CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 17192135.6 which was filed on 20 September 2017 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a radiation source configured to provide EUV radiation. Specifically 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.

BACKGROUND

[0003] 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.

[0004] 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).

[0005] One known type of EUV radiation source directs laser radiation onto fuel targets. This converts the fuel targets into EUV radiation emitting plasma. A radiation source of this type may be referred to as a laser-produced plasma (LPP) source. Known LPP sources have relatively low conversion efficiencies. That is, the power of EUV radiation which they output is small fraction of the power of laser radiation which is incident upon fuel targets.

SUMMARY

[0006] It may be desirable to provide a radiation source which has a better efficiency than a conventional LPP radiation source, or which overcomes some other disadvantage associated with conventional LPP radiation sources.

[0007] According to a first aspect of the invention there is provided 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.

[0008] 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.

[0009] It will be appreciated that a fuel target is intended to mean a discrete (also referred to as: mass limited) quantity of fuel, such as a fuel droplet.

[00010] 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). For example, the laser beam may have a top-hat intensity distribution and therefore may have a relatively well-defined boundary. Alternatively, the laser beam may have a Gaussian-like intensity distribution. For such embodiments, 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%. Alternatively, 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). For such embodiments, 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.

[00011] Similarly, fuel targets typically may not have well-defined sharp edges. It will be appreciated that a fuel target is intended to mean a localised quantity of fluid fuel. For example, in a typical LLP EUV source, 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. For example, upon being hit by the pre-pulse, the fuel target 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. It will be appreciated that in this context 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.

[00012] Within an LPP source, as the radiation of the main pulse is incident on the fuel target, a portion of the fuel target is being ionized and converted into plasma. For example, in case the fuel target is a tin droplet, 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). Typically 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.

[00013] Due to this ionization of fuel within 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). In particular, the laser radiation typically will only penetrate regions with a density below a threshold density. For example, 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 . With an ionization degree of 10, 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. 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).

[00014] 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 .

[00015] In known LPP sources, a beam of laser radiation irradiates fuel targets present at a plasma formation region. Typically, the laser radiation is focused to a focal spot. Typically, 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. It will be appreciated that, as used herein, 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. This shaping of the fuel target using a pre-pulse is discussed below in further detail. In some embodiments, 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. However, note that in some other embodiments 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°.

[00016] In the field of LPP sources, it is generally accepted that in order to achieve relatively high conversion efficiency, the entire fuel target should be illuminated by radiation. Therefore, in known LPP sources, 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). For example, 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).

[00017] 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. In particular, 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. Conventionally, as laser radiation is incident on a fuel target, at least a portion of the laser radiation is scattered outwards. This may be due to grazing incidence reflections from the fuel target and/or due to gradients of refractive index within the fuel target. In particular, when laser radiation is incident on a fuel target, 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. As a result, 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.

[00018] By ensuring that at the fuel target the cross-section of the laser radiation is smaller than the cross-section of the fuel target in a plane perpendicular to a propagation direction of the laser radiation, even though the laser radiation is scattered by the fuel target, the amount of laser radiation which still reaches the plasma conversion portion, and therefore still contributes to the generation of EUV radiation, is increased.

[00019] It will be appreciated that 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. For example, 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). However, it will be appreciated that what is important is that the entire plasma conversion portion receives the laser radiation (in order to optimize conversion efficiency). Therefore, 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. [00020] It will be appreciated that 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. For example, in this region (in front of the beam waist) 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) .Therefore, it will be appreciated that, even in the absence of a fuel target, the cross-section of the laser radiation varies with position along the propagation direction of the laser radiation. However, 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.

[00021] It is known to irradiate a fuel target with a laser pre-pulse before the laser radiation (i.e., the main pulse) is incident on the fuel target so as to shape, or otherwise condition, the fuel target before the (plasma producing) laser radiation of the main pulse is incident on it. For example, 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.

[00022] In an embodiment, 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.

[00023] In a further embodiment, 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.

[00024] For example, the first cross-section is smaller than the fourth cross-section by a factor of 90% or less. Preferably, the first cross-section is smaller than the fourth cross-section by a factor between 65% and 85%.

[00025] In general, the amount of radiation that is scattered away from the plasma conversion portion will be dependent on a number of factors. For example, 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. In addition, 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 μηι.

[00026] According to a second aspect of the present invention there is provided 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. In an embodiment of the method, 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.

[00027] According to a third aspect of the invention, there is provided 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.

[00028] Various aspects and features of the invention set out above or below may be combined with various other aspects and features of the invention as will be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

[00029] 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 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;

- Figure 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; and

- Figure 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).

Throughout the drawings, same or corresponding features are indicated with same reference numerals.

DETAILED DESCRIPTION

[00030] 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.

[00031] 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) may be provided in the radiation source SO. 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. [00032] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source. 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 μηι. Although 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.

[00033] 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.

[00034] 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.

[00035] 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.

[00036] The EUV radiation that is reflected by the collector 5 forms a radiation

beam B. 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.

[00037] 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.

[00038] Following reflection from the patterning device MA 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. Although 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).

[00039] The radiation sources SO shown in Figure 1 may include components which are not illustrated. For example, 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.

[00040] According to embodiments of the present invention 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. In particular, 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.

[00041] The laser beam 2 may have a top-hat intensity distribution in its cross-section and therefore may have a relatively well-defined boundary. Alternatively, the laser beam 2 may have a cross-sectional intensity distribution with a less well-defined boundary. For example, 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. For such embodiments, 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%. Alternatively, 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). For such embodiments, 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.

[00042] Similarly, 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.

[00043] 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. In Figures 2 A and 2B, 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).

[00044] As mentioned above, 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. For example, 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). In other words, 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. In the shown embodiment, the first plane (the x-y plane) is perpendicular to the propagation direction of the laser radiation (the main pulse). However, it will be appreciated that in some embodiments 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°.

[00045] 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. In this context, the term 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.

[00046] The density of the liquid core region 21 may be of the order of 10 ²¹ atoms/cm ³ . In Figures 2A and 2B 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.

[00047] 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.

[00048] Within an LPP source, as the laser beam 2 of the main pulse is incident on the fuel target 20, a portion of the fuel target 20 is being ionized and converted into plasma.

[00049] Due to this ionization of fuel within the fuel target 20, as the laser beam 2 is incident upon a fuel target 20, the radiation of the laser beam 2 will only penetrate part of the fuel target 20 and will not penetrate into the dense central region. In particular, the laser beam 2 typically will only penetrate regions that have a density with a magnitude below a predetermined 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.

[00050] For example, 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. When the laser beam 2 reaches a portion of the fuel target proximate to the boundary 23 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).

[00051] 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 .

[00052] As explained above, 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. In particular, 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.

[00053] In known LPP sources, a beam of laser radiation irradiates fuel targets at a plasma formation region. Typically, 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 μηι. Typically, 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). For example, 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.

[00054] In the field of LPP sources, it is generally accepted that in order to achieve relatively high conversion efficiency the entire fuel target should be illuminated by laser radiation. Therefore, in prior art LPP sources, 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). For example, 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).

[00055] 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.

[00056] However, the inventors of the present invention have realised that, contrary to this conventional wisdom, an increase in the conversion efficiency can be achieved by reducing the cross-section (i.e. the diameter) of the laser beam 2 relative to the cross-section (i.e. the diameter) of the plasma conversion portion 25. This realisation has been prompted by a better understanding of the coupling of the laser beam 2 with a fuel target 20 within an LPP source SO, as now discussed with reference to Figures 3A and 3B.

[00057] 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. To represent the cross-section of the laser beam 2, 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%.

[00058] 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). It will be appreciated that 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.

[00059] 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.

[00060] Under certain conditions, as the laser beam 2 is incident on a fuel target 20, at least a portion of the laser beam 2 is scattered outwards. In particular, as shown in Figure 3A, when the cross-section of the laser beam 2 at the fuel target 20 is matched to the cross-section of the plasma conversion portion 25, such that the lines 31 are incident on the edge of the plasma conversion portion 25, the lower density peripheral portion 24 of the fuel target 20 tends to increase the cross-sectional size of the laser beam 2 before the laser radiation is incident on the plasma conversion portion 25. This scattering may be via refraction (due to gradients of refractive index within the fuel target 20) and/or grazing incidence reflections from the peripheral portion 24 of the fuel target 20. As a result, 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.

[00061] By ensuring that at the fuel target 20 the cross-section of the laser beam 2, in a plane perpendicular to the propagation direction of the laser beam 2, is smaller than the cross-section of the plasma conversion portion 25 in that plane (e.g., the x-y plane of Figures 3A and 3B), even though the laser beam 2 is scattered by the fuel target 20, the amount of laser radiation which still reaches plasma conversion portion 25, and therefore still contributes to the generation of EUV radiation, is increased. In fact, as shown in Figure 3B, when 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 (such that the lines 31 converge to a central portion of the plasma conversion portion 25), 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. Furthermore, as also shown schematically in Figure 3B, as the laser beam 2 is absorbed by the plasma conversion portion 25 and creates the plasma, the high pressure of the plasma tends to distort the shape of the plasma conversion portion 25, making it concave in a central portion. This effect may tend to better support the production of plasma and further increase the EUV conversion efficiency. However, it will be appreciated that as the cross-section of the laser beam 2 decreases such that it is significantly smaller that the cross-section of the plasma conversion portion 25, the portion of the plasma conversion portion 25 which receives EUV radiation will reduce which may reduce the EUV conversion efficiency. It will be appreciated that there is a balance to be struck and there will be an optimum cross-section of the laser beam 2 for maximizing EUV conversion efficiency for a given type of conditioned fuel target.

[00062] It will be appreciated that the expression "cross-section of the laser beam 2 at the fuel target 20", as used above, is intended to mean the cross-section of the laser beam 2 in the absence of any scattering from the fuel target 20. For example, 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. However, it will be appreciated that what is important is that 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. Therefore, 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).

[00063] It will be appreciated that 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). For example, in this region (in front of the beam waist) 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). However, 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.

[00064] 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.

[00065] 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.

[00066] As explained above, when a laser beam 2 is incident on a fuel target 20, 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. As a result, 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.

[00067] It will be appreciated that 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. For example, 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.

[00068] In general, the amount of radiation that is scattered away from the plasma conversion portion 25 will be dependent on a number of factors. For example, 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. In addition, 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).

[00069] How the geometry of the fuel target 20 (i.e. the shape of the fuel target) can affect the scattering of the laser beam 2 is now discussed with reference to Figures 4A, 4B and 4C. [00070] 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. However, 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.

[00071] In the configuration shown in Figure 4B, the side of the core region 21 towards which the laser beam 2 is directed is flat. For example, 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.

[00072] The configurations shown in Figures 4A and 4C show the laser beam 2 having substantially the same cross-section. However, the geometry of the fuel target is different.

[00073] In the configuration shown in Figure 4 A, the side of the core region 21, towards which the laser beam 2 is directed, is convex. With such an arrangement, there is more outward scattering of the laser radiation, as can be seen from the edge rays 30. With such an arrangement, a greater fraction of the laser radiation is directed away from the plasma conversion portion 25. This results in a reduction in the conversion efficiency (relative to the arrangement of Figure 4B). Therefore, for such a configuration wherein the side of the core region 21 towards which the laser beam 2 is directed is convex, an even smaller cross-section of the laser beam 2 should be selected.

[00074] In the configuration shown in Figure 4C, the side of the core region 21, towards which the laser beam 2 is directed, is concave. With such an arrangement, there is more inward scattering of the laser radiation, as can be seen from the edge rays 30. The plasma conversion portion 25 is already concave which, as discussed above (with reference to Figure 3B), may tend to better support the production of plasma and further increase the EUV conversion efficiency. This results in an increase in the conversion efficiency (relative to the arrangement of Figure 4B). Again, it will be appreciated that the optimum cross-section of the laser beam 2 for the concave arrangement shown in Figure 4C will in general differ from the optimum cross-section of the laser beam 2 for the flat arrangement shown in Figure 4B.

[00075] In some embodiments, 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.

[00076] It would be possible to check whether or not the cross-section of the laser beam 2 at the fuel target 20 has been optimized in this way so as to maximize the EUV conversion efficiency of the radiation source SO using the following procedure.

[00077] 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. is at or is proximate to) a local maximum of the EUV conversion efficiency of the radiation source SO with respect to the cross-section of the laser beam 2 at the fuel target 20 (or, equivalently, a distance between the plasma formation region 4 and a focal plane of the laser beam 2).

[00078] 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). In the diagram, 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%. In Figure 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 μηι.

[00079] 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. For fuel targets with a liquid core having a diameter of, say, 250 μιη or above, the optimum value of the diameter of the laser beam 2 at the fuel target is smaller than the liquid fuel target core.

[00080] In Figure 5, it is the region where the EUV conversion efficiency of the radiation source SO is above 5% that is of particular interest.

[00081] Preferably, 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. For example, 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.

[00082] Some embodiments of the present invention relate to methods of generating EUV radiation using the radiation source SO as described above. [00083] Although the above description refers to targets of tin, fuels other than tin may be used.

[00084] In an embodiment, 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.

[00085] In an embodiment, 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.

[00086] 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 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.

[00087] The term "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.

[00088] 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.

[00089] 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 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. 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.

[00090] 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.