Many beetles, like ladybugs, have the ability to stick to things as they walk underwater. They can do this by trapping tiny bubbles within hair-like structures on their feet. Researchers at the National University of Singapore have used a similar bubble trick to develop a way to make high quality graphene films. Their new technique is the first which can accomplish both the growth and transfer steps of graphene onto a silicon wafer.
The researchers are calling their process face-to-face transfer. It differs from the standard dry or wet methods which transfer films in a roll-to-roll fashion. Although stardard methods can grow sheets up to 30 inches in length, they also create many defects. These defects — cracks, folds, and wrinkles — are unavoidable when CVD-grown graphene is transferred from its underlying copper substrate. Ideally, one wants to grow the graphene right on top of a silicon chip, or whatever substrate will be the end product.
In the face-to-face method, the silicon dioxide top layer of a piece of silicon is first bombarded with a nitrogen plasma. This creates a silicon oxy-nitride surface that can trap bubbles that form later during the CVD (chemical vapor deposition) process. As the copper layer that was spun on before the CVD step is later etched away, the graphene is held in place by those bubbles that take up positions to form “capillary bridges,” like those on the setea of some beetles and frogs.
After they fabricated long (up to 1 meter), thin ribbons of graphene, the researchers were able to take images of the process using atomic force microscopy. They also tested the electrical properties of their product using standard four-probe resistivity measurements. To do this they first had to metalize 50nm spots of nickel to act as electrodes. Conductivity was in the range of 4,000 S/cm (compare the conductivity of copper, a bit higher at 6,000 S/cm). Importantly, they were also able to demonstrate uninterrupted electrical continuity of ribbons with length-width ratios of up to 105.
The authors believe their process will help to enable the larger graphene dream many see as the future of microelectronics. The so-called graphene-on-silicon platforms would be a big, or at least interim, part of that future. Such devices are now under development on many fronts, and have been shown to support all manner of useful electrical components — transistors, optical modulators, and even more exotic gate-controlled Schottky barrier triodes. For a batch process, where directed graphene growth can spontaneously attach to an underlying substrate in defect-free form, the technological implications may be huge.
Paper: doi:10.1038/nature12763 – “Face-to-face transfer of wafer-scale graphene films”[Image credit: NUS Faculty of Science]