Semiconductor nanowires have recently attracted a large interest for their use in sensitive, direct electrical detection of biomolecules. The nanowires are usually grown in the gas phase or by the vapor/liquid/solid technique and then deposited onto contacts in a more or less random fashion. Here, we have instead used conventional silicon CMOS technology to produce the nanowires in a top down approach well known to semiconductor industry. To define narrow lines we have used electron-beam lithography combined with plasma etching and subsequent oxidation. The sensitivity of the nanowires for biomolecule detection, has been studied for nanowires in the range 50-150 nm of width and for a microwire ~1 µm wide, and of 1 µm length.
The results show that the nanowires work in a similar way as NMOS devices such that a channel is formed at a certain backgate voltage. The larger surface to volume ratio for smaller wires is shown to have a large impact on the sensitivity of the device due to charged surface states. This behavior is shown experimentally by changing the surface states of the nanowire by coating it with molecules subject to charge changes when buffer solutions of different pH values are added. As the buffer solution is added to the nanowire surface the IDS-VDS characteristics of the nanowire changes dramatically and a shift in VDS is seen. This unexpected behavior is verified through simulation of a cross section along the nanowire.
We conclude that smaller nanowires are more sensitive to surface charge and therefore superior as sensors. We have also demonstrated that CMOS technology in combination with nanolithography can be successfully used to fabricate these wires.
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