LASER POWERED PLASMA BASED EUV GENERATION SYSTEM

Abstract

 A laser powered plasma, LPP, based EUV generation system is configured to generate EUV light by irradiating droplets of a target material with at least one laser beam. The system comprises a metrology module configured to determine a position of a droplet of the target material by comparing a portion of a forward beam with a portion of a reverse beam that is reflected off from the target material. The metrology module comprises a polarization state dependent beam pickup arranged to split a portion from the forward beam and the reverse beam depending on a polarization state, and a polarization state adjuster arranged downstream of a beam path when compared to the polarization state dependent beam pickup, arranged to change a polarization state of at least one of the forward beam and the reverse beam such that the polarization states of the forward and reverse beams incident at the polarization state dependent beam pickup differs.

Description

FIELD

[0001] The present invention relates to a laser powered plasma, LPP, based EUV generation system. Furthermore, the present invention relates to a lithographic apparatus comprising the laser powered plasma, LPP based EUV generation system and to a method of generating EUV by laser powered plasma, LPP.

BACKGROUND

[0002] 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.

[0003] 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.

[0004] It is known for lithographic apparatuses to rely on a plasma for exposing substrates to EUV radiation. This plasma can be generated by vaporizing droplets of tin by means of laser radiation. First, the droplet is irradiated by a pre-pulse to shape the droplet. Then, the droplet is irradiated by a rarefaction pulse to generate a plasma.

[0005] A reflection and/or scattering from the pre-pulse on the droplet may be measured in order to determine a position of the droplet. In order to accurately determine a reflection and/or scattering from the pre-pulse on the droplet, an amplitude of the pre-pulse is measured and compared to an amplitude of the reflection and/or scattering from the pre-pulse on the droplet. Thereto, a beam-pickup transmits a fraction of the pre-pulse to a sensor. Furthermore, the beam-pickup transmits a fraction of the reflection and/or scattering from the pre-pulse to the sensor. An amount of energy reflected and/or scattered on the droplet may be relatively low, causing a signal to noise ratio of the measurement to be low.

SUMMARY

[0006] The invention intends to enable an accurate measurement of the pre-pulse. According to an embodiment of the invention, there is provided a laser powered plasma, LPP, based EUV generation system configured to generate EUV light by irradiating droplets of a target material with at least one laser beam, the system comprising a metrology module configured to determine a position of a droplet of the target material by comparing a portion of a forward beam with a portion of a reverse beam that is reflected off from the target material, wherein the metrology module comprises:

• a polarization state dependent beam pickup arranged to split a portion from the forward beam and a portion from the reverse beam, wherein the portions depend on a polarization state of forward and reverse beam incident on the polarization state dependent beam pickup, and
• a polarization state adjuster arranged downstream of a beam path when compared to the polarization state dependent beam pickup, wherein the polarization state adjuster is arranged to change a polarization state of at least one of the forward beam and the reverse beam such that the polarization state of the reverse beam incident at the polarization state dependent beam pickup differs from the polarization state of the forward beam incident at the polarization state dependent beam pickup.

[0007] According to another embodiment of the invention, there is provided a lithographic apparatus comprising the laser powered plasma, LPP based EUV generation system according to the invention.

[0008] According to yet another embodiment of the invention, there is provided a method of generating EUV by laser powered plasma, LPP, wherein EUV light is generated by irradiating droplets of a target material with at least one laser beam, the method comprising determining by a metrology module a position of a droplet of the target material by comparing a portion of a forward beam with a portion of a reverse beam that is reflected off from the target material, wherein the metrology module comprises:

• a polarization state dependent beam pickup arranged to split a portion from the forward beam and a portion from the reverse beam, wherein the portions depend on a polarization state of forward and reverse beam incident on the polarization state dependent beam pickup, and
• a polarization state adjuster arranged downstream of a beam path when compared to the polarization state dependent beam pickup, wherein the polarization state adjuster is arranged to change a polarization state of at least one of the forward beam and the reverse beam such that the polarization state of the reverse beam incident at the polarization state dependent beam pickup differs from the polarization state of the forward beam incident at the polarization state dependent beam pickup.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] 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 schematic, not-to-scale view of an overall broad conception for a laser-produced plasma EUV radiation source system;
• Figure 2 depicts a schematic, not-to-scale view of a target material metrology module; and
• Figure 3 depicts a schematic view of a part of a metrology module as may be employed in an EUV generation system of a lithographic apparatus.

DETAILED DESCRIPTION

[0010] With initial reference to FIG. 1, there is shown a schematic view of an exemplar. EUV radiation source, e.g., a laser produced plasma EUV radiation source 10 according to one aspect of an embodiment of the presently disclosed subject matter. As shown, the EUV radiation source 10 may include a pulsed or continuous laser source 22, which may for example be a pulsed gas discharge CO2 laser source producing a beam 12 of radiation at a wavelength generally below 20 pm, for example, in the range of about 10.6 pm or to about 0.5 pm or less. The pulsed gas discharge CO2 laser source may have DC or RF excitation operating at high power and at a high pulse repetition rate. The EUV radiation source 10 may also include one or more modules such as a conditioning laser 23 emitting a beam 25 of conditioning radiation as explained above.

[0011] The EUV radiation source 10 also includes a target delivery system 24 for delivering target material in the form of liquid droplets or a continuous liquid stream. In this example, the target material is a liquid, but it could also, for example, be a solid. The target material may be made up of tin or a tin compound, although other materials could be used. In the system depicted the target material delivery system 24 introduces the droplets 14 of the target material into the interior of a vacuum chamber 26 to an irradiation region 28 where the target material may be irradiated to produce plasma. In some cases, an electrical charge is placed on the target material to permit the target material to be steered toward or away from the irradiation region 28. It should be noted that as used herein an irradiation region is a region where target material irradiation is to occur and is an irradiation region even at times when no irradiation is actually occurring. The EUV light source may also include a beam focusing and steering system 32.

[0012] In the system shown, the components are arranged so that the droplets 14 travel substantially horizontally. The direction from the laser source 22 towards the irradiation region 28, that is, the nominal direction of propagation of the beam 12, may be taken as the Z axis. The path the droplets 14 take from the target material delivery system 24 to the irradiation region 28 may be taken as the X axis. The view of FIG. 1 is thus normal to the XZ plane. Also, while a system in which the droplets 14 travel substantially horizontally is depicted, it will be understood by one having ordinary skill in the art that other arrangements can be used in which the droplets travel vertically or at some angle with respect to gravity between and including 90° (horizontal) and 0° (vertical).

[0013] The EUV radiation source 10 may also include an EUV light source controller system 60, which may also include a laser firing control system 65, along with the beam steering system 32. The EUV radiation source 10 may also include a detector such as a target position detection system which may include one or more droplet imagers 70 that generate an output indicative of the absolute or relative position of a target droplet, e.g., relative to the irradiation region 28, and provide this output to a target position detection feedback system 62.

[0014] The target position detection feedback system 62 may use the output of the droplet imager 70 to compute a target position and trajectory, from which a target error can be computed. The target error can be computed on a droplet-by-droplet basis, or on average, or on some other basis. The target error may then be provided as an input to the EUV light source controller 60. In response, the EUV light source controller 60 can generate a control signal such as a laser position, direction, or timing correction signal and provide this control signal to the laser beam steering system 32. The laser beam steering system 32 can use the control signal to change the location and/or focal power of the laser beam focal spot within the chamber 26. The laser beam steering system 32 can also use the control signal to change the geometry of the interaction of the beam 12 and the droplet 14. For example, the beam 12 can be made to strike the droplet 14 off-center or at an angle of incidence other than directly head-on.

[0015] As shown in FIG. 1, the target material delivery system 24 may include a target delivery control system 90. The target delivery control system 90 is operable in response to a signal, for example, the target error described above, or some quantity derived from the target error provided by the system controller 60, to adjust the paths of the target droplets 14 through the irradiation region 28. This may be accomplished, for example, by repositioning the point at which a target delivery mechanism 92 releases the target droplets 14. The droplet release point may be repositioned, for example, by tilting the target delivery mechanism 92 or by shifting the target delivery mechanism 92. The target delivery mechanism 92 extends into the chamber 26 and is preferably externally supplied with target material and with gas from a gas source to place the target material in the target delivery mechanism 92 under pressure.

[0016] Continuing with FIG. 1, the radiation source 10 may also include one or more optical elements. In the following discussion, a collector 30 is used as an example of such an optical element, but the discussion applies to other optical elements as well. The collector 30 may be a normal incidence reflector, for example, implemented as a multilayer mirror (MEM) fabricated by depositing many pairs of Mo and Si layers on a substrate with additional thin barrier layers, for example B4C, ZrC, S 13N4 or C, deposited at each interface between layer pairs to effectively block thermally- induced interlayer diffusion, but the collector 30 may be formed of other layers of material in other embodiments . The collector 30 may be in the form of a prolate ellipsoid, with a central aperture to allow the laser beam 12 to pass through and reach the irradiation region 28. The collector 30 may be, e.g., in the shape of an ellipsoid that has a first focus at the irradiation region 28 and a second focus at a so-called intermediate point 40 (also called the intermediate focus 40) where the EUV radiation may be output from the EUV radiation source 10 and input to, e.g., an integrated circuit lithography scanner or stepper 50. The scanner or stepper 50 uses the radiation, for example, to process a silicon wafer workpiece 52 in a known manner using a reticle or mask 54. The silicon wafer workpiece 52 is then additionally processed in a known manner to obtain integrated circuit devices.

[0017] As mentioned, in general, for a reference coordinate system, Z is the direction along which the laser beam 12 propagates and is also the direction from the collector 30 to the irradiation site 28 and the EUV intermediate focus 40. X is in the droplet propagation plane. Y is orthogonal to the XZ plane. To make this a right-handed coordinate system, the trajectory of the droplets 14 is taken to be in the -X direction.

[0018] In the example shown, the target material 14 is in the form of a stream of droplets released by a target material dispenser 92, which in the example is a droplet generator. The target material droplet 14 can be ionized by a main pulse in this form. Alternatively, the target material 14 can be preconditioned for ionization with a conditioning pulse 25 that can, for example, change the geometric distribution of the target material 14. Thus, it may be necessary both to hit the target material 14 accurately with the conditioning pulse to ensure the target material 14 is in the desired form (disk, cloud, etc.), and to hit the target accurately with the main pulse to promote efficient production of EUV radiation.

[0019] As used herein, the term "irradiation site" is used to connote the position 28 in the chamber 26 where the target material 14 is struck with a main pulse. It may coincide with the primary focus of the collector mirror 30.

[0020] As mentioned, one droplet detection metrology utilizes darkfield illumination, where the backscatter from a droplet passing through a laser curtain is collected near the primary focus. The metrology module detects the droplet crossing at a specific location in space to provide a trigger to the system controls to enable all ensuing sequences to generate EUV light. An example of such a system is shown schematically in FIG. 2, in which a droplet detection controller 122 causes a droplet illumination module (DIM) 124 to illuminate a droplet 14. A droplet detection module (DDM) 126 detects the radiation backscattered by the droplet to permit the droplet detection controller 122 to derive information such as the position of the droplet 14. Note that herein, the form of the target material is referred to as a droplet even if one or more conditioning pulses have altered the target material from a true droplet form. The detection process described above in connection with FIG. 2 may be used to detect the droplets after they have fully coalesced from smaller droplets and tune the operation of the droplet generator.

[0021] In other embodiments, the EUV generation system may be based on laser powered plasma, LPP. The droplet of target material is conditioned in that the droplet of target material is first irradiated by a pre-pulse which changes a shape of the droplet, e.g. into a disk shape, followed by the irradiation of the thus shaped droplet by a rarefaction pulse to generate a plasma of the target material. The main pulse may then be irradiated to the plasma of the target material.

[0022] Figure 3 depicts a schematic view of a metrology module. The metrology module comprises a polarization state dependent beam pickup PDBP and a polarization state adjuster PSA. The polarization state adjuster PSA is arranged downstream of the polarization state dependent beam pickup PDBP to adjust a polarization state of a forward beam PP-FB, as explained below. The forward beam, such as the forward beam of the pre-pulse PP generated by a pre-pulse generator, is incident on the polarization state dependent beam pickup PDBP. A portion of the forward beam is split by the polarization state dependent beam pickup, the portion being provided to a forward beam sensor FBS configured to sense the portion of the forward beam. The remainder of the forward beam is transmitted by the polarization state dependent beam pickup to the polarization state adjuster. The polarization state adjuster is configured to adjust, i.e. change, a polarization state of the forward beam. The forward beam, with changed polarization state, is transmitted to the droplet of target material. A portion of the forward beam is reflected off the droplet of target material, to form a reverse beam PP-RB. The reverse beam is incident on the polarization state adjuster, where the polarization state of the reverse beam is adjusted, i.e. changed, and then the reverse beam PP-RB further propagates to the polarization state dependent beam pickup, where a portion of the reverse beam is split to propagate to the reverse beam sensor RBS. At the polarization state dependent beam pickup, the reverse beam that reaches the polarization state dependent beam pickup has twice been subjected to polarization state change by the polarization state adjuster, namely a change of the polarization state of the forward beam towards the droplet of target material and a change of the polarization state of the reverse beam. The twice changed polarization state of the reverse beam incident at the polarization stated dependent beam pickup may differ from the polarization state of the forward beam incident at the polarization stated dependent beam pickup. As the beam pickup is polarization state dependent, i.e. the portions split from the forward and reverse beams may depend on the polarization state.

[0023] The amount of energy reflected by the droplet of target material may be small. As a consequence, an intensity of the reverse beam may be smaller than an intensity of the forward beam. For example, the intensity of the reverse beam may be substantially smaller than the intensity of the forward beam. In such case a signal to noise ratio of measurement of the portion of the forward beam by the forward beam sensor may for example be substantially higher than a signal to noise ratio of the measurement of the portion of the return beam by the return beam sensor. The ability to split different portions of the forward beam and of the reverse beam, may enable to take account of the (e.g. substantial) difference in intensity of the forward beam and the reverse beam. For example, the portion of the return beam split by the polarization dependent beam pickup may be substantially larger than the portion of the forward beam split by the polarization state dependent beam pickup. As a consequence, an intensity of the portion of the reverse beam as received by reverse beam sensor may be increased. Thereby, a signal to noise ratio of the measurement of the reverse beam at the reverse beam sensor may be enhanced, which may enable to more accurately measure the reverse beam. As the metrology module may determine a position of the droplet of target material using the measurement of the reverse beam reflected from the droplet of target material, a position of the droplet of target material may be determined more accurately.

[0024] Reverting to Figure 3 in some more detail, the polarization state dependent beam pickup comprises two surfaces, indicated by S1 and S2. The forward beam is incident on the first surface S1 of the polarization state dependent beam pickup where it is diffracted and propagates to the second surface S2 of the polarization state dependent beam pickup. The second surface may comprise a polarization dependent coating, which reflects a portion of the forward beam and diffracts the remainder of the forward beam. The portion that is reflected on the second surface propagates back to the first surface, where it is diffracted to propagate to the forward beam sensor. The remainder of the forward beam, which is diffracted at the second surface, propagates to the polarization state adjuster.

[0025] The reverse beam, after having propagated through the polarization state adjuster, where the polarization of the reverse beam is changed, is incident on the second surface S2 of the polarization dependent beam pickup at the second surface, a portion of the reverse beam is reflected and propagates to the reverse beam sensor. The remainder of the reverse beam is diffracted at the second surface, propagates to the first surface, where it is again diffracted and to propagate towards the light source of the forward beam.

[0026] The second surface may be provided with a polarization dependent coating whereby a portion reflected by the coating depends on the polarization state of the beam, i.e. the forward and reverse beam.

[0027] For example, at the polarization state dependent beam pickup, the polarization state of one of the forward beam and the reverse beam may be horizontal and the polarization state of the other one of the forward and the reverse beam may be vertical. The polarization state dependent beam pickup may be arranged to pick up a horizontal polarization state portion of a horizontal polarization state of the beam and a vertical polarization state portion of a vertical polarization state of the beam, wherein the horizontal polarization state portion differs from the vertical polarization state portion. The forward beam may be a laser beam having a vertical or horizontal polarization. For example, the forward beam as emitted by a laser is p (vertically) polarized while the return beam, of which the polarization is changed twice by the polarization state adjuster, is s (horizontally) polarized, as represented by p (denoting vertical polarization) and s (denoting horizontal polarization) in Figure 3. Accordingly, with the opposite polarizations, i.e. vertical and horizontal, a large difference in the properties of the polarization state dependent beam pickup may be achieved: at one of the vertical and horizontal polarization directions, the portion split from the beam may be large while with the other one of the vertical and horizontal polarization directions, the portion split from the beam may be small. Thus, a large difference in the portions split from the forward and reverse beam may be achieved.

[0028] For example, the second surface may exhibit a high reflectance for s polarization and a low reflectance for p polarization. Accordingly, in case the forward beam is p polarized, and the reverse beam is s polarized, a small portion of the forward beam is split (reflected) while a large portion of the reverse beam is split (reflected), thereby enabling to enhance a signal to noise ratio of the measurement of the reverse beam at the reverse beam sensor. The polarization state dependent beam pickup may be configured to split a portion of the forward beam and a portion of the reverse beam by reflecting the respective portions while diffracting the remainders of the forward and reverse beams.

[0029] For example, taking account of the polarization state dependent properties of reflection at a surface of the polarization state dependent beam pickup, the horizontal polarization state portion may exceed the vertical polarization state portion to obtain at the reverse beam sensor a large portion of the reverse beam having a horizontal polarization state and to obtain at the forward beam sensor a small(er) portion of the forward beam having a vertical polarization state. The difference in polarization state dependent reflectivity may be high, e.g. exceeding a factor 10, the reflectivity for s polarization state may e.g. be at least a factor 10 higher than the reflectivity of the p polarization state. As a result, the horizontal polarization state portion may exceed the vertical polarization state portion by a factor of at least 10. As an example, the polarization state dependent beam pickup may be configured to split a portion of 2 % at vertical polarization state (p) and a portion of 60% at horizontal polarization state (s). Thus, with a forward beam having the vertical polarization state, 2% may be split to the forward beam sensor, while a reverse beam having the horizontal polarization state, 60% may be split to the reverse beam sensor.

[0030] The polarization state adjuster may comprise any optical element to capable of changing a polarization state. In an embodiment, the polarization state adjuster comprises a quarter wave plate. The quarter wave pate is configured to change the polarization state of a beam from vertical (p) to circular. Furthermore, the quarter wave plate is configured to change the polarization state of a beam from circular to horizontal (s). Accordingly, quarter wave plate may change the polarization in a well defined way. The quarter wave plate may for example be position in the forward beam path and in the reverse beam path. Reverting to the above example of a forward beam emitted by a forward beam light source, the forward beam having a p polarization state, the quarter wave plate may change the polarization of the forward beam from p (i.e. vertical) to circular, i.e. in the forward beam path, while the quarter wave plate changing the polarization state of the reverse beam reflected from the droplet of target material from circular to s (i.e. horizontal). Thus, the forward beam incident at the polarization state dependent beam pickup exhibits the p polarization state while the reverse beam incident at the polarization state dependent beam pickup exhibits the s polarization state, as explained above.

[0031] The forward beam FB as described above may be a pre-pulse PP configured to condition the droplet of target material. The Laser Powered Plasma based EUV generation system may further be configured to guide a rarefaction pulse through the polarization state dependent beam pickup and the polarization state adjuster, to facilitate alignment of the pre-pulse and the rarefaction pulse. Furthermore, the polarization dependent beam pickup may be configured to split a portion of the rarefaction pulse, which them propagates to the forward beam sensor. The forward beam sensor may thereby be used to further sense the portion of the rarefaction pulse, thus enabling to make use of the same sensor.

[0032] 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, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0033] 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 apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0034] 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.

[0035] 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.

Claims

1. A laser powered plasma, LPP, based EUV generation system configured to generate EUV light by irradiating droplets of a target material with at least one laser beam, the system comprising a metrology module configured to determine a position of a droplet of the target material by comparing a portion of a forward beam with a portion of a reverse beam that is reflected off from the target material, wherein the metrology module comprises:

- a polarization state dependent beam pickup arranged to split a portion from the forward beam and a portion from the reverse beam, wherein the portions depend on a polarization state of forward and reverse beam incident on the polarization state dependent beam pickup, and

- a polarization state adjuster arranged downstream of a beam path when compared to the polarization state dependent beam pickup, wherein the polarization state adjuster is arranged to change a polarization state of at least one of the forward beam and the reverse beam such that the polarization state of the reverse beam incident at the polarization state dependent beam pickup differs from the polarization state of the forward beam incident at the polarization state dependent beam pickup.

2. The LPP based EUV generation system according to any one of the preceding claims, wherein the polarization state dependent beam pickup is arranged to pick up a horizontal polarization state portion of a horizontal polarization state of the beam and a vertical polarization state portion of a vertical polarization state of the beam, wherein the horizontal polarization state portion differs from the vertical polarization state portion.

3. The LPP based EUV generation system according to polarization state dependent beam pickup comprises a surface exhibiting a polarization state dependent reflectivity.

4. The LPP based EUV generation system according to claim 2 or 3, wherein the horizontal polarization state portion exceeds the vertical polarization state portion.

5. The LPP based EUV generation system according to claim 4, wherein the horizontal polarization state portion exceeds the vertical polarization state portion by a factor of at least 10.

6. The LPP based EUV generation system according to any one of claims 2-5, wherein the polarization state dependent beam pickup is arranged to direct the portion from the forward beam to a forward beam sensor and to direct the portion from the reverse beam to a reverse beam sensor.

7. The LPP based EUV generation system according to any one of the preceding claims, wherein the polarization state adjuster comprises a Quarter Wave plate.

8. The LPP based EUV generation system according to any one of the preceding claims, wherein the polarization state adjuster is positioned in the forward beam path of the forward beam and in the reverse beam path of the reverse beam.

9. The LPP based EUV generation system according to any one of the preceding claims, further configured to guide a rarefaction pulse through the polarization state dependent beam pickup and the polarization state adjuster.

10. The LPP based EUV generation system according to any one of the preceding claims, wherein a wavelength of the forward beam is below 1500nm, preferably 1064nm or lower.

11. A lithographic apparatus comprising the laser powered plasma, LPP based EUV generation system according to any one of the preceding claims.

12. A method of generating EUV by laser powered plasma, LPP, wherein EUV light is generated by irradiating droplets of a target material with at least one laser beam, the method comprising determining by a metrology module a position of a droplet of the target material by comparing a portion of a forward beam with a portion of a reverse beam that is reflected off from the target material, wherein the metrology module comprises:

- a polarization state dependent beam pickup arranged to split a portion from the forward beam and a portion from the reverse beam, wherein the portions depend on a polarization state of forward and reverse beam incident on the polarization state dependent beam pickup, and

- a polarization state adjuster arranged downstream of a beam path when compared to the polarization state dependent beam pickup, wherein the polarization state adjuster is arranged to change a polarization state of at least one of the forward beam and the reverse beam such that the polarization state of the reverse beam incident at the polarization state dependent beam pickup differs from the polarization state of the forward beam incident at the polarization state dependent beam pickup.

13. The method of generating EUV by laser powered plasma, LPP according to claim 12, wherein the polarization state dependent beam pickup is arranged to pick up a horizontal polarization state portion of a horizontal polarization state of the beam and a vertical polarization state portion of a vertical polarization state of the beam, wherein the horizontal polarization state portion differs from the vertical polarization state portion.

14. The method of generating EUV by laser powered plasma, LPP according to claim 13, wherein the horizontal polarization state portion exceeds the vertical polarization state portion.

15. The method of generating EUV by laser powered plasma, LPP according to any one of claims 12 or 14, wherein the polarization state adjuster comprises a Quarter Wave plate.

Drawing