Flip-chip Light-emitting Diodes Research Articles

Interfacial reaction and failure mode analysis of the solder joints for flip-chip LED on ENIG and Cu-OSP surface finishes

The interfacial reactions and failure modes of the solder joints for flip-chip light emitting diode (LED) on electroless nickel/immersion gold (ENIG) and Cu with organic solderability preservatives (Cu-OSP) surface finishes were investigated in this study. The experimental results demonstrate that the interfacial reactions in the Au/Sn–Ag–Cu(SAC)/ENIG and Au/SAC/Cu systems are different but the failure mechanisms of the two types of solder joints are similar during the shear test. For the Au/SAC/ENIG system, the Au layer on the surface finish of the diodes dissolved into the molten solder and transformed into a continuous (Au, Ni)Sn4 IMC layer at the diode/solder interface during reflow and the interfacial IMC at the solder/ENIG interface is dendritic Ni3Sn4 IMC grains which are surrounded by (Au, Ni)Sn4. For the Au/SAC/Cu system, however, no IMC layers can be observed at the diode/solder interface. The interfacial IMC at the solder/Cu interface is (Cu, Au)6Sn5 and a Cu3Sn IMC layer at the (Cu, Au)6Sn5/Cu interface. Tiny (Au, Cu)Sn4 IMC grains distribute in the solder layer and surround the (Cu, Au)6Sn5 grains. For the two types of systems, the primary failure mode for the cathode is due to the broken of the Si-based insulation layer which led to a high residue stress and poor connection between the Si-based layer and the solder layer. Meanwhile, the failure of the solder joint for the anode is mainly because of the failure of the solder layer under the conductive via. The crack generally forms at this area and then propagated along the diode or the diode/solder interface.

Microstructure Evolution and Shear Behavior of the Solder Joints for Flip-Chip LED on ENIG Substrate

The microstructure evolution and shear behavior of the solder joints for the flip-chip light-emitting diode on the electroless nickel/immersion gold (ENIG) substrate were investigated in this study. The experimental results reveal that the solder joints for the anode and cathode have different microstructures and failure characteristics during the shear test before and after isothermal aging. For the solder joints for the anode, the interfacial intermetallic compound (IMC) is (Au, Ni)Sn4 at the solder/anode interface but dendritic Ni3Sn4 grains at the solder/ENIG interface after reflow. Meanwhile, the dendritic Ni3Sn4 grains are surrounded by (Au, Ni)Sn4, which suppresses the growth of the Ni3Sn4 grains during aging. For the solder joints for the cathode, a nano scaled Au-rich layer can be observed near the cathode/solder layer interface after reflow. And the Au-rich layer moves toward the bulk solder because of the volume expansion by the transformation from Au into (Au, Ni)Sn4 during reflow and isothermal aging. Due to the diffusion of the Au atom from the Au-rich layer into the bulk solder, the Au-rich layer transformed into an interface inside of the solder joint. The average shear force of the solder joints shows a decrease from 380 gf to 250 gf because of the microstructure evolution during the isothermal aging for 1000 h at 85°C. After long time aging, the primary failure mode of the solder joint for the anode changed from the anode broken to the brittle failure of the solder layer. The delamination between the IMC layer and the insulation layer is suggested to be the dominated failure mode of the solder joint for the cathode after aging.

Dual enhancement of light extraction efficiency of flip-chip light-emitting diodes with multiple beveled SiC surface and porous ZnO nanoparticle layer coating

A porous ZnO nanoparticle layer coating composed of columnar ZnO nanoparticle piles and a multiple-beveled substrate was used to enhance the light extraction efficiency of GaN-based flip-chip light-emitting diodes (FC-LEDs), which were grown on high-purity SiC substrates. The SiC substrate was multiple-beveled by fabricating an ‘X’ pattern on the face of it, followed by a deposition of a porous ZnO nanoparticle layer on the ‘X’-patterned surface. A porous ZnO nanoparticle layer was fabricated with gas phase cluster beam deposition in a glancing incidence. The incident angular-resolved light transmission of the ZnO nanostructure beyond the critical angle of total internal reflection (TIR) was greatly enhanced. The light output power of the LED was improved by nearly 60% compared to the original planar GaN-based LED on an SiC substrate (FC-SLED), which contained a significant enhancement supplemental to the 18% electroluminescence (EL) enhancement realized with the ‘X’-pattern beveling. We demonstrated that a dual enhancement of light extraction efficiency was achieved by using the hierarchical surface consisting of microscale textures (the multiple-beveled surfaces) and nanoscale structures (the ZnO nanoparticle layers).

Inverted tetrahedron-pyramidal micropatterned polymer films for boosting light output power in flip-chip light-emitting diodes.

We report the improved light output power in gallium nitride-based green flip-chip light-emitting diodes (FCLEDs) employed with inverted tetrahedron-pyramidal micropatterned polydimethylsiloxane (ITPM PDMS) films as an encapsulation and protection layer. The micropatterns are transferred into the surface of PDMS films from the sapphire substrate master molds with two-dimensional periodic hexagonal TPM arrays by a soft imprint lithography method. The ITPM PDMS film laminated on the sapphire dramatically enhances the diffuse transmittance (T(D)) in a wavelength (λ) range of 400-650 nm, exhibiting the larger T(D) value of ~53% at λ = 525 nm, (cf., T(D) ~1% for planar sapphire). By introducing the ITPM PDMS film on the outer surface of sapphire in FCLEDs, the light output power is enhanced, indicating the increment percentage of ~11.1% at 500 mA of injection current compared to the reference FCLED without the ITPM PDMS film, together with better electroluminescence intensity and far-field radiation pattern.

Morphological effects boosting light extraction from highly-absorptive light-emitting structures

We studied morphological designs for enhanced light extraction from deep-ultraviolet AlGaN flip-chip light-emitting diodes with an absorptive p-type GaN layer. Ray tracing simulations were performed to investigate the dependence of the extraction efficiency on the thickness of the sapphire substrate, the diameter of mesa columns within arrays, and the refractive index of the cladding layer between the multiple quantum wells and the p-type GaN. The extraction efficiency was significantly improved, compared with reference structure, by tailoring each morphological design. This was due to a reduced interaction between the light and the absorptive p-type GaN, which increased the emission through the sapphire and the AlGaN sides.

Performance enhancement of gallium-nitride-based flip-chip light-emitting diode with through-via structure

For the study of the performance enhancement of a gallium-nitride-based flip-chip light-emitting diode with through-via structure (FC-LED), FC-LED and conventional high-power light-emitting diode (HP-LED) were fabricated simultaneously with the same epitaxial structures and packaging processes. The optoelectronic results showed that the FC-LED's performance of the multiple indices for radiant light and voltage parameters were, respectively, better than that of the conventional HP-LED. One reason for this was that the thermal resistance of the FC-LED chip was about 54% lower, which would induce less heat-flux generation. This was in accordance with the simulation results of the p–n junction temperature. Another reason for this was that the extra parasitic resistance in the conventional HP-LED chip would cause higher voltages and lower multiple indices of radiant power.

Solution-Growth ZnO Nanorods for Light Extraction in GaN-Based Flip-Chip LEDs

Institute of Microelectronics & Department of Electrical Engineering, Institute of Nanotechnology and MicrosystemsEngineering & Department of Materials Science and Engineering, Advanced Optoelectronic Technology Center,Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan

Improving the Vertical Light Extraction Efficiency of GaN-Based Thin-Film Flip-Chip LED With Double Embedded Photonic Crystals

The vertical light-extraction efficiency (LEE) of the thin-film flip-chip light-emitting diodes with the embedded photonic crystal (PhC) is investigated using the finite-difference time-domain method. This paper systematically analyzes the dependence of the vertical LEE on the vertical structure and the embedded PhC parameters. It is found that the introduction of the p-side embedded PhC and n-side embedded PhC does not significantly destroy the microcavity resonant effects. In particular, a twofolds enhancement in the vertical LEE is obtained for the optimized structure by scanning the double embedded PhCs parameters and the physical mechanisms for the enhancement of the vertical LEE are discussed.

Bulk GaN flip-chip violet light-emitting diodes with optimized efficiency for high-power operation

We report on violet-emitting III-nitride light-emitting diodes (LEDs) grown on bulk GaN substrates employing a flip-chip architecture. Device performance is optimized for operation at high current density and high temperature, by specific design consideration for the epitaxial layers, extraction efficiency, and electrical injection. The power conversion efficiency reaches a peak value of 84% at 85 °C and remains high at high current density, owing to low current-induced droop and low series resistance.

Optimized double-sided hemispherical pattern design on patterned sapphire substrate for flip-chip GaN-based LED

Optimized double-sided hemispherical pattern design on patterned sapphire substrate for flip-chip GaN-based LED

Embeded photonic crystal at the interface of p-GaN and Ag reflector to improve light extraction of GaN-based flip-chip light-emitting diode

In this experiment, a flip-chip light-emitting diode with photonic crystal was fabricated at the interface of p-GaN and Ag reflector via nanospheres lithography technique. In this structure, photonic crystal could couple with the guide-light efficiently by reason of the little distance between photonic crystal and active region. The light output power of light emitting diode with embedded photonic crystal was 1.42 times larger than that of planar flip-chip light-emitting diode. Moreover, the embedded photonic crystal structure makes the far-field divergence angle decreased by 18° without spectra shift. The three-dimensional finite difference time domain simulation results show that photonic crystal could improve the light extraction, and enhance the light absorption caused by Ag reflector simultaneously, because of the roughed surface. The depth of photonic crystal is the key parameter affecting the light extraction and absorption. Light extraction efficiency increases with the depth photonic crystal structure rapidly, and reaches the maximum at the depth 80 nm, beyond which light extraction decrease drastically.

White thin-film flip-chip LEDs with uniform color temperature using laser lift-off and conformal phosphor coating technologies.

We fabricated a phosphor-conversion white light emitting diode (PC-WLED) using a thin-film flip-chip GaN LED with a roughened u-GaN surface (TFFC-SR-LED) that emits blue light at 450 nm wavelength with a conformal phosphor coating that converts the blue light into yellow light. It was found that the TFFC-SR-LED with the thin-film substrate removal process and surface roughening exhibits a power enhancement of 16.1% when compared with the TFFC-LED without a sapphire substrate. When a TFFC-SR-LED with phosphors on a Cu-metal packaging-base (TFFC-SR-Cu-WLED) was operated at a forward-bias current of 350 mA, luminous flux and luminous efficacy were increased by 17.8 and 11.9%, compared to a TFFC-SR-LED on a Cup-shaped packaging-base (TFFC-SR-Cup-WLED). The angular correlated color temperature (CCT) deviation of a TFFC-SR-Cu-WLED reaches 77 K in the range of -70° to + 70° when the average CCT of white LEDs is around 4300 K. Consequently, the TFFC-SR-LED in a conformal coating phosphor structure on a Cu packaging-base could not only increase the luminous flux output, but also improve the angular-dependent CCT uniformity, thereby reducing the yellow ring effect.

Optical, spectral, and thermal characteristics of InGaN/GaN green flip-chip light-emitting diodes

We reported the temperature-dependent device characteristics of InGaN/GaN green flip-chip light-emitting diodes (large chip size of 1125×1125μm2) on patterned sapphire substrate. Through the experiments in the temperature range of 298–358K, optical, electrical, and spectral properties were measured and analyzed. At 350mA and 298K, the optical output power of 56.2mW, forward voltage of 3.26V, and emission peak wavelength of 503.6nm were obtained. The characteristic temperature was also estimated at an injection current of 350mA under pulsed mode, indicating a value of 1019K. The junction temperature was experimentally measured by the forward voltage method. For comparison with theoretically calculated results, three-dimensional steady-state heat transfer model based on finite element method was employed. The thermal resistance (∼12.76K/W) extracted from the experimental results exhibited a similar value with the simulated result (~14.22K/W).

Chip-on-Flexible Packaging for High-Power Flip-Chip Light-Emitting Diode by AuSn and SAC Soldering

This paper presents a new chip-on-flexible (COF) packaging structure for flip-chip (FC) light-emitting diode (LED). Two bonding methods, AuSn and Sn-Ag-Cu (SAC) soldering, were compared from thermal, mechanical, and optical performances of the packages. Compared with the package by AuSn soldering method, the package by SAC soldering method showed lower chip temperature due to the avoiding of the large unbounded area. In addition, the SAC soldering package showed a much lower residue stress than the AuSn soldering package. Thus, the possibility for the defects, such as the crack on the reflecting layer and the warping of the pads formation decreased significantly. The reflectivity of the flexible substrate showed a sharp decrease because of the high temperature of AuSn soldering process. Based on the comprehensive evaluation of the thermal properties, residue stress, and optical performances of the LED packages, SAC soldering method is suggested to be more suitable for high-power LED packaging on COF substrates.

Improving the Vertical Light-Extraction Efficiency of GaN-Based Thin-Film Flip-Chip LEDs With p-Side Deep-Hole Photonic Crystals

This study systematically investigates the light-extraction efficiency (LEE) of GaN-based thin-film flip-chip light-emitting diodes (TFFC LEDs) with deeply etched photonic crystals (PhC). The optimal structure can be obtained by scanning the structural parameters using a three-dimensional finite-difference time-domain simulation. The concentration of the internal emission pattern within a limited extraction cone is significantly influenced by several structural parameters. The light-collection enhancement within an extraction cone explains the peak LEE caused by optimized structural parameters such as the p-GaN thickness, the air-hole depth, the lattice constant and the filling factor. The maximum extraction efficiency can reach 65% in the case of a p-GaN thickness of 140 nm, air-hole depth of 240 nm, lattice constant of 350 nm and filling factor of 0.40. Additionally, fabricating the optimal electrical contacts on the planar top surface of the n side can contribute to a higher potential enhancement.

Performance Enhancements of Flip-Chip Light-Emitting Diodes With High-Density n-Type Point-Contacts

Novel gallium nitride-based flip-chip light-emitting diodes (FCLEDs) with high-density distributed n-type point-contacts were designed and fabricated. With high density and uniformly distributed n-type point-contacts, the point-contact (PC) FCLEDs had higher light output power (LOP) by 18% over the reference flip-chip LED with conventional contacts fabricated from the same wafer. The forward voltage of the PC-FCLEDs was 0.16 V lower than the reference FCLED and the wall-plug efficiency was increased by 24% at the same current level. The maximum LOP of the PC-FCLEDs measured at 2.4 A was 43% more than the maximum obtained by the reference LED at 1.8 A. It was also found that the PC-FCLEDs suffered lower efficiency droop. The optical performance improvement of the PC-FCLEDs is attributed to an increase of the light extraction and the uniform carrier distribution, which results from the small and high density deeply etched holes and PCs. The electrical performance was enhanced through a minimized lateral current spreading distance.

Improved Thermal Stability of GaN-Based Flip-Chip Light-Emitting Diodes with TiW-Based Diffusion Barrier

Improved Thermal Stability of GaN-Based Flip-Chip Light-Emitting Diodes with TiW-Based Diffusion Barrier

Measurement of LED chips using a large-area silicon photodiode.

We propose the output power measurement of bare-wafer/chip light-emitting diodes (LEDs) using a large-area silicon (Si) photodiode with a simple structure and high accuracy relative to the conventional partial flux measurement using an integrating sphere. To obtain the optical characteristics of the LED chips measured using the two methods, three-dimensional ray-trace simulations are used to perform the measurement deviations owing to the chip position offset or tilt angle. The ray-tracing simulation results demonstrate that the deviation of light remaining in the integrating sphere is approximately 65% for the vertical LED chip and 53% for the flip-chip LED chip if the measurement distance in partial flux method is set to be 5-40 mm. By contrast, the deviation of light hitting the photodiode is only 15% for the vertical LED chip and 23% for the flip-chip LED chip if the large-area Si photodiode is used to measure the output power with the same measurement distance. As a result, the large-area Si photodiode method practically reduces the output power measurement deviations of the bare-wafer/chip LED, so that a high-accuracy measurement can be achieved in the mass production of the bare-wafer/chip LED without the complicated integrating sphere structure.

Performance enhancement of GaN-based flip-chip ultraviolet light-emitting diodes with a RPD AlN nucleation layer on patterned sapphire substrate

In this work, flip-chip ultraviolet light-emitting diodes (FCUV-LEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the FCUV-LEDs with RPD AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. From the transmission electron microscopy (TEM) image, it can be observed that the tip and incline portion of the pattern was smooth using the RPD AlN nucleation layer. The threading dislocation densities (TDDs) are reduced from 7 × 107 cm−2 to 2.5 × 107 cm−2 at the interface between the u-GaN layers for conventional and AlN PSS devices, respectively. As a result, a much higher light output power was achieved. The improvement of light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence measurement and numerical simulation confirm such increase of output power can be attributed to the improvement of material quality and light extraction.

Image matching technology in high power LED's eutectic welding.

As the integration packaging density of high-power LED (Light Emitting Diode) chip modules become higher and higher, the accuracy and speed of visual inspection require higher demands correspondingly. The accurate position matching of substrates and flip-chip LEDs is one of the key technologies in the automatic eutectic welding process. In this paper we propose a method based on image features to complete the matching of the substrates and the flip-chip LEDs. Firstly, the substrate images and the flip-chip images are pre-processed respectively to obtain binary images. Then we apply Hough transformation to detect straight lines on the binary images, and find out the main linear directions to trigger the mechanical arms to adjust the positions of the substrate and the chip initially. Thirdly, we use eight neighbors interconnected domain algorithm for the first time to locate notable features of the substrate, and pass the located information to the control system to trigger the mechanical arm to adjust the substrate for the second time. At the same time, projection algorithm is applied to locate the anode of the flip-chip to drive the mechanical arm to adjust the position of the flip-chip again. Finally, the position information is used to trigger the mechanical arm to accomplish the matching of the substrate and the flip-chip. The proposed method improves the speed of matching on the basis of the accuracy of matching, which achieves these requirements of real-time and high accuracy applications.