NL2016358A

NL2016358A - A radiation system and method.

Info

Publication number: NL2016358A
Application number: NL2016358A
Authority: NL
Inventors: Theodorus Martinus Jilisen Reinier; Seroglazov Pavel; zhao Chuangxin; Tsigutkin Konstantin; Smeets Dries
Original assignee: Asml Netherlands Bv
Priority date: 2015-03-18
Filing date: 2016-03-03
Publication date: 2016-09-30
Also published as: WO2016146400A1

Abstract

A radiation system for generating a radiation emitting plasma comprises a fuel emitter configured to provide fuel to a plasma formation region, a laser arranged to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma, and an imaging device arranged to obtain at least one image of an imaging region, the image indicating at least one property of the fuel and/or the radiation emitting plasma and/or debris. The imaging region is located between the fuel emitter and the plasma formation region, or the imaging region is located between the plasma formation region and a further component of the radiation system. The system further comprises a controller configured to process the at least one image and to provide an instruction in dependence on said at least one property of the fuel and/or the radiation emitting plasma and/or debris.

Description

A radiation system and method

FIELD

[0001] The present invention relates to methods and systems for generating radiation. BACKGROUND

[0002] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0003] 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 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).

[0004] EUV radiation may be produced using a radiation source arranged to generate an EUV producing plasma. An EUV producing plasma may be generated, for example, by exciting a fuel within the radiation source. In addition to generation of plasma, exciting the fuel may also result in the unwanted generation of particulate debris from the fuel. For example, where a liquid metal, such as tin, is used as a fuel, some of the liquid metal fuel will be converted into an EUV producing plasma, but debris particles of the liquid metal fuel may be emitted at high speeds from the plasma formation region. The debris may be incident on other components within the radiation source, affecting the ability of the radiation source to generate an EUV producing plasma or to provide a beam of EUV radiation from the plasma to other components of the lithographic apparatus. The debris may also travel beyond the radiation source and become incident on other components of the lithographic apparatus.

[0005] The fuel may be in the form of a series of droplets of fuel that are provided by a fuel emitter to a plasma formation region. Errors in the generation of the fuel droplets, for example divergence from desired values of droplet, size, speed, trajectory or spacing can lead to more debris being produced, and less efficient generation of radiation.

SUMMARY

[0006] In a first independent aspect of the invention, there is provided a radiation system for generating a radiation emitting plasma, the radiation system comprising: a fuel emitter configured to provide fuel to a plasma formation region; a laser arranged to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma; and an imaging device arranged to obtain at least one image of an imaging region, the image indicating at least one property of the fuel and/or the radiation emitting plasma and/or debris. The imaging region is located between the fuel emitter and the plasma formation region, or the imaging region is located between the plasma formation region and a further component of the radiation system. The system further comprises a controller configured to process the image and to provide an instruction in dependence on said at least one property of the fuel and/or the radiation emitting plasma and/or debris.

[0007] The at least one property of the fuel and/or the radiation emitting plasma and/or debris may comprise at least one property of the fuel and/or the radiation emitting plasma and/or debris outside the plasma formation region.

[0008] The at least one property may comprise an amount and/or a direction of debris from generation of the radiation emitting plasma.

[0009] It may be that the imaging region does not include the plasma formation region.

[00010] The fuel may comprise a series of droplets of fuel emitted by the fuel emitter.

[00011] The imaging device may be arranged to obtain said at least one image of the fuel in the imaging region located between the fuel emitter and the plasma formation region.

[00012] The imaging device may be configured to obtain said at least one image of the fuel prior to plasma formation using said imaged fuel.

[00013] Said at least one image may comprise at least one image of at least one of a series of droplets of the fuel, and the at least one property may comprise at least one property of said at least one of the droplets of fuel.

[00014] The at least one property may comprise at least one of a size, shape, speed, position or trajectory for at least one of the series of droplets of fuel, and/or a repetition in spacing in time or distance, a repetition frequency, or spread in position or trajectory for a plurality of the series of droplets of fuel.

[00015] The at least one property comprises at least one of: the presence of absence of at least one satellite feature; the number, size or position of at least one satellite feature.

[00016] The imaging device may be arranged to obtain said at least one image of the fuel in the imaging region located between the plasma formation region and the further component of the radiation system.

[00017] The imaging device may be arranged to obtain said at least one image after the fuel and/or plasma and/or debris in the image has been subject to the plasma formation process.

[00018] The imaging device may be arranged to obtain at least one image of debris resulting from the plasma formation and/or spent or unused fuel remaining after the plasma formation.

[00019] The at least one property may comprise at least one of spread, scatter, velocity, quantity, or density of the debris and/or spent or unused fuel.

[00020] The imaging region may be located at or near a surface of the component of the system, and the imaging device may be arranged such that said at least one image comprises an image of fuel and/or debris at or near said surface. The at least one image may comprise an image of fuel and/or debris that has scattered, splashed or otherwise rebounded from, or accreted to, said surface, or that is passing towards said surface.

[00021] The component may comprise at least one of a contamination trap, an optical component, a radiation collector, a laser device.

[00022] The component may comprise a contamination trap, and the at least one instruction may comprise an instruction suitable for causing debris to be emitted substantially in a direction of the contamination trap.

[00023] The component may comprise a contamination trap, and the at least one instruction may comprise an instruction suitable for altering operation of the contamination trap to trap a greater portion of an emitted debris.

[00024] The at least one instruction may comprise a warning signal. A component of the system may be configured to issue a warning to a user, for example an audio and/or visual warning, in response to the warning signal.

[00025] The at least one instruction may comprise an instruction for the fuel emitter. The at least one instruction may comprise an instruction for causing the fuel emitter to modify a property of the fuel. The property may comprise at least one of size, shape, speed, position or trajectory for droplets of the fuel, and/or a repetition in spacing in time or distance, a repetition frequency, or spread in position or trajectory for a series of droplets of the fuel.

[00026] The at least one instruction may comprise an instruction for the fuel emitter to modify an operating parameter of the fuel emitter, and the operating parameter may comprise at least one of: a droplet formation parameter; a rise time, fall time, duration, frequency, amplitude, or gradient of a periodic drive signal for droplet formation, or of a component of such a periodic drive signal; a fuel pressure; a fuel temperature.

[00027] The at least one instruction may comprise an instruction suitable for modifying a laser property of the laser beam. The laser property of the laser beam may comprise at least one of a repetition rate, power, intensity profile, direction of propagation and position of the first laser beam.

[00028] The controller may be configured to process a series of the images obtained at different times and iteratively to provide a series of instructions in response to the series of images, thereby to implement a control loop to control at least one property of the fuel and/or the radiation emitting plasma and/or the debris.

[00029] The system may further comprise a further imaging device arranged to obtain a further image of the imaging region. The controller may arranged to receive the further image, and determine the at least one property of the fuel and/or the radiation emitting plasma and/or the debris from the image and the further image.

[00030] The imaging device may be arranged to obtain images in a first plane and the further imaging device may be arranged obtain images in a second plane substantially orthogonal to the first plane.

[00031] The imaging device may be arranged to obtain images in a plane substantially parallel to a direction of propagation of the laser beam and at 45 or 225 degrees with respect to a direction of propagation of the fuel, and the further imaging device may be arranged to obtain images in a plane substantially parallel to a direction of propagation of the laser beam and at -45 or -225 degrees with respect to the direction of propagation of the fuel.

[00032] The instruction may be suitable for minimizing a quantity of debris generated by generation of the radiation emitting plasma.

[00033] The system may further comprise a focusing assembly having at least one movable optical component, wherein the instruction is suitable for causing movement of the at least one movable optical component.

[00034] The system may further comprise an illumination source arranged to provide first illumination radiation to illuminate the imaging region when the imaging device obtains a first image; wherein the imaging device may be arranged to obtain a second image at a predetermined time after obtaining the first image and the illumination source may be arranged to provide second illumination radiation when the imaging device obtains the second image; wherein the controller may be arranged to process the first and second images to determine at least one of size, speed and direction of at least one particle; and wherein generating said at least one instruction may be based upon said determined at least one of size, speed and direction of said at least one particle.

[00035] The illumination source may comprise a laser arranged to emit an illumination laser beam pulse and conditioning optics arranged to condition the laser beam pulse to provide the first and second illumination radiation.

[00036] The conditioning optics may be arranged to flatten said first and second illumination radiation to provide substantially planar radiation.

[00037] The conditioning optics may be arranged to rotate said first and second radiation through a plurality of planes.

[00038] The conditioning optics may comprise a single rotatable cylindrical lens.

[00039] The conditioning optics may comprise a plurality of rotatable cylindrical lenses.

[00040] The illumination source may be arranged such that the first and second illumination radiation each comprise a volume of illumination.

[00041] The predetermined time between obtaining the first and second images may be less than or equal to approximately 10 ms.

[00042] The controller may be configured to determine a size of the particle emitted from the radiation generated plasma by determining from the first and/or second image a property of photons scattered by the at least one particle.

[00043] The controller may be configured to determine a size of said at least one particle by processing said determined property of photons using the Mie solution for the scattering of electromagnetic radiation by a sphere.

[00044] Determining at least one of a distance and a speed of said at least one particle may comprise cross-correlating the first and second images.

[00045] Determining at least one of a distance and a speed may comprise processing the first and second image using velocimetry techniques to determine a velocity of said particle.

[00046] The imaging device may be arranged to obtain the at least one image using illumination radiation that is generated by an interaction of the laser beam and the fuel. The illumination radiation may be emitted by the radiation emitting plasma. The illumination radiation may be emitted from the plasma formation region. The radiation emitting plasma may provide illumination radiation to illuminate the imaging region when the imaging device obtains the at least one image.

[00047] The system may further comprise a pre-pulse laser arranged to provide a prepulse laser beam. The imaging device may be arranged to obtain the at least one image using illumination radiation that is generated by an interaction of the pre-pulse laser beam and the fuel.

[00048] The imaging device may be arranged to obtain at least one image of debris resulting from the plasma formation and/or spent or unused fuel remaining after the plasma formation.

[00049] The generating of the radiation emitting plasma may comprise generating a first plasma flash from a first droplet of the fuel and generating a second plasma flash from a second droplet of the fuel. The imaging device may be arranged to obtain at least one image of debris resulting from the first plasma flash and/or spent or unused fuel remaining after the first plasma flash, using illumination radiation generated by an interaction of the laser beam with the second droplet of the fuel. The imaging device may be arranged to obtain at least one image of debris resulting from the first plasma flash and/or spent or unused fuel remaining after the first plasma flash, using illumination radiation generated by an interaction of the pre-pulse laser beam with the second droplet of the fuel.

[00050] A frame duration of the imaging device may be such as to capture illumination radiation from a plurality of plasma flashes.

[00051] The illumination radiation emitted from the plasma formation region may comprise at least one of visible light and ultraviolet light.

[00052] The imaging device may comprise a shadography camera. The shadography camera may be provided with an optical filter. The triggering of the shadography camera may be adjusted.

[00053] The light emitted by the radiation emitting plasma may be visible or ultraviolet light.

[00054] In a further, independent aspect of the invention there is provided a radiation system for generating a radiation emitting plasma, the system comprising a fuel emitter configured to provide fuel to a plasma formation region; a laser arranged to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma; and an imaging device arranged to obtain at least one image of an imaging region, the image indicating at least one property of the fuel and/or the radiation emitting plasma and/or debris; wherein the imaging device is arranged to obtain the at least one image using illumination radiation generated by an interaction of the laser beam and the fuel and/or illumination radiation generated by an interaction of a pre-pulse laser beam and the fuel; and the system further comprises a controller configured to process the at least one image and to provide an instruction in dependence on said at least one property of the fuel and/or the radiation emitting plasma and/or debris.

[00055] In a further, independent aspect of the invention there is provided a method of generating a radiation emitting plasma in a radiation system comprising: a fuel emitter configured to provide a fuel target at a plasma formation region; a laser arranged to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma; an imaging device arranged to obtain at least one image of an imaging region, the image indicating at least one property of the fuel, and/or the radiation emitting plasma and/or debris, wherein the imaging region is located between the fuel emitter and the plasma formation region, or the imaging region is located between the plasma formation region and a further component of the system; and a controller; the method comprising at the controller: processing said at least one image; and providing at least one instruction based on said at least one property of the fuel, and/or the radiation emitting plasma and/or debris.

[00056] In another aspect of the invention there is provided a lithographic system comprising a radiation system as claimed and/or described herein.

[00057] In a further, independent aspect of the invention there is provided a radiation source for generating a radiation emitting plasma, the radiation source being arranged to receive a laser beam at a plasma formation region and comprising: a fuel emitter configured to provide fuel to a plasma formation region; and an imaging device arranged to obtain at least one image of an imaging region, the image indicating at least one property of the fuel and/or the radiation emitting plasma and/or debris, wherein the imaging region is located between the fuel emitter and the plasma formation region, or the imaging region is located between the plasma formation region and a further component of the radiation system; and the source further comprises a controller configured to process the image and to provide an instruction in dependence on said at least one property of the fuel and/or the radiation emitting plasma and/or debris.

[00058] In another, independent aspect of the invention there is provided a non-transitory computer readable medium carrying computer readable instructions suitable to cause a computer to: process at least one image of an imaging region, the image indicating at least one property of fuel, and/or radiation emitting plasma and/or debris, wherein the imaging region is located between a fuel emitter and a plasma formation region, or the imaging region is located between the plasma formation region and a further component of a radiation system; and provide at least one instruction based on said at least one property of the fuel, and/or the radiation emitting plasma and/or debris.

BRIEF DESCRIPTION OF THE DRAWINGS

[00059] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

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

Figure 2A schematically depicts a radiation source according to an embodiment of the invention;

Figure 2B schematically depicts an example radiation source according to another embodiment of the invention

Figure 3A depicts an image of fuel processed by a controller of Figure 2A;

Figure 3B depicts an image of fuel and/or debris processed by a controller of Figure 2B;

Figure 4 schematically depicts an alternative radiation source according to an embodiment of the invention;

Figure 5 schematically depicts an alternative radiation source according to an embodiment of the invention;

Figure 6 schematically depicts an alternative radiation source according to an embodiment of the invention;

Figure 7 schematically depicts an alternative radiation source according to an embodiment of the invention; and

Figure 8 schematically depicts an imaging system of the radiation source of Figure 7;

Figure 9 schematically depicts an alternative radiation source according to an embodiment of the invention;

Figure 10 is an example of processed particle data obtained by a Particle Image Velocimetry (PIV) technique;

Figure 11 is a velocity distribution plot of the processed data of Figure 11;

Figure 12 is a size distribution plot of the processed data of Figure 11;

Figure 13 is a schematic view of generated particles, flying from the primary focus and illuminated by a plasma flash;

Figure 14 is a plot showing a calculated dependence of camera signal on droplet size.

DETAILED DESCRIPTION

[00060] Figure 1 shows a lithographic system including a radiation source SO 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.

[00061] 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.

[00062] The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the radiation beam B with a desired cross-sectional shape and a desired angular 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 10 and faceted pupil mirror device 11.

[00063] 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 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 in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[00064] An example of the radiation source SO is shown in Figure 2A. The radiation source SO shown in Figure 2A is of a type which may be referred to as a laser produced plasma (LPP) source). A laser 1, which may for example be a C02 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 3. The laser may be, or may operate in a fashion of, a pulsed, continuous wave or quasi-continuous wave laser. The trajectory of fuel emitted from the fuel emitter is parallel to an x-axis marked on Figure 3. The laser beam 2 propagates in a direction parallel to a y-axis, which is perpendicular to the x-axis. A z-axis is perpendicular to both the x-axis and the z-axis and extends generally into (or out of) the plane of the page.

[00065] Although a tin fuel is described in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, shown in the form of droplets 70, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma. The fuel droplets may be emitted at any desired frequency. In the embodiment of Figure 2A the droplets are emitted at a frequency of around 50 KHz, giving an inter-droplet spacing of around 0.02 ms.

[00066] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[00067] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser 1 and the radiation source SO may together be considered to be a radiation system.

[00068] Radiation that is reflected by the collector 5 forms the radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 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 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source.

[00069] The radiation source SO (or radiation system) further comprises an imaging device in the form of a camera 10 arranged to obtain images of the series of droplets 70 in an imaging region between the fuel emitter 3 and the plasma formation region 4. The camera 10 may comprise a CCD array or a CMOS sensor, but it will be appreciated that any imaging device suitable for obtaining images of the droplets 70 may be used. It will be appreciated that the camera 10 may comprise optical components in addition to a photodetector. The optical components may be selected so that the camera 10 obtains near-field images and/or far-field images. The camera 10 may be positioned within the radiation source SO at any appropriate location from which the camera has a line of sight to the path of the droplets 70. It may be necessary, however, to position the camera 10 away from the propagation path of both the laser beam 2 and the fuel emitted from the fuel emitter 3 so as to avoid damage to the camera 10. The camera 10 is arranged to provide images of the droplets 70 to a controller 11 via a connection 12. The connection 12 is shown as a wired connection, though it will be appreciated that the connection 12 (and other data connections referred to herein) may be implemented as either wired or wireless connections.

[00070] The controller 11 is configured to process the received images of the droplets 70 to automatically determine at least one parameter indicating a property of one or more of the droplets 70. Figure 3 shows a representation of an image 72 of a series of the droplets 70 in an x-y plane (axes are illustrated in Figure 2A for reference) that may be processed by the controller 11. In this figure, the x direction is parallel to a straight line between the fuel emitter 3 and the plasma formation region 7. It will be appreciated that the camera 10 may be arranged to image the droplets 70 in planes other than the x-y plane.

[00071] The controller 11 is configured to process the image 72 to determine any desired parameter relating to the fuel droplets. For instance, any parameter that may have an effect on the plasma formation and/or the radiation generated by the plasma, for example the frequency, amplitude or direction of emission Desired parameters may include one or more of a size, shape, speed, position or trajectory for at least one of the series of droplets of fuel, and/or a repetition in spacing in time or distance, a repetition frequency, or spread in position or trajectory for a plurality of the series of droplets of fuel. Any one of those parameters can affect the plasma formation and consequently generation of radiation. Automated imaging and monitoring of the parameters by the controller 11 can be used to determine whether there are droplet train instabilities or loss of droplet train (which can mean spraying by the fuel emitter). In one mode of operation, the controller 11 processes the images to determine interspacing between droplets and a pixel count of pixels representing material outside the droplet trajectory, which can indicate the presence of satellites.

[00072] In the case of the image 72 of Figure 3, a dotted line 15 joining the fuel emitter 3 and the intended location of the plasma formation region is shown overlaid on the figure for reference. It can be seen that in this case the series of droplets 70 is following a trajectory that does not align well with the line 15, and thus the droplets will not align well with the desired location of the plasma formation region. That may cause the series of fuel droplets 3 to be not optimally synchronised with the laser pulses used to form the plasma, it may mean that conversion of fuel to plasma may not maximised, and/or it may mean that the location of the plasma formation is not precisely in the desired region, each of which can have an effect on the generated radiation and/or the generation or properties of debris. It should be noted that Figure 3 is not to scale, and the divergence of the fuel droplets 70 from the line 15 is exaggerated for clarity. In practice, the plasma formation and radiation generation can be highly sensitive to even small changes in properties of the fuel droplets and their trajectory.

[00073] In addition to the divergence of the fuel droplets 70 from their desired path 15, it can be seen in Figure 3 that smaller amounts of fuel 70a, for example in the form of droplets, have been emitted by the fuel emitter 3 in addition to the main droplets 70. These smaller amounts of fuel can be referred to as satellites 70a. The presence or absence of satellites is another property that is determined from the image 72 in some embodiments. Satellites 70a are generally undesirable and may be considered as debris. The satellites 70a may, for example, be located relative to the series of droplets such that they are not converted to plasma and/or they may follow trajectories that ultimately cause them to collide with surfaces of the apparatus, which may ultimately cause damage or malfunction. The presence of satellites 70a may also indicate that the main droplets 70 are smaller than their desired size, if fuel intended to be part of the droplets 70 is instead being emitted as satellites (the size of droplets 70 is another parameter that may be determined directly from the image 72).

[00074] Generally, it will be appreciated that the controller 11 may be implemented in any appropriate way. For example, the controller 11 comprise one or more digital processors and may be implemented as an FGPA, ASIC or a suitably programmed general purpose computer. Further, processing of the images at the controller 11 may be performed in any appropriate way using any image processing techniques as will be readily apparent to those skilled in the art. For example, image processing techniques such as edge detection, and/or pattern or shape matching may be used to identify droplets, and/or satellites or other debris, and detect a size, shape or location of the droplets 70, satellites 70a or other debris, while image smoothing techniques may be used to reduce noise. Thresholding and/or various morphological operations, for example fill, open or join operations, may be used, for example in determining the presence or absence of satellites. For example, only image features greater than a certain threshold minimum size may be considered and/or adjacent features may be joined or separated, dependent on their size and/or separation, in preliminary image processing stages before determination of a property or properties from the images.

[00075] More than one image may be compared or otherwise processed in combination in order to determine one or more properties. For example, velocities of droplets or other features may be determined from comparisons of the positions of droplets or other features in two or more images obtained at closely spaced times.

[00076] The determined property or properties are used to generate instructions to be provided to components of the radiation system (e.g. the fuel emitter 3, or the radiation source SO and the laser 1). Instructions may then be provided to one or more components of the radiation system in dependence upon the determined properties. For example, the image properties may be used to determine any divergence of properties of the fuel droplets emitted by the fuel emitter 3 from their desired values, or amount, direction and/or quality of debris (such as size of particles, distribution of particles, etc) emanating from the plasma formation region 4 and/or fuel emitter 3.

[00077] The instructions generated by the controller 11 based on the determined image properties and provided to one or more components of the radiation system, may be instructions chosen to adjust those components, or adjust operation of those components, so as to improve plasma formation and/or radiation generation, and/or to reduce one or more detrimental effects of debris. Detrimental effects may include, for example, incidence of debris on mechanical, electrical or optically active components of either the radiation source SO (such as lenses, mirrors, windows etc), or components of an apparatus “downstream” of the radiation source SO.

[00078] In the embodiment of Figure 2A, the controller 11 is connected to the fuel emitter 3 via a connection 14. The controller 11 may be configured to issue one or more instructions to the fuel emitter 3 in order to alter properties of the emitted fuel 70, such as shape, speed, size, trajectory, repetition in spacing in time or distance, repetition frequency, spread etc. The one or more instructions may comprise an instruction to the fuel emitter 3 to halt emission of fuel droplets, in case of significant droplet train instability or other problem indicated by the measured parameter or parameters.

[00079] The fuel emitter 3 and hence the nozzle of the fuel emitter (not shown) may be moveable relative to the other components of the radiation source SO (and in particular relative to the radiation collector CO) by at least one actuator (not shown) mechanically linked to the fuel emitter 3. The fuel emitter 3 may, for example, be moveable by the at least one actuator within the y-z plane in response to instructions received from the controller 11. However, it will be appreciated that in other embodiments of the invention, the fuel emitter 3 may additionally or alternatively be moveable in a direction parallel to the x-axis. Furthermore, in other embodiments of the invention, the fuel emitter 3 may be tilted relative to the x-axis. Further adjustments to fuel provided by the fuel emitter 3 may be made by adjustments to a nozzle (not shown) of the fuel emitter 3, such as expansion, constriction, or change of shape of the nozzle. Indeed, it will be appreciated that any suitable properties of the fuel emitter 3 may be adjusted as appropriate to obtain a desired property of the fuel droplets 70, and/or the plasma 7, and/or the generated radiation, and/or to reduce the presence or effects of debris.

[00080] In the embodiment of Figure 2A, the fuel emitter 3 produces a high pressured liquid metal (e.g. tin) jet and breaks the jet up into the desired series of droplets 70 of a desired size by periodic operation of a piezoelectric actuator. The periodic operation of the piezoelectric actuator may be driven by a drive signal, for example a square wave drive signal. In one mode of operation of the embodiment, the instruction provided by the controller 11 is an instruction to modify an operating parameter of the fuel emitter, and the operating parameter comprises a droplet formation parameter, for example at least one of a rise time, fall time, up time, duration, frequency, amplitude, or gradient of the periodic drive signal for droplet formation, or of a component of the periodic drive signal.

[00081] In other embodiments, or other modes of operation of the embodiment of Figure 2A, the instruction may be an instruction to alter another operating parameter of the fuel emitter 3, for example a pressure or temperature of the fuel used to form the fuel droplets, and/or to cease fuel emission.

[00082] The instruction is not limited to being an instruction for the fuel emitter 3, and in other embodiments or modes of operation the instruction may be an instruction for the laser 1 or other component of the source, or for the lithographic apparatus. For example, if the series of fuel droplets are such that the plasma formation and/or resulting radiation are likely to be significantly affected it may be desired to ensure that the radiation is not used for a lithographic operation on the substrate until issues with the fuel droplets are resolved. Thus, for instance the instruction may be an instruction such as to cease plasma formation or to redirect and/or block generated radiation. Alternatively or additionally, the instruction may be an instruction to log data, for example error data. In response to such logged data further procedures may be performed, for example procedures to check quality of the resulting lithographic structures on a substrate formed using radiation generated using the fuel.

[00083] Upon adjusting a property of the fuel droplets 3 by adjusting an operating parameter of the fuel emitter 3, or other component, the effect of that adjustment is determined based on at least one further image obtained by the camera 10, and provided to the controller 11 which may make additional adjustments on the basis thereof. The controller 11 therefore establishes a control loop in which properties of the fuel droplets 70 may be iteratively controlled in response to feedback indicating changing conditions of the fuel droplets 70 from the camera 10.

[00084] A radiation source according to an alternative embodiment is shown in Figure 2B. The components and arrangement of the embodiment of Figure 2B are the same as those of Figure 2A, except that in this case the camera 10 is arranged to obtain images in an imaging region between the plasma formation region 4 and a further component of the system, for example near a surface of the further component. In the embodiment shown in Figure 2B, the imaging region includes a surface of the collector 4. The timing of operation of the camera 10 may be synchronised with timing of operation of the laser 1 and/or with timing of operation of the fuel emitter 3, optionally with an offset time, to increase the probability that the an image obtained by camera 10 will include debris arising from a particular fuel droplet and/or plasma formation event.

[00085] Figure 3B shows an image obtained by the camera. The image includes a side-view of the surface of the collector 5. The dashed line is a straight line that is included in Figure 3B for reference and that follows part of a straight line path that connects the plasma formation region 7, the fuel emitter 3 and a point on the surface of the collector 5. In this case the dashed line represents part of a path that fuel droplets would follow if they passed from the fuel emitter 3 to the collector 5 without interference for example without being blocked or diverted or without plasma formation taking place.

[00086] The image of Figure 3B includes variously-sized circles that schematically represent spent or unused fuel and/or other debris. In the image shown, the circles are representative of a mixture of unused fuel (tin in this example) and fuel that has been used to form a plasma. In an ideal mode of operation, the unused or spent fuel would be intercepted by a trap, for example a contamination trap, or other component and/or its path would be such that it did not contact the collector 5. Flowever, in the image of Figure 3B it is clear that unused fuel and/or spent fuel and/or other debris has impacted the collector 5 and rebounded. Such undesired impacts and passage of fuel and/or debris along undesired trajectories can cause significant damage to the apparatus and ultimately may require shutdown and cleaning or replacement of parts.

[00087] In the embodiment of Figure 2B, the controller 11 is configured to process the image of Figure 3B to determine any desired parameter relating to the image, for example in similar fashion to that described in relation to Figure 2A. In the case of imaging following plasma formation, such as imaging of a region close to a surface of a component as shown in Figure 3B, suitable parameters can include, but are not limited to, for example spread, scatter, velocity, quantity, or density of spent or unused fuel or other debris. The material may be in-flight or accreted to a surface. In the example of Figure 3B the controller 11 may determine that the presence of any significant amount of unexpected material close to the collector 5 is unacceptable and may automatically issue an instruction in response to the image of Figure 3B.

[00088] In Figure 2B, the controller 11 is shown to be connected to the laser 1 by a connection 13. The controller 11 may therefore provide instructions to the laser 1 over the connection 13 in order to adjust a laser property of the laser beam 2 in response to image properties. By controlling the laser 1 to adjust the laser beam 2, interaction between the laser beam 2 and the fuel target may be changed. For example, a direction and/or angle at which the laser beam 2 is incident on the fuel target may be adjusted. In this way, for example, the laser beam 2 may strike the fuel target at a different location on the surface of the fuel target, or at a different angle. Further examples of laser properties of the laser beam 2 which may be controlled include changes to a total power of the laser beam 2, changes to an intensity distribution in the laser beam 2 (particularly at the plasma formation region 4), and a size/shape of the laser beam 2 at the plasma formation region 4. Where the laser 1 is a pulsed laser such that the laser beam 2 is a laser pulse, the laser 1 may be controlled to vary the pulse repetition rate, the pulse length and the intensity profile of the laser pulse over time (pulse shape). Other modifications to the laser beam 2 will, however, be readily apparent to the skilled person based on the teaching herein.

[00089] By controlling the interaction between the laser beam 2 and the fuel target, properties of the generated plasma may thereby be altered, and consequently, properties of the debris are also altered. For example, the adjustments to the laser beam 2 described above may be used to increase a portion of the fuel target that is within the beam waist of the laser beam 2, thereby increasing the portion of the fuel target that is converted into the plasma 7 and reducing a portion of the fuel target that emanates as debris.

[00090] The instruction in the embodiment of Figure 2B is not limited to being an instruction for the laser 1, and may comprise a warning signal and/or may comprise an instruction for the fuel emitter 3 or other component of the source. For example, the instruction may comprise an instruction to halt or alter fuel production, for instance if a significant amount of debris is being produced. Alternatively or additionally, the instruction may be an instruction for the lithographic apparatus, for example an instruction to halt or alter a lithographic process, or an instruction that radiation production by the source has been or is about to be altered or halted.

[00091] While a plurality of examples are described herein, it will be understood from the teaching herein that detrimental effects of debris may be reduced in any of a plurality of ways and that the invention is not limited to reduction by any particular method. For example, reducing detrimental effects may comprise reducing an amount of debris emitted, altering a direction of emitted debris or altering another quality of the emitted debris, such as particle size or particle distribution. By altering a direction of the debris, for example, a portion of the emitted debris propagating in a direction of debris mitigation devices (not shown in Figure 2A) may be increased. Similarly, debris particle sizes and/or distributions may be controlled so as to remain substantially within a range in which employed debris mitigation mechanisms are most effective.

[00092] Figure 4 schematically illustrates a radiation system including a laser produced plasma (LPP) radiation source SO according to another embodiment, which has an alternative configuration to the radiation source shown in Figure 2A. Where components of the radiation source SO of Figure 4 have equivalent components in the radiation source SO of Figure 2A, like reference numerals have been used. The radiation source SO of Figure 4 includes a fuel emitter 3 which is configured to deliver fuel to a plasma formation region 4. As described above, the fuel may be provided in the form of tin droplets, but fuel of any suitable material or form may be used. A pre-pulse laser 16 emits a pre-pulse laser beam 17 which is incident upon the fuel. The pre-pulse laser beam 17 acts to preheat the fuel, thereby changing a property of the fuel such as its size, shape and/or trajectory. A main laser 18 emits a main laser beam 19 which is incident upon the fuel after the pre-pulse laser beam 17. The main laser beam 18 delivers energy to the fuel and thereby coverts the fuel into an EUV radiation emitting plasma 7.

[00093] A radiation collector 20, which may be a so-called grazing incidence collector, is configured to collect the EUV radiation and focus the EUV radiation at a point 6 which may be referred to as the intermediate focus. Thus, an image of the radiation emitting plasma 7 is formed at the intermediate focus 6. An enclosure structure 21 of the radiation source SO includes an opening 22 which is at or near to the intermediate focus 6. The EUV radiation passes through the opening 22 to an illumination system of a lithographic apparatus (e.g. of the form shown schematically in Figure 1).

[00094] The radiation collector 20 may be a nested collector, with a plurality of grazing incidence reflectors 23, 24 and 25 (e.g. as schematically depicted). The grazing incidence reflectors 23, 24 and 25 may be disposed axially symmetrically around an optical axis O. The illustrated radiation collector 20 is shown merely as an example, and other radiation collectors may be used.

[00095] A contamination trap 26 is located between the plasma formation region 4 and the radiation collector 20. The contamination trap 26 may, for example, be a rotating foil trap, or may be any other suitable form of contamination trap. In some embodiments the contamination trap 26 may be omitted.

[00096] An enclosure 21 of the radiation source SO includes a window 27 through which the pre-pulse laser beam 17 can pass to the plasma formation region 4, and a window 28 through which the main laser beam 19 can pass to the plasma formation region. A mirror 29 is used to direct the main laser beam 19 through an opening in the contamination trap 26 to the plasma formation region 4.

[00097] As in the embodiment of Figure 2A, the radiation source SO of Figure 4 further comprises a camera 10. The camera 10 is shown as imaging an imaging region in between the fuel emitter and the plasma formation region. In variants of the embodiment the camera 10 may be arranged to image any desired imaging region, for example, any desired imaging region between the plasma formation region and a further component of the system. The camera 10 is arranged to transmit images to a controller 11 over a connection 12. The controller 11 is configured to process the received images to automatically determine one or more image properties and to provide instructions to one or more of the components of the radiation system. In particular, the controller 11 is connected to the main laser 18 and the fuel emitter 3 such that instructions may be provided to the main laser 18 and the fuel emitter 3 as described above with reference to the laser 1 and fuel emitter 3 of Figure 2A.

[00098] It will be appreciated that the controller 11 may provide instructions to any suitable components of the radiation source SO in response to the images received from the camera 10. In Figure 4, for example, the controller 11 is connected to the pre-pulse laser 16 via a connection 30 and to the contamination trap 26 via a connection 31. In this way, for example, operation of the pre-pulse laser 16 can be controlled to achieve a desired change in the fuel before the firing of the main laser 18. In this way, properties of the generated plasma 7, and therefore debris emitted by the plasma 7, may be adjusted. Similarly, the controller 11 may provide instructions to the contamination trap 26. For example, where the contamination trap 26 comprises a rotating foil trap comprising a plurality of vanes, instructions may be provided to adjust a speed of rotation and/or an angle of vanes within the rotating foil trap. In this way, the contamination trap 26 may be adjusted as part of the control loop operated by the controller 11 to reduce detrimental effects of debris.

[00099] Figure 5 schematically illustrates a further example of a radiation system including a radiation source SO. The radiation system of Figure 5 is arranged similarly to the radiation source SO of Figure 2A and like components are provide with like reference numerals. In particular, a laser 1 is arranged to deposit energy via a laser beam 2 into a fuel, which is provided from a fuel emitter 3. The laser beam 2 is incident upon the fuel at a plasma formation region 4. The deposition of laser energy into the fuel creates a plasma 7 at the plasma formation region 4.

[000100] In the radiation source SO of Figure 5, components of a focusing assembly, between the laser 1 and the plasma formation region 4, are schematically illustrated. In particular, two fixed reflective elements 40, 41 and a moveable reflective element 42 collectively direct and focus the laser beam 2 towards plasma formation region 4. It will be appreciated that while the reflector elements 40, 41 are fixed in the embodiment of Figure 5, the reflector elements 40, 41 may also be moveable. Indeed, it is to be understood that any appropriate number of fixed reflector elements and/or movable reflector elements may be used to direct and focus the laser beam 2 towards the plasma formation region 4. Furthermore, in other embodiments of the invention, any appropriate focussing element(s) (i.e., other than reflector elements) may be used to focus laser beam 2.

[000101] The moveable reflector element 43 forms part of a radiation directing device. The reflector element 43 of the radiation directing device is located in the path of the laser beam 2. The radiation directing device also comprises at least one reflector actuator that is mechanically linked to the reflector element 43. In this case, the radiation directing device comprises two reflector actuators 44, 45 which are mechanically linked to the reflector 43. Movement of at least one of the reflector actuators 44, 45 changes the orientation and/or position of the reflector 43 relative to the path of the laser beam 2. In this way, each reflector actuator 44, 45 can be actuated in order to adjust the orientation and/or position of the reflector 43 relative to the laser beam 2 so as to alter the focus position of the laser beam 2.

[000102] It will be appreciated that although two reflector actuators 44, 45 are shown in Figure 4, in other embodiments there may be any appropriate number of reflector actuators. Furthermore, it will be appreciated that the actuators may alter any appropriate property of the reflector that will alter the focus position of the radiation beam. For example, the actuator may change the shape of the reflector. Although the radiation directing device of the present embodiment comprises a reflector 43, in other embodiments the radiation directing device may comprise any appropriate directing element that is capable of altering the focus position of the laser beam 2. For example, the radiation directing device may comprise a plurality of lens elements, the properties of each lens element being adjustable.

[000103] As in the embodiments schematically illustrated in Figures 2 and 4, in the embodiment of Figure 5, a camera 10 is arranged to obtain images of an imaging region. The camera 10 is shown as imaging an imaging region in between the fuel emitter and the plasma formation region. In variants of the embodiment the camera 10 may be arranged to image any desired imaging region, for example, any desired imaging region between the plasma formation region and a further component of the system. The camera 10 is connected to a controller 11 via a connection 12. The controller 11 is configured to process images received from the camera 10 to determine image properties. The controller 11 uses the determined image properties to generate instructions to components of the radiation system. In particular, the controller 11 is connected to the laser 1 via a connection 13 and to the fuel emitter 3 by a connection 14. In the embodiment of Figure 5, the controller 11 is further connected the actuators 44, 45 via a connection 46. In this way, the controller 11 can transmit instructions to the actuators 44, 45 in order to adjust the propagation of the laser beam 2 in response to feedback indicating changing image properties from the camera 10.

[000104] It is to be understood that the arrangements schematically illustrated in Figures 2, 5 and 5 are merely exemplary and that features illustrated in one of Figures 2, 4 or 5 may be combined with features illustrated in another of Figures 2, 4 and 5. For example, the embodiment of Figure 4 may utilise a near normal incidence collector in place of the grazing incidence collector 20. Similarly, the embodiments of Figures 2 and 5 may comprise contamination traps such as the contamination trap 26 illustrated in Figure 4. Furthermore, each of the radiation sources SO shown in Figures 2, 4 and 5 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source SO. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[000105] Figure 6 schematically illustrates a further example of a radiation source SO according to an embodiment of the present invention in which two cameras are utilised to image the plasma 7. For clarity, many components of the radiation source SO have been omitted from the schematic illustration of Figure 6. It is to be understood that non-depicted features of the radiation source SO (and the radiation system of which it is a part), such as one or more lasers, a fuel emitter and components of a focussing assembly may be implemented in any appropriate way. For example, the non-depicted components of the radiation source SO of Figure 6 may be arranged according to one or a combination of the examples schematically illustrated in Figures 2, 4 or 5.

[000106] In Figure 6, a first camera 10 and a second camera 50 are provided within the radiation source SO to obtain images of an imaging region 52. The first camera 10 is arranged to obtain images of the imaging region 52 in a first plane, while the second camera 50 is arranged to obtain images of the imaging region 52 in a second plane. The second plane may be substantially orthogonal to the first plane. Example axis are shown on Figure 6, from which it can be seen that the first camera 10 is arranged to obtain images of the imaging region 52 in an x-y plane, while the second camera 50 is arranged to obtain images of the imaging region 52 in a x-z plane.

[000107] The first camera 10 is connected to a controller 11 via a connection 13 while the second camera 50 is connected to the controller 11 via a connection 51. Both the first camera 10 and the second camera 50 are arranged to transmit images to the controller 11.

The controller 11 is configured to calculate one or more image properties based on the images received from each of the first camera 10 and the second camera 50. By providing images in two planes, it is possible to determine a more accurate indication of image properties, and as a result more accurate indications of image properties of, for example, debris and/or fuel. For example, by providing images in two substantially orthogonal planes, a direction of the debris and/or fuel, in three spatial dimensions, may be determined.

[000108] The controller 11 is configured to provide instructions to one or more other components (not shown in Figure 6) of the radiation system in order to mitigate deleterious effects of debris.

[000109] In the embodiments described above, the controller 11 is a digital controller. It is to be understood, however, that the imaging device(s) and/or the controller may be implemented as analogue components. For example, the imaging device(s) may comprise an analogue segmented photo-detector (which may be segmented, for example, in a grid and/or concentric-circular fashion). Each segment of the segmented photo-detector may provide a respective analogue signal to the controller. In one embodiment, for example, the imaging device may be implemented as a quad-cell photo-detector.

[000110] The controller may comprise an analogue signal processor arranged to process analogue signals received from the imaging device(s). In this case, the instructions generated by the controller may take the form of analogue control signals suitable for controlling one or more components. It will be appreciated, therefore, that embodiments may comprise an entirely analogue control loop for reducing a detrimental effect of debris.

[000111] Figure 7 schematically illustrates an alternative embodiment. In particular, the embodiment of Figure 7 uses techniques similar to those used in velocimetry methods such as Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). Generally, velocimetry techniques are used to obtain information relating to the flow of fluids, whereby the fluid under observation is seeded with tracer particles. The tracer particles are then tracked and their movement is used to determine properties of the flow of the fluid within which they are suspended. The present inventors have realised, however, that similar techniques can be used to obtain information about debris and/or fuel of a radiation source SO, which information can be used to control components of the radiation source SO in realtime (as in the embodiments described above with reference to Figures 2, 4, 5 and 6) for example so as to reduce debris and/or to mitigate detrimental effects of the debris and/or to control a fuel droplet train.

[000112] Information about the direction and speed of debris particles and/or fuel, obtained using velocimetry techniques, may be complemented with particle sizing information, based on, for example, Mie scattering of photons from each of the particles.

[000113] In Figure 7 the radiation source SO is shown. As in Figure 6, for reasons of clarity, many components of the radiation source SO are not depicted. It is to be understood that non-depicted features of the radiation source SO (and the radiation system of which it is a part), such as one or more lasers, a fuel emitter and components of a focussing assembly may be implemented in any appropriate way. For example, the non-depicted components of the radiation source SO of Figure 7 may be arranged according to one or a combination of the examples schematically illustrated in Figures 2, 4 or 5.

[000114] In Figure 7, an illumination source is provided. The illumination source 60 is arranged to illuminate an area including and surrounding an imaging region 52, and therefore fuel and/or debris particles, for imaging by the camera 10. In the example embodiment of Figure 7 the illumination source 60 comprises a laser 61 together with conditioning optics 62. The laser 61 is arranged to provide a coherent, low-divergent, pulse of laser radiation. As the fuel and/or debris particles may be traveling at high velocities, each laser pulse provided by the laser 61 lasts only a short time. In some embodiments, the laser pulse duration may be less than 10 ns.

[000115] The laser 61 is operable to provide a pair of laser beam pulses for each fuel target, each pulse in the pair being provided in rapid succession. For example, the laser 61 may be configured to provide a pair of pulses with a delay between each pulse of, or below, 10 ms. Each laser pulse provided by the laser 61 may have the same polarization, and may be of a different wavelength to both a main (initiating) laser beam and, where present, a prepulse laser beam (as described above). In this way, detrimental interference between the laser pulses provided by the laser 61 and laser beams provided by the main or pre-pulse laser may be mitigated.

[000116] The conditioning optics 62 are arranged to condition the laser beam to provide laser radiation with a desired power distribution. In some embodiments, the conditioning optics 62 may comprise a set of lenses (not shown) arranged to expand the laser beam. The set of lenses may comprise a spherical lens. The expanded laser beam may then be provided to a cylindrical lens (not shown) arranged to compress the expanded laser beam to provide illumination radiation in the form of a sheet of laser radiation 63. The illumination source 60 may provide laser beam pulses with a power of the order of 1 mJ to 200 mJ. In the embodiment of Figure 7 the laser beam pulses are of wavelength 532nm, and a band pass filter is placed in front of the camera 10 to ensure capture of light at 532nm and to block light at other wavelengths. In alternative embodiments, any other suitable wavelength may be used.

[000117] It will be appreciated that in other embodiments, the illumination source may take other forms. For example, while laser radiation may be preferable, in other embodiments, alternative radiation sources may be used.

[000118] The camera 10 is arranged to obtain images of the imaging region 52. In some embodiments, however, where a pre-pulse of laser radiation is provided (such as in the embodiment described with reference to Figure 4) the camera 10 may additionally or alternatively be arranged to obtain images of debris arising from incidence of the pre-pulse on the fuel target. In this way, measurements of debris ejected from the fuel target as a result of interaction with the pre-pulse may also be determined. The camera 10 may be provided with an optical filter (not shown), which is substantially transparent to radiation having a wavelength of radiation produced by the laser 61, and substantially opaque to radiation having a wavelength of radiation produced by a pre-pulse or main laser. The filter may also substantially block radiation from plasma formed by the main laser. For example, in some embodiments the laser 61 is arranged to produce laser beams with a wavelength of 532 nm, and a 532 nm bandpass filter may be provided.

[000119] In the embodiment of Figure 7, the camera 10 is arranged to obtain two images, with each being in a different frame. In particular, the camera 10 is arranged to obtain a first image frame to correspond with the first of a pair of pulses of the laser 61 and to obtain a second image frame to correspond with the second of the pair of pulses of the laser 61. It will be appreciated, therefore, that in the embodiment of Figure 7, the camera 10 is able to obtain respective image frames in rapid succession to match the interval between pulses of the laser 61. The camera 10 may take any form suitable for obtaining the pair of image frames, and in some embodiments may be a CCD camera.

[000120] The illumination source 60 is arranged to illuminate the x-z plane at the imaging region 52 at the points in time at which each image frame is obtained by the camera 10. Particles within the x-z plane are illuminated within each image frame obtained by the camera 10. Each of two image frames obtained by the camera 10 therefore provides a snapshot of fuel and/or debris particles at a different point in time within the x-z plane.

[000121] It will be appreciated that while the illumination sheet 63 is described as being within the x-z plane, the illumination sheet 63 may take any orientation so as to image particles in other planes. In some embodiments, the conditioning optics 62 may allow the illumination sheet 63 to be rotated through a plurality of different planes within the exposure of a single frame. For example, where a cylindrical lens is provided to flatten the radiation beam provided by the laser 61, the cylindrical lens may be rotatable. Such rotation of a planar illumination sheet may be referred to as scanning PIV, and may be used to provide a volumetric representation of the imaging region.

[000122] By way of example, Figure 8 schematically illustrates an embodiment of the illumination source 60 and camera 10 in which the conditioning optics 62 are arranged to rotate the radiation sheet 63 through a plurality of angles. As described above, the laser 61 provides laser radiation to the conditioning optics 62. The conditioning optics may comprise one or more cylindrical lenses arranged to focus the laser radiation onto a line, thereby forming the illumination sheet 63. The conditioning optics 62 further comprise rotation means, configured to rotate the one or more cylindrical lenses about the optical axis of the radiation sheet 63.

[000123] Rotation of the cylindrical lenses within the conditioning optics 62 causes the radiation sheet 63 to rotate about its optical axis, thereby illuminating a plurality of planes within the imaging region 52. The camera 10 is configured to obtain a plurality of two-dimensional images as the conditioning optics 62 rotate the radiation sheet 63. It will be appreciated that the radiation sheet 63 may be rotated through 180 degrees, such that the camera is able to obtain a plurality of two-dimensional images, which together cover a three-dimensional volume of the imaging region 52. Alternatively, the radiation sheet 63 may be rotated through a predetermined, non-180 degree angle. In an embodiment, the radiation sheet 63 may be in continuous rotation, therefore rotating through 360 degrees.

[000124] It is described in more detail below that two image frames are compared, in order to track particles within the imaging region. It is to be understood that where the conditioning optics 62 are configured to rotate the radiation sheet 63 through a plurality of angles, it is image frames obtained at corresponding times during different laser pulses that are compared, not image frames obtained during a single laser pulse (or the same rotation). For example, where during rotation of a first laser pulse a first, second and third image may be obtained by the camera 10, and during a second laser pulse, a first, second and third image may be obtained, the two first images may be compared, the two second images may be compared and the two third images may be compared.

[000125] In an embodiment, the conditioning optics 62 comprises a single cylindrical lens arranged to focus a radiation beam onto a line that passes through the imaging region 52 upstream (i.e. closer to the illumination source 60) of the fuel target. In this way, a sheet of radiation is provided that passes through imaging region 52. For example, where a single cylindrical lens is provided, the illumination radiation enters the enclosing structure of the source SO with a generally cylindrical shape, expands towards a line near the plasma formation region.

[000126] In an alternative embodiment, two cylindrical lenses may be provided within the conditioning optics 62, the two cylindrical lenses rotating in synchrony. The cylindrical lenses may be mounted to a stage, or connected together, such that a relative orientation of the two cylindrical lenses with respect to the laser 61 does not change during rotation. The provision of two cylindrical lenses allows the laser radiation to be formed into a sheet (or curtain) before entering the enclosing structure of the source SO. In this way, depth of focus may be improved. Flowever, where two cylindrical lenses are provided, an intensity of the radiation sheet 63 may be greater at positions of the source at which there are optical components such as viewports, which may result in optical damage to such components.

[000127] It will be appreciated that the one or more lenses may be rotated using any suitable mechanism. For example, the one or more cylindrical lenses may be mounted on a rotatable stage within the conditioning optics 62. A motor may be coupled with the rotating stage in order to provide rotational movement.

[000128] By providing a rotating radiation sheet 63, a three-dimensional volume may be imaged with a single camera. This may be advantageous. In particular, use of multiple cameras to image a volume requires additional viewports which may be difficult to provide. Further, it has been observed that interference effects may be present in multi-camera imaging systems, resulting in recording of particles which are not present in the plasma formation region. Further, where images are obtained with multiple cameras, significant processing resources may be required to process each image to generate a three-dimensional volume. Embodiments such as shown in Figure 8 provide imaging of a three-dimensional volume which do not suffer these drawbacks.

[000129] In order to ensure that the timing between the camera 10 and the illumination source 60 is accurate, the illumination source 60 and the camera 10 may be connected to a shared trigger mechanism (not shown). Such a shared trigger mechanism may be implemented in any convenient way. For example, a suitable trigger may be based upon a firing of an initiating (main), or pre-pulse, laser and/or may be based upon signals received from sensors tracking a progression of a fuel target to the plasma formation region 4.

[000130] The source SO of Figure 7 further comprises a radiation dump 64 substantially inline with the direction of propagation of the illumination sheet 63. The radiation dump 64 acts to absorb the radiation of the illumination sheet 63 to prevent reflection from other surfaces within the radiation source SO. The radiation dump 64 therefore helps to provide a substantially dark background to the images obtained by the camera 10.

[000131] The two image frames obtained by the camera 10 are passed to the controller 11 for processing via the connection 13. The controller 11 processes the two images to provide information regarding debris and/or fuel in the imaging region 52. For example, the images may be processed in the same way as images obtained using PIV are processed. Such processing will be known to persons skilled in the art and as such is not described in detail herein.

[000132] In general, however, the first and second image frames may each be split into a plurality of sections, and correlated (using, for example, cross-correlation of the two frames) to calculate a displacement vector for each section. The time delay between the two images, together with the change in position can be used to determine the speed with which those fuel and/or debris particles are travelling. The size of the fuel and/or debris particles may be determined based upon Mie scattering. That is, by measuring the intensity of the images of fuel and/or debris particles imaged by the camera 10 (indicative of the number of photons scattered by those particles in the direction of the camera 10) the controller 11 can determine an indication of the size of the fuel and/or debris particles.

[000133] The processing of images obtained by the camera 10 in the embodiment of Figure 7 enables detection of fuel and/or debris particles or around 0.1 pm or larger. In contrast, prior art methods, such as imaging based on shadowgraph techniques, which illuminate a target with diffuser filtered laser radiation, are generally capable of imaging features with a resolution of only 5 pm and above. Additionally, a wider field-of-view, and a greater depth-of-field, can be achieved in the images obtained using the arrangement of Figure 7 in comparison to those that may be obtained using shadowgraph-based methods.

[000134] While in Figure 7 only a single camera is depicted, it is to be understood that more than one camera may be used. For example, one or more, additional cameras may be arranged to image the imaging region from a different angle to the camera 10 (similarly to as described above with reference to Figure 6). Where a plurality of cameras are provided, each camera may be arranged to image the imaging region at a different angle. The provision of two or more cameras can be used to obtain three-dimensional views of the imaging region.

[000135] Additionally, while it is described above that the conditioning optics are arranged to provide a single sheet of illumination for each image, in other embodiments, the conditioning optics 62 may comprise optics arranged to provide laser beams of different forms, dimensions and orientations. For example, in some embodiments, the illumination source 60 may be arranged to provide a plurality of planar sheets of radiation, each sheet having a different polarization. A plurality of cameras may be provided, each camera comprising a polarisation filter to reflections from only one of the sheets. In other embodiments, a volume of illumination (rather than a sheet) may be provided.

[000136] It is described above that techniques similar to those used in PIV may be utilised to determine a velocity of fuel and/or debris particles. It is to be understood that in other embodiments, other velocimetry techniques may be used in addition to, or in place of, PIV techniques. For example, in some embodiments, Particle Tracking Velocimetry (PTV) may be used by tracking the location of individual particles across a plurality of frames obtained by the camera 10 (or by a plurality of cameras where provided).

[000137] It is described above, with reference to Figure 7 that the camera 10 is operable to obtain two image frames, each frame timed with one of a pair of laser pulses provided by the illumination source 60. In some embodiments, however, the camera 10 may be arranged to obtain the two images in a single frame. That is, where a single frame is obtained, the single frame will comprise a first image of the imaging region for the first of the pair of laser pulses, and a second image of the imaging region for the second of the pair of laser pulses. In such embodiments, additional processing may be required to determine which features depicted in the frame were imaged at the first laser pulse and which features were imaged at the second laser pulse. A single frame comprising two images of imaging region at different times may be auto-correlated to determine a speed and direction of the imaged fuel and/or debris particles.

[000138] In the embodiment described above with reference to Figure 7, illumination radiation is provided by illumination source 60. In other embodiments, illumination radiation may be generated by the interaction of the laser beam and the fuel. For example, illumination radiation may be provided by the radiation emitting plasma. In addition to EUV radiation, which is used for lithography as described above, the radiation emitting plasma may emit other frequencies of electromagnetic radiation which may be used for illumination. A plasma may create light in a broad spectrum comprising EUV, DUV (deep ultraviolet), visible light and/or other frequencies of light. In some cases, light generated by a plasma may have an anisotropic distribution in space.

[000139] Figure 9 schematically illustrates an example of a radiation source SO according to an embodiment of the present invention in which a camera 10 uses visible or ultraviolet light emitted by the radiation induced plasma as an illumination source. In the embodiment of Figure 9, the camera images debris using the visible or ultraviolet light emitted by the radiation induced plasma. The system of Figure 9 may perform a method which may be described as a type of PTV using plasma created light. The system may be used to perform debris mitigation studies.

[000140] For clarity, many components have been omitted from the schematic diagram of Figure 9. Non-depicted components may be arranged in any appropriate way, for example according to one or a combination of the examples schematically illustrated in Figures 2, 4, 5 or 6.

[000141] In the embodiment of Figure 9, a LPP source SO produces EUV by irradiating a tin droplet target with a powerful laser 1.

[000142] A fuel emitter 3 emits tin droplets. For each tin droplet, an explosion caused by the main laser pulse causes target disintegration and pushes debris apart. The laser produced plasma 7 emits debris of several kinds, which may include vapor, ions and particles. Particles are produced in large numbers with typical sizes from around 10 nm to around 1000 nm and with typical speed up to around 1000 m/s. Particles originate from the tin droplet located in the primary focus (PF) of the elliptical collector 5 (the plasma formation region).

[000143] The working principle of the system of Figure 9 is similar to Particle Image Velocimetry (PIV) in that a camera 10 catches the light scattered by debris. Plowever, in the system of Figure 9, the light source that is used is not a PIV laser but light emitted by the plasma itself.

[000144] The LPP source creates plasma flashes with a frequency of 50 kHz and higher. In the case of a 50 kHz frequency, the PF point presents a point light source every 20 ps. If the frequency is higher than 50 kHz, the interval between flashes is less than 20 ps. The point light source presented at the PF may be around 300 pm in diameter.

[000145] In the embodiment of Figure 9, a first plasma flash creates debris. A second flash (20 ps after the first flash) is used to illuminate the debris particles flying away. The camera 10 is used to capture an image of the debris created by the first flash, illuminated by visible or UV light from the second flash. The light that is used for illumination has a different frequency from the frequency of the main laser (and from the frequency of the pre-pulse laser, if used).

[000146] The camera 10 may be used to repeatedly capture images of an area of interest, for example images of part of the collector 5. For example, images may be captured every 20 ps to coordinate with the interval between flashes. A series of images may be acquired in which debris resulting from each flash may be imaged using illumination radiation produced in a subsequent flash. Successive images may be compared to provide velocimetry techniques as described above. More than one camera may be used.

[000147] Images captured by the camera 10 are processed by the controller 11 to provide information about the debris particles, for example the size and velocity of the debris particles. The controller 11 is configured to provide instructions to one or more components (some of which are not shown in Figure 6) of the radiation system in order to mitigate deleterious effects of debris.

[000148] Droplets of debris or of spent or unused fuel may cause contamination. Droplets from the primary focus may travel towards collector 5 and cause direct contamination. Droplets from the primary focus may travel to the source vessel walls, splash or bounce on reaching the source vessel wall, and, if splashes still carry enough momentum or energy, reach the collector and cause indirect contamination of the collector. Droplets may travel towards the intermediate focus (for example, after optional bouncing and/or splashing).

[000149] In the present embodiment, a Dynamic Gas Lock (not shown) is present. If droplets pass through the Dynamic Gas Lock, they may contaminate the illuminator optics and/or may cause possible contamination downstream. Contamination downstream may comprise, for example, contamination to the illuminator, a reticule or ta scanner.

[000150] Hydrogen at a few hundred Pa and flow of a few hundred m/s in the source vessel may be used to stop and/or deflect debris but may be insufficient to stop and/or deflect debris if a product of the size of a droplet of debris and the initial velocity of the droplet of debris is larger than a certain threshold.

[000151] Debris mitigation may be implemented by providing an instruction to, for example, the fuel emitter 3 or the main laser 1, or any other component, for example as detailed above with reference to the embodiments of Figures 2, 4, 5 and 6.

[000152] Experiments using Particle Image Velocimetry (PIV) have been performed on a LPP source SO to acquire information about debris particles created. The experiments were performed on a system having a pre-pulse and a main pulse. Particles of debris were created which had a mean size of around 1 pm and a velocity of around 400 m/s. Bigger particles usually have smaller velocities and vice versa.

[000153] Figure 10 shows an example of processed data that was obtained by a PIV technique. A laser beam travels in the direction indicated by the arrow 80 and interacts with the droplet stream 82, resulting in debris 84. Figure 11 is a histogram showing a velocity distribution of debris particles. The mean velocity is 400 m/s. Figure 12 is a histogram showing a size distribution of debris particles. The mean particle size is 1 pm.

[000154] For a 20 ps interval and a particle velocity of 400 m/s, the main part of the debris will travel 8 mm (20 ps*400 m/s). Slower and bigger particles will travel for a shorter distance. With efficient light intensity, made by plasma, it may be possible to catch with the camera 10 the light scattered by the debris particles. Exemplary photon flux calculations are presented below.

[000155] Figure 13 is a schematic illustration showing a schematic view of some generated particles 90, flying from the primary focus and illuminated by a plasma flash 92. The drive laser is represented by arrow 94 and the direction of the intermediate focus is indicated by arrow 96. The distance from plasma 92 to a centre of the debris is 8 mm.

[000156] The laser produced plasma (LPP) light source spectrum comprises a few bright lines in the visible and ultraviolet (UV) range. Light produced in the visible and/or UV range may be used for debris illumination.

[000157] Flux of scattered photon numbers of a chosen wavelength can be estimated using the formula:

(Equation 1) where Epulse is the total energy of plasma light at the chosen energy, S is the surface area of an illumination volume, Ephot is the energy of a photon at the chosen wavelength, σ is the scattering cross section of the droplet being imaged, is the normalized differential cross-section corresponding to scattering in 1 radian in the direction of the objective lens, Ω is the solid angle of light collected by the objective lens, NA is the numerical aperture of the objective lens, QE is the quantum efficiency of the camera CCD and CE is the A/D (analogue to digital) conversion coefficient. The scattering cross section, σ, of the droplet is a function of the diameter, refraction, and absorption indexes of the fuel material. The scattering cross section is calculated according to Mie scattering theory. The solid angle, Ω, of light collected by the objective lens is defined by the formula:

where r is the radius of the outer lens and FL is the focal length of the objective with diaphragm fully open.

[000158] The maximal spectral radiation from a heated object is set by Planck’s law.

(Equation 2) where h is Planck’s constant, kB is the Boltzmann constant, c is the speed of light, λ is wavelength and T is temperature. Thus, the maximal radiation energy emitted into a wavelength interval Δλ by a heated object with surface area A into a solid angle Ω during time interval τ is:

(Equation 3) [000159] A calculation of radiation limits from a spherical plasma of EUV-relevant temperatures was performed using Planck’s law (Equation 2) for a plasma diameter of 250 microns and for two plasma temperatures, 30eV and 50eV. A pulse duration of 100 ns was assumed. The calculation resulted in a plasma emission limit of around 2 mJ for a wavelength range of 400 to 800 nm.

[000160] A calculation of flux of scattered photon numbers may be performed using Equation 1. The calculated radiation limit of around 2 mJ is divided by 10 to give a number (200 μJ) for the energy of plasma light that may be more realistically obtainable in practice. Therefore, an Epuise of 200 pJ is used in the calculation of Equation 1.

[000161] A spherical plasma is considered, and anisotropy effects are neglected. The illumination volume is considered to be a sphere. In the case of radius 8mm, this gives a surface area S of 8.04cm2.

[000162] 2 mJ of energy in a 4π (spherical) surface results in around 250 pJ/cm2. An energy of 200 pJ gives around 25 pJ/cm2, which is used in Equation 1.

[000163] The chosen wavelength λ is 532 nm, resulting in an Ephot of 2.3 eV.

[000164] Parameters, which may be particularly important for photon flux estimation, are presented below for the calculated embodiment.

Energy in pulse: 200 pJ (10 W, 50 kHz)

Power density: 25 pJ/cm2

Cross section: calculated from Mie scattering theory

Quantum efficiency: 0.4 Conversion efficiency: 0.5

The use of Equation 1 with appropriate parameters may allow the estimation of the camera CCD signal for different sizes of droplet. In other embodiments, different parameters may be used to calculate the obtainable signal. The illumination may be anisotropic. The energy and power density and/or other parameters may differ from those described above. A different wavelength of light may be used.

[000165] Figure 14 is a graph showing the calculated dependence of camera signal on droplet size using the parameters listed above. Figure 14 shows signal, in counts, plotted against droplet diameter, in pm.

[000166] According to the graph of Figure 14, particles having a size of around 1 pm will create a CCD signal of around 150 counts. 150 may be a good and detectable signal, assuming that the noise is low. In some experiments, noise was found to be a maximum of 20 counts.

[000167] Bigger and slower particles may create a higher signal due to increased size and lower velocity. With increased size, the count signal may scale as a square function of the length of a side. Lower velocity may provide a higher light density in the debris. It may be easier to image larger and slower particles than to image smaller and faster particles.

[000168] By using the system of Figure 9 or a similar system, a method similar to Particle Tracking Velocimetry with plasma created light may be used for visualization of big and slow particles using a minimum of external hardware and with minimum impact on the source itself.

[000169] Some existing radiation sources comprise shadography cameras, for example shadography cameras that may be used for target formation investigation (pre-pulse investigation).

[000170] In some embodiments of the present invention, signal detection is performed using shadography cameras that are used for target formation investigation (pre-pulse visualization). To use a shadography camera to detect particles in visible or ultraviolet light using light from the radiation induced plasma, it may be necessary to provide the shadography camera with a corresponding optical filter and to readjust the triggering of the shadography camera.

[000171] In some embodiments, a camera is used that is specially designed for a chosen wavelength. A camera that is specially designed for a chosen wavelength may in some circumstances have a higher QE and CE than a camera that has not been specially designed for that wavelength. Using a higher QE and CE gives a higher signal, increasing sensitivity of the system.

[000172] The wavelength used can be chosen taking into account the fact that a long wavelength (for example, a wavelength in the visible spectrum) may not be able to image smaller particles. A long wavelength may be considered to be blind to the smallest particles. Smaller particles scatter more light with a shorter wavelength. For example, smaller particles may scatter more light with a UV or EUV wavelength than light with a visible wavelength.

[000173] In some embodiments, a camera with a long frame duration is used. Pulses of plasma light may provide a stroboscopic image of flying debris. Such a stroboscopic image may provide information about droplet velocity and direction. For example, a frame duration may be used that is long enough to capture several plasma flashes.

[000174] Although the embodiment described above with reference to Figure 9 uses light from the main pulse as an illumination source, in some embodiments the system comprises a pre-pulse laser in addition to the main laser and the pre-pulse is used for illumination of particles instead of or in addition to the main pulse. The pre-pulse may create some light in a broad spectrum. In some embodiments, the pre-pulse laser interacts with a tin droplet in the plasma formation region and the interaction produces illumination radiation, for example visible or UV light. The light produced by the interaction of the pre-pulse laser and the tin droplet is used by the camera to produce an image of debris.

[000175] In the embodiment described above with reference to Figure 9, the camera 10 obtains images of debris incident on a region of the collector 5. In other embodiment, any suitable imaging region may be imaged. Illumination radiation may be radiation of any suitable wavelength and is not limited to the use of visible or UV radiation.

[000176] Although the embodiment of Figure 9 has been described with reference to imaging droplets of debris, in other embodiments radiation emitted from the plasma formation region (for example, radiation produced by an interaction of the main laser or a pre-pulse laser with a tin droplet) may be used to image spent or unused fuel remaining after plasma formation. In some embodiments, radiation emitted from the plasma formation region may be used to illuminate fuel droplets that have been emitted by the fuel emitter 3 prior to plasma formation using those droplets. In some embodiments, radiation emitted from the plasma formation region may be used to image the plasma formation region itself.

[000177] The method described above with reference to the system of Figure 9 may provide a method for imaging debris, where the debris may cause tin contamination of the collector 5 or other components. Tin contamination in a LPP source may be a huge problem for the lifetime of the collector 5. A Particle Image Velocimetry (PIV) technique, offered as a tool for debris visualization, may be a powerful metrology system but may in some circumstances require expensive hardware and may demand a long preparation period, requiring switching off the source, and making changes in the whole system. Plaving an additional metrology method capable of measuring droplets originating at the primary focus may provide a capability to tune parameters of the source in order to decrease or mitigate production of droplet debris. The system of Figure 9 may in some circumstances be fast and easy as opposed to a PIV system.

[000178] The working principle of the system of Figure 9 is similar to that of PIV but the light source used is not a PIV laser. A PIV laser may be expensive and/or difficult to install and/or difficult to adjust. By using a system that does not use a PIV laser, cost and/or complexity may be reduced when compared to a system using a PIV laser.

[000179] In some embodiments, the system of Figure 9 is used in a system that does not have a PIV laser. In other embodiments, the imaging method described with reference to Figure 9 may be performed as an additional method in a system that does include a PIV laser or other illumination source.

[000180] The system of Figure 9 may provide a low-cost metrology tool. The system may provide online information about particle generation processes. The system may provide real time information about particle generation processes. The system may provide an estimation of particle velocity, direction and size. The system may be used for plasma optimization or debris creation investigation and/or mitigation.

[000181] In an embodiment, the radiation source SO of 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.

[000182] In an embodiment, the radiation source SO 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.

[000183] 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.

[000184] The term “EUV radiation” may be considered to encompass electromagnetic radiation having a wavelength within the range of 5-20 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 5-10 nm such as 6.7 nm or 6.8 nm.

[000185] 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.

[000186] 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.

[000187] 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 for generating a radiation emitting plasma, the radiation system comprising: a fuel emitter configured to provide fuel to a plasma formation region; a laser arranged to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma; and an imaging device arranged to obtain at least one image of an imaging region, the image indicating at least one property of the fuel and/or the radiation emitting plasma and/or debris, wherein the imaging region is located between the fuel emitter and the plasma formation region, or the imaging region is located between the plasma formation region and a further component of the radiation system; and the system further comprises a controller configured to process the at least one image and to provide an instruction in dependence on said at least one property of the fuel and/or the radiation emitting plasma and/or debris. 2. A system according to Clause 1, wherein the at least one property of the fuel and/or the radiation emitting plasma and/or debris comprises at least one property of the fuel and/or the radiation emitting plasma and/or debris outside the plasma formation region. 3. A system according to Clause 1 or 2, wherein the imaging region does not include the plasma formation region. 4. A system according to any preceding clause, wherein the fuel comprises a series of droplets of fuel emitted by the fuel emitter. 5. A system according to any preceding clause, wherein the imaging device is arranged to obtain said at least one image of the fuel in the imaging region located between the fuel emitter and the plasma formation region. 6. A system according to Clause 5, wherein the imaging device is configured to obtain said at least one image of the fuel prior to plasma formation using said imaged fuel. 7. A system according to Clause 5 or 6, wherein said at least one image comprises at least one image of at least one of a series of droplets of the fuel, and the at least one property comprises at least one property of said at least one of the droplets of fuel. 8. A system according to Clause 7, wherein the at least one property comprises at least one of a size, shape, speed, position or trajectory for at least one of the series of droplets of fuel, and/or a repetition in spacing in time or distance, a repetition frequency, or spread in position or trajectory for a plurality of the series of droplets of fuel. 9. A system according to any of Clauses 6 to 8, wherein the at least one property comprises at least one of: the presence of absence of at least one satellite feature; the number, size or position of at least one satellite feature. 10. A system according to any of Clauses 1 to 4, wherein the imaging device is arranged to obtain said at least one image of the fuel in the imaging region located between the plasma formation region and the further component of the radiation system. 11. A system according to any of Clauses 1 to 4 or 10, wherein the imaging device is arranged to obtain said at least one image after the fuel and/or plasma and/or debris in the image has been subject to the plasma formation process. 12. A system according to any of Clauses 1 to 4, 10 or 11, wherein the imaging device is arranged to obtain at least one image of debris resulting from the plasma formation and/or spent or unused fuel remaining after the plasma formation. 13. A system according to any of Clauses 1 to 4, or 10 to 12, wherein the at least one property comprises at least one of spread, scatter, velocity, quantity, or density of the debris and/or spent or unused fuel. 14. A system according to any of Clauses 1 to 4, or 10 to 13, wherein the imaging region is located at or near a surface of the component of the system, and the imaging device is arranged such that said at least one image comprises an image of fuel and/or debris at or near said surface. 15. A system according to any preceding clauses, wherein the component comprises at least one of a contamination trap, an optical component, a radiation collector, a laser device. 16. A system according to any preceding clause, wherein the at least one instruction comprises a warning signal. 17. A system according to any preceding clause, wherein the at least one instruction comprises an instruction for the fuel emitter. 18. A system according to Clause 17, wherein the at least one instruction comprises an instruction for causing the fuel emitter to modify a property of the fuel, or to cease generation of fuel droplets. 19. A system according to Clause 17 or 18, wherein the at least one instruction comprises an instruction for the fuel emitter to modify an operating parameter of the fuel emitter, and the operating parameter comprises at least one of: a droplet formation parameter; a rise time, fall time, duration, frequency, amplitude, or gradient of a periodic drive signal for droplet formation, or of a component of such a periodic drive signal; a fuel pressure; a fuel temperature. 20. A system according to any preceding clause, wherein the at least one instruction comprises an instruction suitable for modifying a laser property of the first laser beam. 21. A system according to Clause 20, wherein the laser property of the laser beam comprises at least one of a repetition rate, power, intensity profile, direction of propagation and position of the first laser beam. 22. A system according to any preceding clause, wherein the controller is configured to process a series of the images obtained at different times and iteratively to provide a series of instructions in response to the series of images, thereby to implement a control loop to control at least one property of the fuel and/or the radiation emitting plasma and/or the debris. 23. A system according to any preceding clause, wherein the imaging device is arranged to obtain the at least one image using illumination radiation that is generated by an interaction of the laser beam and the fuel. 24. A system according to Clause 23, wherein the illumination radiation is emitted by the radiation emitting plasma. 25. A system according to Clause 23 or Clause 24, wherein the illumination radiation is emitted from the plasma formation region. 26. A system according to any of Clauses 23 to 25, wherein the generating of the radiating plasma comprises generating a first plasma flash from a first droplet of the fuel and generating a second plasma flash from a second droplet of the fuel, and wherein the imaging device is arranged to obtain at least one image of debris resulting from the first plasma flash and/or spent or unused fuel remaining after the first plasma flash, using illumination radiation generated by an interaction of the laser beam with the second droplet of the fuel. 27. A system according to any of Clauses 1 to 22, further comprising a pre-pulse laser arranged to provide a pre-pulse laser beam, wherein the imaging device is arranged to obtain the at least one image using illumination radiation that is generated by an interaction of the pre-pulse laser beam and the fuel. 28. A system according to any of Clauses 23 to 27, wherein a frame duration of the imaging device is such as to capture illumination radiation from a plurality of plasma flashes. 29. A system according to any of Clauses 23 to 28, wherein the illumination radiation comprises at least one of visible light, ultraviolet light. 30. A radiation system for generating a radiation emitting plasma, the system comprising: a fuel emitter configured to provide fuel to a plasma formation region; a laser arranged to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma; and an imaging device arranged to obtain at least one image of an imaging region, the image indicating at least one property of the fuel and/or the radiation emitting plasma and/or debris; wherein the imaging device is arranged to obtain the at least one image using illumination radiation generated by an interaction of the laser beam and the fuel and/or using illumination radiation generated by an interaction of a pre-pulse laser beam and the fuel; and the system further comprises a controller configured to process the at least one image and to provide an instruction in dependence on said at least one property of the fuel and/or the radiation emitting plasma and/or debris. 31. A method of generating a radiation emitting plasma in a radiation system comprising: a fuel emitter configured to provide a fuel target at a plasma formation region; a laser arranged to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma; an imaging device arranged to obtain at least one image of an imaging region, the image indicating at least one property of the fuel, and/or the radiation emitting plasma and/or debris, wherein the imaging region is located between the fuel emitter and the plasma formation region, or the imaging region is located between the plasma formation region and a further component of the system; and a controller; the method comprising at the controller: processing said at least one image; and providing at least one instruction based on said at least one property of the fuel, and/or the radiation emitting plasma and/or debris. 32. A lithographic system comprising a radiation system according to Clause 1. 33. A radiation source for generating a radiation emitting plasma, the radiation source being arranged to receive a laser beam at a plasma formation region and comprising: a fuel emitter configured to provide fuel to a plasma formation region; and an imaging device arranged to obtain at least one image of an imaging region, the image indicating at least one property of the fuel and/or the radiation emitting plasma and/or debris, wherein the imaging region is located between the fuel emitter and the plasma formation region, or the imaging region is located between the plasma formation region and a further component of the radiation system; and the source further comprises a controller configured to process the image and to provide an instruction in dependence on said at least one property of the fuel and/or the radiation emitting plasma and/or debris. 34. A non-transitory computer readable medium carrying computer readable instructions suitable to cause a computer to: process at least one image of an imaging region, the image indicating at least one property of fuel, and/or radiation emitting plasma and/or debris, wherein the imaging region is located between a fuel emitter and a plasma formation region, or the imaging region is located between the plasma formation region and a further component of a radiation system; and provide at least one instruction based on said at least one property of the fuel, and/or the radiation emitting plasma and/or debris.

Claims

A lithography device comprising: an illumination device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being 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.