The new roller printing method could be used to rapidly manufacture micro-LED displays. Eleni Margariti, who developed the new technique, is pictured with the clean-room setup used for printing. (Image: Eleni Margariti, University of Strathclyde)

Researchers have demonstrated a continuous roller printing process that can pick up and transfer over 75,000 micrometer-scale semiconductor devices in a single roll with very high accuracy. The new method paves the way to creating large-scale arrays of optical components and could be used to rapidly manufacture micro-LED displays.

“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 from the U.K.’s University of Strathclyde. “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 reports that its new roller technology can match the designed device layout with an accuracy of less than one micron. The setup is also inexpensive and simple enough to be constructed in locations with limited resources.

Here is an exclusive Tech Briefs interview — edited for length and clarity — with Margariti.

Eleni Margariti (Image: Eleni Margariti, University of Strathclyde)

Tech Briefs: What was the biggest technical challenge you faced while developing this approach?

Margariti: The short answer is the huge number. In our case — especially when we are talking about the large displays — we are talking about thousands, even millions, of very small devices. So, the biggest challenge in a way is how you can manipulate these devices, which means how to take them from the native substrate and place them on the target circuit or substrate with high precision.

And then the other important part is how we can inspect these devices, because we need accuracy. It was also very important for us not only to find a way to do this transfer but also to find a way to look at them — assess their position and essentially monitor the accuracy and yield.

Tech Briefs: Can you explain in simple terms how everything works?

Margariti: The easiest way to think about it is that when different materials are in close proximity or in contact, there are some interacting forces between them. So, there is a preferable adhesion between different materials, and this is what we take advantage of. That means that we can somehow use this preferable adhesion both to pick up our devices and then print them to the target surface. You can also enhance this adhesion by using optical coatings or adhesive coatings that can enhance this adhesion to make it easier; in our case, we use an adhesion that is commonly used in the fabrication of electronic circuits so it doesn’t affect the properties of the devices.

Tech Briefs: The team is currently working to further improve the accuracy of the printing process while also scaling up the number of devices that can be transferred at once. How is that coming along? Any updates you can share?

Margariti: What we saw is that the system works in terms of accuracy and yield. Obviously, if you want to reach the point that it will become commercially available, you need to further improve both the scale as well as the accuracy.

I would say that in a way what we are working on at the moment is to do the same things in active devices. What that means is that we have individually addressable microelectronics that we are going to transfer and test to see that we don’t have any issues coming from the printing in the optical or electrical properties.

The big challenge would be to go to a three-color display and maintain good accuracy.

Researchers developed a continuous roller printing process that can pick up and transfer over 75,000 micrometer-scale semiconductor devices in a single roll with very high accuracy. An optical microscopy image of the roller transfer printing results is shown. (Image: Eleni Margariti, University of Strathclyde)

Tech Briefs: Do you have any advice for engineers aiming to bring their ideas to fruition?

Margariti: I would say that it is interesting to see what is going on in the research, but also in the market. What is the impact that your research is going to have on the market or on the industry in general. I would strongly encourage them to go ahead. I mean, obviously, there are challenges that you need to face, but, yes, I think that the most important thing is essentially just to find a way with your research that it is as much as possible compatible with existing industry and existing manufacturing processes. I think that was, in my case, at least, the biggest motivation.

Tech Briefs: Is there anything else you’d like to add that we didn’t touch upon?

Margariti: Because we are talking about micro-LED displays, the first thing that comes to mind is TVs; then it goes to AR/VR systems, and then smartphones and wearable devices. But I would like to mention that we can also think about the micro-LED display for more important things.

For example, even if it’s sunny, you need to be able to read the display. So, for example, let’s say we are talking about displays in cars for navigation reasons; in that case, you want to make sure that the information is readable. For that, you need good contrast.

So, one application is for safety and another, for example, would be AR and VR. In addition to applications like can be used, for example, in the military for training purposes. I just wanted to broaden the applications for micro-LED displays, so that they don’t only apply to TVs.

And another thing that I would like to emphasize is the importance of the inspection method as well. It is very important to be able to see and test that the device has everything in properly place, and also to be able to spot exactly in which area we have any defects.