MicroLED technology has the potential to outperform traditional LCD displays. Each pixel is composed of a microscopic LED, offering exceptional brightness, contrast, and color accuracy. 

As per reports by Mordor Intelligence, the microLED market is expected to be at USD 0.74 billion in 2024. It is expected to reach USD 1.48 billion by 2029. 

But here’s the catch! The process of manufacturing microLED displays is cumbersome due to microLED mass transfer. This transfer includes moving billions of microLEDs from a temporary substrate to their final destinations. 

Various mass transfer techniques can be used to ensure a process is accurate, fast, and efficient.

Let us explore the effective microLED mass transfer techniques, current research and development, and the trajectory of future advancements.

MicroLED Mass Transfer: Exploring Latest Techniques and Innovations

Several techniques have been implemented for mass transfer. Some of them are as follows:

Epitaxial Lift-Off (ELO) Technique

This technique releases large microLED chips from the donor substrate through a precise controlled process. Here are some details:

1. Concept: ELO is a post-growth process used to lift off layers grown on the substrate and transfer them to different surfaces. It amalgamates microLED chips, especially those with dimensions typically below 40 µm. It offers high transfer yields and is suitable for high-density microLED arrays.
2. Patent:
   • WO2009155122A2: This patent is related to ELO thin films and devices and methods used to form such films and devices. These films generally contain epitaxially grown layers. These are formed on a sacrificial layer disposed on or over a substrate, such as a wafer.
   • KR20110015031A: This patent introduces a groundbreaking solution to the problem of handling delicate epitaxial films during the ELO process. The key innovation lies in specially designed support handles that attach to the epitaxial layer. These handles offer controlled compression, preventing cracking during separation from the growth substrate. 
3. Industry Examples:
   • MicroLink Devices, Inc.: MicroLink has developed ELO, a technology for making large areas of thin, flexible, high-efficiency solar cells.
   • High-Throughput MicroLED Manufacturing with LIFT: High-precision automation equipment enables mass production of microLED displays using laser-based transfer techniques. 

Pick-and-Place (PnP) Technique: 

This pick-and-place technique uses a stamp to position and place microLED chips on a receiver substrate. Here are some specific details:

1. Concept: The concept revolves around how a small vacuum chunk picks up the microLED chips from the epitaxial wafer. Then, it places them in the desired location.
2. Patent: This patent titled Fluidic Assembly MicroLED Mass Transfer Method (Application #20240145443) explains a mass transfer stamping system. It includes a stamp substrate with an array of trap sites. Each trap site is configured with a columnar-shaped recess to temporarily secure a keel extended from the bottom surface of a microLED.
3. Industry Examples:
   • Rohinni: Rohinni has developed a high-speed, high-precision method for placing mini and microLEDs onto a substrate. Their technology is capable of placing 50 million LEDs per hour with an accuracy of 10 microns.
   • Aledia: Aledia is a French company that has developed a 3D LED technology based on gallium nitride nanowires. They use a pick-and-place process to transfer the LEDs onto a CMOS wafer.

Contact Micro-Transfer Printing (µTP) Technique

In this technique, the microLED chip is made to contact with the receiving substrate using a special stamp. This method can achieve high transfer yields (>99.99%) but has a low transfer rate. 

This is discussed briefly here:

1. Concept: It uses a stamp to pick up an array of μLEDs. It then transfers it to a display substrate by “stamping” it. 
2. Patent: A patent by Apple Inc. describes a method of forming an array of micro LEDs. The innovation enables reliable massively parallel transfer and heterogeneous integration of arrays of pre-fabricated microLED structures onto receiving substrates. A key aspect is the use of an intermediate electrically conductive bonding layer, such as indium or indium-based alloys, that has a relatively low melting/liquidus temperature below 350°C
3. Industry Examples:
   • X-Celeprint: X-Celeprint has developed a patented micro-transfer printing technology. This technology enables the efficient transfer of microscale devices and systems onto a variety of substrate materials.

Laser Non-Contact Micro-Transfer Printing (µTP) Technique

This approach leverages a laser to separate the selected chip from the stamp by an interfacial thermal mismatch or blister ejection. Here are some key details:

1. Concept: This can achieve high transfer rates (∼100 million h −1) but has an unacceptable success rate (∼90%).
2. Patent: A patent by the University of Illinois describes a transfer printing process. It exploits the mismatch in mechanical or thermo-mechanical response at the interface of a printable micro- or nano-device.
3. Industry Examples:
   • Uniqarta, Inc. has developed a complete wafer-to-panel technology for extremely high-rate assembly of μLEDs. They use a Laser-Enabled Massively Parallel Transfer method with >100M units/hr.
   • X Display Company and ASM/Amicra are also known to produce print tools that use the µTP technique.

Self-Assembly Technique

This technique utilizes fluid as a transfer medium and generates gravitational, hydrophobic, or hydrophilic forces. This is to identify and localize microLED chips with specific dimensions. The self-assembly technique can achieve high transfer rates (∼99.9%, ∼100 million h −1). However, special requirements are needed for microLED chips and receiver panels. 

Here are some key details:

• Concept: One such technique is Fluidic Self-Assembly (FSA). In this case, a collection of microLED chiplets dispersed in an assembly solution is set in motion. The movement causes the chiplets to make repeated contact with a substrate immersed in the fluid. Then, the substrate’s binding sites are coated with molten solder. When a chiplet meets one, surface tension induces an irreversible bond between the solder and a metal electrode.
• Patent: One notable development is the use of controlled viscosity environments to optimize the assembly process. This method focuses on enhancing the yield and efficiency of microLED assembly by manipulating the fluid properties in which the microLED chiplets are dispersed. This approach has been highlighted as a promising way to accelerate the production of microLED displays by achieving high assembly yields.
• Industry Examples:
  • eLux, Inc. has developed fluidic assembly technology to advance manufacturing process developments for microLED displays.
  • VDOM DHTML TML has also created a system that includes a stamp substrate with an array of trap sites.

The Next Frontier: Scaling Up with Innovative MicroLED Mass Transfer Techniques

The future of MicroLED Mass Transfer Techniques beyond 2024 is expected to be promising and transformative. Here are some key insights:

Market Potential

MicroLED has the potential to redefine the $180 billion display market. Forecasts indicate that MicroLED alone could reach a market value of $30 billion by the end of the decade. The total market potential extends beyond displays, hinting at a $1 trillion valuation.

Environmental Stewardship

MicroLED has a green advantage over OLED. Research shows that MicroLED fabs consume 30% to 200% less water, power, and materials, significantly reducing their environmental footprint. Further, with 50% to 200% lower power consumption, MicroLED emerges as a sustainable choice in the quest for eco-friendly technologies.

Novel Techniques

Novel techniques of mass transfer for large-scale and high-density MicroLED arrays are being explored. These include the epitaxial Lift-off technique and pick-and-place technique.

End Note

Considering the evolution of MicroLED mass transfer techniques, the epitaxial lift-off (ELO) and pick-and-place (PnP) techniques are the most promising. Future endeavors should focus on refining these methods for enhanced accuracy, speed, and scalability. 

Exploring new approaches and investing in automation can further propel MicroLED technology toward widespread adoption and market dominance.