December 3, 2009 – Researchers at Cornell say they’ve come up with a way to make graphene devices more simply, by growing the material directly onto a silicon wafer.
Their work, published in the journal Nano Letters, aims to solve a key challenge with utilizing graphene and its unique properties (strength even with atomic thinness, and excellent electrical capabilities): figuring out a reproducible large-scale way to manufacture and integrate it into device circuitry, compatible with current processes. (Some efforts have involved methods as crude as stripping a layer of graphene away from graphite with scotch tape, they note.)
|A conceptual illustration of an array of single atom-thick graphene transistors. (Source: Shivank Garg/Cornell)|
Drawing from previous efforts to grow graphene on copper foil, they deposited evaporated copper film onto a silicon wafer and then grew graphene on top of the wafer (the journal paper’s supplemental info more precisely describes the processes). The graphene films could then be cut using standard methods (e.g., photolithography) and the underlying copper removed via chemical solution, leaving just the graphene film draped over the silicon wafer. At that point, "you can apply any thin-film processing technique," notes lead researcher Jiwoong Park, Cornell assistant professor of chemistry and chemical biology, in a statement. To build the devices, a 100nm SiO2 layer was deposited followed by litho steps, and deposition of Cr and Au to define gate electrodes; to facilitate electrical contacts with the graphene/Ci/Ni electrode pads, windows in the SiO2 layer were opened using litho and wet etch.
From the journal paper abstract:
A novel fabrication method was used to directly pattern these graphene sheets into devices by simply removing the underlying copper film. Raman and conductance measurements show that the mechanical and electrical properties of our single layer graphene are uniform over a large area [...] which leads to a high device yield and successful fabrication of ultra long (>0.5 mm) graphene channels. Our graphene based devices present excellent electrical properties including a promising carrier mobility of 700 cm2/V*s and current saturation characteristics similar to devices based on exfoliated graphene.
The researchers say they have moved on to working with full-scale 4-in. graphene wafers, as a demo for manufacturing potential. Their work is supported by DARPA and the school’s Center for Materials Research.
|Side-by-side comparison of growths on a Cu foil (left) and an evaporated Cu thin film (right). Feature sizes appear to be much larger for Cu foils than for Cu thin films. (Source: Nano Letters)|