Optical systems have evolved from large, cumbersome setups to sophisticated, compact systems thanks to the field of silicon photonics, which uses light instead of electricity to transmit and process data on semiconductor chips. But these silicon-photonics devices are usually opaque and stiff.

Scientists at MIT have now discovered a scalable method for making silicon-photonics chips transparent and flexible, paving the way for the development of sophisticated microchips that may find use in things like transparent augmented reality displays that fit the curve of a pilot's helmet or covert health monitors that conform to the body.

While scientists have recently performed lab demonstrations of chips that were flexible or transparent, they could only fabricate a few devices at a time, reports Adam Zewe in MIT News.

The MIT researchers, in close collaboration with engineers at NY Creates at the Albany NanoTech Complex, created a fabrication process that uses standard semiconductor manufacturing techniques to generate flexible and transparent silicon-photonics chips on large-scale wafers.

To validate this platform, the researchers bent a single chip thousands of times around cylinders with various diameters — down to the width of a small screw — with no drop in performance. They also determined that looking through the chips would not cause much haze or distortion.

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“We’ve now developed a wafer-scale process that produces wafers that are mechanically flexible and optically transparent, enabling novel applications that weren’t previously possible with silicon photonics. We hope that, by working closely with our colleagues at NY Creates and using the foundry at the Albany NanoTech Complex, there’s the potential for us to make the platform accessible to other groups within our research community and open these new application areas to the field of silicon photonics as a whole,” says Jelena Notaros, the Robert J. Shillman Career Development Associate Professor of Electrical Engineering and Computer Science (EECS) at MIT, a member of the Research Laboratory of Electronics, and senior author of a paper on this fabrication platform.

Flexible and transparent

In the past, researchers have created translucent or flexible single silicon-photonics circuits, but these methods were not scalable. Notaros' team recently presented a foundry-scale method for producing silicon-photonics devices on a flexible substrate in order to overcome this scalability issue.

With a scalable method that yields translucent and flexible 300-millimeter silicon-photonics wafers, the team has now advanced these breakthroughs even farther.

They start the fabrication process just like they would with a conventional, hard silicon wafer. On this stiff silicon substrate, the researchers meticulously place and design tiny optical lines called waveguides.

A temporary silicon wafer is then bonded on top. All of the original silicon substrate is removed from what is now the wafer's top by flipping the wafer over. After that, they are left with a thin layer of material that is thinner than a tenth of a human hair.

They use an adhesive to stick a thin, transparent polyester film on top of these ultrathin layers and “de-bond” the temporary silicon wafer from the bottom to remove it.

This leaves them with a flexible, transparent wafer only a few microns thick that contains the oxide and waveguide layers needed to capture and transport light for silicon photonics.

The researchers performed three experiments to test different functionalities of these flexible, transparent silicon-photonics wafers.

They also evaluated transparency by setting up a bionic eye and testing whether the chip would distort the user’s vision when placed in front of the eye. They found that the chip causes only minimal haze for the viewer and would not noticeably distort images the eye perceives when looking through it.

In the future, the researchers want to add more complex components and functionality to the chips as they move toward enabling these and other new applications. They also want to refine the design to further improve waveguide efficiency and boost transparency performance.