Patent Publication Number: US-2022221636-A1

Title: Energy-efficient window coatings transmissible to wireless communication signals and methods of fabricating thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application 63/135,214, filed on 2021 Jan. 8 (Docket No. LFINP005P2), U.S. Provisional Patent Application 63/182,077, filed on 2021 Apr. 30 (Docket No. LFINP006P), all of which are incorporated herein by reference in their entirety for all purposes. 
    
    
     BACKGROUND 
     Windows tend to be the least energy-efficient component in buildings. For example, radiation-based heat transfer represents about 60% of the total energy loss through standard windows. Energy-efficient windows utilize special coatings to reduce this heat transfer, e.g., by blocking the IR (infrared) radiation, corresponding to wavelengths between 5 micrometers to 50 micrometers. However, energy-efficient windows also tend to block wireless communication signals with wavelengths longer than 50 micrometers or even longer than 0.5 millimeters. This signal blocking negatively impacts cellular reception, Wi-Fi access, and the like. Conventional approaches use external antennas to rebroadcast signals inside the building. However, such systems are complex, expensive, and provide minimal coverage inside the buildings. Furthermore, covering all areas inside the building with such systems can be difficult. 
     SUMMARY 
     Provided are novel energy-efficient signal-transparent window assemblies and methods of fabricating thereof. These window assemblies are specifically configured to allow selective penetration of millimeter waves, representing current and future wireless signal spectrum. This signal penetration is provided while IR blocking properties are retained. Furthermore, the windows assemblies remain substantially transparent within the visible spectrum with no specific features detectable to the naked eye. This unique performance is achieved by patterning conductive layers such that the conductive layer edges remain protected during most fabrication steps and the fabrication. As such, the conductive layers are encapsulated and being separated from the environment while retaining separation between individual disjoined structures of these layers. For example, a barrier layer and/or a dielectric layer may extend over the conductive layer edge. The patterning is achieved by forming spacers on the substrate and depositing a stack over these photoresist structures. The spacers are removed thereafter. 
     In some examples, an energy-efficient signal-transparent window assembly comprises a window substrate, a first dielectric layer disposed over the window substrate, and a conductive layer disposed over the first dielectric layer such that the first dielectric layer is disposed between the conductive layer and the window substrate. The conductive layer is formed by multiple disjoined structures separated by gaps. The energy-efficient signal-transparent window assembly also comprises a barrier layer disposed over the conductive layer such that the conductive layer is disposed between the first dielectric layer and the barrier layer. The energy-efficient signal-transparent window assembly comprises a second dielectric layer disposed over the barrier layer such that the barrier layer is disposed between the second dielectric layer and the conductive layer. Each of the above-referenced layers (e.g., a first dielectric layer, a conductive layer, a barrier layer, and a second dielectric layer) can be a multilayered structure, in which different sublayers are formed from different materials. The first dielectric layer, the conductive layer, the barrier layer, and second dielectric layer form a plurality of primary stacks and a plurality of secondary stacks over at least a portion of the window substrate. The plurality of secondary stacks forms a pattern, defined by pattern lines extending between each adjacent pair of the plurality of primary stacks such that each of the pattern lines has a width smaller than 100 micrometers. Each of the plurality of primary stacks has an enclosed shape, defined by the pattern, with a dimension smaller than 10 millimeters in any direction parallel to the window substrate. 
     In some examples, each of the plurality of secondary stacks is spaced away or partially spaced away from the window substrate by one or more of residual spacers and air. In the same or other examples, each of the plurality of secondary stacks is spaced away from the window substrate by less than 200 nanometers. For example, on average, the plurality of secondary stacks protrudes further away from the window substrate more than the plurality of primary stacks. In some examples, each of the plurality of secondary stacks directly interfaces the window substrate. 
     In some examples, the first dielectric layer, the conductive layer, the barrier layer, and the second dielectric layer are disjoined among the plurality of primary stacks and the plurality of secondary stacks. In the same or other examples, the width of each stack in the plurality of secondary stacks is between 1 micrometer and 20 micrometers. 
     In some examples, the energy-efficient signal-transparent window assembly further comprises an additional first dielectric layer disposed over the stack, an additional conductive layer disposed over the additional first dielectric layer and formed by additional multiple disjoined structures separated by additional gaps, an additional barrier layer disposed over the additional conductive layer, and an additional second dielectric layer disposed over the additional barrier layer. 
     In some examples, the energy-efficient signal-transparent window assembly further comprises a protective layer disposed over the stack, comprising a transparent material with an extinction coefficient of less than 0.1 at 550 nanometers. For example, the protective layer has a conductivity of less than 1 S/M. The protective layer can be configured to bond to an additional window substrate. 
     In some examples, the pattern lines comprise pattern line segments, more than half of which have a curvature of more than 1 m −1 . In the same or other examples, the area of the enclosed shape of each of the plurality of primary stacks, defined by the pattern, is varied throughout the pattern such that a ratio of the area of the 25% largest ones to the 25% smallest ones is between 1.01 to 4.0. For example, the enclosed shape is a polygon or, more specifically, a hexagon. 
     In some examples, a method of forming an energy-efficient signal-transparent window assembly comprises forming a pattern of spacers on a window substrate and depositing a stack on the window substrate and the spacers. The stack comprises a first dielectric layer, a conductive layer, a barrier layer, and a second dielectric layer, wherein the conductive layer comprises multiple disjoined structures defined by the pattern of the spacers. The method proceeds with removing the spacers such that portions of the stack, previously positioned over the spacers, are disposed on the window substrate while maintaining the conductive layer as the multiple disjoined structures. 
     In some examples, the conductive layer is deposited using a high-pulsed plasma source with a power-on-cycle duration of less than 1 microsecond. In the same or other examples, the spacers have a tapered shape such that a substrate-interfacing surface of each of the spacers is smaller than a top surface, opposite of the substrate-interfacing surface. Furthermore, in some examples, the substrate-interfacing surface is narrower than the top surface by between 200 nanometers and 1500 nanometers. The spacers may have a height of less than 1500 nanometers. 
     These and other examples are described further below with reference to the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic cross-sectional view of an energy-efficient signal-transparent window assembly, comprising a conductive layer formed by multiple disjoined structures, in accordance with some examples. 
         FIG. 1B  is a schematic expanded view of a portion of the energy-efficient signal-transparent window assembly in  FIG. 1A . 
         FIG. 1C  is a schematic cross-sectional view of another example of the energy-efficient signal-transparent window assembly, comprising a protective layer. 
         FIG. 1D  is a schematic cross-sectional view of an energy-efficient signal-transparent window assembly with multiple low-E stacks formed on top of each other, in accordance with some examples. 
         FIG. 1E  is a perspective view of an energy-efficient signal-transparent window assembly, illustrating a pattern, formed by gaps between the multiple disjoined structures, in accordance with some examples. 
         FIGS. 2A-2F  are schematic top views of different patterns used for an energy-efficient signal-transparent window assembly, in accordance with some examples. 
         FIG. 2G  is a schematic top view of an energy-efficient signal-transparent window assembly, showing patterned portions and non-patterned portions. 
         FIG. 3  is a process flowchart of a method for forming an energy-efficient signal-transparent window assembly, in accordance with some examples. 
         FIGS. 4A-4D  are schematic cross-sectional views of various stages of the method while forming an energy-efficient signal-transparent window assembly, in accordance with some examples. 
         FIG. 4E  is a schematic cross-sectional view of a spacer, showing an undercut, in accordance with some examples. 
         FIGS. 5A-5F  are schematic views of additional stages of the method while forming an energy-efficient signal-transparent window assembly, in accordance with some examples. 
         FIG. 6A  illustrates an example of a sputtering apparatus illustrating relative positions of the sputtering target and edges of disjoined structures. 
         FIG. 6B  is an expanded view of one disjoined structure showing the edge position relative to the material flux direction. 
         FIG. 7  illustrates test results of energy-efficient signal-transparent window assemblies. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are outlined to provide a thorough understanding of the presented concepts. The presented concepts may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail to not unnecessarily obscure the described concepts. While some concepts will be described in conjunction with the specific embodiments, it will be understood that these embodiments are not intended to be limiting. 
     INTRODUCTION 
     Energy-efficient windows are becoming more popular in commercial and residential buildings as well as other applications. An energy-efficient window may comprise one or more silver-based layers, responsible for blocking IR radiation, in addition to various dielectric layers and barrier layers. These silver-based layers may be also referred to as metal layers or conductive layers. However, energy-efficient windows or, more specifically, silver-based layers tend to interfere with the wireless signal transmission (e.g., cellular signal) due to signal attenuation. As noted above, conventional solutions involve the installation of distributed antenna systems (DAS) within buildings to promote signal propagation. However, this approach requires special equipment, additional power consumption, and additional cost. 
     It has been found that using separating a conductive layer into multiple disjoined structures helps to reduce signal attenuation. It should be noted that the wavelength of electromagnetic waves, which can pass through this patterned conductive layer, depends on the opening size between the disjoined structures. More specifically, the wavelength depends on the opening width between pairs of the adjacent disjoined structures, e.g., the largest opening width is smaller than the wavelength. For example, a continuous conductive layer may be formed over the substrate and subsequently patterned, e.g., removing small portions of this conductive layer and forming top-to-bottom/through openings (e.g., extending to the substrate). However, the patterning process and subsequent exposure of conductive layer edges (within the openings) cause various durability issues with these silver-based conductive layers as well as aesthetics issues (e.g., unsightly visible line marks). As a result, patterning methods have not been widely adopted. Furthermore, patterning becomes very challenging when dealing with the 5 th  generation (5G) networks using wavelengths greater than 1 millimeter. Such wavelengths require openings smaller than 0.1 millimeters in width to achieve adequate signal transmission. Future generation networks are expected to use even shorter wavelength requires smaller openings, which may be challenging to achieve with conventional laser scribing techniques. 
     Described herein are various examples of energy-efficient signal-transparent window assemblies and methods of fabricating thereof. These assemblies are transparent in the visible light region, allowing penetration of the electromagnetic waves at set wavelengths (e.g., carrying wireless communication signals), and are configured to block the IR radiation. For example, the transparency in the visible light region (e.g., wavelength 380-780 nanometers) may be between 10% and 100% transmission. In the same or other examples, the energy-efficient signal-transparent window assemblies allow penetration of the electromagnetic waves having a wavelength of 12.5 centimeters (corresponding to 2.4 GHz frequency) at only around 5 dB extra loss than that of an uncoated window substrate. Furthermore, the IR-blocking/emissivity is less than 0.15 in some examples. This value indicates that more than 85% of spectra between wavelength 5 micrometers to 50 micrometers are blocked by an energy-efficient signal-transparent window assembly. For comparison, conventional low-E windows (e.g., a sample from AGC Glass North America Alpharetta, Ga.) were reported around 30 DB signal loss measured from 1 GHz to 5 GHz. 
     Furthermore, the energy-efficient signal-transparent window assemblies described herein do not have unsightly visible marks and have a pleasant aesthetic appearance, unlike laser-patterned low-E windows. For example, when a window assembly is inspected at an angle of 90° to its surface with a uniform backlight simulating daylight (e.g., light intensity 10,000 lux or above), no visible marks can be observed without magnification (i.e., not observable with the “naked eye”). Furthermore, a digital photo, with a pixel density of 150,000 pixels per centimeter-square also does not show any visible marks. 
     The energy-efficient signal-transparent window assemblies described herein also have long-term durability. For example, an accelerated durability test, which involves dipping a sample into boiling water for one hour, does not reveal any visible marks with the inspection criteria presented above (e.g., the “naked eye” inspection and digital photo). Furthermore, no additional defects, which are attributable to this accelerated durability test, were detected under the microscope. Another accelerated durability was performed by baking a sample in a 650° C. oven for 8 minutes. Likewise, the microscope inspection did not reveal any additional defects. 
     Finally, the energy-efficient signal-transparent window assemblies allow wireless signal propagation of 5G signals (frequency of 6 GHz corresponding to 50-millimeter wavelength) and other like signals (e.g., future generation using higher frequencies and smaller wavelengths). In some examples, the opening width is 0.1 millimeters or even less, which much smaller than the wavelength of these communication technologies. 
     Examples of Energy-Efficient Signal-Transparent Window Assemblies 
       FIG. 1A  illustrates one example of energy-efficient signal-transparent window assembly  100 , comprising window substrate  110 , first dielectric layer  120 , conductive layer  130 , barrier layer  140 , and second dielectric layer  150 . First dielectric layer  120 , conductive layer  130 , barrier layer  140 , and second dielectric layer  150  can form stacks  170 , which may also be referred to as a low-E stack. In some examples, stacks  170  comprise primary stacks  171  and secondary stack  172 , separated from each other by stack gaps  175 . These stack gaps  175  or, more specifically, secondary stack  172  form pattern  179 , various examples of which are further described below with reference to  FIGS. 2A-2G . More specifically, secondary stacks  172  form pattern  179 , defined by pattern lines extending between each adjacent pair of primary stacks  171 . 
     For example, pattern  179  can be such that each of primary stacks  171  has an enclosed shape to ensure the electrical separation between primary stacks  171  or, more specifically, between conductive layers  130  in these primary stacks  171  to enhance the permeability of energy-efficient signal-transparent window assembly  100  to wireless communication signals. In more specific examples, the dimension in each enclosed shape is less than 10 millimeters in any direction parallel to window substrate  110 , again to enhance the permeability of energy-efficient signal-transparent window assembly  100 . At the same time, secondary stacks  172  form pattern lines of pattern  179 . These pattern lines extend between each adjacent pairs of primary stacks  171  and have a width of less than 100 micrometers to ensure that these pattern lines are not visible on energy-efficient signal-transparent window assembly  100 . In other words, when a person casually looks at energy-efficient signal-transparent window assembly  100 , energy-efficient signal-transparent window assembly  100  appears (to the “naked eye”) as a homogeneous structure without any patterns. 
     While primary stacks  171  and secondary stack  172  may have the same structure and composition of individual components, primary stacks  171  and secondary stack  172  are formed in a different manner. Specifically, primary stacks  171  are formed right away on window substrate  110 , while secondary stack  172  are formed over spacers, which are removed thereby causing secondary stack  172  to be positioned on window substrate  110 . Additional details of forming primary stacks  171  and secondary stack  172  are described below with reference to  FIG. 3 . 
     Referring to  FIG. 1A , in some examples, each primary stack  171  directly interfaces window substrate  110 . As noted above, primary stacks  171  are formed right away on window substrate  110 . Secondary stacks  172  can also directly interface window substrate  110  or be spaced away from window substrate  110 . In general, at least some of secondary stacks  172  are spaced away from window substrate  110  by less than 200 nanometers or, more specifically, less than 100 nanometers or even less than 30 nanometers. In some examples, at least some of secondary stacks  172  are spaced away from window substrate  110  by gap  190 , which may include one or more of residual spacers  194  and air. In some examples, gap  190  is free from any solid residual component and is filled with air. In other words, the gap is filled by only air. In the same or other examples, at least some of secondary stacks  172  is partially spaced away from window substrate  110 . For example, a portion (e.g., a corner) of secondary stack  172  can directly interface window substrate  110  while another portion of the same secondary stack  172  is positioned away from window substrate  110 . 
     In some examples, primary stacks  171  are wider than secondary stacks  172 . One having ordinary skills would understand how a width can be defined for irregular shapes. For example, the largest dimension of a shape can be defined as a length. Width is defined as the largest dimension that is perpendicular to the length. It should be noted that both length and width are measured within a plane parallel to the surface of window substrate  110 . As noted below, the width of secondary stacks  172  is determined by spacers, used to create a pattern. The width of these spacers can be selected to ensure signal transmission through energy-efficient signal-transparent window assembly  100 . Additional width aspects are described below with reference to  FIG. 1E . 
     Referring to  FIG. 1A , in some examples, on average, secondary stacks  172  protrude further away from window substrate  110  more than primary stacks  171 . In these examples, primary stacks  171  can directly interface window substrate  110  while secondary stacks  172  can be spaced away or at least partially spaced away from window substrate  110 . Alternatively, both primary stacks  171  and secondary stacks  172  are spaced away or at least partially spaced away from window substrate  110 . 
     First dielectric layer  120  is disposed over window substrate  110 . In some examples, first dielectric layer  120  may comprise multiple disjoined portions that belong to different stacks  170 , e.g., as schematically shown in  FIG. 1A . Conductive layer  130  is disposed over first dielectric layer  120  such that first dielectric layer  120  is positioned between conductive layer  130  and each of window substrate  110 . Conductive layer  130  is formed by multiple disjoined structures  132 , defined by gap pattern  179 . It should be noted that multiple disjoined structures  132  allow transmission of electromagnetic waves through energy-efficient signal-transparent window assembly  100  as noted above. Barrier layer  140  is disposed over conductive layer  130  such that conductive layer  130  is disposed between first dielectric layer  120  and barrier layer  140 . Similar to first dielectric layer  120  and conductive layer  130 , in some examples, barrier layer  140  comprises disjoined portions. Finally, second dielectric layer  150  is disposed over barrier layer  140  such that barrier layer  140  is positioned between second dielectric layer  150  and conductive layer  130 . Similar to other components of energy-efficient signal-transparent window assembly  100 , second dielectric layer  150  comprises disjoined portions. 
     The composition and other structural features of each component will now be described in more detail. In some examples, window substrate  110  comprises glass, plastics, or any materials that can support at least first dielectric layer  120 , conductive layer  130 , barrier layer  140 , and second dielectric layer  150 . In some examples, window substrate  110  is transparent. 
     In some examples, first dielectric layer  120  and second dielectric layer  150  are formed from the same material. Alternatively, first dielectric layer  120  and second dielectric layer  150  are formed from different materials. In general, materials suitable for first dielectric layer  120  and second dielectric layer  150  include, but are not limited to, transparent dielectric materials such as a zinc-tin oxide (Zn x Sn y O z ) and a silicon nitride (Si 3 N 4 ). In some examples, the dielectric conductivity of the material forming first dielectric layer  120  and/or second dielectric layer  150  is smaller than 1000 S/M (Siemens per meter) or, more specifically, smaller than 1 S/M. In some examples, the extinction coefficient is less than 0.1 at 550 nm. These materials may be selected for color tuning, e.g., to make the boundary of the discontinuous layer invisible. Additional color tuning may be achieved by controlling the thickness of first dielectric layer  120  and second dielectric layer  150 . For example, first dielectric layer  120  and/or second dielectric layer  150  may have a thickness of 10 nm to 100 nm. In some examples, first dielectric layer  120  and/or second dielectric layer  150  allows for vacuum break during fabrication of energy-efficient signal-transparent window assembly  100 . 
     In some examples, each of first dielectric layer  120  and second dielectric layer  150  is formed entirely from the same material (e.g., having the same composition through the entire volume of the layer). Alternatively, one or both first dielectric layer  120  and second dielectric layer  150  are multilayered structures. At least one layer in these multilayered structures is formed using a dielectric material. 
     In some examples, conductive layer  130  is configured to provide IR-blocking for energy saving while allowing penetration of signal-carrying electromagnetic waves. Some examples of materials suitable for conductive layer  130  include, but are not limited to, silver, silver alloys, copper, gold, ITO (indium tin oxide), and the like. In some examples, the sheet resistance of conductive layer  130  is smaller than 100 Ohm/square. In some examples, the thickness of conductive layer  130  is between 5 nanometers and 40 nanometers. 
     In some examples, conductive layer  130  is patterned or, more specifically, formed by multiple disjoined structures  132 . The size of these multiple disjoined structures  132  and spacing between two adjacent ones of multiple disjoined structures  132  are set by pattern  175 . The size of these disjoined structures  132  depends on the stack type as e.g., schematically shown in  FIG. 1E . For example, disjoined structures  132  in primary stacks  171  may have a width (W 1 ) between about 0.05 millimeters and 5 millimeters or, more specifically, between about 0.1 millimeters and 2 millimeters. In the same or other examples, disjoined structures  132  in secondary stacks  172  may have a width (W 2 ) between about 0.1 micrometers and 20 micrometers or, more specifically, between about 1 micrometer and 8 micrometers. These parameters define the transmissibility of energy-efficient signal-transparent window assembly  100  to signal-carrying electromagnetic waves. 
     In some examples, barrier layer  140  is used to protect conductive layer  130  from the environment and degradation (e.g., to protect silver in conductive layer  130  from oxidation). The materials suitable for barrier layer  140  include, but are not limited to metals or metal oxides, such as NiCr, NiCrO x , TiO x , NiTiNb, NiTiNbO x . In some examples, the thickness of barrier layer  140  is between about 1 nanometers and 15 nanometers or, more specifically, between 2 nanometers and 10 nanometers. It should be noted that all thicknesses values defined above refer to the planar portions of these components and away from stack gaps  175 . 
     Referring to  FIGS. 1A and 1B , in some examples, stacks  170  comprise sidewalls  160  at least one of barrier layer  140  and second dielectric layer  150 . Sidewalls  160  can be formed in-situ while depositing barrier layer  140  and second dielectric layer  150 . In some examples, each of two adjacent sidewalls  160  is formed by both barrier layer  140  and second dielectric layer  150 . Sidewalls  160  protect conductive layer  130  from the environment, e.g., when stacks gaps  175  have openings extending below the level of second dielectric layer  150 . It should be noted that in some examples, stacks gaps  175  are at least partially filled or even fully filled. In some examples, sidewalls  160  extend to window substrate  110 . 
     Specifically, sidewalls  160  are formed by specifically tuning the deposition processes of conductive layer  130 , barrier layer  140 , and second dielectric layer  150 . In some examples, the total sidewall thickness (identified as T t  in  FIG. 1B ) is between 2 nanometers and 100 nanometers or, more specifically, between 2 nanometers and 20 nanometers. In the same or other examples, the contribution of barrier layer  140  to the total sidewall thickness (identified as T 1  in  FIG. 1B ) is between about 0.1 nanometers and 2 nanometers or, more specifically, between 0.2 nanometers and 1 nanometer. Alternatively, barrier layer  140  is not a part of sidewalls  160 . In the same or other examples, the contribution of second dielectric layer  150  to the total sidewall thickness (identified as T 2  in  FIG. 1B ) is between about 2 nanometer and 100 nanometers or, more specifically, between 2 nanometers and 15 nanometers. Alternatively, second dielectric layer  150  is not a part of sidewalls  160 . It should be noted that the thickness of sidewall-forming portions of barrier layer  140  and/or second dielectric layer  150  can be different (e.g., smaller) than the corresponding thickness of barrier layer  140  and/or second dielectric layer  150  away from sidewalls  160  due to the deposition angle. 
       FIG. 1C  illustrates another example of energy-efficient signal-transparent window assembly  100 , which comprises protective layer  198 . In this example, protective layer  198  conforms to the entire surface of second dielectric layer  150 , and over both primary stacks  171  and secondary stacks  172 , thereby forming an entire surface (on one side) of energy-efficient signal-transparent window assembly  100 . Specifically, protective layer  198  extends over all stacks  171  and over stack gaps  175 . By extending over stack gaps  175 , protective layer  198  also forms adjacent sidewalls  160 . In some examples, protective layer  198  is the only component forming sidewalls  160 . Alternatively, sidewalls  160  are formed by protective layer  198  and one or both of barrier layer  140  and second dielectric layer  150 . The thickness of protective layer  198  may be from 10 nanometers to 100 micrometers (or higher). In some examples, protective layer  198  comprises a transparent material with an extinction coefficient of less than 0.1 at 550 nanometers. In the same or other examples, protective layer  198  has a conductivity of less than 1 S/M. 
     In some examples, protective layer  198  can be used for bonding to additional glass substrate  199 , e.g., as schematically shown in  FIG. 1C . Protective layer  198  can be formed, e.g., from polyvinyl butyral (PVB), thermoplastic polyurethane (TPU), and/or ethylene-vinyl acetate (cross-linked EVA), to form a laminated glass assembly. 
     In some examples, energy-efficient signal-transparent window assembly  100  is placed into an insulated glass unit (IGU) window, which features multiple panes of glass, separated by an inert gas or vacuum, widely used in buildings. For example, protective layer  198  is configured to bond to an additional window substrate. 
     Referring to  FIG. 1D , in some examples, energy-efficient signal-transparent window assembly  100  comprises multiple low-E stacks, formed on the top of each other, such as stacks  170  and additional stacks  173 , disposed over stacks  170 . Each low-E stack comprises first dielectric layer  120 , conductive layer  130 , barrier layer  140 , and second dielectric layer  150 . In some examples, one or more characteristics (e.g., composition, thickness, morphology) of at least one of first dielectric layer  120 , conductive layer  130 , barrier layer  140 , and second dielectric layer  150  can be different in different low-E stacks. Stacks  170  and additional stacks  173  can have the same dimensions or, more specifically, the footprints of stacks  170  and additional stacks  173  can coincide. Similar to stacks  170 , additional stacks  173  can also include primary stacks  171  and secondary stacks  172 . While  FIG. 1D  illustrates an example with two low-E stacks in the Z direction, one having ordinary skill in the art would understand that any number of low-E stacks can be arranged in this manner (e.g., three, four, five, or more). 
       FIG. 1E  illustrates a perspective view of energy-efficient signal-transparent window assembly  100  showing different types of stacks, e.g., primary stacks  171  and secondary stacks  172 . Secondary stacks  172  form pattern  179 . In this example, the pattern is a rectangular grid, which is also schematically shown in  FIG. 2A . In this example, two sets of parallel lines crossed each other, extending at a 90° angle relative to each other. Other angle values are also within the scope.  FIGS. 2B-2F  illustrate other examples of pattern  179 . 
     Although straight parallel pattern lines are not visible in many cases, certain conditions (e.g., sunshine or spotlight) can generate a diffraction pattern, in part caused by the light reflection at various edges and surfaces formed by pattern  179  in energy-efficient signal-transparent window assembly  100 . More specifically, straight parallel pattern lines cause the amplification of diffraction. Diffraction is typically caused by periodic structures, such as a diffraction grating, that splits and diffracts light into several beams traveling in different directions. Such diffraction should be avoided for aesthetic and other reasons in low-E glass applications. In general, pattern  179  shown in  FIGS. 2G-2F  is expected to produce less visible lines than pattern shown in  FIG. 2A . 
     The diffraction can be reduced by using different types of pattern  179  that limit the periodic structure in the pattern design, more specifically, limit the length of straight-line portions and/or limit the level of parallelism among the pattern lines. In other words, these patterns  179  are specifically selected to have fewer and/or smaller periodic structures that do not amplify the diffraction effects as much as, e.g., a set of parallel lines. For example, any 1-square centimeter surface area of patterned energy-efficient signal-transparent window assembly  100  may contain 100 or fewer periodic structures, e.g. parallel straight lines, or even 2 or fewer periodic structures, e.g. parallel straight-line segments. For purposes of this disclosure, a straight line is defined as a line with (1) a connection line between two intersection points in pattern  179  and (2) a curvature (the reciprocal of the curvature radius) of less than 1 m −1 . Also, for purposes of this disclosure, the term “parallel” is defined as having an angle between two lines of less than 0.1 degrees. 
     In some examples, pattern  179 , which is formed by secondary structures  172 , comprises enclosed shapes of primary structure  171 . These enclosed shapes can be repeated shapes (e.g., polygons) and/or random shapes. For example, pattern  179  may be formed by different kinds of shapes. In some examples, each enclosed shape of the primary stacks  171  has a dimension of less than 10 millimeters in any direction parallel to window substrate  110  or, more specifically, less than 5 millimeters or even less than 2 millimeters. As noted above, this enclosed shape dimension is used to enhance the permeability of energy-efficient signal-transparent window assembly  100  to wireless communication signals. In some examples, the enclosed shape area can be varied with a distribution from small to large area, and the area ratio of the largest 25% shapes relative to the smallest 25% shapes is between 1.01 to 4.0. 
     Referring to  FIG. 2B , in some examples, pattern  179  is formed by a pattern line, more than half of which comprise pattern line segments with a curvature of more than 1 m −1 . A pattern line segment is defined as a portion of the pattern line that extends between two intersection points in pattern  179 . In other words, pattern  179  is formed by lines that are not straight. These lines may be referred to as curved lines. These lines may be continuous, e.g., extending to edges of energy-efficient signal-transparent window assembly  100 . In the same or other examples, pattern lines comprise segments, which form various enclosed shapes, neighboring each other as, e.g., is shown in  FIGS. 2C-2E . An enclosed shape can be a polygon with straight edges or an irregular shape with one or more curved edges (with or without straight edges). The curvature of these curved edges is greater than 1 m −1 . In some examples, pattern  179  is random/non-repetitive and/or formed by irregular shapes, which helps to minimize optical artifacts that are common with repetitive patterns and/or parallel lines. The Voronoi pattern is an example of a suitable pattern for this application. 
     In some examples, stack  171  is formed by enclosed shapes, such as an equilateral hexagon. Because stack  171  is enclosed, a portion/island of the stack, defined by this shape, is physically and electrically isolated from adjacent stacks. More specifically, there is no electrical contact between the conductive layer portions in the stacks of two adjacent portions/islands, which helps with the transmission of signal-carrying electromagnetic waves. 
     In some examples, stack  171  is formed by polygons with variable-length edges (e.g., hexagons) that have length variations to achieve pattern variations as, e.g., is schematically shown in  FIG. 2C . For example, each edge length among all the pattern lines may vary by between −5% and 5% or even between −30% and 30% to the average edge length. Furthermore, this variation may be random, e.g., any variation randomly between −5% and 5% or even between −30% and 30%. This random length variation reduces the detectability of the diffraction phenomenon, which is described above. 
     Regardless of the shape of these pattern units, in some examples, the ratio between the square root of the unit area over the unit perimeter (i.e., the SqRt-of-Area-over-Perimeter ratio) is maximized. In some examples, this SqRt-of-Area-over-Perimeter ratio is greater than 0.1, or more specifically, greater than 0.2, or even greater than 0.24. The reason for maximizing this SqRt-of-Area-over-Perimeter ratio is as follows. The overall length of the pattern line (for each unit) is defined by the perimeter. Multiplying the perimeter by the line width provides the pattern line area. The proportion of this pattern line area to the enclosed unit area should be as small as possible, such as less than 10% or even less than 1% to ensure low-E properties. The smaller pattern line area translates into aesthetic improvements. Furthermore, large pattern line areas cause the emissivity of the overall low-E coating to increase. Thus, large SqRt-of-Area-over-Perimeter ratios allow for wider pattern lines. Increasing the width of the pattern lines simplifies the manufacturing process and reduces the cost. The most effective geometric shape is a hexagon with an SqRt-of-Area-over-Perimeter ratio of 0.268. However, other polygon shapes are also within the scope, e.g., a square has an SqRt-of-Area-over-Perimeter ratio of 0.25. Overall, the equilateral polygons (e.g., square, pentagon, hexagon, heptagon, octagon, and so on) are beneficial from the SqRt-of-Area-over-Perimeter ratio perspective. It should be noted that the SqRt-of-Area-over-Perimeter ratio maximization is coupled with the random edge length variation (described above) to achieve the best aesthetic performance. As such, in some examples, pattern  179  comprises hexagonal units with the edge length variation randomly between −5% and 5% of the original equilateral hexagon edge length, further with the edge length variation randomly between −30% and 30% of the original equilateral hexagon edge length. 
     In some examples, the angle between two intersecting pattern lines is controlled, e.g., a lower limit is set. Small angles may pose a challenge for deposition step coverage, which may result in defects. In some examples, the angle between two intersecting patterns is at least about 30 degrees or even at least about 60 degrees or even at least 80 degrees or even at least 100 degrees. Thus, a cross-pattern pattern or polygon with more edges (e.g., hexagon) may be used such as shown in  FIG. 2C . For example, using a hexagon as a unit pattern allows having an angle of about 120 degrees. It should be noted that angles will vary within each unit and among different units due to the random edge length variation. 
     In some examples, the distance between any pair of adjacent points (where each point is formed by two or more intersecting lines) among more than 90% enclosed units is less than 2 millimeters or, more specifically, less than 0.7 millimeters or even less than 0.2 millimeters. This distance helps to maximize the wireless signal transmission at frequencies of 5 GHz and above. As the wavelength becomes smaller, the transmission of these wavelengths requires smaller structures. 
     In some examples, the pattern line width is less than 10 micrometers or, more specifically, less than 5 micrometers, or even less than 2 micrometers. The smaller line width improves the aesthetics and the proportion of this pattern line area to the enclosed unit area for better maintaining the Low-E properties. 
     Referring to  FIG. 2G , it should be noted that energy-efficient signal-transparent window assembly  100  does not need to be covered entirely by patterned low-E coating  102  and some areas of energy-efficient signal-transparent window assembly  100  may have un-patterned low-E coating  104 . In this example, energy-efficient signal-transparent window assembly  100  is still able to transmit signal-carrying electromagnetic waves through patterned low-E coating  102 , while un-patterned low-E coating  104  may only block partial of these electromagnetic waves. 
     Processing Examples 
       FIG. 3  is a process flowchart corresponding to method  300  of forming an energy-efficient signal-transparent window assembly  100 , in accordance with some examples. 
     Method  300  may commence with forming (block  310 ) a pattern of spacers  180  on window substrate  110 . This spacer pattern defines gap pattern  179 , which is formed at a later operation when spacers  180  are removed. The pattern of spacers  180  may be formed, for example, using photolithography as, e.g., is schematically shown in  FIGS. 4A-4D . Specifically,  FIG. 4A  illustrates a processing stage during which under-layer  410  is formed as a continuous coating on window substrate  110 . Under-layer  410  is an optional component and, in some examples, under-layer  410  is not formed.  FIG. 4B  illustrates a processing stage during which photoresist layer  420  is formed over under-layer  410 . In some examples (e.g., when under-layer  410  is not formed), photoresist layer  420  is directly on window substrate  110  (not shown). Photoresist layer  420  can be also formed as a continuous coating. Photoresist layer  420  may be formed from positive or negative photoresist, which corresponds to whether the exposed portion of photoresist layer  420  is soluble or insoluble to a photoresist developer. It should be noted that under-layer  410  is an optional layer. In some examples, photoresist layer  420  is formed directly on window substrate  110 .  FIG. 4C  illustrates a processing stage during which photoresist layer  420  is exposed, e.g., using photolithographic mask  430 . Finally,  FIG. 4D  illustrates a processing stage after etching and cleaning photoresist layer  420  and, if present, under-layer  410 . Specifically, photoresist layer  420  is converted into spacer head  189 , while under-layer  410  is converted into spacer base  188 . The materials of photoresist layer  420  and under-layer  410  may be selected such that the etching rate of under-layer  410  is faster than that of photoresist layer  420 . As a result, spacer base  188  has a smaller width than spacer head  189 . In some examples, the width difference on each side (W d  in  FIG. 4D ), which may be also referred to as an overhang, is between 200 nanometers and 1,500 nanometers or, more specifically, between 500 nanometers and 1,000 nanometers. In the same or other examples, the height of spacer base  188  (H B  in  FIG. 4D ) is between 200 nanometers and 800 nanometers or, more specifically, between 300 nanometers and 600 nanometers. Furthermore, the total height of spacer  180  (H T  in  FIG. 4D ) is between 300 nanometers and 1,500 nanometers or, more specifically, between 400 nanometers and 1,200 nanometers. Collectively, spacer head  189  and spacer base  188  form spacers  180 . 
     While  FIGS. 4A-4D  illustrate an example in which two layers are used to form spacers  180 , one having ordinary skill in the art would understand that a single layer or more than two layers may be used. 
     In some examples, one or more components of each spacer  180  (or the entire spacer  180 ) are formed from glass fiber and/or photoresist. In the same or other components, spacers  180  include any non-conductive materials, such as plastics, glass, polymers, resins, and the like. In some examples, the electrical conductivity of one or more materials forming spacers  180  is less than 1000 S/M (Siemens per meter) or, more specifically, less than 1 S/M. While spacers  180  are removed in later operations some residual parts of spacers  180  may remain in energy-efficient signal-transparent window assembly  100 . If the remaining portion of spacers  180  is conductive, this portion may interfere with the penetration of wireless communication signals. For example, experimental results have shown that when a material with a conductivity of 3000 S/M was used for spacer  180 , the penetration of wireless communication signals drops down substantially. In some examples, spacers  180  are transparent. For example, the extinction coefficient of spacers  180  material is at least smaller than 0.3 at the visible region at 550 nm, specifically, smaller than 0.1 at 550 nm. As noted above, a portion of spacers  180  may remain even after the spacer-removal operation. Experimental results have shown that using a material with a higher extinction coefficient (e.g., &gt;0.3) resulted in more obviously noticeable (e.g., by human eyes) pattern lines than samples, in which the spacer materials had a lower extinction coefficient (e.g., &lt;0.1). 
     Various methods of forming spacers  180  (e.g, according to a set pattern) protruding on a substrate and later removing these spacers  180  (e.g., to replace the glass fiber in the previous sample) are within the scope. These methods should be differentiated from laser ablation, which disturbs the substrate surface, therefore, weakening the window substrate. Specifically, the glass substrate surface loss along the pattern line could potentially reduce the glass substrate mechanical strength, and such weakened mechanical strength could potentially make the tempered glass window prone to brake, which is not acceptable for many applications (e.g., large glass windows). 
     In some examples, the complexity and cost of large size lithography equipment can be reduced by using a plurality of smaller sized modules (e.g., half size of the maximum substrate width in the production or smaller). The lithography pattern from different modules can be overlapped, and the light intensity non-uniformity crossing the whole lithography pattern area can be more than 20% calculated using the formula: 
       (MAX value−MIN value)/(2×AVERAGE value).
 
     It should be noted that there is a tradeoff between the equipment complexity/cost and the uniformity of the light intensity in the lithography equipment. Described are novel methods of using multiple module exposures, with a tolerance of &gt;20% of non-uniformity of light intensity, which can significantly reduce the lithography equipment cost. 
     In some example, multiple exposure light sources and optical apparatus are used (e.g., for cost reduction with light dose uniformity trade-off). For example, a tube-shaped lamp can replace a point lamp. In another example, a collimated ultraviolet light-emitting diode (UV-LED) array light source can be used with or without a photo-mask. The spectrum of the light can be in a wavelength 365-nanometer narrow band. 
     Integrating multiple modules in a lithography process may involve portions of energy-efficient signal-transparent window assembly  100  in which portions of low-E coating stack are not patterned.  FIG. 2G , described above, illustrates patterned portions  102  positioned within non-patterned portions  104 . The size and relative areas of these portions are selected based on the signal transmission requirements. In some examples, the average width of non-patterned portions  104  is smaller than 50 centimeters or, more specifically, smaller than 10 centimeters. In some examples, patterned portions  102  represent 10% or 50% of the substrate total area with only 10 DB or 3 DB additional losses on the signal transmission. The lithography pattern area larger than 90% substrate area only introduces less than 1 DB additional loss on the cellphone transmission in comparing that of the whole substrate patterned. In this example with some areas without patterns, the positive photoresist is used for the lithography. More specifically, areas without lithography patterned are cleaned out after the PR post-development process, to leave clean glass surfaces for the rest of the glass coating processes. 
     Furthermore, in some examples, spacers  180  have a tapered structure, defined by an undercut. One such example is schematically shown in  FIG. 4E . As described above, the undercut helps with forming the separation between multiple disjoined structures  132  as further described below. 
     Specifically, each of spacers  180  has substrate-interfacing surface  181  and top surface  182 , opposite of substrate-interfacing surface  181 . The width of dielectric-interfacing surface  182  is larger than the width of top surface  181 . In some examples, the difference between the width of top surface  182  and the width of substrate-interfacing surface is larger than 100 nm. 
     Overall, a pattern of spacers  180  may be formed using lithography, materials extrusion, nozzle jetting, or indirect deformation mechanically, such as using a mold, stamp, or by laser, UV source, or electron beam curing or other heating source hardening, or combined those techniques. 
     Returning to  FIG. 3 , method  300  proceeds with depositing (block  320 ) stack  170  over window substrate  110  and spacers  180 . As described above, stack  170  comprises first dielectric layer  120 , conductive layer  130 , barrier layer  140 , and second dielectric layer  150 . Each layer is formed in a separate operation using, e.g., physical vapor deposition (PVD).  FIGS. 5A-5D  illustrate different stages during this stack forming operation. As shown in  FIG. 5B , conductive layer  130  comprises multiple disjoined structures  132  defined by the pattern spacers  180 . Disjoined structures  132  are formed due to spacers  180  protruding over the substrate. In some examples, conductive layer  130  is deposited using a high-pulsed plasma source with a power-on-cycle duration of less than 1 microsecond. 
     For the deposition of barrier layer  140  and second dielectric layer  150 , various ways of controlling the deposition extension (of each layer forming a stack in) the undercut area of the photoresist (PR) is within the scope. For example, increasing the pressure of barrier layer  140  and second dielectric layer  150  in a sputtering deposition chamber makes deposition more isotropic or, in other words, less directional. Thus, there is more encroachment of sputtered materials in the PR undercut region. For example, when the pressure is lower during deposition of conductive layer  130  than that during the deposition of barrier layer  140  and also than that during the deposition of second dielectric layer  150 , the edge sidewall of conductive layer  130  is covered by barrier layer  140  and second dielectric layer  150 . The higher the pressure difference, the thicker sidewall protection is provided by each barrier layer  140  and second dielectric layer  150 . For example, a low-pressure processing condition (such as 0.5 ˜ 2 milli-Torr) is for deposition of conductive layer  130 . At a such low-pressure level, a very limited amount of material will reach the undercut region. On the other hand, a high-pressure condition (such as 2-300 milli-Torr) is used for deposition of barrier layer  140  and second dielectric layer  150 , providing more material into the undercut region. In some examples, additional and/or alternative techniques are used to enhance the directional deposition of conductive layer  130 , such as including (1) ionized sputtering technique with the high ionization rate plasma from a special sputter source to enhance the sputtering directional feature, (2) the second bias source under the glass to enhance the directional sputter deposition, (3) collimators for sputtering, and/or (4) evaporation method can enhance the direction deposition. 
     In some examples, stack  170  is positioned away from the edge of window substrate  110  at the side from the moving direction in the production line as shown in  FIGS. 6A and 6B , e.g., spaced by at least 1 centimeter, such as 10 centimeters. Thus, the discontinued conductive layer at the pattern line away from the side edge direction can be protected during the isotropic deposition of barrier layer  140  and second dielectric layer  150  (e.g., from the edge of the sputtering targets  610 ). 
     For example, stacks  170  partially (e.g., 5%-95% based on the total substrate area) cover window substrate  110  as, e.g., is shown with patterned area  102  in  FIG. 2G . Stacks  170  are located away from areas of window substrate  110  that are particularly challenging for processing, e.g., around the edges of window substrate  110 . Furthermore, forming stacks  170  on window substrate  110  or, more specifically, forming stack gaps  175  can reduce the mechanical durability of the window-substrate portions positioned under and around stacks  170 . Leaving portions of window substrate  110  free from stacks  170  and/or stack gaps  175  helps to maintain the mechanical durability of these portions, e.g., for robot picking or loading energy-efficient signal-transparent window assembly  100  during transportation. In some examples, portions of window substrate  110  free from stacks  170  and/or stack gaps  175 , which may be referred to as unpatterned areas  104 , extend at least 1 centimeter from the edge of window substrate  110  or, more specifically, at least 2 centimeters, e.g., as schematically shown in  FIGS. 6A and 6B . In some examples, some unpatterned area  104  on window substrate  110  is located away from the edges (e.g., in the center region of window substrate  110 ), which can be used for robot loading area. 
     In some examples, the sputtering direction during the conductive layer deposition is controlled (e.g. improved) through a high-density plasma. For example, a high frequency (&gt;10 k Hz) power source may be used to generate plasma or, more specifically, a 13.56-Mhz power source. It should be noted that such high-frequency power sources have not been adopted for conducting metal layer deposition on glass substrates in low-E production lines. The main reason is that a DC (direct current) power source and a low-frequency AC (&lt;10 kHz) power source provide high production efficiency and low manufacturing cost. However, a high-frequency power source can generate highly ionized (ionization rate &gt;50%) plasma. In some examples, the ionization rate is at or greater than 90%. In such a case, the plasma potential could be 10 Volts or more, and a self-bias phenomenon occurs near the glass surface. This phenomenon guides the ion deposition direction at a substantially normal angle, referring to the incidence angle deposition relative to the glass surface. 
     In some examples, sputtering target  610  is wider than the area in which stacks  170  are formed on window substrate  110  as, e.g., is schematically shown in  FIGS. 6A and 6B . This feature may be referred to as a target offset and ensures that the outermost edges of the outermost stacks  170  receive materials from sputtering target  610  thereby assuring that sidewalls  160  are formed. 
     In some examples, the sputtering direction is controlled (e.g., improved) using a high-density plasma, generated by a high-power impulse magnetron sputtering (HIPIMS) method. This method can generate plasma with an ionization rate of more than 50% and even fully ionized plasma. Specifically, this method may be used for the directional deposition of conducting layer  130 . Other layers of the same assembly may be deposited using other deposition methods (other than highly ionized plasma). These other methods are less directional and cover the sidewalls of conducting layer  130 . The pulse duration length of the HIPIMS power source could be smaller than 1000 microseconds, more specifically 100 or 10 microseconds or 1 microsecond. The duty cycle factor is less than 10%, more specifically 1% or less. The duty cycle factor is defined as the ratio of time that the circuit power is ON compared to the time the circuit power is OFF. 
     The conducting layer sidewall protection can also be achieved by a CVD or ALD or PECVD deposition of barrier layer  140 , such as Ni, Ti, Al, Y, Cr, or other protection metal materials or their alloys with one or more of those elements. CVD stands for chemical vapor deposition, PECVD stands for plasma-enhanced chemical vapor deposition, and ALD stands for atomic layer deposition. By CVD, PECVD, or ALD method, the sidewall thickness of barrier layer  140  can be easily achieved at &gt;1 nm, or at least &gt;0.3 nm. 
     The sidewall protection of conductive layer  130  (with barrier layer  140  and second dielectric layer  150 ) has demonstrated excellent environmental and thermal durability. The environmental durability was tested by dipping a sample for one hour into a boiling water container. The thermal durability was tested using 650° C. baking for 8 minutes for 3 mm glass coating, or 650° C. baking for 7 minutes for 0.5 mm glass coating. There were no noticeable defects under the microscope inspection. These sidewall conductive layer protection designs and methods apply to any number of layers in a stack. 
     In some examples, depositing stack  170  also comprises forming two adjacent sidewalls  160  around each of spacers  180  as well as on edges of stack  170  positioned over each spacer  180 . Various ways of controlling the deposition extension (of each layer forming a stack in) the undercut area of the photoresist (PR) are within the scope. For example, increasing the pressure in a sputtering deposition chamber makes deposition more isotropic or, in other words, less directional. Thus, there is more encroachment of sputtered materials in the PR undercut region. For example, when the pressure is lower during deposition of conductive layer  130  than that during the deposition of barrier layer  140  and also than that during the deposition of second dielectric layer  150 , the edge sidewall of conductive layer  130  is covered by barrier layer  140  and second dielectric layer  150 . The higher the pressure difference, the thicker sidewall protection is provided by each barrier layer  140  and second dielectric layer  150 . For example, a low-pressure processing condition (such as 0.5 ˜ 2 milliTorr) is for deposition of conductive layer  130 . At a such low-pressure level, a very limited amount of material will reach the undercut region. On the other hand, a high-pressure condition (such as 3-300 milliTorr) is used for deposition of barrier layer  140  and second dielectric layer  150 , providing more material into the undercut region. In some examples, additional and/or alternative techniques are used to enhance the directional deposition of conductive layer  130 . One example is an ionized sputtering technique with a high ionization rate plasma from a special sputter source to enhance the sputtering directional feature. Another example is using a second bias source under the glass to enhance the directional sputter deposition. Additional examples include collimators for sputtering, and/or evaporation method can enhance the direction deposition. 
     The sidewall protection of conductive layer  130  (with a barrier layer and a second dielectric layer) has demonstrated excellent environmental and thermal durability. The environmental durability was tested by dipping a sample for one hour into a boiling water container. The thermal durability was tested using 650° C. baking for 8 minutes. There were no noticeable defects under the microscope inspection. 
     These sidewall conductive layer protection designs and methods apply to any stacks and any number of layers in each stack. Both highly non-directional processes (e.g., high-pressure processes) and directional processes (e.g., low-pressure processes) are within the scope. 
     One issue of the undercut profile of single-layer photoresist is influenced by the non-uniformity of the light intensity with lithography equipment. There is a bi-layer method that minimizes this influence, where the bottom layer material having a dissolution rate in the developer much different from that of the photoresist above the bottom materials, so the dissolution rate is more dependent on time and other processing parameters instead of exposing light intensity. Thus, the undercut amount is more dependent on the photoresist materials and less dependent on the light intensity. As such, a large non-uniformity of exposing light intensity has a very small influence on the undercut amount, so that it can be acceptable in this application. 
     In some examples, method  300  comprises depositing (block  330 ) one or more additional stacks over stack  170 , which is disposed over window substrate  110  and spacers  180 . 
     In some examples, method  300  comprises removing (block  340 ) spacers  180  from energy-efficient signal-transparent window assembly  100 . For example, energy-efficient signal-transparent window assembly  100  may be tempered (e.g., subjected to high temperatures) turning spacers  180  into volatile species, which are removed from the environment with oxygen, such as air. The spacer materials can also be removed or partially removed by using a plasma process using a gas containing oxygen and/or nitrogen gas. Non-conductive spacers  180  can comprise materials that could be burned out completely in an oxygen-containing environment (e.g., air) without any residues. In some examples, spacers  180  are consist of one or more of the following five elements: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S). The absence of other elements from the spacer composition ensures the residual-free removal of spacers  180 . 
     It should be noted that stacks  170  positioned over spacers  180 , which may be referred to as secondary stacks  172 , are effectively lowered onto window substrate  110  as, e.g., is schematically shown in  FIG. 5E . 
     In some examples, method  300  comprises forming (block  350 ) protective layer  198  over stacks  170  as, e.g., is schematically shown in  FIG. 5F . 
     Experimental Results 
     Various tests have been conducted to evaluate the performance of energy-efficient signal-transparent window assemblies prepared in accordance with various examples listed above. Uncoated glass samples and conventional low-E coating samples were used as references. The results are presented in  FIG. 7 . 
     Specifically, all samples used a glass substrate that was 0.5 millimeters thick. Furthermore, conventional low-E coating samples and energy-efficient signal-transparent window assemblies with patterned conductive layers utilized the same low-E stack, i.e., a first dielectric layer formed from ZnSnO, a conductive layer formed from a silver, a barrier layer formed from NiCr alloy, and a second dielectric layer formed from ZnSnO. The overall coating size was 55 millimeters×55 millimeters. The pitch size of the pattern was 0.5 millimeters, the pattern line width is 5 micrometers. 
     IR blocking characteristics of various test samples have been demonstrated using an IR lamp and a light-mill radiometer. Specifically, when IR radiation is present, the vanes of the radiometer spin. Without IR radiation, the vanes do not spin. Sample 1 described above showed exceptional IR blocking characteristics. When Sample 1 was inserted between the IR lamp and the radiometer, the vanes of the radiometer stopped spinning completely, which demonstrates effective IR blocking. The emissivity was measured as 0.06 in this case. 
     Referring to  FIG. 7 , the electromagnetic wave penetration of wireless signal (e.g., cellular phone signal) was tested using the following method. A heavy-duty 16 GA welded steel box leaves a window open with a size of 50 millimeters by 50 millimeters. The box simulates a building, which electromagnetic waves cannot penetrate. Only the window in the box allows electromagnetic wave penetration. The signal source, used for this experiment, was a router with 5 GHz (wavelengths of 60 millimeters). A cell-phone, APPLE® IPHONE® 7, was used as a signal receiver. The phone was equipped with a software application “AirPort Utility” to measure the Wi-Fi signal intensity, recorded the Wi-Fi signal data every 5 seconds, as received by the phone. The phone was placed inside the box facing the window. A reference test with a blank substrate glass on the window of the box has shown the signal strength of around −45 DBm, which is a reference baseline for the Wi-Fi signal in the box. A low-E coated glass was tested on the window as another reference, and the signal strength in the box dropped to around −75 DBm to −72 DBm. As such, two different glass samples have shown about nearly 30 DBm difference or around 1000 time signal intensity reduction due to the low-E coating on the glass. Then, two invention prototype samples with the pattern on low-E coated glass was tested on the window, and the signal strength in the box only slightly drop, as around −50 DBm, or the difference from the pure glass only around 5 DBm. These results clearly demonstrated that the prototype sample of patterned low-E coating glass, significantly improve the Wi-Fi signal transmission. 
     CONCLUSION 
     Although the foregoing concepts have been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems, and apparatus. Accordingly, the present examples are to be considered illustrative and not restrictive.