Radiation source for generating EUV light

Abstract

A radiation source for generating EUV light from plasma, comprises a plasma generation region to hold a plasma, a first reflecting body facing the plasma generation region and having a primary focus and a secondary focus, configured to reflect EUV light generated in the plasma generation region to the secondary focus, a second reflecting body facing the plasma generation region, wherein the second reflecting body is configured to reflect the EUV light from the plasma generation region to or near the primary focus and via the first reflecting body to the secondary focus. The radiation source is configured in that in use a self image of the plasma generation region reflected by the second reflecting body is offset relative to the plasma generation region.

Description

RADIATION SOURCE FOR GENERATING EUV LIGHTCROSS REFERENCE TO EARLIER APPLICATION

[0001] The present application claims benefit of EP Application No. 24174852.4, filed 8 May 2024.FIELD

[0002] The present invention relates to a radiation source for generating EUV light from plasma, an exposure apparatus comprising such a radiation source, a method of generating radiation, a semiconductor device manufacturing method, a semiconductor device manufactured by the semiconductor device manufacturing method, and a second reflecting body.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, 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 (e.g., a photoresist or resist) provided on a substrate. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses a reflective surfaces instead of a lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector, the multilayers may be altematingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a massproduction process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patternedmask inspection and can be carried out for the quality control of EUV patterned masks. For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Forth, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0006] A radiation source for generating EUV light from plasma comprises a plasma generation region configured to hold a plasma. The plasma may for example be generated by providing droplets of a fuel, such as droplets of a molten metal, and irradiating the droplets by a laser to form a plasma. The plasma in the plasma generation region may be irradiated by a laser beam to generate EUV radiation. An EUV source collector mirror may reflect the EUV radiation emitted from and / or scattered from the laser produced plasma to converge at a focus point.

[0007] The laser produced plasma generated from a droplet of molten metal may emit EUV radiation in a wide angle, such as in 4 pi steradians. The collector mirror may however only capture a part of this radiation, such as less than half of the radiation, i.e. less than a hemisphere of the emitted radiation. A remainder of the emitted EUV radiation may get lost.SUMMARY

[0008] It is desirable to enhance an efficiency of the radiation source.

[0009] According to an aspect of the invention, there is provided a radiation source for generatingEUV light from plasma, comprising:- a plasma generation region configured to hold a plasma;- a first reflecting body facing the plasma generation region and having a primary focus and a secondary focus, the first reflecting body being configured to reflect EUV light generated in the plasma generation region to the secondary focus; and- a second reflecting body facing the plasma generation region, wherein the second reflecting body is configured to reflect the EUV light from the plasma generation region to or near, preferably near, the primary focus and via the first reflecting body to the secondary focus, wherein the radiation source is configured in that in use a self-image of the plasma generation region reflected by the second reflecting body is offset relative to the plasma generation region.

[0010] According to an aspect of the invention, there is provided an exposure apparatus comprising the radiation source according to the previous aspect.

[0011] According to an aspect of the invention, there is provided a method of generating radiation comprising:- generating a plasma in a plasma generation region,- generating EUV light from the plasma,- reflecting by a first reflecting body facing the plasma generation region EUV light generated in the plasma generation region to a secondary focus of the first reflecting body; and - reflecting by a second reflecting body the EUV light from the plasma generation region to or near, preferably near, the primary focus and via the first reflecting body to the secondary focus, wherein a self image of the plasma generation region reflected by the second reflecting body is offset relative to the plasma generation region.

[0012] According to an aspect of the invention, there is provided a semiconductor device manufacturing method comprising:- receiving a substrate with a photoresist layer;- directing EUV radiation from the above-defined radiation source to transfer a pattern from a mask onto the photoresist layer;- processing the substrate by removing a portion of the photoresist layer to form the pattern over the substrate, and- manufacturing a semiconductor device from the processed substrate.

[0013] According to an aspect of the invention, there is provided a semiconductor device manufactured by the semiconductor device manufacturing method according to the preceding aspect of the invention.

[0014] According to an aspect of the invention, there is provided a second reflecting body comprising a ridged mirror array, configured to be used in the above-defined radiation source.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 depicts a system for (actinic) mask inspection;Figure 3A and 3B depict highly schematic views of a radiation source according to an embodiment of the present invention;Figure 4 A to 4D depict highly schematic, detailed views of a part of a radiation source;Figure 5 shows a graph of normalized transmission at secondary focus as function of distance between the center of plasma generation region and primary focus; andFigure 6 A to 6B depicts a highly schematic view of the radiation source according to Figures 3A and 3B.DETAILED DESCRIPTION

[0016] Figure 1 shows a 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 tosupply 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 PD (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0017] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device PD. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0018] After being thus conditioned, the EUV radiation beam B interacts with the patterning device PD. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise 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’, thus forming an image with features that are smaller than corresponding features on the patterning device PD. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0019] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0020] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0021] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0022] Figure 2 depicts a system for (actinic) mask inspection. A mask inspection system can be used to identify or inspect defects in a mask to be used in a lithographic process by means of an apparatus described in figure 1 . The mask inspection system comprises an radiation source SO and an illumination system IS and a detection system DS. A mask MA is placed on a support structure MT (e.g. a mask stage) and illuminated by the illumination system (IS) reflecting radiation incident from the radiationsource. The radiation coming from the illuminated mask is reflected by the detection system. In this way an image is formed on a detector DET.

[0023] The radiation source SO shown in Figures 1 and 2 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. Shown in Figure 1, a laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator 3. Although tin is referred to 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 generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0024] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal -incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0025] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0026] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts 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 to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0027] Figure 3A schematically depicts a radiation source according to an embodiment of the invention. The radiation source is configured to generate Extreme Ultra Violet, EUV, light from plasma. The radiation source comprises a plasma generation region PGR configured to hold a plasma, such as the plasma generated from a molten metal or molten alloy as described above. A first reflecting body FRB, such as a collector mirror, faces the plasma generation region for reflecting EUV light emitted by or scattered from the plasma generation region. The first reflecting body provides for a primary focusof the reflected EUV radiation and a secondary focus of the reflected EUV radiation, the secondary focus being more remote from the first reflecting body than the primary focus . The first reflecting body is configured to reflect the EUV light generated in the plasma generation region to the secondary focus. Accordingly, a part of the EUV light emitted by or scattered from the plasma generation region is incident on the first reflecting body, where it is reflected to the secondary focus of the first reflecting body.

[0028] The plasma generation region may emit EUV light in 4 pi steradian. The first reflecting body extends along approximately a hemisphere, thereby capturing a half, or less, of the emitted EUV light.

[0029] Embodiments of present invention seeks to enhance a collection efficiency of collecting EUV light emitted by the plasma generation region.

[0030] As depicted in Figure 3A, the radiation source further comprises a second reflecting body SRB facing the plasma generation region, such as in the present example a plurality of tiled mirrors TM. The least one second reflecting body is configured to reflect the EUV light from the plasma generation region via the first reflecting body to or near the primary focus. Accordingly, a part of the remaining light emitted by the plasma generation region, that would otherwise be absorbed or lost, may be reflected into a light path along which the light reflected by the first reflecting body propagates.

[0031] An illustration of EUV light by the second reflecting body is illustrated in Figure 3B. Figure 3B depicts the radiation source as depicted in Figure 3A, illustrating light reflected by the second reflecting body. Light emitted or scattered from the plasma generation region to the second reflecting body is indicated by 21. The light reflected by the second reflecting body, in the present example by one of the tiled mirrors, is indicated by 22. In the embodiment as depicted in Figure 3A and 3B, the light reflected by the second reflecting body propagates to or near the primary focus and then, as indicated by 23, to the first reflecting body in Figure 3B, where, as indicated by 24, it is reflected by the first reflecting body FRB towards the secondary focus SF of the first reflecting body.

[0032] In case the second reflecting body would reflect the EUV light from the second reflecting body back to the plasma generation region, i.e. in case a self image of the plasma generation region would substantially overlap with the plasma generation region, the EUV light would be reflected into the plasma at the plasma generation region. A transmissivity or other optical properties of the plasma at the plasma generation region may be unfavorable, i.e. may adversely affect a transmission of the EUV light reflected from the second reflecting body into the plasma generation region. Therefore, according to embodiments of the invention, an offset is provided between the plasma generation region and the self-image of the plasma generation region. Thus, the location of the self-image of the plasma generation region is offset in respect of the location of the plasma generation region. Accordingly, the radiation source is configured in that in use a self-image of the plasma generation region reflected by the second reflecting body is offset relative to the plasma generation region.

[0033] The offsetting of the self-image of the plasma generation region in respect of the plasma generation region is further explained below with reference to Figures 4A - 4D.

[0034] Figure 4 A depicts the first reflecting body FRB and plasma generation region PGR. A part of the EUV light emitted by the plasma at the plasma generation region is reflected by the first reflecting body towards the secondary focus SF.

[0035] Further to the first reflecting body and the plasma generation region as depicted in Figure 4A, Figure 4B depicts one of the tiled mirrors TM of the secondary reflecting body SRB. A part of the EUV light emitted in use by the plasma at the plasma generation region reflects on the mirror of the second reflecting body. The mirror of the secondary reflecting body may comprise a spherical mirror. The part of the EUV light reflected by the mirror of the secondary reflecting body is reflected to form a selfimage of the plasma generation region SI-PGR. In the explanatory figure 4B, the self-image of the plasma generation region SI-PGR substantially overlaps with the plasma generation region PGR. The EUV light reflected by the secondary reflecting body then propagates to the first reflecting body where it is reflected towards the secondary focus of the first reflecting body.

[0036] As described above, a transmissivity or other optical properties of the plasma at the plasma generation region may be unfavorable, i.e. may adversely affect a transmission of the EUV light reflected from the second reflecting body into the plasma generation region. Therefore, according to embodiments of the invention, an offset is provided between the plasma generation region and the selfimage of the plasma generation region. Thus, the location of the self-image of the plasma generation region is offset in respect of the location of the plasma generation region.

[0037] The above described offset may be implemented in various ways. Embodiments will be described below with reference to Figures 4C and 4D.

[0038] Figure 4C depicts a schematic view of a radiation source, wherein the primary focus PF of the first reflecting body FRB is in the plasma generation region PGR and wherein the self-image of the plasma generation region SI-PGR is offset from the primary focus PF by offset OFF. As depicted in Figure 4C, the self-image of the plasma generation region is offset from the primary focus towards the secondary focus, which may provide a defocus of the image at the secondary focus SF, possibly causing the resulting image at SF to be a bit larger due to this defocus, resulting in a spherical aberration.

[0039] As an alternative, Figure 4D depicts a schematic view of a radiation source, wherein both the plasma generation region PGR and the self-image of the plasma generation region SI-PGR are offset from the primary focus PF a combined offset being indicated by OFF. An offset of the plasma generation region PGR from the primary focus PF is indicated by OFF1 and an offset of the self-image of the plasma generation region SI-PGR from the primary focus PF is indicated by OFF2, whereby the offsets OFF1 and OFF2 combined provide for a combined offset being indicated by OFF. Similarly to Figure 4C, the offsets may provide a defocus or spherical aberration at the secondary focus SF, however the defocus or spherical aberration may possibly be smaller in magnitude than in the embodiment described with reference to Figure 4C. As depicted in Figure 4D, the plasma generation region and the self-image of the plasma generation region are on opposite sides of the primary focus, seen along a line extending from the primary focus to the secondary focus. Offsetting on both sides may provide that thedistance, i.e. the magnitude of offset OFF1 of the plasma generation region PGR and the magnitude of the offset OFF2 of the self-image of the plasma generation region SI-PGR may be smaller than the offset OFF in the embodiment in accordance with Figure 4C, which may result in less spherical aberration on the image at the secondary focus SF.

[0040] When the tiled mirrors TM of the secondary reflecting body SRB are spherical with their focal points coinciding exactly with the primary focus PF of the first reflecting body FRB, OFF1 may generally be equal to OFF2. However, in some embodiments OFF1 may advantageously be smaller than OFF2. Asymmetry between OFF1 and OFF2 favoring the alignment of the plasma generation region with the primary focus of the first reflecting body FRB over the alignment of the self-image of the plasma generation region with the primary focus of the first reflecting body FRB, has been found to result in more EUV in the secondary SF. This may be achieved for example by an adjustment of the SRB.

[0041] The inventors have devised that a small offset of the self-image of the plasma generation region relative to the plasma generation region may suffice, more specifically, the inventors have devised that a distance between a center of the plasma generation region and a center of the self-image of the plasma generation region of 50 micrometers - 300 micrometers may enhance an efficiency of the radiation source.

[0042] Alternatively, the distance between the center of the plasma generation region and the center of the self-image of the plasma generation region is selected in a range between 300 micrometers and 600 micrometers, whereby a first offset (OFF1) between the center of the plasma generation region and a primary focus of the first reflecting body is in the region of 50 micrometers - 300 micrometers. Preferably, the distance between the center of the plasma generation region and the center of the selfimage of the plasma generation region is selected in a range between 310 micrometers and 600 micrometers, whereby the first offset is in the region of 50 micrometers - 300 micrometers. More preferably, the distance between the center of the plasma generation region and the center of the selfimage of the plasma generation region is selected in a range between 310 micrometers and 600 micrometers, whereby the first offset is in the region of 50 micrometers - 150 micrometers. Within these ranges, the first offset of the plasma generation region from the primary focus seen along the line extending from the primary focus to the secondary focus is always smaller than the second offset of the self-image of the plasma generation region from the primary focus seen along that line. In each situation described in this paragraph, the plasma generation region and the self image of the plasma generation region are on opposite sides of the primary focus.

[0043] Figure 5 shows a graph of normalized transmission (Tr) at secondary focus, as function of distance between the center of plasma generation region and primary focus in the direction of the first reflection body (OFF1). The transmission at secondary focus is normalized relative to the radiation from the plasma generation region coinciding with the primary focus (OFF1 = 0) without any contribution from a self image from the secondary reflecting body. Calculations have been made with assumptionsabout reflectivity coefficients from the first and second reflecting bodies, and plasma density distribution around the center of the plasma generation region. Curve 31 shows the relative transmission of the plasma in the plasma generation region, as a function of OFF 1 of the plasma generation center. Curve 32 shows the relative transmission of the self image of the plasma generation region, which is established by the secondary reflecting body at a fixed offset OFF of 400 microns relative to the plasma generation region (center-to-center). Curve 33 shows the relative transmission of the plasma and the self image combined. The relative transmission of the plasma generation region and the self image combined would be about 1.135 when the plasma generation region would coincide with the primary focus. This is 13.5% higher than the would be obtained by the plasma generation region only, without the contribution of the self image formed by the secondary reflection body. The relative transmission gain can be even higher by shifting the center of the plasma formation region toward the first reflection body along the Z-axis. At OFF1 of about 115 microns, a maximum transmission gain of 1.151 is reached. For reference, the top axis reflects OFF2, which is defined as OFF minus OFF1.

[0044] In the embodiments described above, the reflective element which reflects the EUV light reflected from the second reflecting body is the first reflecting body. Thereby, the light reflected by the second reflecting body may be added to the EUV light reflected by the first reflecting body, without adding additional optical components. Accordingly, in an embodiment, the second reflecting body is arranged to reflect the EUV light to the first reflecting body.

[0045] Figure 6 A depicts a further embodiment of the radiation source according to embodiments of the invention. In the present embodiment, likewise to Figures 3A, 3B, 4A to 4D, the plasma generation region PGR and the first reflecting body FRB are depicted, the first reflecting body configured to provide the primary focus PF and the secondary focus SF. In the present embodiment, the second reflecting body SRB comprises a ridged mirror array RMA, providing plural ridges comprising plural mirrors. The ridged mirror array may comprise plural mirror segments configured to reflect EUV light from the plasma generation region towards a same spatial location to form the self-image of the plasma generation region. The mirror segments may capture a relatively large part of the EUV light, i.e. the EUV radiation, emitted in the upper hemisphere (i.e. in the hemisphere not reflected on the primary reflecting body, thereby further enhancing an efficiency of the radiation source. Moreover, the shape of the ridged mirror array may be compatible with a shape of existing EUV radiation sources, thus enabling to increase an efficiency within a form factor of an existing EUV radiation source.

[0046] An optimization may be performed in that a position and / or orientation of the mirror segments of the ridged mirror array may be adjusted to enhance an output of the radiation source. For example, an output intensity or output intensity spatial distribution may be measured, and the position and / or orientation of the mirrors of the ridged mirror array may be adjusted on the basis of the measured output intensity or output intensity spatial distribution. The adjustment of the position and / or orientation of the mirrors of the ridged mirror array may be performed for all mirror segments simultaneously, e.g. byadjustment of the position of the second reflecting body, or may be adjusted per mirror segment, thereby enhancing an output of the radiation source on a per mirror segment basis.

[0047] In order to individually adjust the mirror segments of the ridged mirror array, the second reflecting body may comprises a plurality of mirror segment actuators, each of the mirror segment actuators configured to actuate a position of a respective one of the mirror segments of the ridged mirror array.

[0048] In order to adjust the ridged mirror array in its entirety, the second reflecting body may further comprise a ridged mirror array actuator configured to actuate the position of the ridged mirror array.

[0049] The mirror segment actuators and / or the ridged mirror array actuator may be driven by a control device of the radiation source, e.g. responsive to the above described measurement of intensity and / or intensity distribution.

[0050] Figure 6B depicts a further embodiment of the radiation source according to embodiments of the invention. In the present embodiment, likewise to Figure 6A, the plasma generation region PGR and the first reflecting body FRB are depicted, the first reflecting body configured to provide the primary focus PF and the secondary focus SF. The second reflecting body SRB comprises a ridged mirror array RMA, providing plural ridges comprising plural mirrors. The ridged mirror array may comprise plural mirror segments configured to reflect EUV light from the plasma generation region towards a same spatial location to form the self-image of the plasma generation region. Edges between the mirror segments may not reflect EUV radiation. These edges may e.g. be perpendicular to the reflecting surface of the mirror segments and may be identified as dark edges DE. The radiation source in accordance with Figure 6B may further comprise the mirror segment actuators and / or the ridged mirror array actuator as described above with reference to Figure 6A.

[0051] A part of the second reflecting body is depicted at a right side of Figure 6B in an enlarged view. As depicted in Figure 6B, in order to be able to clean the second reflecting body from particles of the metal or alloy as may be applied to generate the plasma at the plasma generation region, the second reflecting body comprises gas nozzles GN to provide a purging gas or cleaning gas, e.g. comprising a Hydrogen gas. The nozzles may be arranged at the dark edges DE of the ridged mirror array RMA, thereby on the one hand keeping the mirror segments MSE usable for collecting EUV radiation available for collecting EUV radiation, while providing a steam of the gas along a surface of these mirror segments.

[0052] The radiation source as described above may be comprised in an exposure apparatus, such as a lithographic system as described above with reference to Figure 1.

[0053] According to an aspect of the present invention, there is provided a method of generating radiation comprising: generating a plasma in a plasma generation region, generating EUV light from the plasma, reflecting by a first reflecting body facing the plasma generation region EUV light generated in the plasma generation region to a secondary focus of the first reflecting body, reflecting by a second reflecting body the EUV light from the plasma generation region to or near the primary focus and viathe first reflecting body to the secondary focus, wherein a self image of the plasma generation region reflected by the second reflecting body is offset relative to the plasma generation region. With the method of generating radiation according to embodiments of the invention, the same or similar effects may be achieved as described above with reference to the radiation source according to embodiments of the invention. Furthermore, the same or similar preferred embodiments of the method may be provided as described with reference to the radiation source according to embodiments of the invention.

[0054] According to a further aspect of the invention, there is provided a semiconductor device manufacturing method comprising: receiving a substrate with a photoresist layer; directing EUV radiation from a radiation source according to embodiments of the invention to transfer a pattern from a mask onto the photoresist layer; processing the substrate by removing a portion of the photoresist layer to form the pattern over the substrate, and manufacturing a semiconductor device from the processed substrate.

[0055] According to a further aspect of the invention, there is provided a semiconductor device manufacturing method comprising: receiving a substrate with a photoresist layer; generating EUV radiation by the method of generating radiation according to embodiments of the invention, directing the generated EUV radiation to transfer a pattern from a mask onto the photoresist layer; processing the substrate by removing a portion of the photoresist layer to form the pattern over the substrate, and manufacturing a semiconductor device from the processed substrate.

[0056] With the semiconductor device manufacturing method according to embodiments of the invention, the same or similar effects may be achieved as described above with reference to the radiation source according to the invention. Furthermore, the same or similar preferred embodiments of the method may be provided as described with reference to the radiation source according to embodiments of the invention.

[0057] According to a further aspect of the invention, there is provided a semiconductor device manufactured by the semiconductor device manufacturing method according to embodiments of the invention.

[0058] According to a further aspect of the invention, there is provided a second reflecting body, e.g. comprising a ridged mirror array, configured to be used in the radiation source according to embodiments of the invention. The second reflecting body may comprise gas nozzles at dark edges of the ridged mirror array.

[0059] 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 the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0060] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus.Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0061] 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, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0062] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may 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 read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical, and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0063] Clauses1. A radiation source for generating EUV light from plasma, comprising:- a plasma generation region configured to hold a plasma- a first reflecting body facing the plasma generation region and having a primary focus and a secondary focus, the first reflecting body being configured to reflect EUV light generated in the plasma generation region to the secondary focus;- a second reflecting body facing the plasma generation region, wherein the second reflecting body is configured to reflect the EUV light from the plasma generation region to or near the primary focus and via a reflective element to the secondary focus, wherein the radiation source is configured in that in use a self image of the plasma generation region reflected by the second reflecting body is offset relative to the plasma generation region.2. The radiation source according to clause 1, wherein the offset is along a line between the primary focus and the secondary focus of the first reflecting body.3. The radiation source according to any one of the preceding clauses, wherein the primary focus is in the plasma generation region and wherein the self image of the plasma generation region is offset from the primary focus.4. The radiation source according to clause 3, wherein the self image of the plasma generation region is offset from the primary focus towards the secondary focus.5. The radiation source according to clause 1 or 2, wherein both the plasma generation region and the self image of the plasma generation region are offset from the primary focus.6. The radiation source according to clause 5, wherein the plasma generation region and the self image of the plasma generation region are on opposite sides of the primary focus, seen along a line extending from the primary focus to the secondary focus.7. The radiation source according to clause 5 or 6, wherein a distance between a center of the plasma generation region and a center of the self-image of the plasma generation region is selected in a range of between 300 micrometers and 600 micrometers, whereby a first offset between the center of the plasma generation region and a primary focus of the first reflecting body is in the region of 50 micrometers - 300 micrometers.8. The radiation source according to clause 7, wherein the distance between the center of the plasma generation region and the center of the self-image of the plasma generation region is selected in a range of between 310 micrometers and 600 micrometers.9. The radiation source according to clause 8, wherein the first offset is in the region of 50 micrometers - 150 micrometers.10. The radiation source according to any one of the preceding clauses, wherein a distance between a center of the plasma generation region and a center of the self image of the plasma generation region is 50 micrometers - 300 micrometers.11. The radiation source according to any one of the preceding clauses, wherein the reflective element is the first reflecting body.12. The radiation source according to any one of the preceding clauses, wherein the second reflecting body is arranged to reflect the EUV light to the first reflecting body.13. The radiation source according to any one of the preceding clauses, wherein the second reflecting body comprises a ridged mirror array.14. The radiation source according to clause 13, wherein the second reflecting body comprises gas nozzles at dark edges of the ridged mirror array.15. An exposure apparatus comprising the radiation source according to any one of the preceding clauses.16. A method of generating radiation comprising:- generating a plasma in a plasma generation region,- generating EUV light from the plasma,- reflecting by a first reflecting body facing the plasma generation region EUV light generated in the plasma generation region to a secondary focus of the first reflecting body;- reflecting by a second reflecting body the EUV light from the plasma generation region to or near the primary focus and via a reflective element to the secondary focus, wherein a self image of the plasmageneration region reflected by the second reflecting body is offset relative to the plasma generation region.17. A semiconductor device manufacturing method comprising:- receiving a substrate with a photoresist layer;- directing EUV radiation from a radiation source according to any one of clauses 1 to 14 to transfer a pattern from a mask onto the photoresist layer;- processing the substrate by removing a portion of the photoresist layer to form the pattern over the substrate, and- manufacturing a semiconductor device from the processed substrate.18. A semiconductor device manufactured by the method according to clause 17.19. A second reflecting body comprising a ridged mirror array, configured to be used in the radiation source according to any one of clauses 1 to 14.20. The second reflecting body according to clause 19, wherein the ridged mirror array provides plural ridges comprises plural mirror segments configured to reflect EUV light from the plasma generation region towards a same spatial location to form the self-image of the plasma generation region.21. The second reflecting body according to clause 20, wherein the shape of the ridged mirror array is compatible with the shape of the EUV radiation source to enable to increase EUV collection efficiency within a form factor of the EUV radiation source.22. The second reflecting body according to clause 20 or 21, wherein edges between the mirror segments extend perpendicular to the reflecting surface of the mirror segments and do not reflect EUV radiation.23. The second reflecting body according to clause 20 or 21, wherein edges between the mirror segments extend as truncated cone sections originating from the primary focus.24. The second reflecting body according to any of clauses 19 to 23, further comprising gas nozzles.25. The second reflecting body according to clause 19, wherein the ridged mirror array comprises a plurality of tiled mirrors.26. The second reflecting body according to clause 25, wherein each tiled mirror comprises a spherical mirror.27 The second reflecting body according to clause 26, wherein each spherical mirror comprises reflecting surface which is equidistant from a central focal point.28. The second reflecting body according to clause 27, wherein the central focal point is arranged at or near the primary focus of the first reflecting body.

[0064] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

[0065] The lithographic apparatus LA and radiation source SO described herein can be used in method for manufacturing a semiconductor device. A semiconductor device manufacturing method comprises receiving a substrate W with a photoresist layer. The method further comprises directing EUV radiation from radiation source SO to transfer a pattern from a mask onto the photoresist layer; and removing a portion of the photoresist layer to form the pattern over the substrate W.

[0066] The substrate W may be made of silicon or other semiconductor materials. Alternatively or additionally, the substrate W may include other semiconductor materials such as germanium (Ge) or carbon (C). In some embodiments, the semiconductor substrate is made of a compound semiconductor such as III-V compound semiconductors, II-V compound semiconductors, and / or any suitable integration of Group IV materials. In some embodiments, the substrate W may be a silicon-on-insulator (SOI) or a germanium -on-insulator (GOI) substrate.

[0067] The semiconductor device made from the substrate W may have various device elements. Examples of semiconductor device elements that are formed over the substrate W include transistors (e.g., planar or non-planar metal oxide semiconductor field effect transistors (MOSFET), bipolar junction transistors (BJT), high-voltage transistors, high-frequency transistors, etc.), diodes, CMOS image sensors, passive devices, and / or other applicable elements. Various processes may be performed to form the semiconductor device elements, such as deposition, etching, implantation, epitaxial growth, polishing, thermal treatment, and / or other suitable processes. In some embodiments, the substrate W is coated with a photoresist layer sensitive to the EUV light.

Claims

CLAIMS1. A radiation source for generating EUV light from plasma, comprising:- a plasma generation region configured to hold a plasma;- a first reflecting body facing the plasma generation region and having a primary focus and a secondary focus, the first reflecting body being configured to reflect EUV light generated in the plasma generation region to the secondary focus; and- a second reflecting body facing the plasma generation region, wherein the second reflecting body is configured to reflect the EUV light from the plasma generation region to or near the primary focus and via the first reflecting body to the secondary focus, wherein the radiation source is configured in that in use a self image of the plasma generation region reflected by the second reflecting body is offset relative to the plasma generation region.

2. The radiation source according to claim 1, wherein the offset is along a line between the primary focus and the secondary focus of the first reflecting body.

3. The radiation source according to any one of the preceding claims, wherein the primary focus is in the plasma generation region and wherein the self image of the plasma generation region is offset from the primary focus.

4. The radiation source according to claim 3, wherein the self image of the plasma generation region is offset from the primary focus towards the secondary focus.

5. The radiation source according to claim 1 or 2, wherein both the plasma generation region and the self image of the plasma generation region are offset from the primary focus.

6. The radiation source according to claim 5, wherein the plasma generation region and the self image of the plasma generation region are on opposite sides of the primary focus, seen along a line extending from the primary focus to the secondary focus.

7. The radiation source according to claim 6, wherein a first offset of the plasma generation region from the primary focus seen along the line extending from the primary focus to the secondary focus is smaller than a second offset of the self-image of the plasma generation region from the primary focus seen along said line.

8. The radiation source according to any one of the preceding claims, wherein a distance between a center of the plasma generation region and a center of the self image of the plasma generation region is in a range of from 50 micrometers to 300 micrometers.

9. The radiation source according to any one of the preceding claims, wherein the second reflecting body is arranged to reflect the EUV light to the first reflecting body.

10. The radiation source according to any one of the preceding claims, wherein the second reflecting body comprises a ridged mirror array.

11. The radiation source according to claim 10, wherein the second reflecting body comprises gas nozzles at dark edges of the ridged mirror array.

12. An exposure apparatus comprising the radiation source according to any one of the preceding claims.

13. A method of generating radiation comprising:- generating a plasma in a plasma generation region,- generating EUV light from the plasma,- reflecting by a first reflecting body facing the plasma generation region EUV light generated in the plasma generation region to a secondary focus of the first reflecting body;- reflecting by a second reflecting body the EUV light from the plasma generation region to or near the primary focus and via the first reflecting body to the secondary focus, wherein a self image of the plasma generation region reflected by the second reflecting body is offset relative to the plasma generation region.

14. A semiconductor device manufacturing method comprising:- receiving a substrate with a photoresist layer;- directing EUV radiation from a radiation source according to any one of claims 1 to 11 to transfer a pattern from a mask onto the photoresist layer;- processing the substrate by removing a portion of the photoresist layer to form the pattern over the substrate, and- manufacturing a semiconductor device from the processed substrate.

15. A semiconductor device manufactured by the method according to claim 14.

16. A second reflecting body comprising a ridged mirror array, configured to be used in the radiation source according to any one of claims 1 to 11.

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