A novel low temperature fabrication approach of multichannel zinc oxide nanowire field effect transistors for biosensing applications

Abstract

Sensing of bacteria, viruses and biomolecules is increasingly important for environmental monitoring and healthcare applications. Electronic devices as transducer elements, that are scaled down to nanometre dimensions offer a sensitive electrical detection of bioanalytes. In this work, nanowire field effect transistors (NWFET) made from zinc oxide (ZnO) offer high sensitivity and low thermal budget fabrication compared to silicon nanowire sensors.

The novelty of this work is a new low temperature top-down fabrication process, which makes it possible to define ZnO NWFET arrays with different numbers of nanowires simultaneously and systematically compare their electrical performance. The main feature of this process is a developed bilayer photoresist pattern with a retrograde profile, which enables the modification of the nanowire in width, length, height and the number of transistor channels. The approach is compatible with low cost manufacture without electron beam lithography and benefits from process temperatures below 150ºC. Process reliability has been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD) and atomic force microscopy (AFM). The nanowires exhibit a cross section dimension of 30.9 nm height and 257.4 nm width and varying lengths from 5 µm to 45 µm and show a very smooth top surface with a root mean squared (rms) roughness of only 1.2 nm. Electrical measurements demonstrate enhancement mode transistors, which show a scalable correlation between the number of nanowires and the electrical characteristics. Thereby, devices with 100 nanowires exhibit the best performance with a high field effect mobility of 11.0 cm²/Vs, on/off current ratio of 4 × 10⁷ and subthreshold swing of 660 mV/dec.

The fabricated multichannel ZnO NWFET have been investigated for their potential bio-sensing capabilities. The ZnO NWFET passivated with Al₂O₃ is able to operate 16 hours continuously in phosphate buffered saline (PBS) solution with a very small current drift of 1.3 % per hour. It was found, that an Al₂O₃ passivation layer of 30 nm gives the best electrical performance of the ZnO NWFETs. Hereby, the ZnO NWFET shows a very good recovery behaviour up to 81.8 % of its original signal output current. The output current of the ZnO NWFET shifts to different ionic strengths in aqueous solutions and changes during exposure with 10x, 100x and 1000x diluted PBS of up to 23.7%. Investigations on the sensing capabilities on proteins show that the ZnO NWFET responds at a very low drain voltage of 5 mV to varying charges within liquid solutions containing lysozyme and bovines serum albumin (BSA). An output current signal change between these two proteins of 295.5 % was measured, indicating a very good sensitivity of the ZnO nanowire channel to the presence of surrounding charges.

After evidence was provided that the fabricated ZnO NWFET are capable for bio-sensing experiments, a mask design was developed, which allows to package the ZnO NWFET with gold wire bonding onto a polychlorinated biphenyl (PCB) board to enable statistical bio-sensing experiments. Hereby individual ZnO NWFETs can be addressed and measured by flushing laser cut poly(methyl methacrylate) (PMMA) micro fluidic channels with analytes solutions.

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