Substrate and method for high power laser systems

Abstract

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

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to European application 24167233.6, filed on 28 March 2024, which is incorporated herein by reference in its entirety. Technical Field

[0003] This invention relates to substrates and methods for beam paths suitable for high-power lasers. High power, as used herein, can refer to a laser beam with an energy content exceeding 500 W. The high-power laser can be used as a seed laser for extreme ultraviolet (EUV) radiation. This radiation can be used in state-of-the-art photolithography equipment. The substrate can be part of reflective optics within the beam path. Background Technology

[0004] A lithography apparatus is a machine configured to apply a desired pattern onto a substrate. For example, a lithography apparatus can be used in the manufacture of integrated circuits (ICs). A lithography apparatus can, for example, project a pattern from a patterning apparatus (e.g., a mask) onto a radiation-sensitive material (resist) layer disposed on a substrate.

[0005] To project a pattern onto a substrate, a photolithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Compared to a photolithography apparatus that uses radiation with a wavelength of, for example, 193 nm, a photolithography apparatus that uses extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0006] EUV light naturally appears in outer space. However, state-of-the-art engineering is required to make EUV lithography possible. Typically, in a laser-generated plasma (LPP) source, droplets of molten tin, about 25 micrometers in diameter, are ejected from the generator at a speed of 70 meters per second. As the tin droplets fall, they are first struck by a first low-intensity laser pulse, which flattens the droplet into a pancake shape. Then, a more powerful second laser pulse evaporates the flattened droplet to generate plasma that emits EUV light. To generate enough light to fabricate microchips, this process is repeated 50,000 times per second.

[0007] Although considered "low-intensity" pulses, both the first laser pulse ("pre-pulse") and the second laser pulse ("main pulse") are relatively high-power pulses from a laser perspective. For example, to generate EUV light with approximately 500W of electromagnetic energy, the first laser pulse typically needs to have at least approximately 500W of electromagnetic energy, while the second laser pulse may need to have approximately tens of kW of electromagnetic energy. The beam path, equipped with optical elements, guides the corresponding laser pulses from the laser seed to the tin droplets in the LPP. Given the significant power of the two pulses, safety measures are provided for the beam path and the optical elements within it to prevent the laser pulses from deviating from the beam path.

[0008] The beam path typically includes multiple reflective optical elements arranged between the laser source and the exit point. These reflective elements are usually provided with a substrate and a coating. The substrate thickness can range from millimeters to centimeters.

[0009] Both the first and second pulses can use lasers with a wavelength of approximately 10 μm. To improve the throughput of the EUV lithography system, for example, to increase the power of the EUV source, it may be necessary to adjust one or both of the pre-pulse and the main pulse, for example, by increasing the power or changing the wavelength. For example, the wavelength of the first laser pulse can be reduced to approximately 1 μm. The power of the first laser pulse can be increased, for example, to the range of 700 W to 1.5 kW.

[0010] However, since even a 500W laser pulse can burn through metal and potentially cause serious injury, careful safety measures are typically required to contain both laser pulses and prevent any laser from leaving the beam path. The substrate of a reflective element is usually coated with a protective coating. This protective coating is typically a reflective coating. However, when the coating is damaged, the laser can reach the substrate of the reflector. Here, even a 500W laser can burn through the substrate of the reflective optics within seconds and subsequently reach the housing of the beam path. Warning systems can typically be installed to detect any damage to the substrate and, in the event of damage, shut down the laser system to prevent damage to the housing. However, given the high power of the laser, the warning system needs to operate much faster than the time required for the laser to burn through the substrate, which is relatively difficult to guarantee in practice.

[0011] Furthermore, the beam path typically includes one or more apertures comprising absorbing metal rings with multiple thermal switches connected to the rings for detecting stray laser beams. When a beam strikes the absorbing ring, the temperature of the ring rises, triggering the switches. However, once a stray beam strikes a corresponding aperture, the aperture is damaged and needs to be replaced. This results in downtime for the lithography equipment, leading to reduced production. Additionally, a laser beam striking an aperture can introduce metal debris into the beam path, potentially causing secondary problems in other parts of the lithography system, such as damage to the diamond window and other safety issues.

[0012] US20220269162A1 discloses an optical element and an EUV device for reflecting EUV radiation. The optical element includes a substrate formed of a ceramic material such as alumina. The light source of the device can be a plasma source for generating laser-induced plasma.

[0013] JP2013113980A discloses a substrate for mounting optical elements. The optical elements may be optical scanning elements used to reflect light emitted from a light source. The optical element mounting substrate may include an insulating base made of ceramic, such as an alumina-based sintered body.

[0014] US20130301151A1 discloses a substrate for a reflector used in EUV lithography. The reflector may be made of aluminum oxide dispersed in a metal matrix.

[0015] With increased laser power and shorter wavelengths, the aforementioned safety issues become significantly more difficult to overcome. None of the publicly available documents cited above provide solutions for laser pulse systems with power exceeding that of current systems and / or for laser pulses with wavelengths below 10 μm.

[0016] This disclosure aims to provide one or more improvements to the beam path of a high-power laser to avoid at least one of the aforementioned disadvantages. Summary of the Invention

[0017] This disclosure provides a laser system, including:

[0018] A transmission system, arranged to transmit a laser beam from the entrance to the target area along the beam path,

[0019] The beam path is provided with a plurality of reflective optical elements, the plurality of reflective optical elements comprising a substrate having a reflective coating thereon.

[0020] The substrate is made of porous or sintered ceramic, such as alumina.

[0021] In one embodiment, the alumina is in the form of a sintered ceramic, wherein the alumina is polycrystalline and diffusely reflective to visible light.

[0022] In one embodiment, the beam path is provided with one or more loops having holes for the laser beam to pass through, wherein the loops comprise silicon carbide (SiC).

[0023] In one embodiment, the ring is made of SiC.

[0024] In the embodiment, the SiC is doped with a concentration of 10. 19 cm -3 N on the order of magnitude d .

[0025] In an embodiment, the laser beam comprises a laser with a wavelength of approximately 1 μm.

[0026] In one embodiment, the beam path is provided with one or more amplifiers for amplifying the electromagnetic power of the laser beam.

[0027] In one embodiment, the system includes a housing that encloses at least a portion of the beam path.

[0028] According to another aspect, this disclosure provides a lithography system including the laser system described above.

[0029] According to another aspect, this disclosure provides a radiation source including the laser system described above.

[0030] According to one aspect, this disclosure provides a method comprising the following steps:

[0031] Provide laser beam;

[0032] A transmission system is used to transmit a laser beam along a beam path from the inlet to the target area, the beam path being provided with multiple reflective optical elements to reflect the laser beam along the beam path;

[0033] The reflective optical element includes a substrate having a reflective coating thereon, wherein the substrate is made of porous or sintered ceramic such as alumina.

[0034] In one embodiment, the alumina is in the form of a sintered ceramic, wherein the alumina is polycrystalline and diffusely reflective to visible light.

[0035] In one embodiment, the beam path is provided with one or more loops, each loop having a hole for the laser beam to pass through, wherein the loops are made of SiC, wherein the SiC is doped with a concentration of 10. 19 cm -3 N on the order of magnituded .

[0036] In an embodiment, the laser beam comprises a laser with a wavelength of approximately 1 μm.

[0037] In one 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.

[0038] This disclosure includes a reflective substrate. For example, in a 1 μm beam path, the substrate comprises sintered and / or porous alumina, i.e., aluminum oxide. In the sintered form, aluminum oxide is diffusely reflective of light. As a result, even if the optical reflective coating is locally damaged, the substrate does not heat up significantly, and the transmitted light is sufficiently diffusely reflected, thereby avoiding further damage to the substrate. Attached Figure Description

[0039] Embodiments of the invention will now be described by way of example only with reference to the accompanying illustrative drawings, in which:

[0040] - Figure 1 A lithography system, including lithography equipment and a radiation source, is described;

[0041] - Figure 2 The beam path of the extreme ultraviolet (EUV) radiation source is schematically depicted;

[0042] - Figure 3 A cross-section of an exemplary material is depicted, schematically indicating surface scattering and volume scattering;

[0043] - Figure 4A , Figure 4B and Figure 4C Side views of a SiSic substrate, a quartz substrate, and a porous Al2O3 substrate are schematically depicted when exposed to high-power laser radiation, respectively.

[0044] - Figure 5 A top view of the substrate of this disclosure is depicted when exposed to laser radiation;

[0045] - Figure 6 A schematic diagram depicts the relationship between the transmittance T [%] of sintered alumina ceramics with different pore sizes r [µm] and the incident radiation wavelength λ [µm].

[0046] - Figure 7 A perspective view depicting the aperture used for laser radiation; and

[0047] - Figure 8 A schematic diagram is depicted indicating the absorption coefficients of various forms of SiC relative to photon energies with different dopant concentrations. Detailed Implementation

[0048] Figure 1 A lithography system including a radiation source SO and a lithography apparatus LA is shown. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an irradiation system IL, a support structure MT configured to support a pattern forming apparatus MA (e.g., a mask), a projection system PS, and a substrate stage WT configured to support a substrate W.

[0049] The irradiation system IL is configured to adjust the EUV radiation beam B before it is incident on the pattern forming apparatus MA. Furthermore, the irradiation system IL may include a faceted field mirror assembly 10 and a faceted pupil mirror assembly 11. The faceted field mirror assembly 10 and the faceted pupil mirror assembly 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. In addition to or in place of the faceted field mirror assembly 10 and the faceted pupil mirror assembly 11, the irradiation system IL may include other mirrors or devices.

[0050] After such adjustment, the EUV radiation beam B interacts with the patterning apparatus MA. Due to 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 this purpose, the projection system PS may include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate stage WT. The projection system PS can apply a reduction factor to the patterned EUV radiation beam B' to form an image with features smaller than the corresponding features on the patterning apparatus MA. For example, a reduction factor of 4 or 8 can be applied. Although the projection system PS... Figure 1 The image is shown with only two mirrors 13 and 14, but the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0051] The substrate W may include a previously formed pattern. In this case, the photolithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0052] A relative vacuum, i.e., a small amount of gas (e.g., hydrogen) at a pressure far below atmospheric pressure, can be provided in the radiation source SO located in the irradiation system IL and / or the projection system PS.

[0053] Figure 1The radiation source SO shown is, for example, of the type that can be referred to as a laser-generated plasma (LPP) source. A laser system 1, which may include, for example, a CO2 laser, is arranged to deposit energy into a fuel, such as tin (Sn), supplied from, for example, a fuel emitter 3, via a laser beam 2. Although tin is mentioned in the description below, any suitable fuel can be used. The fuel may be, for example, in liquid form and may be, for example, a metal or alloy. The fuel emitter 3 may include a nozzle configured to guide tin, for example, in droplet form, along a trajectory toward the plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. The laser energy deposited into the tin generates tin plasma 7 at the plasma formation region 4. During the de-excitation and recombination of electrons with ions from the plasma, radiation, including EUV radiation, is emitted from the plasma 7.

[0054] EUV radiation from the plasma is collected and focused by collector 5. Collector 5 includes, for example, a near-normal incident radiation collector 5 (sometimes more generally referring to a normal incident radiation collector). Collector 5 may have a multi-layer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation with a desired wavelength such as 13.5 nm). Collector 5 may have an elliptical configuration with two foci. The first of the foci may be located at the plasma formation region 4, and the second of the foci may be located at an intermediate focal point 6, as discussed below.

[0055] The laser system 1 can be spatially separated from the radiation source SO. In this case, the laser beam 2 can be transmitted from the laser system 1 to the radiation source SO by means of a beam delivery system (not shown), which includes, for example, suitable directional mirrors and / or beam expanders, and / or other optical devices. The laser system 1, the radiation system SO, and the beam delivery system can be collectively considered as a radiation system.

[0056] The radiation reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at an intermediate focal point 6 to form an image of the plasma presented in the plasma formation region 4 at the intermediate focal point 6. The image at the intermediate focal point 6 serves as a virtual radiation source for the irradiation system IL. The radiation source SO is arranged such that the intermediate focal point 6 is located at or near the opening 8 in the enclosure structure 9 of the radiation source SO.

[0057] Although Figure 1The radiation source SO is described as a laser-generated plasma (LPP) source, but any suitable source such as a discharge-generated plasma (DPP) source or a free-electron laser (FEL) can be used to generate EUV radiation.

[0058] Figure 2 The laser system 1 of the radiation source SO is illustrated in more detail. The laser system 1 may include, for example, one or more of the following: a laser source 20, an oscillator 22, a preamplifier 24, one or more power amplifiers (PAs) 26, and a focusing optics 28. Figure 2 The laser pulse 30 is generally indicated. The laser source 20 can be adapted to provide one or both of the first and second laser pulses.

[0059] Beam path 40 guides laser pulse 30 from laser source 20 to focusing optics 28. The beam path may be provided with a housing 42. Housing 42 may include multiple components or sections. Beam path 40 may extend partially below and partially above factory floor 44. Beam path 40 includes one or more optical elements 46 to guide laser pulse 30 along a predetermined trajectory. A portion of the beam path between amplifier 26 and focusing optics 28 may be referred to as beam transmission system 47. The beam path may be provided with one or more apertures 48. Apertures 48 may have openings for laser pulse 30 to pass through and may be adapted to provide an alarm signal when a laser pulse deviates from the predetermined trajectory and unintentionally hits one of the apertures.

[0060] Beam path 40 and beam delivery system 47 typically include one or more active elements to control beam position and beam focusing. This is, for example, compared to beam delivery in welding lasers used in the automotive industry. Figure 2 The examples illustrate the relatively demanding nature of these applications. In conventional welding applications, the laser beam is unfocused or shifted within the transmission system. However, in applications such as... Figure 2 In the application depicted, beam path 40 includes an active tilting and focusing mirror 46 to allow the molten tin droplet in region 4 to be struck. However, this makes safety calculations more difficult. One must calculate the possible beam position and focus if any actuator in the actuator reaches an unwanted extreme position. This control of beam 30 is required to strike the droplet, which further complicates safety calculations. The indestructible aperture 48 or the intrinsically safe substrate for the optics 46, as disclosed below, makes it easier to construct an intrinsically safe system 1.

[0061] Figure 3Details of optical element 46 are shown. Optical elements typically guide laser 30 by reflecting the light. Here, optical elements typically include a substrate 60 with a reflective coating 62. If the coating conforms to its design specifications, the coating 62 reflects light at its surface, thereby providing so-called surface reflection 64. Figure 3 The reflection shown is exemplary and illustrates a surface reflection of the diffuse type. In a practical embodiment, the coating 62 of the optical element 64 will provide a more concentrated reflection, wherein the incident laser beam 30 is substantially reflected as a reflected beam.

[0062] Overall reference Figure 4A Problems may arise when the reflective coating 62 is damaged. For example, the coating 62 could be locally reduced in thickness or even have an opening 66 provided. Here, the incident laser pulse 30 could hit the substrate 60 instead of being reflected by the coating.

[0063] refer to Figure 3 Ideally, substrate 60 can provide so-called volume scattering. In volume scattering, incident light is reflected within the volume of the material, thus providing internal reflection 68. In practical embodiments, in addition to showing volume scattering, the absorptivity of the medium is also very low. This is because if the material has an excessively high absorptivity during volume scattering, the material effectively becomes an absorber. If the material absorbs too much, it may exceed the local thermal gradient, leading to ceramic braking. In practical embodiments, the absorptivity is less than 0.1% for the laser pulse power as discussed herein to avoid substrate breakage.

[0064] A typical mirror substrate 60 for an optical element 46 suitable for a laser beam path with a wavelength of approximately 1 μm comprises molten silicon dioxide or quartz. These materials have advantages due to their relatively low coefficient of thermal expansion (CTE), relative ease of polishing, and relative availability of high-quality materials.

[0065] Reflective coatings can comprise multiple layers of high-refractive-index and low-refractive-index materials. Typically, SiO2 is used as the low-refractive-index material. High-refractive-index materials include Ta2O5, TiO2, HfO2, and Al2O3, used for high-power laser applications. For low-power applications, virtually any metal oxide will be acceptable, and some fluorides are also possible.

[0066] Figure 4A An example is shown: a substrate 60A made of a material that absorbs a portion of the laser energy. An absorption rate of 5% is sufficient to melt a reflector at the laser power discussed herein. Figure 4AThis refers to any material that absorbs the laser wavelength. Materials that absorb laser wavelengths can be SiC, or (uncoated) aluminum (5% absorptivity), or iron (40% absorptivity at 1µm). If sufficiently pure, SiC can even be transparent.

[0067] For example, silicon carbide (SiSiC / SiC) will absorb a portion of the laser pulse energy. Silicon carbide behaves almost like diamond. It is not only the lightest but also the hardest ceramic material, and possesses excellent thermal conductivity, low thermal expansion, and is highly acid-resistant. However, tests on a SiSiC substrate 60A with a 1μm beam path have shown that, in the case of a damaged surface of coating 62, a 700W laser beam 30 can penetrate the silicon dioxide within seconds. This is much faster than expected. Detection systems used for burn-through and safety checks are often insufficient to detect burn-through in a timely manner. The latter can lead to significant damage to optical components before the error is detected, and in more serious cases, the laser pulse 30 may strike the housing 42 with the possibility of leaving the beam path.

[0068] Figure 4A This illustrates the mechanism by which a laser so rapidly burns through silicon dioxide. When laser 30 strikes the absorbing substrate material, the laser beam energy is absorbed in a relatively small region 70 at and near the material surface. Therefore, all the laser energy is concentrated in a relatively small volume of material, resulting in a significant and relatively rapid temperature rise in that volume. Consequently, the material in this volume may evaporate, causing the laser beam to strike other segments of the substrate material until the laser beam has completely burned through the substrate.

[0069] Figure 4B An example is a substrate 60B made of quartz. Quartz is transmissive to laser light with wavelengths of approximately 0.2 μm to 1.1 μm. For wavelengths of 10 μm, other substrate materials, such as ZnSe or CVD diamond, may be preferred. Figure 4B Primarily for 1μm wavelengths. For 10μm, metal mirrors can be used. Due to their ease of manufacture, mirrors made by applying a metal coating to glass (quartz) are commonly used for 1µm lasers (and shorter wavelengths).

[0070] For a laser pulse with a wavelength of approximately 1 μm, the laser beam 30 will penetrate almost entirely through the quartz substrate 60B. Therefore, when the laser 30 strikes the quartz 60B, the laser beam energy will not be absorbed, and virtually all the energy will pass through the substrate 60B and could strike anything located behind the substrate, such as the housing 42. Unless the damaged section 66 of the coating is detected sufficiently early, a strike of the laser beam 30 into the housing 42 poses a significant safety risk, including a significant risk of injury to personnel.

[0071] Extensive testing has been conducted in search of a secure, impenetrable substrate. Virtually no material offers the right trade-off between relatively low density, the ability to machine the material into specific shapes, and security. Security here is either inherent or imposed through burn-through detection. One of the best compromises found is a substrate made of aluminum, where the laser beam still burns through the material relatively slowly, allowing for sufficiently fast detection.

[0072] Surprisingly, the inventors have discovered that substrate 60C, made of or comprising at least sintered Al2O3 ceramic, provides excellent behavior in the event of damage to the front surface mirror coating. See also Figure 4C Sintered Al2O3 ceramics are suitable as substrates 60 for reflectors used to reflect high-power laser beams with a wavelength of 1 μm.

[0073] In its crystalline form, Al₂O₃ is a mineral known as sapphire. Crystalline Al₂O₃ is transparent to light with a wavelength of 1 μm. However, in its sintered ceramic form, alumina is polycrystalline and diffusely reflective to visible light, including wavelengths with a wavelength of 1 μm. (Reference) Figure 4C The laser beam 30, reaching the sintered alumina substrate 60C through the opening 66 in coating 62, will be scattered. Alumina provides volume scattering, such as... Figure 3 As indicated in the instructions. This means that even if the optical coating 62 on the front surface of the optical element is locally damaged (which is a major safety risk when using, for example, SiC as a substrate), the sintered ceramic alumina will not heat up significantly.

[0074] Transmitted and reflected light will be sufficiently diffused to avoid any further damage. For example... Figure 4C As illustrated, approximately 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 also Figure 5 In the example of the uncoated side of substrate 60C, the transmitted laser 72 is scattered. Although beam shape 74 can still be visible and includes more energy than the wider scattered beam 72, beam 74 has a significantly reduced energy concentration, thus limiting safety concerns. Because the electromagnetic energy of the laser beam is diffusely reflected over a wider area and because only partially transmitted, the transmitted light will not damage machine parts and housing 42 located behind the corresponding substrate 60C, thus significantly limiting safety risks. This behavior allows for the use of alumina ceramics in intrinsically safe mirrors for lasers with a wavelength of 1µm, avoiding complex beam penetration detection schemes.

[0075] like Figure 4CAs illustrated, backscattering on the ceramic surface is also diffuse reflection. Unlike many conventional substrates, backscattering can also damage equipment; however, it is often difficult to define reflections from molten surfaces, making accurate safety calculations challenging. By its very nature, backscattering from partially molten surfaces is undefined; however, it can have a focusing effect on distant surfaces and is difficult to calculate and predict to ensure it remains within safety thresholds. Similarly, diffuse backscattering from the ceramic substrate 60C provides a controlled diffuse reflection surface, which is superior. It allows for better safety calculations. Preventing substrate melting is a significant advantage.

[0076] For alumina ceramics, solid-state sintering involves densifying the green body into a dense, solid object at temperatures above 1500°C to 1800°C without melting (alumina has a melting point of 2050°C). Sintering to full density can be achieved under atmospheric pressure (reduced pressure sintering).

[0077] For high-power laser beams, thermal dissipation of the mirror must be considered. The thermal conductivity of sintered alumina is approximately 30 W / m / K, which is nearly 10 times better than quartz (~3 W / m / K), fused silica (~1 W / m / K), or other high-melting-temperature ceramics such as ZrO2 (melting temperature around 3000°C; ~3 W / m / K). The CTE is also higher, at 2.5 times, so the net result means that thermal deformation is also about 4 times lower than on SiO2-based substrates. Common dielectric coating stacks can be used to produce substrates made of sintered alumina ceramic. To achieve polishing and planarization, polishable layers, such as sputtered alumina, can be added to obtain specific optical flatness. Polishing standard alumina can achieve as low as approximately 5 nm (RMS). For mirrors in high-power laser applications with a 1 µm diameter, additional layers may be needed to improve flatness.

[0078] Alumina is relatively easy to process into complex shapes, which may be necessary for some mirrors in beam paths. Furthermore, alumina is a very cost-effective material available from multiple suppliers.

[0079] The optical properties of sintered alumina ceramics can depend on characteristics such as pore size and particle size. Figure 6 Test results depicting the relationship between the transmittance T [%] of incident radiation and the dominant wavelength λ0 [μm] of sintered alumina ceramic samples with various fixed pore sizes r [μm] are presented. The test samples used had a thickness t of 0.5 mm and a porosity V of 0.002. p Alumina ceramic samples. Wavelengths varied from 0.5 μm to 5 μm.

[0080] The sintering process for manufacturing alumina ceramics can produce different particle sizes and pore sizes. Figure 6The graph shows the net (unscattered) transmittance through a sample with a thickness of 0.5 mm. Figure 6 The curves indicate that transmittance is strongly dependent on pore size (each line is indicated by r=xx µm). The transmittance results are surprising because for radiation with a wavelength of 1 μm, based on a refractive index of about 1.7, one would expect a transmittance of about 85%.

[0081] Figure 6 The graphs indicate that alumina ceramics with a pore size of approximately 0.5 μm will have a very limited transmittance of about 5% for radiation with a wavelength of 1 μm. Alumina ceramics with a pore size of approximately 1.0 μm will have a transmittance of about 20% for radiation with a wavelength of 1 μm. Alumina ceramics with a pore size of approximately 0.1 μm will have a transmittance of about 30% for radiation with a wavelength of 1 μm.

[0082] In a practical embodiment, the substrate for the beam path of a laser pulse with a wavelength of 1 μm can be composed of alumina ceramic having a pore size in the range of 0.3 μm to 1.0 μm (e.g., about 0.5 μm).

[0083] Alumina ceramics exhibit relatively low absorption of electromagnetic energy. For temperatures T ranging from 300 K to 2300 K, the absorption coefficient α can be approximately 25 ppm / cm or 2.5 × 10⁻⁶. -5 cm -1 Other properties of alumina ceramics, such as tensile strength and CTE, are comparable to or better than those of glass. For alumina ceramics with 10-micron particles, the fracture strength is approximately 200 MPa, which is significantly higher than the fracture strength of glass, which is typically around 100 MPa. Furthermore, alumina ceramics are polishable and are suitable substrates for reflective optical coatings.

[0084] For details on the optical properties of sintered alumina, please refer to pages 162-170 of Infrared Physics and Technology 77 (2016), Aleksey V. Liitsyn et al., “Near-infrared optical properties of a porous alumina ceramics produced by hydrothermal oxidation of aluminum”.

[0085] Figure 7 An example of hole 48 is shown. (See example.) Figure 7One or more apertures 48 shown may be positioned along the beam path 40. Apertures 48 may include external elements 80 with openings 82 to allow laser pulses 30 to pass through. The openings may have windows 84, for example made of quartz or glass, which are substantially transmissive to the corresponding laser pulse 30. External elements 80 are generally impermeable to the corresponding laser pulse. Elements 80 may be structural elements, such as ring structures.

[0086] The aperture 48 enhances safety, for example, by providing an early warning signal if the laser pulse deviates from its predetermined trajectory. A sensor connected to element 80 can detect when the laser pulse engages structural element 80, thereby triggering a warning signal to indicate that the laser pulse has deviated from its trajectory.

[0087] like Figure 7 The hole illustrated is relatively basic. Typically, a security hole 48 may include a serrated profile with very sharp angles on its surface, such that light reflected from the surface is reflected into a second surface within the hole, rather than being reflected out of the hole. Furthermore, the surface may typically be treated to be more absorbent.

[0088] As mentioned in the introduction, once the laser beam hits a conventional aperture, the conventional aperture must be replaced. Furthermore, windows 84 or structural elements 80 are typically susceptible to damage upon laser impact, and such damage often results in fragmentation. This, in turn, can lead to secondary problems such as contamination of nearby diamond windows. For laser pulses with a wavelength of 1µm, backscattering during the melting of aperture 48 triggers numerous uncertainties in the safety assessment. This is problematic because the surface shape is undefined, as backscattering can reach any angle and any random focus. This makes safety calculations very difficult; there is always a spot where you cannot tolerate an intermediate focus, and for molten surfaces, it is relatively difficult to guarantee that the light remains within safety specifications.

[0089] In this embodiment, element 80 comprises silicon carbide (SiC). SiC can be used as the substrate for aperture 48, particularly for its structural component 80. Furthermore, the structural component can be made of other materials, such as a metal that allows for the detection of laser light. In a practical embodiment, aperture 48 comprising SiC is suitable for beam paths of laser pulses with a wavelength of approximately 1 μm.

[0090] Compared to other ceramic materials or metals, silicon carbide has two unique advantages. Silicon carbide has no liquid phase, and its absorption depth into the skin can be adjusted.

[0091] First, SiC lacks a liquid phase. When typical materials begin to melt, the absorption rate of electromagnetic energy typically increases by an order of magnitude, triggering thermal runaway when struck by a laser pulse. This is the basis for laser processing or welding of many materials. In contrast, SiC does not melt, so it will not initiate this runaway. This is a very unique characteristic that, until now, has not been recognized or explicitly considered advantageous in laser-safe and non-destructive situations.

[0092] SiC begins to sublimate at 2700°C. This temperature is much higher than typical infrared mirror substrates used for such applications, such as Al (~600°C), Cu (~1100°C), Si (~1400°C), or molten silicon dioxide (~2000°C). Conventional coating materials for radiation with a wavelength of 1 μm typically consist of a combination of SiO2 (melting at 1400°C) and, for example, refractory oxides such as hafnium(IV) oxide (HfO2; melting at 2800°C), Al2O3 (2000°C), or Ta2O5 (~1800°C). Once the substrate begins to melt, a single layer of coating on the optics, such as tungsten (3400°C) or molybdenum (2600°C), will not protect the surface. In laser welding, molybdenum mirrors are sometimes used because they are very scratch-resistant and easy to remove weld debris, and they do not melt when molten metal (e.g., iron) splashes onto the front surface. However, for light with a wavelength of 1µm, the reflectivity is only 70%, making this material unsuitable. For CO2 lasers, Mo has a reflectivity exceeding 98%, making it a viable option.

[0093] Materials containing SiC compounds such as SiSiC typically do not have high melting temperatures, where SiC particles are contained within a Si matrix. The composite material also has an unsuitable liquid phase. In a preferred embodiment, the material used for the porous element 80 is pure SiC.

[0094] Another advantage of silicon carbide is that the material's absorptivity can be tuned by controlling the dopant level. Controlling the absorptivity prevents laser ablation. Preventing laser ablation is a property of creating "impenetrable" materials for high-power lasers. In contrast, laser ablation of materials in the nanosecond pulse length range depends on the limited absorption depth during the nanosecond laser pulse. The thermal diffusion length within a few nanoseconds is at most a few micrometers. Therefore, during nanosecond-scale pulse lengths, the entire laser pulse energy is absorbed in a small volume. This small volume is then extremely heated, causing it to evaporate or rapidly melt (see also the section above regarding...). Figure 4A(As described above). In contrast, silicon carbide (SiC) can be completely transparent to light with wavelengths of approximately 1 μm. Absorption is limited to the ppm level. By controlling the amount of doping, absorption can be extended to, for example, millimeters in length. Therefore, the energy in the laser pulse can be much higher during the pulse, before the material melts. In a 1 μm beam path, this means that the laser is never actually sufficiently focused to achieve ablation, i.e., ablation can be virtually completely avoided. This is a unique advantage.

[0095] Tests have shown that samples made of SiC are indestructible to radiation with wavelengths of approximately 1 µm. The latter supports a wide range of laser powers, including those up to 700 W and 1.5 kW. However, samples made of SiSiC burned through within 1 to 2 seconds. This difference is due to the fact that SiC has no melting point. Silicon carbide only has a sublimation temperature of approximately 2700°C. SiSiC will melt at approximately 1000°C to 1200°C, similar to pure silicon.

[0096] Under irradiation with light at a wavelength of approximately 1 μm and a power of about 100 kW / cm², the optical coating on the substrate may begin to deteriorate. Tests indicate that the SiC substrate remains intact even when the coating is damaged. Conversely, samples made of other materials, such as Si substrates, suffer catastrophic damage. In silicon, the laser pulse completely burns a hole through the material within one second.

[0097] Figure 8 The graph shows that the absorption rate of silicon carbide is a function of the dopant. Adding selective dopant allows for tuning or selection of the optimal SiC composition to be used. Figure 8 The absorption coefficient of 3C-SiC at 300K is shown [cm]. -1 The relationship between [eV] and photon energy. Line 90 indicates the result of relatively pure crystalline SiC. Line 92 indicates the relationship between [eV] and photon energy. d =10 19 cm -3 The results of SiC doping.

[0098] The absorption temperature during the pulse is calculated; if absorption occurs only at the inlet surface, the surface temperature will exceed 2700°C. For a 700W CW laser with 100% absorption at a 0.5mm spot size, representing the worst-case scenario with a 1µm beam path, the surface temperature rises to 1000°C within 10 microseconds and to 2700°C within 30 microseconds. For a 50kHz laser with a pulse energy of 14mJ, approximately 5J / cm² is calculated. 2The flux was increased to reach a temperature of 2700°C. The latter could imply a risk of ablation, i.e., the removal of material by the laser pulse. This risk can be avoided if the absorption extends over a larger volume and is not limited to the surface. This can be achieved by adjusting the impurity level of the SiC. In practical embodiments, doped SiC is chosen. Therefore, the depth of light penetration is reduced.

[0099] While alumina has been mentioned above, other materials can also be used as the substrate for the mirror. In a practical embodiment, substrate 60C is an optically transparent ceramic that is sintered to make the material more robust. The following is a list of possible materials, but it is not exhaustive. Alumina is the cheapest, but a range of other materials are available that can also be used. For example, modified materials can be doped. Examples of other materials that can also possess the properties of surface and volume scattering for substrate 60C include:

[0100] •- Zirconia ceramics

[0101] • Magnesium oxide ceramics

[0102] •-Magnesium aluminum silicate

[0103] •-Aluminum oxynitride

[0104] •- Zirconium nitride ceramics

[0105] •-Silicon nitride ceramics

[0106] •-Boron nitride ceramics

[0107] Other materials that can be used for aperture 48 include zinc sulfide, which is a material that does not melt or sublimate. Zinc sulfide is also transparent at both 1µm and 10µm apertures, which may offer some unique advantages. Zinc sulfide can also be doped to give it sufficient absorption similar to SiC. However, unlike SiC, zinc sulfide has very low thermal conductivity, so it may be more prone to fracture. Zinc sulfide is easier to machine than SiC, making this material suitable for practical applications.

[0108] While this article provides specific references to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein can have other applications. Possible other applications include the fabrication of integrated optical systems, patterning for guiding and detecting magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0109] While embodiments of the invention are specifically referred to herein in the context of a photolithography apparatus, these embodiments can be used with other apparatuses. Embodiments of the invention can form part of a mask inspection apparatus, a measurement apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning apparatuses). These apparatuses are generally referred to as photolithography tools. Such photolithography tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0110] Although specific reference has been made above to the use of embodiments of the present invention in the context of optical lithography, it will be understood that the present invention is not limited to optical lithography and can be used in other applications, such as imprint lithography, where circumstances permit.

[0111] The following are part of this disclosure.

[0112] 1. A laser system, comprising:

[0113] A transmission system, configured to transmit a laser beam along a beam path from the inlet to the target area,

[0114] The beam path is provided with a plurality of reflective optical elements, the plurality of reflective optical elements comprising a substrate having a reflective coating thereon.

[0115] The substrate is made of ceramic.

[0116] 2. The laser system according to aspect 1, wherein the substrate is made of porous or sintered alumina.

[0117] 3. The laser system according to aspect 2, wherein the alumina is in the form of sintered ceramic, wherein the alumina is polycrystalline and diffusely reflective to visible light.

[0118] 4. The laser system according to any of the foregoing aspects, wherein the beam path is provided with one or more loops, the one or more loops having holes for the laser beam to pass through, wherein the loops comprise silicon carbide (SiC).

[0119] 5. The laser system according to aspect 4, wherein the ring is made of SiC.

[0120] 6. The laser system according to aspect 5, wherein the SiC is doped with a concentration of 10. 19 cm -3 N on the order of magnitude d .

[0121] 7. The laser system according to any one of the foregoing aspects, wherein the laser beam comprises a laser having a wavelength of approximately 1 μm.

[0122] 8. A laser system according to any one of the foregoing aspects, wherein the beam path is provided with one or more amplifiers for amplifying the electromagnetic power of the laser beam.

[0123] 9. The laser system according to any one of the foregoing aspects, comprising a housing enclosing at least a portion of the beam path.

[0124] 10. A photolithography system comprising the laser system according to aspect 1.

[0125] 11. A radiation source comprising a laser system according to any one of aspects 1 to 9.

[0126] 12. A method comprising the following steps:

[0127] Provide laser beam;

[0128] A transmission system is used to transmit a laser beam along a beam path from the inlet to the target area, the beam path being provided with multiple reflective optical elements to reflect the laser beam along the beam path;

[0129] The reflective optical element includes a substrate having a reflective coating thereon, wherein the substrate is made of ceramic.

[0130] 13. The method according to aspect 12, wherein the ceramic is alumina in the form of sintered ceramic, wherein the alumina is polycrystalline and diffusely reflective to visible light.

[0131] 14. The method according to aspect 12 or 13, wherein the beam path is provided with one or more loops, the one or more loops having holes for allowing the laser beam to pass through, wherein the loops are made of SiC, wherein the SiC is doped with a concentration of 10. 19 cm -3 N on the order of magnitude d .

[0132] 15. The method according to any one of aspects 12 to 14, wherein the laser beam comprises a laser having a wavelength of approximately 1 μm.

[0133] 16. The method according to any one of aspects 12 to 15, comprising the step of amplifying the electromagnetic power of the laser beam using one or more amplifiers disposed in the beam path.

[0134] While specific embodiments of the invention have been described above, it will be understood that the invention may be practiced in ways different from those described. The description above is intended to be exemplary and not restrictive. Therefore, those skilled in the art will understand that modifications can be made to the invention as described without departing from the scope of the claims set forth below.

Claims

1. A laser system, comprising: A transmission system, arranged to transmit a laser beam from the entrance to the target area along the beam path, The beam path is provided with a plurality of reflective optical elements, the plurality of reflective optical elements comprising a substrate having a reflective coating thereon. The substrate is made of ceramic.

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

3. The laser system according to claim 2, wherein, The alumina is in the form of sintered ceramic, wherein the alumina is polycrystalline and diffusely reflective to visible light.

4. The laser system according to any one of the preceding claims, wherein, The beam path is provided with one or more loops, the loops having holes for the laser beam to pass through, wherein the loops are made of silicon carbide (SiC).

5. The laser system according to claim 4, wherein, The ring is made of SiC.

6. The laser system according to claim 5, wherein, The SiC is doped with a concentration of 10. 19 cm -3 N on the order of magnitude d .

7. The laser system according to any one of the preceding claims, wherein, The laser beam comprises a laser with a wavelength of approximately 1 μm.

8. The laser system according to any one of the preceding 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 according to any one of the preceding claims, comprising a housing enclosing at least a portion of the beam path.

10. A photolithography system comprising the laser system according to claim 1.

11. A radiation source comprising a laser system according to any one of claims 1 to 9.

12. A method comprising the following steps: Provide laser beam; A transmission system is used to transmit a laser beam along a beam path from the inlet to the target area, the beam path being provided with multiple reflective optical elements to reflect the laser beam along the beam path; The reflective optical element includes a substrate having a reflective coating thereon, wherein the substrate is made of ceramic.

13. The method according to claim 12, wherein, The ceramic is alumina in the form of sintered ceramic, wherein the alumina is polycrystalline and diffusely reflective to visible light.

14. The method according to claim 12 or 13, wherein, The beam path is provided with one or more loops, each loop having a hole for the laser beam to pass through, wherein the loops are made of SiC, wherein the SiC is doped with a concentration of 10. 19 cm -3 N on the order of magnitude d .

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

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