Printing process holds promise for bendable displays

A new process for creating flexible large area electronics could lead to breakthroughs in technologies including prosthetics, high-end electronics and fully bendable digital displays.

This is the claim of a team of engineers from Glasgow University’s Bendable Electronics and Sensing Technologies (BEST) group who have developed a new method for manufacturing electronics that prints high-performance silicon directly onto flexible materials. The EPSRC-supported research is described in a paper published in NPJ Flexible Electronics.

Until now, the most advanced flexible electronics have been mainly manufactured via a three-stage stamping process called transfer printing.

In the process, a silicon-based semiconductor nanostructure is first designed and grown on a substrate. In the second stage, the nanostructure is picked up from the substrate by a soft polymeric stamp. In the final stage, the nanostructure is transferred from the stamp to another flexible substrate, ready for use in bendable devices like health monitors, soft robotics, and bendable displays.

According to BEST, the transfer printing process has a number of limitations which have made it challenging to create more large-scale, complex flexible devices. Precisely controlling critical variables such as the speed of transfer, and the adhesion and orientation of the nanostructure, makes it difficult to ensure each stamp is identical to the last. An incomplete or misaligned polymeric stamp onto the final substrate can lead to substandard electronic performance or even prevent devices from working.

Processes have been developed to make the stamping transfer more effective, but they often require additional equipment like lasers and magnets, which adds extra manufacturing cost.

The Glasgow team said they have eliminated the second stage of the conventional transfer printing process and replaced it with ‘direct roll transfer’ to print silicon straight onto a flexible surface.

The process begins with the fabrication of a silicon nanostructure of less than 100nm. The receiving polyimide substrate is then covered in an ultrathin layer of chemicals to improve adhesion.

Image: NPJ Flexible Electronics/ Glasgow University

The prepared substrate is wrapped around a metal tube, and a computer-controlled machine developed by the team then rolls the tube over the silicon wafer, transferring it to the flexible material.

By optimising the process, the team created highly-uniform prints over an area of about 10cm2, with around 95 per cent transfer yield, which they said is significantly higher than most conventional transfer printing processes at the nanometre scale.

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In a statement, Professor Ravinder Dahiya, leader of the BEST group at Glasgow University’s James Watt School of Engineering, said: “Although we used a square silicon wafer sample of 3cm on each side in the process we discuss in this paper, the size of the flexible donor substrate is the only limit on the size of silicon wafers we can print. It’s very likely that we can scale up the process and create very complex high-performance flexible electronics, which opens the door to many potential applications.

“The performance we’ve seen from the transistors we’ve printed onto flexible surfaces in the lab has been similar to the performance of comparable CMOS devices – the workhorse chips which control many everyday electronics.

“That means that this type of flexible electronics could be sophisticated enough to integrate flexible controllers into LED arrays, for example, potentially allowing the creation of self-contained digital displays which could be rolled up when not in use. Layers of flexible material stretched over prosthetic limbs could provide amputees with better control over their prosthetics, or even integrate sensors to give users a sense of ‘touch’.

“It’s a simpler process capable of producing high-performance flexible electronics with results as good as, if not better, than conventional silicon based electronics. It’s also potentially cheaper and more resource-efficient, because it uses less material, and better for the environment, because it produces less waste in the form of unusable transfers.”