A NANOGAP-EMBEDDED NANOWIRE FIELD EFECT TRANSISTOR FOR SENSOR APPLICATIONS: IMMUNOSENSOR AND HUMIDITY SENSOR

Jae-Hyuk Ahn, Jee-Yeon Kim, Maesoon Im, Jin-Woo Han, and Yang-Kyu Choi KAIST, Republic of Korea

ABSTRACT

A nanogap-embedded nanowire (NW) field effect transistor (FET) is demonstrated for sensor applications. The feasi-bility of the device as an immunosensor is presented via detection of C-reactive protein (CRP). Application of the pro-posed device as a humidity sensor is also demonstrated. Biomolecules or water molecules confined in the nanogaps in-crease the gate dielectric constant, consequently leading to a shift of the threshold voltage and, accordingly, variation of the drain current. A numerical simulation is carried out to support the validity of the experimental results. The nanogap-embedded NWFET can provide a route for producing multi-functional sensors with label–free electrical detection. KEYWORDS: Nanogap, Nanowire, Transistor, Immunosensor, Humidity sensor

INTRODUCTION

Due to their compatibility with electrical label-free methods, FET-based sensors have strong potential for the detec-tion of biomolecules [1,2]. The feasibility of employing nanogap-embedded FETs for biosensors has recently been dem-onstrated [3,4]. The sensors were simply fabricated by a conventional complementary metal-oxide-semiconductor (CMOS) process, and thus could be monolithically integrated with CMOS readout circuits through the same process steps. In previous studies, however, ensuring that the biomolecules can completely fill nanogaps has proved challenging, as the nanogap is a fluidic channel composed of an one-sided closed wall. For improvement of gap filling, and hence possible enhancement of sensitivity, this work reports a novel NWFET with nanogaps that consist of two-side opened walls. The sensing characteristics of the devices are confirmed by both label-free electrical immunodetection of heart a disease marker, i.e., CRP, and humidity sensing.

THEORY

A schematic view of the nanogap-embedded NWFET is presented in Figure 1. The two gates and the single NW are se-parated by nanogaps. Filling the nanogap via antigen-antibody binding or with water molecules increases the gate dielectric constant within it, which leads to a negative shift of the threshold voltage (VT) and increment of the drain current (ID). In oth-er words, it is possible to detect specific interactions in the nanogap by monitoring changes of either the VT or ID.

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(a) (b) (c) Figure 1: (a) A schematic view of a nanowire field effect transistor with nanogaps. The nanogap is filled with antigen-antibody or water molecules. The device can be applied to (b) an immunosensor, and (c) a humidity sensor. Filling the nanogap causes a shift of the threshold voltage (VT) and a change in the drain current (ID).

EXPERIMENTAL

The fabrication flow is shown in Figures 2 (a)-(d). The device was fabricated using a conventional CMOS process. The starting substrate was a silicon-on-insulator (SOI) wafer with a 100 nm thick top silicon (p-type, 1015 cm–3) layer and a 140 nm thick buried oxide (BOX). Silicon nitride (Si3N4) of 50 nm thickness was deposited by low-pressure chemical-vapor-deposition (LPCVD), and thereafter the nanowires were patterned by the use of UV lithography and subsequent plasma etching. Afterwards, a sacrificial layer of tetraethyl orthosilicate (TEOS) oxide of 30 nm thickness and n+ in-situ poly-crystalline silicon (poly-Si) of 110 nm thickness were sequentially deposited. Chemical mechanical polishing (CMP) was applied to separate the gate, which tends to be a double-gate. CMP etched the poly-Si gate until the Si3N4 layer was exposed. After gate patterning, the sacrificial TEOS layer was removed by diluted hydrofluoric acid (HF) solution to pro-duce the nanogaps.

Figures 2 (e)-(f) show SEM images of the fabricated device. The nanogap thickness corresponds to the thickness of the deposited sacrificial TEOS oxide (i.e., 30 nm). The width and length of the nanowire are 90 nm and 1000 nm, respec-tively.

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A semiconductor parameter analyzer (HP 4156C) was used to analyze the electrical characteristics of the NWFETs upon filling the nanogap with biomolecules or water molecules. VT is defined at the gate voltage (VG) to induce a specific current level of ID, i.e., 10 nA. Prior to detection of the CRP antigen, its antibody was immobilized onto the gate and nanowire sur-face by employment of a self-assembled monolayer used in a previous work [4]. The concentration of the antigen and the an-tibody was 100 µg/mL, respectively. All biomolecules were dissolved in a phosphate-buffered silane (PBS, pH 7.4) solution. A drop of 100 μL of the target solution was cast on the devices and incubation was performed for 1 h at room temperature. The devices were washed several times with deionized water and dried with N2 gas before electrical measurement.

In order to investigate the feasibility of the device as a humidity sensor, a humidifier was used to introduce water mole-cules after calibration. The relative humidity was then measured with a commercialized electrical hygrometer.

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Figure 2: The fabrication flow and SEM images of the device. (a) Starting SOI wafer with top silicon (100 nm) and BOX (140 nm) (b) Silicon nanowire formation after silicon nitride (Si3N4) deposition (50 nm) (c) CMP after deposition of the sacrificial TEOS (30 nm) and poly-Si (300 nm). CMP is stopped at the Si3N4 layer. (d) Nanogap formation after gate pat-terning. (e) Top view (f) Cross-sectional view along the dashed line in (e). The nanogap thickness corresponds to the thickness of the deposited sacrificial TEOS, i.e., 30 nm.

RESULTS AND DISCUSSION

As shown in Figure 3 (a), specific antigen-antibody CRP interaction leads to a negative VT shift due to increment of the gate dielectric constant in the nanogaps. As a control experiment, PBS solution without the CRP antigen was treated on the CRP antibody-immobilized device. A negligible VT shift was observed, because no CRP binding occurred. This result veri-fies that the proposed devices are suitable to detect specific CRP binding.

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Figure 3: Detection of CRP. (a) Electrical characteristics before and after CRP antigen binding, leading to a VT shift. (b) Comparison of the VT shift between the work group with CRP antigen binding and the control group without CRP an-tigen binding.

The humidity response of the proposed devices was also investigated, as shown in Figure 4. Figure 4 (a) shows that the devices are insensitive to humid air before the formation of the nanogaps, because they are completely filled with the sacrifi-cial TEOS. After the formation of the nanogaps, the device shows high sensitivity to water molecules in air, as shown in Fig-ure 4 (b). Normalized ID increases after the injection of a humid air flow and returns to its initial level after outgasing. Figure 4 (c) demonstrates humidity responsivity, extracted from (a) and (b) for fair comparison between before and after the nano-gap formation. The device also exhibits a different response depending on the relative humidity condition. Figure 4 (d) shows the trend that the normalized ID is increased as the relative humidity is increased.

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Figure 4: Humidity response (a) before and (b) after the nanogap formation. The device with the nanogaps is highly

sensitive to water molecules. ID increases after the injection of humid air and returns to its initial baseline after outgas-ing. (c) Error bar incorporating plots extracted from (a) and (b). (d) Humidity response depending on the relative hu-midity.

The results of the VT and ID change upon filling of the nanogaps were validated by a semiconductor device simulation

tool (SILVACO) by increasing the gate dielectric constant of the nanogap. In the simulation, a gate dielectric material used to fill the nanogaps plays a role of introduced biomolecules or water molecules. Figure 5 (a) shows that the simulated electron density inside the NW is dramatically increased after the gate dielectric constant is increased. As shown in Figures 5 (b)-(c), the value of the VT shift and ID change, respectively, increases as the gate dielectric constant increases. This is consistent with the trend observed from the experimental results.

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Figure 5: Simulation results (a) VT shift and (b) ID change according to the gate dielectric constant. (c) Electron den-

sity in a cross-sectional NW filled with air and the gate dielectric material in the nanogaps. The simulated electron den-sity inside the NW increases dramatically after the gate dielectric constant changes to 2 from unity, corresponding to the dielectric constant of air.

CONCLUSIONS

Multi-functionality of a nanogap-embedded NWFET was demonstrated, with application to both immunosensors and humidity sensors. This work showed that the proposed device could act as a multi-purpose sensor with a single sensor unit and be monolithically integrated onto the same chip to embed readout CMOS circuitry by virtue of the matured CMOS process. ACKNOWLEDGEMENTS

This research was supported by Grant 08K1401-00210 from the Center for Nanoscale Mechatronics and Manufactur-ing, one of the 21st Century Frontier Research Programs supported by the Korea Ministry of Education, Science and Technology (MEST). It was also supported by the National Research and Development Program (Grant 2009-0065615) for the development of biomedical function monitoring biosensors, and the National Research Foundation of Korea funded by the Korean government (Grant 2009-0083079). REFERENCES [1] E. Stern, J. F. Klemic, D. A. Routenberg, P. N. Wyremebak, D. B. Turner-Evans, A. D. Hamilton, D. A. LaVan, T.

M. Fahmy, and M. A. Reed, Label-free immunodetection with CMOS-compatible semiconducting nanowires, Na-ture, 445, 519 (2007).

[2] N. Elfström, A. E. Karlström, and J. Linnros, Silicon Nanoribbons for Electrical Detection of Biomolecules, Nano Letters, 8, 945 (2008)

[3] H. Im, X.-J. Huang, B. Gu, and Y.-K. Choi, A dielectric-modulated field effect transistor for biosensing, Nature Na-notechnology, 2, 430 (2007).

[4] J.-H. Ahn, M. Im, Y.-K. Choi, Label-free electrical detection of PSA by a nanogap field effect transistor, Proc. Mi-cro Total Analysis Systems 2008, 979 (2008) .

CONTACT: Y.-K. Choi, tel: +82-42-350-3477; ykchoi@ee.kaist.ac.kr

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