Reliable processing of graphene using metal etchmasks
© Kumar et al; licensee Springer. 2011
Received: 5 November 2010
Accepted: 18 May 2011
Published: 18 May 2011
Graphene exhibits exciting properties which make it an appealing candidate for use in electronic devices. Reliable processes for device fabrication are crucial prerequisites for this. We developed a large area of CVD synthesis and transfer of graphene films. With patterning of these graphene layers using standard photoresist masks, we are able to produce arrays of gated graphene devices with four point contacts. The etching and lift off process poses problems because of delamination and contamination due to polymer residues when using standard resists. We introduce a metal etch mask which minimises these problems. The high quality of graphene is shown by Raman and XPS spectroscopy as well as electrical measurements. The process is of high value for applications, as it improves the processability of graphene using high-throughput lithography and etching techniques.
Graphene has many potential applications including micro-nanoelectronics, sensors and transparent electronics. For applications in electronics, the reliability of processing of graphene is a major obstacle. The processing of graphene requires a transfer or growth on an insulating substrate, its patterning and subsequent contacting. With recent development of large scale synthesis of graphene layers [1–4], its use in high volume applications has become a serious option. Especially suitable for electronics is large scale chemical vapour deposition (CVD) growth of graphene on metal surfaces, as good quality graphene in an acceptable thermal budget has been reported [5, 6]. Recently, large-scale transfer and patterning of graphene have been shown by [7, 8]. In order to fabricate graphene-based devices, lithographic patterning is used to make etch masks, using standard positive or negative resists. This is followed by oxygen-based plasma to remove graphene, and subsequent removal of the residual resist.
Each of these processing steps may affect the quality of the graphene as defects can be created, and contaminants can be introduced. While contaminants or solvent residues may be reduced by annealing and/or cleaning procedures, polymers residues are difficult to remove with these techniques. Harsh cleaning conditions may cause introduction of defects to the graphene layers or its delamination due to the absence of interfacial bonds to the substrate. Recently, the substrate effects and possible capture of contaminants under the graphene layers have been discussed .
In this article, we show reliable processing of graphene on insulating substrates to produce high quality graphene field effect transistor (FET) devices. CVD graphene grown on copper was used, which was analysed by various methods after transfer. The patterning of graphene is of note, as this step was found to be unreliable using conventional methods, i.e. by the use of polymers as etch masks [7, 8]. In our experiments, delamination of graphene occurred when removing the mask after etch treatment. This may be attributed to the low adhesion of graphene to the substrate in absence of chemical bonds. In order to deal with this problem, we have introduced metal patterns as etchmask. The graphene is covered with a Ni mask which is later removed by non-oxidizing acids. The process flow further avoids the exposure of the active graphene layers with polymers during plasma processing, reducing the possibility of polymer residues.
Results and discussion
CVD growth has been shown to give large area mono- or bilayer graphene. We produced FET devices arrays using optical lithography. Using metal hardmask, we were able to minimize delamination of underlying graphene films, which often occured when using standard resist. Further, the introduction of metal hardmask seems to have a minimal impact on the graphene and does prevent contamination by polymer residues. The large area processing shown here will open opportunities for graphene in industrial settings.
Graphene was produced by CVD on copper foils at a temperature of 1000°C using methane as a carbon precursor . The foils get coated with carbon on both sides. One side was spin-coated with PMMA, while on the other side, the carbon coating was mechanically removed. The foil was placed in an etchant (1 M FeCl3) to remove copper. The resulting film was cleaned with DI water. The graphene layer on the film was then pressed onto substrates while heated at the same time in a mechanical press. After 45 s, the support layer was dissolved in acetone and substrates were placed in chloroform for further cleaning. These substrates were then analysed with XPS, AFM and Raman spectroscopy. The XPS spectrometer used a monochromatised Al Kα X-ray source with a resolution of 0.7 eV. For AFM an Asylum MFP-3D with standard silicon tip was employed. The Raman spectra were taken with a laser excitation of 633 nm on a Horiba Jobin-Yvon Labram spectrometer using a 100 × magnification. Silicon substrate with 300 nm of oxide on top was used for characterising transferred graphene.
Electrical measurements were done on heavily doped silicon with 60 nm of Al2O3 on top as substrates. Liftoff pattern were applied on top of graphene using a resist bilayer. Nickel (30 nm) was evaporated on the substrates and followed by a liftoff. This produced an etchmask of Ni sitting on top of graphene. The samples were then etched in O2 plasma in a barrel etcher to transfer the pattern on Ni to graphene underlayer. Nickel coating was finally removed using dilute HCl (1 M, 2 h). Another liftoff pattern was made on top for contacting graphene, using same process as above. Nickel contacts were deposited and a liftoff was performed again. The overall process is shown in Figure 2. The electrical characterisation of devices was done on a Suss mechanical four probe station, with Keithley 2400 sourcemeters.
chemical vapour deposition
field effect transistor.
This work was supported by the SFI under Contract No. 08/CE/I1432. SK is supported by SFI IRCSET fellowship. KHL and HYK are supported by National Research Foundation of Korea (NRF, grant 2010K000981, WCU R32-2008-000-10082-0, MEST).
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