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Improving the pH sensitivity of ISFET arrays with reactive ion etching

In this paper, we report a method to improve sensitivity for CMOS ISFET arrays using Reactive Ion Etching (RIE) as a post-processing technique. The process etches away the passivation layers of the commercial CMOS process, using an oxygen (O2) and sulfur hexafluoride (SF6) plasma. The resulting attenuation and pH sensitivity are characterised for five dies etched for 0 to 15 minutes, and we demonstrate that capacitive attenuation is reduced by 196% and pH sensitivity increased by 260% compared to the non-etched equivalent. The spread of trapped charge is also reduced which relaxes requirements on the analogue front-end. The technique significantly improves the performance of the fully-integrated sensing system for applications such as DNA detection.