A COMPREHENSIVE STUDY ON P+ POLYSILICON-GATE MOSFETS INSTABILITY WITH FLUORINE INCORPORATION

A device characteristics instability in MOSFET’s associated with fluorine incorporation in the p + -gate is presented. MOSFETs with BF2 or boron-implanted polysilicon gates were processed identically except at gate implantation. A substantial shift and fluctuation in threshold voltage of MOSFETs with BF2-implanted gate were observed even under moderate annealing conditions, while the boron-implanted gate devices still exhibited normal characteristics. The threshold voltage was found to shift more positively and the subthreshold swing shifted to a large value as the fluorine concentration increased in the gate. The physical causes accounting for the threshold voltage shift are identified to be the fluorine-enhanced boron penetration and/or negative charge generation. A detailed examination on the p+ -gate MOS-FET devices with various gate lengths, which received the same amount of fluorine dose at gate implantation, revealed that the long gate-length device had an abnormal abrupt turn-on Id−Vg characteristics, while the submicrometer gate-length devices appeared to be normal. The abnormal turn-on Id−Vg characteristics associated with long gate-length p+-gate devices vanished by subjecting the device to the X-ray irradiation and/or to a high voltage dc stressing at source/drain. The C-V characteristics of MOS structure of various gate dopants, processing ambients, doping concentration, and annealing conditions were studied. Based on all experimental results, the degradation model of p +-gate devices is presented. The incorporation of fluorine into p + -gate is to enhance boron penetration through thin gate oxide into silicon substrate and create negative-charge interface states. The addition of H/OH species into F-rich gate oxide will further aggravate the extent of F-enhanced boron penetration by annealing out the negative-charge interface states. The proposed model explains the subtle difference in the electrical characteristics observed in the long and short BF2-implanted devices. © 1990 IEEE