Effects of CF4 Plasma Treatment on pH and pNa Sensing Properties of Light-Addressable Potentiometric Sensor with a 2-nm-Thick Sensitive HfO2 Layer Grown by Atomic Layer Deposition

We investigated the effect of the carbon tetrafluoride (CF4) plasma treatment on pH and pNa sensing characteristics of a light-addressable potentiometric sensor (LAPS) with a 2-nm-thick HfO2 film grown by atomic layer deposition (ALD). An inorganic CF4 plasma treatment with different times was performed using plasma enhance chemical vapor deposition (PECVD). For pH detection, the pH sensitivity slightly decreased with increasing CF4 plasma time. For pNa detection, the proposed fluorinated HfO2 film on a LAPS device is sensitive to Na+ ions. The linear relationship between pNa sensitivity and plasma treatment time was observed and the highest pNa sensitivity of 33.9 mV/pNa measured from pNa 1 to pNa 3 was achieved. Compared with that of the same structure without plasma treatment, the sensitivity was improved by twofold. The response mechanism of the fluorinated HfO2 LAPS is discussed according to the chemical states determined by X-ray photoelectron spectroscopy (XPS) analysis. The analysis of F 1s, Hf 4f, and O 1s spectra gives evidence that the enhancement of pNa sensitivity is due to the high concentration of incorporated fluorine in HfO2 films by CF4 plasma surface treatment.