Impedance Characterization of the Capacitive field-Effect pH-Sensor Based on a thin-Layer Hafnium Oxide Formed by Atomic Layer Deposition

Abstract

As a sensing element, silicon dioxide (SiO2) has been applied within ion-sensitive field effect transistors (ISFET). However, a requirement of increasing pH-sensitivity and stability has observed an increased number of insulating materials that obtain high-k gate being applied as FETs. The increased high-k gate reduces the required metal oxide layer and, thus, the fabrication of thin hafnium oxide (HfO2) layers by atomic layer deposition (ALD) has grown with interest in recent years. This metal oxide presents advantageous characteristics that can be beneficial for the advancements within miniaturization of complementary metal oxide semiconductor (CMOS) technology. In this article, we describe a process for fabrication of HfO2 based on ALD by applying water (H2O) as the oxygen precursor. As a first, electrochemical impedance spectroscopy (EIS) measurements were performed with varying pH (2-10) to demonstrate the sensitivity of HfO2 as a potential pH sensing material. The Nyquist plot demonstrates a high clear shift of the polarization resistance (Rp) between pH 6-10 (R2 = 0.9986, Y = 3,054X + 12,100). At acidic conditions (between pH 2-10), the Rp change was small due to the unmodified oxide gate (R2 = 0.9655, Y = 2,104X + 4,250). These preliminary results demonstrate the HfO2 substrate functioned within basic to neutral conditions and establishes a great potential for applying HfO2 as a dielectric material for future pH measuring FET sensors.

Keywords

• Hafnium oxide
• Field-effect transistor
• pH
• Electrochemical impedance spectroscopy.