Sensing Property of Horizontally Aligned Carbon Nanotube Field-Effect Transistor on Quartz Substrate Satoshi Okuda, Shogo Okamoto, Yasuhide Ohno, Kenzo Maehashi, Koichi Inoue and Kazuhiko Matsumoto The Institute of Scientific and Industrial Research, Osaka Univ. 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Phone: +81-6-6879-8412 E-mail: okuda11@sanken.osaka-u.ac.jp 1. Introduction In recent years, highly sensitive and label-free biosen- sors are required in various fields such as early diagnosis, home care, and practical pharmacy. Carbon nanotube field-effect transistors (CNTFETs) are one of the most promising platforms for electronic bio-detection owning to their unique electrical and physical properties. The detec- tion of biomolecules such as DNA and proteins using the CNTFETs has been successfully performed [1, 2]. However, high bias can not be applied between electrodes because of the oxidation-reduction reaction of biomolecules in solu- tions. Hence, the current that flows in one CNT channel is less than tens of nA, and it is comparable to the noise level. It is the reason why the current changes are small when biomolecules are adsorbed onto the CNT channels. The use of multiple nanotube channels in a FET offers several ad- vantages such as higher uniformity, lower noise, and higher reproducibility [3]. In this abstract, horizontally-aligned CNTs were grown on quartz substrates and CNTFETs with multi CNT chan- nels were fabricated. Aligned arrays of CNTs increase the drain current (I D ) of CNTFETs. Therefore, the performance of CNTFET based biosensors will be improved. In this study, we carried out solution pH sensing and label-free immunosensing based on CNTFETs. 2. Experimental Procedure Quartz substrates were annealed at 900ºC for 8 hours in air to re-crystalize the surfaces. 0.5-nm-thick Fe catalysts were patterned by conventional photolithography and liftoff process. CNTs were synthesized by alcohol CVD. Ethanol as a carbon source gas was introduced for 20 min at 900 ºC. Then, source and drain electrodes of Ti (2 nm)/Au (30 nm) were formed. To measure pH value and protein adsorption in solution, the device was surrounded by a silicone rubber barrier attached to the substrate. Figure 1 shows a schemat- ic image of the fabricated CNTFET based sensor on quartz substrate. A Ag/AgCl reference electrode was used as a top-gate electrode to minimize the effects of environment. Fig. 1. Schematic illustration of CNTFET-based sensor. 3. Results and Discussion Figures 2 (a) and (b) show scanning electron microsco- py (SEM) images of CNTs grown on Si/SiO 2 and quartz substrates, respectively. The random networks of CNTs were observed on the Si/SiO 2 substrate (Fig. 2 (a)). In con- trast, aligned CNTs were observed on the quartz substrate (Fig. 2 (b)). It has been reported that CNTs are aligned along the specific crystalline directions of quartz substrate [4]. This alignment enables fabricating highly density CNT arrays between electrodes and minimizes the tube-to-tube contact resistance associated with network devices. There- fore, it is expected that the performance of the CNTFET is improved using aligned CNTs grown on quartz substrate. (a) (b) Fig. 2. SEM images of CNTs grown on Si/SiO 2 substrate (a) and quartz substrate (b). -1256- Extended Abstracts of the 2011 International Conference on Solid State Devices and Materials, Nagoya, 2011, pp1256-1257 K-1-4