WO2025252529A1

WO2025252529A1 - Method for producing a display, display substrate and display

Info

Publication number: WO2025252529A1
Application number: PCT/EP2025/064580
Authority: WO
Inventors: Martin Hetzl; Devendra Pareek
Original assignee: Ams Osram International GmbH
Current assignee: Ams Osram International GmbH
Priority date: 2024-06-05
Filing date: 2025-05-27
Publication date: 2025-12-11
Anticipated expiration: 2026-12-05

Abstract

A method for producing a display (1) is specified herein, including the steps of: • providing a display substrate (2) with a main surface (21) comprising a plurality of electric contact pads (3) for electrically contacting a plurality of individually controllable light sources (4), • modifying a surface structure of the main surface (21) of the display substrate (2) in at least one area (22) adjacent to at least one electric contact pad (3), and • soldering the plurality of light sources (4) onto the plurality of electric contact pads (3), wherein • the surface structure is modified for controlling a flow of molten solder (5) during the soldering step. Further, a display substrate and a display are specified herein.

Description

METHOD FOR PRODUCING A DISPLAY, DISPLAY SUBSTRATE AND DISPLAY

A method for producing a display, a display substrate and a display are speci fied herein .

At least one obj ect of certain embodiments is to provide a method for producing a display, wherein an alignment of individual pixel light sources is improved . At least one further obj ect of certain embodiments is to provide a display substrate that enables a reduced misalignment of individual pixel light sources . At least one further obj ect of certain embodiments is to provide a display, wherein an alignment of individual pixel light sources is improved .

According to at least one aspect of the method for producing a display, a display substrate with a main surface comprising a plurality of electric contact pads for electrically contacting a plurality of individually controllable light sources is provided . For example , the main surface corresponds to a main extension plane of the display substrate . For example , the display substrate is rigid or flexible . In particular, the flexible display substrate is bendable and/or rollable in a reversible manner . For example , the flexible display substrate comprises or consists of a film, such as a polymer film, or of a laminate .

The display substrate can be at least partially transparent , in particular for electromagnetic radiation in a visible spectral range . For example , a transmittance of the display substrate for electromagnetic radiation in the visible spectral range is at least 50% , or at least 70% , or at least 90%. For example, the display substrate comprises or consists of sapphire, a glass, a ceramic, a polymer, a printed circuit board, and/or a semiconducting material, such as silicon.

For example, the display substrate is a complementary metal oxide semiconductor substrate (short: CMOS substrate) . For example, the display substrate comprises a wafer, one or more undoped semiconductor layers, one or more doped semiconductor layers, one or more metal layers, and/or one or more oxide layers. An integrated circuit for controlling and/or driving the plurality of individually controllable light sources can be formed in or on the display substrate, or can be embedded into the display substrate. For example, the main surface of the display substrate is a CMOS backplane. In other words, the integrated circuit is formed or embedded on a side of the display substrate opposite to the main surface, where the plurality of electrical contact pads for electrically contacting the plurality of light sources are arranged.

In the following, features of a single electric contact pad are specified. One or more of the features of a single electric contact pad can apply to at least one, a majority, or all of the electric contact pads. In particular, the electric contact pad comprises or consists of an electrically conductive material, such as a metal, an alloy, and/or a conductive oxide. For example, the electrically conductive material is arranged on the display substrate in the form of a layer or a layer sequence. For example, the electric contact pad comprises or consists of copper, gold, and/or aluminum. For example, a shape of the electric contact pad in plan view of the main surface is circular, elliptic, oval, square, rectangular, triangular, or polygonal. In particular, the electric contact pad is configured for soldering one of the plurality of light sources onto the display substrate . For example , soldering establishes a mechanically stable and electrically conductive contact between the electric contact pad and the light source . For example , the light source comprises or consists of a light emitting diode and the plurality of electric contact pads are configured for individually contacting an anode and/or a cathode of each light emitting diode .

For example , the electric contact pad is electrically connected to an electronic circuit , such as the integrated circuit for controlling and/or driving the plurality of light sources , by a conductor track or by a via, such as a plated through hole in the display substrate . For example , the conductor track is arranged on the main surface of the display substrate or is embedded into the display substrate , in particular into the CMOS substrate . For example , the conductor track is covered by a passivation layer comprising a dielectric material , in particular an oxide such as silicon dioxide . For example , the surface of the passivation layer forms the main surface of the display substrate .

According to at least one further aspect of the method, a surface structure of the main surface of the display substrate is modi fied in at least one area adj acent to at least one electric contact pad . For example , the surface structure is modi fied in at least one area surrounding and/or directly adj acent to at least one electric contact pad . For example , the surface structure in the at least one area is modi fied by removing atoms or molecules from the main surface of the display substrate , or by adding atoms or molecules to the main surface of the display substrate in the at least one area .

For example , the surface structure of the modi fied main surface of the display substrate has a higher surface roughness than the unmodi fied main surface of the display substrate . It is also possible that the surface structure of the modi fied main surface of the display substrate has a lower surface roughness than the unmodi fied main surface of the display substrate . Here and in the following, the surface roughness refers to a root mean square ( short : RMS ) deviation of a height of the main surface relative to a mean height of the main surface , for example . Here , the height refers to a local position of the main surface in a direction perpendicular to the mean main extension plane of the main surface . For example , the surface roughness of the at least one area with the modi fied surface structure is at least two times , at least five times , or at least ten times higher than the surface roughness of the unmodi fied main surface of the display substrate . For example , the surface roughness of the modi fied main surface can be increased or decreased relative to the unmodi fied main surface depending on the chosen solder alloy configuration and a wettability of the molten solder alloy on the main surface of the surface substrate .

For example , the surface structure of the modi fied main surface of the display substrate has a di f ferent chemical composition compared to the unmodi fied main surface of the display substrate . For example , the at least one area with the modi fied surface structure has a higher oxygen content or a lower oxygen content than the unmodi fied main surface of the display substrate . In particular, the modi fied surface structure of the at least one area of the main surface of the display substrate has a lower af finity to molten solder than the unmodi fied main surface of the display substrate and/or than the electric contact pad . In particular, the af finity refers to a binding af finity between the molten solder and the display substrate , or to a binding af finity between the molten solder and the electric contact pad . For example , the binding af finity corresponds to a strength of a binding interaction, such as an electrostatic interaction, between the molten solder and the display substrate or the electric contact pad . For example , the af finity of the electric contact pad and/or of the unmodi fied main surface of the display substrate to molten solder is at least two times , at least five times , or at least ten times higher than the af finity of the modi fied surface structure of the main surface to molten solder .

According to at least one further aspect of the method, the plurality of light sources is soldered onto the plurality of electric contact pads . In particular, before soldering the light sources are arranged on the electric contact pads by trans fer printing or by a pick-and-place process . For example , the light sources are provided on a temporary carrier . For example , a solder paste is disposed on the electric contact pads of the display substrate . In particular, the solder paste comprises metallic particles , such as tin, copper, and/or silver particles . For example , at least a part of the plurality of light sources is detached from the temporary carrier, e . g . using a stamp, and arranged on the solder paste that is disposed on the electric contact pads . For example , the plurality of light sources is soldered onto the plurality of electric contact pads by means of a reflow soldering process . For example , upon reflow soldering the display substrate with the electric contact pads and the solder paste as well as the light sources arranged thereon is heated above a melting point of the solder paste .

For example , the plurality of light sources is arranged on the main surface of the display substrate in the form of an array, in particular in the form of a two-dimensional array . For example , the light sources are arranged at vertices of a regular two-dimensional lattice , such as a quadratic, rectangular, or triangular lattice . For example , the plurality of light sources forms a plurality of individually controllable pixels . For example , each pixel of the display comprises or is formed by one , two three or more light sources of the plurality of light sources . In other words , the light sources are pixel light sources . Alternatively, it is also possible that the plurality of light sources is used for backlighting the display, such as a liquid crystal display, for example .

For example , each light source comprises or consists of an individual light emitting semiconductor chip, in particular a micro-LED chip . In particular, each light emitting semiconductor chip forms a pixel or part of a pixel of the display . For example , upon soldering the plurality of light sources onto the plurality of electric contact pads , a plurality of individual light emitting semiconductor chips is soldered onto the plurality of electric contact pads . For example , each electric contact pad is soldered to a corresponding light emitting semiconductor chip . For example , the light emitting semiconductor chips are flip chips . For example , for each of the plurality of light emitting semiconductor chips the substrate comprises two electric contact pads for electrically contacting the corresponding light emitting semiconductor chip .

According to at least one further aspect of the method, the surface structure is modi fied for controlling a flow of molten solder during the soldering step . For example , the modi fied surface structure enhances a solderability on designated areas , in particular on the electric contact pads , and/or improves a quality of a solder connection between the light source and the respective electric contact pad . For example , the modi fied surface structure reduces or minimi zes a spreading of molten solder and/or reduces a probability of forming unwanted solder bridges between adj acent contact pads . Further, the modi fied surface structure can improve a sel f-alignment of the plurality of light sources during the soldering step . Here and in the following, sel f-alignment refers to a parallel or almost parallel alignment of a light outcoupling surface of the light source to the main surface of the display substrate during soldering, for example . For example , the modi fied surface structure reduces an unwanted tilting of at least some of the plurality of light sources during the soldering process , wherein the light outcoupling surfaces of at least some of the plurality of light sources are no longer parallel or almost parallel to the main surface after the soldering step . For example , the tilting is caused by an unwanted spreading of molten solder, or by an unwanted runof f of molten solder from the electric contact pad .

According to an embodiment , the method for producing a display includes the following steps :

- providing the display substrate with the main surface comprising the plurality of electric contact pads for electrically contacting the plurality of individually controllable light sources ,

- modi fying the surface structure of the main surface of the display substrate in at least one area adj acent to at least one electric contact pad, and

- soldering the plurality of light sources onto the plurality of electric contact pads , wherein

- the surface structure is modi fied for controlling the flow of molten solder during the soldering step .

Preferably, the steps of the method are performed in the order speci fied above . Alternatively, it is also possible that the surface structure of the main surface of the display substrate is modi fied before the plurality of electric contact pads is arranged on the main surface of the display substrate .

The method for producing a display speci fied herein can be used for producing a micro-LED display, for example , where individual micro-LEDs form pixels of the display . However, micro-soldering millions of individual micro-LEDs onto the electric contact pads of the display substrate poses several challenges , such as achieving a precise pixel placement , mitigating issues like tilt and deformation of individual micro-LEDs during the soldering process , and/or meticulously controlling the flow of molten solder to avoid short circuits .

Conventional soldering techniques encompassing pick-and-place machinery, traditional solder pastes and conveyor belt furnaces can fall short when applied to micro-LEDs . These conventional techniques can suf fer from the following limitations : Inaccurate placement: difficulty in manipulating miniscule light sources can lead to misaligned pixels, resulting in suboptimal display quality.

- Tilt and deformation: the heat involved in conventional soldering can induce undesirable physical changes in the micro-LEDs, compromising their integrity and functionality .

Inconsistent solder joint formation: achieving reliable and uniform solder connections is challenging with current methods, potentially impacting electrical performance and lifespan. Inefficient mass soldering: scalable production currently faces bottlenecks due to limitations in handling large quantities of micro-LEDs simultaneously.

- Defect detection and repair challenges: the small size and intricate nature of micro-LEDs make identifying and rectifying faults a significant hurdle.

Furthermore, conventional reflow soldering presents additional risks for micro-LEDs, specifically the reflow connecting same-type electric contact pads in neighboring pixels. During solder reflow, there is a risk of inadvertently connecting electrodes of the same type, e.g. anodes or cathodes of the micro-LEDs, between neighboring pixels, thereby creating unintended electrical pathways and compromising a display functionality.

The method for producing a display substrate specified herein addresses at least some of the problems outlined above. In particular, the method specified herein is based on the idea to precisely control the flow of molten solder by modifying the surface structure of the main surface of the display substrate adjacent to the electric contact pads. This approach also facilitates a sel f-leveling and/or sel falignment of micro-LED pixels during the soldering process , thereby ensuring a precise flat leveling of the micro-LEDs resulting in a uni form display with a uni form directional light emission of the plurality of pixels .

According to at least one further aspect of the method, the modi fied surface structure inhibits the formation of solder bridges between adj acent electric contact pads during the soldering step . In particular, the modi fied surface structure reduces a probability of electrical short-circuits due to unwanted solder bridges between adj acent electric contact pads . For example , the modi fied surface structure physically restricts the flow of molten solder between adj acent electric contact pads . For example , the modi fied surface structure comprises pillars and/or trenches on or in the at least one area of the main surface of the display substrate . For example , the modi fied surface structure comprises at least one microchannel that is formed around at least one electric contact pad for guiding the flow of molten solder .

According to at least one further aspect of the method, each light source comprises or consists of a micro-LED . In particular, the micro-LED is a semiconductor light emitting diode ( short : LED) comprising a pn-j unction configured for at least partially converting an electric operating current into electromagnetic radiation . Micro-LEDs (micro light emitting diodes , short : micro-LEDs ) are semiconductor light emitting diodes with a particularly small si ze . For example , a growth substrate for an epitaxial growth of a semiconductor layer sequence of the micro-LED is removed from the micro-LED . In other words , the micro-LED does not comprise the growth substrate . For example , a thickness or height of the micro- LED in a growth direction of the semiconductor layer sequence is between 1,5 micrometer and 10 micrometer.

A light emission surface of the micro-LED can be rectangular or can have a different shape, such as triangular, hexagonal, or circular, for example. In particular, each lateral extension of the light emission surface is at most 100 micrometer or at most 70 micrometer in plan view of layers of the semiconductor layer sequence. For example, if the micro- LED has a rectangular shape, an edge length of the micro-LED - in particular in plan view of layers of the semiconductor layer sequence - is at most 70 micrometer or at most 50 micrometer. For example, the rectangular shaped light emission surface of the micro-LED has a width between 1 micrometer and 10 micrometer, inclusive, and a length between 10 micrometer and 70 micrometer, inclusive.

For example, micro-LEDs are provided on wafers with detachable holding structures, such that the micro-LED can be detached from the wafer non-destructively . Micro-LEDs may also be referred to as pLEDs, p-LEDs, uLEDs, u-LEDs, or Micro Light Emitting Diodes.

According to at least one further aspect of the method, the modified surface structure reduces or prevents a tilt, a deformation and/or a misalignment of the micro-LED during the soldering step. For example, the modified surface structure facilitates a self-leveling of the light sources, in particular of the micro-LEDs, during the soldering step. For example, the modified surface structure prevents an uncontrolled flow or runoff of molten solder off at least one electric contact pad, or off all electric contact pads. Such an uncontrolled runoff can cause a tilt or misalignment of the light source , for example . In particular, the modi fied surface structure ensures a precise flat leveling of the light sources , such that the main emission direction of each light source is perpendicular or almost perpendicular to the main surface of the display substrate within manufacturing tolerances , for example . For example , the flat leveling of the plurality of light sources advantageously results in a uni form display with a uni form directional light emission of the plurality of light sources .

According to at least one further aspect of the method, the modi fied surface structure has a lower surface wettability for molten solder than the electric contact pad and/or than the unmodi fied main surface of the display substrate . For example , the surface wettability is characteri zed by a contact angle of a droplet of molten solder that is in direct contact with the main surface of the display substrate or with the electric contact pad, respectively . In particular, the contact angle refers to an angle at which a liquid-vapor interface of the molten solder droplet meets the main surface of the display substrate or the main extension plane of the electric contact pad, respectively . Alternatively or in addition, the modi fied surface structure has a lower adhesion for molten solder than the electric contact pad and/or than the unmodi fied main surface of the display substrate .

For example , the contact angle of a molten solder droplet on the modi fied surface structure of the main surface of the display substrate is at least 10 ° larger, or at least 40 ° larger, or at least 90 ° larger than the contact angle of the molten solder droplet on the unmodi fied main surface of the display substrate and/or than the contact angle of the molten solder droplet on the electric contact pad . According to at least one further aspect of the method, each electric contact pad is completely surrounded by a corresponding area of the main surface with the modi fied surface structure . For example , the main surface comprises a plurality of annular or ring-shaped areas with the modi fied surface structure , wherein each electric contact pad is enclosed by a corresponding annular or ring-shaped area .

According to at least one further aspect of the method, at least one area with a modi fied surface structure is directly adj acent to a corresponding electric contact pad or directly adj oins a corresponding electric contact pad .

According to at least one further aspect of the method, the surface structure of the entire main surface of the display substrate aside from the plurality of electric contact pads is modi fied . For example , the surface structure of the entire main surface of the display substrate is modi fied before or after the electric contact pads are formed on the main surface of the display substrate .

According to at least one further aspect of the method, the surface structure is modi fied by exposing the at least one area of the main surface of the display substrate to a plasma . For example , the main surface of the display substrate is exposed to an oxygen plasma, a hydrogen plasma or an argon plasma in the at least one region . For example , the temperature of the plasma is at most 400 ° C . For example , the at least one area can be exposed to the plasma while the remainder of the main surface and/or the electric contact pads can be protected from the plasma by using a lithographic mask . For example , exposing the main surface in the at least one area to an oxygen plasma can increase the surface wettability of the at least one area, whereas an exposure to a hydrogen plasma can decrease the surface wettability .

According to at least one further aspect of the method, the surface structure is modi fied by irradiating the at least one area of the main surface of the display substrate with electromagnetic laser radiation . For example , the surface structure in the at least one area is modi fied by laser ablation . For example , the main surface is roughened in the at least one area using laser irradiation .

According to at least one further aspect of the method, the surface structure is modi fied by applying a superhydrophobic coating to the at least one area of the main surface of the display substrate . For example , the superhydrophobic coating repels molten solder due to its low surface energy . For example , the superhydrophobic coating comprises or consists of a polymer, polytetrafluoroethylene ( short : PTFE ) and/or a fluorinated silane . Alternatively or in addition, the at least one area of the main surface can also be coated with a thin layer photo-resist or with similar polymers , or with an oxide . For example , the contact angle of a molten solder droplet on the superhydrophobic coating is at least 90 ° , or at least 120 ° , or at least 150 ° .

According to at least one further aspect of the method, the modi fied surface structure comprises at least one nanopatterned area and/or at least one micro-patterned area for physically restricting the flow of molten solder . For example , the nano-patterned area and/or the micro-patterned area is formed by etching, photolithography, and/or laser ablation of the main surface in the at least one area . For example , the micro-patterned area comprises surface structures with a characteristic or average si ze in the range between 1 micrometer and 100 micrometer, inclusive . For example , the nano-patterned area comprises surface structures with a characteristic or average si ze in the range between 10 nanometer and 1000 nanometer, inclusive .

For example , i f the main surface of the display substrate is hydrophobic, increasing the surface area of the main surface by means of the nano- or micro-patterned area can result in a reduced wettability of the main surface for molten solder . For example , i f the main surface of the display substrate is hydrophilic, increasing the surface area of the main surface by means of the nano- or micro-patterned area can result in an increased wettability of the main surface for molten solder .

According to at least one further aspect of the method, the nano-patterned area comprises nano-pillars and/or nanotrenches . For example , at least one nano-pillar and/or at least one nano-trench has a circular, quadratic, rectangular, triangular or polygonal cross-section in plan view of the main surface of the display substrate . For example , the cross-section has a maximal diameter or an edge length between 10 nanometer and 1000 nanometer, inclusive . For example , a height of at least one nano-pillar and/or a depth of at least one nano-trench is between 10 nanometer and 1000 nanometer, inclusive , wherein the height or depth refers to a spatial extension in a direction perpendicular to the main surface of the display substrate .

According to at least one further aspect of the method, the micro-patterned area comprises micro-pillars and/or micro-trenches . For example , at least one micro-pillar and/or at least one micro-trench has a circular, quadratic, rectangular, triangular or polygonal cross-section in plan view of the main surface of the display substrate . For example , the cross-section has a maximal diameter or an edge length between 1 micrometer and 100 micrometer, inclusive . For example , a height of at least one nano-pillar and/or a depth of at least one nano-trench is between 1 micrometer and 500 micrometer, inclusive , wherein the height or depth refers to a spatial extension in a direction perpendicular to the main surface of the display substrate .

According to at least one further aspect of the method, a lithographic mask for defining the at least one area is disposed on the main surface of the display substrate before modi fying the surface structure . For example , the lithographic mask has at least one opening aligned with the at least one region of the main surface , where the surface structure is to be modi fied . For example , the lithographic mask protects the electric contact pads from the plasma, the laser radiation and/or an etchant during formation of the modi fied surface structure in the at least one area .

According to at least one further aspect , the method comprises a further step of modi fying a surface of at least one electric contact pad, at least in places , such that a surface wettability of the at least one electric contact pad for molten solder is increased, at least in places . For example , the surface of all electric contact pads is modi fied by exposing the electric contact pads to a plasma, in particular to an oxygen plasma, and/or by laser irradiation . For example , the contact angle of a molten solder droplet on the modi fied surface of the electric contact pad is at most 90 ° , or at most 60 ° , or at most 30 ° . For example , a lithographic mask is used for selectively modi fying the surface of the at least one electric contact pad at least in places . Alternatively or in addition, modi fying the surface of the at least one electric contact pad increases an adhesion of solder on the electric contact pad .

Further, a display substrate is speci fied herein . All features of the method for producing a display are also disclosed for the display substrate and vice versa .

According to at least one aspect , the display substrate has the main surface comprising the plurality of electric contact pads for electrically contacting the plurality of individually controllable light sources .

According to at least one further aspect of the display substrate , the main surface of the display substrate has the modi fied surface structure in at least one area adj acent to at least one electric contact pad .

According to at least one further aspect of the display substrate , the at least one area with the modi fied surface structure has a lower af finity for solder than the unmodi fied main surface and/or than the electric contact pads . For example , the at least one area with the modi fied surface structure has a lower surface wettability for molten solder than the unmodi fied main surface and/or than the electric contact pads .

According to an embodiment of the display substrate , the display substrate has a main surface comprising a plurality of electric contact pads for electrically contacting a plurality of individually controllable light sources , wherein

- the main surface of the display substrate has a modi fied surface structure in at least one area adj acent to at least one electric contact pad, and

- the at least one area with the modi fied surface structure has a lower af finity for solder than the unmodi fied main surface and/or than the electric contact pads .

According to at least one further aspect of the display substrate , the display substrate comprises a semiconductor material , sapphire , glass , and/or a polymer . For example , the polymer comprises or consists of poly methyl methacrylate ( short : PMMA) , polyimide or polycarbonate . Alternatively or in addition the display substrate can comprise or consist of a ceramic or a printed circuit board, for example .

According to at least one further aspect of the display substrate , a center-to-center distance between adj acent electric contact pads is at most 200 micrometer, or at most 100 micrometer, or at most 5 micrometer . For example , the center-to-center distance between adj acent electric contact pads is in the range between 5 micrometer and 400 micrometer, inclusive . For example , the plurality of electric contact pads is arranged on the main surface in the form of a regular array .

According to at least one further aspect of the display substrate , the modi fied surface structure comprises a surface roughening and/or a di f ferent molecular composition than the unmodi fied main surface of the display substrate . For example , an oxygen content of the modi fied surface structure is higher than an oxygen content of the unmodi fied main surface of the display substrate .

Further, a display is speci fied herein . In particular, the display can be produced according to the method for producing a display as speci fied above . All features of the method for producing a display are also disclosed for the display and vice versa .

According to an embodiment of the display, the display comprises a display substrate as speci fied above , and a plurality of light sources are soldered onto the plurality of electric contact pads .

According to a further aspect of the display, solder is not in direct contact with the modi fied surface structure of the main surface of the display substrate . For example , the solder is confined to the electric contact pads . For example , the modi fied surface structure repels molten solder during the soldering of the plurality of light sources onto the electric contact pads .

According to a further aspect of the display, at least one of the plurality of light sources comprises or consists of a micro-LED . For example , each light source is a micro-LED . In particular, each micro-LED is individually addressable and controllable via the plurality of electric contact pads . For example , the display comprises a plurality of individually controllable pixels and one , two , three or four micro-LEDs form one pixel of the display .

According to a further aspect of the display, the display has a plurality of individually controllable pixels , and each pixel comprises a red light emitting micro-LED, a green light emitting micro-LED and a blue light emitting micro-LED as light sources .

Further advantageous embodiments and further embodiments of the method for producing a display, the display substrate and the display become apparent from the following exemplary embodiments described in connection with the figures .

Figures 1 to 6 show schematic cross-sections of a state of a display after di f ferent steps of a method for producing a display according to an exemplary embodiment .

Figure 7 shows a schematic cross-section of a display according to an example .

Figure 8 shows a schematic cross-section of a display substrate according to an exemplary embodiment .

Figure 9 shows a schematic plan view of a display according to an exemplary embodiment .

Elements that are identical , similar, or have the same ef fect , are denoted by the same reference signs in the figures . The figures and the proportions of the elements shown in the figures are not to be regarded as true to scale . Rather, individual elements , may be shown exaggeratedly large for better representability and/or better understanding .

Figure 1 shows a first step of a method for producing a display according to an exemplary embodiment , wherein a display substrate 2 is provided . The display substrate 2 is rigid or flexible . For example , the display substrate 2 comprises or consists of silicon, sapphire , glass , or a polymer . The display substrate 2 has a main surface 21 where a plurality of electric contact pads 3 are arranged . The electric contact pads 3 comprise a metal layer or a metal layer sequence that is configured for soldering light sources 4 (not shown in Figure 1 ) thereon . The electric contact pads 3 are configured for providing individually controllable electric operating currents to the light sources 4 . A center- to-center distance D between adj acent electric contact pads 3 is between 5 micrometer and 400 micrometer, inclusive . The electric contact pads 3 are arranged on the main surface 21 of the display substrate 1 in the form of a regular two- dimensional array .

Figure 2 shows a second step of the method for producing a display according to an exemplary embodiment , wherein a layer of photo-resist for forming a lithographic mask 7 is disposed on the main surface 21 of the display substrate 2 . In particular, the entire main surface 21 including the electric contact pads 3 is covered by the layer of photo-resist .

Figure 3 shows a second step of the method for producing a display according to an exemplary embodiment , wherein the layer of photo-resist is structured by exposure to light and subsequently removing unwanted photo-resist , such that a lithographic mask 7 is formed that covers each electric contact pad 3 while the remainder of main surface 21 is free of the lithographic mask 7 . The lithographic mask 7 can also be structured such that it has a ring-shaped, rectangular, hexagonal or polygonal-shaped opening around each electric contact pad 3 , such that the main surface 21 of the display substrate 2 is accessible through the ring-shaped opening . Figure 4 shows a fourth step of the method for producing a display according to an exemplary embodiment , wherein a surface structure of the main surface 21 of the display substrate 2 in areas 22 adj acent to the electric contact pads 3 is modi fied by exposing it to an oxygen plasma .

Alternatively or in addition, the surface structure can also be modi fied by exposure to a di f ferent plasma, such as a hydrogen or an argon plasma, by laser irradiation, such as laser ablation, by etching such as wet etching or dry etching, and/or by coating the main surface 21 in the area 22 with a superhydrophobic coating .

In particular, the areas 22 are defined by the openings in the lithographic mask 7 . The areas 22 can be ring-shaped areas that are directly adj acent to the electric contact pads 3 and/or fully enclose the electric contact pads 3 . It is also possible that the surface structure of the entire main surface 21 of the display substrate 2 apart from the electric contact pads 3 is modi fied in the fourth step of the method . In other words , the area 22 can correspond to the entire main surface 21 of the display substrate 2 that is not covered by the electric contact pads 3 .

The process for modi fying the surface structure in the areas 22 of the main surface 21 can change a molecular arrangement or a molecular composition of the main surface 21 . For example , in the areas 22 an oxygen content of the main surface 21 is increased or decreased compared to the unmodi fied main surface 21 . Alternatively or in addition, the process for modi fying the surface structure in the areas 22 of the main surface 21 can change an ef fective surface area of the main surface 21 . In other words , the processes for modi fying the surface structure can give rise to an increased or a decreased surface roughness in the areas 22 compared to the unmodi fied main surface 21 . For example , the areas 22 with the modi fied surface structure comprise or consist of micro- and/or nano-structured areas . In particular, the areas 22 can comprise micro-channels , micro- or nano-pillars , and/or micro- or nano-trenches .

The process for modi fying the surface structure in the areas 22 preferably decreases the surface wettability of the main surface 21 for molten solder 5 (not shown in Figure 4 ) compared to the unmodi fied main surface 21 . The modi fied surface structure in the areas 22 is configured to precisely control the flow of molten solder 5 during a subsequent soldering step . For example , the modi fied surface structure in the areas 22 prevents or reduces the formation of solder bridges and thus electrical short-circuits between adj acent electrical contact pads 3 . Moreover, the modi fied surface structure in the areas 22 prevents or reduces an uncontrolled spreading of solder 5 that can give rise to an unwanted tilting of the light sources 4 with respect to the main surface 21 during the subsequent soldering step .

As a further optional step, a surface structure of the electric contact pads 3 can be modi fied at least in places before or after modi fying the surface structure in the areas 22 of the main surface 21 of the display substrate 2 . In particular, the surface wettability of the electric contact pads 3 for molten solder 5 can be increased at least in places , for example by exposing the electric contact pads 3 to a plasma and/or by irradiating the electric contact pads 3 with laser radiation . This allows for a further increased control of the flow of molten solder during the subsequent soldering step . Figure 5 shows a fi fth step of the method for producing a display according to an exemplary embodiment , wherein a plurality of light sources 4 are arranged on the plurality of electric contact pads 3 . Each light source 4 is a micro-LED comprising a semiconductor layer sequence with a pn-j unction for at least partially converting an electric operating current into electromagnetic radiation, preferably in the visible and/or in the infrared spectral range . In particular, for each light source 4 the display substrate 2 comprises two corresponding electric contact pads 3 for electrically contacting an anode and a cathode of each light source 4 , for example . A drop of solder 5 , such as solder paste , is arranged on each electric contact pad 3 . Alternatively, the drop of solder 5 can also be arranged on the light sources 4 before arranging the light sources 4 on the electric contact pads 3 . Subsequently, the light sources 4 are trans ferred from a temporary carrier to the display substrate 2 and placed on the electric contact pads 3 using a pick-an-place method or by a trans fer printing method using a stamp 8 , for example .

Figure 6 shows a sixth step of the method for producing a display according to an exemplary embodiment , wherein the light sources 4 are soldered onto the electric contact pads 3 and the production of the display 1 is completed . For example , soldering is performed using a reflow process , wherein the display substrate 2 , the electric contact pads 3 with the solder 5 disposed thereon and the light sources 4 arranged thereon is heated above a melting point of the solder 5 . The modi fied surface structure in the areas 22 of the main surface 21 around the electric contact pads 3 prevents an uncontrolled spreading of the molten solder 5 . Consequently, the light sources 4 are precisely levelled, giving rise to a uni form display where the main emission directions of the plurality of light sources 4 are parallel or almost parallel within manufacturing tolerances .

Figure 7 shows an example of a display . In contrast to the display according to the exemplary embodiment shown in Figure 6 , the display substrate 2 in the example of Figure 7 does not comprise the areas 22 with a modi fied surface structure around the electric contact pads 3 . Consequently, the solder 5 can spread in an uncontrolled manner during the soldering step, leading to solder-bridges and thus short-circuits between adj acent electric contact pads , for example . Further, the uncontrolled spreading of the solder can lead to a tilt of light sources 4 upon soldering, whereby a light outcoupling surface of the light source 4 is no longer parallel to the main surface 21 of the display substrate 2 . The tilting can be characteri zed by a tilt angle cp between a surface normal of the main surface 21 and a surface normal of the main extension plane of the light source 4 . For example , the modi fied surface structure in the areas 22 of the main surface 21 of the display substrate 2 described in connection with Figure 6 allows to limit the tilting angle cp to at most 5 ° , or to at most 1 ° . For example , the tilting angle cp is between 1 ° and 20 ° , inclusive .

The display substrate 2 according to the exemplary embodiment in Figure 8 is a CMOS substrate comprising silicon . A plurality of electric contact pads 3 for electrically contacting a plurality of light sources 4 , in particular micro-LEDs , is arranged on the main surface 21 of the display substrate 2 . An integrated circuit 9 for controlling and/or driving a plurality of light sources 4 is integrated into the display substrate 2 . The main surface 21 of the display substrate 2 corresponds to a CMOS backplane , i . e . the integrated circuit 9 is formed on a side of the display substrate 2 opposite to the main surface 21 . The integrated circuit 9 is electrically connected to the electric contact pads 3 by through connections 10 , such as plated through- holes or vias .

The main surface 21 of the display substrate 2 comprises a plurality of areas 22 with a modi fied surface structure . Speci fically, each electric contact pad 3 has a corresponding area 22 that is directly adj acent to and completely surrounds the respective electric contact pad 3 . The areas 22 comprise a micro- and/or nano-structured region of the main surface 21 of the display substrate 2 . The micro- and/or nano-structured region is formed using a photolithographic process , for example .

The micro-structured region comprises micro-pillars 61 and/or micro-trenches 62 , such as micro-channels . A height or depth of one micro-pillar 61 or one micro-trench 62 in a direction perpendicular to the main surface 21 is between 1 micrometer and 500 micrometer, inclusive . A lateral spatial extension of one micro-pillar 61 or one micro-trench 62 in a direction parallel to the main surface 21 is between 1 micrometer and 100 micrometer, inclusive . The micro-pillars 61 can also be conically shaped or non-uni form, for example . The nanostructured region comprises nano-pillars 61 as well as nanotrenches 62 . A height or depth of one nano-pillar 61 or one nano-trench 62 in a direction perpendicular to the main surface 21 is between 10 nanometer and 1000 nanometer, inclusive . A lateral spatial extension of one nano-pillar 61 or one nano-trench 62 in a direction parallel to the main surface 21 is between 10 nanometer and 1000 nanometer, inclusive . The nano-pillars 61 can also be conically shaped or non-uni form, for example . The micro- and/or nanostructured region in the area 22 physically restricts the flow of molten solder of f the electric contact pads 3 during a soldering step, for example .

Figure 9 shows a plan view on the main surface 21 of the display substrate 2 of a display 1 according to an exemplary embodiment . A plurality of light sources 4 , in particular micro-LEDs , are soldered onto electric contact pads 3 (not shown in Figure 9 ) that are arranged on the main surface 21 of the display substrate 2 . The light sources 4 are arranged in the form of a regular quadratic or rectangular array . Each pixel 11 of the display 1 is formed by four adj acent light sources 4 . For example , each pixel 11 consists of one red- light emitting light source 4 , one blue-light emitting light source 4 and two green-light emitting light sources 4 , wherein each light source 4 is independently controllable . It is also possible that each pixel 11 consists of three or more than four light sources 4 .

Around each electric contact pad 3 the main surface 21 comprises a ring-shaped area 22 with a modi fied surface structure . The modi fied surface structure has a reduced af finity and/or a reduced surface wettability for molten solder 5 (not shown in Figure 9 ) compared to the unmodi fied main surface 21 and compared to the electric contact pads 3 . In particular, molten solder 5 is repelled from the areas 22 during a soldering step . Accordingly, solder 5 is not in direct contact with the areas 22 on the main surface 21 of the display substrate 2 . This patent application claims the priority of German patent application DE 102024115643 . 0 , the disclosure content of which is hereby incorporated by reference . The invention is not restricted to the exemplary embodiments by the description on the basis of said exemplary embodiments . Rather, the invention encompasses any new feature and also any combination of features , which in particular comprises any combination of features in the patent claims and any combination of features in the exemplary embodiments , even i f this feature or this combination itsel f is not explicitly speci fied in the patent claims or exemplary embodiments .

References

I display

I I pixel 2 display substrate

21 main surface

22 area

3 electric contact pad

4 light source 5 solder

61 pillar

62 trench

7 mask

8 st amp 9 integrated circuit

10 through connection

D distance cp tilt angle

Claims

1. A method for producing a display (1) , including the steps of :

- providing a display substrate (2) with a main surface (21) comprising a plurality of electric contact pads (3) for electrically contacting a plurality of individually controllable light sources (4) ,

- modifying a surface structure of the main surface (21) of the display substrate (2) in at least one area (22) adjacent to at least one electric contact pad (3) , and

- soldering the plurality of light sources (4) onto the plurality of electric contact pads (3) , wherein

- the surface structure is modified for controlling a flow of molten solder (5) during the soldering step.

2. The method according to the previous claim, wherein the modified surface structure inhibits the formation of solder bridges between adjacent electric contact pads (3) during the soldering step.

3. The method according to any of the previous claims, wherein

- each light source (4) comprises a micro-LED, and

- the modified surface structure reduces or prevents a tilt, a deformation and/or a misalignment of the micro-LED during the soldering step.

4. The method according to any of the previous claims, wherein the modified surface structure has a lower surface wettability for molten solder (5) than the electric contact pad (3) and/or than the unmodified main surface (21) of the display substrate (2) .

5. The method according to any of the previous claims, wherein the surface structure of the entire main surface (21) of the display substrate (2) aside from the plurality of electric contact pads (3) is modified.

6. The method according to any of the previous claims, wherein the surface structure is modified by exposing the at least one area (22) of the main surface (21) of the display substrate (2) to a plasma.

7. The method according to any of the previous claims, wherein the surface structure is modified by irradiating the at least one area (22) of the main surface (21) of the display substrate (2) with electromagnetic laser radiation.

8. The method according to any of the previous claims, wherein the surface structure is modified by applying a superhydrophobic coating to the at least one area (22) of the main surface (21) of the display substrate (2) .

9. The method according to any of the previous claims, wherein the modified surface structure comprises at least one nano-patterned area (22) and/or at least one micro-patterned area (22) for physically restricting the flow of molten solder ( 5 ) .

10. The method according to the previous claim, wherein

- the nano-patterned area (22) comprises nano-pillars (61) and/or nano-trenches (62) , and/or

- the micro-patterned area (22) comprises micro-pillars (61) and/or micro-trenches (62) .

11. The method according to any of the previous claims, wherein a lithographic mask (7) for defining the at least one area (22) is disposed on the main surface (21) of the display substrate (2) before modifying the surface structure.

12. The method according to any of the previous claims, comprising a further step of modifying a surface of at least one electric contact pad (3) such that a surface wettability of the at least one electric contact pad (3) for molten solder (5) is increased.

13. A display substrate (2) with a main surface (21) comprising a plurality of electric contact pads (3) for electrically contacting a plurality of individually controllable light sources (4) , wherein

- the main surface (21) of the display substrate (2) has a modified surface structure in at least one area (22) adjacent to at least one electric contact pad (3) , and

- the at least one area (22) with the modified surface structure has a lower affinity for solder (5) than the unmodified main surface (21) and/or than the electric contact pads ( 3 ) .

14. The display substrate (2) according to the previous claim, wherein the display substrate (2) comprises a semiconductor material, sapphire, glass, or a polymer.

15. The display substrate (2) according to any of claims 13 or 14, wherein a center-to-center distance (D) between adjacent electric contact pads (3) is at most 200 micrometer.

16. The display substrate (2) according to any of claims 13 to 15, wherein the modified surface structure comprises a surface roughening and/or a different molecular composition than the unmodified main surface (21) of the display substrate ( 2 ) .

17. A display (1) , comprising:

- a display substrate (2) according to any of claims 13 to 16,

- a plurality of light sources (4) soldered onto the plurality of electric contact pads (3) .

18. The display (1) according to the previous claim, wherein at least one of the plurality of light sources (4) comprises a micro-LED.

19. The display (1) according to any of claims 17 or 18, wherein

- the display (1) has a plurality of individually controllable pixels (11) , and

- each pixel (11) comprises a red light emitting micro-LED, a green light emitting micro-LED and a blue light emitting micro-LED as light sources (4) .