Patent Publication Number: US-2017371087-A1

Title: Displays with Ramped Light Guide Layers and Multidirectional Light-Emitting Diodes

Description:
The application claims the benefit of provisional patent application No. 62/353,510, filed Jun. 22, 2016, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices, and more particularly, to electronic devices with displays. 
     Electronic devices often include displays. For example, cellular telephones and portable computers often include displays for presenting information to a user. 
     Displays such as liquid crystal displays may include backlight units. A backlight unit may include a light source and a light guide plate for distributing light from the light source across the display. 
     It can be challenging to form a satisfactory backlight. If care is not taken, a backlight unit may exhibit undesirable hotspots or may not allow backlight illumination intensity to be locally adjusted. 
     SUMMARY 
     A display such as a liquid crystal display may have an array of pixels that is illuminated using backlight illumination from a backlight. The backlight may have a light guide plate that distributes light from light-emitting diodes. 
     The light-emitting diodes in the backlight may be overlapped by the light guide plate and may emit light laterally into portions of the light guide plate that have ramped profiles. Each light-emitting diode may supply light to an elongated light distribution region. The light distribution regions may extend parallel to horizontal or vertical edges of the display. 
     Light-emitting diodes for the backlight may have multiple light-emitting diode dies mounted on common package substrates. Reflective walls may be formed between the light-emitting diode dies on a substrate. Phosphor may cover the dies. The light-emitting diodes may each contain two light-emitting diode dies or four light-emitting diode dies that emit light respectively in two or four different directions. Two or more of the dies may emit light of different colors. Light may be emitted into the corners of rectangular light distribution regions of a light guide plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view of an illustrative backlit display in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of an illustrative light guide layer having ramped portions in accordance with an embodiment. 
         FIG. 3  is a top view of an illustrative display showing how individually adjustable light-emitting diodes may supply illumination to different regions of a light guide layer with ramped portions in accordance with an embodiment. 
         FIGS. 4 and 5  are cross-sectional side views of illustrative light guide layers with ramped portions in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of an illustrative backlight unit having a light guide layer that is illuminated by multidirectional light-emitting diodes in accordance with an embodiment. 
         FIG. 7  is a diagram showing how four light-emitting diode dies may be packaged in a common package to create a multidirectional light-emitting diode in accordance with an embodiment. 
         FIG. 8  is a diagram showing how two light-emitting diode dies may be packaged in a common package to create a multidirectional light-emitting diode in accordance with an embodiment. 
         FIG. 9  is a top view of an illustrative light guide layer that is being provided with illumination from packaged light-emitting diodes that each include four light-emitting diode dies in accordance with an embodiment. 
         FIG. 10  is a top view of an illustrative light guide layer that is being provided with illumination from packaged light-emitting diodes that each include two light-emitting diode dies in accordance with an embodiment. 
         FIG. 11  is a top view of an illustrative light distribution region in a light guide layer showing how the density of light extraction features on the light guide layer may be varied to as a function of position within the light distribution region to ensure that uniform backlight illumination is produced in accordance with an embodiment. 
         FIGS. 12 and 13  are diagrams showing how packaged multidirectional light-emitting diodes may be fabricated in accordance with an embodiment. 
         FIGS. 14 and 15  are perspective views of illustrative packaged multidirectional light-emitting diodes in accordance with an embodiment. 
         FIG. 16  is a cross-sectional side view of an illustrative light guide layer with ramped portions that is being provided with light from a multidirectional light-emitting diode in accordance with an embodiment. 
         FIG. 17  is a cross-sectional side view of an illustrative flip-chip light-emitting diode in accordance with an embodiment. 
         FIG. 18  is a cross-sectional side view of an illustrative light-emitting diode with side microstructures in accordance with an embodiment. 
         FIG. 19  is a cross-sectional side view of an illustrative light-emitting diode with a top metal layer in accordance with an embodiment. 
         FIGS. 20 and 21  are cross-sectional side views of illustrative light-emitting diodes with slanted edge surfaces in accordance with an embodiment. 
         FIG. 22  is a cross-sectional side view of an illustrative light-emitting diode with a patterned distributed Bragg reflector layer in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as cellular telephones, computers, wristwatches, media players, televisions, and other electronic devices may include displays. The displays may be used to display images for a user and may be backlit. 
     A cross-sectional side view of an illustrative backlit display for an electronic device is shown in  FIG. 1 . Display  14  of  FIG. 1  may produce images for viewing in direction  30  by a viewer such as user  28 . 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  may include backlight structures such as backlight unit  18  for producing backlight illumination  26 . Pixels P may be formed in an array in a display module such as display module  16 . Display module  16 , which may sometimes be referred to as a display layer or a display, may be an electrophoretic display, a liquid crystal display, or other display that has an array of individually controlled light modulating pixels. With one illustrative configuration display module  16  may be a liquid crystal display module having upper and lower polarizers, layers such as a color filter layer and a thin-film transistor layer between the upper and lower polarizers, and a layer of liquid crystal material between the color filter layer and thin-film transistor layer. Pixel electrodes on the thin-film transistor layer may be used to apply electric fields to portions of the liquid crystal layer associated with pixels P and thereby control light transmission (i.e., transmission of backlight illumination  26 ) through layer  16 . In general, display module  16  may be formed from any suitable backlit display panel with an array of pixels for presenting images to user  28 . The use of a liquid crystal display arrangement for forming display  14  is merely illustrative. 
     Backlight unit  18  (sometimes referred to as a backlight) may include a light guide layer such as light guide layer  22 . Light guide layer  22 , which may sometimes be referred to as a light guide plate, may be formed from a transparent material such as molded plastic (e.g., polymethylmethacrylate or other suitable polymer). Light guide plate  22  may have a planar shape (e.g., a shape that lies in the X-Y plane of  FIG. 1 ). Light from one or more light-emitting diodes or other suitable light sources may be emitted into light guide plate  22  and may be distributed laterally (e.g., in dimensions X and Y in the example of  FIG. 1 ) in accordance with the principal of total internal reflection. 
     The upper and/or lower surfaces of light guide plate  22  may include light scattering features such as bumps, ridges, or other protrusions, pits, grooves, or other recesses, printed ink light scattering features, embedded light scattering structures such as bubbles or light-scattering particles, or other structures that help scatter light out of layer  22 . Light that is scattered out of layer  22  and that travels upwards in direction Z may serve as backlight illumination  26 . Reflector  24  may be located under light guide plate  22  and may be used to reflect light that has scattered downward out of layer  22  in direction −Z back in the upward direction (+Z) to serve as backlight illumination  26 . Optical films  20  in backlight unit  18  may be interposed between light guide layer  22  and display module  16 . Films  20  may include one or more layers such as a diffuser layer to homogenize backlight illumination  26 , prism films for collimating backlight illumination  26 , and compensation films for improving off-axis viewing performance. If desired, these films may be incorporated into other portions of display  14 . For example, a compensation film may be incorporated into a polarizer layer in display module  16 , etc. 
       FIG. 2  is a cross-sectional side view of an illustrative light guide plate of the type that may be used in backlight  18 . As shown in  FIG. 2 , light-emitting didoes  40  may be mounted under light guide plate  22 . For example, light guide plate  22  may have a rectangular shape that extends under a rectangular array of pixels P and light-emitting diodes  40  (or nearly all of light-emitting diodes  40 ) may be overlapped by light guide plate  22 . 
     Light guide plate  22  may have ramped regions (e.g., regions in which the upper and lower surfaces of light guide plate  32  are not parallel to each other so that they exhibit tapered profile) such as ramped regions (ramps)  36 . Light guide plate  22  may have thicker regions such as regions  32  that receive light  42  via edge surfaces  38  and may have thinner regions  34  that help laterally distribute the received light over display  14 . Ramped regions  36  may have a thickness that tapers from the larger thickness T 1  of regions  32  to the smaller thickness T 2  of regions  34 . With one illustrative configuration, surfaces  38  of light guide plate may be formed on thicker portions of light guide plate  22  such as thicker regions  32 . Ramped regions  36  may have thickness T 1  where ramped regions  36  join regions  32  and may have thickness T 2  where ramped regions  36  join regions  34 . Ramped regions  36  may have planar surfaces, concave surfaces, convex surfaces, or surfaces of other suitable shapes (e.g., regions  36  may be characterized by other straight and/or curved profiles). The illustrative configuration of  FIG. 2  in which regions  36  have tapered profiles with straight edges is merely illustrative. 
     Thicknesses T 1  and T 2  may have any suitable values. As an example, T 1  may be 2 mm, more than 1 mm, less than 3 mm, or other suitable value, thickness T 2  may be 0.7 mm, more than 0.2 mm, less than 1.2 mm, or other suitable value, and the lateral dimensions of regions  32 ,  36 , and  34  may be about 5 mm, more than 1 mm, 4-20 mm, less than 2 cm, less than 3 cm, or other suitable sizes. 
     If desired, ramped regions  36  of light guide plate  22  may include ramps that taper in opposing directions. For example, some of ramps  36  may taper towards the right and other ramps (e.g., ramps  36 ′ in light guide plate portion  22 ′ of  FIG. 2 , which are receiving light from light-emitting diodes  40 ′) may taper to the left. 
     The areas of light guide plate  22  that receive light  42  from light-emitting diodes  40  may be elongated, as shown by illustrative light distribution regions  35  of  FIG. 3 . Each light-emitting diode  40  may be individually controlled to implement a local dimming scheme for display  14  (e.g., to increase dynamic range) and/or to conserve power by dynamically depowering unneeded portions of backlight  18 . 
     There may be any suitable number of light-emitting diodes  40  and associated light distribution regions  35  in light guide plate  22  (e.g., 2-100 rows of regions  35 , more than 2 rows of regions  35 , fewer than  40  rows of regions  35 , fewer than  1000  rows of regions  35 ,  2 - 100  columns of regions  35 , more than 2 columns of regions  35 , fewer than 40 columns of regions  35 , fewer than 1000 columns of regions  35 , etc.). In the illustrative example of  FIG. 3 , all light-emitting diodes  40  are emitting light  42  in the same direction (the positive X direction). If desired, tapered regions  36  may be oriented in both the X and −X directions and light  42  may be emitted into light guide plate  22  in both the X and −X directions. Regions  35  may be elongated along the X dimension or may be elongated along the Y dimension. 
       FIGS. 4 and 5  are cross-sectional side views of illustrative light guide plates showing how the locations of light-emitting diodes  40  and the shapes of the portions of light guide plate  22  that receive light  42  may have various different configurations. In the example of  FIG. 4 , light  42  from diodes  40 A is emitted into vertical edge surfaces  38 A that extend partway into light guide plate  22  from the lower surface of light guide plate  22  and light  42  from diode  40 B is emitted into vertical portion  38 B of a notch in the upper surface of light guide plate  22  at the end of light guide plate  22 . In the example of  FIG. 5 , light  42  from diodes  40 A is emitted into vertical edge surfaces  38 A that extend partway into light guide plate  22  from the lower surface of light guide plate  22  and light  42  from diode  40 B is emitted into edge surface  38 B at the end of light guide plate  22 . Other arrangements and/or combinations of these arrangements may also be used in forming edge surfaces for receiving light  42  from a light guide plate with ramped portions. The arrangements of  FIGS. 2, 4, and 5  are illustrative. 
     If desired, light-emitting diodes  40  may emit light in multiple directions. For example, each light-emitting diode  40  may contain multiple crystalline semiconductor light-emitting diode dies mounted in a common package (e.g., multiple dies that are soldered to a common printed circuit board package substrate). Each of the light-emitting diode dies in the package of the light-emitting diode may be oriented in a different direction. This allows light  42  to be emitted into light guide plate  22  at the corners of rectangular light distribution regions (e.g., rectangular regions that extend in an array across plate  22 ). The light distribution regions may be formed from tiled light guide plate members (separate plates arranged in a tiled pattern) or may be formed from a single light guide plate that has an array of openings, ramped portions, or other structures for receiving light from light-emitting diodes  40 . 
     As shown in  FIG. 6 , for example, light guide plate  22  may have a plurality of light-distribution regions  22 T that receive light  42  from multidirectional light-emitting diodes  40  that are located in an array of respective openings  21  in light guide plate  22 . Regions  22 T may be formed from separate tiled light guide members (plates) or may be portions of an integral light guide plate. 
       FIGS. 7 and 8  are top views of illustrative multidirectional light-emitting diodes  40 . In the illustrative configuration of  FIG. 7 , light-emitting diode  40  has four light-emitting diode dies  40 D, each of which emits light  42  in a different one of four directions D. The direction in which light  42  is emitted by each light-emitting diode die  40 D may be separated from the direction in which light  42  is emitted by the next light-emitting diode die  40 D by 90° , so that light-emitting diode  40  emits light over 360° . Dies  40 D may be mounted to a common printed circuit board substrate  40 C. 
     Reflective walls  40 W may be used to help reflect light  42  outwardly into light guide plate  22  from each die  40 D, as illustrated by reflected light rays. Wall structures such as walls  40 W may sometimes be referred to as reflectors or reflective wall structure and may be formed from white polymer (e.g., a resin with titanium dioxide particles or other reflective material) or other suitable reflective structures. A photoluminescent material (e.g., a phosphor, quantum dots, etc.) may be formed over light-emitting diode dies  40 D and substrate  40 C to adjust the color and/or intensity of light  42  (e.g., to produce white light from a colored light-emitting diode, to adjust the white point of light  42 , etc.). As shown in  FIG. 8 , packaged light-emitting diode  40  may, if desired, have a wall such as wall  40 W that runs straight across substrate  40 C between a pair of light-emitting diode dies  40 D. Dies  40 D in  FIG. 8  may emit light  42  in opposite directions D. 
       FIG. 9  shows how light guide plate  22  may have multiple tiled square (rectangular) light distribution regions  22 T (formed from separate planar members or formed as an integral light guide plate). Packaged multidirectional light-emitting diodes  40  such as four-direction light-emitting diode  40  of  FIG. 7  may be located at square-shaped openings at the corners of regions  22 T and may supply light  42  to light guide plate  22 . 
     The dies  40 D in light-emitting diodes  40  may all emit the same color of light or may have different colors. The different colors may include, for example, different colors of white (e.g., CIE Standard Illuminant D65, D50, etc.), and/or colors such as red, blue, green, yellow, etc. In configurations in which light-emitting diodes  40  have dies  40 D of different colors, the color of background illumination produced by each region  22 T may be adjusted by varying the relative contribution of light from each die  40 D. As shown in  FIG. 9 , for example, the color of backlight produced by region  22 T′ may be adjusted by adjusting the relative intensity of light from the four dies  40 D associated with the diodes  40  at the four corners of region  22 T′. In the  FIG. 9  example, these dies  40 D produce light of four different respective colors C 1 , C 2 , C 3 , and C 4 , but, if desired, two or three of the dies may produce light of the same color or dies  40 D may all be of the same color. 
       FIG. 10  shows how regions  22 T of light guide plate  22  may be supplied with light  42  from packaged multidirectional light-emitting diodes  40  such as two-direction light-emitting diode  40  of  FIG. 8 . Each of the dies  40 D in diode  40  of  FIG. 8  may produce light  42  of the same color or each diode  40  may have first and second dies  40 D that produce first and second respective different colors. 
     To help create uniform backlight illumination  26 , the light-scattering structures in light guide plate  22  may be denser at locations where the light from light-emitting diodes  40  has decreased in intensity due to propagation through layer  22  and associated scattering of light out of layer  22  to serve as backlight illumination  26 . The light-scattering structures may be less dense at locations where the intensity of the light from light-emitting diodes  40  is greatest (e.g., at the exit of each light-emitting diode  40 ). As shown in the configuration of  FIG. 11 , for example, light-scattering features may be most dense at the centers of regions  22 T and may be less dense near the corners of regions  22 T where light  42  is received from light-emitting diodes  40 . 
     An illustrative arrangement for forming multidirectional light-emitting diodes  40  is shown in  FIGS. 12 and 13 . Initially, a printed circuit board substrate such as substrate  52  of  FIG. 12  may be populated with an array of light-emitting diode dies  40 D (i.e., dies  40 D may be soldered to metal traces in substrate  52 ) and may be coated with capping material  54  (e.g., an encapsulating photoluminescent material such as white phosphor and/or a clear encapsulation material). Laser drilling, mechanical cutting (e.g., using a grinding tool, saw, or other groove formation equipment) may then be used to form grooves in layer  54 . As shown in  FIG. 13 , these grooves may have a diagonal lattice pattern and may be filled with white polymer or other suitable reflective material to form reflective walls  40 W. The reflective material may optionally be formed on the top of the diodes in addition to forming reflective walls  40 W to help confine emitted light  42 . Substrate  52  may then be singulated to form individual light-emitting diodes  40  by die cutting or laser cutting substrate  52  along lines  56 . 
     Perspective views of illustrative multidirectional packaged light-emitting diodes  40  are shown in  FIGS. 14 and 15 . The example of  FIG. 14  shows how encapsulant  54  (e.g. phosphor, clear encapsulation material, etc.) may be used to cover dies  40 . If desired, some of the white polymer that forms reflective walls  40 W or other reflective material may be formed on top of phosphor or other encapsulant. As shown in  FIG. 15 , for example, capping layer  54  may be formed from encapsulant  54 - 1  (e.g., phosphor, etc.) and from white reflective layer  54 - 2  on the upper surface of encapsulant  54 - 1 . 
     If desired, ramped regions  36  may be incorporated into light guide plate  22  in configurations in which light guide plate  22  receives light from multidirectional light-emitting diodes  40  (e.g., two-direction diodes  40 , four-direction diodes  40 , etc.). A cross-sectional side view of a portion of a light guide plate of this type is show in  FIG. 16 . As shown in  FIG. 16 , light guide plate  22  may have edges  38  and ramped portions  36  that receive light  42  that is emitted from light-emitting diode  40  in multiple different directions (e.g., two different directions or four different directions). Light-emitting diode  40  may, for example, be mounted in a recess such as recess  60  in a thick region of light-guide plate  22 . 
     Light-emitting diodes  40  may be formed using a flip-chip configuration of the type shown in  FIG. 17 . Diode  40  of  FIG. 17  has sapphire substrate  50 . A layer of n-type GaN such as GaN layer  54  is deposited on a patterned surface of sapphire substrate  50 . P-type GaN layer  56  is formed on layer  54 . Passivation layer  58  is formed on layer  54 . Lower distributed Bragg reflector layer  60  is formed on passivation layer  58  and upper distributed Bragg reflector layer  52  is formed on substrate  50 . Passivation layer  58  and layer  60  are patterned and opening  68  is formed through layer  56 . A patterned metal layer may be used to form p-type contact  62  along the side of diode  40  and may be used to form n-type contact  66  through opening  68 . 
     After fabrication, diode  40  may be flipped (into the flipped orientation of  FIG. 17 ), so that layer  52  is on the top T of diode  40  and so that contacts  62  and  66  are on the bottom B of diode  40 . Contacts  62  and  66  may be soldered to a printed circuit or other suitable substrate. When current is applied to diode  40  through contacts  62  and  66 , light  64  is produced and is emitted out of the edge surfaces E of diode  40 . Distributed Bragg reflector layers  60  and  52  on bottom surface B and top surface T of diode  40 , respectively, prevent light  64  from escaping in the up or down directions. By concentrating light  64  out of the edges of diode  40 , diodes  40  can effectively emit light into the edges of light-guide plate  22 . 
     If desired, coatings, treated surfaces, and/or other microstructures may be added to edge surfaces E of diode  40 , as illustrated by microstructures  70  of  FIG. 18 . Microstructures  70  may be formed by roughening or otherwise patterning edge surfaces E, by coating edge surfaces E with thin-film layers such as passivation layers, antireflection coatings, photoluminescent coatings, etc., or by otherwise modifying edge surfaces E. Microstructures  70  may help reduce internal reflections to enhance light emission, may be used to modify the color of emitted light  64 , etc. 
     As shown in the example of  FIG. 19 , a metal layer such as metal layer  72  may be formed on top of distributed Bragg reflector layer  52 . Distributed Bragg reflector layers  52  and  60  may be formed from stacks of thin-film dielectric layers with alternating high and low refractive indices (and/or thin-film layers with other refractive index values). Examples of materials that may be used in forming the thin-film layers of reflectors  52  and  60  include titanium oxide, silicon oxide, aluminum oxide. A reflector may, for example, be formed from a stack of alternating titanium oxide and silicon oxide thin-film layers or may be formed from a stack of aluminum oxide, titanium oxide, and silicon oxide thin-film layers. Metal layer  72  may help enhance the confinement of light  64  within diode  40  and may therefore help prevent light  64  from being emitted upwardly through top surface T of diode  40 . 
     To help direct emitted light  64  in a desired direction, edge surfaces E of diode  40  may be tilted (slanted). In the example of  FIG. 20 , the surface normal n of planar edge surfaces E has been tilted downwardly to help direct light  64  upwardly. In the example of  FIG. 21 , the surface normal of planar edge surfaces E has been tilted upwardly to help direct light  64  downwardly. 
       FIG. 22  is a cross-sectional side view of diode  40  in an illustrative configuration in which distributed Bragg reflector  52  has been patterned (e.g., to form reflecting portions  52 R and transparent portions  52 T). The presence of transparent portions  52 T may allow some of light  64  to be emitted through top surface T (e.g., to help backlight display  14  directly through the thickness of light-guide plate  22 ). 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.