Planar compatible polymer technology for packaging of chemical microsensors

By applying special thick film photolithography it is possible to realize a highly automative ion-sensitive field effect transistor (ISFET) packaging at the wafer level. In this paper two approaches based on photolithographic processing of encapsulating layers are presented. A lift-off method of commercial thermocurable encapsulants has been investigated, as well as direct photopolymerization of ultraviolet-curable encapsulant compositions, containing either epoxy acrylate or polyurethane acrylate oligomers together with monomers and photoinitiators. Lift-off has been developed using either thin or thick photoresist sacrificial layers. The best compatibility implies thick photosensitive polyimide layers as a sacrificial photoresist together with the encapsulant material based on alumina-filled epoxy. However, better results are obtained by using photopatternable encapsulant polymers that permit application of thin or thick polymer layers on a wafer substrate containing ISFET chips. Windows over the gate region and contact pads are opened by exposure to ultraviolet light in a standard mask aligner system. Compositions based on epoxy acrylate oligomers are proved to be more reliable in a packaging process. Lifetime of encapsulated microsensors for pH measurements is presented.