/, :i ~ii/i!i ~i'/ii~ ¸¸ i B ELSEVIER Sensors and Actuators B 35-36 (1996) 136-140 ~HEMItP~I Multilayer ISFET membranes for microsystems applications C. Can6*, A. GOtz, A. Merlos, I. Gr~cia, A. Errachid, P. Losantos, E. Lora-Tamayo Centre Nacional de Microelectrbnica, IMB-CSIC, Campus UAB, 08193 Bellaterra, Spain Abstract The fabrication of microsystems in which chemical measurements play a role make the on-chip combination of chemical sensors and signal processing circuitry highly desirable. Chemical sensors use sensitive layers which are not a part of MOS devices and in this sense, the determination of the best sensors and technological processes to achieve good integrated smart sensors is very important, in this paper a study of compatibility of ISFET chemical sensors and CMOS circuitry is presented for application to biomedical microsys- terns. Keywonts: Muitilayer ISFET membranes: Micmsystem applications 1. Introduction Much of the literature concerning fabrication tech- nologies for ion sensitive chemical sensors (ISFETs) has concentrated on the compatibility of sensitive materials for the monolitical integration of chemical sensors and signal processing circuitry for smart applications. As an ISFET is a MOS-based device, the most straightforward approach for a compatible technology using MOS-based circuitry, is CMOS, the technology of most interest for microsystems integration [1-3], In this case, the main issue when fabricating ion sensitive gates and MOS gates on the same substrate is the coexistence of dielectric and polysilicon gates. In this paper a complete comparative study of the sensitivity of the different possible CMOS-compatible ISFET gates is presented. Despite the fact that better re- suits can be obtained with A!203 or Ta2Os, only Si3N4 and SiOaNy layers have been considered for ISb'ET gates, as this material has also shown near-Nemstian response and is fully compatible with a CMOS production line. Differ- ent gate structures for ISFETs have been fabricated con- sidering that a CMOS process permits two approaches: first, some slight changes on the CMOS flow chart are accepted and second, only CMOS gate structures with no additional layers are accepted for ISFETs. In the first • * Conesponding author. case, the variations in the process consist of the introduc- tion of additional layers and selective etching, but as they do not introduce high-temperature steps, the CMOS ther- mal budget is not affected. With the two approaches a lot of applications can be covered, from the definition of a compatible process that favours the sensing performances for special multisensing applications to a process that is fully focused on a CMOS technology from a standard foundry for the development of a sensor cell library for application specific integrated sensors (ASIS). The final application is a biomedical microsystem for drug delivery that requires a multisensing chip for on-line characteriza- tion of pH, pressure and temperature. Complete com- patibility of the different sensor technologies with a CMOS process is necessary and in this paper, the results concerning the chemical sensor part are presented. !. I. The full custom approach A customized technology was developed for the fabri- cation of ISFETs combined with differential measurement circuitry for its application to a multisensor module that is a part of a biomedical microsystem for drug delivery. This microsystem is the main demonstrator of an Esprit- III project entitled BARMINT (Basic Research on Micro- systems Integration). The technology taken as the base line for this development consists of a standard CMOS technology of CNM laboratory with minimum dimen- 0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved PII S0925-4005(96)02027-8