High polarization and low-repulsion HfO₂ thin film for alkali metal ion detections by plasma system with a complementary filter

A new plasma system with a complementary filter for low-damage carbon tetrafluoride (CF4) plasma treatment is proposed to incorporate fluorine (F) atoms into a hafnium dioxide (HfO2) thin-film sensing membrane in an electrolyte-insulator-semiconductor structure for alkali metal ion (Na+ and K+) detection. Hf 4f and F 1s X-ray photoelectron spectra confirmed the incorporation of fluorine (F) atoms into the HfO2 sensing membrane after low-damage CF4 plasma treatment. As the duration of the plasma treatment is increased, the F 1s intensity and the amount of Hf-F bonds increased, and the corresponding Na+ and K+ sensitivities drastically increased (pNa: 121.6 mV/pNa; pK: 98.1 mV/pK). The super- Nernst phenomenon is attributed not only to the formation of higher dipole property between Hf and F than that between Hf and O but also to the reduction of plasma bombardment and UV irradiation, which damage the HfO2 sensing membrane. A positive charge model is proposed to explain the improvements by fluorine incorporation with complementary filter.