Fabrication and testing of ISFET based pH sensor for microliter scale solution targets

In recent years, there has been an increasing interest in the monitoring and controlling of pH. It has become an important aspect of many industrial wastewater and water quality treatment processes. At the same time, the demand for smaller electronic devices used for various industrial, commercial, and research applications has greatly increased. In this paper, we propose a fabrication method of Ion-Sensitive Field Effect Transistor (ISFET) using MEMS techniques for pH sensing application. The novelty of this device lies in the detection of target solution with volumes in the sub-micro liter range. This achievement has the potential to satisfy the research demands in various areas including chemistry, biology and medicine. Nanomaterials, such as Carbon Nanotubes (CNTs) with excellent electrical, mechanical, and thermal properties can be incorporated to these small ISFET devices through certain nano techniques including Atomic Force Microscopy (AFM) based surface nanomachining and Dielectrophoresis (DEP). With proper electrical packaging, our ISFET chip has been able to detect the pH values of 2.5 μl solutions. The results reveal a linearity of pH measurement with a corresponding sensitivity of 10.7 mV/pH.