Full Color Micro-LED Display Technology
May 27, 2021
Micro-LEDs were first proposed around 2000. In 2004, a research team at the University of Strathclyde in the UK produced GaN-based blue micro-LED arrays with a pixel size of 20μm and a pixel number of 128×96 using dry etching technology. In 2006, Liang Jingqiu research team of Changchun Institute of Optics and Machine Technology prepared AlGaIP red micro-LED array by layered wet etching, with pixel size of 16μm×20μm and pixel number of 1000×818. In 2011, a research team at the University of Texas produced GaN-based green light micro-LED array with pixel size of 15μm and pixel number of 640×480.
In 2014, Apple acquired Lux Vue and officially entered the technical research of micro-LED, and micro-LED technology also entered the public's vision.
In the development process of micro-LED devices, the research work in the world mostly focuses on the structure, fabrication process, device characteristics and other aspects of monochrome micro-LED devices such as red light, blue and green light and ultraviolet light. Fig. 1 takes a sapphire substrate epitaxial chip as an example to show the structure and brief preparation process of micro-LED. The epitaxial layer usually includes GaN buffer layer, N-type GaN, InGaN/GaN multi-quantum well, AlGaN electron barrier layer and p-GaN layer, etc. The fabrication process of Micro LED usually includes ICP mesa etching, N electrode metal deposition, SiO2 layer deposition, P electrode deposition and annealing, etc.
With the increasingly urgent application demand for micro-LED display devices, the fabrication technology of full-color micro-LED devices has gradually become a research hotspot. Conventional red, green, and blue LEDs generally cannot be grown on the same substrate at the same time. So far, most research has focused on monochrome displays. To achieve a full-color micro-LED display, researchers in Liang Jingqiu's team made the following explorations.
1. Tri-color integrated full color micro-LED Micro display
By transferring red, green and blue micro-LED light emitting chip to silicon based drive circuit, a full-color independently addressable Micro display device is fabricated. Using silicon wafer as the substrate, the metallized pattern is produced by lithography and magnetron sputtering. The metallized pattern consists of metal pads and wires. Each pixel of the display device consists of red, green and blue micro-LED chips.
Red, green and blue micro-LED chips are flip chips, the size and shape of which are the same or similar. They are made by flip bonding process, which has good process consistency, short production cycle and high efficiency. Since no lead bonding is required, the anode pad can be placed under the micro-LED, which further reduces the size of the full color pixel unit. Characterized by high brightness and wide operating temperature range, this microdisplay has great potential for indoor/outdoor applications.
2. Monolithic integrated quantum dot micro-LED micro-display device
By using quantum dot materials, a quantum dot color conversion layer matching blue light micro-LED array was prepared, and the micro-LED array devices were fully colored. The full-color device is composed of blue micro-LED, red quantum dot and green quantum dot.
The preparation process of quantum dot color conversion layer substrate is as follows.
First, the long wave pass filter film was evaporated on the surface of transparent glass substrate, and the filter film was etched by femtosecond laser.
Then, PDMS polymer film was prepared on the surface of the filter film.
Finally, red and green quantum dot pixel units are prepared on them.
The prepared color conversion layer is integrated with blue Micro-LED array to achieve full color display. When there is no filter film, a large amount of blue light penetrates the QD color conversion layer and generates a large amount of blue background light, which reduces the color purity of the QD pixel unit and affects the display ability of the device. However, the filter film can effectively suppress the blue emission and improve the purity of the red and green pixel units.
By using non-toxic InP/ZnS core-shell quantum dot materials to realize full-color display devices based on blue light micro-LED array, the excellent photoluminescence properties of environment-friendly quantum dot materials can be utilized to improve the display quality of full-color micro-LED display devices.
3. Tri-color synthetic micro-LED micro-projection display
The single-chip micro-LED display arrays of red, green and blue were made respectively, and the light was closed by the optical system and projected onto the receiving screen to achieve the full-color micro-projection display of micro-LED.
The red light micro-LED display array is fabricated by AlGaInP-LED.
The red micro-LED display array was made by etching, filling the front and back isolation channels, deposition of P electrode, N electrode and annealing, green light and blue light micro-LED display arrays are both made of GaN epitaxial wafers with sapphire substrates.
As the key of Micro-LED projection display, optical system plays a vital role in imaging effect. The micro projection system can restore the image on the display chip and project a clear picture on the projection screen.
Challenges and Prospects
Some challenges faced by the implementation of micro-LED full-color micro-display:
(1) How to meet the requirements of batch manufacturing, transfer, integration technology and equipment of micro-LEDs;
(2) How to improve the yield of micro-LEDs;
(3) Efficient and low-cost active drive scheme suitable for micro-LED micro-display;
(4) How to improve the life of micro-LED devices based on quantum dots.
Although micro-LED micro-display technology may not become the mainstream display technology in a short time, more and more technologies will be breakthrough through the joint efforts of researchers, and micro-LED micro-display technology will eventually promote the continuous vigorous development of the display industry.
