NL2011514A - Beam delivery system for an euv radiation source, euv radiation source and method of generating euv radiation. - Google Patents

Description

BEAM DELIVERY SYSTEM FOR AN EUV RADIATION SOURCE, EUV RADIATION SOURCE AND METHOD OF GENERATING EUV RADIATION

HELD

[0001] The present invention relates to a beam delivery system for an EUV radiation source, such as those used in EUV optical apparatuses (for example a lithographic apparatus) and a method of generating EUV radiation.

BACKGROUND

[0002] 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.

[0003] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

[0004] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

(1) where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, kl is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of kl.

[0005] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. 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. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0006] EUV radiation 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 module 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. The source collector module 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.

[0007] LPP sources typically require a beam delivery apparatus to deliver the beam from the laser to plasma generation site. Such a beam delivery apparatus typically comprises a number of mirrors, perhaps ten or more, to guide the beam. Heating of the mirrors caused by absorption of beam energy, causes mirror deformations which in turn lead to divergence of the beam and a focus shift at the plasma generation site.

[0008] It is desirable to reduce the impact of the mirror deformations on the beam shape and/or focus at the plasma generation site.

[0009] According to an aspect of the invention, there is provided a beam delivery apparatus for a laser produced plasma source, the beam delivery apparatus comprising: a plurality of mirrors operable to direct a beam of radiation to a plasma generation site; and a compensatory optical device being operable to compensate for defocus of said beam of radiation, said defocus being resultant from deformation of said mirrors due to them absorbing energy from said beam of radiation.

[0010] According to a further aspect of the invention, there is provided a method of generating plasma for use as a radiation source, comprising: generating a beam of radiation; directing the beam of radiation along a predetermined beam path to a plasma generation site using a plurality of mirrors; and compensating for defocus of said beam of radiation, said defocus being resultant from deformation of said mirrors due to them absorbing energy from said beam of radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0012] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention;

[0013] Figure 2 is a more detailed view of the apparatus 100;

[0014] Figure 3 is a more detailed view of the source collector module SO of the apparatus of Figures 1 and 2;

[0015] Figure 4 depicts a lithographic apparatus according to a further embodiment of the invention;

[0016] Figure 5 depicts a mirror such as that which may form part of a beam delivery system, and illustrates the issue of mirror heating and deformation.

[0017] Figure 6 depicts schematically a beam delivery system according to a first embodiment of the invention;

[0018] Figure 7 depicts a first wavefront reversal apparatus usable in the beam delivery system of Figure 6;

[0019] Figure 8 depicts a second wavefront reversal apparatus usable in the beam delivery system of Figure 6;

[0020] Figure 9 depicts part of a third wavefront reversal apparatus usable in the beam delivery system of Figure 6;

[0021] Figure 10 depicts schematically a beam delivery system according to a second embodiment of the invention; and

[0022] Figure 11 depicts a beam focusing element usable in the beam delivery system of Figure 10.

DETAILED DESCRIPTION

[0023] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector module SO according to one embodiment of the invention. The apparatus comprises:

[0024] an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation).

[0025] 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;

[0026] a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a rcsist-coatcd wafer) W and connected to a second positioner PW configured to accurately position the substrate; and

[0027] 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.

[0028] 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.

[0029] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0034] 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.

[0035] Referring to Figure 1, the illuminator EL receives an extreme ultra violet radiation beam from the source collector module 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 source collector module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a C02 laser is used to provide the laser beam for fuel excitation.

[0036] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source collector module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0037] 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 TL 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.

[0038] 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, die radiation beam B passes through die 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 PS1 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.

[0039] The depicted apparatus could be used in at least one of the following modes:

[0040] 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.

[0041] 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.

[0042] 3. In another mode, the support structure (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.

[0043] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0044] Figure 2 shows the apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector module SO. An EUV radiation emitting plasma 210 may be formed by a discharge produced plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the very hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma 210 is created by, for example, an electrical discharge causing an at least partially ionized plasma. 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, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0045] The radiation emitted by the hot plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap) which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 may include a channel structure. Contamination trap 230 may also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure, as known in the art.

[0(346] The collector chamber 211 may include a radiation collector CO which may be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module is arranged such that the intermediate focus IF is located at or near an opening 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[0(347] Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the wafer stage or substrate table WT.

[0048] More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 may optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in the Figures, for example there may be 1- 6 additional reflective elements present in the projection system PS than shown in Figure 2.

[0049] Collector optic CO, as illustrated in Figure 2, is depicted as a nested collector with grazing incidence reflectors 253, 254 and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254 and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

[0050] Alternatively, the source collector module SO may be part of an LPP radiation system as shown in Figure 3. A laser LA is arranged to deposit laser energy into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), creating the highly ionized plasma 210 with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near normal incidence collector optic CO and focused onto the opening 221 in the enclosing structure 220.

[0051] Figure 4 shows an alternative arrangement for an EUV lithographic apparatus in which the spectral purity filter SPF is of a transmissive type, rather than a reflective grating. The radiation from source collector module SO in this case follows a straight path from the collector to the intermediate focus IF (virtual source point). In alternative embodiments, not shown, the spectral purity filter 11 may be positioned at the virtual source point 12 or at any point between the collector 10 and the virtual source point 12. The filter can be placed at other locations in the radiation path, for example downstream of the virtual source point 12. Multiple filters can be deployed. As in the previous examples, the collector CO may be of the grazing incidence type (Figure 2) or of the direct reflector type (Figure 3).

[0052] As already mentioned, an LPP source uses a high power C02 radiation beam, which is transported to and focused on a tin droplet via a beam delivery system. This beam delivery system may comprise ten or more mirrors, such that the beam is reflected ten or more times. These mirrors are typically copper having a high reflectivity coating and internal water cooling. The lowest possible absorption per mirror is 0.2%. With a typical beam power of 10kW-40kW, this results in 20W-80W absorption per mirror. In reality, typical coatings are worse than this and more power is absorbed.

[0053] The absorption of beam power leads to a deformation of the mirror surface and change of its curvature. This change is mostly pronounced in the regions of the highest power load, which is normally at the center of the mirror. As a result, unwanted aberrations are induced, with the largest aberration being defocus. The normally flat mirrors of the beam transport system become convex, convex mirrors become “more convex” and concave mirrors “less concave, which results in beam divergence. Beam divergence leads to a focus shift (defocus) of the focused beam. This is illustrated in Figure 5, which shows a mirror surface 400, with incident 410 and reflected 420 beams when the mirror is cold and therefore undeformed. Also shown is the mirror surface 400’ and consequent divergent reflected rays 420’ when the mirror is heated and deformed. The effect of the deformation and divergence is exaggerated for illustration.

[0054] Furthermore a transient occurs when the source switches from off to on, and vice versa. The consequence of this transient is that, for the duration of the transition, the system is not in a stable state, and the output of the system (e.g. dose and focus) is not stable.

[0055] The effect of each mirror acts in the same direction such that no compensation occurs. Consequently, the focus position of the radiation beam depends on the absorption within the beam delivery system, and the focus shift is such that the beam does not always hit the tin droplet precisely. This results in a poor overlap of the tin droplet with the beam waist and as a consequence, reduced EUV generation efficiency. Dose variations can occur as a result. Furthermore, beam divergence can result in the beam growing to a size too large for the optics, resulting in further losses.

[0056] Figure 6 shows schematically a beam delivery arrangement according to a first embodiment, which aims to address these issues. It shows a laser source 500 (such as a C02 laser), beam delivery system 510 and plasma generation site 520. The beam delivery system comprises mirrors 540 and a wavefront reversal device 550. The wavefront reversal device may comprise an array retroreflectors or a phase-conjugate mirror. Also shown is the beam path 560 through the beam delivery system 510 (the arrow head of the beam indicating the direction of propagation) and a further arrows 570 indicating the wavefront direction at each stage of the beam path 560.

[0057] The wavefront reversal device 550 reverses the sign of the wavefront 570 of the beam, such that a converging beam becomes diverging beam and vice versa. This allows thermal induced effects of a first portion of the path 560 to compensate the effects in a second (subsequent) portion of the path 560. In one embodiment the wavefront reversal device 550 is located approximately halfway along the beam path 560, with the same (or similar) number of mirrors 540 either side of it. Alternatively, or in addition, the location of the device may be selected (or experimentally found) such that an optimum cancellation wavefront contribution occurs for the first and second portions of the beam path 560.

[0058] In an embodiment, the wavefront reversal device 550 comprises a retroreflector array. Retroreflectors reflect light back to its source with a minimum of scattering, the electromagnetic wavefront being reflected back along a vector that is parallel to but opposite in direction from the wave's source. They are simple passive devices comprising (for example) an array of small prisms or other optical elements in which entering light undergoes total internal reflection one or more times. Their simplicity results in a passive, cheap and simple method for compensating for thermal transients, thereby ensuring that the source C02 beam is intrinsically more stable.

[0059] In a specific example, the prism may comprise two, three or four orthogonal surfaces, each at 45 degrees with respect to the incident beam: the beam undergoing total internal reflection off two or more surfaces.

[0060] Because the reflected beam of a retroreflector follows essentially the same path than that of the incident beam, a beam splitter and polarization state changing optics may be used. Such an arrangement is shown in Figure 7. This shows retroreflector array 600, comprising prisms 610 and polarizing beam splitter 630 (The polarization optics are not shown for clarity). Also shown is beam path 660 (the arrow head of the beam indicating the direction of propagation) and further arrows 670 indicating the wavefront direction at each stage of the beam path 660. It should be noted that the use of a retroreflector results in a focus shift in comparison to using a normal mirror.

[0061] An oblique detail of a particular embodiment of retroreflector prism 620 which may be used is also shown in Figure 7. This prism comprises four reflecting surfaces arranged such that they resemble the inside surfaces of a pyramid, with light entering and exiting through the “base” of the pyramid.

[0062] Figure 8 shows a variation on the Figure 7 embodiment, which replaces the retroreflector array 600 with two orthogonal ID retroreflector structures 680a, 680b located at different points along the beam path 660. Retroreflector structures 680a, 680b may resemble grating stmetures.

[0063] Figure 9 illustrates a further embodiment which dispenses with the need for a beam splitter and polarization optics. Instead of using a more conventional retroreflector array, it is proposed to use what is referred to herein as a “semi-retroreflector” array. Such a semi-retroreflector array does not reflect an incident beam back in its original direction. Instead the beam is reflected at an angle related to its angle of incidence in a similar way to that of a mirror, except that the angle of incidence and angle of reflection are not equal.

[0064] Figure 9 shows a detail of a single optical element of the semi-retroreflector array. The single optical element may (as with the example of Figure 7) comprise four reflecting faces, of which two faces 700a, 700b are shown. Also shown is the beam path, comprising incident beam 710, intermediate beam 720 and reflected beam 730. The main difference between the semi-retroreflector array and conventional retroreflector array is that the angle γ of the reflecting faces 700a, 700b of the semi- retroreflector is not 45 degrees. As with the example of Figure 8, the semi-retroreflector array of this embodiment can be split into two ID structures.

[0065] The angle of reflection β can be determined from the angle of incidence a and the face angle γ. It can be shown that: β=180-α-4γ

[0066] This arrangement allows for a setup more like that shown in Figure 6, where the beams incident on and reflected from the scmi-rctrorcflcctor array follow different paths. This is more practical for high energy infrared beams, since the beam splitter and waveplates are not required.

[0067] Figure 10 illustrates an alternative method to address the issues discussed above, particularly in relation to Figure 5. Similarly to Figure 6, it shows a laser source 900 (such as a C02 laser), beam delivery system 910 and plasma generation site 920. The beam deliver}' system comprises mirrors 940. Also shown is the beam path 960 through the beam delivery system 910. The embodiment differs from that illustrated in Figure 6 by it not comprising a wavefront reversal device, but instead comprising a beam focusing element 950. While the beam focusing element 950 can be located at any point in the beam delivery system 910, it is preferably located at or near the beginning of the beam path 960 where beam diameter is not yet too large. The location of the device may be selected (or experimentally found) such that an optimum cancellation effect is achieved.

[0068] The beam focusing element 950 should focus the beam such that the degree of focusing (i.e. its focal point) is dependent on the impinging laser power (heat load). In one embodiment the degree of focusing is proportional to the impinging laser power, such that its focal length becomes shorter with increased power. In this way mechanical deformations are dynamically compensated, and beam size variations during power ramp-up and down (transient effects) are minimized.

[0069] Figure 11 illustrates a beam focusing element 950 which may be used in the beam delivery system of Figure 10. The effect of the deformation and focusing is exaggerated for illustration. The beam focusing element 950 comprises a mirror having a front surface 1060a which is transparent (e.g. antireflective (AR) coated) to an incident beam 1070, while the back surface 1060b is reflective (e.g. high reflective (HR) coated). In this arrangement, as the laser power is increased, the mirror becomes increasingly concave, as illustrated by the heated back surface of the mirror 1060b’, shown dotted. In this way, the reflected beam 1080, 1080’ is focused to a degree dependent on the absorbed laser power, which compensates for the defocus introduced by other mirrors in the beam delivery system 1010. Reflected beam 1080 is the beam reflected from a cold (undcformcd) beam focusing clement 950, and reflected beam 1080’ is the beam reflected from a deformed beam focusing element 950.

[0070] The mirror material for beam focusing element 950 should be transparent in the IR (10.6 pm) region. Preferably, the thermal deformation time constant matches that of the rest of the system. The time constant and amplitude of deformation may be tuned by selecting appropriate material, mirror thickness and cooling conditions. Some materials (used for high-power C02 laser windows and lenses) are listed in Table 1.

TABLE 1

[0071] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, the invention is more generally applicable to any EUV optical device requiring a plasma source, for example an aerial image measurement system or a reticle inspection system. Where lithographic apparatuses are described, they may have applications other than 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.

[0072] 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.

[0073] 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 arc set out as in the following numbered clauses: 1. A beam delivery apparatus for a laser produced plasma source, the beam delivery apparatus comprising: a plurality of mirrors operable to direct a beam of radiation to a plasma generation site; and a compensatory optical device being operable to compensate for defocus of said beam of radiation, said defocus being resultant from deformation of said mirrors due to them absorbing energy from said beam of radiation.

2. A beam delivery apparatus as claimed in clause 1 wherein said compensatory optical device comprises wavefront reversal optics located between two of said plurality of mirrors, said wavefront reversal optics being operable to reverse the wavefront direction of the beam of radiation.

3. A beam delivery apparatus as claimed in clause 2 wherein the wavefront reversal optics is located approximately half way along the path of said beam of radiation through the beam delivery apparatus.

4. A beam delivery apparatus as claimed in clause 2 or 3 wherein the difference between the number of mirrors on said path on one side of said wavefront reversal optics and the number of mirrors on said path on the other side of said wavefront reversal optics is not greater than one.

5. A beam delivery apparatus as claimed in clause 2, 3 or 4 wherein said wavefront reversal optics comprises an array of retroreflectors.

6. A beam delivery apparatus as claimed in clause 5 wherein each retroreflector comprises four reflector surfaces.

7. A beam delivery apparatus as claimed in clause 2, 3 or 4 wherein said wavefront reversal optics comprises two orthogonal retroreflector structures.

8. A beam delivery apparatus as claimed in clause 7 wherein each of said two orthogonal retroreflector structures comprises a grating structure.

9. A beam delivery apparatus as claimed in any of clauses 5 to 8 wherein said beam delivery apparatus comprises a beamsplitter for separating the paths of said wavefront reversal optics’ incident and reflected beams.

10. A beam delivery apparatus as claimed in clause 2, 3 or 4 wherein said wavefront reversal optics comprises at least one array of optical elements, each optical element comprising two or more reflector surfaces, each of said surfaces being arranged at an obtuse angle to the other surface(s).

11 A beam delivery apparatus as claimed in clause 10 wherein said obtuse angle is between 91 degrees and 150 degrees, and more preferably between 91 degrees and 120 degrees.

12. A beam delivery apparatus as claimed in clause 10 or 11 wherein said optical elements comprise prisms and said reflector surfaces are total internal reflection surfaces.

13. A beam delivery apparatus as claimed in clause 12 wherein each optical element comprises four reflector surfaces 14. A beam delivery apparatus as claimed in clause 10 or 11 wherein said wavefront reversal optics comprises two of said arrays of optical elements, arranged orthogonally.

15. A beam delivery apparatus as claimed in clause 14 wherein each of said two orthogonal arrays of optical elements comprises a grating structure.

16. A beam delivery apparatus as claimed in clause 1 wherein said compensatory optical device comprises a focusing element for focusing said beam of radiation.

17. A beam delivery apparatus as claimed in clause 16 wherein said beam focusing element is such that its focal length decreases as the amount of energy absorbed by it increases.

18. A beam delivery apparatus as claimed in clause 16 or 17 wherein said beam focusing element comprises a mirror having a transmissive front surface and a reflective rear surface.

19. A beam delivery apparatus as claimed in clause 18 wherein said mirror is comprised of a material substantially transparent to infra-red radiation, and comprises an anti-reflective coating on its front surface and a high-reflective coating on its rear surface.

20. A beam delivery apparatus as claimed in any of clauses 16 to 19 wherein said focusing element is located at the beginning of said beam delivery apparatus.

21. An EUV radiation source comprising: a laser device operable to emit a beam of radiation; a plasma generation site located at a position in which a fuel will be contacted by said beam of radiation to form a plasma; and a beam delivery apparatus as claimed in any of clauses 1 to 20, being operable to direct said beam of radiation from said laser device to said plasma generation site.

22. A lithographic apparatus, comprising: an EUV radiation source as claimed in clause 21 configured to generate a beam of EUV radiation; an illumination system configured to condition the beam of radiation; a support constructed to support a patterning device, the patterning device being capable of imparting the beam of radiation with a pattern in its cross-section to form a patterned beam of radiation; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned beam of radiation onto a target portion of the substrate.

23. A method of generating EUV radiation, comprising: generating a beam of radiation; directing the beam of radiation along a predetermined beam path to a plasma generation site using a plurality of mirrors; and compensating for defocus of said beam of radiation, said defocus being resultant from deformation of said mirrors due to them absorbing energy from said beam of radiation.

24. The method of clause 23 further comprising an initial step of determining an optimum location for performing said compensation for defocus, said initial step comprising: determining the location which provides the greatest cancellation of the effects resultant from mirror deformation of the mirrors on said path prior to where the compensation step is performed, by the effects resultant from mirror deformation of the mirrors on said path subsequent to where the compensation step is performed.

25. The method of clause 23 or 24 wherein the step of compensating for defocus comprises reversing the wavefront direction of the beam of radiation at a location along said predetermined beam path, between two of said mirrors.

26. The method of clause 25 wherein said wavefront reversal is performed using an array of retroreflectors.

27. The method of clause 25 wherein said wavefront reversal is performed using an array of optical elements, each optical element comprising two or more reflector surfaces, each of said surfaces being arranged at an obtuse angle to the other surface(s).

28. The method of clause 25 wherein said wavefront reversal is performed in two stages using two orthogonal 1-dimensional structures.

29. The method of clause 23 or 24 wherein the step of compensating for defocus comprises using a focusing element to focus said beam of radiation at a point along said predetermined path.

30. The method of clause 29 wherein said focusing of said beam is performed such that the degree of focusing decreases as the power of said beam of radiation increases.

Claims (1)

A lithography device comprising: an exposure device adapted to provide a radiation bundle; 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 for projecting 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.