PATENT DOCUMENT

Publication Number: US-9983423-B2
Application Number: US-201615148833-A
Country: US
Kind Code: B2

Title: Touch sensitive display with graded index layer

Abstract:
An electronic device may include multiple display layers. The display layers may include a matching layer implemented using multiple graded index of refraction sublayers to help minimize reflections. The matching layer may include a first sublayer having a monotonically increasing index of refraction, a second (reverse matching) sublayer having a monotonically decreasing index of refraction, and a third sublayer having a monotonically increasing index of refraction. The second reverse matching sublayer may serve to induce an optical path difference that results in destructive inference at one or more specific wavelengths. The thickness of the reverse matching sublayer may be tuned to center the destructive interference at the desired wavelength(s). If desired, multiple matching layers each having their own reverse matching layers may be stacked on top of one another to provide reflectance suppression at multiple wavelengths.

Claims:
What is claimed is: 
     
       1. Display circuitry, comprising:
 a first display layer having a first index of refraction that is substantially fixed; 
 a second display layer having a second index of refraction that is substantially fixed, wherein the second index of refraction is different than the first index of refraction; and 
 a matching layer interposed between the first and second display layers, wherein the matching layer has a first sublayer having a graded index of refraction that monotonically increases from the first display layer to the second display layer and a second sublayer having a graded index of refraction that monotonically decreases from the first display layer to the second display layer, wherein the first sublayer contacts the second sublayer. 
 
     
     
       2. The display circuitry defined in  claim 1 , wherein the matching layer further includes a third sublayer having a graded index of refraction that monotonically increases from the first display layer to the second display layer. 
     
     
       3. The display circuitry defined in  claim 2 , wherein the second sublayer of the matching layer is interposed between the first and third sublayers of the matching layer. 
     
     
       4. The display circuitry defined in  claim 1 , wherein the second sublayer of the matching layer has a thickness that determines what wavelength additional reflection suppression is provided. 
     
     
       5. The display circuitry defined in  claim 1 , wherein the first sublayer of the matching layer has a first surface with an index of refraction that matches with the first index of refraction. 
     
     
       6. The display circuitry defined in  claim 5 , wherein the first sublayer of the matching layer has a second surface with another index of refraction, and wherein the second sublayer of the matching layer has a surface with an index of refraction that matches the another index of refraction. 
     
     
       7. The display circuitry defined in  claim 1 , further comprising:
 an additional matching layer interposed between the first and the second display layers. 
 
     
     
       8. The display circuitry defined in  claim 7 , wherein the additional matching layer comprises:
 a first sublayer having a graded index of refraction that monotonically increases from the first display layer to the second display layer; and 
 a second sublayer having a graded index of refraction that monotonically decreases from the first display layer to the second display layer. 
 
     
     
       9. The display circuitry defined in  claim 1 , wherein the first index of refraction is less than the second index of refraction. 
     
     
       10. The display circuitry defined in  claim 1 , wherein incoming light traverses the first display layer before traversing the second display layer. 
     
     
       11. An electronic device, comprising:
 a housing; and 
 a display in the housing, wherein the display comprises:
 a first layer having a substantially fixed refractive index; 
 a second layer having a substantially fixed refractive index; and 
 a matching layer that is interposed between the first and second layers and that includes a first sublayer having gradually increasing refractive indices and a second sublayer having gradually decreasing refractive indices, wherein the first sublayer is in direct contact with the second sublayer. 
 
 
     
     
       12. The electronic device defined in  claim 11 , wherein the matching layer of display further comprises:
 a third sublayer having gradually increasing refractive indices. 
 
     
     
       13. The electronic device defined in  claim 12 , wherein the second sublayer is interposed between the first and third sublayers. 
     
     
       14. The electronic device defined in  claim 13 , wherein first sublayer is in direct contact with the first layer, and wherein the third sublayer is in direct contact with the second layer. 
     
     
       15. The electronic device defined in  claim 11 , wherein first sublayer is in direct contact with the first layer, and wherein the second sublayer is in direct contact with the second layer. 
     
     
       16. Apparatus, comprising:
 a first layer having a first index of refraction that is substantially fixed; 
 a second layer having a second index of refraction that is substantially fixed; and 
 a graded index of refraction matching layer interposed between the first and second layers, wherein the graded index of refraction matching layer includes an embedded reverse matching sublayer, wherein the graded index of refraction matching layer includes a sublayer having an index of refraction that monotonically increases in a given orientation, and wherein the sublayer is directly on the embedded reverse matching sublayer. 
 
     
     
       17. The apparatus defined in  claim 16 , wherein the first index of refraction is different than the second index of refraction. 
     
     
       18. The apparatus defined in  claim 16 , wherein the embedded reverse matching sublayer has an index of refraction that monotonically decreases in the given orientation. 
     
     
       19. The apparatus defined in  claim 16 , further comprising:
 an additional graded index of refraction matching layer that is interposed between the first and second layers, wherein the additional graded index of refraction matching layer also includes an embedded reverse matching sublayer. 
 
     
     
       20. The apparatus defined in  claim 19 , wherein the embedded reverse matching sublayer in the graded index of refraction matching layer has a first thickness, and wherein the embedded reverse matching sublayer in the additional graded index of refraction matching layer has a second thickness that is different than the first thickness.

Description:
This application claims the benefit of provisional patent application No. 62/221,901, filed Sep. 22, 2015, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices with touch screen displays. 
     Electronic devices often include displays. For example, cellular telephones and portable computers often include displays for presenting information to a user. 
     Some conventional displays include a silicon nitride layer that is formed on a glass substrate layer. The index of refraction of silicon nitride is relatively high compared to that of the glass substrate material. For example, silicon nitride may have a refractive index of 1.9, whereas the glass substrate may have a refractive index of 1.5. As a result, there is a significant index-of-refraction mismatch between the silicon nitride layer and the substrate. If care is not taken, the index-of-refraction mismatch may give rise to increased reflection from the display. 
     It would therefore be desirable to be able to provide improved touch screen displays for electronic devices. 
     SUMMARY 
     An electronic device that includes display circuitry is provided. In accordance with an embodiment, the display circuitry may include a first display layer having a first index of refraction that is substantially fixed, a second display layer having a second index of refraction that is substantially fixed, wherein the second index of refraction is different from the first index of refraction, and a matching layer interposed between the first and second display layers. In particular, the matching layer may include a first portion having a graded index of refraction that monotonically increases from the first display layer to the second display layer and a second portion having a graded index of refraction that monotonically decreases from the first display layer to the second display layer. The second portion of the matching layer has a thickness that determines what wavelength additional reflection suppression is provided. 
     In some arrangements, the matching layer may further include a third portion having a graded index of refraction that monotonically increases from the first display layer to the second display layer. The second portion of the matching layer may be interposed between the first and third portions of the matching layer. 
     Incoming light may traverse the first display layer before traversing the second display layer. The first index of refraction may be less than the second index of refraction. The first portion of the matching layer may have a first surface with an index of refraction that matches with the first index of refraction of the first display layer. The first portion of the matching layer may have a second surface with another index of refraction. The second portion of the matching layer may have a surface with an index of refraction that matches the another index of refraction. 
     In accordance with another embodiment, an apparatus is provided that includes a first layer having a first index of refraction that is substantially fixed, a second layer having a second index of refraction that is substantially fixed, and a graded index of refraction matching layer interposed between the first and second layers, where the graded index of refraction matching layer includes an embedded reverse matching sublayer. 
     In particular, the embedded reverse matching sublayer has an index of refraction that monotonically decreases in a given orientation. The graded index of refraction matching layer may include another sublayer having an index of refraction that monotonically increases in the given orientation (i.e., one sublayer has refractive indices that increase while the other sublayer has refractive indices that decrease). 
     The apparatus may also include an additional graded index of refraction matching layer that is interposed between the first and second layers, where the additional graded index of refraction matching layer also includes an embedded reverse matching sublayer. The embedded reverse matching sublayer in the graded index of refraction matching layer may have a first thickness, and the embedded reverse matching sublayer in the additional graded index of refraction matching layer may have a second thickness that is different than the first thickness. Configured in this way, the two matching layers serve to provide enhanced reflection suppression in at least two separate wavelengths. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device such as a laptop computer with a display in accordance with an embodiment. 
         FIG. 2  is a perspective view of an illustrative electronic device such as a handheld electronic device with a display in accordance with an embodiment. 
         FIG. 3  is a perspective view of an illustrative electronic device such as a tablet computer with a display in accordance with an embodiment. 
         FIG. 4  is a perspective view of an illustrative electronic device such as a computer or other device with display structures in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of an illustrative display in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of a conventional display stack-up. 
         FIG. 7  is a cross-sectional side view of illustrative display layers including a gradual refractive index matching layer in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view of illustrative display layers including a reverse matching layer in accordance with an embodiment. 
         FIG. 9  is a diagram showing how the index of refraction may vary within a matching layer of the type shown in  FIG. 8  in accordance with an embodiment. 
         FIG. 10  is a plot of reflection versus wavelength showing how reflections can be suppressed at a selected wavelength in accordance with an embodiment. 
         FIG. 11  is a plot showing how the thickness of the reverse matching layer of  FIG. 8  can be varied to adjust the selective reflection suppression wavelength in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of illustrative display layers having a reverse matching layer in accordance with another embodiment. 
         FIG. 13  is a diagram showing how the index of refraction may vary within a matching layer of the type shown in  FIG. 12  in accordance with an embodiment. 
         FIG. 14  is a diagram showing how multiple matching layers may be stacked to provide reflection suppression at more than one wavelength in accordance with another embodiment. 
         FIG. 15  is a plot of reflection versus wavelength showing how reflections can be suppressed at multiple different wavelengths in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may include displays. The displays may be used to display images to a user. Illustrative electronic devices that may be provided with displays are shown in  FIGS. 1, 2, 3, and 4 . 
       FIG. 1  shows how electronic device  10  may have the shape of a laptop computer having upper housing  12 A and lower housing  12 B with components such as keyboard  16  and touchpad  18 . Device  10  may have hinge structures  20  that allow upper housing  12 A to rotate in directions  22  about rotational axis  24  relative to lower housing  12 B. Display  14  may be mounted in upper housing  12 A. Upper housing  12 A, which may sometimes referred to as a display housing or lid, may be placed in a closed position by rotating upper housing  12 A towards lower housing  12 B about rotational axis  24 . 
       FIG. 2  shows how electronic device  10  may be a handheld device such as a cellular telephone, music player, gaming device, navigation unit, watch, or other compact device. In this type of configuration for device  10 , housing  12  may have opposing front and rear surfaces. Display  14  may be mounted on a front face of housing  12 . Display  14  may, if desired, have openings for components such as button  26 . Openings may also be formed in display  14  to accommodate a speaker port (see, e.g., speaker port  28  of  FIG. 2 ). In compact devices such as wrist-watch devices, port  28  and/or button  26  may be omitted and device  10  may be provided with a strap or lanyard. 
       FIG. 3  shows how electronic device  10  may be a tablet computer. In electronic device  10  of  FIG. 3 , housing  12  may have opposing planar front and rear surfaces. Display  14  may be mounted on the front surface of housing  12 . As shown in  FIG. 3 , display  14  may have an opening to accommodate button  26  (as an example). 
       FIG. 4  shows how electronic device  10  may be a display such as a computer monitor, a computer that has been integrated into a computer display, or other device with a built-in display. With this type of arrangement, housing  12  for device  10  may be mounted on a support structure such as stand  30  or stand  30  may be omitted (e.g., to mount device  10  on a wall). Display  14  may be mounted on a front face of housing  12 . 
     The illustrative configurations for device  10  that are shown in  FIGS. 1, 2, 3, and 4  are merely illustrative. In general, electronic device  10  may be a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device, or other wearable or miniature device, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. 
     Housing  12  of device  10 , which is sometimes referred to as a case, may be formed of materials such as plastic, glass, ceramics, carbon-fiber composites and other fiber-based composites, metal (e.g., machined aluminum, stainless steel, or other metals), other materials, or a combination of these materials. Device  10  may be formed using a unibody construction in which most or all of housing  12  is formed from a single structural element (e.g., a piece of machined metal or a piece of molded plastic) or may be formed from multiple housing structures (e.g., outer housing structures that have been mounted to internal frame elements or other internal housing structures). 
     Display  14  may be a touch sensitive display that includes a touch sensor or may be insensitive to touch. Touch sensors for display  14  may be formed from an array of capacitive touch sensor electrodes, a resistive touch array, touch sensor structures based on acoustic touch, optical touch, or force-based touch technologies, or other suitable touch sensor components. 
     Display  14  for device  10  may include pixels formed from liquid crystal display (LCD) components. A display cover layer may cover the surface of display  14  or a display layer such as a color filter layer or other portion of a display may be used as the outermost (or nearly outermost) layer in display  14 . The outermost display layer may be formed from a transparent glass sheet, a clear plastic layer, or other transparent member. 
     A cross-sectional side view of an illustrative configuration for display  14  of device  10  (e.g., for display  14  of the devices of  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4  or other suitable electronic devices) is shown in  FIG. 5 . As shown in  FIG. 5 , display  14  may include backlight structures such as backlight unit  42  for producing backlight  44 . During operation, backlight  44  travels outwards (vertically upwards in dimension Z in the orientation of  FIG. 5 ) and passes through display pixel structures in display layers  46 . This illuminates any images that are being produced by the display pixels for viewing by a user. For example, backlight  44  may illuminate images on display layers  46  that are being viewed by viewer  48  in direction  50 . 
     Display layers  46  may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in housing  12  or display layers  46  may be mounted directly in housing  12  (e.g., by stacking display layers  46  into a recessed portion in housing  12 ). Display layers  46  may form a liquid crystal display or may be used in forming displays of other types. 
     Display layers  46  may include a liquid crystal layer such a liquid crystal layer  52 . Liquid crystal layer  52  may be sandwiched between display layers such as display layers  58  and  56 . Layers  56  and  58  may be interposed between lower polarizer layer  60  and upper polarizer layer  54 . 
     Layers  58  and  56  may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers  58  and  56  may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers  58  and  56  (e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers  58  and  56  and/or touch sensor electrodes may be formed on other substrates. 
     With one illustrative configuration, layer  58  may be a thin-film transistor layer that includes an array of pixel circuits based on thin-film transistors and associated electrodes (pixel electrodes) for applying electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Layer  56  may be a color filter layer that includes an array of color filter elements for providing display  14  with the ability to display color images. If desired, layer  58  may be a color filter layer and layer  56  may be a thin-film transistor layer. Configurations in which color filter elements are combined with thin-film transistor structures on a common substrate layer in the upper or lower portion of display  14  may also be used. 
     During operation of display  14  in device  10 , control circuitry (e.g., one or more integrated circuits on a printed circuit) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed to a display driver integrated circuit such as circuit  62 A or  62 B using a signal path such as a signal path formed from conductive metal traces in a rigid or flexible printed circuit such as printed circuit  64  (as an example). 
     Backlight structures  42  may include a light guide plate such as light guide plate  78 . Light guide plate  78  may be formed from a transparent material such as clear glass or plastic. During operation of backlight structures  42 , a light source such as light source  72  may generate light  74 . Light source  72  may be, for example, an array of light-emitting diodes. 
     Light  74  from light source  72  may be coupled into edge surface  76  of light guide plate  78  and may be distributed in dimensions X and Y throughout light guide plate  78  due to the principal of total internal reflection. Light guide plate  78  may include light-scattering features such as pits or bumps. The light-scattering features may be located on an upper surface and/or on an opposing lower surface of light guide plate  78 . Light source  72  may be located at the left of light guide plate  78  as shown in  FIG. 5  or may be located along the right edge of plate  78  and/or other edges of plate  78 . 
     Light  74  that scatters upwards in direction Z from light guide plate  78  may serve as backlight  44  for display  14 . Light  74  that scatters downwards may be reflected back in the upwards direction by reflector  80 . Reflector  80  may be formed from a reflective material such as a layer of plastic covered with a dielectric mirror thin-film coating. 
     To enhance backlight performance for backlight structures  42 , backlight structures  42  may include optical films  70 . Optical films  70  may include diffuser layers for helping to homogenize backlight  44  and thereby reduce hotspots, compensation films for enhancing off-axis viewing, and brightness enhancement films (also sometimes referred to as turning films) for collimating backlight  44 . Optical films  70  may overlap the other structures in backlight unit  42  such as light guide plate  78  and reflector  80 . For example, if light guide plate  78  has a rectangular footprint in the X-Y plane of  FIG. 5 , optical films  70  and reflector  80  may have a matching rectangular footprint. If desired, films such as compensation films may be incorporated into other layers of display  14  (e.g., polarizer layers). 
     The example of  FIG. 5  in which display  14  is a liquid crystal display is merely illustrative and does not serve to limit the scope of the present invention. In other suitable embodiments, display  14  may be an organic light-emitting diode display, a plasma display, an electrophoretic display, an electrowetting display, a display using other types of display technology, or a display that includes display structures formed using more than one of these display technologies. 
     Regardless of the type of display technology on which an electronic device display is based, the display typically includes multiple layers formed from different materials.  FIG. 6  is a cross-sectional side view of a conventional display stack-up. As shown in  FIG. 6 , the display includes a first outermost cover layer  100 , a second layer  102  on which first layer  100  is formed, and a third layer  104  on which second layer  102  is formed. Other display layers can be formed beneath layer  104 . 
     In the configuration of  FIG. 6 , layer  100  is a transparent glass substrate having a refractive index of 1.5; layer  102  is a silicon-based spin-on glass (SOG) layer having a refractive index of 1.51; and layer  104  is a silicon nitride layer having a refractive index of 1.9. Due to the index-of-refraction mismatch between SOG layer  102  and silicon nitride layer  104 , there is a potential for undesired reflections and visible artifacts. In the example of  FIG. 6 , incoming light ray  110  entering the display layers can potentially be reflected back towards user  48  (see, light  112  being reflected at interface  114  between layers  102  and  104 ), which can cause glare and specular reflections at the surface of the display and thereby reduces the visibility of the content that is being displayed. 
     To minimize such types of undesired reflections due to index-of-refraction mismatch, the material surrounding the high index of refraction material can be configured to have a graded index of refraction. The presence of the graded index in the vicinity of the high index of refraction material such as a silicon nitride layer helps to reduce index mismatch at the interface between the silicon nitride layer and adjoining layers that are formed from lower index of refraction materials. Such types of layers with a graded refractive index are sometimes referred to as a “matching layer.” 
       FIG. 7  shows a cross-section of a display that includes a gradual refractive index matching layer  103  that is interposed between low index of refraction layer  102  and high index of refraction layer  104 . The graded index of refraction may be produced by using a continuously or stepwise varying mixture of high and low index materials. As an example, the graded index of refraction may be produced by depositing a continuously varying (or stepwise varying) mixture of silicon oxide (SiO 2 ) and niobium oxide (Nb 2 O 5 ). The deposited mixture may have an index of refraction of close to that of glass by adjusting the mixture to include mostly silicon oxide (n=1.51). Near the interface between the deposited mixture and the silicon nitride layer  104 , more niobium oxide may be incorporated into the mixture to raise the index of refraction to match that of silicon nitride (n=1.9). Configured in this way, incoming light ray  120  entering the display layers and traversing through matching layer  103  may exhibit reduced reflections  122  relative to that observed in  FIG. 6 . 
     In accordance with an embodiment,  FIG. 8  shows a cross-sectional side view of display  14  with illustrative display layers that may include a matching layer having an embedded reverse matching layer that can help provide improved reflection suppression. As shown in  FIG. 8 , display  14  may include a first outer layer such as layer  200 , a second layer such as layer  202  that is formed under layer  200 , a third layer under such as layer  204  that is formed under layer  202 , and a matching layer such as graded refractive index matching layer  210  that is interposed between layers  202  and  204 . Layer  200  may, for example, be a transparent substrate layer, a cover glass layer, or any suitable protective layer that serves as an external cover for the face of display  14 . Layer  202  may be an underlying layer such as a silicon-based spin-on-glass (SOG) layer that has a substantially similar index of refraction as layer  200 . As an example, layer  200  may have a refractive index n 1  of 1.5, whereas layer  202  may have a refractive index n 1 ′ of 1.51. 
     Layer  204  may be a passivation layer (e.g., a silicon nitride passivation layer), a touch sensor electrode layer (e.g., an indium tin oxide layer with embedded electrodes for a touch sensor display), or other suitable types of layers having an index of refraction n 2  that is substantially different than that of layer  202 . As an example, layer  204  may have a refractive index n 2  that is equal to 1.9. In general, layers  202  and  204  may represent any two adjacent layers with substantially mismatching indices of refraction within an electronic display. Two layers may be considered to have substantially different/mismatched indices of refraction when their indices of refraction differ by at least 10%, at least 20%, at least 30%, etc. 
     As described above, matching layer  210  may be interposed between layers  202  and  204  to minimize reflections at the interface between the two layers. In particular, matching layer  210  may have a first matching sublayer  212 , a second matching sublayer  214 , and a third matching sublayer  216 . Sublayer  212  may be a graded index layer having an index of refraction of n 1 ′ at its top surface to match with that of adjoining display layer  202  and an index of refraction of n 3  at its bottom surface (in the orientation of  FIG. 8 ), where n 3  is greater than n 1 ′ (i.e., the refractive index increases with depth). 
     Sublayer  214  may be a graded index layer having an index of refraction of n 3  at its top surface and an index of refraction of n 4  at its bottom surface, where n 4  is actually less than n 3 . Sublayer  214  having refractive indices that decreases with depth is sometimes referred to herein as a “reverse” matching sublayer. 
     Sublayer  216  may be a graded index layer having an index of refraction of n 4  at its top surface and an index of refraction of n 2  at its bottom surface to match with that of adjoining display layer  204 , where n 2  is greater than n 4 . Formed in this way, matching layer  210  has a refractive index profile that helps minimize reflections at the interface between the upper surface of sublayer  212  and layer  202  and at the interface between the bottom surface of sublayer  216  and layer  204 . 
     Sublayers  212 ,  214 , and  216  may be formed from transparent dielectric material such as a mixture of silicon oxide and dielectric that has a higher index of refraction than silicon oxide. Silicon oxide has an index of refraction of 1.5. Dielectric materials that have an index of refraction higher than silicon oxide include niobium oxide, tantalum oxide, titanium oxide, other metal oxides, oxynitrides, silicon nitride, etc. In other words, an increasing amount of silicon oxide (relative to the amount of niobium oxide, for example) may be used in the mixture to gradually lower the index of refraction, whereas a decreasing amount of silicon oxide may be used in the mixture to gradually increase the index of refraction. 
       FIG. 9  is a graph showing how the index of refraction n may vary as a function of vertical position Z through the display layers of  FIG. 8 . Because cover layer  200  is often considered to be one of the layers in display  14 , outer layer  200  may also be referred to as a display layer. 
     Matching layer  210  may have a continuously graded profile as illustrated by continuously varying index of refraction profile  60  or may have a stepwise varying index of refraction profile. There may be any suitable number of discrete steps in a stepped index of refraction profile (e.g., two or more, three or more, four or more, five or more, six or more, ten or more, twenty or more, etc.). Continuous profile  60  and/or a stepped profile may by formed using deposition techniques such as sputtering. The total thickness of layer  210  may be, for example, 200 nm (e.g. 10 nm to 1 micron, etc.). 
     Display layer  202  (e.g., a glass or other transparent layer) may have an upper surface at position Z 1  and a lower surface at position Z 2 . Between Z 1  and Z 2 , layer  202  has an index of refraction of n 1 ′, as illustrated by index of refraction profile segment  62 . At point  64 , the index of refraction of matching layer  210  is exactly or approximately matched to the index of refraction n 1 ′ of layer  202 . In a continuously variable graded index configuration, for example, the index of refraction of matching layer  210  is preferably close to or equal to n 1 ′ at point  64 , as shown by line  60 . The value of n 1 ′ may be 1.5 (e.g., for glass or plastic), 1.6, 1.7 (e.g., for sapphire), less than 1.75, 1.6 to 1.8, less than 1.7, less than 1.6, 1.4 to 1.6, less than 1.8, etc. 
     Sublayer  212  may have an upper surface at position Z 2  and a lower surface at position Z 3 . Between heights Z 2  and Z 3 , the index of refraction in sublayer  212  preferably increases monotonically (i.e., the index of refraction is ever increasing from point  64  to point  66 ). At point  66 , the index of refraction may be equal to n 3 . Index n 3  may be equal to 1.7 (as an example). 
     Sublayer  214  may have an upper surface at position Z 3  and a lower surface at position Z 4 . Between heights Z 3  and Z 4 , the index of refraction in sublayer  214  preferably decreases monotonically (i.e., the index of refraction is ever decreasing from point  66  to point  68 ). At point  68 , the index of refraction may be equal to n 4 . Index n 4  may be equal to 1.6 (as an example). 
     Sublayer  216  may have an upper surface at position Z 4  and a lower surface at position Z 5 . Between heights Z 4  and Z 5 , the index of refraction in sublayer  216  preferably increases monotonically (i.e., the index of refraction is ever increasing from point  68  to point  70 ). At point  70 , the index of refraction of sublayer  216  is exactly or approximately matched to the index of refraction n 2  of adjoining layer  204 . Display layer  204  (e.g., a silicon nitride layer, indium tin oxide layer, or other display layers with a relatively elevated refractive index compared to silicon oxide) may have an upper surface at position Z 5  and a lower surface at position Z 6 . Between Z 5  and Z 6 , layer  204  has a fixed index of refraction of n 2 , as illustrated by index of refraction profile segment  72 . 
     Continuously varying profile  60  of  FIG. 9  is merely illustrative and does not serve to limit the scope of the present invention. In other suitable arrangements, matching layer  210  may have a profile such as profile  60 ′. As illustrated by profile  60 ′, the index of refraction of sublayer  212  may increase from n 1 ′ to n 2  between heights Z 2  and Z 3 ; the index of refraction of sublayer  214  may decrease from n 2  back down to n 1 ′ between heights Z 3  and Z 4 ; and the index of refraction of sublayer  216  may increase from n 1 ′ back up to n 2  between heights Z 4  and Z 5 . In general, the graded profile of matching layer  210  can vary up and down to between any desired levels of index of refraction (i.e., n 3  and n 4  can be any suitable value, where n 3  is greater than n 1 ′ and n 4 ), as long as the profile is substantially matched to n 1 ′ at point  64  and to n 2  at point  70 . 
     With the arrangement of  FIG. 9 , index of refraction discontinuities, which can lead to undesired reflections and visible artifacts, are minimized. For example, index mismatch between layer  202  and layer  210  is minimized by reducing mismatch at position Z 2  (point  64 ). The smooth gradually varying index values between points  64  and  70  avoid abrupt large index of refraction discontinuities and thereby avoid reflections. Index mismatch between layer  210  and layer  204  is minimized by reducing mismatch at position Z 5  (point  70 ). 
     Referring back to  FIG. 8 , matching layer  210  includes an additional reverse matching layer  214  in comparison to matching layer  103  of  FIG. 7 . The insertion of this reverse matching layer between sublayers  212  and  216  may be advantageous since it can be used to help induce a desired amount of optical path displacement  4  for light traveling through reverse matching sublayer  214 . As shown in the example of  FIG. 8 , incoming light  250  traversing through layer  214  may be deflected by an amount Δ (see portion  250 ′), while the corresponding reflected light  260  may again be deflected by amount Δ (see portion  260 ′) when traversing through  214  for the second time. The total effective optical path difference may therefore be equal to 2Δ, which can be tuned to result in a destructive optical interference between the incoming light and the reflected light to further reduce the reflectance at one or more wavelengths. 
       FIG. 10  is a plot of reflection versus wavelength showing how reflections can be suppressed at a selected wavelength in accordance with an embodiment. Curve  400  represents the amount of reflectance provided by a display without a reverse matching layer, whereas curve  402  represents the amount of reflectance provided by a display that includes a reverse matching layer. As shown in  FIG. 10 , curve  402  offers a noticeable amount of reflectance suppression at wavelength λ X  relative to curve  400 . The general use of a graded index matching layer can already substantially reduce the amount of reflection on a display. Implementing an additional reverse matching layer in this way can further minimize reflections at selected wavelength(s) by an additional 10% or more. 
     As shown in both  FIGS. 8 and 9 , the reverse matching layer  214  may have a thickness d. In accordance with another embodiment, thickness d of sublayer  214  may be tuned to adjust the wavelength at which the destructive interference is provided.  FIG. 11  is a plot showing how thickness d of sublayer  214  can be varied to adjust the wavelength λ X  at which the enhanced reflection suppression is centered. As shown by curve  450  of  FIG. 11 , wavelength λ X  may decrease as thickness d is increased. For example, a 5000 Å (angstrom) reverse matching sublayer may yield additional reflectance suppression at 680 nanometers (nm), whereas a 9000 Å reverse matching sublayer may yield additional reflectance suppression at 550 nm. If it is desired to center λ X  at some wavelength between 550 and 680, a corresponding point on curve  450  may be selected. This example is merely illustrative. In other arrangements, thickness d may be decreased to lower wavelength λ X . 
     The configuration of  FIG. 8  in which matching layer  210  includes three sublayers is merely illustrative.  FIG. 12  shows another suitable arrangement in which a matching layer such as matching layer  500  includes only two sublayers, one of which is serves as a reverse matching layer. As shown in  FIG. 12 , matching layer  500  may include a first matching sublayer  502  and a second matching sublayer  504 . Sublayer  502  may be a graded index layer having an index of refraction of n 1 ′ at its top surface to match with that of adjoining display layer  202  and an index of refraction of n LOW  at its bottom surface (in the orientation of  FIG. 12 ), where n LOW  is less than n 1 ′ (i.e., the refractive index decreases with depth). Sublayer  502  configured in this way may therefore serve as a reverse matching layer. Reverse matching sublayer  502  may have a thickness d that can also be tuned to adjust the wavelength at which additional reflectance suppression is provided, as discussed in connection with  FIG. 11 . 
     Sublayer  504  may be a graded index layer having an index of refraction of n LOW  at its top surface and an index of refraction of n 2  at its bottom surface to match with that of adjoining display layer  204 , where n 2  is greater than n LOW . Formed in this way, matching layer  500  has a refractive index profile that helps minimize reflections at the interface between the upper surface of sublayer  502  and layer  202  and at the interface between the bottom surface of sublayer  504  and layer  204 . 
       FIG. 13  is a graph showing how the index of refraction n may vary as a function of vertical position Z through the display layers of  FIG. 12 . Matching layer  500  may have a continuously graded profile as illustrated by continuously varying index of refraction profile  590  or may have a stepwise varying index of refraction profile. There may be any suitable number of discrete steps in a stepped index of refraction profile (e.g., two or more, three or more, four or more, five or more, six or more, ten or more, twenty or more, etc.). Continuous profile  590  and/or a stepped profile may by formed using deposition techniques such as sputtering. The total thickness of layer  500  may be, for example, 100 nm (e.g. 10 nm to 1 micron, etc.). 
     Display layer  202  (e.g., a glass or other transparent layer) may have an upper surface at position Z 1  and a lower surface at position Z 2 . Between Z 1  and Z 2 , layer  202  has an index of refraction of n 1 ′, as illustrated by index of refraction profile segment  580 . At point  592 , the index of refraction of matching layer  500  is exactly or approximately matched to the index of refraction n 1 ′ of layer  202 . In a continuously variable graded index configuration, for example, the index of refraction of matching layer  500  is preferably close to or equal to n 1 ′ at point  590 , as shown by line  590 . 
     Sublayer  502  may have an upper surface at position Z 2  and a lower surface at position Z 3 . Between heights Z 2  and Z 3 , the index of refraction in sublayer  502  preferably decreases monotonically (i.e., the index of refraction is ever decreasing from point  592  to point  594 ). At point  594 , the index of refraction may be equal to n LOW . Index n LOW  may be equal to 1.3 (as an example). 
     Sublayer  504  may have an upper surface at position Z 3  and a lower surface at position Z 4 . Between heights Z 3  and Z 4 , the index of refraction in sublayer  214  preferably increases monotonically (i.e., the index of refraction is ever increasing from point  594  to point  596 ). At point  596 , the index of refraction may be exactly or approximately matched to the index of refraction n 2  of adjoining layer  204 . Between Z 5  and Z 6 , layer  204  has a fixed index of refraction of n 2 , as illustrated by index of refraction profile segment  582 . 
     Continuously varying profile  590  of  FIG. 13  is merely illustrative and does not serve to limit the scope of the present invention. In general, the matching layer interposed between layers  202  and  204  may include any number of sublayers having one or more reverse matching layers, as long as the profile is substantially matched to n 1 ′ at point  592  and to n 2  at point  596 . In other words, some matching layers can include two or more non-adjacent reverse matching sublayers, three or more non-adjacent reverse matching sublayers, etc. With the arrangement of  FIG. 12 , index of refraction discontinuities, which can lead to undesired reflections and visible artifacts, are minimized. The smooth gradually varying index values between points  592  and  596  avoid abrupt large index of refraction discontinuities and thereby avoid reflections. 
     In accordance with another suitable arrangement, multiple matching layers may be formed between display layers  200  and  204 . Display layer  202  need not be formed.  FIG. 14  is a diagram showing how multiple matching layers may be stacked to provide reflection suppression at more than one wavelength. As shown in  FIG. 14 , a first matching layer  210 - 1  that includes its own reverse matching sublayer  214 - 1  may be formed under layer  200 , whereas a second matching layer  210 - 2  that includes its own reverse matching sublayer  214 - 2  may be formed between layers  210 - 1  and  204 . Matching layers  210 - 1  and  210 - 2  may be implemented using the three-layer configuration of  FIG. 8 , using the two-layer configuration of  FIG. 12 , or using any number of sublayers having at least one reverse matching sublayer. 
     The reverse matching sublayer  214 - 1  of matching layer  210 - 1  may have a thickness d 1 , whereas the reverse matching sublayer  214 - 2  of matching layer  210 - 2  may have a thickness d 2 . Thickness d 2  may generally be different than thickness d 1 . If desired, thickness d 1  may be set equal to thickness d 2 . Superimposed in this way, thickness d 1  of reverse matching layer  214 - 1  may be selected to provide enhance reflection reduction at a first visible wavelength while thickness d 2  of reverse matching layer  214 - 2  may be selected to provide enhanced reflection reduction at a second visible wavelength that is different than the first visible wavelength. 
       FIG. 15  is a graph in which the amount of reflection from a display has been plotted as a function of wavelength. The graph of  FIG. 15  covers visible light wavelengths ranging from 390 nm to 800 nm. Line  700  corresponds to reflection from a display that includes a matching layer of  FIG. 7 . Line  702  illustrates the result when display  14  is provided with multiple matching layers in the way shown in  FIG. 14 . As shown by line  702 , this type of arrangement can result in additional reflection reduction of ΔR 1  at wavelength λ X1  (which is set by thickness d 1 ) and of ΔR 2  at wavelength λ X2  (which is set by thickness d 2 ). In general, any number of matching layers can be stacked in this way to help reduce reflections at any desired number of wavelengths. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20160506
Publication Date: 20180529
Grant Date: 20180529
Priority Date: 20150922
Inventors: HSIEH, WEN-I
Assignee: APPLE INC
CPC Classifications: [{"code": "G02F1/133", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133502", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133502", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0011", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13338", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0011", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13338", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 58282568