Thin-film Flip-chip Research Articles

Increased Light Extraction of Thin-Film Flip-Chip UVB LEDs by Surface Texturing.

Ultraviolet light-emitting diodes (LEDs) suffer from a low wall-plug efficiency, which is to a large extent limited by the poor light extraction efficiency (LEE). A thin-film flip-chip (TFFC) design with a roughened N-polar AlGaN surface can substantially improve this. We here demonstrate an enabling technology to realize TFFC LEDs emitting in the UVB range (280-320 nm), which includes standard LED processing in combination with electrochemical etching to remove the substrate. The integration of the electrochemical etching is achieved by epitaxial sacrificial and etch block layers in combination with encapsulation of the LED. The LEE was enhanced by around 25% when the N-polar AlGaN side of the TFFC LEDs was chemically roughened, reaching an external quantum efficiency of 2.25%. By further optimizing the surface structure, our ray-tracing simulations predict a higher LEE from the TFFC LEDs than flip-chip LEDs and a resulting higher wall-plug efficiency.

High conductivity n-Al0.6Ga0.4N by ammonia-assisted molecular beam epitaxy for buried tunnel junctions in UV emitters

Highly doped n-Al0.6Ga0.4N can be used to form tunnel junctions (TJs) on deep ultraviolet (UVC) LEDs and markedly increase the light extraction efficiency (LEE) compared to the use of p-GaN/p-AlGaN. High quality Al0.6Ga0.4N was grown by NH3-assisted molecular beam epitaxy (NH3 MBE) on top of AlN on SiC substrate. The films were crack free under scanning electron microscope (SEM) for the thickness investigated (up to 1 µm). X-ray diffraction reciprocal space map scan was used to determine the Al composition and the result is in close agreement with atom probe tomography (APT) measurements. By varying the growth parameters including growth rate, and Si cell temperature, n-Al0.6Ga0.4N with an electron density of 4×1019 /cm3 and a resistivity of 3 mΩ·cm was achieved. SIMS measurement shows that a high Si doping level up to 2×1020 /cm3 can be realized using a Si cell temperature of 1450 °C and a growth rate of 210 nm/hr. Using a vanadium-based annealed contact, ohmic contact with a specific resistance of 10−6 Ω·cm2 was achieved as determined by circular transmission line measurement (CTLM). Finally, the n-type AlGaN regrowth was done on MOCVD grown UVC LEDs to form UVC TJ LED. The sample was processed into thin film flip chip (TFFC) configuration. The emission wavelength is around 278 nm and the excess voltage of processed UV LED is around 4.1 V.

Laser Lift-Off of the Sapphire Substrate for Fabricating Through-AlN-Via Wafer Bonded Absorption Layer Removed Thin Film Ultraviolet Flip Chip LED

In this study we report chip fabrication process that allows the laser lift-off of the sapphire substrate for the transfer of the GaN based thin film flip chip to the carrier wafer. The fabrication process includes 365-nm ultraviolet flip chip LED wafer align bonding with through-AlN-via wafer and sapphire laser lift-off. n-holes with the diameter of 100 µm were etched on the GaN epilayers for accessing n-type GaN. Through-AlN-via size was 110-µm and filled by Cu electroplating method for the electrical connection. Mechanical stabilization to prevent the GaN epilayers cracking and fragmentation during laser lift-off was achieved by utilizing epoxy based SU-8 photoresist support.

Superlattice hole injection layers for UV LEDs grown on SiC

AlGaN-based germicidal UV LEDs show promise in fighting the COVID-19 pandemic through disinfection of air, water, and surfaces. We report UV LEDs grown by MOCVD on SiC substrates, fabricated into thin-film flip chip devices. Replacing the uniform p-Al x Ga1-xN layer (x = 0.2) with a short-period-superlattice of alternating (x = 0.1 and 0.8) Al-composition improved EQE from 1.3% to 2.7% (3.2% with encapsulation) at 20 A/cm2. Peak EQE and WPE values of 4.8% and 2.8% (287 nm) were measured at current densities below 2 A/cm2, and maximum output power of 7.4 mW (76 mW/mm2) was achieved at 284 nm. Further WPE improvements are expected with both superlattice and uniform layer optimization, improved p-contact metallization, and active region optimization.

Thin-film flip-chip UVB LEDs realized by electrochemical etching

We demonstrate a thin-film flip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviolet B (UVB) at 311 nm, where substrate removal has been achieved by electrochemical etching of a sacrificial Al0.37Ga0.63N layer. The electroluminescence spectrum of the TFFC LED corresponds well to the as-grown LED structure, showing no sign of degradation of structural and optical properties by electrochemical etching. This is achieved by a proper epitaxial design of the sacrificial layer and the etch stop layers in relation to the LED structure and the electrochemical etch conditions. Enabling a TFFC UV LED is an important step toward improving the light extraction efficiency that limits the power conversion efficiency in AlGaN-based LEDs.

Ambient Temperature–Corrected Mechanical Stress Mapping of Gallium Nitride and Aluminum Indium Gallium Phosphide Films by Raman Scattering Spectroscopy for Characterization of Light‐Emitting Diodes

Raman spectroscopy can provide a detailed analysis of mechanical stress in crystalline materials through the measurement of shifts in vibrational frequency. However, the ambient temperature drift during measurements can lead to inaccurate mechanical stress values. Herein, the 2D Raman mapping analysis of gallium nitride (GaN) and aluminum indium gallium phosphide (AlInGaP) epitaxial films within packaged light‐emitting diode (LED) devices reveals micrometer‐scale localized stress distributions. A temperature‐corrected measurement method is developed to evaluate the mechanical stress of InGaN LED dies at various LED operating conditions. The impact of die design on mechanical stress is studied for both GaN on Al2O3 (chip scale package, CSP) and GaN thin‐film flip chip (TFFC) designs. Raman mapping of AlInGaP LEDs is also demonstrated and shows comparable resolution with the GaN LED results.

Light Extraction Analysis of AlGaInP Based Red and GaN Based Blue/Green Flip-Chip Micro-LEDs Using the Monte Carlo Ray Tracing Method.

Micro-scale light emitting diodes (micro-LEDs) commonly employ a thin-film flip-chip (TFFC) structure whose substrate is lifted off by an excimer laser. However, flip-chip (FC) micro-LEDs with a substrate can provide a sharp rise on sidewall emission by increasing the sidewall area. Here, we investigate the influence of substrate thickness, encapsulation, surface texture, microstructures between the substrate and epilayer, as well as the size, cutting shape, and angle of the chip on the light extraction efficiencies (LEEs) of FC micro-LEDs by using the Monte Carlo ray tracing method. We find that the LEE of the blue FC micro-LED chip increases by 46.5% over that of the blue TFFC micro-LED chip. After the encapsulation with the epoxy lens is applied, the LEEs of the blue TFFC micro-LED and blue FC micro-LED increase by 129% and 110.5%, respectively. The underlying mechanisms for the use of surface texture, patterned sapphire substrate, air-void array, and chip shaping technologies to improve the LEEs of FC micro-LEDs are also investigated in detail. We find that the LEEs AlGaInP based red FC micro-LED and GaN based blue/green FC micro-LEDs exhibit a sharp rise when the chip size drops from 30 to 10 µm. The inverted trapezoid FC micro-LED with patterned sapphire substrate (PSS) and encapsulation shows extraordinarily strong top emission and high collimation. We believe that our study offers a promising and practical route for obtaining high efficiency micro-LEDs.

Thermally stable lead-free phosphor in glass enhancement performance of light emitting diodes application.

A co-sintering method for preparing PIG is proposed, a Ce3+ doped yttrium aluminum garnet (YAG:Ce) phosphor in glass (PIG) slice is fabricated through co-sintering polycrystalline Ce:YAG phosphor, and a new, to the best of our knowledge, homemade matrix glass SiO2-K2O-BaO-B2O3 glass powder is detected to confirm crystal phase by X-ray powder diffractions (XRD) and micromorphology by scanning electron microscope (SEM). The YAG:Ce phosphor particles dispersed in the matrix are vividly observed and their distributions are relatively uniform. The PIG packaged light-emitting diode (LED) sample shows a relatively lower correlated color temperature (CCT) than the conventional phosphor-dispensing LED sample. White light can be achieved in the PIG slice under 465nm LED excitation by controlling the phosphor concentration or thickness. The highest luminous efficacy of the PIG slice packaged white LED is 94.67lm/W at 100mA. Controllable geometrical and optical properties of the YAG:Ce PIG can generate evident improvements in the manufacturing of identical CCTs for high-power white LEDs. The combination of PIG with a blue LED chip of a specific wavelength, if measured before final assembly, enables targeted production of specific CCT white LEDs. PIG technology allows the realization of high stability and thermal conductivity for the phosphor layer. This simple method provides many possibilities for LED packaging, including thin film flip chip and remote phosphor technology.

Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ∼46 μm hr–1 at a high RF bias (400 W), and ∼7 μm hr–1 at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ∼3× after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency and power conversion efficiency UV-C LEDs.

Numerical analysis of light extraction enhancement of GaN-based thin-film flip-chip light-emitting diodes with high-refractive-index buckling nanostructures

Abstract In this work, the light extraction efficiency enhancement of GaN-based thin-film flip-chip (TFFC) light-emitting diodes (LEDs) with high-refractive-index (TiO2) buckling nanostructures was studied using the three-dimensional finite difference time domain method. Compared with 2-D photonic crystals, the buckling structures have the advantages of a random directionality and a broad distribution in periodicity, which can effectively extract the guided light propagating in all azimuthal directions over a wide spectrum. Numerical studies revealed that the light extraction efficiency of buckling-structured LEDs reaches 1.1 times that of triangular lattice photonic crystals. The effects of the buckling structure feature sizes and the thickness of the N-GaN layer on the light extraction efficiency for TFFC LEDs were also investigated systematically. With optimized structural parameters, a significant light extraction enhancement of about 2.6 times was achieved for TiO2 buckling-structured TFFC LEDs compared with planar LEDs.

High-luminous efficacy white light-emitting diodes with thin-film flip-chip technology and surface roughening scheme

High-luminous efficacy white light-emitting diodes (LEDs) were realized by using GaN-based thin-film (TF) flip-chip (FC) LEDs with phosphor–silicone encapsulation. The TFFC-LEDs were fabricated by electrode isolation, FC configuration, copper electroplating, and laser lift-off (LLO) techniques. During the fabrication process, the high-defect undoped GaN layer was eliminated by inductively coupled plasma (ICP) etching to lower the absorption loss. Then, the exposed N-face n-GaN surface formed after the ICP etching was systematically studied through control of the temperature, time and concentration of the KOH solution to acquire hexagonal cones with high extraction efficiency. It is found that the external quantum efficiency was improved by a maximum value of 169% for the TFFC-LEDs with optimized surface hexagonal cones compared to TFFC-LEDs with flat surfaces. To further improve the output power, the chip size and n-contact via holes of the TFFC-LEDs were increased. A maximum luminous efficacy of 139 lm W−1 was realized for white LEDs (5700 K, 350 mA, 2.98 V) using these TFFC-LEDs with phosphor–silicone encapsulation. In addition, these white LEDs also have a lower junction temperature of 87 °C even at 700 mA. These results indicate that the proposed TFFC-LEDs are promising for use in automotive and solid-state lighting applications.

Photoelectrochemical liftoff of LEDs grown on freestanding c-plane GaN substrates.

We demonstrate a thin-film flip-chip (TFFC) process for LEDs grown on freestanding c-plane GaN substrates. LEDs are transferred from a bulk GaN substrate to a sapphire submount via a photoelectrochemical (PEC) undercut etch. This PEC liftoff method allows for substrate reuse and exposes the N-face of the LEDs for additional roughening. The LEDs emitted at a wavelength of 432 nm with a turn on voltage of ~3 V. Etching the LEDs in heated KOH after transferring them to a sapphire submount increased the peak external quantum efficiency (EQE) by 42.5% from 9.9% (unintentionally roughened) to 14.1% (intentionally roughened).

Resonant cavity effect optimization of III-nitride thin-film flip-chip light-emitting diodes with microsphere arrays.

Comprehensive studies were carried out to investigate the light extraction efficiency of thin-film flip-chip (TFFC) light-emitting diodes (LEDs) with anatase TiO(2) microsphere arrays by employing the finite-difference time-domain method. The quantum well position and the resonant cavity effect were studied to obtain optimum light extraction for the planar TFFC LED. Further enhancement in light extraction was achieved by depositing microsphere arrays on the TFFC LED. The calculation results showed that the sphere diameter, packing density, and packing configuration have significant effects on the light extraction efficiency. A maximum light extraction efficiency of 75% in TFFC LEDs with microsphere arrays has been achieved. This study demonstrates the importance of optimizing the quantum well position, cavity thickness, sphere diameter, sphere packing density, and packing configuration for enhancing the light extraction efficiency of TFFC LEDs with microsphere arrays.

Visible Light-Emitting Diodes With Thin-Film-Flip- Chip-Based Wafer-Level Chip-Scale Package Technology Using Anisotropic Conductive Film Bonding

Demonstrated is advanced device and packaging architecture of visible GaN-based light-emitting diodes (LEDs) combining thin-film flip-chip devices and wafer-level chip-scale package with through-silicon-via (TSV) and wafer-to-wafer alignment bonding. In addition, a new interconnect technique for LEDs is introduced using an anisotropic conductive film with metal balls. Thermal rollover in light output versus current characteristics is not observed up to 700 mA. A forward voltage at 350 mA is 3.06 V. The architecture can facilitate excellent heat removal through a TSV-formed Si wafer in addition to expected benefits of easy integration of Si-based devices in lighting modules. Light-output power at 350 mA increases by 11.1% compared with that of conventional flip-chip LEDs. A Lambertian-like emission pattern is also achieved.

Enhancement of light extraction efficiency in sub-300 nm nitride thin-film flip-chip light-emitting diodes

We report on the improved light extraction efficiency of 287nm small periphery AlGaN/AlGaN thin-film flip-chip (TFFC) light-emitting diodes (LEDs) fabricated using the laser lift-off technique. After sapphire substrate removal, the exposed AlN N-face was etched in a KOH solution. The result was a 1.5-fold increase in the TFFC device output power at 20mA dc current injection, which was attributed to the AlN surface texturing. TFFC die encapsulation process produced an additional 1.34-fold enhancement of the emitted light power. More importantly, the encapsulated thin-film LEDs exhibited a similar reliability performance compared to encapsulated conventional FC LEDs processed from the same epiwafer, and with no noticeable degradation of the encapsulating material optical properties.

Preparation of a YAG:Ce phosphor glass by screen-printing technology and its application in LED packaging

A simple and practical method for preparing phosphor glass is proposed. Phosphor distribution and element analysis are investigated by optical microscope and field emission scanning electron microscope (FE-SEM). The phosphor particles dispersed in the matrix are vividly observed, and their distributions are uniform. Spectrum distribution and color coordinates dependent on the thickness of the screen-printed phosphor layer coupled with a blue light emitting diode (LED) chip are studied. The luminous efficacy of the 75 μm printed phosphor-layer phosphor glass packaged white LED is 81.24 lm/W at 350 mA. This study opens up many possibilities for applications using the phosphor glass on a selected chip in which emission is well absorbed by all phosphors. The screen-printing technique also offers possibilities for the design and engineering of complex phosphor layers on glass substrates. Phosphor screen-printing technology allows the realization of high stability and thermal conductivity for the phosphor layer. This phosphor glass method provides many possibilities for LED packing, including thin-film flip chip and remote phosphor technology.

Analysis of light extraction efficiency enhancement for thin-film-flip-chip InGaN quantum wells light-emitting diodes with GaN micro-domes

The enhancement of light extraction efficiency for thin-film flip-chip (TFFC) InGaN quantum wells (QWs) light-emitting diodes (LEDs) with GaN micro-domes on n-GaN layer was studied. The light extraction efficiency of TFFC InGaN QWs LEDs with GaN micro-domes were calculated and compared to that of the conventional TFFC InGaN QWs LEDs with flat surface. The three dimensional finite difference time domain (3D-FDTD) method was used to calculate the light extraction efficiency for the InGaN QWs LEDs emitting at 460nm and 550 nm, respectively. The effects of the GaN micro-dome feature size and the p-GaN layer thickness on the light extraction efficiency were studied systematically. Studies indicate that the p-GaN layer thickness is critical for optimizing the TFFC LED light extraction efficiency. Significant enhancement of the light extraction efficiency (2.5-2.7 times for λ(peak) = 460nm and 2.7-2.8 times for λ(peak) = 550nm) is achievable from TFFC InGaN QWs LEDs with optimized GaN micro-dome diameter and height.

Blue-emitting InGaN–GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200A∕cm2

Auger recombination is determined to be the limiting factor for quantum efficiency for InGaN–GaN (0001) light-emitting diodes (LEDs) at high current density. High-power double-heterostructure (DH) LEDs are grown by metal-organic chemical vapor deposition. By increasing the active layer thickness, DH LEDs can reach a maximum in quantum efficiency at current densities above 200A∕cm2. Encapsulated thin-film flip-chip DH LEDs with peak wavelength of 432nm have an external quantum efficiency of 40% and output power of 2.3W at 2A.

High performance thin-film flip-chip InGaN–GaN light-emitting diodes

Data are presented on the operation of thin-film flip-chip InGaN∕GaN multiple-quantum-well light-emitting diodes (LEDs). The combination of thin-film LED concept with flip-chip technology is shown to provide surface brightness and flux output advantages over conventional flip-chip and vertical-injection thin-film LEDs. Performance characteristics of blue, white, and green thin-film flip-chip 1×1mm2 LEDs are described. Blue (∼441nm) thin-film flip-chip LEDs are demonstrated with radiance of 191mW∕mm2sr at 1A drive, more than two times brighter than conventional flip-chip LEDs. An encapsulated thin-film flip-chip blue LED lamp is shown to have external quantum efficiency of 38% at forward current of 350mA. A white lamp based on a YAG:Ce phosphor coated device exhibits luminous efficacy of 60lm∕W at 350mA with peak efficiency of 96lm∕W at 20mA and luminance of 38Mcd∕m2 at 1A drive current. Green (∼517nm) devices exhibit luminance of 37Mcd∕m2 at 1A.