Massimo Vanzi

Accelerated Life Test of High Brightness Light Emitting Diodes

Short-term accelerated life test activity on high brightness light emitting diodes is reported. Two families of 1-W light-emitting diodes (LEDs) from different manufacturers were submitted to distinct stress conditions: high temperature storage without bias and high dc current test. During aging, degradation mechanisms like light output decay and electrical property worsening were detected. In particular, the degradation in light efficiency induced by thermal storage was found to follow an exponential law, and the activation energy of the process was extrapolated. Aged devices exhibited a modification of the package epoxy color from white to brown. The instability of the package contributes to the overall degradation in terms of optical and spectral properties. In addition, an increase in thermal resistance was detected on one family of LEDs. This increase induces higher junction temperature levels during operative conditions. In order to correlate the degradation mechanisms and kinetics found during thermal stress, a high dc current stress was performed. Results from this comparative analysis showed similar behavior, implying that the degradation process of dc current aged devices is thermal activated due to high temperatures reached by the junction during stress. Finally, the different effects of the stress on two families of LEDs were taken into account in order to identify the impact of aging on device structure.

High temperature electro-optical degradation of InGaN/GaN HBLEDs

This paper presents a study of the high temperature degradation of high brightness light emitting diodes (HBLEDs) on gallium nitride. Two different families of devices, from two leading manufacturers, have been submitted to thermal stress: during treatment, the optical and electrical characteristics of the devices have been analyzed. Degradation modes detected after stress have been (i) operating voltage increase, (ii) output power decrease, (iii) modifications of the spectral properties. The degradation of the electrical and optical characteristics of the devices were found to have different kinetics: this fact indicates that optical power (OP) loss is not strongly related to the degradation of the electrical parameters of the LEDs. On the other hand, spectral analysis indicated that OP loss is strongly related to the decrease of the phosphors-related yellow emission band. Microscopic analysis showed that this effect can be ascribed to the carbonization of the package and phosphorous material. A degradation of the transparency of the top-side ohmic contact has been also detected after stress: these mechanisms are thought to be responsible for the detected OP decrease. OP decay process has been found to be thermally activated, with activation energy equal to 1.5 eV.

Failure Analysis-assisted FMEA

In this paper, ordinary FMEA (Failure Mode and Effects Analysis) was applied during the design phase of an electric motor control system for vehicle HVAC (Heating/Ventilation/Air Conditioning). The analysis of the field data from the second year forces to review FMEA. The corrective actions, planned on the basis of the sole failure mode, as usual in FMEA, proved to be inadequate and Failure Analysis was performed to understand the failure mechanism of the indicted component and integrate. New proper corrective actions were devised and successfully implemented.

High brightness GaN LEDs degradation during dc and pulsed stress

This paper reports an analysis of InGaN/GaN LEDs degradation under dc and pulsed current conditions. The analysis was carried out by means of current-voltage and optical power measurements, scanning electron microscopy and EDS maps. Identified degradation modes were leakage and generation/recombination current increase, series resistance increase and output power decrease, related to the high temperatures reached by the devices during stress. Failure analysis revealed degradation of anode contacts and Ti-W reflector, which can be related to the measured series resistance increase. Comparison between dc and pulsed stress carried out with the same average current indicated that pulsed driving does not imply an acceleration in the degradation rate, except for the lowest duty cycles.

Thermal stability analysis of high brightness LED during high temperature and electrical aging

In this paper we report the analysis of thermal stability of High Brightness Light Emitting Diode subjected to thermal and bias ageing. The degradation mechanisms of several families of commercial available devices were investigated. In the first part of the work we estimated thermal resistance and thermal behaviour under dc bias condition. After this thermal characterisation two different ageing tests were carried out on devices: thermal aging at high temperature levels without biasing the devices and accelerated dc stress at nominal current value (400mA). At each step a complete electrical and optical characterisation of aged devices was performed, in order to find a correlation between different aging and a better understanding of degradation mechanism. This characterisation included I-V measurements, optical power vs current characteristics and spectral analysis. During thermal stress we observed the increase of forward voltage at nominal current and the degradation of optical power with nearly exponential kinetics. We found that lifetimes were well correlated with stress temperature: therefore it was possible to find an activation energy of degradation mechanism of about 1.5eV. Moreover, modifications of spectral properties during electrical and thermal stress were found. Thus, a package level analysis was carried out in order to clarify the role of modification in optical properties of reflector cup and the efficiency of phosphors. Finally, evaluation of differential structure functions indicated that stress induces also the worsening of the properties of the chip-to-package thermal path: this phenomenon has been attributed to the partial detachment and degradation of the ohmic contacts.

Failure modes and mechanisms of DC-aged GaN LEDs

This paper presents failure modes observed in long-term aging of high-brightness GaN/InGaN LEDs. The blue LEDs submitted to DC aging test present large decrease of emitted optical power and increase of diode reverse leakage current. Increase of parasitic series resistance, suggesting contact degradation, has also been found in stressed sample, together with apparent carrier density increases and reduction of the junction depletion width. Furthermore stressed LEDs present modification of a specific trap property: trap activation energy decreases from 340 meV in the virgin sample down to 75 meV in the stressed sample. Generation of non-radiative recombination centers seems to be one of the dominant failure mechanisms responsible for the observed electrical and optical LED degradations.

Chip and package-related degradation of high power white LEDs

With this paper we present an analysis of the degradation of state-of-the-art high power LEDs. Three different kinds of commercially available samples, from leading manufacturers, were submitted to stress under various current and temperature levels. Based on an accurate estimation of the thermal resistance of the devices, iso-thermal and iso-current stress tests have been carried out, with the aim of separately evaluating the role of current and temperature in determining the degradation of the LEDs. Results indicate that state-of-the-art LEDs can show a significant degradation of their electrical and optical characteristics, when they are operated close to their current/temperature limits. In particular, data reveal the presence of two different degradation mechanisms: (i) the degradation of the blue semiconductor chip, due to the increase in non-radiative recombination, or to the decrease in the acceptor dopant concentration at the p-side of the diodes; (ii) the chemical degradation of the package, with subsequent worsening of its optical properties. Results suggest that even high-performance LEDs can suffer from limited lifetime: thermal management and driving conditions must be carefully optimized with the aim of achieving high reliability for LEDs to be adopted in high efficiency lighting systems.

Sulfur-contamination of high power white LED.

Silver–sulfur compounds have been detected at the internal lead frame of several commercial plastic packaged white LEDs that passed a standard incoming lot inspection but resulted all electrically open or degraded at the switching-on after mounting on the final boards.

An Automatic Alignment Procedure for a Four-Source Photometric Stereo Technique Applied to Scanning Electron Microscopy

This paper presents an automatic alignment procedure for a four-source photometric stereo (PS) technique for reconstructing the depth map in the scanning electron microscope (SEM). PS, which is based on the so-called reflectance map, used several images of a surface to estimate the surface depth at each image point, in which the Lambertian reflectivity function is the simplest. In the SEM, the backscattered electron emission, which is one of the most important signals, is nearly Lambertian, and to simplify matters, SEM images are intrinsically grayscale maps. The possibility of having electron-PS at the SEM is assumed, taking advantage of one of the most exciting features of the technique, which returns true numerical 3-D models instead of some depth illusion from ordinary pictures.

Observation of latch-up phenomena in CMOS ICs by means of digital differential voltage contrast

The latch-up phenomenon in CMOS ICs is studied by means of a SEM observation technique that is based on capacitively coupled voltage contrast and is characterized by digital beam control and image acquisition. Passivated devices are studied at low beam energies, without interfering with their electrical behavior. The comparison between images taken in the latched and nonlatched state allows reliable identification of the latch-up current paths and thus of the latched site. The performance of the technique is demonstrated by three examples which refer to one standard and two custom CMOS ICs.