CN115485835A

CN115485835A - System and method for selectively collecting light emitting elements

Info

Publication number: CN115485835A
Application number: CN202180032747.3A
Authority: CN
Inventors: 佐佐木健司; 葛特鄂孟; 保罗·舒勒; 李宗霑
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2020-05-16
Filing date: 2021-05-17
Publication date: 2022-12-16
Also published as: KR20220165284A; WO2021236552A1

Abstract

A method for selectively collecting micro led devices from a carrier substrate is provided. A carrier substrate includes a predetermined defective area of a plurality of adjacent defective micro led devices thereon. The solvent-resistant gum material is covered over the predetermined defect area and the exposed adhesive is dissolved by the adhesive-dissolving solvent. The defect-free micro led devices that are outside the predetermined defective region are separated from the carrier substrate while maintaining an adhesive attachment between the micro led devices within the predetermined defective region and the carrier substrate. A method of dispensing micro led devices on a light emitting display panel by optically measuring a suspension of micro leds to determine suspension uniformity and counting the number of micro leds per unit volume is also provided. If the number of micro led devices collected in the suspension is known, the number of micro led devices per unit volume of the suspension can be calculated.

Description

System and method for selectively collecting light emitting elements

Technical Field

The present invention relates generally to Light Emitting Diodes (LEDs) having a size of less than 100 microns, and more particularly to a process and system for fabricating micro-LEDs.

Background

A red/green/blue (RGB) display consists of pixels that emit light at three wavelengths corresponding to the visible colors red, green and blue. The RGB components of a pixel, each referred to as a sub-pixel, are excited in a systematic manner to produce colors in the visible spectrum. There are several display types that generate RGB images in different ways. Liquid Crystal Displays (LCDs) are the most popular technology, which produce RGB images by illuminating a white light source (usually a phosphor-produced white LED) through a color filter of a sub-pixel. A part of the wavelength of the white light is absorbed, and a part of the wavelength of the white light is transmitted through the color filter. Thus, the efficiency of LCD displays may be below 4%, and the contrast ratio is limited by light leakage through the liquid crystal. Organic Light Emitting Diode (OLED) displays produce RGB light by directly emitting each of these wavelengths of light at the pixel level within an organic light emitting material. OLED materials are directly emissive and therefore display high contrast, but organic materials may suffer from long term degradation, leading to image degradation.

A third display technology and discussed herein is a micro LED display that uses inorganic LEDs of micron size (5 to 100 microns (μm) in diameter) to emit light directly at the sub-pixel level. Inorganic microLED displays have some advantages over competing displays. In comparison to LCD displays, micro led displays have over 50,000:1, very high contrast and higher efficiency. Unlike OLED displays, inorganic LEDs do not suffer from aging effects and can achieve significantly higher brightness.

Micro LEDs are made from Metal Organic Chemical Vapor Deposition (MOCVD) wafers, similar to wafers used to manufacture general illumination LEDs, which makes the cost per device very low, but also raises issues unique to some micro LED technologies. The structure for the fluid assembly of the micro led has been described extensively in the parent patent US10,643,981, which is incorporated herein by reference. For use in general lighting, the most important feature of the device is the low cost production of each photon 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 sorting process involves testing all LEDs after packaging and placing LEDs with similar efficiency and emission wavelength characteristics on the same component while discarding the defective devices. The sorting process allows MOCVD fabrication to be cheaper as the cost of reducing defects and process control is minimized.

Recently characterized by the manufacturing success of a 40 μm typical gallium nitride (GaN) micro led, we found that the micro led defects had 0.25% device short and 0.75% device open. These defects can result in non-emissive pixels, which is unacceptable for display products. The micro leds are not encapsulated and the device size, especially the electrodes, is very small, making handling and functional testing of the device difficult. Since Ultra High Definition (UHD) displays require at least 2480 million micro leds (3 × 3840 × 2160), the test time becomes very surprising. Therefore, conventional sorting techniques are not practical for identifying and discarding defective micro leds. Therefore, new structures and methods are needed to prevent defective micro leds from generating defective sub-pixels. It is possible to remove and replace defective micro leds as described in parent application serial numbers 16/125,671, 16/595,623 and 16/693,674, which have been incorporated by reference herein, but mechanical pick and place tools are expensive to purchase and operate. It is more desirable to identify defective micro leds and prevent them from entering the suspension for fluid assembly.

The LEDs used for general illumination are much larger than those used for micro LED displays (up to 3 to 4 mm per side, 5 to 100 microns in diameter), so the pattern and electrode requirements are very different. Micro LEDs are bonded to substrate electrodes using solder or anisotropic conductive film, while large general purpose light emitting LEDs are typically bonded by wire bonding or solder paste on a lead frame. Since the micro LEDs are very small, the technology of handling the devices, especially the technology of assembling the micro LED display is very different from the technology developed for the very large LEDs used in general lighting.

To fabricate a micro led display, green and blue GaN micro leds are fabricated on a sapphire substrate, and red aluminum gallium arsenic phosphorous (AlGaAsP) micro leds are fabricated on a GaAs substrate. After fabrication and singulation, the micro leds must be transferred to a second substrate that becomes the light emitting display. The second substrate may be a silicon (Si) wafer (or chip) with built-in control circuitry, or it may be a glass or flexible plastic substrate with thin film transistors. The conventional transfer method is a mechanical pick-and-place system that uses a pick-up head to capture and position the device on the display substrate. Other mechanical transfer methods that use a die or the like to simultaneously transfer a block of micro leds are known as bulk transfer. As described herein, one alternative technique utilizes a fluid assembly process to position the micro leds.

Briefly, a fluid assembly process applies a suspension of micro leds onto a substrate having an array of trapping trap sites (wells), and moves the suspension to assemble the micro leds in the trap sites. For the fluid assembly to be successful, it is necessary to collect the defect-free micro leds from the growth substrate, formulate a suspension with known concentrations of micro leds, and then distribute the suspension uniformly over the display substrate.

The processing of suspensions of micron-sized particles has gained good acceptance in systems such as cell culture in bioscience or abrasive slurries in industrial applications. In all cases, the goal of the suspension treatment system is to achieve a highly uniform suspension and to transfer the suspension to the target process with a high degree of control over volume and concentration. Suspension homogeneity is typically achieved by direct mechanical mixing using submerged impellers or by active circulation of the pump. The transfer of the well-mixed suspension to the target process is usually accomplished by piping pumped downstream of the supply tank or by pressurizing a sealed tank. The volume control of a well-suspended system is achieved by controlling the flow with differential pressure and metering the net flow with a valve timing. It may be desirable to control the suspension concentration, particularly when the suspension is being reused, and therefore the transfer tube typically includes a fitting with multiple inputs so that the pure carrier liquid can be equilibrated with the suspension.

Unfortunately, conventional suspension processing techniques are not compatible with the characteristics of micro leds or the requirements of fluid assembly techniques. In particular, the microLED suspensions have the significant features described below, and it is desirable to develop an alternative method.

Fig. 1 depicts a microLED suspension with a uniform distribution after stirring, which settles to about half the height of the liquid column after time t1 and completely settles after time t 2. Unlike abrasive slurries, which are formulated to prolong uniform mixing and have settling times measured in months, micro led suspensions are formulated to have relatively short settling times. The micro leds must be fixed to the target substrate surface for assembly, so the micro led suspension for fluid assembly usually settles completely within a few minutes and loses homogeneity very soon after mixing has ceased. By way of example, a disc-shaped microLED of diameter 42 μm and thickness 5 μm fabricated according to the center mesa design set forth in parent application Ser. No. 16/406,080 (incorporated herein by reference) has a hydrodynamic diameter of 18.9 μm. An object of this size has a terminal velocity in the liquid when gravity balances the viscosity of the liquid, according to the following equation:

wherein D _LED Is the hydrodynamic diameter, ρ is the liquid density, ρ _LED Is the density of the micro led and μ is the liquid viscosity. For water, the terminal velocity is 1.1 millimeters per second (mm/sec), so in a typical container such as a 50 milliliter (ml) Falcon tube (Falcon tube), the micro led sinks completely in about one minuteAnd then the process goes down.

Micro leds typically have a surface that includes metals, inorganic materials, and organic materials. Therefore, it is almost impossible to prevent temporary blocking of the solid surface in contact with the micro led suspension. Therefore, the container containing the micro led suspension is typically made of a hydrophobic material, such as acetal homopolymer, polytetrafluoroethylene (PTFE), polypropylene, etc., to minimize stickiness. The final state in fig. 1 shows the effect of the interaction of the micro leds with the container wall, where the bottom cone is smaller than the angle of repose, so that some micro leds stick to the container wall and do not settle on a uniform layer at the bottom.

Microorganisms in biological applications are generally sufficiently robust to not lyse when internally mixed (e.g., using a stir bar), while industrial abrasive suspensions (such as Chemical Mechanical Polishing (CMP) slurries) are suspended by drum circulation or impeller mixing without damage. In contrast, micro leds are fragile and can be broken by direct mechanical mixing or pumping. Damaged microleds are similar in major dimension to good microleds and therefore cannot be removed from the suspension by filtration and can interfere with fluid assembly by partially blocking trap sites.

Micro LEDs represent a significant portion of the cost of manufacturing displays, and the low utilization and recycling efficiency of the micro LEDs severely impacts the cost. The components in the suspension are more valuable than in biological and industrial abrasive applications.

Unlike conventional suspensions, the performance characteristics of each individual micro led are important because each device constitutes a sub-pixel. It is necessary to tightly control the number of micro leds available for assembly to control the light emission profile of the complete display. Therefore, suspension handling must be designed to prevent cross-contamination.

The characteristics of the micro led and the stringent requirements for display manufacturing preclude traditional industrial systems and suspension processing methods. Controlled and efficient dispensing of clean, high quality components is critical to fluid assembly because the forces involved in fluid assembly are limited by the threshold at which the assembled components disengage. Rapid fluid assembly then relies on a short travel path on the substrate between the micro led and its final assembly (trap) location. Therefore, the optimal distribution of the micro led suspension on the display substrate must not only be low loss and damage free, but also fast and highly uniform.

It would be advantageous if there were a collection and dispensing method specifically for inorganic micro leds used in process fluid assembly.

Disclosure of Invention

Systems and methods for formulating and operating suspensions of fluid-assembled micro light emitting diodes (micro leds) suitable for use in micro led displays are described herein. One selective collection method produces a suspension of micro LEDs consisting of LEDs known to be good in a determined concentration in a suitable liquid. The micro led suspension is supplied to the display substrate using a dispensing system that minimizes damage and loss of micro leds while evenly distributing the devices on the display substrate at a controlled density. This optimal initial condition is crucial for a successful fluid assembly of a micro led display.

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

The defect-free micro led devices located outside the predetermined defective area are separated from the carrier substrate while maintaining an adhesive attachment between the micro led devices within the predetermined defective area and the carrier substrate. In response to the micro led device being separated from the carrier substrate, the active micro led devices are collected in a collection container. In one variation, only certain portions of the carrier substrate are exposed to the binder-dissolving solvent, such that the micro leds are separated from only the selectively exposed portions of the carrier substrate.

In addition, the carrier substrate may be inspected to locate defective micro led devices in non-predetermined defective areas, and a solvent-resistant gel material may also be formed on these non-predetermined defective areas. In one aspect, inspection can locate micro led devices that are not pre-determined individual defects, and a laser trimming process can be used to eject micro led devices that are individual defects. The inspection may be performed using optical comparison, electroluminescent, photoluminescent or cathodoluminescent tests.

The micro led devices collected in the collection container are typically a suspension of effective micro led devices with an average cross-sectional dimension s. However, impurities are also present in the suspension. In one aspect, the filtration step is performed using a mechanical screen, elution, fractionation, or a combination thereof to remove impurities having a maximum cross-sectional dimension greater than t, where t > s. Also, a single filtration step can remove impurities having a maximum cross-sectional dimension less than p, where p < s. In one aspect, 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 prior to filtration. Alternatively, or after filtration, the fluid in the collected microLED suspension may 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 of the foregoing may be added.

A method for distributing micro led devices on a light emitting display panel is also presented. The suspension of the collected micro led devices is transferred to a transparent first container and stirred. Some examples of agitation processes include external vibration of the first vessel, creating a fluid flow in the suspension, and flowing a gas through the first vessel. The opacity of the suspension is optically measured at a plurality of first container heights to determine the homogeneity of the suspension. When the determined uniformity is greater than the uniformity minimum threshold, the suspension may be dispensed on a top surface of the light emitting display panel. Some examples of dispensing processes include single step macro decantation, multi-step pipette translation, nozzle restricted container translation, and translating tubes.

If the number of micro LED devices collected in the suspension is known, the number of micro LED devices per unit volume of the suspension can be calculated. As a result, a known first number of micro led devices may be deposited on the light emitting display panel in response to dispensing the first volume of the suspension. Advantageously, after determining the number of assembly locations in the first area of the top surface of the light emitting display panel, the number of known first number of micro led devices deposited is at least equal to the number of assembly locations in the first area.

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

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

Further details of the above-described method, as well as a system for separating regions of a micro led carrier substrate, a system for collecting micro leds, a system for characterizing a micro led suspension, will be presented below.

Drawings

Fig. 1 depicts a microLED suspension with a uniform distribution after stirring, which settles to about half the height of the liquid column after time t1 and completely settles after time t 2.

Fig. 2 is a partial cross-sectional view of a system for characterizing a microLED suspension.

Fig. 3A and 3B are schematic partial cross-sectional views of components in a system for selectively collecting micro led devices from a carrier substrate.

Fig. 4 is a partial cross-sectional view of a system for selectively isolating carrier substrate regions of a micro led.

Fig. 5 is a schematic diagram depicting the preparation of a micro led cluster for fluid assembly consisting of three sequential steps.

Fig. 6 is a partial cross-sectional view of a typical micro led on a carrier wafer after device processing is complete.

Fig. 7A and 7B depict a wafer map of known defect locations from micro led fabrication (fig. 7A) and a wafer map of known defect locations of typical alignment structures (fig. 7B).

Fig. 8 is a graph depicting the luminescence spectrum of the GaN microLED cathode.

FIG. 9 is a composite defect map that may be used to guide the defect control process.

Fig. 10 is a plan view of a wafer in which the emission wavelengths are shown as having a uniform width profile.

Fig. 11A and 11B are partial cross-sectional views depicting some representative micro led defects and corrective measures, respectively.

Fig. 12A, 12B and 12C are partial cross-sectional views of a micro led collected in a solvent.

Fig. 13 is a partial cross-sectional view showing a suitable storage container.

Fig. 14 is a graph showing exemplary micro leds and the principal diameters of contaminants.

FIG. 15 is a schematic of an elution flow cell.

FIG. 16 is a schematic of a continuous flow staged filtration process.

Fig. 17A to 17C are graphs depicting exemplary measurements of optical transmittance versus time for a suspension of 42 μm diameter micro leds in 20mL isopropyl alcohol (IPA), 130 million micro leds in different volumes of IPA, and a calibration curve of concentration versus optical transmittance for a suspension of micro leds for the system, respectively.

Fig. 18A and 18B compare dispense density gradients for single and double pass/double speed dispense paths, respectively.

Fig. 19A-19C are schematic diagrams depicting the transfer of a well-mixed suspension directly from an initial source container to an assembly baseplate by decanting from the container, nozzle, and tube, respectively.

Figure 20 is a schematic showing controlled volume pipetting from well-mixed suspensions to assembly substrates.

Figure 21 is a schematic showing the corrected suspension in the intermediate tank and subsequent aspiration and dispensing through a bubble mix dispensing head.

Fig. 22 is a schematic diagram of a parallel dispensing method using an array of dispensing heads.

Fig. 23 is a flow chart illustrating a method for selectively collecting micro led devices from a carrier substrate.

Fig. 24A to 24C are flowcharts illustrating a first method for distributing micro led devices on a light emitting display panel.

Fig. 25 is a flow chart illustrating a second method for distributing micro led devices on a light emitting display panel.

Detailed Description

Fig. 2 is a partial cross-sectional view of a system for characterizing a microLED suspension. The system 200 includes a transparent container 201 having a vertical central axis 202. A plurality of light emitting devices 204a to 204n (LED arrays) are shown, each having a predetermined output light intensity, directed towards the central axis 202 of the container 200 and spaced from each other a first predetermined distance 206 along a vertical first axis 208 parallel to the central axis 202, wherein (n) is an integer greater than 1. The plurality of photodetectors 210 a-210 n (photodetector arrays) are spaced apart from one another by a first predetermined distance 206 along a vertical second axis 212 that is parallel to the central axis 202. Photodetectors 210a through 210n each have a light input directed to the output of the corresponding light emitting device and an output on lines 214a through 214n, respectively, to provide an electro-optical density signal responsive to the measured light intensity. The monitoring device 216 has inputs for receiving the optical density signals 214a through 214 n. The monitoring device 216 compares the light intensity associated with the optical 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 that they have the same pitch. In addition, the light emitting device outputs light of the same intensity. However, it should be understood that once calibrated, similar systems may be enabled without these explicit limitations.

The container 201 comprises a suspension 218 of micro leds. The monitoring device 216 can provide a microLED uniformity measurement or a calculation of the microLED count per unit volume of suspension on line 218 as determined from the uniformity (density) measurement. In one aspect, the monitoring device 216 includes a non-transitory memory 220 having a stored calibration curve 222. In this case, the monitoring device 216 can provide a micro led count per unit volume of suspension on line 218 in response to a comparison of the optical density signals on lines 214a to 214n to the calibration curve 222. As part of the calibration curve, in one aspect, the monitoring device 216 is capable of receiving and storing data regarding the volume of the container 201.

In another aspect, the monitoring device 216 has an input on line 224 to accept a calibration input signal indicative of the total number of micro leds in the suspension, in which case the monitoring device is capable of providing a count of micro leds per unit volume of suspension in response to comparing the optical density signals 214a to 214n with the total number of micro leds. For example, the total number of micro leds can be known by counting the number of active micro leds collected from the carrier substrate. The input on line 224 may alternatively or additionally accept a running measurement of the volume of suspension.

In one variation, the monitoring device 216 accepts a set of optical density signals 214a through 214n collected over a period of time and provides a micro led settling time or micro led size output on line 218. Further, the container 200 may be divided by a plurality of scales 226a to 226n and may include a uniform suspension of micro leds (the suspension 218 is not shown to be uniform). In this case, the photodetectors 210a to 210n detect changes in suspension level, as measured for the container scales 226a-226 n. The monitoring device provides an output on line 218 of the number of micro led devices dispensed from the vessel 200 or the volume of suspension dispensed from the vessel. Advantageously, the output may be provided in real time.

To assist in the above measurements, a stirring device may be used to homogenize the suspension. Various homogenization mechanisms are described in more detail below. In one aspect shown, the suspension may be mixed using a solution (solvent) or a gas. Knowing the suspension volume, the stirring mechanism can be adjusted to optimize mixing.

Fig. 3A and 3B are schematic partial cross-sectional views of components in a system for selectively collecting micro led devices from a carrier substrate. As shown in fig. 3A, the system 300 includes a turntable or vacuum chuck 302 having a rotating (spindle) interface for mounting a carrier substrate 304, the carrier substrate 304 including an inorganic micro led device 306 attached to the carrier substrate by an adhesive 308. The elbow 310 connects the rotating vacuum chuck 302 and has a number of optional settings for determining the angle (tilt) 312 by which the turntable rotates in the plane of the x-axis and z-axis. A gantry 314 is coupled to the elbow 310 and has a plurality of optional settings for determining the height of the turntable along the z-axis. The tray 316 includes an adhesive dissolving solvent 318 and has a top opening to receive the carrier substrate 304. The controller 320 has outputs connected to the gantry 314 and the elbow 310 on

lines

322 and 324, respectively, and provides height and angle settings. As shown, the system 300 allows for exposure of selected radial portions of the carrier substrate 304 to the adhesive-dissolving solvent 318 in response to the positioning of the gantry 314 and elbow 310. The micro led devices 306 separated from the selectively exposed portions of the carrier substrate 304 are collected in a tray 316. In this example, λ of the carrier substrate ₁ The region is being collected (see fig. 10).

In one aspect, the controller 320 has an input on line 326 to accept a first map of the micro led performance area and provides, in response to the first map, an arrangement of the gantry 314 and an arrangement of the elbow 310 that select a radial region of the carrier substrate 304 exposed to the adhesive dissolving solvent.

As shown in fig. 3B, the system 300 may include an inspection subsystem 328, the inspection subsystem 328 having a light input 330 and an output on a line 332 connected to the controller 320 for identifying individual defective micro led devices 306 on the carrier substrate 304. The trim laser 334 has an input on line 336 connected to the controller 320 for accepting the second map of defective micro led devices and an output 338 for ejecting defective micro led devices from the carrier substrate 304 by laser radiation in response to the second map.

Fig. 4 is a partial cross-sectional view of a system for selectively isolating carrier substrate regions of a micro led. The system 400 includes a controller 402 having an output on line 404 to provide a first map of predetermined defective areas 406 on a carrier substrate 408. The printer 410 has an input on line 404 for receiving the first drawing and a nozzle 412 for applying a solvent-resistant gel material 414 to selected areas of the carrier substrate 408 in response to the first drawing. The micro leds 415 in the selected regions 406 remain attached to the carrier substrate 408 despite exposure to an adhesive dissolving solvent (not shown). Some examples of the solvent-resistant glue material 414 include SU-8, epoxy, polyethylene terephthalate (PET), acrylonitrile Butadiene Styrene (ABS), and polyimide.

Optionally, the system 400 may further include an inspection subsystem 416 having a light input 418 and an output on line 420 connected to the controller for identifying areas 422 of non-predetermined defective micro led devices on the carrier substrate 408. The printer input on line 404 can accept a second map of areas 422 of non-predetermined defective micro led devices from the controller 402 and apply a solvent-resistant gel material to the detected areas 422 of defective micro led devices in response to the second map (the solvent-resistant gel material has not been applied in fig. 4).

Fig. 5 is a schematic diagram depicting the preparation of a micro led cluster for fluid assembly consisting of three sequential steps. First, the micro led wafer is inspected to determine the location of defective micro leds. According to the defect map, defective micro leds and other debris particles are either removed from the carrier substrate or encapsulated on the carrier substrate to prevent them from being collected into the suspension. The carrier substrate is then immersed in a solvent to dissolve the adhesive that secures the known good micro leds on the substrate and the micro leds are rinsed into a holding vessel. After allowing the micro led to settle, the solvent with the dissolved adhesive was carefully poured out and several solvent exchanges were performed to remove the remaining adhesive residues. The resulting suspension was filtered to remove particles of a size significantly different from the micro led, and the amount of solvent was adjusted to make the density of the micro led in the suspension suitable for the subsequent mixing and dispensing operation.

The suspension of micro leds is dispensed on the display substrate using three alternative dispensing systems with different tradeoffs in terms of dispensing speed, volume control and complexity. In each case, several separate equal transfers are performed to cover the substrate. The equal transfer can be directly to the substrate or can be through a controlled volume intermediate in the form of a "suspension tank" that either dilutes the suspension or actively mixes the micro led suspension before dispensing onto the substrate using a dispensing head. For suspensions with sufficiently long settling times, direct transfer is preferred. Finally, the uniformity of the substrate can be checked and additional facet dividers can be used to fill the low density areas.

Fig. 6 is a partial cross-sectional view of a typical micro led on a carrier wafer after device processing is complete. Micro leds are typically fabricated on sapphire substrates and transferred to carrier wafers using Laser Lift Off (LLO) as described in parent patent US10,643,981, which is incorporated herein by reference. As shown, the resulting wafer has millions of micro leds embedded in an adhesive layer on a carrier wafer. Unfortunately, wafers also have several different defect types (including process control structures) that adversely affect subsequent processing, and therefore selective collection is used to ensure that the fluid assembly process can be performed using only known good micro leds.

Fig. 7A and 7B depict a wafer map of known defect locations from micro led fabrication (fig. 7A) and a wafer map of known defect locations of a typical alignment structure (fig. 7B). The device fabrication to fabricate the micro led has several known defects or process control structures that appear systematically at the same location due to the nature of the process as shown. Each wafer has an identifying mark near the edge of the wafer, which can create pits in the substrate surface, thus damaging the micro led pattern. The process steps for forming the micro led structure all use a photoresist coating to transfer a pattern to the wafer. The photoresist coating is not perfect and ring-shaped pattern defects will result, especially at the wafer edge. The lithographic process uses a series of marks to locate each layer relative to the previous layer. The alignment marks and metrology structures used (fig. 7B) are necessary for the micro led fabrication, but they are defects if they are contained and collected in the solution of the micro led. These defects are much larger than typical micro led dimensions, and wafer edge ring pattern defects can be as wide as 2 to 3 millimeters. The first component of the wafer defect map is the location of these large system placement structures.

The second type of defects are random processing defects such as Chemical Mechanical Polishing (CMP) scratches on the substrate, large residual gallium nitride (GaN) chunks due to particles falling during isolation etching, metal electrode defects, and the like. These larger defects may be identified by optical scanning, which compares the differences in adjacent images that do not match the expected micro led pattern. This portion of the defect map consists of a series of coordinates outlining the area and defect location of each defect.

One of the most important types of defects is functional defects that affect the electrical performance and light emission of the micro led. Mapping these defects can be done by four different complementary techniques.

l) perhaps the most desirable technology is the Electroluminescence (EL) test, which detects each micro led and measures the resulting luminescence. The test directly identifies weak devices with low emission, as well as short or open devices. The disadvantage is that the technique is slow and it is difficult to probe small electrodes, especially without damaging the electrode surface. This technique can be used to measure some representative devices due to low energy density or off-target emission wavelength, and one area of the wafer can be added to the defect map.

2) Laser luminescence (PL) employs a light source (typically a laser) with a wavelength that excites transitions in the LED structure, and measures the wavelength and intensity of the light produced. The technology can identify Metal Organic Chemical Vapor Deposition (MOCVD) defects and cracked or shorted micro LED devices, but cannot identify missing or open contacts of metal.

3) For optical comparison, the usual method is to compare two images and look for differences between them, differences being defects. The optical image may also be compared to a pattern (die-to-database).

4) Cathodoluminescence (CL) is described below.

Fig. 8 is a graph depicting a GaN micro led cathodoluminescence spectrum. Micro-cathodoluminescence 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, gaN microleds have multiple characteristic emission lines, so spectral differences can identify different defect mechanisms. The weak or absence of the emission peak at 455nm indicates that the LED is shorted or that there is a problem with the Multiple Quantum Well (MQW). Higher emission in the broad peak around 570nm may indicate etch damage or poor quality of n-GaN. The exciton peak of low intensity may also indicate poor initial growth quality or lack of dopant. Any significant deviation from the typical microLED spectrum indicates that the device may perform poorly in a display. FIG. 9 is a composite defect map that may be used to guide the defect control process. The functional test may identify individual defective micro LEDs, so this part of the defect map consists of the X-Y coordinates of each defective LED. Electroluminescence and laser electroluminescence can also generate a map of LED performance characteristics including emission wavelength, efficiency, and threshold voltage. Using these techniques, regions with different properties can be identified, so selective collection techniques can be applied to collect smaller regions of the wafer to produce a micro led suspension with a tighter distribution of uniformity of properties of the micro led devices.

Fig. 10 is a plan view of a wafer in which the emitted light wavelengths are shown as having a uniform width profile. In some cases, it may be desirable to acquire each radial band of the carrier substrate separately so that the individual suspensions (e.g., the four bands as 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 having the same wavelength of emitted light.

Fig. 11A and 11B are partial cross-sectional views depicting some representative micro led defects and corrective measures, respectively. Using the defect size information in the composite defect map, the wafer is processed in two ways as shown in fig. 11B to eliminate defects. For large defect areas, such as wafer edge and area defect patterns, a material resistant to the solvents used during the collection process is applied so that 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, patterned by inkjet means, or even applied with a brush or pen. The wafer may be baked after the coating process to cure the retention material. For individual devices identified as small defects, a pulsed laser beam having a size smaller than the diameter of the micro led may be used to impinge on the individual devices of small defects, thereby removing the defective micro led. The laser wavelength and energy are chosen such that defective micro led absorption results in rapid heating and thus ejection from the adhesive layer. Secondary vacuum nozzles near the laser target area may be used to capture and process the ejected devices, which may prevent defective micro leds from being redeposited on the carrier wafer.

Fig. 12A, 12B and 12C are partial cross-sectional views of a micro led collected in a solvent. In fig. 12A, a carrier wafer with known good micro leds (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 micro leds. The solvent may be acetone, toluene, trichloroethane, N-methylpyrrolidone (NMP), xylene, cyclohexanone, butyl acetate, etc., and the solvent may be heated and gently stirred to accelerate binder dissolution and micro led collection. Care is taken in the process to prevent the containers from coming into mechanical contact with the micro leds on the carrier, which could damage the micro led devices. Maintaining the vertical position of the carrier wafer in the container allows for good solvent circulation and allows the micro leds to settle to the bottom of the container as the adhesive dissolves.

Another method of collection is shown in fig. 12B, where the carrier wafers are placed horizontally in a shallow container with the carrier wafer edges supported by a narrow ledge of the container. It can be seen that the solvent resistant gum material printed or coated can protect the adhesive from the solvent, so that the micro leds in the defect capture area beyond a certain size remain on the carrier wafer. When all the micro leds are released and settle to the bottom of the container, the carrier wafer is removed and it can be checked whether there are any remaining good micro leds. In some cases, if the carrier substrate is not immediately removed from the solvent, the solvent may intrude from beneath the capture area coated with the capture material defects and dissolve the adhesive, and release a bulk of the capture medium into the container.

The "complete" wafer collection method described above is both fast and simple, but in some cases selective collection techniques may be used. The micro leds collect only in the areas that are in contact with the adhesive dissolving solvent, so a simple selective collection can be performed by placing a small droplet of solvent on the horizontal carrier wafer, as shown in fig. 12C. After the adhesive dissolves, the collection area is rinsed to remove the released micro leds from the carrier wafer and collect them.

Another system for selectively collecting radial regions of a wafer may use the same principle of exposing controlled regions of a carrier wafer to an adhesive dissolving solvent, this method being depicted in the system of fig. 3A. For example, based on the emission wavelength contour diagram of fig. 10, the inclination of the carrier substrate and the height of the z-axis are adjusted so that only the region λ ₁ (see fig. 10) is immersed in the solvent. The carrier substrate is slowly rotated to expose the radial bands to the solvent, and the released micro leds settle to the bottom of the solvent tank. When the first zone has been collected, the carrier is dissolvedTaken out of the agent and will have a lambda ₁ The wavelength distributed micro leds are removed from the solvent. The carrier can then be repositioned to collect lambda ₂ Area, and sequential micro led collection is performed in the same manner.

Typically, the collection solvent is heavily contaminated by the adhesive remaining and covering the micro led, and therefore the micro led suspension needs to be decontaminated by a series of solvent exchanges. One solvent exchange cycle was performed as follows:

1) The solvent is indirectly stirred by a vortex mixer or ultrasonic bath to thoroughly homogenize the suspension and break down any remaining binder agglomerates;

2) Several times of settling time is required, so all the micro leds are collected at the bottom of the container;

3) Carefully pour out 80% to 90% of the solvent without disturbing the already settled micro led;

4) Adding new solvent to the vessel; and

5) Repeating steps 1) to 4).

Typically, three or more solvent circulation exchanges are performed to ensure that the binder components are removed. The solvent chosen for removing the adhesive is based entirely on the ability to dissolve the adhesive without compromising the micro led electrodes, and therefore it is not the optimal choice for subsequent filtration and fluid assembly operations. The solvent exchange step may be performed in step 4) above by replacing the new solvent selected for cleaning or fluid assembly. The solvent cycle exchange may be performed at least 3 times to ensure that the solvent has washed the micro led off the vessel wall. After solvent exchange, the microLED solution is transferred from the collection vessel to a clean vessel that can be used for storing and transporting the suspension. The container should be chemically stable to the suspending solution and hydrophobic to minimize sticking of the micro led to the surface. Some suitable materials are acetal homopolymers, polytetrafluoroethylene (PTFE), polypropylene, polystyrene, and the like.

Fig. 13 is a partial cross-sectional view showing a suitable storage container. Typically, the container is tall and thin and has vertical walls and large angled surfaces to minimize stacking and to stack the micro leds in a single desired location. This control of the point of accumulation allows the micro leds to be efficiently transferred in the container. If the container is transparent, optical density measurements can be conveniently used to monitor the homogeneity of the stirred suspension and adjust the micro led concentration by adding or removing solvent.

Fig. 14 is a graph showing the major diameters of exemplary micro leds and contaminants. The quality of the micro led suspension can be further improved by filtering to remove particles and debris. An exemplary micro led has a diameter of 42 micrometers (μm). Small particles below about 10 μm may be fragments of electrode metal pieces, interlayer dielectrics (ILDs), or dirt in the air from the electrode stripping process. These particles can be trapped in the well structure during fluid assembly, which can interfere with the micro led assembly or affect the bonding between the micro led electrodes and the substrate electrodes resulting in yield loss. The large fragments may be GaN fragments or smaller defects wrapped in the trapping medium released upon collection. The filtering process will remove all objects outside the size band centered on the micro led diameter.

As is clear from the figure, broken micro leds, which are comparable in size to good ones, cannot be removed by filtration, so it is important to selectively collect the broken micro leds before collection or to remove them. It is also important that the suspension of micro leds does not generate new broken micro leds due to excessive mechanical interaction between micro leds or between micro leds and the container and fixture.

A simple filtering method uses a mesh filter developed for cell collection to create the required band pass around the micro led size. First, the suspension is filtered through a 40 μm mesh to remove large debris including any trapping medium that escapes from the carrier wafer upon collection. The suspension was then filtered using a 20 μm mesh to capture the micro led and let the small particles pass into the waste container. The micro led is back flushed into a clean vessel by the cleaning solvent in the filter.

FIG. 15 is a schematic of an elution flow cell. The mesh filtration method is cheap and efficient, but there is a significant mechanical interaction between the micro LED and the filter, and during fine filtration the device is affected by shear forces in the build-up on the filter membrane. Fluid flow based filtration methods help to avoid potential mechanical damage. Since the hydrodynamic diameter of the micro led and the viscosity of the suspension solvent are known, an elution flow cell can be made to function as a filter as shown in the figure. The microLED suspension is introduced into the top of the first filter column (the μ LED supply), the solvent flows in from the bottom of the column and at a rate (stream i) that pushes all particles in the column below the critical size of about 50 μm upwards, while the larger particles settle to the bottom of the column where they can be collected and discarded. The small size fraction flows through the transfer channel to the top of the second elution column. In the second column, flow 2 is adjusted so that particles smaller than about 30 μm are forced upwards out of the waste channel, while the micro leds settle at the bottom of the column where they can be collected for micro led display screen assembly.

FIG. 16 is a schematic of a continuous flow staged filtration process. The continuous flow fractionation method is similar to an elution flow-splitting cell, and utilizes the difference of the sedimentation rates of different particles to separate good micro LEDs from small particles. As shown, the carrier flow is adjusted so that particles smaller than about 30 μm flow out of the upper waste gate, while the higher drop rate micro led flows out of the lower port.

Efficient fluid assembly requires a uniform distribution of the micro leds on the display substrate and the number of micro leds must be sufficient to fill all available assembly sites (also referred to as trap sites or wells). In practice, the optimal number of micro leds is greater than the number of assembly positions. If the number of micro LEDs is below the optimum, the assembly time will increase because the micro LEDs must go farther to reach the empty well site for assembly. However, if the number of micro leds is higher than optimal, the devices tend to come together, interfering with the assembly process. Furthermore, all excess micro leds must be removed after assembly, so if too many micro leds are dispensed, the clean up time increases and more micro leds are included in the recycling process. Therefore, it is important that the suspension dispensing process is based on a microLED suspension with a known and well controlled number of microleds per unit volume.

Since the variation in the number of micro leds in an aliquot (aliquot) increases with suspension concentration and non-uniformity, the concentration of micro leds in the suspension must be adjusted to ensure that the correct number of micro leds is transferred to the display substrate. By calculating the collection area after removing the defect region, the number of micro leds collected from the carrier wafer can be well determined. The concentration of the suspension can then be set simply by adding the appropriate volume of solvent during the final exchange. However, the concentration variation is caused by solvent evaporation, taking an aliquot for dispensing, and returning the recovered micro led to the suspension. A system for accurately determining the concentration of a suspension in order to control the concentration is necessary.

Returning to fig. 2, for a radially symmetric transparent container, the concentration varies only in the z-axis, so to quantify the micro LED concentration for several heights in the suspension container, pairs of collimated LEDs (or laser diodes) and photodetectors are used to emit light through the suspension and measure the density (optical opacity), expressed as log (I) in terms of intensity _in /I _out ). The amount of light attenuation at different heights in the suspension is directly proportional to the concentration of the micro led at these heights, so a calibration curve based on the size of the micro led is used to convert the optical density measurements to calculated concentrations. Optical density measurements were started immediately after stirring to produce a homogeneous suspension to determine the concentration. The measurement of the relationship to time and height in the vessel gives directly the sedimentation rate.

After about half the settling time, where detectors 210a and 210b receive full intensity, detector 210c returns to an intensity indicating 50% -60% of the uniform microLED density, and detector 210n sees a microLED density near the uniform state. When the suspension is undisturbed for a long time, all the micro leds are collected at the bottom of the vessel compared to the settling time, so that light scattering is minimal and each intensity measurement is maximal. If the suspension is well stirred, the micro leds will be evenly distributed throughout the liquid column with minimal light scattering at each height. The micro leds start to fall under the influence of gravity after the stirring stops until they reach the terminal speed. With increasing time, the micro led concentration at the top of the fluid column decreases and the detector intensity increases.

Fig. 17A to 17C are graphs depicting exemplary measurements of optical transmittance versus time for suspensions of 42 μm diameter micro leds in 20mL isopropyl alcohol (IPA), graphs of exemplary measurements of optical transmittance versus time for 130 million micro leds in different volumes of IPA, and calibration curves of concentration versus optical transmittance for suspensions of micro leds of the system, respectively. Optical density is defined herein as the inverse of transmittance, and the data is divided by the final intensity for normalization purposes. In fig. 17A, approximately 120 ten thousand micro leds were suspended in a cylindrical translucent tube having a diameter of 27.5 millimeters (mm), and the optical density was measured at five different vertical positions. At time zero, the tube is mechanically vibrated to agitate the suspension, thereby producing a uniform distribution of micro leds in the fluid column. During stirring, and the first few seconds after stirring, significant noise is generated due to air bubbles in the liquid. The light intensity measured after stirring was about 60% lower than that of the settled suspension. When all the micro leds settle out of the sensing hole, the recovery to full intensity is 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, t ₁ 23 seconds and the bottom sensor, located 33 mm from the top of the liquid column, is at 64.5 seconds (t) ₅ ) And (4) internal recovery. Thus, the terminal velocity of these micro leds in IPA is 0.51 mm/s to 0.56 mm/s, so the settling time of a 45 mm liquid column is about 85 seconds, which takes several seconds to settle after dispensing into a thin liquid layer.

The transmittance measurement system can also be used to determine the number density of micro leds in the suspension, which is key information for accurate processing of the suspension. In fig. 17B, the transmittance was measured as a function of time after stirring five different suspension dilutions. A microLED 42 μm in diameter was measured at a 13mm position in 20mL of IPA and then a measured amount of liquid was added to prepare a less dense suspension. In FIG. 17B, the intensity of light after stirring for the highest concentration (C) ₁ ) Is the lowest and increases with each successive decrease in density. Due to the increase of the height of the liquid column, the liquid column is in the containerThe settling time at a fixed location on the vessel also increases.

Fig. 17C is a graph of a number of experiments showing a calibration curve for determining the number of micro leds per ml of liquid based on the optical transmittance of a homogeneous suspension. It can also be seen that the homogeneity of the suspension is high within a few seconds after stirring and that the duration of this state decreases with decreasing distance to the top of the fluid column. It is desirable to withdraw an aliquot from the system a few seconds after stirring and more than 20 mm below the top surface. The characterization system can be used to select the optimal process parameters for different micro led sizes, container sizes and container shapes and suspensions.

The mixing of the micro led suspension is a balance between the force required to achieve high suspension homogeneity and the shear force to prevent breakage. Mixing can be performed by external stirring, where a smooth container wall impacts the suspension creating a fluid flow inside the suspension container that agitates the micro led. Internal agitation may be achieved by introducing a solvent or gas stream into the suspension vessel to induce turbulent fluid flow. Mixing may also be performed by, for example, a pipette by quickly removing and injecting liquid. The aim is of course to produce a uniform distribution of the micro leds on the vertical column of the container without damaging the micro led devices.

From the well-mixed suspension, a controlled volume containing the number of LEDs for a single-pass dispensing path can be extracted. The distribution path may be a single point, a single line segment, a serpentine path, or some combination of paths. Multiple distribution paths are used to ensure a complete and uniform distribution over the display element area. Since the settling time of LEDs is very short, especially in thin layer fluids for assembly, the lateral diffusion of the distribution path is limited to the millimeter level. Therefore, a uniform distribution of the micro leds requires a plurality of relatively close distribution paths.

Fig. 18A and 18B compare dispense density gradients for single and double pass/double speed dispense paths, respectively. The exact method of transferring the dispensed amount from a well-mixed suspension depends on the characteristics of the system and product requirements: in particular, the size of the assembled substrate, the speed of travel of the dispensing head, the thickness of the assembled fluid above the substrate, the uniformity of mixing, and control of the volume transferred are important. These considerations are typically a tradeoff between uniformity and cost, including system expense, processing time, and product yield. Defects in dispensing uniformity can also be compensated for during assembly, and a number of methods have been proposed. For example, a given path may quickly trace back a single line segment multiple times to compensate for uneven dispense rates (e.g., as may be caused by settling within the dispense head) and achieve uniform density along the path as shown in fig. 18B.

Fig. 19A-19C are schematic diagrams depicting the transfer of well-mixed suspension directly from an initial source container to an assembly baseplate by decanting from the container, nozzle, and tube, respectively. Transfer of well-mixed suspension, limiting or completely avoiding the use of tubing and fittings, may use direct transfer from a source tank or withdrawal of a discrete aliquot of the source suspension by a pipette tip. This approach avoids the build up of dead zones in the micro led in the fitting and limits the interaction of the micro led with the suspension to the following surfaces: a) small, b) cleanable, c) possibly disposable. Pipette tips may draw suspensions from the canisters by volumetric displacement (e.g., by a plunger) or by actively applied vacuum. The advantage of applying a continuous vacuum is that once the aliquot of suspension is no longer in contact with the source suspension, ambient atmosphere is drawn into the aliquot as bubbles, thereby actively mixing the suspension during transfer.

To limit yield loss, high concentration suspensions are preferred until just before introduction of the substrate. In this regard, the above-described aliquoting method may 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 by standard systems (including tubing, valves and fittings) since the pure liquid is not limited by the micro led suspension. Suspension aliquots can be corrected by drawing liquid into the dispensing head or by depositing the suspension into a suspension tank and then drawing the mixture back into the dispensing head. The diluted suspension may then be transferred to a substrate.

The above described processes for mixing, transferring and diluting are general and their variants can be chosen to constitute a unified system and application method optimized for the type of micro led display being produced. Several complete allocation procedures are described in detail below as examples.

In fig. 19A, the desired number of LEDs can be dispensed directly from a well-mixed container to an assembly substrate. This method has a minimum of transfer steps and may have a minimum of surface exposure to undesirable stiction of the micro led. Since the fully settled state of the suspension is not uniform, the system mixes the suspension to a uniform suspension density, so that the transfer of the volume of suspension corresponds to the transfer of the number of micro leds. The well-mixed suspension may then be transferred directly onto a substrate. Pouring the micro LEDs onto the substrate creates a significant flow throughout the fluid on the substrate that can quickly transfer the LEDs over the full range of large areas. However, uniformity can be poor, requiring additional assembly time to compensate.

In fig. 19B, a small area nozzle paired with appropriate pressure control in the vessel headspace can transfer the suspension in a more controlled manner, although this approach is significantly slower than pouring. This method also requires translating the micro led container on the assembly substrate more accurately than the pouring method.

In fig. 19C, rather than translating the container, a tube may be used with one end submerged in the suspension and the output end translated onto the assembly substrate. The advantage of this method is that the suspension container can be actively mixed during dispensing and that it is much easier to translate the tip than the container. Multiple tubes can be from the same micro led suspension vessel, thereby significantly increasing the area of distributed coverage in a single pass. The disadvantage of this method is that the surface area with which the suspension interacts is very large and thus micro leds are trapped in the tubing. This method of using limited piping is the best option in applications where cross-contamination is not a concern.

Figure 20 is a schematic showing controlled volume pipetting from a well-mixed suspension to an assembly substrate. As shown schematically, a pipette may draw an aliquot from the well-mixed suspension, rather than dispensing directly from the container for direct dispensing onto the substrate. This method utilizes mature 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 pouring spouts, nozzles, etc.

When precise volume and prevention of cross-contamination are important, it is preferable to use a pipette transfer method, for example, when the use of redundant micro leds in assembly is minimized, or when different, proprietary-sized micro leds are assembled sequentially. The trade-off is that pipettes dispense slower than other methods because pipettes need to return to the suspension container after each dispense path. Multi-headed pipettes exist, but they are not suitable for aspiration from a single source given the concentration of the suspension vessel and the mixing limitations of the suspension vessel.

Figure 21 is a schematic showing the corrected suspension in the intermediate tank and subsequent aspiration and dispensing through a bubble mix dispensing head. Another limitation of the direct transfer method and the pipette transfer method is that the dispensed volume is equal to the volume withdrawn from the source suspension container. To limit the size of the suspension container for large dispensing areas, it may be desirable to reduce the concentration of the suspension dispensed on the assembly substrate. As shown, one way to achieve this dilution is to draw a concentrated aliquot from the suspension supply vessel and modify it with neat liquid in the dispense head or intermediate tank.

Fig. 22 is a schematic diagram of a parallel dispensing method using an array of dispensing heads. The use of an intermediate tank has the potential advantage of separating the dispensing head pickers from the suspension container pickers. Thus, the system shown in the figures can be scaled up to an array of dispense heads that are pulled from an array of suspension tanks. The creation of the suspension tank chassis with precisely controlled concentrations of micro leds can be done in a separate step, and then the chassis is loaded into the assembly tool together with the assembly substrate.

All three described methods are variants of the core concept of the efficient transfer of micro leds. The fluidic assembly can be used for various micro led sizes, assembly areas, and pixel pitches. The following are some examples of how variations in assembly requirements affect method selection:

for monochrome assembly of small area substrates, direct dispensing may be required.

Batch assembly of multiple substrates in parallel from a single suspension vessel suggests the use of a pouring (tumbling) method.

The assembly of successive iterations of the substrate may be most suitable for the nozzle method.

Large-area assembly of large volumes with well-suspended micro leds has low batch-to-batch wavelength variation, and it may be most economical to use tube-mediated transfer from the source container of the suspension.

Pipette dispensing may be preferred for medium sized substrates (a few centimeters on a side) with valuable micro leds and low cross-contamination tolerance, such as color displays of sequentially assembled RGB three emitters. This is particularly the case when the dispensing time of the pipettor is not a significant part of the production time.

For oversized substrates beyond the 2 nd generation size (360 x 465 mm), single head dispense becomes exceptionally slow, and throughput requirements dictate parallel dispensing of arrays of multiple heads in a dispense system. For fast settling suspensions, the ability to mix the suspension in vacuum is important to improve the dispensing uniformity. Furthermore, for large substrates, the total dispense volume requirement becomes very high, and modifying the concentrated source suspension can improve the uniformity of processing and mixing.

Some of the key requirements for dispensing the micro leds from the suspension to the substrate are to limit the waste of micro leds due to breakage, surface adhesion and application non-uniformity. Thus, during the entire collection, filtration, mixing, distribution and recovery operation, the suspension preferably does not encounter valves, pumps or fittings. Inevitably, there will be some loss from the suspension vessel itself, but this loss is greatly mitigated by the thorough flushing, combined capture and recovery process. For the pipette transfer method, only the pipette tip is in contact with the suspension. The pipettor can be flushed inside and outside to recover the microLED, and can also be discarded to prevent microLED cross contamination.

Vacuum mixing dispense heads using disposable tips, and intermediate suspension tanks can be flushed for recycling, reused if cross contamination is not an issue, or replaced to prevent cross contamination. The embodiment depicted in fig. 19C uses a tube that is unlikely to be cleaned and this method is only as an option without considering cross contamination and micro leds are a good suspension to minimize contact with the tube sidewall.

Fig. 23 is a flow chart illustrating a method for selectively collecting micro led devices from a carrier substrate. For clarity, the method is described as a series of numbered steps, but the numbering does not necessarily indicate the order of the steps. It should be understood that these steps may be skipped, performed in parallel, or performed without the requirement of maintaining a strict order of sequence. In general, however, the method follows the numerical sequence of the depicted steps. The method starts at step 2300.

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

In one aspect, step 2305a inspects the carrier substrate to locate defective micro led devices, and step 2305b locates non-predetermined defective regions comprising a plurality of adjacent defective micro led devices. The inspection process may be performed by optical comparison, electroluminescent, photoluminescent or cathodoluminescent tests. Step 2306 then forms a solvent-resistant gel material covering the non-predetermined defect regions. In another aspect, in response to the inspection of step 2305a, step 2305c locates individual defective micro led devices that are not predetermined. Step 2307 then uses a laser trimming process to pop out individual defective micro led devices.

In one aspect, step 2309 applies additional power, such as fluid circulation, thermal energy, gravity, vibration, or a combination thereof, and step 2310 at least partially separates the micro led devices from the carrier substrate in response to the additional power.

In one aspect, dissolving the exposed adhesive in step 2308 includes selectively exposing the carrier substrate portion to an adhesive dissolving solvent. Then, separating the micro led device from the carrier substrate in step 2310 includes separating the micro led device from the selectively exposed portion of the carrier substrate. More specifically, selectively exposing the carrier substrate portion to a solvent may include the following substeps. Step 2308a rotates the carrier substrate in a solvent. Step 2308b exposes radial portions of the carrier substrate having a radius greater than d to a solvent. Then, separating the micro led device from the exposed portion of the carrier substrate at 2310 includes separating the micro led device from the radial portion of the carrier substrate.

In another aspect, collecting the valid microLED devices in the collection container in step 2314 includes replacing the adhesive dissolution solvent with a different liquid. If the valid microLED devices collected into the collection vessel 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 method of

steps

2315a and 2315b may use mechanical screens, elution, fractionation, or a combination of the above. For example, to perform high-pass filtering and low-pass filtering simultaneously, mechanical filtering may use two different mesh sizes. For elution and fractionation, the output port may be product or waste, based on the flow rate needs to be changed. Furthermore, there is no reason to use the same filtering method for both types of filtering. For example, a mesh filter may be used to remove large contaminants, followed by a classification tank to remove small particulate contaminants.

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 in step 2315 using the filtered solution to remove impurities from the filtered solution.

In another aspect, step 2314 replaces the adhesive-dissolving solvent with an assembly solution having a lower polarity or a higher evaporation rate than the adhesive-dissolving solvent. Surfactants may also be added to the assembly solution, such as anionic, cationic, nonionic surfactants, or combinations of the foregoing.

Fig. 24A to 24C are flowcharts illustrating a first method for spreading micro led devices on a light emitting display panel. The method starts in step 2400. Step 2402 adds the collected suspension of micro led devices to a transparent first container. Step 2404 stirs the suspension. Some examples of the agitation process include external vibration of the first vessel, creating a fluid flow in the suspension, and flowing a gas through the first vessel. Step 2406 optically measures the opacity of the suspension at multiple heights in the first vessel. In response to the optical measurement, step 2408 determines the homogeneity of the suspension. In response to determining that the uniformity is greater than the uniformity minimum threshold, step 2410 disperses the suspension on the top surface of the light emitting display panel.

In one aspect, step 2401a determines the number of micro led devices collected. For example, the number of micro leds collected from the carrier substrate may be known. Step 2409a counts a number of micro led devices per unit volume of the suspension, and dispersing the suspension on the top surface of the light emitting display panel in step 2410 includes depositing a known first number of micro led devices in response to dispersing the first volume of the suspension.

In one aspect, optically measuring the opacity of the suspension at the plurality of heights in the first vessel in step 2406 includes the substep. Step 2406a arranges a plurality of light emitting devices having a predetermined output light intensity, directed toward a central axis of the first receptacle, and spaced apart from each other a first predetermined distance along a first vertical axis. Step 2406b arranges a plurality of photodetectors spaced apart from each other by a first predetermined distance along a second vertical axis, each photodetector having an input directed toward an output of a corresponding light emitting device. Step 2406c compares the light intensity received by the photodetector.

Another alternative starts with a known number of micro leds (step 2401 a), step 2409a calculates a first number of micro led devices per unit volume of suspension, and determines the homogeneity of the suspension in step 2408. Step 2412 changes the ratio of fluid to LED devices in the suspension by changing a predetermined amount of fluid, and step 2414 optically measures the opacity of the suspension to calculate a second number of micro LED devices per unit volume of the suspension.

Dispersing the suspension on the light emitting display panel in step 2410 includes using one of the following dispersion processes: single step bulk dumping, multi-step pipette translation, nozzle-constrained container translation, and translating tubes. The multi-step pipette lateral dispersion process includes the following sub-steps. Step 2410a maintains homogeneity of the suspension above a minimum threshold for homogeneity in the first vessel. Step 2410b repeatedly withdrawing a predetermined aliquot volume from the first container using a pipette. After each aliquot draw, 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.

In another aspect, the first vessel is pressure controlled and includes a nozzle, and the nozzle-limited vessel translation dispersion process includes the following sub-steps. Step 2410e maintains homogeneity of the suspension greater than a minimum threshold for homogeneity in the first vessel. Step 2410f translates the first container a predetermined distance relative to the top surface of the light emitting panel, and step 2410g releases a predetermined amount of suspension from the nozzle per second during the translation.

In one aspect, the first vessel is pressure controlled and includes an output port connected to one or more delivery tubes, and the translating tube dispersion process includes the following sub-steps. Step 2410h maintains uniformity of the suspension greater than a minimum threshold of uniformity in the first vessel. 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.

A single-step mass registration dispersion process includes the following sub-steps. Step 2410k maintains the uniformity of the suspension greater than a minimum critical value for uniformity 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.

In one aspect, step 2401b determines the number of assembly locations in the first area of the top surface of the light emitting display panel. Then, dispersing the suspension on the top surface of the light emitting display panel in step 2410 includes depositing a first number of micro led devices at least equal to the number of assembly locations in the first area.

In another aspect, step 2409b determines the number of panning path iterations for the first region of the emissive display panel, and step 2409c determines the panning speed. Then, dispersing the suspension in step 2410 includes calculating a rate of dispersion of the suspension for the first volume in response to the number of iterations of the path and the translation speed to produce a uniform suspension density over the first area of the emissive display panel.

Fig. 25 is a flow chart illustrating a second method for distributing micro led devices on a light emitting display panel. The method begins at step 2500. Step 2502 adds the collected suspension of micro led 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 vessel heights. In response to the optical measurement, step 2508 calculates a first number of micro led devices per unit volume of the suspension. Step 2510 dispenses an aliquot volume 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 micro led devices per unit volume of suspension.

In one aspect, step 2509a transfers a known aliquot volume of the suspension to the 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 micro led 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.

In another aspect, step 2516 modifies the amount of suspension in the first container and, after the suspension is stirred, step 2518 optically measures the suspension density to calculate a fourth number of micro led devices per unit volume of suspension.

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

Claims

1. A method for selectively collecting micro led devices from a carrier substrate, wherein the method comprises:

providing an inorganic microLED device attached to a carrier substrate by an adhesive;

predetermining a defect region, wherein the defect region comprises a region consisting of a plurality of adjacent defective micro LED devices or a region consisting of a process control structure;

forming a solvent-resistant gum material covering the predetermined defect area;

dissolving the exposed adhesive with an adhesive dissolving solvent;

separating the micro LED device outside the predetermined defect area from the carrier substrate; and

maintaining the adhesive attachment of the micro LED devices within the predetermined defect area to the carrier substrate.

2. The method for selective collection of micro led devices from a carrier substrate according to claim 1, further comprising:

inspecting the carrier substrate to locate defective micro led devices;

locating a non-predetermined defective area comprising a plurality of adjacent defective micro led devices; and the number of the first and second groups,

wherein forming the solvent-resistant gel material comprises covering the solvent-resistant gel material in non-predetermined defect areas.

3. The method for selective collection of micro led devices from a carrier substrate according to claim 2, further comprising:

in response to the inspection, locating individual defective micro led devices that are not predetermined; and the number of the first and second groups,

and using a laser trimming process to knock out the single defective micro LED device.

4. The method for selective collection of micro led devices from a carrier substrate according to claim 2, wherein the method of inspecting to locate defective micro led devices comprises using an inspection process selected from optical comparison, electroluminescence, photoluminescence or cathodoluminescence tests.

5. The method for selectively collecting micro led devices from a carrier substrate as recited in claim 2, further comprising:

in response to separating the micro LED devices from the carrier substrate, collecting the active micro LED devices in a collection container.

6. The method for selectively collecting micro LED devices from a carrier substrate as recited in claim 5, further comprising:

applying additional power selected from the group consisting of fluid circulation, thermal energy, gravity, vibration, and combinations thereof; and the number of the first and second groups,

wherein detaching the micro LED device from the carrier substrate comprises at least partially detaching the micro LED device in response to the additional driving force.

7. The method for selective collection of micro LED devices from a carrier substrate according to claim 5, wherein collecting active micro LED devices in the collection vessel comprises replacing the adhesive dissolving solvent with another liquid.

8. The method for selectively collecting micro LED devices from a carrier substrate as recited in claim 7, wherein collecting active micro LED devices in the collection vessel comprises creating a suspension of active micro LED devices having an average cross-sectional physical dimension s, and the fluid has impurities therein;

the method further comprises the following steps:

after collection of the active micro led devices, filtration is performed to remove impurities having a maximum cross-sectional physical dimension greater than t, where t > s.

9. The method for selective collection of micro LED devices from a carrier substrate according to claim 7, wherein collecting the active micro LED devices in the collection vessel comprises creating a suspension of active micro LED devices having an average cross-sectional physical dimension s, and impurities are in the fluid;

the method further comprises the following steps:

filtering to remove impurities with a maximum cross-sectional physical dimension less than p, wherein p < s.

10. The method for selective collection of micro LED devices from a carrier substrate according to claim 8, wherein the filtration process is selected from the group consisting of mechanical screening, elution, fractionation, or combinations thereof.

11. The method for selectively collecting micro LED devices from a carrier substrate of claim 7, wherein replacing the binder dissolving solvent with another liquid comprises replacing the binder dissolving solvent with a filtered solution having a lower viscosity than the binder dissolving solvent; and the number of the first and second groups,

the method further comprises the following steps:

filtering to remove impurities from the filtered solution.

12. The method for selective collection of micro led devices from a carrier substrate of claim 7, wherein replacing the adhesive dissolution solvent with another liquid comprises replacing the adhesive dissolution solvent with an assembly solution having properties selected from the group consisting of: an assembly solution having a polarity lower than that of the binder dissolving solvent or an assembly solution having an evaporation rate higher than that of the binder dissolving solvent.

13. The method for selectively collecting micro led devices from a carrier substrate of claim 12, wherein replacing the binder dissolution solvent with the assembly solution comprises adding a surfactant selected from the group consisting of anionic, cationic, non-ionic surfactants, or combinations thereof, to the assembly solution.

14. The method for selectively collecting micro led devices from a carrier substrate of claim 1, wherein dissolving the exposed adhesive comprises selectively exposing portions of the carrier substrate to the adhesive dissolving solvent; and the number of the first and second groups,

wherein separating the micro LED device from the carrier substrate includes separating the micro LED device from the selectively exposed portion of the carrier substrate.

15. The method for selectively collecting micro led devices from a carrier substrate of 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 the number of the first and second groups,

wherein separating the micro LED device from the exposed portion of the carrier substrate comprises separating the micro LED device from the radial portion of the carrier substrate.

16. The method for selective collection of micro led devices from a carrier substrate according to claim 1, wherein exposing the micro led devices to the binder-dissolving solvent comprises the binder-dissolving solvent being selected from the group consisting of acetone, toluene, trichloroethane, N-methylpyrrolidone, xylene, cyclohexanone, butyl acetate, or combinations thereof.

17. A system for selectively collecting micro led devices from a carrier substrate, wherein the system comprises:

a vacuum chuck having a rotary interface for mounting a carrier substrate comprising an inorganic micro LED device attached to the carrier substrate by an adhesive;

an elbow connected to the vacuum chuck, the elbow having a plurality of selectable settings for determining an angle of rotation of the turntable in a plane in which the x-axis and z-axis lie;

a gantry coupled to the elbow, the gantry having a plurality of selectable settings for determining a height of the turntable along a z-axis;

a tray of adhesive dissolving solvent, the tray having a top opening to receive the carrier substrate;

a controller having an output connected to the gantry and the elbow to provide the setting of the height and the setting of the angle, respectively;

wherein, responsive to the gantry and the elbow being disposed, a selected radial portion of the carrier substrate is exposed to the adhesive-dissolving solvent; and the number of the first and second groups,

wherein the micro LED devices separated from the selectively exposed portion of the carrier substrate are collected in the tray.

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

19. A system for selectively collecting micro led devices from a carrier substrate as recited in claim 17, further comprising:

an inspection subsystem having a light input and an output connected to the controller for identifying individual defective micro LED devices on the carrier substrate; and, the system further comprises:

a trim laser having an input connected to the controller for accepting the second map of defective microLED devices and an output for ejecting defective microLED devices from the carrier substrate by laser radiation in response to the second map.

20. A system for selectively isolating regions of a carrier substrate of a micro led, wherein the system comprises:

a controller having an output for providing a first map of predetermined defect regions in the carrier substrate;

a printer having an input for receiving the first map and a nozzle for printing solvent-resistant gum material onto selected areas of the carrier substrate in response to the first map; and;

wherein the micro LEDs in the selected regions remain attached to the carrier substrate despite exposure to a binder-dissolving solvent.

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

22. The system for selectively isolating regions of a carrier substrate for a microLED according to claim 20, further comprising:

an inspection subsystem having a light input and an output connected to the controller for identifying areas of non-predetermined defective micro LED devices on the carrier substrate; and the number of the first and second groups,

wherein the printer receives a second map of the areas of non-predetermined defective micro LED devices from the controller and prints the solvent-resistant gel material to the areas of detected defective micro LED devices in response to the second map.