EUV Collector Mirror

Abstract

The EUV collector mirror has a reflective surface (16) that reflects usable EUV light impinging on the reflective surface (16) from a source region (17) to subsequent EUV optics. The reflective surface (16) carries a pump light grating structure (19) that is designed to retro-reflect pump light (22) impinging on the pump light grating structure (19) from the source region (17) back to the source region (17). The pump light (22) has a wavelength that is offset from the wavelength of the usable EUV light. Such an EUV collector mirror allows for high conversion efficiency between the energy of the pump light of a laser discharge produced plasma (LDPP) EUV light source, on the one hand, and the resulting usable EUV energy, on the other hand.

Description

[Technical field]

[0001] The content of German patent application DE 10 2019 215 829.3 is incorporated by reference.

[0002] The present invention relates to an EUV collector mirror for use in an EUV projection exposure apparatus. [Background technology]

[0003] Illumination optical units using such collectors are known from DE 10 2013 002 064 A1 and US 2019 / 0094699. A collector mirror is known from US 10,101,569 B2. Another EUV collector mirror embodiment is known from DE 10 2019 200 698 A1.

[0004] The aim of the present invention is to develop an EUV collector mirror which allows a higher conversion efficiency between the energy of the pump light of a Laser Discharge Produced Plasma (LDPP) EUV light source on the one hand and the resulting usable EUV energy on the other hand. Summary of the Invention

[0005] According to the invention, this object is achieved by an EUV collector mirror comprising the features specified in claim 1.

[0006] In accordance with the present invention, it has been realized that a grating structure designed to retroreflect pump light emanating from the source region of an LDPP light source allows for high conversion efficiency of pump light energy to usable EUV light energy, which is then used back in the source region for conversion to usable EUV light.

[0007] In particular, such a retro-reflective pump light lattice structure of the EUV collector mirror can eliminate the need for a pre-pulse / main pulse scheme of the pump light source known from the prior art. This makes it easier to build a pump light laser source. In particular, the target of the LDPP light source, e.g. a tin droplet, does not need to be pre-shaped by a pre-pulse in order to increase the efficiency of converting the main pulse into usable EUV light. This leads to a reduction in the costs of building the source collector module (source collector module) containing the light source on the one hand and the EUV collector mirror on the other hand. The effectively used pump light energy compared to the conventional pre-pulse / main pulse scheme can be increased significantly, e.g. by about 50% or even 100%.

[0008] In particular, a spherical target can be used for impinging with the pump light without any need for further shaping.

[0009] The pump light grating structure may be a blazed grating. The blaze angle may be optimized for retroreflective diffusion of the pump light wavelengths of 0th or ±1st orders. The pitch of the pump light grating structure varies across the reflective surface of the EUV collector mirror to match the retroreflective properties to the incidence angle conditions of the pump light beam impinging on the pump light grating structure.

[0010] If the reflective surface of the EUV collector mirror is rotationally symmetric with respect to an axis of rotational symmetry, the pitch may increase with increasing distance of the grating structure to such axis of symmetry. Alternatively, in another embodiment, the pitch may decrease with increasing distance. In another embodiment, the dependence between the pitch and such distance of the grating structure to the axis of symmetry may be non-monotonic.

[0011] The pump light grating structure may include two or more different height levels, ie, may be realized as a two-level grating or as a multi-level grating.

[0012] Pump light wavelengths according to claims 2 and 3 have proven to be particularly suitable for efficiently generating usable EUV illumination light.

[0013] The pitch, e.g., grating period, according to claim 4 satisfies the retroreflection condition for the pump light wavelength. The pitch depends on the angle of incidence of the pump light on the reflecting surface. Such angle of incidence depends on the distance between the point where the respective pump light ray strikes the reflecting surface and an axis of rotational symmetry. As a result, the pitch depends on the distance of the reflecting surface area carrying the pump light grating structure to such axis of symmetry. If the distance to the axis of symmetry increases, the pitch can decrease.

[0014] Typical pitch dimensions may be in the range between 0.1 mm and 2 mm, particularly in the range between 0.2 mm and 1.0 mm, and more particularly in the range between 0.5 mm and 0.9 mm.

[0015] The reflectivity as claimed in claim 5 can be achieved by using a suitable pump light grating structure. The reflective surface of the EUV collector mirror can carry a highly reflective coating, which is of course optimized to be primarily highly reflective of the usable EUV illumination light. Such a reflective coating can also be optimized for the pump light wavelength of high reflection.

[0016] The advantages of the source collector module as claimed in claim 6 are as explained above for the EUV collector mirror.

[0017] The same applies to the pump light source recited in claim 7.

[0018] A pulse duration as claimed in claim 8 and / or a pulse rise time as claimed in claim 9 have been demonstrated to exhibit particularly high conversion efficiencies.

[0019] The advantages of the illumination optics as claimed in claim 10, the projection exposure apparatus as claimed in claim 11, the manufacturing method as claimed in claim 12 and the micro- and / or nanostructured components as claimed in claim 13 have been discussed above in relation to the EUV collector mirror, the source collector mirror module and the pump light source.

[0020] In particular, semiconductor components, such as memory chips, may be manufactured using projection exposure apparatus.

[0021] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. [Brief description of the drawings]

[0022] [Figure 1] 1 is a schematic diagram of a projection exposure apparatus for EUV microlithography; [Diagram 2] FIG. 2 is a meridional cross section of an EUV collector mirror including the optical path of the pump light coming from the pump light source. [Diagram 3] FIG. 1 is a close-up of a reflective surface showing a pump light grating structure designed to retro-reflect pump light impinging on the pump light grating structure from a source region back to the source region, with retro-reflecting pump light paths shown for +1 diffraction order and also for −1 diffraction order. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The projection exposure apparatus 1 for microlithography comprises a light source module 2 for EUV illumination light and / or imaging light 3, which will be described in more detail below. Such a light source module 2 is also designated light source collector module. The light source of the light source module 2 is an EUV light source, which generates light for example in a wavelength range between 5 nm and 30 nm, in particular between 5 nm and 15 nm. The illumination light and / or imaging light 3 is also referred to below as used EUV light.

[0024] In particular, the EUV light source may be a source with an EUV wavelength of 13.5 nm or a source with an EUV wavelength of 6.9 nm or 7 nm. Other EUV wavelengths are also possible. The beam path of the illumination light 3 is shown very diagrammatically in FIG.

[0025] An illumination optical unit 6 serves to guide illumination light 3 from a light source to an object field 4 in an object plane 5. Said illumination optical unit comprises a field plane mirror FF, which is shown very diagrammatically in Fig. 1, and a pupil plane mirror PF, which is arranged downstream in the beam path of the illumination light 3 and is also drawn very diagrammatically. A field-forming mirror for grazing incidence 6b (GI mirror) is arranged in the beam path of the illumination light 3 between the pupil plane mirror PF, which is arranged in a pupil plane 6a of the illumination optical unit, and the object field 4. Such a GI mirror 6b is not essential.

[0026] The pupil surface of the pupil plane mirror PF (not described in more detail) is part of a transfer optical unit, which transfers the field surface of the field plane mirror FF (also not described), in particular the images, superimposed on one another into the object field 4. An embodiment known from the prior art may be used for the field plane mirror FF on the one hand and the pupil plane mirror PF on the other hand. By way of example, such an illumination optical unit is known from DE 10 2009 045 096 A1.

[0027] Using a projection or imaging optical unit 7, the object field 4 is imaged to a defined scale into an image field 8 in an image plane 9. Projection optical units that can be used for this purpose are known, for example, from DE 10 2012 202 675 A1.

[0028] In order to facilitate the description of the projection exposure apparatus 1 and the various embodiments of the projection optical unit 7, a Cartesian xyz coordinate system is shown in the figures, from which the respective positional relationships of the components shown in the figures are clear. In figure 1, the x-direction runs perpendicular to the plane of the drawing, into the plane. The y-direction runs towards the left in figure 1 and the z-direction runs towards the top in figure 1. The object plane 5 runs parallel to the xy-plane.

[0029] The object field 4 and the image field 8 are rectangular. Alternatively, the object field 4 and the image field 8 can be in a bent or curved embodiment, i.e. in particular partially ring-shaped. The object field 4 and the image field 8 have an x / y aspect ratio greater than 1. The object field 4 therefore has a longer object field dimension in the x direction and a shorter object field dimension in the y direction. These object field dimensions extend along the field coordinates x and y.

[0030] One of the exemplary embodiments known from the prior art can be used for the projection optical unit 7. What is imaged as object in this case is a part of a reflective mask 10, also called a reticle, which coincides with the object field 4. The reticle 10 is held by a reticle holder 10a. The reticle holder 10a is moved by a reticle movement drive 10b.

[0031] Imaging by the projection optical unit 7 is realised on the surface of a substrate 11 in the form of a wafer which is held by a substrate holder 12. The substrate holder 12 is moved by a wafer or substrate movement drive 12a.

[0032] 1 shows diagrammatically a ray beam 13 of illumination light 3 entering the projection optical unit 7 between a reticle 10 and the projection optical unit 7, and a ray beam 14 of illumination light 3 emerging from the projection optical unit 7 between the projection optical unit 7 and a substrate 11. The image field-side numerical aperture (NA) of the projection optical unit 7 is not reproduced to scale in FIG.

[0033] The projection exposure apparatus 1 is of the scanner type. During operation of the projection exposure apparatus 1, both the reticle 10 and the substrate 11 are scanned in the Y direction. A stepper type projection exposure apparatus 1 is also feasible, in which a stepwise movement in the y direction of the reticle 10 and the substrate 11 is performed between the individual exposures of the substrate 11. These movements are performed synchronously with one another by suitable activation of the movement drives 10b and 12a.

[0034] Figure 2 shows a meridional section of an EUV collector mirror 15, which is part of the light source module 2. The reflecting surface 16 of the EUV collector mirror 15 serves to reflect the illumination light impinging on it from a source area 17, not shown in Figure 2, to the subsequent EUV illumination optical unit 6. For this purpose, the reflecting surface 16 has an elliptical shape that is rotationally symmetrical with respect to an optical axis 18. The source area 17 is arranged in one focal area of ​​the elliptical shape of the reflecting surface 16. At the other focal point of this elliptical shape, an intermediate focal point, not shown in Figure 2, is arranged, which serves to distinguish the usable EUV illumination light 3 from other wavelengths and also from debris.

[0035] 3 shows an enlarged cross-sectional view of the reflective surface 16 of the EUV collector mirror 15. The enlarged view shows, in schematic form, a pump-beam grating structure 19 that includes periodically alternating positive structures 20 ("peaks") and negative structures 21 ("valleys"). Such periodicity of the grating structure 19 is described by the grating pitch p.

[0036] The pump light grating structure 19 is designed to retro-reflect pump light 22 (compare also FIG. 2) impinging on the pump light grating structure 19 from the source region 17 back to the source region.

[0037] The pump light 22 is emitted from a pump light source 23, as shown in FIG. 2. The pump light source 23 is a CO2 laser light source that generates pump light having a wavelength of about 10 μm, for example, a pump light wavelength of 10.6 μm. Alternatively, the pump light source 23 may be a Nd-based solid-state laser light source, for example, a Nd:YAG laser, that generates pump light having a wavelength of about 1 μm, for example, a pump light wavelength of 1.064 μm. The wavelength of the pump light 22 is shifted from the wavelength of the illumination light 3, i.e., from the wavelength of the usable EUV light.

[0038] The pump light 22 is pulsed. The pump light source 23 is a MOPA (Master Oscillator Power Amplifier) ​​laser light source.

[0039] The parallel pump light 22 passes through a through hole 24 arranged on the reflecting surface 16 of the EUV collector mirror 15 and hits a tin droplet 25 arranged on the light source region 17 to generate illumination light 3 (not shown in Fig. 2). The optical path of the pump light 22 from the pump light source 23 to the light source region 17 is collinear with the optical axis 18 of the reflecting surface 16 of the EUV collector mirror 15.

[0040] The tin droplets 25 are spherical when struck by the pump light 22, ie, not pancake shaped.

[0041] A portion of the pump light 22 that strikes the tin droplet 25 is absorbed by the tin droplet 25. Another portion of the impinging pump light 22 is reflected from the tin droplet 25. The beam path of such reflected pump light 22 is shown in FIG. r Since the tin droplet 25 is spherical, the reflected pump beam 22 r The pump light impinges over its wide area on a reflective surface 16 carrying a pump light grating structure 19, shown diagrammatically in FIG.

[0042] FIG. 3 also illustrates one exemplary reflected pump beam 22. r The retroreflection state of such pump beam 22 is shown in FIG. r is applied to the reflecting surface 16, and θr The incident angle α is also written as θ r is the pump ray 22 on the reflecting surface 16. r 3 depends on the radial distance of the impact point IP from the optical axis 18. Such distance between the impact point IP and the optical axis 18 in FIG.

[0043] The pitch p of the pump light grating structures 19 varies across the reflecting surface 16 depending on the distance r of the respective positive and negative structures 20 / 21 to the optical axis according to the following equation:

number

[0044] Where: p(r) represents the pitch of the pump light grating structure 19 depending on the distance r between the respective impingement points IP and the optical axis 18; λ PL represents the wavelength of the pump light 19, θ r (In the above formula for p(r),

number

[0045] According to the above formula, the incident angle θ r If the angle of incidence θ is small, the pitch p becomes large. r The larger the θ, the smaller the pitch p. This leads to the following change in the dependence of the pitch on the distance r to the optical axis 18 of the respective rating structure 20 / 21 depending on the entrance geometry, i.e., on the arrangement of the reflecting surface 16, its curvature, and the position of the source area: The change in pitch p may be such that the pitch increases as the distance r increases. The change in pitch p may be such that as the distance r increases, the pitch decreases. The pitch p may depend non-monotonically on the distance r.

[0046] The pitch of the pump light grating structure 19 is such that for the +1st diffraction order, the pump light 22 r As a result, pump light ray 22 impinging on pump light grating structure 19 on reflective surface 16 of EUV collector mirror 15 r 2, and pump beam 22 r 3, which illustrates one of the optical fibers 100 and 102.

[0047] In addition, FIG. 3 also shows the diffracted beam of the −1st diffraction order.

[0048] The pump light grating structure 19 is irradiated with pump light 22, not shown in FIG. r Therefore, the +1st diffraction order is blazed relative to the +1st diffraction order of the incident pump beam 22. r holds almost all of its energy.

[0049] Retroreflected pump beam 22 r The light again hits the tin droplets 25 , thereby increasing the pumping efficiency of the light source module 2 .

[0050] Individual pulses of the pump light 19 have a pulse duration (full width at half maximum) of less than 50 ns, preferably less than 40 ns, preferably less than 30 ns, preferably less than 20 ns, preferably less than 10 ns, preferably less than 8 ns, preferably less than 5 ns.

[0051] In particular, the rise time of the pump light pulse between a low irradiance level, which is less than 10% of the maximum pulse intensity, and a high irradiance level, which is more than 80% of the maximum pulse intensity, is less than 15 ns, less than 10 ns, or even less than 5 ns. Such a short pulse duration and / or such a small rise time results in good conversion efficiency of pump light energy to usable EUV illumination light 3 energy.

[0052] The pump light grating structure 19 has a reflectivity of more than 50% for the pump light 22. Specifically, such reflectivity may be in the range between 50% and 90%, in the range between 60% and 85%, or in the range between 65% and 75%.

[0053] By interaction of the pump light 19 with the tin droplets 25 usable EUV illumination light 3 is generated, for example at wavelengths of 6.5 nm or 13 nm.

[0054] To produce micro- or nanostructured components, the projection exposure apparatus 1 is used as follows: First, a reflective mask 10, i.e. a reticle, and a substrate, i.e. a wafer 11, are provided. Subsequently, the structure on the reticle 10 is projected onto a photosensitive layer of the wafer 11 by means of the projection exposure apparatus 1. Then, the microstructures or nanostructures on the wafer 11, and therefore the structured components, are produced by developing the photosensitive layer.

Claims

1. An EUV collector mirror (15) having a reflective surface (16) for reflecting usable EUV light (3) impinging on said reflective surface (16) from a region (17) where illumination light is generated to a subsequent EUV optical section (6, 7), The reflective surface (16) carries a pump light grating structure (19) designed to reflect pump light (22) impinging on the pump light grating structure (19) back to the illumination light generating region (17) from the illumination light generating region (17), the pump light (22) having a wavelength (λ) that is offset from the wavelength of the usable EUV light (3). PL ) An EUV collector mirror, wherein the pitch of the pump beam grating structure (19) varies across a reflective surface (16) of the EUV collector mirror (15).

2. The pump light grating structure (19) is irradiated with light having a wavelength (λ PL 2. The EUV collector mirror according to claim 1, wherein the EUV collector mirror is designed for a pump light (22) having a .mu.m / .mu.m.

3. The pump light grating structure (19) is irradiated with light having a wavelength (λ PL 2. The EUV collector mirror according to claim 1, wherein the EUV collector mirror is designed for a pump light (22) having a .mu.m / .mu.m.

4. The grating pitch p(r) of the pump light grating structure 19 obeys the following equation: [0010] Where: p(r) represents the pitch of the pump light grating structure (19), which depends on the distance r between each impingement point on the reflecting surface (16) and the axis of symmetry rotation (18) of the reflecting surface (16); λ PL represents the wavelength of the pump light (19), θ r are the respective pump beams (22 r ) to the reflecting surface (16) 4. An EUV collector mirror according to claim 1, characterized in that it comprises a first insulating layer and a second insulating layer.

5. The EUV collector mirror according to any one of claims 1 to 4, wherein the pump light grating structure (19) has a reflectivity between 50% and 90% for the pump light (22).

6. A pump light source (23) for generating pump light (22), A source collector module (2) comprising an EUV collector mirror (15) according to any one of the preceding claims.

7. 7. A pump light source (23) as part of a light source collector module (2) according to claim 6, which is designed for a spherical target.

8. The pump light source of claim 7 , which generates pump light pulses having a pulse duration of less than 50 ns.

9. 9. A pump light source according to claim 7 or 8, which generates pump light pulses with a pulse rise time of less than 5 ns.

10. An EUV collector mirror according to any one of claims 1 to 5; illumination optics for directing the usable EUV illumination light from the EUV collector mirror towards an object field in an object plane of an EUV projection exposure apparatus, in which an object to be imaged can be located; A lighting system including:

11. - illumination optics (15, 6) for directing the usable EUV illumination light (3) from the EUV collector mirror (15) according to any one of claims 1 to 5 towards an object field (4) in an object plane (5) in which an object (10) to be imaged can be located; a projection optical unit (7) for imaging said object field (4) into an image field (8) in which a substrate (11) can be arranged; a holder (10a) for an object (10) to be imaged and / or a holder (12) for said substrate (11); A projection exposure apparatus comprising:

12. 1. A method of fabricating a structured component, comprising the steps of: Providing a reticle (10) and a wafer (11); - projecting a structure on a reticle (10) onto a photosensitive layer of a wafer (11) using a projection exposure apparatus as claimed in claim 11; Fabricating microstructures and / or nanostructures on the wafer (11); The method includes:

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