Optical system and optical material

Abstract

The performance of F-containing optical materials can degrade rapidly in the extreme ultraviolet (EUV), VUV, deep ultraviolet (DUV) and / or UV spectral ranges as a result of radiation degradation, leading to reduced optical performance and lifetime of optical applications and / or systems.SOLUTION: The optical system comprises a bulk material comprising an optical material comprising fluorine (F). When a bulk material is under extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV) and / or UV radiation, the bulk material is exposed to an environment with a pressure ranging from atmospheric pressure to vacuum. The environment includes at least one type of gas or vapor. At least one type of gas or vapor comprises polar molecules.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to protecting optical materials of optical components from radiation degradation, for example, protecting fluorine (F)-containing optical materials of optical components from degradation under radiation. [Background technology]

[0002] REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 070,842 (August 27, 2020), the contents of which are incorporated herein by reference in their entirety.

[0003] F-containing optical materials are typically used in broadband optical applications and / or systems ranging from vacuum ultraviolet (VUV) to near infrared (NIR) optical applications and / or systems. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent Application Publication No. 2014 / 0158914 [Patent Document 2] U.S. Patent Application Publication No. 2012 / 0250144 Summary of the Invention [Problem to be solved by the invention]

[0005] The performance of F-containing optical materials can degrade rapidly in the extreme ultraviolet (EUV), VUV, deep ultraviolet (DUV) and / or UV spectral ranges as a result of radiation degradation, leading to reduced optical performance and lifetime of optical applications and / or systems. [Means for solving the problem]

[0006] According to one embodiment, an optical system is disclosed. The optical system may include a bulk material including a fluorine (F)-containing optical material. The bulk material may be exposed to an environment at a pressure ranging from atmospheric pressure to vacuum when the bulk material is exposed to extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV), and / or UV radiation. The environment may include at least one type of gas or vapor. The at least one type of gas or vapor may include polar molecules.

[0007] According to another embodiment, an optical system is disclosed. The optical system may include a bulk material including a fluorine (F)-containing optical material. The bulk material may be exposed to an environment at a pressure ranging from atmospheric pressure to vacuum when the bulk material is exposed to extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV), and / or UV radiation. The environment may include at least one type of gas or vapor. The at least one type of gas or vapor may include polar molecules. The optical system may further include a coating layer disposed on the bulk material.

[0008] In yet another embodiment, an optical material is disclosed. The optical material includes a bulk material, and the bulk material includes a fluorine (F)-containing optical material. The F-containing optical material can be selected from the group consisting of magnesium fluoride (MgF), calcium fluoride (CaF), lanthanum trifluoride (LaF), strontium fluoride (SrF), barium fluoride (BaF), lithium fluoride (LiF), and combinations thereof. The optical material can further include a coating layer disposed on the bulk material, the coating layer including a coating material, and the coating material can have a reflectivity of at least 20%. [Brief explanation of the drawings]

[0009] [Figure 1] 1 shows a schematic diagram of a first embodiment of an exemplary optical system according to the present disclosure. [Figure 2] Figure 1 shows the reflectance spectrum of MgF2 and the spectral change when MgF2 is under VUV radiation. [Figure 3] FIG. 2 is a schematic diagram of a second embodiment of an exemplary optical system according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] Embodiments of the present disclosure are described herein. However, it should be understood that the disclosed embodiments are merely examples, and that other embodiments may take various alternative forms. The drawings are not necessarily to scale. Some features may be exaggerated or minimized to show details of particular components. Therefore, 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 how to use the embodiments in various ways. As will be understood by those skilled in the art, various features shown and described with reference to any one of the figures can be combined with features shown in one or more other figures to produce embodiments not explicitly shown or described. The illustrated combination of features provides a representative embodiment for a typical application. However, various combinations and modifications of features consistent with the teachings of the present disclosure may be desired for a particular application or implementation.

[0011] An optical component may include only bulk material. The optical component may include a substrate having an optical surface and a film layer formed on the optical surface, with a protective layer deposited on the film layer to protect the optical component from optical degradation. Alternatively, the optical component may include bulk material, with a protective layer deposited on the bulk material to protect the optical component from optical degradation. Optical degradation may be caused by extreme ultraviolet (EUV), VUV, deep ultraviolet (DUV), and / or UV radiation. VUV radiation generally refers to UV light having a wavelength in the range of 100 to 190 nm. DUV radiation generally refers to UV light having a wavelength in the range of 190 to 280 nm. EUV radiation generally refers to light having a wavelength in the range of 10 to 100 nm. Either the film layer or the bulk material may include an optical material containing fluorine (F) (also referred to as an F-containing optical material). Unfortunately, F-containing optical materials can be degraded by humidity, oxidation, contamination, radiation, and other environmental conditions. Specifically, when an optical component operates in an operating environment for EUV, VUV, DUV, and / or UV radiation (e.g., a vacuum containing residual gases or an inert atmosphere containing impurities), the F-containing optical material is susceptible to degradation by EUV, VUV, DUV, and / or UV radiation, resulting in degradation of the optical performance of the optical component and a shortened lifetime of the optical component.

[0012] Upon exposure to EUV, VUV, DUV, and / or UV radiation, fluorine (F) atoms in F-containing optical materials may migrate from their original positions within the F-containing optical material. Some of these F atoms may leave the F-containing optical material. The migration and / or loss of F atoms creates defects, such as fluorine vacancies or interstitial F atoms, in the F-containing optical material. The defects can be surface or bulk defects. Either surface or bulk defects can adversely affect the optical performance of optical components. For example, the migration and / or loss of F atoms can induce surface or subsurface oxidation of the F-containing optical material and can also cause oxidation of film layers when they are deposited on the optical component. Such oxidation can lead to degradation of the optical performance of the optical component over time. Calculations based on density functional theory (DFT) also suggest that it is energetically favorable for oxygen to occupy fluorine vacancies, which can lead to degradation of the optical performance of the optical component.

[0013] Optical components, including those applied in EUV, VUV, DUV, and / or UV optical applications and / or systems, are expected to have long service lives, such as 10 years or more. However, various harmful optical degradations often require replacement of degraded optical components. Such replacements can be expensive. To mitigate radiation degradation on optical components, optical polishing techniques have been employed to improve or smooth the rough or degraded optical surfaces of optical components. While the use of optical polishing techniques can produce highly accurate optical surfaces, some drawbacks of these optical polishing techniques are that they can introduce contaminants and residues onto the polished surface and require long processing times. Furthermore, optical polishing techniques can affect the mechanical properties of the optical surface, leading to variations in the optical performance of optical components. Therefore, there is a need for more efficient protection of optical components.

[0014] Aspects of the present 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 present disclosure relates to an optical system including a bulk material having an F-containing optical material, wherein the bulk material is exposed to an environment having a pressure ranging from atmospheric pressure to a vacuum when the bulk material is exposed to EUV, VUV, DUV, and / or UV radiation to protect the bulk material from EUV, VUV, DUV, and / or UV radiation. The environment may include at least one type of gas or vapor. In another embodiment, the present disclosure relates to an optical system including a bulk material and a coating layer formed on the bulk material, wherein the bulk material includes an F-containing optical material and is exposed to an environment having a pressure ranging from atmospheric pressure to a vacuum when the bulk material is exposed to EUV, VUV, DUV, and / or UV radiation to protect the bulk material from EUV, VUV, DUV, and / or UV radiation. The environment may include at least one type of gas or vapor. In yet another embodiment, the present disclosure is directed 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 at a pressure ranging from atmospheric pressure to a vacuum so as 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 type of gas or vapor.

[0015] FIG. 1 shows a schematic diagram of a first embodiment of an exemplary optical system 10 according to the present disclosure. The optical system 10 may include an optical material. The optical material may be a bulk material 12 that protects the optical system 10 from optical degradation. The optical degradation may be caused by EUV, VUV, DUV, and / or UV radiation. The intensity of the EUV, VUV, DUV, and / or UV radiation may be 5 mW / cm. 2 ~20W / cm 2The bulk material 12 may be an F-containing optical material. The F-containing optical material may be, but is not limited to, magnesium fluoride (MgF), calcium fluoride (CaF), lanthanum trifluoride (LaF), strontium fluoride (SrF), barium fluoride (BaF), lithium fluoride (LiF), or combinations thereof. The optical system 10 may be, but is not limited to, an optical window, a beam splitter, a mirror, a charge-coupled device (CCD), a detector, or a time-delay integration (TDI) CCD. Upon exposure to EUV, VUV, DUV, and / or UV radiation, the bulk material 12 may undergo radiation degradation, including migration and / or loss of F atoms within the bulk material 12 and / or surface reconstruction. Such radiation-induced degradation may consequently adversely affect the optical performance of the optical system 10.

[0016] To protect the bulk material 12 of the optical system 10 from radiation degradation, particularly EUV, VUV, DUV, and / or UV radiation degradation, the optical system 10 or the bulk material 12 of the optical system 10 may be exposed to an environment with a pressure ranging from atmospheric pressure to vacuum (e.g., subatmospheric pressure) when exposed to optical radiation. The environment may include at least one type of gas or vapor. The at least one type of gas or vapor may condense on a surface of the bulk material 12. The at least one type of gas or vapor may include polar molecules. In some embodiments, the at least one type of gas or vapor may include water (H2O). In some other embodiments, the at least one type of gas or vapor may include other polar molecules with dipole moments comparable to the dipole moment of H2O. As described herein, the dipole moment of a molecule is a measure of the molecular polarity of the chemical bonds within the molecule. The dipole moment of H2O is approximately 1.85 Debye (D). The dipole moment of the polar molecules may range from 1.3 to 2.5 D. Other polar molecules may be heavy water (DO), methanol (CHOH), ethylene glycol ((CHOH)), ethanol (CHCHOH), ammonia (NH), or combinations thereof. The concentration of the at least one type of gas or vapor exposed to optical system 10 or the bulk material 12 of optical system 10 may be less than 20 parts per million (ppm) by volume of the environment. The concentration of the at least one type of gas or vapor may be controlled at a level such that the presence of the at least one type of gas or vapor does not interfere with optical radiation directed at optical system 10 (e.g., attenuates the optical radiation intensity) while still protecting optical system 10 from the optical radiation.

[0017] 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, such as by forming bonds on the surface of the bulk material 12. The polar molecules can quench dangling bonds (i.e., broken bonds) in the bulk material 12. The dangling bonds may be pre-existing or may be created by surface polishing. By physically or chemically interacting with the bulk material 12, the surface energy of the bulk material 12 can be reduced, making the bulk material 12 more stable. Furthermore, because the bulk material 12 is surrounded by polar molecules, it is energetically unfavorable for the bulk material 12 to undergo surface changes, such as surface reconstruction or phase transitions.

[0018] A purge gas may be used to expose the bulk material 12 of the optical system 10 to the environment. The purge gas may be mixed with at least one type of gas or vapor. To protect the bulk material 12 from contamination, the at least one type of gas or vapor as well as the purge gas may be purified before being introduced into the bulk material 12. The purge gas may be, but is not limited to, nitrogen (N), helium (He), carbon monoxide (CO), carbon dioxide (CO), krypton (Kr), argon (Ar), xenon (Xe), hydrogen (H), compressed dry air, or a combination thereof. The amount of the at least one type of gas or vapor introduced by the purge gas may be controlled at a certain level so that the presence of the at least one type of gas or vapor does not interfere with the optical radiation directed toward the optical system 10 (e.g., attenuates the radiation intensity) while still protecting the optical system 10 from the optical radiation.

[0019] Figure 2 shows the reflectance spectrum and spectral change of MgF2 when MgF2 is exposed to VUV radiation. In particular, plot I represents the spectral change when MgF2 is not exposed to VUV radiation, while plots II, III, and IV represent the spectral change when MgF2 is exposed to VUV radiation. Figure 2 shows the reflectance spectrum in the wavelength range of 100-250 nm. The intensity of VUV radiation is 100 mW / cm. 2 The duration of VUV irradiation is one week. When exposed to VUV radiation, the MgF2 can be exposed to an environment with a pressure ranging from atmospheric pressure to vacuum (e.g., subatmospheric pressure). The environment can include H2O. The concentration of H2O in the environment can vary. Purified Ar gas is used to introduce H2O into the MgF2.

[0020] Radiation on an optical material can cause surface defects in the optical material. Such surface defects can result in a decrease or increase in the reflectance value of the optical material. Referring to FIG. 2, plot I represents a scenario in which VUV radiation is not directed at MgF2 (e.g., before optical radiation). Plot IV represents a scenario in which VUV radiation is directed at MgF2 while the MgF2 is exposed to an environment with a relatively low HO concentration (e.g., 0.1 ppm by volume of the environment). Plots II and III represent scenarios in which VUV radiation is directed at MgF2 while the MgF2 is exposed to environments with relatively high HO concentrations (e.g., 10 ppm by volume and 1 ppm by volume of the environment), respectively. As shown by plot IV, the reflectance value of MgF2 appears to decrease over one week of VUV radiation. However, after one week of VUV irradiation, the amount of spectral change of MgF2 in plots II and III is negligible compared to that in plot I (i.e., when VUV radiation is not directed at MgF2).

[0021] Continuing to refer to Figure 2, plots II and III appear to indicate that when MgF2 is exposed to an environment with a relatively high HO concentration (e.g., in the range of 1 to 10 ppm by volume of the environment), the presence of HO does not interfere with the optical radiation directed at the MgF2 (e.g., attenuates the radiation intensity) and may nonetheless protect the MgF2 from radiation-induced decomposition. In contrast to plots II-III, the reflectance values ​​of MgF2 for plot IV appear significantly lower. This seems to suggest that exposing MgF2 to a certain amount of HO when the MgF2 is under VUV radiation may protect the MgF2 from VUV radiation. However, when the amount of HO introduced into the MgF2 is relatively small, e.g., 0.1 ppm by volume of the environment, exposing the MgF2 to a small amount of HO may not effectively protect the MgF2 from VUV radiation.

[0022] 3 shows a schematic diagram of a second embodiment of an exemplary optical system 30 according to the present disclosure. The optical system 30 may include an optical material. The optical material may be a bulk material 32 that protects the optical system 30 from optical degradation. The optical degradation may be caused by EUV, VUV, DUV, and / or UV radiation. The intensity of the EUV, VUV, DUV, and / or UV radiation may be 5 mW / cm. 2 ~20W / cm 2 The bulk material 32 may be an F-containing optical material, including, but not limited to, MgF2, CaF2, LaF3, SrF2, BaF2, LiF, or combinations thereof. The optical system may be, but is not limited to, an optical window, a beam splitter, a mirror, a charge-coupled device (CCD), a detector, or a 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 migration and / or loss of F atoms in the bulk material 32. Such radiation degradation may consequently adversely affect the optical performance of the optical system 30.

[0023] Referring to FIG. 3 , the bulk material 32 may include a first surface 34 and a second surface 36 opposite the first surface 34. The optical system 30 may include a coating layer 38 formed on the second surface 36 of the bulk material 32. It is contemplated that the coating layer 38 may also be formed on the first surface 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 a combination 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.

[0024] To protect the bulk material 32 of the optical system 30 from radiation degradation, particularly EUV, VUV, DUV, and / or UV radiation degradation, the optical system 30 or the bulk material 32 of the optical system 30 may be exposed to an environment at a pressure ranging from atmospheric to vacuum (e.g., subatmospheric pressure) when exposed to optical radiation. The environment may include at least one type of gas or vapor. The at least one type of gas or vapor may condense on a surface of the bulk material 32, for example, on the first side 34 of the bulk material 32. The at least one type of gas or vapor may include polar molecules. In some embodiments, the at least one type of gas or vapor may include water (H2O). In some other embodiments, the at least one type of 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 the chemical bonds within the molecule. The dipole moment of H2O is approximately 1.85 Debye (D). The dipole moment of the polar molecules may range from 1.3 to 2.5 D. Other polar molecules may be heavy water (DO), methanol (CHOH), ethylene glycol ((CHOH)), ethanol (CHCHOH), ammonia (NH), or a combination thereof. The concentration of the at least one type of gas or vapor exposed to the optical system 30 or the bulk material 32 of the optical system 30 may be less than 20 parts per million (ppm) by volume of the environment. The concentration of the at least one type of gas or vapor may be controlled at a level such that the presence of the at least one type of gas or vapor does not interfere with the optical radiation directed at the optical system 30 (e.g., attenuates the radiation intensity) while still protecting the optical system 30 from the optical radiation.

[0025] 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, such as by forming bonds to atoms on the surface of the bulk material 32. The polar molecules can quench dangling bonds (i.e., broken bonds) in the bulk material 32. The dangling bonds may be pre-existing or may be created by surface polishing. By physically or chemically interacting with the bulk material 32, the surface energy of the bulk material 32 can be lowered, making the bulk material 32 more stable. Furthermore, because the bulk material 32 is surrounded by polar molecules, it is energetically unfavorable for the bulk material 32 to undergo surface changes, such as surface reconstruction or phase transitions.

[0026] A purge gas may be used to expose the bulk material 32 of the optical system 30 to the environment. The purge gas may be mixed with at least one type of gas or vapor. To protect the bulk material 32 from contamination, the at least one type of gas or vapor used as the purge gas may be purified before being introduced into the bulk material 32. The purge gas may be, but is not limited to, nitrogen (N), helium (He), carbon monoxide (CO), carbon dioxide (CO), krypton (Kr), argon (Ar), xenon (Xe), hydrogen (H), compressed dry air, or a combination thereof. The amount of the at least one type of gas or vapor introduced by the purge gas may be controlled at a certain level so that the presence of the at least one type of gas or vapor does not interfere with the optical radiation directed toward the optical system 30 (e.g., attenuates the radiation intensity) while still protecting the optical system 30 from the optical radiation.

[0027] While exemplary embodiments have been described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The terms used herein are terms of description rather than limitation, and it is to be understood that various changes can be made without departing from the spirit and scope of the present disclosure. As noted above, features of various embodiments can be combined to form further embodiments of the present invention that may not be explicitly described or illustrated. While various embodiments have been described as offering advantages or being preferred over other embodiments or prior art implementations with respect to 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 attributes that depend on the particular application and implementation. These attributes may include, but are not limited to, cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, maintainability, weight, manufacturability, ease of assembly, and the like. Thus, to the extent that any embodiments are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics, these embodiments may be desirable for particular applications without departing from the scope of the present disclosure.

Claims

1. 1. An optical system comprising: a bulk material comprising a fluorine (F)-containing optical material, the bulk material being exposed to an environment at a pressure ranging from atmospheric pressure to vacuum when the bulk material is under extreme ultraviolet (EUV), vacuum ultraviolet (VUV), deep ultraviolet (DUV) and / or UV radiation, the environment comprising at least one type of gas or vapor, the at least one type of gas or vapor comprising polar molecules; a coating layer disposed on the bulk material; and An optical system comprising:

2. The F-containing optical material is magnesium fluoride (MgF ２ ), calcium fluoride (CaF ２ ), lanthanum trifluoride (LaF ３ ), strontium fluoride (SrF ２ ), barium fluoride (BaF ２ ), lithium fluoride (LiF), and combinations thereof.

3. The at least one type of gas or vapor is water (H ２ 10. The optical system of claim 1, wherein

4. The at least one type of gas or vapor is H ２ 10. The optical system of claim 1, comprising a polar molecule having a dipole moment similar to the dipole moment of O.

5. The polar molecule is heavy water (D ２ O), methanol (CH ３ OH), ethylene glycol ((CH ２ OH) ２ ), ethanol (CH ３ CH ２ OH), ammonia (NH ３ 5. The optical system of claim 4, selected from the group consisting of:

6. 10. The optical system of claim 1, wherein the concentration of the at least one type of gas or vapor is less than 20 parts per million (ppm) by volume of the environment.

7. The optical system of claim 1 , wherein the coating layer comprises a coating material, the coating material having a reflectivity of at least 20%.

8. The coating material may be aluminum (Al), gold (Au), silver (Ag), copper (Cu), chromium (Cr), titanium (Ti), ruthenium (Ru), magnesium fluoride (MgF ２ ), calcium fluoride (CaF ２ ), lanthanum trifluoride (LaF ３ ), strontium fluoride (SrF ２ ), barium fluoride (BaF ２ 8. The optical system of claim 7, wherein the fluorine-containing compound is selected from the group consisting of lithium fluoride (LiF), lithium fluoride (LiF), and combinations thereof.

9. The intensity of the EUV, VUV, DUV and / or UV radiation is 5 mW / cm ２ ~20 W / cm ２ 10. The optical system of claim 1, wherein the optical system is in the range of .mu.m.sup.-10.mu.m.sup.-10.times. ...

10. An optical material, Bulk materials including fluorine (F)-containing optical materials, such as magnesium fluoride (MgF ２ ), calcium fluoride (CaF ２ ), lanthanum trifluoride (LaF ３ ), strontium fluoride (SrF ２ ), barium fluoride (BaF ２ a bulk material comprising a fluorine (F)-containing optical material selected from the group consisting of: fluorine (F), lithium fluoride (LiF), and combinations thereof; a coating layer disposed on the bulk material; and An optical material comprising:

11. The coating layer may be made of aluminum (Al), gold (Au), silver (Ag), copper (Cu), chromium (Cr), titanium (Ti), ruthenium (Ru), magnesium fluoride (MgF ２ ), calcium fluoride (CaF ２ ), lanthanum trifluoride (LaF ３ ), strontium fluoride (SrF ２ ), barium fluoride (BaF ２ ), lithium fluoride (LiF), and combinations thereof.

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