CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 22167287.6 which was filed on April 08, 2022 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to an electro-optical device for use in a laser system of a radiation system and associated systems and methods.

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] A lithographic system may comprise a radiation system and a lithographic apparatus. The radiation system may comprise a laser system configured to generate a laser beam and a EUV radiation source. EUV radiation may be produced, for example, by directing the laser beam into the radiation source so that the laser beam is incident on a fuel in the radiation source. The laser beam may deposit energy into the fuel to create a plasma. Radiation, including EUV radiation, may be emitted from the plasma.

SUMMARY

[0006] According to a first aspect of the present invention there is provided an electro-optical device for use in a laser system of a radiation system, the device comprising an electro-optical element having a first surface and a second surface, the electro-optical element being formed from a material that is optically transparent to a laser beam generated by the laser system and an antireflection structure formed from the material on at least one of the first surface and the second surface of the electro-optical element, the antireflection structure comprising a plurality of nanostructures.

[0007] By forming the antireflection structure from the material of the electro-optical element on at least one of the first and second surfaces of the electro-optical element, delamination of the antireflection structure from the electro-optical element may be prevented. The antireflection structure may be integrally formed from the material. The antireflection structure may be formed directly on at least one of the first surface and the second surface of the electro-optical element, e.g. without any intermediate layers. Additionally or alternatively, the formation of the antireflection structure from the material may allow for a wider range of materials to be used for the formation of an electro-optical element. The wider range of materials may include one or more materials, which previously may have been rarely used or not at all.

[0008] The first and second surfaces of the electro-optical element may be arranged on opposite sides of the electro-optical element.

[0009] The device may be arranged or be arrangeable in a path of the laser beam generated by the laser system.

[00010] The material may be selected such that the electro-optical element changes or modulates a property of the laser beam generated by the laser system, e.g. when a voltage is applied to the device. The property of the laser beam may comprise at least one of: a phase and a polarisation of the laser beam. The material may be selected such that a refractive index of the material changes, e.g. when a voltage is applied to the device. The material may comprise a crystalline material. The material may comprise a material that exhibits an electro-optical effect, e.g. the Pockels effect, e.g. when a voltage is applied to device.

[00011] The antireflection structure may comprise a Moth eye structure.

[00012] The plurality of nanostructures may be formed or arranged to be periodic in at least one direction.

[00013] The plurality of nanostructures may comprise at least one of: a plurality of grooves, a plurality of pillars and a plurality of holes. At least one or each pillar of the plurality of pillars may comprise a conical shape, e.g. substantially conical shape. For example, the at least one or each pillar of the plurality of pillars may comprise a truncated cone shape, e.g. substantially truncated cone shape. [00014] One or more properties of the plurality of nanostructures may be selected such that a refractive index of the antireflection structure is lower than a refractive index of the material. The one or more properties of the plurality of nanostructures may be selected such that the refractive index of the antireflection structure is higher than a reflective index of an optical medium surrounding the device. Expressed differently, the one or more properties of the plurality of nanostructures may be selected such that the refractive index of the antireflection structure is between the refractive index of the material and the refractive index of the optical medium surrounding the device. The optical medium surrounding the device may comprise air. The one or more properties of the plurality of nanostructures may comprise at least one of: a size or dimension of each nanostructure of the plurality of nanostructures, a shape of each nanostructure of the plurality of nanostructures and a periodicity of the plurality of nanostructures. The plurality of nanostructures may be arranged in an array. The periodicity of the plurality of nanostructure may comprise or be defined by a period of the array. [00015] According to a second aspect of the present invention there is provided a laser system for use in a radiation system, the laser system comprising a laser configured to generate a laser beam comprising a plurality of laser pulses and an optical modulation system comprising an electro-optical device according to the first aspect.

[00016] The optical modulation system may be configured to modulate one or more laser pulses of the plurality of laser pulses generated by the laser. Each of the one or more modulated laser pulses may comprise a portion of at least one laser pulse generated by the laser. The optical modulation system may be configured to modulate or control one or more properties of the one or more modulated laser pulses. The one or more properties of the one or more modulated laser pulses may comprise at least one of: a timing or sequence of the one or more modulated laser pulses, a temporal duration of each of the one or more modulated laser pulses and an intensity of each of the one or more modulated laser pulses.

[00017] The laser system may comprise an amplification system. The amplification system may be configured to amplify the one or more modulated laser pulses. The optical modulation system may be arranged between the laser and the amplification system.

[00018] The optical modulation system may comprise two or more electro-optical devices. The two or more electro-optical devices may be arranged in series.

[00019] According to a third aspect of the present invention there is provided a radiation system comprising an EUV radiation source and a laser system according to the second aspect.

[00020] According to a fourth aspect of the present invention there is provided a lithographic system comprising a radiation system according to the third aspect and a lithographic apparatus.

[00021] According to a fifth aspect of the present invention there is provided method of forming an antireflection structure of an electro-optical device, the method comprising providing an electro-optical element having a first surface and a second surface, applying a pattern of the antireflection structure on at least one of the first surface and the second surface, the antireflection structure comprising a plurality of nanostructures, and using a material removal process to transfer the pattern into the at least one of the first surface and the second surface, thereby forming the antireflection structure.

[00022] Various aspects and features of the invention set out above or below may be combined with various other aspects and features of the invention as will be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

[00023] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation system;

Figure 2 depicts a first part of an exemplary laser system for use in the radiation system of

Figure 1; Figure 3 depicts a second part of an exemplary laser system for use in the radiation system of Figure 1;

Figure 4 depicts an exemplary laser pulse generated by each of the first part of the laser system of Figure 2 and the second part of the laser system of Figure 3;

Figure 5 depicts an exemplary electro-optical device for use in the first part of the laser system of Figure 2 and/or the second part of the laser system of Figure 3;

Figure 6A depicts a plan view of an exemplary first antireflection structure of the electro- optical device of Figure 5;

Figure 6B depicts a plan view of an exemplary second antireflection structure of the electro- optical device of Figure 5;

Figure 7 depicts another an exemplary electro-optical device for use in the first part of the laser system of Figure 2 and/or the second part of the laser system of Figure 3;

Figure 8A depicts a plan view of an exemplary first antireflection structure of the electro- optical device of Figure 7;

Figure 8B depicts a plan view of alternative exemplary first antireflection structure of the electro-optical device of Figure 7;

Figure 9 depicts a cross-sectional view of exemplary nanostructures of the antireflection structure of Figure 8A; and

Figure 10 depicts an exemplary flow diagram outlining the steps of a method of forming an antireflection structure of an electro-optical device.

DETAILED DESCRIPTION

[00024] Figure 1 shows a lithographic system comprising a radiation system RS and a lithographic apparatus LA. The radiation system RS comprises a radiation source SO. 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.

[00025] 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.

[00026] 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).

[00027] 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.

[00028] 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.

[00029] 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 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.

[00030] 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.

[00031] 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 and the radiation source SO may together be considered as the radiation system RS. The radiation system RS may comprise the beam delivery system. [00032] 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.

[00033] As described above, the laser system 1 may be configured to generate the laser beam 2. The laser beam 2 generated by the laser system 1 may comprise a plurality of laser pulses. The laser system 1 may be configured to generate a pre-pulse and a main pulse. The pre-pulse may be configured to condition a droplet of fuel for receipt of the main pulse. For example, the pre-pulse may be configured to heat, deform, expand, gasify, vaporise and/or ionise the droplet and/or to generate a weak plasma. The main pulse may be configured to convert the conditioned droplet of fuel into a plasma 7 producing the EUV radiation. For example, the main pulse may be configured to convert most or all of the conditioned droplet of fuel into the plasma 7.

[00034] The laser system 1 may comprise a first part la. The first part la of the laser system 1 may be configured to generate the pre-pulse. The first part la of the laser system 1 may be considered as a first subsystem of the laser system 1. The laser system 1 may comprise second part lb. The second part lb of the laser system 1 may be configured to generate the main pulse. The second part lb of the laser system 1 may be considered as a second subsystem of the laser system 1.

[00035] Figure 2 shows the first part la of an exemplary laser system 1 for use in the radiation system RS shown in Figure 1. The laser system 1 may comprise a first laser 16a. The first laser 16a may be part of or comprised in the first part la of the laser system 1. In some embodiments, the first laser 16a may be provided in the form of a CO2 laser. In such embodiments, the first laser 16a may be configured to generate a first laser beam 2a having a wavelength of about 10.207 pm.

[00036] It will be appreciated that in other embodiments another type of laser may be used. For example, in other embodiments, the first laser 16a may be provided in the form of a solid-state laser, such as yttrium-aluminum-garnet (YAG) laser. In such other embodiments, the first laser 16a may be configured to generate a first laser beam 2 having a wavelength of about 1 pm.

[00037] The laser system 1 may comprise a first optical modulation system 18a. The first optical modulation system 18a may be part of or comprised in the first part la of the laser system 1. The first optical modulation system 18a may be arranged in a path of the first laser beam 2a. The optical modulation system 18 may be configured to modulate one or more laser pulses of the plurality of laser pulses generated by the first laser 16a. For example, each modulated laser pulse may comprise a portion of one or more laser pulse generated by the first laser 16a.

[00038] The first optical modulation system 18a may be configured to control one or more properties of the modulated laser pulses. For example, the properties of the modulated laser pulse may comprise at least one of: a timing or sequence of the modulated laser pulses, a temporal duration of each modulated laser pulses and an intensity of each more modulated laser pulses. For example, the first optical modulation system 18a may be configured to control the timing or sequence of the modulated laser pulses so that one or more of the modulated laser pulses interact with the droplets of fuel.

[00039] The laser system 1 may comprise a first amplification system 20a. The first amplification system 20a may be configured to amplify the modulated laser pulses, e.g. before interaction of the modulated laser pulses with the droplets of fuel. As can be seen from Figure 2, the first optical modulation system 18a is arranged between the first laser 16a and the first amplification system 20a. The first amplification system 20 may comprise a Radio Frequency (RF) controller (not shown). The RF controller may be configured to apply RF power to the first amplification system 20a to cause amplification of the modulated laser pulses.

[00040] The first optical modulation system 18a may comprise a first electro-optical device 22a. The first electro-optical device 22a may be provided in the form of an electro-optical modulator (EOM). The first electro-optical device 22a may be arranged in a path of the first laser beam 2a of the first laser 16a. The first electro-optical device 22a may be configured to change or modulate a property of the first laser beam 2a. The property may include at least one of a phase and polarisation of the first laser beam 2a. For example, a change or modulation of polarisation of the first laser beam 2a may comprise a change or modulation of a polarisation state of the first laser beam 2a. For example, the polarisation state of the first laser beam may be changed or modulated between an p-polarised state and an s- polarised state. In the p-polarised state, an electric field component of the light of the first laser beam 2a is parallel to a plane of incidence. In the s-polarised state, the electric field component of the light of the first laser beam 2a is perpendicular to the plane of incidence.

[00041] The first optical modulation system 18a may be operable between a first configuration and a second configuration. When the first optical modulation system 18a is in the first configuration, the first optical modulation system 18a may be configured to prevent transmission of the first laser beam 2a therethrough. For example, the first optical modulation system 18a may be configured to block the first laser beam 2a. When the first optical modulation system 18a is in the second configuration, the first optical modulation system 18a may be configured to transmit the first laser beam 2a, e.g. to the first amplification system 20a.

[00042] The first optical modulation system 18a may comprise one or more optical elements. In the exemplary embodiment shown in Figure 2, the first optical modulation system 18a comprises a first optical element 24a and a second optical element 24b. However, it will be appreciated that in other embodiments, the first optical modulation system may comprise more or less than two optical elements. The first and second optical elements 24a, 24b may each be provided in the form of an optical filter, such as a polariser or polarising filter.

[00043] The first and second optical elements 24a, 24b may be arranged in the path of the first laser beam 2a. The first electro-optical device 22a may be arranged between the first and second optical elements 24a, 24b. The first and second optical elements 24a, 24b may be arranged such that transmission of the first laser beam 2a through the first optical modulation system 18 is prevented, e.g. when the first optical modulation system 18a is operated in the first configuration. The first and second optical elements 24a, 24b may be arranged such that the first laser beam 2a is transmitted through the first optical modulation system 18a, e.g. when the first optical modulation system 18a is operated in the second configuration. Each of the first and second optical elements 24a, 24b may comprise a polarising axis. The term “polarising axis” may be understood as a direction along which each of the first and second optical elements 24a, 24b passes the electric field component of the first laser beam 2a. The first and second optical elements 24a, 24b may be arranged such the polarising axes of the first and second optical elements 24a, 24b are perpendicular, e.g. substantially perpendicular, relative to each other. For example, when the first optical modulation system 18a is operated in the second configuration, the first electro-optical device 22a may be configured to modulate or change the property of the first laser beam 2a such that the first laser beam 2a transmits through the first electro-optical device 22a and each of the first and second optical elements 24a, 24b. For example, when the first optical modulation system 18a is operated in the second configuration, the first electro-optical device 22a may be configured to modulate or change the polarisation of the first laser beam 2a such that the electric field component of the first laser beam 2a is in the same direction as the polarising axis of the second optical element 24b. For example, the first electro-optical device 22a may change or modulate the polarisation state of the first laser beam 2a between the p-polarised state and the s-polarised state.

[00044] The laser system 1 may comprise a first control system 21a. The first control system 21a may be part of or comprised in the first part la of the laser system 1. The first control system 21a may be part of or comprised in the first optical modulation system 18a. The first control system 21a may be configured to operate the first optical modulation system 18a between the first and second configurations. For example, the first control system 21a may be configured to apply a voltage to the first electro-optical device 22a. The first control system 21a may be configured to connect the first optical modulation system 18a to a voltage source (not shown). For example, the first control system 21a may be configured to connect the first electro-optical device 22a to the voltage source. When a voltage is applied to the first electro-optical device 22a, the first electro-optical device 22a may be configured to change or modulate the property of the first laser beam 2a. The voltage applied to the first electro-optical device 22a may be between about 1 to 10 kV, such as about 4kV. The first control system 21a may be configured to coordinate the application of the voltage with a pulse repetition rate of the first laser 16a. For example, the first control system 21a may be configured to operate the first optical modulation system 18a from the first configuration to the second configuration at the same time as a laser pulse generated by the first laser 16 is incident on the first electro-optical device 22a.

[00045] Figure 3 shows the second part lb of an exemplary laser system 1 for use in the radiation system RS shown in Figure 1. As described above, the second part lb of the laser system 1 may be configured to generate the main pulse. The second part Ibof the laser system 1 shown in Figure 3 is similar to the first part la of the laser system 1 shown in Figure 2. As such, the second part lb of the laser system 1 may comprise any features of the first part la of the laser system 1 shown in Figure 2. In the following description, only difference between the first and second parts la, lb of the laser systems 1 shown in Figures 2 and 3, respectively, will be described.

[00046] The laser system 1 comprises a second laser 16b. The second laser 16b may be part of or comprised in the second part lb of the laser system 1. The second laser 16b may be provided in the form of a CO2 laser. The second laser 16b may be configured to generate a second laser beam 2b having a wavelength that differs from the wavelength of the first laser beam 2a generated by the first laser 16a. For example, the second laser 16b may be configured to generate a second laser beam 2b having a wavelength of about 10.6 pm.

[00047] The laser system 1 may comprise a second optical modulation system 18b. The second optical modulation system 18b may be part of or comprised in the second part lb of the laser system 1. In the embodiment shown in Figure 3, the second optical modulation system 18 comprises two electro- optical devices 22b, 22c. Each of the two electro-optical devices 22b, 22c shown in Figure 3 may comprise any of the features described above in relation to the first electro-optical device 22a. By providing the second optical modulation system 18b with two electro-optical devices 22b, 22c, suppression of one or more unwanted portions of each modulated laser pulse may be increased. The electro-optical devices 22b, 22c are arranged in series. Each electro-optical device 22b, 22c is arranged between respective first and second optical elements 24c, 24d, 24e, 24f. The respective first and second elements 24c, 24d, 24e, 24f may comprise any of the features of the first and second optical elements 24a, 24b described above in relation to Figure 2. Although Figure 3 shows the second optical modulation system 18b as comprising two electro-optical devices 22b, 22c, it will be appreciated that in other embodiments, the second optical modulation system may comprise more than two electro- optical devices, such as three electro-optical devices.

[00048] The laser system 1 may comprise a second control system 21b. The second control system 21b may be part of or comprised in the second part lb of the laser system 1. As described above in relation to the first control system 21a shown in Figure 2, the second control system 21b may be configured to operate the second optical modulation system 18b between the first and second configurations. The second control system 21b may be configured to connect each electro-optical device 22b, 22c to the voltage source. The second control system 21b may be configured to simultaneously apply the voltage to each electro-optical device 22b, 22c, e.g. to operate the second optical modulation system 18b into the second configuration.

[00049] The laser system 1 may comprise a second amplification system 20b. The second amplification system 20b may be part of or comprised in the second part lb of the laser system 1. The second amplification system 20b may comprise any of the features of the first amplification system 20a described above in relation to Figure 2. Alternatively, the laser system may be arranged such that one or more modulated laser pulses transmitted by the first and second optical modulation systems are amplified by the same amplification system, such as the first or second amplification system disclosed herein.

[00050] Figure 4 illustrates an exemplary laser pulse 2c that may be generated by each of the first and second parts la, lb of the laser system 1 shown in Figures 2 and 3, respectively. Figure 4 shows the intensity of the laser pulse 2c as a function of time. The laser pulse 2c has a temporal profile (intensity versus time) that is approximately Gaussian. As described above, each of the first and second optical modulation systems 18a, 18b may be configured to modulate a laser pulse 2c generated by the respective first and second lasers 16a, 16b, to form a modulated laser pulse 2d. Each of the first and second optical modulation systems 18a, 18b may be configured to select or extract a particular portion 2e of the respective laser pulse 2c. For example, at a first time t=ta of the respective laser pulse 2c, each of the first and second optical modulation systems 18a, 18b may be operated in the second configuration and at a second time t=tb of the respective laser pulse 2c, each of the first and second optical modulation systems 18 a, 18b may be operated in the first configuration. Each of the first and second optical modulation systems 18a, 18b may be configured to transmit light of the respective portion 2e, e.g. the light of the respective modulated laser pulse 2d between the first and second times ta, tb. For example, each of the first and second control systems 21a, 21b may be configured to operate each of the respective first and second optical modulation systems 18a, 18b into the second configuration to transmit light at the first time ta of the respective laser pulse 2c by applying the voltage to the respective electro-optical devices 22a, 22b, 22c such that the electric field component of the light is in the same direction as the polarising axis of the respective second optical element(s) 24b, 24d, 24f. Each of the first and second control systems 21a, 21b may be configured to operate each of the respective first and second optical modulation systems 18a, 18b into the first configuration to stop transmitting light at the second time tb of the respective laser pulse by removing the voltage from the respective electro-optical devices 22a, 22b, 22c. Each of the first and second control systems 21a, 21b may be configured to operate each of the respective first and second optical modulation systems 18 between first and second configurations such that each respective modulated laser pulse comprises a pre-determined temporal duration and/or intensity. Each of the first and second control systems 21a, 21b may be configured to operate each of the respective first and second optical modulation systems 18a, 18b between first and second configurations such that the respective modulated laser pulses 2d are transmitted at a pre-determined timing or sequence. In this embodiment, the selected or extracted portion 2e corresponds to a central portion of the laser pulse 2c. It will be appreciated that in other embodiments, the selected or extracted portion may correspond to another portion of the laser pulse. In such other embodiments, an intensity of the selected or extracted portion may be decreased relative to an intensity of the central portion of the laser pulse. Although the above description describes the modulation of the laser pulse 2c in relation to each of the first and second optical modulation system 18a, 18b, it will be appreciated that the abovedescribed features may also apply to only one of the first and second optical modulation systems 18a, 18b. [00051] Figure 5 shows an exemplary electro-optical device 22 for use in the laser system 1, e.g. the first part lb of the laser system 1 shown in Figure 2 and/or the second part 2b of the laser system 1 shown in Figure 3. In the following description, the electro-optical device is indicated by reference numeral 22. However, it will be appreciated that the electro-optical device 22 may be or comprise any of the electro-optical devices 22a, 22b, 22c described above. The control system 21 is indicated in Figure 5 by reference numeral 21. However, it will be appreciated that the control system 21 may be or comprise any of the control systems 21a, 21b described above. The electro-optical device 22 comprises an electro-optical element 26 having a first surface 26a and a second surface 26b. The first and second surfaces 26a, 26b may be arranged or formed on opposite sides of the electro-optical element 26. The electro-optical element 26 may be formed from a material that is optically transparent, e.g. substantially optically transparent, to the first and/or second laser beams 2a, 2b generated by the first and/or second lasers 16a, 16b, respectively. Expressed differently, the material may be selected to transmit the first and/or second laser beams 2a, 2b. The material may comprise a crystalline material, such as a semiconductor material. For example, the semiconductor material may include cadmium zinc telluride (CdZnTe or CZT), cadmium telluride (CdTe), zinc telluride (ZnTe), and/or gallium arsenide (GaAs). It will be appreciated that in other embodiments, the electro-optical element may be formed from another material, such as an insulator.

[00052] The electro-optical device 22 may comprise a first electrode 30a and a second electrode 30b. The first and second electrodes 30a, 30b may be arranged on opposite sides of the electro-optical element 26, e.g. so that the electro-optical element 26 is positioned between the first and second electrodes 30a, 30b. The first and second electrode 30a, 30b may be arranged to be perpendicular, e.g. substantially perpendicular, to the first and second surfaces 26a, 26b of the electro-optical element 26. When the voltage is applied to the electro-optical device 22, e.g. the first and second electrodes 30a, 30b, the material of the electro-optical element 26 may be exposed to an electric field.

[00053] The material may be selected such that the electro-optical element 26 changes or modulates the property of the first and/or second laser beams 2a, 2b, for example, when the voltage is applied to the electro-optical device 22. As described above, the property of the first and/or second laser beams 2a, 2b may comprise at least one of a phase and polarisation of the first and/or second laser beams 2a, 2b. For example, the material may be selected such that a refractive index of the material changes, when the voltage is applied to the electro-optical device 22. The change in refractive index may be proportional to the electrical field. This electro-optical effect may also be referred to as the Pockels effect. As such, the material may comprise a material, such as a crystalline material, that exhibits the Pockels effect, when the voltage is applied to the electro-optical device 22.

[00054] The electro-optical device 22 comprises an antireflection structure. The antireflection structure may be formed from the material on at least one of the first surface 26a and the second surface 26b of the electro-optical element 26. [00055] The inventors have found that some antireflective coatings applied to a material used in electro-optical devices, such as electro-optical modulators, may delaminate or become damaged, when a voltage is applied to the electro-optic devices. This may be due to stress being induced between the material of the electro-optical devices and the antireflective coating, when the voltage is applied to the electro-optical devices. By forming the antireflection structure from the material of the electro-optical element on at least one of the first and second surfaces 26, delamination of the antireflection structure from the electro-optical element may be prevented. The antireflection structure may be integrally formed from the material. The antireflection structure may be formed directly on at least one of the first surface 26a and the second surface 26b, e.g. without any intermediate layers. Additionally or alternatively, the formation of the antireflection structure from the material of the electro-optical element may allow for a wider range of materials to be used for the formation of an electro-optical element. The wider range of materials may include one or more materials, which previously may have been rarely used or not at all, e.g. due to the possible delamination of the antireflective coating.

[00056] The antireflection structure may comprise a plurality of nanostructures. For example, the antireflection structure may comprise a Moth eye structure. The nanostructure may be periodically arranged. For example, the nanostructures may be formed to be periodic in at least one direction. For example, the nanostructures may be arranged in an array. The array may be periodic in the at least one direction. However, it will be appreciated that in other embodiments the nanostructures may be differently arranged, such as semi-periodically or randomly.

[00057] In the embodiment shown in Figure 5, the electro-optical device comprises a first antireflection structure 32a formed on the first surface 26a and a second antireflection structure 32b formed on the second surface 26b. However, it will be appreciated that in other embodiments, the antireflection structure may be formed only on one of the first and second surfaces of the electro-optical element.

[00058] Figure 6A shows a plan view of an exemplary first antireflection structure 32a of the electro-optical device 22 shown in Figure 5. Figure 6B shows a plan view of an exemplary second antireflection structure 32b of the electro-optical device 22 shown in Figure 5. In this embodiment, the first and second antireflection structures 32a, 32b each comprise a plurality of grooves 34. It will be appreciated that in other embodiments, the first and/or second antireflection structures may comprise a plurality of different nanostructures, such as a plurality of pillars, plurality of holes or the like.

[00059] The grooves 34 may be formed to be periodic in at least one direction. The grooves 34 may be arranged in an array. For example, the grooves 34 may be arranged to parallel, e.g. substantially parallel, relative to each other. The grooves 34 may be formed to be equidistant relative to each other. The array of grooves 34 may define a grating 36, such as a one-dimensional grating. Each of the first and second antireflection structures 32a, 32b may be provided in the form of the grating 36.

[00060] The grooves 34 may be formed on the first surface 26a of the electro-optical element 26 to extend in a first direction x. The grooves 34 may be formed on the second surface 26b of the electro- optical element 26 to extend in a second direction y. The first and second directions x, y may be first and second lateral directions. The first and second directions x, y may be the same or different. The first and second directions x, y may be selected based on the polarisation, e.g. the polarisation state, of the first and/or second laser beams 2a, 2b. For example, as shown in Figures 6A and 6B, the first and second directions x, y may be perpendicular, e.g. substantially perpendicular, relative to each other. By forming the grooves 34 on the first and second surfaces 26a, 26b of the electro-optical element 26 to extend in perpendicular, e.g. substantially perpendicular, directions x, y relative each other, transmission of the first and/or second laser beams 2a, 2b through the electro-optical element 26 may be increased or maximised and/or a reflection loss of the first and/or second laser beam 2a, 2b may be minimised or decreased.

[00061] One or more properties of the grooves 34 may be selected such that a refractive index of each of the first and second antireflection structures 32a, 32b is between a refractive index of the material of the electro-optical element 26 and a refractive index of an optical medium surrounding the electro-optical device 22. In this embodiment, the material of the electro-optical element 26 comprises gallium arsenide, which has a refractive index of nl = 3.2. The optical medium that surrounds the electro-optical device 22 is air, which has a refractive index of about n2=l . The properties of the grooves 34 may be selected such that a refractive index n3 of the each of the first and second antireflection structures 32a, 32b is about 1.8. This value corresponds to an optimal value of the refractive index n3 of the first and second antireflection structures 32a, 32b, which can be determined by: n3 = nTn2 (Equation 1).

[00062] At the optimal value of the refractive index n3 of the first and second antireflection structures 32a, 32b, transmission of the first and/or second laser beams 2a, 2b may be maximised and/or a reflection loss of the first and/or second laser beams 2a, 2b may be minimised or decreased. Expressed differently, by selecting the properties of the grooves 34 such that the refractive index of each of the first and second antireflection structures 32a, 32b is between the refractive index of the material of the electro-optical element 26 and the refractive index of the optical medium surrounding the electro-optical device 22, transmission of the first and/or second laser beams 2a, 2b through the electro-optical element 26 may be increased, while the reflection loss of the first and/or second laser beams 2a, 2b, may be decreased. The properties of grooves 34 may comprise a size or dimension of each groove 34 and/or a periodicity of the grooves 34.

[00063] The periodicity of the plurality of grooves 34 may be defined by a period P of the array of grooves 34, e.g. the grating 36. The period P of the grating 36 may be smaller than a wavelength of the first and/or second laser beams 2a, 2b. For example, the period of the grating 36 may be equal to or smaller than a tenth of the wavelength of the first and/or second laser beams 2a, 2b. For example, when the wavelength of the first laser beam 2a is 10.207 pm and/or the wavelength the second laser beam 2b is 10.6 pm, the period of the grating 36 of each of the first and second antireflection structures 32a, 32b may be about 1 pm or smaller than 1 pm. It will be appreciated that in other embodiments the period of the grating may be selected to be larger than one tenth of the wavelength of the laser beam, but smaller than the wavelength of the first and/or second laser beams.

[00064] The size or dimension of each groove 34 may comprise a width W of each groove 34 and a depth or height D of each groove 34. The depth or height D of each groove 34 may depend on a difference between the refractive index of the material of the electro-optical element 26 and the refractive index of the optical medium surrounding the electro-optical device 22. For example, a minimum depth of each groove 34 may be inversely proportional to the difference between the refractive index of the material of the electro-optical element and the refractive index of the optical medium surrounding the electro-optical device 22. The width W of each groove 34 may be selected to be smaller than the wavelength of the first and/or second laser beam 2a, 2b. For example, the width W of each groove 34 of the grating 36 may be in the region between about 300nm and 700nm. The depth or height D of each groove 34 may be in the region between about 800nm and 1.5 pm. It will be appreciated that in other embodiments, the depth of each groove may be larger or smaller than the exemplary values disclosed herein. It will be appreciated that the properties of the grooves 34 may be selected based on the polarisation, e.g. the polarisation state, of the first and/or second laser beams 2a, 2b. For example, the width W of each grooves may be selected based on the polarisation state of the first and/or second laser beams 2a, 2b.

[00065] Figure 7 shows another an exemplary electro-optical device 22 for use in laser system 1, e.g. the first part of the laser system 1 shown in Figure 2 and/or the second part of the laser system 1 shown in Figure 3. Figure 8A shows a plan view of an exemplary first antireflection structure 32a of the electro-optical device 22 shown in Figure 7. Figure 8B shows a plan view of alternative exemplary first antireflection structure 32a of the electro-optical device 22 shown in Figure 7. The first and second electrodes 30a, 30b have been omitted from Figures 8A and 8B for sake of clarity. Figure 9 shows a cross-sectional view of the exemplary nanostructures of the first antireflection structure 32a shown in Figure 8A. The electro-optical device 22 shown in Figure 7 is similar to the electro-optical device 22 shown in Figure 5. As such, the electro-optical device 22 shown in Figure 7 may comprise any of the features of the electro-optical device 22 shown in Figure 5. In the following description, only differences will be described.

[00066] In the embodiment shown in Figure 7, the electro-optical device 22 comprises a first antireflection structure 32a formed on the first surface 26a of the electro-optical element 26. The electro- optical device 22 comprises a second antireflection structure 32b formed on the second surface 26b of the electro-optical element 26. It will be appreciated that in other embodiments, an antireflection structure may be formed on only one of the first and second surfaces. The first and second nanostructures of the first and second antireflection structure 32a, 32b may be arranged as shown in Figure 8A or Figure 8B. [00067] In this embodiment, the nanostructures are provided in the form of a plurality of pillars 38. By providing the nanostructures in the form of pillars 38, a decrease of optical losses, e.g. due to reflection of one or more parts of the first and/or second laser beams 2a, 2b, and/or an increase in the transmission of the first and/or second laser beams 2a, 2b, which may be due to the first and/or second antireflection structures 32a, 32b, may be substantially independent of the polarisation, e.g. the polarisation state, of the first and/or second laser beams 2a, 2b.

[00068] As shown in Figure 8A and 8B, the pillars 38 may be arranged in an array of pillars 38. The pillars 38 may be formed to periodic in at least two lateral directions, such as a first direction x and a second direction y, which are indicated in Figures 8A and 8B. The first and second directions x, y may be perpendicular, e.g. substantially perpendicular, relative to each other. The pillars 38 may be formed to define a two-dimensional array of pillars 38. In the embodiment shown in Figure 8A, the pillars 38 are shown as being formed or arranged in a cubic arrangement. In the embodiment shown in Figure 8B, the pillars are shown as being formed or arranged in a hexagonal arrangement, such as a closed packed hexagonal arrangement. It will be appreciated that in other embodiments, the pillars may be arranged in another arrangement, such as an oblique, rectangular arrangement or the like.

[00069] One or more properties of the pillars 38 may be selected such that a refractive index of the first and second antireflection structures 32a, 32b is between a refractive index of the material of the electro-optical element 26 and a refractive index of an optical medium surrounding the electro-optical device 22. In this embodiment, the material of the electro-optical element 26 comprises gallium arsenide, which has a refractive index nl of 3.2. The optical mediums that surrounds the electro-optical device is air, which has a refractive index n2 of about 1. The properties of the pillars 38 may be selected such that a refractive index of each of the first and second antireflection structures 32a, 32b is about 1.8. As described above, this value corresponds to the optimal value of the refractive index n3 of each of the first and second antireflection structures 32a, 32b. By selecting the properties of the pillars 38 such that the refractive index of each of the first and second antireflection structures 32a, 32b is between the refractive index of the material of the electro-optical element 26 and the refractive index of the optical medium surrounding the electro-optical device 22, transmission of the first and/or second laser beams 2a, 2b through the electro-optical element 26 may be increased, while the reflection loss of the first and/or second laser beams 2a, 2b, may be decreased. The properties of the pillars 38 may comprise a size or dimension of each pillar 38, a shape of each pillar 38 and a periodicity of the pillars 38, e.g. a period of the array of pillars 38.

[00070] In the embodiments shown in Figures 7 to 9, each pillar 38 comprises a conical shape, such as a truncated cone shape. By forming each pillar 38 with a conical shape, the first and/or second antireflection structures 32a, 32b may introduce a gradual change in refractive index from the optical medium surrounding the electro-optical device 22 to the material 26. It will be appreciated that in other embodiments, one or more pillars may comprise a different shape, such as a cylindrical shape, pyramidal shape or the like. [00071] The size or dimension of each pillar 38 may comprise a height H, a diameter DI of a top portion of each pillar and a diameter D2 of a base portion of each pillar 38.

[00072] The height of each pillar 38 may depend on a difference between the refractive index of the material of the electro-optical element 26 and the refractive index of the optical medium surrounding the electro-optical device 22. For example, a minimum height of each pillar 38 may be inversely proportional to the difference between the refractive index of the material of the electro-optical element and the refractive index of the optical medium surrounding the electro-optical device 22. For example, the height of each pillar 38 may be selected to be between about 1.4 pm and 1.8 pm, such about 1.6 pm. The diameter DI of the top portion of each pillar 38 may be selected to be between about 1.9 pm and 2.3 pm, such as about 2 pm. The diameter D2 of the base portion of each pillar 38 may be selected to be between about 2 pm and 3 pm, such as about 2.5 pm. It will be appreciated that in other embodiments, each pillar may comprise a different size or dimension than the sizes or dimensions disclosed herein. For example, it may be desirable to minimise the diameter of the top portion of each pillar such that each pillar comprises a sharp tip. This may lead to a more gradual change in the refractive index from the optical medium surrounding the electro-optical device to the material. This in turn may increase transmission of the first and/or second laser beams through the electro-optical element, while reducing the reflection loss of the first and/or second laser beams. Additionally or alternatively, the height of each pillar may be selected to be larger than 1.8 pm.

[00073] A period of the array of pillars 38 may be the same or different in the first and second directions x, y. The period of the array of pillars may be defined as a distance in the first direction x and a distance in the second direction at which a pattern of the array repeats itself. The period is indicated in Figures 8A and 8B by reference numerals Px and Py, whereby Px indicates the period in the first direction x and Py indicated the period in the second direction y.

[00074] For example, in the embodiment shown in Figure 8A, the period Px, Py of the array of pillars 38 may be defined as a distance between the central axes C of two adjacent pillars 38. The period Px, Py is the same in the first and second directions x, y. For example, the period Px, Py may be selected to be between 2.5 pm and 3.5 pm, such as 3 pm.

[00075] In the embodiment shown in Figure 8B, the period Px, Py of the array of pillars 38 is different in the first and second directions x, y. For example, in the first direction x, the period Px may be selected to be between 2 pm and 3 pm, such as about 2.5 pm. In a second direction y, the period Py may be selected to be between 4 pm and 6 pm, such as about 5 pm. However, it will be appreciated that the array of pillars is not limited to having the exemplary periods disclosed herein and that in other embodiments, the period may be selected to be larger or small than the exemplary periods disclosed herein.

[00076] Figure 10 shows an exemplary flow diagram outlining the steps of a method 100 of forming an antireflection structure 32 of an electro-optical device 22. The electro-optical device 22 may comprise any of the features of the electro-optical devices 22 described above. In step 105, the method 100 comprises providing an electro-optical element 26 having a first surface 26a and a second surface 26b. The electro-optical element 26 may be formed from the material described above.

[00077] In steps 110 and 115, the method may comprise applying a pattern 40 of the antireflection structure 32 on at least one of the first and second surfaces 26a, 26b. The antireflection structure 32 comprises a plurality of nanostructures. In the embodiment shown in Figure 10, the pattern 40 is applied on the first surface 26a of the electro-optical element 26. The step of applying the pattern on at least one of the first and second surfaces 26a, 26b may comprise applying a layer of resist 42 on the first surface 26a of the electro-optical element 26 (step 110). For example, the layer of resist 42 may be applied using a coating process, such as spin coating or the like. The material of the layer of resit may be selected to be radiation sensitive or to be sensitive to an electron beam. A lithographic system, such as the lithographic system shown in Figure 1 may be used to transfer the pattern 42 of the antireflection structure 32 from a patterning device (e.g. a mask) onto the layer of resist 42 (step 115). It will be appreciated that in other embodiments, another lithographic system, such as an electron-beam lithographic system, an interference or holographic lithographic system or the like, may be used to transfer the pattern of the antireflection structure on the layer of resist. It will be appreciated that in other embodiments, the step of applying a pattern of the antireflection structure on at least one of the first and second surfaces may comprise using a Langmuir Blodgett process.

[00078] In step 120, the method 100 may comprise using a material removal process to transfer the pattern 42 into the first surface 26a, thereby forming the antireflection structure 32. The material removal process may comprise an etching process, such as a wet etching process. For example, an etch or etch mixture, such as silicon tetrachloride/chloride (SiCl i/CF) or the like, may be used in embodiments where the material comprises gallium arsenide.

[00079] The antireflection structure 32 may comprise the grooves 24 or the pillars 38, as described above. It will be appreciated that in other embodiments, the antireflection structure may comprise other structures, such as a plurality of holes or the like.

[00080] Although the formation of the antireflection structure 32 has been described above in relation to the first surface 26a of the electro-optical element 26, it will be appreciated that the method 100 may be used to additionally or alternatively form the antireflection on the second surface 26b of the electric-optic element 26.

[00081] The electro-optical element 26 may be arranged, e.g. clamped, between the first and second electrode 30a, 30b, e.g. subsequent to forming the antireflection structure 32.

[00082] The method 100 may comprise texturing or roughening of a surface of at least some or all nanostructures. This may minimise reflection loss and/or increase transmission of the first and/or second laser beam 2a, 2b. The surface of some or all nanostructures may be textured or roughened using an etching process, a laser texturing process or the like.

[00083] It will be appreciated that in other embodiments, one or more of the above-described steps may be omitted from the method. For example, in such other embodiments, the material removal process may comprise an ion beam milling or sputtering process, e.g. to transfer a pattern of nanostructures into the first surface.

[00084] Although the laser system is described as comprising the disclosed first and second parts, it will be appreciated that in other embodiments the laser system may comprise only one of the first and second parts disclosed herein. For example, in such other embodiments, the laser system may comprise the second part disclosed herein to generate the main pulse and a different part or subsystem to generate the pre -pulse.

[00085] 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.

[00086] 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.

[00087] In order to illustrate certain possible combination of embodiments, the following clauses are given, without limiting the combinations thereof which arise directly and unambiguously from the above description.

CLAUSES:

1. An electro-optical device for use in a laser system of a radiation system, the device comprising: an electro-optical element having a first surface and a second surface, the electro-optical element being formed from a material that is optically transparent to a laser beam generated by the laser system; and an antireflection structure formed from the material on at least one of the first surface and the second surface of the electro-optical element, the antireflection structure comprising a plurality of nanostructures.

2. The device of clause 1, wherein the antireflection structure is integrally formed from the material and the first and second surfaces of the electro-optical element are arranged on opposite sides of the electro-optical element.

3. The device of clause 1 or 2, wherein the device is arrangeable in a path of the laser beam generated by the laser system and wherein the material is selected such that the electro-optical element changes or modulates a property of the laser beam generated by the laser system, when a voltage is applied to the device.

4. The device of clause 3, wherein the property of the laser beam comprises at least one of a phase and a polarisation of the laser beam.

5. The device of any preceding clause, wherein the material is selected such that a refractive index of the material changes, when a voltage is applied to the device.

6. The device of any preceding clause, wherein the material comprises a crystalline material that exhibits the Pockels effect, when a voltage is applied to device.

7. The device of any preceding clause, wherein the plurality of nanostructures comprises at least one of: a plurality of grooves, a plurality of pillars and a plurality of holes.

8. The device of clause 7, wherein at least one or each pillar of the plurality of pillars comprises a truncated cone shape.

9. The device of any preceding clause, wherein one or more properties of the plurality of nanostructures are selected such that a refractive index of the antireflection structure is lower than a refractive index of the material and, such that the refractive index of the antireflection structure is higher than a reflective index of an optical medium surrounding the device.

10. The device of clause 9, wherein the one or more properties of the plurality of nanostructures comprises at least one of: a size or dimension of each nanostructure of the plurality of nanostructures, a shape of each nanostructure of the plurality of nanostructures and a periodicity of the plurality of nanostructures.

11. A laser system for use in a radiation system, the laser system comprising: a laser configured to generate a laser beam comprising a plurality of laser pulses; and an optical modulation system comprising an electro-optical device according to any preceding clause.

12. The laser system of clause 11, wherein the optical modulation system is configured to modulate one or more laser pulses of the plurality of laser pulses generated by the laser, each of the one or more modulated laser pulses comprising a portion of at least one laser pulse generated by the laser. 13. The laser system of clause 12, wherein the optical modulation system is configured to modulate or control one or more properties of the one or more modulated laser pulses, the one or more properties of the one or more modulated laser pulses comprising at least one of: a timing or sequence of the one or more modulated laser pulses, a temporal duration of each of the one or more modulated laser pulses and an intensity of each of the one or more modulated laser pulses.

14. A radiation system comprising: an EUV radiation source; and a laser system according to any one of clauses 11 to 13.

15. A lithographic system comprising a radiation system according to clause 14 and a lithographic apparatus.

16. A method of forming an antireflection structure of an electro-optical device, the method comprising: providing an electro-optical element having a first surface and a second surface; applying a pattern of the antireflection structure on at least one of the first surface and the second surface, the antireflection structure comprising a plurality of nanostructures; and using a material removal process to transfer the pattern into the at least one of the first surface and the second surface, thereby forming the antireflection structure.