Patent Publication Number: US-2022216386-A1

Title: Structures and methods for electrically connecting printed horizontal components

Description:
PRIORITY APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 63/050,732, filed on Jul. 10, 2020, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to structures and methods for electrically connecting devices (e.g., printed devices) to destination substrates. 
     BACKGROUND 
     Electronic systems typically comprise a substrate, for example a backplane, such as a printed circuit board, on which are assembled electronic components such as integrated circuits, resistors capacitors, inductors, and connectors. The electronic components can be surface mount devices (SMDs) that are typically placed on the backplane together with solder bumps using mechanical pick-and-place equipment and then heated to reflow the solder, thereby adhering the electronic components to the backplane and electrically connecting the electronic components to contact pads or other electrical conductors on the backplane. At present, the smallest surface mount components have dimensions of 600 μm by 300 μm and, for very small and simple electronic devices such as resistors, dimensions of 400 μm by 200 μm. The size of the electrical connection and spacing between contact pads or pins of the electronic devices likewise has a lower limit, for example small solder bumps can have a diameter of 75-150 μm and in extreme cases, solder bumps as small as 30 μm in diameter have been tried. However, there is a demand for increasing electronic system miniaturization with even smaller electronic components and electrical connections. 
     Methods for transferring active small components, for example components having a size less than the smallest surface mount devices, from one substrate to another are described in U.S. Pat. No. 7,943,491. In examples of these approaches, small integrated circuits are formed on a native semiconductor source wafer. The small unpackaged integrated circuits, or chiplets, are released from the native source wafer by etching a layer formed beneath the circuits. A PDMS stamp is pressed against the native source wafer and the process side of the chiplets is adhered to individual stamp posts. The chiplets are removed from the native source wafer and pressed against a destination substrate or backplane with the stamp to adhere the chiplets to the destination substrate. 
     In other examples, U.S. Pat. No. 8,722,458 teaches transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate or backplane. The chiplets are then electrically connected using conventional photolithographic methods (e.g., forming patterned metal wires using blanket metal evaporation and photoresist coating, patterned mask exposure, cure, pattern-wise etching, and photoresist stripping). However, these steps can be slow, complex, and relatively expensive for certain applications. 
     There remains a need, therefore, for disposing small electronic components on a backplane (or other substrates) and electrically connecting the small electronic components to conductors (e.g., contact pads or wires) formed on the backplane. 
     SUMMARY 
     The present disclosure provides, inter alia, structures, materials, and methods that provide electrical connections between small electronic components (e.g., having at least one of a length and a width no greater than 200 μm) disposed on a substrate, for example by transfer printing (e.g., micro-transfer printing). 
     According to some embodiments of the present disclosure, a printed structure comprises a device comprising device electrical contacts disposed on a common side of the device and a substrate non-native to the device comprising substrate electrical contacts disposed on a surface of the substrate. At least one of the substrate electrical contacts has a rounded shape. The device electrical contacts are in physical and electrical contact with the corresponding substrate electrical contacts (e.g., each of the device electrical contacts is in contact with a corresponding substrate electrical contact of the substrate electrical contacts). The rounded shape can be at least a portion of a sphere, a portion of a hemisphere, or have one or more side walls with a first curvature and a top with a second curve that has a larger curvature than the first curve, e.g., a flattened top on an opposite side of the rounded shape from the substrate. 
     According to some embodiments, the device electrical contacts are substantially planar and are disposed in a common plane. According to some embodiments, the device electrical contacts are substantially planar and are disposed in different planes (e.g., each in its own plane). Each of the at least one of the substrate electrical contacts can conform to the shape of a corresponding device electrical contact of the device electrical contacts and a contact electrical conductor on a substrate electrical contact can wick along the device electrical contact. 
     According to some embodiments of the present disclosure, at least one of the substrate electrical contacts having a rounded shape comprises a polymer core coated with a contact electrical conductor on a surface of the polymer core. The polymer core can be compliant, conformal, flexible, or reflowable. The polymer core can be soft cured, reflowed, and then hard cured. The polymer core can comprise an electrically conductive polymer. The contact electrical conductor can be an electrically conductive surface layer that comprises a metal, a metal alloy, a solder, a transparent conductive oxide, or an electrically conductive polymer. The contact electrical conductor can be reflowable and can wick along another electrically conductive surface. The contact electrical conductor comprising a conductive surface layer can have a thickness no more than 25% of a lateral extent of the polymer core over the surface of the substrate. The conductive surface layer can have a thickness of no more than 250 nm. The contact electrical conductor can be, is, or has been wicked along the device electrical contact. According to some embodiments, each of the at least one of the substrate electrical contacts has a lateral extent over the substrate of no more than 10 μm. 
     According to some embodiments of the present disclosure, the device comprises one or more active layers through which current flows when current is provided from the substrate electrical contacts through the device electrical contacts. According to some embodiments, the device is tilted with respect to the destination substrate. According to some embodiments, the substrate electrical contacts are at least partially transparent to visible light or light emitted by the device, e.g., at least 50 percent transparent. According to some embodiments, the device is a light-emitting diode, an inorganic light-emitting diode, or an organic light-emitting diode. According to some embodiments, the device has a surface of a second side opposite the common side and the surface of the second side is roughened. 
     According to some embodiments of the present disclosure, the substrate is an intermediate substrate and the printed structure further comprises a system substrate comprising substrate conductors disposed on or in the system substrate. The device is electrically connected to the substrate conductors through the substrate electrical contacts. Printed structures of the present disclosure can comprise a second device comprising second device electrical contacts disposed on a common side of the second device, wherein the substrate further comprises second substrate electrical contacts disposed on the surface of the substrate and wherein at least one of the second substrate electrical contacts has a rounded shape and each of the second device electrical contacts is in electrical and physical contact with one of the second substrate electrical contacts. The substrate electrical contacts can be electrically connected to one or more substrate conductors disposed in or on the substrate. 
     According to some embodiments of the present disclosure, the device has at least one of a width, a length, and a thickness of no more than 100 μm, or the device has a length to width ratio of at least 1:1, 2:1, or 4:1, and/or the device has a length and/or width to thickness aspect ratio of at least 1:1, 2:1, 5:1, or 10:1. 
     According to some embodiments of the present disclosure, printed structures can comprise an adhesive disposed between the device and the substrate that adheres the device to the substrate. The adhesive can have a higher Young&#39;s modulus than at least one of the substrate electrical contacts. The adhesive can be cured, soft cured, or hard cured. The adhesive can be or can be in an unpatterned adhesive layer. The adhesive can be a patterned layer. The adhesive can have a thickness less than a thickness of at least one of the substrate electrical contacts such that each of the at least one of the substrate electrical contacts protrudes above the adhesive. At least one of the substrate electrical contacts can have a thickness or height that is greater than a distance between surfaces of ones of the device electrical contacts in a direction orthogonal to at least one of the surfaces. In such embodiments, ones of the device electrical contacts can be disposed in different planes. 
     According to some embodiments of the present disclosure, all of the substrate electrical contacts have rounded shapes. According to some embodiments of the present disclosure, the at least one of the substrate electrical contacts is a plurality of the substrate electrical contacts and the plurality comprises ones of the substrate contacts having different heights or sizes (e.g., thicknesses). According to some embodiments of the present disclosure, the device is an unpackaged bare die. 
     According to embodiments of the present disclosure, a method of making electrical connections comprises providing a substrate comprising substrate electrical contacts disposed on a surface of the substrate, wherein at least one of the substrate electrical contacts has a rounded shape, providing a device comprising device electrical contacts disposed on a common side of the device, and printing the device to the destination substrate such that each of the device electrical contacts is in electrical contact with one of the substrate electrical contacts. 
     Providing the substrate comprising substrate electrical contacts disposed on a surface of the destination substrate can comprise providing a substrate, patterning a polymer on the substrate, heating (e.g., reflowing) the patterned polymer to form one or more rounded shapes of the polymer, and coating (and optionally patterning) the one or more rounded shapes with a conductive material, wherein each of the at least one of the substrate electrical contacts comprises one of the one or more rounded shapes coated with the conductive material to form a conductive surface layer that is a contact electrical conductor. According to some embodiments, coating the one or more rounded shapes with the conductive material comprises depositing a layer of the conductive material and patterning the layer such that the one or more rounded shapes remain coated with the conductive material. 
     According to some embodiments, methods of the present disclosure comprise disposing adhesive on the destination substrate prior to printing the device, wherein printing the device comprises contacting the device to the adhesive. 
     According to some embodiments, methods of the present disclosure comprise curing the adhesive after the device has been printed. 
     According to some embodiments, methods of the present disclosure comprise baking the device and the destination substrate. 
     According to some embodiments, methods of the present disclosure comprise reflowing the conductive material. 
     According to some embodiments, methods of the present disclosure comprise reflowing the rounded shape. 
     According to some embodiments, methods of the present disclosure comprise conforming the substrate electrical contact to the device electrical contact. 
     Embodiments of the present disclosure provide methods and structures for electrically connecting small electronic devices to a substrate using efficient and inexpensive manufacturing techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, aspects, features, and advantages of the present disclosure will become more apparent and better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a cross section of a device comprising device electrical contacts in a common plane printed onto a substrate with rounded contacts, according to illustrative embodiments of the present disclosure; 
         FIG. 2  is a cross section of a tilted horizontal LED comprising device electrical contacts in different planes printed onto a substrate with rounded contacts, according to illustrative embodiments of the present disclosure; 
         FIG. 3  is a cross section of a horizontal LED with a roughened surface and comprising device electrical contacts in different planes printed onto a substrate with rounded contacts of different sizes, according to illustrative embodiments of the present disclosure; 
         FIG. 4  is a cross section of multiple horizontal LEDs with roughened surfaces printed onto a substrate in contact with two rounded contacts where the devices are tilted, according to illustrative embodiments of the present disclosure; 
         FIGS. 5A and 5B  are each a cross section of an LED printed onto a substrate in contact with one rounded contact and one planar contact, according to illustrative embodiments of the present disclosure; 
         FIGS. 6-8  are flow diagrams of methods for printing and electrically connecting a device, according to illustrative embodiments of the present disclosure; 
         FIGS. 9A-9H  are successive cross sections illustrating methods of making a printed structure according to illustrative embodiments of the present disclosure; 
         FIGS. 10A-10D  are successive cross sections illustrating methods of adhering a printed device to make a printed structure according to illustrative embodiments of the present disclosure; 
         FIG. 11  is a cross section of an LED printed onto a substrate in contact with a conformal rounded contact according to illustrative embodiments of the present disclosure; 
         FIG. 12  is plan schematic view of a display comprising printed structures according to illustrative embodiments of the present disclosure; 
         FIG. 13  is a cross section of a pixel with a pixel substrate according to illustrative embodiments of the present disclosure; 
         FIG. 14  is a cross section of pixels of  FIG. 13  on a display substrate according to illustrative embodiments of the present disclosure; and 
         FIG. 15  is a plan view of a display comprising pixels according to illustrative embodiments of the present disclosure. 
     
    
    
     Features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The figures are not necessarily drawn to scale. 
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
     Structures and methods of embodiments of the present disclosure enable electrically connecting printed devices, such as light-emitting devices including inorganic light-emitting diodes, to substrate electrical contacts disposed on a substrate. Each device likewise comprises a device electrical contact. One or more substrate electrical contacts can comprise a heat-reflowable material (such as a polymer, resin, epoxy, or soft metal, for example) disposed on and protruding from a surface of a substrate and coated with a surface conductive layer and patterned to form a contact electrical conductor. Substrate electrical contacts can have a rounded shape (e.g., can be bumps, can be hemispherical, can have other rounded shapes, or can have contact angles less than 180 degrees) and can be formed by heating (e.g., reflowing) a patterned layer of polymer so that surface energy (surface tension or capillary) forces form rounded shapes at a high resolution to which devices can be printed and electrically connected without using photolithographic methods to form electrical connections between the device and the substrate. In particular, according to some embodiments of the present disclosure, photolithographic processing to form electrical connections between a substrate and a printed device can be unnecessary (reducing photolithographic processing steps) and problems with forming electrical connections over the device edge (e.g., a step height) are avoided. 
     The patterned electrical conductor coating (the contact electrical conductor) can also be heat reflowable (e.g., comprise a solder). A device can be printed to a substrate (e.g., using an elastomeric stamp to micro-transfer print the device, for example as described in U.S. patent application Ser. No. 16/532,591, filed Aug. 6, 2019, the disclosure of which is hereby incorporated by reference herein in its entirety) with each device electrical contact in electrical (e.g., and physical) contact with a corresponding substrate electrical contact. After the device is printed, substrate electrical contacts can be heated, for example by heating the substrate. The heat can cause the heat-reflowable material to flow, which can cause substrate electrical contact(s) to conform with the shape of the device electrical contact(s) and the contact electrical conductor to wick along the device electrical contact(s) to improve electrical contact by increasing the contact area between the device electrical contact and the substrate electrical contact and to firmly adhere the device to the substrate. Thus, in some embodiments, a substrate electrical contact is conformable and conforms to a device electrical contact. In some embodiments, the substrate electrical contact can be cured after conforming to the device electrical contact. A substrate electrical contact can be compliant. The printed structure can then be cooled, for example to room temperature, integrated into an electrical system, and operated. 
     As shown in  FIG. 1 , a printed structure  99  comprises a device  20  comprising device electrical contacts  22  disposed on a common side  23  of device  20  and a substrate  10  non-native to device  20  comprising substrate electrical contacts  30  disposed on a surface  11  of substrate  10 . At least one of substrate electrical contacts  30  has a rounded shape. Common side  23  of device  20  can be adjacent to surface  11  so that device electrical contacts  22  face substrate electrical contacts  30 . Device electrical contacts  22  are in physical and electrical contact with corresponding substrate electrical contacts  30 . 
     Device  20  can be any functional device, for example an electrical or optical device such as an integrated circuit and can comprise active components (e.g., transistors, diodes, inorganic light-emitting diodes, organic light-emitting diodes, or sensors) and passive components (e.g., electrical conductors such as wires, optical conductors such as light pipes, electrical resistors, electrical capacitors, and electrical inductors). Device  20  can be electrically insulated by patterned dielectric structure  24  that exposes device electrical contacts  22 . Device  20  can comprise one or more active layers (e.g., semiconductor layers, for example forming one or more quantum wells) through which electrical current flows when electrical current is provided by device electrical contacts  22 . Device  20  can comprise one or more of a device substrate (e.g., a dielectric substrate or a semiconductor substrate), patterned dielectric structures  24 , and semiconductor structures or layers (e.g., silicon or compound semiconductors). Device  20  can comprise a semiconductor component disposed on a dielectric device substrate or a semiconductor layer of device  20  can comprise a device substrate. Device  20  can comprise multiple components, such as optical or electrical components or a combination of one or more optical and one or more electrical components (e.g., an LED and a controller). Device  20  can be or comprise one or more of one or more inorganic or organic light-emitting diodes, one or more control circuits, and one or more electrical conductors. Device  20  can comprise a silicon device substrate with a control circuit disposed in or on the silicon device substrate and light-emitting diodes or other compound semiconductor devices disposed on the silicon device substrate, for example by micro-transfer printing, that can be controlled by the control circuit. In some embodiments, a control circuit is disposed on a dielectric device substrate, for example by micro-transfer printing. 
     Embodiments of the present disclosure can be applied to high-resolution and dense micro-circuits with small devices  20 . According to some embodiments, device  20  is a micro-device and has at least one of a length and a width no greater than  200  μm (e.g., no greater than 100 μm, no greater than 50 μm, no greater than 20 μm, no greater than 10 μm, or no greater than 5 μm). Device  20  can have various aspect ratios, for example (i) a length to width and/or (ii) a length and/or width to thickness aspect ratio of at least 1:1, at least 2:1, at least 4:1, at least 8:1, or at least 10:1. 
     Device  20  can be a flip chip, with active components provided on common side  23  and disposed in an inverted configuration with respect to substrate  10 . In some embodiments, device  20  comprises active components provided on a side of device  20  opposite common side  23 . Active components can be connected to device electrical contacts  22  using, for example vias such as through-silicon vias (TSVs). Active components in device  20  can be formed in or on a surface of a semiconductor device substrate or disposed on a semiconductor or dielectric device substrate. Components can be disposed on device  20  substrate by micro-transfer printing and can comprise fractured or separated component tethers as a consequence of transfer printing (not shown). Moreover, devices  20  can be micro-transfer printed onto substrate  10 , for example using elastomeric stamps, and can comprise a fractured or separated device tether  26  as a consequence of transfer printing. 
     Device  20  can have two or more device electrical contacts  22 . The two or more device electrical contacts  22  can be substantially planar and can be disposed in a common plane on common side  23 , for example as shown in  FIG. 1 , and can be electrically connected to components in device  20 . Device electrical contacts  22  can comprise a patterned metal contact pad or patterned transparent conductive oxide contact pad. Device  20 , device electrical contacts  22 , and dielectric structures  24  can be constructed using photolithographic methods and materials, for example as found in the integrated circuit and display industries. 
     Device electrical contacts  22  and substrate conductors  12  (e.g., wires) can comprise metals such as aluminum, gold, or silver or combinations (e.g., alloys) thereof and can be deposited, for example by evaporation or sputtering, and, in some embodiments, patterned using pattern-wise exposed, cured, and etched photoresists, e.g., constructed using photolithographic methods and materials, imprinting methods and materials, or inkjet printers and materials, for example comprising cured conductive inks deposited on a surface or provided in micro-channels. 
     Substrate  10  can be any suitable substrate, for example comprising any one or more of glass, polymer, ceramic, sapphire, quartz, or a semiconductor, having surface  11  suitable for forming substrate electrical contacts  30  and contacting device  20 , for example a substantially planar surface (excluding rounded substrate electrical contacts  30 ) within the limitations of a manufacturing process. Substrate  10  can be a backplane, a pixel substrate, a display substrate, or a printed circuit board. Substrate  10  can be patterned with substrate conductors  12 , such as wires or traces, that for example, may be electrically connected and disposed to be used to conduct electrical current, ground, or electrical control signals. Conductors  12  can be disposed in or on substrate  10 . Substrate  10  can be constructed using methods and materials known in the integrated circuit and display industries. 
     Substrate electrical contact  30  can have a rounded shape that is at least a portion of a sphere, ellipsoid, teardrop, or hemisphere, a solid with an oval or elliptical cross section, that has no sharp angles or corners, or that has one or more side walls with a first curvature and a top with a second curve that has a larger curvature than the first curvature (e.g., a flattened sphere with a flat or flatter portion disposed on the top of substrate electrical contact  30 ). When substrate electrical contact  30  has a rounded shape, substrate electrical contact  30  is proud of (e.g., is above, protrudes from, or extends from) substrate  10  and can have a contact angle with respect to substrate  10  that is no greater than 180 degrees. According to some embodiments, at least one of substrate electrical contacts  30  conforms to the shape of a corresponding device electrical contact  22 . For example, a flat portion of substrate electrical contact  30  can conform to and follow the contours of a portion of a substantially planar device electrical contact  22 . 
     According to some embodiments of the present disclosure, and as shown in  FIG. 1 , at least one of the rounded substrate electrical contacts  30  comprises a polymer core  32  coated with a patterned conductive surface layer forming a contact electrical conductor  34 . Polymer core  32  can be rigid or can be compliant, conformal, flexible, or reflowable. For example, polymer core  32  can be a partially cured, compliant polymer (or a softened polymer, such as one at a temperature above its glass transition temperature) that is placed into contact with device  20  and device electrical contacts  22  to conform substrate electrical contacts  30  to device electrical contacts  22  and then subsequently hard-cured to form a rigid polymer core  32  that conforms to device electrical contact  22 . Polymer core  32  can comprise an electrically conductive polymer (e.g., polythiophene or other electrically conductive polymers) and can conduct electrical current. Polymer core  32  can be photoactive and patternable using photolithographic methods and materials. Polymer core  32  can be a thermoplastic material or a thermoset material. 
     Contact electrical conductor  34  comprises a conductive material that can be coated on a non-conductive core. For example, contact electrical conductor  34  can be a surface layer film comprising a thin film of physically deposited metal or conductive oxide, or a combination of these. An electrical conductor can be coated (e.g., deposited) and patterned such that the outer surface of each polymer core  32  is separately electrically conductive. Contact electrical conductor  34  can be in electrical and physical contact with device electrical contact  22 . Contact electrical conductor  34  (or device electrical conductor  22 , or both) can comprise a metal, a metal alloy, a solder, a transparent conductive oxide (e.g., indium tin oxide or aluminum zinc oxide), or an electrically conductive polymer. According to some embodiments, contact electrical conductor  34  is transparent, polymer core  32  is transparent, and substrate electrical contact  30  is transparent (or device electrical conductor  22 , or both), for example 50% transparent to visible light or light emitted from device  20  (e.g., not less than 70% transparent, not less than 80% transparent, or not less than 90% transparent to visible light or light emitted from device  20 ). Contact electrical conductor  34  can be reflowable and, in some embodiments, can wick along device electrical contact  22  by heating once device  20  has been printed to substrate  10 . For example, heat can be applied to cure polymer core  32  and contact electrical conductor  34  that causes polymer core  32  to reflow (e.g., soften and morphologically equilibrate) such that it conforms to device  20  and device electrical contact  22 . In some embodiments, heating can alternatively or additionally cause contact electrical conductor  34  to reflow and wick along the surface of device electrical contact  22 , thereby improving and strengthening the physical and electrical contact between substrate electrical contact  30  and device electrical contact  22  and between substrate  10  and device  20 . By reflowing a conductive surface layer of substrate electrical contact(s)  30 , contact with device electrical contacts  22  can be improved, especially for device electrical contacts  22  in different planes where contact area between substrate electrical contacts  30  and device electrical contacts  22  can be small initially (e.g., after printing prior to heating). Contact electrical conductor  34  can be a surface layer. As an example, in some embodiments, contact electrical conductors  34  have a thickness no more than 25% of a lateral extent of polymer core  32  over surface  11  of substrate  10 . As another example, contact electrical conductors  34  can have a thickness of no more than one μm (e.g., no more than 500 nm, no more than 250 nm, or no more than 100 nm). 
     According to embodiments of the present disclosure, printed structure  99  can provide devices  20  electrically connected to substrate conductors  12  in a dense configuration and at a high resolution over substrate  10 , for example by micro-transfer printing devices  20  using a stamp that has at least hundreds, at least thousands, at least tens of thousands, or at least hundreds of thousands of posts that each pick up a respective device  20  from a source wafer. Substrate conductors  12  can be wires and can be deposited and patterned in a common step with contact electrical conductors  34 . If substrate  10  is a backplane, contact electrical conductors  34  can be electrically connected to backplane wiring levels through a via using traditional routing techniques. 
     According to some embodiments, substrate electrical contacts  30  have a lateral extent (e.g., diameter  35 ) over substrate  10  of no more than 50 μm, no more than 20 μm, no more than 10 μm, or no more than 5 μm. Furthermore, substrate electrical contacts  30  can be disposed close to each other at a high resolution, for example separated by separation distance  38  of no more than 50 μm, no more than 20 μm, no more than 10 μm, or no more than 5 μm. Substrate electrical contacts  30  can protrude orthogonally from surface  11  of substrate  10  to a similar distance of height  36  (e.g., have similar thicknesses), for example to a height  36  of (e.g., have a thickness of) no more than 50 μm, no more than 20 μm, no more than 10 μm, or no more than 5 μm. For example, height  36  can be 0.5 to 5 μm or 1-2 μm, diameter 35 can be 2 to 10 μm or 3-5 μm, planar electrical contacts can be 5-10 μm wide and/or long, and device  20  can have a length and/or width of 2-20 μm, for example 3-5 μm. 
       FIG. 1  illustrates a device  20  with substantially planar device electrical contacts  22  disposed substantially in a common plane. According to embodiments of the present disclosure, and as shown in  FIG. 2 , device  20  can comprise a device  20  comprising substantially planar device electrical contacts  22  disposed on common side  23  of device  20  but in different planes. The different planes can be, but are not necessarily, substantially parallel (e.g., to within 10 degrees). A device  20  having multiple device electrical contacts  22  on a common side  23  is a horizontal device. For example, device  20  can be a horizontal inorganic light-emitting diode. In embodiments in which substrate electrical contacts  30  have a common height  36  and device electrical contacts  22  are in different planes, device  20  disposed on substrate  10  can be tilted with respect to surface  11  of substrate  10 , for example as shown in  FIG. 2 . A tilted arrangement can be one in which a major surface of device  20  is not substantially parallel to surface  11 , for example a surface of device  20  on an opposite side of device  20  from device electrical contacts  22  or substrate  10 . As shown in  FIG. 3 , and according to some embodiments of the present disclosure, substrate electrical contacts  30  on a substrate  10  can have different sizes, for example different lateral extents over (e.g., diameters  35 ) or different heights  36  above surface  11  of substrate  10 . In some such embodiments, devices  20  with device electrical contacts  22  in different planes can have a major surface that is parallel to surface  11 , as also shown in  FIG. 3 . 
     According to some embodiments of the present disclosure and as shown in  FIG. 3 , device  20  is or comprises a light-emitting diode and a surface of device  20 , e.g., a second side  25  surface opposite common side  23  and a surface of second side  25  is roughened. Roughening can be accomplished, for example, by exposure to a plasma such as an oxygen plasma and the roughened surface can at least partially mitigate or eliminate total internal reflection of light emitted by active layers in the light-emitting diode, thereby improving the efficiency, appearance, and angular distribution of light emission from the light-emitting diode, for example to widen the angle of view for emitted light, as illustrated in  FIG. 3 , or compensate for a tilted light-emitting diode, as illustrated in  FIG. 2 . Furthermore, for light-emitting devices, a roughened second side  25  opposite device electrical contacts  22  provides a diffuse light output that is not dependent on the orientation angle of device  20  with respect to the substrate  10 . 
     As shown in  FIG. 4 . and according to embodiments of the present disclosure, multiple devices  20  can be disposed on and in physical and electrical contact with multiple corresponding substrate electrical contacts  30  on a substrate  10 . Devices  20  can be arranged randomly over substrate  10 , in an unstructured arrangement over substrate  10  or in a structured arrangement over substrate  10  (e.g., in an array over substrate  10 ). Some devices  20  can be electrically connected together through substrate electrical conductors  30 , for example by substrate conductors  12 . The arrangement of devices  20  can form a display or detector (or a combination display and detector). 
     According to some embodiments, and as shown in  FIG. 4 , substrate  10  can have a rounded substrate electrical contact  30  provided for each device electrical contact  22 . In some embodiments, and as shown in  FIGS. 5A and 5B , a substrate  10  comprises a planar electrical contact  16  and a device electrical contact  22  is in electrical (e.g., and physical) contact with planar electrical contact  16 . As shown in  FIGS. 5A and 5B , devices  20  have multiple device electrical contacts  22  and less than all of the device electrical contacts  22  are electrically connected to substrate conductors  12  through rounded substrate electrical contacts  30 . As shown, some device electrical contacts  22  can be electrically connected to substrate conductors  12  through substantially planar electrical contacts  16  (electrical contact pads on substrate  10 ) that can be electrically connected to substrate conductors  12 . Devices  20  can be tilted, as shown in  FIG. 5A  or can be flat (not tilted), as shown in  FIG. 5B , for example if substrate electrical contact  30  has a height  36  substantially equal to the orthogonal difference between the different device electrical contact  22  planes. 
     As shown in the flow diagram of  FIG. 6  and with reference to the successive structures illustrated in  FIGS. 9A-9H , embodiments of the present disclosure can be constructed by providing a substrate  10  in step  100 , as shown in  FIG. 9A . As shown in  FIG. 9B , substrate  10  is coated with a polymer  31  (e.g., an unpatterned reflowable layer of polymer  31  disposed by slot coating, spin coating, or spray coating) in step  110 . Polymer  31  can optionally be partially cured (e.g., soft cured) and can be patterned in step  120  as shown in  FIG. 9C , for example by coating with a photomask, exposing the photomask with a pattern, etching masked or unmasked portions of polymer  31  to form a patterned layer of polymer  31 , and stripping the photomask, as is known in photolithography. In step  130  and as shown in  FIG. 9D , patterned polymer  31  can be heated with heat  80  to reflow polymer  31 . Because of the relative surface energies of polymer  31  and surface  11  of substrate  10 , reflowing polymer  31  can form droplets (beads) on surface  11  to form polymer cores  32  in a rounded shape. Polymer cores  32  can then be hard cured or a hard cure can be performed later. Polymer  31  can comprise, for example, a thermoplastic material. Optionally, substrate  10  can be initially coated with a material selected to provide a suitable surface energy and enhance or enable the formation of rounded polymer cores  32 . Substrate  10  and polymer cores  32  can then be coated with an electrical conductor coating  33  as shown in  FIG. 9E  (e.g., by evaporation, sputtering, or vapor deposition) in step  140  and patterned in step  150 , as shown in  FIG. 9F , to form contact electrical conductor  34  and substrate conductors  12  (and any planar electrical contacts  16  shown in  FIGS. 5A and 5B ). Surface  11  of substrate  10  is then coated with adhesive  40  in step  160  as shown in  FIG. 9G . Substrate electrical contact  30  can have or be treated to have a surface energy selected to reduce the amount of adhesive  40  that coats substrate electrical contact  30 . Devices  20  with device electrical contacts  22  are provided in step  170  and disposed (e.g., micro-transfer printed) onto substrate electrical contacts  30  in step  180  and as shown in  FIG. 9H . Adhesive  40  is cured in step  190  to form printed structure  99 . The resulting printed structure  99  comprises a mass-transferred device  20  that is mechanically attached to substrate  10  through the cured polymer resin adhesive  40 , like a die attach, and the same device  20  is electrically connected to the underlying backplane (substrate  10 ) through the physical bond between the underside device electrical conductor  22  (device contact pad) and the substrate electrical conductor  30  (substrate contact electrical conductor  34  or substrate contact pad disposed on the top-side conductive surface layer of polymer core  32  in a rounded configuration such as a bump). 
     In some embodiments, device electrical contacts  22  are brought into physical and electrical contact with contact electrical conductors  34  by micro-transfer printing, for example as shown in  FIG. 9H . In some embodiments, adhesive  40  can be disposed between device electrical contacts  22  and contact electrical conductors  34 . In either case, curing adhesive  40  shrinks adhesive  40  and pulls device  20  toward substrate  10  so that device electrical contacts  22  are brought more closely into physical and electrical contact with contact electrical conductor  34  and substrate electrical contacts  30 . Polymer core  32  of substrate electrical contacts  30  can have a lower Young&#39;s modulus than adhesive  40 . 
     According to some methods of the present disclosure, and as illustrated in the flow diagram of  FIG. 7  and in  FIG. 9G , a relatively thin layer of adhesive  40  is disposed over substrate  10  in step  162  that has a depth less than height  36  of substrate electrical contacts  30 . When devices  20  are transfer printed (e.g., micro-transfer printed with an elastomeric stamp) in step  180 , a portion of devices  20  contacts adhesive  40  and, when adhesive  40  is cured and shrinks in step  190 , devices  20  are brought into closer proximity to substrate  10  and device electrical contacts  22  are brought into closer proximity and electrical contact with substrate electrical contacts  30 . Optionally, contact electrical conductor  34  of substrate electrical contacts  30  reflows to wick onto device electrical contact  22  to improve electrical contact with device electrical contact  22  when heated (e.g., during an adhesive curing step). In some embodiments, device electrical contact  22  comprises reflowable material, such as a solder that wicks onto contact electrical conductor  34  (or both). In some embodiments, reflowing a conductor (whether from device electrical contact  22  or substrate electrical contact  30 ) can occur as a part of curing adhesive  40 . Similarly, in some embodiments, polymer core  32  can reflow and conform to or comply with device electrical contact  22  as a part of curing adhesive  40 . 
     According to some methods of the present disclosure, and as shown in  FIG. 10A , a relatively thick layer of adhesive  40  is disposed over substrate  10  that has a depth greater than height  36  of substrate electrical contacts  30 . If devices  20  are transfer printed (e.g., micro-transfer printed with an elastomeric stamp) in step  180  onto such a thick adhesive  40  layer, devices  20  can be disposed on (e.g., float on) adhesive  40  (although in some embodiments adhesive  40  is pushed aside and device electrical contacts  22  are disposed in contact with substrate electrical contacts  30 ). In some embodiments, curing adhesive  40  can bring device electrical contacts  22  into physical and electrical contact with substrate electrical contacts  30 , as described above with respect to  FIG. 7 . Adhesive  40  is then cured in step  190 . 
     According to some methods of the present disclosure, and as illustrated in the flow diagram of  FIG. 8  and successive cross sections of  FIGS. 10A-10D , a relatively thick layer of adhesive  40  is disposed over substrate  10  in step  164  that has a depth greater than height  36  of substrate electrical contacts  30 , as shown in  FIG. 10A . Adhesive  40  is partially cured (e.g., soft cured) in step  166  and patterned in step  168 , as shown in  FIG. 10B . Devices  20  are transfer printed (e.g., micro-transfer printed with an elastomeric stamp) in step  180  and disposed on adhesive  40  (e.g., float on adhesive  40 ) as shown in  FIG. 10C . In some such embodiments, when adhesive  40  is heated or cured, adhesive  40  reflows and spreads over local portions of substrate  10  in step  192  so that devices  20  are brought into closer proximity to substrate  10  and device electrical contacts  22  are brought into closer proximity and electrical contact with substrate electrical contacts  30 , as shown in  FIG. 10D . Adhesive  40  is then cured in step  190 . The cure step  190  can be done in stages so as to initially reflow polymer core  32  as in step  130  to conform to device electrical contacts  22  and wick contact electrical conductor  34  in step  194  and then finally to hard cure adhesive  40  and polymer core  32 . 
     Adhesive  40  can comprise a layer of resin, polymer, or epoxy, either curable or non-curable and can be disposed, for example by coating or lamination as an unpatterned layer or a patterned layer. In some embodiments, the layer of adhesive  40  is disposed in a pattern, for example using inkjet, screen printing, or photolithographic techniques. In some embodiments, a layer of adhesive  40  is coated, for example with a spray or slot coater, and then patterned, for example using photolithographic techniques. 
     According to some embodiments of the present disclosure and as illustrated in  FIG. 11 , substrate electrical contacts  30  can conform to device electrical contacts  22  after device  20  is transfer printed to substrate  10 , for example by heating. For example polymer core  32  can reflow (for example when heated can reflow, soften, and conform to device electrical contact  22  as a consequence of surface tension and relative surface energies, e.g., capillary forces), and then be hard cured (e.g., baked) to permanently set polymer core  32  into a conformal configuration. Contact electrical conductor  34  can also reflow when heated and the material can wick along a surface of device electrical contact  22  and then harden when the printed structure  99  is cooled. According to some embodiments of the present disclosure, polymer  31  layer is soft-baked (heated to a lower temperature), cooled, patterned using photolithography, and then reheated to reflow the patterned polymer  31  to form polymer cores  32 . After transfer printing device  20 , polymer cores  32  are again heated, together with any adhesive  40 , to conform polymer cores  32  to device  20  and device electrical contacts  22  and wick contact electrical conductor  34  along device electrical contacts  22 . Printed structure  99  is then cooled for use. 
     According to embodiments of the present disclosure, and as illustrated in  FIG. 12 , pixels  60  in a display  50  can comprise printed structures  99 , for example in an active-matrix-controlled array. Substrate  10  of printed structures  99  can be common to all printed structures  99  (e.g., as in  FIG. 4 ) and can be a display substrate  62 . Each pixel  60  (and printed structure  99 ) can be controlled through row wires  56  and column wires  58  (e.g., substrate conductors  12 ) connected to a row controller  52  and a column controller  54 , respectively. Row controller  52  and column controller  54  can be driven through buses  70  from a display controller (not shown). 
     According to some embodiments of the present disclosure, a printed structure  99  comprises a single device  20  disposed on a substrate  10  (e.g., as shown in  FIGS. 1-3 ) or a printed structure  99  comprises multiple devices  20  are disposed on a substrate  10  (e.g., as shown in  FIG. 4 ). In either case, printed structure  99  can itself be a device  20  that can then be disposed as a printed structure on another substrate  10  (e.g., a backplane). Thus, printed structures  99  can be employed multiple times at different levels in micro-electronic systems. For example, a printed structure  99  can comprise a device  20  disposed on a substrate  10  that in turn is disposed on another, different substrate  10  to make a larger printed structure  99 . Hence, a micro-electronic system according to embodiments of the present disclosure can comprise a printed structure  99  that comprises another printed structure  99  that comprises yet another printed structure  99 , at successively smaller scales and with smaller or fewer components. Each printed structure  99  can be or include a device  20  that is comprised in another, larger printed structure  99 . 
     As shown in  FIG. 13 , a printed structure  99  comprises three devices  20  disposed on a common substrate  10 . For example, the three devices  20  can be three different micro-LEDs, each emitting a different color of light disposed on a pixel substrate  64 . Printed structure  99  can therefore be a pixel  60 . In some embodiments, pixel substrate  64  can be a semiconductor substrate (e.g., a silicon substrate) that comprises a pixel control circuit  66  (e.g., a CMOS circuit) that controls LED devices  20 . In some embodiments a separate control circuit with a separate substrate (e.g., an integrated circuit) is transfer printed onto pixel substrate  64 ,  10  (not shown). 
     According to some embodiments of the present disclosure, each of the printed structures  99  of  FIG. 13  can be a device  20  that is, in turn, printed onto a substrate  10 , for example a display substrate  62  to make a display  50 , as shown in  FIGS. 14 and 15 .  FIG. 15  is a plan view of display  50  having pixels  60 , each pixel  60  a printed structure  99  as shown in  FIG. 13 . As shown in  FIGS. 13 and 14 , each pixel  60  (comprising three devices  20 ) comprises a pixel substrate  64  and forms a printed structure  99 . Four device electrical contacts  22  electrically connect to display substrate  62 ,  10  to form another, larger printed structure  99 . Conductors  12  corresponding to a row wire  56 , power wire  57 , column wire  58 , and ground wire  59 , conduct electrical signals to control each pixel  60 , as shown in  FIG. 15 .  FIGS. 14 and 15  thus illustrate a display  50  comprising a printed structure  99  having a substrate  10  (display substrate  62 ) and devices  20  (each a printed structure  99  corresponding to  FIG. 13 ). 
     Therefore, according to embodiments of the present disclosure, a substrate  10  of a printed structure  99  can be an intermediate substrate  64  (e.g., pixel substrate  64 ) and printed structure  99  can further comprise a system substrate  62  (e.g., a display substrate  62 ) comprising substrate conductors  12  disposed on or in system substrate  62 . Device  20  can be electrically connected to substrate conductors  12  through contact electrical conductors  34  of substrate electrical contacts  30 . An intermediate substrate  64  can be originally formed or disposed on a source wafer (e.g., a pixel source wafer) and then printed to a larger substrate  10 , such as a backplane or printed circuit board. For example, a dense array of printed structures  99  can be assembled on an intermediate source substrate and printed to a larger substrate (e.g., a backplane) in an array (e.g., having a lower areal density) to form a display or detector. 
     Embodiments of the present disclosure provide a mechanically bonded and electrically connected device  20  and substrate  10  at a high resolution that does not rely on solder bumping (e.g., at a lower resolution than printed structures  99 ) or that is solder free. It can be difficult to form small, micro-sized solder structures (e.g., solder bumps). 
     Typical solder bumps have a size no less than 100 microns and, in recent, advanced solder systems, have a size no less than 30 microns in diameter. In contrast, according to embodiments of the present disclosure, electrical connections on the order of 1-5 microns (or smaller) are readily, efficiently, and effectively constructed with good electrical connection in high-volume processes and electrically connected to devices  20  with lengths and widths in the tens of microns (or smaller or larger). Moreover, forming rounded substrate electrical contacts  30  as disclosed in various embodiments herein can be less expensive (e.g., due to reduced processing steps and/or reduced material costs) than alternative methods of forming robust electrical connections, such as using devices with connection posts (e.g., as disclosed in U.S. patent application Ser. No. 14/822,864). For example, due to relatively lower photolithographic resolution requirements and the relative inexpensiveness of compliant, flexible, or reflowable polymers, it can be easier to form rounded polymer shapes coated with electrically conductive material than to form a connection post. In some embodiments, device electrical contacts  22  include connection posts. 
     Micro-transfer printing processes and structures suitable for disposing devices  20  onto substrates  10  are described in  Inorganic light - emitting diode displays using micro - transfer printing  (Journal of the Society for Information Display, 2017, DOI # 10.1002/jsid.610, 1071-0922/17/2510-0610, pages 589-609), U.S. Pat. No. 8,722,458 entitled Optical Systems Fabricated by Printing-Based Assembly, U.S. patent application Ser. No. 15/461,703 entitled Pressure-Activated Electrical Interconnection by Micro-Transfer Printing, U.S. Pat. No. 8,889,485 entitled Methods for Surface Attachment of Flipped Active Components, U.S. patent application Ser. No. 14/822,864 entitled Chiplets with Connection Posts, U.S. patent application Ser. No. 14/743,788 entitled Micro-Assembled LED Displays and Lighting Elements, and U.S. Pat. No. 10,153,256, entitled Micro-Transfer Printable Electronic Component, the disclosure of each of which is incorporated herein by reference in its entirety. 
     For a discussion of micro-transfer printing techniques, see also U.S. Pat. Nos. 7,622,367 and 8,506,867, each of which is hereby incorporated by reference in its entirety. Micro-transfer printing using compound micro-assembly structures and methods can also be used with the present disclosure, for example, as described in U.S. patent application Ser. No. 14/822,868, filed Aug. 10, 2015, entitled Compound Micro-Assembly Strategies and Devices, which is hereby also incorporated by reference in its entirety. In some embodiments, micro-transfer printed structure  99  is a compound micro-assembled structure (e.g., a macro-system). Devices  20 , in certain embodiments, can be made using integrated circuit photolithographic techniques having a relatively high resolution and cost and substrates  10 , for example a printed circuit board, can be made using printed circuit board techniques having a relatively low resolution and cost, thereby reducing manufacturing costs. 
     In certain embodiments, substrate  10  comprises a member selected from the group consisting of polymer, plastic, resin, polyimide, PEN, PET, metal, metal foil, glass, a semiconductor, a compound semiconductor, and sapphire. In certain embodiments, substrate  10  has a thickness from 5 microns to 20 mm (e.g., 5 to 10 microns, 10 to 50 microns, 50 to 100 microns, 100 to 200 microns, 200 to 500 microns, 500 microns to 0.5 mm, 0.5 to 1 mm, 1 mm to 5 mm, 5 mm to 10 mm, or 10 mm to 20 mm). 
     Devices  20 , in certain embodiments, can be constructed using foundry fabrication processes used in the art. Layers of materials can be used, including materials such as semiconductors, doped semiconductors, metals, oxides, nitrides and other materials used in the integrated-circuit art. Each device  20  can be or include a complete semiconductor integrated circuit and can include, for example, one or more of a transistor, a diode, a light-emitting diode, and a sensor. Devices  20  can have different sizes, for example, 50 square microns or larger, 100 square microns or larger, 1000 square microns, larger or 10,000 square microns or larger, 100,000 square microns or larger, or 1 square mm or larger. Devices  20  can have variable aspect ratios, for example between 1:1 and 10:1 (e.g., 1:1, 2:1, 5:1, or 10:1). Devices  20  can be rectangular or can have other shapes. In some embodiments, transferring, printing, or transfer printing occurs by micro-transfer-printing. In some embodiments, micro-transfer printing involves using a transfer device (e.g., an elastomeric stamp, such as a PDMS stamp) to transfer a device  20  using controlled adhesion. For example, an exemplary transfer device can use kinetic or shear-assisted control of adhesion between a transfer device and device  20 . It is contemplated that, in certain embodiments, where a method is described as including micro-transfer-printing a device  20 , other analogous embodiments exist using a different transfer method. In some examples, transferring a device  20  (e.g., from a source wafer to a substrate  10 ) can be accomplished using any one or more of a variety of known techniques. For example, in certain embodiments, a pick-and-place method can be used. As another example, in certain embodiments, a flip-chip method can be used (e.g., involving a handle or carrier substrate). In methods according to certain embodiments, a vacuum tool, electrostatic pick-up tool, or other transfer device is used to transfer a device  20 . 
     As is understood by those skilled in the art, the terms “over” and “under” are relative terms and can be interchanged in reference to different orientations of the layers, elements, and substrates included in the present disclosure. Furthermore, a first layer “on” a second layer is a relative orientation of the first layer to the second layer that does not preclude additional layers being disposed therebetween. For example, a first layer on a second layer, in some implementations, means a first layer directly on and in contact with a second layer. In other implementations, a first layer on a second layer includes a first layer and a second layer with another layer therebetween (e.g., and in mutual contact). 
     It should be understood that the order of steps or order for performing certain action is immaterial so long as the disclosed technology remains operable. Moreover, two or more steps or actions in some circumstances can be conducted simultaneously. Throughout the description, where apparatus and systems are described as having, including, or comprising specific elements, or where processes and methods are described as having, including, or comprising specific steps, it is contemplated that, additionally, there are apparatus and systems of the disclosed technology that consist essentially of, or consist of, the recited elements, and that there are processes and methods according to the disclosed technology that consist essentially of, or consist of, the recited processing steps. It is contemplated that structures, devices, methods, and processes of the disclosure encompass variations and adaptations developed using information from the embodiments described herein. Adaptation and/or modification of the devices, methods, and processes described herein may be performed by those of ordinary skill in the relevant art. 
     Certain embodiments of the present disclosure are described above. It is, however, expressly noted that the present disclosure is not limited to those embodiments, but rather the intention is that additions and modifications to what was expressly described in the present disclosure are also included within the scope of the disclosure. Moreover, it is to be understood that the features of the various embodiments described in the present disclosure were not mutually exclusive and can exist in various combinations and permutations, even if such combinations or permutations were not made express, without departing from the spirit and scope of the disclosure. Having described certain implementations of structures and methods for electrically connecting printed horizontal devices, it will now become apparent to one of skill in the art that other implementations incorporating the concepts of the disclosure may be used. Therefore, the disclosure should not be limited to certain implementations, but rather should be limited only by the spirit and scope of the following claims. 
     PARTS LIST 
     
         
           10  substrate 
           11  surface 
           12  substrate conductor/wire 
           16  planar electrical contact 
           20  device 
           22  device electrical contacts 
           23  common side 
           24  dielectric structure 
           25  second side 
           26  device tether 
           30  substrate electrical contact 
           31  polymer 
           32  contact non-conductive core/contact polymer core 
           33  electrical conductor coating 
           34  contact electrical conductor 
           35  diameter 
           36  height 
           38  separation distance 
           40  adhesive 
           50  display 
           52  row controller 
           54  column controller 
           56  row wire 
           57  power wire 
           58  column wire 
           59  ground wire 
           60  pixel 
           62  display substrate/system substrate 
           64  pixel substrate/intermediate substrate 
           66  pixel control circuit 
           70  bus 
           80  heat 
           90  printed structure 
           100  provide substrate step 
           110  coat substrate with polymer step 
           120  pattern polymer step 
           130  reflow patterned polymer step 
           140  coat substrate with electrical conductor step 
           150  pattern electrical conductor step 
           160  coat substrate with adhesive layer step 
           162  coat substrate with thin adhesive layer step 
           164  coat substrate with thick adhesive layer step 
           166  partial cure adhesive layer step 
           168  pattern adhesive layer step 
           170  provide device step 
           180  transfer print device step 
           190  cure adhesive step 
           192  reflow adhesive step 
           194  optional reflow electrical conductor step