Deep UV hardening of photoresist for shaping of graphene and lift-off fabrication of back-gated field effect biosensors by ion-milling and sputter deposition

Abstract

The development of microfabrication techniques compatible with scalable production processes of semiconductor industry is of crucial importance for the technological exploitation of graphene. We report here a novel technique for the photolithographic fabrication of back-gated field-effect transistor immunosensors using chemical vapor deposited (CVD) graphene. The CVD graphene was transferred following polymer assisted method and shaped by ion-milling rather than conventional oxygen plasma etching. A deep UV hardening technique was applied, respectively, before the ion milling shaping of graphene channels and before the deposition of Cr/Au layers for the lift-off fabrication of FET electrodes to ensure the removability of the photoresist. The quality of resulting graphene FET was confirmed by Raman spectroscopy, optical spectroscopy and the detection towards an ultralow concentration of human chorionic gonadotropin. SEM cross-sectional analysis revealed that the deep UV (DUV) hardening process prevents the formation of a post-etching residue shell at the high local temperature during the ion-milling process or sputtering deposition and hence maintain the integrity and significantly improve the removability of the photoresist without the need of using sonication. By using this ion-milling shaped and lift-off fabricated graphene immunosensor, a limit-of-detection of 6 pg/ml has been achieved with a detection range from 1 pg/ml to 100 ng/ml for the label-free detection of hCG.