Thermal/luminescence characterization and degradation mechanism analysis on phosphor-converted white LED chip scale packages

According to the requirements on minimizing the package size, guaranteeing the performance uniformity and improving the manufacturing efficiency in LEDs, a Chip Scale Packaging (CSP) technology has been developed to produce white LED chips by impressing a thin phosphor film on LED blue chips. In this paper, we prepared two types of phosphor-converted white LED CSPs with high color rendering index (CRI > 80, CCT ~ 3000 K and 5000 K) by using two mixed multicolor phosphor materials. Then, a series of testing and simulations were conducted to characterize both short- and long-term performance of prepared samples. A thermal analysis through both IR thermometry and electrical measurements and thermal simulation were conducted first to evaluate chip-on-board heat dissipation performance. Next, the luminescence mechanism of multicolor phosphor mixtures was studied with the spectral power distribution (SPD) simulation and near-field optical measurement. Finally, the extracted features of SPDs and electrical current-output power (I-P) curves measured before and after a long-term high temperature accelerated aging test were applied to analyze the degradation mechanisms. The results of this study show that: 1) The thermal management for prepared CSP samples provides a safe usage condition for packaging materials at ambient temperature; 2) The Mie theory with Monte-Carlo ray-tracing simulation can be used to simulate the SPD of Pc-white LEDs with mixed multicolor phosphors; 3) The degradation mechanisms of Pc-white LEDs can be determined by analyzing the extracted features of SPDs collected after aging.     

Improving the light output power of GaN-based light-emitting diodes through the use of SiO2 cones

We introduced a simple wet-etching process to form SiO₂ cones and investigated the effect of the size and coverage of the SiO₂ cones on the output power of GaN-based light-emitting diodes (LEDs). The diameter of the cones varies from 2.8 to 17.1 μm and the height from 0.6 to 2.0 μm. It is shown that regardless of the sizes of the cones, all of the LEDs exhibit a same forward-bias voltage of 3.31 V at an injection current of 20 mA. As the size of the cones increases, the light output increases, reaches maximum at cone #3 (12.4 μm in diameter and 2.0 μm in height), and then decrease slightly. For example, the LEDs fabricated with different SiO₂ cones exhibit 11.4–35.9% higher light output power (at 20 mA) than do the LEDs without the cones. The electroluminescence (EL) intensity (at 20 mA) also exhibits cone size dependence similar to that of light output power. For example, the LEDs fabricated with different cones exhibit 7.7–36.3% higher EL intensity than the LEDs without the cones.     

Optimizing n-type contact design and chip size for high-performance indium gallium nitride/gallium nitride-based thin-film vertical light-emitting diode

The effects of the n-contact design and chip size on the electrical, optical and thermal characteristics of thin-film vertical light-emitting diodes (VLEDs) were investigated to optimize GaN-based LED performance for solid-state lighting applications. For the small (chip size: 1000×1000 µm²) and large (1450×1450 µm²) VLEDs, the forward bias voltages are decreased from 3.22 to 3.12 V at 350 mA and from 3.44 to 3.16 V at 700  mA, respectively, as the number of n-contact via holes is increased. The small LEDs give maximum output powers of 651.0–675.4 mW at a drive current of 350 mA, while the large VLEDs show the light output powers in the range 1356.7–1380.2 mW, 700 mA, With increasing drive current, the small chips go through more severe degradation in the wall-plug efficiency than the large chips. The small chips give the junction temperatures in the range 51.1–57.2 °C at 350  mA, while the large chips show the junction temperatures of 83.1–93.0 °C at 700  mA, The small LED chips exhibit lower junction temperatures when equipped with more n-contact via holes.     

Electrical and optical characterization of GaN-based light-emitting diodes fabricated with top-emission and flip-chip structures

We investigated the electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with top-emission and flip-chip structures. Compared with top-emission LEDs, flip-chip LEDs exhibited a 0.25 V smaller forward voltage and an 8.7 Ω lower diode resistance. The light output power of the flip-chip LED was also larger than that of the top-emission LED by factors of 1.72 and 2.0 when measured before and after packaging, respectively. The improved electrical and optical output performances of flip-chip LEDs were quantitatively analyzed in terms of device resistance and ray optics, respectively.     

Low temperature molten salt synthesis of YAG: Ce spherical powder and its thermally stable luminescent properties after post-annealing treatment

Spherical YAG: Ce particles were successfully synthesized at 350 °C by the molten-salt method. The effect of temperature and amount of salt on the crystallization and particle size of YAG: Ce were investigated thoroughly. The results demonstrated that the powders prepared at 350 °C in salt to reactant ratio 2:1 were pure YAG: Ce phase with 200–300 nm in particle size. The as-synthesized phosphors were later post annealed at 1200 °C in O₂, air and N₂, respectively. The results showed that the emission intensity of YAG: Ce sensitively depended on the post-treated atmosphere and the phosphor annealed in N₂ showed the highest emission intensities and a good thermal stability.     

Accelerated life test of high power white light emitting diodes based on package failure mechanisms

Accelerated life tests of high-power white light emitting diodes (LEDs) were conducted under an unbiased highly accelerated temperature and humidity test (HAST) and a normal aging test. The conditions in the unbiased HAST were 110 °C-85% RH, 130 °C-85% RH without bias. During the aging, the degradation mechanisms of optical power reduction and degradation of 455 mm blue wavelengths that were better than phosphors related yellow emission bands were observed. The microscopy analysis showed that this effect could be ascribed to the bubbling and discoloration of the silicone encapsulating material of the package. It is thought that these features are also responsible for the optical power reduction and thermal resistance increase.     

Effects of silicone coating degradation on GaN MQW LEDs performances using physical and chemical analyses

This work presents a physics of failure (POF) methodology coupling failure signatures with physico-chemical analyses. The aim is to work out electro-optical failure signatures located in packaged InGaN/GaN Multiple Quantum Wells Light Emitting Diodes (MQW LEDs). Electrical and optical characteristics performed after accelerated ageing tests (30 mA/85 °C/1500 h), confirm a 65% drop of optical power and an increase of one decade of leakage current spreading at the silicone oil/chip interfaces. Through measurements of silicone coating fluorescence emission spectra, we demonstrate that the polymer enlarges the LED emission spectrum and shifts central wavelength. This shift is related to silicone oil spectral instability and the central wavelength of packaged LED appears to be temperature insensitive. In this paper, we discriminate the degradation of bulk silicone oil responsible for optical losses from the polymer/chip interface inducing larger leakage current.     

Degradation mechanisms of high-power white LEDs activated by current and temperature

This paper presents a study of the degradation mechanisms that limit the reliability of commercially-available white LEDs. Purely thermal stress and biased iso-thermal stress were carried out for several thousands hours on 1W-power LEDs, produced by a leading manufacturer. Results reveal that temperature and operating current have different roles in determining the optical degradation of these devices: (i) pure thermal stress induces a short-term optical power decay, strictly correlated to the decrease in the reflectivity of the package/reflector system and with no effects on the electrical characteristics of the blue chip; the activation energy of thermally-induced degradation is equal to 1.8 eV; (ii) constant current stress induces a long-term degradation process, with a degradation rate which is strongly dependent on the stress current level. In this latter case, optical degradation is ascribed to the degradation of the blue semiconductor chip: details are provided through the analysis of forward voltage and wavelength shift during stress time.     

Multifunctional homoleptic iridium(III) dendrimers towards solution-processed nondoped electrophosphorescence with low efficiency roll-off

A series of new iridium dendrimers comprised of bifunctional charge transport peripheral groups have been designed and facilely synthesized. The relationship between the structures and their photophysical, electrochemical and electrophosphorescent performances is investigated. Through the incorporation of rigid electron-transporting phosphine oxide groups and/or hole-transporting arylamine units, the new complexes all have good thermal stabilities with high glass-transition temperature up to 284 °C. Besides, the peripheries sufficiently shield the emissive core from the intermolecular interactions and prevent luminance quenching in neat films. Solution-processed phosphorescent organic light-emitting device (PhOLED) based on bipolar phosphor 2 as neat emitter achieves a maximum current efficiency of 12.4 cd A⁻¹ with Commission Internationale de l’Eclairage coordinates of (0.57, 0.42), and the value remains at 11.5 cd A⁻¹ at a practical luminance of 1000 cd m⁻². This low roll-off can be attributed to the bipolar nature of the emitter. This indicates that rational incorporation of charge-transporting moieties into the sphere of iridium(III) core is a simple and effective approach to develop efficient host-free phosphors for solution-processable nondoped PhOLEDs.     

Electrical and thermal stability of Ag ohmic contacts for GaN-based flip-chip light-emitting diodes by using an AgAl alloy capping layer

We have investigated Ag(200 nm)/AgAl(100 nm) ohmic contacts to p-type GaN for near-UV (405 nm) flip-chip light-emitting diodes (LEDs). It is shown that the use of an AgAl alloy capping layer (with 8 at% Al) results in better electrical and optical properties as compared to single Ag contacts when annealed at 430 °C. For example, Ag/AgAl (8 at% Al) contacts give specific contact resistance of 4.6×10^–4 Ω cm² and reflectance of 90% at a wavelength of 405 nm. However, use of an AgAl (with 50 at% Al) layer is not effective. LEDs fabricated with the Ag/AgAl (8 at% Al) reflectors produce higher light output as compared with the ones with single Ag reflectors. Ohmic mechanisms of the Ag/AgAl (8 at% Al) contacts are described and discussed.     

Thermal characterization of multicolor LED luminaire

The LED based dynamic lighting scheme, require compact and thermally efficient luminaire. This paper presents the thermal investigation on the conceptual design of 36 W multicolor light emitting diode (LED) luminaire. The developed prototype design includes configuration and placement of the multichip LED package, RGBW and single die amber LEDs in a 5 × 3 array on the heat sink. LED configurations with low power input are placed between the LEDs having the high power input. The proposed configuration and placement of LEDs reduces the local heat concentration in the centre region of the heat sink. The temperature of 72 °C at LED chip base plate is reduced to 32.1 °C on the heat sink surface. The numerical results are experimentally validated. The proposed method contributes to a reduction in the size of the luminaire and also enhancement of heat dissipation for improving the longevity of the multicolor based LED luminaire.     

Degradation behaviors of GaN light-emitting diodes under high-temperature and high-current stressing

A number of commercially available multiple-quantum well (MQW) InGaN/GaN blue LEDs with wavelengths of about 460 nm and a power of 1 mW were stressed at temperatures ranging from 25 to 120 °C at several accelerated DC currents. Both the forward and reverse current voltage characteristics as well as the electroluminescent spectra of the LEDs were monitored. These effects also resulted in the pronounced degradation of light efficiency and device operation lifetime. We found that the degradation of photonic characteristics, correlated very well with the generation-recombination current which is governed by the defect density. The device degradation is much faster at high temperatures. At nominal operation current and at room temperature, the light intensity degradation reaches a saturation level before the light dyes out. These results shed new lights upon the design and lifetime specifications for the emerging commercial solid-state lighting devices.     

Formation of low-resistance and transparent indium tin oxide ohmic contact for high-brightness GaN-based light-emitting diodes using a Sn–Ag interlayer

We report on the formation of low-resistance and highly transparent indium tin oxide (ITO) ohmic contacts to p-GaN using a Sn–Ag alloy interlayer. Although the as-deposited Sn–Ag(6 nm)/ITO(200 nm) contacts show non-ohmic behaviors, the scheme becomes ohmic with specific contact resistance of 4.72×10⁻⁴ Ω cm² and produce transmittance of ∼91% at wavelength of 460 nm when annealed at 530 °C. Blue light-emitting diodes (LEDs) fabricated with the Sn–Ag/ITO contacts give forward-bias voltage of 3.31 V at injection current of 20 mA. LEDs with the Sn–Ag/ITO contacts show the improvement of the output power by 62% (at 20 mA) compared with LEDs with Ni/Au contacts.     

Enhanced efficiency of organic photovoltaic cells with Sr2SiO4:Eu2+ and SrGa2S4:Eu2+ phosphors

We report the effect of yellow Sr₂SiO₄:Eu²⁺ and green SrGa₂S₄:Eu²⁺ phosphors on the efficiency of organic photovoltaic (OPV) cells. Each phosphor was coated on the back side of indium tin oxide (ITO)/glass substrates by spin coating with poly(methyl methacrylate) (PMMA). The maximum absorption wavelength of the active layer in the OPV cells was ～512 nm. The emission peaks of Sr₂SiO₄:Eu²⁺ and SrGa₂S₄:Eu²⁺ were maximized at 552 nm and 534 nm, respectively. The short circuit current density (J_sc) and power conversion efficiency (PCE) of the OPV cells with Sr₂SiO₄:Eu²⁺ (8.55 mA/cm² and 3.25%) and with SrGa₂S₄:Eu²⁺ (9.29 mA/cm² and 3.3%) were higher than those of the control device without phosphor (7.605 mA/cm² and 3.04%). We concluded that phosphor tuned the wavelength of the incident light to the absorption wavelength of the active layer, thus increasing the J_sc and PCE of the OPV cells.     

Modulation instability initiated high power all-fiber supercontinuum lasers and their applications

High average power, all-fiber integrated, broadband supercontinuum (SC) sources are demonstrated. Architecture for SC generation using amplified picosecond/nanosecond laser diode (LD) pulses followed by modulation instability (MI) induced pulse breakup is presented and used to demonstrate SC sources from the mid-IR to the visible wavelengths. In addition to the simplicity in implementation, this architecture allows scaling up of the SC average power by increasing the pulse repetition rate and the corresponding pump power, while keeping the peak power, and, hence, the spectral extent approximately constant. Using this process, we demonstrate >10 W in a mid-IR SC extending from ～0.8 to 4 μm, >5 W in a near IR SC extending from ～0.8 to 2.8 μm, and >0.7 W in a visible SC extending from ～0.45 to 1.2 μm. SC modulation capability is also demonstrated in a mid-IR SC laser with ～3.9 W in an SC extending from ～0.8 to 4.3 μm. The entire system and SC output in this case is modulated by a 500 Hz square wave at 50% duty cycle without any external chopping or modulation. We also explore the use of thulium doped fiber amplifier (TDFA) stages for mid-IR SC generation. In addition to the higher pump to signal conversion efficiency demonstrated in TDFAs compared to erbium/ytterbium doped fiber amplifier (EYFA), the shifting of the SC pump from ～1.5 to ～2 μm is pursued with an attempt to generate a longer extending SC into the mid-IR. We demonstrate ～2.5 times higher optical conversion efficiency from pump to SC generation in wavelengths beyond 3.8 μm in the TDFA versus the EYFA based SC systems. The TDFA SC spectrum extends from ～1.9 to 4.5 μm with ～2.6 W at 50% modulation with a 250 Hz square wave. A variety of applications in defense, health care and metrology are also demonstrated using the SC laser systems presented in this paper.     

Low power high gain CMOS LNA based on inverter cell and self-body bias for UWB receivers

A low-power low-noise amplifier (LNA) utilized a resistive inverter configuration feedback amplifier to achieve the broadband input matching purposes. To achieve low power consumption and high gain, the proposed LNA utilizes a current-reused technique and a splitting-load inductive peaking technique of a resistive-feedback inverter for input matching. Two wideband LNAs are implemented by TSMC 0.18 μm CMOS technology. The first LNA operates at 2–6 GHz. The minimum noise figure is 3.6 dB. The amplifier provides a maximum gain (S₂₁) of 18.5 dB while drawing 10.3 mW from a 1.5-V supply. This chip area is 1.028×0.921 mm². The second LNA operates at 3.1–10.6 GHz. By using self-forward body bias, it can reduce supply voltage as well as save bias current. The minimum noise figure is 4.8 dB. The amplifier provides a maximum gain (S₂₁) of 17.8 dB while drawing 9.67 mW from a 1.2-V supply. This chip area is 1.274×0.771 mm².     

B-Spline X-Ray Diffraction Imaging — Rapid non-destructive measurement of die warpage in ball grid array packages

Next generation “More than Moore” integrated circuit (IC) technology will rely increasingly on the benefits attributable to advanced packaging (www.itrs.net [1]). In these increasingly heterogeneous systems, the individual semiconductor die is becoming much thinner (25 to 50 μm, typically) and multiple dies can be stacked upon each other. It is difficult to assess non-destructively, non-invasively and in situ the stress or warpage of the semiconductor die inside these chip packages and conventional approaches tend to monitor the warpage of the package rather than the die.This paper comprises an account of a relatively new technique, which we call B-Spline X-Ray Diffraction Imaging (B-XRDI) and its application, in this instance, to the non-destructive mapping of Si semiconductor die lattice misorientation inside wire bonded encapsulated Low-profile Fine-pitch Ball Grid Array (LFPGA) packages. B-XRDI is an x-ray diffraction imaging technique which allows the user to reconstruct from a series of section x-ray topographic images a full profile of the warpage of the silicon semiconductor die inside such a chip package. There is no requirement for pre-treatment or pre-processing of the chip package and we show that synchrotron-based B-XRDI mapping of wafer warpage can be achieved with angular tilt resolutions of the order of 50 μrad ≈ 0.003° in times as short as 9–180 s (worst case X–Y spatial resolution = 100 μm) for a full 8.7 mm × 8.7 mm semiconductor die inside the fully encapsulated LFBGA packages. We confirm the usefulness of the technique by correlating our data with conventional warpage measurements performed by mechanical and interferometric profilometry and finite element modelling (FEM). We suggest that future developments will lead to real-time, or near real-time, mapping of thermomechanical stresses during chip packaging processes, which can run from bare wafer through to a fully encapsulated chip package.     

3.1–10.6 GHz ultra-wideband LNA design using dual-resonant broadband matching technique

This paper presents an ultra-wideband low noise amplifier design using the dual-resonant broadband matching technique. The proposed LNA achieves a 10.2 dB gain with ±0.9 dB gain flatness over a frequency range of 3.1–10.6 GHz and a ?3-dB bandwidth of 2.4–11.6 GHz. The measured noise figure ranges from 3.2 to 4.7 dB over 3.1–10.6 GHz. At 6.5 GHz, the measured IIP3 and input-referred P1dB are +6 dBm and ?5 dBm, respectively. The proposed LNA occupies an active chip area of 0.56 mm² in a TSMC 0.18 μm RF-CMOS process and consumes 16 mW from a 1.8 V supply.     

A compact broadband MMIC sub-harmonic mixer using quasi-lumped transmission lines

A compact broadband monolithic microwave integrated circuit (MMIC) sub-harmonic mixer using an OMMIC 70 nm GaAs mHEMT technology is demonstrated for 60 GHz down-converter applications. The present mixer employs an anti-parallel diode pair (APDP) to fulfill a sub-harmonic mixing mechanism. Quasi-lumped components are employed to broaden the operational bandwidth and minimize the chip size to 1.5×0.77 mm². The conversion gain is optimized by a quasi-lumped 90° phase shift stub. Experimental results show that from 50 GHz to 70 GHz, the conversion gain varies between −12.1 dB and −15.2 dB with a LO power level of 10 dBm and 1 GHz IF. The LO-to-RF, LO-to-IF and RF-to-IF isolations are found to be greater than 19.5 dB, 21.3 dB and 25.8 dB, respectively. The second harmonic component of the LO signal is suppressed. The proposed mixer has an input 1 dB compression point of -2 dBm and exhibits outstanding figure-of-merits.     

Simulation of LED based luminaires by using multi-domain compact models of LEDs and compact thermal models of their thermal environment

Fast and accurate prediction of hot lumens of LEDs installed in luminaires is an important step in the design of robust and reliable products. A possible approach to this is to create a multi-domain circuit model of a complete LED chip + package + luminaire system that can be simulated by any Spice-like circuit simulator with electro-thermal capabilities. Many LED chip and LED package models and modeling techniques have been published recently, but compact thermal modeling of luminaires as multi heat-source system was not yet dealt with in the literature. This paper aims to fill this gap be describing a systematic approach for system (luminaire) level analysis aimed at solving the combined thermal, electrical and light output simulation problem consistently by describing a method for creating a compact thermal model of LED luminaries with an approach borrowed from the layout based electro-thermal simulation of analog ICs. The applicability of the described method is demonstrated with a real life example, including the validation of the results with thermal measurements.