Titanium Nitride Membrane Application to Extended Gate Field Effect Transistor pH Sensor Using VLSI Technology

Abstract

A new process for the fabrication of the extended gate field effect transistor (EGFET) together with complementary metal oxide semiconductor (CMOS) circuits on the same chip is reported. The sensing membrane of the EGFET is titanium nitride (TiN) conducting material and it is fabricated using the r.f. sputtering method. The chips are fabricated using the standard submicron 0.5 µm double poly double metal (DPDM) N-well CMOS IC process. No extra mask is used in the post-process. An instrument amplifier circuit is described that provides an output voltage dependent on the threshold-voltage variations in the sensing membrane. According to the experimental results, the high linear sensitivity approaches 57 mV/pH. The hysteresis voltage is 0.5 mV per cycle of buffer solutions of pH7→pH4→pH7→pH10→pH7. This structure is also insensitive to light. This EGFET is fabricated using the standard technology and no difficulty is experienced in realizing this multi species device. The EGFET and readout circuits are produced using VLSI technology, achieving reduced area and low cost. This device has the advantages of mass production.