Flexible display and method of formation with sacrificial release layer

A flexible display panel and method of formation with a sacrificial release layer are described. The method of manufacturing a flexible display system includes forming a sacrificial layer on a carrier substrate. A flexible display substrate is formed on the sacrificial layer, with a plurality of release openings that extend through the flexible display substrate to the sacrificial layer. An array of LEDs and a plurality of microchips are transferred onto the flexible display substrate to form a flexible display panel. The sacrificial layer is selectively removed such that the flexible display panel attaches to the carrier substrate by a plurality of support posts. The flexible display panel is removed from the carrier substrate and is electrically coupled with display components to form a flexible display system.

BACKGROUND

The present invention relates to display systems. More particularly embodiments of the present invention relate to flexible display systems having semiconductor microchips and LEDs on a flexible display substrate.

2. Background Information

Display panels are critical components in modern mobile electronic devices, such as smartphones, tablets, and laptop/notebook computers. Through recent development, flexible display panels are becoming a viable replacement for conventional rigid display panels. Flexible display panels are display panels that are not formed with a rigid substrate so that they can be curved and bent. Currently, organic light emitting diode (OLED) technology is widely adopted for forming flexible display panels. Typical OLED display panels are constructed from a glass substrate, on top of which are a circuit containing thin-film transistors and a capacitor, then the light emitting OLED devices and, finally, a transparent, protective layer on top. The thin-film transistor circuit is formed within the OLED display substrate and is subjected to constricting forces during curving and bending of the display. Furthermore, OLEDs need to be hermetically sealed because they are hypersensitive to oxygen and water.

SUMMARY OF THE INVENTION

A method and apparatus for flexible light emitting diode (LED) display panels are described. In one embodiment, the method includes forming a sacrificial layer on a carrier substrate. The method also includes forming a flexible display substrate on the sacrificial layer where the flexible display substrate includes a plurality of release openings that extend through the flexible display substrate to the sacrificial layer. Furthermore, the method includes transferring an array of LEDs and a plurality of microchips onto the flexible display substrate. In an embodiment, the flexible display substrate is formed by spinning on a photo-definable material. Additionally, in an embodiment, forming the flexible display substrate includes forming at least one photo-definable polymer layer and at least one metal layer. Forming the at least one metal layer may be performed by sputtering.

In an embodiment, the method further includes etching a plurality of openings in the sacrificial layer and forming the flexible display substrate on the sacrificial layer and within the openings to form a plurality of posts extending through the sacrificial layer. Additionally, in an embodiment, the method further includes selectively removing the sacrificial layer and separating the flexible display substrate from the carrier substrate. Selectively removing the sacrificial layer may be performed by a process selected from the group consisting of a vapor etching process and a plasma etching process. Additionally, in an embodiment, the method further includes forming a transparent contact for each LED in the array of LEDs, forming a black matrix layer on the flexible display substrate where the black matrix layer surrounds the array of LEDs, and covering the array of LEDs and the plurality of microchips with a protective material. Covering the array of LEDs may be performed by a process selected from the group consisting of a slit-coating process and a laminating process.

In an embodiment, a flexible display panel includes a flexible substrate including a front surface, a back surface, and a display area on the front surface. The flexible display panel also includes a plurality of interconnects that extend at least partially through the flexible substrate from the front surface to the back surface where the flexible substrate and the plurality of interconnects form a build-up structure. Further, the flexible display panel includes an array of light emitting diodes (LEDs) and a plurality of microchips on the front surface of the flexible display substrate in the display area and electrically coupled to the plurality of interconnects. A plurality of release openings may extend through the flexible substrate from the front surface to the back surface.

Each microchip of the plurality of microchips may include a driving circuit to drive one or more LEDs to emit light. In an embodiment, the plurality of microchips are electrically coupled to the array of LEDs. Additionally, in an embodiment, the flexible display panel further includes at least one display component electrically coupled to the array of microchips on the front surface of the flexible substrate through the plurality of interconnects, where the display component comprises a chip selected from the group consisting of a sense controller chip, a scan driver chip, a data driver chip, a processor chip, and a power supply. The display component may be on the back surface of the flexible substrate. Further, the display component may be on the front surface of the flexible substrate outside of the display area. In an embodiment, the build-up structure includes at least one layer of polymer and at least one layer of metal.

In an embodiment, a structure includes a carrier substrate, a flexible substrate on the carrier substrate where the flexible substrate includes a plurality of electrical interconnects that extend at least partially between a front surface and a back surface of the flexible substrate, and an array of LEDs and a plurality of microchips on the front surface of the flexible display substrate. The structure also includes a sacrificial release layer between the back surface of the flexible substrate and the carrier substrate within a display area on the front surface, and a plurality of release openings that extend through the flexible substrate from the front surface to the back surface and expose the sacrificial release layer. Additionally, in an embodiment, the back surface of the flexible substrate includes a plurality of support posts. Each support post of the plurality of support posts may be laterally surrounded by the sacrificial release layer. Further, the sacrificial layer may include a material selected from the group consisting of an oxide and a nitride.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention describe flexible display systems and methods of manufacture thereof. In an embodiment, a method of manufacturing a flexible display system includes forming a sacrificial layer on a carrier substrate. A flexible display substrate is formed on the sacrificial layer, with a plurality of release openings that extend through the flexible display substrate to the sacrificial layer. In an embodiment, the flexible display substrate is formed using a photo-definable polymer. An array of light emitting diodes (LEDs) and a plurality of microchips are transferred onto the flexible display substrate to form a flexible display panel. The sacrificial layer is selectively removed such that the flexible display panel attaches to the carrier substrate by a plurality of support posts. The flexible display panel is removed from the carrier substrate and is electrically coupled with display components to form a flexible display system.

Embodiments of the invention enable the fabrication of flexible display panels whose operation does not require them to be held in a rigid structure. In an embodiment, the flexible display panel described herein includes a flexible display substrate having an array of LEDs and a plurality of microchips on a front surface of the flexible display substrate within a display area. In an embodiment, the flexible display substrate is a build-up structure that has more than one layer of insulating material and more than one layer of conductive material. At least one layer of conductive material within the flexible substrate electrically couples the array of LEDs to the plurality of microchips. In an embodiment, bond pads are exposed on a back surface of the flexible display substrate to which display components electrically connect. Alternatively, in an embodiment, bond pads are exposed on the front surface of the flexible display substrate outside of a display area. The bond pads are electrically coupled to the plurality of microchips on the front surface of the flexible display substrate. The conductive material within the flexible substrate electrically couples the bond pads to the plurality of microchips.

In an embodiment, the flexible display panel is fabricated by forming a layer of sacrificial material on a carrier substrate. A plurality of openings is formed in the sacrificial layer, within which a portion of the flexible display substrate is formed. The portion of the flexible display substrate in the plurality of openings forms a plurality of posts that extends through the sacrificial layer. In an embodiment, the flexible display substrate is constructed by forming at least one layer of insulating material and one layer of conductive material. An array of LEDs and a plurality of microchips are then transferred onto a front surface of the flexible display substrate. In an embodiment, the array of LEDs and the plurality of microchips are transferred onto the flexible display substrate by mass transfer tools operating using electrostatic principles to pick up and transfer large arrays of LEDs and microchips. Electrostatic transfer enables driving circuitry to be located on the front surface of the flexible display substrate, rather than embedded within the flexible display substrate. The array of LEDs and the plurality of microchips are covered with a transparent material to protect it from physical, environmental, and/or electrical disturbance while allowing for the visualization of light emitted from the array of LEDs. In an embodiment, the flexible substrate containing the array of LEDs and the plurality of microchips is separated from the carrier substrate by selectively removing the sacrificial layer and pulling the display substrate away from the carrier substrate, resulting in a flexible display panel that can be integrated with additional display components to form a display system.

In an embodiment, a flexible display system includes a flexible display panel having an array of LEDs and a plurality of microchips on a front surface of the flexible display substrate. A plurality of display components is electrically coupled to the plurality of microchips through the flexible display substrate. In an embodiment, the plurality of display components is located on the back surface of the flexible display substrate directly behind the display area. The plurality of display components can include, but are not limited to, scan drivers, data drivers, sense controllers, write controllers, microcontrollers, and power supplies. Alternatively, in an embodiment, the plurality of display components is located on the front surface of the flexible display substrate outside of a display area. In an embodiment, the flexible display substrate is formed with one or more layers of insulating material and one or more layers of conductive material. The layered structure of the flexible display substrate allows the flexible display panel to bend in various directions and to various degrees while maintaining electrical connectivity between the display components, microchips, and LEDs. As such, the flexible display system is enabled to display images or sense light while being bent in various directions.

In accordance with some embodiments, the interactive display panel described herein is a micro LED active matrix display panel formed with semiconductor-based micro LEDs. Such a micro LED active matrix display panel utilizes the performance, efficiency, and reliability of semiconductor-based LEDs for emitting light. Furthermore, a micro LED active matrix display panel enables a display panel to achieve high resolutions, pixel densities, and subpixel densities due to the small size of the micro LEDs and microchips. In some embodiments, the high resolutions, pixel densities, and subpixel densities are achieved due to the small size of the micro LEDs and microchips.

For example, the term “micro” as used herein, particularly with regard to LEDs and microchips, refers to the descriptive size of certain devices or structures in accordance with embodiments. The term “micro” is meant to refer to the scale of 1 to 300 μm or, more specifically, 1 to 100 μm. In some embodiments, “micro” may even refer to the scale of 1 to 50 μm, 1 to 20 μm, or 1 to 10 μm. However, it is to be appreciated that embodiments of the present invention are not necessarily so limited, and that certain aspects of the embodiments may be applicable to larger, and possibly smaller size scales. For example, a 55 inch interactive television panel with 1920×1080 resolution, and 40 pixels per inch (PPI) has an approximate RBG pixel pitch of (634 μm×634 μm) and subpixel pitch of (211 μm×634 μm). In this manner, each subpixel contains one or more micro LEDs having a maximum width of no more than 211 μm. Furthermore, where real estate is reserved for microchips in addition to micro LEDs, the size of the micro LEDs may be further reduced. For example, a 5 inch interactive display panel with 1920×1080 resolution, and 440 pixels per inch (PPI) has an approximate RBG pixel pitch of (58 μm×58 μm) and subpixel pitch of (19 μm×58 μm). In such an embodiment, not only does each subpixel contain one or more micro LEDs having a maximum width of no more than 19 μm, in order to not disturb the pixel arrangement, each microchip may additionally be reduced below the pixel pitch of 58 μm. Accordingly, some embodiments combine with efficiencies of semiconductor-based LEDs for emitting light with the scalability of semiconductor-based LEDs, and optionally microchips, to the micro scale for implementation into high resolution and pixel density applications.

FIG. 1Ais a cross-sectional side view illustration of a plurality of flexible display panels103with covered release openings111mounted on a carrier substrate101in accordance with an embodiment of the invention. The illustrated embodiment depicts the plurality of flexible display panels103after removal of the sacrificial layer and before separation from the carrier substrate101. As shown inFIG. 1A, the flexible display panels103have multiple sections102that will be described in more detail when discussing the method of forming the flexible display panel103below. The plurality of flexible display panels103is on the carrier substrate101. In an embodiment, the carrier substrate101is any suitable substrate, such as glass, upon which the flexible display panel103can be formed. In embodiments, the carrier substrate101is rigid enough to withstand process forces associated with the transfer of the array of LEDs and the plurality of microchips to the flexible display panel103with an electrostatic transfer head. In an embodiment, the carrier substrate101is formed of a material that can be reused for making new batches of flexible display panels103. Each flexible display panel103is separated from adjacent flexible display panels by a trench109. The trench109physically separates each flexible display panel103so that each flexible display panel103can be removed individually without interfering or damaging an adjacent flexible display panel. In an embodiment, the flexible display panel103includes a flexible substrate105formed from at least one layer of insulating material and at least one layer of conductive material. In an embodiment, the insulating material is a polymer. Alternatively, in an embodiment, the insulating material is a photo-definable polymer, such as an acrylic or an SU-8 photoresist (i.e., an epoxy photoresist). In a particular embodiment, the flexible substrate105is formed of at least one layer of photo-definable polyimide and at least one layer of metal. Although any insulating material may be used in embodiments, polyimide and metal may be a viable combination because of its ease of use and cost effectiveness. An array of LEDs106and a plurality of microchips108are located on the flexible display substrate105. A transparent protective layer107is formed over the array of LEDs, plurality of microchips, and exposed top surfaces of the flexible display substrate105. In an embodiment, the transparent protective layer107is polymethyl methacrylate (PMMA) or acrylic glass. Furthermore, a plurality of release openings111extends through the flexible display substrate105. The release openings111and trenches109provide channels within which an etchant may flow to remove the sacrificial layer as will be discussed further below. In this embodiment, the release openings111are covered by the transparent protective layer107so that holes do not extend through the flexible display panel103.

FIG. 1Bis a cross-sectional side view illustration of an array of flexible display panels103with exposed release openings111mounted on a carrier substrate101in accordance with an embodiment of the invention. The illustrated embodiment depicts the plurality of flexible display panels103after removal of the sacrificial layer and before separation from the carrier substrate101. The plurality of flexible display panels103is on the carrier substrate101. Each flexible display panel103is separated from adjacent flexible display panels by a trench109. The trench109physically separates each flexible display panel103so that each flexible display panel103can be removed individually without interfering or damaging an adjacent flexible display panel. An array of LEDs106and a plurality of microchips108are located on the flexible display substrate105. A transparent protective layer107, such as PMMA, is formed over the array of LEDs, plurality of microchips, and exposed top surfaces of the flexible display substrate105. Furthermore, a plurality of release openings111extends through the flexible display substrate105. The release openings111provide a channel within which an etchant may flow to remove the sacrificial layer as will be discussed further below. In this embodiment, the transparent protective layer107does not cover the release openings111. Rather, an opening is formed through the transparent protective layer107and the release openings111. Accordingly, the plurality of exposed release openings111creates a perforated flexible display panel103.

FIG. 1Cis a schematic top view illustration of an array of flexible display panels103with release openings111mounted on a carrier substrate101in accordance with an embodiment of the invention. Each flexible display panel103is separated from another flexible display panel by vertical and horizontal trenches109. Interspersed within the array of LEDs106is the plurality of microchips108. The plurality of microchips108controls the emission and/or sensing of the array of LEDs. The transparent protective layer107covers the array of LEDs106and the plurality of microchips108to protect them from damage or electrical interference. Furthermore, the array of LEDs106are covered with a transparent protective layer107to allow light to be emitted or sensed from the array of LEDs106. The plurality of release openings111is located within the inner area of the flexible display panels103. In an embodiment, the release openings111are covered release openings. Alternatively, in an embodiment, the release openings111are uncovered release openings. Release openings111allow etchants to remove the sacrificial layer located directly below the flexible display panel103. In an embodiment, the release openings111are equidistant from one another to so that etchants have the same amount of distance to travel between each release opening111. Alternatively, the release openings111may be designed to have a higher concentration or larger size in areas that are more difficult for etchants to reach, e.g., at locations farther away from trenches109. In an embodiment, release openings111enable the complete removal of sacrificial material below the flexible display substrates103.

FIG. 2is a cross-sectional side view illustration of a flexible display substrate105with LEDs106and microchips108on a front surface of the flexible display substrate105and back component bond pads213on a back surface225of the flexible display substrate105in accordance with an embodiment of the invention. The illustration inFIG. 2depicts a section102of the flexible display panel103and does not show a cross-section of the whole flexible display103. In an embodiment, the flexible display substrate105is formed on a carrier substrate101, which may be formed of glass.

In an embodiment, the flexible display substrate105is formed from at least one layer of insulating material and at least one layer of conductive material. In an embodiment, the insulating material is a polymer. Alternatively, in an embodiment, the insulating material is a photo-definable polymer, such as an acrylic or an SU-8 photoresist. In a particular embodiment, the insulating material is a photo-definable polyimide and the conductive material is a metal. As depicted inFIG. 2, the flexible display substrate105is formed from more than one insulating layers203,205,207, and209and more than one conductive layer211,217, and218. Although the embodiment depicted inFIG. 2illustrates four layers of insulating materials and three layers of conductive materials, embodiments of the present invention are not limited to such arrangements. The insulating layers203,205,207, and209are layered with the conductive layers211,217, and218to form a build-up structure in one embodiment. The build-up structure is a series of insulating layers with interconnect structures and conductive lines formed within. The interconnect structures electrically couple conductive lines to one another to form larger interconnect systems that span multiple layers. In an embodiment, the insulating layers203,205,207, and209are in the range of 2 to 2.5 μm thick to provide structural strength and sufficient electrical isolation between conductive layers when the flexible display panel is bent. The conductive layers211,217, and218are structured so that the back surface225of the flexible display substrate105is electrically coupled to the front surface223of the flexible display substrate105.

In an embodiment, the front surface223of the flexible display substrate105includes a plurality of wells with side surfaces221in which the LEDs106and microchips108are transferred. AlthoughFIG. 2depicts the LEDs106and the microchips108in wells, embodiments are not limited to such arrangements. For example, the front surface223of the flexible display substrate105may not have a plurality of wells, but rather have a flat surface upon which the LEDs and microchips are transferred. In an embodiment, the microchips108are electrically coupled to the LEDs through at least one of the conductive layers211,217, and218within the flexible display substrate105. In an embodiment, the microchips108are electrically coupled to the back surface225of the flexible display substrate105through the conductive layers211,217, and218. The back surface225of the flexible display substrate105includes back component bond pads213having back component bonding surfaces215for electrical coupling to display components as will be discussed further below. A transparent top contact229is located on the LEDs106to form an electrical connection between the LEDs106and a ground electrode (Vss)233. The transparency of the top contact229allows light emitting to or from the LEDs106to easily pass through the top contact229. The transparent contact229may be formed from any suitable transparent and conductive material, such as indium tin oxide (ITO) in one embodiment. As such, during operation, positive voltage may be applied by the microchip108to forward bias the LEDs106, whose cathode electrode is grounded by the transparent top contact229and the metal ground electrode223. It is to be appreciated that forward biasing the LEDs is but only one exemplary operation, to which other embodiments are not limited. For instance, the LEDs106may be reverse biased to sense light.

To ensure stability and protection of the electric connection to the LEDs106, a sidewall passivation material227is located between the sidewalls221of the wells and the LEDs106. The sidewall passivation material227stabilizes the LEDs106and prevents particles from falling underneath the LEDs106. Additionally, the sidewall passivation material227passivates sidewalls of the LEDs to prevent shorting of an active layer as well as provides step coverage for structures (e.g., metal contacts, transparent acrylics, transparent oxides, and/or transparent polymers, such as those that may form top contact229) formed upon it. In an embodiment, a black matrix layer231is formed over the front surface225of the flexible display substrate105. The black matrix layer231may absorb all wavelengths of visible light to prevent light from bleeding between adjacent LEDs. Accordingly, the black matrix layer231may mitigate any self-generated light disturbance within the flexible display panel103while the flexible display panel103is displaying an image.

In an embodiment, a sacrificial layer201is formed in between the carrier substrate101and the flexible display substrate105. The sacrificial layer201may be formed from any suitable material that can be etched selective to the flexible display substrate105and the carrier substrate101. In an embodiment, the sacrificial layer201is formed from silicon dioxide. The sacrificial layer201acts as a support layer for the fabrication of the flexible display panel103as well as an adhesive to secure the flexible display substrate105during fabrication. The sacrificial layer201may be selectively removed to allow separation of the flexible display panel103from the carrier substrate101. In an embodiment, a plurality of posts202extends through the sacrificial layer201to support the flexible display panel103after removal of the sacrificial layer201. The posts202are a portion of the flexible display substrate105extending from the back surface225of the flexible display substrate105. The bottom surface204of the posts202adhere to the carrier substrate101until the flexible display panel103is separated. In an embodiment, the structure of the posts202affects adhesion strength between the flexible display panel103and the carrier substrate101as well as the amount of force required to separate the flexible display panel103from the carrier substrate101. Wider posts202increase the adhesion strength and the required separation force due to an increase in surface area that makes contact with the carrier substrate101. In addition to the size of the posts202, the number of posts202affects adhesion strength and separation force as well. An increase in the number of posts202increases the surface area adhered to the carrier substrate101. As such, an increase in posts202increases the adhesion strength and the required force to separate the flexible display panel103from the carrier substrate101.

Trenches109are at the ends of the flexible display panel103. In an embodiment, the sacrificial layer201extends from underneath the flexible display substrate105and forms a layer across the bottom of the trench109. Alternatively, in an embodiment, the sacrificial layer201does not extend from underneath the flexible display substrate105. The trenches109expose the sacrificial layer201such that etchants may reach the sacrificial layer201. Furthermore, in an embodiment, release openings111are formed through the flexible display substrate105to expose the sacrificial layer201. The release openings111form a passageway for etchants to reach the sacrificial layer201.

FIG. 3is a cross-sectional side view illustration of a flexible display substrate105with LEDs106and microchips108on a front surface223of the flexible display substrate105and front component bond pads301on the front surface223of the flexible display substrate105in accordance with an embodiment of the invention. In an embodiment, the front component bond pads301are electrically coupled to the microchip108for sending electrical signals to the microchips108. A top surface303of the front component bond pad301is exposed to allow a display component to make electrical connection to the microchip108through at least one of the conductive layers211,217, and218. Having the front component bond pads301on the front surface223of the flexible display substrate105allow display components to be placed on the front surface223of the flexible display panel103outside of a display area.

Trenches109are at the ends of the flexible display panel103. The trenches109expose the sacrificial layer201such that etchants may reach the sacrificial layer201. Furthermore, in an embodiment, release openings111are formed through the flexible display substrate105to expose the sacrificial layer201. The release openings111form a passageway for etchants to reach the sacrificial layer201.

FIGS. 4A-4Sillustrate a method of fabricating a flexible display panel103including a flexible display substrate105with LEDs106and microchips108on a front surface223of the flexible display substrate in accordance with embodiments of the invention.FIGS. 4A-4Pillustrate a method of fabricating a flexible display panel103with LEDs106and microchips108on a front surface223of the flexible display substrate105with covered release openings111.FIGS. 4Q-4Sillustrate a method of fabricating a flexible display panel103with LEDs106and microchips108on a front surface223of the flexible display substrate105with exposed release openings111as continued fromFIG. 4N.

With reference toFIG. 4A, a sacrificial layer201is formed on a carrier substrate101. In an embodiment, the carrier substrate101is glass. The sacrificial layer201may be formed from any material that can be etched selective to the insulating material used to form the flexible display substrate105and the carrier substrate101. In an embodiment, the sacrificial layer is formed from silicon dioxide. Furthermore, in an embodiment, the thickness of the sacrificial layer201ranges from 0.5 to 1.5 μm. The sacrificial layer may be formed by a deposition process such as, but not limited to, chemical vapor deposition (CVD) or physical vapor deposition (PVD).

As shown inFIG. 4B, openings401are etched into the sacrificial layer201. The openings401may be spaced evenly apart from one another or arranged in a specific pattern. In an embodiment, openings401are patterned so that there is an equal distribution of openings dispersed around the area within which the flexible display panel103is to be formed. The placement of openings401determines the locations of posts202as shown inFIG. 4C.

InFIG. 4C, a first layer203of the flexible display substrate105is formed on the sacrificial layer201and within the trench109. In an embodiment, the first layer203is formed from an insulating material. The insulating material may be a polymer or a photo-definable insulating material, such as a photo-definable polymer. In an embodiment, the photo-definable polymer may be an acrylic or an SU-8 photoresist. In a particular embodiment, the first layer203is formed from a photo-definable polyimide, so that exposure to electromagnetic radiation chemically modifies the molecular structure of the polyimide to allow solubility in a developer solution. The photo-definable insulating material enables patterning without forming a separate mask layer, such as a photoresist. Therefore, having the photo-definable insulating material may reduce patterning operations and cost. During its formation, the first layer203fills in the openings401to form a plurality of posts202. The posts202are essentially an extension of the first layer203of insulating material such that the posts202and the first layer203form one integrated structure. The first layer203of the flexible display substrate105may be formed by spinning on or spray coating a layer of insulating material. When the insulating material is spun on, it fills in the openings401and subsequently forms the plurality of posts202.

As shown inFIG. 4D, via openings403, release openings111, and trenches109are etched into the first layer203of the flexible display substrate105. In an embodiment, the trenches109, via openings403, and release openings111extend through the first layer203and expose the sacrificial layer201underneath. In an embodiment, the via openings403are openings used for forming an interconnect via to electrically couple structures above and below the first layer203. In an embodiment, the release openings111and trenches109are openings used to provide a passageway for etchant chemicals to remove the sacrificial layer201. The via openings403, release openings111, and trenches109may be formed by conventional patterning and developing techniques. In an embodiment, patterned electromagnetic radiation, such as visible or ultraviolet light, is exposed onto the first layer203. Parts of the first layer203that are exposed to the electromagnetic radiation become cross-linked. As such, when the first layer203is submerged in a developer solution, unexposed regions are removed to form the patterned openings403,111, and109. Alternatively, if the first layer203is not formed from photo-definable material, then another suitable patterning technique may be used to form the openings403,111,109in the first layer203.

As shown inFIG. 4E, a first conductive layer405is formed over the first layer203and within the via openings403, release openings111, and trenches109. The first conductive layer405may be formed from a conductive material, such as a metal or a metal alloy, or any combination of multiple layers of conductive materials. In an embodiment, the first conductive layer405is a titanium-gold-titanium (Ti—Au—Ti) layer stack where a layer of gold is sandwiched between two thin layers of titanium. One reason why the first conductive layer405may be formed with Ti—Au—Ti is because although gold is an excellent conductor and is highly resistant to oxidation, it does not adhere well with insulating materials, such as the first layer203. As such, adding the outer layers of titanium, which adheres well with insulating material, allows the gold layer to be securely attached to the first layer203. It is to be appreciated that the thickness of gold may be much greater than the thickness of titanium. In an embodiment, the gold to titanium layer thickness ratio ranges from 5:1 to 10:1. Formation of the first metal layer405may be performed by a conformal deposition technique. In one embodiment, the first conductive layer405, having multiple layers of conductive materials, is formed by sputtering.

As shown inFIG. 4F, openings407are etched in the first conductive layer405to form back component bond pads213having back component bonding surfaces215and a first conductive layer211. The back component bond pads213conform to the surfaces of the opening403and first layer203upon which they are formed. In an embodiment, first conductive layer211is a part of an electrical connection between two semiconductor devices within the flexible display substrate105. The conductive layer formed within the release openings111and trenches109is removed to expose the sacrificial layer201in order for an etchant to reach the sacrificial layer201by access through the release openings111and trench109. The openings407in the first conductive layer405may be etched by a mask and etch process, such as an anisotropic dry or plasma etch process.

FIG. 4Gillustrates the flexible display substrate105subsequent to iterative formation of second and third insulating layers205and207, respectively, and second and third conductive layers217and218, respectively, with process techniques and conductive materials discussed inFIGS. 4C-4Faccording to an embodiment of the invention. The third conductive layer218includes various device bond pads, such as a ground pad411, LED pads413, and microchip pads415. The second conductive layer217is an interconnect layer designed to form various interconnections between back component bond pads213and the ground pad411, LED pads413, and microchip pads415. The conductive layers217and218may be a single conductive layer or multiple conductive layers formed from any conductive material, such as but not limited to, amorphous silicon, conductive oxides, conductive polymers, metals, and metal alloys. For example, in an embodiment, the conductive layers217and218are formed from aluminum, titanium, or an aluminum and titanium alloy. Additionally, the conductive layers217and218may be formed from more than one metal layer, such as a titanium-tungsten alloy and gold layer (TiW—Au) or a titanium and aluminum (Ti—Al) layer. In an embodiment, the back component bond pads213are electrically coupled to the microchip pads415of the flexible display substrate105through the second and third conductive layers217and218, respectively. Although the embodiment depicted inFIG. 4Gillustrates three insulating layers and three conductive layers, embodiments are not so limited. Various openings409in the top conductive layer218have been etched to form the ground, LED, and microchip pads411,413, and415, respectively. Throughout the processes up to this point, release openings111and trenches109have continuously been etched to expose the sacrificial layer201such that etchants may access the sacrificial layer201through the release openings111and trenches109.

InFIG. 4H, a fourth layer209is formed on a portion of the exposed pads411,413, and415and on exposed top surfaces of the third layer207using any of a variety of techniques such as inkjet printing, screen printing, lamination, spin coating, spray coating, CVD, and PVD. The fourth layer209may be formed of a variety of insulating materials such as, but not limited to, photo-definable acrylic, photoresist, silicon dioxide, silicon nitride, poly(methyl methacrylate) (PMMA), benzocyclobutene (BCB), polyimide, SU-8 photoresist, acrylate, epoxy, and polyester. In an embodiment, the fourth layer209is formed of an opaque material such as a black matrix material. Exemplary insulating black matrix materials include organic resins, glass pastes, and resins or pastes including a black pigment, metallic particles such as nickel, aluminum, molybdenum, and alloys thereof, metal oxide particles (e.g., chromium oxide), or metal nitride particles (e.g., chromium nitride). In an embodiment, the fourth layer209is formed from the same material as the first through third layers203,205, and207, respectively, such as a photo-definable insulating material, or any other protective, flexible material.

In an embodiment, the fourth layer209has openings417that expose the ground pad411, LED pads413, and microchip pads415to which electrical devices may be electrically coupled. As shown inFIG. 4H, the openings417in the fourth layer209have oblique sidewalls221that slope downward to form a well or a bank structure. In an embodiment, the well may be used for optical separation of adjacent LEDs to prevent optical interference. Additionally, in an embodiment, a surface of the well may be used to form mirrors to aid in light extraction. Furthermore, in an embodiment, the well may provide a structure for pooling sidewall passivation material227to passivate sidewalls of the LEDs to prevent shorting of an active layer, and may provide a structure for providing better step coverage for structures (e.g., top contact229) formed upon it.

As shown inFIG. 4I, conductive ground electrodes233are formed on the fourth layer209and on the ground pad411. Conductive ground electrodes233provide electrical connections to ground (Vss) for any device to which it is coupled. In an embodiment, the ground electrodes233are electrically coupled with at least the second conductive layer217through the ground pad411to form a connection to ground. The conductive ground electrodes233may be formed by a deposition and etch technique. In an embodiment, the conductive ground electrodes233are formed by sputtering followed by an anisotropic etch process.

InFIG. 4J, device bonding layers222are formed on the exposed LED pads413and microchip pads415to facilitate bonding of electrical devices. In an embodiment, the device bonding layers222are selected for its ability to be interdiffused with a bonding layer on the electrical devices (that are to be placed on the pads) through bonding mechanisms such as eutectic alloy bonding, transient liquid phase bonding, or solid state diffusion bonding. In an embodiment, the device bonding layers222have a melting temperature of 250° C. or lower. For example, the device bonding layers222may include a solder material such as tin (232° C.) or indium (156.7° C.), or alloys thereof. Device bonding layers222may also be in the shape of a post. In accordance with some embodiments of the invention, taller device bonding layers222may provide an additional degree of freedom for system component leveling, such as planarity of the electrical devices with the pad during device transfer operations and for variations in height of the devices, due to the change in height of the liquefied bonding layers as they spread out over the pad during bonding. The width of the device bonding layers222may be less than a width of a bottom surface of the electrical devices to prevent wicking of the device bonding layers222around the sidewalls of the electrical devices which can cause electrical shorting. The device bonding layers222may be formed by a photoresist lift-off technique or electroplating.

As shown inFIG. 4K, LEDs106and microchips108are transferred onto the device interconnect layers222such that the LEDs106and microchips108are electrically coupled to the LED pads413and microchip pads415, respectively. In an embodiment, the LEDs106are micro LEDs having a device size of 1-20 μm. The LEDs106may be any color-emitting LED, such as a red-, green-, blue-, infrared-, cyan-, white-, yellow-, or any other color-emitting LED. The microchips108may contain circuitry to receive signals from display components extraneous to the flexible display substrate105as well as circuitry to operate the LEDs106according to the received signals. In an embodiment, the microchips108contain driving circuitry to drive the LED in forward bias mode for emitting light. Optionally, the microchips108may also contain a selection device, such as a multiplexer, to disconnect the LED from the driving circuit and connect to a sensing circuit to operate the LED in reverse bias mode for sensing light. AlthoughFIG. 4Killustrates only two LEDs and one microchip108, embodiments of the present invention are not limited to such configurations. Rather, any number of LEDs106and any number of microchips108may transferred onto the flexible display substrate105. More specifically, the number and size of the LEDs and microchips108may scale according to the resolution or size of the flexible display panel103. Higher numbers and smaller sizes of LEDs and microchips108may be formed in flexible display panels that require higher resolutions and/or smaller flexible display panels103. The LEDs106and microchips108are electrostatically transferred onto the device bonding layers222by a pickup-and-placement method. In one embodiment, an electrostatic transfer head uses electrostatic force to pick up the LEDs106and microchips108and place them on the device bonding layers222.

Thereafter, inFIG. 4L, gaps between the LEDs106and microchips108and the sidewalls221of the wells in which they are bonded are filled to form sidewall passivation structures227. In embodiments, the sidewall passivation structures227pool around the LEDs106and microchips108within the wells in openings417. The sidewall passivation structures227attach to sidewalls of the LEDs106and microchips108and to the sidewalls221of the wells in openings417. Additionally, the sidewall passivation structures227fill gaps underneath the LEDs106and microchips108. In accordance with embodiments of the invention, the sidewall passivation structures227are transparent or semi-transparent to the visible wavelength so as to not significantly degrade light extraction efficiency of the LED. Sidewall passivation structures227may be formed of a variety of materials such as, but not limited to epoxy, poly(methyl methacrylate) (PMMA), benzocyclobutene (BCB), polyimide, and polyester. The sidewall passivation structures227may be formed by a precision deposition technique such as, but not limited to, inkjet printing.

The sidewall passivation structures227may secure the LEDs106and microchips108in place to prevent electrical disconnection from the device bonding layers222. Electrical disconnection from the device bonding layers222may render the LEDs and microchips108inoperable. Additionally, the sidewall passivation structures227may provide a surface for better step coverage for structures (e.g., top contact229) formed on top of the sidewall passivation structures227. Furthermore, the sidewall passivation structures227may insulate exposed sidewalls of the LEDs106in order to prevent short circuiting of active layers.

As shown inFIG. 4M, top contacts229are formed over the LEDs106to electrically couple the LEDs106to the metal ground electrodes233. Depending upon the particular application, top contacts229may be opaque, reflective, transparent, or semi-transparent to the visible wavelength spectrum. Exemplary transparent conductive materials include amorphous silicon, transparent conductive oxides (TCO) such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO), carbon nanotube film, or a transparent conductive polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline, polyacetylene, polypyrrole, and polythiophene. In an embodiment, the top conductive contact layer155includes nanoparticles such as silver, gold, aluminum, molybdenum, titanium, tungsten, ITO, and IZO. In an embodiment, the top contacts229are approximately 50 nm to 1 μm thick. Methods of formation include CVD, PVD, spray coating, or spin coating depending upon the desired area to be coated and any thermal constraints. In some embodiments, the top contacts229are formed by inkjet printing or screen printing. In an embodiment, inkjet printing or screen printing provides a practical approach for patterning the individual top contacts229without requiring separate mask layers.

InFIG. 4N, a black matrix layer231having openings419is formed over the exposed front surface223of the flexible display substrate105, surrounding the LEDs106. Exemplary black matrix materials include carbon, metal films (e.g., nickel, aluminum, molybdenum, and alloys thereof), metal oxide films (e.g., chromium oxide), metal nitride films (e.g., chromium nitride), organic resins, glass pastes, and resins or pastes including a black pigment or silver particles. The black matrix layer231prevents light from bleeding between LEDs and/or being absorbed by adjacent LEDs. Presence of the black matrix layer231, therefore, improves the contrast of images displayed on the flexible display panel103. In an embodiment, portions of the black matrix layer231in the release openings111and trenches109are removed to maintain the release openings111and trenches109. The black matrix layer231can be formed from a method that is appropriate based upon the material used. For example, black matrix layer231can be applied using inkjet printing, sputter and etching, spin coating with lift-off, lamination, or a printing method.

As shown inFIG. 4O, the sacrificial layer201between the flexible display substrate105and carrier substrate101is removed by etching with an etchant419the sacrificial layer201selective to the flexible display substrate105and carrier substrate101. The etchant419reaches the sacrificial layer201through the release openings111and the trenches109. The sacrificial layer201is removed by an etchant that can penetrate through the small dimensions between the flexible display substrate105and the carrier substrate101such as a vapor or plasma etch process. In an embodiment, the sacrificial layer201is removed by a vapor etch process utilizing vaporized HF as the etchant. The etchant is selective of the sacrificial layer201relative to the flexible display substrate105and the carrier substrate101such that the sacrificial layer201is substantially etched away while the flexible display substrate105and the carrier substrate101is not substantially etched away. In an embodiment, the flexible display substrate105and the carrier substrate101remain after removing the sacrificial layer101. After removing the sacrificial layer201, the flexible display panel103rests upon the carrier substrate101by the plurality of posts202. The plurality of posts202are laterally surrounded by voids423, which were previously occupied by the sacrificial layer201. In an embodiment, the thin Ti layer of the first metal layer405for the back component bonding pads213formed of Ti—Au—Ti is simultaneously removed by the selective etch process. Accordingly, the gold layer is exposed to make electrical contact with any display component that electrically couples to it. The gold layer is an excellent conductor and is highly resistive to oxidation.

As shown inFIG. 4P, a protective topcoat107is deposited over the display panel103, including within the release openings111. The protective topcoat107may be formed by lamination, slit coating, inkjet printing, or any deposition and etch techniques. If deposited by a non-precise deposition technique, the protective topcoat107formed within the trenches109may be removed to maintain the trenches109to separate adjacent flexible display panels103. The protective topcoat107may be any suitable transparent, flexible, and protective material to seal the devices and structures that form the display panel103. Transparency allows light to pass through the protective topcoat107to and from the LEDs106. Furthermore, flexibility allows the flexible display panel103to bend and flex in a variety of positions without fracturing the protective topcoat107. Additionally, the protective property of the protective topcoat107seals the devices and structures of the flexible display panel103from the environment and protects it from physical intrusion and/or electrical interference. In an embodiment, the protective topcoat107is formed from a variety of materials such as, but not limited to, epoxy, acrylic (polyacrylate) such as benzocyclobutene (BCB), polyimide, and polyester. In a specific embodiment, the protective topcoat107is formed of poly(methyl methacrylate) (PMMA). Although thick layers of PMMA are rigid and inflexible, the thickness of the protective topcoat107is substantially thin to allow flexibility. In an embodiment, the thickness of the protective topcoat ranges from 15 to 20 μm. In an embodiment, the protective topcoat107partially fills the release opening111, or completely fills the release opening111such that a bottom surface421of the protective topcoat107reaches the back surface225of the flexible display substrate105. Once the protective topcoat107is formed, the flexible display panel103is now ready to be separated from the carrier substrate101.

FIG. 4Qillustrates an alternative method of fabricating the flexible display panel103according to embodiments of the invention.FIG. 4Qcontinues fromFIG. 4N, where the black matrix layer231was formed and where the sacrificial layer201is still intact. Following formation of the black matrix layer231, inFIG. 4Q, the protective topcoat107may be deposited over the flexible display substrate103. Portions of the protective topcoat107that may be formed directly above or within the release openings111and trenches109are removed to expose the sacrificial layer201such that the sacrificial layer201can be accessed by a chemical etchant. Thereafter, inFIG. 4R, the sacrificial release layer201may be removed by selectively etching the sacrificial release layer201with a selective etchant419. Selectively removing the sacrificial layer201may be performed as discussed above withFIG. 4O. As shown inFIG. 4S, the flexible display panel103is now ready to be separated from the carrier substrate101. The flexible display panel103rests upon the carrier substrate101with the plurality of posts202, where each post202is laterally surrounded by voids423that were previously filled with sacrificial layer201. Release openings111remain exposed within the flexible display panel103.

FIG. 5is a schematic cross-sectional side view illustration of a flexible display panel103being separated from a carrier substrate101in accordance with an embodiment of the invention. In one embodiment, the flexible display panel103is separated by lifting a side of the flexible display panel103and peeling off the flexible display panel103as shown inFIG. 5. The posts202may not break or shear when the flexible display panel103is separated from the carrier substrate101, and may remain intact following the separation. Alternatively, in an embodiment, the flexible display panel103is vacuumed or electrostatically transferred off of the carrier substrate101. Harsh, chemical etchants are not needed to remove the flexible display panel103because the adhesion force between the plurality of posts202and the carrier substrate101is low enough to allow physical, dry separation. However, using wet chemical solutions to separate the flexible display panel103from the carrier substrate101is a viable method of separation that is envisioned in embodiments of the invention.

FIG. 6is a perspective view of a flexible display panel103illustrating an arrangement of LEDs and microchips in accordance with an embodiment of the invention. The flexible display substrate105inFIG. 6is transparent to better illustrate the layout of the flexible display panel103, and is not intended to be limiting. The array of LEDs106and the plurality of microchips108are on a front surface223of the flexible display panel103. The conductive layers211and218are formed to electrically couple the array of LEDs106and the plurality of microchips108to one another. The conductive layers211and218may be arranged horizontally and vertically as shown inFIG. 6, although embodiments are not limited to such arrangements for interconnecting the plurality of microchips108with the array of LEDs106. Additionally, the top contact229for each LED106in the array of LEDs106is transparent to allow transmission of light to and from the LEDs106.

FIG. 7Ais a cross-sectional side view illustration of a flexible display panel103with back component bond pads213on the back surface225of the flexible display panel103after separation from a carrier substrate101in accordance with an embodiment of the invention. The flexible display panel103has been separated from the carrier substrate101and is ready to be integrated into a flexible display system. The flexible display panel103includes a front surface223that has an array of LEDs106and a plurality of microchips108. Furthermore, the flexible display panel103includes a back surface225that includes exposed back component bond pads213and a plurality of posts202. The array of LEDs106is electrically coupled to the plurality of microchips108such that the plurality of microchips108can control the operations of the LEDs106. In an embodiment, the back component bond pads213located on the back surface225of the flexible display panel103are electrically coupled to the plurality of microchips108such that the microchips108are capable of receiving operating signals from the back component bond pads213. Accordingly, the back component bond pads213are electrically coupled to the plurality of microchips108through the conductive layers211,217, and218in the flexible display substrate105. As a result, the back surface225is electrically coupled with the front surface223.

FIG. 7Bis a cross-sectional side view illustration of a flexible display system800including a flexible display panel103and a plurality of display components803mounted on a back surface225of the flexible display panel103in accordance with an embodiment of the invention. The flexible display panel103includes covered release openings111so that holes do not extend through the flexible display panel103. The release openings111extend from the front surface223to the back surface225of the flexible display substrate105. In an alternative embodiment, the release openings111are opened to form holes through the flexible display panel103. A plurality of display components803is electrically coupled to the back component bond pads213. In an embodiment, the display components803are electrically coupled to the back component bond pads213through solder bumps805so that signals can be sent from the display components803to the microchips108. The conductive layers211,217, and218form the necessary interconnection between the display components803and the microchips108as well as between the microchips108and the LEDs106. In an embodiment, the back component bonding surfaces215of the back component bond pads213are formed from gold following the release etch process discussed inFIG. 4Oabove. In an embodiment, the conductive layers211,217, and218extend from the front surface223to the back surface225of the flexible display substrate105. The display components803may be any microchip or microcontroller with circuitry or program instructions used to operate the flexible display panel103. For example, in an embodiment, the display component803is a scan driver chip, a sense controller chip, a data driver chip, a processor chip, or a power supply. In an embodiment, the power supply is a battery.

FIG. 8Ais a cross-sectional side view illustration of a flexible display panel103with front component bond pads301on the front surface223of the flexible display panel103after separation from a carrier substrate101in accordance with an embodiment of the invention. The flexible display panel103has been separated from the carrier substrate101and is ready to be integrated into a flexible display system. The flexible display substrate103includes a front surface223that has an array of LEDs106, a plurality of microchips108, and a plurality of front component bond pads301on the front surface223. The array of LEDs106is electrically coupled to the plurality of microchips108such that the plurality of microchips108can control the operations of the LEDs106. The front component bond pads301are electrically coupled to the plurality of microchips108through at least one of the conductive layers211,217, and218in the flexible display substrate105.

FIG. 8Bis a cross-sectional side view illustration of a flexible display system800including a flexible display panel103and a plurality of display components803mounted on a front surface223of the flexible display panel103in accordance with an embodiment of the invention. The flexible display panel103includes covered release openings111so that holes do not extend through the flexible display panel103. The release openings111extend from the font surface223to the back surface225of the flexible display substrate105. In an alternative embodiment, the release openings111are opened to form holes through the flexible display panel103. A plurality of display components803is electrically coupled to the front component bond pads301. Accordingly, in an embodiment, the display components803are electrically coupled to the microchips108through solder bumps805so that signals can be sent from the display components803to the microchips108. The conductive layers211,217, and218form the necessary interconnection between the display components803and the microchips108as well as between the microchips108and the LEDs106. The conductive layers211,217, and218extend at least partially through the flexible display substrate105. As shown inFIG. 8B, the conductive layers211,217, and218extend more than half way through the flexible display substrate105. The conductive layers211,217, and218do not necessarily extend completely through the flexible display substrate105because there are no back component bond pads513, although embodiments that do extend completely through the flexible display substrate105are envisioned in embodiments of the present invention. The display components803may be any microchip or microcontroller with circuitry or program instructions used to operate the flexible display panel103. For example, in an embodiment, the display component803is a scan driver chip, a sense controller chip, a data driver chip, a processor chip, or a power supply. In an embodiment, the power supply is a battery.

FIG. 9Ais a schematic top view illustration of a back surface225of a flexible display system800including a flexible display panel103and a plurality of display components803mounted on a back surface225of the flexible display panel103in accordance with an embodiment of the invention. The side view illustration of the display system800shown inFIG. 9Ais illustrated inFIG. 7Bdiscussed above. As shown inFIG. 9A, the array of LEDs106and plurality of microchips108are illustrated with dotted gray lines to indicate that these devices are located on the front surface223of the display panel103. The area of the flexible display panel103where the LEDs106and the microchips108are located forms a display area901. The display area901is delineated by the dotted gray line formed around the perimeter of the LEDs106and microchips108. The display area901is the area where the flexible display system800emits and senses light. The plurality of display components803may be bonded to the back surface225of the flexible display panel103in any suitable orientation. As shown inFIG. 9A, the display components803are oriented in horizontal and vertical orientations. Furthermore, the display components803are electrically coupled with the back component bond pads213(not shown, as they are covered by the display components803). It is to be appreciated that placing the display components803on the backside of the display panel103allows the construction of a display panel system800to have a smaller surface area footprint than the surface area footprint of a display panel system800with display components803mounted on the font surface223of the flexible display panel103shown inFIG. 9B. AlthoughFIG. 9Adepicts the display components803mounted on the back surface225of the flexible display panel103on the opposite side of the display area901, the display components may be mounted on the back surface225of the flexible display panel103outside of the display area901.

FIG. 9Bis a schematic top view illustration of a front side223of a flexible display system800including a flexible display panel103and a plurality of display components803mounted on a front surface223of the flexible display panel103outside of a display area901in accordance with an embodiment of the invention. The schematic side view illustration of the display system800shown inFIG. 9Bis illustrated inFIG. 8Bdiscussed above. A marked difference between mounting the display components803on a front surface223of the flexible display panel103is the increase in surface area footprint of the flexible display system800. The display components803cannot be placed on the front surface223of the flexible display panel103because doing so would block the LEDs106from emitting light. However, placing the display components on the front surface223of the flexible display substrate103outside of the display area901may allow the flexible display system800to have a thinner profile. It is to be appreciated that the display components803may be arranged in any orientation, not just in the horizontal and vertical orientations as illustrated inFIG. 9B.

In utilizing the various aspects of this invention, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for emitting light with a flexible display panel. Although the present invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as particularly graceful implementations of the claimed invention useful for illustrating the present invention.