Fabrication and characterization of a pH sensor

Ion Sensitive Field Effect Transistors (ISFETs) revolutionized pH and chemical sensing technology with its small size and compatibility with high integrated fabrication technologies. We have developed a methodology to fabricate and characterize ISFET probes for pH measurement at UFRGS Microelectronics Laboratory. NMOS depletion mode devices with a silicon dioxide sensing layer were fully fabricated with standard CMOS technology. Sensors with aluminum gate were fabricated to allow standard C-V and I-V curves parameter extraction to monitor the process. Encapsulation was entirely performed with photoresist. Probes were made with regular Printed Circuit Board (PCB) methods. Devices were immersed into different pH solutions and connected to a Semiconductor Parameter Analyzer to perform pH measurements. Output voltage response at a defined current of 5 mA was of 32 mV/pH for acids and 20 mV/pH for bases.