Investigation on Sensing and Stability of Nitrogen-doped ITO Thin-film Strain Gauges at High Temperatures

Abstract

Nitrogen-doped indium tin oxide (ITO) has been applied in the thin-film strain gauges (TFSGs) due to their high stability, excellent piezoresistivity and antioxidation at elevated temperatures. However, the mechanism on the sensing and stability of the nitrogen-doped ITO TFSGs at high temperatures was not comprehensively clarified. In this work, various ITO TFSGs were fabricated by RF magnetron sputtering with different nitrogen partial pressures (NPPs) of 5%~40%. The elemental composition and band structures of the ITO TFSGs were examined by the energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), respectively. Results show that the Fermi energy level shifts closer to the valence band maximum energy (Ev) gradually with the growth of NPPs, causing a reduction in the number of electrons ionized to the conduction band. The smallest content change rates of nitrogen (3.8%) and oxygen (1.6%) after subjecting to the thermal strain test were observed in the 20%N2 ITO TFSG. In consequence, the 20%N2 ITO TFSG exhibits the lowest resistance drift rate (DR) at high temperatures due to its stable elemental composition. Moreover, we found that the band structures and elemental composition of the ITO TFSGs are the main factors affecting their piezoresistive response at different temperatures. The band structures play a major role in the gauge factors (GFs) of the ITO TFSGs at room temperature and 600 ℃. The element variation takes responsibility for the different GFs of the ITO TFSGs at 800℃, 900℃ and 1000℃. In addition, the piezoresistive stability is also dependent on the elemental composition affected by the dynamic equilibrium between the diffusion amount of oxygen and the escape number of the nitrogen in the ITO thin films at high temperatures.