High Polarization and Low-Repulsion ${\rm HfO}_{2}$ 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.