NL2008963A - Ion capture apparatus, laser produced plasma radiation source, lithographic apparatus. - Google Patents

Description

Ion capture apparatus, Laser Produced Plasma radiation source, Lithographic

Apparatus

FIELD

The present invention relates to an ion capture apparatus for a laser produced plasma (LPP) radiation source apparatus, a laser produced plasma (LPP) radiation source apparatus comprising the ion capture apparatus, and a lithographic apparatus comprising the LPP radiation source.

BACKGROUND

[0001] 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.

[0002] 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.

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.

[0003] 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.

[0004] 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 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. 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.

[0005] For LPP type EUV sources the use of a magnetic field to collect and confine tin ions emitted from the plasma is well known. The purpose is to collect the ions using an ion capture apparatus including ion catcher devices. Such an ion capture apparatus is arranged to prevent ions reaching the mirrored surface of the radiation collector, or contaminating the enclosing structure with tin. It is well known that ions can seriously decrease the lifetime of a collector when they hit the optical surface.

SUMMARY

[0006] In order to provide an ion capture apparatus featuring an improved ion collection yield, an ion capture apparatus for a laser produced plasma (LPP) radiation source apparatus according to an aspect of an embodiment of the present invention comprises a magnetic field generator and two magnetic cores. The two magnetic cores are arranged to channel a magnetic field generated by said magnetic field generator, such that said magnetic field is largely confined to, and approximately homogeneous within, a region within the vicinity of the plasma of said LPP radiation source.

According to an aspect of the invention the two magnetic cores of said magnetic field generator comprise two respective coils, such that with each magnetic core there is associated a respective coil.

In an embodiment of the invention each of said magnetic cores extends from each respective coil towards the plasma.

According to an aspect of the invention there is provided a laser produced plasma (LPP) radiation source apparatus comprising an ion capture apparatus as described above.

[0007] According to a further aspect of the invention there is provided a lithographic apparatus, comprising the source apparatus as described above.

[0008] 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

[0009] 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;

Figure 3 illustrates a prior art ion capture arrangement;

Figure 4 illustrates magnetic field lines of the prior art ion capture arrangement shown in Figure 3, and

Figure 5 illustrates an ion capture arrangement according to an embodiment of the invention.

[0010] 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

[0011] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector apparatus SO according to one embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation).

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

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0018] 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.

[0019] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source collector apparatus 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 apparatus 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 apparatus. The laser and the source collector apparatus may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0020] 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 apparatus 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 apparatus, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0021] 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.

[0022] 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.

[0023] 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 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.

[0024] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0025] Figure 2 shows the apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector apparatus SO. The systems IL and PS are likewise contained within vacuum environments of their own. An EUV radiation emitting plasma 210 may be formed by a laser produced LPP plasma source. The function of source collector apparatus SO is to deliver EUV radiation beam 20 from the plasma 210 such that it is focused in a virtual source point. The virtual source point is commonly referred to as the intermediate focus (IF), and the source collector apparatus is arranged such that the intermediate focus IF is located at or near an aperture 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[0026] From the aperture 221 at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called “fly’s eye” illuminator, which is 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 21 at the patterning device MA, held by the support structure (mask table) 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. To expose a target portion C on substrate W, pulses of radiation are generated on substrate table WT and masked table MT perform synchronized movements 266, 268 to scan the pattern on patterning device MA through the slit of illumination.

[0027] Each system IL and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures similar to enclosing structure 220. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there may be more mirrors present than those shown in the Figures. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in Figure 2.

[0028] Considering source collector apparatus SO in more detail, laser energy source comprising laser 223 is arranged to deposit laser energy 224 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. Higher energy EUV radiation ay be generated with other fuel materials, for example Tb and Gd. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near-normal incidence collector CO and focused on the aperture 221. The plasma 210 and the aperture 221 are located at first and second focal points of collector CO, respectively.

[0029] To deliver the fuel, which for example is liquid tin, a droplet generator 226 is arranged within the enclosure 220, arranged to fire a high frequency stream 228 of droplets towards the desired location of plasma 210. In operation, laser energy 224 is delivered in a synchronism with the operation of droplet generator 226, to deliver impulses of radiation to turn each fuel droplet into a plasma 210. The frequency of delivery of droplets may be several kilohertz, for example 50 kHz. In practice, laser energy 224 is delivered in at least two pulses: a pre pulse with limited energy is delivered to the droplet before it reaches the plasma location, in order to vaporize the fuel material into a small cloud, and then a main pulse of laser energy 224 is delivered to the cloud at the desired location, to generate the plasma 210. A trap 230 is provided on the opposite side of the enclosing structure 220, to capture fuel that is not, for whatever reason, turned into plasma.

[0030] 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 20, 21, 26. 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 optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source collector apparatus, the X axis coincides broadly with the direction of fuel stream 228, 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.

[0031] Numerous additional components critical to operation of the source collector apparatus and the lithographic apparatus as a whole are present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector CO and other optics. Other features present but not described in detail are sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithographic apparatus.

[0032] Figures 3 and 4 illustrate a present arrangement for preventing deposits of fuel material damaging or impairing the performance of collector. It shows collector CO, a plasma 210 and an ion capture apparatus. At two opposing points on the periphery of the collector there are disposed respective ion catchers 300, being part of the ion capture apparatus. To accelerate the ions 310 towards the ion catchers 300, the ion capture apparatus includes magnetic coils 320 arranged to produce a magnetic field (illustrated by lines of flux 330) in the region of the plasma 210 and the ion catchers 300.

[0033] One problem with this arrangement of the ion capture apparatus is that it features a large gradient in the magnetic field strength when moving from the plasma 210 location (where the field strength may, for example, be 0.6T) to the catcher 300 position (where the field strength may, for example, be 1.0T). As a result, only a small fraction of the ions (less than 30%) is able to reach the ion catcher surfaces due to the mechanism of magnetic reflection (the magnetic mirror effect, see Fundamentals of plasma physics, J.A.Bittencourt), thereby reducing the ion collection yield of the ion capture apparatus.

[0034] Another problem with this arrangement is that, since an LPP source vessel uses gas (e.g. hydrogen) for thermalizing tin atoms, the interaction of tin ions with gas molecules causes scatter which further decreases the ion collection yield of the ion capture apparatus.

[0035] Furthermore, with the ion catchers 300 placed close to the mirror surface of the collector CO, serious contamination of the collector will occur as a result of sputtering of material from the ion catcher surfaces. Sputter originates from the impact of tin ions hitting an ion catcher surface.

[0036] Figure 5 shows an ion capture apparatus according to an aspect of the invention, including a magnet configuration for an LPP EUV source which addresses these drawbacks of arrangements depicted in Figures 3 and 4. Again, it shows collector CO and plasma 210. Outside of the chamber wall 340, which may be a portion of the enclosure 220, are magnetic coils 320. Magnetic field guides or cores 350 extend from the magnetic coils 320 into the chamber and towards plasma 210. Shields 360 are attached to the cores 350.

[0037] The two magnetic coils 320 in combination with the two cores 350 of a material having high magnetic permittivity (like iron) guide the magnetic field such that it is strongly confined to, and highly homogeneous within, a region directly around the plasma 210. To provide a homogenous field, the cores are preferably of solid (i.e. not hollow) cross-section. The end surface 370 of each core, which is facing the plasma, serves as ion catcher. Flereinafter, the end surfaces 370 may also be referred to as the ion catch surfaces 370.

[0038] Since the magnetic field is largely homogeneous (with a magnetic field strength gradient of less then 0.4T between the location of the plasma 210 and the ion catch surfaces 370, or with a substantially vanishing magnetic field strength gradient) magnetic reflection will not occur or will be substantially absent, facilitating a greatly improved ion collection yield, which may be at or approaching 100%. Additionally, as the catcher surfaces 370 are closer to the plasma compared to the arrangement shown in Figure 3, the impact of ion scatter when encountering hydrogen gas molecules will decrease.

[0039] It is envisaged that a cooling function should be included in the core structure to keep its temperature below Curie temperature so as to ensure confinement of the magnetic field during high power operation of the source.

[0040] The side of the cores 350 facing the collector CO further includes a shielding feature 360 to avoid direct line of sight from the ion catch surface 370 to the collector CO to mitigate risk of collector contamination by sputtering material present at the ion catch surface 370.

[0041] This arrangement allows operation of the source vessel at two different pressure regimes of hydrogen gas: a “hot” region with low pressure (for example, less than lOPa), confined to the region between the two ion catch surfaces 370 and a “cold” region of high pressure (for example, 20-100Pa) in front of the collector mirror CO for thermalizing tin neutrals (and any ions not collected by the magnetic field).

[0042] A further advantage over prior arrangements is that, due to the magnetic field confinement, the magnetic coils may be significantly smaller and therefore the volume clause for the magnet coil structure can be significantly reduced. Also, a smaller magnet coil structure reduces magnetic stray fields in the area around the source.

[0043] 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.

[0044] 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.

[0045] 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 he 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. An ion capture apparatus for a laser produced plasma (LPP) radiation source apparatus comprising a magnetic field generator and two magnetic cores, wherein said magnetic cores are arranged to channel a magnetic field generated by said magnetic field generator, such that said magnetic field is largely confined to, and approximately homogeneous within, a region within the vicinity of the plasma of said LPP radiation source.

2. The ion capture apparatus of clause 1 wherein the two magnetic cores of said magnetic field generator comprise two respective coils, such that with each magnetic core there is associated a respective coil.

3. The ion capture apparatus of clause 2 wherein each of said magnetic cores extends from each respective coil towards the plasma.

4. The ion capture apparatus of clause 1, 2 or 3 wherein said apparatus is operable to capture ions on each magnetic core’s end surface adjacent to the plasma.

5. The ion capture apparatus of any preceding clause comprising means for cooling said magnetic cores.

6. The ion capture apparatus of any preceding clause wherein said magnetic cores are of solid cross-section.

7. A laser produced plasma (LPP) radiation source apparatus comprising: an ion capture apparatus of any preceding clause; a radiation collector and a chamber, said radiation source apparatus being arranged such that said radiation collector, said plasma and said region within the vicinity of the plasma in which the magnetic field is largely confined is comprised within said chamber.

8. The radiation source apparatus of clause 7 wherein shielding is provided between each magnetic core’s end surface adjacent to the plasma, and said radiation collector.

9. The radiation source apparatus of clause 7 or 8 being operable such that in use, said chamber contains a gas, said gas being at a lower pressure in the region of the magnetic field than in front of the collecting/reflecting surface of said radiation collector.

10. The radiation source apparatus of clause 9 wherein said gas is at a pressure in front of the collecting/reflecting surface of said radiation collector that is at least twice that in the region of the magnetic field.

11. A lithographic apparatus, comprising: a source apparatus as claimed in any of clauses 7 to 10 configured to generate a beam of EUV radiation; an illumination system configured to condition the radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate.

Claims (1)

A lithography device comprising: an exposure 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.