NL2015136A - Radiation systems and associated methods. - Google Patents

Description

RADIATION SYSTEMS AND ASSOCIATED METHODS

Field [0001] The present invention relates to radiation systems and associated methods of generating a plasma, and in particular radiation systems which use laser radiation to excite a fuel to produce a plasma. Such radiation systems may be used as a radiation source in a lithographic apparatus.

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

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

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

[0005] When using a plasma based radiation system, contamination particles are created as a by-product of the plasma generation. Generally, such contamination particles are undesired because they adhere (for example) to reflective surfaces within the radiation system. Accumulation of these contamination particles on reflective surfaces of the radiation system (directly or via other non-reflecting surfaces) reduces the reflectivity of those surfaces, reducing the radiation system efficiency. In particular contamination of the radiation collector causes the radiation system efficiency to fall significantly. This contamination may be the result of, at least in part, unconsumed fuel used to generate the plasma. When such a radiation system is used as a radiation source in a lithographic apparatus, any reduction in efficiency reduces the achievable throughput of the lithographic apparatus.

SUMMARY

[0006] It is desirable to reduce contamination within a radiation system.

[0007] The invention in a first aspect provides a radiation system comprising: a monitoring device for monitoring a target area where, in operation, a fuel droplet is conditioned by conditioning radiation to form a conditioned fuel droplet and the conditioned fuel droplet is excited by a beam of excitation radiation so as to form a plasma; and a control module operable to determine from an output of said monitoring device, one or both of: a size attribute of the conditioned fuel droplet within the target area; and/or a time attribute relating to fuel droplet debris persistence within the target area following said excitation of said conditioned fuel droplet by excitation radiation to form the plasma; the control module being further operable to use one or both of the time attribute and the size attribute in monitoring of the operation of said radiation system.

[0008] The invention in a second aspect provides a method of generating a plasma comprising: monitoring a target area where, in operation, a fuel droplet is conditioned by conditioning radiation to form a conditioned fuel droplet and the conditioned fuel droplet is excited by a beam of excitation radiation so as to form a plasma; and determining from an output of said monitoring device, one or both of: a size attribute of the conditioned fuel droplet within the target area; and/or a time attribute relating to fuel droplet debris persistence within the target area following said excitation of said conditioned fuel droplet by excitation radiation to form the plasma; using one or both of the time attribute and the size attribute in monitoring of the operation of said radiation system.

[0009] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0010] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. Embodiments of the invention are described, by way of example only, with reference to the accompanying drawings, in which:

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

Figure 2 is a more detailed view of the apparatus of Figure 1, illustrating a radiation system in accordance with embodiments of the invention;

Figure 3a illustrates schematically the beam and droplet arrangement of a radiation system illustrated in Figure 2;

Figure 3b shows in more detail the beam and droplet arrangement of Figure 3a when the radiation system is operating within specification, at plural different instances of time; and

Figure 3c shows in more detail the beam and droplet arrangement of Figure 3 a when the radiation system is operating out of specification, at plural different instances of time.

[0011] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0012] Figure 1 schematically depicts a lithographic apparatus 100, which includes a radiation system according to an embodiment of the invention being employed as radiation source SO. The lithographic apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation). a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g. a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0013] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0014] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support stmcture can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support stmcture may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0015] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0016] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0017] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment, may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0018] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0019] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0020] Referring to Figure 1, the illuminator IL receives an extreme ultraviolet radiation beam from the radiation source SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The radiation source SO may be part of a radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the radiation source. The laser and the radiation source may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0021] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the radiation source with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander.

[0022] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0023] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0024] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. 2. In scan mode, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. 3. In another mode, the support stmcture (e.g. mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0025] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0026] Figure 2 shows an embodiment of the lithographic apparatus in more detail, including a radiation system 42 according to an embodiment, the illumination system IL, and the projection system PS. The radiation system 42 as shown in Figure 2 is of the type that uses a laser-produced plasma as a radiation source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which a hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The hot plasma is created by causing a at least partially ionized plasma by, for example, optical excitation using CCF laser light. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, Sn is used to create the plasma in order to emit the radiation in the EUV range.

[0027] The radiation system 42 embodies the function of radiation source SO in the apparatus of Figure 1. Radiation system 42 comprises a source chamber 47, in this embodiment, which not only substantially encloses a source of EUV radiation, but also radiation collector 50 which, in the example of Figure 2, is a normal-incidence collector, for instance a multi-layer mirror.

[0028] A laser radiation system 61 (described in more detail below) is constructed and arranged to provide a laser beam 63 which is delivered by a beam delivery system 65 through an aperture 67 provided in the radiation collector 50. Also, the radiation system includes a target material, such as Sn or Xe, which is supplied by droplet generator 71. The beam delivery system 65, in this embodiment, is arranged to establish a beam path focused substantially upon a predetermined plasma formation location 73 at the primary focus of the radiation collector 50.

[0029] In operation, the target material, which may also be referred to as fuel, is supplied by the droplet generator 71 in the form of fuel droplets 69. The droplet generator 71 is arranged within the source chamber 47, to fire a stream of fuel droplets 69 towards the plasma formation location 73. When such a fuel droplet 69 reaches the plasma formation location 73, the laser beam 63 impinges on the fuel droplet 69 and an EUV radiation-emitting plasma forms inside the source chamber 47. In the case of a pulsed laser beam 63, this involves timing the pulse of laser radiation to coincide with the passage of the fuel droplet 69 through the plasma formation location 73. As mentioned, the fuel may be for example xenon (Xe), tin (Sn) or lithium (Li) in liquid state. These create a highly ionized plasma with electron temperatures of several 10's of eV. Higher energy EUV radiation may be generated with other fuel materials, for example Tb and Gd. The energetic radiation generated during de-excitation and recombination of these ions includes the wanted EUV radiation which is emitted from the plasma at plasma formation location 73. The plasma formation location 73 and the aperture 52 are located at first and second focal points of radiation collector 50, respectively and the EUV radiation is focused by the (normal-incidence) radiation collector 50 onto the intermediate focus point IF.

[0030] The frequency of delivery of fuel droplets 69 may be several kilohertz, or even several tens or hundreds of kilohertz. In practice, laser beam 63 may be delivered by a laser radiation system 61 in at least two pulses: a pre-pulse PP with limited energy is delivered to the droplet before it reaches the plasma location, in order to condition the fuel droplet 69 by either vaporizing the fuel material into a small cloud or deforming it into a flattened “pancake” shape, and then a main pulse MP of laser energy is delivered to the conditioned fuel droplet at the desired location, to generate the plasma. In a typical example, the diameter of the conditioned fuel droplet, e.g., the “pancake”, is about 0.2-0.5 mm. A trap 72 is provided on the opposite side of the source chamber 47, to capture fuel that is not, for whatever reason, turned into plasma.

[0031] Radiation passed by radiation collector 50 passes in this example through a transmissive filter spectral purity filter SPF, located near the aperture 52. The spectral purity filter SPF blocks unwanted radiation wavelengths also produced by the plasma and infrared radiation from the laser beam 63. Other methods to block such unwanted radiation may be employed, in addition or as an alternative.

[0032] The beam of radiation emanating from the source chamber 47 traverses the illumination system IL via so-called normal incidence reflectors 53, 54, as indicated in Figure 2 by the radiation beam 56. The normal incidence reflectors direct the beam 56 onto a patterning device (e.g. reticle or mask) positioned on a support structure (e.g. reticle or mask table) MT. A patterned beam 57 is formed, which is imaged by projection system PS via reflective elements 58, 59 onto a substrate carried by wafer stage or substrate table WT. More elements than shown may generally be present in illumination system IL and projection system PS. For example there may be one, two, three, four or even more reflective elements present than the two elements 58 and 59 shown in Figure 2. Radiation collectors similar to radiation collector 50 are known from the prior art.

[0033] As the skilled reader will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 55, 56, 57. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction of optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the radiation system 42, the X axis coincides broadly with the direction of stream of fuel droplets 69, while the Y axis is orthogonal to that, pointing out of the page as indicated in Figure 2. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram Figure 2, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0034] Referring to laser radiation system 61 in more detail, the laser in the illustrated example is of the ΜΟΡΑ (Master Oscillator Power Amplifier) type, although other lasers may be used. The laser radiation system 61 includes a main laser having two stages: a “master” laser or “seed” laser, labeled MO in the diagram, followed by a power amplifier system PA, for firing the main pulse of laser energy towards an expanded (i.e., conditioned) fuel droplet or fuel droplet cloud. The laser radiation system 61 also includes a conditioning laser, referred to herein as a pre-pulse laser, for firing the pre-pulse of laser energy towards a droplet, so as to condition said fuel droplet, forming said expanded fuel droplet. In practice, the pre-pulse element of the laser energy may be delivered by a separate laser.

[0035] Laser radiation system 61, droplet generator 71 and other components can be controlled by a control module 20. Control module 20 may perform many control functions, and may have many sensor inputs and control outputs for various elements of the system. Sensors may be located in and around the elements of radiation system 42, and optionally elsewhere in the lithographic apparatus. In one embodiment of the present invention, the main pulse and the pre-pulse are derived from a same laser. In another embodiment of the present invention, the main pulse and the pre-pulse are derived from different lasers which are independent trom each other. Control module 20 may be a single module or be distributed across plural locations.

[0036] Many measures can be applied in the control module 20. Such measures include monitoring to check that the virtual source point IF is aligned with the aperture 52, at the exit from the source chamber 47. Other measures include monitoring of droplet formation metrics, which include: a size attribute relating to the fuel droplet size, a time attribute relating to fuel droplet debris persistence (or time to transparency) following excitation of the conditioned droplet to form the plasma and fuel droplet position. To achieve this, control module 20 may be supplied with monitoring data from one or more monitoring devices, which may include sensors and/or cameras. These monitoring devices provide feedback on the droplet formation metrics to the control module 20. In response, control module 20 may control (for example) the injection of the fuel droplets and the timing of the pulses, both main pulse and pre-pulse, from laser radiation system 61.

[0037] One of the monitoring devices may comprise, for example, an imaging device. Such an imaging device may be configured to image a target area which includes the plasma formation location 73 and its immediate vicinity, and is referred to herein as a droplet formation camera 66. The droplet formation camera 66 can be used to determine metrics relating to fuel droplet debris contamination. To achieve this, droplet formation camera 66 may monitor the formation of a fuel droplet before, during and after it is impacted by the laser radiation system 61 pre-pulse laser beam. Additionally, the droplet formation camera 66 can monitor the target area immediately after impact of the main beam, so as to monitor fuel droplet debris persistence, sometimes referred to as time to transparency. The resultant images captured by droplet formation camera 66, sometimes referred to as “shadowgrams”, can be used to obtain the metrics relating to fuel droplet debris contamination.

[0038] The fuel droplet size should be maintained such that the main laser beam is “underfilled”, i.e., the beam should have a diameter greater than (or equal to) the droplet size at the plasma formation location 73. At the same time, the laser beam irradiance should be maximized in order to consume all of the fuel, and therefore the beam width cannot be too large. Also fuel droplet debris persistence should preferably be less than 500ns.

[0039] It has been observed that fuel droplet size and fuel droplet debris persistence are important metrics for maximizing the radiation collector lifetime. When these metrics are out of specification, radiation collector contamination occurs at a significantly increased rate, causing loss of EUV radiation output efficiency. For example, when the fuel droplet size is greater than the width of the main laser beam, much faster radiation collector contamination is observed. This is presumably because the periphery of the conditioned (flattened) fuel droplet (following conditioning by the pre-pulse laser beam) is not heated and therefore not consumed by the main laser beam. This unconsumed fuel remains within the system as a contaminant. Also, it has been observed that when fuel droplet debris persistence (or time to transparency) is long, which is indicative of low quality plasma, the radiation collector is contaminated faster.

[0040] Figures 3 a to 3c illustrate the above issue. Figure 3a shows schematically a side view of conditioning radiation in the form of a pre-pulse laser beam 310, excitation radiation in the form of a main laser beam 320, a focusing unit 330 and fuel droplets 69. The fuel droplets 69 are formed by the droplet generator 71 (Figure 2) and are delivered towards the target area, where they are first conditioned by the pre-pulse laser beam 310 to form a flattened disk shape (or vapour cloud), before being excited by pulse of the main laser beam 320 at the plasma formation location. Figure 3b shows a more detailed side view of the prepulse laser beam 310 and the main laser beam 320. A fuel droplet 69, 69’, 69” is shown at three different times: fuel droplet 69 is shown prior to being conditioned, fuel droplet 69’ is shown during conditioning by the pre-pulse laser beam 310 and conditioned fuel droplet 69” is shown being excited by main laser beam 320. Here the size of conditioned fuel droplet 69” is smaller than the diameter of the main laser beam 320, and therefore underfills the main laser beam 320. This optimizes the amount of the fuel droplet 69” which is converted to plasma and consumed. Figure 3c shows that same arrangement as that of Figure 3b but with the size of conditioned fuel droplet 69’ ’ being larger than the diameter of the main laser beam 320. The periphery of the conditioned fuel droplet 69’ ’ is not within diameter of the main laser beam 320, and is therefore not properly heated and not converted to plasma, remaining unconsumed. The unconsumed fuel at the periphery of the conditioned fuel droplet 69” forms contamination particles 350.

[0041] It is therefore proposed that the droplet formation camera 66 (or other suitable monitoring device) is used monitor a size attribute relating to the size of the conditioned fuel droplet 69”, on a real-time basis during conditioning of the fuel droplet. The resultant images can then be processed by the control module 20. The control module 20 could, in one embodiment, monitor the size of the conditioned fuel droplet 69”, comparing it to a size threshold value. Should the size of the conditioned fuel droplet 69” be greater than the size threshold (in any dimension, one or more predetermined dimensions or according to another criterion e.g., area or volume) then a fault state may be entered. The size threshold may be selected according to the known main laser beam size at the plasma formation location. It may be based on the main laser beam diameter, such that if the diameter or any other dimension of the conditioned fuel droplet 69” is greater than the main laser beam diameter, then a fault state is entered. The size threshold may be a preselected value, for example below 300pm, below 250pm, below 200pm or below 100pm. In another embodiment, the radiation system may monitor the main laser beam size and select a threshold accordingly. This could be done in real-time as part of a feedback loop, such that the threshold is maintained at an appropriate level.

[0042] It is also proposed that, either as an alternative or in addition to the fuel droplet size attribute monitoring of the above paragraph, the droplet formation camera 66 (or other monitoring device) is used monitor a time attribute relating to the fuel droplet debris persistence (or time to transparency) on a real-time basis. Fuel droplet debris persistence is a measure of the time period between the time that the conditioned fuel droplet 69” is excited by the main laser beam 320 to form the plasma and the time when there is no (or minimal) visible trace of the fuel in the target area following consumption of the conditioned fuel droplet 69” in providing EUV radiation. The resultant images can then be processed by control module 20. The control module 20 could, in one embodiment, monitor the fuel droplet debris persistence, comparing it to a time threshold value. Should the fuel droplet debris persist for longer than the threshold then a fault state may be entered. The time threshold may be a preselected value, for example below 500ns, below 400ns or below 300ns.

[0043] Tn either example, the fault state may take the form of an alarm being raised. Such an alarm may take any form, whether visual, audible, both visual and audible or by any other means. In this way an operator can intervene and take the necessary action to prevent severe collector contamination. Alternatively, or in addition, the fault state may automatically cease operation of the radiation system.

[0044] As an alternative or in addition to entering a fault state, the radiation system (and more specifically the control module 20) can determine corrections for one or more aspects of control of the radiation system from the output images of the droplet formation camera, and use these in a feedback loop. The corrections can be based upon measurements of the size attribute (e.g. the size of the conditioned fuel droplet) and/or of the time attribute (e.g., fuel droplet debris persistence) as determined from the output images. The corrections can be determined from any measured deviation of these measurements from a reference value or range. The corrections can then be applied to the control of one or more aspects of the radiation system so as to maintain (or at least attempt to maintain) one or both of the time attribute and size attribute within operational limits. Should a threshold value still be exceeded, then the radiation system may still enter a fault state. Any aspect of the radiation system may be controlled to maintain droplet size and/or fuel droplet debris persistence within the relevant operational limits, and may include one or more of: pre-pulse laser power, pre-pulse laser timing, pre-pulse laser beam size, pre-pulse focus position, main laser power, main laser timing, main laser beam size, main-pulse focus position, fuel droplet timing, droplet generator nozzle characteristics, fuel droplet velocity, fuel droplet expansion velocity, fuel droplet size (prior to conditioning), plasma position.

[0045] Merits of the concepts disclosed herein include: the lifetime of the radiation collector is extended; errors are actively monitored and reported immediately, which can prevent severe damage; availability of the radiation system is higher, thus reducing costs and downtime; errors and their effects can be logged and (automated) data analysis can help identify the root causes of these errors; EUV power is higher with improved targeting of the fuel droplet.

[0046] Although specific reference may be made in this text to the provision and operation of an EUV radiation source in a lithographic apparatus, it should be understood that the EUV radiation apparatus described herein may have other applications in EUV optical apparatus. Further in the case of a lithographic apparatus, this may have other applications besides the manufacture of ICs, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0047] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0048] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A radiation system comprising: a monitoring device for monitoring a target area where, in operation, a fuel droplet is conditioned by conditioning radiation to form a conditioned fuel droplet and the conditioned fuel droplet is excited by a beam of excitation radiation so as to form a plasma; and a control module operable to determine from an output of said monitoring device, one or both of: a size attribute of the conditioned fuel droplet within the target area; and/or a time attribute relating to fuel droplet debris persistence within the target area following said excitation of said conditioned fuel droplet by excitation radiation to form the plasma; the control module being further operable to use one or both of the time attribute and the size attribute in monitoring of the operation of said radiation system. 2. A radiation system as in clause 1 wherein said size attribute comprises a dimension of the conditioned fuel droplet in a plane perpendicular to an axis of said beam of said excitation radiation; and at a position where said excitation radiation excites the conditioned fuel droplet to form the plasma. 3. A radiation system as in clause 2 wherein said size attribute comprises a furthest distance between opposite points at a periphery of said conditioned fuel droplet, in said plane perpendicular to an axis of said beam of said excitation radiation. 4. A radiation system as in clause 1, 2 or 3 wherein said time attribute is a measure of a time between excitation of the conditioned fuel droplet to form the plasma and a time when there is no visible trace of the conditioned fuel droplet in the target area, or when a visible trace of the conditioned fuel droplet in the target area falls below a threshold. 5. A radiation system as in any preceding clause wherein said monitoring of the operation of said radiation system comprises comparing the size attribute to a size threshold; and should the size attribute exceed the size threshold, entering a fault state. 6. A radiation system as in clause 5 wherein said size threshold is related to a diameter of said beam of excitation radiation, such that said size threshold is exceeded if said size attribute is greater than said diameter of said beam of excitation radiation. 7. A radiation system as in clause 6 wherein said control module is operable to monitor diameter of said beam of excitation radiation and determine said size threshold accordingly. 8. A radiation system as in clause 5 wherein said size threshold is 300pm. 9. A radiation system as in any preceding clause wherein said monitoring of the operation of said radiation system comprises comparing the time attribute to a time threshold; and should the time attribute exceed the time threshold, entering a fault state. 10. A radiation system as in clause 9 wherein said time threshold is 500ns. 11. A radiation system as in any of clauses 5 to 10 wherein said fault state comprises raising an alarm. 12. A radiation system as in any of clauses 5 to 11 wherein said fault state ceases operation of the radiation system. 13. A radiation system as in any preceding clause wherein said monitoring of the operation of said radiation system comprises determining a correction for one or more aspects of control of the radiation system, and applying said correction in control of the radiation system so as to maintain the time attribute and/or the size attribute within operational limits. 14. A radiation system as in clause 13 wherein said operational limits are such that said application of the correction in control of the radiation system acts to maintain said size attribute of the conditioned droplet to be smaller than a diameter of said beam of excitation radiation. 15. A radiation system as in clause 14 said control module is operable to monitor ths diameter of said beam of excitation radiation and determine said operational limits accordingly. 16. A radiation system as in clause 14 or 15 wherein said operational limits are such that said application of the correction in control of the radiation system acts to maintain said size attribute below 300μπι. 17. A radiation system as in any of clauses 13 to 16 wherein said operational limits are such that said application of the correction in control of the radiation system acts to maintain said time attribute below 500ns. 18. A radiation system as in any of clauses 13 to 17 comprising a droplet generator for generating said fuel droplet, a conditioning laser for providing said conditioning radiation, and a main laser for providing said excitation radiation; and said application of said correction in control of the radiation system comprises applying said correction in controlling one or more of: output power of said conditioning laser, timing of said conditioning laser, output beam size of said conditioning laser, a focus position of the conditioning laser, output power of said main laser, timing of said main laser, output beam size of said main laser, a focus position of the main laser, timing of fuel droplet generation, nozzle characteristics of said droplet generator, fuel droplet velocity, fuel droplet expansion velocity, fuel droplet size prior to conditioning, plasma position. 19. A radiation system as in any preceding clause wherein said monitoring device is an imaging device and said output is an image of said target area. 20. A radiation system as in any preceding clause being operable to provide high frequency radiation having a wavelength in the EUV range or smaller. 21. A method of generating a plasma comprising: monitoring a target area where, in operation, a fuel droplet is conditioned by conditioning radiation to form a conditioned fuel droplet and the conditioned fuel droplet is excited by a beam of excitation radiation so as to form a plasma; determining from an output of said monitoring device, one or both of: a size attribute of the conditioned fuel droplet within the target area; and/or a time attribute relating to fuel droplet debris persistence within the target area following said excitation of said conditioned fuel droplet by excitation radiation to form the plasma; and using one or both of the time attribute and the size attribute in monitoring of the operation of said radiation system. 22. A method as in clause 21 wherein said size attribute comprises a dimension of the conditioned fuel droplet in a plane perpendicular to an axis of said beam of said excitation radiation; and at a position where said excitation radiation excites the conditioned fuel droplet to form the plasma. 23. A method as in clause 22 wherein said size attribute comprises a furthest distance between opposite points at a periphery of said conditioned fuel droplet, in said plane perpendicular to an axis of said beam of said excitation radiation. 24. A method as in clause 21, 22 or 23 wherein said time attribute is a measure of a time between excitation of the conditioned fuel droplet to form the plasma and a time when there is no visible trace of the conditioned fuel droplet in the target area, or when a visible trace of the conditioned fuel droplet in the target area falls below a threshold. 25. A method as in any of clauses 21 to 24 wherein said monitoring of the operation of said radiation system comprises: comparing the size attribute to a size threshold; and should the size attribute exceed the size threshold, entering a fault state. 26. A method as in clause 25 wherein said size threshold is related to a diameter of said beam of excitation radiation, such that said size threshold is exceeded if said size attribute is greater than said diameter of said beam of excitation radiation. 27. A method as in clause 26 comprising monitoring the diameter of said beam of excitation radiation and determining said size threshold accordingly. 28. A method as in clause 25 wherein said size threshold is 300μπι. 29. A method as in any of clauses 21 to 28 wherein said monitoring of the operation of said radiation system comprises: comparing the time attribute to a time threshold; and should the time attribute exceed the time threshold, entering a fault state. 30. A method as in clause 29 wherein said time threshold is 500ns. 31. A method as in any of clauses 25 to 30 wherein said fault state comprises raising an alarm. 32. A method as in any of clauses 25 to 31 wherein said fault state ceases operation of the radiation system. 33. A method as in any of clauses 21 to 32 wherein said monitoring of the operation of said radiation system comprises determining a correction for one or more aspects of control of the radiation system, and applying said correction in control of the radiation system so as to maintain the time attribute and/or the size attribute within operational limits. 34. A method as in clause 33 wherein said operational limits are such that said application of the correction in control of the radiation system acts to maintain said size attribute of the conditioned droplet to be smaller than a diameter of said beam of excitation radiation. 35. A method as in clause 34 comprising monitoring a diameter of said beam of excitation radiation and determine said operational limits accordingly. 36. A method as in clause 33, 34 or 35 wherein said operational limits are such that said application of the correction in control of the radiation system acts to maintain said size attribute below 300pm. 37. A method as in any of clauses 33 to 36 wherein said operational limits are such that said application of the correction in control of the radiation system acts to maintain said time attribute below 500ns. 38. A method as in any of clauses 33 to 37 wherein application of said correction in control of the radiation system comprises applying said correction in controlling one or more of: power of the conditioning radiation, timing of the conditioning radiation, output beam size of the conditioning radiation, focus position of the conditioning radiation, power of the excitation radiation, timing of the excitation radiation, output beam size of the excitation radiation, focus position of the excitation radiation, timing of fuei dropiet formation, dropiet nozzle characteristics, fuel droplet velocity, fuel droplet expansion velocity, fuel droplet size prior to conditioning, plasma position.

Claims (1)

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.