Buried and graded/buried LDD structures for improved hot-electron reliability

New buried and graded/buried lightly doped drain (LDD) structures have been demonstrated, for the first time, to improve significantly the hot-electron reliability of NMOS devices. Both LDD structures have peak doping of the n- spacer implant approximately 1000 A below the Si-SiO2interface forming a "buried" n- spacer near the drain region. In the graded/buried LDD structure the junction of the "buried" n- spacer is further graded by an additional low-dose phosphorus spacer implant.