I. Background of Application
Micro LED technology, a key advancement in next-generation display technology, has shown significant promise in various industries. However, it faces several challenges in mass adoption.
With the increasing consumer demand for higher display quality in consumer electronics such as smartphones and televisions, there is a growing need for clearer, more realistic, and energy-efficient displays. Micro LED meets these demands with superior color performance, high contrast, fast response time, high brightness, and energy efficiency, providing users with an exceptional visual experience.
In emerging fields like wearable devices, automotive displays, and AR/VR technologies, the need for miniaturization, high resolution, reliability, and flexible displays is rising. The characteristics of Micro LED make it the ideal choice for these applications, such as providing precise information displays in smartwatches, clear driving information on automotive dashboards, and enhanced immersion in AR/VR devices.
However, the manufacturing process of Micro LED is highly complex, involving precision chip fabrication, micro/nano-scale etching, and transfer technologies. These processes require advanced equipment and technology, and defects are common, resulting in lower production yields and higher manufacturing costs.
Currently, large-scale production of Micro LED faces significant obstacles, such as issues with chip transfer and yield limitations, restricting the overall output. Additionally, the high complexity of the manufacturing process and limited production volume keep Micro LED costs elevated, hindering its widespread adoption in the market.
II. Application of Laser Technology in Micro LED
i. Laser Epitaxial Growth (Substrate Epitaxy)
Principle: Laser heating and material deposition grow Micro LED chips on substrates. Techniques like Pulsed Laser Deposition (PLD) utilize laser pulses to irradiate target materials, energizing atoms and molecules to form plasma clouds, which are then deposited onto substrates to form thin films. Laser Molecular Beam Epitaxy (LMBE) uses laser evaporation to convert metal gallium and nitrogen gas into high-energy atomic and molecular beams, which are precisely controlled to deposit on substrates.
a. Pulsed Laser Deposition (PLD)
- Material Precision Control: PLD allows for micrometer-scale localized growth and deposition, providing more precise control over the structure. For instance, when fabricating optical films and GaN thin films, PLD enables precise control over the growth position and thickness of the material.
- Lower Growth Temperature: Compared to traditional Metal Organic Chemical Vapor Deposition (MOCVD), PLD allows for material growth at lower temperatures, reducing thermal decomposition and non-uniform growth issues.
- Wide Applicability: PLD can be used with various materials and substrates, including silicon and sapphire.
However, its slower growth rate and difficulty controlling uniformity in large-scale production make it more suitable for research and small-scale production, with the ability to quickly prepare samples.
b. Laser Molecular Beam Epitaxy (LMBE)
- High-Quality Growth: LMBE’s precise control over atomic and molecular beams enables high-quality crystal growth, enhancing the performance and reliability of Micro LED devices.
- Variable Growth Parameters: Parameters such as growth temperature, nitrogen flow rate, and laser scanning frequency significantly influence the process. The growth temperature affects material structure and quality, nitrogen flow determines the nitrogen content in nitride materials, and laser scanning frequency impacts growth rate and crystal quality.
- Mass Production Advantage: LMBE is suitable for large-scale production of Micro LED epitaxial layers, as it offers higher growth rates, uniformity, and repeatability. For instance, when growing GaN on sapphire substrates, LMBE effectively reduces the effects of polarization, producing high-quality, low-stress GaN layers.
ii. Laser Etching
Principle: High-energy lasers are focused on specific areas of the chip’s surface to induce chemical or physical changes that result in material removal.
a. Laser-Assisted Dry Etching
- Performance advantage: Compared to traditional Inductively Coupled Plasma (ICP) or Reactive Ion Etching (RIE), laser-assisted etching has a higher etching rate (approximately 16 times that of ICP/RIE), better etching uniformity (space uniformity of 1-3%, compared to 3-5% for ICP/RIE), higher throughput (50-100 wafers per hour, compared to 10-20 for ICP/RIE), and superior sidewall quality (sidewall verticality up to 8-80°, surface roughness RMS down to 0.5-1nm, outperforming ICP/RIE).
- Complex process: However, the process is complex. Photolithography is a critical step in semiconductor micro/nano structure fabrication. In laser-assisted etching, the photolithography process includes exposure, development, etching, and stripping, requiring precise alignment of masks and patterns. The UV laser energy absorption characteristics limit etching depth, which may require multiple treatments for Micro LED applications with deeper etching needs.
b. Laser Direct Writing (LDW)
- High Precision, Mask-Free Etching: LDW enables high-precision, mask-free etching, directly controlling the laser beam to create patterns on the material’s surface. This technique allows for single-step, high-precision, and efficient micro/nano-scale fabrication, ideal for GaN thin film patterning.
- High Efficiency: LDW provides fast processing speeds and is suitable for a wide range of materials. Femtosecond lasers used in ultra-fast LDW offer higher precision with minimal thermal damage, making it a new tool for GaN thin film processing.
iii. Laser Lift-Off (LLO)
Principle: Short-wavelength lasers are directed onto the sapphire side of the substrate. The laser energy is absorbed by the GaN epitaxial layer, causing thermal decomposition and nitrogen gas and liquid gallium generation, which induces interface stress and releases the epitaxial layer from the substrate.
- High Efficiency: LLO is widely used in LED and Micro LED production for substrate removal, with transfer efficiencies of up to 99.9%. However, transfer accuracy is slightly coarse (±10μm).
iv. Laser Mass Transfer
Principle: Laser irradiation on a transparent substrate’s Dynamic Release Layer (DRL) generates localized energy absorption, ablation, and decomposition, producing gas pressure that transfers materials and attached devices to a target substrate.
a. Laser-Induced Forward Transfer (LIFT)
DRL materials must exhibit suitable adhesion and release properties under laser exposure. For instance, single-layer metal films like Au-DRL have been used for transferring phosphor powders but may cause contamination. At the same time, polymer materials such as photo-decomposable triazine polymer TP or polyimide PI reduce contamination, though PI’s decomposition generates gases and mechanical energy for efficient, high-precision transfer.
v. Laser-Assisted Bonding
Principle: High-intensity lasers are used to heat specific areas of metal surfaces, causing them to melt and create electrical connections. The laser’s precision and focus allow for selective heating and bonding, improving bonding accuracy and stability.
Advantages: This process is particularly effective for small-pitch Micro LED bonding, reducing thermal stress and wafer warping risks, improving production efficiency, and ensuring reliable bonding with minimal chip damage.
vi. Laser Detection and Repair
a. Laser Detection
Based on photoluminescence (PL), high-energy laser excitation causes electrons to jump from the conduction band to the valence band, emitting photons. The characteristics of these photons, such as wavelength and intensity, help assess the Micro LED‘s performance.
- Non-Contact: Laser detection does not physically contact the chip, ensuring chip integrity and performance.
- High Precision: Laser detection allows for accurate analysis of Micro LED emission properties, with the ability to detect even minor defects by adjusting the laser spot size (down to 2μm or less).
- Laser Repair: High-energy ultraviolet lasers are used to eliminate or repair defects in Micro LED chips. This method is fast and efficient, improving chip yield and overall product quality, while reducing production costs.
b. Laser Repair
The use of high-energy ultraviolet laser in the defect area to eliminate, remove or repair the defect in the Micro LED, the method is fast, efficient, and can remove the chip defect in a short time.
By repairing the defective chip, the yield of the Micro LED chip is improved, and the scrap rate is reduced, so as to improve the quality and reliability of the entire product, reduce the production cost, and enhance the competitiveness of the product.
III. Conclusion
Laser technology has broad applications in the Micro LED field, primarily using ultraviolet lasers with wavelengths of 355nm, 266nm, and 248nm. Traditional red lasers (1064nm) are less commonly used due to the unique characteristics of Micro LED chips and the manufacturing process.
The application of laser technology in this domain is a refinement of traditional laser use, with significant improvements. Collaboration with other fields is key to solving practical challenges in Micro LED manufacturing.
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