TWI900850B - Fluidic assembly carrier substrate system for microled mass transfer and microled mass transfer method - Google Patents

Abstract

A microLED mass transfer stamping system includes a stamp substrate with an array of trap sites, each configured with a columnar-shaped recess to temporarily secure a keel extended from a bottom surface of a microLED. In the case of surface mount microLEDs, the keel is electrically nonconductive. In the case of vertical microLEDs, the keel is an electrically conductive second electrode. The stamping system also includes a fluidic assembly carrier substrate with an array of wells having a pitch separating adjacent wells that matches the pitch separating the stamp substrate trap sites. A display substrate includes an array of microLED pads with the same pitch as the trap sites. The stamp substrate top surface is pressed against the display substrate, with each trap site interfacing a corresponding microLED site, and the microLEDs are transferred. Fluidic assembly stamp substrates are also presented for use with microLEDs having keels or axial leads.

Description

Fluid assembly carrier substrate system for mass transfer of micro-light-emitting diodes and method for mass transfer of micro-light-emitting diodes

This application relates to the field of micro-light emitting diode (micro-LED) displays, and more particularly to a system and method for mass transfer of micro-LEDs during display manufacturing.

A red-green-blue (RGB) display consists of multiple pixels that emit three wavelengths of light, corresponding to red, green, and blue in visible light. The RGB parts of these pixels (each part is called a subpixel) are energized in a systematic way, and when superimposed, they produce the colors in the visible light spectrum. Different displays produce RGB images in different ways. Liquid Crystal Displays (LCDs), the most popular technology, produce RGB images by illuminating color filters on the subpixels with a white light source (usually a fluorescent white LED). Some wavelengths of light contained in the white light are absorbed, while others are transmitted by the color filters. As a result, the efficiency of an LCD display can be less than 4%, and its contrast is limited by the light that leaks out of the liquid crystal cells. Organic Light Emitting Diode (OLED) displays generate RGB light by stimulating the organic light-emitting material in each subpixel, causing it to directly emit light of the corresponding wavelength. Because OLED pixels emit light directly, the display has a higher contrast ratio. However, the organic material degrades over time, causing image burn-in.

The third type of display technology, and the one covered by this application, is microLED displays, which use tiny (5-150 micrometers (μm) in diameter) inorganic LEDs as subpixels to directly emit light. Inorganic microLED displays offer numerous advantages over other display technologies. Compared to LCDs, they boast contrast ratios exceeding 50,000:1 and higher efficiency. Unlike OLED displays, inorganic LEDs do not experience aging and can achieve significantly higher brightness.

The mainstream high-definition television (HDTV) resolution standard is known to have two million pixels (or six million subpixels), while the higher-resolution 4K and 8K standards have eight and 33 million pixels, respectively. Even the relatively small displays used in tablets and mobile phones have millions of pixels, with resolutions exceeding 600 pixels per inch (ppi). Therefore, the manufacture of displays using microdiodes requires the low-cost assembly of large-area microdiode arrays with varying pixel pitches, allowing the production of displays of various sizes and resolutions. The most traditional microdiode array assembly technology is called pick-and-place technology, because each microdiode is individually picked from a carrier and placed on a substrate, as described below. Because each micro-LED is handled individually, the assembly process is very slow.

Figures 1A-1C depict cross-sections of a gallium nitride (GaN)-based LED stack (Figure 1A), two fully processed vertical micro-LEDs (Figure 1B), and a surface-mount micro-LED (Figure 1C) (known technology). The widespread adoption of GaN-based high-brightness LEDs for general lighting has created a complex manufacturing system, so micro-LEDs for display applications build on existing investments in the industry. GaN-based LEDs emitting in the blue (approximately 440 nanometers (nm)) are fabricated in a complex series of high-temperature metal-organic chemical vapor deposition (MOCVD) steps to produce the vertical LED structure shown in the cross-section of Figure 1A. Fabrication takes place on a polished sapphire, silicon, or silicon carbide (SiC) substrate with a diameter of 50-200 mm. The surface is prepared by depositing undoped GaN and, optionally, an aluminum nitride (AlN) buffer layer, resulting in a surface with low defects and a high GaN lattice constant. Because the initial deposition thickness and temperature must be adjusted to compensate for the lattice mismatch between the substrate and the GaN, increasing the thickness to improve surface quality leads to high-efficiency devices with thicknesses exceeding approximately 3μm. Because the MOCVD deposition process is complex and expensive, optimizing the microLED process to most efficiently utilize the entire area of the growth wafer is crucial.

After initial growth to form a crystalline GaN surface, the first LED layer is generated by adding silicon doping to form n+GaN for the cathode. Optionally, the stack can include layers tuned for electron injection and hole blocking. Next, a multiple quantum well (MQW) structure with alternating layers of indium gallium nitride (InxGa1-xN) and GaN is deposited, where the indium content and the thickness of the layer determine the emission wavelength of the device. Increasing the indium content shifts the emission peak to longer wavelengths, but also increases the stress caused by lattice mismatch, making it impossible to produce high-efficiency GaN devices for red light emission, and the efficiency of green LEDs is lower than that of blue LEDs. After forming the MQW, the stack structure can 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 the microdiodes used in microdiode displays (5-150μm in diameter), resulting in significantly different patterning and electrode requirements. Microdiodes require soldering materials or asymmetric conductive film (ACF) to be bonded to substrate electrodes, while large LEDs are typically wire-bonded or soldered to a lead frame. Due to the very small size of microdiodes, a large 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 increases costs. Therefore, it is crucial to use the light-emitting area as efficiently as possible to minimize the material cost of micro-LED displays.

In the simplest process flow, a transparent conductive electrode is formed on the MOCVD stack by depositing a thin layer (a few nanometers) of nickel oxide (NiOX) to match the p+ GaN work function, followed by a 50-300nm thick layer of indium tin oxide (ITO). The deposited stack is then patterned and etched, typically using a chlorine (Cl2)-based reactive ion etch (RIE) process, to produce individual microLEDs with the smallest practical size and pitch. When producing microLEDs at high efficiency, the thickness of the LED structure is only 3-5μm, so the minimum space that can be successfully etched is limited by the thickness of the LED structure.

As schematically shown in Figure 1C, after etching the outline of the LED, additional processing is performed to form the electrode on the anode. To prevent leakage and etch openings for connecting to the ITO layer, a passivation layer is typically applied. The passivation layer is typically composed of plasma-enhanced chemical vapor deposition (PECVD) silicon dioxide (SiO2) or optionally includes a thin atomic layer deposition (ALD) aluminum oxide (Al2O3) layer on the surface. The anode structure is completed by depositing an electrode stack structure comprising materials such as indium/tin (In/Sn) or gold and germanium (Au/Ge) alloys.

Figure 2A illustrates the process of removing a microLED from a sapphire substrate using laser lift-off (LLO). Figure 2B depicts the pick-and-place process for moving and placing the device from a carrier wafer onto a display substrate. Figure 2C illustrates the connection between the microLED anode and the substrate electrode (known practice). Specifically, in Figure 2A, the fabricated microLED is bonded to the carrier wafer via an adhesive layer and simultaneously removed from the sapphire substrate using LLO. In Figure 2B, a microLED is removed from the carrier using a pick-up head and placed onto a subpixel, electrically connecting its anode to the corresponding electrode on the substrate. The pixel is completed by coating the micro-LED with a suitable dielectric (such as photolithographically patternable polyimide), while connecting the cathode of the micro-LED to an electrode on the substrate. Metal interconnects are deposited and patterned to form the connections shown in Figure 2C.

LEDs emitting red light at a wavelength of approximately 630 nm are typically made of aluminum gallium indium phosphide (AlGaInP) grown on gallium arsenide (GaAs). Because GaAs is opaque, laser lift-off cannot be used to remove the LED from the GaAs substrate. Therefore, to remove the red LED from the substrate, the substrate can be completely etched, or a selective etch (typically using hydrogen chloride (HCl):acetic acid) can be used to undercut and remove the LED. The LED's dimensions (cross-section) are similar to those of GaN general-purpose lighting LEDs (150-1000 μm). The AlGaInP LED process is more fully described in US Patent No. 10,804,426, which is incorporated herein by reference.

The pick-and-place assembly process described above has several significant issues that result in high costs and low throughput. Specifically, the assembly process is serial in nature, so assembling millions of microLEDs takes a long time and is very costly. The small size of the microLEDs themselves makes the gripper heads difficult to manufacture, and the edges of the gripper are likely to interfere with adjacent microLEDs during the gripping process or with the reflector structures between pixels during assembly. The single pick-and-place method described above can be expanded to a parallel process by using a mass transfer head to simultaneously grasp and transfer multiple microLEDs. However, the quality of this mass transfer method can be poor because defective components may be present in a group of micro-LEDs transferred simultaneously, and the pitch between each micro-LED is determined by the pitch of the components grown on the wafer.

Figures 3A-3H illustrate an example of a mass transfer method (known in the art). Mass transfer involves transferring a plurality of microLEDs arranged in an array as a whole onto a display substrate. This method has been widely developed to address the low throughput issues of serial pick-and-place assembly. In the simplest mass transfer process, a rectangular stamp picks up a rectangular array of microLEDs from a carrier and presses them against the display substrate, bonding each microLED to a corresponding electrode. Because RGB displays require micro-LEDs of different colors, the transfer stamp is arranged to pick up every three micro-LEDs, leaving space for micro-LEDs of the other two sub-pixel colors. For the surface-mount micro-LED shown in Figure 2C, the assembly process proceeds as follows:

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

2) Multiple sets of cathode and anode electrodes are arranged on the display substrate (Figure 3B). The spacing between each set of electrodes is several times the spacing between each micro-LED on the wafer, so that the electrodes match the positions of the micro-LEDs on the transfer stamp. This spacing determines the ultimate resolution of the display. The electrodes can be made of copper, indium tin oxide/aluminum (ITO/Al), gold, or solder such as tin/indium (Sn/In). An ACF film can also be used to cover the electrodes. By determining the materials of the electrodes and micro-LEDs on the display panel, ohmic contacts can be formed through the subsequent bonding process in step 5 below.

3) Prepare the stamp with the pickup points aligned with the subpixel pitch on the display. Known pickup mechanisms used to secure each microLED include elastomers, tape, electrostatics, and magnetic fields. Figure 3C depicts a 3x3 pixel stamp, but actual stamps typically accommodate hundreds of pixels.

4) Referring to Figure 3D, the embossing stamp is aligned with the carrier substrate carrying the first color of micro-LEDs, and the embossing stamp contacts the carrier substrate, allowing the fixing structure to grasp the multiple micro-LEDs and remove them from the carrier substrate.

5) Refer to Figure 3E. The filled stamp is aligned with the electrodes on the first set of display substrates.

6) Referring to Figure 3F, the embossing stamp is pressed against the display substrate and is typically heated to form a bond between the micro-LED electrodes and the electrodes on the display substrate. After the bond is formed and sufficiently cooled to secure the micro-LEDs, the embossing stamp is removed for reuse.

7) As shown in Figures 3G and 3H, the same operation is performed on the second and third color micro-LEDs, respectively, to form an RGB display array.

While the mass transfer method described above is feasible for assembly and has already been applied in display manufacturing, it still presents several challenges, resulting in low product yields and high costs. First, in Figure 3B, the display's x- and y-pitch spacing can only be an integer multiple of the pitch between the microLEDs on the MOCVD wafer—3x2 in the example shown. A comprehensive display manufacturing technology must be able to produce screens of varying sizes that meet industry standards, such as 4K (3840x2160 pixels). Therefore, a technique that can vary the pitch of the microLEDs on the stamp (pitch expansion) is required. Alternatively, the size of the microLEDs on the MOCVD wafer can be customized for each display based on size and resolution, but this adds unnecessary cost. Secondly, the pickup device must strike a balance in terms of the strength of the connection. If the connection strength is too weak, some micro-LEDs will not detach from the carrier substrate, leaving gaps in the array. Conversely, if the connection strength is too strong, the micro-LEDs will be forcibly removed after being soldered to the substrate. In both cases, the brightness of the sub-pixel will be reduced, which is unacceptable in a display. Finally, the structure of the transfer stamp is complex and difficult to manufacture. The connection point must be smaller than the distance between the micro-LEDs to avoid the stamp interfering with adjacent micro-LEDs. This is difficult for complex fixing methods that require the generation of local fields (such as electrostatic or magnetic forces). Embossed stamps are also susceptible to contamination and damage, especially those made of elastomers such as polydimethylsiloxane (PDMS). Therefore, effective cleaning of embossed stamps for reuse is crucial.

To illustrate the defects of the mass transfer imprint process, Figure 3H depicts several possible failure scenarios: Fault a: Missing microLED due to poor adhesion of the imprint stamp during pickup; Fault b: Misplaced microLED due to contamination on the imprint stamp; Fault c: Particles introduced by contamination from the transfer imprint stamp; Fault d: Broken microLED; Fault e: Short circuit of the microLED due to defects in the MOCVD process; Fault f: Damaged electrodes due to the microLED being forcibly removed by the imprint stamp.

Figures 4A and 4B depict exemplary area coverage for emboss pick-up using a 14 mm embossing stamp (Figure 4A) on a 100 mm wafer. Of these, 20% of the micro-LEDs ultimately remained on the wafer, with three of the embossing stamps containing defective micro-LEDs. Another limitation of the mass transfer process described above is the square shape of the embossing stamp, which does not match the round wafers used to grow LEDs using MOCVD. Figure 4A shows a typical arrangement using a 14*14 mm embossing stamp on a 100 mm wafer. Using a larger embossing stamp can increase assembly speed, at the expense of leaving more micro-LEDs on the growth wafer. Because the requirement to fill all embossing stamps is met, large areas on the wafer cannot be used with the stamps. In the above example, approximately 20% of the qualified micro-LEDs are discarded, directly increasing costs. Furthermore, defective micro-LEDs must be repaired or the affected stamps discarded. The above example describes three random defects for illustrative purposes only. If the defective stamps were discarded in this example, only approximately 70% of the initial micro-LEDs would be usable in display manufacturing.

A significant advantage of the mass transfer method is that the bonding process is performed under pressure on the microLED, resulting in good mechanical contact between the two bonded electrodes. This ensures contact over a large area between the electrodes. Mechanical contact also breaks down insulating oxides on the surface, improving the wettability of the solder material. ACF bonding similarly requires pressure to ensure that the conductive filler material forms a hard contact with the microLED and the electrodes on the display substrate.

It would be advantageous to have a structure and method for filling the carrier substrate for mass transfer assembly of micro-LED displays that can improve assembly flexibility and yield in the following ways: 1. Any display resolution can be achieved through simple pitch expansion; 2. A series of micro-LEDs (known good chips) can be manufactured without device defects such as missing, damaged, or shorted components; 3. The mass transfer assembly speed can be increased by filling and transferring the embossing stamps through a large-scale parallel transfer method; 4. A simple transfer embossing stamp can be used, which has low manufacturing costs and can be reused through strong cleaning; 5. A simple embossing mechanism can be employed that does not damage the display substrate; and 6. Excess micro-LEDs can be recovered from defective embossing stamps.

This application provides a method and related structure for fabricating an array of microLEDs by fluid assembly onto a carrier substrate or transfer stamp. The assembled microLEDs can be applied to a display substrate and bonded using a mass transfer method. The microLEDs are fabricated on a wafer using conventional MOCVD methods, with their shape chosen to facilitate fluid assembly and stamping onto the display substrate.

Therefore, the present application provides a micro-LED mass transfer imprinting system, comprising an imprinting stamp substrate having a top surface. An array of imprinting stamp substrate capture locations is formed on the top surface, each of which is provided with a columnar groove for temporarily securing a keel extending from the bottom surface of the micro-LED. When the micro-LED is a surface mount type, it has a planar top surface, which includes a first electrode and a second electrode. When the micro-LED is a vertical type, it has a planar top surface, which includes the first electrode, and the keel is a conductive second electrode. The imprinting system also includes a fluid assembly carrier substrate having an array of wells formed on its planar top surface. The array of wells has a spacing between adjacent wells that matches the spacing between adjacent capture locations on the imprinting stamp substrate.

A related method for mass-transferring micro-LEDs includes providing the aforementioned fluid assembly carrier substrate with a well array and the aforementioned embossing stamp substrate, wherein each trapping location in the array is configured with a columnar recess that matches a well on the carrier substrate. The method utilizes a fluid assembly process to fill the wells of the carrier substrate with micro-LEDs. The method then presses the top surface of the embossing stamp substrate against the top surface of the carrier substrate, aligning each trapping location with a corresponding well, thereby transferring the micro-LEDs from the carrier substrate to the embossing stamp substrate. Each trapping site groove supports a keel extending from the bottom surface of the microLED. By constraining the keel, the microLED is secured to the imprinting stamp substrate. The use of a carrier substrate eliminates the limitations imposed by the spacing between microLEDs on the MOCVD wafer, allowing a variety of imprinting stamp substrate pitches to be used for 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, the electrode being electrically connected to an underlying matrix of column and row control lines. The connection pads have a spacing separating adjacent locations that matches the spacing separating adjacent trapping locations in the embossing stamp substrate. The method presses the top surface of the embossing stamp substrate against the top surface of the display substrate, connecting each trapping location to a corresponding micro-LED location, and transferring a plurality of micro-LEDs from the embossing stamp substrate to the plurality of connection pads on the display substrate. In one aspect, 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 an RGB display, the method can sequentially laminate an embossing stamp substrate having micro-LEDs of a first wavelength, a second wavelength, and a third wavelength at the capture locations, or a single embossing stamp substrate can be used for each micro-LED of a wavelength.

This application also provides a method for mass-transferring micro-LEDs, which utilizes a fluid assembly stamp substrate having a planar top surface, on which a plurality of trapping sites having a first peripheral shape, a depth, and a planar bottom surface are formed. Through a fluid assembly process, the trapping sites can be filled with micro-LEDs having a first peripheral shape, a thickness greater than the trapping site depth, a bottom surface in contact with the trapping site bottom surface, a planar top surface, a first electrode extending beyond the trapping site, and a securing mechanism. In one aspect, the fixing mechanism is a keel formed on the top surface of the microLED. This keel can be either a conductive keel connected to the first electrode or a temporary non-conductive keel that is removed after the microLED is fixed to the stamp substrate. In another aspect, the fixing mechanism is a first component comprising a conjugated biomolecule pair disposed on the bottom surface of each microLED. In this case, a second component comprising a conjugated biomolecule pair is disposed on the bottom surface of each trapping site.

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 being electrically connected to a matrix of column and row control lines beneath the substrate. The spacing between adjacent wells in the display substrate matches the spacing between adjacent trapping sites in the embossing stamp substrate. The method embosses the top surface of the stamp substrate against the top surface of the display substrate, filling each trapping site with a micro-LED, and transferring the plurality of micro-LEDs from the embossing stamp substrate to the micro-LED connection pads on the display substrate. Likewise, the display substrate can be heated during the transfer process to facilitate electrode bonding.

The present application also provides a method for mass transfer of axial micro-LEDs. The method provides a fluid assembly stamp substrate having a planar top surface, wherein a plurality of capture locations are formed on the top surface, wherein the capture locations have a first peripheral shape, a central portion having a planar first depth, a distal end having a planar second depth (the second depth is less than the first depth), and a proximal end having a planar second depth. Through a fluid assembly process, the method fills the capture locations with axial micro-LEDs, wherein each micro-LED occupying a corresponding capture location has the first peripheral shape, a main body connected to the central portion, and a vertical planar main body, wherein the main body thickness is greater than the first depth but less than twice the first depth. A distal electrode horizontally bisects the main body and contacts the distal end of the capture site. The electrode thickness of the distal electrode in a vertical plane is greater than the second depth of the capture site but less than twice the second depth. A proximal electrode has the same thickness as the distal electrode, bisects the main body horizontally, and contacts the proximal end of the capture site.

The method provides a display substrate having a planar top surface and an array of microLED connection pads. Each microLED connection pad includes 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 spacing separating adjacent wells that matches the spacing separating adjacent trapping sites on the imprinting stamp substrate. The method presses the top surface of the imprinting stamp substrate against the top surface of the display substrate, bringing each trapping site into contact with a corresponding microLED, and transfers the microLEDs from the imprinting stamp substrate to the display substrate. Heating is typically applied to facilitate electrode bonding.

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

900, 900a, 900b, 900c, 1300, 1500: Embossed seals

902, 1302, 1502: Top surface of the embossed seal

904, 1304, 1504: Capture locations

906: Keel

908: Micro-LED bottom surface

910: Micro LED

910a, 910b, 910c: Surface mount micro-LEDs

912: Micro-LED top surface

914, 1316: First electrode

916, 1324: Second electrode

918, 1315, 1318, 1525: Display substrate

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

1002: Carrier bottom and top surfaces

1004: Trap

1006: Spacing

1008: Bottom surface of carrier substrate

1010: Heating device

1100: Vertical Micro-LED

1102: Vertical micro-LED top surface

1104: Vertical micro-LED first electrode

1106: Insulation layer

1200, 1400: Static Electricity Generator

1202, 1402: Magnetic generator

1306: Depth

1308: Bottom surface of the capture location

1310: Micro-LED thickness

1312: Micro-LED bottom surface

1314: Micro-LED top surface

1320: Groove

1322: Thiol-biotin bifunctional molecule/first component

1325:ACF

1326: Silicon dioxide film

1327: Streptavidin molecule/first component

1506: Center

1508: First Depth

1510: Remote

1512: Second Depth

1514: Proximal

1516: Axial Micro-LED

1518: Subject

1520:Main body thickness

1522: Distal Electrode

1524: Electrode thickness

1526: Proximal electrode

1528: Electrode

1530: Dielectric Film

1532: Main body groove

1534:P Connector Pad

1536:N connection pad

1538: First Groove

Figures 1A-1C are cross-sectional views of a GaN LED (Figure 1A), a cross-sectional view of two vertical micro-LEDs (Figure 1B), and a cross-sectional view of a surface-mount micro-LED (Figure 1C) (known technology).

Figure 2A shows the process of removing micro-LEDs from a sapphire growth substrate using laser ablation technology (known technology).

Figure 2B illustrates the pick-and-place process for moving devices from a carrier wafer and positioning them on a display substrate (known technique).

Figure 2C shows the process of connecting the anode of a micro-LED to the substrate electrode (known technology).

Figures 3A-3H illustrate the steps of an exemplary mass transfer process (known in the art).

Figures 4A and 4B show an example of the coverage area of a 100mm wafer using a 14mm stamp for pick-and-place (Figure 4A). At this point, 20% of the LEDs remain on the wafer, and three of the LEDs on the stamp have defects (Figure 4B). (Known technology)

Figure 5 is a partial cross-sectional view of a typical backplane layout showing surface-mounted microLEDs and the power transistors that control the brightness of the microLEDs.

Figures 6A and 6B are top views and cross-sectional views, respectively, of a surface-mount microLED for fluid assembly.

Figure 7 is a schematic diagram of a micro-LED wafer after selective picking.

Figure 8 provides a brief description of the fluid effect, which allows 100% of the micro-LEDs to be assembled in the correct orientation with the electrodes facing downward.

Figures 9A-9D illustrate the steps of using a micro-LED mass transfer imprinting system.

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

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

Figures 12A-12B are partial cross-sectional views of an attractive force generator used to assist in securing a micro-luminescent diode to a capture position on a carrier substrate.

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

Figures 14A and 14B are schematic diagrams of using an electrostatic force generator and a magnetic force generator as auxiliary devices, respectively, to assist in securing the micro-LED to the fluid assembly capture position.

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

Figure 16 is a flow chart of the micro-LED mass transfer method corresponding to the system shown in Figures 9A-9D.

Figure 17 is a flow chart of the method for mass transfer of micro-LEDs using a fluid assembly stamp substrate as shown in Figures 13A-13K.

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

Figure 19 is a flow chart of the method for extending the transmission time span of a micro-LED.

U.S. Patents 9,825,202 and 10,418,527, which are incorporated herein by reference, have reported general processes for fabricating micro-LED displays using inorganic LEDs and fluidic assembly on display backplanes. Specifically, U.S. Patent 9,825,202 describes a process flow for fabricating a suitable display backplane, starting at column 13 and row 26, as shown in Figure 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 a well layer, as a mass transfer stamp sets the position of the micro-LEDs.

Figure 5 is a partial cross-sectional view of a typical backplane layout showing surface-mounted microLEDs and the power transistors that control the brightness of the microLEDs.

The fluidic assembly techniques proposed in U.S. Patents 9,825,202, 10,418,527, and 10,543,486 (cited herein) are suitable for direct random assembly of low-cost micro-LED displays. Here, the same assembly techniques are used to create an embossing stamp, which is used to alternately bond micro-LEDs to electrodes on a display substrate. This approach has the advantage over direct fluidic assembly strategies in that the application of pressure by the embossing stamp during the bonding process facilitates the formation of ohmic contact between the micro-LEDs and the display. As used herein, the transfer embossing stamp is configured with an array of trapping sites, with the spacing between the trapping sites matching the spacing between display pixels. The embossing stamp can be made of glass, quartz, or single-crystal silicon, and the trapping sites (also known as wells) can be formed by etching the embossing stamp or by placing a film, such as patterned polyimide, on the embossing stamp and patterning the wells using photolithography. The trapping sites have the same shape as the micro-LEDs and can be slightly larger than shown in FIG8 of U.S. Patent 10,804,426, which is incorporated herein by reference. A unique feature of the system described herein is that the depth of the trapping sites can be less than at least one point of the thickness of the micro-LEDs, so that the micro-LEDs can contact the assembly tool or display substrate without being disturbed by the top surface of the embossing stamp. Wells (trapping sites) etched into an embossing stamp are more stable, allowing for more thorough cleaning, but controlling the depth of the trapping sites becomes more difficult. In contrast, the depth of trapping sites formed on polyimide or a deposited film can be controlled by the film thickness, but the film is more susceptible to damage.

The imprinting system described in this application is compatible with a variety of micro-LED configurations, but the conventional LED structure shown in FIG2C is not suitable because it lacks a device for positioning in the fluid assembly, and therefore the electrode cannot be properly positioned on the display substrate for bonding. The disc-shaped surface-mount micro-LED described in U.S. Patent 10,804,426 was designed as a solution within a certain range, constrained by the fluid assembly described in U.S. Patent 9,825,202, as shown in 12 columns and 56 rows and FIG16, so these devices are 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, as described in FIG. 8 of U.S. Patent 9,825,202 and FIG. 4 of U.S. Patent 10,516,084 (incorporated herein by reference), can be used in the same manner. Similarly, the embossing system is not limited to surface-mount microLEDs. Vertical microLEDs can also be fabricated using this method, employing a single bottom electrode and fabricating the top electrode after assembly. These variations will be readily apparent to those skilled in the art and, for the sake of brevity, will not be further described in this application.

Figures 6A and 6B depict, respectively, a top view and a cross-sectional view of a surface-mount microLED for fluidic assembly. The device structure typically has a diameter of 20-100 micrometers (μm), a thickness of 4-6 μm, and includes a keel with a height of 5-10 μm. In this case, the well depth is typically 3.5-4.5 μm to accommodate the thickness of the microLED. A detailed fabrication process flow can be found in columns 8 and 56 of U.S. patent 10/804,426 and in Figure 6. The shape of the disk matches the cylindrical shape of the trapping site, which is typically less than the thickness of the microLED and slightly larger in diameter. Surface mount electrodes are usually made of solder such as tin/indium or gold/germanium. 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 microLED wafer after selective picking. Defects are identified using optical microscopes, scanning electron microscope (SEM) images, cathodoluminescence, or photoluminescence. The goal is to identify all defects that could cause display pixel errors so that defective products can be removed from the microLED suspension used for manufacturing. By combining the defect map with known patterns such as edge beading maps and alignment structures, the locations of all known defective microLEDs can be determined. Using a printing process, defective microLEDs are covered with a capture material to prevent them from being picked up. As shown in the figure, the selective pick-up stamping process captures all qualified microLEDs while leaving behind defective ones. Combining high utilization with the prevention of defective devices is a significant advantage of fluidic assembly technology. The selective pick-up method is described in more detail in unpublished application No. 16,875,994, which is incorporated herein by reference.

After the micro-LEDs are fabricated, the grown wafer is attached to a carrier wafer via an adhesive layer. Laser lift-off (LLO) technology is used to lift the micro-LEDs off the sapphire wafer, and a keel is patterned on the bottom surface of the micro-LEDs.

A suspension of micro-LEDs is dispersed onto a carrier substrate and assembled as described in Figure 7 of U.S. Patent No. 10,418,527 and U.S. Patent No. 10,804,426. For mass transfer, it is crucial to prevent surface contaminants from interfering with the exposed surface of the micro-LEDs and the target surface. Therefore, any excess unassembled micro-LEDs on the surface are removed and recovered after assembly, making effective cleaning methods crucial.

Figure 8 provides a simplified overview of the fluidic effect that enables 100% of the micro-LEDs to be assembled with the correct electrode-down orientation. The assembled substrate is inspected, and if some wells are not filled or other defects such as excess, unassembled micro-LEDs are present, the micro-LEDs are removed by cleaning the stamp with a solvent, and the solvent is captured in a reservoir for micro-LED recovery. The empty stamp is further cleaned, dried, and inspected to ensure that there is no surface contamination or residue at the capture location. This capability is particularly important compared to traditional stamping stamps that use elastomers or adhesives to secure the micro-LEDs, which are difficult to clean and reuse. Under traditional technology, stamps with contaminated or missing LEDs are usually discarded, making it impossible to recycle the intact LEDs on the stamps.

9A-9D illustrate the steps of using a micro-LED mass transfer imprinting system. The system includes an imprinting stamp substrate 900 having a top surface 902. An array of imprinting stamp substrate capture locations 904 is formed on the top surface 902. Each capture location 904 is configured as a columnar groove to temporarily secure a keel 906 extending from a bottom surface 908 of a micro-LED 910. As shown, the micro-LEDs 910 are surface mount micro-LEDs, each of which includes a planar top surface 912 having a first electrode 914 and a second electrode 916 disposed thereon. In this case, the keel 906 is non-conductive. In this particular example, as shown in FIG6A , the second electrode is a full or partial ring surrounding the first electrode. For the systems of FIG9A-9D or FIG11A-11D (see below), an adhesive or elastomer can be patterned on the top surface 902 of the stamp substrate to facilitate attachment of the microLEDs to a trapping location.

The filled carrier substrate 1000 is the basis for mass transfer using the embossing stamp substrate 900 to the display substrate 918, illustrated in the figure with a single micro-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 operate the micro-LEDs. For details, please refer to U.S. Patent 9,825,202. In this case, the carrier substrate 1000 is a flat surface substrate with wells that allow local protrusions (optionally adhesive or elastomer) near the embossing stamp head capture location 904 to contact each micro-LED (as shown in Figure 9B). Since the micro-LED is typically held in place in the carrier solely by gravity, the relatively weak adhesion forces allow the micro-LED to be removed from the carrier during the transfer process using an optional adhesive or elastomer. The embossing stamp is aligned with the electrodes on the display substrate and pressed to create a hard contact between the electrodes of the micro-LED and the electrodes on the display substrate, while heat is applied to form a solder bond (Figure 9C). In another embodiment, the connection can be made with an additional ACF film (not shown). When the bond is complete, the transfer embossing stamp is recovered and released from the micro-LED (Figure 9D). The transfer head 900 and carrier substrate 1000 are cleaned and reused, and the cycle is repeated to fill the entire area of the display substrate 918.

Figures 10A-10D are cross-sectional views illustrating the process of transferring microluminescent diodes from a carrier substrate to a display substrate. The system includes a fluidic assembly carrier substrate 1000a-1000c having a planar top surface 1002 and an array of wells 1004 arranged on the carrier substrate top surface 1002. Adjacent wells are spaced 1006 apart, and the spacing 1006 matches the spacing separating adjacent capture locations on the imprinting stamp substrate. The wells 1004 of the carrier substrate have a first peripheral shape (circular in this embodiment) and a planar well bottom surface 1008. Surface-mount microLEDs 910a-910c each have a first peripheral shape and a planar top surface 912, thereby contacting the well bottom surface 1008 via a first electrode 914 and a second electrode 916 (as shown in FIG. 9A ).

In the case of RGB display, the imprint system can further include a first fluid assembly carrier substrate 1000a and an array of wells disposed on the top surface of the carrier substrate, with adjacent wells spaced 1006 apart to match the capture locations of the imprinting stamp substrate (Figure 10B). The micro-LEDs 910a are configured to emit light of a first wavelength, with each micro-LED occupying a corresponding well in the first carrier substrate 1000a. Similarly, the second fluid assembly carrier substrate 1000b includes an array of wells disposed on the top surface of the carrier substrate, with adjacent wells spaced 1006 apart to match the capture locations of the imprinting stamp substrate. Micro-LEDs 910b are configured to emit light at a second wavelength, with each micro-LED occupying a corresponding well in the second carrier substrate 1000b. The third fluid assembly carrier substrate 1000c includes wells arranged in an array on the top surface of the carrier substrate, with adjacent wells spaced 1006 apart to match the capture locations of the imprint stamp substrate. Micro-LEDs 910c are configured to emit light at a third wavelength, with each micro-LED occupying a corresponding well in the third carrier substrate 1000c.

In order to manufacture the three colors required for an RGB display, micro-LEDs of the three colors need to be assembled and embossed sequentially, as shown in Figures 10A-10D. The design of the array of capture sites on the three carrier substrates is spaced according to the pitch 1006 of the display pixels. The process flow of micro-LEDs of different colors or the performance gap of LEDs are likely to determine that the micro-LEDs of different colors have different sizes and/or shapes. For example, the red micro-LED can be made of aluminum indium gallium phosphide (AlInGaP), as described in U.S. Patent 10,804,426. In this case, the red micro-LED may be thicker than the blue and green devices based on GaN. Because blue and green microLEDs have different quantum efficiencies, and the human visual system is more sensitive to green, it may be necessary to manufacture blue and green microLEDs with different emission areas. An example of these differences is shown in FIG10A , where each carrier substrate is tuned to accommodate the needs of the corresponding color of microLED. The embossing stamp substrate 900a captures the array of blue microLEDs 910a from the carrier substrate and moves it to the display substrate 918, aligning the embossing stamp 900a with the vacant area on the display substrate and physically contacting the electrodes of the microLEDs with the matching electrodes on the display substrate ( FIG10B ). Pressure and heating device 1010 is used to strengthen the contact between the electrodes, melting the metal material and forming a solder bond. In Figures 10C and 10D, green micro-LED 910b and red micro-LED 910c are transferred and bonded in the same manner (stamping stamp 900b, stamping stamp 900c). Bonding between the micro-LED and the connection pad can use materials such as gold/germanium to copper, indium/zinc to copper, and gold/ACF/copper. If ACF is used, a wider selection of display electrode materials is available, such as Mo/Al/Mo.

The fluid assembly used in this application achieves several improvements over the conventional simple imprint process: 1) There are no gaps in the array pattern due to defects or missing micro-LEDs; 2) Selective pick-up and fluid assembly fully utilize all intact micro-LEDs on a wafer; 3) Micro-LEDs are recovered during the assembly process and on defective carrier substrates to prevent waste; 4) The carrier substrate is manufactured based on the distance between capture locations on the display, allowing for simple pitch expansion.

Figures 11A-11D illustrate an imprinting system in which the micro-LEDs are vertical micro-LEDs. Each vertical micro-LED 1100 has a planar top surface 1102 with a first electrode 1104 and a conductive keel 906 serving as a second electrode. Similar to surface-mount micro-LEDs, the well 1004 on the substrate has a first peripheral shape (e.g., circular) and a planar well bottom surface 1008. Each vertical micro-LED 1100 has a first peripheral shape and a planar top surface 1102, which contacts the corresponding well bottom surface 1008 via the first electrode 1104.

For smaller microLEDs, which don't have the space to fabricate two electrodes on the same surface as surface-mount microLEDs, the same assembly process can be applied to vertical microLEDs. In this case, the microLED is configured with a single anode electrode on the top surface and a cathode electrode on the bottom surface, either as a conductive post (keel) or electroplated with gold or copper. The conductive post also serves as the keel for fluidic assembly on a carrier board (substrate).

The assembly and bonding sequence of conductive keel vertical micro-LEDs is demonstrated. A micro-LED suspension is prepared by the selective capture method described above and dispensed onto the surface of a carrier substrate provided with wells having a display spacing, followed by assembly according to conventional procedures. The embossing stamp is aligned with the carrier substrate, and the micro-LED is removed from the carrier substrate, as shown in FIG11A . The filled embossing stamp is aligned with the display substrate, and pressure is applied to establish mechanical contact between the cathode electrode on the micro-LED and the P-connection pad electrode on the display substrate (as shown in FIG11B ). A heating device 1010 is used to form the solder bond, after which the stamp is removed, cleaned, and reused. An insulating layer 1106, such as polyimide, is used to fill the gap between the microLED and the reflective well to prevent shorting and planarize the surface for metal deposition (Figure 11C). A keel protrudes from the insulating layer 1106, forming a self-aligned contact to each microLED. Removing a portion of the insulating layer with a short O2 plasma etch improves contact quality. The conductive posts of the microLEDs are connected to Vss (power) via patterned metal as shown in Figure 11D, completing the circuit.

Figures 12A and 12B are partial cross-sectional views of a force generator used to assist in securing a microLED to a capture location on a carrier substrate. Figure 12A illustrates the use of an electrostatic force generator 1200, while Figure 12B illustrates the use of a magnetic force generator 1202. Although shown for surface-mount microLEDs, these force generators can also be applied to vertical microLEDs.

Figures 13A-13K illustrate the steps of a micro-LED mass transfer imprint system using a fluid assembly imprint stamp substrate. To further simplify the assembly process, the imprint stamp can be directly filled using fluid assembly, thereby omitting the carrier substrate. The micro-LED shown in Figure 6 utilizes a keel structure on the bottom surface, hereinafter referred to as a fixing mechanism, to secure the micro-LED to the electrode at the fluid assembly capture location. For the direct assembly process, the electrode location must be "upward" in the imprint stamp, so the keel structure is fabricated on the top surface of the micro-LED, as shown in Figure 13A. The fluid assembly is performed in a conventional manner, and the micro-LEDs are arranged in an array in the trapping position with the keel structure and the electrode facing upward. The material used to make the keel structure is usually a photosensitive polyimide, which can be removed using a solvent or etched using an oxygen plasma. After assembly and drying, the keel is removed (as shown in Figure 13B) to facilitate bonding the electrode to the display substrate. The method for making the imprint stamp is the same as the above 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 etch directly on the substrate to form the trapping position structure. Gravity and van der Waals forces allow the stamp to hold the microLEDs. If the stamp is inverted, the microLEDs will fall out. Therefore, during heating, the stamp must be placed with the surface facing upward for transfer assembly and bonding, and the display substrate must be pressed down onto the stamp (Figure 13C).

The fluid assembly stamp substrate 1300 has a planar top surface 1302. An array of capture locations 1304 is formed on the stamp substrate top surface 1302, each capture location having a first peripheral shape, a depth 1306, and a planar capture location bottom surface 1308. As in the aforementioned embodiment, the first peripheral shape is circular, but the system is not limited to this shape. A micro-light emitting diode 910 is disposed in the capture location 1304, having a first peripheral shape, a thickness 1310 greater than the capture location depth 1306, a planar bottom surface 1312 in contact with the bottom surface 1308, a planar top surface 1314 having a first electrode 1316 and extending beyond the capture location, and a protective mechanism (see explanation below). The microLED has the same electrical connections as vertical microLED 1100, which has a second electrode formed on bottom surface 1312 (as shown in FIG. 13D ), or as surface-mount microLED 910, which has a first electrode 1316 and a second electrode 1324 formed on surface 1314 (see FIG. 13A and FIG. 13E ).

As shown in FIG13A , the fixing mechanism is a keel 906 formed on the top surface of the micro-LED. Keel 906 is a temporary, non-conductive keel that is removed before the micro-LED contacts the display substrate 1315. Alternatively, as shown in FIG13D and FIG13E , the fixing mechanism can be a conductive keel 906 connected to the first electrode 1316. In FIG13D , the micro-LED is a vertical micro-LED 1100.

In another embodiment, a conductive center post is used in place of a non-conductive keel during direct imprint transfer, allowing the structure to serve as both a keel during the fluidic assembly process and as an anode electrode ( FIG13E ). In this case, the imprint stamp is a simple plate with an array of trapping sites spaced the same as the display pixel pitch. In the display substrate 1318, the P-connection pad electrode is located below the N-connection pad electrode, leaving space for the conductive post that forms the anode electrode on the micro-LED ( FIG13F ). Due to process variations, there may be differences in the height of the conductive pillars and the depth of the P-connection pad grooves. Therefore, the ACF1325 can be used to connect the micro-LEDs and the display substrate to compensate for these differences.

Thus, the micro-LED in FIG13E is a surface-mount micro-LED 910a, and the display substrate 1318 in FIG13F includes a recess 1320 for receiving the conductive keel 906.

Another mechanism for positioning and securing the microLED within the transfer stamp utilizes preferential bonding between conjugated biomolecule pairs (e.g., streptavidin-biotin pairs). As shown in Figure 13G , after LLO, a functionalized microLED is fabricated by depositing a thin silicon dioxide film 1326 on the backside of device 1312. The surface of the microLED is exposed to hydrogen ions or alkaline compounds, which then react with amine-terminated molecules such as 3-aminopropyltrimethoxysilane, resulting in silanization. The surface is washed with a streptavidin solution, allowing the streptavidin molecules 1327 to bind to the amine ends, thereby forming a streptavidin-functionalized microLED (as shown in Figure 13H). Prior to assembly, the trapping sites on the transfer stamp can be similarly treated with biotin-terminated ligands, or the bottom surface of the well can be a gold surface exposed to thiol-biotin bifunctional molecules 1322, as shown in Figure 13I.

Thus, Figures 13G-13K illustrate microLEDs using conjugated biomolecule pairs as "anchoring mechanisms." A first component 1322 comprising the conjugated biomolecule pairs is coated on the bottom surface 1308 of the stamp substrate. The microLED anchoring mechanism is a second component 1327 comprising the conjugated biomolecule pairs coated on the bottom surface 1312 of each microLED. During assembly, the relatively low trapping site depth (approximately 1 μm) allows for easier removal of misoriented microLEDs by fluid agitation, while correctly oriented microLEDs can be chemically bound to the trapping site bottom surface and better marked by being confined within the trapping site. In Figure 13J , the bioconjugate bond is shown with a greatly magnified z-scale to illustrate the binding effect. In reality, the binding layer is very thin, as shown more accurately in Figure 13K . Alternative chemistries to the biotin-streptavidin system, such as thiol-maleimide and azide-alkyne, may have advantages in terms of stability and ease of processing, but the preparation sequence is similar.

Figures 14A and 14B illustrate the use of an electrostatic force generator 1400 and a magnetic force generator 1402, respectively, as auxiliary mechanisms for securing a microLED within a fluid assembly capture location (with or without a keel). In Figures 14A and 14B , the primary securing mechanism may be gravity. Alternatively, in Figure 14A , the securing mechanism may be a cohesive biomolecule (not shown). In other embodiments (not shown), the force generator in Figure 14A may also be a magnetic force generator, and the force generator in Figure 14B may also be an electrostatic force generator. Although Figures 14A and 14B illustrate only a fluid assembly embossing stamp substrate, it should be understood that the force generator can also be used with the groove configuration embossing stamp substrates shown in Figures 9B-9D and 11A-11B.

At the expense of increased complexity, some fixing structures can be added to the stamp structure to prevent the micro-LED from detaching from the trapping site when the stamp is inverted. Since the fixing structure can be removed from the micro-LED after binding, the use of adhesives for binding is unattractive. By providing a porous layer between the substrate support surface and the trapping site forming layer, vacuum conditions are introduced into the stamping system, but the liquid of the fluid assembly may flow into the porous layer, making it impossible to dry. The most practical structures for fixing the micro-LED in the stamp are magnetic or electrostatic structures. For electrostatic mounting, the microLED has a dielectric film deposited on the surface corresponding to the surface-mount electrode (i.e., the bottom surface), and the embossing stamp includes a dynamic electrode below the trapping location structure. For magnetic mounting, the microLED electrode structure can include a magnetic material such as nickel, while the embossing stamp contains a permanent magnet or electromagnet.

These fixtures are switchable at independent points in the array, allowing the following process to repair defective stamps: 1) Inspect the stamp to identify defective microLEDs; 2) Activate the fixture for all good microLEDs; 3) Remove defective microLEDs by rinsing; 4) Place additional microLED suspension and proceed with assembly.

In one aspect, the stamp can include a photosensor that, when pressed against the display substrate, activates all capture locations (simultaneously or sequentially) that are temporarily electrically connected to the micro-LEDs on the stamp. The stamp and associated driver circuitry are connected to a system that records which micro-LEDs are intact. A fixture on the stamp is activated to hold the intact micro-LEDs in the capture locations, and assembly continues until all micro-LEDs have been tested as intact, as described in steps 2)-4) above. The binding process then proceeds.

Figures 15A-15I illustrate a microLED mass transfer imprint system using a fluid imprint stamp substrate and axially oriented microLEDs. This hybrid fluid assembly mass transfer method can also be applied to the axially oriented microLEDs described in Application No. 16/846,493. To reduce cost and increase density, the microLEDs are configured as vertical devices with a light-emitting area of 5 by 8 μm, as shown in Figure 15G. The blade-shaped microLED electrodes can be electroplated copper or gold. The dimensions of all these features are adjustable, but their relative shapes are important to facilitate fluid assembly into oriented arrays.

Figures 15A-15C illustrate the fabrication process of an axially oriented micro-LED display substrate 1525. Electrodes 1528 are deposited and patterned from a conductive material (e.g., molybdenum/copper (Mo/Cu)) to form contact pads for accommodating the cathode and anode of the micro-LED. A dielectric film 1530, which can be made of silicon dioxide, silicon nitride (Si3N4), or polyimide, is deposited on the electrodes. The dielectric film 1530 is patterned and etched to create contact openings, as shown in Figure 15B. Using the metal electrode as a hard mask, a body recess 1532 is etched to accommodate the micro-LED body, as shown in Figure 15B. Finally, N-connection pad 1536 and P-connection pad 1534 are formed by electroplating, sputtering, or evaporation, as shown in FIG15C.

For the shape of an axial micro-LED, the stamping process is more complex, requiring two trapping locations at different depths. As shown in Figure 15D , a first recess 1538 is etched into the substrate. This recess 1538 has a depth and profile suitable for accommodating the micro-LED body protruding below the surface of the axial electrode. In Figure 15E , a second recess 1504 is formed by etching to accommodate the axial electrode. This second recess can also be formed from a thin film material (such as photolithographic polyimide) after the first recess 1538 is formed.

A suspension of known good axial micro-LEDs is applied to an embossing stamp and assembled into a micro-LED array (see Figure 15F). The assembled embossing stamp is inspected, aligned with the display substrate, and pressed together, bonding the LED electrodes to the electrodes on the display substrate (see Figure 15I). After bonding, the embossing stamp is retrieved, cleaned, and inspected for reuse.

Thus, the system includes a fluid assembly embossing stamp substrate 1500 having a planar top surface 1502. An array of capture locations 1504 formed on the top surface 1502 of the embossing stamp substrate includes a first peripheral shape (largely rectangular), a central portion 1506 having a planar first depth 1508, a distal end 1510 having a planar second depth 1512, the second depth 1512 being less than the first depth 1508, and a proximal end 1514 having a planar second depth 1512.

Referring to Figures 15F and 15G , an axial micro-LED 1516 occupies the corresponding trapping site 1504 and has the first peripheral shape, a body 1518 in contact with the central portion 1506 of the trapping site, and a vertical planar portion thickness 1520 greater than the first trapping site depth 1508 but less than twice the first trapping site depth 1508. A distal electrode 1522 horizontally bisects the body 1518 and in contact with the distal end 1510 of the trapping site. The distal electrode 1522 has a vertical electrode thickness 1524 greater than the second trapping site depth 1512 but less than twice the second trapping site depth 1512. A proximal electrode 1526 horizontally bisects the main body 1518 and contacts the proximal end 1514 of the capture location. The proximal electrode 1526 has an electrode thickness 1524.

As shown in Figure 15I , the process for transferring the microLEDs to the display substrate is similar to that described in Figure 13C . The aligned display substrates are pressed down onto the fluid assembly stamp substrate, bringing the electrodes of the microLEDs into contact with the corresponding electrodes on the display substrate. Transfer and bonding are accomplished by applying heat to the solder while applying pressure. Optionally, an ACF film (not shown) can be inserted between the corresponding electrodes to achieve both electrical and mechanical connections without requiring a metal phase change.

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

FIG16 is a flow chart describing a micro-LED mass transfer method corresponding to the system shown in FIG9A-9D. Although the method is described as comprising a series of numbered steps for ease of understanding, the numbers do not necessarily indicate the order of these steps. It should be understood that some steps may be skipped, performed simultaneously, or performed in no strict order. However, the method can generally be performed in numerical order. The method begins at step 1600.

In step 1602, an embossing stamp substrate is provided, the substrate having a planar top surface and an array of trapping locations formed on the top surface, each of which is configured as a columnar recess. In one aspect, in step 1603a, the top surface of the embossing stamp substrate is patterned using an adhesive material or an elastomer. In step 1604, each of the trapping locations is recessed to accommodate a keel extending from the bottom surface of a micro-LED. By restraining the keel of each micro-LED, the micro-LED is secured to the embossing stamp substrate in step 1606. Step 1606 may utilize additional electrostatic or magnetic forces to secure the micro-LED to the embossing stamp substrate.

In one aspect, constraining the keel in step 1604 includes constraining a surface-mount LED having a non-conductive keel, which includes a planar surface having a first electrode and a second electrode. In another aspect, in step 1604, constraining a conductive keel connected to the second electrode, the vertical LED includes a planar surface having a first electrode (i.e., the keel serves as the second electrode).

In one aspect, step 1602 provides an embossing stamp substrate having spaced-apart trapping locations. In step 1601a, a fluid assembly carrier substrate is provided having a planar top surface and a plurality of wells arranged in an array on the carrier substrate top surface, with the spacing between adjacent wells matching the spacing between trapping locations on the embossing stamp substrate. In step 1601b, micro-LEDs are filled into the wells of the carrier substrate via a fluid assembly process. In one aspect, step 1601b can utilize electrostatic or magnetic forces to secure the micro-LEDs to the wells. In step 1603b, the top surface of the embossing stamp substrate is pressed against the top surface of the carrier substrate, so that each capture site contacts a corresponding well. In step 1603c, the micro-LEDs are transferred in bulk from the carrier substrate to the embossing stamp substrate.

Specifically, step 1601a may include providing a carrier substrate having a plurality of wells having a first peripheral shape and a planar well bottom surface. Subsequently, in step 1601b, each well is filled with a micro-LED. The surface-mount micro-LEDs filled into the wells have a first peripheral shape, a planar top surface in contact with the well bottom surface, and include a first electrode and a second electrode. In other embodiments, the vertical micro-LEDs filled into the wells in step 1601b have a first peripheral shape, a planar top surface in contact with the well bottom surface, and include a first electrode.

In the case of RGB display, the carrier substrate provided in step 1601a includes: a first fluid assembly carrier substrate comprising a well array on the top surface of the carrier substrate, wherein the distance between adjacent wells matches the spacing between adjacent capture locations on the embossed stamp substrate; a second fluid assembly carrier substrate comprising a well array on the top surface of the carrier substrate, wherein the distance between adjacent wells matches the spacing between adjacent capture locations on the embossed stamp substrate; and a third fluid assembly carrier substrate comprising a well array on the top surface of the carrier substrate, wherein the distance between adjacent wells matches the spacing between adjacent capture locations on the embossed stamp substrate. Then, the process of filling the wells of the carrier substrate in step 1601b includes: filling the wells on the first carrier substrate with a first microLED configured to emit light of a first wavelength; filling the wells on the second carrier substrate with a second microLED configured to emit light of a second wavelength; and filling the wells on the third carrier substrate with a third microLED configured to emit light of a third wavelength. Transferring the microLEDs from the carrier substrates to the embossing stamp substrate in step 1603c includes transferring the microLEDs from the first, second, and third carrier substrates to the corresponding embossing stamp substrates. As shown in Figures 10A and 10B, for RGB microLEDs having 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 be used to fill them with micro-LEDs of different wavelengths and then transferred to the stamp substrate separately.

In step 1608, a display substrate is provided 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 an underlying matrix of column and row control lines. The spacing between adjacent connection pads on the display substrate matches the spacing between adjacent trapping sites on the imprinting stamp substrate, which is the same spacing as the spacing between adjacent wells on the carrier substrate. In step 1610, the top surface of the imprinting stamp substrate is pressed against the top surface of the display substrate, with each trapping site contacting a corresponding micro-LED connection pad. In step 1612, the micro-LEDs are transferred in bulk from the stamp substrate to the micro-LED pads on the display substrate. In one aspect, step 1612 heats the display substrate, thereby binding the micro-LEDs to the micro-LED pads.

In the case of an RGB display, the display substrate in step 1608 includes a plurality of connection pads for a first micro-LED for emitting light of a first wavelength; a plurality of connection pads for a second micro-LED for emitting light of a second wavelength; and a plurality of connection pads for a third micro-LED for emitting light of a third wavelength. Subsequently, in step 1610, the top surface of the embossing stamp substrate is pressed against the top surface of the display substrate, including pressing the embossing stamp substrates filled with the first, second, and third micro-LEDs, respectively. One stamp substrate can be used for each wavelength of micro-LEDs, or if all micro-LEDs have similar shapes, the same substrate can be populated with micro-LEDs of different wavelengths and then transferred to the display substrate.

FIG17 is a flow chart of a method for mass transfer of micro-LEDs using a fluid assembly stamp substrate as shown in FIG13A-FIG13K. The method starts at step 1700. Step 1702 provides a fluid assembly stamp substrate having a planar top surface, wherein a capture location provided on the top surface has a first peripheral shape, a depth, and a planar capture location bottom surface. Under the fluid assembly process, the micro-LEDs filled into the capture location in step 1704 have: a first peripheral shape, a thickness greater than the depth of the capture location, a planar bottom surface in contact with the capture location bottom surface, and a planar top surface having a first electrode extending out of the capture location. The micro-LEDs also include a fixing mechanism. In step 1704, a micro-LED with a second electrode on the bottom surface can be used, or a surface-mount micro-LED with a first electrode and a second electrode on the top surface can be used to fill the trapping location.

In one aspect, providing an imprinting stamp substrate in step 1702 includes providing an imprinting stamp substrate having spaced-apart capture locations. A display substrate is provided in step 1706 having a planar bottom surface and an array of micro-LED connection pads, each of which includes a first electrode formed on a top surface and electrically connected to an underlying matrix of column and row control lines. The spacing between adjacent connection pad locations on the display substrate matches the spacing between adjacent capture locations on the imprinting stamp substrate. In step 1708, the top surface of the imprinting stamp substrate is pressed against the top surface of the display substrate so that each capture location contacts a micro-LED connection pad. In step 1710, the micro-LEDs on the stamp substrate are transferred in bulk to the micro-LED connection pads on the display substrate. Step 1710 may include applying heat to cause the micro-LEDs to form a bond with the connection pads on the display substrate.

On the one hand, step 1704 provides a fixing mechanism in the form of a keel formed on the top surface of the micro-LED. The keel can 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 capture location of the stamp substrate provided in step 1702 is coated with a first component including a conjugated biomolecule pair. Then, the fixing mechanism mentioned in step 1704 is a second component having a conjugated biomolecule pair, which coats the bottom surface of each micro-LED. Examples of conjugated biomolecule pairs include biotin-streptavidin, thiol-maleimide, and azide-alkyne. The stamp substrate can also be further provided with an electrostatic or magnetic force generator, as shown in Figures 14A and 14B.

FIG18 is a flow chart of a method for mass transfer of axial micro-LEDs for the system shown in FIG15A-FIG15I. The method begins at step 1800. Step 1802 provides a fluid assembly stamp substrate having a planar top surface, with a plurality of trapping locations formed on the top surface. Each trapping location has a first peripheral shape, a center portion having a planar first depth, a distal end having a planar second depth less than the first depth, and a proximal end having the second depth. Under the fluid assembly process, step 1804 fills the trapping locations with axial micro-LEDs. Each micro-LED occupies a corresponding trapping location and has the first peripheral shape and a main portion aligned with the center portion. The main portion has a vertical body thickness greater than the first depth of the trapping location but less than twice the first depth. The micro-LED also has a distal electrode that horizontally bisects the main body portion. The distal electrode is aligned with the distal portion of the trapping location, and the electrode thickness of the vertical surface of the distal electrode is greater than the second depth of the trapping location but less than twice the second depth. The micro-LED also has a proximal electrode that horizontally bisects the main body portion, is aligned with the proximal portion of the trapping location, and has an electrode thickness. In one aspect, the stamp electrode can further include an electrostatic or magnetic force generator, as shown in Figures 14A and 14B.

In one aspect, providing an imprinting stamp substrate in step 1802 includes providing an imprinting stamp substrate having spaced-apart capture locations. In step 1806, providing a display substrate having a planar top surface and an array of micro-LED connection pads is performed, each micro-LED connection pad including a first electrode and a second electrode formed on the top surface, the plurality of electrodes being electrically connected to an underlying matrix of column and row control lines. The display substrate includes a plurality of connection pads separated by a spacing that matches a spacing that separates the capture locations on the imprinting stamp substrate. In step 1808, pressing the top surface of the imprinting stamp substrate onto the top surface of the display substrate aligns each capture location with a micro-LED connection pad. In step 1810, the micro-LEDs are mass-transferred from the stamp substrate to the micro-LED connection pads on the display substrate. Optionally, heat may be applied to facilitate bonding between the micro-LEDs and the display substrate connection pad electrodes.

Figure 19 is a flow chart of a method for pitch expansion during micro-LED transfer. The method begins at step 1900. Step 1902 provides a micro-LED MOCVD wafer, wherein adjacent micro-LEDs have a first spacing between them. Step 1904 releases the micro-LEDs into a fluid assembly suspension. Step 1906 provides a carrier substrate having wells arranged in an array, wherein adjacent wells have a second spacing between them, which is different from the first spacing. In step 1908, the micro-LEDs are filled into the wells of the carrier substrate through a fluid assembly process. In step 1910, an embossing stamp substrate is provided that includes an array of trapping sites, with adjacent trapping sites separated by a second spacing. In step 1912, the top surface of the embossing stamp substrate is pressed against the top surface of a carrier substrate, such that each trapping site contacts a corresponding well. In step 1914, microluminescent diodes are mass-transferred from the carrier substrate to the embossing stamp substrate.

In step 1916, a display substrate is provided having an array of micro-LED connection pads, each of which includes 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 locations on the display substrate are separated by the second spacing. In step 1918, the top surface of the embossing stamp substrate is pressed against the top surface of the display substrate, such that the capture locations contact a corresponding micro-LED connection pad. In step 1920, the micro-LEDs are mass-transferred from the embossing stamp substrate to the micro-LED connection pads on the display substrate. Optionally, heat is applied to cause the micro-LEDs to bond to the electrodes of the display substrate connection pads.

In one aspect, steps 1906, 1908, 1912, and 1914 are bypassed, and an additional step 1911 is performed to directly fill the micro-LEDs into the capture locations of the stamp substrate using a fluid assembly process.

This application provides systems and methods for mass transfer of micro-LEDs. Examples of specific LEDs, carrier substrates, and embossing stamp substrate structures are provided to illustrate this application. However, this application is not limited to the above examples. Other variations and embodiments of this application will be apparent to those skilled in the art.

Claims (17)

A fluid assembly carrier system for mass transfer of micro-LEDs is improved in that it comprises: a fluid assembly carrier substrate having a planar top surface; an array of capture locations formed on the top surface of the carrier substrate, each capture location being configured as a recessed well for temporarily fixing a fluid-deposited micro-LED; and micro-LEDs filling the wells on the carrier substrate; wherein the distance between adjacent wells on the carrier substrate is less than or equal to the distance between adjacent capture locations on the corresponding mass transfer stamp; the carrier system further comprises the The mass transfer stamp comprises: an stamp substrate having a top surface; and an array of capture locations formed on the top surface of the stamp substrate, each capture location being configured to temporarily receive a corresponding micro-LED from a well on a carrier substrate; wherein each well on the carrier substrate has a planar bottom surface; each LED has a top surface in contact with the bottom surface of a corresponding well, and a keel extending from its bottom surface; and the stamp substrate capture location is configured to receive the keel of the LED. The carrier system as described in claim 1, wherein the carrier substrate does not include conductive traces and electronic components. The carrier system of claim 1, wherein each well of the carrier substrate has a planar bottom surface; and the micro-light-emitting diode is a surface-mounted micro-light-emitting diode, each of the micro-light-emitting diodes includes a planar top surface having a first electrode and a second electrode, and the first electrode and the second electrode are in contact with the bottom surface of a corresponding carrier substrate well. The carrier system of claim 3, wherein each of the micro-LEDs further comprises a non-conductive keel extending from a bottom surface of the micro-LED. The carrier system of claim 1, wherein each of the carrier substrate wells has a planar bottom surface; and the micro-LEDs are vertical micro-LEDs, each of the micro-LEDs including a planar top surface having a first electrode in contact with a corresponding carrier substrate well, and a second electrode located on the bottom surface of the micro-LED. The carrier system of claim 5, wherein the second electrode of each light-emitting diode is a conductive keel extending from the bottom surface of the light-emitting diode. The carrier system of claim 1, wherein the carrier bottom well has a first peripheral shape; and the micro light-emitting diode has the first peripheral shape. The carrier system as claimed in claim 1, further comprising: a first fluid assembly carrier substrate having an array of wells formed on the top surface of the first fluid assembly carrier substrate; a second fluid assembly carrier substrate having an array of wells formed on the top surface of the second fluid assembly carrier substrate; a third fluid assembly carrier substrate having an array of wells formed on the top surface of the third fluid assembly carrier substrate; a plurality of a plurality of micro-LEDs configured to emit light of a first wavelength, each occupying a corresponding well in the substrate of the first fluid assembly carrier; a plurality of micro-LEDs configured to emit light of a second wavelength, each occupying a corresponding well in the substrate of the second fluid assembly carrier; and a plurality of micro-LEDs configured to emit light of a third wavelength, each occupying a corresponding well in the substrate of the third fluid assembly carrier. The carrier system as described in claim 1 further includes: an attraction force generator located below the carrier substrate, the attraction force generator being selected from a group consisting of a magnetic force generator and an electrostatic force generator, and being used to temporarily fix the micro-light-emitting diode in the well of the carrier substrate. A carrier system as described in claim 1, wherein the bottom surface of each well of the carrier substrate is coated with a first component comprising a conjugated biomolecule pair; and wherein each of the micro-light-emitting diodes includes a top surface, the top surface is coated with a second component comprising a conjugated biomolecule pair, and the top surface is in contact with the bottom surface of a corresponding well of the carrier substrate. A method for mass-transferring micro-LEDs is improved in that it includes: manufacturing micro-LEDs on a wafer; releasing the micro-LEDs from the wafer into a suspension; fluid-depositing the micro-LEDs onto a carrier substrate; transferring the micro-LEDs from the carrier substrate onto a mass-transfer stamp; and transferring the micro-LEDs from the mass-transfer stamp onto a display substrate. The method further includes: forming a keel on each micro-LED before fluid-depositing the micro-LEDs onto the carrier substrate, wherein the keel extends over the exposed bottom surface of the LED. A method for mass transfer of micro-luminescent diodes as described in claim 11, wherein the carrier substrate has a planar top surface and an array of wells formed on the top surface of the carrier substrate, the wells being filled with the micro-luminescent diodes; and the step of transferring the micro-luminescent diodes from the carrier substrate to a mass transfer stamp comprises: pressing the top surface of the mass transfer stamp against the top surface of the carrier substrate so that the array of mass transfer stamp capture bits formed on the top surface of the mass transfer stamp are bonded to the corresponding micro-luminescent diodes in the wells of the carrier substrate. A method for mass transfer of micro-LEDs as described in claim 12, wherein the well of the carrier substrate has a first peripheral shape and a planar bottom surface of the well; and the step of transferring the micro-LED fluid deposition onto the carrier substrate includes filling the micro-LEDs having the first peripheral shape into the well. A method for mass transfer of micro-light-emitting diodes as described in claim 12, wherein adjacent wells in the array of wells on the carrier substrate have a spacing between them; and the spacing between adjacent capture positions in the array of capture positions of the mass transfer stamp is greater than or equal to the spacing between adjacent wells in the array of wells on the carrier substrate. The method for mass transfer of microluminescent diodes as described in claim 11, wherein the step of transferring the microluminescent diodes from the carrier substrate to a mass transfer stamp includes configuring a capture position of the mass transfer stamp to receive the keel of the microluminescent diode. The method for mass transfer of micro-luminescent diodes as described in claim 11 further includes: before the step of transferring the micro-luminescent diode fluid deposition onto the carrier substrate, coating the bottom surface of the well of the carrier substrate with a first component having a conjugated biomolecule pair; and coating the micro-luminescent diode with a second component having a conjugated biomolecule pair in the suspension. The method for mass transfer of micro-LEDs as described in claim 11 further includes: before the step of transferring the micro-LED fluid deposition onto the carrier substrate, using an attraction force generator below the carrier substrate, wherein the attraction force generator is selected from a group consisting of a magnetic force generator and an electrostatic force generator, and is used to temporarily fix the micro-LEDs in the well of the carrier substrate.