Laser Lift-off Research Articles

Sacrificial layer for laser lift-off process for flexible-display production

In this study, we developed a new sacrificial layer (SL) for laser transfer process. Metallic substrate i.e. invar foil was temporarily docked to a glass substrate using glass powder. To ensure successful delamination, the SL was pre-deposited between metal foil and glass substrate. For the first time, the SLs were amorphous gallium nitride and non-stoichiometric gallium oxide which were implemented for laser lift off (LLO) processes of metal foil. Bonding of metal foil to glass sheet was performed using heat treatment while debonding was achieved by LLO method. The laser wavelength was 355 nm which was the best fit for full absorption from SL layers. Transmission electron microscopy, element mapping, and energy dispersive X-ray spectroscopy analyses were performed for investigating elements’ migration and bonding-debonding mechanism.

Theoretical and experimental studies of laser lift-off of nonwrinkled ultrathin polyimide film for flexible electronics

This paper theoretically and experimentally studies an innovative laser lift-off (LLO) technique that utilizes a line-shaped laser to scan the interfacial polyimide (PI) film through the transparent substrate to peel off the thin-film electronics. A comprehensive illustration of the mechanism of the occurrence of wrinkles in ultrathin PI films is provided, based on which the LLO process for ultrathin flexible electronics is optimized. A mechanical model is established to describe the deformation of the laser-induced blister and validated by experimental results. The results show that the wrinkles are due to the plastic deformation inside the film, which is caused by the instantaneous pressure of gas products generated by laser ablation. By measuring the instantaneous pressure through piezoelectric sensors, the maximum allowable laser fluence without inducing plastic deformation has been found, which decreases linearly with the film thickness. Accordingly, the process window of the traditional single-pass LLO has been obtained, which confirms that the wrinkles in ultrathin PI films (<5 μm) are inevitable through traditional LLO process. To obtain non-wrinkled ultrathin films, two optimization approaches are proposed, which are also verified through the tests of strain sensors.

High-Power GaN-Based Vertical Light-Emitting Diodes on 4-Inch Silicon Substrate.

We demonstrate high-power GaN-based vertical light-emitting diodes (LEDs) (VLEDs) on a 4-inch silicon substrate and flip-chip LEDs on a sapphire substrate. The GaN-based VLEDs were transferred onto the silicon substrate by using the Au–In eutectic bonding technique in combination with the laser lift-off (LLO) process. The silicon substrate with high thermal conductivity can provide a satisfactory path for heat dissipation of VLEDs. The nitrogen polar n-GaN surface was textured by KOH solution, which not only improved light extract efficiency (LEE) but also broke down Fabry–Pérot interference in VLEDs. As a result, a near Lambertian emission pattern was obtained in a VLED. To improve current spreading, the ring-shaped n-electrode was uniformly distributed over the entire VLED. Our combined numerical and experimental results revealed that the VLED exhibited superior heat dissipation and current spreading performance over a flip-chip LED (FCLED). As a result, under 350 mA injection current, the forward voltage of the VLED was 0.36 V lower than that of the FCLED, while the light output power (LOP) of the VLED was 3.7% higher than that of the FCLED. The LOP of the FCLED saturated at 1280 mA, but the light output saturation did not appear in the VLED.

Performance analysis of GaN-based micro light-emitting diodes by laser lift-off process

In this study, a monochromatic GaN-based micro-light-emitting-diode (µLED) array was fabricated using flip-chip technology. The laser lift-off (LLO) process was employed to decrease the light divergence caused by the differing refractive indexes of sapphire (n = 1.77) and GaN (n = 2.4). The LLO-µLEDs considerably improve light collimation, compared with conventional flip-chip µLEDs containing a sapphire substrate. We highlight, in particular, the importance of the optical characteristics before and after LLO. Collimation of light was discovered to be 12% higher after removal of the sapphire substrate. The results are of high importance for understanding the optical properties of µLED arrays after LLO.

Dual-Structured Flexible Piezoelectric Film Energy Harvesters for Effectively Integrated Performance.

Improvement of energy harvesting performance from flexible thin film-based energy harvesters is essential to accomplish future self-powered electronics and sensor systems. In particular, the integration of harvesting signals should be established as a single device configuration without complicated device connections or expensive methodologies. In this research, we study the dual-film structures of the flexible PZT film energy harvester experimentally and theoretically to propose an effective principle for integrating energy harvesting signals. Laser lift-off (LLO) processes are used for fabrication because this is known as the most efficient technology for flexible high-performance energy harvesters. We develop two different device structures using the multistep LLO: a stacked structure and a double-faced (bimorph) structure. Although both structures are well demonstrated without serious material degradation, the stacked structure is not efficient for energy harvesting due to the ineffectively applied strain to the piezoelectric film in bending. This phenomenon stems from differences in position of mechanical neutral planes, which is investigated by finite element analysis and calculation. Finally, effectively integrated performance is achieved by a bimorph dual-film-structured flexible energy harvester. Our study will foster the development of various structures in flexible energy harvesters towards self-powered sensor applications with high efficiency.

Efficient performance enhancement of GaN-based vertical light-emitting diodes coated with N-doped graphene quantum dots

Novel vertical light-emitting diodes (VLEDs) decorated with N-doped graphene quantum dots (N-GQDs) have been fabricated, based on the metal organic chemical vapor deposition method (MOCVD), wafer bonding technique, laser lift-off (LLO) technique, inductively coupled plasma (ICP) etching and impregnation processing. There is an obvious luminescence enhancement and resistance reduction for VLEDs after coating N-GQDs. The results show an improvement of light output power (LOP), dependent on the improvement of optical extraction efficiency and current spreading. The improvement of external quantum efficiency (EQE) is mainly owing to the roughened surfaces. Meanwhile, an enhancement of electroluminescence (EL) intensity and photoluminescence (PL) intensity has been achieved when the VLEDs were immersed in 0.9 mg/mL of N-GQDs solution, 4 times that of bare VLEDs. After coating N-GQDs, the performance of VLEDs is greatly improved, which could be attributed to the photons recovered by extracting light from waveguide modes.

Achieving high-resolution pressure mapping via flexible GaN/ ZnO nanowire LEDs array by piezo-phototronic effect

Simulating human tactile sensing through high-resolution electronics is a significant and challenging topic in artificial intelligence. A flexible p-GaN film/n-ZnO nanowire light-emitting diode (LED)-based pressure sensor array, with the merits of flexibility, high resolution, good transparency, fast response, stability, and lightweight, is fabricated through laser lift-off (LLO) process and hydrothermal growth of ZnO nanowires directly on a flexible GaN film, which is used for acquiring the two-dimensional (2D) pressure distribution mapping by reading the illumination intensities from all LED pixels parallelly. The intensity of each pixel composed by a GaN/ZnO nanowire heterostructure LED can be enhanced by the local compressive strain based on piezo-phototronic effect. A high spatial resolution of 2.6 μm and a fast response time of 180 ms were obtained, and also the sensor arrays can still function well after 4000 bending circles. The promising flexible LEDs-based pressure sensor array gains some technological innovations in the field of tactile sensing, with potential applications in smart skin, biomedicine, optical MEMS, and touchpad technology.

Experimental study of laser lift-off of ultra-thin polyimide film for flexible electronics

It is increasingly crucial for flexible electronics to efficiently and reliably peel large-area, ultra-thin flexible films off from rigid substrate serving as substrates of flexible electronics device, especially in industrial production. This paper experimentally investigated the mechanism and technologic characteristics of laser lift-off (LLO) process of ultra-thin (~ 2 μm) polyimide (PI) film. It was found increasingly difficult to obtain desirable ultra-thin PI film by LLO with the decrease of the film thickness. The optimal process parameters were achieved considering laser fluence and accumulated irradiation times (AIT), which were found to be strongly correlative to the thickness of PI film. The process mechanism of LLO of PI film was disclosed that laser ablation of interfacial PI will result in the formation of gas products between the PI and glass substrate, enabling the change of interface microstructures to reduce the interface bond strength. The amount of gas products mainly determines the result of LLO process for ultra-thin PI film, from residual adhesion to wrinkles or cracking. The strategy of multi-scanning based on multiple irradiations of low-energy laser pulses was presented to effectively achieve a reliable LLO process of ultra-thin PI film. This study provides an attractive route to optimize the LLO process for large-scale production of ultra-thin flexible electronics.

Low-threshold ultraviolet stimulated emissions from large-sized single crystalline ZnO transferable membranes.

Wide-bandgap inorganic semiconductors based ultraviolet lasers bring versatile applications with significant advantages including low-power consumption, high-power output, robustness and long-term operation stability. However, flexible membrane lasers remain challenging predominantly due to the need for a lattice matched supporting substrate. Here, we develop a simple laser liftoff process to make freestanding single crystalline ZnO membranes that demonstrate low-threshold ultraviolet stimulated emissions together with large sized dimension (> 2 mm), ultralow-weight (m/A<15 g/m2) and excellent flexibility. The 2.6 μm-thick crack-free ZnO membrane exhibits well-retained single crystallinity and enhanced excitonic emissions while the defect-related emissions are completely suppressed. The inelastic exciton-exciton scattering stimulated emissions with increased spontaneous emission rate is obtained with a reduced threshold of 0.35 MW/cm2 in the ZnO membrane transferred onto a flexible polyethylene naphthalate substrate. Theoretical simulations reveal that it is a synergetic effect of the increased quantum efficiency via Purcell effect and the improved optical gain due to vertical directional waveguiding of the membrane, which functions as a Fabry-Perot photonic resonator due to the refractive index contrast at ZnO-air boundaries. With simple architecture, efficient exciton recombination and easy fusion with waveguide system, the ZnO membranes provide an alternative platform to develop compact low-threshold ultraviolet excitonic lasers.

Light- and space-adaptable display

ABSTRACTPresented is a flexible dual-mode display operable in reflective and emissive mode according to ambient light for optimal visibility on a nonplanar surface. Flexible-backplane-embedding organic light-emitting diodes (OLEDs), thin-film transistors (TFTs), and control electrodes for liquid crystal (LC) shutter are realized by the laser lift-off (LLO) method and the newly developed polyimide (PI) delamination technique. A novel color-filterless LC shutter with color dyes is merged with this flexible backplane for operation in reflective mode. This work opens up a promising approach to building displays that are adaptive to the surrounding environment for better usability.

21.2: Invited Paper: The application of AMOLED in smart wearable technology

Two approaches of fabrication of a 1.2 inch round AMOLED panel were discussed in this paper, in which one is glass+thin film encapsulation(TFE) and the other is polyimide(PI)+TFE. The innovative glass+TFE approach has the distinctive feature of ultra‐thinness compared to traditional glass+glass panel structure, and reduces yield loss in the PI and laser lift off(LLO) process compared to PI+TFE panel. The flexible PI+TFE approach can realize the bending of bottom border. Thus it fulfills true circular requirements for wearable displays.

Super flexible GaN light emitting diodes using microscale pyramid arrays through laser lift-off and dual transfer.

We demonstrated a method to obtain super flexible LEDs, based on high quality pyramid arrays grown directly on sapphire substrates. Laser lift-off (LLO) and dual transfer processes were applied to transfer pyramid arrays face up onto the flexible substrates, which is more efficient than back light emission. Ag grid and Ag nanowires were employed as the electrical connection. No significant performance reduction appeared until the device reached a curvature radius of 0.5 mm. The performance reduction results from cracks appearing at the junction of the Ag grid, which can be improved by replacing the Ag grid with a strip electrode.

Control of Bowing in Freestanding-GaN Substrate Using Electrochemical Etching Methods

In this study, we demonstrated the effect of electrochemical etching on freestanding GaN (FS-GaN) with convex and concave bowing. FS-GaN substrates were grown by hydride vapor phase epitaxy (HVPE) and then separated by the laser lift-off (LLO) process. Electrochemical etching was applied to the exposed Ga- and N-polar faces of the FS-GaN substrates using C2H2O4 as the electrolyte and high voltage. The N-polar faces on the concave and convex FS-GaN substrates were selectively etched, yielding a nano-porous structure. However, the bowing and strain of the FS-GaN substrates only decreased for the convex type. The electrochemical etching was observed to eliminate a nonhomogeneous layer on the N-polar face. The electrochemical etching caused the convex FS-GaN substrate to be more flat, and its bowing curvature decreased from 0.08 m−1 to 0.06 m−1.

Chip design for thin-film deep ultraviolet LEDs fabricated by laser lift-off of the sapphire substrate

We report on a chip design which allows the laser lift-off (LLO) of the sapphire substrate sustaining the epitaxial film of flip-chip mounted deep ultraviolet light emitting diodes. A nanosecond pulsed excimer laser with a wavelength of 248 nm was used for the LLO. A mechanically stable chip design was found to be the key to prevent crack formation in the epitaxial layers and material chipping during the LLO process. Stabilization was achieved by introducing a Ti/Au leveling layer that mechanically supports the fragile epitaxial film. The electrical and optical characterization of devices before and after the LLO process shows that the device performance did not degrade by the LLO.

Effect of atmospheric-pressure plasma treatment on the adhesion properties of a thin adhesive layer in a selective transfer process

Abstract The adhesion between a stamp and thin film devices is crucial for their transfer on a flexible substrate. In this paper, a thin adhesive silicone layer on the stamp was treated by atmospheric pressure plasma to locally control the adhesion strength for the selective transfer. The adhesion strength of the silicone layer was significantly reduced after the plasma treatment, while its surface energy was increased. To understand the inconsistency between the adhesion strength and surface energy changes, the surface properties of the silicone layer were characterized using nanoindentation and X-ray photoelectron spectroscopy. These techniques revealed that a thin, hard, silica-like layer had formed on the surface from plasma-enhanced oxidation. This layer played an important role in decreasing the contact area and increasing the interfacial slippage, resulting in decreased adhesion. As a practical application, the transfer process was demonstrated on GaN LEDs that had been previously delaminated by a laser lift-off (LLO) process. Although the LEDs were not transferred onto the treated adhesive layer due to the reduced adhesion, the untreated adhesive layer could readily pick up the LEDs. It is expected that this simple method of controlling the adhesion of a stamp with a thin adhesive layer would enable a continuous, selective and large-scale roll-to-roll selective transfer process and thereby advance the development of flexible, stretchable and wearable electronics.

Tunable InGaN quantum dot microcavity light emitters with 129 nm tuning range from yellow-green to violet

An electrically pumped wavelength-tunable InGaN quantum dot (QD) based microcavity (MC) lighter emitter with a large tuning range of 129 nm was demonstrated. The multi-mode emission spectrum was tuned by injected current from 564 nm (yellow-green) to 435 nm (violet). The MC light emitter is featured with a double dielectric distributed Bragg reflector structure and a copper substrate fabricated using substrate transfer and laser lift off techniques. By utilizing an InGaN QD active layer with a tunable broad emission spectrum and a Fabry-Pérot cavity which allows multi-longitudinal mode resonating, the emission spectrum could be tuned among several particular cavity modes, which are decided by the gain enhancement factor. In addition, both the enhancement and suppression of MC emission modes caused by the gain enhancement factor were observed in a single MC device. As the first electrically driven III-V nitride semiconductor based tunable MC light emitter with a tuning range of 129 nm, the device is promising for applications such as in wide-gamut compact displays and projectors.

Threshold Fluence Measurement for Laser Liftoff of InP Thin Films by Selective Absorption

We explore conditions for achieving laser liftoff in epitaxially grown heterojunctions, in which single crystal thin films can be separated from their growth substrates using a selectively absorbing buried intermediate layer. Because this highly non‐linear process is subject to a variety of process instabilities, it is essential to accurately characterize the parameters resulting in liftoff. Here, we present an InP/InGaAs/InP heterojunction as a model system for such characterization. We show separation of InP thin films from single crystal InP growth substrates, wherein a ≈10 ns, Nd:YAG laser pulse selectively heats a coherently strained, buried InGaAs layer. We develop a technique to measure liftoff threshold fluences within an inhomogeneous laser spatial profile, and apply this technique to determine threshold fluences of the order 0.5 J cm−2 for our specimens. We find that the fluence at the InGaAs layer is limited by non‐linear absorption and InP surface damage at high powers, and measure the energy transmission in an InP substrate from 0 to 8 J cm−2. Characterization of the ejected thin films shows crack‐free, single crystal InP. Finally, we present evidence that the hot InGaAs initiates a liquid phase front that travels into the InP substrate during liftoff.

Effect of Wafer and Damaged Layer Thickness on Residual Stress and Bow of Free-Standing Gallium Nitride Wafers during Wafering Process

In recent years, free-standing gallium nitride (FS-GaN) wafers were researched due to high demand for power device, high brightness LED (HB-LED) and laser diode (LD). The GaN wafers grown by hydride vapor phase epitaxy (HVPE) have been used for the substrate of power device, HB-LED and LD because they demonstrated lower dislocation density (DD) than that of GaN wafers grown by metal-organic chemical vapor deposition (MOCVD). However, they still have residual stress and wafer bow, which lower the device productivity in mass production. To overcome these issues, the control of crystallographic quality and wafer bow are the key technologies. The crystallographic quality of the GaN wafer is mostly determined at the growth process. The bow of the wafer is determined by the difference in thermal expansion coefficient (TEC) between sapphire substrate and GaN, and it can be controlled at the wafering process. In this study, therefore, we investigated how the bow of the FS-GaN wafer is changed during wafering process. In particular, the effect of the abrasive size on the damaged layer thickness during lapping process was investigated and residual stress was calculated theoretically. Two inch GaN wafers were fabricated on (0001) sapphire substrates by HVPE and separated by the laser lift-off (LLO) process. The bow of as-grown GaN wafer after LLO process was ranged from -300 to -100 um, which was determined by the residual thermal stress. The GaN wafer bow is changed during the wafering process, especially the lapping process. From the Stoney’s formula, it is notable that the thickness of substrate (tS) and sub-surface damaged layer (tSSD) can change the wafer bow. Fig. 1 shows the wafer bow (black triangle), residual stress (blue square) of experimented GaN wafer and wafer bow calculated by the Stoney’s formula (red triangle). It was confirmed that the wafer bow was reduced as the thickness of GaN wafer reduced. In particular, the bow was inverted from concave to convex at the thickness of around 180 um. As the lapping process progressed, tS is reduced and tSSD is generated, which led to the change of bow to convex. Fig. 2 shows the sub-surface damaged layer of the GaN wafer after lapping with 2-um diamond abrasives. It exhibits that the crystalline GaN layer was damaged by the diamond abrasives and changed to polycrystalline phase at the top surface. In addition, it indicates that the microcracks propagated into the wafer under the polycrystalline layer. Total thickness of polycrystalline layer and microcracks, tSSD, was around 1.09 um. Thus, the tSSD should be removed for achieving the damage-free GaN wafer. In our presentation, we report how we achieved zero-bow FS-GaN wafer shown in Fig. 3, and how to control residual stress and optimize wafering process. * This work was financially supported by the Brain Korea 21 Plus Program in 2017. Figure 1

Enhanced performance of large‐area vertical light‐emitting diodes treated by laser irradiation

In this work, a comparative analysis of vertical light-emitting diodes (VLEDs) examining the device properties is performed with different surface treatment schemes. The VLEDs on graphite substrate are fabricated by the laser lift-off and the wafer bonding processes. The significance of KrF laser irradiation to form protrusions on the surfaces of undoped gallium nitride (u-GaN) and n-GaN is analysed in detail. The light output power of the fabricated VLEDs is significantly increased through the formation of micro-sized protrusions by the laser irradiation. For VLED irradiated with an energy density of 600 mJ/cm2 on n-GaN surface with a thin u-GaN layer, the light output power is improved by 24.8% at 350 mA without degradation of electrical properties compared with that of the conventional VLED.

Self-powered flexible touch sensors based on PZT thin films using laser lift-off

Touch screens have become an inherent part of the user interface in many electronics applications such as smartphones. The two types of developed touch sensors, the resistive and capacitive sensing devices, may face several difficulties when applied to flexible device applications such as touch signals arising from bending motions. In this study, we assess the feasibility of flexible touch sensors based on piezoelectric PbZr0.52Ti0.48O3 (PZT) thin films. Piezoelectric ceramic based flexible touch sensors possess unique advantages including scalable fabrication, fast response time, durability, and being self-powered. A demonstration device has been fabricated with a sandwich structure consisting of Pt electrode/functional PZT/Pt electrode/flexible substrate structure using laser lift-off (LLO) method. In order to anneal the functional PZT layer at high temperature (600°C), the device was first fabricated on the sapphire substrate and transferred via melting sacrificial PZT layer with an excimer laser. We demonstrate the detection of x- and y-axis touch location via piezoelectric materials and confirm that the flexible piezoelectric touch sensors can distinguish between touch-induced and bending-induced signals via signal location, signal shape, and duration time. A notable feature of this fabrication technique involves its possibility to be fabricated in high resolution. This device may potentially achieve high resolution with suitable fabrication techniques, thus, providing the possibility for the next generation touch sensors.