AGENDA

​

​

Laser technologies are essential for display fabrication today. Several laser processes and laser types are required for state-of-the-art display manufacturing. With the move from OLED to microLED displays some processes remain and several new manufacturing processes are required. The success of microLED displays is mainly driven by costs and availability of volume manufacturing equipment. Thus, microLEDs must become very small and need to be processed with highest throughput and yield. Lasers have proven their capability already in many applications but also in display fabrication. In this presentation, we will provide an overview of the microLED display process chain and highlight the individual laser processes.

​

MicroLED displays are poised to play a significant role in the future of the display market due to their advanced features, which may fill the gap of lack of luminance of incumbent technologies and market demand for energy efficient displays.This talk introduces designs, use cases, challenges and technical solutions for automotive products opportunities.

​

The developments of high performance InGaN based RGB micro-light-emitting diodes (µLEDs) are discussed. Through novel epitaxial growth and processing, and transparent packaging we have achieved external quantum efficiencies as high as 58% EQE at blue wavelengths (450nm) and 21% for green (520nm) for microLEDs. The critical challenges of µLEDs, namely full-color scheme, decreasing pixel size and mass transfer technique, and their potential solutions are explored. Recently, we have demonstrated efficient microLEDs emitting in the blue to red at dimensions as small of 1 micron. Using metalorganic chemical vapor deposition (MOCVD) and strain relaxation methods we have also extending the wavelength range of the InGaN alloys as into the red with emission as long as 640nm. Red InGaN based red MicroLEDs with efficiencies of 6% has been fabricated, and they display superior temperature performance in comparison to AlGaInP based devices. Recently, we have employed tunnel junction technology to vertically stack blue and green MicroLEDs monolithically on the same wafer. Independent control of the BG colors with high efficiency is demonstrated with tunnel junctions. This work was supported by the Solid State Lighting and Energy Electronics Center(SSLEEC) at UC Santa Barbara.

Micro-LEDs are promising for next-generation displays such as AR/VR. InGaN material can generate emissions in red, green, and blue. In the same material system, the LED devices can be stacked continuously, making it possible to fabricate monolithic RGB micro-LEDs on the same wafer. The low efficiency of red LEDs is the bottleneck for RGB micro-LED development. In this presentation, we will discuss the growth technology for InGaN-based red LEDs and the realization of monolithic RGB micro-LEDs.

​

​

​

Our novel bottom-up concept based on InGaN/GaN uLEDs offers solutions to several challenges that the uLED development is facing right now, namely miniaturization of the die without efficiency droop, sufficient small pitch to reach FHD resolution, and focused light emission to reach sufficient incoupling efficiency into waveguide optics. By using selective area growth, the dies can be placed deterministically onto the lithographically patterned SiN-masked GaN templates and die sizes down to 300nm have been achieved. As no etching of the die itself is needed the efficiency of the InGaN quantum wells, which are the active emitters, stay intact. The geometic structure of the uLED, a hexagonal pyramid, facilitates the focused emission and a sub-lambertian emission was obtained.

One of the intriguing yet largely unexplored technological approaches to fabricating microLED displays involves utilizing UV micro LEDs alongside colloidal quantum dots as light-converting materials. A main difference from traditional blue LEDs backlighting lies in the necessity of integrating hard-to-make and hard-to-get blue QDs in addition to red and green QDs. Despite this challenge, this approach offers several significant advantages, such as the lack of blue light leakage or better absorption efficiency of red and green QDs in the UV range as to name the most important ones. In this presentation, we will show the properties of our UV curable inks, which are based on heavy metal-free, blue light-emitting QDs known as PureBlue.dots, which can be used for UV light conversion to high quality 455 nm blue light which can be used in microLED displays. Furthermore, we will showcase our recent findings obtained from electroluminescent devices utilizing PureBlue.dots as the active material.

Quantum dot (QD) materials, known for their high photoluminescence quantum yield (PLQY) and narrow emission linewidths, offer a promising solution to the efficiency challenges faced by micro-LED chips. The green gap refers to the lower efficiency in the spectral range of 500-570 nm compared to blue and red LEDs due to the material performance. The red gap refers to the lower efficiency of red LEDs compared to blue ones due to the efficiency drop at elevated temperatures and sidewall effect. The short talk will focus on Fraunhofer IAP’s advancements in the fields of QD material and ink development for high-resolution EHD-Jet printing in the sub 10 µm range, where the efficiency gaps get especially relevant. Here, QD color conversion is a promising technology for the evolution of high-resolution micro-LED RGB displays.

Light-matter interactions at scales much smaller than the wavelength of the light opens new possibilities to control light. This field is called nano-photonics and enables improvements and new applications in micro-LEDs that are not possible with classical optics and current micro-structuring methods.Micro-LEDs have 3 challenges that can be “overcome” by making use of wafer-scale sub- 100nm patterns with single nm reproducibility. First, by making use of templated growth a full RGR LED system can be made on one substrate. For high resolution displays, the recombination process is not required anymore. Second, the light generated in the high refractive index semiconductor needs to be couple to air-modes. In macroscopic LEDs this is achieved by micro-patterns, light re-direction and recycling. This method is not possible to use in micro-LEDs as the micro-patterns are the size of the LED size. Photonic crystals can extract the light and also shape the extraction to either beam-like or bat-wing, compared to Lambertian in conventional LEDs. This creates directly more usable light by the enhanced out-coupling and directionality. Last, depending on the application, LED size and use of photonic crystals ,additional beam shaping might be required. The fast growing field and adoption of meta-lenses can help to keep the whole optical system efficient and compact. Metalenses function by precisely positioning nano-resonators that control the phase of the light and thereby can shape the wave-front and the (freeform) lens. These are robust, flat, thin (lens < 1micron) and therefore allow for easier integration. All the above mentioned applications of nano-photonics require feature sizes below 50nm with 1-5nm absolute size control to achieve the desired functions. The patterning method that can achieve this in a cost efficiency production method is based on nanoimprinting to form inorganic hard masks or functional devices. In the contribution the technology and nano-photonic applications will be discussed.

​

Methods to measure subpixel luminance & chromaticity for correction (Demura) & Quality Control for MicroLED displays. This inorganic emissive technology offers many benefits over other display technologies including high brightness, contrast, wide colour gamut, longevity, and high pixel density, improving visual performance in various ambient-light conditions from total darkness to full daylight and from multiple viewing angles. What are the challenges to efficiently control the quality of microLED displays and how to enable display correction?

In this presentation we will introduce our latest technology in micro-LED wafer inspection based on photoluminescence analysis. We are introducing our new imaging module mounted in our full wafer micro-LED inspection machine. This imaging module can simultaneously capture intensity and wavelength of light emitted from micro-LEDs by illuminating them with a stable light source. It allows a rapid acquisition of the photoluminescence over the whole wafer surface. By detecting the intensity and wavelength of the micro-LED emission the system makes quick pass/fail decisions to find abnormalities on the surface and semiconductor level.

The talk provides a brief overview of how laser-based processes revolutionize µLED production. It highlights the use of LIFT & line beam solutions to enhance yield and productivity. The talk also showcases a novel process chain involving LIFT and bonding on the backplane receiver, as the development and optimization of backplane materials alongside the laser process are crucial for successful bonding processes.

In the past decades the color gamut and resolution of displays was increased, as well as the energy efficiency. Under the current circumstances and with respect to their size microLEDs offer benefits for light weight smart glasses, transparent displays and automotive interior displays. Even the implementation in automotive rear lamps seems to be interesting for designers and engineers. An important material class are adhesives or so called functional polymers. Directional conductive adhesives can be coated over lager areas enabling the electrical and mechanical connection, while intrinsically preventing short circuits. Moreover functional polymers can be tailored in terms of color, transmission and viscosity to name only a few important parameters. It will be shown, that a reliable electrical and mechanical connection of miniLEDs with adhesives is possible after automated pick and place and thermal curing as an alternative to conventional solder materials.

Inkjet printing of has become a manufacturing standard for obtaining high-end QD-enhanced OLED displays. To extend the viability of this approach towards microLED manufacturing, the conventional inkjet process is too limiting though in terms of droplet size and precision. Also, the low viscosity of inks is a major concern, since layer thickness of printed QD layers needs to be minimized, in order to reduce light-losses at the black matrix. All these problems can be coped with by using EHD printing instead of conventional piezo-based ejection, as the process not only delivery sub-micron resolution but is also compatible with much higher ink viscosities. So far, the limitation, like so often, is not quality but quantity though. Scrona offers multinozzle based EHD printheads that combine the benefits of the EHD process with MEMS-based throughput-scaling that conceptually mimics that of conventional piezo-based inkjet heads, thereby finally paving the way for the technology to mature into production environments.

There are three main types of mass transfer technology in Micro LED. 1) Pick & Place method, 2) Laser Galvano scanning, 3) Line laser scanning. Each method has its advantages and challenges, and I will give the current status of these methods.

​