PATENT DOCUMENT

Publication Number: US-9494178-B2
Application Number: US-201414183224-A
Country: US
Kind Code: B2

Title: Methods for bonding substrates using liquid adhesive

Abstract:
Structures in an electronic device such as substrates associated with a display may be bonded together using liquid adhesive. Fiber-based equipment may be used to apply ultraviolet light to peripheral edges of an adhesive layer during bonding. There-dimensional adhesive shapes may be produced using nozzles with adjustable openings, computer-controlled positioners, and other adhesive dispensing equipment. Ultraviolet light may be applied to liquid adhesive through a mask with an opacity gradient. Adjustable shutter structures may control adhesive exposure to ultraviolet light. Ultraviolet light exposure may be used to create an adhesive dam that helps create a well defined adhesive border. Multiple layers of adhesive may be applied between a pair of substrates.

Claims:
What is claimed is: 
     
       1. A method for bonding substrates, comprising:
 applying liquid adhesive to at least one of the substrates; 
 applying ultraviolet light to an edge of the liquid adhesive using fiber-based equipment, wherein the fiber-based equipment includes a fiber bundle containing a strip of fibers, wherein applying the ultraviolet light comprises applying the ultraviolet light using the strip of fibers, wherein each fiber in the fiber bundle emits light, and wherein a light modulator is interposed within the path of each fiber; and 
 with a controller, sending control signals to the light modulator associated with each fiber to individually adjust a magnitude of light emitted by each fiber. 
 
     
     
       2. The method defined in  claim 1  wherein the substrates comprise display substrates for an electronic device display and wherein applying the adhesive comprises applying the adhesive to at least one of the display substrates. 
     
     
       3. The method defined in  claim 2  wherein the display substrates include a polarizer layer and wherein applying the adhesive comprises applying the adhesive to the polarizer layer. 
     
     
       4. The method defined in  claim 2  wherein the display substrates include a display cover glass layer and wherein applying the adhesive comprises applying the adhesive to the display cover glass layer. 
     
     
       5. The method defined in  claim 1  wherein applying ultraviolet light comprises applying ultraviolet light to the fiber bundle using a plurality of light-emitting diodes. 
     
     
       6. The method defined in  claim 5 , wherein each light-emitting diode is aligned with a respective fiber in the fiber bundle. 
     
     
       7. The method defined in  claim 1  wherein the fiber bundle contains side-firing fibers and wherein applying the ultraviolet light comprises applying the ultraviolet light at a right angle relative to a longitudinal axis of at least one of the fibers. 
     
     
       8. The method defined in  claim 1 , wherein at least one of the light modulators comprises a mechanical shutter. 
     
     
       9. The method defined in  claim 1 , wherein at least one of the light modulators comprises a liquid crystal shutter. 
     
     
       10. The method defined in  claim 1 , wherein at least one of the light modulators a microelectromechanical attenuator. 
     
     
       11. A method for bonding a display module to a display cover glass layer, comprising:
 applying liquid adhesive between the display module and the display cover glass layer, wherein a housing structure is positioned adjacent to the display cover glass layer, and wherein there is a gap between the display module and the housing structure; and 
 applying ultraviolet light to an edge of the liquid adhesive using fiber-based equipment, wherein the fiber-based equipment includes a fiber bundle containing a strip of side-firing fibers, and wherein applying the ultraviolet light to the edge of the liquid adhesive comprises inserting the fiber bundle into the gap and applying the ultraviolet light at a right angle relative to a longitudinal axis of the side-firing fibers. 
 
     
     
       12. The method defined in  claim 11  wherein each side-firing fiber in the fiber bundle emits light, the method further comprising individually controlling the light emitted by each side-firing fiber. 
     
     
       13. The method defined in  claim 12  wherein individually controlling the light comprises using light modulators in fiber paths associated with the side-firing fibers to modulate how much of the light is emitted from each side-firing fiber. 
     
     
       14. The method defined in  claim 12  wherein individually controlling the light comprises individually controlling each of a plurality of light-emitting diodes that are each aligned with a respective side-firing fiber. 
     
     
       15. The method defined in  claim 11 , wherein the display cover glass layer bridges the gap between the display module and the housing structure. 
     
     
       16. The method defined in  claim 11 , wherein the gap between the display module and the housing structure is less than 300 microns. 
     
     
       17. The method defined in  claim 11 , wherein each side-firing fiber includes a prism that directs light at a right angle relative to the longitudinal axis of the side-firing fiber.

Description:
This application claims priority to U.S. provisional patent application No. 61/771,590 filed Mar. 1, 2013, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to adhesives and, more particularly, to using liquid adhesives to assemble components. 
     Electrical devices such as computers and cellular telephones contain components that are attached together using adhesive. For example, liquid optically clear adhesive may be used to attach a display cover glass layer to a liquid crystal display module. If care is not taken, adhesive borders may be poorly defined and voids may be formed within the adhesive. This can degrade adhesive performance and can create visible defects. 
     It would therefore be desirable to be able to form improved adhesive bonds between structures in an electronic device. 
     SUMMARY 
     An electronic device may be provided that includes structures that are attached together using adhesive. Liquid adhesive such as liquid optically clear adhesive may be used to bond together substrates such as display layer substrates for a display. 
     Ultraviolet light may be applied to the border of an adhesive layer using fiber-based equipment. The fiber-based equipment may include a strip of fibers that apply the ultraviolet light along an exposed adhesive edge. Light may be applied to one or more edges simultaneously using multiple bundles of fibers. Side-firing fibers may be used to allow the fiber-based equipment to be introduced into confined areas. 
     Slit-based adhesive dispensers and other adhesive dispensers may be provided with adjustable nozzles. Computer-controlled positioners and adjustable nozzles may be used to create three-dimensional adhesive layers. The three-dimensional adhesive layers may have protruding portions that create well-defined initial contact points between the adhesive and substrate layers to minimize voids during bonding. 
     Localized and global energy may be applied to adhesive to form adhesive protrusions and other features that facilitate the formation of satisfactory adhesive bonds. Energy may be applied to adhesive during precuring operations and during substrate bonding. Bonding stage electrodes or other structures may be used in applying localized and global energy in the form of heat, light, static electric fields, static magnetic fields, radio-frequency signals, etc. 
     Light masks with transmission gradients may be used in applying light to adhesive to cure the adhesive. Adhesive dam structures with tapered inner edges may be formed using light masks with light transparency gradients. Movable shutter systems may also be used in controlling the application of light to an adhesive layer. 
     Further features, their nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device of the type that may be provided with structures attached to each other using liquid adhesive in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of a portion of an electronic device having structures such as layers of material associated with a display that are bonded using adhesive in accordance with an embodiment. 
         FIG. 3  is a diagram showing how lamination equipment may be used in bonding together structures in an electronic device such as substrate layers in accordance with an embodiment. 
         FIG. 4  is a diagram showing how adhesive may be dispensed onto a substrate in a three-dimensional shape to ensure initial contact is made between the adhesive and a substrate at a desirable location for minimizing adhesive voids during substrate bonding in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of electronic device structures such as electronic device display structures being bonded using adhesive and an associated fiber-based system for applying ultraviolet light to a peripheral edge portion of the adhesive in accordance with an embodiment. 
         FIG. 6  is a side view of a side-firing fiber with a prism structure for directing light at a right angle with respect to the longitudinal axis of the fiber to facilitate application of ultraviolet light to adhesive in accordance with an embodiment. 
         FIG. 7  is a perspective view of a fiber bundle of the type that may be used in applying ultraviolet light to adhesive when assembling structures together for an electronic device in accordance with an embodiment. 
         FIG. 8  is a diagram of fiber bundle equipment for applying light to adhesive in accordance with an embodiment. 
         FIG. 9  is a perspective view of a portion of a fiber bundle having individually positioned fibers in accordance with an embodiment. 
         FIG. 10  is a cross-sectional side view of a portion of a display containing adhesive that is being illuminated with fiber-based equipment such as a system having a bundle of side-firing fibers arranged in a strip along the exposed edge of the adhesive in accordance with an embodiment. 
         FIG. 11  is a perspective view of an adhesive curing arrangement in which a pair of strip-shaped fiber bundles is being used to illuminate adhesive along opposing edges of a display or other electronic device structure in accordance with an embodiment. 
         FIG. 12  is a perspective view of a layer of adhesive that has been dispensed on a substrate in a pyramidal shape in accordance with an embodiment. 
         FIG. 13  is a perspective view of a layer of adhesive that has been dispensed on a substrate in a shape that is characterized by a raised ridge with a central peak in accordance with an embodiment. 
         FIG. 14  is a cross-sectional side view of computer-controlled adhesive dispensing equipment having a movable head for dispensing adhesive onto the surface of a substrate in accordance with an embodiment. 
         FIG. 15  is a graph showing how the speed of travel of the head of the adhesive dispensing tool of  FIG. 14  may be varied as a function of linear position along the length of a substrate in accordance with an embodiment. 
         FIG. 16  is a graph showing how the thickness of adhesive dispensed using a speed profile of the type shown in  FIG. 15  may vary as a function of position along the length of a substrate in accordance with an embodiment. 
         FIG. 17  is a diagram of an illustrative nozzle in a slit dispenser for applying adhesive to a substrate in accordance with an embodiment. 
         FIG. 18  is a diagram of an illustrative nozzle with a triangular central portion in a slit dispenser for applying adhesive to a substrate in accordance with an embodiment. 
         FIG. 19  is a diagram of an illustrative adhesive dispensing structures such as an adhesive dispensing nozzle with a pattern of openings for creating an adhesive layer with a protruding central portion in accordance with an embodiment. 
         FIG. 20  is a diagram of a slit dispenser with multiple individually controlled nozzle slits in accordance with an embodiment. 
         FIG. 21  is a perspective view of an illustrative screen printing tool for patterning adhesive in accordance with an embodiment. 
         FIG. 22  is a perspective view of an illustrative computer-controlled needle dispenser for applying adhesive in a pattern on a substrate in accordance with an embodiment. 
         FIG. 23  is a top view of an illustrative substrate coated with a layer of adhesive in a rectangular pattern accordance with an embodiment. 
         FIG. 24  is a top view of an illustrative substrate coated with a layer of adhesive in an oval pattern in accordance with an embodiment. 
         FIG. 25  is a top view of an illustrative substrate coated with a layer of adhesive in a double-Y pattern in accordance with an embodiment. 
         FIG. 26  is a top view of an illustrative substrate coated with two overlapping layers of adhesive to create adhesive with a raised central portion in accordance with an embodiment. 
         FIG. 27  is a cross-sectional side view of a roller-based lamination system for attaching a flexible substrate to another substrate with adhesive in accordance with an embodiment. 
         FIG. 28  is a diagram of a light-based system for curing the edge of an adhesive layer to control the boundary of the adhesive layer in accordance with an embodiment. 
         FIG. 29  is a diagram of a substrate having a surface with a patterned material for helping to control the boundary of the adhesive in accordance with an embodiment. 
         FIG. 30  is a cross-sectional side view of bonding equipment with electrodes or other structures for applying localized energy to a layer of adhesive between a pair of opposing substrates in accordance with an embodiment. 
         FIG. 31  is a flow chart of illustrative steps involved in locally applying energy to adhesive during precuring and substrate bonding operations in accordance with an embodiment. 
         FIG. 32  is a diagram of a light-mask-based system for applying light in a desired pattern to a layer of adhesive on a substrate in accordance with an embodiment. 
         FIG. 33  is a graph showing how mask density in a system of the type shown in  FIG. 32  may vary as a function of distance from the center of the mask in accordance with an embodiment. 
         FIG. 34  is a cross-sectional side view of substrates such as display layers that have been attached using two layers of adhesive in accordance with an embodiment. 
         FIG. 35  is a cross-sectional side view of substrates such as display layers that have been attached using three layers of adhesive in accordance with an embodiment. 
         FIG. 36  is a flow chart of steps involved in bonding substrates together using one or more layers of locally or globally modified adhesive in accordance with an embodiment. 
         FIG. 37  is a top view of a substrate having a layer of adhesive that has been illuminated with patterned light to control lateral adhesive spread and having adhesive that has been illuminated with patterned light to produce a protruding central portion in accordance with an embodiment. 
         FIG. 38  is a top view of a substrate that has been covered using a layer of adhesive in accordance with an embodiment. 
         FIG. 39  is a top view of the substrate of  FIG. 38  following deposition of a patterned additional layer of adhesive in accordance with an embodiment. 
         FIG. 40  is a cross-sectional side view of bonding stage equipment being used to bond substrates together using upper and lower layers of adhesive in accordance with an embodiment. 
         FIG. 41  is a cross-sectional side view of the bonding stage equipment of  FIG. 40  after pressing together the substrates and compressing the adhesive between the substrates until the upper adhesive layer flows sufficiently to cover the lower adhesive layer in accordance with an embodiment. 
         FIG. 42  is a cross-sectional side view of bonding stage equipment being used to bond substrates together using upper and lower layers of adhesive in accordance with an embodiment. 
         FIG. 43  is a cross-sectional side view of the bonding stage equipment of  FIG. 42  after pressing together the substrates and compressing the interposed layer of adhesive until the upper adhesive layer flows outward and covers the edges of the lower adhesive layer in accordance with an embodiment. 
         FIG. 44  is a side view of light-based equipment for curing a ring of adhesive on a substrate to form an adhesive dam with a tapered inner edge in accordance with an embodiment. 
         FIG. 45  is a cross-sectional side view of an adhesive dam with a tapered inner edge and an associated laterally confined layer of liquid adhesive in accordance with an embodiment. 
         FIG. 46  is a flow chart of illustrative steps involved in forming a structure of the type shown in  FIG. 45  in accordance with an embodiment. 
         FIG. 47  is a top view of a substrate layer on which a layer of adhesive has been patterned with light of varying intensity to form a smoothly varying edge for an adhesive dam in accordance with an embodiment. 
         FIG. 48  is a top view of a movable shutter system of the type that may be used in applying light to a layer of adhesive to form an adhesive dam with a tapered edge in accordance with an embodiment. 
         FIG. 49  is a top view of the movable shutter system of  FIG. 48  in a configuration in which the shutters have been moved into an expanded configuration in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Adhesive may be used in attaching structures formed from plastic, glass, ceramic, fiber-based composites such as carbon fiber composites and fiberglass, metal, and other structures. These structures may form part of electrical components in an electronic device. As an example, adhesive may be used in coupling together planar substrate layers such as planar layers of glass or plastic that are associated with a display. The substrates that are joined with adhesive in this way may include layers such as a display cover layer, a touch sensor array substrate that is part of a display or that is separate from a display, a thin-film transistor layer, a color filter layer, and/or other substrates associated with a display or electronic device. Arrangements in which adhesive is used in bonding together layers in an electronic device such as layers associated with a display may sometimes be described herein as an example. This is, however, merely illustrative. Any suitable structures may be attached to each other using adhesive if desired. 
     An illustrative electronic device having structures such as substrates for a display or other structures that are bonded using adhesive is shown in  FIG. 1 . Electronic devices such as device  10  of  FIG. 1  may be cellular telephones, media players, other handheld portable devices, somewhat smaller portable devices such as wrist-watch devices, pendant devices, or other wearable or miniature devices, gaming equipment, tablet computers, notebook computers, desktop computers, televisions, computer monitors, computers integrated into computer displays, or other electronic equipment. 
     In the example of  FIG. 1 , device  10  includes a display such as display  14 . Display  14  has been mounted in a housing such as housing  12 . Housing  12 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). 
     Display  14  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. 
     Display  14  may include an array of display pixels formed from liquid crystal display (LCD) components, an array of electrophoretic display pixels, an array of plasma display pixels, an array of organic light-emitting diode display pixels, an array of electrowetting display pixels, or display pixels based on other display technologies. The brightness of display  14  may be adjustable. For example, display  14  may include a backlight unit formed from a light source such as a lamp or light-emitting diodes that can be used to increase or decrease display backlight levels and thereby adjust display brightness. Display  14  may also include organic light-emitting diode pixels or other pixels with adjustable intensities. In this type of display, display brightness can be adjusted by adjusting the intensities of drive signals used to control individual display pixels. 
     Display  14  may be protected using a display cover layer such as a layer of transparent glass or clear plastic. Openings may be formed in the display cover layer. For example, an opening may be formed in the display cover layer to accommodate a button such as button  16 . An opening may also be formed in the display cover layer to accommodate ports such as speaker port  18 . 
     A cross-sectional side view of device  10  taken along line  20  and viewed in direction  22  is shown in  FIG. 2 . As shown in the illustrative configuration of  FIG. 2 , device  10  may include one or more printed circuits such as printed circuit  28 . Integrated circuits and other electrical components  30  may be mounted on printed circuit  28  and may be interconnected with display module  26 . Display module  26  may be a liquid crystal display module, an organic light-emitting diode display, an electrophoretic display module, or a display module using other types of display technology. Display module  26  may contain multiple display layers  32 . Display layers  32  may, for example, include polarizer layers, thin-film transistor layers, and color filter array layers. Optional display cover layer  24  may form an additional display layer for display  14 . Display cover layer  24  may be formed from a clear layer of glass, a transparent plastic layer, or other transparent member. 
     Adhesive may be used in assembling the structures of device  10 . For example, liquid optically clear adhesive may be interposed between the layers of display  14  such as display cover layer  24  and/or layers  32 . The use of optically clear adhesive may allow light from display pixels in display  14  to be viewed by a user of device  10 . Other types of adhesive (e.g., adhesive that is not clear) may be used in assembling structures in device  10  if desired. Configurations in which display layers in display  14  are laminated to each other using optically clear adhesive are sometimes described herein as an example. 
     The adhesive that is used in bonding the structures of device  10  such as display layers in display  14  may be thermally cured (e.g., by applying heat form a hot plate, heated member, heat gun, oven, or other heat source), may be chemically cured (e.g., by exposing the adhesive to catalyst), and/or may be cured by exposure to light such as ultraviolet light or other sources of energy (e.g., electromagnetic energy). 
       FIG. 3  is a system diagram showing how adhesive bonding operations may be performed on the structures of device  10 . As shown in  FIG. 3 , bonding (lamination) equipment  34  may receive structures to be bonded such as substrate layers  36 . Substrate layers  36  may include display layers in display  14  such as cover layer  24  and/or display module layers  32  such as a thin-film transistor layer, color filter layer, polarizer layers (e.g., an upper polarizer and/or a lower polarizer), backlight layers, organic-light-emitting diode substrate layers, etc. 
     Adhesive  38  may be used in attaching substrate layers  36 . Adhesive  38  may be liquid adhesive such as liquid optically clear adhesive. Adhesive  38  may be cured by ultraviolet light (i.e., adhesive  38  may be ultraviolet light cured adhesive), may be cured by application of heat (i.e., adhesive  38  may be thermally cured adhesive), and/or may be cured by application of catalyst (as examples). Curing operations may involve full curing to form a strong bond between respective substrate layers and/or partial curing. Partial curing may be used to render adhesive  38  tacky to facilitate assembly operations or may be used to thicken adhesive  38  (e.g., to increase the viscosity of adhesive  38  sufficiently to allow adhesive  38  to form a barrier for liquid adhesive flow or to form a protrusion that facilitates contact between a substrate layer and adhesive  38  at a predefined location). Adhesive curing operations may also be used to form structures such as peripheral adhesive dams that help laterally contain liquid adhesive and thereby prevent the formation of irregular adhesive borders. 
     Bonding equipment  34  may include computing equipment such as one or more networked computers, stand-alone computing equipment, computing equipment embedded into a bonding tool, positioner, sensor, or other equipment, or other processing and storage circuitry that serves as a controller for bonding operation. The computing equipment may serve as control circuitry that generates control signals for components in equipment  34 . The components in bonding equipment  34  may include computer-controlled positioners, shutters for blocking the passage of light, adhesive flow control structures such as computer-controlled valves, fiber positioners in a fiber-based light source, heating equipment for selectively heating adhesive  38 , equipment for applying localized and global energy to adhesive  38  (e.g., equipment for applying light, radio-frequency signals, electrostatic magnetic and/or electric fields), liquid catalyst dispensing equipment, computer-controlled adhesive dispensers (e.g., adhesive dispensers whose position is controlled by associated computer-controlled positioners), bonding stage vacuum chucks, upper and lower bonding stage plates for applying force to the upper and lower surfaces of substrates being bonded together with adhesive  38 , equipment for controlling adhesive flow, vacuum chamber equipment for creating a vacuum during vacuum lamination operations with adhesive  38 , individually controlled light sources and light modulators, and other components. 
     During adhesive bonding operations involving planar structures such as display substrate layers in display  14 , it can be challenging to avoid incorporating voids (e.g., bubbles) within adhesive  38 . As an upper substrate is brought into contact with adhesive on a lower substrate, there is an opportunity for voids to be created. In arrangements in which vacuum chamber equipment is not being used, for example, there is a potential for undesired air bubbles to be incorporated into the adhesive when substrates with planar layers of adhesive are being sandwiched together. 
     The likelihood of void formation can be minimized by creating a protrusion in the adhesive before the substrates are joined. For example, an adhesive protrusion may be formed within the adhesive on the lower substrate. As shown in  FIG. 4 , for example, adhesive application equipment  42  may be used to apply adhesive  38  to the upper surface of lower substrate  44  so that portion  38 ′ forms a protrusion that protrudes upwards from surface  46  and layer  38  in direction Z. 
     Bonding equipment  48  (sometimes referred to as lamination equipment) may be used in bonding together upper substrate  58  and lower substrate  44 . Bonding equipment  48  may include computer controlled positioners such as positioners  50  and  56 . Computer-controlled positioner  50  may be used in controlling upper bonding stage plate  52 . Computer-controlled positioner  56  may be used in controlling lower bonding stage plate  54 . Positioners  50  and  56  may be used in making 6-axis adjustments to ensure alignment between plates  52  and  54  and thereby ensure alignment between upper substrate  58  and lower substrate  44 . Vacuum chucks or other holding mechanisms may be incorporated into plates  52  and  54  to hold substrates  58  and  44  in place. As bonding stage plates  52  and  54  are brought together, adhesive protrusion  38 ′, which is higher than surrounding portions of adhesive  38 , makes initial contact with lower surface  60  of upper substrate  58 . The position and shape of protrusion  38 ′ ensures that contact will be made between protrusion  38 ′ and upper substrate  58  before other portions of adhesive  38 . As substrates  58  and  44  are brought further together to complete the bonding process, adhesive  38  is compressed between substrates  58  and  44 . Due to the presence of protrusion  38 ′, the adhesive that is being compressed flows outward from the center of substrates  44  and  58  in a predefined pattern, thereby avoiding void formation. The thinner portions of adhesive  38  that cover substrate  44  may help to pre-wet substrate  44  and thereby ensure even adhesive flow. Protrusion  38 ′ may have the shape of a pyramid, a star, an oval, a square, a double Y, or other suitable shape that ensures satisfactory adhesive coverage over the surface areas of substrates  58  and  44 . 
     As adhesive  38  is compressed in the gap between substrates  58  and  44 , peripheral portions of adhesive  38  will move laterally outward from the center of substrates  44  and  58 . To ensure that the border of adhesive  38  is well defined, it may be desirable to apply ultraviolet light to the edge of adhesive  38  (e.g., in a scenario in which adhesive  38  is ultraviolet light cured adhesive). By applying the ultraviolet light to the exposed edge of adhesive  38 , the exposed edge of adhesive  38  may be cured at a defined location, thereby forming a uniform border (e.g., by preventing excess adhesive leakage along one or more of the four edges of rectangular substrates such as substrates  44  and  58 ). 
     As shown in  FIG. 5 , fiber-based adhesive curing equipment  62  may be used to cure exposed peripheral edge  72  of adhesive layer  38  between substrates  44  and  58 . Fiber-based adhesive curing equipment  62  may include light source  64  and fiber  66 . Light source  64  may be an ultraviolet light-emitting diode, an ultraviolet light lamp, a laser, or other source of curing light. Fiber  66  may be a single mode fiber, a multimode fiber, a fiber bundle, a plastic rod, or other light guiding structure for conveying light from light source  64  to edge  72  of adhesive  38 . In the example of  FIG. 5 , fiber structure  66  has an end such as end  68  at which ultraviolet light  74  is emitted. End  68  is preferably sufficiently compact to allow end  68  to be placed adjacent to edge  72 . 
     The position of end  68  may be controlled using computer-controlled positioner  70 . For example, positioner  70  may be used to run end  68  along edge  72  of adhesive  38  during curing operations, so that the entire desired length of edge  72  is cured. As adhesive  38  cures along edge  72 , it will harden, thereby forming an adhesive dam that prevents excess liquid adhesive leakage. The use of equipment  62  to cure the edge of adhesive  38  can thereby help form a well-defined straight border for adhesive  38 . 
     If desired, end  68  of fiber  66  may be provided with lenses, prisms, gratings, or other structures to redirect light  74  in a desired direction (e.g., using light reflection, refraction and/or diffraction).  FIG. 6  is a cross-sectional side view of fiber  66  in an illustrative configuration in which end  68  of fiber  66  has been provided with a light directing structure  78  such as a prism that directs light  74  at a right angle with respect to longitudinal axis  76  of fiber  66  (i.e., a structure that directs light at a 90° angle to create a side-firing fiber structure). This type of configuration for fiber  66  may help fiber  66  apply ultraviolet light  74  to adhesive  38  in partially assembled structures where access to edge  72  might otherwise be difficult or impossible. In some situations, for example, there may only be tens or hundreds of microns of clearance between adjacent structures. Fiber-based structures may have fibers  66  that are sufficiently narrow to fit within these relatively confined spaces (e.g., spaces with openings for fiber  66  that are less than 300 microns, as an example). 
     As shown in  FIG. 7 , fiber-based equipment  62  may include a fiber bundle  82  formed from an array of individual fibers  66 . Fibers  66  may, for example, be configured so that ends  68  of fibers  66  extend along a straight line such as line  80  (i.e., fibers  66  may be configured to form a strip-shaped bundle). This allows fibers  66  to produce a strip of light  74  to cure an entire edge  72  of adhesive  38  at once. 
       FIG. 8  shows how fiber-based adhesive curing equipment  62  may be provided with components that allow the amount of light  74  that is provided at each fiber end  68  to be individually controlled. As shown in  FIG. 8 , fiber bundle  82  contains multiple fibers  66 . Each fiber  66  may receive ultraviolet light from a respective light source  64 . Optional modulators  84  may be interposed within the paths of fibers  66 . Modulators  84  may contain mechanical shutters, liquid crystal shutters, microelectromechanical system (MEMS) attenuators, or other controllable light modulators. During operation, controller  86  may provide controls signals to modulators  84  over paths  90  and/or may provide control signals to light sources  64  over paths  88 . The control signals generated by control circuitry such as controller  86  (e.g., computing equipment or other circuitry) may be used in adjusting the magnitude of light  74  that is exiting the end  68  of each fiber  66  in fiber bundle  82 . This allows controller  86  to generate a strip of output light  74  with any desired intensity profile. As an example, controller  86  may adjust light  74  so that the intensity of light  74  is evenly distributed across the ends  68  of fibers  66  in fiber bundle  82 . If desired, controller  86  may also issue control signals over control paths  92  to positioners such as positioners  70  of  FIG. 9  to control the relative position of each fiber  66  in bundle  82 . 
       FIG. 10  is a cross-sectional side view of a portion of device  10  in which fiber bundle  82  is being used to cure edge  72  of adhesive  38 . In the  FIG. 10  example, display  10  has a display module such as display module  26 . Lowermost substrate layer  58  of display module  26  may be a polymer polarizer layer (as an example). Substrate  44  may be a display cover layer (see, e.g., layer  24  of  FIG. 2 ). Housing structure  94  may be attached to substrate  44  along the edge of substrate  44 . Opaque masking material  96  such as a layer of black ink may be formed in an inactive border portion of display  14  (i.e., in a strip-shaped region  98  along the edge of layer  44 ). Housing structure  94  may be formed from an opaque material such as opaque plastic. Due to the presence of structures such as opaque masking layer  96  and housing structure  94 , it can be difficult to shine light  74  on edge  72  of adhesive  38 . By using a fiber bundle such as fiber bundle  82  of  FIG. 10  in which the ends of each fiber  66  have been provided with respective light redirecting structures  78 , fiber bundle  82  may be used to apply ultraviolet light  74  effectively to edge  72  of adhesive  38 . 
     There may be one fiber bundle  82  in fiber-based adhesive curing equipment  62  or there may be two or more fiber bundles in equipment  62 . Each fiber bundle may be configured to form a straight edge that emits light  74 . In a configuration in which the substrates that are being attached to one another have rectangular outlines with four peripheral edges, it may be desirable to use fiber bundles for curing adhesive  38  along one edge, along two edges, along three edges, or along all four of edges  72 . In the illustrative configuration of  FIG. 11 , equipment  62  has been provided with two fiber bundles  82 A and  82 B. Fiber bundle  82 A may be used in curing one edge  72  of adhesive  38  (i.e., a right-hand edge of a display) and fiber bundle  82 B may be used in curing an opposing edge  72  of adhesive  38  (i.e., the opposing left-hand edge of the display). It may be less critical to limit adhesive overflow on the top and bottom edges of the display than on the side edges of the display in this example, because the top and bottom edges may be hidden from view from a user when the display is assembled into device  10 , whereas the side edges may be more visible. 
       FIG. 12  is a diagram of an illustrative layer of adhesive  38  having a pyramidal shape to create a central raised point (point  100 ) to serve as an initial contact point when substrates  58  and  44  are brought together using bonding equipment. Adhesive shapes such as the pyramid shape of adhesive  38  of  FIG. 13  help prevent voids from being created during bonding. 
     Another illustrative adhesive shape that may be used to prevent void formation is shown in  FIG. 13 . In the illustrative configuration of  FIG. 13 , adhesive  38  has been formed with raised ridge  102 . The middle of raised ridge  102  along dimension Y is raised more than the ends of ridge  102 , creating initial contact point  100 . 
     Shapes such as the shape of  FIG. 13  and the shape of  FIG. 12  have protruding portions which create predefined contact locations between adhesive  38  and substrate  58  during bonding to prevent voids, as described in connection with protrusion  38 ′ of  FIG. 4 . These shapes are sometimes referred to as three-dimensional adhesive shapes. In general, any suitable three-dimensional adhesive shape may be used for adhesive  38  to prevent void formation. The examples of  FIGS. 4, 12, and 13  are merely illustrative. 
     Adhesive dispensing equipment for dispensing adhesive  38  in two-dimensional and three-dimensional shapes may be based on slit-coating equipment, screen printing (stencil) equipment, squeegee-based equipment, needle dispensing equipment, jet printing equipment, spin coating equipment, and physical vapor deposition equipment (as examples). Computer-controlled positioners may be used to control the position of the adhesive dispensing equipment relative to substrates  44  and  58  (e.g., to create locally thickened regions of adhesive by locally slowing movement of an adhesive dispensing nozzle, etc.). 
     A side view of illustrative slit-coating adhesive dispensing equipment  104  is shown in  FIG. 14 . Equipment  104  includes adhesive dispenser  106 . Adhesive dispenser  106  contains adhesive  38 . Computer-controlled positioner  108  may be used to move adhesive dispenser  106  relative to substrate  44 . Adhesive dispenser  106  has a nozzle with an opening (sometimes referred to as a slit) such as opening  112 . As adhesive dispenser  106  is moved in direction  110  by positioner  108 , adhesive  38  is dispensed onto the surface of substrate  44  through opening  112 . 
     As shown in the graph of  FIG. 15 , a controller may control the movement of opening  112  relative to the surface of substrate  44  by controlling movement of adhesive dispenser  106  with positioner  108  (or by controlling a stage on which substrate  44  is supported) so that the speed of opening  112  along the surface of substrate  44  varies. The speed with which opening  112  travels along the surface of substrate  44  may, for example, be slowed as opening  112  passes over the middle of substrate  44 . As shown in  FIG. 16  in which dispensed adhesive thickness is plotted as a function of linear position along the surface of substrate  44 , varying the speed of opening  112  as shown in  FIG. 15  may result in more adhesive  38  being deposited in the middle of substrate  44  than at the ends of substrate  44 . By using a nozzle opening  112  for slit dispenser  106  of  FIG. 14  that has a shape of the type shown in  FIG. 17 , an adhesive shape of the type shown in  FIG. 13  may be created. As shown in  FIG. 17 , opening  112  has a triangular upper portion  114  that can be used to create ridge  102  of  FIG. 13 . During operation, dispenser  106  may be used to move slit opening  112  over substrate  44  so that lower slit edge  112 E travels along the surface of substrate  44 . In the example of  FIG. 18 , slit opening  112  has a first triangular portion  112 A and a more sharply angled second triangular portion  112 B for creating a protruding adhesive structure on the surface of adhesive  38 . 
     Adhesive opening  112  may, if desired, be formed from multiple sub-openings such as openings  112 S of  FIG. 19 . The density of openings  112 S (i.e., the number of openings  112 S per unit area on the surface of the nozzle), the sizes of openings  112 S, and/or the shapes of openings  112 S may be varied across opening  112 . For example, openings  112 S may be configured (in density, size, and/or shape), so that more adhesive  38  is dispensed in the middle of opening  112  (in dimension X) than along the edges of opening  112 . This allows openings such as illustrative opening  112  of  FIG. 19  to form adhesive shapes with upwardly protruding portions to serve as initial substrate contact points during substrate bonding. 
     Opening  112  may have openings  112 S that are used to dispense individually controlled flows of adhesive. This type of arrangement is shown in  FIG. 20 . As shown in  FIG. 20 , adhesive reservoir structure  114  may contain multiple adhesive reservoirs such as reservoirs  114 A,  114 B, and  114 C. Reservoirs  114 A,  114 B, and  114 C may each contain a separate type of adhesive or may dispense the same type of adhesive. For example, reservoirs  114 A,  114 B, and  114 C may contain adhesive  38  of different viscosities, different chemical compositions, etc. Adhesive flow control equipment such as valves  118 A,  118 B, and  118 C may be controlled by an adhesive dispensing equipment controller. For example, valve  118 A may be used to control how much adhesive  38  from reservoir  114 A flows to nozzle opening  112 S- 1  via adhesive passageway  116 A, valve  118 B may be used to control how much adhesive  38  from reservoir  114 B flows to nozzle opening  112 S- 2  via adhesive passageway  116 B, and valve  118 C may be used to control how much adhesive  38  from reservoir  114 C flows to nozzle opening  112 S- 3  via adhesive passageway  116 C. By controlling the flow to each portion of opening  112  separately, the shape of adhesive  38  (e.g., the thickness of adhesive  38  on substrate  44 ) can be controlled in three dimensions. Openings  112  may be associated with a slit dispenser, a screen in a screen printing tool, a portion of a squeegee, needle dispensing equipment, jet printing equipment, etc. 
       FIG. 21  is a perspective view of screen printing equipment  120  having screen openings  112 S. Openings  112 S may have the same density, size, and shape when it is desired to produce a uniform layer of adhesive  38  on substrate  44 ) or may have different densities, sizes and/or shapes. Computer-controlled positioner  124  may move blade  122  across the surface of the screen in direction  122  to force adhesive  38  through openings  112 S. The pattern of openings  112 S (density, size, and/or shape) may be selected so that adhesive  38  has a desired two-dimensional or three-dimensional shape (e.g., a shape with a protruding region). As illustrated by the dashed line on blade  122 , blade  122  may, if desired, be provided with a shape that facilitates formation of a desired adhesive profile. 
       FIG. 22  shows how computer-controlled positioner  126  may be used to control the position of an adhesive dispenser such as adhesive dispensing needle  128 . One or more needle dispensers such as needle  128  may be used in dispensing adhesive  38  onto the surface of substrate  44 . The needles may have different sizes and shapes of openings to help define a desired pattern of deposited adhesive. If desired, adhesive dispensing needles such as needle  128  and other adhesive dispensing equipment may be used to introduce adhesive  38  between a pair of closely spaced substrates (i.e., needle  128  may dispense adhesive  38  into a gap between substrates  44  and  58 ). 
     Before bringing substrates  44  and  58  together during bonding, adhesive  38  may be deposited in a pattern on the surface of one or both of substrates  44 . In the example of  FIG. 23 , adhesive  38  has been deposited in a rectangular pattern. In the example of  FIG. 24 , adhesive  38  has been deposited in an oval pattern. In the  FIG. 25  example, a double Y pattern has been used when depositing adhesive  38  on the surface of the substrate. Other patterns for adhesive  38  may be used if desired. The patterns of  FIGS. 23, 24, and 25  are merely illustrative. 
     It may be desirable to deposit multiple layers of adhesive  38 . For example, it may be desirable to deposit a first layer of adhesive  38  such as a rectangular layer that covers some or all of substrate  44  to provide an initial wetting of the surface of substrate  44 , followed by deposition of a second layer of adhesive  38  to form a desired three-dimensional adhesive shape. The second layer of adhesive  38  may, for example, form protrusion  38 ′ of  FIG. 4 .  FIG. 26  is a top view of substrate  44  in a configuration in which a first layer of adhesive  38 - 1  has been covered with a second layer of adhesive  38 - 2 . The patterns used for the first and second adhesive layers of  FIG. 26  are ovals, but this is merely illustrative. The first and second adhesive layers (and, if desired, subsequent adhesive layers) may have any suitable shapes. 
     The substrates that are being joined by adhesive  38  in device  10  may be rigid layers or may be flexible. When bonding flexible layers, it may be desirable to use roller-based equipment such as bonding equipment  128  of  FIG. 27 . In the  FIG. 27  configuration, equipment  128  is being used to bond substrate  136  to substrate  138 . Substrate  136  has been coated with adhesive layer  38 . Substrate  138  may be a rigid substrate such as a layer of glass or plastic. Substrate  136  may be a flexible substrate such as a flexible sheet of polyimide or other flexible polymer sheet. During operation of equipment  128 , substrate  138  and substrate  136  may travel in direction  140 . Rollers such as rollers  130  may each rotate about a rotational axis  132  in direction  134  to help guide and bond together substrates  136  and  138 . 
       FIG. 28  is a diagram of an illustrative substrate  44  that has been coated with adhesive  38 . Light source  142  is being used to produce light  144 . Light source  142  may be a lamp or light-emitting diode that emits light  144  such as ultraviolet light. Adhesive  38  of  FIG. 28  may be ultraviolet light cured adhesive. Light source  142  may apply light  144  to cure a peripheral edge portion of adhesive  38  and thereby form an adhesive-containing adhesive dam structure. 
     In the configuration of  FIG. 28 , a strip of light  144  cures adhesive portion  38 B of adhesive  38  as the edge of adhesive  38  flows outward under light  144 . Exposure to light  144  will therefore create an adhesive dam that prevents liquid adhesive  38  from flowing further outward past light  144 . By patterning light  144  with an appropriate shape (e.g., a rectangular ring), excess flow of adhesive  38  may be prevented (i.e., light  144  may cure adhesive  38 B sufficiently to form a rectangular ring-shaped adhesive dam that prevents the remainder of liquid adhesive  38  from flowing past the rectangular ring). In the  FIG. 28  example, energy for locally curing adhesive  38  to form cured adhesive barrier  38 B along the periphery of adhesive  38  (and along the periphery of display  14  or other structure in which substrate  44  and adhesive  38  are formed) is provided in the form of ultraviolet light. In general, adhesive curing energy may be applied locally and/or globally to adhesive  38  using electrostatic fields (electrical and magnetic), using radio-frequency energy, using light, using conducted or radiated heat, or using other types of energy. The example of  FIG. 28  is merely illustrative. 
       FIG. 29  is a side view of substrate  44  showing how a surface structure such as surface structure  146  may be formed on the surface of substrate  44 . Structure  146  may have the shape of a rectangular ring or other shape that serves as an adhesive dam for preventing excess outward movement in direction  148  of adhesive  38 . Structure  146  may be formed from a hydrophobic (moisture-repelling) material (as shown in  FIG. 29 ), may be formed from a hydrophilic material, may be formed by depositing and patterning material on the surface of substrate  44 , may be formed by surface treatment of the surface of substrate  44  using light, ion bombardment, surface roughening, may be formed by patterning a catalyst onto the surface of substrate  44 , or may be formed from other patterned materials on substrate  44 . 
       FIG. 30  is a cross-sectional side view of substrates  44  and  58  in a configuration in which bonding stage  120  has been provided with structures  150  on plates  52  and/or  54 . Structures  150  may be used to provide localized energy to adhesive  38  (e.g., static electric fields, static magnetic fields, radio-frequency signals, heat, etc.). For example, structures  150  may be metal electrodes for applying an electrostatic field that causes adhesive  38  to protrude sufficiently to form protrusion  38 ′ before plates  52  and  54  of bonding stage equipment  48  press upper substrate  58  downwards in direction  152  against lower substrate  44 . The energy that is applied to the localized portion of adhesive layer  38  may adjust the viscosity of layer  38 , the amount of curing (polymer cross-linking) in layer  38 , the temperature of layer  38 , and other physical parameters that may affect the point of contact between layer  38  and the way in which layer  38  flows and coats the surfaces of substrates  44  and  58 . Structures such as structures  150  may also be used in applying energy globally (e.g., to all of the surfaces of substrates  44  and/or  58 ). 
       FIG. 31  is a flow chart of illustrative steps involved in bonding substrates  44  and  58  using localized energy deposition operations with structures such as structures  150  of bonding stage equipment  48  of  FIG. 30 . At step  154 , substrates  44  and/or  58  may be subjected to surface treatment (e.g., cleaning). Adhesive  38  may then be deposited to the surface of substrate  44 , the surface of substrate  58 , or to the surfaces of both substrates  44  and  58 . Adhesive  38  may be applied using a nozzle or other adhesive dispenser in one or more layers using computer-controlled adhesive dispenser equipment. 
     At step  156 , energy may be locally applied to deposited adhesive  38  as described in connection with  FIG. 30 . For example, light, heat, or static or dynamic electromagnetic signals, may be applied to the boundary of adhesive  38  to form an adhesive dam from portions of adhesive  38  and thereby laterally confine adhesive  38  during bonding. If desired, the localized surface treatments and/or structures such as structures  146  of  FIG. 29  may be used to laterally confine adhesive  38 . Localized energy may also be applied to create adhesive protrusions to serve as predefined substrate contact points during substrate bonding. 
     At step  158 , substrates  58  and  44  may be bonded together using bonding stage equipment  48 . If desired, structures  150  or other structures may be used to apply localized energy to adhesive  38  as part of the bonding process (e.g., to help form protrusion  38 ′ by electrostatic attraction, etc.). 
     Light masking equipment such as equipment  160  of  FIG. 32  may be used in defining a pattern of applied light during precuring operations such as the operations of step  156  and/or during bonding operations such as the operations of step  158  of  FIG. 31 . As shown in  FIG. 32 , equipment  160  may include a light source such as light source  162 . Light source  162  may be an ultraviolet light source such as an ultraviolet lamp, ultraviolet light-emitting diode source, or ultraviolet laser. Light source  162  may provide uniform ultraviolet light  166  to mask  164 . Mask  164  contains transparent and opaque regions and, if desired, regions with a gradient in density (e.g., graded density areas that vary from dark to light across a range of grey tones). The graded density regions may include continuous zones of gray or may contain discrete clear and dark regions with varying densities to create a transmission gradient. In the example of  FIG. 32 , mask  164  has a central portion that is more opaque than its edge portions, so transmitted light  166 C in the center of mask  164  is less bright than transmitted light  166 E near the edge of mask  164 . A transmission gradient (i.e., a mask opacity gradient) may be provided in mask  164  between the edge and center regions, so that transmitted light  166 M that lies between center light  166 C and edge light  166 E has an intensity that is greater than the light intensity of transmitted light  166 C and that is less than the intensity of transmitted light  166 E. 
     By using a mask such as mask  164  of  FIG. 32 , adhesive  38  may be exposed to desired local and/or global patterns of light  166 . The applied light may be used in modifying the properties of adhesive  38  in a desired pattern (e.g., by curing and thereby cross-linking the polymer of layer  38  to change its viscosity, etc.). 
     A graph showing how the density of mask  164  may vary as a function of distance R from its center towards its edge is shown in  FIG. 33 . Solid line  168  corresponds to a pattern in which the edge regions of adhesive  38  are being heavily exposed to ultraviolet light (e.g., to create an adhesive dam by curing the outer periphery of adhesive  38 ) whereas the central portion of adhesive  38  is being left unexposed (e.g., to ensure low viscosity so that adhesive  38  can flow over the surface of substrate  44  satisfactorily). Dashed line  170  is an illustrative mask gradient that may be used where a more gradual transition between exposed and unexposed portions of adhesive  38  is desired. 
     Adhesive  38  may be deposited in one or more layers. As an example, adhesive  38  may include an upper layer such a layer  38 A and a lower layer such as layer  38 B, as shown in  FIG. 34 . The properties of each sublayer of adhesive that is used in bonding substrates  44  and  58  together may be tailored to accommodate different bonding requirements. For example, in a configuration in which upper substrate  58  is a display cover layer formed from a hard material such as glass and in which lower substrate  44  is a relatively soft polymer layer such as an upper polarizer in a liquid crystal display module, it may be desirable to form adhesive layer  38 A of a material that cures to a harder state than that of adhesive layer  38 B. When cured, adhesive layer  38 A in this type of configuration will be compatible with adjacent hard layer  58  (e.g., glass) and adhesive layer  38 B will be compatible with adjacent softer layer  44  (e.g., plastic). By choosing adhesives that match adjacent substrate layers, delamination of adhesive and bond failures can be minimized. In the configuration of  FIG. 35 , three layers of adhesive  38  have been provided. Layer  38 C in the  FIG. 35  configuration may serve as an interface between layers  38 A and  38 B (e.g., layer  38 C may be formed from a material that forms satisfactory bonds with both layer  38 A and layer  38 B). 
       FIG. 36  is a flow chart of illustrative steps involved in bonding substrates  44  and  58  using one or more layers of adhesive  38 . At step  172 , adhesive application equipment may be used to dispense a layer of adhesive  38  (e.g., by applying adhesive to substrate  44  and/or substrate  58 ). The layer of deposited adhesive may be applied globally and/or may be patterned using slit dispensing, needle dispensing, squeegee dispensing, jet dispensing, dispensing techniques with individually controlled nozzle openings, adhesive deposition techniques using one or more passes of a dispensing head across the surface of a substrate, adhesive deposition techniques involving nozzle speed and/or flow control adjustment to adjust adhesive thickness, etc. 
     At step  174 , energy may be locally and/or globally applied to the deposited adhesive  38 . For example, light, electrostatic fields, radio-frequency signals, heat, or other forms of energy may be applied using masks, scanning stages, lenses, waveguides, electrodes, etc. Catalyst may also be locally and/or globally applied. If more layers are to be applied, processing may loop back to step  172 , as illustrated schematically by line  176 . 
     Once all desired layers of adhesive  38  have been deposited, processing may proceed to step  178 . During the operations of step  178 , bonding stages  48  may be used to complete the bonding process by pressing substrates  58  and  44  together. Local and/or global energy may be applied to adhesive  38  to help form well defined adhesive borders, to facilitate curing, etc. 
       FIG. 37  is a top view of an illustrative substrate  44  on which a layer of adhesive  38  has been deposited. By applying energy in the form of ultraviolet light or by otherwise creating adhesive flow barrier  38 R, outward adhesive flow may be controlled and a well-defined rectangular border for adhesive  38  may be created. Region  38 C may contain protruding adhesive  38 C for forming an initial contact point with upper substrate  58 . Region  38 C may be created using localized application of energy, using multiple passes of adhesive application equipment  42  to deposit an upper adhesive layer on a lower adhesive layer, or by using other application techniques for dispensing adhesive  38  in a three-dimensional pattern. 
     In an illustrative two-pass configuration, adhesive application equipment  42  ( FIG. 4 ) makes a first pass to deposit blanket lower adhesive layer  38 BL on substrate  44  ( FIG. 38 ). Adhesive application equipment  42  ( FIG. 4 ) may then make a second pass to deposit patterned adhesive  38 PT. During each pass of adhesive application equipment  42 , computer-controlled positioning equipment may move an adhesive dispensing nozzle such as nozzle  106  across the surface of substrate  44  (while optionally varying nozzle speed to locally thicken the adhesive), may use a screen printing tool, may use a jet printer, may use a squeegee, may use needle adhesive dispensing equipment, etc. 
     As shown in  FIG. 40  adhesive  38  may be deposited in the form of two layers such as lower adhesive layer  38 L and upper adhesive layer  38 U. Upper adhesive layer  38 U may have a footprint that lies within the footprint of lower adhesive layer  38 L (when viewed in direction  180 ). Adhesive  38 L may be more viscous than adhesive  38 U. The pattern of adhesive  38 U may be defined by application of a rectangle of ultraviolet light, use of adhesive dispensing equipment that creates a rectangular pattern for adhesive  38 L, by depositing adhesive using a rectangular adhesive dam, or using other adhesive patterning techniques. During bonding of upper substrate  58  to lower substrate  44 , upper substrate  58  may be pressed downwards in direction  180 . This may cause upper adhesive  38 U to flow outwards in directions  184  until each of the four peripheral edges  182  of upper adhesive  38 U are aligned with each of the four peripheral edges  186  of adhesive layer  38 L. Surface tension may help prevent adhesive  38 U from overflowing edges  186 , so that the final footprints of upper layer  38 U and lower layer  38 L will match. 
     In the illustrative configuration of  FIGS. 42 and 43 , adhesive  38 U is initially deposited so that the outline of adhesive  38 U lies within the outline of adhesive  38 L ( FIG. 42 ). When upper substrate  58  is pressed downwards in direction  180  during bonding, adhesive  38 U flows outwards and downwards in directions  188  so that edges  186  of lower adhesive layer  38 L are covered by portions of upper adhesive layer  38 U. When bonding is complete, edges  182  of upper adhesive layer  38 U will lie outside of edges  186  of lower adhesive layer. This may cause upper adhesive layer  38 U to have a larger footprint than the footprint of lower adhesive layer  38 L (i.e., the footprint of upper adhesive layer  38 U will overlap the footprint of lower adhesive layer  38 L). If desired, ultraviolet light may be applied to the edge of adhesive  38 U to create a well-defined border for adhesive  38 U. Adhesives  38 U and  38 L may be selected to ensure compatibility with adjacent substrates  58  and  44 , respectively. 
       FIG. 44  is a system diagram of light-based equipment that produces adhesive dam structures with smoothly varying edges. As shown in  FIG. 44 , light source  190  may produce light  192 . Light source  190  may be an ultraviolet lamp, an ultraviolet laser, an ultraviolet light-emitting diode source, or other light source for producing light  192 . Light source  190  may contain an array of individually controlled light producing elements or other structures for producing a gradually changing light intensity pattern in emitted light  192 . For example, light source  190  may contain a halftone mask (e.g., a mask using pixilated metal patterns), may contain a gradient mask with continuously varying opacity, may contain an array of individually controlled light producing cells, may contain an array of individually controlled light modulators, may contain shutters, or may contain other structures for varying the intensity of emitted light  192  as a function of lateral position across the surface of substrate  44 . 
     Adhesive  38  may be used to form an adhesive dam. In areas of adhesive  38  that are exposed to the full strength of light  192 , adhesive  38  will cure to form a rigid dam structure (e.g., in the shape of a rectangular ring). To avoid creating an unsightly line along the inner edge of the adhesive dam (where the adhesive dam is joined by liquid adhesive), the intensity of light  192  may be gradually tapered off. For example, light  192  may be decreased in intensity as a function of increasing distance towards the center of substrate  44 , thereby creating a gradient in light intensity  192  for inner peripheral edge region  381  of adhesive ring  38 . As shown in the cross-sectional side view of  FIG. 45 , liquid adhesive  380  may be deposited in the center of the adhesive dam formed by adhesive  38  following exposure of adhesive  38  to light  192 . Inner edge  194  of the adhesive dam formed from adhesive  38  has a tapered shape with a smoothly varying height, rather than a sharp vertical wall shape, so adhesive  380  forms a gradual interface with adhesive dam  38  and the visibility of edge  194  of adhesive dam  38  is reduced sufficiently to be unnoticeable to a user. 
     Illustrative steps involved in forming adhesive dams with gradually tapered inner edges to reduce edge visibility are shown in  FIG. 46 . 
     At step  195 , adhesive  38  is deposited in the shape of an adhesive dam. For example, when it is desired to bond two rectangular substrates such as two display layers in display  14  of device  10 , a bead of adhesive  38  may be dispensed in the shape of a rectangular ring running around the rectangular periphery of the display using a needle dispenser or other adhesive dispenser. 
     At step  196 , light  192  with an intensity gradient for forming a tapered inner dam surface may be applied to the deposited adhesive. For example, light  192  may be applied to the deposited rectangular ring of adhesive with inwardly decreasing intensity so that the inner edge of the adhesive dam is cured with a correspondingly decreasing gradient. 
     At step  198 , liquid adhesive is deposited within the center of the rectangular ring-shaped adhesive dam formed from the cured portions of adhesive  38 . Because of the gradient in light intensity used for the curing light applied to adhesive dam  38 , the inner peripheral surface of the rectangular ring-shaped adhesive dam has an edge that is not abrupt (e.g., an edge with a tapered thickness). After substrates  58  and  44  are bonded together, the non-abrupt nature of the interface between the adhesive dam and the liquid adhesive that is contained by the dam (i.e., the formation of an inner edge for the adhesive dam that is less abrupt than the outer edge) will ensure that the inner edge of the adhesive dam is not readily visible. 
       FIG. 47  is a top view of a system in which movable shutters have been used to form an exposure gradient when curing a rectangular adhesive dam to form an adhesive dam with a tapered inner edge. As shown in  FIG. 47 , adhesive on substrate  44  includes an adhesive dam formed from outer rectangular ring portion  202  (which is more heavily cured) and inner ring portion  204  (which is less heavily cured and is therefore tapered or otherwise forms an inner edge that is not abrupt). Adhesive  206  fills the rectangular interior of the adhesive dam. Due to the lower exposure of adhesive ring  204  relative to adhesive ring  202  and the resulting gradient formed in the inner edge of the adhesive ring, the adhesive ring does not form a sharp interface with adhesive  206 . The example of  FIG. 47  includes an outer ring that has been exposed to a first amount of curing light and an inner ring that has been exposed to a second amount of curing light that is less than the first amount of curing light. Configurations in which an adhesive dam has three or more different amounts of exposure or continuously varying levels of exposure may be used if desired. 
     The exposure variations of the adhesive dam of  FIG. 47  may be achieved using a rectangular shutter with an adjustable outer periphery. This type of arrangement is shown in  FIGS. 48 and 49 .  FIG. 48  shows how a rectangular shutter may have a rectangular outline that has been adjusted to form a first footprint. The shutter of  FIG. 48  has four movable blades  208 . Blades  208  may be formed from an opaque material such as metal that blocks ultraviolet light. When it is desired to change the size of the shutter, blades  208  may be moved outward in directions  210 . Blades  208  overlap in the center of the shutter, so that the center of the shutter is never opened (in this example). 
       FIG. 49  is a top view of the rectangular shutter of  FIG. 48  following expansion of the size of the shutter from initial size  212  (i.e., an outline produced when shutter blades  208  are in the position of  FIG. 48 ) to final size  214 . Using a rectangular shutter with an adjustable size, rectangular ring-shaped adhesive dams with inner edges of the type described in connection with  FIG. 47  may be formed. During exposure of the adhesive of the adhesive dam to ultraviolet light, the size of the rectangular shutter may be adjusted in steps or continuously, thereby exposing the inner edge of the adhesive dam to light with a gradient in intensity and creating a cured inner edge to the adhesive dam with a tapered shape or other non-abrupt shape. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20140218
Publication Date: 20161115
Grant Date: 20161115
Priority Date: 20130301
Inventors: LIU CYRUS Y.
SUNG KUO-HUA
CHEN PO-JUI
GRESPAN SILVIO
Assignee: APPLE INC
CPC Classifications: [{"code": "B32B2457/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J2205/31", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B2457/20", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F2202/28", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B5/26", "inventive": true, "first": false, "tree": "[]"}, {"code": "F16B11/006", "inventive": true, "first": true, "tree": "[]"}, {"code": "B32B7/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y10T156/10", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J5/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "F16B11/006", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F2202/28", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B7/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y10T156/10", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J2301/416", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B2457/20", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J5/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B5/26", "inventive": true, "first": false, "tree": "[]"}, {"code": "C09J5/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B5/26", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B2457/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B2457/20", "inventive": false, "first": false, "tree": "[]"}, {"code": "F16B11/006", "inventive": true, "first": true, "tree": "[]"}, {"code": "B32B2457/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F2202/28", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B7/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y10T156/10", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J2301/416", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 51420333