Fabrication Process of Carbon Nanotube FETs Using ALD Passivation for Biosensors

In this study, we have applied atomic layer deposition (ALD) for the fabrication of CNT-FET biosensors.Biosensor operation was confirmed by CNT-FETs fabricated using the ALD passivation film. CNTs were grown on a SiO₂(100 nm)/p+-Si substrate by alcohol catalytic chemical vapor deposition. Following the fabrication of the CNT-FETs, HfO₂ film (50 nm) with a large dielectric constant was deposited on the device surface by ALD at 250degC. HfO₂ was chosen as a passivation/top-gate insulator to realize a large transconductance of the device. SEM image of the HfO₂-deposited sample, indicates that the step coverage is very good in the case of the ALD. The devices with the top gate were used for biosensors. The self-assembled-monolayer (SAM) film with carboxyl group was formed on the top-gate FET surface by immersing the device in 3-Mercaptopropionic acid. Biosensor measurement was performed by measuring the I_D when the enzyme Cytochrome c (pl= 10.25, 2.7mg/ml) containing solution was dropped into the phosphate-buffer (PB) solution (pH=7.0) where the SAM-coated device was immersed. Thus we have successfully fabricated CNT-FET biosensors using ALD HfO₂ as a passivation film with little degradation of the I_D.