EP4460876A1

EP4460876A1 - Laser beam amplification system

Info

Publication number: EP4460876A1
Application number: EP22838861.7A
Authority: EP
Inventors: Petrus Antonius Johannes VAN MELICK
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2022-01-07
Filing date: 2022-12-21
Publication date: 2024-11-13
Also published as: CN118511402A; WO2023131536A1

Abstract

A laser beam amplification system comprising a plurality of laser amplifiers configured to amplify a laser beam in series, wherein at least one of the laser amplifiers is supported by a plurality of actuators which are configured to move the laser amplifier in a controlled manner and thereby apply a desired movement to the laser beam.

Description

LASER BEAM AMPLIFICATION SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 22150540.7 which was filed on January 7, 2022 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a laser beam amplification system. The amplification system may form part of a laser system, which may in turn form part of a laser produced plasma (LPP) radiation source. The LPP radiation source may produce extreme ultraviolet (EUV) radiation and may form part of a lithography system.

BACKGROUND

[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 may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] EUV radiation for a lithographic apparatus may be produced by a laser produced plasma (LPP) radiation source. Within an LPP radiation source, a laser beam may be used to irradiate fuel droplets so as to produce a plasma which will emit EUV radiation.

[0006] It is desirable that the laser beam which is used to illuminate the fuel droplets has a high power. In addition, it is desirable that the position of the laser beam is controlled sufficiently accurately that hits fuel droplets in order to generate the EUV radiation.

[0007] It may be desirable to control the position of the laser beam in a manner which is not disclosed or suggested by the prior art.

SUMMARY

[0008] According to a first aspect of the invention there is provided a laser beam amplification system comprising a plurality of laser amplifiers configured to amplify a laser beam in series, wherein at least one of the laser amplifiers is supported by at least one actuator, the at least one actuator being configured to move the laser amplifier in a controlled manner and thereby apply a desired movement to the laser beam.

[0009] Advantageously, this may provide a simple and robust way of adjusting the laser beam.

[00010] The at least one laser amplifier may be supported by a plurality of actuators configured to move the laser amplifier in a controlled manner.

[00011] The plurality of actuators may be configured to provide movement in only one direction.

[00012] The plurality of actuators may be configured to provide movement only in the vertical direction.

[00013] Each of the laser amplifiers may be provided with a plurality of actuators.

[00014] The at least one actuator may be thermally controlled.

[00015] The system may further comprise a cooling fluid system which provides cooling fluid to an actuator, the cooling fluid being actively controlled in order to control the temperature of the actuator.

[00016] The actuator may be generally rectangular with a central hole.

[00017] The at least one actuator may be electro-mechanical.

[00018] The at least one laser amplifier may be supported by three actuators.

[00019] The laser beam amplification system may further comprise an optical sensor configured to determine a position of the laser beam.

[00020] The laser beam amplification system may further comprise a control system configured to compare a sensed position of the laser beam with a desired position, and to operate the actuators to move the laser beam closer to the desired position.

[00021] An output of the at least one laser amplifier may be vertically displaced relative to an input of the at least one laser amplifier.

[00022] According to a second aspect of the invention there is provided a laser produced plasma radiation source comprising a fuel emitter operable to produce a fuel target at a plasma formation region, a seed laser configured to emit a laser beam, and the laser beam amplification system of the first aspect, arranged to amplify the laser beam before it is incident upon the fuel target at the plasma formation region.

[00023] Advantageously, the second aspect of the invention may provide a more efficient radiation source, due to more accurate pointing of the laser beam at fuel droplets. The radiation source may be more robust than prior art radiation sources.

[00024] According to a third aspect of the invention there is provided a lithographic system comprising the laser produced plasma radiation source of the second aspect of the invention, and a lithographic apparatus.

[00025] Advantageously, the third aspect of the invention may provide a higher throughput lithographic system, due to more efficient generation of radiation (which may provide higher intensity radiation). [00026] According to a fourth aspect of the invention there is provided a laser amplifier configured to amplify a laser beam, wherein the laser amplifier is supported by a plurality of actuators which are configured to move the laser amplifier in a controlled manner and thereby apply a desired movement to the laser beam.

[00027] Advantageously, the laser amplifier of the fourth aspect of the invention may provide easier adjustment of the laser beam than prior art laser amplifiers.

[00028] According to a fourth aspect of the invention there is provided a method of compensating for drift of a laser beam in a laser system comprising a seed laser and a series of laser amplifiers, the method comprising determining a deviation of the laser beam from a desired position, and using one or more actuators to move at least one of the laser amplifiers and thereby move the laser beam closer to the desired position.

[00029] Advantageously, this may provide a simple and robust method of adjusting the laser beam. Features of different aspects of the invention may be combined together.

BRIEF DESCRIPTION OF THE DRAWINGS

[00030] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and a radiation source according to an embodiment of the invention;

Figure 2 schematically depicts a laser system according to an embodiment of the invention; Figure 3 schematically depicts a laser system according to an embodiment of the invention; Figure 4 schematically depicts a laser amplifier according to an embodiment of the invention; Figure 5 schematically depicts an effect on a laser beam of a movement of a laser amplifier;

Figure 6 schematically depicts a different effect on a laser beam of the movement of the laser amplifier;

Figure 7 depicts a control loop which may be used by an embodiment of the invention;

Figure 8 depicts an actuator which may support a laser amplifier according to an embodiment of the invention; and

Figure 9 depicts a control loop which may be used by an embodiment of the invention.

DETAILED DESCRIPTION

[00031] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic 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. [00032] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with 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 faceted pupil mirror device 11.

[00033] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are 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’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[00034] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[00035] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[00036] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma.

[00037] The laser beam 2 which is incident upon the tin at the plasma formation region 4 may be a pulsed laser beam. The laser beam 2 which is incident upon the tin at the plasma formation region 4 may be referred to as a main laser beam and individual pulses of this laser beam 2 may be referred to as main pulses. [00038] Before the main laser beam 2 is incident upon the tin at the plasma formation region 4, another pre-pulse laser beam may be incident on the tin. The pre-pulse laser beam may act to change a shape of the tin so as to increase the conversion efficiency when the main pulse is (subsequently) incident on the tin. One or more additional pulses may be used.

[00039] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[00040] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[00041] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[00042] The laser system 1 may comprise a seed module (which may be referred to as a high- power seed module) and an amplification system. The seed module may be a laser operable to produce the pre-pulse laser beam and the main pulse laser beam. The amplification system may be operable to receive the pre -pulse laser beam and the main pulse laser beam from the seed module and to increase the power of each of the pre -pulse laser beam and the main pulse laser beam. The amplification system may comprise a series of laser amplifiers.

[00043] In an alternative arrangement, the laser system 1 may comprise a first seed module and first associated amplification system, and a second seed module and second associated amplification system. In a further alternative arrangement, the laser system 1 may comprise first and second seed modules and a single associated amplification system. In general, at least one seed module (which may be a laser) and at least one associated amplification system may be provided. For simplicity, this document describes a single seed module and associated amplification system.

[00044] Figure 2 schematically depicts the laser system 1 of Figure 1. The laser system 1 comprises a seed module 20 and four laser amplifiers 21-24. The laser amplifier 21-24 may alternatively be referred to as laser beam amplifiers or optical amplifiers. The laser amplifiers 21-24 are provided in series such that a first laser amplifier 21 amplifies a laser beam output by the seed module 20, the second laser amplifier 22 provides further amplification to the laser beam output from the first laser amplifier 21, the third laser amplifier provides further amplification to the amplified laser beam output from the second laser amplifier 22, and the fourth laser amplifier 24 provides further amplification to the laser beam output from the third laser amplifier 23. Thus, four amplification stages are applied to the laser beam by the four laser amplifiers 21-24, the amplification stages being provided in series and each increasing the power of the laser beam. The laser beam output from the fourth and final laser amplifier 24 passes to a beam steering system 26. The beam steering system 26 steers the amplified laser beam to the plasma formation region 4 (see Figure 1), where the amplified laser beam is incident upon fuel droplets and thereby generates EUV radiation.

[00045] Although four laser amplifiers 21-24 are depicted in Figure 2, a different number of laser amplifiers may be provided. In general, a plurality of laser amplifiers may be used.

[00046] Figure 3 schematically depicts the laser system 1 in a particular arrangement. In the arrangement depicted in Figure 2 the laser amplifiers 21-24 are aligned with each other in one direction (this may be referred to as an in-line arrangement). In the arrangement in Figure 3, the laser amplifiers 21-24 have an arrangement which allows them to extend over a shorter distance. In the depicted arrangement the first to third amplifiers 21-23 are all adjacent to the fourth amplifier 24. This arrangement may be referred to as a T-shape arrangement). This arrangement advantageously reduces the maximum dimension of the amplification system 1, allowing it to be more easily housed within a lithographic fabrication plant. Other arrangements may be used.

[00047] In the arrangement depicted in Figure 3, beam directing mirrors 30 are used to direct the laser beam between laser amplifiers 21-24. In order to avoid over complicating Figure 3 only two of the reflectors 30 are labelled. Figure 3 depicts the amplification system 1 in two dimensions. In practice, the system 1 is three dimensional. In an embodiment, the amplifiers 21-24 may be arranged such that the laser beam travels around each laser amplifier twice. In this arrangement, the laser beam travels around a laser amplifier two times. However, in other arrangements the laser beam may travel around the laser amplifier once, three times or more.

[00048] The passage of the laser beam 55 around the first laser amplifier 21 is schematically depicted by a dashed line 21a. The laser amplifier 21 may be provided with a gas to which energy is provided from an electrical source. The energy provides a population inversion in the gas, and the laser beam 55 is amplified as it passes through the gas, extracting energy from the gas. Although one loop 21a of the laser beam 55 around the laser amplifier 21 is depicted in Figure 3, in practice two loops of the laser amplifier may occur. The second loop may be displaced vertically relative to the first loop. The same may apply for each of the other laser amplifiers 22-24. This z-direction aspect of the amplification system is not depicted in Figure 3 in order to avoid over complicating Figure 3. In other embodiments different numbers of loops of the laser amplifier may occur. [00049] A camera 44 (or other optical sensor) is provided downstream of the fourth amplifier 24. The camera 44 is configured to determine the position of the output laser beam 2. The camera may be used as an input for a controller 46 which controls the position of at least one of the amplifiers 21-24.

[00050] Figure 4 schematically depicts, in a perspective view from underneath, the final laser amplifier 24 of the laser system 1. Because Figure 4 is three dimensional, a z-direction (vertical) separation between an input 40 and an output 42 of the laser amplifier is shown. In Figure 4 the vertical (z) direction is upwards on the page and the x-direction is across the page. The y-direction is diagonally into the page.

[00051] The laser amplifier 24 is supported by three actuators 51-53. The actuators 51-53 may be mounted on a common base (not depicted). The common base may be substantially invariant (fixed) over time. As a result, the spatial position of the laser amplifier 24 may depend upon the actuated positions of the actuators 51-53 (together with any thermal expansion or contraction of the laser amplifier which may take place).

[00052] As depicted, the actuators 51-53 are configured to provide the movement in one direction only. In the figure this direction is the z (vertical) direction. This advantageously allows the actuators to have a simple construction (compared for example with actuators which are configured to move in multiple directions). Although the actuators only provide movement in the z-direction, the actuated positions of the actuators, taken in combination with the position of the input 40 and the output 42 of the laser amplifier 24, are such that a variety of movements of the output laser beam 2 may be achieved via operation of the actuators. Examples of the movement are now explained.

[00053] In a first example movement, all of the actuators 51-53 are actuated in unison (e.g. all of the actuators are lengthened). This provides a z-direction (vertical) movement of the laser amplifier 24. This movement may provide a z-direction (vertical) movement of the output laser beam 2. This vertical movement of the output laser beam 2 may be provided without a modification of the direction (which may be referred to as beam-pointing) of the output laser beam 2. In other words, vertical movement of the output laser beam 2 is provided without changing the x and y directions of propagation of the laser beam.

[00054] Figure 5 schematically depicts the effect of z-direction movement of the laser amplifier 24 depicted in Figure 4. Figure 5 schematically depicts two mirrors 60, 62 which form part of the laser amplifier. The mirrors 60, 62 direct the laser beam 55 from the end of a lower loop of the laser amplifier to the start of an upper loop of the laser amplifier. To achieve this, the mirrors may be facing each other at 45° relative to the horizontal (as depicted). When a z-direction (vertical) movement of the laser amplifier occurs, the mirrors 60, 62 move upwards from a first position (depicted with solid lines) to a second position (depicted with dotted lines). The z-direction movement is denoted as l*z. The laser beam 55 also moves in the z-direction, as depicted by the dotted line. Due to the mirror configuration, the z-direction movement of the laser beam is 2*z. [00055] Figure 6 schematically depicts a different effect of z-direction movement of a laser amplifier 24 which may occur. The output laser beam 2 may pass through a lens 64. If the output laser beam 2 is moved in the vertical direction (z-direction), then effect of the lens may be to change the beam pointing direction of the output laser beam. In the depicted example, the output laser beam initially passes through the centre of the lens 64, and so the lens 64 has no effect upon the direction of the output laser beam. When the output laser beam 2 is moved upwards, it now passes through a different part of the lens 64, and the lens bends the output laser beam such that it now points downwards. The same effect applies when a laser amplifier containing a lens is moved with respect to the laser beam.

[00056] In general, vertical movement of a laser amplifier may have one or more effects (e.g. displacement and change of pointing direction) upon the output laser beam 2. These effects may be calibrated by applying vertical movements to the laser amplifier and measuring movement of the output laser beam 2 (e.g. using an optical detector).

[00057] In another example of movement of the laser amplifier 24, only the first actuator 51 is actuated (e.g. lengthened), with the other actuators 52, 53 remaining unchanged. As may be appreciated from Figure 4, this will provide a rotation of the laser amplifier 24 about an axis which extends in the y-direction and is approximately midway (in the x-direction) between the first actuator and the second and third actuators. Since the input 40 and the output 42 of the laser amplifier 24 are vertically displaced relative to this axis of rotation, the position of the input 40 relative to the laser beam 55 will move through an arc. The position of the output 42 will also move through an arc (the arcs may have different shapes to each other). This movement of the laser amplifier 24 will provide a movement of the output laser beam 2. The movement of the output laser beam may be in the x and z directions (and may include a y direction component).

[00058] A similar movement of the output laser beam 2, but with the arc having a shape which is mirrored through the z-plane, will be provided if the first actuator 51 is fixed and the second and third actuators 52, 53 are moved in unison.

[00059] In another example, the second actuator 52 may be operated without the third actuator 53 being operated. This will raise an input and output face of the laser amplifier 24 (the face which includes the input 40 and the output 42) relative to an opposite side of the laser amplifier. The first actuator 51 may be actuated through half of the distance of the second actuator 52 in order to avoid adding a rotation about a y-direction axis to the movement. The combination of actuation of the second actuator 52 and the first actuator 51 will provide a rotation about an x-direction axis which bisects a bottom surface of the laser amplifier 24. When this movement is applied to the laser amplifier 24 it may rotate the beam pointing direction of the output laser beam about an axis which extends in the x-direction. In other words, the beam pointing direction of the output laser beam 2 may be raised or lowered in the z-direction. [00060] Other movements of the output laser beam 2 may be obtained via combinations of different movements of the actuators 51-53. Referring again to Figures 2 and 3, two or more of the laser amplifiers 21-24 may be supported by actuators. Operation of the actuators will allow for a combination of spatial displacement of the output laser beam 2 and changes of the beam pointing direction of the output laser beam 2. In other words, the position of the output laser beam 2 at the output 42 of the fourth laser amplifier 24 may be modified, and in addition the beam pointing direction of the laser beam 2 may be modified.

[00061] In general, actuators may be used to move a laser amplifier and thereby apply a desired movement to the laser beam. The movement of the laser amplifier is controlled by the actuators. These effect of the movement of each laser amplifier may be calibrated by applying movements to that laser amplifier and measuring movement of the output laser beam 2 (e.g. using an optical detector).

[00062] Embodiments of the invention advantageously allow a correction for undesired movement of the output laser beam 2 to be achieved. For example, drift of the output laser beam 2 which may occur can over time may be corrected. This advantageously may allow improved performance of the EUV radiation source SO, by allowing the laser beam 2 to be more accurately directed to the plasma formation location 4 over time (see Figure 1). Additional or alternatively, embodiments of the invention may allow for the output laser beam 2 to be positioned with a desired accuracy using a system which is less complicated and/or more reliable than the existing systems, particularly with respect to slowly varying errors such as drift (e.g. drift caused by temperature changes - so called thermal drift). Embodiments of the invention may provide beam steering and positioning in a relatively robust and straightforward manner via actuators which are external to the laser amplifiers 21-24, instead of for example requiring actuation of actuators which are positioned within laser amplifiers. This is advantageous because actuators external to the laser amplifiers may experience a less harsh environment than actuators within laser amplifiers, and they may be easier to access and therefore easier to maintain.

[00063] A camera 44 (or other optical sensor) may be provided downstream of the output 42 of the fourth amplifier 24 (see Figure 3). The camera 44 may be used to determine the position of the output laser beam 2. The camera may be used as an input for a controller 46. The controller 46 may control actuators of the laser amplifiers 21-24 using a feedback loop which receives as inputs a desired position P (which may be referred to as a set point) of the output laser beam 2 at the camera 44, And an actual position of the output laser beam at the camera. One or more cameras (or other optical sensors) may be provided at other locations in the laser amplification system. For example, a camera (or other optical sensor) may be provided between two laser amplifiers and may be configured to determine the position of the laser beam between the two laser amplifiers. A camera (or other optical sensor) may be provided between each laser amplifier. Outputs from the camera (or cameras) may be used by a control system to control the laser beam. [00064] An example feedback loop which may be implemented by a control system is depicted in Figure 7. A first input of the feedback loop is a set point P, i.e. the desired position of the laser beam. A second input C is the position of the laser beam as measured by the camera 44. A difference between the desired position P and the actual position C is determined. If the actual position C corresponds with the desired position P to within a predetermined threshold, then no actuation of the laser amplifier actuators is performed. If the difference between positions is greater than the threshold, then actuation of actuators is performed, as indicated by A. The resulting position of the output laser beam is determined by a transfer function TA which links the actuator positions to the position of the laser beam and the camera C. The transfer function TA may be determined via a calibration process in which different actuators, and combinations of actuators, are operated and the resulting movement of the output laser beam 2 at the camera C is determined.

[00065] Also shown in the control loop of Figure 7 is a cross D which represents drift that causes movement of the position of the output laser beam 2 on the camera C over time. The drift D may for example be caused by a change of temperature of the environment in which the laser amplifiers are provided. Thermal expansion or contraction of laser amplifiers or components of the laser amplifiers, etc. may occur. This drift D causes movement of the output laser beam 2. A transfer function TD links the drift D to movement of the position of the output laser beam 2 on the camera C. The movement of the output laser beam 2 caused by the drift D may be corrected for in the control loop via actuation of the actuators A.

[00066] In an embodiment, the actuators may be thermally controlled. That is, an actuator may have a z-direction (vertical) dimension which changes as a function of the temperature of the actuator. The actuator may be actively cooled, for example using water or other circulated coolant. Active control of the cooling allows the temperature of the actuator to be controlled. Additionally, or alternatively, a heater element may be provided as part of the actuator (e.g. at an upper end or lower end of the actuator). The actuator may also include a temperature sensor.

[00067] An example of a thermally controlled actuator is schematically depicted in Figure 8. The thermally controlled actuator 80 is depicted on the left-hand side of Figure 8 at a higher temperature than on the right-hand side of Figure 8. The thermally controlled actuator 80 has a generally rectangular shape (with an opening 81 at its centre). The actuator 80 may also be referred to as an actuating support. In general actuators of embodiments of the invention may be referred to as actuating supports or support actuators. The actuator 80 comprises a bottom member 82, two side members 84, 85 and a top member 86. An attachment member 88 is provided at an uppermost end of the actuator 80, for attachment to a laser amplifier. A bottom attachment member 90 is provided at a lowermost end of the actuator 80 for attachment to a base or other support. Each attachment member may for example provide a single point of attachment.

[00068] The side members 84, 85 may extend in the vertical direction (z-direction) such that expansion or contraction of the side members will cause a vertical expansion or contraction of the actuator. That is a vertical separation between the top and bottom attachment members 88, 90 is increased or decreased. Because the attachment members 88, 90 are centrally positioned on the actuator 80, thermal expansion or contraction of the actuator will not cause a movement of the attachment members in the x or y directions (or may cause a relatively small movement in the x and/or y direction e.g. due to temperature gradient across the actuator).

[00069] Arrows schematically depict one possible flow of coolant fluid through the actuator 80 (for ease of illustration this is shown only on the left-hand side of Figure 8). The temperature of the actuator 80 may be determined by a temperature sensor mounted on the actuator and/ or by a temperature sensor which is configured to measure the temperature of the coolant fluid.

[00070] A fluid cooling system and/or a fluid heating system may be used to change the temperature of the coolant fluid which is passed to the actuator 80, thereby changing the temperature of the actuator. Additionally, or alternatively, a pump which circulates the cooling fluid through the actuator may be controllable to pump at a greater rate (in which case more cooling is provided) or at a reduced rate (in which case less cooling is provided). In general, the temperature of the actuator 80 may be controlled by actively controlling the fluid used to cool the actuator. When the temperature of the actuator 80 is lowered, the actuator contracts as depicted on the right-hand side of Figure 8. This causes downwards movement of the attachment member 88 which supports a laser amplifier. No horizontal movement of the attachment member 88 takes place.

[00071] Although the depicted thermally controlled actuator 80 has a particular shape, the thermally controlled actuator may have any suitable shape. The thermally controlled actuator may for example be a cylinder or a rectangular pillar. Other forms of actuator may have any suitable shape, e.g. cylinder or rectangular pillar.

[00072] Figure 9 depicts a control loop which may be used for embodiments of the invention in which the actuators are thermally controlled (e.g. an embodiment such as depicted in Figure 8). The control loop corresponds with the control loop of Figure 7, but in this embodiment the actuation step A comprises changing the temperature of the actuators. When the temperature of an actuator is changed A then a transfer function TA determines the resulting movement of the output laser beam 2 on the camera C (or other optical sensor).

[00073] The cross D represents drift that causes movement of the position of the output laser beam 2 on the camera C over time. The drift D may for example be caused by a change of temperature of the environment in which the laser amplifiers are provided. A transfer function TD links the drift D to movement of the position of the output laser beam 2 on the camera C.

[00074] The control loop includes a nested loop relating to the measured temperature MT of the actuator. Dashed lines indicate inputs which affect the measured temperature MT of the actuator. A first input is the drift D. That is, a thermal drift of the laser amplifier may cause the temperature of the actuator to change. The result of this change as measured by a thermal sensor is determined by a transfer function TDMT. The measured temperature MT of the actuator will change with thermal drift D according to the transfer function TDMT. The second input is the controlled change of temperature of the actuator (i.e. the actuation step A). The result of this change as measured by a thermal sensor is determined by a transfer function TAMT. The measured temperature, which is a result of both transfer functions TDMT, TAMT, is fed back to the actuation step A. This nested loop allows the actuator to be maintained at desired temperatures.

[00075] The control system may include additional control loops which adjust the position of the laser beam 55 between different laser amplifiers.

[00076] Various different forms of actuator may be used. The actuator schematically depicted in Figure 8 is operated via control of the coolant fluid which passes through the actuator. In addition, or alternatively, heating or cooling elements may be provided on the actuator itself. In another example, mechanical actuation e.g. via electrically driven motors, may be used. In another example linear actuators may be used. Solenoid actuators, Lorentz motors, stepper motors, conventional rotary motors, gears/pinions. In general, the actuators may comprise any suitable electro-mechanical systems. The actuators may be electrical, e.g. piezo-electric actuators. The actuators may for example be hydraulic or pneumatic.

[00077] In the embodiment depicted in Figure 8 the actuator 80 comprises a generally rectangular support with a central opening. In other embodiments the same or similar configuration of support may be connected to a separately provided actuator. For example, a linear actuator may be connected between a base and the lowermost attachment point 90 of a support 80. In this example the support 80 may be no longer thermally controlled but may still nevertheless be provided with pumped coolant fluid.

[00078] Although Figures 7 and 9 depict specific control loops which may be implemented as control systems, any suitable control system may be used. The control system may for example take into account the position of the laser beam 55 as measured by cameras (or other optical sensors) located between laser amplifiers. The control system may for example have a fixed set point P which does not vary over time. In another example, the set point P may be modified over time for example to correct for disturbances (e.g. thermal disturbances) that are occurring elsewhere in the EUV radiation source SO.

[00079] In other examples, feed-forward control may be used. In one example, movement of the output laser beam 2 due to thermal drift when the laser system has been switched on may follow a known path. This path may be corrected via pre-determined operation of the actuators.

[00080] In one example, the actuator 80 may have a height of around 0.3 meters and may have a co-efficient of thermal expansion of around 23 e-6 [1/K]. In this example a change of temperature of the actuator of up to 5K may be provided. A change of 5K would provide an expansion of around 30 microns. A separation between the actuator 80 and other actuators of the laser amplifier may be 1 meter. Where this is the case, an end of the laser amplifier which is supported by the actuator will rotate through an angle of approximately 30 grad. This may provide a vertical displacement of the beam by around 1mm at a downstream position (e.g. multiple meters downstream). Smaller changes of temperature may be used to obtain smaller displacements. The actuator 80 may have a relatively low thermal mass, such that the change of temperature of the actuator may take place in a period of the order of minutes. This may be sufficiently fast to correct for thermal drift for example. This is merely an illustrative example of an actuation distance. Other actuation distances, e.g. smaller distances, may be used.

[00081] Other actuators may provide the same or other actuation distances over similar or different time periods.

[00082] An electro-mechanical actuator may be used to provide actuation in a similar manner. For example, a linear actuator may be used to provide an actuation of up to around 30 microns (or actuations of other amounts).

[00083] Actuators may be configured to provide actuations of the order of the tens or hundreds of microns. Other order of magnitude of actuations may be provided.

[00084] In described embodiments of the invention the actuators are located beneath the laser amplifiers. However, in an alternative arrangement the actuators may be provided at a different location. For example, the laser amplifiers may be held by actuators which are located above the laser amplifiers (e.g. the actuators may extend down from a frame, ceiling or other support).

[00085] Although the depicted embodiment shows a laser amplifier supported by three actuators, in other embodiments a laser amplifier may be supported by a different number of actuators. For example, a last amplifier may be supported by four or more actuators.

[00086] One or more of the laser amplifiers may be provided with no actuators (i.e. provided on supports which do not include actuators).

[00087] Although the depicted actuators are only capable of actuation in one direction, in other embodiments actuators may be provided which are capable of actuation in more than one direction.

[00088] Although embodiments of the invention have been described in the context of EUV radiation generation, the invention may be used in other application areas. However, the invention is particularly advantageous for EUV radiation generation, because EUV radiation generation benefits from highly accurate control of the position of the laser beam 2 at the plasma formation location 4 (inaccurate laser beam positioning will reduce the amount of EUV radiation which is generated). In addition, the invention allows for simple and robust actuators to be used. This is particularly advantageous for EUV radiation generation because it is desirable to operate the EUV radiation source continuously without interruption for as long as possible (in order to maximize throughput of the lithographic apparatus).

[00089] Embodiments of the invention may provide improved control of the positioning of the laser beam 2 at the plasma formation location 4 (see Figure 1). This may provide an increase of EUV radiation power provided by the source SO. [00090] Adjustment of the position of the laser beam 2 at the plasma formation location 4 (see Figure 1) using the invention may allow back-reflections of the laser beam from fuel droplets to be reduced. A feedback loop may be established with a transfer function which indicates the power of back reflected laser light, with actuators being controlled to control the power of back reflected laser light (e.g. keep it below a predetermined threshold).

[00091] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[00092] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[00093] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[00094] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

[00095] In order to illustrate certain possible combinations of embodiments following clauses are given, without limiting the combinations thereof which arise directly and unambiguously from the above description. 1. A laser beam amplification system comprising a plurality of laser amplifiers configured to amplify a laser beam in series, wherein at least one of the laser amplifiers is supported by at least one actuator, the at least one actuator being configured to move the laser amplifier in a controlled manner and thereby apply a desired movement to the laser beam.

2. The laser beam amplification system of clause 1, wherein the at least one laser amplifier is supported by a plurality of actuators configured to move the laser amplifier in a controlled manner.

3. The laser beam amplification system of clause 2, wherein the plurality of actuators are configured to provide movement in only one direction.

4. The laser beam amplification system of clause 2, wherein the plurality of actuators are configured to provide movement only in the vertical direction.

5. The laser beam amplification system of any preceding clause, wherein each of the laser amplifiers is provided with a plurality of actuators.

6. The laser beam amplification system of any preceding clause, wherein the at least one actuator is thermally controlled.

7. The laser beam amplification system of clause 5, wherein the system further comprises a cooling fluid system which provides cooling fluid to an actuator, the cooling fluid being actively controlled in order to control the temperature of the actuator.

8. The laser beam amplification system of clause 6 or clause 7, wherein the actuator is generally rectangular with a central hole.

9. The laser beam amplification system of any of clauses 1 to 5, wherein the at least one actuator is electro-mechanical.

10. The laser beam amplification system of any preceding clause, wherein the at least one laser amplifier is supported by three actuators.

11. The laser beam amplification system of any preceding clause, further comprising an optical sensor configured to determine a position of the laser beam.

12. The laser beam amplification system of clause 11, further comprising a control system configured to compare a sensed position of the laser beam with a desired position, and to operate the actuators to move the laser beam closer to the desired position.

13. The laser beam amplification system of any preceding clause, wherein an output of the at least one laser amplifier is vertically displaced relative to an input of the at least one laser amplifier.

14. A laser produced plasma radiation source comprising: a fuel emitter operable to produce a fuel target at a plasma formation region; a seed laser configured to emit a laser beam; and the laser beam amplification system of any preceding clause, arranged to amplify the laser beam before it is incident upon the fuel target at the plasma formation region. A lithographic system comprising: the laser produced plasma radiation source of clause 14; and a lithographic apparatus. A laser amplifier configured to amplify a laser beam, wherein the laser amplifier is supported by a plurality of actuators which are configured to move the laser amplifier in a controlled manner and thereby apply a desired movement to the laser beam. A method of compensating for drift of a laser beam in a laser system comprising a seed laser and a series of laser amplifiers, the method comprising: determining a deviation of the laser beam from a desired position; and using one or more actuators to move at least one of the laser amplifiers and thereby move the laser beam closer to the desired position.

Claims

1. A laser beam amplification system comprising a plurality of laser amplifiers configured to amplify a laser beam in series, wherein at least one of the laser amplifiers is supported by at least one actuator, the at least one actuator being configured to move the laser amplifier in a controlled manner and thereby apply a desired movement to the laser beam.

2. The laser beam amplification system of claim 1, wherein the at least one laser amplifier is supported by a plurality of actuators configured to move the laser amplifier in a controlled manner.

3. The laser beam amplification system of claim 2, wherein the plurality of actuators are configured to provide movement in only one direction.

4. The laser beam amplification system of claim 2, wherein the plurality of actuators are configured to provide movement only in the vertical direction.

5. The laser beam amplification system of any preceding claim, wherein each of the laser amplifiers is provided with a plurality of actuators.

6. The laser beam amplification system of any preceding claim, wherein the at least one actuator is thermally controlled.

7. The laser beam amplification system of claim 5, wherein the system further comprises a cooling fluid system which provides cooling fluid to an actuator, the cooling fluid being actively controlled in order to control the temperature of the actuator.

8. The laser beam amplification system of any of claims 1 to 5, wherein the at least one actuator is electro-mechanical.

9. The laser beam amplification system of any preceding claim, further comprising an optical sensor configured to determine a position of the laser beam.

10. The laser beam amplification system of claim 9, further comprising a control system configured to compare a sensed position of the laser beam with a desired position, and to operate the actuators to move the laser beam closer to the desired position.

11. The laser beam amplification system of any preceding claim, wherein an output of the at least one laser amplifier is vertically displaced relative to an input of the at least one laser amplifier.

12. A laser produced plasma radiation source comprising: a fuel emitter operable to produce a fuel target at a plasma formation region; a seed laser configured to emit a laser beam; and the laser beam amplification system of any preceding claim, arranged to amplify the laser beam before it is incident upon the fuel target at the plasma formation region.

13. A lithographic system comprising: the laser produced plasma radiation source of claim 12; and a lithographic apparatus.

14. A laser amplifier configured to amplify a laser beam, wherein the laser amplifier is supported by a plurality of actuators which are configured to move the laser amplifier in a controlled manner and thereby apply a desired movement to the laser beam.

15. A method of compensating for drift of a laser beam in a laser system comprising a seed laser and a series of laser amplifiers, the method comprising: determining a deviation of the laser beam from a desired position; and using one or more actuators to move at least one of the laser amplifiers and thereby move the laser beam closer to the desired position.