Methods to characterize graphene films have the tough job of keeping up with the pace of developments in graphene growth and transfer. As graphene production is decisively moving towards wafer-scale integration into existing silicon fab processes, there is a growing demand for fast, non-contact characterization techniques that could be implemented on graphene production sites. Optical characterization methods immediately come to mind, with terahertz time-domain spectroscopy (THz TDS) having emerged as a dominant method for studying graphene.
Graphene researchers from eleven countries have now authored a publication summarizing the state of the art of graphene characterization with THz TDS. The study was published in the journal 2D Materials under open access. THz TDS has been proven to work on various substrates with a spatial resolution of 400 micrometers.
Image: Map of graphene conductivity obtained with THz TDS. From: Patrick Rebsdorf Whelan et al 2021 2D Mater. in press https://doi.org/10.1088/2053-1583/abdbcb.
Transmission mode THz TDS has been used to characterize graphene on silicon, silicon carbide, sapphire, and many polymeric materials. In reflection mode, the graphene can sit on virtually any substrate. The method is especially good at mapping any defects that may occur in graphene, such as tears, rips, folds, wrinkles or adlayers, but also lithographically defined edges and grain boundaries. With this novel characterization tool, researchers can measure both doping and carrier mobility in a single rapid surface scan.
Especially interesting is the fact that the tool can be used to characterize graphene that has been encapsulated under protective layers, which is a common way of shielding graphene from the surrounding environment. Not only that, but the tool can be used in between steps in complex device fabrication procedures, providing quality analysis checkpoints. Rapid mapping of graphene quality at each step of the way also assists in disentangling the effects of defects due to growth from defects that are related to transfer from the growth substrate or other post-processing. As opposed to a competing method of micro four-point probe (M4PP) measurements, THz TDS works on graphene of any conductivity, including areas with no graphene at all, where M4PP fails.
In a production scenario, THz TDS can characterize graphene on a 12-inch silicon wafer in 1 hour, with 1 mm resolution. The quality of testing is virtually the same as in the case of a 12-hour scan of the same area. The researchers predict that if state-of-the-art affordable spectroscopy technology was used, a 12-inch wafer could be scanned in 5-10 minutes.
Image: Map of graphene on a 12-inch silicon wafer obtained in one hour. From: Patrick Rebsdorf Whelan et al 2021 2D Mater. in press https://doi.org/10.1088/2053-1583/abdbcb.
To conclude, THz TDS is a valid method of characterizing graphene on a production site at various production stages and on various substrates. The method is rapid and contact-free, both necessary requirements for integration in existing silicon fab lines. The researchers emphasize that the development of international metrology standards is now necessary for further advancement of this technology.