A new ISFET technology with back contacts using deep diffusions

Abstract

A new fabrication method for ion-sensitive field-effect transistors (ISFETs) with back contacts has been developed. The connections between the drain and the source diffusions and the back contacts are achieved by diffusing impurities from both sides of the wafer, until a conductive path has been formed through the thickness of the wafer. After these conductive paths are built, an FET structure is constructed on the front face of the chip. The back access allows for a clean face where the chemically active gate is placed. This contrasts with more traditional devices where the gate and the contacts are placed on the same face of the transistor. It is also set apart from devices featuring back contacts with anisotropically etched pits that require a different processing laboratory separated from the main clean-room facilities. The aim of the present work is to produce ISFET-based chemical sensors with back contacts featuring a clean technology using standard silicon foundry facilities. These sensors are more compact, easier to encapsulate and with a longer lifetime when compared to devices produced with more traditional structures.