By Graphenea
Graphene can be contacted by metallisation. The most widely used contacts are Ti/Au, Cr/Au and Ni, because they result in relatively low contact resistances compared to other metals. The lowest contact resistances are obtained with side-contacts, due to the better coupling of the metal to the graphene film. Once the contacts are fabricated, an annealing step can help to further reduce the contact resistance: either thermal annealing in vacuum or in an inert atmosphere, or a current annealing with a relatively large current (10^8 A/cm 2 ). Graphene can be coated by a thin layer of Al (2-3 nm) prior to processing to avoid polymeric residues. This Al layer can be removed later by using some developers (for example, MF-319 developer). However, MF-319 and similar TMAH-based developers must be carefully used because they can result on graphene detachment from the substrate (see next question). - We highly recommend carrying out device fabrication (especially the developing step) in an environment with less than 40% of humidity to try to avoid detachment. Besides that, thermal annealing just before the fabrication of the device (300 ºC, 9h in inert atmosphere) can also be helpful. Regarding the fabrication process, using freshly coated resist is very important. The use of basic developers, such as TMAH-based ones, can result on graphene detachment from the substrate if the process is not properly controlled. For instance, the developing time is a crucial parameter on this process: in order to minimize the probability for detachment, the developing time must be reduced as much as possible. We strongly recommend a previous optimization on a bare substrate in order to know the exact developing time for each resist and developer. When working with 1x1 cm 2 samples, this issue becomes critical: due to the small area of the sample, the resist coating might not be uniform, ending up with slightly thicker resist at the edges of the substrate. During the developing in these samples, the resist on the edges might still be present once the structure on the central part of the sample are already well-developed. At this stage, if the sample is kept on the developer for a longer time, the probability for detachment increases. Therefore, we recommend to keep the developing time needed for the central part of the sample. In case the presence of resist at the edges of the sample is critical for the process, we recommend using an extra step before the structure patterning for removing the excess of resist at the edges. This process works in a similar way for developers diluted in H 2 O: in this case, the developing time must be increased with respect to the non-diluted developer, which in some cases makes the developing process more controllable.
