CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US provisional application 61/293,139 which was filed on 07 January 2010, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to an EUV radiation source and to a lithographic apparatus.

BACKGROUND

[0003] 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] 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.

[0005] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

CD = k _l *—

NA (1)

where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k _\ 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 X, by increasing the numerical aperture NA or by decreasing the value of k _\ .

[0006] 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, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0007] 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 droplets 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.

[0008] The intensity of EUV radiation which is generated by an LPP source may suffer from unwanted fluctuations. These unwanted fluctuations may have a detrimental effect on the accuracy with which a pattern is imaged onto a substrate by a lithographic apparatus.

[0009] It is desirable to provide an EUV radiation source and lithographic apparatus which suffers from smaller fluctuations of EUV radiation intensity than at least some prior art EUV radiation sources and lithographic apparatus.

SUMMARY

[00010] According to an aspect of the invention, there is provided an EUV radiation source that includes a fuel supply configured to supply fuel to a plasma formation location. The fuel supply includes a reservoir configured to hold fuel at a temperature that is sufficiently high to maintain the fuel in liquid form, and a pressure vessel configured to contain the reservoir, the pressure vessel being at least partially thermally isolated from the reservoir. The EUV radiation source also includes a laser radiation source configured to irradiate fuel supplied by the fuel supply at the plasma formation location.

[00011] According to an aspect of the invention, there is provided a method of generating EUV radiation that includes holding a fuel in a reservoir at a temperature that is sufficiently high to maintain the fuel in liquid form; applying a pressure to the fuel using a pressure vessel which holds the reservoir, the pressure vessel being at least partially thermally isolated from the reservoir; ejecting a droplet of fuel from the reservoir via a nozzle; and directing a laser beam at the droplet of fuel such that the droplet of fuel vaporizes and generates EUV radiation.

[00012] According to an aspect of the invention there is provided a lithographic apparatus that includes an EUV radiation source configured to generate EUV radiation. The EUV radiation source includes a fuel supply configured to supply fuel to a plasma formation location. The fuel supply includes a reservoir configured to hold fuel at a temperature that is sufficiently high to maintain the fuel in liquid form, and a pressure vessel configured to contain the reservoir, the pressure vessel being at least partially thermally isolated from the reservoir. The EUV radiation source also includes a laser radiation source configured to irradiate fuel supplied by the fuel supply at the plasma formation location, a support configured to support a patterning device, the patterning device being configured to pattern the EUV radiation to create a pattemed radiation beam, and a projection system configured to project the pattemed radiation beam onto the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[00013] 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:

[00014] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention;

[00015] Figure 2 is a more detailed view of the apparatus of Figure 1 , including an LPP source collector module; and

[00016] Figure 3 schematically depicts a fuel supply of an EUV radiation source of the lithographic apparatus of Figures 1 and 2. DETAILED DESCRIPTION

[00017] Figure 1 schematically depicts a lithographic apparatus 100 according to an embodiment of the invention. The lithographic apparatus includes an EUV radiation source according to an embodiment of the invention. The apparatus comprises an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation); a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g. a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[00018] 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.

[00019] 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.

[00020] 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.

[00021] 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.

[00022] 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.

[00023] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[00024] 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.

[00025] Referring to Figure 1 , the illuminator IL receives an extreme ultraviolet (EUV) radiation beam from the source collector module SO. Methods to produce EUV radiation 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 of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation source 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.

[00026] The laser and the source collector module may be separate entities, for example when a C0 ₂ laser is used to provide the laser beam for fuel excitation. In such cases, 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. The laser and a fuel supply may be considered to comprise an EUV radiation source.

[00027] 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.

[00028] 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 PSl 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.

[00029] The depicted apparatus could be used in at least one of the following modes:

[00030] 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.

[00031] 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. [00032] 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 a programmable patterning device, such as a programmable mirror array of a type as referred to above.

[00033] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[00034] Figure 2 shows the lithographic 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.

[00035] A laser LA is arranged to deposit laser energy via a laser beam 205 into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li) which is provided from a fuel supply 200. This creates a highly ionized plasma 210 at a plasma formation location 21 1 which has electron temperatures of several l O's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected and focussed by a near normal incidence radiation collector CO. The laser LA and fuel supply 200 may together be considered to comprise an EUV radiation source.

[00036] Radiation that is reflected by the radiation collector CO is focused at a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module SO is arranged such that the intermediate focus IF is located at or near to an opening 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[00037] 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.

[00038] More elements than shown may generally be present in the illumination system IL and projection system PS. Furthermore, 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.

[00039] Figure 3 shows schematically the fuel supply 200 in more detail. The fuel supply comprises a reservoir 300 which contains a fuel liquid 302 (for example liquid tin), and a nozzle 304 which is configured to eject droplets of the fuel liquid towards the plasma formation location 211 (see Figure 2). The droplets of fuel liquid may be ejected from the nozzle 304 by a combination of pressure within the reservoir and a vibration applied to the nozzle by a piezoelectric actuator. Two droplets of fuel 306 are shown in Figure 3, together with arrows which indicate the direction of travel of the droplets of fuel. The reservoir 300 is located within a pressure vessel 308. The pressure vessel 308 is connected to a source of high pressure gas (e.g. Argon) via a connector 310.

[00040] The reservoir 300 includes heaters (not shown) which are configured to heat the fuel to a temperature that is sufficiently high to maintain the fuel in a liquid form. For example, if the fuel is tin then it may be heated to at temperature which is above 232°C (e.g. around 270°C). The heaters may for example be located in one or more walls of the reservoir 300 and/or in the base of the reservoir. Alternatively, the heaters may be provided at any other suitable location.

[00041] Walls 312 of the pressure vessel are at least partially thermally isolated from the reservoir 300. The thermal isolation may be provided via one or more thermal isolation features and/or apparatus. A thermal isolation feature may comprise providing a gap between the reservoir 300 and walls 312 of the pressure vessel 308, such that heat is not conducted directly from the reservoir to the walls of the pressure vessel. A thermal isolation apparatus may comprise a thermally insulating heat shield 314 which is provided around the reservoir 300. The thermally insulating heat shield 314 may include an active cooling apparatus (e.g. apparatus which facilitates circulation of a cooling fluid through the heat shield). Additionally or alternatively, a thermal isolation apparatus may comprise constructing supports 316, which are used to support the reservoir 300 in the pressure vessel 308, from a material which acts as a thermal insulator. [00042] The at least partial thermal isolation of the pressure vessel 308 from the reservoir 300 allows walls 312 of the pressure vessel 308 to be maintained at a low temperature. In this context, the term 'low temperature' is intended to mean a temperature which is significantly less than the temperature of the fuel liquid 302.

[00043] The reservoir 300 has an open construction, thereby ensuring that there is no difference between the pressure inside the reservoir 300 and the pressure outside of the reservoir.

[00044] The fuel supply 200 of Figure 3 allows the fuel 302 to be maintained at a temperature which is sufficiently high to keep the fuel liquid, while at the same time allowing a pressure of for example 400, 600, 800, 1000 bar or higher to be applied to the fuel liquid. The fuel supply 200 allows a fuel liquid pressure to be achieved which may not be achievable using a conventional fuel supply (this may for example be limited to 200 bar).

[00045] The fuel supply 200 allows a combination of high temperature and high pressure to be achieved because the reservoir 300 which is used to maintain the high temperature of the fuel liquid 302 is at least partially thermally isolated from the walls 312 of the pressure vessel 308. In prior art fuel supplies, a fuel reservoir is formed by walls of a pressure vessel, and consequently the walls of the pressure vessel have a similar temperature to the fuel liquid. It is difficult to maintain a fuel liquid at a high temperature and high pressure (e.g. at 270°C and at 1000 bar) in prior art fuel supplies, since seals of the pressure vessel are prone to failure when both the temperature and the pressure are high.

[00046] The fuel supply 200 thus allows the fuel liquid 302 to be held at a pressure which is higher than the pressure that is achievable using conventional prior art fuel supplies, while maintaining the fuel liquid at a sufficiently high temperature to keep it in a liquid form.

[00047] Since the fuel liquid 302 is held at a higher pressure than is usual, the speed at which droplets of fuel 306 are projected from the nozzle 304 is increased. This increased speed of the fuel droplets 306 may provide two potential advantages.

[00048] The first potential advantage relates to the fact that a fuel droplet generates a shockwave when it is vaporized by the laser beam 205. This Shockwave will be incident upon a subsequent fuel droplet which is travelling towards the plasma formation location 211. The shockwave may modify the direction of travel of the fuel droplet such that the fuel droplet will not pass through an optimally focussed portion of the laser beam 205 at the plasma formation location 21 1 (see Figure 2), and thus may not be vaporized in an optimum manner. The increased speed of fuel droplets generated by the fuel supply 200 increases the separation between the fuel droplets (for a given EUV plasma generation frequency). The Shockwave is spherical, and has an energy which decreases quadratically as a function of distance from the plasma formation location. Thus, increasing the separation between fuel droplets reduces the force of the Shockwave on a subsequent fuel droplet. Furthermore, since the subsequent fuel droplet is travelling more quickly, it has higher momentum and thus is affected less by the shockwave. Both of these effects reduce the extent to which the direction of travel of the subsequent fuel droplet is modified by the shockwave, and consequently the subsequent fuel droplet passes closer to the optimally focussed portion of the laser beam 205 at the plasma formation location. Therefore, the fuel droplet may be vaporized more consistently and efficiently.

[00049] The second potential advantage relates to the fact that the laser beam 205 exerts force on each fuel droplet, which pushes each fuel droplet away from the plasma formation location 21 1. Deviation of the fuel droplet away from the plasma formation location 21 1 is undesirable, because the fuel droplet will not pass through an optimally focussed portion of the laser beam 205, and thus the fuel droplet will not be vaporized in an optimum manner. Increasing the speed of the fuel droplets reduces the deviation of fuel droplets from the plasma formation location 21 1 caused by the laser beam 205. As a result, the fuel droplet may pass closer to an optimally focussed portion of the laser beam 205, and thus the fuel droplet may be vaporized more consistently and efficiently.

[00050] Both of the above potential advantages may allow the fuel droplets 306 to be delivered to the plasma formation location with improved accuracy. This in turn may allow vaporization of the fuel droplets to be achieved more consistently and efficiently. Thus, EUV radiation may be provided with a more consistent intensity.

[00051] As can be seen from Figure 3, the reservoir 300 is open at an upper end. In an alternative arrangement, the reservoir 300 may be partially closed at an upper end. This may allow some thermal insulation to be provided at the upper end of the reservoir. The reservoir is not fully closed, and consequently the pressure in the pressure vessel is equal to the pressure in the reservoir.

[00052] Although the reservoir 300 and pressure vessel 308 shown in Figure 3 are both rectangular in shape, and are both provided with vertical sides and a horizontal bottom surface, they may have any suitable shape or orientation. For example, they may be orientated at an angle relative to the vertical, as is shown schematically in Figure 2.

[00053] The above description refers to fuel droplets. This may include for example clusters of fuel material, or fuel material provided in other discrete pieces.

[00054] The above description refers to the reservoir being at least partially thermally isolated from the pressure vessel. The term 'at least partially thermally isolated' is not intended to mean that no heat passes from the reservoir to the pressure vessel. Instead, it may be interpreted as meaning that at least some heat does not pass from the reservoir to the pressure vessel. This allows the temperature of walls of the pressure vessel to be significantly lower than the temperature of the reservoir.

[00055] 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, 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.

[00056] 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.

[00057] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.