Sensitivity Optimization of Silicon Nanowire ISFET Sensor for Detecting Fluoride Ions

Abstract

We investigate the fluoride ion sensitivity dependence on the width of silicon nanowire (SiNW) ion-sensitive field effect transistor (ISFET) sensor with polycrystalline lanthanum fluoride (poly-LaF3) sensing membrane. The poly-LaF3 sensing membrane was deposited using thermal evaporation under 600 °C substrate temperature. The device shows improved electrical characteristics (threshold voltage (Vth) and subthreshold swing (SS)) as the diameter shrinks. On the other hand, the fluoride ion sensitivity, which is significantly affected by SS, is not improved unconditionally even though the diameter decreases. The sensor exhibits the highest sensitivity in the nanowire width of 200 nm. The sensitivity is degraded when the nanowire diameter is less than 200 nm. This degradation is caused by decreased thermal conductivity in narrow nanowire since the deposition temperature of the poly-LaF3 film is important parameter to determine sensitivity. These results show that the optimization of nanowire geometry is essential for highly sensitive fluoride ion ISFET sensor.