Fabrication and testing of ISFET based pH sensors for microliter target solutions

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 micro–electro-mechanical systems (MEMS) techniques for pH sensing application. The novelty of this device lies in the detection of target solution with volumes in the sub-microliter range. 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. Furthermore, Nernst equation is employed to analyze the performance, and a promising scheme to improve the sensitivity while measuring tiny solutions is proposed. This achievement has the potential to satisfy the research demands in various areas including chemistry, biology and medicine.

► ISFETs are fabricated using MEMS to measure ion concentrations in solutions.
► An ISFET has been tested as pH sensor for microliter solution target.
► The sensitivity is obtained and analyzed by modeling.
► Promising methodologies are discussed to further improve sensor performance.