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Nov 18, 2023

The Future is MicroLEDs, Given the Long Gestation of Display Technologies

There have been numerous reports recently about Apple’s adoption of MicroLEDs,

There have been numerous reports recently about Apple's adoption of MicroLEDs, and recently there was the news of Samsung and LG, in collaboration with the Korean government, looking at inorganic LEDs (ILEDs) as the means to regain the company's premier position in the display industry. So, we have an interesting confluence of events that signals a rush to deliver the next generation of display technologies and, more importantly, own the means of production.

Therefore, I thought it would be nice to try and get some historical perspective to try and find another way to analyze the future path of display technologies. A lot of data is from the pages of Display Daily, too many to mention, but I have also added some references below, all of which are quite readable and relevant. I hope you find something in here that helps you with your insights. And thanks to the boss for letting me have a shot at this. Blame him if everything goes wrong.

Nothing is meant to be definitive, but more of an overview, It starts with a historical perspective on OLEDs, how the US lost the display industry after CRTs, and the part that I think is most relevant, the path of MicroLEDs to a ubiquitous future.

Since its discovery nearly a century ago, electroluminescence has evolved from a mere scientific curiosity to a powerful and versatile technology. Initially confined to niche markets such as industrial, military, and medical equipment:

In the early stages of electroluminescent technology, thin-film electroluminescent (TFEL) displays and inorganic LEDs dominated the market. TFEL displays were primarily used in specialized applications, such as military and medical equipment, while inorganic LEDs found their place in various industrial and commercial devices. However, the limitations in achieving sufficient efficiencies for full-color applications and reducing the cost of TFELs hindered their widespread adoption. TFEL displays were relegated to niche markets, focusing mainly on military, medical, and automotive applications.

Over time, significant progress has been made in various aspects of OLED technology. Improvements have been achieved in carrier injection, doping methods, phosphor development, device design, and encapsulation processes. The efficiency of OLEDs has reached impressive levels, approaching 20%. OLEDs have demonstrated potential in inkjet-printed devices, full-color large-area panels, flexible displays, and high-resolution active-matrix (AM) displays.

OLED technology was first discovered in the early 1960s by André Bernanose and his co-workers at the Nancy-Université in France. They observed electroluminescence in organic materials. However, it wasn't until 1987 when Ching W. Tang and Steven Van Slyke at Eastman Kodak demonstrated the first practical OLED device. This marked a significant milestone in the development of OLED technology.

Early OLED materials had a limited lifespan, especially for the blue color component, which degraded faster than red and green. Researchers and companies had to develop more stable materials and better encapsulation techniques to extend the lifetime of OLEDs.

The commercial production of OLED panels began with small-molecule passive-matrix monochrome OLED panels in the late 1990s. Pioneer Corp. and Tohoku Pioneer Corp. led the way by producing OLED panels for automotive audio systems. The development continued, culminating in Sony's creation of the world's first full-color small-molecule AM display in 2001. The OLED technology advanced further with the introduction of a polymer-based proof-of-concept display by Toshiba Matsushita Display and the development of a prototype driven by super-a-Si thin-film transistors (TFTs) by IBM.

While OLED technology holds great promise, there are technical challenges to overcome for large-scale production. Issues such as device lifespan, color patterning, crosstalk, and yield rates remain areas of focus. However, experts believe that OLED technology, with its inherent advantages, has a bright future and the potential to gradually replace LCDs as the dominant flat-panel display (FPD) technology. It is crucial to address these challenges to ensure the successful transition from research breakthroughs to commercial products.

OLEDs offer several advantages over traditional display technologies like LCDs, such as higher contrast ratios, wider viewing angles, faster refresh rates, thinner and more flexible form factors, and lower energy consumption. These advantages made OLEDs an attractive option for various applications, including mobile devices, TVs, and wearable technology.

Manufacturing OLEDs, particularly on a large scale, posed significant challenges. Early manufacturing processes were expensive and yielded low production volumes. Over time, manufacturing techniques like vacuum deposition and inkjet printing were refined and improved, leading to cost reductions and higher production volumes.

OLEDs were initially expensive compared to LCDs. However, as manufacturing processes became more efficient and economies of scale were achieved, the cost of OLEDs decreased, making them more competitive with other display technologies.

In recent years, OLED technology has seen significant advancements, including the development of flexible and foldable OLED displays, which have led to innovative products like foldable smartphones and rollable TVs. These breakthroughs were facilitated by continued research into suitable organic materials and improvements in device architectures.

The history of the Cathode Ray Tube (CRT) is closely tied to the development of television and computer monitors. This technology has evolved over several decades, with researchers and companies overcoming numerous challenges to make CRTs commercially viable.

Invention and early development: German physicist Karl Ferdinand Braun invented the first CRT, known as the "Braun Tube," in 1897. This early device displayed images by deflecting an electron beam onto a phosphorescent screen, which produced visible light when struck by the electrons.

Advancements and challenges: One of the significant advancements in CRT technology was the development of the iconoscope by Vladimir Zworykin in 1929. This camera tube greatly improved image quality and marked the beginning of the electronic television era. However, CRTs faced several challenges, such as size, weight, power consumption, and manufacturing costs, which needed to be addressed for commercial success.

Commercial adoption and breakthroughs: The first commercially available CRT television sets appeared in the 1940s, and their adoption continued throughout the following decades. As technology advanced, CRTs were also used in computer monitors, providing a high-quality display option for early computers. However, the rise of alternative display technologies, such as LCDs and OLEDs, led to a decline in the popularity of CRTs.

The US’ failure to capitalize on its lead in CRTs and transition to newer display technologies like LCDs, PDPs, and OLEDs can be attributed to a combination of factors, including a shift in focus, competition from Asian companies, manufacturing cost advantages, lack of foresight, and fragmented research efforts.

In the early days of Liquid Crystal Display (LCD) and Plasma Display Panel (PDP) development, several companies and countries played significant roles in pioneering and advancing these technologies. Japan was at the forefront of both LCD and PDP development, with leading electronics companies like Sharp, Toshiba, and Hitachi heavily involved in the research and commercialization of LCDs. Meanwhile, other Japanese companies like Fujitsu, NEC, and Panasonic were instrumental in PDP development.

Sharp Corporation, a Japanese company, was an early innovator in LCD technology. In 1973, Sharp produced the first commercially viable LCD, which was used in calculators and digital watches. They continued to develop LCD technology throughout the 1980s and 1990s, eventually producing the first full-color TFT LCD panels, which became widely used in laptop computers and television screens.

Another Japanese electronics company, Toshiba, was also involved in early LCD development. They focused on producing small- and medium-sized LCD panels for laptops, mobile phones, and other portable electronic devices. Hitachi, yet another Japanese company, contributed to LCD research and development in the early days, focusing on improving image quality and refresh rates. They also manufactured LCD panels for a variety of applications, including televisions and computer monitors.

Fujitsu, a Japanese multinational company, was a pioneer in PDP development. They began their research in the 1960s and introduced the first commercial PDP in 1989. Fujitsu continued to develop PDP technology throughout the 1990s, focusing on improving image quality, color reproduction, and energy efficiency. NEC, another Japanese electronics company, was an early player in the PDP market. They invested in research and development of PDP technology and introduced their first PDP product in the early 1990s. Panasonic, a Japanese multinational corporation, was also involved in early PDP development. They introduced their first PDP product in 1997 and continued to invest in PDP technology until the early 2000s, when the focus shifted towards LCD and OLED technologies.

While US companies initially led the development of CRTs, they eventually shifted their focus to other areas, such as computers and software development. As a result, US companies did not invest as heavily in display technology research and development, allowing other countries, especially Japan and later South Korea, to take the lead in the development of LCDs and PDPs.

Asian companies, particularly those from Japan, were more agile and quick to invest in and adopt new display technologies. They heavily invested in research, development, and manufacturing infrastructure for LCDs and PDPs. These companies were able to create high-quality, cost-competitive products, making it difficult for US companies to compete in the market.

Asian countries, particularly Japan and South Korea, had lower manufacturing costs, which allowed them to produce LCDs and PDPs at a lower cost than their US counterparts. This cost advantage enabled them to dominate the market, making it challenging for US companies to compete.

US companies may have underestimated the potential of LCDs, PDPs, and OLEDs to replace CRTs in the market. By the time they realized the potential of these new technologies, Asian companies had already established a significant lead in research, development, and market share.

While the US had some notable research efforts in display technologies, they were often fragmented and spread across different institutions and companies. This made it difficult for the US to consolidate and coordinate efforts to develop and commercialize new display technologies effectively. You only have to look at the effort Korean companies are putting into display manufacturing to realize the amount of commitment required from all stakeholders.

Maybe that explains what Apple is doing. First of all, we can start by acknowledging that MicroLED manufacturing is a semiconductor industry. Secondly, Apple ditched Intel very comfortably when it moved to its own Mx processors. To become a display manufacturer is not going to be a stretch for Apple. If anything is going to accelerate the volume of MicroLED production for all display form factors, it is going to be Apple. Given that, it's not a stretch to think to assume that MicroLED development is going to receive an acceleration in interest and deployment. The future is MicroLEDs and the future is going to get closer.

MicroLED is a type of display technology, an evolution of the existing LED technology. It has been a significant topic in scientific research for more than two decades, and already in 2023, the volume of research papers coming out on the device is going to exceed the previous year, and continue a 4–5 year run of increasing scientific output. With good reason, MicroLEDs offer several benefits over traditional displays—they are more efficient, use less energy, are brighter, and last longer than both liquid crystal and OLED displays.

However, MicroLEDs are smaller than their predecessors, with pixel sizes often less than 100 or even 50 micrometers. This small size presents substantial challenges when it comes to arranging these LEDs into a usable display. To build a high-quality MicroLED display, these tiny LEDs need to be integrated or assembled together in a precise way. This integration process is currently seen as the biggest obstacle in developing high-performance MicroLED displays.

There are three key aspects of integration technology in MicroLED manufacturing:

From a manufacturing perspective, there are three methods for integrating MicroLED displays:

This section describes the process of "Transfer Integration" for MicroLEDs, which is crucial for the production of large-scale, high-definition displays, like a 55-inch 4K TV. Given the sheer number of LED dies needed for such a display, this process is also known as "mass transfer."

The transfer integration process uses various techniques that depend on different physical mechanisms. Methods have been developed by leading companies and research institutes worldwide. These include:

Overall, the transfer integration process involves three technical steps: substrate release, pick-and-place, and electrical interconnection.

In the substrate release phase, the MicroLEDs, typically grown on silicon, sapphire, or GaAs substrates, need to be released or removed. This step is necessary because the substrate is thick and can create issues with electrical interconnection and thermal management once the MicroLEDs are transferred.

The substrate release technique depends on the physical and chemical properties of the material used. Common release techniques for different substrates are shown in Figure 2 (which is not provided in the text).

In general, substrates can be released physically or chemically:

Each of these techniques has its own advantages and constraints, and their choice depends on the specific requirements of the MicroLED production process.

The process of substrate release for MicroLEDs is a vital step in the transfer integration process. The specific technique used for substrate release varies depending on the material properties of the substrate.

For substrates made of silicon or GaAs, mechanical grinding and wet chemical etching techniques are more suitable. However, mechanical grinding can cause a significant mechanical impact on the MicroLEDs, requiring them to firmly adhere to a temporary substrate, which can complicate subsequent transfer steps. Thus, the mechanical grinding process is primarily used for transferring the substrate of vertical structure LEDs. Dawson et al. used a KOH solution to etch silicon in a process called anisotropic etching to achieve substrate release.

After the release of the substrate, the next crucial step in the transfer integration of MicroLEDs is the picking and placing of the MicroLEDs. This process needs to be both swift and precise to achieve cost-effective and high-resolution displays. Two methods have shown exceptional performance in this regard: the elastomer stamp method and laser selective release.

This technique was developed by Rogers’ group, who researched how to control the adhesion strength between a stamp and a film reversibly. Essentially, they found that the energy-release rate of the stamp-film interface is proportional to the peeling speed. They used this to control the adhesion strength between the stamp and LED membranes to allow for transfer printing of a MicroLED array. In 2009, Park et al. used a flat polydimethylsiloxane (PDMS) stamp to transfer an AlInGaP-based MicroLED array onto a polyurethane and a glass substrate, creating a flexible and semi-transparent display.

To improve the transfer yield and repeatability, researchers created a microstructured PDMS stamp in 2010, which had a more distinct reversibility window. However, the process involves an adhesion-enhancement layer on receiving substrates, which can impact thermal management and luminous efficiency due to their poor thermal conductivities and change in refractive index. To mitigate these problems,

This technique was inspired by laser-induced forward transfer. In this method, a laser selectively irradiates the backside of a transparent donor substrate. The energy from the laser is absorbed by a dynamic release layer (DRL), a polymer sacrificial layer located between the substrate and the film to be transferred. This absorption leads to partial ablation of the DRL, generating a repulsive force that causes delamination between the microstructure membrane and the donor, allowing for the transfer of dies onto a receiving substrate. Saeidpourazar et al. further refined this technique by using a PDMS stamp as the DRL, making use of the fact that PDMS stamps are transparent to infrared lasers, which are used to facilitate the transfer of MicroLEDs.

Meanwhile, a massively parallel laser-enabled transfer technology that utilizes an arrayed UV laser has been suggested. When the DRL is partially ablated, a blister is formed in the DRL. The expanding blister, combined with gravitational force, allows for the transfer of MicroLED dies to a receiving substrate across a gap of 10 to 300 micrometers.

After the MicroLEDs are assembled, they need to be interconnected to allow for addressable driving of MicroLED displays. This interconnection process typically involves forming a metal mesh by using photolithographic patterns and metal deposition. For matrix-addressable driving, the p-electrodes of each MicroLED are connected in rows or columns, and the n-electrodes are connected in the opposite orientation.

The first step in the process is to fabricate column wires and cover them with a dielectric film before transferring the MicroLED dies onto the receiving substrate. Vias are then opened through the dielectric layers for the connection of MicroLEDs and column wires using standard photolithography and reactive ion etching techniques. The electrode mesh can then be achieved with one metal patterning and deposition.

However, this method only allows for passive matrix interconnection of the MicroLEDs. To achieve active-matrix driving of MicroLED displays, the MicroLEDs need to be directly transferred onto substrates with micro-CMOS circuit arrays, or micro-integrated circuit (micro-IC) units, that drive the MicroLED can be integrated via transfer printing. Electrical connections between the MicroLEDs and micro-ICs are achieved through photolithography and metal deposition processes, and each MicroLED can be controlled by its corresponding micro-CMOS circuit in an integrated subpixel.

While the introduction of CMOS circuits into subpixels can reduce resolution, the active-matrix driving mode can significantly increase the brightness of the MicroLED display and reduce pixel crosstalk.

Transfer integration is a highly effective method for integrating most inorganic micro-devices and their arrays, such as MicroLEDs, microsensors, and micro-CMOS. This method is especially useful for MicroLED displays and is considered essential for futuristic large-area flat-panel MicroLED displays due to its ability to expand the MicroLED array through multiple printing processes. It also allows for full-color displays with wider color gamuts and higher efficiency and is well-suited to flexible displays.

However, the challenges facing transfer integration include a high cost due to the limited yield of mass transfer and the need for repair and redundancy, and difficulty in designing driving circuits due to the differing operating currents required by MicroLEDs with different light wavelengths. Despite these challenges, with advanced equipment and innovative technology, future breakthroughs in high resolution, high yield, and low cost are expected to be realized.

MicroLED displays integrate MicroLEDs with active-matrix CMOS circuits to control each LED pixel individually. Different techniques exist to achieve this integration, each with its advantages and challenges:

As we traverse the promising landscape of MicroLED technology, several critical challenges must be addressed to unlock its potential for mass production and widespread adoption in display systems. A central theme revolves around materials, techniques, and integration processes that could make manufacturing more efficient and cost-effective.

Foremost, specific materials used for LEDs and driver circuitry play a pivotal role. Research indicates the promise of large-sized heterogenous epitaxy for the creation of large-scale, full-color monolithic displays. The potential use of 2D materials that can be mass-prepared also emerges as a promising frontier for driver transistor fabrication. Additionally, compound quantum dots (QDs), such as ZnS, InP, and perovskite QDs, are under consideration as color-conversion materials when integrated with GaN-based MicroLEDs.

Robust methods for quality assurance and testing are also essential. Though not explicitly discussed, the required coordination of multiple integration techniques implies an inherent need for comprehensive and effective quality controls. Ensuring the consistent and reliable production of MicroLEDs is paramount for commercial viability.

The issue of heat management and power efficiency is also integral. Growth integration is one technique that holds promise in this regard, potentially enabling more compact MicroLED displays with high efficiency and low energy consumption. Yet, more specific techniques for managing heat dissipation within these compact, power-dense systems are still a subject of ongoing research.

Scalability and cost-effectiveness pose another significant hurdle. The advancement of transfer integration, notable for its capability in fabricating large-area flat-panel displays, offers the potential in increasing yield and reducing cost. This suggests a promising pathway toward the broader commercialization of MicroLED displays. Future displays are expected to require updates in multiple aspects, including materials and processes, all of which can impact scalability and cost.

Regarding long-term reliability and failure modes, the need for multiple integration processes suggests an underlying focus on these areas. The durability and longevity of MicroLED displays in the face of continual use and potential component failure is crucial for consumer confidence and industry uptake.

A look towards the future sees the emergence of intelligent displays. By integrating diverse devices and components, such as optical waveguides, photodetectors, sensors, actuators, logic and analog circuits, radio-frequency devices, and energy harvesters, MicroLED displays could find broader applications. The goal is to create versatile, functional systems that extend beyond traditional display functions, promising exciting new possibilities in visible light communication, the Internet of Things (IoT), and biomedical and micro-nano manufacturing. All these areas would be of significant value to Apple and could be incorporated into iPhones and smartwatches.

As such, the evolution of MicroLED technology is not solely about enhancing display technology but also about how it integrates with other technologies, driving toward a future of more sophisticated and intelligent systems.