Laser-pumped plasma source bulb

Abstract

A bulb for a laser-pumped plasma light source, the bulb having an inner surface defining a cavity; a plasma-sustaining location in the cavity for sustaining a plasma in use; and a shield interposed between the plasma-sustaining location and a portion of the inner surface, wherein the shield is configured to redirect gas moving from the plasma-sustaining location to the portion of the inner surface around the shield.

Description

LASER-PUMPED PLASMA SOURCE BULBCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24218630.2 which was filed on 10 December 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a bulb for a laser-pumped plasma light source.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 (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] A Laser-Pumped Plasma Source (LPPS) may be used to produce light (such as UV light) for e.g. a level sensor in a lithographic apparatus or metrology system. For example, the level sensor may be used for measuring a position of a surface of a substrate in the lithographic apparatus.

[0006] A LPPS may comprise a bulb configured to have a plasma sustained therein. The plasma may be triggered in the bulb by electrical discharge or laser excitation, and then sustained in the bulb by focusing an infrared laser onto the plasma inside the bulb. Sustaining plasma in the bulb by a laser may be referred to as laser pumping. The plasma may emit a broad wavelength spectrum (including UV light), and the emitted light may be used in metrology applications, e.g. for a level sensor. The emitted light may comprise a range of frequencies.

[0007] It is desirable that the light emitted by a LPPS has a high temporal stability, i.e. a high uniformity of intensity output over time. Light emitted by known LPPS may have relatively low temporal stability.

[0008] It is further desirable that the light emitted by a LPPS has a high spectral stability, i.e. a high uniformity of intensity over a broad wavelength range.

[0009] Temporal and spectral instabilities in a light source of a level sensor may limit the resolution of the level sensor and may lead to an inaccurate measurement of a position of a surface of the substrate. Similar problems may be encountered in a metrology system comprising a light source with low temporal stability and / or noise in its wavelength spectrum.

[0010] It may be desirable to reduce or alleviate the presence of temporal and / or spectral noise in the light emitted by an LPPS. It may be desirable to improve the temporal and / or spectral stability of the light output of LPPS.SUMMARY

[0011] According to a first aspect of the present invention, there is provided a bulb for a laser- pumped plasma light source, the bulb having an inner surface defining a cavity; a plasma-sustaining location in the cavity for sustaining a plasma in use; and a shield interposed between the plasmasustaining location and a portion of the inner surface, wherein the shield is configured to redirect gas moving from the plasma-sustaining location to the portion of the inner surface around the shield.

[0012] According to a second aspect of the present invention, there is provided a laser-pumped plasma source comprising a bulb according to the present invention. The laser-pumped plasma source may comprise a power source configured to power the electrode or the pair of electrodes, and a laser configured to sustain a plasma in the plasma-sustaining location. The laser-pumped plasma source may comprise a laser configured to trigger a plasma in the plasma-sustaining location and to sustain a plasma in the plasma-sustaining location.

[0013] According to a third aspect of the present invention, there is provided a sensor for measuring substrate surface topography comprising: a projection unit comprising a radiation source providing a radiation beam, wherein the projection unit is configured to project a radiation beam to a measurement location on the substrate, a detection unit configured to detect radiation reflected from the measurement location, wherein the radiation source comprises a laser-pumped plasma source according to the present invention.

[0014] According to a fourth aspect of the present invention, there is provided a lithographic apparatus configured to apply a pattern to a substrate, the lithographic apparatus comprising a sensor according to the present invention for measuring the substrate surface topography.

[0015] According to a fifth aspect of the present invention, there is provided a metrology system comprising a sensor according to the present invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0017] Figure 1 depicts a schematic overview of a lithographic apparatus;

[0018] Figure 2 depicts a schematic overview of a level or height sensor.

[0019] Figure 3 depicts an example of a known laser-pumped plasma source.

[0020] Figure 4 is a cross-section of a known bulb that may be used in a laser-pumped plasma source, such as the laser-pumped plasma source shown in Figure 3.

[0021] Figure 5 shows simulated density and gas velocity profiles of a bulb having a plasma sustained therein.

[0022] Figure 6 is a cross-section of a bulb according to the present invention comprising a shield.

[0023] Figure 7 is a cross-section of a bulb according to the present invention comprising a shield.

[0024] Figure 8 shows a portion of a cross-section of the bulb taken along line A-A shown inFigures 6 and 7.

[0025] Figures 9a, 9b, 9c and 9d show possible configurations of the shield.

[0026] Relative dimensions of components in drawings may be exaggerated for clarity. Within the following description of drawings the same or like reference numbers refer to the same or like components or entities, and only the differences with respect to the individual embodiments are described.DETAILED DESCRIPTION

[0027] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0028] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0029] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) Wand connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0030] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0031] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0032] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0033] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0034] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0035] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aidof the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0036] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axes. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry -rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0037] The lithographic apparatus may comprise a sensor LS used for e.g. measuring the topography of a top surface of the substrate. Sensor LS may be disposed in the vicinity of substrate support WT so that it may measure a property of the substrate. Sensor LS may use light emitted by the LPPS (e.g. UV light).

[0038] A topography measurement system, level sensor or height sensor (such as the sensor LS) may be integrated in the lithographic apparatus and may be arranged to measure a topography of a top surface of the substrate. A map of the topography of the substrate, also referred to as height map, may be generated from these measurements indicating a height of the substrate as a function of the position on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in a properly focused position on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis). Typically, the level or height sensor performs measurements at a fixed location (relative to its own optical system) and a relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[0039] An example of a level or height sensor LS as known in the art is schematically shown in Figure 2, which illustrates only the principles of operation. In this example, the level sensor LS comprises an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a beam of radiation LSB which isimparted by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO may include a plurality of radiation sources having different colors, or wavelength ranges, such as a plurality of LEDs and / or LPPS. The radiation source LSO of the level sensor LS is not restricted to visible radiation, but may additionally or alternatively encompass UV (ultra-violet) and / or IR (infra-red) radiation and any range of wavelengths suitable to reflect from a surface of a substrate.

[0040] The projection grating PGR is a periodic grating comprising a periodic structure resulting in a beam of radiation BE1 having a periodically varying intensity. The beam of radiation BE1 with the periodically varying intensity is directed towards a measurement location MLO on a substrate W having an angle of incidence ANG with respect to an axis perpendicular (Z-axis) to the incident substrate surface between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement location MLO, the patterned beam of radiation BE1 is reflected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD.

[0041] In order to determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET produces a detector output signal indicative of the light received, for example indicative of the intensity of the light received, such as a photodetector, or representative of a spatial distribution of the intensity received, such as a camera. The detector DET may comprise any combination of one or more detector types.

[0042] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal strength as measured by the detector DET, the signal strength having a periodicity that depends, amongst others, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0043] The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned beam of radiation between the projection grating PGR and the detection grating DGR (not shown).

[0044] In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0045] In order to cover the surface of the substrate W effectively, a level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0046] Various height sensors of a general type are disclosed for example in US7265364 and US7646471, both incorporated by reference. A height sensor using UV radiation instead of visible orinfrared radiation is disclosed inUS2010233600Al, incorporated by reference. In W02016102127A1, incorporated by reference, a compact height sensor is described which uses a multi-element detector to detect and recognize the position of a grating image, without needing a detection grating.

[0047] The radiation source LSO may comprise an LPPS. Optionally, light emitted by an LPPS may be optically processed by means of lenses and / or gratings to form beam of radiation LSB.

[0048] Figure 3 depicts an example of a known LPPS 10 comprising a bulb 12. Figure 4 shows a cross-section of the bulb 12. The LPPS 10 includes a bulb 12 having a gas disposed therein. The bulb 12 comprises electrodes 22 which are configured to trigger a plasma in the gas inside the bulb. The bulb 12 has an inner surface 13 defining a cavity. The electrodes 22 extend into the cavity from opposite ends of the cavity, i.e. from a first end of the cavity 34 and from a second end of the cavity 36. A proximal portion 22a of the electrode is located toward the inner surface 13 and a distal portion 22b of the electrode is located toward the cavity. The electrodes 22 are connected to a power source (not shown). Applying a potential difference between the electrodes 22 triggers a plasma in a space between the electrodes. The potential difference applied between the electrodes 22 may be the breakdown voltage of the gas inside bulb 12.

[0049] The LPPS further comprises an infra-red laser 14 (IR laser 14), configured to emit IR beam 16. The plasma may be sustained by focusing the IR beam 16 into the plasma. The region where the plasma is sustained is the plasma-sustaining location 32. In use, a portion of the inner surface 31 is disposed above the plasma-sustaining location 32. Sustaining of the plasma by a laser may be referred to as laser pumping. Sustaining of a plasma means continuous or pseudo-continuous irradiation of the plasma with a IR beam 16 to prevent the plasma from extinguishing.

[0050] The IR beam 16 may be reflected from a mirror 18 before passing through a focusing lens 20, which is configured to focus the IR beam 16 at the plasma-sustaining location 32. Operating parameters of the IR laser 14 may be selected so that the IR beam 16 sustains the plasma. Parameters of the IR laser 14 include but are not limited to the following: the wavelength emitted by the IR laser 14 and the power of the IR laser 14. Additionally, the focusing power of focusing lens 20 may be selected to focus the IR beam 16 in the plasma-sustaining location 32. The selected values of the operating parameters of the IR laser 14 and / or the focusing lens 20 may depend on the gas inside the bulb 12. For example, different wavelengths / powers of the IR laser 14 may be required to sustain a plasma of xenon compared to a plasma of helium. Instead of IR laser 14, a laser configured to emit other wavelengths may be provided.

[0051] The plasma may emit radiation, such as light. The properties of the radiation emitted by the plasma depend on the gas in the bulb 12. For example, the wavelength of light emitted by the plasma may depend on the gas that is contained within the bulb 12. The energy of emitted light may correspond to a possible energy transition in the gas between an excited state and a relaxed state. For example, when the gas inside bulb 12 is xenon, UV light is expected to be output. Plasma of other media mayalso emit UV light and / or light having other wavelengths. In LPPS 10, the light emitted by the plasma may pass through a light output window 24.

[0052] The emitted light (e.g. UV light) may be focused and used for applications such as the beam of radiation LSB in the level sensor LS. An LPPS thus effectively converts energy from high- wavelength IR beam 16 emitted by IR laser 14 the into low-wavelength UV light.

[0053] The light output of a known LPPS may have low temporal stability. In other words, the wavelength of light output from known LPPS may change over time. The light output of a known LPPS may also have a low spectral stability. In other words, higher or lower intensities of the output may occur at certain wavelengths. Another problem which may be faced by a known LPPS is that the light output may have relatively high levels of noise. For example, the light output may comprise one or more wavelengths which are not emitted by the plasma itself but come from sources of noise in the LPPS system.

[0054] Low temporal stability, low spectral stability and / or noise may be undesirable. For applications such as the radiation source LSO in level sensor LS, it may be desirable to precisely and accurately know the wavelength of beam of radiation LSB. Thus, it may be desirable to reduce the presence of noise and improve the light output stability of an LPPS.

[0055] US20150034838A1 discloses a bulb with flow control elements that encourage a flow of gas through the location where the plasma is sustained, from the bottom of the bulb to the top of the bulb.

[0056] EP2766919 A2 discloses a bulb comprising radiation shields which redirect convective flow of gas so that damage of the bulb is avoided. The radiation shields and electrodes of EP2766919A2 cover an entire cross section of the cavity to block passage of air to the top / bottom portions of the bulb.

[0057] US9099292B1 discloses a bulb comprising a cell formed of a continuous tube. InUS9099292B 1, convection currents through the cell are directed through the location where the plasma is sustained in use.

[0058] US20040189206A1 discloses a bulb which encourages convection flows throughout the bulb.

[0059] The present inventors have discovered a configuration of the bulb which reduces the temporal instabilities in the light output and reduces noise. The present inventors have discovered that configurations of a bulb which encourage convection flows of gas result in temporal instabilities and noise in the light output.

[0060] An unstable / noisy LPPS system may have pronounced noise peaks at particular frequencies in its intensity spectrum. Such peaks in noise may be termed “ripple peaks” because they occur at distinct harmonic frequencies, thereby presenting a ripple-like noise effect. Noise in the LPPS output can in turn lead to inaccuracies when using the LPPS as a light source in various metrology applications. Accordingly, the presence of these ripple peaks may be undesirable.

[0061] The present inventors have discovered that these ripple peaks in the intensity spectrum are caused by gas dynamics inside the bulb. The plasma acts as a heat source, which heats the surrounding gas. As the gas is heated its density decreases, and buoyancy causes it to rise to the top of the bulb. As the heated gas hits the top surface of the bulb, the gas cools and gravity acts to pull the cooler, denser gas from the top to the bottom. This process may cause instabilities in the flow of gas within the bulb. These instabilities may be referred to as convective instabilities. In other words, convective instabilities inside the bulb may cause ripple peaks in the intensity output of an LPPS.

[0062] In the frame of reference of the bulb as shown in the figures, the direction in which the buoyancy forces act are from the bottom of the bulb to the top of the bulb, and the direction in which gravity acts is from the top of the bulb to the bottom of the bulb.

[0063] Figure 5 shows simulated gas density and gas velocity profiles in a bulb of an LPPS system, and the development of the gas density profile and gas velocity profile over time. To describe the development of the gas density profile over time, reference is made to the top row of Figure 5. In the density profiles shown on Figure 5, the regions of low density appear darker than the regions of higher density. The top row of Figure 5 shows that there is a plume of gas having relatively low density (i.e. having a relatively high temperature) based in the centre of the bulb (i.e. in the plasma sustaining location). This is indicated by the plume-shaped dark region of the bulb, surrounded by lighter regions. The shape of the plume changes over time, i.e. its shape is different at t = 0 ms, t = 8 ms and t = 16 ms. This shape-change is due to a bubble of hot gas which forms at t = 0 ms in the centre of the bulb (i.e. in the plasma-sustaining location). Here, the term hot gas bubble is used to refer to a volume of gas which is heated by the plasma to form an approximately spherical region of hot gas that rises due to buoyancy forces. The hot gas bubble is surrounded by cooler gas. The hot gas bubble is initiated by plasma which heats the surrounding gas. The bubble of hot gas rises to the top of the bulb over time (via convection currents), as indicated by the density profile at t = 8 ms. When the bubble of hot gas reaches the top surface of the bulb, it spreads across the top surface of the bulb as shown at t= 8 ms and t = 16 ms before the gas in the hot gas bubble cools and is pulled down to the bottom of the bulb by gravity. In other words, once the hot gas bubble reaches to the top of the bulb, the hot gas bubble cools down and dissipates.

[0064] The lower row of Figure 5 shows the velocity profile of gas in the bulb at t = 0 ms, 8 ms and 16 ms. The upwards movement of the hot gas bubble can be seen from the high-velocity region at the centre of the bulb which gradually moves towards the top of the bulb. The high-velocity is indicated by lighter lines, and the surrounding areas of stationary gas are shown by dark lines.

[0065] The present inventors have discovered that these gas convective instabilities cause instabilities in light emitted by the plasma.

[0066] The cycle described above (involving generation of a rising heated bubble which is then separates) repeats at a certain frequency, which causes instabilities in the light output of the plasma and the ripples in the light output of the plasma.

[0067] The fluid mechanics of the gas within the bulb may be characterised by the Rayleigh number(Ra):wherein: p is the average mass density of the gas,P is the thermal expansion coefficient of the gas, AT is the temperature difference across distance L, L is a characteristic length, g is the acceleration due to gravity,T] is the dynamic viscosity of the gas, and a is the thermal diffusivity of the gas.

[0068] The Rayleigh number (Ra) is a dimensionless number which indicates how strong buoyancy forces are relative to viscous forces. The ratio of these two forces determines whether the fluid motion is stable or unstable. The Ra number at which the transition to unstable behaviour occurs is the critical Ra number. When Ra is below the critical Ra value, the formation of convective instabilities is suppressed. Thus, for a bulb filled with gas, the Ra number indicates how strong buoyancy forces in the gas are relative to viscous forces in the gas. Below the critical Ra value of the gas, the formation of convective instabilities in the gas is suppressed.

[0069] The Ra is a function of a number of thermo-physical properties of the gas (p, P, r], a), as well as characteristic dimension L. For a bulb having a plasma sustained therein, characteristic dimension L may be defined as the shortest distance between the location for sustaining a plasma source in use and the surface of the bulb above the location for sustaining a plasma source in use. The local value of Ra may be calculated by using the local values of thermo-physical gas properties (p, p, r], a) based on determined temperature and pressure of the bulb at a given point. A suitable approximation for the local values of thermo-physical gas properties (p, p, r], a) may be the average values of these thermophysical gas properties over characteristic dimension L, which may be used to obtain an average value of Ra over the characteristic dimension L.

[0070] It is not feasible to change the properties of the gas while maintaining a useful light output (e.g. the average mass density of xenon at a particular temperature is fixed, and cannot be changed to influence the value of Ra). However, a parameter that may be manipulated is characteristic dimension L. The present inventors have developed a novel configuration of the bulb which limits L, without substantially changing the volume of the cavity of the bulb (which can detrimentally impact the lifetime of the bulb).

[0071] According to the present invention, there is provided a bulb 12 for a laser-pumped plasma light source 10, the bulb having an inner surface 13 defining a cavity. A plasma-sustaining location 32 in the cavity for sustaining a plasma in use; and a shield 30 interposed between the plasma-sustaining location 32 and a portion of the inner surface 31, wherein the shield 30 is configured to redirect gas moving from the plasma-sustaining location 32 to the portion of the inner surface 31 around the shield 30. In other words, the gas moves from the plasma-sustaining location 32 to a portion of the cavity above shield 30 in a path that moves around the shield 30. An example of a bulb 12 according to the present invention is shown in Figure 6.

[0072] The bulb 12 shown in Figure 6 additionally comprises electrodes 22 extending into the cavity, which are optional. As discussed in further detail below, the bulb 12 may comprise only one electrode or no electrodes.

[0073] In use, the bulb 12 is filled with a gas. In the present embodiment, the bulb is filled with xenon gas.

[0074] The plasma-sustaining location 32 is a volume of the bulb where a plasma of the gas is sustained in use. The plasma has a higher temperature than the same matter in a state of gas, due to the higher thermal energy of the plasma state when compared to the gas state. For example, a plasma of xenon has a higher temperature than xenon in the gas state. In use, the maximum temperature inside the bulb is located in the plasma-sustaining location 32. The maximum temperature may be the location where a laser (e.g. an IR laser 14) is focused to sustain the plasma. Gas surrounding the plasma is cooler, meaning there is a temperature gradient inside the bulb 12.

[0075] The plasma-sustaining location 32 may be defined as a spherical volume having a center at the maximum-temperature location and having a radius extending from the maximum-temperature location to the closest point at which the temperature is between one quarter and one third of the maximum temperature. The plasma-sustaining location 32 has a much higher temperature than the surrounding gas, and a sharp change in the gas temperature may be observed between the plasmasustaining location and the surrounding gas. The exact shape of the temperature profile within the bulb 12 may depend on factors such as the size of the bulb 12, gas pressure, the power of the laser focused in the plasma-sustaining location 32. For certain configurations of the bulb and / or the LPPS, the plasma-sustaining location 32 may be a non-spherical volume. The plasma-sustaining location 32 may alternatively be a volume of any other suitable shape, including conical, ovoidal, tear-drop etc.

[0076] The plasma-sustaining location 32 may be disposed in the centre of the bulb 12. The plasmasustaining location 32 may be off-centre. In use, the highest temperature in the cavity may be in the plasma-sustaining location 32. It may be preferable to sustain the plasma in a centre of the bulb, with gas between the plasma and the inner surface 13. The gas between the plasma and the inner surface 13 may protect the bulb 12 from the plasma and may prevent the bulb 12 from becoming damaged due to high temperatures of the plasma.

[0077] The plasma-sustaining location 32 of the bulb 12 is a region where the plasma may be triggered. For example, the electrodes 22 shown in Figure 6 may have a potential difference applied between them which causes electrical breakdown of the gas such that the plasma is triggered in a region between the electrodes 22. A laser (e.g. IR laser 14 or a different laser not shown in the figures) may be configured to focus light (e.g. IR light) in the plasma-sustaining location 32, to sustain the plasma.

[0078] However, the presence of electrodes 22 is not required to define the plasma-sustaining location 32. Embodiments of the bulb which do not have electrodes 22 may have a plasma triggered therein by means of a laser (e.g. IR laser 14 or a different laser not shown in the figures) wherein a pulse of light from the laser is focused in the bulb 12. The laser may provide a high-energy pulse of light to trigger the plasma . The same laser or a different laser may then sustain the plasma.

[0079] The plasma-sustaining location 32 is where the plasma is sustained when the LPPS is in use. In use, the plasma causes hot bubbles of gas to form in the plasma-sustaining location 32.

[0080] By providing the shield 30 which is interposed between the plasma-sustaining location 32 and the portion of the inner surface 31, gas is redirected around the shield 30 from the plasma-sustaining location 32 to the portion of the inner surface 31. In use, the plasma-sustaining location 32 is below the shield. If the shield 30 were not there, the bubbles of hot gas would rise directly upwards to the portion of the inner surface 31 above the plasma-sustaining location 32 as described with reference to Figure 4.

[0081] For determining the Ra value in the region between the plasma-sustaining location 32 and the shield 30, the relevant characteristic length is Li, which is the distance between a centre of the plasmasustaining location 32 and a surface of the shield 30 facing the plasma-sustaining location 32. Distance L may be defined as the shortest distance between the maximum temperature point and the surface of the shield 30 facing the plasma-sustaining location 32. As it can be seen from Figure 6, distance Li for a bulb 12 comprising shield 30 is much smaller than distance L2for a bulb 12 having no shield. The local values of p, p, q, a of the gas inside the plasma-sustaining location 32 may be used. For determining the average Ra over the characteristic distance L, a suitable approximation for the values of of p, , q, a within the plasma-sustaining location 32 may the average-values of these thermo-physical parameters taken over distance L.

[0082] The portion of the inner surface 31 means the portion of the surface of the cavity which is above the plasma-sustaining location 32 in use. As convection currents are driven by buoyancy and gravity, “above” the plasma-sustaining location 32 means in the direction in which buoyancy acts relative to the plasma-sustaining location 32. For completeness, “below” the plasma-sustaining location 32 means in the direction in which gravity acts.

[0083] Provision of the shield 30 above the plasma-sustaining location 32 reduces the value of length L, meaning that Ra is reduced (when compared to a bulb 12 without a shield 30 having the same gas dispose therein), ideally to be less than the critical Ra. When the value of Ra in the region between the plasma-sustaining location 32 and the shield 30 is less than the critical Ra, convective instabilities and ripple peaks in the light output of the plasma are suppressed.

[0084] When Ra is below the critical Ra value, the formation of convective instabilities is suppressed. Thus, provision of the shield 30 reduces (or ideally prevents) the formation of convective instabilities between the plasma-sustaining location 32 and the shield 30. Accordingly convective currents of gas which interfere with the plasma are reduced (or ideally avoided) and the stability of the plasma light output is improved.

[0085] The shield 30 redirects the hot gas around the shield 30, which prevents the cycle of hot gas bubble formation, rising, separation and dissolving as described with reference to Figure 5. As the formation of convection currents is reduced or prevented, instabilities of the plasma are reduced. Thus, providing a shield 30 above the plasma-sustaining location 32 reduces the presence of instabilities and ripple peaks from the light output of the plasma.

[0086] The shield 30 is configured to restrict the development of convection currents in a space between the plasma-sustaining location 32 and the portion of the inner surface 31.

[0087] The convective currents may also be described as Rayleigh-Bemard-type instabilities. In fluid thermodynamics, Rayleigh-Bernard convection is a type of convection in which a fluid develops a regular pattern of convection cells. The shield 30 reduces or prevents the formation of convective instabilities within the bulb 12.

[0088] To avoid the development of convective instabilities, the shield 30 may cover a volume of the cavity where the gas is so hot that convective currents may otherwise develop i.e. over the plasmasustaining location 32.

[0089] The cavity may comprise- the first end 34, the second end 36 and a longitudinal axis B therebetween. The plasma-sustaining location 32 may be disposed between the first end 34 and a second end 36. The shield 30 may have a length extending from the first end 34 towards the second end 36 along longitudinal axis B, such that the length of the shield 30 at least reaches and surpasses the plasmasustaining location 32. By reaching and surpassing the plasma-sustaining location 32, the shield 30 redirects gas from the plasma-sustaining location 32 to the portion of the inner surface 31 in a path around the shield 30.

[0090] In Figure 6, the longitudinal axis extends between first end 34 and the second end 36 of the bulb 12. As shown in Figure 6, the plasma-sustaining location 32 may be disposed between electrodes 22. The distal ends 22b of electrodes 22 may be separated by distance d, and the shield 30 may extend into the cavity so that it covers distance d. The shield 30 extends above the region between the electrodes 22, meaning that the plasma-sustaining location 32 is covered by the shield 30 and convection flows of gas from the plasma-sustaining location 32 are redirected around the shield 30. In some embodiments, only one electrode 22 may be provided inside bulb 12.

[0091] An electrode 22 may extend along the longitudinal axis B from the first end 34 or from the second end 36. When there are two electrodes 22, one electrode 22 may extend from the first end 34 and the other electrode 22 may extend from the second end 22. The electrode(s) 22 may be configured to be connected to a (high) power supply, and may be further configured to trigger the plasma in theplasma-sustaining location 32. Applying a potential to electrode(s) 22 may set up a potential difference across the gas in the plasma-sustaining location 32, triggering creation of a plasma.

[0092] There may be provided a pair of electrodes 22, as shown in Figure 8 for example. One electrode 22 may extend from the first end of the cavity and the other electrode 22 extending from the second end of the cavity. The pair of electrodes 22 are configured to be connected to a power supply and trigger the plasma in the plasma-sustaining location 32.

[0093] An embodiment of the bulb 12 of the present invention may have all features shown in Figure 6 except for electrodes 22. The shield 30 may extend into the cavity as shown in Figure 6, so that the plasma-sustaining location 32 is covered by the shield 30.

[0094] The shield 30 at least partially extends from the first end 34 of the cavity to the second end 36 of the cavity, so that at least a portion of the shield 30 is interposed between the plasma-sustaining location 32 and the portion of the inner surface 31.

[0095] The shield 30 may extend from the first end of the cavity to the second end of the cavity, as shown in Figure 7. In other words, the shield 30 may be connected to the first end 34 and second end 36 of the bulb 12 in the direction of the longitudinal axis. When the shield 30 extends from the first end of the cavity to the second end of the cavity, it redirects gas around the shield 30 to a portion of the cavity above shield 30. The shield 30 may extend over the whole longitudinal length of the cavity, or may extend over most of the most of the longitudinal length of the cavity.

[0096] The shield 30 may extend along longitudinal axis B without being attached to the inner surface 13 at either of the first end 34 or the second end 36. In an example, the shield 30 may be platform which is fixed above the plasma-sustaining location 32. For example, the platform may be fixed to the inner surface 13 so that the platform extends above the plasma-sustaining location 32 without contacting the first end 34 or the second end 36. For example, the platform may be attached across the width of the bulb 12 or could be attached via projections from the platform to the top of the cavity of the bulb 12.

[0097] Figure 8 is a portion of the cross-section through A-A in either of Figures 6 or 7. A width w of the shield 30 may be greater than or equal to the width of the plasma-sustaining location 32. The width w may be perpendicular to the longitudinal axis B. The width w may be in a plane normal to the direction of the buoyancy force in the cavity of the bulb 12.

[0098] In Figure 8 the shield 30 is shown as having a smaller width than the width of the electrode 22. The width of the electrode does not necessarily impact the size of the plasma-sustaining location 32, so the value of w is not tied to the width of the electrodes 22. The width w may be equal to or greater than the width of the plasma-sustaining location 32, i.e. the diameter of the volume of the plasma-sustaining location 32.

[0099] The width w of the shield 30 may be such that the shield 30 extends across an entire width of the cavity. When the shield 30 extends across the entire width of the cavity, the length of the shield of the shield is less than the length of the cavity so that gas may be redirected from below the shield 30 toabove the shield 30. In other words, the shield 30 may extend over most of an entire cross-section of the cavity to extend over and beyond the plasma-sustaining location 32, but so that gas may be redirected from below the shield 30 to above the shield 30. The width w is not necessarily uniform along the length of the shield 30.

[0100] The surface of the shield 30 facing the plasma-sustaining location 32 may be flat, curved or angular. A plurality of possible shapes of the shield 30 are shown in Figures 9a, 9b, 9c and 9d. A flat surface may be completely flat, or it may have a textured or patterned surface. For example, in Figure 9d, a surface of the shield 30 facing the obstruction is flat.

[0101] The surface may be curved, for example, it may be concave or convex. An example of a concave surface is shown in Figure 9a.

[0102] An angular surface may comprise a plurality of surfaces, formed in a geometric arrangement. For example, Figures 9b and 9c show examples of the shield 30 having a triangular surface.

[0103] Any of the shields shown in Figures 9a, 9b, 9c and 9d may be used as the shield 30 in a bulb described with reference to Figures 6-8. For example, any of the shields shown in in Figures 9a, 9b, 9c and 9d may extend from a first end of the cavity to reaching and surpassing the plasma-sustaining location 32, optionally, extending to the second end of the cavity. Figures 9a, 9b, 9c and 9d show some possible examples of shield 30 shapes, but other shapes of the shield 30 which redirect gas moving from the plasma-sustaining location 32 to the portion of the inner surface around the shield are possible.

[0104] The shape, length and width of the shield 30 may be selected so that gas moving from the plasma-sustaining location 32 to the portion of the inner surface is redirected around the shield 30. The shield extends over the plasma-sustaining location 32 so that the development of convention currents in a region between the plasma-sustaining location 32 and a portion of the shield 30 facing the plasmasustaining location 32 is reduced.

[0105] As the size of the plasma-sustaining location 32 depends on a variety of operating parameters such as the pressure of the gas, the intensity of the laser that is pumping the plasma, the type of gas used, the temperature, and the shape of the bulb 12 a variety of possible configurations of the shield 30 are possible.

[0106] In an example, the distance L may be in the region of 2-10 mm. However, for some configurations of the bulb 12 and operating parameter values, L may fall outside this range.

[0107] The gas may comprise one or more of xenon, helium and hydrogen. Where the present disclosure refers to a bulb 12 comprising xenon gas, it will be appreciated that the bulb 12 may be used with other gases such as helium or hydrogen (or a mix thereof with xenon).

[0108] The shield may comprise quartz, glass, ceramic or a metal. Glass, quartz or ceramic shields 24 may be particularly suitable for bulbs 12 having electrodes 22 disposed therein. This is because in some embodiments, a metal shield 30 may lead to a plasma being triggered between the metal shield 30 and the electrode 22.

[0109] The bulb 12 may comprise quartz, glass, ceramic or metal. It may be convenient to manufacture the bulb 12 and the shield 30 from the same material. Although the bulb and shield could be made of different material.

[0110] The shape of the bulb 12 is not particularly limited and may be spherical, ovoid, cylindrical, or cuboidal.

[0111] A bulb according to the present invention may have any of the features described above. A bulb 12 having any of the features disclosed above may be provided in a laser-pumped plasma source. When the bulb 12 comprises at least one electrode 22, a power source may be provided to configured to power the electrode 22 or the pair of electrodes 22. A laser (e.g. IR laser) may be provided to sustain a plasma in the plasma-sustaining location 32.

[0112] The bulb 12 may be provided in a laser-pumped plasma source (LPPS) which uses a laser to trigger a plasma. In that case, the bulb 12 may not comprise electrodes 22 and so a power source to power the electrodes 22 may not be required.

[0113] The level sensor LS comprising the bulb 12 may be provided in a lithographic apparatus or in a metrology system.

[0114] Although specific reference has been made to use of the bulb and LPPS of the present invention for a level sensor LS, the bulb and LPPS of the present application may be used in any other applications where a stable source of UV radiation is desirable.

[0115] Although specific reference may be made in this text to the use of a 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, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0116] 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.

[0117] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0118] 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 thatmodifications may be made to the invention as described without departing from the scope of the claims set out below.Exemplary embodiments of the invention are set out in the following numbered clauses:1. A bulb for a laser-pumped plasma light source, the bulb having an inner surface defining a cavity; a plasma-sustaining location in the cavity for sustaining a plasma in use; and a shield interposed between the plasma-sustaining location and a portion of the inner surface, wherein the shield is configured to redirect gas moving from the plasma-sustaining location to the portion of the inner surface around the shield.2. The bulb according to clause 1, wherein the shield is configured to restrict the development of convective instabilities in a space between the plasma-sustaining location and the portion of the inner surface.3. The bulb according to any of the preceding clauses, wherein the plasma-sustaining location is a spherical volume comprising a maximum-temperature location in the cavity, wherein in use, the plasma exhibits a maximum temperature at the maximum-temperature location, and the spherical volume is centered on the maximum-temperature location and has a radius extending from the maximum-temperature location to the closest point at which the temperature is between one quarter and one third of the maximum temperature.4. The bulb according to any of the preceding clauses, wherein a width of the shield is greater than or equal to the width of the plasma-sustaining location.5. The bulb according to any of the preceding clauses, wherein the shield has a surface facing the plasma-sustaining location, the surface being flat, curved or angular.6. The bulb according to any of the preceding clauses wherein the cavity is filled with the gas.7. The bulb according to any of the preceding clauses, wherein the gas comprises one or more of xenon, helium and hydrogen.8. The bulb according to any of the preceding clauses, wherein the shield comprises glass, quartz, ceramic or a metal.9. The bulb according to any of the preceding clauses, wherein the bulb comprises quartz, glass, metal or ceramic.10. The bulb according to any of the preceding clauses, wherein the bulb is spherical, ovoid, cylindrical, or cuboidal.11. The bulb according to any of the preceding clauses; wherein the cavity has a first end, a second end and a longitudinal axis therebetween; the plasma-sustaining location is disposed between the first end and the second end;the shield has a length extending from the first end of the cavity towards the second end of the cavity along the longitudinal axis; the length of the shield at least reaching and surpassing the plasma-sustaining location.12. The bulb according to clause 11 , wherein the shield at least partially extends from the first end of the cavity to the second end of the cavity.13. The bulb according to clause 11 or clause 12, wherein the shield extends from the first end of the cavity to the second end of the cavity.14. The bulb according to one of clauses 11 to 13, wherein there is provided at least one electrode extending along the longitudinal axis from the first end or the second end, wherein the electrode is configured to be connected to a power supply, and is further configured to trigger the plasma in the plasma-sustaining location.15. The bulb according to one of clauses 11 to 13 , wherein there is provided a pair of electrodes, one electrode extending from the first end and the other electrode extending from the second end, wherein the pair of electrodes are configured to be connected to a power supply and trigger the plasma in the plasma-sustaining location.16. A laser-pumped plasma source comprising a bulb according to any one of clauses 14 or 15, further comprising: a power source configured to power the electrode or the pair of electrodes, and a laser configured to sustain a plasma in the plasma-sustaining location.17. A laser-pumped plasma source comprising a bulb according to any one of clauses 1 to 13, further comprising: a laser configured to trigger a plasma in the plasma-sustaining location and to sustain a plasma in the plasma-sustaining location.18. A sensor for measuring substrate surface topography comprising: a projection unit comprising a radiation source providing a radiation beam, wherein the projection unit is configured to project a radiation beam to a measurement location on the substrate, a detection unit configured to detect radiation reflected from the measurement location, wherein the radiation source comprises the laser-pumped plasma source of clause 16 or clause 17.19. A lithographic apparatus configured to apply a pattern to a substrate, the lithographic apparatus comprising the sensor of clause 18 for measuring the substrate surface topography.20. A metrology system comprising a sensor according to clause 18.

[0119] The following reference numerals are used throughout the description and drawings:10 Laser-Pumped Plasma Source (LPPS)12 bulb13 inner surface14 IR laser fibre16 IR radiation beam18 mirror20 focusing lens22 electrodes 22a proximal end22b distal end24 light output window30 shield31 portion of the inner surface 32 plasma-sustaining location34 first end36 second end

Claims

CLAIMS1. A bulb for a laser-pumped plasma light source, the bulb having an inner surface defining a cavity; a plasma-sustaining location in the cavity for sustaining a plasma in use; and a shield interposed between the plasma-sustaining location and a portion of the inner surface, wherein the shield is configured to redirect gas moving from the plasma-sustaining location to the portion of the inner surface around the shield.

2. The bulb according to claim 1, wherein the shield is configured to restrict the development of convective instabilities in a space between the plasma-sustaining location and the portion of the inner surface.

3. The bulb according to any of the preceding claims, wherein the plasma-sustaining location is a spherical volume comprising a maximum-temperature location in the cavity, wherein in use, the plasma exhibits a maximum temperature at the maximum-temperature location, and the spherical volume is centred on the maximum-temperature location and has a radius extending from the maximum-temperature location to the closest point at which the temperature is between one quarter and one third of the maximum temperature.

4. The bulb according to any of the preceding claims, wherein a width of the shield is greater than or equal to the width of the plasma-sustaining location.

5. The bulb according to any of the preceding claims, wherein the shield has a surface facing the plasma-sustaining location, the surface being flat, curved or angular.

6. The bulb according to any of the preceding claims wherein the cavity is filled with the gas, and wherein the gas comprises one or more of xenon, helium and hydrogen.

7. The bulb according to any of the preceding claims, wherein the shield comprises glass, quartz, ceramic or a metal.

8. The bulb according to any of the preceding claims, wherein the bulb comprises quartz, glass, metal or ceramic.

9. The bulb according to any of the preceding claims; wherein the cavity has a first end, a second end and a longitudinal axis therebetween; the plasma-sustaining location is disposed between the first end and the second end; the shield has a length extending from the first end of the cavity towards the second end of the cavity along the longitudinal axis; the length of the shield at least reaching and surpassing the plasma-sustaining location.

10. The bulb according to claim 9, wherein the shield at least partially extends from the first end of the cavity to the second end of the cavity.

11. The bulb according to claim 9 or claim 10, wherein there is provided at least one electrode extending along the longitudinal axis from the first end or the second end, wherein the electrode is configured to be connected to a power supply, and is further configured to trigger the plasma in the plasma-sustaining location.

12. The bulb according to claim 9 or 10, wherein there is provided a pair of electrodes, one electrode extending from the first end and the other electrode extending from the second end, wherein the pair of electrodes are configured to be connected to a power supply and trigger the plasma in the plasma-sustaining location.

13. A laser-pumped plasma source comprising a bulb according to any one of claims 1 to 12, further comprising: a laser configured to trigger a plasma in the plasma-sustaining location and to sustain a plasma in the plasma-sustaining location.

14. A sensor for measuring substrate surface topography comprising: a projection unit comprising a radiation source providing a radiation beam, wherein the projection unit is configured to project a radiation beam to a measurement location on the substrate, a detection unit configured to detect radiation reflected from the measurement location, wherein the radiation source comprises the laser-pumped plasma source of claim 13.

15. A lithographic apparatus configured to apply a pattern to a substrate, the lithographic apparatus comprising the sensor of claim 14 for measuring the substrate surface topography.

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