CN111279267B

CN111279267B - Apparatus for receiving conductive fuel

Info

Publication number: CN111279267B
Application number: CN201880069896.5A
Authority: CN
Inventors: 约翰内斯·克里斯蒂安·里昂纳多斯·弗兰肯; M·R·德巴尔
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2017-10-26
Filing date: 2018-09-05
Publication date: 2023-08-29
Anticipated expiration: 2038-09-05
Also published as: WO2019081105A1; CN111279267A; KR20200068679A; NL2021562A

Abstract

An apparatus for receiving conductive fuel comprising: a main body; a current generating mechanism; and a magnetic generating mechanism. The body defines a surface for receiving fuel. The current generating means is adapted to generate a current in the body, the current having a component at least parallel to the surface. The magnetic generating means is arranged to generate a magnetic field having a component at least perpendicular to said surface.

Description

Apparatus for receiving conductive fuel

Cross Reference to Related Applications

The present application claims priority from european/us application 17198469.3 filed on 10.26 2017, which is incorporated herein by reference in its entirety.

Technical Field

The present application relates to an apparatus for receiving conductive fuel. The application relates in particular to a device for receiving an electrically conductive fuel, said device being provided with means for exerting a force on the received fuel to move it. The device may be a fuel collector adapted to receive and at least partially contain the electrically conductive fuel it receives. The apparatus may form part of a laser produced plasma radiation source.

Background

A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). The lithographic apparatus may, for example, project a pattern from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate.

The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the smallest dimension of a feature that can be formed on the substrate. Lithographic apparatus using EUV radiation (electromagnetic radiation having a wavelength in the range of 4-20 nm) may be used to form smaller features on a substrate than conventional lithographic apparatus (which may, for example, use electromagnetic radiation having a wavelength of 193 nm).

EUV radiation may be generated using a plasma. The plasma may be generated, for example, by directing a laser beam at fuel in the radiation source. The generated plasma may emit EUV radiation. Such radiation sources are known as Laser Produced Plasma (LPP) radiation sources. In such LPP radiation sources, at least a portion of the fuel may be incident on surfaces within the radiation source. It may be desirable to limit the amount of such fuel incident on the surface of at least an optical element (such as, for example, a radiation collector) within the LPP source. Contamination of the surface of such optical elements in the radiation source by fuel may lead to reduced performance of the radiation source and, in turn, may lead to associated degradation of lithographic apparatus performance. Eventually, this may lead to a significant downtime of the lithographic apparatus, while cleaning or replacing parts of the radiation source.

It is an object of the present invention to obviate or mitigate at least one problem of the prior art.

Disclosure of Invention

According to a first aspect of the present invention there is provided an apparatus for receiving an electrically conductive fuel, the apparatus comprising: a body defining a surface for receiving fuel; a current generating mechanism for generating a current in the body, the current having a component at least parallel to the surface; and a magnetic generating mechanism arranged to generate a magnetic field having a component at least perpendicular to the surface.

The device may be a fuel collector. The fuel collector may be an electrically conductive fuel adapted to receive and at least partially contain its receipt.

The conductive fuel may be received by a surface defined by the body. At least a portion of the body proximate the surface is formed of a conductive material to support an electrical current. When the current generating mechanism generates an electrical current in the body, at least a portion of the electrical current may flow through any conductive fuel deposited on the surface and thus in contact with the surface. When an electric current flows through the fuel, the current will exert a force on the fuel.

The applied force is given by the lorentz force formula. In particular, the force is perpendicular to both the magnetic field and the current.

The device according to the first aspect of the invention is advantageous in that it provides a surface for receiving the electrically conductive fuel and the current generating means and the magnetic generating means together provide a means for applying a force to the fuel. This can for example allow guiding the fuel away from the area or zone of the surface on which it is incident and towards a collection container or reservoir of the fuel, for example.

The device according to the first aspect of the invention has the advantage that it provides a mechanism for applying a force to the fuel incident thereon which is capable of preventing accumulation of fuel on the surface. Typically, when the fuel is incident on the surface, a first portion of the fuel may be deposited on the surface and a second portion of the fuel may scatter or bounce off the surface. It may be convenient, for example, to arrange the surface such that in use fuel is incident on the surface at a glancing incidence angle. With this arrangement, the portion of fuel incident on the surface from the inlet aperture or the like and bouncing off the surface will tend to have a trajectory pointing away from the inlet aperture as a whole. However, if a sufficiently large amount of fuel accumulates on the surface, at least a portion of the fuel directed toward the surface may bounce back toward the inlet aperture.

The first aspect of the invention is thus able to retrieve the amount of fuel backscattered from the surface in general, i.e. the amount of fuel bouncing in a direction substantially antiparallel to the initial trajectory of the incident fuel.

The body may include a non-conductive support portion and a layer of conductive material defining the surface.

This arrangement is advantageous, as will now be discussed. Providing the support portion allows the thickness of the layer of conductive material to be reduced relative to the thickness of material required if the conductive portion of the body is self-supporting. Further, by forming the support portion from a non-conductive material, the current generated in the body is restricted from flowing in the layer of conductive material, which may be relatively thin.

As explained above, when the current generating mechanism generates an electric current in the body, at least a portion of the electric current may flow through any conductive fuel deposited on the surface. The magnitude of the current flowing through the deposited conductive fuel depends on the relative resistance of: a conductive fuel and a layer of conductive material forming a portion of the body.

The resistance of the layer of conductive material depends on its thickness (and on its specific resistance). In particular, by reducing the thickness of the conductive material layer, the resistance of the conductive material layer increases. In turn, this increases the fraction of the current flowing through the deposit of conductive fuel on the surface. This has the effect of increasing the forces exerted on such deposits, thereby increasing the efficiency of moving them away from the area or region of the surface from which they are incident, reducing the tendency of the fuel to backscatter from the body.

The resistance of the deposit of conductive fuel is also dependent on its thickness (and on its specific resistance). The larger the deposit of fuel, the lower the resistance of the deposit and therefore the greater the magnitude of the current flowing through the deposit of conductive fuel. In turn, a larger current will result in a greater force being applied to the deposit. In this way it can be seen that the apparatus according to the first aspect of the invention is advantageously self-adjusting in that it applies more force to larger deposits (which may tend to cause more backscatter).

At least a portion of the layer of conductive material comprises a material that is readily wettable by tin.

It should be appreciated that a material that is readily wettable by tin may be defined as one such material: for this material, the contact angle at which the liquid-vapor interface of the tin droplet deposited on the material intersects the interface between tin and the material is less than 90 °. Suitable materials include, for example, stainless steel ANSI 316L.

At least a portion of the layer of conductive material includes a layer of liquid tin. For example, the apparatus may be provided with a mechanism for wetting the surface with tin.

The apparatus further comprises a heater arranged to heat the surface. The heater may be arranged to heat the surface to a temperature above the melting point of the conductive fuel (such as, for example, tin) that is collected by the device in use.

The apparatus may further comprise a container for collecting at least a portion of the fuel incident on the surface.

In use, the conductive fuel may strike a surface and may then be directed toward the container. The current generating means may be arranged to generate an electrical current and the magnetic generating means may be arranged to generate a magnetic field such that a force is exerted on a medium supporting the electrical current, the force being directed generally towards the container.

At least a portion of the current generated by the current generating mechanism may be supported by the conductive fuel deposited on the surface. With the arrangement in which the force applied to the medium supporting the current is directed generally towards the vessel, any conductive fuel deposited on the surface is directed towards the vessel.

The current generation mechanism includes a power source connected to the body via a physical link to enable generation of the current in the body. It will be appreciated that such a power supply will be arranged to transmit current to and receive current from the body via the physical link, thereby forming a circuit with the body. For example, the power source may be operable to generate a voltage across at least a portion of the surface.

Alternatively, the current generating mechanism may be operable to generate an electric current in the body via electromagnetic induction. The current generating mechanism may include a magnet operable to generate a time-varying magnetic field for generating eddy currents within the body.

For example, the magnet may comprise an electromagnet that can generate a time-varying magnetic field by varying the current supplied to the magnet. Additionally or alternatively, the magnet may be moved relative to the surface such that the magnetic field generated by the magnet at a given point on the surface varies over time. The time-varying magnetic field will induce eddy currents in the body (and any conductive fuel deposited thereon). For such an embodiment, the same magnet may also form part of the magnetic field generating mechanism.

The current generating means may be arranged to generate a current flowing across the surface in a generally first linear direction. With this arrangement, the force exerted by the magnetic field on the medium supporting the current is substantially perpendicular to the first linear direction. That is, the force exerted by the magnetic field on the medium supporting the current is in substantially the same direction as all positions in the plane of the surface. For arrangements in which the magnetic field is perpendicular to the surface, the force applied to the medium supporting the current is in a second linear direction, which is substantially parallel to the surface.

The magnetic generating mechanism may comprise an electromagnet. Such an arrangement may be beneficial because such an electromagnet may be operable to apply a magnetic field over a greater range of ambient temperatures. For example, the apparatus may form part of a laser produced plasma radiation source SO and may be operated at a temperature exceeding the melting point of a fuel such as, for example, tin. That is, in use, the apparatus may be operated at temperatures in excess of 232 ℃. Another advantage of using electromagnets may be that they are operable to also function as current generating mechanisms.

Alternatively, the magnetic generating mechanism may comprise a permanent magnet. For such embodiments, the permanent magnet preferably has a curie temperature that is higher than the melting point of the conductive fuel desired to be collected with the device. For example, the apparatus may form part of a laser produced plasma radiation source and may operate at temperatures exceeding the melting point of fuels such as, for example, tin. It may therefore be preferable that the permanent magnet has a curie temperature above the melting point of tin (232 ℃).

The apparatus may further comprise a wall defining the inlet aperture. The wall may, for example, separate the body from a source of conductive fuel such as, for example, a fuel droplet generator within a laser-produced plasma radiation source. The conductive fuel may pass through the inlet aperture and strike the surface. The wall may thus provide a barrier between the body receiving the fuel and the region where such fuel is desired to be relatively absent (e.g., a container of a laser-produced plasma radiation source).

The wall may form part of a housing defining a cavity within which the body is disposed. Such a housing may better contain fuel received through the inlet aperture.

According to a second aspect of the present invention there is provided a radiation system comprising: a fuel emitter configured to provide fuel to the plasma formation region; an excitation means arranged to provide an excitation beam at the plasma formation region to convert at least a first portion of the fuel into a radiation-emitting plasma; and the apparatus of the first aspect of the invention is configured to collect at least a second portion of the fuel.

The second portion of the fuel may comprise fuel that passes through the plasma formation region without being converted to a radiation-emitting plasma.

The surface of the body may be arranged such that the second portion of the fuel is incident on the surface at a glancing incidence angle. This arrangement reduces the amount of fuel "back-scattered" from the surface. That is, less conductive fuel is scattered or bounced off the surface in a direction generally antiparallel to the initial trajectory of the fuel.

It should be appreciated that the term "glancing incidence angle" herein refers to the angle between the direction of propagation of the electro-fuel and the surface upon which it is incident. This angle is complementary to the angle of incidence, i.e. the glancing angle of incidence plus the angle of incidence is a right angle. It will also be appreciated that fuel incident on a surface at a glancing incidence angle is intended to mean that the glancing incidence angle is relatively small such that the path of the fuel is nearly parallel to the surface. For example, the glancing incidence angle may be less than 30 °. In some embodiments, the glancing incidence angle may be in the range of 2 ° to 12 °, for example about 7 °.

According to a third aspect of the invention, there is provided a lithographic system comprising: a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate; and a radiation system according to a second aspect of the invention, arranged to provide at least some of the radiation to the lithographic apparatus.

The lithographic apparatus may include an illumination system configured to condition at least some of the radiation so as to form a radiation beam. The lithographic apparatus may include a support structure configured 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. The lithographic apparatus may include a substrate table configured to hold a substrate. The lithographic apparatus may include a projection system configured to project a patterned radiation beam onto a substrate.

According to a fourth aspect of the present invention there is provided a method of moving deposits of conductive fuel on a surface of a body, the method comprising: generating a magnetic field having a component at least perpendicular to the surface; an electrical current is generated in the body, the electrical current having a component at least parallel to the surface such that at least a portion of the electrical current flows through the deposit of conductive fuel.

When current flows through the deposits of conductive fuel, the current will exert a force on the fuel. The applied force is given by the lorentz force formula. In particular, the force is perpendicular to both the magnetic field and the current.

The method according to the fourth aspect of the invention is advantageous in that it provides a simple arrangement for applying a force to the deposit of fuel. This can for example allow guiding the fuel away from the area or zone of the surface on which it is incident and towards a collection container or reservoir of the fuel, for example.

The resistance of the deposit of conductive fuel depends on its thickness. The larger the deposit of fuel, the lower the resistance of the deposit and therefore the greater the magnitude of the current flowing through the deposit of conductive fuel will be. In turn, a larger current will result in a larger force being exerted on the deposit by the magnetic field. In this way it can be seen that the method according to the fourth aspect of the invention is advantageously self-adjusting in that it applies more force to larger deposits (which may tend to cause more backscatter).

The magnetic field and the current may be continuously generated. This may allow the conductive fuel to move continuously over the surface, thereby limiting the size of deposits of conductive fuel that can form on the surface. Alternatively, the magnetic field and current may be generated intermittently or periodically. This may allow the conductive fuel to accumulate on the surface when no magnetic field and current are being generated, and then move the accumulation each time a magnetic field and current are generated.

It will be apparent to those skilled in the art that the various aspects and features of the invention set forth above or below may be combined with various other aspects and features of the invention.

Drawings

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

FIG. 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source incorporating a fuel collector according to an embodiment of the invention;

FIG. 2 is a first view of a fuel collector, which may form part of the lithography system shown in FIG. 1, according to an embodiment of the invention;

FIG. 3 is a second view of a fuel collector, which may form part of the lithography system shown in FIG. 1, FIG. 3 showing a deposit of fuel on a surface of a body, according to an embodiment of the invention;

FIG. 4 is a third view of a fuel collector, which may form part of the lithography system shown in FIG. 1, according to an embodiment of the invention, FIG. 4 showing the magnetic field generated by the magnet and the Lorentz force acting on the deposit of fuel;

fig. 5 shows a plan view of the body of the fuel collector shown in fig. 2, 3 and 4, fig. 5 showing the current generated by the power supply and the lorentz forces acting on the deposits of fuel; and is also provided with

Fig. 6 is a cross-sectional view of a portion of the body of the fuel collector shown in fig. 2, 3 and 4.

Detailed Description

FIG. 1 depicts a lithography system including a fuel collector 15 according to one embodiment of the invention. The lithographic system includes a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an Extreme Ultraviolet (EUV) radiation beam B. The lithographic apparatus LA comprises: an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS, and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident on the patterning device MA. The projection system is configured to project a radiation beam B (now patterned by a mask MA) onto a substrate W. The substrate W may include a previously formed pattern. In this case, the lithographic apparatus aligns the patterned beam of radiation B with a pattern previously formed on the substrate W.

The radiation source SO, the illumination system IL, and the projection system PS may be constructed and arranged SO that they are isolated from the external environment. A gas (e.g. hydrogen) at a sub-atmospheric pressure may be provided in the radiation source SO. A vacuum may be provided in the illumination system IL and/or the projection system PS. A small amount of gas (e.g., hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

The radiation source SO shown in fig. 1 is of a type which may be referred to as a Laser Produced Plasma (LPP) source. The laser 1 (which may be CO, for example ₂ A laser) is arranged to deposit energy into the fuel, such as tin (Sn) provided from the fuel emitter 3, via the laser beam 2. Although tin is mentioned in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, for example in the form of droplets, along a trajectory 16 towards the plasma formation zone 4. The laser beam 2 is incident on tin at the plasma formation region 4. Laser energy is deposited into the tin, generating a plasma 7 at the plasma formation region 4. During de-excitation and recombination of ions of the plasma, radiation, including EUV radiation, is emitted from the plasma 7.

EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multi-layered structure arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration with two elliptical foci. The first focus may be at the plasma formation region 4 and the second focus may be at the intermediate focus 6, as described below.

The radiation source SO further comprises a fuel collector 15. The fuel collector 15 may be arranged to collect at least a portion of the fuel that passes through the plasma formation region 4 of the radiation source SO (i.e., along the trajectory 16) without being converted into a radiation-emitting plasma.

The laser 1 may be separated from the radiation source SO. In this case, the laser beam 2 may be transferred from the laser 1 to the radiation source SO by means of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or beam expanders, and/or other optics. The laser 1 and the radiation source SO may together be considered as a radiation system.

The supply of hydrogen may also be provided through the opening 1 substantially along the same axis as the laser beam. Hydrogen may also be supplied around the collector 5 and/or optionally through a supply port. Hydrogen has a variety of purposes including minimizing contamination of the collector 5 (and optionally also the metrology module, not shown), acting as a source of hydrogen radicals for purification, and conditioning the plasma to keep the ionized gas away from the collector CO and metrology module.

The radiation reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at a point 6 to form an image of the plasma formation zone 4, which serves as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as an intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near an opening 8 in a closed structure 9 of the radiation source.

The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. The illumination system IL may include a facet field mirror device 10 and a facet pupil mirror device 11. Together, facet field mirror device 10 and facet pupil mirror device 11 provide a desired cross-sectional shape and a desired angular distribution for radiation beam B. The radiation beam B is delivered from the illumination system IL and is incident on the patterning device MA, which is held by the support structure MT. The patterning device MA reflects the radiation beam B and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or in place of facet field mirror device 10 and facet pupil mirror device 11.

After reflection from patterning device MA, patterned radiation beam B enters projection system PS. The projection system includes a plurality of mirrors configured to project a radiation beam B onto a substrate W held by the substrate table WT. The projection system PS can apply a demagnification factor to the radiation beam to form an image having features smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 may be applied. Although the projection system PS has two mirrors in fig. 1, the projection system may include any number of mirrors (e.g., six mirrors).

The radiation source SO shown in fig. 1 may comprise components not shown. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive to EUV radiation, but substantially blocking radiation of other wavelengths, such as infrared radiation.

Fig. 2 and 3 schematically illustrate a fuel collector 20 according to an embodiment of the present invention. The fuel collector 20 may be suitable for use as the fuel collector 15 shown in fig. 1. The fuel collector 20 may thus be arranged to collect at least a portion of the fuel that passes through the plasma formation region 4 of the radiation source SO without being converted into a radiation-emitting plasma. Such fuel may generally travel along a trajectory 21 that may coincide with the trajectory 16 shown in fig. 1.

It should be understood that the term "fuel" may be considered to include fuel droplets or fuel droplets. At least a portion of the fuel delivered to the fuel collector may comprise droplets or droplets that have not been converted into a plasma and/or that have not been impinged by the laser beam 2 (see fig. 1).

The fuel collector 20 includes a body 22, the body 22 defining a surface 24 for receiving an electrically conductive fuel (e.g., tin).

The fuel collector 20 further includes a housing 26, the housing 26 defining a cavity 28, the body 22 being disposed in the cavity 28. The wall 30 of the housing defines an inlet aperture 32 of the fuel collector 20. In use, the fuel collector 20 is arranged such that fuel propagating generally along the trajectory 21 passes through the inlet aperture 32 into the cavity 28 and is incident on the surface 24 of the body 22.

The wall 30 of the housing 26 separates the body 22 from a source of electrically conductive fuel such as, for example, the fuel emitter 3 within the LPP radiation source SO. The wall 30 thus provides a barrier between the body 22 receiving fuel and the area where such fuel is desired to be relatively absent (e.g., a container of LPP radiation source SO). The housing 26 is adapted to at least partially contain fuel received through the inlet aperture 32.

In use, the fuel collector 20 is arranged such that any fuel incident on the surface 24 of the body 22 (which fuel generally propagates along the trajectory 21) is incident on the surface 24 at a glancing incidence angle. This arrangement reduces the amount of fuel "back-scattered" from the surface 24. That is, less conductive fuel is scattered or bounced off the surface 24 in a direction generally antiparallel to the initial trajectory of the fuel (i.e., back along the trajectory 21 and out of the inlet aperture 32). Rather, as indicated by arrow 34, fuel scattered or bouncing off surface 24 tends to travel generally away from inlet orifice 32.

It should be appreciated that the term "glancing incidence angle" herein refers to the angle between the direction of propagation of the electro-fuel (i.e., trace 21) and the surface 24 upon which it is incident. This angle is complementary to the angle of incidence, i.e. the glancing angle of incidence plus the angle of incidence is a right angle. It will also be appreciated that fuel incident on the surface 24 at a glancing incidence angle is intended to mean that the glancing incidence angle is relatively small such that the path of the fuel is nearly parallel to the surface 24. For example, the glancing incidence angle may be less than 30 °. In some embodiments, the glancing incidence angle may be in the range of 2 ° to 12 °, for example about 7 °.

The fuel collector 20 further comprises a heater arranged to heat the surface 24. In particular, the heater may be arranged to heat the surface 24 to a temperature above the melting point of the conductive fuel (such as, for example, tin) that is collected by the device in use. For example, the body 22 may be heated to a temperature between 250 and 350 ℃.

It should be appreciated that such a heater may be implemented in a variety of ways. For example, the heater may include a heating element configured to heat the body 22, which may be embedded in the body 22, for example. Alternatively, the heating element may be provided separately from the main body 22. Such heating elements may include: tungsten or carbon, such as tungsten wire or carbon wire. Alternatively, the body 22 may be inductively heated. It should be appreciated that additionally or alternatively, heating can also be achieved by an electric current driven through the body 22.

When fuel is incident on the surface 24, the fuel tends to accumulate on the surface 24. As explained, arranging the fuel collector 20 such that fuel is incident on the surface 24 of the body 22 at a glancing incidence angle reduces the amount of fuel "back-scattered" from the surface 24. However, if a sufficiently large amount of fuel accumulates on the surface 24, at least a portion of the fuel directed toward the surface may bounce back toward the inlet aperture. This is illustrated in fig. 3, fig. 3 showing a deposit 36 of fuel on the surface 24. Deposit 36 may be in the form of droplets of fuel on surface 24, which may have been formed, for example, from a plurality of droplets received from fuel emitter 3. When such deposits 36 form on the surface, as indicated by arrows 38, a majority of the fuel scattered or bouncing off the surface 24 tends to propagate generally toward the inlet aperture 32.

The fuel collector 20 also includes a magnet 40. As shown in fig. 4, the magnet 40 is arranged to generate a magnetic field substantially perpendicular to the surface 24 at least in the region of the surface 24 where the fuel is incidentB(depicted by the dashed line). The area of the surface 24 on which the fuel is incident may be referred to as the landing zone and may have dimensions on the order of 2mm by 2 mm. The magnet 40 may be arranged to generate a magnetic field at least over a region of the surface 24, the magnetic field being of the order of 5cm by 5cm in size and centered on the landing zone. The magnet 40 may be regarded as a magnetic field generating mechanism.

It will be appreciated that, at least for the arrangement shown in figures 2 to 4, in order for the magnets 40 to generate a magnetic field in the landing zone on the surface 24BThe body 22 may be made of a substantially non-magnetizable material. That is, the body 22 may be formed of a material having a relatively low magnetic susceptibility so as to minimize the magnetic field imparted by the body 22BIs included in the shielding. Suitable materials for the body 22 include, for example, stainless steel.

In one embodiment, the magnet 40 comprises an electromagnet. Such an arrangement may be beneficial because such an electromagnet may be operable to apply a magnetic field over a greater range of ambient temperatures. For example, in use, the LPP radiation source SO may be operated at a temperature exceeding the melting point of a fuel such as, for example, tin. That is, in use, the fuel collector 20 may operate at temperatures in excess of 232 ℃. Alternatively, the magnet 40 may include a permanent magnet. For such embodiments, the permanent magnet preferably has a curie temperature that is higher than the melting point of the conductive fuel (e.g., tin) desired to be collected using the fuel collector 20.

As shown in fig. 5, the fuel collector also includes a power source 42 connected to the body 22 via a physical link, such as a wire 44. The power supply 42 is operable to generate and deliver an output current such that an output current I flows through the body 22 (depicted by the dashed line). The power supply 42 is thus arranged to deliver current to the body 22 via the wire 44 and to receive current from the body 22, thereby forming an electrical circuit with the body 22. The power source 42 is operable to generate a voltage across at least a portion of the surface 24. As shown in fig. 5, current I flows across and parallel to surface 24. The power source 42 and the wire 44 may be considered to constitute a current generating mechanism operable to generate an electrical current in the body 22.

The fuel collector 20 is adapted to receive and at least partially contain the electrically conductive fuel (e.g., tin) it receives.

In use, the conductive fuel may be received by the surface 24 defined by the body 22. At least a portion of the body 22 proximate the surface 24 is formed of a conductive material to support an electrical current. When the power source 42 generates an electrical current in the body 22, at least a portion of the current I may flow through any conductive fuel deposited on the surface 24 and thus in contact with the surface 24. Due to the magnetic field generated by the magnet 40 BSo that a force will be exerted on the fuel when an electric current flows through the fuel. The applied force is given by the lorentz force formula. In particular, the force is perpendicular to both the magnetic field and the current.

The fuel collector 20 is advantageous in that it provides a surface 24 for receiving conductive fuel, and the power source 42 and magnet 40 together provide a mechanism for applying a force to the fuel. This can, for example, allow directing the fuel away from the area or region of the surface 24 upon which it is incident, and directing the fuel toward, for example, a collection container or reservoir of the fuel.

Since the fuel collector 20 has a mechanism that applies a force to the fuel incident thereon, the fuel collector 20 can prevent the fuel from accumulating on the surface 24. In turn, this can generally reduce the amount of fuel backscattered from the surface 24 (i.e., bouncing in a direction generally antiparallel to the initial trajectory 21 of the incident fuel) by reducing the accumulation of fuel that can cause the fuel to be backscattered from the surface 24 (see fig. 3).

As described above, a current driven through the body 22 (such as, for example, a current I driven by the power source 42) can heat the body 22, for example, to heat the surface 24 to a temperature above the melting point of the conductive fuel collected by the device when in use. Although a separate heat source or heater may be provided if the mechanism for applying lorentz force to such fuel incident on the surface 24 is used intermittently (i.e., only at repeated time intervals) during use.

The fuel collector 20 also includes a container 46 for collecting at least a portion of the fuel incident on the surface 24. In use, the conductive fuel may strike the surface 24 and may then be directed toward the receptacle 46.

The power supply 42 is arranged to generate a current I and the magnet 40 is arranged to generate a magnetic field such that a force is exerted on the medium supporting the current, said force being directed generally towards the container 46, as will now be discussed.

The surface 24 defined by the body 22 may be substantially planar. Hereinafter, a direction substantially perpendicular to the surface 24 may be referred to as a z-direction. The direction in which the plane of surface 24 lies may be referred to as the x-y plane.

As shown in fig. 4, the magnetic field generated by the magnet 40 is at least in the vicinity of the region of the surface on which the conductive fuel is incidentBPerpendicular to surface 24, i.e. magnetic fieldBGenerally in the z-direction. As shown in fig. 5, a current I generated by the power source 42 flows through the body 22 in a negative y-direction substantially parallel to the surface 24. At least a portion of the current I generally generated by the power source 42 may be supported by the conductive fuel deposited on the surface 24, also generally in the negative y-direction. With this arrangement, the force exerted on the medium supporting the current I is directed substantially in the negative x-direction. As can be seen from fig. 4, this directs the fuel towards the edge 25 of the surface 24 provided above the container 46. The fuel can pass or drip from the edge 25 of the surface 24 and fall under gravity into the container 46. In this way, any conductive combustion deposited on surface 24 The material is directed to a receptacle 46.

It should be appreciated that as the size of the fuel deposit 36 increases, its resistance will decrease, and thus the portion of the current flowing through the fuel deposit 36 increases. Thus, although not shown in fig. 5 (which is merely illustrative), in practice the amount of current supported by the deposit 36 of fuel may be significantly greater than the amount of current supported by the surrounding surface 24 (or the conductive fuel layer disposed thereon).

The volume of the container 46 may be selected such that the radiation source SO or one or more components thereof may need to be replaced before the fuel collected in the container 46 may need to be removed. In other words, the volume of the container 46 may be selected such that the amount of fuel incident during the period of time between maintenance operations of the radiation source SO can be collected and/or retained in the second container. For example, the container 46 may include a volume of 1 liter to 8 liters, such as 2 liters to 5 liters. Alternatively or additionally, a suction device (not shown), such as a pump, may be arranged to draw fuel from the container 46, for example when the container 46 is filled or nearly filled. The suction means may be arranged to transfer fuel from the container 46 to the outside of the radiation source SO.

The container 46 may be provided with a heater (not shown) configured to heat the container 46 to a temperature equal to or greater than the melting temperature of the fuel. By heating the container 46 to a temperature equal to or greater than the melting temperature of the fuel, the fuel in the container 46 may be evenly spread and/or stalagmite-like accumulation may be prevented. The container 46 may be heated frequently or infrequently to disperse the fuel therein and/or to prevent stalagmite accumulation.

As shown in fig. 6, the main body 22 includes a non-conductive support portion 22a and a conductive portion 22b. Further, the conductive portion 22b includes a layer 48 of material that is readily wettable by a conductive fuel (e.g., tin) and a layer 50 of conductive fuel (e.g., tin). The conductive portion 22b may be referred to as a conductive material layer 22b.

Note that in general, the magnetic field will beBLorentz forces are generated in all materials through which the current flows. Thus, the lorentz force is applied to the magnetic fieldBIs provided on the entire conductive portion 22b.

It should be appreciated that a material that is readily wettable by tin can be defined as one such material: for this material, the liquid-vapor interface of the tin droplet deposited on the material (i.e., the edge of the tin droplet) intersects the interface between tin and the material at a contact angle of less than 90 °. Such materials may be referred to as "tin philic". Suitable materials include, for example, stainless steel ANSI 316L. Tungsten may be treated to render it tin philic.

Stainless steel ANSI 316L has a resistivity (specific resistance) of 74 μΩ cm and tin has a resistivity (specific resistance) of 1100 μΩ cm. Thus, for embodiments in which the layer of material 48 that is readily wetted by tin comprises stainless steel ANSI 316L and the thickness of layer 48 is approximately the same as the thickness of tin layer 50, the magnitude of the current through the tin will be approximately 14 times greater than the magnitude of the current through the stainless steel.

It should be appreciated that the fuel collector 20 may be provided with a mechanism for wetting the surface of the layer 48 with tin in order to provide a bulk tin layer 50. The layer of conductive material 22b defines a surface 24.

Providing the support portion 22a allows the thickness of the layer of conductive material 22b to be reduced relative to the thickness of material required if the conductive portion 22b of the body is self-supporting. Further, by forming the support portion 22a from a non-conductive material, the flow of current generated in the body is restricted in the potentially relatively thin conductive material layer 22b.

As explained above, when the power source 42 generates a current I in the body 22, at least a portion of the current may flow through any conductive fuel deposited on the surface 24. The magnitude of the current I flowing through the conductive fuel deposit depends on the relative resistance of: conductive fuel and a conductive material layer 22b forming a portion of the body 22.

The resistance of the conductive material layer 22b depends on its thickness. In particular, by reducing the thickness of the conductive material layer 22b, the resistance of the conductive material layer 22b increases. In turn, this increases the fraction of current flowing through the deposits of conductive fuel on surface 24. This has the effect of increasing the forces exerted on such deposits, thereby increasing the efficiency of moving them away from the area or region of the surface 24 from which they are incident, reducing the tendency of the fuel to backscatter from the body 22.

The resistance of the deposit of conductive fuel is also dependent on its thickness. The larger the deposit of fuel, the lower the resistance of the deposit and therefore the greater the magnitude of the current flowing through the deposit of conductive fuel will be. In turn, a larger current will result in a greater force being applied to the deposit. In this way, it can be seen that the fuel collector 20 is advantageously self-regulating in that it exerts a greater force on larger deposits (which may tend to cause more backscatter).

The power supply is arranged to generate a current I flowing across the surface 24 of the body 22 generally in a first linear direction (y-direction in fig. 5). With this arrangement, the magnetic fieldBThe force exerted on the medium supporting the current I is substantially perpendicular to the first linear direction. I.e. magnetic fieldsBThe force applied to the medium supporting the current I is in substantially the same direction for all positions in the plane of the surface 24. Due to magnetic fieldsBPerpendicular to the surface 24, the force F applied to the medium supporting the current I is thus in a second linear direction, which is substantially parallel to the surface (negative x-direction in fig. 4 and 5). Note that the current I and the magnetic fieldBIs selected such that the force F exerted on the medium supporting the current I is directed towards the edge 25 of the surface 24 arranged above the container 46.

For embodiments in which the magnet 40 applies a substantially constant magnetic field strength of 0.2T across the landing zone and the current driving 10A passes through the body 22 with a length (in the y-direction) of 0.05m, the current-carrying force is 0.1N. If a large part of this current is carried by a mass of 1g, the mass will experience 100m/s ² Acceleration of the magnitude.

In accordance with an embodiment of the present invention, a method is provided for moving deposits of conductive fuel on the surface 24 of the body 22, as now discussed.

The method comprises the following steps: generating a magnetic field having a component at least perpendicular to surface 24BThe method comprises the steps of carrying out a first treatment on the surface of the A current I is generated in the body 22, the current having a component that is at least parallel to the surface 24 such that at least a portion of the current I flows through the deposit of conductive fuel. When a current I flows through the deposition of the conductive fuelAt that time, the current I will exert a force on the fuel. The applied force is given by the lorentz force formula. In particular, the force is perpendicular to the magnetic fieldBAnd also perpendicular to the current I.

This method is advantageous because it provides a simple arrangement for applying a force to the deposit of fuel. This can, for example, allow directing fuel away from the area or region of the surface 24 upon which it is incident, and directing the fuel toward, for example, a collection container 46 or reservoir of the fuel.

The resistance of the deposit of conductive fuel depends on its thickness. The larger the deposit of fuel, the lower the resistance of the deposit and therefore the greater the magnitude of the current I flowing through the deposit of conductive fuel will be. Further, a larger current will result in a magnetic fieldBSo that a greater force is exerted on the deposit. In this way it can be seen that the method is advantageously self-regulating in that it applies more force to larger deposits (which may tend to cause more backscatter).

Magnetic fieldBAnd the current I may be continuously generated. This may allow the conductive fuel to continuously move over the surface 24, thereby limiting the size of deposits of conductive fuel that can form on the surface 24. Alternatively, a magnetic fieldBAnd the current I may be generated intermittently or periodically. This may allow for the absence of a magnetic fieldBAnd the conductive fuel accumulates on the surface as the current I is being generated, and then each time a magnetic field is generatedBAnd moving the accumulation at a current I.

In alternative embodiments, the magnet 40 may be operable to produce both: (a) a magnetic field having a component at least perpendicular to surface 24; and (b) the current in the body 22, as now discussed.

In this second embodiment, the magnet 40 is operable to generate a time-varying magnetic field having a component at least perpendicular to the surface 24. For example, the magnet 40 may include an electromagnet that may generate a time-varying magnetic field by varying the current supplied to the magnet 40. Additionally or alternatively, the magnet may be moved relative to the surface 24 such that the magnetic field at a given point on the surface 24 varies over time. The time-varying magnetic field will induce eddy currents in the body 22 (and any conductive fuel deposited thereon). Thus, in such an embodiment, the magnet 40 may be considered to constitute a current generating mechanism operable to generate an electrical current in the body 22 (in addition to the magnetic field generating mechanism). In turn, these eddy currents interact with the magnetic field such that lorentz forces will act on the medium supporting the eddy currents (e.g., deposits of conductive fuel).

Note that in this second embodiment, the current I flowing through the body 22 is not typically in a linear direction, but will be circulated in a closed loop. Thus, forces on the carrier medium that are typically applied in different locations on the surface 24 will experience forces in different ranges of directions. It will be appreciated that the magnets 40 are arranged such that the time-varying magnetic field causes conductive fuel deposited on a surface on the landing zone (i.e. the area of the surface 24 on which the fuel is incident during use) to be transported away from the landing zone. For example, the time-varying magnetic field may be arranged such that the conductive fuel deposited on the landing zone is pushed generally outward from the center of the landing zone.

In such an embodiment, the lorentz force will not generally direct fuel toward the edge 25 of the surface 24 disposed above the container 46. However, as shown, the body 22 may be arranged such that the surface 24 is inclined relative to the horizontal. This may allow the conductive fuel to flow generally under gravity to the rim 25 to fall under gravity into the container 46.

Such an embodiment does not include a power source 42 (although it may include a power source for an electromagnet) connected to the body 22 via a physical link.

While the specific embodiment described above includes a fuel collector 20 provided with a mechanism for applying a force to the conductive fuel incident on the surface, alternative embodiments may include other devices. Such other devices may for example comprise components of a laser generated plasma radiation source SO. Such other devices may include, for example, radiation collectors. In general, embodiments of the invention may relate to any surface from which it is desired to remove conductive fuel (e.g., tin).

Although specific reference may be made herein to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part of a mask inspection apparatus, metrology apparatus or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

The term "EUV radiation" may be considered to include electromagnetic radiation having a wavelength in the range of 4-20nm, for example in the range of 13-14 nm. EUV radiation may have a wavelength of less than 10nm, for example a wavelength in the range of 4-10nm, such as a wavelength of 6.7nm or 6.8 nm.

Although fig. 1 depicts the radiation source SO as a laser produced plasma LPP source, any suitable source may be used to generate EUV radiation. For example, EUV emitting plasma may be generated by converting a fuel (e.g., tin) into a plasma state using an electrical discharge. This type of radiation source may be referred to as a discharge-generated plasma (DPP) source. The discharge may be generated by a power supply, which may form part of the radiation source, or may be a separate entity connected to the radiation source SO via an electrical connection.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include integrated optical systems for the fabrication of guidance and detection patterns for magnetic domain memories, flat panel displays, liquid Crystal Displays (LCDs), thin film magnetic heads, etc.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography, topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist after the resist is cured, leaving a pattern in it.

While specific embodiments of the invention have been described above, it should be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative and not restrictive. Accordingly, it will be apparent to those 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.

Claims

1. An apparatus for receiving conductive fuel, the apparatus comprising:

a body defining a surface for receiving fuel;

a current generating mechanism for generating a current in the body, the current having a component at least parallel to the surface; and

a magnetic generating means arranged to generate a magnetic field having a component at least perpendicular to the surface, wherein the current generating means and the magnetic generating means are configured to apply a force to the fuel in use.

2. The apparatus of claim 1, wherein the body comprises a non-conductive support portion and a layer of conductive material defining the surface.

3. The apparatus of claim 2, wherein at least a portion of the layer of conductive material comprises a material that is readily wettable by tin.

4. A device according to claim 2 or claim 3, wherein at least a portion of the layer of conductive material comprises a layer of liquid tin.

5. The apparatus of any preceding claim, further comprising a heater arranged to heat the surface.

6. The apparatus of any one of the preceding claims, further comprising a container for collecting at least a portion of fuel incident on the surface.

7. The apparatus of claim 6, wherein the current generating mechanism is arranged to generate an electrical current, the magnetic generating mechanism being arranged to generate a magnetic field such that a force is applied to a medium supporting the electrical current, the force being directed generally towards the container.

8. Apparatus according to any preceding claim, wherein the current generating means is arranged to generate a current flowing across the surface in a generally first linear direction.

9. The apparatus of any one of the preceding claims, wherein the magnetic generating mechanism comprises an electromagnet.

10. The apparatus of any one of the preceding claims, further comprising a wall defining an inlet aperture.

11. The apparatus of claim 10, wherein the wall forms part of a housing defining a cavity within which the body is disposed.

12. A radiation system, comprising:

a fuel emitter configured to provide fuel to the plasma formation region;

an excitation means arranged to provide an excitation beam at the plasma formation region to convert at least a first portion of the fuel into a radiation-emitting plasma; and

the apparatus of any preceding claim, configured to collect at least a second portion of the fuel.

13. The radiation system of claim 12, wherein a surface of the body is arranged such that the second portion of the fuel is incident on the surface at a glancing incidence angle.

14. A lithography system, comprising: a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate; and a radiation system according to any of claims 12-13, arranged to provide at least some of the radiation to the lithographic apparatus.

15. A method for moving deposits of conductive fuel on a surface of a body, the method comprising:

generating a magnetic field having a component at least perpendicular to the surface;

an electrical current is generated in the body, the electrical current having a component that is at least parallel to the surface such that at least a portion of the electrical current flows through the deposit of conductive fuel.