Tien Li Chang

Analysis of thermal characteristics and mechanism of degradation of flip-chip high power LEDs

Abstract The purpose of this study is to investigate the thermal behavior at the die-attached interfaces of flip-chip GaN high-power light emitting diodes (LEDs) using a combination of theoretical and experimental analyses. The results indicate that contact thermal resistance increased dramatically at the die-attached interfaces with aging time and stress, degrading the luminous flux. The junction temperature and thermal uniformity of the flip-chip structure both strongly depend on the arrangement of gold bumps. Local hot spots effectively reduce light output under high electric and thermal stress, influencing the long-term performance of the LED device. The results were validated using finite element analysis and in experiments using an infrared and an emission microscope. A two-step thermal transient degradation mode was identified under various aging stresses. A simulation further optimized the bump configuration that was associated to yield a low junction temperature and high temperature uniformity of the LED chip. Accordingly, the results are helpful in enhancing the performance and reliability of high-power LEDs.

Transient Analysis of Partial Thermal Characteristics of Multistructure Power LEDs

The performance of a high-power light-emitting diode (LED) strongly depends on the effectiveness of thermal management. Electrical, thermal, and optical measurements were combined to analyze the complex thermal structure and interface effects inside the LED package. Thermal network synthesis and 3-D finite element modeling simulations are used to simulate the heat flow path and temperature gradient from junction to environment. The exponential curve is applied to model the radiant flux and thermal resistance as a function of heating power. In addition, the results of peak wavelength obviously indicate the blue-shifted phenomena with driving current due to band filling effect and tend to be saturated when the injection current is higher than 0.5 A. The radiant flux showed the opposite behavior as driving current increases. Results demonstrated that partial thermal characteristics of the chip, die-attached layer, and heat slug can be determined individually in the LED packages.

Micro/nano structures induced by femtosecond laser to enhance light extraction of GaN-based LEDs

Surface texturing has been widely adopted to enhance the light extraction efficiency of light-emitting diodes (LEDs), and chemical etching is a technique commonly used to produce surface texturing. This study employed femtosecond lasers to apply ITO films directly onto the surface of LEDs to generate periodic micro/nanostructures and roughen the surface without contact or chemical substances. As a result, photons emitted in the active region escape into the free space, due to the scattering effect produced by texturing. This study discovered that light-emitting efficiency increases with surface roughness, and achieved an improvement of 18%. Caution regarding laser fluence was required during laser processing to avoid damaging the LED beneath the ITO film, which could detract from the electrical characteristics.

A novel fabrication method for forming inclined groove-based microstructures using optical elements

Backlight modules with inclined groove-based microstructures, such as V-cuts for liquid crystal display (LCD) applications, have been developed. This study presents a novel photolithography technology which utilizes an inclined exposure method. An inclined microstructure surface fabricated with a controlled optical path is exposed using refracted ultraviolet (UV) light. A weight-bearing point of inclined exposure uses an optical element prism which is placed between the UV light source and the photoresist. This approach utilizes a prism-based exposure which complies with Snells law. Different from conventional photolithography, the angle needed and the smooth surfaces of microstructures fabricated using this approach can be easily controlled. The aim of this study is to fabricate inclined groove-based microstructures on optical film to form structures ranging from 30 to 90°. This research made use of microstructure plates with dimensions of 30×30 mm2 and with surface roughness less than 20 nm. The results from this study could provide an important foundation of knowledge for the fabrication of next-generation displays.

Thermal Analysis of Eutectic Flip-Chip Light-Emitting Diodes Fabricated Using Copper-Coated Ceramic Substrate

Die attachment quality plays a remarkable role in producing highly reliable solid-state lighting fixtures by providing a dominant impact on thermal resistance. Color rendering, efficacy, and lifetime are strongly related to junction temperature. This paper investigated the effects of varying the thickness of sputter-coated copper on an Al2O3 ceramic substrate on the thermal resistance and luminous intensity of flip-chip light-emitting diode (LED) devices. Eutectic bonding was applied to provide excellent bonding strength and low void content between the chip and ceramic substrate. The thermal resistance dramatically decreased as the copper thickness was reduced because of a substantial reduction in the conduction impedance for heat dissipation from the junction to the ambient. The luminous intensity was improved by reducing the copper thickness as the driving current was increased from 50 to 700 mA. The results demonstrated that reducing the copper thickness effectively reduced the junction temperature and improved the performance of the eutectic flip-chip bonding LED devices.

Femtosecond laser nonlinear ablation process of biliary nitinol stent for cholangiocarcinoma

Recently, the different types of biliary stent are widely used to provide a longer patency and are more cost-effective for patients surviving. In this study, the design of biliary stent can be fabricated by femtosecond laser ablation in wavelength of 1035 nm. Here the pulsewidth of this laser is smaller than heat diffusion time to reduce heat-affected zone (HAZ). And then, the smart material of nitinol (Ni-Ti) alloy is used to fabricate the stent for medicine application of cholangiocarcinoma. The experimental results show the good quality edge of stent can be obtained via the nonlinear ablation process; however, the kerf surface of stent still can generate some nano-scale structures after this process.

Applications of magnetic nanoparticles in engineering and biomedical science

This study mainly employs magnetic nanoparticles (MNPs) for an amazing variety of engineering and biomedical applications. Herein MNPs are fabricated from fine ferromagnetic particles of iron ferrite by chemical co-precipitation technique, and their average size is about 27 nm via HR-TEM micrograph and XRD analysis to investigate. In this study, MNPs have been demonstrated their excellent properties of heat transfer, electric conductivity, magnetism within the applications for multi-loop pulsating heat pipe (MLPHP), switch-based nanodevice, microfluidic on-chip system and nanogap-based DNA sensor. Based on the effect of magnetic field for MNPs, MLPHP can enhance thermal performance itself at different heating power. In addition, the switch-based nanodevice with MNPs can efficiently add and remove an electrical function of electron charging with current shift. Furthermore, the microfluidic chip utilizing MNPs is demonstrated that can be suited for drug delivThis study mainly employs magnetic nanoparticles (MNPs) for an amazing variety of engineering and biomedical applications. Herein MNPs are fabricated from fine ferromagnetic particles of iron ferrite by chemical co-precipitation technique, and their average size is about 27 nm via HR-TEM micrograph and XRD analysis to investigate. In this study, MNPs have been demonstrated their excellent properties of heat transfer, electric conductivity, magnetism within the applications for multi-loop pulsating heat pipe (MLPHP), switch-based nanodevice, microfluidic on-chip system and nanogap-based DNA sensor. Based on the effect of magnetic field for MNPs, MLPHP can enhance thermal performance itself at different heating power. In addition, the switch-based nanodevice with MNPs can efficiently add and remove an electrical function of electron charging with current shift. Furthermore, the microfluidic chip utilizing MNPs is demonstrated that can be suited for drug delivery. Finally, we use MNPs to develop an electrical approach to detect femtomolar DNA that can amplify the low target DNA concentration for a clinical gene diagnostic system.ery. Finally, we use MNPs to develop an electrical approach to detect femtomolar DNA that can amplify the low target DNA concentration for a clinical gene diagnostic system.

Study of metals by femtosecond laser processing for electro-optics applications

Femtosecond laser (FS-laser) microstructuring of metals has become a promising tool because of its non-contact nature, which allows the micromachining and direct processing of materials with a minimized volume of heat-affected zone for electro-optics applications such as light emitting diodes (LED) and solar photovoltaic (PV) lighting. This study presents ultra-short pulse (10-15 sec) FS-laser processing. Through integrating the laser source, optical system and dynamic control modules, the materials of metals with micro-scale or nanoscale structures can be fabricated. In traditional processing such as semiconductor processing, development, exposure and etching necessitate expensive equipment and time-consuming tasks. With FS-laser processing, high-precision patterns are obtained, which will be a great benefit to keeping costs down. In this study, the wavelengths of FS-laser ablation are employed using visible and infrared light. To make a breakthrough in electro-optics processes, the CIGS thin-film of solar cells in metal process can be easily produced by the FS-laser. The ablation speed of the FS-laser for thin film layer CIGS solar cells can reach 2000 mm/s which is faster than the current Nd:YAG laser machine (~1000 mm/s). On the other hand, the minimum size of metal lines can be controlled to a value that is lower than 40 µm. Furthermore, green energy can be effectively developed for the future.

Novel concept design for complementary metal oxide semiconductor capacitive Z-direction accelerometer

The purpose of this study is to provide a novel concept design for complementary metal oxide semiconductor (CMOS) capacitive Z-direction accelerometers. In this design, capacitance-sensing parallel plates and thin single-metal-layer springs are fabricated by chemical plasma etching. This plasma etching process utilizes a rough vacuum to shorten the mean free path of the reactive gases and increase lateral etching and undercut. Moreover, the thin single-metal-layer springs have the characteristic of a lower spring constant to increase the displacement of the proof mass. Thus, the sensitivity can be further improved. This study has established a post-CMOS chemical plasma etching process to release the accelerometer using sensing electrodes of the capacitance-sensing parallel plates. The Z-direction accelerometer has been fabricated by the standard CMOS 0.35 µm two-polycrystalline silicon four-metal (2P4M) process and a post-CMOS process of chemical plasma etching. The measurement results indicate that the sensitivity of the accelerometer is about 0.8 mV/g and the total noise floor is 4.2 µV/√Hz.

Design of self-alignment devices with fluidic self-assembly for flip chip packages in batch processing

An advanced LED multi-die-bonding integration using a fluidic self-assembly technique is proposed in the field of flip chip packages. Different form the conventional pick-and-place methods for a single LED die bonding, the fluidic approach is a relatively new design and a batch process, which can achieve not only die self-alignment but die self-assembly. Here, the size of LED die is 1-mm-square chip with the thickness of 0.3 mm. Due to the smaller size of LED die, the die-bonding process is still in need of finding a suitable approach and breakthrough. In this study, our design of fluidic self-assembly device is based on the experimental test and simulation results. The device design is the gas-flow channels with the magnetism. The width, height and length of each gas-flow channel are 1.1 mm, 0.5 mm, and 1 cm, respectively. With the restriction of the channel width, this structure design can control well to die self-alignment. In addition, the design of two circular structures in the channel can form a flat rim to achieve the die self-assemble. This mechanism of fluidic approach can be useful to the LED die self-alignment and self-assembly in the future batch processing.