ELSEVIER Sensors and Actuators B 26-27 (1995) 336-340 B CHEMICAL Application of nickel electroless plating to the fabrication low-cost backside contact ISFETs A. Merlos, J. Esteve, M.C. Acero, C. Can6, J. Bausells Centre Nacional de Microelectrdnica CSIC, Campus Universitat AutOnoma de Barcelona, 08193 Bellaterra, Barcelona, Spain of Abstract In this work an alternative, very simple, high-yield and low-cost technology for the fabrication of backside contacts ISFET sensors is presented. This new technological approach is based on the use of a nickel electroless plating technique for the selective metallization of the silicon electrical contacts. The need of any photolithographic step after the silicon anisotropic etch is avoided in this way and, therefore, the yield and the reliability of the whole fabrication process remains unaffected. The fabricated backside contacts ISFET devices has been tested successfully in a prototype of a whole chemical analysis system dedicated to the monitoring of the environment. Keywords: Nickel electroless plating; ISFET; Backside contact ISFET 1. Introduction Research and technology developments in the field of silicon chemical sensors based on ISFETs are still of great interest because of their rapidly growing accep- tance in the European market [1]. One of the most important application fields of ISFET devices is the control of the environment. The robustness and low cost of ISFET-based sensors as well as the possibility of obtaining CHEMFETs selective to different ions (Na +, K ÷, Ca 2 +, NH4 +, etc.) [2--4] make them specially suitable for the monitoring of waste waters, rivers, etc. ISFET-based sensors are also very attractive for the fabrication of instruments and measurement systems which should be compact, simple and reliable in order to allow their application to the continuous flow analysis of chemical parameters in complex systems like the monitoring of the environment or the control of pro- cesses. The most important factors that limit the commer- cialization of ISFET-based sensors are the problems related to the sensor packaging and their automatization. In order to facilitate the encapsulation process backside contacts (BSC)-ISFET devices are very promising. Sev- eral technologies have been proposed for the fabrication of BSC-ISFETs [5-7]. The yield and reliability of these technologies are seriously affected due to the important problems related with the realization of photolitographic 0925-4005/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-4005 (94)01613-M processes on to the deeply microstructured wafer sur- faces that are obtained after the silicon anisotropic etch needed for the definition of the BSCs. In previous work [8,9] we have made an effort in order to eliminate that kind of problems by using a standard metallization technology with aluminium and avoiding the need of making photolithographical pro- cesses on to the microstructured wafer side. All these technologies are very promising due to their simplicity and high yield, and because of the possibility of their compatibilization with a CMOS process. In these technologies some photolithographic steps have to be made once the silicon wafer has been microstructured by the anisotropic etching and, there- fore, it is necessary to the take special care in order to not affect the yield of the technology. In this work an alternative, very simple, high-yield and low-cost technology for the fabrication of BSC- ISFET sensors is presented. This new technological approach is based on the use of a nickel electroless plating technique [10,11] for the selective metallization of the silicon electrical contacts. The need of any photolithographic step after the silicon anisotropic etch is avoided in this way and, therefore, the yield and the reliability of the whole fabrication process remains unaffected. The electrical and chemical characteristics of these BSC-ISFETs have been measured in a flow-injection