Nafion as a patternable high-speed ionomer for electronics applications

“We've shown that Nafion is viable as a device material and can be used in a sophisticated way in that context. It can be patterned down to the nanoscale, incorporated as a functional element in integrated circuits.”Adam Micolich

Nafion is an ionomer already much used as a proton conductor in proton exchange membrane fuel cells due to its exceptionally high protonic conductivity, and increasingly as a possible component in artificial synapse devices. A constraint on such new applications is a lack of ways to scalably pattern Nafion films into device structures and integrated circuits. However, a new study has demonstrated a scalable method for electron beam processing of Nafion films spin-coated on solid substrates to give nanoscale patterns with linewidths as low as 125 nm.

As reported in Materials Horizon [Gluschke et al. Mater. Horiz. (2020) DOI: 10.1039/D0MH01070G], this new Nafion patterning capability allows it to be used as an ion conducting element in complex nanoscale devices and integrated circuits in bioelectronics. Proof-of-concept was provided by a hybrid organic-inorganic ion-to-electron transducer circuit giving a DC gain exceeding 5 and frequency response up to 2 kHz, which demonstrates its potential as a high-speed device-level ion-transport material.

The research builds on previous research into patterning polyethylene oxide with an electron-beam for use as a nanoscale ionic-gating structure for nanowires, which they extended here to build an inverter circuit that converted ionic signals into electronic signals. The key breakthrough was producing it as a fully integrated package with a whole active circuit inside 100 square microns, as well as increasing the gain up over 5 and the frequency response out into the kHz.

The latter findings are significant for applications in neural interfaces, while use of Nafion as a nanoscale-patternable device element was also significant. As researcher Adam Micolich told Materials Today, “We've shown that Nafion is viable as a device material and can be used in a sophisticated way in that context. It can be patterned down to the nanoscale, incorporated as a functional element in integrated circuits.”

To make Nafion compatible with nanoscale devices, working out the patterning was crucial. The material was suspended in alcohol and then spin-coated, before being directly patterned with an electron-beam as they had with polyethylene oxide. A mix of acetone in 2-propanol allowed them to get linewidths down to a bit over 100nm. Moving fabrication of the Nafion components to this early in the process was essential since if any of the metal interconnects are already on the device when the Nafion if patterned, it would eat the nanowires.

Nafion was also shown to be soluble enough to get contrast in lithography but sufficiently insoluble to survive other device fabrication processes. They now hope to gain a better understanding of how electron-beam exposure changes Nafion, particularly with respect to protonic conductivity, and to explore reducing the structure in size to assess if it still provides a synaptic response or behaves differently.