CN116632125A - Fluid-Assembled Carrier Substrate System for Mass Transfer of Micro-LEDs - Google Patents

Abstract

A micro-LED macro-transfer imprint system includes an imprint template having an array of capture locations, each configured with a columnar recess to temporarily secure a keel extending from a bottom surface of a micro-LED. For surface mount micro LEDs, the keels are non-conductive. In the case of a vertical micro LED, the keel is the second electrode that is electrically conductive. The imprint system further includes a fluid-assembled carrier substrate having an array of wells with a pitch separating adjacent wells that matches the pitch separating the capture locations of the stamp substrate. The display substrate includes an array of micro LED connection pads at the same pitch as the capture locations. The top surface of the stamp substrate is pressed against the display substrate, each capture location is connected to a respective micro LED location, and then the micro LEDs are transferred. A fluid assembled impression substrate for use with micro LEDs having a keel or axial is also provided.

Description

Fluid-Assembled Carrier Substrate System for Mass Transfer of Micro-LEDs

This application is a divisional application of a patent with the application number 202111275465.4, the application date is 2021-10-29, and the invention title is "Mass transfer method of micro light-emitting diode display based on fluid assembly".

technical field

The present application relates to the field of micro-light emitting diode (micro-LED) displays, in particular to a mass transfer system and method for micro-light emitting diodes in the display manufacturing process.

Background technique

Red-green-blue (RGB) displays consist of pixels that emit light at three wavelengths, corresponding to red, green, and blue in visible light. The RGB portions of these pixels (each called a sub-pixel) are energized in a systematic fashion to add up to produce colors in the visible spectrum. Different monitors generate RGB images in different ways. Liquid Crystal Displays (LCDs) are currently the most popular technology, which generate RGB images by illuminating color filters on sub-pixels with a white light source (usually a fluorescent white LED). Some wavelengths of light contained in white light are absorbed, and other wavelengths are transmitted through the color filter. As a result, an LCD display can be less than 4 percent efficient, and its contrast ratio is limited by light leaking from the liquid crystal cells. An Organic Light Emitting Diode (OLED) display directly emits light of a corresponding wavelength by exciting an organic light-emitting material in each sub-pixel, thereby generating RGB light. OLED pixels emit light directly, so the display has a high contrast ratio, but the organic material degrades over time, causing image burn-in.

The third display technology that this application is concerned with is micro-LED displays, which use tiny (5-150 micron (μm) body diameter) inorganic LEDs as sub-pixels and emit light directly. Inorganic LED displays have many advantages over other displays. Compared with LCD displays, LED displays have a contrast ratio of more than 50,000:1 and higher efficiency. Unlike OLED displays, inorganic LEDs do not age and can achieve significantly higher brightness.

The current mainstream high-definition television (High Definition Television, HDTV) resolution standard TV has two million pixels (or six million sub-pixels), and the higher-resolution 4K and 8K standards are eight million and three million pixels, respectively. Thirteen megapixels. Even the relatively small displays used in tablets and cell phones have millions of pixels, with displays having a resolution of more than six hundred pixels per inch (ppi). Therefore, the manufacture of display screens using micro-LEDs requires the assembly of large-area micro-LED arrays with different pixel pitches at a lower cost, so that displays of various sizes and resolutions can be manufactured. The most traditional micro-LED array assembly technology is called pick-and-place technology, because each micro-LED is individually picked from a carrier and placed on a substrate, as described below. Since each tiny LED is handled individually, the assembly process is very slow.

Figures 1A-1C depict a cross-sectional view of a Gallium Nitride (GaN)-based LED stack (Figure 1A), a cross-sectional view of two fully fabricated vertical micro-LEDs (Figure 1B), and a surface-mounted micro-LED Cross-sectional view of a diode (Fig. 1C) (prior art). The already widespread adoption of GaN-based high-brightness LEDs for general lighting has created a complex manufacturing system, so miniaturized LEDs for use in displays are based on investments already made by the industry. GaN-based LEDs emitting at blue (about 440 nanometers (nm)) wavelengths were fabricated in a complex series of high-temperature metal-organic chemical vapor deposition (MOCVD) steps to produce the vertical LED structure shown in cross-section in Figure 1A. The fabrication process takes place on polished sapphire, silicon, or silicon carbide (SiC) substrates with a diameter of 50-200 millimeters (mm). The surface is prepared by depositing undoped GaN and optionally depositing an aluminum nitride (AlN) buffer layer, resulting in a lattice surface with low defects and GaN lattice constant. Since it is necessary to adjust the thickness and temperature of the initial deposition to compensate for the lattice mismatch between the substrate and GaN, that is, to increase the thickness to improve the surface quality, the thickness of high-efficiency devices is higher than about 3 μm. Since the MOCVD deposition process is complex and expensive, it is important to optimize the micro-LED process to most efficiently utilize the entire area of the growth wafer.

After the initial growth to form the crystalline GaN surface, the first LED layer is created by adding silicon doping to form n+ GaN for the cathode. Optionally, the stack may include layers tuned for electron injection and hole blocking. Next, a Multiple Quantum Well (MQW) structure with alternating layers of Indium Gallium Nitride (In _x Ga _1-x N) and GaN is deposited, where the indium content and the thickness of the layers determine the emission wavelength of the device . Increasing the indium content shifts the emission peak to longer wavelengths, but also increases the stress due to lattice mismatch, so high-efficiency GaN devices cannot be made for red emission, and green LEDs are less efficient than Blue LEDs. After forming the MQWs, the stack structure may also include layers tuned for electron blocking and hole injection. The MOCVD layer sequence is completed by depositing magnesium (Mg) doped GaN to form the p+ anode layer.

LEDs used for general lighting (up to 3-4mm per side) are much larger than micro-LEDs (5-150μm in diameter) used in micro-LED displays, so the requirements for patterning and electrodes are significantly different. Miniature light-emitting diodes need to be bound to substrate electrodes with soldering materials or Asymmetric Conductive Film (ACF), while large-scale LEDs are usually bound to lead frames by wire bonding or solder paste. Due to the very small size of micro-LEDs, most of the area on the MOCVD wafer is removed during the patterning process, reducing the available light-emitting area per wafer. LED wafers are relatively expensive, and the high resolution required to manufacture micro-LEDs further drives up the cost, so it is important to use the light-emitting area as efficiently as possible to minimize the material cost of micro-LED displays.

In the simplest process flow, by depositing a thin (several nanometers) nickel oxide (NiO _X ) to match the p+GaN work function, and depositing a layer of Indium Tin Oxide (ITO) with a thickness of 50-300nm , to form transparent conductive electrodes on the MOCVD stack. The deposited stack structure is then patterned and etched, usually using a chlorine (Cl ₂ )-based reactive ion etch (Reactive Ion Etch, RIE) process to produce individual miniature light-emitting diodes with the smallest practical size and spacing, Especially when producing miniature light-emitting diodes with high efficiency, the thickness of the LED structure is only 3-5 μm, so the thickness of the LED structure limits the smallest space that can be successfully etched.

After etching the outline of the LED, as schematically shown in Figure 1C, additional processing is performed to form electrodes on the anode. In order to prevent leakage and etch out the opening for connecting the ITO layer, a passivation layer is usually provided, and the passivation layer is usually made of plasma-enhanced chemical vapor deposition (Plasma-enhanced Chemical Vapor Deposition, PECVD) silicon dioxide (Silicon Dioxide, SiO ₂ ) or optionally a thin atomic deposition (Atomic Layer Deposition, ALD) aluminum oxide (Aluminum Oxide, Al ₂ O ₃ ) layer on the surface. The anode structure is accomplished by depositing an electrode stack comprising materials such as indium/tin (In/Sn) or gold, germanium (Au/Ge) alloys.

Figure 2A depicts the process of removing micro LEDs from a sapphire substrate using laser lift off (Laser Lift Off, LLO). Figure 2B depicts the pick-and-place process in which devices are moved from the carrier wafer and placed on the display substrate. Figure 2C depicts the connection of the micro LED anode to the substrate electrode (prior art). Specifically, in FIG. 2A , the fabricated miniature light-emitting diodes are bonded to the carrier wafer through an adhesive layer, and removed from the sapphire substrate by laser lift-off. In FIG. 2B , a micro LED can be removed from the carrier by a pick-up head and placed on a sub-pixel with its anode electrically connected to the corresponding electrode on the substrate. Pixels are implemented by coating the micro-LEDs with a suitable dielectric (such as photo-patternable polyimide), while connecting the cathodes of the micro-LEDs to electrodes on the substrate. Metal interconnects are deposited and patterned to form connections as shown in Figure 2C.

LEDs that emit red light with a wavelength of about 630nm, which are usually made of aluminum gallium indium phosphide (AlGaInP) grown on gallium arsenide (GaAs). Since GaAs is opaque, it cannot be removed from the GaAs substrate using laser lift-off technology. Peel off the LEDs. So if you want to lift the red LED off the substrate, you can either etch the substrate completely, or use a selective etch (usually using hydrogen chloride (HCl): acetic acid) to undercut and lift off the LED. The dimensions (cross-section) of the LEDs are similar to GaN general lighting LEDs (150-1000 μm in size). The AlGaInP LED process is more fully described in patent: US 10,804,426, which is incorporated herein by reference.

There are some important issues with the pick-and-place assembly process described above that lead to higher costs and lower yields. Specifically, the assembly process is serial in nature, so assembling millions of tiny LEDs takes a long time and is expensive. The small size of the micro-LEDs themselves makes the gripper head difficult to fabricate, and the edges of the gripping instrument are likely to interfere with adjacent micro-LEDs during gripping or with reflector structures between pixels during assembly. The single pick-and-place method described above can be extended to a parallel process by using a mass transfer head to simultaneously pick and transfer multiple miniature LEDs. However, the quality of this mass transfer method can be poor due to the presence of defective components in a group of micro-LEDs transferred at the same time, and the spacing between each micro-LED is determined by the growth process on the wafer. determined by the spacing of the components on the

3A-3H depict an example of a mass transfer method (prior art). The mass transfer method is to transfer a plurality of micro light-emitting diodes arranged in an array to a display substrate as a whole, and has been widely developed to solve the low throughput problem of serial pick-and-place assembly. In the simplest mass transfer process, a rectangular imprint stamp picks up a rectangular micro-LED array from a carrier, and presses the micro-LEDs against the display substrate so that each micro-LED is combined with a corresponding electrode. Since different colors of LEDs need to be considered in making an RGB display, the transfer stamp stamp is arranged to be picked up every third LED, thus leaving room for the other two sub-pixel colored LEDs. For the surface-mount miniature LEDs shown in Figure 2C, the assembly process proceeds in the following order:

1) Prepare a separate MOCVD wafer for each color of micro-LEDs, with appropriate size and spacing between each micro-LED. The gap between adjacent micro LEDs is called pitch. See Figure 3A. Each tiny LED has a cathode and anode for connection to the display substrate. The array of micro LEDs is removed from the growth wafer by laser lift-off and held on a carrier substrate (not shown).

2) Multiple sets of cathode electrodes and anode electrodes are arranged on the display substrate (Figure 3B). The distance between each set of electrodes is several times the distance between each micro-LED on the wafer. The positions of the tiny LEDs will match each other. This spacing determines the final resolution of the display. The electrodes may be copper, indium tin oxide/aluminum (ITO/Al), gold, or solder such as tin/indium (Sn/In). It is also possible to use an ACF membrane to cover the electrodes. By determining the materials of the electrodes on the display panel and the micro light emitting diodes, ohmic contacts can be formed through the subsequent bonding process in step 5 below.

3) Prepare the embossed stamp based on where the pick-up point matches the sub-pixel pitch on the display. Pickup mechanisms currently used to hold each tiny LED include elastomers, tape, static electricity, and magnetic fields. Figure 3C depicts an embossed stamp with a size of 3*3 pixels, but actual imprinted stamps usually assemble hundreds of pixels.

4) See Figure 3D, the embossed stamp is placed in alignment with the carrier substrate carrying the micro-LEDs of the first color, and the imprinted stamp is in contact with the carrier substrate, so that the fixing structure can grab multiple micro-LEDs and remove it from the carrier substrate.

5) See Figure 3E, the filled embossed stamp is placed in alignment with the electrodes on the first set of display substrates.

6) Referring to FIG. 3F , the embossed stamp is pressed against and contacts the display substrate, and is usually heated together so that the electrodes of the micro-LEDs and the electrodes on the display substrate form a binding structure. After the bond has formed and cooled sufficiently to secure the miniature LEDs, the imprint stamp is removed for reuse.

7) As shown in FIG. 3G-FIG. 3H, the same operation is performed on the micro light-emitting diodes of the second color and the third color respectively, so as to form an RGB display array.

The above-mentioned mass transfer method for assembly is feasible and has been applied in the manufacture of displays, but there are still some problems resulting in low product yield and high product cost. First, in FIG. 3B , the pitch of the display in the x-direction and y-direction can only be an integer multiple of the pitch between the micro light-emitting diodes on the MOCVD wafer, and the example in the figure is 3*2. And a complete display manufacturing technology must be able to manufacture screens of different sizes that meet industry standards, such as 4K (3840*2160 pixels), so a technology that can change the spacing of micro-LEDs on the embossed stamp is needed (pitch expansion) . It is also possible to customize the size and resolution of the tiny LEDs on the MOCVD wafer for each display, but this adds unnecessary cost. Second, the pick-up device must strike a balance in the size of the connection strength. If the connection strength is too small, some micro LEDs will not detach from the carrier substrate, leaving gaps in the array. On the contrary, if the connection strength is too high, the micro LEDs will be forcibly removed after being soldered on the substrate. In both cases, there is a reduction in the brightness of the sub-pixels, which cannot be tolerated in a display. Finally, the structure of transfer imprint stamps is complex and difficult to manufacture. The connection points must be smaller than the spacing between the micro-LEDs so as to avoid the embossed stamp from interfering with adjacent micro-LEDs. This is difficult for complex fixation methods that require the generation of localized fields such as electrostatic or magnetic forces. Imprint stamps are also susceptible to contamination and damage, especially those made of elastomers such as polydimethylsiloxane (PDMS), so how to effectively clean the imprint stamps for repeated use is also very important.

To illustrate the drawbacks of the mass transfer imprinting process, Figure 3H depicts several possible failure scenarios:

Fault a: Missing micro-LEDs due to poor adhesion of the imprinted stamp when picked up;

Fault b: Misplacement of micro-LEDs due to contamination on the embossed stamp;

Fault c: Particles due to contamination of the transfer imprint stamp;

Fault d: Broken micro-LED;

Fault e: Micro LED short circuit due to defects in MOCVD process:

Fault f: Electrode damage due to forcible removal of micro-LEDs by embossed stamp.

4A and 4B depict exemplary area coverage for imprint pick-up using a 14 mm imprint stamp ( FIG. 4A ) on a 100 mm wafer. Of these, 20% of the micro-LEDs remained on the wafer, and there were defective micro-LEDs on three imprinted stamps. Another limitation of the mass transfer process described above is the square shape of the imprint stamp, which does not match the circular wafers used to generate LEDs with MOCVD. Figure 4A shows a typical arrangement of 14*14mm embossed stamps on a 100mm wafer. Using large-area imprint stamps can speed up assembly, at the expense of leaving more tiny LEDs on the growing wafer. There are large areas on the wafer where imprint stamps cannot be used due to the requirement to fill all imprint stamps. In the above example, discarded mini LEDs of acceptable quality account for about 20% of the total, which directly increases the cost. Furthermore, defective micro-LEDs must be repaired or the affected stamp stamps discarded. The above example depicts three random defects for illustrative purposes only, and if the defective imprint stamp is discarded in this example, only about 70% of the initial miniature LEDs can be used for the manufacture of the display.

A significant advantage of the mass transfer method is that the bonding process takes place under pressure on the micro-LEDs, so that there is good mechanical contact between the two bonding electrodes. This ensures a large area of contact between the electrodes. Mechanical contact also breaks insulating oxides on the surface, which improves the wetting of the solder material. ACF bonding also requires pressure to make hard contact between the conductive fill material and the micro LEDs and electrodes on the display substrate.

It would be advantageous to have a structure and method that can fill a carrier substrate for mass transfer assembly of micro-LED displays, and that can increase assembly flexibility and yield in the following manner:

1. Any display resolution can be realized through simple spacing expansion;

2. It is possible to manufacture a series of miniature light-emitting diodes (chips known to be good) without device defects such as missing, damaged or short-circuited;

3. The assembly speed of mass transfer can be improved by filling and transferring imprinted stamps by means of massive parallel transfer;

4. Use simple transfer embossing stamps, which have low manufacturing costs and can be reused through strong cleaning;

5. A simple imprinting mechanism that does not damage the display substrate can be used;

6. Excess micro-LEDs can be recovered from defective embossed stamps.

Contents of the invention

The present application provides a method of fabricating micro-LED arrays on carrier substrates or transfer imprint stamps by fluid assembly and related structures. The assembled miniature light-emitting diodes can be applied to the display substrate and bonded by mass transfer method. Micro-LEDs are fabricated on wafers by conventional MOCVD methods, and their shape is chosen in a pattern that facilitates fluid assembly and imprinting onto display substrates.

Therefore, the present application provides a micro-LED mass transfer imprinting system, which includes an embossing stamp substrate having a top surface. An array of embossed stamp substrate capturing positions are formed on the top surface, each of the capturing positions is provided with a columnar groove for temporarily fixing the keel extending from the bottom surface of the micro light emitting diode. When the micro light emitting diode is a surface mount type, it has a planar top surface, and the planar top surface includes a first electrode and a second electrode. When the micro light emitting diode is vertical, it has a planar top surface with a first electrode, and at this time the keel is a conductive second electrode. The imprinting system also includes a fluid assembled carrier substrate having an array of wells formed on a planar top surface of the carrier substrate, the array of wells having a spacing separating adjacent wells, the spacing being different from the The pitch separating adjacent said capture locations on the imprinted stamp substrate is matched.

A related micro-LED mass transfer method includes: providing the above-mentioned fluid-assembled carrier substrate with an array of wells, and providing the above-mentioned embossed stamp substrate, wherein each trapping position in the array is configured with the carrier substrate. The wells on the bottom match the columnar grooves. The method uses a fluidic assembly process such that micro-LEDs are filled into wells of the carrier substrate. The method transfers the micro LEDs from the carrier substrate to the embossed stamp substrate by pressing the top surface of the embossed stamp substrate against the top surface of the carrier substrate so that each of said trapping sites corresponds to a corresponding well superior. The groove at each trapping position bears a keel extending from the bottom surface of the micro-LED, and by binding the keel, the micro-LED is fixed to the embossed stamp substrate. The use of a carrier substrate removes the limitation imposed by the pitch between micro-LEDs on the MOCVD wafer, allowing various imprinted stamp substrate pitches to be applied to different display substrate sizes and resolutions.

The method also provides a display substrate having an array of micro LED connection pads, wherein each micro LED connection pad includes at least one electrode formed on the top surface, said electrode being electrically connected to an underlying matrix of column and row control lines. The connection pads have a pitch separating adjacent locations that matches a pitch separating adjacent capture locations in the stamp substrate. The method presses the top surface of the embossed stamp substrate onto the top surface of the display substrate, each capture location is connected to a corresponding micro LED location, and transfers a plurality of micro LEDs from the embossed stamp substrate to the display substrate. connection pads. In one aspect, the step of transferring the micro-LEDs to the connection pads of the display substrate includes heating the display substrate to bind the plurality of micro-LEDs to the plurality of connection pads. In the case of RGB display, the method can sequentially press the embossed stamp substrates with the micro-LEDs of the first wavelength, the micro-LEDs of the second wavelength, and the micro-LEDs of the third wavelength on the capture position, or a separate The imprinted stamp on the substrate corresponds to a wavelength of microscopic LEDs.

The present application also provides a method for mass transfer of miniature light-emitting diodes, which adopts a fluid-assembled imprinted stamp substrate having a planar top surface, on which a plurality of bottoms having a first peripheral shape, depth and plane are formed. The capture location on the surface. Through a fluid assembly process, the trapping site may be filled with micro-LEDs having a first perimeter shape, the micro-LED having a thickness greater than the depth of the trapping site, a bottom surface in contact with a bottom surface of the trapping site, a plane The top surface and the first electrode extending out of the capture location, and a securing mechanism. On the one hand, the fixing mechanism is a keel formed on the top surface of the miniature light-emitting diode. The keel can be a conductive keel connected to the first electrode, or it can be fixed on the substrate of the embossed stamp. Temporary non-conductive studs that are later removed. On the other hand, the fixing mechanism is a first component including a pair of conjugated biomolecules disposed on the bottom surface of each micro LED. In this case, the bottom surface of each capture site is provided with a second component comprising pairs of conjugated biomolecules.

As described above, the method provides a display substrate having a planar top surface and an array of micro-LED connection pads, each micro-LED connection pad includes a first electrode formed on the top surface, the first electrode and the column below the substrate and the matrix of row control lines are electrically connected. The spacing between the adjacent wells of the display substrate matches the spacing of the trapping sites adjacent to the embossed stamp substrate. The method embossing presses the top surface of the stamp substrate against the top surface of the display substrate, fills each trapping site with a micro LED, and transfers the micro LEDs from the embossed stamp substrate to the micro light emitting diodes of the display substrate. Diode connection pad. Also, during the transfer process, the display substrate can be heated to facilitate electrode binding.

The present application also provides a mass transfer method of axial miniature light emitting diodes. The method provides a fluid assembled imprint stamp substrate having a planar top surface, the top surface having a plurality of capture sites formed thereon, the capture sites having a first perimeter shape, a first depth having a planar surface A central portion, a distal end having a planar second depth, the second depth being less than the first depth, and a proximal end having a planar second depth. Through a fluid assembly process, the method enables axial micro-LEDs to be filled into trapping locations, each micro-LED occupying a respective trapping location having said first peripheral shape, a body connected to said central portion, and It has a vertical plane main body, and the thickness of the main body is greater than the first depth but less than twice the first depth. a distal electrode that bisects the body horizontally and is in contact with the distal end of the trapping site, the distal electrode having an electrode thickness in a vertical plane that is greater than a second depth of the trapping site but less than twice the second depth of the trapping site. depth. A proximal electrode has the same thickness as the distal electrode, horizontally bisects the body and contacts the proximal end of the capture site.

The method provides a display substrate having a planar top surface and an array of micro-LED connection pads, each micro-LED connection pad comprising a pair of electrodes formed on the top surface and electrically connected to a matrix of column and row control lines below it . The display substrate has a pitch separating adjacent wells that matches the pitch separating adjacent trapping sites on the imprinted stamp substrate. The method presses the top surface of the embossed stamp substrate against the top surface of the display substrate, brings each capture site into contact with a corresponding micro LED, and transfers the micro LEDs from the embossed stamp substrate to the display substrate Above, heating is usually also required to facilitate binding of the electrodes.

The above-mentioned system and method will be described in detail below.

Description of drawings

Figures 1A-1C are a cross-sectional view of a GaN-based LED (Fig. 1A), a cross-sectional view of two vertical micro-LEDs (Fig. 1B), and a cross-sectional view of a surface-mount micro-LED (Fig. 1C) (prior art) .

FIG. 2A is a process of removing micro-LEDs from a sapphire growth substrate using laser lift-off technology (prior art).

FIG. 2B is a pick-and-place process for moving and positioning devices from a carrier wafer onto a display substrate (prior art).

FIG. 2C is a process of connecting the anode of the micro LED to the substrate electrode (prior art).

3A-3H are steps of an exemplary bulk transfer process (prior art).

Figures 4A-4B are examples of footprints for imprint pick-up using 14mm imprint stamps on a 100mm wafer (Figure 4A), leaving 20% of the micro LEDs on the wafer and three imprint stamps The micro-LEDs on the chip have defects (FIG. 4B) (prior art).

5 is a partial cross-sectional view of a typical backplane arrangement showing a surface mount micro-LED and a power transistor controlling the brightness of the micro-LED.

6A-6B are respectively a top view and a cross-sectional view of a surface-mount micro-LED for fluid assembly.

FIG. 7 is a schematic diagram of a micro-LED wafer after selective pickup.

Figure 8 is a short description of the fluid effect that allows 100% of micro LEDs to be assembled in the correct orientation with electrodes down.

9A-9D are the steps of using the micro-LED mass transfer imprinting system.

10A-10D are cross-sectional views of the process of transferring micro-LEDs from a carrier substrate to a display substrate.

11A-11D are schematic diagrams of an imprinting system, wherein the micro LEDs are vertical micro LEDs.

12A-12B are partial cross-sectional views of an attractive force generator used to assist in securing a micro LED to a trapping location on a carrier substrate.

13A-13K are schematic diagrams of the steps of the micro-LED mass transfer imprinting system using fluid assembly imprinting stamp substrates.

14A and 14B are schematic diagrams of using an electrostatic force generator and a magnetic force generator as an auxiliary device, respectively, to assist in fixing the micro LEDs to the fluid assembly and trapping position.

15A-15I are schematic diagrams of a micro-LED mass transfer system using fluid imprinting and axial micro-LEDs.

FIG. 16 is a flow chart of a mass transfer method for micro-LEDs corresponding to the system shown in FIGS. 9A-9D .

FIG. 17 is a flow chart of the mass transfer method for micro-LEDs shown in FIGS. 13A-13K using fluidic assembly of imprinted stamp substrates.

FIG. 18 is a flow chart of the axial (lead) micro-LED mass transfer method shown in FIGS. 15A-15I .

FIG. 19 is a flow chart of a method for extending the transmission time distance of micro light emitting diodes.

Description of main component symbols

Embossed stamp 900, 900a, 900b, 900c, 1300, 1500

Imprint Stamp Top Surface 902, 1302, 1502

Trap position 904, 1304, 1504

Keel 906

Micro LED bottom surface 908

Micro LED 910

Surface Mount Miniature Light Emitting Diodes 910a, 910b, 910c

Micro LED top surface 912

First electrode 914, 1316

Second electrode 916, 1324

Display Substrate 918, 1315, 1318, 1525

Carrier substrate 1000, 1000a, 1000b, 1000c

Carrier substrate top surface 1002

Well 1004

Pitch 1006

Carrier substrate bottom surface 1008

Heating device 1010

Vertical Micro LED 1100

Vertical Micro LED Top Surface 1102

Vertical Micro LED First Electrode 1104

Insulation layer 1106

Electrostatic Force Generator 1200, 1400

Magnetic generator 1202, 1402

Depth 1306

Capture location bottom surface 1308

Micro LED thickness 1310

Micro LED bottom surface 1312

Micro LED top surface 1314

groove 1320

Thiol Biotin Bifunctional Molecule/First Component 1322

ACF 1325

Silica film 1326

Streptavidin molecule/first component 1327

Centerpiece 1506

First Depth 1508

remote 1510

Second Depth 1512

proximal 1514

Axial Micro LED 1516

Subject 1518

Body Thickness 1520

Distal electrode 1522

Electrode thickness 1524

Proximal electrode 1526

Electrodes 1528

Dielectric film 1530

Body groove 1532

P connection pad 1534

N connection pad 1536

First Groove 1538

The following specific embodiments will further illustrate the present application in conjunction with the above-mentioned drawings.

Detailed ways

US Patents 9,825,202 and 10,418,527, which are hereby incorporated by reference, have reported a general process for fabricating miniature light-emitting diode displays using inorganic LEDs and fluidic assembly on a display backplane. In particular, US 9,825,202 describes the process flow for manufacturing a suitable display backplane starting from column 13 and line 26, as shown in FIG. 17 . Its electrical requirements are described in unpublished patent application 16/727,186, which is also incorporated herein by reference. The display substrate used here has the same row and column arrangement and thin-film transistor (TFT) circuitry as described in Figures 14B and 14C of patent 9,825,202, but without the well layer because the mass transfer imprint stamp sets the micro-LED's Location.

5 is a partial cross-sectional view of a typical backplane arrangement showing a surface mount micro-LED and a power transistor controlling the brightness of the micro-LED.

The fluidic assembly techniques proposed in US Patent Nos. 9,825,202, 10,418,527 and 10,543,486 (herein cited) are suitable for direct random assembly of low-cost miniature LED display fabrication. Here the same assembly technique was used to prepare an embossed stamp for in turn bonding micro-LEDs to the electrodes of the display substrate. The advantage of this approach over direct fluidic assembly strategies is that it facilitates the formation of ohmic contacts between the micro-LEDs and the display by using an imprint stamp and applying pressure during the bonding process. As used herein, a transfer stamp is configured to have capture sites distributed in an array such that the pitch between capture sites matches the pitch between pixels of a display. The embossed stamp can be made of glass, quartz or single crystal silicon, and the trapping sites (also called wells) can be formed by etching the imprinted stamp or placing a film on the imprinted stamp, such as a patterned polymer. imide and pattern the wells using photolithographic techniques. The trapping sites have the same shape as the miniature LEDs and may be slightly larger than that shown in Figure 8 of US Patent 10,804,426, which is hereby incorporated by reference. The system described here is unique in that the depth of the trapping location can be less than at least one point in the thickness of the micro-LED, so that the micro-LED can contact the assembly tool or display substrate without interference from the top surface of the embossed stamp . The traps (trap sites) etched on the imprinted stamp can be more robust, allowing for more thorough cleaning, but control of the depth of the trap sites becomes more difficult. In contrast, the depth of trapping sites formed on polyimide or a deposited film can be controlled by the thickness of the film, but is more susceptible to damage.

The imprinting system described in this application is compatible with various configurations of micro-LEDs, but the traditional LED structure shown in Figure 2C is not suitable because it lacks a device for positioning in fluid assembly, so Electrodes are not properly positioned on the display substrate for bonding. The disk-shaped surface-mounted micro-LEDs described in US Patent 10,804,426 are designed as a solution within a certain range, constrained in the fluid assembly described in US Patent 9,825,202, as shown in 12 columns and 56 rows and Figure 16, so These devices were used to describe the imprinting system described in this application. It should be understood that other microLED shapes, such as square, rectangular and triangular devices, such as Figure 8 of US Patent 9,825,202 and Figure 4 of US Patent 10,516,084 (herein incorporated by reference), can be used in the same manner. Likewise, the stamping system is not limited to surface mount miniature LEDs. Vertical micro-LEDs can also use this approach, using a single bottom electrode and machining the top electrode after assembly. These changes are obvious to those skilled in the art, and in consideration of brevity, the present application will not describe them too much.

6A and 6B respectively depict a top view and a cross-sectional view of a surface-mount micro-LED for fluid assembly. The structure of the device is typically 20-100 micrometers ([mu]m) in diameter, 4-6 [mu]m thick, and includes a keel with a height of 5-10 [mu]m. In this case, the well depth is usually 3.5-4.5 μm to accommodate the thickness of the micro LEDs. For the detailed manufacturing process flow, please refer to the 8 columns and 56 rows of US Patent No. 10/804,426 and FIG. 6 . The shape of the disk matches the cylindrical shape of the trapping site, the depth of the trapping site is usually smaller than the thickness of the micro-LEDs, and its diameter is slightly larger than that of the micro-LEDs. Surface mount electrodes are usually made of solder such as tin/indium or gold/germanium, and the bonding surfaces of the P connection pad and the N connection pad must be on the same plane for easy contact.

Figure 7 depicts a micro LED wafer after selective pickup. Defects are identified by optical microscopy, Scanning Electron Microscope (SEM) images, cathodoluminescence or photoluminescence. The aim is to identify all defects that could cause errors in display pixels, so that defective products can be removed from the suspension of micro-LEDs used for manufacturing. Combining the defect map with known patterns such as edge bead patterns and alignment structures, all known locations of defective micro-LEDs can be obtained. Using a printing process, the tiny LEDs with defects are covered with a trapping material to prevent them from being picked up. As shown, the selective pickup imprint stamp process will get all good Micro LEDs and leave the defective Micro LEDs. Combining high utilization with the prevention of defective device mix-in is a significant advantage of fluidic assembly technology. Selective picking methods are described in more detail in unpublished application number 16,875,994, which is incorporated herein by reference.

After the micro-LEDs are fabricated, the growth wafer is attached to a carrier wafer through an adhesive layer, and the micro-LEDs are peeled off from the sapphire wafer by laser lift-off (LLO) technology, and placed on the micro-LEDs. Patterned keel on bottom surface.

The micro LED suspension was dispersed on a carrier substrate and assembled as described in Figure 7 of US Patent 10,418,527 and US Patent 10,804,426. For mass transfer methods, it is important to avoid surface contamination from interfering with the exposed surface of the micro-LEDs and the surface at the target site. Therefore, any unassembled superfluous micro-LEDs on the surface are removed and recycled after assembly, so effective cleaning methods are also very important.

Figure 8 provides a simple overview of the fluidic effect that enables 100% of micro LEDs to be assembled in the correct orientation with electrodes down. Assembled substrates are inspected, and if some wells are not filled, or there are other defects such as redundant unassembled micro-LEDs, simply clean the imprint stamp with solvent to remove the micro-LEDs and trap the solvent into the Storage to recover micro LEDs. Empty imprinted stamps are further cleaned, dried and inspected to ensure that there is no surface contamination or residue at the capture location. This capability is important for traditional imprint stamps that use elastomers or adhesives to hold micro-LEDs, which are difficult to clean and reuse. Under traditional technology, imprinted stamps with contamination or missing micro-LEDs are usually discarded, resulting in the inability to recycle the intact micro-LEDs on the imprinted stamp.

9A-9D describe the steps of using the micro-LED mass transfer imprinting system. The system includes an imprint stamp substrate 900 having a top surface 902 . An array of embossed stamp substrate capture locations 904 is formed on the top surface 902 . Each trapping location 904 is configured as a columnar groove to temporarily hold the keel 906 extending from the bottom surface 908 of a micro LED 910 . As shown in the figure, the micro LEDs 910 are surface mount micro LEDs, and each micro LED 910 includes a planar top surface 912 , and a first electrode 914 and a second electrode 916 are disposed on the top surface 912 . In this case, the keel 906 is not conductive. In this particular example, as shown in Figure 6A, the second electrode is a full or partial ring surrounding the first electrode. For the systems of Figures 9A-9D or Figures 11A-11D (see below), adhesive glue or elastomers can be used to pattern the stamp substrate top surface 902 to facilitate attachment of the miniature LEDs to a trap. position.

The filled carrier substrate 1000 is the basis for mass transfer to the display substrate 918 using the imprint stamp substrate 900, illustrated in the figure as a single miniature LED. Although not explicitly shown in the figure, it is obvious that the electrode connection pads of the display substrate are connected to a network of row and column lines to make the miniature LEDs work. For details, please refer to US Patent 9,825,202. In this case, the carrier substrate 1000 is a planar surface substrate with wells that allow localized protrusions (optionally adhesive or elastomer) near the capture location 904 of the imprint stamp head to contact each Micro LEDs (as shown in Figure 9B). Since the micro-LEDs are usually fixed in the carrier only by gravity, the relatively weak adhesion will cause the micro-LEDs to be removed from the carrier by optional adhesive glue or elastomer during the transfer process. The embossed stamp is aligned with the electrodes on the display substrate and press-fitted to form a hard contact between the electrodes of the micro-LEDs and the electrodes on the display substrate, while heating forms a solder bond (Figure 9C). In another embodiment, the connection can be made by an additional ACF membrane (not shown). When binding is complete, the transfer imprint stamp is recovered and detached from the micro-LEDs (Fig. 9D). The transfer head 900 and the carrier substrate 1000 are cleaned for reuse and cycled to fill the entire area of the display substrate 918 .

10A-10D are cross-sectional views describing the process of transferring micro-LEDs from a carrier substrate to a display substrate. The system includes a fluid assembled carrier substrate 1000a-1000c having a planar top surface 1002 and an array of wells 1004 arranged on the top surface 1002 of the carrier substrate with a spacing 1006 between adjacent wells, the spacing 1006 being separated from The spacing of adjacent trapping sites on the embossed stamp substrate was matched. The well 1004 of the carrier substrate has a first peripheral shape (circular in the exemplary embodiment) and a planar well bottom 1008 . Surface mount miniature LEDs 910a-910c each have a first perimeter shape and a planar top surface 912 to contact the well bottom surface 1008 through a first electrode 914 and a second electrode 916 (as shown in FIG. 9A).

In the case of an RGB display, the imprinting system may further include a first fluid assembly carrier substrate 1000a, and an array of wells disposed on the top surface of the carrier substrate with a spacing 1006 between adjacent wells so as to be compatible with the imprinted stamp substrate The trapping position matches that of the (Fig. 10B). The micro light emitting diodes 910a are configured to emit light at a first wavelength, each micro light emitting diode occupying a corresponding well in the first carrier substrate 1000a. Likewise, the second fluid-assembled carrier substrate 1000b includes an array of wells disposed on the top surface of the carrier substrate with a spacing 1006 between adjacent wells to match the trapping positions of the embossed stamp substrate. The micro light emitting diodes 910b are configured to emit light of the second wavelength, each micro light emitting diode occupying a corresponding well in the second carrier substrate 1000b. The third fluidic assembled carrier substrate 1000c includes an array of wells disposed on the top surface of the carrier substrate with a spacing 1006 between adjacent wells to match the trapping locations of the embossed stamp substrate. The micro light emitting diodes 910c are configured to emit light of the third wavelength, each micro light emitting diode occupying a corresponding well in the third carrier substrate 1000c.

In order to manufacture the three colors required for RGB displays, the micro-LEDs of the three colors need to be assembled and imprinted sequentially, as shown in FIGS. 10A-10D . The design of the array of trapping sites on the three carrier substrates will be spaced according to the pitch 1006 of the display pixels. The process flow of micro light emitting diodes of different colors or the performance gap of LEDs may determine that the micro light emitting diodes of different colors have different sizes and/or shapes. For example, red micro-LEDs can be made of aluminum indium gallium phosphide (AlInGaP), as described in US Patent 10,804,426, in which case the red micro-LEDs may be more efficient than GaN-based blue and Green devices are thicker. Since blue and green micro-LEDs have different quantum efficiencies, and the human visual system is more sensitive to green, it may be desirable to fabricate blue and green micro-LEDs with different emission regions. An example of these differences is shown in Figure 10A, where each carrier substrate is tuned to meet the requirements of the corresponding color micro-LEDs. The embossed stamp substrate 900a captures the blue micro-LEDs 910a arranged in an array from the carrier substrate, and moves them to the display substrate 918, aligning the embossed stamp 900a with the vacant area on the display substrate and aligning the micro-LEDs 910a. The electrodes make physical contact with matching electrodes on the display substrate (FIG. 10B). Pressure and heating means 1010 are used to enhance the intimate contact between the electrodes, thereby melting the metallic material and forming a solder bond. In FIG. 10C and FIG. 10D , green micro-LEDs 910b and red micro-LEDs 910c are transferred and bonded in the same way (stamp 900b, stamp 900c). Bonding between micro LEDs and connection pads may use materials such as gold/germanium to copper, indium/zinc to copper, and gold/ACF/copper. If ACF is used, the material of the display electrode can have a wider choice, such as Mo/Al/Mo.

The fluid assembly used in this application achieves several improvements over the simple imprinting process of the prior art:

1) There are no gaps in the array pattern due to defects or lack of micro-LEDs;

2) Selective pick-up and fluid assembly fully utilizes all intact micro-LEDs on a wafer;

3) Recycling micro-LEDs during assembly and on defective carrier substrates can prevent waste;

4) The carrier substrate is manufactured according to the distance of the capture position components on the display, and the expansion of the distance can be simply completed.

Fig. 11A-Fig. 11D have described an embossing system, wherein micro light emitting diode is vertical micro light emitting diode, and each vertical micro light emitting diode 1100 has the top surface 1102 of the plane that is provided with first electrode 1104, and has as second electrode conductive keel 906. Same as the surface-mounted micro-LEDs, the well 1004 of the carrier substrate has a first peripheral shape (such as a circle) and a planar well bottom 1008 . Each vertical micro LED 1100 has a first peripheral shape and a planar top surface 1102 , the top surface 1102 is in contact with the corresponding well bottom surface 1008 via the first electrode 1104 .

For smaller Micro LEDs, there is not enough space to make two electrodes on the same surface like surface mount Micro LEDs, the same assembly process can also be applied to vertical Micro LEDs. In this case, the micro LEDs are configured with a single anode electrode on the top surface and the cathode electrode on the bottom surface as a conductive post (keel) or gold or copper electroplated on the bottom surface. The conductive post can also serve as a keel during fluid assembly on a carrier plate (substrate).

The assembly and binding sequence of the conductive keel vertical micro-LEDs is demonstrated. Micro-LED suspensions are prepared by the above-mentioned selective acquisition method and distributed on the surface of the carrier substrate provided with wells with display spacing, and then assembled according to the conventional procedure. The embossed stamp is aligned with the carrier substrate, and the micro LEDs are removed from the carrier substrate, as shown in FIG. 11A. The filled embossed stamp is aligned with the display substrate, and mechanical contact is formed between the cathode electrode on the micro LED and the P connection pad electrode on the display substrate by applying pressure (as shown in FIG. 11B ). The heating device 1010 is used to form the solder bond, and the imprint stamp is then removed and cleaned and reused. An insulating layer 1106, such as polyimide, is used to fill the gap between the micro LEDs and the reflective wells to prevent short circuits and planarize the surface for metal deposition (FIG. 11C). The keel protrudes from the insulating layer 1106 to form a self-aligned contact point connected to each micro-LED, and the contact effect can be improved by removing a part of the insulating layer by short _O2 plasma etching. A circuit is formed by connecting the conductive pillars of the micro LEDs to Vss (power supply) through patterned metal as shown in FIG. 11D .

12A and 12B are partial cross-sectional views of a force generator used to assist in securing a micro-LED to a capture location on a carrier substrate. Wherein FIG. 12A uses an electrostatic force generator 1200 , and FIG. 12B uses a magnetic force generator 1202 . Although shown as surface mount miniature LEDs, the force generators described above can also be applied to vertical miniature LEDs.

13A-13K depict the steps of a micro-LED mass transfer imprinting system using fluidic assembly of imprinted stamp substrates. To further simplify the assembly process, the imprinted stamps can be filled directly using fluidic assembly, thereby omitting the carrier substrate. The micro-LED shown in FIG. 6 uses a keel structure on the bottom surface, which is referred to as a fixing mechanism below, so that the micro-LED and the electrode at the fluid assembly and trapping position are fixed. For the direct assembly process, the electrode position must be "up" in the stamping stamp, so the keel structure is fabricated on the top surface of the micro-LEDs, as shown in Figure 13A. Fluid assembly is carried out in a conventional manner, and the micro LEDs are assembled in an array arrangement in the trapping position, and the keel structure and the electrodes are both upward. The material used to make the keel structure is usually a photosensitive polyimide, which can be removed using solvents or etched using oxygen plasma. After assembly and drying, the keel is removed (as shown in FIG. 13B ) to facilitate bonding the electrodes to the display substrate. The manufacturing method of the embossed stamp is the same as the previous embodiment, but the well structure must not be affected by the removal of the keel, so organic thin films cannot be used. A preferred solution is to directly etch on the substrate to form the trapping position structure. Under the action of gravity and van der Waals forces, the imprinted stamp can accommodate micro-LEDs, which will fall out of the stamped stamp if it is turned upside down. Therefore, it is necessary to turn the surface of the embossed stamp up for transfer assembly and binding when heating, and press the display substrate down on the embossed stamp ( FIG. 13C ).

The fluid assembled imprint stamp substrate 1300 has a planar top surface 1302 . An array of trapping sites 1304 is formed on the top surface 1302 of the imprint stamp substrate, each trapping site has a first peripheral shape, a depth 1306 , and a planar trapping site bottom surface 1308 . As in the previous embodiments, the first peripheral shape is circular, but the system is not limited to this shape. The miniature light emitting diode 910 is arranged in the trap location 1304, has a first peripheral shape, a thickness 1310 greater than the trap location depth 1306, a planar bottom surface 1312 in contact with the bottom surface 1308, a first electrode 1316 extending out of the trap location. The planar top surface 1314 of the set position, and a protection mechanism (see explanation below). The micro LEDs have the same electrical connections as the vertical micro LEDs 1100, the vertical micro LEDs 1100 have a second electrode formed on the bottom surface 1312 (as shown in FIG. With the same electrical connection relationship, the surface mount micro LED 910 has a first electrode 1316 and a second electrode 1324 formed on the surface 1314 (see FIG. 13A and FIG. 13E ).

As shown in FIG. 13A , the fixing mechanism is a keel 906 formed on the top surface of the micro-LEDs. The keel 906 is a temporary non-conductive keel. The keel 906 will be removed before the micro-LEDs are in contact with the display substrate 1315 . Alternatively, as shown in FIG. 13D and FIG. 13E , the fixing mechanism can be a conductive keel 906 connected to the first electrode 1316 . In FIG. 13D , the micro LEDs are vertical micro LEDs 1100 .

In another embodiment, a conductive center post is used in place of a non-conductive keel in a direct imprint transfer process, allowing the structure to serve both as a keel in a fluidic assembly process and as an anode electrode (FIG. 13E). In this case, the embossed stamp is a simple plate with an array of trapping sites arranged at the same pitch as the display pixels. In the display substrate 1318, the P connection pad electrode is located below the N connection pad electrode, leaving space for the conductive column forming the anode electrode on the micro LED (FIG. 13F). Due to process changes, there may be differences in the height of the conductive pillars and the depth of the groove of the P connection pad. Therefore, these differences can be compensated by setting the ACF1325 to connect the micro LEDs and the display substrate.

Thus, the micro-LEDs in FIG. 13E are surface-mounted micro-LEDs 910a, while the display substrate 1318 in FIG.

Another mechanism for positioning and immobilizing micro-LEDs in transfer imprint stamps utilizes preferential linkage between pairs of conjugated biomolecules such as streptavidin-biotin pairs. As shown in FIG. 13G , after LLO, a thin silicon dioxide film 1326 is deposited on the backside of the device 1312 to fabricate a functionalized micro-LED. The surface of the micro-LEDs is exposed to hydrogen ions or alkaline compounds, and then interacts with amine terminal molecules such as 3-aminepropyl-trimethoxysilane (3-aminepropyl-trimethoxysilane) to be silanized. The surface was washed with a streptavidin solution to bind the streptavidin molecule 1327 to the amine end, thereby forming a streptavidin-functionalized miniature LED (as shown in FIG. 13H ). The trapping sites on the transfer imprint stamp can be similarly treated with biotin-terminal ligands prior to assembly, or the bottom of the well can be a surface made of gold and exposed to the thiol-biotin bifunctional molecule 1322 , as shown in Figure 13I.

Thus, Figures 13G-13K describe miniaturized LEDs using pairs of conjugated biomolecules as "anchor mechanisms". Wherein, the bottom surface 1308 of the imprint stamp substrate is coated with a first component 1322 comprising a pair of conjugated biomolecules. The micro-LED fixing mechanism is the second component 1327 coated on the bottom surface 1312 of each micro-LED, which includes a pair of conjugated biomolecules. During the assembly process, the relatively low depth of the trapping site (about 1 μm) can easily remove the wrongly oriented micro-LEDs by fluid perturbation, while the correctly oriented micro-LEDs can be chemically bound to the trapping site. The bottom surface is better marked by constraints in the trapping position. In Figure 13J, the bioconjugated bonds are shown on a greatly enlarged z scale to illustrate binding effects. In fact, the binding layer is very thin and is more accurately shown in Figure 13K. Alternative chemical pairings of exemplary biotin-streptavidin systems, such as thiol-maleimide and azide-alkyne, may have advantages in terms of stability and ease of processing, but the sequence of preparation is similar.

Figures 14A and 14B depict the use of an electrostatic force generator 1400 and a magnetic force generator 1402, respectively, as an auxiliary mechanism for assisting in securing the micro LEDs in a fluid assembly capture location (with or without keel). In Figures 14A and 14B, the primary fixation mechanism may be gravity. Additionally, in Figure 14A, the immobilization mechanism is a conjugated biomolecule (not shown). In other embodiments (not shown), the force generator in FIG. 14A can also be a magnetic force generator, and the force generator in FIG. 14B can also be an electrostatic force generator. Although FIGS. 14A and 14B only illustrate the fluid assembly of stamp substrates, it should be understood that the force generators can also be used with stamp substrates in the groove configurations of FIGS. 9B-9D and 11A-11B .

As a price for increased complexity, some fixing structures can be added to the embossed stamp structure to prevent the micro light emitting diodes from detaching from the trapping position when the embossed stamp is turned upside down. Bonding with adhesives is not attractive since the fixing mechanism can be removed from the micro LEDs after bonding. Vacuum conditions are introduced into the imprint system by providing a porous layer between the substrate-carrying surface and the capture site forming layer, but the fluid assembly liquid may flow into the porous layer, making drying work impossible. The most practical structure for fixing micro-LEDs in embossed stamps is a magnetic or electrostatic force structure. For electrostatic force immobilization, the micro LEDs have a dielectric film deposited on the surface corresponding to the surface mount electrodes (i.e. the bottom surface), and the embossed stamp includes the kinetic electrodes below the trap site structure. For magnetic fixation, the micro-LED electrode structure can contain a magnetic material, such as nickel, while the embossed stamp has permanent magnets or electromagnets.

These fixing mechanisms are switchable at individual points in the array, so the following procedure can be used to repair imprinted stamps with defects:

1) Check the embossed stamp for micro LEDs with defects;

2) Open the fixing mechanism for all good miniature light-emitting diodes;

3) removing defective micro-LEDs by rinsing;

4) Place additional micro-LED suspensions and assemble.

In one aspect, the imprint stamp may include a light sensor that, when pressed onto the display substrate, activates (simultaneously or sequentially) all trapping sites that are temporarily electrically connected to the micro LEDs on the imprint stamp. The embossed stamp, with associated driver circuitry, is connected to a system that records which micro LEDs are good. The fixing device on the embossed stamp is activated to keep the intact micro-LEDs in the trapping position, and the assembly is continued until all the micro-LEDs are tested intact, as shown in the above process 2)-4). Then proceed to the binding process.

Figures 15A-15I depict a micro-LED mass transfer imprinting system that uses a fluidic imprint stamp substrate along with axial micro-LEDs. The mixed fluid assembly mass transfer method can also be applied to the axial micro-LEDs described in Application No. 16/846,493. In order to reduce cost and increase density, micro LEDs are configured as vertical devices with a light emitting area of 5*8 μm, as shown in FIG. 15G . The electrodes of the blade-shaped mini LEDs can be electroplated copper or gold. The dimensions of all of the features described above are adjustable, but their relative shapes are important to facilitate fluid assembly into directional arrays.

15A-15C describe the fabrication process of the axial micro-LED display substrate 1525 . Electrodes 1528 are deposited and patterned from a conductive material such as molybdenum/copper (Mo/Cu) to form connection pads for accommodating the cathode and anode of the micro LEDs. Deposit a dielectric film 1530 on the electrode, its material can be silicon dioxide, silicon nitride (Si ₃ N ₄ ) or polyimide, and pattern and etch a contact opening on the dielectric film 1530, as shown in Figure 15B . Using the metal electrode as a hard mask, a body groove 1532 for accommodating the body of the micro LED is etched, as shown in FIG. 15B . Finally, an N connection pad 1536 and a P connection pad 1534 are formed by electroplating, sputtering or evaporation, as shown in FIG. 15C .

For the shape of the axial MLEDs, the fabrication process of the embossed stamp would be more complicated, requiring two trapping locations with different depths. As shown in FIG. 15D, a first recess 1538 is etched into the substrate, the first recess 1538 having a depth and profile for accommodating the body of the miniature LED protruding below the axial electrode surface. In FIG. 15E, a second recess 1504 for receiving the axial electrodes is formed by etching. The second groove can also be made of thin film material (such as photolithographic polyimide) after the first groove 1538 is formed.

A known good axial MLED suspension was applied to the embossed stamp and assembled into an array of MLEDs (as shown in Figure 15F). The assembled embossed stamp is inspected and pressed together after being matched with the display substrate, thereby binding the electrodes of the LED to the electrodes on the display substrate (as shown in FIG. 15I ). After binding is complete, the debossed stamp is retrieved, cleaned and inspected for reuse.

Thus, the system includes a fluid assembled stamp substrate 1500 having a planar top surface 1502 . The arrayed capture sites 1504 formed on the top surface 1502 of the imprint stamp substrate include a first perimeter shape (largely rectangular), a central portion 1506 having a planar first depth 1508, a planar second depth 1512 A distal end 1510 having a second depth 1512 less than the first depth 1508, and a proximal end 1514 having a planar second depth 1512.

Please refer to Fig. 15F and 15G together, an axial miniature light-emitting diode 1516 occupies the corresponding capture position 1504, and has the first peripheral shape, the main body 1518 is in contact with the central part 1506 of the capture position, and a vertical Planar portion thickness 1520 is greater than first depth of trap location 1508 but less than twice first depth of trap location 1508 . A distal electrode 1522 bisects the body 1518 horizontally and contacts the distal end 1510 of the capture site. The distal electrode 1522 has a vertical electrode thickness 1524 that is greater than the second capture location depth 1512 but less than twice the capture location second depth 1512 . A proximal electrode 1526 bisects the body 1518 horizontally and contacts the capture site proximal end 1514 , the proximal electrode 1526 has an electrode thickness 1524 .

As shown in Figure 15I, the process of transferring micro-LEDs to the display substrate is similar to that described in Figure 13C, the aligned display substrate is pressed down onto the fluid assembly imprinted stamp substrate so that the electrodes of the micro-LEDs are aligned with the corresponding Electrode contacts on the substrate are shown. Transfer and bonding are accomplished by heating the solder while applying pressure. Optionally, an ACF film (not shown) can be interposed between the corresponding electrodes to achieve electrical and mechanical connections without the need for a metallic phase transition.

Although not explicitly shown, the imprint stamp substrate of this embodiment may include electrostatic force or magnetic force generators as shown in FIGS. 14A and 14B .

FIG. 16 is a flowchart describing a micro LED mass transfer method corresponding to the system shown in FIGS. 9A-9D . Although the method is described as comprising a series of numbered steps for ease of understanding, the numbering does not necessarily indicate the order of the steps. It should be understood that some steps may be skipped, performed concurrently, or performed in no exact order. However, the method can generally be performed in numerical order of steps. The method starts at step 1600 .

Step 1602 provides an embossing stamp substrate having a planar top surface and an array of trapping sites formed on the top surface, each trapping site being configured as a columnar groove. In one aspect, step 1603a patterns the top surface of the imprint stamp substrate using an adhesive material or elastomer. In step 1604, each capture location is recessed to accommodate a keel extending from the bottom surface of the micro-LED, and by constraining the keel of each micro-LED, the micro-LED is secured to the embossed stamp in step 1606. on the substrate. Step 1606 may use additional electrostatic or magnetic forces to secure the micro LEDs to the embossed stamp substrate.

In one aspect, constraining the keel at step 1604 includes constraining the surface mount LED with a non-conductive keel comprising a planar surface having a first electrode and a second electrode. On the other hand, step 1604 defines a conductive keel and connects to the second electrode, the vertical LED includes a surface with the plane of the first electrode (ie the keel is the second electrode).

In one aspect, step 1602 provides an imprint stamp substrate having spaced apart capture locations. Step 1601a provides a fluidically assembled carrier substrate having a planar top surface and a plurality of wells disposed in an array on the top surface of the carrier substrate, the spacing between adjacent wells being the same as the spacing between trapping sites on the imprinted stamp substrate match. In step 1601b, micro LEDs are filled into the wells of the carrier substrate through a fluid assembly process. In one aspect, step 1601b can use electrostatic force or magnetic force to fix the micro LEDs in the well. Step 1603b presses the top surface of the embossed stamp substrate onto the top surface of the carrier substrate, each trapping site is in contact with the corresponding well, and step 1603c mass transfers the micro LEDs from the carrier substrate to the embossed stamp substrate .

Specifically, step 1601a may provide a carrier substrate having a plurality of wells including a first peripheral shape and a planar well bottom surface. Afterwards, in step 1601b, micro light emitting diodes are filled into each well, wherein the surface mount micro light emitting diodes filled in the wells have a first peripheral shape, a planar top surface in contact with the bottom surface of the well, which includes first electrode and second electrode. In other embodiments, the vertical micro light emitting diodes filled into the well in step 1601b have a first peripheral shape, a planar top surface contacting the bottom surface of the well, which includes a first electrode.

In the case of an RGB display, the carrier substrate provided in step 1601a includes:

A first fluidly assembled carrier substrate comprising an array of wells disposed on the top surface of the carrier substrate, the distance between adjacent wells matching the spacing between adjacent trapping sites on the imprinted stamp substrate;

a second fluidly assembled carrier substrate comprising an array of wells disposed on the top surface of the carrier substrate, the distance between adjacent wells matching the spacing between adjacent capture sites on the imprinted stamp substrate;

A third fluid assembled carrier substrate includes an array of wells disposed on the top surface of the carrier substrate, the distance between adjacent wells matching the pitch of adjacent trapping sites on the imprint stamp substrate. Then, the process of filling the well of the carrier substrate in step 1601b includes:

filling a well on the first carrier substrate with a first micro light emitting diode configured to emit light at a first wavelength;

filling a well on a second carrier substrate with a second micro light emitting diode configured to emit light at a second wavelength; and

A well on a third carrier substrate is filled with a third micro light emitting diode configured to emit light at a third wavelength. In step 1603c, transferring the micro LEDs from the carrier substrate to the embossed stamp substrate includes transferring the micro LEDs from the first, second and third carrier substrates to the corresponding embossed stamp substrates. As shown in FIG. 10A and FIG. 10B , for RGB micro-LEDs with different shapes, it is necessary to use carrier substrates of different sizes. In addition, if the diameters of the RGB micro-LEDs are equal, a carrier substrate can also be used to fill micro-LEDs of different wavelengths and transfer them to the embossed stamp substrate respectively.

Step 1608 provides a display substrate having a planar top surface and an array of micro-LED connection pads, each micro-LED connection pad includes at least one electrode formed on the top surface, and is electrically connected to a lower column and row control line. matrix. The spacing between adjacent connection pads on the display substrate matches the spacing between adjacent trapping sites on the imprinted stamp substrate, which is the same as the spacing between adjacent wells on the carrier substrate. In step 1610, the top surface of the embossed stamp substrate is pressed against the top surface of the display substrate, and each capture location is in contact with a corresponding micro-LED connection pad. In step 1612, the micro-LEDs are mass-transferred from the embossed stamp substrate to the micro-LED connection pads of the display substrate. On the one hand, in step 1612, the display substrate is heated to bind the micro-LEDs to the connection pads of the micro-LEDs.

In the case of RGB display, the display substrate in step 1608 includes a plurality of connection pads for the first micro light emitting diodes for emitting light of the first wavelength; a plurality of connection pads for the second micro light emitting diodes for emitting light of the second wavelength; and a plurality of connection pads for the third micro light emitting diodes for emitting light of the third wavelength. Afterwards, the process of pressing the top surface of the embossed stamp substrate to the top surface of the display substrate in step 1610 includes pressing the embossed stamp substrate filled with the first micro-LEDs, the second micro-LEDs and the third micro-LEDs respectively. One imprinted stamp substrate can be used for each wavelength of the LEDs, or if all the LEDs are similar in shape, the same substrate can be used to fill the different wavelength LEDs and transfer them to the display substrate.

FIG. 17 is a flow chart of the mass transfer method for micro-LEDs shown in FIGS. 13A-13K using a fluid-assembled imprinted stamp substrate. The method starts at step 1700 . Step 1702 provides a fluid assembled imprint stamp substrate having a planar top surface with capture sites disposed on the top surface having a first perimeter shape, a depth, and a planar capture site bottom surface. Under the fluid assembly process, the micro-LEDs filled into the trapping location in step 1704 have: a first peripheral shape, a thickness greater than the depth of the trapping location, a planar bottom surface in contact with the bottom surface of the trapping location, and a first The top surface of the electrode extends out of the plane of the trapping site. The miniature LED also includes a fixing mechanism. In step 1704, micro LEDs with a second electrode on the bottom surface may be used, or surface-mounted micro LEDs with a first electrode and a second electrode on the top surface may be used to fill the trapping positions.

In one aspect, providing an imprint stamp substrate in step 1702 includes providing an imprint stamp substrate having spaced apart capture locations. The display substrate provided in step 1706 has a flat bottom surface and micro-LED connection pads arranged in an array, each micro-LED connection pad includes a first electrode formed on the top surface, and is electrically connected to the lower column and row control lines matrix. The spacing between adjacent connection pad locations on the substrate was shown to match the spacing between adjacent trapping locations on the imprinted stamp substrate. In step 1708, press the top surface of the embossed stamp substrate onto the top surface of the display substrate, so that each capture site is in contact with a micro LED connection pad. In step 1710, the micro-LEDs on the embossed stamp substrate are mass-transferred to the micro-LED connection pads of the display substrate. Step 1710 may include applying heat to form a bond between the micro LEDs and the connection pads of the display substrate.

In one aspect, step 1704 provides a fixing mechanism in the form of a keel formed on the top surface of the micro-LED, the keel may be a conductive keel connected to the first electrode (as shown in Figures 13D and 13E), or a temporary (removable) Non-conductive keel (as shown in Figure 13A). On the other hand, the bottom surface of each trapping position of the imprinted stamp substrate provided in step 1702 is coated with the first component comprising a pair of conjugated biomolecules. Then, the immobilization mechanism mentioned in step 1704 is a second component with pairs of conjugated biomolecules, which coats the bottom surface of each micro LED. Examples of conjugated biomolecular pairs include biotin-streptavidin, thiol-maleimide, and azide-alkyne. The embossed stamp substrate can also be further provided with an electrostatic force or magnetic force generator, as shown in FIG. 14A and FIG. 14B .

FIG. 18 is a flow chart of the axial micro-LED mass transfer method of the system shown in FIGS. 15A-15I . The method starts at step 1800 . Step 1802 provides a fluid assembled stamp substrate having a planar top surface on which a plurality of capture sites are formed, each capture site having a first perimeter shape, a central portion having a planar first depth, a A distal end having a second depth that is less than a plane of the first depth and a proximal end having a second depth. Under the fluid assembly process, step 1804 fills the trapping locations with axial micro-LEDs, each micro-LED occupying a corresponding trapping location and having the first peripheral shape and a main body portion conforming to the central portion, the main body portion The vertical body thickness of is greater than the first depth at the capture location but less than twice the first depth. The miniature light-emitting diode also has a distal electrode that horizontally bisects the main body, and the distal electrode is bonded to the distal portion of the trapping position, and the electrode thickness of the vertical surface of the distal electrode is greater than the second depth of the trapping position, But less than twice the second depth. The miniature light emitting diode also has a near-end electrode that horizontally bisects the main body, which is attached to the near-end part of the trapping position and has an electrode thickness. In one aspect, the imprinting stamp electrode may also include an electrostatic force or magnetic force generator, as shown in FIGS. 14A and 14B .

In one aspect, providing an imprint stamp substrate in step 1802 includes providing an imprint stamp substrate having spaced apart capture locations. In step 1806, a display substrate having a planar top surface and an array of micro-LED connection pads is provided, each micro-LED connection pad includes a first electrode and a second electrode formed on the top surface, and the plurality of electrodes are electrically connected to a Square's columns and rows control the lines of the matrix. The display substrate includes a plurality of connection pads separated by a pitch that matches the pitch that separates the capture locations on the imprinted stamp substrate. In step 1808, the top surface of the embossed stamp substrate is pressed onto the top surface of the display substrate so that each capture site is aligned with a micro LED connection pad. In step 1810, the micro-LEDs are mass-transferred from the embossed stamp substrate to the micro-LED connection pads of the display substrate. Optionally, the bonding between the micro light emitting diodes and the connection pad electrodes of the display substrate can be promoted by heating.

FIG. 19 is a flow chart of a pitch extension method for micro-LED transfer. The method begins at step 1900 . Step 1902 provides a micro LED MOCVD wafer with a first distance between adjacent micro LEDs. Step 1904 releases the micro LEDs into the fluid assembly suspension. Step 1906 provides a carrier substrate having wells arranged in an array with a second spacing between adjacent wells, and the second spacing is different from the first spacing. Through a fluidic assembly process, micro LEDs are filled in the wells of the carrier substrate in step 1908 . Step 1910 provides an imprint stamp substrate comprising an array of capture locations, adjacent capture locations being separated by a second spacing. In step 1912, the top surface of the imprint stamp substrate is pressed against the top surface of the carrier substrate so that each trapping site is in contact with a corresponding well. In step 1914, the micro LEDs are mass transferred from the carrier substrate to the embossed stamp substrate.

Step 1916 provides a display substrate with an array of micro-LED connection pads, each micro-LED connection pad including at least one electrode formed on the top surface and electrically connected to an underlying matrix of column and row control lines. Adjacent connection pad positions on the display substrate are separated by the second distance. In step 1918, the top surface of the embossed stamp substrate is pressed against the top surface of the display substrate, so that the capture site is in contact with a corresponding micro-LED connection pad. In step 1920, the micro-LEDs are mass-transferred from the embossed stamp substrate to the micro-LED connection pads of the display substrate. Optionally, heat is used to promote the formation of bonds between the micro light emitting diodes and the electrodes of the connection pads of the display substrate.

In one aspect, step 1906, step 1908, step 1912 and step 1914 are bypassed and an additional step 1911 is performed to fill the micro LEDs directly into the trapping sites of the embossed stamp substrate using a fluidic assembly process.

The present application provides a system and method for mass transfer of miniature light emitting diodes. Examples of specific LED, carrier substrate and embossed stamp substrate structures are given to illustrate the application. However, the present application is not limited to the above examples. Other variations and embodiments of the application may occur to those skilled in the art.

Claims (22)

1. A micro-led mass transfer fluid assembly carrier system comprising:

a fluid assembly carrier substrate having a planar top surface; and

an array of trapping sites formed on a top surface of the carrier substrate, each trapping site configured as a recessed well to temporarily hold a fluid deposited micro light emitting diode.

2. The carrier system of claim 1, further comprising: and the miniature light-emitting diode is filled in the well on the carrier substrate.

3. The carrier system of claim 1, wherein a distance between adjacent wells on the carrier substrate is less than or equal to a distance between adjacent trapping sites on a corresponding bulk transfer stamp.

4. The carrier system of claim 3, further comprising:

the bulk transfer stamp comprising:

an imprint stamp substrate having a top surface; and

An array of capture locations is formed on the top surface of the imprint stamp substrate, each capture location being configured to temporarily receive a corresponding micro-led from a well of a carrier substrate.

5. The carrier system of claim 4, wherein the well of each of the carrier substrates has a planar bottom surface;

The carrier system further comprises:

micro light emitting diodes filling said wells on said carrier substrate, each of said light emitting diodes having a top surface in contact with a bottom surface of a corresponding well, and a keel extending from the bottom surface thereof; the method comprises the steps of,

the imprint stamp substrate capture station is configured to receive a keel of the light emitting diode.

6. The carrier system of claim 1, wherein the carrier substrate does not include conductive traces and electronic components.

7. The carrier system of claim 2, wherein the well of each of the carrier substrates has a planar bottom surface; and

the micro light emitting diodes are surface mount micro light emitting diodes, each comprising a planar top surface having a first electrode and a second electrode, the first electrode and the second electrode being in contact with a bottom surface of a corresponding carrier substrate well.

8. The carrier system of claim 7, wherein each of the micro-leds further comprises a non-conductive keel extending from a bottom surface of the micro-led.

9. The carrier system of claim 2, wherein each of the carrier substrate wells has a planar bottom surface; and

The micro light emitting diodes are vertical micro light emitting diodes, each including a planar top surface with a first electrode in contact with a corresponding well of the carrier substrate, and a second electrode on the bottom surface of the micro light emitting diode.

10. The carrier system of claim 9, wherein the second electrode of each of the light emitting diodes is a conductive keel extending from a bottom surface of the light emitting diode.

11. The carrier system of claim 2, wherein the carrier substrate well has a first perimeter shape; the method comprises the steps of,

the micro light emitting diode has the first perimeter shape.

12. The carrier system of claim 2, further comprising:

a first fluid assembly carrier substrate having an array of wells formed in a top surface of the first fluid assembly carrier substrate;

a second fluid assembly carrier substrate having an array of wells formed in a top surface of the second fluid assembly carrier substrate;

a third fluid assembly carrier substrate having an array of wells formed in a top surface of the third fluid assembly carrier substrate;

a plurality of micro light emitting diodes configured to emit light at a first wavelength, each occupying a corresponding well in the first fluid assembly carrier substrate;

A plurality of micro light emitting diodes configured to emit light at a second wavelength, each occupying a corresponding well in the second fluid assembly carrier substrate; and

A plurality of micro light emitting diodes configured to emit light at a third wavelength, each occupying a corresponding well in the third fluid assembly carrier substrate.

13. The carrier system of claim 2, further comprising:

an attractive force generator below the carrier substrate, the attractive force generator selected from the group consisting of a magnetic force generator and an electrostatic force generator, for temporarily fixing the micro light emitting diode in the well of the carrier substrate.

14. The carrier system of claim 2, wherein a bottom surface of the wells of each of the carrier substrates is coated with a first component comprising a conjugated biomolecule pair; the method comprises the steps of,

wherein each of the micro light emitting diodes comprises a top surface coated with a second component comprising a conjugated biomolecule pair, the top surface being in contact with a bottom surface of a well of a corresponding one of the carrier substrates.

15. A method for mass transfer of a micro light emitting diode, comprising:

Manufacturing a micro light emitting diode on a wafer;

releasing the micro light emitting diode from the wafer into a suspension;

transferring the micro light emitting diode fluid deposition onto a carrier substrate;

transferring the micro light emitting diode from the carrier substrate to a bulk transfer stamp; and

and transferring the miniature light emitting diode from the huge transfer stamping seal to a display substrate.

16. The micro light emitting diode macro-transfer method of claim 15, wherein the carrier substrate has a planar top surface and an array of wells formed on the carrier substrate top surface, the wells being filled with the micro light emitting diodes; and

the step of transferring the micro light emitting diode from the carrier substrate to a bulk transfer stamp comprises: pressing the top surface of the bulk transfer stamp against the top surface of the carrier substrate such that an array of bulk transfer stamp capture bits formed on the top surface of the bulk transfer stamp are inter-engaged with corresponding micro light emitting diodes in the wells of the carrier substrate.

17. The micro light emitting diode macro-transfer method of claim 16, wherein the wells of the carrier substrate have a first perimeter shape and a bottom surface of the planar wells; and

The step of fluid deposition transferring the micro light emitting diode onto a carrier substrate comprises filling micro light emitting diodes having the first perimeter shape into the wells.

18. The method of claim 16, wherein adjacent wells in the array of wells of the carrier substrate have a spacing therebetween; and

the spacing between adjacent trapping sites in the array of trapping sites of the bulk transfer stamp is greater than or equal to the spacing between adjacent wells in the array of wells of the carrier substrate.

19. The micro light emitting diode macro-transfer method of claim 15, further comprising:

prior to the step of fluid depositing and transferring the micro-leds onto a carrier substrate, a keel is formed on each micro-led, the keel extending over the exposed bottom surface of the led.

20. The method of mass transfer of micro-leds of claim 19, wherein transferring the micro-leds from the carrier substrate to a mass transfer stamp comprises configuring a capture location of the mass transfer stamp to receive a keel of the micro-leds.

21. The micro light emitting diode macro-transfer method of claim 15, further comprising:

coating a first component having a conjugated biomolecule pair on a bottom surface of a well of a carrier substrate prior to the step of fluid deposition transferring the micro light emitting diode onto the carrier substrate; and

in the suspension, a second component having a conjugated biomolecule pair is coated on the micro light emitting diode.

22. The micro light emitting diode macro-transfer method of claim 15, further comprising:

before the step of transferring the micro light emitting diode fluid deposit onto a carrier substrate, an attractive force generator is used below the carrier substrate, the attractive force generator being selected from the group consisting of a magnetic force generator and an electrostatic force generator for temporarily fixing the micro light emitting diode in a well of the carrier substrate.