Micro-LED Micro-Lens Bonding for Higher Light Extraction
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Solution Overview
Problem
Micro-LEDs, particularly AlGaInP-based red light-emitting LEDs, face challenges in achieving high light extraction efficiency due to factors like total internal reflection, surface recombination, and Auger recombination, which reduce their internal and external quantum efficiencies as their size decreases.
Innovation Solution
Incorporating a micro-lens bonded to the micro-LED through a thin bonding layer with a refractive index close to or greater than the semiconductor layer, and optimizing the bonding layer's thickness to control emission angles, thereby reducing frustrated total internal reflection and enhancing light collimation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If micro-LED size is reduced to increase packing density, then resolution and brightness improve, but light extraction efficiency deteriorates due to total internal reflection and surface recombination
Solution Approach 1:
A micro-lens array is introduced as an intermediary optical element between the micro-LEDs and the external environment. The micro-lenses have a higher refractive index than the semiconductor material and are designed with specific curvature radii to refract and extract light that would otherwise be trapped by total internal reflection, thereby improving light extraction efficiency without requiring larger LED sizes
Solution Approach 2:
The refractive index parameter is changed by introducing a micro-lens layer with refractive index n2 > n1 (semiconductor refractive index). The curvature radius of the micro-lens (R1, R2) is optimized to control the refraction angle and improve light extraction. Additionally, a bonding layer with specific thickness (10-20 μm) and refractive index is introduced to further optimize optical coupling and reduce frustrated total internal reflection
2Measurement precision
If micro-LED size is reduced to increase packing density, then resolution improves, but external quantum efficiency deteriorates due to surface recombination and Auger recombination
Solution Approach 1:
The micro-lens array acts as an optical intermediary that enhances the extraction of photons generated within the micro-LEDs. By optimizing the lens curvature and refractive index, more photons are directed outward, effectively increasing the external quantum efficiency without changing the micro-LED active region size or material composition
Solution Approach 2:
The micro-lenses are designed with spherical or aspherical curvature (with radii R1 and R2) to optimize light refraction and extraction. The curved surface geometry enables better control over light emission angles and improves the collection of light that would otherwise be trapped, thereby enhancing external quantum efficiency
3Loss of energy
If a bonding layer is introduced between micro-LED and micro-lens, then light extraction efficiency improves, but device complexity increases
Solution Approach 1:
A thin bonding layer with specific refractive index and controlled thickness (10-20 μm) is introduced as an intermediary between the micro-LED and micro-lens. This layer optimizes optical coupling by reducing frustrated total internal reflection and improving light transmission, while its thin profile minimizes the increase in overall device complexity
Solution Approach 2:
The bonding layer parameters (thickness, refractive index) are optimized to achieve the best light extraction performance. The thickness is controlled to be less than 20 μm to minimize optical path differences and maintain manufacturing feasibility, while the refractive index is selected to provide optimal optical coupling between the semiconductor and lens materials
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 approach significantly improves the collected light extraction efficiency of micro-LEDs by promoting more light to enter the micro-lens, resulting in higher external quantum efficiency and better light emission directionality.
Implementation Method 1
Incorporating a micro-lens bonded to the micro-LED through a thin bonding layer with a refractive index close to or greater than the semiconductor layer, and optimizing the bonding layer's thickness to control emission angles, thereby reducing frustrated total internal reflection and enhancing light collimation
Implementation Method 2
optimizing the bonding layer's thickness to control emission angles, thereby reducing frustrated total internal reflection and enhancing light collimation
Data Source
AI summary
A light source includes an array of micro-light emitting diodes (micro-LEDs), an array of micro-lenses, and a bonding layer bonding the array of micro-lenses to the array of micro-LEDs. Each micro-LED of the array of micro-LEDs includes a first mesa structure formed in a plurality of semiconductor layers. The array of micro-lenses is bonded to a first semiconductor layer of the plurality of semiconductor layers by the bonding layer. The first semiconductor layer includes an array of second mesa structures formed therein. The first mesa structure and the second mesa structure are on opposite sides of the plurality of semiconductor layers. Each second mesa structure of the array of second mesa structures is aligned with a respective micro-lens of the array of micro-lenses and the first mesa structure of a respective micro-LED of the array of micro-LEDs.