Patent Application
20140307207
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.
Liquid crystal displays contain a layer of liquid crystal material. Display pixels in a liquid crystal display contain thin-film transistors and electrodes for applying electric fields to the liquid crystal material. The strength of the electric field in a display pixel controls the polarization state of the liquid crystal material and thereby adjusts the brightness of the display pixel.
Substrate layers such as color filter layers and thin-film transistor layers are used in liquid crystal displays. The thin-film transistor layer contains an array of the thin-film transistors that are used in controlling electric fields in the liquid crystal layer. The color filter layer contains an array of color filter elements such as red, blue, and green elements. The color filter layer provides the display with the ability to display color images.
In an assembled display, the layer of liquid crystal material is sandwiched between the thin-film transistor layer and the color filter layer. Polyimide passivation layers cover the inner surface of the color filter layer and the upper surface of the thin-film transistor layer. An array of column spacers is formed on the inner surface of the color filter layer to maintain a desired gap between the color filter layer and the thin-film transistor layer. Column spacers are typically formed from hard organic materials such as photoresist.
There are typically two types of column spacers in a liquid crystal display. A relatively sparse set of main column spacers extends between the color filter layer and the thin-film transistor layer. The thickness of the column spacers and their associated landing pads establishes the amount of separation between the color filter layer and the thin-film transistor layer. Another set of column spacers, referred to as subspacers, has structures that extend only partway between the color filter layer and the thin-film transistor layer. Subspacers are used to prevent the thin-film transistor layer and column spacer from contacting one another. The subspacers do not extend all the way between the color filter layer and thin-film transistor layer to accommodate deformation of the color filter layer relative to the thin-film transistor upon a drop in ambient temperature for the display.
There are tradeoffs involved when determining an appropriate number column spacers to use in a given display. If too few of the main column spacers are provided, there will be insufficient support for the display. This will make the display susceptible to an undesirable visual effect called pooling mura. If too many of the main column spacers are provided, the display will become overly stiff. This will make the display prone to stress-induced birefringence when deformed, leading to undesired light leakage effects. With existing column spacer designs, it can be challenging to identify an acceptable tradeoff between pooling and light leakage. Displays are often sensitive to manufacturing variations and may exhibit more pooling and light leakage effects than desired.
It would therefore be desirable to be able to provide a display with an improved column spacer configuration.
A display may have a color filter layer with opposing upper and lower surfaces and a thin-film transistor layer with opposing upper and lower surfaces. A layer of liquid crystal material may be located between the lower surface of the color filter layer and the upper surface of the thin-film transistor layer.
Column spacer structures may be formed between the color filter layer and the thin-film transistor layer to maintain a desired separation between the color filter layer and the thin-film transistor layer. The column spacer structures may be formed from polymer structures such as photoresist pillars and may include pads such as metal pads. The metal pads may be formed on the upper surface of the thin-film transistor layer or the lower surface of the color filter layer. The photoresist pillars may be formed on a surface in the display such as the lower surface of the color filter layer.
Column spacer structures may include main spacer structures, subspacer structures, and one or more different types of intermediate thickness spacer structures. The use of the intermediate thickness spacer structures may simultaneously improve pooling mura performance and light leakage performance.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.
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
The illustrative configurations for device 10 that are shown in
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 includes display pixels formed from liquid crystal display (LCD) components or other suitable image pixel structures.
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
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.
In a configuration in which display layers 46 are used in forming a liquid crystal display, 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 56 and 58 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 thin-film transistors and associated electrodes (display 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.
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 62A or 62B 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 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 white plastic or other shiny materials.
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
To maintain a desired gap for the liquid crystal material between the lower surface of color filter layer 56 and the upper surface of thin-film transistor layer 58, display 14 may be provided with column spacer structures (sometimes referred to as post spacers). The column spacer structures may be formed from column structures (e.g., cylindrical posts) and/or planar structures such as metal pads on the surfaces of color filter layer 56 and/or thin-film transistor layer 58.
Column spacer structures 100 of
If desired, column spacer structures 100 may be formed in display 14 using a configuration in which a pad (e.g., metal pad 104) is formed on lower surface 114 of color filter layer 56, as shown in
In the illustrative arrangement of
As shown in
Column spacer structures 100A, 100B, and 100C may be formed by depositing column spacers on surface 114 of color filter layer 56 such as column spacers 102A, 102B, and 102C. One or more masks (e.g., binary masks, halftone masks, and/or grayscale masks) may be used in forming photoresist pillars (column spacers) of different thicknesses. Landing pads such as landing pad 104 and other pad structures may overlap column spacers such as column spacer 102A and may be used to prevent scratches in the surfaces of the display layers and/or to make desired thickness adjustments in the column spacer structures. Metal or other materials may be used in forming pads.
In display 14, there are generally numerous column spacer structures such as column spacer structures 100A, numerous column spacer structures such as column spacer structures 100B, and numerous column spacer structures such as column spacer structures 100C and structures 100A, 100B, and 100C are generally distributed uniformly across the surface of display 14. The portion of display 14 shown in
Column spacers 102A, 102B, and 102C have different thicknesses (sometimes referred to as heights). For example, column spacer 102A of
Column spacer structures 100A (and column spacers 102A) may sometimes be referred to as main column spacer structures (or main column spacers). As shown in
Column spacer structures 100B do not extend all the way between surface 114 on color filter layer 56 and surface 116 on thin-film transistor layer 58 and are therefore sometimes referred to as subspacers. As shown in
Column spacer structures 100C form a gap ΔH′ that is intermediate in size between the size of gap ΔH associated with subspacer column spacer structures 100B and the zero gap size associated with main column spacer structures 100A. The thickness of column spacer structures 100C also lies between the thickness of main column spacer structures 100A and the thickness of subspacer column spacer structures 100B. Column spacer structures 100C may therefore sometimes be referred to as intermediate column spacer structures, intermediate thickness column spacer structures, or transitional column spacer structures.
Intermediate column spacer structures 100C are thicker than subspacer structures 100B (e.g., intermediate column spacers 102 are thicker than subspacer column spacers 102B) and therefore provide more support for the layers of display 14 than subspacer column spacers 100B. This can help display 14 resist undesired pooling mura. As shown in
There are generally tradeoffs to be considered between light leakage performance and pooling performance in a display such as display 14 of
Pooling mura curve 140 illustrates how pooling performance tends to degrade as the concentration of main column spacers in a display decreases. This is because the column spacer structures in a display help to prevent layers 56 and 58 from coming into contact with each other. By providing a sufficient number of main column spacers, pooling performance can be improved, as indicated by the downward slope of curve 140 in of
Light leakage curve 142 illustrates how stress-induced birefringence and therefore light leakage tends to become worse as the number of main column spacers in a display increases. For a given deformation in the planarity of display 14, stress tends to rise in proportion to the stiffness of the display. Displays with fewer main column spacers are more flexible than displays with more column spacers. As a result, displays with fewer main column spacers develop less stress when deformed and produce correspondingly less stress-induced birefringence and light leakage (undesired localized brightening of the display). This behavior is reflected by the upwards slope of curve 142. When fewer main column spacers are present (near the left-hand side of curve 142 in
The inclusion of intermediate thickness column spacer structures such as column spacer structures 100C that have thicknesses greater than that of subspacer structures 100B enhances pooling mura performance by providing additional structural support for the layers of display 14 during temperature changes and other forces that exert bending pressure on layers such as color filter layer 56 without causing excessive stiffness of the type that may result by increasing the number of main column spacers 102A in display 14. The benefit of including intermediate thickness column spacer structures such as column spacer structures 100C of
As illustrated by arrows 146, curve 144 represents an improvement over curve 140 resulting from the inclusion of intermediate column spacers. When trading off light leakage performance against pooling mura performance in a display without intermediate column spacer structures 100C, a display might be configured to use the number of main column spacers associated with point 148 of the graph of
If desired, column spacer structures can use upper and/or lower pads (e.g., metal pads) and/or column spacers of different thicknesses to achieve desired overall thicknesses for the column spacer structures. Consider, as an example, the arrangement of
In the example of
The example of
The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.
