Multi-Layer Glass for Foldable Devices

An electronic device may have a display and may be configured to fold about a bend axis that overlaps the display. The display may have a display cover layer formed from three layers of glass fused together. The display cover layer may have outer glass layers that have lower coefficients of thermal expansion than a core inner layer that is formed between the outer glass layers. This places the outer glass layers into deep compressive stress relative to the inner layer, which is under tensile stress. A notch may be formed in one of the outer glass layers to enhance flexibility of the display cover layer in a strip-shaped notch region that extends along the bend axis. An exposed portion of the inner glass layer in the notch may be chemically strengthened.

FIELD

This relates generally to electronic devices, and, more particularly, to glass for foldable electronic devices.

BACKGROUND

Electronic devices may have displays. An electronic device display may be used to present information for a user. In some devices, displays may be covered with protective cover glass.

SUMMARY

An electronic device may have a display and may be configured to fold about a bend axis that overlaps the display. The display may have a display cover layer formed from three layers of glass fused together. The display cover layer may have a flexible portion that overlaps the bend axis and allows the display cover layer to bend.

The layers of glass material in the display cover layer may have different compositions with different properties. In an illustrative configuration, a core inner layer is sandwiched between a pair of outer glass layers that have lower coefficients of thermal expansion than the core inner layer. As the laminated glass layers cool during fabrication, this arrangement places the outer glass layers into compressive stress relative to the inner layer, which is under tensile stress. The outer glass layers may be relatively thick, so that the compressive stress penetrates deeply into the display cover layer to provide protection from damage due to exposure from sharp objects.

A notch may be formed in an inwardly facing one of the outer glass layers to enhance flexibility of the display cover layer in a strip-shaped flexible portion that extends along the bend axis. An exposed portion of the inner glass layer in the notch may be chemically strengthened so that this portion of the inner glass layer is placed under compressive stress rather than tensile stress. This helps prevent cracking in the notch when the display cover layer is folded.

DETAILED DESCRIPTION

Electronic devices may be provided with displays. Displays may be used for displaying images for users. Displays may be formed from arrays of light-emitting diode pixels or other pixels. For example, a device may have a flexible organic light-emitting diode display or a flexible display formed from an array of micro-light-emitting diodes (e.g., diodes formed from crystalline semiconductor dies). Electronic devices with flexible displays may be provided with hinges so that the devices can be folded for storage.

FIG.1is a side view of an illustrative folding electronic device in a partly folded configuration. Device10ofFIG.1may be a portable device such as a cellular telephone, tablet computer, or laptop computer, or may be any other suitable electronic device with a display.

Device10may include control circuitry. The control circuitry may include storage and processing circuitry for supporting the operation of device10. The storage and processing circuitry may include storage such as nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in the control circuitry may be used to gather input from sensors and other input devices and may be used to control output devices. The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors and other wireless communications circuits, power management units, audio chips, application specific integrated circuits, etc. During operation, the control circuitry of device10may use a display such as display14and other output devices in providing a user with visual output and other output.

Display14may have an array of pixels such as pixel array22(sometimes referred to as a display or display layer) that is configured to display images for a user. The pixels may be formed as part of a display panel that is bendable. This allows device10to be folded and unfolded about a bend axis. For example, a flexible (bendable) display in device10may be folded so that device10may be placed in a compact shape for storage and may be unfolded when it is desired to view images on the display. Pixel array (display)22may be covered by a transparent protective layer such as display cover layer20. Display cover layer20may be formed from glass, glass-ceramic, transparent ceramic, transparent crystalline materials such as sapphire, or other transparent protective material.

As shown inFIG.1, device10may have a housing such as housing12in which display14is mounted. Hinge16may be used to attach first and second portions of housing12together. Hinge16allows the first and second portions (left and right sides) of housing12to rotate with respect to each other about a hinge axis (sometimes referred to as a fold axis or bend axis). Display14may have planar portions such as portions14A and a bendable portion such as bendable portion14B that is located between the planar portions. When device10is in an unfolded (planar) configuration, portion14B is unbent (planar). When the left and right sides of device10are folded together as shown inFIG.1, portion14B bends. To prevent display cover layer20from becoming damaged (e.g., cracking) when device10is folded, display cover layer20may have a strip-shaped bendable portion such as portion20B that overlaps hinge16and extends along its associated bend axis (see, e.g., bend axis B ofFIG.2, which shows a top view of layer20). The width of portion20B orthogonal to bend axis B may be 20 mm, 10-30 mm, 1-50 mm, at least 0.5 mm, less than 60 mm, or other suitable width.

Because portion20B of layer20must bend during folding and unfolding operations, it is desirable for portion20B to be highly flexible and to exhibit high compressive surface stress. This helps enhance the bendability and bending strength of portion20B. Planar portions20A of display cover layer20may be supported by planar portions of housing12and need not be as flexible as bendable portion20B. Accordingly, it may be desirable to provide portions20A with deep compressive stress so that portions20A are highly resistant to puncture and other damage from contact with sharp objects.

In an illustrative embodiment, these requirements may be satisfied using a laminated arrangement for display cover layer20. A cross-sectional side view of display cover layer20in an illustrative arrangement in which layer20has multiple laminated sublayers of different materials is shown inFIG.3. As shown inFIG.3, layer20may have a first layer such as layer30(e.g., a layer that faces outwardly from device10and that faces away from pixel array22ofFIG.1), a second layer such as layer32, and a third layer such as layer34(which may face pixel array22ofFIG.1). Some or all of layer34may be removed in a strip that runs under strip-shaped portion20B and extends along bend axis B (FIG.2), as shown by notch (removed area)24ofFIG.3. This recess in layer20may be filled with index-matched polymer26or other clear flexible material, so that images on pixel array22are not optically distorted when a viewer views pixel array22through layer20.

Outer layers30and34of layer20may have a thickness of 40 microns, at least 20 microns, at least 30 microns, less than 80 microns, less than 70 microns, 20-80 microns, or other suitable thickness. Inner layer32may have a thickness of 120 microns, 10-240 microns, or other suitable thickness. The total thickness of layer20(the combined thickness of layers30,32, and34in portions20A, away from notch24) may be 50-350 microns, 100-300 microns, 150-250 microns, at least 75 microns, at least 100 microns, at least 150 microns, at least 160 microns, at least 200 microns, less than 300 microns, or other suitable thickness. Layers30,32, and/or34may be formed from transparent materials such as glass, glass-ceramic, transparent ceramic, or crystalline material. In an illustrative configuration, which may sometimes be described as an example, layers30,32, and34may be formed from glass.

Outer layers30and34may be formed from the same type of glass or may be formed from different types of glass. In an illustrative embodiment, outer layers30and34are formed from the same type of glass. To provide deep compressive stress to the outer surfaces of layer20, outer layers30and34may be formed from a first type of glass that has a first coefficient of thermal expansion (or two different materials with different respective coefficients of expansion), whereas layer32may be formed from a second type of glass that has a second coefficient of thermal expansion that is greater than the first coefficient of expansion (or that is greater than the coefficients of thermal expansion of each of layers30and34in arrangements in which layers30and34are formed from different materials). As an example, the second coefficient of thermal expansion may be at least 10%, at least 30%, or at least 50% greater than the coefficient of thermal expansion of layer30and/or may be at least 10%, at least 30%, or at least 50% greater than the coefficient of thermal expansion of layer32.

During fabrication of layer20, layers30,32, and34may be laminated together in a stack by heating and fusing layers30,32, and34together at elevated temperatures or otherwise fabricating a single glass layer by joining layers30,32, and34directly to each other (without intervening adhesive) at an elevated temperature. As layers30,32, and34cool, the higher coefficient of thermal expansion of layer32relative to layers30and34causes layer32to contract more than layers30and34. This places layers30and34under compressive stress (e.g., 250-900 MPa of stress or other suitable amount of compressive stress) and places layer32under tensile stress. Due to the relatively large thicknesses of layers30and34, the compressive stress profile of layer20penetrates relatively deeply into layer20(e.g., tens of microns), which helps ensure that layer20(e.g., portions20A) will be satisfactorily resistant to sharp damage. As an example, the compressive stress on each of the outer surfaces of layer20may penetrate at least 15%, at least 20%, at least 25%, at least 30%, or at least 35% into the total thickness of layer20. If, as an example, the total thickness of layer20is 200 microns, layer20may be compressively stressed at least 40 microns, at least 50 microns, or at least 66 microns into layer20from each of the surfaces of layer20, whereas the remaining core portion of layer20may be under tensile stress. By ensuring that the depth of the compressively stressed portion of layer20is sufficiently large (e.g., at least 20%, at least 25%, at least 30%, or at least 35% of the total thickness of layer20), layer20(e.g., portions20A) may be satisfactorily resistant to damage when exposed to sharp objects during use of device10.

If desired, the flexibility of strip-shaped portion20B of layer20may be enhanced by removing some or all of layer34in portion20B (see, e.g., notch24in layer34ofFIG.3) and notch24may be filled with flexible index-matched polymer26. In this type of arrangement, the total thickness remaining in glass layer20(e.g., the combined thickness of layers30and32in the portion of layer20that overlaps the bend axis and notch24) may be 30-150 microns, 50-125 microns 75-90 microns, at least 60 microns, less than 50 microns, less than 95 microns, less than 100 microns, less than 125 microns, less than 150 microns, or other suitable thickness that is sufficiently small to allow portion20B to bend as device10is folded and unfolded. Notch24may be formed only in layer34(without penetrating into layer32) or may, if desired, penetrate partway into layer32. Chemical etching or other notch formatting techniques may be used to form notch24.

To ensure sufficient compressive surface stress in the exposed surface of layer32in notch24of portion20B, layer20may be chemically strengthened following formation of notch24. For example, the exposed surface of layer32may be placed in compressive stress using an ion-exchange process. During the ion-exchange process, smaller ions in the glass may be replaced with larger ions. For example, sodium in the glass at the exposed surfaces of layer32may be replaced by potassium. This creates compressive stress within the treated surface layers of layer32in notch24and prevents tensioned portions of layer20from being exposed at the surface of layer20. The presence of chemically strengthened glass at the surface of layer32in notch24thereby helps to ensure that portion20B can bend satisfactorily without experiencing cracking or other damage (e.g., when glass layer20is bent to form a bend radius of 4-6 mm (as an example). During chemical strengthening, layer20may be immersed in an ion-exchange bath for a relatively short amount of time so as to avoid excessively exposing layers30and34to elevated temperatures that could cause the compressive stress in layers30and34to relax. The depth of the chemically strengthened layer at the surface of layer32in notch24may be relatively shallow (e.g., 5-15 microns), because this exposed surface of layer32faces the interior of device10when device10is folded and will therefore not be exposed to scratching or other damage due to exposure to sharp objects.

To help protect the privacy of users, any personal user information that is gathered by electronic devices may be handled using best practices. These best practices including meeting or exceeding any privacy regulations that are applicable. Opt-in and opt-out options and/or other options may be provided that allow users to control usage of their personal data.