WO2025201772A1

WO2025201772A1 - Substrate and method for a high power laser system

Info

Publication number: WO2025201772A1
Application number: PCT/EP2025/054997
Authority: WO
Inventors: Adriaan Roelof VAN ZWOL
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2024-03-28
Filing date: 2025-02-25
Publication date: 2025-10-02
Anticipated expiration: 2026-09-28

Abstract

The present disclosure provides a laser system comprising: a transport system arranged to transport a laser beam along a beam path from an inlet to a target region, wherein the beam path is provided with a plurality of reflective optical elements comprising a substrate with a reflective coating thereupon, wherein the substrate is composed of porous or sintered ceramic, preferably alumina.

Description

SUBSTRATE AND METHOD FOR A HIGH POWER LASER SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24167233.6 which was filed on 28 March 2024 which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a substrate and a method suitable for use in the beam path of a high power laser. High power herein may refer to a laser beam having an energy content exceeding 500 W. The high power laser may be used as a seed laser for a source of Extreme Ultraviolet (EUV) radiation. Said radiation may be used in a state of the art lithography apparatus. The substrate may be part of a reflective optical element in said beam path.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] EUV light occurs naturally in outer space. But to make EUV lithography possible, state of the art engineering is required. Typically, in a laser-produced plasma (LPP) source, molten tin droplets of around 25 microns in diameter are ejected from a generator at a speed of 70 meters per second. As the tin droplets fall, the droplets are hit first by a first, low-intensity laser pulse that flattens the droplets into a pancake shape. Then, a more powerful second laser pulse vaporizes the flattened droplet to create a plasma that emits EUV light. To produce enough light to manufacture microchips, this process is repeated 50,000 times every second.

[0006] Despite being regarded as a 'low-intensity' pulse, the first laser pulse (the "pre-pulse") and the second laser pulse (the "main pulse") are both relatively high power pulses from a laser light perspective. For instance, to generate EUV light having an electromagnetic energy in the order of 500 W, typically, the first laser pulse may need to have an electromagnetic energy in the order of at least 500 W while the second laser pulse may require an electromagnetic energy in the order of tens of kW. A beam path provided with optical elements guides the respective laser pulses from a seed of the laser light to the tin droplets in the LPP. Given the significant power of both pulses, the beam path and the optical elements in the beam path are provided safety measures to obviate the laser pulse to exit the beam path.

[0007] The beam path typically comprises a number of reflective optical elements arranged between a laser source and an outlet. The reflective elements are typically provided with substrates and coatings. The thickness of the substrates may be in the range of millimeters to centimeters.

[0008] Both the first pulse and the second pulse may use laser light having a wavelength in the order of 10 pm. To improve throughput of the EUV lithography systems, for instance to increase the power of the EUV light source, one or both of the pre-pulse and the main pulse may have to be adapted, for instance by increasing the power or changing the wavelength. For instance, the wavelength of the first laser pulse may be reduced, to the order of 1 pm. The power of the first laser pulse may be increased, for instance to the range of 700 W to 1.5 kW.

[0009] However, as even a laser pulse of 500 W may bum through metal, and can cause serious harm to personnel, elaborate safety measures are typically required to contain both laser pulses and avoid any laser light from leaving the beam path. The substrates of the reflective elements typically are provided with a protective coating. Such a protective coating is often a reflective coating. However, when the coating is damaged, the laser light may reach the substrates of the reflectors. Herein, the laser light even at 500 W may bum through the substrates of the reflective optical elements in a matter of seconds, and may subsequently be able to reach an enclosure of the beam path. A warning system may typically be installed to detect damage to any of the substrates, and in case of damage to shut down the laser system to avoid damage to the enclosure. However, given the high power of the laser, the warning system needs to be faster than the time required for the laser to bum through the substrate, which in practice is relatively difficult to guarantee.

[00010] In addition, the beam path may typically comprise one or more apertures, comprising an absorbing metal ring with a number of thermal switches connected to the ring in order to detect a stray laser beam. When the beam hits the absorbing ring, the temperature of the ring will rise, tripping a switch. However, once a stray beam has hit the respective aperture, the aperture is damaged and needs to be replaced. The latter results in downtime for the lithographic apparatus, resulting in reduced throughput. Furthermore, a laser beam striking the aperture can introduce metal debris into the beam path, potentially causing secondary problems in other parts of the lithographic system, such as damage to diamond windows and other safety concerns.

[00011] US20220269162A1 discloses an optical element for reflecting EUV radiation and an EUV apparatus. The optical element includes a substrate formed from a ceramic material, such as aluminum oxide. The light source of the apparatus may be a plasma light source for generating a laser-induced plasma.

[00012] JP2013113980A discloses a substrate for mounting an optical element. The optical element may be an optical scanning element for reflecting light emitted from a light source. The optical element mounting substrate may include an insulating base made of a ceramic such as an aluminum oxide -based sintered body.

[00013] US20130301151 Al discloses a substrate for a mirror for EUV lithography. The mirror may be made of aluminum oxide dispersed in a metallic matrix.

[00014] The safety concerns outlines above are significantly more difficult to overcome with increasing laser power and smaller wavelength. None of the disclosures referenced above provides a solution suitable for a laser pulse system exceeding the laser power of current systems, and/or for laser pulses having a wavelength below 10 pm.

[00015] The present disclosure aims to provide one or more improvements to the beam path of a high power laser, to obviate at least one of the disadvantages referenced above.

SUMMARY

[00016] The present disclosure provides a laser system comprising: a transport system arranged to transport a laser beam along a beam path from an inlet to a target region, wherein the beam path is provided with a plurality of reflective optical elements comprising a substrate with a reflective coating thereupon, wherein the substrate is composed of porous or sintered ceramic, such as alumina.

[00017] In an embodiment, the alumina is in the form of sintered ceramics, wherein the alumina is poly-crystalline and diffusive for visible light.

[00018] In an embodiment, the beam path is provided with one or more rings having an aperture for passage of the laser beam, wherein the rings are composed of silicon carbide (SiC).

[00019] In an embodiment, the rings are made of SiC.

[00020] In an embodiment, the SiC is doped in a concentration Nd in the order of 10¹⁹ cm'³.

[00021] In an embodiment, the laser beam includes laser light having a wavelength in the order of

1 pm.

[00022] In an embodiment, the beam path is provided with one or more amplifiers for amplifying the electromagnetic power of the laser beam.

[00023] In an embodiment, the system comprises an enclosure enclosing at least part of the beam path.

[00024] According to another aspect, the disclosure provides a lithographic system comprising the laser system as described above.

[00025] According to another aspect, the disclosure provides a radiation source comprising the laser system as described above.

[00026] According to yet another aspect, the disclosure provides a method comprising the steps of: providing a laser beam; using a transport system to transport the laser beam from an inlet to a target region along a beam path, the beam path being provided with a plurality of reflective optical elements to reflect the laser beam along the beam path; wherein the reflective optical elements comprise a substrate with a reflective coating thereupon, wherein the substrate is composed of porous or sintered ceramic, such as alumina.

[00027] In an embodiment, the alumina is in the form of sintered ceramics, wherein the alumina is poly-crystalline and diffusive for visible light.

[00028] In an embodiment, the beam path is provided with one or more rings having an aperture for passage of the laser beam, wherein the rings are made of SiC, wherein the SiC is doped in a concentration Nd in the order of 10¹⁹ cm'³.

[00029] In an embodiment, the laser beam includes laser light having a wavelength in the order of 1 pm.

[00030] In an embodiment, the method includes the step of amplifying the electromagnetic power of the laser beam using one or more amplifiers provided in the beam path.

[00031] The disclosure comprises mirror substrates. The substrates, for instance in the 1 pm beam path, comprise sintered and/or porous alumina, i.e. aluminum oxide. In sintered form, alumina is diffusive for light. As a result, even if an optical reflective coating is locally damaged, the substrate will not heat up significantly, and the transmitted light is diffused sufficiently, obviating further damage to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[00032] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;

Figure 2 schematically depicts a beam path of a radiation source of Extreme Ultraviolet (EUV) radiation;

Figure 3 depicts a cross section of an exemplary material, schematically indicating surface scattering and volume scattering;

Figures 4A, 4B and 4C schematically depict side views of a SiSic substrate, a quartz substrate, and a porous AI2O3 substrate respectively, when exposed to high power laser radiation;

Figure 5 depicts a top view of a substrate of the present disclosure when exposed to laser radiation;

Figure 6 depicts a diagram indicating test results of transmission T [%] versus wavelength of incident radiation X [pm] for sintered alumina ceramic having different pore size r [pm];

Figure 7 depicts a perspective view of an aperture for laser radiation; and Figure 8 depicts a diagram indicating an absorption coefficient of various forms of SiC with respect to photon energy for different dopant concentrations.

DETAILED DESCRIPTION

[00033] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[00034] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[00035] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[00036] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[00037] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[00038] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.

[00039] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal -incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[00040] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[00041] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[00042] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source or a free electron laser (FEL) may be used to generate EUV radiation.

[00043] Figure 2 schematically depicts the laser system 1 of the radiation source SO in more detail. The laser system 1 may comprise, for instance, one or more of a laser source 20, an oscillator 22, a preamplifier 24, one or more power amplifiers (PA) 26, and focusing optics 28. Figure 2 generically indicates a laser pulse 30. The laser source 20 may be adapted to provide one or both of the first laser pulse and the second laser pulse.

[00044] A beam path 40 guides the laser pulse 30 from the laser source 20 to the focusing optics 28. The beam path may be provided with an enclosure 42. The enclosure 42 may comprise multiple parts or sections. The beam path 40 may extend in part under and in part above a factory floor 44. The beam path 40 comprises one or more optical elements 46 to direct the laser pulse 30 along a predetermined trajectory. Part of the beam path between the amplifiers 26 and the focusing optics 28 may be referred to as a beam transport system 47. The beam path may be provided with one or more apertures 48. The apertures 48 may have an opening for passage of the laser pulse 30 and may be adapted to provide an alarm signal when the laser pulse deviates from the predetermined trajectory and inadvertently hits one of the apertures.

[00045] The beam path 40 and the beam transport system 47 typically comprise one or more active elements to control the beam position and the beam focus. This is what makes the application as exemplified in Fig. 2 relatively demanding, for instance compared to the beam delivery of a welding laser in, for instance, the auto industry. In a conventional welding application, the laser beam is not focused or displaced in the transport system. However, in the application as depicted in Figure 2, the beam path 40 includes active tilting and focusing mirrors 46, to allow to hit the tin droplets in the area 4. This however makes the safety calculations way more difficult. One has to calculate the possible beam position and focus for when any of the actuators goes to an unwanted extreme position. The need for having this kind of control over the beam 30, to be able to hit the droplet, makes the safety calculations way more difficult. An indestructible aperture 48, or an intrinsically safe substrate for the optical devices 46, as disclosed herein below, make it easier to construct an intrinsically safe system 1. [00046] Figure 3 shows a detail of an optical element 46. The optical elements typically guide the laser light 30 by reflecting the light. Herein, the optical elements typically comprise a substrate 60 provided with a reflective coating 62. If the coating adheres to its design specifications, the coating 62 reflects the light at the surface of the coating, providing so-called surface reflection 64. The reflections shown in Fig. 3 are exemplary, and show a diffuse kind of surface reflection. In a practical embodiment, the coating 62 of the optical elements 64 will provide a more concentrated reflection, wherein the incident laser light beam 30 is substantially reflected as a reflected beam.

[00047] Generally referring to Figure 4A, a problem may arise when the reflective coating 62 becomes damaged. For instance, the coating 62 may locally reduce in thickness or even provide an opening 66. Herein, the incident laser pulse 30 may hit the substrate 60 rather than being reflected by the coating.

[00048] Referring to Figure 3, in an ideal situation the substrate 60 may provide so called volume scattering. In volume scattering, the incident light is reflected inside the volume of a material, providing internal reflections 68. In a practical embodiment, the medium is also low in absorption, in addition to showing volume scattering. This is because if the material has too high absorption during the volume scattering process, the material in effect becomes an absorber. If the material absorbs too much, the material may exceed a local thermal gradient, causing the ceramic to brake. In a practical embodiment, absorption is smaller than 0. 1% for the laser pulse powers as discussed herein, to avoid fracturing the substrate.

[00049] Typical mirror substrates 60 suitable for the optical elements 46 of a beam path suitable for laser light having a wavelength in the order of 1 pm comprise fused silica or quartz. These materials have advantages due to their relatively low coefficient of thermal expansion (CTE), the relative ease of polishing these materials, and the relative ease of obtaining high quality materials.

[00050] Reflective coatings may include multiple layer pairs of high and low refractive index materials. Typically, SiCE is used as low refractive index material. High refractive index materials include Ta2Os, TiC , HfC AI2O3, for high power laser applications. For low power applications, basically any metal oxide will do, and also some fluorides.

[00051] Figure 4A exemplifies a substrate 60A made of a material absorbing a part of the laser energy. 5% absorption is enough to melt the mirror at the laser powers discussed herein. Figure 4A represents any material absorbing the laser wavelength. It could be SiC, or (uncoated) Aluminum (5% absorption), or Iron (40% absorption at 1 pm). SiC can be even transparent if pure enough.

[00052] For instance a Silicon Carbide (SiSiC/SSiC) will absorb part of the laser pulse energy. Silicon carbide behaves almost like a diamond. It is not only the lightest, but also the hardest ceramic material and has excellent thermal conductivity, low thermal expansion and is very resistant to acids. However, tests of the SiSiC substrate 60A for the 1 pm beam path have indicated that in the event of a damaged surface of the coating 62, a 700 W laser beam 30 could penetrate through the silica in a few seconds. This was way faster than expected. Detection systems for bum through and safety detection are typically insufficient to detect the bum through in a timely manner. The latter may result in significant damage to optical elements before the error is detected, as well as more severe cases wherein the laser pulse 30 may hit the enclosure 42 with a potential to exit the beam path.

[00053] Figure 4A exemplifies the mechanism of the laser light burning through the silica so fast. When the laser light 30 hits the absorbing substrate material, the laser beam energy is absorbed in a relatively small area 70 at and near the surface of the material. All the laser energy is thus concentrated in a relatively small volume of material, causing the volume to heat up significantly and relatively fast. As a result, the material in said volume may evaporate, causing the laser beam to hit other sections of the substrate material, until the laser beam has burned through the substrate altogether.

[00054] Figure 4B exemplifies a substrate 60B made of quartz. Quartz is transmissive for laser light having a wavelength in the order of 0.2 to 1.1 pm. For 10 pm other substrate materials like ZnSe or CVD diamond may be preferred. Figure 4B is mainly for the 1 pm wavelength. For 10 pm, metal mirrors may be used. The use of a metal coating on glass (quartz) to make a mirror is typically used for 1 pm lasers (and shorter wavelengths) due to ease of manufacturing.

[00055] For a laser pulse having a wavelength in the order of 1 pm, the laser beam 30 will pass through the quartz substrate 60B virtually intact. Thus, when the laser light 30 hits the quartz 60B, the laser beam energy will not be absorbed and virtually all the energy passes the substrate 60B and may hit whatever is located behind the substrate, such as the enclosure 42. Unless the damaged section 66 of the coating would be detected sufficiently early, the laser beam 30 hitting the enclosure 42 poses significant safety risks, including a significant risk of harm to personnel. [00056] Significant testing has been conducted in search of a safe, impenetrable substrate. Virtually no material provided the correct compromise with respect to relatively low specific weight, ability to machine the material to a specific shape, and safety. Safety herein is either intrinsic, or through the application of bum through detection. One of the best compromises found was a substrate made of aluminum, wherein the laser beam still bums through the material yet relatively slowly, thus enabling sufficiently fast detection.

[00057] Surprisingly, the inventor has found that a substrate 60C made of, or at least comprising, sintered AI2O3 ceramic offers superior behavior in case the front surface mirror coating is damaged. See Fig. 4C. Sintered AI2O3 ceramics are suitable as a substrate 60 for mirrors to reflect a high power laser beam having a wavelength of 1 pm.

[00058] In crystalline form, AI2O3 is a mineral known as Sapphire. Crystalline AI2O3 is transparent for light having a wavelength of 1 pm. However, in sintered ceramics form, alumina is poly crystalline and diffusive for visible light, including for light having a wavelength of 1 pm. Referring to Figure 4C, a laser beam 30 reaching the sintered alumina substrate 60C through an opening 66 in the coating 62 will scatter. The alumina provides volume scattering, as indicated in Fig. 3. This means that even if the optical coating 62 on the front surface of an optical element is locally damaged (which is the main safety risk for using for instance SiC as a substrate), the sintered ceramic alumina will not heat up significantly. [00059] The transmitted and reflected light will be sufficiently diffused to avoid any consecutive damage. As exemplified in Fig. 4C, for instance about 85% of the incident laser beam 30 will be diffusely reflected due to volume scattering in the alumina. About 15% of the incident light will be transmitted, but also in a relatively diffuse form. See Fig. 5 for an example of the non-coating side of the substrate 60C, wherein the transmitted laser light 72 is scattered. Although the beam shape 74 may still be visible, and comprise more energy than the wider scattered beam 72, the beam 74 has a significantly reduced energy concentration thus limiting safety concerns. As the electromagnetic energy of the laser beam is diffused over a wider area, and as only part is transmitted, the transmitted light will not damage machine parts and enclosures 42 located behind the respective substrate 60C, thus limiting safety risks significantly. This behavior allows alumina ceramic to be used for intrinsically safe mirrors for laser light having a 1 pm wavelength. Complicated beam break through detection schemes are obviated.

[00060] As exemplified in Figure 4C, also the back -reflection on the surface of the ceramic is diffuse. Unlike many conventional substrates, back -reflection may also damage equipment, yet it is typically very difficult to define reflection from a molten surface, which makes it very hard to make accurate safety calculations. By its nature the back reflection from a partly molten surface is undefined, yet it can create focused impact at distant surfaces, and is very hard to calculate and predict to ensure staying within safety thresholds. Also in this respect, the diffuse backscattering from the ceramic substrate 60C provides a controlled diffusive surface, which is superior. It allows to do better safety calculations. Preventing melting of the substrate is a significant advantage. [00061] For alumina ceramic, solid-state sintering involves densifying a green body into a dense solid object at a high temperature above 1500 to 1800 °C without melting (the melting point of alumina is 2050 °C). Sintering to full density can be achieved at atmospheric pressure (pressure less sintering). [00062] For a high power laser beam, the thermal divergence of a mirror is taken into account. The thermal conductivity of sintered alumina is in the order of 30 W/m/K, which is almost 10 times better than quartz (~3 W/m/K) or fused silica (~1 W/m/K), or other high melting temperature ceramics like ZrC>2 (about 3000 °C melting temperature; ~3 W/m/K). The CTE is 2.5 times higher, so in net result means that the thermal deformation is also some four times lower than for Si O2 based substrates. Normal dielectric coating stacks can be used to produce substrates made of sintered alumina ceramic. To enable polishing and flattening, a polishable layer can be added, like sputtered alumina, to get to a specific optical flatness. Polishing standard alumina may enable down to some 5 nm (RMS). For a mirror in a 1 pm high power laser application, an additional layer to improve the flatness may be needed.

[00063] Alumina is relatively easy to process into complex shapes, which may be required for some of the mirrors in the beam path. Also, it is a very cost effective material that can be obtained from multiple vendors.

[00064] Optical properties of sintered alumina ceramics may depend on characteristics, such as pore size and grain size. Figure 6 depicts test results of transmission T [%] of incident radiation versus the main wavelength of the incident radiation Xo [pm] for samples of sintered alumina ceramic having various fixed pore sizes r [pm] . The test samples used alumina ceramic samples having a thickness t of 0.5 mm and a porosity V_p of 0.002. The wavelength was varied in the range of 0.5 to 5 pm.

[00065] The sintering process to create the alumina ceramic can result in different grain size and pore sizes. The graph of Figure 6 shows the net (un-scattered) transmission through a sample having a thickness of 0.5 mm. The graph of Figure 6 indicates that the transmission strongly depends on the pore size (indicated per line with r=xx pm). Transmission results are surprising, as one would expect a transmission of some 85% based on the refractive index of about 1.7 for radiation having a 1 pm wavelength.

[00066] The graph of Figure 6 indicates that alumina ceramic having a pore size of about 0.5 pm would have a very limited transmission of about 5% for radiation having a 1 pm wavelength. Alumina ceramic having a pore size of about 1.0 pm would have a transmission of about 20% for radiation having a 1 pm wavelength. Alumina ceramic having a pore size of about 0.1 pm would have a transmission of about 30% for radiation having a 1 pm wavelength.

[00067] In a practical embodiment, substrates for the beam path of a laser pulse having a 1 pm wavelength can be comprised of alumina ceramic having a pore size in the range of 0.3 to 1.0 pm, for instance about 0.5 pm.

[00068] Absorption of electromagnetic energy in the alumina ceramic is relatively low. For a temperature T in the range of 300 to 2300 K, the absorption coefficient a can be in the order of 25 ppm / cm, or 2.5* IO^-5 cm'¹. Other properties of alumina ceramic like tensile strength, CTE etc. are all equal or beter than glass. For alumina ceramic having 10 micron grains, the breaking strength is in the order of 200 MPa, which is much higher than the breaking strength for glass, which is typically about 100 Mpa. Also, alumina ceramic is polishable and is a suitable substrate material for reflective optical coatings.

[00069] For details on optical properties on sintered alumina oxide, reference is made to Infrared Physics & Technology 77 (2016) 162-170, "Near-infrared optical properties of a porous alumina ceramics produced by hydrothermal oxidation of aluminum", Aleksey V. Lisitsyn et al..

[00070] Figure 7 shows an example of an aperture 48. One or more apertures 48 as shown in Fig. 7 may be positioned along the beam path 40. An aperture 48 may comprise an outer element 80 provided with an opening 82 to allow passage of the laser pulse 30. The opening may be provided with a window 84, for instance made of quartz or glass, which is substantially transmissive for the respective laser pulse 30. The outer element 80 is typically non-permeable for the respective laser pulse. The element 80 may be a structural element, for instance a ring-shaped structure.

[00071] The apertures 48 improve safety, for instance by providing early warning signals in case the laser pulse deviates from a predetermined trajectory. Sensors connected to the element 80 may be able to detect when a laser pulse engages the structural element 80, seting off a warning signal to indicate that the laser pulse has deviated from its trajectory.

[00072] The aperture as exemplified in Figure 7 is relatively rudimentary. In general, a safety aperture 48 may include a saw tooth like profile on the surface with very sharp angles so that light that reflects of the surface is reflected to a second surface in the aperture, and not outside the aperture. Also the surface may typically be treated to be more absorbing.

[00073] As referenced in the introduction, conventional apertures must be replaced once a laser beam hits them. Also, typically either the windows 84 or the structural element 80 may be damaged when hit by the laser, and the occurring damage typically causes debris. The later in turn can cause secondary problems, such as contamination of nearby diamond windows. For a laser pulse at a 1 pm wavelength, the back scater during melting of the aperture 48 triggers many uncertainties in a safety evaluation. It causes problems because the shape of the surface is undefined, as the backscatering can go to any angle and at any random focus. This makes safety calculations very difficult; there is always a spot where you cannot tolerate an intermediate focus, and with molten surfaces it is relatively difficult to guarantee to stay within safety specifications.

[00074] In an embodiment, the element 80 is comprised of silicon carbide (SiC). Herein, the SiC can be used as a substrate for the apertures 48, in particular for the structural part 80 thereof. The structural part may, in addition, be provided with other materials, for instance a metal of another material allowing detection of the laser light. In a practical embodiment, the aperture 48 comprising SiC is suitable for the beam path of the laser pulse having a wavelength in the order of 1 pm.

[00075] The silicon carbide has two unique advantages over other ceramic materials or metal. Silicon carbide does not have a liquid phase, and the absorption skin depth can be tuned. [00076] Firstly, SiC does not have a liquid phase. When typical materials start melting, the absorption of electromagnetic energy typically increases with an order of magnitude, triggering thermal runaway when hit by a laser pulse. This is the basis for laser processing or welding of many materials. SiC to the contrary does not melt, so it will not start this runaway. In the context of laser safety and indestructability, this is a very unique feature, that until now has not been recognized or explicitly been regarded as advantageous.

[00077] SiC starts sublimating at 2700 °C. This temperature is much higher than typical infrared mirror substrates like Al (-600 °C) , Cu (-1100 °C), Si (-1400 °C), or Fused silica (-2000 °C) that are typically used for such applications. Conventional coating materials for radiation having a 1 pm wavelength typically include combinations of SiC>2 (melting at 1400 °C) and for instance a refractory oxide, such as Hafnium(IV) oxide (HfCh; 2800 °C melting) , or AI2O3 (2000 °C), or Ta2Os (-1800 °C). Coating the optic with a single layer of for instance tungsten (3400 °C) or molybdenum (2600 °C) does not protect a surface, once the substrate starts melting. In laser welding sometimes molybdenum mirrors are used as they are very scratch resistant and easy to clean from welding debris, and do not melt when liquid metal (e.g. iron) splashes arrive at the front surface . But, for 1 pm wavelength light the reflectivity is only 70%, rendering this material unsuitable. For CO2 lasers reflectivity of Mo exceeds 98%, so for CO2 laser light Mo is an option.

[00078] Materials comprising a compound of SiC, such as SiSiC, wherein SiC particles are contained in a Si matrix, typically do not have the high melting temperature. Compound materials also have an unsuitable liquid phase. In a preferred embodiment, the material for the aperture element 80 is pure SiC.

[00079] Another advantage of silicon carbide is that the absorption of the material can be tuned by controlling the dopant level. Controlling the absorption can prevent laser ablation. Preventing laser ablation is an attribute for making an ‘impenetrable’ material for high power lasers. For comparison, laser ablation of materials in the nanosecond pulse length regime relies on a limited skin depth of absorption during the nanosecond laser pulse. The thermal diffusion length in a few nanoseconds is only a few microns at most. The full laser pulse energy is thus absorbed in a small volume during the nanoseconds of the pulse length. This small volume is then heated so extremely that it evaporates, or melts quickly (see also the description above with respect to Figure 4A). In contrast, silicon carbide (SiC) can be fully transparent for light having a wavelength of about 1 pm. Absorption is limited to a ppm level. By controlling the amount of doping, it is possible to spread the absorption over a length of, for instance, millimeters. Thus, the energy in a laser pulse can be much higher before melting of the material occurs during a pulse. In the 1 pm beam path it would mean that the laser is virtually never focused sufficiently to achieve ablation, i.e. ablation can be substantially obviated entirely. This is a unique advantage.

[00080] Tests have indicated that a sample made of SiC was indestructible for radiation having a wavelength of about 1 pm. The latter held for a wide range of laser power, including the range up to 700 W and 1.5 kW. Yet, a sample made of SiSiC was burned through in a matter of 1 to 2 seconds. The difference is due to the fact that SiC does not have a melting temperature. Silicon carbide only has a sublimation temperature, at around 2700 °C. SiSic will melt at about 1000 to 1200 °C, similar to pure silicon.

[00081] Under irradiation with 1 pm wavelength light with around lOOkW/cm², the optical coating on a substrate may start to damage. Tests have indicated that even when the coating is damaged, a substrate made of SiC remains intact. To the contrary, samples of other materials, such as a Si substrate, are catastrophically damaged. In a silicon material, the laser pulse bums a hole entirely through the material within a second.

[00082] The chart in Figure 8 shows how the absorption of the silicon carbide is a function of dopants. Adding selective dopants allows to tune or select the best SiC composition to be used. Figure 8 shows the absorption coefficient [cm ¹] versus the photon energy [eV] for 3C-SiC, at 300 K. Line 90 indicates results for a relatively pure crystalline SiC. Line 92 indicates results for doped SiC at Nd = 10¹⁹ cm^-3.

[00083] Calculating the temperature of absorption during a pulse indicates that the surface temperature will exceed 2700 degrees if the absorption only happens at the entrance surface. For a 700 W CW laser, and 100% absorption, on a 0.5 mm spot size, which is a worst case 1 pm beam path situation, the surface temperature rises to 1000 °C in 10 microseconds, and to 2700 °C in under 30 microseconds. For a 50 kHz laser with a 14 mJ per pulse energy, a fluence of about 5 J/cm² is calculated to reach the temperature of 2700 °C. The latter could mean there is a risk for ablation, i.e. a laser pulse that removes material. This risk is obviated if the absorption is spread over a larger volume and absorption is not only at the surface. This can be achieved by tuning the impurity level of the SiC. In a practical embodiment, doped SiC is selected. Thus, the penetration depth of the light decreases.

[00084] Although reference above has been made to alumina, other materials can be used as a mirror substrate. In a practical embodiment, the substrate 60C is an optically transparent ceramic that is sintered to render the material tougher. Below is a list of possible materials, but the list is not exhaustive. Alumina is the cheapest, but there is a whole range of other materials that one could use. For instance, it is possible to dope of modify materials. For the substrate 60C with surface and volume scattering, examples of other materials that may also have such properties:

• -Zirconia Oxide Ceramics

• -Magnesium Oxide ceramics

• -Magnesium Aluminate Spinael

• -Aluminum Oxi-Nitride

• -Zirconium Nitride ceramics

-Silicon Nitride ceramics

-Boron Nitride ceramics [00085] Other materials that could be used for an aperture 48 include zinc sulfide, which is a material that also does not melt and sublimates. Zinc sulfide is also transparent for both 1 pm and 10 pm, which might give some unique advantages. One could also dope that to have sufficient absorption similar to SiC. Though, unlike SiC heat conduction is quite low, so it might fracture easier. It is more machinable than SiC, rendering the material suitable for a practical application.

[00086] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[00087] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[00088] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[00089] The following clauses are part of the present disclosure.

1. A laser system comprising: a transport system arranged to transport a laser beam along a beam path from an inlet to a target region, wherein the beam path is provided with a plurality of reflective optical elements comprising a substrate with a reflective coating thereupon, wherein the substrate is composed of a ceramic.

2. The laser system of clause 1, wherein the substrate is made of porous or sintered alumina.

3. The laser system of clause 2, wherein the alumina is in the form of sintered ceramics, wherein the alumina is poly-crystalline and diffusive for visible light.

4. The laser system of one of the previous clauses, wherein the beam path is provided with one or more rings having an aperture for passage of the laser beam, wherein the rings are composed of silicon carbide (SiC).

5. The laser system of clause 4, wherein the rings are made of SiC.

6. The laser system of clause 5, wherein the SiC is doped in a concentration Nd in the order of 10¹⁹ cm'³. 7. The laser system of one of the previous clauses, wherein the laser beam includes laser light having a wavelength in the order of 1 pm.

8. The laser system of one of the previous clauses, wherein the beam path is provided with one or more amplifiers for amplifying the electromagnetic power of the laser beam.

9. The laser system of one of the previous clauses, comprising an enclosure enclosing at least part of the beam path.

10. A lithographic system comprising the laser system of clause 1.

11. A radiation source comprising the laser system according to any of clauses 1 - 9.

12. A method comprising the steps of: providing a laser beam; using a transport system to transport the laser beam from an inlet to a target region along a beam path, the beam path being provided with a plurality of reflective optical elements to reflect the laser beam along the beam path; wherein the reflective optical elements comprise a substrate with a reflective coating thereupon, wherein the substrate is composed of a ceramic.

13. The method of clause 12, wherein the ceramic is alumina in the form of sintered ceramic, wherein the alumina is poly -crystalline and diffusive for visible light.

14. The method of clause 12 or 13, wherein the beam path is provided with one or more rings having an aperture for passage of the laser beam, wherein the rings are made of SiC, wherein the SiC is doped in a concentration Nd in the order of

10¹⁹ cm^-3.

15. The method of one of clauses 12 to 14, wherein the laser beam includes laser light having a wavelength in the order of 1 pm.

16. The method of one of clauses 12 to 15, including the step of amplifying the electromagnetic power of the laser beam using one or more amplifiers provided in the beam path.

[00090] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

2. The laser system of claim 1, wherein the substrate is made of porous or sintered alumina.

3. The laser system of claim 2, wherein the alumina is in the form of sintered ceramics, wherein the alumina is poly-crystalline and diffusive for visible light.

4. The laser system of one of the previous claims, wherein the beam path is provided with one or more rings having an aperture for passage of the laser beam, wherein the rings are composed of silicon carbide (SiC).

5. The laser system of claim 4, wherein the rings are made of SiC.

6. The laser system of claim 5, wherein the SiC is doped in a concentration Nd in the order of 10¹⁹ cm'³.

7. The laser system of one of the previous claims, wherein the laser beam includes laser light having a wavelength in the order of 1 pm.

8. The laser system of one of the previous claims, wherein the beam path is provided with one or more amplifiers for amplifying the electromagnetic power of the laser beam.

9. The laser system of one of the previous claims, comprising an enclosure enclosing at least part of the beam path.

10. A lithographic system comprising the laser system of claim 1.

11. A radiation source comprising the laser system according to any of claims 1 - 9.

13. The method of claim 12, wherein the ceramic is alumina in the form of sintered ceramic, wherein the alumina is poly -crystalline and diffusive for visible light.

14. The method of claim 12 or 13, wherein the beam path is provided with one or more rings having an aperture for passage of the laser beam, wherein the rings are made of SiC, wherein the SiC is doped in a concentration Nd in the order of 10¹⁹ cm^-3.

15. The method of one of claims 12 to 14, including the step of amplifying the electromagnetic power of the laser beam using one or more amplifiers provided in the beam path.