Anion detection in air using silicon nanoFET

Abstract

Abstract This study aims to produce a nano-field-effect-transistor (nanoFET) with a top-down approach, and quantitatively determine the concentration of negative ions in the air. The operational principles of this device are experimentally evaluated, by measuring the changes in channel conductance caused by the field-effect generated on the devices nanochannel by electric charges. The development of such sensors has been hindered by various noise signals occurring in nanoFET experiments, including interference caused by ion concentration, pH, and the electrical potential of the buffer solution. The measurement of airborne anions using nanoFETs, the approach proposed in this study, can be clearly explained in terms of the chemical gate phenomenon, a phenomenon caused by the adsorption of electrically charged particles. Moreover, by evaluating the performance of existing commercial devices to measure airborne anion levels, the possibility was investigated in commercializing the nanoFET-based airborne anion sensor. The existing methods to measure airborne anions detect changes in electric fields, caused by the negative charged particles in the air, using Gerdien tubes, and air ventilators to draw air into the tubes. Commercial airborne anion measurement devices using Gerdien tubes have larger sizes then nanoFET-based solutions, Moreover, it is difficult to integrate those devices with air ionizers (that produce anions), which have recently started to be used in many aspects of everyday life, or with air quality related devices. In contrast, the nanoFET-based anion measurement system can be integrated with anion suppliers in small home appliances, such as air purifiers, electric fans, and refrigerators

this system can therefore be used as an indicator, to effectively control airborne anions, manage air quality, and sustain healthy living conditions. Other studies address the manufacturing process. In the beginning, this research was focused on the fundamental characteristics of nanoFETs, including the electrical characteristics of nanowires and CNT

therefore, studies were conducted to confirm the possibility of developing nanoFET sensors with a bottom-up approach. In contrast to this early nanoFET research, studies are currently focused on reproducible manufacturing methods, capable of enabling mass production of nanoFETs for commercial nanoFET device development. Even in devices using outstanding single-walled carbon nanotubes (SWCNT) or single crystal nanowires, the productivity of the bottom-up approach declines in comparison with the top-down approach, when structures are dispersed on wafers and electrodes are formed. Recently, a strong desire to obtain better controlled device characteristics at wafer scale led to a top-down approach using conventional microfabrication processes to create nanowire-like structures. Until now, research on the top-down approach has focused on the uniformity of the nanochannel length and the reproducibility of signals, by first producing nanowire structures according to the crystal orientation of the silicon wafer through a wet-etching process, and then producing source, drain, and gate electrodes. However, even devices produced with this conventional top-down approach have problems to overcome

the manufacturing process is difficult, and thus not suitable for the production of commercial sensors, and its production yield should also be improved. In this study, the complex nanoFET manufacturing process is simplified, using a typical microelectromechanical system (MEMS) process to enable mass production, by implementing a batch process for nanoFET channel placement on 8-inch ultra-thin silicon-on-insulator (UT-SOI) wafers, up to surface treatment. The nanochannel, gate oxide, floating gate electrode (Ti/Au), and buried oxide (BOX) silicon layer were 20, 15, 10/100, and 145 nm, respectively. The length and width of the channels were 5 μm and 1 μm, respectively. The obtained results confirmed the chemical gate phenomenon, caused by the adsorption of electrically charged particles to the channels of the nanoFET. By measuring the operational characteristics of the nanoFET repeatedly, in different air anionic concentrations, it was clearly demonstrated that the electric charges adsorbed onto the channel surface operated via the chemical gate phenomenon. For quantitative analysis, the change was measured the change in the gradient of the drain-source electric current (Ids), which occurs when the anions produced by anion ionizers diffuse into the air and adsorb onto the gate surface, and acts, therefore, as the device detection parameter. Moreover, the introduction of an additional gold top-gate electrode on top of the gate oxide film of the nanoFET, not only increased the size of the detector, but also provided a mechanism to reinitialize the accumulation of charges, thus improving the efficiency of the nanoFET as an airborne anion sensor. These results lead us to believe that nanoFET sensors with a top-gate electrode will find efficient applications as airborne anion measurement sensors. It was expected that the understanding of the operational principles of nanoFET devices produced by these basic experimental results will help solve the problems that are currently preventing the use of nanoFETs as biosensors.