Protecting optical materials of optical components from radiation degradation

Abstract

An optical system includes a bulk material that includes a fluorine (F)-containing optical material. The bulk material is exposed to an environment having a pressure ranging from atmospheric pressure to a vacuum when the bulk material is under an extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV), and / or UV radiation. The environment includes at least one gas or vapor. The at least one gas or vapor includes polar molecules.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to U.S. Provisional Patent Application 63 / 070,842, filed August 27, 2020, the contents of which are incorporated herein by reference in their entirety. Technical Field

[0003] This invention relates to protecting optical materials of optical components from radiation degradation, for example, protecting fluorine (F)-containing optical materials of optical components from radiation degradation. Background Technology

[0004] Fluorine-containing optical materials are commonly used in broadband optical applications and / or systems, ranging from vacuum ultraviolet (VUV) to near-infrared (NIR). Due to radiation degradation, the properties of fluorine-containing optical materials degrade rapidly across the extreme ultraviolet (EUV), VUV, deep ultraviolet (DUV), and / or UV spectral ranges, leading to reduced optical performance and lifetime in optical applications and / or systems. Summary of the Invention

[0005] According to one embodiment, an optical system is disclosed. The optical system may include a bulk material comprising a fluorine (F) optical material. When the bulk material is exposed to extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV), and / or UV radiation, the bulk material may be exposed to an environment with pressure ranging from atmospheric pressure to vacuum. The environment may include at least one gas or vapor. At least one gas or vapor may include polar molecules.

[0006] According to another embodiment, an optical system is disclosed. The optical system may include a bulk material comprising a fluorine (F) optical material. When the bulk material is exposed to extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV), and / or UV radiation, the bulk material may be exposed to an environment with pressure ranging from atmospheric pressure to vacuum. The environment may include at least one gas or vapor. The at least one gas or vapor may include polar molecules. The optical system may also include a coating layer disposed on the bulk material.

[0007] According to another embodiment, an optical material is disclosed. The optical material may include a bulk material comprising a fluorine (F)-containing optical material. The F-containing optical material may be selected from magnesium fluoride (MgF2), calcium fluoride (CaF2), lanthanum trifluoride (LaF3), strontium fluoride (SrF2), barium fluoride (BaF2), lithium fluoride (LiF), and combinations thereof. The optical material may also include a coating layer comprising a coating material disposed on the bulk material, wherein the coating material may have a reflectivity of at least 20%. Attached Figure Description

[0008] Figure 1A schematic diagram of a first embodiment of an exemplary optical system according to the present disclosure is depicted.

[0009] Figure 2 The reflectance spectrum of MgF2 and the spectral changes when MgF2 is exposed to VUV radiation are depicted.

[0010] Figure 3 A schematic diagram of a second embodiment of an exemplary optical system according to the present disclosure is depicted. Detailed Implementation

[0011] This document describes embodiments of the present disclosure. However, it should be understood that the disclosed embodiments are merely examples, and other embodiments may take various alternative forms. The drawings are not necessarily drawn to scale; some features may be enlarged or reduced to show details of specific components. Therefore, the specific structural and functional details disclosed herein should not be construed as limiting, but merely as a representative basis for teaching those skilled in the art to use the embodiments in various ways. As will be understood by those skilled in the art, various features illustrated and described with reference to any of the drawings may be combined with features illustrated in one or more other drawings to produce embodiments that are not explicitly illustrated or described. The combinations of features shown provide representative embodiments for typical applications. However, for a particular application or implementation, various combinations and modifications of features consistent with the teachings of this disclosure may be required.

[0012] Optical components may consist only of bulk material. Optical components may include a substrate having an optical surface and a film formed on the optical surface, and a protective layer may be deposited on the film to protect the optical component from optical degradation. Alternatively, optical components may include bulk material, and a protective layer may be deposited on the bulk material to protect the optical component from optical degradation. Optical degradation can be caused by extreme ultraviolet (EUV), VUV, deep ultraviolet (DUV), and / or UV radiation. VUV radiation typically refers to UV light with wavelengths in the range of 100 to 190 nm. DUV radiation typically refers to UV light with wavelengths in the range of 190 to 280 nm. EUV radiation typically refers to light with wavelengths in the range of 10 to 100 nm. The film or bulk material may include fluorine (F)-containing optical materials, also known as F-containing optical materials. Unfortunately, F-containing optical materials may degrade due to moisture, oxidation, contamination, radiation, and other environmental conditions. Specifically, when optical components operate in environments with EUV, VUV, DUV and / or UV radiation (e.g., a vacuum containing residual gases or an inert atmosphere containing impurities), F-containing optical materials are susceptible to degradation by EUV, VUV, DUV and / or ultraviolet radiation, which degrades the optical performance of the optical components and shortens their lifespan.

[0013] When exposed to EUV, VUV, DUV, and / or UV radiation, fluorine (F) atoms in fluorine-containing optical materials migrate from their original positions within the material. Some of these F atoms may leave the material. This migration and / or loss of F atoms creates defects in the material, such as fluorine vacancies or interstitial F atoms. These defects can be surface or bulk defects. Both surface and bulk defects can negatively impact the optical performance of optical components. For example, migration and / or loss of F atoms can cause surface or subsurface oxidation of the fluorine-containing optical material, and can also cause oxidation of the film when it is deposited onto the optical component. Over time, this oxidation leads to degradation of the optical performance of the component. Calculations based on density function theory (DFT) also indicate that oxygen occupying fluorine vacancies (which leads to degradation of the optical performance of the component) is energy-favorable.

[0014] Optical components, including those used in EUV, VUV, DUV, and / or UV optical applications and / or systems, are expected to have a long service life, such as ten years or more. However, due to various harmful optical degradations, degraded optical components often need to be replaced. This replacement can be expensive. To mitigate radiation degradation of optical components, optical polishing techniques have been employed to improve or smooth rough or degraded optical surfaces. While optical polishing techniques can produce highly precise optical surfaces, some drawbacks of these techniques include the introduction of contaminants and residues onto the polished surface and the requirement of long processing times. Furthermore, optical polishing techniques can affect the mechanical properties of the optical surface, leading to changes in the optical properties of the optical components. Therefore, there is a need for more effective ways to protect optical components.

[0015] Various aspects of this disclosure relate to protecting optical materials of optical components from radiation degradation, particularly EUV, VUV, DUV, and / or UV radiation degradation. In one embodiment, the invention relates to an optical system comprising a bulk material having an F-containing optical material, wherein when the bulk material is exposed to EUV, VUV, DUV, and / or UV radiation, the bulk material is exposed to an environment with a pressure range from atmospheric pressure to vacuum to protect the bulk material from EUV, VUV, DUV, and / or UV radiation. The environment may include at least one gas or vapor. In another embodiment, the invention relates to an optical system comprising a bulk material and a coating layer formed on the bulk material, wherein the bulk material comprises an F-containing optical material, and when the bulk material is exposed to EUV, VUV, DUV, and / or UV radiation, the bulk material is exposed to an environment with a pressure range from atmospheric pressure to vacuum to protect the bulk material from EUV, VUV, DUV, and / or UV radiation. The environment may include at least one gas or vapor. In another embodiment, the present invention relates to an optical material comprising a bulk material and a coating layer formed on the bulk material, wherein the bulk material comprises an F-containing optical material and is exposed to an environment with pressure ranging from atmospheric pressure to vacuum to protect the bulk material from EUV, VUV, DUV and / or UV radiation when the bulk material is exposed to EUV, VUV, DUV and / or UV radiation. The environment may include at least one gas or vapor.

[0016] Figure 1 A schematic diagram of a first embodiment of an exemplary optical system 10 according to this disclosure is depicted. The optical system 10 may include optical materials. The optical materials may be bulk material 12 that protects the optical system 10 from optical degradation. Optical degradation may be caused by EUV, VUV, DUV, and / or UV radiation. The intensity of EUV, VUV, DUV, and / or UV radiation may be as high as 5 mW / cm². 2 Up to 20W / cm 2 Within the specified range. The bulk material 12 can be an F-containing optical material. F-containing optical materials can be, but are not limited to, magnesium fluoride (MgF2), calcium fluoride (CaF2), lanthanum trifluoride (LaF3), strontium fluoride (SrF2), barium fluoride (BaF2), lithium fluoride (LiF), and combinations thereof. The optical system 10 can be, but is not limited to, an optical window, beam splitter, mirror, charge-coupled device (CCD), detector, or time-delay integration (TDI) CCD. Upon exposure to EUV, VUV, DUV, and / or UV radiation, the bulk material 12 may undergo radiation degradation, including the migration and / or loss of F atoms and / or surface reconstruction in the bulk material 12. This radiation-induced degradation may therefore adversely affect the optical performance of the optical system 10.

[0017] To protect the bulk material 12 of the optical system 10 from radiation degradation, particularly EUV, VUV, DUV, and / or UV radiation degradation, when the optical system 10 or its bulk material 12 is subjected to optical radiation, it may be exposed to an environment with pressure ranging from atmospheric pressure to vacuum (e.g., below atmospheric pressure). The environment may include at least one gas or vapor. At least one gas or vapor may condense on the surface of the bulk material 12. At least one gas or vapor may include polar molecules. In some embodiments, at least one gas or vapor may include water (H2O). In some other embodiments, at least one gas or vapor may include other polar molecules having a dipole moment comparable to that of H2O. As described herein, the dipole moment of a molecule is a measure of the molecular polarity of intramolecular chemical bonds. The dipole moment of H2O is approximately 1.85 Debye (D). The dipole moments of polar molecules can range from 1.3 to 2.5D. Other polar molecules can be heavy water (D2O), methanol (CH3OH), ethylene glycol ((CH2OH)2), ethanol (CH3CH2OH), ammonia (NH3), and combinations thereof. The concentration of at least one gas or vapor exposed to the optical system 10 or the bulk material 12 of the optical system 10 can be less than 20 parts per million (ppm) by ambient volume. The concentration of at least one gas or vapor can be controlled at a level such that the presence of at least one gas or vapor does not interfere with optical radiation directed at the optical system 10 (e.g., attenuate the intensity of optical radiation), while still protecting the optical system 10 from optical radiation.

[0018] To protect the bulk material 12 of the optical system 10 from radiation degradation, particularly EUV, VUV, DUV, and / or UV radiation degradation, polar molecules in the environment can physically or chemically interact with the bulk material 12, for example, by forming bonds on the surface of the bulk material 12. Polar molecules can quench dangling bonds (i.e., break bonds) in the bulk material 12. Dangling bonds may be pre-existing or generated due to surface polishing. Through physical or chemical interactions with the bulk material 12, the surface energy of the bulk material 12 can be reduced, making the bulk material 12 more stable. Furthermore, due to being surrounded by polar molecules, it is energy-wise unfavorable for the bulk material 12 to undergo surface changes, such as surface reconstruction or phase transitions.

[0019] To expose the bulk material 12 of the optical system 10 to the environment, a purge gas can be used. The purge gas can be mixed with at least one gas or vapor. To prevent contamination of the bulk material 12, at least one gas or vapor and the purge gas can be purified before being introduced into the bulk material 12. The purge gas can be, but is not limited to, nitrogen (N2), helium (He), carbon monoxide (CO), carbon dioxide (CO2), krypton (Kr), argon (Ar), xenon (Xe), hydrogen (H2), compressed dry air, and combinations thereof. The amount of at least one gas or vapor introduced by the purge gas can be controlled at a level such that the presence of at least one gas or vapor does not interfere with optical radiation directed at the optical system 10 (e.g., attenuate radiation intensity) while still protecting the optical system 10 from optical radiation.

[0020] Figure 2 The reflectance spectrum of MgF2 and its spectral changes when MgF2 is exposed to VUV radiation are depicted. Specifically, curve I represents the spectral changes when MgF2 is not exposed to VUV radiation, while curves II, III, and IV represent the spectral changes when MgF2 is exposed to VUV radiation. Figure 2 The reflectance spectrum is shown in the wavelength range of 100 to 250 nm. The intensity of VUV radiation is 100 mW / cm². 2 The duration of VUV radiation is one week. When MgF2 is subjected to VUV radiation, it may be exposed to environments ranging from atmospheric pressure to vacuum (e.g., below atmospheric pressure). The environment may include H2O. The concentration of H2O in the environment may vary. Purified Ar gas is used to introduce H2O into MgF2.

[0021] Radiation on optical materials can induce surface defects. These defects can lead to a decrease or increase in the reflectivity of the optical material. (Reference) Figure 2 Curve I represents the case where no VUV radiation is directed towards MgF2 (e.g., before optical radiation). Curve IV represents the case where VUV radiation is directed towards MgF2, and MgF2 is exposed to an environment with a relatively low H2O concentration, for example, 0.1 ppm by ambient volume. Curves II and III represent the case where VUV radiation is directed towards MgF2, and MgF2 is exposed to an environment with a relatively high H2O concentration, for example, 10 ppm and 1 ppm by ambient volume, respectively. As shown in Curve IV, the reflectance of MgF2 indicates a decrease during a week of VUV radiation. However, after a week of VUV radiation, the spectral changes of MgF2 in Curves II and III are negligible compared to Curve I (i.e., where no VUV radiation is directed towards MgF2).

[0022] Continue to refer to Figure 2Curves II and III show that when MgF2 is exposed to an environment with a relatively high H2O concentration, such as 1 to 10 ppm by ambient volume, the presence of H2O does not interfere with optical radiation directed at MgF2 (e.g., attenuating radiation intensity) and can still protect MgF2 from radiation-induced degradation. Compared to curves II to III, the reflectance value of MgF2 in curve IV is significantly lower. This indicates that when MgF2 is under VUV radiation, exposing it to a certain amount of H2O can protect it from VUV radiation; however, when the amount of H2O introduced into MgF2 is relatively low, such as 0.1 ppm by ambient volume, exposing MgF2 to low amounts of H2O may not effectively protect it from VUV radiation.

[0023] Figure 3 A schematic diagram of a second embodiment of an exemplary optical system 30 according to this disclosure is depicted. The optical system 30 may include optical materials. The optical materials may be bulk material 32 that protects the optical system 30 from optical degradation. Optical degradation may be caused by EUV, VUV, DUV, and / or UV radiation. The intensity of EUV, VUV, DUV, and / or UV radiation may be as high as 5 mW / cm². 2 Up to 20W / cm 2 Within the specified range. The bulk material 32 can be an F-containing optical material. F-containing optical materials can be, but are not limited to, MgF2, CaF2, LaF3, SrF2, BaF2, LiF, or combinations thereof. The optical system can be, but is not limited to, an optical window, beam splitter, mirror, charge-coupled device (CCD), detector, or time-delay integration (TDI) CCD. Upon exposure to EUV, VUV, DUV, and / or UV radiation, the bulk material 32 of the optical system 30 may undergo radiation degradation, including the migration and / or loss of F atoms in the bulk material 32. This radiation degradation may therefore adversely affect the optical performance of the optical system 30.

[0024] refer to Figure 3 The bulk material 32 may include a first side 34 and a second side 36 opposite to the first side 34. The optical system 30 may include a coating layer 38 formed on the second side 36 of the bulk material 32. It is contemplated that the coating layer 38 may also be formed on the first side 34 of the bulk material 32. The coating layer 38 may include a coating material. The coating material may have a reflectivity of at least 20%. The coating material may be, but is not limited to, aluminum (Al), gold (Au), silver (Ag), copper (Cu), chromium (Cr), titanium (Ti), ruthenium (Ru), MgF2, CaF2, LaF3, SrF2, BaF2, LiF, or combinations thereof. The coating layer 38 may be a single layer or multiple layers. By forming the coating layer 38 on the bulk material 32, the bulk material 32 can act as a radiation reflector, such as a VUV radiation reflector.

[0025] To protect the bulk material 32 of the optical system 30 from radiation degradation, particularly EUV, VUV, DUV, and / or UV radiation degradation, when the optical system 30 or its bulk material 32 is subjected to optical radiation, it may be exposed to an environment with pressure ranging from atmospheric pressure to vacuum (e.g., below atmospheric pressure). The environment may include at least one gas or vapor. At least one gas or vapor may condense on the surface of the bulk material 32, for example, on a first side 34 of the bulk material 32. At least one gas or vapor may include polar molecules. In some embodiments, at least one gas or vapor may include water (H2O). In some other embodiments, at least one gas or vapor may include other polar molecules having a dipole moment comparable to that of H2O. As described herein, the dipole moment of a molecule is a measure of the molecular polarity of intramolecular chemical bonds. The dipole moment of H2O is approximately 1.85 Debye (D). The dipole moments of polar molecules can range from 1.3 to 2.5D. Other polar molecules can be heavy water (D2O), methanol (CH3OH), ethylene glycol ((CH2OH)2), ethanol (CH3CH2OH), ammonia (NH3), and combinations thereof. The concentration of at least one gas or vapor exposed to the optical system 30 or the bulk material 32 of the optical system 30 can be less than 20 parts per million (ppm) by ambient volume. The concentration of at least one gas or vapor can be controlled at a level such that the presence of at least one gas or vapor does not interfere with the optical radiation directed by the optical system 30 (e.g., attenuate radiation intensity) while still protecting the optical system 30 from optical radiation.

[0026] To protect the bulk material 32 of the optical system 30 from radiation degradation, particularly EUV, VUV, DUV, and / or UV radiation degradation, polar molecules in the environment can physically or chemically interact with the bulk material 32, for example, by forming bonds with atoms on the surface of the bulk material 32. Polar molecules can quench dangling bonds (i.e., break bonds) in the bulk material 32. Dangling bonds may be pre-existing or generated due to surface polishing. Through physical or chemical interactions with the bulk material 32, the surface energy of the bulk material 32 can be reduced, making the bulk material 32 more stable. Furthermore, due to being surrounded by polar molecules, it is energy-adverse for the bulk material 32 to undergo surface changes (e.g., surface reconstruction or phase transition).

[0027] To expose the bulk material 32 of the optical system 30 to the environment, a purge gas can be used. The purge gas can be mixed with at least one gas or vapor. To prevent contamination of the bulk material 32, at least one gas or vapor, as well as the purge gas, can be purified before being introduced into the bulk material 32. The purge gas can be, but is not limited to, nitrogen (N2), helium (He), carbon monoxide (CO), carbon dioxide (CO2), krypton (Kr), argon (Ar), xenon (Xe), hydrogen (H2), compressed dry air, and combinations thereof. The amount of at least one gas or vapor introduced by the purge gas can be controlled at a level such that the presence of at least one gas or vapor does not interfere with the optical radiation of the guiding optical system 30 (e.g., attenuate radiation intensity) while still protecting the optical system 30 from optical radiation.

[0028] While exemplary embodiments have been described above, this does not mean that these embodiments describe all possible forms included in the claims. The language used in this specification is descriptive and not restrictive, and it should be understood that various changes may be made without departing from the spirit and scope of this disclosure. As previously stated, features of various embodiments may be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments may have been described as providing an advantage or superiority over other embodiments or prior art implementations in one or more desired characteristics, those skilled in the art will recognize that one or more features or characteristics may be compromised to achieve desired overall system properties depending on the particular application and implementation. These properties may include, but are not limited to, cost, strength, durability, lifecycle cost, merchantability, appearance, packaging, size, suitability, weight, manufacturability, ease of assembly, etc. Therefore, with respect to one or more features, if any embodiment is described to the extent that it is less desirable than other embodiments or prior art implementations, these embodiments are not outside the scope of this disclosure and may be ideal for a particular application.

Claims

1. An optical system, comprising: Bulk materials containing fluorine (F) optical materials, wherein when the bulk material is exposed to extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV) and / or UV radiation, the bulk material is exposed to an environment with a pressure range from atmospheric pressure to vacuum, the environment comprising at least one gas or vapor comprising polar molecules, wherein the dipole moment of said polar molecules is in the range of 1.3 to 2.5D.

2. The optical system according to claim 1, wherein, The fluorine (F) optical material is selected from magnesium fluoride (MgF2), calcium fluoride (CaF2), lanthanum trifluoride (LaF3), strontium fluoride (SrF2), barium fluoride (BaF2), lithium fluoride (LiF), and combinations thereof.

3. The optical system according to claim 1, wherein, The at least one gas or vapor includes water (H2O).

4. The optical system according to claim 1, wherein, The at least one gas or vapor includes polar molecules having a dipole moment comparable to that of H2O.

5. The optical system according to claim 4, wherein, The polar molecules are selected from heavy water (D2O), methanol (CH3OH), ethylene glycol ((CH2OH)2), ethanol (CH3CH2OH), ammonia (NH3), and combinations thereof.

6. The optical system according to claim 1, wherein, The concentration of the at least one gas or vapor is less than 20 parts per million (ppm) based on the ambient volume.

7. The optical system according to claim 1, wherein, At least one gas or vapor is introduced into the bulk material using a purging gas.

8. The optical system according to claim 7, wherein, The purging gas is selected from nitrogen (N2), helium (He), carbon monoxide (CO), carbon dioxide (CO2), krypton (Kr), argon (Ar), xenon (Xe), hydrogen (H2), compressed dry air, and combinations thereof.

9. The optical system according to claim 1, wherein, The intensity of the extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV), and / or UV radiation is within 5 mW / cm². 2 Up to 20W / cm 2 Within the range.

10. An optical system comprising: A bulk material containing fluorine (F) optical materials, wherein, when the bulk material is exposed to extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV), and / or UV radiation, the bulk material is exposed to an environment with a pressure range from atmospheric pressure to vacuum, the environment comprising at least one gas or vapor comprising polar molecules, wherein the dipole moment of said polar molecules is in the range of 1.3 to 2.5D; and A coating layer applied to a bulk material.

11. The optical system according to claim 10, wherein, The fluorine (F) optical material is selected from magnesium fluoride (MgF2), calcium fluoride (CaF2), lanthanum trifluoride (LaF3), strontium fluoride (SrF2), barium fluoride (BaF2), lithium fluoride (LiF), and combinations thereof.

12. The optical system according to claim 10, wherein, The at least one gas or vapor is water (H2O).

13. The optical system according to claim 10, wherein, The at least one gas or vapor includes polar molecules having a dipole moment comparable to that of H2O.

14. The optical system according to claim 13, wherein, The polar molecules are selected from heavy water (D2O), methanol (CH3OH), ethylene glycol ((CH2OH)2), ethanol (CH3CH2OH), ammonia (NH3), and combinations thereof.

15. The optical system according to claim 10, wherein, The concentration of the at least one gas or vapor is less than 20 parts per million (ppm) based on the ambient volume.

16. The optical system according to claim 10, wherein, The coating layer includes a coating material having a reflectivity of at least 20%.

17. The optical system according to claim 16, wherein, The coating material is selected from aluminum (Al), gold (Au), silver (Ag), copper (Cu), chromium (Cr), titanium (Ti), ruthenium (Ru), magnesium fluoride (MgF2), calcium fluoride (CaF2), lanthanum trifluoride (LaF3), strontium fluoride (SrF2), barium fluoride (BaF2), lithium fluoride (LiF), and combinations thereof.

18. The optical system according to claim 10, wherein, The intensity of the extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV), and / or UV radiation is within 5 mW / cm². 2 Up to 20W / cm 2 Within the range.

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