A NEW ISFET TECHNOLOGY WITH BACK CONTACTS USING DEEP DIFFUSIONS

A new fabrication method for ion-sensitive field-effect transistors (I SFETs) 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 conduct ive path has been formed through the thickness of the wafer. After the se 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 traditi onal devices where the gate and the contacts are placed on the same fa ce of the transistor. It is also set apart from devices featuring back contacts with anisotropically etched pits that require a different pr ocessing laboratory separated from the main clean-room facilities. The aim of the present work is to produce ISFET-based chemical sensors wi th back contacts featuring a clean technology using standard silicon f oundry facilities. These sensors are more compact, easier to encapsula te and with a longer lifetime when compared to devices produced with m ore traditional structures. (C) 1997 Elsevier Science S.A.