WO2019192841A1

WO2019192841A1 - Spatial modulation of a light beam

Info

Publication number: WO2019192841A1
Application number: PCT/EP2019/056898
Authority: WO
Inventors: Kevin Weimin ZHANG; Michael Anthony PURVIS; Cory Alan STINSON
Original assignee: Asml Netherlands B.V.
Priority date: 2018-04-03
Filing date: 2019-03-20
Publication date: 2019-10-10
Also published as: CN111955058A; TWI820102B; NL2022769A; KR20200138728A; JP7356439B2; TW201945792A; JP2021517662A

Abstract

A system includes a spatial modulation device configured to interact with a light beam to create a modified light beam, the modified light beam including a spatial pattern of light that has a non-uniform intensity along a direction that is perpendicular to a direction of propagation of the modified light beam, the spatial pattern of light including one or more components of light; and a target supply system configured to provide a target to a target region, the target including target material that, when in a plasma state, emits EUV light. The target region overlaps with the beam path such that at least some of the one or more components of light in the modified beam interact with a portion of the target.

Description

SPATIAL MODULATION OF A LIGHT BEAM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 62/651,928 which was filed on April 3, 2018 and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] This disclosure relates to techniques for spatially modulating a light beam. The techniques may be used in, for example, an extreme ultraviolet (EUV) light source. The light beam may be, for example, a light beam that illuminates a target material or a fuel material.

BACKGROUND

[0003] Extreme ultraviolet (“EUV”) light, for example, electromagnetic radiation having wavelengths of 100 nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of, for example, 20 nm or less, between 5 and 20 nm, or between 13 and 14 nm, may be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers, by initiating polymerization in a resist layer.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a target material into a plasma that emits EUV light. The target material includes an element, for example, xenon, lithium, or tin, that has an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma may be produced by irradiating a target that includes target material with an amplified optical beam that may be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment. The target material may be in the form of a droplet, plate, tape, stream, or cluster.

SUMMARY

[0005] In one general aspect, a system includes a spatial modulation device configured to interact with a light beam to create a modified light beam, the modified light beam including a spatial pattern of light that has a non-uniform intensity along a direction that is perpendicular to a direction of propagation of the modified light beam, the spatial pattern of light including one or more components of light; and a target supply system configured to provide a target to a target region, the target including target material that, when in a plasma state, emits EUV light. The target region overlaps with the beam path such that at least some of the one or more components of light in the modified beam interact with a portion of the target.

[0006] Implementations may include one or more of the following features. The spatial modulation device may be a diffractive optical element. The diffractive optic may be a spatial light modulator (SLM), an adaptive optic, a reticle, and/or a grating. The spatial modulation device may be a refractive optical element. The refractive optical element may be a lens, a lenslet array, and/or a reticle.

[0007] The spatial pattern of light may include two or more components of light, and each of the two or more components of light may have substantially the same intensity. The spatial pattern of light may include two or more components of light arranged in a rectilinear grid. The spatial modulation device may include at least one Dammann grating.

[0008] The spatial modulation device also may be configured to interact with a second light beam to create a second modified light beam, the second modified light beam including a second spatial pattern of light that has a non-uniform intensity along a direction that is perpendicular to a direction of propagation of the second modified light beam, the second spatial pattern of light including one or more second components of light.

[0009] In some implementations, the system also includes a first light generation module configured to emit the light beam, and a second light generation module configured to emit a second light beam.

[0010] In another general aspect, a method of forming a target for an extreme ultraviolet (EUV) light source includes directing a light beam onto a beam path; interacting the light beam with a spatial modulation device positioned on the beam path to form a modified light beam, the modified light beam including a spatial pattern of light that has a non-uniform intensity along the direction that is perpendicular to a direction of propagation of the modified light beam, the spatial pattern of light including one or more components of light; and interacting the modified light beam with a target that includes target material that emits EUV light when in a plasma state. At least some of the one or more components of light of the spatial pattern interacts with a region of the target to modify a property of that region of the target. [0011] Implementations may include one or more of the following features. The property may be density, and, in these implementations, modifying the property includes decreasing the density. The property may be a surface area of the target, and, in these implementations, modifying the property of any portion of the target includes increasing the surface area of the entire target. An amount of increase of the surface area may be related to a number of light components in the modified light beam.

[0012] The spatial pattern of light may include two or more components of light. All of the components of light may have the same intensity. The components of light may be arranged in a grid, and the regions of the target that interact directly with a component of light may be arranged in a grid. The components of light may be spatially separated and spatially discrete such that a portion of the target between any two components of light does not interact with any of the components in the modified light beam.

In some implementations, the method further includes interacting the modified beam with a focusing assembly prior to interacting the modified light beam with the target.

[0013] In some implementations, the method further includes interacting an initial target with a second light beam to form an altered target, the altered target having a greater extent in a first direction than the initial target and a smaller extent in a second direction than the initial target, the first and second directions being orthogonal to each other. In these implementations, interacting the modified light beam with a target that includes target material that emits EUV light when in a plasma state includes interacting the modified light beam with the altered target, and each of the one or more components of light interacts with a region of the altered target to modify a property of that region of the altered target. Furthermore, in some implementations, after interacting the modified light beam with the altered target, the altered target interacts with a third light beam, the third light beam having an energy sufficient to convert at least some of the target material in the altered target to the plasma that emits EUV light.

[0014] The method also may include, after interacting the target with the modified light beam, interacting the target with another light beam, the other light beam having an energy sufficient to convert at least some of the target material in the second altered target to the plasma that emits EUV light. The light beam and the other light beam may be temporally connected and part of a single pulse of light. The property may include a conversion efficiency related to an amount of EUV light emitted and the energy of the other light beam, and modifying the property of a portion of the target includes increasing the conversion efficiency associated with the entire target.

[0015] Interacting the modified light beam with a target that includes target material that emits EUV light when in a plasma state may include interacting the modified light beam with a target is that has a substantially spherical shape.

[0016] Implementations of any of the techniques described above may include an EUV light source, a system for an EUV light source, instructions stored on a non-transient electronic storage medium, a method, a process, a device, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1A is a block diagram of an example of an EUV light source.

[0018] FIG. 1B is a plot of intensity of an example of a light beam as a function of position along a direction perpendicular to a direction of propagation prior to spatial modulation.

[0019] FIG. 1C is a plot of intensity of the light beam of FIG. 1B as a function of position along a direction perpendicular to a direction of propagation after spatial modulation.

[0020] FIG. 1D is a block diagram of an example of a target that interacts with the light beam of FIG. 1C.

[0021] FIG. 2A is a block diagram of another example of an EUV light source.

[0022] FIG. 2B is a block diagram of another example of a target.

[0023] FIG. 2C is a plot of intensity of another example of a light beam as a function of position along a direction perpendicular to a direction of propagation after spatial modulation.

[0024] FIG. 2D is a block diagram of an example of a modified target.

[0025] FIGS. 2E-2D show examples of light patterns in a target region.

[0026] FIGS. 3-6 are block diagrams of additional examples of EUV light sources.

[0027] FIG. 7 is an illustration of target region over time.

[0028] FIG. 8 is an illustration of intensity of light in the target region of FIG. 7 over the time scale of FIG. 7.

[0029] FIG. 9 is a block diagram of an example of a lithographic apparatus.

[0030] FIG. 10 is a block diagram of an example of an EUV lithographic system. [0031] FIG. 11 is a block diagram of an example of an EUV light source.

DETAILED DESCRIPTION

[0032] Techniques for spatially modulating a light beam are disclosed. The spatially modulated light beam is used to irradiate a target material or fuel material.

[0033] Referring to FIG. 1A, a side view of an extreme ultraviolet (EUV) light source 100 is shown. The EUV light source 100 includes a light- generation module 105 that emits a light beam 106 onto a beam path 107 and toward a spatial modulation device 120 that includes a modulation element 122. An interaction between the modulation element 122 and the light beam 106 forms a modified light beam 132. The modified light beam 132 interacts with a target 140 at a target region 142. The light beam 106 may be a pulsed light beam that includes pulses of light separated from each other in time. In these implementations, the modified light beam 132 is also a pulsed light beam.

[0034] The target 140 includes a target material or a fuel material that, when in a plasma state, emits EUV light. The target material includes a target substance and may also include impurities such as non-target particles. The target substance is the substance that is converted to a plasma state that has an emission line in the EUV range. The target substance, may be, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the target substance may be the element tin, which can be used as pure tin (Sn); as a tin compound, for example, SnBr₄, SnBr₂, SnH₄; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. In the situation in which there are no impurities, the target material includes only the target substance.

[0035] The target 140 may take any form that is conducive to the production of the plasma that emits EUV light. The target 140 may be, for example, a droplet of liquid or molten metal, a portion of a liquid stream, solid particles or clusters, solid particles contained within liquid droplets, a foam of target material, or solid particles contained within a portion of a liquid stream. The target 140 may take other forms. For example, the target 140 may be a contiguous segment of molten metal that is substantially disk shaped. The target 140 may be a collection of particles that occupy a substantially disk-shaped volume or a hemisphere-shaped volume. The target 140 is a continuous segment of target material that does not have gaps or voids, a mist of nano- or micro-particles, or a cloud of atomic vapor.

[0036] The present technique relates to spatially modulating the light beam 106 prior to the light beam interacting with the target 140. As discussed below, spatially modulating the light beam 106 may lead to increased conversion efficiency (CE) and/or reduced debris production.

[0037] The modulation element 122 is an optical element capable of spatially modulating the light beam 106 to form a modified light beam 132. The modulation element 122 may be a diffractive optical element, which is any structure capable of modulating the light beam 106 by diffraction. The diffractive optical element may be, for example, a grating, a spatial light modulator (SLM), an acousto-optic modulator (AOM), an acoustic-optic deflector (AOD), an aperture or collection of apertures arranged in a plane to create a specific diffraction pattern, and/or a reticle. The modulation element 122 may be a refractive optical element, such as a two- dimensional array of lenses or another arrangement of lenses, a phase plate, a deformable mirror, and/or a refractive reticle. The modulation element 122 may include more than one instance of a particular type of modulation element or a collection of various different modulation elements.

In these implementations, more complex spatial modulations may be achieved by combining the effects of more than one modulation element. For example, two identical diffraction gratings may be placed in series on the beam path 107 and rotated about the beam path 107 relative to each other to form a more complex diffraction pattern that corresponds to a more complex spatial modulation of the light beam 106. In some implementations, the modulation element 122 includes both refractive and diffractive optical elements. Furthermore, the modulation element 122 may be any sort of adaptive optic, such as, for example, a deformable mirror.

[0038] Moreover, the modulation element 122 may be static or dynamic. A static modulation element is one in which the structure of the modulation element 122 is fixed at the time of manufacture of the modulation element 122 and is not changed after the modulation element 122 is formed. A dynamic modulation element is one in which the spatial modulation imparted by the interaction between the light beam 106 and the modulation element 122 may be changed or adjusted during the lifetime of the modulation element 122. For example, the spatial modulation imparted by an AOM onto an incident light beam depends on the properties of an acoustic wave that propagates in a medium (such as quartz) through which the incident light beam passes.

Thus, by changing the amplitude and/or period of the acoustic wave, the characteristics of the modulation provided by the AOM also may be changed. As such, an AOM may be considered to be a dynamic modulation element. An SLM also may be used as a dynamic modulation element. A deformable mirror or any other type of adaptive optical element also may be used as a dynamic modulation element. On the other hand, a lenslet array and a blazed diffraction grating formed from classical or traditional refractive and/or reflective materials such that the optical properties and mechanical properties are not intended to be changeable by an end user are examples of static modulation elements.

[0039] Due to the spatial modulation, the spatial profile of the modified light beam 132 is different than the spatial profile of the light beam 106. The spatial profile of a light beam is a property (for example, intensity and/or phase) of the light beam as a function of position along a direction that is in a plane perpendicular to the direction of propagation. Thus, the modified light beam 132 may have a different intensity and/or phase profile than the light beam 106. The characteristics of the profile of the modified light beam 132 depend on the characteristics and/or arrangement of the modulation element 122. For example, in implementations in which the modulation element 122 is a diffraction grating, the angles at which the diffractive orders propagate away from the modulation element 122 (and thus the positions of the diffractive orders at the target region 142) depend on the spacing between grooves on the diffraction element.

[0040] In the example of FIG. 1 A, the light beam 106 and the modified light beam 132 propagate generally along a Z direction. FIG. 1B is a plot of intensity (in arbitrary units) of the light beam 106 as a function of position along a direction X, which is perpendicular to the direction Z. FIG. 1C is a plot of intensity (in arbitrary units) of the modified light beam 132 as a function of position along the direction X at the target region 142. In the example of FIGS. 1A- 1C, the intensity profile of the light beam 106 is substantially Gaussian. The intensity profile of the modified light beam 132 is different from the intensity profile of the light beam 106 because of the interaction between the light beam 106 and the modulation element 122. The intensity profile shown in FIG. 1C is along a single line in the X-Y plane. The intensity profile of the modified light beam 132 in the X-Y plane at other lines along the X direction may be the same as the intensity profiles shown in FIG. 1C or may be different. In other words, the spatial profile of the modified light beam 132 may vary along the X direction, along the Y direction, or along both the X and Y direction. [0041] The nature of the spatial profile of the modified light beam 132 depends on the configuration of the modulation element 122. For example, the modified light beam 132 may have a profile in which the intensity of the modified light beam 132 varies continuously as a function of position in the X-Y plane and without regions that do not include light, such as the example shown in FIG. 1C. In some implementations, the modified light beam 132 is formed from discrete components that are separated from each other such that the profile of the modified light beam 132 includes regions that substantially lack light. An example of such a modified light beam is shown in FIG. 2C. Regardless, the profile of the modified light beam 132 is different from the profile of the light beam 106 due to the interaction between the modulation element 122 and the light beam 106.

[0042] The modified light beam 132 irradiates the target 140. The intensity profile of the modified light beam 132 shown in FIG. 1C interacts with the target 140 at the line C-C’ (FIG. 1D). Because the spatial profile of the modified light beam 132 is different than the profile of the light beam 106, the modified light beam 132 interacts with the target 140 in a different manner than the light beam 106 would. For example, as compared to the light beam 106, the modified light beam 132 provides relatively more light to portions l40a and l40c near the outer edge of the target 140 than to the portion l40b near the center of the target 140.

[0043] As discussed below, using the modulation element 122 allows the spatial profile of the light that interacts with the target 140 to be tailored to consume more of the target material in the target 140. This, in turn, increases the amount of EUV light produced from the interaction between the modified light beam 132 and the target 140 and/or results in more effective preparation of the target 140 prior to interaction with a separate light beam that converts the target material in the target 140 to the plasma that emits EUV light. Moreover, by increasing the amount of target material that is consumed, using the modified light beam 132 also decreases the debris produced by an interaction between the target 140 and a light beam.

[0044] Furthermore, the modulation element 122 may be configured to cause the modified light beam 132 to have a profile that is optimized based on known, assumed, or estimated properties of the target. For example, the target 140 may be known to contain more target material near a lower edge of the target 140 than in the center or near the top edge. For such a target, the modulation element 122 may be configured to produce the modified light beam having a spatial profile such as shown in FIG. 1C. [0045] Only a single light beam 106 is illustrated in the example of FIG. 1A. However, the light source 100 may use more than one light beam, and each of the more than one light beams may interact with different forms of the target 140. For example, the light source 100 may use one or more“pre -pulse” light beams that shape, change the density, or otherwise modify one or more properties of the target 140 (without necessarily producing the plasma that emits EUV light) to produce a modified target or an intermediate target. These implementations also may use a“main pulse” light beam that has an energy sufficient to convert the target material in the modified or intermediate target to the plasma that emits EUV light. FIGS. 3, 4, and 6 show examples of implementations of the EUV light source 100 that use more than one pulsed light beam. Any or all of the light beams used in the EUV light source 100 may interact with the modulation element 122 to modify the spatial profile of that light beam. In implementations in which the light source 100 uses more than one light beam, the light source 100 may include more than spatial modulation device 120. For example, in these implementations, the light source 100 may include a separate spatial modulation device 120 positioned to interact with each of the more than one light beams.

[0046] Referring to FIG. 2A, a side view of an EUV light source 200 is shown. The EUV light source 200 is an example of an implementation of the EUV light source 100 (FIG. 1A).

[0047] The EUV light source 200 includes a modulation device 220. The modulation device 220 includes a modulation element 222 that spatially modulates the light beam 106 to produce a modified light beam 232 that includes components 233 (FIG. 2B), each of which is also a light beam. The components 233 are separated from each other such that a region of no light (or significantly reduced light) is between each component and its neighboring component or components. The modified light beam 233 includes many individual components, and the individual components are collectively referred to as the components 233. Components 233a, 233b, 233c, 233d, 233e are shown in FIG. 2A. The modulation element 222 may be a diffraction grating. In these implementations, each component 233a, 233b, 233c, 233d, 233e is a diffractive order. The component 233c may be the zeroth order that is not diffracted by the modulation element 222. In these implementations, the component 233c propagates in generally the same direction as the light beam 106. In the example of FIG. 2A, each of the components 233a, 233b, 233c, 233d, 233e propagate away from the diffraction grating in different directions. [0048] The modified light beam 232 also includes other components that, with the components 233a, 233b, 233c, 233d, and 233e, form a two-dimensional grid pattern in the X-Y plane at the target region 242. FIG. 2B shows the target region 242 in the X-Y plane. In FIG. 2B, the solid circles represent components 233 that are in the target region 242. In the example of FIG. 2B, all of the components 233 interact with a respective portion of the target 140. The element labeled 243 represents a portion that interacts with the component 233a. For simplicity, only the one portion 243 is labeled in FIG. 2B. However, other of the components 233 interact with other portions of the target 140. The portion 243 is illustrated as a circular region on the target 140. However, the interaction between the component 233a and the target 140 can impact parts of the target 140 other than what is labeled as 243 in FIG. 2B, and the portion 243 is not necessarily a circular region.

[0049] FIG. 2C is a plot of the intensity of the components 233a-233e of the modified light beam 232 as a function of position along the direction X. In the example of FIG. 2C, the intensity of components 233a-233e vary. In other implementations, the modulation element 222 is a diffractive element that produces diffractive orders of equal intensities. For example, in these implementations, the modulation element 222 may be a Dammann grating.

[0050] The interaction between the components 233 and the target 140 produces a modified target 245 (FIG. 2D). The modified target 245 has modified regions 244 that are formed by interacting the components 233 with the target 140. For simplicity, only the interaction between target material in the portion 243 and the component 233a is discussed, and only one modified region 244 is labeled. However, other of the components 233 interact with target material in other portions of the target 140 in a similar manner and form other modified regions. FIG. 2D shows the other modified regions as dashed circular areas.

[0051] The interaction between the target material in the portion 243 and the component 233a changes a physical characteristic of the portion 243. For example, the interaction may change a geometric distribution of the portion 243 by removing some of the target material from the portion 243 to thereby form a recessed region. In this example, the modified region 244 is the recessed region. The recessed region is a region that lacks target material. The recessed region may be a void. The target material may be removed by, for example, ablation, ejection, and/or conversion into a plasma that does not emit EUV light or emits only minimal EUV light. The recessed region may be a hole, pocket, or opening in the modified target 245. The recessed region may pass through the modified target 245. Moreover, the recessed region may have any shape. For example, the recessed region may be a cone or rectangular slit that extends into the modified target 245 but does not pass all the way through the modified target 245. The characteristics (for example, shape, depth, and cross-section) of the recessed region depend on the intensity and diameter of the component 233a and on the properties of the target material in the portion 243. The interaction between the component 233a and the portion 243 may change the characteristics of the portion 243 in other ways. For example, the interaction may reduce the density of the portion 243. In this example, the modified region 244 is a region of reduced density that may include target material.

[0052] As discussed above, although only one modified region 244 is labeled in FIG. 2D, other modified regions are formed to produce the modified target 245. The various modified regions on the modified target 245 may have different characteristics from each other.

[0053] Regardless of how the interaction changes the specific characteristics of the portion 243 (and other portions that are not labeled), the interaction between the components 233 and the target 140 forms the modified target 245, which is more easily converted into the plasma that emits EUV light. For example, forming recessed regions results in the modified target 245 having a greater surface area than the target 140. The larger surface area corresponds to a greater amount of target material that is exposed to an incident light beam, thereby allowing more target material to be converted into the plasma that emits EUV light.

[0054] The two-dimensional grid pattern that the components 233 form in FIG. 2B is just one possible pattern that may be formed in the target region 242. Other patterns may be produced depending on the configuration and characteristics of the modulation element 222. FIGS. 2E-2G show examples of components that are in other patterns. FIG. 2E includes components 233_E that are concentric rings separated by regions of no light. The components 233_E may be formed, for example, in implementations in which the modulation element 222 is a circular aperture. FIG. 2F shows components 233_F that are arranged in a one-dimensional array. In the example of FIG. 2F, the components 233F have a rectangular cross-section. FIG. 2G illustrates yet another example of an arrangement of components 233_G. The components 233_G are represented as solid circles. FIG. 2G shows the target region 142 in the X-Y and X-Z planes. In the example of FIG. 2G, the components 233_G propagate generally along the Z direction and the -X direction. Thus, the components 233_G arrive in the target region 142 from more than one direction and interact with the target 140 at surfaces in the X-Y plane and in the X-Z plane.

[0055] FIG. 3 is a block diagram of an EUV light source 300. The EUV light source 300 is an example of an implementation of the light source 100 of FIG. 1 A. The EUV light source 300 includes a first light- generation module 305a and a second light- generation module 305b. The light- generation module 305a emits a first light beam 306a onto a beam path 307a, and the light- generation module 305b emits a second light beam 306b onto a beam path 307b. The first light beam 306a is used to form a modified light beam 332a. The modified light beam 332a interacts with a target 340 to form a modified target 345 but generally does not form a plasma that emits EUV light (or forms a plasma that emits only a small or negligible amount of EUV light). The first light beam 306a may be referred to as a“pre-pulse” light beam. The second light beam 306b is a light beam has energy sufficient to convert target material in the modified target 345 to a plasma that emits EUV light 399. The second light beam 306b may be referred to as a“main pulse” light beam or a heating light beam. The first light-generation module 305a and/ the second light- generation module 305b are controlled such that, for a particular pair of pre-pulse and main pulse where the pre-pulse forms the modified target 345 and the main pulse converts that modified target 345 to the plasma that emits EUV light, the pre-pulse occurs prior to main pulse.

[0056] The light- generation module 305b may be, for example, a carbon dioxide (C0₂) laser, and the wavelength of the second light beam 306b may be, for example, 10.59 microns (pm).

The first light- generation module 305a may be, for example, a solid state laser, such as an erbium-doped fiber (Englass) laser or a Q-switched Nd:YAG laser. In these implementations, the wavelength of the first light beam 306a may be, for example, 1.06 pm. In some

implementations, the first light generation module 305a and the second light-generation module 305b are the same type of optical source. For example, the first and second light- generation modules 305a, 305b may both be C0₂ lasers. In these implementations, the first and second light beams 306a, 306b may have the same spectral content. For example, both the first and second light beams 306a, 306b may have a wavelength of 10.59 pm. In yet a further example, both the first and second light generation modules 305a, 305b may be solid state lasers. In these implementations, both the first and second light beams 306a, 206b may have a wavelength of, for example, 1.06 pm. [0057] In some implementations, the same type of optical source is used for the first and second light- generation modules 305a, 305b, but the spectral content of the first and second light beams 306a, 306b differs. For example, the first and second light- generation modules 305a,

305b may be implemented as a single module that includes two C0₂ seed laser subsystems and one amplifier. One of the seed laser subsystems produces the first light beam 306a at a wavelength of, for example, 10.26 pm, and the other seed laser subsystem produces the second light beam 306b at a wavelength of, for example, 10.59 pm. These two wavelengths may come from different lines of the C0₂ laser.

[0058] Moreover, wavelengths other than the examples provided above may be used. For example, either or both of the first light beam 306a and the second light beam 306b may have a wavelength of less than 1 pm. Using a relatively short wavelength (such as a wavelength of less than 1 pm) may be advantageous in some circumstances. For example, a relatively short wavelength enables a smaller focus size, allowing for improved control of beam shaping.

[0059] The first light beam 306a interacts with a modulation element 322 to produce the modified light beam 332a. The modulation element 322 is any optical component or collection of components that is capable of spatially modulating the first light beam 306a. The modified light beam 332a has a different spatial profile than the first light beam 306a. The spatial profile of the modified light beam 332a depends on the configuration of the modulation element 322.

The modified light beam 332a may have a spatial profile that varies continuously as a function of position (for example, as shown in FIG. 1C), or the modified light beam 332a may include components of light that are separated by a region of no light (for example, as shown in FIGS.

2B and 2E-2G). In implementations in which the spatial profile varies continuously as a function of position, the components are not discrete and may be considered to be any portion of the spatial profile.

[0060] The EUV light source 300 also includes a beam combiner 324 that is positioned to direct the modified light beam 332a and the second light beam 306b toward a beam transport system 325. The beam combiner 324 is any optical element or collection of optical elements that is capable of interacting with the modified light beam 332a and the second light beam 306b. For example, the beam combiner 324 may include one or more mirrors and/or one or more beam splitters, some of which are positioned to direct the modified light beam 332a toward the beam transport system 325 and others of which are positioned to direct the second beam 306b toward the beam transport system 325. In implementations in which the modified light beam 332a and the second light beam 306b have different spectral content, the beam combiner 324 may be a dichroic element (such as a dichroic beam splitter) that is configured to transmit the wavelengths in the second light beam 306b and reflect the wavelengths in the modified light beam 332a. In the example of FIG. 3, the beam combiner 324 directs the modified light beam 332a and the second light beam 306b toward the beam transport system 325 on spatially separate beam paths.

[0061] The beam transport system 325 also includes a focusing system 326. The focusing system 326 includes any combination of optical elements arranged to focus the modified light beam 332a and the second light beam 306b. For example, the focusing system 326 may include lenses and/or mirrors. The modified light beam 332a is focused at or near an initial target region 342a, and the second light beam 306b is focused at or near a modified target region 342b. In the example illustrated in FIG. 3, the focusing system 326 focuses the modified light beam 332a and the second light beam 306b even though these beams do not follow the same beam path through the focusing system 326. However, in some implementations, the optical elements that focus the modified light beam 332a are separate from the optical elements that focus the second light beam 306b. For example, separate optical components may be used when the spectral content of the modified light beam 306a is different from that of the second light beam 306b.

[0062] The initial target region 342a receives a target 340 from a target material supply system 350. In the example of FIG. 3, the target 340 is a spherical droplet of molten metal. The components in the modified light beam 332a interact with the target 340 to form a modified target 345. The interaction between the modified light beam 332a and the target 340 changes one or more properties of the target 340. For example, the modified target 345 may have recessed regions and/or regions of reduced density such as discussed with respect to the modified target 245 of FIG. 2D. The modified target 345 travels to the modified target region 342b and interacts with the second light beam 306b. The interaction between the second light beam 306b converts at least some of the target material in the modified target 345 to a plasma that emits EUV light 399.

[0063] FIG. 4 is a block diagram of an EUV light source 400. The EUV light source 400 is another example of an implementation of the EUV light source 100. The EUV light source 400 is similar to the EUV light source 300, except the EUV light source 400 uses two“pre-pulse” light beams to produce a modified target 445. [0064] The EUV light source 400 includes light-generation modules 405a, 405b, and 405c.

The light- generation module 405a emits a first light beam 406a. The light- generation module 405b emits a second light beam 406b. The light-generation module 405c emits a third light beam 406c. The light- generation module 405c may be, for example, a C0₂ laser. All of the light beams 405a, 405b, 405c may have the same spectral content, or the spectral content of at least one of the light beams 405a, 405b, 405c may be different from the other light beams. The light beams 405a and 405b may be two different emission lines of the C0₂ laser. The emission lines of a C0₂ laser include, for example, light at 9.4 pm, 10.26 pm, and 10.59 pm. In some implementations, either of the light beam 405a or the light beam 405b is a beam formed by an emission line of a C0₂ laser at 10.26 pm. In these implementations, the other of the light beams 405a and 405b is a beam having a wavelength of 1.06 pm produced by a solid-state laser (such as, for example, a Q-switched Nd:YAG laser). In other implementations, both the light beam 405a and the light beam 405b are produced by a solid-state laser.

[0065] The first and second light beams 406a, 406b change one or more physical properties of a target 440 to produce the modified target 445. In the implementation shown in FIG. 4, the first light beam 406a interacts with the target 440 to spatially expand of the target 440 and form an intermediate target 447. The intermediate target 447 may be a disk- shaped segment of molten metal that has a greater extent along the X axis (which includes the X direction and a -X direction that is opposite the X direction) than the target 440. Additionally, the intermediate target 447 has a smaller extent along the Z axis than the target 440. The intermediate target 447 moves in the X direction.

[0066] The EUV light source 400 also includes a modulation element 422 that is positioned to interact with the second light beam 406b. The modulation element 422 is any optical element or a collection of elements capable of spatially modulating the second light beam 406b. For example, the modulation element 422 may be similar to the modulation element 122 discussed with respect to FIG. 1A or the modulation element 222 discussed with respect to FIG. 2A. The interaction between the modulation element 422 and the second light beam 406b produces a modified light beam 432b. The modified light beam 432b has a different spatial profile than the second light beam 406b. The modified light beam 432b and the first light beam 406a are directed toward a focusing system 425a by a beam combiner 424. The beam combiner 424 may be any optical element or collection of optical elements that is able to direct the modified light beam 432b and the first light beam 406a toward the focusing system 425a. The focusing system 425a focuses the first light beam 406a at or near a target region 442a, which receives the target 440 from a target supply system 450, and focuses the modified light beam 432b at or near a target region 442b, which receives the intermediate target 447 from the target region 442a.

[0067] The intermediate target 447 and the modified light beam 432b interact at the target region 442b to form the modified target 445. In the example shown, the interaction between the modified light beam 432b and the intermediate target 447 forms recessed regions 444 on the modified target 445. After interacting with the modified light beam 432b, the modified target 445 moves into a target region 442c, which receives the third light beam 406c. The third light beam 406c is focused by a focusing system 426c and has an energy sufficient to convert at least some of the target material in the modified target 445 to plasma that emits EUV light. The recessed regions 444 may cause more of the target material in the modified target 445 to be converted to the plasma. Thus, as compared to a target that lacks the recessed regions 444 (such as the target 440 or the intermediate target 447), more EUV light and less debris may be produced by the interaction between the modified target 445 and the third light beam 406c.

[0068] FIG. 5 is a block diagram of an EUV light source 500. The EUV light source 500 is another example of an implementation of the EUV light source 100 of FIG. 1A. In the EUV light source 500, a single light beam is used to produce a modified target 545 and to form a plasma that emits EUV light from the modified target 545.

[0069] The EUV light source 500 includes a light- generation module 505. The light- generation module 505 may be, for example, a C0₂ laser. The light- generation module 505 emits a light beam 506 onto a beam path 507 toward a modulation device 520. The modulation device 520 includes a modulation element 522. The modulation element 522 is any optical element that is capable of spatially modulating the light beam 506 into components that do not all have the same intensity. The modulation element 522 may be similar to the modulation element 222 (FIG. 2A). An interaction between the modulation element 522 and the light beam 506 modulates the light beam 506 and produces components 533a-533g. The components 533a- 533g are separated from each other by regions that do not have light.

[0070] The components 533a-533g propagate away from the modulation element 522 in different directions. The components 533e-533g propagate toward a beam dump 528 and do not leave the modulation device 520. The components 533a-533c propagate toward a target region 542a, which receives a target 540. The target 540 may be a spherical droplet provided by a target material supply system, such as the system 450 of FIG. 4, or the target 540 may be an intermediate target (such as the intermediate target 447 of FIG. 4) that is produced by a prior interaction with another light beam. The components 533a-533c have an intensity sufficient to modify a property of the target 540 without producing the plasma that emits EUV light. Thus, regardless of the form of the target 540, an interaction between the components 533a-533c produces a modified target 545. In the example of FIG. 5, three recessed regions 544 are illustrated. Each recessed region 544 is formed from the interaction between one of the components 533a-533c and the target 540.

[0071] The component 533d propagates toward a focusing system 525. The focusing system 525 focuses the component 533d near or at a modified target region 542b, which receives the modified target 545. The focusing system 525 also includes an optical delay 529 that causes the component 533d to arrive at the modified target region 542b at a time after the components 533a-533c and at a time when the modified target 545 is in the modified target region 542b. The optical delay 529 may be, for example, an arrangement that includes a plurality of reflective elements (such as mirrors) that fold the component 533d into many passes in a relatively compact volume. Such an optical delay 529 may cause the component 533d to travel an additional few hundred meters such that a delay of a few hundred nanoseconds may be achieved.

[0072] The component 533d has a greater intensity than the components 533a-533c, and the component 533d has an energy that is sufficient to convert at least some of the target material in the modified target 545 to the plasma that emits EUV light. For example, the total energy of the components may be about 2 kiloWatts (kW), whereas the energy in the component 533d may be greater than 100 kW.

[0073] FIG. 6 is a block diagram of an EUV light source 600. The EUV light source 600 is another example of an implementation of the EUV light source 100. The EUV light source 600 is similar to the EUV light source 300 (FIG. 3), except the EUV light source 600 includes a modulation device 620 that spatially modulates a portion of the second light beam 306b instead of modulating the first light beam 306a. In the implementation shown in FIG. 6, a target region 642 receives a target 640 from the target material supply 350. FIG. 7 is an illustration of the target region 642 over time. FIG. 8 is an illustration of intensity of light in the target region 642 over the time scale of FIG. 7. [0074] The beam combiner 324 directs the first light beam 306a toward the target 640. An interaction between the first light beam 306a and the target 640 forms an intermediate target 647. The second light beam 306b passes through the beam combiner 324 and interacts with the modulation device 620 to form a single pulse of light 604. The modulation device 620 includes a temporal modulation device 662 and a spatial modulation element 622. The spatial modulation element 622 is a dynamic modulation element that is controllable by a controller 660. The temporal modulation device 662 is also controllable by the controller 660. The controller 660 may include an electronic processor and an electronic memory or storage. The electronic processor may store instructions, perhaps as a computer program, that instruct the processor to perform actions. For example, the electronic processor may generate signals that, when provided to the modulation device 620, the modulation element 622, and/or the temporal modulation device 662 by the controller 660, cause the modulation device 620, the modulation element 622, and/or the temporal modulation device 662 to perform particular actions.

[0075] The temporal modulation device 662 is any optical element that is capable of controlling a temporal profile (intensity as a function of time) of the second light beam 306b.

For example, the temporal modulation device 662 may be an electro-optic modulator (EOM).

The temporal modulation device 662 is controlled to form the pulse of light 604, which includes a pedestal 608 and a heating portion 609, from the second light beam 605b. The pedestal 608 and the heating portion 609 are shown in FIG. 8. The pedestal 608 is temporally connected to the heating portion 609 such that the pedestal 608 and the heating portion 609 are part of a single pulse of light 604 (FIGS. 6 and 8).

[0076] The temporal profile of the pedestal 608 may have any shape. For example, the intensity of the pedestal 608 may increase and decrease over time, such that the pedestal 608 has a shape somewhat similar to a pulse. The example of FIG. 8 illustrates an example of such a pedestal. In other examples, the intensity of the pedestal 608 may increase and decrease over time without having a profile that resembles a pulse. In yet other examples, the pedestal 608 may have an intensity that monotonically increases until the heating portion 609 begins.

Regardless of the shape of the pedestal 608, the pedestal 608 and the heating portion 609 together form the single pulse 604. That is, there is not a region without light between beginning of the pulse 604 and the end of the pulse 604. [0077] The spatial modulation element 622 is controlled such that only the pedestal 608 is spatially modulated. Thus, the spatial profile of the pedestal 608 is changed. In some implementations, the pedestal focus size in the x-y plane is modulated to achieve different intensities. For example, the focus size may be modulated to heat the outer rim of the target before heating material located closer to the center of the target.

[0078] As shown in FIGS. 7 and 8, the pedestal 608 arrives in the target region 642 before the heating portion 609. The spatially modulated pedestal 608 interacts with the intermediate target 647 and forms the modified target 645. The heating portion 609 arrives in the target region 642 after the pedestal 608. The heating portion 609 has a much greater intensity than the pedestal 608 and is able to convert the target material in the modified target 645 to plasma that emits EUV light 399. Thus, the EUV light source 600 uses two separate pulses, the first light beam 306a and the second light beam 306b. However, the second light beam 306b is acted upon by the modulation device 620 such that the second light beam 306b produces the pulse 608, which modifies the intermediate target 647 and converts the modified target 645 to plasma that emits EUV light 399.

[0079] FIGS. 9 and 10 discuss an example of an EUV photolithography system 900. The EUV light generated by any of the EUV light sources 100, 200, 300, 400, 500, and 600 may be used with the photolithography system 900. Moreover, a system that includes any of the EUV light sources 100, 200, 300, 400, 500, and 600 also may include a photolithography system such as the photolithography system 900. FIG. 11 discusses an example of an EUV optical source. The EUV light sources 100, 200, 300, 400, 500, and 600 may include additional components and systems such as discussed with respect to FIGS. 10 and 11. For example, the EUV light sources 100, 200, 300, 400, 500, and 600 include a vacuum chamber such as the vacuum chamber 1130 discussed with respect to FIG. 11.

[0080] FIG. 9 schematically depicts a lithographic apparatus 900 including a source collector module SO according to one implementation. The lithographic apparatus 900 includes:

• an illumination system (illuminator) IL configured to condition a radiation beam B (for example, EUV radiation).

• a support structure (for example, a mask table) MT constructed to support a patterning device (for example, a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; • a substrate table (for example, a wafer table) WT constructed to hold a substrate (for example, a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and

• a projection system (for example, a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (for example, including one or more dies) of the substrate W.

[0081] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0082] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0083] The term“patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0084] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase- shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0085] The projection system PS, like the illumination system IL, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0086] As here depicted, the apparatus is of a reflective type (for example, employing a reflective mask). The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such“multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0087] Referring to FIG. 9, the illuminator IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, for example, xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line- emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in FIG. 9, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, for example, EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a carbon dioxide (C0₂) laser is used to provide the laser beam for fuel excitation.

[0088] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source collector module with the aid of a beam delivery system including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0089] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator IL may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0090] The radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (for example, 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 aid of the second positioner PW and position sensor PS2 (for example, an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, for example, so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (for example mask) MA with respect to the path of the radiation beam B. Patterning device (for example mask) MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0091] The depicted apparatus could be used in at least one of the following modes:

1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (that is, a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (that is, a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (for example, mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0092] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0093] FIG. 10 shows an implementation of the lithographic apparatus 900 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum

environment can be maintained in an enclosing structure 1020 of the source collector module SO. The systems IL and PS are likewise contained within vacuum environments of their own. An EUV radiation emitting plasma 2 may be formed by a laser produced LPP plasma source.

The function of source collector module SO is to deliver EUV radiation beam 20 from the plasma 2 such that it is focused in a virtual source point. The virtual source point is commonly referred to as the intermediate focus (IF), and the source collector module is arranged such that the intermediate focus IF is located at or near an aperture 1021 in the enclosing structure 1020. The virtual source point IF is an image of the radiation emitting plasma 2.

[0094] From the aperture 1021 at the intermediate focus IF, the radiation traverses the illumination system IF, which in this example includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called“fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA (as shown by reference 1060). Upon reflection of the beam 21 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the substrate table WT. To expose a target portion C on substrate W, pulses of radiation are generated while substrate table WT and patterning device table MT perform synchronized movements to scan the pattern on patterning device MA through the slit of illumination.

[0095] Each system IF and PS is arranged within its own vacuum or near-vacuum

environment, defined by enclosing structures similar to enclosing structure 1020. More elements than shown may generally be present in illumination system IF and projection system PS.

Further, there may be more mirrors present than those shown. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in FIG. 10.

[0096] Considering source collector module SO in more detail, a laser energy source including a laser 1023 is arranged to deposit laser energy 1024 into a fuel that includes a target material. The target material may be any material that emits EUV radiation in a plasma state, such as xenon (Xe), tin (Sn), or lithium (Li). The plasma 2 is a highly ionized plasma with electron temperatures of several lO's of electron volts (eV). Higher energy EUV radiation may be generated with other fuel materials, for example, terbium (Tb) and gadolinium (Gd). The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near-normal incidence collector 3 and focused on the aperture 1021. The plasma 2 and the aperture 1021 are located at first and second focal points of collector CO, respectively.

[0097] Although the collector 3 shown in FIG. 10 is a single curved mirror, the collector may take other forms. For example, the collector may be a Schwarzschild collector having two radiation collecting surfaces. In an embodiment, the collector may be a grazing incidence collector which comprises a plurality of substantially cylindrical reflectors nested within one another.

[0098] To deliver the fuel, which, for example, is liquid tin, a droplet generator 1026 is arranged within the enclosure 1020, arranged to fire a high frequency stream 1028 of droplets towards the desired location of plasma 2. In operation, laser energy 1024 is delivered in a synchronism with the operation of droplet generator 1026, to deliver impulses of radiation to turn each fuel droplet into a plasma 2. The frequency of delivery of droplets may be several kilohertz, for example 50 kHz. In practice, laser energy 1024 is delivered in at least two pulses: a pre pulse with limited energy is delivered to the droplet before it reaches the plasma location, in order to vaporize the fuel material into a small cloud, and then a main pulse of laser energy 1024 is delivered to the cloud at the desired location, to generate the plasma 2. A trap 1030 is provided on the opposite side of the enclosing structure 1020, to capture fuel that is not, for whatever reason, turned into plasma.

[0099] The droplet generator 1026 comprises a reservoir 1001 which contains the fuel liquid (for example, molten tin) and a filter 1069 and a nozzle 1002. The nozzle 1002 is configured to eject droplets of the fuel liquid towards the plasma 2 formation location. The droplets of fuel liquid may be ejected from the nozzle 1002 by a combination of pressure within the reservoir 1001 and a vibration applied to the nozzle by a piezoelectric actuator (not shown).

[0100] As the skilled reader will know, reference axes X, Y, and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 20, 21, 26. At each part of the apparatus, a local reference frame of X,

Y and Z axes may be defined. In the example of FIG. 10, the Z axis broadly coincides with the direction optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source collector module, the X axis coincides broadly with the direction of fuel stream 1028, while the Y axis is orthogonal to that, pointing out of the page as indicated in Figure 10. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram FIG. 10, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0101] Numerous additional components used in the operation of the source collector module and the lithographic apparatus 900 as a whole are present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector 3 and other optics. Other features present but not described in detail are all the sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithographic apparatus 900.

[0102] Referring to FIG. 11, an implementation of an LPP EUV optical source 1100 is shown. The optical source 1100 may be used as the source collector module SO in the lithographic apparatus 900. Furthermore, the light- generation module 105 of FIGS. 1A and 2A, the light- generation module 505 of FIG. 5, the light- generation module 305b of FIG. 3, or the light- generation module 405b of FIG. 4 may be part of the drive laser 1115. The drive laser 1115 may be used as the laser 1023 (FIG. 10).

[0103] The LPP EUV optical source 1100 is formed by irradiating a target mixture 1114 at a plasma formation location 1105 with an amplified optical beam 1110 that travels along a beam path toward the target mixture 1114. The plasma formation location 1105 is within an interior 1107 of a vacuum chamber 1130. When the amplified optical beam 1110 strikes the target mixture 1114, a target material within the target mixture 1114 is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 1114. These characteristics may include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma.

[0104] The optical source 1100 also includes the supply system 1125 that delivers, controls, and directs the target mixture 1114 in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The target mixture 1114 includes the target material such as, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the element tin may be used as pure tin (Sn); as a tin compound, for example, SnBr₄, SnBr₂, SnH₄; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. The target mixture 1114 may also include impurities such as non-target particles. Thus, in the situation in which there are no impurities, the target mixture 1114 is made up of only the target material. The target mixture

1114 is delivered by the supply system 1125 into the interior 1107 of the chamber 1130 and to the plasma formation location 1105.

[0105] The optical source 1100 includes a drive laser system 1115 that produces the amplified optical beam 1110 due to a population inversion within the gain medium or mediums of the laser system 1115. The optical source 1100 includes a beam delivery system between the laser system

1115 and the plasma formation location 1105, the beam delivery system including a beam transport system 1120 and a focus assembly 1122. The beam transport system 1120 receives the amplified optical beam 1110 from the laser system 1115, and steers and modifies the amplified optical beam 1110 as needed and outputs the amplified optical beam 1110 to the focus assembly 1122. The focus assembly 1122 receives the amplified optical beam 1110 and focuses the beam 1110 to the plasma formation location 1105.

[0106] In some implementations, the laser system 1115 may include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system 1115 produces an amplified optical beam 1110 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 1115 may produce an amplified optical beam 1110 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 1115. The term“amplified optical beam” encompasses one or more of: light from the laser system 1115 that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system 1115 that is amplified and is also a coherent laser oscillation.

[0107] The optical amplifiers in the laser system 1115 may include as a gain medium a filling gas that includes C0₂ and may amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 800 times. Suitable amplifiers and lasers for use in the laser system 1115 may include a pulsed laser device, for example, a pulsed, gas-discharge C0₂ laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, lOkW or higher and high pulse repetition rate, for example, 40 kHz or more. The pulse repetition rate may be, for example, 50 kHz. The optical amplifiers in the laser system 1115 may also include a cooling system such as water that may be used when operating the laser system 1115 at higher powers.

[0108] The optical source 1100 includes a collector mirror 1135 having an aperture 1140 to allow the amplified optical beam 1110 to pass through and reach the plasma formation location 1105. The collector mirror 1135 may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation location 1105 and a secondary focus at an intermediate location 1145 (also called an intermediate focus) where the EUV light may be output from the optical source 1100 and may be input to, for example, an integrated circuit lithography tool (not shown). The optical source 1100 may also include an open-ended, hollow conical shroud 1150 (for example, a gas cone) that tapers toward the plasma formation location 1105 from the collector mirror 1135 to reduce the amount of plasma- generated debris that enters the focus assembly 1122 and/or the beam transport system 1120 while allowing the amplified optical beam 1110 to reach the plasma formation location 1105. For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation location 1105. [0109] The optical source 1100 may also include a master controller 1155 that is connected to a droplet position detection feedback system 1156, a laser control system 1157, and a beam control system 1158. The optical source 1100 may include one or more target or droplet imagers 1160 that provide an output indicative of the position of a droplet, for example, relative to the plasma formation location 1105 and provide this output to the droplet position detection feedback system 1156, which may, for example, compute a droplet position and trajectory from which a droplet position error may be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system 1156 thus provides the droplet position error as an input to the master controller 1155. The master controller 1155 may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 1157 that may be used, for example, to control the laser timing circuit and/or to the beam control system 1158 to control an amplified optical beam position and shaping of the beam transport system 1120 to change the location and/or focal power of the beam focal spot within the chamber 1130.

[0110] The supply system 1125 includes a target material delivery control system 1126 that is operable, in response to a signal from the master controller 1155, for example, to modify the release point of the droplets as released by a target material supply apparatus 1127 to correct for errors in the droplets arriving at the desired plasma formation location 1105.

[0111] Additionally, the optical source 1100 may include optical source detectors 1165 and 1170 that measures one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The optical source detector 1165 generates a feedback signal for use by the master controller 1155. The feedback signal may be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production.

[0112] The optical source 1100 may also include a guide laser 1175 that may be used to align various sections of the optical source 1100 or to assist in steering the amplified optical beam 1110 to the plasma formation location 1105. In connection with the guide laser 1175, the optical source 1100 includes a metrology system 1124 that is placed within the focus assembly 1122 to sample a portion of light from the guide laser 1175 and the amplified optical beam 1110. In other implementations, the metrology system 1124 is placed within the beam transport system 1120. The metrology system 1124 may include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that may withstand the powers of the guide laser beam and the amplified optical beam 1110. A beam analysis system is formed from the metrology system 1124 and the master controller 1155 since the master controller 1155 analyzes the sampled light from the guide laser 1175 and uses this information to adjust components within the focus assembly 1122 through the beam control system 1158.

[0113] Thus, in summary, the optical source 1100 produces an amplified optical beam 1110 that is directed along the beam path to irradiate the target mixture 1114 at the plasma formation location 1105 to convert the target material within the mixture 1114 into plasma that emits light in the EUV range. The amplified optical beam 1110 operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system 1115. Additionally, the amplified optical beam 1110 may be a laser beam when the target material provides enough feedback back into the laser system 1115 to produce coherent laser light or if the drive laser system 1115 includes suitable optical feedback to form a laser cavity.

[0114] Other implementations are within the scope of the claims. The modulation elements discussed above may be implemented to produce any type of pattern on the target. For example, the modulation element 222 may produce a different pattern of components than what is shown in the example of FIG. 2A. In some implementations, the modulation element 222 is

implemented such that all of the components are on one side of the zeroth order. Moreover, the modulation element 522 also may be implemented in this manner such that all of the components other than the component 533d propagate toward the target region 542a and the beam dump 528 is not needed.

[0115] Other aspects of the invention are set out in the following numbered clauses.

1. A system comprising:

a spatial modulation device configured to interact with a light beam to create a modified light beam, the modified light beam comprising a spatial pattern of light that has a non-uniform intensity along a direction that is perpendicular to a direction of propagation of the modified light beam, the spatial pattern of light comprising one or more components of light; and a target supply system configured to provide a target to a target region, the target comprising target material that, when in a plasma state, emits EUV light, wherein the target region overlaps with the beam path such that at least some of the one or more components of light in the modified beam interact with a portion of the target.

2. The system of clause 1, wherein the spatial modulation device comprises a diffractive optical element.

3. The system of clause 2, wherein the diffractive optic comprises a spatial light modulator (SLM), an adaptive optic, a reticle, and/or a grating.

4. The system of clause 1, wherein the spatial modulation device comprises a refractive optical element.

5. The system of clause 4, wherein the spatial modulation device comprises a lens, a lenslet array, and/or a reticle.

6. The system of clause 1, wherein the spatial pattern of light comprises two or more components of light, and each of the two or more components of light has substantially the same intensity.

7. The system of clause 1, wherein the spatial pattern of light comprises two or more components of light arranged in a rectilinear grid.

8. The system of clause 2, wherein the spatial modulation device comprises at least one Dammann grating.

9. The system of clause 1, further comprising:

a first light generation module configured to emit the light beam; and

a second light generation module configured to emit a second light beam.

10. The system of clause 1, wherein the spatial modulation device is further configured to interact with a second light beam to create a second modified light beam, the second modified light beam comprising a second spatial pattern of light that has a non-uniform intensity along a direction that is perpendicular to a direction of propagation of the second modified light beam, the second spatial pattern of light comprising one or more second components of light.

11. A method of forming a target for an extreme ultraviolet (EUV) light source, the method comprising:

directing a light beam onto a beam path; interacting the light beam with a spatial modulation device positioned on the beam path to form a modified light beam, the modified light beam comprising a spatial pattern of light that has a non-uniform intensity along the direction that is perpendicular to a direction of propagation of the modified light beam, the spatial pattern of light comprising one or more components of light; and

interacting the modified light beam with a target that comprises target material that emits EUV light when in a plasma state, wherein at least some of the one or more components of light of the spatial pattern interacts with a region of the target to modify a property of that region of the target.

12. The method of clause 11, wherein the property comprises a density, and modifying the property comprises decreasing the density.

13. The method of clause 11, wherein the spatial pattern of light comprises two or more components of light.

14. The method of clause 13, wherein all of the components of light have the same intensity.

15. The method of clause 13, wherein the components of light are arranged in a grid, and the regions of the target that interact directly with a component of light are arranged in a grid.

16. The method of clause 11, further comprising interacting the modified beam with a focusing assembly prior to interacting the modified light beam with the target.

17. The method of clause 13, wherein the components of light are spatially separated and spatially discrete such that a portion of the target between any two components of light does not interact with any of the components in the modified light beam.

18. The method of clause 11, further comprising:

interacting an initial target with a second light beam to form an altered target, the altered target having a greater extent in a first direction than the initial target and a smaller extent in a second direction than the initial target, the first and second directions being orthogonal to each other, and wherein

interacting the modified light beam with a target that comprises target material that emits EUV light when in a plasma state comprises interacting the modified light beam with the altered target, and each of the one or more components of light interacts with a region of the altered target to modify a property of that region of the altered target. 19. The method of clause 18, further comprising, after interacting the modified light beam with the altered target, interacting the altered target with a third light beam, the third light beam having an energy sufficient to convert at least some of the target material in the altered target to the plasma that emits EUV light.

20. The method of clause 11, further comprising, after interacting the target with the modified light beam, interacting the target with another light beam, the other light beam having an energy sufficient to convert at least some of the target material in the second altered target to the plasma that emits EUV light.

21. The method of clause 20, wherein the property comprises a conversion efficiency related to an amount of EUV light emitted and the energy of the other light beam, and modifying the property of a portion of the target comprises increasing the conversion efficiency associated with the entire target.

22. The method of clause 20, wherein the light beam and the other light beam are temporally connected and part of a single pulse of light.

23. The method of clause 11, wherein the property comprises a surface area of the target, modifying the property of any portion of the target comprises increasing the surface area of the entire target.

24. The method of clause 23, wherein an amount of increase of the surface area is related to a number of light components in the modified light beam.

25. The method of clause 11, wherein interacting the modified light beam with a target that comprises target material that emits EUV light when in a plasma state comprises interacting the modified light beam with a target is that has a substantially spherical shape.

[0116] Other implementations are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A system comprising:

a spatial modulation device configured to interact with a light beam to create a modified light beam, the modified light beam comprising a spatial pattern of light that has a non-uniform intensity along a direction that is perpendicular to a direction of propagation of the modified light beam, the spatial pattern of light comprising one or more components of light; and

a target supply system configured to provide a target to a target region, the target comprising target material that, when in a plasma state, emits EUV light, wherein the target region overlaps with the beam path such that at least some of the one or more components of light in the modified beam interact with a portion of the target.

2. The system of claim 1, wherein the spatial modulation device comprises a diffractive optical element.

3. The system of claim 2, wherein the diffractive optic comprises a spatial light modulator (SLM), an adaptive optic, a reticle, and/or a grating.

4. The system of claim 1, wherein the spatial modulation device comprises a refractive optical element.

5. The system of claim 4, wherein the spatial modulation device comprises a lens, a lenslet array, and/or a reticle.

6. The system of claim 1, wherein the spatial pattern of light comprises two or more components of light, and each of the two or more components of light has substantially the same intensity.

7. The system of claim 1, wherein the spatial pattern of light comprises two or more components of light arranged in a rectilinear grid.

8. The system of claim 2, wherein the spatial modulation device comprises at least one Dammann grating.

9. The system of claim 1, further comprising:

a first light generation module configured to emit the light beam; and

a second light generation module configured to emit a second light beam.

10. The system of claim 1, wherein the spatial modulation device is further configured to interact with a second light beam to create a second modified light beam, the second modified light beam comprising a second spatial pattern of light that has a non-uniform intensity along a direction that is perpendicular to a direction of propagation of the second modified light beam, the second spatial pattern of light comprising one or more second components of light.

directing a light beam onto a beam path;

interacting the light beam with a spatial modulation device positioned on the beam path to form a modified light beam, the modified light beam comprising a spatial pattern of light that has a non-uniform intensity along the direction that is perpendicular to a direction of propagation of the modified light beam, the spatial pattern of light comprising one or more components of light; and

12. The method of claim 11, wherein the property comprises a density, and modifying the property comprises decreasing the density.

13. The method of claim 11, wherein the spatial pattern of light comprises two or more components of light.

14. The method of claim 13, wherein all of the components of light have the same intensity.

15. The method of claim 13, wherein the components of light are arranged in a grid, and the regions of the target that interact directly with a component of light are arranged in a grid.

16. The method of claim 11, further comprising interacting the modified beam with a focusing assembly prior to interacting the modified light beam with the target.

17. The method of claim 13, wherein the components of light are spatially separated and spatially discrete such that a portion of the target between any two components of light does not interact with any of the components in the modified light beam.

18. The method of claim 11, further comprising:

interacting the modified light beam with a target that comprises target material that emits EUV light when in a plasma state comprises interacting the modified light beam with the altered target, and each of the one or more components of light interacts with a region of the altered target to modify a property of that region of the altered target.

19. The method of claim 18, further comprising, after interacting the modified light beam with the altered target, interacting the altered target with a third light beam, the third light beam having an energy sufficient to convert at least some of the target material in the altered target to the plasma that emits EUV light.

20. The method of claim 11, further comprising, after interacting the target with the modified light beam, interacting the target with another light beam, the other light beam having an energy sufficient to convert at least some of the target material in the second altered target to the plasma that emits EUV light.

21. The method of claim 20, wherein the property comprises a conversion efficiency related to an amount of EUV light emitted and the energy of the other light beam, and modifying the property of a portion of the target comprises increasing the conversion efficiency associated with the entire target.

22. The method of claim 20, wherein the light beam and the other light beam are temporally connected and part of a single pulse of light.

23. The method of claim 11, wherein the property comprises a surface area of the target, modifying the property of any portion of the target comprises increasing the surface area of the entire target.

24. The method of claim 23, wherein an amount of increase of the surface area is related to a number of light components in the modified light beam.

25. The method of claim 11, wherein interacting the modified light beam with a target that comprises target material that emits EUV light when in a plasma state comprises interacting the modified light beam with a target is that has a substantially spherical shape.