Radiant System

Abstract

The radiation system comprises a radiation source (SO) and a plasma generator (16). The radiation source comprises a housing (9) provided with an exit aperture (8); and a radiation generator operable to generate output radiation in the housing and to direct at least a portion of the output radiation through the exit aperture. The plasma generator is operable to generate a plasma (18) extending at least partially across the at least a portion of the output radiation directed through the exit aperture.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from European application number 19156096.0 filed on February 8, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a radiation system comprising a radiation source. The radiation source may be, for example, an extreme ultraviolet (EUV) laser produced plasma (LPP) source. In particular, the present invention relates to a radiation system provided with a mechanism for at least partially filtering out one or more spectral components and / or particle debris. The radiation system may find application in the field of lithography. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. For example, a lithographic apparatus may be used in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate.

[0005] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of a feature that can be formed on a substrate. A lithographic apparatus using extreme ultraviolet (EUV) radiation (having a wavelength in the range of 4 nm to 20 nm, such as 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate compared to a lithographic apparatus using radiation having a wavelength of, for example, 193 nm.

[0006] A lithography system may include a radiation source, which is a radiation source operable to generate a radiation beam, and a lithography apparatus operable to receive the radiation beam and form an image (e.g., a diffraction limited image) of a patterning device on a substrate using the radiation beam. One known type of radiation source used in the EUV lithography field is an LPP radiation source.

[0007] Whether otherwise identified herein or not, it may be desirable to provide new radiation systems, and associated methods of operation, that at least partially address one or more problems associated with prior art arrangements. Summary of the invention

[0008] According to a first aspect of the present invention, there is provided a system comprising: a radiation source, the radiation source comprising: a housing, the housing being provided with an exit aperture; and a radiation generator operable to generate output radiation in the housing and to direct at least a portion of the output radiation through the exit aperture; and a plasma generator operable to generate plasma, the plasma at least partially extending across the at least a portion of the output radiation directed through the exit aperture.

[0009] The output radiation may comprise extreme ultraviolet (EUV) radiation.The system according to the first aspect of the invention is advantageous as now discussed.

[0010] The system may form part of a lithography system. It may be desirable that the radiation output by the radiation source has a desired wavelength and bandwidth. For example, it may be desirable that the radiation output by the radiation source comprises EUV radiation. Furthermore, it may be desirable to suppress any out-of-band radiation having a wavelength outside of the desired bandwidth.

[0011] Since the plasma generated by the plasma generator at least partially extends across at least a portion of the output radiation directed through the exit aperture, at least a portion of the output radiation directed through the exit aperture will be incident on the plasma. The plasma generator may be configured such that the plasma filters out at least some out-of-band radiation. In this way, the plasma may be configured to increase the spectral purity of radiation output by the radiation source.

[0012] For EUV lithography apparatus using a laser produced plasma (LPP) source to generate EUV radiation, it is known to use a spectral purity filter downstream of the LPP source (e.g., a spectral purity filter downstream of the intermediate focus of the LPP source) to filter out out-of-band radiation, however, it has been found that these filters do not survive due to the high power density and inhomogeneity of the light. Advantageously, since the system according to the first aspect of the invention uses a plasma as a filter, there is no such damage threshold problem.

[0013] Furthermore, the plasma is only transparent to radiation having a frequency greater than the plasma frequency, which in turn depends on the plasma electron density and the effective mass of the electrons in the plasma. The plasma frequency is proportional to the square root of the ratio of the plasma electron density to the effective mass of the electrons in the plasma. Thus, by adjusting these parameters of the plasma, the system according to the first aspect of the invention can be provided with an adjustable spectral filter.

[0014] Additionally or alternatively, the plasma may be arranged to intercept particles that may be present within the enclosure (and which particles may, for example, be produced as a by-product of producing the output radiation).

[0015] The radiation generator may be operable to generate a laser produced plasma (LPP).

[0016] The radiation generator may include: a target generator operable to generate a target including a target material at a plasma formation region, the plasma formation region being arranged to receive laser radiation for irradiating the target material to thereby generate a second plasma emitting the output radiation; and an optical element arranged to direct at least a portion of the output radiation toward the exit aperture.

[0017] Such an LPP radiation source may include a plurality of out-of-band radiation sources. For example, the output radiation emitted by the second plasma may include an undesirable out-of-band radiation component. In addition, a portion of the laser radiation used to generate the second plasma may be scattered toward the exit aperture. Furthermore, it will be appreciated that the housing of the radiation source may be maintained at a relatively high temperature (e.g., exceeding the melting point of the target material (which may include tin)). Components of the radiation source maintained at such a high temperature will emit portions of infrared radiation that may be directed toward the exit aperture.

[0018] The radiation generator may be arranged to focus at least a portion of the output radiation at an intermediate focus, the intermediate focus being arranged at or near the exit aperture.

[0019] For such embodiments, arrangements in which the plasma generator is operable to generate a plasma that extends at least partially across the exit aperture are particularly advantageous, as will now be discussed. In embodiments in which the radiation generator is arranged to focus at least a portion of the output radiation at the intermediate focus, the volume of the plasma that should be generated at the exit aperture to ensure that the plasma extends across the propagation path of the output radiation can be minimized. This, in turn, minimizes the amount of power used by the plasma generator.

[0020] Furthermore, such an arrangement differs from existing EUV radiation sources (e.g. LPP radiation sources) which are provided with a spectral filter, as such an arrangement does not place the spectral filter near the intermediate focus of the radiation source, since the skilled person will appreciate that at the intermediate focus the power density is maximum and therefore the spectral filter is likely to be damaged.

[0021] By using plasma as the spectral filter (eg generated by the plasma generator), the system according to the first aspect of the invention is able to provide such a spectral filter close to the intermediate focus.

[0022] The plasma generator may be operable to form the plasma from hydrogen gas.

[0023] Alternatively, the plasma may be formed from, for example, an inert gas such as, for example, argon. In some embodiments, the plasma may be formed from a gas mixture, for example, a mixture of hydrogen and argon.

[0024] The plasma is opaque to radiation having a frequency less than the plasma frequency, which in turn depends on the plasma electron density and the effective mass of electrons in the plasma.

[0025] The plasma generator may be operable to form a plasma having a value greater than 10 19 cm -3 The electron density of the plasma.

[0026] Using such an arrangement, the plasma will be opaque to infrared (IR) radiation, for example radiation having a wavelength of the order of 10 μm or more. In contrast, for EUV radiation, such a plasma will behave like a lossless dielectric, and the EUV radiation can propagate through such a plasma without attenuation.

[0027] The plasma generator may be operable to form a plasma having a value greater than 6.7×10 22 cm -3 The electron density of the plasma.

[0028] Using such an arrangement, the plasma will be opaque to infrared DUV radiation, for example radiation having a wavelength of the order of 130 nm to 400 nm or more.

[0029] The plasma generator may be operable to form a plasma having a value less than 6.7×10 24 cm -3 The electron density of the plasma.

[0030] Using such an arrangement, the plasma will be transparent to EUV radiation having a wavelength of 13.5 nm or less. In contrast, radiation having a wavelength greater than 13.5 nm will decay exponentially with distance into the plasma, and for a sufficiently large volume of plasma, radiation having a wavelength greater than 13.5 nm will be blocked.

[0031] It will be appreciated that radiation having a frequency less than the plasma frequency will decay exponentially with distance into the plasma. It will also be appreciated that it is desirable that the plasma be sufficiently massive in the direction of propagation of the output radiation that substantially all radiation having a frequency less than the plasma frequency is blocked by the plasma.

[0032] The plasma generator may be operable to form plasma in a region having an extent greater than 300 μm along a direction of propagation of the output radiation.

[0033] In some embodiments, the plasma may be formed in a region having an extent of about 1 cm along a propagation direction of the output radiation.

[0034] It will be appreciated that the desired size of the plasma in a plane perpendicular to the direction of propagation of the output radiation may depend on the size of the exit aperture. In some embodiments, the exit aperture may have a diameter of the order of 6 mm and the plasma may have a size of approximately 1 cm in a plane perpendicular to the direction of propagation of the output radiation.

[0035] The plasma generator may be operable to generate the plasma using an arc discharge.

[0036] Alternatively, any other method for producing plasma may be used, including, for example, an arrangement using any of: laser radiation for producing plasma (i.e., laser-produced plasma); a plasma railgun; a hypervelocity plasma source; a microwave-driven plasma source; or any other compact plasma source.

[0037] It will be appreciated that the plasma generator may be located at or near the exit aperture. For example, the plasma may be formed at the exit aperture. Alternatively, the plasma generator may be located away from the exit aperture and the plasma may be directed transverse to the direction of propagation of the output radiation.

[0038] The plasma generator may be operable to direct plasma generated by the plasma generator to flow at least partially across the exit aperture.

[0039] Advantageously, by providing a plasma stream across the exit aperture, the efficiency of removing particulate debris may be improved compared to, for example, a static plasma, as the particulate debris may be entrained in the plasma stream and prevented from passing through the plasma stream and forming part of the output of the radiation source.

[0040] The plasma generator may be operable to direct plasma generated by the plasma generator to flow at least partially through the exit hole and into the housing.

[0041] Advantageously, by providing a plasma flow across the exit aperture and into the housing, the efficiency of removing particulate debris can be further improved, as the plasma flow will tend to force particulate debris back into the housing. In this way, the plasma generator can be considered to form part of a dynamic plasma lock.

[0042] The plasma generator may include at least one plasma generating unit arranged to generate plasma and direct the plasma at least partially across the exit hole.

[0043] The or each plasma generating unit may comprise: a first electrode and a second electrode; a voltage source operable to apply a voltage between the first electrode and the second electrode; and an airflow generator operable to generate an airflow between the first electrode and the second electrode and in a direction at least partially transverse to the exit aperture.

[0044] The voltage source may include one or more high voltage power supplies. The voltage source may include one or more resistors. The voltage source may include one or more energy storage capacitors. The voltage source may include one or more power conversion devices.

[0045] It will be appreciated that a direction at least partially traversing the exit aperture may be at least partially transverse to a main direction of the output radiation directed towards the exit aperture. It will be appreciated that the output radiation may be focused at or near the exit aperture and may therefore comprise a converging radiation beam or a diverging radiation beam. In this context, it will be appreciated that the main direction of the output radiation is intended to mean an average direction of the output radiation (the average direction of the output radiation may be the direction of a chief ray of the output radiation).

[0046] The gas flow generated by the gas flow generator may include hydrogen. The hydrogen may be provided at a pressure of about 1 atmosphere or about 1 bar.

[0047] The voltage source may be operable to generate an electric field strength sufficient to ionize the gas flow to form a plasma flow between the first electrode and the second electrode. The voltage source may be operable to apply a larger initial voltage between the first electrode and the second electrode to initiate the plasma formation. Once the plasma has been formed, the resistance between the first electrode and the second electrode may drop significantly, and a smaller voltage may be applied between the first electrode and the second electrode to maintain the plasma. An electric field strength of the order of 10 kV / cm may be used for the initial ignition process for the plasma formation. For example, at least initially, the voltage source may be operable to apply a voltage of the order of 10 kV between the first electrode and the second electrode, and the first electrode and the second electrode may be separated by a distance of the order of 1 cm. In order to maintain the plasma, the voltage source may be operable to generate an electric field strength of at least 100 V / m between the first electrode and the second electrode.

[0048] The first electrode may be hollow and the second electrode may be disposed within the first electrode.

[0049] Using such an arrangement, the first electrode may be generally tubular and may form part of a conduit for gas flow. The second electrode may be generally coaxial with the first electrode. The first electrode and the second electrode may together provide a channel, the cross-section of the channel being generally annular, through which the gas flow may flow between the first electrode and the second electrode.

[0050] The or each plasma generating unit may comprise a nozzle.

[0051] The nozzle may be used to focus the plasma into a smaller volume. This in turn may increase the electron density of the plasma at the exit aperture.

[0052] The plasma generator may include a plurality of the plasma generating units.

[0053] Each plasma generating unit may be operable to provide a plasma stream from a different direction at least partially across the exit aperture. All of these plasma streams may be combined together to form a plasma extending at least partially across the exit aperture and having a greater electron density than any individual plasma stream.

[0054] The plasma generator may be provided with one or more adjustable parameters. One or more properties of the plasma may be controlled by controlling the one or more adjustable parameters.

[0055] The one or more adjustable parameters may, for example, comprise a voltage between the first electrode and the second electrode which may be applied by the voltage source.Additionally or alternatively, the one or more adjustable parameters may, for example, comprise a pressure or an airflow generated by the airflow generator.

[0056] The output radiation generated by the radiation generator may be pulsed and the plasma generated by the plasma generator may be pulsed. The radiation generator and the plasma generator may be synchronized such that the plasma exists when each pulse of the output radiation propagates through the region where the plasma is formed. For example, the radiation generator and the plasma generator may be synchronized such that the plasma exists when each pulse of the output radiation propagates through the exit aperture.

[0057] According to a second aspect of the present invention, there is provided a lithography system comprising: a system according to the first aspect of the present invention; and a lithography device operable to receive output radiation from the radiation source and to form an image of a pattern forming device on a substrate using the output radiation.

[0058] The lithographic system according to the second aspect of the invention may comprise any of the features of the system according to the first aspect of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0060] - Figure 1 A lithography system including a lithography apparatus and a radiation source is depicted;

[0061] - Figure 2 An embodiment of a plasma generating unit is depicted in cross section, which plasma generating unit may form a part of a plasma generator, which part of the plasma generator may form a Figure 1 A portion of a lithography system shown in;

[0062] - Figure 3A The radiation source is shown in cross section and in a plane containing the optical axis of the radiation source. Figure 2 A first embodiment of a plasma generator having multiple plasma generating units is shown in FIG. Figure 1 A portion of the enclosed structure of the radiation source shown in;

[0063] - Figure 3B The radiation source is shown in cross section and in a plane perpendicular to the optical axis. Figure 3A The first embodiment of the plasma generator shown in FIG.

[0064] - Figure 4 The radiation source is shown in cross section and in a plane containing the optical axis of the radiation source. Figure 2 A second embodiment of a plasma generator having multiple plasma generating units is shown in FIG. Figure 1 A portion of the enclosing structure of the radiation source is shown in FIG. DETAILED DESCRIPTION

[0065] Figure 1 A lithographic system according to an embodiment of the invention is shown, the lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and for supplying the EUV radiation beam B to the lithographic apparatus LA. 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.

[0066] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the pattern forming device MA. In addition, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and the faceted pupil mirror device 11 receive the EUV radiation beam B, and the faceted field mirror device 10 and the faceted pupil mirror device 11 are arranged together to provide an EUV radiation beam B' having a desired cross-sectional shape and a desired intensity distribution. In addition to the faceted field mirror device 10 and the faceted pupil mirror device 11 or instead of the faceted field mirror device 10 and the faceted pupil mirror device 11, the illumination system IL may include other mirrors or devices.

[0067] After being so adjusted, the EUV radiation beam B' interacts with the pattern forming device MA. As a result of this interaction, a patterned EUV radiation beam B". The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To this end, the projection system PS may include a plurality of mirrors 13, 14, which are configured to project the patterned EUV radiation beam B" onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B", thereby forming an image having features that are smaller than corresponding features on the pattern forming device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is Figure 1 Although illustrated in FIG. 1 as having only two mirrors 13, 14, the projection system PS may include any number of mirrors (eg, six or eight mirrors).

[0068] The substrate W may include a previously formed pattern. In this case, the lithographic apparatus LA aligns an image formed by the patterned EUV radiation beam B with a pattern previously formed on the substrate W.

[0069] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure much less than atmospheric pressure, may be provided in the radiation source SO in the illumination system IL and / or the projection system PS.

[0070] Figure 1 The radiation source SO shown is of a type which may be referred to as a laser produced plasma (LPP) source, for example. The laser system 1 (which may include, for example, a CO 2 A laser) is arranged to deposit energy into a fuel, such as tin (Sn) provided from, for example, a fuel emitter 3, via a laser beam 2. The laser beam 2 may be referred to as the laser radiation 2. Although tin is mentioned in the following description, any suitable fuel may be used. Such a fuel may, for example, be in liquid form and may, for example, be a metal or an alloy. The fuel emitter 3 may include a nozzle configured to direct tin, for example, in the form of droplets, along a trajectory toward a plasma formation region 4. Thus, the fuel emitter 3 may be considered a target generator, which is operable to generate a target (e.g., tin droplets) comprising a target material (e.g., tin) at the plasma formation region 4. The laser beam 2 is incident on tin at the plasma formation region 4. The fuel emitter 3 may be operable to generate a plurality of targets (e.g., a stream of tin droplets) at the plasma formation region 4. The laser beam 2 may be pulsed, for example, each pulse being incident on a different one of the plurality of targets (e.g., tin droplets) at the plasma formation region 4. The deposition of laser energy into tin generates a tin plasma 7 at the plasma formation region 4. During de-excitation and recombination of electrons with ions of the plasma, radiation including EUV radiation is emitted from the plasma 7 .

[0071] The laser radiation 2 may have any suitable wavelength. In some embodiments, the laser radiation 2 has a wavelength of about 10 μm, which may be emitted by a CO 2 Laser Generation In some embodiments, the laser radiation 2 has a wavelength of about 1 μm, which may be generated by a YAG (yttrium aluminum garnet) based laser.

[0072] EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 includes, for example, a near normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multilayer mirror 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 having two focal points. A first of the focal points may be located at the plasma formation region 4, and a second of the focal points may be located at an intermediate focus 6, as discussed below.

[0073] The collector 5 may be regarded as an optical element arranged to focus at least a portion of the radiation generated by the plasma 7 at the intermediate focus 6 .

[0074] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be transferred from the laser system 1 to the radiation source SO by means of a beam delivery system (not shown) comprising, for example, suitable directional mirrors and / or beam expanders, and / or other optical devices.

[0075] The radiation reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at an intermediate focus 6 to form an image at the intermediate focus 6 of the plasma located at the plasma formation region 4. The image at the intermediate focus 6 serves as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near an opening 8 in an enclosed structure 9 of the radiation source SO. The opening 8 in the enclosed structure 9 may be referred to as an exit aperture 8, and the enclosed structure 9 may be referred to as a housing 9 provided with the exit aperture 8. The collector 5 may be considered as an optical element arranged to direct at least a portion of the output radiation (i.e., the EUV radiation beam B) towards the opening 8.

[0076] The fuel emitter 3 and the plasma formation region 4 arranged to receive the laser beam 2 for irradiating the target material may together be considered to form a radiation generator operable to generate output radiation in the enclosed structure 9 and to direct at least a portion of the output radiation (i.e., EUV radiation beam B) through the opening 8. The collector 5 may also be considered to form a part of the radiation generator.

[0077] Although Figure 1The radiation source SO is depicted as a laser produced plasma (LPP) source, but any suitable source, such as a discharge produced plasma (DPP) source or a free electron laser (FEL), may be used to generate EUV radiation.

[0078] like Figure 1 Schematically indicated in the lithography system, the lithography system further comprises a plasma generator 16 operable to generate a plasma 18 extending at least partially across the opening 8 of the radiation source SO. Such an arrangement is advantageous, as will now be discussed. The radiation source SO and the plasma generator 16 may be considered to provide a system which may be referred to as a radiation system. It will be appreciated that the laser system 1 and / or the beam delivery system (when provided) may be considered to form part of the radiation system.

[0079] It will be appreciated that the radiation system may form part of a lithography system (eg Figure 1 ) or may be used for some other purpose. It may be desirable that the radiation output by the radiation source SO has a desired wavelength and bandwidth. For example, in order to accurately control the lithography process, it may be desirable that the radiation output by the radiation source SO includes only EUV radiation. In addition, it may be desirable to suppress any out-of-band radiation having a wavelength outside the desired bandwidth.

[0080] Since the plasma 18 generated by the plasma generator 16 extends at least partially across the opening 8, at least a portion of the radiation directed through the opening 8 will be incident on the plasma 18. The plasma generator 16 can be configured so that the plasma filters out at least some out-of-band radiation. In this way, the plasma can be configured to increase the spectral purity of the radiation output by the radiation source.

[0081] For EUV lithography apparatus using an LLP source to generate EUV radiation, it is known to use a spectral purity filter downstream of the LPP source (e.g. downstream of the intermediate focus 6 of the LPP source) to filter out out-of-band radiation. However, it has been found that these filters cannot survive due to the high power density and inhomogeneity of the light. Advantageously, since the radiation source SO uses the plasma 18 as a filter, there is no such damage threshold problem.

[0082] Figure 1The LPP radiation source of the type shown in FIG. 1 may include multiple out-of-band radiation sources. For example, the output radiation emitted by the radiation generating plasma 7 may include undesired out-of-band radiation components (e.g., DUV radiation). In addition, a portion of the laser radiation 2 used to generate the radiation generating plasma 7 may be scattered toward the opening 8. Such scattering may be promoted, for example, by tin droplets and tin debris, which may efficiently reflect such laser radiation 2.

[0083] Furthermore, it will be appreciated that the enclosed structure 9 of the radiation source SO may be maintained at a relatively high temperature (e.g. above the melting point of tin). Components of the radiation source SO maintained at such a high temperature will emit infrared radiation portions which may be directed towards the opening 8.

[0084] The inventors of the present invention have recognized that the plasma 18 can be arranged so as to filter out at least some of these out-of-band radiation sources. The plasma is only transparent to radiation having a frequency greater than the plasma frequency, which in turn depends on the plasma electron density and the effective mass of the electrons in the plasma. The plasma frequency, ω p , is given by:

[0085]

[0086] Where, e is the electron charge, n is the plasma electron density, ε 0 is the dielectric constant of free space, and m e is the effective mass of the electrons in the plasma. Thus, the plasma frequency is proportional to the square root of the ratio of the plasma electron density to the effective mass of the electrons in the plasma. Thus, by adjusting these parameters of the plasma, the radiation source according to the first aspect of the invention may be provided with an adjustable spectral filter. In practice, the effective mass of the electrons in the plasma may be relatively fixed and the main control parameter that may be used to influence the plasma frequency may be the plasma electron density.

[0087] The plasma 18 may be opaque to radiation having a frequency less than the plasma frequency.

[0088] In some embodiments, the plasma generator 16 is operable to form a plasma 18 having an electron density greater than a threshold electron density value. This ensures that the plasma is opaque to radiation having a frequency less than a corresponding threshold frequency value (the corresponding threshold frequency value can be found from the threshold electron density value using equation (1)).

[0089] In some embodiments, the plasma generator 16 is operable to form a plasma having a 19 cm -3 With such an arrangement, the plasma will be opaque to infrared (IR) radiation, for example radiation having a wavelength of the order of 10 μm or more. In at least some embodiments, the laser radiation 2 has a wavelength of about 10 μm, and thus, for such embodiments, a plasma having a wavelength of greater than 10 μm is formed. 19 cm -3 A plasma 18 with an electron density of at least 100 nm will prevent (or at least reduce) the amount of stray laser radiation received by the lithographic apparatus LA. In contrast, for radiation with a larger frequency (e.g., EUV radiation), such a plasma 18 will behave like a lossless dielectric, and the EUV radiation can propagate through such a plasma without attenuation.

[0090] In some embodiments, the plasma generator 16 may be operable to form a plasma having a 21 cm -3 With such an arrangement, the plasma will be opaque to infrared (IR) radiation, for example radiation having a wavelength of the order of 1 μm or greater. In at least some embodiments, the laser radiation 2 has a wavelength of about 1 μm, and thus, for such embodiments, a plasma having a wavelength of greater than 10 μm is formed. 21 cm -3 The plasma 18 having an electron density of greater than 10 will prevent (or at least reduce) the amount of stray laser radiation received by the lithographic apparatus LA. 21 cm -3 Such an embodiment of a plasma 18 having an electron density of 100 Å (such that the plasma 18 is opaque to infrared radiation) will prevent (or at least reduce) the amount of infrared radiation emitted by components of the radiation source SO and received by the lithographic apparatus LA.

[0091] In some embodiments, the plasma generator 16 may be operable to form a plasma having a mass greater than 6.7×10 22 cm -3 The plasma 18 has an electron density of . Using such an arrangement, the plasma will also be opaque to deep ultraviolet (DUV) radiation, for example radiation having a wavelength of the order of 130 nm to 400 nm or more.

[0092] In some embodiments, the plasma generator 16 may be operable to form a plasma having a mass less than 6.7×1024 cm -3 With such an arrangement, the plasma will remain transparent to EUV radiation, for example radiation having a wavelength of 13.5 nm or less (whereas the plasma 18 will block radiation having a wavelength greater than 13.5 nm).

[0093] It will be appreciated that radiation having a frequency less than the plasma frequency cannot propagate through the plasma 18. Rather, in the plasma 18, radiation having a frequency less than the plasma frequency will decay exponentially (i.e., the amplitude of the radiation will decay exponentially with distance into the plasma 18). Such radiation may be referred to as evanescent or cutoff. It will also be appreciated that it is desirable that the plasma 18 be sufficiently massive in the direction of propagation of the output radiation that substantially all radiation having a frequency less than the plasma frequency is blocked by the plasma 18 so that energy from such radiation is not transferred through the plasma 18. Radiation having a frequency less than the plasma frequency will decay exponentially within a characteristic depth, L, which is given by:

[0094]

[0095] Here, ω is the frequency of the radiation propagating into the plasma 18 .

[0096] For example, in the case where the plasma generator 16 is operable to form a plasma having a value greater than 10 19 cm -3 In an embodiment of a plasma 18 having an electron density of greater than 10 μm, the characteristic depth for infrared radiation (having a wavelength of about 10 μm) will be about 100 μm. It will be understood that the characteristic depth is the distance of the plasma at which the amplitude of the radiation will decay by a factor or 1 / e. In some embodiments, the plasma generator 16 is operable to generate a plasma having a density of greater than 10 μm. 19 cm -3 The plasma 18 is formed in a region having an electron density of 100 μm and having an extent of more than 300 μm along the propagation direction of the EUV radiation beam B. With such an arrangement, about 95% of the infrared radiation will be filtered out by the plasma. It is desirable to form the plasma 18 in a region having an extent of about 1 cm or more along the propagation direction of the EUV radiation beam B.

[0097] It will be appreciated that the desired size of the plasma 18 in a plane perpendicular to the propagation direction of the EUV radiation beam B may depend on the size of the opening 8. In some embodiments, the opening 8 may have a diameter of the order of 6 mm, and the plasma may have a size of approximately 1 cm in a plane perpendicular to the propagation direction of the output radiation.

[0098] As discussed above, the plasma 18 may act as a spectral filter. Additionally or alternatively, the plasma 18 may be arranged to intercept particles that may be present within the enclosure 9 (and which particles may, for example, be produced as a byproduct of generating the output radiation).

[0099] exist Figure 1 In the embodiment shown in , the plasma generator 16 is operable to generate a plasma 18 that extends at least partially across the opening 8. In alternative embodiments, the plasma generator 16 may be operable to generate the plasma 18 at another location within the lithography system (preferably upstream of the radiation source SO and upstream of the optics within the lithography apparatus LA). However, an arrangement in which the plasma generator 16 is operable to generate a plasma 18 that extends at least partially across the opening 8 is particularly advantageous, as will now be discussed. As discussed above, the collector 5 is arranged to focus at least a portion of the output radiation B at the intermediate focus 6 (the intermediate focus 6 being located near the opening 8). For such an embodiment, by generating the plasma 18 so as to extend at least partially across the opening 8, the volume of the plasma 18 that should be generated to ensure that the plasma 18 extends across the propagation path of the output radiation B can be minimized. This, in turn, minimizes the amount of power used by the plasma generator 16.

[0100] Furthermore, such an arrangement is different from existing EUV radiation sources (e.g., LPP radiation sources) that are provided with a spectral filter. Such an arrangement does not place the spectral filter close to the intermediate focus 6 of the radiation source SO, as the skilled person will recognize that the power density is maximum at the intermediate focus 6 and therefore the spectral filter is likely to be damaged. However, by using the plasma 18 as the spectral filter (as generated by the plasma generator 16), the radiation system (including the radiation source SO and the plasma generator 16) is able to provide such spectral filtering functionality near the intermediate focus 6.

[0101] The plasma generator 16 may be operable to form the plasma from any gas. The gas may include, for example, hydrogen. Additionally or alternatively, the gas may include, for example, an inert gas (such as, for example, argon).

[0102] In some embodiments, the plasma generator 16 may be operable to use an arc discharge to generate the plasma 18. Alternatively, any other method for generating plasma may be used, including, for example, an arrangement using any of: laser radiation for generating plasma (i.e., laser-generated plasma); a plasma railgun; a hypervelocity plasma source; a microwave-driven plasma source; or any other compact plasma source.

[0103] It will be appreciated that the plasma generator 16 may be located at or near the opening 8. For example, the plasma 18 may be formed at the opening 8. Alternatively, the plasma generator 16 may be located away from the opening 8, and the plasma 18 may be directed transversely to the propagation direction of the output radiation B.

[0104] In some embodiments, the plasma generator 16 may be operable to direct a plasma 18 generated by the plasma generator 16 to flow at least partially across the opening 8. Advantageously, by providing a plasma flow 18 across the opening 8, the efficiency of removing particulate debris may be improved compared to, for example, a static plasma, because the particulate debris may be entrained in the plasma flow and prevented from passing through the plasma flow and from forming part of the output of the radiation source SO.

[0105] In some embodiments, the plasma generator 16 may include at least one plasma generating unit arranged to generate plasma 18 and direct the plasma 18 at least partially across the path of the output radiation beam B. For example, one or more plasma generating units may be arranged to direct the plasma flow 18 at least partially across the opening 8. Figure 2 An embodiment of a plasma generating unit 20 which may form part of the plasma generator 16 is shown schematically and in cross-section in FIG.

[0106] The plasma generating unit 20 includes a first electrode 22, a second electrode 24, a voltage source 26 and a gas flow generator 28. The first electrode 22 is a hollow member having a central opening. The second electrode 24 is disposed in the opening of the hollow first electrode 22.

[0107] The voltage source 26 is operable to apply a voltage between the first electrode 22 and the second electrode 24. Figure 2As schematically shown in FIG. 1 , this is achieved by connecting the voltage source 26 to two connection lines 32, 34, respectively, of the first electrode 22 and the second electrode 24. As will be appreciated by the skilled person, the voltage source 26 may include any combination of: one or more high voltage power supplies; one or more resistors; one or more energy storage capacitors; and one or more power conversion devices.

[0108] The gas flow generator 28 is operable to generate a gas flow 36 between the first electrode 22 and the second electrode 24. The gas flow 36 generated by the gas flow generator 28 may include hydrogen gas. The hydrogen gas may be provided at a pressure of about 1 atmosphere or about 1 bar.

[0109] The voltage source 26 is operable to generate an electric field strength between the first electrode 22 and the second electrode 24 sufficient to ionize the gas flow 36 to form a plasma flow 38. The voltage source 26 may be operable to apply a large initial voltage between the first electrode 22 and the second electrode 24 to initiate the formation of the plasma 18. Once the plasma 18 has been formed, the resistance between the first electrode 22 and the second electrode 24 drops significantly, and a smaller voltage may be applied between the first electrode 22 and the second electrode 24 to maintain the plasma 18. The initial ignition process for the formation of the plasma 18 may use an electric field strength of the order of 10 kV / cm. For example, at least initially, the voltage source 26 may be operable to apply a voltage of the order of 10 kV between the first electrode 22 and the second electrode 24, and the first electrode 22 and the second electrode 24 may be separated by a distance of the order of 1 cm. In order to maintain the plasma 18, the voltage source 26 may be operable to generate an electric field strength of at least 100 V / m between the first electrode 22 and the second electrode 24.

[0110] exist Figure 2 In the arrangement shown in FIG. 1 , the first electrode 22 is generally tubular and may form part of a conduit for an airflow 36 from the airflow generator 28. The second electrode 24 is generally coaxial with the first electrode 22. The first electrode 22 and the second electrode 24 may together define a channel 30 having a generally annular cross-section through which the airflow 36 may flow between the first electrode 22 and the second electrode 24.

[0111] Although the first electrode 22 is generally tubular, the first electrode 22 is provided with a generally truncated cone portion at the end of the first electrode 22 away from the gas flow generator 28. The truncated cone portion serves as a nozzle. The nozzle is used to focus the plasma stream 38 into a smaller volume. This in turn can increase the electron density of the plasma stream 38 (the plasma stream 38 can be at least partially directed across the opening 8).

[0112] Figure 1 The plasma generator 16 shown in FIG. 1 may include a generally Figure 2 A plurality of plasma generating units 20 of the form shown in FIG. 3A to 3B are now described. Figure 4 An embodiment of such a plasma generator 16 will be described.

[0113] exist Figure 3A and Figure 3B A first embodiment of a plasma generator 16 including a plurality of plasma generating units 20 is shown in FIG.

[0114] Figure 3A A portion of the enclosing structure 9 of the radiation source SO close to the opening 8 is shown in cross-section in a plane containing the optical axis 40 of the radiation source SO. Figure 3A and Figure 3B , the optical axis 40 of the radiation source SO is the z-direction. It will be appreciated that the optical axis 40 of the radiation source SO may coincide with a main direction of the output radiation B, which may be an average direction of the output radiation B (the average direction may be the direction of a chief ray of the output radiation B). Figure 3B An opening 8 of the radiation source SO is shown in cross section in a plane perpendicular to the optical axis 40 of the radiation source SO.

[0115] like Figure 3B As can be seen in Figure 3A and Figure 3B In the embodiment shown in FIG. 1 , the plasma generator 16 comprises three plasma generating units 20 distributed around the opening 8 and three plasma outlets 42 distributed around the opening 8. Each of the plasma generating units 20 is arranged to guide the plasma flow 38 generated thereby to cross the opening 8. Each of the plasma outlets 42 is arranged to be opposite to a different one of the plasma generating units 20 and is arranged to receive the plasma flow 38 generated thereby when the plasma flow 38 has propagated across the opening 8.

[0116] Each plasma generating unit 20 is operable to provide a plasma stream 38 from a different direction at least partially across the opening 8. All of these plasma streams 38 combine together to form a plasma 18 extending at least partially across the opening 8 and having a greater electron density than any individual plasma stream 36.

[0117] Although in Figure 3A and Figure 3B The example shown in FIG. 4 includes three plasma generating units 20 and three corresponding plasma outlets 42, but it will be appreciated that in alternative embodiments, more or fewer plasma generating units 20 may be provided. Typically, the plasma outlets 42 may be provided on the side of the opening 8 opposite each of the plasma generating units 20.

[0118] exist Figure 4 A second embodiment of a plasma generator 16 including a plurality of plasma generating units 20 is shown in FIG.

[0119] Figure 4 A portion of the enclosing structure 9 of the radiation source SO close to the opening 8 is shown in cross-section in a plane containing the optical axis 40 of the radiation source SO. Figure 4 , the optical axis 40 of the radiation source SO is the z-direction. It will be appreciated that the optical axis 40 of the radiation source SO may coincide with a main direction of the output radiation B, which may be an average direction of the output radiation B (the average direction may be the direction of a chief ray of the output radiation B).

[0120] In the described embodiment, Figure 4 As shown in FIG. 1 , the plasma generator 16 includes six plasma generating units 20 distributed around the opening 8. Figure 4 The example shown in FIG. 8 includes six plasma generating units 20 , but it will be appreciated that in alternative embodiments, more or fewer plasma generating units 20 may be provided. It will also be appreciated that the plasma generating units 20 may be substantially evenly distributed around the opening 8 .

[0121] Each of the plasma generating units 20 is arranged to guide the plasma flow 38 generated by the plasma generating unit 20 partially across the opening 8. Figure 4 As can be seen in FIG. 4 , each plasma generating unit 20 is arranged to direct the plasma flow 38 generated thereby so as to flow at least partially through the opening 8 and into the enclosing structure 9 of the radiation source SO.

[0122] Each plasma generating unit 20 is operable to provide a plasma stream 38 from a different direction at least partially across the opening 8. All of these plasma streams 38 combine together to form a plasma 18 extending at least partially across the opening 8 and having a greater electron density than any individual plasma stream 36.

[0123] Advantageously, by providing a plasma flow 18 across the opening 8 and into the enclosure 9 of the radiation source SO, the efficiency of removing particle debris may be further improved, as the plasma flow 18 will tend to force particle debris back into the enclosure 9. This particle debris is thus (at least partially) prevented from travelling through the opening 8 and reaching optics in the lithographic apparatus LA. In this way, the plasma generator 16 may be considered to form part of a dynamic plasma lock.

[0124] In some embodiments, the plasma generator 16 is provided with one or more adjustable parameters, and one or more properties of the plasma 18 can be controlled by controlling the one or more adjustable parameters. The one or more adjustable parameters may, for example, include a voltage that can be applied between the first electrode 22 and the second electrode 24 by a voltage source 26 of one or more plasma generating units 20. Additionally or alternatively, the one or more adjustable parameters may, for example, include a pressure or gas flow 36 generated by a gas flow generator 28 of one or more plasma generating units 20.

[0125] In some embodiments, the output radiation B generated by the radiation source SO is pulsed. For such embodiments, the plasma 18 generated by the plasma generator 16 may be pulsed. Furthermore, the radiation source SO and the plasma generator 16 may be synchronized so that the plasma 18 is present when each pulse of the output radiation propagates through the opening 8.

[0126] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include 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, and the like.

[0127] Although specific reference is made herein to embodiments of the invention in the context of lithographic equipment, embodiments of the invention may be used in other equipment. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other pattern forming device). These apparatus may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0128] Where context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium that can be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include a read-only memory (ROM); a random access memory (RAM); a magnetic storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.), and the like. In addition, herein, firmware, software, routines, instructions may be described as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and that these actions are actually generated by a computing device, a processor, a controller, or other device that executes firmware, software, routines, instructions, etc., and that doing so may cause an actuator or other device to interact with the physical world.

[0129] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in a manner different from that described. The above description is intended to be exemplary rather than restrictive. Therefore, it will be appreciated by those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A system comprising: A radiation source, the radiation source comprising: a housing, the housing being provided with an exit hole; and a radiation generator operable to generate output radiation in the housing and to direct at least a portion of the output radiation through the exit aperture; and a plasma generator operable to generate plasma extending at least partially across the at least a portion of the output radiation directed through the exit aperture, wherein the plasma generator comprises at least one plasma generating unit arranged to generate the plasma and to direct the plasma to flow at least partially across the exit aperture.

2. The system according to claim 1, wherein: The radiation generator comprises: a target generator operable to generate a target comprising a target material at a plasma formation region, the plasma formation region being arranged to receive laser radiation for irradiating the target material to thereby generate a second plasma emitting the output radiation; and An optical element is arranged to direct at least a portion of the output radiation towards the exit aperture.

3. A system according to claim 1 or claim 2, wherein: The radiation generator is arranged to focus at least a portion of the output radiation at an intermediate focus, which is arranged at or near the exit aperture.

4. A system according to claim 1 or claim 2, wherein: The plasma generator is operable to form the plasma from hydrogen gas.

5. The system according to claim 1 or claim 2, wherein: The plasma generator is operable to form a plasma having a value greater than 10 19 cm -3 The electron density of the plasma.

6. The system of claim 1 or claim 2, wherein: The plasma generator is operable to form plasma in a region having an extent greater than 300 μm along a direction of propagation of the output radiation.

7. The system of claim 1 or claim 2, wherein: The plasma generator is operable to generate the plasma using an arc discharge.

8. The system of claim 1 or claim 2, wherein: The plasma generator is operable to direct the plasma generated by the plasma generator to flow at least partially across the exit aperture.

9. The system according to claim 1, wherein: The or each plasma generating unit comprises: a first electrode and a second electrode; a voltage source operable to apply a voltage between the first electrode and the second electrode; and An airflow generator is operable to generate an airflow between the first electrode and the second electrode and in a direction at least partially transverse to the exit aperture.

10. The system according to claim 9, wherein: The first electrode is hollow and the second electrode is disposed within the first electrode.

11. A system according to claim 9 or claim 10, wherein: The or each plasma generating unit comprises a nozzle.

12. The system according to any one of claims 1, 9 to 10, wherein: The plasma generator includes a plurality of the plasma generating units.

13. The system according to any one of claims 1, 2, 9 to 10, wherein: The output radiation generated by the radiation generator is pulsed and the plasma generated by the plasma generator is pulsed, and wherein the radiation generator and the plasma generator are synchronized such that the plasma exists as each pulse of the output radiation propagates through a region where the plasma is formed.

14. A photolithography system comprising: A system according to any preceding claim; and A lithographic apparatus is operable to receive output radiation from the radiation source and to form an image of a patterning device on a substrate using the output radiation.

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