Ion sensitive field effect transistor (ISFET) has been developed for more than 30 years. It shows high potential to replace the conventional pH glass electrode, but there are still some drawbacks need to be improved. Therefore, many high-k materials have been investigated to be a perfect sensing membrane. In this paper, a binary metal oxide layer, HfTaO, was first proposed to be the sensing membrane of electrolyte insulator semiconductor (EIS) structure. The HfTaO layer was deposited by reactive rf sputtering with a binary metal target (Hf:Ta = 1:1). Afterwards, a post rapid thermal annealing (RTA) was used at 500, 700, 900^oC in O₂ ambient for 1 min. For the evaluation of sensing properties, the pH sensitivity and light immunity were improved with higher RTA temperature.

For the cleaning-in-process (CIP) test, the HfTaO layer was optimized to 20 cycles at 700^oC but degraded to 2 cycles at 900^oC. According to the X-ray diffraction (XRD) spectrum, it started to crystallize at 900^oC. It results the formation of grain boundary as the weak point to be etched by NaOH and HNO₃ solution. To check the HfTaO-EIS annealed at 700^oC after 20 CIP cycles, the surface morphology was observed by optical microscope, and the pH sensitivity was changed from 48.4 mV/pH to 52.0 mV/pH. A slight increase of capacitance could be come from the etching mechanism to decreas the thickness of HfTaO layer. Finally, the HfTaO layer with 700^oC RTA is concluded as the best condition of sensing membrane for food industry.

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