Comparison between direct current and sinusoidal current stressing of gate oxides and oxide/silicon interfaces in metal-oxide-silicon field-effect transistors

It was recently reported that plasma process-induced damage to metal-oxide-silicon field-effect transistors (MOSFETs) comprises a damage mechanism that involves alternating-current (ac) stressing of the oxide and the oxide/silicon interface. The study reported herein is aimed at establishing signatures of MOSFET damage induced by ac stressing applied at conditions that emulate plasma processing environment. We apply sinusoidal ac voltage stress signals to 0.5 μm n-channel or p-channel MOSFETs with 90-Å-thick gate oxides. We assess damage on MOSFETs by measuring transconductance, threshold voltage, and subthreshold swing. We find that the onset of damage to devices subjected to ac stressing occurs at voltage amplitudes as low as 4 V, whereas in dc stressing, applied for the same time, damage becomes significant only at dc voltages larger than 10 V. We also show that damage from ac stressing attains a maximum at frequencies in the range 1-100 kHz and decreases at frequencies above 5 MHz. It is proposed that carrier hopping is primarily responsible for oxide current and, hence, device damage observed following the ac stress. This hopping current is insignificant during high-field dc stress when Fowler-Nordheim tunneling becomes the dominant conduction mechanism.