System and method for selectively collecting light emitting elements

Abstract

A method for selectively collecting microLED devices from a carrier substrate is provided. The carrier substrate includes a predetermined defect region comprising a plurality of adjacent defective microLED devices. A solvent-resistant colloid material is applied to cover the predetermined defect region, and exposed adhesive is dissolved by an adhesive-dissolving solvent. Non-defective microLED devices located outside the predetermined defect region are separated from the carrier substrate, while adhesive attachment is maintained between the microLED devices within the predetermined defect region and the carrier substrate. A method for distributing microLED devices on a light-emitting display panel by optically measuring a suspension of microLEDs to determine the uniformity of the suspension and calculating the number of microLEDs per unit volume is also provided. If the number of microLED devices collected in the suspension is known, the number of microLED devices per unit volume of the suspension can be calculated.

Description

Technical Field

[0001] The present invention generally relates to light emitting diodes (LEDs) with a size less than 100 microns, and more particularly to a process and system for preparing micro-light emitting diodes (microLEDs). Background Art

[0002] A red / green / blue (RGB) display consists of pixels that emit three wavelengths of light corresponding to the visible colors red, green, and blue. The RGB components of a pixel (each called a subpixel) are excited in a systematic manner to produce colors across the visible spectrum. Several display types generate RGB images in different ways. Liquid crystal displays (LCDs) are the most popular technology. They produce RGB images by shining a white light source (typically a phosphor-generated white LED) through color filters in the subpixels. Some of the white light wavelengths are absorbed, while some are transmitted through the color filters. As a result, LCD displays can have efficiencies below 4%, and contrast is limited by light leakage through the liquid crystal. Organic light-emitting diode (OLED) displays produce RGB light by emitting each of these wavelengths directly at the pixel level within an organic light-emitting material. The direct emission of OLED materials results in high display contrast, but the organic materials can be subject to long-term degradation, resulting in image burn-in.

[0003] The third display technology, discussed in this article, is microLED displays, which use inorganic LEDs with micrometer-sized dimensions (5 to 100 micrometers (μm) in diameter) to emit light directly at the sub-pixel level. Inorganic microLED displays offer several advantages over competing displays. Compared to LCD displays, microLED displays offer very high contrast ratios exceeding 50,000:1 and higher efficiency. Unlike OLED displays, inorganic LEDs do not suffer from aging effects and can achieve significantly higher brightness.

[0004] MicroLEDs are made from metalorganic chemical vapor deposition (MOCVD) wafers similar to those used to make LEDs for general lighting, which results in very low cost per device, but also introduces some issues unique to microLED technology. The structure of the fluid assembly used for microLEDs is described in detail in the parent patent US10,643,981, which is incorporated herein by reference. For use in general lighting, the most important characteristic of the device is to produce each photon at a low cost to minimize the cost per bulb. This limitation has led to LED manufacturing using a process called binning to deal with process variability and defects. In short, the binning process involves testing all LEDs after packaging and placing LEDs with similar efficiency and emission wavelength characteristics into the same group, while discarding defective devices. The binning process allows MOCVD manufacturing to be cheaper because the cost of defect reduction and process control is minimized.

[0005] Recent characterization of typical 40μm gallium nitride (GaN) microLED manufacturing results revealed microLED defects ranging from 0.25% of devices to shorts and 0.75% to opens. These defects result in non-luminous pixels, which is unacceptable for display products. MicroLEDs are not packaged, and the device dimensions, especially the electrodes, are very small, making device handling and functional testing difficult. Since ultrahigh definition (UHD) displays require at least 24.8 million microLEDs (3×3840×2160), testing time becomes prohibitive. Consequently, conventional sorting techniques are impractical for identifying and discarding defective microLEDs. Therefore, new structures and methods are needed to prevent defective microLEDs from generating defective subpixels. Removing and replacing defective microLEDs is possible, as described in parent application serial numbers 16 / 125,671, 16 / 595,623, and 16 / 693,674, which are incorporated herein by reference, but mechanical pick-and-place tools are expensive to purchase and operate. Even more desirable would be to identify defective microLEDs and prevent them from entering the suspension used for fluidic assembly.

[0006] LEDs used for general lighting are much larger than those used for microLED displays (up to 3 to 4 mm per side and 5 to 100 microns in diameter), so the patterning and electrode requirements are very different. MicroLEDs are bonded to substrate electrodes using solder or anisotropic conductive film, while large general-purpose light-emitting LEDs are typically bonded via wire bonding or solder paste on a lead frame. Because microLEDs are so small, the technology for processing the devices, especially for assembling microLED displays, is very different from the technology developed for the very large LEDs used in general lighting.

[0007] To make microLED displays, green and blue GaN microLEDs are fabricated on sapphire substrates, and red aluminum gallium arsenide phosphide (AlGaAsP) microLEDs are fabricated on GaAs substrates. After fabrication and singulation, the microLEDs must be transferred to a second substrate that becomes the light-emitting display. The second substrate can be a silicon (Si) wafer (or chip) with built-in control circuitry, or it can be a glass or flexible plastic substrate with thin-film transistors. The traditional transfer method is a mechanical pick-and-place system that uses a pick head to capture the device and position it on the display substrate. Other mechanical transfer methods that use a stamp to transfer a block of microLEDs at the same time are called mass transfer. As described in this article, an alternative technology uses a fluid assembly process to position the microLEDs.

[0008] In short, the fluidic assembly process applies a suspension of microLEDs to a substrate with an array of trapping sites (wells) and moves the suspension to assemble the microLEDs in the traps. For fluidic assembly to succeed, it is necessary to collect defect-free microLEDs from the growth substrate, prepare a suspension with a known concentration of microLEDs, and then evenly distribute the suspension on the display substrate.

[0009] The processing of suspensions of micron-sized particles is well established in systems such as cell culture in the biosciences or abrasive slurries in industrial applications. In all cases, the goal of the suspension handling system is to achieve a highly homogenous suspension and transfer the suspension to the target process with a high degree of control over volume and concentration. Suspension homogeneity is typically achieved through direct mechanical mixing using a submerged impeller or active circulation via a pump. Transfer of the well-mixed suspension to the target process is typically accomplished through piping pumped downstream from a supply tank or by pressurizing a sealed tank. Volumetric control in a good suspension system is achieved by controlling flow with differential pressure and metering the net flow with timed valves. Control of suspension concentration may be necessary, especially when reusing the suspension, so transfer piping often includes fittings with multiple inputs to allow for equilibration of the pure carrier fluid with the suspension.

[0010] Unfortunately, conventional suspension processing techniques are not compatible with the properties of microLEDs or the requirements of fluidic assembly techniques. Specifically, microLED suspensions have significant characteristics described below, necessitating the development of an alternative approach.

[0011] Figure 1 Depicted is a microLED suspension with a uniform distribution after stirring, which settles to approximately half the height of the liquid column after time t1 and is completely settled after time t2. Unlike abrasive slurries, which are formulated for prolonged uniform mixing and have settling times measured in months, microLED suspensions are formulated to have relatively short settling times. MicroLEDs must be fixed to the target substrate surface for assembly, so microLED suspensions used for fluidic assembly typically completely settle within minutes and lose uniformity quickly after mixing stops. As an example, a 42 μm diameter, 5 μm thick disk-shaped microLED fabricated according to the center mesa design proposed in parent application serial number 16 / 406,080 (incorporated herein by reference) has a hydrodynamic diameter of 18.9 μm. When gravity balances the viscosity of the liquid, an object of this size has a terminal velocity in a liquid according to the following equation:

[0012]

[0013] Among them D LED is the hydrodynamic diameter, ρ is the liquid density, and ρ LED is the density of the microLED, and μ is the viscosity of the liquid. For water, the terminal velocity is 1.1 millimeters per second (mm / sec), so in a typical container such as a 50 ml Falcon tube, the microLED will completely settle in about one minute.

[0014] MicroLEDs typically have surfaces that include metals, inorganic materials, and organic materials. Therefore, it is nearly impossible to prevent temporary sticking to solid surfaces that come into contact with microLED suspensions. Therefore, containers containing microLED suspensions are typically made of hydrophobic materials, such as acetal homopolymer, polytetrafluoroethylene (PTFE), and polypropylene, to minimize stickiness. Figure 1 The final state in Figure 3 shows the effect of the microLEDs interacting with the container walls, where the bottom cone is smaller than the angle of repose, so some microLEDs stick to the container walls and do not settle into a uniform layer on the bottom.

[0015] Microorganisms in biological applications are generally robust enough to survive internal mixing (e.g., using a stir bar) without lysis, while industrial abrasive suspensions such as chemical mechanical polish (CMP) slurries can be suspended without damage by drum circulation or impeller mixing. In contrast, microLEDs are fragile and can be broken by direct mechanical mixing or pumping. Damaged microLEDs are similar in major dimensions to good microLEDs and therefore cannot be removed from the suspension by filtration and can interfere with fluidic assembly by partially blocking capture traps.

[0016] MicroLEDs account for a significant portion of display manufacturing costs, and low utilization and recycling efficiency of μLEDs significantly impact costs. Compared to biological and industrial abrasive applications, the components in the suspension are more valuable.

[0017] Unlike conventional suspensions, the performance characteristics of each individual microLED are critical, as each device constitutes a subpixel. Tight control over the number of microLEDs available for assembly is necessary to manage the emission distribution of the complete display. Therefore, suspension processing must be designed to prevent cross-contamination.

[0018] The properties of microLEDs and the stringent requirements for display manufacturing preclude conventional industrial systems and suspension handling methods. Controlled and efficient dispensing of clean, high-quality components is crucial for fluidic assembly, as the forces involved in fluidic assembly are limited by the critical value at which the assembled components break away. Rapid fluidic assembly, however, relies on a short travel path on the substrate between the microLED and its final assembly (catch trap) location. Optimal dispensing of the microLED suspension onto the display substrate must therefore not only be low-loss and damage-free, but also fast and highly uniform.

[0019] It would be advantageous if collection and distribution methods existed specifically for handling inorganic microLEDs used in fluidic assemblies. Summary of the Invention

[0020] This paper describes systems and methods for preparing and manipulating suspensions of micro-light-emitting diodes (microLEDs) suitable for fluidic assembly of microLED displays. A selective collection method produces a microLED suspension composed of known-good LEDs at a defined concentration in a suitable liquid. The microLED suspension is supplied to a display substrate using a dispensing system that minimizes damage and loss of the microLEDs while uniformly distributing the devices at a controlled density across the display substrate. Such optimal initial conditions are critical for successful fluidic assembly of microLED displays.

[0021] Thus, a method for selectively collecting microLED devices from a carrier substrate is provided. The method provides an inorganic microLED device attached to a carrier substrate by an adhesive. The defect area is predetermined (e.g., the edge of a wafer) and includes a plurality of adjacent defective microLED devices. A solvent-resistant colloid material is applied to the predetermined defect area, and the exposed adhesive is dissolved by an adhesive-dissolving solvent. Some examples of adhesive-dissolving solvents include acetone, toluene, trichloroethane, N-methylpyrrolidone (NMP), xylene, cyclohexanone, butyl acetate, or combinations thereof.

[0022] Non-defective microLED devices located outside the predetermined defect region are separated from the carrier substrate, while microLED devices within the predetermined defect region maintain adhesive attachment to the carrier substrate. In response to the separation of the microLED devices from the carrier substrate, valid microLED devices are collected in a collection container. In one variation, only certain portions of the carrier substrate are exposed to the adhesive-dissolving solvent, such that the microLEDs are separated from only the selectively exposed portions of the carrier substrate.

[0023] Furthermore, the carrier substrate can be inspected to locate defective microLED devices in non-predetermined defect areas, and a solvent-resistant colloid material can be formed on these non-predetermined defect areas. In one aspect, the inspection can locate microLED devices with individual defects that are not predetermined, and a laser trimming process can be used to eject the individual defective microLED devices. Inspection can be performed using optical comparison, electroluminescence, photoluminescence, or cathodoluminescence testing.

[0024] The microLED devices collected in the collection container are typically a suspension of active microLED devices having an average cross-sectional dimension s. However, impurities are also present in the suspension. In one aspect, a filtration step is performed using mechanical sieving, elution, classification, or a combination thereof to remove impurities having a maximum cross-sectional dimension greater than t, where t>s. Similarly, a separate filtration step can remove impurities having a maximum cross-sectional dimension less than p, where p<s. In one aspect, prior to filtration, the binder dissolving solvent in the collected microLED suspension has been replaced with a filtration solution having a lower viscosity than the binder dissolving solvent. Alternatively, or after filtration, the fluid in the collected microLED suspension can be replaced with an assembly solution having a lower polarity or a higher evaporation rate. In one aspect, a surfactant, such as an anionic, cationic, nonionic surfactant, or a combination thereof, can be added.

[0025] A method for distributing microLED devices on a light-emitting display panel is also proposed. The above-mentioned suspension of collected microLED devices is transferred to a transparent first container and stirred. Some examples of the stirring process include external vibration of the first container, generating fluid flow in the suspension, and flowing a gas through the first container. The opacity of the suspension is optically measured at multiple first container heights to determine the uniformity of the suspension. When the determined uniformity is greater than a minimum uniformity threshold, the suspension can be distributed on the top surface of the light-emitting display panel. Some examples of the dispensing process include single-step bulk decantation, multi-step pipette translation, nozzle-limited container translation, and translation tube.

[0026] If the number of microLED devices collected in the suspension is known, the number of microLED devices per unit volume of the suspension can be calculated. Consequently, in response to dispensing a first volume of the suspension, a known first number of microLED devices can be deposited on the light-emitting display panel. Advantageously, after determining the number of assembly locations in the first region of the top surface of the light-emitting display panel, the number of the known first number of microLED devices deposited is at least equal to the number of assembly locations in the first region.

[0027] Optical measurement of the opacity of the suspension is performed by arranging a plurality of light-emitting devices having predetermined output light intensities, directed toward the central axis of the first container, and spaced apart from each other by a first predetermined distance along a first perpendicular axis. A plurality of photodetectors are spaced apart from each other by the first predetermined distance along a second perpendicular axis, each photodetector having an input terminal directed toward the output terminal of a corresponding light-emitting device. The light intensities received by the photodetectors are then compared.

[0028] In one aspect, in response to determining the opacity of the suspension at multiple first container heights, a first number of microLED devices per unit volume of the suspension can be calculated. After dispersing an equal volume of the suspension onto the top surface of the light-emitting display panel, the optical measurement can be repeated to calculate a second number of microLED devices per unit volume of the suspension. If the known equal volume of the suspension is transferred to a second container and a predetermined amount of fluid is added to (or removed from) the second container, a third number of microLED devices per unit volume of the suspension in the second container can be calculated. If a fluid is added to (or removed from) the suspension in the first container, then after stirring the suspension, the suspension density can again be optically measured to calculate a fourth number of microLED devices per unit volume of the suspension.

[0029] Additional details of the above-described method, as well as a system for separating regions of a microLED carrier substrate, a system for collecting microLEDs, and a system for characterizing a microLED suspension, are presented below. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Depicted is a microLED suspension with a uniform distribution after stirring, which settles to approximately half the height of the liquid column after time t1 and completely settles after time t2.

[0031] Figure 2 A partial cross-sectional view of the system used to characterize microLED suspensions.

[0032] Figure 3A and Figure 3B is a schematic partial cross-sectional view of components in a system for selectively collecting microLED devices from a carrier substrate.

[0033] Figure 4 A partial cross-sectional view of a system used to selectively isolate regions of a carrier substrate for microLEDs.

[0034] Figure 5 Schematic diagram depicting the preparation of microLED clusters for fluidic assembly consisting of three consecutive steps.

[0035] Figure 6 A partial cross-sectional view of a typical microLED on a carrier wafer after device processing.

[0036] Figure 7A and Figure 7B Wafer map depicting known defect locations from microLED manufacturing ( Figure 7A ) and wafer maps of known defect locations for typical alignment structures ( Figure 7B ).

[0037] Figure 8 is a graph depicting the cathodoluminescence spectrum of GaN microLEDs.

[0038] Figure 9 It is a composite defect map that can be used to guide the defect control process.

[0039] Figure 10 is a plan view of the wafer, where the emission wavelengths are shown as contours of equal width.

[0040] Figure 11A and Figure 11B Partial cross-sectional views depicting some representative microLED defects and corrective measures.

[0041] Figure 12A、 Figure 12B and Figure 12C A partial cross-sectional view of a microLED collected in a solvent.

[0042] Figure 13 is a partial cross-sectional view showing a suitable storage container.

[0043] Figure 14 is a chart showing exemplary microLEDs and the principal diameters of contaminants.

[0044] Figure 15 is a schematic diagram of the elution diversion pool.

[0045] Figure 16 Schematic diagram of the continuous flow fractional filtration method.

[0046] 17A to 17C Graphs depicting exemplary measurements of optical transmittance versus time for a suspension of 42 μm diameter microLEDs in 20 mL of isopropanol (IPA), graphs depicting exemplary measurements of optical transmittance versus time for 1.3 million microLEDs in different volumes of IPA, and a calibration curve of concentration versus optical transmittance of the system's microLED suspension.

[0047] Figure 18A and Figure 18B The partitioning density gradients were compared for single-pass and double-pass / twice-speed partitioning paths.

[0048] 19A to 19C are schematic diagrams depicting the transfer of a well-mixed suspension from an initial source container directly to an assembled substrate by decanting from a container, a nozzle, and a tube, respectively.

[0049] Figure 20 is a schematic diagram showing controlled volume pipetting from a well-mixed suspension to an assembled substrate.

[0050] Figure 21 is a schematic diagram showing the suspension corrected in the intermediate tank and the subsequent aspiration and distribution through the bubble mixing distribution head.

[0051] Figure 22 is a schematic diagram of a parallel allocation method using an array of allocation heads.

[0052] Figure 23 is a flow chart illustrating a method for selectively collecting microLED devices from a carrier substrate.

[0053] Figures 24A to 24C is a flow chart illustrating a first method for distributing microLED devices on a light-emitting display panel.

[0054] Figure 25 is a flow chart illustrating a second method for distributing microLED devices on a light-emitting display panel. DETAILED DESCRIPTION

[0055] Figure 2 2 is a partial cross-sectional view of a system for characterizing a microLED suspension. System 200 includes a transparent container 201 having a vertical central axis 202. A plurality of light emitting devices 204a through 204n (LED array) are shown, each having a predetermined output light intensity, directed toward central axis 202 of container 200 and spaced apart from one another by a first predetermined distance 206 along a first vertical axis 208 parallel to central axis 202, where (n) is an integer greater than 1. A plurality of photodetectors 210a through 210n (photodetector array) are spaced apart from one another by the first predetermined distance 206 along a second vertical axis 212 parallel to central axis 202. Each photodetector 210a through 210n has an optical input directed toward the output of the corresponding light emitting device and an output on lines 214a through 214n, respectively, to provide an electrical optical density signal responsive to the measured light intensity. A monitoring device 216 has an input for receiving the optical density signals 214a through 214n. Monitoring device 216 compares the light intensity associated with the light density signal and provides an output on line 218 in response to the comparison. For simplicity, the figure shows that the number of light emitting devices is equal to the number of photodetectors and has the same spacing. In addition, the light emitting devices output light of the same intensity. However, it should be understood that once calibrated, similar systems can be implemented without these explicit limitations.

[0056] Container 201 includes a suspension 228 of microLEDs. Monitoring device 216 can provide a microLED uniformity measurement, or a calculation of a microLED count per unit volume of the suspension on line 218 determined based on the uniformity (density) measurement. In one aspect, monitoring device 216 includes non-transitory memory 220 having a stored calibration curve 222. In this case, monitoring device 216 can provide a microLED count per unit volume of the suspension on line 218 in response to a comparison of the optical density signals on lines 214a to 214n with the calibration curve 222. As part of the calibration curve, in one aspect, monitoring device 216 can receive and store data regarding the volume of container 201.

[0057] In another aspect, monitoring device 216 includes an input on line 224 for accepting a calibrated input signal representing the total number of microLEDs in the suspension. In this case, the monitoring device can provide a microLED count per unit volume of the suspension in response to comparing optical density signals 214a to 214n to the total number of microLEDs. For example, the total number of microLEDs can be determined by counting the number of active microLEDs collected from the carrier substrate. The input on line 224 can alternatively or additionally accept a running measurement of the suspension volume.

[0058] In one variation, monitoring device 216 receives a set of optical density signals 214a to 214n collected over a period of time and provides an output on line 218 of microLED settling time or microLED size. Furthermore, container 200 may be divided by a plurality of graduations 226a to 226n and may include a uniform suspension of microLEDs (suspension 228 shown is not uniform). In this case, photodetectors 210a to 210n detect changes in the level of the suspension, as measured against container graduations 226a-226n. The monitoring device provides an output on line 218 of the number of microLED devices dispersed from container 200 or the volume of suspension dispersed from the container. Advantageously, this output may be provided in real time.

[0059] To aid in performing the above measurements, a stirring device can be used to homogenize the suspension. A variety of homogenization mechanisms are described in more detail below. In one aspect shown, a solution (solvent) or a gas can be used to mix the suspension. Once the suspension volume is known, the stirring mechanism can be adjusted to optimize mixing.

[0060] Figure 3A and Figure 3B FIG. 1 is a schematic partial cross-sectional view of components of a system for selectively collecting microLED devices from a carrier substrate. Figure 3AAs shown, system 300 includes a turntable or vacuum chuck 302 having a rotational (spindle) interface for mounting a carrier substrate 304, which includes an inorganic microLED device 306 attached to the carrier substrate via an adhesive 308. An elbow 310 is coupled to the rotary vacuum chuck 302 and has multiple selectable settings for determining the angle (tilt) 312 of rotation of the turntable in the plane of the x- and z-axes. A gantry 314 is coupled to the elbow 310 and has multiple selectable settings for determining the height of the turntable along the z-axis. A tray 316 includes an adhesive dissolving solvent 318 and has a top opening for receiving the carrier substrate 304. A controller 320 has outputs connected to the gantry 314 and elbow 310 on lines 322 and 324, respectively, and provides height and angle settings. As shown, system 300 allows selected radial portions of the carrier substrate 304 to be exposed to the adhesive dissolving solvent 318 in response to the settings of the gantry 314 and elbow 310. The MicroLED devices 306 separated from the selectively exposed portions of the carrier substrate 304 are collected in a tray 316. In this example, the λ1 region of the carrier substrate is being collected (see Figure 10 ).

[0061] In one aspect, controller 320 has an input on line 326 to accept a first map of microLED performance areas and, responsive to the first map, provides settings for gantry 314 and elbow 310 that select radial areas of carrier substrate 304 exposed to the adhesive dissolving solvent.

[0062] like Figure 3B As shown, system 300 can include an inspection subsystem 328 having an optical input 330 and an output on line 332 connected to controller 320 for identifying individual defective microLED devices 306 on carrier substrate 304. A trimming laser 334 has an input on line 336 connected to controller 320 for receiving a second pattern of defective microLED devices, and an output 338 for ejecting the defective microLED devices from carrier substrate 304 by laser radiation in response to the second pattern.

[0063] Figure 4FIG4 is a partial cross-sectional view of a system for selectively isolating regions of a carrier substrate of microLEDs. System 400 includes a controller 402 having an output on line 404 for providing a first map of predetermined defect regions 406 on a carrier substrate 408. A printer 410 has an input on line 404 for receiving the first map and a nozzle 412 for applying a solvent-resistant adhesive material 414 to selected regions of the carrier substrate 408 in response to the first map. Despite exposure to an adhesive-dissolving solvent (not shown), microLEDs 415 in the selected regions 406 remain attached to the carrier substrate 408. Some examples of solvent-resistant adhesive material 414 include SU-8, epoxy, polyethylene terephthalate (PET), acrylonitrile butadiene styrene (ABS), and polyimide.

[0064] Optionally, the system 400 can further include an inspection subsystem 416 having an optical input 418 and an output connected to the controller on line 420 for identifying areas 422 of non-predetermined defective microLED devices on the carrier substrate 408. The printer input on line 404 can receive a second map of the areas 422 of non-predetermined defective microLED devices from the controller 402 and, in response to the second map, apply a solvent-resistant adhesive material to the areas 422 of the detected defective microLED devices (to which the solvent-resistant adhesive material has not yet been applied). Figure 4 middle).

[0065] Figure 5 The figure depicts a schematic diagram depicting the preparation of microLED clusters for fluidic assembly, consisting of three sequential steps. First, a microLED wafer is inspected to determine the locations of defective microLEDs. Based on the defect map, the defective microLEDs and other debris particles are either removed from the carrier substrate or encapsulated on the carrier substrate to prevent them from being collected in the suspension. The carrier substrate is then immersed in a solvent to dissolve the adhesive securing the known-good microLEDs to the substrate and rinse the microLEDs into a holding container. After allowing the microLEDs to settle, the solvent with the dissolved adhesive is carefully decanted, and several solvent exchanges are performed to remove any remaining adhesive residue. The resulting suspension is filtered to remove particles whose size differs significantly from the microLEDs, and the amount of solvent is adjusted to achieve a suitable density of microLEDs in the suspension for subsequent mixing and dispensing operations.

[0066] Using a suspension of microLEDs, three alternative dispensing systems are employed, each offering different trade-offs in terms of dispensing speed, volume control, and complexity, to dispense the microLED suspension onto a display substrate. In each case, several separate aliquot transfers are performed to cover the substrate. This aliquot transfer can be performed directly onto the substrate or through a controlled volume intermediate in the form of a "suspension tank," which dilutes the suspension, or a dispensing head is used to actively mix the microLED suspension before dispensing onto the substrate. Direct transfer is preferred for suspensions with sufficiently long settling times. Finally, the substrate can be inspected for uniformity, and an additional small-area dispenser can be used to fill low-density areas.

[0067] Figure 6 Figure 1 is a partial cross-sectional view of a typical microLED on a carrier wafer after device processing is completed. MicroLEDs are typically fabricated on sapphire substrates and transferred to a carrier wafer using laser lift off (LLO), as described in the parent patent US10,643,981, which is incorporated herein by reference. As shown, the resulting wafer has millions of microLEDs embedded in an adhesive layer on the carrier wafer. Unfortunately, wafers can also have several different defect types (including process control structures) that can adversely affect subsequent processes, so selective collection is used to ensure that the fluid assembly process can be performed only with known good microLEDs.

[0068] Figure 7A and 7B Wafer map depicting known defect locations from microLED manufacturing ( Figure 7A ) and wafer maps of known defect locations for typical alignment structures ( Figure 7B ). The device processing used to manufacture microLEDs has several known defects or process control structures that systematically appear in the same location due to the nature of the process as shown in the figure. Each wafer has an identification mark near the edge of the wafer, which creates a pit in the substrate surface that disrupts the microLED pattern. The processing steps that form the microLED structure use a photoresist coating to transfer the pattern to the wafer. The photoresist coating is not perfect, especially at the edge of the wafer, which will cause ring pattern defects. The photolithography process uses a series of marks to locate the position of each layer relative to the previous layer. The alignment marks and metrology structures used ( Figure 7B) are necessary for microLED manufacturing, but if these structures are contained and collected in the microLED solution, they are defects. These defects are much larger than the typical microLED size, and wafer edge ring pattern defects can be as wide as 2 to 3 mm. The first component of the wafer defect map is the location of these large systems to place the structures.

[0069] The second type of defect is random processing defects, such as chemical mechanical polishing (CMP) scratches on the substrate, large residual gallium nitride (GaN) blocks caused by particles dropped during isolation etching, and missing metal electrodes. These larger defects can be identified by optical scanning, which compares differences in adjacent images that do not match the expected microLED pattern. This part of the defect map consists of a series of coordinates that outline the area and location of each defect.

[0070] The most important class of defects are functional defects that affect the electrical performance and light emission of microLEDs. Mapping these defects can be accomplished using four different complementary techniques.

[0071] l) Perhaps the most promising technique is electroluminescence (EL) testing, which probes each microLED and measures the resulting light. This test directly identifies weak devices with low light emission, as well as those that are shorted or open. However, the technique is slow and has difficulty probing small electrodes, especially without damaging the surface. However, this technique can be used to measure a few representative devices.

[0072] 2) Photoluminescence (PL) uses a light source (usually a laser) with a wavelength that excites transitions in the LED structure and measures the resulting wavelength and intensity. This technique can identify metal organic chemical vapor deposition (MOCVD) defects and cracked or shorted microLED devices, but cannot identify missing metal or open contacts.

[0073] 3) For optical comparison, the usual approach is to compare two images and look for differences between them, which are defects. Optical images can also be compared with patterns (die-to-database).

[0074] 4) Next, cathode luminescence (CL) will be described.

[0075] Figure 8This is a graph depicting the cathodoluminescence spectrum of a GaN microLED. Microcathodoluminescence uses an electron beam from a scanning electron microscope (SEM) to excite transitions in the LED structure and measures the wavelength and intensity of the light produced. As shown in the figure, GaN microLEDs have multiple characteristic emission lines, so differences in the spectrum can identify different defect mechanisms. A weak or absent emission peak at 455nm indicates a short circuit in the LED or a problem with the multiple quantum well (MQW). Higher emission in a broad peak near 570nm may indicate etching damage or poor quality of the n-GaN. Low intensity of the exciton peak may also indicate poor initial growth quality or missing dopants. Any significant deviation from the typical microLED spectrum indicates that the device may perform poorly in a display. Figure 9 A composite defect map can be used to guide defect control processes. Functional testing identifies individual defective microLEDs, so this portion of the defect map consists of the XY coordinates of each defective LED. Electroluminescence and laser-induced luminescence can also generate maps of LED performance characteristics, including emission wavelength, efficiency, and threshold voltage. Using these techniques, areas with different performance can be identified, so selective acquisition techniques can be applied to smaller areas of the wafer to produce microLED suspensions with more tightly distributed microLED device performance uniformity.

[0076] Figure 10 This is a plan view of a wafer, showing the emission wavelengths as contours of equal width. In some cases, it is necessary to sample each radial strip of the carrier substrate separately so that the individual suspensions (e.g., the four strips shown) each have a narrow wavelength or efficiency distribution. This simple sorting technique can be used to produce displays with no color spots by collecting suspensions from different wafers with the same emission wavelength.

[0077] Figure 11A and Figure 11B The following are partial cross-sectional views depicting some representative microLED defects and corrective measures. Using the defect size information in the composite defect map, Figure 11BTwo methods are shown for processing wafers to remove defects. Large defect areas, such as wafer edges and patterns of regional defects, are coated with a material resistant to the solvents used during the collection process, so the defective areas remain trapped on the carrier wafer after collection. Suitable coating materials are SU8, epoxy, polyethylene terephthalate (PET), acrylonitrile butadiene styrene (ABS), or polyimide, which can be sprayed on, patterned using an inkjet method, or even applied with a brush or pen. The wafer can be baked after the coating process to cure the retention material. For individual devices identified as smaller defects, a pulsed laser beam smaller than the diameter of the microLED can be used to impact the smaller defective individual devices, thereby removing the defective microLEDs. The appropriate laser wavelength and energy are selected so that absorption by the defective microLED causes rapid heating, resulting in ejection from the adhesive layer. An auxiliary vacuum nozzle close to the laser target area can be used to capture and process the ejected devices, which prevents the defective microLEDs from redepositing on the carrier wafer.

[0078] Figure 12A 、 Figure 12B and Figure 12C This is a partial cross-sectional view of a microLED collected in a solvent. Figure 12A In the process, a carrier wafer with known good microLEDs (a carrier wafer that has been treated with corrective measures) is placed in a solvent-resistant container and immersed in a solvent or solvent mixture that can dissolve the adhesive holding the microLEDs. The solvent can be acetone, toluene, trichloroethane, N-methylpyrrolidone (NMP), xylene, cyclohexanone, butyl acetate, etc. The solvent can be heated and gently stirred to accelerate the dissolution of the adhesive and the collection of the microLEDs. During the process, care must be taken to prevent mechanical contact between the container and the microLEDs on the carrier, which may damage the microLED devices. Maintaining the vertical position of the carrier wafer in the container can well allow the solvent to circulate and allow the microLEDs to settle to the bottom of the container when the adhesive dissolves.

[0079] Another collection method is Figure 12BAs shown, a carrier wafer is placed horizontally in a shallow container, supported by its edges on the container's narrow ledges. As can be seen, the printed or coated solvent-resistant adhesive protects the adhesive from the solvent, so that microLEDs in defective capture areas exceeding a certain size remain on the carrier wafer. Once all microLEDs have been released and settled to the bottom of the container, the carrier wafer is removed and inspected for any remaining good microLEDs. In some cases, if the carrier substrate is not immediately removed from the solvent, the solvent may intrude from beneath the capture areas where the capture material was applied, dissolving the adhesive and releasing large chunks of the capture medium into the container.

[0080] The “whole” wafer collection method described above is quick and simple, but in some cases selective collection techniques can be used. MicroLEDs are collected only in areas that came into contact with the binder-dissolving solvent, so simple selective collection can be performed by placing a small drop of solvent on a horizontal carrier wafer, such as Figure 12C After the adhesive dissolves, the collection area is rinsed to remove the released microLEDs from the carrier wafer and collect them.

[0081] Another system for selectively collecting radial areas of a wafer can use the same principle of exposing controlled areas of a carrier wafer to an adhesive dissolving solvent. Figure 3A For example, based on Figure 10 The emission wavelength contour map is obtained by adjusting the inclination of the carrier substrate and the height of the z-axis so that only the region λ1 (see Figure 10 ) is immersed in a solvent. The carrier substrate is slowly rotated to expose the radial strips to the solvent, and the released microLEDs settle to the bottom of the solvent tank. When the first region has been collected, the carrier is removed from the solvent, and the microLEDs with the λ1 wavelength distribution are removed from the solvent. The carrier can then be repositioned to collect the λ2 region, and successive microLED collections are performed in the same manner.

[0082] Typically, the collection solvent is heavily contaminated by the adhesive that remains and covers the microLEDs, so the microLED suspension needs to be removed through a series of solvent exchanges. A solvent exchange cycle is performed as follows:

[0083] 1) The solvent is indirectly stirred using a vortex mixer or ultrasonic bath to thoroughly homogenize the suspension and break up any remaining adhesive clumps;

[0084] 2) It takes several times the settling time to collect all the microLEDs at the bottom of the container;

[0085] 3) Carefully pour off 80% to 90% of the solvent without disturbing the settled microLEDs;

[0086] 4) adding new solvent to the container; and

[0087] 5) Repeat steps 1) to 4).

[0088] Typically, three or more solvent cycle exchanges are performed to ensure that the adhesive component is removed. The solvent selected for adhesive removal is based solely on its ability to dissolve the adhesive without damaging the microLED electrodes, so it will not be the best choice for subsequent filtration and fluidic assembly operations. The solvent exchange step can be performed in step 4) above to replace the new solvent selected for cleaning or fluidic assembly. Solvent cycle exchanges can be performed at least 3 times to ensure that the solvent has rinsed the microLEDs from the container walls. After the solvent exchange, the microLED solution is transferred from the collection container to a clean container that can be used for storage and transportation of the suspension. The container should be chemically stable to the suspension solution and hydrophobic to minimize microLED adhesion to the surface. Some suitable materials are acetal homopolymer, polytetrafluoroethylene (PTFE), polypropylene, polystyrene, etc.

[0089] Figure 13 The figure shows a partial cross-section of a suitable storage container. Typically, the container is tall and slender with vertical walls and steeply angled surfaces to minimize packing and concentrate the microLEDs in a single, desired location. This control over the packing point allows for efficient transfer of microLEDs within the container. If the container is transparent, optical density measurements can be used to monitor the homogeneity of the stirred suspension and adjust the microLED concentration by adding or removing solvent.

[0090] Figure 14 is a chart showing the main diameters of exemplary microLEDs and contaminants. The quality of the microLED suspension can be further improved by filtering to remove particles and debris. The exemplary microLED has a diameter of 42 micrometers (μm). Small particles below about 10 μm may be electrode metal flakes from the electrode stripping process, fragments of the interlevel dielectric (ILD), or dirt in the air. These particles can be trapped in the trap structure during fluid assembly, which can interfere with the microLED assembly or affect the bonding between the microLED electrode and the substrate electrode, resulting in yield loss. Large fragments can be GaN fragments or smaller defects wrapped in the capture medium that is released during collection. The filtration process removes everything outside a size band centered on the microLED diameter.

[0091] It is clear from the figure that broken microLEDs comparable in size to good microLEDs cannot be removed by filtration, so selective collection is very important to capture or remove broken microLEDs before collection. It is also important that the microLED suspension does not generate new broken microLEDs due to excessive mechanical interaction between microLEDs or between microLEDs and the container and fixture.

[0092] A simple filtration method uses a mesh filter developed for cell collection to create the desired bandpass around the microLED size. First, the suspension is filtered through a 40μm mesh to remove large debris, including any capture medium that escaped from the carrier wafer during collection. The suspension is then filtered using a 20μm mesh to capture the microLEDs and allow small particles to pass into a waste container. The microLEDs are backwashed with a cleaning solvent in the filter into a clean container.

[0093] Figure 15 Figure 1 is a schematic diagram of an elution splitter cell. Mesh filtration methods are inexpensive and effective, but they involve significant mechanical interactions between microLEDs and the filter, and during fine filtration, devices are subject to shear forces as they accumulate on the filter membrane. Filtration methods based on fluid flow help avoid potential mechanical damage. Because the hydrodynamic diameter of microLEDs and the viscosity of the suspension solvent are known, an elution splitter cell can be constructed to function as a filter, as shown in the figure. The microLED suspension is introduced to the top of the first filtration column (μLED supply). Solvent flows from the bottom of the column at a rate (stream 1) that pushes all particles smaller than a critical size of approximately 50 μm upward, while larger particles settle to the bottom of the column, where they can be collected and discarded. The small-size fraction flows through a transfer channel to the top of a second elution column. In this second column, stream 2 is adjusted so that particles smaller than approximately 30 μm are forced upward and out the waste channel, while the microLEDs settle to the bottom of the column, where they can be collected for microLED display assembly.

[0094] Figure 16 This is a schematic diagram of a continuous flow fractionation filtration method. Similar to an elution splitter, the continuous flow fractionation method utilizes the differences in sedimentation rates of different particles to separate high-quality microLEDs from smaller particles. As shown in the figure, the carrier flow is adjusted so that particles smaller than approximately 30 μm flow out the upper waste gate, while faster-falling microLEDs flow out the lower port.

[0095] Efficient fluidic assembly requires a uniform distribution of microLEDs on the display substrate and the number of microLEDs must be sufficient to fill all available assembly sites (also called trap sites or wells). In practice, the optimal number of microLEDs is greater than the number of assembly sites. If the number of microLEDs is lower than the optimal value, the assembly time increases because the microLEDs have to travel farther to reach the empty well sites for assembly. However, if the number of microLEDs is higher than the optimal value, the devices tend to clump together, which interferes with the assembly process. In addition, all excess microLEDs must be removed after assembly, so if too many microLEDs are dispensed, the cleanup time increases and more microLEDs are included in the recycling process. Therefore, it is crucial that the suspension dispensing process is based on a microLED suspension with a known and well-controlled number of microLEDs per unit volume.

[0096] Since the variation in the number of microLEDs in an aliquot increases with the concentration and inhomogeneity of the suspension, the concentration of microLEDs in the suspension must be adjusted to ensure that the correct number of microLEDs are transferred to the display substrate. The number of microLEDs collected from the carrier wafer can be well determined by calculating the collection area after removing the defective area. The concentration of the suspension can then be simply set by adding an appropriate volume of solvent during the final exchange process. However, variations in concentration are caused by solvent evaporation, removing aliquots for distribution, and returning recovered microLEDs to the suspension. In order to control the concentration, a system is necessary to accurately determine the concentration of the suspension.

[0097] Back to Figure 2 For a radially symmetric transparent container, the concentration varies only along the z-axis, so to quantify the microLED concentration at several heights in the suspension container, pairs of collimated LEDs (or laser diodes) and photodetectors are used to transmit light through the suspension and measure the density (optical opacity), expressed as log(I in / I out The amount of light attenuation at different heights in the suspension is directly proportional to the concentration of microLEDs at those heights, so a calibration curve based on microLED size is used to convert optical density measurements into calculated concentrations. Optical density measurements are taken immediately after stirring to create a homogeneous suspension to determine concentration. The relationship between these measurements and time and height in the container directly gives the sedimentation rate.

[0098] After approximately half the settling time, where detectors 210a and 210b receive full intensity, detector 210c returns to an intensity of 50%-60%, indicating uniform microLED density, and detector 210n sees a near-uniform microLED density. When the suspension is undisturbed for an extended period, compared to the settling time, all microLEDs collect at the bottom of the container, resulting in minimal light scattering and a maximum intensity value for each intensity measurement. If the suspension is sufficiently agitated, the microLEDs are evenly distributed throughout the liquid column, with minimal light scattering at every height. Once agitation ceases, the microLEDs begin to fall under the influence of gravity until they reach terminal velocity. As time increases, the microLED concentration at the top of the fluid column decreases, and the detector intensity increases.

[0099] 17A to 17C A graph depicting exemplary measurements of optical transmittance versus time for a suspension of 42 μm diameter microLEDs in 20 mL of isopropyl alcohol (IPA), a graph depicting exemplary measurements of optical transmittance versus time for 1.3 million microLEDs in different volumes of IPA, and a calibration curve of concentration versus optical transmittance for the system's microLED suspension. Optical density is defined herein as the inverse of transmittance, and the data are divided by the final intensity for normalization purposes. Figure 17A In this study, approximately 1.2 million microLEDs were suspended in a cylindrical, translucent tube with a diameter of 27.5 millimeters (mm), and the optical density was measured at five different vertical positions. At time zero, the tube was mechanically vibrated to agitate the suspension, creating a uniform distribution of microLEDs throughout the fluid column. During the agitation process, and for the first few seconds afterward, significant noise was generated due to bubbles in the liquid. The light intensity measured after agitation was approximately 60% lower than that of the settled suspension. Once all microLEDs had settled from the sensing well, recovery to full intensity was a function of the distance from the sensor to the top of the fluid column. Thus, for the upper sensor, located 12 mm from the top of the liquid column, t1 was 23 seconds, while the bottom sensor, located 33 mm from the top of the liquid column, recovered within 64.5 seconds (t5). Therefore, the terminal velocity of these microLEDs in IPA ranged from 0.51 mm / s to 0.56 mm / s, resulting in a settling time of approximately 85 seconds for a 45 mm liquid column, requiring several seconds for them to settle after being dispensed into the thin liquid layer.

[0100] The transmittance measurement system can also be used to determine the number density of microLEDs in the suspension, which is crucial information for accurate processing of the suspension. Figure 17BIn the , the transmittance was measured as a function of time after stirring for five different suspension dilutions. A microLED with a diameter of 42 μm was measured at a position of 13 mm in 20 mL of IPA, and then a measured amount of liquid was added to prepare a suspension with a lower density. Figure 17B The light intensity after stirring is lowest for the highest concentration (C1) and increases with each successive decrease in density. As the height of the liquid column increases, the settling time at a fixed position on the container also increases.

[0101] Figure 17C Figure 2 shows a calibration curve for determining the number of microLEDs per milliliter of liquid based on the optical transmittance of a homogeneous suspension, obtained from multiple experiments. It can be seen that the homogeneity of the suspension is high within a few seconds of stirring, and that this state decreases with distance from the top of the fluid column. It is ideal to extract an aliquot from the system a few seconds after stirring and at least 20 mm below the top surface. This characterization system can be used to select optimal process parameters for different microLED sizes, container sizes and shapes, and suspensions.

[0102] Mixing a microLED suspension is a balance between the forces required to achieve high suspension uniformity and the shear forces required to prevent breakage. Mixing can be achieved through external agitation, where the smooth walls of a container impact the suspension, creating a fluid flow that agitates the microLEDs within the suspension container. Internal agitation can be achieved by introducing a solvent or gas stream into the suspension container to induce turbulent fluid flow. Mixing can also be achieved by rapidly withdrawing and injecting liquid through a pipette, for example. The goal, of course, is to produce a uniform distribution of microLEDs across the vertical column of the container without damaging the microLED devices.

[0103] From a well-mixed suspension, a controlled volume containing the number of LEDs required for a single dispensing path can be drawn. The dispensing path can be a single point, a single line segment, a serpentine path, or some combination of paths. Multiple dispensing paths are used to ensure complete and uniform distribution across the display assembly area. Due to the extremely short settling time of LEDs, especially in the thin layers of fluid used for assembly, the lateral spread of the dispensing path is limited to the millimeter scale. Therefore, uniform distribution of microLEDs requires multiple dispensing paths in relatively close proximity.

[0104] Figure 18A and Figure 18BComparison of dispensing density gradients for single-pass and double-pass / twice-speed dispensing paths, respectively. The precise method for transferring the dispensed volume from a well-mixed suspension depends on the characteristics of the system and the product requirements: in particular, the size of the assembly substrate, the travel speed of the dispensing head, the thickness of the assembly fluid above the substrate, the uniformity of mixing, and the control of the transferred volume are all important. These considerations are generally a trade-off between uniformity and cost (including system expense, processing time, and product yield). Imperfections in dispensing uniformity can also be compensated for during the assembly process, and a range of methods have been proposed. For example, a given path can be rapidly retraced multiple times on a single line segment to compensate for uneven dispensing rates (which may be caused by settling within the dispensing head, for example), and along e.g. Figure 18B The path shown achieves uniform density.

[0105] 19A to 19C Schematic diagrams depicting the transfer of a well-mixed suspension from an initial source container directly to an assembled substrate by decanting from a container, a nozzle, and a tube, respectively. Transfer of a well-mixed suspension that limits or completely avoids the use of tubes and fittings can be performed using direct transfer from the source tank or removal of discrete aliquots of the source suspension via a pipette tip. This approach avoids the accumulation of dead zones in the fittings for the microLEDs and limits the interaction of the microLEDs with the suspension to surfaces that are: a) small, b) cleanable, and c) potentially disposable. The pipette tip can withdraw the suspension from the tank by volumetric displacement (e.g., via a plunger) or by an actively applied vacuum. The advantage of applying a continuous vacuum is that once the aliquoted suspension is no longer in contact with the source suspension, ambient atmosphere is drawn into the aliquot as bubbles, actively mixing the suspension during the transfer process.

[0106] To limit yield losses, a highly concentrated suspension is preferred until just before introduction to the substrate. At this point, the aliquoting method described above can be combined with an intermediate small volume container in which the dispensed suspension is supplemented with additional liquid. This can be done by filling the suspension tank with a known volume of pure liquid using a standard system (including tubing, valves, and fittings), as pure liquids are not limited by the microLED suspension. Suspension aliquots can be corrected by aspirating the liquid into the dispensing head or by depositing the suspension into the suspension tank and then pumping the mixture back into the dispensing head. The diluted suspension can then be transferred to the substrate.

[0107] The processes described above for mixing, transferring, and diluting are universal, and variations of them can be selected to form a unified system and application method that is optimized for the type of microLED display being produced. Several complete dispensing processes are described in detail below as examples.

[0108] exist Figure 19AIn this method, the desired number of LEDs can be dispensed directly from a well-mixed container onto an assembly substrate. This approach has the fewest transfer steps and potentially the least exposure of surfaces to unwanted stiction of the microLEDs. Because the fully settled state of the suspension is non-uniform, the system mixes the suspension to a uniform suspension density, so that the volume of suspension transferred corresponds to the number of microLEDs transferred. The well-mixed suspension can then be transferred directly onto the substrate. Pouring the microLEDs onto the substrate creates a significant flow throughout the fluid on the substrate, which can quickly transfer the LEDs across a large area. However, uniformity can be poor, requiring additional assembly time to compensate.

[0109] exist Figure 19B In the present work, a small-area nozzle paired with appropriate pressure control in the container headspace can transfer the suspension in a more controlled manner, although this method is significantly slower than pouring. This method also requires more precise translation of the microLED container on the assembled substrate than the pouring method.

[0110] exist Figure 19C In this approach, instead of translating the container, a tube can be used with one end immersed in the suspension and the output end translated onto the assembly substrate. The advantage of this approach is that the suspension container can be actively mixed during the dispensing process, and translating the tube end is much easier than translating the container. Multiple tubes can be drawn from the same container of microLED suspension, significantly increasing the area covered by the dispensing in a single pass. The disadvantage of this approach is that the surface area with which the suspension interacts is very large, thus trapping microLEDs in the tube. In applications where cross-contamination is not a concern, this approach using limited tubes is the best choice.

[0111] Figure 20 This is a schematic diagram showing controlled-volume pipetting from a well-mixed suspension onto an assembled substrate. As shown in the schematic, the pipette can draw aliquots from the well-mixed suspension, rather than dispensing directly from the container, onto the substrate. This method leverages established techniques in the biological sciences to reliably transfer aliquots with high precision. Furthermore, this method does not require any additional configuration of the suspension container, such as pour spouts or nozzles.

[0112] Pipette transfer is best used when precise volume and preventing cross-contamination are top priorities, such as when minimizing excess microLEDs used in assembly or when sequentially assembling microLEDs of different proprietary sizes. The trade-off is that pipette dispensing is slower than other methods because the pipette needs to return to the suspension container after each dispensing pass. Multi-tip pipettes exist, but they are not suitable for pipetting from a single source due to the concentration of the suspension container and the mixing limitations of the suspension container.

[0113] Figure 21 Schematic diagram showing the suspension being corrected in an intermediate tank and subsequently aspirated and dispensed by a bubble-mixing dispensing head. Another limitation of both direct and pipette transfer methods is that the volume dispensed is equal to the volume removed from the source suspension container. To limit the size of the suspension container for large dispensing areas, it may be necessary to reduce the concentration of the suspension dispensed onto the assembled substrate. As shown in the figure, one way to achieve this dilution is to withdraw a concentrated aliquot from the suspension supply container and correct it with pure liquid in the dispensing head or intermediate tank.

[0114] Figure 22 The figure shows a schematic diagram of a parallel dispensing method using a dispensing head array. The use of an intermediate tank has the potential advantage of separating the dispensing head pickup from the suspension tank pickup. Therefore, the system shown can be scaled up to an array of dispensing heads drawn from a suspension tank array. The generation of a suspension tank rack with a precisely controlled microLED concentration can be accomplished in a separate step, and the rack is then loaded into the assembly tool along with the assembled substrate.

[0115] All three methods described are variations on the core concept of efficient microLED transfer. Fluidic assembly can be used for a variety of microLED sizes, assembly areas, and pixel pitches. Here are some examples of how variations in assembly requirements influence method selection:

[0116] For single-color assembly of small-area substrates, direct dispensing may be necessary.

[0117] A decanting method has been proposed for parallel batch assembly of multiple substrates from a single suspension container.

[0118] Serially repeated assembly of substrates may be best suited to the nozzle approach.

[0119] Large-area assembly of large volumes of well-suspended microLEDs with low batch-to-batch wavelength variation may be most economical using tube-mediated transfer from a source container of the suspension.

[0120] For medium-sized substrates (a few centimeters on a side) with valuable microLEDs and low cross-contamination tolerance—for example, a sequentially assembled RGB triple-emitter color display—pipette dispensing may be preferred, especially when pipette dispensing time is not a significant portion of production time.

[0121] For very large substrates exceeding the second generation size (360×465mm), single-head dispensing becomes extremely slow, and production volume requirements dictate the need for multiple heads in the dispensing system to dispense in parallel. For fast-settling suspensions, the ability to vacuum mix the suspension is important for improving dispensing uniformity. In addition, for large substrates, the total dispensing volume requirements become very high, and modifying the concentrated source suspension can improve processing and mixing uniformity.

[0122] Key requirements for dispensing microLEDs from a suspension onto a substrate are limiting microLED waste due to breakage, surface adhesion, and uneven application. Therefore, ideally, the suspension should not encounter valves, pumps, or fittings throughout the collection, filtration, mixing, dispensing, and recovery operations. Inevitably, some loss occurs in the suspension container itself, but thorough rinsing, combined with the capture and recovery processes, greatly mitigates this loss. For pipette transfer methods, only the pipette tip comes into contact with the suspension. The pipette can be rinsed inside and out to recover the microLEDs, or it can be discarded to prevent cross-contamination of the microLEDs.

[0123] The vacuum mixing dispensing head uses a disposable tip, and the intermediate suspension tank can be flushed for recycling and reused if cross contamination is not an issue, or replaced to prevent cross contamination. Figure 19C The embodiments described use tubes that are unlikely to be cleaned, and this approach is only an option if cross contamination is not a concern and the microLEDs are well suspended, minimizing contact with the tube sidewalls.

[0124] Figure 23 is a flow chart illustrating a method for selectively collecting microLED devices from a carrier substrate. For clarity, the method is depicted as a series of numbered steps, but the numbering does not necessarily indicate the order of the steps. It should be understood that some of these steps may be skipped, performed in parallel, or performed without maintaining a strict order. However, in general, the method follows the numerical order of the depicted steps. The method begins at step 2300.

[0125] Step 2302 provides an inorganic microLED device attached to a carrier substrate by an adhesive. In step 2304, defect regions are predetermined, wherein each defect region includes a plurality of adjacent defective microLED devices or process control structures (e.g., CMP scratches). Step 2306 forms a solvent-resistant colloid material covering the predetermined defect regions. Step 2308 dissolves the exposed adhesive with an adhesive dissolving solvent. The adhesive dissolving solvent includes acetone, toluene, trichloroethane, N-methylpyrrolidone (NMP), xylene, cyclohexanone, butyl acetate, or a combination thereof. Step 2310 separates the microLED devices located outside the predetermined defect regions from the carrier substrate. Step 2312 maintains the adhesive attachment of the microLED devices within the predetermined defect regions to the carrier substrate. In response to the separation of the microLED devices from the carrier substrate, step 2314 collects the valid microLED devices in a collection container.

[0126] In one aspect, step 2305a inspects the carrier substrate to locate defective microLED devices, and step 2305b locates a non-predetermined defect region comprising a plurality of adjacent defective microLED devices. The inspection process can be performed through optical comparison, electroluminescence, photoluminescence, or cathodoluminescence testing. Then, step 2306 forms a solvent-resistant colloid material covering the non-predetermined defect region. In another aspect, in response to the inspection in step 2305a, step 2305c locates individual non-predetermined defective microLED devices. Then, step 2307 uses a laser trimming process to eject the individual defective microLED devices.

[0127] In one aspect, step 2309 applies additional motive force, such as fluid circulation, heat, gravity, vibration, or a combination thereof, and in response to the additional motive force, step 2310 at least partially separates the microLED device from the carrier substrate.

[0128] In one aspect, dissolving the exposed adhesive in step 2308 includes selectively exposing a portion of the carrier substrate to an adhesive-dissolving solvent. Separating the microLED device from the carrier substrate in step 2310 then includes separating the microLED device from the selectively exposed portion of the carrier substrate. More specifically, selectively exposing the portion of the carrier substrate to the solvent can include the following sub-steps: Step 2308a rotates the carrier substrate in the solvent. Step 2308b exposes a radial portion of the carrier substrate having a radius greater than d to the solvent. Separating the microLED device from the exposed portion of the carrier substrate in step 2310 then includes separating the microLED device from the radial portion of the carrier substrate.

[0129] In another aspect, collecting the active microLED devices in the collection container in step 2314 includes replacing the binder-dissolving solvent with a different liquid. If the active microLED devices collected in the collection container in step 2314 have an average cross-sectional physical dimension s and there are impurities in the fluid, step 2315a filters to remove impurities having a maximum cross-sectional physical dimension greater than t, where t>s. Alternatively, or in addition, step 2315b filters to remove impurities having a maximum cross-sectional physical dimension less than p, where p<s. The filtration methods of steps 2315a and 2315b can use mechanical screening, elution, classification, or a combination thereof. For example, to perform both high-pass and low-pass filtration, mechanical filtration can use two different mesh sizes. For both elution and classification, the output port can be either product or waste, depending on the desired flow rate. Furthermore, there is no reason to use the same filtration method for both types of filtration. For example, a mesh filter can be used to remove large contaminants, and a classification tank can then be used to remove small particle contaminants.

[0130] In one aspect, replacing the binder dissolving solvent with a different liquid in step 2314 includes replacing the binder dissolving solvent with a filtered solution having a lower viscosity than the binder dissolving solvent, and filtering the filtered solution in step 2315 to remove impurities from the filtered solution.

[0131] In another aspect, step 2314 replaces the binder dissolving solvent with an assembly solution having a lower polarity or a higher evaporation rate than the binder dissolving solvent. Surfactants can also be added to the assembly solution, such as anionic, cationic, nonionic surfactants, or combinations thereof.

[0132] Figures 24A to 24C 2 is a flow chart illustrating a first method for distributing microLED devices on a light-emitting display panel. The method begins at step 2400. Step 2402 adds a suspension of collected microLED devices to a transparent first container. Step 2404 agitates the suspension. Some examples of agitation include externally vibrating the first container, generating a fluid flow in the suspension, and flowing a gas through the first container. Step 2406 optically measures the opacity of the suspension at multiple heights within the first container. In response to the optical measurements, step 2408 determines the uniformity of the suspension. In response to the determined uniformity being greater than a minimum uniformity threshold, step 2410 distributes the suspension over the top surface of the light-emitting display panel.

[0133] In one aspect, step 2401a determines the number of collected microLED devices. For example, the number of microLEDs collected from the carrier substrate may be known. Step 2409a calculates the number of microLED devices per unit volume of the suspension, and step 2410 of dispensing the suspension onto the top surface of the light-emitting display panel includes depositing the known first number of microLED devices in response to dispensing the first volume of the suspension.

[0134] In one aspect, optically measuring the opacity of the suspension at multiple heights within the first container in step 2406 includes substeps. Step 2406a comprises arranging a plurality of light emitting devices having predetermined output light intensities, directed toward a central axis of the first container, and spaced apart from each other by a first predetermined distance along a first vertical axis. Step 2406b comprises arranging a plurality of photodetectors spaced apart from each other by a first predetermined distance along a second vertical axis, each photodetector having an input terminal directed toward an output terminal of a corresponding light emitting device. Step 2406c comprises comparing the light intensities received by the photodetectors.

[0135] Another alternative approach begins with a known number of microLEDs (step 2401a), calculates a first number of microLED devices per unit volume of the suspension in step 2409a, and determines the uniformity of the suspension in step 2408. Step 2412 varies the ratio of fluid to LED devices in the suspension by varying a predetermined amount of fluid, and step 2414 optically measures the opacity of the suspension to calculate a second number of microLED devices per unit volume of the suspension.

[0136] In step 2410, dispersing the suspension on the light-emitting display panel includes using one of the following dispersion processes: single-step large-volume pouring, multi-step pipette translation, nozzle-limited container translation, and translation tube. The multi-step pipette lateral dispersion process includes the following sub-steps. Step 2410a maintains the uniformity of the suspension greater than the minimum uniformity critical value in the first container. Step 2410b repeatedly extracts a predetermined aliquot volume from the first container using a pipette. After each aliquot is extracted, step 2410c translates the pipette a predetermined distance relative to the top surface of the light-emitting panel. Step 2410d releases a predetermined amount of aliquots per second during the transfer.

[0137] In another aspect, the first container is pressure-controlled and includes a nozzle, and the nozzle-constrained container translational dispersion process includes the following sub-steps: Step 2410e maintains the uniformity of the suspension above a minimum uniformity threshold in the first container; Step 2410f translates the first container relative to the top surface of the light-emitting panel by a predetermined distance, and Step 2410g releases a predetermined amount of the suspension from the nozzle per second during the translation.

[0138] In one aspect, the first container is pressure controlled and includes an output port connected to one or more delivery tubes, and the translation tube dispersion process includes the following sub-steps: Step 2410h maintains the uniformity of the suspension above a minimum uniformity threshold in the first container; Step 2410i translates the one or more delivery tubes a predetermined distance relative to the top surface of the light emitting panel; and Step 2410j releases a predetermined amount of suspension from the one or more delivery tubes per second during the translation.

[0139] The single-step mass decantation dispersal process includes the following sub-steps: Step 2410k maintains the uniformity of the suspension greater than a minimum uniformity threshold in the first container, and Step 2410m releases the suspension from the first container to the top surface area of ​​the light-emitting panel using a fixed position center area release or an area translation release.

[0140] In one aspect, step 2401b determines a number of assembly locations in a first region of a top surface of the light-emitting display panel. Then, dispensing the suspension over the top surface of the light-emitting display panel in step 2410 includes depositing a first number of microLED devices that is at least equal to the number of assembly locations in the first region.

[0141] In another aspect, step 2409b determines the number of translation path iterations for the first region of the light-emitting display panel, and step 2409c determines the translation speed. Then, dispersing the suspension in step 2410 includes calculating a rate at which the first volume of suspension is dispersed in response to the number of path iterations and the translation speed to produce a uniform suspension density across the first region of the light-emitting display panel.

[0142] Figure 25 2 is a flow chart illustrating a second method for distributing microLED devices on a light-emitting display panel. The method begins at step 2500. Step 2502 adds a suspension of collected microLED devices to a transparent first container. Step 2504 stirs the suspension. Step 2506 optically measures the density of the suspension at a plurality of first container heights. In response to the optical measurements, step 2508 calculates a first number of microLED devices per unit volume of the suspension. Step 2510 dispenses an aliquot of the suspension onto the top surface of the light-emitting display panel. Step 2512 repeats the optical measurements, and step 2514 calculates a second number of microLED devices per unit volume of the suspension.

[0143] In one aspect, step 2509a transfers a known aliquot of the suspension to a second container. Step 2509b modifies the amount of liquid in the second container to a predetermined amount, and step 2509c calculates a third number of microLED devices per unit of suspension in the second container. Step 2509d dispenses the suspension in the second container onto the top surface of the light-emitting display panel.

[0144] In another aspect, step 2516 modifies the amount of suspension in the first container, and after stirring the suspension, step 2518 optically measures the density of the suspension to calculate a fourth number of microLED devices per unit volume of the suspension.

[0145] Systems and methods for collecting and dispersing microLEDs have been provided. Examples of specific process steps and hardware elements have been provided to illustrate the present invention. However, the present invention is not limited to these examples. Other variations and embodiments of the present invention will occur to those skilled in the art.

Claims

1. A method for selectively collecting microLED devices from a carrier substrate, wherein: The method comprises: providing an inorganic microLED device attached to a carrier substrate by an adhesive; Predetermining a defective region, wherein the defective region includes a region consisting of a plurality of adjacent defective microLED devices or a region consisting of a process control structure; forming a solvent-resistant colloidal material covering a predetermined defect area; Dissolve the exposed adhesive with an adhesive dissolving solvent; separating the microLED device located outside the predetermined defect region from the carrier substrate; and The microLED device within the predetermined defect area is kept adhesively attached to the carrier substrate.

2. The method for selectively collecting microLED devices from a carrier substrate according to claim 1, wherein: Also includes: inspecting the carrier substrate to locate defective microLED devices; locating a non-predetermined defect region comprising a plurality of adjacent defective microLED devices; as well as, The forming of the solvent-resistant colloid material includes covering the non-predetermined defect area with the solvent-resistant colloid material.

3. The method for selectively collecting microLED devices from a carrier substrate according to claim 2, wherein: Also includes: responsive to the inspecting, locating non-predetermined individually defective microLED devices; as well as, A laser trimming process is used to eject the individual defective microLED devices.

4. The method for selectively collecting microLED devices from a carrier substrate according to claim 2, wherein: Methods of inspection to locate defective microLED devices include using an inspection process selected from optical comparison, electroluminescence, photoluminescence, or cathodoluminescence testing.

5. The method for selectively collecting microLED devices from a carrier substrate according to claim 2, wherein: Also includes: In response to separating the microLED devices from the carrier substrate, valid microLED devices are collected in a collection container.

6. The method for selectively collecting microLED devices from a carrier substrate according to claim 5, wherein: Also includes: applying additional power selected from the group consisting of fluid circulation, thermal energy, gravity, vibration, and combinations thereof; and, Wherein, separating the microLED device from the carrier substrate includes at least partially separating the microLED device in response to the additional driving force.

7. The method for selectively collecting microLED devices from a carrier substrate according to claim 5, wherein: Collecting active microLED devices in the collection container includes replacing the adhesive dissolving solvent with another liquid.

8. The method for selectively collecting microLED devices from a carrier substrate according to claim 7, wherein: Collecting the active microLED devices in the collection container comprises generating a suspension of active microLED devices having an average cross-sectional physical dimension s, wherein the suspension has impurities therein; The method further comprises: After collecting the active microLED devices, they are filtered to remove impurities with a maximum cross-sectional physical dimension greater than t, where t>s.

9. The method for selectively collecting microLED devices from a carrier substrate according to claim 7, wherein: Collecting the active microLED devices in the collection container comprises generating a suspension of active microLED devices having an average cross-sectional physical dimension s, wherein the suspension has impurities therein; The method further comprises: Filter to remove impurities with a maximum cross-sectional physical dimension less than p, where p < s.

10. The method for selectively collecting microLED devices from a carrier substrate according to claim 8, wherein: The filtration process is selected from the group consisting of mechanical sieving, elution, classification or a combination thereof.

11. The method for selectively collecting microLED devices from a carrier substrate according to claim 7, wherein: Replacing the binder dissolving solvent with another liquid includes replacing the binder dissolving solvent with a filtered solution having a lower viscosity than the binder dissolving solvent; as well as, The method further comprises: Filtration is performed to remove impurities from the filtered solution.

12. The method for selectively collecting microLED devices from a carrier substrate according to claim 7, wherein: Replacing the binder dissolving solvent with another liquid includes replacing the binder dissolving solvent with an assembly solution having a property selected from the group consisting of an assembly solution having a lower polarity than the binder dissolving solvent or an assembly solution having a higher evaporation rate than the binder dissolving solvent.

13. The method for selectively collecting microLED devices from a carrier substrate according to claim 12, wherein: Replacing the binder dissolving solvent with the assembly solution includes adding a surfactant to the assembly solution, the surfactant being selected from the group consisting of anionic, cationic, nonionic surfactants, or combinations thereof.

14. The method for selectively collecting microLED devices from a carrier substrate according to claim 1, wherein: dissolving the exposed adhesive includes selectively exposing portions of the carrier substrate to the adhesive dissolving solvent; as well as, Wherein, separating the microLED device from the carrier substrate includes separating the microLED device from the selectively exposed portion of the carrier substrate.

15. The method for selectively collecting microLED devices from a carrier substrate according to claim 14, wherein: Selectively exposing portions of the carrier substrate to the solvent comprises: rotating the carrier substrate in a solvent bath; exposing a radial portion of the carrier substrate having a radius greater than d to the solvent bath; and, Wherein, separating the microLED device from the exposed portion of the carrier substrate includes separating the microLED device from the radial portion of the carrier substrate.

16. The method for selectively collecting microLED devices from a carrier substrate according to claim 1, wherein: Exposing the microLED device to the binder-dissolving solvent includes the binder-dissolving solvent being selected from the group consisting of acetone, toluene, trichloroethane, N-methylpyrrolidone, xylene, cyclohexanone, butyl acetate, or a combination thereof.

17. A system for selectively collecting microLED devices from a carrier substrate, wherein: The system comprises: a vacuum chuck having a rotational interface for mounting a carrier substrate comprising an inorganic microLED device attached to the carrier substrate by an adhesive; an elbow connected to the vacuum cup, the elbow having a configuration for determining an angle of rotation of the vacuum cup in a plane containing an x-axis and a z-axis; a gantry connected to the elbow, the gantry having a configuration for determining the height of the vacuum cup along the z-axis; a tray of adhesive dissolving solvent, the tray having a top opening to receive the carrier substrate; a controller having outputs connected to the gantry and the elbow to provide a setting for the height and a setting for the angle, respectively; wherein, in response to the positioning of the gantry and the elbow, selected radial portions of the carrier substrate are exposed to the adhesive dissolving solvent; and, The microLED devices separated from the selectively exposed portions of the carrier substrate are collected in the tray.

18. The system for selectively collecting microLED devices from a carrier substrate according to claim 17, wherein: The controller has an input for accepting a first map of microLED performance areas, and in response to the first map the controller provides gantry and elbow settings that select radial areas of the carrier substrate exposed to an adhesive dissolving solvent.

19. The system for selectively collecting microLED devices from a carrier substrate according to claim 17, wherein: Also includes: an inspection subsystem having an optical input and an output connected to the controller for identifying individual defective microLED devices on the carrier substrate; and A trimming laser has an input connected to the controller for receiving a second pattern of defective microLED devices and an output for ejecting the defective microLED devices from the carrier substrate by laser radiation in response to the second pattern.

20. A system for selectively isolating regions of a carrier substrate of microLEDs, wherein: The system comprises: a controller having an output for providing a first map of predetermined defect areas in the carrier substrate; a printer having an input for accepting the first pattern and a nozzle for printing a solvent-resistant colloidal material onto selected areas of the carrier substrate in response to the first pattern; and; Wherein, the microLEDs in the selected areas remain attached to the carrier substrate despite exposure to the adhesive dissolving solvent.

21. The system for selectively isolating regions of a microLED carrier substrate of claim 20, wherein: The solvent-resistant colloid material is selected from the group consisting of SU-8, epoxy resin, polyethylene terephthalate, acrylonitrile butadiene styrene or polyimide.

22. The system for selectively isolating regions of a microLED carrier substrate of claim 20, wherein: Also includes: an inspection subsystem having a light input and an output coupled to the controller for identifying areas of the carrier substrate that are not predetermined defective microLED devices; and The printer receives a second map of the area of ​​the non-predetermined defective microLED device from the controller and prints the solvent-resistant colloid material to the area of ​​the detected defective microLED device in response to the second map.

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