Researchers at the University of Exeter have used CMOS-compatible processes to create graphene-based humidity sensors with performance on par with commercial ones. The process was demonstrated on an array of sensors produced on a 4” silicon wafer as well as on transparent and flexible PET. The work paves the way to practical use of 2D materials in low-cost, low-power, wearable and flexible sensors.

The work, published in the Wiley journal Advanced Science, makes use of Graphenea graphene oxide (GO). One of the key achievements demonstrated is lithographic technology to produce features of graphene with a size of several tens of micrometers, starting from a water-based solution and using low temperatures and processes compatible with CMOS technology. This set of processing steps can thus be incorporated in silicon foundries that produce all the world’s microchips. In fact, the process is also compatible with flexible electronics technology and can be applied to typical polymer substrates such as PET or PEN. The core advance is to employ graphene transfer developed earlier by the same group, which yields nearly transparent graphene films on any target substrate using facile and readily available processing.

Figure from E. Torres Alonso et al, Advanced Science 2019, 1802318 (Creative Commons 4.0).

The technological achievement is demonstrated by producing interdigitated electrical contacts made of graphene, with a width between 20 and 200 micrometers. It was found that a width of 50 micrometers was sufficient for good electrical contact, which is on par with the resolution of graphene inkjet printing. The electrodes were subsequently covered with GO which acted as a sensing layer. Graphenea’s GO was used as purchased, without any further processing, which highlights the material’s industrial readiness. Sensing capability was tested by changing environmental humidity between 40% and 90% while measuring electrical conductance. The performance is on par with commercial humidity sensors, with added possibility of integration on flexible substrates.

The authors, led by Elias Torres Alonso who is now a researcher at Graphenea, went on to pattern a 4” Si/SiO2 wafer with an array of such sensors. The sensors all show similar performance, within experimental error. The sensors on flexible PET, commonly used as a substrate for printed electronics, do not show any wear after 2,000 bending cycles. As opposed to other available sensors, the range of sensitivity of the graphene sensors is of interest for low power, Internet of Things (IoT) applications, where simple electronics are needed and can be attached in a small packaging.

The technique shown may become a cornerstone when integrating solution‐processed 2D materials into real‐world, CMOS‐based applications in the future, with massive potential for applications where cost‐effective and disposable electronics are needed.