Physical Science Laboratory
Ion-sensitive field-effect transistors (ISFETs) are one of the most powerful
tools for biotechnology and molecular engineering because they promise
high integrability, good miniaturization, and high-speed sensing. More
interestingly and importantly, nanoscale ISFETs have sensitivity high enough
to detect single electrons at room temperature [1]. On the other hand,
a serious issue with ISFETs, which are usually used in ionized and/or charged
impurities distributed randomly in a liquid, is background noise. The background
noise disturbs the precise detection of small amounts of material, which
hinders precise molecular detection. In this study, in an attempt to distinguish
a target signal from background noise, we fabricated nanoscale Si-based
ISFETs, which are arrayed in parallel and share one floating gate [2].
The ISFET’s channels with a constriction part were patterned on a silicon-on-insulator (SOI) wafer (Fig. 1). A subsequent oxidation process shrunk the constriction of the channel, which gives ISFETs high charge sensitivity with single-electron resolution [1]. Au and Ti wires were formed simultaneously on three constrictions by a well-aligned lift-off process using electron-beam lithography. As a substance for detection, we used octadecane thiol (ODT) solved in a tetrahydrofuran (THF). THF with an ODT was dripped onto the ISFETs with a dropper and the characteristics of the current, I1, I2, I3, and I4, flowing through the four constrictions in one device were measured.
After complete evaporation of the THF droplet, current characteristics as a function of back-gate voltage shifted according to the concentration of ODT. This means that the fabricated devices work as an ISFET to detect negatively charged ODT on the devices.
Next, we monitored I1, I2, I3, and I4 in real time when THF with an ODT was supplied (Fig. 2). Although all curves seem to change individually, I1, I2, and I3 show mutually synchronized step-wise changes as indicated by arrows, whereas
I1 changes individually. This means that the Au wire covering the channels for I2, I3, and I4 is covered with ODT and therefore gives those synchronized changes due
to its capacitive coupling to the channels. That is, although each curve
individually has background noise or current caused by the THF, ODT, and
FETs themselves, real-time monitoring of I1, I2, I3, and I4 allows us to make a distinction between such background noise and an effect
caused by materials attached to the Au wire. The presented schame is promising
for a noise-robust sensor.
[1] K. Nishiguchi, Jpn. J. Appl. Phys. 47 (2008) 8305.
[2] K. Nishiguchi, Appl. Phys. Lett. 94 (2009) 163106.
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