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.