Nanoscale Fabrication of Microwave Detectors from Commercially-Available CVD-Grown Monolayer Graphene

Using commercially-available monolayer graphene, synthesized by means of chemical vapor deposition, microwave power sensing elements have been nanofabricated and integrated with microwave-grade test structures suitable for on-wafer probing. The graphene, situated on a thermal oxide, was first cleaned of stray contaminants in a forming gas environment briefly held at 250 degrees Celsius using a rapid thermal annealer. Immediately following this step, the graphene was passivated with a protective aluminum (Al) oxide layer (approximately 5 nm in thickness). Micrometer-scale Corbino disc test structures were then fabricated in direct contact with the graphene using a self-aligned process, which relies on the fact that tetramethylammonium hydroxide develops the photoresist while removing the Al-oxide. Graphene nanoribbons (with widths as small as 400 nm) were then fabricated across the Corbino disc gaps using electron-beam writing in conjunction with a negative tone resist. The same developer exposed the majority of the graphene while defining nanometer-scale lines of photoresist stacked upon the Al-oxide layer. These stacks served as etch-stops while the unprotected remnants of Al-oxide and graphene were etched in a CF ₄ / O ₂ plasma. Finally, the photoresist was removed leaving behind passivated graphene nanoribbons. Damage caused by the fabrication was evaluated by comparing the Raman spectra of the graphene before and after microfabrication. Current-versus-voltage traces of the nanodevices exhibited the characteristic conductance minima corresponding to the charge-neutrality point of graphene. Radio frequency power detection experiments at this DC biasing point revealed a detection sensitivity of 0.14 mV/mW.