Effects of thickness effect and rapid thermal annealing on pH sensing characteristics of thin HfO₂ films formed by atomic layer deposition

In this article, thin hafnium oxide (HfO₂) films deposited by atomic layer deposition (ALD) were investigated as a sensing layer on an electrolyteinsulator-semiconductor (EIS) structure for pH sensor applications. Compared with sputtering, ALD provides the possibility of shrinking the thickness of the HfO₂ sensing layer down to 3.5 nm with a low drift coefficient (<0.2mV/h). To increase the pH response, an additional rapid thermal annealing was performed on a 3.5-nm-thick ALD-HfO₂ film. After annealing at 900 °C, the pH sensitivity could be effectively increased to that near the Nernstian response (59.6 mV/pH), and the drift coefficient (<1 mV/h) and hysteresis width (4.26 mV) are still sufficient. On the basis of the results of atomic force microscopy (AFM) analysis, the increase in surface roughness is possibly the reason for the increase in pH response after annealing. For compatibility with advanced complementary metal-oxide- semiconductor (CMOS) technology, the thin ALD-HfO₂ film is a promising candidate for pH sensor fabrications.