Fabrication platform could enable flexible, transparent next-generation photonic chips

The field of silicon photonics, which employs light instead of electricity to transfer and process data on semiconductor chips, has progressed from large-scale setups to compact and sophisticated systems. However, these silicon-photonics chips are typically rigid and opaque. Scientists at MIT have developed a scalable method to create flexible and transparent silicon-photonics chips, potentially paving the way for advanced microchips with applications such as wearable health monitors and transparent augmented reality displays.

Researchers from MIT and NY Creates at the Albany NanoTech Complex have devised a fabrication process that utilizes standard semiconductor manufacturing techniques to produce flexible and transparent silicon-photonics chips on large-scale wafers. The team demonstrated that their platform can bend a single chip thousands of times around various-sized cylinders without any loss in performance. They also confirmed that the chips do not introduce significant haze or distortion when viewed through them.

MIT's Jelena Notaros, a professor of electrical engineering and computer science, leads the research and emphasizes that their wafer-scale process yields flexible and optically transparent chips, enabling novel applications that were previously unattainable with silicon photonics. Her co-authors include MIT graduate students Tal Sneh and Andres Garcia Coleto, as well as Kevin Fealey and Milica Notaros from NY Creates. The findings were published in the journal Optica.

Silicon-photonics chips have been designed at scale using advanced microelectronics foundry processes, resulting in 300-millimeter-diameter wafers containing billions of nanoscale optical devices. However, these chips are rigid and opaque. Notaros notes that several applications would benefit from flexible and transparent chips, but previous attempts to achieve this were not scalable.

To overcome the scalability challenge, the MIT researchers developed a wafer-scale fabrication process that produces flexible and transparent silicon-photonics wafers. The process begins similarly to traditional rigid silicon wafer fabrication, with the deposition and patterning of tiny optical wires called waveguides onto a rigid silicon substrate.

The researchers then create a temporary silicon wafer on top, flip the wafer over, and remove the original silicon substrate, leaving behind a flat layer of material approximately a tenth of a human hair thick.

The ultra-thin material is further bonded with a transparent polyester film and has the temporary silicon wafer removed to yield a flexible, transparent wafer containing the oxide and waveguide layers necessary for capturing and transporting light for silicon photonics. The primary challenge in developing this fabrication process was removing enough material from a large 300-millimeter-diameter silicon wafer to leave just a few microns of material behind.

The researchers managed this challenge by employing low-temperature processes and employing a combination of industrial thinning methods and a more precise selective chemical etch for the final removal step.