micro-LED Display Research Articles

Monolithically integrated high-resolution full-color GaN-on-Si micro-LED microdisplay

Full-color micro-LED displays are being widely developed and regarded as a primary option in current microdisplay technologies to fulfill the urgent demands of metaverse applications in the next decade. In this paper, a monolithic full-color micro-LED microdisplay with a resolution of 423 pixels per inch is demonstrated through the integration of a blue GaN-on-Si display module and a quantum dots photoresist (QDs-PR) color conversion module. The 400 × 240 active-matrix blue micro-LED display with a dominant wavelength of 440 nm was monolithically fabricated using GaN-on-Si epiwafers and flip-chip bonded on a custom-designed complementary metal-oxide semiconductor backplane. A color conversion module was independently fabricated on a 4-in. sapphire substrate by applying red and green QDs-PR arrays and a color filter array through the standard lithography process. Combining the blue GaN-on-Si micro-LED display module and the lithography-based QDs-PR color conversion module, a full-color micro-LED display was achieved with a wide color gamut up to 104% of the standard red, green, and blue and a maximum brightness of over 500 nits. The influence of blue light leakage resulting from the possible misalignment of flip-chip bonding and crosstalk in the bottom GaN-on-Si display was investigated in which the percentages of efficient pumping light for the blue, green, and red subpixels are around 95%, 89%, and 92%, respectively. This prototype demonstrates potential scalability and low-cost volume production of high-resolution full-color micro-LED microdisplays soon.

Improved Optical and Electrical Characteristics of GaN-Based Micro-LEDs by Optimized Sidewall Passivation.

GaN-based Micro-LED has been widely regarded as the most promising candidate for next generation of revolutionary display technology due to its advantages of high efficiency, high brightness and high stability. However, the typical micro-fabrication process would leave a great number of damages on the sidewalls of LED pixels, especially for Micro-LEDs, thus reducing the light emitting efficiency. In this paper, sidewall passivation methods were optimized by using acid-base wet etching and SiO2 layer passivation. The optical and electrical characteristics of optimized Micro-LEDs were measured and analyzed. The internal quantum efficiency (IQE) of Micro-LED was increased to 85.4%, and the reverse leakage current was reduced down to 10-13 A at -5 V. Optimized sidewall passivation can significantly reduce the non-radiative recombination centers, improving the device performance and supporting the development of high-resolution Micro-LED display.

Manufacturing MicroLED Display by PixeLED Solution

Manufacturing MicroLED Display by PixeLED Solution

Three-dimensional Monolithic Micro-LED Display Driven by Atomically Thin Transistor Matrix

Three-dimensional Monolithic Micro-LED Display Driven by Atomically Thin Transistor Matrix

Technological Advancements and Manufacturing Readiness of Micro-LED Displays

Technological Advancements and Manufacturing Readiness of Micro-LED Displays

Challenges and Solutions for the Fabrication of CMOS-driven Microled Displays

Challenges and Solutions for the Fabrication of CMOS-driven Microled Displays

Vertical stack integration of blue and yellow InGaN micro-LED arrays for display and wavelength division multiplexing visible light communication applications.

In this work, we demonstrated a convenient and reliable method to realize the vertical stack integration of the blue and yellow InGaN micro-LED arrays. The standard white and color-tunable micro-light sources can be achieved by adjusting the current densities injection of the micro-LEDs. The spectra cover violet, standard white, cyan, etc., showing an excellent color-tunable property. And the mixed standard white light can be separated into red-green-blue three primary colors through the color filters to realize full-color micro-LED display with a color gamut of 75% NTSC. Besides, the communication capability of the integrated micro-LED arrays as visible light communication (VLC) transmitters is demonstrated with a maximum total data rate of 2.35 Gbps in the wavelength division multiplexing (WDM) experimental set-up using orthogonal frequency division multiplexing modulation. In addition, a data rate of 250 Mbps is also realized with the standard white light using on-off keying (OOK) modulation. This integrated device shows great potential in full-color micro-LED display, color-tunable micro-light sources, and high-speed WDM VLC multifunctional applications.

Inkjet Printed Quantum Dots Color Conversion Layers for Full-Color Micro-LED Displays

With the ever-growing demands for larger size and high resolution displays, Micro-light-emitting diode (Micro-LED) display with quantum dots (QDs) film as color conversion layers (CCLs) has become one of the most promising candidates of future display for its advantages in low power consumption and wide color range. In this study, we report a novel full-color display based on blue Micro LED, which has patterned red and green QDs color conversion (QDCC) layers fabricated by inkjet printing (IJP). A structure of double-layer bank was designed to reduce color deviation, prevent crosstalk, and flatten the QDCC layer. By optimizing the thickness of the red/green QDCC layers and the wavelength of blue Micro LED backlights, a full-color QDCC-LED display with 228 PPI resolution and size of 1.11-inch was successfully fabricated and showed superb performance. We not only effectively reduced crosstalk, but also improved the color conversion efficiency of QDs. In addition, this QDCC-LED display prepared by embedded bonding process shows a color gamut of 107.53% NTSC.Graphical

Micro LED Display Pixel Circuit Using Amorphous Indium-Gallium-Zinc Oxide Thin-Film Transistor

Micro LED (μ-LED) refers to a micro-light emitting diode with a chip size of 5 to 100 μm. Since micro-LED displays use individual LED chips as a light source of the pixel, they do not need backlight and color filters, which are suitable for implementing flexible displays. In addition, micro-led displays have many advantages over OLED displays, such as excellent color gamut, power consumption, lifetime, and response time. With these advantages, micro-LED displays are attracting attention as next-generation displays, and accordingly, interest in micro-LED driving technology is increasing. While existing OLED displays express grayscale by controlling the amount of current flowing through the light-emitting device, micro-LED can have a problem of color change depending on the grayscale because the light-emitting wavelength shifts when the amount of current changes. Therefore, in order to express a certain color, it is necessary to express the grayscale by adjusting the emission time at the same current density. That is, it is more suitable to use the PWM(pulse width modulation) for each pixel than to use the PAM(pulse amplitude modulation) for the grayscale representation of the OLED display. Therefore, unlike OLED, the micro-LED display requires a new pixel circuit because it must use a PWM driving. In addition, unlike LCDs, micro-led displays are sensitive to changes in electrical characteristics of TFTs depending on the location of each pixel or external stress. Thus, threshold voltage compensation is essential because changes and degradation of the characteristics of the thin film transistor backplane, such as threshold voltage shift, cause non uniform brighteness. Therefore, we propose a PWM compensation pixel circuit based on a-IGZO TFT that can be fabricated on a flexible substrate and has a lower process cost than LTPS and greater mobility than a-Si TFT.The operation of proposed pixel is divided into (1) reset 1, (2) PWM data compensation, (3) reset 2, (4) CCG (constant current generation)compensation, and (5) emission. The circuit is divided into a CCG unit that generates a constant current and a PWM unit that adjusts emission time by a ramping signal. Figure 1 (a) shows the output current simulation result according to PWM data voltage variation. When the PWM data voltages vary from -1 to ~ - 5V, the currents were constant at 70.9 μA, and the smaller the PWM data voltage, the longer the emission time was. In addition, Figure 1 (b) shows the effect of the threshold voltage compensation. The current and emission time are constant for various threshold voltages such as 1, 2, and 3 V. Figure 1

(Invited) InGaN/AlGaN Red-Emitting Nanowire LEDs for Monolithic Micro-LED Displays

High efficiency, high color rendition and low-cost light-emitting diodes (LEDs) with low power consumption, long lifetime and high reliability are highly expected for general lighting illumination, smartphones, smartwatches, virtual reality (VR), augmented reality (AR) headsets, and micro-display applications. Nonetheless, the achievement of deep green to red-emitting LEDs using conventional III-nitride quantum well heterostructures has been difficult, due to the presence of large densities of dislocations, strong polarization fields, poor hole transport, and carrier delocalization [1]. The external quantum efficiency (EQE) of the blue and green InGaN LEDs surpassed 80% and 53%, respectively. However, due to the aforementioned issues, the current InGaN red LEDs with high Indium composition exhibit extremely low EQE which is less than 3% [2]. In this regard, LEDs using nanowire structures offer dramatically reduced strain-induced polarization fields and dislocation densities, providing ideal material structure for high efficiency full-color and even white light emission without using phosphor-converters. In this study, we have successfully designed, and fabricated high efficiency red-emitting nanowire heterostructures and demonstrated micro-LEDs with stable and strong emission at ~645 nm. Moreover, the micro-LEDs have high internal quantum efficiency of >40%.The InGaN/AlGaN nanowire micro-LED structures are grown by RF plasma-assisted molecular beam epitaxy (MBE) under nitrogen-rich condition. The LED structure consists of a 250 nm n-GaN nanowire template, 10 couples of 3 nm AlGaN quantum barrier (QB)/ 3 nm InGaN quantum well (QW) served as the active region, and a 200 nm p-GaN. During the epitaxial growth of AlGaN barriers, an AlGaN shell spontaneously forms, enabling a unique InGaN/AlGaN core-shell structure [3]. The emission color of the micro-LEDs can be defined by controlling the ratio of Ga/In flux and the substrate temperature during the MBE growth process. Detailed growth conditions and the device fabrication can be found elsewhere [3-5]. The nanowires are uniformly arranged on Si substrates, as illustrated in Figure 1(a). Figure 1(b) shows the schematic structure of the fabricated micro-LEDs. Strong red emissions were measured from the InGaN/AlGaN core-shell LEDs, as shown in Figure 1(c). At injection current of 400mA, the peak emission wavelength is at ~645nm. The red-emitting micro-LEDs exhibit stable emissions with a blue-shift of only ~ 4nm under injection current from 50 mA to 400 mA, attributed to the significantly reduced quantum-confined Stark effect (QCSE) in the nanowire structures. Moreover, full color micro-LEDs with device size from 10x10 µm2 to 100x100 µm2 have been fabricated using similar approach. Such high efficiency, high color rendering properties, and low power consumption micro-LEDs are promising candidates for emerging AR/VR devices and micro-LED display applications.

46.4: Laser Processes for MicroLED Display Manufacturing

The market for MicroLED displays is strongly growing in all segments from very small AR/VR to very large TV and signage displays. For a successful market adoption costs of MicroLED displays must be significantly reduced. The entire supply chain is working on different proposals. Roadmaps have already shown where MicroLED sizes need be reduced to 5 μm while the processing times of such displays must be reduced by factors. It is obvious that more precision processing equipment with increased throughput is needed to fulfil future market requirements. Laser are known for their precision and scalability and therefore a key to successful commercialization of MicroLED displays. In this paper we will describe the two critical processes during MicroLED manufacturing, the Lift‐Off process where the Gallium Nitride (GaN) MicroLED's need to be released from the growth wafer, and later be transferred to a substrate.

46.1: Invited Paper: MicroLED Technology and Applications by PixeLED Solutions

MicroLED display is an emerging technology with high brightness, high contrast ratio, and wide color gamut. This technology can be used for all current display applications and new scenarios, such as transparent, seamless tiling, and AR glasses for metaverse. Based on our proprietary PixeLED display, PixeLED Matrix, μ-PixeLED, and SMAR-Tech technologies, we could utilize different process for each application with MicroLED displays.

Red, green and blue InGaN micro-LEDs for display application: temperature and current density effects.

Micro-LED has attracted tremendous attention as next-generation display, but InGaN red-green-blue (RGB) based high-efficiency micro-LEDs, especially red InGaN micro-LED, face significant challenges and the optoelectronic performance is inevitably affected by environmental factors such as varying temperature and operating current density. Here, we demonstrated the RGB InGaN micro-LEDs, and investigated the effects of temperature and current density for the InGaN RGB micro-LED display. We found that temperature increase can lead to the changes of electrical characteristics, the shifts in electroluminescence spectra, the increase of full width at half maximum and the decreases of light output power, external quantum efficiency, power efficiency, and ambient contrast ratios, while current density increase can also give rise to different changing trends of the varieties of parameters mentioned just above for the RGB micro-LED display, creating great challenges for its application in practical scenarios. Despite of the varying electrical and optical charateristics, relatively high and stable colour gamut of the RGB display can be maintained under changing temperature and current density. Based on the results above, mechanisms on the temperature and current density effects were analyzed in detail, which would be helpful to predict the parameters change of micro-LED display caused by temperature and current density, and provided guidance for improving the performance of InGaN micro-LED display in the future.

Fabrication and Mechanical Properties Improvement of Micro Bumps for High-Resolution Micro-LED Display Application

Recently, many significant strides have been made for the integration of micro-light-emitting diodes (Micro-LEDs) in display. However, the reflow process is usually accomplished under formic acid or hydrogen ambient, which is irritating, flammable, explosive, and very expensive. For reliable micro bumps in the flip-chip package, it is essential that the correlations among mechanical properties, fracture mechanisms, and interfacial reactions of micro bumps formed on a silicon substrate be understood. In this article, a novel under bump metallurgy (UBM) structure based on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> openings was described, as a result of which the indium micro bump arrays with a pitch of 40 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> and a diameter of 15 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> were successfully fabricated by the reflow process via glycerol. The effect of reflow time and reflow temperature on the reliability of indium micro bumps was also investigated. Experimental results showed that the average shear strength of indium micro bumps peaked at the reflow conditions of 200 °C for 180 s and decreased with further increasing reflow time and temperature. It was considered that the stable AuIn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> intermetallic compounds (IMCs) formed toward the solder joint improved the shear strength after reflowed at 200 °C for 180 s. As the reflow time and temperature increased, the grain size of IMCs increased, which weakened the interfacial connection. In addition, the transfer and bonding reliability of Micro-LED chips through the flip-chip bonding process have also been discussed.

Mass transfer, detection and repair technologies in micro-LED displays

Micro-light emitting diode (micro-LED) is an emerging display technology with excellent performance of high contrast, low power consumption, long lifetime, and fast response time compared with the current display (e.g., liquid crystal and organic LED (OLED)). With technological advantages, micro-LED holds promise to be widely applied in augmented reality (AR), flexible screens, etc. and is thus regarded as the next generation of display technology. In the process flow of micro-LED, the step known as mass transfer that requires transferring millions of micro-LEDs from a growth substrate to a display plane, is one of the key challenges limiting the commercialization of micro-LED from laboratory. Worldwide academic and industrial efforts have been devoted to developing mass transfer strategies with purposes of improving yield and reducing cost. Herein we review three main categories of mass transfer technologies for micro-LED display (pick-and-place, fluid self-assembly and laser-enabled advanced placement) and the coupled detection and repair technologies after transfer. Discussions and comparisons have been provided about the underlying general principle, history, and representative parties, advantages, and disadvantages (yield/efficiency/cost) of these technologies. We further envision the application prospect of these transfer technologies and the promise of the future display of micro-LED.

50‐4: Invited Paper: Development of MicroLED Display Technology and Applications

MicroLED display is an emerging technology with high brightness, high contrast ratio, and wide color gamut. This technology can be used for all current display applications and new scenarios, such as transparent, seamless tiling, and AR glasses for metaverse. Based on our proprietary PixeLED display, SMAR·Tech, PixeLED Matrix, and μ‐PixeLED technologies, we could utilize different process for each application with MicroLED displays.

50‐1: Invited Paper: Designs and Manufacturing Processes for microLED Displays in Handsets, Smartwatches, and Personal Computers.

Industry‐wide efforts to develop microLED technology for displays brought focused attention to the importance of mass transfer micro‐assembly. Processes that are compatible with high‐volume manufacturing that can rapidly manipulate millions of discrete micron‐scale semiconductor objects per product unit are a new technological capability and required for making microLED displays accessible to mainstream consumer applications. As those assembly processes become mature, new attention shifts to the microLED devices themselves and to electronic driving schemes to operate them. Furthermore, mature mass transfer processes make possible new semiconductor constituents to the backplane, including singlecrystal devices with levels of integration density that are not natively available to display panel manufacturing. This confluence of new requirements and new possibilities sets a stage for far‐reaching innovation in consumer displays. This paper describes designs for microLED displays that use micron‐scale full color emitter packages and micron‐scale driver elements (microICs) that control clusters of pixels. A combination of mass transfer and conventional backplane fabrication processes is suitable for making displays of these designs, and the resulting products will support more than 300‐400 pixels per inch with advantageous power consumption characteristics.

57‐3: MircoLED Display Integration on 300mm Advanced CMOS Platform

We show for the first time a microLED display integration flow in a 300mm CMOS manufacturing platform resulting a pixel density of 9150dpi with a 3μm pixel pitch and a brightness for blue μLED in excess of 600kcd/m2 @100A/cm2.

P‐27: A Novel PWM Driving Pixel Circuit with Metal‐Oxide TFTs for MicroLED Displays

A pixel circuit for micro‐LED display has been developed with IGZO TFT device. By using pulse width modulation method, the pixel circuit can operate at high peak current and low duty ratio to keep micro‐LED at high emission efficiency and stable color performance.

Synthesis of SiO2-coated CdSe/ZnS quantum dots using various dispersants in the photoresist for color-conversion micro-LED displays

Quantum dots (QDs) are a promising technology for next-generation display screens owing to their well-defined and tunable emission wavelengths. However, QDs can chemically react with external environment, decreasing their photoluminescence efficiency and reliability. In addition, further work is needed on lithography techniques to make QDs compatible with semiconductor manufacturing processes. In this study, silicon dioxide (SiO2)-coated cadmium selenide/zinc sulfide QDs are synthesized using tetraethoxysilane. The ratio of cadmium oxide to zinc acetate was tuned before coating the SiO2 passivation layer to adjust the emission wavelength of the QDs between 520 and 625 nm. The SiO2 passivation layer not only enhanced the photoluminescence intensity (PLI) by 69% but also effectively decelerated the fluorescence decay rate (<1% PLI decay after 100 h). In addition, the hydrophilicity of the SiO2-coated QDs enabled them to integrate with appropriate dispersants and photoresist for creating a 50-μm × 50-μm pixel on the color-conversion layer of a blue microlight emitting diode (micro-LED) display. Finally, a structure of a distributed Bragg reflector (DBR) was designed using TFCalc software. Subsequently, a DBR layer was deposited on the red–green QDs’ color-conversion layer to fabricate a full-color micro-LED display.