficient to compensate for deviations in the position of fuel droplets from the nominal position. The position-controllable mirror may have a diameter of, for example, up to 2 cm, or, for example, up to 4 cm. This is a relatively small diameter (compared to some prior art mirrors), which helps to keep the mass of the position-controllable mirror relatively low.

[0117] [000116] In one embodiment, the actuator yoke 518 and / or stator 530 may be formed from a soft magnetic composite material (SMC). The actuator yoke 518 may be formed as a single monolithic component. The stator 530 may be formed as a single monolithic component (including, for example, pillars 534). The use of SMC is advantageous because it reduces the formation of eddy currents in the yoke actuator 518 and stator 530 (compared to conventional ferromagnetic materials). Eddy currents degrade the performance of the position-controllable mirror 501 (i.e., reduce the operating bandwidth). The SMC may be formed from ferromagnetic powder coated with an electrical insulating material. The SMC has lower conductivity than ferromagnetic materials (therefore reducing eddy currents) but has coercivity, permeability, and magnetic flux density that enable operation in bandwidths of 1 kHz or higher. The SMC may be, for example, Somaloy, available from Hoganas AB in Sweden.

[0118] [000117] In an alternative configuration, the actuator yoke 518 and / or stator 530 may be formed from a laminated steel material (e.g., laminated cobalt ferrite). An example of this configuration is illustrated in Figure 5. As shown in Figure 5, the actuator yoke 518 comprises multiple layers of cobalt ferrite fixed to each other using an insulating adhesive. The insulating adhesive may have relatively low gas emission characteristics compared to, for example, conventional adhesives. The adhesive may be, for example, Remisol EB 549, available from Rembrandtin Coatings GmbH in Vienna, Austria. The lamination is perpendicular to axis A of the position-controllable mirror 501. Thus, in the illustrated configuration, the lamination of the actuator yoke 518 is horizontal. The actuator yoke 518 may be formed, for example, by constructing a laminated structure wider than required and then removing the material using laser cutting, electrical discharge machining, or some other machining process.

[0119] [000118] The stator 530 comprises stator arms 532 and pillars 534. The stator arms 532 and the lamination of the central part of the stator are perpendicular to the central axis A of the position-controllable mirror 501. Similar to the actuator yoke 518, the lamination of the stator 530 may be formed from cobalt ferrite layers bonded together using an insulating adhesive. Here again, the insulating adhesive may have relatively low gas emissions. An example of an insulating adhesive is Remisol EB 549. The stator 530 may be formed, for example, by constructing a lamination structure wider than required and then removing the material using laser cutting or some other machining process.

[0120] [000119] The pillars 534 are stacked in a direction parallel to the central axis A of the position-controllable mirror 501. Thus, the stacking of the pillars 534 is perpendicular to the stacking of the stator arms 532. The pillars 534 may be formed, for example, by constructing a stacked pillar that is wider than required and then removing the material using laser cutting or some other machining process.

[0121] [000120] The pillars 534 are housed in openings 535 of the stator arm 532. The openings 535 penetrate the stator arm 532 completely so that a portion of each pillar 534 can protrude from the bottom surface of the stator arm 532. This is advantageous as it allows the height of each pillar 534 to be adjusted during construction to provide a desired gap to the actuator yoke 518. The pillars 534 may be bonded in place within the stator arm 532.

[0122] [000121] The central pillar 540 is also formed as a laminated structure. The lamination of the central pillar 540 is parallel to the central axis A of the position-controllable mirror 501. As with the other configurations, the central pillar 540 is annular in order to house the bendable stem 542. A permanent magnet 541 is provided on the central pillar 540.

[0123] [000122] One or more components of the position-controllable mirror 501 may be constructed using 3D printing (also known as additive manufacturing). For example, the substrate 504 may contain one or more hollow volumes. This advantageously reduces the mass of the moving part 544 of the mirror. Reducing the mass of the moving part 544 of the mirror will increase the natural frequency of the position-controllable mirror 501. The bandwidth of operation of the position-controllable mirror 501 may be increased. Additionally or alternatively, position control may be achieved using a reduced amount of power.

[0124] [000123] Additive manufacturing may be used to form one or more hollow volumes in the encoder support 510. This advantageously reduces the mass of the movable part 544 of the mirror. One or more hollow volumes may be formed in other components of the movable part 544 of the mirror.

[0125] [000124] If a hollow volume is formed in the movable portion 544 component of the position-controllable mirror 501, the hollow volume may be completely sealed. This is advantageous as it provides better rigidity than when the hollow volume is open, thereby allowing for more precise control of the mirror's tilt.

[0126] [000125] To provide better rigidity, internal structures such as ribs may be provided within the hollow volume.

[0127] [000126] Additive manufacturing may be used to provide one or more undercuts. Additive manufacturing may be used to provide variable wall thickness.

[0128] [000127] An encoder scale may be applied to the sidewall of the mirror substrate 504 (for example, by using laser engraving and / or etching). If this is done, the encoder support 510 is no longer required. Since the encoder support 510 gives the position controllable mirror 501 significant mass, omitting the encoder support reduces the mass of the position controllable mirror. This can increase the natural frequency and bandwidth of the position controllable mirror 501.

[0129] [000128] In other embodiments, the encoder scale may be provided on the sidewall of the mirror substrate 504 using other techniques. In one example, the encoder scale may be engraved on the sidewall of the mirror substrate 504.

[0130] [000129] An encoder head (not shown) may be located adjacent to the encoder scale. The encoder head may have a conventional structure and is configured to emit radiation through a grating. The grating pattern of the radiation forms an interference pattern with the encoder grating, and the optical detector of the encoder head uses this interference pattern to measure the position.

[0131] [000130] In the embodiments illustrated in Figures 3 and 4, the encoder scale 516 is provided on a glass piece. In an alternative configuration, the encoder scale may be formed directly on the encoder arm 514. This advantageously eliminates thermal instability that may be caused by the adhesive provided between the glass encoder scale and the encoder arm. It also reduces the mass of the movable portion 544 of the position-controllable mirror 501.

[0132] [000131] The encoder scale 516 may be flat as shown in the figure. Alternatively, the encoder scale may be convex. The encoder scale may have a radius of curvature that is approximately corresponding to the radial distance from the encoder scale to the central axis A of the position-controllable mirror 501. A convex encoder scale is advantageous because it reduces or eliminates changes in the distance between the encoder scale and the encoder head that would occur due to the tilting motion of the position-controllable mirror 501. This allows for more precise and / or greater range of motion control of the position of the movable part 544 of the mirror.

[0133] [000132] Furthermore, as described above, the encoder scale may be provided on the side wall of the mirror substrate 504. The side wall of the mirror substrate 504 may be convex. The side wall of the mirror substrate 504 may have a radius of curvature that roughly corresponds to the radial distance from the encoder scale to the central axis A of the position-controllable mirror 501. This allows the position of the movable part 544 of the mirror to be controlled more accurately and / or over a larger range of movement.

[0134] [000133] Four encoders may be provided, each having an encoder scale 516 and an encoder head (not shown). The four encoders may be separated from each other at 90° angles, thereby providing fourfold symmetry. Providing four encoders is advantageous because it allows the controller to distinguish and account for encoder signals resulting from undesirable movement of the movable portion 544 of the position-controllable mirror 501. For example, movement along the direction of the central axis A may be distinguished from tilting. Using four encoders also allows the controller to distinguish and account for thermal expansion and contraction of the mirror components that may occur during use.

[0135] [000134] Fewer encoders than four may be used. However, using four encoders may provide a measurement stability of 25 μrad (compared to a measurement stability of 250 μrad when three encoders are used). Two encoders may also be used, but this will further reduce the measurement stability.

[0136] [000135] A position-controllable mirror can provide an angular resolution accuracy of approximately 0.1 μrad or more.

[0137] [000136] Since encoders can provide a response bandwidth of 1 kHz or more, it is advantageous to use encoders to monitor the position of the movable part 544 of the position-controllable mirror 501. Such a response bandwidth may not be achievable when, for example, an eddy current sensor is used (as conventionally). In addition, encoders can provide better accuracy and / or range than eddy current sensors.

[0138] [000137] In one embodiment (not shown), the position-controllable mirror may include a heat damper. The heat damper may comprise a metal sheet provided between the mirror substrate 504 and the actuator yoke 518. The metal sheet may assist in the thermal management of the movable portion 544 of the position-controllable mirror 501. A flexible metal connector may extend from the mirror substrate 504 to the metal sheet. The flexible metal connector conducts heat from the mirror substrate 504 to the metal sheet.

[0139] [000138] Figure 6 shows a cross-sectional view of the movable portion 544 of the position-controllable mirror 501, and Figure 7 is a perspective view of the flexure 546 located between the encoder support 510 and the radial projection 506.

[0140] [000139] The features of the movable part 544 of the position-controllable mirror 501 are described below, which can be more easily understood by referring to Figures 6 and 7 than to the preceding figures.

[0141] [000140] The bolt 526 extends through the second clamp plate 524, the actuator yoke 518, the encoder support 510, the flexure 546, and the radial projection 506 of the mirror body 503. The bolt 526 is housed in the first clamp plate 508. The first clamp plate 508 is threaded so that the bolt 526 can be screwed into the first clamp plate. The other components 524, 518, 510, 546, and 506 through which the bolt 526 passes are not threaded. Instead, there is a gap between the bolt and the components. This gap allows for some thermal expansion and contraction of the components, but the bolt does not transmit its thermal expansion or contraction to the other components. For example, the radial projection 506 may expand relative to the flexure 546. The radial projection 506 can slide on the flexure 546, but the bolt 526 does not transmit its expansion to the flexure.

[0142] [000141] The flexible stem 542 includes a reduced diameter portion 550. The reduced diameter portion 550 is located adjacent to the upper end of the flexible stem. The flexible stem 542 bends preferentially at the reduced diameter portion 550. The center of gravity 552 of the movable portion 544 of the position-controllable mirror 501 is identified by the disk. The center of gravity 552 is aligned with the central axis A of the movable portion 544 of the position-controllable mirror 501. The center of gravity 552 is located in the reduced diameter portion 550 of the flexible stem 542. In one embodiment, the center of gravity 552 may be aligned with the flexure 546 (for example, as shown).

[0143] [000142] The center of gravity 552 of the movable part 544 of the position-controllable mirror 501 may correspond to the elastic center of the movable part (i.e., the position around which the movable part 544 tilts). Setting the elastic center at the center of gravity 552 is advantageous because it separates the translational mode of the movable part 544 from the undesirable rotational mode of the moving part.

[0144] [000143] The flexure 546 comprises a plate. The plate may be formed from, for example, metal, such as Stavax stainless steel. The inner portion 554 of the flexure 546 is fixed between the encoder support 510 and the radial projection 506 of the mirror 503 body. The outer portion 556 of the flexure engages with the actuator housing (the housing is fixed and is not part of the movable portion 544 of the position-controllable mirror 501). The flexure 546 may be configured to eliminate or reduce undesirable movement of the mirror 503 body.

[0145] [000144] In one example, the flexure 546 can reduce the tendency of the movable part 544 to rotate around the central axis A when a tilt is applied (this may be caused by thermal expansion of the components of the movable part). This is achieved by a U-shaped connector 552 extending from the inner part 554 of the flexure to the outer part 556 of the flexure. If a straight connector were provided between the inner part 554 and the outer part 556 of the flexure 546, thermal expansion may cause undesirable rotation of the mirror body 503 during tilting. The U-shaped connector 552 prevents or reduces rotation when tilting occurs. Although a U-shaped connector 552 is shown, a connector including a bend may be used.

[0146] [000145] A gap is provided between the bendable stem 542 and the actuator yoke 518. Similarly, a gap is provided between the bendable stem 542 and the encoder support 510. The gaps allow the inclination of the movable portion 544 to occur without the inner wall of the actuator yoke 518 or the encoder support 510 coming into contact with the bendable stem 542.

[0147] [000146] The permanent magnet 541 provides magnetic fields extending along the actuator arm 522, along the pillar 534, and along the stator arm 532. The electromagnet formed by the coil 536 increases or decreases the magnetic field provided by the permanent magnet 541, thereby tilting the mirror body 503 (as further described below). The permanent magnet 541 may provide a magnetomotive force of, for example, about 7000 A. The permanent magnet 541 may have a magnetoresistance of, for example, 1.5e8 1 / He.

[0148] [000147] The length of the flexible stem 542 may be selected such that a gap exists between the lowest surface of the arm 518 of the yoke actuator 510 and the highest surface of the pillar 534 of the stator 530. The gap may be selected to provide a desired amount of angular rotation to be achieved (e.g., a maximum of + / - 10 mrad). The gap may be, for example, at least 0.2 mm. The gap may be, for example, up to 0.7 mm. A gap greater than 0.7 mm is not very preferable because an increase in the gap reduces the force applied by the stator 530 for a given current supplied to the coil 536.

[0149] [000148] A gap also exists between the permanent magnet 541 and the lowest surface of the actuator yoke 518.

[0150] [000149] In one embodiment, the position-controllable mirror may include an adjustable end stop (not shown). The adjustable end stop may comprise, for example, a bolt extending upward from a non-movable portion of the position-controllable mirror 501. The upper end of the bolt may be positioned to align with the encoder arm 514. The upper end of the bolt may be positioned such that the lowest surface of the encoder arm contacts the upper end of the bolt when the inclination of the movable portion 544 reaches the end of a desired range. The position of each end of the bolt may be adjusted by screwing the bolt upward and downward in the non-movable portion.

[0151] [000150] Figure 8 shows an encoder head 600 and a locking system 602 in a perspective view, which may form part of one embodiment of the present invention. The encoder head 600 and the locking system 602 may form part of a position-controllable mirror 501 according to one embodiment of the present invention. The encoder head 600 may be one of four encoder heads, each encoder head being located adjacent to its respective encoder scale 516 (which are illustrated, for example, in Figure 4).

[0152] [000151] The vertical position of the encoder head 600 is adjustable. This allows the encoder head to be positioned in a desired vertical position relative to the associated encoder scale 516.

[0153] [000152] Figure 8 also shows a locking system 602. The locking system 602 is configured to lock the encoder head 600 in place when the encoder head is moved to a desired vertical position. Locking the encoder head 600 in place is desirable because, once locked in place, the encoder head will not move vertically and will therefore maintain measurement accuracy. In addition, locking the encoder head 600 in place gives the encoder head greater robustness compared to when the encoder head is not locked in place. Greater robustness is advantageous in achieving a desired control bandwidth of at least 500 Hz. If the encoder head 600 had lower robustness, it may have a natural frequency of less than 500 Hz. In such a case, this may reduce the control bandwidth of the position-controllable mirror 501 to less than 500 Hz.

[0154] [000153] The locking system 602 includes a bolt 604 into which the chamfered block 606 is screwed. The chamfered block 606 and the bolt 604 form an actuator (other forms of actuators may be used). The chamfered block 606 has a vertical side 608 facing toward the encoder head 600 and an inclined side 610 facing away from the encoder head 600. The bolt 604 and the chamfered block 606 are housed in an opening 626 of a housing 612. The housing may be the housing of a position-controllable mirror 501. The opening 626 of the housing 612 includes a vertical wall 614 opposite to the vertical side 608 of the chamfered block 606. The housing 612 further includes an inclined wall 616 opposite to the inclined side 610 of the chamfered block 606.

[0155] [000154] The locking system 602 further comprises a locking member 618. The locking member 618 comprises a body 622 fixed to the housing 612 by bolts 620. The locking member 618 further comprises a leaf spring 624 extending into the opening 626 of the housing 612. The leaf spring 624 lies in a plane substantially parallel to the central axis A of the position-controllable mirror 501 (see Figure 4). The leaf spring 624 may extend substantially radially with respect to the central axis A of the position-controllable mirror 501.

[0156] [000155] The encoder head 600 is fixed to the encoder head holder 627. The encoder head 600 is secured to the holder 627 by bolts 628. When the vertical position of the encoder head 600 is selected, the holder 627 and the encoder head 600 move together. The holder 627 may include one or more protrusions that are housed in one or more grooves (not shown) of the housing 612.

[0157] [000156] The encoder head holder 627 includes a plate 629 that extends into the opening 626 of the housing 612. The plate 629 is located in a plane substantially parallel to the central axis A of the position-controllable mirror. The plate 629 may also be located in a plane extending radially from the central axis A of the position-controllable mirror. The plate 629 is sandwiched between a leaf spring 624 and the vertical wall 614 of the housing.

[0158] [000157] When in use, turning the bolt 604 clockwise pulls the chamfer block 606 upward. The inclined side 610 of the chamfer block 606 cams against the inclined wall 616 of the housing. This camming action of the inclined side 610 against the inclined wall 616 moves the chamfer block 606 toward the encoder head 600. The vertical side 608 of the chamfer block 606 moves toward the vertical wall 614 of the housing. The vertical side 608 of the chamfer block 606 presses the leaf spring 624 and plate 628 toward the vertical wall 614 of the housing. This clamps the encoder head holder 627 toward the vertical wall 614 of the housing, thereby securing it firmly in place. Thus, the encoder head 600, which is fixed to the encoder head holder 627, is also securely fixed in place.

[0159] [000158] Figures 9A to 9C illustrate the adjustment of the height of the encoder head 600 and the locking of the encoder head in a predetermined position.

[0160] [000159] Figure 9A shows the encoder head 600, encoder head holder 627, housing 612, and locking system 602 viewed from one side. In Figure 9A, the encoder head is below the desired vertical position. The bolt 604 is rotated so that the chamfer block 606 does not press against the leaf spring 624 or plate 629. There is a gap 630 between the chamfer block 606 and the leaf spring 624.

[0161] [000160] Referring to Figure 9B, the encoder head 600 and the encoder head holder 627 have been moved upward to the desired position. The chamfering block 606 is still not pressing against the leaf spring 624 or the plate 629. A gap 630 remains between the chamfering block 606 and the leaf spring 624.

[0162] [000161] Referring to Figure 9C, bolt 604 is being rotated. By rotating bolt 604, the chamfer block 606 is being pulled upward. The inclined side 610 of the chamfer block 606 engages with the inclined wall 616 of the housing 612, thereby pushing the chamfer block toward the encoder head 600 by a cam motion. Bolt 604 is rotated until the vertical side 608 of the chambered block 606 presses the leaf spring 624 and the encoder head holder plate 629 against the vertical wall 614 of the housing. This locks the encoder head holder 627 and the encoder 600 into place.

[0163] [000162] Advantageously, although the chamfered block 606 moves vertically when it engages with the leaf spring 624, it does not transmit parasitic vertical forces to the encoder head 600. This is because, when the leaf spring 624 is engaged with the chamfered block 606, it acts to separate the horizontal and vertical forces.

[0164] [000163] According to one aspect of the present invention, an encoder head and a locking system are provided, the locking system comprising a leaf spring that is bendable in a direction perpendicular to the adjustment direction of the encoder head.

[0165] [000164] The position-controllable mirror may be configured for use in a vacuum environment. The components of the position-controllable mirror may be formed from metal, ceramic, or glass that do not suffer from significant gas release. Furthermore, as mentioned above, the adhesive may have relatively lower gas release characteristics compared to conventional adhesives.

[0166] [000165] Although this text specifically refers to the use of lithography equipment in the manufacture of ICs, it should be understood that lithography equipment described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, and the like.

[0167] [000166] Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored in a machine-readable medium that can be read and executed by one or more processors. The machine-readable medium may include any mechanism for storing or transmitting information in a form that can be read by a machine (e.g., a computing device). For example, the machine-readable medium may include read-only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, electrical, optical, acoustic or other forms of propagating signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Furthermore, firmware, software, routines, and instructions may be described herein as performing specific actions. However, it should be understood that such descriptions are merely for convenience and that such actions are actually obtained from computing devices, processors, controllers, or other devices that execute firmware, software, routines, instructions, etc.

[0168] [000167] While specific embodiments of the present invention have been described above, it will be understood that the present invention may be carried out in ways other than those described. The above description is intended to be illustrative and not limiting. It will therefore be apparent to those skilled in the art that modifications to the described invention may be made without departing from the following claims.

[0169] [000168] Clause 1. An EUV radiation source comprising a laser configured to emit a laser beam and an optical system configured to control the position of the laser beam so that the laser beam is incident on a fuel droplet, wherein the optical system comprises an optical path including a position-controllable mirror having a control bandwidth of at least 500 Hz. 2. The position-controllable mirror is an EUV radiation source according to Clause 1, having a control bandwidth of at least 1 kHz. 3. An EUV radiation source according to Clause 1 or Clause 2, further comprising a controller configured to control the tilt of a position-controllable mirror, the controller configured to control the position-controllable mirror to at least partially compensate for laser beam position errors caused by an operable optical device in the optical path having a lower control bandwidth than the position-controllable mirror. 4. A position-controllable mirror equipped with multiple optical encoders, an EUV radiation source according to any of clauses 1 to 3. 5. The optical encoder includes an encoder scale mounted on a radially extending arm of the encoder support, and is an EUV radiation source as specified in Clause 4. 6. The optical encoder is an EUV radiation source according to Clause 4, including an encoder scale provided on the side wall of the mirror substrate of the position-controllable mirror. 7. The encoder scale is convex, and the EUV radiation source is one of the types specified in clauses 4 to 6. 8. The encoder scale is an EUV radiation source according to Clause 7, having a radius of curvature that approximately corresponds to the radial distance from the encoder scale to the central axis of the position-controllable mirror. 9. A position-controllable mirror comprising a hybrid reluctance actuator formed from an actuator yoke and a stator, each of which comprises multiple radially extending arms, is an EUV radiation source according to any of clauses 1 to 8. 10. An EUV radiation source according to Clause 9, wherein at least one of the actuator yoke and the stator is formed from a soft magnetic composite material. 11. The stator is formed from a soft magnetic composite material, and the stator further comprises pillars extending perpendicularly from radially extending stator arms, the pillars being integrally formed with the radially extending arms, the EUV radiation source of clause 10. 12. An EUV radiation source according to Clause 9, wherein at least one of the actuator yoke and the stator is formed from laminated steel material. 13. The stack of radially extending arms of the stator is perpendicular to the central axis of the position-controllable mirror, according to clause 12 of the EUV radiation source. 14. The stator further comprises a pillar extending vertically from a radially extending arm, the stack of pillars being parallel to the central axis of the position-controllable mirror, an EUV radiation source according to clause 13. 15. The pillar is an EUV radiation source of clause 14, extending through an opening in a radially extending stator arm. 16. An EUV radiation source according to any of clauses 1 to 15, wherein the position-controllable mirror further comprises a clamp configured to clamp together an actuator yoke and a radial projection protruding from the stem of the mirror. 17. The clamp also clamps the encoder support, along with the radial projections protruding from the actuator yoke and mirror stem, to the EUV radiation source of Clause 16. 18. An EUV radiation source according to any of the clauses 1 to 17, wherein the mirror comprises a substrate having a reflective surface, and the mirror substrate comprises at least one of one of one or more hollow volumes, one or more undercuts, and a variable wall thickness. 19. A position-controllable mirror further comprises a bendable stem connecting the mirror to a base, according to any EUV radiation source of clauses 1 to 18. 20. An EUV radiation source according to clause 19, having a flexible stem with a reduced diameter portion. 21. The center of gravity of the movable part of the position-controllable mirror is located at the reduced diameter portion of the bendable stem, according to the EUV radiation source of Clause 20. 22. The center of gravity of the movable part of the position-controllable mirror corresponds to the elastic center of the movable part of the position-controllable mirror, and is an EUV radiation source of any of the terms 1 to 21. 23. The position-controllable mirror further comprises a flexure located below the mirror and above the actuator yoke, according to any EUV radiation source of clauses 1 to 22. 24. The flexure is an EUV radiation source according to Clause 23, comprising multiple connection points between an inner portion and an outer portion, the connection points including bends. 25. The flexure is an EUV radiation source according to Clause 24, comprising multiple connection points between an inner portion and an outer portion, the connection points being U-shaped. 26. A position-controllable mirror is configured for use in a vacuum environment with any of the EUV radiation sources described in Clauses 1 through 25. 27. The mirror body of the position-controllable mirror is formed from a ceramic material, and is an EUV radiation source according to any of the clauses 1 to 26. 28. A position-controllable mirror, including an adjustable end stopper, is an EUV radiation source as specified in any of Clauses 1 to 27. 29. Position-controllable mirrors include heat dumping and are EUV radiation sources as specified in any of clauses 1 through 28. 30. An encoder head and locking system further comprising a leaf spring and an actuator, the actuator configured to engage the leaf spring with the encoder head holder, the leaf spring being located in a plane substantially parallel to the central axis of the position-controllable mirror, an EUV radiation source according to any of the clauses 1 to 29. 31. The encoder head holder further comprises a plate configured to be sandwiched between a leaf spring and the housing wall of a position-controllable mirror, the EUV radiation source of clause 30. 32. The actuator comprises a chamfered block engaged with bolts and screws, the chamfered block being located within an opening in the housing wall, the position-controllable mirror of clause 31. 33. A position-controllable mirror for use with an EUV radiation source, having a control bandwidth of at least 500 Hz. 34. A method for guiding a laser beam onto a fuel droplet in an EUV radiation source, comprising controlling the position of the laser beam using a position-controllable mirror having a control bandwidth of at least 500 Hz. 35. The method of Clause 34, wherein a position-controllable mirror is located in the optical path together with at least one operable optical device, and the position-controllable mirror is tilted to at least partially compensate for a laser beam position error caused by a lower control bandwidth of the operable optical device compared to the control bandwidth of the position-controllable mirror.

Claims

1. An EUV radiation source comprising a laser configured to emit a laser beam and an optical system configured to control the position of the laser beam so that the laser beam is incident on a fuel droplet, wherein the optical system includes an optical path comprising a position-controllable mirror having a control bandwidth of at least 500 Hz.

2. The EUV radiation source according to claim 1, wherein the position-controllable mirror has a control bandwidth of at least 1 kHz.

3. The EUV radiation source according to claim 1 or 2, further comprising a controller configured to control the tilt of the position-controllable mirror, the controller configured to control the position-controllable mirror to at least partially compensate for laser beam position errors caused by an operable optical device in the optical path having a lower control bandwidth than the position-controllable mirror.

4. The EUV radiation source according to any one of claims 1 to 3, wherein the position-controllable mirror comprises a plurality of optical encoders, the optical encoders include encoder scales provided on radially extending arms of encoder supports, and the optical encoders include encoder scales provided on the side walls of the mirror substrate of the position-controllable mirror.

5. The EUV radiation source according to claim 4, wherein the encoder scale is convex such that it has a radius of curvature that is approximately corresponding to the radial distance from the encoder scale to the central axis of the position-controllable mirror.

6. The EUV radiation source according to any one of claims 1 to 5, wherein the position-controllable mirror comprises a hybrid reluctance actuator formed from an actuator yoke and a stator, and each of the actuator yoke and the stator comprises a plurality of radially extending arms.

7. The EUV radiation source according to claim 6, wherein at least one of the actuator yoke and the stator is formed from a soft magnetic composite material, and the stator further comprises a pillar extending perpendicularly from the radially extending stator arm, the pillar being integrally formed with the radially extending arm.

8. The EUV radiation source according to claim 6, wherein at least one of the actuator yoke and the stator is formed from a laminated steel material, the lamination of the radially extending arms of the stator is perpendicular to the central axis of the position-controllable mirror, the stator further comprises pillars extending perpendicularly from the radially extending arms, the lamination of the pillars is parallel to the central axis of the position-controllable mirror, and the pillars further extend through openings provided in the radially extending stator arms.

9. The EUV radiation source according to any one of claims 1 to 8, wherein the position-controllable mirror further comprises a clamp configured to clamp together the actuator yoke and a radial projection protruding from the stem of the mirror, the clamp also clamps an encoder support together with the actuator yoke and the radial projection protruding from the stem of the mirror.

10. The EUV radiation source according to any one of claims 1 to 9, wherein the mirror comprises a substrate having a reflective surface, and the mirror substrate comprises at least one of one or more hollow volumes, one or more undercuts, and a variable wall thickness.

11. The EUV radiation source according to any one of claims 1 to 10, wherein the position-controllable mirror further comprises a bendable stem connecting the mirror to a base, the bendable stem having a reduced diameter portion, and the center of gravity of the movable portion of the position-controllable mirror is located at the reduced diameter portion of the bendable stem.

12. The EUV radiation source according to any one of claims 1 to 11, wherein the center of gravity of the movable part of the position-controllable mirror corresponds to the elastic center of the movable part of the position-controllable mirror.

13. The EUV radiation source according to any one of claims 1 to 12, wherein the position-controllable mirror further comprises a flexure located below the mirror and above the actuator yoke, the flexure having a plurality of connection points between an inner portion and an outer portion, the connection points including bends, and the flexure having a plurality of connection points between an inner portion and an outer portion, the connection points being U-shaped.

14. The position-controllable mirror is • Designed for use in a vacuum environment. Includes adjustable end stoppers, - Including heat dumping An EUV radiation source according to any one of claims 1 to 13, wherein it is one or more of the following.

15. The EUV radiation source according to any one of claims 1 to 14, wherein the mirror body of the position-controllable mirror is formed from a ceramic material.

16. An EUV radiation source according to any one of claims 1 to 15, further comprising an encoder head and a locking system, the locking system comprising a leaf spring and an actuator, the actuator configured to engage the leaf spring with an encoder head holder, the leaf spring being in a plane substantially parallel to the central axis of the position-controllable mirror, and the encoder head holder further comprising a plate configured to be sandwiched between the leaf spring and the housing wall of the position-controllable mirror.

17. A position-controllable mirror for use with an EUV radiation source, having a control bandwidth of at least 500 Hz.

18. A method for guiding a laser beam onto a fuel droplet in an EUV radiation source, comprising controlling the position of the laser beam using a position-controllable mirror having a control bandwidth of at least 500 Hz.

19. The method of claim 18, wherein the position-controllable mirror is located in the optical path together with at least one operable optical device, and the position-controllable mirror is tilted to at least partially compensate for a laser beam position error caused by a control bandwidth of the operable optical device that is lower than the control bandwidth of the position-controllable mirror.

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