WINDOW HAVING METAL LAYER THAT TRANSMITS MICROWAVE SIGNALS AND REFLECTS INFRARED SIGNALS

A window structure includes a metal layer that transmits microwave signals and reflects infrared signals. A microwave signal is a signal that has a frequency in the microwave spectrum of frequencies (a.k.a. the microwave frequency spectrum). The microwave frequency spectrum extends from 300 megahertz (MHz) to 300 gigahertz (GHz). An infrared signal is a signal that has a frequency in the infrared spectrum of frequencies (a.k.a. the infrared frequency spectrum, which extends from 300 GHz to 430 terahertz (THz)). The metal layer may be a discontinuous metal layer that's an electrically discontinuous metal layer and/or a physically discontinuous metal layer.

FIELD OF THE INVENTION

Generally, the present disclosure is directed towards window structure embodiments and related methods of making and using embodiments, wherein the window structures are configured to transmit microwave signals and reflect (e.g., rejects) infrared signals. More specifically, the present disclosure is directed towards window structures that include a glass layer and a metal layer, where the metal layer is formed on the glass layer such that the method layer is configured to transmit signals having frequencies in a range from 28 gigahertz to 60 gigahertz and further configured to reflect signals having infrared frequencies.

BACKGROUND

Recent innovations in window design have led to windows having greater energy efficiency. A window may have a single sheet (e.g., pane) of glass or multiple sheets of glass. Each sheet may include a single layer of glass or multiple layers of glass that are attached using an adhesive. The energy efficiency of modern windows often is increased by covering a surface of at least one of the sheets with a low thermal emissivity coating (a.k.a. low-E coating) and/or by filling a space between the sheets with an inert gas having relatively low thermal conductivity. Each low-E coating manages electromagnetic (EM) radiation that is incident on the coating.

Low-E coatings often are metallic. For instance, silver is commonly used as a low-E coating. Accordingly, low-E coatings typically reflect frequencies that are used in cellular communications in addition to infrared frequencies that are intended to be blocked for greater energy efficiency. A low-E coating may attenuate microwaves having a frequency of greater than 1.0 gigahertz (GHz) up to 40 dB. Building materials typically allow frequencies in the range of 0.6 GHz to 2.7 GHz, which are used by 3G and 4G cellular systems, to pass through with relatively low attenuation. Thus, attenuation of 3G and 4G frequencies by low-E coatings in windows traditionally has not been a significant issue. However, the same building materials typically attenuate frequencies in the range of 6 GHz to 100 GHz, which are used by 5G systems, quite substantially (e.g., nearly 100% in some instances). Accordingly, the reflection of microwave frequencies by traditional windows that have low-E coatings has become an even more pressing concern with the advent of 5G systems.

SUMMARY

Various window structures are described herein that are configured to include a metal layer that transmits microwave signals and reflects (e.g., rejects) infrared signals. A microwave signal is a signal that has a frequency in the microwave spectrum of frequencies (a.k.a. the microwave frequency spectrum). The microwave frequency spectrum extends from 300 megahertz (MHz) to 300 GHz. An infrared signal is a signal that has a frequency in the infrared spectrum of frequencies (a.k.a. the infrared frequency spectrum). The infrared frequency spectrum extends from 300 GHz to 430 terahertz (THz). The metal layer may or may not be a discontinuous metal layer. A discontinuous metal layer is a metal layer that is an electrically discontinuous metal layer and/or a physically discontinuous metal layer. Accordingly, the direct current (DC) conductivity of the discontinuous metal layer may be vanishingly small.

A physically discontinuous metal layer is a metal layer that includes multiple metal portions disposed in a plane such that the metal portions do not form a continuous path of metal between opposing sides of the metal layer in the plane. For example, the metal portions may not form a continuous path of metal between any two opposing sides of the metal layer in the plane. In another example, any one or more (e.g., all) of the metal portions may not be in direct physical contact with any of the other metal portions. A metal portion that is not in direct physical contact with any of the other metal portions is defined herein to be a metal island structure. For instance, the metal island structure may be separated from the other metal portions by a non-metal substance, such as a gas (e.g., air, noble gas(es), hydrogen, or nitrogen).

An electrically discontinuous metal layer is a metal layer in which one or more boundaries inhibit flow of electrons from a first side of the metal layer to a second, opposing side of the metal layer for at least a portion of the microwave frequency spectrum. In one example, the metal layer may include metal island structures, each of which is electrically isolated from the other metal island structures in the metal layer. In accordance with this example, gaps between the metal island structures may constitute boundaries that inhibit the flow of electrons to adjacent metal island structures. In further accordance with this example, each metal island structure may be electrically conductive; however, the metal layer as a whole may have a DC conductivity that is substantially less than the DC conductivity of the individual metal island structures because the metal island structures are electrically isolated from the other metal island structures. In another example, the chemical composition of the metal layer may cause the metal layer to be electrically discontinuous.

A first example window structure includes a glass layer and a metal layer. The metal layer is formed on the glass layer. The metal layer is configured to transmit signals having frequencies in a range from 28 gigahertz to 60 gigahertz and further configured to reflect signals having infrared frequencies.

A second example window structure includes a glass substrate and a discontinuous metal layer. The discontinuous metal layer is configured to reflect infrared wavelengths. The discontinuous metal layer comprises metal island structures having a thickness and a lateral dimension disposed adjacent to the glass substrate. The thickness of the metal island structures is in a range from 1 nanometer and 7 nanometers. The lateral dimension of the metal island structures averages at least 15 nanometers.

In an example method of making a window structure, a glass layer is provided. A metal layer is formed on the glass layer. Forming the metal layer comprises configuring the metal layer to transmit signals having frequencies in a range from 28 gigahertz to 60 gigahertz and to reflect signals having infrared frequencies.

In an example method of using a window structure having a glass layer and a metal layer formed on the glass layer, infrared signals having infrared frequencies are received at the metal layer. Microwave signals having frequencies in a range from 28 gigahertz to 60 gigahertz are received at the metal layer. The microwave signals are transmitted through the metal layer based at least in part on a configuration of the metal layer. The infrared signals are reflected from the metal layer based at least in part on the configuration of the metal layer.

DETAILED DESCRIPTION

Descriptors such as “first”, “second”, “third”, etc. are used to reference some elements discussed herein. Such descriptors are used to facilitate the discussion of the example embodiments and do not indicate a required order of the referenced elements, unless an affirmative statement is made herein that such an order is required.

Example window structures described herein are configured to include a metal layer that transmits microwave signals and reflects (e.g., rejects) infrared signals. A microwave signal is a signal that has a frequency in the microwave spectrum of frequencies (a.k.a. the microwave frequency spectrum). The microwave frequency spectrum extends from 300 megahertz (MHz) to 300 GHz. An infrared signal is a signal that has a frequency in the infrared spectrum of frequencies (a.k.a. the infrared frequency spectrum). The infrared frequency spectrum extends from 300 GHz to 430 terahertz (THz). The metal layer may or may not be a discontinuous metal layer. A discontinuous metal layer is a metal layer that is an electrically discontinuous metal layer and/or a physically discontinuous metal layer.

A physically discontinuous metal layer is a metal layer that includes multiple metal portions disposed in a plane such that the metal portions do not form a continuous path of metal between opposing sides of the metal layer in the plane. For example, the metal portions may not form a continuous path of metal between any two opposing sides of the metal layer in the plane. In another example, any one or more (e.g., all) of the metal portions may not be in direct physical contact with any of the other metal portions. A metal portion that is not in direct physical contact with any of the other metal portions is defined herein to be a metal island structure. For instance, the metal island structure may be separated from the other metal portions by a non-metal substance, such as a gas (e.g., air, noble gas(es), hydrogen, or nitrogen).

An electrically discontinuous metal layer is a metal layer in which one or more boundaries inhibit flow of electrons from a first side of the metal layer to a second, opposing side of the metal layer for at least a portion of the microwave frequency spectrum. In one example, the metal layer may include metal island structures, each of which is electrically isolated from the other metal island structures in the metal layer. In accordance with this example, gaps between the metal island structures may constitute boundaries that inhibit the flow of electrons to adjacent metal island structures. In further accordance with this example, each metal island structure may be electrically conductive; however, the metal layer as a whole may have a conductivity that is substantially less than the conductivity of the individual metal island structures because the metal island structures are electrically isolated from the other metal island structures. In another example, the chemical composition of the metal layer may cause the metal layer to be electrically discontinuous.

Example window structures described herein have a variety of benefits as compared to conventional window structures. For instance, the example window structures may provide a relatively high energy efficiency (e.g., by attenuating infrared frequencies) while transmitting one or more microwaves frequencies (e.g., 5G frequencies). For instance, the microwave frequencies may include 28 GHz, 37 GHz, 39 GHz, and/or 60 GHz. Accordingly, 5G devices may be able to communicate with a base station (or vice versa) through the example window structures.

The example window structures may be fabricated using conventional fabrication techniques with the addition of one extra step (e.g., annealing to form metal island structures in the metal layer). The window structures may be fully compatible with existing 4G (e.g., frequency <2.7 GHz) and developed 5G (e.g., 28 GHz, 37 GHz, 39 GHz, 60 GHz) frequency standards. The frequency response of the example window structures may be flat up to at least 10 THz. Conventional anti-reflective and barrier layers may work substantially as well on metal layers that include metal island structures as on continuous metal films, for example, because the metal island structures may be flat and have a width of tens of nanometers, which is substantially smaller than the wavelength of light.

FIG. 1is a cross-section of an example window structure100having a microwave-transmissive (mw-transmissive) infrared-reflective (IR-reflective) metal layer110in accordance with an embodiment. As shown inFIG. 1, the window structure100includes the following layers in order: a glass substrate102, an under layer104, a first dielectric layer106, a first blocker layer108, the mw-transmissive IR-reflective metal layer110, a second blocker layer112, a second dielectric layer114, and an over layer116.

The glass substrate102is a glass layer on which the other layers of the window structure100may be formed. The glass layer may be a glass material such as soda-lime glass (SLG), Eagle XG (EXG™) glass, or High Purity Fused Silicon™ (HPFS™) glass. It is noted that the loss tangent of SLG is approximately ten times the loss tangent of EXG™ glass (e.g., at 5G frequencies, such as 28 GHz, 37 GHz, 39 GHz, and/or 60 GHz). The loss tangent of EXG™ glass is approximately ten times the loss tangent of HPFS™ glass (e.g., at 5G frequencies, such as 28 GHz, 37 GHz, 39 GHz, and/or 60 GHz). EXG™ glass and HPFS™ glass are made and distributed by Corning Inc.

Each of the under layer104and the over layer116includes an oxide that is resistant to moisture. Accordingly, the under layer104and the over layer116may inhibit moisture from reaching (e.g., penetrating) the mw-transmissive IR-reflective metal layer110. The under layer104may increase adhesion between the substrate102and the first dielectric layer106and/or increase transmittance of visible light through the mw-transmissive IR-reflective metal layer110. The under layer104may include a metal nitride, a metal oxide, and/or a metal oxynitride. The over layer116may increase scratch resistance of the window structure100. The first dielectric layer106includes an oxide that electrically isolates the mw-transmissive IR-reflective metal layer110from the under layer104. The second dielectric layer114include an oxide that electrically isolates the mw-transmissive IR-reflective metal layer110from the over layer116. Each of the first and second dielectric layers106and114may include Si3N4, SnO, SnO2, ZnO:Al, WO, LaB6, and/or other dielectric material(s). Each of the first and second blocker layers108and112includes an anti-reflective material that is configured to mitigate reflection of visible light from the window structure100. Each of the first and second blocker layers108and112may include TiO2, SnO, WO, LaB6, and/or other anti-reflective material(s).

The mw-transmissive IR-reflective metal layer110is configured to transmit microwave signals and to reflect infrared signals. For example, the mw-transmissive IR-reflective metal layer110may be configured to transmit signals having frequencies in one or more portions of the microwave frequency spectrum. For instance, the mw-transmissive IR-reflective metal layer110may be configured to transmit signals having frequencies in a range from 6 GHz to 80 GHz, in a range from 28 GHz to 60 GHz, and/or in other range(s) in the microwave frequency spectrum.

In another example, the mw-transmissive IR-reflective metal layer100may provide a transmittance that is greater than or equal to a threshold transmittance across one or more portions of the microwave frequency spectrum. For instance, the threshold transmittance may be 40%, 50%, 60%, 70%, 80%, or 90%. The transmittance of the mw-transmissive IR-reflective metal layer100may be greater than or equal to the threshold transmittance across a range of frequencies from 28 GHz to 60 GHz, across a range of frequencies from 6 GHz to 80 GHz, and/or across other range(s) in the microwave frequency spectrum. For instance, the transmittance of the mw-transmissive IR-reflective metal layer100may be up to 100% across one or more ranges in the microwave frequency spectrum.

In yet another example, the mw-transmissive IR-reflective metal layer100may provide a transmittance in a range between 35% and 100%, in a range between 40% and 100%, in a range between 50% and 100%, or in a range between 60% and 100% across one or more portions of the microwave frequency spectrum. For instance, the mw-transmissive IR-reflective metal layer100may provide the aforementioned transmittance for signals having frequencies in a range between 28 GHz and 60 GHz, in a range between 6 GHz and 80 GHz, and/or in other range(s) in the microwave frequency spectrum.

In still another example, the mw-transmissive IR-reflective metal layer100may have a resistance that is greater than or equal to a threshold resistance with regard to signals having frequencies in one or more portions of the microwave frequency spectrum. For instance, the threshold resistance may be 5 megaohms (MΩ), 10 MΩ, 20 MΩ, 50 MΩ, 100 MΩ, or 200 MΩ. The resistance of the mw-transmissive IR-reflective metal layer100may be greater than or equal to the threshold resistance with regard to signals having frequencies in a range from 6 GHz to 80 GHz, in a range from 28 GHz to 60 GHz, and/or in other range(s) in the microwave frequency spectrum.

In another example, the mw-transmissive IR-reflective metal layer100may have a conductivity that is less than or equal to a threshold conductivity with regard to signals having frequencies in one or more portions of the microwave frequency spectrum. For instance, the threshold conductivity may be 10−4siemens per meter (S/m), 10−5S/m, or 10−6S/m. The conductivity of the mw-transmissive IR-reflective metal layer100may be less than or equal to the threshold conductivity with regard to signals having frequencies in a range from 6 GHz to 80 GHz, in a range from 28 GHz to 60 GHz, and/or in other range(s) in the microwave frequency spectrum.

In yet another example, the mw-transmissive IR-reflective metal layer100may be configured to reflect at least a threshold proportion of the infrared signals. For instance, the threshold proportion may be 15%, 20%, 25%, 30%, or 40%.

In still another example, the mw-transmissive IR-reflective metal layer100may provide a reflectance in a range between 20% and 70%, in a range between 25% and 65%, in a range between 30% and 60%, or in a range between 35% and 55% across one or more portions of the infrared frequency spectrum. For instance, the mw-transmissive IR-reflective metal layer100may provide the aforementioned reflectance for signals having frequencies in a range between 25 THz and 80 THz, in a range between 30 THz and 75 THz, in a range between 35 THz and 70 THz, or in a range between 40 THz and 65 THz.

In another example, the metal layer may be a discontinuous metal layer. For instance, the metal layer may be an electrically discontinuous metal layer and/or a physically discontinuous metal layer. In an aspect of this example, the discontinuous metal layer may include metal island structures disposed in a plane. Shape and/or size of the metal islands may be random, thought the scope of the example embodiments is not limited in this respect. A layer projection area of the plane is an area of the plane that is defined by a projection of the discontinuous metal layer on the plane. An island projection area of the plane is an area of the plane that is defined by projections of the respective metal island structures on the plane. An areal coverage of the discontinuous metal layer is defined to be the island projection area divided by the layer projection area. The areal coverage may be greater than or equal to a lower threshold. For instance, the lower threshold may be 25%, 30%, 35%, 40%, or 45%. The areal coverage may be less than or equal to an upper threshold. For instance, the upper threshold may be 45%, 50%, 55%, 60%, or 65%. The areal coverage may be in a range between the lower threshold and the upper threshold.

The mw-transmissive IR-reflective metal layer110may include any suitable metal(s), including but not limited to gold, silver, aluminum, copper, or any combination thereof.

The mw-transmissive IR-reflective metal layer110is shown inFIG. 1to have a thickness T. Accordingly, if mw-transmissive IR-reflective metal layer110includes metal islands, the metal islands have the thickness T. The thickness T may be greater than or equal a lower thickness threshold. For instance, the lower thickness threshold may be 0.5 nm, 1 nm, 1.5 nm, 2 nm, or 3 nm. The thickness T may be less than or equal to an upper thickness threshold. For instance, the upper thickness threshold may be 5 nm, 6 nm, 7 nm, 8 nm, or 10 nm. The thickness T may be in a range between the lower thickness threshold and the upper thickness threshold. If the mw-transmissive IR-reflective metal layer110includes metal islands, each of the metal islands may have a lateral dimension that is perpendicular to an axis along which the thickness T is measured. For instance, the metal islands may be configured such that an average of the lateral dimensions of the metal islands is greater than or equal to a threshold dimension. For example, the threshold dimension may be 10 nm, 12 nm, 15 nm, 20 nm, or 25 nm. For instance, the metal islands having lateral dimensions or an average lateral dimension greater than or equal to 20 nm may reduce absorption of microwave signals by the mw-transmissive IR-reflective metal layer110. Each of the metal islands may be configured to have a lateral dimension that is substantially greater than the thickness T of the metal island.

The example layers shown inFIG. 1are provided for illustrative purposes and are not intended to be limiting. The window structure100may not include one or more of the layers shown inFIG. 1. Moreover, the window structure100may include layer(s) in addition to or in lieu of one or more of the layers shown inFIG. 1.

FIG. 2illustrates example concentrations of elements with respect to etch time, which may be used to fabricate a window structure (e.g., window structure100shown inFIG. 1), in accordance with an embodiment.

FIG. 3is a graph300including example plots302and304of transmission and reflection, respectively, with respect to wavelength for a mw-transmissive IR-reflective metal layer110shown inFIG. 1. For plot302, transmittance is represented along the right Y-axis of the graph300, and wavelength is represented along the X-axis of the graph300. For plot304, reflectance is represented along the left Y-axis of the graph300, and wavelength is represented along the X-axis of the graph300.

Over the wavelengths shown inFIG. 3, the low-E window functions as a bandpass filter with peak transmission of approximately 90% for wavelengths in the visible spectrum306, while substantially reflecting wavelengths in the infrared spectrum. The visible spectrum306includes wavelengths in a range from approximately390nanometers (nm) to 700 nm. The infrared spectrum includes wavelengths in a range from 700 nm to one millimeter (mm). Wavelengths in the infrared spectrum are referred to as “infrared wavelengths”. The example embodiments described herein may be capable of causing a low-E window to function as a bandpass filter that includes multiple passbands. For instance, the bandpass filter may include a passband that includes the visible spectrum and one or more additional passbands that include respective portion(s) of the microwave spectrum, while still substantially reflecting the infrared wavelengths. The microwave spectrum includes wavelengths in a range from one mm to one meter (m). Wavelengths in the microwave spectrum are referred to as “microwave wavelengths”.

FIG. 4is a graph400including example plots402,404, and406of spectral intensity with respect to wavelength for three different black bodies having respective temperatures. Plot402corresponds to a black body (e.g., the sun) having a temperature of 6000 K. Plot404corresponds to a black body having a temperature of 3000 K. Plot406corresponds to a black body (e.g., a room in a building) having a temperature of 300 K. In the 300 K black body, radiation starts at a wavelength of approximately four micrometers (μm) and peaks at a wavelength of approximately 10 μm. The example embodiments described herein may be capable of reflecting the radiation associated with plot406while enabling radiation in the microwave spectrum to be transmitted. For instance, a window structure described herein may cause the radiation associated with plot406to be reflected back into a room while allowing radiation in the microwave spectrum to be transmitted into and/or out of the room through the window structure.

FIG. 5is a graph500including example plots502,504,506,512,514, and516of loss with respect to frequency for microwave signals that pass through various structures. Plots502and512depict computer-modelled loss and measured loss, respectively, of a microwave signal that passes through a wall. Plots504and514depict computer-modelled loss and measured loss, respectively, of a microwave signal that passes through a low-E glass that includes a metal film. Plots506and516depict computer-modelled loss and measured loss, respectively, of a microwave signal that passes through standard glass (i.e., glass that does not include a low-E coating). The loss was modelled and measured over a frequency range from 0.8 GHz to 40 GHz.

As depicted by plots504and514, 4G signals (e.g., signals at 2.7 GHz) are blocked by the low-E glass with 26 dB loss; however, as depicted by plots502and512, 4G signals travel unhindered through the wall. As further depicted by plots504and514, 5G signals (e.g., signals at 28-40 GHz) are blocked by the low-E glass with 26-37 dB loss; also, as depicted by plots502and512, 5G signals are substantially completely blocked by the wall (e.g., ˜100 dB loss). The blocking behavior of the low-E glass appears to be due to the metal film therein hindering the transmission of microwaves. For instance, a relatively simple calculation of transmittance is provided as follows:

where Rs=1/(σd) [Ω/sq.] is the resistance per unit square of the metal film; σ is the conductivity of the metal film; d is a thickness of the metal film; and Z0/2=188Ω is half of the free space impedance. In some industry standard metal films for low-E windows, Rs=2-5 [Ω/sq.], so that Tx≈2Rs/Z0«1 and the response is flat in the microwave frequency spectrum covering 4G, 5G, and up to the THz region.

FIG. 6is a graph600that includes example plots602and604of transmission loss with respect to frequency for a window without a low-E coating and a window with a metal film low-E coating. The window with the metal film low-E coating in the embodiment ofFIG. 6includes three layers of the metal film low-E coating on glass having a thickness of 30 nm for non-limiting illustrative purposes. As shown inFIG. 6, the window with the metal film low-E coating provides a relatively flat 20 dB transmission loss from 25 GHz to 45 GHz, as compared to a substantially negligible loss for the window without a low-E coating.

Electron scattering contributes substantially to the relatively high transmission loss that occurs with respect to microwave frequencies for conventional metal film low-E coatings. For instance, the electron scattering contributes to shortening the effective mean free path of electrons through a metal film and defines the metal film's response to microwaves and light. Electron scattering in a thin film, such as a metal film low-E coating, may be caused by grain boundary scattering and/or surface roughness scattering. The example window structures described herein may mitigate the effect of such grain boundary scattering and/or surface roughness scattering.

FIG. 7is an example illustration of a metal film700in which grain boundary scattering occurs. As shown inFIG. 7, metal film700includes multiple grains. The grains include a first grain702, a second grain704, a third grain706, and a fourth grain708. A first electron716in the first grain702scatters from a first surface710between the first and second grains702and704. The first electron716then scatters from a second surface712at an outer boundary of the metal film700. A second electron718in the third grain706scatters from a third surface714between the third and fourth grains706and708. The scattering of the first electron716from the first and second surfaces710and712inhibits transmission of the first electron716through the metal film700. The scattering of the second electron718from the third surface714inhibits transmission of the second electron718through the metal film700.

The grains in the metal film700may be any suitable sizes and may vary by any suitable amount. For instance, if the metal film700is 50 nm thick, the grain size may vary 19 nm. If the metal film700is 20 nm thick, the grain size may vary 10.8 nm. If the metal film700is 12 nm thick, the grain size may vary 8.4 nm, and so on. The example thicknesses and variations described herein are provided for non-limiting illustrative purposes.

The example embodiments described herein may be capable of reducing grain boundary scattering that electrons encounter in a metal layer. For example, the metal layer may be a physically discontinuous metal layer that includes metal islands. In accordance with this example, each metal island may have relatively few grains as compared to the metal layer as a whole, which may facilitate transmission of the electrons through the metal islands. In further accordance with this example, electrons may travel between the metal islands, which may facilitate their transmission through the metal layer.

FIG. 8is an example illustration of a metal film800in which surface roughness scattering occurs. The surface roughness scattering in the metal film800may be modelled using the “Fuchs-Sondheimer model,” for example. In accordance with the Fuchs-Sondheimer model, electrons have a limited mean free path as a result of phonon and impurity scattering. In further accordance with this model, a specularity coefficient, p, may be used to specify a fraction of electrons that are scattered at a surface806of the metal film800. First electrons802and second electrons804are shown to be incident on the surface806of the metal film800to illustrate differences in the amount of scattering, as indicated by respective values of the specularity coefficient. In a first example, the specularity coefficient having a value of one (i.e., p=1) results in all (i.e., 100%) of the first electrons802scattering at the surface806. In a second example, the specularity coefficient having a value of zero (i.e., p=0) results in none (i.e., 0%) of the second electrons804scattering at the surface806.

The example embodiments described herein may be capable of reducing surface roughness scattering that electrons encounter in a metal layer. For instance, in a physically discontinuous metal layer, electrons may travel between metal islands therein, which may reduce a number of electrons that encounter surface roughness scattering in the metal layer.

Resistivity of a metal film varies with the thickness of the metal film.FIG. 9is an example plot900of resistivity with respect to thickness for an unannealed metal film. The plot900shows example contributions of various scattering mechanisms to the resistivity in the unannealed metal film. The contributions include a bulk resistivity contribution902and a grain boundary scattering contribution904.

FIG. 10is an example plot1000of resistivity with respect to thickness for an annealed metal film. The plot1000shows example contributions of various scattering mechanisms to the resistivity in the annealed metal film. The contributions include a bulk resistivity contribution1002, a grain boundary scattering contribution1004, and an interface scattering contribution1006.

The dependence of the resistivity of a metal film may be defined by the following equation:

where ρbulkis the resistivity of the bulk metal; p is the Fuchs-Sondheimer specularity factor (p=0); S is the surface roughness factor, which is in a range between 1 and 2; R is the reflectance of the grain boundaries, which is in a range between 0.07 and 0.10; l is the bulk mean free path; and g is the grain size. See S. M. Rossnagel and T. S. Kuan, “Alteration of Cu conductivity in the size effect regime,”J. Vac. Sci. Technol. B22 (1), pp. 240-247, January/February 2004. It is noted that the product of the resistivity of the metal film, ρ, and the scattering time, τ, is constant (i.e., ρτ=constant). For example, in silver, ρτ=59±2 μΩ·cm·fs. The scattering time defines the frequency dependence of the dielectric function of the film through a simple Drude formula that is applicable for microwave and optical frequencies:

where ε∞=4 for silver; ωpis the bulk metal plasma frequency; and τ is the scattering time, as mentioned above.

The transmittance calculated with the above parameters for silver films having respective thicknesses of 30 nm and 5 nm is shown inFIG. 11. More particularly,FIG. 11shows example plots1100and1150of transmittance, reflectance, and absorptance with respect to frequency for silver films having the thicknesses of 30 nm and 5 nm, respectively. As shown inFIG. 11, the transmittance is relatively low and relatively flat across the microwave frequency spectrum and the portion of the infrared frequency spectrum from 300 GHz to approximately 10 THz and then increases for the remaining portion of the infrared frequency spectrum and the visible frequency spectrum. For the 5 nm-thick silver film, the transmittance is approximately 0.03 across the microwave frequency and approaches 100% in the visible frequency spectrum. The losses on the 5 nm-thick silver film are approximately 15 dB, which is substantially less than the 20+ dB loss that is associated with the 30 nm-thick silver film.

FIG. 12Aillustrates behavior of a window structure1200having a mw-transmissive IR-reflective metal layer1210coupled to a glass layer1202in accordance with an embodiment. As depicted inFIG. 12A, the mw-transmissive IR-reflective metal layer1210allows at least some microwave signals1204to propagate through the window structure1200. For instance, if the mw-transmissive IR-reflective metal layer1210is a discontinuous metal layer that includes metal islands, the mw-transmissive IR-reflective metal layer1210may allow the microwave signals1204to propagate through openings between the metal islands.

FIG. 12Billustrates behavior of a window structure1250having a low-E metal film1260coupled to a glass layer1252. As depicted inFIG. 12B, the low-E metal film1260does not allow microwave signals1254to propagate through the window structure1250. Rather, the low-E metal film1260reflects the microwave signals1254.

Referring toFIGS. 12A and 12B, the mw-transmissive IR-reflective metal layer1210and the low-E metal film1260respond to microwave signals qualitatively differently, even if the mw-transmissive IR-reflective metal layer1210and the low-E metal film1260have the same amount of metal per unit area. It is noted that reflectance increases rather sharply with thickness of continuous films. For instance, reflectance of a microwave signal having a frequency of 9.8 GHz by a silver film may exceed 65% for a thickness of 20 nm. At optical frequencies, skin depth becomes frequency-independent and equal to c/ωp≈20 nm, where c=3*1010cm/s (i.e., the speed of light). In one example, the aforementioned skin depth may be larger than an average size of metal islands in a mw-transmissive IR-reflective metal layer (e.g., mw-transmissive IR-reflective metal layer1210) and comparable to the thickness of the metal islands; whereas, the skin depth for microwave frequencies is in the micrometer range. Despite interest in scattering on metallic subwavelength features that dates back to the early 1900s, a complete microscopic theory does not exist. Because the average size of metal islands in a mw-transmissive IR-reflective metal layer is likely to be substantially less than wavelengths of incident microwave radiation, brute force numerical methods may not be helpful. Even a simpler case relating to subwavelength metallic gratings with account for finite conductivity is a matter of debate. For a mw-transmissive IR-reflective metal layer, solving for transmittance for a given realization of a random structure and then performing an average with respect to all possible realizations of disorder may not be possible. Accordingly, it may be desirable to replace the mw-transmissive IR-reflective metal layer with an intended “equivalent” continuous film. However, such an equivalency is not currently known and may not exist.

However, because the forward scattering is dominant in the sub-wavelength geometry, one could go beyond the usual quasi-static approximation and first find the field distribution in metallic islands and dielectric background separately, averaged over the thickness of the mw-transmissive IR-reflective metal layer and without any averaging over the lateral extend of the mw-transmissive IR-reflective metal layer. The electric and magnetic fields (E and H) in the mw-transmissive IR-reflective metal layer may be expressed via incident and scattered fields via generalized Ohmic parameters (u, v) separately for metallic (um, vm) and dielectric (ud, vd) areas. The parameter u represents the incident field intensity. The parameter v represents the scattering field intensity. The effective parameters (ue, ve) then may be determined via a relevant ensemble averaging of the parameters (um, vm) and (ud, vd), and the transmittance of the mw-transmissive IR-reflective metal layer may be determined based on the effective parameters (ue, ve). For instance, the parameters (um, vm) and (ud, vd) may be averaged using Effective Medium Theory described in classical paper by D. A. G. Bruggeman, “Berechnung verschiedener physikalischer konstanten von heterogenen substanzen,”Annals of Physics, vol. 24, pp. 636-679, 1935.

FIG. 13shows example plots1304,1304, and1306of transmittance, reflectance, and absorptance with respect to metal areal filling fraction for a discontinuous metal layer in accordance with an embodiment. The metal areal filling fraction is the proportion of the metal layer that is metal. The plots1304,1304, and1306represent the transmittance, the reflectance, and the absorptance, respectively, for a fixed frequency of incident radiation. As shown inFIG. 13, the transmittance approaches 100% in the discontinuous metal layer for a metal areal filling fraction that is less than a percolation threshold1308. The percolation threshold1308may correspond to a metal areal filling fraction of approximately 0.5, though the scope of the example embodiments is not limited in this respect. It should be noted that the transmittance for microwave frequencies changes from 0% to 100% more abruptly than the transmittance for infrared frequencies as the metal areal filling fraction is reduced. This difference may be most pronounced in a range of metal areal filling fractions near (e.g., just below or including) the percolation threshold1308. Designing a window to have a metal layer with a metal areal filling fraction in this range may enable the window to transmit microwave frequencies while reflecting IR frequencies. The aforementioned difference will be discussed in further detail below with reference toFIGS. 15 and 17.

FIGS. 14A-14Care example scanning electron microscope (SEM) images1400,1430, and1460of discontinuous gold layers having respective thicknesses (d) of 4 nm, 7 nm, and 10 nm in accordance with embodiments. The SEM images1400,1430, and1460may correspond to different metal areal filling fractions. Each of the discontinuous gold layers may include gold islands that are randomly arranged in the discontinuous gold layer. Gaps between the randomly arranged gold islands may provide holes through the discontinuous gold layer, enabling microwave signals to pass through.

FIG. 15is an example plot1500of static conductivity of a discontinuous gold layer with respect to metal areal filling fraction in accordance with an embodiment. As shown inFIG. 15, the conductivity of the discontinuous gold layer drops by several orders of magnitude when the metal areal filling fraction drops below the percolation threshold, pth. The metal areal filling fraction being less than the percolation threshold, pth, corresponds to the discontinuous gold layer including gold particles that are separated by gaps, thereby forming gold islands. The gaps between the gold islands reduce the conductivity of the discontinuous gold layer. This reduced conductivity may result in higher transmittance of microwave frequencies. When the metal areal filling fraction goes above the percolation threshold, pth, the gold islands are more likely to be in physical contact, resulting in clusters of the metal particles, which increases the conductivity of the discontinuous gold layer. Such an increase in conductivity may result in lower transmittance of microwave frequencies. Accordingly, it can be seen that designing the discontinuous gold layer to have a metal areal filling fraction that is less than the percolation threshold, pth, may be desirable to achieve transmission of microwave frequencies while reflecting infrared frequencies.

FIG. 16shows plots1600of conductivity with respect to frequency for gold films and for gold layers that include gold island structures in accordance with an embodiment. The gold films have a metal areal filling fraction greater than the percolation threshold; whereas, the gold layers that include the gold island structures have a metal areal filling fraction less than the percolation threshold. The plots1600include plots1602,1604,1606, and1608, which represent conductivity of the respective gold films with respect to a frequency range from 800 MHz to 20 GHz. The plots1600further include plots1610,1612, and1614, which represent conductivity of the respective gold layers that include gold island structures with respect to the frequency range from 800 MHz to 20 GHz. As illustrated inFIG. 16, the gold films, which correspond to plots1602,1604,1606, and1608, have greater conductivities than the gold layers that include the gold island structures, which correspond to plots1602,1604, and1606, across the frequency range from 800 MHz to 20 GHz. Noteworthy is the flat response versus frequency, as expected from the model inFIG. 11. Conductivity may be inversely proportional to microwave transmittance, though the example embodiments are not limited in this respect.

FIG. 17shows plots1702,1704,1752, and1754of transmittance and reflectance with respect to metal areal filling fraction of a discontinuous metal layer for a microwave frequency of 10 GHz and for a near-infrared (NIR) optical wavelength of 2.5 μm in accordance with an embodiment. Plots1702and1704correspond to the microwave frequency of 10 GHz. In particular, the plot1702represents the transmittance with respect to the metal areal filling fraction for the microwave frequency, and the plot1704represents the reflectance with respect to the metal areal filling fraction for the microwave frequency. Plots1752and1754correspond to the NIR optical frequency. In particular, plot1752represents the transmittance with respect to the metal areal filling fraction for the NIR optical wavelength, and the plot1754represents the reflectance with respect to the metal areal filling fraction for the NIR optical wavelength.

As depicted by plot1702, the transmittance for the microwave frequency increases abruptly for a metal areal filling fraction less than the percolation threshold, pth; whereas, as depicted by plot1752, the transmittance for the NIR optical wavelength increases more gradually for a metal areal filling fraction less than the percolation threshold, pth. Accordingly, the difference between the transmittance for the microwave frequency and the transmittance for the NIR optical wavelength is relatively large (e.g., reaches a maximum) just below the percolation threshold, pth, and this difference becomes less as the metal areal filling fraction is further reduced. A working region may be defined for a range of metal areal filling fraction values based on design requirements. For example, an upper limit of the range may be selected to be near (e.g., just below, the same as, or just above) the percolation threshold, pth, and a lower limit on the range may be selected to be a metal areal filling fraction value at which a difference between the transmittance of the microwave frequency and the transmittance of the NIR optical wavelength reaches a threshold difference. In another example, the upper and lower limits of the range may be predetermined values. InFIG. 17, a working region for a range of metal areal filling fraction values is selected to be from 35% to 55% for non-limiting illustrative purposes. It will be recognized that the working region may be any suitable range of metal areal filling fraction values in which the relationship between reflecting IR signals and reflecting microwave signals is disconnected. In the working region ofFIG. 17the transmittance of the microwave frequency is approximately 90%, and the reflectance of the NIR optical wavelength is approximately 35-40%.

FIG. 18shows example steps of a process1800to fabricate a window having a discontinuous metal layer in accordance with an embodiment. In step1of the process1800, a continuous metal layer is deposited onto an underlayer, which is on a substrate. In step2of the process1800, the continuous metal layer is modified to obtain the discontinuous metal layer. For example, if the continuous metal layer is deposited at a relatively low temperature in step1, the continuous metal layer may be dewetted in step2to obtain the discontinuous metal layer. In another example, if the continuous metal layer is deposited at a relatively high temperature in step1, the relatively high temperature may cause the continuous metal layer to form islands to obtain the discontinuous metal layer. In step3of the process1800, an anti-reflection layer is deposited onto the discontinuous metal layer. Because the thickness and the lateral size of the metal islands are substantially less than the wavelengths in the microwave spectrum, the anti-reflection layer may be substantially same as the anti-reflection layers used with continuous metal films. In step4of the process1800, an over layer is placed on the anti-reflective layer to provide a first structure of the window. As shown inFIG. 18, a second, alternative structure of the window may be achieved by placing a dielectric layer between the under layer and the discontinuous metal layer.

FIG. 19depicts a flowchart1900of an example method for making a window structure in accordance with an embodiment. Flowchart may be performed by any suitable fabrication machinery. As shown inFIG. 19, the method of flowchart19begins at step1902. In step1902, a glass layer is provided.

At step1904, a metal layer is formed on the glass layer. Forming the metal layer includes configuring the metal layer to transmit signals having frequencies in a range from 28 GHz to 60 GHz and to reflect signals having infrared frequencies. The metal layer may be formed on the glass layer by depositing metal on or adhering the metal to the glass layer. For instance, the metal may be spray coated or sputter coated onto the glass layer.

In an example embodiment, forming the metal layer at step1904includes configuring the metal layer to transmit signals having frequencies in a range from 6 GHz to 60 GHz, in a range from 28 GHz to 80 GHz, or in a range from 6 GHz to 80 GHz.

In another example embodiment, forming the metal layer at step1904includes configuring the metal layer to have a resistance of at least a threshold resistance with regard to the signals having the frequencies in the range from 28 gigahertz to 60 gigahertz. For instance, the threshold resistance may be 10 MΩ or 100 MΩ.

In yet another example embodiment, forming the metal layer at step1904includes configuring the metal layer to reflect at least a threshold percentage of the signals having the infrared frequencies. For instance, the threshold percentage may be 30%, 35%, 40%, or 45%.

In still another example embodiment, forming the metal layer at step1904includes configuring the metal layer to have a conductivity less than or equal to a threshold conductivity with regard to the signals having the frequencies in the range from 28 gigahertz to 60 gigahertz. For instance, the threshold conductivity may be 10−6siemens per meter, 10−5siemens per meter, or 10−4siemens per meter.

In another example embodiment, forming the metal layer at step1904includes configuring the metal layer to provide a transmittance of at least 80% across a range of frequencies from 6 GHz to 80 GHz, across a range of frequencies from 6 GHz to 60 GHz, across a range of frequencies from 28 GHz to 80 GHz, or across a range of frequencies from 28 GHz to 60 GHz.

In yet another example embodiment, forming the metal layer at step1904includes configuring the metal layer to be a discontinuous metal layer. For example, configuring the metal layer to be the discontinuous metal layer may include configuring the metal layer to be an electrically discontinuous metal layer. In another example, configuring the metal layer to be the discontinuous metal layer may include configuring the metal layer to be a physically discontinuous metal layer.

In a first aspect of this embodiment, configuring the metal layer includes configuring the discontinuous metal layer to have an areal coverage in a range between 35% and 55%.

In a second aspect of this embodiment, forming the metal layer at step1904includes disposing the metal layer on the glass layer to provide a thickness of the metal layer that is less than a threshold thickness. In accordance with the second aspect, the thickness being less than the threshold thickness causes the metal layer to become discontinuous (e.g., electrically discontinuous and/or physically discontinuous).

In some example embodiments, one or more steps1902and/or1904of flowchart1900may not be performed. Moreover, steps in addition to or in lieu of steps1902and/or1904may be performed. For instance, in an example embodiment, the method of flowchart1900further includes applying heat to the metal layer to adhere the metal layer onto the glass layer. In another example embodiment, the method of flowchart1900further includes removing portions of the metal layer in response to forming the metal layer on the glass layer. In accordance with this embodiment, removing the portions of the metal layer causes the metal layer to become discontinuous (e.g., electrically discontinuous and/or physically discontinuous).

FIG. 20depicts a flowchart2000of an example method for using a window structure having a glass layer and a metal layer formed on the glass layer in accordance with an embodiment. Flowchart2000may be performed by window structure100shown inFIG. 1, for example. For illustrative purposes, flowchart2000will be described with reference to window structure100. Further structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the discussion regarding the flowchart2000.

As shown inFIG. 20, the method of flowchart2000begins at step2002. In step2002, infrared signals having infrared frequencies are received at the metal layer. In an example implementation, the mw-transmissive IR-reflective metal layer110receives the infrared signals.

At step2004, microwave signals having frequencies in a range from 28 gigahertz to 60 gigahertz are received at the metal layer. In an example implementation, the mw-transmissive IR-reflective metal layer110receives the microwave signals.

At step2006, the microwave signals are transmitted through the metal layer based at least in part on a configuration of the metal layer. In an example implementation, the mw-transmissive IR-reflective metal layer110transmits the microwave signals based at least in part on the configuration of the mw-transmissive IR-reflective metal layer110.

At step2008, the infrared signals are reflected from the metal layer based at least in part on the configuration of the metal layer. In an example implementation, the mw-transmissive IR-reflective metal layer110reflects the infrared signals based at least in part on the configuration of the mw-transmissive IR-reflective metal layer110.

In an example embodiment, transmitting the microwave signals at step2006includes transmitting the microwave signals through the metal layer based at least in part on the metal layer being a discontinuous metal layer. For example, the microwave signals may be transmitted through the metal layer based at least in part on the metal layer being an electrically discontinuous metal layer. In another example, the microwave signals may be transmitted through the metal layer based at least in part on the metal layer being a physically discontinuous metal layer.

In some example embodiments, one or more steps2002,2004,2006, and/or2008of flowchart2000may not be performed. Moreover, steps in addition to or in lieu of steps2002,2004,2006, and/or2008may be performed.

III. Further Discussion of Some Example Embodiments

A first example window structure comprises a glass layer and a metal layer.

The metal layer is formed on the glass layer. The metal layer is configured to transmit signals having frequencies in a range from 28 gigahertz to 60 gigahertz and is further configured to reflect signals having infrared frequencies.

In a first aspect of the first example window structure, the metal layer is configured to transmit signals having frequencies in a range from 6 gigahertz to 80 gigahertz.

In a second aspect of the first example window structure, the metal layer has a resistance of at least 10 megaohms with regard to the signals having the frequencies in the range from 28 gigahertz to 60 gigahertz. The second aspect of the first example window structure may be implemented in combination with the first aspect of the first example window structure, though the example embodiments are not limited in this respect.

In a third aspect of the first example window structure, the metal layer has a resistance of at least 100 megaohms with regard to the signals having the frequencies in the range from 28 gigahertz to 60 gigahertz. The third aspect of the first example window structure may be implemented in combination with the first and/or second aspect of the first example window structure, though the example embodiments are not limited in this respect.

In a fourth aspect of the first example window structure, the metal layer is configured to reflect at least 20% of the signals having the infrared frequencies. The fourth aspect of the first example window structure may be implemented in combination with the first, second, and/or third aspect of the first example window structure, though the example embodiments are not limited in this respect.

In a fifth aspect of the first example window structure, the metal layer has a conductivity less than or equal to 10−5siemens per meter with regard to the signals having the frequencies in the range from 28 gigahertz to 60 gigahertz. The fifth aspect of the first example window structure may be implemented in combination with the first, second, third, and/or fourth aspect of the first example window structure, though the example embodiments are not limited in this respect.

In a sixth aspect of the first example window structure, the metal layer provides a transmittance of at least 80% across a range of frequencies from 28 GHz to 60 GHz. The sixth aspect of the first example window structure may be implemented in combination with the first, second, third, fourth, and/or fifth aspect of the first example window structure, though the example embodiments are not limited in this respect.

In a seventh aspect of the first example window structure, the metal layer provides a transmittance of at least 80% across a range of frequencies from 6 GHz to 80 GHz. The seventh aspect of the first example window structure may be implemented in combination with the first, second, third, fourth, fifth, and/or sixth aspect of the first example window structure, though the example embodiments are not limited in this respect.

In an eighth aspect of the first example window structure, the metal layer is an electrically discontinuous metal layer. The eighth aspect of the first example window structure may be implemented in combination with the first, second, third, fourth, fifth, sixth, and/or seventh aspect of the first example window structure, though the example embodiments are not limited in this respect.

In an implementation of the eighth aspect of the first example window structure, the electrically discontinuous metal layer has an areal coverage in a range between 35% and 55%.

A second example window structure comprises a glass substrate and a discontinuous metal layer. The discontinuous metal layer is configured to reflect infrared wavelengths. The discontinuous metal layer comprises metal island structures having a thickness and a lateral dimension disposed adjacent to the glass substrate. The thickness of the metal island structures is in a range from 1 nanometer and 7 nanometers. The lateral dimension of the metal island structures averages at least 15 nanometers.

In a first aspect of the second example window structure, the discontinuous metal layer has an areal coverage in a range between 35% and 55%.

In a second aspect of the second example window structure, the discontinuous metal layer provides a transmittance in a range between 0.4 and 1.0 for signals having frequencies in a range between 6 GHz and 80 GHz and a reflectance in a range between 0.3 and 0.6 for signals having frequencies in a range between 30 terahertz and 75 terahertz. The second aspect of the second example window structure may be implemented in combination with the first aspect of the second example window structure, though the example embodiments are not limited in this respect.

In a third aspect of the second example window structure, the discontinuous metal layer includes at least one of gold, silver, aluminum, or copper. The third aspect of the second example window structure may be implemented in combination with the first and/or second aspect of the second example window structure, though the example embodiments are not limited in this respect.

In a fourth aspect of the second example window structure, the second example window structure further comprises a dielectric layer that includes at least one of Si3N4, SnO, WO, or LaB6. In accordance with the fourth aspect, the discontinuous metal layer is between the dielectric layer and the glass layer. The fourth aspect of the second example window structure may be implemented in combination with the first, second, and/or third aspect of the second example window structure, though the example embodiments are not limited in this respect.

In a fifth aspect of the second example window structure, the second example window structure further comprises an anti-reflective layer between the glass substrate and the discontinuous metal layer, the anti-reflective layer including at least one of TiO2, SnO, WO, or LaB6. The fifth aspect of the second example window structure may be implemented in combination with the first, second, third, and/or fourth aspect of the second example window structure, though the example embodiments are not limited in this respect.

In an example method of making a window structure, a glass layer is provided. A metal layer formed on the glass layer. Forming the metal layer comprises configuring the metal layer to transmit signals having frequencies in a range from 28 gigahertz to 60 gigahertz and to reflect signals having infrared frequencies.

In a first aspect of the example method, forming the metal layer comprises configuring the metal layer to transmit signals having frequencies in a range from 6 gigahertz to 80 gigahertz.

In a second aspect of the example method, forming the metal layer comprises configuring the metal layer to have a resistance of at least 10 megaohms with regard to the signals having the frequencies in the range from 28 gigahertz to 60 gigahertz. The second aspect of the example method may be implemented in combination with the first aspect of the example method, though the example embodiments are not limited in this respect.

In a third aspect of the example method, forming the metal layer comprises configuring the metal layer to have a resistance of at least 100 megaohms with regard to the signals having the frequencies in the range from 28 gigahertz to 60 gigahertz. The third aspect of the example method may be implemented in combination with the first and/or second aspect of the example method, though the example embodiments are not limited in this respect.

In a fourth aspect of the example method, forming the metal layer comprises configuring the metal layer to reflect at least 30% of the signals having the infrared frequencies. The fourth aspect of the example method may be implemented in combination with the first, second, and/or third aspect of the example method, though the example embodiments are not limited in this respect.

In a fifth aspect of the example method, forming the metal layer comprises configuring the metal layer to have a conductivity less than or equal to 10−5siemens per meter with regard to the signals having the frequencies in the range from 28 gigahertz to 60 gigahertz. The fifth aspect of the example method may be implemented in combination with the first, second, third, and/or fourth aspect of the example method, though the example embodiments are not limited in this respect.

In a sixth aspect of the example method, forming the metal layer comprises configuring the metal layer to provide a transmittance of at least 80% across a range of frequencies from 28 GHz to 60 GHz. The sixth aspect of the example method may be implemented in combination with the first, second, third, fourth, and/or fifth aspect of the example method, though the example embodiments are not limited in this respect.

In a seventh aspect of the example method, forming the metal layer comprises configuring the metal layer to provide a transmittance of at least 80% across a range of frequencies from 6 GHz to 80 GHz. The seventh aspect of the example method may be implemented in combination with the first, second, third, fourth, fifth, and/or sixth aspect of the example method, though the example embodiments are not limited in this respect.

In a first implementation of the eighth aspect of the example method, configuring the metal layer comprises configuring the electrically discontinuous metal layer to have an areal coverage in a range between 35% and 55%.

In a second implementation of the eighth aspect of the example method, forming the metal layer comprises disposing the metal layer on the glass layer to provide a thickness of the metal layer that is less than a threshold thickness. In accordance with the second implementation, the thickness being less than the threshold thickness causes the metal layer to become electrically discontinuous.

In a third implementation of the eighth aspect of the example method, the example method further comprises removing portions of the metal layer in response to forming the metal layer on the glass layer. In accordance with the third implementation, removing the portions of the metal layer causes the metal layer to become electrically discontinuous.

In an example method of using a window structure having a glass layer and a metal layer formed on the glass layer, infrared signals having infrared frequencies are received at the metal layer. Microwave signals having frequencies in a range from 28 gigahertz to 60 gigahertz are received at the metal layer. The microwave signals are transmitted through the metal layer based at least in part on a configuration of the metal layer. The infrared signals are reflected from the metal layer based at least in part on the configuration of the metal layer.

In a first aspect of the example method, transmitting the microwave signals comprises transmitting the microwave signals through the metal layer based at least in part on the metal layer being an electrically discontinuous metal layer.