A novel fabrication method for co-integrating ISFET with damage-free sensing oxide and threshold voltage-tunable CMOS read-out circuits

A novel fabrication method using a top SiO2-SiN-bottom SiO2 (ONO) dielectric stack is proposed and implemented to obtain ion sensitive field effect transistor (ISFET) with damage-free sensing oxide and threshold voltage (Vth)-tunable devices in CMOS read-out circuits. By wet-etching the top SiO2 and the SiN sequentially, the ISFET with damage-free sensing oxide is obtained due to the high selectivity between them. Also, the Vth-tunable circuit devices with the ONO stacks are simultaneously achieved by protecting the ONO stacks from the wet-etching. Through the measurements of pH and biomolecule responses, it is confirmed that the pH and biomolecule can be detected stably because the drain current (ID) is stabilized to a predetermined value more quickly and the ID fluctuation during the ID stabilization is significantly reduced compared with devices having damaged sensing oxide. Additionally, it is verified that the Vth of devices for circuits can be fine-controlled by injecting charges into the SiN via Fowler-Nordheim tunneling. Furthermore, it is demonstrated that the biomolecule-induced Vth shift of ∼150 mV in the proposed ISFET is successfully amplified to the output voltage change of ∼370 mV in common source amplifier (CSA) voltage-readout circuit consisting of one p-type ISFET and one Vth-tuned n-type MOS.