EXTRAPOLATION OF HIGH-VOLTAGE STRESS MEASUREMENTS TO LOW-VOLTAGE OPERATION FOR THIN SILICON-OXIDE FILMS

MOS capacitors fabricated with 10 nm thick silicon oxide films on p-type silicon have been stressed high voltages and the high voltage stress induced changes in the oxide properties were extrapolated to low operating voltages. The stress voltages ranged from -7.5 V to -14.5 V. The fluence during the stress was systematically varied from 2 x 10(-5) C/cm2 to 6 C/cm2 by varying the stress time at each voltage. The number of interface traps and surface charges induced by the stress were both fluence and voltage dependent. The number of interface traps generated by the stress increased as the stress voltage and fluence increased. However, the interface trap generation rate decreased as the fluence increased. The trap generation rate at low operating voltages was very high, but because the current through the oxide was small, the total number of traps generated was low. The trap generation rate was proportional to the inverse square root of the fluence with a voltage dependence that decreased as the fluence increased. Extrapolation of the high voltage stress measurements to 5 V showed that easily detectable changes in the oxide properties would only occur after several years of 5 V operation. Extrapolation of charge-to-breakdown and time-to-breakdown data to 5 V operation indicated that breakdown would occur after hundreds of years of device operation. The decreasing interface trap generation rate was interpreted to indicate that the trap generation process involved the breakdown of bonds having a continuous energy spectrum. The trap generation-energy dispersion relationship explans the physical origin of the voltage dependent field acceleration factor. Stress induced threshold voltage shifts should not be used to describe oxide wearout since the number and sign of the charges populating the stress induced interface traps was determined by the sign and number of charges at the silicon-oxide interface at the end of the stress measurement.