LED Dome With Index-Matched Pillar for Light Extraction
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Solution Overview
Problem
The light extraction efficiency of Flip-Chip LED devices, such as those with Patterned Sapphire Substrate (PSS) architectures, is limited due to total internal reflections (TIR) and wide emission angles, which are not effectively addressed by increasing substrate thickness, as it complicates fabrication and increases costs.
Innovation Solution
A transparent rectangular pillar with a refractive index matching the sapphire substrate is positioned on top of the LED die, surrounded by a cylinder and dome, all made of materials with carefully selected indices to minimize TIR and enhance light extraction, narrowing the emission field and improving efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the substrate thickness is increased to improve light extraction efficiency, then more light can be extracted from the substrate, but fabrication becomes more complicated and costs increase
Solution Approach 1:
The patent introduces an index-matched transparent material (such as epoxy or silicone) with refractive index between 1.6-1.9 as an intermediary layer between the GaN substrate and the encapsulation dome. This intermediary material acts as a mediator that facilitates light extraction from the substrate without requiring increased substrate thickness, thereby resolving the contradiction between improving light extraction efficiency and maintaining ease of manufacture
Solution Approach 2:
The patent changes the refractive index parameter of the encapsulation material to match that of the sapphire substrate (1.6-1.9), creating an index-matched interface. This parameter change enables efficient light extraction from the substrate without needing to increase substrate thickness, thus improving light extraction efficiency while keeping fabrication simple
2Loss of energy
If high refractive index encapsulation dome lenses are used to mitigate total internal reflections, then light extraction efficiency is boosted by up to 20%, but the emission field becomes wider with more downward side light
Solution Approach 1:
The patent applies local quality by creating an index-matched interface specifically at the substrate-dome interface where light extraction is needed, while maintaining a lower refractive index in the encapsulation material to control the emission angle. This localized application of high refractive index only where necessary allows efficient light extraction without creating excessive downward side light
Solution Approach 2:
The patent optimizes the refractive index parameter of the encapsulation material to be between 1.6-1.9, which is higher than conventional materials but carefully selected to balance light extraction efficiency with emission angle control. This parameter optimization resolves the contradiction between boosting light extraction and minimizing harmful downward side light
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration reduces side light reflection, attenuates less light, and increases light extraction efficiency by up to 6% compared to conventional LED devices, while maintaining a thinner substrate for easier fabrication and lower costs.
Implementation Method 1
The light extraction efficiency (ExE) of LED devices is limited by total internal reflections (TIR) in the high refractive index (n) epitaxial layers
Implementation Method 2
A transparent rectangular pillar (a rectangular prism), having a footprint approximately the same size as the LED die, is positioned directly on the top substrate surface of the LED die. The pillar is formed of a material (e.g., silicone) that has an index (n) approximately equal to that of the sapphire substrate (approximately 1.8)
Data Source
Figure 1~2
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Figure 6~7
AI summary
Affixed over a transparent growth substrate (34) of an LED die (30) is a transparent rectangular pillar (40), having a footprint approximately the same size as the LED die. The pillar height is greater than a length of the LED die, and the pillar has an index (n) approximately equal to that of the substrate (e.g., 1.8), so there is virtually no TIR at the interface due to the matched indices. Surrounding the pillar and the LED die is a lens portion (42) having a diameter between 1.5-3 times the length of the LED die. The index of the lens portion is about 0.8 times the index of the substrate. The lens portion may have a dome shape (46). A large portion of the light exiting the substrate is internally reflected off the lateral pillar/cylinder interface and exits the top surface of the pillar. Thus, the emission is narrowed and light extraction efficiency is increased.