A thin hafnium oxide (HfO2) layer (8 nm), for hydrogen ion sensors, was deposited directly on a silicon substrate without the buffer oxides. Post deposition annealing (PDA) was performed to improve the sensitivity. The as-deposited HfO2 sensing dielectric functions from pH 4 to pH 12, and the sensitivity is 46.2 mV/pH. For the samples with 900 degrees C PDA, the sensitivity is increased to 58.3 mV/pH from pH 2 to pH 12. From atomic force microscope analysis, the improvement is related to the surface morphology. A physical model was proposed to explain PDA effects by the surface site density (surface area) and dissociation constants. (c) 2006 The Electrochemical Society.