Laser Lift-off Process Research Articles

Advanced Mechanical Transfer of Micro-LEDs Enabled by Structurally Modified Wide Sapphire Nanomembranes through Thermal Reflow of Photoresist.

Micro light-emitting diodes (micro-LEDs) are pivotal in next-generation display technologies, driven by the need for high pixel density. This study introduces a novel methodology utilizing wide sapphire nanomembranes (W-SNM) as a dual-purpose template for high-quality epitaxial growth and the mechanical lift-off of individual micro-LEDs. Micro-LEDs grow individually on W-SNM, obviating the chip singulation process. By employing mechanical fracturing of the thin W-SNM, our method facilitates the transfer of micro-LEDs without the conventional laser lift-off (LLO) process. Previously introduced sapphire nanomembranes (SNM) have shown promise in enhancing epitaxial layer quality; however, they encountered challenges in managing micro-LED size variation and achieving efficient mechanical transfer. Here, we apply simple yet effective adjustments to the SNM structure, specifically, its elevation and widening. This strategic modification allows micro-LEDs to endure applied forces without incurring cracks or defects, ensuring that only the targeted W-SNM are selectively fractured. The mechanically transferred vertical 15 × 15 μm2 micro-LED device operates at an optimal turn-on voltage of 3.3 V. Finite element simulations validate the mechanical strain distribution between the W-SNM and GaN when pressure is applied, confirming the efficacy of our design approach. This pioneering methodology offers a streamlined, efficient pathway for the production and mechanical transfer of micro-LEDs, presenting new avenues for their integration into next-generation, high-performance displays.

Vertical InGaN Light-Emitting Diode with Hybrid Distributed Bragg Reflectors.

A vertical-type InGaN light-emitting diode with a resonant cavity was demonstrated with a 9 μm aperture size and a short cavity formed by hybrid distributed Bragg reflectors (DBRs). The approach involved designing epitaxial structures and utilizing an electrochemical etching process to convert heavily doped n-type gallium nitride (n+-GaN) layers into porous GaN layers as a porous-GaN DBR structure. Thirteen pairs of the conductive porous-GaN:Si/GaN:Si DBR structure provided a vertical current path in a vertical-type light-emitting diodes (LED) structure. The LED epitaxial layers were separated from sapphire for membrane-type LED structures through a laser lift-off process. During the free-standing membrane fabrication process, the dielectric DBR deposited on ITO/p-GaN:Mg layers was inverted from top to bottom, thereby establishing the concept of higher reflectivity for the bottom DBR compared to the porous-GaN DBR. The physical cavity length was reduced from about 2.3 μm for the LED membrane to 0.74 μm for the membrane-type LED with the embedded porous-GaN DBR structure. The divergent angles and line width of EL emission light were reduced from 124°/31.7 nm to 44°/3.3 nm due to the resonant cavity effect. The membrane-type LED structures with hybrid DBRs consisted of small divergent angles, narrow line width, and vertical current injection properties that have potential for directional emission light sources and vertical-cavity surface-emitting diode laser applications.

Flexible arrays of GaN-based Micro-LEDs fabricated on different substrates by a laser lift-off process

Fabrication of flexible Micro-LEDs is becoming an important technology for flexible displays, which plays a significant role in the field of visual communication in the upcoming Internet-of-Things era and metaverse. Here, we explore high-quality laser lift-off (LLO) methods for gallium nitride (GaN)-based Micro-LED arrays using LLO technology to separate Micro-LED arrays from sapphire substrates via a KrF excimer laser system (λ = 248 nm). At the same time, the damage of GaN under high laser energy density was analyzed. Ultimately, we used viscoelastic stamp transfer technology to successfully transfer Micro-LED arrays to various types of substrates, including tape and copper foil (Cu), thus enabling applications in different scenarios. The results show that the threshold voltage and electroluminescence (EL) intensity of Micro-LEDs on the tape and Cu substrates increased slightly, and the peak of the EL spectrum was basically stable at 458 nm. Furthermore, Micro-LEDs on polyethylene terephthalate (PET) can still work normally when the bending radius of PET is 4 cm.

Study of Thin Film Blue-Light MicroLEDs by Laser Lift-Off and Texture Process

Study of Thin Film Blue-Light MicroLEDs by Laser Lift-Off and Texture Process

Comparative Analysis of Optoelectrical Performance in Laser Lift-Off Process for GaN-Based Green Micro-LED Arrays.

This work explores the pivotal role of laser lift-off (LLO) as a vital production process in facilitating the integration of Micro-LEDs into display modules. We specifically investigate the LLO process applied to high-performance gallium nitride (GaN)-based green Micro-LED arrays, featuring a pixel size of 20 × 38 μm on a patterned sapphire substrate (PSS). Scanning electron microscopy (SEM) observations demonstrate the preservation of the GaN film and sapphire substrate, with no discernible damage. We conduct a comprehensive analysis of the optoelectrical properties of the Micro-LEDs both before and after the LLO process, revealing significant enhancements in light output power (LOP) and external quantum efficiency (EQE). These improvements are attributed to more effective light extraction from the remaining patterns on the GaN backside surface. Furthermore, we examine the electroluminescence spectra of the Micro-LEDs under varying current conditions, revealing a slight change in peak wavelength and an approximate 10% decrease in the full width at half maximum (FWHM), indicating improved color purity. The current-voltage (I-V) curves obtained demonstrate the unchanged forward voltage at 2.17 V after the LLO process. Our findings emphasize the efficacy of LLO in optimizing the performance and color quality of Micro-LEDs, showcasing their potential for seamless integration into advanced display technologies.

BaTiO3-assisted inorganic laser lift-off process for flexible micro-light-emitting diodes

BaTiO3-assisted inorganic laser lift-off process for flexible micro-light-emitting diodes

Laser Lift‐Off Technologies for Ultra‐Thin Emerging Electronics: Mechanisms, Applications, and Progress

AbstractDue to the advantages of non‐contact processing, high efficiency, high precision, and superior controllability, laser lift‐off (LLO) technologies exhibit excellent material applicability and process compatibility in the lift‐off of ultra‐thin electronic devices. This brings new opportunities for large‐scale manufacturing of emerging electronics that meet ultra‐thin, fragile, and poor high‐temperature resistance. However, this opportunity also comes with formidable challenges from the complex mechanisms of laser‐matter interactions, which hinder comprehensive exploration and fundamental understanding. By elaborating the laser‐material interactions, the photothermal effect, “cold” processing, shockwave effect, and cavitation effect in LLO process from multiple perspectives, this review is devoted to discussing the latest progress and sustainable application of LLO technologies in emerging electronics that are developing toward ultra‐thin, miniaturization, and integration, such as wafer level packaging, displays, energy harvesters, sensors, and memories. Finally, starting from the fundamental principles of material design, light field regulation, and intelligent control system, the challenges and new approaches faced by LLO technologies are explored and fully integrated with future innovative manufacturing concepts and device structures.

Mechanisms and Performance Analysis of GaN-Based Micro-LED Grown on Pattern Sapphire Substrate by Laser Lift-Off Process

The laser lift-off (LLO) process was employed to fabricate a monochromatic GaN-based green micro-light-emitting-diode (Micro-LED) 14 × 14 arrays grown on pattern sapphire substrate (PSS) in this research. A new experimental phenomenon and internal mechanism of some epitaxial residues on the top and sidewalls of PSS protrusion after laser stripping were deeply discussed. In addition, the reasons for the formation of micro lattice dislocation of epitaxial layer crystal caused by laser were also analyzed. The leakage current of the array devices after LLO was 200 times higher that before LLO under the same voltage, but the light output current efficiency was discovered to be 109.7% higher under the 500 cd m−2 brightness. The results indicate peak wavelength was blue shift 3 nm once the LLO process had been performed. Furthermore, the optical pattern from a “heart-shaped” distribution before LLO was changed into the maximum intensity directly above the array devices. The luminous angle was also reduced from ±75° to ±65°, which indicates that the collimation of the light after LLO process was increased. The results are high important for understanding the optical properties of Micro-LED array devices after LLO process.

Experimental and modeling study of controllable laser lift-off via low-fluence multiscanning of polyimide-substrate interface

Fabricating large-area thin-film flexible electronics rely on the peeling process from rigid prefabricated carriers. The well-established laser lift-off (LLO) employs a shaped excimer laser to scan the interfacial polyimide (PI) for peeling. However, using the traditional LLO (high-fluence single scanning), nondestructive peeling of ultra-thin fragile devices remains challenging. Here, an enhanced LLO strategy based on multiple low-fluence irradiations with an excimer laser (m-LLO) is studied to perform noninvasive and controllable peeling. We first construct a model predicting the core parameter of the peeling effect, the amount of gas accumulated at the interface generated by multiple backside irradiations. The specific influence of gas on the peeling effect is obtained through the accurate assessment of the interface bonding state by profile analysis, micro-observations, and strain sensing. We establish a deterministic peeling method by providing the peeling index that can precisely evaluate the interface bonding state under different process parameters, showing advantages over traditional LLO. The characterizations of chemical modifications after peeling further confirmed a different process mechanism of m-LLO. These results would be helpful to develop a non-destructive and controllable LLO process for the mass production of the next generation of flexible electronics.

Strain-induced piezotronic effects in nano-sized GaN thin films

Flexible devices have attracted considerable attention because of their very productive applications in the diverse areas such as healthcare, artificial intelligence, and robotics. Accordingly, in this study, we fabricated a flexible nano-sized GaN thin-film device via a double-transfer method using a laser lift-off process from sapphire with thermal tape and then with carbon tape onto a polycarbonate substrate. With the application of compressive/tensile strains, the flexible nano-sized GaN thin-film device can be mechanically controlled by the piezotronic effect. The device has excellent electromechanical performance with a gauge factor of 2206 at a compressive strain of 0.32%, which is nearly 11.11 times greater than that of a conventional strain-gauge value. Furthermore, the device shows an effective ON condition in the compressive direction and OFF condition in the tensile direction at an applied bias voltage of 0.5 V. The fabricated device has potential for innovative and advanced applications such as human–machine interfaces, biomedical diagnoses, prostheses, intelligently controlled rooms, and active flexible electronics.

Advances in strain engineering on oxide heteroepitaxy

Advances in strain engineering on oxide heteroepitaxy

Laser lift-off of polyimide thin-film from glass carrier using DPSS laser pulses of top-hat square profiles

Laser lift-off (LLO) process using a diode pumped solid state (DPSS) laser with wavelength of 355 nm and pulse width of 37 ns was investigated experimentally and theoretically for non-contact delamination of the polyimide (PI) substrate from the glass carrier. Each pulse emitted from the DPSS laser was shaped into a top-hat square beam by an optical system including a diffractive optical element lens and a spatial filter. The typical FWHM size of the top-hat beam was 315μm×328μm. The train of pulses was irradiated on the specimen in the form of a quasi-line beam through a scanner system for two representative pulse repetition rates (PRR) of 1 and 150 kHz. Experimental observations and theoretical simulations using the photothermal ablation model clarified that the intensive laser pulse induced extremely fast ablation inside the PI substrate, which caused the PI to delaminate internally along the PI/glass interface, leaving a residual PI layer of about 100 nm in thickness on the glass carrier. The successful non-overlap LLO by each pulse was confirmed optically by the Fizeau fringes at the threshold fluence of 240 mJ/cm2. The overlapping irradiation of laser pulses, which minimized overlay or stitch errors, was able to lower the threshold fluence of each pulse by about 50% compared to the non-overlapping irradiation by a single pulse. With increasing PRR for the same number of irradiation overlaps, the threshold fluence was reduced considerably due to the thermal accumulation effect, but the thickness of the residual PI layer remaining on the glass carrier increased slightly.

Buckling on Fe‐Doped AlGaN/GaN High Electron Mobility Transistor Films after Laser Liftoff Process: Phenomena and Mechanism

The influence of Fe in the buffer layer on the laser liftoff (LLO) of GaN high electron mobility transistors is presented herein. LLO is performed on Fe‐doped GaN films using a KrF excimer LLO system. The morphology, crystal characteristics, and 2D electron gas (2DEG) characteristics of the GaN films after LLO are characterized using a 3D optical microscope, X‐ray diffraction, and Hall methods. The present work is different from previous methods because of the large‐area buckling instead of cracks on the Fe‐doped GaN films. The results show that the crystal quality and 2DEG characteristics of the buckled parts are significantly affected. Furthermore, a modified simulation model of the temperature and thermal‐induced stress fields in the films is used to explain the buckling of the films, whose results are consistent with the characterization results. The additional absorption centers induced by the Fe impurity energy levels increase the gradient of temperature field, thereby increasing the compressive stresses in the films at the edges of the laser spots. Moreover, a higher temperature is observed at the interface between the film and the bonding glue, which results in the debonding. Therefore, Fe‐doped GaN films are more prone to buckling than unintentionally doped GaN films.

Miniaturized LED light source with an excitation filter for fluorescent imaging

In this study, we developed a miniaturized InGaN blue μLED-based excitation source intended for an implantable lens-less brain imaging system. To enhance its implantability, the μLED was thinned further through the laser lift-off process. A filtering component comprising a short-pass interference filter and a low-NA fiber optic plate (FOP), was loaded onto the μLED. The addition of an interference filter reduced normal incident green-end band emissions from the μLED by two orders of magnitude. The application of FOP enables the resolution of the angle-dependent problem of the interference filter. Further, a fluorescent imaging experiment was carried out on the proposed excitation source. The proposed device produced bright fluorescence images that are sufficient for implantable in vivo application. The thickness of the proposed excitation device was 180 μm, making it suitable for implantable brain fluorescent imaging applications.

Investigation of the Chemical Structure of Ultra-Thin Polyimide Substrate for the Xenon Flash Lamp Lift-off Technology.

Lift-off is one of the last steps in the production of next-generation flexible electronics. It is important that this step is completed quickly to prevent damage to ultrathin manufactured electronics. This study investigated the chemical structure of polyimide most suitable for the Xe Flash lamp–Lift-Off process, a next-generation lift-off technology that will replace the current dominant laser lift-off process. Based on the characteristics of the peeled-off polyimide films, the Xe Flash lamp based lift-off mechanism was identified as photothermal decomposition. This occurs by thermal conduction via light-to-heat conversion. The synthesized polyimide films treated with the Xe Flash lamp–Lift-Off process exhibited various thermal, optical, dielectric, and surface characteristics depending on their chemical structures. The polyimide molecules with high concentrations of –CF3 functional groups and kinked chemical structures demonstrated the most promising peeling properties, optical transparencies, and dielectric constants. In particular, an ultra-thin polyimide substrate (6 μm) was successfully fabricated and showed potential for use in next-generation flexible electronics.

Wafer‐Scale Micro‐LEDs Transferred onto an Adhesive Film for Planar and Flexible Displays

AbstractThe development of micro‐sized light emitting diode (LED) displays has driven the research of micro‐LED mass‐transfer technology. To date, various transfer technologies are proposed, but ample room for improvements in the transfer yield and transfer accuracy still remains. Furthermore, whether these techniques are suited for the subsequent bonding process is not well investigated, which is essential for achieving a good electric connection between micro‐LEDs and driver electronics. Here a systematical solution, termed as “tape‐assisted laser transfer,” which is not only suited for high‐yield micro‐LED transfer but also well compatible with subsequent bonding process, is developed. Using a low‐cost adhesive tape as the support substrate, the method allows fast and wafer‐level transfer of micro‐LED with extremely high yield (≈99.8%) and minimized transfer displacement (<0.5 µm), based on a laser lift‐off (LLO) process. Combined with a shadow mask, the LLO process also allows the selective transfer of micro‐LED to the tape. Such thin film micro‐LEDs are well compatible with the subsequent “bumpless” bonding process using low‐melting point solder. Representative display devices including planar display and deformable display are further developed, suggesting the method has a good potential for developing high‐resolution micro‐LED display panels for the applications in VR/AR, wearables, and smart glasses.

Strain-Controlled Flexible Graphene/GaN/PDMS Sensors Based on the Piezotronic Effect.

Using simple graphene transfer and the laser lift-off process for a non-centrosymmetric GaN layer on a flexible polydimethylsiloxane (PDMS) substrate, the piezotronic effect by strain-induced current-voltage measurements at the two end points is studied. By inducing compressive strain on the flexible graphene/GaN/PDMS sensor, the Schottky barrier between the graphene and GaN/PDMS heterojunction can be electro-mechanically modulated by the piezotronic effect. It is observed that the flexible graphene/GaN/PDMS sensor is sensitive to various applied compressive and tensile strains in the positive/negative bias scans. The sensor is extremely sensitive to a compressive strain of -0.1% with a gauge factor of 13.48, which is 3.7 times higher than that of a standard metal strain gauge. Furthermore, the sharp response of the flexible graphene/GaN/PDMS sensor under the -0.1% compressive strain is also investigated. The results of this study herald the development of commercially viable large-scale flexible/wearable strain sensors based on the strain-controlled piezotronic effect in future investigations.

Photoassisted chemical smoothing of AlGaN surface after laser lift-off

To fabricate AlGaN-based ultraviolet (UV) vertical cavity surface-emitting laser diodes, a pair of distributed Bragg reflectors (DBRs) having a smooth surface is desired to have a high quality factor (Q). In this work, photoassisted chemical etching (PCE) was attempted to smoothen the -c(000-2) AlN surface after removing the sapphire substrate by means of the laser lift-off process. First, the conditions for PCE were optimized using an Xe lamp and KOH solution. The root-mean-square roughness of the -c(000-2) AlN surface was reduced from 30.5 to 5.6 nm, which enables us to fabricate an improved HfO2/SiO2 DBR on the AlN surface. Then, using the optimized PCE technique, improved UV (Al,Ga)N planar microcavities were fabricated and distinct cavity-mode-related emissions were observed using the photoluminescence (PL) technique. By comparing the PL spectra between the samples with and without PCE treatment, the Q value at 303 nm for the case of the improved DBR was determined to increase from 174 to 270. The increase in the Q value is mainly attributed to the reduction of scattering losses in optical cavities. Furthermore, the discussion on the mechanism of improved surface during the PCE treatment is given. Consequently, PCE is demonstrated to be a feasible approach to refine the quality of ultraviolet nitride microcavities.

Damage-Free Transfer of GaN-Based Light-Emitting Devices and Reuse of Sapphire Substrate

Gallium nitride (GaN) based semiconductors have been gaining worldwide attention in optoelectronics application, notably information displays and solid-state lighting. GaN light-emitting diodes (LEDs) epilayers are typically grown on sapphire substrates with metal-organic chemical vapor deposition (MOCVD) technique due to the absence of native nitride substrates. In this paper, the mesh-patterned sapphire substrates are fabricated to speed up the laser lift-off efficiency for the separation of GaN films from sapphire substrates. Damage-free GaN thin films fabricated on the sapphire substrates were successfully released and transferred onto Si substrates with sufficient quality through a Ti/Pt/Au/In bonding and subsequent laser lift-off processes. The GaN film mounted on Si substrates exhibits almost no attenuation in PL intensity before and after its transfer. The released sapphire substrate can be reprocessed by annealing & chemical mechanical polishing (CMP) process for reuse as epitaxial substrate with considerable cost reduction, featuring a comparable photoluminescence performance with fresh sapphire substrates.

Ablation of polyimide thin-film on carrier glass using 355 nm and 37 ns laser pulses

Ablation of the polyimide (PI) thin-film deposited on glass substrate was investigated theoretically and experimentally to find out the important characteristics during the laser-lift off (LLO) process for the fabrication of flexible electronics. Instead of using conventional ablation induced by the front irradiation on the specimen, the LLO process employs the backside irradiation on the specimen transferred through the glass substrate, assuming that the ablation occurs volumetrically in the narrow region inside the PI film near the PI and glass interface. Experimentally the PI film on glass was exposed to a pulse laser beam emitted from UV laser of 355 nm in wavelength and 37 ns in duration. Depths and morphologies of the ablation spots were observed by the surface profiler and the FE-SEM. Theoretically the ablation process was estimated by the photothermal mechanism, using the one-dimensional heat conduction equation with volumetric absorption of the laser beam. It was shown that the experimental ablation could be described through the theoretical analysis based on the surface and the bulk photothermal models. While the former assumed that the ablation occurred only on the ablation front surface, the latter assumed the ablation was accompanied by the thermal and volumetric bond breakage. In general, the theoretical calculations agreed well with the experimental observations. In particular, the experimental observations of the ablation for the backside irradiation case that was the core phenomenon in the LLO process was explained appropriately by the features predicted using the bulk model.