Optical And Thermal Structure Optimization Design Of Light-emitting Diodes

Light Emitting Diode (LED) whose advantages are energy saving, environmentalprotection and long life, is the new generation of green light source after incandescent lampand fluorescent lamp, and now it is gradually replacing traditional light sources andinfiltrating into people’s daily life. But the luminous efficiency of LED is still quite low now,the reason of which is that firstly total reflection occurs at the chip surface and most of thelights are eventually absorbed and converted into heat due to the high refractive index ofsemiconductor material, secondly the generated heat caused the reduction of internal quantumefficiency because of its temperature sensibility. Therefore, in this dissertation the method ofmanufacturing light extraction microstructure (LEM) at the chip surface by microcutting wasproposed, microstructure parameters were optimized to improve LED light extractionefficiency (LEE), and LED packaging and system-level thermal structure was researched andoptimized. The main contents are as follows:(1) LED Chip Surface Light Extraction Microstructure Optimization DesignBased on the analysis of the micro mechanical properties of GaN, the frustum pyramidLEM of0.5μm-2μm size which is manufactured by micromachining at n-type GaN surface ofthe vertical LED chips was proposed. Micromachining experimental system was built, and thefeasibility of LEM microcutting was verified. Frustum pyramid structure parametersincluding groove width w, platform width wtand groove depth d were optimized, by ways ofthe light simulation orthogonal experiment and mathematical modeling and calculation.Theoptimal LEM parameters with w=2.6μm，wt=0.35μm，d=0.95μm were obtained, finally theLED light extraction efficiency was improved by51%after LEM machining was discoveredby light extraction simulation and test.(2) LED Packaging Optical and Thermal OptimizationIt was found that the die-bonding layer structure is the key factor affecting LED devicethermal property, and the void distribution in the die-bonding layer will affect LED junctiontemperature directly. The die-bonding layer model was rebuilt by X-ray reconstitution, andthe LED device thermal property finite element modeling under different die-bonding layerstructures was processed by ANASY Icepak, it was found that temperature distribution of epitaxial layer is same with the void distribution in die-bonding layer, there are hotspotsabove the void elements, and the qualified Au/Sn die-bonding LED device has optimumthermal property. It was also found that the die-bonding layer contact thermal resistanceaccounts for more than50%of die-bonding layer total thermal resistance. LED luminous fluxand life testing under different die-bonding structures were carried out, and disqualifieddie-bonding may led to the reducing of luminous efficiency and reliability of LED device.(3) High-power LED Lamp Cooling System OptimizationAccording to the cooling system thermal resistance network analysis, it was concluded thatheat spreading and fin radiating are key factors affecting the property. A multi-artery vaporchamber was designed and manufactured to solve the heat spreading problem, it was foundthat its minimum thermal resistance is0.033K/W, and it can work normally when the heatflux density reaches300W/cm2, but it is only suitable for the LED chip concentrated packagedevice with highly concentrated heat. Three heat sinks with different fin structures weredesigned, fin gap was optimized by simulation method, and the influence of the angle ofinclination to the heat sink property was studied. With the optimum heat sink, the junctiontemperature of the180W LED lighting system was only72℃.(4) LED Lighting System Optimized Property AnalysisAccording to the optical and thermal structure optimization results from chip to systemlevel of LED lighting system, the actual measurement results on the influence of junctiontemperature to the LED light extraction property, and simulated junction temperature in theLED lighting system under different light extraction efficiency, the overall light extractionproperty of LED lighting system after the combination of LEM and optimized cooling systemstructure was analyzed. The luminous efficiency of LED with optimal optical and thermalstructure is121lm/W, and the total thermal resistance of LED lighting system is only0.29K/W.