A comprehensive study on p/sup +/ polysilicon-gate MOSFET's instability with fluorine incorporation

Abstract

It is reported that fluorine can jeopardize p/sup +/-gate devices under moderate annealing temperatures. MOSFETs with BF/sub 2/ or boron-implanted polysilicon gates were processed identically except at gate implantation. Evidence of boron penetration through 12.5-nm oxide and a large quantity of negative charge penetration (10/sup 12/ cm/sup -2/) by fluorine even at moderate annealing conditions is reported. The degree of degradation is aggravated as fluorine dose increases. A detailed examination of the I-V characteristics of PMOSFET with fluorine incorporated p/sup +/-gate revealed that the long gate-length device had abnormal abrupt turn-on I/sub d/-V/sub g/ characteristics, while the submicrometer-gate-length devices appeared to be normal. The abnormal turn-on I/sub d/-V/sub g/ characteristics associated with long-gate-length p/sup +/-gate devices vanished when the device was subjected to X-ray irradiation and/or to a high-voltage DC stressing at the source/drain. The C-V characteristics of MOS structures of various gate dopants, processing ambients, doping concentrations, and annealing conditions were studied. Based on all experimental results, the degradation model of p/sup +/-gate devices is presented. The incorporation of fluorine in the p/sup +/ gate enhances boron penetration through the thin gate oxide into the silicon substrate and creates 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. >