WO2019057584A1 - Source de rayonnement - Google Patents

Source de rayonnement Download PDF

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Publication number
WO2019057584A1
WO2019057584A1 PCT/EP2018/074606 EP2018074606W WO2019057584A1 WO 2019057584 A1 WO2019057584 A1 WO 2019057584A1 EP 2018074606 W EP2018074606 W EP 2018074606W WO 2019057584 A1 WO2019057584 A1 WO 2019057584A1
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WO
WIPO (PCT)
Prior art keywords
fuel target
radiation
cross
section
laser
Prior art date
Application number
PCT/EP2018/074606
Other languages
English (en)
Inventor
Andrei Mikhailovich Yakunin
Johannes Adrianus Cornelis Maria PIJNENBURG
Christian Gerardus Norbertus Hendricus Marie CLOIN
Original Assignee
Asml Netherlands B.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Asml Netherlands B.V. filed Critical Asml Netherlands B.V.
Priority to JP2020512670A priority Critical patent/JP2020534562A/ja
Priority to KR1020207008047A priority patent/KR20200055719A/ko
Priority to CN201880060612.6A priority patent/CN111108815A/zh
Publication of WO2019057584A1 publication Critical patent/WO2019057584A1/fr

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Classifications

    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05G—X-RAY TECHNIQUE
    • H05G2/00—Apparatus or processes specially adapted for producing X-rays, not involving X-ray tubes, e.g. involving generation of a plasma
    • H05G2/001—Production of X-ray radiation generated from plasma
    • H05G2/008—Production of X-ray radiation generated from plasma involving an energy-carrying beam in the process of plasma generation

Definitions

  • the present invention relates to a radiation source configured to provide EUV radiation.
  • the radiation source is of a type known as a laser produced plasma radiation source.
  • the radiation source may form part of a lithographic system.
  • 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 from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.
  • a patterning device e.g. a mask
  • a layer of radiation-sensitive material resist
  • the wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate.
  • a lithographic apparatus which uses extreme ultraviolet (EUV) radiation being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).
  • EUV extreme ultraviolet
  • LPP laser-produced plasma
  • a radiation source configured to provide EUV radiation.
  • the radiation source comprises a fuel emitter configured to provide a fuel target to a plasma formation region.
  • the radiation source also includes a laser system configured to illuminate an exposed surface of the fuel target with laser radiation when the fuel target is at the plasma formation region in order to convert the fuel target into plasma.
  • the laser radiation incident on the exposed surface has a first cross-section in a plane perpendicular to a propagation direction of the laser radiation.
  • the laser system is configured such that the first cross-section is smaller than a second cross-section of exposed surface in the plane.
  • the radiation source according to the first aspect of the present invention provides a laser produced plasma (LPP) source with improved conversion efficiency relative to known LPP sources, as now discussed.
  • LPP laser produced plasma
  • a fuel target is intended to mean a discrete (also referred to as: mass limited) quantity of fuel, such as a fuel droplet.
  • the laser radiation may be of the form of a laser beam (main pulse) and may have any of various different spatial intensity distributions in the laser beam's cross-section (i.e., the first cross-section referred to above).
  • the laser beam may have a top-hat intensity distribution and therefore may have a relatively well-defined boundary.
  • the laser beam may have a Gaussian-like intensity distribution.
  • the boundary or edge of the first cross-section of the laser beam may be defined as a central region of the first cross-section that contains a certain percentage of the intensity of the laser beam, for example 90%.
  • the boundary or edge of the first cross- section may be defined by a pre-determined intensity threshold (with the region wherein the intensity is above the intensity threshold being within the boundary and the region wherein the intensity is below the intensity threshold being outside the boundary).
  • the diameter of the laser beam's first cross-section may be understood to mean the diameter of the boundary of the central region of the first cross-section containing a certain percentage of the intensity or a threshold-intensity.
  • a fuel target is intended to mean a localised quantity of fluid fuel.
  • the fuel target (or: fuel droplet) is hit by a first laser pulse, referred to in the literature as: "a pre-pulse", and subsequently by a second laser pulse, referred to in the literature as: "a main pulse”.
  • the pre-pulse serves to condition the fuel target for receipt of the main pulse, e.g., by shaping the fuel target and/or affecting the mass density of the fuel target.
  • the fuel target upon being hit by the pre-pulse, is formed so as to have a core region of liquid fuel surrounded by a cloud of fuel vapor.
  • the density of the liquid core may be of the order of 10 21 atoms/cm 3 and the density of fuel within the fuel target typically decreases as the distance away from the core region increases.
  • vapor may include both atomic vapor of the fuel and also a cloud of small fuel droplets with the size of the order of 1-5000 nm. Since the term "vapor" is used to encompass small droplets of liquid fuel, it will be appreciated that in the following any reference to a density of the fuel target should be interpreted as an average density.
  • tin atoms within the plasma may have an ionization degree of around 10 (i.e. 10 electrons are stripped from each tin atom upon being hit by a barrage of photons of the main pulse).
  • this stripping away of electrons may occur on a time scale of a few picoseconds after the main pulse of radiation starts hitting the fuel target.
  • the laser radiation of the main pulse will only penetrate part of the fuel target and will not penetrate into the dense central region (i.e., the liquid core of the fuel target).
  • the laser radiation typically will only penetrate regions with a density below a threshold density.
  • the laser radiation may only penetrate to a depth wherein the density of the fuel target stays below a threshold density of the order of 10 18 atoms/cm 3 .
  • this threshold density corresponds to an electron density of the order of 10 19 electrons/cm 3 .
  • the laser radiation propagates through regions of the fuel target that are below the threshold density, which are effectively transparent to the laser radiation.
  • the laser radiation When the laser radiation reaches a portion of the fuel target proximate to the density threshold region a first portion of the laser radiation is absorbed. This heats this portion of the fuel target, converting it into plasma which, in turn, emits EUV radiation. A second portion of the laser radiation is reflected from the portion of the fuel target proximate to the density threshold region (due to the ionization of tin within the fuel target).
  • the portion of the fuel target, which is proximate to the density threshold region and which absorbs the laser radiation, may be referred to as the plasma conversion portion of the fuel target, or simply: the plasma conversion portion.
  • This plasma conversion portion is the portion of the fuel target that is converted into plasma and may, for example, comprise a portion of the fuel target with a density in the range of 10 17 -10 18 atoms/cm 3 .
  • a beam of laser radiation irradiates fuel targets present at a plasma formation region.
  • the laser radiation is focused to a focal spot.
  • the fuel target that the laser radiation of the main pulse illuminates has a larger diameter (also in a plane perpendicular to a propagation direction of the laser radiation) than the diameter of the focal spot of the laser radiation.
  • the diameter (or: the second cross-section) of the fuel target in a particular plane is intended to mean the diameter (or: the second cross-section) of the projection of the fuel target onto that particular plane.
  • the fuel target, conditioned by the pre-pulse is typically not spherical but rather elongate.
  • the plane in which the second cross-section of the fuel target is maximised may be generally perpendicular to the propagation direction of the main pulse.
  • the plane in which the second cross-section of the fuel target is maximised may be tilted relative to a plane perpendicular to the propagation direction of the main pulse, for example by up to 30° or 45°.
  • a beam of laser radiation is provided such that at the fuel target the diameter of the laser beam is the same or larger than the diameter of the fuel target.
  • This matching of the diameter of the laser beam to the diameter of the fuel target is achieved, for example, by controlling the distance between the fuel target and the focal plane of the laser radiation (i.e. the position of the beam waist).
  • the plasma formation region may be of the order of, say, a few mm in front of the focal plane of the laser radiation (i.e. the laser radiation is incident on the fuel target before it has converged to the focal spot).
  • the inventors of the present invention have realised that an increase in the conversion efficiency can be achieved by reducing the size of the cross-section of the laser radiation relative to the size of the cross-section of the conditioned fuel target.
  • This realisation has been prompted by a better understanding of the coupling of the laser radiation with a fuel target within an LPP source.
  • the inventors of the present invention now believe that when laser radiation is incident on a conditioned fuel target, a lower density peripheral portion of the fuel target acts as a refractive optical element that alters the shape, and in particular the cross-sectional size, of the laser radiation before the laser radiation is incident on the plasma conversion portion.
  • a lower density peripheral portion of the fuel target acts as a refractive optical element that alters the shape, and in particular the cross-sectional size, of the laser radiation before the laser radiation is incident on the plasma conversion portion.
  • the lower density peripheral portion of the fuel target tends to increase the cross-sectional size of the laser radiation before the laser radiation is incident on the plasma conversion portion.
  • the fuel target if the fuel target is hit by a laser beam with a cross-section that matches the size of the fuel target, the fuel target will widen the laser beam and will thus direct energy away from the volume occupied by the plasma conversion portion. This leads to a loss of energy and therefore to a lowering of the conversion efficiency.
  • the cross-section of the laser radiation at the fuel target is intended to mean the cross-section of the laser radiation in the absence of any scattering from the fuel target.
  • this may mean the cross-section of the radiation beam immediately before it enters the low density vapor cloud of the fuel target (the boundary being defined, for example, by a density threshold).
  • the entire plasma conversion portion receives the laser radiation (in order to optimize conversion efficiency).
  • a better definition of the cross-section of the laser radiation at the fuel target may be the cross-section that the laser radiation would have had at a position along the propagation direction of the laser radiation that corresponds to the position of the plasma conversion portion if the fuel target had been present, i.e., the cross-section that the laser radiation has at that position in the absence of any scattering effects caused by the presence of the conditioned fuel target.
  • the laser radiation is focused to a focal spot and that the plasma formation region may be in front of the focal plane of the laser radiation (i.e. the laser radiation is incident on the fuel target before it has converged to the focal spot).
  • the cross-section of the laser beam at the focal spot is also referred to as the "waist" of the laser beam.
  • the laser radiation may have a relatively small angle of convergence (i.e. an angle between the cone of laser radiation and a chief ray of the laser radiation) .
  • the cross-section of the laser radiation varies with position along the propagation direction of the laser radiation.
  • the typical dimensions of a fuel target and the typical angles of convergence are such that, in practice, the change in the cross- section of the laser radiation (in the absence of the fuel target) between a position that corresponds to the start of the fuel target (as defined by a density threshold) if the fuel target had been there and a position that corresponds to the plasma conversion portion if it had been there may be a relatively small percentage change. Therefore, the two above-mentioned definitions of what is meant by the cross- section of the laser radiation at the fuel target may be generally equivalent.
  • the pre-pulse may spread the fuel target out such that it is relatively larger in a first plane and relatively smaller in a second, perpendicular plane.
  • the first plane may be perpendicular to the propagation direction of the laser radiation or inclined thereto by up to 30°, or by up to 45°, for example.
  • the first cross-section of the laser radiation may be smaller than the second cross-section of the fuel target in a plane perpendicular to a propagation direction of the laser beam by an amount so as to at least partially reduce the amount of laser radiation that is scattered so that it is not incident on the plasma conversion portion.
  • the fuel target has a portion with a mass density configured to absorb the laser radiation, also referred to as the plasma conversion portion, and the portion has a third cross- section the plane.
  • the first cross-section is smaller than the third cross-section.
  • a liquid core of the fuel target has a fourth cross-section in the plane; and the first cross-section is smaller than the fourth cross-section.
  • the first cross-section is smaller than the fourth cross-section by a factor of 90% or less.
  • the first cross-section is smaller than the fourth cross-section by a factor between 65% and 85%.
  • the amount of radiation that is scattered away from the plasma conversion portion will be dependent on a number of factors.
  • the amount of radiation that is scattered away from the plasma conversion portion may be dependent on properties of the conditioned fuel target such as the optical properties of the fuel (for example the (spatially varying refractive index of the fuel target for the particular wavelength of the radiation), the geometry of the fuel target (i.e. the shape of the fuel target), and the density distribution of the fuel target.
  • the amount of radiation that is scattered away from the plasma conversion portion may be dependent on properties of the laser radiation such as the wavelength of the laser radiation, the relative size of the laser radiation and the fuel target, the direction and divergence of the laser beam (for example whether the fuel target is at, in front of, or behind a focus position of the laser beam).
  • the wavelength of the laser beam may be approximately 10 ⁇ .
  • a method of operating a radiation source configured to provide EUV radiation.
  • the radiation source comprises a fuel emitter configured to provide a fuel target to a plasma formation region; and a laser system configured to illuminate an exposed surface of the fuel target with laser radiation when the fuel target is at the plasma formation region in order to convert the fuel target into plasma.
  • the laser radiation incident on the exposed surface has a first cross-section in a plane perpendicular to a propagation direction of the laser radiation.
  • the method comprises controlling the first cross-section to be smaller than a second cross- section of the exposed surface in the plane.
  • the fuel target has a portion with a mass density configured to absorb the laser radiation. This portion has a third cross-section the plane.
  • the method comprises controlling the first cross-section to be smaller than the third cross- section.
  • a lithographic system comprising a radiation source according to the invention as addressed supra, and further comprising a lithographic apparatus configured to image a pattern onto a substrate using the EUV radiation.
  • Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source according to an embodiment of the invention
  • Figure 2A is a schematic representation of a cross-section of a fuel target at the plasma formation region of the lithographic system shown in Figure 1 in a plane containing the propagation direction of the laser beam;
  • Figure 2B is a schematic representation of a cross-section of the fuel target shown in Figure 2A in a plane perpendicular to the propagation direction of the laser beam 2;
  • Figure 3A is a schematic representation of a cross-section of part of a fuel target and a laser beam at the plasma formation region of the lithographic system shown in Figure 1 in a plane perpendicular to the propagation direction of the laser beam wherein the laser beam has a first cross- section
  • Figure 3B is a schematic representation of a cross-section of part of a fuel target and a laser beam at the plasma formation region of the lithographic system shown in Figure 1 in a plane perpendicular to the propagation direction of the laser beam wherein the laser beam has a second cross- section;
  • FIG. 4A is a schematic representation of a cross-section of part of a fuel target and a laser beam at the plasma formation region of the lithographic system shown in Figure 1 in a plane perpendicular to the propagation direction of the laser beam wherein the fuel target has a first shape;
  • Figure 4B is a schematic representation of a cross-section of part of a fuel target and a laser beam at the plasma formation region of the lithographic system shown in Figure 1 in a plane perpendicular to the propagation direction of the laser beam wherein the fuel target has a second shape;
  • Figure 4C is a schematic representation of a cross-section of part of a fuel target and a laser beam at the plasma formation region of the lithographic system shown in Figure 1 in a plane perpendicular to the propagation direction of the laser beam wherein the fuel target has a third shape;
  • FIG. 5 is a contour plot of the EUV conversion efficiency of the radiation source of Figure las a function of: the diameter of the fuel target in microns (on the vertical axis) and the diameter of the laser beam at the location of the fuel target in microns (lower scale on the horizontal axis).
  • Figure 1 shows a lithographic system which comprises a radiation source SO according to an embodiment of the invention and a lithographic apparatus LA.
  • the radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B.
  • the lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS and a substrate table WT configured to support a substrate W.
  • the illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA.
  • the projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W.
  • the substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.
  • the radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment.
  • a gas at a pressure below atmospheric pressure e.g. hydrogen
  • a vacuum may be provided in illumination system IL and/or the projection system PS.
  • a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.
  • the radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source.
  • LPP laser produced plasma
  • the radiation source SO includes a laser 1, which may for example include a CO 2 laser, that is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 3.
  • the laser beam 2 may be referred to as laser radiation.
  • the wavelength of the laser radiation may be approximately 10 ⁇ .
  • tin is referred to in the following description, any suitable fuel may be used. At least a part of the fuel may for example be in liquid form, and may for example be a metal or alloy.
  • the fuel emitter 3 may comprise a nozzle configured to direct fuel, e.g. in the form of discrete fuel targets (also referred to as "droplets”), along a trajectory 16 towards a plasma formation region 4.
  • the fuel emitter 3 generates then a stream of discrete liquid targets (droplets).
  • a target will generally be hit first by a pre -pulse from a pre-pulse laser in order to condition the target for receipt of the main pulse supplied by a main-pulse laser.
  • the main pulse converts the conditioned target into plasma.
  • the effect of conditioning may be a variation in density across the thus conditioned target.
  • Laser 1 may be the main-pulse laser.
  • Each conditioned fuel target may have a core region of liquid fuel surrounded by a cloud of fuel vapor. It will be appreciated that a fuel target is intended to mean a discrete quantity of fuel.
  • the laser beam 2 is incident upon the fuel at the plasma formation region 4.
  • the deposition of laser energy into the fuel creates 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 the ions of the plasma.
  • the EUV radiation emitted by the plasma is collected and focused by 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 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 focal point may be at, or close to, the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.
  • the laser beam 2 propagates through a central opening in the collector 5 towards the plasma formation region 4.
  • the laser 1 may be located at some distance from the plasma formation region 4. Where this is the case, the laser beam 2 may be passed from the laser 1 to the plasma formation region 4 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 1 and the beam delivery system may be considered to be a laser system configured to illuminate a fuel target with laser radiation when the fuel target is at the plasma formation region 4 in order to convert a portion of the fuel target into plasma.
  • the radiation beam B is focused at a location 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL.
  • the location 6 at which the radiation beam B is focused may be referred to as the intermediate focus.
  • 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 that helps to preserve a vacuum.
  • the radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam.
  • the illumination system IL may include a 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 radiation beam B with a desired cross-sectional shape and a desired angular distribution of intensity in the beam's cross-section.
  • the radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT.
  • the patterning device MA reflects and patterns the radiation beam B.
  • 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.
  • the patterned radiation beam B enters the projection system PS.
  • the projection system comprises a plurality of mirrors which are configured to project the radiation beam B onto a substrate W held by the substrate table WT.
  • the projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA.
  • a reduction factor of 4 may, for example, be applied.
  • the projection system PS is shown as to have two mirrors in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).
  • the radiation sources SO shown in Figure 1 may include components which are not illustrated.
  • a spectral filter may be provided in the radiation source.
  • the spectral filter may serve to provide substantially EUV radiation to the intermediate focus 6 and substantially prevent radiation of other wavelengths, such as infrared radiation, from reaching the intermediate focus 6.
  • the laser system i.e. the laser 1 and any beam delivery system configured to deliver laser beam 2 to the plasma formation region 4 of radiation source SO is configured so as to provide an increased conversion efficiency relative to known LPP radiation sources.
  • the laser system is configured such that, in a plane perpendicular to a propagation direction of the laser beam 2, a cross-section of the laser beam 2 at each of the fuel targets produced by the fuel emitter 3 is smaller than a cross-section of the fuel target or even smaller than a portion of the fuel target that is converted into plasma.
  • the laser beam 2 may have a top-hat intensity distribution in its cross-section and therefore may have a relatively well-defined boundary.
  • the laser beam 2 may have a cross-sectional intensity distribution with a less well-defined boundary.
  • the laser beam 2 may have a Gaussian-like intensity distribution, wherein the intensity of the laser beam 2 decays exponentially beyond a certain distance from the axis of the laser beam 2.
  • a boundary or edge of the laser beam 2 may be defined as a central region of the laser beam that contains a certain percentage of the intensity of the laser beam 2, for example 90%.
  • the boundary or edge of the laser beam 2 may be defined by a pre-determined intensity threshold (with the region wherein the intensity is above the intensity threshold being within the boundary, and the region wherein the intensity is below the intensity threshold being outside the boundary).
  • the cross-sectional area of the laser beam 2 may be understood to mean the area defined by the boundary and the diameter of the laser beam 2 may be understood to mean the diameter of the boundary as defined by a certain percentage of the laser intensity or by a pre-determined threshold intensity.
  • the conditioned fuel targets may not have a well-defined boundary, e.g., as a result of a radially varying mass density. It will be appreciated that a conditioned fuel target is intended to mean a localised quantity of fluid fuel. Typically, such a conditioned fuel target has some internal structure, as now discussed with reference to Figures 2 A and 2B.
  • Figures 2A and 2B are schematic representations of cross-sections of a typical conditioned fuel target 20 at the plasma formation region 4, the cross-sections being given in two different planes.
  • the propagation direction of the laser beam 2 (for example as defined by a chief ray or axis of the laser beam 2) is labelled as the z direction.
  • Figure 2A is a schematic representation of a cross-section of the fuel target 20 in a plane containing the propagation direction of the laser beam 2 (i.e., in the x-z plane).
  • Figure 2B is a schematic representation of a cross-section of the fuel target 20 in a plane perpendicular to the propagation direction of the laser beam 2 (i.e., in the x-y plane).
  • a fuel target is typically irradiated with a laser pre-pulse before the laser radiation (i.e., the main-pulse) is incident on the fuel target.
  • the pre-pulse serves to shape or otherwise condition the fuel target before the (plasma producing) laser beam 2 (the main pulse) is incident on the fuel target.
  • the pre-pulse is configured to illuminate a fuel target with pre-pulse laser radiation before the fuel target is at the plasma formation region 4 proper.
  • the pre-pulse may spread the fuel target out such that the fuel target is relatively larger, relative to the unconditioned target as emitted by the fuel emitter 3, in a first plane (e.g., the x-y plane in Figures 2A and 2B) and relatively smaller in a second plane which is perpendicular to the first plane (e.g. in the x-z plane).
  • the conditioned fuel target may have the shape of a disk (a three-dimensional rounded volume substantially thinner in one dimension than in the other two dimensions), with or without a well-defined boundary as mentioned above.
  • the first plane (the x-y plane) is perpendicular to the propagation direction of the laser radiation (the main pulse).
  • the disk may be tilted with respect to the propagation direction of the laser beam 2.
  • the first plane may then be inclined relative to the plane perpendicular to the propagation direction of the laser radiation by up to, for example, 30° or 45°.
  • the fuel target 20 has a core region 21 of liquid fuel surrounded by a region of lower mass density, e.g., a cloud of fuel vapor.
  • a region of lower mass density e.g., a cloud of fuel vapor.
  • vapor includes atomic, and/or ionized, vapor of the fuel and/or a cloud of small fuel droplets with the size of the order of 1-5000 nm.
  • the ionized vapor may have resulted from interaction of the pre-pulse with the fuel target.
  • the density of the liquid core region 21 may be of the order of 10 21 atoms/cm 3 .
  • the fuel target 20 is depicted as being generally ellipsoidal. It will be appreciated that Figures 2A and 2B are rather schematic for illustrative purposes and that a fuel target may have a different shape.
  • the mass density of fuel within the fuel target 20 typically decreases as the distance away from the core region 21 increases.
  • An edge 22 of the fuel target 20 may be defined as a region within which a magnitude of the density of fuel is above a certain threshold.
  • Reference numeral 23 indicates a two-dimensional surface between a three-dimensional region with a density whose magnitude is below the threshold, and another three-dimensional region with a density whose magnitude is above the threshold. The region with a density, whose magnitude is below the threshold, is outside the surface 23.
  • the laser beam 2 may only penetrate to a depth wherein the density of the fuel target is below a threshold density of the order of 10 18 atoms/cm 3 . With an ionization degree of 10, this corresponds to an electron density of the order of 10 19 electrons/cm 3 .
  • the surface 23 then represents a boundary of the region wherein the density of the conditioned fuel target has reached a magnitude of 10 18 atoms/cm 3
  • the laser beam 2 propagates through a region 24 of the fuel target 20 wherein the density has a magnitude below the threshold (i.e. the region 24 between the edge 22 of the fuel target 20 and the boundary 23), which is effectively transparent to the laser beam 2.
  • a first portion of the laser beam 2 is absorbed. This first portion of the laser beam 2 heats this portion of the fuel target 20, converting it into plasma which, in turn, emits EUV radiation. A second portion of the laser beam 2 is reflected from the portion of the fuel target 20 proximate to the density threshold region 23 (due to the ionization of tin within the fuel target).
  • the portion 25 of the fuel target 20 proximate to the density threshold 23 which absorbs the laser radiation may be referred to as a plasma conversion portion.
  • This plasma conversion portion is the portion of the fuel target 20 that will be converted into plasma and may, for example, comprise a portion of the fuel target 20 with a density in the region of 10 17 -10 18 atoms/cm 3 .
  • the radiation source SO comprises a laser system (i.e. the laser 1 and any beam delivery system) that is configured to illuminate a fuel target 20 generated by the fuel emitter 3 with laser radiation when the fuel target 20 is at the plasma formation region 4 in order to convert a portion of the fuel target 20 into plasma.
  • the radiation source SO according to an embodiment of the present invention comprises a laser system which is configured such that a cross- section of the laser beam 2 at the fuel target 20 is smaller than a cross-section of the portion of the fuel target 20 that is going to be converted into plasma (i.e. the portion 25 of the fuel target 20 proximate to the surface of the density threshold 23) in a plane perpendicular to a propagation direction of the laser beam 2.
  • a beam of laser radiation irradiates fuel targets at a plasma formation region.
  • the laser radiation is focused to a focal spot (in a plane perpendicular to a propagation direction of the laser radiation).
  • the focal spot may have a diameter of the order of, say, 80 ⁇ .
  • the cross-section of the conditioned fuel target that the laser radiation illuminates has a larger diameter (also in a plane perpendicular to a propagation direction of the laser radiation).
  • the liquid core of the fuel target may have a diameter in a plane perpendicular to a propagation direction of the laser radiation of the order of 300-500 nm. It will be appreciated that, as used herein, the diameter of the fuel target 20 in a particular plane is intended to mean the diameter of the projection of the fuel target 20 onto that particular plane.
  • a beam of laser radiation is provided such that at the fuel target the diameter of the laser beam is the same or larger than the diameter of the fuel target.
  • Conventional wisdom is it that maximum conversion efficiency is achieved when, at the fuel target, the diameter of the laser beam is equal to the diameter of the fuel target.
  • This matching of the diameter of the laser beam to the diameter of the fuel target may be achieved by controlling the distance between the fuel target and the focal plane of the laser radiation (i.e. the position of the beam waist).
  • the plasma formation region may be of the order of a few mm in front of the focal plane of the laser radiation (i.e. the laser radiation is incident on the fuel target before it has converged to the focal spot where the laser beam has minimum diameter).
  • the skilled person would expect that any reduction in the diameter of the laser beam 2, relative to the diameter of the fuel target, would result in a reduction in conversion efficiency since not the entire fuel target receives laser radiation. That is, the skilled person would expect at least some of the fuel target to not be heated and therefore not to produce plasma which emits EUV radiation. For this reason, the skilled person would not have contemplated an arrangement wherein at the fuel target the diameter of the laser radiation is smaller than the diameter of the fuel target or even smaller than the diameter of the plasma conversion portion in a plane perpendicular to a propagation direction of the laser radiation. Rather, if seeking to increase the conversion efficiency of an LPP source, the teachings available to the skilled person would lead him/her to consider an arrangement wherein at the fuel target the diameter of the laser radiation is the same or even slightly larger than the diameter of the fuel target.
  • Figures 3A and 3B are schematic representations of a cross-section of part of a fuel target 20 and a laser beam 2 at the plasma formation region 4 in a plane perpendicular to the propagation direction of the laser beam 2 (i.e. the x-y plane). For simplicity and ease of understanding, only the liquid core region 21 and the plasma conversion portion 25 are shown, rather schematically, in Figures 3 A and 3B.
  • two edge rays 30 which represent the boundary or edge of the laser beam 2 are shown in Figures 3 A and 3B.
  • the edge rays 30 mark the boundary of a region in the laser beam 2 that contains a certain percentage of the intensity of the laser beam 2, for example 90%.
  • FIGS 3 A and 3B Also shown (as dashed lines) in Figures 3 A and 3B are two lines 31 which represent a path that the edge rays 30 would have followed in the absence of any scattering effects caused by the surrounding fuel vapor cloud (not shown in Figures 3 A and 3B for simplicity and ease of understanding).
  • the plasma formation region 4 is in front of the focal plane of the laser beam 2 (i.e. such that the laser beam 2 is incident on the fuel target 20 before it has converged to the focal spot). Accordingly, the lines 31 are convergent.
  • the inventors of the present invention now believe that when a laser beam 2 is incident on a conditioned fuel target 20, the lower density peripheral portion 24 (see Figs. 2A and 2B) of the fuel target 20 acts as an optical element that alters shape, and in particular the cross-sectional size, of the laser beam 2 before the laser beam 2 is incident on the plasma conversion portion 25.
  • the fuel target 20 if the fuel target 20 is hit by a laser beam 2 with a cross-section that matches the size of the plasma conversion portion 25, the fuel target 20 will widen the laser beam 2 and will thus direct energy away from the volume occupied by the plasma conversion portion 25. This leads to a loss of energy and therefore to a lowering of the EUV conversion efficiency.
  • the lower density peripheral portion 24 of the fuel target 20 may tend to decrease the cross-sectional size of the laser beam 2 before the laser radiation is incident on the plasma conversion portion 25. This can ensure that substantially all of the energy is directed towards the plasma conversion portion 25, therefore increasing the EUV conversion efficiency.
  • the high pressure of the plasma tends to distort the shape of the plasma conversion portion 25, making it concave in a central portion.
  • cross-section of the laser beam 2 at the fuel target 20 is intended to mean the cross-section of the laser beam 2 in the absence of any scattering from the fuel target 20.
  • this may mean the cross-section of the radiation beam 2 immediately before the laser beam 2 enters the low density vapor cloud 24 of the fuel target 20, i.e. before the laser beam 2 crosses the edge 22 of the fuel target 20 as defined, for example, by a density threshold as mentioned above.
  • a density threshold as mentioned above.
  • the surface of the entire plasma conversion portion 25, the surface facing the laser beam 2 is irradiated by the laser beam 2 in order to optimize conversion efficiency.
  • a better definition of the cross-section of the laser beam 2 at the fuel target 20 may be the cross-section that the laser beam 2 would have at the plasma conversion portion 25 in the absence of any scattering effects caused by the surrounding fuel vapor cloud (i.e. the region 24 of the fuel target 20 that is below the threshold density 23).
  • the laser beam 2 is focused to a focal spot and that the plasma formation region 4 may be in front of the focal plane of the laser beam 2 (i.e. such that the laser beam 2 is incident on the fuel target 20 before it has converged to the focal spot).
  • the laser beam 2 may have an angle of convergence (i.e. an angle between the cone of laser beam 2 and a chief ray of the laser beam 2). Therefore, it will be appreciated that, even in the absence of a fuel target 20, the cross-section of the laser beam 2 varies with position along the propagation direction of the radiation beam 2 (i.e. in the z direction in Figure 2A).
  • the typical dimensions of a fuel target 20 and the typical angles of convergence of the laser beam 2 are such that, in practice, the change in the cross-section of the laser beam 2 (in the absence of the fuel target 20) between a position that corresponds to the start of the fuel target 20 (i.e. the edge 22) if the fuel target had been present, and a position that corresponds to the plasma conversion portion 25, if the fuel target had been present, may be a relatively small percentage change. Therefore, the two above-mentioned definitions of what is meant by the cross-section of the laser beam 2 at the fuel target 20 may be generally equivalent.
  • the cross-section of the laser beam 2 may be smaller than the cross-section of the fuel target 20 in a plane perpendicular to a propagation direction of the laser beam 2 by an amount so as to at least partially reduce the amount of laser radiation that is scattered so that it is not incident on the plasma conversion portion 25.
  • the laser system of the radiation source SO may be configured such that the cross- section of the laser beam 2 at the fuel target 20 is smaller than the cross-section of the plasma conversion portion 25 (i.e. the portion of the fuel target 20 that is going to be converted into plasma) in the plane perpendicular to the propagation direction of the laser beam 2 (i.e. the x-y plane) by an amount such that substantially all of the laser beam 2 is incident on the plasma conversion portion 25.
  • the lower density peripheral portion (i.e. region 24) of the fuel target 20 tends to increase the cross-sectional size of the laser beam 2 before the laser beam 2 is incident on the plasma conversion portion 25.
  • the fuel target 20 if the fuel target 20 is hit by a laser beam 2 with a cross-section that matches the cross-section of the plasma conversion portion 25, the fuel target 20 will widen the laser beam 2 and will thus direct energy away from the volume occupied by the plasma conversion portion 25 such that at least some of the laser radiation is not incident on the plasma conversion portion 25.
  • substantially all of the laser radiation being incident on the plasma conversion portion 25 may mean that at least a certain percentage of the energy of the laser radiation is incident on the plasma conversion portion 25.
  • substantially all of the laser radiation being incident on the plasma conversion portion 25 may mean that more than 80%, preferably more than 85%, or more preferably more than 90% of the energy of the laser radiation is incident on the plasma conversion portion 25.
  • the amount of radiation that is scattered away from the plasma conversion portion 25 will be dependent on a number of factors.
  • the amount of radiation that is scattered away from the plasma conversion portion 25 may be dependent on properties of the fuel target such as the optical properties of the fuel (for example the (varying) refractive index) of the fuel target 20, the geometry of the fuel target 20 (i.e. the shape of the fuel target), and the density distribution of the fuel target 20.
  • the amount of radiation that is scattered away from the plasma conversion portion may be dependent on properties of the laser radiation such as the wavelength of the laser beam 2, the relative size of the laser beam 2 with respect to the fuel target 20, the direction and angle of convergence or divergence of the laser beam 2 (for example whether the fuel target is at, in front of, or behind a focus position of the laser beam 20).
  • Figures 4A, 4B and 4C are schematic representations of the cross sections of part of a conditioned fuel target 20 and of a laser beam 2 at the plasma formation region 4, both in a plane perpendicular to the propagation direction of the laser beam 2 (i.e. the x-y plane).
  • Figures 4 A, 4B and 4C are similar to Figures 3 A and 3B in that they schematically depict the liquid core region 21, the plasma conversion portion 25, the two edge rays 30 and the two lines 31.
  • Figures 4A, 4B and 4C schematically show three different topologies for the central liquid core region 21: in Figure 4 A the side of the core region 21, towards which the laser beam 2 is directed, is convex; in Figure 4B the side of the core region 21, towards which the laser beam 2 is directed, is flat; and in Figure 4C the side of the core region 21, towards which the laser beam 2 is directed, is concave.
  • the side of the core region 21 towards which the laser beam 2 is directed is flat.
  • the core region 21 may be generally of the form of a flat disc.
  • the laser system is configured such that a cross-section of the laser beam 2 at the fuel target 20 is smaller than a cross-section of the plasma conversion portion 25, such that a relatively high EUV conversion efficiency is achieved. With this arrangement, substantially all of the laser radiation is incident on the plasma conversion portion. Some fraction of this laser radiation will be reflected. However, the remainder will be absorbed and will contribute to the generation of an EUV radiation emitting plasma.
  • the laser system of the radiation source SO is configured such that the cross-section of the laser beam 2 at the fuel target 20 is smaller than the cross-section of the fuel target 20, or even smaller than the plasma conversion portion 25 (i.e. the portion of the fuel target 20 that is going to be converted into plasma) in the plane perpendicular to the propagation direction of the laser beam 2 by an amount such that an EUV conversion efficiency of the radiation source SO is maximised.
  • the radiation source SO may be operated at a range of different values of the cross-section of the laser beam 2 at the fuel target 20 while the EUV conversion efficiency of the radiation source SO is being monitored.
  • the variation in the cross-section of the laser beam 2 at the fuel target 20 may be achieved, for example, by varying the distance between the plasma formation region 4 and a focal plane of the laser beam 2.
  • a cross-section of the laser beam 2 at the fuel target 20 that maximizes the EUV conversion efficiency of the radiation source SO may be defined as a cross-section of the laser beam 2 at the fuel target 20 that corresponds to (i.e.
  • Figure 5 is a diagram with contour plots of the EUV conversion efficiency of the radiation source SO as a function of: the diameter of the fuel target in microns (on the vertical axis) and the diameter of the laser beam 2 at the location of the fuel target in microns (lower scale on the horizontal axis).
  • the contours are labelled with numerical values of the EUV conversion efficiency. For example, the contour labelled "5" in the upper left-hand corner is associated with an EUV conversion efficiency of 5%, and the contour labelled "4.5" is associated with an EUV conversion efficiency of 4.5%.
  • the diameter of the fuel target on the vertical axis is actually the diameter of the liquid core region 21.
  • the diameter of the plasma conversion portion 25 is typically greater than the diameter of the liquid core region 21, e.g., by 100 ⁇ .
  • FIG. 5 Also shown in Figure 5 is a curve (as a dotted line) illustrating an approximate optimum value of the diameter of the laser beam 2 at the fuel target for a given the diameter of the fuel target core.
  • the optimum value of the diameter of the laser beam 2 at the fuel target is smaller than the liquid fuel target core.
  • the diameter of the laser beam 2 at the fuel target has a magnitude of 90% or less of the magnitude of the diameter of a liquid core 21 of the fuel target 20 in a plane perpendicular to a propagation direction of the laser beam 2.
  • the diameter of the laser beam 2 at the fuel target 20 is between 65% and 85% of the diameter of the liquid core 21 of the fuel target 20 in the plane perpendicular to a propagation direction of the laser beam 2.
  • Some embodiments of the present invention relate to methods of generating EUV radiation using the radiation source SO as described above.
  • the above description refers to targets of tin, fuels other than tin may be used.
  • the radiation source may form part of a mask inspection apparatus.
  • the mask inspection apparatus may use EUV radiation to illuminate a mask and use an imaging sensor to monitor radiation reflected from the mask. Images received by the imaging sensor are used to determine whether or not defects are present in the mask.
  • the mask inspection apparatus may include optics (e.g. mirrors) configured to receive EUV radiation from an EUV radiation source and form it into a radiation beam to be directed at a mask.
  • the mask inspection apparatus may further include optics (e.g. mirrors) configured to collect EUV radiation reflected from the mask and form an image of the mask at the imaging sensor.
  • the mask inspection apparatus may include a processor configured to analyse the image of the mask at the imaging sensor, and to determine from that analysis whether any defects are present on the mask.
  • the processor may further be configured to determine whether a detected mask defect will cause an unacceptable defect in images projected onto a substrate when the mask is used by a lithographic apparatus.
  • the radiation source may form part of a metrology apparatus.
  • the metrology apparatus may be used to measure alignment of a projected pattern formed in resist on a substrate relative to a pattern already present on the substrate. This measurement of relative alignment may be referred to as overlay.
  • the metrology apparatus may for example be located immediately adjacent to a lithographic apparatus and may be used to measure the overlay before the substrate (and the resist) has been processed.
  • 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 substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.
  • EUV radiation may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.
  • 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).
  • a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others.
  • 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.

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  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • X-Ray Techniques (AREA)
  • Exposure And Positioning Against Photoresist Photosensitive Materials (AREA)

Abstract

La présente invention concerne une source de rayonnement configurée pour fournir un rayonnement EUV, la source de rayonnement comprenant un émetteur de combustible et un système laser. L'émetteur de combustible est configuré pour fournir une cible de combustible à une région de formation de plasma. Le système laser est configuré pour éclairer la cible de combustible à l'aide d'un rayonnement laser lorsque la cible de combustible se trouve au niveau de la région de formation de plasma afin de convertir une partie de la cible de combustible en plasma. En particulier, le système laser est configuré de telle sorte qu'une section transversale du rayonnement laser au niveau de la cible de combustible est plus petite qu'une section transversale de la partie de la cible de combustible qui est convertie en plasma dans un plan perpendiculaire à une direction de propagation du rayonnement laser. Une telle source de rayonnement fournit une source de plasma produit par laser (LPP) présentant une meilleure efficacité de conversion par rapport aux sources LPP connues. FIG. 5 : AA%%%Diamètre de noyau de gouttelette de combustible (μm) BB%%% Diamètre de faisceau laser au niveau d'une gouttelette de combustible (μm)
PCT/EP2018/074606 2017-09-20 2018-09-12 Source de rayonnement WO2019057584A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2013029906A1 (fr) * 2011-09-02 2013-03-07 Asml Netherlands B.V. Source de rayonnement
WO2016012192A1 (fr) * 2014-07-21 2016-01-28 Asml Netherlands B.V. Source de rayonnement
WO2017103991A1 (fr) * 2015-12-15 2017-06-22 ギガフォトン株式会社 Dispositif de génération de lumière dans l'ultraviolet extrême (uve)

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Publication number Priority date Publication date Assignee Title
US20130015373A1 (en) * 2010-04-08 2013-01-17 Asml Netherlands B.V. EUV Radiation Source and EUV Radiation Generation Method
NL2009372A (en) * 2011-09-28 2013-04-02 Asml Netherlands Bv Methods to control euv exposure dose and euv lithographic methods and apparatus using such methods.
WO2013131693A1 (fr) * 2012-03-05 2013-09-12 Asml Netherlands B.V. Appareil de capture d'ions, source de rayonnement plasma produit par laser, appareil lithographique
WO2015110238A1 (fr) * 2014-01-27 2015-07-30 Asml Netherlands B.V. Source de rayonnement
US9826615B2 (en) * 2015-09-22 2017-11-21 Taiwan Semiconductor Manufacturing Co., Ltd. EUV collector with orientation to avoid contamination

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Publication number Priority date Publication date Assignee Title
WO2013029906A1 (fr) * 2011-09-02 2013-03-07 Asml Netherlands B.V. Source de rayonnement
WO2016012192A1 (fr) * 2014-07-21 2016-01-28 Asml Netherlands B.V. Source de rayonnement
WO2017103991A1 (fr) * 2015-12-15 2017-06-22 ギガフォトン株式会社 Dispositif de génération de lumière dans l'ultraviolet extrême (uve)
US20180255631A1 (en) * 2015-12-15 2018-09-06 Gigaphoton Inc. Extreme ultraviolet light generating apparatus

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