Design and fabrication of n-ISFET using Si₃N₄/SiO₂ structure for pH measurement

The design and fabrication of n-ISFET using Si₃N₄/SiO₂ structure for pH measurement has been carried out. In general Ion Sensitive Field Effect Transistor (ISFET) is a potentiometric pH sensor which widely used in chemical, biochemical and biomedical applications due to its advantages such as small size, low power consumption, robustness, and fast response time over the ion-selective electrode (ISE). In this study, the ISFET was designed and fabricated in-house in Micro Fabrication Cleanroom Laboratory using a standard Complementary Metal Oxide Semiconductor (CMOS) processes fabrication except the gate area was replaced by reference electrode, sensing membrane and electrolyte under test. The main objective of this study is to present a concept, the design, fabrication and testing appropriate to process flow in fabricating the n-ISFFET on silicon wafer, which will finally be characterized using a suitable test methodology. Hence, fabrication on p- type <100> 4 inch silicon wafer by photolithography, wet chemical etching, thermal oxidation, diffusion and metallization with focus on a pH measurement has been executed.The n-ISFET was operated when the surface absorption of the charges in the electrolyte under test simultaneously interact with both reference electrode and surface sensing membrane. Overall process has 5 mask levels consist of source mask and drains mask, gate mask, contact mask and metallization mask. The fifth mask was used to find the best thickness of silicon nitride for sensing membrane layer and 50 nm was selected. For sensing material, SiO2 insulator layer was used and later deposited on top with Si3N4 insulator layer by Plasma Enhanced Chemical Vapor Deposition (PECVD). The latter layer serves as a pH sensitive membrane. The electrical tests were performed using buffer solutions with varying pH values, indicated that the transistor can be employed to measure the pH of solutions at room temperature. The interaction between these methods will modulate a threshold voltage and simultaneously will extracted the output (IdVd) and transfer (IdVg) characteristic curves at three differences channel length; 250μm, 300 μm and 500 μm respectively.The best pH sensitivity achieved at the channel length 500 μm with the measurement value equal to 54.43 milivolt per pH (mV/pH).