Printing process developed for microLED displays

Technology News | July 17, 2023

By Peter Clarke

Researchers from the University of Strathclyde in Scotland have demonstrated a continuous roller process that can pick up and transfer over 75,000 micrometer-scale LEDs in a single roll with high accuracy.

The process could form the basis of a method for high-volume, rapid manufacture of microLED displays.

MicroLEDs are of great interest because they are emissive and provide high-resolution while using little power. They are believed to be applicable to augmented and virtual reality devices, smartphone screens all the way up to large TV displays. For some applications it is possible that millions of individual LEDs will need to be placed on with high accuracy on an independent electronic control backplane substrate.

Inkjet printing for microLED quantum dots

“Transferring micrometer-scale semiconductor devices from their native substrate to a variety of receiving platforms is a challenge being tackled internationally by both academic research groups and industries,” said research team leader Eleni Margariti. “Our roller-based printing process offers a way to achieve this in a scalable manner while meeting the demanding accuracy necessary for this application.”

The team has published Continuous roller transfer-printing and automated metrology of >75,000 micro-LED pixels in a single shot in the learned publication Optical Materials Express.

The paper reports that the method transferred an array of 320 by 240 microLEDs in a single shot with spatial location error of less than one-micron deviation from the as-designed layout. The setup is also simple and inexpensive the team reports.

“This printing process could also be used for other types of devices including silicon and printed electronics such as transistors, sensors and antennas for flexible and wearable electronics, smart packaging and radio-frequency identification tags,” said Margariti, who developed the new printing process. “It could also be useful for making photovoltaics and for biomedical applications such as drug delivery systems, biosensors and tissue engineering.”

It’s sticky

The method starts with an array of devices that are loosely attached to their growth substrate. The surface of a cylinder containing a slightly sticky silicone polymer film is then rolled over the suspended array of devices, allowing adhesive forces between the silicone and semiconductor to detach the devices from their growth substrate and array them on the cylinder drum. Because the printing process is continuous it can be used to simultaneously print numerous devices, which makes it highly efficient for large-scale production.

“By carefully selecting the properties of the silicone and receiving substrate surface and the speed and mechanics of the rolling process, the devices can be successfully rolled over and released onto the receiver substrate while preserving the spatially arrayed format they had on the original substrate,” said Margariti. “We also developed a custom analysis method that scans the printed sample for defects and provides the printing yield and positioning accuracy in just minutes.”

The method shows similarities to that used by Terecircuits Corp. (Mountain View, Calif.) (see Photopolymer improves IC, display assembly, says Terecircuits). Terecircuits uses the additional refinement of ultraviolet light to provide fine-control of release of the die.

The University of Strathclyde researchers used GaN-on-Si devices to test their method.