The development of scalable sensor arrays using standard CMOS technology

This paper describes an approach to developing MOSFET-based scalable sensor arrays in an unmodified standard CMOS process. The multiplexed design can be used as either a single-ended or differential circuit to make potentiometric measurements in each cell of the array. The FET-based sensors employ a floating gate electrode structure and use the nitride passivation layer as a pH-sensitive membrane. An implementation of a single-chip 2 x 2 array fabricated in an unmodified commercial 0.35 mum CMOS process is presented. All signal acquisition is performed in-situ and all readout circuitry is located on-chip. On return from the foundry, the devices are exposed to ultraviolet light to eliminate any difference in threshold voltage. The circuit provides a sufficient linear range that allows the FET devices to operate as pH sensors in the array. A double layer of SU-8 photoresist is used to provide both a biocompatible and waterproof package for the chip. The biocompatibility of the chip surface is investigated using a well-established cell line. (C) 2004 Elsevier B.V. All rights reserved.