CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application no. 17173481.7, which was filed on 30 May 2017 and which is incorporated herein its entirety by reference.

FIELD

[0002] The present invention relates to a radiation source. The radiation source maybe an extreme ultraviolet radiation source. The extreme ultraviolet radiation source may form part of a lithographic system.

BACKGROUND

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

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

[0005] One known type of EUV radiation source directs laser radiation onto fuel droplets. This converts the fuel droplets into EUV radiation emitting plasma. A radiation source of this type may be referred to as a laser-produced plasma (LPP) source. Known LPP sources have a relatively low conversion efficiency. That is, the power of EUV radiation which they output is a relatively small fraction of the power of laser radiation being input.

SUMMARY

[0006] It may be desirable to provide an EUV radiation source which has a better conversion efficiency than a conventional LPP radiation source, or which overcomes some other disadvantage associated with conventional LPP radiation sources.

[0007] According to a first aspect of the invention there is provided a radiation source comprising a fuel emitter configured to provide droplets of fuel to a plasma formation region, and a laser system configured to supply a laser beam, wherein the laser system comprises a delay line configured to delay a primary portion of the laser beam relative to a subsidiary portion of the laser beam, such that a pulse of the laser beam subsidiary portion is incident at the plasma formation region before a pulse of the laser beam primary portion.

[0008] Advantageously, directing a pulse of the laser beam subsidiary portion onto a fuel droplet before a pulse of the laser beam primary portion increases the conversion efficiency with which

EUV radiation is generated.

[0009] Optionally, the pulse of the laser beam subsidiary portion may form a pedestal which precedes the pulse of the laser beam primary portion.

[00010] Optionally, the pulse of the laser beam subsidiary portion does not temporally overlap with the pulse of the laser beam primary portion.

[00011] The delay line may be configured to delay pulses of the laser beam primary portion by between 100ns and 300ns relative to pulses of the laser beam subsidiary portion.

[00012] Pulses of the subsidiary laser beam portion pulse may have a duration which is between

30ns and 150ns.

[00013] The delay line may comprise an optical amplifier.

[00014] The laser system may be configured such that the laser beam primary portion passes forwards and backwards multiple times within the optical amplifier and the laser beam subsidiary portion travels directly through the optical amplifier.

[00015] Advantageously, this provides significantly more amplification of the laser beam primary portion than the laser beam subsidiary portion.

[00016] The optical amplifier may comprise an entrance window an exit window and a series of mirrors, and wherein the laser system is configured such that the laser beam subsidiary portion passes directly from the entrance window to the exit window, whereas the laser beam primary portion passes from the entrance window via the series of mirrors to the exit window.

[00017] The laser beam may be separated into the primary portion and the secondary portion using a laser beam splitting apparatus.

[00018] The laser system may further comprise a pulse shaping device configured to modify pulses of the laser beam subsidiary portion.

[00019] The laser system may further comprise an amplification system configured to amplify the laser beam primary portion and the laser beam subsidiary portion before they are incident at the plasma formation region.

[00020] According to a second aspect of the invention there is provided a lithographic system comprising the radiation source of the first aspect of the invention, and further comprising an illumination system configured to condition a radiation beam received from the radiation source, a support structure constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto the substrate.

[00021] Advantageously, directing a pulse of the laser beam subsidiary portion onto a fuel droplet before a pulse of the laser beam primary portion increases the conversion efficiency with which EUV radiation is generated. This provides a higher intensity radiation beam, thereby allowing more substrates to be patterned per hour using the lithographic apparatus.

[00022] According to a third aspect of the invention there is provided a laser system configured to supply a laser beam for an EUV radiation source, wherein the laser system comprises a delay line configured to delay a primary portion of the laser beam relative to a subsidiary portion of the laser beam, such that a pulse of the laser beam subsidiary portion is output from the laser system before a pulse of the laser beam primary portion.

[00023] According to a fourth aspect of the invention there is provided a method of generating EUV radiation comprising using a pulsed laser system to provide a pulsed laser beam primary portion and a pulsed laser beam subsidiary portion, the primary portion being delayed by a delay line relative to the subsidiary portion, and directing the pulsed laser beam primary portion and the pulsed laser beam subsidiary portion onto fuel droplets to generate EUV radiation emitting plasma.

[00024] Advantageously, directing a pulse of the laser beam subsidiary portion onto a fuel droplet before a pulse of the laser beam primary portion increases the conversion efficiency with which EUV radiation is generated.

[00025] Optionally, pulses of the laser beam subsidiary portion form pedestals which precede pulses of the laser beam primary portion.

[00026] Optionally, pulses of the laser beam subsidiary portion does not overlap with pulses of the laser beam primary portion.

[00027] The delay line may delay the laser beam primary portion by between 100ns and 300ns relative to the laser beam subsidiary portion.

[00028] Pulses of the subsidiary laser beam portion pulse may have a duration which is between 30ns and 150ns.

BRIEF DESCRIPTION OF THE DRAWINGS

[00029] 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 radiation source and a lithographic apparatus according to an embodiment of the invention;

Figure 2 depicts an embodiment of a laser system which may form part of the radiation source of figure 1 ; and

Figure 3 depicts an alternative embodiment of a laser system which may form part of the radiation source of figure 1. DETAILED DESCRIPTION

[00030] Figure 1 shows a lithographic system including a mirror array according to one embodiment of the invention. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. 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. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[00031] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[00032] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source. The radiation source comprises a laser system 2, which may be referred to as a main pulse laser system. The radiation source may optionally comprise an additional laser system 1. The additional laser system may be referred to as a pre-pulse laser system 1. Laser beams 2, 3 from the laser systems 1, 2 are combined using beam combination optics 5 (e.g. a dichroic mirror) and then deposit energy into a fuel, such as tin (Sn) which is provided from a fuel emitter 6. 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 6 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 7. The laser beam 2 is incident upon the tin at the plasma formation region 7. The deposition of laser energy into the tin creates a plasma 8 at the plasma formation region 7. Radiation, including EUV radiation, is emitted from the plasma 8 during de-excitation and recombination of ions of the plasma.

[00033] The EUV radiation is collected and focused by a near-normal incidence radiation collector 9 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 9 may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 9 may have an ellipsoidal configuration, having two focal points. A first focal point may be at the plasma formation region 7, and a second focal point may be at an intermediate focus 10, as discussed below.

[00034] The laser systems 1, 2 may be remote from other parts of the radiation source SO. Where this is the case, the laser beams 3, 4 may be passed from the laser systems 1, 2 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 pre-pulse laser system 1 and the main pulse laser system 2 taken together may be referred to as a combined laser system CS.

[00035] Radiation that is reflected by the collector 9 forms a radiation beam B. The radiation beam B is focused at point 10 to form an image of the plasma formation region 7, which acts as a virtual radiation source for the illumination system IL. The point 10 at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus 10 is located at, or near to, an opening 11 in an enclosing structure 12 of the radiation source.

[00036] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. The illumination system IL includes a faceted field mirror device 13 and may include a faceted pupil mirror device 14. The faceted field mirror device 10 is a mirror array made up of individually controllable mirrors. A mirror of the array together with an associated actuator and sensing apparatus may be referred to as a mirror assembly. A controller CT controls the orientations of the mirrors (as is described further below). The faceted field mirror device 13 and faceted pupil mirror device 14 together provide the radiation beam B with a desired cross-sectional shape and a desired angular intensity distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 13 and faceted pupil mirror device 14.

[00037] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 15, 16 which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors 15, 16 in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[00038] The radiation source SO shown in Figure 1 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be configured to receive the electromagnetic radiation produced by the plasma and separate the EUV radiation from radiation other than EUV, for example, from infrared radiation. The spectral filter may be a transmissive filter that is substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation. Alternatively, the spectral filter may be a reflective filter that reflects incident EUV radiation into a specific direction and non-EUV radiation into other directions.

[00039] The pre-pulse laser system 1 may be configured to provide pulses of radiation which, when incident upon fuel droplets, condition the fuel droplets but do not generate significant amounts of EUV radiation emitting plasma. These pulses, which may be referred to as pre-pulses, may for example modify the shape of the fuel droplets to a pancake shape and/or may ablate some material from the fuel droplets. The main pulse laser system 2 may be configured to provide pulses of radiation which, when incident upon fuel droplets, convert the fuel droplets into EUV radiation emitting plasma. These pulses of radiation may be referred to as main pulses. The main pulses of radiation may have significantly more energy than the pre-pulses of radiation.

[00040] The main pulses of radiation may have a longer wavelength than the pre-pulses of radiation.

For example, the main pulses of radiation may be infrared, and may have a wavelength of around 10 microns (e.g. 10.6 microns). Alternatively, the main pulses of radiation may have a shorter infrared wavelength, e.g. around 1 micron. The pre-pulses of radiation may also be infrared, and may also have a wavelength of around 10 microns (e.g. 10.3 microns). Alternatively, the pre-pulses of radiation may have a shorter infrared wavelength, for example around 1 micron. The pre-pulses of radiation may have a wavelength in a range of around 1 micron to around 10 microns.

[00041] An embodiment of the main pulse laser system 2 is schematically depicted in figure 2. The main pulse laser system 2 comprises a laser 20 configured to emit a pulsed laser beam 4, which may be referred to as the main beam of radiation. The laser 20 may for example be a CO ₂ laser. The laser beam 4 passes through optics 22 (e.g. a polarizer, beam condition optics, etc) and is then incident upon a first partial reflector 24. The first partial reflector 24 may for example have a reflectivity of around 10% or less. The first partial reflector 24 may for example have a reflectivity of around 1% or more. The majority of the laser beam 4, which may be referred to as a primary portion 4a, passes through the first partial reflector 24 and enters a delay line 26. The delay line may comprise mirrors 25 arranged to reflect the laser beam primary portion 4a back and forth until it has travelled a desired distance (e.g. tens of meters e.g. between 30 and 100 meters). On leaving the delay line 26 the primary portion of the laser beam 4a travels to a second partial reflector 32.

[00042] The remainder of the laser beam 4, which may be referred to as a subsidiary portion 4b, is reflected by the first partial reflector 24 and travels via a reflector 28 to a pulse shaping device

30. The pulse shaping device 30 is configured to modify pulses of the subsidiary portion of the laser beam 4b. The pulse shaping device 30 may for example be an electro-optic modulator (EOM). The pulse shaping device 30 may for example remove an unwanted sharp peak at the beginning of a pulse of the subsidiary portion of the laser beam 4b. This may provide a flatter pulse shape, which may be desirable for a pedestal pulse. Other forms of pulse shaping may be applied by the pulse shaping device 30. The pulse shaping device 30 may for example be used to shorten pulses of the subsidiary portion of the laser beam 4b. The pulse shaping device

30 may also attenuate pulses of the subsidiary portion 4b. The pulse shaping device 30 may be an acousto-optic modulator (AOM). The AOM may be used to attenuate pulses of the subsidiary portion of the laser beam 4b. The AOM might not have a sufficiently fast response to remove a peak from the beginning of a pulse or to shorten a pulse. On leaving the pulse shaping device 30, the subsidiary portion of the laser beam 4b is reflected by another reflector

31 to the second partial reflector 32. In an embodiment the pulse shaping device 30 may be omitted.

[00043] The second partial reflector 32 may for example have a reflectivity of around 10% or less.

The second partial reflector 32 may for example have a reflectivity of around 1% or more. The second partial reflector 32 may have the same reflectivity as the first partial reflector 24 (this may provide maximum transmission of the primary portion of the laser beam 4a). The delayed primary portion of the laser beam 4a is incident upon the partial reflector 32 and the undelayed subsidiary portion 4b of the laser beam is also incident upon the partial reflector 32. Since the second partial reflector 32 has a low reflectivity, most of the primary portion of the laser beam 4a is transmitted by the second partial reflector 32. The primary portion of the laser beam 4a then travels to an amplification system 34. The amplification system 34 may for example be a series of optical amplifiers. A reflected part of the primary portion of the laser beam 4a is incident upon a beam dump 36. Most of the subsidiary portion of the laser beam 4b is transmitted by the second partial reflector 32 and is incident upon the beam dump 36. A reflected part, e.g. around 10% or less, of the subsidiary portion of the laser beam 4b travels to the amplification system.

[00044] Thus, part of the primary portion 4a and part of the subsidiary portion 4b of the laser beam are recombined at the second partial reflector 32 and travel together through the amplification system 34. On leaving the amplification system 34 the primary portion 4a and the subsidiary portion 4b of the laser beam travel together to the plasma formation region 7 (see figure 1).

[00045] In an embodiment in which the reflectivity of the first and second partial reflectors 24, 32 is 10%, the primary portion of the laser beam 4a will have a power of 81% of the initial power of the laser beam 4 and the subsidiary portion 4b will have a power of 1 % of the initial power of the laser beam. Hence, 18% of the laser beam will be incident upon the beam dump 36. In other embodiments the relative proportions of the powers may be different. In general however, the subsidiary portion 4b of the laser beam will have a power which is significantly lower than the primary portion 4a of the laser beam. The power of the subsidiary portion 4b may for example be less than 5% of the power of the primary portion 4a, and may for example be less than 1% of the power of the primary portion.

[00046] Because the subsidiary portion 4b of the laser beam has not passed through the delay line 26 it arrives at the partial reflector 32 before the primary portion of the laser beam 4a. As previously mentioned, the laser beam 4 is pulsed. Thus, a laser pulse of the subsidiary portion

4b arrives at the partial reflector 32 before a laser pulse of the primary portion 4a. Consequently, following recombination by the partial reflector 32 the primary portion of the laser beam 4a and the subsidiary portion of the laser beam propagate together, with pulses of the subsidiary portion preceding associated pulses of the primary portion (associated pulses is intended here to mean two pulses which have been generated from the same initial pulse). A delay between the subsidiary pulse and the primary pulse is determined by an optical path length of the delay line 26. For example, if the delay line 26 has an optical path length of around 30m then the subsidiary pulse will precede the primary pulse by around 100 ns. The optical path length of the delay line 26 may be selected or may be adjustable to provide a desired temporal separation between the subsidiary pulse and the primary pulse. The optical path length of the delay line 26 may be adjustable (e.g. by modifying a separation between reflectors of the delay line). The temporal separation may for example be between 100ns and 300ns. In this document, the temporal separation between two pulses may for example be measured as the temporal separation between centres of the two pulses (which may be halfway between ends of the pulses), or may be measured as the temporal separation between leading edges of the two pulses. The temporal separation is therefore a result of rearranging in the time-dimension different temporal portions of the laser beam.

[00047] In an embodiment, the duration of the subsidiary pulse may be significantly less than the separation between the subsidiary pulse and the primary pulse. Where this is the case the subsidiary pulse may be separated from the primary pulse. In an embodiment, the duration of the subsidiary pulse may be similar to or longer than the separation between the subsidiary pulse and the primary pulse. Where this is the case the subsidiary pulse may merge with the primary pulse. In such a situation the subsidiary pulse may be referred to as a pedestal.

[00048] The primary pulse may for example have a duration which is between 30ns and 150ns. The subsidiary pulse may for example have a duration which is between 30ns and 150ns. The primary pulse and the subsidiary pulse may have the same duration. Measurements of pulse duration may refer to the full width at half-maximum of the pulse. A significant proportion of the pulse may extend beyond this duration. In an embodiment, the subsidiary pulse and the primary pulse may each have a duration of 75ns and may have a temporal separation of 100ns (as applied by the delay line 26). Although the full width at half-maximum points of the subsidiary pulse and primary pulse will not meet each other, the subsidiary and primary pulse will still overlap with each other because a significant proportion of the pulses extends beyond the full width half-maximum duration. The subsidiary pulse may still be considered to form a pedestal. The same applies to other delay line lengths and other pulse durations.

[00049] It has been found that providing a subsidiary pulse which precedes the primary pulse, either as a pedestal or as a separate pulse, increases the conversion efficiency with which EUV emitting plasma is generated using the fuel. This improved conversion efficiency may be achieved in addition to improved conversion efficiency provided by using a pre-pulse laser beam 3 and a primary pulse laser beam 4 (compared with the conversion efficiency achieved if a subsidiary pulse is not used).

[00050] An alternative embodiment of the main laser system 2 is depicted in figure 3. In this embodiment an optical amplifier 40 acts as a delay line. The optical amplifier 40 is depicted schematically in cross section viewed from one side and, in addition, the ends of the optical amplifier are also schematically depicted. The optical amplifier 40 comprises an annular chamber 42 filled with gas which is excited using for example a radio frequency voltage applied across the annular chamber 42. A laser beam passing through the gas receives energy from the excited gas and is thereby amplified. An entrance window 44 and an exit window 46 are provided at opposite ends 43, 45 of the annular chamber 42. Mirrors 48 are also provided at the opposite ends 43, 45 of the annular chamber 42. The mirrors 48 are oriented to cause an incident laser beam to reflect from one mirror at one of the ends 43, 45 to a next mirror at the other one of the ends 43, 45 along an optical path which travels backwards and forwards between ends 43, 45 of the annular chamber 42 and at the same time progresses around the annular chamber 42. The optical amplifier 40 may for example be a Tru-Coax optical amplifier, available from Trumpf of Stuttgart, Germany.

[00051] A laser beam primary portion 4a to be amplified enters the optical amplifier 40 via the entrance window 44, is reflected by the mirrors 48 and travels around the annular chamber 42 whilst at the same time being amplified. The amplified laser beam primary portion 4a then leaves via the exit window 46. A subsidiary portion of the laser beam 4b (indicated by a dashed line) which does not require significant amplification also passes through the optical amplifier 40. However, the subsidiary portion of the laser beam 4b passes directly from the entrance window 44 to the exit window 46 and is not reflected by the mirrors 48 around the annular chamber 42. The subsidiary portion of the laser beam 4b thus only receives a small amount of amplification from the gas in the annular chamber, and travels over a much shorter distance within the annular chamber 42 than the laser beam primary portion 4a.

[00052] In order to cause the laser beam primary portion 4a to travel around the annular chamber 42 whilst the subsidiary portion of the laser beam 4b passes directly through the optical amplifier, the laser beam primary portion 4a and subsidiary portion of the laser beam 4b are not coaxial when they enter the optical amplifier 40. In an embodiment, an acousto-optic modulator (AOM) 50 is used to separate the subsidiary portion of the laser beam 4b such that it is non- coaxial with respect to the laser beam primary portion 4a when it enters the optical amplifier 40. The acousto-optic modulator 50 diffracts the incident laser beam 4 to form a zeroth order and a first order. The first order beam is the laser beam primary portion 4a. The laser beam primary portion 4a passes through a first partial reflector 52 and then enters the optical amplifier 40. Part of the laser beam primary portion 4a is reflected by the partial reflector 52 and is incident upon a beam dump 56.

[00053] The first order beam is the subsidiary portion of the laser beam 4b. The first order beam is incident upon a reflector 54. The reflector 54 may have a reflectivity of 100% or may be a partial reflector, for example with a reflectivity of around 50% or more. If the reflector 54 is a partial reflector, then the portion of the subsidiary portion of the laser beam which passes through the reflector 54 is incident upon a second beam dump 57. The reflected portion is incident upon the first partial reflector 52 and travels from that partial reflector to the optical amplifier 40 (with part of the beam passing through the first partial reflector 52 and being incident upon the beam dump 56). The first partial reflector 52 may for example have a reflectivity of around 1 %. Thus, the subsidiary portion of the laser beam 4b may have a power which is less than the power of the laser beam primary portion 4a by a factor of around 100 before it enters the optical amplifier 40.

[00054] The second partial reflector 52 is oriented such that the subsidiary portion of the laser beam 4b is non-coaxial with the laser beam primary portion 4a when they enter the optical amplifier 40. The orientation of the partial reflector 52 is such that the subsidiary portion of the laser beam 4b passes through the entrance window 44 but is not incident upon a mirror 48 of the optical amplifier 40. Instead, the subsidiary portion of the laser beam 4b passes out of the exit window 46.

[00055] Thus, the laser beam primary portion 4a travels around the annular chamber 42 of the optical amplifier 40 but the subsidiary portion of the laser beam 4b does not. The optical amplifier 40 acts as a delay line and, at the same time, amplifies the laser beam primary portion 4a. Thus, the optical amplifier 40 advantageously provides two functions simultaneously.

[00056] The embodiment depicted in figure 3 may include a pulse shaping device 60 which is configured to modify pulses of the subsidiary portion of the laser beam 4b. The pulse shaping device 60 may for example be an electro-optic modulator (EOM). The pulse shaping device

60 may for example remove an unwanted sharp peak at the beginning of a pulse of the subsidiary portion of the laser beam 4b. This may provide a flatter pulse shape, which may be desirable for a pedestal pulse. Other forms of pulse shaping may be applied by the pulse shaping device 60. The pulse shaping device 60 may for example be used to shorten pulses of the subsidiary portion of the laser beam 4b. The pulse shaping device 60 may also attenuate pulses of the subsidiary portion 4b. The pulse shaping device 60 may be an acousto-optic modulator (AOM). The AOM may be used to attenuate pulses of the subsidiary portion of the laser beam 4b. The AOM might not have a sufficiently fast response to remove a peak from the beginning of a pulse or to shorten a pulse. In an embodiment the pulse shaping device 60 may be omitted.

[00057] In the embodiment depicted in figure 3, in common with the embodiment of figure 2, the laser system 1 further comprises an amplification system 62 through which the subsidiary pulse

4b and the primary pulse 4a pass before being incident at the plasma formation location (see figure 1). The amplification system 62 may for example be a series of optical amplifiers.

[00058] In the embodiment of figure 3 the subsidiary portion of the laser beam 4b is non-collinear with the primary portion of the laser beam 4a when it enters the optical amplifier 40. This non- collinearity allows the subsidiary portion of the laser beam 4b to travel directly from the entrance window 44 to the exit window 46 instead of being reflected by mirrors 48 of the optical amplifier 40. Because the subsidiary portion of the laser beam 4b and the primary portion of the laser beam 4a are non-collinear when they enter the optical amplifier 40, they are also non- collinear when they exit the optical amplifier. The non-collinear nature of the subsidiary and primary laser beam portions 4a, 4b may be a combination of different spatial positions of the laser beams and different angular orientations of the laser beams (which may be referred to as beam pointing). The partial reflector 52 and the reflector 54 may be adjusted to modify the spatial position and the beam pointing of the subsidiary portion of the laser beam 4b relative to the primary portion of the laser beam 4a in order to obtain a desired combination of beam position and beam pointing. For example, the subsidiary portion of the laser beam 4b may be incident upon the partial reflector 52 at a different spatial position from the primary portion of the laser beam 4a, and may be pointed such that it subsequently intersects with the primary portion of the laser beam. In figure 3 this approach has been used to cause the subsidiary portion of the laser beam 4b to intersect with the primary portion of the laser beam 4a within the amplification system 62. In other embodiments the subsidiary portion of the laser beam 4b may intersect with the primary portion of the laser beam 4a at some other position (e.g. at a position where a spatial filter is provided). In other embodiments the subsidiary portion of the laser beam 4b may not intersect with the primary portion of the laser beam 4a. In some embodiments the spatial position and the beam pointing of the primary portion of the laser beam 4a relative to the subsidiary portion of the laser beam 4b may be adjusted.

[00059] Although the optical amplifier depicted in figure 3 has an entrance window 44 at one end and an exit window 46 at an opposite end, in another embodiment the entrance and exit windows may be at the same end.

[00060] Although the optical amplifier 40 depicted in figure 3 is annular the optical amplifier may have some other shape. For example, the optical amplifier may be cuboid. The optical amplifier may for example have an entrance window at one end face and an exit window at an opposite end face. The optical amplifier may for example have an entrance window and an exit window at the same end face.

[00061] Although the embodiment depicted in figure 3 uses an acousto-optic modulator (AOM) 50 to split the laser beam 4 into the subsidiary laser beam portion 4b and the primary laser beam portion 4a, other apparatus may be used to split the laser beam (e.g. a partially reflective mirror).

Although in the embodiment depicted in figure 2 uses a partially reflective mirror 24 to split the laser beam 4 into the subsidiary laser beam portion 4b and the primary laser beam portion 4a, other apparatus may be used to separate the laser beam portions (e.g. an acousto-optic modulator (AOM)). In general, any suitable laser beam splitting apparatus may be used to separate the laser beam into the subsidiary portion and the primary portion, e.g. a partially reflective mirror or a modulator.

[00062] In general, the subsidiary portion of the laser beam 4b may have a power which is 5% or less of the power of the laser beam primary portion 4a, e.g. 1% or less. In one example, after amplification by the amplification system 34, 62, a pulse of the subsidiary portion of the laser beam 4b may have a power of a few mJ whereas a pulse of the laser beam primary portion 4a may have a power of a few hundred mJ (e.g. around 400 mJ).

[00063] In an embodiment, the invention may form part of a mask inspection apparatus. The mask inspection apparatus may use EUV radiation to illuminate a mask and use an imaging sensor to monitor radiation reflected from the mask. Images received by the imaging sensor are used to determine whether or not defects are present in the mask. The mask inspection apparatus may include optics (e.g. mirrors) configured to receive EUV radiation from an EUV radiation source and form it into a radiation beam to be directed at a mask. The mask inspection apparatus may further include optics (e.g. mirrors) configured to collect EUV radiation reflected from the mask and form an image of the mask at the imaging sensor. The mask inspection apparatus may include a processor configured to analyse the image of the mask at the imaging sensor, and to determine from that analysis whether any defects are present on the mask. The processor may further be configured to determine whether a detected mask defect will cause an unacceptable defect in images projected onto a substrate when the mask is used by a lithographic apparatus.

[00064] In an embodiment, the invention may form part of a metrology apparatus. The metrology apparatus may be used to measure alignment of a projected pattern formed in resist on a substrate relative to a pattern already present on the substrate. This measurement of relative alignment may be referred to as overlay. The metrology apparatus may for example be located immediately adjacent to a lithographic apparatus and may be used to measure the overlay before the substrate (and the resist) has been processed.

[00065] 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.

[00066] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4- 10 nm such as 6.7 nm or 6.8 nm.

[00067] 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.

[00068] 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 disk 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.

[00069] 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.