CMOS 3D-Extended Metal Gate ISFETs with Near Nernstian Ion Sensitivity

Over the past few decades, Ion Sensitive Field Effect Transistor (ISFET) has been fabricated using a variety of technologies [1] based on the idea of an ion-sensitive gate. In the effort to bring ISFETs into Point-of-Care applications and wearable products [2], their compatibility with industrial CMOS processes is crucial for realizing miniaturized integrated sensor system. The challenges are high, knowing that it is very difficult to change any processing steps in established CMOS Front-End-of-the-Line. Moreover, the majority of ISFETs fabricated in commercial CMOS processes and reported to date showed low sensitivity, especially when using oxynitrides as sensing gate insulator [3]. In this work, we report for the first time, to the best of our knowledge, a post-processed 0.18 μm commercial CMOS chip where the transducing transistor and the sensing gate electrode are vertically co-integrated, resulting a 3D-Extended Metal Gate ISFET (3D-EMG-ISFET). The top electrode is made of Al with an Al
₂
O
₃
native oxide connected to the transistor gate by using vertical vias. With this approach, a sensitivity of 56.8 mV/pH is achieved. The proposed ISFET is validated as a full-scale pH sensor and it can be fabricated in a non-modified commercial CMOS process.