NL2012718A - Radiation systems and associated methods. - Google Patents

Description

RADIATION SYSTEMS AND ASSOCIATED METHODS

FIELD

[0001] The present invention relates to radiation systems such as radiation systems which use laser radiation to excite a fuel to produce a plasma.

BACKGROUND

[0002] Extreme ultraviolet (EUV) radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, and may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector apparatus to contain the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.

[0003] One application of an EUV radiation source is in lithography. A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0004] In order to reduce the minimum printable size, imaging may be performed using radiation having a short wavelength. It has therefore been proposed to use an EUV radiation source providing EUV radiation within the range of 13-14 nm, for example. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation.

SUMMARY

[0005] In a LPP source, to maximize or increase efficiency, the laser beam should hit the fuel droplet as accurately as possible. Accuracy may be reduced in practice due to the difficulty in measuring laser accuracy on the droplet and/or due to one or more causes of any inaccuracy over the whole of the laser beam path from laser to fuel, particularly during times when no EUV radiation is being produced.

[0006] Tt is desirable to increase accuracy when targeting fuel with a laser beam in a LPP source.

[0007] According to an aspect, there is provided a radiation system comprising: a first pulsed laser source operable to emit first pulses of radiation for the generation of a plasma in a source chamber; a measurement laser source operable to emit measurement radiation at least during a time that the radiation system is not generating plasma, the measurement radiation being of insufficient power for the generation of plasma; and a metrology apparatus operable to measure reflected measurement radiation, the reflected measurement radiation comprising measurement radiation which arrives at the metrology apparatus subsequent to being reflected in the source chamber.

[0008] According to an aspect, there is provided a radiation system comprising: a pulsed laser source operable to emit pulses of radiation; a beam transport system operable to transport the pulses of radiation towards a source chamber for generation of a plasma; a measurement laser source operable to emit measurement radiation towards the source chamber via the beam transport system; and a metrology apparatus operable to measure the measurement radiation prior to reflection in the source chamber and/or measure the reflected radiation subsequent to reflection in the source chamber.

[0009] According to an aspect, there is provided a method of generating a plasma comprising: emitting first pulses of radiation towards a fuel droplet at a source chamber such that the first pulses of radiation excite the fuel droplet to generate the plasma; emitting measurement radiation at least during a time that no plasma is being generated, the measurement radiation being of insufficient power for the generation of plasma; and measuring reflected measurement radiation subsequent to it being reflected from a fuel droplet in the source chamber.

[0010] According to an aspect, there is provided a method of generating a plasma comprising: emitting pulses of radiation; using a beam transport system to transport the pulses of radiation towards a fuel droplet at a source chamber such that the pulses of radiation excite the fuel droplet to generate the plasma; emitting measurement radiation towards the source chamber via the beam transport system; and measuring the measurement radiation prior to reflection in the source chamber and/or measuring the reflected radiation subsequent to reflection in the source chamber.

[0011] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0012] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate embodiments of the present invention and, together with the description, further serve to explain the principles of embodiments of the invention and to enable a person skilled in the relevant art(s) to make and use embodiments of the invention. Embodiments of the invention are described, by way of example only, with reference to the accompanying drawings, in which:

[0013] Figure 1 depicts schematically a lithographic apparatus having reflective projection optics;

[0014] Figure 2 is a more detailed view of the apparatus of Figure 1, illustrating a laser radiation system and EUV radiation system in accordance with embodiments of the invention;

[0015] Figure 3 shows a further configuration for an EUV radiation system in the apparatus of Figures 1 and 2;

[0016] Figure 4 shows schematically a laser produced plasma radiation system including a pointing laser and corresponding sensor;

[0017] Figure 5 shows schematically the laser produced plasma radiation system of Figure 4 with the laser switched on, but the system not producing plasma radiation;

[0018] Figure 6 shows schematically the laser produced plasma radiation system of Figure 4 during production of plasma radiation;

[0019] Figure 7 shows schematically a laser produced plasma radiation system according to an embodiment of the invention; Γ00201 Figure 8 shows schematically a laser produced plasma radiation system according to an embodiment of the invention;

[0021] Figure 9 shows schematically a laser produced plasma radiation system according to an embodiment of the invention; and

[0022] Figures 10a to 10c show a main laser beam, pre-pulse laser beam and further beam so as to illustrate timing control of the laser beams in accordance with an embodiment of the invention.

[0023] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0024] Figure 1 schematically depicts a lithographic apparatus 100 including a source module SO according to one embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation).

a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g. a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0025] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0026] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0027] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0028] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0029] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0030] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0031] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0032] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source module SO. Methods to produce EUV radiation include, hut are not necessarily limited to, converting a material into a plasma state that has at least one clement, e.g., xenon, lithium or tin, with one or more emission lines in the EUY range. In one such method, often termed laser produced plasma ("LPP") the plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam. The source module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source module. The laser and the source module may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0033] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0034] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0035] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0036] The depicted apparatus could be used in at least one of the following modes:

[0037] 1. In step mode, the support stmeture (e.g. mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

[0038] 2. In scan mode, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0039] 3. In another mode, the support stmeture (e.g. mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0040] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0041] Figure 2 schematically shows an embodiment of the lithographic apparatus in more detail, including a radiation system 42, the illumination system IL, and the projection system PS. The radiation system 42 as shown in Figure 2 is of the type that uses a laser-produced plasma as a radiation source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma is created by causing an at least partially ionized plasma by, for example, optical excitation using CO2 laser radiation. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, Sn is used to create the plasma in order to emit the radiation in the EUV range.

[0042] The radiation system 42 embodies the function of source SO in the apparatus of Figure 1. Radiation system 42 comprises a source chamber 47, in this embodiment not only substantially enclosing a source of EUV radiation, but also a collector mirror 50 which, in the example of Figure 2, is a normal-incidence collector, for instance a multi-layer mirror.

[0043] As part of an LPP radiation source, a laser radiation system 61 (described in more detail below) is constructed and arranged to provide a laser beam 63 which is delivered by a beam delivering system 65 through an aperture 67 provided in the collector mirror 50. Also, the radiation system includes a target material 69, such as Sn or Xe, which is supplied by target material supply 71. The beam delivering system 65, in this embodiment, is arranged to establish a beam path focused substantially upon a predetermined plasma position 73.

[0044] In operation, the target material 69, which may also be referred to as fuel, is supplied by the target material supply 71 in the form of droplets. When such a droplet of the target material 69 reaches the plasma formation position 73, the laser beam 63 impinges on the droplet and an EUV radiation-emitting plasma forms inside the source chamber 47. In the case of a pulsed laser, this involves timing the pulse of laser radiation to coincide with the passage of the droplet through the position 73. As mentioned, the fuel may be for example xenon (Xe), tin (Sn) or lithium (Li). These create a highly ionized plasma with electron temperatures of several 10's of eV. Higher energy EUV radiation may be generated with other fuel materials, for example Tb and Gd. The energetic radiation generated during de-excitation and recombination of these ions includes the wanted EUV radiation which is emitted from the plasma at position 73. The plasma formation position 73 and the aperture 52 are located at first and second focal points of collector 50, respectively and the EUV radiation is focused by the normal-incidence collector 50 onto the intermediate focus point IF.

[0045] Radiation passed by collector 50 passes in this example through a transmissive filter spectral purity filter SPF, located near the aperture 52.

[0046] The beam of radiation emanating from the source chamber 47 traverses the illumination system IL via so-called normal incidence reflectors 53, 54, as indicated in Figure 2 by the radiation beam 56. The normal incidence reflectors direct the beam 56 onto a patterning device (e.g. reticle or mask) positioned on a support (e.g. reticle or mask table) MT. A patterned beam 57 is formed, which is imaged by projection system PS via reflective elements 58, 59 onto a substrate carried by wafer stage or substrate table WT. More elements than shown may generally be present in illumination system IL and projection system PS. For example there may be one, two, three, four or even more reflective elements present than the two elements 58 and 59 shown in Figure 2. Radiation collectors similar to radiation collector 50 are known from the art.

[0047] As the skilled reader will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 55, 56, 57. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction of optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source collector module 42, the X axis coincides broadly with the direction of fuel stream (69, described below), while the Y axis is orthogonal to that, pointing out of the page as indicated in Figure 2. On the other hand, in the vicinity of the support structure MT that holds the patterning device MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram Figure 2, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0048] In addition to the wanted EUV radiation, the plasma produces other wavelengths of radiation, for example in the visible, UV and DUV range. There is also infrared (IR) radiation present from the laser beam 63. The non-EUV wavelengths are not wanted in the illumination system IL and projection system PS and various measures may be deployed to block the non-EUV radiation. As schematically depicted in Figure 2, a transmissive SPF may be applied upstream of virtual source point IF. Alternatively or in addition to such a filter, filtering functions can be integrated into other optics. For example a diffractive filter can be integrated in collector 50 and/or mirrors 53, 54, etc., by provision of a grating structure tuned to divert the longer, IR radiation away from the virtual source point IF. Filters for IR, deep ultraviolet (DUV) and other unwanted wavelengths may thus be provided at one or more locations along the paths of beams 55, 56, 57, within source module (radiation system 42), the illumination system IL and/or projection system PS.

[0049] Figure 3 schematically shows a LPP source arrangement which may be used in place of that illustrated in Figure 2. A main difference is that the main pulse laser beam is directed onto the fuel droplet from the direction of the intermediate focus point IF, such that the collected EUV radiation is that which is emitted generally in the direction from which the main laser pulse was received. Figure 3 shows the main laser 30 emitting a main pulse beam 31 delivered to a plasma generation site 32 via at least one optical element (such as a lens or folding mirror) 33. The EUV radiation 34 is collected by a grazing incidence collector 35 such as those used in discharge produced plasma (DPP) sources. Also shown is a debris trap 36, which may comprise one or more stationary foil traps and/or a rotating foil trap, and a pre-pulse laser 37 operable to emit a pre-pulse laser beam 38.

[0050] To deliver the fuel, which for example is liquid tin, a droplet generator or target material supply 71 is arranged within the source chamber 47, to fire a stream of droplets towards the plasma formation position 73. In operation, laser beam 63 may be delivered in a synchronism with the operation of target material supply 71, to deliver impulses of radiation to turn each fuel droplet into a plasma. The frequency of delivery of droplets may be several kilohertz, or even several tens or hundreds of kilohertz. In practice, laser beam 63 may be delivered by a laser radiation system 61 in at least two pulses: a pre-pulse PP with limited energy is delivered to the droplet before it reaches the plasma location, in order to vaporize the fuel material into a small cloud (or deforms it into a flattened “pancake” shape), and then a main pulse MP of laser energy is delivered to the cloud at the desired location, to generate the plasma. In a typical example, the diameter of the plasma is about 0.2-0.5 mm. A trap 72 is provided on the opposite side of the enclosing structure 47, to capture fuel that is not, for whatever reason, turned into plasma.

[0051] Referring to laser radiation system 61 in more detail, the laser in the illustrated example is of the ΜΟΡΑ (Master Oscillator Power Amplifier) type, although it may be any laser, for example a no master oscillator (ΝΟΜΟ) laser. The laser radiation system 61 includes a “master” laser or “seed” laser, labeled MO in the diagram, followed by a power amplifier system PA, for firing a main pulse of laser energy towards an expanded droplet cloud, and a pre-pulse laser for firing a pre-pulse of laser energy towards a droplet. A beam delivery system 65 is provided to deliver the laser energy 63 into the source chamber 47. In practice, the pre-pulse element of the laser energy may be delivered by a separate laser. Laser radiation system 61, target material supply 71 and other components can be controlled by a control module 20. Control module 20 may perform many control functions, and have many sensor inputs and control outputs for various elements of the system. Sensors may be located in and around the elements of radiation system 42, and optionally elsewhere in the lithographic apparatus. In one embodiment, the main pulse and the pre-pulse arc derived from a same laser. In another embodiment, the main pulse and the pre-pulse are derived from different lasers which are independent from each other.

[0052] Many measures can be applied in the controller 20. Such measures include monitoring to check that the virtual source point IF is aligned with the aperture 52, at the exit from the source chamber 47. In systems based on LPP sources, control of alignment is generally achieved by controlling the location of the plasma formation position 73, rather than by moving the collector optic 50. The collector optic, the exit aperture 52 and the illuminator IL are aligned accurately during a set-up process, so that aperture 52 is located at the second focal point of collector optic. However, the exact location of the virtual source point IF formed by the EUV radiation at the exit of the source optics is dependent on the exact location of the plasma, relative to the first focal point of the collector optic. To fix this location accurately enough to maintain sufficient alignment generally requires active monitoring and control.

[0053] For this purpose, controller 20 in this example may control the location of the plasma 73 (the source of the EUV radiation), by controlling the injection of the fuel, and also for example the timing of energizing pulses from laser. By appropriate control, it can be maintained that the EUV radiation beam is focused by collector optic 50 precisely on the aperture 52. If this is not achieved, all or part of the beam will impinge upon surrounding material of the enclosing structure. In that case, a heat dissipation mechanism can be used to absorb the EUV radiation incident on the enclosing structure.

[0054] Control module 20 may be supplied with monitoring data from one or more arrays of sensors (not shown) which provide a first feedback path for information as to the location of the plasma. The sensors may be of various types, for example as described in U.S. Patent Application Publication No. US20050274897A1. The sensors may be located at more than one position along the radiation beam path, for example on or near a controlled optical element, such as a controlled mirror forming part of the beam delivery system 65. A main pulse laser fired by laser radiation system 61 may be incident on the controlled mirror and directed by the mirror towards a droplet. The one or more sensors can monitor a tilting angle of the controlled mirror, and the relevant monitoring data relating to the tilting angle can then be fed back to control module 20. Control module 20 can use the relevant monitoring data to trigger the actuator AC to adjust the tilting angle of the controlled mirror, thereby keeping the laser beam aligned onto the droplet with very high accuracy (2-10pm).

[0055] Figure 4 schematically depicts in greater detail a possible laser and beam delivery system which may form the laser 61 and beam delivery system 65 shown in Figure 3. The laser comprises a pulsed seed laser module 400 and an amplifier module 405 (for example a CO2 laser amplifier) mounted on a support frame 410. In this embodiment, the beam delivery system comprises a beam transport system 415, a controlled mirror 420 and a focusing unit 430. The beam transport system 415 and controlled mirror 420 direct the laser radiation to the focusing unit 430 and the focusing unit 430 focuses this radiation onto a fuel droplet 425 in the source chamber 47. Beam transport system 415 may comprise a number of mirrors for directing the beam along a predefined path.

[0056] Also shown are one or more metrology apparatuses. These include a pointing laser 435 and corresponding pointing sensor 440, a forward beam detector module (FBD) 445 and a reverse beam detector module (RBD) 450. In some embodiments the controlled mirror 420 may be controlled using information obtained by one, some or all of these metrology apparatuses. Which modules are used at a particular time may be dependent on the mode of operation of the system.

[0057] In Figure 4 the apparatus is shown during a period where no EUV is being generated, hereafter referred to as an “EUV off’ state. The beam path (had the laser been producing radiation) is shown by narrow broken lines 455. However, in this example, the laser system 400, 405 is not outputting any radiation. Consequently neither FBD 445 nor RBD 450 receives any information and therefore cannot be used to monitor the system. Pointing laser 435 outputs a beam 460 which follows a similar path to the main laser beam path 455, before being detected by pointing sensor 440. Pointing sensor can only be positioned to detect the beam outside of the main beam path 455. While the pointing beam path 460 is shown here to be parallel to the main beam path 455, it may instead cross along the diagonal of the main beam path 455.

[0058] The pointing laser system 435, 440 references the laser system 400, 405 output to the source vessel (where the EUV is generated) during the EUV off state. Therefore it reduces global laser movements with respect to the controlled mirror 420. During this state (for example during periods between exposure of dies, substrates, lots) the laser is not measured with respect to the droplet. Also the internal laser system 400, 405 information/metrology is lost. Before exposing dies, the laser 400, 405 needs to be switched on and then locked onto the droplet 425. This step takes time and produces debris. The controlled part of the laser beam path is only that depicted within box 427. Laser amplifiers and droplet stream dynamics arc not measured or controlled.

[0059] Figure 5 schematically depicts the same apparatus as that depicted in Figure 4. However, despite still being in an EUV off state, the laser apparatus 400, 405 is kept on, producing a pulsed output 500 between dies which fires between droplets (so that no EUV is produced). This laser may be kept on so as to keep mirror heating and deformation constant within the beam delivery system. It may also allow forward beam pointing errors to be directly measured and controlled using (for example) FBD 445, therefore enabling correction for drift in the system optics. Return beam errors (laser onto droplet) are not measured, as the laser beam radiation 500 is not incident on the droplet 425, and no radiation is reflected. Here, the controlled part of the laser beam path is only that depicted within box 527.

[0060] Figure 6 schematically depicts the same apparatus as that depicted in Figure 4 or 5, but in an EUV on state, such that EUV is being generated. Here the output laser beam radiation 600 is incident on the droplet 425 via controlled mirror 420. Some of this radiation 610 is reflected back along the same path, after which it may be measured by RBD 450. As before FBD 445 measures some of the forward radiation from the laser system 400, 405. As a consequence the measured radiation from both the FBD 445 and RBD 450 can be compared, and the resulting control data used to control the controlled mirror 420 within a control loop to accurately target droplet 425. Therefore, only when this system is in an EUV on state, is the whole of the laser beam path controlled from laser to droplet 425.

[0061] Figure 7 schematically depicts an apparatus which is operable to continuously measure and control the beam path between laser amplifier 405 and droplet 425. The system may dispense with the pointing laser and pointing sensor. An additional laser 700 is provided, hereafter referred to as measurement laser 700, which may be a pulsed laser or continuous wave laser (CW laser). In some embodiments (described later), the pre-pulse laser may be used as this additional laser. In the embodiment shown here, a beam 710 from measurement laser 700 is continuously injected between seed laser 400 and amplifier 405. Consequently beam 710 passes through the amplifier 405, through the beam transport system 415, onto controlled mirror 420 and then onto the droplet 425. The reflected beam 720 is measured by the RBD 450. As the measurement laser 700 is on and emitting (or pulsing) radiation continuously, a signal is received by RBD 450 during the EUV off state for every droplet 425. In this way, the main beam focus position is locked to the droplet position even when in an EUV off state.

[0062] In addition to this, the whole beam path is measured and controlled at all times that a droplet passes. Therefore thermal lensing, global drift and dynamics are all continuously measured from laser to droplet. For the laser 405 and beam transport system 415, these variables may be measured from the forward beam by the FBD 445. This may be either in addition to, or instead of, measurement of these variables from the return (reflected) beam using the RBD 450. As with the Figure 5 example, the main laser 400, 405 may be kept on during the EUV off state (beams not shown on this diagram) so as to keep the mirror heating constant, but pulsed so as to fire between droplets to prevent EUV generation.

[0063] Figure 8 shows schematically a system according to a further embodiment. It comprises seed laser 800, laser amplifier 810, beam transport system 820, metrology apparatus (FBD and RBD) 830, controlled mirror 840, focusing unit 850 and fuel droplet 860. Also shown is the measurement laser 870, 870’ 870” injecting a beam at three alternative locations in accordance with three embodiments. In the first embodiment, measurement laser 870 is injected between seed laser 800 and laser amplifier 810. This is the same as the Figure 7 example. In a second embodiment the measurement laser 870’ is injected at the laser amplifier 810 output, before the beam transport system 820. In a third embodiment the measurement laser 870” is injected at the output of the beam transport system 820. Metrology apparatus 830 is operable to measure the forward beam, the return beam or both forward and return beams.

[0064] It should be understood that, depending on the embodiment, the concepts described herein may provide two distinct advantages. A first advantage is to keep the laser beam control loop on the droplet closed when operating in an EUV off state. This loop is illustrated by the dotted arrows indicating information feedback from the droplet 860 to the metrology apparatus 830 and, based upon this, the control information sent from the metrology apparatus 830 to the controlled mirror 840. This first advantage is achieved for all three of the embodiments illustrated in Figure 8. Where there is a separate pre-pulse laser, this continuously closed loop can be used for control of the pre-pulse laser in the same way as described in relation to the main laser. Therefore, both the pre-pulse laser and main laser may be controlled in this manner. As the pre-pulse to droplet accuracies are much stricter (small beam waist size on a 35 pm droplet) than for the main pulse (large beam waist size), control of the pre-pulse laser in this manner can be particularly advantageous.

[0065] A second advantage is that the proposed system, according to the first and second embodiments, monitors the relevant optical properties better than using a pointing laser. In the case of the first embodiment using measurement laser 870, thermal lensing, global drift and dynamics are all measured from laser 810 to droplet 860. In the case of the second embodiment using measurement laser 870’ global drift and dynamics are measured, as is the thermal lensing of the beam transport system 820. Appropriate corrections can then be made to the system or elements thereof to allow for or compensate for this thermal lensing, drift etc.

[0066] In the case of the third embodiment with measurement laser 870” at the beam transport system 820 output, the forward beam from the laser system 800, 810 and/or a pointing laser could still be measured in the metrology apparatus. In fact the second advantage may be obtained from measurement of the forward beam only in any of the arrangements depicted herein.

[0067] The measurement laser may be any kind of laser such as, for example, a He-Ne laser, low power CO2 laser or the (e.g. Nd:Yag) pre-pulse laser. The benefit of a He-Ne laser is that high bandwidth detectors are widely available and are of low cost. The downside is that such a laser has poor transmission through the beam transport system (optimized for a CO2 laser) when following the whole beam path. A low power CO2 laser has the benefit that the transmission is good (>99% per reflection).

[0068] A CO2 laser injected before the amplifier 810 (Figure 7 embodiment), may take two forms. It may be a low power laser, for example a detuned seed laser, which is sufficiently amplified by the CO2 amplifier. This will result in a small amount of gain stripping, that is the gain of the amplifier on the main pulse will be slightly reduced. Alternatively a higher power laser (high enough for sensors) may be used. This should lase at a wavelength where the CO2 medium has low or zero gain (amplifier gain is a function of wavelength). In this way there should be (virtually) no gain stripping. Detuning can be performed in various ways, for example with an acoustic optical modulator (AOM). Alternatively a quantum cascade laser could be used.

[0069] Figure 9 schematically shows an arrangement comprising a pre-pulse laser 900 to condition a fuel droplet 905. In this embodiment pre-pulse laser 900 is also used as the measurement laser, with measurements taken along both the pre-pulse 910 and main pulse beam paths 915. The pre-pulse path 910 is separate to the main pulse path 915 and should be separately controlled. The pre-pulse beam 920 has its own dedicated controlled optical element (e.g. mirror or lens, not shown here) for aiming at the correct droplet position, and may also have a “zoom” element to manipulate focus position in the z-direction. This controlled optical element operates in a similar manner to that described in relation to control of the main pulse beam. Also shown is main pulse laser 925 which emits main pulse beam 930 for exciting conditioned fuel droplet 935, pre-pulse RBD (PP-RBD) 940, main pulse RBD (MP-RBD) 945, pre-pulse FBD (PP-FBD) 950 and main pulse FBD (MP-FBD) 955 arranged as shown.

[0070] In this embodiment, when in an EUV on state, RBD measurements are taken from reflected radiation 960, which is radiation originating from the pre-pulse laser 900 that has reflected off the droplet 905 and has travelled along the pre-pulse beam path 910 to the PP-RBD 940. Also, RBD measurements are taken from reflected radiation 965, which is radiation originating from the pre-pulse laser 900 that has reflected off the droplet 905 and has travelled along the main pulse beam path 915 to the MP-RBD 945. A benefit of measuring pre-pulse return beam 965 over the main pulse beam path 915 is that the two paths 910, 915 are referenced to each other.

[0071] When in an “EUV off’ state the pre-pulse laser 900 may be used in a low power pulse mode so as not to fragment or significantly deform the droplet 905, with measurements taken in the same way as in the “EUV on” state described in the previous paragraph. As such, RBD measurements are also taken along both pre-pulse and main pulse paths 910, 915 using radiation reflected from the droplet 905. In this way the beam control system can be maintained at all times for both the pre-pulse beam 920 and main pulse beam 930. Alternatively a separate measurement laser (not shown) may be used in this “EUV off’ state, instead of using the prepulse laser 900, such that the measurement laser is used to make measurements for control of the pre-pulse beam path 910 and main pulse beam path 915, the measurement paths being much the same as shown in Figure 9.

[0072] The pre-pulse laser 900 may be located in any of the locations of the measurement laser 870, 870’, 870’ ’ in Figure 8. In a particular embodiment, the pre-pulse laser is located just before the metrology apparatus, and therefore its beam is injected at the location equivalent to that of measurement laser 870” in Figure 8.

[0073] The main laser may continue to fire between droplets when in an EUV off state, possibly with low duty cycle. In this way, measurements relating to some or all of thermal lensing, global drift and dynamics of the beam transport system and/or laser amplifier may be obtained directly from the resultant forward beam 930 using MP-FBD 955. Alternatively, an additional measurement laser (for example a Hc-Nc laser or low power CO2 laser) laser may be employed in order to measure these variables from the forward beam, again using the MP-FBD 955 in the manner already described. Depending on the type of laser employed, this measurement laser may be located in either of the locations of the measurement laser 870, 870’ in Figure 8. Measurements may also be taken from the forward pre-pulse beam 920 using PP-FBD 950.

[0074] In a further embodiment, a timing sensor may be implemented. Using forward and return beam diagnostics for laser to droplet control in a feedback configuration effectively means always acting after the event, with the result that the error is chased. The effects occur at high frequencies (>300Hz or even higher per droplet) with relatively high errors.

[0075] The fuel droplet may be affected by hydrogen flow profiles (droplet Y and Z position) and plasma force (X position). Typical positional errors may be in the region of 5 pm in the Y direction, 100 pm in the Z direction and 15 pm in the X direction. The effect in the X direction occurs per droplet and is related to EUV pulse power; the effect in the Y and Z directions occur at millisecond timescales.

[0076] While the error in the Z direction can be absorbed in the depth of focus, pre-pulse laser to droplet accuracy in the X and Y directions should be maintained within 1 or 2 pm. Effects in Y are seen at frequencies greater than 800 Hz. These frequencies cannot be suppressed sufficiently by mechanical actuation (tilt mirror).

[0077] It is therefore proposed to measure the upcoming droplet velocity and position just before impact of the droplet by the pre-pulse laser (within a few microseconds), and to adjust the timing (X position) and Y position of the pre-pulse laser beam accordingly. The same measurements can also be used for timing and positional control of the main pulse beam. Consequently, impact timing and position is responsive to actual droplet position instead of measurements obtained from a previous droplet.

[0078] Beam actuation in the X direction may be performed by laser timing triggering (per droplet). Beam actuation in the Y direction may be performed using an acousto optical deflector (AOD), per droplet. Such a device may have a suitably fast response time: less than 1 ps, where time between droplets is about 10 ps. In one embodiment the AOD may take the place of the controlled mirror 420, 840 of previous embodiments. However, it may be desirable to have a controlled mirror for long range (slow) and an AOD for short range (fast per droplet) control. In the latter example, the AOD may be placed between the metrology apparatus 860 and the controlled mirror 840. In such an arrangement the forward beam is picked up by the metrology apparatus, travels through the AOD and controlled mirror into the focussing optics and towards the droplet.

[0079] Metrology of the droplet position and velocity may be performed using the measurement laser previously described. Alternatively a separate illumination laser may be used (e.g., a 1 pm wavelength fiber laser). It may be desirable to illuminate (overfill) the droplet. In this way the pointing of the illumination laser will not disturb the measurement.

[0080] Detection of the reflected illumination radiation, and therefore droplet position, may be achieved using one or more position sensitive devices, such as a position sensitive diode. In one embodiment, two (or more) spaced-apart position sensitive devices may be used. In an embodiment the position sensitive devices may comprise bi-cells. In an embodiment, the two position sensitive devices may be separated by 500 pm, which should provide sufficiently accurate velocity information. The second position sensitive device of the pair provides positional information. Together the two position sensitive devices provide sufficient information to accurately predict the position at the pre-pulse position, which may be approximately 200 pm further along the droplet’s path. This should allow for time for calculating and reacting to expected droplet position for the pre-pulse (including delays).

[0081] Alternative detection methods may use a laser Doppler vibrometer. Such a detector directly measures the speed by phase difference of the reflected beam.

[0082] It is possible to integrate this metrology into the metrology apparatus. In this way the metrology is automatically referenced to the pre-pulse laser. The illumination radiation (from illumination laser or other source to droplet) and resultant reflected radiation (from droplet to metrology apparatus, in particular RBD) should therefore follow the pre-pulse metrology path.

[0083] By positioning the measurement laser beam before the actual plasma position, timing (X) position can be measured close to the plasma position using the measurement laser. This timing information can be used to trigger pulses from the pre-pulse and/or main pulse lasers to increase timing accuracy. Also the measurement laser may be operable to provide a slit illumination profile in the Y direction. In this way, the Y position of the droplet can also be measured close to the plasma position using the measurement laser.

[0084] Figures 10a-10c schematically illustrate both of these concepts, although they may be implemented independently. Figure 10a shows schematically a side view of the measurement laser beam 1000, the pre-pulse laser beam 1010 and the main pulse laser beam 1020, the focusing unit 1030 and droplets 1040. Figure 10b shows a more detailed side view of the measurement laser beam 1000, the pre-pulse laser beam 1010 and the main pulse laser beam 1020 waists. Again droplets 1040 are shown, and also a conditioned droplet 1050 following conditioning by the pre-pulse laser. Figure 10c shows that same arrangement as Figure 10b in cross-section.

[0085] As the measurement laser beam is reflected off a droplet, the metrology module can directly measure its X and Y position from the reflected radiation. The X direction is the droplet’s direction of travel and therefore is a measure of droplet timing. The Y direction is the direction perpendicular to, but in the same plane, as the X direction, and is the direction of the slit illumination. Timing can be used for laser trigger control. It predicts the actual droplet timing instead of reacting to the previous droplet (baseline).

[0086] Although specific reference may be made in this text to the provision and operation of an EUV radiation source in a lithographic apparatus, it should be understood that the EUV radiation apparatus described herein may have other applications in EUV optical apparatus. Further in the case of a lithographic apparatus, this may have other applications besides the manufacture of ICs, such as 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. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0087] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0088] 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 clauses set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A radiation system comprising: a first pulsed laser source operable to emit first pulses of radiation for the generation of a plasma in a source chamber; a measurement laser source operable to emit measurement radiation at least during a time that the radiation system is not generating plasma, the measurement radiation being of insufficient power for the generation of plasma; and a metrology apparatus operable to measure reflected measurement radiation, the reflected measurement radiation comprising measurement radiation which arrives at the metrology apparatus subsequent to being reflected in the source chamber.

2. The radiation system of clause 1, further comprising a first controlled optical element operable to controllably direct the first pulses of radiation to the source chamber.

3. The radiation system of clause 2, further comprising a beam transport system configured to transport the first pulses of radiation from the first pulsed laser source to the first controlled optical element.

4. The radiation system of clause 3, operable such that the measurement radiation is injected between the output of the beam transport system and the first controlled optical element.

5. The radiation system of clause 3, operable such that the measurement radiation is injected between the output of the first pulsed laser source and the input of the beam transport system such that the measurement radiation is transported by the beam transport system.

6. The radiation system of clause 3, wherein the first pulsed laser source comprises a seed laser and a laser amplifier, and the measurement radiation is injected between the seed laser and the laser amplifier such that the measurement radiation passes through the laser amplifier and the beam transport system.

7. The radiation system of clause 6, wherein the measurement radiation is amplified by the laser amplifier.

8. The radiation system of clause 6, wherein the measurement radiation has a wavelength which is not amplified or minimally amplified by the laser amplifier.

9. The radiation system of any of clauses 3 to 8, wherein the metrology apparatus is operable to measure a system stability parameter within the beam transport system and/or first pulsed laser source.

10. The radiation system of clause 9, wherein the system stability parameter comprises one or more selected from: thermal lensing, global drift and/or dynamics.

11. The radiation system of clause 9 or clause 10, wherein the metrology apparatus is operable to measure the system stability parameter by measuring the measurement radiation prior to reflection in the source chamber and/or measuring the reflected measurement radiation subsequent to reflection in the source chamber.

12. The radiation system of any of clauses 9 to 11, wherein the metrology apparatus is operable to measure the system stability parameter by measuring the first pulses of radiation prior to their arrival in the source chamber and/or measuring reflected radiation resultant from the first pulses of radiation being reflected in the source chamber.

13. The radiation system of any of clauses 9 to 12, operable to use the measured system stability parameter to make a system correction when generating the first pulses of radiation.

14. The radiation system of any of clauses 2 to 13, further comprising a controller operable to control the first controlled optical element in dependence of the measured reflected measurement radiation.

15. The radiation system of any of clauses 2 to 13, further comprising a second pulsed laser source operable to condition a fuel droplet prior to the fuel droplet being contacted by a pulse of radiation from the first pulsed laser source in order to generate the plasma, wherein the system is operable to: deliver the first pulses of radiation from the first pulsed laser source to the source chamber via a first beam path, the first beam path comprising the first controlled optical element, and deliver second pulses of radiation from the second pulsed laser source to the source chamber via a second beam path, the second beam path comprising a second controlled optical element; and wherein the metrology apparatus is operable to: measure reflected measurement radiation which arrives at the metrology apparatus via the first beam path subsequent to reflection in the source chamber, and measure reflected measurement radiation which arrives at the metrology apparatus via the second beam path subsequent to reflection in the source chamber.

16. The radiation system of clause 15, wherein the metrology apparatus is operable to measure reflected conditioning radiation which arrives at the metrology apparatus via the first and second beam paths following conditioning of the droplet by the second pulsed laser source during a time that the radiation system is generating plasma.

17. The radiation system of clause 15 or clause 16, comprising a controller operable to control the first controlled optical element in dependence of the reflected radiation measured by the metrology apparatus after having traveled the first beam path and control the second controlled optical element in dependence of the reflected radiation measured by the metrology apparatus after having traveled the second beam path.

18. The radiation system of any of clauses 15 to 17, wherein the second pulsed laser source and the measurement laser source comprise a single laser source operable to emit both the second pulses of radiation and the measurement radiation.

19. The radiation system of clause 18, wherein the single laser source is operable to emit the measurement radiation to the source chamber at a power insufficient to condition the fuel droplet during the time that the radiation system is not generating the plasma.

20. The radiation system of any preceding clause, wherein the reflected measurement radiation comprises radiation reflected off a fuel droplet in the source chamber.

21. The radiation system of clause 20, operable to determine from radiation reflected from the fuel droplet, the position and/or velocity of the fuel droplet in a first dimension, the first dimension comprising the direction of travel of the droplet.

22. The radiation system of clause 21, operable to use the determined fuel droplet position in the first dimension for timing of the first pulses of radiation from the first pulsed laser source and/or pulses of radiation from a conditioning laser source, the conditioning laser source operable to condition the fuel droplet prior to it being excited by radiation from the first pulsed laser source.

23. The radiation system of clause 22, operable to determine from radiation reflected from the fuel droplet, the position of the fuel droplet in a second dimension, the second dimension being perpendicular to, and in the same plane as, the first dimension.

24. The radiation system of clause 23, operable to use the determined fuel droplet position in the second dimension for aiming of the first pulses of radiation from the first pulsed laser source and/or the pulses of radiation from the conditioning laser source.

25. The radiation system of clause 24, further comprising an acousto optical deflector configured to position the first pulses of radiation from the first pulsed laser source and/or the pulses of radiation from the conditioning laser source.

26. The radiation system of any of clauses 23 to 25, operable such that: the aiming and timing of pulses of radiation from the conditioning laser source used to condition a particular fuel droplet, and/or the aiming and timing of the first pulses of radiation from the first pulsed laser source used to excite a particular fuel droplet, is determined from the position and/or velocity in the first dimension and/or position in the second dimension of that same fuel droplet.

27. The radiation system of any of clauses 23 to 26, further comprising a position sensitive device configured to determine the position and/or velocity in the first dimension and/or position in the second dimension of that same fuel droplet.

28. The radiation system of clause 27, wherein the position sensitive device comprises a bicell diode device.

29. The radiation system of clause 23, wherein the measurement radiation comprises a beam having an elongate cross sectional profile in the second dimension to enable determination of the position of the fuel droplet in this dimension.

30. The radiation system of any of clauses 20 to 29, further comprising a fuel source operable to provide droplets of fuel to the source chamber and the system is operable to direct the first pulses of radiation so as to contact the fuel droplets to form a plasma.

31. The radiation system of any preceding clause, operable such that the measurement laser source continuously emits the measurement radiation and the metrology apparatus continuously measures the reflected radiation, including during times when no plasma is being generated.

32. The radiation system of any preceding clause, wherein the first pulsed laser source and the measurement laser source each comprise a CO2 laser source.

33. The radiation system of any of clauses 1 to 31, wherein the measurement laser source comprises a He-Ne laser source.

34. A radiation system comprising: a pulsed laser source operable to emit pulses of radiation; a beam transport system operable to transport the pulses of radiation towards a source chamber for generation of a plasma; a measurement laser source operable to emit measurement radiation towards the source chamber via the beam transport system; and a metrology apparatus operable to measure the measurement radiation prior to reflection in the source chamber and/or measure the reflected radiation subsequent to reflection in the source chamber.

35. The radiation system of clause 34, wherein the metrology apparatus is operable to measure measurement radiation emitted from the measurement laser source prior to reflection in the source chamber and to measure the reflected radiation subsequent to reflection in the source chamber.

36. The radiation system of clause 34 or clause 35, wherein the metrology apparatus is operable to measure a system stability parameter within the beam transport system and/or pulsed laser source.

37. The radiation system of clause 36, wherein the system stability parameter comprises one or more selected from: thermal lensing, global drift and/or dynamics.

38. The radiation system of any of clauses 34 to 37, wherein the pulsed laser source comprises a seed laser and a laser amplifier, and the measurement radiation is injected between the seed laser and laser amplifier such that the measurement radiation passes through the laser amplifier and the beam transport system.

39. The radiation system of clause 38, wherein the measurement radiation is amplified by the laser amplifier.

40. The radiation system of clause 38, wherein the measurement radiation has a wavelength which is not amplified or minimally amplified by the laser amplifier.

41. The radiation system of any of clauses 34 to 40, wherein the pulsed laser source and the measurement laser source each comprise a CO2 laser source.

42. The radiation system of any of clauses 34 to 40, wherein the measurement laser source comprises a He-Ne laser source.

43. The radiation system of any of clauses 34 to 42, operable to determine from radiation reflected from a fuel droplet in the source chamber, the position and/or velocity of the fuel droplet in a first dimension, the first dimension comprising the direction of travel of the droplet.

44. The radiation system of clause 43, operable to use the determined fuel droplet position in the first dimension for timing of the first pulses of radiation from the first pulsed laser source and/or pulses of radiation from a conditioning laser source, the conditioning laser source operable to condition the fuel droplet prior to it being excited by radiation from the first pulsed laser source.

45. The radiation system of clause 44, operable to determine from radiation reflected from the fuel droplet, the position of the fuel droplet in a second dimension, the second dimension being perpendicular to, and in the same plane as, the first dimension.

46. The radiation system of clause 45, operable to use the determined fuel droplet position in the second dimension for aiming of the first pulses of radiation from the first pulsed laser source and/or the pulses of radiation from the conditioning laser source.

47. The radiation system of clause 46, further comprising an acousto optical deflector for the positioning of the first pulses of radiation from the first pulsed laser source and/or the pulses of radiation from the conditioning laser source.

48. The radiation system of any of clauses 45 to 47, operable such that: the aiming and timing of pulses of radiation from the conditioning laser source used to condition a particular fuel droplet, and/or the aiming and timing of the first pulses of radiation from the first pulsed laser source used to excite a particular fuel droplet, is determined from the position and/or velocity in the first dimension and/or position in the second dimension of that same fuel droplet.

49. The radiation system of any of clauses 45 to 48, further comprising a position sensitive device configured to determine the position and/or velocity in the first dimension and/or position in the second dimension of that same fuel droplet.

50. The radiation system of clause 49, wherein the position sensitive device comprises a bicell diode device.

51. The radiation system of any of clauses 43 to 50, further comprising a fuel source operable to provide droplets of fuel to the source chamber and the system is operable to direct the first pulses of radiation so as to contact the fuel droplets to form a plasma.

52. The radiation system of any preceding clause, operable to provide high frequency radiation having a wavelength in the EUV range or smaller.

53. A method of generating a plasma, the method comprising: emitting first pulses of radiation towards a fuel droplet at a source chamber such that the first pulses of radiation excite the fuel droplet to generate the plasma; emitting measurement radiation at least during a time that no plasma is being generated, the measurement radiation being of insufficient power for the generation of plasma; and measuring reflected measurement radiation subsequent to the measurement radiation being reflected from a fuel droplet in the source chamber.

54. The method according to clause 53, further comprising controllably directing the first pulses of radiation to the source chamber.

55. The method according to clause 54, further comprising transporting the first pulses of radiation using a beam transport system.

56. The method according to clause 55, wherein the measurement radiation is injected at the output of the beam transport system.

57. The method according to clause 55, wherein the measurement radiation is injected at the input of the beam transport system such that the measurement radiation is transported by the beam transport system.

58. The method according to clause 55, wherein the first pulses of radiation are generated using a seed laser and a laser amplifier, and the measurement radiation is injected between the seed laser and laser amplifier such that the measurement radiation passes through the laser amplifier and the beam transport system.

59. The method according to clause 58, further comprising amplifying the measurement radiation using the laser amplifier.

60. The method according to clause 58, wherein the measurement radiation has a wavelength that is not amplified or minimally amplified by the laser amplifier.

61. The method according to any of clauses 55 to 60, further comprising measuring a system stability parameter within the beam transport system and/or the source of the first pulses of radiation.

62. The method according to clause 61, wherein the system stability parameter comprises one or more selected from: thermal lensing, global drift and/or dynamics.

63. The method according to clause 61 or clause 62, further comprising measuring the system stability parameter by measuring the measurement radiation prior to reflection in the source chamber and/or measuring the reflected measurement radiation subsequent to reflection in the source chamber.

64. The method according to any of clauses 61 to 63, further comprising measuring the system stability parameter by measuring the first pulses of radiation prior to their arrival in the source chamber and/or measuring reflected radiation subsequent to their reflection in the source chamber.

65. The method according to any of clauses 61 to 64, further comprising using the measured system stability parameter to make a system correction when generating the first pulses of radiation.

66. The method according to any of clauses 54 to 65, wherein the controllably directing of the first pulses of radiation to the source chamber is performed in dependence of the measured reflected radiation.

67. The method according to any of clauses 54 to 66, further comprising: delivering first pulses of radiation to the source chamber via a first beam path so as to excite fuel droplets; and delivering second pulses of radiation to the source chamber via a second beam path, so as to condition each fuel droplet prior to the excitation by the first pulses of radiation; measuring reflected measurement radiation that has traveled the first beam path following reflection in the source chamber; and measuring reflected measurement radiation that has traveled the second beam path following reflection in the source chamber.

68. The method according to clause 67, further comprising measuring reflected conditioning radiation that has traveled the first and second beam paths following conditioning of the droplet during a time that the plasma is being generated.

69. The method according to clause 67 or clause 68, further comprising controllably directing the first pulses of radiation to the source chamber in dependence of the measured reflected radiation that traveled the first beam path and controllably directing the second pulses of radiation to the source chamber in dependence of the measured reflected radiation that traveled the second beam path.

70. The method according to any of clauses 67 to 69, wherein the same laser source is used to emit the second pulses of radiation and to emit the measurement radiation.

71. The method according to clause 70, further comprising emitting measurement radiation to the source chamber at a power insufficient to condition the fuel droplet during the time that no plasma is being generated.

72. The method according to any of clauses 53 to 71, further comprising: determining from radiation reflected from the fuel droplet, the position and/or velocity of the fuel droplet in a first dimension, the first dimension comprising the direction of travel of the droplet; and using the determined fuel droplet position in the first dimension for timing of the first pulses of radiation and/or conditioning pulses of radiation, the conditioning pulses of radiation conditioning the fuel droplet prior to it being excited by the first pulses of radiation.

73. The method according to clause 72, further comprising: determining from radiation reflected from the fuel droplet, the position of the fuel droplet in a second dimension, the second dimension being perpendicular to, and in the same plane as, the first dimension; and using the determined fuel droplet position in the second dimension for aiming of the first pulses of radiation and/or conditioning pulses of radiation.

74. The method according to any of clauses 53 to 73, wherein the measurement radiation is emitted as a beam having an elongate cross sectional profile in a dimension that is perpendicular to, and in the same plane as, a direction of travel of the fuel droplet and further comprising determining the position of the fuel droplet in the dimension.

75. The method according to any of clauses 53 to 74, further comprising continuously emitting the measurement radiation and continuously measuring the reflected radiation, including at times when no plasma is being generated.

76. A method of generating a plasma, the method comprising: emitting pulses of radiation; using a beam transport system to transport the pulses of radiation towards a fuel droplet at a source chamber such that the pulses of radiation excite the fuel droplet to generate the plasma; emitting measurement radiation towards the source chamber via the beam transport system; and measuring the measurement radiation prior to reflection in the source chamber and/or measuring the reflected radiation subsequent to reflection in the source chamber.

77. The method according to clause 76, comprising measuring the measurement radiation prior to reflection in the source chamber and measuring the reflected radiation subsequent to reflection in the source chamber.

78. The method according to clause 76 or clause 77, further comprising measuring a system stability parameter within the beam transport system and/or the source of the first pulses of radiation.

79. The method according to clause 78, wherein the system stability parameter comprises one or more selected from: thermal lensing, global drift and/or dynamics.

80. The method according to any of clauses 76 to 79, further comprising using the measured system stability parameter to make a system correction when generating the first pulses of radiation.

81. The method according to any of clauses 76 to 80, wherein the first pulses of radiation are generated using a seed laser and a laser amplifier, and the measurement radiation is injected between the seed laser and laser amplifier such that the measurement radiation passes through the laser amplifier and the beam transport system.

82. The method according to clause 81, further comprising amplifying the measurement radiation using the laser amplifier.

83. The method according to clause 81, wherein the measurement radiation has a wavelength not amplified or minimally amplified by the laser amplifier.

84. The method according to any of clauses 76 to 83, further comprising: determining from radiation reflected from the fuel droplet, the position and/or velocity of the fuel droplet in a first dimension, the first dimension comprising the direction of travel of the droplet; and using the determined fuel droplet position in the first dimension for timing of the first pulses of radiation and/or conditioning pulses of radiation, the conditioning pulses of radiation conditioning the fuel droplet prior to it being excited by the first pulses of radiation.

85. The method according to clause 84, further comprising: determining from radiation reflected from the fuel droplet, the position of the fuel droplet in a second dimension, the second dimension being perpendicular to, and in the same plane as, the first dimension; and using the determined fuel droplet position in the second dimension for aiming of the first pulses of radiation and/or conditioning pulses of radiation.

86. The method according to any of clauses 53 to 85, wherein the plasma emits high frequency radiation having a wavelength in the EUV range or smaller.

Claims (1)

A lithography device comprising: an exposure device adapted to provide a radiation beam; a carrier configured to support a patterning device, which patterning device is capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.