Introduction
The mass production of microLED displays is still a great challenge due to the complexity of the manufacturing process, which involves transferring and bonding microLED chips onto a backplane with high precision and yield. The bonding process requires attaching individual microLEDs onto a substrate to form a display panel.
According to research by Semiconductor Engineering, microLED displays are expected to use sub-pixels of native red, green, and blue microLEDs.
However, other businesses are pursuing the usage of blue LEDs with quantum dots to create blue and red sub-pixels. This down-converting approach simplifies the complicated pick-and-place transfer of red, green, and blue microLEDs, but at the expense of luminosity loss.
According to a recent market research report, the global microLED market is expected to grow at a compound annual growth rate (CAGR) of 80.1% from 2021 to 2026, reaching USD 21,002.9 Million by 2026.
Let’s discuss these innovative microLED bonding techniques, their alternatives, and their implications, if any, for the future of display technology.
MicroLED Bonding Techniques
The following are the microLED bonding methods that are transforming the microLED bonding sector:
1. Laser-assisted bonding
This technique helps the MicroLEDs bond together by using a laser. The support substrate is a cheap adhesive tape, and the microLED is transferred onto the tape using the Laser Lift-Off (LLO) method.
A company that has been working with this technology is Bolite, which applies laser technology in its microLED manufacturing process.
Recent research has led to the development of a systematic solution named “tape-assisted laser transfer” (TALT).
2. Fluidic self-assembly
This method involves dispersing microLED chiplets in a fluid and agitating them, so they make repeated contact with a substrate immersed in the fluid. The substrate’s binding sites are coated with molten solder; when a chiplet meets one, surface tension induces an irreversible bond.
LG and Seoul National University have been working on this technology. Recent research has demonstrated a microLED lighting panel consisting of more than 19,000 disk-shaped GaN chiplets, assembled in 60 seconds by a simple agitation-based, surface-tension-driven fluidic self-assembly (FSA) technique.
3. Paste-and-cut method
This method starts with temporarily bonding the LEDs on a process/handle wafer onto a glass substrate (the pasting step). The LEDs are then released (cut) to the flexible substrate.
The University of Waterloo in Canada has developed this new transfer and bonding method to deposit a flexible microLED array on plastic substrates.
4. Simultaneous Transfer and Bonding (SITRAB) adhesives
SITRAB is a process where transfer and bonding occur at the same time. When a homogenized laser is applied to the SITRAB film, micro-LEDs can be attached in seconds.
The Electronics and Telecommunications Research Institute (ETRI) in Korea has developed this process.
Recent research has used a unique substance known as SITRAB adhesive to create a novel procedure in which bonding and transfer happen simultaneously.
MicroLED Bonding: Patent Review
There has been an increase in the number of patents in the sector of microLED displays, with around 9,000 patents from 480 organizations.
This increase in intellectual property (IP) filings offers a clearer picture of the prospects of microLED technologies.
The patents highlighting the innovative approaches being taken to overcome the challenges associated with microLED bonding are given here.
1. Die Bonding Method for Micro-LED
A patent filed by Shenzhen Lehman Optoelectronics Technology Co., Ltd. and Huizhou Lehman Optoelectronics Technology Co., Ltd. describes a die-bonding method for a micro-LED.
The method involves performing tin plating at a die bonding position of a PCB board to obtain a tin plating layer, sequentially adding a protective layer and a flux layer on the tin plating layer to obtain a pretreated PCB board.
Subsequently, a flip-chip mounted micro-LED chip was transferred onto the pretreated PCB board, and reflux and die bonding were performed to complete the die bonding of the micro-LED.
2. Laser-assisted Bonding Process
A patent discusses a new laser-assisted bonding process of pure metallic Au/Sn for 62×78 full-color micro-LED arrays using the Sn–Au layers.
This process is characterized by high productivity, low cost, and no use of flux underfill.
3. Metal Interconnection Process with Micro-LED
A patent filed by Kansas State University discusses the development of micro-sized LEDs (microLEDs) and various interconnection schemes between microLEDs.
Companies Involved with MicroLED Bonding Innovation
Here are some examples of companies and industries using innovative microLED bonding approaches for next-generation displays:
Samsung, LG, and BOE
These companies have showcased prototype TVs and monitors with MicroLED. They are among the more than 130 companies working in the microLED display supply chain.
These companies are investing in various innovative microLED bonding approaches to overcome the challenges associated with microLED fabrication and to meet the increasing demand for high-resolution VR/AR and transparent displays.
Apple (LuxVue) and Oculus (InfiniLED)
These companies are also part of the microLED display supply chain. They are exploring innovative microLED bonding approaches to enhance the performance of their devices.
For instance, Apple acquired LuxVue, a company known for its innovative microLED-based displays for consumer electronics applications.
Here’s the gist of the deal:
- LuxVue’s Expertise: LuxVue was known for developing microLED displays that consumed less energy while offering brighter screens compared to traditional displays. This technology had the potential to significantly improve battery life in various consumer electronics.
- Apple’s Interest: Apple likely acquired LuxVue to develop its display technology in-house. This would give them more control and potentially lead to better displays for their iPhones, iPads, and other devices.
- LuxVue’s Patents: LuxVue held patents related to microLED displays, which would be valuable assets for Apple’s display development efforts.
The Shanghai Team
This team has enhanced the flip-chip bonding process by using a layered gold-indium-gold (Au/In/Au) metal sandwich in place of conventional pure indium bumps.
Flip-chip bonding then joins the microLED to the silicon at mild 200°C temperatures, avoiding damage from heating and cooling mismatches while forming highly conductive bonds.
Some Notable Alternatives to MicroLED Bonding
There are some alternatives to microLED bonding being explored by researchers and companies:
300mm Advanced CMOS Platform
Micledi, a spin-out of IMEC, is the only startup for microLED in 300mm. They are leveraging the most advanced equipment in the semiconductor industry to meet AR display requirements.
Their technology uniqueness lies in advanced node driving ASIC (<45nm), advanced litho for wafer level optics (CD<100nm, overlay<20nm), and tight W2W hybrid bonding overlay specs (<200nm).
Improving Yield
Yield is a significant issue for microLEDs. Engineers are tackling the challenge of yield improvement with early test results indicating yield issues at chip transfer, array-to-driver bonding, and other relatively new processes.
Dialog Semiconductor, a Renesas Company, developed a testing scheme for a white adaptive headlight module containing a 20,000-microLED array with a 40µm pitch.
End Note
Looking ahead, it is recommended that we continue investing in the research and development of microLED bonding techniques to propel the evolution of display technology further.
It is worth noting that collaborative efforts among industry players, academic institutions, and research organizations can foster innovation and accelerate the adoption of novel bonding approaches.
In the future, emphasis should be placed on enhancing yield rates, optimizing manufacturing processes, and reducing production costs to make microLED displays more accessible to a wider consumer base.