Henk van Zeijl

Increasing the reliability of solid state lighting systems via self-healing approaches: A review

Reliability issues in solid state lighting (SSL) devices based on light emitting diodes (LED) is of major concern as it is a limiting factor to promote these optoelectronic devices for general lighting purposes. This postulate is even truer for high power devices in which high current and thus high thermal load are involved. In order to increase reliability and lighting efficacy, LED designs related to thermal management are evolving parallel to LED research and development. However there are still some issues mainly related to the degradation of LED’s constituents with time involving a faster decay of the lightning efficacy. In order to increase reliability of SSL devices, components presenting self-repairing properties could be implemented. In this review we will first briefly expose the state of the art on inorganic semiconductor based LED research and development, trends and challenges that lead to an increase of lighting efficiency. In a second part the different failure mode occurring for SSL devices have been compiled highlighting what are the main mechanism influencing and limiting LED reliability. Strong from this knowledge, in the last part, self-healing concepts will be proposed to further improve LED’s reliability.

A review of passive thermal management of LED module

Recently, the high-brightness LEDs have begun to be designed for illumination application. The increased electrical currents used to drive LEDs lead to thermal issues. Thermal management for LED module is a key design parameter as high operation temperature directly affects their maximum light output, quality, reliability and life time. In this review, only passive thermal solutions used on LED module will be studied. Moreover, new thermal interface materials and passive thermal solutions applied on electronic equipments are discussed which have high potential to enhance the thermal performance of LED Module.

Thermal Transient Effect and Improved Junction Temperature Measurement Method in High-Voltage Light-Emitting Diodes

The diode forward voltage method with pulsed currents was widely used for monitoring junction temperature (Tj) of light-emitting diodes (LEDs). However, a thermal transient effect (TTE) was observed by the pulsed currents and consequent errors were introduced. Thermoelectric physics was conducted to explain this phenomena and a group of experiments was used to reveal the TTE during Tj measurement for high-voltage (HV) LEDs. In addition, an improved pulse-free direct junction temperature measurement method was conducted for HV LEDs to reduce the errors and to achieve in situ Tj measurement with dc currents, simpler setups, and a less step sequence.

Self-healing thermally conductive adhesives:

Thermally conductive composites with a temperature-triggered self-healing response were produced by dispersing boron nitride or graphite particles into two types of polysulphide-based thermoset matrices. The composites produced exhibit recovery of both cohesion and adhesion properties upon thermally activated healing. Using a mild healing temperature (65°C), the materials show full recovery of their initial adhesive strength during multiple healing cycles. The composites behave differently regarding the cohesion recovery: 20%–100% recovery is achieved depending on the filler type, filler loading and the type of matrix. The thermal conductivity of the composites increases with the amount of filler. Values of 1 and 2 W/m K can be achieved for the boron nitride and graphite-based composite, respectively. The results presented in this work clearly show that multifunctional materials with different functionalities and mechanical self-healing responses can be designed using this strategy.

A polymer based miniature loop heat pipe with silicon substrate and temperature sensors for high brightness light-emitting diodes

Solid State Lighting (SSL) systems, powered by light-emitting diodes (LEDs), are revolutionizing the lighting industry with energy saving and enhanced performance compared to traditional light sources. However, around 70%-80% of the electric power will still be transferred to heat. As the elevated temperature negatively affects the maximum luminous output, efficiency, light quality, reliability and the lifetime of the SSL systems, thermal management is a key design aspect for LED products. In this work, an innovative thermal management with a package, a silicon substrate with temperature sensors and a polymer based loop heat pipe (LHP) was designed, manufactured and assembled. It can supply a low and relatively stable temperature to maintain higher optical power, more luminous flux and less color shift. In a word, the novel design can provide LEDs with the efficient thermal management and temperature monitoring with reduced weight, easy fabrication, less energy consumption and better light quality. cop. 2014 Elsevier Ltd. All rights reserved.

Backwafer optical lithography and wafer distortion in substrate transfer technologies

A method has been developed by which, after removal of the bulk silicon in a substrate transfer process, the backside of a wafer can be processed with the same lithography as the front side of the wafer. To achieve an accurate front-to-backwafer alignment accuracy, mirror symmetric alignment markers for an ASML PAS5000 waferstepper have been developed and applied in a Silicon-on- Anything process. In this manner minimum dimension low-ohmic contacts were fabricated on the backwafer. The mirror symmetric alignment markers are used in combination with standard overlay test procedures to characterize the front-to backwafer overlay accuracy. The measured overlay errors are divided up in non- mirror symmetric lens distortions and wafer distortion as a result of the substrate transfer process. The practical minimum device feature that can be realized on the backwafer is limited to 0.9-1.2 micrometers as a result of front-to-backwafer overlay errors.

Output blue light evaluation for phosphor based smart white LED wafer level packages.

This study presents a blue light detector for evaluating the output light of phosphor based white LED package. It is composed of a silicon stripe-shaped photodiode designed and implemented in a 2 μm BiCMOS process which can be used for wafer level integration of different passive and active devices all in just 5 lithography steps. The final device shows a high selectivity to blue light. The maximum responsivity at 480 nm is matched with the target blue LED illumination. The designed structure have better responsivity compared to simple photodiode structure due to reducing the effect of dead layer formation close to the surface because of implantation. It has also a two-fold increase in the responsivity and quantum efficiency compared to previously similar published sensors.

An integrated large SSL system with wireless communications

In this paper we present our preliminary results obtained from implementing a SSL system on the facade of EWI faculty building in Delft University of Technology as shown in Figure 1. Two hundred intelligent SSL modules are planed to be deployed. All the modules are controlled via wireless links. We propose solutions for challenges such as hardware and software designs. Experiences learned are also presented.

Monolithically Integrated Light Feedback Control Circuit for Blue/UV LED Smart Package

Given the performance decay of high-power light-emitting diode (LED) chips over time and package condition changes, having a reliable output light for sensitive applications is a point of concern. In this study, a light feedback control circuit, including blue-selective photodiodes, for blue/ultraviolet (UV) LED, has been designed and implemented using a low-cost seven-mask BiCMOS process. The feedback circuit was monolithically integrated in a package with four high-power blue LED chips. For sensing the intensity of exact colored blue/UV light in the package, selective photodiodes at 480-nm wavelength were implemented. An opamp-based feedback circuit combined with a high-power transistor controls the output light based on real-time sensor data. The whole system is a low-cost integrated package that guarantees a stable and reliable output light under different working conditions. Output light can be also controlled linearly by a reference input voltage.

High aspect ratio lithography for litho-defined wire bonding

To overcome some challenges and reliability issues of wire bonding, a study of a new interconnect technique is done. High aspect ratio interconnect called “litho defined wire bonding” can be a promising method to resolve these problems in many of the applications. Using contact aligner and resist multilayer spray-coating over high steps up to 150 μm, we are able to achieve AL interconnect lines with different parameters. Further, using multi step imaging on a ASM PAS 5500/80 projection aligner a resolution down to 2 μm on the bottom of a 150 μm deep cavity was attained. With simulation based on aerial image calculation, different multi step imaging schemes are studied to get higher resolution over topographies.