EUV excitation light source and EUV light source

Abstract

The present invention relates to an EUV excitation light source, wherein the EUV excitation light source comprises a laser source (B) configured to emit a laser beam (1) — the laser beam (1) comprises two partial beams (1', 1") having different wavelengths (λ1, λ2) — a separating optical element (C) for separating the two partial beams (1', 1") of the laser beam (1) into two separated beams (2', 2"), and a superposition unit (D) for superimposing the two separated beams (2', 2") at a predefined superposition position (3) at a predefined superposition angle (α), wherein the separating optical element is a first reflection diffraction grating (C). The present invention also relates to an EUV light source (S) comprising an EUV excitation light source (A) as described above, a vacuum chamber (T) comprising an irradiation area (4), and a focusing unit (U) for focusing the first separated beam (2') and the second separated beam (2") in the irradiation area (4).

Description

Technology Field

[0001] The present invention relates to an EUV excitation light source comprising a laser source configured to emit a laser beam—the laser beam comprises two partial beams having different wavelengths—a separating optical element for separating the two partial beams of the laser beam into two separate beams, and a superposition unit for superimposing the two separate beams at a predefined superposition position and a predefined superposition angle. The present invention also relates to an EUV light source comprising such an EUV excitation light source. Background Technology

[0002] The trend toward much smaller structures in the semiconductor industry necessitates the use of light sources with much shorter wavelengths in the microlithographic fabrication of structures. Extreme ultraviolet (EUV) light is generally defined as electromagnetic radiation with wavelengths ranging from about 10 to about 120 nanometers. In microlithography, EUV light refers more specifically to a wavelength range of about 5 to 20 nanometers. Unlike current industry standards that generally use light with a wavelength of 193 nanometers, the use of EUV light enables the stable fabrication of much smaller structures, thereby improving device performance accordingly.

[0003] One method of generating EUV light is based on irradiating a material such as tin with a high-intensity laser. Irradiation causes a plasma (so-called laser-generated plasma, LPP) to form, and this plasma emits light in the EUV band. As described in US8681427B2, a droplet stream of liquefied irradiated material may be formed, and each droplet may be struck sequentially by two laser pulses. First, the droplet is struck by a first laser pulse, so-called a "pre-pulse," which deforms, expands, and / or vaporizes the droplet. Subsequently, the droplet is struck by a second laser pulse, so-called a "main pulse," to form a plasma that emits EUV light.

[0004] When the droplet moves, the first laser pulse must be focused at a first position, and the second laser pulse must be focused at a second position. Generally, the two pulses are focused using the same focusing optical unit. This means that the two pulses must overlap at an overlapping position at or near the focusing optical unit. Subsequently, different focusing positions can be obtained by selecting an appropriate overlap angle of the first and second laser pulses at the overlapping position.

[0005] However, generally, the first and second laser pulses emitted by the laser source propagate collinearly along the same beam path, that is, they form two partial beams of the laser beam emitted by the laser source. Therefore, it is necessary to separate the two partial beams before focusing them to corresponding focusing positions.

[0006] To separate the main pulse and the free pulse, the EUV excitation source described in US8681427B2 has a dichroic beam splitter module. The dichroic beam splitter module consists of a dichroic element configured to pass the free pulse but reflect the main pulse. Subsequently, the reflected main pulse is further reflected through two mirrors from the first dichroic element to the second dichroic element, and then to the irradiation position.

[0007] A similar EUV excitation source is described in US9129717B2. In this source, the free pulse and the main pulse are separated by a beam splitter and superimposed on a superimposing mirror, and the superimposing mirror operates in a transmission mode for laser beams striking one side of the mirror and in a reflection mode for laser beams striking the other side. The problem to be solved

[0008] The present invention aims to provide a cost-effective EUV excitation light source suitable for high laser output, and to provide an EUV light source having such an EUV excitation light source. means of solving the problem

[0009] The first aspect of the present invention relates to an EUV excitation light source as described in the introduction, wherein the separating optical element is a (first) reflection diffraction grating.

[0010] According to this embodiment, a laser beam comprising two partial beams having different wavelengths is incident on a separating optical element in the form of a first reflection diffraction grating. The diffraction grating is a diffraction optical element having a periodic structure that diffracts light into one or more diffraction orders due to interference effects. Since these interference effects depend on wavelength, light having different wavelengths is diffracted at different angles in the diffraction grating. Thus, two partial beams of a laser beam having different wavelengths are separated into two separate beams in the diffraction grating. Subsequently, these separated beams can be manipulated through an overlapping unit to overlap at a predefined overlap position and at a predefined overlap angle.

[0011] According to this embodiment, a reflective optical element in the form of a reflective diffraction grating is used as a separating element rather than a partially transmitted optical element as described, for example, in US8681427B2 and US9129717B2. This is advantageous because the use of transmitted optical elements in EUV excitation sources has several disadvantages. For example, to avoid blocking the beam path, the transmitted optical element can only be cooled through a small area at its edge. This fact reduces the cooling efficiency of areas further from the edge and limits the scaling of the maximum laser output depending on their diameter. This increases the cost of substrate manufacturing, and the application of coatings, contact elements, and holders must be designed to ensure efficient cooling, which is difficult to achieve at laser outputs in the kW range.

[0012] Furthermore, for the laser wavelengths currently used in light sources, the substrates of the transmission optical elements are generally made of very expensive diamond. In addition, the manufacturing cost of diamond substrates increases disproportionately with the diameter of the substrate.

[0013] In contrast, in a reflection diffraction grating, the diffracted beam is reflected and does not pass through the grating. This offers several advantages compared to using transmission dichroic optical elements, particularly when operating at high laser powers, such as the typical output used in EUV excitation sources of approximately 10 kW or more. First, the fabrication of a reflection diffraction grating is generally simpler and less expensive than that of a transmission dichroic optical element. Second, since reflection diffraction gratings can be cooled from the back, they can be cooled more effectively. This allows for operation at smaller apertures, ultimately reducing costs. Third, because reflection diffraction gratings can be cooled more effectively, they are less susceptible to damage from the laser beam, which increases their lifespan and enables operation at higher laser powers.

[0014] It is advantageous for the superposition unit to include only reflective optical elements, for example, one or more reflective diffraction gratings and / or one or more mirrors.

[0015] In one embodiment, the overlapping unit includes a second reflection diffraction grating, two separated beams are incident on the second reflection diffraction grating, and the separated beams reflected by the second reflection diffraction grating are overlapped at an overlapping angle at the overlapping position.

[0016] As with the case of separated optical elements, it is advantageous to use reflective optical elements in the superposition unit. One method of superimposing separated beams is based on a second reflective diffraction grating. Two separated beams are incident on the second reflective diffraction grating, diffracted at different angles, and superimposed at the superposition location. By appropriately selecting the distance between the first reflective diffraction grating and the second reflective diffraction grating, the distance between the second reflective diffraction grating and the superposition location can be set to a desired value. By appropriately selecting the grating period for the first diffraction grating and the second diffraction grating, the superposition angle can be set to a desired value.

[0017] In one developmental form, the overlapping unit includes a tilting unit configured to tilt a second diffraction grating to adjust the overlapping position and / or overlapping angle.

[0018] By tilting the second diffraction grating using a tilting unit, the angle at which the separated beam is incident on the second grating can be changed. This changes the overlap angle, which can be continuously adjusted in this manner. At the same time, this change in the angle of incidence results in a change in the overlap position and a change in the alignment of the separated beam. These effects may be undesirable and can be corrected by an additional deflection mirror.

[0019] In an alternative developmental form, the first grating period of the first reflection diffraction grating is the same as the second grating period of the second reflection diffraction grating, the surface normal of the first reflection diffraction grating is parallel to the surface normal of the second reflection diffraction grating, the superposition unit further includes a third reflection diffraction grating, the separated beams reflected by the second reflection diffraction grating are incident on the third reflection diffraction grating, and the separated beams reflected by the third reflection diffraction grating are superimposed at a superposition angle at the superposition location.

[0020] Another method for superimposing separated beams is based on a superposition unit comprising second and third reflection diffraction gratings. The second reflection diffraction grating is selected such that its grating period is equal to the grating period of the first reflection diffraction grating. Additionally, the surface normal of the second reflection diffraction grating is parallel to the surface normal of the first reflection diffraction grating. If the same diffraction order is used for the first and second reflection diffraction gratings, this means that the separated beams reflected by the second reflection diffraction grating are parallel. Then, superposition of the separated beams can be achieved through the third reflection diffraction grating.

[0021] In this form of development, the angle of incidence of the laser beam on the first reflection diffraction grating can be set to zero so that the laser beam strikes the first reflection diffraction grating parallel to the surface normal. In this case, the second reflection diffraction grating and, if possible, the third reflection diffraction grating are aligned parallel to the first reflection diffraction grating.

[0022] In an additional developmental form, the overlapping unit includes a tilting unit configured to tilt a third diffraction grating to adjust the overlapping angle and / or overlapping position.

[0023] By tilting the third diffraction grating using a tilting unit, the overlap angle can be continuously adjusted. Any undesirable change in the overlap position and / or the alignment of the separated beams resulting therefrom can be corrected by an additional deflection mirror.

[0024] In one embodiment, the overlap unit includes an adjustment unit for adjusting the optical path length between the first reflection diffraction grating and the second reflection diffraction grating to adjust the overlap position.

[0025] By using an adjustment unit to adjust the optical path length between the first reflection diffraction grating and the second reflection diffraction grating, the spatial separation between the first separated beam and the second separated beam on the second diffraction grating can be adjusted. In this way, the spatial separation between the first partial beam and the second partial beam on the last diffraction grating within the beam path, and thus the distance between the last diffraction grating and the overlap position, can be continuously adjusted.

[0026] In one developmental form, an adjustment unit for adjusting the optical path length between a first reflective diffraction grating and a second reflective diffraction grating includes a first pair of mirrors and a second pair of mirrors, and the adjustment unit is configured to adjust the optical path length by changing the distance between the first pair of mirrors and the second pair of mirrors. The adjustment unit can be designed in a traditional optical delay line manner, which allows the optical path length to be adjusted, for example, by changing the distance between the first pair of mirrors and the second pair of mirrors forming a retroreflector.

[0027] Using this traditional optical delay line, the separated beam is first incident on the first mirror of the first pair of two mirrors and is reflected from the first mirror of the retroreflector. Subsequently, the separated beam retroreflected from the second mirror of the retroreflector is incident on the second mirror of the first pair of mirrors and is reflected from thereon by the second reflection diffraction grating. Subsequently, the optical path length between the first reflection diffraction grating and the second reflection diffraction grating is adjusted by changing the distance between the retroreflector and the two mirrors. This change in distance can be achieved, for example, through a motorized or piezo-actuated linear translational stage. Using such an adjustment unit, in addition to changing the spatial distance between the two separated beams on the second diffraction grating, the angle between the two partial beams is not changed while the optical path length is changed.

[0028] In an alternative embodiment, the overlapping unit includes a first mirror and a second mirror, a first separated beam is incident on the first mirror and a second separated beam is incident on the second mirror, and the first mirror and the second mirror are arranged to overlap the separated beams reflected by the mirrors at an overlapping angle at an overlapping position.

[0029] Another method for superimposing separated beams is based on a superposition unit containing two (or more) mirrors rather than one or more reflective diffraction gratings (and possibly one or more mirrors). Compared to using a second reflective diffraction grating, this is a particularly simple and cost-effective solution. However, the distance or spatial separation between the separated beams is determined by their wavelength separation and the optical path length between the first reflective diffraction grating and the two mirrors. Since the first separated beam must be incident on the first mirror while the second separated beam must be incident on the second mirror, a specific minimum distance is required between the two separated beams striking the first and second mirrors. Due to this fact, this approach is limited to laser beams containing partial beams with sufficiently separated wavelengths or large distances between the first diffraction grating and the mirrors. The longer the optical path length, the smaller the required wavelength separation becomes.

[0030] In an additional developmental form, the superposition unit includes an extension unit for extending the optical path length between the first reflection diffraction grating and the first and second mirrors.

[0031] By extending the optical path length between the first reflection diffraction grating and the two mirrors, the required wavelength separation can be reduced. The corresponding extension unit may include two or more additional mirrors. For example, two separated beams may be incident on a mirror, reflected from another mirror, and reflected back and forth between these mirrors multiple times, so that their optical path lengths can be substantially extended.

[0032] In one developmental form, the overlapping unit includes a tilting unit configured to tilt the first mirror and / or second mirror to adjust the overlapping position and / or overlapping angle.

[0033] By using a tilting unit to tilt the first mirror and / or the second mirror, the overlap position and overlap angle can be continuously adjusted. However, without any additional optical element, these amounts generally cannot be adjusted independently. Any change in one or both of the tilting angles causes a change in both the overlap position and the overlap angle.

[0034] In one embodiment, the overlap unit further includes at least one deflection mirror for deflecting two separate beams to adjust the overlap position and a tilting unit for tilting the deflection mirror.

[0035] A tilting unit used to tilt the reflection diffraction grating or the first and / or second mirrors enables adjustment of the overlap position and the overlap angle, but complete independent control over both of these is generally achieved only when the overlap unit additionally includes a deflection mirror and a tilting unit for tilting the deflection mirror. In this case, two separate beams are incident on the deflection mirror. By tilting the deflection mirror, the overlap position can be adjusted, while the distance between the deflection mirror and the overlap position, as well as the overlap angle, remains constant.

[0036] In one embodiment, at least one of the first, second, and third reflection diffraction gratings is a blazed grating or a resonant waveguide grating.

[0037] In traditional diffraction gratings, the laser output is dispersed among different diffraction beams corresponding to different diffraction orders. This fact generally leads to a reduction in the efficiency of EUV excitation sources when using traditional diffraction gratings.

[0038] Blazed gratings are a well-established specific type of diffraction grating. Blazed gratings have a sawtooth cross-section, which causes near-optimal concentration of laser output of the desired diffraction order.

[0039] Resonant waveguide gratings are a more recent form of development capable of achieving the same purpose of focusing a very large portion of the laser output of a desired diffraction order. Their operating principle relies on resonance resulting from interaction with pseudo-induction modes (see, for example, paper “Radially polarized 3 kW beam from a CO2 laser with an intracavity resonant grating mirror”, MA Ahmed et al., Optics Letters, 32(13), 1824-1826).

[0040] In additional embodiments, at least one of the first, second, and third reflection diffraction gratings comprises a substrate made of a material selected from the group consisting of Cu, SiC, Si, and diamond. These (and other) materials have high thermal conductivity and can effectively dissipate heat. Furthermore, the manufacturing process for forming a reflection diffraction grating based on these substrate materials is relatively simple. For example, the diffraction grating can be formed using a lithography process on a substrate made of SiC or Si. When Cu is used as the substrate material, the diffraction grating can be created directly on the substrate through diamond turning. When diamond is used as the substrate material, the diffraction grating is generally applied in the form of a coating. Using diamond as the substrate material is advantageous because (synthetic) diamond has very high thermal conductivity and can be cooled through the entire back surface when used as the substrate for a reflection diffraction grating. Therefore, when using a diamond substrate, the aperture or diameter of the reflection optical element can be significantly reduced, thereby reducing the installation space of the optical assembly. In this way, production costs can be reduced and the availability of the optical element can be increased.

[0041] According to a second embodiment, this objective is achieved by an EUV light source having such an EUV excitation light source, a vacuum chamber including an irradiation area, and a focusing unit for focusing a first partial beam and a second partial beam in the irradiation area. The EUV light source further includes a target material delivery system that generally delivers a drop of a target material, such as tin, to the irradiation area. The first partial beam (pre-pulse) and the second partial beam (main pulse) are generally focused at different focusing positions in the irradiation area to irradiate a drop of the target material delivered to the vacuum chamber by the target material delivery system. To focus the partial beams in the irradiation area, the first and second partial beams are superimposed at an overlapping position in or near the focusing unit, which may include one or more reflective or transmissive optical elements.

[0042] The spatial separation of the two focusing positions of the irradiation area is generally very small, for example, in the range of several hundred micrometers, for example, from 100 μm to 300 μm. Based on the typical path length of the partial beam from the overlapping position to the irradiation area, the overlap angle at the overlapping position is also generally very small, typically less than 50 mrad, less than 30 mrad, or less than 3 mrad.

[0043] Further advantages of the present invention may be discovered from the description and drawings. Likewise, the features mentioned above and the features mentioned below may be used independently, or some of them may be used in any desired combination. The embodiments illustrated and described should not be interpreted as an exhaustive list, but rather are of an exemplary nature for illustrating the present invention. Brief explanation of the drawing

[0044] In the drawings: FIG. 1 illustrates a schematic diagram of an EUV excitation light source having a laser source, a separated optical element in the form of a first reflection diffraction grating, and a superposition unit including one reflection diffraction grating. FIG. 2 shows a schematic diagram of an EUV excitation light source similar to that shown in FIG. 1, and the superposition unit includes two reflection diffraction gratings. FIG. 3 illustrates a schematic diagram of an EUV excitation light source similar to that shown in FIG. 1, and the superposition unit includes two mirrors. Figure 4 illustrates a schematic diagram of an EUV light source having such an EUV excitation light source. Specific details for implementing the invention

[0045] In the following drawing descriptions, the same references are used for identical or functionally equivalent components.

[0046] FIG. 1 illustrates an EUV excitation source (A). The EUV excitation source (A) comprises a laser source (B), a separating optical element (C), and an overlapping unit (D). The laser source (B) is designed to emit a laser beam (1) comprising two partial beams (1', 1") having different wavelengths (λ1, λ2). The two partial beams (1', 1") of the laser beam (1) are separated into two separate beams (2', 2") using the separating optical element (C). The two separate beams (2', 2") are overlapped using the overlapping unit (D) at a predefined (desired) overlapping position (3) at a predefined (desired) overlapping angle (α). The overlapping angle (α) is typically very small, for example, less than 50 mrad, 30 mrad, or 3 mrad.

[0047] The laser source (B) is designed to generate two partial beams (1', 1') having different wavelengths (λ1, λ2) by using a single laser resonator or two laser resonators. In either case, the partial beams (1', 1') forming the laser beam (1) overlap and propagate collinearly along the same beam path when striking the separating optical element (C). In this example, the laser source (B) is a CO2 laser source that generates a first partial beam (1') having a first wavelength (λ1) of 10.59 μm and a second partial beam (1") having a second wavelength (λ2) of 10.207 μm. In addition to the laser source (B) of the EUV excitation light source (A), other types of laser sources, such as solid-state laser sources or fiber laser sources, may also be used.

[0048] A laser beam (1) comprising two partial beams (1', 1") having different wavelengths (λ1, λ2) is incident on a separating optical element forming a first reflection diffraction grating (C). According to the grating formula, the wavelength ( Light containing ) is diffracted at the following angles:

[0049]

[0050] Here, d1 is the grating period of the first diffraction grating (C), m is the diffraction order, and in this example where the laser beam (1) has two wavelengths (λ1, λ2), a = 1, 2. Thus, different wavelengths (λ a Light having ) is at different angles (β) at the first diffraction grating (C). a It is diffracted into ). This wavelength dependence is used to split two partial beams (1', 1") of the laser beam (1) into two separate beams (2', 2").

[0051] The angular separation of the two separated beams (2', 2") diffracted from the first diffraction grating (C) is given as follows.

[0052]

[0053] The superposition unit (D) includes a second reflection diffraction grating (E) into which separated beams (2', 2") are incident. If L1 is the distance between the first reflection diffraction grating (C) and the second reflection diffraction grating (E), the angular separation (δ1) of the two separated beams (2', 2") results in the following spatial separation of the two separated beams (2', 2") in the second reflection diffraction grating (E).

[0054]

[0055] In the second reflection diffraction grating (E), the two separated beams (2', 2") are again at different angles according to the following equation ( 1, 2) It is diffracted:

[0056]

[0057] Here, d2 is the grating period of the second diffraction grating (E), n is the diffraction order, and γ a θ and θ are the respective angles of incidence, and a = 1, 2. Condition γ a = β a This applies when the first reflection diffraction grating (C) and the second reflection diffraction grating (E) are arranged parallel to each other, that is, when the surface normal (n1) of the first reflection diffraction grating (C) and the surface normal (n2) of the second reflection diffraction grating (E) are arranged parallel to each other, as in the case of Fig. 1.

[0058] The second angle separation (δ2) of the two separated beams (2', 2") diffracted from the second diffraction grating (E) is given as follows.

[0059]

[0060] By appropriately selecting the grating period (d1) for the first diffraction grating (C) and the grating period (d2) for the second diffraction grating (E), the second angular separation (δ2) of the two separated beams (2', 2") diffracted from the second diffraction grating (E) can be set to a desired value. The two separated beams (2', 2") overlap at an overlap position (3) with an overlap angle (α) (i.e., α = δ2) that corresponds to the second angular separation (δ2).

[0061] The distance between the second reflection diffraction grating (E) and the superposition position (3) is given as follows.

[0062]

[0063] Accordingly, by appropriately selecting the distance (L1) between the first reflection diffraction grating (C) and the second reflection diffraction grating (E), the distance (L2) between the second reflection diffraction grating (E) and the overlap position (3) can be set to a desired value. Thus, two partial beams (1', 1") can be overlapped at any distance (L2) from the second reflection diffraction grating (E).

[0064] The superposition unit (D) further includes a tilting unit (F) for tilting the second reflection diffraction grating (E) to adjust the superposition position (3) and the superposition angle (α). Tilting the second reflection diffraction grating (E) is done to adjust the incident angles (γ) of the two separated beams (2', 2") to the second reflection diffraction grating (E). a It causes a change in ) so that they no longer coincide with the diffraction angles (β1, β2) in Equation 4. As is evident from Equation 4, the angle of incidence (γ a The change in ) is the diffraction angle ( a It causes a change in ). Consequently, the overlap angle (α) can be adjusted by tilting the second reflection diffraction grating (E). At the same time, tilting the second reflection diffraction grating (E) causes a change in the overall tilt and overlap position (3) of the two separate beams (2', 2").

[0065] As illustrated in FIG. 1, the superposition unit (D) is designed to align the first separated beam (2') at the superposition position (3) parallel to the laser beam (1) incident on the first reflection diffraction grating (C) when the second diffraction grating (E) is arranged parallel to the first diffraction grating (C). The tilt of the second diffraction grating (E) can be compensated and the parallel alignment of the first separated beam (2') with respect to the laser beam (1) can be maintained, and additional control over the superposition position (3) can be achieved using one or more (possibly tiltable) deflection mirrors not depicted herein.

[0066] FIG. 2 illustrates an EUV excitation light source (A) similar to that shown in FIG. 1, and the superposition unit (B) includes a third reflection diffraction grating (G) in addition to a second reflection diffraction grating (E). A laser beam (1) comprising two partial beams (1', 1") is incident on a separating optical element (C) forming a first reflection diffraction grating (C). The diffraction angle (β) at the first reflection diffraction grating (C) a Due to the wavelength dependence of ), different wavelengths (λ a Two partial beams (1', 1") having ) are separated into two separated beams (2', 2"). The two separated beams (2', 2") are incident on a second reflection diffraction grating (E), and they are at angles ( 1, 2) It is diffracted. The grating period (d2) of the second reflection diffraction grating (E) is the same as the grating period (d1) of the first reflection diffraction grating (C) (d1 = d2). . In addition, the second reflection diffraction grating (E) is arranged parallel to the first reflection diffraction grating (C), and this is condition γ a = β a and implies that the same diffraction orders are used (n = m). Consequently, the following result is derived from Equations 4 and 5:

[0067]

[0068] This means that the two separated beams (2', 2") reflected by the second reflection diffraction grating (E) are parallel to each other and parallel to the laser beam (1) incident on the first reflection diffraction grating (C). Thus, the spatial separation x of the two separated beams (2', 2") in the second reflection diffraction grating (E) is maintained.

[0069] Two separate beams (2', 2") reflected by the second reflection diffraction grating (E) are incident on the third reflection diffraction grating (G), and are diffracted again at different angles (ε1, ε2) according to the following equation.

[0070]

[0071] Here, d3 is the grating period of the third diffraction grating (G), and is the diffraction order, and ζ a θ and θ are the respective angles of incidence, and a = 1, 2.

[0072] After diffraction at the third diffraction grating (G), the two separated beams (2', 2") have an angle separation (δ3) given as follows.

[0073]

[0074] Next, the two separated beams (2', 2") are deflected by a deflection mirror (H) and overlapped at an overlap angle (α) (i.e., α = δ3) that corresponds to the third angle separation (δ3) at the overlap position (3).

[0075] The optical path length (L3) between the third reflection diffraction grating (G) and the overlap position (3) is given as follows.

[0076]

[0077] Accordingly, similar to the situation of the overlapping unit (D) illustrated in FIG. 1, by appropriately selecting the distance (L1) between the first reflective diffraction grating (C) and the second reflective diffraction grating (E), the optical path length (L3) between the third reflective diffraction grating (G) and the overlapping position (3) can be set to a desired value. This is conveniently achieved through an adjustment unit (I) for adjusting the optical path length (L1) between the first reflective diffraction grating (C) and the second reflective diffraction grating (E). In this example, the adjustment unit (I) includes a first mirror pair (J) and a second mirror pair (K). Then, the optical path length (L1) between the first reflective diffraction grating (C) and the second reflective diffraction grating (E) is adjusted by changing the distance between the first mirror pair (J) and the second mirror pair (K).

[0078] The superposition unit (D) further includes a tilting unit (F) for tilting a third reflection diffraction grating (G) having a function similar to the tilting unit (F) for tilting the second reflection diffraction grating (E) shown in FIG. 1.

[0079] An additional tilting unit (L) is used to tilt the deflection mirror (H). By tilting the deflection mirror (H), the overlap position (3) can be adjusted, and the overall tilt of the two separate beams (2', 2") can be compensated for or adjusted, while the optical path length (L3) between the third reflection diffraction grating (G) and the overlap position (3), as well as the overlap angle (α), remains constant. Generally, more additional deflection mirrors and corresponding tilting units may be used for this purpose.

[0080] In the example illustrated in FIG. 2, it will be understood that the adjustment unit (I) is not required. Additionally, the angle of incidence (γ) of the laser beam (1) on the first reflection diffraction grating (C) can be set to 0 (γ = 0) so that the laser beam (1) strikes the first reflection diffraction grating (C) parallel to the surface normal (n1). In this case, since both the second reflection diffraction grating (E) and the third reflection diffraction grating (G) can be arranged parallel to the first reflection diffraction grating (C), the surface normals (n1, n2, n3) of all three diffraction gratings (C, E, G) are aligned parallel, for example, in the horizontal direction in FIG. 2.

[0081] FIG. 3 illustrates an EUV excitation light source (A) similar to that shown in FIG. 1, and the superposition unit (D) includes a first mirror (M) and a second mirror (N). A laser beam (1) containing two partial beams (1', 1") is separated into two separated beams (2', 2") by a separating optical element (C) forming a first reflection diffraction grating (C). The first separated beam (2') is incident on the first mirror (M), and the second separated beam (2") is incident on the second mirror (N). After being reflected from each mirror (M, N), the two separated beams (2', 2") are superimposed at a superposition position (3) at a superposition angle (α).

[0082] By appropriately selecting the position and orientation of the first mirror (M) and the second mirror (N), the overlap position (3) and overlap angle (α) can be set to a desired value.

[0083] Since the first separated beam (2') is incident on the first mirror (M) and the second separated beam (2") is incident on the second mirror (N), a minimum spatial separation of two separated beams (2', 2") is required, which is at least equal to the diameter of the separated beams (2', 2") or the diameter of the laser beam (1). For example, if the beam diameter of the laser beam (1) is 30 mm and the distance between the first reflection diffraction grating (C) and the overlap position (3) is about 1 m, the separation angle (δ1) is about 30 mrad. If the distance between the first reflection diffraction grating (C) and the overlap position (3) is about 10 m, the separation angle (δ1) is about 3 mrad. Thus, in the embodiment of FIG. 3, a large distance between the first reflection diffraction grating (C) and the overlap position (3) and / or a relatively large overlap angle exceeding, for example, 10 mrad is advantageous.

[0084] Additionally, when the difference between the wavelength (λ1) of the first partial beam (1') and the wavelength (λ2) of the second partial beam (1") is small, a large optical path length between the first diffraction grating (C) and the mirrors (N, M) is advantageous, which can be achieved by using an extension unit (P) to extend the optical path length. In this example, the extension unit (P) includes a first additional mirror (Q) and a second additional mirror (R). Two separate beams (2', 2") are incident on the first additional mirror (Q) and reflected from the second additional mirror (R) before being incident on the mirrors (N, M), which causes an extension of the optical path length between the first diffraction grating (C) and the mirrors (N, M). A similar extension unit may be used to extend the beam path between the first mirror (M) and / or the second mirror (N) and the overlap position (3).

[0085] The overlapping unit (D) of FIG. 3 also includes a tilting unit (O). By using the tilting unit (O) to tilt the first mirror (M) and / or the second mirror (N), the overlapping position (3) and the overlapping angle (α) can be continuously adjusted as described above. High flexibility for adjusting the overlapping angle (α) is provided by tilting the first and second mirrors (M, N) individually.

[0086] The three diffraction gratings (C, E, G) illustrated in FIGS. 1 through 3 are preferably configured to maximize the ability to diffract to a desired diffraction order (m, n, etc.). Suitable types of diffraction gratings (C, E, G) that meet these requirements include blazed gratings and resonant waveguide gratings. Blazed gratings include structures having one or more blaze angles to provide nearly 100% diffraction efficiency to a desired diffraction order (see, for example, "https: / / en.wikipedia.org / wiki / Blazed_grating"). Resonant waveguide gratings can also be optimized for this purpose. In particular, when using wavelengths in the near-infrared (NIR) range, such as the light source described herein, diffraction efficiencies of (nearly) 100%, e.g. 99% or 98%, were observed. For example, in the paper by MA Ahmed et al. cited above, the suitability of a resonant wavelength grating for high laser powers exceeding 3 kW was demonstrated for a CO2 laser having an in-cavity resonant mirror configured to generate radially polarized laser radiation. In contrast, with a traditional diffraction grating, the laser power is dispersed among different diffraction beams corresponding to different diffraction orders. Therefore, the use of a traditional diffraction grating in an EUV excitation source (A) results in a loss of laser power that is not diffracted to the desired diffraction order.

[0087] Three diffraction gratings (C, E, G) are formed on a substrate made of a material suitable for high laser outputs exceeding 10 kW. Such materials generally have high thermal conductivity, which enables efficient cooling. Suitable materials include, in particular, Cu, SiC, Si, and (synthetic) diamond. It is advantageous to use diamond as a substrate material because (synthetic) diamond has very high thermal conductivity and can be cooled through the entire back surface when used as a substrate for the reflection diffraction gratings (C, E, G). Therefore, using a diamond substrate allows the diameter of the reflection diffraction gratings (C, E, G) to be significantly reduced compared to using a diamond substrate for the transmission optical element, thereby reducing the installation space of the EUV excitation light source.

[0088] FIG. 4 illustrates an EUV light source (S) having an EUV excitation light source (A) as illustrated in FIG. 1 through 3. The EUV light source (S) comprises a vacuum chamber (T) having an irradiation area (4) and a focusing unit (U). A first separated beam (2') and a second separated beam (2") are superimposed at an overlapping position (3) at an overlapping angle (α). A focusing unit (U), more specifically, a first optical element of the focusing unit (U), such as a mirror, is arranged at the overlapping position (3). The first separated beam (2') and the second separated beam (2") are focused using the focusing unit (U) in the irradiation area (4) at the first focusing position (4') and the second focusing position (4"), respectively. Although the focusing unit (U) is arranged inside the vacuum chamber (T) in FIG. 4, it is not necessarily so: the focusing unit (U) or at least a part of the focusing unit (U) may be arranged outside the vacuum chamber (T).

[0089] The EUV light source (S) further includes a target material delivery system (V), which delivers a drop of target material to an irradiation area (4). Each target material drop is first irradiated at a first focusing position (4') by a first separated beam (2'), and then irradiated at a second focusing position (4") by a second separated beam (2"). Upon irradiation of the target material drop, a plasma is formed and emits EUV radiation. The EUV radiation is collected and focused through an optical element (W) at the EUV focusing position (5). For example, the optical element (W) may be an elliptical mirror.

Claims

Claim 1 EUV excitation light source (A) comprising a laser source (B) configured to emit a laser beam (1) - said laser beam (1) comprises two partial beams (1', 1") having different wavelengths (λ1, λ2) - a separating optical element (C) for separating said two partial beams (1', 1") of said laser beam (1) into two separated beams (2', 2"), and a superposition unit (D) for superimposing said two separated beams (2', 2") at a predefined superposition position (3) at a predefined superposition angle (α), wherein said separating optical element is a first reflection diffraction grating (C). Claim 2 An EUV excitation light source according to claim 1, wherein the superposition unit (D) includes a second reflection diffraction grating (E), the two separated beams (2', 2") are incident on the second reflection diffraction grating (E), and the separated beams (2', 2") reflected by the second reflection diffraction grating (E) are superimposed at the superposition position (3) at the superposition angle (α). Claim 3 An EUV excitation light source according to claim 2, wherein the overlapping unit (D) comprises a tilting unit (F) configured to tilt the second reflection diffraction grating (E) to adjust at least one of the overlapping position (3) and the overlapping angle (α). Claim 4 An EUV excitation light source according to claim 2, wherein the first grating period (d1) of the first reflection diffraction grating (C) is the same as the second grating period (d2) of the second reflection diffraction grating (E), the surface normal (n1) of the first reflection diffraction grating (C) is parallel to the surface normal (n2) of the second reflection diffraction grating (E), the superposition unit (D) further includes a third reflection diffraction grating (G), the separated beams (2', 2") reflected by the second reflection diffraction grating (E) are incident on the third reflection diffraction grating (G), and the separated beams (2', 2") reflected by the third reflection diffraction grating (G) are superimposed at the superposition position (3) at the superposition angle (α). Claim 5 An EUV excitation light source according to claim 4, wherein the overlapping unit (D) comprises a tilting unit (F) configured to tilt the third reflection diffraction grating (G) to adjust at least one of the overlapping angle (α) and the overlapping position (3). Claim 6 An EUV excitation light source characterized in that, in any one of claims 2 to 5, the overlapping unit (D) includes an adjustment unit (I) for adjusting the optical path length between the first reflection diffraction grating (C) and the second reflection diffraction grating (E) to adjust the overlapping position (3). Claim 7 An EUV excitation light source according to claim 6, wherein the adjustment unit (I) for adjusting the optical path length between the first reflection diffraction grating (C) and the second reflection diffraction grating (E) comprises a first mirror pair (J) and a second mirror pair (K), and the adjustment unit (I) is configured to adjust the optical path length by changing the distance between the first mirror pair (J) and the second mirror pair (K). Claim 8 An EUV excitation light source according to claim 1, wherein the superposition unit (D) comprises a first mirror (M) and a second mirror (N), a first separated beam (2') is incident on the first mirror (M), a second separated beam (2") is incident on the second mirror (N), and the first mirror (M) and the second mirror (N) are configured to superimpose the separated beams (2', 2") reflected by the first and second mirrors (M, N) at the superposition position (3) at the superposition angle (α). Claim 9 An EUV excitation light source according to claim 8, wherein the superposition unit (D) comprises an extension unit (P) for extending the optical path length between the first reflection diffraction grating (C) and the first and second mirrors (M, N). Claim 10 EUV excitation light source according to claim 8 or 9, wherein the overlapping unit (D) comprises a tilting unit (O) configured to tilt at least one of the first mirror (M) and the second mirror (N) to adjust at least one of the overlapping position (3) and the overlapping angle (α). Claim 11 An EUV excitation light source according to any one of claims 1 to 5, 8, and 9, wherein the overlapping unit (D) further comprises at least one deflection mirror (H) for deflecting the two separated beams (2', 2") to adjust the overlapping position (3) and a tilting unit (L) for tilting the deflection mirror (H). Claim 12 An EUV excitation light source according to claim 4 or 5, wherein at least one of the first, second, and third reflection diffraction gratings (C, E, G) is a blazed grating or a resonant waveguide grating. Claim 13 An EUV excitation light source according to claim 4 or 5, wherein at least one of the first, second, and third reflection diffraction gratings (C, E, G) comprises a substrate made of a material selected from the group consisting of Cu, SiC, Si, and diamond. Claim 14 EUV light source (S), comprising an EUV excitation light source (A) according to any one of claims 1 to 5, 8, and 9, a vacuum chamber (T) including an irradiation area (4), and a focusing unit (U) for focusing a first separated beam (2') and a second separated beam (2") in the irradiation area (4).

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