A novel ultrathin (equivalent oxide thickness, EOT = 2.1 nm) atomic layer deposited (ALD) Si-nitride/SiO2 stack gate dielectrics annealed in NH3 at a moderate temperature of 550 degreesC is presented. Metal-oxide-semiconductor (MOS) capacitors are fabricated using the proposed dielectrics. Excellent performances in reduction of leakage currents and electrical stress induced degradations are exhibited by these capacitors. An interesting feature of suppressed soft breakdown events is observed in ramped-voltage-stressing experiments and constant voltage induced time-dependent dielectric breakdown measurements. The dielectrics also exhibited better interface quality, lower bulk trap density, lower trap generation rate and higher reliability in comparison with the ALD Si-nitride/SiO2 stack dielectrics without NH3-annealing as well as conventional thermal SiO2 dielectrics. The proposed stack gate dielectric appears to be very promising for future ULSI devices. (C) 2002 Elsevier Science Ltd. All rights reserved.