Conformal transfer of graphene on a prepatterned substrate is a viable technology for reproducible fabrication of graphene devices. Such is the conclusion of a recent study by a team of scientists from Germany, the Netherlands, Spain and Saudi Arabia.
Reliable fabrication of graphene devices for electronics has been a technological challenge for the graphene community for years. Large area high quality graphene, required for high-tech applications, is typically grown by chemical vapor deposition (CVD) on metal foils. Following growth, the graphene film is transferred to an electronically useful substrate, which is commonly SiO2 on Si. Standard procedure follows these steps up with photolithographic patterning of devices and evaporation of gold contacts.
Image: Reliable wafer scale production of graphene devices.
In the past, this sequence of steps failed to result in devices of consistently high quality, as each fabrication step introduced unpredictable defects. The film growth was the first step to be perfected by a global network of busy scientists. Graphene transfer was in parallel mastered by the Spanish company Graphenea, a participant of this most recent breakthrough, as evidenced by their patent award earlier this year. Still, lithography and evaporation would cause film breaks, cracks and wrinkles, which were unpredictable and which lowered the quality of the final devices. Now, researchers have turned the fabrication upside down by performing lithography and contact evaporation prior to graphene transfer. The result is a reliable method for wafer-scale fabrication of graphene devices.
The paper, recently published in Applied Materials & Interfaces (a publication of the American Chemical Society), starts by considering the values reported in literature for contact and sheet resistance obtained with the standard graphene fabrication and transfer method. Focusing on graphene grown by CVD, the researchers find that there is significant scatter in the reported values, spanning nearly an order of magnitude. The scientists then performed the standard procedure themselves, finding that the resulting graphene sheet is inhomogeneous, with defects appearing in random places.
Following the improved process, in which the contact patterning and metal deposition are performed prior to the transfer, yields a much better result. The graphene sheet is smooth and uniform across the wafer, conformally covering the electrode structures. Electrical measurements indicate good device reproducibility, with sheet resistance low enough to consider using these graphene devices in radio frequency electronics. The potential application of the graphene channels is additionally confirmed by steady device performance over a wide range of applied current, up to 0.5 A. The careful fabrication led to devices that support the highest current density ever reported in transferred CVD graphene.
Over 600 devices were tested, showing a very narrow spread in the measured parameters. Such statistical similarity not only opens a gate to mass production of graphene devices for radio frequency applications, but immediately allows the testing of fundamental device physics of monolayer materials.