WO2025224582A1

WO2025224582A1 - Interference filter and method of manufacturing thereof

Info

Publication number: WO2025224582A1
Application number: PCT/IB2025/054082
Authority: WO
Inventors: Kenneth Jarefors
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2024-04-22
Filing date: 2025-04-17
Publication date: 2025-10-30
Anticipated expiration: 2026-10-22

Abstract

An interference filter includes an optical stack disposed on an optically transparent substrate. The optical stack includes a plurality of first layers alternating with a plurality of second layers. Each second layer includes a plurality of discrete clusters extending along a plane of the optical stack and a thickness of the second layer. The plurality of discrete clusters is spaced apart from each other, such that each second layer further includes a plurality of voids extending through the thickness of the second layer and having a total void area in the plane of the optical stack. An average largest cluster dimension of the plurality of discrete clusters of each second layer measured along the plane of the optical stack is less than 70 nanometers and the total void area each second layer is less than 1% of a total stack area of the optical stack.

Description

INTERFERENCE FILTER AND METHOD OF MANUFACTURING THEREOF

Technical Field

The present disclosure relates generally to an interference filter for an automatic darkening filter and a method of manufacturing the interference filter.

Background

Automatic darkening filters are often provided on a protective headgear, where protection from high intensity light is desired. Automatic darkening filters generally include a switchable filter and an interference filter. The interference filter may include a plurality of dielectric layers and a plurality of metallic layers alternating with the plurality of dielectric layers . The interference filter may attenuate infrared radiation, blue light, and ultraviolet radiation. Fora desirable performance of the interference filter, each of the plurality of metallic layers may need to be uniform while being thin (e.g., less than 15 nanometers in thickness). However, it may be difficult to produce metallic layers that are uniform as well as thin.

Summary

In a first aspect, the present disclosure provides an interference filter for an automatic darkening filter. The interference filter includes a substrate that is optically transparent. The interference filter further includes an optical stack disposed on the substrate. The optical stack includes a plurality of first layers and a plurality of second layers alternating with the plurality of first layers. The optical stack defines a plane and a total stack area in the plane of the optical stack. Each first layer of the plurality of first layers is dielectric. Each second layer of the plurality of second layers is metallic. Each second layer defines athickness normal to the plane. The thickness of each second layer is less than 40 nanometers (run). Each second layer includes a plurality of discrete clusters extending along the plane of the optical stack and the thickness of the second layer. The plurality of discrete clusters is spaced apart from each other, such that each second layer further includes a plurality of voids extending through the thickness of the second layer and having a total void area in the plane of the optical stack. An average largest cluster dimension of the plurality of discrete clusters of each second layer measured along the plane of the optical stack is less than 70 nm. The total void area of each second layer is less than 1% of the total stack area of the optical stack.

In a second aspect, the present disclosure provides an automatic darkening filter. The automatic darkening filter includes a switchable filter that changes from a light transmission state to a dark transmission state in response to an activation signal. The automatic darkening filter further includes the interference filter of the first aspect. The interference filter is disposed on the switchable filter. In a third aspect, the present disclosure provides a protective headgear. The protective headgear includes the automatic darkening filter of the second aspect.

In a fourth aspect, the present disclosure provides a method of manufacturing an interference fdter for an automatic darkening fdter. The method includes providing a substrate that is optically transparent. The method further includes forming an optical stack on the substrate by depositing a plurality of second layers on a plurality of first layers, such that the plurality of second layers alternate with the plurality of first layers to form the optical stack on the substrate.

The optical stack defines a plane and a total stack area in the plane of the optical stack. Each first layer of the plurality of first layers is dielectric. Each second layer of the plurality of second layers is metallic. Each second layer defines a thickness normal to the plane. The thickness of each second layer is less than 40 nanometers (nm). Each second layer includes a plurality of discrete clusters extending along the plane of the optical stack and the thickness of the second layer. The plurality of discrete clusters is spaced apart from each other, such that each second layer further includes a plurality of voids extending through the thickness of the second layer and having a total void area in the plane of the optical stack. An average largest cluster dimension of the plurality of discrete clusters of each second layer measured along the plane of the optical stack is less than 70 nm. The total void area of each second layer is less than 1% of the total stack area of the optical stack.

The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.

Brief Description of Drawings

Exemplary embodiments disclosed herein are more completely understood in consideration of the following detailed description in connection with the following figures. The figures are not necessarily drawn to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labelled with the same number.

FIG. 1 is a schematic cross-sectional view of an interference filter for an automatic darkening filter according to an embodiment of the present disclosure;

FIG. 2A is a schematic top view of a second layer of the interference filter of FIG. 1 in isolation according to an embodiment of the present disclosure;

FIG. 2B is a schematic partial cross-sectional view of the interference filter of FIG. 1 depicting a first layer and an adjacent second layer in isolation according to an embodiment of the present disclosure; FIG. 3 is a schematic exploded perspective view of an automatic darkening fdter according to an embodiment of the present disclosure;

FIG. 4 is a schematic perspective view of a protective shield including the automatic darkening fdter of FIG. 3 according to an embodiment of the present disclosure;

FIG. 5 is a flowchart depicting various steps of a method of manufacturing an interference fdter according to an embodiment of the present disclosure;

FIG. 6A is a photograph of a first sample that was produced by depositing pure silver on a base layer;

FIG. 6B is a photograph of a second sample that was produced by depositing copper doped silver on a base layer;

FIG. 7A illustrates SEM images of the first sample of FIG. 6A obtained using a scanning electron microscope;

FIG. 7B illustrates SEM images of the second sample of FIG. 6B obtained using a scanning electron microscope;

FIG. 8 is a graph depicting transmittance versus wavelength for various first samples and second samples;

FIG. 9 is a graph depicting photopic transmittance versus deposition rate;

FIG. 10A is a graph depicting transmittance versus wavelength for various first samples after heat treatment; and

FIG. 10B is a graph depicting transmittance versus wavelength for various second samples after heat treatment.

Detailed Description

In the following description, reference is made to the accompanying figures that form a part thereof and in which various embodiments are shown by way of illustration. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense.

In the following disclosure, the following definitions are adopted.

As used herein, “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably.

As used herein as a modifier to a property or attribute, the term “generally,” unless otherwise specifically defined, means that the property or attribute would be readily recognizable by a person of ordinary skill but without requiring absolute precision or a perfect match (e.g., within +/- 20 % for quantifiable properties). The term “substantially,” unless otherwise specifically defined, means to a high degree of approximation (e.g., within +/- 10% for quantifiable properties) but again without requiring absolute precision or a perfect match.

As used herein, all numbers should be considered modified by the term “about.” The term “about,” unless otherwise specifically defined, means to a high degree of approximation (e.g., within +/- 5% for quantifiable properties) but again without requiring absolute precision or a perfect match.

As used herein, the terms “first” and “second” are used as identifiers. Therefore, such terms should not be construed as limiting of this disclosure. The terms “first” and “second” when used in conjunction with a feature or an element can be interchanged throughout the embodiments of this disclosure.

As used herein, when a first material is termed as “similar” to a second material, at least 90 weight % of the first and second materials are identical and any variation between the first and second materials comprises less than about 10 weight % of each of the first and second materials. As used herein, “at least one of A and B” should be understood to mean “only A, only B, or both A and B.”

As used herein, the term “transmittance” broadly refers to the percentage of energy in a given region of the electromagnetic spectrum (e.g., visible, infrared, or any other range) that is transmitted across a surface.

As used herein, the term “optically transparent” refers to the ability of light to transmit through a material. An element that is optically transparent has a high light transmittance (e.g., greater than 80%) over at least a portion of the visible light spectrum (from about 400 nm to about 700 nm).

As used herein, the terms “luminous transmittance” and “photopic transmittance” both refer to a weighted mean of the transmittance in the visible wavelength range, i.e., from about 380 nm to about 780 nm.

As used herein, the term “dielectric” refers to non-metallic and non-electrically conducting materials. A layer that is dielectric is made of one or more dielectric materials.

As used herein, the term “refractive index” broadly refers to the absolute refractive index of a material which is understood to be the ratio of the speed of electromagnetic radiation in free space to the speed of electromagnetic radiation in that material. The refractive index can be measured using known methods and is generally measured using an Abbe Refractometer in the visible light region.

As used herein, the term “between about,” unless otherwise specifically defined, generally refers to an inclusive or a closed range. For example, if a parameter X is between about A and B, then A < X < B. The present disclosure relates to an interference filter for an automatic darkening filter. The interference filter includes a substrate that is optically transparent. The interference filter further includes an optical stack disposed on the substrate. The optical stack includes a plurality of first layers and a plurality of second layers alternating with the plurality of first layers. The optical stack defines a plane and a total stack area in the plane of the optical stack. Each first layer of the plurality of first layers is dielectric. Each second layer of the plurality of second layers is metallic. Each second layer defines a thickness normal to the plane. The thickness of each second layer is less than 40 nanometers (nm). Each second layer includes a plurality of discrete clusters extending along the plane of the optical stack and the thickness of the second layer. The plurality of discrete clusters is spaced apart from each other, such that each second layer further includes a plurality of voids extending through the thickness of the second layer and having a total void area in the plane of the optical stack. An average largest cluster dimension of the plurality of discrete clusters of each second layer measured along the plane of the optical stack is less than 70 nm. The total void area of each second layer is less than 1% of the total stack area of the optical stack.

Each second layer having the average largest cluster dimension less than 70 nm and the total void area less than 1% of the total stack area may be highly uniform (i.e., have a substantially uniform thickness with reduced defects, such as discrete clusters and void areas). The high uniformity of each second layer may improve an optical performance (i.e., improved luminous transmittance, reduced light scattering, and improved attenuation of near infrared radiation) of the interference filter.

In some examples, each second layer may be made from doped silver, that is, silver and a dopant. The dopant may include copper (Cu), aluminum (Al), zinc (Zn), silicon carbide (SiC), or combinations thereof.

Making each second layer from the doped silver may improve the uniformity of the second layer. Specifically, making each second layer from the doped silver may reduce both the average largest cluster dimension of the plurality of discrete clusters and the total void area of the plurality of voids of the second layer. As discussed above, the high uniformity of each second layer may improve the optical performance of the interference filter.

Furthermore, making each second layer from the doped silver can allow reducing the thickness of the second layer while maintaining the high uniformity of the second layer. For example, the thickness of each second layer may be in a range of from 6 nm to 12 nm. The doped silver may therefore allow forming ultrathin and uniform second layers. Moreover, a mixture of the silver and the dopant may be deposited at a higher deposition rate as compared to pure silver without compromising the uniformity of the resulting second layer. As a result, the mixture of the silver and the dopant may provide a much greater flexibility in the deposition rate as compared to pure silver, and in some cases, may expedite manufacturing of the interference fdter.

Referring now to the figures, FIG. 1 illustrates a schematic cross-sectional view of an interference filter 100 for an automatic darkening filter according to an embodiment of the present disclosure. The interference filter 100 shown in FIG. 1 may be alternatively referred to as an “induced transmittance filter.”

The interference filter 100 includes a substrate 102 that is optically transparent. In some embodiments, the substrate 102 includes glass or aluminum oxide (A12O3). In some embodiments, the substrate 102 may include soda-lime glass, borosilicate glass, aluminosilicate glass, or any other suitable glass based on desired application attributes. The substrate 102 may have any suitable thickness depending on desired application attributes.

The interference filter 100 further includes an optical stack 104 disposed on the substrate 102. The optical stack 104 defines a plane 110 (schematically depicted by a line in FIG. 1) and a total stack area in the plane 110 of the optical stack 104. The optical stack 104 includes a plurality of first layers 106 and a plurality of second layers 108 alternating with the plurality of first layers 106.

Each first layer 106 of the plurality of first layers 106 is dielectric. In some embodiments, each first layer 106 includes silicon dioxide (SiO2), titanium dioxide (TiO2), tantalum pentoxide (Ta2O5), aluminum oxide (A12O3), hafnium oxide (HfO2), zinc sulfide (ZnS), or combinations thereof.

Each first layer 106 may define a thickness 106T normal to the plane 110. The thickness 106T of each first layer 106 may be selected based on desired application attributes. The thicknesses 106T of the plurality of first layers 106 may be same or different. Further, the plurality of first layers 106 may have same or different refractive indices. In some embodiments, one or more first layers 106 of the plurality of first layers 106 may be multi-layered. In other words, in some embodiments, the one or more first layers 106 may be made up of a plurality of dielectric layers.

Each second layer 108 of the plurality of second layers 108 is metallic. Each second layer 108 defines a thickness 108T normal to the plane 110. The thickness 108T of each second layer 108 is less than 40 nanometers (nm).

FIG. 2A illustrates a schematic top view of one second layer 108 of the plurality of second layers 108 in isolation according to an embodiment of the present disclosure. FIG. 2B illustrates a schematic partial cross-sectional view of the interference filter 100 of FIG. 1 depicting one second layer 108 and one first layer 106 adjacent to the one second layer 108 in isolation according to an embodiment of the present disclosure. The plane 110 of the optical stack 104 (shown in FIG. 1) is also depicted in FIGS. 2A and 2B.

Referring to FIGS. 1, 2A, and 2B, each second layer 108 includes a plurality of discrete clusters 112 (schematically depicted as circles in FIG. 2A) extending along the plane 110 of the optical stack 104 and the thickness 108T of the second layer 108. Each discrete cluster 112 of the plurality of discrete clusters 112 may include a largest cluster dimension 112L in the plane 110 of the optical stack 104. The plurality of discrete clusters 112 of each second layer 108 may define an average largest cluster dimension. The average largest cluster dimension may be a mean or an average of the largest cluster dimensions 112L of the plurality of discrete clusters 112 of the respective second layer 108.

The plurality of discrete clusters 112 is spaced apart from each other, such that each second layer 108 further includes a plurality of voids 114 (schematically depicted as dashed ovals in FIG. 2A) extending through the thickness 108T of the second layer 108 and having a total void area in the plane 110 of the optical stack 104. The total void area may be a sum of the areas of the plurality of voids 114 of the respective second layer 108 in the plane 110. In other words, the total void area of each second layer 108 may be the total area of the plurality of voids 114 ofthe second layer 108. The total void area may correspond to an area ofan adjacent first layer 106 that is not covered by the second layer 108. The average largest cluster dimension and the total void area may be determined using a Scanning Electron Microscope (SEM).

The average largest cluster dimension of the plurality of discrete clusters 112 of each second layer 108 measured along the plane 110 of the optical stack 104 is less than 70 nm.

Further, the total void area of each second layer 108 is less than 1% of the total stack area of the optical stack 104.

Each second layer 108 having the average largest cluster dimension less than 70 nm and the total void area less than 1% of the total stack area may be highly uniform (i.e., have a substantially uniform thickness with reduced defects, such as discrete clusters and void areas). The high uniformity of each second layer 108 may improve an optical performance (i.e., improved luminous transmittance, reduced light scattering, and improved attenuation of near infrared radiation) of the interference filter 100.

In some embodiments, each second layer 108 includes silver in an amount of from 95% to 99.5%, by weight. Each second layer 108 further includes a dopant in an amount of from 0.5% to 5%, by weight. In some embodiments, the dopant includes copper (Cu), aluminum (Al), zinc (Zn), silicon carbide (SiC), or combinations thereof.

Making each second layer 108 from the aforementioned composition of silver and the dopant (also referred to as “doped silver” herein) may improve the uniformity of the second layer 108. Specifically, making each second layer 108 from the doped silver may reduce both the average largest cluster dimension of the plurality of discrete clusters 112 and the total void area of the plurality of voids 114 ofthe second layer 108. As discussed above, the high uniformity of each second layer 108 may improve the optical performance of the interference fdter 100.

Furthermore, making each second layer 108 from the doped silver can allow reducing the thickness 108T of the second layer 108 while maintaining the high uniformity of the second layer 108. In some embodiments, the thickness 108T of each second layer 108 may be less than 12 nm. In some embodiments, the thickness 108T of each second layer 108 may be in a range of from 6 nm to 12 nm. The doped silver may thus allow forming ultrathin and uniform second layers 108. In some embodiments, the average largest cluster dimension may be from 48 nm to 68 nm.

In some embodiments, the total void area of each second layer 108 may be less than 0.1% of the total stack area of the optical stack 104, such that the second layer 108 substantially covers an adjacent first layer 106 of the plurality of first layers 106. This configuration of the plurality of second layers 108 may further improve the optical characteristics of the interference filter 100.

In some embodiments, the interference filter 100 may further include a dielectric stack (not shown). In some embodiments, the dielectric stack may be disposed on the optical stack 104, such that the optical stack 104 is disposed between the dielectric stack and the substrate 102. In some other embodiments, the dielectric stack may be disposed between the optical stack 104 and the substrate 102. In some other embodiments, the dielectric stack may be disposed between two layers of the optical stack 104. The dielectric stack may include a plurality of dielectric layers stacked on top of each other. The plurality of dielectric layers may have same or different refractive indices. In some cases, alternating dielectric layers from the plurality of dielectric layers may have alternating high and low refractive indices. For example, a first dielectric layer from the plurality of dielectric layers may have a high refractive index and a second dielectric layer adjacent to the first dielectric layer may have a low refractive index. The interference filter 100 may therefore employ a combination of the dielectric stack and the optical stack 104, based on desired application attributes.

The interference filter 100 may be configured to attenuate the infrared (IR), blue light, and ultraviolet (UV) wavelength components from a high intensity light. In some embodiments, the interference filter 100 may eliminate hazardous IR, blue light, and UV wavelength components from the high intensity light.

FIG. 3 illustrates a schematic exploded perspective view of an automatic darkening filter 10 according to an embodiment of the present disclosure.

The automatic darkening filter 10 includes a switchable filter 40 that changes from a light transmission state to a dark transmission state in response to an activation signal. The automatic darkening filter 10 further includes the interference filter 100. The interference filter 100 is disposed on the switchable filter 40. In some embodiments, the interference filter 100 may be removably disposed on the switchable filter 40, such that the interference filter 100 is replaceable. In the illustrated embodiment of FIG. 3, the switchable filter 40 includes a first polarization filter 14, a first optically rotating liquid crystal cell 16, a second polarization filter 18, a second optically rotating liquid crystal cell 20, a third polarization filter 22, and a fourth polarization filter 26. The polarization filters 14, 18, 22 may have substantially orthogonal polarization directions. In other words, the polarization direction of the second polarization filter 18 may be substantially orthogonal to the polarization direction of the first polarization filter 14, and the third polarization filter 22 may have substantially the same polarization direction as the first polarization filter 14. Further, the third polarization filter 22 and the fourth polarization filter 26 may have substantially parallel polarization directions.

In the illustrated embodiment of FIG. 3, the switchable filter 40 further includes a low twist color-tunable liquid crystal cell 24 disposed between the third polarization filter 22 and the fourth polarization filter 26. The low twist color-tunable liquid crystal cell 24 may be similar in design as the liquid crystal cells 16, 20, but its operation is different because it is sandwiched between parallel polarizers 22, 26, as opposed to crossed polarizers.

The liquid crystal cells 16, 20, 24 may be provided with connectors 30, 32, 34, respectively, by which control voltages can be applied. Each of the liquid crystal cells 16, 20 may include liquid crystal defining surfaces. Nematic crystal molecules may take specific angular positions so that the nematic crystal molecules are twisted through their respective twist angle between the defining surfaces.

The application of a voltage to connectors 30 and 32 may create an electric field between the defining surfaces. The nematic liquid crystal molecules may align with the electric field perpendicular to the defining surfaces, rather than parallel to them, and the cell achieves a darkened state. Thus, when a control voltage is applied to the low twist cells 16, 20, a filter effect may be obtained. The degree of rotation of the nematic crystal molecules may be controlled by varying the control voltage, and thus the corresponding filter effect may also be controlled. The result is that liquid crystal cells 16, 20 may be in the light transmission state in the absence of an applied voltage, and in the dark transmission state in the presence of an applied voltage. The voltage levels may be different for varying cell designs, depending on the liquid crystal materials used, different cell gap geometries, etc. Further, the low twist color-tunable liquid crystal cell 24 may be dark and exhibit a certain color, which appears predominately blue to the user, when no voltage is applied to the connectors 34. The low twist color-tunable liquid crystal cell 24 may become optically transparent when a voltage greater than around 4.0 V is applied via the connectors 34. FIG. 4 illustrates a schematic perspective view of a protective headgear 50 according to an embodiment of the present disclosure.

The protective headgear 50 includes the automatic darkening filter 10. In the illustrated embodiment of FIG. 4, the protective headgear 50 is a shield 60 (e.g., safety shield, also known as helmet). The shield 60 includes a shield body 62. The automatic darkening filter 10 may be positioned in the shield body 62 such that the automatic darkening filter 10 is directly in front of the eyes of a user when the shield 60 is worn by the user. The automatic darkening filter 10 may intercept electromagnetic radiation (e.g., visible light, UV light, IR, etc.). In some embodiments, the automatic darkening filter 10 may be removably connected to the shield body 62.

In some embodiments, the automatic darkening filter 10 may be provided in any other suitable equipment or articles and for other applications. For example, the automatic darkening filter 10 may be supplied as part of protective eyewear (e.g. goggles) rather than the fullcoverage shield 60. Alternatively, the automatic darkening filter 10 may be provided in a handheld device, or in a window or aperture allowing inspection of a room, enclosure, machinery space etc., in which high intensity light may be present. The automatic darkening filter 10 may be used in connection with industrial operations, for example welding (e.g. arc welding, torch welding, acetylene welding), cutting (e.g. laser cutting, acetylene cutting), brazing, soldering and the like. The automatic darkening filter 10 may also be used in connection with medical procedures involving high intensity light, for example, laser surgery, hair removal, tattoo removal, light-curing of dental resins.

FIG. 5 illustrates a flowchart of a method 200 of manufacturing an interference filter (e.g., the interference filter 100 of FIG. 1) for an automatic darkening filter (e.g., the automatic darkening filter 10 of FIG. 3) according to an embodiment of the present disclosure. The method 200 will be described with further reference to FIGS. 1 to 4.

At step 202, the method 200 includes providing a substrate that is optically transparent. Referring to FIG. 1, for example, the method 200 may include providing the substrate 102.

At step 204, the method 200 further includes forming an optical stack on the substrate by depositing a plurality of second layers on a plurality of first layers, such that the plurality of second layers alternate with the plurality of first layers to form the optical stack on the substrate . The optical stack defines a plane and a total stack area in the plane of the optical stack. Each first layer of the plurality of first layers is dielectric. Each second layer of the plurality of second layers is metallic. Each second layer defines athickness normal to the plane. The thickness of each second layer is less than 40 nm. Each second layer includes a plurality of discrete clusters extending along the plane of the optical stack and the thickness of the second layer. The plurality of discrete clusters is spaced apart from each other, such that each second layer further includes a plurality of voids extending through the thickness of the second layer and having a total void area in the plane of the optical stack. An average largest cluster dimension of the plurality of discrete clusters of each second layers measured along the plane of the optical stack is less than 70 nm. The total void area of each second layer is less than 1% of the total stack area of the optical stack.

Referring to FIGS. 1, 2A, and 2B, for example, the method 200 may include forming the optical stack 104 on the substrate 102 by depositing the plurality of second layers 108 on the plurality of first layers 106, such that the plurality of second layers 108 alternate with the plurality of first layers 106 to form the optical stack 104 on the substrate 102.

In some embodiments, the average largest cluster dimension is from 48 nm to 68 nm. In some embodiments, the total void area of each second layer is less than 0.1% of the total stack area of the optical stack, such that the second layer substantially covers an adjacent first layer of the plurality of first layers.

In some embodiments, each second layer includes silver in an amount of from 95% to 99.5%, by weight. Each second layer further includes a dopant in an amount of from 0.5% to 5%, by weight.

In some embodiments, the dopant includes copper (Cu), aluminum (Al), zinc (Zn), silicon carbide (SiC), or combinations thereof. In some embodiments, the thickness of each second layer is less than 12 nm.

In some embodiments, each first layer includes silicon dioxide (SiO2), titanium dioxide (TiO2), tantalum pentoxide (Ta2O5), aluminum oxide (A12O3), hafnium oxide (HfO2), zinc sulfide (ZnS), or combinations thereof.

In some embodiments, the substrate includes glass or aluminum oxide (A12O3).

In some embodiments, each second layer is deposited on the adjacent first layer by physical vapor deposition (PVD) using athermal resistive source or electron gun evaporation. Referring to FIGS. 1 and 2B, for example, each second layer 108 may be deposited on the adjacent first layer 106 by physical vapor deposition (PVD) using a thermal resistive source or electron gun evaporation. PVD may be suitable and preferable process for depositing each second layer 108.

In some embodiments, each second layer is deposited by evaporating a mixture of the silver and the dopant. Referring to FIG. 1 and 2B, for example, each second layer 108 may be deposited by evaporating a mixture of silver and the dopant.

In some embodiments, a deposition rate of each second layer on the adjacent first layer is greater than 8 angstroms per second. Referring to FIG. 1, for example, a deposition rate of each second layer 108 on the adjacent first layer 106 may be greater than 8 angstroms per second.

The mixture of the silver and the dopant may be deposited at a higher deposition rate as compared to pure silver without compromising the uniformity of the resulting second layer 108. As a result, the mixture of the silver and the dopant may provide a much greater flexibility in the deposition rate as compared to pure silver, and in some cases, may expedite manufacturing of the interference filter.

Experimental Results

Various samples were prepared by coating an 11 run metallic layer on an 80 nm base layer disposed on glass substrate using physical vapor deposition, such that each sample had the following structure : {metallic layer/base layer/glass substrate } . The physical vapor deposition was carried out in a vacuum chamber (of type often referred to as “box coater”) with planetary rotation of the {base layer/glass substrate} structure.

A plurality of first samples was prepared by coating pure silver on the base layer. Further, a plurality of second samples was prepared by coating a mixture of 98.5% silver and 1.5% Cu (also referred to as “copper doped silver” herein) on the base layer. Granulates of size ranging from 2 millimeter (mm) to 5 mm were used to deposit the mixture (copper doped silver) on the base layer.

Various experiments were conducted on the samples to test their optical performance, uniformity, and robustness to heat.

FIG. 6A is a photograph of one first sample. FIG. 6B is a photograph of one second sample. Referring to FIGS. 6A and 6B, it was observed that the second sample (shown in FIG. 6B) with the copper doped silver coating exhibited reduced light scattering as compared to the first sample (shown in FIG. 6A) with the pure silver coating. Specifically, the second sample exhibited reduced “wide angle forward scattering,” which was visually observable. The first sample and the second sample were then tested in accordance with ISO 18526-2:2020, 14.2 (Test methods for eye and face protection). About 2% to 4% improvement in the second sample with respect to the first sample was observed.

FIG. 7A illustrates SEM images of different sections of the first sample obtained using a scanning electron microscope. FIG. 7B illustrates SEM images of different sections of the second sample obtained using the scanning electron microscope.

Referring to FIG. 7A, an average largest cluster dimension of discrete clusters formed in the pure silver layer from three sections was calculated to be from 84 nm to 99 nm. A noncovered area (i.e., area of the base layer not covered by the pure silver layer) of the three sections was calculated to be from 1.7% to 4.5% of the total area of the base layer. Referring now to FIG. 7B, an average largest cluster dimension of discrete clusters formed in the copper doped silver layer from three sections was calculated to be from 48 nm to 68 nm. Further, the copper doped silver layer substantially covered the base layer. That is, a non-covered area of the base layer was zero. The copper doped silver layer was smoother, more uniform, and had much smaller discrete clusters as compared to the first sample (shown in FIG. 7A).

FIG. 8 illustrates a graph 300 depicting transmittance (y-axis, in %) versus wavelength (x- axis, in nm) for some first samples and some second samples.

The graph 300 includes a calculated curve 301 (dashed) depicting an ideal or desirable relation between transmittance and wavelength for the samples produced for experimental purposes and having the structure: {metallic layer/base layer/glass substrate}. The graph 300 further includes a first curve 302 (long-dash short-dashed) depicting a variation of transmittance with respect to wavelength for one first sample (having pure silver layer) and a second curve 304 (long -dash double short-dashed) depicting a variation of transmittance with respect to wavelength for one second sample (having copper doped silver layer). The graph 300 also includes other curves depicting variations of transmittance with respect to wavelength for other samples, but they are not designated by reference characters for clarity purposes. The best performing first and second samples (relative to the calculated curve 301) correspond to first and second curves 302, 304, respectively. It was noted that the second curve 304 was closer to the calculated curve 301 than the first curve 304.

The second sample corresponding to the second curve 304 transmitted greater than 60% of a substantially normally incident light for each wavelength in a visible wavelength range extending from about 400 nm to about 520 nm. When compared to the first sample corresponding to the first curve 304, the second sample exhibited higher transmittance in the visible wavelength range extending from about 400 nm to about 520 nm. The second sample exhibited better luminous transmittance than the first sample.

Furthermore, the second sample corresponding to the second curve 304 transmitted less than 30% of the substantially normally incident light for each wavelength in an infrared wavelength range extending from about 960 nm to about 1000 nm. When compared to the first sample corresponding to the first curve 302, the second sample exhibited lower transmittance (or higher reflectance) in the infrared wavelength range extending from about 960 nm to about 1000 nm. In other words, the second sample exhibited better infrared reflectance (or infrared attenuation) than the first sample. It was also observed that the second sample corresponding to the second curve 304 exhibited a higher ratio of luminous transmittance (in a range of from 400 nm to 700 nm) versus near infrared transmittance (greater than 780 nm) than the first sample corresponding to the first curve 302, which is desirable for interference filters. FIG. 9 illustrates a graph 350 depicting photopic transmittance (y-axis, in %) versus deposition rate (x-axis, in angstroms/second) for the samples prepared by the pure silver and the copper doped silver. The graph 350 includes a first curve 352 depicting a variation of photopic transmittance with respect to deposition rate of the pure silver on the base layer for the first samples. The graph 350 further includes a second curve 354 depicting a variation of photopic transmittance with respect to deposition rate of copper doped silver layer on the base layer for the second samples. As depicted by the first curve 352, the photopic transmittance of the first samples was very sensitive to the deposition rate of the pure silver. On the other hand, as depicted by the second curve 354, the photopic transmittance of the second samples was less sensitive to the deposition rate of the copper doped silver. It was concluded that the copper doped silver allowed a much greater flexibility in the deposition rate as compared to pure silver. Also, the copper doped silver could be deposited at a higher deposition rate (e.g., greater than 8 angstroms/seconds) without negatively affecting the photopic transmittance of the produced second sample.

The first samples and the second samples were also tested for robustness to heat. Some first samples and some second samples were heat treated at 100 degrees Celsius for 10 minutes. Further, some first samples and some second samples were heat treated at 290 degrees Celsius for 10 seconds.

The first samples that were heat treated at 100 degrees Celsius for 10 minutes exhibited minor visual degradation. On the other hand, the second samples that were heat treated at 100 degrees Celsius for 10 minutes exhibited no visual change.

The first samples that were heat treated at 290 degrees Celsius for 10 seconds also exhibited minor visual degradation. On the other hand, the second samples that were heat treated at 290 degrees Celsius for 10 seconds exhibited no visual change.

It was noted that the pure silver layer of the first samples degraded after undergoing both the heat treatments, while the copper doped silver layer of the second samples showed no visual degradation after undergoing both the heat treatments. The first samples and the second samples that underwent the aforementioned heat treatments were then tested for transmittance versus wavelength characteristics.

FIG. 10A illustrates a graph depicting transmittance (y-axis, in %) versus wavelength (x- axis, in nm) for the first samples that were heat treated as well as the first samples that were not heat treated.

The graph 400 includes reference curves 401A, 401B, 401C, 401D (solid) depicting variation of transmittance versus wavelength for four first samples that were not heat treated. The graph 400 further includes first curves 402A, 402B (long-dash double short-dashed) depicting variation of transmittance with respect to wavelength for two first samples that were heat treated at 100 degrees Celsius for 10 minutes. The graph 400 further includes second curves 404A, 404B (dashed) depicting variation of transmittance with respect to wavelength for two first samples that were heat treated at 290 degrees Celsius for 10 seconds.

As depicted by the first curves 402A, 402B, the first samples that were heat treated at 100 degrees Celsius for 10 minutes exhibited significantly degraded photopic transmittance as compared to those first samples (corresponding to the reference curves 401A, 401B, 401C, 40 ID) that were not heat treated. Furthermore, as depicted by the second curves 404A, 404B, the first samples that were heat treated at 290 degrees Celsius for 10 seconds also exhibited substantially degraded photopic transmittance as compared to those first samples (corresponding to the reference curves 401A, 401B, 401C, 401D) that were not heat treated. It was noted that the pure silver layers degraded significantly when exposed to high temperatures, which detrimentally affected the photopic transmittance of the first samples.

FIG. 10B illustrates a graph 450 depicting transmittance (y-axis, in %) versus wavelength (x-axis, in nm) for the second samples that were heat treated as well as the second samples that were not heat treated.

The graph 450 includes reference curves 451A, 45 IB (solid) depicting variation of transmittance with respect to wavelength for two second samples that were not heat treated. The graph 400 further includes a first curve 452 (long-dash double short-dashed) depicting a variation of transmittance with respect to wavelength for a second sample that was heat treated at 100 degrees Celsius for 10 minutes. The graph 400 further includes second curves 454A, 454B (dashed) depicting variation of transmittance with respect to wavelength for two second samples that were heat treated at 290 degrees Celsius for 10 seconds.

As depicted by the first curve 452, the second sample that was heat treated at 100 degrees Celsius for 10 minutes exhibited similar photopic transmittance as compared to those second samples (corresponding to the reference curves 451 A, 45 IB) that were not heat treated. Furthermore, as depicted by the second curves 454A, 454B, the second samples that were heat treated at 290 degrees Celsius for 10 seconds also exhibited similar photopic transmittance as compared to those second samples (corresponding to the reference curves 451 A, 45 IB) that were not heat treated. It was noted that the copper doped silver layers were robust to high temperatures, and as a result, the photopic transmittance of the second samples exposed to high temperatures was not negatively affected.

Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

CLAIMS What is claimed is:

1. An interference filter for an automatic darkening filter, the interference filter comprising: a substrate that is optically transparent; and an optical stack disposed on the substrate, the optical stack comprising a plurality of first layers and a plurality of second layers alternating with the plurality of first layers, the optical stack defining a plane and a total stack area in the plane of the optical stack; wherein each first layer of the plurality of first layers is dielectric; wherein each second layer of the plurality of second layers is metallic, each second layer defining a thickness normal to the plane, wherein the thickness of each second layer is less than 40 nanometers (nm); wherein each second layer comprises a plurality of discrete clusters extending along the plane of the optical stack and the thickness of the second layer, the plurality of discrete clusters being spaced apart from each other, such that each second layer further comprises a plurality of voids extending through the thickness of the second layer and having a total void area in the plane of the optical stack, wherein an average largest cluster dimension of the plurality of discrete clusters of each second layer measured along the plane of the optical stack is less than 70 nm, and wherein the total void area of each second layer is less than 1% of the total stack area of the optical stack.

2. The interference filter of claim 1, wherein the average largest cluster dimension is from 48 nm to 68 nm.

3. The interference filter of claim 1, wherein the total void area of each second layer is less than 0.1 % of the total stack area of the optical stack, such that the second layer substantially covers an adjacent first layer of the plurality of first layers.

4. The interference filter of claim 1, wherein each second layer comprises: silver in an amount of from 95% to 99.5%, by weight; and a dopant in an amount of from 0.5% to 5%, by weight.

5. The interference filter of claim 4, wherein the dopant comprises copper (Cu), aluminum (Al), zinc (Zn), silicon carbide (SiC), or combinations thereof.

6. The interference filter of claim 1, wherein the thickness of each second layer is less than 12 nm.

7. The interference filter of claim 1, wherein each first layer comprises silicon dioxide (SiO2), titanium dioxide (TiO2), tantalum pentoxide (Ta2O5), aluminum oxide (A12O3), hafnium oxide (HfO2), zinc sulfide (ZnS), or combinations thereof.

8. The interference filter of claim 1 , wherein the substrate comprises glass or aluminum oxide (A12O3).

9. An automatic darkening filter comprising: aswitchable filter that changes from a light transmission state to a dark transmission state in response to an activation signal; and the interference filter of claim 1, wherein the interference filter is disposed on the switchable filter.

10. A protective headgear comprising the automatic darkening filter of claim 9.

11. A method of manufacturing an interference filter for an automatic darkening filter, the method comprising: providing a substrate that is optically transparent; and forming an optical stack on the substrate by depositing a plurality of second layers on a plurality of first layers, such that the plurality of second layers alternate with the plurality of first layers to form the optical stack on the substrate, the optical stack defining a plane and a total stack area in the plane of the optical stack; wherein each first layer of the plurality of first layers is dielectric; wherein each second layer of the plurality of second layers is metallic, each second layer defining a thickness normal to the plane, wherein the thickness of each second layer is less than 40 nanometers (nm); wherein each second layer comprises a plurality of discrete clusters extending along the plane of the optical stack and the thickness of the second layer, the plurality of discrete clusters being spaced apart from each other, such that each second layer further comprises a plurality of voids extending through the thickness of the second layer and having a total void area in the plane of the optical stack, wherein an average largest cluster dimension of the plurality of discrete clusters of each second layer measured along the plane of the optical stack is less than 70 nm, and wherein the total void area of each second layer is less than 1% of the total stack area of the optical stack.

12. The method of claim 11, wherein the average largest cluster dimension is from 48 nm to 68 nm.

13. The method of claim 11 , wherein the total void area of each second layer is less than 0.1% of the total stack area of the optical stack, such that the second layer substantially covers an adjacent first layer of the plurality of first layers.

14. The method of claim 11, wherein each second layer comprises: silver in an amount of from 95% to 99.5%, by weight; and a dopant in an amount of from 0.5% to 5%, by weight.

15. The method of claim 14, wherein the dopant comprises copper (Cu), aluminum (Al), zinc (Zn), silicon carbide (SiC), or combinations thereof.

16. The method of claim 11, wherein the thickness of each second layer is less than 12 nm.

17. The method of claim 11, wherein each first layer comprises silicon dioxide (SiO2), titanium dioxide (TiO2), tantalum pentoxide (Ta2O5), aluminum oxide (AI2O3), hafnium oxide (HfO2), zinc sulfide (ZnS), or combinations thereof.

18. The method of claim 11 , wherein the substrate comprises glass or aluminum oxide (A12O3).

19. The method of claim 11, wherein each second layer is deposited on an adjacent first layer by Physical Vapor Deposition (PVD) using a thermal resistive source or electron gun evaporation.

20. The method of claim 11, wherein each second layer is deposited by evaporating a mixture of silver and a dopant.

21. The method of claim 11 , wherein a deposition rate of each second layer on an adjacent first layer is greater than 8 angstroms per second.