Ultrahigh-Sensitive CMOS pH Sensor Developed in the BEOL of Standard 28 nm UTBB FDSOI

Abstract

This paper reports ultrahigh-sensitive and ultralow-power CMOS compatible pH sensors that are developed in the back-end-of-line (BEOL) of industrial 28-nm ultrathin body and buried oxide (UTBB) fully depleted silicon-on-insulator (FDSOI) transistors. Fabricating the sensing gate and the control gate of the sensors in a capacitive divider circuit, CMOS compatible pH sensors are demonstrated where the front gate bias is applied through a control gate rather than a bulky reference electrode. On the other hand, the strong electrostatic coupling between the front gate and the back gate of FDSOI devices provide an intrinsic signal amplification feature for sensing applications. Utilizing an atomic layer deposited aluminum oxide (Al2O3) as a pH sensing film, pH sensors having a sensitivity of 475 mV/pH and 730 mV/pH in the extended gate and BEOL configuration, respectively, are reported. Sensitivities of both configurations are superior to state-of-the-art low power ion-sensitive field-effect transistors. The small sensing area and the FDSOI-based low power technology of the device make the sensors ideal for the IoT market. The proposed approach has been validated by TCAD simulation, and demonstrated through experimental measurements on proof-of-concept extended gate pH sensors and on sensors that are developed in the BEOL of industrial UTBB FDSOI devices.

Keywords

• Aluminum oxide
• back-end-of-line (BEOL)
• capacitive coupling
• extended gate
• fully-depleted silicon-on-insulator (FDSOI)
• ion-sensitive field-effect transistor (ISFET)