Flip-chip Light-emitting Diodes Research Articles

High-Performance Green Flip-Chip LEDs with Double-Layer Electrode and Hybrid Reflector

High-performance green flip-chip light-emitting diode (FC-LED) was realized by combining double-layer electrode and highly reflective hybrid reflector. The introduction of double-layer electrode improved uniformity of current spreading over the entire active region. The highly reflective hybrid reflector, composed of SiO2 total internal reflection (TIR) layer and Nb2O5/SiO2 distributed Bragg reflector (DBR), offer higher reflectance and wider bandwidth compared to conventional DBR. Numerical simulation and experimental results indicated that the green FC-LED has advantages of uniform current spreading and high light extraction efficiency (LEE) compared to green top-emitting LED (TE-LED). Due to uniform current spreading, the forward voltage of the green FC-LED is 0.28 V lower than that of the TE-LED at 20 mA. Compared to TE-LED, the light emission intensity of the green FC-LED at 20 mA is enhanced by 18% due to the higher reflectance of hybrid reflector and increased utilization ratio of active region area caused by double-layer electrode structure.

Effect of Dielectric Distributed Bragg Reflector on Electrical and Optical Properties of GaN-Based Flip-Chip Light-Emitting Diodes.

We demonstrated two types of GaN-based flip-chip light-emitting diodes (FCLEDs) with distributed Bragg reflector (DBR) and without DBR to investigate the effect of dielectric TiO2/SiO2 DBR on optical and electrical characteristics of FCLEDs. The reflector consisting of two single TiO2/SiO2 DBR stacks optimized for different central wavelengths demonstrates a broader reflectance bandwidth and a less dependence of reflectance on the incident angle of light. As a result, the light output power (LOP) of FCLED with DBR shows 25.3% higher than that of FCLED without DBR at 150 mA. However, due to the better heat dissipation of FCLED without DBR, it was found that the light output saturation current shifted from 268 A/cm2 for FCLED with DBR to 296 A/cm2 for FCLED without DBR. We found that the use of via-hole-based n-type contacts can spread injection current uniformly over the entire active emitting region. Our study paves the way for application of DBR and via-hole-based n-type contact in high-efficiency FCLEDs.

Effect of Different Welding Methods on Flip-Chip LED (FC-LED) Filament Properties

This paper investigates the effect of two different welding methods, direct welding (DW) and vacuum furnace welding (VFW), on flip-chip light-emitting diode (FC-LED) filament properties. Shearing force, SEM, steady-state voltage, steady-state luminous flux, and change of photoelectric performance with aging time were employed to characterize the differences in filament properties between the two welding methods. The shearing test revealed that the average shearing force of the VFW group was higher than that of the DW group, but the two groups followed the standard. Furthermore, the microstructure of the VFW group fault was more smoother, and the voids were fewer and smaller based on the SEM test results. The steady-state voltage and luminous flux revealed that the VFW group had a more concentrated voltage and a higher luminous flux. The aging data revealed that the steady-state voltage change rate of both groups was not very different, and both luminous flux maintenance rates of the VFW group were higher than those of the DW group, but all were within the standard range. In conclusion, if there is a higher requirement for filament in a practical application, such as the filament is connected in series or in parallel and needs a higher luminous flux, it can be welded using vacuum furnace welding. If the focus is on production efficiency and the high performance of filaments is not required, direct welding can be used.

Optical and thermal performance of nitride-based thin-film flip-chip light-emitting diodes

Nitride-based light-emitting diodes (LEDs) were fabricated based on initial patterned sapphire substrate by thin film (TF) and flip-chip (FC) techniques. An enhancement of 7.7% in light output power was first obtained by optimizing the p-type contact layer. Then, periodic patterned surface was optimized by wet etching after removal of the sapphire substrate, and the light output power of the TFFC-LEDs was enhanced by 19.8% as compared to that of the TFFC-LEDs with original patterned surface. The device performance characteristic including illuminant pattern, emission wavelength movement and junction temperature variation of the optimized TFFC-LEDs were shown as compared with commercialized LEDs. The results indicate that the TFFC-LEDs possess uniform current distribution and lower junction temperature, thus show promising applications in various areas such as automotive lighting, illegal capture and solid-state lighting.

Analysis of high-temperature aging and microstructure of FC LED solder joints of a silver conductive layer bonded using SAC solder

Flip-chip (FC) light-emitting diode (LED) filaments with a silver conductive layer are impeded by heat dissipation, with the flip-chip junction temperature of reaching up to 423.15 K. To overcome this problem, we simulated aging of SAC305 solder joints at a high temperature of 433.15 K. Scanning electron microscopy was performed to analyze the changes in the microstructure of the solder joints with aging time. Moreover, energy-dispersive X-ray spectroscopy was performed to analyze changes in the element content of the solder joints with aging time. The shear force of solder joints was measured using a strength tester. Results revealed that the Ag/Au content of the solder joints increased under the high-temperature conditions, resulting in the formation of cracks and holes in the solder joints. Furthermore, the reliability of the FC LED filaments was affected by the maximum shear stress that can be tolerated by the solder joints.

Optical and thermal properties of flip-chip LED bulbs using chip-scale package

A kind of LED bulb which is made by the school and enterprise is introduced in this paper. Fluorescent film is used for chip-scale packaging on flip-chip LED. The film is made by spin coating method, bonded to the surface of the chip and cut off by laser and then finished by a simple process. The optical properties and thermal properties of the LED bulbs were measured by various methods. The experimental results showed that with the increasing of the voltage, color temperature and the color rendering index changing 0.522% and 0.200% respectively which can be seen as a relatively stable data. The highest surface temperature of the flip-chip LEDs using chip-scale package we estimated which can be kept between 133.8°C and 135 °C can also be accepted. These results have a certain guiding significance for the industrialization of the LED bulbs.

Light Extraction Efficiency Analysis of Flip-Chip Ultraviolet Light-Emitting Diodes With Patterned Sapphire Substrate

This study investigates polarization-dependent light extraction efficiency (η extraction ) of AlGaN-based flip-chip ultraviolet (UV) light-emitting diodes (LEDs) emitting at 230 nm and 280 nm with microdome-shaped patterning on sapphire substrate based on 3-D finite-difference time-domain simulations. Three types of patterned sapphire substrates (PSS) have been analyzed: bottom-side PSS, top-side PSS, and double-sided PSS. Our results show that microdome-shaped patterning on sapphire substrate is predominantly beneficial in enhancing transverse-magnetic (TM)-polarized output. Specifically, TM-polarized η extraction enhancement of up to ~4.5 times and ~2.2 times can be obtained for 230 nm and 280 nm UV LEDs with bottom-side PSS, respectively, and ~6.3 times and ~1.8 times for 230 nm and 280 nm UV LEDs with top-side PSS, respectively. By employing double-sided PSS, up to ~11.2 times and ~2.6 times enhancement in TM-polarized η extraction can be achieved for 230 nm and 280 nm UV LEDs, respectively. In contrast, the microdome-shaped PSS act as a reflector for transverse-electric-polarized photons which leads to severe limitation in light extraction for both 230 nm and 280 nm flip-chip UV LEDs. Thus, it is expected that this study will serve as a guidance in designing PSS for high-efficiency mid- and deep-UV LEDs.

Light-extraction enhancement of GaN-based 395 nm flip-chip light-emitting diodes by an Al-doped ITO transparent conductive electrode.

The distinct ultraviolet (UV) light absorption of indium tin oxide (ITO) limits the performance of GaN-based near-UV light-emitting diodes (LEDs). Herein, we report an Al-doped ITO with enhanced UV transmittance and low sheet resistance as the transparent conductive electrode for GaN-based 395nm flip-chip near-UV LEDs. The thickness dependence of optical and electrical properties of Al-doped ITO films is investigated. The optimal Al-doped ITO film exhibited a transmittance of 93.2% at 395nm and an average sheet resistance of 30.1 Ω/sq. Meanwhile, at an injection current of 300mA, the forward voltage decreased from 3.14 to 3.11V, and the light output power increased by 13% for the 395nm near-UV flip-chip LEDs with the optimal Al-doped ITO over those with pure ITO. This Letter provides a simple and repeatable approach to further improve the light extraction efficiency of GaN-based near-UV LEDs.

Light extraction efficiency enhancement of flip-chip blue light-emitting diodes by anodic aluminum oxide.

We produced an anodic aluminum oxide (AAO) structure with periodic nanopores on the surface of flip-chip blue light-emitting diodes (FC-BLEDs). The nanopores had diameters ranging from 73 to 85 nm and were separated by distances ranging from approximately 10 to 15 nm. The light extraction efficiency enhancement of the FC-BLEDs subjected to different durations of the second pore-widening process was approximately 1.6–2.9%. The efficiency enhancement may be attributed to the following mechanism: periodic nanopores on the surface of FC-BLEDs reduce the critical angle of total reflection and effective energy transfer from a light emitter into a surface plasmon mode produced by AAO.

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.

(Electronics and Photonics Division Award Address) Technological Issues and Design Rules of Electrodes for High-Efficiency GaN-Based Light-Emitting Diodes

Group III-nitride-based LEDs are of great technological importance because of their applications including display, solid-state lighting, automotive headlight, and water/air purification. For these applications, the attainment of high external quantum efficiency (EQE) of LEDs is essential. On the one hand, their applications necessitate the fabrication of different geometry LEDs, such as lateral-type, vertical-type, and flip-chip LEDs, which require different design rules of electrodes and current spreaders. To maximize the EQE, it is important to increase the light extraction efficiency (LEE). In addition, the improvement of current injection (by forming low contact resistivity) and current spreading is vital for the fabrication of high-efficiency LEDs. It is well known that lateral LEDs suffer from the light extraction limit due to the absorption of the metal contacts, such as n and p contacts, and bond pads. In addition, p-GaN has characteristic problems, which make it difficult to achieve device-quality ohmic contacts with the specific contact resistance lower than ~10–4 Ωcm2 because of the difficulty in growing a highly doped p-GaN and the absence of appropriate metals or conducting oxides having work function larger than that of p-GaN. Thus, for the formation of ohmic and reflective contacts to p-GaN, also serving as light extractor and/or current spreader, transparent conducting oxides (TCOs)-based and Ag-based schemes have been extensively investigated. In this talk, we present methods of enhancing current injection efficiency through the formation of transparent and reflective ohmic electrodes. Design rules are suggested to enhance current injection efficiency, such as the control of surface Fermi-level through surface treatment, barrier tunneling by forming interfaces with inhomogeneous Schottky barriers, the modification of Schottky barrier heights (SBHs), control of native defects in GaN surface regions. Furthermore, nanostructures, such as Ag nano-dots (NDs) and Ag nano-wires (NWs), are used to enhance the light output powers of GaN-based LEDs. Finally, on the basis of scanning transmission electron microscopy, X-ray photoemission spectroscopy, secondary ion mass spectroscopy and Auger electron spectroscopy results, possible ohmic formation and electrical degradation mechanisms are described and discussed.

GaN-based mid-power flip-chip light-emitting diode with high -3 dB bandwidth for visible light communications.

By directly flip-chip soldering three mid-power light-emitting diode (LED) chips with periodic micro-via-holes on ceramic substrates, -3 dB modulation bandwidths of 49.9MHz, 58.8MHz, and 25MHz are obtained at the driving current of 170mA. To the best of our knowledge, these are the reported highest -3 dB bandwidth values for flip-chip power-type LEDs (FC-LEDs) at the low-bias current levels. Moreover, good radiant powers are also achieved: 180.2mW, 168.8mW, and 233.8mW at 150mA, respectively. With a gold wire-free feature, the fabricated FC-LEDs could offer an opportunity to miniaturize the package volume of a LED module for illumination and free-space high-speed VLC dual-usage applications.

Residual free solder process for fluxless solder pastes

Purpose For soldering, flux is essential because it enables the wetting of the molten solder. Fluxless soldering, i.e. residue-free soldering with the aid of gaseous activators, has been known for many years, but is only well established in the field of opto- and microwave electronics where the solder is applied as preform. In high-volume SMD applications where solder paste is printed, this technology is rarely used until now. The reducing effect of a gaseous activator like formic acid vapor on certain solder alloys is known in practice. However, the corresponding reactions which occur under soldering conditions in nitrogen atmosphere have so far not been systematically investigated for different solder alloys. This study aims to analyze the different chemical reaction channels which occur on the surface of different solders, i.e. catalytical dissociation of formic acid on the pure or oxidized metal surface and the formation and evaporation of metal formates. Based on this analysis, a residue-free solder process under formic acid is developed for solder paste applications. Design/methodology/approach In this paper, different solder alloys (SnAgCu, SnPb, BiSn, In) were analyzed with thermal gravimetric analysis (TGA) under formic acid flow. Details on mass change depending on the soldering temperature are presented. Activation temperatures are estimated and correlated to the soldering processes. Based on the analysis, fluxless solder pastes and suitable soldering processes are developed and presented. Major paste properties such as printability are compared to a commercial flux solder paste. High-power flip chip LEDs which can be assembled directly on a printed circuit board are used to demonstrate the fluxless soldering. Likewise, the soldering results of standard paste and fluxless paste systems after a reflow process are evaluated and compared. Findings The experimental results show that TGA is an efficient way to gain deeper understanding of the redox processes which occur under formic acid activation, i.e. the formation of metal formates and their evaporation and dissociation. It is possible to solder residue-free not only with preforms but also with a fluxless solder paste. The resulting solder joints have the same quality as those for standard solder paste in terms of voids detected by X-ray and mechanical shear strength. Originality/value In the fluxless soldering process, the reduction of oxide layers, and therefore the wetting of the solder spheres, is enabled by gaseous formic acid. After the soldering process, no cleaning process is necessary because no corrosive residues are left on the circuit boards and components. Therefore, soldering using solder paste without aggressive chemical ingredients has a high market potential. Expensive preforms could be replaced by paste dispensing or paste printing.

Study of Laser Lift-Off Process for Fabrication of GaN-Based 365-nm Ultraviolet Absorption Layer Removed Flip Chip LED

The objective of this study was to develop a fabrication process to realize feasible n-type GaN thinning on a 365-nm flip chip ultraviolet LED for enhancement of light output power. The fabrication process included isolation trench filling using epoxy-based SU-8 photoresist, aligned wafer bonding, and sapphire laser lift-off techniques. Study of laser lift-off process for successful transfer of the flip chip LED GaN epilayers to the carrier wafer using different SU-8 trench filling methods are in progress. The first method of SU-8 trench filling was implemented on a flip chip LED wafer with chip isolation defined up to n-type GaN, whereas the second method of SU-8 trench filling was implemented on a flip chip LED wafer with chip isolation defined down to the sapphire substrate. SU-8 2 and SU-8 2025 series were used for the first and second trench filling methods, respectively. In addition to SU-8 trench filling, the second method comprised Cu electroplating and planarization polishing.

Design of patterned sapphire substrates in flip-chip LEDs for improvement of light-extraction efficiencies

Patterned sapphire substrates (PSSs) in flip chip light-emitting diodes (FC-LEDs) were designed to improve the light extraction efficiencies (LEEs). Two typical types were considered in the cross-section of the PSS pattern; an extruded type and a subtracted type. For pattern shapes in PSS, various polygonal pyramid pattern shapes were also considered, such as triangular-pyramid, square-pyramid, hexagonal-pyramid, circular-pyramids, in addition to hemispheres. Ray-tracing-based numerical simulations were performed for the LEEs of FC-LEDs with various PSSs. The optimal pattern shape for the high LEE was found to be the subtracted type triangular-pyramid shape, which is different from that of the optimal PSS patterns in conventional planar chip LEDs.

RGB-Stack Light Emitting Diode Modules with Transparent Glass Circuit Board and Oil Encapsulation

The light emitting diode (LED) is widely used in modern solid-state lighting applications, and its output efficiency is closely related to the submounts’ material properties. Most submounts used today, such as low-power printed circuit boards (PCBs) or high-power metal core printed circuit boards (MCPCBs), are not transparent and seriously decrease the output light extraction. To meet the requirements of high light output and better color mixing, a three-dimensional (3-D) stacked flip-chip (FC) LED module is proposed and demonstrated. To realize light penetration and mixing, the mentioned 3-D vertically stacking RGB LEDs use transparent glass as FC package submounts called glass circuit boards (GCB). Light emitted from each GCB stacked LEDs passes through each other and thus exhibits good output efficiency and homogeneous light-mixing characteristics. In this work, the parasitic problem of heat accumulation, which caused by the poor thermal conductivity of GCB and leads to a serious decrease in output efficiency, is solved by a proposed transparent cooling oil encapsulation (OCP) method.

Comparative experimental and simulation studies of high-power AlGaN-based 353 nm ultraviolet flip-chip and top-emitting LEDs

Experimental and simulation studies of high-power AlGaN-based 353 nm ultraviolet (UV) flip-chip (FC) and top-emitting (TE) light-emitting diodes (LEDs) are performed here. To improve the optical and electrical properties of ultraviolet LEDs, we fabricate high-power FC-UV LEDs with Ta2O5/SiO2 distributed Bragg reflectors (DBRs) and a strip-shaped SiO2 current blocking layer (CBL). The reflectance of fourteen pairs of Ta2O5/SiO2 DBRs is 96.4% at 353 nm. The strip-shaped SiO2 CBL underneath the strip-shaped p-electrode can prevent the current concentrating in regions immediately adjacent to the p-electrode where the overlying opaque p-electrode metal layer absorbs the emitted UV light. Moreover, two-level metallization electrodes are used to improve current spreading. Our numerical results show that FC-UV LED has a more favorable current spreading uniformity than TE-UV LED. The light output power of 353 nm FC-UV LED was 23.22 mW at 350 mA, which is 24.7% higher than that of TE-UV LED.

Parameter driven monitoring for a flip-chip LED module under power cycling condition

In this paper, a parameter driven monitoring model is introduced, in which a flip-chip LED module was investigated during a power cycling test. This approach was investigated to develop a monitoring model to describe thermally induced solder fatigue as root cause of flip-chip failure in a power cycling test. As monitoring parameter the thermal resistance of the LED module was used, which was determined by thermal impedance measurements of the whole LED module, as well as for each LED chip itself. Further analyses of the occurring temperature at the LED junction recorded of each chip during the power cycling test were used to generate a prediction model. The evaluation of the temperature change allowed to forecast the number of cycles until failure.

Comparative Study of Highly Reflective ITO/DBR and Ni/Ag ohmic Contacts for GaN-Based Flip-Chip Light-Emitting Diodes

Heat removal and current spreading uniformity are important for the realization of highly efficient light-emitting diodes (LEDs). Here we have compared two types of GaN-based flip-chip LEDs (FCLEDs) with highly reflective indium-tin oxide (ITO)/distributed Bragg reflector (DBR) and Ni/Ag p-type ohmic contacts. The via-based n-type contact structure was implemented to increase the utilization ratio of active region area and improve current spreading over the entire LEDs. Compared with FCLED with Ni/Ag, the FCLED with ITO/DBR shows a 6.3% higher light output power (LOP) at low current density of 31 A/cm2 (90 mA) due to higher reflectance of ITO/DBR ohmic contact. However, at high injection currents, the Ni/Ag ohmic contact in FCLED alleviates self-heating issue by means of its superior heat dissipation performance. We also found that the less area coverage between ITO and metallization layer have a negative influence on heat dissipation performance of FCLED with ITO/DBR. As a result, the maximum light output power (LOP) of the FCLED with Ni/Ag obtained at 292.8 A/cm2 is 1.3 times larger than that of FCLED with ITO/DBR obtained at 196.3 A/cm2.

高光提取效率倒装发光二极管的设计与优化

高光提取效率倒装发光二极管的设计与优化