Airtight seal to protect titania-containing optical devices from light damage

Abstract

This invention provides a method for extending the lifespan of titania-containing resins and titania nanoparticle-containing resins with a high refractive index in sunlight. [Solution] Optical devices coated with a high refractive index (RI) titania-containing resin are hermetically sealed to protect them from degradation by short-wavelength visible light (less than 425 nm), including sunlight, and ultraviolet light in the 300-400 nm range. The method includes placing the optical device in an assembly that can be hermetically sealed. The method further includes introducing an inert atmosphere or vacuum and hermetically sealing the assembly so that oxygen and water are excluded. The manufactured hermetically sealed optical device obtains improved resistance to photochemical degradation due to exposure to sunlight and more common light having wavelengths in the 300-425 nm range. The method of this disclosure significantly improves the operating life of optical devices using high refractive index titania-containing resin exposed to sunlight.

Description

[Technical Field]

[0001] This disclosure relates generally to the protection of optical devices, and more specifically to methods for improving the durability of titania-containing coatings in optical applications. [Background technology]

[0002] Optical devices for professionals and individuals, waveguides, and lenses for cameras and detectors often employ high refractive index (RI) materials to enable thin lenses and wide field of view (FOV) for the devices. High refractive index glass and plastics (refractive index greater than 2.0 at 589 nm) are available, and etching patterns onto the substrate is a costly method when features are required on high refractive index materials, such as waveguides. An economical alternative to etching is a technique called nanoimprint lithography (NIL), which involves imprinting nanometer-scale patterns. In this technique, a resin is coated onto the substrate, imprinted with a master or stamper, and cured by heat or light. Generally, this curing is done with UV light, and this process is called UV-NIL. To avoid reflection, the NIL resin should match the refractive index (RI) of the substrate as closely as possible. NIL resins with a refractive index close to or exactly 2.0 are commercially available, and those with higher refractive indices may soon be commercially available.

[0003] High refractive index resins may also be used as submicron-thick layers within devices, such as in anti-reflective layers of optical devices. Anti-reflective layers are formed from thin films of high-refractive-index and low-refractive-index materials to cancel out the waves of reflected light.

[0004] High refractive index resins used in NIL processes or as thin films generally consist of polymerizable raw materials containing small (less than 50 nm) titanium dioxide-based nanoparticles (NPs). This high refractive index is given by titania (TiO2 or titanium dioxide), and in the most common forms, the refractive index at 589 nm is 2.5–2.7. The titania-based nanoparticles are formed by the sol-gel method or by pulverization, and the nanoparticles used in high refractive index resins contain organic "capping" agents to prevent aggregation and, in some cases, polymerize with the raw materials.

[0005] While the design objective is simple, high refractive index resins containing titania or titania-based nanoparticles have significant drawbacks. The band gap of TiO2 is 3.0–3.2 eV, and TiO2 absorbs short-wavelength visible light (less than 425 nm) and some of the ultraviolet component (i.e., 300–400 nm) in sunlight reaching the Earth's surface. Therefore, light in the 300–425 nm range is absorbed by titania, facilitating electron transitions from the valence band to the conduction band. The excited state of titania thus formed can either relax back to its ground state (a process called recombination) or undergo electron transfer or redox reactions that lead to radical formation. The photoreactivity of TiO2 typically causes rapid degradation of high refractive index resin films when exposed to short-wavelength visible light and ultraviolet light in sunlight. This degradation manifests as a decrease in mass, a change in refractive index, or often a shift from a solid to a sticky or semi-solid state in the cured material. In reality, high-refractive-index titania-containing resins are damaged by average exposure to sunlight over several days, which makes them unsuitable for use in equipment exposed to sunlight. [Overview of the project] [Problems that the invention aims to solve]

[0006] Therefore, it is desirable to employ methods to extend the lifespan of high refractive index titania-containing resins and titania nanoparticle-containing resins in sunlight, so that devices employing high refractive index titania resin can be used even in locations exposed to short-wavelength visible light and ultraviolet light from sunlight or other sources. [Means for solving the problem]

[0007] One aspect of the present disclosure provides a method for hermetically sealing an optical device coated with a high refractive index (RI) titania-containing resin to prevent degradation induced by short-wavelength visible light and ultraviolet light of sunlight. The method includes placing the optical device in an assembly configured for hermetically sealing, and ensuring that the assembly containing the optical device is in an oxygen- and moisture-free environment having an atmosphere consisting of at least one of an inert gas, a group of inert gases, or a vacuum. The method further includes sealing the assembly to form an hermetically sealed area.

[0008] Another aspect of the present disclosure discloses a method for protecting a device having a thin film containing titanium dioxide from degradation induced by short-wavelength visible light (less than 425 nm) and ultraviolet light in the range of 300-400 nm. The method includes placing the optical device in an assembly configured for hermetically sealed and ensuring that the assembly containing the optical device is in an oxygen- and moisture-free environment having an atmosphere consisting of at least one of an inert gas, a group of inert gases, or a vacuum. The method further includes sealing the assembly to form an hermetically sealed environment.

[0009] Another aspect of the present disclosure discloses a method for protecting optical devices employing high refractive index (RI) titania-containing resins used in nanoimprint lithography (NIL) from damage by short-wavelength visible light (less than 425 nm) and ultraviolet light in the range of 300-400 nm. The method includes placing the optical device in an assembly configured for hermetically sealed and ensuring that the assembly containing the optical device is in an oxygen- and moisture-free environment having an atmosphere consisting of at least one of an inert gas, a group of inert gases, or a vacuum. The method further includes sealing the assembly to form an hermetically sealed environment.

[0010] The methods and other aspects and features disclosed herein will be better understood by reading the following detailed description with reference to the accompanying drawings. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a schematic side view of a hermetically sealed device according to an embodiment of the present disclosure. [Figure 2] Figure 2 is a schematic plan view of the hermetically sealed device shown in Figure 1, according to an embodiment of the present disclosure. [Figure 3] Figure 3 is a model of the excitation of titania by sunlight, followed by the reduction of oxygen and oxidation of water, according to an embodiment of this disclosure. [Figure 4] Figure 4 shows the refractive index of a resin having unshielded titania-containing nanoparticles exposed to artificial sunlight, according to an embodiment of the present disclosure. [Figure 5] Figure 5 shows the behavior of LuxNIL® cured resin exposed to artificial sunlight according to an embodiment of this disclosure. [Figure 6] Figure 6 shows the behavior of LuxNIL® cured resin exposed to artificial sunlight according to an embodiment of this disclosure. [Figure 7] Figure 7 shows the behavior of a LuxNIL® cured resin exposed to artificial sunlight according to an embodiment of this disclosure.

[0012] The drawings are merely illustrative of one embodiment of the invention presented for illustrative purposes. Those skilled in the art will readily understand from the following description that alternative embodiments of the structures and methods described herein may be employed without departing from the principles described herein. [Modes for carrying out the invention]

[0013] Referring here to the drawings, particularly with respect to the hermetically sealed optical apparatus shown in Figure 1, an optical device 100 is disclosed, which is coated with a resin with a high refractive index (RI) and sealed within an assembly 102 using a hermetically sealed agent 104. Figure 1 shows a side cross-sectional view of the optical device 100 in the assembly 102 according to an embodiment of the present disclosure. Figure 2 shows a plan cross-sectional view of the optical device 100 in the assembly 102 of Figure 1 according to an embodiment of the present disclosure. The assembly 102 may be a case configured for hermetically sealing.

[0014] Figures 1 and 2 show an optical device 100 coated with a high refractive index titania-containing resin or titania nanoparticle-containing resin, housed in an assembly 102 between a first plate 106 and a second plate 108. Plates 106 and 108 may be made from materials that impede the movement of oxygen and water (i.e., glass, silicon, plastic, metal, or ceramic), but the only limitation is that one of the plates must be transparent or translucent so that the image inside the case is visible. An hermetic sealant 104 surrounds the optical device 100 or multiple optical devices between the first plate 106 and the second plate 108. The hermetic sealant 104 is applied to surround the optical device 100 to prevent the ingress of oxygen or water into the casing.

[0015] Short-wavelength visible light and the ultraviolet component of sunlight are absorbed by titania-containing materials, such as high-refractive-index titania resin coated on optical device 100. In these high-refractive-index titania resins, excited titania can react with any redox-sensitive materials, including organic raw materials and organic capping agents on nanoparticles (NPs) of the high-refractive-index titania resin. This phenomenon causes photodegradation of the high-refractive-index resin by "sunlight" through reactions that occur mainly between excited titania species and oxygen and / or water molecules, namely, the generation of an oxygen radical anion, which is then protonated to produce a hydroperoxyl radical and / or a protonated hydroxyl radical, and then deprotonated to produce a hydroxyl radical. The radicals thus formed can react with organic components in the solvent, damaging the cured high-refractive-index resin.

[0016] Figure 3 shows the photocatalytic behavior of titanium dioxide (TiO2) films or nanoparticles under ultraviolet (UV) light exposure. TiO2, with a band gap of 3.0–3.2 eV, absorbs short-wavelength visible light (less than 425 nm) and ultraviolet light in the 300–400 nm wavelength range, causing electrons to transition from the valence band to the conduction band. The excited electrons (eV) - ) can react with oxygen molecules (O2), forming superoxide radicals (O2 - ) forms a proton (H + It further reacts with the other components to produce a hydroperoxyl radical (·OOH). As a result, the holes oxidize water (H2O) to produce a hydroxyl radical (·OH) and a proton. The radicals thus formed are highly reactive and can react with organic components in the resin.

[0017] Optical device 100 exhibits an extended lifespan in sunlight when the hermetic seal of optical device 100, which includes a resin layer based on high refractive index titania or titania nanoparticles, excludes oxygen and water. The high refractive index titania-containing material is stable against oxygen and water in the absence of visible light with short wavelengths (less than 425 nm) and ultraviolet light of 300 - 400 nm, but not in the presence of such light. In the presence of such light, the hermetic seal confers stability to the high refractive index material. The hermetic seal of the high refractive index titania-containing or titania nanoparticle-containing resin within the optical device improves the stability in sunlight by orders of magnitude, i.e., the sunlight stability is improved by three or more orders of magnitude.

[0018] The hermetic seal for titania-containing NIL and resin was tested by the following procedure. The high refractive index titania nanoparticle resin was coated on a glass plate, also called a glass wafer. Both open samples and hermetically sealed samples conditioned in a nitrogen atmosphere were exposed to artificial sunlight in a xenon arc lamp test chamber. After various irradiation periods, the samples were analyzed and changes in refractive index and film thickness were measured.

[0019] More specifically, two types of high refractive index UV-NIL resins containing titania nanoparticles were studied. One type of resin contained unshielded nanoparticles, and the other type had nanoparticles containing an inorganic shield designed to protect the excited state titania at the molecular level around the nanoparticles. The latter type is a commercially available LuxNIL® U-series resin manufactured by Addison Clear Wave Coatings, Inc.

[0020] The resin containing the solvent was spin-coated on the glass wafer, heated at 80 - 100 °C for 1 minute for solvent removal, and irradiated with 250 mW / cm using a 365 nm LED flood illumination system under a nitrogen atmosphere. 2The samples were irradiated for 100 seconds and then treated at 150°C for 1–4 hours. These high refractive index resin films had a thickness of 600–900 nm. The refractive index and thickness of these samples were analyzed using a prism coupler.

[0021] Several optical devices were housed in glass cases sealed with Addison Clear Wave Coatings, Inc.'s UV-curing epoxy resin A1450-TX. This epoxy sealant has been used for hermetically sealed devices. The installation procedure was performed in a glove box under a dry nitrogen atmosphere with a measured oxygen concentration of less than 0.1%.

[0022] Both unprotected and hermetically sealed optical devices were irradiated in a Q-Sun® model Xe-1 xenon arc lamp test chamber (manufactured by Q-Lab Corp.) using a Daylight Q filter. The resulting irradiation spectra closely match the spectrum of sunlight reaching the Earth's surface. The irradiance was 0.35 W / (m²) at 340 nm. 2 The setting is 365 W / (m) in the range of 300-800 nm, which is the irradiance used in ASTM Method 5701 for testing photodegradable plastics using a xenon arc lamp (Reference 1). 2 The sample provided a value of (nm) (Reference 1), which is slightly greater than approximately twice the average amount of sunlight reaching the Earth's surface. Pieces were taken from the test chamber at any time, and the hermetically sealed samples were crushed and opened. The RI value and film thickness of the "opened" hermetically sealed samples were measured.

[0023] Referring to Figures 4-7, these show the results for unprotected, non-hermetically sealed samples. Figure 4 shows the refractive index of a resin containing shielded titania nanoparticles when exposed to artificial sunlight, at which point TiO2 in the resin has an anatase-type crystalline structure. Figures 5-7 show the behavior of these cured LuxNIL® resins when exposed to artificial sunlight, at which point TiO2 in the resin has a rutile-type crystalline structure. Figures 5-7 clearly show the change in refractive index and the ratio of resin layer thickness with respect to irradiation time. Figure 5 shows the results for LuxNIL® P283-U. Figure 6 shows the results for LuxNIL® P285-U. Figure 7 shows the results for LuxNIL® P288-U.

[0024] As shown in Figure 4, within several hours of irradiation, the sample containing shielded titania nanoparticles showed an initial increase in RI followed by a decrease in RI. As shown in Figures 5-7, the sample with shielded nanoparticles exhibited the same refractive index behavior, but the change in refractive index was clearly more gradual than that observed in the unshielded nanoparticle sample. In addition, Figures 5-7 clearly show the tracking of resin thickness for the shielded nanoparticle sample, and a considerable amount of material loss was observed during the irradiation process. In many cases, the sample became tacky when the refractive index decreased by approximately 0.5 to 1.0 refractive index units. The final reductions in refractive index and thickness, and the final formation of tacky or semi-solid material, serve as indicators and signs of degradation of the high refractive index titania resin 106.

[0025] The behavior of the high-refractive-index titania-containing resin differed significantly from that of the hermetic seal samples. In these cases, the changes in refractive index and resin thickness were very small even after hundreds of hours of exposure to artificial sunlight. Table 1 shows the results of hermetic sealing test samples made with titania-containing high-refractive-index resin. Even after hundreds of hours of exposure to artificial sunlight, only a small increase in refractive index and a slight decrease in resin thickness were observed. Using the initial rate of increase in refractive index of the open sample as a baseline, it is clear that the sunlight reactivity decreased by several orders of magnitude compared to the "open" sample, and a rough estimate suggests that the sunlight reactivity would decrease by more than three orders of magnitude.

[0026] Several features of the artificial sunlight exposure studies in Table 1 are noteworthy. One feature is that hermetic seal stability was observed in both resins containing anatase-type crystalline TiO2 and resins containing rutile-type crystalline TiO2. In addition, both uncoated titania and titania coated with layers of inorganic material exhibited similar behavior. Another important point is that the irradiance of the artificial sunlight used in the Table 1 studies was greater than twice the average irradiance of sunlight reaching the Earth's surface, and this irradiation was continuous, in contrast to the fact that sunlight typically only averages 12 hours a day. This means that the 600 hours of irradiation conducted in this study is equivalent to 100 days of exposure to average sunlight on the Earth's surface. Finally, it is noteworthy that the degree of degradation observed was so small that it may be possible to extend the lifespan of the titania-containing resin to the lifespan of the hermetic seal, which is known to be several years in some cases.

[0027] Table 1 shows the results of irradiation experiments on hermetically sealed samples. [Table 1] a Resins beginning with XX are test resins containing anatase-type TiO2 nanoparticles, while LuxNIL® P283U, P285U, and P288U resins contain rutile-type TiO2 nanoparticles. b Irradiation time (h) c Wavelength in refractive index measurement d Value measured after irradiation

Claims

1. A method for protecting an optical device coated with a high refractive index (RI) titania-containing resin from damage caused by short-wavelength visible light (less than 425 nm) and ultraviolet light in the range of 300 to 400 nm, Placing the optical device within an assembly configured for hermetically sealed, The assembly including the optical device is placed in an oxygen- and moisture-free environment having an atmosphere consisting of at least one of an inert gas, a group of inert gases, or a vacuum. To seal the aforementioned assembly, and A method comprising forming an airtight seal.

2. The method according to claim 1, further comprising obtaining a photo-stabilized optical device containing a high refractive index titania-containing resin that maintains a refractive index change of 0.10 units or less and at least 95% of the initial thickness even after at least 1,000 hours of average sunlight exposure on the Earth's surface.

3. The method according to claim 1, wherein the high refractive index titania-containing resin has a refractive index greater than 1.7 at 589 nm.

4. The method according to claim 1, wherein the crystal structure of the titania is anatase type or rutile type.

5. The method according to claim 1, wherein the titania is either uncoated or coated with one or more inorganic materials.

6. The method according to claim 1, wherein the high refractive index titania-containing resin comprises titanium oxide nanoparticles having an average particle size of less than 100 nm dispersed in an organic medium.

7. A method for protecting a device having a thin film containing titanium dioxide from degradation induced by short-wavelength visible light (less than 425 nm) and ultraviolet light in the range of 300 to 400 nm, Placing the device within an assembly configured for hermetically sealed, The assembly, including the optical device, is kept in an oxygen- and moisture-free environment having an atmosphere consisting of at least one of an inert gas, a group of inert gases, or a vacuum. To seal the aforementioned assembly, and A method comprising forming an airtight seal.

8. The method according to claim 7, further comprising obtaining a photo-stabilized optical device containing a high refractive index titania-containing resin that maintains a refractive index change of 0.10 units or less and at least 95% of the initial thickness even after at least 1,000 hours of average sunlight exposure on the Earth's surface.

9. The method according to claim 7, wherein the high refractive index titania-containing resin has a refractive index greater than 1.7 at 589 nm.

10. The method according to claim 7, wherein the crystal structure of the titania is anatase type or rutile type.

11. The method according to claim 7, wherein the titania is either uncoated or coated with one or more inorganic materials.

12. The method according to claim 7, wherein the high refractive index titania-containing resin comprises titanium oxide nanoparticles having an average particle size of less than 100 nm dispersed in an organic medium.

13. The method according to claim 7, wherein the inert gas is nitrogen, argon, or helium.

14. A method for protecting optical devices using high refractive index (RI) titania-containing resins used in nanoimprint lithography (NIL) from damage caused by short-wavelength visible light (less than 425 nm) and ultraviolet light in the range of 300 to 400 nm, Placing an optical device within an assembly configured for hermetically sealed conditions, The assembly including the optical device is placed in an oxygen- and moisture-free environment having an atmosphere consisting of at least one of an inert gas, a group of inert gases, or a vacuum. To seal the aforementioned assembly, and A method comprising forming an airtight seal.

15. The method according to claim 14, further comprising obtaining a photo-stabilized optical device containing a high refractive index titania-containing resin that maintains a refractive index change of 0.10 units or less and at least 95% of the initial thickness even after at least 1,000 hours of average sunlight exposure on the Earth's surface.

16. The method according to claim 14, wherein the high refractive index titania-containing resin has a refractive index greater than 1.7 at 589 nm.

17. The method according to claim 14, wherein the crystal structure of the titania is anatase type or rutile type.

18. The method according to claim 14, wherein the titania is either uncoated or coated with one or more inorganic materials.

19. The method according to claim 14, wherein the high refractive index titania-containing resin comprises titanium oxide nanoparticles having an average particle size of less than 100 nm dispersed in an organic medium.

20. The method according to claim 14, wherein the inert gas is nitrogen, argon, or helium.

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