MicroLED displays offer futuristic potential, but their microscopic size poses unique challenges. Unlike traditional LEDs, MicroLEDs require a specialized foundation known as MicroLED substrates.  

This substrate must be incredibly stable and precise to support and align the minuscule LEDs. It needs to be extremely durable to withstand the fabrication process and the rigors of daily usage. Furthermore, it must be smooth enough to ensure maximum light extraction and color accuracy. Not only that, its display’s long-term reliability also depends on material properties like thermal conductivity.

This article highlights the intense developmental activities in the development of microLED substrates.

Novel Substrate Materials and Their Impact on Performance and Reliability

There has been an increase in research on MicroLED technology in recent years, as evidenced by the increasing number of patents filed. 

As of last year, nearly 5,500 patents in µLED technology have been filed by more than 350 different organizations. 

Creating new microLED substrates for high-performance printed circuit boards (PCBs) requires achieving functionality, more streamlined designs, and higher performance.

Conventional substrates such as FR-4 (Flame Retardant 4) have become widespread because they are affordable and generally perform well. 

However, as electronics become more compact and complicated, the shortcomings of conventional substrates—like low signal integrity at high frequencies and restricted heat conductivity—have become apparent.

Choosing the Right MicroLED Substrate for Your Application

Advanced microLED substrates are rising in the electronics industry due to their extraordinary thermal stability and flexibility. Some of them are as follows:

High-performance contenders

• Polyimide: Excellent heat dissipation and dimensional stability, but expensive and harder to manufacture/solder.
• Rogers Laminates: Designed for high-speed applications with improved heat conductivity and less signal loss, but more expensive and requires specialized manufacture.
• Metal Core PCBs: Their metal core’s superior heat conductivity makes them ideal for high-power applications, but they are substantially heavier and bulkier.

Futuristic options

• Graphene: Unparalleled thermal qualities, but still in early development with high costs and manufacturability issues.
• LCP substrates: These substrates offer great performance and affordability. They boast good thermal conductivity and are easy to make using standard methods.

Other considerations

• DuPont’s Pyralux: Its thermal qualities are outstanding, yet it is expensive and prone to brittleness.
• 3D-printed substrates: While they offer the potential for customization, factors like material characteristics, resolution constraints, and cost considerations hinder their effectiveness.

Microled Substrates Engineering Techniques for Improved Light Extraction, Thermal Management, and Transfer Efficiency

The improvement in light extraction efficiency (LEE) of GaN-based LEDs is one of the most important areas for increasing the external quantum efficiency for solid-state lighting applications.

Several novel techniques have been developed to overcome the low escape angle or avoid the wave-guiding effect, including photonic crystal (PC) structures, photoelectrochemical (PEC) wet etching, natural lithography, graded-refractive index layer (GRIN), and patterned sapphire substrate (PSS).

The droplet-based microfluidic cooling methods have shown an attractive capability for microscale liquid manipulation and relatively high heat flux removal for hot spots. 

Key aspects of droplet-based microfluidic cooling methods:

• Operates on the basic theory of electrocapillarity, cooling applications of continuous electrowetting (CEW), electrowetting (EW), and electrowetting-on-dielectric (EWOD).
• Addresses the challenges of high heat flux removal and nonuniform temperature distribution in confined spaces for high-performance electronic devices.

Integration Methods for Mounting MicroLED Substrates:

The companies who have come forward to contribute to microLED substrates include:

Microsoft

Microsoft has reportedly filed a new patent to develop a microLED display solution to enhance virtual, augmented, and mixed reality (VR/AR/MR) experiences. The patent seeks to enhance the capabilities of devices such as the HoloLens 2, hinting at a possible resurgence in Microsoft’s research and development (R&D) efforts.

It expands on previous breakthroughs (US11688333B1) and outlines a display setup consisting of two parts: a display base holding arrays of micro-LEDs forming pixels and a backplane base accommodating hardware modules for pixel logic.

Microsoft intends to investigate microLED displays to address some of the difficulties of waveguide technologies, such as their size and poor visual quality. Using microLED screens can minimize HoloLens device size while improving imaging quality.

This patent application comes amid a struggle between the world’s leading mixed-reality headset manufacturers. It also follows other patents in which Microsoft filed for hot-swappable batteries for an anticipated AR smart glasses gadget. This indicates a drive to revitalize its rumored AR head-mounted displays.

University of Strathclyde

Researchers at the University of Strathclyde in the UK, led by Eleni Margariti, have devised a printing technique using rollers. This method allows for the seamless and precise incorporation of MicroLEDs on a large scale by effectively transferring micrometer-scale semiconductor devices onto different platforms.

The roller method demonstrated in the journal Optical Materials Express has an outstanding accuracy of less than 1 micron in matching the device’s planned pattern. The setup is inexpensive and simple, making it usable even in areas with low resources.

The method uses a loosely attached array of small devices on a growing substrate. A cylinder covered in a slightly sticky silicone polymer coating is then wrapped over the suspended device array. The adhesive forces between the silicone and semiconductor separate the devices from their growing substrate and position them on the cylinder drum.

This method runs continuously, enabling the printing of multiple devices at once, which boosts efficiency for mass production. Beyond just MicroLEDs, it is versatile and applicable to silicon-based electronics, transistors, sensors, antennas for flexible and wearable gadgets, smart packaging, and RFID tags.

Dutch Organisation for Applied Scientific Research

TNO, a Dutch organization for applied scientific research, has developed a groundbreaking microfluidic, two-phase cooling technology for high-performance thermal management. This cutting-edge technology aims to improve the dependability and efficiency of high-powered microelectronics. 

It introduces an ultra-thin cooling system for both wafers and PCBs, offering outstanding heat transfer with minimal flow requirements. It can easily attach to silicon wafers or chips, providing significantly better cooling performance compared to traditional methods. 

Furthermore, it is designed for seamless integration directly into heat sources. This innovative cooling solution has wide-ranging applications, spanning from data centers and electric vehicles to LEDs and fuel cell management. It can also serve as an evaporator or boiler for waste heat recovery in heavy-duty engines. 

Addressing the challenge of improving performance while reducing weight and size, including cooling, presents a cost-effective solution that can be produced using various efficient techniques.

Future Patentable Areas in MicroLED Substrates

Choosing the right microLED substrates depends on the specific needs, balancing performance, cost, and manufacturability. 

Some of the emerging patents in microLED substrates include:

• Microfluidic integration for active thermal management: One route to address this problem is microfluidic cooling in a heat sink built within or attached directly to the silicon chip.
• Flexible displays: These are exciting developments because of their physical and performance attributes and their capability to enable new products requiring displays with unique form factors that the current rigid glass substrate-based displays cannot support.
• Ultra-thin glass (UTG): This glass, with excellent flexibility and transparency, has been used to overcome the low surface hardness of the typical polymer substrate used in existing foldable substrates.
• Flexible electrochromic devices (ECDs): These have generated widespread interest due to their attractive application prospects in emerging wearable and portable electronics, electronic papers/billboards, and other advanced display applications.

Final Note

The continued development of microLED substrates promises remarkable advancements in display technology. 

As researchers and manufacturers tackle challenges and optimize materials, we can expect even brighter, more efficient, and flexible microLED displays, paving the way for a future of extraordinary visual experiences.