Introduction

MicroLED integration often delivers the best results, like reaching over 10,000 nits of brightness, embedding sensors directly into the display, upgrading the gaming experience, and so on. All these are possible with technologies like epitaxy technology, chip integration, and high electron mobility transistors (HEMTs). 

Over 130 companies are currently associated with microLED integration research and development.

Over the decade, companies like PlayNitride, TCL/CSoT, Jasper Display, Sony, Samsung, LG, Tianma, Jade Bird Display, Plessey Semiconductors, and Ostendo Technologies have made progress in microLED integration. 

With tiles called “cabinets” that can be tiled to build a huge display of any size and whose resolution increases with size, Samsung and Sony’s current research and advancements in microLED integration are one for the books. 

All these companies are paving their way into microLED integration with unmatched performance and design flexibility, and this article will explore that. 

Key Technologies in MicroLED Integration

MicroLED integration involves some fascinating technologies that are developing with time. Some of these are:

Chip Technology

Chip technology particularly refers to the techniques used in the manufacture of individual microLEDs. Some of these techniques are as follows:

Epitaxy Techniques for Uniform Growth

Epitaxy is a process of growing a crystalline layer on a substrate with a specific crystal structure. It ensures uniform growth of the semiconductor material (typically gallium nitride, GaN) on the substrate. For each individual microLED to perform consistently and emit light in the same color, it is imperative that they are uniform.

Chip Fabrication Challenges and Structures

MicroLEDs are tiny, often less than 100 micrometers in size. Fabricating such small structures requires precision lithography and etching techniques. For microLED integration using this technique, two types of structures can be formed. 

• Flip-Chip Structure: This is formed by directly bonding the microLED chip to the substrate, allowing efficient heat dissipation.
• Vertical Structure: This is formed by stacking multiple layers (e.g., red, green, blue) for full-color displays.

Integration Approaches (Full-Color Display, Backplane Bonding, Driving)

Although the focus of chip technology is on producing individual microLEDs, this section looks at how these chips are integrated into a working display. 

Full-Color Display

As mentioned earlier, microLEDs typically emit a single color (red, green, or blue). Combining these individual microLEDs creates a single pixel capable of displaying any color, and this process is often known as wafer bonding. 

Backplane Bonding

This explains how the microLED array (made of many individual chips) is attached to a backplane containing electronic circuits. These circuits are responsible for controlling the brightness and color of each microLED.

Driving

Here, the focus is on integrating driving circuits directly onto the backplane itself. These circuits regulate the individual microLEDs, eliminating the need for separate control circuitry.

The pixel driver output controls the current and voltage for color and brightness. The driving circuit module is integrated with the microLED array using techniques such as pick-and-place, bonding, or metal interconnection. This forms a two-dimensional (2D) or three-dimensional integrated structure, enabling efficient control of the pixels.

Mass Transfer

Large-scale, high-density, and effective heterogeneous integration of microLED arrays onto target substrates is made possible via mass transfer techniques. Some techniques under mass transfer are:

Pick and Place Technique

In microLED integration, the pick-and-place method utilizes robotic micromanipulators equipped with microgrippers to perform selective transfer of individual red, green, and blue (RGB) microLEDs from a source wafer or patterned substrate.

High-precision alignment systems ensure accurate placement onto a receiving substrate, typically a TFT backplane, with pre-defined landing sites for each microLED. Subsequent bonding via solder reflow, metal deposition, or strong Van der Waals interactions creates secure electrical and mechanical connections. 

Fluidic Assembly 

Fluidic assembly is another method that uses liquid carriers to transport microLEDs from the growth substrate to the target substrate. The suspension flows over a substrate patterned with microfluidic channels and wells corresponding to the desired pixel layout. 

Surface tension and controlled fluid flow guide the microLEDs into these pre-defined traps, achieving self-assembly of the microLED array. 

Research Progress on MicroLED Integration

The concept of micro light-emitting diodes or microLEDs was first proposed in 2000. Research groups like that of Hongxing Jiang and Jingyu Lin at Texas Tech University pioneered the invention and demonstration of inorganic semiconductor microLED technology. 

Further, the first high-resolution and video-capable InGaN microLED microdisplay in VGA format (640 × 480 pixels, each 12 μm in size) was realized in 2009 by the Texas Tech University team through the heterogeneous integration of a microLED array with a CMOS integrated circuit.

In terms of patents in this field, countries like Taiwan, the United States, China, South Korea, Japan, and many more are also contributing to microLED integration.

A Snapshot of Recent Progress

Recent progress in microLED integration includes the following:

Quantum Dots

Quantum dots (QDs) are semiconductor nanocrystals with unique size-dependent light emission properties.

MicroLED integration with a QD layer allows for:

• Wider Color Gamut: QDs can extend the range of colors a microLED display can produce, bringing it closer to the theoretical human color vision range (BT.2020 standard).
• Enhanced Color Purity: QDs can eliminate unwanted color bleed, resulting in more vibrant and realistic images.
• Potentially Higher Light Output: Certain QD materials offer higher photoluminescence efficiency, potentially leading to brighter displays.

Perovskite Materials

Perovskite based microLEDs are being explored as an alternative to traditional GaN-based microLEDs. This includes depositing perovskite layers on the microLED array and then connecting the array to a driving circuit. The use of perovskite materials also allows for the fabrication of microLEDs with smaller pixel sizes and higher pixel densities.

US Patent 11677472B2: Hybrid Integration of MicroLED Interconnects with ICs

The patent was filed by Avicenatech Corp. and assigned to Nokia Solutions and Networks Oy. 

Key Points

• Hybrid Integration: The patent describes a hybrid integration method that combines microLED interconnects with ICs. This involves integrating microLEDs with ICs to create a single package that includes both optical and electrical interconnects.
• MicroLED Interconnects: The patent focuses on the use of microLED interconnects, which are small, high-speed optical interconnects that can be used to connect ICs.
• IC Integration: This involves attaching the microLED interconnects to the ICs using a variety of methods, including flip-chip bonding and wire bonding.
• Optical Interconnects: The patent also highlights the use of optical interconnects, which are designed to transmit data between ICs. The microLED interconnects are used to connect the ICs and provide high-speed data transmission.

Key Takeaway

MicroLED integration has advanced significantly, leveraging key technologies like epitaxy techniques for uniform semiconductor growth, precise chip fabrication methods, and sophisticated mass transfer techniques such as pick-and-place and fluidic assembly. 

Recent patents, such as the hybrid integration of microLED interconnects with ICs, underscore the ongoing evolution in the field. Future developments will likely focus on refining these technologies for even greater efficiency and expanding their applications in various industries.