An implantable ion sensor transducer

This paper describes the design and construction of an ion-sensor transducer optimized for implant-able applications. The unique features of this chip include (1) borosilicate glass as a pH- sensor material with a more linear and closer to nernstian response than nitride, (2) an on-chip Ag/AgCl electrode, (3) polysilicon contacts to source and drain to replace the light-sensitive diffused interconnections found on other ISFET´s, (4) Ti/Ag corrosion-resistant metallization, (5) a long narrow probe, and (6) junction isolation with the device in a p well on an n-type substrate to simplify encapsulation. The transducer can sense both pCL and pH and is suitable for pH measurement where more are negligible variations in the chloride concentration, as frequently occur in respiratory and metabolic acidosis.