Share full-text access Please review our Terms and Conditions of Use and check box below to share full-text version of article. I have read and accept the Wiley Online Library Terms and Conditions of Use Shareable Link Use the link below to share a full-text version of this article with your friends and colleagues. Learn more. Copy URL High-precision automation equipment enables mass production of microLED displays using laser-based transfer techniques. MICROLED TECHNOLOGY IS REVOLUTIONIZING THE display industry by offering superior image quality, energy efficiency, and longevity compared to traditional LCD- and OLED-based displays. MicroLEDs, composed of inorganic materials, are robust and resistant to degradation over time. They provide the potential for exceptional brightness, a wider color gamut, and higher contrast ratios, resulting in vibrant and lifelike visuals. Manufacturing microLEDs typically involves depositing semiconductor materials onto a growth substrate. Once fabricated, microLEDs must be transferred from individual red, green, and blue (RGB) growth substrates to the final display backplane via a series of mass-transfer techniques. Conventional pick-and-place manufacturing techniques are ineffective for die with dimensions of 50 µm or less; however, alternative approaches overcome the mechanical limitations of manipulating such small devices. This article summarizes the laser-induced forward transfer (LIFT) process (that enables high-volume mass transfer of microLEDs), identifies commercially available LIFT manufacturing equipment's technical challenges, and discusses how to meet the display industry's future needs. Enabling MicroLEDs’ Mass Transfer To make a full-color microLED display, each pixel must contain at least one RGB microLED along with the associated driving circuitry on the display's backplane. The fundamental physics behind lasing semiconductors drives the need for a mass transfer step in the manufacturing process. At a high level, direct bandgap semiconductor materials lase more easily than indirect bandgap materials, where the wavelength of the emitted photon, λ, is inversely proportional to the bandgap energy, E (given by E = hv = hc/λ, where h is Plank's constant and c is the speed of light in the medium). To achieve a specific color of emitted light, a semiconductor material with the corresponding bandgap energy must be used to fabricate a given LED, resulting in three distinct Group III–V wafers (one for each display color: red, green, and blue). Once the die are singulated, they must be transferred from high-density spacing on a wafer to lower-density pixel spacing on the display's backplane. Transferring microLEDs with dimensions of 50 µm per side or less cannot rely on traditional electronics packaging processes that use flip-chip techniques or pick-and-place systems. While mature, these automation processes are designed for assembly and packaging of electronic devices with dimensions of 100 µm or larger. They rely on calibrated mechanics, machine vision, and active alignments to achieve micrometer-level placement accuracies over small distances. More importantly, they use vacuum wands, grippers, and pick-and-place technologies that are rendered ineffective for die with dimensions <50 µm, where stiction forces between the die and tooling are dominant compared to the force of gravity and motion-induced forces. Fortunately, industry and academia have developed numerous methods for mass transfer of semiconductor layers with dimensions <50 µm and thicknesses on the nm to single-µm level. These methods also are effective for transferring microLEDs on display backplanes; however, throughput and yield must be dramatically improved for microLED transfer to become a cost-feasible solution before consumer electronics companies begin designing microLED displays into their products. A survey of microLED transfer technologies 1 displays the relative throughput and yield of competing microLED transfer methods (Table 1 ). It includes some of the most promising manufacturing methods, such as laser-based transfer (laser lift off [LLO] and LIFT), roll-to-roll transfer, and elastomer stamp transfer technologies. Table 1. Summary of mass-transfer techniques and their relative performance. * 1 Transfer Method Laser-based Transfer yield 99.99%. The transfer rate is ≈ 100 million per hour. No impurities transfer on substrate surface. Laser source can damage their transfer stability. > 1 µm Optovate/Uniqarta 7 Flexible to use and have perfect repeatability. Due to electrostatic, the charges are induced which can degrade device performance. 1-100 µm Apple/Luxvue 8 Fluidic-based assembly Transfer yield ≈ 65%. The transfer rate is ≈ 50 million per hour. Economical, easy to operate and minimum parasitic effect. 5 Inefficient, the probability of pixel damages during transfer is high. > 20 µm Foxconn/eLux 9 Elastomer stamp (van der Waals) Transfer yield 99.99%. The transfer rate is ≈ 1 million per hour. It can be transferred by efficient and economical way because of its stickiness (elastomer stamp) nature. It has poor repeatability because the stamp adhesion force is controlled by peeling speed. It is optimized by a magnetorheological stamp. 6 > 10 µm X-Celeprint 10 Transfer yield 99.99%. The transfer rate is ≈ 10,000 per second. Economical, high efficiency and high throughput Probability of device damages is high. ≈ less than 100 µm 2 Korean Institute of Machinery and Materials 11 , 12 * The detail about performance (transfer rate) for each mass transfer technology is given, 2 , 3 along with chip size. 4 Since the publication of Table 1 in 2022, several leading microLED technology companies have made dramatic improvements in their throughput rates, making elastomer stamp and laser-based transfer technologies more promising for high-throughput transfer of microLEDs to display backplanes. Improvements in throughput generally have resulted from process automation improvements, including parallel transfer of arrays of microLEDs onto larger substrates with more than 1 million die transferred in a single step. However, as substrate size increases, placement accuracy becomes more difficult, and yield tends to suffer. Therefore, replacing and repairing individual microLEDs across the backplane also must be considered when developing an automation process. That said, laser-based processes have an advantage over other transfer methods because individual die can be targeted to avoid transferring known bad die and to repair defects on the individual pixel level. Employing Laser-Based Mass Transfer Laser-based mass transfer for microLED display manufacturing typically occurs in two steps. The first step relies on LLO, a process that removes the microLEDs from their sapphire growth substrate, leaving them attached—at their original high-density spacing—to a sacrificial layer called the dynamic release layer (DRL) and a secondary substrate. To effectively release the LEDs from the growth substrate, the laser wavelength must be transparent to the sapphire substrate yet absorbed by the microLEDs because of their differences in band gap energy. Laser ablation of the LED and sapphire interface causes separation of the materials. 2 Once the microLEDs have been released from the growth substrate through LLO, the wafer can be flipped and prepared for transferring the microLEDs from their high-density spacing to the display backplane's low-density pixel spacing using the LIFT process. To accomplish this, the donor microLED wafer must be positioned directly above the display backplane such that microLEDs on the donor are aligned with their final placement positions on the backplane. Next, a laser beam is positioned such that the DRL is selectively irradiated behind each microLED to be transferred. When lased, sublimation of the DRL generates a pressure differential that propels the chip away from the donor, transferring it onto the backplane below (Fig. 1 ). This process is repeated until each pixel on the backplane is fully populated with at least one RGB microLED. 13 The process is directional and takes advantage of gravity in the direction of transfer. Fig 1 Considering Capital Equipment Solutions Currently, only a handful of companies are publicly offering turnkey systems for the transfer of microLEDs to display backplanes using the LIFT process. One system is limited to single-die transfer with dimensions >50 × 50 µm. This system uses a dual-bridge gantry design that moves the donor substrate and laser scanner above a stationary display backplane. The original equipment manufacturer's (OEM's) design minimizes the machine footprint but sacrifices LED placement accuracy because of error motions associated with the mechanical-bearing gantry configuration. Other similar designs offer transfer of microLEDs with minimum dimensions of 5 µm and placement accuracies on the order of +/−2.5 µm. These systems are capable of executing LLO, LIFT, and single-die repair processes on display backplanes up to Gen 2 (360 × 465 mm). Although it is unclear how these systems’ accuracy specifications are measured, the tight placement tolerances achieved are certainly the result of moving the substrate and wafer in plane using high-accuracy motion systems. Once the backplane and donor are aligned, they are exposed to a UV laser through a mask, allowing selective irradiation and transfer of microLED arrays from a single laser exposure. Such systems can transfer tens to hundreds of millions of microLEDs per hour. However, die sizes will begin to shrink below 5 µm, and display backplanes will increase up to at least Gen 6 (1,500 × 1,850 mm) to support mass production, leaving a large gap in precision automation capabilities between currently available solutions and the requirements for achieving micrometer-level placement of die on production-size backplanes. Technical Considerations for Increased Substrate Size As microLED sizes shrink and backplane sizes increase to support production-level manufacturing, precision motion systems will take center stage, enabling alignment and transfer of microLEDs with placement accuracies at the single-micrometer scale. This challenge will push the boundaries of existing precision motion technologies. Understanding the depth of this challenge requires addressing the motion requirements of each component in the LIFT process. First, consider positioning a laser beam with µm-level spot size onto the backside of the donor substrate. The spot's two-dimensional positional accuracy requirement depends on the beam shape (top-hat, Gaussian), microLED size, and laser-material interaction with the DRL. One spot-positioning technique places a mask in the beam path between the laser source and donor substrate (Fig. 2a ). A mask provides consistent spot size from run to run, allowing parallel transfer of hundreds of thousands of microLEDs at once. However, each product in manufacturing needs a unique mask because the microLED spacing may change based on product design. Additionally, a unique mask must be used for the repair process. Fig 2 Positioning a laser beam with µm-level spot size onto a donor substrate's backside. (a) Depicting a mask in the beam path. (b) Depicting a laser scan head with a diffractive optic for arrayed beam scanning. (c) Lens distortions over a laser scan head FoV. From a positioning perspective, the mask solution requires two-dimensional motion of the donor substrate to align the beam with the microLED pattern before transfer. This limits the total throughput to the motion stages’ dynamic capabilities. Alternatively, a two-dimensional laser scan head can be used to position the beam on the donor substrate. Precision laser scan heads use lightweight mirror optics mounted on motors to position the beam through a lens and onto the donor substrate with high-dynamic performance. Laser scan heads offer orders of magnitude higher accelerations and velocities relative to positioning stages. They also are used with diffractive optics to position arrays of beams (Fig. 2b ), allowing beam positioning at a higher throughput than stage positioning can deliver for LLO, LIFT, and repair processes. 14 However, the f-theta lenses used on laser scan heads produce spot-size distortions at the edges of the field of view (FoV), limiting the working range of the scanner to the center of the FoV for the most stringent applications (Fig. 2c ). Second, consider the positioning of the backplane with respect to the donor substrate. When a new backplane and donor substrate are loaded, some misalignment occurs between their coordinate frames and respective XY motion stages. These alignment errors typically are measured with a vision system and compensated for in software. Next, the backplane and donor coordinate frames must be physically aligned using a rotation stage. This is typically a vision-guided process that minimizes angular misalignments between the two coordinate frames. Once the donor and backplane coordinate frames are aligned, the donor must be brought within <100 µm of the backplane. In some cases, a fly height of as little as 10 µm is required for accurate die placement. Tip, tilt, and z motion must be used to maintain parallelism between the donor and backplane during the LIFT process. Errors in parallelism between the two surfaces can lead to inaccurate final die placement and, in the worst case, collisions between the donor substrate and backplane. Motion system performance selection is critical here, as geometric error motions from the backplane motion system are amplified over the distance between the motion system carriage and the glass sheet's edge (Fig. 3a ). Fig 3 (a) Exaggeration of pitch of stage affecting parallelism. (b) Exaggeration of effects of gantry axis torsion on donor substrate positioning. (c) Image of a large-format planar air bearing. Mechanical bearings and gantries are not sufficient for achieving tight flatness, pitch, and roll tolerances needed to minimize parallelism error between the substrate and donor. As gantry axes move, torsion of the unsupported gantry axis can cause many micrometers of motion at the donor substrate on even the stiffest of gantries (Fig. 3b ). This results in parallelism and placement error between the donor substrate and backplane; thus, gantries typically are used on systems with larger microLED device sizes and looser placement accuracy requirements. Planar air-bearing systems typically are used to minimize vertical straightness, pitch, and roll error motions that influence parallelism over the length of the sheet of glass (Fig. 3c ). This effect is further pronounced as glass sizes increase from Gen 2 to Gen 3, 4, 5, and 6. Planar air bearings minimize geometric errors by creating a thin film of air between the stage body and a granite surface, providing a stiff, frictionless bearing surface that is fully supported throughout the XY range of motion. Furthermore, planar air bearings can provide high payload capacities of up to 100 kg and more for large glass sheets and can be manufactured with large travels over a meter in length. Although air-bearing solutions provide optimal geometric positioning, they are not sufficient to maintain single-µm level parallelism between the donor and backplane for large backplanes because of the amplification of errors over the glass sheet. Therefore, active tip, tilt, and z alignments that servo off of distance sensors between the donor and backplane are typically required. Once parallelism is established, the donor and backplane stages must position themselves such that the die are directly above their attachment position before the laser can fire, transferring the microLED onto the backplane. Machine vision typically is used to establish the initial relative position of the donor and backplane. Lower accuracy positioning systems can produce high-accuracy final microLED placement by using machine vision, if they are capable of making steps ∼10× smaller than the final microLED placement tolerance. However, this vision identification and alignment process is time consuming and will not allow for dynamic, on-the-fly transfer of microLEDs. Therefore, positioning systems with industry-leading two-dimensional accuracy are desired for blindly positioning the backplane beneath the donor. Again, planar air-bearing stages are the natural choice for highly accurate long-travel positioning because of the error-averaging effect of the thin film of air on a precision granite baseplate (Fig. 4 ). Horizontal straightness and yaw errors also affect two-dimensional position accuracy on the glass substrate. Horizontal straightness and yaw error calibration over stage travel can result in µm-level two-dimensional position accuracies over a Gen 5 glass substrate. Fig 4 Traditional methods require motion systems to settle into position before lasing. Transferring microLED via LIFT while the stages are in motion (or on the fly) will further improve throughput by eliminating settle times associated with the traditional step-and-settle approach. Processing on the fly requires coordinating many servo and scanner axes, position-based triggering of the laser, and minimization of following error throughout the stage and scanner move profiles. This need places increased importance on the controls system coordinating each element's motion. Modern motion controllers use high-speed processing to achieve higher precision performance when coordinating the motion of these devices. Certain OEMs also provide enhanced functionality allowing for direct coordination in positional space between the laser spot and motion of the stages, making processing on the fly a possibility. The Future of MicroLED Automation As microLED sizes shrink to 5 µm and smaller and backplane dimensions grow larger than Gen 2, existing mass-transfer solutions designed to populate backplanes with microLEDs will no longer be sufficient to meet manufacturers’ accuracy requirements. Large-scale adoption of microLED displays will require a breakthrough in precision automation equipment to enable LIFT population and repair processes. Large-travel, high-precision stages paired with laser scan heads and high-performance control systems will be required to support mass production of microLED displays. To break the barrier to microLED display manufacturing, motion suppliers and machine builders must demonstrate and continue to improve on their large-format precision stages’ dynamic performance capabilities and provide tools to improve laser scanner FoV with position-based laser triggering while maintaining a high level of coordination between servo and scanner axes. Biography Brett Heintz is the group leader of Aerotech's Applied Technologies. His development team focuses on programming precision manufacturing and inspection processes using the company's precision motion products. Heintz earned an MS in electrical and computer engineering from the University of Pittsburgh. He can be reached at [email protected] . 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